Effect of mechanical fractionation and torrefaction on the biomass composition

In the Netherlands, green residues such as verge grass are a possible alternative to (partially) replace traditional fossil fuels. A feasible way of doing this, without large modification costs on existing power plants, is biomass co-firing. However, grass has high moisture content, poor grindability properties and high ash content, which have prevented it from being directly co-combusted. Pre-treatments consisting of mechanical dewatering and thermal treatment (torrefaction) could greatly improve the feedstock properties in an energy efficient way. This study firstly investigates the effect of mechanical fractionation on the biomass composition, with focus on the mass losses of carbohydrates, lignin, extractives and inorganic matters during pressing. Secondly, an experimental bench-top, batch torrefaction setup was built. This test rig could provide valuable data from drying and torrefaction experiments, which could be used for modeling purposes and operating experiences for designing a larger-scale torrefaction plant in the future. Also, a preliminary study on the biomass torrefaction behavior was done by analyzing the experimental products. Mechanical dewatering was found to be quite effective for handling herbaceous biomass feedstock as it removes approximately 30% of the moisture; this pre-treatment also improves the biomass quality by removing about half of the inorganic matters, which could cause slagging and fouling problems during combustion. Besides this, chemical analysis incorporating extraction, hydrolysis and High Performance Liquid Chromatography (HPLC) showed that pressing had removed about 10wt% of the carbohydrates and 20wt% of acid insoluble lignin. In addition, studies on extractive free samples proved that extractives had a catalytic effect on the thermal reactivity of biomass, which means that removal of extractives could lead to (slightly) higher thermal decomposition temperature. Depending on the process conditions, dried biomass will suffer a 20% - 50% mass loss during torrefaction. Results from the chemical analysis on the torrefied grass had shown the reduction of carbohydrates content at different torrefaction temperatures. The resulted solid product, biochar, has a higher energy density than the primary feedstock and it is easier to store and transport. Also, torrefaction makes the biomass feedstock more brittle and less fibrous, which would benefit the fuel preparation for co-firing.Energy TechnologyProcess and EnergyMechanical, Maritime and Materials Engineerin