An instrument to promote the CO2 emission reductions, taking the Kyoto Protocol goal
into account, can be the assignment to energy conversion plants of a monetary charge
linked to their specific emission intensity. Once the choice of a charge is defined, the
next problem is the choice of a strategy to determine the amount of the imposed charge,
named Carbon Tax (CT).
In this paper an analytical procedure for the Carbon Tax evaluation is proposed and
applied. This approach is based on the concept of Efficiency Penalty of the energy
system, that represents the evaluation of the cost of the exergy destroyed inside the
system and the cost of the exergy rejected in the biosphere with the plant wastes; the
Efficiency Penalty term is coupled with the evaluation of the Index of CO2 Emission,
which connects the amount of the CO2 emitted by the plant with the Second Law
efficiency of the plant itself. The evaluated charge on the CO2 emissions is defined as
Carbon Exergy Tax (CET). The procedure is applied here to the analysis of a 700 MW
combined plant burning fossil fuels in two different configurations: a typical natural gas
fired combined plant, and a coal fired combined plant burning coal in a Pressurised
Fluidised Bed Combustor (PFBC).
