【作者】 闫桂焕；

【导师】 孙奉仲；

【作者基本信息】 山东大学 ， 热能工程， 2011， 博士

【摘要】 气化是生物质能主要利用方向之一,但是传统气化技术存在燃气中焦油含量高、废水难以处理等问题,燃气净化不彻底是阻碍生物质气化技术大规模商业化应用的至关因素,开发稳定高效的低焦油气化工艺成为生物质气化行业公认的难题。热裂解脱除焦油是一种简单有效且易于工程化的方法,但是目前还没有成熟的研究成果及工业化运行装置,因此研究开发完善简单高效的低焦油气化工艺及装置对于推动生物质能大规模应用具有重要的工程价值。由于焦油成分的复杂性,目前对生物质气化过程所做的数值模拟计算较少有考虑焦油组分的,因此无法分析到焦油在炉内的裂解状况,无论从热力学还是动力学角度,建立含有焦油的生物质气化过程数学模型对于研究低焦油气化工艺具有重要的理论指导意义。本文以下吸式固定床为对象,借助试验和数值计算两种手段研究了农林类生物质实现低焦油气化关键问题,包括低焦油气化装置、考虑焦油裂解过程的数学模型等。热解是生物质气化过程非常重要的一个环节,热解过程生成焦油,热解产物为后续氧化还原反应过程提供物质来源。为了更好地组织氧化还原过程、获得低焦油气化工艺,就需要掌握焦油生成特性及热解产物分布规律,本文首先在所建小型常压热解试验台上,研究了农林类生物质在移动床中的热解特性,得出了生物质在不同反应温度和反应时间下气、液、固三种热解产物的产率及其分布规律,并分析了组成成分。结果表明,在小型装置常规热解气化过程中,对热解产物分布影响最大的是反应温度,反应时间影响很小；随着反应温度的升高,固体、液体产物产率明显下降,气体产物产率显著上升,固、液、气三种产物产率400℃时分别约占40-45%、45-50%、7-10%,800℃时分别约占20-30%、12-18%和55-60%；常温下热解气态产物主要由H2、CO、CO2、CH4、轻烃CmHn组成,随着热解温度从400℃提高800℃,H2含量显著上升,CO和CO2含量明显下降,CH4含量有所上升,CmHn含量变化不大；热解温度在500-600℃时,焦油产量最大,随着温度的提高,焦油产量明显降低。为实现低焦油气化过程,就必须提供焦油裂解所需的稳定高温环境,使焦油裂解环节与热解还原过程在空间上分开,即利用分步式方法实现低焦油气化过程。本文借助下吸式固定床反应器,提出了一种简单易操作的分步式低焦油气化工艺,即将干燥热解和燃烧还原过程在物理空间上分开进行,并建立了相应气化装置,通过试验验证了低焦油气化工艺的可行性。结果表明,在分步式固定床气化装置中,燃气组分、热值、燃气中焦油含量与氧化区温度密切相关,热解温度、当量比ER等因素又直接决定着氧化区的温度；在其它条件不变的情况下,热解温度从390℃提高到550℃,氧化区温度也随之升高,反应加剧,焦油分解彻底,出灰率降低,碳转化率升高；空气气化、ER等于0.25、热解温度超过450℃、氧化区温度超过950℃时,产品气焦油含量小于20mg/Nm3,燃气热值约为5MJ/Nm3,气化效率大于72%,碳转化效率超过90%；热解温度450℃,ER从0.23提高到0.3,氧化区温度升高,燃气中CO、CH4含量下降,H2含量略有增加,燃气热值降低,焦油含量减少；当量比0.25-0.3、热解温度400-500℃,空气气化燃气热值为4.2-5.3MJ/Nm3,富氧气体(氧体积浓度90%)气化燃气热值为7-9.5MJ/Nm3。当以空气为气化剂时,水蒸汽加入后氧化区温度有所降低,燃气中H2含量提高,CO含量下降,CH4含量有所增加,气体热值略有升高。氧化区是焦油裂解的关键环节,为了更加清楚掌握反应条件对焦油裂解过程的影响,建立了氧化区热解产物燃烧及焦油裂解过程的动力学数学模型,实现了氧化区反应过程的可视化研究,得到了氧化区内各种物质的变化规律。计算结果表明,在其它条件不变时,ER对氧化区温度场、浓度场有较大影响,随着ER从0.17提高到0.32,氧化区内整体温度提高,氧化区出口C(S)、H2O、C2H4比例下降,N2和C02比例上升,CO与H2含量先增加后减少,CH4含量变化不大,焦油含量明显降低；热解温度高,氧化区内整体温度也高,焦油裂解速度越快；气化剂入口速度越大,氧化区气流扰动越大,燃烧越充分,焦油裂解越彻底；在热解温度和ER相同时,空气气化和富氧气体(氧体积浓度90%)气化两种方式对焦油裂解程度影响不大,但对浓度场影响较大；从氧化区轴向截面来看,各种组分沿轴向方向都在不断发生变化,燃烧及焦油裂解反应主要集中在气化剂入口向下约200mm范围内,氧化区喇叭口及下部区域主要是H20与各种物质发生的一些重整反应；焦油在燃烧初期裂解较快,之后速度逐渐变慢,焦油裂解速度实质上主要受温度影响。为了能够从理论上系统地考察气化炉输入与输出物质的关系,本文从热力学角度,基于物质平衡、能量平衡和化学反应平衡建立了生物质气化过程数学模型,并引入了焦油模化物基本概念。模型计算结果与前人所建模型计算结果基本吻合,说明模型计算准确；但与分步式固定床气化试验结果相比时发现在CO、CO2组成上存在较大差别,其它成分基本接近,这主要是由于试验过程中受多方面因素的影响,很多反应实际达不到平衡,模型计算是完全实现平衡时的理想结果,它给出了各种成分的最终变化趋势,因此热力学计算结果可以从宏观上指导工程实践。计算结果表明,空气气化、当量比0.25、反应温度1000℃,达到平衡状态时,燃气中CO、H2体积之和在40%以上,CH4约占0.8-2.5%,焦油含量很少,燃气热值为6-7.7MJ/Nm3;空气预热温度提高,燃气中CO、H2体积含量增加,CO2、H2O、CH4、N2、焦油体积含量均有所下降,气体热值明显增加；原料水分增加,燃气中CO、H2、N2含量下降,CO2、H2O、CH4含量上升,焦油含量微增,气体热值先微降后微升；随着水蒸汽的加入,燃气中H2、CH4、H2O、CO2体积含量均略有增加,CO、焦油含量略有下降,燃气热值增加；ER从0.2提高到0.3,CO含量明显下降,CO2、N2含量显著上升,H2O、CH4、焦油含量下降,H2含量变化不大,气体热值明显下降。综上所述,本课题从理论分析、试验研究和数值计算等方面比较全面系统地研究了农林类生物质低焦油气化关键技术问题,利用分步式气化方法找到了实现低焦油气化的工艺参数,获得了考虑焦油在内的氧化区反应过程动力学数学模型和整个气化过程的热力学数学模型,得出了各种反应物质在炉内的变化规律,这些成果不仅具有重要的理论意义,还可以指导工程实践。更多还原

【Abstract】 Gasification is one of the most promising conversion routes for biomass utilization, but traditional gasification technology is flawed in some ways, such as high tar content in fuel gas and difficulty in waste water treatment. A pivotal factor that prevents biomass gasification technology from massive commercial application is incomplete purification of fuel gas. To develop stable and efficient low-tar gasification process becomes an acknowledged difficulty. Heat destruction for tar removal is a simple and effective method which is easy to be engineered, but now it still lack perfect research result and industrialized running device, therefore, developing and perfecting effective low-tar gasification technology and apparatus is of significant engineering value for promoting the massive application of biomass energy. Due to the complexity of the tar composition, current numerical computation on biomass gasification process seldom takes the tar composition into consideration, which makes it impossible to analyze the cracking of tar in the gasifier. Hence, building up a mathematical model of biomass gasification process that contains tar as an impact parameter will make theoretical guidance to study on low-tar gasification technology. In this paper, to take downdraft fixed bed for an example, many critical issues in making plant biomass achieve low-tar gasification were investigated by means of experiment and numerical computation, such as low-tar gasification device, mathematical model with tar crack process and so on.Pyrolysis plays a very important role in biomass gasification process, for tar is produced during pyrolysis process and pyrolysis products are reactants of the following oxidiation and reduction processes. In order to organize redox processes well and gain low-tar gasification technology, it is neccesary to matser the rules of tar production and distribution properties of pyrolysis products. A small pyrolysis experiment platform was built up. The pyrolysis property of plant biomass was studied on the platform. The distribution rules of the gaseous, liquid and solid pyrolysis products at different reaction temperature and time were obtained, and their compositions were analyzed. The results showed that the reaction temperature had a strong impact on the products distribution while the reaction time showed very little. It was also found that the yields of liquid products and solid products decreased obviously as the reaction temperature rising, while the yields of gaseous products clearly increased. The mass ratios of solid, liquid and gaseous products took the percentages of 40-45%,45-50%,7-10% respectively at about 400℃, and they went to 20-30%,12-18% and 55-60% respectively as the reaction temperature rose to 800℃. It could be concluded that the uncondensable gaseous products mainly contained H2, CO, CO2, CH4 and light hydrocarbon (CmHn). As the temperature increased from 400℃to 800℃, the volume content of H2 rose obviously, those of CO content and CO2 both decreased distinctly, the CH4 volume content increased slightly, and there was no visible change in CmHn content. The yield of tar reached its peak at the pyrolysis temperature of 500-600℃and decreased as the temperature rose.In order to achieve the low-tar gasification process, a stable high-temperature condition which should separate from pyrolysis and reduction stages is essential. A step-by-step method can carry out the low-tar gasification. In this paper, with the use of downdraft fixed bed reactor for example, a simple and maneuverable low-tar step-by-step gasification technology was proposed, which made pyrolysis and oxidation become two separate processes by physical space. Besides, a corresponding device was built up, and the low-tar gasification technology was validated through extensive experiments. As the results showed, in the step-by-step fixed gasification device, the component, heat value, and tar content of the fuel gas were nearly bound up with the temperature of oxidation zone which can be directly influenced by pyrolysis temperature and equivalence ratio (ER). With other conditions remaining the same, as the pyrolysis temperature rose from 390℃to 550℃, the temperature of oxidation zone increased, the reaction intensified, tar decomposed more completely, the residual ash content decreased and the carbon conversion ratio increased. Under the conditions that air ER was 0.25, pyrolysis temperature was above 450℃and the temperature of oxidation zone was above 950℃, the raw tar content of the fuel gas was less than 20 mg/Nm3, and the low heat value(LHV) was about 5MJ/Nm3, while the gasification efficiency was higher than 72%, and the carbon conversion ratio was over 90%. As ER increased from 0.23 to 0.3 at the pyrolysis temperature 450℃, the temperature of oxidation zone rose, the volume contents of CO and CH4 in the fuel gas decreased while the H2 content increased slightly, both the gas calorific value and the tar content reduced. Under the conditions that ER was from 0.25 to 0.3 and pyrolysis temperature varied from 400℃to 500℃, the gas calorific value ranged from 4.2 to 5.3 MJ/Nm3 while using air as the gasification agent, and the value changed from 7 to 9.5 MJ/Nm3 while using oxygen-enriched gas in which the volume concentration of oxygen was 90% as the gasification agent. When air was used as the gasification agent, as adding the steam, the temperature of oxidation zone reduced, the H2 content increased and the CO content decreased, while the CH4 content and the gas calorific value added up slightly.Oxidation zone is the key to tar crack. In order to know the influence of reaction conditions on the tar crack process, a dynamic mathematical model of reaction process in oxidation zone was established, which made the reaction processes visualize, and the changing rules of various reactants in oxidation zone could be analyzed. It was concluded from the calculation results that ER had a great effect on both the temperature value and the substance concentration in oxidation zone. As ER growing from 0.17 to 0.32, the average temperature of the oxidation zone increased, while the mole ratios of C(S), H2O, C2H4 and tar all decreased at the outlet of oxidation zone, in contrast to the increase of the ratios of N2 and CO2. The content of CO and H2 declined after their initial growth. The growth of ER showed a little effect on the content of CH4. The results also indicated that the higher the pyrolysis temperature was, the higher the average temperature of the oxidation zone was and the faster the tar cracked. With the increase of the inlet velocity of gasification agent, the reactions in the field worked more perfect and tar crack became more complete for intensification of gasification agent disturbance. While the pyrolysis temperature and ER remained unchanged, the two gasification methods with separately air and oxygen-enriched gas as the gasifying agent, in which the volume concentration of oxygen was 90%, showed slight influence on tar crack but great effect on concentration field. Seen from the axial cross-section of the oxidation zone, it showed that all the components changed along the axial direction. Combustion reactions and tar crack reactions mainly took place at the gasifying agent inlet surface down to about 200mm and it was some reforming reactions in the horn part and the lower area of the oxidation zone. The cracking speed of tar was mainly influenced by temperature.In order to systematically investigate the relationship between reactants and products of gasifier in theory, this paper established a thermodynamic mathematical model of biomass gasification processs with tar on the bases of mass balance, energy balance and chemical reaction balance. The results were in good accord with those the predecessors got, but had some difference in the content of CO and CO2 with the experiment results of step by step fixed gasification bed. The difference was mainly resulted from that some reactions couldn’t achieve the balance in experiments while in numerical simulations they were supposed to be balanced. The simulation results actually indicated the ideal trends. Therefore they could be used to make theoretical guidance to the practice on the view of macroscopic. The results showed that under the conditions that air ER was 0.25 and reaction temperature was 1000℃, after the reactions achieved their balance, the volume of CO and H2 in the gas added up to more to 40% while CH4 took about 0.8% to 2.5%. The tar content was really a little and the LHV of fuel gas ranged from 6 to 7 MJ/Nm3. As the air preheating temperature went up, the volume ratios of CO and H2 increased, while the volume contents of CO2, H2O, CH4, N2 and tar all reduced, as a result, LHV of the fuel gas improved. Increasing moisture content in raw material could bring down the volume ratios of CO, H2 and N2, and raise the ratios of CO2, H2O, CH4, while making little effect on the content of tar and the gas calorific value. The entry of water vapor could increase the volume contents of H2, CO2, H2O and CH4 while decrease the contents of CO and tar, as a result, which could make LHV of the fuel gas rise. As ER increased from 0.2 to 0.3, the contents of CO, H2O, CH4 and tar decreased clearly, and the contents of CO2 and N2, increased obviously, while the H2 content varied inconspicuously and the gas calorific value decreased.In summary, this subject comprehensively and systematically studied the key issues in plant biomass low-tar gasification technology with the use of theoretical analysis, experiment research and numerical calculation. In this paper, the low-tar gasification processing parameters were found through a step-by-step gasification method. Besides, a dynamic mathematical model of oxidation zone in downdraft fixed bed and a thermodynamics mathematical model for the whole gasification process were established, in which the tar was both considered as effect parameters, and the changing rules of various reactants in the furnace were obtained. The work done by this research can offer theoretical support and practice guidance to the plant biomass gasification industry.更多还原