煤热解与挥发份燃烧时SO2与NO的交互作用

为研究煤热解与挥发份燃烧过程中SO2对NO的影响，在燃烧综合实验台上研究了硫含量、煤种等因素在煤热解与挥发份燃烧过程中对SO2生成量、NO生成量等的影响。结果表明，SO2的生成对燃烧过程中NO的生成具有一定影响和作用。但不同的煤种硫的析出与氧化对NO的生成影响和作用有所不同，挥发份含量高、活化能低的煤，与NO之间的交互作用较强。铜川贫煤和宜宾无烟煤，原煤中的固有硫和添加硫总量分别达到4.34％和5．17％以上时，硫的热解和氧化过程才对NO的生成有较明显的影响；而对神木烟煤，原煤中的固有硫和添加硫总量达到2．0％时，硫的热解和氧化就对NO的生成起到了明显的作用。图10表3参6     

一维火焰炉中混煤燃烧氮析出特性的试验研究

在一维煤粉燃烧试验台上,对贫煤掺混不同煤种的混煤在一维火焰燃烧过程中氮的析出特性进行了试验研究.结果表明:煤中析出氮主要以NO存在,NO2生成量很少;贫煤掺混不同煤种在相同工况下燃烧时氮释放差别较大,掺配比、氮存在形态对氮析出产物的形式和浓度有较大影响;混煤燃烧氮析出曲线一般具有双峰结构,混煤析出氮与组分单煤的加权平均有一定差异,氮析出量与混煤掺烧比例有关.     

焦炭流化床燃烧条件下氧化亚氮生成过程的实验研究

在小型流化床实验台上对一种无烟煤焦炭燃烧过程中氧化亚氮的生成途径进行了实验研究,实验结果表明,在焦炭燃烧过程中HCN的氧化反应是焦炭氮向N2O转化的一条途径。同时,NO与焦炭表面的多相反应也是N2O的一条生成途径。HCN的析出是焦炭燃烧过程中进一步脱挥发份的结果,当焦炭脱挥发份过程结束后,N2O来源于NO和焦炭表面的气固多相反应。     

循环流化床煤气化细粉灰硫氮转化分析

利用热重质谱联用(TG-MS)研究气化细粉灰燃烧过程SO2和NOx的生成特性,利用X射线光电子能谱仪(XPS)对比分析煤及相应气化细粉灰中硫、氮的赋存形态,考察煤在循环流化床气化转化为气化细粉灰的过程中硫、氮赋存形态及含量的变化。结果表明:与煤相比,气化细粉灰中的硫、氮元素结合能高,燃烧过程中SO2、NO释放温度升高;气化细粉灰中硫化物硫、硫酸盐硫减少,噻吩硫、亚砜硫增加;各形态氮含量均降低,不同形态氮元素析出率不同。     

不同煤种挥发氮析出过程的数值模拟与试验研究

采用数值模型研究了烟煤、贫煤和无烟煤等不同煤种热解、燃烧过程中挥发分氮的析出、中间含氮产物HCN的生成以及转变为NO的过程。利用有限体积方法对质量、化学组分、动量和热量守恒方程进行离散求解。计算获得了不同煤种的颗粒着火时间、热解过程、孔隙率、HCN和NO生成率等数据,并与沉降炉试验结果进行了比较,分析。     

浓淡煤粉燃烧的试验与理论研究

对浓淡煤粉燃烧技术在四角布置燃烧器锅炉上进行了比较深入的试验研究和理论研究。试验煤种为晋东南无烟煤和铜川贫煤。试验结果和理论分析表明浓淡煤粉燃烧存在最佳煤粉浓淡比。此外，浓淡比对于飞灰含碳量和ＮＯｘ排放量也有较大影响。     

模型化合物吡啶加热氧化规律研究

为研究煤中吡啶型氮的氧化规律，选取吡啶为煤的含氮模型化合物，采用傅立叶红外光谱仪(Ft-Ir)和烟气分析仪连用，在600—1400℃温度范围内，对其氧化产物进行研究．实验结果表明，吡啶氧化产物主要是NO、NO2、N2O、CO和CO2．N2O在750～900℃生成量最大；氧气量大于或等于理论氧气量的76％时，CO在650℃存在一个高峰，NOx随着温度升高而增加；氧气量等于理论氧气量的59％时，生成大量CO，抑制了NOx的生成．     

温度及氧煤比对煤预燃过程中NO/HCN转化的影响

为探究煤粉预燃-燃烧耦合技术的低氮机理,通过在一维管式炉上对煤粉预燃过程中温度和氧煤比对NO/HCN的转化特性进行详细研究.实验结果表明在氧煤比为0和0.1时,随着温度升高,NO的含量下降,而HCN含量上升;当氧煤比进一步升高到0.3及以上时,随温度的升高NO的含量会升高,HCN的含量先升高后降低.同时,利用NO生成与还原的总包反应的速率常数随温度的变化趋势,得出在高温低氧情况下HCN还原NO占主导地位,在高氧情况下氧化生成NO占主导地位,以此来揭示NO的不同变化趋势.     

无烟煤与贫煤混煤燃烧和NO_x排放特性的实验研究

利用热天平和小型煤粉燃烧实验台对无烟煤、贫煤及其三种配比混煤的燃烧特性、不同配风下NOx的生成规律和燃尽特性进行了实验研究。通过对试验数据的整理和分析,认为无烟煤与贫煤在燃烧性能上略有差异,混煤的燃烧特性介于两者之间,合适的选取过量空气系数可实现不同掺烧比无烟煤与贫煤混煤高效低NO燃烧。并针对三种掺烧比的混煤提出了其高效低污染燃烧的过量空气系数范围,为燃用上述混煤的电厂经济清洁运行提供一些参考数据。     

生物质挥发分燃烧NO生成规律研究

生物质燃料在燃烧过程中燃料氮会转化为NO释放,而生物质中绝大部分的燃料氮在热解过程中以挥发分氮的形式释放,因此挥发分氮对于NO的生成极为重要。为探究生物质挥发分燃烧生成NO的规律,在水平管式炉反应器及Chemkin仿真模拟上研究了不同生物质、不同温度、不同氧浓度对挥发分NO生成的影响。试验结果表明:生物质含氮量越高,挥发分NO的转化率越低;挥发分NO的转化率随温度上升先增加,在800℃达到峰值后略微减小;氧浓度越高,挥发分NO的转化率越高。该结论有助于理解生物质挥发分燃烧生成NO的规律,对采取合理措施降低NO排放有积极意义。     

链条炉区段烟气再循环对锅炉运行及NOx排放特性影响的工业试验研究

针对一台采用尽早配风方式的29MW链条炉进行分区段烟气再循环对锅炉运行及NOx排放特性影响的工业试验。在挥发分析出及燃烧区段煤层下的一次风室混入再循环烟气将有效强化该区段煤层燃烧,降低该区段煤层以上燃烧空间的氧浓度,控制及消减挥发分N向NOx的转化,同时降低了穿过该区段煤层一次风的氧浓度,抑制焦炭N向NOx转化,NO消减效果最高达到25%。在焦炭燃烧区段煤层下的一次风室混入再循环烟气,能够降低穿过床层气流的氧浓度,抑制焦炭氮向NO的转化过程,该区段烟气再循环低氮效果有限,最大降幅9%。再循环烟气可以替代部分一次风,以维持足够的风室风压,进而降低穿过煤层气流的O2浓度,从而强化链条炉区段燃烧特性的低氮特征,实现链条炉的NOx减排。随着工业锅炉NOx排放指标的不断提高,烟气再循环作为一项有效的前置低氮环节,能有效降低整个低氮系统的投资,进而取得较好的经济性。     

Experimental and in-situ estimation on hydrogen and methane emission from spontaneous gasification in coal fire

In order to quantitatively understand the wasteful and combustible gas resources which are released from underground coal fires caused by spontaneous combustion, the emission characteristics of hydrogen and methane in the thermochemical process of coal oxidation are investigated by both laboratory tests and on-site measurements. Employing an adiabatic oxidation test, the releasing rules of index gases in limited space were estimated with programmed temperature rising up to 200 °C. Experimental results demonstrate that the releasing concentrations of methane and hydrogen preform an exponential trend with oxidation temperature, while the release rates are significantly influenced by the metamorphic degrees and oxygen supplement conditions. Field survey was also operated to trace the gaseous products in a typical coal fire area in Xinjiang Region, China via gas monitoring at surface emission vents and fractures. Measurement data illustrate a good consistency between the index gases and the stage of coal spontaneous combustion, and the exhaust hydric gases are estimated at more than 1000 tons per year. The presented method and results could provide a useful reference to gaseous products estimation for coal spontaneous combustion.     

Combustion of particles in a large pulverized brown coal flame

A model of the ignition of a polydisperse cloud of brown coal particles, in a known gas environment, is presented and used to predict the behavior of the particles in a burner jet of a utility boiler. The model allows for drying, devolatilization, and char combustion of the particles. It is assumed that the volatiles burn in the free stream so that char combustion can occur during volatiles evolution, the diffusion of oxygen to the particle surface being inhibited due to the net outflow of volatiles. The model is used to calculate the behavior of a cloud of p.f. size particles along the centerline of a brown coal burner jet in which the gas temperature and composition have been measured. Rates of volatile release and char combustion are calculated and shown to be in agreement with measurements of volatile material in the flame. It is found that particles smaller than about 80 μm contribute most to the ignition of the jet and that they closely follow the local gas temperature. The unique character of brown coal of combustion, its high volatile evolution on rapid heating, the high activity of its char at low temperature, and the demonstrated ignition of its char without a jump in temperature make the overlap of devolatilization and char combustion more likely than with other coals. The mathematical formulation that allows this overlap gives oxygen consumption levels consistent with measurement. An analysis is made of the relative importance of radiation from the flame front to the particle, and entrainment of hot combustion gases into the jet. It is found that the radiation is of secondary importance compared to the effect of entrainment which is the controlling mechanism in the initial heating of the particles. Also, the significance of the assumption that the volatiles burn in the free stream is discussed.     

Nitrogen oxide formation from australian coals

The evolution of fuel nitrogen during devolatilization and the formation of NO_x during combustion were studied for two Australian coals in crucible, thermobalance, and rapid heating (drop-tube furnace) experiments. The evolution of coal nitrogen during devolatilization was dependent on both temperature and mode of heating. Under near stoichiometric combustion, 20–30% of coal nitrogen was converted to NO_x, Conversion increased markedly with increased fuel-lean conditions. The NO_x formed from volatiles was proportional to the fraction of coal nitrogen evolved as HCN and NH₃. The combustion of char at various temperatures and stoichiometries showed that the conversion of char nitrogen to NO_x depended primarily on char burnout. The contribution of char nitrogen to NO_x formation was greater than that of volatile nitrogen under fuel-rich conditions.     

An experimental study of propane combustion in a fluidized-bed gasifier

This paper reports an experimental study of the combustion of volatiles from coal, simulated by propane, and its interaction with char gasification reactions in a fluidized-bed coal gasifier (FBG). Experiments were performed under conditions of propane pyrolysis (in a bed fluidized by nitrogen and steam), propane reacting with oxygen and steam (in a bed fluidized by air and steam), char gasification only (in a bed fluidized by air and steam without propane) and in char gasification (in a bed fluidized by air and steam with propane). Axial concentration profiles of various species were obtained at 750, 850 and 950 °C. It was observed that the combustion of propane in an FBG, but without char, behaves as reported in the literature for fluidized-bed combustion (FBC). However, upon introducing char into the bed to simulate the reducing atmosphere in an FBG, oxygen was rapidly consumed within a short distance of the distributor, by significant partial oxidation of both propane and its decomposition products to carbon monoxide. The char was found to aid the pyrolysis of propane, limiting the amount of hydrocarbons surviving into the freeboard. The experimental results reported here are believed to be the first observations on the combustion of volatiles under conditions in an FBG.     

分级燃烧降低燃煤锅炉NOx排放的机理及影响因素分析

再燃燃料在还原性气氛下对主燃区煤粉燃烧生成的氮氧化物的还原反应中 ,再燃燃料中产生的中间产物氰基、氨基和烃根等起到分解氮氧化物的作用。同一再燃燃料中烃类物质在富燃料和贫燃料气氛中所起作用截然不同。实际应用中应使再燃区内各处处于弱还原性气氛下以保证再燃降低NOx 排放的效果 ,并尽量采用气体燃料作为再燃燃料 ,同时在获取所需NOx 排放水平前提下尽量选取较高的空气过量系数(化学当量比 ) ,以同时降低飞灰中的含碳量、减轻高温腐蚀的程度。     

The devolatilization of coal and a comparison of chars produced in oxidizing and inert atmospheres in fluidized beds

This is a study of the devolatilization of coal in a laboratory-scale bed of silica sand, fluidized with either air or N₂ and electrically heated to 750 or 900°C. Coal particles (diameter 1.4–1.7 or 2.0–2.36 mm) were fed in batches to the surface of the bed and allowed to devolatilize in either an oxidizing atmosphere of air or inert N₂. In the first case, combustion of the volatiles occurred, but there was only thermal decomposition (pyrolysis) in the second situation. The resulting chars were recovered and analyzed for composition and structure, so that comparisons could be made between the effects of devolatilization with combustion and of pyrolysis in an inert atmosphere. It was found that the fractions of C and H in the char were only slightly sensitive to the type of fluidizing gas used. The amount of nitrogen in the recovered char and also the devolatilization time showed no dependence on the type of fluidizing gas, whereas BET areas were slightly larger after combustion in air. It is concluded that these effects are small relative to other errors, inherent in experiments on coal combustion, so that chars prepared in a bed fluidized by hot N₂ are very similar to those formed during coal combustion at nominally the same temperature. Equally, the overall composition of the volatile matter released during combustion in a fluidized bed is the same as in pyrolysis in nitrogen. The effects of other parameters, such as the temperature of the bed, the flow rate of the fluidizing gas and the size of the coal particles are also discussed in detail. It is concluded that most of the volatiles burn in a fluidized bed (at or less than 900°C) far away from the original coal particle. Also, NO_x is in effect a primary product of devolatilization, being produced in appreciable amounts when coal is heated in inert N₂. The ratio of C/N in the volatiles is found to be a constant during the latter stages of devolatilization, but beforehand at lower temperatures, carbon species are preferentially released. Overall, devolatilization of small particles (< 2.4 mm) in a fluidized bed at 900°C is kinetically controlled. The activation energy is small, being 15 ± 6 kJ/mol.     

Mineral transformation during combustion of coal blends

Mineral behaviour for two individual coals (I, J) and their two‐component coal blends and 800°C ash blends heating were studied. Ash samples were heated progressively from 800°C to IT (initial deformation temperature) at 100°C intervals under different conditions. Coal samples were heated from room temperature to the corresponding temperature. Mineral transformation at each temperature was determined by X‐ray diffraction and SEM measurements. The results show that Si, Al, Fe and Ca compounds have a great form variation during heating. Their forms at different temperatures depend on the chemical composition of the ash, the blending ratio and the atmosphere. For different coal ashes, the main mineral matters at 800°C were quartz, anhydrite, hematite, calcite and feldspar. As the temperature increased, oxidation, thermal decomposition, transformation and reaction occurred between the components. Comparing a 40% I+60% J ash blend with individual ashes, fayalite was formed at 1100°C for the blend; the reaction product existed in a glassy phase at 1300°C. For a coal blend having the same ash ratio as the ash blend, FeO reacted with amorphous SiO₂ or Al₂O₃ to form fayalite and hercynite at 1000°C. As the temperature increased to 1100°C, fayalite and hercynite increased obviously. At 1200°C, some iron inclusion compounds melted to become glassy phase matter. Compared with the ash blend, iron species undergo a different change during coal blend heating: fayalite and hercynite formed earlier, iron compounds melted to form a glassy phase at lower temperature. This may be caused by early combustion of the more reactive coal (J coal) in the blend inducing local variation in oxygen concentration gradients around the less reactive coal and consequently affecting the reaction atmosphere and Fe mineral behaviour and interaction. That is to say, for coal blends, the mineral transformation was affected by both the mineral species interaction and the combustion behaviour. The calculations were performed to examine the fate of mineral matter under different combustion conditions using a thermodynamic chemical equilibrium calculation program. Calculations from coal blends were comparable with experiments from ash blends, this is because the calculation program only considers the interaction among the mineral species but does not consider the combustion reaction. It indicates that combustion and the relative volatiles also affected the mineral behaviour and slagging during coal blend combustion. Meanwhile, the mineral species evaporations were measured at high temperature: the main evaporated species were Na, K pure species and compounds, Fe, FeO, SiO and SiO₂. The evaporation of Fe has an important effect on initial deposition. Calculations were comparable with the experiments. Copyright © 1999 John Wiley & Sons, Ltd.