Staged combustion properties for pulverized coals at high temperature

Staged combustion properties for pulverized coals have been investigated by using a new-concept drop-tube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. Reaction temperature (1800–2100 K) and time (1–2 s) were similar to those used in actual boilers. When coal was burnt at the same stoichiometric ratio as in actual boilers, similar combustion performance values as for actual boilers were obtained regarding NO_x emission and carbon in ash. The most important factor for low NO_x combustion was to raise the combustion temperature above the present range (1800–2100 K) in the fuel-rich zone. The NO_x emission was significantly increased with decrease of burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. But, NO_x emission was cut to around 100–150 ppm, for sub-bituminous coal and hv-bituminous coal, in the latest commercial plants by forming this high-temperature fuel-rich region in the boilers. If the temperature and stoichiometric ratio could be set to the most suitable conditions, and, burning gas and air were mixed well, it would be possible to lower NO_x emission to 30–60 ppm (6% O₂). The most important NO_x reduction reaction in the fuel-rich zone was the NO_x reduction by hydrocarbons. The hydrocarbon formation rate in the flame was varied with coal properties and combustion conditions. The NO_x was easily reduced when coals which easily formed hydrocarbons were used, or, when burning conditions which easily formed hydrocarbons were chosen. Effects of burning temperature and stoichiometric ratio on NO_x emission were reproduced by the previously proposed reaction model. When solid fuel was used, plant performance values varied with fuel properties. The proposed drop-tube furnace system was also found to be a useful analysis technique to evaluate the difference in combustion performance due to the fuel properties.     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

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.     

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.     

煤粉炉的分级燃烧技术研究

分析了煤粉燃烧过程NOx的形成机制和特点,研究了减少NOx生成量的基本途径和分级燃烧的基本原理,并进行了数值分析。在乌拉山电厂100MW煤粉炉上进行了分级燃烧改造,将以轴向空气分级和径向空气分级为核心的现代低NOx燃烧技术引入了传统的煤粉炉燃烧系统中,考察了炉膛轴向和径向分级风,过量空气系数,锅炉负荷等因素对炉内NOx形成的影响。NOx排放浓度降低了250－500mg/m^3(干烟气,70％O2）。     

空气深度分级对低挥发分煤燃烧过程影响的研究

通过数值模拟研究了在一维燃烧炉上燃用低挥发分煤的条件下,空气深度分级和煤粉细度变化对煤粉燃尽过程和NO_x排放的影响,得到了沿炉膛轴线方向上的温度、氧浓度和NO_x的分布,表明空气深度分级后燃烧后期的氧量增加,炉膛温度水平提高,而煤粉细度的提高使得上述效果更加明显,因而燃烧效率提高和NO_x排放降低,并通过实际燃烧试验验证了数值模拟结果.研究结果表明,对燃用低挥发分煤,采用空气深度分级技术和提高煤粉细度的措施,可以同时取得高效低NO_x排放的效果.     

Comparison of nitrogen oxide emissions from boilers for a wide range of coal qualities

This paper presents the results of experimental researches on nitrogen oxide emissions from coal-fired boilers. Two Chinese lean coals have been fired in two full scale boilers (1 025 t · h ⁻¹ ) and in a pilot scale test furnace (Drop-Tube Furnace) to study the influence of nitrogen content in coal on nitrogen oxide emission. The nitrogen oxide emission was found to correlate well with the fuel nitrogen content. The test results of the drop-tube furnace and the single burner furnace with three Chinese coals show that the staged combustion can greatly reduce the nitrogen oxide emission. Identical trends in nitrogen oxide emission as a function of the volatile matter of the coals have been obtained under different combustion conditions. The principle of low nitrogen oxide emissions of the wide range burner (biased combustion or internal air staging) and the cases with over fired air port (furnace air staging) is introduced in this paper. In addition, the influence of excess oxygen content on nitrogen oxide emission has been tested in the utility boilers and test furnaces.     

Preliminary study of trace element emissions and control during coal combustion

Hazardous trace element emissions have caused serious harm to human health in China. Several typical high-toxic trace element coals were collected from different districts and were used to investigate the emission characteristics of toxic trace elements (As, Se, Cr, Hg) and to explore preliminary control methods. Coal combustion tests were conducted in several bench-scale furnaces including drop tube furnace (DTF), circulating fluidized bed (CFB) combustion furnace, and fixed-bed combustion furnace. Calcium oxide was used to control the emission of arsenic and selenium. The granular activated carbons (AC) and activated-carbon fibers (ACF) were used to remove mercury in the flue gas from coal combustion. The chemical composition and trace element contents of ash and particulate matter (PM) were determined by X-ray fluorescence (XRF) spectrometry and inductively coupled plasma-atomic emission spectrometry (ICP-AES), respectively. The speciation and concentration of mercury were investigated using the Ontario-Hydro method. X-ray diffraction spectrometry (XRD) was used to determine the mineral composition of production during combustion experiments. With the addition of a calcium-based sorbent, arsenic concentration in PM₁ sharply decreased from 0.25–0.11 mg/m³. In fixed-bed combustion of coal, the retention rates of selenium volatiles were between 11.6% and 50.7% using lime. In the circulating fluidized-bed combustion of coal, the content of selenium in ash from the chimney was reduced to one-fourth of its original value and that in leaching water from the chimney decreased by two orders of magnitude using lime. Calcium-based sorbent is an effective additive to control the emission of As and Se during coal combustion. The emission of chromium is influenced by the occurrence mode of Cr in coal. Chromium emission in PM_2.5 during coal combustion is 55.5 and 34.7 μg/m³ for Shenbei coal and mixed Pingdingshan coal, respectively. The adsorptive capacity of granular activated carbon for Hg⁰ is significantly enhanced through ZnCl₂-impregnation. The activated carbon fibers showed decent efficiency in mercury adsorption, on which surface oxygen complex showed positive effects on mercury adsorption.     

Hydrothermal preparation of low-rank coal-water fuel slurries

Lignite and sub-bituminous coals are by nature high in inherent moisture and oxygen content and low in calorific heating value. As-mined low-rank coals when mixed with water generally produce slurries with low solids content and with heating values generally less than 11.6 MJ/kg. These same slurries are usually unstable and form hard-pack sediments quickly, unless chemical additives or constant agitation are added. The low heating value and poor storage and flow characteristics of these coal-water mixtures discourage the use of raw lignite and subbituminous coals for preparation of slurries for fuel purposes.Hydrothermal conditioning, in a water slurry at temperatures above 230 °C and pressures above 552 MPa, is one method that can significantly aid in the preparation of low-rank coal-water fuels. High-pressure hot-water thermal conditioning of lignites and sub-bituminous coals has been found not only to change both the chemical and physical characteristics of the coal but also to alter the coal slurry's rheological properties. These changes are controlled by process variables (i.e. temperature, residence time, particle size, and mode of processing) and result because of decarboxylation, mild pyrolysis, extraction, dehydration, and surface modification; all of which occur during hydrothermal treatment. Using the hydrothermal process, concentrated low-rank coal-water slurries with heating values approaching or exceeding the heating value of the as-mined coal have been achieved with pseudoplastic flow behavior and stability towards settling, without the use of additives.Pilot-scale studies using a 90-kg/hr process development unit (PDU) are currently under way to produce hydrothermally treated low-rank coal fuel slurries for combustion tests in a pilot-scale, slurry-fed test furnace.     

高温空气燃烧技术的特点及其应用前景

高温空气燃烧技术（HTAC）是九十年代初在日本开发的一项新的燃烧技术,该项技术具有节约燃烧、低NOx排放、热利用率高和减设备尺寸等特点。分析了HTAC的火焰温度分布特征及其可降低NOx排放的原理。高炉煤气、焦炉煤气及94．5％高炉煤气与5．5％焦炉煤气的混合煤气应用于HTAC技术的理论计算结果表明,理论燃烧温度随着预热空气和燃气温度的升高而升高,因此HTAC技术可燃用传统燃烧方式不能使用的低热值燃气。此外,空气、燃气双预热可奖排烟温度将得更低,热利用率提高更高。最后,指出了在燃油、燃气锅炉及煤（或可燃固体废弃物）气化系统中采用HTAC技术的可能性。     

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.     

MMT对煤燃烧飞灰含碳量和NOx排放的影响

在一维沉降炉上对神混l号煤样添加MMT后燃烧的飞灰含碳量和NOx排放特性进行了试验研究,并对其燃尽率进行了分析.试验结果表明:所用沉降炉设计合理,添加MMT后,煤燃烧的飞灰含碳量和NOx排放同时降低.与原煤相比,最大降低量分别为18.39％和14.29％.原因是MMT的含锰燃烧产物能够促进C的氧化,同时还能作为催化剂促...     

工业炉窑燃烧过程中节能减排问题的研究进展与发展方向

工业炉窑是建材、冶金、化工等流程工业中至关重要的用能装备,也是化石能源消耗和环境污染的主要源头之一.工业炉窑燃料中煤炭占70％,存在着能耗高排放大的问题,需要研发高能效低排放的新技术.对于典型的工业炉窑,其生产过程具有多工艺目标、多品种交叉生产,以及大负荷调节比、宽阈度负荷变化的复杂工艺特点.通过综述在节能管控、富氧燃...     

不同煤种高温燃烧时NOx和SO2生成影响因素的实验

在一维沉降炉上进行了在煤粉高温燃烧时NOx和SO2生成影响因素的实验研究,系统考察了煤粉高温燃烧时NOx和SO2生成的影响因素.试验表明,SO2和NOx生成与炉内温度,煤粉细度,给粉量,煤种,二次风温,一、二次风比值及过量空气系数等有密切关系.通过SO2和NOx生成规律的研究,对煤粉炉的优化运行有一定的指导意义.     

锅炉燃烧调整对NOx排放和锅炉效率影响的试验研究

在2台典型的1025 t/h锅炉上进行了燃烧调整降低NOx排放浓度的试验研究,通过改变过量空气系数、辅助风配风方式、运行负荷和制粉系统运行方式等,测定了锅炉尾部烟道NOx排放浓度,分析了锅炉运行工况、运行方式对NOx排放的影响.结果表明:降低过量空气系数,烟煤锅炉NOx减排效果比贫煤锅炉好得多;降低负荷,烟煤锅炉的NOx排放量降低值较大;缩腰式配风的NOx排放浓度比均匀配风方式约降低10%,制粉系统的运行方式影响炉内燃料的燃烧状态和温度分布,也影响NOx的生成和排放.在不降低锅炉效率的前提下,调整燃烧工况,可降低锅炉排放NOx浓度1O%～20%.     

Influence of division cone angles between the fuel-rich and the fuel-lean ducts on gas–particle flow and combustion near swirl burners

Radial bias combustion pulverized coal swirl burners are used in boilers when burning low-grade coals, which are often low-volatility anthracite and lean coals that do not burn stably. In a gas–particle test facility, a three-dimensional particle-dynamics anemometer was used to measure gas–particle flows in the near-burner region. Division cones between the fuel-rich and the fuel-lean ducts of the burner were set at angles of 43.2° and 0°. Velocity and particle volume flux profiles were obtained and the influence of the cones on combustion was analyzed. Burners with a division cone angle of 43.2° were used in a lean-coal fired boiler (volatility 13.2% as received, ash 27.46% as received), rated at 670 t/h. Pulverized coal was conveyed by cold moisture-laden exhaust air from the pulverizer. In situ aerodynamic field measurements in cold state were made and combustion tests were carried out. Low NO_x emission and high thermal efficiency resulted and the turn-down ratio was 50%.     

A comprehensive study on NOx emission and fuel nitrogen conversion of solid biomass in bubbling fluidized beds under staged combustion

Despite the relatively low emissions in fluidized-bed combustion, NOx emission for biomass combustion is still a major concern because of increasingly stricter regulations. To realize NOx emission behavior in fluidized beds comprehensively, the effects of bed temperature, excess oxygen, staged combustion, and flue gas recirculation (FGR) are investigated in this study. In particular, three different types of operation are applied in staged combustion to find out the key parameter. The results indicate that NOx emissions increase with both bed temperature and excess oxygen, in which the influence of excess oxygen is greater than the other. Lowering bed temperature by water addition seems to be able to simultaneously reduce NOx emission and agglomerate formation, especially for fuels with high nitrogen content, but the pros and cons should be considered. The results in staged combustion infer that the residence time is much more critical than the stoichiometry in the bed. As for FGR, its impact appears to depend on the type of fuel. The correlation between NOx emission behavior and fuel characteristics is also scrutinized; it is concluded that the fuel-N conversion to NOx is essentially related to some features of fuels.     

分级燃烧最佳一次风空气系数的实研究

在一维煤粉燃烧炉上进行了不同煤种、不同细度的分级燃烧试验。实验发现,分级燃烧对高挥发份 煤种以及同一煤种的细煤粉的Nox排放浓度的降低效果更显著,而且在分级燃烧条件下,同一煤种细煤粉的 飞灰含量较粗煤粉低。另外,还得到了不同煤种在分级燃烧条件下的最佳一次风空气系数。图9表2参3     

煤颗粒热膨胀破碎特性的试验研究

煤颗粒的热膨胀破碎特性直接影响流化床锅炉的运行效率。利用热机械分析仪(TM A)测定了不同种类不同密度的型煤受热膨胀的特性,并对部分破碎微观形貌利用场发射扫描电子显微镜(FSEM)进行了观察;通过马弗炉内的燃烧试验研究了热膨胀特性和燃烧破碎的关系。研究表明,煤颗粒在燃烧过程中其热膨胀破碎主要发生在挥发分析出阶段;内部挥发分的析出会使颗粒内压增大而产生膨胀,进而产生细小裂纹并破碎;挥发分越高,颗粒密度越大,其热膨胀形变率越大,越容易发生破碎现象;主要挥发分析出后热膨胀引起的破碎可以忽略。