The carbon content of the primary energy supply

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The primary energy supply comprises all the energy sources which contribute to the overall supply of primary energy: nuclear energy, coal, oil, natural gas, hydroelectricity, biomass, wind, solar and geothermal energy.

The diversification of energy sources is essential. It helps to reduce the dependence on oil imports and thus to promote the security of supplies. However, it is not necessarily beneficial in terms of climate change. Thus, in the absence of appropriate measures, the comeback of coal might lead to a significant increase in CO2 emissions.

From this point of view, the situation can be improved only by increasing the share of low carbon energy (nuclear and renewable), or by using as energy vectors electricity or hydrogen produced from fossil fuels with CO2 capture and storage (see Chapter 8).

It is also possible to use biomass as an energy source. As the carbon contained in biomass has been extracted from the atmosphere during the photosynthesis process, it is considered as recycled when it is emitted as a result of biomass combustion. The carbon balance of biomass combustion is therefore carbon neutral (except if fossil energy has been used for biomass production). A complete carbon balance has to take into account all CO2 emissions resulting from biomass production (agricultural machines, fertilisers, etc.), harvesting and transport to the final user site.

When energy is produced from a fossil fuel, the quantity of CO2 emitted per unit of energy produced depends on the hydrogen/carbon ratio of the initial fuel. The energy carbon content can be expressed in kg of CO2 (or only carbon) per unit of energy produced (GJ). The carbon content of natural gas is therefore much lower than that of coal, as shown in Table 6.1.

Table 6.1 Comparison of CO2 emissions for various fossil energies (Source: ADEME)

Heating

CO2 emissions per

CO2 emissions

value (GJ/t)

unit mass (t/t)

per toe (t/toe)

Natural gas

57 (LHV)a

3.26

2.394

Oil

42

3.066

3.066

Coal

26

3.14

3.990

The lower heating value (LHV) or net heating value does not take into account the heat supplied through the condensation of steam contained in the flue gases.

The lower heating value (LHV) or net heating value does not take into account the heat supplied through the condensation of steam contained in the flue gases.

In addition, a fuel such as natural gas offers high energy efficiencies, especially through the use of combined cycles. In this type of cycle, steam is produced by recovering heat from the exhaust gases of the gas turbine and used to drive a steam turbine. The energy efficiency of a natural gas combined cycle therefore reaches a value of nearly 60%, whereas the efficiency of the present coal-fired power stations is in the range of45-47%.

By substituting natural gas for coal, the CO2 emissions per unit of energy produced can therefore be reduced by a factor of 2-3. Substitution of biomass for coal can lead to even better results, since the carbon emitted is then considered as recycled (provided, however, that the CO2 emissions associated with the production and transport of biomass are sufficiently reduced).

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