Assessment of the energetic and mechanical properties of pellets produced from agricultural biomass

This paper presents an assessment of the energetic and mechanical properties of pellets produced from agricultural biomass. For the production of pellets the following raw materials were used: wheat straw, rape straw, and maize straw. Additionally, the mixtures of wheat-rape straw, wheat-maize straw, and rape-maize straw (each accounting for 50% of the mass) were applied. The studied resources were ground with the use of a universal shredder driven by a 7.5 kW electric engine. A pelleting machine fitted with a fixed flat matrix with two driven thickening rolls was used to produce the pellets. Analyses of the moisture and calorific value of resources as well as the bulk density and mechanical strength of pellets were performed according to biding standards. The moisture of resources ranged from 16.5% to 18.5% for rape and maize straw, respectively. The average calorific value fluctuated between 15.3 MJ kg⁻¹ for a mixture of wheat and rape straw to 16.2 MJ kg⁻¹ for maize straw. The bulk density and mechanical strength of pellets depended on the type of resources used. The lowest bulk density was recorded for wheat straw pellets (386–420 kg m⁻³), and the highest (561–572 kg m⁻³) for maize straw pellets. The lowest mechanical strength of pellets was noted for rape (95.4–96.8%), whereas the highest was for pellets made from a wheat and maize straw mixture (96.8–98.9%).     

Influence of particle size and moisture content on tendency to bridge in biofuels made from willow shoots

Different systems for harvesting and storage of wood fuel from willow shoots give fuels with different particle size distribution, particle shape and moisture content. These factors influence the tendency for wood fuel particles to form a stable bridge over openings, which prevents the feeding of the fuel. The influence of cutting and storage method on the tendency to bridge was studied for chips and chunks made from 3 to 5 year old willow shoots harvested in January and December. Shoots were cut with four different machines to produce five fuel assortments with nominal particle length from 28 to 200 mm, and stored outdoors, in central Jutland, Denmark, in 160 m³ loose volume piles. Some piles were uncovered, some covered with plastic and two were sealed in an airtight silage plastic film enclosure. The bridging tendency was measured at the end of May and September by determination of how wide a “bridge” of fuel over a slot opening could be before it collapsed. With a 500 mm thick layer of fuel above the slot opening, the bridge width varied between 58 mm for the small chips and 977 mm for the large chunks. Most of the variation was due to two fuel properties, the proportion of particles longer than 100 mm and the moisture content of the fuel.     

利用近红外光谱技术测定生物质的水分含量

含水量是衡量生物质燃烧的一个重要指标,目前还没有快速、高效地测定生物质水分含量的方法。文章利用傅立叶近红外漫反射光谱(NIRS)技术,采用偏最小二乘法(PLS)建立了木质纤维素类生物质水分含量的校正模型。试验结果表明,当主成分数为6时,预测值与实测值之间的相关系数(Corr.Coeff)可达到最大值0.999 74,校正集均方根误差(RMSEC)达到最小值0.179,预测值和实测值之间的预测均方根误差(RMSEP)为0.518。可见,近红外光谱技术对于快速测定生物质水分含量是可行的。     

Effect of grinder configuration on forest biomass bulk density,particle size distribution and fuel consumption

The effect of the grinder configuration, bit type and screen size, on bulk density and fuel consumption when processing forest harvest residues for energy purposes is analyzed. Residues were divided in three size classes based on the piece diameter and length and were processed in a six treatment structured randomized test using a horizontal grinder. For each treatment the basic density, moisture content, bulk density, particle size distribution, fuel consumption and bark and other non-wood substances content was estimated. No effect of bit type or screen size on bulk density was found when processing branches-and-tops size class residue. For the pulpwood and butt-log-chunks size classes, the knife-edge bits tend to produce a denser material explained in part by their cutting capabilities across the grain compared to the normal hammering process using carbide hammer bits. Fuel consumption was only affected by screen size when processing the branches-and-tops size class. For pulpwood and butt-log-chunks size classes, the use of carbide hammer bits for processing increased fuel consumption between 42 and 48% compared to knife-edge bits. Bark and other non-wood substances content accounted for 11% of the total grinding mixture in the branches-and-tops size class compared to 2.5% in grindings from pulpwood and butt-log-chunks size classes. The branches-and-tops size class residue produced denser bulk material compared to the other classes and consumed less fuel due in part to the higher basic density and increasing amount of fine particles compared to the other analyzed size classes.     

Industrial processes for biomass drying and their effects on the quality properties of wood pellets

This paper contributes to the discussion of how different kinds of industrial scale dryers for biomass influence the quality properties of wood pellets. It also discusses how the drying technique can affect the environment. The most common biomass drying processes in use, i.e., convection dryers are discussed. The discussion of drying techniques is based on advantages and disadvantages with a focus on the drying medium, temperature and residence time. The choice of drying technique is particularly important if the end-user’s choice of pellets is made due to the specific requirements for the heating system used. Some specific parameters were tested in order to investigate how the choice of drying technique affects the pellet quality. The parameters tested were moisture content and the emissions of volatile hydrocarbons. Pellets available on the market were chosen for the tests. The amount of volatile hydrocarbons left in sawdust after drying vary with drying technique, as emissions of terpenes are larger in dryers with long residence times. Low emissions of volatile hydrocarbons would improve the energy content of the sawdust, and by decreasing air pollution improve the work environment and the environment in the surroundings of the dryers.     

燃料含水率对于生物质半气化炉性能影响研究

生物质能源的清洁利用作为解决能源短缺的有效途径,受到了国际上越来越多的关注。目前,世界各国的生物质炉灶热效率测试方法中均未规定燃料的含水率,为了探究燃料的含水率对于生物质炉灶性能的影响,文章采用一台生物质半气化炉对不同含水率的生物质燃料进行测试,对最终所得的炉灶热性能及污染情况进行比对。结果显示,燃料含水率越高,炉灶火力强度越低,排放的一氧化碳、颗粒物及多环芳烃等污染物越少。     

Optimisation of the manufacturing variables of sawdust pellets from the bark of Pinus caribaea Morelet: Particle size, moisture and pressure

Different variables affect the processes of compacting biomass, and these variables depend on the raw material, such as the type of wood or type of biomass being compacted, its moisture, size, as well as on other aspects relating to the manufacturing process, such as the pressure applied and the temperature reached during compaction.This work analyses the influence of the size of the sawdust particles from the bark of the species Pinus caribaea var. Morelet, which ranges from 0.63 to 2 mm; it had a moisture content in dry basis of 6; 9.5; 13 and 20%; and the force applied in the laboratory was 8000, 12,000, 16,000 and 20,000 N. These values are relatively high given that they do not take into account the heat caused by the matrixes chafing with the material to be compacted, as occurs in industrial processes.The different experimental treatments were processed and statistically analysed using SSPS version 12.0, and Statgraphics version 5.0. Once the data had been tested in the ANOVA and in various comparison tests, it was observed that the results did not show any significant differences between the pellets obtained with forces of 16,000 and 20,000 N, or between pellets obtained with 9.5 and 13% moisture, and that the pellets with higher qualities were obtained from particle sizes of between 1 and 2 mm.     

Effect of cycling rate,particle size and transport properties on lithium-ion cathode performance

Much progress has been made in modeling the lithium-ion battery technology. There exists a critical need to establish a framework to assess the role of various physical, geometrical, and operating parameters and their relative influence on the energy and power capability of batteries. In this study, a surrogate modeling framework has been introduced to map the effect of design-related parameters on the performance of a lithium-ion cell. In particular, the effects of cycling rate, cathode particle size, and diffusion coefficient and electrical conductivity of the solid cathode material, on the specific energy and power have been studied using a cell-level model in conjunction with tools such as kriging, polynomial response, and radial-basis neural networks. Through global sensitivity analysis the relative impact of the various parameters are quantified under different scenarios. Specifically, the design space can be split into distinct regions based on the discharge and diffusion time scales for separate, more refined analysis. It is shown that the cathode performance becomes independent of the diffusion coefficient above a critical value. A Pareto-optimal front was constructed to quantify the tradeoff between maximum achievable energy and power levels. Such an analysis can provide guidelines for the optimization of the positive electrode design.     

Effect of compressive load and particle size on compression characteristics of selected varieties of wheat straw grinds

In this study, the effect of compressive load and particle size on compression characteristics of four varieties (Strongfield, Blackbird, DT773 and DT818) of wheat straw grown at two different fields was investigated. Particle size, bulk and particle densities of all wheat straw samples were determined after grinding. Ground wheat straw samples were densified in a cylindrical die at 90 °C using an Instron testing machine. The wheat straw samples with 9% moisture content were compressed at five levels of compressive pressures (31.6, 63.2, 94.7, 126.4 and 138.9 MPa) and two levels of particle sizes (1.6 and 3.2 mm). Dimensions and mass of all compressed samples were measured to calculate the pellet density. The specific energy required to compress and eject the pellets was calculated from force-displacement data. Applied compressive force and particle size significantly affected the pellet density of wheat straw samples. The pellet density was in the range of 699–1064 kg m⁻³ increasing with pressure and particle size. The total specific energy required for compression and ejection of pellets varied from 4.35 to 33.64 MJ t⁻¹ that increased with compressive load and particle size. Higher compressive forces and particle size increased the durability of pellets to more than 95%. Blackbird variety was the most compressible of the four varieties of wheat straw.     

Percolation theory in SOFC composite electrodes: Effects of porosity and particle size distribution on effective properties

A percolation model, accounting for polydispersion of powders and presence of pore formers (i.e. porosity), is presented to predict effective properties of composite electrodes for solid oxide fuel cells, such as the three-phase boundary length and the mean hydraulic radius. Porosity affects both numbers of contacts (so probabilities of connection) and number of particles per unit volume. Both these effects, together with granulometric distribution, are accounted for the estimation of effective properties. As a consequence, the theory can predict numbers of contacts, coordination numbers and therefore effective properties of the electrode for multicomponent polydisperse mixtures.Model simulations show that the three-phase boundary length sharply decreases as porosity increases while the effects of polydispersion of powders are less pronounced, although significant, suggesting that these features should be considered in SOFC electrode models.     

Effect of process parameters and raw material characteristics on physical and mechanical properties of wood pellets made from sugar maple particles

The aim of the current study was to investigate the influence of process parameters and raw material characteristics on physical and mechanical properties of wood pellets made from particles of sugar maple trees of different vigor. Pellets were made in a single pelletizer while controlling temperature (75, 100 and 125 °C), moisture content (8.1, 11.2 and 17.2%), compression force (1500, 2000 and 2500 N) and particle size (<0.25, 0.25–0.5 and 0.5–1.0 mm). Particle size was the most important factor influencing friction in the die, followed by moisture content, compression force and temperature. Moisture content was the most important factor affecting pellet density, followed by temperature, compression force and raw material particle size. Temperature was the most important factor for pellet compression strength, followed by compression force, particle size and moisture content. Friction in the die decreased with increasing particle size and moisture content of the material and increased with increasing compression force. It decreased initially with increasing temperature from 75 °C to 100 °C, and then increased with temperature. Density and strength of pellets increased with temperature and compression force, decreased with increasing particle size, and decreased with increasing moisture content. Pelletizing should be performed at 100 °C to minimize friction and a moisture content of 11.2% to maximize density and compression strength of the pellets. Wood particles from sugar maple trees of low vigor were more suitable for making wood pellets in terms of friction in the pelletizer and compression strength than those from vigorous trees.     

Estimation of the effect of biomass moisture content on the direct combustion of sugarcane bagasse in boilers

Many of the large-scale biomass combustion systems for producing heat, hot water, or steam accept biomass fuels containing relatively large amounts of moisture. Dry biomass burns at higher temperatures and thermal efficiencies than wet biomass. Flame temperature is directly related to the amount of heat necessary to evaporate the moisture contained in the biomass, the lower the moisture content, the lower the amount of energy needed to remove the water and the higher the boiler efficiency. In this article, a simple predictive tool is developed to estimate boiler efficiency as a function of stack gas temperature and sugarcane bagasse moisture content. The method quantitatively illustrates the effect of moisture content on the performance of a thermochemical process, for the direct combustion of sugarcane bagasse in a conventional boiler. The results are found to be in excellent agreement with reported data in the literature with average absolute deviation being around 1%. The tool developed in this study can be of immense practical value for engineers to have a quick check on biomass moisture content on the boiler performance at various conditions without opting for any experimental trials. In particular, engineers would find the approach to be user-friendly with transparent calculations involving no complex expressions.     

Influence of moisture content and hammer mill screen size on the physical quality of barley,oat, canola and wheat straw briquettes

In order to determine the briquetting characteristics of biomass in a commercial setting, a hydraulic briquetter was used to study the compaction behavior of biomass grinds from barley, oat, canola and wheat straw. The selected straw samples were ground with a hammer mill using screen sizes of 19.05, 25.40 and 31.75 mm and conditioned to three moisture content levels of 0.09, 0.12 and 0.15 (w.b.). The residence time was about 6–10 min before being extruded from the briquetter. The specific energy, throughput, as well as the density, and durability of manufactured briquettes were measured during or after briquetting. The applied compression pressure at different parameter combinations ranged from 7 to 14 MPa. Higher pressure resulted at higher biomass moisture content. Hammer mill grinding of biomass with a large screen size (31.75 mm) resulted in high energy consumption and low throughput during briquetting. The increase in moisture content decreased the total energy consumption and increased the throughput of the briquetter. Briquette densities were of consistently higher value when biomass samples were compressed at a lower moisture level. The moisture content and hammer mill screen size indirectly influenced the briquette densities by affecting the pressure and residence time in commercial briquette production. Briquettes were successfully formed without adding a binder.     

A study of bonding and failure mechanisms in fuel pellets from different biomass resources

Pelletization of biomass reduces its handling costs, and results in a fuel with a greater structural homogeneity. The aim of the present work was to study the strength and integrity of pellets and relate them to the quality and mechanisms of inter-particular adhesion bonding. The raw materials used were: beech, spruce and straw, representing the most common biomass types used for fuel pellet production, i.e. hardwoods, softwoods and grasses, respectively. The results showed that the compression strengths of the pellets were in general higher for pellets produced at higher temperatures, and much higher for wood pellets than for straw pellets. Scanning electron microscopy of the beech pellets fracture surfaces, pressed at higher temperatures, showed areas of cohesive failure, indicating high energy failure mechanisms, likely due to lignin flow and inter-diffusion between adjacent wood particles. These were absent in both spruce and straw pellets. Infrared spectroscopy of the fracture surfaces of the straw pellets indicated high concentrations of hydrophobic extractives, that were most likely responsible for their low compression strength, due to presence of a chemical weak boundary layer, limiting the adhesion mechanism to van der Waals forces. Electron micrographs indicating interfacial failure mechanisms support these findings. Infrared spectra of the fracture surface of wood pellets, pressed at elevated temperatures, showed no signs of hydrophobic extractives. It has been shown that both temperature and chemical composition, i.e. the presence of hydrophobic extractives, have a significant influence on the bonding quality between biomass particles during the pelletizing process.     

Effect of moisture content on oil extraction from spent coffee grounds

Spent coffee grounds (SCG) were used in this study for oil extraction. It was observed that oil extraction increased with water content up to 20 wt% for non-polar solvents and up to 40 wt% for polar solvents. At 80 wt% moisture, no oil was observed instead a black gel was formed. Oil extraction was observed to increase with time but it was concluded that increasing extraction time beyond 60 min had no significant benefit for both solvents (ethanol and toluene). The highest amount of oil extracted was observed when using ethanol at 40 wt% moisture and it was around 20%. The saponification values for oil extracted from SCG were observed to be between 175 and 180 but were lower than those obtained for soybean (193). The SCG oil was observed to contain the following fatty acids in this quantity order: palmitic> linoleic acid > stearic acid.     

The properties of pellets from mixing bamboo and rice straw

Rice straw pellets are the main type of biomass solid fuel and have great potential as a bioenergy resource of the future in China. But it also showed important problems because of its high content of ashes and its low gross calorific value, reducing the possibility to be used in domestic heating. It was certified that mixing different types of biomass materials was helpful to improve the properties of pellets. To improve properties of rice straw pellets and investigate the effect of mixing bamboo and rice straw on the pellet properties, some properties of pellets, manufactured using different mixing ratio of bamboo and rice straw particles, were determined in this research. It can be concluded from this research that physical properties of all pellets meet the requirements of Pellet Fuels Institute Standard Specification for Residential/Commercial Densified except for bulk density of pellets, manufactured using mixing ratio (≤3:2) of bamboo and rice straw. The inorganic ash and gross calorific value of rice straw pellets cannot meet the requirement of Pellet Fuels Institute Standard Specification for Residential/Commercial Densified (8.0%) and the minimum requirement for making commercial pellets of DIN 51731 (>17,500 J/g). Both properties are improved through mixing bamboo particles and rice straw particles. It is significant that inorganic ash content and gross calorific value of pellets, manufactured using mixing ratio (≥3:2) of bamboo and rice straw, were lower than 8.0% and higher than 17,500 J/g, respectively. This also shows that mixing different biomass materials is an effective way to optimize properties of biomass solid fuel. All pellets after improvement are proposed as biomass solid fuel and have the potential to be developed as commercial pellets on an industrial scale in China.     

A study of spontaneous combustion characteristics of a turkish lignite: particle size, moisture of coal, humidity of air

This study evaluated the spontaneous combustion characteristics of Askale lignite from Turkey. The effect of the gas flow rate, the moisture of the piles of coal, the humidity of the air and particle size on the spontaneous combustion characteristics of coal samples were examined using Crossing Point Methods adapted to our laboratories conditions. The amounts of three predominant oxygen functional groups (carboxyl, hydroxyl and carbonyl) in untreated and moist coal samples were also determined with wet chemical methods. The amounts of oxygen functional groups in moist coal samples do not differ significantly from that of untreated coal. The liability of spontaneous combustion of this lignite was increased with decreasing particle size, increasing moisture content of the coal and decreasing humidity of the air.     

生物质电厂燃料灰分含量与元素分析指标间相关分析研究

文章利用偏相关分析法研究了湛江生物质电厂生物质燃料的灰分含量与元素分析等指标间的相关性;以灰分含量为因变量,元素分析指标为自变量,建立起具有定量关系的多元线性回归方程;利用误差分析方法,对所建立的回归方程预测效果进行评估。结果表明,生物质灰分含量与其C,H和O含量有着显著负相关关系,同时在以灰分为因变量,C,H和O为自变量的回归方程中,若待测试的生物质各性质指标变量在适用范围内,则预测结果误差较小,预测效果较为理想。     

Pt–Ir binary hydrophobic catalysts: Effects of Ir content and particle size on catalytic performance for liquid phase catalytic exchange

Pt/C and Pt–Ir/C catalysts with different atomic ratios were synthesized in a closed PTFE vessel by a microwave-irradiated polyol method. The characterization results indicated that the average Pt–Ir particle size changed only slightly and the face-centered crystalline structure gradually became more amorphous with increasing iridium content. The particle size could be controlled by altering the pH of the synthesis solution and the microwave heating rate. The carbon-supported catalysts and polytetrafluoroethylene were then loaded together on a foamed nickel (FN) carrier to obtain hydrophobic catalysts. The reaction mechanisms were presented for liquid phase catalytic exchange reaction, which could explain that a hydrophobic catalyst with a Pt/Ir molar ratio of 4/1 exhibited the best catalytic activity. Furthermore, the effect of particle size on catalytic performance was investigated. The activity of the Pt₄Ir₁/C/FN catalyst was enhanced by decreasing the average particle size, in the range of 2.5–3.8 nm.