Densification of selected agricultural crop residues as feedstock for the biofuel industry
Date
2011
Authors
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Journal ISSN
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Degree Level
Doctoral
Abstract
The two main sources of biomass for energy generation are purpose-grown energy crops and waste materials. Energy crops, such as Miscanthus and short rotation woody crops (coppice), are cultivated mainly for energy purposes and are associated with the food vs. fuels debate, which is concerned with whether land should be used for fuel rather than food production. The use of residues from agriculture, such as barley, canola, oat and wheat straw, for energy generation circumvents the food vs. fuel dilemma and adds value to existing crops. In fact, these residues represent an abundant, inexpensive and readily available source of renewable lignocellulosic biomass.
In order to reduce industry’s operational cost as well as to meet the requirement of raw material for biofuel production, biomass must be processed and handled in an efficient manner. Due to its high moisture content, irregular shape and size, and low bulk density, biomass is very difficult to handle, transport, store, and utilize in its original form. Densification of biomass into durable compacts is an effective solution to these problems and it can reduce material waste. Upon densification, many agricultural biomass materials, especially those from straw and stover, result in a poorly formed pellets or compacts that are more often dusty, difficult to handle and costly to manufacture. This is caused by lack of complete understanding on the natural binding characteristics of the components that make up biomass.
An integrated approach to postharvest processing (chopping, grinding and steam explosion), and feasibility study on lab-scale and pilot scale densification of non-treated and steam exploded barley, canola, oat and wheat straw was successfully established to develop baseline data and correlations, that assisted in performing overall specific energy analysis. A new procedure was developed to rapidly characterize the lignocellulosic composition of agricultural biomass using the Fourier Transform Infrared (FTIR) spectroscopy. In addition, baseline knowledge was created to determine the physical and frictional properties of non-treated and steam exploded agricultural biomass grinds.
Particle size reduction of agricultural biomass was performed to increase the total surface area, pore size of the material and the number of contact points for inter-particle bonding in the compaction process. Predictive regression equations having higher R2 values were developed that could be used by biorefineries to perform economic feasibility of establishing a processing plant. Specific energy required by a hammer mill to grind non-treated and steam exploded barley, canola, oat and wheat straw showed a negative power correlation with hammer mill screen sizes.
Rapid and cost effective quantification of lignocellulosic components (cellulose, hemicelluloses and lignin) of agricultural biomass (barley, canola, oat and wheat) is essential to determine the effect of various pre-treatments (such as steam explosion) on biomass used as feedstock for the biofuel industry. A novel procedure to quantitatively predict lignocellulosic components of non-treated and steam exploded barley, canola, oat and wheat straw was developed using Fourier Transformed Infrared (FTIR) spectroscopy. Regression equations having R2 values of 0.89, 0.99 and 0.98 were developed to predict the cellulose, hemicelluloses and lignin compounds of biomass, respectively. The average absolute difference in predicted and measured cellulose, hemicellulose and lignin in agricultural biomass was 7.5%, 2.5%, and 3.8%, respectively.
Application of steam explosion pre-treatment on agricultural straw significantly altered the physical and frictional properties, which has direct significance on designing new and modifying existing bins, hoppers and feeders for handling and storage of straw for biofuel industry. As a result, regression equations were developed to enhance process efficiency by eliminating the need for experimental procedure while designing and manufacturing of new handling equipment.
Compaction of low bulk density agricultural biomass is a critical and desirable operation for sustainable and economic availability of feedstock for the biofuel industry. A comprehensive study of the compression characteristics (density of pellet and total specific energy required for compression) of ground non-treated and steam exploded barley, canola, oat and wheat straw obtained from three hammer mill screen sizes of 6.4, 3.2 and 1.6 mm at 10% moisture content (wb) was conducted. Four preset pressures of 31.6, 63.2, 94.7 and 138.9 MPa, were applied using an Instron testing machine to compress samples in a cylindrical die. It was determined that the applied pressure (60.4%) was the most significant factor affecting pellet density followed by the application of steam explosion pre-treatment (39.4%). Similarly, the type of biomass (47.1%) is the most significant factor affecting durability followed by the application of pre-treatment (38.2%) and grind size (14.6%). Also, the applied pressure (58.3%) was the most significant factor affecting specific energy required to manufacture pellets followed by the biomass (15.3%), pre-treatment (13.3%) and grind size (13.2%), which had lower but similar effect affect on specific energy. In addition, correlations for pellet density and specific energy with applied pressure and hammer mill screen sizes having highest R2 values were developed. Higher grind sizes and lower applied pressures resulted in higher relaxations (lower pellet densities) during storage of pellets.
Three compression models, namely: Jones model, Cooper-Eaton model, and Kawakita-Ludde model were considered to determine the pressure-volume and pressure-density relationship of non-treated and steam exploded straws. Kawakita-Ludde model provided the best fit to the experimental data having R2 values of 0.99 for non-treated straw and 1.00 for steam exploded biomass samples. The steam exploded straw had higher porosity than non-treated straw. In addition, the steam exploded straw was easier to compress since it had lower yield strength or failure stress values compared to non-treated straw.
Pilot scale pelleting experiments were performed on non-treated, steam exploded and customized (adding steam exploded straw grinds in increments of 25% to non-treated straw) barley, canola, oat and wheat straw grinds obtained from 6.4, 3.2, 1.6 and 0.8 mm hammer mill screen sizes at 10% moisture content (wb). The pilot scale pellet mill produced pellets from ground non-treated straw at hammer mill screen sizes of 0.8 and 1.6 mm and customized samples having 25% steam exploded straw at 0.8 mm. It was observed that the pellet bulk density and particle density are positively correlated. The density and durability of agricultural straw pellets significantly increased with a decrease in hammer mill screen size from 1.6 mm to 0.8 mm. Interestingly, customization of agricultural straw by adding 25% of steam exploded straw by weight resulted in higher durability (> 80%) pellets but did not improve durability compared to non-treated straw pellets. In addition, durability of pellets was negatively correlated to pellet mill throughput and was positively correlated to specific energy consumption. Total specific energy required to form pellets increased with a decrease in hammer mill screen size from 1.6 to 0.8 mm and also the total specific energy significantly increased with customization of straw at 0.8 mm screen size. It has been determined that the net specific energy available for production of biofuel is a significant portion of original agricultural biomass energy (89-94%) for all agricultural biomass.
Description
Keywords
Density, Fourier Transform Infrared Spectroscopy, Steam Explosion Pretreatment, Wheat Straw, Oat Straw, Energy Analysis, Compression and Compaction Characteristics, Coefficient of Internal Friction, Hammer Mill, Grinding, Densification, Durability, Lignocellulose, Agricultural Biomass Residue, Barley Straw, Canola Straw, Pelleting
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Agricultural and Bioresource Engineering
Program
Agricultural and Bioresource Engineering