Effect of cellulose, lignin, alkali and alkaline earth metallic species on biomass pyrolysis and gasification

Fundamental pyrolysis/gasification characteristics of natural biomass and acid-washed biomass without alkali and alkaline earth metals (AAEM) were investigated by a thermogravimetric analyzer (TGA) and a fixed-bed reactor. In these experiments, six types of biomass were used and the contents of cellulose, lignin and AAEM species in the biomass were measured. It was observed that the characteristic of biomass pyrolysis and gasification was dependent on its components and AAEM species on the basis of TGA experiments. During biomass pyrolysis, the tar and gas yields increased with the growth of cellulose content, but the char yield decreased. There were two reactions indicating two major decomposition mechanisms. The first stage of decomposition showed rapid mass decrease due to the volatilization of cellulose, while the second stage became slow attributed to the lignin decomposition. The higher the cellulose content, the faster the pyrolysis rate. In contrast, the pyrolysis rate of biomass with higher lignin content became slower. In addition, the rises of cellulose content elevated the peak temperature of gasification and prolonged the gasification time. Meanwhile, the effect of AAEM species on gasification behavior was studied by comparing unwashed and acid-washed biomass. AAEM species increased the peak gasification value, whereas decreased initial gasification temperature. It revealed that the activity of biomass gasification was attributed to the interaction between AAEM-cellulose/lignin.     

Performance assessment of drop tube reactor for biomass fast pyrolysis using process simulator

Biomass pyrolysis process from a drop tube reactor was modelled in a plug flow reactor using Aspen Plus process simulation software. A kinetic mechanism for pyrolysis was developed considering the recent improvements and updated kinetic schemes to account for different content of cellulose, hemicellulose, and lignin. In this regard, oak, beechwood, rice straw, and cassava stalk biomasses were analyzed. The main phenomena governing the pyrolysis process are identified in terms of the characteristic times. Pyrolysis process was found to be reaction rate controlled. Effects of pyrolysis temperature on bio-oil, gases, and char yields were evaluated. At optimum pyrolysis conditions (i.e., 500°C), a bio-oil yield of 67.3, 64, 43, and 52 wt.% were obtained from oak, beechwood, rice straw, and cassava stalk, respectively. Oak and beechwood were found to give high yields of bio-oil, while rice straw produced high gas and char yields compared to other biomasses. Although temperature is the main factor that plays a key role in the distribution of pyrolysis products, the composition of cellulose, hemicellulose, and lignin in the feedstock also determines the yield behaviour and composition of products. With the rise in pyrolysis temperature, further decomposition of intermediate components was initiated favouring the formation of lighter fractions. Comparably, species belonging to the aldehyde chemical family had the highest share of bio-oil components in all the investigated feedstocks. Overall, the present study shows a good agreement with the experimental study reported in the literature, confirming its validity as a predictive tool for the biomass pyrolysis process.     

Co-pyrolysis of pine cone with synthetic polymers

Biomass from pine cone (Pinus pinea L.) was co-pyrolyzed with synthetic polymers (PE, PP and PS) in order to investigate the effect of biomass and plastic nature on the product yields and quality of pyrolysis oils and chars. The pyrolysis temperature was of 500 °C and it was selected based on results from thermogravimetric analysis of the studied samples. Co-pyrolysis products namely gases, aqueous and tar fraction coming from biomass, oils from synthetic polymers and residual char were collected and analyzed. Due to the synergistic effect in the pyrolysis of the biomass/polymer mixtures, higher amounts of liquid products were obtained compared to theoretical ones. To investigate the effect of biomass content on the co-pyrolysis, the co-pyrolysis of pure cellulose as model natural polymer for biomass with polymer mixture was also carried out. In the presence of cellulose, degradation reaction leading to more gas formation and less char yield was more advanced than in the case of co-pyrolysis with pine cone. Co-pyrolysis gave polar oxygenated compounds distributed between tar and aqueous phase and hydrocarbon oils with composition depending on the type of synthetic polyolefin. Co-pyrolysis chars had higher calorific values compared to pyrolysis of biomass alone.     

热解温度对纤维素和木质素成炭结构的影响

从纤维素、木质素和半纤维素热解转化特征及分子重构建行为着手,利用TG、TEM、Raman、XRD、FT-IR等分析手段探究这3种物质的热解炭化机理。实验结果表明：半纤维素在炭化过程中几乎完全分解;链状结构的纤维素热分解脱除氢氧后,形成的碳自由基发生芳构化重排,大部分构成生物质热解炭中的结晶区;木质素分子结构复杂,呈交联态,在热解过程中同时发生软化熔融,大部分构成了生物质热解炭中的无定形区。在炭化过程中,纤维素在200 ℃之前主要发生脱水反应,200~400 ℃是热解的主要阶段;木质素在研究温度范围（200~500 ℃）内结构相对稳定,在软化熔融的同时仅发生部分结构转变。     

Influence of mineral matter on biomass pyrolysis characteristics

Studies on wood and twelve other types of biomass showed that in general, deashing increased the volatile yield, initial decomposition temperature and rate of pyrolysis. However, coir pith, groundnut shell and rice husk showed an increase in char yield on deashing, which is attributed to their high lignin, potassium and zinc contents. These results were supported by studies on salt-impregnated, acid-soaked and synthetic biomass. A correlation was developed to predict the influence of ash on volatile yield. On deashing, liquid yield increased and gas yield decreased for all the biomass studied. The active surface area increased on deashing. The heating value of the liquid increased, whereas the increase in char heating value was only marginal.     

Structural evolution of chars from biomass components pyrolysis in a xenon lamp radiation reactor

The structural evolution of the chars from pyrolysis of biomass components(cellulose, hemicellulose and lignin)in a xenon lamp radiation reactor was investigated. The elemental composition analysis showed that the C content increased at the expense of H and O contents during the chars formation. The values of ΔH/C/ΔO/Cfor the formation of cellulose and hemicellulose chars were close to 2, indicating that dehydration was the dominant reaction. Meanwhile, the value was more than 3 for lignin char formation, suggesting that the occurrence of demethoxylation was prevalent. FTIR and XRD analyses further disclosed that the cellulose pyrolysis needed to break down the stable crystal structure prior to the severe depolymerization. As for hemicellulose and lignin pyrolysis, the weak branches and linkages decomposed firstly, followed by the major decomposition. After the devolatilization at the main pyrolysis stage, the three components encountered a slow carbonization process to form condensed aromatic chars. The SEM results showed that the three components underwent different devolatilization behaviors, which induced various surface morphologies of the chars.     

The influence of temperature on the yields of compounds existing in bio-oils obtained from biomass samples via pyrolysis

The influence of temperature on the compounds existing in liquid products obtained from biomass samples via pyrolysis were examined in relation to the yield and composition of the product bio-oils. The product liquids were analysed by a gas chromatography mass spectrometry combined system. The bio-oils were composed of a range of cyclopentanone, methoxyphenol, acetic acid, methanol, acetone, furfural, phenol, formic acid, levoglucosan, guaiacol and their alkylated phenol derivatives. Thermal depolymerization and decomposition of biomass structural components, such as cellulose, hemicelluloses, lignin form liquids and gas products as well as a solid residue of charcoal. The structural components of the biomass samples mainly affect the pyrolytic degradation products. A reaction mechanism is proposed which describes a possible reaction route for the formation of the characteristic compounds found in the oils. The supercritical water extraction and liquefaction partial reactions also occur during the pyrolysis. Acetic acid is formed in the thermal decomposition of all three main components of biomass. In the pyrolysis reactions of biomass: water is formed by dehydration; acetic acid comes from the elimination of acetyl groups originally linked to the xylose unit; furfural is formed by dehydration of the xylose unit; formic acid proceeds from carboxylic groups of uronic acid; and methanol arises from methoxyl groups of uronic acid     

The effect of alkali metals on combustion and pyrolysis of Lolium and Festuca grasses,switchgrass and willow

The effect of alkali metals on the thermal degradation of biomass during combustion and pyrolysis has been investigated for 19 Lolium and Festuca grass varieties. These samples have been grown under the same conditions, but has been genetically mutated to give varying lignin contents in the range 2–6% measured by Klason. These grasses also have a high alkali metal content resulting in a high ash content. In order to compare the Lolium and Festuca grasses willow chip and switchgrass were also studied to act as a reference fuels. All samples were subjected to different washing conditions to investigate the effect of decreasing the metal content. The resulting biomass samples were studied for pyrolysis characteristics using thermogravimetric analysis (TGA) and pyrolysis gas chromatography–mass spectrometry (pyroprobe-GC/MS) and for combustion characteristics by TGA. A strong catalytic effect of metals, particularly potassium, was observed in both pyrolysis and combustion. Also, it was found that as the lignin content increases, the metal content (especially potassium and sodium) decreases. Furthermore, the char yield from pyrolysis (measured at 773 K from TGA pyrolysis traces) increases as metals increase, and hence char yield increases as the lignin content decreases. Py-GCMS showed that peak intensities varied for untreated and treated samples; in particular the levoglucosan yield is higher and the hydroxyacetaldehyde yield is lower for treated (low metal content) samples. This supports previous work mechanisms by Liden et al. in which alkali metals promote an ionic route that favours ring-scission and hydroxyacetaldehyde formation.     

Effects of organic and inorganic metal salts on thermogravimetric pyrolysis of biomass components

Thermogravimetric analyzer (TGA) was employed to elucidate the catalytic effects of organic and inorganic metal salts (K₂CO₃, KAc, Na₂CO₃ and NaAc) on the pyrolysis of three biomass components (cellulose, hemicellulose and lignin). In case of cellulose, TG analysis results showed that all the four metal salts increased the yield of char products and decreased the weight loss rates of cellulose pyrolysis, which followed the order of Na₂CO₃>K₂CO₃>NaAc>KAc. In contrast to cellulose, the four organic and inorganic salts employed had no significant effects on the remaining two biomass components:, hemicellulose and lignin. However, the four metal salts led to the devolatilization reaction of hemicellulose to occur at lower temperature region, and the dehydration reaction of lignin was promoted more or less. An increase in the heating rate might augment the maximum degradation rate. Different mixing ratios had little influence on the progress of catalytic pyrolysis. Based on the observations, the potential mechanism of the catalytic pyrolysis of biomass components with metal salts was discussed.     

基于PY-GC/MS的生物质组分间相互作用的热解实验

为研究生物质三组分间热裂解过程中的相互作用,利用热裂解-气相色谱/质谱联用仪（PY-GC/MS）联用的方法,对纤维素、木聚糖（半纤维素的模化物）和木质素进行单独热裂解及两两组分混合热裂解实验。单组分实验结果表明,在热解温度600℃、热解时间10s条件下纤维素的热解产物主要以左旋葡聚糖为主,木聚糖以乙酸和糠醛为主,而木质素主要以酚类物质为主。组分混合热裂解实验结果表明,纤维素促进了木聚糖热裂解生成更多的乙酸和糠醛,而木聚糖和木质素对纤维素热解生成左旋葡聚糖具有强烈的抑制作用;纤维素和木聚糖的存在大大促进了木质素热裂解生成酚类物质,而木质素抑制了木聚糖热裂解生成乙酸和糠醛。此外,研究还发现混合组分热解的相互作用受到热解温度和停留时间的影响。     

木塑复合材料热解特性

通过热重分析(TGA)和裂解/气质联用(Py-GC/MS)对杨木/高密度聚乙烯(HDPE)木塑复合材料(WPC)进行热解,考察了木粉和聚烯烃塑料热解过程中的相互作用。结果表明:生物质和塑料热解过程中存在明显的协同作用,杨木在较低的温度下即开始发生热解,其提供的自由基参与了聚烯烃热解反应,产生了更多的轻质烃类产物。而聚烯烃分解产生的碳氢化合物向生物质分解产生的自由基提供氢,促进挥发性物质生成,部分抑制了活性自由基进一步聚合结焦,得到了更多的挥发性产物和减少了固体残炭。     

生物质三组分在O2/CO2气氛下的着火行为研究

生物质的富氧燃烧技术结合了生物质燃烧与富氧燃烧的优点,既能减少化石燃料的使用,又易实现CO₂捕集。富氧燃烧的最显著特点是气氛中的氧气体积分数大于21%,其对生物质着火行为的影响至关重要。纤维素、半纤维素和木质素是生物质的3种主要组分,研究其在富氧条件下的着火及燃烧行为,可为生物质的着火及燃烧行为研究提供重要依据。利用滴管炉结合高速摄像机,研究了粒径74~154μm的纤维素、半纤维素和木质素在温度1273 K,氧气体积分数21%、30%、50%、70%和100%的O₂/CO₂气氛中的着火行为,并利用辐射能测温技术计算着火图片中的颗粒温度。结果表明,随着O₂体积分数增加,纤维素、半纤维素由联合着火以及木质素由均相着火均转为非均相着火,纤维素、半纤维素、木质素着火机理发生转化的O₂体积分数分别为30%、70%和50%。纤维素着火对O₂体积分数变化敏感,氧气体积分数超过30%时,纤维素焦率先发生着火。半纤维素和木质素的升温速率随氧气体积分数的升高而提高,半纤维素是由于挥发分在燃烧过程中随着氧气体积分数的增加,其燃烧比例减弱,焦燃烧比例增加,而木质素因为氧气体积分数的升高强化了木质素焦燃烧。半纤维素和木质素燃烧时间均随氧气体积分数的升高而缩短,两者都是由于氧气体积分数升高强化了焦的燃烧。另外,在较高氧气体积分数下木质素焦会发生熔融并膨胀,形成明显的膨胀火焰。     

Comparison of the thermal reactivities of isolated lignin and holocellulose during pyrolysis

Woody shells of Turkish hazelnuts which are rich in lignin content offer an important potential as a renewable energy source. Hence, this study focuses on the investigation of the thermal reactivities of the real macromolecular ingredients of this biomass species. Hazelnut shells were treated with chemicals to isolate its holocellulose (hemicelluloses + cellulose) and lignin. Scanning Electron Microscopy (SEM) images revealed the significant differences between the physical features of the untreated biomass and its isolated ingredients. Thermal properties of the biomass and these ingredients were examined by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) techniques under non-isothermal pyrolysis conditions from ambient to 900 °C. It was found that unlike holocellulose, lignin slowly decomposes in a wider temperature range, and its decomposition is associated with exothermic heat flow. It was also concluded that the hemicellulosics in holocellulose have very important effects with respect to the char yield and the exothermicity of the process. Besides, inorganics in biomass play a catalytic role during pyrolysis. The activation energies calculated according to Borchardt-Daniels' kinetic model were 64.8 and 51.8 kJ/mol for the pyrolysis of holocellulose and lignin, respectively, and each of them is higher than that for the untreated biomass.     

Characteristics of hemicellulose,cellulose and lignin pyrolysis

The pyrolysis characteristics of three main components (hemicellulose, cellulose and lignin) of biomass were investigated using, respectively, a thermogravimetric analyzer (TGA) with differential scanning calorimetry (DSC) detector and a pack bed. The releasing of main gas products from biomass pyrolysis in TGA was on-line measured using Fourier transform infrared (FTIR) spectroscopy. In thermal analysis, the pyrolysis of hemicellulose and cellulose occurred quickly, with the weight loss of hemicellulose mainly happened at 220–315 °C and that of cellulose at 315–400 °C. However, lignin was more difficult to decompose, as its weight loss happened in a wide temperature range (from 160 to 900 °C) and the generated solid residue was very high (∼40 wt.%). From the viewpoint of energy consumption in the course of pyrolysis, cellulose behaved differently from hemicellulose and lignin; the pyrolysis of the former was endothermic while that of the latter was exothermic. The main gas products from pyrolyzing the three components were similar, including CO₂, CO, CH₄ and some organics. The releasing behaviors of H₂ and the total gas yield were measured using Micro-GC when pyrolyzing the three components in a packed bed. It was observed that hemicellulose had higher CO₂ yield, cellulose generated higher CO yield, and lignin owned higher H₂ and CH₄ yield. A better understanding to the gas products releasing from biomass pyrolysis could be achieved based on this in-depth investigation on three main biomass components.     

Influence of particle size on the analytical and chemical properties of two energy crops

Two energy crops (switchgrass and reed canary grass) have been processed using ball mills and divided into two size fractions (<90 μm and 90–600 μm) and analysed using an array of analytical techniques including proximate and ultimate analysis, metal analysis, calorific value determination, and plant component analysis (cellulose, lignin and hemicellulose contents). The results indicate that smaller particles of the two grasses have a significantly higher concentration of inorganic matter and moisture content than larger particles. In contrast the larger size fractions had a higher carbon content, and lower nitrogen content, with a resulting higher calorific value. The volatile content was also higher in the larger size fraction. The composition of the organic content varied between the two size fractions, most noticeable was the difference in cellulose concentration which was approximately 50% higher in the >90 μm sample. Two laboratory scale techniques, thermogravimetric analysis (TGA) and pyrolysis–GC–MS (py–GC–MS), were used to study the significance of these differences in thermal conversion. In py–GC–MS of reed canary grass, and switchgrass to a lesser extent, the amounts of cellulose and lignin decomposition products were higher for the larger particle size fraction. The differences in cellulose contents were also apparent from the TGA studies, where different mass losses were seen in the cellulose decomposition region of the two size fractions. From the results of these two techniques it was concluded that the differences in ash, and therefore catalytic metal contents, between the two size fractions, resulted in lower pyrolysis temperatures, lower char combustion temperatures, and higher yields of catalytic pyrolysis decomposition products for the smaller size fractions. The implications of the results are discussed in terms of the bio-oil quality in fast pyrolysis and the predicted behaviour of the ash in combustion. It is suggested that pre-treatment by milling is one route that might be used routinely as a feedstock quality improvement strategy in integrated biomass conversion processes.     

Revealing low temperature microwave‐assisted pyrolysis kinetic behaviors and dielectric properties of biomass components

The kinetic characteristics of microwave‐assisted pyrolysis (MAP) of biomass components were investigated in a self‐designed microwave thermogravimetric analysis using the KAS model and the master plot method. Compared with conventional pyrolysis, the initial decomposition temperatures of biomass components were reduced by 50–100°C and the fastest weight loss regions were shifted to lower temperatures. The average apparent activation energies of cellulose, hemicellulose, and lignin were 47.82, 44.81, and 51.54 kJ/mol, respectively. Analysis with master plot method suggested the MAP of cellulose followed the 2‐D diffusion reaction model, while hemicellulose and lignin could be interpreted by third order‐based and 3‐D diffusion model. The change of dielectric properties was consistent with the weight loss behaviors of biomass components during the pyrolysis process. The increase of dielectric properties with temperature can lead to a thermal gradient and “hot spots” within biomass, which accelerated the pyrolysis process at low temperatures and reduced the apparent activation energy. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2124–2134, 2018     

Correlations of kinetic parameters in biomass pyrolysis with solid residue yield and lignin content

A kinetic analysis of the pyrolysis of various types of biomass (trunk, bark, leaf, shell, herbage, food dregs, and polysaccharide) as well as synthetic biomass consisting of cellulose and lignin was performed using thermogravimetric analysis data. The reaction rates of biomass pyrolysis were found to be expressed simply by a single nth-order reaction model. The kinetic parameters (frequency factor k₀, activation energy E, and reaction order n) were estimated first by differentiating the thermogravimetric curves and then by the nonlinear estimation method. The rate parameters of the pyrolysis of both 38 biomass samples and 9 synthetic biomass samples were successfully correlated in terms of the solid residue yield ω; charts are presented showing the correlations. Furthermore, a linear correlation was found between ω and the lignin content L in the woody biomass. This allows the kinetic parameters of biomass pyrolysis to be estimated using the value of ω, which is obtained from thermogravimetric measurements or estimated from the value of L for the biomass feedstock.     

Is it possible to predict gas yields of any biomass after rapid pyrolysis at high temperature from its composition in cellulose, hemicellulose and lignin?

Ligno-cellulosic biomass from different sources presents very variable compositions. Consequently, there is a wide variation in the nature and quantities of gaseous products obtained after thermal treatment of biomasses.The objective of this work is to establish a link between the composition of a biomass and its pyrolysis gas yields and composition. Experimental flash pyrolysis of several biomasses at a temperature of 950 °C and a gas residence time of about 2 s was carried out. An attempt was then made to predict gas yields of any biomass according to its composition. We show that an additivity law does not allow the gas yields of a biomass to be correlated with its fractions of cellulose, hemicellulose and lignin. Several potential explanations are then offered and quantitatively demonstrated: it is shown that interactions occur between compounds and that mineral matter influences the pyrolysis process.     

Detailed CFD modelling of fast pyrolysis of different biomass types in fluidized bed reactors

In the present study, an Eulerian‐Eulerian computational fluid dynamics (CFD) model, combined with a comprehensive biomass reaction scheme, was used to simulate fast pyrolysis of four different biomass types in the fluidized bed reactors. The study focuses on the influence of biomass components of different biomass types on the yields, formations, and contents of compositions of pyrolysis products. The result showed that the bio‐oil yield of cellulose‐rich biomass was higher than other biomass types, and char was mainly produced by the fast pyrolysis of LIG‐C of biomass. Moreover, the contents of bio‐oil components were affected by the fast pyrolysis of biomass components. Further, the energy recovery coefficient (ERC) of bio‐oil obtained from pyrolysis of different biomass types was also calculated and analyzed in this paper.