燃煤过程中砷的赋存形态及其挥发特性

选取3个国内煤样,利用自制恒温热重装置研究了燃烧过程中砷的赋存特性及其挥发规律。通过测定不同停留时间下燃烧样品中砷的含量,拟合得到砷的挥发曲线和挥发速率曲线,并采用逐级化学提取的方法对原煤和不同停留时间下的燃烧样品进行形态分析。实验结果表明:温度是影响砷挥发的重要因素,700~1000℃是砷挥发的主要温度区间。煤粉燃烧过程中,砷的挥发速率与煤粉的失重速率具有同步性;伴随着煤中水分和挥发分的快速析出,砷也具有较高的挥发速率;随着燃烧过程的深入进行,砷的挥发速率变得缓慢。煤粉燃烧结束,3种煤(五里庄、红岩和梅花井)砷的挥发比例分别为49.5%、80.7%、65.0%,且在燃烧过程中煤中残渣态、硫化物结合态和可交换态砷相互作用迁移。     

温度和赋存形态对燃煤过程中砷迁移和释放的影响

选取3个煤阶共6个国内典型煤种,利用水平管式炉在不同温度下进行煤的燃烧实验,研究燃煤过程中砷的迁移和释放特性。利用热分析的相关理论和方法,将煤的热重分析手段运用于煤燃烧过程中砷的质量变化,通过对实验结果进行拟合得到砷的失重曲线和失重速率曲线,并采用逐级化学提取的方法对原煤及不同温度下煤灰中砷的赋存形态进行分析。25～1100℃的实验结果表明:随着温度升高,煤中砷的释放比例逐渐增大,1100℃下砷的释放比例变化范围为30%～67%。不同温度区间下砷的失重速率存在差异,800～900℃区间出现显著的砷失重峰,主要原因是以硫化物形式存在的砷在800～900℃区间发生剧烈的分解/氧化分解。此外,相同温度下褐煤的失重比例和失重速率较大,无烟煤的失重比例和失重速率较小,烟煤则介于无烟煤和褐煤之间。温度升高后,煤中的有机物结合态砷向气相迁移,酸溶态砷和残渣态砷共同作用,减少的砷主要进入气相中,还有一部分向可交换态砷迁移。     

富氧燃烧方式下煤中砷的挥发行为

选取SJS烟煤,利用高温管式炉模拟富氧燃烧,在600~1400℃温度范围内研究了O₂浓度、CO₂浓度及温度对砷挥发的影响,并进行了空气燃烧模式下的对比实验。对不同工况下的灰样进行FTIR表征并结合化学热力学软件模拟进行分析,结果表明:富氧气氛和空气气氛下煤中砷的挥发比例均随温度升高不断增大,并在低温区间(<900℃)和高温区间(>900℃)分别出现了砷的剧烈失重,但O₂浓度和CO₂浓度影响了不同气氛下砷的具体挥发行为。低温下(<900℃)O₂浓度是影响砷挥发的主要因素,O₂浓度越高,砷的挥发比例越大;相同O₂浓度下,CO₂浓度越高,砷的挥发比例越低,CO₂的存在抑制了煤中砷的挥发。高温下(>900℃)CO₂浓度是影响砷挥发的主要因素,富氧气氛下高CO₂浓度对热量的阻碍导致相同条件下砷酸盐发生分解需要更高的温度,因此富氧气氛下砷的挥发较空气模式滞后;此外CO₂在煤颗粒表面形成还原性气氛,高价态砷化合物向不稳定的低价态砷化合物转变,低价态砷化合物的快速分解导致高温下富氧气氛中砷的挥发速率较常规空气模式快。     

神府煤粉燃烧特性

为了给煤粉锅炉用户提供煤粉优选理论依据,以煤粉锅炉主要用煤神府煤制备的煤粉为研究对象,采用TG-DTG对煤粉的燃烧特性进行研究,分析了不同煤粉及升温速率对煤粉燃烧特性的影响。结果表明:在空气气氛下,升温速率提高,TG、DTG曲线向高温方向移动,煤粉的着火温度升高,最大质量变化速率增大,最大失重温度提高,燃尽指数增大;随着灰分和粒径改变,升温速率为10或20℃/min时,煤粉的着火温度变化不显著,燃尽指数及综合燃烧特性指数均有影响。灰分减小,粒径不变时,D煤粉的综合燃烧指数为1.51,优于粒径74μm、灰分9.5%的P煤粉。     

转炉除尘灰与纳米催化剂对不同煤阶燃料燃烧特性的影响

利用热分析法研究了转炉除尘灰与纳米催化剂对高挥发分煤、低挥发分煤、无烟煤、石墨等四种不同煤阶样品燃烧过程中着火温度、燃尽指数、最大燃烧速率和最大燃烧速率所对应的温度等燃烧特征参数的影响。结果表明:催化剂对高挥发分煤和低挥发分煤催化效果不明显,对无烟煤和石墨起到了降低着火温度、提高燃尽指数和最大燃烧速率等催化效果。催化剂添加量为4%(质量分数,下同)时,Fe_2O_3对无烟煤催化效果最佳,转炉除尘灰对石墨催化效果最佳,而CaO对无烟煤和石墨的催化效果均最差。对于无烟煤,分别添加2%CaO、4%转炉除尘灰和10%Fe_2O_3时催化效果达到最佳。对于无烟煤和石墨,与纳米催化剂相比,转炉除尘灰催化效果更加明显。无烟煤和石墨在加入催化剂后,整个燃烧过程中的放热量均有所增加,但过高添加量对释放热量有明显的抑制作用。     

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。     

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°C)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。     

吐哈盆地褐煤的热解和燃烧特性及动力学分析

吐哈盆地褐煤的热解和燃烧特性研究利于煤的清洁高效利用及煤炭地下气化的开展,为探究吐哈盆地褐煤煤粉颗粒的热解特性和燃烧特性及动力学特性,通过热重实验、热解特征指数计算、综合燃烧指数计算及动力学软件Kinetics Neo模型拟合法,研究了煤粉在不同升温速率(5℃/min, 10℃/min, 20℃/min)和不同粒径(大于0.8 mm, 0.2 mm～0.6 mm,小于0.1 mm)下分别在氮气气氛中的热解特性和空气气氛中的燃烧特性,获得了不同条件下煤粉颗粒热解和燃烧过程的动力学参数。结果表明：升温速率升高有利于煤粉颗粒热解和燃烧,显著提升了热解和燃烧性能;煤粉颗粒粒径增大有利于煤粉热解,不利于煤粉燃烧;不同粒径煤样热解和燃烧焦产率没有明显区别,粒径增加对于挥发分释放的影响不大;热解与燃烧过程中活化能与指前因子分别在一次热解阶段和干燥挥发阶段较高,说明在这两个阶段反应速率较慢,单位时间内化学反应程度较高。     

水煤浆在双锥逆喷燃烧器内燃烧过程的数值分析

为了研究煤粉和煤浆在双锥燃烧器内燃烧过程的区别,应用计算流体软件Fluent,对14 MW逆喷双锥燃烧器建立了燃烧过程的热态模型,分别进行煤粉和水煤浆的燃烧过程模拟研究。结果发现:水煤浆和煤粉的速度场基本类似;水煤浆中水分汽化增大了燃烧器的阻力,使离散相颗粒在燃烧器内的停留时间增加;由于水分高,水煤浆着火位置较煤粉延后110 mm;燃用水煤浆的燃烧器内平均温度和出口温度分别比燃用煤粉低162 K和199 K;水煤浆火炬的核心温度区比煤粉火炬提前438mm,且水煤浆火炬刚性更强,但衰减速度较快。水煤浆的燃烧特性处于逆喷双锥燃烧组织控制的有效范围,证明该燃烧组织原理是解决水煤浆燃烧温度与效率之间矛盾的有效方法之一。     

不连沟煤热解半焦燃烧特性研究

煤热解产生具有高利用价值的煤气和焦油,并伴随产生大量的热解半焦,燃烧是半焦的主要利用途径之一。本文采用非等温热重分析法研究了热解条件(热解温度和停留时间)、热解气氛和燃烧升温速率对热解半焦燃烧行为的影响,并利用Coats-Redfern积分法对半焦燃烧过程进行动力学计算。结果表明:热解温度对甲烷二氧化碳重整与煤热解耦合过程半焦的燃烧反应特性有重要影响。随热解温度升高,半焦燃烧反应性呈下降趋势,反应活化能逐渐增加,这与半焦中较低的挥发分成正相关。热解停留时间和热解气氛对半焦燃烧影响较小。与在氮气中热解半焦相比,加氢热解和耦合热解半焦表现出几乎相同的燃烧特征和反应活化能。燃烧升温速率显著影响半焦的燃烧特性,提高燃烧升温速率促使半焦燃烧反应在更高温度下进行。     

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。     

燃煤过程中水蒸气对砷释放特性的影响

为了研究水蒸气对煤燃烧过程中砷释放特性的影响,在800~1 300℃下对庆华煤进行燃烧试验,并结合XRD分析了水蒸气对砷释放的作用机理。气氛中有水蒸气时,可以明显加快煤的失重;相同温度下,气氛中存在水蒸气时,煤中砷的逃逸有一定的增加,随着水蒸气浓度的增加,逃逸率增加趋势放缓;随着温度的增加,水蒸气对砷的逃逸率的提升效果减弱。在水蒸气气氛下灰的矿物质组成结构发生变化,石膏等硫酸盐增多,主要是水蒸气促进了石膏的形成,缩减了高活性CaO的停留时间,抑制了CaO对砷的捕捉,造成砷吸附率降低,逃逸率增加。     

Pulverized coal combustion characteristics of high-fuel-ratio coals

It is strongly desired for coal-fired power plants in Japan to utilize not only low-rank coals with high moisture and high ash contents, but also high-rank coals with high fuel ratio for diversifying fuel sources and lowering cost. In this study, pulverized coal combustion characteristics of high-fuel-ratio coals are experimentally investigated using an approximately 100 kg-coal/h pulverized coal combustion test furnace. The combustion characteristics are compared to those for bituminous coal. The coals tested are six kinds of coal with fuel ratios ranging from 1.46 to 7.10. The results show that under the non-staged combustion condition, the minimum burner load for stable combustion rises as fuel ratio increases. To improve the stability, it is effective to lengthen the residence time of coal particles in the high gas temperature region close to the burner outlet by using a recirculation flow. The conversion ratio of fuel nitrogen to NOx and unburned carbon fraction increases with increasing the fuel ratio. In addition, as the fuel ratio increases, NOx reduction owing to the staged combustion becomes small, and unburned carbon fraction increment becomes significant. The numerical simulations conducted under the staged combustion condition show that although the numerical results are in general agreement with the experimental ones, there remains room for improvement in NOx reduction model for high-fuel-ratio coals.     

Direct modification of solid residues during co-firing of coal sludge and coal

In order to utilize the solid residues simultaneously with clean and high-combustion efficiency when firing high sulfur and ash coal sludge generated in coal washing process in China, the mineral composition and usability of combustion solid products were investigated when injecting compound additives into series of coals or coals blended with sludges. In this study, a single burner furnace was used for combustion test and the solid residues were analyzed by means of X-ray powder diffraction method. The influences of temperature, residence time, and compound additives on mineral characteristics of the residues have been discussed. The results indicate that, the compositions of solid products are principally free oxides and a small amount of gehlenite, dicalcium silicate, etc. below 1000 °C; when the firing temperature increases above 1100 °C, the major minerals are gehlenite and dicalcium silicate. Also it can be found that several seconds is not enough for mineral complete reaction, and suitable calcium oxide content and particles fineness are recommended. The result of combustion test shows that there is no evident effect on ignition and combustion when the additive/coal (a/c) ratio is below 30% by weight; and the normal stable combustion with ignition delay can be achieved when the a/c ratio is ranging between 30 and 50%; but the a/c ratio exceeding 50% will damage combustion and even result in fire extinction. Industrial test results indicated that the solid residues was composed of silicate mineral rich in belite by adopting compound additives, but the comprehensive reaction mechanism needs further studies as well as the effect of additives on combustion should be concerned.     

CCSEM analysis of ash from combustion of coal added with limestone

Combustion of three Chinese coals, mixed with limestone physically, was carried out in drop tube furnace. The drop tube furnace consisted of two parts, the top side has a length of about 1.0 m and kept at 1573 K in all the runs, while the bottom-side has a length of 0.5 m and kept at 1173 K. SO₂ removal efficiency of about 80 and 73% were obtained in the combustion of Yanzhou with high and low sulfur, respectively. In contrast, for Datong coal, the De-S efficiency was only about 50% at the molar Ca/S ratio of 2.0; increasing Ca/S ratio to 3.0 had little effect on De-S efficiency. The combustion ashes were analyzed by several techniques including XRD, SEM-EDX and CCSEM (computer-controlled SEM). A novel calcium-based phase definition, based on CCSEM data was developed to investigate the modes of occurrence of added limestone in the ashes. Additionally, the mixture of limestone with kaolinite was injected into the furnace to study their transformation behavior under simulated coal combustion conditions. The governing mechanisms for limestone capturing sulfur and its reaction with the inherited minerals were correspondingly revealed. It was found that under the given coal combustion conditions, the calcium distribution in the ash varied with coal type and residence time. Briefly, more calcium was used for desulfurization or fixed into mineral; as time progressed, the inherited aluminosilicate, small sized excluded particles in the coal matrix, facilitated its reaction with limestone; it also reacted quickly compared to sulfation of limestone in coal combustion. This in turn hampered the efficient utilization of limestone in coal combustion.     

Behaviour of elements and minerals during preparation and combustion of the Pernik coal,Bulgaria

The chemical and mineral composition, including major (Al, Ca, Fe, K, Mg, S, Si, Ti), minor (Na, P) and trace (Br, Cl, Co, Cr, Cu, Li, Mn, Ni, Pb, Rb, Sr, Zn) elements and different minerals, of the Pernik subbituminous coals and their preparation and combustion solid waste products were studied. Feed coals, upgraded coals (high-grade and low-grade coals) and their waste products, namely coal slimes and host rocks generated from the Pernik coal preparation plant, as well as combustion waste products such as bottom ashes, fly ashes and lagooned ashes resulted from the Republica coal-fired thermoelectric power station were characterized. The occurrence and behaviour (partitioning, volatilization, condensation, capture and retention) of the above-mentioned elements and various minerals during coal preparation and combustion are described. The results indicate some technological problems and possible environmental pollution of the air, water, soil and vegetation with certain elements in the areas surrounding both thermoelectric power station and coal preparation plant.     

煤中氟分布与燃烧排放特性

采用高温水解 -离子选择电极法对我国典型煤种的氟含量进行测定 ,得到了煤中氟分布规律 ,分析了煤中氟含量与灰分的关系及氟化物的赋存形态 .首次通过燃烧实验探索了燃煤过程中氟化物生成规律 ,确定了燃煤氟排放的影响因素 ,得到了燃煤氟排放与煤种、燃烧温度、停留时间、燃烧气氛等因素的影响规律 ,提出了燃煤过程中氟化物的生成机理 .研究结果对煤中氟的燃烧转化和污染治理有指导意义