Thermochemical conversion processes include combustion, gasification, and pyrolysis. Potential energy types include heat, steam, electricity, and liquid fuels (biofuels if the feedstock is biomass). Liquid fuel products from thermochemical conversion routes include, ethanol, methanol, mixed alcohols, Fischer-Tropsch (FT) liquids, other renewable gasolines and diesels, pyrolysis oils, and others. There currently are no commercial facilities producing liquid fuels from municipal solid waste (MSW)-derived feedstocks. (There are facilities producing electricity, heat, and steam).
Waste and Biomass to Energy
Waste to energy, also known as combustion, is oxidation of the fuel for the production of heat at elevated temperatures without generating useful intermediate fuel gases, liquids, or solids. Combustion normally employs excess oxidizer (air) to ensure maximum fuel conversion. Products of combustion processes include heat, oxidized species (e.g. carbon dioxide, water), products of incomplete combustion (e.g. carbon monoxide and hydrocarbons), other reaction products (most as pollutants), and ash. Electricity can be produced using boilers and steam-driven engines or turbines.
In 2014 there were 84 facilities in the United States that combust MSW for energy recovery (three are in California), consuming about 30 million tons of waste per year. In Europe, more than 400 combustion-based MSW energy plants are operating, consuming about 82 million tons of material annually.
In California, there are about two dozen active biomass to energy plants in operation. These plants are fueled primarily by wood waste and agricultural residues. For additional information check the California Biomass Energy Alliance website.
Gasification typically refers to conversion in an oxygen- or air-deficient environment to produce fuel gases (e.g. synthesis gas, producer gas). The fuel gases are principally carbon monoxide, hydrogen, methane, and lighter hydrocarbons, but depending on the process used, can contain significant amounts of carbon dioxide and nitrogen, the latter mostly from air. Gasification processes also produce liquids (tars, oils, and other condensates) and solids (char, ash) from solid feedstocks. The combustion of gasification-derived fuel gases generates the same categories of products as direct combustion of solids, but pollution control and conversion efficiencies may be improved.
Electricity and heat can be produced by burning the synthesis gas in a steam boiler and turbine plant, a gas turbine, or an internal combustion or stirling engine generator. Synthesis gases can produce fuel products and other chemicals by chemical reactions such as Fischer-Tropsch synthesis.
U.S. Department of Energy’s National Energy Technology Laboratory tracks proposed gasification projects in the United States and world-wide.
Pyrolysis is the thermal degradation of a material usually without the addition of any air or oxygen. The process is similar to gasification but generally optimized for the production of fuel liquids or pyrolysis oils (sometimes called bio-oils if biomass feedstock is used). Pyrolysis also produces gases and a solid char product. One such char product, known as biochar, may be used as a soil or compost additive to sequester carbon in the soil.
Pyrolysis liquids can be used directly (e.g. as boiler fuel and in some stationary engines) or refined for higher quality uses such as motor fuels, chemicals, adhesives, and other products. Direct pyrolysis liquids may be toxic or corrosive.
Plasma arc refers to a specific device used to provide heat for gasification, pyrolysis, or combustion, depending on the amount of oxygen fed to the reactor. Plasma arc processes use electricity passing through electrodes to produce a discharge converting the surrounding gas to an ionized gas or plasma. Gases heated in plasmas typically reach temperatures of 7000° F or higher.
The Gasification Technologies Council has more information on their website about plasma arc gasification.
Additional information on thermochemical conversion technologies can be found in New and Emerging Conversion Technologies: Report to the Legislature, June 2007.