Energy conversion of biomass char: Oxidation rates in mixtures of O2/CO2/H2O

• Published in: Energy (Oxford) 2019-08, Vol.181, p.615-624
• Main Authors: Karlström, Oskar, Hupa, Leena
• Format: Article
• Language: English
• Subjects: Biomass; Biomass energy production; Carbon dioxide; Char; Combustion; Conversion; Energy conversion; Gasification; High temperature; Low temperature; Oxidation; Oxidizing agents; Particulates; Thermal energy
• ISSN: 0360-5442; 1873-6785
• DOI: 10.1016/j.energy.2019.05.192

In thermal energy conversion of biomass, the char conversion rate influences the design of industrial systems. Yet there is no consensus on the relative influences of O2, CO2 and H2O on the char conversion rate. The present study aims at clarifying the roles of these oxidizers on biomass char conversion. Single particles of different particles sizes were oxidized in mixtures of O2/N2, CO2/N2, H2O/N2 and O2/CO2/H2O/N2 at various temperatures. The results show that for large particles and high temperatures, CO2 and H2O gasification reactions play a dominant role, while for sufficiently small particles and low temperatures, the O2 oxidation reaction plays a dominant role in mixtures of O2/CO2/H2O/N2. Two different particle models were used to compute char conversions: a detailed single particle model, and a conventional particle model assuming that rates in mixtures of oxidizers can be predicted based on rates of single oxidizers. The results show that the char oxidation rates are strongly overestimated when predicting rates based on single oxidizers, while the detailed model shows a good agreement to the experimental measurements. •Biomass char oxidation model for the first time validated in mixtures of O2/CO2/H2O.•Influences of CO2 and H2O on char conversion depend strongly on the particle size.•Biomass char conversion in O2/CO2/H2O/N2 occurred more rapidly than in O2/N2.•Rates cannot be predicted based on rates of single oxidizers.