An investigation of the grindability of two torrefied energy crops

The process of torrefaction alters the physical properties of biomass, reducing its fibrous tenacious nature. This could allow increased rates of co-milling and therefore co-firing in coal fired power stations, which in turn would enable a reduction in the amount of coal used and an increase in the use of sustainable fuels, without the need for additional plant. This paper presents an experimental investigation of the pulverisation behaviour of two torrefied energy crops, namely: willow and Miscanthus. A multifactorial method approach was adopted to investigate the three process parameters of temperature, residence time and particle size, producing fuels treated using four different torrefaction conditions. The untreated and torrefied fuels were subjected to standard fuel analysis techniques including ultimate analysis, proximate analysis and calorific value determination. The grindability of these fuels was then determined using a laboratory ball mill and by adapting the Hardgrove Grindability Index (HGI) test for hard coals. After grinding, two sets of results were obtained. Firstly a determination similar to the HGI test was made, measuring the proportion of sample passing through a 75 μm sieve and plotting this on a calibrated HGI chart determined using four standard reference coals of known HGI values. Secondly the particle size distributions of the entire ground sample were measured and compared with the four standard reference coals. The standard fuel tests revealed that temperature was the most significant parameter in terms of mass loss, changes in elemental composition and energy content increase. The first grindability test results found that the untreated fuels and fuels treated at low temperatures showed very poor grindability behaviour. However, more severe torrefaction conditions caused the fuels to exhibit similar pulverisation properties as coals with low HGI values. Miscanthus was found to have a higher HGI value than willow. On examining the particle size distributions it was found that the particle size distributions of torrefied Miscanthus differed significantly from the untreated biomass and had comparable profiles to those of the standard reference coals with which they had similar HGI values. However, only the torrefied willow produced at the most severe conditions investigated exhibited this behaviour, and the HGI of torrefied willow was not generally a reliable indicator of grindability performance for this energy crop. Overall it was concluded that torrefied biomass can be successfully pulverised and that torrefied Miscanthus was easier to grind than torrefied willow.     

烟气烘焙对玉米秆可磨性的影响规律研究

可磨性是生物质在现有燃煤机组规模化燃烧利用中必须考虑的问题之一。本文基于固定床反应器对玉米秆进行烘焙预处理,针对生物质单烧和与煤混烧两种技术路线,利用臼式研磨仪、全自动粒径筛分仪、纤维素分析仪和傅里叶红外光谱,研究了不同烘焙气氛、温度对燃料可磨性的影响。结果表明,相比于氮气,烟气能够在更低的烘焙温度下使玉米秆可磨性提升至接近于典型动力用煤,主要是由于烟气中的氧化性组分促进了纤维素、半纤维素的分解。当共磨时,煤颗粒表现出助磨的作用提升了玉米秆的可磨性,在较高温度或烟气烘焙条件下,混样的可磨性近似甚至优于煤。     

烘焙提升纤维素类生物质热解气化性能的研究进展

生物质作为唯一含碳的可再生能源受到广泛关注。由于生物质具有含水率高、氧含量高、热值低等特性,在生物质热解气化中存在热转化效率低、焦油含量高、产品气热值低等问题。烘焙预处理对于改善生物质原料特性和提升热解气化性能具有积极的影响。本文阐述了烘焙预处理技术对于纤维素类生物质原料的疏水性、可磨性、元素组成、能量密度以及热解气化中产生的产品气组分、焦油组分、产品气热值等方面的影响。原料经烘焙预处理后疏水性、可磨性增强,热值增加,提升了原料品质。同时,经烘焙预处理的纤维素类生物质原料可明显提高热解气化性能,产品气中可燃气体组分含量、产量以及热值得到提升,焦油含量明显下降,提高了热解气化的产品气燃烧性能和利用品质。下一步应开展烘焙与热解气化耦合工艺及应用模式研究,提高生物质热解气化的整体经济性和产品附加值。     

Torrefaction of non ‐lignocellulose biomass waste

Torrefaction of some non‐lignocellulose waste biomass was attempted to examine if such materials could benefit from this process as conventional lignocellulose biomass does. Experiments were conducted on chicken litter, digested sludge, and undigested sludge from a municipality in Canada. Effects of two important torrefaction process parameters: temperature and residence time on the torrefaction yield were studied. For reference, torrefaction of three lignocellulose biomass (switch grass, coffee husk, and wood pellet) was also carried out in the same apparatus under identical conditions. A comparison of torrefaction yield and other properties of these biomass showed that in spite of the large difference in their constitution the torrefaction behaviour of non‐lignocellulose and lignocellulose biomass were similar. The increase in energy density after torrefaction and the effect of temperature and residence time on torrefaction were also similar for these two types. The present research made an important addition to the existing database on torrefaction of biomass by adding missing information on torrefaction of sludge and poultry litter. Additionally, this work unearthed a potential option for production of composite pellets of waste (e.g., sludge) mixed with biomass (e.g., switch grass). © 2011 Canadian Society for Chemical Engineering     

烘焙对生物质理化性质及气化特性的影响

生物质资源丰富廉价,因清洁可再生、碳中和等优点备受研究者的关注,但是其能量密度低、水分和氧含量高等缺点也限制了其规模化应用;另外,生物质直接气化产生的合成气热值较低,且会产生大量焦油。本文阐述了烘焙预处理对生物质燃料品质的提升以及对气化过程积极的调控作用。文章指出,生物质烘焙后,氧元素含量、H/C和O/C下降,固定碳含量和高位热值增加;可磨性和疏水性得以提高,在一定程度上弥补了烘焙过程的耗能。文中从微观角度对生物质燃料品质的提升进行了解释,并简述了微波烘焙的特点与优势。使用烘焙生物质气化,产生的合成气可燃成分高,且焦油产量有所下降。文章总结后续工作可以考虑从以下三个方面展开,即对“烘焙-利用”过程进行全生命周期评价、利用微波技术更准确地探索温度对烘焙效果的的影响机制、结合烘焙与焦油催化重整技术进一步降低焦油产量。     

Effects of torrefaction process parameters on biomass feedstock upgrading

Biomass is a primary source of renewable carbon that can be utilized as a feedstock for biofuels or biochemicals production in order to achieve energy independence. The low bulk density, high moisture content, degradation during storage and low energy density of raw lignocellulosic biomass are all significant challenges in supplying agricultural residues as a cellulosic feedstock. Torrefaction is a thermochemical process conducted in the temperature range between 200 and 300 °C under an inert atmosphere which is currently being considered as a biomass pretreatment. Competitiveness and quality of biofuels and biochemicals may be significantly increased by incorporating torrefaction early in the production chain while further optimization of the process might enable its autothermal operation. In this study, torrefaction process parameters were investigated in order to improve biomass energy density and reduce its moisture content. The biomass of choice (corn stover) was torrefied at three moisture content levels (30%, 45% and 50%), three different temperatures (200, 250 and 300 °C), and three unique reaction times (10, 20 and 30 min). Solid, gaseous, and liquid products were analyzed, and the mass and energy balance of the reaction was quantified. An overall increase in energy density (2–19%) and decrease in mass and energy yield (3–45% and 1–35% respectively) was observed with the increase in process temperature. Mass and energy losses also increased with an increase in the initial biomass moisture content.     

Isothermal and Morphological Studies of the Torrefaction of Spruce Wood

The replacement of fossil fuels by biofuel for decreasing the action of greenhouse gases on the global climate is encouraged in industrially developed countries. A promising trend in the refining of waste biomass is torrefaction—a mild pyrolysis process in which biomass is heated to 250–350°C without the access of oxygen at low heating rates; as a result, biocoal with improved chemical and physical properties is formed. The torrefaction (mild pyrolysis at 250–300°C) of spruce stem wood was studied in a fixed-bed reactor at different temperatures. The mass and energy yields of biocoal, its specific heat of combustion, and morphological changes in the biomass structure in the course of spruce wood torrefaction were determined. It was established that the torrefied samples began to decompose at higher temperatures, as compared with the nontorrefied biomass. The torrefied fuel had a higher heat of combustion, which increased with the temperature of torrefaction. Conclusions on the restructuring of test samples and the formation of a porous structure at different temperatures depending on exposure time were made.     

Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties

Torrefaction is a treatment which serves to improve the properties of biomass in relation to thermochemical processing techniques for energy generation; for example, combustion, co-combustion with coal or gasification. The topic has gathered interest in the past two decades but further understanding is required for optimisation of the process thus enhancing economic efficiency, which is crucial to the success of the treatment commercially and within industry. In particular there is a noticeable gap in current literature regarding the combustion properties of torrefied biomass. This study examines torrefaction in nitrogen of two energy crops, reed canary grass and short rotation willow coppice (SRC), and a residue, wheat straw. Product evolution and mass and energy losses during torrefaction are measured using a range of laboratory scale methods. Experiments at different torrefaction conditions were undertaken to examine optimization of the process for the three fuels. Progress of torrefaction was also followed by chemical analysis (C, H, N, O, ash), and it was seen that the characters of the biomass fuels begin to resemble those of low rank coals in terms of the van Krevelen coal rank parameter. In addition, the results indicate that the volatile component of biomass is both reduced and altered producing a more thermally stable product, but also one that produces greater heats of reaction during combustion. The difference between the mass and energy yield was shown to improve for the higher torrefaction temperatures investigated. The combustion behaviour of raw and torrefied fuels was studied further by differential thermal analysis (DTA) and also, for willow, by suspending individual particles in a methane-air flame and following the progress of combustion by high-speed video. It is shown that both volatile and char combustion of the torrefied sample become more exothermic compared to the raw fuels, and that depending on the severity of the torrefaction conditions, the torrefied fuel can contain up to 96% of the original energy content on a mass basis. Upon exposure to a methane-air flame, torrefied willow ignites more quickly, presumably because its low moisture content means that it heats faster. Torrefied particles also begin char combustion quicker than the raw SRC particles, although char combustion is slower for the torrefied fuel.     

生物质烘焙处理的能量平衡分析

基于烘焙过程分析,建立了生物质烘焙处理的能量平衡计算方法,结合固定床烘焙实验结果,研究了烘焙温度和保温时间对杨树枝烘焙特性、烘焙所需热及3种烘焙产物能量分布的影响,探讨了杨树枝烘焙处理的能量平衡. 结果表明,烘焙温度是影响生物炭得率、热值和能量分布的主要因素,随烘焙温度提高,杨树枝炭得率显著降低,液体产物和气体产物逐渐增加,杨树枝炭和不凝气体产物的热值显著增加,而液体产物热值变化不大. 烘焙产物中,杨树枝炭的能量显著高于液体和气体的能量. 随烘焙温度升高,杨树枝烘焙的能量平衡率先快速升高后趋于平稳,在温度225℃、保温时间30 min的条件下,烘焙可获得64.0%的杨树枝炭,能量平衡率约为2.5,烘焙所得液体和气体产物的燃烧热完全能满足杨树枝烘焙能量自给的需要.     

Impact of torrefaction on syngas production from wood

Torrefaction is a way to treat biomass before transportation or thermochemical conversion. It can be used to increase the energy content of wood or to facilitate grinding. The purpose of this paper was to quantify the impact of such a treatment on the behaviour of wood during gasification by steam at high temperature to produce syngas. The aspects of both gas yields and reaction kinetics were considered.Beechwood was submitted both to light torrefaction and severe torrefaction, using a specially designed crossed fixed bed reactor. The initial wood and the torrefied woods were first characterised, then gasified in a new laboratory high-temperature entrained flow reactor (HT-EFR) at 1400 °C for 2 s in an atmosphere containing 20 vol% steam in N₂. The syngas produced was then analysed. The experiments were modelled using a thermo-dynamical equilibrium approach.It was confirmed that torrefaction decreased the O/C ratio. The quantity of syngas produced increased with the severity of the torrefaction. The equilibrium approach describes the results satisfactorily.Gasification experiments carried out at a lower temperature - 1200 °C - indicated that the chars from torrefied woods are less reactive towards steam than the char from wood.     

NMR analysis of the transformation of wood constituents by torrefaction

The injection of biomass in a pressurised entrained flow reactor is challenging. Biomass preparation by torrefaction before gasification could be a suitable option to improve it. Transformation of the material induced by this treatment lead to interesting features: increased brittleness, improved fluidisation properties of the powder, hydrophobicity, higher energy content. The major biomass constituents, cellulose, hemicelluloses and lignin are variously affected by torrefaction, depending on their respective reactivity. The objective of this work is to investigate the transformation of the biomass constitutive polymers induced by this thermal treatment. For that purpose, both solid-state NMR and EPR investigations have been performed on wood samples (beech) torrefied at different temperatures ranging from 200 °C to 300 °C. The results of these investigations have been compared with data obtained on untreated wood. These characterizations have brought to light different transformations of the polymers: de-acetylation of hemicelluloses, demethoxylation of lignin, changes in the cellulose structure. Furthermore, the temperature at which depolymerisation of the different components begins to occur has been identified.     

微波烘焙预处理降解玉米秸秆

用微波可高效对生物质烘焙预处理,考察了不同微波烘焙过程对玉米秸秆主要组分的降解作用及酸、碱、甘油催化剂对纤维素转化效率的影响,并对预处理的玉米秸秆进行酶解实验。结果表明,单纯的微波预处理对玉米秸秆中主要组分纤维素、半纤维素和木质素均有强烈的转化作用。无催化剂微波烘焙后,样品中纤维素含量降低了30%。在微波烘焙中添加酸、碱、甘油催化剂,可选择性降解玉米秸秆中的半纤维素或木质素,有效提高预处理后玉米秸秆中的纤维素含量,添加NaOH后纤维素含量增加最明显,由33%增至42%,纤维素最高转化率达65%。     

Improvement of grinding characteristics of Indian coal by microwave pre-treatment

The influence of microwave pretreatment on the grindability of high-ash Indian coal was investigated. Scanning electron microscope analysis characterized the micro fractures in microwave treated coal samples, which may be due to the thermal stress cracking resulting from the microwave pretreatment. X-ray diffraction analysis showed that the microwave treated coal had a higher peak compared to the untreated coal indicates that the crystallinity increased with the microwave exposure time. The calculation of the Hardgrove grindability index indicated an increase in the ease of grinding or an increase in the grindability index of the microwave treated coal. Bond grindability tests showed that the work index decreased 15.4% after a microwave exposure time of 120 s. Grindability tests showed that the microwave treated coal ground much more rapidly initially than the untreated coal, with the specific rate of breakage (S₁) increasing by an average of 15%. The results showed that the rates of breakage of both microwave untreated and treated coal samples were dependent on particle size.     

原料烘焙预处理对生物质气化的影响综述

简述了生物质原料的物理和化学特性,介绍了现有的生物质气化原料预处理方法,着重阐述了生物质烘焙技术研究现状和烘焙预处理对生物质气化的影响。众多研究结果表明：生物质烘焙预处理能够显著改善生物质的磨粉性能,有效提高生物质的能量密度和堆积密度,同时还能降低生物质的O／C比。最后对原料烘焙预处理在生物质气化中的应用做出了展望。     

烘焙生物质燃烧过程中钾的赋存形态及析出迁移特性

选取稻秆和棉秆为原料,在烘焙预处理后,通过固定床燃烧实验结合HSC Chemistry热力学平衡计算,获得了烘焙生物质燃烧过程中碱金属K的析出和迁移规律。结果表明：烘焙过程中存在着少量水溶性K的释放及其向醋酸铵溶态K的转化,而其转化和释放能力与Cl/K呈正相关。烘焙生物质的成灰率随着燃烧温度的升高而下降,而K的释放率随温度的变化则相反,其主要释放形式为K的氯化物和KOH;此外,600℃时,水溶性K和醋酸铵溶态K主要转化为char-K,700~900℃时其主要转化为K的硅酸盐,随着温度的上升,其转化量不断增加。相比于原生生物质,烘焙生物质有更高的成灰率;此外,烘焙促进了生物质燃烧过程中醋酸铵溶态K向酸溶态或残渣态K转化,同时抑制了水溶性K的释放,这些使得烘焙生物质燃烧过程中K的释放率更低,而烘焙对生物质燃烧过程中K释放的抑制与烘焙过程中Cl的释放率呈正相关。     

Effect of torrefaction on the density and volume changes of coarse biomass particles

This work provides preliminary data on the effect of torrefaction on the apparent density of biomass. It also examines how the shape of wood is affected by torrefaction. Experiments conducted on cylinders of poplar wood showed that torrefaction reduced both density and volume of the wood, and the extent of reduction increased with increase in severity of torrefaction. The shape of the wood appears to have some effect on the extent of density change. The shrinkage in radial direction was 3–4%, while reduction in longitudinal direction was 6.5–8.8%. The mass yield decreased with torrefaction severity. © 2013 Canadian Society for Chemical Engineering     

Effect of carbonization temperature on the grindability of carbonaceous material produced from different coals

Carbonization experiments were conducted on four kinds of sub‐bituminous coal particles at a temperature range of 450–1200 °C. The effect of treating temperature on the grindability of produced carbonaceous materials was investigated, and its mechanism was analyzed through various means. Results show that the grindability of all coals, whether caking or slightly‐caking, exhibit the same variation trend with an increase in carbonization temperature. Moreover, the entire process can be divided into four stages. (1) After the most intense devolatilization stage, the grindability of carbonaceous material exhibited different degrees of increase compared with that of raw coal because of the development of a pore structure. (2) The first significant decrease in the grindability occurred from the plastic stage to the complete resolidification of the coal matrix. (3) After the aromatic polycondensation stage accompanied by a large amount of H₂ release, the coal molecular structure became compact, such that the grindability of semi‐coke considerably decreased again. (4) At high temperatures, the coal matrix underwent graphitization, which changed semi‐coke to coke. The molecular structure of coal became ordered, and the grindability decreased again. The analysis shows that a change in the internal chemical structure of carbonaceous material has a much more pronounced effect on grindability than a change in its pore structure, except in the first stage. The constant compaction and regularization of the coal molecular structure continued happening throughout the entire process and play a decisive role in the change in grindability.     

Gasification performance of torrefied Timothy hay and spruce wood chars in a CO2 environment

Timothy hay abundantly available in New Brunswick, Canada, is mostly used for animal feed and bedding. Upgrading biomass using Torrefaction method can offer benefits in its waste management, energy density and energy conversion efficiency. Temperature and residence time play an important role in the torrefaction process. Meanwhile, CO₂ gasification is also a promising thermochemical conversion process due to its potential to reduce net GHG emissions and tune syngas composition. This study investigates the impact of torrefaction parameters on isothermal and non-isothermal CO₂ gasification of Timothy hay and spruce chars. Timothy hay chars exhibited higher CO₂ gasification reactivity than chars from spruce. The physicochemical properties analysis indicated that higher reactivity of Timothy hay char was mainly attributed to the high amount of alkali and alkaline earth metal (AAEM) content, relatively large BET surface area, a high number ofactive sites, and a low crystalline index. Moreover, in both experimental cases, char derived through a high heating rate and high residence time conditions exhibited improved gasification performance, which was attributed to the generation of large amounts of AAEM (Ca and K) and high specific surface area. Co-gasification results during non-isothermal processes under CO₂ showed the presence of larger interactions in coal char/Timothy hay char blends than that of coal char/spruce char blends. For both experimental conditions, interactions were enhanced once the char prepared from high heating rate and high residence time was gasified with coal char. Thus, the proposed approach is a sustainable way of conversion of Timothy hay under CO₂ environment.     

Optimization of pre-treatment selection for the use of woody waste in co-combustion plants

This work is focused on the use of biomass waste to feed already existing coal combustion plants as a part of paving the way toward the reduction of the environmental impact. The biomass waste supply chain optimization is critical to conceive long-term viable projects and deal with the biomass heterogeneous nature and drawbacks to be used with coal, i.e. principally high moisture content and low bulk density. This paper studies biomass transportation, storage and change of properties (moisture content and hence dry matter, energy density and bulk density) through the use of different pre-treatments: (i) torrefaction, (ii) torrefaction combined with pelletization, (iii) pelletization, (iv) fast pyrolysis and (v) fast pyrolysis combined with char grinding, which produce a range of very different pre-treated biomass. The optimization problem is formulated as a mixed integer linear program (MILP) that evaluates the net present value and the environmental impact through a life cycle assessment (LCA). The results propose location–allocation decision together with the selection/capacity of pre-treatment technologies for each scenario. The scenarios contemplate different biomass characteristics, availability and distribution for a supply chain case study located in Spain: forest and agricultural woody residues used to replace at least 10% of the total thermal inlet power provided by coal in the existing network of thermal plants.     

An investigation into the effect of biomass particle size on its torrefaction

Present work examines an important practical aspect of torrefaction that is, the effect of size and shape of biomass on torrefaction. Experiments were conducted on different sizes of Poplar and Oak wood in directly heated torrefaction reactors. Experiments were conducted with 13, 19 and 25 mm diameter Poplar wood with lengths varying from 8 to 65 mm long. Most of the experiments were conducted at 250°C for 60 min in a directly heated convective reactor with limited few in a fluidised bed reactor. Results showed increase in mass and energy yield with increasing particle length but opposite with particle diameter. © 2012 Canadian Society for Chemical Engineering