Prediction of burning rate of coal in fluidized-bed combustion

A theory has been proposed to evaluate the burning rate of a single carbon particle in a continuously operated coal-fired fluidized bed. Experimental verification was carried out in a laboratory scale 200 mm × 200 mm combustor. The burning rate increases with the increase of the fluidization velocity and the size of the bed material. The predicted data on burning rate agree fairly well with the experimental values. The gas concentration in the bed and freeboard has also been measured and it is seen that the consumption of oxygen mostly takes place in the bed.     

Reactivity of bituminous coal chars during the initial stage of pulverized-coal combustion

The reactivities of pyrolysed and partially burned char particles prepared in an entrained-flow reactor have been investigated. The results indicate that chars collected at the end of the active devolatilization stage are more reactive than those collected before or after this stage. Deactivation of the pyrolysed chars was accompanied by the development of micropores. The chars produced from a liptinite-rich fraction of an HVA bituminous coal showed higher reactivities than those generated from an inertinite-rich fraction. It is suggested that residual volatiles play a more important role in determining char reactivity than the microporosity and the optical anisotropy of the chars. A new expression for TGA reactivity is suggested for use in deriving char combustion kinetics. Relatively constant activation energies of ≈ 125 kJ mol⁻¹ were obtained for chars prepared from a wide range of coal precursors. Calculated char combustion rates at high temperatures extrapolated from such reactivity parameters were in agreement with experimentally determined rates.     

A model on chemical looping combustion of methane in a bubbling fluidized-bed process

We developed a mathematical model to discuss the performance of chemical looping combustion (CLC) of methane in continuous bubbling fluidized-beds. The model considers the particle population balance, oxidation and reduction rate of particles in fluidized beds. It also considers utilization efficiency of oxygen carrier (OC) particles, residence time of particles in each reactor, and particle size in reaction rate. The model was applied for a bubbling coreannulus fluidized-bed process. The core bed was the fuel reactor (0.08 m-i.d., 2.1 m-height) and the annulus bed was the air reactor (0.089 m-i.d., 0.15 m-o.d., 1.6 m-height). The process employed a type of Ni-based OC particles. The present model agrees reasonably well with the combustion efficiency measured in the process. Simulation was performed to investigate the effects of some variables for the process. The present model revealed that the range of circulation rate of OC particles for achieving complete combustion determined the operating range of the CLC system. The minimum circulation rate of OC particles for complete combustion decreased in the considered operating range as temperature or bed mass increased in the fuel reactor. A large mass of the fuel bed was necessary to obtain complete combustion at low fuel reactor temperature. The fresh feed rate of OC particles for steady state operation increased in complete combustion condition as temperature or static bed height or gas velocity increased.     

流化连续造粒过程传热传质分析

根据连续造粒机造粒室内的温度分布状况 ,将流化连续造粒过程沿高度方向分成四个区域 ,并对各区域粉粒的传热传质进行了分析 ,得出了适合于连续造粒机的传热计算式 ,这对该类机型的研制开发具有一定的指导意义。     

Influence of mineral matter in coal on decomposition of NO over coal chars and emission of NO during char combustion

NO-char reaction and char combustion in the presence and absence of mineral matter were studied in a quartz fixed bed reactor. Eight chars were prepared in a fluidized bed at 950 °C from four Chinese coals that were directly carbonized without pretreatment or were first deashed before carbonization. The decomposition of NO over these coal-derived chars was studied in Ar, CO/Ar and O₂/Ar atmospheres, respectively. The results show that NO is more easily reduced on chars from the raw coals than on their corresponding deashed coal chars. Mineral matter affects the enhancement both of CO and O₂ on the reduction of NO over coal chars. Alkali metal Na in mineral matter remarkably catalyzes NO-char reaction, while Fe promotes NO reduction with CO significantly. The effect of mineral matter on the emission of NO during char combustion was also investigated. The results show that the mineral constituents with catalytic activities for NO-char reaction result in the decrease of NO emission, whereas mineral constituents without catalytic activities lead to the increase of NO emission. Correlation between the effects of mineral matter on NO-char reaction and NO emission during char combustion was also discussed.     

Thermogravimetric assessment of combustion characteristics of blends of a coal with different biomass chars

In connection to future energy demand and fossil fuel crisis particularly in India, biomass is gaining its importance for possible use as co-fuel. In India varieties of biomass products are available which do have tremendous potentiality for co-combustion with pulverized coal. Based on the emerging need, detailed investigations are felt necessary to examine the compatibility of different kind of biomass with coal and to select suitable blend composition(s) before utilizing those biomass products in utility operation as co-fuels. This study elaborates the lab scale findings of combustion experiments in DSC-TGA apparatus with a typical Indian coal, two biomass samples and low temperature biomass chars (300 and 450 °C) as well as with ‘blends of low temperature chars and coal’. Conventional TGA parameters, activation energy and ignition index of different blends were estimated which provided elaborate information on their basic combustion features. Results of non-isothermal combustion studies in general depict that blends containing less than 50% biomass char are better performing as compared those with higher biomass char content. Lowering of activation energy and improvement of reactivity in major combustion zone were also observed in the coal/biomass-char blends. Improvement of ignition index of the blends of coal with 300 °C chars over expected weighted mean values was noticed. Such attempts may help to identify appropriate biomass-type, blend proportion for a given coal and to derive some specific advantages with respect to particular combustion practice.     

Leaching of ashes and chars for examining transformations of trace elements during coal combustion and pyrolysis

One sub-bituminous coal and two bituminous coals were subjected to the combustion and pyrolysis by slow heating to a temperature ranging 550-1150 °C. Leaching of raw coals, ashes and chars with dilute HCl and HNO₃ was carried out, and leachate concentrations of major and trace elements were determined. Such a comparative leaching method was validated for characterizing the modes of occurrence of trace elements in coal and their transformations upon heating. Leaching results suggested that Be, V, Co, Cr and Ni were partially associated with organic matter, and As was partially associated with pyrite. During the ashing at 550-750 °C, the organically associated trace elements in coal formed some acid-soluble species. After the ashing at 1150 °C, Be, Co, Cr and Ni, together with Mn, Zn, and Pb, were immobilized in ash against leaching, whereas As was not immobilized. After pyrolysis, the organically associated trace elements in chars remained insoluble in both acids, and some HNO₃-soluble As in coal turned to a HNO₃-insoluble species.     

Enhancement of sorbent utilization in fluidized-bed combustion

A new, simple and non-corrosive method is described which can substantially reduce the lime requirement in fluidized-bed combustion of coal. The method entails spraying the limestone particles before they are fed to the combustor with an aqueous solution of iron (ferric and/or ferrous) sulphate and/or sulphite. These compounds rapidly decompose in the environment of the combustor and Fe₂O₃ is deposited on the lime surface. Small amounts of Fe₂O₃ deposited in this manner substantially catalyse the rate of SO₂ uptake. For example, 1.4% Fe₂O₃ on Greer limestone nearly doubles the rate. The catalysis would translate into a calculated reduction of the lime requirement by over 40%, although direct translation would require large-scale testing. The basis of the catalysis is that Fe₂O₃ catalyses the oxidation of SO₂ to form SO₃. The SO₂ capacity of the limestone is also enhanced. This simple technique may also be applied to various hot-gas desulphurization processes, such as those with the dry scrubbers, and to other SO₂ sorbents.     

无烟煤化学链燃烧过程燃料氮转化释放特性

基于赤铁矿石载氧体,在小型单流化床反应器上,开展煤挥发分和焦炭的化学链燃烧研究,探讨挥发分氮和焦氮在化学链燃烧过程中的转化特性。研究表明:燃料氮释放的中间产物HCN和NH₃与铁矿石载氧体具有较高的化学反应亲和性,易于被载氧体氧化生成N₂和NO。淮北无烟煤挥发分氮转化过程中,NO是唯一的氮氧化物,反应器出口中间产物NH₃的释放份额略高于HCN。在煤焦化学链燃烧还原过程中,部分燃料氮释放的中间产物HCN和NH₃被铁矿石氧化导致少量NO的生成,还原过程中无N₂O的释放;较高的还原反应温度加速了NO的生成。减少进入载氧体氧化再生过程的焦炭量可减少空气反应器NO和N₂O的生成。     

The role of fuel reactivity in fluidized-bed combustion

This paper reports the use of the two-phase bubbling fluidized-bed model for calculations of the conditions in fluidized-bed combustors. The model takes account of the sequential steps of oxygen transport from bubble to dense phases, transport through the dense phase, and finally reaction with the fuel particle. At 1200 K, for practical conditions of pressure, fluidizing velocity, particle size, bed voidage and bubble size, fuel reactivity was sufficiently high, and bubble-to-dense phase transport sufficiently rapid, to ensure that neither of these steps had a notable rate-controlling effect. Transport through the dense phase was of most influence. However, some effect of reactivity was shown at 1000 K, and a strong effect occurred at 800 K. The model proved to be markedly sensitive to the carbon concentration in the bed, bubble size, and particle size.     

Robust model predictive control of an industrial partial combustion fluidized-bed catalytic cracking converter

It is presented here in the study of the application of a robust model predictive control to an industrial partial combustion fluidized-bed catalytic cracking (FCC) converter. This particular type of FCC converter shows an interesting dynamics in which most of the system outputs are integrating with respect to the manipulated inputs. Time delays are also present and the model parameters can change depending on the operating point. Then, the system model should be represented by a set of possible plants, which can stand for different operating conditions of this process system. Moreover, one needs to include a comprehensive model formulation in order to accommodate time-delays for both stable and integrating outputs. The proposed control strategy was tested through simulation for the disturbances commonly found in the FCC converter unit, taking into consideration the plant/model mismatch. Results obtained from the simulated scenarios point out a fine prospective method. The robust controller shows a good potential to be implemented in the real process.     

Migration and emissions of selenium during chemical looping combustion of coal

Selenium is one of the most volatile toxic elements in coal, and its emissions must be strictly controlled. Chemical looping combustion (CLC) is a clean and efficient technology for coal. Herein, the iron-based oxygen carrier (OC) was used as an adsorbent to study the migration and emissions of selenium during the CLC of coal. Due to the oxidation and adsorption of selenium by iron-based OC, most of the selenium was retained in OC or distributed in the CO₂ stream. The proportion of gaseous selenium released into the atmosphere was less than 10%—significantly lower than that from the traditional combustion process of coal, which had a value of 91.79%. The presence of OC increased the distribution phase of selenium, promoted the conversion of gaseous selenium to solid selenium, and reduced selenium emissions in flue gas. During CLC of coal, the fuel reactor (FR) temperature and the number of OC re-oxidation cycles played an important role in the emissions and retention of selenium. The increasing FR temperature increased the gaseous selenium in the CO₂ stream, reduced the particulate selenium absorbed by OC, and reduced the selenium emissions in the atmosphere. After 10 continuous CLC cycles, the selenium concentration in OC increased from 0.889 to 8.20 mg kg⁻¹. The continuous cycling of CLC could realize the enrichment of selenium from coal to OC. Furthermore, the migration and transformation mechanism of selenium during CLC was deduced by experiments and thermodynamic simulation. This research provides a suitable reference for reducing selenium emissions and developing CLC technology.     

煤化学链燃烧载氧体研究进展

煤的化学链燃烧是清洁煤燃烧的重要技术之一。化学链中载氧体的使用可以避免煤和空气直接接触,从而避免氮氧化物等污染物的产生并提高能量转化效率。一般来说,煤的化学链燃烧有2种反应途径:煤气化化学链燃烧和氧解耦化学链燃烧;不同反应途径将极大影响载氧体组分以及结构设计。详细论述了2015-2020年煤化学链燃烧中固态金属载氧体的研究进展,包括铁基、锰基、铜基、镍基、硫酸钙以及其他复合金属载氧体。总结了不同金属载氧体的优缺点、反应路径、气-固和固-固反应机理、金属与载体的相互作用以及载氧体失活原理。铁基载氧体被广泛应用于气化化学链燃烧中,但单一铁基载氧体的反应速率较低。适量添加碱金属或碱土金属可以提升载氧体的反应活性。锰基载氧体在化学链燃烧中具有两面性:一方面可以在高温缺氧气氛中释放气态氧,另一方面也可以与还原性气体发生气-固反应。通过使用惰性载体以及碱金属添加剂可以提高锰基载氧体的机械强度和氧解耦能力。含铜载氧体具有出色的氧解耦能力和反应活性而被广泛关注,然而铜及其氧化物低熔点所带来的金属聚集导致载氧体的失活问题亟需克服。研究发现使用铁、锰和铜矿石制得的载氧体具有良好的反应性能。硫酸钙载氧体具有较好的反应活性,但煤的化学链燃烧时潜在的二氧化硫和硫化氢副产物需要引起重视。镍基载氧体虽然在煤的化学链燃烧中反应性能较好,但硫毒化、成本较高和环保性能不佳等缺点导致近年来镍基载氧体的研究较少。新型双金属或多金属载氧体可以同时结合2种金属的反应特性,从而显著提高载氧体的整体反应活性。基于载氧体的研究现状,对未来的发展方向提出了4点建议:结合2种煤的化学链燃烧机理设计新型氧解耦辅助化学链燃烧载氧体;发展新型材料和金属组分的载氧体;利用冶金工业废料制得载氧体;开发新型结构的载氧体。     

Experimental and mechanistic study on chemical looping combustion of caking coal

Under high-temperature batch fluidized bed conditions and by employing Juye coal as the raw material,the present study determined the effects of the bed material,temperature,OC/C ratio,steam flow and oxygen carrier cycle on the chemical looping combustion of coal.In addition,the variations taking place in the surface functional groups of coal under different reaction times were investigated,and the varia-tions achieved by the gas released under the pyrolysis and combustion of Juye coal were analyzed.As revealed from the results,the carbon conversion ratio and rate were elevated significantly,and the vol-ume fraction of the outlet CO2 remained more than 92％ under the oxygen carriers.The optimized reac-tion conditions to achieve the chemical looping combustion of Juye coal consisted of a temperature of 900℃,an OC/C ratio of 2,as well as a steam flow rate of 0.5 g·min-1.When the coal was undergoing the chemical looping combustion,volatiles primarily originated from the pyrolysis of aliphatic-CH3 and-CH2,and CO and H2 were largely generated from the gasification of aromatic carbon.In the CLC process,H2O and CO2 began to separate out at 270 ℃,CH4 and tar began to precipitate at 370 ℃,and the amount of CO2 was continuously elevated with the rise of the temperature.     

Mercury control technologies for coal combustion and gasification systems

Development and testing of mercury control technologies have largely focused on coal-fired combustion systems, with less emphasis on advanced power systems. Mercury control is influenced by coal properties and chemistry, plant configuration, pollution control devices, flue gas conditions, and plant operations, which differ between combustion and gasification systems. Sorbents such as treated activated carbons have shown promising results in low-temperature environments; however, elevated temperature and reducing environments of many advanced systems remain challenging, requiring research and development to obtain acceptable mercury control levels. Concurrent pollutant/multipollutant control strategies that include CO₂ control are critically needed for both conventional and advanced power systems.     

Mathematical model for heat transfer and combustion in a pulverized coal flame

A mathematical model for the heat transfer and combustion of a single coal particle is presented. The model correctly predicts the time of the reaction steps and the temperature of the coal particle during ignition and combustion. Results are given of calculations of the combustion of single coal particles of the hard coal and brown Nazarovo coal at various oxygen concentrations and ambient temperatures. The results of combustion experiments with Irsha-Borodino coal on a combustion test facility are in good agreement with the results of three-dimensional numerical modeling of a pulverized coal flame.     

化工计算传质学的一个改进模型

计算传质学模型由浓度输运方程及其封闭方程和相应的计算流体力学模型组成。使用这一模型可同时计算出湍流传质扩散系数、三维浓度场和速度场分布。但刘伯潭提出的传质封闭方程c2-cε模式,形式复杂而且其模型常数均直接借鉴于传热研究;此外,由于计算流体力学模型对最终浓度场的预测结果有着重要的影响,所以有对其进一步改进的必要。文中对传质封闭方程进行了简化并重新确定了模型常数,而且对计算流体力学模型进行了改进使其预测结果与文献中的实验值符合得更好。通过与文献中的湍流传质扩散系数及浓度值的比较,证明了这一简化的可行性。