The basics of bioenergy

Expert Q&A  |  Published:

What is bioenergy?

Bioenergy is energy that is derived from biological sources – living things and their metabolic products. It can be in the form of heat, light, electricity or fuel for transport. Another term that is commonly used (often interchangeably) is biofuel – fuel derived from biological sources.

What sources of bioenergy are there?

The biological sources that are used to create bioenergy are known as biomass. There are many different types, with more constantly in development. Broad categories include:

  • wood (including wood chips, sawdust and forestry by-products)
  • animal fat waste cooking oil
  • algae purpose-grown plants (energy crops)
  • livestock effluents (including dairy)
  • human waste organic fraction of municipal solid wastes

Bioenergy sources are diverse because production needs can be tailored for each region or country’s specific environment, land availability, population, energy needs and so on.

For example, most bioenergy in the US comes from soybeans and corn; in Brazil sugarcane is used and in China, cassava and sorghum are common sources. New Zealand’s bioenergy industry is still developing and companies and researchers are looking into the use of a range of sources including animal tallow, industrial waste gases and woody biomass.

Biodiesel and bioethanol – what is the difference?

Two of the most widely used types of bioenergy are biodiesel and bioethanol, both of which can be used as fuel for transportation (for example, in cars).

Biodiesel

Biodiesel is made from oils or fats extracted directly from plants or animals. This is chemically converted into esters, which behave similarly to ‘regular’diesel. Biodiesel is either used on its own or blended with regular diesel. The most popular sources of oil for biodiesel are from specifically grown plants, or waste cooking oil.

The US, for example, grows soybeans for the purpose. New Zealand does not grow a large amount of oil-producing crops, nor do we have enough waste cooking oil to make more than a small amount of biodiesel. However, we do have a good supply of animal-based fat. Most is currently sold overseas, but if it were used to make biodiesel, it could provide about 5% of our annual diesel demands.

At least two companies are using animal-based fat in R&D with the aim of scaling up to commercial levels.

Bioethanol

For bioethanol production, feedstocks that have significant sugar and starch contents which can be fermented to produce ethanol are often purpose grown. Processes for cellulosic production of ethanol are also available. For example, ethanol can be produced from other sources (such as milk sugars) as well as by-products from anaerobic digestion processes of organic wastes. This process is the same as that used to make the alcohol in alcoholic beverages.

The bioethanol is then blended with petrol to make a fuel source for transportation. There have also been reports of un-blended bioethanol being used as a fuel source. One of the problems with biofuels is that they blend easily with water, meaning that it usually has be mixed with petrol immediately before delivery to petrol stations so water does not have a chance to enter.

Overseas, corn and sugarcane are popular choices for making bioethanol, but these are not grown in large amounts here. In New Zealand, bioethanol is made from fermenting lactose, but supplies are limited. Other sources are being investigated, such as salix – a woody crop which is also useful for a variety of other purposes. Research is also being carried out into using microorganisms to convert carbon monoxide from industrial waste into ethanol.

What are the advantages of bioenergy?

Bioenergy can help reduce reliance on fossil fuels such as oil and coal. This has a number of advantages: First, unlike fossil fuels, bioenergy sources are renewable. Second, use of some sources of bioenergy (particularly plant-based sources) can mean a reduction in carbon emissions compared to fossil fuels. This is because while burning these sources of bioenergy releases carbon dioxide, the next batch of plants grown absorbs an equivalent amount of carbon dioxide during photosynthesis, meaning a zero net output of carbon.

However, energy is still needed to produce and process the plants, so the reduction in carbon emissions is not total. Some types of bioenergy require more energy in their production than others. Third, the biomass needed to make bioenergy can be grown locally, meaning each country can manufacture its own energy sources rather than relying on imported ones.

What environmental, economic and ethical issues are associated with bioenergy?

There are a number of environmental, economic and social issues associated with bioenergy production and use: Bioenergy that comes from waste products such as wood chips and dairy effluent is desirable, as it produces valuable energy from waste.

However, even if all our waste were used to make bioenergy, this still would not produce enough energy to meet demand. Therefore, crops need to be purpose grown for bioenergy.

Depending on management practices, growing and processing bioenergy sources can cause pollution. This can come things like the erosion of topsoil and the application of chemicals such as pesticides or fertilisers. Some bioenergy comes from crops that can also be used as food (for example, soy, sugar cane and corn).

As the use of bioenergy rises, food prices can increase because energy production is competing for the same resource. Like other areas of farming, there is the risk that natural habitats could be destroyed in order to make way for bioenergy production. An example of this is replacing native tropical forests with palm oil plantations.

What sources of bioenergy does New Zealand have?

New Zealand has a wide variety of current and potential sources of bioenergy. Some of these are: wood and wood wastes from forestry and wood processing animal tallow and used cooking oil algae lactose industrial waste gas methane from livestock effluent human waste organic fraction of municipal solid waste (OFMSW)

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This Science Byte was reviewed by Gerry Carrington, Department of Physics, University of Otago