高体积分数CO_2气氛下NO-煤焦反应特性

以李家塔烟煤和锦界烟煤煤焦为研究对象,利用固定床反应器系统,研究了高体积分数CO2气氛下煤焦异相还原NO的反应特性,分析了反应气氛、CO2体积分数、O2体积分数、NO初始体积分数和热解温度等对煤焦还原NO的影响.结果表明:反应气氛中高体积分数CO2的存在不利于煤焦还原NO,且CO2体积分数越大,越不利于NO还原;当反应温度较低时,反应气氛中O2对煤焦还原NO具有一定的促进作用,但当反应温度较高时则不利于NO还原;随着NO初始体积分数增大,NO还原率降低;随着热解温度的升高,煤焦还原NO的能力下降.     

煤焦O_2/CO_2燃烧方式下NO_x生成与异相还原特性

在微型流化床反应器上,对几种典型煤焦在O2/CO2燃烧方式下不同燃烧气氛、氧气体积分数、初始循环NOx体积分数对NOx生成的影响以及煤焦/NO的异相还原进行了实验研究.实验结果表明,同一种气氛下CO2焦的N转化率大于N2气氛焦转化率;氧气体积分数提高对煤焦还原NOx既有促进也有抑制作用;初始NOx体积分数的提高对NOx还原量、还原速率均有提高.     

O_2/CO_2气氛下煤粉燃烧NO排放特性

在沉降炉上对O_2/CO_2气氛下超细煤粉(0~20,μm)、大粒径煤粉(70~100,μm)以及二者混合煤粉NO的生成特性进行了研究,结合红外光谱分析研究了煤焦表面含氧官能团与NO生成之间的关系.结果表明:随着挥发分析出燃烧和煤焦燃烧的进行,NO沿炉子长度方向分布呈现为"M"型.入口气氛中氧体积分数提高,NO生成速度加快,入口附近NO体积分数峰值提高,炉子出口附近NO体积分数并未同步提高.煤粉粒径分布不同,沿炉子方向NO分布曲线形态不同,NO峰值出现位置发生改变.入口氧体积分数不同,煤焦表面含氧官能团生成规律不同,煤焦表面含氧官能团增加,有利于降低NO的生成.     

分解炉内贫氧高CO2燃烧条件下挥发分NO生成机理

基于GRI3.0详细动力学反应机理数据库,采用生成速率分析方法,从化学反应动力学角度分析水泥分解炉内煤粉挥发分贫氧燃烧时低温、高浓度CO2条件下挥发分NO生成机理及挥发分中HCN转化生成NO的主要转化路径.分析结果表明,煤粉在过量空气系数为0.8的贫氧燃烧条件下,分解炉内高浓度CO2气氛会促进NO生成,增大NO的排放浓度;850～950℃温度范围内,CO2体积分数为0%～35%条件下挥发分NO生成的主要机理反应式为N+O2(→)NO+O、HNO+H(→)H2+NO和N+CO2(→)NO+CO;高体积分数CO2通过推进反应FINO+H(→)H2+NO和N+CO2(→)NO+CO促进NO生成;其中HNO是NO生成过程中最重要的活性含氮中间产物,对NO生成起主要的贡献作用;HCN氧化生成NO的主要反应路径为HCN先转化生成NHi,再进一步转化生成HNO活性含氮中间体,最终生成NO.     

高温条件下N_2O分解转换对NO生成的影响

根据50 kW下行燃烧炉中燃烧过程中的NO和N_2O生成及还原规律,采用高温固定床模拟了空气分级燃烧过程的还原及燃尽阶段的气氛对N_2O高温分解及对NO生成影响的实验,测量了多种气氛模拟工况下的N_2O高温分解等特性,得到了N_2O高温分解及对NO生成转化的影响规律.实验结果显示,在CO+O_2+平衡气的气氛条件下,N_2O分解生成大量NO,有别于传统的N_2O分解理论.利用基于MB89机理的化学动力学模拟方法模拟了对应气氛下N_2O分解规律及产物特性,分析N_2O分解生成NO的基元反应.根据模拟结果发现,生成的大量NO一是来源于CO燃烧产生大量的O自由基与N_2O反应的产物,二是N_2O逆向生成的NCO进一步分解成NO.     

微富氧条件下煤粉燃烧及NO生成特性的研究

利用热重分析技术对微富氧条件下煤粉的燃烧特性进行了研究,并与富氧条件下煤粉的燃烧特性进行了对比,利用固定床测定了燃煤NO的生成规律,分析了反应气氛和煤种的影响．结果表明：随着氧体积分数增加,微富氧条件下煤粉的燃烧向低温区移动,综合燃烧特性指数S逐渐增大;在相同的氧体积分数下,由于N2和CO2的物性差异,煤粉的微富氧燃烧特性优于富氧燃烧特性,但当氧体积分数升高到40％时,两种气氛的燃烧特性差别不大;反应气氛和煤种均对燃料氮的转化率影响显著;氧体积分数升高或N2的参与会使反应温度上升,影响燃料氮的转化率;煤的挥发分和元素氮的质量分数也会影响燃料氮的转化率．     

煤粉再燃过程中主要气体体积分数的变化规律

在一维炉上对煤粉再燃过程中烟气内主要气体体积分数随再燃区初始氧体积分数的变化规律进行了研究.再燃温度为1 273 K时,脱硝效率及H2、CO体积分数曲线随再燃区氧体积分数升高均呈不规则"M"型.脱硝效率与H2体积分数出现峰值时的氧体积分数一致,CO体积分数峰值处的氧体积分数高一些.脱硝效率及H2、CO体积分数的第一次下降是由气相着火引起的.氧体积分数进一步上升时,煤焦被气相燃烧热引燃,颗粒温度的大幅跃升促使异相脱硝反应增强,脱硝效率及H2、CO、CH4体积分数再次上升.氧体积分数更高时煤焦燃烧开始受扩散控制,火焰自焦表面外移,颗粒升温趋缓,能够到达焦表面的氧气减少,必须有氧参与的异相还原反应减弱,脱硝效率再次下降.再燃温度更高时,脱硝效率及H2、CO体积分数的变化趋势与1 273 K时基本类似,但稍平缓.     

O_2/CO_2/水蒸气气氛下煤的燃烧对石灰石分解行为的影响

钙基CO_2吸收剂再生过程中,石灰石的分解需要大量能量.本文提出用煤的燃烧为石灰石分解提供能量,并研究了O_2/CO_2/水蒸气气氛下煤的燃烧对石灰石分解行为的影响.实验结果表明,过量氧气系数增大可以抑制CO和H_2的生成,促进硫化反应,从而降低SO_2的排放,然而也会促进NO的排放.煤的燃烧会使分解产物CaO活性降低,但水蒸气的添加又提升了产物的活性.水蒸气的添加还会促进硫化反应,使SO_2排放得到控制.     

煅烧气氛对石灰石循环分离CO2过程特性的影响

在热重分析仪上研究了煅烧气氛对石灰石循环分离CO2特性的影响。结果表明:煅烧气氛中CO2体积分数和循环次数的增加皆导致化学反应控制阶段提前结束,致使产物层扩散控制阶段提前,最终导致CaO最终转化率随CO2体积分数和循环次数的增加而降低。CaO活性随循环次数的增加而降低主要发生在化学反应控制阶段,相同循环次数下煅烧气氛中CO2体积分数越大CaO活性越低。     

再燃烧条件下煤焦结构其对NO还原的影响

在携带流反应装置中测量了再燃条件下煤粉在高温烟气环境中迅速热解时的质量损失,通过扫描电镜观察和分析了煤焦的显微结构;分析了煤种、热解温度、热解气氛和煤粉粒径等因素对煤粉热解特性的影响;探讨了煤焦形成条件对NO还原的影响.结果表明,随着煤的挥发分含量增加,煤的质量损失份额和煤焦还原NO的能力增加;热解温度升高,将导致煤焦还原NO能力下降;在一次燃烧区空气过量系数SR1＝1.0～1.2范围内,煤粉质量损失变化不大.     

CaO和Fe（N03）3复合催化锦界煤焦-C02气化的实验研究

在搭建的固定床实验台上对CaO和Fe（NO3）3复合催化锦界煤焦-CO2常压等温气化的反应特性进行了研究．结果表明：复合催化剂最佳质量添加比例为1％Ca、29／6Fe,且在最佳添加量下气化时间比原煤焦和单组分催化剂下的气化时间分别缩短了103rain和18rain,催化强度系数分别是原煤焦、单组分催化剂CaO和Fe（N03）3的5．71倍、1．65倍和2．04倍,气化温度降低了100K,气化温度降低程度介于单组分催化剂CaO和Fe（NO3）3之间;复合催化过程生成的部分Ca（NO3）2与CaO的活性不同,即Ca的催化强度与其前驱物的形式有一定关系;添加单组分催化剂的煤焦活化能与原煤焦的活化能基本相同,不同添加比例的复合催化剂的煤焦活化能增加的程度不同,且在最佳添加比例下增加程度最小,其值为15．4％．     

Research on the dynamic process of NO heterogeneous and homogeneous reduction with cement raw meal in vertical tubular reactor

A certain amount of coal is supplied at the outlet of cyclone suspension preheater C5 to reduce NO_x through homogenous and heterogenous reduction. Subsequently, char enters into precalciner and burnt out, while volatiles are burnt out after being exposed to over-fire air. Such process, i.e. the high-order reduction of NO_x, has been verified as a high-efficiency control mean for NO_x emission in cement industry. The dynamic processes of NO reduction by volatiles and coal in absence of O₂ have been studied in the vertical tubular reactor. The effects of cement raw meal and reaction temperature on NO heterogenous and homogeneous reduction were analyzed. The results suggested that the reactions were classified into two types (the homogeneous and the heterogeneous reactions), characterizing the reactions in the gas phase and the reactions between the gas phase and the particle surfaces, respectively. Homogeneous reduction was the critical mechanism during the earlier stage of reaction. Then, the char reduction mechanism became more efficient. Raw meal could promote NO heterogeneous reduction because CaO, as the carrier, could accelerate the transfer of oxygen atom. However, raw meal could significantly inhibit NO homogeneous reduction. CaO, as reactant, could change the pathways of cleavage reaction; the products of new reaction path contained a neutral molecule and an anion, instead of unstable free radicals which are actually vital to reduce NO in the gas phase. High temperature could facilitate homogeneous and heterogenous NO reduction. The catalysis of CaO on heterogenous had the optimum temperature (850 °C), and it could restrain the effect of heat inactivation. And CaO could significantly inhibit NO homogeneous reduction at the whole operation temperature.     

Two-step steam pyrolysis of biomass for hydrogen production

In this study, different char based catalysts were evaluated in order to increase hydrogen production from the steam pyrolysis of olive pomace in two stage fixed bed reactor system. Biomass char, nickel loaded biomass char, coal char and nickel or iron loaded coal chars were used as catalyst. Acid washed biomass char was also tested to investigate the effect of inorganics in char on catalytic activity for hydrogen production. Catalysts were characterized by using Brunauer–Emmet–Teller (BET) method, X-ray diffraction (XRD) analyzer, X-ray fluorescence (XRF) and thermogravimetric analyzer (TGA). The results showed that the steam in absence of catalyst had no influence on hydrogen production. Increase in catalytic bed temperature (from 500 °C to 700 °C) enhanced hydrogen production in presence of Ni-impregnated and non-impregnated biomass char. Inherent inorganic content of char had great effect on hydrogen production. Ni based biomass char exhibited the highest catalytic activity in terms of hydrogen production. Besides, Ni and Fe based coal char had catalytic activity on H₂ production. On the other hand, the results showed that biomass char was not thermally stable under steam pyrolysis conditions. Weight loss of catalyst during steam pyrolysis could be attributed to steam gasification of biomass char itself. In contrast, properties of coal char based catalysts after steam pyrolysis process remained nearly unchanged, leading to better thermal stability than biomass char.     

煤粉再燃过程对煤焦异相还原NO的影响

利用高温携带流反应装置,研究了煤种(包括褐煤、烟煤和贫煤)、再燃区内反应温度、煤粉粒径、一次燃烧区空气过量系数SR1和再燃区空气过量系数SR2对煤焦异相还原NO作用的影响,探讨了煤焦异相还原NO的机理.结果表明:随着SR2和煤粉粒径的减小以及再燃区反应温度的提高,煤粉NO还原效率增加;在相同的SR2下,随着煤中挥发分含量的提高,煤粉粒径的增加和再燃区反应温度的降低,煤焦异相还原NO贡献上升;对于相同再燃燃料份额:SR1=1.0和SR1=1.2时煤焦异相还原NO的贡献均大于SR1=1.1时的异相还原NO的贡献.     

高水分煤在流化床中燃烧时NO_x的排放特性

通过数值计算,研究了高水分煤在流化床中燃烧时ＮＯ的排放特性及其形成的主要影响因素。结果表明煤中的水分,床内的ＣＯ浓度和空－燃比等都对于ＮＯ转换率有较大影响。     

CO_2浓度对煤焦燃烧及污染物排放特性影响的试验研究

在热重分析仪上进行了燃烧特性试验,考察了CO2浓度对徐州烟煤及其煤焦燃烧特性的影响,并在固定床反应器上研究了CO2浓度对烟煤燃烧过程中气态污染物排放特性的影响.结果表明:在相同CO2浓度下,随着CO2浓度的提高,煤及煤焦的着火温度略有上升,燃尽温度明显升高,煤焦的燃尽时间显著延长;SO2排放浓度随时间增加呈双峰析出,其排放量随CO2浓度的提高而减少;NO排放量随CO2浓度的提高而减少;CO排放量随CO2浓度的提高而增加;当温度低于烟煤的燃尽温度时,CO2对煤焦的气化作用使CO排放量大幅增加.     

不同催化剂对脱矿煤焦还原NO的催化能力比较

通过实验研究了几种在煤灰中常见的金属 (K ,Na,Cu ,Fe ,Ca)氧化物对煤焦还原NO的催化作用 ,为了比较这些金属氧化物对焦炭还原NO反应的催化能力 ,对煤焦进行了脱矿处理。实验是在高温沉降炉内完成的。实验中使用的煤粉及其煤焦来自神府褐煤。实验结果表明 ,煤灰中的矿物质对煤焦异相还原NO具有催化作用。在实验条件下不同金属氧化物对煤焦还原NO的催化能力有如下顺序 :K >Na>Ca >Cu >Fe。实验结果还表明 ,实验工况 (温度 ,SR数 )和催化剂的添加量对NO的还原都具有不同程度的影响     

停留时间对微细化煤焦再燃还原NO效率的影响

以4种细度的混煤(烟煤与褐煤)煤焦作为再燃燃料,用N_2、O_2、CO_2和NO配制模拟烟气,在1300℃的立式管式携带炉中,对停留时间与再燃还原NO效率的关系进行了实验研究,分析了停留时间对再燃还原NO效率的影响机制。结果表明,随着再燃停留时间的延长,NO还原效率增大;煤焦再燃还原NO的适宜停留时间约为0.8～1.0s.     

NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.     

Nitric oxide destruction during coal and char oxidation under pulverized-coal combustion conditions

A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oxidation of chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide (α_NO). The results confirm previous findings by other investigators that α_NO decreases as the weight of char burned increases. α_NO for coal was the same as (at 4% O₂) or lower than (at 20% O₂) that for an equal mass of char during oxidation. Since coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichiometry surrounding the particle is responsible for the observed reduction in α_NO as sample size increases. The analysis of the exhaust gases showed increases in HCN concentration and a decrease in CO₂/CO ratio as sample size increased, suggesting that local stoichiometry influences α_NO. Additional experiments showed that α_NO decreased as the background NO concentration was increased, at rates that diminished as the oxygen concentration increased, independent of particle size. The steep reduction in NO production as the background NO concentration increased was explained by the destruction of NO in the gas phase.