Primary energy sources such as coal, crude oil, natural gas, oil sands and biofuels are converted into wide array of fuels. A typical crude oil refinery, for example, produces a dozen or more fuels.

Similarly, electricity can be produced from thermal power plants using fossil fuels, biofuels or enriched uranium, or it can come from solar panels, wind turbines or hydroelectric plants.

The diversity of the energy system is important because each energy supply system has a unique greenhouse gas (GHG) intensity: the quantity of GHG emissions per kilowatt-hour of electricity generated or fuel produced. Many energy systems release multiple greenhouse gases, so aggregate metrics are used to compare the emissions from various greenhouse gases based upon their global warming potential (GWP).  GWPs compare the abilities of different greenhouse gases to trap heat in the atmosphere. The GWP of the emission of a greenhouse gas is the ratio of global warming, or radiative forcing (both direct and indirect), from one kilogram of a greenhouse gas to one kilogram of CO2 over a specified period of time.

Using this approach, greenhouse gas emissions are commonly expressed either in terms of carbon-dioxide equivalent (CO₂-eq).  GHG emissions from electricity generation are measured in CO₂-eq/kwh.

A complete picture of the GHG emissions from an energy supply system includes both direct and indirect emissions. Direct emissions refer to those from the generation process itself, often referred to as “stack emissions.”  For example, a coal-fired power plant releases CO₂ up the stack from the combustion of the fuel itself.  A wind turbine produces zero stack emissions.

Indirect emissions refer to GHGs released from stages other than electricity generation. In the case of coal, this refers to the GHG emissions in mining coal and transporting it to the power plant. Indirect GHG emissions in the case of wind include those produced in the manufacture and transport of the turbine.

Electricity generation from fossil fuels has substantially larger GHG intensities compared to nuclear, solar, and wind.  Coal technologies release 800 to 1,400 tonnes CO₂-eq/Gwh (gigawatt hour).  Electricity from natural gas has the lowest emissions among the fossil fuels, ranging from 400 to 550 tonnes CO₂-eq/Gwh. Most of the emissions from fossil fuel plants are stack emissions.  The emissions from renewable power generation and nuclear power are in some cases more than one order of magnitude lower than fossil fuels, and principally in the form of indirect emissions.

Liquid fuels produced from conventional and unconventional oil sources exhibit a wide range of GHG emissions (Table 1). In general, alternatives to gasoline refined from conventional oil production release more GHG, and in some cases the difference is very large.  Oil sands, for example, are abundant, but they release 14 to 40 percent more GHGs per liter of refined fuel. Coal-to-liquids is an established technology, but in a world where GHGs were significantly constrained, it would be difficult to establish without a viable method to capture and store emissions.

Source: Farrell, A. E. and A. R. Brandt.  2006. Risks of the oil transition, Environmental Research Letters, 1: 1404

Soybean-based biodiesel and renewable fuels, produced from seed oils or animal fats via the transesterification process, have been the focus of biofuel production because of their potential environmental benefits and because it is made from renewable biomass resources. Compared to conventional fuels derived from crude oil, soybean-derived fuels achieve a significant reduction (52–107%) in fossil energy use, and modest to significant reductions (57–174%) in well-to-wheels GHG emissions.

The impact on climate from corn-to-ethanol in the United States is the subject of debate.  Well-to-wheels analyses performed at the Argonne National Laboratory indicate that ethanol produced from corn reduce GHG emissions by at least 20 percent relative to conventional motor gasoline from crude oil.  Some ecologists question this result due to the impacts that ethanol production may have on global land use patterns. A diversion of cropland in Iowa from food to fuel production could cause forest land in Brazil (or elsewhere) to be converted to cropland. The resulting release of carbon from that land use change could negate some of GHG offset produced from ethanol replacing conventional gasoline. Careful analyses will be needed to capture all of the direct and indirect effects of biofuels production.

Sources

• Farrell, A. E. and A. R. Brandt.  2006. Risks of the oil transition, Environmental Research Letters, 1: 1404.
• Huo, H. M. Wang, C. Bloyd, and V. Putsche, Life-Cycle Assessment of Energy and Greenhouse Gas Effects of Soybean-Derived Biodiesel and Renewable Fuels, Argonne National Laboratory, March 12, 2008
• http://www.transportation.anl.gov/modeling_simulation/GREET/publications.html
• R.E.H. Sims, R.N. Schock, A. Adegbululgbe, J. Fenhann, I. Konstantinaviciute, W. Moomaw, H.B. Nimir, B. Schlamadinger, J. Torres-Martínez, C. Turner, Y. Uchiyama, S.J.V. Vuori, N. Wamukonya, X. Zhang, 2007: Energy supply. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.