【作者】 任枫；

【导师】 李争起；

【作者基本信息】 哈尔滨工业大学 ， 热能工程， 2010， 博士

【摘要】 我国难燃煤占动力用煤的40%,是世界上为数较少难燃煤储量丰富的国家之一,W火焰锅炉作为燃用难燃煤的重要炉型已在我国得到了很大的发展,其中应用最多的为福斯特惠勒(FW)型W火焰锅炉。在实际运行过程中,这种锅炉存在着燃烧不稳、飞灰可燃物含量高、NOx排放量高等问题。本文在国家高新技术发展计划(863计划)的资助下,立足于解决这些问题,一步步的提出了二次风下倾技术、FW型W火焰锅炉高效燃烧技术,最后形成了完整的FW型W火焰锅炉高效低NOx燃烧技术。通过冷态单相空气动力场试验、示踪试验、气固两相流动特性试验、数值模拟等手段对这些技术的机理进行了深入研究,并进行参数优化。最后将这些技术分别应用660MW和300MW容量的FW型W火焰锅炉上,通过工业试验研究其热态工作性能,验证其应用效果。首先在1:15缩小的单相及气固两相试验台上对原FW型W火焰锅炉的运行状态进行了研究。通过对E层二次风速和拱上淡煤粉气流百分比的改变,发现目前状况下无论运行参数如何调整,原有锅炉上拱上一次煤粉气流始终无法穿透F层二次风气流区,从而造成煤粉气流在下炉膛的停留时间偏短。通过在300MW机组上的工业试验研究则可以发现,在运行时,燃烧系统运行阻力大,着火点远离喷口,火焰中心出现在上炉膛,炉内温度分布不合理,飞灰可燃物含量高,NOx排放量高的问题突出。依靠其现有调整手段,包括对淡煤粉气流挡板、二次风挡板、消旋叶片位置和油二次风挡板的调节,能从一定程度上改善燃烧,但无法从根本上解决问题;针对上述问题,提出了二次风下倾技术,同样进行了冷态单相和气固两相的试验研究。无论从冷态单相还是气固两相试验结果来看,随着F层二次风下倾角度增加,浓煤粉气流在炉内达到的下冲深度和F层气流区域的湍流强度均增加。综合考虑煤粉形成和壁面结渣的问题,可将F层二次风角度设为25°。在300MW机组锅炉上应用二次风下倾技术,发现F层二次风下倾后,NOx排放由2101mg/m3降至1926mg/m3(折算氧量6%),飞灰可燃物含量从7.84%降至4.91%,锅炉效率由91.08%提高到93.25%。在二次风下倾技术基础上发展起FW型W火焰锅炉高效燃烧技术。在冷态单相模化试验台上进行了将淡煤粉气流通过后半个油二次风喷口通入炉膛的试验,发现淡煤粉气流位置的改变不会对炉膛里的气流流动状况产生本质上的影响。将FW型W火焰锅炉高效燃烧技术应用于660MW机组锅炉后,燃烧系统阻力由原来的3734Pa降为1692Pa,排烟温度由136℃降为124℃,飞灰可燃物含量由原来的9.55降低到3.82,锅炉效率从84.54%增大至92.17%,燃烧时可掺烧无烟煤比例大幅上升。但同时发现,该技术对抑制NOx生成无能为力。在高效燃烧技术的基础上,进一步对锅炉进行低NOx改造,提出了FW型W火焰锅炉高效低NOx燃烧技术。冷态单相试验结果表明,随着OFA风率的增加,下炉膛一次风射流衰减变慢,穿透深度增加,煤粉行程增大。利用示踪试验的方法,在实验室试验台上研究了不同风率、不同摆放角度和不同喷口结构下,OFA气流在W火焰上炉膛内的流动特性,最终确定:OFA风率需至少在20%以上,OFA喷口倾角选在30°,喷口结构的选择可采用内直流外旋流OFA喷口,外层旋流部分喷口所安装的叶片应选用弯曲叶片。在气固两相试验台上的试验也表明,OFA喷口倾角也应选在30°。对FW型W火焰锅炉流动燃烧进行数值预测,发现应用FW型W火焰锅炉高效低NOx燃烧技术后,锅炉性能优于原锅炉状态。在300MW的W火焰锅炉上进行了FW型W火焰锅炉高效低NOx燃烧技术的应用。尽管OFA的引入使煤粉长时间处于贫氧燃烧有可能提升飞灰可燃物含量,但二次风下倾以及淡煤粉气流由油风喷口通出的措施促进了浓煤粉气流的点燃,并改善了炉内的温度分布,抵消了OFA的不利影响,改造后飞灰略有降低,锅炉效率升高。NOx排放量降低了50%,说明FW型W火焰锅炉高效低NOx燃烧技术能在不降低锅炉效率的基础上大幅降低氮氧化物。更多还原

【Abstract】 China is one of the few countries that has an abundance of hard-to-burn coals taking up 40 % of the total coal power reserves. The main furnace types burning hard-to-burn coals are down-fired boilers, which have seen rapid development in China. Among these the most widely used is the Foster-Wheeler (FW) type. However, in the practical operation of these boilers, the problems of unstable combustion, high carbon content in fly ash and high NOx emission exist. Sponsored by the Hi-Tech Research and Development Program of China (863 program), a series of technology improvements for the retrofit of FW type down-fired boilers were developed. The technologies of declining secondary air (DSA) and high efficiency combustion (HEC) were progressively improved and finally a high-efficiency and low-NOx combustion (HELNC) technology emerged. By means of cold single-phase aerodynamic experiments, trajectory experiments, gas/solid two-phase flow experiments and numerical simulations, the mechanisms of these technologies were investigated and the operating parameters of the boilers were optimized. Finally, these technologies were applied to 660 MW and 300 MW down-fired boilers. By performing industrial experiments, their effects were evaluated and validated.Initially the characteristics of the original FW type down-fired boiler were investigated on a single phase and a gas/solid two phase 1:15 small scale model test facility. By regulating the E tier secondary air velocity and the fuel lean flow ratio, it was found that whatever the setting of these parameters, the fuel-rich flow could not penetrate the F-tier airflow zone and this lead to a relatively shorter coal residence time in the furnace. The industrial experimental results on a full scale 300 MW boiler unit show that the down-fired boilers of the original FW type had several problems such as the large resistance in the combustion system, the distance of the ignition point far from the nozzle, the flame kernel appearing in the upper furnace, the high level of unburnt carbon in the fly ash and the high NOx emissions. The current regulating method, including the regulation of the fuel lean air valve, secondary air damper, adjustable vane position and oil secondary air damper, was able to improve the combustion conditions over a limited range but could not solve the above problems entirely. As a consequence of the above problems, the application of the DSA technology was proposed. The results of cold single-phase and two-phase experiments on the small scale test facilities show that the rise of the secondary air angle increases the depth of reach of the fuel rich flow in the lower furnace. The turbulence intensity also increases. Considering both the coal residence time and the wall slagging, the angle of the F-tier secondary air was selected at 25°, and the technology was then applied to a 300MW boiler unit. It caused the NOx content to fall from 2101mg/m3 to 1926mg/m3 (at 6 % O2), the unburnt carbon in the fly ash to fall from 7.84% to 4.91% and the boiler efficiency to rise from 91.08 % to 93.25%.The HEC technology for FW type down-fired boilers was improved with the application of DSA. The results of the single phase experiments show that eliminating the original fuel-lean ports and guiding the flow through oil ports into the furnace does not significantly influence the flow field in the furnace. This technology was then applied to a 660 MW boiler unit. It caused the resistance of the combustion system to fall from 3724 pa to 1692 pa, the gas temperature at the furnace exit to fall from 136°C to 124°C, the unburnt carbon in the fly ash to fall from 9.55 % to 3.82 % and the boiler efficiency to rise from 84.54 % to 92.17 %. However, this technology had minimum influence on restricting the NOx emission.On the basis of the HEC technology, the low NOx retrofit was implemented on the down-fired boiler, thus creating the HELNC technology for FW type down-fired boilers. The results of the single phase experiments show that with the increase of the OFA ratio, the decay in fuel-rich flow slows down, the depth of reach for the fuel-rich flow in the lower furnace increases and the residence time for the coal in the furnace rises. By means of trajectory experiments, the flow characteristics for the OFA in the down-fired furnace under the condition of different OFA ratios, different nozzle angles and different nozzle structures were investigated. We determined that the OFA ratio should be at least 20 % and the nozzle angle 30°, the structures of the nozzles chosen were the inner direct and outer swirl types, and the vanes should be the chosen bending type located in the outer swirl nozzle. The results from the experiments on a two phase test facility indicate that 30°should be chosen as the optimized OFA nozzle angle in the furnace. Using Fluent 6.3, a numerical simulation was carried out for the combustion and NOx emission in the FW type down fired furnace. The results indicate that the boiler performance can be improved by applying the HELNC technology to the FW type down-fired boiler. Finally, this technology was applied to a 300 MW boiler unit. The results show that although the introduction of the OFA may increase the quantity of unburnt carbon in the fly ash, the decline of the secondary air and the guide of fuel lean flow through the oil port into the furnace promote the ignition of a fuel rich flow and improve the gas temperature distribution in the furnace. They eliminate the disadvantage of the OFA. After the retrofit, the amount of unburnt carbon in the fly ash reduced slightly and the boiler thermal efficiency increased. The NOx emission lowered by 50 %, which means that the HELNC technology for down-fired boilers can lower the NOx emission without reducing the efficiency of the boiler.更多还原