Plasma pyrolysis provides high temperature and high energy for reaction as the reaction sample is rapidly heated up to a high temperature. This review also covers the experimental and modeling study status of plasma-assisted pyrolysis.
Biofuels produced via gasification routes include direct gasoline and diesel substitutes made from gas-to-liquid processes (i.e., the Fischer-Tropsch process), methanol, ethanol, mixed alcohols, and hydrogen. Gas-to-liquids technologies are utilized commercially using natural gas or stranded natural gas as feedstock. Coal was used extensively by Germany in WWII and is still used in the Sasol (South Africa) facilities for gasoline and diesel fuel synthesis along with a wide variety of other products.
In a biomass-to-liquids process, the feedstock undergoes a pretreatment or selection (sizing, drying, and sorting) and is then gasified in a reactor. The gas product (carbon monoxide, hydrogen, low-boiling hydrocarbon derivatives, tars, and particulate material) undergoes extensive cleanup to remove catalyst poisons and other undesirable components. This is followed by gas processing/reforming where the hydrogen/carbon monoxide ratio is adjusted before entering the (Fischer-Tropsch) synthesis reactor. The liquid synthesis reactor contains catalyst material and operates at elevated pressure and temperature forming hydrocarbon compounds or alcohols from the synthesis gas. The liquids can be further refined to the desired end product.
See also: Biomass – Gasification, Biomass to Syngas, Synthesis Gas.
Biomass Waste
Biomass is the material derived from plants that use sunlight to grow which include plant and animal material such as wood from forests, material left over from agricultural and forestry processes, and organic industrial, human, and animal wastes. Biomass comes from a variety of sources which include (alphabetically and not by preference or use):
Agricultural residues such as straw, stover, cane trash and green agricultural wastes
Agro-industrial wastes, such as sugarcane bagasse and rice husk
Animal wastes
Food processing wastes
Forestry plantations
Forestry residues
Municipal solid wastes (MSW)
Industrial wastes, such as black liquor from paper manufacturing
Sewage
Wood from natural forests and woodlands
The energy contained in biomass originally came from the sun. Through photosynthesis, carbon dioxide in the air is transformed into other carbon-containing molecules (e.g., sugars, starches, and cellulose) in plants. The chemical energy that is stored in plants and animals (animals eat plants or other animals) or in their waste is called bio-energy.
When biomass is burned, it releases its energy, generally in the form of heat. The biomass carbon reacts with oxygen in the air to form carbon dioxide. If fully combusted, the amount of carbon dioxide produced is equal to the amount which was absorbed from the air while the plant was growing.
In nature, if biomass is left lying around on the ground, it will break down over a long period of time, releasing carbon dioxide and its store of energy slowly. By burning biomass, its store of energy is released quickly and often in a useful way. So converting biomass into useful energy imitates the natural processes but at a faster rate.
Biomass wastes can be transformed into clean energy and/or fuels by a variety of technologies, ranging from conventional combustion process to state-of-the-art thermal depolymerization technology. Besides recovery of substantial energy, these technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal, which can be better managed for safe disposal in a controlled manner while meeting the pollution control standards.
Biomass waste-to-energy conversion reduces greenhouse gas emissions in two ways. Heat and electrical energy are generated which reduces the dependence on power plants based on fossil fuels. The greenhouse gas emissions are significantly reduced by preventing methane emissions from landfills. Moreover, waste-to-energy plants are highly efficient in harnessing the untapped sources of energy from wastes.
Biomass energy projects provide major business opportunities, environmental benefits, and rural development. Feedstocks can be obtained from a wide array of sources without jeopardizing the food and feed supply, forests, and biodiversity in the world and include (i) agricultural residues, (ii) animal waste, (iii) forestry residue, (iv) wood waste, (v) industrial waste, (vi) municipal solid waste, and (vii) sewage.
Agricultural residues encompasses all agricultural wastes such as bagasse, straw, stem, stalk, leaves, husk, shell, peel, pulp, stubble, etc. Large quantities of crop residues are produced annually worldwide, and are vastly underutilized. Rice produces both straw and rice husks at the processing plant which can be conveniently and easily converted into energy. Significant quantities of biomass remain in the fields in the form of cob when maize is harvested which can be converted into energy. Sugar cane harvesting leads to harvest residues in the fields, while processing produces fibrous bagasse, both of which are good sources of energy. Harvesting and processing of coconuts produces quantities of shell and fiber that can be utilized. The current farming practice is usually to plough these residues back into the soil, or they are burnt, left to decompose, or grazed by cattle. These residues could be processed into liquid fuels or thermochemically processed to produce electricity and heat. Agricultural residues are characterized by seasonal availability and have characteristics that differ from other solid fuels such as wood, charcoal, and char briquette. The main differences are the high content of volatile matter and lower density and burning time.
Animal waste represents a wide range of biomass that can be used as sources of biomass energy. The most common sources are animal and poultry manures. In the past, this waste was recovered and sold as a fertilizer or simply spread onto agricultural land, but the introduction of tighter environmental controls on odor and water pollution means that some form of waste management is now required, which provides further incentives for waste-to-energy conversion. The most attractive method of converting these waste materials to useful form is anaerobic digestion which gives biogas that can be used as a fuel for internal combustion engines, to generate electricity from small gas turbines, burnt directly for cooking, or for space and water heating.
Forestry residues are generated by operations such as thinning of plantations, clearing for logging roads, extracting stem-wood for pulp and timber, and natural attrition. Harvesting may occur as thinning in young stands, or cutting in older stands for timber or pulp that also yields tops and branches usable for biomass energy. Harvesting operations usually remove only 25 to 50% of the volume, leaving the residues available as biomass for energy. Stands damaged by insects, disease, or fire are additional sources of biomass. Forest residues normally have low density and fuel values that keep transport costs high, and so it is economical to reduce the biomass density in the forest itself.
Wood wastes generated by the wood processing industries primarily include sawmilling, plywood, wood panel, furniture, building component, flooring, particle board, molding, jointing, and craft industries. Wood wastes generally are concentrated at the processing factories, e.g., plywood mills and sawmills. The amount of waste generated from wood processing industries varies from one type of industry to another depending on the form of raw material and finished product. Typically, the waste from wood industries such as saw millings and plywood, veneer, and others are sawdust, off-cuts, trims, and shavings. Sawdust arises from cutting, sizing, re-sawing, and edging, while trims and shaving are