【作者】 易其国；

【导师】 肖波；

【作者基本信息】 华中科技大学 ， 环境工程， 2013， 博士

【摘要】 生物质能作为一种可再生能源,其大规模利用有可能满足各种能源需求,同时能减少CO2的排放。目前,生物质直接燃烧是生物质能应用最简单可行的直接利用,但长期以来,生物质直接燃烧温度偏低和能量利用效率不高的缺点使得其推广应用受到了一定限制。将生物质破碎成微米燃料(BMF),开发生物质微米燃料(BMF)高温高效燃烧技术,可进一步促进生物质的工业化应用和温室气体的减排。本文选取生物质微米燃料锯末及其半焦作为原料,为进一步提高其旋风燃烧温度,先添加其热值较高的半焦与之混燃,然后通过预热空气提高炉膛温度模拟烟气再循环技术,并探讨了生物质微米燃料煅烧水泥的可行性；同时,建立的燃烧动力学模型为进一步建立综合的燃烧数学模型打下了基础。论文具体研究工作如下：(1)生物质的挥发分高,着火性能和燃尽性能较好,但能量密度低；半焦固定碳含量高,热值高,但挥发分低,着火性能较差。生物质与其半焦混燃有助于各补其短,半焦可以补充生物质的较低热值,而生物质将有助于半焦提前着火。(2)采用热重分析仪对生物质微米燃料在不同升温速率下的热解、燃烧失重特性进行了分析。研究结果表明：生物质微米燃料综合反应活性随升温速率增加而提高。最大反应速率随着升温速率的增大呈线性增大趋势。在此基础上,建立了生物质微米燃料热解和燃烧的动力学模型。该模型通过三个主要组分半纤维素、纤维素和木质素热解的独立平行反应,可以比较准确的描述生物质微米燃料的二段燃烧过程：第一段类似于其热解过程,第二段是木质素热解残焦燃烧,其活化能较高。研究发现,实验结果与通过该模型的非线性回归法拟合获得的结果基本一致,证实了BMF的热解、燃烧过程中确实存在着上述假定的反应机理。(3)通过热重分析研究了生物质微米燃料和其半焦混燃特性,研究结果表明混燃各单组分间有协同促进作用,能改善单一组分的可燃性。在综合热分析基础上,考察了空气当量比、粒径、含水率、生物质半焦添加比例对生物质微米燃料旋风炉燃烧炉膛温度、烟气及灰分的影响。试验研究发现空气当量比为1.2,粉体粒径在0.177mm(即80目)以下,生物质含水率控制在8.1%以下,生物质半焦添加比例为20%,燃烧效果更好,燃烧经济成本合理,燃烧效率高于成型燃料的燃烧效率,燃烧烟气中NOx和S02等有害气体的含量较少。(4)在上述实验研究的基础上,初步开展了基于生物质微米燃料煅烧水泥的应用研究。相应条件下,通过空气预热,炉膛温度可提高到1360℃以上,可满足水泥煅烧的基本温度要求。并初步获得了成分符合要求的水泥熟料,验证了其可行性。但对该系统的能量评估表明：装置的燃烧效率较低,主要是由于排烟热损失导致的。(5)为了降低排烟热损失,可通过循环烟气加热空气,提高生物质燃烧温度和能量利用效率。为此,利用TG-DTA、TG-MS、GC等技术,初步研究生物质燃烧烟气再循环技术。先利用TG-DTA对比分析了O2/CO2与空气气氛(不同氧气浓度条件下)对生物质着火模式、燃烧特性的影响。结果表明：在O2/CO2气氛下,随着氧气的浓度增加,BMF的着火模式是从联合着火转变为均相着火模式；最大燃烧速率增大,着火点提前,且燃烧时间缩短。同时,CO2气氛有助于抑制NOx生成。进一步在管式固定床上考察了O2/CO2燃烧的初级阶段——气化的影响因素。实验结果表明：高温、较小粒径以及CO2气氛有利于生物质0气化反应的进行。气化比热解气氛下产生的可燃气体多,半焦产量少,有利于推进整个燃烧过程的进行,从而有望提高生物质燃烧温度,减少环境污染。故O2/CO2燃烧有望提高生物质燃烧温度,减少环境污染。更多还原

【Abstract】 The widespread use of biomass fuels, as a renewable energy, could meet the demand of all kinds of energy and reduce carbon dioxide emissions. Combustion is the most simple and direct technology nowadays available for biomass utilization. But its lower burning temperature and energy utilization efficiency has long limited its wide application. Biomass fuel was broken into microns named as biomass micron fuel (BMF). It is necessary to develop BMF’ efficient combustion technology for high temperature, which can further promote the industrialization of biomass application and greenhouse gas reductions.In this dissertation, BMF(sawdust) and its biochar was used as raw material. In order to further improve the cyclone combustion temperature, the biochar with the higher heating value was added to co-combustion with BMF, and then the air was preheated to simulate flue gas recycling technology. At the same time, the combustion kinetics model is set up in order to further establish a comprehensive combustion mathematical model. The following work was carried out in this dissertation:(1) Biomass has high volatile and its ignition and burnout performance is good, but its energy density is low. Biochar contains higher fixed carbon content and calorific value than raw biomass, but its ignition performance is worse due to lower volatile than raw biomass. Thus, biomass and biochar seem to have important potential to compensate the disadvantages coming from each other. Biochar may contribute to the calorific value of biomass, and biomass may promote early ignition of biochar.(2) The pyrolysis and combustion characteristics of BMF were obtained at different heating rates in a thermo gravimetric analyzer. The results showed Comprehensive reaction activity increased and the maximum reaction rate increased linearly with the increase of heating rate. A mechanism based on three independent parallel reactions has been used to model the pyrolysis process of hemicellulose, cellulose and lignin. Combustion process can be divided into two distinct stages, with first stage coinciding with pyrolysis process and the second one concerning lignin char combustion with higher activation energy. The fitting curve of TG obtained from nonlinear regression method is coincident with experiment curves, which confirmed the presumed reaction mechanism does exist during the process of BMF pyrolysis and combustion.(3) The co-combustion of biomass and biochar was investigated by thermo gravimetric analysis. The results showed that synergy exists between the two components and better combustibility was feasible by co-firing biochar with biomass. On the basis of comprehensive thermal analysis, the effects of equivalence ration (ER), particle size, moisture ratio of BMF and biochar blending ratio on combustion performance were studied. The results showed the optimal value of ER and biochar blending ratio obtained was respectively1.2and20%. Smaller particles (below0.177mm) and lower moisture ratio ((below8.1%) results in not only better combustion performances but also economy cost. Combustion efficiency of BMF was higher than that of the biomass briquette. The harmful gas content, such as NOx and SO2, was less in flue gas.(4) Moreover, a pilot study of cement calcining with BMF demonstrated the maximum temperature can be raised to above1360℃through the air preheating. The maximum temperature could meet the demand of cement calcining. However, the combustion efficiency was relatively low for this kiln due to heat losses of flue gas. As for components, calcined raw meal was basically similar to industrial cement clinker. It was feasible to calcine cement by the technology. But the energy evaluation of the system showed that the low combustion efficiency of the device was mainly caused by the exhaust heat loss.(5) To reduce exhaust heat loss, the recycled flue gas was used to preheat air, which can improve the biomass combustion temperature and energy utilization efficiency.Therefore, the flue gas recycling technology for biomass combustion was basically investigated via TG-DTA, TG-MS and GC. By using TG-DTA, the effect of the O2/CO2atmosphere and air (under the conditions of different oxygen concentration) on the biomass ignition mode and the combustion characteristic was analysed. The results showed that with the increasing oxygen concentration, the ignition mode of BMF from joint fire was transformed into homogeneous ignition mode; Maximum combustion rate increased, the ignition advanced and burning time was shortened under O2/CO2atmosphere. At the same time, CO2atmosphere helped to inhibit NOx generation. Morever, under O2/CO2the combustion initial stage-CO2gasification experiments of BMF were carried out in tubular fixed-bed reactor. The results showed that higher temperature, smaller size and CO2atmosphere favored gasification process. Gasification produced more combustible gas and less biochar than pyrolysis atmosphere. CO2atmosphere promoted the whole combustion process, which was expected to increase biomass combustion temperature and reduce the pollution of the environment under O2/CO2atmosphere.更多还原