Combustion of lignin-rich residues with coal in a pilot-scale bubbling fluidized bed reactor

The production of second-generation bioethanol generates a waste stream consisting of lignin-rich residues whose valorization has to be found. Fluidized bed combustion technology can be considered as a promising and viable option to recover thermal power from lignin-rich residues of a second-generation bioethanol production plant. This work aims at investigating the combustion of lignin-rich residues with coal in a pilot-scale bubbling fluidized bed combustor (FBC). An experimental campaign was carried out to study gaseous and particulate emissions and thermal regimes during the combustion of mixtures of coal with 30%w of lignin-rich residues varying bed temperature, excess air and fluidization velocity. A few experimental tests were carried out for comparison using only coal, a mixture of coal with 40%w of lignin-rich residues and a mixture of coal with 20%w of wood chips. The analysis of the experimental results mainly highlights that: 1) the gaseous emissions did not significantly change with respect to coal or to reference biomass-coal mixture at least until the content of lignin-rich residues in the mixture was 30-40%w; 2) the particulate emissions increased with the percentage of residues content, but, at the same, the particulate carbon concentration was significantly reduced. Bottom bed particles were analyzed at the end of each experiments, highlighting the absence of agglomerates. However, a significant enrichment of metals like Fe, Mg, Na, Ca and K, most of them probably coming from the ash of lignin-rich residues, was observed when the FBC was operated under conditions which emphasize ash accumulation inside the bed.