Densification of selected agricultural crop residues as feedstock for the biofuel industry

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

Compression Characteristics of Selected Ground Agricultural Biomass

Agricultural biomass such as barley, canola, oat and wheat straw has the potential to be used as feedstock for bioenergy. However, the low bulk density straw must be processed and densified in order to facilitate handling, storage and transportation. It is important to understand the fundamental mechanism of the biomass compression process, which is required in the design of energy efficient compaction equipment to mitigate the cost of pre-processing and transportation of the product. Therefore, a comprehensive review of various compression models was performed and the compression behavior of selected ground agricultural biomass was studied. Five compression models were considered to determine the pressure-volume and pressure-density relationship to analyze the compression characteristics of biomass samples, namely: Jones (1960), Heckle (1961), Cooper-Eaton (1962), Kawakita-Ludde (1971) and Panelli-Filho (2001) models. Densification studies were conducted on four selected biomass samples at 10 % moisture content (w.b.) and 1.98 mm grind size using four pressure levels of 31.6, 63.2, 94.7 and 138.9 MPa. The mean densities of barley, canola, oat and wheat straw increased from 907 to 977 kg/m3, 823 to 1003 kg/m3, 849 to 1011 kg/m3 and 813 to 924 kg/m3, respectively. The Kawakita-Ludde model provided an excellent fit having R2 values of 0.99 for selected agricultural straw samples. It was also concluded that the ground oat and canola straw had the highest level of porosity and failure stress, respectively. The parameters of Cooper-Eaton model indicated that the ground straw samples were densified easily by the particles rearrangement method and Jones model indicated that canola and oat straw were more compressible as compared to barley and wheat straw

Potential Applications of Infrared and Raman Spectromicroscopy for Agricultural Biomass

The low bulk density agricultural biomass should be processed and densified making it suitable for biorefineries. However, many agricultural biomass (lignocellulosic) especially those from straw and stover results in poorly formed pellets or compacts that are more often dusty, difficult to handle and costly to manufacture. The binding characteristics of biomass can be enhanced by modifying the structure of lignocellulose matrix (cellulose-hemicellulose-lignin) by different pre-processing and pre-treatment methods. However, it is not well understood as to how various pre-processing and pre-treatment methods affect the lignocellulosic matrix at the molecular level. Therefore, it is essential to determine chemical composition of agricultural biomass and the distribution of lignin relative to cellulose and hemicellulose before and after application of various treatment methods and after densification process. In this paper, the structural characteristics of lignocellulosic plant biomass and applications of Infrared (IR) and Raman spectromicroscopy methods are reviewed. The IR and Raman methods have good potential to determine the structural characteristics and distribution of chemical components in lignocellulosic biomass. Both methods have their own advantages and drawbacks, and should be used as complementary techniques

Performance study of a heat pump assisted dryer system for specialty crops

This thesis is concerned with the technology of heat pump assisted drying of specialty crops, the benefits and potential of the process in the agriculture sector. The purpose of using a heat pump for drying of specialty crops is to dry them at lower temperature than would otherwise be possible using conventional drying techniques. Low temperature drying of specialty crops reduces the risk of loss in nutrient content and damage to physical properties which are important aspects considering their high commercial value. Heat pump drying is potentially more energy efficient since it is possible to recover latent heat from the humid dryer exhaust air. In this research, cabinet and prototype continuous bed heat pump dryers were studied. Simulation models for the heat pump dryers were developed to predict the performance of the dryer, i.e. the drying rate of the material and psychrometric conditions of the air, and also to determine the refrigerant mass flow rate and corresponding temperatures at the condenser and evaporator coils of the heat pump system, based on the psychrometirc conditions of process air inside the drying chamber. The accuracy of the predicted results is later verified with the experimental results. Chopped alfalfa was dried in a cabinet dryer in batches and by emulating the continuous bed drying using two household dehumidifiers. The reason for using alfalfa instead of specialty crops like ginseng, herbs, echinacea, feverfew, etc. was that the material and its drying properties were readily available. Also the structure of alfalfa leaves and stems is similar to that of many herbs and specialty crops. Results showed that alfalfa was dried from an initial moisture content of 70% (wb) to a final moisture content of 10% (wb ). It was noticed that batch drying took about 4.5 h while continuous bed drying took 4 h to dry the material. The initial weight of alfalfa in each tray was 400 g. The average air velocity inside the dryer was 0.36 m/s. Low temperatures (30-45°C) for safe drying of specialty crops were achieved experimentally. Specific moisture extraction rate was maximum when relative humidity stayed above 40%. The household type dehumidifiers used in this study were about 50% more efficient in recovering the latent heat from the dryer exhaust compared to the conventional dryers. It was concluded that continuous bed drying is potentially a better option than batch drying because high process air humidity ratios at the entrance of the evaporator and constant moisture extraction rate and specific moisture extraction rate values can be maintained. Simulation results for a prototype heat pump continuous bed dryer system suggested that the change in dryer inlet temperatures of the process air has an insignificant effect on drying of the material. Therefore, based on the results, it was concluded that the dryer inlet air temperature could be kept as low as 30°C, if required, to maintain product quality. The material was dried to a safe limit of 10% moisture content. The material mass flow rate was only 3-4.5 kg/h, indicating that it might be advisable to use the heat pump dryer in combination with some other technique. ****Disk with program codes was included with the original thesis***