Effect of biomass high-frequency pre-treatment on combustion characteristics

Combustion characteristics of microwave (2.45 GHz) pre-treated biomass (wood) pellets are investigated experimentally with the aim to find out the effect of pellets pre-treatment on the biomass composition and the feasibility of their thermo-chemical conversion. A complex study of the main combustion characteristics of the pre-treated biomass samples was carried out using a thermo-gravimetric method and a pilot-scale combustion test facility. The study includes time dependent measurements of mass loss at the primary stage of biomass gasification, variations of the flame temperature and heat production rate, combustion efficiency and composition of pollutants. The results have shown that the low-temperature microwave pre-treatment of wood pellets enriches the biomass carbon content from 50 % to ∼ 60 % (d.b.) for biomass pre-treated for 180 s accompanied by the increasing higher heating values (HHV) from 19.9 up to 23.4 MJ/kg. The microwave pre-treatment promotes a faster thermal decomposition of biomass pellets with an enhanced heat energy production (up to 30 %) and a correlating increase of the volume fraction of CO ₂ in the products up to 25-28 %, while the mass fraction of the main volatiles (CO and H₂) in the products decreases, indicating a more complete combustion of the volatiles. The shear of the char combustion stage increases with the increasing pre-treatment time. The data of thermo-gravimetric analysis coincide with the results of the pilot-scale tests.