深度空气分级下混煤未燃尽碳与NOx协同优化数值模拟

为探究掺混方式及配风方式对混煤燃尽率和NO_x排放量的影响,以一台660 MW四角切圆锅炉为研究对象,开展混煤燃烧过程未燃尽碳和NO_x排放的协同优化数值研究。结果表明,在深度空气分级条件下,未燃尽碳主要受停留时间、掺混方式、配风方式等因素共同影响;合理搭配掺混方式和配风方案可降低混煤未燃尽碳水平和炉内NO_x总生成量,达到两者的协同优化。与炉外掺混相比,炉内掺混更加灵活,可通过优化低挥发分煤的燃尽程度来改善混煤整体的未燃尽碳水平。在炉内掺混方式下,均等配风将低挥发分煤置于上部燃烧器或正宝塔配风将低挥发分煤置于下部燃烧器,均有利于低挥发分煤的燃尽;而将高挥发分煤置于上部燃烧器更有利于NO_x减排。综合考虑混煤未燃尽碳和NO_x排放特性,将高挥发分煤置于上部燃烧器且采用正宝塔配风可为下部低挥发分煤的燃烧提供相对充足的氧量,提高低挥发粉煤的燃尽率,降低混煤未燃尽碳水平;上部高挥发分煤析出的挥发分中含有大量含氮中间产物HCN,可将已生成的NO_x还原,有利于降低炉内混煤NO_x生成量。     

用于混煤燃烧控制砷排放模型研究

混煤掺烧是控制燃煤砷排放的有效方式,但由于缺乏相关的配煤模型,限制了该技术的应用。根据燃煤过程中砷的挥发释放机制,提出一种采用砷的释放指数P表征煤燃烧过程砷的释放特性的配煤模型。该模型综合考虑煤的灰分、灰中主要矿物元素含量、各矿物元素对砷的固定系数以及煤中砷含量等因素。研究结果表明,随着煤灰固定系数由23.12增至50.90,煤灰的气相砷吸附量由3.39 mg/g增至6.14 mg/g;随着释放指数P增大,砷的固定率减小,且随着温度升高,两者相关性由900℃的0.67增至1 300℃的0.86。根据P值筛选煤种进行掺烧,当掺混煤种P值差异较大时,掺烧低P值煤种不仅可降低混煤中的砷含量,还能促进高P值煤的砷在灰中富集,促进率达77.14%;掺混煤种P值差异较小时,掺烧低P值煤种会促进砷的释放。本模型可较好地筛选煤种,为混煤掺烧控制砷等痕量元素的排放提供了新的思路。     

生物质与煤直接耦合燃烧试验研究

为了研究生物质不同掺烧位置、掺烧比例、生物质一次风温和风率以及不同煤种掺烧后对锅炉NO排放以及飞灰可燃物的影响,基于50 kW下行炉煤粉综合试验台进行了生物质颗粒掺烧热态试验。试验结果表明,生物质输入热量掺烧比例为6%时,生物质从不同位置掺入,NO排放都有一定程度下降,不同位置降幅不同,生物质从还原区送入时NO排放下降幅度最大,烟煤下降幅度为23.47%,贫煤下降幅度为13.64%;烟煤掺烧生物质时从燃烧前期加入利于煤粉燃尽,贫煤掺烧生物质时从燃烧后期加入利于煤粉燃尽,总体上掺入生物质后,炉膛出口烟气温度都有不同程度上升;生物质输入热量掺烧比例从6%提高到12%后,与烟煤耦合燃烧时NO排放降幅从23.31%增至39.5%,与贫煤耦合燃烧时NO排放的降幅从13.61%降至10%,但贫煤作为主燃料时,NO下降绝对值高于烟煤;不同生物质对NO下降幅度与生物质中氮含量相关,氮含量越低,NO下降幅度越大;提高生物质的一次风温,飞灰可燃物含量从13.88%降至7.62%,不影响安全的情况下,应尽量提高生物质一次风温度;生物质一次风率的变化对燃烧效率影响较小。     

半焦空气分级燃烧NOx排放试验研究

半焦是低阶煤经低温热解后的产物,其中半焦粉与煤粉工业锅炉常用煤种烟煤相比价格低廉。若能将半焦粉用作煤粉工业锅炉的燃料,既可拓宽煤粉工业锅炉的适用燃料范围,又可增强煤粉工业锅炉的市场竞争力。由于半焦挥发分低、固定碳高,实现其着火和稳定燃烧需要更高的温度,同时,降低NOx初始排放也是一个技术难题。为了实现半焦在煤粉工业锅炉中的稳定燃烧及NOx排放的降低,采用两段式滴管炉开展半焦空气分级燃烧NOx排放规律研究。笔者对半焦空气不分级燃烧NOx排放规律进行了研究,主要探究了主燃区温度(1 000~1 400℃)及过量空气系数的影响,为后续空气分级燃烧降低NOx的效果提供对比依据。半焦空气分级燃烧试验主要研究了主燃区温度(1 000~1 400℃)及二次风比例(0.4~0.8)的影响,并从燃尽率、NOx减少比例、灰样微观孔隙和形貌等方面进行论证,试验结果表明,在空气不分级燃烧条件下,即燃尽风配风比例为0时,随着主燃区温度升高,NOx排放浓度随之迅速升高;随着过量空气系数增加,NOx浓度先迅速增加,过量空气系数大于1.15时,NOx浓度增速变缓;在空气分级燃烧中,相同主燃区温度条件下,二次风比例由高到低变化时,NOx排放呈先迅速下降后缓慢回升的变化趋势,燃尽率先快速升高而后趋于平缓。二次风比例为0.56时(即燃尽风率为0.39),燃尽率达90%,NOx排放浓度降至最低,为120 mg/m^3以下,此时是试验条件下的最佳二次风比例。     

贫煤和无烟煤及混煤燃烧硫析出特性研究

实验研究了贫煤、无烟煤及其混煤燃烧时硫析出的动态特性 .结果表明 :贫煤 HZ,YQ及进行实验的各种混煤燃烧时 SO2 动态析出速率曲线呈现典型“双峰”结构 ,而无烟煤 FF呈现出较平坦的单峰结构 .二元或三元混煤实测硫析出速率曲线与按质量配比加权计算硫析出速率曲线相比存在较大差异 .混煤硫析出速率曲线的实测第一峰对应时间比加权计算对应时间有所延迟 .多种煤混烧有促进中低温硫析出的作用 ,随掺配煤种数目增加作用越明显 .混煤燃烧实测硫的最终析出率低于、接近或高于加权计算最终析出率与组分煤的最终析出率、掺配比有密切关系     

空气分级燃烧技术中两级燃尽风技术试验研究

以神木烟煤和阳泉贫煤为试验对象,在一维火焰炉上进行了空气分级燃烧技术模拟试验研究。燃尽风输入方式(即两级燃尽风)试验结果表明,与通常的空气分级技术相比较,燃尽风分两股送入炉内不仅能有效降低NO_x的排放量,还能很好的降低飞灰可燃物含量,取得更佳的脱硝效果和燃尽效果。     

300 MW四角切圆锅炉低负荷下燃烧特性数值模拟研究

随着环保标准提高,电站锅炉NO_x排放量控制日益严格。低氮改造可以有效降低NO_x生成,而对于改造后低负荷下炉内燃烧特性研究有限。对某电厂低氮改造后的一台300 MW四角切圆煤粉锅炉进行了低负荷下多工况燃烧特性的数值模拟,研究了过量空气系数、燃尽风量和一次风喷口给煤量对炉内速度场、温度场、组分浓度场的影响。通过改进网格系统,提高模拟结果的准确性。数值模拟结果和试验测量值偏差较小,说明其数值模拟结果可信。结果表明:随着过量空气系数的增加,炉内燃烧温度升高,还原性物质减少,NO_x排放量增加,当过量空气系数从1.20增加到1.30时,NO_x排放从221.12 mg/m~3增加到196.26 mg/m~3;随着燃尽风量增加,主燃区温度降低,燃尽区温度升高,主燃区温度的降低抑制了热力型NO_x的生成,NO_x排放量降低,当燃尽风量从20%增加到30%时,NO_x从231.21 mg/m~3降低到180.95 mg/m~3;一次风喷口给煤量变化对炉膛内温度场、组分浓度场和NO_x生成影响较小。     

300MW四角切圆贫煤锅炉三次风布置对NOx排放的影响

低氮燃烧改造是燃煤电厂降低氮氧化物排放最主要的策略之一。空气分级燃烧技术因其技术成熟、成本低廉等优势在燃用烟煤的锅炉中得到广泛应用。然而,随着煤/风比的进一步增加,NO_x降幅减小,未燃尽碳含量显著变大。与燃用烟煤的锅炉相比,燃用低挥发分煤种锅炉的低氮改造工作更加困难和复杂。四角切圆贫煤锅炉的三次风会影响风煤混合、燃烧气氛和温度,这些都会对煤粉燃烧过程和NO_x生成产生显著影响,若仅采用空气分级技术,并不能满足NO_x排放标准。因此,在低氮燃烧改造方案设计过程中,需寻求最佳的三次风布置方案以实现低氮高效燃烧。将一台300 MW四角切圆贫煤燃烧锅炉作为研究对象,采取CFD数值模拟方法,考察了三次风布置方式对锅炉燃烧特性的影响。结果表明:当三次风布置在燃烧区下部时,下层一次风和三次风中的煤粉迅速着火燃烧,温度攀升,火焰中心上移; NO_x还原区变长,此时炉膛出口NO_x浓度最低,为405 mg/Nm~3;三次风的下移导致炉膛主燃区中上部氧量较少,煤粉不充分燃烧,燃尽率降低。当三次风布置在主燃区中部时,由于三次风风温较低,导致炉膛燃烧温度下降,一定程度上抑制了热力型NO_x的生成,炉膛出口NO_x排放量减少;三次风的喷入增加了主燃区过量空气系数,有利于煤粉的充分燃烧,燃尽率提高。当三次风布置在主燃区上部时,随着三次风位置的升高,三次风煤粉整体燃烧燃尽区域上移,折焰角附近温度依次升高;三次风位置的上移增加了NO_x还原区的长度,三次风喷口位置越高,炉膛出口NO_x浓度越低;三次风上移导致三次风煤粉在炉膛的停留时间变短,造成燃烧不充分,飞灰含碳量增加,燃尽率降低。此外,对改造后飞灰及大渣含碳量,炉膛出口烟温和NO_x浓度等参数进行现场测量,NO_x排放浓度模拟值和测量值分别为445和448 mg/Nm~3,飞灰含碳量分别为1. 92%和1. 48%,数值模拟结果与现场测量结果吻合较好。     

神华煤锅炉低氮燃烧特性研究

神华煤极易着火、燃尽,燃烧性能优良,且煤中氮、硫含量低,使神华煤在低氧和低氮结合的燃烧条件下,保持了较高的燃尽性能和较一般烟煤偏低的NOx生成量。神华煤的低硫特性保证了在贫氧(没有足够的氧气保证充分燃烧)条件下,炉内H2S含量不是太高,没有明显的高温腐蚀倾向。采用低氮燃烧后,燃烧器区燃烧强度降低,缓解了炉内结渣,部分锅炉屏区的结渣也有所缓解,提高了具有严重结渣倾向的神华煤的掺烧比例。神华煤采用低氧燃烧和低氮燃烧相结合的技术,保证了锅炉的燃烧经济性、安全性和低NOx生成特性,指标明显优于国内其他典型烟煤。     

混煤粒度影响HT-L炉燃烧动力的原因及工况纠正

针对HT-L炉烧神木煤与晋城无烟煤混煤出现的渣口压差波动频繁(渣口堵塞)、粗渣中块状渣比例高、细渣含碳量高等不良情况,通过研究不同粒度混煤的密度、混煤各煤种在不同粒度范围的比例、不同粒度混煤的燃烧动力学、混煤粒度对结渣性的影响,得出HT-L炉烧混煤发生渣口压差波动(渣口堵塞)时,尽量采用辅助手段对HT-L炉的燃烧工况进行纠正,尽可能避免采用降低负荷、切气至火炬、停车等影响收益的处理手段,从源头上提高HT-L炉运行的经济效益与抗风险能力。     

Experimental study on interaction and excess heat release under oxy-fuel combustion of blended coals

The combustion behavior and excess heat release during the oxy-fuel combustion of blended coals were investigated experimentally using a non-isothermal thermogravimetric analyzer. The atmospheres were set to 10%O₂/90%CO₂, 21%O₂/79%N₂, 30%O₂/70%CO₂, and 50%O₂/50%CO₂, and Arthur coal (bituminous coal, BA) and KPU (sub-bituminous coal, SK) were selected as fuel with blending ratios of BA25%/SK75%, BA50%/SK50%, and BA75%/SK25%. The purpose of this study is to investigate the interaction between the blended coals and the effects of blending ratio and oxygen concentration on the excess heat release under oxy-fuel combustion. The results showed that as the oxygen concentration and proportion of sub-bituminous coal increased, the peak value in the differential thermal analysis curve increased by the enhanced reaction rate. A higher oxygen concentration led to excess heat release. The ignition temperatures depended on the volatile matter content of the sub-bituminous coal, whereas the burnout temperature was largely affected by the fixed carbon content of the bituminous coal. For interaction behaviors on characteristic temperatures, the volatile release temperature shows an additive behavior; however, ignition and burnout temperatures show non-additive behaviors for blended coals.     

百万塔式炉机组神华煤掺烧试验研究

为了研究塔式锅炉掺烧神华煤的适应性,采用预混和分磨2种掺烧方式,不断提高神华煤掺烧比例,评估锅炉运行的安全性、经济性和环保性。试验结果表明,随着神华煤掺烧比例的升高,灰渣含碳量降低,锅炉效率升高。神华煤预混掺烧比例低于75%时,锅炉汽水参数正常,锅炉没有出现结渣现象,锅炉对掺配煤种具有良好的适应性;神华煤掺烧比例高于75%时,锅炉局部出现结渣。分磨掺烧最高掺烧比例达到60%,锅炉局部出现结渣,并且在相同掺烧比例下,分磨掺烧的锅炉效率高于预混掺烧。长期高比例掺烧神华煤时,应注意加强燃烧器区域和一级过热器区域的监测,避免形成大渣块。     

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.     

The possible role of fissure formation in the prevention of coking pressure generation

Fissure patterns have been studied for both high-volatile, low-coking pressure coals and low-volatile, high-coking pressure coals. The high-volatile coals form an extensive pattern of interconnected fissures, which seem to form early on and extend further toward the plastic region than the low-volatile coals, which form only a few fissures that do not extend very far into the charge. It is proposed that the combination of high-fluidity and extensive fissure network present for high-volatile coals may assist in allowing continual release of volatiles throughout the coking process and play a part in preventing the generation of high gas pressures. It was also found that a higher proportion of the volatiles for the low-volatile, high-coking pressure coals is released after the coal has been converted to semi-coke, which may play a role in preventing the effective release of gas.     

Effect of moisture on the methane capacity of American coals

An investigation of methane sorption in bituminous coals ranging from low-volatile to high-volatile B has been carried out. Equilibrium sorption isotherms for dry and moist coal were measured at 30 °C and at pressures up to 60 atm. The natural oxygen content of a coal plays a major role in determining its methane capacity. Capacities of high-oxygen coals undergo a much greater reduction when saturated with moisture than do low-oxygen coals. Measurements of adsorbed-water saturation capacities clearly suggest that only adsorbed water affects the equilibrium capacity of a coal for methane; water present in excess of the adsorbed water has no effect on methane sorption. Excellent agreement has been found between the methane sorption data reported here and field measurements of methane emission from coal samples obtained during borehole drillings.     

A three-dimensional numerical study of the combustion of coal blends in blast furnace

The practice of blending coals for pulverized coal combustion is widely used in ironmaking blast furnace. It is desirable to characterize the combustion behaviour of coal blends and their component coals. A three-dimensional numerical model is described to simulate the flow and combustion of binary coal blends under simplified blast furnace conditions. The model is validated against the experimental results from a pilot-scale combustion test rig for a range of conditions, which features an inclined co-axial lance. The overall performance of coal blend and the individual behaviours of their component coals are analysed, with special reference to the influences of particle size and coal type. The synergistic effect of coal blending on overall burnout is examined. The results show that the interactions between component coals, in terms of particle temperature and volatile content, are responsible for the synergistic effect. Such synergistic effect can be optimized by adjusting the blending fraction. The model provides an effective tool for the design of coal blends.     

Effect of biomass blending on coal ignition and burnout during oxy-fuel combustion

Oxy-fuel combustion is a GHG abatement technology in which coal is burned using a mixture of oxygen and recycled flue gas, to obtain a rich stream of CO₂ ready for sequestration. An entrained flow reactor was used in this work to study the ignition and burnout of coals and blends with biomass under oxy-fuel conditions. Mixtures of CO₂/O₂ of different concentrations were used and compared with air as reference. A worsening of the ignition temperature was detected in CO₂/O₂ mixtures when the oxygen concentration was the same as that of the air. However, at an oxygen concentration of 30% or higher, an improvement in ignition was observed. The blending of biomass clearly improves the ignition properties of coal in air. The burnout of coals and blends with a mixture of 79%CO₂-21%O₂ is lower than in air, but an improvement is achieved when the oxygen concentration is 30 or 35%. The results of this work indicate that coal burnout can be improved by blending biomass in CO₂/O₂ mixtures.     

Combustion modelling of coal volatiles in the freeboard of a calorimetric fluidized bed combustor

A simplified kinetic approach, based on functional groups of the parent coal, was coupled with the bed hydrodynamics and a volatiles evolution region within the bed to conduct a parametric study with the experimental results obtained from a calorimetric fluidized bed combustor (FBC). The model results revealed that, for high-volatile coals with particle diameters of 1–3 mm, the fraction of the original volatiles burnt above the bed may be as high as 0.44-0.20, 0.36-0.09 and 0.30-0.02 for excess air levels ranging from 0 to 40% and bed temperatures of 800, 850 and 900°C respectively. For a low-volatile coal, the computed fractions were found to be in the ranges 0.35-0.08, 0.29-0.01 and 0.25-0.00 for similar operating conditions to the above. Good agreement between the model and experimental data suggests that the evolution of volatiles for coal particle diameters <5 mm is mechanistically controlled by both diffusion and chemical kinetics, while their combustion is largely governed by the mixing of volatiles and oxygen in the bed region.     

高砷褐煤与低砷烟煤混燃砷的挥发特性及模型

选取典型的高砷褐煤和低砷烟煤,在一维等温燃烧实验台上进行混燃实验,研究温度（600～1100℃）和掺混比（3:1、1:1、1:3）对高砷褐煤混燃砷挥发的影响。实验结果表明：随着温度的升高,单煤及混煤燃烧砷的挥发比例逐渐增大,不同温度下混煤燃烧砷的挥发比例介于两个单煤之间,但砷的挥发比例并不是简单的加权平均,不同温度和掺混比下混煤砷的挥发比例均高于加权值,高砷褐煤中较高的挥发分含量在影响混煤焦炭燃烧的同时也促进了混煤中砷的挥发。因此,提出了综合考虑温度、掺混比和高砷褐煤影响的混煤砷挥发模型,不同温度和掺混比下的模型计算结果与实验值吻合度较好。