Influence of biomass moisture content on pellet properties - Review

Abstract

Seeking to become more climate-friendly and less energy-consuming, the European Union has pledged to cut its greenhouse gas emissions and milestones to achieve this are set to 20 % by 2020, 40 % by 2030, 60 % by 2040 and 80 % by 2050. Due to its abundance, biomass is gaining more and more importance both for the production of thermal energy by direct combustion or gasification of vegetal materials, for electricity and for the production of biofuels. Direct combustion of biomass generates CO2, but the process is neutral in terms of greenhouse gas emissions, because the same amount of CO2 was absorbed by plants from the air during their life cycle. Ecological solid fuels such as pellets have become rapidly a viable alternative to fossil fuels, due to their high energy content, which makes them suitable for use by both small households and industrial consumers. Pellets are obtained from a variety of raw materials such as: agricultural residues, energy crops, forestry and wood residues, used exclusively or mixed and having different physical-chemical properties. This paper presents a summary of literature on the effect of the moisture content on the properties of pellets obtained from various types of biomass. Moisture content of raw material is one of the most important factors that influence negatively the properties of pellets, such as bulk density or mechanical durability during storage and transportation. Energy consumption increases during pelletizing of high moisture biomass, as moisture is a lubricant that lowers friction in the die. Other studies found a positive correlation between pellets durability and optimal moisture (10 %), because water together with the die temperature, pressure and chemical composition of raw material acts like a binding agent that increases pellet quality. Pellets with 5 % moisture have low strength, become brittle, and large amounts of dust are produced during their storage and transportation. Moisture higher than 15 % damages pellets during storage.