A review on biomass classification and composition, co-firing issues and pretreatment methods

Abstract

Presently, around the globe, there is a significant interest in using biomass for power generation as power generation from coal continues to raise environmental concerns. Using just biomass for power generation can bring a lot of environmental benefits. However the constraints of using biomass alone can include high investments costs for biomass feed systems and also uncertainty in the security of the feedstock supply due to seasonal variations, and in most countries, limited infrastructure for biomass supply. Alternatively, co-firing biomass along with coal offers advantages like a) reducing the issues related to biomass quality and buffers the system when there is insufficient feedstock quantity and b) costs of adapting the existing coal power plants will be lower than building new systems dedicated only to biomass. However, with the above said advantages there exists some technical constrains including low heating and energy density values, low bulk density, lower grindability index, higher moisture and ash content. In order to successfully cofire biomass with coal, biomass feedstock specifications need to be established to direct pretreatment options that may include increasing the energy density, bulk density, stability during storage and grindability. Impacts on particle transport systems, flame stability, pollutant formation and boiler tube fouling/corrosion must also be minimized by setting feedstock specifications including composition and blend ratios if necessary. Some of these limitations can be overcome by using preprocessing methods. This paper discusses the impact of feedstock preprocessing methods like sizing, baling, pelletizing, briquetting, washing/leaching, torrefaction, torrefaction and pelletization and steam explosion in attainment of optimum feedstock characteristics to successfully cofire biomass with coal.