Production of power using underground coal gasification

Underground coal gasification (UCG) is a process that converts deep, un-mineable coal resources into syngas, which can then be converted into valuable end products such as electric power. This paper provides a summary of the options to combine UCG with electric power production and focuses on commercial-scale applications using a combined-cycle power plant including integration options and syngas cleanup steps. Simulation results for a UCG power plant with carbon capture are compared against the results for an equivalent Integrated Gasification Combined Cycle (IGCC) plant using the same feedstock. Relative capital cost savings for a UCG power plant are estimated based on published IGCC process unit costs. The UCG power plant with carbon capture is shown to provide a higher thermal efficiency, lower CO₂ intensity, and lower capital cost than an equivalent IGCC plant. Finally, the potential of UCG as a method for producing cost-effective, low-emissions electrical power from deep coal is discussed and some of the challenges and opportunities are summarized.     

Underground coal gasification: From fundamentals to applications

Underground coal gasification (UCG) is a promising option for the future use of un-worked coal. UCG permits coal to be gasified in situ within the coal seam, via a matrix of wells. The coal is ignited and air is injected underground to sustain a fire, which is essentially used to “mine” the coal and produce a combustible synthetic gas which can be used for industrial heating, power generation or the manufacture of hydrogen, synthetic natural gas or diesel fuel. As compared with conventional mining and surface gasification, UCG promises lower capital/operating costs and also has other advantages, such as no human labor underground. In addition, UCG has the potential to be linked with carbon capture and sequestration. The increasing demand for energy, depletion of oil, and gas resources, and threat of global climate change have lead to growing interest in UCG throughout the world. The potential for UCG to access low grade, inaccessible coal resources and convert them commercially and competitively into syngas is enormous, with potential applications in power, fuel, and chemical production. This article reviews the literature on UCG and research contributions are reported UCG with main emphasis given to the chemical and physical characteristic of feedstock, process chemistry, gasifier designs, and operating conditions. This is done to provide a general background and allow the reader to understand the influence of operating variables on UCG. Thermodynamic studies of UCG with emphasis on gasifier operation optimization based on thermodynamics, biomass gasification reaction engineering and particularly recently developed kinetic models, advantages and the technical challenges for UCG, and finally, the future prospects for UCG technology are also reviewed.     

Movable injection point–based syngas production in the context of underground coal gasification

Underground coal gasification (UCG) has been proven as a viable technology for the generation of high calorific value syngas using deep mine coal seams. The use of multiple injection points/movable injection point method could be an alternate technique for efficient gasification of high ash Indian coals. In this context, the present study is focused on evaluating the heating value of syngas using a variety of gasifying agents such as pure O₂, air, humidified O₂, and CO₂-O₂ dual-stage gasification under movable injection method for high ash coals. It is found that the use of movable injection point method had significantly increased the heating value of the product gas, compared with the fixed point injection method. For high and low ash coal under pure O₂ gasification, the calorific value of syngas obtained using movable injection point is 123.2 and 153.9 kJ/mol, which are 33.5% and 24.3% higher than the syngas calorific value obtained using fixed injection point, respectively. Further, the air as a gasification agent for high ash coals had increased the gross calorific value of the syngas by 24%, using this technology. The results of high ash coal gasification using humidified oxygen at optimum conditions (0.027-kg moisture/kg dry O₂) and CO₂-O₂ gas had enhanced the syngas calorific value by 12.6% and 5%, respectively. Humidified O₂ and CO₂-O₂ gasifying agents produced a high-quality syngas with the calorific value of 190 kJ/mol, among the gasifying agents used. The experimental results had shown that the movable injection point method is found to be a better alternative for the generation of calorific value-enriched syngas using high ash-based Indian coals.     

Integration of underground coal gasification with a solid oxide fuel cell system for clean coal utilization

Underground coal gasification (UCG) is a promising clean coal technology. Typically, the syngas obtained from UCG is used for power generation via the steam turbine route. In the present paper, we consider UCG as a hydrogen generator and investigate the possibility of coupling it with a solid oxide fuel cell (SOFC) to generate electrical power directly. We show, through analysis, that integration with SOFC gives two specific advantages. Firstly, because of the high operating temperature of the SOFC, its anode exhaust can be used to produce steam required for the operation of UCG as well as for the reforming of the syngas for the SOFC. Secondly, the SOFC serves as a selective absorber of oxygen from air which paves the way for an efficient system of a carbon-neutral electrical power generation from underground coal. Thermodynamic analysis of the integrated system shows considerable improvement in the net thermal efficiency over that of a conventional combined cycle plant.     

Public perceptions of underground coal gasification in the United Kingdom

There is growing interest internationally in the technology of Underground Coal Gasification (UCG) as a means of accessing the energy contained within inaccessible coal reserves. One of the potential obstacles to UCG deployment is adverse public perceptions and reactions, either stopping or delaying proposed applications. This paper explores the public perceptions of UCG in the UK through a detailed case-study and focus group discussion. A failed proposal for a UCG drill site at Silverdale (Staffordshire) provides an opportunity to understand the influence of local social, cultural and institutional factors on the manner in which the risks and benefits associated with UCG are perceived. The participants of the focus group recognised the potential of UCG as a secure source of energy for the UK in the future, provided that it is safe to humans and the environment and cost-effective. The group discussed potential benefits to the local community, potential risks, the role of carbon dioxide capture and storage, and links to the hydrogen economy. The group recommended that an open, transparent and consultative process of decision-making and operation should be adopted by the developer, operator and regulator; and that UCG should be developed at a remote site, preferably on land, before applying it in coal seams close to populated areas.     

Thermal–mechanical modelling around the cavities of underground coal gasification

Underground coal gasification (UCG) is an efficient method for the conversion of the deep coal resources into energy. This paper is concerned with a feasibility study of the potential of deeply lying coal seams (>1200 m) for the application of UCG combined with subsequent storage of CO₂ for a site located in Bulgaria. A thermal–mechanical coupled model was developed using the ABAQUS software package to predict the heat transfer, the stress distributions around the UCG and the consequent surface subsidence. Material properties of rocks and coal were obtained from existing literature and geomechanical tests which were carried out on samples derived from the demonstration site in Bulgaria. Three days of gasification has been simulated by assigning a moving heat flux on a cell of 2 m × 2 m × 2 m at a velocity of 2 m/day. Results of temperature and stress distribution showed that the developed numerical model was able to simulate the heat propagation and the stress distribution around cavities under a thermal–mechanical coupled loading during the UCG process. Also, the surface subsidence was found to be 0.08 mm after three days of gasification for the case studied. It is anticipated that the results of this paper can be used for the prediction and optimization of the UCG process in deep coal seams.     

Exergy analysis in the assessment of hydrogen production from UCG

The demand for H₂ increases rapidly with the gradual recognition of the potential of H₂ 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 H₂ production pathway. This paper presents comparative study of energy utilization and resource consumption in H₂ production by UCG and typical surface coal gasification (SCG) technology, namely Lurgi fixed bed gasification, with 1.2 billion Nm³/a throughput of H₂ 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 H₂ production from UCG and SCG. The result shows that although the H₂ production from UCG has higher CExC, it can significantly reduce the resource consumption of equipment comparing with H₂ production from SCG, indicating its advantage in total investment. It is found that the exergy efficiency increases with the rise in H₂O-to-O₂ and O₂-to-CO₂ ratio, while the value of CExC decreases with the appreciation of H₂O-to-O₂ ratio yet increases as the O₂-to-CO₂ 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.     

Evaluation method of the energy conversion efficiency of coal gasification and related applications

Traditional gasification parameters, such as cold gas efficiency, hot gas efficiency, or thermal efficiency, only evaluate the heat energy utilisation efficiency of gasifiers and do not take into account the gasification processes expending electricity and other types of energies. Therefore, the energy conversion efficiency cannot be assessed using these parameters. The calculation process on the energy conversion efficiency of underground coal gasification (UCG) is the basis for obtaining quantitative data of carbon emission reduction and establishing the carbon trading methodology of UCG. Moreover, the energy conversion efficiency both for surface coal gasification and UCG is a key research topic because it directly affects the economic and environmental benefits of gasification projects. This study proposed that two parameters, the integrated gasification efficiency (h_com) and the hot gas integrated gasification efficiency ( ), should be included into the coal gasification parameters and used to evaluate the energy conversion efficiency of coal gasification. In addition, the calculation methods of these two parameters for both surface gasification and UCG were established. Using the method, h_com and , of the UCG and Texaco gasification under the same scale was compared and that of various UCG processes was calculated. The results proved the necessity and reasonability of the two parameters and suggested that a certain amount of CO₂ was favourable to improve h_com and of UCG. However, a certain amount of pure O₂ can improve h_com of UCG without direct influences on . Under the condition of each process, to maximise h_com and , there must be an optimal steam (CO₂) to O₂ rate. Copyright © 2015 John Wiley & Sons, Ltd.     

The effect of sludge from coal to oil process on the stability of coal/sludge water slurries

Sludge produced from coal to oil process contains larger content of tar, phenols, ammonia components, and ashes. It cannot be treated by using traditional disposal methods as landfill or incineration. A promising solution is to blend sludge with coal for preparation of sludge/coal-water slurries. The new slurry fuels could be used in commercial gasifiers for syngas generation. In this process, the first goal is to form a stable slurry. In the research project, the addition of sludge on the stability of coal-water slurries was investigated. Results show that the addition of sludge can improve the static stability for slurries prepared by lean coal, coking coal, and lignite. The effect on stability of coking coal-water slurry is the most significant. The proportion of sludge to lean coal added in the slurry can be maintained at 10% to 15%.     

煤质对水煤浆气化性能的影响研究

煤质与气流床气化炉的匹配性至关重要,其不但影响气化炉的运行条件,也影响气化性能。本文选择了10种来自新疆和陕西北部的煤样进行了工业分析、元素分析、灰组成分析、灰熔点分析以及成浆性测试,并筛选出适合水煤浆气化的煤样。同时借助Aspen Plus软件对适合水煤浆气化的煤样在相同的煤浆浓度、碳转化率及操作压力条件下开展煤质对水煤浆气化性能影响的模拟分析。结果表明煤中灰含量越高,冷煤气效率和有效气含量越低,比氧耗和比煤耗越高;煤中O/C质量比和H/C质量比的增加也会导致冷煤气效率和有效气含量降低,比氧耗和比煤耗增加。因此从水煤浆气化经济性考虑,建议水煤浆气化煤质灰含量小于9.0wt%,煤中O/C质量比小于0.173,H/C质量比小于0.065。     

整体煤气化联合循环(IGCC)发电技术介绍

整体煤气化联合循环（IGCC）发电技术是煤气化和蒸汽联合循环的结合,是当今国际正在兴起的一种先进的洁净煤（CCT）发电技术,具有高效、低污染、节水、综合利用好等优点。它的原理是：煤经过气化和净化后,除去煤气中99％以上的硫化氢和接近100％的粉尘,将固体燃料转化成燃气轮机能燃用的清洁气体燃料,以驱动燃气轮机发电,再使燃气发电与蒸汽发电联合起来。     

Simulation of cavity formation in underground coal gasification using bore hole combustion experiments

Studies on the growth of three-dimensional cavity geometries in underground coal gasification (UCG) are important in exploiting the large fraction of coal that is present in underground coal seams. In the present study, the cavity formation in UCG has been simulated using experiments carried out in three configurations: (i) sublimation experiments in camphor simulating primarily the heat transfer aspects, (ii) bore hole combustion in Acacia nilotica wood bringing in chemical reaction into play, and (iii) bore hole combustion a coal block bringing into consideration the effect of ash on the cavity formation. In all the three cases, the time-evolution of the cavity shape has been monitored under constant oxygen flow rate conditions by measuring the cavity shape and size at periodic intervals. Results show that the cavity formation rates as well as the shape of the cavity are significantly affected by the oxidant flow rate. The importance of the ash present in the coal on the cavity growth has also been brought out. A fair amount of gasification leading to the formation of H₂, CO and CH₄ was observed; this is shown to depend both on the inherent moisture as well as on the reaction zone temperature.     

Transient gasification and syngas formation from coal particles in a fixed‐bed reactor

An experimental investigation on gasification and syngas formation from coal particles in a fixed‐bed reactor is conducted; particular attention is paid to the transient reaction dynamics. Three different coals, including two high‐volatile coals and a low‐volatile coal, are taken into consideration. In the initial reaction period, a two‐stage reaction is clearly observed; specifically, an exothermic reaction followed by an endothermic reaction is exhibited. Meanwhile, seeing that the devolatilization and pyrolysis reactions are pronounced, the initial concentrations of H₂ and CH₄ are relatively high, especially for the former. With increasing time, the interaction between coal and char particles is dominated by the latter, the concentrations of CO and CO₂ thus become higher. From the observation of syngas combustion, the entire gasification intensity proceeds from intensified growth, rapid decay, and then to progressive decay with increasing reaction time. For the two high‐volatile coals, the mass depletion is enhanced markedly once the reaction temperature is as high as 1000°C, whereas it is insensitive to the temperature for the low‐volatile coal. Nevertheless, it is found that, based on the weights of moisture and volatile matter, their relative release ratio from the low‐volatile coal is better than that from the high‐volatile coals. This implies that the final devolatilization and pyrolysis extent is not determined by coal grade. Copyright © 2006 John Wiley & Sons, Ltd.     

积极推进煤炭地下气化技术的试验研究和示范应用

本文简要介绍了国内外煤炭地下气化的发展概况。与传统的煤炭地下开采相比,煤炭地下气化技术具有安全、高效、污染少等优点,是我国开展节能减排、调整能源结构和发展绿色经济的重要途径。积极推进煤炭地下气化技术研究和示范应用具有战略意义。提出我国煤炭地下气化技术需要尽快由目前的工业化试验阶段转入示范应用阶段。本文总结了影响我国煤炭地下气化技术发展的3个方面的主要问题,提出了积极推进煤炭地下气化技术的试验研究和示范应用的6项建议。     

Parametric techno‐economic studies of coal/biomass co‐gasification for IGCC plants with carbon capture using various coal ranks,fuel‐feeding schemes,and syngas cooling methods

Because of biomass's limited supply (as well as other issues involving its feeding and transportation), pure biomass plants tend to be small, which results in high production and capital costs (per unit power output) compared with much larger coal plants. Thus, it is more economically attractive to co‐gasify biomass with coal. Biomass can also make an existing plant carbon‐neutral or even carbon‐negative if enough carbon dioxide is captured and sequestered (CCS). As a part of a series of studies examining the thermal and economic impact of different design implementations for an integrated gasification combined cycle (IGCC) plant fed with blended coal and biomass, this paper focuses on investigating various parameters, including radiant cooling versus syngas quenching, dry‐fed versus slurry‐fed gasification (particularly in relation to sour‐shift and sweet‐shift carbon capture systems), oxygen‐blown versus air‐blown gasifiers, low‐rank coals versus high‐rank coals, and options for using syngas or alternative fuels in the duct burner for the heat recovery steam generator (HRSG) to achieve the desired steam turbine inlet temperature. Using the commercial software, Thermoflow^®, the case studies were performed on a simulated 250‐MW coal IGCC plant located near New Orleans, Louisiana, and the coal was co‐fed with biomass using ratios ranging from 10% to 30% by weight. Using 2011 dollars as a basis for economic analysis, the results show that syngas coolers are more efficient than quench systems (by 5.5 percentage points), but are also more expensive (by $500/kW and 0.6 cents/kW h). For the feeding system, dry‐fed is more efficient than slurry‐fed (by 2.2–2.5 points) and less expensive (by $200/kW and 0.5 cents/kW h). Sour‐shift CCS is both more efficient (by 3 percentage points) and cheaper (by $600/kW or 1.5 cents/kW h) than sweet‐shift CCS. Higher‐ranked coals are more efficient than lower‐ranked coals (2.8 points without biomass, or 1.5 points with biomass) and have lower capital cost (by $600/kW without using biomass, or $400/kW with biomass). Finally, plants with biomass and low‐rank coal feedstock are both more efficient and have lower costs than those with pure coal: just 10% biomass seems to increase the efficiency by 0.7 points and reduce costs by $400/kW and 0.3 cents/kW h. However, for high‐rank coals, this trend is different: the efficiency decreases by 0.7 points, and the cost of electricity increases by 0.1 cents/kW h, but capital costs still decrease by about $160/kW. Copyright © 2016 John Wiley & Sons, Ltd.     

Theory of reverse combustion linking

The present work suggests a theory of reverse combustion linking (RCL). RCL is a central unit operation of underground coal gasification (UCG) technology. The theory is based on analyzing the stability of different branches of the propagation speed curves and determining the regime that is responsible for propagation of the flame during RCL. The theory is in good qualitative and quantitative agreement with the data obtained in practical use of RCL in UCG operations.     

赤泥-铜矿石复合载氧体用于煤化学链气化性能研究

煤化学链气化制合成气是一种资源利用率高、环境污染低、节能环保的新型气化技术,而高效载氧体的设计开发是化学链气化技术的关键。本文以铜矿石和赤泥为原料采用挤出滚圆法制备R-Cu-10M（蒙脱石质量分数为10%）复合金属载氧体,实现载氧体颗粒内粉末的物理均匀混合、颗粒一次成型以及活性组分间的协同效应。围绕反应温度、氧煤比、水蒸气输入量三个关键操作变量,测试了R-Cu-10M载氧体与褐煤气化反应特性。表征结果表明,R-Cu-10M载氧体具有较好的还原性,赤泥与铜矿石中Cu-Fe金属间的协同效应有助于晶格氧释放以及还原性的提升。R-Cu-10M载氧体与褐煤发生气化反应的最佳温度为950℃,在氧煤比为3∶1、水蒸气通量为0.08 mL/min的最优工况下,合成气产量可以达到50 mmol/g载氧体,合成气选择性和碳转化率分别为75.9%和71.2%。     

Life cycle energy consumption and GHG emissions of hydrogen production from underground coal gasification in comparison with surface coal gasification

Developing underground coal gasification (UCG)-based hydrogen production (UCG-H₂) 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-H₂ and typical surface coal gasification (SCG)-based hydrogen production (SCG-H₂) is carried out using life cycle assessment method. Result shows energy consumption of UCG-H₂ is only 61.2% of that of SCG-H₂, 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-H₂ and SCG-H₂ are roughly equivalent, which are 207,582 and 197,419 kg CO₂-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.     

我国煤炭清洁利用战略探讨

中国在一定时期内能源结构仍将以煤为主,而煤炭的清洁利用将是我国未来能源战略的关键问题。目前煤炭清洁利用单元技术主要有整体煤气化联合循环(IGCC)、煤炭地下气化技术(UCG)以及以煤气化为龙头,以碳一化工技术为基础,合成、制取化工产品和燃料油的现代煤化工。现代煤化工包括煤制油、煤制天然气、煤制烯烃、煤制乙二醇、煤制乙炔等,其产品大多属于石油化工产品的替代品。现代煤化工在世界范围内还没有完全成熟的技术和成功的工业化经验可借鉴,尚处在探索、验证阶段,发展前景仍存在不确定性。为此有学者提出,煤化工项目应与IGCC发电等一起发展多联产系统,从而实现经济效益最大化、环境污染最小化。煤基多联产比单纯的IGCC发电具有更好的经济性、更高的能量效率以及更加灵活的操作性,而作为先进的洁净煤技术,IGCC将成为未来能源系统的核心技术和重要基础之一。煤气化是IGCC的核心技术,煤炭地下气化技术开辟了煤炭高效、清洁、低碳开发利用的新途径,是从根本上解决传统开采方法存在的一系列技术和环境问题的重要途径。近年来,我国现代煤化工产业发展迅速,但同时也呈现出过热和无序发展态势。目前煤化工的经济性并没有得到充分论证和认可,国内当前正在运营的项目较大部分仍处于试点阶段。以煤气化为龙头的IGCC多联产是我国煤炭清洁利用的战略方向。目前我国IGCC和多项现代煤化工技术已具备技术推广的条件,国家应该从管理体制、政策法规、融资等方面给予大力扶持。     

Laboratory studies on cavity growth and product gas composition in the context of underground coal gasification

Systematic laboratory scale experiments on coal blocks can provide significant insight into the underground coal gasification (UCG) process. Our earlier work has demonstrated the various features of the early UCG cavity shape and rate of growth through lab-scale experiments on coal combustion, wherein the feed gas is oxygen. In this paper, we study the feasibility of in situ gasification of coal in a similar laboratory scale reactor set-up, under conditions relevant for field practice of UCG, using an oxygen-steam mixture as the feed gas. By performing the gasification reaction in a cyclic manner, we have been able to obtain a product gas with hydrogen concentrations as high as 39% and a calorific value of 178 kJ/mol. The effect of various operating parameters such as feed temperature, feed steam to oxygen ratio, initial combustion time and so on, on the product gas composition is studied and the optimum operating conditions in order to achieve desired conversion to syngas, are determined. We also study the effect of various design and operating parameters on the evolution of the gasification cavity. Empirical correlations are proposed for the change in cavity volume and its dimensions in various directions. The results of the previous study on the combustion cavity evolution are compared with this gasification study.