Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO₂ is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO₂ capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH₄ concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO₂ capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO₂ capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.     

The generation of high temperature gas from fluidized bed furnaces

The work described relates to the development of a coal-fired fluidized bed furnace for the generation of hot gases for industrial heating processes. Following a programme of coal model studies of the internal solids circulation rates between adjacent beds, a furnace test facility was developed in the form of two interconnected fluidized beds. One bed is operated as a partial gasifier/pyrolyser and the other as a char combustor. The gas produced by pyrolysis is mixed above the bed with the oxygen-rich gas from the char combustor and burns to give hot gas at temperatures of up to 1600°C. The use of low ash washed singles grade coal (13–25 mm size range) gives an overall combustion efficiency better than 98 per cent.     

Physicochemical properties and gasification reactivity of the ultrafine semi-char derived from a bench-scale fluidized bed gasifier

Zhundong coalfield is the largest intact coalfield worldwide and fluidized bed gasification has been considered as a promising way to achieve its clean and efficient utilization.The purpose of this study is to investigate the physieochemical properties and gasification reactivity of the ultrafine semi-char,derived from a bench-scale fluidized bed gasifier,using Zhundong coal as fuel.The results obtained are as follows.In comparison to the raw coal,the carbon and ash content of the semi-char increase after partial gasification,but the ash fusion temperatures of them show no significant difference.Particularly,76.53％ of the sodium in the feed coal has released to the gas phase after fluidized bed gasification.The chemical compositions of the semi-char are closely related to its particle size,attributable to the distinctly different natures of diverse elements.The semi-char exhibits a higher graphitization degree,higher BET surface area,and richer meso-and macropores,which results in superior gasification reactivity than the coal char.The chemical reactivity of the semi-char is significantly improved by an increased gasification temperature,which suggests the necessity of regasification of the semi-char at a higher temperature.Consequently,it will be considered feasible that these carbons in the semi-char from fluidized bed gasifiers are reclaimed and reused for the gasification process.     

温度对中试规模的喷动流化床煤部分气化行为的影响

在构建的热输入2MW的中试规模加压喷动流化床部分气化试验装置上,对徐州烟煤的加压部分气化行为进行了研究.重点考察了气化温度对煤气成分、煤气热值、碳转化率和煤气产率等气化指标的影响.研究结果表明,煤气各组分的浓度,特别是甲烷浓度对气化温度非常敏感,碳转化率和煤气产率随温度的升高而增加,在试验的温度区间内,温度对煤气热值影响不大.部分气化炉所产生的煤气和半焦的热值均满足第二代增压流化床联合循环发电系统的要求.     

A proposed scheme for coal fired combined cycle and its concise performance

For IGCC, the primary investment is too high due to the demand of high gasification efficiency. For PFBCC, the thermal efficiency is too low due to the relatively low turbine inlet temperature and the hot working medium of the gas turbine is not easy to clean. A new scheme is proposed for coal fired combined cycle to overcome the main drawbacks of IGCC and PFBCC. The research targets are developing a new cycle construction of coal fired combines cycle to raise the efficiency and reduce the primary investment. Actually, the new scheme is a synthesis of some existing proposals. It adopts partial gasification to reduce the primary investment of the gasification equipment. The un-gasified surplus solid is then feed to a pressurized fluidized bed boiler, but adopting Curtiss Wright type external combustion to lower the ash content in the working medium. The gas fuel from the partial gasifier is combusted in a top combustor to further increase the working medium temperature. An extremely concise performance estimation method for the new scheme and its equations is proposed in order to easily understand the basic physical meaning of the new system. Some typical calculations based on this concise method are given. Then, a more detailed computation is accomplished with Aspen Plus code. The basic feasibility of this scheme is proven to be favorable. The efficiency is higher than the existing coal fired IGCC plants. The advantage of the new scheme comes from the better utilization of coal energy. Almost all the energy of coal is first utilized in the top cycle, and then the bottom cycle, just like the gas fueled combined cycle does. The primary investment is lower than the ordinary IGCC due to the lack of air separation unit and the adoption of partial gasification. The ash content is much lower than that of the existing PFBCC plants. If no any harmful ash in working medium is required, the atmospheric fluidized bed can be applied rather than the pressurized fluidized bed. A similar proposal with atmospheric fluidized bed and its performance estimation are also given. However, its efficiency will be lower than the pressurized fluidized bed scheme.     

Bench-scale bubbling fluidized bed systems around the world - Bed agglomeration and collapse: A comprehensive review

Thermochemical conversion by gasification process is one of the most relevant technologies for energy recovery from solid fuel, with an energy conversion efficiency better than other alternatives like combustion and pyrolysis. Nevertheless, the most common technology used in the last decades for thermochemical conversion of solid fuel through gasification process, such as coal, agriculture residues or biomass residues are the fluidized bed or bubbling fluidized bed system. For these gasification technologies, an inert bed material is fed into reactor to improve the homogenization of the particles mixture and increase the heat transfer between solid fuel particles and the bed material. The fluidized bed reactors usually operate at isothermal bed temperatures in the range of 700–1000 °C, providing a suitable contact between solid and gas phases. In this way, chemical reactions with high conversion yield, as well as an intense circulation and mixing of the solid particles are encouraged. Moreover, a high gasification temperature favours carbon conversion efficiency, increasing the syngas production and energy performance of the gasifier. However, the risk of eutectic mixtures formation and its subsequent melting process are increased, and hence the probability of bed agglomeration and the system collapse could be increased, mainly when alkali and alkaline earth metals-rich biomasses are considered. Generally, bed agglomeration occurs when biomass-derived ash reacts with bed material, and the lower melting temperature of ash components promotes the formation of highly viscous layers, which encourages the progressive agglomerates creation, and consequently, the bed collapse and system de-fluidization. Taking into account the relevance of this topic to ensure the normal gasification process operating, this paper provides several aspects about bed agglomeration, mostly for biomass gasification systems. In this way, chemistry and mechanism of bed agglomeration, as well as, some methods for in-situ detection and prediction of the bed agglomeration phenomenon are reviewed and discussed.     

Transformation characteristics of Na and K in high alkali residual carbon during circulating fluidized bed combustion

In this study, the transformation characteristics of sodium (Na) and potassium (K) during combustion of Zhundong coal gasification fly ash in circulating fluidized bed (CFB) reactors were investigated by examining gasification fly ash (TCf) from a 0.1-MW CFB test system. Experimental results indicated that TCf was rich in Na and K, with water-soluble and insoluble Na the main Na forms. Insoluble K was the major K form in TCf, accounting for 70.6% of total K. Reactor bed temperature exerted important effects on Na release during combustion such that, as bed temperature increased, the proportions of Na in bottom and circulating ash decreased while the Na in fly ash increased. Hydrochloric acid-soluble and insoluble Na in ash accounted for a large fraction of total Na. However, insoluble K was the principle K form in ash and bed temperature showed little influence on K release and distribution in ash during combustion. With decreased flue gas temperature, the Na content in deposition ash initially increased, then decreased, and eventually stabilized, while the K content in deposition ash was basically unchanged. Agglomeration of ash particles occurred during combustion, being more apparent at higher gas temperatures, and the agglomerates were rich in Na, K, sulfur (S), chlorine (Cl), and calcium (Ca). Deposition ash Na was mainly contained NaCl and Ca/Na sulfates. The enrichment of these salts as well as of Ca sulfate in ash was the main cause of ash agglomeration and deposition.     

煤基化学链燃烧串行流化床的冷态试验研究

采用“湍动床＋快速床”作为煤基化学链燃烧（CLC）系统的空气反应器（AR）,鼓泡床作为燃料反应器（FR）,设计了流动密封阀和旋风分离器,分别用于隔绝2个反应器之间的气氛和进行气固分离,在冷态试验装置上分析研究了CLC系统的压力分布、固体循环流量、气体泄漏率及煤灰与循环载体的分离效果．结果表明：该串行流化床反应器之间气氛隔绝性良好,气体泄漏率较低,固体循环流量达到甚至超过设计标准,FR二级旋风分离器的分离效率接近100％,FR中煤灰进入AR的质量分数小于1．55％,煤灰分离效果良好;装置可以长时间连续稳定运行,且操作气速范围较广,自行设计建造的循环流化床作为煤基化学链燃烧试验装置是可行的．     

Description,applications and numerical modelling of bubbling fluidized bed combustion in waste-to-energy plants

The use of the fluidized bed combustor (FBC) has increased. It began in the 20th century as coal combustion and gasification, which then developed into catalytic reactions. Only recently, the application field has been extended to the incineration of biomass and pre-treated waste, for either power generation or waste disposal. The success of fluidized bed combustion is due to high combustion efficiency, great flexibility when it comes to the heating value of the fuel and reduction in pollutants emitted with the flue gas.     

Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier

Chemical looping combustion is a novel technology that can be used to meet the demand on energy production without CO₂ emission. To improve CO₂ capture efficiency in the process of chemical looping combustion of coal, a prototype configuration for chemical looping combustion of coal is made in this study. It comprises a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and a loop-seal. The loop-seal connects the spout-fluid bed with the fast fluidized bed and is fluidized by steam to prevent the contamination of the flue gas between the two reactors. The performance of chemical looping combustion of coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier in a 1 kW_th prototype. The experimental results show that the configuration can minimize the amount of residual char entering into the air reactor from the fuel reactor with the external circulation of oxygen carrier particles giving up to 95% of CO₂ capture efficiency at a fuel reactor temperature of 985 °C. The effect of the fuel reactor temperature on the release of gaseous products of sulfur species in the air and fuel reactors is carried out. The fraction of gaseous sulfur product released in the fuel reactor increases with the fuel reactor temperature, whereas the one in the air reactor decreases correspondingly. The high fuel reactor temperature results in more SO₂ formation, and H₂S abatement in the fuel reactor. The increase of SO₂ in the fuel reactor accelerates the reaction of SO₂ with CO to form COS, and COS concentration in the fuel reactor exit gas increases with the fuel reactor temperature. The SO₂ in the air reactor exit gas is composed of the product of sulfur in residual char burnt with air and that of nickel sulfide oxidization with air in the air reactor. Due to the evident decrease of residual char in the fuel reactor with increasing fuel reactor temperature, it results in the decrease of residual char entering the air reactor from the fuel reactor, and the decrease of SO₂ from sulfur in the residual char burnt with air in the air reactor.     

Gasification of lignocellulosic biomass in fluidized beds for renewable energy development: A review

A literature review on gasification of lignocellulosic biomass in various types of fluidized bed gasifiers is presented. The effect of several process parameters such as catalytic bed material, bed temperature and gasifying agent on the performance of the gasifier and quality of the producer gas is discussed. Based on the priorities of researchers, the optimum values of various desired outputs in the gasification process including improved producer gas composition, enhanced LHV, less tar and char content, high gas yield and enhanced carbon conversion and cold gas efficiency have been reported. The characteristics and performance of different fluidized bed gasifiers were assessed and the obtained results from the literature have been extensively reviewed. Survey of literature revealed that several industrial biomass gasification plants using fluidized beds are currently conducting in various countries. However, more research and development of technology should be devoted to this field to enhance the economical feasibility of this process for future exploitations.     

An experimental study of propane combustion in a fluidized-bed gasifier

This paper reports an experimental study of the combustion of volatiles from coal, simulated by propane, and its interaction with char gasification reactions in a fluidized-bed coal gasifier (FBG). Experiments were performed under conditions of propane pyrolysis (in a bed fluidized by nitrogen and steam), propane reacting with oxygen and steam (in a bed fluidized by air and steam), char gasification only (in a bed fluidized by air and steam without propane) and in char gasification (in a bed fluidized by air and steam with propane). Axial concentration profiles of various species were obtained at 750, 850 and 950 °C. It was observed that the combustion of propane in an FBG, but without char, behaves as reported in the literature for fluidized-bed combustion (FBC). However, upon introducing char into the bed to simulate the reducing atmosphere in an FBG, oxygen was rapidly consumed within a short distance of the distributor, by significant partial oxidation of both propane and its decomposition products to carbon monoxide. The char was found to aid the pyrolysis of propane, limiting the amount of hydrocarbons surviving into the freeboard. The experimental results reported here are believed to be the first observations on the combustion of volatiles under conditions in an FBG.     

焦油主要组份在循环灰下催化裂解的实验研究

在以循环流化床锅炉循环灰为热载体，部分气化产生的半焦为锅炉燃料，煤气为燃气轮机燃料的煤的部分气化联合循环中，降低焦油产率，提高煤气产率是一个技术关键，以焦油的两种主要组份苯和甲苯为研究对象，利用固定床实验台实验研究了一种混煤形成的循环灰条件下的裂解反应特性，测定了裂解反应动力学参数，探讨了循环灰对焦油裂解的催化机理。实验结果表明，与石英砂条件相比，循环灰极大地促进了焦油的裂解程度，气态裂解产物总量     

循环流化床燃煤过程汞控制性能的实验研究

在热态循环流化床实验台上进行了不同工况燃煤过程汞控制特性的研究,得出如下结论:循环流化床燃煤过程对燃煤中汞的排放具有一定的控制作用;多煤种混烧在汞的控制方面优于单煤种燃烧;煤中掺入石灰石可以有效地减少汞向大气的排放;燃烧的煤种不同,汞的排放特性也不相同.     

褐煤在鼓泡流化床和循环流化床燃烧的汞迁移试验研究

对褐煤在小型电加热鼓泡流化床和小型电加热循环流化床中燃烧时的汞迁移特性进行了对比试验研究,重点考察了不同燃烧工况对汞迁移特性的影响。试验结果表明,炉膛温度和给煤量增加,鼓泡流化床和循环流化床的烟气总汞HgT均增加,飞灰颗粒汞含量Hg(p)均减少,并且循环流化床的烟气总汞HgT值均低于相同燃烧工况的鼓泡流化床值,循环流化床的飞灰颗粒汞含量Hg(p)值均高于相同燃烧工况的鼓泡流化床的值;流化风速增加,循环流化床的烟气总汞HgT减少,飞灰颗粒汞含量Hg(p)增加,鼓泡流化床烟气总汞HgT增加,飞灰颗粒汞含量Hg(p)减少。     

410 t/h循环流化床锅炉煤焦混烧热力性能试验研究

在额定蒸发量为410t/h的循环流化床锅炉上进行以烟煤、石油焦混合物为燃料的锅炉热力性能试验,根据ASME锅炉性能规程计算了锅炉热效率,并对温度、汽水流量、气态污染物排放、灰渣含碳量等多个参数进行了测试,整理得到了一些规律性的结果。结果表明,锅炉以烟煤、石油焦为燃料,添加石灰石脱硫,采用尾部飞灰再循环,额定负荷下锅炉热效率可达92.8%;炉内密相区温度分布均匀;床温、分离器入口温度、排烟温度和排渣温度等各温度稳定;主蒸汽流量、给水流量、减温水流量等汽水流量波动小;气态污染物均可控制在较低水平。这对我国循环流化床锅炉燃用煤焦混合物的设计和运行工作有一定借鉴意义。     

Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation

A new process integrating a circulating fluidized bed (CFB) reactor and an entrained bed reactor was proposed for gasification of preheated coal. The CFB reactor as a preheater was successfully used in clean coal combustion. In this study, gasification of preheated coal was tested in a bench-scale test rig, which consisted of a CFB preheater and a down flow bed (DFB) gasifier. The effects of operating parameters of the preheater and gasifier were revealed via thermodynamic equilibrium calculations. A stable preheating process was obtained in the CFB preheater at the O₂/C molar ratio of 0.31 and higher gasification reactivity was gained in preheated char owing to the improvement in intrinsic reactivity, specific surface area and total pore volume. Effective gasification of preheated char was achieved in the DFB gasifier at 1100 °C and the total O₂/C molar ratio of 0.67, meanwhile the CO + H₂ yield and carbon conversion increased. Thermodynamic equilibrium calculations revealed when the gasification reaction rates varied little above 1100 °C and the same carbon conversion was achieve in gasifier, lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O₂ demand.     

流化床部分煤气化影响因素研究

在流化床部分气化炉上系统研究了流化风量、给煤量、水蒸气量、床层温度、静止床层高度、煤种、催化剂等因素对煤气成分和热值的影响,研究结果表明:流化风量、给煤量、水蒸气量、静止床层高度对煤气成分的影响较为复杂,4者都存在最佳范围;床层温度是影响煤气成分的主要因素,煤气热值与温度成正比;增加床层高度,有利于H2、CO生成和CH4分解;烟煤的煤气中含有的可燃成分(H2、CO、CH4)含量比无烟煤高,优质烟煤比劣质烟煤更适合于气化;Ca、Na、K等碱土金属化合物对煤气化具有催化作用,且Na2CO3和K2CO3的催化能力比CaO强。     

Experimental study of wood char gasification kinetics in fluidized beds

During gasification two steps take place. The first one is pyrolysis and the second one is gasification of the char that remains back after pyrolysis. The second step is slower than the first one, so this step is the limiting factor in designing fluidized beds. Kinetic data for designing fluidized beds are necessary. The paper describes gravimetric measurements directly applied to fluidized bed with large sample sizes. The samples are char of 6 mm wood pellets and 10–40 mm wood cubes in order to directly measure ”apparent kinetics”. The parameters examined in this paper are particle size, product gases (= hydrogen) in the gasification medium, type of wood and differences in CO₂/steam gasification. The results are presented as Arrhenius diagrams and half-value period diagrams. The most important parameters are the temperature and product gases (hydrogen) in the gasification agent. The particle size seems to be less important for large wood particles as the measurements do not show significant differences for gasifying char of wood cubes 10–40 mm. The half-value periods for gasification of char from wood cubes (10 mm - 40 mm) with 100% steam at atmospheric pressure lie between 1000 s at 1023 K and 300 s at 1173 K. For char of 6 mm wood pellets the half-value periods lie between 1900 s at 1023 K and 250 s at 1173 K. The reaction is most likely in pore diffusion regime.     

A review of oxy‐fuel combustion in fluidized bed reactors

Presently, there is no detailed review that summarizes the current knowledge status on oxy‐fuel combustion in fluidized bed combustors. This paper reviewed the existing literature in heat transfer, char combustion and pollutant emissions oxy‐fuel combustion in fluidized beds, as well as modelling of oxy‐fuel in FB boiler and gaps were identified for further research direction. Copyright © 2016 John Wiley & Sons, Ltd.