October 20, 2021

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Steam/Oxygen Gasification of Biomass for Synthetic Fuels Production

Gasification is the heating of carbonaceous feedstock to very high temperatures (>650°C) in an oxygen-limited environment.  Restricting the amount of oxygen below the stoichiometric requirements for complete combustion results in conversion of the solids to a flammable synthetic gas, known as syngas.  Raw syngas is composed of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2), methane (CH4), water vapor (H2O), and multiple contaminants derived from the process or impurities in the feedstock.  Fuel synthesis processes typically tolerate very low levels of contamination and hence require extensive syngas cleaning. 

The primary objectives for this project are gasifying biomass feedstock and purifying the syngas to contaminant levels tolerated by synthetic fuels production, such as hydrogen or Fischer-Tropsch liquids.  (A more complete description of synfuels production is available at the following project web page: Renewable Hydrogen Production from Biomass Gasification [add link].) This project will ultimately demonstrate renewable fuels production through a continuous biomass to synfuels process development unit (PDU) that utilizes commercially-ready cleaning and processing technology.

Figure 1. Syngas to synfuels process overview. Phase 1 of the project is currently underway, and involves two major process areas: steam and oxygen gasification (blue) and syngas cleanup (green). The purple components are projections for phase 2 of the project involving syngas utilization.

The PDU for this project features a 20 kg/h fluidized bed gasification reactor (gasfier) that can be operated at pressures up to 20 psig. It is specially designed to be adiabatic in order to accurately simulate the behavior of larger industrial gasifiers. The gasifier is operated at 850°C using either air or steam/oxygen mix as a fluidizing agent. 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). The primary gasification feedstock for this project is switchgrass, but other possibilities include corn stover, wood fiber, corn fiber, and red oak.

Figure 2. Semi-pressurized biomass feed system (two vessels at left) and fluidized bed gasification reactor (right) located at Iowa State University’s BioCentury Renewable Farm (Boone, IA).

The gasification process also generates minor amounts of higher molecular weight components (tars), contaminant compounds (H2S, NH3, HCl, etc.) and char. These components inhibit downstream applications by fouling pipes or deactivating catalysts used in fuels production. Removing them is necessary to apply the gas stream in multiple applications from power generation in turbines to liquid fuels production via Fischer-Tropsch synthesis.

The cleaning system downstream of the gasifier removes the contaminants using several techniques. Tar and residual particulate matter are removed by oil scrubbing. Sulfur removal is performed through fixed-bed adsorption. Water scrubbing removes ammonia via absorption.

Figure 3. Gas cleaning process under construction at the BioCentury Renewable Farm. The order of unit operations proceeds from left as follows: tar and residual particulate matter removal, sulfur removal, ammonia removal, syngas polishing reactor for residual contaminant removal.

These methods were selected to effectively demonstrate the removal of contaminants while minimizing waste and using commercially available technology.  For example, tar compounds condense or dissolve from the gas stream into the heavy gas oil utilized in the oil scrubber.  The oil scrubber also removes residual particulate matter missed by the upstream cyclones.  Conventional techniques for removing tar and particulate matter use water as the scrubbing liquid, which requires treatment and disposal.  Using oil enables further processing of the waste stream for applications such as heat and power generation, in which the tars and particulate matter serve as additional carbon feedstock rather than contamination.

The second phase of the project tentatively includes Fischer-Tropsch conversion of the gas stream into liquid fuels.  The FT process transforms the carbon monoxide and hydrogen gas stream into synthetic hydrocarbons that are essentially identical to fossil-based hydrocarbons. These fuels are thus suitable for direct application to the current fuels transport and distribution infrastructure, demonstrating biomass to biofuel technology in a present-day bioeconomy.

Principal Investigators:

Dr. Robert C. Brown, Iowa State University, rcbrown3@iastate.edu

Research By:

Karl Broer, Iowa State University, kbroer@iastate.edu
Patrick J. Woolcock, Iowa State University, woolcock@iastate.edu

Participating and/or Sponsoring Organizations:

U.S. Department of Energy