Integrated vacuum pressure swing adsorption and Rectisol process for CO_(2) capture from underground coal gasification syngas

An integrated vacuum pressure swing adsorption(VPSA) and Rectisol process is proposed for CO_(2) capture from underground coal gasification(UCG) syngas. A ten-bed VPSA process with silica gel adsorbent is firstly designed to pre-separate and capture 74.57% CO_(2) with a CO_(2) purity of 98.35% from UCG syngas(CH_(4)/CO/CO_(2)/H_(2)/N_(2)= 30.77%/6.15%/44.10%/18.46%/0.52%, mole fraction, from Shaar Lake Mine Field,Xinjiang Province, China) with a feed pressure of 3.5 MPa. Subsequently, the Rectisol process is constructed to furtherly remove and capture the residual CO_(2)remained in light product gas from the VPSA process using cryogenic methanol(233.15 K, 100%(mass)) as absorbent. A final purified gas with CO_(2) concentration lower than 3% and a regenerated CO_(2) product with CO_(2) purity higher than 95% were achieved by using the Rectisol process. Comparisons indicate that the energy consumption is deceased from 2.143 MJ·kg^(-1) of the single Rectisol process to 1.008 MJ·kg^(-1) of the integrated VPSA & Rectisol process, which demonstrated that the deployed VPSA was an energy conservation process for CO_(2) capture from UCG syngas. Additionally, the high-value gas(e.g., CH_(4)) loss can be decreased and the effects of key operating parameters on the process performances were detailed.

Groundwater Pollution from Underground Coal Gasification

In situ coal gasification poses a potential environmental risk to groundwater pollution although it depends mainly on local hydrogeological conditions. In our investigation,the possible processes of groundwater pollution origi-nating from underground coal gasification (UCG) were analyzed. Typical pollutants were identified and pollution con-trol measures are proposed. Groundwater pollution is caused by the diffusion and penetration of contaminants generated by underground gasification processes towards surrounding strata and the possible leaching of underground residue by natural groundwater flow after gasification. Typical organic pollutants include phenols,benzene,minor components such as PAHs and heterocyclics. Inorganic pollutants involve cations and anions. The natural groundwater flow after gasification through the seam is attributable to the migration of contaminants,which can be predicted by mathematical modeling. The extent and concentration of the groundwater pollution plume depend primarily on groundwater flow ve-locity,the degree of dispersion and the adsorption and reactions of the various contaminants. The adsorption function of coal and surrounding strata make a big contribution to the decrease of the contaminants over time and with the distance from the burn cavity. Possible pollution control measures regarding UCG include identifying a permanently,unsuitable zone,setting a hydraulic barrier and pumping contaminated water out for surface disposal. Mitigation measures during gasification processes and groundwater remediation after gasification are also proposed.

Method of oxygen-enriched two-stage underground coal gasification

Two-stage underground coal gasification was studied to improve the caloric value of the syngas and to extend gas production times.A model test using the oxygen-enriched two-stage coal gasification method was carried out.The composition of the gas produced,the time ratio of the two stages,and the role of the temperature field were analysed.The results show that oxygen-enriched two-stage gasification shortens the time of the first stage and prolongs the time of the second stage.Feed oxygen concentrations of 30%, 35%,40%,45%.60%,or 80%gave time ratios（first stage to second stage） of 1：0.12,1：0.21.1：0.51,1：0.64, 1：0.90.and 1：4.0 respectively.Cooling rates of the temperature field after steam injection decreased with time from about 19.1-27.4℃/min to 2.3-6.8℃/min.But this rate increased with increasing oxygen concentrations in the first stage.The caloric value of the syngas improves with increased oxygen concentration in the first stage.Injection of 80%oxygen-enriched air gave gas with the highest caloric value and also gave the longest production time.The caloric value of the gas obtained from the oxygenenriched two-stage gasification method lies in the range from 5.31 MJ/Nm^3 to 10.54 MJ/Nm^3.

A contrast study on different gasifying agents of underground coal gasification at Huating Coal Mine

To optimize the technological parameter of underground coal gasification （UCG）, the experimental results of air gasification, air-steam gasification, oxygen-enrichment steam gasification, pure oxygen steam gasification and two-stage gasification were studied contrastively based on field trial at the Huating UCG project. The results indicate that the average low heat value of gas from air experiment is the lowest （4.1 MJ/Nm3） and the water gas from two-stage gasification experiment is the highest （10.72 MJ/Nm3）. The gas productivity of air gasification is the highest and the pure oxygen steam gasification is the lowest. The gasification efficiency of air gasification, air-steam gasification, oxygen-enriched steam gasification, pure oxygen steam gasification and two-stage gasification is listed in ascending order, ranging from 69.88% to 84.81%. Described a contract study on results of a field test using steam and various levels of oxygen enrichment of 21%, 32%, 42% and 100%. The results show that, with the increasing of O2 content in gasifying agents, the gas caloricity rises, and the optimal O2 concentration range to increase the gas caloricity is 30%-40%. Meanwhile, the consumption of O2 and steam increase, and the air consumption and steam decomposition efficiency fall.

Environmental benefits of underground coal gasification

Environmental benefits of underground coal gasification are evaluated. The results showed that through underground coal gasification, gangue discharge is eliminated, sulfur emission is reduced, and the amount of ash, mercury, and tar discharge are decreased. Moreover, effect of underground gasification on underground water is analyzed and CO 2 disposal method is put forward.

A remining technology of underground coal gasification at Zhongliangshan Coal Mine

The field trail used a mixture of steam and air with various levels of oxygen en- richment.Steady conditions were achieved in the field trail which produced high quality hydrogen-enriched syngas.To understand and optimize the UCG process,a simplified heat and mass transfer model was presented,providing a predictive tool for temperature and the major constituents of the syngas production.The model is compared with the field trail measurements for air and two levels of oxygen enrichment,showing reasonable agreement for the channel temperature and product syngas concentration profile.

Large-scale laboratory study on the evolution lawof temperature fields in the context of underground coal gasification

This article presents the evolution law of temperature fields in a large-scale laboratory Underground Coal Gasification reactions using Ulanqab lignite under actual conditions.The results show that in the cultivation stage of oxidation zone,the main direction of the temperature field expansion is consistent with the crack direction of the coal seam.In the gasification stabilization stage,the main direction of the temperature field expansion is along the channel.The temperature of the coal seam and the overlying rock mass at its interface with the furnace directly above the gasification channel is equivalent to that of the coal seam temperature,and this temperature is much greater than the temperatures observed near both side walls of the gasification channel at the interface.However,temperatures perpendicular to the axis of the gasification channel are similar at a vertical distance of 40 cm away from the interface.The temperature distributions indicate that the transmission of heat through the overlying rock mass is more rapid in the vertical direction than in the horizontal direction.Moreover,some degree of thermal dispersion is observed in the vertical direction near the outlet.The thermal dispersion coefficient is 0.72 and dispersion angle γ is 78.7°.

Reaction Kinetic Equation for Char Combustion of Underground Coal Gasification

Based on the quasi-steady-state approximation, the dynamic equation of char combustion in the oxidation zone of underground coal gasification (UCG) was derived. The parameters of the dynamic equation were determined at 900℃ using a thermo-gravimetric (TG) analyzer connected to a flue gas analyzer and this equation. The equation was simplified for specific coals, including high ash content, low ash content, and low ash fusibility ones. The results show that 1) the apparent reaction rate constant increases with an increase in volatile matter value as dry ash-free basis,2) the effective coefficient of diffusion decreases with an increase in ash as dry basis, and 3) the mass transfer coefficient is independent of coal quality on the whole. The apparent reaction rate constant, mass-transfer coefficient and effective coefficient of diffusion of six char samples range from 7.51×104 m/s to 8.98×104 m/s, 3.05×106 m/s to 3.23×106 m/s and 5.36×106 m2/s to 8.23×106 m2/s at 900℃, respectively.

Prediction of cavity growth rate during underground coal gasification using multiple regression analysis

During underground coal gasification （UCG）, whereby coal is converted to syngas in situ, a cavity is formed in the coal seam. The cavity growth rate （CGR） or the moving rate of the gasification face is affected by controllable （operation pressure, gasification time, geometry of UCG panel） and uncontrollable （coal seam properties） factors. The CGR is usually predicted by mathematical models and laboratory experiments, which are time consuming, cumbersome and expensive. In this paper, a new simple model for CGR is developed using non-linear regression analysis, based on data from 1 l UCG field trials. The empirical model compares satisfactorily with Perkins model and can reliably predict CGR.

A back analysis of the temperature field in the combustion volume space during underground coal gasification

The exact shape and size of the gasification channel during underground coal gasification(UGC) are of vital importance for the safety and stability of the upper parts of the geological formation.In practice existing geological measurements are insufficient to obtain such information because the coal seam is typically deeply buried and the geological conditions are often complex.This paper introduces a cylindrical model for the gasification channel.The rock and soil masses are assumed to be homogeneous and isotropic and the effect of seepage on the temperature field was neglected.The theory of heat conduction was used to write the equation predicting the temperature field around the gasification channel.The idea of an excess temperature was introduced to solve the equations.Applying this model to UCG in the field for an influence radius,r,of 70 m gave the model parameters,u1,2,3...,of 2.4,5.5,8.7...By adjusting the radius(2,4,or 6 m) reasonable temperatures of the gasification channel were found for 4 m.The temperature distribution in the vertical direction,and the combustion volume,were also calculated.Comparison to field measurements shows that the results obtained from the proposed model are very close to practice.

Changes in properties of tar obtained during underground coal gasification process

In this study,the composition of tars collected during a six-day underground coal gasification(UCG)test at the experimental mine‘Barbara’in Poland in 2013 was examined.During the test,tar samples were taken every day from the liquid product separator and analysed by the methods used for testing properties of typical coke oven(coal)tar.The obtained results were compared with each other and with the data for coal tar.As gasification progressed,a decreasing trend in the water content and an increasing trend in the ash content were observed.The tars tested were characterized by large changes in the residue after coking and content of parts insoluble in toluene and by smaller fluctuations in the content of parts insoluble in quinoline.All tested samples were characterized by very high distillation losses,while for samples starting from the third day of gasification,a clear decrease in losses was visible.A chromatographic analysis showed that there were no major differences in composition between the tested tars and that none of the tar had a dominant component such as naphthalene in coal tar.The content of polycyclic aromatic hydrocarbons(PAHs)in UCG tars is several times lower than that in coal tar.No light monoaromatic hydrocarbons(benzene,toluene,ethylbenzene and xylenes—BTEX)were found in the analysed tars,which results from the fact that these compounds,due to their high volatility,did not separate from the process gas in the liquid product separator.

An exsitu underground coal gasification experiment with a siderite interlayer:course of the process,production gas,temperatures and energy efficiency

A 72-h ex situ hard coal gasification test in one large block of coal was carried out.The gasifying agent was oxygen with a constant flow rate of 4.5 m^(3)/h.The surroundings of coal were simulated with wet sand with 11%moisture content.A 2-cm interlayer of siderite was placed in the horizontal cut of the coal block.As a result of this process,gas with an average flow rate of 12.46 m^(3)/h was produced.No direct influence of siderite on the gasification process was observed;however,measurements of CO_(2)content in the siderite interlayer before and after the process allow to determine the location of high-temperature zones in the reactor.The greatest influence on the efficiency of the gasification process was exerted by water contained in wet sand.At the high temperature that prevailed in the reactor,this water evaporated and reacted with the incandescent coal,producing hydrogen and carbon monoxide.This reaction contributes to the relatively high calorific value of the resulting process gas,averaging 9.41 MJ/kmol,and to the high energy efficiency of the whole gasification process,which amounts to approximately 70%.

A thermo-mechanical simulation for the stability analysis of a horizontal wellbore in underground coal gasification

The stability analysis of horizontal wells is essential for a successful underground coal gasification(UCG)operation.In this paper,a new 3D coupled thermo-mechanical numerical modeling is proposed for analyzing the stability of UCG horizontal wells.In this model,the effect of front abutment stresses,syngas pressure,syngas temperature and thermal stresses is considered to predict the mud weight window and drilling mud pressure during UCG process.The results show that the roof caving in UCG panel has a greatest impact on the stability of horizontal well.Moreover,when the time of coal gasification is increased,the well convergence increases and for more stability it is necessary to increase the drilling mud pressure.This research was carried out on the M2 coal seam in Mazino coal deposit(Iran).The results showed that the mud weight window for horizontal well drilling is between 0 and 33 MPa.The appropriate stress for the maximum stability of the horizontal well,taking all the thermal and mechanical parameters into account,is 28 MPa.The suggested numerical method is a comprehensive and consistent way for analyzing the stability of horizontal wells in UCG sites.

Coal decarbonization:A state-of-the-art review of enhanced hydrogen production in underground coal gasification

The world is endowed with a tremendous amount of coal resources,which are unevenly distributed in a few nations.While sustainable energy resources are being developed and deployed,fossil fuels dominate the current world energy consumption.Thus,low-carbon clean technologies,like underground coal gasification(UCG),ought to play a vital role in energy supply and ensuring energy security in the foreseeable future.This paper provides a state-of-the-art review of the world's development of UCG for enhanced hydrogen production.It is revealed that the world has an active interest in decarbonizing the coal industry for hydrogen-oriented research in the context of UCG.While research is ongoing in multiple coal-rich nations,China dominates the world's efforts in both industrial-scale UCG pilots and laboratory experiments.A variety of coal ranks were tested in UCG for enhanced hydrogen output,and the possibilities of linking UCG with other prospective technologies had been proposed and critically scrutinized.Moreover,it is found that transborder collaborations are in dire need to propel a faster commercialization of UCG in an ever-more carbon-conscious world.Furthermore,governmental and financial support is necessary to incentivize further UCG development for large-scale hydrogen production.

Production of synthetic hydrocarbons from coal through its underground gasification

The problem of the high-level processing of coal into synthetic motor fuels assumes worldwide actual meaning nowadays. Thereat, it is important especially for countries and regions which possess extensive coal resources and are forced to be guided by the import of liquid and gas hydrocarbons. However, a greater emphasis is paid to the given issue in Russia-The development of the federal program for highlevel processing of coal into synthetic motor fuels was initiated. This article describes options of underground coal gasification (UCG) use for the generation of hydrocarbons from UCG gas in the process of the Fischer-Tropsch synthesis (FTS). The technical and economic analysis of the integrated UCG-FTS powerchemical factories has detected their investment attractiveness and practicability of experimental-industrial testing at coal deposits of the Russian Federation.

Economic overview of a two-agent process for underground coal gasification with Fischer-Tropsch- based poly-generation

Underground Coal Gasification(UCG)is a mining-gasification process that produces synthesis gas from in situ coal by injecting air(or oxygen and steam)and extracting the produced gas via boreholes.The resulting synthesis gas(‘syngas’)produced by UCG can be used directly as a fuel-gas or deep-cleaned to produce many other products like chemicals,liquid fuels,hydrogen and synthetic natural gas.UCG has significant environmental,safety and practical advantages over conventional coal mining and utilization,foremost being cost and emissions reductions.In South Africa,facilities of an industrial-scale Fischer-Tropsch(FT)have been operating for decades on coal-to-liquid and gas-to-liquid conversion.Oxygen-blown UCG produces syngas,which contains a mixture of carbon monoxide(CO)and hydrogen,and these molecules can be conditioned to serve as feed for synthetic-crude-oil(‘syncrude’)conversion technologies based on the FT process.African Carbon Energy(Pty)Ltd.(Africary)completed a study integrating its UCG technology with mini gas-to-liquids plant.Syngas from Africary’s Theunissen Underground Coal Gasification(TUCG)project will be used for power production and synthesis of liquid fuels in a unique poly-generation configuration.The TUCG process will consist of two parallel gasifiers,operated on different agents,making the component parts of UCG and coal-to-(any product)(CtX)tightly linked and interdependent,but reducing both cost and emissions.UCG-CtX offers a lower capital investment to conventional underground mining and surface gasification due to the removal of the surface gasifier and coal-mine operations.The gas clean-up systems will remove undesirable components from each gasifier and blend the cleaned syngas for H2:CO ratio control and provide implementation of carbon capture and sequestration.This study has minimized the complexity and optimized the process flow to provide own-use electricity and about 4000 barrels/day equivalent fuels at a capital cost estimate(2017)of about US$350 million and operating cost of around 28 US$/barrel.

Enhanced-hydrogen gas production through underground gasification of lignite

Underground coal gasification is one of the clean technologies of in-situ coal utilization.Hydrogen production from underground gasification of lignite was investigated in this study based on simulation experiments.Pyrolysis of lignite, gasification activity, oxygen-steam gasification and the effect of groundwater influx were studied.As well, the advantages of lignite for stable underground gasification were analyzed.The results indicate that lignite has a high activity for gasification.Coal pyrolysis is an important source of hydrogen emission.Under special heating conditions, hydrogen is released from coal seams at temperatures above 350 °C and reaches its maximum value between 725 and 825 °C.Gas with a hydrogen concentration of 40% to 50% can be continuously obtained by oxygen-steam injection at an optimum ratio of steam to oxygen, while lignite properties will ensure stable gasification.Groundwater influx can be utilized for hydrogen preparation under certain geological conditions through pressure control.Therefore, enhanced-hydrogen gas production through underground gasification of lignite has experimentally been proved.

An analysis of new generation coal gasification projects

The global trends of increasing oil and gas costs have compelled coal possessing countries to start long term underground coal gasification （UCG） projects. These enhance national energy security and are among the cleanest, ecologically safest coal utilization technologies. This paper delineates the major characteristics of such technologies and analyzes technical solutions. Highlighting the desire to develop large scale industrial UCG plants, pilot level projects are presented using a new UCG method developed in Russia by Joint Stock Company Gazprom Promgaz. This method is distinct for its high controllability, stability, and energy efficiency. New, efficient technical solutions have been developed over the last 10-15 years and are patented in Russia. They guarantee controllability and stability of UCG gas produc- tion. Over one hundred iniection and gas production wells have been operated simultaneously.

Experimental forward and reverse in situ combustion gasification of lignite with production of hydrogen-rich syngas

This research focused on the feasibility of applying the forward and reverse combustion approach to the in situ gasification of lignite with the production of hydrogen-rich syngas(H_(2)and CO).The so-called forward combustion gasification(FCG)and reverse combustion gasification(RCG)approach in which oxygen and steam are simultaneously fed to the simulated system of underground coal gasification(UCG)was studied.A simulated system of UCG was designed and established.The underground conditions of the coal seam and strata were simulated in the system.The combustion gasification of lignite has been carried out experimentally for almost 6.5 days.The average effective content(H_(2)+CO)of syngas during the FCG phase was 62.31%and the maximum content was 70.92%.For the RCG phase the corresponding figures are 61.33%and 67.91%.Thus,the feasibility of using RCG way for UCG has been demonstrated.The temperature profiles have been provided by using of 85 thermocouples during the model experiment,which portrayed the several nephograms of thermal data in the gasifier were of significance for the prospective gasification processes.

Polycyclic aromatic hydrocarbon formation under simulated coal seam pyrolysis conditions

Coal seam pyrolysis occurs during coal seam fires and during underground coal gasification. This is an important source of polycyclic aromatic hydrocarbon （PAH） emission in China. Pyrolysis in a coal seam was simulated in a tubular furnace. The 16 US Environmental Protection Agency priority controlled PAHs were analyzed by HPLC. The effects of temperature, heating rate, pyrolysis atmosphere, and coal size were investigated. The results indicate that the 3-ring PAHs AcP and AcPy are the main species in the pyrolysis gas. The 2-ring NaP and the 4-ring Pyr are also of concern. Increasing temperature caused the total PAH yield to go through a minimum. The lowest value was obtained at the temperature of 600℃. Higher heating rates promote PAH formation, especially formation of the lower molecular weight PAHs. The typical heating rate in a coal seam, 5 ℃/min, results in intermediate yields of PAHs. The total PAHs yield in an atmosphere of N2 is about 1.81 times that seen without added N2, which indicates that an air flow through the coal seam accelerates the formation of PAHs. An increase in coal particle size reduces the total PAHs emission but promotes the formation of 5- and f-ring PAHs.