Retracted Article: Challenges and opportunities of hydrothermal carbonisation in the UK; case study in Chirnside

Abstract

The latest research and development in hydrothermal carbonisation (HTC) processes are reviewed and the feasibility of application to small towns in the UK is assessed. The HTC process designed in this report is theoretically evaluated for the biodegradable municipal waste and sewage waste produced by the small-town Chirnside, in the Scottish Borders. Calculation of mass and energy balances of the process are carried out alongside the evaluation of challenges and environmental, social and economic opportunities presented. The hypothetical HTC plant is capable of processing Chirnside's waste at a rate of 72.5 kg h⁻¹ and has a positive net energy. The hydrochar produced is capable of producing 1452 MW h per year which equates to 35.6% of Chirnside's predicted energy demand in 2041. Both the expected opportunities and challenges for the application of HTC are discussed, sheding light on the associated research on sustainable technology.

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1. Introduction

Fossil fuels, as an energy source, accounted for 80.8% of the world's total energy consumption in 2014.¹ This demonstrates a reduction in consumption when compared to the start of the industrial revolution where alternative energy sources were scarce. However, this reliance on fossil fuels in the 21^st century is unsustainable as the world's reserves are limited and are continually depleting. This depletion of reserves demonstrates the need for alternative energy sources and that the investment into the development of their technology being paramount for sustainable development.

In order to minimise the reliability on fossil-based energy sources, there is a requirement for the continuation of research into technology that drives the renewable energy sector. One such renewable resource includes biomass, the official term denoted to organic matter that can be optimised as an energy source. Although biomass technologies are relatively new to modern societies, the energy that can be harvested from biomass has been used by humankind as a heat source since the dawn in our discovery of fire, approximately 4–500 000 years ago.² Despite our daily and world-wide consumption of this fuel it has only been until the 21^st century that large scale, industrial harvesting of this energy is being introduced into countries worldwide. Harvesting of energy from biomass has been coined bioenergy, in which the state of matter defines three broad categories of biofuels: solid biomass (e.g., wood, harvesting residues, pellets), liquid biofuel (e.g. bioethanol, biodiesel) and gaseous biofuels (e.g., biogas). Comparing to the world's fossil fuel consumption, bioenergy contributes approximately 10% of the world's total energy production. However, it is the largest renewable energy source that is presently used.¹ The following shares of this contribution by region have been estimated as the following; North America (44.1%), South and Central America (28.7%), Europe and Eurasia (16.5%), Asia Pacific (10.6%), Middle East (∼0%) and Africa (∼0%).³

As global energy demands grow exponentially with time, the number of research projects into various, large-scale biomass processes increases.⁴ Besides the traditional thermal conversion of biomass (combustion), there are currently three main process technologies currently available: bio-chemical, thermo-chemical and physio-chemical. Bio-chemical conversion encompasses two primary process options: anaerobic digestion (to biogas) and fermentation (to ethanol) where enzymes or microorganisms break down the biomass into liquid fuels. Physio–chemical conversion consists principally of extraction (with esterification) where oilseeds are crushed to extract oil. Thermo–chemical conversion processes include gasification, pyrolysis and hydrothermal carbonisation (wet pyrolysis).⁵

The main focus of this review is hydrothermal carbonisation (HTC), which was first studied over a century ago by Nobel Prize winner Friedrich Bergius (1913).⁶ This technology presents a relatively new, renewable and innovative process that has only started to be applied on an industrial scale. HTC processing of biomass is similar to the previously mentioned thermo–chemical processes, as they are all operated by exposing organic substrates to elevated temperatures; however, contrary to the other biomass processes, HTC operates at an elevated pressure and is capable of processing feeds with a moisture content of 75–90%.⁷ This includes (but not limited to) agricultural waste such as alongside horse manure,⁸ municipal waste, organic waste from the industrial food sector, sewage sludge,⁷ green waste⁹ to fiber sludge derived from the paper industry.¹⁰ The final product of this biomass reformation process is a carbon-based solid, referred to as ‘hydrochar’. Due to the compactness of nutrients in the hydrochar pellets, which can be produced without binders or expensive drying procedures,¹¹ they can be applied in agricultural practices for soil amendment.¹² the more beneficial application which is igniting the interest of researchers worldwide is its ability to act as a neutral combustible being an energy-dense source of carbon. There are various wet biomass sources for hydrochar production the calorific value and quality of hydrochar pellets is dependent on the biomass feedstock.^13,15 In addition, as the severity of carbonisation increases (higher temperature, longer residence times), carbon, fixed carbon, and the higher heating value of the resulting hydrochar increase.¹⁴ However, the net energy produced by the overall process is positive.¹⁵ Therefore, HTC technology simultaneously presents a solution to the waste management of biomass by turning it into a valuable resource for the production of renewable energy.¹⁶

The research presented in this review first details the different thermochemical processes alongside the possible reaction mechanisms that occur in the reactor. The challenges currently faced in the hydrothermal carbonisation industry alongside the opportunities this technology presents are assessed. More specifically, in order to explore the opportunity of HTC technology, the implementation of a HTC plant capable of processing both the municipal and sewage waste of a small village (Chirnside in Berwickshire) will be assessed (approx. 2250 residents). Collected data on the current and predicted energy demands alongside waste figures and waste disposal techniques will be used to determine if the implementation of a HTC plant can provide a feasible, sustainable source of energy and efficient waste disposal system in Chirnside. More specifically, the feasibility will be determined by calculating the overall energy balance of the process and demand of the village in 2041. In addition, the current developments made in HTC are also explored for Europe, the United Kingdom, America and Asia.

2. Thermochemical processes

In the 21^st century, biomass is being converted into a renewable energy source through the global application of numerous industrial technologies and processes. Besides thermal conversion of biomass (combustion), there are currently three main process technologies currently available: bio-chemical, thermo-chemical and physio-chemical. The main reason behind recent interest in bioenergy production is the potentially unlimited supply of biomass available, due to its renewability. Thus, biomass is the only naturally occurring carbon resource that is available in large enough quantities to substitute for the world's primary energy-containing resource (fossil fuels).¹⁷ Due to this regeneration potential, the carbon-cycle connecting the production and combustion of products is favourable when compared to fossil fuels which are finite.¹⁸ Therefore, the variety of biomass energy conversion processes present and a viable opportunity to combat both global warming and climate change.

As previously described, thermo-chemical processes use the application of both heat and chemical processing to produce an energy product (biofuel) from biomass. In literature, these processes are often referred to under varying synonymous names, with the reactor conditions terming the specific type. Table 1 summarises the typical process conditions and product distribution of the various thermo-chemical processes.^7,19–21 However, it should be noted that the reactor conditions employed will vary depending on the reactor size, feedstock type, product application and technology manufacturer. The significant difference between the thermo-chemical processes identified is the ability of reactors to process feedstocks with a moisture content of 75–90%. In comparison, dry pyrolysis, gasification and torrefaction are unlikely to be driven economically by a moisture content above approximately 50–70%.⁷ Previous to wet pyrolysis, any feedstocks with high moisture contents would require a significant amount of energy to thermally dry the feed before processing. As a result of this unfavourable energy used on intensive pre-treatment of the biomass, the more viable option would be discarding any high moisture biomass feeds. This highlights the importance of hydrothermal carbonisation: a bioenergy process that is capable of processing feedstocks with an elevated moisture content.

Table 1 Comparison of thermochemical processes for biomass transformation^7,19–21

Process	Temperature (°C)	Residence time	Pressure (bar)	Other conditions	Typical product distribution (weight%)
					Solid	Liquids	Gases
Gasification	900–1500	10–20 s	1	Limited oxygen supply	10	5	85
				Moisture content 10–20%
Dry torrefaction (mild pyrolysis)	200–300	1 h	1	No oxygen	80–90	5–10	0–10
				Moisture content < 10%
Slow pyrolysis	350–400	5 min–12 h	1	No oxygen	25–35	20–50	20–50
				Moisture content 15–20%
Intermediate pyrolysis	350–450	4 min	1	No oxygen	30–40	35–45	20–30
				Moisture content < 10%
Fast pyrolysis	450–550	1–5 s	1	No oxygen	10–25	50–70	10–30
				Moisture content < 10%
Wet pyrolysis (HTC)	180–250	0.5–8 h	10–40	Moisture content 75–90%	50–80	5–20	2–5

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2.1 Pyrolysis

Derived from the Greek word ‘pyro’ meaning fire and ‘lysis’ meaning ‘to unbind’, this process describes the thermal decomposition of organic material under anaerobic conditions. During a pyrolysis operation, the biomass feed decomposes under high temperatures and pressures to produce an energy dense and carbon rich stream. Pyrolysis can be completed under a variety of process conditions which redefines the process as slow, fast or intermediate, which determines the product yields. For sixty experimental feedstocks, the typical mass yields obtained for biochar, bio-oils and biogases for each slow, intermediate and fast pyrolysis can be found in Table 1.¹⁹ Feed moisture contents below 10% is recommended for fast pyrolysis to ensure that the rate of temperature rise is not restricted by the evaporation of water.¹⁹ Whereas slow pyrolysis is more tolerant at a moisture content of 15–20%. However, the main concern associated with slow pyrolysis is the effect of longer residence time on the process energy requirement.²⁰ The operating pressure of pyrolysis will strongly influence the yield of biochar produced; experimental data demonstrates that as reactor pressure increases, the product yield increases, independent of feedstock used.²² This being said, pyrolysis is often carried out at atmospheric pressure²³ to minimise energy consumption and associated costs.

2.2 Wet pyrolysis (hydrothermal carbonisation)

When pyrolysis is carried out in the presence of subcritical liquid water, the process is redefined as ‘hydrous’ or ‘wet’ pyrolysis, industrially known as hydrothermal carbonisation. In comparison to dry pyrolysis, the moisture content of the biomass feed is typically between 75–90%.⁷ This allows HTC to process a variety of non-traditional biomass sources when compared to pyrolysis, such as municipal solid wastes, animal manure and sewage sludge, alongside traditional biomass sources, e.g., wood and grass.^7,24

In an operational HTC process, the wet biomass is transformed into pellets known as hydrochar through thermal treatment in a pressurised vessel. HTC is distinguished from hydrothermal liquefaction as the hydrochar product is solid, as opposed to a liquid bio-oil.

In comparison to pyrolysis which typically takes place at higher temperatures and atmospheric pressure, HTC reactor conditions are typically within in the operating range of 180–250 °C and take place at elevated pressures, typically between 10–40 bar.²¹

In addition to the feedstock type, HTC reactor conditions also affect the property of the resulting hydrochar.²⁵ For example, one study conducted HTC of paper sludge over an experimental range of 180–300 °C. The maximum heating value (9.7 MJ kg⁻¹) and highest energetic recovery efficiency (90.12%) of the experimental trials was at a temperature of 210 °C. This implies that final application of the hydrochar as fuel source would be most optimally produced at this temperature. However, this study further found that hydrochar had lower nitrogen and sulphur contents as the reactor temperature was increased.²⁶ This implies that a lower reactor temperature would be favoured for hydrochar that is to be applied as a soil conditioner (for a paper sludge feedstock). Furthermore, nitrogen content in hydrochar has been shown to have a significant impact on its specific applications.²⁷ By identifying the application of the hydrochar and by analysing the composition of the feedstock, research has shown that the ideal reactor conditions can be determined. In turn, the resulting hydrochar can be optimised for a variety of applications, currently including:

• An independent or co-generative heat and power fuel source.^13,16,28

• A soil conditioner.⁹

• An adsorbent.¹⁶

• A supercapacitor electrode material.²⁹

• Replacing biomass in co-fired coal plants (preventing fuel segregation in boilers, burnout, inefficiencies and fouling).³⁰

3. Hydrothermal carbonisation: fundamentals and reaction mechanisms

Fig. 1 represents the typical process units as defined by the NEWAPP project.¹⁵ Typically, alongside the elevated temperatures, the closed reactor vessel within a HTC plant is subject to elevated pressures and residence time. The process route, unit dimensions and conditions will vary depending upon technology licencing of the original equipment manufacturers. As shown on Fig. 1, the reactor effluent is subjected to a downstream filter press unit in order to increase the concentration of the carbon content through reduction of the moisture content to near 50 wt%. After that, the process water is removed through filtration and is partially recycled back into the reactor to increase the energy efficiency.¹⁵ The carbonaceous produced is then subjected to thermal drying, which is an extremely energy intensive process to remove excess moisture before pelletization of the solid hydrochar.

Fig. 1 Typical hydrothermal carbonisation process.

A secondary product amongst the hydrochar pellets is the process water stream that is released from the filter press and thermal drying stages. The process water contains short-chained carboxylic acids and inorganic ions such as potassium and phosphate, both of which are beneficial to plant growth. However, out of 680 organic pollutants tested for in the process water, traces of 13 were detected. These initial results are ‘un-alarming’ for fertiliser applications. However, in the long term, tests on the impact of irrigation with HTC process water in agricultural soils have been recommended.¹⁵ Alongside the liquid and solid phase products, approximately 5 wt% of the raw materials dry mass will be accounted for the gaseous effluent which consists mainly of carbon dioxide with traces of carbon monoxide and methane.¹⁶

3.1 Structure of biomass

Biomass cannot be defined as a specific reactant due to its high degree of chemical complexity and heterogeneity.⁸ Lignocellulose (plant) biomass consists mainly of three carbohydrate polymers: cellulose, hemicellulose and lignin. Chemical structures of these compounds are shown in Fig. 2. Small quantities of pectin, protein, extractives and ash have also been detected and the composition of all constituents vary among plant species. Cellulose is the main constituent of the plant cell wall and chains of 20–300 monomers group together to form microfibrils. Hemicellulose is the second most abundant polymer, which is not chemically homogeneous and contains branches with short lateral chains of different sugar types (xylan is presented in Fig. 2). Lignin is the third most abundant polymer in nature. Its molecular structure contains cross-linked polymers of phenolic monomers.³¹

Fig. 2 Chemical structures of cellulose, hemicellulose and lignin.

3.2 Reaction mechanisms

The thermo-chemical conversion of biomass into lignite coal-type hydrochar is a complex reaction network, the exact details of which is unknown.³² In order to reach a clearer understanding of the reaction mechanisms that are involved in hydrothermal carbonisation, infrared (IR) spectroscopy of both biomass feedstock and resulting hydrochar has been utilised.³³ This appliance promotes the identification of possible reaction mechanisms through the detection of functional groups present in both the feedstocks and product samples; however, it is time-consuming expensive and can require the use of additional chemicals. More recently, hyperspectral imagining has been used to provide a robust and reliable alternative for quantitative determination of polysaccharides in biomass and biomass chars.³⁴ Hyperspectral imagining is both fast and non-destructive, and in storing data on-line, decompositions of polysaccharides (and thus resulting qualities of hydrochar) can be predicted from feedstock analysis and comparison.³⁴

However and so far, only separate discussions of general reaction mechanisms have been identified to provide useful information about the possibilities of manipulating the reaction. The reaction mechanisms that have been identified for pyrolysis in the presence of subcritical water include hydrolysis, dehydration, decarboxylation, condensation polymerisation and aromatization.⁷ These do not represent consecutive reaction steps but rather form a parallel network of simultaneous reaction paths.³²

Using cellulose chains as a model biomass substance, the following reaction equations under hydrothermal carbonisation conditions have been deduced from experimental results:

(C₆H₁₂O₅)_n → nC_5.25H₄O_0.5 + 0.75nCO₂ + 3nH₂O (1)

ΔH_R = −1.65 kJ kg⁻¹ cellulose (2)

Eqn (1) approximates the stoichiometric ratios of reactants to products within an HTC reactor.⁶ However, these approximations have a large margin for error and should be treated with care, as the chemical pathway is not fully defined. Additionally, eqn (1) does not account for the liquid organic reaction by-products that represent an important fraction.³⁵ As described by eqn (2), the HTC process is exothermic (negative heat of reaction) for a pure cellulose feed. However, the heat released is highly dependent on feed composition and the reactor conditions. Although eqn (1) and (2) cannot accurately describe the treatment of a biomass stream, these equations can offer an insight of what is to be expected from HTC of lignocellulosic biomass. Thus, the reaction pathways identified for the pyrolysis of the three lignocellulosic carbohydrate polymers can be predicted. This being said experiments by Volpe determined that pure cellulose remained unaltered at temperatures up to 220 °C, yet significantly decomposed at 230 °C to produce recalcitrant aromatic and high energy-dense material.³⁶

3.2.1 Hydrolysis. Hydrolytic reactions occur on the surface of solid biomass, where water reacts with biomacromolecules by breaking both ester and ether bonds to produce a wide range of products.⁴ Liquid water enters through surface pores and hydrolyses the components, after which the hydrolysed products may proceed to exit out of the same pore. The biomacromolecules (cellulose and hemicellulose chains) are initially hydrolysed into soluble oligomer products. With increased reaction time, the oligomers further hydrolyse into simple monosaccharide or disaccharides.⁴ Fig. 3 shows the reaction pathway during further hydrolysis of the oligomers to produce glucose and xylose from cellulose and hemicellulose, respectively. However, the quantity of different fragments formed from the hydrolysis of these polymers is very high and is not limited to the reaction pathway shown in Fig. 3. Alongside this, hydrolysis of lignin is known to produce guaiacol, phenol and catechol.³⁷

Fig. 3 The hydrolysis reaction pathway from cellulose and hemicellulose to glucose and xylose.

Through forced convection, hydrolysis can be completed within a few minutes with the rate being determined by the adjusted flowrate, not only the reaction temperature.³⁸ Although hydrolysis of lignocellulosic biomass can take place at lower temperatures, significant hydrolysis of cellulose has been found to occur above 220 °C and lignin is most likely realizable at 200 °C due to the high number of ether bonds. And, hemicellulose has been found to readily hydrolyse at around 180 °C.³² IR spectroscopy graphs for lignocellulose hydrochar contain no evidence of the presence of hemicellulose, suggesting that hemicellulose is fully hydrolysed at elevated temperatures.⁴ The fragments formed are highly reactive and will quickly undergo condensation reactions to form precipitates.³⁹ The rate of hydrolysis during HTC is primarily determined by diffusion and thus limited by transport phenomena within the matrix of the biomass. This may lead to condensation of fragments within the matrix at high temperatures.⁴⁰

3.2.2 Dehydration. Dehydration of biomass is the formation of water molecules via the elimination of branched hydroxyl (–OH) groups, also known as dihydroxylation. This reaction produces hydrochar with a lower O/C and H/C ratio when compared to the feed and replicates the ratios present in natural coal. However, the complete chemical structure varies significantly between these two fuels. The ratio of O/C and H/C bonds is inversely dependant on the temperature, and more significantly so for O/C bonds.⁴¹

The products resulting from the hydrolysis of cellulose and hemicellulose are dehydrated to form 5-hydroxymethylfurfural (HMF) and furfural, respectively, as shown in Fig. 3. Dehydration of water during the cleavage of both phenolic monomers and hydroxyl functional groups may occur during HTC at temperatures above 150 °C and 200 °C, respectively. The dehydration of catechol, formed from the hydrolysis of lignin, may also occur.³²

Dehydration (and decarboxylation) occurs in the HTC process as both residence time and temperature increase.⁴² Alongside the manipulation of these conditions for improved efficiency, additives that promote the rate of reaction can be combined into the feedstock to support and accelerate this reaction mechanism. For example, alkaline conditions give the highest reaction rates for hydrolysis whereas further degradation reactions of simple mono- or disaccharides are highly enhanced under acidic conditions⁴³ using most commonly mineral acids such as sulphuric and hydrochloric acids.⁴⁴

3.2.3 Decarboxylation. IR spectroscopy graphs for hydrochar demonstrate no peak detection around wavenumber 1725 cm⁻¹.⁴ This suggests complete carbonyl (–C=O) and carboxyl (–COOH) degradation and can be associated to the formation of carbon monoxide (CO) and carbon dioxide (CO₂), respectively.⁴ Carbonyl and carboxyl degradation occur rapidly at temperatures above 150 °C to produce minor concentrations of the gases mentioned prevsiouly.⁴⁵ The carbonyl functional group is presented on both 5-HMF and furfural molecules (Fig. 3), and the likely source of the carboxyl functional group is the formation of both formic acid and levulinic acid (the hydrolysis products of furfural and 5-HMF, respectively).⁴⁶ Research publications confirmed the major portion of gaseous products from HTC to be CO₂.^47,48 However, more CO₂ is produced than can be explained by the elimination of carboxyl groups alone.⁴¹ This suggests that other mechanisms are involved in the process; likely carboxyl group sources have been identified as products from condensation reactions⁴¹ and the cleavage of intermolecular bonds.⁴⁹ Alongside this, experimental evaluations of the carbon monoxide produced during HTC is insufficient to account for the loss of all carbonyl groups. This suggests that carbon dioxide may be formed from their degradation.⁴⁵ It should be noted that dehydration and decarboxylation occur simultaneously with significant decarboxylation appearing after a significant amount of water has been formed.⁵⁰

3.2.4 Condensation polymerisation. Some of the fragments formed from the degradation of biomacromolecules are highly reactive (e.g., anhydroglucose, 5-HMF, aldehydes, lignin fragments).⁴ The unsaturated carboxyl and hydroxyl groups polymerise easily⁵¹ and this leads to the formation of a water molecule (condensation) and ether bonds (–COC–).³² Condensation polymerisation is most likely governed by step-growth polymerisation which is enhanced at higher temperatures and reaction time.⁵² Highly reactive lignin fragments have been reported to polymerise in several minutes at 300 °C, whereas at room temperature polymerization can continue for months.⁵³ The rate of polymerisation during HTC is similarly temperature dependant; ether bond in IR graphs of HTC hydrochar becomes more pronounced as the reactor temperature is increased.⁴ Thus, the formation of the lignite-structure of hydrochar is mainly characterised by condensation polymerisation.³⁸ However, the knowledge about the detailed polymerization sequences during the course of hydrothermal carbonisation is essentially missing.³²

3.2.5 Aromatization. Lignin is naturally composed of many stable aromatic rings, as shown in Fig. 3. These aromatic structures exhibit high stability under hydrothermal carbonisation conditions and are considered to be the basic building block of the resulting hydrochar.³² The IR spectra graph for lignocellulose's hydrochar product have shown that the peak corresponding to aromatics (1694 cm⁻¹) is enhanced when compared to that of the raw feedstock.⁴ Alongside this, experiments have shown that increasing the reactor residence time and/or reactor temperature leads to an increased percentage of lignin, which is more than that of raw feed. To quantify, one experiment measured a percentage mass of lignin in the raw biomass feed as 7%. Operating the pilot HTC reactor at 200 °C for 1 and 6 hours found a percentage mass for pseudo lignin of 25.1 and 38.3%, respectively. Whereas operating at 250 °C for the same residence time found 44.4% and 58.3%, respectively.¹² Therefore, conclusions have been drawn that the lignin-like substances that are formed (pseudo lignin) during HTC conditions result from the aromatization of cellulose and hemicellulose, despite there being linear carbohydrate polymer chains.^4,52 The structure of hydrochar is concluded to be in agreement with natural coal, as the cross-linking condensation of aromatic rings makes up its major constituent.⁵⁴ As shown, aromatization and the concentration of pseudo-lignin in hydrochar is significantly dependant on temperature.⁵¹ In addition, it has been determined that the formation of aromatic structures have been enhanced by alkaline conditions.⁵⁵

3.2.6 Other mechanisms. Other minor mechanisms that may occur under the hydrothermal carbonisation of biomass include:

• Demethylation.⁵⁶

• Pyrolytic reactions.^32,57–59

• Fischer–Tropsch reactions.⁶⁰

• Transformation reactions.^42,61

• Secondary char formation.¹⁴

The catechol-structure of the coal is thought to be explained by the demethylation of phenol.⁵⁶ This is commonly the replacement of a methyl group (–CH₃) with a hydrogen atom. This mechanism is supported by the production of minor amounts of methane that has been observed over several experiments.³²

Alongside this, pyrolytic reactions have been reported to be competing reactions when under hydrothermal conditions.⁵⁷ In general, they might become more significant above 200 °C,⁵⁸ though typical products from pyrolysis have not been reported to be formed in significant amounts during hydrothermal carbonisation.³² They are thought to occur due to fragments of the feedstock that have not come into contact with water due to being trapped within the biomass matrix by the precipitation of condensed fragments.⁵⁹

Fischer–Tropsch reactions have also been observed under hydrothermal conditions.⁶⁰ A high amount of CO₂ is formed during hydrothermal carbonisation and the Fisher–Tropsch reactions may play a role in the production of this gas that has not been investigated in detail so far.

Transformation reactions within the lignin may occur when the hydrolysis and subsequent condensation (polymerisation) cannot take place. This is mainly for stable compounds with a crystalline structure and oligomer fragments as these do not hydrolyse.⁶¹ However, given the high rate of fragmentation by degradation due to hydrolysis above 180 °C, it appears unlikely that transformation reactions play a key role under hydrothermal conditions.³²

In addition, solid secondary chars have been determined to form from the liquid depolymerized cellulose anhydro-oligomers formed in pyrolysis.⁶² Similary, Lucian et al. writes that the formation of hydrochars from the hydrothermal carbonisation of the organic fraction of municipal solid waste forms a reactive secondary chars on the surface of the primary hydrochar, suggested from the thermal stability and reactivity of the intermediate hydrochars.¹⁴ Extracting and experimenting, the HHV of the secondary chars in this study was found to be significantly higher than those of the primary char that was formed.¹⁴

4. Research, development and application

The support for the research, development and application of HTC technology is based on the promising positive contribution the technology is expected to have, within both fields of renewable energy production and waste biomass disposal. There are an estimated 200 companies and organisations distributed worldwide that are currently involved in the research, development and application of HTC technology. In 2013, 150 patents concerning the hydrothermal carbonisation process were processed, of which 39% were from cross-country collaborations, China (27%), America (14%), Germany (10%) and others (10%).¹⁵

4.1 Europe

4.1.1 Incentive in Europe. Before the development of HTC, wet biomass feedstock was sent to landfill, directly incinerated or transformed by the alternative thermochemical methods outlined in Section 2. However, new approaches to waste management are proceeding in compliance to the requirements of the Landfill Directive (1999/31/EC). This directive is composed by the European Commission who proposes legislations for the EU member countries. In 2014, the European Commission outlined landfilling as the least preferable option of waste disposal.⁶³ Alternatively, direct incineration of biomass with a high moisture content should be avoided due to the low energy efficiency that results from the large quantity of energy required for the evaporation of water. Alongside diversion of waste from landfills, the European Commission have set a ‘binding’ target to achieve 20% of the EU's final energy consumption to be from renewable sources by 2020. The 2030 target was originally at 27%. However, recent revision of the Renewable Energy Directive increased the target to 32% as the EU aims to be a global leader in renewable energy production.⁶⁴

4.1.2 Support for HTC research and development in Europe. The European Union is actively involved in the support and funding of innovation within HTC research, development and its multi-market application. Programmes such as Horizon 2020 and the EUs 7^th Framework were established by the European Union with the aim to tackle the biggest challenges within transportation and energy sectors that currently face modern society.⁶⁵

Alongside this, the European Biomass Industry Association has coordinated projects such as the ‘new technological applications for wet biomass waste stream products’,¹⁵ which received a contribution of €1.76 million from the EU. One of the main targets of this research was to produce a draft of quality standards for hydrochar that is to be used as a solid fuel and as a soil conditioner (in cooperation with the organisation for standardization (ISO)).¹⁵ The formation of these standards was deemed necessary in order to prove the viability of hydrochar in commercial applications. Establishing standards allows hydrochar manufacturers to receive certification based upon the quality of their product and in turn market growth is stimulated as the product and technology is trusted by investors/clients.

In preparation of these standards, project NEWAPP identified the following 5 substrate streams as feedstock which were then tested and analysed from potential suppliers to assess its suitability to the HTC process:¹⁵

• Sewage sludge – from wastewater treatment plants.

• Digestate – from anaerobic digestion plants.

• Green waste – vegetables, pruning etc.

• Household food waste.

• Organic fraction of municipal solid waste.

The standards established from the experimental testing of these streams are taken as the basis of calculation for the energy balance produced in Section 6. Alongside the many experimental trials performed, project NEWAPP conducted a comparative economic analysis of hydrochar to other fuel sources and a comparative economic analysis of HTC to other waste disposal methods. The results are considered when discussing the opportunity and challenges HTC presents to the UK in Sections 7 and 8, respectively. In addition, the impact assessment of the comparative environmental life cycle assessment study concluded that application of hydrochar as a fuel source is more suitable than application as a soil conditioner.¹⁵

4.1.3 Research, development and application in Europe. Europe is leading the way through commercial and industrial application of HTC technology. Recent studies have investigated the hydrothermal carbonisation of olive mill waste, resulting from the production of olive oil, which has demonstrated very positive results; high heating values of 32.3 MJ kg⁻¹, alongside improved fouling and slagging properties than the direct combustion of olive mill waste.¹¹

European researchers have been collaborating internationally to assess the viability of implementation in alternative markets. For example, researchers from Berlin have investigated the feasibility of the hydrothermal carbonisation of empty fruit bunches (EFB) that result from the production of palm oil in Indonesia and Malaysia.¹¹ Similarly, researchers from Switzerland have worked with academics in Thailand to characterise the hydrochar produced from the HTC of bamboo.¹⁴

Noticeable companies developing a HTC process in Europe include Ingelia (Spain), C-Green (Sweden), HTCycle (Germany), SunCoal (Germany) and AVA-CO₂ based in Switzerland with subsidiaries in Germany.

Ingelia is one of a handful of recent companies founded with the purpose of providing the technology for hydrothermal carbonisation. This is the first industrial HTC plant worldwide capable of carbonizing wet biomass in a continuous process.⁶⁶ The HTC process design produced by Ingelia is modular, which allows scalability for a client's specific needs and future plant expansion.

In mid-2018, C-Green €2.2 developed a full-scale HTC plant in Heinola, Finland, capable of processing 25 000 tonnes of residual biomass per year that is currently produced by StoraEnso's corrugated board mill.⁶⁷

HTCycle and SunCoal are based in Germany where they too are collaborating with partners and clients to commercialise their patented HTC technology. Alongside offering services for HTC technology, SunCoal have developed an entrained-flow gasifier for the production of syngas from hydrochar.⁷⁹

In 2010, AVA-CO₂ had claim to the world's largest HTC demonstration plant based in Karlsruhe, Germany, with a production capacity of 1000 tonnes of hydrochar per year.⁶⁸ After which, AVA-CO₂ constructed and commenced operation in an industrial-sized multi-batch HTC plant in 2012, with production capacity of 8000 tonnes of hydrochar per year.⁸¹

4.2 The United Kingdom

4.2.1 Research, development and application in the United Kingdom. The contributions to research within the field of HTC technology continue to increase from academics based at universities across the UK. Noticeable contributions come from the University of Edinburgh, Queen Mary University of London, the University of Nottingham and Loughborough University.

Uniquely, academics from Loughborough University have progressed beyond experimental research as they have developed a small-scale HTC toilet system.^88–90

Noticeable companies in th UK include clean-tech start-ups such as Antaco and Valmet. Due to the commercial potential of their patented process, Antaco completed construction on its pilot plant in 2014 making it the first HTC plant in the UK (not of commercial scale).⁶⁹

Valmet and previously discussed German-based company SunCoal have joined forces with the focus on the HTC processing of sludge derived from the paper and pulp industry for.

4.3 Research and development in Asia

As mentioned previously, besides those from multiple-country-collaborations, the majority of applications for HTC patents, come from China (27%). Research conducted by Zhou et al. (2018) has shown that the weight percentage of food waste in municipal solid waste (MSW) in cities throughout China ranges from 30–60%. This range is larger than that in the following individual countries: USA, Germany, England, Japan and Singapore.⁷⁰

Application of HTC in China has already begun; an HTC plant that processes 14 000 tonnes of sewage sludge per year is operated in Jining.⁷¹

However, Asia has been exploring the HTC processing of alternative wastes compared to the UK, such as waste textiles (China's),⁹⁸ coconut fibre and eucalypts leaves (Singapore)³⁹ and seaweed (Japan and Indonesia)²⁹ due to the high production potential of both biomass sources there.

5. Current methods for biodegradable municipal waste, sewage waste and final treatment waste

As discussed, research, development and application of hydrothermal carbonisation is continuing to grow. In the 21^st century, HTC technology companies worldwide are being founded and industrial-sized plants have commenced operation. Alongside this, commercial plants within a multitude of markets have been established through collaboration with the companies who have patented their technology. This section provides the estimate data of biodegradable municipal waste (BMW) and sewage waste produced in the UK alongside current waste disposal methods used. From this, an in-site into the potential supply of BMW and sewage waste biomass for HTC processing in the UK is assessed.

5.1 Biodegradable municipal waste

The enforcement of the environmental policies set by the EC directives is covered by the Scottish Environmental Protection Agency (SEPA) in Scotland under the Department for Environmental Food & Rural Affairs (DEFRA). DEFRA, in compliance to the EC directives are required to release yearly statistics of relevant data to prove compliance with the established standards and targets. The most recent available data for the UKs Statistics on Waste is the 2016 report produced by the Government Statistical Service.⁷² The key points of relevance in this report are defined as follows:

• UK Biodegradable Municipal Waste (BMW) sent to landfill has continued to reduce and in 2015 was 7.7 million tonnes. This represents 22 percent of the 1995 baseline value. There is an EU target to restrict BMW landfilled to 35 per cent of the 1995 baseline by 2020.

• Of the 209.0 million tonnes of all waste that entered final treatment in the UK in 2014, 44.5% was recovered (including recycling and energy recovery). The proportion that went to landfill was 23.1 percent.

The Scottish Government launched Scotland's first zero-waste policy on the 9th of June 2010. This plan envisions a zero-waste society in which all waste is acknowledged as a viable resource.⁷³ From this, waste produced by Scotland's residents and businesses is to be minimised and valuable resources are not to be disposed via landfill sites. This initiative action defines that new measures are to be taken by local councils. These measures include:

The banning of specific waste types from landfills in order to capture the value these resources hold.

Restrictions on the energy input to municipal waste facilities (incineration) to encourage waste prevention, reuse and recycling.

Application of HTC technology could be beneficial to the achievement of these measures. However, to date, there has been no investigation by the Scottish Government into the employment of HTC technology in the country. The findings presented in this report will be the first.

5.1.1 Biodegradable municipal waste sent to landfill. Biodegradable Municipal Waste (BMW) is defined in the Landfill Directive (1999/31/EC) as household waste that is capable of undergoing anaerobic or aerobic decomposition. Here, biodegradable fractions are noted to include paper, card, green waste, food waste, miscellaneous combustibles and fines. The aim of the Landfill Directive is to prevent or reduce, as far as possible, the negative impacts on groundwater, soil, air and human health that are associated with the landfilling of waste. This is achieved through the stringent technical requirements established for the UK to achieve. One of which is to reduce the amount of BMW sent to landfill as uncontrolled decomposition of BMW leads to the production of landfill gases.⁷⁴ This gas mainly consists of carbon dioxide and methane, both of which are greenhouse gases. And, methane gas is 20 times more potent than carbon dioxide in its impact.⁷⁵

The 2010 target defined in the Landfill Directive states that UK should aim to reduce the tonnage of BMW sent to Landfill to 35% of the baseline by 2020. Table 2 (data from Department for Environment, Food and Rural Affairs)⁷⁶ shows the percentage of the 1995 target baseline of BMW sent to Landfill for each country in the UK from 2010 to 2015. It should be noted that biodegradable municipal waste for each country (bar Northern Ireland) represents approximately half of the overall municipal waste sent to landfills in the UK. Table 2 demonstrates that the UK has achieved and even improved upon the target established in 2010 set to control BMW sent to landfill; the overall percentage in 2015 has been reduced to 22% whereas the target was to reach 35% of the 1995 baseline by 2020. This demonstrates that the UK has significantly reduced the amount of BMW produced and/or took affirmative action for BMW diversion from landfills. This being said, 7682 ktonnes of BMW that could have been treated through HTC was sent to landfill in 2015.

Table 2 BMW sent to Landfill in the UK and country split and the % representation in comparison to the 1995 baseline⁷⁶

Year	Mass of BMW sent to landfill per year (ktonnes per year)					Percentage value to baseline (%)
	UK	England	NI	Scotland	Wales	UK	England	NI	Scotland	Wales
1995	35 688	29 030	1225	3595	1837	—	—	—	—	—
2010	12 982	10 339	558	1484	600	36	36	46	41	33
2011	11 719	9360	464	1358	538	33	32	38	38	29
2012	10 337	8129	394	1292	522	29	28	32	36	28
2013	9326	7347	299	1183	497	26	25	24	33	27
2014	8711	6843	322	1122	424	24	24	26	31	23
2015	7682	5980	307	1084	311	22	21	25	30	17

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Although surpassing the 35% target established by the EU by 5%, Scotland is the lowest performing country in the UK at reducing the amount of BMW sent to landfill. The linear trend shown in Fig. 4 indicates a future prediction of BMW sent to landfill in Scotland based on previous data.⁷⁶ By 2020 the quantity of BMW sent to landfill is predicted to be approximately 6 million tonnes if efforts for its reduction are continued.

Fig. 4 A graph to show the current data and predicted trend line for the BMW to Landfill in Scotland.¹⁰⁶

5.1.2 Biodegradable municipal waste sent to incineration. The largest reduction in BMW sent to landfill was in Wales which saw a 6% drop in the years 2014 to 2015. The UK Statistics on Waste identified this considerable reduction to be attributed to an energy-from-waste plant becoming fully operational in Cardiff. The type of energy-from-waste plant was found to be an incineration plant owned by Viridor Ltd Proven to aid in the reduction of BMW sent to landfill, the incineration of biomass is a source of energy. More specifically, Viridor's thermo-chemical incineration plant is capable of generating 30 MW of electricity for the national grid (∼50 000 households) and handles 350 000 tonnes of residual waste per year. The carbon footprint calculated in association with Viridor's incineration plant is lower than that produced by landfill. It is also lower than the carbon footprint produced from the conventional fossil-based electricity and heat generation.⁷⁷

DEFRA recognises that a Municipal Solid Waste (MSW) stream will contain both carbon-based (biomass derived material) and fossil-fuel based products. Incineration of biomass in MSW is a renewable source of energy, as this biogenic portion is ‘capable of being replenished, not depleted by its utilization’ (OED). However, incineration of fossil-fuel based products is not a renewable source of energy as the emissions released from their combustion contribute to the greenhouse effect and global warming.⁷⁸ In turn, defining the overall process of incineration as ‘renewable’ is incorrect. Alongside greenhouse gas emissions, incineration of waste has the potential to release various harmful and carcinogenic emissions including acid gases, nitrogen oxide, heavy metals (lead), particulates, dioxins and furans.⁷⁹ Thus, some air-pollution control techniques are implemented in plant designs (NOx control, acid gas scrubber, continuous emission monitors, etc.). However, emissions from incineration is inevitable. Alongside this, data required for necessary health-effect assessments, specifically data on the most harmful emissions (dioxins, furans, heavy metals and particulates) are not readily available from operating plants.⁸⁰ Therefore, the escape of these carcinogenic compounds cannot be overlooked when considering incineration of waste alongside sustainable future development. From this, it can be concluded that incineration of biomass is renewable, while current incineration methods are not sustainable due to the combined processing of biomass with fossil-fuel based products. Therefore, when comparing incineration with hydrothermal carbonisation of waste, HTC presents a more sustainable energy-from-waste process as there is no association with the release of harmful/carcinogenic emissions.

5.2 Sewage treatment

Urban waste water, commonly referred to as sewage, is composed of domestic waste water from baths, sinks, washing machines and toilets, alongside industrial waste and rainwater machines and toilets, alongside industrial waste and rainwater collected from drains.⁸¹ The sewer system in the UK collects over 11 billion litres a day which is equivalent to 4400 Olympic swimming pools. This water is treated at one of the 9000 sewage treatment plants in the UK before being discharged into inland waters. Through extraction of organic substrates from wastewater, the discharged water will have a concentration of biological oxygen demand (BOD) deemed safe by DEFRA standards for aquatic life to survive.⁸²

There are four types of treatment that waste water can be subjected to:

• Preliminary treatment – removal of grit, gravel and larger solids.

• Primary treatment – settling out of any solid matter (removes ∼60% of solids and ∼35% of BOD).

• Secondary treatment – the use of digestate bacteria to breakdown organic substances (removes ∼85% of BOD and solids).

• Tertiary treatment – disinfecting/denitrification of the treated effluent (to protect sensitive water environments from eutrophication).

Typically, sewage waters contain less than 0.1% of solid matter. And once separated in the primary treatment, the resulting ‘sludge’ contains organic matter, dead bacteria from the treatment process and any particulates.⁸³ It is this biomass-rich sludge that can be processed in a HTC reactor. Historically, a quarter of the sludge was dumped at sea or discharged to surface waters. However, the EC Directive required the cessation of these practices in 1999. Increasingly, sewage sludge is being processed under anaerobic digestion in which bacteria consumes some of the organic matter in the sludge to produce biogas, a renewable energy source which can be used in combined heat and power plants for electricity generation.⁸²

Fig. 5 displays the percentage split of sewage sludge across its current disposal methods including landfill, incineration and the reusable disposal techniques that include soil and agricultural applications and others.⁷⁶ Clearly, the majority of the UKs sewage sludge is currently reused as a soil enhancer to fertilise agricultural lands, which is considered to be the ‘environmentally favoured option' by DEFRA. Due to the direct application of sewage sludge as a soil enhancer, processing this waste through a HTC reactor to produce hydrochar pellets for soil enhancement applications would therefore be an inappropriate use of energy.

Fig. 5 Sewage sludge disposal techniques in the UK (2010).⁷⁶

Incineration accounts for 18% of the disposal of the UKs sewage sludge. This is the only energy generating application of sewage sludge currently in the UK. As previously discussed, incineration of wet biomass is energetically inefficient as the water content in the sludge requires a large energy input (latent heat) for evaporation. This demonstrates both the advantage and opportunity application of HTC technologies can have in the UK due to the ability to process high moisture feeds. Therefore, HTC of sewage sludge compared to its direct incineration should be considered for sustainable future development in the UK, as energy consumption can be reduced.

5.3 Final treatment of waste

DEFRA currently identifies eleven categories of waste in the UK and six final treatment methods. The percentage split of the waste generated in the UK (2014) over the eleven subcategories of waste materials are displayed on Fig. 6.⁷⁶ The waste generated from mining-and-quarrying extractions (mineral wastes) and from soils accounts for two thirds of the overall waste generated in the UK. Considering the other categories presented on Fig. 4, hydrothermal carbonisation is capable of processing household, paper & cardboard, wood and vegetal wastes which accounts for a total of 6.2% of the UKs overall waste; a total tonnage of 29.7 million. It should be noted that households and similar wastes are not solely generated by households and this figure does not account for sewage waste.

Fig. 6 Percentage split of the UKs waste generation by waste material (2014).⁷⁶

The six final treatment methods, in order of majority percentage are defined by DEFRA as recycling and other recovery, landfill, land treatment and release into water bodies, backfilling and incineration and energy recovery. Fig. 7 is a visual representation of the percentage split amongst these final treatment methods.⁸² Although the majority of waste in the UK is recycled, there is clearly little investment into energy recovery and incineration processes (total 4.5% in 2014).

Fig. 7 Percentage split of the final treatment method optimised for the overall waste generated in the UK in 2014.⁸²

6. HTC application in Chirnside, Scotland

6.1 Project brief

In order to make progress for sustainable future development, we rely on the continual research and commitment from dedicated scholars into alternative energy production and waste disposal processes. The following section will assess the feasibility of operating a HTC plant in Chirnside, a small village in Scotland. This plant will be capable of processing the biodegradable municipal waste (BMW) and sewage waste produced by the estimated village population in 2041. Calculation of the associated energy balance around the HTC plant design, alongside comparisons to the current waste disposal methods employed in Chirnside, will determine the feasibility of implementing a HTC plant in this village.

6.2 Chirnside: research

6.2.1 Population. Chirnside is a small village located in the Scottish Borders and is operated by the Scottish Borders Council. The most recent population count in 2011 identified 1459 residents. However, the difference in this figure and the resident numbers recorded in the 2001 consensus and the approval of recent housing developments in Chirnside by the Scottish Borders Council (46 houses approved in 2010;⁸⁴ 25 houses approved in 2017;⁸⁵ 57 houses applied for planning permission in 2018 (ref. 86)) demonstrates the need to estimate an appropriate population growth. Fig. 8 represents the linear relationship between the population of Chirnside and the years.

Fig. 8 Predicted population growth in Chirnside.⁷³

In order to appropriately size a HTC module for Chirnside that is capable of processing the towns waste, the population at the time of decommissioning must be estimated. Assuming construction of the plant is completed in 2021, and assuming a 20 year life expectancy of the reactor unit,¹⁵ the population of Chirnside is estimated to be 2250 in 2041. Various factors can influence this estimate such as fertility, mortality, housing developments and house prices, which in turn will impact the estimated processing rate and plant size. However, the population estimate is appropriate as if the plant is not at capacity, it is assumed that waste from neighbouring municipalities or the agricultural sector can be processed here.

6.2.2 Chirnside: BMW management.
6.2.2.1 Current BMW disposal techniques. Chirnside is classified as a ‘rural’ area by the Scottish Borders Council and therefore their municipal waste, both food and garden, is not collected for disposal. The Scottish Borders Council suggests each homeowner composts these wastes, and they provide a free home composter to households which do not receive garden and food waste collection. General waste and recycling is collected once every two weeks and recycling centres are located at larger communities in the Scottish Borders, such as Peebles or Eyemouth.⁸⁷ The percentage of Chirnside's residents who use a home compost and those who put their food waste in the general waste bin is unclear. However, for those who do not home compost, it is estimated that more than 30% of the waste in an average bin will be food. This presents an inefficient use of potential resources in Chirnside, unlike several villages in the Scottish borders (Galashiels, Hawick, Jedburgh, Peebles, Selkirk and Tweedbank) where the food waste is collected separately for recycling.⁸⁶ This demonstrates that HTC of waste could provide a more sustainable solution to the disposal of food waste in Chirnside, diverting food waste that is currently sent to landfill or composted by transforming the waste into renewable energy.
6.2.2.2 Current BMW figures. Data on the municipal waste produced in Chirnside is minimal. In order to approximate a municipal waste figure for Chirnside the overall household food and drink waste figure and the 2015 population figure for Scotland is used. In 2014, the Government of Scotland estimated the total food waste produced in Scotland to be 600 000 tonnes.⁸⁸ In addition, the most recent population count in Scotland, conducted in 2015, estimated a count of 5.3476 million.⁸⁹ Using these values, a figure for the food waste produced per resident of Scotland/Chirnside and the total food waste produced in Chirnside can be estimated (estimate data highlighted in Tables 3–8).

Table 3 Estimate data (*) to calculate the current municipal food waste in Chirnside

Estimate data	Value	Unit
Food waste in Scotland 2014 (ref. 88)	600 000	Tonnes
Population of Scotland 2015 (ref. 88)	5 347 600	Persons
Food waste per person in Scotland per year (average)	112.2*	kg per year
Population of Chirnside in 2041 (estimate)	2250*	Persons
Food waste in Chirnside in 2041 (estimate)	252.45*	Tonnes per year

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6.2.3 Chirnside: sewage waste management.
6.2.3.1 Current sewage waste disposal techniques. Chirnside's wastewater is currently treated by Scottish Water at their small wastewater treatment site, located in Chirnside. This wastewater treatment (WWT) plant is licensed to discharge 318 m³ of treated final effluent per day to a standard of less than 25 mg L⁻¹ BOD and 10 mg L⁻¹ of suspended solids (A. Edmund, personal communications, December 17, 2017).

The Scottish Borders Council stated in the 2016 Chirnside Local Development Plan that ‘Chirnside has a limited capacity in respect to the waste water treatment works located here and contributions by developers may be required where upgrades are necessary’.⁸⁷ From this, it can be interpreted that necessary investments could be assigned to the potential upgrading and expansion of the current waste water treatment site, and/or contributed to the construction of a HTC plant in Chirnside for sewage sludge processing.

6.2.3.2 Current sewage waste figures. It is assumed that the maximum allowable discharge rate of 318 m³ per day is reached each day. Considering the population growth from 1459 to 2250 residents by 2041 (as estimated in Section 6.2.1), a maximum volume of water in the sewage waste is estimated to be 490 m³ per day, in 2041. This equates to 178 tonnes of wastewater produced in Chirnside per year. However, this figure does not account for the solid waste separated at the treatment site and additional calculations based on faecal production are required. It has been estimated that the average person produces a median wet faecal mass of 128 g per day.⁹⁰ From this data, the amount of faecal sludge produced in Chirnside in 2041 is estimated to be 105.1 tonnes per year. The total influent of wastewater to Chirnside's WWT plant in 2041 is therefore the sum of both solid waste and fluid waste (328.1 tonnes). Estimate data is presented in Table 4 and calculations are shown in eqn (3)–(6).

490 m³ per day × 1 kg m⁻³ × 365 days = 178 tonnes per year (4)

2250 residents × 128 g per day × 365 days = 105.1 tonnes per year (5)

178 tonnes per year + 150.1 tonnes per year = 328.1 tonnes per year (6)

Table 4 Estimate data (*) to calculate the total influent of wastewater to Chirnside's wastewater treatment site in 2041

Estimate data	Value	Unit
Maximum allowable discharge from Chirnside's WWT site (per day)	318*	m^3 per day
Discharge from Chirnside's WWT site in 2041 (per day)	490*	m^3 per day
Discharge from Chirnside's WWT site per year in 2041 (per year)	178*	Tonnes per year
Faecal mass produced by an average person per day^90	128	g per day
Faecal sludge produced by Chirnside residents in 2041	105.1*	Tonnes per year
Total influent of wastewater to Chirnside's WWT site in 2041	328.1*	Tonnes per year

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6.2.4 Chirnside: energy demand. Considering that there are many different types and sizes of houses in Chirnside, it is clear that varying amounts of energy will be consumed by the different households. Research undertaken by the World Energy Council estimated the average residential electricity consumption per person in the UK is 1985 kW h per year.³ Assuming that the energy consumption per person remains the same in the future, the approximate energy demand of Chirnside is estimated to be 4.47 GW h per year in 2041, as shown by eqn (7).

1985 kW h per year (pp) × 2250 residents ≈ 4470 MW h per year (7)

6.3 HTC design

6.3.1 Location. The map displayed on the left side of Fig. 9 shows the current residential layout of Chirnside. The ‘built up area’ indicated by the grey regions of the map represents local housing in the area. The right-hand map shows the boundary of current and predicted development areas, with ‘structural planting/landscaping’ and ‘mixed use’ development mainly occurring North East from the town centre. These areas have been identified in the Local Development Plan published by the Scottish Borders Council in 2016 and represents the area that would thus be unsuitable for the location of a hydrothermal carbonisation plant.

Fig. 9 Left: map of Chirnside, Scotland. Right: map of future development plans in Chirnside.⁸⁷

However, the area identified by the blue cross on the North-West of the left-hand map is deemed suitable for the placement of an HTC plant. This area is currently used for agricultural purpose; however, it provides an appropriate location for the plant site as it is currently uninhabited and there are no known plans for future development. This location is also convenient in terms of transporting Chirnside's biodegradable municipal and sewage waste to the site. Close proximity to the village would result in fewer emissions from biodegrade municipal waste transportation vehicles. Additionally, the capital costs for pipe-line construction, associated with the removal of sewage waste, would be significantly lower when compared to a plant located several miles outside of Chirnside.

6.3.2 Module sizing. From Sections 6.2.2.2 and 6.2.3.2, the total tonnage of waste (sum of BMW and sewage) produced by Chirnside and available for processing in a HTC plant is estimated to be 581 tonnes per year. Assuming a continuous operation of the plant for 8000 operating hours per year, the single reactor module must be capable of processing the combined waste at an estimated rate of 72.5 kg h⁻¹. Estimate data is presented in Table 5 and calculations are shown by eqn (8) and (9).

252.45 tonnes per year + 178 tonnes per year + 150.1 tonnes per year ≈ 581 tonnes per year (8)

581 tonnes per year × 1000 kg ÷ 8000 h = 72.5 kg h⁻¹ (9)

Table 5 Estimated data (*) for module sizing

Estimate data	Value	Unit
Total tonnage of waste (BMW and sewage) available for HTC in Chirnside	581*	Tonnes per year
HTC plant operating hours	8000*	Hours per year
Continuous mass flowrate into HTC reactor	72.5*	kg per hour

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HTC company Ingelia recently implemented their patented HTC process in UK. Therefore, the Chirnside plant in Scotland is assumed to implement their technology. Ingelia's singular continuous HTC reactor has the capacity of processing 6000 tonnes of wet biomass per year.¹⁵ As mentioned in Section 3.1, Ingelia's reactor design can be scaled depending on the processing requirement. Therefore, an HTC plant operating in Chirnside would require a singular reactor unit at approximately 1/10th the size of Ingelia's singular continuous reactor module.

Using Ingelias HTC process means that in the event of an increased waste feedstock, or collaboration with neighbouring municipalities/agricultural industries, the plant can be easily expanded upon.

6.3.3 Mass and energy balance.
6.3.3.1 Relevant data from previous studies for balance calculations. Project NEWAPP conducted a study for the potential business prospects in HTC technology. The values presented in Table 6 are taken from the study conducted for a German municipality with a population of 75 000 residents. The plant designed for this municipality was constructed around 4 large HTC reactors capable of processing 22 000 tonnes of wet biomass per year and is capable of producing 10 920 tonnes of pelletized hydrochar. Based on the assumption that the production of hydrochar can be scaled using this data, the singular (and much smaller) reactor based in Chirnside that is to process 580 tonnes of wet biomass would be capable of producing 288 tonnes of pelletized hydrochar.

Table 6 Data for the NEWAPP projects 4 module reactor and estimated data (*) for a single reactor in Chirnside

HTC plant	NEWAPP^13	Chirnside
Reactor units	4	1*
Wet biomass processed (tonne per year)	22 000	580*
Hydrochar produced (tonnes per year)	10 920	288*
Capital expenditure (million €)	4.7	∼1–2*
Operational expenditure (€ per year)	572 615	∼150 000*

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As mentioned in Section 2, an energy input is necessary in order to pressurize and heat the reactor unit, thermally dry the resulting effluent and pelletize the resulting hydrochar. A positive net energy is calculated when completing the energy balance of the entire HTC process, which is paramount to concluding whether the construction is energetically viable and economically feasible. The energy input over the process units have been previously evaluated for a 1000 kg feedstock of anaerobically digested sewage sludge.⁹¹ The energy inputs into the process units in this example have been used to estimate the energy balance associated to the Chirnside plant (Fig. 10).

Fig. 10 Mass and energy balance for the HTC plant in Chirnside processing sewage and food waste.

In order to determine if a net positive energy balance occurs over the plant, the net calorific value (NCV) of the hydrochar produced must be evaluated. The average NCV of hydrochar produced from biodegradable municipal waste and sewage sludge were evaluated in the NEWAPP project, at Ingelia's HTC plant. However, the average values calculated are from the multiple experimental hydrochar pellets produced; fluctuations can be expected due to variations in feedstock composition and reactor conditions.^15,92 Alongside evaluating the average NCV of various hydrochar produce, project NEWAPP created quality standards from their experimental data. The standard NCV for BMW and sewage waste was determined to be greater than 19 and 17 MJ kg⁻¹, respectively.

As previously calculated in Sections 6.2.2.2 and 6.2.3.2, the Chirnside plant must be capable of processing 252.45 tonnes BMW and 328.1 tonnes sewage waste per year. Fractionally, this represents a mass percentage of 43.45% and 56.55% for each waste stream respectively. From this, the tonnage of hydrochar produced by each respective stream is estimated at 125.1 and 162.9 tonnes per year for BMW and sewage waste, respectively. This data is shown in Table 7 and calculated by eqn (10) and (11).

Table 7 The Net Calorific Value of the hydrochar produced from respective waste streams. Estimated data denotes*

Estimate data	BMW	Sewage waste*	Unit
Average net calorific value (NCV)	22.09	18.22	MJ kg^−1
Quality standard NCV^15	>19	>17	MJ kg^−1
Mass processed at Chirnside	251.25*	328.1*	Tonnes per year
Fraction of total waste to process at Chirnside	43.43	56.56	Percentage, %
Mass of pellets produced at Chirnside	125.1*	162.9*	Tonnes per year

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BMW hydrochar pellets:

43.45% × 288 tonnes = 125.1 tonnes (10)

Sewage waste hydrochar pellets:

56.55% × 288 tonnes = 162.9 tonnes (11)

6.3.3.2 Mass and energy balance for Chirnside HTC plant. The mass percentage and average NCV data of both waste streams presented in Table 7 can be used to estimate the average NCV of the plants mixed feedstock hydrochar pellet. This is estimated to be 19.90 MJ kg⁻¹. The total average NCV and the total tonnage of hydrochar pellets produced allows for calculation of the total potential energy production of the pellets. This is estimated to be 5731.2 GJ per year, or 1592 MW h per year. These figures are presented in Table 8.

Table 8 Estimate energy data (*) for the Chirnside plant

Estimate data	Value	Units
Average NCV of mixed hydrochar pellets	19.90*	MJ kg^−1
Mass of hydrochar produced at Chirnside	288*	Tonnes per year
Total energy produced from pellets	5731.2*	GJ per year
Total energy produced from pellets	1592*	MW h per year

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The mass and energy balance of the Chirnside plant is calculated through scaling process data (M. Child 2014; Fig. 32)⁹¹ to the data associated to the Chirnside plant presented in Tables 7 and 8 The overall mass and energy balance (per operational year) associated with the HTC plant based in Chirnside is shown on Fig. 10.

The net energy is defined as the output energy produced by the hydrochar pellets less of the input energy for the total process operation. The net energy of the Chirnside plant is calculated using eqn (12):

Therefore, the net energy for the HTC process implemented in Chirnside is positive, with a potential to generate 1452 MW h per year. The approximated energy consumption of Chirnside is estimated as 4470 MW h per year (Section 6.2.4). Therefore, a HTC plant based in Chirnside would have the potential to contribute 35.6% of Chirnside's energy demand, as shown by eqn (13).

1452 MW h per year/4470 MW h per year × 100% = 35.6% (13)

6.3.4 Cost analysis. Capital expenditure associated with construction of a HTC plant includes equipment costs, land acquisition, grid connection, construction fees, etc. And the operational expenditure includes operation and maintenance, technical services, and general expenditures.¹⁵ The capital and operational expenditure associated with a 22 000 tonne processing plant has been estimated as €4.7 million and €570 000 per year, respectively (Table 6). Scaling of this data to the Chirnside plant leads to estimates for the total capital and operational expenditure of €1–2 million and €150 000, respectively. The operational expenditure represents an appropriate income of 2 full time workers at the plant and approximate costs of electricity/general maintenance. These costs are estimated to be larger than the figures generated from direct factor scaling in order to account for the benefits of economies of scale associated with processing larger quantities of waste. These figures are rough estimations and additional costs may be incurred during construction and operation. It is recommended that a full economic analysis is performed in order to evaluate the feasibility of a smaller HTC plant; it may be more economically feasible to construct and operate a larger plant/module size.

Revenue of a HTC plant in Chirnside can be obtained through sales of hydrochar, sales of liquid products for fertiliser applications and gates fees for biomass disposal (depending on the plant operator; government, council, business). Solid hydrochar has a range of price evaluations that depend on the its modification for final applications. As discussed in Section 4, some final applications of hydrochar include pellets (bio-coal) for electricity and/or heat generation, activated carbon and as supercapacitor electrode material. Therefore, several alternative final applications of the hydrochar produced in Chirnside are evaluated:

• Solid hydrochar has a price evaluation of €130–200 per tonne.¹⁵ Therefore, direct sales pellets can generate an estimate total revenue of €37–57 000 (£33–50 000) per operational year at the Chirnside plant.

• Assuming that the plant has an on-site combined heat and power (CHP) system with an electrical efficiency of 22% and a heat efficiency of 50%. A price per kWhr of £0.125 (ref. 93) would generate a revenue of £43 780 per operational year for electrical sales, and £99 500 in heat (in the form of steam/hot water) sales.

• Activation of the carbon would increase the sale price of hydrochar to €500–1,500, depending on the quality. This could generate a revenue of €144–432 000 (£128–385 000) per operational year.

In order to assess the economic viability of the plant, the total revenue over the project lifetime should be larger than the total operational and capital expenditure. The 22 000 tonne plant achieved a payback period of 5.5 years, after which a net profit is generated by the plant.¹⁵ For the Chirnside plant that is predicted to generate £143 280 in revenue per year, the summation of expenditure (capital and total operational) to achieve a similar payback period of 5.5 years must total approximately £788 000. Considering this and the estimate capital and operation costs for the Chirnside plant stated in th Table 6, it may be more feasible to construct a large plant in order to process more waste or activate the hydrochar. However, it is reiterated that a detailed cost analysis is performed to accurately assess economic feasibility and application options. In addition, no waste removal fees have been accounted for.

6.4 Chirnside conclusions

The need for improvements on the villages wastewater treatment site has already been acknowledged by the Scottish Borders council. As housing developments commence in Chirnside and the population is forecast to increase to 2250 by 2041, a suitably sized HTC plant in Chirnside would not only provide a solution to the management of increased waste production as it would also be capable of providing 35.6% of the total village energy demand.

In addition, the HTC plant would be capable of processing any green waste that Chirnside produces alongside any additional waste supplied by neighbouring municipalities until 2041, when the predicted maximum capacity of the reactor modelled in this review will be reached. However, the residents of Chirnside would only benefit from the local construction of a HTC plant if the electricity produced (from the hydrochar) is priced competitively to their current energy expenditure, and/or if local residents are employed at the site.

The plant designed in this report is capable of producing revenue from the sales of the 288 tonnes of hydrochar produced per year, independent of its final application. Thus, an estimated £143 280 in revenue can be achieved when producing electricity from the hydrochar for the residents of Chirnside. However, the payback period and profitability of the plant is highly dependent on the capital and operational expenditure, estimating of which from scaling has a large margin for error. Therefore, a complete cost analysis of all expected expenditures is required before a conclusion on economic feasibility can be achieved.

The following sections explore in greater detail the opportunities and challenges that are currently presented within the hydrothermal carbonisation industry, alongside the specific application of a plant based in Chirnside, UK.

7. Opportunity

7.1 Introduction to opportunities identification

The research conducted into renewable HTC technology has the main incentive of advancing humankind into a more sustainable future, specifically when comparing to the current energy production and waste disposal methods. In order to assess the opportunities that HTC technology can present in greater detail, the positive impacts that future implementation can have socially, environmentally and economically for the European Union will be explored. In addition, the opportunities presented to a small village that operates a HTC plant is explored, through application to Chirnside in the UK.

7.2 Policy opportunities

The European Commission, as an institution of the European Union, devised the environmental policy to outline targets for member countries to achieve. This have the main aim of protecting the health and wellbeing of EU member citizens, through environmental protection. The targets defined under the waste management section of this policy include a commitment to limit energy recovery (incineration) to non-recyclables by 2020.⁹⁴ In order to achieve this objective, it is clear that investment into alternative waste disposal and/or energy-from-waste processes for renewable/biomass materials is required. This implies that all member countries of the EU should prioritise investment into these technologies if the target is to be achieved by 2020.

In 2014, 80.8% of the world's energy was derived from fossil fuel sources.¹ The 7th Environment Action Programme (EAP) established by the European Commission aims to phasing out subsidies to environmentally harmful projects by 2020.⁹⁵ This implies phasing out to zero of subsidies provided to fossil fuel-based projects. The EU does not publish an inventory of fossil fuel subsides and this absence of inventory reduces the ability to monitor progress. However, one study conducted with the purpose of monitoring Europe's fossil fuel subsides has made claims that the EU, through the EU budget, European public banks and related financial instruments continue to provide financial aid to the fossil fuel industry. According to the Climate Action Network⁹⁶ an average contribution was made by the EU to the oil and gas industries of €515 million. In addition, €2 million is believed to have been provided by EU public banks for coal production in the years 2014–2016 (both inside and outside the EU). Although subsidies to fossil-fuel based industries is still occurring, the complete halt of them is unrealistic as market demand of these commodities still exists. Alongside this, the subsidies provided may be lower (phasing-out) when compared to those provided before the 7th EAP was established. The CEE Bankwatch Network have advised the European Investment Bank (EIB) to end its support for coal and non-renewable lignite power plants, as they should favour projects involving demand side energy efficiency and renewable energy sources.⁹⁷ It is unclear if the EIB support renewable initiatives, however, the information gathered in this review (Section 4) has identified that the EU is actively investing into the development of renewable-energy technology. They do so through 7th EAP and Horizon 2020 project, which had/have a budget of €50.5 and €77 billion, respectively. This budget is significantly greater than the acclaimed subsidy amount to the oil and gas industry. More specifically, the EU has invested in multiple HTC companies besides HTC research and development projects (HTCycle, Ingelia, NEWAPP).^15,66 The support and financial aid contributed by the EU has allowed development of HTC technology to reach a stage of commercialisation, alongside the formation of hydrochar standards which aim to increase product marketability.¹⁵ Therefore, despite contributions to fossil fuel industries, the EU are continuing to advance contributions towards technology that will decrease the market demand for fossil fuels. The EU's investment into the development of alternative, renewable energy solutions such as HTC today will contributes to the phasing-out of subsidies to environmentally harmful projects in the future.

7.3 Environmental opportunities

Hydrochar is the product of HTC with a renewable feedstock, therefore, the carbon dioxide (CO₂) produced from combustion of this product does not contribute to a net increase in atmospheric greenhouse gas (GHG) concentrations. However, the combustion of non-renewable fossil fuels for energy generation does contribute a net increase in these emissions. Research identifies that increased atmospheric GHG emissions are the cause of many negative global impacts, such as ocean acidification, melting of polar ice caps and glaciers, rising sea levels and sea temperature, global warming and agricultural impacts, etc.⁹⁸ Therefore, when comparing the two sources of energy, hydrochar is favourable when aiming to achieve sustainable future development. Alongside this, local production of hydrochar for energy applications can lead to reduction of carbon footprint (carbon emissions), as fossil fuel imports are reduced.

In addition to the opportunity for renewable energy production, HTC presents the opportunity of an effective waste disposal solution of biodegradable waste biomass when diverted from landfilling or incineration. Diversion from these practices can prevent the release of harmful emissions and in turn can prevent health risks whilst minimising the negative effects of global warming.⁷⁴ Diversion of renewable biomass waste from incineration plants (by 2020 as defined in the EC environmental policy) to HTC plants would prevent the release of harmful and carcinogenic emissions that are known to be produced by incineration of waste. This includes production of acid gases, nitrogen oxide, heavy metals, particulates, dioxin and furans.⁷⁹ To some degree, preventing the atmospheric release of these chemicals could be achieved via several control techniques within the incineration process. However, there is still risk associated with those that are not currently controlled, as well as accidental release in the case of equipment failure. Alongside this, health-effect assessments on the emissions released from incinerator plants for several hazardous compounds identified has not been completed due to emission data being ‘not readily available’.⁸⁰ Comparatively, the production of effluent gases in a HTC process is extremely minimal (2–5%).⁷ The majority of the gaseous effluents that is produced consists mainly of CO₂ (∼90%), with the remaining composition being a collection of hydrocarbon gases, H₂ and CO.⁹¹ To date, there have been no studies into the collection, separation and utilization of the gaseous effluent produced in the HTC process. However, it has been acclaimed that there is the opportunity to produce a pure form of CO₂, hydrocarbon gases and syngas.⁹¹ Therefore, a HTC plant would negate production and release of large, uncontrolled volumes of hazardous/greenhouse gases that are produced via incineration and/or landfilling of renewable biomass. Alongside this, there is potential to decrease the CO₂ emissions associated with the transportation of waste if fewer transportation miles accumulate when transferring to an HTC plant over a landfill or incineration site.

The liquid product stream from the HTC process also presents an opportunity in valuable material recovery. As the effluent contains favourable amounts of beneficial organic and inorganic compounds, such as nitrogen and phosphorous, the reuse of the water on agricultural lands can enrich soils as a natural fertilizer. There are various fertilisers utilised for crop production, the choice of which depends on both the crop and the farmer. In 2013, it was estimated that the application rate of total nitrogen on the crops and grasslands in both England and Scotland was 95 kg ha⁻¹ and 87 kg ha⁻¹, respectively (not including phosphate, potash and sulphur).⁹⁹ Chemical fertilizers are known to be damaging to both the environment and human health. And long-term use can change soil pH, upset beneficial microbial ecosystems, increase pests and contribute to greenhouse gas emissions. In addition, the toxic build-up of chemicals (including arsenic, cadmium and uranium) in soil escalate up the food chain into the bodies of consumers.¹⁰⁰ Therefore, the production and application of a natural fertiliser, as achieved through hydrothermal carbonisation, can lead to a decrease in the application of chemical fertilizers on agricultural lands. Additionally, chemical fertilizers are primarily made from fossil fuels; the hydrogen used in the production of ammonia (Haber–Bosch process) is obtained from methane steam reforming, coal gasification or partial oxidation of oil (totalling ∼96% of worldwide hydrogen production).¹⁰¹ Therefore, natural fertiliser production via HTC would result in a decreased reliance on fossil fuels. In turn, application of natural fertiliser presents the prospect of progression towards sustainable future development.

In order to realise sustainable future development, it is necessary to compare the environmental impacts associated with the energy sources that are currently available. Fig. 11 compares the use of alternative fuel sources that can be used to power a domestic oven. When comparing the environmental impacts (sustainability) of HTC pellets and fossil-fuels (coal, diesel and natural gas), the utilisation of hydrochar is more environmentally favourable.

Fig. 11 Comparison of fuel products for domestic oven use.¹⁵

7.4 Social opportunities

The construction and operation of localised HTC plants within the European Union would create and provide long-term employment opportunities for residents in member countries. In turn, this could increase national employment. Furthermore, diversion of biomass from landfill and/or incineration plants to HTC plants can aid in the reduction of carbon dioxide emissions. Alongside mitigating the of global warming and climate change, ground level air pollution could be reduced. Exposure to carbon dioxide and other emissions released from the combustion of non-renewable fossil fuel sources have been proven to have a negative impact of human health. The ‘health bill’ associated with the combustion of coal is estimated to total €43 billion in the EU per year.¹⁰¹ Therefore, phasing-out of subsidies towards fossil-fuel based industries and directing investment into renewable energy sources, more specifically towards the development of HTC in the EU, creates social opportunities in the form of potential employment prospects and human health benefits.

7.5 Economic opportunities

The production of hydrochar through HTC of waste biomass can provide energy security in regions where coal, and other fossil fuels, are currently imported. The EU currently imports the majority of their coal demand from Russia, Columbia and Australia.¹⁰² Therefore, operating HTC plants in member countries that currently consume imported coal would provide energy security in the event of interruption/termination of supply. Alongside this, fossil-fuel reserves continue to deplete with time whilst population and energy demand continue to grow. Therefore, a countries energy security via renewable technologies will contribute to greater economic stability in the future.

Although the capital and operational expenditure associated with an HTC plant can be high due to the technology being relatively new to the market, the implementation of HTC has associated several monetary gains. As previously described, revenue can be generated from the direct sales of hydrochar pellets (coal), application of these for electricity generation, activated carbon production (used as supercapacitor electrode material). In turn, the profitability of the company depends on the quality of hydrochar produced, the final application, and the capital and operational expenditures. Moreover, the process water can be sold for fertilization of crops and gate fees could be collected from the disposal of biomass (depending on the market in which HTC is applied). A profitable HTC company can improve the local economy if the plant is owned, constructed and operated by local companies. Alongside this, an HTC can increase potential employment prospects and therefore improve the local economy. Additionally, exportation (of hydrochar) can improve the gross domestic product (GDP).¹⁰³

In order to assess the economic opportunity HTC presents, the costs associated with the consumption of common household fuel products and HTC pellets (hydrochar) must be compared. In addition to a comparison of sustainability, Fig. 11 shows how the price, price variability and energy content of the fuels have been ranked for domestic oven use. As shown, the combination of the rankings from these categories forms an average ranking position, which places hydrochar pellets in 1^st place. Coal is shown to have the cheapest fuel price at £2.22 per Giga Joule (GJ), whereas hydrochar pellets are priced at £8 per GJ. Despite this, comparison of fuel price to the alternative fuel types presented in Fig. 11 demonstrates that hydrochar pellets are competitive within the fuel product market, due to their relatively low cost. Alongside the low-cost evaluation, the pellets present a valuable economic opportunity to those countries who produce it, as their price variability is the highest. This is due to pellet price being independent of both political and economic policies, varying only slightly with energy content, which is dependent on the biomass feedstock. Stable prices of hydrochar pellets can prove a beneficial opportunity to economy, as stable commodity prices contribute to a country achieving high levels of economic activity and employment.¹⁰⁴

7.6 Conclusion on the opportunities HTC presents

The implementation of both industrial and commercial HTC plants throughout the EU is a viable solution to achieving the target to phase-out environmentally harmful subsidies by 2020, as defined in the environmental policy.⁹⁵ Through funding businesses established within the field of HTC for energy production, via initiatives established by the EU such as Horizon 2020, the reliance on fossil-fuel energy production in the EU would be reduced. Increased investment and/or re-direction of subsidies to research and development within HTC will not only aid in achieving the phasing-out of subsidies to fossil-fuel industries, but also aid in phasing-out of fossil fuel reliance. With this comes the opportunity to combat global warming, climate change and the negative impacts associated. In addition, optimising HTC as an alternative biomass waste disposal method will lead to diversion of biodegradable municipal waste from landfills (the least preferable option of waste disposal as defined in the Landfill Directive (1999/31/EC)⁶³) and/or incineration plants, in which combustion of high moisture feeds is inefficient and is to be limited to only non-recyclables by 2020.⁹⁴ Diversion of waste biomass from both of these practices can also prevent the release of harmful (carcinogenic) compounds and greenhouse gas emissions (carbon dioxide and methane). The stable prices of pellets (due to unpolitical pricing) and competitively priced hydrocar can contribute to achieving high levels of economic activity and employment.¹⁰⁴ Alongside this, hydrochar pellets are competitively priced when compared to other fuel sources.¹⁵ Member countries who import coal can benefit from localised/industrial plants as hydrochar pellets are capable of replacing coal, benefiting from energy security. However, the EU's switch from fossil fuels to renewable sources is proving to be a slow, transitional process. Overall, HTC plants present many valuable opportunities for the UK and all the countries of the EU which, with continued research and investment, would benefit the most as we aim towards a more sustainable state of living in line with the UNs sustainable development targets.

8. Challenges in HTC technology

Alongside the many positive opportunities that hydrothermal carbonisation can present, the identification of potential challenges to overcome is necessary to secure its implementation in future projects. The sections below identify some of the more concerning challenges currently faced in HTC, alongside any potential solutions that are being/can be further explored to overcome them.

8.1 New technology

As explored in Section 4, HTC plants are currently being operated worldwide. Although interest in its application is increasing, it should be noted that HTC technology is new and in the early stages of implementation. The National Sludge Strategy, conducted by Scottish Waters, classified the pyrolysis of wastewater sludge as ‘relatively high risk’, as they are ‘commercially unproven technologies with planning and procurement time equal to or greater than an incineration plant’. They also stated that ‘They [includes gasification] typically have high capital and operating costs’.¹⁰⁵ However, since these statements were published in 2006, numerous research projects and advancements have been conducted for the development and industrial application of HTC technology. In addition, high capital and operational costs can be expected from the construction of any new technology-based processing plant. Therefore, in order to achieve efficient and sustainable development, a re-evaluation of the waste processing techniques outlined in the 2006 National Sludge Strategy is due if implementation of HTC is to be encouraged.

In recent years, there has been significant amounts of funding in support of the research and development into HTC technology. However, there are other energy-from-waste processes that have already established themselves within the market and are rapidly expanding. One of which is Anaerobic Digestion (AD: the treatment of biodegradable (food) waste and sewage waste using microbes for biogas (methane) production. In 2014, the UK recorded over 100 plants in operation which rapidly grew to 640 plants in 2018.¹⁰⁶ Investments made towards AD development and application is contributing to sustainable future development. Thus, the plentiful investments into an established business model can make it challenging for a HTC company to compete with. Based on this, HTC has great potential to rival AD due to the following comparative advantages.

In comparison to a solid fuel produced by HTC, the enzymes in AD produce biofuel in the form of a gaseous product. Storage and transport of a gaseous fuel can, in some instances, be challenging, costly and pose a greater risk due to storage equipment requirements, high pressures and potential leaks. Moreover, the AD of municipal biodegradable waste must be complete free of any food waste packaging to avoid operational challenges. Whereas, HTC is capable of depolymerising plastics if not completely removed from the feedstock. In addition, AD requires large land requirements whereas HTC can process large masses of waste over a small plant footprint.³⁵ What is more, the cost of AD is greater than the cost associated with HTC (Fig. 12). However, the most noticeable difference between these two technologies is efficiencies: the efficiency of HTC is approximately 5 times greater than that of AD, and almost double the carbon efficiency.¹⁰⁷ To conclude, when debating between investment into AD and HTC development, the main point to consider is the less efficient, more expensive production of a gaseous fuel, or the cheaper, more efficient production of a solid fuel.

Fig. 12 Economic comparison of current waste disposal techniques.¹⁵

This being said investigations into the coupling of the two technologies to form AD–HTC hybrids can solve the problem of by-product use for the other. Investigations include utilising AD technology to produce biogas from the upstream HTC reactors process water.¹⁰⁸ However, one HTC company claims to produce clean process water through their HTC technology/process, thus eliminating the requirement of AD for the treatment of process water.¹⁰⁷ The reverse operation has likewise been investigated, where HTC is used to process the digestate remaining after AD.¹⁰⁹ Reza et al. found that processing AD digestate through HTC results in a greater amount of energy per 1 kg of raw biomass, which is 20% and 60% more than that of HTC alone and AD alone, respectively.¹² Therefore applications of HTC in conjunction with existing AD plants may grow over the following decades as companies previously invested in AD aim for greater energy outputs. Investment by AD operators can thus aid in overcoming the challenge of technology marketability.

Another energy-from-waste treatment method in which HTC must compete with is incineration of waste; the UK Government's Department for Environment Food & Rural Affairs recorded that 83 incinerator plants were operational in 2014.¹¹⁰ However, HTC has been proven to be more energetically viable than incineration of wet biomass for moisture contents greater than 10%.²⁴ Concluding that albeit HTC is a new technology, the process should be the new choice to avert such inefficiencies experienced in both anaerobic digestion and the incineration of high moisture biomass feeds.

As HTC is a new technology, there are still many unknowns about the exact performance details of the process. This arises due to each lignocellulose biomass feed capable of being processed through HTC having a different percentage of hemicellulose, cellulose and lignin. This allows for kinetic modelling to be completed; however, it is specific to the type of feedstock which it is completed for.¹¹¹ Even then, exact reaction mechanisms are unknown and there will be discrepancies in the properties of the hydrochar produced. To some degree, not knowing exact process details can correlate to a client's insecurity towards the technology. However, to combat any concern potential, HTC companies such as Antaco (UK) and Ingelia (Spain) that have patented their process should provide support and reassurance to potential clients. For example, Antaco offers a wide range of services that include organic waste assessment, site assessment, feasibility and costings.¹⁰⁷ And, Ingelia have stated that they will ‘establish cooperation, framework and joint venture agreements with international partners to support the deployment of [our] HTC plants all over the world’.⁷⁴ The method of a joint venture business entity with the HTC specialist demonstrates the company's collaboration method and confidence in the ability of their process to perform. In turn, this provides clients and investors with assurance in the new technology. Additionally, NEWAPP is producing a standardised quality database for experimental data recorded on the different feedstocks with the aim of providing assurance and encouraging marketability.

8.2. Logistics

Implementing a HTC plant alongside a process that currently generates a biomass rich waste stream leads to semi-straightforward calculations for the logistics and reactor module sizes. However, the logistics for feed transportation from homeowner to plant can become challenging. In terms of both economic and environmental costs, it would be inefficient to collect the municipal waste from each household via heavy duty vehicles every day of the week. Therefore, the logistics of waste transport and the associated cost are dependent on the operational capacity of the plant and the average household's municipal waste production over a week (as to prevent adverse side effects from storing food waste). From this, efficient waste collection logistics can be achieved through computer simulations that account for these dependencies. This would allow for the most efficient routes and collection days to be calculated with easy revision each year. This method may be costly to initiate, however, most towns within the UK have already established waste disposal logistics which can be analysed to adapt to the location of the HTC plant. Implementing heavy-duty vehicles for waste collection in areas which do not currently operate waste collection can have negative environmental impacts due to the release of carbon dioxide emissions. Thus, emissions could be reduced if the distance travelled to a HTC plant is shorter in comparison to the established waste processing site. Moreover, the use of heavy-duty vehicles may be reduced and even completely eliminated if food waste disposal systems (sink shredder) are applied to new (and old) households. This would mean that shredded food waste would flow in the wastewater stream (in the sewage system) towards the HTC plant.

The transport of sewage sludge in the UK is achieved through an underground waste water sewage system which is transported towards the ‘sewage works’ or ‘wastewater treatment’ plant.¹¹² Therefore, diversion of the underground sewage system that encourages flow towards a HTC plant can be challenging and costly to achieve. Additionally, wastewater systems typically contain around 0.1% of solid matter.¹¹³ Therefore, preliminary and primary treatment methods would need to be located close to the HTC plant in order to separate the organic fraction (unless a separate toilet sewage system is constructed). Therefore, location of the HTC plant in accordance to the established sewage system and WWT plant containing both primary and pre-treatment methods would be most beneficial when processing waste waters. This demonstrates the practicality of operating AD–HTC hybrid plants for wastewater treatment as described in Section 8.1. The HTC plant would be capable of processing the by-product of AD (digestate) and the AD could process (purify) the HTC process water.

As demonstrated, choosing the right logistic methods for a HTC town plant, alongside implementing the chosen method, can be challenging and time consuming, especially for a HTC plant with a continuous onsite feed stream that requires minimal logistic consideration. However, by efficiently completing the above, the biodegradable municipal and sewage waste produced by the residents of a town in the UK can provide just over a third of its entire energy demand.

8.3. Economics

8.3.1 Capital and operational expenditure. The associated expenditures for construction of any new plant or processing facility can be expected to be high, with HTC being no exception. A HTC plant capable of processing 22 000 tonnes of biomass has been estimated to cost €7.8 million until the payback period of 5.5 years is reached.¹⁵ Alternatively, a 20 000 tonne plant has been estimated to cost £10 million until the payback period of 10 years is reached.²¹ Thus, an economic comparison study of various waste disposal methods has ranked the HTC business model in 2nd place overall. This comparison method is shown in Fig. 12, where current waste disposal methods have been ranked on their average costs, capital expenditure (CAPEX) and operating expenditure (OPEX). HTC is the highest ranked energy-from-waste disposal process as it only ranks below the average cost associated with landfilling and composting. Neither of these methods produces energy and therefore they produce little to no profits. Comparing the energy-from-waste processes in Fig. 12shows that the CAPEX of HTC is ranked as ‘affordable low’, whereas anaerobic digestion and incineration are ranked ‘expensive’ and ‘very expensive’, respectively. The OPEX associated with HTC is ranked as ‘expensive’ which can be associated to the cost of high pressure and temperature reactor coupled with the new type of technology when compared to older waste treatment methods. Comparatively, a correlation between the investment cost into an incineration plant and the processing tonnage has been established by Waste to Energy International; for a 20 000 tonne plant the investment cost is estimated at £16.5 million.¹¹⁴

8.3.2 Hydrochar pricing. Due to the small number of operational HTC plants, the current supply situation causes the hydrochar pellets to be less competitively priced (£8 per GJ) when compared to the prices of coal and woodchips (£2.22 and £4.53 per GJ, respectively). Nevertheless, future prices of competing fossil-based fuels can be expected to increase due to the oil supply policy change in producer countries, increased upstream production costs, decreased reserves and stricter environmental policies.¹¹⁵ Besides, increasing the investments towards the development of HTC technology can decrease the CAPEX and OPEX of future HTC projects. In turn, operators can choose to reduce the price of hydrochar or improve their profitability.

8.3.3 Biomass feed. Dependant on the industry in which it is implemented, a challenge in the concern of potential investors could arise from a predictable depletion of their biomass feedstock. For example, operation of a unit that is to process Brussel sprout or orange-peel waste would mean that the feedstock quantities are highly dependent on the given season. Other factors that can lead to an unpredicted depletion of biomass feedstock include climate change, crop failure and changing farming strategies. Some of the factors contributing to feedstock fluctuations are beyond the control of unit operators and should be considered by potential investors. Thus, the HTC plant should be flexible, having a wide variety of biomass feedstock to be processed if there was ever a decline in feedstock supply. Therefore, if intended feedstock were to become scarce for any circumstance, alternative biomass feeds can be (pre)sourced for processing to ensure revenue and investment security.

A challenge concerned with the varying ‘pumpability’ of the feed slurry is that the lower the moisture content of the biomass entering the reactor, the more difficult the pumping operation becomes. However, this challenge can be circumvented by adding a recycle stream of the process water to the feed, as shown in Fig. 11 and 12.¹⁵

8.3.4 Product variability and complications. Overall mass and energy balance calculations over a HTC plant used to estimate hydrochar production, quality and heating value (etc.) have margins for error, even based on experimental data. This is due to the heterogeneity of the biomass feed composition coupled with the unknown details of the reaction mechanisms. However, the increasing interest into HTC by academics, investors and governing bodies are propelling the research required to reduce the error and uncertainty. To understand the processing of different feeds under different conditions and minimise the error in HTC feasibility calculations, a standardised quality criteria of the products from alternative feedstock is currently being developed by the European Biomass Industry Association (EUBIA) in association with the International Organisation for Standardisation (ISO).¹⁵ Once these standards have been established and are industrially recognised, consistency in the quality of hydrochar produced can be achieved and marketability can be improved.

Inorganic materials such as stones, pieces of metal, dust and sand have reportedly been found in the product streams. When present in the biomass feedstock, unlike the organic compounds, they are not destroyed during the process reactions. Their presence can lead to penalties on the process energy balance, as energy can be wasted from the unit trying to process and heating the inert material. However, experimental research conducted on a waste stream with a high amount of inorganic material (>20%) demonstrated that HTC can still proceed smoothly. This means that HTC is a robust process and that technical problems due to chemical composition are of minor importance.⁹ However, it should be noted that the presence of any large solid particles can also damage valves and process pumps. Thus, any suppliers of biomass feed to a HTC plant should be notified to prevent contamination from large inorganic materials as separation is difficult and inviable.

The contamination of the biomass feed with any heavy metals such as mercury, lead and chromium will lead to their persisting presence in the product process water and hydrochar. These can be introduced from printed paper or batteries entering the process. Due to the high toxic risk factor associated with heavy metals present in the feed even at extremely low concentrations, the extraction from the products is paramount. Special attention should be paid on the extraction of heavy metals from any process water that is to have agricultural applications, such as a fertiliser, or if the solid hydrochar is to be alternatively applied as a soil conditioner. Hydrochar contaminated with heavy metals with applications for fuel can still be valorised energetically but must be done under controlled conditions.¹⁵ Another potential contaminant of the HTC process water is that of persistent organic pollutants (POPs) which are resistant to environmental degradation. Very little is known about their presence in the HTC process water but to ensure they are not. Preliminary tests should be conducted on new sources of biomass feed.

9. Conclusion and perspectives

HTC is an effective method for the treatment of biodegradable municipal waste and sewage waste. Compared with the traditional landfill and incineration method, it can greatly decrease the emission of harmful gases. In addition, the energy produced is renewable and is generated with no net CO₂ production, making it sustainable. The major product, hydrochar, can also bring good profit. The assessment of HTC plant in a small village (Chirnside) further confirms the promising application of this technology. Both social and economic benefit could be expected. However, there are still some challenges for HTC replacing the current process:

(1) As a new technology, there are many unknown mechanisms in HTC process. In addition, it has to compete with current waste disposal methods, as well as other renewable energy technologies.

(2) The logistics system for HTC can be both time consuming and costly.

(3) The associated expenditures for construction of HTC can be expected to be high. The price of hydrochar is competitive, however; the price is currently higher than the price of coal in equivalent Joules of energy.

(4) There might be some uncertainty in the resulting quality of hydrochar due to the complexity of different biomass sources used and possible contamination of the biomass feed.

Conflicts of interest

There are no conflicts to declare.

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