Producing pellets from torrefied herbaceous biomass in a commercial capacity

Abstract

The global temperature rise has pushed governments more into finding ways to reduce CO2 emissions, by increasing the use of renewable fuels. Lignocellulosic biomass (wood, forest residues, agricultural residues) is a renewable fuel that has still not been utilized to its full potential. That is because it lacks properties such as homogeneity, high volumetric energy density and low moisture that its main fossil fuel competitor -coal- has. Therefore, a type of pre-treatment is needed for these disadvantages. Torrefaction, a process of heating biomass (200-300 oC) in an inert environment (no combustion), provides a product with improved physicochemical properties: reduced hydrophobicity, easier grindability, homogeneity, reduced microbial activity and increased calorific value. However, it is not enough to torrefy the biomass, as it needs to be densified in order to be utilized. With the process of pelletizing, a type of densification, torrefied biomass becomes compact and can be considered as bio-coal. While pelletizing is influenced by many parameters (moisture content, die pressure, torrefaction temperature etc), herbaceous types of biomass such as wheat straw, hay, reeds and grass, still cannot produce quality pellets without the addition of a binder. Typical organic binders such as starch, lignin and sawdust can either be expensive or biologically degrade in storage conditions. That is why addition of plastic binder should be considered. The aim of this report was to investigate how quality torrefied pellets from herbaceous biomass can be produced with the addition of plastic binder in a commercial capacity. Wheat straw and woodchips were used as feedstock, while polyethylene resin was the plastic binder. The biomasses were torrefied at 240 oC and 270 oC on the Torrgreen facilities, via a pilot-scale packed bed reactor, which recycles the volatiles from torrefaction for inertization and pelletized with a 100 kg/h pellet mill. For the torrefied wheat straw, a design of experiments was formed, to investigate how 4 parameters for pelletizing (torrefaction temperature, moisture content, plastic binder addition and pellet diameter) improved mechanical durability. Results indicated that 5% of plastic improved durability and pellet formation in total, while higher torrefaction temperature weakens the pellets and aggravates pellet formation. The highest durability achieved was 90.3%. Torrefied wood chips were pelletized for reference and showed low durability, mostly due to inability of the pelletizer. Water immersion tests on the straw pellets showed that higher torrefaction temperature increased hydrophobicity a lot, while plastics also had a positive effect. Comparison of the straw pellets produced with conventional wood pellets revealed that the former lack in mechanical durability and are high in ash content, making them incompliant with current standards. Overall, inclusion of plastics in pelletizing of torrefied herbaceous biomass, demonstrated very positive results and improved their quality, while making the operation smoother, but for scaling up and keeping the whole process sustainable a polyethylene (or other plastic) waste stream should be utilized.