煤燃烧过程中NOx的生成和还原

燃煤过程中ＮＯｘ 的排放是一个复杂的过程, 既包括ＮＯｘ 的生成过程又包括生成的ＮＯｘ 进行均相和多相还原反应． 简述了煤燃烧过程中ＮＯｘ 生成和还原的机理． 认为燃料氮是ＮＯｘ的主要来源,ＮＯｘ 的排放与煤阶、煤中氮含量以及温度等因素有关;ＮＯｘ 与半焦的多相反应是ＮＯｘ还原的主要原因, 其中包括ＮＯｘ 在半焦表面的化学吸附、表面络合物的生成以及产物的生成和脱附等过程     

Effects of gas temperature fluctuation on the instantaneous char reaction of pulverized coal particle

The heterogeneous char reaction processes of pulverized coal particle in a hot gas flow with temperature fluctuation are investigated. The instantaneous mass variations and char reaction rates of the particles with initial diameters of 10-50 μm are calculated under different conditions. The gas temperature fluctuation has evident influences on the instantaneous char reaction processes of the pulverized coal particles. The instantaneous char reaction rates with the gas temperature fluctuation are different from those without the gas temperature fluctuation. The gas temperature fluctuation leads to more rapid char reaction and faster mass loss of the particles. The effects of fluctuation amplitude of the gas temperature and particle Reynolds number on the instantaneous char reaction processes are delineated.     

Modeling of in-line low-NOx calciners—a parametric study

Simulations with a heterogeneous model of an in-line low-NO_x calciner, based on non-isothermal diffusion-reaction models for char combustion and limestone calcination combined with a kinetic model for NO formation and reduction, are reported. The analysis shows that the most important hydrodynamic parameter is the mixing rate of preheated combustion air into the sub-stoichiometric suspension leaving the reducing zone and the most important combustion parameter is the char reactivity. Also, the calcination rate modifies very much the temperature in the calciner, char and limestone conversion and NO emission. Carbon monoxide is a key component for the reduction of NO and reliable data for the kinetics of NO reduction by CO over CaO are very important for the prediction of the NO emission. The internal surface area of char and limestone particles influences the combustion and calcination rates and thereby the char and limestone conversion and the NO emission.     

煤燃烧过程中NOx的生成和还原

燃煤过程中NO     

煤焦燃烧生成NO的特性研究

对小屯煤焦中氮的存在形态进行了X射线光电子能谱(XPS)分析。并对小屯煤焦在模拟分解炉中NO释放特性进行了实验研究,考察了气氛、温度、生料对焦炭氮释放特性的影响。研究结果表明:小屯煤焦中氮主要以吡咯五元环形式存在;氧气体积分数对NO的生成速率有明显影响;在无生料催化下,生成量基本随温度的升高而加大,但是,当生料存在时,生成量随温度的升高而减少;生料的加入大大加速了NO的生成速率和生成量,表明生料对NO生成有显著的催化作用。焦炭氮转化为NO的几率主要由2个互相竞争的反应决定:包括N的氧化反应和NO的还原反应。     

Heterogeneous reduction of nitric oxide on synthetic coal chars

Model compounds, with a controlled heteroatoms content and well-defined functionalities, were used to study the release of nitrogen compounds from char combustion. In the present work, the mechanisms involved in NO-char heterogeneous reduction were studied with a synthetic coal (SC) char as carbon source. Another synthetic char (SN) without any nitrogen in its composition was also employed in these studies. Temperature programmed reduction (TPR) tests with a gas mixture of 400 ppm NO in argon and with isotopically labelled nitric oxide, ¹⁵NO (500 ppm ¹⁵NO in argon), were carried out. The gases produced were quantitatively determined by means of MS and FTIR analysers.Under the conditions of this work the main products of the NO-C reaction were found to be N₂ and CO₂. The main path of reaction involves the formation of surface nitrogen compounds that afterwards react with nitrogen from the reactive gas to form N₂. It was observed that fuel-N also participates in the overall heterogeneous reduction reaction, although to a lesser extent.     

Formation and reduction of nitric oxide in fixed-bed combustion of straw

Mechanisms of nitric oxide (NO) formation and reduction in fixed-bed combustion of straw have been modeled mathematically and verified experimentally. The model for the straw combustion and nitrogen chemistry consists of sub-models for evaporation, pyrolysis, tar and char combustion, nitrogen conversion, and energy and mass conservation. Twenty chemical reactions are included, of which 12 belong to the fuel nitrogen reaction network. Volatile nitrogen is assumed to be NO, NH₃, HCN and HNCO, and char nitrogen is converted to NO during char oxidation. The model predictions are in qualitative agreement with the measurements during the ignition phase, i.e. when the combustion front passes through the un-burnt fuel. The yield of NO can be reduced considerably by using a low primary air flow due to the longer gas residence in the fixed-bed, while the NO exhaust concentration is insensitive to the bed temperature. The NO exhaust concentration initially reaches a maximum and then decreases towards a stable value after the straw bed is ignited. Variations of NO, NH₃, HCN, and HNCO concentrations in the ignition flame front indicate that a large quantity of NO can be reduced in the thin flame front zone. The developed model is further validated by separate experiments in which NO or NH₃ was added at the middle through tubes or at the bottom of the bed with the primary air flow. Both the simulations and measurements showed that the variation of the NO exhaust concentration is small as compared with the injected NO or NH₃ concentration. According to the simulations and experiments, it is proposed that flue gas recirculation may be a very effective method of reducing NO emissions from flue gas in the fixed-bed combustion of straw. Calculations indicated that about 20% of the flue gas may be recirculated without significantly affecting the combustion behavior.     

Simulation of coal combustion by AUSM turbulence-chemistry char combustion model and a full two-fluid model

An algebraic unified second-order moment (AUSM) turbulence-chemistry model of char combustion is introduced in this paper, to calculate the effect of particle temperature fluctuation on char combustion. The AUSM model is used to simulate gas-particle flows, in coal combustion in a pulverized coal combustor, together with a full two-fluid model for reacting gas-particle flows and coal combustion, including the sub-models as the k-ε-k_p two-phase turbulence model, the EBU-Arrhenius volatile and CO combustion model, and the six-flux radiation model. A new method for calculating particle mass flow rate is also used in this model to correct particle outflow rate and mass flow rate for inside sections, which can obey the principle of mass conservation for the particle phase and can also speed up the iterating convergence of the computation procedure effectively. The simulation results indicate that, the AUSM char combustion model is more preferable to the old char combustion model, since the later totally eliminate the influence of particle temperature fluctuation on char combustion rate.     

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.     

Devolatilization in the temperature range 300-600 K of liquids derived from wood pyrolysis and gasification

A thermogravimetric system, previously developed for solid fuel degradation, has been modified to examine liquids obtained from conventional pyrolysis and updraft gasification of beech wood. Thermogravimetric curves in air show two main reaction stages. The first (temperatures ≤600 K) concerns evaporation, formation and release of gases and formation of secondary char (coke). Then, at higher temperatures, heterogeneous combustion of secondary char takes place. A reliable procedure has been developed to carry out the first stage under assigned temperature using a PID controller and the applied heat flux as the manipulated variable. It has been found that the pyrolysis temperature does not affect significantly weight loss dynamics and amount of secondary char (approximately equal to 20% of the liquid on a dry basis). The thermogravimetric curves are well predicted by a global mechanism consisting of three parallel first-order reactions (activation energies of 66, 32 and 36 kJ/mol, respectively). Due to strong physico-chemical transformations (sample swelling and solidification) associated with secondary char formation, it is not possible to avoid ignition during heterogeneous combustion. Therefore, this reaction stage should be investigated separately after collection and adequate re-preparation of the charred sample.     

NO reduction capacity of four major solid fuels in reburning conditions - Experiments and modeling

The combustion of solid fuels in the rotary kiln and in the calciner of a cement plant generates fuel and thermal NO. This NO can be reduced inside the reducing zone of the calciner. This occurs in two different ways: homogeneous reduction by hydrocarbons and heterogeneous reduction by char. The purpose of this paper is to identify the relative contribution of volatile matters or char on the NO reduction process, which largely depends on the nature of the solid fuel used for reburning.Experiments were undertaken in an Entrained Flow Reactor (EFR), at three temperatures: 800, 900 and 1000 °C. Four major fuels used in the cement production process were studied: a lignite, a coal, an anthracite and a petcoke. Specific experiments were undertaken to determine (i) their devolatilisation kinetics and the gas species released. A wide range of species influencing the NO chemistry was carefully analyzed. Then, (ii) the char oxidation and (iii) the char NO reduction kinetics were characterized. Finally, (iv), the “global” NO reduction capability of each fuel was quantified through experiments during which all phenomena could occur together. This corresponds to the situation of an industrial reactor in reducing conditions. Anthracite and petcoke reduce only very small quantities of NO whereas lignite and coal reduce, respectively, 90% and 80% of the initially present 880 ppm of NO (at 1000 °C after 2 s).The four types of experiments described above were then modeled using a single particle thermo-chemical model that includes heterogeneous reactions and detailed chemistry in the gas phase. This model reveals that both NO reduction on char and NO reduction by volatiles mechanisms contribute significantly to the global NO reduction. After short residence times (several tenth of a second), gas phase reactions reduce NO efficiently; after long residence times (several seconds) the char reduces larger quantities of NO.     

Modeling the temperature in coal char particle during fluidized bed combustion

The temperatures of a coal char particle in hot bubbling fluidized bed (FB) were analyzed by a model of combustion. The unsteady model includes phenomena of heat and mass transfer through a porous char particle, as well as heterogeneous reaction at the interior char surface and homogeneous reaction in the pores. The parametric analysis of the model has shown that above 550 °C combustion occurs under the regime limited by diffusion. The experimental results of temperature measurements by thermocouple in the particle center during FB combustion at temperatures in the range 590-710 °C were compared with the model predictions. Two coals of different rank were used: lignite and brown coal, with particle size in the range 5-10 mm. The comparisons have shown that the model can adequately predict the histories of temperatures in char particles during combustion in FB. In the first order, the model predicts the influence of the particle size, coal rank (via porosity), and oxygen concentration in its surroundings.     

Fully resolved simulations of single char particle combustion using a ghost‐cell immersed boundary method

A novel ghost‐cell immersed boundary method for fully resolved simulation of char particle combustion has been developed. The boundary conditions at the solid particle surface, such as velocity, temperature, density, and chemical species concentration, are well enforced through the present method. Two semiglobal heterogeneous reactions and one homogeneous reaction are used to describe the chemical reactions in the domain, and the Stefan flow caused by the heterogeneous reactions is considered. A satisfactory agreement can be found between the present simulation results and experimental data in the literature. The method is then used to investigate the combustion property of a char particle and the interaction between CO₂ gasification and O₂ oxidation. Furthermore, combustion effect on the exchange of mass, momentum and energy between gas‐ and solid‐ phase is explored. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2851–2863, 2018     

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.     

CFD study of a sudden-expanding coal combustor using Euler–Euler and Euler–Lagrange models

In this study, the Euler–Euler (E–E) and Euler–Lagrange (E–L) models designed for the same chemical mechanism of heterogeneous reactions were used to predict the performance of a typical sudden-expanding coal combustor. The results showed that the current E–E model underestimated the coal burnout rate because the particle temperature fluctuation on char combustion is not adequately considered. A comparison of the E–E and E–L simulations showed the underestimation of heterogeneous chemical reaction rates by the E–E model.     

DEM-LES simulation of coal combustion in a bubbling fluidized bed Part II: coal combustion at the particle level

The discrete element method-large eddy simulation (DEM-LES) is used to model coal combustion at the particle level in a bubbling fluidized bed. The gas phase is modelled as a continuum and the solid phase is modeled by DEM. Chemical reactions consist in the heterogeneous reactions of char with O₂, CO, CO₂, NO, and N₂O, and in the homogeneous reactions involving CO, O₂, NO, and N₂O. The colliding particle-particle heat transfer is based on the analysis of the elastic deformation of the spheres during their contact. The model predicts the effects of the particle heterogeneous flow structure on the thermal characteristics of coal particles when heating and burning, and the gaseous emissions from a fluidized sand-coal binary mixture. The heating rates are 1627 and for, respectively, 0.8 and diameter coal particles fed into the fluidized bed. The instantaneous contribution of the collision heat transfer is weak, less than 5.0% of the total power exchanges (coal combustion, radiation, convection and collision) during the heating and 1.5% during the combustion. The temperature of the coal particles exceeds the bed temperature, which is in qualitative agreement with experimental data from literature. The effects of the diameter of coal particles, of the bed temperature, and of the inlet gas velocity on the thermal characteristics are also studied.     

燃煤过程中颗粒物的形成机理研究进展

介绍了煤粉燃烧过程中颗粒物的形成机理，包括亚微米飞灰和残灰颗粒的主要形成途径．亚微米颗粒主要来自无机物的气化-凝结过程，在高温条件下无机矿物首先以氧化物、次氧化物或原子的形式气化，当温度降低时，无机蒸气通过均相成核、异相冷凝、凝并、团聚等过程形成细微颗粒．残灰由残留在焦炭颗粒中的矿物转化而成，焦炭破碎和表面灰的聚合是决定残灰最终粒径分布的主要过程，除此之外，对于含外来矿物较多的煤种，矿物破碎对残灰颗粒的形成也有十分重要的影响．最后对燃煤过程中颗粒物的形成机理研究提出了建议．     

Conversion of volatile-nitrogen and char-nitrogen to NO during combustion

The design of low emission combustion chambers using low NO_x strategies involving staged burning or stratified combustion requires a detailed understanding of the combustion processes of the fuel volatiles and char burning. In this paper some aspects of the combustion of coal-volatiles and char are considered. The extreme cases of volatile combustion, namely premixed and diffusive burning are examined in order to consider the range of NO_x reduction options available to the combustion chamber designer. A similar set of situations is examined for char burning and the release of the fuel-nitrogen to form NO.

The implications of the processes are considered in two practical applications, those of the high temperature combustion found in pulverised coal burning and in a lower temperature regime of the conditions under fluidised bed combustion. In the case of pulverised coal flame the degree of mixedness of the volatiles played a dominant part in determining the extent of NO formation whilst the role of char-nitrogen is only to form NO and NO reduction is limited because of the short residence time and low char concentrations at the end of the reaction zone. In a circulating fluidised bed combustor it was concluded that a different situation can arise. If the bed is sufficiently large enough to give a residence time of several seconds, then the NO initially formed in the fluidised bed is reduced by the carbon in the top of the bed and the riser under steady state conditions and its concentration at the exit can be estimated by equilibrium calculations.     

煤焦燃烧中氢氧化物生成因素的研究

在一固定床反应器上研究了3种煤焦在不同氧浓度、不同温度下燃烧时NO及N_2O生成特性,还研究了燃烧后气体停留时间对NO和N_2O生成的影响,以及NO的加入对N_2O对生成的影响.研究表明,煤焦燃烧中NO主要由氧气吸附于(-CN)基上形成(-CNO)基后而生成,而N_2O的生成机理则有3个,本文比较了这3个反应机理的相对重要性.     

煤焦燃烧中氢氧化物生成因素的研究

在一固定床反应器上研究了3种煤焦在不同氧浓度、不同温度下燃烧时NO及N_2O生成特性,还研究了燃烧后气体停留时间对NO和N_2O生成的影响,以及NO的加入对N_2O对生成的影响.研究表明,煤焦燃烧中NO主要由氧气吸附于(-CN)基上形成(-CNO)基后而生成,而N_2O的生成机理则有3个,本文比较了这3个反应机理的相对重要性.