by Dr. Peter Wylie
As the world debates climate change, it is clear that renewable fuels are needed if we are serious about halting global warming.
The big question is: how to do this without clearing more trees or causing food shortages?
Producing ethanol from crop residues is not an answer because it will reduce soil carbon and the future productivity of agriculture. Biodiesel is on a rocky road, with several production facilities in Australia closed due to the high price of feedstocks, which include tallow and canola.
Planting oil palms to produce biodiesel has attracted worldwide criticism because large areas of rainforest have been cleared in South East Asia for plantations. Australia is not suited to oil palms, except on the northern coastline with irrigation in the dry season.
Jatropha is a large shrub which is more suited to arid lands. The fruit contains seeds with oil which can be converted into biodiesel. Jatropha is a toxic plant and requires careful hand-harvesting.
In Queensland there has been interest in the “diesel tree”– copaifera langsdorfii – from Brazil. A latex-like oil can be extracted and converted to biodiesel. Copaifera does not need as much rain as oil palm but still grows in rainforest areas, which in Queensland are already cleared for sugarcane or have high conservation value.
The tree takes 15 years to mature and must be hand-harvested like a rubber tree. Although labour is cheaper in Brazil, it has not taken off there, where an expansion of sugarcane for ethanol is still going on.
More promising is an Australian native, pongamia milletia pinnata, sometimes known as native wisteria because it produces purple flowers. It has been grown as an ornamental in Brisbane and will tolerate drought, frost and salty soils and produces seeds with a high oil content.
The tree is fast growing but it takes five or six years until the first harvest. With moderate summer rain, a yield of 3000 litres a hectare of biodiesel is possible. The by-products would be 10 tonnes of pod shell and seven tonnes of high-profile meal, which could be worth more than the diesel.
An advantage of a legume is that energy inputs are low because it produces its own nitrogen and, being a tree, it does not need to be planted each year. This means it has a high net energy balance for biodiesel.
If grown on pasture land, the by-product may – with some modification – be valuable for feeding to dairy cattle or as a high-protein ingredient in fish food.
It could be possible to produce all of Queensland’s fuel requirements from about 1.5 million hectares of legume trees; and rather than compete with food production, more beef, milk and fish could be produced from this area at the same time.
At a bio-energy conference on the Gold Coast late last year, Professor Peter Gresshoff reported on research at the University of Queensland to improve the quality and quantity of oil from pongamia.
However, despite some trial plantings, the success of this crop has yet to be proven in the field and there is a lot of work to be done to develop a biodiesel industry based on legume trees.
Dr Peter Wylie is a researcher and consultant specialising in environmental issues. Peter can be reach at peter@horizonrural.com.au. Read Peter Wylie’s perspective on the Food v Fuel debate.