纤维素微波热解基础研究

为了改善生物质热解油品质,有必要开展微波选择性热解的基础研究。以微晶纤维素为模型,研究微波热解温度、微波吸收剂(催化剂)等条件对产物分布组成规律的影响,确定了呋喃类和糖类是裂解的主要产物。通过分析几种主要产物的变化行为和转化途径,认为微波特有的"体加热"机制促进了3,6-脱水吡喃糖开环路径、左旋葡萄糖二次脱水以及通过C2和C5的缩合形成呋喃结构葡萄糖苷。在此基础上,探讨了纤维素微波热裂解机理模式。     

钾元素对纤维素热解特性的影响

为考察钾元素及其存在形态对生物质热解特性的影响,实验用不同浓度的KAc和KCl溶液浸渍微晶纤维素,其热重结果表明,钾能促进纤维素低温分解、降低热解反应速率并使固体焦产率增加,能降低纤维素热解的表观活化能,活化能随钾添加量的增加而降低。通过对KCl、KAc浸渍纤维素的热重-红外(TG-FTIR)分析结果表明,钾能使纤维素热解向低小分子产物转化,但其作用能力受到添加盐种类的影响,KAc对热解反应温度、产物的影响显著大于KCl,使纤维素热解分为两段,而KCl的作用能力易受到添加量的限制。生物质中以有机结合态存在的钾对热解的过程的影响大于以无机态存在的钾。     

钾盐对纤维素热解特性的影响

利用微型热解系统,研究500℃终温条件下,钾盐(K2SO4,KNO3,K2HPO4,KOH)对纤维素热解特性的影响。收集炭、焦、重油、轻油、气5种产物,计算各产物产率,分析结果表明,钾盐添加作用下,钾对热解存在主要影响,盐本身理化特性存在次要影响。利用气质联用仪(GC-MS)对重油和轻油进行分析,发现糖、苯类物质主要集中于重油,而醛、酮类物质主要集中于轻油,钾盐显著降低了糖类产物产率,增加了酮类、环烷烃类物质产率。利用X射线光电子能谱仪(XPS)对浸渍样、热解炭、焦含碳官能团进行分析,结果表明,热解后,热解炭与热解焦具有类似的含碳官能团分布,但热解炭中以芳香环中的C=C官能团为主,含氧官能团主要为羰基-CO-而热解焦中另外含有-COO-官能团。     

有机钙盐预处理纤维素的热解试验研究

摘要：在管式炉上进行了预处理纤维素（CaFA纤维素）的热解实验,研究了预处理对纤维素热解特性的影响。样品红外压片分析显示预处理影响了纤维素组成单元吡喃环的稳定性,且CaFA纤维素出现了明显的羧酸根官能团振动。热解实验表明：预处理使得纤维素的半焦和气体产率增加,生物油产率下降。CaFA纤维素最大生物油产率为0.496(g/g),相比未处理纤维素最大生物油产率降低19.1%。CaFA纤维素的气体产物中,CO含量减少,而CO2、CH4和H2含量增加,一定程度上提高了热解气相产物中的氧含量。GC-MS分析表明预处理对纤维素生物油组分具有明显的选择性,CaFA纤维素生物油中,大分子糖类及其衍生物的相对含量显著减少,而小分子酮类物质明显增加。     

纤维素热解的TG-FTIR分析

以生物质主要组分纤维素为原料,在热重-红外光谱联用仪上对纤维素分别以5,10,20,40℃/min的升温速率进行了热解实验研究,考察了纤维素的热解特性及轻质气体析出规律。结果表明:较高的升温速率能促进热解反应的进行,升温速率可作为影响最大热解失重速率对应温度(Tp)的一个重要因素,Tp会随着升温速率的增大而升高;纤维素热解过程中,热解气体的最大析出峰都对应于给定升温速率下的DTG失重峰;4种主要轻质气体(H2O,CO,CO2和CH4)均表现为双峰特性,且CO气体在热解后期的析出规律与CO2,H2O和CH4气体的析出规律不同;不同官能团键的断裂和重整,致使小分子气体组分和析出量的差异很大,热解过程中,羰基(C=O)和醚键(C-O-C)的断裂对CO2的生成影响显著;在低温区间CO的析出主要源于C-O-C的断裂,而在高温区间二芳基醚的分解则是CO产生的主要原因;CH4气体的析出主要由甲氧基(CH3O-)的伸缩振动引起。     

纤维素的加压热解特性及动力学研究

利用加压热重仪对纤维素进行了热重分析实验,获得了不同升温速率(5,10,20 K/min)和不同压力(0.1,0.5,1,1.5,2 MPa)条件下的热重曲线TG和失重速率曲线DTG,并通过热分析数学方法获得了热解动力学参数。结果表明,在各压力条件下,提高升温速率,纤维素主热解区间均往高温区移动,热解略有加深;在各升温速率条件下,增大压力,主热解区间均往低温区移动,热解时间缩短,剩余残渣百分比增大;在同一升温速率下,随着压力的增大,热解活化能增大,且升温速率越大,活化能随压力增大越明显;在同一压力下,随着升温速率的提高,热解活化能增大,且压力越大,活化能随升温速率增大趋势越明显;在各条件下热解活化能和指前因子存在着较好的补偿效应。     

流化床中单颗粒纤维素热解模型研究

为了研究生物质热解过程，该文对纤维素这种生物质中主要组份的流化床热解过程进行了数值模拟。模型在合理选取动力学模型的基础上考虑了单颗粒纤维素在流化床热解过程中由扩散和对流所引起的热量传递，包括了各种重要的气、液相热解产物的质量传递以及颗粒内部压力对过程的影响。计算结果显示，即使是对非常小的颗粒，热解反应热对热解过程的影响也至关重要；而无论是在大颗粒还是小颗粒中，热解液相中间产物流动对能量、质量传递的影响以及挥发份参加颗粒内二次反应的份额则可以忽略。计算还得到不同粒径颗粒热解的产物分布。总体来说，该模型为我们提供了一个探究纤维素热解细节的机会。计算结果可以为实际热解反应器的设计和运行提供依据。     

各种因素对煤加压热解影响的试验研究

在新研制的双炉膛加压热重分析仪上进行了韩桥烟煤和阳泉无烟煤的加压热解试验,研究了各种因素（如温度,粒径,加热速率等）在压力下对煤热解行为的影响,并对此作出新的合理的解释。     

常压及加压条件下生物质热解特性的热重研究

在常压热重分析仪和自行研制的加压热重分析仪上进行了生物质热解特性的系统研究 ,得到了升温速率、压力等因素对生物质热解过程的影响规律。对不同试验条件下的反应动力学参数进行了求解和比较 ,并作了机理分析。试验表明 :与常压热解相比 ,在加压条件下 ,生物质的反应速率有明显提高 ;随着升温速率的增加 ,热解反应趋于更加激烈。上述研究结果为生物质的合理利用提供了一定的理论基础。     

Evaluating the effect of potassium on cellulose pyrolysis reaction kinetics

This paper proposes modifications to an existing cellulose pyrolysis mechanism in order to include the effect of potassium on product yields and composition. The changes in activation energies and pre-exponential factors due to potassium were evaluated based on the experimental data collected from pyrolysis of cellulose samples treated with different levels of potassium (0–1% mass fraction). The experiments were performed in a pyrolysis reactor coupled to a molecular beam mass spectrometer (MBMS). Principal component analysis (PCA) performed on the collected data revealed that cellulose pyrolysis products could be divided into two groups: anhydrosugars and other fragmentation products (hydroxyacetaldehyde, 5-hydroxymethylfurfural, acetyl compounds). Multivariate curve resolution (MCR) was used to extract the time resolved concentration score profiles of principal components. Kinetic tests revealed that potassium apparently inhibits the formation of anhydrosugars and catalyzes char formation. Therefore, the oil yield predicted at 500 ^°C decreased from 87.9% from cellulose to 54.0% from cellulose with 0.5% mass fraction potassium treatment. The decrease in oil yield was accompanied by increased yield of char and gases produced via a catalyzed dehydration reaction. The predicted char and gas yield from cellulose were 3.7% and 8.4%, respectively. Introducing 0.5% mass fraction potassium treatment resulted in an increase of char yield to 12.1% and gas yield to 33.9%. The validation of the cellulose pyrolysis mechanism with experimental data from a fluidized-bed reactor, after this correction for potassium, showed good agreement with our results, with differences in product yields of up to 5%.     

纤维素热裂解试验研究及分析

利用自行设计的固定床热裂解试验系统,在不同压力条件下,研究了纤维素的热解行为,在常压下分别考察热解温度、N2流量对热解产物的影响。研究结果表明,热解温度为450℃时,可得到较高收率的液体产物,并且液体产物的收率随着N2流量的增加而降低。当压力降低时,在450℃热解温度下液体产物的收率最高,为58.6%,比常压热解提高12.3%,生物油中水分含量随着热解温度的升高而升高。试验对在不同真空度下热解得到的液体产物进行了元素分析。     

The kinetics of vapour-phase cellulose fast pyrolysis reactions

Biomass fast pyrolysis reactions consist of primary activation and fragmentation reactions, followed by secondary vapour-phase cracking reactions. Kinetic data derived from in-house experiments and published literature have clearly indicated that under true fast pyrolysis conditions, the primary reaction rates exceed those of the secondary reactions by several orders of magnitude. Therefore, since the cracking reactions are rate-limiting, an estimation of the rate of conversion of biomass to secondary products is in fact an estimation of the secondary reaction rate. This paper focuses on the determination of the key kinetic parameters (rate constants, pre-exponential constant and activation energy) for the vapour-phase cracking reactions which occur during cellulose pyrolysis. The parameters were determined using a first-order kinetic model and a non-linear regression routine. The experimental work was conducted in the Ultrapyrolysis equipment at the University of Western Ontario in London, Canada.     

The oil bath thermal cycling absorption process design and separation process research

Thermal cycling absorption process (TCAP) has been developed for years to support the separation of hydrogen isotopes, which has the characteristics of high separation efficiency and high recovery rate. The design of separation column structure, heating and cooling (H&C) system and technological parameters are the basis of TCAP technical process study and are the key points of TCAP engineering research. In this work, an improved separation system has been designed and built based on an oil bath H&C system for the first time. The separation column in this facility is 45 m long and the packing weight in the column is up to 8 kg. The separation experiments were carried out based on this facility, and the process parameters were adjusted according to the size of the separation column, which proved the superior performance of this facility. The separation experiments show that for 50% D₂ - 50% H₂ feed gas, the deuterium abundance can reach to 99% and the steady state extraction can be realized in production mode with the processing capacity over 400 standard L per day. Another experiment has been carried out with 1% D₂ - 99% H₂ feed gas, and the deuterium abundance exceeded 10%, verifying the separation ability at low abundance deuterium feed gas. Furthermore, the extraction rate can reach to 25% column capacity when the deuterium abundance in production gas is 5%.     

Numerical simulation of cellulose pyrolysis

A mathematical model of transport phenomena and chemical processes of the thermal degradation of cellulose is presented. The kinetic model developed by Bradbury et al. (J. Appl. Polym. Sci. 23, 3271, 1979) for primary pyrolysis is extended to include secondary reactions of volatiles:

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From the physical point of view, the model describes convective, conductive and radiative heat transfer, mass convection and diffusion and velocity and pressure variations interior to the porous solid (Darcy law). Furthermore, porosity, mass diffusivity, permeability and thermal conductivity vary with the composition of the reacting medium. Time and space evolution of the main variables, and reaction product distribution, are simulated by varying the reactor temperature and the reactor heating rate.     

Evaluation of process conditions for methane pyrolysis applying the triple thermal plasma system

Among various thermal plasma sources for CH₄ pyrolysis, singular direct current (DC) thermal plasma has limited capacity, erosion, and efficiency. However, the triple thermal plasma system has been applied for nanomaterial synthesis and exhibited results overcoming these limitations. This study used the triple thermal plasma system to investigate the most suitable conditions for CH₄ pyrolysis. CH₄ conversion rate and selectivity of H₂ and C₂H₂ were analyzed by varying the CH₄ flow rate and quenching conditions at a fixed power supply of approximately 30 kW, and the specific energy requirement (SER) per 1 kg H₂ was compared with that of previous works. The maximum conversion rate was 97% at 50 L/min of CH₄, which is approximately 7% higher than earlier studies under conditions with similar process enthalpy. In addition, the conversion of CH₄ to C₂H₂ and further to heavier hydrocarbons proceeded one order of magnitude faster than the reaction time expected by the gas-phase reaction. This result is attributed to the easy penetration of CH₄ into the core region with the highest temperature and the strong interaction between the processing gas and graphite surface due to the arrangement of the torches in the triple plasma system. C₂H₂ selectivity was relatively high, while it was less affected by the increase in the quenching gas than generally expected. This finding was attributed to the naturally fast quenching rate without quenching gas due to the structure expanding from the first to the second graphite. While quenching can enhance selectivity by stabilizing the radicals as intermediates such as H₂ or C₂H₂, it depressed the following reaction with dehydrogenation. Thus, the quenching conditions must be optimized. Finally, we demonstrated that the triple thermal plasma enhanced CH₄ pyrolysis regarding H₂ production efficiency.