Combustion
Most fossil fuels contain carbon, hydrogen, and sometimes oxygen. Combusting these fuels with oxygen creates carbon dioxide and water.
Every carbon molecule combusted creates a carbon dioxide molecule.
Combustion By-Products
The atmosphere is not pure oxygen and contains mostly nitrogen. Combusting fossil fuels in the atmosphere also creates nitrogen oxides that lead to acid rain.
Fossil fuels also contain other elements. Combusting fossil fuels containing sulfur releases sulfur oxides that also lead to acid rain.
Carbon Emission Intensities
There are several fractions that quantify carbon emissions. In general, we want to minimize the amount of carbon dioxide released, while maximizing the services delivered (electricity, transportation, heat, or others). These are also called emission intensities.
- Carbon Intensity of Electricity
- Mass of carbon released per unit of electricity delivered to the grid
- Carbon Emissions from Fuel Burning
- Carbon released divided by heat energy released
- also called carbon intensity of combustion
- Electricity plant efficiency
- ratio of electrical energy out to heat energy input
- Carbon Intensity of Transportation
- Mass of carbon released per passenger mile of transportation
Carbon Intensity of Electricity
Note some of these are for technologies and some are averaged over a supply of electricity.
Coal-fired electricity carbon intensity (EIA) | 2.21 lb CO2 / kWh |
Natural Gas electricity carbon intensity (EIA) | 0.91 lb CO2/ kWh |
PG&E electricity carbon intensity | 0.524 lb CO2 / kWh |
Sonoma Clean Power Clean Start | 98.8 lb CO2 / MWh |
Sonoma Clean Power EverGreen | 46.0 lb CO2 / MWh |
Carbon Intensity of Combustion
Coal carbon intensity of combustion | 210 lb CO2/MM BTU |
Natural gas carbon intensity of combustion | 120 lb CO2/MM BTU |
Natural gas carbon intensity of combustion (EPA) | 0.0549 MT CO2/Mcf |
Power Plant Efficiencies
Coal plant efficiency | 30–45% |
Natural gas plant | 42% |
Natural gas combined cycle | 60% |
Transportation
Van Pool | 0.22 lb CO2/passenger-mile |
Heavy Rail Transit | 0.22 lb CO2/passenger-mile |
Light Rail | 0.36 lb CO2/passenger-mile |
Bus Transit | 0.64 lb CO2/passenger-mile |
Private auto | 0.96 lb CO2/passenger-mile |
Source: DOT Report
Units
Common units of energy for carbon intensities are the kilowatt-hour, megawatt-hour, and gigawatt hour.
Note that:
- 1 MWh = 1000 kWh
- 1 GWh = 1000 MWh
3 MWh \frac{1000\; kWh}{1\; MWh} = 3000\; kWh
A common unit of mass for carbon dioxide emissions is the metric ton.
- 1 metric ton = 1000 kg