Co-pyrolysis of low rank coals and biomass: Product distributions

Pyrolysis and gasification of combined low rank coal and biomass feeds are the subject of much study in an effort to mitigate the production of green house gases from integrated gasification combined cycle (IGCC) systems. While co-feeding has the potential to reduce the net carbon footprint of commercial gasification operations, success of this strategy requires investigation of the effects of coal/biomass co-feeding on reaction kinetics and product distributions. Southern yellow pine was pyrolyzed in a semi-batch type drop tube reactor with either Powder River Basin sub-bituminous coal or Mississippi lignite at several temperatures and feed ratios. Product gas composition of expected primary constituents (CO, CO2, CH4, H-2, H2O, and C2H4) was determined by in situ mass spectrometry while minor gaseous constituents were determined using a GC-MS. Product distributions are fit to linear functions of temperature, and quadratic functions of biomass fraction, for use in computational co-pyrolysis simulations. The results are shown to yield significant nonlinearities, particularly at higher temperatures and for lower ranked coals. The co-pyrolysis product distributions evolve more tar, and less char, CH4, and C2H4, than an additive pyrolysis process would suggest. For lignite co-pyrolysis, CO and H-2 production are also reduced. The data suggests that rapid pyrolysis of biomass produces hydrogen that stabilizes large radical structures generated during the early stages of coal pyrolysis. Stabilization causes these structures to be released as tar, rather than crosslinking with one another to produce secondary char and light gases. Finally, it is shown that, for the two coal types tested, co-pyrolysis synergies are more significant as coal rank decreases, likely because the initial structure in these coals contains larger pores and smaller clusters of aromatic structures which are more readily retained as tar in rapid co-pyrolysis. (C) 2013 Elsevier Ltd. All rights reserved.