Introduction
Biomass is the material derived from plants that use sunlight to grow, which include plant and animal materials 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:
- Wood from natural forests and woodlands
- Forestry plantations
- Forestry residues
- Agricultural residues such as straw, stover, cane trash and green agricultural wastes
- Agro-industrial wastes, such as sugarcane bagasse and rice husk
- Animal wastes
- Industrial wastes, such as black liquor from paper manufacturing
- Sewage
- Municipal Solid Wastes (MSW)
- Food processing wastes
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 is 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.
Conversion Technologies
Biomass energy technology is inherently flexible. The variety of technological options available means that it can be applied at a small, localized scale primarily for heat, or it can be used in much larger base-load power generation capacity whilst also producing heat. Biomass generation can, thus, be tailored to rural or urban environments, and utilized in domestic, commercial or industrial applications.
A host of technologies are available for realizing the potential of biomass waste as an energy source, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste.
Biomass can be converted into energy by simple combustion, by co-firing with other fuels, or through some intermediate process such as gasification. The energy produced can be electrical power, heat, or both (combined heat and power, or CHP). The advantage of utilizing heat as well as or instead of electrical power is the marked improvement of conversion efficiency – electrical generation has a typical efficiency of around 30%, but if heat is used, efficiencies can rise to more than 85%.
Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. In addition, wastes can also yield liquid fuels, such as cellulosic ethanol, which can be used to replace petroleum-based fuels. Algal biomass is also emerging as a good source of energy because it can serve as natural source of oil, which conventional refineries can transform into jet fuel or diesel fuel.
Major Types of Biomass 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.
Agricultural Residues
Crop 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 under-utilised. 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. Sugarcane 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 fibre that can be utilized.
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 thermo-chemical 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, char briquette. The main differences are the high content of volatile matter and lower density and burning time.
