A systems analysis of biomass densification process
Creators: name: Mani, Sudhagar
Publisher: name: University of British Columbia
Year: 2009

description: Pelletizing is a method of densifying biomass. Pellets have low moisture content (about 8% wet basis) for safe storage and a high bulk density (more than 500 kg/m3) for efficient transport. Biomass pellets that are usually up to 6 mm in diameter and 12 mm long, are uniform in moisture content. They can be handled, transported and fed to boilers and furnaces easily. Manufacturing of pellets involves energy intensive drying, grinding, and pelleting processes. In a typical operation, manufacturing one ton of dried pellets may use 300-3500 MJ for drying, 100-180 MJ for grinding, and 100-300 MJ for densification. The present study investigates the entire densification process using the systems analysis approach and on finding out the best alternative fuel source for biomass drying application with the lowest cost, emissions and energy consumptions. In this study, biomass drying, size reduction and compaction were studied in detail theoretically and experimentally. Drying of biomass is performed in a direct contact, co-current type rotary drum dryer. Single and triple pass rotary dryers were modeled using the lumped parameter approach. The developed models predicted the temperature and moisture profiles of hot flue gas and biomass particles and the results were in agreement with commercial rotary dryer outlet conditions. Five heating fuel sources for the dryer were compared: natural gas, coal, wet sawdust, dry sawdust and wood pellets. The combustion of fuels was modeled to predict the hot flue gas compositions and fuel requirement for the given dryer inlet conditions. A series of experiments were conducted where biomass samples were ground using a laboratory hammer mill at different screen sizes and moisture contents. Specific energy consumption of grinding biomass was estimated and used to develop the relationship between hammer mill screen size and specific grinding energy data. The laboratory hammer mill energy data were compared with commercial hammer mill data. The ground samples were analysed for particle size distribution, geometric mean particle size, bulk density and particle density. Biomass grinds' were compacted into pellets using a single pelleter unit. Compression data from the experiment were analyzed using several compaction models. Particle rearrangement and elastic and plastic deformation were the predominant compaction mechanisms during the pelleting process. Particle interlocking or local melting of constituents could have occurred at high pressures and temperatures during compaction, although this phenomenon was not examined in detail. The force-displacement data were collected and analyzed to estimate the specific energy required to compress and extrude biomass materials. It was found that more than 60% of the total energy spent during the extrusion of pellet was to overcome the wall friction. The pelleting energy could be reduced if some processing aids are used without losing the quality of compacted pellets. Or a new compaction unit may be designed and developed to eliminate the friction energy consumed during the compaction process. To conduct a systems analysis of the entire biomass densification process, a typical wood pelleting plant was chosen to evaluate the total energy consumption, environmental emissions and cost of pellet production using different alternative fuels. The process models developed in the thesis were used to predict the energy consumption and emissions during combustion process. Average emission factors were used from published literature sources to estimate the emissions of trace metals and toxic pollutants. The environmental impacts of the emissions were evaluated based on greenhouse gases, acid rain formation, smog formation and human toxicity impact potentials. A detailed engineering cost analysis was conducted to estimate the pellet production cost using different process options and fuel sources. A multi-criteria decision making method, Preference Ranking Organization Method for Enrichment Evaluation (PROMETHEE) was used to rank fuel alternatives. The best fuel source was selected based on the four main criteria - energy, environmental impacts, economics and fuel quality. It was found that wood pellet or dry sawdust may be the best alternative to natural gas followed by coal and wet sawdust, if all the criteria are weighed equally. The ranking was changed to: 1) coal; 2) dry sawdust; 3) wet sawdust; 4) wood pellet; and 5) natural gas, when the weighting factor for cost was doubled.