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Combustion of fixed fuel beds in grate furnaces is common within production of heat and power from solid fuels. Available theoretical and experimental experience provides a solid base of knowledge on how a conversion model of a fuel bed, using Computational Fluid Dynamics (CFD), needs to be structured and solved. Most existing models, however, handle the conversion in one single dimension of constant bed properties; when observing a burning fuel bed in a grate furnace it becomes apparent that the fuel bed is neither homogeneous nor uni-dimensional. In this study, a two-dimensional model of the combustion of fixed fuel beds has been developed for the purpose of studying the influence of heterogeneous fuel-bed properties on the conversion. In the model, the available experience from fuel-bed modelling by means of the sub-models for fixed-bed conversion was structured into a fluid-flow scale and into a fuel-particle scale, in which new formulations describing the shrinkage of the fuel bed on a multi-particle scale was introduced. Both available and new sub-models were introduced into a pre-existing CFD-platform, in which the framework for simulating fluid flow in porous media was used to solve also the conversion related processes acting within the particle scales as well as within the multi-particle scales. The complete model was validated with good correspondence between available measurements of temperature and species concentration in a wood-char combustor. In addition, the modelled shrinkage was found to well describe the observed shrinkage of the fuel bed in a combustion experiment. Results of model simulations by using heterogeneous bed porosity show that a porous passage through the bed risks causing channelling in the fuel bed – a phenomenon common in modern grate furnaces and suspected to cause increased emissions of nitric oxides and unburned carbon compounds. The channelling tendency could, however, to a large extent be reduced by grates of higher flow resistance. The natural porosity increase attributable to the packing of particles onto a wall was shown to concentrate combustion disturbances close to the surface of the grate. Thus, larger changes in the porosity than caused by natural fuel packing against a wall are needed to give rise to channels that emerge through the fuel bed. 相似文献
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The present study investigates how sudden changes in fuel moisture affected the combustion characteristics of the fuel bed in a 4-MW reciprocating-grate furnace. The moisture content of the fuel fed to the furnace was monitored online using a near-infrared spectroscopy device, and the water vapor concentration in the flue gas was measured continuously. To obtain experimental data on fuel-bed conditions, the temperature and gas composition in the bed were measured using a probe. A simplified drying model was developed using the measured gas composition values as inputs. The model was then used to estimate the drying rate and to simulate the extent of the drying zone along the grate. Measurements indicated that a change in the moisture content of the fuel fed to the furnace was detected as a change in water vapor concentration in the flue gas with a delay of about 2 h. The model predicted that a portion of wet fuel would need about 2 h to become dry, in line with the measured time delay of the water vapor concentration change in the flue gas. Overall, there was good alignment between the measured and simulated results, supporting the validity of the model and the assumed mechanisms. 相似文献
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典型生物质燃料层燃燃烧特性的试验研究 总被引:4,自引:0,他引:4
在小型单元体炉中进行了不同形状尺寸及种类的生物质燃料的层燃燃烧试验.采用着火锋面传播速率及着火锋面温度研究了生物质燃料在同一给风量条件下的层燃燃烧特性,并分析了不同给风量对层燃燃烧的影响.结果表明:尺寸较小的燃料颗粒,完全燃尽需要的时间较长,燃烧过程中床层温度较高,而经过压缩的成型生物质燃料,燃烧稳定性好,适合层燃燃烧;对于不同种类的生物质,挥发分含量越高,其燃尽时间越短,灰分含量越高,燃烧稳定性越差;着火锋面传播速率与着火锋面温度都随着给风量的增加而提高. 相似文献
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《能源学会志》2020,93(1):324-334
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. 相似文献
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在天然气锅炉中引入柔和燃烧技术将大大降低NOx排放,高速未燃气卷吸高温烟气回流并与之快速掺混再燃烧是柔和燃烧的重要特征,因此,开展天然气锅炉关键结构参数优化设计以组织流场形成柔和燃烧所需的高温低氧反应气氛非常必要。基于天然气锅炉的工况特征,设计了热负荷15kW的模型燃烧室,采用数值模拟手段详细研究了燃烧室高度、喷嘴孔径、喷嘴相对位置及烟气出口尺寸对燃烧室流场、组分场及关键参数——烟气回流比的影响规律,并最终确定了燃烧室结构优选方案,对天然气锅炉柔和燃烧机设计提供理论基础数据。 相似文献
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The influence of bed-region stoichiometric ratio and fuel nitrogen content on the formation of gaseous species formed during grate combustion of biomass fuels is reported based on gas measurements made within the fuel bed. Three fuels were studied: two mixtures of pelletized bark and wood chips and one of pelletized straw. Experiments were performed in a vertical, cylindrical, laboratory-scale grate-furnace with 0.245 m i.d. and 1.8 m height. Primary air was supplied through a grate consisting of a steel plate with 340 holes of 3.7 mm diameter. Secondary air was supplied 0.66 m above the grate. Gas analysis was performed for O2, CO2, CO, H2 and NO. Results were compared with values calculated using a computer program for thermochemical equilibrium conditions. The measured contents of O2, CO2, CO and H2 show good agreement with calculated equilibrium conditions at all bed region stoichiometries. A higher formation of NO was found for the straw fuel (0.58% fuel nitrogen) than for the bark/wood chip fuels (≈0.25% fuel nitrogen). This is not in accordance with the thermochemical equilibrium calculations indicating that the formation of nitric oxide does not attain thermochemical equilibrium and that the nitrogen content of the fuel has an influence on the amount of NO that is formed. The fuel nitrogen conversion to NO ranged from 3 to 20% at reducing conditions and from 20 to 40% at bed region stoichiometries between 1.00 and 1.25. 相似文献
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为了响应政府业及民用天然气锅炉达到超低氮排放,要求绝大多数天然气锅炉采用低氮燃烧器+烟气再循环系统的技术路线,实施后普遍出现NOx、CO含量偏高、炉膛振动较大等问题。借助116 MW天然气锅炉进行试验研究,研究了燃烧器燃料配比、燃烧火焰长度、助燃空气氧含量三个因素对NOx及CO的影响,并对投入烟气再循环前后炉膛振动情况进行了检测。试验表明:燃烧器燃料内外配比对NOx、CO生成影响较大,两者呈现相反趋势变化;燃烧火焰长度对NOx生成影响较大,对CO含量影响较小;助燃空气氧含量对NOx、CO生成以及锅炉振动影响较大。三种影响因素相比,助燃空气氧含量影响更为突出。 相似文献
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《能源学会志》2020,93(2):739-751
Because of their saponin content, Camellia oleifera shells cannot be directly discarded. However, this eco-unfriendly agricultural waste is suitable for use as a fuel. A comparison between the combustion of crushed C. oleifera shell (CCS) and pelletized C. oleifera shells (PCS) was carried out in a pilot-scale fluidized-bed. Both directly combustion and flue gas recirculation (FGR) combustion modes were employed. The effects of particle size and the FGR ratio on the combustion behavior and pollutant emissions characteristics were also investigated. Results show that the combustion efficiencies for both CCS and PCS are higher than 99%. The combustion fraction in the bed region of CCS and PCS-6 are 55% and 85%, respectively, resulting in different combustion and pollutant emission characteristics. CCS has smaller CO and NO emissions and higher PCDD/Fs concentration of the fly ash compared with those of pelletized fuels. However, considering the material bridging in the feeding process of CCS, using pellets as the fuel is a better choice. The CO emissions increase while dioxin and furan emissions decrease with an increase in FGR ratio. The lowest NO emissions (about 150 ppm) of the three pelletized fuels combustion were achieved at an FGR ratio of 42%. In addition, all the pollutant emissions are lower than the minimum standards required for municipal solid waste (MSW) incinerators in Taiwan. 相似文献
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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。 相似文献
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对29 MW燃煤链条炉排锅炉增加烟气循环系统,让烟气在风室中与新风混合,从而降低风室中氧浓度(体积分数),创造炉内低氮燃烧的氛围,促进NO_x减排。通过试验与系统流程计算的两种方法,对比了改造前后链条炉锅炉出口的NO_x和氧气浓度,从而得出了烟气循环的最佳循环量。研究表明:改造后,随着烟气循环比例增加,烟气中氧气浓度降低,伴随NO_x排放浓度降低,在研究条件下,由实验得出最佳的烟气循环率约为12%;系统流程计算是基于系统质量守恒,将烟气循环、布袋除尘器内漏风等因素加以考虑,程序可预报烟气氧浓度变化的规律,证明与实测一致;烟气循环比率存在最大值,超过后会严重影响炉内火焰稳定性;该锅炉在12%烟气循环比率条件下,NO_x排放浓度降低22%,是低氮燃烧获得较好效果的烟气循环工况。 相似文献
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《Energy》2005,30(8):1429-1438
Experiments with a newly designed controlled multiple air staging technology (CMAST) in grate firings show a considerable reduction in NOx emissions. The applicability of the CMAST depends on fuel parameters. Fuels with high moisture content cause a drop in the heat output during full load operation due to the reduced fuel conversion. Due to reduced temperatures in the furnace, the emissions of products of incomplete combustion rise during part-load operation. More primary air is necessary to decrease incomplete combustion products, thus preventing the successful implementation of the multi-air-staging technique. Experiments in the laboratory, and test and commercial firings assist in understanding the influence of different fuel characteristics on the combustion system, and detect practical potentials and limits of air staging. On this basis, concepts have been developed for an optimised operation of grate firings dependent on the fuel characteristics. These results promise a further improvement of combustion technology using wood fuels. 相似文献
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基于多孔介质非热平衡的方法,考虑了床层高度的变化及颗粒内部温度梯度的影响,建立了一维非稳态燃烧模型来模拟炉排上移动床层的生物质燃烧。模拟计算结果与实验值对比分析表明,总体上数值计算结果与实验数据吻合较好。通过对不同一次风参数下床层燃烧的模拟结果分析得到,随着一次风风量的增加,床层剩余质量先减小后增大;在燃烧前期,床层出口气体温度上升速度减慢,挥发分析出速率降低,焦炭燃烧速率增大;在燃烧中期,床层出口气体温度先上升后下降,焦炭燃烧速率下降。一次风风温相比于一次风风量对床层燃烧影响较小,增大一次风风温可以提高挥发分析出速率,降低床层出口气体温度和床层剩余质量。 相似文献