Shahab Sokhansanj

ECONOMICS OF PRODUCING FUEL PELLETS FROM BIOMASS

An engineering economic analysis of a biomass pelleting process was performed for conditions in North America. The pelletization of biomass consists of a series of unit operations: drying, size reduction, densifying, cooling, screening, and warehousing. Capital and operating cost of the pelleting plant was estimated at several plant capacities. Pellet production cost for a base case plant capacity of 6 t/h was about

The Potential of C4 Perennial Grasses for Developing a Global BIOHEAT Industry

51/t of pellets. Raw material cost was the largest cost element of the total pellet production cost followed by personnel cost, drying cost, and pelleting mill cost. An increase in raw material cost substantially increased the pellet production cost. Pellet plants with a capacity of more than 10 t/h decreased the costs to roughly

Bulk Density of Wet and Dry Wheat Straw and Switchgrass Particles

40/t of pellets. Five different burner fuels – wet sawdust, dry sawdust, biomass pellets, natural gas, and coal were tested for their effect on the cost of pellet production. Wet sawdust and coal, the cheapest burner fuels, produced the lowest pellet production cost. The environmental impacts due to the potential emissions of these fuels during the combustion process require further investigation.

Torrefaction of sawdust in a fluidized bed reactor

Unprecedented opportunities for biofuel development are occurring as a result of rising fossil fuel prices, the need to reduce greenhouse gases, and growing energy security concerns. An estimated 250 million hectares (ha) of farmland could be utilized globally to develop a bioenergy industry if efficient and economical perennial biomass crops and bioenergy conversion systems are employed. In temperate zones, C4 or warm-season grass research and development efforts have found switchgrass (Panicum virgatum) and Miscanthus capable of producing biomass yields of 10 to 20 oven dried tonnes (ODT)/ha/yr, while in tropical areas Erianthus and napier grass (Pennisetum purpureum) are producing 25 to 35 ODT/ha/yr. The potential to annually produce 100 barrels of oil energy equivalent/ha with a 25:1 energy output to input ratio appears achievable with high-yielding, N-fixing warm-season grasses grown on marginal lands in the tropics. Commercialization of densified herbaceous plant species has been slow because of the relatively high alkali and chlorine contents of the feedstocks, which leads to clinker formation and the fouling of boilers. This challenge can be overcome by improving biomass quality through advances in plant breeding and cultural management to reduce the chlorine, alkali, and silica content and through the use of new combustion technologies. Warm-season grasses can be readily densified provided suitable grinding and densification equipment and pressure are utilized. The major advantages of producing densified warm-season grasses for BIOHEAT include: it is the most efficient strategy to use marginal farmlands in most temperate and tropical climates to collect solar radiation; it has an excellent energy balance; the feedstocks can be used conveniently in a variety of energy applications; and it is relatively environmentally friendly. Densified warm-season grass biofuels are poised to become a major global fuel source because they can meet some heating requirements at less cost than all other alternatives available today.

Process Conditions Affecting the Physical Quality of Alfalfa Pellets

Bulk density is a major physical property in designing the logistic system for biomass handling. The size, shape, moisture content, individual particle density, and surface characteristics are few factors affecting the bulk density. This research investigates the effects of true particle lengths ranging from 6 to 50 mm and moisture contents ranging from 8% to 60% wet basis (wb) on the bulk density of wheat straw and switchgrass. Three types of particle densities of straw and switchgrass measured were: a hollow particle density assuming a hollow cylindrical geometry, a solid particle density assuming a solid cylindrical geometry, and a particle density measured using a gas pycnometer at a gas pressure of 40 kPa. The bulk density of both loose-fill and packed-fill biomass samples was examined. The calculated wet and dry bulk density ranged from 24 to 111 kg m-3 for straw and from 49 to 266 kg m-3 for switchgrass. The corresponding tapped bulk density ranged from 34 to 130 kg m-3 for straw and 68 to 323 kg m-3 for switchgrass. The increase in bulk density due to tapping the container was from 10% for short 6-mm particles to more than 50% for long 50-mm particles. An equation relating the bulk density of stems as a function of moisture content, dry bulk density, and particle size was developed. After the validation of this bulk density equation, the relationship would be highly useful in designing the logistics system for large-scale transport of biomass to a biorefinery. The bulk density and particle density data of uniform particles would be important, if straw and switchgrass is used for pulping and paper making.

Binderless Pelletization of Biomass

In the present work, stable fluidization of sawdust was achieved in a bench fluidized bed with an inclined orifice distributor without inert bed materials. A solids circulation pattern was established in the bed without the presence of slugging and channeling. The effects of treatment severity and weight loss on the solid product properties were identified. The decomposition of hemicelluloses was found to be responsible for the significant changes of chemical, physical and mechanical properties of the torrefied sawdust, including energy content, particle size distribution and moisture absorption capacity. The hydrophobicity of the torrefied sawdust was improved over the raw sawdust with a reduction of around 40 wt.% in saturated water uptake rate, and enhanced with increasing the treatment severity due to the decomposition of hemicelluloses which are rich in hydroxyl groups. The results in this study provided the basis for torrefaction in fluidized bed reactors.

Oxidative torrefaction of biomass residues and densification of torrefied sawdust to pellets

The physical quality of alfalfa pellets can be optimized by control of the manufacturing process. Process conditions studied were conditioning and pellet temperature, die geometry (length-to-diameter or l/d ratio), hammer mill screen size used in grinding the dehydrated hay chops, and die speed. As conditioning temperature increased, the durability of the pellets also increased. Pellet temperature increased with conditioning temperature. Pellet temperature has a significant effect on pellet durability. The l/d ratio also directly affects pellet durability. The die with a high l/d ratio produced more durable pellets. Differences due to the screen size of the hammer mill used in grinding the chops and the die speeds were not significant. A correlation between energy consumption and pellet durability was developed.

Bulk density and compaction behavior of knife mill chopped switchgrass, wheat straw, and corn stover.

A cost-effective pelletized biomass is a key to the success of bio-based industry. Low production costs along with safe handling of biomass will make biomass competitive with fossil fuels. Recent advances in biochemical treatments and force-deformation research point to a possible breakthrough in the age-old pelleting/cubing. The structure of lignocellulosic biomass consists of complex molecules of cellulose, hemicellulose and lignin. Modifying the structure of cellulose-hemicelluloselignin matrix can enhance binding characteristics of lignocellulosic biomass. Furthermore. The research reported in this paper demonstrates that by optimizing a combination of physico-chemical treatments of biomass before and during its densification will improve the hardness and durability of pelletized biomass.

Fast classification and compositional analysis of cornstover fractions using Fourier transform near-infrared techniques.

Oxidative torrefaction of sawdust with a carrier gas containing 3-6% O(2) was investigated in a TG and a fluidized bed reactor, with the properties of the torrefied sawdust and pellets compared with traditional torrefaction without any O(2), as well as the dry raw material. It is found that the oxidative torrefaction process produced torrefied sawdust and pellets of similar properties as normally torrefied sawdust and corresponding pellets, especially on the density, energy consumption for pelletization, higher heating value and energy yield. For moisture absorption and hardness of the torrefied pellets, the oxidative torrefaction process showed slightly poor but negligible performance. Therefore, it is feasible to use oxygen laden combustion flue gases as the carrier gas for torrefaction of biomass. Besides, torrefied sawdust can be made into dense and strong pellets of high hydrophobicity at a higher die temperature than normally used in the production of traditional control pellets.

Characterization and kinetics study of off-gas emissions from stored wood pellets

Bulk density of comminuted biomass significantly increased by vibration during handling and transportation, and by normal pressure during storage. Compaction characteristics affecting the bulk density of switchgrass, wheat straw, and corn stover chopped in a knife mill at different operating conditions and using four different classifying screens were studied. Mean loose-filled bulk densities were 67.5+/-18.4 kg/m(3) for switchgrass, 36.1+/-8.6 kg/m(3) for wheat straw, and 52.1+/-10.8 kg/m(3) for corn stover. Mean tapped bulk densities were 81.8+/-26.2 kg/m(3) for switchgrass, 42.8+/-11.7 kg/m(3) for wheat straw, and 58.9+/-13.4 kg/m(3) for corn stover. Percentage changes in compressibility due to variation in particle size obtained from a knife mill ranged from 64.3 to 173.6 for chopped switchgrass, 22.2-51.5 for chopped wheat straw and 42.1-117.7 for chopped corn stover within the tested consolidation pressure range of 5-120 kPa. Pressure and volume relationship of chopped biomass during compression with application of normal pressure can be characterized by the Walker model and Kawakita and Ludde model. Parameter of Walker model was correlated to the compressibility with Pearson correlation coefficient greater than 0.9. Relationship between volume reduction in chopped biomass with respect to number of tappings studied using Sones model indicated that infinite compressibility was highest for chopped switchgrass followed by chopped wheat straw and corn stover. Degree of difficulty in packing measured using the parameters of Sones model indicated that the chopped wheat straw particles compacted very rapidly by tapping compared to chopped switchgrass and corn stover. These results are very useful for solving obstacles in handling bulk biomass supply logistics issues for a biorefinery.