褐煤提质技术浅析

褐煤因其热值低、含水量高、易风化自燃等特点,给其燃烧、运输、储存等方面带来了很多困难,如何实现褐煤的高效利用已成为亟待解决的问题。褐煤提质是指褐煤通过干燥或热解工艺降低水分、转化成具有烟煤性质的提质煤。提质技术已成为近年来褐煤应用方面的主要研究方向之一。     

The effect of air staged combustion on NOx emissions in dried lignite combustion

Experiments were carried out in a multi-path air inlet one-dimensional furnace to assess NOx emission characteristics of the staged combustion of BRXL lignite and its dried coals. The impact of moisture content, multiple air staging, pulverized coal fineness and burnout air position on NOx emissions under deep, middle and shallow air-staged combustion conditions. Moreover, the impact of blending coals on NOx emissions was investigated in this paper. The unburned carbon concentration in fly ash was also tested. Experimental results based on the combustion of BRXL lignite and its dried coals show that NOx emissions can be reduced drastically by air-staged combustion. NOx emissions reduce with the increase of the air that is staged and the distance between the burner and burnout air position. Dried coal of BRXL lignite emits a smaller amount of NOx than that of BRXL lignite. However, the dried degree of BRXL lignite is closely related to R₉₀ fineness. Dried coal with optimal moisture content yields least NOx emissions. When deep or middle staged combustion was adopted, the application of multi-staged combustion is conducive to NOx reduction. However, when shallow staged combustion was adopted, NOx emissions are higher in multi-staged combustion than that in single-staged combustion with M_S = 0.54. Thus, the existence of a certain concentration of O₂ in reduction zone would significantly reduce NOx emissions. The blending coals that dried coals of BRXL lignite were blended with bituminous coals emit a larger amount of NOx than that of the dried coal alone. NOx emissions decrease with the increase of the proportion of dried coal in the blending coal. Moreover, the unburned carbon concentration in fly ash of dried coal in staged combustion is lower than that of BRXL lignite in staged combustion. On the whole, the dried coal of BRXL lignite is conducive to NOx reduction in staged combustion.     

Combustion of single coal particles in a jet

Fluidized bed combustion has attracted much interest in recent years, but there is very little data on the behavior of coal particles at these new conditions. Coal of much larger diameter (1–10 mm), much lower furnace temperatures (~850 °C), and different fluid mechanical conditions exist compared to pulverized coal furnaces. This paper presents experimental data on the behavior and combustion rates of individual coal particles aerodynamically suspended in a heated jet, to stimulate flow conditions in a fluidized bed.Tests of bituminous, sub-bituminous and lignite coals from 2 to 12 mm at jet temperatures of 705 and 816 °C in air and air diluted with equal parts of nitrogen were conducted. The ignition delay time varied from 2 to 44 sec. The devolatilization time extended up to 80 sec and was dependent mainly on particle size. The total burn time was independent of coal type and temperature, and varied as the square of the size and inversally with the oxygen concentration. The total turn time varied from 25 to 740 sec independently of coal type. The square law for the char burning rate was investigated.     

Influence of functional group structures on combustion behavior of pulverized coal particles

The ignition and combustion behavior of pulverized coal was studied with respect to coal rank in a custom-designed visual drop tube furnace. The results showed that low-rank coals were ignited in a shorter time, mainly due to the presence of larger amounts of functional groups, while the ignition delay time of high-rank coals was longer. With increasing temperature and particle size, the ignition mode of coals shifted from heterogeneous into homogeneous, which was related to the increased yield of volatile matter. The chemical percolation devolatilization analysis results showed a clear relationship between the yield and composition of volatile matter and the amount and type of functional groups in coal. In addition, the tar yield was consistent with the amount of aliphatic hydrocarbons and the length of aliphatic chains, which explained the tailing combustion mode of the bituminous coal. The findings of the study showed that the yield and composition of volatiles in coal had a significant impact on the ignition behavior, which depended on the composition of functional groups, particle size, and the combustion environment.     

Effects of high-temperature oxygen-deficient oxidation on the surface properties of sub-bituminous coal

The air oxidation of coal releases heat which increases coal temperature, as a result, the spontaneous combustion of coal happens. Generally, coal spontaneous combustion needs to be prevented artificially, however, most of the coal particles still suffer a high-temperature heating process under the oxygen-deficient condition. This paper aims to investigate the effect of high temperature (500°C, 600°C, and 700°C) oxygen-deficient (3% oxygen and 97% nitrogen) oxidation on the surface properties of sub-bituminous coal. SEM and XPS were applied to show the changes of surface properties of sub-bituminous coal. SEM results showed the number of pores and cracks on coal surface increased with the increasing heating temperature, and XPS results showed the content of hydrophobic functional groups on coal surface reduced whereas the content of hydrophilic oxygen-containing functional groups increased after the oxygen-deficient oxidation.     

Comparison of bituminous coal and lignite during combustion: Combustion performance,coking and slagging characteristics

The combustion characteristics such as combustion performance, coking, and slagging—at high temperatures (700–1300 °C) of bituminous coal and lignite were investigated and compared. The results show that the ignition temperature and the activation energy of lignite are lower than those of bituminous coal, and the combustion index and the burnout index are less than those of bituminous coal. Lignite has almost no coking while bituminous coal tends to coke at high temperatures. The larger the content and reflectivity of the vitrinite, the more severe the degree of coking. In the range of 700–1300 °C, the increase of temperature has little influence on the coking characteristics of lignite and bituminous coal. The low-rank lignite has larger amounts of mineral content which tend to form low-fusion-temperature eutectics. Furthermore, there is a connection between the combustion performance, coking and slagging characteristics through the maceral compositions: the coal which is hard to ignite but easy to burn out is more likely to have strong coking ability. Meanwhile, coking tends to keep alkaline oxides stay in the char and reinforce slagging at high temperatures.     

Combustion Characteristics of 24 Lignite Samples

Abstract

In this study, the combustion characteristics such as thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA), burning profile, ignition temperature, and peak temperature were analyzed for 24 lignite samples from different areas of Turkey. The samples were heated up to 900°C at a constant rate of 10°C/min in a 5 mL/min flow of dry air. The burning profiles of the samples studied, combined with proximate, sulfur analysis and calorimetry results, contribute to a clearer identification of lignite samples' structure and a better understanding of the coalification process. The lignite samples have been tested with particle size of 0–0.05 mm. Ignition temperatures of the samples have been determined from their burning profiles.     

煤粉细度对燃烧特性影响的实验研究

对冷水江和斗笠山两种煤粉,采用热天平进行了热重分析实验,分析了煤粉样在不同细度下的着火特性、燃烧特性、燃尽性能及动力学参数,给出了反映煤粉燃尽性能的综合判别指数Hj并进行了对比.结果表明:在相同的升温速度下,随着煤粉粒径的减小,挥发分析出量及DTG峰值增大,出现最大燃烧速率的时间提前;煤粉粒径减小,活化能降低,着火温度降低,着火提前,煤粉的燃烧特性也随之变好.     

Lignite‐fired air‐blown IGCC systems with pre‐combustion CO2 capture

Detailed analyses based on mass and energy balances of lignite‐fired air‐blown gasification‐based combined cycles with CO₂ pre‐combustion capture are presented and discussed in this work. The thermodynamic assessment is carried out with a proprietary code integrated with Aspen Plus^® to carefully simulate the selective removal of both H₂S and CO₂ in the acid gas removal station. The work focuses on power plants with two combustion turbines, with lower and higher turbine inlet temperatures, respectively, as topping cycle. A high‐moisture lignite, partially dried before feeding the air‐blown gasification system, is used as fuel input. Because the raw lignite presents a very low amount of sulfur, a particular technique consisting of an acid gas recycle to the absorber, is adopted to fulfill the requirements related to the presence of H₂S in the stream to the Claus plant and in the CO₂‐rich stream to storage. Despite the operation of the H₂S removal section representing a significant issue, the impact on the performance of the power plant is limited. The calculations show that a significant lignite pre‐drying is necessary to achieve higher efficiency in case of CO₂ capture. In particular, considering a wide range (10–30 wt.%) of residual moisture in the dried lignite, higher heating value (HHV) efficiency presents a decreasing trend, with maximum values of 35.15% and 37.12% depending on the type of the combustion turbine, even though the higher the residual moisture in the dried coal, the lower the extraction of steam from the heat recovery steam cycle. On the other hand, introducing the specific primary energy consumption for CO₂ avoided (SPECCA) as a measure of the energy cost related to CO₂ capture, lower values were predicted when gasifying dried lignite with higher residual moisture content. In particular, a SPECCA value as low as 2.69 MJ/kg_CO2 was calculated when gasifying lignite with the highest (30 wt.%) residual moisture content in a power plant with the advanced combustion turbine. Ultimately, focusing on the power plants with the advanced combustion turbine, air‐blown gasification of lignite brings about a reduction in HHV efficiency equal to almost 1.5 to 2.8 percentage points, depending on the residual moisture in the dried lignite, if compared with similar cases where bituminous coal is used as fuel input. Copyright © 2016 John Wiley & Sons, Ltd.     

热重-FTIR法分析不粘煤氧化特性参数

针对不粘煤粉生产过程的氧化自燃灾害,采用TG分析实验研究煤粉在空气条件下的氧化参数与动力学特征.利用FTIR-850红外光谱仪对煤样做红外光谱图分析,研究不同氧化温度下煤化学结构的官能团变化.结果表明:TG/DTG曲线上存在7个特征温度点,特征温度与升温速率呈正相关.将煤粉氧化过程分为失水失重(T1～T2)、吸氧增重(...     

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.     

Co-combustion of low rank coal/waste biomass blends using dry air or oxygen

Biomass species such as the rice husk and the olive milling residue, and a low quality Turkish coal, Soma Denis lignite, were burned in a thermal analyzer under pure oxygen and dry air up to 900 °C, and differential thermal analysis (DTA) and derivative thermogravimetric (DTG) analysis profiles were obtained. Co-combustion experiments of lignite/biomass blends containing 5–20 wt% of biomass were also performed. The effects of the oxidizer type and the blending ratio of biomass were evaluated considering some thermal reactivity indicators such as the maximum burning rate and its temperature, the maximum heat flow temperature, and the burnout levels. FTIR (Fourier transform infrared) spectroscopy and SEM (scanning electron microscopy) were used to characterize the samples, and the variations in the combustion characteristics of the samples were interpreted based on the differences in the intrinsic properties of the samples.     

Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse

Comparative combustion studies were performed on particles of pulverized coal samples from three different ranks: a high-volatile bituminous coal, a sub-bituminous coal, and two lignite coals. The study was augmented to include observations on burning pulverized woody biomass residues, in the form of sugarcane bagasse. Fuel particles, in the range of 75–90 μm, were injected in a bench-scale, transparent drop-tube furnace, electrically-heated to 1400 K, where they experienced high-heating rates, ignited and burned. The combustion of individual particles in air was observed with three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time–size histories. Based on combined observations from these techniques, in conjunction to morphological examinations of particles, a comprehensive understanding of the combustion behaviors of these fuels was developed. Observed differences among the coals have been striking. Upon pyrolysis, the bituminous coal chars experienced the phenomena of softening, melting, swelling and formation of large blowholes through which volatile matter escaped. Combustion of the volatile matter was sooty and very luminous with large co-tails forming in the wake of the particle trajectories. Only after the volatile matter flames extinguished, the char combustion commenced and was also very luminous. In contrast, upon pyrolysis, lignite coals became fragile and experienced extensive fragmentation, immediately followed by ignition of the char fragments (numbering in the order of 10–100, depending on the origin of the lignite coal) spread apart into a relatively large volume. As no separate volatile matter combustion period was evident, it is likely that volatiles burned on the surface of the chars. The combustion of the sub-bituminous coal was also different. Most particles experienced limited fragmentation, upon pyrolysis, to several char fragments, with or without the presence of brief and low-luminosity volatile flames; other particles did not fragment and directly proceeded to char combustion. Finally combustion of bagasse was once again very distinctive. Upon pyrolysis, long-lasting, low-luminosity, nearly-transparent spherical flames formed around slowly-settling devolatilizing particles. They were followed by bright, short-lived combustion of the chars. Both volatiles and chars experienced shrinking core mode of burning. For all fuels, flame and char temperature profiles were deduced from pyrometric data and burnout times were measured. Combustion rates were calculated from luminous carbon disappearance measurements, and were compared with predictions based on published kinetic expressions.     

流化床中高水分煤的燃烧与排放试验研究

通过在一小型流化床中进行高水分煤的燃烧与排放的试验研究，表明水分含量和空气－燃料比对于高水分煤的燃烧与排放有较大影响。随着水分增加，流化床床温下降，ＮＯｘ、ＳＯｘ排放量也下降。空气－燃料比存在一最佳值，这时床温最高，而偏离此值，床温下降，随着空气量的增加，ＮＯｘ、ＳＯｘ排放量也增加。当空气－燃料比变化时，燃烧干煤与燃烧高水分煤有着类似的试验结果。     

The Effects of Bed Height and Initial Moisture Concentration on Drying Lignite in a Batch Fluidized Bed

In this study, drying of a lignite from Erzurum Ispir-Karahan province, with air in a batch fludized bed, was investigated. In the experiments, effects of bed height and initial moisture concentration of the coal on the drying behavior were studied. It was found that the drying rate increased with a decrease in bed height. Also, coal particles with an initial lower moisture content showed a marginal decrease of drying rate compared with higher moisture content particles. It was also observed that the equilibrium moisture content, and the time taken to reach the equilibrium moisture content, decreased with decreasing initial moisture concentration, while the critical moisture content generally decreased. Also, it fixed with decreasing bed height, while the critical moisture content increased.     

Factors influencing the transformation of minerals during pulverized coal combustion

The transformation of minerals and dispersed inorganic constitutents during pulverized coal combustion has been examined by burning utility sized coals (70% less than 200 mesh) in a laboratory-scale combustor. Experiments were conducted with four U.S. coals possessing different mineralogies. Size and composition of the initial minerals and the resulting ash were measured by a variety of techniques, including computer controlled SEM, low temperature ashing, deposition on a cascade impactor, and optical (Malvern) particle size analysis. Results for a Kentucky # 11 coal with a large amount of fine, included silicate minerals suggest that coalescence amongst illite, kaolinite, and quartz minerals was the dominant process, with occasional iron incorporation into the resulting glass. Pyrite was found to fragment to a limited extent. Illinois # 6 bituminous coal, possessing a similar mineralogy, yielded similar results. For a Beulah lignite coal containing large pyrite grains, mineral fragmentation was inferred from the data, increasing with increasing oxygen level. A high ash content San Miguel Texas lignite containing zeolite minerals demonstrated little mineral interaction during combustion. Differences in results obtained for the different coals highlighted the importance of understanding individual mineral transformations in predicting the formation and behavior of ash.     

Combustion,cofiring and emissions characteristics of torrefied biomass in a drop tube reactor

The study investigates cofiring characteristics of torrefied biomass fuels at 50% thermal shares with coals and 100% combustion cases. Experiments were carried out in a 20 kW, electrically heated, drop-tube reactor. Fuels used include a range of torrefied biomass fuels, non-thermally treated white wood pellets, a high volatile bituminous coal and a lignite coal. The reactor was maintained at 1200 °C while the overall stoichiometric ratio was kept constant at 1.15 for all combustion cases. Measurements were performed to evaluate combustion reactivity, emissions and burn-out.Torrefied biomass fuels in comparison to non-thermally treated wood contain a lower amount of volatiles. For the tests performed at a similar particle size distribution, the reduced volatile content did not impact combustion reactivity significantly. Delay in combustion was only observed for test fuel with a lower amount of fine particles. The particle size distribution of the pulverised grinds therefore impacts combustion reactivity more.Sulphur and nitrogen contents of woody biomass fuels are low. Blending woody biomass with coal lowers the emissions of SO₂ mainly as a result of dilution. NO_X emissions have a more complex dependency on the nitrogen content. Factors such as volatile content of the fuels, fuel type, furnace and burner configurations also impact the final NO_X emissions. In comparison to unstaged combustion, the nitrogen conversion to NO_X declined from 34% to 9% for air-staged co-combustion of torrefied biomass and hard coal. For the air-staged mono-combustion cases, nitrogen conversion to NO_X declined from between 42% and 48% to about 10%–14%.     

流化床O_2_CO_2燃烧_氧浓度对粒径的影响

为研究氧浓度对燃料粒径的影响,在15 kW循环流化床试验系统和0.15 MW循环流化床试验系统上进行不同氧气浓度、不同燃料、不同粒径的燃烧试验。试验结果表明,燃烧高挥发分的煤时,氧气浓度对燃料的热破碎有较大影响,进而影响燃料粒径。50%左右氧气浓度下时,燃烧高挥发分燃料,平均粒径要比在常规空气下燃烧增加约35%。低挥发分的神木半焦在燃烧过程中热破碎较弱,氧气浓度对热破碎特性影响不大,粒径和空气燃烧条件下无明显差异。     

600MW亚临界锅炉褐煤燃烧系统设计与运行

本文通过对华能某电厂600MW亚临界燃煤机组风扇磨高水份褐煤燃烧系统的分析，结合燃烧调整和性能考核试验报告，介绍了燃煤发电机组的超临界风扇磨高水份褐煤燃烧系统设计，为今后的系统设计与运行提供了宝贵的经验。     

超细煤粉燃烧氮氧化物释放特性的研究

通过试验和数值模拟，对超细煤粉在一维热态煤粉炉内燃烧时煤粉粒度、炉膛温度、过量空气系数、煤种等因素对NOx释放特性的影响规律进行了研究。研究结果表明：超细煤粉NOx的排放浓度低于常规粒度煤粉；NOx的排放浓度，随过量空气系数的增加而明显增加；煤种不同，NOx释放规律不同，煤粉超细化后，龙口褐煤的排放量明显减少，晋城无烟煤则变化不大；NOx的排放浓度随温度的升高而升高，但温度升高到一定值后，NOx的排放浓度却呈现下降趋势。以超细煤粉作为再燃燃料，NOx的还原率将比常规粒度煤粉再燃有所提高，褐煤作为再燃燃料时，效果更明显。模拟计算与试验结果较为吻合。图6表2参2