The effect of birch-bark addition on the elemental composition and combustion characteristics of different types of biomass pellets

The main aim of this paper is improvement of the combustion characteristics of different biomass types (wood and agriculture residues, herbaceous biomass) by adding birch outer bark or bark (up to 20 %) to raw biomass with densification of the produced mixture. Produced pellets are experimentally characterized by elemental analysis determining the composition, higher and lower heating values, bulk and energy density. The results of elemental analysis of the pelletized mixture show that for all biomass types an addition of birch outer bark or birch bark to raw biomass results in an increase of the carbon content and heating values of produced samples relative to original biomass pellets with direct impact on the gasification and combustion characteristics of swirling flame flow. A more pronounced increase of the heating values (up to 18.8 %) after addition of outer birch bark or bark to raw biomass (20 %) is detected for wheat straw pellets indicating a lower carbon and lignin content in raw biomass. Less effective variations of the heating values at birch bark or outer birch bark addition (up to 13 %) are detected for woody biomass - grey alder pellets with a higher carbon and lignin content in raw biomass. In accordance with the variations of the elemental composition and heating values of the produced samples with birch bark or birch outer bark addition, a correlating increase of the heating values, heat power and total produced heat energy at thermo-chemical conversion of pelletized mixtures downstream the swirling flame flow with the high level of the correlation coefficient (up to 96 %) are observed. Moreover, the increase of the total amount of the produced heat energy results in the correlating increase of the volume fraction of CO₂ and combustion efficiency, while the air excess, the mass fraction of CO and free hydrogen in the products decrease indicating that the birch-outer bark or bark addition to biomass promotes the enhanced combustion of the volatiles and can be used for more effective and cleaner heat energy production at thermo-chemical conversion of different biomass types.