我国水泥行业NOx排放与污染控制现状分析

1 水泥行业NOx产生机理 根据目前的认识水平,在水泥熟料煅烧过程中,存在三种NO形成方式,即热NO形成、瞬时NO形成和燃料NO形成,其中瞬时NO形成方式必须有碳氢原子团存在,形成量很少,在工业窑炉中一般不予考虑.由燃料氮形成的NO量主要与挥发性的氮含量和反应条件如温度、过剩空气系数等因素有关.水泥窑产生的NO主要来源于助燃空气中的氮以及燃料、原料中的氮经氧化而成,常称为热力型氮氧化物,它的生成与温度、氧气过剩量和反应时间等因素有关,通常情况下,燃烧温度越高,生成的NO愈多;氧分子浓度愈高,NO生成速度愈快,NO愈多;高温区停留时间愈长,NO生成量愈多.在水泥回转窑系统中主要生成NO,NO2的量很少.     

气化细粉灰预热无焰燃烧煤氮转化与NOx排放特性

为实现煤化工固废——气化细粉灰的清洁高效利用,采用先进的煤粉自预热燃烧技术,在30 kW固体碳基燃料预热无焰燃烧试验平台上,针对不同预热温度、不同预热燃烧器当量比下的烟煤经循环流化床气化后,细粉灰中氮的转化及NO_x排放特性进行试验研究。结果表明,气化细粉灰能在该试验系统上实现稳定的无焰燃烧。预热可明显改善气化细粉灰的燃烧特性改善具有重要作用。几乎全部挥发分氮在预热燃烧器内的强还原性气氛下提前脱除,主要向N_2、NH_3与HCN三种含氮物质转化,焦炭氮为后续燃烧中NO_x的主要来源。预热温度对预热过程中煤氮向N_2的转化率影响显著,预热燃烧器空气当量比直接关系煤氮向N_2和NH_3的转化率,且与焦炭氮析出情况密切相关。预热温度和预热燃烧器空气当量比对NO_x排放浓度及燃料氮向NO_x转化率的影响效果差别明显。在预热温度为902℃、预热燃烧器空气当量比为0.45的条件下,NO_x排放浓度和燃料氮向NO_x转化率最低,分别为83.02 mg/m~3(6%O_2)和5.94%。     

富氧燃烧系统中NO的还原及其排放

富氧燃烧方式具有排烟中的CO2浓度高、CO2捕捉回收处理容易等优点,是很有前途的新型清洁煤燃烧方式.通过实验得出了富氧燃烧过程中燃料氮生成NO的转化率随CO2浓度的变化、循环NO在挥发分燃烧火焰中的还原率、循环NO对燃料氮生成NO的转化率的影响,并得出了相应的定量关系式.结合实验结果和系统物质平衡理论分析,得出了整个系...     

生物质燃烧NO生成的影响因素实验研究

以稻壳、木屑、甘蔗渣和秸秆等4种典型生物质颗粒为研究对象,在自主设计的十字形管式炉内,研究了燃料种类、温度、氧量和气氛因素对NO生成的影响.实验结果表明:燃料含N量越高,NO生成总量亦越高,最高可达0.9848 mg·g-1,而此时燃料N向NO的转化率是下降的,仅为7.18％;NO生成总量随着温度升高而升高,但不同生物...     

燃煤增压流化床氧化亚氮排放特性试验

在小型增压流化床内研究N2 O和NO的排放特性 ,研究运行条件包括压力、氧浓度以及床层温度对N2 O和NO排放量的影响 .结果表明 ,在增压燃烧的条件下 ,随着床温的增加 ,N2 O的排放量减少得很快 ;而床温对NO的排放影响很小 ,此结果与常压下的结果不同 .随着氧浓度的增加 ,N2 O和NO的排放量均增加 ,但N2 O增幅不如NO强 .NO的排放量随着压力的增高有明显的降低 ,在氧浓度较低的条件下 ,压力越高 ,NO的降低幅度越显著 ;而压力对N2 O排放影响则相反     

贫煤直接混烧低氮稳燃研究

针对贫煤难稳燃,NO_x生成量大的突出问题,采用热重分析法结合卧式炉燃烧研究,通过Testo 350在线烟气分析仪考察了贫煤及其与生物质等直接混烧的燃烧特性和NO_x排放特性。结果表明:贫氧气氛下燃烧,氧浓度对焦炭氮的影响明显大于对挥发分氮的影响,氧浓度低于21%时,NO_x转化率由27.4%降低至15%左右;在1 100℃以下,温度升高促进燃料氮与挥发分同时释放,还原性气氛下含N中间体更容易向N_2转化,NO_x排放浓度及转化率降低。生物质可以促进燃烧,减少NO_x排放,但容易出现挥发分与焦炭分段燃烧现象,影响焦炭稳定燃烧。为此,以烟煤煤矸石为调节燃料,将贫煤、玉米芯、煤矸石三者混配,配比为80∶5∶15(质量比)时,着火温度较贫煤单独燃烧降低100℃左右,最大失重峰温由670℃降低至600℃;氮转化率由28.5%下降至16.7%。     

无烟煤化学链燃烧过程燃料氮转化释放特性

基于赤铁矿石载氧体,在小型单流化床反应器上,开展煤挥发分和焦炭的化学链燃烧研究,探讨挥发分氮和焦氮在化学链燃烧过程中的转化特性。研究表明:燃料氮释放的中间产物HCN和NH₃与铁矿石载氧体具有较高的化学反应亲和性,易于被载氧体氧化生成N₂和NO。淮北无烟煤挥发分氮转化过程中,NO是唯一的氮氧化物,反应器出口中间产物NH₃的释放份额略高于HCN。在煤焦化学链燃烧还原过程中,部分燃料氮释放的中间产物HCN和NH₃被铁矿石氧化导致少量NO的生成,还原过程中无N₂O的释放;较高的还原反应温度加速了NO的生成。减少进入载氧体氧化再生过程的焦炭量可减少空气反应器NO和N₂O的生成。     

矿物质对煤焦燃烧过程中NO释放规律的影响

在石英固定床反应器上研究了煤焦燃烧过程中矿物质在不同燃烧条件下对NO释放规律的影响.结果表明：煤中的矿物质对燃料氮转化为NO有显著的影响,其影响与矿物质的组成和燃烧条件有关,碱金属Na、K催化半焦氮的氧化在较低的温度下进行并降低半焦氮对于NO的转化率,而Ca、Fe在低温燃烧条件下增加NO的排放,高温时使NO的排放降低;矿物质惰质组分的存在使NO的排放增加;随着煤阶的升高,半焦的反应性降低,燃料氮对于NO的转化率增大;燃料氮的转化率随燃烧温度的升高而增加,但达到极大值后又趋于降低;矿物质对于NO排放量的影响决定于矿物质对于半焦氮的氧化以及半焦还原NO反应催化作用的相对大小.     

流化床煤燃烧过程不同气氛下的气态氮释放特征

利用微型流化床反应装置,结合快速过程质谱仪,在850~940℃操作温度下,研究了三种不同粒度分布烟煤和无烟煤在热解、气化和燃烧反应条件下四种主要气态氮产物HCN、NH₃、NO和NO₂的释放规律。结果表明,微型流化床可以实时检测挥发分氮和焦炭氮的动态释放序和类型,热解、气化和燃烧反应气氛的改变主要影响HCN和NH₃的释放量。热解产物的气态氮主要是来自于挥发分,燃烧反应的HCN和NH₃的释放量与温度有明显关系,而气化反应的各类气态氮释放量随温度变化波动不大。煤颗粒尺寸和温度变化对烟煤和无烟煤中各类气态氮释放量产生影响比较复杂,其中NH₃的释放特性是区分挥发分N释放和半焦N释放的重要特征。     

含氮模型化合物用于化学链燃烧的氮氧化物释放特性

以谷氨酸、甘氨酸和苯丙氨酸作为生物质的含氮模型化合物,进行化学链燃烧实验,主要考察了反应温度、氨基酸种类、碱金属钾元素等对化学链燃烧还原反应过程中氮氧化物释放特性的影响。结果显示,挥发分氮的释放迅速,温度的升高有利于NO和NO₂的生成,N₂O的生成会出现波动。模型化合物的氮含量越高,可能越不利于氮向氮氧化物的转化。钾元素对苯丙氨酸的化学链燃烧过程中NO的释放抑制作用较强,而对其他氨基酸化学链燃烧过程中氮氧化物释放的影响则不太显著。     

The NO and N2O formation mechanism under circulating fluidized bed combustor conditions: From the single particle to the pilot-scale

The NO and N₂O formation mechanism is studied starting from a single fuel particle burning under well-defined conditions up to a pilot-scale circulating fluidized bed combustor (CFBC). The fuel, petroleum coke, was the same in all tests and care has been taken to obtain chemical similarity between the different units: a formation rate unit, a laboratory-scale and a pilot-scale CFBC. A detailed single particle NO/N₂O formation model has been developed and incorporated in a CFBC NO/N₂O emission model. To thoroughly test the modeled NO/N₂O mechanism, the iodine addition method has been used in all units.     

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.     

一种石油焦与CO2高温气化原位反应特性

采用高温热台显微镜原位研究了气化温度和颗粒粒度对石油焦气化反应的影响,并对比了神府烟煤和石油焦的反应特性。研究发现,石油焦在气化过程中颗粒收缩,表面结构发生变化,反应速率的改变由低温下逐渐减小,到高温下1300℃时先增大后减小。相同转化率下,反应时间随温度的升高而减少,随颗粒粒度的减小而减少。对比神府煤焦和石油焦的气化反应发现,石油焦反应活性低,为神府烟煤焦平均气化反应速率的1/6。     

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.     

Effect of the cooling and reheating during coal pyrolysis on the conversion from char-N to NO/N2O

Coal chars employed in the previous studies were usually prepared from coal pyrolysis at high temperatures and then cooled down to the room temperature. As these chars were burned out, they had undergone an additional process: cooling and reheating. The influence of this process on the conversion from char-N to nitrogen oxides had mostly been neglected. In this paper, the influence of cooling and reheating during coal pyrolysis on the conversion from char-N to NO and N₂O is studied on a drop tube/fixed bed reactor. Results indicate that the process of cooling and reheating can cause a fall of NO emission during the coal char combustion. The discrepancy between the experiments with and without cooling and reheating at low temperatures is greater than that at high temperatures. For a high rank coal sample, the difference can be up to 20% in the temperature range of 973–1073 K. In addition, the discrepant nitrogen, which would have been converted into NO in the experiments without cooling and reheating, does not retain in the char during the char combustion. Most of this nitrogen is oxidized into N₂O through the reduction of NO.     

Investigation of combustion reactivity and NO emission characteristics of chars obtained from the devolatilization of raw and partially dried lignite

In an attempt to achieve the clean and efficient utilization of lignite, drying pre-treatment was performed in this study before lignite combustion. The combustion reactivity and NO emission characteristics of the partially dried lignite samples in the char combustion stage were investigated by means of TG and isothermal combustion tests, and the reactivity could be summarized as the following order: L1>L0.5>raw>L3>LT>L5 (chars obtained from the devolatilization of the raw and partially dried coals at 378 K for 0.5, 1, 3, 5, and 120 minutes) and the NO conversion ratio of L1 was the lowest. When the moisture content in the coal particles was relatively high (19.68%-35.84%), the drying treatment could increase the combustion reactivity and inhibit NO emission in the char combustion stage; When the moisture content was within a relatively low range (0.17%-19.68%), the moisture removal had negative effects on the reactivity and NO emission in the char combustion stage. The surface behaviour and microstructure of the raw coal char and chars derived from the partially dried coals were clarified by temperature programmed desorption/reduction (TPD/TPR) and Raman spectroscopy. The results illustrated that L1 derived from L_c1 (19.68%) was the most reactive sample with the largest amount of C(O) on the particle surface. There were also more reactive aromatic structures in L1 than other samples. Compared with direct combustion or excessive drying treatment, lignite should be dried to a certain degree (19.68%) for optimized lignite combustion.     

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.