For the production of energy from biomass, the term feedstock is used to refer to whatever type of organic material will be used to produce a form of energy. The feedstock must then be converted to a usable energy form through one of many processes.
Feedstock + Process -> Usable Energy Form
Some common biomass conversion processes include:
- Combustion: the process by which flammable materials are burned in the presence of air or oxygen to release heat. It is the simplest method by which biomass can be used for energy. In its rudimentary form, combustion is used for space heating (i.e. a fire for warmth) but can also be used to heat steam for electricity generation.
- Gasification: is the conversion of biomass into a combustible gas mixture referred to as Producer Gas (CO+H2+CH4) or Syngas. The gasification process uses heat, pressure and partial combustion to create syngas, which can then be used in place of natural gas.
- Pyrolysis: Consists of thermal decomposition in the absence of oxygen. It is the precursor to gasification, and takes place as part of both gasification and combustion. The products of pyrolysis include gas, liquid and a sold char, with the proportions of each depending upon the parameters of the process.
- Anaerobic digestion (or biodigestion): is the process whereby bacteria break down organic material in the absence of air, yielding a biogas containing methane and a solid residue. The methane can then be captured to produce energy. Similarily, the solid residue can also be burned to produce energy.
- Fermentation: involves the conversion of a plant’s glucose (or carbohydrate) into an alcohol or acid. Yeast or bacteria are added to the biomass material, which feed on the sugars to produce ethanol (an alcohol) and carbon dioxide. The ethanol is distilled and dehydrated to obtain a higher concentration of alcohol to achieve the required purity for the use as automotive fuel. The solid residue from the fermentation process can be used as cattle-feed and in the case of sugar cane can be used as a fuel for boilers or for subsequent gasification.
Some feedstocks are more conducive for certain biomass conversion processes than others. The determination of which feedstocks and processes will be used is determined largely by the availability of resources and the desired end form of energy.
Prior to the industrial revolution, biomass was the primary source of energy. Biomass now makes up only a small percentage of total world energy use. However, for approximately 2.5 billion people, it remains the primary source of energy for cooking and heating. The use of biomass is highly contextual to the region in which it is used – availability of resources, availability of technology and economic viability are all drivers of biomass use.
Some jurisdictions – especially those with sustainable forestry initiatives – have declared biomass a “carbon neutral” energy source. This is based upon the logic that carbon emissions from burning biomass will be recaptured by the plants grown to feed biomass reactors in the future, thus forming a carbon cycle for the plant.
The environmental benefits and costs are highly contextual depending on the technology and feedstocks used. While some biomass processes such as waste-to-energy are touted for their lower CO2 emissions, some processes, such as combustion, release carbon dioxide and particulate matter that is a significant concern for human health.
The world’s most energy-poor peoples and regions still rely on biomass for the majority of their energy needs. The lack of appropriate ventilation mechanisms for burning biomass is a major health concern and contributes to short life expectancies in much of the developing world.
Concerns associated with biomass go beyond human health. Depending upon the source of biomass used, deforestation, cropland degradation (due to diverting agricultural residues), and land use alteration can all be relevant issues associated with biomass.