• “Save the planet - by taking your car on an alcohol-fuelled jaunt( Robin McKie. The Observer 26th Feb 2006)
• “A sweet solution to fuel troubles” (Ian Sample. The Guardian 16th Feb 2006)
• “Bioethanol is produced from a wide variety of vegetable matter including forest residue, sugar cane and sugar beet, which makes it a renewable and sustainable fuel source. .... a planet-fiendly fuel .. “ (Martin Love, The Observer 12th March 2006)
The reality
Bioethanol - what is it and where does it come from?
Bioethanol is simply ethanol. It is the same chemical as the alcohol in beer, wine or any spirit. Indeed it is made in the same way as those alcoholic drinks using a technology that dates back thousands of years - the fermentation of sugars using a yeast. The use of the term bioethanol is just an attempt at giving “alcohol” a makeover.
For every 133 units over energy one can get from burning bioethanol, 100 units of energy have been used in its production!
Add yeast to a solution of sugars and you get ethanol. The sugars can come from any plant capable of making them in significant amount - sugar cane and sugar beet have been bred to do just that and husbandry methods are well established. If there are surpluses of sugar in the world, making ethanol from that sugar and calling the ethanol bioethanol is relatively easy.
As an alternative, the sugars can be derived from starch. For thousands of years the technique of malting has used the ability of seeds to break down their starch to give sugars during the early stages of germination as a way of converting starch to sugar for subsequent fermentation. During the malting process, hydrolytic enzymes such as alpha amylase produced by the germinating seed, convert the starch to soluble sugars but it is now possible to use other organisms to make hydrolytic enzymes to carry out this conversion. Consequently, there are now various routes available to the production of sugars for fermentation and a wider range of plant products can be channelled towards ethanol production.
Bioethanol - inherent inefficiencies
What those hyping bioethanol are often reluctant to address is the very poor overall efficiency of energy capture of solar energy via the fermentation of plant-derived sugars.
The basic problem is that modern agriculture is often energy intensive and the distillation of ethanol from the fermented solution requires large amounts of energy. If one combines these two factors, bioethanol production is far from promising.
For every 133 units over energy one can get from burning bioethanol, 100 units of energy have been used in its production! Lets look more closely at why these problems exist and whether further research might overcome them.
Energy use in agriculture
• Modern, high output agriculture, has high inputs of energy. The fuel used for cultivation and harvesting of the crops are the most visible energy inputs but it is the energy needed to make the fertilisers that is the biggest problem. Every kg of plant material taken away from a farm will take with it some minerals that the plant took up from the soil. If those minerals are not replaced, the yield of the crops on that farm will decline every year. In the developed world, the 20th C saw that problem solved by addition of fertilisers to the fields every year.
• The biggest energy demand for arable agriculture comes from providing the crop with chemically fixed nitrogen - in the form of nitrate, urea or ammonia. All these chemicals are made by making the very inert nitrogen gas in the air react with hydrogen and the only way that can be achieved is to use very high pressures and temperatures during the reaction stage. The huge amounts of energy needed to “fix” atmospheric nitrogen means that modern agriculture uses very large amounts of energy - currently mainly obtained from fossil fuels.
• But surely some plants, like legumes, can fix their own nitrogen? Why not use them? Well organism have the same difficulty fixing nitrogen that humans have experienced. Thus leguminous plants, such as beans or peas, use sugars to feed the microbes in their root nodules which carry out the nitrogen fixation. This means that legumes can grow well without nitrogen fertilisers additions but the energy saving achieved is offset by the loss of carbon production by the crop.
• Finally, in the more humid regions of northern latitudes harvested grains often have to be dried in order to store without spoilage. This increases the energy use again.
Energy use in distillation
• The distillation process is energy intensive. In some schemes for producing bioethanol, plant waste is burned to produce some of this energy. For example after the sugar is squeezed from sugar cane stems, the stems can be burned on site to provide energy for the sugar processing - a practice that is hundred of years old.
Bioethanol - sugar cane vs sugar beet
Plants species differ in their overall carbon fixing efficiency. There are two main classes of agricultural crops - the C3 and the C4 species.
• C3 crops. These species are less efficient in fixing carbon dioxide than C4 plants and are also less efficient in terms of water use efficiency. The major C3 crops are wheat, barley, soybean, potato and sugar beet.
• C4 crops. These more productive species include maize (corn) and sugar cane.
These inherent properties mean that all other things being equal, it is likely that sugar from sugar cane is likely to be cheaper to produce than sugar from sugar beet. Starch from maize is likely to be cheaper than starch produced from wheat. It is thus predictable that regions of the world with a long growing season, with high light levels and where C4 crops such as sugar cane and maize grow well, will be able to produce bioethanol more cheaply than much of the UK.
Green energy - more than just energy inputs need to be taken into account
So far I have concentrated on the energy efficiency of bioethanol production - the message is that there is only a small gain. However, there are other inputs and outputs that need to be taken into account.
Water availability
Invisible to most consumers, crops distil off huge amounts of water as they grow. For every kg of carbon fixed by a plant, the plant will have lost 250-500 kg of water. In a world with increasingly uncertain water supplies, basing any fuel production methodology on a system that needs huge amounts of water needs requires very serious consideration.
Brazil - leaders in bioethanol production and leaders in rainforest destruction
How sad that the current hype about bioethanol is so uncritical about the likely source of the majority of the bioethanol to be sold for the next few years. Even if burning bioethanol in a car releases less carbon than burning petrol if the growing of the crop to produce the bioethanol is not environmentally friendly then there could be a overall negative effect of using bioethanol. (Somehow those who proclaim the virtue of using fuels containing 5% bioethanol simply ignore that fact using a car with a 6 % better fuel consumption on pure petrol might be even better.) As has been shown, the clearing of forest to produce agricultural land releases huge amount of carbon so that carbon release needs to be accounted for in any increase in bioethanol production before it can simply be stated that bioethanol is a “sustainable green fuel”.
So what is the future for bioethanol?
The bottom line is that the world simply does have enough spare agricultural land to produce enough sugar (and then bioethanol) to provide a sustainable source of fuel. Any attempts to increase the supply of bioethanol will inevitably carry with them environmental costs:
• If the area of land under cultivation is increased, the fraction of the world’s carbon that is being used by humans will increase further and other organisms will suffer. Many people seem to ignore, or choose to ignore, the fact that feeding the projected world population in 2025 will place huge environmental loads on the world (water, minerals, land area, etc.). It is almost certain that the world simply does not possess the capacity to produce enough food, to feed 9 billion people by 2050 with a diet rich in meat. If that is true, then diverting plant material to feed cars as well as humans is unsustainable.
• If humans develop methodologies to economically convert the cellulose in agricultural “waste” into sugars (hence into bioethanol), it is predicable that soil carbon in agricultural soils will decline because much of that “agriculturable waste” currently is used by other organisms in the soil.
However, during periods of food surpluses, there will indeed some limited opportunities to use the surplus as an energy source. Yet even when trying to use such surpluses, making bioethanol might not be the best option.
Surplus grain - burn it or make bioethanol?
Grain possesses a certain amount of energy, not only in the starch but also in the oils, fats and cell walls. Grain can be burned directly in some stoves designed to burn wood pellets. Simple thermodynamics suggest to me that growing yeast on grain starch must yield less energy than burning the grains directly. The yeast is using the sugar as an energy source and they convert the carbon in the sugar into many compounds other than ethanol. Finally the is a large energy demand in the distillation process needed to produce ethanol pure enough to mix with petrol.
If this simple analysis is valid, then it would make much more sense environmentally and energetically to burn any surplus grain in modified pellet stoves for space or water heating. In rural areas this would involve little transport of the grain and the ash from burning (which will contain minerals) could be discarded on soils to achieve mineral recycling. The technologies exist to achieve this NOW.
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