Reducing the Energy Carbon Content - Energy and Climate: How to Achieve a Successful Energy Transition - page 68

When energy is produced froma fossil fuel, the quantity of CO 2 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 CO 2 (or
only carbon) per unit of energy produced (GJ). The carbon content of
natural gas is thereforemuch lower than that of coal, as shown inTable6.1.

Table 6.1 Comparison of CO 2 emissions for various fossil energies ( Source :

ADEME)

Heating
value (GJ/t)

CO 2 emissions per
unit mass (t/t)

CO 2 emissions
per toe (t/toe)

57 (LHV) a

Natural gas 3.26 2.394

Oil 42 3.066 3.066

Coal 26 3.14 3.990

a 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-firedpower stations is in the range of 45-47%.
By substituting natural gas for coal, the CO 2 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 CO 2 emissions
associated with the production and transport of biomass are sufficiently
reduced).

Carbon intensity

Carbon dioxide emissions per inhabitant in the world are highly variable.
They range from 1 tonne per year per inhabitant for India up to 18 tonnes
per year per inhabitant for North America.
They are directly correlated to the Gross Domestic Product (GDP) per
inhabitant, as shown by the graph in Figure 6.1. Some countries are above
the correlation interval (United States, Canada, Russia, Saudi Arabia,
Poland, South Africa). In contrast, France and Japan, which are below the
interval, may be considered as comparatively 'virtuous'.
The carbon intensity is defined as the level of CO 2 emissions related to
the GDP. Between 1990 and 2005, the carbon intensity dropped in several