Kinetics of combustion of lignocellulosic biomass: recent research and critical issues

Predicting biomass combustion at temperatures of practical interest requires kinetic expression robust and reliable over a large temperature range. The present work examines experimental works on combustion kinetics of lignocellulosic biomass published since 2015 and reports the experimental methods used and the kinetic parameters obtained therein. The most common experimental strategy, used in 39 referenced works, was non-isothermal thermogravimetric analysis, yet the kinetic parameters obtained varied by order of magnitude. The large variability of the kinetic parameters found in literature cannot be only explained by the feedstock heterogeneity or the different data analysis methods. One possible and apparently neglected source of error lies in the fact lignocellulosic biomasses are complex materials, constituted by different components, thus thermogravimetric curves are the resultant of multiple and partly overlapping stages of mass loss. Kinetic analysis in this case easily generates errors. Another criticality that the work wants to emphasize is the fact that biomass pyrolyses, burns and even thermally anneals throughout a TGA experiment. These progressive structural changes are a further problem for application of conventional kinetic analysis methods. Altogether, it can be concluded that it is difficult to obtain from conventional TGA analysis reliable kinetic expressions of biomass combustion to be used for boilers modelling. For complex materials like biomass, it is recommended to adopt expert-case sensitive approaches. As an example, the case of walnut shells combustion is presented. For this material, four components with different combustion reactivity have been identified and simplified single rate kinetics are proposed according to the temperature interval of interest.