Australasian Science Magazine

Blue agave at Kalamia Estate, Queensland, in March 2010 during the crop’s first wet season. Photo: Don Chambers

By Daniel Tan

Agave is most popularly known for its use in tequila, but it could also usher in the dawn of a sustainable biofuel industry that does not compete with food crops for arable land.

Travellers in rural Australia are familiar with Agave americana. The blue-green leaves of the century plant are often the only reminders of long-abandoned farmhouses.

Agave was introduced into Australia for ornamental and possibly medicinal purposes, and has proved to be a successful survivor in the often arid Australian landscape. Now, researchers are asking if that capability could be the basis of a sustainable biofuel industry.

The agave plant has not yet been widely cultivated as a fuel source, but it promises some significant advantages over existing sources of ethanol such as sugarcane and corn. It can grow in arid areas without irrigation, and it doesn't compete with food crops or put demands on limited water supplies.

Food or Fuel?
Due to its rich reserves of coal, natural gas and uranium, Australia is nominally self-sufficient in energy, except for transport fuels and heavy oils. At the moment, bioethanol and biodiesel are being imported as renewable replacements for petrol and diesel, respectively.

Attempts to reduce our dependence on petrol have established sugarcane and corn as popular sources of biofuel, but not without considerable cost. Not only do they compete with food crops for arable land, they can affect water quality through excessive fertiliser use and lead to undesirable land-use changes such as deforestation. In a world where arable land and water resources are increasingly scarce, conflicts between food and fuel needs are likely to intensify given the expected scale of biofuel production required to meet the world’s energy needs.

There is a serious ethical debate about using food crops and arable land for biofuel production, particularly in times of global concern about food security. In the USA, almost one-third of the corn crop is grown for bioethanol production.

In the Australian context, Brian Fleay of ASPO Australia calculated that if the entire sugarcane and wheat crop in Australia is converted to ethanol, it would only supply 20% of the Australian transport fuels. This means that by burning our food for fuel, we would have very little fuel and no bread or sugar.

One solution would be the sustainable cultivation of biofuel crops on land that is not suited to food crop production so that the two systems were complementary rather than competitive. Agave production may be quite suitable for this.

Why Agave?
Agave belongs to a group of succulent plants with a special type of photosynthesis known as crassulacean acid metabolism (CAM).These plants include cacti, Aloe vera and pineapple, and are well-adapted to arid and semi-arid habitats. They open their stomata at night and take up carbon dioxide in the dark to form malic acid, which is then metabolised to release carbon dioxide for photosynthesis during the following day. By closing the stomata during the day, less water is lost, resulting in high water use efficiencies with a trade-off of lower growth rates.

Agave plants are well-suited for bio­energy production as they can be grown in sandy soil with little or no irrigation, and are less likely to be weedy.

Many cultivated varieties of agave are sterile clones and are not usually invasive. Agaves are monocotyledons (like grasses and lilies), and contain about 20% soluble sugars, which can be converted to ethanol. They are tolerant of high temperatures (up to 60°C), so they can adapt to global warming.

Aboveground biomass productivity of agave averages 25 t/ha/year in regions where annual rainfall is about 1000 mm (without irrigation).

A study by plant physiologist Park Nobel suggested that blue agave can achieve strong growth rates by switching from CAM to turbo-charged C3 photosynthesis if there is sufficient water supply. This makes agave very adaptable to Australia’s wet–dry tropics as it should be able to survive the dry season and take-off and grow quickly during the wet season.

Life Cycle Analysis
Life cycle analysis takes into account the inputs and outputs of a bioenergy crop production system, including the growing of the crop and its subsequent processing into ethanol. The technique is also used to assess the energy efficiency and impact of bioenergy feedstocks on greenhouse gases.

Life cycle analyses have shown that ethanol from Brazilian sugarcane could achieve substantial greenhouse gas offsets, while corn in the USA and China offers modest or no offsets. In recent years there have been increasing concerns about the impacts associated with large-scale biofuel production on land use change, such as deforestation and water resources, not to mention the impacts on global food production.

I have been part of a team consisting of Xiaoyu Yan, Oliver Inderwildi, Andrew Smith and Sir David King at the University of Oxford that produced the first life cycle energy and greenhouse gas analysis for agave-derived bioethanol. The main life cycle stages include agave cultivation, harvesting, transport and ethanol production.

Our analysis was based on a first-generation ethanol production facility in Mexico using blue agave as the sugar source. Our analysis was based on the experiences in the Mexican tequila and Brazilian sugarcane industry, as ethanol production from agave is not an established process.

It was assumed that the sugar extracted from agave juice was used for ethanol production while the cellulosic residue is combusted in a cogeneration system to provide the process energy, with excess electricity exported to the electrical grid. Hence, the ethanol-processing facility is self-fuelling, with the plant’s woody by-products (bagasse and residue) fuelling the production facility’s energy requirements.

Ethanol derived from agave has a positive energy balance. The energy created is five times the amount required to produce it (Fig. 1). This compares favourably to highly efficient sugarcane and to the less efficient corn as a source of biofuel. It also compares favourably to sugarcane-derived ethanol for its ability to offset greenhouse gas emissions, which we calculated at 7.5 tonnes of carbon dioxide per hectare per year (Fig. 1).

Hence ethanol derived from agave is likely to be superior, or at least comparable, to ethanol derived from corn, switchgrass and sugarcane in terms of energy, greenhouse gas balances and ethanol output.

Agave Trials in Australia
In 2003, Don Chambers of Ausagave identified blue agave as a crop with potential for areas of seasonally-limited rainfall in Australia. Varieties were sourced from Mexico and imported into Australia. Ausagave developed a tissue culture protocol where plants were deflasked into a plug to enable efficient mechanised planting (Fig. 2).

In June 2009 Chambers and Joseph Holtum of James Cook University planted the first trial of blue agave in the Burdekin River Irrigation Area. The plants are now 2 years old, and have survived two wet seasons as well as Cyclone Yasi. Growth rates have been slightly better than agave planted at the same time in Mexico.

Blue agave has yet to be grown commercially in Australia and has not been grown for biofuel feedstock, but Holtum, Chambers, Terry Morgan and I selected it as a trial species because it was likely to produce ethanol under conventional extraction and fermentation technologies.

For agave, the main soils to avoid are those prone to water-logging, so the Australian field trial was planted into raised beds to facilitate drainage, especially during the wet season. Nevertheless, some mortality occurred in the Australian trial as a result of crown rot during the wet season, when the plants experienced high humidity for 3 months.

So far, no insecticides or fungicides have been used. Fortunately, the agave snout weevil is currently absent from Australia due to strict quarantine regulations.

Blue agave grows relatively slowly in its early years, and competes poorly with weeds as it does not form a closed canopy. Hence, the surfaces of the beds in the first Australian trial were covered with plastic sheeting to minimise competition from weeds (Fig. 3).

Conclusion
The transport sector uses 60% of global oil production and has relied on fossil-based liquid fuels for more than a century. Large-scale biofuel production has been criticised due to concerns about algal blooms from excessive fertiliser use/runoff and for driving undesired land use change such as deforestation.

Unlike other biofuel feedstocks, agave has the potential to grow on marginal semi-arid agricultural land and would therefore have limited impact on global food production and biodiversity. The challenge now is to develop an agronomic production system for agave bioenergy production.

An agave biofuel industry is more likely to be viable if it is close to existing infrastructure, such as a sugar mill with spare capacity. With sugar prices high it is unlikely that cane farmers would turn over their land to agave, but it’s possible they could consider it for land that is too marginal for sugar cane.

Box: Biofuel Production in Australia
Biodiesel is being produced in Australia from used cooking oil, tallow and canola seed, but availability of feedstock is the main limitation. There is the potential for new crops, such as Indian mustard and pongamia, to be developed for biodiesel production in marginal agricultural land.

Bioethanol is produced locally in Australia from sugarcane molasses, grain sorghum, wheat and waste wheat starch, but feedstock availability is also the main limitation here. Many have high hopes for ethanol produced from second-generation cellulosic feedstocks such as crop residues and grass and tree crops, but the production technology is yet to be widely commercialised.