五彩湾煤中钠在燃烧过程中的迁移释放规律

采用不同萃取液对新疆准东高钠煤进行逐级萃取实验, 分析了煤中钠的存在形式。分别检测了不同温度下五彩湾煤原煤以及815℃下不同萃取方式处理的煤燃烧后煤灰中钠的含量, 研究了五彩湾煤燃烧过程中钠的迁移释放规律。并且对传统灰分分析方法和微波消解-电感耦合等离子体原子发射光谱仪分析法(微波消解-ICP-AES分析法)测量煤中钠含量进行了比较。实验结果表明, 新疆高钠煤中钠主要为水溶钠。五彩湾煤燃烧过程中, 钠的释放主要发生在815℃之前, 并且以水溶钠和有机钠的释放为主。不同存在形式的钠在燃烧中存在转化, 主要表现为水溶钠向不可溶钠的转化以及不可溶钠向有机钠的转化。通过比较, 微波消解-ICP-AES分析法对煤中钠含量的分析更准确。     

Ash formation during pulverized coal combustion : 1: Aerodynamic influences

The elemental composition of pulverized fuel boiler deposits often differs markedly from that of the corresponding fly ash and coal ash. Evidence is given here to support the hypothesis that such chemical segregation could be induced by competition between aerodynamic drag and inertial forces on a particle in a curved streamline; this increases the probability of impacting the boiler walls for large particles of high density, leading to preferential deposition. A conventional air classifier was used to separate mono-sized samples of pulverized coal and char particles into aerodynamically different fractions, and to prepare vitrinite-char and inertinite-char concentrates. The relevance of such aerodynamic segregation is considered by comparing the enrichment (or depletion) of non-volatile elements in boiler ashes with that in ashes derived from air-classified char fractions. Common trends, particularly for iron (the major fluxing element) have been identified (i) between the highest density fractions and furnace deposits, and (ii) between the lower density fractions and cyclone ash.     

四角切圆煤粉锅炉燃烧工况评判

针对现有锅炉燃烧工况评判方法的不足,结合某300MW四角切圆燃烧煤粉锅炉实际运行工况,创新性地利用因子分析法,对锅炉燃烧工况影响因素进行降维分析。按公共因子各自的方差贡献率加权相加计算综合评价得分,从而评价各因素影响作用的大小,避免了各因素之间不可比性给评判决策造成的困难,综合评判的结果正确反映了锅炉实际运行工况,相对现有锅炉燃烧工况评判方法,具有一定的优越性。     

煤粉无焰富氧燃烧的数值模拟方法进展

无焰富氧燃烧是煤粉清洁燃烧技术的前沿发展方向之一,可在捕集高浓度CO₂的同时显著降低NOx排放,并提升富氧燃烧稳定性和热力性能。计算流体力学(CFD)作为燃烧研究的重要手段之一,具有快捷、成本低和数据丰富等优点,有效促进了无焰富氧燃烧技术发展。基于笔者团队对煤粉富氧燃烧和无焰燃烧的多年研究积累,对近十几年来煤粉无焰富氧燃烧CFD模拟方法和模拟研究进展进行了总结:首先强调了煤粉无焰燃烧的试验和数学定义,其由于存在非均相反应而区别于气体燃料无焰燃烧;然后详述了煤粉无焰富氧燃烧CFD模拟方法进展,包括模拟流动、传热、燃烧和污染物生成方面的子模型和机理,其中考虑强烈烟气卷吸的可实现k-ε湍流模型、P1或DO辐射模型及针对富氧气氛修正的WSGG气体辐射模型、CPD挥发分析出模型、考虑湍流与化学反应交互的有限速率EDC均相燃烧模型、针对无焰及富氧燃烧开发验证的均相反应机理、考虑气化反应的多步表面焦炭非均相燃尽模型、含氮化学详细反应机理氮转化模拟、动态自适应反应机理加速算法等可显著提高煤粉无焰富氧燃烧的模拟精度和计算效率。总结了煤粉无焰富氧燃烧在基准对照试验、微观反应区域分析、宏观反应特征、污染物生成及大型化锅炉概念设计方面的模拟研究情况;最后以大涡模拟、燃烧模型、高精度反应机理及动态自适应反应机理、工业应用优化等角度展望了煤粉无焰富氧燃烧CFD研究的发展方向。     

Ash deposition behavior of upgraded brown coal in pulverized coal combustion boiler

Ash with a low melting point causes slagging and fouling problems in pulverized coal combustion boilers. Ash deposition on heat exchanger tubes reduces the overall heat transfer coefficient due to its low thermal conductivity. The purpose of this study is to evaluate the ash deposition for Upgraded Brown Coal (UBC) and bituminous coal in a 145 MW practical coal combustion boiler. The UBC stands for Upgraded Brown Coal. The melting temperature of UBC ash is relatively lower than that of bituminous coal ashes. Combustion tests were conducted on blended coal consisting 20 wt.% of UBC and 80 wt.% of bituminous coal. Before actual ash deposition tests, the molten slag fractions in those coal ashes were estimated by means of chemical equilibrium calculations. The calculation results showed the molten slag fraction for UBC ash reached approximately 90% at 1523 K. However, that for blended coal ash decreased to 50%. These calculation results mean that blending UBC with bituminous coal played a role in decreasing the molten slag fraction. This phenomenon occurred because the coal blending led to the formation of alumino-silicates compounds as a solid phase. Next, ash deposition tests were conducted using a practical pulverized coal combustion boiler. A water-cooled stainless-steel tube was inserted in locations at both 1523 K and 1273 K in the boiler to measure the amount of ash deposits. The results showed that the mass of ash deposition for blended coal did not greatly increase, compared with that for bituminous coal alone. Therefore, appropriately blending UBC with bituminous coal enabled the use of UBC without any ash deposition problems in practical boilers.     

煤粉高压着火特性及影响因素

高压气化炉内煤粉的着火特性对煤粉烧嘴和气化室的设计与运行调控具有重要意义。笔者采用加压热重分析法对3个煤样的着火特性进行研究,根据升温过程中的能量守恒原理和谢苗诺夫着火理论提出了一种新的处理PTG曲线求取着火温度的拐点法,并与传统经验切线法进行对比;讨论了压力、氧气体积分数、升温速率、挥发分和颗粒粒径对着火温度的影响。研究结果表明,煤粉着火温度区间为从初始着火温度(Ti)到极限着火温度(Tig),环境换热条件所决定的切点位置是唯一定解条件,高温工业炉高加热速率对应的为极限着火温度;与常压条件下相比,加压下固定床煤粉的着火为异相着火,着火温度随挥发分的增加而略有增加;在0.1~1.0 MPa和3~5 MPa的压力范围内,随压力的升高,着火温度下降,且比常压下低很多,虽然在1~3 MPa的着火温度略有增加;氧气体积分数对着火温度的影响规律与常压的类似,随氧气体积分数的增加,着火温度降低;虽然加压条件下煤粉的快速反应,拐点法与切线法得到的着火温度相近,但切线法无法响应环境条件的变化,且常压下,经验的切线法无法给出令人满意的结果。     

低挥发分煤粉燃烧新技术发展与应用

介绍了国内外低挥发分煤粉燃烧技术及发展。对具有代表性的燃烧器及炉膛结构进行简要分析,说明热回流、煤粉浓缩、延长火焰长度等关键技术在实现低挥发分难燃煤粉快速着火、稳定燃烧中的应用。指出尽管燃烧器在应用中取得一定的效果,但仍然存在一些问题,因而对于低挥发分煤种还需要同时选择合理的燃烧方式,如切向燃烧、对冲燃烧、W型火焰燃烧及CUF火焰燃烧等技术。其中,W型火焰燃烧方式对难燃无烟煤的燃烧稳定性优于四角和对冲燃烧方式,是目前主要采用的燃烧结构。高温空气燃烧技术对低挥发分煤具有火焰稳定、热效率高、再循环分级燃烧,低NOx排放等优点,将成为更有前景的燃烧技术。     

Fate of coal nitrogen during combustion

A total of 21 coals covering all ranks have been burned under a wide variety of conditions to ascertain the impact of coal properties on the fate of fuel nitrogen. Fuel NO was identified with a nitrogen-free oxidant consisting of Ar-O₂CO₂. In general, under fuel-lean conditions fuel NO formation increases with increasing fuel nitrogen content; however, other fuel properties also significantly affect the fate of fuel-bound nitrogen during combustion. In particular, fuel nitrogen conversion appears to be greater with coals containing a high fraction of volatile reactive nitrogen. Under fuel-rich conditions measurements of first-stage and exhaust-species concentrations suggest that the optimum stoichiometry for minimum emissions is a function of fuel composition. As first-stage stoichiometry is decreased, the NO formed in the first stage decreases, but other oxidizable gas nitrogen species increase as does nitrogen retention in the char. Total fixed nitrogen generally increases with increasing fuel nitrogen and correlates well with excess air exhaust emissions. The distribution of the total fixed nitrogen species leaving the first stage is strongly dependent upon the coal composition. Of the 12 coals tested in detail, only 1 (the high-volatile B bituminous from Utah) produced high HCN concentrations. The low-volatile Pennsylvania anthracite formed almost no HCN or NH₃ even under extremely fuel-rich conditions. In general, the first stage NO percentage decreased significantly with decreasing coal rank from anthracite to lignite. Conversely, the relative importance of NH₃ grew with decreasing rank. HCN was greater than NH₃ in all bituminous tests, but less than NH₃ with all subbituminous and lignite coals.     

Investigation of particle dynamics and pulverized coal combustion in a cavity bluff-body burner

Mean and fluctuating velocity components and turbulence characteristics of the flow in the outlet of a cavity bluff-body burner were measured using three-dimensional laser particle dynamics anemometry (3D PDA). Results are also given on the choice of model particles, particle diameter distribution in and out of the recirculation zone, particle number density and volume flux distribution, and the combustion process for low-volatile coals. The experimental results not only provide an explanation of the mechanism of flame stabilization of the cavity bluff-body burner, but also demonstrate the potential of this kind of flame stabilization in industrial use.     

煤粉工业锅炉燃烧兰炭试验研究

为提高兰炭在煤粉工业锅炉上的燃烧效率,以陕西煤业化工集团生产的兰炭为原料,进行煤粉工业锅炉燃烧试验,分析了兰炭着火、稳燃、燃烬情况;针对兰炭燃烧过程中存在的问题提出解决方案。结果表明:高效煤粉工业锅炉双锥燃烧器的独特结构和浓相燃烧的方式,为兰炭的着火和稳燃提供了良好条件。在过量空气系数1.2,一、二、三次风比例分别为0.11、0.47、0.42,预热时间3 min,伴燃时间4 min的条件下,实现了兰炭粉的着火和自维持稳定燃烧,燃烧期间后部温度保持在550℃,炉膛中部温度大于800℃。针对兰炭燃烧存在燃烧器内燃点靠后、着火区域温度低和兰炭燃烧不完全等问题,提出可通过调整燃烧室的结构和尺寸,使燃烧器蓄热能力增强,延长煤粉预热时间,产生更多高温回流烟气,使兰炭在燃烧器中快速着火并稳定传播到炉膛,降低兰炭灰残炭率,提高燃烧效率。     

Numerical simulation and application experiment of the spontaneous combustion tendency of a coal stockpile covered with pulverized coal

In this paper, we developed a new method for preventing the spontaneous combustion of a coal stockpile covered by pulverized coal. This technique is based on the numerical-simulation analysis of endothermic/exothermic balance in coal stockpile. Depth and height are confirmed to be the main factors influencing the endothermic/exothermic balance in coal stockpiles and the hot-spot region is easily formed at a coal stockpile height of 1-1.5 m and a depth of 2-3 m, where there is the highest tendency for spontaneous combustion. The numerical simulation and the 120-day application test both confirm that the diffusion process of oxidation can be prevented and the coal oxidation reaction can be hindered by covering the surface of the hot region with pulverized coal. As a result, the coal spontaneous combustion is prevented effectively.     

Analysis of low NO emission in high temperature air combustion for pulverized coal

High temperature air combustion experiments for pulverized coal in a large-scale furnace have been done before and shown that the NO emission in the high temperature air combustion is significantly lower than that in the normal temperature air combustion. This paper numerically studies the NO evolution in the large-scale experiments with a simplified chemical reaction model. Through an analysis of numerical results a low NO emission mechanism in the high temperature air combustion has been presented. If the HCN concentration is high, the NO generation is fast. But, the high HCN and NO concentrations together will make NO destruction fast. It is found that, by properly arranging flow patterns, the high HCN and NO concentrations can be obtained in the vicinity of primary air nozzle. Thus, the generation and destruction of NO can reach an equilibrium point so that the net NO emission rate is low.     

Physical structure and combustion properties of super fine pulverized coal particle

Physical structures and combustion properties of super fine pulverized coal particles of eight Chinese coals, Heshan subbituminous coals and Jincheng lean coals from two areas of China, have been investigated using accelerated surface area and porosimetry, thermobalance (TGA), and Fourier transform infrared spectrometer. Results showed that the particle specific surface area and pore volume increased greatly when the coal particle size was reduced. The higher the carbon content on a dry ash-free basis is, the larger the particle specific surface area and pore volume are. When the coal particle size decreases, the combustion process can be largely improved, ignition temperature is reduced, and SO₂ emission from coal combustion is also lower.     

A mathematical model of ash formation during pulverized coal combustion

A mathematical model of ash formation during high-rank pulverized coal combustion is reported in this paper. The model is based on the computer-controlled scanning electron microscope (CCSEM) characterization of minerals in pulverized coals. From the viewpoint of the association with coal carbon matrix, individual mineral grains present in coal particles can be classified as included or excluded minerals. Included minerals refer to those discrete mineral grains that are intimately surrounded by the carbon matrix. Excluded minerals are those liberated minerals not or at least associated with coal carbon matter. Included minerals and excluded minerals are treated separately in the model. Included minerals are assumed to randomly disperse between individual coal particles based on coal and mineral particle size distributions. A mechanism of partial-coalescence of included minerals within single coal particles is related to char particulate structures formed during devolatilization. Fragmentation of excluded minerals, which is important particularly for a coal with a significant fraction of excluded minerals, is simulated using a stochastic approach of Poisson distribution. A narrow-sized sample of an Australian bituminous coal was combusted in a drop-tube furnace under operating conditions similar to that in boilers. The particle size distribution and chemical composition of experimental ash were compared to those predicted with the model. The comparisons indicated that the model generally reflected the combined effect of coalescence of included minerals and fragmentation of excluded minerals, the two important mechanisms governing ash formation for high-rank coals.     

空气冷却式煤粉燃烧室数值模拟研究

双锥煤粉燃烧室在小容量工业锅炉中广泛采用水冷却方式,但随着市场对高容量锅炉需求的增加,双锥燃烧室体积增大、数量增多,如仍采用水冷却的方式将导致安装困难、水系统复杂等问题,亟需开发新的冷却方式。空气冷却形式具有结构简单、预热后的空气可以增加煤粉的着火稳定性等优点,需要考察其首次应用于双锥煤粉燃烧室中的效果。为了确定空气冷却式燃烧室燃烧和壁面冷却情况,采用数值模拟技术对14 MW工业锅炉燃烧室和炉膛进行三维建模,得到50%和100%两种负荷下不同内外二次风配风比例下燃烧室内部燃烧情况、金属壁面温度、出口火焰形状和炉膛充满度。结果表明:控制总空气过量系数不变,随着内二次风比例的逐渐增加,燃烧室内的平均温度逐渐降低;50%负荷下金属壁面温度随二次风比例的增加逐渐降低,100%负荷下金属壁面温度先降低后升高,这是内二次风助燃燃烧和外二次风的冷却共同作用的结果。随着内二次风比例的增加,金属壁面的高温区域逐渐后移,集中于后锥出口区域;在50%负荷下内二次风量占总空气量比例为0.4时,金属壁面具有最高温度930 K,100%负荷下内二次风量占总空气量比例为0.2时,壁面金属最高温度835K,2个最高温度均出现在后锥收缩段,据最高温度推荐壁面材料选取0Cr18Ni9,2种负荷下最高温度出现时燃烧室内的内二次风配风量为2 600 Nm3/h,应尽量使内二次风远离此配风量;50%负荷下燃烧室平均温度、金属壁面平均温度及最高温度均高于100%负荷,是空气冷却结构需要重点考察的工况。随着内二次风比例的逐渐增加,火焰长度先增加后减小,当内二次风过小时,出口气速较小,外二次风具有向中心的速度分量,火焰主要集中在炉膛前部。随着内二次风比例的增加,出口速度增大,火焰变长变细。但随着比例的继续增加,外二次风的轴向速度变小,出口火焰的旋流强度完全由二次风决定,出口旋流强度的增大导致了火焰的变短变粗,在2种负荷下,火焰长度较长时,内二次风比例为0.4~0.5。内外二次风比例为0.5∶0.5时,燃烧室内燃烧情况和壁面温度均匀稳定,火焰在炉膛内的充满度最好,是2个考察负荷下均较适合的运行参数。     

合成煤中钠对煤燃烧及灰特性影响

准东煤中钠含量高，燃用时锅炉会出现严重结渣问题。通过向准东煤为原料制取的超纯煤中添加灰的模型化合物，得到合成煤。并在此基础上利用热重-差示扫描量热分析法（TG/DTG/DSC）、X射线衍射仪（XRD）和灰熔融温度测定分析手段，研究Na₂O含量对煤燃烧特性和灰熔融性的影响。结果表明：钠主要影响合成煤的着火温度（T_i）与焦炭燃烧阶段，钠含量增加使T_i升高，并且Na₂O在灰中质量分数由5%升高至8%后，钠含量增加使焦炭燃烧速率先减小后加快，并能够改善煤粉燃尽特性。钠能够降低灰熔融温度，并在Na₂O质量分数高于5%后，温度下降更加明显。在三元相图中钠对莫来石的助熔作用是造成灰熔融温度降低的重要原因。XRD分析表明Na₂O含量增加，充当骨架作用的石英在钠的助熔作用下与难熔矿物硅钙石、MgO等生成低熔点长石类矿物，这类矿物在高温下有助熔作用，能够降低灰熔融温度。同时还生成助熔性含钠矿物霞石，加剧了灰熔融。     

The influence of fragmentation on the behavior of pyrite particles during pulverized coal combustion

Coal combustion is a complex series of reactions, dominated by transport mechanisms. A poorly understood aspect is the influence that fragmentation of excluded pyrite minerals has on the combustion thermal history and sulfur release. To explore this effect, fragmentation was incorporated into a mathematical model consisting of several stages, namely: heating up the particle, decomposition to pyrrhotite, fragmentation, and oxidization. A computational fluid dynamic (CFD) model was used to verify the heat and mass transfer predictions of the model. The model was then solved to simulate the particle heat and mass balance of a pyrite particle in a combustion environment.Fragmentation was found to increase particle temperature and sulfur release rate compared with a no fragmentation case, speeding up the particle temperature increase towards its melting point. Increased oxygen content in the bulk environment decreases the time required for a particle to reach its melting point and for complete sulfur release due to increased oxidation reaction rates. Model results also indicate that small pyrite particles need less time to reach both decomposition and melting temperatures; suggesting that smaller particles will spend more time in a molten state and react more completely.     

城市污泥对五彩湾高钠煤灰熔融特性的影响

通过X射线衍射（XRD）谱图分析结合灰熔融温度测定,研究了五彩湾高钠煤灰中矿物转化机理及城市污泥对高钠煤灰分特性的影响。试验结果表明：当污泥添加质量比为S/C（sludge/coal）=4时,结渣指数（F_s）_sludge=1150℃,污泥提高灰熔融温度程度较小。对污泥改性使其（SiO₂/Al₂O₃）物质的量比为2,添加至煤中,当S/C > 1时,F_s > 1235℃,灰熔融温度明显提高。R_b/a、R_b/a（+P）、R_b/a×Na等指标可良好预测高钠煤与污泥混合物燃烧的沾污、结渣倾向。煤在空气气氛下,燃烧温度800～1100℃,灰中主要矿物钠长石、钙铁辉石、蓝方石等熔点低,高熔点矿物霞石等助融性强、消失温度低。而添加污泥后,混合物灰中新生成Ca₃（PO₄）₂、Ca₂P₂O₇、CaAl₂Si₂O₈等高熔点物质,有利于提高煤灰熔融温度。而污泥中（SiO₂/Al₂O₃）比、Fe含量高,会对硅铝酸盐等产生助熔作用,抑制污泥中P提高灰熔融温度的作用。     

高汞煤燃烧过程中汞的析出规律试验研究

针对3种中国典型的高汞煤,利用马弗炉和管式试验炉进行了燃烧温度、燃烧时间等条件因素影响燃煤汞的析出及形态迁移转化的试验研究。研究表明,燃烧条件下加热时间对典型高汞煤中汞的析出率的影响不明显;高汞煤燃烧后,煤中汞在很短时间内即以气态方式进入烟气,并主要以二价汞(Hg2 )和零价汞(Hg0)形式存在,其形态分布与燃烧温度和煤种有关;随着燃烧温度的升高,烟气中的二价汞含量有逐渐升高和零价汞含量逐渐降低的趋势。