Agave - Biofuel of the future?
Harvesting blue agave for the tequila industry
18 October 2010 by Andrew Leitch, Theodosios Korakianitis and Manuel Robert
Traditionally grown for tequila and fibre, agave could also become an important source of energy in the dry regions where it thrives. Andrew Leitch, Theodosios Korakianitis and Manuel Robert describe their team's efforts to investigate this plant group's energy potential.
The trend towards replacing fuels derived from oil with cleaner renewable ones generated from living organisms is a very attractive proposition, but it's full of potential problems that need to be addressed in detail.
Recent events in the Gulf of Mexico make biofuels even more relevant, in the light of the environmental problems associated with the oil industry. But we need to make the new methods as efficient and environmentally friendly as possible, and to find the right strategy for different regions of the world so that new fuels are economically competitive.
Producing new fuels locally would reduce the very high costs of transporting them from one place to another and the risks of contaminating the environment. Also, crops used to produce biofuels must not affect the production of food or alter its markets. This has already happened to Zea mays (maize) production in the Americas, where demand for maize as a biofuel, food and fodder crop led to higher prices.
All this means we will need more than one strategy to satisfy an energy-hungry world while taking account of the threat of climate change, the market laws of price competition and the specific needs of different countries. Agaves could play an important role.
For many years, these plants have been a source of products including sugars for producing alcoholic drinks like tequila, and hard fibres such as henequen and sisal for making products including ropes, twine and bags. But these same raw materials could become an important source of biofuels, whether bioethanol or biodiesel.
Agaves are perennial plants that produce large leaves in a rosette form. Their size and lifespan vary enormously between species, from 20 to 200cm in height and between 8 and 30 years old. Cultivated agaves benefit from adequate water from rain, but most are well adapted to arid conditions, and tolerate high temperatures and water shortages. This means they can be grown on land that would not be suitable for other purposes, and where soils are easily degraded by disturbance.
Agave fourcroydes, the main species cultivated in Yucatán, Mexico, growing in a nursery
It is not clear whether these plants can become an economically competitive alternative source of biofuels, but their biomass and growth characteristics make it worth looking into the possibility, particularly given the dry conditions that climate change may create in many parts of the world.
How to exploit the plant depends on the type of agave and the final product aimed for. Alcohol is made by fermenting the sugars stored in the plant's 'bole', or stem, after many years of growth, while biodiesel could be produced using fast pyrolysis, burning the biomass harvested regularly from fibrous agave leaves.
The most efficient alcohol-producing agave is Agave tequilana Weber, best known as the blue agave from which tequila is made. The industry generates an average of 120 tons of boles per hectare every six years, from which 20,000 litres of tequila (46 per cent alcohol) are produced.
One of the most important questions is how to transport the raw material to the processing plants. This calls for small facilities near the industry's centres of operation. This is nothing new; in Germany, hundreds of small plants that make methane from agricultural waste are being strategically placed near farms, and the production facilities of companies that use fast pyrolysis to generate crude biodiesel are all found near where the crops are grown.
Agaves produce considerable biomass, though not nearly as much as annual crops. A key advantage would be that no new planting is needed, and it takes relatively little work to maintain existing or new plantations.
It is also possible to use waste leaves left by the tequila industry, or the stems and short fibre discarded during henequen or sisal production. This might not generate very much biodiesel, but it would not require any extra expenditure on establishing and running new plantations, or on fuel to move products long distances.
Another alternative for biofuel production has already been implemented in Tanzania - a plant that makes biogas from the controlled fermentation of the liquid waste generated when leaves are decorticated - their outer layers removed and their fibres extracted. The gas, methane, is burnt on site to generate electricity. This in turn powers the decorticating plant and the small town nearby. Any that is left over is sold to the national network.
The best fuel will be suitable for combustion engines. We now need to examine different species and varieties of agave to determine how best to produce biofuels for this use. We will soon be seeking funding to let us select fuel production processes, engine materials and fuel mixtures suitable for combustion engines, taking into consideration engine performance and the emissions of agave-derived biofuels.
Improving the crop
The main problem when considering agaves for industrial purposes is that they have not been studied in detail. There are many taxonomical studies, classifying different agave species according to where they fit into the wider group, but only a small number of papers have been published on functional aspects of their biology such as genetics, biochemistry and physiology.
Agave amaniensis x A. angustifolia, grown here in a plantation at Kwaraguru, Tanzania
We have made a start on this study by analysing the genome organisation of commercially grown agave species and generating physical and genetic maps. These maps can be used to find agave lines most suitable for using targeted breeding to create new varieties with particular desired characteristics, using strategies already well developed in breeding new varieties of other crops.
However, most agaves spread vegetatively through rhizomes - underground root-stalks. This is an advantage when producing planting material, as this can be done simply by taking cuttings. But it presents us with a challenge for genetic improvement, as it's hard to combine the genes of two different plants by breeding them. So far, the only successful programme to genetically improve agaves was carried out in Tanzania during the first half of the twentieth century. Then, it took George Lock around 30 years to produce a family of hybrids that produce long fibre. We hope to make progress more quickly than that.
New, more efficient and faster-growing varieties will be needed, and we plan to use new molecular techniques, such as the use of genetic markers to help selectively breed plants with desired characteristics, together with new methods to grow plant tissues efficiently. These advances will shorten the time needed to generate new plant materials. A programme for the genetic improvement of Agave tequilana using these techniques is already under way in Mexico. However, much more work is needed.
The best way to use agaves will depend on the special circumstances of the place where they will be grown, and a combination of options may be called for. However, since agaves have not been genetically improved in a consistent way, the most important initiative to consider is a large-scale, long-term programme for the selection and generation of new agave types that will be more suitable for biofuel production.
Even using the best modern genetic techniques, this process of selective breeding will be long and difficult. But in the end it could provide us with new and useful sources of renewable, carbon-neutral energy that can thrive in hot, dry conditions. It could be grown across large tracks of land that currently have little agriculture, or only subsistence farming, and often limited conservation value. This means the industry doesn't just offer cleaner energy; it could also bring wealth to people who suffer from extreme poverty.
Andrew Leitch is professor of plant genetics and Theodosios Korakianitis is professor and chair of engineering, both at Queen Mary University of London. Dr Manuel Robert is a member of the biotechnology department of the Centro de Investigación Científica de Yucatán in Mexico.
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'Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions' - T Korakianitis, A Namasivayam, and RJ Crookes, (2010). Progress in Energy & Combustion Science, doi:10.1016/j.pecs.2010.04.002.
'Wild and agronomically important Agave species (Asparagaceae) show proportional increases in chromosome number, genome size, and genetic markers with increasing ploidy' - ML Robert, KY Lim, L Hanson, F Sanchez-Teyer, MD Bennett, AR Leitch and IJ Leitch (2008). Botanical Journal of the Linnean Society, 158, 215-22.