Effect of Gasification Agent on Underground Coal Gasification

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Marek Laciak	--	2466	2023-09-18 11:52:45	\|
2	references update	Fanny Huang	-12 word(s)	2454	2023-09-20 08:13:46	\|

This entry is adapted from the peer-reviewed paper 10.3390/en16176250

The underground coal gasification (UCG) technology converts coal into product gas and provides the option of environmentally and economically attractive coal mining. Obtained syngas can be used for heating, electricity, or chemical production. Numerous laboratory coal gasification trials have been performed in the academic and industrial fields. Lab-scale tests can provide insight into the processes involved with UCG. Many tests with UCG have been performed on ex-situ reactors, where different UCG techniques, the effect of gasification agents, their flow rates, pressures, and various control mechanisms to improve gasification efficiency and syngas production have been investigated.

underground coal gasification gasification agent syngas laboratory experiments

1. Introduction

The underground coal gasification (UCG) is a technology that converts coal into a synthetic gas (i.e., syngas) by heating. Currently, more than 909 trillion tons of coal are in stock worldwide, and only 15% of the available coal is for traditional mining methods. UCG technology is still evolving and provides an alternative to conventional underground coal mining. This technology can be attractive from an environmental and economic point of view and may have extensive use in the future. The technology is also less expensive than conventional coal mining. UCG allows the use of coal resources that would otherwise be economically or technically unfeasible to extract through conventional mining methods. UCG can lead to lower emissions compared to traditional coal mining and combustion because the process takes place underground and can include carbon capture and storage (CCS) technologies. UCG can potentially reduce the carbon footprint associated with traditional coal utilization. By converting coal in situ, UCG can facilitate the capture and storage of carbon dioxide (CO

_{2}

) produced during the process, reducing greenhouse gas emissions.

Additionally, syngas production through UCG can be combined with the technology of carbon capture and storage (CCS), which reduces the emission of CO

_{2}

from industrial processes. Although countries worldwide are placing increased emphasis on reducing greenhouse gas emissions, mitigating climate change, and diversifying their energy mix away from fossil fuels, UCG is discussed as a potential technology that could provide a bridge between traditional coal-based energy systems and cleaner energy alternatives. UCG can be integrated with renewable energy sources to enhance overall energy efficiency. For instance, the syngas produced from UCG can be a backup fuel for renewable energy systems, ensuring a continuous and reliable energy supply. Additionally, surplus renewable energy can be utilized in UCG operations during periods of low demand, optimizing energy utilization. The main benefits of UCG compared to conventional coal utilization methods are listed in ^[1].

On the other hand, it must be said that there is also criticism of UCG, which is based on the fear of groundwater contamination, subsidence, and the release of harmful gases during the UCG process. The environmental risks associated with UCG should have regulatory frameworks which need to be stringent to ensure safety and environmental protection. Moreover, the global energy situation is shifting towards renewable energy sources due to increasing concerns about climate change. Solar and wind energy have grown substantially and have become more economically viable. Governments, industries, and investors are focusing more on sustainable energy solutions and reducing their reliance on fossil fuels. In this context, the prospects for UCG can be uncertain, as it remains an intermediate solution with potential environmental risks, competing against the rapid advancements in cleaner and more sustainable energy technologies.

However, it is likely that there may be a lack of energy in times of energy crisis, and countries may return to using fossil fuels. In addition, some countries have stopped using nuclear power, and renewable sources are insufficient to meet the population’s energy needs. Moreover, renewable energy technologies depend on the weather (e.g., sun and wind) to generate energy. They are still significantly new to the market and lack the much-needed efficiency. Setting up renewable energy generation facilities requires a substantial financial outlay.

In a chemical view, gasification is the conversion of bigger organic macromolecules of solid fuel to smaller volatile or gas molecules consisting of syngas fuel. This conversion is obtained by heating the solid fuel to temperatures above 750 °C. Such temperatures are achieved by partial combustion of the solid fuel or indirectly by heating it with an overheated mean. UCG is performed as an auto-thermal process, in which, with the help of an injected gasification agent from an injection well, heat is generated in the coal deposit through combustion reactions with coal. When coal is heated, it releases volatile substances, leading to the production of combustible gases. Raw-unprocessed fuel (i.e., solid coal) is converted into combustible syngas containing CO, CH

_{4}

, and H

_{2}

. The gasification process also generates heat, CO

_{2}

, and H

_{2}

O. The primary chemical reactions in coal gasification include drying, pyrolysis, combustion, and gasification of solid hydrocarbons. UCG essentially represents the acquisition of a spatially and thermally distributed reaction zone in a coal seam, in which regions of coal oxidation, coal reduction, and coal pyrolysis occur.

The principle of UCG technology was well illustrated in ^[2], and the description of the leading chemical processes of UCG has been well described in ^[3]^[4]^[5].

In an in situ UCG test, the coal is converted into syngas underground in its natural location within a georeactor. A typical in situ UCG test involves drilling injection and production wells into the coal seam. The coal is ignited, and the injection well supplies oxidants (such as air, oxygen, or steam) to the coal seam to initiate the gasification process. Next, the syngas formed is extracted through the production well and can be further processed for various applications. In situ UCG tests provide insights into the behavior of coal, gasification reactions, and the potential for underground resource utilization.

Ex-situ UCG involves performing UCG tests aboveground, typically in laboratory gasifiers or reactors. In this method, coal samples are taken from underground coal seams and brought to the surface for gasification experiments. The coal bedding in the ex-situ reactor usually corresponds to the underground seam under conditions of geometric similarity. Ex situ UCG tests allow researchers to study and analyze the gasification process in a more controlled and manageable setting. The results obtained from ex-situ tests help us to understand the fundamental aspects of coal gasification and optimize the process parameters. In addition, valuable data from laboratory tests can be used to design mathematical models of UCG and perform simulations ^[6].

Both approaches are valuable in the development and understanding of UCG technology. In situ UCG tests are crucial for evaluating the viability of underground coal gasification in specific geological formations and providing site-specific data. Ex situ UCG tests, on the other hand, help in fundamental research, process optimization, and developing efficient gasification techniques.

2. Effect of Gasification Agent on Underground Coal Gasification

The choice and composition of the gasification agent, which can be air, oxygen, or steam, can significantly impact the gasification process. Optimizing the gasification agent involves determining the optimal oxygen ratio to coal, or steam to coal, to achieve the desired gasification reactions and syngas composition. Controlling the oxidant ratio (i.e., air, oxygen, or steam) to coal is crucial for optimizing gasification. The stoichiometric balance of the oxidant-to-fuel ratio influences the gasification efficiency, syngas composition, and heat release. Optimizing this ratio allows for efficient utilization of the available energy in the coal.

2.1. Effect of Additional Oxygen on the Calorific Value of Syngas

Air is usually used as the primary oxidizer during gasification experiments on ex-situ reactors. Because, in actual gasification operations, oxygen-enriched air is used to improve syngas production, Kačur et al. ^[8]^[9] performed three ex-situ trials where the impact of additional oxygen injected on the syngas calorific value was investigated. The lignite blocks from the Cigel mine (Slovakia) were gasified in the experimental reactor. The received coal had a total moisture of 22.25% and a calorific value of 13.74 MJ/kg.

Feng et al. ^[10] confirmed that gasification with oxygen, unlike gasification with air, brings higher temperatures in the oxidation zone and a higher quality of syngas, and the reaction zone is closer upstream. In addition, they found that, although an increased gasifier flow can improve syngas quality, it can also cause coal cooling. Moreover, they experimented with the initial gasification channel length (

L_{0}

). Their findings revealed that igniting the gasification at

3 / 4 L_{0}

results in higher temperatures than ignition at

1 / 2 L_{0}

, leading to a shorter effective syngas production time (

t_{e}

). However, this time increases as the gasification channel increases. The reaction zone is closer to the upstream with a wider gasification channel. In addition, they found that, although an increased gasifier flow can improve syngas quality, it can also cause coal cooling. The increased oxygen concentration in the oxidation mixture was evaluated as an effective tool for increasing the calorific value while increasing the flow rate of the input oxidizer will extend the effective time of syngas production. By increasing the oxygen flow, it was possible to increase the temperature of the oxidation zone up to 1300 °C and the syngas’s heating value to 12.1 MJ/m

^{3}

. When air was injected, the oxidation zone’s temperature was lower, resulting in a lower calorific value of syngas. Other researchers, e.g., Zagorscak et al. ^[11] found that when the flow rate increased from 6 Nm^{$^{3}$}/h to 10 Nm

^{3}

/h during air gasification, the proportion of CO and CO

_{2}

in syngas increased. And the average heating value and maximum heating value of gas produced under air at 10 m

^{3}

/h were lower than those under air 6 m

^{3}

/h. Stanczyk et al. ^[12] found that compared with air gasification, in oxygen gasification, the proportion of CO, CO_{$_{2}$}, H_{$_{2}$}, and CH_{$_{4}$} syngas and heating value is higher, which is consistent with the results obtained in ^[10].

2.2. Effect of Gasification Agent on Tar Concentration

The tars from UCG are black, viscous liquids with visible inclusions of dust and high-molecular-weight agglomerates. Smaller-scale experiments are useful for studying the tar evolution mechanisms in coal gasification. Researchers usually sample from the syngas or the neighboring water. Wiatowski et al. ^[13]^[14] carried out a series of measurements on the yields, composition, heating value, density, and viscosity of tar samples in the UCG trial at Mine “Wieczorek” and “Barbara” (Poland). Xu et al. ^[15] investigated the relationship between tar behaviors, including its yields, viscosity, and composition at low pyrolysis temperature, and tar formation in a fixed bed reactor. Xu et al. ^[16] found the tar yield decreased with the increase in pyrolysis temperature on a high-temperature tube furnace in UCG conditions.

In their study, Dong et al. ^[17] investigated tar's spatial and temporal changes during ex-situ coal gasification. They carried out multiple experiments, varying the flow rates of gasification agents (i.e., oxygen and air), and analyzed the tar composition at different locations and time intervals. The artificial coal bed consisted of bituminous coal cut into blocks with gasification channels and channels for tar removal. The coal moisture was 14.39%, and the ash content was 5.67%. They found that the concentration of tar in the reaction zone decreased during gasification with oxygen. Also, the percentage of PAHs (i.e., polycyclic aromatic hydrocarbons) fell when the oxygen flow rate increased from 10 to 15 L/min.

Moreover, with this increase in oxygen flow, a decrease in carbon emissions in the gases and an increase in the percentage of tar were observed. The tar concentration was found to be much lower in gasification with air than in gasification with oxygen. In addition, when the airflow rate increased (i.e., from 10 to 50 L/min), a more even distribution of concentration and tar composition occurred. Carbon emissions also decreased, but the percentage of tar-polluting substances increased. The highest temperature of more than 1300 °C was reached during gasification with pure oxygen. With air, this temperature was only reached at a flow rate of 50 L/min.

Pankiewicz-Sperka et al. ^[18] have found that the higher values of PAHs were in the case of wastewater from semi-anthracite while from bituminous coal gasification PAHs values are in lower ranges. Studies have shown that concentrations of phenols, BTEX, and PAHs decrease with increasing pressure. Other results showed that the yield and viscosity of tar increased with the increase in heating rate and pyrolysis temperature ^[15]. In addition, the tar yield under a hydrogen atmosphere was observed to be higher than that under a nitrogen, carbon dioxide, methane, and carbon monoxide atmosphere. The tar yield increased gradually with the increase of H_{$_{2}$} flow rate ^[16].

2.3. Effect of Gasification Agent on Cavity Growth and Syngas Production

The experiments reported in ^[19]^[20] showed the cavity growth under certain operating conditions in a horizontal channel of a coal block, through which the flow of gas takes place. It was assumed that the cavity size and shape are likely to substantially impact the gasification extent. The effect of the gasification agent on cavity enlargement during coal gasification was investigated by Daggupati et al. ^[21]. Moreover, they studied various UCG operating parameters required to convert coal to syngas (i.e., initial burn time, steam-to-oxygen ratio, feed water temperature). They gasified an artificial coal seam with a moisture of 40%. The mixture of steam and oxygen was injected to support ex-situ coal gasification. The experiments have shown that the optimum oxygen-to-steam ratio depends on the type of coal being gasified. Several experiments showed that this optimal ratio is 2.5. During the test, they produced syngas with a heating value of 178 kJ/mol and a content of H

_{2}

in the syngas of up to 38%. Moreover, the gasification cavity’s growth rate was observed to be relatively higher than that of the cavity of coal combustion. It was mainly due to the higher speed of the reaction gases in the case of gasification. The speed of the cavity enlargement also significantly affects the coal spalling, increasing the reaction surface. It was also found that the cyclic steam injection into the cavity could cause thermal shocks due to structural failures of the coal. Shu-qin et al. ^[22] have also reported that the optimum value of the injected steam and oxygen ratio for lignite is circa 2.5. Hettema et al. ^[23] have experimentally demonstrated that the surface material cracks and breaks due to local temperature variations, and they have also shown the effect of steam pressure on thermal spalling using laboratory-scale experiments.

3. Conclusions

The impact of experimental coal gasification in ex-situ reactors on in situ underground gasification (UCG) is significant, as it allows researchers and engineers to gain valuable insights and data that can be applied to improve the technology of UCG. Conducting experimental research in ex-situ reactors is generally safer and more cost-effective compared to full-scale in situ UCG operations. It allows researchers to test and optimize gasification conditions without the risks associated with underground operations. The choice and ratio of the gasification agent, which can be air, oxygen, or steam, can significantly impact the gasification process. Optimizing the gasification agent involves determining the optimal oxygen ratio to coal or steam to coal, to achieve the desired gasification reactions and required syngas composition.

References

Bhutto, A.W.; Bazmi, A.A.; Zahedi, G. Underground coal gasification: From fundamentals to applications. Prog. Energy Combust. Sci. 2013, 39, 189–214.
Perkins, G.M.P. Mathematical Modelling of Underground Coal Gasification, Academic Dissertation. Ph.D. Thesis, University of New South Wales, Faculty of Science and Engineering, Sydney, Australia, 2005.
Littlewood, K. Gasification: Theory and application. Prog. Energy Combust. Sci. 1977, 3, 35–71.
Hobbs, M.L.; Radulovic, P.T.; Smoot, L.D. Combustion and gasification of coals in fixed-beds. Prog. Energy Combust. Sci. 1993, 19, 505–586.
Thimsen, D.P.; Maurer, R.E. Coal Devolatilization in a Moving-Bed Gasifier; Technical Report GS-6797, Research Project 2525-14; Technical Report; EPRI—Electric Power Research Institute: Palo Alto, CA, USA, 1990.
Ranade, V.; Mahajani, S.; Samdani, G. Laboratory Studies on Underground Coal Gasification. In Computational Modeling of Underground Coal Gasification; CRC Press: Boca Raton, FL, USA, 2019; pp. 121–149.
Kačur, J.; Kostúr, K. Approaches to the Gas Control in UCG. Acta Polytechnica 2017, 57, 182-200.
Kačur, J. Optimálne Riadenie Procesov Splyňovania Uhlia v Podzemí (en: Optimal Control of Coal Gasification Processes in Underground). Ph.D. Thesis, Technical University of Košice, Faculty BERG, Košice, Slovakia, 2009.
Kačur, J.; Durdán, M.; Laciak, M.; Flegner, P. Impact analysis of the oxidant in the process of underground coal gasification. Measurement 2014, 51, 147–155.
Feng, L.; Zhou, S.; Xu, X.; Qin, B. Importance evaluation for influencing factors of underground coal gasification through ex situ experiment and analytic hierarchy process. Energy 2022, 261, 125116.
Zagorščak, R.; Sadasivam, S.; Thomas, H.R.; Stańczyk, K.; Kapusta, K. Experimental study of underground coal gasification (UCG) of a high-rank coal using atmospheric and high-pressure conditions in an ex situ reactor. Fuel 2020, 270, 117490.
Stańczyk, K.; Howaniec, N.; Smoliński, A.; Świądrowski, J.; Kapusta, K.; Wiatowski, M.; Grabowski, J.; Rogut, J. Gasification of lignite and hard coal with air and oxygen enriched air in a pilot scale ex situ reactor for underground gasification. Fuel 2011, 90, 1953–1962.
Wiatowski, M.; Kapusta, K.; Świądrowski, J.; Cybulski, K.; Ludwik-Pardała, M.; Grabowski, J.; Stańczyk, K. Technological aspects of underground coal gasification in the Experimental “Barbara” Mine. Fuel 2015, 159, 454–462.
Wiatowski, M.; Kapusta, K.; Muzyka, R. Study of properties of tar obtained from underground coal gasification trials. Fuel 2018, 228, 206–214.
Xu, Y.; Wang, J.; Zhang, G.; Zhang, X.; Zhang, Y. The characteristics and mechanism for the formation of tars from low temperature pyrolysis of lignite. J. Energy Inst. 2021, 99, 248–255.
Xu, M.; Xin, L.; Liu, W.; Hu, X.; Cheng, W.; Li, C.; Wang, Z. Study on the physical properties of coal pyrolysis in underground coal gasification channel. Powder Technol. 2020, 376, 573–592.
Dong, M.; Feng, L.; Zhou, Q.; Zhou, S.; Xu, X.; Qin, B. Spatial and temporal evolution of tar during ex situ underground coal gasification. Fuel 2022, 317, 123423.
Pankiewicz-Sperka, M.; Kapusta, K.; Basa, W.; Stolecka, K. Characteristics of Water Contaminants from Underground Coal Gasification (UCG) Process—Effect of Coal Properties and Gasification Pressure. Energies 2021, 14, 6533.
Shannon, M.; Thorsness, C.; Hill, R. Early Cavity Growth during Forward Burn; 5:28302, Report no. UCRL-84584. U.S. DOE, Livermore, CA; Technical Report; Lawrence Livermore National Laboratory: Livermore, CA, USA, 1980.
Thorsness, C.; Hill, R. Coal Block Gasification: Laboratory Results and Field Plans; 81:12021, Report no. UCRL-85906. U.S. DOE, Livermore, CA; Technical Report; Lawrence Livermore National Laboratory: Livermore, CA, USA, 1981.
Daggupati, S.; Mandapati, R.N.; Mahajani, S.M.; Ganesh, A.; Sapru, R.K.; Sharma, R.K.; Aghalayam, P. Laboratory studies on cavity growth and product gas composition in the context of underground coal gasification. Energy 2011, 36, 1776–1784.
Liu, S.Q.; Wang, Y.Y.; Zhao, K.; Yang, N. Enhanced-hydrogen gas production through underground gasification of lignite. Min. Sci. Technol. 2009, 19, 389–394.
Hettema, M.; Wolf, K.H.; DePater, C. The influence of steam pressure on thermal spalling of sedimentary rock: Theory and experiments. Int. J. Rock Mech. Min. Sci. 1998, 35, 3–15.

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Ján Kačur

Marek Laciak

Milan Durdán

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Update Date: 20 Sep 2023

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