Energy & Fuels, Vol.34, No.3, 3661-3670, 2020
Extension of the Thermal Annealing Concepts Developed for Coal Combustion to Conversion of Lignocellulosic Biomass
During the lifetime of coal and biomass particles in a reactor, severe changes in the carbonaceous structure occur. In the early stages of heat treatment, transformations at both the structural and chemical level are dramatic and well-recognized in the literature under the name of pyrolysis. Further heat treatment, even in parallel with heterogeneous reaction, produces less evident changes but is still very impactful on the char reactivity in the late stages of burnoff, which are recognized in the literature under the name of thermal annealing. Thermal annealing of biomass has often been neglected and underestimated. In the present work, a large experimental campaign has been carried out to measure the effects of heat treatment on the reactivity of a lignocellulosic material, namely, walnut shells, and, for comparison, a bituminous coal. The campaign included experiments in a thermogravimetric analyzer, fixed bed reactor, drop tube reactor, heated strip reactor, and flat flame burner, spanning over a very wide range of temperatures (700-2300 K), heating rates (0.1-10(5) K/s), and residence times (0.02-20 000 s). Results provide a unique set of data useful for testing pyrolysis-annealing models on lignocellulosic materials. Thermal annealing is very relevant also for lignocellulosic biomass and reduces the reactivity of char up to 1 order of magnitude. However, the distinction between pyrolysis and thermal annealing is made complex by the presence of multiple components with different inertia to thermal treatment. The applicability of the concepts developed for thermal annealing of coal to lignocellulosic biomass is therefore open to discussion. It is observed that the boarder line between the stage of pyrolysis and the stage of thermal annealing can be reasonably set when approximately 70-80% of the (ASTM) volatile matter content has been released. The early stages of thermal annealing, involving aromatization and graphitization of the carbon structure, occur in parallel with pyrolysis tails.