Heat of wood pyrolysis

The heat of pyrolysis of beech and spruce wood was investigated by means of a differential scanning calorimeter. Wide variations were found for the heat of the primary pyrolysis process, depending on the initial sample weight and on the conditions used in the measurements. However, reporting the heat of the primary pyrolysis process versus the final char yield resulted in a linear correlation. This strong dependency of the heat of wood pyrolysis on the final char yield, that is in turn highly sensitive to the experimental conditions, can explain the uncertainty of the data for the heat of wood pyrolysis reported in the literature. A possible explanation for the variability of the heat of wood pyrolysis depending on the final char yield seems to be an exothermic primary char formation process competing with an endothermic volatile formation process.     

Flash pyrolysis of wood in a cyclone reactor

This paper reports the first results of an experimental study of the continuous flash pyrolysis of wood sawdust in a cyclone reactor between 893 and 1330 K. The reaction produces low fractions of char (4%) and the gasification yield increases from 0% at about 800 K to 90% at around 1330 K with a constant volume fraction of CO and H₂ (≈73%) and an increasing fraction of light hydrocarbons (up to 50% mass fraction). The heating value of the gas reaches 19 000 kJ m⁻³ STP for the highest temperatures. The wood particles mainly heated by radiation and solid convection react in less than 1 s while the carrier gas (residence time of the order of 0.05 s) seems to be only weakly heated. The 46.2 × 10⁻⁶ m³ cyclone reactor can operate with excellent stability for wood flow rates up to 0.35 kg h⁻¹ at a wall temperature of 1330 K. The cyclone seems to be very efficient for carrying out reactions of the solid → fluids type but more accurate determination of process parameters such as gas and solid residence times and heat transfer efficiencies are required to gain a better understanding of the behaviour of such a high temperature reactor.     

Analysis and modelling of wood pyrolysis

In many industrial processes wood is treated as big fragments or pellets. In such conditions kinetics and yields are different with respect to the case of particles with few mg weight. However most of published kinetic models were based on experimental data obtained with very small samples. In this work pyrolysis of wood pellets was investigated by using a special experimental device which allowed to determine kinetics of total weight loss, gas and tar production. Two different heating rates, 0.05 and 1 K/s, were employed to determine kinetic parameters. Dynamic and isothermal pyrolysis tests were carried out on beech and pine wood previously dried in an oven. A simple but realistic kinetic scheme was proposed able to take into account the phenomena that happen in big wood particles. The numerical parameters were determined from the results of experimentation on beech wood. The proposed kinetic model takes into account the presence of two different stages during pyrolysis: a first one involving only unreacted wood and a second one where the products not yet escaped from the solid matrix react further. This kinetic model allowed to fit the experimental data quite well. The model was successfully validated with tests performed at an elevated heating rate (approximately 60–100 K/s) of the external surface of the pellets. In these conditions, the pellets showed a marked gradient of temperature inside, which was suitably considered.     

Catalytic decomposition of methane over a wood char concurrently activated by a pyrolysis gas

The catalytic activity of a wood char towards CH₄ decomposition in a pyrolysis gas was investigated in a fixed bed reactor for maximising hydrogen production from biomass gasification. Wood char is suggested to be the cheapest and greenest catalyst for CH₄ conversion as it is directly produced in the pyrolysis facility. The conversion of methane reaches 70% for a contact time of 120 ms at 1000 °C. Because steam and CO₂ are simultaneously present in the pyrolysis gas, the carbon catalyst is continuously regenerated. Hence the conversion of methane quickly stabilises. Such a phenomenon is shown to be possible through the oxidation of the char by CO₂ and H₂O at high temperature, which prevents the blocking of the mouth of pores by the concurrent pyrolytic carbon deposition. In the experimental conditions, oxygenated functional surface groups are continuously formed (by steam and CO₂ oxidation) and thermally decomposed. The active sites for CH₄ chemisorption and decomposition are suggested to be the unsaturated carbon atoms generated by the evolution of the oxygenated functions at high temperature.     

Dimensional modelling of wood pyrolysis using a nodal approach

New gasification installations and techniques are being tested today but they all struggle with mainly the same drawbacks such as removal of various pollutants in the producer gas or clogging of material pathways.This work is oriented on developing a new model for the non-oxidative pyrolysis step of a gasification process as a part of a wider research conducted on the overall gasification of wood waste. A batch reactor is modelled by means of nodal modelling, a technique widely used for simple heat transfer processes. Additionally to the heat transport inside the batch reactor the model uses a simple and versatile generic chemistry and simplified mass transfer principles. Thermal data from modelling is compared with data obtained from an experimental batch pyrolysis reactor using wood sawdust and cutter shavings. Experimental and theoretical results regarding thermal phenomena are in good agreement.     

水热预水解分离相思木半纤维素及其组分与结构分析

通过水热预水解法处理相思木制得预水解液,采用乙醇沉淀回收预水解液中的半纤维素,通过条件实验优化了最佳回收半纤维素的预水解条件,然后对回收的半纤维素进行热稳定性、成分和结构分析。结果发现,获得最大半纤维素回收率的预水解条件为170℃/20min,原料中碳水化合物回收率为3.93%,回收的半纤维素中木糖为主要成分占75.56%,葡萄糖占10.71%,半乳糖占7.80%。离子色谱分析表明160℃/40min,170℃/15min、170℃/25min和180℃/15min水解条件下的回收半纤维素组分与170℃/20min时的回收半纤维素化学组分相同,但含量上略有差别;热重分析表明水解温度提高和时间延长使回收半纤维素热稳定性降低;核磁共振和红外光谱分析结果表明回收半纤维素的聚木糖主链上残留有乙酰基、阿拉伯糖和半乳糖连接。     

The flash pyrolysis of aspen-poplar wood

A continuous fluidized bed bench scale flash pyrolysis unit operating at atmospheric pressure and feed rates of about 15 g/h has been successfully designed and operated. A unique solids feeder capable of delivering constant low rates of biomass has also been developed. Extensive pyrolysis tests with hybrid aspen-popular sawdust (105–250 μm) have been carried out to investigate the effects of temperature, particle size, pyrolysis atmosphere and wood pretreatment on yields of tar, organic liquids, gases and char. At optimum pyrolysis conditions high tar yields of up to 65% of the dry wood weight fed are possible at residence times of less than one second.     

Further aspects of powdered poplar wood liquefaction by aqueous pyrolysis

The liquefaction of powdered poplar wood by rapid pyrolysis in water at 350°C has been studied further. It differs significantly from the liquefaction of wood chips in that oil yields are improved by pressurisation of the reactor. Oil yields (chloroform soluble) are about 45 per cent and are sensitive to water/wood mass ratios and residence time. Gas, measured in a new handling system, is produced in 5–12 per cent mass yields and contains 86–90 per cent carbon dioxide and 10–13 per cent carbon monoxide. The oil and aqueous phases contain carboxylic acids (particularly acetic), phenols, cyclohexanones, cyclopentanones, benzyl alcohols and furfural.     

预水解强度对相思木硫酸盐法制浆黑液性质的影响

经水热预水解技术处理木质纤维素,木片的主要成分发生变化,对后续的化学制浆性能及制浆黑液性质带来影响。本实验采用不同预水解强度（P因子）处理相思木片,并对其硫酸盐制浆行为和黑液性质进行分析。研究结果表明,随着P因子的增大,粗浆得率和细浆得率逐渐降低;浆料的卡伯值逐渐升高。当P因子＜200时,水解后木片的黑液固形物含量由空白组的138.48 g/L升高到162.86 g/L;木素含量由58.57 g/L升高到72.68 g/L;残碱量由4.37 g/L迅速降低到2.07 g/L。当200＜P因子＜600时,黑液固形物含量由162.86 g/L降低到142.71 g/L;黑液中木素含量由72.41 g/L降低到59.16 g/L;残碱量由2.07 g/L降低到1.26 g/L。当P因子＞600时,黑液中固形物含量由142.71 g/L升高到156.95 g/L;木素含量由59.16 g/L升高到62.16 g/L;残碱量由1.26 g/L升高到1.37 g/L。不同条件下,黑液中无机物含量变化不大;黑液固形物的热值由对照组的13.71 MJ/kg提高到14.63~15.09 MJ/kg。     

Numerical investigation of turbulent non-premixed combustion of a wood pyrolysis gas

A fully compressible database of turbulent non-premixed flames of a wood pyrolysis gas is developed by means of direct numerical simulation (DNS). A reduced kinetic mechanism is used to model the combustion of a pyrolysis gas-air mixture. The instantaneous flame surface density evolution equation based on the concept of a displacement speed is examined. The normal component of the displacement speed is nearly constant with respect to curvature, while the curvature-related component tries to restore the flame front to a planar shape. The strain-rate term is mainly a source as the flame is mostly extended. The normal displacement is responsible for both positive and negative contributions to the flame area. The displacement/curvature term is primarily a sink, since it is dominated by its curvature component. Effects of strain and curvature are analyzed by considering their correlations with reaction rates. Reaction rates are enhanced with increased positive strain rates owing to an increase in the flame surface area and to a decrease in curvature. The analyzed results aid in the development of turbulent combustion models. Finally, a new model for a mean variance of the scalar dissipation rate, based on a scale similarity approach, is proposed and examined. A comparison with DNS results shows that the proposed model provides a significant improvement over existing models. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 15–34, May–June, 2007.     

Thermal-balanced integral model for pyrolysis and ignition of wood

The pyrolysis and ignition of wood is of great importance to understand the initial stage of combustion, helping control the occurrence and spread of unwanted building and forestry fires. The development of a thermal-balanced model is introduced for examining the analytical relationship between the ignition time and external heat flux. The critical heat flux, one of the important fire-retardant characteristics of combustible solid, is determined from a correlation study between the ignition time and external heat flux. One of the thermal-balanced integral models, considering the effect of surface heat losses, average absorptivity and moisture content, is employed to give the prediction of surface temperature rise, ignition time and ignition temperature of the Aspen. The results show that the model readily and satisfactorily predicts ignition temperature and ignition time of wood with different moisture contents.     

Decomposition and carbonisation of wood biopolymers—a microstructural study of softwood pyrolysis

Pyrolysis of softwood (spruce and pine) is investigated in the temperature range between 298 K and 1673 K within narrow intervals. A heating rate of 2 K/min and the use of thin wood sections allow one to fully preserve the original cellular wood structure without the formation of cracks. Thermal degradation of the wood biopolymers and evolution of the atomic/molecular and mesoscopic structure of the carbonaceous material is studied by wide-angle X-ray scattering, small-angle X-ray scattering and Raman spectroscopy. An isotropic and almost structureless material marks a clear transition region between 580 K and 620 K, where the crystal structure of the cellulose microfibrils is completely degraded and the scattering contrast based on the density difference between cellulose microfibrils and polyoses/lignin has fully disappeared. After the complete decomposition of the wood nanocomposite structure, the charring process commences with the formation of aromatic structures, and a strongly increasing scattering signal at small angles indicates the formation and growth of nanopores. The developing graphene sheets show a slight preferred orientation with respect to the oriented cellular structure of the material. It is concluded that the unique microfibril orientation of cellulose in the native wood cell wall might affect the carbonaceous material, in agreement with recent predictions in the literature.     

Phenolic compounds from vacuum pyrolysis of wood wastes

Various softwood and hardwood bark residues, primary sludges and softwood sawdust residues were processed by vacuum pyrolysis in a laboratory scale batch reactor. The pyrolysis oil, water, charcoal, and gas were recovered and analyzed. The pyrolysis oils were analyzed in details for their content in phenolic compounds after derivatization to their acetyl derivatives. The influence of temperature, heating rate, feedstock bed thickness, particle size and feedstock water pretreatment on the yield of phenols was investigated. The highest yield of phenols was obtained when hardwood bark was soaked in water for 48 hours and pyrolyzed at a temperature of 450°C and a heating rate of 10°C/min. Pyrolysis performance was evaluated in terms of total phenolic yield and composition.     

Heterogeneous and homogeneous reactions of pyrolysis vapors from pine wood

To maximize oil yields in the fast pyrolysis of biomass it is generally accepted that vapors need to be rapidly quenched. The influence of the heterogeneous and homogeneous vapor‐phase reactions on yields and oil composition were studied using a fluidized‐bed reactor. Even high concentrations of mineral low char (till 55 vol %) appeared not to be catalytically active. However, the presence of minerals, either in biomass or added, does influence the yields, especially by the occurrence of vapor‐phase charring/polymerization reactions. Contradictory, in the absence of minerals, homogeneous vapor‐phase cracking reactions were dominant over polymerization/charring reactions (400–550°C, 1–15 s). With increasing vapor residence time, the oil yield reached an asymptotic value, which decreased with temperature. At a vapor temperature of 400°C no decrease in oil yield was observed, but dedicated analysis showed that homogeneous vapor to vapor reactions had occurred. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char

Pine wood was pyrolyzed in a fixed bed reactor at a heating rate of 10 °C and a final temperature of 700 °C, and the resultant volatiles were allowed to be secondarily cracked through a tubular reactor in a temperature range of 500-700 °C with and without packing a bed of char. The thermal effect and the catalytic effect of char on the cracking of tar were investigated. An attempt was made to deconvolute the intermingled contributions of the char-catalyzed tar cracking and the char gasification to the yields of gaseous and liquid products. It was found that the wood char (charcoal) was catalytically active for the tar cracking at 500-600 °C, while at 650-700 °C, the thermal effect became a dominant mode of the tar cracking. Above 600 °C, the autogenerated steam gasified the charcoal, resulting in a marked increase in the yield of gaseous product and a significant change in the gas composition. An anthracite char (A-char), a bituminous coal char (B-char), a lignite char (L-char) and graphite also behaved with catalytic activities towards the tar cracking at lower temperature, but only L-char showed reactivity for gasification at higher temperature.     

A model for primary and heterogeneous secondary reactions of wood pyrolysis

A chain growth model for heterogeneous secondary reactions is developed for the pyrolysis of large wood particles and the parameters determined by nonlinear optimization. The model takes both the volatile retention time and cracking and repolymerization reactions of the vapours with the decomposing solid as well as sutocatalysis into consideration. The extent of the secondary reactions is strongly influenced by the time and the ratio of the autocatalytic (propagation) reaction rate to noncatalytic (initiation) reaction rate. The wood which has a higher value of the autocatalytic/noncatalytic ratio also has a higher exothermic heat of reaction and yields a higher amount of final char residue. This fact confirms the heterogeneous secondary reactions lead to carbon enrichment of the final residue and are accompanied with an exothermic heat of reaction. The lower activation energies of the initiation and propagation reactions as compared to primary reactions (competitive reaction model consisting of weight loss and char forming reactions) confirm autocatalysis in large particles. The sealed reactor studies of small quantities of fine wood samples show that heterogeneous secondary reactions and not lower heating rates in large particles are the main source of char formed during the thermal decomposition of large wood particles. The model predictions are in agreement with the weight loss and temperature versus time curves over a wide range of particle size and furnace temperatures.     

Fast pyrolysis of wood: direct measurement and study of ablation rate

Quantitative measurements of the apparent rate of reaction of wood rods undergoing fast pyrolysis by contact with a hot spinning steel disc are reported. The variations in the apparent volatilization rate and in the thickness of the reacting layer were studied as a function of disc velocity, rod diameter, contact pressure and disc temperature. The results show that the rate of reaction is mainly limited by heat transfer outside and inside the wood (ablation conditions). The external heat transfer coefficient, studied as a function of contact pressure, is much more important than the heat transfer coefficient calculated on the basis of simple radiation transfer. The conclusion is that fast pyrolysis of wood in the ablation regime can be observed if two necessary conditions are fulfilled : high available heat flux and efficient removal of the primary products of the reaction.     

CO2 gasification of wood charcoals derived from vacuum and atmospheric pyrolysis

The gasification of biomass derived char obtained via vacuum and atmospheric pyrolysis of Populus tremuloides has been studied in the ranges of 725–960°C and 0.1 to 6 MPa. CO₂ was used as the oxidizing gas. The results show that char reactivity is influenced by the preheating rates and that pressure effects are significant between 850°C and 950°C. A correlation based on the expression: df/dt = k₀{exp(-E/RT)}(1 - f)^af^βP^yCO₂ was used to fit the experimental data. In general, vacuum pyrolysis derived char showed a higher reactivity than atmospheric pyrolysis chars. An explanation based on a higher oxygen content of the vacuum pyrolysis char is suggested.     

生物质热解油在木材工业胶粘剂中的应用研究进展

介绍了生物质热解油的来源和组成,综述了热解油在酚醛树脂(PF)、脲醛树脂(UF)、淀粉胶粘剂以及结构胶等方面的应用研究进展,并总结了其在目前研究和应用中存在的问题。最后对热解油基木材工业胶粘剂的发展方向进行了展望。     

Flaming pyrolysis model of the fixed bed cross draft long-stick wood gasifier

The future industrial development of biomass energy depends on the application of renewable energy technology in an efficient manner. Of all the competing technologies under biomass, gasifiers are considered to be one of most viable applications. The use of biomass fuel, especially biomass wastes, for distributed power production can be economically viable in many parts of the world through gasification of biomass. Since biomass, is a clean and renewable fuel, gasification gives the opportunity to convert biomass into clean fuel gas or synthesis gas for industrial uses. The preparation of feedstock for a gasifier requires time, energy and labour and this has been a setback for gasifier technology development. The present work is focused on gasification of long-stick wood as a feed material for gasifiers. This application makes reduction not only in the cost but also on the power consumption of feed material preparation. A 50 m³/h capacity gasifier was fabricated in the cross draft mode. The cross draft mode makes it possible to produce low tar content in producer gas. This cross draft mode operates with 180 W of blower supply for air to produce 10 kW of thermal output. The initial bed heights of the long-stick wood and charcoal are 58 cm and 48 cm respectively. Results were obtained for various flow conditions with air flow rates ranging from 20 to 30 m³/h. For modelling, the flaming pyrolysis time for long-stick wood in the gasifier is calculated to be 1.6 min. The length of the flaming pyrolysis zone and char gasification zone is found to be 34 cm and 30 cm respectively. The rate of feed was between 9 and 10 kg/h. Continuous operation for 5 h was used for three runs to study the performance. In this study we measured the temperature and pressure in the different zones as a function of airflow. We measured the gas flow and efficiency of the gasifier in order to determine its commercial potential for process and power industries.