Liquids from BiomassThis category includes landfill gas, gasses from sewage and animal slurries (manure lagoons), as well as the much hyped ethanol.
Landfill, Sewage, and Slurry GassesAll of these gasses involve the capture of methane produced by anaerobic bacteria feeding in these conditions and then burning this methane to produce energy. According to the EPA total methane emissions into the atmosphere from all sources amount to 5.66*10^11 kilograms per year. Combustion of methane releases roughly 5.55*10^7 Joules per kilogram burned. This means even if we were able to capture all of the methane leaking into the atmosphere around the world this would amount to 3.1*10^19 Joules per year, or 6.2% of the world demand. Capturing methane might make sense in some situations, but it does not have large scale potential.
EthanolEthanol is the same alcohol found in the alcoholic beverages we consume. It is produced from the fermentation of a number of foods, primarily corn, but also sugarcane, other cereals, and more recently even cellulose. These plants gather energy from the sun, so in a sense ethanol production is an inefficient, but convenient form of solar power collection.
Measuring the energy gained from producing ethanol is very difficult to determine due to all the factors that go into producing it: energy to use the farm equipment, energy to produce fertilizers, energy to transport crops, energy to refine the ethanol. Partially due to the large extent the United States has been pushing ethanol fuels, a great deal of effort has been put into calculating this energy gained from ethanol.
Early studies on producing ethanol from corn were unclear as to whether there was any net benefit, putting it somewhere between a couple times more energy produced, down to significantly less energy gained than originally used to produce the ethanol. It now appears that producing ethanol from corn has a marginal benefit of somewhere between 6% and 67% more energy extracted from the ethanol than was used to produce it. One study on biofuels soberly put the case as, “The picture for ethanol from corn is particularly depressing. The entire global harvest of corn (700 million tons) converted to ethanol with current technology would yield enough transportation fuels to supply only 6% of the global gasoline and diesel demand.” Gasoline and diesel for transport being only a fraction of the world's overall energy use.
The outlook for producing ethanol from sugarcane has a much better outlook than corn, producing somewhere in the range of 8-10 times as much energy as is used to produce it. Nonetheless, our overall potential from all ethanol fuels is still quite small as the previous biofuels study notes, “Converting 100% of the global harvest of corn, sugarcane, soy and palm oil into liquid fuels, using the current technology, would provide fuel energy of 3% of global primary energy from fossil fuel combustion and net energy (after subtracting the energy required to produce the fuels) of 1.2% of the global primary energy from fossil fuel combustion.” Converting all of our major food crops into ethanol still only has the potential to replace a small fraction of our fossil fuel dependence.
Industrial and Municipal WastesBoth of these are comprised of burning the solid and liquid wastes which would otherwise be thrown away. If we assume that municipal trash in Denmark is fairly typical of the energy content of wastes around the world then incineration of waste yields roughly 1.05*10^7 Joules per kilogram.
No worldwide figures exist for total garbage production, but the United States Environmental Protection Agency estimates that 2.3*10^11 kilograms of garbage is thrown away in the United States each year. Considering both the proportion of greenhouse gas emissions that come from the United States and the percentage of the world's population that lives there, I think a reasonable estimate for the world total would be 10-20 times the US total. Taking the most optimistic of all our assumptions, this still yields a maximum energy potential from burning wastes of about 4*10^19 Joules or less than 10% of the world's current demand.
Environmental ImpactThe incineration of municipal and industrial wastes used to contribute a significant number of pollutants into the environment. Newer standards for cleaning the flue gasses combined with newer technologies have turned incineration from one of the dirtiest energy forms to one of the cleanest. The United States Environmental Protection Agency estimates that dioxin emissions from incineration are 1/1000 of their levels in 1987. Emissions of many other pollutants have also decreased by upwards of 90%.
Solid Biomass and Animal ProductsThis category involves the direct burning wood or other plant matter for fuel or turning it into another form before burning it for fuel. Also included in this category is the burning of animal products or wastes; although this is not a significant source of energy. At present, the burning of wood and other plant materials is still a significant source of heat for warmth as well as cooking especially in developing countries.
Burning these fuels directly is much dirtier than turning it into cleaner fuels such as ethanol, and it has little more energy potential at this stage.