Co-pyrolysis of torrefied biomass and coal: Effect of pressure on synergistic reactions

Abstract

The co-utilization of torrefied biomass and coal in thermochemical conversion technologies is an attractive process for the transition to green energy and chemicals. The main advantage of this process is the production of high quality oil products (closer resembling crude-oil) via synergistic interaction of primary products from the two feedstocks. These synergistic reactions often involve secondary reactions, which are promoted at high pressures. This paper reports quantitative results on the extent of synergistic reactions and the role of pressure on these reactions during co-pyrolysis of torrefied biomass and coal. Torrefied biomass was produced at 280 °C in a pilot rotary kiln and subsequently both pyrolysis and co-pyrolysis of torrefied biomass and coal was investigated in a fixed bed reactor (heating rate 7 °C/min) at temperatures of 400–600 °C and pressures of 1, 15 and 30 bar. The results show that the prior removal of hemicellulose during torrefaction maximized the potential of hydrogen transfer from cellulose/lignin-derived products to depolymerized coal fragments. Furthermore, the dehydration and condensation reactions of depolymerized fragments were suppressed during co-pyrolysis in favour of synergistic reactions between the fragments. These reactions occurred predominantly in the molten/liquid phase and their rates could be indirectly controlled by pressure (through changing the evaporation rate), resulting in substantial changes in product distribution. In the presence of coal and its released vapours, the methoxyphenols (guaiacols) and furanics yields were significantly enhanced (positive deviation >54% and >40%, respectively, compared to additive predictions), whereas the phenol yields were inhibited (negative deviation >20%); suggesting the inhibition of demethoxylation reactions in the presence of coal depolymerized fragments. To date no studies have reported the molten phase synergistic reaction pathways occurring during co-pyrolysis of torrefied biomass and coal. Based on the findings in this paper, reaction pathways were proposed for these molten phase synergistic reactions.