水泥工业煤粉富氧燃烧NO释放特性实验研究

在回转炉实验台架上,分别研究了掺石灰石及掺CaO的情况下,N2/O2、CO2/O2气氛及氧浓度对典型煤种在富氧燃烧条件下燃烧释放NO的影响规律.研究结果表明:CO2/O2气氛下煤粉燃烧NO释放总量较N2/O2气氛下少;CO2/O2气氛下氧浓度的增加对煤粉燃烧NO释放有促进作用;石灰石及CaO对煤粉燃烧NO的生成均具有催化作用,且CaO的催化作用更强.     

四角切圆锅炉富氧燃烧对NO_x生成影响的数值模拟

采用RFG富氧燃烧方法在新疆某电厂350 MW机组锅炉上进行数值模拟,对燃烧时炉内温度场、CO与O2及NOx排放进行分析。结果表明:在21%、25%、29%富氧燃烧工况下,NOx排放浓度均低于空气燃烧时的浓度;四角切圆燃烧煤粉锅炉采用富氧燃烧后,炉膛出口NOx浓度由空气燃烧时的359 mg/m3分别降低到235 mg/m3、272 mg/m3、305 mg/m3;高浓度的CO2与煤粉反应生成CO,形成还原性氛围,有助于抑制NOx生成以及增大对已生成NOx还原的概率;在氧气含量为21%的浓度下,通过增加循环烟气中NO含量可以减少NOx的生成和排放。     

废物燃料对燃料型NOx排放的影响

研究了废皮革、废轮胎与煤在管式电炉中燃烧对燃料型NOx排放的影响。结果表明,在900℃富氧条件下,废皮革中的氨基酸氮比烟煤中的六元杂环氮更易于转化为燃料型NOx,而废轮胎生成的燃料型NOx浓度较低。废皮革与烟煤混烧产生的燃料型NOx浓度低于两者各自燃烧所生成的NOx排放浓度总和。废轮胎与烟煤混烧,不仅低于两者各自燃烧所生成的燃料型NOx排放浓度总和,而且还低于烟煤单独燃烧所生成的燃料型NOx排放浓度。另外,水泥生料会促进烟煤、废皮革及废轮胎生成燃料型NOx。     

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.     

SO2对氮氧化物生成的影响

研究了燃料燃烧过程中 N- S间的相互作用 .分别从实验和数值计算的角度总结 SO2对 NOx 及含氮中间产物生成和释放的影响 ,并试图通过反应机理来解释 .SO2 在贫燃料状态下能降低热力型 NO的释放 ,抑制富燃料状态下快速型 NO的生成 ,对贫燃料及富燃料状态下燃料型NO释放的影响不能得到统一定论 ,但 N- S间确实发生了复杂的相互作用 .数值计算的结果与实验相差较大 ,但具有一定参考价值 ,且能得到实验难以测量的中间产物的浓度变化趋势     

富氧富水蒸气条件下燃烧高氮燃料的NO_x排放特性

在富氧富水蒸气条件下,研究了富含氮的燃料白酒糟在流化床中燃烧时NO_x的排放特性。结果表明,在过量空气系数1.2条件下,水蒸气和O_2对NO_x的生成相互影响。当O_2浓度低于约35%时,向燃烧气中加入水蒸气能抑制NO_x生成,使烟气中NO_x的排放浓度和燃料N转化为NO_x的转化率降低;而当氧气浓度高于约35%时,加入水蒸气促进了NO_x生成,表明提高氧气浓度使得氧化作用起到主导地位。NO_x生成量随温度的升高先增加后减少,在较高氧气浓度下,NO_x生成量随温度升高而降低的转折点发生在较低的温度;燃烧气氛中添加水蒸气延迟了转折点的发生,使转折点发生在较高温度。     

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。     

流化床中燃烧高水高氮酒糟的NO排放特性

采用流化床反应器,研究富水蒸气条件下酒糟燃烧的NO排放特性。结果表明,增加过量空气系数和升高燃烧温度,NO排放浓度升高;对送入燃烧反应器的气体中添加水蒸气模拟高水分燃料燃烧有效地降低了酒糟燃烧的NO排放浓度及总排放量,且在适当条件下可减少NO排放约46%(质量)。酒糟灰分中的金属氧化物对NO的还原有催化作用,且随着温度的升高变强。在含H₂或CO的N₂气氛中,灰分对NO催化还原作用更明显。水蒸气本身对NO没有明显还原作用,说明水蒸气是通过与碳氢化合物反应生成还原性气体,如H₂和CO,从而在酒糟灰催化作用下强化NO的还原。     

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.     

二氧化碳减排及利用的现状及发展

化石燃料的燃烧是人类排放CO2的主要来源。对近几年新兴的CO2减排及利用技术进行了总结和分析,并重点介绍了一种新型CO2减排技术——基于循环载氧体的化学链燃烧技术。化学连燃烧通过不同品位能的梯级利用,具有比传统燃烧方式更高的能源利用效率,而且在CO2富集、污染物协同控制方面具有优越性。     

Effects of operation parameters on NO emission in an oxy-fired CFB combustor

Oxy-fuel Circulating Fluidized Bed (CFB) combustion technology, a very promising technology for CO₂ capture, combines many advantages of oxy-fuel and CFB technologies. Experiments were carried out in a 50 kW_th CFB facility to investigate how operation parameters influence the NO emission in O₂/CO₂ atmospheres. The simulated O₂/CO₂ atmospheres were used without recycling the flue gas. Results show that NO emission in 21% O₂/79% CO₂ atmosphere is lower than that in air atmosphere because of lower temperature and higher char and CO concentrations in the dense bed. Elevating O₂ concentration from 21% to 40% in O₂/CO₂ atmosphere enhances fuel-N conversion to NO. Increasing bed temperature or oxygen/fuel stoichiometric ratio brings higher NO emission in O₂/CO₂ atmosphere, which is consistent with the results in air-fired CFB combustion. As primary stream fraction increases, NO emission increases more rapidly in O₂/CO₂ atmosphere than that in air atmosphere. Stream staging is more efficient for controlling NO emission in oxy-CFB combustion than that in air combustion. Oxygen staging provides an efficient way to reduce NO emission in oxy-CFB combustion without influencing the hydrodynamic characteristic in the riser.     

Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation

Coal combustion with O₂/CO₂ is promising because of its easy CO₂ recovery, extremely low NO_x emission and high desulfurization efficiency. Based on our own fundamental experimental data combined with a sophisticated data analysis, its characteristics were investigated. It was revealed that the conversion ratio from fuel-N to exhausted NO in O₂/CO₂ pulverized coal combustion was only about one fourth of conventional pulverized coal combustion. To decrease exhausted NO further and realize simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x, a new scheme, i.e. O₂/CO₂ coal combustion with heat recirculation, was proposed. It was clarified that in O₂/CO₂ coal combustion, with about 40% of heat recirculation, the same coal combustion intensity as that of coal combustion in air could be realized even at an O₂ concentration of as low as 15%. Thus exhausted NO could be decreased further into only one seventh of conventional coal combustion. Simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x could be realized with this new scheme.     

Conversions of fuel-N,volatile-N,and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N₂O during combustion of a single coal particle in O₂/N₂ and O₂/H₂O were studied in a tube reactor at low temperature. In O₂/N₂, NO reaches the maximum value in the devolatilization stage and N₂O reaches the maximum value in the char combustion stage. In O₂/H₂O, both NO and N₂O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N₂O in O₂/N₂ are obviously higher than those in O₂/H₂O, due to the reduction of H₂O on NO and N₂O. Temperature changes the trade-off between NO and N₂O. In O₂/N₂ and O₂/H₂O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N₂O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at < O₂ > = 30 vol% in O₂/N₂. In O₂/H₂O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at < O₂ > = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N₂O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N₂O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N₂O in both atmospheres. This work is to develop efficient ways to understand and control NO and N₂O emissions for a clean and sustainable atmosphere.     

Experimental investigation of NOx emissions in oxycoal combustion

This work presents the results of an experimental investigation on NO_x emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO₂/O₂, and O₂ enriched recirculated flue gas. NO_x emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O₂ concentration on NO_x formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NO_x emissions has been obtained for air and CO₂/O₂ in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NO_x emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO₂/O₂. No significant impact of the oxidiser O₂ concentration in the CO₂/O₂ mixture on NO_x emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NO_x formation and reduction mechanisms showed, that the observed reduction of NO_x emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NO_x alone, but also to a reduced conversion of fuel-N to NO.     

Hydrolysis of HCN as an important step in nitrogen oxide formation in fluidised combustion. Part II: heterogeneous reactions involving limestone

Limestone addition in fluidised bed combustion of coal is a well established technique for sulphur dioxide control. However, calcium oxide interferes with fuel-N conversion by enhancing the NO emission while strongly reducing N₂O release. In laboratory experiments, the role of Ca-compounds on combustion and hydrolysis of HCN was studied. It could be shown that CaCN₂ is an intermediate product that directs the fuel-N conversion towards the formation of NH₃ which is subsequently oxidised to NO.The catalysed hydrolysis of HCN besides its relevance for nitrogen oxide formation is also an explanation for the observed favoured release of NH₃ during pyrolysis of low rank coals such as lignites. Because of the increased content of calcium compounds in the ashes of many lignites the transformation of HCN with water into NH₃ is favoured.     

DETERMINATION OF DILUTE SLURRY DENSITIES IN A VERTICAL ANNULUS USING ISOKINETIC SAMPLING

The effects of coal properties on N₂O and NO_x formation from circulating fluidized bed combustion of coal was examined through burning nine typical coals and a coal shale, widely used in China over a wide range of coal ranks, in a bench-scale circulating fluidized bed. It was found that N₂O and NO_x formation had similar dependence on coal rank. Fixed carbon content and nitrogen content were the most important coal properties to influence N₂O and NO_x emissions from circulating fluidized bed combustion of coal. A coal with high fixed carbon content had high conversion ratio of fuel-N into N₂O and NO_x. The conversion ratio of fuel-N into N₂O or NO_x increased with nitrogen content of coal, whereas it decreased with O/N ratio. No significant correlation between conversion ratio of fuel-N into N₂O or NO_x and C/N ratio was identified. To clarify the coal property effect, investigation of a wide range of coal rank, is important.     

Heterogeneous reduction of nitric oxide on synthetic coal chars

Model compounds, with a controlled heteroatoms content and well-defined functionalities, were used to study the release of nitrogen compounds from char combustion. In the present work, the mechanisms involved in NO-char heterogeneous reduction were studied with a synthetic coal (SC) char as carbon source. Another synthetic char (SN) without any nitrogen in its composition was also employed in these studies. Temperature programmed reduction (TPR) tests with a gas mixture of 400 ppm NO in argon and with isotopically labelled nitric oxide, ¹⁵NO (500 ppm ¹⁵NO in argon), were carried out. The gases produced were quantitatively determined by means of MS and FTIR analysers.Under the conditions of this work the main products of the NO-C reaction were found to be N₂ and CO₂. The main path of reaction involves the formation of surface nitrogen compounds that afterwards react with nitrogen from the reactive gas to form N₂. It was observed that fuel-N also participates in the overall heterogeneous reduction reaction, although to a lesser extent.     

Theoretical and experimental study on the emission characteristics of waste plastics incineration by modified O2/RFG combustion technology

For mitigating the emission of greenhouse gas CO₂ from general air combustion systems, a clean combustion technology O₂/RFG is in development. The O₂/RFG combustion technology can significantly enhance the CO₂ concentration in the flue gas; however, using almost pure oxygen or pure CO₂ as feed gas is uneconomic and impractical. As a result, this study proposes a modified O₂/RFG combustion technology in which the minimum pure oxygen is mixed with the recycled flue gas and air to serve as the feed gas. The effects of different feed gas compositions and ratios of recycled flue gas on the emission characteristics of CO₂, CO and NO_x during the plastics incineration are investigated by theoretical and experimental approaches.Theoretical calculations were carried out by a thermodynamic equilibrium program and the results indicated that the emissions of CO₂ were increased with the O₂ concentrations in the feed gas and the ratios of recycled flue gas increased. Experimental results did not have the same trends with theoretical calculations. The best feed gas composition of the modified O₂/RFG combustion was 40% O₂ + 60% N₂ and the best ratio of recycled flue gas was 15%. As the O₂ concentration in feed gas and the ratio of recycled flue gas increased, the total flow rates and pressures of feed gas reduced. The mixing of solid waste and feed gas was incomplete and the formation of CO₂ decreased. Moreover, the emission of CO was decreased as the O₂ concentration in feed gas and the ratio of recycled flue gas increased. The emission of NO_x gradually increased with rising the ratio of recycled flue gas at lower O₂ concentration (<40%) but decreased at higher O₂ concentration (>60%).     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Modelling of NO and N2O emissions from biomass-fired circulating fluidized bed combustors

A model for NO and N₂O emissions from biomass-fired circulating fluidized bed (CFB) combustors has been developed and evaluated in this study. All the model parameters were chosen for a typical woody biomass-pinewood chips. Both drying and devolatilization of biomass particles were modelled with limited rates, which were selected from the literature based on woody biomass fuels. The partition of fuel-nitrogen between volatiles and char was also specifically chosen for pinewood based on available experimental data from the literature. Volatile nitrogen was assumed to consist of NH₃, HCN and N₂ with the distribution between three species as input parameters to the model. Twenty-five homogenous and heterogeneous global chemical reactions were included in the model, of which 20 reactions represents the global fuel-nitrogen reactions. Both gaseous and solid phase were assumed to be in plug flow. The model has been applied to the modelling of a 12 MW_th CFB boiler. The predicted N₂O emissions were always less than 5 ppmv for pinewood combustion, which was consistent with the experimental results. The predicted NO emissions increased with the total excess air of the riser and the fuel-N content while the predicted percentage conversion of fuel-N to NO decreased with increasing fuel-N content. The NO emissions were also predicted to decrease with increasing primary zone stoichiometry. These predictions agree with the experimental results. The predicted NO emissions decreased slightly with increasing bed temperature, whereas experiments showed that NO emissions slightly increased with bed temperature for birch chips combustion and did not change with bed temperature for fir chips combustion. Sensitivity analyses reveal that the reaction between NO and char is the key reaction to determine the NO emissions.