Biomass – the growing energy resource

Key text

What is biomass?

Biomass is a general term for living material – plants, animals, fungi, bacteria. Taken together, the Earth's biomass represents an enormous store of energy. It has been estimated that just one eighth of the total biomass produced annually would provide all of humanity's current demand for energy. And, since biomass can be regrown, it is a potentially renewable resource.

One of the most appealing things about biomass energy is that it doesn't contribute to the enhanced greenhouse effect, provided that the biomass is harvested sustainably (Box 1: Biomass and greenhouse). Coal, gas, oil and other fossil fuels – the main greenhouse culprits – don't qualify as biomass, even though they are derived from living material. The time required for the formation of these fuels – millions of years – means that they can't be counted as renewable.

Where does the energy come from?

The original source of the energy present in biomass is the sun. Small 'factories' in plant-leaves called chloroplasts use solar energy (in the form of light energy, or photons), together with carbon dioxide from the air and water from the soil, to manufacture a range of compounds. These compounds include sugars, starches and cellulose – collectively called carbohydrates. The original solar energy is now stored in the chemical bonds of these compounds.

Some of this stored energy is passed on to animals when they eat plants (or eat other animals). So plants, animals and animal excretions – biomass – can be seen as storehouses of solar energy (Box 2: Introduction to food chains).

How biomass is used

Scientists are busy developing different ways of converting biomass into a form that meets our energy needs, while making best use of the available energy. There are five different ways of extracting biomass energy: solid fuel combustion, gasification, pyrolysis, digestion and fermentation. Research into each of these is producing dramatic advances (Box 3: Ways of extracting biomass energy).

Making haste with waste

One source of biomass material is waste. Human society produces a veritable compost heap of organic waste. Kitchen scraps, sewage, the leftovers of the food processing industries, paper, sawdust, lawn clippings...the list is long. One of the reasons that energy from biomass is receiving so much attention is that it represents an opportunity to convert waste into something very valuable.

In Australia, the potential value of organic waste as an energy source is only just starting to be tapped, with the sugar industry leading the way. It burns the residual fibre waste from raw sugar processing – called bagasse – to produce steam, which in turn is used to work the machines that process the cane and to drive electricity generators. The present installed electricity generating capacity of all the sugar mills in Australia is about 250 megawatts, 60-70 megawatts of which is sold to the electricity grid. According to the Sugar Research Institute, this is only a fraction of the potential capacity should more efficient systems be installed.

One way of improving efficiency is called cogeneration, the practice of producing both electricity and useful heat. Some sawmills, for example, uses excess heat from boilers fired by sawdust to supply energy to their kiln-drying operations. But excess heat can also be used to gasify the biomass fuels so they can be used in a gas turbine, which is more efficient than a simple boiler that produces steam. Combined cycle technology can produce extra savings by using any additional waste heat from the gas turbine to power a steam-driven turbine.

Landfill waste is a largely untapped resource in Australia. According to the Department of Industry, Science and Resources, the present installed capacity for landfill gas in Australia is about 72 megawatts from just over a dozen council tips. The scope for expansion is considerable.

Nor is sewage so much on the nose any more, as we start to make use of it as a biofuel. The Department of Primary Industries and Energy estimates that the installed electricity generation capacity of sewage farms around Australia is about 7.5 megawatts - electricity production from this source could triple by 2010.

Biomass farming

Making better use of our waste could contribute significantly to our energy needs but it won't satisfy them completely. Some analysts have suggested that we should grow biomass specifically for energy production. One has even suggested that by committing about 2.5 per cent of the world's land area to energy crops (as well as by improving the recovery of energy from waste) we could meet about half of the world's current energy needs.

In Australia, the Landcare movement is currently engaged in a large-scale tree-planting program in an attempt to arrest environmental degradation. Some of these plantations may produce valuable timber, but the commercial value of others is questionable.

Perhaps some of the plantations could be used to generate electricity, thereby helping to meet the country's energy needs while also making a dollar for farmers. In Esperance, on the south coast of Western Australia, such a scheme has already been suggested. A power company there is considering plans to generate electricity by biomass gasification, using locally grown plantation trees as its biomass material. In suitable regions, biomass could be grown close to existing coal-fired power stations and used to supplement the fossil fuel supply.

Future growth

Ultimately, the success of biomass as an energy alternative will be determined by economics. Industries that use their waste biomass for energy simultaneously solve a waste disposal problem and save money on their energy needs (and, sometimes, earn money by selling excess electricity). As biomass technology becomes more efficient, the chances of biomass energy competing in the wider market place will increase. The government has announced that an additional 2 per cent of electricity (about 9500 gigawatts) will be provided by renewable energy sources by 2010. Biomass is likely to provide about half of this increase.

Related Nova topics:

• Harnessing direct solar energy

• Wind power gathers speed

• Fuelling the 21st century

• Enhanced greenhouse effect – a hot international issue

Box 1. Biomass and greenhouse

Energy from biomass is one such alternative. This source is usually considered to be greenhouse-neutral, which means it neither adds to nor reduces greenhouse gases. This is because, theoretically at least, all the carbon dioxide that was removed from the atmosphere by the growing plants is later released when biomass fuel is burnt.

But there are scenarios in which biomass could be greenhouse-positive. Biomass could contribute to the greenhouse effect if, for example, forests were harvested and used but not replaced. Burning biomass would have the net effect of adding greenhouse gases to the atmosphere, because the carbon formerly stored in the biomass would be released as carbon dioxide

On the other hand, biomass could be greenhouse-negative and help reduce greenhouse gases in two ways. First, by substituting a renewable resource for fossil fuels – for example, using ethanol produced from biomass, instead of petroleum – we could achieve a net reduction in greenhouse gas emissions. Second, if biomass is left to rot – as in a landfill site – it produces methane, which traps heat more efficiently than carbon dioxide and is therefore a more potent greenhouse gas. By capturing the methane and using it for energy we produce carbon dioxide, thereby reducing (but not eliminating) the impact on the enhanced greenhouse effect.

Related sites

• Greenhouse gases (Nova: Science in the news, Australian Academy of Science)

• Global warming and the Third World: Biomass as an energy source (Tiempo Climate Cyberlibrary, UK)

Box 2. Introduction to food chains

Animals cannot use the sun's energy in this way. They are consumers and must get the energy they need in order to survive by eating plants and extracting the chemical energy stored in plant material. They then use the energy to keep alive – to pump their blood, to move their muscles, and to operate their nerves. These plant-eating animals are called herbivores. A high, but variable, proportion of the energy in food is not extracted at all but is passed out in an animal's wastes, which are a food source for other creatures (often microscopic ones). Of the energy that is absorbed from an animal's food some goes to build up, maintain or repair its body and some may be stored as fat that can later be used as an energy source. But much of the energy is lost as heat directly from the animal, even if it is not warm-blooded.

Because the bodies of animals are made from complex chemicals, they too represent stored chemical energy. Other animals take advantage of this available food by eating animals that feed on plants. These meat-eating animals are called carnivores. Still higher up the food chain we have other predators that may feed on these meat-eaters.

As a rough rule, each level in the food chain can support about a tenth of its own weight of animals feeding upon it and still survive. We therefore have a food and energy pyramid that looks something like this:

Box 3. Ways of extracting biomass energy

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)

Activities

• Energy Quest (California Energy Commission, USA)
  • Peanut power – how to use the energy in a peanut to heat water

• Newton's Apple (USA)
  • Ethanol – fermenting different plant substances
  • Photosynthesis – the effect of lack of sunlight on leaves

• Daily Lesson Plan (New York Times, USA)
  Not just a corny idea – students explore ways in which ethanol can be created using alternate energy sources.

Further reading

Useful sites

A fact sheet about biomass energy that includes sources of biomass, biomass applications and electricity generation.
http://www.aie.org.au/national/factsheet/FS8_BIOMASS.pdf

Biomass energy (Queensland Government Environment Protection Agency and Education Queensland, Australia)

A brief introduction to biomass energy including the use of bagasse in steam turbine power stations.
http://www.sustainableenergy.qld.edu.au/fact/factsheet_10.html

Biomass energy (Queensland Department of Education)

Describes bioenergy and opportunities to use biomass for energy generation in Australia, particularly the use of bagasse in Queensland.
http://www.sustainableenergy.qld.edu.au/pdfs/factsheet_10.pdf

Wood residue as an energy source for the forest products industry (Australian National University)

Describes the energy value of wood, conversion technologies, combustion technologies and future prospects for an Australian forest products industry.
http://sres.anu.edu.au/associated/fpt/nwfp/woodres/woodres.html

Newsletters (Bioenergy Australia)

Up-to-date articles on the use of biomass for energy production.
http://www.users.bigpond.net.au/bioenergyaustralia/Newsletters.htm

Research sparks bright future for bio-energy (Rural Industries Research and Development Corporation, Australia)

Covers plans for future biomass energy systems in Australia and provides background information about the benefits and potential use of bio-energy.
http://www.rirdc.gov.au:80/pub/media_releases/9feb00.html

Australian Greenhouse Office

• Map of operating renewable energy generators in Australia
  Provides maps of proposed and operational renewable energy generators across Australia.
  http://www.agso.gov.au/renewable/

• Renewable energy commercialisation in Australia
  Lists the renewable energy projects that are being commercialized in Australia, including solar thermal and solar photovoltaics projects.
  http://www.greenhouse.gov.au/renewable/recp/

Australian Broadcasting Corporation

• Ethanol part 2 – cellulosic ethanol (The Science Show, 10 February 2007)
  Reports on the use of cellulosic ethanol in Ottawa, Canada.
  http://www.abc.net.au/rn/scienceshow/stories/2007/1844229.htm

• Ethanol part 1 – corn ethanol (The Science Show, 3 February 2007)
  Reports on the use of corn ethanol as a fuel in Minnesota, USA.
  http://www.abc.net.au/rn/scienceshow/stories/2007/1839140.htm

• The ethanol alternative (Catalyst, 12 October 2006)
  Describes the research of two Australian scientists that may lead to ethanol becoming a viable alternative to oil. http://www.abc.net.au/catalyst/stories/s1763365.htm

• Ethanol more efficient than we think (News in Science, 27 January 2006)
  Re-evaluates the energy efficiency of ethanol.
  http://www.abc.net.au/science/news/enviro/EnviroRepublish_1556439.htm

• Fuelling a revolution (Catapult, 24 November 2005)
  Looks at the incresing use of biodiesel in Australia.
  http://www.abc.net.au/catapult/indepth/s1515906.htm

• The hidden costs of biofuels (News in Science, 27 September 2005)
  Looks at ethanol and biodiesel as fuel alternatives.
  http://abc.net.au/science/news/stories/s1468698.htm

Glossary

chemical bonds. The attractions that hold atoms together to form molecules.

chloroplasts. Small organelles found in plant cells. They contain the green pigment chlorophyll which captures solar energy from the sun and is essential for photosynthesis in plants.

enhanced greenhouse effect. An increase in the natural process of the greenhouse effect, brought about by human activities, whereby greenhouse gases such as carbon dioxide, methane, chlorofluorocarbons and nitrous oxide are being released into the atmosphere at a far greater rate than would occur through natural processes and thus their concentrations are increasing. Also called anthropogenic greenhouse effect or climate change.

fossil fuels. Carbon or hydrocarbon fuels, derived from what was living material, and found underground or beneath the sea. The most common forms are coal, oil and natural gas. They take millions of years to form. Their energy is only released upon burning, when the carbon and hydrogen within them combine with the oxygen in air to form carbon dioxide (CO₂ ), or carbon monoxide (CO) and water (H₂O). Other elements within the fuels (such as sulfur or nitrogen) are also released into the air after combining with oxygen, causing further pollution with SO₂ and nitrogen oxide gases. In the case of coal, ash particles are also a problem.

hydrocarbon. Compound containing only the two elements, carbon and hydrogen.

kilowatt, megawatt, gigawatt. The basic unit of power (the rate at which energy is used) in the metric system is the watt (W); a kilowatt is 1000 watts. A watt is a very small amount of power and in most mechanical applications we count power in kilowatts. A kilowatt is about equal to the heat energy put out by a single bar radiator, and is also about equal to the power expended by a person running up stairs. A car engine typically produces 50 to 100 kilowatts.

When we consider power generation, we use larger units. A megawatt is 1,000,000 watts or 1000 kilowatts. A typical coal-burning power station produces about 1 gigawatt (1000 megawatts) of power.

When we consider power generation, we use larger units. A megawatt is 1,000,000 watts or 1000 kilowatts. A typical coal-burning power station produces about 1 gigawatt (1000 megawatts) of power.

photosynthesis. The biochemical process in which green plants (and some microorganisms) use energy from light to synthesise carbohydrates from carbon dioxide and water.