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Purpose: Synthetic ammonia is not only the basis of the fertilizer industry in China but also has the highest energy consumption and pollution emissions in the chemical industry. The objective of this study was to conduct a cradle-to-gate life cycle assessment (LCA) of ammonia production based on different raw materials to identify the crucial processes and parameters and to provide suggestions for clean and sustainable development of the ammonia industry in China. Methods: Based on actual industrial data, this study comprehensively evaluated the resource consumption and pollution emissions caused by different raw material routes and coal-to-ammonia technologies from a life cycle perspective according to the LCA standards ISO 14040 series and using the CML 2001 method and identified the key environmental impact categories and stage contributions. In addition, the effects of various input parameters on the environmental burden were specified through sensitivity analysis. Accordingly, suggestions for improving the environmental performance of ammonia production are proposed. Results: The environmental burdens of the coal-based and coke oven gas-based routes were 1.43 and 1.7 times higher than that of the natural gas-based route, respectively. The significant differences were mainly reflected in the greenhouse effect, acidification, and fossil energy depletion. Advanced coal-to-ammonia technology, represented by coal water slurry gasification, showed a lower environmental burden than the traditional intermittent gasification technology, especially in terms of greenhouse gas (GHG) emissions and energy consumption. The GHG emissions involved in producing ammonia decreased from 3.88 to 2.18 kg per 1 kg of ammonia, and energy consumption decreased by approximately 17%, from 5.69 to 4.71 MJ. Conclusions: Coal is the main raw material used for ammonia production in China, and the results showed that the application of advanced coal gasification with energy-saving technologies can effectively improve the environmental performance of synthetic ammonia production in China. [ABSTRACT FROM AUTHOR]

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]

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]

China is the largest coal gasification market in the world. The gasifiers have been commercialized for producing syngas for coal-to-liquids, coal-to-olefin, and coal-to-substitute natural gas. During coal gasification at high temperature and pressure, the differences in reactivity of organic matter in coal are almost negligible; however, the transformation behaviour of minerals is important to the stability of the process. Detailed knowledge of the mineralogical properties of ash and slag is essential for optimizing the operational parameters. It is also significant for determining the influence of the solid wastes on the environment: for example, trace elements carried in bottom slag, fly ash, condensate water, and gaseous products migrate to natural water and soil and into the atmosphere. With this in mind, the mineralogy of ash and slag from typical coal gasification processes in China are reviewed, including the transformation of minerals typically found in coal, and the effects of operating conditions on mineral transformation and melting/slagging behaviour of minerals during gasification process. In addition, trace element migration behaviour in commercialized gasifiers is discussed. [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]

Coal gasification plays an irreplaceable role in promoting the sustainable development of coal chemical industry. The characteristics of ash have become the key factor to reflect the status and influence of the stable operation of coal gasification process. Introduce the current development and working principle of traditional characterization analysis of the composition, structure and morphology in gasification ash. Investigate the application of some new technologies, such as HTXRD, which can directly achieve the diffraction pattern of minerals at different temperature. Mössbauer spectroscopy can determine the iron materials in coal and ash through the qualitative and quantitative analysis, it also can test Fe2+/Fe3+, crystal structure parameters, solid solution types and content. Microanalysis can test the micro-constitution of minerals using SEM and X-ray. The choice of analysis technologies should depend on the composition features of ash. [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]

• 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]

A coal containing high concentrations of hazardous trace elements F, V, Cr, Se, Mo, Cd, and U was selected for the gasification experiment based on the Lurgi fixed-bed pressurized gasifier. The feed coal was a super-high organic sulfur coal sampled from the Ganhe Mine, Yanshan Coalfield, Yunnan Province, China. Fluorine, Ga, Se, Cd, In, Tl, Pb, and Bi were volatilized during gasification, whereas V, Mo, and U were significantly enriched in the residue. The erosion of the gasifier internals resulted in the abnormal enrichment of Cr and Ni in the residue, with the maximum concentrations of Cr and Ni of 26,073 μg/g and 23,982 μg/g, respectively. Except for quartz, other minerals in the feed coals were transformed. The newly formed phases such as mullite, hercynite, magnetite, Fe aluminosilicate, and Al-Si-O-glass containing Fe-Ca-Mg-V-K-Ti were identified in the gasification residue. Pyrite was transformed to elemental iron in some cases. Pyrrhotite with a magnetite rim, representing the high temperature transformation product of pyrite, was observed in the gasification residue by scanning electron microscopy. [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]

• Improvement potential of deep IGCtH are first assessed by advanced exergy analysis. • Efficiency of deep IGCtH are measured and compared by CExC per kg hydrogen as index. • CExC per kg hydrogen reduction by 16.4% for deep IGCtH compared with Lurgi SGCtH. • Lowest cost by deep IGCtH and better ability to resist market risk than Lurgi SGCtH. • Carbon tax of 132 RMB/t CO 2 for deep IGCtH with 80% CR equalize cost of without CCS. The deep in-situ gasification based coal-to-hydrogen (deep IGCtH) is attracting attention for its unique advantage in utilizing unrecoverable coal seam. In this paper, its improvement scope in energy conversion and utilization is analyzed using advanced exergy analysis, and the efficiency from cumulative exergy consumption (CExC) perspective and economic competitiveness with Lurgi surface gasification based coal-to-hydrogen (Lurgi SGCtH), coke-oven gas-to-hydrogen (COGtH) and natural gas-to-hydrogen (NGtH) are compared. Results indicate 39.9 % and 25.1 % of the exergy destruction of deep IGCtH and Lurgi SGCtH are unavoidable, while steam methane reforming and coal gasification possess the largest improvable potential, with avoidable endogenous/exogenous destruction of 96.63 MW/81.58 MW and 162.52 MW/151.19 MW, suggesting thermodynamic improvement can also be achieved by improving other components or optimizing process structure in addition to increasing its own efficiency. The CExC of deep IGCtH outputting 1 kg hydrogen is only 83.6 % that of Lurgi SGCtH, indicating it is more efficient route with significantly energy consumption reduction and can better demonstrate its efficiency advantage under large production capacity condition. The deep IGCtH shows only 68.7 % that of Lurgi SGCtH in investment and also presents significant advantage in cost. However, its profitability advantage over NGtH loses with hydrogen price increasing and there exists lower limit on capacity of 0.46 billion Nm3 to maintain cost and profitability competitiveness over COGtH. Furthermore, deep IGCtH shows emission reduction cost advantage over Lurgi SGCtH when capture rate exceeds 80 %, and profitability will not be limited by carbon tax and coal seam thickness within the considered range. [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]