Published by Australian Academy of Science
Biomass – the growing energy resource Box 3 | Ways of extracting biomass energy

1. Solid fuel combustion

Perhaps the simplest and most common way of extracting energy from biomass is by direct combustion of solid matter. For example, more than a million households in Australia use firewood to provide at least some of their heating needs. Overall, wood provides about 2.4 per cent of Australia's total primary energy needs. Developing countries such as Nepal, Ethiopia and Kenya are said to obtain the majority of their energy needs from the burning of wood, animal dung and other biomass.

But burning can be inefficient. An open fireplace may let large amounts of heat escape, while a significant proportion of the fuel may not even get burnt. Up to three-quarters of the energy in biomass fuels may be contained in volatile matter – compounds released as the fuel heats up. If the fireplace is inefficient, much of this volatile matter may simply 'go up in smoke' without burning.

2. Gasification

Gasification is a process that exposes a solid fuel to high temperatures and limited oxygen, to produce a gaseous fuel. This is a mix of gases such as carbon monoxide, carbon dioxide, nitrogen, hydrogen and methane.

Gasification has several advantages over burning solid fuel. One is convenience – one of the resultant gases, methane, can be treated in a similar way as natural gas, and used for the same purposes.

Another advantage of gasification is that it produces a fuel that has had many impurities removed and will therefore cause fewer pollution problems when burnt. And, under suitable circumstances, it can produce synthesis gas, a mixture of carbon monoxide and hydrogen. This can be used to make almost any hydrocarbon (eg, methane and methanol) which can then be substituted for fossil fuels. But hydrogen itself is a potential fuel of the future. Some scientists and policy makers predict that hydrogen will one day perform the role that oil and petroleum perform today – but without the pollution.

3. Pyrolysis

Pyrolysis is an old technology with a new lease of life. In its simplest form it involves heating the biomass to drive off the volatile matter, leaving behind the black residue we know as charcoal. This has double the energy density of the original material. This means that charcoal which is half the weight of the original biomass contains the same amount of energy – making the fuel more transportable. The charcoal also burns at a much higher temperature than the original biomass, making it more useful for manufacturing processes. More sophisticated pyrolysis techniques have been developed recently to collect the volatiles that are otherwise lost to the system. The collected volatiles produce a gas rich in hydrogen (a potential fuel) and carbon monoxide. These compounds, if desired, can be synthesised into methane, methanol and other hydrocarbons. 'Flash' pyrolysis can be used to produce bio-crude – a combustible fuel.

4. Digestion

Biomass digestion works by the action of anaerobic bacteria. These microorganisms usually live at the bottom of swamps or in other places where there is no air, consuming dead organic matter to produce, among other things, methane and hydrogen.

We can put these bacteria to work for us. By feeding organic matter such as animal dung or human sewage into tanks – called digesters - and adding bacteria, we can collect the emitted gas to use as an energy source. This can be a very efficient means of extracting usable energy from such biomass – up to two-thirds of the fuel energy of the animal dung is recovered.

Another, related, technique is to collect gas from landfill sites. A large proportion of household biomass waste, such as kitchen scraps, lawn clippings and prunings, ends up at the local tip. Over a period of several decades, anaerobic bacteria are at work at the bottom of such tips, steadily decomposing the organic matter and emitting methane. The gas can be extracted and used by 'capping' a landfill site with an impervious layer of clay and then inserting perforated pipes that collect the gas and bring it to the surface.

5. Fermentation

Like many of the other processes described here, fermentation isn't a new idea. For centuries, people have used yeasts and other microorganisms to ferment the sugar of various plants into ethanol. Producing fuel from biomass by fermentation is just an extension of this old process, although a wider range of plant material can now be used, from sugar cane to wood fibre. For instance, the waste from a wheat mill in New South Wales has been used to produce ethanol through fermentation. This is then mixed with diesel to produce 'diesehol', a product used by some trucks and buses in Sydney and Canberra.

Technological advances will inevitably improve the method. For example, scientists in Australia and the United States have substituted a genetically engineered bacterium for yeast in the fermentation process, vastly increasing the efficiency by which waste paper and other forms of wood fibre can be fermented into ethanol.

Related site

• Wood energy conversion (Regional Wood Energy Development Programme in Asia, United Nations Food and Agriculture Organization)

Other boxes

Box 1. Biomass and greenhouse

Box 2. Introduction to food chains