The world’s airlines face a painful challenge; of all the main energy sources, aviation fuel is going to be the most difficult to replace with low carbon equivalents. As the number of flights increases in the industrialising world, it is not far-fetched to see aviation using up the entire global CO2 budget in 2050. Some of the more progressive airlines can see the clear need to experiment with making an equivalent liquid fuel made from biological sources. British Airways is to be congratulated for examining the feasibility of using a gasification process to create a kerosene-like fuel from domestic waste. Unfortunately its sums are wrong and the amount of energy available from municipal rubbish (garbage in US terminology) is only a few percent of what BA rcentlly claimed to The Guardian. According to Damian Carrington writing in his blog on the Guardian web site, the airline thinks that the UK produces about 200 million tonnes of waste that is usable for conversion into aviation fuel. BA’s head of environment says that half a million tonnes of this rubbish used in its new gasification plant can produce about 50,000 tonnes of aviation fuel – a ratio of about ten to one. In addition to the liquid fuel, the new BA unit will generate about 33 megawatts of electricity.
These numbers aren’t right. The UK does produce about 200 million tonnes of waste a year, but only a small fraction of this is in the form of hydrocarbons that can be converted to energy-laden fuels. Very roughly, about half the waste is from construction and demolition sites. This is mostly used concrete and stone. Not even the world’s most advanced energy conversion technology can take an inert lump of concrete (composed largely of calcium, silicon and oxygen) and turn it into molecules of carbon and hydrogen.
To make a hydrocarbon fuel¸ BA needs waste material of containing the right chemical elements. Potential sources of liquid fuel include food waste, rubber, textiles, paper and other products containing carbon and hydrogen. This type of waste very largely arises from household collections and to a much lesser extent from garbage from restaurants and cardboard from shops. In the last financial year to April 2011, the UK’s households produced about 23.5 million tonnes of waste, not much more than 10% of the total national figure. About 9.5 million tonnes of this was recycled, composted or reused, leaving about 14 million tonnes of true waste.
In addition to this, just under 4 million tonnes of other waste collections, not from households, were of animal or vegetable origin. (If it isn’t of this origin, it won’t contain usable amounts of carbon or hydrogen for fuel). So the absolute maximum amount of UK waste available to be converted into complex hydrocarbons for fuel is about 13.5 million tonnes. This number is tending to fall quite rapidly as households produce less waste each year and, second, this rubbish is increasingly recycled or reused. But even today’s maximum figure of 13.5 million tonnes is less than 7% of BA’s claims for the weight of available UK feedstock for its plant.
The second problem is the efficiency of conversion. The energy value of municipal waste is generally thought to be between 6 and 7 gigajoules per tonne. This is about a seventh of the value of aviation fuel. In other words, for every seven tonnes of waste, we can only conceivably get one tonne of aviation fuel. This is a law of physics; we cannot create energy. Moreover the process of changing waste into fuel must involve losses of energy – all energy conversion processes result in the production of low grade waste heat. The very best gasification technologies only capture 50% of the energy in the feedstock and the BA plant is probably much less. So the ratio of tonnes of waste in to tonnes of fuel out will be, at best, about fourteen to one and probably far worse. In other words, instead of the BA fuel production process producing 50,000 tonnes of aviation kerosene from half a million tonnes of rubbish, it can only possibly produce 30,000 tonnes. This is still a worthwhile amount, but significantly below what BA says.
These two adjustments – the actual amount of waste available and the lower efficiency of conversion – will reduce the possible yield from UK rubbish from 20 million tonnes to about 1 million tonnes of fuel. This lower figure is about 8% of the UK’s total use of aviation fuel. Moreover, we are reducing domestic waste every year and are getting systematically better at recycling. Recycling an object is almost always more efficient in energy terms than converting it into fuel. We therefore can’t discourage recycling just because BA needs feedstock for its waste plant. In a few years it is not inconceivable that the UK’s total amount of carbon-based waste falls to well 10 million tonnes. Concomitantly, the absolute maximum fuel output will fall to not much more than 5% of aviation needs.
These numbers should not be a surprise to us. The false promise of biofuels (such as aviation fuel from municipal waste or ethanol from corn) is that we will get low-carbon energy from a plentiful supply of biological material, whether it be waste or US corn crops. The promise always fails when it hit biological limits. Our needs for transport fuels are simply far too great - by between one and two orders of magnitude -ever to be met from organic sources such as waste or agricultural crops. We cannot both feed the world and power our airplanes with biofuels.