Carbon dioxide, SO2 and NOx
The concept of ‘fixing’ atmospheric CO2 by planting trees on a very large scale has attracted much attention. Halting deforestation would bring many environmental benefits, as would re-planting large areas with trees. However, absorption of carbon dioxide by a new forest plantation is a once-and-for-all measure, ‘buying time’ by fixing atmospheric CO2 while the trees mature, say for 30–60 years, after which the CO2 would probably be released. A wider bioenergy strategy, concentrating on the substitution of biofuels for fossil fuels, may be a more effective lasting solution.
To analyse the benefits of substitution, it is essential to assess all the effects in a life-cycle analysis (LCA). Table 3 shows a LCA for electricity generating plants that are either operating or near to commercial implementation. It shows the emissions of carbon dioxide and two of the main contributors to acid rain: sulfur dioxide and nitrogen oxides.
|Emissions 1/t GW h-1|
|Combustion, steam turbine||CO2||SO2||NOx|
|Imported wood pellets||50||0.1||1.58|
|Anaerobic digestion, gas engine|
|Natural gas: CCGT2||446||0.0||0.5|
|Coal: with minimal pollution abatement||955||11.8||4.3|
|Coal: Flue Gas Desulfurization and low NOx3 burner||987||1.5||2.9|
Even the best systems are not entirely carbon-neutral, but all the bioenergy systems, even MSW combustion, have lower CO2 emissions than any of the fossil fuel plants.
Nitrogen and sulfur oxides
Nitrogen oxides (NOx) are an inevitable product of the combustion in air of any fuel, because four-fifths of the air is nitrogen. High temperatures – in furnaces or internal combustion engines increase NOx production, and bioenergy systems will need to meet the same ‘clean-up’ requirements as those using fossil fuels. This also applies to the removal of particulates. The emissions of sulfur oxides depend on the sulfur content of the biofuel, which will vary with the particular characteristics of the feedstock concerned.
Dioxins are complex, carcinogenic compounds formed during combustion, and are a continuing source of public concern. However, it has been estimated that EfW accounts for only 0.1% of UK dioxin emissions. The UK and the EU are enforcing increasingly stringent emission standards and the installation of pollution control technology.
Methane (CH4) is a powerful greenhouse gas produced from the anaerobic breakdown of biomass. A molecule of CH4 is about 22 times as effective as a molecule of CO2 in trapping the Earth’s radiated heat.
With careful storage and efficient combustion, the methane emissions from MSW should be low. However, with landfill it is never possible to collect all the gas, and there are inevitably methane emissions to the atmosphere. This is the case whether the gas is being collected and used or not, so the most important consideration is whether the methane emissions from anaerobic digestion plants are greater or less than those that occur would occur naturally in landfill anyway.