A biorefinery is a plant that converts biomass into useful products such as fuels, chemicals and power. Three types of refineries are being developed. 1) The sugar biorefinery, which in widespread use, is based on the fermentation of sugars. 2) A close coupled system that primarily produces fuel that is used to produce power or heat from either syngas or pyrolysis oil. 3) This post discusses a thermochemical refinery that is more analogous to a petroleum refinery and can produce an array of products in addition to fuel and power. This process is referred to as the Biomass to Liquid (BTL) process. This process consists of:
- Feed processing and handling to prepare the biomass for gasification.
- Gasification of the biomass, producing syngas.
- Syngas pretreatment prior to liquefaction to remove undesirable components.
- Gas to liquids (Fischer-Tropsch) technology.
- Product Refining of the liquids into fuels and byproducts.
Feed processing and handling technologies are necessary to provide a uniform supply of feedstock. The system should incorporate reliable feed preparation, storage, and handling systems. Current generation feed supply systems require a narrow range of physical properties including size and moisture content. Feedstock supply systems that can handle a wider variety of feedstocks or on site system that can convert feedstock to a consistent form that can be handled by existing feeders would be desirable and are the subject of ongoing research.
Biomass Gssification uses high temperatures (greater than 650 oC) to thermally degrade the feedstock into a mixture of gases, primarily composed of CO, H20, H2 and CO2. In Europe biomass gasifiers are widely used to power engines and to provide gas for district heating. In the U.S. biomass gasifiers have been developed and tested on a pilot and demonstration scale. The Vermont Gasifier Demonstration Project has demonstrated, on a 200 MW per day scale, that consistent quality syngas can be produced from biomass on a continuous basis. Research is continuing to determine operating conditions that minimize tar loadings and still maintain quality syngas. Pearsons (see below) has demonstrated gasifiers as part of their projects.
Gas Cleanup is a major challenge in the development of of a thermochemical refinery, integrating the gasifier and the downstream gas to liquids step. In particular, particulate removal and tar reforming are the most pressing problems. These contaiminates are not as a severe problem in close coupled gasifiers but present a more serious problem in BTL processes. Various filtration processes are being tested and evaluated by NREL. Several catalysts are being evaluated for use in the steam reforming catalytic tar-conditioning process. Pearson Technologies of Mississippi, Inc. has constructed a 30 ton per day BTL wood waste to ethanol facility in Mississippi. They have employed off the shelf gas cleanup equipment in their process with great success. A paper presented at the Pyrolysis and Gasification of Biomass and Waste, Expert meeting, Sept, 30, 2002, in Strasbourg, France discusses gas cleanup requirements as encountered in their testing. A paper and presentation "Large-Scale Production of Fischer Tropsch Diesel from Biomass", 2004, updates this project.
Fischer Tropsch (FT) synthesis of the syngas to liquids is fairly straight foreword. DOE has all but stopped development of this part of the process, focusing its efforts on FT implementation in coal liquefaction technologies. Pearsons uses FT synthesis with a proprietary catalyst they have developed. In addition to the project referred to above, Pearsons has conducted pilot plant studies converting rice straw to ethanol for the city of Gridley, CA, and has proposed building a demonstration plant using their technology.
Product Refining - Since the liquids are quite clean, having no significant contaminants they can often be used directly, as fuels, without further processing. Production of byproducts from the FT liquids has not been demonstrated to any great extent. The use of petroleum refining practices to produce byproducts is expected to be fairly straightforward.
A study conducted by Eindenhoven University of the Netherlands indicates that the cost of producing ethanol and electricity from a thermochemical biorefinery is comparable to current diesel prices.