恒温热重法单颗粒煤焦燃烧动力学

引言 煤焦燃烧动力学的研究对确定燃煤特性、燃烧效率、锅炉设计及环境排放等具有重要意义与应用价值.煤焦的燃烧为典型的多孔颗粒的气固反应过程,其主要受到外扩散控制、内扩散控制和化学反应控制的影响.要研究化学反应本身对燃烧过程的影响就需要在实验设计中消除内、外扩散对燃烧过程的影响,或者通过理论或实验的定量计算将内、外扩散的影响予以扣除[1-5];但是,进行理论或实验定量计算内、外扩散的影响实际是建立在经验公式的基础上,对不同体系及物料会引起难以预测的误差.一般说来,实验设计中消除内、外扩散的影响可通过减小煤粒大小和提高气流速度等方式实验确定[4,5].本研究拟采用热失重分析技术,基于煤焦颗粒在热天平中燃烧反应过程的分析,寻找确定燃烧过程中已消除内、外扩散影响的化学反应控制阶段的方法,以获得煤焦颗粒燃烧过程本征化学反应常数及其简便计算方法.     

Boundary layer model for char combustion and gasification

A non-steady boundary layer model is developed for numerical simulation of combustion and gasification of a single shrinking char particle. The model considers mass and energy conservation coupled with heterogeneous char reactions producing CO and homogeneous oxidation of CO to CO₂ in the boundary layer surrounding the char particle. Mass conservation includes accumulation, molecular diffusion, Stefan flow and generation by chemical reaction. Energy conservation includes radiation transfer at the particle surface and heat accumulation within the particle. Simulation results predict experimentally measured conversion and temperature profiles of a burning Spherocarb particle in a laminar flow reactor. Effects of bulk oxygen concentration and particle size on the combustion process are addressed. Predicted particle temperature is significantly affected by boundary layer combustion of CO to CO₂. With increasing particle size, char gasification to char combustion ratio increases, resulting in decreasing particle temperature and increasing peak boundary layer temperature.     

Identification of the mechanism of coal char particle combustion by porous structure characterization

Two limiting models—the shell progressive mechanism and the homogeneous mechanism—can describe combustion of a single coal particle. Some information about the real mechanism can be obtained from investigation of the porous structure development during combustion. Using the principles of gas adsorption and mercury penetration, the porous structure of a partially combusted particle was estimated. Experiments were carried out in an equipment by applying the thermogravimetric method and using a single devolatilized coal particle. The inlet concentration of oxygen was 5 and 15 mol%. The initial temperature of combustion was in a range from 450 to 800 °C. The mechanism of coal char particle combustion depends on the initial temperature and the inlet concentration of oxygen. At low temperature and low inlet concentration of oxygen, the rate of principal chemical reactions is comparable with the rate of diffusional transport of oxygen inside the particle. Combustion is governed by the diffusion mechanism. This is evident from the values of the specific surface area of pores and proportional representation of individual pore types. At higher temperatures and low inlet concentrations of oxygen, combustion proceeds by the shell progressive mechanism. The specific surface area is lower in comparison with the previous case. There is a sharp interface between the particle core and the ash shell. The core exhibits a higher value of specific surface area than in the case of a non-combusted coal char particle. This fact can be explained by the consecutive reaction of carbon dioxide with carbon in the core of the particle. The rate of this reaction is sufficiently high at temperatures above 800 °C.     

包含反应阶数变化的分形煤焦颗粒燃烧模型的建立与实验验证

为了深入研究煤焦燃烧的机理并提高对煤焦燃烧过程的预测精度,建立了一个综合的煤焦燃烧模型。该模型考虑了煤焦颗粒孔隙内二次反应与扩散的耦合作用、煤焦燃烧反应阶数的变化和反应过程中CO/CO₂比例等问题。使用热天平(TGA)对11种煤焦的燃烧特性进行分析,测得各种煤焦的表观活化能与指前因子,以确定模型中的待定参数。在沉降炉(DTF)中对这11种煤焦做燃烧实验,用TGA基于灰分守恒测得DTF炉管出口处的煤焦样品的转化率。运用建立的模型模拟这些煤焦的燃烧过程,预测的转化率与实验结果有较好的吻合度,相比传统的本征动力学模型,该模型预测的精度有了较大提高,证明了该模型能适用于从褐煤到无烟煤的较广煤焦范围。研究还发现,煤焦燃烧的表观反应阶数在燃烧过程中不断减小并最终趋于稳定。     

Modelling of combustion characteristics of high ash coal char particles at high pressure: Shrinking reactive core model

Combustion of a single-particle high ash coal char at elevated pressure has been analyzed. A fully transient shrinking reactive core model incorporating a simple mechanistic kinetic scheme is used to study the combustion characteristics of high ash coal char. The model includes heat and mass transfer phenomena, reaction kinetics and intra-particle details. Finite volume method (FVM) has been used to solve partial differential equations representing fully transient conservation equations. The char combustion model predicts the mass-loss profile and burnout time of the char particle at different temperature and oxygen concentration. The computed results are found to agree well with the published experimental findings of pressurized combustion of high ash coal char. The effects of bulk temperature, total pressure and initial particle size on combustion characteristic and burnout time have been examined through model simulation.     

高温下煤焦气化反应特性(Ⅰ)灰分熔融对煤焦气化反应的影响

在900～1500℃灰分的熔融温度范围内和常压下,研究了煤焦与二氧化碳的气化反应。实验研究发现,活化能随气化过程而变化,除反应初期外,气化反应严重受内扩散过程的影响。高温下的煤焦气化存在一个特性温度,在该温度下,反应中、后期气化速率显著降低,其影响范围在特性温度上下约200～300℃。特性温度与煤种及灰分性质等因素有关,一般低于其相应灰分的软化点温度。     

褐煤焦中的矿物质对气化动力学的影响

利用热重分析仪研究了水蒸气气氛下霍林河褐煤焦和脱灰褐煤焦的气化动力学特性,并考察了脱灰前后褐煤焦孔结构的变化。结果表明:褐煤原焦气化反应速率在反应初始阶段(转化率30%)高于脱灰褐煤焦,但在反应后期低于脱灰焦,这是因为煤焦中灰分的脱除一方面去除了矿物质的催化作用,另一方面增大了煤焦的孔径,因而减小了气化剂的扩散阻力。灰层扩散控制的缩核模型可以描述褐煤焦水蒸气气化过程,而脱灰后褐煤焦水蒸气气化过程用均相模型可以很好地表示。     

Heat transfer in the rotary ash cooler with residual char combustion considered

In circulating fluidized-bed (CFB) boilers, some residual char is left in the ash when the ash is drained into the rotary ash cooler (RAC). The residue char will combust in the RAC given oxygen is present. It is important to understand and predict the influence of residual char combustion on the heat transfer process in the RAC in order to avoid slagging and fouling inside the RAC. In this paper, an improved heat transfer model of the RAC was developed in which the residual char combustion process was considered. Based on the analysis, the shrinking sphere model was selected to describe the residual char combustion. The improved model was validated by the final ash temperature and cooling water temperature data measured from a few RACs equipped with the coal fired CFB boilers with different steam output. The predictions of the improved model agree well with the field measurement data, within an error range of ± 10%. The simulation results also show that the heat released from residual char combustion counts for about 60-80% of the total heat released by ash in the inlet section of the RAC, and the influence of residual char combustion decreases rapidly along the ash flow. The final ash temperature increases accordingly with the increase of inlet carbon content, and decreases with the increases of residual char size. As particle size is larger than 3 mm, the final ash temperature changes little at a given carbon content due to a low mass burning rate. It is suggested that combustion of residual char should be considered when the inlet carbon content is over 2.5%; otherwise, the combustion of residual char in the RAC could be neglected.     

单颗粒煤焦燃烧反应动力学研究方法

采用热天平研究了福建龙岩和加福的两种煤焦的单颗粒燃烧过程。探讨其燃烧过程中的气体扩散、灰层以及反应阻力的影响,建立了单颗粒煤焦燃烧过程分析方法,并由此建立了单颗粒煤焦热天平测定化学反应本征动力学常数的新方法。研究发现,随着单颗粒煤焦燃烧的进行,燃烧总阻力逐渐减小,当反应趋于结束时,燃烧阻力不再随反应时间变化,而是趋于稳定,此时燃烧阻力即为化学反应本征动力学的阻力,由此测定化学反应本征动力学常数。通过对不同温度、不同粒径煤焦以及不同空气流量下的实验与分析,表明该测定方法稳定性好,且测得的煤焦燃烧本征动力学常数和活化能与文献报道一致。     

Modeling and experimental studies on single particle coal devolatilization and residual char combustion in fluidized bed

Single particle devolatilization followed by combustion of the residual coal char particle has been analyzed in a batch-fluidized bed. The kinetic scheme with distributed activation energy is used for coal devolatilization while multiple chemical reactions with volume reaction mechanism are considered for residual char combustion. Both the models couple kinetics with heat transfer. Finite Volume Method (FVM) is employed to solve fully transient partial differential equations coupled with reaction kinetics. The devolatilization model is used to predict the devolatilization time along with residual mass and particle temperature, while the combined devolatilization and char combustion model is used to predict the overall mass loss and temperature profile of coal. The computed results are compared with the experimental results of the present authors for combustion of Indian sub-bituminous coal (15% ash) in a fluidized bed combustor as well as with published experimental results for coal with low ash high volatile matter. The effects of various operating parameters like bed temperature, oxygen mole fraction in bulk phase on devolatilization time and burn-out time of coal particle in bubbling fluidized bed have been examined through simulation.     

Phosphorus‐containing silica gel‐coated ammonium polyphosphate: Preparation,characterization, and its effect on the flame retardancy of rigid polyurethane foam

A phosphorus‐containing silica gel was synthesized via a reaction between phenyl dichlorophosphate, poly(ether polyol), and γ‐aminopropyltriethoxysilane. Ammonium polyphosphate (APP) was modified by the synthesized phosphorus‐containing silica gel (MAPP) and then incorporated into the rigid polyurethane foam (PU). Results showed that APP had a smaller particle size, lower initial decomposition temperature, better heat resistance at high temperature, and better compatibility with PU matrix after the modification. The cone calorimeter test results showed that the incorporation of MAPP obviously reduced the values including peak of heat release rate, total heat release, average effective heat of combustion, and total smoke release, and increased the char yield of PU composite comparing with APP. The improved flame retardancy of PU/MAPP composite was attributed to the quenching effect of PO· and PO₂· free radicals released by MAPP in the early stage and the improved thermal stability of phosphorus‐ and silicon‐containing char layer formed in the later stage. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46334.     

Application of percolation model to particulate matter formation in pressurized coal combustion

Coal is an important energy resource for meeting the future demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are studied. The devolatilization rate of coal is given by the first-order reaction model with FLASHCHAIN^® model [Niksa, S., Combust. Flame, 100, (1995) 384-394.]. The characteristics of a burning coal particle are investigated under the atmospheric and high pressure conditions. The results show that in the atmospheric pressure condition, the characteristics of the burning coal particle obtained by the percolation model are in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. As the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. The porosity in the char-combustion-dominant process decreases with decreasing the initial coal size. It is also observed that the effect of ambient pressure is prominent in the char-combustion-dominant process. The particle diameter and porosity in the pressurized condition are greater than those in the atmospheric pressure condition. These behaviors can be explained by the interaction between char reaction and ash agglomeration.     

Modeling study for the effect of particle size on char gasification with CO2

A high temperature stage microscope to investigate the temperature effect caused by particle size on char gasification is applied in this study. Experiments were carried out with different particle sizes for raw chars and chars on molten slag surface, respectively. Heat transfer models were built for the raw char of two temperature distributions and char particle on molten slag, respectively. Results showed that reaction layer temperature of raw char decreased in the reaction dominant while char on molten slag had higher temperature. Temperature difference between two distributions increased with the initial particle size, indicating the temperature effect on large particles was obvious. Shrinking core model was applied and modified herein coupled with the modification of reaction layer temperature and reaction area. Model prediction and experimental data showed good agreements of carbon conversion and reactivity index for raw char and char on molten slag, respectively. © 2016 American Institute of Chemical Engineers AIChE J, 63: 716–724, 2017     

低阶煤低温热解半焦在模拟高炉喷吹条件下的燃烧性能

采用自制固定床热解装置在隔绝空气的条件下制备神木长焰煤热解终温分别为400℃、450℃、500℃及550℃的热解半焦,利用管式沉降炉模拟高炉喷吹条件研究神木长焰煤低温热解半焦的燃烧性能,并考察了热解终温、半焦喷吹粒径以及燃烧反应温度对半焦燃烧性能的影响。研究表明：低温热解半焦的燃烧性能优于实验所选用无烟煤的燃烧性能,半焦的燃烧性能与其燃料比之间存在负相关关系,即燃料比越高,燃烧性能越差;降低热解终温、减小半焦喷吹粒径以及提高燃烧反应温度均能改善半焦的燃烧性能,当热解终温为400℃、喷吹粒径100～200目、燃烧反应温度为1100℃时半焦的燃尽度最佳为96%。本实验半焦制备及燃烧条件与现有低温热解和高炉喷吹工艺相符,且热解半焦各项性能均符合喷吹用煤指标。     

The kinetics of combustion of chars derived from sewage sludge

Experiments have been conducted to determine the combustion characteristics of sewage sludge chars in electrically heated beds of silica sand fluidised by air. The effects of the initial size of the char particles, the temperature of the bed and [O₂] in the fluidising gas were investigated. Also, the temperatures of burning particles were measured with embedded thermocouples. The kinetics of combustion were measured at temperatures low enough for the CO formed by initial reaction between the carbon and oxygen to burn at some distance away from the particle. Accordingly, the particle is only heated by the enthalpy of the reaction C+0.5O₂→CO. The activation energy for the intrinsic kinetics of combustion of the char was estimated to be 130–144 kJ/mol. The former value makes allowance for the fact that the particles are at a temperature in excess of that of the bed (determined by a heat balance on a reacting particle), whilst the latter value assumes that the particles are at the same temperature of the bed. It is probable that the lower value is closer to the actual value, thought to be 135±15 kJ/mol, reflecting the catalytic nature of the ash skeleton on which the carbon is supported. It was possible to obtain good agreement between measured burnout times and those predicted using the grain model of Szekely J, Evans JW, Sohn HY. Gas–solid reactions. New York: Academic Press; 1976, for the case where the kinetics are controlled by a combination of: (i) external mass transfer of oxygen from the particulate phase to the external surface of the burning char particle, (ii) diffusion of oxygen from the external surface into the porous matrix to the surfaces of grains, of which the solid is composed, and (iii) diffusion of oxygen into the microporous grains, where reaction occurs with the carbon. It was found that, for particles with diameters of 2 mm or larger, the initial rates of reaction, for bed temperatures in excess of 750 °C, are dominated by external mass transfer. This explains the dependence of the rate of oxidation of unit mass of char on 1/d_p, and the relatively small influence of temperature on these rates. Particles of char from sewage sludge are so reactive that it is essential to make allowance for a difference in temperature between the particle and the bed. Thus, experimental determinations on particles with d_p∼6.5 mm, suggested a difference in temperature of ∼150 K, in line with calculations using a steady-state heat balance.     

Effects of fuel devolatilisation on the combustion of wood chips and incineration of simulated municipal solid wastes in a packed bed

Detailed mathematical simulations as well as experiments have been carried out for the combustion of wood chips and the incineration of simulated municipal solid wastes in a bench-top stationary bed and the effects of devolatilisation rate and moisture level in the fuel were assessed in terms of ignition time, burning rate, reaction zone thickness, peak flame temperature, combustion stoichiometry and unburned gas emissions at the bed top. It is found that devolatilisation kinetic rate has a noticeable effects on the ignition time, peak flame temperature, CO and H₂ emissions at the bed top and the proportion of char burned in the final stage (char burning only) of the combustion. However, it has only a minor effect on the other parameters. Reaction zone thickness ranges from 20 to 55 mm depending on the moisture level in fuel and an increase in the moisture level causes a shift of the combustion stoichiometry to more fuel-lean conditions.     

A mathematical model of dilute phase transport combustion of spent oil shale

A model for the combustion of a mixture of spent oil shale and shale ash for a range of particle sizes in a vertical dilute phase transport contactor is presented. The “grain” model for a sphere of unchanging size is used to describe the combustion of a single particle of spent shale. The rate of gas-solid heat transfer is assumed to be controlled by gas film forced convection. The aerodynamic properties, namely terminal and slip velocities of particles and choking conditions, are taken into account. Realistic conditions of inlet air and solids to the combustor are determined from the mass and energy balances for a hypothetical, 50,000 barrels per day (7.95 10⁶1/day), energy self-sufficient oil shale processing plant.The fraction of the largest particle size is shown to govern the aerodynamic characteristics and hence the design of the combustor. The sensitivity of the profile of average particle temperature to a number of shale-related properties is evaluated; for a typical material such as Condor brown spent shale, the ranking in descending order is as follows: heat of combustion, particle specific heat capacity, initial char content, particle terminal velocity, particle density, intrinsic combustion rate coefficient, pore effective diffusivity and choking velocity of the top size particle. The rate of combustion of the Condor spent shale/shale ash mixture as a whole in the contactor is controlled by the intrinsic kinetics.     

NaOH亚熔盐法分解钾长石矿

以高浓度NaOH溶液为亚熔盐介质分解钾长石矿精粉,考察了矿物粒径、NaOH溶液浓度、搅拌速度、反应时间、反应温度、液固比对K+溶出率的影响,并对分解过程动力学进行分析. 结果表明,100 mm粒径钾长石矿精粉的最佳反应条件为：NaOH初始浓度60%(w)、反应温度约160℃、搅拌速度400 r/min、液固质量比4:1、反应140 min,该条件下K+溶出率大于98%. 钾长石的分解符合粒径恒定的缩核模型,反应初期固相产物层内扩散为速控步骤. 80~140℃下,反应的表观活化能为110.42 kJ/mol.     

Intraparticle diffusion and char combustion

A diffusion and reaction model describing the combustion of porous char particles is developed and used to study the effects of intraparticle diffusional limitations on their pseudo-steady state and dynamic behaviour. The model allows for intraparticle temperature gradients and shrinkage of the reacting particles. Extensive computations over a wide range of ambient temperature and particle size reveal a strong dependence of the ignition and extinction phenomena observed in the combustion of porous particles, and hence of the predicted burning times, on the initial mass transport resistances and on their evolution with the extent of reaction.