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Innovating coal development and utilization technology to reduce carbon emissions during the whole life cycle of coal from production to utilization is a coal-based clean energy route that meets the characteristics of China's energy endowments. Underground gasification is one of the potential methods of deep coal in-situ mining, coupled with CCS/CCUS (Carbon Capture and Storage/Carbon Capture, Utilization and Storage) deep coal underground gasification hydrogen production technology route can not only utilize the rich deep coal resources, but also is expected to become an ideal of coal-based low-cost hydrogen production route. Based on the world's only kilometer-level deep coal underground gasification test data, combined with Aspen Plus process simulation, the carbon emission accounting and carbon emission reduction potential analysis of deep coal underground gasification hydrogen production were carried out. Compared with the commercial ground Lurgi fixed bed pressurized gasification hydrogen production route, the life cycle assessment method is used to establish the whole life cycle carbon emission calculation model of the two processes, compared the life cycle carbon emissions of the two hydrogen production routes.The CCS/CCUS path and carbon emission reduction potential of the underground gasification of deep coal are further evaluated.The results show that the carbon emissions during the life cycle of underground gasification of deep coal and ground gasification of Lurgi coal are 3.29×109 t CO2-eq and 3.93×106 t CO2-eq in the case of hydrogen production capacity of 1.2 billion Nm3/a, respectively, and the carbon dioxide directly emitted into the atmosphere in the form of exhaust gas is 2.09×106 t and 2.24×106 t, respectively. The main carbon emitters in the hydrogen production phase are exhaust gases, including exhaust gases from acid gas removal units and flue gases from methane reforming units. The high methane content characteristics brought about by the underground gasification process of deep coal lead to the CO2 emissions contributed by the flue gas of the methane reforming unit to 8.84 kg when producing 1 kg of hydrogen.In terms of indirect emissions, deep coal underground gasification directly uses liquid water as a gasifier without consuming external steam, so the carbon emissions caused by steam consumption are lower than that of ground gasification. The upstream of Lurgi coal surface gasification includes the coal mining and processing and coal transportation stages, and although the carbon emissions in these two phases are only 6.7%, but still bring 2.63×105 t CO2-eq. If the deep coal underground gasification cavity is used for CO2 geological storage under the premise of geological safety risk assessment, the storage capacity can reach 61.8% of the total carbon emissions, and if the urea device is supported by 470,000 tons / year, the remaining CO2 can be effectively used,to form a near-zero emission deep coal underground gasification hydrogen production and urea co-production technical route. The research results provide a scientific basis for quantitative evaluation of carbon emissions for hydrogen production from deep coal underground gasification.

China has rich reserves of deep coal resources, and underground coal gasification can transform coal into gas and transport it to the ground, which is an important way for in-situ fluidized mining of deep coal.This article introduces the development history and technical status of underground coal gasification technology (UCG), as well as typical cases of underground coal gasification in the middle and deep parts.Based on the modern underground coal gasification technology system, the key technologies and technical research directions of deep underground coal gasification was analyzed and the prospect of deep coal gasification mining with the goal of natural gas production was outlooked.The development trend of UCG is from the mine type to the drilling type, from shallow coal seams to deep coal seams, from single power generation to comprehensive utilization.The practice of UCG in the middle and deep coal seams has verified the technical feasibility.The key technologies of deep underground coal gasification mainly include geological evaluation and scientific site selection, gasifier construction, deep coal seam high-pressure ignition, controllable mobile injection and deep fire zone geophysical detection.Deep coal gasification to produce natural gas can not only realize the clean and efficient use of deep coal resources, but also solve the contradiction between supply and demand caused by the increase in natural gas consumption.

The mineralogical characteristics of underground coal gasification (UCG) residuals are closely related to the gasification conditions. The study of the mineral composition and transformation of ash and slag under different gasification conditions has important scientific significance for understanding the real gasification conditions, improving the utilization efficiency of coal, and identifying and controlling the environmental impact. In this paper, the sequential transformation of minerals under different temperature and atmosphere are studied by X‐ray diffraction (XRD) and FactSage software, and the elemental composition of the surface of the UCG ash and slag is characterized by SEM‐EDX. The results show that the main minerals in the semicoke are quartz (SiO2), illite (K1.5Al4(Si6.5Al1.5)O20(OH)4), and sanidine (KAlSi3O8). In the 900–1300°C reduction, ash, quartz, sanidine, hercynite (FeAl2O4), anorthite (CaAl2Si2O8), and mullite (Al6Si2O13) are the main minerals. The minerals in the oxidation residuals (1100–1500°C) are quartz, anorthite, hematite (Fe2O3), hercynite, cordierite (Fe2Al4Si5O18), and cristobalite. The reaction temperature has a significant effect on the changes in mineralogy. The reaction temperature is low during the pyrolysis process, and most changes involve only structural changes. With the reaction temperature increases, a series of reactions occur between different minerals. The reaction atmosphere also has a significant effect on the mineralogy. The research on the transformation of minerals in UCG residues can indirectly reflect the actual process of UCG, which can provide suggestions for the stable operation of the UCG process. [ABSTRACT FROM AUTHOR]

Developing underground coal gasification (UCG)-based hydrogen production (UCG-H 2) is expected to alleviate hydrogen supply and demand contradiction, but its energy consumption and environmental impact need to be clarified. In this paper, comparative study of energy consumption and greenhouse gas (GHG) emissions between UCG-H 2 and typical surface coal gasification (SCG)-based hydrogen production (SCG-H 2) is carried out using life cycle assessment method. Result shows energy consumption of UCG-H 2 is only 61.2% of that of SCG-H 2 , which is 1,327,261 and 2,170,263 MJ respectively, reflecting its obvious energy saving advantage. 80% capture rate can achieve an appropriate balance between energy consumption and emissions. Under this capture rate, emissions of UCG-H 2 and SCG-H 2 are roughly equivalent, which are 207,582 and 197,419 kg CO 2 -eq respectively. Scenario analysis indicates energy consumption in hydrogen industry can reduce by 38.8% when hydrogen production is substituted by UCG with CCS to fully meet demand of 21 Mt in 2030. • Consumption and emissions of UCG-H 2 and SCG-H 2 are first compared using LCA method. • Energy consumption of UCG-H 2 is only 61.2% of that of SCG-H 2. • Implementing CCS reduces GHG emissions of UCG-H 2 to be roughly equivalent to SCG-H 2. • 80% capture rate can achieve a balance between energy consumption and emissions. • Applying UCG-H 2 can reduce energy consumption by 38.8% in selected scenario. [ABSTRACT FROM AUTHOR]

The demand for H 2 increases rapidly with the gradual recognition of the potential of H 2 as an important secondary energy. At present, coal gasification is the main way to obtain hydrogen on a large scale and at a low cost in China. The underground coal gasification (UCG), as a kind of in-situ utilization technology that can exploit the unreachable deep coal resources, could become an alternative H 2 production pathway. This paper presents comparative study of energy utilization and resource consumption in H 2 production by UCG and typical surface coal gasification (SCG) technology, namely Lurgi fixed bed gasification, with 1.2 billion Nm3/a throughput of H 2 as example, to offer corresponding data support. The efficiency and the amount of resources consumed in constructing and operating each coal-to-hydrogen system under different conditions have been researched from exergetic point of view, which is not reported in existing literatures. In this paper, the exergy efficiency is calculated to be 40.48% and 40.98% for hydrogen production using UCG and SCG. The result indicates the competitiveness of UCG in the field of hydrogen production comparing with widely used coal gasification technology. The resource consumption is measured by cumulative exergy consumption (CExC), which is 8.17E+10 MJ and 6.57E+10 MJ for H 2 production from UCG and SCG. The result shows that although the H 2 production from UCG has higher CExC, it can significantly reduce the resource consumption of equipment comparing with H 2 production from SCG, indicating its advantage in total investment. It is found that the exergy efficiency increases with the rise in H 2 O-to-O 2 and O 2 -to-CO 2 ratio, while the value of CExC decreases with the appreciation of H 2 O-to-O 2 ratio yet increases as the O 2 -to-CO 2 ratio rises. In addition, the sensitivity analysis of production capacity reveals that the exergy efficiency gap and CExC gap between hydrogen production by UCG and SCG diminishes at smaller scale production capacities, showing that UCG is more suitable for small-scale hydrogen production. • A resource consumption comparison of H 2 production from UCG and SCG is conducted. • Exergy is analyzed with 1.2 billion Nm3/a throughput of H 2 as example. • Exergy efficiency of UCG-H 2 reaches 40.48%, comparable to 40.98% for SCG-H 2. • UCG-H 2 significantly reduces CExC of equipment compared to SCG-H 2. • Effects of key operational parameters on system's performances are investigated. [ABSTRACT FROM AUTHOR]

Underground coal gasification (UCG) can be adopted to convert unminable coal resources into synthesis gas for use in the production of electricity, fuels, and chemicals. UCG is a promising technology with many health, safety, and environmental advantages over conventional mining techniques. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of the gasification processes and improve the utilization efficiency of coal seams. The presence of coal partings has an important impact on mineral transformation. In this paper, the UCG process has been subdivided into pyrolysis, reduction, and oxidation stages, and the progressive coal conversion products were prepared. Ulanqab lignite was used as the test coal. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and scanning electron microscopy coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic equilibrium simulation software, FactSage 7.2, was used to calculate the relevant phase diagram to assist in understanding the mineral transformations. The experimental results indicate that the different iron-bearing minerals occur in raw coal and non-coal partings which are nontronite (Na 0.3 Fe 2 Si 4 O 10 (OH) 2.4H 2 O) and glauconite (K (Fe, Al) 2 (Si, Al) 4 O 10 (OH) 2) respectively. Pyrrhotite is the main iron-bearing mineral in the pyrolysis products. In the reduction stage, the relative content of Al 2 O 3 is increased due to the presence of non-coal partings, which reacts with FeO to produce hercynite (FeAl 2 O 4) at a temperature above 1100 °C. When the temperature increases to 1300 °C, hercynite is converted to the thermodynamically-stable cordierite (Fe 2 Al 4 Si 5 O 18). In the oxidation stage, mullite (Al 6 Si 2 O 13) is produced when the oxidation temperature reaching 1300 °C, which could be used to reflect the underground reaction conditions in the coal seam. The phase diagram of (SiO 2) 6.2 Al 2 O 3 -CaO-Fe 2 O 3 proves that mullite and anorthite (CaAl 2 Si 2 O 8) are the dominant minerals when the mole fraction of (SiO 2) 6.2 Al 2 O 3 is above 0.7. In addition, the ash melting point rises with an increase of Si and Al content in the presence of non-coal partings. Unlabelled Image • The different iron-bearing minerals occur in raw coal and non-coal partings which are nontronite (Na 0.3 Fe 2 Si 4 O 10 (OH) 2.4H 2 O) and glauconite (K (Fe, Al) 2 (Si, Al) 4 O 10 (OH) 2) respectively. • In the reduction stage, the relative content of Al 2 O 3 is increased due to the presence of non-coal partings, which reacts with FeO to produce hercynite (FeAl 2 O 4) at a temperature above 1100 °C. • In the oxidation stage, mullite (Al 6 Si 2 O 13) is produced when the oxidation temperature reaching 1300 °C, which could be used to reflect the underground reaction conditions in the coal seam. • The phase diagram of (SiO 2) 6.2 Al 2 O 3 -CaO-Fe 2 O 3 proves that mullite and anorthite (CaAl 2 Si 2 O 8) are the dominant minerals when the mole fraction of (SiO 2) 6.2 Al 2 O 3 is above 0.7. [ABSTRACT FROM AUTHOR]

Detailed mineralogical information from underground coal gasification (UCG) is essential to better understand the chemical reactions far below the surface. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of gasification processes and improve the utilization efficiency of coal seams. The transformation of iron-bearing minerals has the typical characteristics during the UCG process and is expected to indicate the process parameters. In this paper, UCG progress was subdivided into pyrolysis, reduction and oxidation stages, and the progressive coal conversion products were prepared. Two types of lignite with different iron contents, Ulankarma and Ulanqab coals, were used in this study. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation performed using the phase diagram of FactSage 7.1 was used to help to understand the transformation of minerals. The results indicate that the transformation behavior of iron-bearing minerals in the two lignites are similar during the pyrolysis process, in which pyrite (FeS2) in the raw coal is gradually converted into pyrrhotite (Fe1-xS). In the reduction stage, pyrrhotite is transformed into magnetite (Fe3O4) and then changes to FeO. The reaction of FeO and Al2O3 in the low iron coal produces hercynite above 1000 °C because of the difference in the contents of Si and Al, while in the high iron coal, FeO reacts with SiO2 to generate augite (Fe2Si2O6). When the temperature increases to 1400 °C, both hercynite and augite are converted to the thermodynamically-stable sekaninaite. [ABSTRACT FROM AUTHOR]

Abstract: Mercury, arsenic and selenium are trace elements well-known for their high volatility in underground coal gasification (UCG) which can lead to environmental and technical problems during gas utilization. In this paper, the volatilization of mercury, arsenic and selenium from coal in a seam during the process of UCG were investigated, based on comparison of their volatility during the transformation of coal to char and the conversion of char to ash. Three types of coal were involved in this study. The results indicate that the volatility of mercury, arsenic and selenium during UCG in the seam follows the sequence of Hg>Se>As. Mercury and selenium show volatility higher than 90% from coal to ash. The volatility of arsenic is lower than 60% as confirmed by arsenic enrichment in UCG ash. Arsenic volatilization during UCG is also enhanced by increasing temperature, which is different from the result during the combustion of crushed coal. Coal type has obvious effect on element volatilization. The higher the coal reactivity, the easier is the evaporation of the elements from coal in the seam. At the same time, thermodynamic equilibrium calculations using MTDATA program were performed to predict the possible species in UCG gas. With regard to UCG gas at the production well, mercury presents as Hg(g), H2Se(g) is the main gaseous species of selenium, whereas arsenic occurs in condensed phase as As2S3 and As. The effect of the pressure on the equilibrium composition of the gas results in major changes of the proportions of the species. High pressure leads to the formation and enhancement of the reduced species and increases the condensation temperature of the volatile elements. [Copyright &y& Elsevier]

Abstract: In order to provide theoretical basis for gas cleaning and pollution control thermodynamic equilibrium calculations were performed to predict the partitioning of trace element species under special conditions for underground coal gasification, including both oxygen-steam gasification and air blown gasification under elevated pressures. The trace elements studied include As, Se, Pb, Ni, Cd, Cr, Sb. The results indicate, in the condition of large-section UCG process with oxygen-steam injection, all the elements studied present in the gas phase during gasification stage. Ni and Cr are hardly volatile and tend to condense below 1000 °C. Most of them will be enriched in bottom ash. As, Pb, Cd, Sb totally or partially occur in gas phase in underground gas cleaning system. In cold cleaning system, they exist in condensed phases and tend to be enriched in fly ash, which is beneficial to trace element removal. Se presents in gas phase in the form of H2Se(g) even in ground cold gas cleaning system. The presence of potassium makes arsenic less volatile due to the formation of K3AsO4 and selenium is not affected. Also the amount of gaseous antimony chloride is reduced because of prior formation of alkali metal chloride. Pressure shows a remarkable effect on equilibrium partitioning of As, Se, Sb. With rising pressure, increasing quantities of hydrides of these trace elements are generated due to the enhancement of the reducing atmosphere. At the same time, the condensation points of all the trace elements sharply increase with pressure. It is found that for underground air blown gasification, the gaseous species of trace element sulfide can be easily formed, and the trace elements have lower condensation points than those for oxygen-steam gasification. [Copyright &y& Elsevier]

• The chemical speciation of hazardous trace elements was studied. • The existing forms of hazardous trace elements were studied. • The risk assessment code (RAC) was introduced to evaluate the risk level. Underground coal gasification (UCG) is a new coal utilization technology with advantages of high safety and high efficiency. However, the risk of hazardous trace elements (HTEs) in the UCG residue to the underground environment has always been an important issue. In this paper, the UCG sequential transformation products were prepared, and the chemical speciation of hazardous trace elements (Hg, As, Se, Pb, Cr, and Cd) in the UCG products were studied by using a combination of chemical analysis and thermodynamic calculation. Moreover, the risk assessment code (RAC) was introduced to evaluate the risk level of hazardous trace elements to underground environment. The results indicate that during the UCG pyrolysis process, Hg, Pb, Se, and Cd in the Ulanqab coal react with S to form HgS, PbS, SeS, and CdS, respectively. The oxidizable form of Hg (50.00 ± 0.18–66.67 ± 0.23%) and As (78.34 ± 0.37–83.36 ± 0.46%) is the main speciation in 900–1300 °C reduction ash. For the oxidation ash and slag samples, Hg is loaded on the mineral system of Na-Si-O in the form of HgO, and As is combined with the FeAl 2 O 4. Based on the risk assessment code (RAC), Hg, Se and Pb have medium risk level in the coal and semi-coke. Cr and Se in the 900–1200 °C reduction ash can pose high risk on the underground environment. In the oxidation ash and slag, the risk level of HTEs to the environment decreases. The results can provide scientific guidance for the control and treatment of residues in the later-stage of underground coal gasification. [ABSTRACT FROM AUTHOR]

Calcium-bearing minerals are one of the main typical minerals in coal and coal ash. In the process of coal thermal conversion, calcium-bearing minerals undergo different morphological transformation in which the reaction temperature, pressure, and atmosphere are important factors affecting their transformation. The reaction process of underground coal gasification (UCG) could be clearly divided into pyrolysis, reduction, and oxidation and the typical calcium-bearing minerals are expected to indicate the actual reaction conditions of UCG. A high-calcium coal, Zhundong coal, was used in this research. The products of UCG were prepared and the minerals were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation was used to assist in understanding the transformation behaviors of calcium-bearing minerals. The experimental results show that the calcium-bearing mineral is gradually converted from gypsum (CaSO4·2H2O) in the raw coal into anhydrite (CaSO4) during the pyrolysis process. In the reduction stage, anhydrite reacts with the reducing gas (CO) to produce oldhamite (CaS), and the oldhamite is stably present in the reduction ash. During the oxidation process, oldhamite is first transformed into CaSO4, and then CaSO4 is converted into CaO. Finally, CaO reacts with Al2O3 and SiO2 to produce gehlenite (Ca2Al2SiO7) at 1100 °C. As the oxidation temperature rises to 1400 °C, gehlenite is transformed into the thermodynamically stable anorthite (CaAl2Si2O8). With the further progress of the reaction, anorthite will co-melt with iron-bearing minerals above 1500 °C. The ternary phase diagram of SiO2–Al2O3–CaO system proves that anorthite and gehlenite are the typical high-temperature calcium-bearing minerals when the mole fraction of SiO2 is higher than 0.6. Moreover, the gehlenite is converted to anorthite with the temperature rise, which is consistent with experimental results. This study provides a scientific basis for understanding the UCG reaction conditions. [ABSTRACT FROM AUTHOR]

Underground coal gasification (UCG) produces less pollution and is safer than traditional coal mining. In order to investigate the effects of different gasifying agents or comprehensive analyses of the characteristics of the gas components in the three zones for the reverse underground coal gasification process, a model test was carried out. The results showed that the oxygen concentration of a gasifying agent is recommended to be higher than 21%, which will lead to more combustible gases and a higher calorific value of gas. Higher flow rates and oxygen content generally afforded more desirable gas compositions and calorific values, with the latter as high as 1430.19 kcal/Nm3. For the enriched oxygen gasifying agent in the reverse gasification process, the flow increase from 10 to 20 Nm3/h affords a rapid increase in the growth rate of the flame front, from 1.80 to 4.88 m/day, which is much faster than that for the air gasifying agent. Increasing the gas injection rate and oxygen concentration will increase the growth rate of the flame front. This affects the distribution of the three zones and further leads to different characteristics of the gas components. [ABSTRACT FROM AUTHOR]

Underground coal gasification (UCG) hydrogen production (UCG-H 2) can convert deep coal resources difficult to mine into hydrogen, its economic performance needs to be investigated so the application prospect can be clarified. This paper take hydrogen production with capacity of 1.2 billion Nm3/a as objective, based on simulation results, comparative techno-economic analysis was conducted between UCG-H 2 and Lurgi-based surface coal gasification (SCG) hydrogen production (SCG-H 2). Benefiting from lower gasification equipment investment, capital investment of UCG-H 2 without CCS is only 77.1% of that of SCG-H 2 , product cost is only 0.73 RMB/Nm3, lower than 1.69 RMB/Nm3 of SCG-H 2 , and will not be affected by price fluctuation. With 80% capture rate (CR), capital investment ratio increases to 81.8%, product cost of UCG-H 2 increases to 1.05 RMB/Nm3, still possesses great cost advantage. If hydrogen market price less than 1.5 RMB/Nm3, IRR of UCG-H 2 with 90% CR cannot reach the standard of 12%, possesses no economic feasibility, while SCG-H 2 can only be economically feasible when hydrogen market price less than 2.2 RMB/Nm3 without CCS. By conducting sensitivity analysis, competitive carbon tax between UCG-H 2 and SCG-H 2 with 80% CR and without CCS are determined, as well as correlation between price and carbon tax when SCG-H 2 is cost-competitive. [Display omitted] • Techno-economic performance of UCG-H 2 and SCG-H 2 is conducted and compared. • UCG-H 2 shows better performance on capital investment and product cost. • UCG-H 2 performs better in terms of resistance to price fluctuation and market risk. • Competitive carbon tax between UCG-H 2 with 80% CR and without CCS is 124 RMB/t CO 2. • Established correlation between price and carbon tax when SCG-H 2 is cost-competitive. [ABSTRACT FROM AUTHOR]

• Pressurized pyrolysis of dense coal cores. • Compared with 0.1 MPa, the CH 4 yield of 3 MPa of increased from 32.60 to 77.31 L/kg. • With the increase of pyrolysis pressure, the oxy-quaternary carbon chain decreased. • Pyrolysis pressure can promote the conversion of aliphatic groups to CH 4. Deep underground coal gasification (UCG) is characterized by high reaction pressure (dependent on hydrostatic pressure of coal seam) and high CH 4 content. The formation mechanism of CH 4 in product gases has always been an interesting topic. In this paper, pressurized pyrolysis characteristics of coal core were studied through a pressurized pyrolysis device. The formation mechanism of CH 4 and structural evolution of semi-coke during the pressurized pyrolysis process were identified by X-ray diffractometer (XRD), pressurized thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG-FTIR) and carbon-13 nuclear magnetic resonance (13C NMR). The results indicate that the yield of pyrolysis gas and carbon emission rate rise with an increase of pyrolysis pressure, while the yield of tar decreases. Compared with atmospheric pressure, the pyrolysis gas yield increases from 148.6 L/kg to 172.6 L/kg at 3.0 MPa, the CH 4 yield increases from 32.6 L/kg to 77.3 L/kg. The CH 4 begins to be released at around 400 °C at different pyrolysis pressures, and the generation rate reaches its maximum at around 500 °C. The analysis of 13C NMR shows that the relative content of the oxy-quaternary carbon in the aliphatic group decreases significantly from 9.2% to 1.8% with an increasing pyrolysis pressure. Therefore, pressure promotes the reaction of aliphatic carbon with hydrogen radicals to form CH 4 , which is one of the main reactions for the generation of CH 4 by pressurized pyrolysis. [ABSTRACT FROM AUTHOR]

Underground coal gasification (UCG) is considered to be an important means of coal utilization. However, the stability of underground coal gasification residuals has always been the focus of people's attention. In this paper, based on the sequential chemical extraction and thermodynamic equilibrium calculation, the chemical forms and transformation mechanism of the toxic elements (TEs) were investigated. Moreover, the risk level of toxic elements to underground environment was also studied in this paper. Results showed that Zn and Ni are thiophilic elements and favored to react with S to generate ZnS and NiS in the UCG pyrolysis stage. In the 900–1300 °C reduction ash, the toxic elements (Zn, Be, Ni, Ba, and U) in the residual speciation (F4) have the large percentage. For the oxidation residuals, Be and U are intimately related with the Fe–Si–Al–O–Be–U mineral system, and F exists in the form of CaF 2. According to the risk assessment code (RAC), the harmful degree of toxic elements to the underground environment decreases with rises of reaction temperature. When the reaction temperature reaches 1200 °C, the harmful degree of toxic elements to the underground environment is range in the Low risk, indicating that the underground residuals have reached the stability. The results can provide suggestions for future handing of the underground coal gasification residuals. [Display omitted] • The chemical speciation of toxic elements was studied. • The existing forms of toxic elements were studied. • The risk assessment code (RAC) was introduced to evaluate the risk level. [ABSTRACT FROM AUTHOR]