In 2007 the UK emitted 545.7 million tons of the greenhouse gas CO2 into the atmosphere. Nearly 40% of this gas was produced by the generation of electricity as is shown in the following graph of sources of CO2 in the UK.
The UK has made the commitment to reduce these CO2 emissions to less than 60% of the 1990 levels (589 million tons in 1990) by 2050 with significant progress made by 2020. Additionally, the UK will need around 30-35GW of new electricity generation capacity over the 20 years as many of the current coal and nuclear power stations, built in the 1960s and 1970s, reach the end of their lives and are set to close. Given the expected increase in energy needs in the UK by 2050, the reduction in CO2 from power plants in reality must be reduced by 90% from current levels.
To achieve such drastic decreases in the emission of CO2 gas every opportunity for clean technology must be investigated, both in conventional thermal power plants, automobiles, homes, industry and most importantly in renewable energy.
Natural Gas and Coal fired power plants are a major contributor to CO2 emission. These two types of power plants make up 73% of all electricity production in the UK as shown in the following graph.
CO2 per unit electricity (coal and natural gas)
Natural gas and coal generate 40% and 33% of the UK’s total electricity respectively. Both of these fuels emit CO2 when burned and coal additionally produces significant volumes of other harmful pollutants as shown below.
Pounds of Air Pollutants Produced per Billion Btu of Energy
- Table from EIA – “Natural Gas 1998: Issues and Trends”
- No post combustion removal of pollutants. Bituminous coal burned in a spreader stoker is compared with No. 6 fuel oil burned in an oil-fired utility boiler and natural gas burned in uncontrolled residential gas burners. Conversion factors are: bituminous coal at 12,027 Btu per pound and 1.64 percent sulfur content; and No. 6 fuel oil at 6.287 million Btu per barrel and 1.03 percent sulfur content—derived from Energy Information Administration, Cost and Quality of Fuels for Electric Utility Plants (1996).
Pounds of pollutant per BTU is only part of the story. Natural Gas and Coal fired power plants are not 100% efficient at converting thermal energy from burning into electricity.
From the US National Petroleum Council, “ELECTRIC GENERATION EFFICIENCY”, report of 2007, the average US coal fired plant required 10,400 BTU of energy per kW-H of electricity produced. The average US natural gas fired plant required 7,920 BTU per kW-H. Under perfect conversion circumstances, it only requires 3412 BTU per kW-H. This means that the average thermal efficiencies of US coal and gas fired plants are 32.8% and 43.1% respectively.
To calculate the pounds of CO2 emitted per MWh:
In 2007, the UK demand for electricity was 400,160 GWh (http://stats.berr.gov.uk/energystats/dukes52.xls).
CO2 Generated in the UK from Electricity Generation (2007)
This translates into 132,053 GWh produced with Coal and 160,064 GWh produced by Natural Gas. Finally, this can be reduced to tons of CO2 emitted by coal plants and natural gas fired power plants in 2007 as follows:
Coal: 142,815,320 tons CO2
Gas: 8,483,392 tons of CO2
TOTAL: 151,298,712 tons of CO2 from coal and gas power generation
The above number is calculated based on the average efficiency of US coal and gas fired plants, so the measured number of tons of CO2 released by UK power plants is slightly different.
Where does CO2 come from in Combustion
The emission of CO2 per unit of fossil fuel burned is a function of the chemical reaction of the fuel with oxygen. The energy released per unit fuel increases with the number of molecular bonds that are broken during the combustion process. Fuels with high ratios of hydrogen to carbon have relatively more bonds and less mass, and with consequently release less CO2 per unit of energy released.
To show this, consider the three chemical reactions governing the combustion of goal, natural gas and oil. These are balanced reactions (no excess air, etc…), so they are not strictly accurate as most combustion processes are not stoichiometrically balanced, but the following does illustrate the points made earlier.
Coal: C + O2 >> CO2 + 30 MJ/kg
Gas: CH4 + 202 >> CO2 + H2O + 50 MJ/kg
Oil (gasoline): C8H18 + 12.5O2 >> 8CO2 +9H2O + 50 MJ/kg
The energy released is given per unit of fuel combusted and not of CO2. The amount of CO2 released per unit energy can be calculated from the reactions by comparing the molecular mass of fuel going into the reaction to the mass of CO2 emitted.
The carbon “intensity” of methane (gas – CH4) is calculated below in units of MJ/Kg CO2 produced.
Molecular mass of Carbon (C ) = 12
Molecular mass of Oxygen (O) = 16
Molecular mass of Hydrogen (H) = 1
Mass of a Kg-Mole of CH4 = 16 kg
Mass of Kg-Mole of CO2 = 44 kg
Therefore, the amount of energy released per unit of CO2 emitted is:
50 Mj/kg X (16/44) = 18.2 Mj/kg CO2
Similarly for oil (gasoline) is 16.1 Mj/ kg CO2 and for coal 8.18 Mj/kg CO2.
From these results, one can see that for a given amount of CO2 produced, coal produces the least amount of energy (Mj).
NOTE: MJ = 1000 joules of energy (950 BTU)