A Streamlined Life Cycle Analysis of Biomass Densification

Mechanical densification is a process of transforming loose biomass into dense pellets. In this study, a wood pelleting plant was chosen to evaluate the total energy consumption, environmental emissions and cost of pellet production using different alternative fuels for the drying process. The fuels compared were natural gas, coal, dry and wet sawdust, and ground wood pellets. The process models were developed and applied to predict the energy consumption and emissions during combustion process. A streamlined life cycle analysis approach was used to quantify emissions. Average emission factors from published literature were used 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. Preference Ranking Organization Method for Enrichment Evaluation (PROMETHEE) was used to rank fuel alternatives. The best fuel source was selected based on four main criteria – energy, environmental impacts, economics and fuel quality. The results showed that wood pellet or dry sawdust might be the best alternative when compared to natural gas followed by coal and wet sawdust, when all the criteria were weighed equally. If the weighting factor for cost was doubled, coal ranked highest followed by dry sawdust, wet sawdust, wood pellet and natural gas, respectively.