Study on pulverized coal gasification using waste heat from high temperature blast furnace slag particles

A combined species transport and reaction-discrete phase model was established to numerically study pulverized coal gasification using waste heat from high temperature slag particles. The effects of slag particles temperature, coal/gasification agent mass ratio and water content in gasification agent on the gasification characteristics were discussed. The results indicate that higher particle temperature leads to better gasification reaction efficiency. Compared to the maximum syngas productivity (67.9%) and carbon conversion efficiency (91.7%) at 1500 K, they are respectively reduced to about 45% and 60% when temperature drops to 1000 K. Excessive or insufficient pulverized coal would have a negative effect on the syngas production for a specific flow rate of gasification agent, and the appropriate proportion range is 0.8–0.84. The CO yield declines with the increase of particles diameter, while H₂ firstly increases and then declines attributing to the lower gasification agent temperature and higher flow velocity gained at larger diameter. The raise of water content in gasification agent is beneficial to H₂ production, but CO yield continues to decline after the water content exceeds 5% for the reason that the incomplete combustion of volatiles and the gasification reaction of coke are inhibited. The diameter of slag particles and the water content suitable for coal gasification reaction are 2.0–2.5 mm and 5%–10%, respectively.     

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO₂ and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO₂ and H₂O reaction rate coefficients for a 100 μm coal char particle reacting in environments of varying O₂, H₂O, and CO₂ concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO₂ gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O₂ concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.     

我国发展煤制天然气误区分析

从煤炭中的C转化成CH4,需要进行煤气化、脱硫、CO变换、脱除CO2,然后甲烷化反应。在这一生产过程中,碳的利用率和热能转换率均约为1/3,制取1000m3的CH4要放出约3.34t的二氧化碳。按照我国拟建和在建的煤制天然气规模360×108m3/a、碳的利用率1/3计,将浪费煤炭5664×104t标煤,排放二氧化碳1.2×108t,总投资需2100亿元。据测算,煤制天然气生产成本约为3元/m3CH4,与管输进口天然气相比,价格上没有竞争性,并带来环境污染。由于煤制天然气投资费用高(1000m3/a天然气的投资费用约合5833元)、碳与热能利用率低、污染源处理费用高,所以煤制天然气不应该是煤清洁利用的发展方向。我国常规天然气储量和产量迅速增加,预计到2020年天然气产量将达到2000×108m3(约合2×108t油当量),而有关机构预测我国2020年天然气消费量为1.46×108t油当量,国产常规天然气产量就可满足国内燃料消费需求,为此我国完全没有必要大规模建煤制天然气项目。     

我国天然气供需现状及煤制天然气工艺技术和经济性分析

我国天然气消费市场持续增长,2008年天然气消费量达807×10^8m3,比上年增长10．1％;2020年天然气需求将增至2500×10^8m3,供应缺口达1000×10^8m3。与国际天然气价格相比,我国天然气价格水平仍然偏低。煤制天然气可以作为液化石油气和常规天然气的替代和补充,缓解我国天然气供应缺口。其竞争力主要源于可采用低价劣质煤．需要选择的主要是煤气化及甲烷化技术。含水含灰高、低热值的褐煤比较适于碎煤加压固定床或流化床气化。鲁奇煤气化工艺是煤制天然气项目首选的煤气化技术,此外还有流化床气化炉技术、BGL块／碎煤熔渣气化技术。鲁奇甲烷化技术是世界上首个商业化业绩,此外还有托普索公司甲烷化循环工艺技术和Davy甲烷化技术。以某年产10×10^8m3（标准）煤制天然气项目为例,其投资利润率16．16％（平均）,全部投资内部收益率16．21％（所得税后）,投资回收期7．72年,在经济上是可行的。目前一些地方和企业对煤制天然气项目的风险认识不足,首先应正确评价煤制天然气的能源效率和CO2排放,过分强调和夸大煤制天然气这个单一过程的高能源效率是不客观的：其次应认识到原料煤及产品价格是制约煤制天然气项目的关键因素;同时此类项目产品关联度低,并会受到天然气管网建设和管理的制约。     

Numerical Simulation of Pressure Effects on the Gasification of Australian and Indian Coals in a Tubular Gasifier

This article presents a numerical study on the effect of pressure on the gasification performance of an entrained flow tubular gasifier for Australian and Indian coals. Gasification using a substoichiometric amount of air, with or without steam addition, is considered. The model takes into account phenomena such as devolatilization, combustion of volatiles, char combustion, and gasification. Continuous-phase conservation equations are solved in an Eulerian frame and those of the particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The numerical results obtained show that the gasification performance increases for both types of coal when the pressure is increased. Locations of devolatilization, combustion, and gasification zones inside the gasifier are analyzed using the temperature plots, devolatilization plots, and mass depletion histories of coal particles. With increase in pressure, the temperature inside the gasifier increases and also the position of maximum temperature shifts upstream. For the high-ash Indian coal, the combustion of volatiles and char and the gasification process are relatively slower than those for the low-ash Australian coal. The mole fractions of CO and H₂ are found to increase with increase in pressure, in all the cases considered. Further, the effects of pressure on overall gasification performance parameters such as carbon conversion, product gas heating value, and cold gas efficiency are also discussed for both types of coals.     

Gasification of a pulverized sub-bituminous coal in CO2 at atmospheric pressure in an entrained flow reactor

Char gasification by CO₂ may play an important role in oxy-fuel applications and affect particle temperature histories and overall reaction rates during combustion. This paper presents the results of a complete set of experiments of char gasification in CO₂ performed with a pulverized Indonesian sub-bituminous coal in an entrained flow reactor under realistic conditions; series of burnout curves at different reactor temperatures (1040–1300 °C) and CO₂ concentrations (0.7–100%) reveal consistent trends in the gasification rates. The study included also devolatilization and oxidation tests with this coal in the same experimental facility. The data are used to derive apparent kinetics for the three processes, in a manner similar to that followed in a previous work for the oxidation of a pulverized coal. The gasification kinetic parameters and reaction rates measured are then compared with values taken or derived from previous works by others, obtained by thermogravimetric analysis or experiments in entrained flow reactors. Finally, the relevance of char gasification in the overall reaction rate under conditions representative of those in an industrial boiler is explored, in particular for the case of oxy-coal combustion.     

CO_2气氛下劣质煤气化及动力学特性实验研究

利用综合热分析仪以非等温热重法研究了升温速率及粒径对于两种劣质煤粉在CO2气氛下气化反应特性的影响规律,考察了灰分对于两种劣质煤气化反应性的影响,并采用均相反应模型(HM),利用Freeman-Carroll法计算拟合得到各条件下气化反应动力学参数。结果表明:两种劣质煤CO2气化反应级数都是1.0级。反应条件对两种煤样的反应活化能产生了相似的影响:CO2气氛下,在900～1 300℃的样品气化反应区间,当其它条件不变时,随着煤样粒径由150～400μm减小到0～75μm,两种劣质煤样表观活化能呈明显下降趋势;而随着升温速率由30℃/min降至10℃/min,两种煤样反应活化能则在上升。在不同样品粒径及升温速率下,两种煤粉的气化活化能和对应的指前因子之间存在着动力学补偿效应。     

Effect of the type of gasifying agent on gas composition in a bubbling fluidized bed reactor

It is commonly accepted that gasification of coal has a high potential for a more sustainable and clean way of coal utilization. In recent years, research and development in coal gasification areas are mainly focused on the synthetic raw gas production, raw gas cleaning and, utilization of synthesis gas for different areas such as electricity, liquid fuels and chemicals productions within the concept of poly-generation applications. The most important parameter in the design phase of the gasification process is the quality of the synthetic raw gas that depends on various parameters such as gasifier reactor itself, type of gasification agent and operational conditions. In this work, coal gasification has been investigated in a laboratory scale atmospheric pressure bubbling fluidized bed reactor, with a focus on the influence of the gasification agents on the gas composition in the synthesis raw gas. Several tests were performed at continuous coal feeding of several kg/h. Gas quality (contents in H₂, CO, CO₂, CH₄, O₂) was analyzed by using online gas analyzer through experiments. Coal was crushed to a size below 1 mm. It was found that the gas produced through experiments had a maximum energy content of 5.28 MJ/Nm³ at a bed temperature of approximately 800 °C, with the equivalence ratio at 0.23 based on air as a gasification agent for the coal feedstock. Furthermore, with the addition of steam, the yield of hydrogen increases in the synthesis gas with respect to the water–gas shift reaction. It was also found that the gas produced through experiments had a maximum energy content of 9.21 MJ/Nm³ at a bed temperature range of approximately 800–950 °C, with the equivalence ratio at 0.21 based on steam and oxygen mixtures as gasification agents for the coal feedstock. The influence of gasification agents, operational conditions of gasifier, etc. on the quality of synthetic raw gas, gas production efficiency of gasifier and coal conversion ratio are discussed in details.     

Hydrogen-rich gas production from biomass catalytic gasification using hot blast furnace slag as heat carrier and catalyst in moving-bed reactor

A new concept that a heat recovery system from blast furnace (BF) slag which would generate hydrogen-rich gas was proposed and a continuous moving-bed biomass gasification reactor was designed to evaluate the feasibility. The influences of temperature and particle size of BF slag on gasification product distribution and gas characterization were discussed. The results show that BF slag demonstrated better catalytic performance in improving tar cracking, enhancing char gasification and reforming of hydrocarbons; higher BF slag temperature and smaller particles size can produce more light gases, less char and condensate; as temperature of BF slag being 1200 °C and its size below 2 mm, gas yield and H₂ content achieved the maximum being 1.28 N m³/kg and 46.54%, respectively.     

Coal gasification in CO2 atmosphere and its kinetics since 1948: A brief review

Numerous coal gasification studies have been found in the literature those employed various kinds of gasifying agents such as steam and carbon dioxide. These studies are featured with wide variations in the parametric conditions and the usage of equipments. Steam is frequently employed as a gasifying agent, however, in several studies carbon dioxide has also been used as a gasifying agent either pure or in combination with other gasifying agents (H₂O, O₂, CO, H₂). This paper is a brief review of the coal gasification with CO₂ as a diluent. Different factors were studied over the coal gasification with CO₂ such as coal rank, pressure, temperature, gas composition, catalyst and the minerals present inside the coal, heating rate, particle size, and diverse reactor types. It also deals with the application of the gas-solid models developed in the literature and the combustion and gasification mechanisms for O₂/CO₂ streams. Moreover, it reviews the kinetics and the reaction rate equations (Arrhenius and Langmuir-Hinshelwood types) for coal-char gasification both in the reaction kinetic control region (low temperature) and the diffusion control region (high temperature) and at both low and high pressures.     

Comparing prospective hydrogen pathways with conventional fuels and grid electricity in India through well-to-tank assessment

The present study uses Greenhouse Gases, Regulated Emissions, and Energy Use in Technologies Model (GREET), to compare hydrogen generated via multiple pathways (Natural gas, methanol reforming; coal, petcoke, biomass gasification etc) with the conventional fuels like diesel and compressed natural gas and grid electricity under Indian context through a comprehensive well to tank assessment based on net CO₂ equivalent emission and energy consumption. Limited availability of customized studies comparing hydrogen production and supply with other energy options in India distinguishes the present work as it provides a fresh insight into potential pathways for hydrogen production while assessing feedstock availability and raw water consumption. The study reveals that biomass gasification and solar electrolysis are among the least GHG emitting pathways to fill one unit of energy equivalent in the tank. Hydrogen produced through natural gas reforming is 70% less emission intensive and 38% more energy efficient than Indian grid electricity.     

Modelling and experimental investigations on gasification of coarse sized coal char particle with steam

The steam gasification characteristics of coal char produced two sub-bituminous coals of different origin have been investigated through modelling and experiments. The gasification experiments are carried out in an Isothermal mass loss apparatus over the temperature range of 800–900 °C using a gas mixture of 65% steam and 35% N₂. A fully transient single particle gasification model, based on the random pore model, is developed incorporating reaction kinetics, heat and mass transport inside the porous char particle and the gas film. Stefan-Maxwell equation and Knudson diffusion are incorporated in the multi-component diffusion of species and pore diffusion. The model is validated with the experimental data of the present authors as well as that reported in the literature. The particle centre temperature is found to increase, then decrease and increase again to reach the reactor temperature finally, and the trend is more prominent for the larger particles. The pore opening phenomenon is more evident in SBC2 char, leading to a final char porosity of 0.65 vis-à-vis 0.52 in SBC1 and making it more reactive. Temporal evolution of contours of carbon conversion and concentration of other gaseous species like steam, H₂O, H₂, CO and CO₂ in the particle are computed to investigate the gasification process. A higher temperature is found to favour both the rate peak and the total production of H₂ for both the chars. The total H₂ production from SBC2 char is found to be 0.0189 mol and 0.0236 mol at 800 and 850 °C, while the same for SBC1 char is0.0232 mol and 0.0290 mol respectively. The reaction follows the shrinking core model at the outset, shifting to the shrinking reactive core model subsequently.     

三维喷动流化床流动特性数值模拟

为了研究喷动流化床煤部分气化炉的气-固流动特性,采用三维欧拉多相流模型和颗粒动能理论相结合的数学模型,对一台直径100 mm的喷动流化床试验台进行了数值模拟研究.研究内容包括喷动流化床不同工况下内部射流的发展、气-固流动特性、典型工况下气体速度分布、颗粒速度分布以及由于颗粒碰撞引起的颗粒相压力分布.模拟结果表明:典型工况下,当喷动风与总风的比例为50%时,流场有利于煤气化;气体曳力和颗粒碰撞对环形区颗粒特别是靠墙区颗粒的运动影响很大.为了验证模型的合理性,采用文献中的试验工况进行计算,计算结果和文献中的测量值吻合较好.     

Coal char particle secondary fragmentation in an entrained-flow coal-water slurry gasifier

Coal char particle size in the gasifier has an influence on the carbon conversion, gasifier slagging as well as the particle matter content in raw syngas. The particle size distribution in a bench-scale opposed multi-burner (OMB) gasifier illustrated that the secondary fragmentation behavior exists in the entrained-flow gasifier after the particles moving from high temperature impinging flame region to the gasification chamber outlet region. Particles larger than 200 μm in the impinging flame region have porosity structures with fragile shapes. Particle size distributions under different oxygen to carbon ratios (O/C) also indicate that there is an obvious fragmentation while the particle size is larger than 200 μm. As long as the coal char particles move from the impinging flame region towards the gasification chamber outlet region, the secondary fragmentation is probably taken place as a result of percolative fragmentation, undergoing gasification reactions and thermal stress. However, thermal stress fragmentation only has an influence on the particle sizes larger than 350 μm according to the calculation by a simplified mathematical model.     

空气鼓风流化床煤部分气化炉煤气成分与热值试验

在内径100mm，高4．2m的常压流化床气化炉试验装置上进行试验，考察了气化炉温度，空煤比，汽煤比等影响因素对煤气成分和热值的影响，并对其气化机理进行了分析。结果表明，流化床煤部分气化炉内存在较佳气化温度，汽煤比和空煤比区域。图7表3参4     

Molecular dynamics investigation on the gasification of a coal particle in supercritical water

Coal gasification technology in supercritical water provides a clean and efficient way to convert coal to H₂. In the present paper, the whole supercritical water（SWC）gasification process of a coal particle is studied with the reactive force field (ReaxFF) molecular dynamics (MD) method for the first time. First, the detailed reaction mechanism which can't be clearly illustrated in experiments, such as the evolution of the carbon structure during the gasification process and the detailed reaction mechanism of the main products, is obtained. According to the generation mechanism of H₂, it is found that the supercritical water gasification process of a coal particle can be divided into two stages with different reaction mechanisms, namely the rapid reaction stage and the stable reaction stage. Then, the effects of temperature and coal concentration in the reaction system on the yield of H₂ are studied. Finally, the transition of N in the coal particle is revealed, in which the precursors of NH₃ such as CN, CHN, and CHON are the basic molecular structures for nitrogen atoms during the gasification process at high temperature.     

CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification

For the solar thermochemical gasification of coal coke to produce CO + H₂ synthetic gas using concentrated solar radiation, a windowed reactor prototype is tested and demonstrated at laboratory scale for CO₂ gasification of coal coke using concentrated Xe light from a 3-kW_th sun simulator. The reactor was designed to be combined with a solar reflective tower or beam-down optics. The results for gasification performance (CO production rate, carbon conversion, and light-to-chemical efficiency) are shown for various CO₂ flow rates and ratios. A kinetics analysis based on homogeneous and shrinking core models and the temperature distributions of the prototype particle bed are compared with those for a conventional fluidized bed reactor tested under the same Xe light irradiation and CO₂ flow-rate conditions. The effectiveness and potential impacts of internally circulating fluidized bed reactors for enhancing gasification performance levels and inducing consistently higher bed temperatures are discussed in this paper.     

煤制天然气产业发展前景分析

我国天然气供不应求的局面将长期存在,而利用煤炭资源相对丰富的特点发展煤制天然气产业,是缓解我国天然气供求矛盾的一条有效途径。煤制天然气产品的低热值比国家天然气质量标准规定的低热值高17.8%～21%,能量转化效率高。当石油价格为80美元/bbl时,与进口天然气、进口LNG相比,煤制天然气价格具有竞争力。固定床鲁奇炉加压气化技术是煤制天然气较好的选择,而耐硫变换、低温甲醇洗、硫回收、丙烯制冷、压缩干燥等工艺单元不存在技术问题。如果我国企业开发的低温甲烷化技术获得成功,那么煤制天然气项目就可以完全实现国产化,使我国煤制天然气技术居于世界前列。利用低品质的褐煤、采用碎煤固定床鲁奇炉加压气化技术,粗煤气中含有8%～12%的甲烷,气化单元投资仅为气流床气化技术的一半,电耗低,同时可实现焦油、轻油、酚、硫磺、硫酸铵等多联产。但对于排出的"黑水"问题,应采取积极措施尽快予以解决。煤制天然气项目最好在示范装置取得圆满成果之后,特别是环境保护问题解决之后,再进行项目的研究和建设。厂址最好设在坑口或煤炭产地附近,并且要考虑输送问题。