This encompasses all forms of processing waste material to produce energy. In practise it tends to be limited to biological materials. The is typically sewage and garbage, both domestic and industrial, together with waste paper and cardboard
This means incinerating sewage and garbage so that power can be generated power from the waste heat. It happens, but the NIMBYs[2] are against it in the United Kingdom, so it does not look like a goer here. I guess that is the same in the USA.
LanzaTech, a New Zealand biotech start-up, announced a scheme in late 2011 to capture and ferment the carbon monoxide in flue gas. They have had a pilot plant running since 2008. This consists of two 500 litre fermentation reactors and uses flue gas from a steel mill as its feedstock, so development is well past the lab-scale experimentation stage. A demonstration system, scaled to produce 23 million litres of ethanol annually, is scheduled to be operational in Shanghai before the end of 2011, with full commercial operation expected in 2014. Virgin airlines has signed a contract to run its airliners on this fuel.
LanzaTech believe their plant can be connected to 65% of the steel plants in the world, but there are no predictions of how much of the world's oil requirement this system has the ability to replace, or what other flue gas sources it can work with apart from the statement that it can work with a variety of flue gasses provided they contain carbon monoxide and that these do not impact food production.
The availability of flue gas is presumably the principal limit to ethanol production from this source because this process is primarily recycling the energy locked up in the carbon monoxide content of flue gas. Since the flue gas feedstocks are mostly a byproduct of fossil fuel burning plant, it strikes me that this may well be a time-limited technology. By this I mean that as and when fossil fuels are exhausted there will be much less flue gas to convert. Its not obvious what sources of non-fossil flue gas will exist at that point: very little indeed if we all go down a solar/tidal/nuclear path as seems likely.
That said, it sounds like a good wheeze and I hope it goes well.
Methane is emitted by domestic waste that has been buried in landfill sites and by sewage treatment plants. Much of this is simply emitted into the atmosphere, where it becomes a potent greenhouse gas. Some is flared off. This converts an otherwise harmful geenhouse gas into carbon dioxide. This is carbon neutral: it is merely returned to the atmosphere from where it was originally extracted by photosynthesis.
Increasingly, methane from these sources is being burnt in internal combustion engines to drive generators. Physically these are quite similar to large diesel generator sets.
Generating electricity from farmed animal waste is a recent American development. For example, a large dairy herd in Vermont produces 9000 gallons of milk a day - along with 35,000 gallons of animal waste. The farm installed an anaerobic digestor which outputs enough methane to run a 200 kW generator set. This covers all the farm's electricity needs as well as supplying 300 to 400 houses by selling electricity to the local electricity utility. This represents an average load of about 600 watts per house. By comparison the typical British house averages 400 watts, with climactic differences making up a lot of the difference. The exported power represents additional farm income of $US 120,000 per year at 2006 prices. The system does help with heating, presumably by using the generator's waste heat to heat the barn and supply hot water to the dairy, but doesn't cover the entire heating requirement. As another bonus, the liquid effluent from the digestor is suitable as fertiliser and the solid waste, a fluffy brown material, makes a hygenic substitute for sawdust as bedding for the cows with an additional saving of $US62,000 per year. At a capital cost of $US1.2 million, the payback period of just under 7 years looks pretty reasonable. This type of scheme makes both economic and environmental sense and so looks likely to spread. More details can be found in a Live Science article, A Dairy Farm Where Milk and Manure Pay the Bills dated 30th June, 2006.
The viability of this type of system is very dependent on the local approach to farming. It probably would not work in New Zealand, where dairy herds live outside all year round and only need fodder to be imported during the winter. As a consequence, all the manure produced by a herd is absorbed by the farm's pastureage on a sustainable basis. Obviously "intensive dairy farming" in New Zealand means a much lower carrying capacity per hectare than in the UK or America. Otherwise a dairy herd in New Zealand would produce more manure than the pastures could tolerate. "Intensive dairy farming" in the UK and America tends to mean keeping cattle under cover, particularly during American winters. Add the functional separation of buying all the cattle feed in from, say, the Midwest corn belt rather than growing it on the farm and you end up with large herds on small farms with insufficient land to either carry them or spread the ordure. Under these conditions you really have to do something like using the animal waste to run a generator. It seems likely that intensive farming New Zealand-style just doesn't generate the excessive volumes of waste that make a co-genetration system practical.
Smaller scale domestic systems are available that supply methane for heating, etc. from domestic and farm sewage. These have been around for 20 or 30 years but I have no idea of how widespread or popular they are.
I have no methane usage figures for the UK, where most of the methane from landfill and farming is simply vented into the atmosphere with apparently little effort to make any use of it or to neutralise its impact on global warming. The USA is generating some power from landfill methane and is starting to use methane generated from farm waste and emitted by city sewage plants, e.g. in Los Angeles. In New Zealand there are three landfill sites which use the methane on site to power generator sets: this amounts to 50.8 Mw of installed capacity and contributes 218.9 Gwh (0.6%) to the country's annual electricity generation capacity.
A variation on incineration is pyrolysis. It has been suggested that almost any organic waste, ranging from wood waste to sewage, can be subjected to destructive pyrolysis. The products are said to be:
In The Charcoal Vision: A Win-Win-Win Scenario for Simultaneously Producing Bioenergy, Permanently Sequestering Carbon, while Improving Soil and Water Quality David A. Laird calculates that a distributed network of fast pyrolysers could replace 25% of the USA's oil consumption while permanently sequestrating carbon equivalent to 10% of its carbon dioxide emissions, using 2008 as the baseline year. However, this paper glosses over three rather important points:
However it should be noted that James Lovelock thinks that charcoal sequestration is the most viable approach to removing carbon from the biosphere. I have a feeling that its a much better approach than pumping carbon dioxide into disused oil wells or (heaven help us) the deep ocean.
A New Zealand based start-up, Carbonscape, has a pilot system running, based on microwave heated pyrolysis, that is designed purely to create carbon for burial and soil improvement. The plant is said to convert anything from wood waste to sewage into charcoal, so it should not impact forest or farm production. Nonetheless there would seem to be a few questions that need answering: