Recent advances of sodium borohydride reduction in coal water slurry desulfurization: integration of chemical and electrochemical reduction

Abstract

Clean fuel technologies have been widely developed in current society because fuel combustion can directly bring about the emission of hazardous gasses, i.e. SO₂, sulfate particulate matter (SPM), resulting in environmental pollution and other related problems that endanger public heath and community property; as well as reducing the life of the engine due to corrosion. The research efforts for developing conventional hydrodesulfurization (HDS) and alternative desulfurization methods such as selective adsorption, biodesulfurization, oxidation/extraction (oxidative desulfurization), etc. for removing these refractory sulfur compounds from liquid fuel products are on the rise. The reductive desulfurization method under ambient conditions has rarely been researched. This paper reviews the current status of various desulphurization techniques being studied worldwide. It presents an overview of novel emerging technologies of chemical and electrochemical reduction (CECR) for deep desulfurization of coal water slurry (CWS) so as to provide a new way of thinking about ultra-clean fuel technology.

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Yafei Shen

Yafei Shen (Josh) got his M.S. (2005) degree in applied chemistry from Nanjing University of Information Science & Technology. Then, he worked as a M.S. (2009–2012) student in Shanghai Jiaotong University (SJTU) on the major of environmental engineering. At present, he undertakes research work on environmental chemistry, energy & chemical engineering, especially in clean coal technology, flue gas desulfurization.

Xunyue Liu

Xunyue Liu was born in 1987, Shandong, China. She got her B.S. (2009) degree in Biotechnology from Weifang University. Currently, she is diligently studying Ph.D. degree on the major of environmental toxicology at Institution of Nuclear Agricultural Science in Zhejiang University. She is interested in environmental biochemistry & agricultural biotechnology.

Tonghua Sun

Tonghua Sun received his Ph.D. (2004) degree in environmental engineering from SJTU, China. Currently, he is a research professor at the School of Environmental Science & Engineering in SJTU. His research interests include industrial pollutants treatment, energy & chemical engineering and flue gas desulfurization. In the past, he directed the National “863” Project, the Baosteel COREX gas desulfurization project, etc.

Jinping Jia

Jinping Jia earned his B.S. (1983) & M.S. (1986) degrees in applied chemistry from University of Science & Technology of China (USTC). Then, he received Ph.D. (1999) degree in material science from SJTU. He is currently a professor at the School of Environmental Science & Engineering in SJTU. He is interested in the related works on the study of environmental electrochemical method, the study of new technology in monitoring and toxicity of the environmental pollutants, new technology of clean production and crafts, TiO2 photocatalytic technology.

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1. Introduction

Sulfur dioxide (SO₂) emission from coal-fired power plants, refinery operations and vehicles has been implicated as a cause of acid rain and other air pollution related problems.¹Fig. 1 presents the formation of acid precipitation (rain, snow, etc.) from SO₂ and NO_x. It can cause significant damage to landmarks (e.g. historical relics), modern constructions, plants (e.g. forests), animals (e.g. fish), and so on. Meanwhile, acid rain always falls in the economically developed and densely populated region at the southeast of China.

Fig. 1 The formation of acid rain from SO₂ and NO_x.

According to the report on the state of the environment in China by the Ministry of Environmental Protection (MEP), national SO₂ emissions decreased continuously from a maximum of 2.59 billion tons in 2006 to 2.17 billion tons in 2010, that is, a 14% reduction had been achieved in relation to the 2.55 billion tons in 2005, mainly through flue gas desulfurization (FGD) in more than 70% of coal-fired power plants.² In the recent years, coal is still the predominant fossil fuel. Clean coal technology is being applied and popularized at an unprecedented rate in China.³ Coal water slurry (CWS) is a coal-based clean liquid fuel, which is currently prepared with refined coal. In the course of coal washing, most of the ash and inorganic sulfur can be removed, so the concentration of ash and SO₂ will be lower than that of the original coal by combustion and gasification. But with the more restricted environment legislation and more complex coals being used to make the slurry, the release of sulfur is greater, sometimes beyond the requirements of environment protection. As a clean liquid coal, desulfurization of CWS prior to combustion can save significant costs in follow-up FGD.⁴

A number of physical, microbial, chemical, pyrolytic, and radiation assisted oxidative desulfurization methods have been reported. Physical processes such as froth flotation and magnetic force separation can only remove a portion of pyritic sulfur but cannot reduce organic sulfur.^5–8 Besides, some physical processes are not cost effective methods.⁹ Microbial desulfurization of coal is usually carried out with chemolithoautotrophic bacteria such as T. thiooxidans and T. ferrooxidans.¹⁰ However, both of the microorganisms for leaching organic sulfur from coal are not effective.¹¹ Coal pyrolysis can remove most sulfurs, including inorganic and organic forms, but requires rigorous operating conditions, e.g., the treatment temperature is generally higher than 600 °C.^12,13 Ultrasonic, microwave, and γ-ray radiolysis assisted oxidative desulfurization of coal, operated at low temperature conditions (<100 °C), have been reported by many workers with organic sulfur reduction in the 30–60% range.^4,14–16 However, these processes involve some drawbacks that are hard to overcome, such as large energy consumption and the obvious loss of caloric value. Thus, chemical oxidative methods at low temperature conditions have been extensively studied (Table 1). Exciting as these researches are, they also have their problems. Firstly, all these chemical desulfurization approaches mentioned above have involved oxidation reactions. As shown in Table 1, little literature has been published on the reductive desulfurization of coal, especially at mild conditions. Secondly, these treatments are tedious and costly because of their long duration, high reaction temperature, large reagent consumption and miscellaneous approach. Thirdly, some chemical reagents, such as hydrogen peroxide and nitric acid are poisonous and highly corrosive. Finding a simple, effective, technologically feasible, economically viable and commercially exploitable method of coal desulfurization, capable of simultaneously removing inorganic and organic sulfur from coal without any loss of calorific value is of immediate industrial and environmental significance.¹⁶

Table 1 Various chemical desulfurization methods of coal ⁴¹

Author	Reagent	Time	Sulfur removal	Ref.
Mukherjee	H2O2 + H2SO4	4 h	Using 15% (vol.) H2O2 and 0.1N H2SO4: 45% of total sulfur removed (complete removal of inorganic sulfur and 31% removal of organic sulfur)	20
Ozdemmir	Chlorine in CCl4 + H2O	6 h	Using 0.033 L min^−1 chlorine flow rate at ambient temperature and pressure: All pyritic and sulphate sulfur removed and 30% organic sulfur removed	21
Ahnonkitpanit and Prasassarakich	Aqueous H2O2 + H2SO4	2 h	Using 15% H2O2 and 0.1N H2SO4 at 40 °C: 48.7% total sulfur removed (97% pyritic, 89% sulphate and 7.1% organic sulfur removed)	22
Krzymien	Aqueous CuCl2	48 h	Using 10 mL of 10% (vol.) CuCl2 at 200 °C, 100% sulfur removed	23
Sonmez and Giray	Peroxy acetic acid	72 h	45% sulfur removed from Gediz lignite; 85% sulfur removed from Cayirhan lignite	24
Chandra	Atmospheric oxidation	106 days	44% sulfur removed (36% organic sulfur removal)	25
Steinberg	O3 + O2	1 h	Using a flow rate of 200 mL min^−1, 1% O3 at 25 °C, 20% sulfur removed.	26
Chaung	Dissolved oxygen and alkalis:NaHCO3, Na2CO3 and Li2CO3	1 h	0.2M alkali solution with 3.4 atm O2 partial pressure at 150 °C. Na2CO3:72% of sulfur removed; Li2CO3: 73.1% of sulfur removed; At 0.4 M NaHCO3: 77% of sulfur removed.	27
Liu	Aeration + NaOH, HCl	5 h	Using 0.25 M NaOH at 90 °C with aeration rate of 0.136 m3/h and 0.1 N HCl solution: 73% organic sulfur removed; 83% sulfide sulfur removed; 84% pyritic sulfur removed.	28
Zaidi	NaOH	3 h	Using 0.2 N NaOH at 70 °C, 36.2% sulfur removed	29
Aarya	NaOH	8 h	Using 100 g dm^−3 NaOH at 80 °C, 30% sulfur removed	30
Rodriguez	HNO3	2 h	Using 20% HNO3 at 90 °C, 90% inorganic and 15% organic sulfur removed	31
Prasassarakich and Thaweesri	Sodium benzoxide	90 min	Using 600 mL sodium benzoxide at 205 °C, 45.9% sulfur removed (83.7% sulphate, 68.6% pyritic, 33.3% organic sulfur removed)	32
Yang	NaOH	60 min	Using 10 wt% NaOH at 250 °C: 55% sulfur removed (95% pyritic and 33% organic sulfur removed)	33
Kara and Ceylan	Molten NaOH at different temperatures	30 min	Using 20 wt% NaOH at 450 °C: 83.5% sulfur removed	34
Ratanakandilok	Methanol/water and methanol/KOH	90 min	Using 2% methanol and 0.025 g KOH/g coal at 150 °C: 58% total sulfur removed (77% sulphate, 47% pyritic and 42% organic sulfur removed)	35
Aacharya	Thio-bacillas, ferro-oxidants	30 days	91.81% sulfur removed from lignite, 63.17% sulfur removed from polish bituminous coal, 9.41% sulfur removed from Assam coal	36
Ali	H2O2, NH4OH, K2Cr2O7 and CH3COOH	30 min	50–90% of sulfur removed, depending on concentration and solvent.	37
Baruah	Water	120 h	77.59% pyritic sulfur removed with aqueous leaching at 45 °C.	38
Hamamci	Fe(NO3)3·9H2O	12 h	Using 50 mL of 1 M solvent at 70 °C, 72.2% total sulfur (96.6% pyritic sulfur) removal	39
Li and Cho	NaClO	2 h	Pittsburgh No.8 coal: More than 70% of pyritic sulfur removal was achieved at the optimum conditions of 0.4 M hypochlorite, 0.4 M NaOH, and 90 °C.	40
Li	NaBH4	1 min	Using 1.6 mM of NaBH4 concentration, −109 μm of particle size, neutral pH of initial media, 1 min of treated time, 100 rpm of shaking rate, 30 °C of temperature: 23.8% and 59.0% reduction in the pyritic sulfur, 70.4% and 100% reduction in the sulfate sulfur, 11.0% and 15.0% reduction in the organic sulfur, and 31.3% and 40.8% reduction in the total sulfur for the YZ coal and the YS coal, respectively.	19

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However, a method for removing sulfur compounds from gasoline was reported involving the conversion of sodium metaborate (NaBO₂) to sodium borohydride (NaBH₄) by electrochemical reduction and the incorporation of a type of metal compound. The desulfurization efficiency was 48.1%.¹⁷ Guo¹⁸ had performed research on borohydride potassium (KBH₄) as a reducing agent in order to remove benzothiophene sulfur in gasoline. Li¹⁹ conducted a new desulfurization process of NaBH₄ reduction for high-sulfur coals of Yanzhou and Yanshan in China. The combustion characteristics were slightly improved after treatment. This research revealed the potential of metal borohydride (KBH₄, NaBH₄, etc.) for reductive desulfurization. This review will present chemical and electrochemical works for CWS desulfurization via sodium borohydride reduction.

2. Chemical reduction

2.1. Hydrogenation reduction

The current widely utilized method of industrial reductive desulfurization of fuels is hydrodesulfurization (HDS), which is a catalytic process under high temperature and pressure conditions.⁴² The HDS reaction in refineries is carried out in trickle-bed reactors. These reactors are commonly operated at temperatures in the range 300–450 °C, and at H₂ pressures of 3.0–5.0 MPa, usually with CoMo/Al₂O₃ or NiMo/A1₂O₃ catalysts.^43,48 This makes HDS a very costly option for deep desulfurization. Scheme 1 shows the HDS mechanism of dibenzothiophene (DBT) at 300 °C and 102 atm in the presence of CoMo/Al₂O₃. Moreover, HDS is not effective for removing heterocyclic sulfur compounds such as DBT and its derivatives, especially 4, 6-dimethyldibenzothiophene (4, 6-DMDBT).⁴⁹ Interest in HDS was initially stimulated by the availability of hydrogen from catalytic reformers.⁴⁴ Typically, the HDS process involves catalytic treatment with hydrogen to convert the various sulfur compounds to H₂S and sulfur-free organic compounds at high temperature and partial pressure of hydrogen.^45,46 Conventional catalytic HDS method for reducing sulfur content requires severe conditions of operation. In refineries, the H₂S resulting from the HDS reaction is eventually converted to elemental sulfur (S) by a modified version of the Claus process.⁴⁷Fig. 2 presents the Claus process for elemental S production from waste H₂S gasses.

Fig. 2 The modified version Claus process for elemental S production from H₂S gasses.

Scheme 1 HDS of DBT at 300 °C and 102 atm in the presence of CoMo/Al₂O₃.⁴⁹

Under these harsh conditions, olefins get hydrogenated, leading to a loss of octane rating and excess hydrogen consumption. Under mild HDS conditions, H₂S can react with olefins in the reactor to create recombinant mercaptans which are linear or branched thiols of typically 5–12 carbons. The formation of recombinant mercaptans causes sulfur to be retained in the product, limiting the effectiveness of the HDS process. Further research in HDS catalysis and process designs are being carried out so as to increase the sulfur removal and still maintain the fuel quality at some minimum specification.⁴⁴ In general, when the unpaired electrons of the sulfur can resonate with the pi electrons of the organic structure, the energy of the carbon-sulfur bond (C–S) becomes practically identical with that of the carbon-carbon (C–C) bond.⁵⁰ It leads to a reduction in the selectivity of the HDS process, and hydrogenation of carbon-carbon bonds happens. Saturated hydrocarbons lead to a lower-grade fuel, and require additional processing steps.

There are several constraints in the HDS process. Meeting the sulfur requirement for gasoline is believed to be the greatest challenge for the refining business requiring substantial revamps to equipment or even construction of new units. This is due to the fact that most of the gasoline production in the market today comes from cracked stocks that contain a larger concentration of compounds with aromatic rings and high olefin content, thus making sulfur removal more difficult. The need to desulfurize the cracked stocks in addition to the straight-run streams is forcing the refiners to choose the most cost-effective technology.^51–56 Although HDS processes have dominated desulfurization of FCC gasoline/diesel in the past their cost and the requirements of the upcoming strict fuel specifications combine to motivate the development of innovative process technologies.

2.2. NaBH₄ reduction

Reductant costs, strict equipment and operation conditions have seriously restricted the development and utilization of chemical reduction methods by HDS. Sodium borohydride (NaBH₄) is a versatile reducing agent used in many industrial processes. It releases hydrogen as a consequence of hydrolysis. This is a quick reaction at normal pressure and temperature (NPT). Four moles of protonic hydrogen come from borohydride, and four moles of protons come from water.⁵⁷ Also, it is the cheapest metal hydride and easy to store and dispose of.⁵⁸

As mentioned above, Li proposed a new desulfurization process with sodium borohydride reduction for high-sulfur coals.¹⁹ After that, we researched the use of NaBH₄ for deep desulfurization of CWS and gasoline in depth and the common influencing factors such as reductant (NaBH₄) concentration, treatment time, coal particle size, reaction temperature, shaking rate, and initial pH value have been taken into account. Ultimately, optimized reaction conditions were obtained. Fig. 3 shows the general desulfurization procedure of coal slurry via NaBH₄ reduction by Ni catalysis under ambient conditions. At first, NaBH₄ can react with NiCl₂ to generate the Ni₂B catalyst. Secondly, NaBH₄ can rapidly hydrolyse with the Ni₂B catalyst under ambient conditions. Finally, the produced H₂ or active hydrogen atom can translate sulfur compounds into H₂S by catalytic reduction, releasing from coal particles. Fig. 4 presents our research work on CWS desulfurization by NaBH₄ reduction. It could be found that the chemical reaction easily occurred in the aqueous phase and the electrolysis of NaBH₄ was a key step to the desulfurization of CWS (Fig. 4A). Significantly, compared with catalysis, there was no evident improvement when using ultrasonics to enhance the desulfurization efficiency. Meanwhile, the desulfurization efficiency of CWS depended on the hydrogen ratio and hydrolysis rate of borohydrides. In Fig. 4B, the hydrolysis rate of KBH₄ under Ni catalysis was slower and more stable than NaBH₄, resulting in a lower desulfurization efficiency. After treatment, the removed sulfur compounds were converted into H₂S (Fig. 4C). The hydrogenation and deashing exceeded to the decarbonization, which contributed to the slightly improvement of combustion characteristics (i.e. ignition temperature, calorific value).

Fig. 3 General procedure of liquid coal desulfurization via NaBH₄ catalytic reduction.

Fig. 4 Mechanism (A), results (B), and GC analysis (C) of NaBH₄ reduction for CWS desulfurization.^4,41

Moreover, the reaction details of NaBH₄ hydrolysis by Ni catalysis^41,59 are shown as the following eqn. (1)–(3):

4NaBH₄ + 9H₂O + 2NiCl₂ → Ni₂B↓ (black precipitates) + 3H₃BO₃ + 4NaCl + 12.5H₂↑ (1)

NaBH₄ + 2H₂O → NaBO₂ + 8H* (4H₂, catalytic hydrolysis) (2)

4NaBO₂ + 11H₂O → Na₂B₄O₅ (OH)₄·8H₂O (sodium borate) + 2NaOH (3)

In the process of desulfurization, pyritic sulfur (FeS₂) widely existing in the coals is removed according to below eqn. (4)–(6).¹⁹

FeS₂ + 2H* → Fe + H₂S↑ + S (4)

H₂S + 2NaOH → Na₂S + 2 H₂O (5)

Since NaBH₄ had high reducing property, the oxygen functional groups of OS (S=O, –SO₂⁻, etc.) were reduced into the non-oxygen functional groups (C–S, etc.) by catalytic hydrogenation and hydrolysis. After that, the activated hydrogen (H*) produced by NaBH₄ hydrolysis attacked the C–S bond, accordingly organic sulfur compounds were converted into H₂S and released from the coal (Fig. 5).

Fig. 5 FT-IR analysis of the solid CWS samples before and after NaBH₄ reduction.⁴¹

As mentioned above, NaBH₄ reduction can be better utilized for oil fuel desulfurization due to the good water solubility of NaBH₄. The reducing reaction occurred in the aqueous phase, so the catalysts and by-products can be easily separated and recycled from the fuels after treatment. As Fig. 6A shows, catalytic hydrolysis of the reductants occurs in the aqueous phase, while catalytic reduction takes place in the oil phase. The desulfurization procedure very conveniently includes the two steps of reduction and separation (Fig. 6C). In addition, the desulfurization efficiency of gasoline is proportional to the hydrolysis ratio of NaBH₄ and hydrogenation ratio (Fig. 6B).

Fig. 6 Mechanism (A), results (B), and procedure (C) of NaBH₄ reduction for gasoline desulfurization.⁵⁹

3. Electrochemical reduction

Electrochemical desulfurization (ECDS) technology has been explored to remove sulfur by electrochemical oxidation or the reduction of sulfur compounds in fossil fuels.^60–65 This technology is able to remove sulfur at relatively low temperatures and pressures, which potentially makes the process much less energy intensive and more economical than conventional technologies. In ECDS, there is also a degree of freedom to control products by adjusting the applied potential.

Promising results have been reported using this technique. However, the ECDS technology is still in its early development stage, and further research is needed to push this technology toward commercialization. With the intension to facilitate the research and development, we reviewed the progress of this technology, and discussed the principles and technology development for the electrochemical desulfurization of fossil fuels. As the organic compositions of coal, petroleum products, and bitumen are similar, a method for one can provide insight into methods for the others.

Several methods have been employed for electrochemical desulfurization, which can be generally grouped into two categories. The methods that facilitate electrochemical desulfurization by applying a fixed potential or drawing a fixed current can be categorized as static methods. Other methods where the applied potential changes over time, such as linear or square wave voltammetry can be categorized as dynamic methods. Bio-electrochemical desulfurization (BECDS) , which uses microorganisms such as bacteria to mitigate the sulfur removal in fossil fuels, is classified as an independent method.⁶⁶

3.1 Common electroreduction

As early as 1960s, Sterber proposed electrochemical reduction (ECR) for coal desulfurization. However, ECR methods were inefficient and rigid without reasonable reductants. Later on, Lalvani^62,67 studied the electrochemical oxidation (ECO) for coal desulfurization. It can be easy to find excellent oxidizers and get a high desulfurization efficiency. After that, ECO has become the prevailing ECDS method in the world, while ECR is rarely researched. Oxidation in the anode is the key step for ECDS. During the oxidation process, the coal structure is usually destroyed to an appreciable, or significant, extent.^68–70 While ECR methods can utilize cathodic reduction to change some chemical compounds into strong reducing agents which convert S into H₂S and S²⁻, thus releasing S from coals. Meanwhile, ECR can proceed under mild conditions, as well as producing high-purity hydrogen.^60,61,71 Thus, this method has a prospective future for coal desulfurization and hydrogen production, if outstanding reductants are found.

ECDS is based on the reduction and/or oxidation of sulfur-containing compounds in fossil fuels.^72–77 The electrochemical cathodic reduction of organic sulfur compounds (expressed as R–SH) leads to the formation of H₂S, as shown in eqn. (7):

R–SH + 2H⁺ +2e⁻ = R–H + H₂S (7)

The resulting H₂S can be removed by a gas/liquid separation process or the aformentioned Claus process. The electrochemical anodic oxidation of organic sulfur compounds is expressed in eqn. (8):

R–SH + 4OH⁻ − 4e⁻ = R–H + SO₂ + 2H₂O (8)

In eqn. (8), SO₂ represents RSH oxidation products, which might be O-containing RSH molecules (e.g. RSO_xH). The addition of oxygen to sulfur-containing compounds increases the polarity of the sulfur compounds so that they can be removed by extraction with polar solvents or by adsorption with polar adsorbents.

Electrooxidation of CWS occurred on the interface between coal particles and electrode, thus sulfur macromolecules inside the coal were not removed by electrochemical desulfurization. Decreasing the size of the coal particles can improve electrolysis desulfurization effects,⁷⁸ however, there are still some organic sulfur (OS) structures inside coal particles, e.g. thioether, thiophenol, thiophene, and so on, leading to the low removal ratio of OS in coal.⁷⁹ Some researchers found that CWS electrolysis in water-organic solvents such as methanol, ethanol and acetone increased the desulfurization effect.⁸⁰ The desulfurization efficiency of CWS was improved with the addition of organic solvents as the organic solvent used would extract some of the OS, and lower the energy of some chemical bonds.⁷⁹ The conductivity of these organic solvents was less than that of water with electrolyte, so that the electrolysis process was also influenced by them. On the other hand, CWS electrolysis generates pure hydrogen at the cathode. If this hydrogen reacted with the OS, electrolysis HDS was achieved on the cathode. But there is no appropriate hydrogen donating solvent at the cathode, and the hydrogen donating effects of water is not obvious. Thus, it is necessary to seek the appropriate hydrogen donating solvents or electrodes for hydrogen evolution at the cathode to achieve the HDS reaction. At present, we investigated the ECDS of CWS in the mixed solvent water–alcohol (H₂O–C₂H₅OH). Compared with alcohol, water is the prime substance providing the protons. A mass of protons in the electrolyte is beneficial for coal hydrogenation in the cathode, while NaBH₄ reacts more easily with water than with sulfides in coal. Therefore, the water content in the electrolyte has a significant effect on desulfurization efficiency (Fig. 7a). Oxygen in water molecules and sulfur in sulfides have one electron pair, but the polarity of a water molecule is stronger than that of a sulfide, so NaBH₄ is prone to react with water. It can be concluded that a low water content of the electrolyte is beneficial for the ERD process. On the contrary, the hydrogen yield shows a positive correlation with water content (Fig. 7b).⁸¹

Fig. 7 Desulfurization of CWS under atmospheric conditions via sodium metaborate (NaBO₂) electroreduction in the isolated system. (a. Effect of water content on desulfurization efficiency; b. Effect of water content on hydrogen yield). Process conditions: 12 g L⁻¹ of NaBO₂, 3.0 V of electrolytic voltage, 5 h of electrolytic time, 50 g L⁻¹ of CWS, 140 mesh of coal particle size, and 1.0 g L⁻¹ of NaOH.⁸¹

3.2. NaBO₂ electroreduction

On the whole, NaBH₄ reduction shows incomparable advantages: mild conditions, short reaction time, high hydrogen donating effect and high efficiency for reductive desulfurization,^4,19 but it is also an expensive, incomplete reaction with excessive waste. NaBO₂,⁸² the end product of both the hydrolysis and electrooxidation of NaBH₄,¹⁷ is much cheaper and more stable. It is of great importance to convert NaBO₂ into NaBH₄. Electrochemical methods can realize boron recycling only by the process of discharge and recharge,⁸¹ that is to say, it is a good idea to combine chemical and electrochemical methods of realizing boron (B) recycling for coal desulfurization by NaBH₄ reduction. It can resolve the problem of deoxidizer costs and realize the recycling of the filtrate. Besides, this kind of clean reduction method only uses electricity and green solutions (H₂O), so it reduces additional pollution, realizing reasonable application and sustainable development of resources (Scheme 2). NaBO₂ electroreduction for fuels desulfurization is based on converting NaBO₂ into NaBH₄ by electrolysis in the cathode. Then, sulfur (S) can be reduced into H₂S or S²⁻, separated from fuels.

Scheme 2 Green chemical pathway of converting NaBO₂ into NaBH₄ to realize B recycling

3.2.1 Non-isolated system. A non-isolated system can be widely used in the electrochemical reaction, which is simply operated. The cathode and anode can simultaneously present reductive and oxidative performance. In addition, parts of the sulfur compounds in fuels can be oxidated into SO₄²⁻ and removed easily by washing and filtering. As Scheme 3 and Fig. 7 show, the reductive electrode basically removes OS from the mercaptan (R–SH) and the thioether (R–S–R¢) (eqn. 9–14). Electrolysis in alkaline systems allows OH⁻ lose charges on the anode surface and create active oxygen free radicals (H₂O˙, O˙, H₂O₂˙, O₂˙, etc.). These active free radicals attack organic sulfur (OS) in coals, which is oxidated into polysulfide. Furthermore, the polysulfide is oxidated into sulfoxide (R–S=O–R¢) and sulphone (R–O=S=O–R¢). Then sulphone is converted into sulfonic compounds or SO₄²⁻ by hydrolysis (eqn. 15–19).^83,84 The oxidizing electrode removes fat sulfur and thiophenic sulfur. The participation of anodic oxidation is beneficial for OS reduction. The desulfurization efficiency of CWS increased with the increase of electrolytic time (Fig. 8)

Cathode: BO₂⁻ + 6H₂O + 8e⁻ → BH₄⁻ + 8OH⁻ (9)

BH₄⁻ + 2H₂O → BO₂⁻+ 8H* (active hydrogen atom) (10)

(1) Pyrite sulfur (PS) removal: FeS₂ + 2H* → Fe + S + H₂S↑ ^4,19 (11)

(2) Fat-sulfur removal: 2H* + R–SH → R–H + H₂S↑ (12)

4H* + R–S–R¢ → R–H* + R¢–H* + H₂S↑ (13)

H₂S + H₂O₂ → S↓ + 2H₂O (14)

Anode: 4OH⁻ − 4e⁻ → O₂[O] (active oxygen free radicals) + 2H₂O (15)

(3) Fat-sulfur removal: 2R–SH + [O] → R–S–S–R + H₂O (16)

[O] + R–S–S–R → R–S–S(O)R¢ (17)

4[O] + R–S–S–R → R–S(O₂)–S(O₂)R¢ (18)

R–S(O₂)–S(O₂)R¢ + 6H₂O → R–OH + R¢–OH +10H⁺ + 2SO₄²⁻ (19)

Fig. 8 Schematic diagram of ECD process for CWS in the non-isolated alkaline NaBO₂ system.⁸⁴

Scheme 3 Electrolysis desulfurization of CWS without membrane⁸³

(4) Dibenzothiophene (DBT) sulfur removal (special example inScheme 4)

Scheme 4 Sulfur removal pathway of DBT by electrooxidation in the alkaline NaBO₂ electrolyte

3.2.2 Isolated system. According to the principle of different electric charges attracting each other, the negative charge of BO₂⁻ will move to the anode, resulting in a decrease of BO₂⁻ concentration near the cathode, which needs to be adjusted for. Meanwhile, a produced negative charge (BH₄⁻) may be attracted to the anode, resulting in BH₄⁻ being consumed because of anodic oxidation.⁸² Before Fairbridge et al.⁸⁵ described a desulfurization process using a polymer electrolyte membrane system. The process involved electrocatalytic hydrogenation of the organic compound through electroreduction in the cathode compartment, proton production through water electrolysis in the anode compartment, and proton transfer from anode to cathode through the membrane, as shown in Fig. 9. In the process, sulfur is freed from the organic compound through electroreduction and then combines with hydrogen to form hydrogen sulfide that can be easily removed. An advantage of this system is that it can operate at low temperatures and pressures relative to conventional HDS conditions. Consequently, the isolated apparatus 1 or 2 in Fig. 9 & 10 is designed for the ERD. It can avoid anions (BH₄⁻, BO₂⁻, etc.) moving to the anode and can also decrease the coal calorific value loss caused by anodic oxidation.⁸² The innovative isolated apparatus can surmount the above disadvantages and completely utilize the oxidation. The disadvantage of isolated apparatus 1 is that it can not be utilized for high concentrations of CWS due to blockage of the ion exchange membrane, although it is used for gasoline desulfurization.⁵⁹ However, the isolated apparatus 2, without an ion exchange membrane, designed by us is an alternative for any liquid fuel desulfurization. We found that the NaBH₄ concentration and desulfurization efficiency of CWS increased with the electrolytic time, while the relationship of NaBH₄ yield and electrolytic time accorded with a first order kinetic reaction (Fig. 10).⁸²

Fig. 9 ECDS in the alkaline NaBO₂ electrolyte with an ion exchange membrane.

Fig. 10 ECDS in the alkaline NaBO₂ electrolyte without an ion exchange membrane.

In the cathode, NaBO₂ was transformed into NaBH₄ by electrochemical reduction, and the sulfur compounds in CWS were converted into H₂S and released from the coal particles. In the anode, the removed sulfur or H₂S was partially converted into S or SO₄²⁻, leaching and filtering away from the electrolytic cell (Fig. 11).

Fig. 11 Schematic of ECDS in the alkaline NaBO₂ system without a cation-exchange membrane.⁸²

4. Integrative process of CECR

As described above, no matter whether chemically reductive desulfurization by sodium borohydride (NaBH₄) reduction or electrochemical reductive desulfurization by sodium metaborate (NaBO₂) electroreduction was performed, the desired desulfurization efficiency for CWS was obtained. Obviously, it is necessary to combine the NaBH₄ reduction with the NaBO₂ electroreduction for CWS desulfurization to realize boron (B) recycling and the integrative process of chemical and electrochemical reduction (CECR), described as follows. At first, NaBH₄ reduction can be used for fuel desulfurization. Then, the by-product of NaBO₂ can be reduced into NaBH₄ once again by electroreduction. This desulfurization system can be easily realized, while some problems, such as operation expenses, electrochemical efficiency, which should be resolved in further works. According to our research, the integrative process of CECR for coal or CWS can be carried out as in the following industrial schematic. Firstly, raw coal can be crushed to a certain particle size. Then, pulverized coal is made into raw CWS by adding water. After that, ECDS of the prepared CWS is carried out by the CECR apparatus. Significantly, the filtrate can be reused for coal washing at the first step of the industrial process (Fig. 12).

Fig. 12 Schematic diagram of integrative chemical and electrochemical desulfurization process of CWS via NaBH₄ reduction (ECDS mechanism, ECDS experimental apparatus, electrode preparation and industrial procedure).

5. Future outlook

(1) Ionic liquids (utilization and recycle)

Ionic liquids (ILs) are organic salts which are typically a liquid at ambient temperature resulting from the combination of organic cations and various anions, and have aroused increasing interest owing to their unique properties such as negligible vapor pressure, good dissolving ability, a wide electrochemical work window and high ionic conductivity, thus they have been utilized as a green solvent in many fields.^86–88 In the patent and academic literature, the term “ionic liquids” now refers to liquids composed entirely of ions that are fluid around or below 100 °C.⁸⁹ Recent literature describes their potential applications for embalming fluids,⁹⁰ in ion drives for space travel,⁹¹ for desulfurization of fuels.⁹²

Gong⁹³ has investigated three kinds of ILs including 1-butyl-3-methylimidazolium chloride (C₈H₁₅N₂Cl), 1-butyl-3-methylimidazolium bromide (C₈H₁₅N₂Br) and N-butylpyridinium bromide (C₉H₁₄NBr) for ECDS of CWS in the HCl electrolyte. The desulfurization reaction occurred in the electrolysis apparatus with or without a membrane, as shown in Fig. 13A. With the addition of three ILs, electrolysis desulfurization efficiency increased, improving organic sulfur removal. CWS electrolysis on the cathode achieved hydrogenation of coal under mild conditions, and electrolysis desulfurization of ILs with a hexatomic ring as an additive was higher than that of the five-membered ring (Fig. 13B, 13C). Firstly, ILs used as solvent extracted parts of the sulfur structure from solid to liquid phase, thus the desulfurization reaction occurred easily on the electrode surface. Secondly, during electrolysis desulfurization on the cathode, the ILs acted as a kind of hydrogen donating solvent and accelerated hydrogenation of the organic sulfurs’ structure.

Fig. 13 Desulfurization of electrolyzed coal water slurry in HCl system with ionic liquid addition ⁹⁴

Meanwhile, ILs have been examined for possible applications related to green chemical processes, such as liquid/liquid extraction, gas separations, electrochemistry and catalysis.^94–102 These liquids are easy to handle because of their non-volatility, non-flammability, and high thermal stability. The ILs have been applied for the selective removal of sulfur from fuels in recent years because ILs are easily regenerated from adsorbed S-containing compounds by distillation or by dissolution in water, where ILs are air- and moisture-stable at low temperature and non-corrosive. Hence, ILs can be used appropriately under the conditions of ECDS for liquid fuels.

(2) Electrodes (alloy or BDD, etc.)

The electrode is a significant sub-unit of electrolysis apparatus. Commonly, the oxidation and reduction reactions occur near the electrodes by electrical transfer. It is attractive for researchers to seek a high-performance electrode material (environmental friendly, low energy consumption, negative potential, etc.), in the electrochemistry field. Furthermore, alloy electrodes can be developed for ECDS because of their hardness, low expense, corrosion resistance, and so on.

Boron-doped Diamond (BDD) is another well-characterised electrode material with well known attributes, such as hardness, chemical inertness, optical transparency, high thermal conductivity and electrical conductivity (depending on the doping level).^103–109 The electrochemistry on BDD is not complicated by extraneous change-transfer reactions and is stable over a wide electrochemical window,¹¹⁰ thereby making it an ideal substrate for electrolysis at a very negative potential in ILs.¹¹¹ In the future, it is a priority to try BDD as a basal electrode material for ECDS in the alkaline NaBO₂ system, reducing the electrode potential and formation of hydrogen.

Once the basal electrode is chosen, some single or composite catalysts (Ni, Co, Mo, NiMo, CoMo, etc.) can be mingled in the surface of the alloy or BDD electrode. On one hand, the development of the catalyst doped electrode will realize the catalyst recycling, reducing the process expense and secondary pollution due to the wastage of catalysts. On the other hand, it can improve the activity of catalyst and conversion efficiency.

(3) Hydrogen production

As an important energy in the current or prospective world, hydrogen is mainly a by-product of ECDS, derived from electroreduction at the cathode and hydrolysis of NaBH₄, which can be easily separated and collected. The hydrogen storage properties could be enhanced by promoting the NaBH₄ yield.

Acknowledgements

This work is financially supported by the National High Technology Research & Development Program of China (“863” program, Grant No. 2009AA062603). The thoughtful comments provided by anonymous editor and reviewers are greatly appreciated.

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