Pilot Scale Production of Mixed Alcohols from Wood

An integrated thermochemical biomass to ethanol process was demonstrated at the pilot scale at the National Renewable Energy Laboratory (NREL). A total of 233 h of pilot scale mixed alcohol production was achieved, comprising 81 h of continuous operation in methanol-derived syngas followed by 152 h of continuous operation in biomass-derived syngas. During this period the system generated 20 L of mixed alcohol product. The fully integrated biomass to mixed alcohol process was comprised of a solids feeder, fluidized bed indirect steam gasifier, thermal cracker, char collector, fluidized bed steam reformer, packed bed polishing steam reformer, scrubber, pressure-swing CO₂ adsorber, and gas-phase continuously stirred tank gas-to-liquid reactor (CSTR). Additional pumps, compressors, and blowers were used to convey gases, solids, and liquids. Tars and methane were reformed using sequential steps: first in a fluidized bed using an NREL-developed Ni-based catalyst followed by a fixed bed reactor loaded with pelletized, precious metal catalyst developed by Johnson Matthey. Mixed alcohols (a mixture of methanol, ethanol, 1-propanol, etc.) were produced using a metal sulfide catalyst developed at NREL. Under steady state conditions, the steam reformers converted >99.9, 97.0, and 86% of tars, benzene, and methane, respectively, in the producer gas. A simulated partial recycle of carbon dioxide to the gasifier was used to reduce the H₂:CO ratio of the reformed syngas to 3:1 without adding water gas shift reactors to the process or coking the reforming catalysts. When operating on biomass-derived syngas in a CSTR, the fuel synthesis catalyst produced as much as 31 g of EtOH·kg of catalyst^–1·h^–1 at a CO₂-free ethanol selectivity of 27% at 2000 psi, 300 °C, and 27% CO conversion. A bench scale packed bed reactor operated under analogous conditions produced 39 g of EtOH·kg of catalyst^–1·h^–1 at a CO₂-free ethanol selectivity of 28% showing reasonable parity between bench scale and pilot scale