Bioenergy is renewable energy made from material of recent biological origin derived from plant or animal matter, known as biomass. This can be used to generate electricity with steam turbines and gasifiers, or produce heat, usually by direct combustion. There are two forms of biomass:
Dry biomass – this is burned to produce heat and/or make steam to drive a turbine to produce electricity
Wet biomass – this is anaerobically digested to produce a flammable biogas that can be used for heat or electricity generation.
Biomass sources include virgin wood, wood residues from sawmills, agricultural residues and agricultural energy crops, such as miscanthus (a type of grass). Municipal solid waste and wet waste can also be used. Bioenergy includes biofuels, a type of fuel derived from biomass conversion, as well as solid biomass, liquid fuels and various biogases. Conventional biofuels are made from vegetable oil and sugar, and include biodiesel and bioalcohol.
Bioenergy helps to mitigate climate change by reducing greenhouse gas emissions and provides a cost-effective energy supply, relative to other renewable technologies. When biomass is burned to create energy, they return the same amount of CO2 to the atmosphere as if they decompose naturally. This means that properly managed biomass does not disturb what is known as the carbon balance and does not contribute to climate change. However, there are various concerns associated with biomass. Using biomass as a fuel produces air pollution in the form of carbon monoxide, nitrogen oxides and other pollutants, in some cases at levels above those from traditional fuel sources. Biofuel production is particularly contentious. For example, it has been suggested that biofuel production was accountable for 3-30% of the increase in food prices in 2008. In addition, the payback period (the time it will take biofuels to pay back the carbon debt that they acquire due to land-use change) has been estimated to be between 100-1000 years.
There is another issue with biomass, as the carbon savings from biomass can vary widely because the savings are offset by the fossil energy that is used for cultivation (such as fertilisers), harvesting, processing and transportation. Major land use change, particularly deforestation, can completely negate the carbon saving, as well as cause damage to biodiversity and the ecosystem. The UK has a target to source 15% of our overall energy from renewable sources by 2020 and bioenergy has the potential to provide about 30% of our 2020 target. Some of this can be utilised by co-firing coal with biomass feedstocks in order to aid the transition to a low-carbon energy generation. Drax power station, for example, uses this method of co-firing to produce electricity. However, it can be argued that biomass and biofuels are not, at this time, the “greenest” form of renewable energy available because of food vs. fuel issues. In addition, bioenergy is more expensive than some other renewable and traditional forms, costing $0.10/kWh as of 2015.