Reengineering of the carbon-to-acetylene process featuring negative carbon emission

Abstract

Various sources of carbon can be converted into acetylene (C₂H₂) by using the key intermediate calcium carbide (CaC₂). However, the production of CaC₂ is a typical energy-intensive process, accompanied by considerable carbon dioxide (CO₂) emissions and a large amount of industrial solid waste. In this study, a sustainable methodology for carbon-to-acetylene and carbon monoxide (CO) co-production as well as CO₂ capture based on BaCO₃−BaC₂−Ba(OH)₂−BaCO₃ looping was first established, in which BaC₂ replaced CaC₂ as the key intermediate of the carbon-to-acetylene process to generate C₂H₂. The kinetic behavior investigation of the BaC₂ formation indicated that the solid-phase synthesized BaC₂ is a promising intermediate for the carbon-to-acetylene conversion owing to its faster kinetics, lower formation temperature, and no carbon dioxide release compared with those observed for the CaC₂ production. Moreover, the lab-scale recovery of barium to carbide formation was conducted as the proof-of-concept to validate the coupling process of carbon-to-acetylene with CO₂ capture based on Ba looping, resulting in less carbide slag waste and negative carbon emission. The facile co-production of carbon monoxide, environmentally friendly process, and convenience of large-scale production, as well as possible independent manufacturing of fossil resources, make barium carbide-based carbon-to-C₂H₂ -CO a promising key chemical platform for sustainable development. The proposed technology would provide new insights into the reengineering process of carbon to chemicals.

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Introduction

While artificial synthetic chemicals are rapidly gaining importance as a cornerstone of modern society, they rely heavily on unsustainable fossil resources and pose a serious threat to the environment. Currently, one of the key global concerns in industrial chemistry is to design and invent green reactions and processes to sustainably prepare chemicals by using renewable resources, clean technologies, and revalorization of waste.^1–4 With a thermodynamically unstable carbon–carbon triple bond, acetylene can undergo diverse coupling/addition reactions, serving as a versatile building block for the syntheses of organic compounds, including vinyl chloride, acrylonitrile, vinyl acetate, 1,4-butanediol (BDO), etc.^5–8 C₂H₂ is also widely used in welding, brazing, cutting, hardening, texturing, and thermal spraying.⁹ Recently, C₂H₂ has been identified as a primary core for new drugs and biologically active molecules, with potential access to novel carbon nanomaterials and is promising for molecular 3D printing.^10–13 In particular, with the increasing limitations on the fossil resource supply, C₂H₂, which can be conveniently produced from renewable biomass instead of fossil feedstocks, could usher in new opportunities to be a key chemical platform for sustainable development.¹⁴

Since 1892, C₂H₂ has been industrially produced by the calcium carbide method using coal as a carbon source.¹⁵ In fact, inexpensive and available sources of carbon materials, including coke,^16,17 anthracite,^18,19 biochar,^20,21 pyrolysis char,²²etc., can react with lime (CaO) to produce calcium carbide (3C + CaO = CaC₂ + CO), which is a key intermediate that reacts with water to produce acetylene (CaC₂ + H₂O = Ca(OH)₂ + C₂H₂). In the coal chemical industry, the coal-to-acetylene process is shorter and lower equipment investment is required, compared with the coal-to-olefin process led by coal gasification. However, because of the highly endothermic nature of CaC₂ formation, this seemingly simple chemical reaction has to be performed industrially in an electric arc furnace at 2000–2300 °C for approximately 2 h to overcome dynamic and thermodynamic restrictions (poor mass and heat transfer as well as a slow reaction rate).^23–26 In addition, considerable carbon dioxide (CO₂) emissions generated from the conversion of limestone (CaCO₃) to CaO, as well as a large amount of solid waste of the by-product CaC₂ slag (the main component is insoluble Ca(OH)₂),²⁷ originated from C₂H₂ generation, have exacerbated the calcium-based carbon-to-acetylene process.^28,29 The disadvantages of high temperature, high energy consumption, and high waste emissions in calcium carbide-based C₂H₂ production have seriously restricted the whole acetylene chemical industry. In the past few decades, the plasma reactor,^30,31 the rotary kiln process,³² the microwave heating method,^33,34 the oxygen-thermal method,^35,36 and catalytic synthesis^37,38 have been tried to produce CaC₂. However, these efforts, mainly focused on the dynamic condition optimization and the new reactor development for CaC₂ synthesis, have made limited progress in achieving green manufacturing of calcium carbide-based C₂H₂.

As mentioned above, the drawbacks in calcium carbide-based C₂H₂ production are inherently related to the corresponding chemical reactions associated with the calcium cycling of CaCO₃−CaO–CaC₂−Ca(OH)₂. That is, on the basis of calcium carbide, it should be very difficult to achieve a sustainable acetylene chemical industry, as shown in Fig. 1a.

Fig. 1 Schematic of the carbon-to-acetylene process. (a) Calcium-based carbon-to-acetylene process. (b) Coupling process of carbon-to-acetylene with CO₂ capture based on Ba looping.

Bearing the same C₂²⁻ anion as CaC₂, barium carbide can react with water to release C₂H₂,^39,40 indicating its potential as the key intermediate in carbon-to-acetylene production. As early as 1892, a product containing approximately 38 wt% BaC₂ was first synthesized by heating a mixture of barium carbonate, carbon, and magnesium at 700–800 °C (BaCO₃ + 3Mg + C = BaC₂ + 3MgO + CO).⁴¹ Subsequently, Thomson confirmed the formation of BaC₂ by the reaction of BaO with carbon black in a vacuum at 1400 °C (BaO + 3C = BaC₂ + CO).⁴² Later, Napier reported that barium carbide could be produced at approximately 1650 °C under normal pressure.⁴³ The above studies indicated a more moderate synthesis temperature and less energy consumption for barium carbide. Unfortunately, owing to the global prevailing petrochemical processes based on the cost-effectiveness of ethylene and propene, the research on the coal-to-acetylene process has been interrupted considerably, and barium carbide has not received sufficient attention, not to mention its consideration as a commercial intermediate for carbon-to-acetylene production. Recently, with the expanding market demand for degradable plastics, polyvinylidene fluoride, etc., the importance of carbon-to-acetylene chemistry has been reassessed, especially in areas containing more coal and less oil. For example, more than 42 million metric tons of CaC₂ were produced in 2021 in China. Hence, the carbon-to-acetylene process based on barium carbide must be re-evaluated.

In this study, a proof-of-concept of the negative carbon emission technology for the carbon-to-acetylene process and carbon dioxide utilization based on the barium cycle was first proposed (Fig. 1b). Pellets of a BaCO₃ and coconut shell carbon mixture were used as the reactants to demonstrate the novel methodology of C₂H₂ production based on the solid-state synthesis of barium carbide. Emphasis was placed on the dynamic behaviours during the solid-phase formation of carbide, which was investigated using a thermogravimetric (TG) analyser and a high-temperature fixed-bed reactor equipped with an online gas chromatography (GC) instrument, respectively. In addition, the barium recovery and reuse for the carbide production were explored to demonstrate the coupling process of carbon-to-acetylene with CO₂ capture based on practical, not conceptual, barium looping, resulting in less carbide slag waste. This work indicates that the barium-based carbon-to-acetylene process was more economical, more environmentally friendly than that based on calcium carbide, and would be executable with bright application prospects.

Results and discussion

Reaction behaviours of BaCO₃ and C in the thermogravimetric analyser

A milligram-scale reaction was conducted in the simultaneous thermal analyser to explore the reaction behaviours of C and BaCO₃ and the result is shown in Fig. 2a and Table S1.† A two-stage process was clearly observed for the mass loss at temperatures ranging from room temperature to 1300 °C, implying that two reactions occurred during the heating process. The first stage started at approximately 580 °C and ended at 950 °C, where a maximum mass loss rate was observed at approximately 800 °C. In this stage, the mass loss was 22.6 wt%, which was very close to the theoretical mass loss of 22.8 wt% in the thermodynamically advantageous reaction of barium carbonate with carbon to form barium oxide and carbon monoxide (BaCO₃ + C = BaO + 2CO), as predicted by thermodynamic analysis.⁴⁴ The result indicated that there should be less CO₂ emissions caused by direct BaCO₃ decomposition (BaCO₃ = BaO + CO₂), benefiting from the noticeable distinction in thermodynamic reaction temperatures between 1047 °C (BaCO₃ = BaO + CO₂) and 700 °C (CO₂ + C = CO).

Fig. 2 Reaction behaviours of BaCO₃ and C. (a) TG/DTG curves of a mixture of C/BaCO₃. (b) TG/DTG curves of a mixture of C/CaCO₃.

When the heating temperature was greater than 950 °C, the first mass loss process ended, while the second mass loss process started slowly, due to the formation of BaC₂ from the prepared BaO and the remaining C. In this stage, the DTG curve displayed a comparable trend, but with lower values in terms of the loss rate than those in the first stage. The result suggested a lower gas release amount and a slower reaction rate of BaC₂ formation than those of BaCO₃ decomposition. With an increase in the heating temperature, the mass loss rate constantly increased and then peaked at approximately 1065 °C, where the maximum formation rate of BaC₂ was observed. Subsequently, the mass loss rate progressively decreased because of the constant consumption of BaO and C. At a temperature of 1300 °C, the mass loss was 9.6 wt% in the second stage of the reaction. According to the theoretical mass loss of 14.8 wt% for the BaC₂ formation reaction (BaO + 3C = BaC₂ + CO), 64.9% of the BaO obtained in the first stage should be converted into BaC₂ during the second process.

Different from that observed in the barium system, the mass loss of the CaCO₃ and C mixture with a C/Ca molar ratio of 4 occurred mainly in the low temperature range, shown in Fig. 2b. The DTG curve clearly exhibited only one major mass loss peak in the whole investigated temperature range, which started at 480 °C and peaked at 685 °C with a mass loss of 31.4 wt%, and no clear DTG peak was observed at higher temperatures. The result implied that the main mass loss was attributed to the decomposition of CaCO₃, and that CaC₂ was hardly formed during the heating process. As shown in Table S1,† if CaCO₃ was completely decomposed into CO₂ and CaO (CaCO₃ = CaO + CO₂), the weight loss should be 29.7 wt%, while if CaCO₃ was decomposed to CO and CaO (CaCO₃ = CaO + CO₂, CO₂ + C = 2CO), the weight loss should be 37.8 wt%. The experimental mass loss related to the decomposition of CaCO₃ was 31.7 wt%, which was between 29.7 and 37.8 wt%, signifying that approximately 60.9% CO and 39.1% CO₂ were produced during CaCO₃ decomposition. The result should be due to the insufficient distinction in the thermodynamic reaction temperature between CaCO₃ decomposition (886 °C) and the Boudouard reaction (700 °C), showing that a large amount of produced CO₂ from the CaCO₃ decomposition would release directly from the reaction system before reacting with C to convert to CO. The above comparative TG investigation results indicated that BaC₂, as a key intermediate in the carbon acetylene process, has advantages over CaC₂ due to its lower formation temperature and no CO₂ emissions.

Reaction behaviours of BaCO₃ and C in the fixed-bed reactor

To confirm the inference obtained by TG analysis, BaC₂ and CaC₂ were synthesized by using their corresponding pellet reactants in the high-temperature fixed-bed reactor, and the gas-phase products obtained during the heating processes were monitored by online GC. Fig. 3a and b show the temperature dependence of the gas by-products by time-on-stream in barium and calcium systems, respectively. Not surprisingly, Fig. 3a clearly shows two separate CO peaks centred at 1025 °C and 1500 °C, respectively. Their integral area ratio is 1.96 : 1, close to the theoretical value of 2 : 1 (BaCO₃ + C = BaO + 2CO and BaO + 3C = BaC₂ + CO), conforming the two-step reaction route for the formation of BaC₂ from BaCO₃ and C. CO₂ was not detected in the outlet gas of the barium system during the entire reaction, confirming the absence of CO₂ emissions in the barium system. Compared with the TG analysis result, the two CO peaks appear at higher temperatures. This should be due to the negative effects of mass and heat transfer in a large dosage reaction system, where pellet reactants replaced the powder materials used in TG analysis. Moreover, the solid products obtained at the ends of the above two reaction stages were separately collected and characterized by XRD analysis, and their XRD spectra are shown in Fig. 4a. The XRD pattern of the solids obtained at the end of the first reaction stage (heating for 250 min) revealed characteristic peaks of BaO (JCPDS 65-2923) with the Fm3̄m space group, and no characteristic peaks of BaCO₃ were detected. The results indicated that BaCO₃ was completely transformed into BaO when the reactants were heated to approximately 1150 °C. In the XRD pattern of the solids obtained after heating for 600 min, i.e., the end of the second reaction stage, the characteristic diffraction peaks of BaO disappeared; meanwhile, new diffraction peaks were observed at 23.8°, 25.2°, 28.6°, 38.5°, 40.9°, 43.5°, and 47.9°, which are indexed to BaC₂ (JCPDS 65-2661) with the I4/mmm space group. The BaC₂ content in it was 97.5% by measuring the corresponding C₂H₂ amount from the hydrolysis of the solid products (BaC₂ + H₂O = C₂H₂ + Ba(OH)₂).

Fig. 3 Gas phase product analysis of the Ca system and Ba system. Temperature dependence of the gas products by time-on-stream in the (a) barium system and (b) calcium system.

Fig. 4 XRD patterns of reactants and their corresponding products. (a) Ba system. (b) Ca system. (1) reactants and (2) heating products at 250 min (Ba system) and 325 min (Ca system), and (3) heating products at 600 min.

Fig. 3b shows the temperature dependence of the gas by-products by time-on-stream in the calcium system. It is obvious that in the lower temperature region, the CO peak overlapped significantly with the CO₂ peak, corresponding to CaCO₃ decomposition. The integral area ratio of CO to CO₂ was 2.5 : 1, indicating 71.4% of CO and 28.6% of CO₂ in the outlet gas. In the high temperature range, with a continuous increase in the temperature to 1550 °C, the CO concentration increased, but still remained at a low level, and no clear CO peak was observed in the outlet gas. Fig. 4b shows the XRD pattern of the solid products obtained by heating for 325 min and 600 min. Both the solid products only exhibited obvious characteristic peaks of CaO (JCPDS 37-1497), and the CaC₂ characteristic diffraction peaks were not clearly detected, indicating that although a small amount of CO was observed when the temperature was beyond 1150 °C, the amount of CaC₂ formed was very small. The results are in good agreement with those obtained from TG analysis, confirming that BaC₂ was a promising candidate to replace CaC₂ as the key intermediate of carbon-to-acetylene for the lower manufacturing temperature without CO₂ emission.

To explore the effects of kinetic conditions on the formation of BaC₂, a series of experiments were conducted at 1400–1550 °C with different temperatures and holding times in a high-temperature fixed-bed reactor. Fig. 5a shows the BaC₂ yields while Fig. 5b shows the corresponding average reaction rates. As shown in Fig. 5a, the BaC₂ yield exhibited a strong dependence on the temperature and reaction time. Notably, the reaction proceeded rather rapidly in the first 30 min, then gradually slowed down, and finally reached a steady state after approximately 1 h of preservation. Moreover, with an increase in the reaction temperature, a shorter heating time was needed to reach the same BaC₂ yield. For example, a 75.2% BaC₂ yield was obtained after heating for 60 min at 1400 °C, and a 75.8% BaC₂ yield was obtained after heating for 30 min at 1500 °C. At 1550 °C, an 81.6% BaC₂ yield was achieved after heating for 20 min, and up to a 97.5% yield after heating at 60 min. Pillai et al.³³ reported that only a 14.1% CaC₂ yield was obtained after heating for 30 min at 1700 °C when 10 g of pressed pellets of the graphite and CaO fine powder mixtures were used as the reactant. These results confirmed that the reaction conditions for the synthesis of barium carbide were considerably milder than those utilized for the synthesis of CaC₂.

Fig. 5 The effects of kinetic conditions on the formation of BaC₂.(a) Dependence of the BaC₂ yield and Ba mass balance on the reaction temperature and time; (b) BaC₂ reaction rate dependence on the reaction temperature and time.

Considering the fact that calcium would be lost as calcium vapor during the production of CaC₂,⁴⁵ the mass balance of barium before and after barium carbide formation was investigated further. In our experimental ranges, elemental Ba was not detected in the outlet gas, and the loss of Ba, defined as the relative difference between the total amount of barium in the reactants and solid products, was less than 0.3% (Table S2†); this value was possibly related to a systematic error in the experimental operation. The results were in good agreement with the thermodynamic prediction that barium carbide cannot react further with barium oxide to form barium vapor and carbon monoxide (BaC₂ + 2BaO = 3Ba + 2CO) at temperatures less than 2175 °C,⁴⁴ indicating the good stability of BaC₂ at high temperatures.

Morphology of the synthesized solid BaC₂

Fig. 6 shows the SEM images and Fig. S1† shows the corresponding EDS mapping images of the reactants and the heated solid products obtained under different reaction conditions. As shown in Fig. 6a and a₁, the rod-shaped BaCO₃ particles and the irregularly shaped coconut shell carbon particles were physically mixed together before the reaction. These particles exhibited clear sharp edges, and the particle size of the majority of BaCO₃ was about 5–10 μm and the carbon was about 5–30 μm. After heating at 1150 °C, the rod-shaped BaCO₃ particles disappeared (Fig. 6b and b₁), and some new aggregated fine particles were observed and easily distinguished from the larger carbon particles with no obvious morphological changes. The result was in good agreement with the TG and XRD analysis results that BaCO₃ was decomposed into BaO first. Fig. 6c and c₁ show the SEM images of the heated solid products with a BaC₂ content of 74.1 wt% obtained at 1400 °C for 60 min. The aggregated fine BaO particles, mainly observed in the solids heated at 1150 °C, seemed to have migrated to carbon particles and developed into new larger ones with an irregular shape. Moreover, the less isolated carbon particles were observed. The particle surface with several small pores was flatter compared with the BaO particles. It can be speculated that a partial liquid-phase formed from a microscopic perspective, during the BaC₂ formation process, and the pores could be due to the release of CO from the reactive interfaces. The results provided evidence to propose the diffusion of barium into the carbon materials and the formation of BaC₂ on the internal and external surfaces of carbon particles, which was similar to the mass-transfer behaviours of CaO and C during CaC₂ formation.¹⁷Fig. 6d and d₁ show the SEM images of the BaC₂ product with a 91.4 wt% content obtained by heating at 1550 °C for 30 min. The isolated carbon particles that were clearly in the other samples disappeared, which was in good agreement with the high BaC₂ content. The particles developed into new larger ones rather than that formed at 1400 °C and all particles in these samples exhibited clear shapes and edges. As shown in Fig. S2,† all of the heated pellets rather retained their original shape and did not melt or stick together, confirming the solid conversion mechanism of BaC₂, from a macroscopic perspective. Differing from the liquid-state synthesis of the industrial production of CaC₂, the solid conversion mechanism of BaC₂ was confirmed facilitating continuous operation during future large-scale industrial production. Moreover, Fig. 6c₁ and d₁ clearly show that the obtained barium carbide products are very porous, and would be very easy to react with water to generate C₂H₂, and would greatly facilitate the industrial operation in the carbon-to-C₂H₂ process. We know that in the calcium-based carbon-to-acetylene process, the liquid-phase formed CaC₂ products are microstructure-densified and must be broken into small pieces of several millimetres before reacting with water to generate C₂H₂. The CaC₂ dust generated in the crushing process is so hazardous that this process is typically carried out underground.

Fig. 6 SEM images of the reactant and products: (a and a₁) reactant and (b and b₁) heating products at 1150 °C, (c and c₁) at 1400 °C for 60 min, and (d and d₁) at 1550 °C for 30 min.

Ba recovery to carbide synthesis

In the industrial calcium-based C₂H₂ generation process, each ton of industrial CaC₂ (about 80% pure) would produce 1.2 tons of dry CaC₂ slag, composed primarily of Ca(OH)₂ and other water-insoluble inorganic salts originating from the ashes of carbon raw materials. In practice, CaC₂ slag barely can be recycled for the production of carbide. One reason is the lower reactivity of the recovered CaO from the slag than that of the fresh one.⁴⁶ The other important reason is no economic method to remove the inorganic impurities from the recovered CaO, which would accumulate in the arc furnace and lead to increased energy consumption and operational risks, as well as a lower quality of the carbide product.

Fortunately, the carbon-to-C₂H₂ process based on barium looping is another matter. Different from Ca(OH)₂, the soluble concentration of Ba(OH)₂ in water was strongly dependent on the temperature (Fig. S4†). In cold water, the solubility of barium hydroxide was almost as low as that of Ca(OH)₂, but in hot water of 80 °C, the solubility of Ba(OH)₂ reached 100 g/100 g water. Hence, in the barium system, water-insoluble inorganic impurities can be easily separated by filtration from the hot Ba(OH)₂ solution, which was obtained from the exothermic reaction of the generation of C₂H₂ gas. This excellent water solubility of Ba(OH)₂ suggested that less solid waste would be produced in the barium-based carbon-to-acetylene process.

Considering that Ba(OH)₂ can easily absorb CO₂ to form BaCO₃, the recovered Ba(OH)₂ can be used directly as a CO₂ adsorbent to regenerate BaCO₃. Here, BaCO₃ was regenerated from its carbide product by simple exposure to water-saturated CO₂ gas, because there were little impurities in the reactants. As can be observed from Fig. S5a,† the regenerated BaCO₃ samples exhibited a smaller particle size than that of the fresh one. When the regenerated BaCO₃ was used as the reactant under the same conditions, the BaC₂ yield reached 97.0%, similar to that of the fresh one (97.2%), indicative of the good activity of the recovered BaCO₃.

On this basis, the carbon-to-C₂H₂ process based on barium looping is proposed and shown in Fig. 7. The entire system of the new process can be divided into three units: the barium carbide production unit, the C₂H₂ production unit, and the CO₂ capture and barium carbonate recovery unit. In the barium carbide production unit, the powder mixture of barium carbonate and carbon-rich materials (biochar, coke, pyrolytic char, etc.) would be granulated into small pellets using a pellet mill. After drying, the dried pellets would be put into a high-temperature reactor to produce barium carbide. In this unit, the off-gas mainly containing CO and dust would be delivered into a separator and a heat exchanger to obtain dust-free cooled CO gas, which would be transferred to the next process, while the recovered energy would be used to preheat the reactants. The hot barium carbide solids would be cooled by a heat exchanger either directly or indirectly; the as-obtained cooled products would be transported to storage bins. In the C₂H₂ production unit, the carbide from the storage bins would be delivered into the C₂H₂ generator, where crude C₂H₂ and the hot barium hydroxide aqueous solution with some insoluble impurities would be formed. In this unit, the obtained barium carbide is porous and can react quickly with water to generate acetylene, so the industrial acetylene generator for the calcium-based process could be fit for the new process after small modifications. The crude C₂H₂ would be transferred to the refining process to obtain pure C₂H₂, while the hot slurry comprising barium hydroxide and the insoluble salts would be fed into a hot solid–liquid separator to obtain a hot barium hydroxide solution. In the CO₂ capture process, CO₂-rich gas can be bubbled into the hot barium hydroxide solution to accelerate the conversion of barium hydroxide to barium carbonate, which would be fed to the barium carbide production unit and used as a raw material. Notably, an appropriate amount of fresh barium carbonate should be supplied to the reaction system to compensate for the loss of barium, caused by the side-reaction of barium oxide with acidic impurities, mainly silicon dioxide, and is generally present in various carbon sources, to produce insoluble barium salts, which are important raw materials for producing anti-electromagnetic radiation concrete.

Fig. 7 Schematic diagram of the carbon-to-acetylene process with the co-production of carbon monoxide using BaC₂ as an intermediate.

We know well that gasification is one of the most proven approaches to large-scalely transform carbonaceous fuels into syngas, which mainly consists of a mixture of H₂ and CO, and then into a wide range of chemicals via the F–T (Fischer–Tropsch) process, the MTO (Methanol to Olefin) process and so on. In the coal chemical industry, syngas production by coal gasification is regarded as the leading process among all and has been used on a large scale for over 150 years. Shell Coal gasification technology, one of the world-renowned gasification technology, can process petroleum coke, anthracite, bituminous coal, lignite and biomass into syngas at 4.0 MPa operating pressure and 1400–1700 °C gasification temperature.

Compared with the gasification technology, the barium-looping based new process, shown in Fig. 8, has the ability to directly convert a large variety of carbonaceous materials into CO and the advanced acetylene but not H₂ under even mild reaction conditions. The novel method for carbon-to-chemical conversion based on barium-looping is more competitive with the shorter and simpler process flow, which would provide new insights for the coal chemical industry and organic synthesis industry.

Fig. 8 Schematic of the barium-based carbon-to-acetylene process with the co-production of carbon monoxide.

Conclusions

The carbide-based carbon-to-acetylene process proposes a pathway for converting various sources of carbon into acetylene directly, which has an irreplaceable place in the acetylene chemical industry. However, the traditional calcium carbide production is a typical energy-intensive and heavy waste discharge process, which greatly limited its development. In this study, a novel sustainable coupling technology based on barium looping for carbon-to-C₂H₂ conversion and CO₂ capture is designed and implemented for the first time to realize the simultaneous production of C₂H₂ and CO with a lower energy consumption and lower waste discharge. The research results demonstrate that this method is technically simple, feasible, and environmentally friendly, which is expected to achieve the sustainable reconstruction of the carbon-to-C₂H₂ process. Moreover, because of the high efficiency of co-production of acetylene and carbon monoxide, this technology is expected to have the prospect of partially replacing coal gasification, which is usually the first process in the modern coal chemical industry to produce CO and H₂ as platform chemicals. Our work would provide new insights into the conversion of carbon into chemicals by a simpler, greener, and more efficient method.

Finally, it should be noted that, unlike the denser, more transportable calcium carbide produced by electric arc furnaces, solid-state synthesized granular barium carbide is porous and easily reacts with water to form acetylene. As a result, it is unstable in air, and the use of barium carbide far from the production site may lead to unsafe transportation over long distances. Thus, it is better to produce acetylene on site.

Experimental

Preparation of reaction powder and pellets

As a carbon resource, coconut shell carbon (99.8% fixed carbon, Ningxia Guanghua Activated Carbon Co., Ltd) was ground and sieved to obtain a carbon powder with a particle size of 150–180 μm. Barium carbonate (BaCO₃, >99.0%, Shanghai Titan Scientific Co., Ltd) with a particle size of 5–10 μm was used as the barium resource. A mixture of the sieved coconut shell carbon powder and barium carbonate powder in a C/Ba molar ratio of 4 was completely mixed and then dried at 120 °C for 12 h to obtain the reactant powder.

BaCO₃-C pellets prepared by wet granulation were used as the reactants for the synthesis of BaC₂ in a fixed-bed reactor (Fig. 9a). First, a mixture of the sieved coconut shell carbon powder and barium carbonate powder in a C/Ba molar ratio of 4 was completely mixed, followed by the addition of 20 wt% deionized water and continuous stirring to obtain a reactant slurry. After the slurry was granulated in a disc granulator, wet reactant pellets with a diameter of 5 mm were obtained. Next, the wet pellets were dried at 120 °C for 12 h to obtain the reactant pellets.

Fig. 9 Schematic of the (a) fixed-bed reactor and (b) lab-made acetylene gas generator equipment.

Reaction behaviour investigation of BaCO₃-C

The reaction behaviours of the BaCO₃-C reactants were first observed using a simultaneous thermal analyser SDT Q600 (TA, USA). First, 10 mg of the BaCO₃–C reactant mixture with a C/Ba molar ratio of 4 : 1 was loaded in a TG crucible, and the temperature was increased from room temperature to 1300 °C at a rate of 5 °C min⁻¹ under an Ar flow of 120 mL min⁻¹. Blank experiments (without a sample) were conducted under the same conditions to correct the TG data by eliminating the buoyancy effect caused by the ramp in the temperature.

The reaction behaviours of the BaCO₃-C reactants were further explored in a fixed-bed reactor (Fig. 9a). The fixed-bed reactor used for barium carbide synthesis was adapted from a high-temperature furnace reactor (Y-Feng Shanghai Co., Ltd), and it was equipped with an on-line GC instrument. The length and internal diameter of the reaction tube were 1200 mm and 50 mm, respectively, which was fitted with a corundum tube lined with graphite. Molybdenum silicide was used as the heating element, and the reaction temperature was regulated using an automatic temperature controller. Ar was used as the carrier gas, which was precisely regulated using a mass-flow meter (Beijing Seven Star Co., Ltd). In this section, 10 g of the reactant pellets placed in a uniform temperature zone were heated to the target temperature (1550 °C) at a rate of 2 °C min⁻¹. After the reaction, the heated solid samples were cooled to a temperature of less than 100 °C and rapidly transferred to a glove compartment filled with N₂ for subsequent analysis and characterization.

Barium carbide synthesis

Typically, 200 g of the reactant pellets placed in a uniform temperature zone were heated to the target temperature (1400–1550 °C) at a rate of 2 °C min⁻¹, which were maintained at the target temperature for different times (20–60 min). After the reaction, the heated solid samples were cooled to a temperature less than 100 °C and rapidly transferred to a glove compartment filled with N₂ for subsequent analysis and characterization.

Product analysis

The crystalline phase structures of the heated solid samples were detected by X-ray diffraction (XRD, Mini Flex 600) with Cu Kα radiation (40 kV, 15 mA) at a scan rate of 3° min⁻¹. The powder sample was sealed in an airtight sample bin to avoid possible air contact during the XRD tests. The morphology and microstructure of the synthesized solid samples were monitored using a field-emission scanning electron microscope (FE-SEM, Gemini SEM 300). The barium balance before and after the reaction was determined by inductively coupled plasma spectrometry (ICP, Thermo iCAP7600 USA). The yields of barium carbonate to carbide were calculated by measuring the corresponding C₂H₂ amounts from the hydrolysis of the synthesized solid samples in lab-made C₂H₂ gas generator equipment (Fig. 9b), which was qualified with standard CaC₂ samples.

The gas by-products obtained during BaC₂ synthesis were analysed online on a SHIMADZU 2014C GC system equipped with a thermal conductivity detector (TCD) and a Porapak Q column (2 m × 4 mm).

Author contributions

Conceptualization: Biao Jiang and Hong Zhao; data curation: Hong Zhao, Miao Li, Siyuan Chen, and Chen Hou; formal analysis: Hong Zhao and Miao Li; funding acquisition: Hong Zhao and Long Yan; investigation: Hong Zhao and Miao Li; methodology: Hong Zhao and Miao Li; project administration: Hong Zhao; resources: Hong Zhao; software: Miao Li, Siyuan Chen, and Siyuan Liu; supervision: Hong Zhao; validation: Hong Zhao and Miao Li; visualization: Hong Zhao and Miao Li; writing – original draft: Miao Li; writing – review & editing: Hong Zhao.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the Grant YLU-DNL Fund (2021013, 2021014) and the National Natural Science Foundation of China (22278417). The authors acknowledge the user experiment assistance system of SSRF.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3gc01775c

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