Co-Pyrolysis of Low Rank Coals and Biomass At Transport Gasifier Conditions

Given the EPA’s proposed rules on limiting CO₂ emissions from new coal power plants to match those of natural gas, blending of carbon-neutral biomass with coal is an increasingly attractive option for power producers.  Further, gasification technologies that can capture and store CO₂ prior to combustion are also being considered, and efforts to reduce the cost of electricity from such plants include the consideration of cheaper, lower rank coals and lignites, a significant and under-utilized resource within the United States.  Transport gasifiers hold significant promise for achieving the desired CO₂ emissions reductions in this manner, particularly in regard to low rank fuels.  As a result, the United States Department of Energy’s National Energy Technology Laboratory (NETL) has undertaken a significant research effort to advance the technology of transport gasification through state-of-the-art multi-phase computational modeling. 

In order to most accurately model the effects of co-feeding low rank coal and biomass into such a reactor, an experimental effort has been carried out to obtain kinetic rates and product distributions for co-pyrolysis and co-gasification of coal and biomass feedstocks currently in use at the Transport Integrated Gasifier (TRIG) at the National Carbon Capture Center in Wilsonville, AL.  The focus of this study is to evaluate of the complex pyrolysis reaction and to provide fundamental co-pyrolysis kinetic data under transport gasifier conditions for inclusion into NETL’s Carbonaceous Chemistry for Computational Modeling (C3M) platform, which supplies kinetic information to multiphase CFD codes and other computational models. 

The experimental test matrix includes sub-bituminous Powder River Basin (PRB) coal, Mississippi lignite, Southern Yellow Pine, multiple PRB/pine blends, and an 80/20 lignite/pine blend, at temperatures ranging from 600 – 975 °C, and pressures of 1, 4 and 16 atm.  Co-pyrolysis tests were performed in an isothermal drop reactor, where rapid sampling via in-situ mass spectrometry yields kinetic rates as well as quantitative product gas measurements of the primary syngas constituents CO, CO₂, CH₄, H₂, H₂O, H₂S and C₂H₄.  Product distributions of these species, as well as char and tar, are found to be approximately linear with respect to temperature and biomass blend ratio.  These results are generalized to all blend ratios, temperatures, and pressures within the ranges studied through curve fit coefficients generated by regression analysis.  Both kinetic activation energies and pre-exponential factors are shown to be non-linear with respect to biomass blend ratio, indicating a significant reaction synergy in co-pyrolysis of low rank coals and biomass that must be accounted for in accurate modeling of transport gasifier systems.  Comparisons are also made to kinetics and product distributions available in other kinetic modeling packages used for coal reaction chemistry.