Center for Sustainable Environmental Technologies

Steam/Oxygen Gasification of Biomass for Fischer-Tropsch Synthetic Fuels

This project will include essentially three stages: a steam and oxygen blown gasification process, a cleanup train to remove contaminants such as char, tars, hydrogen sulfide, hydrochloric acid, and ammonia, and the final FT-synthesis process unit.  The first stage utilizes thermochemical conversion processes to transform ground biomass of low moisture content into a hot gas stream at a rate of around 20 kg/hr. Typical feedstock is switchgrass of 5-15% moisture, but possibilities include corn stover, corn fiber, red oak, etc.  Typical operating parameters for this system are estimated to be temperatures around 850C at 15 psig pressure, with a steam and oxygen mixture to serve as the fluidizing gas and oxidant.

The thermochemical process of gasification uses the energy released by partial combustion of the biomass to heat the remaining biomass and decompose the complex solid carbonaceous formations into gaseous compounds.  A steam and oxygen mixture is used in this application to provide fluidization and the oxidizing agent for partial biomass combustion.  High temperatures within the reactor result in rapid chemical reactions, yielding a producer gas stream composed mostly of lower molecular weight compounds (H2, CO, CO2, CH4, and H2O).  Added benefits of the steam and oxygen mixture when compared with the more simplistic air gasification approach include less dilution by the inert nitrogen component of air and a more beneficial final composition of the gas stream for applications in FT synthesis.  Also created are minor amounts of higher molecular weight components (tars), contaminant compounds (H2S, NH3, HCl, etc.) and char (ash and elutriated carbon resulting from incomplete reactions).  Once cleaned of these minor contaminants, the clean gas stream can be utilized in multiple applications from power generation in turbines to liquid fuels through Fischer-Tropsch synthesis. 

The cleaning system connected to this gasifier will remove the problematic chars, tars, and other contaminant compounds which cause pipe blockages, inhibit downstream applications, or deactivate catalysts used in fuels production.  Rather than using in-situ methods of gas cleanup which manipulate the gas stream products while still within the reaction vessel, this project focuses on methods that clean the resulting gas downstream of the reaction vessel.  These secondary methods will include oil scrubbing for tar and residual particulate removal, sulfur removal through fixed-bed adsorption, and final ammonia absorption with water scrubbing.  These methods were selected to effectively demonstrate the reduction of contaminants while minimizing waste and using commercially available technology. 

For example, the heavy tar compounds will be soluble in the heavy gas oil utilized in the oil scrubber, and will therefore be removed from the gas stream into the scrubbing fluid along with any remaining particulates.  This approach has the additional benefit of reducing potential waste streams which occur with typically practiced water scrubbing.  Traditional water scrubbing requires water treatment to remove the impurities from the contaminated scrubbing fluid.  However, the scrubbing oil shows potential for further application in fuel processing or heat and power generation with the tars and particulates serving as added carbon feedstock materials. 

The final stage involves the Fischer-Tropsch conversion of the gas stream into liquid fuels.  The FT process will utilize a currently available catalyst and reactor configuration to transform the carbon monoxide and hydrogen gas stream into synthetically produced hydrocarbon fuels.  These fuels are suitable for direct application to our society’s infrastructure and will ultimately demonstrate this biomass to biofuel technology in a present-day bioeconomy.   

Principal Investigators:

Dr. Robert Brown, Iowa State University, rcbrowniastate.edu
Dr. Samuel Jones, Iowa State University, sjonesiastate.edu

Research By:

Karl Broer, Iowa State University, kbroeriastate.edu
Patrick J. Woolcock, Iowa State University, woolcockiastate.edu

Participating and/or Sponsoring Organizations:

ConocoPhillips
U.S. Department of Energy