【作者】 付鹏；

【导师】 向军； 胡松；

【作者基本信息】 华中科技大学 ， 热能工程， 2010， 博士

【摘要】 随着化石能源的日益紧缺和环境污染的日趋严重,生物质能的资源化利用引起了全世界的广泛关注。农业废弃物作为我国主要的生物质能资源,其洁净高效转化技术的研究和开发对于建立可持续发展的能源系统,促进社会经济的发展和生态环境的改善具有重大意义。基于国内外在生物质热解和气化方面的研究进展及不足,本文以中国4种典型农业废弃物玉米秆、稻草、棉秆和谷壳为研究对象,深入研究热解和气化过程中气体产物的释放规律、形成机理及焦颗粒结构的演变行为,对于深刻揭示生物质热解和气化机理具有重要的意义。首先采用TG/DTG技术研究农业生物质的热解特性,获得生物质在挥发分析出阶段的特征参数,定量描述了升温速率对生物质热解特性的影响,建立了生物质热解的反应动力学模型。结果表明,农业生物质的热解表现出相似的规律,热分解主要集中200-500℃。随着升温速率的增大,挥发分析出阶段的起始和终止温度、峰值温度均向高温侧轻微移动,并且主热解反应温度区间也在增大。最大热解速率随着升温速率的增大呈线性增大趋势。三组分模型可以很好地模拟木质纤维类生物质在不同升温速率下的热解行为。半纤维素、纤维素和木质素的热解活化能分别在98-114kJ/mol、132-186kJ／mol和21-26kJ／mol内变化。采用热解反应器与Gasboard-3100气体分析仪／Gasmet Dx-4000红外气体分析仪联用技术开展了生物质固定床热解的实验研究,探讨了主要气相组分的释放特性和形成机理以及热解产物分布的影响因素。结果表明农业生物质热解的主要气体产物有CO2、CO、CH4、C2H6、C2H4和一些有机物(如甲醇、甲醛、甲酸、丙酮和酚类化合物等)。CO2和CO的释放主要是羰基、羧基和醚基等断裂和重整所致。CH4主要通过甲氧基、亚甲基和芳香环等断裂而产生。甲醇的形成主要源自于芳香甲氧基和烷基侧链γ位上的脂肪-CH2OH等基团的断裂和重整。甲醛的形成与含有-CH2OH或羧酸基的烷基侧链上C-C键的断裂有关。热解温度和升温速率对热解产物分布及气体释放特性有明显影响。随着温度的升高,热解气中CO2含量显著减小,CO含量明显增大,CH4与CO含量的变化趋势比较相似,C2H6的含量一直在减小,而C2H4含量的变化不太明显。低升温速率有利于CH4和CO2的形成,而高升温速率有利于CO和C2H4的形成。利用元素分析、FTIR、In-situ DRIFTS、ESEM、真密度法、氮气等温吸附/脱附法等方法研究了热解过程中农业生物质焦物理化学结构的演化规律,并采用分形维数定量描述了焦颗粒内部孔隙表面形态的复杂程度。结果表明,热解温度对生物质焦的化学组成及结构特性有明显影响。高温下生物质焦中热不稳定的羟基、脂肪C-H键、烯属C=C键和羰基等基团已基本消失,而醚结构的含量也很少,生物质焦变得更具芳香性和碳质化。高温下生物质焦颗粒发生软化变形、熔融和碳结构有序化等现象,碳结构有序化进而导致了热退火和热失活现象的发生。此外热解过程中生物质焦颗粒还发生了结构收缩和孔窄化现象。热解过程中农业生物质的比表面积变化规律基本相近,在慢速热解条件下,比表面积都经历一个先增大后减小的过程,而在快速热解条件下,比表面积都经历一个先缓慢增大后急剧增大的过程。生物质/焦颗粒的孔隙表面具有分形特性,其分形特征与热解温度密切相关,慢速热解过程中分形维数与比表面积的变化趋势比较相似,而快速热解过程中分形维数与比表面积的变化趋势呈现出较大差异。最后在固定床反应器上开展了生物质焦与水蒸气气化的实验研究,实时在线分析了主要气体产物的释放特性,基于吉布斯自由能最小化原理开展了生物质焦水蒸气气化的热力学模拟研究,详细探讨了气化温度、压力、水蒸气加入量和CaO/C摩尔比对气化效果的影响,并通过元素分析、FESEM、氮气等温吸附/脱附法、XRD等方法研究了气化过程中生物质焦结构的演化规律。研究表明,生物质焦水蒸气气化的主要气体产物有H2、CO、CO2和CH4,其中H2的含量最多,达到50%以上,而CH4的含量很少。热力学模拟结果表明气化反应器的最佳运行工况为温度850℃左右,H2O/C摩尔比1.5左右,CaO/C摩尔比应在2.0以上。生物质气化焦颗粒的孔隙表面具有分形特性,气化过程中分形维数与比表面积的变化趋势比较相似,均呈现出先增大后减小的变化规律。随着气化反应的进行,焦中无定形碳和脂肪侧链的含量在减小,其结构变得更加有序化,且颗粒内部晶粒直径也变得更大。更多还原

【Abstract】 With the excessive use of fossil fuels and the concerns over environmental protection, the resource utilization of biomass energy has attracted increasing worldwide interest. Agricultural residues are the main biomass resources available in China. However, at present most of agricultural residues are disposed in open fields, causing environment and public health problems. The research and development of pyrolysis and gasification technologies which can convert agricultural residues to high-quality energy contribute to establish sustainable energy system, promote the development of social economy and improve ecological environment. Based on the research progress and deficiency in biomass pyrolysis and gasification fields, maize stalk, rice straw, cotton straw and rice husk were used in this study as the representatives of Chinese typical agricultural biomass residues and the gas evolution patterns and formation mechanisms and char structural evolution during pyrolysis and gasification were studied in detail, which were essential to understand the fundamentals and mechanisms involved in biomass pyrolysis and gasification.Firstly, the pyrolysis characteristics of typical Chinese agricultural biomass were studied using thermogravimetric and derivative thermogravimetric (TG/DTG) analysis. The effect of heating rate was evaluated in the range of 5-50℃/min providing significant parameters for the fingerprinting of the fuels. Thus, the three-pseudocomponent model was used to simulate the pyrolysis behaviors of the materials studied. Model parameters of pyrolysis were given. The results showed that the thermal decomposition of agricultural biomass mainly occurred between 200 and 500℃. Higher heating rate increased the onset temperature and offset temperature of devolatilization and peak temperature for biomass pyrolysis. The maximum pyrolysis rate increased almost linearly with increasing heating rate. The three-pseudocomponent model with first-order kinetics had capability of predicting the pyrolysis behaviors of lignocellulosic biomass at different heating rates. Activation energy values varied between 98 and 114 kJ/mol for hemicellulose,132-186 kJ/mol for cellulose and 21-26 kJ/mol for lignin.The pyrolysis of agricultural biomasses was studied using a bench scale fixed bed reactor coupled with a Gasboard-3100 gas analyzer and a Gasmet Dx-4000 FTIR multicomponent gas analyzer. The release properties and formation mechnisms of gases produced during biomass pyrolysis were investigated. The influences of pyrolysis temperature, heating rate and residence time on the product yields and gas composition were analyzed in detail. The results showed that the major pyrolysis gases for agricultural residues were similar, including CO2, CO, methane, ethane, ethylene and some organics such as methanol, formaldehyde, formic acid, acetone, etc. The release of CO2 and CO was mainly caused by the cracking and reforming of carbonyl, carboxyl and ether groups. The formation of methane was mainly attributed to the rupture of methoxyl, methylene and aromatic rings. The aliphatic-CH2OH groups in-γposition of the alkyl side chains and aromatic methoxyl groups are the main source of methanol. Moreover, the reforming of free hydroxyl groups and C-0 bonds could also generate methanol. The formation of formaldehyde was probably caused by the Cβ-Cγcleavage in alkyl side chains that have-CH2OH groups or carboxylic acid groups in the-γposition. The products yields and gas composition showed a clear dependence on pyrolysis temperature and heating rate. The results showed that higher temperature led to the increase in gas yield and the decrease in char and liquid yields. As the pyrolysis temperature increased from 600 to 900℃, the CO and CH4 contents in the product gases increased obviously, while the CO2 content exhibited the opposite trend. Lower heating rate favoured the formation of CH4 and CO2, while higher heating rate favoured the formation of CO and C2H4.The structural evolution of biomass chars during pyrolysis was studied. The original samples and chars were characterized by ultimate analysis, FTIR, In-situ DRIFTS, ESEM, helium density measurement and N2 isothermal adsorption/desorption method. The results showed that the chemical composition and physicochemical structure of biomass chars were greatly affected by pyrolysis temperature. At high temperatures, the hydroxyl, aliphatic C-H, olefinic C=C and carbonyl groups in biomass chars were almost lost and the content of ether structure was very low, suggesting that the chars became progressively more aromatic and carbonaceous with increasing temperature. High temperature led to the occurrence of softening, melting and carbon structural ordering. Char structure ordering was responsible for thermal annealing and thus for thermal deactivation. Moreover, the structural shrinkage and pore narrowing occurred during biomass pyrolysis. The change of BET specific surface area during pyrolysis was similar for different agricultural biomass residues. The fractal dimension calculated by fractal Frenkl-Halsey-Hill (FHH) equation could represent pore structure satisfactorily. The fraction dimension was closely related to pyrolysis temperature. The change trends of the fractal dimension and BET specific surface area were similar during slow pyrolysis. However, the change trends of the two showed relatively large difference during fast pyrolysis.Finally, the steam gasification of biomass chars was studied in a bench scale fixed bed reactor. The release properties of gas products were analyzed on-line. Based on chemical equilibrium calculation, the influences of temperature, pressure, H2O/C molar ratio and CaO/C molar ratio on gasification process were studied. Then the structural evolution of biomass chars during gasfication was also investigated by applying ultimate analysis, FESEM, N2 isothermal adsorption/desorption method and XRD. The results showed that the major gases produced during biomass steam gasification were H2, CO, CO2 and CH4. H2 content in the product gases is the highest and it reached more than 50%, while CH4 content is the lowest. Gasification temperature had a notable impact on the release patterns and distribution of gas products. As the gasification temperature rised from 800 to 1000℃, the CO concentration in the produced gases significantly increased, the CO2 concentration decreased, and the CH4 concentration had little change. The change of H2 concentration mainly depended on the gasification reactions between biomass char and steam. The proper temperature, pressure, H2O/C molar ratio and CaO/C molar ratio of gasifier were 850℃, 0.1 MPa,1.5 and 2.0 seperately. The change trends of the fractal dimension and BET specific surface area were similar duing gasification. With the development of gasification, the content of amorphous structure and aliphatic side chains in biomass chars decreased and the crystallite diameter in these solids became bigger, indicating that biomass chars became more ordered with increasing carbon conversion.更多还原