两种玉米秸秆热解过程动力学模型的比较

介绍了生物质热解的简单一级动力学模型和分布活化能模型,并利用这两种模型分别对玉米秸秆在15、25、30K/min升温速率下的热重分析数据进行了研究。利用简单一级动力学模型计算的活化能数值在7～54kJ/mol之间,指前因子在2.8×10~4～3.3×10~4min~(-1)之间;利用分布式活化能模型计算的活化能数值在65～80kJ/mol之间,指前因子在1.9×10~5～3.0×10~5min~(-1)之间。同时,分析了产生上述差异的原因,并通过研究分布活化能模型的计算结果得出分布活化能模型更能反映生物质热解动力学过程的结论。     

Pyrolytic characteristics and kinetic studies of agricultural wastes—Four kinds of grasses

Several grasses are among the agricultural wastes generated annually in a large quantity during farming activities. In this study, thermogravimetric analysis was used to evaluate the fuel properties of a number of grasses, including Aeluropus sinensis, Conyza canadensis, Imperata cylindrica, and Setaria viridis. The pyrolysis behavior was also compared with the other biomasses. The thermal reaction systems fitted well with the distributed activation energy model and the global kinetic model and it is obvious that the values of E and k₀ calculated by the distributed activation energy model were much higher than that of the global kinetic model.     

Isothermal and non-isothermal kinetic study on CO2 gasification of torrefied forest residues

CO₂ gasification of torrefied forest residues (birch and spruce branches) was investigated by means of a thermogravimetric analyser operated non-isothermally (400–1273 K) and isothermally (1123 K) under the kinetic regime, followed by kinetic analyses assuming different models. For the non-isothermal gasification, the distributed activation energy model (DAEM) with four or five pseudo-components was assumed. It is found that the severity level of torrefaction had great influences on gasification behaviour as well as devolatilization step. The activation energy of non-isothermal gasification step of three samples varied in the range of 260–290 kJ/mol. The char reactivity decreased with increased torrefaction temperature. For the isothermal gasification, the random pore model (RPM), shrinking core model (SCM), and homogeneous model (HM) were tested. The result has confirmed the trend of decrease in char reactivity with increased torrefaction temperature observed from the non-isothermal gasification. However, different trends in char reactivity due to different wood types were observed by the two methods of gasification.     

Thermal decomposition behavior of tobacco stem Part II: Kinetic analysis

As a continuation of the previous study on the thermal degradation behavior of tobacco stem, this work is focused on the kinetics of pyrolytic decomposition. Thermogravimetric analysis of tobacco stem samples was conducted under nitrogen atmosphere at different heating rates of 5, 10, 15, and 20°C/min at a temperature range of 25–1,000°C. The kinetic parameters, such as activation energy, pre-exponential factor, and reaction order, were determined by applying the Coats–Redfern method for the main pyrolysis occurred in the second zone by means of the decomposition of hemicellulose, cellulose, and lignin at a temperature range 180–540°C. In addition, the activation energy was calculated using various degradation models, including Kissinger, Friedman (FR), Flynn–Wall–Ozawa (FWO), and Kissinger–Akahira–Sunose (KAS). The average activation energy of tobacco stem was calculated to be 150.40, 230.76, 216.97, and 218.56 kJ/mol by the Kissinger, FR, FWO, and KAS models, respectively.     

Kinetic and thermodynamic analysis of iron oxide reduction by graphite for CO2 mitigation in chemical-looping combustion

Chemical-looping combustion (CLC) provides a platform to generate energy streams while mitigating CO₂ using iron oxide as a carrier of oxygen. Through the reduction process, iron oxide experiences phase transformation to ultimately produce metallic iron. To understand iron oxide reduction characteristics and optimally design the fuel reactor, kinetic and thermodynamic analyses were proposed, utilizing graphite. This study aims to evaluate the reduction behavior under the non-isothermal process of various mixture ratios of hematite and graphite via thermogravimetric analysis with simultaneously evaluating evolved gases using a Fourier transform infrared spectrometer. The Coats-Redfern model was employed to approximate the kinetic and thermodynamic parameters which assessed the different reaction mechanisms together with the distributed activation energy model (DAEM). The results revealed that the hematite-to-graphite ratio of 4:1 had the highest reduction degree and had three distinct peaks representing three iron oxide reduction phases. The zero-order reaction mechanism agreed with the experimental results compared with other reaction models. The thermodynamic analysis showed an overall endothermic spontaneous reaction for the three phases which signified the direct reduction of the iron oxides. The DAEM result validated a stepwise reduction of iron oxides to metallic iron. The study aids the optimal design of the CLC fuel reactor for enhanced system performance.     

废弃物干酒糟生物质热解特性及动力学研究

酒糟（DG）的组成成分以及在50~900 ℃范围内的热解进行研究。TG/DTG实验结果表明,DG的开始热解温度为137 ℃,热解温度在305 ℃时热解速率最快,为6%/min。傅里叶变换红外光谱（FTIR）数据表明,DG的主要气态产物为CO₂、CH₄、酮、醛、酸和胺。通过分布式活化能模型（DAEM）与无模型积分（Flynn-Wall-Ozawa,FWO）方法对DG的动力学行为分析发现,DG在热解初始阶段活化能为76.49 kJ/mol,平稳阶段活化能为160 kJ/mol。随着热解反应的进行,DG的热解活化能逐渐升高。     

猪粪热解特性及其动力学研究

在程序控温热重分析仪上进行了不同升温速率(10,20,30,50℃/min)的猪粪热解失重试验,获得了猪粪热解特性参数;采用分布活化能模型(DAEM)进行动力学分析,计算得到整个热解过程的活化能和频率因子的分布规律。结果表明,猪粪热解过程呈现失水干燥段、热解过渡段、挥发分析出段和碳化段,升温速率对猪粪的热解有一定的影响,表现为随升温速率的升高,DTG曲线向高温侧移动;动力学分析表明,猪粪热解活化能在52~113 kJ/mol变化,低于锯末、稻壳、稻秆、椰壳热解的活化能,说明猪粪较其他生物质易受热分解;同时猪粪热解的活化能和频率因子之间存在动力学补偿关系,但整个热解过程中这种补偿关系呈分段趋势。     

A weighted average global process model based on two−stage kinetic scheme for biomass combustion

This research study combustion kinetics of four biomass samples in China, red pine (Pinus tabulaeformis), corn straw (hybrid corn Zheng Dan-958), Bermuda grass and bamboo (Phyllostachys heterocycla var), using thermogravimetric analysis (TGA). Three stages of combustion process are identified as water evaporation, removal and combustion of volatile matters and combustion of char. Thermal kinetic parameters of each sample are calculated by using 1st order Coats–Redfern method based on the TGA data. It is found that the activation energy of the global process is in the range of 53.6–65.2 kJ mol⁻¹ with a poor linear correlation. The experimental data are then used to develop a two−stage reaction kinetic scheme with low temperature region (2nd stage) and high temperature region (3rd stage). The activation energy of the second stage is in the range of 123.5–140.5 kJ mol⁻¹, and that of the third stage was in the range of 59.4–93.4 kJ mol⁻¹, both of which were based on the 1st order Coats–Redfern method. Because the global process of actual combustion is different from the TGA, a modified weighted average model is proposed based on the two−stage reaction kinetic scheme. According to the modified model, the kinetic parameters of the global process for actual combustion are calculated and are all found a little smaller than that of the 2nd stages. That will benefit for the combustion simulation and the design of facility of biomass fuel.     

TGA and kinetic study of different torrefaction conditions of wood biomass under air and oxy-fuel combustion atmospheres

Combustion and oxy-fuel combustion characteristics of torrefied pine wood chips were investigated by Thermogravimetric Analysis (TGA). Three torrefaction temperatures (250, 300, and 350 °C) and two residence times (15 and 30 min) were considered. Experiments were carried out at three heating rates of 10, 20, and 40 °C/min. The isoconversional kinetic methods of FWO, KAS, and Friedman were employed to estimate the activation energies. The assessment of uncertainty in obtaining the activation energy values was also considered. The obtained results indicated that due to torrefaction, the O/C and H/C atomic ratios decreased, resulting the 300ºC-30 min and 350ºC-15 min torrefied biomass to be completely embedded in lignite region in van-Krevelen's diagram. Oxy-fuel combustion affected the decomposition of cellulose and lignin components of biomass while the impact on the hemicellulose component was negligible. The kinetic analysis revealed that with the evolution of conversion degree, the activation energy values increased during hemicellulose degradation, remained approximately constant during cellulose decomposition and showed a sharp decrease for lignin decomposition. The activation energy trends were comparable in both air and oxy-fuel combustion conditions, however slight changes in activation energy values were noticed. The highest activation energy value was obtained for 250ºC-30 min torrefied biomass at 183.40 kJ/mol and the lowest value was 72.93 kJ/mol for 350ºC-15 min biomass. The uncertainty values related to FWO method were lower than KAS and Friedman methods. The uncertainty values for FWO and KAS methods were at the range of 5–15%.     

Devolatilization Characteristics of Shenmu Coal Macerals and Kinetic Analysis

The pyrolysis of Shenmu coal macerals was performed in TG151 thermobalance. The volatile matter evolved in primary and secondary devolatilization and devolatilization kinetics were studied. The volatile matter evolved during primary devolatilization is the major part of the total volatile matter, especially for vitrinite. The percentage of volatile matter evolved during primary and secondary devolatilization suggested that inertinite have higher thermal stability. Though the heating rate can affect the percentage of volatile matter evolved during primary and secondary devolatilization, the order of volatile matter in all the temperature range is the same: vitrinite > parent coal > inertinite. The kinetic analysis of devolatilization using distributed activation energy model (DAEM) shows that the activation energy existed relatively large error at the conversion of initial 10% and final 10%. And the conversion of 10% to 90% was used to describe the variation of activation energy during pyrolysis. The activation energy of vitrinite appeared a minimum of about 50% conversion, but that of inertinite always increased as pyrolysis went on, indicating the different structure and chemical composition between them. Inertinite has higher activation energy and lower pyrolysis rate than vitrinite.     

Kinetics of the thermal decomposition of Eucommia ulmoides Oliver leaves and its fermentation products

The pyrolysis characteristics of Eucommia ulmoides Oliver leaves untreated and treated by biological fermentation were investigated using thermogravimetric analysis. The derivative thermogravimetry curves, kinetic characteristics, and activation energy of the samples were significantly different from other biomass. The average activation energies (253.40 and 268.77 kJ/mol, 257.71, and 270.82 kJ/mol) were close almost similar to that obtained from Kissinger–Akahira–Sunose and Flynn–Wall–Ozawa methods and were higher due to the presence of gutta-percha. Meanwhile, the decomposition and kinetic characteristics could be changed by fermentation technology. Besides, Coats–Redfern revealed that second-order model (f(α) = (1–α)²) could be used to better describe the decomposition mechanism of these samples.     

A comparative study of kinetics for combustion versus pyrolysis of Mesua ferrea husk,soya husk,and Jatropha curcas husk using thermogravimmetry and different methods

The combustion and pyrolysis kinetics of three selected biomasses generated as co-products of the oil and biodiesel industry – namely soya husk (SH), jatropha husk (JH), and Mesua ferrea husk (MH) – using thermogravimetric and differential thermal analysis techniques have been reported. These biomasses were initially characterized for basic fuel property, proximate analysis, ultimate analysis, and fiber analysis. The activation energy calculated using three different kinetic equations, viz. Coats and Redfern, Differential, and Friedman methods for combustion, were found higher than that for pyrolysis for all biomasses at the maximum airflow rate (20ml/min). However, the kinetic parameters are not sufficient alone to explain the suitability of the biomass.     

Pyrolysis and kinetic behavior of banana stem using thermogravimetric analysis

Pyrolysis and kinetic behavior of banana stem (BS) were investigated using thermogravimetric analysis (TGA). The behavior of mass loss demonstrated that pyrolysis process of BS appeared in three stages with a conversion range of 0–0.20, 0.20–0.90, and 0.90–1, respectively. The reaction mechanism of BS pyrolysis followed the 3D diffusion model, with an apparent activation energy range of 130.63–192.10 kJ/mol and a pre-exponential factor range of 2.42×10⁷–4.10 × 10¹⁰ s^–1. Stages 1, 2, and 3 were mainly attributed to pyrolysis of hemicellulose, cellulose, and lignin with mean activation energies of 139.09, 155.41, and 188.71 kJ/mol, respectively. The experimental data obeyed the iso-conversional model well with correlation coefficients (R²) over than 0.9928.     

Alkaline pyrolysis of anaerobic digestion residue with selective hydrogen production

Hydrogen (H₂) production from biomass has attracted the research attention as it is renewable and clean. This work investigates the alkaline pyrolysis (AP) of corn stover digestate (CSD) with sodium hydroxide (NaOH) to promote the production of H₂ and suppress carbon dioxide (CO₂) at moderate conditions. It is observed that the H₂ production is affected by the mass ratio of CSD to NaOH and reaction temperature. The H₂ yield is enhanced from 1:1 to 1:2 ratio of CSD to NaOH (10.9–25.9 mmol g⁻¹) with the purity of 81.21–84.98% at 500 °C, whereas a slight increase in H₂ production at 1:3 ratio of CSD to NaOH is observed which may attribute to the mass transfer matter. The possible mechanism of AP is identified. Through the thermogravimetric analysis (TGA), distributed activation energy model (DAEM) was applied which evidences the catalytic ability of NaOH via the reduced activation energies.     

玉米芯水解残渣热解焦ZnCl2活化动力学分析

为优化玉米芯水解残渣低温热解、ZnCl2活化制备活性炭的工艺过程,获取活化过程的优化工艺参数,在热分析仪上对残渣热解焦的ZnCl2活化过程进行了研究。通过对比ZnCl2溶液浸渍前后残渣热解焦的热重特性,分析了ZnCl2活化机理,采用分布活化能模型(DAEM)对浸渍热解焦活化过程进行了动力学分析。结果表明,经ZnCl2浸渍后的残渣热解焦热失重区间集中在450~650℃,且失重峰明显增强,固体残留质量大幅降低。DAEM计算结果表明,浸渍热解焦在活化过程中的活化能分布为190~280 kJ/mol,在E=210 kJ/mol时活化能分布函数达到最大值。     

Combustion characteristics of Turkish lignites at oxygen-enriched and oxy-fuel combustion conditions

Combustion and oxy-fuel combustion characteristics of two Turkish lignites (Orhaneli and Soma) were investigated by Thermogravimetric Analysis (TGA) method. Experiments were carried out under oxygen-enriched air and oxy-fuel combustion conditions with 21, 30, 40% oxygen concentrations. Three heating rates of 5, 10, and 20 °C/min were considered and the isoconversional kinetic methods of FWO, KAS, and Friedman were employed to estimate activation energies. The uncertainty assessment in obtaining the activation energy values was also considered. The obtained results indicated that the combustion of volatiles at both air and oxy-fuel conditions were approximately identical. However, at air combustion conditions, the decomposition of CaCO₃ took place at temperatures above 700 °C. This decomposition process was independent of the oxygen concentration and took place when the temperature reached to a certain threshold. The decomposition of CaCO₃ did not accomplish in oxy-fuel conditions as far as the temperature was higher than 900 °C. Combustion in oxy-fuel conditions had higher activation energy values comparing to conventional combustion atmosphere. The activation energy values were approximately unchanged at the start of combustion regardless of oxygen concentration or combustion atmosphere at about 165 kJ/mol and 150 kJ/mol for Orhaneli and Soma lignites, respectively. The apparent activation energies were higher at elevated oxygen concentrations. The uncertainties values related to FWO method were lower than KAS and Friedman methods. The calculated average uncertainty values were found to be at the range of 5–15% for most of the cases.     

Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review

The past decades have seen increasing interest in developing pyrolysis pathways to produce biofuels and bio-based chemicals from lignocellulosic biomass. Pyrolysis is a key stage in other thermochemical conversion processes, such as combustion and gasification. Understanding the reaction mechanisms of biomass pyrolysis will facilitate the process optimization and reactor design of commercial-scale biorefineries. However, the multiscale complexity of the biomass structures and reactions involved in pyrolysis make it challenging to elucidate the mechanism. This article provides a broad review of the state-of-art biomass pyrolysis research. Considering the complexity of the biomass structure, the pyrolysis characteristics of its three major individual components (cellulose, hemicellulose and lignin) are discussed in detail. Recently developed experimental technologies, such as Py-GC–MS/FID, TG-MS/TG-FTIR, in situ spectroscopy, 2D-PCIS, isotopic labeling method, in situ EPR and PIMS have been employed for biomass pyrolysis research, including online monitoring of the evolution of key intermediate products and the qualitative and quantitative measurement of the pyrolysis products. Based on experimental results, many macroscopic kinetic modeling methods with comprehensive mechanism schemes, such as the distributed activation energy model (DAEM), isoconversional method, detailed lumped kinetic model, kinetic Monte Carlo model, have been developed to simulate the mass loss behavior during biomass pyrolysis and to predict the resulting product distribution. Combined with molecular simulations of the elemental reaction routes, an in-depth understanding of the biomass pyrolysis mechanism may be obtained. Aiming to further improve the quality of pyrolysis products, the effects of various catalytic methods and feedstock pretreatment technologies on the pyrolysis behavior are also reviewed. At last, a brief conclusion for the challenge and perspectives of biomass pyrolysis is provided.     

Thermogravimetric characteristics and kinetics of sawdust pyrolysis catalyzed by potassium salt during the process of hydrogen preparation

Pyrolysis experiments of sawdust with KOH and K₂CO₃ catalysts were carried out under different heating rate in nitrogen atmosphere using thermogravimetric analyzer. The distributed activation energy model (DAEM) was used to analyze pyrolysis kinetics of sawdust. The results showed that both KOH and K₂CO₃ had strong catalytic effect on sawdust pyrolysis, which reduced the pyrolysis temperature of sawdust and increased the yield of char. There was only one main peak in DTG curve, which means that the pyrolysis behavior of cellulose and hemicellulose in sawdust was greatly changed. The catalytic performance of KOH was found to be more excellent in sawdust pyrolysis. Also KOH could catalyze the pyrolysis of sawdust at low temperature. The kinetic analysis results showed that the two kinds of catalysts could reduce the activation energy of sawdust pyrolysis and maintain a similar catalytic trend, but KOH had a more stable catalytic performance.     

模式搜索法在生物质热解动力学中的应用

生物质热解动力学模型参数的传统求解方法-积分法和微分法存在着理论上的限制,且对复杂模型存在困难。引入一种直接搜索全局优化方法-模式搜索法求解动力学模型参数,该方法十分有效。利用模式搜索法,提出采用经验二步连续反应模型描述小麦秸秆的热解动力学过程。因为考虑了中间产物的生成,使用了经验动力学模式函数,经验二步连续反应模型比单步全局模型及n阶二步连续反应模型拟合实验数据要好得多。图3表1参15     

Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis

The CO₂ gasification reactivity of pine sawdust chars (PS char) obtained from the different high-temperature pyrolysis is studied based on non-isothermal thermogravimetric method. Results show that the order of gasification reactivity is PS char-1073 > PS char-1273 > PS char-1473. Under the effect of high-temperature pyrolysis, the surface structure of biomass char is gradually destroyed and the pore structure parameters of specific surface area, total pore volume and average pore diameter increase. By means of the N₂ adsorption-desorption isotherms, it is seen that biomass char has more micro- and mesoporous at higher pyrolysis temperature. Besides, the PS char-1073 mostly has rich closed cylinder pores and parallel plate pores, and the PS char-1273 and PS char-1473 have plentiful open cylinder pores and parallel plate pores. An increase of pyrolysis temperature contributes to the development of porosity and improves diffusion path, which promotes the gasification reactivity. But, its effect on the decline of active site hinders the gasification reactivity. What's more, the kinetic model of distributed activation energy model (DAEM) is applied to calculate activation energy and pre-exponential factor with the integral and differential methods. The calculation results of integral method is more accurate and precise because the differential method is more sensitive than integral method for experimental noise. There is a compensation effect in the CO₂ gasification process.