Gasifier: Gasifiers for Special Applications
Hydrogasification is gasification in a hydrogen-rich environment, often used for the production of synthetic natural gas (SNG) from coal or other gasifier feedstocks. Hydrogasification for SNG production from coal and biomass has been used since the 1930s. Hydrogasification tends to have low carbon conversions and product yields and slower reaction rates (without the use of catalysts).
Steam hydrogasification uses both steam and hydrogen to affect the reaction. For example, steam hydrogasification of wood substantially increases the production rate of methane (the key component of SNG) over simple hydrogasification. This process requires more energy to bring the reactor to gasification temperatures because of the high latent heat of water, but if the steam produced can be used later (say, for steam methane reformation) this may not be a disadvantage. Steam hydrogasification also requires a supply of hydrogen, which can be supplied by various processes, such as reformation of natural gas or oxygen-steam gasification of char. The addition of steam into the gasifier promotes additional hydrogen formation. This hydrogen can be separated and reused but the advantages of this may be offset by the cost of gas separation.
Hydrogasification does not require an oxygen plant, which can be a substantial cost to a gasification facility. It also minimizes exothermic reactions (better thermal balance of reactions) so it can be more thermally efficient. The addition of steam to hydrogasification significantly increases reaction rates which lowers residence times allowing for, amongst other things, smaller reactors. Since the feed will be gasified with water (steam), it does not need to be dried beforehand and could potentially be fed as a slurry. Although steam hydrogasification has been studied only recently (a project at the University of California – Riverside is described below), it appears to be compatible with all the typical gasification feedstocks, from coal to renewable sources like wood, agriculture residues, green wastes, municipal solid wastes, food and animal waste, and sewage sludge. Some studies have shown that coal can be mixed with dead wood, agricultural residues, and animal waste without significantly affecting the process efficiency.
University of California – Riverside CE-CERT Project
The research team, through the University, has filed 11 patent applications for the technology and, in 2006, scaled up the process (tested at bench level) into a design for a pilot-scale plant. Viresco Energy, LLC, a local company and primary investor in the research with a global option on the process, is working to build a pilot plant in Utah. As currently conceived, the pilot plant would process 5 tons per day feed of coal and/or coal-biomass mixtures and provide information necessary for further commercialization and understanding of the science behind steam hydrogasification.
The Bourns College of Engineering – Center for Environmental Research and Technology (CE-CERT) lab at UC-Riverside has worked to further develop the steam hydrogasification process. Their method consists of four highly coupled steps: steam hydrogasification, hot gas cleanup, steam reforming, and a fuel processing step to produce clean liquid fuels and other useful synthesized products.