A percentage improvement in heat rate is nearly equivalent to an equal percentage
improvement in the emissions rate in terms of the change in CO2 emissions. The difference
stems from the small variation in carbon per Btu across coal varieties. The heterogeneity in
heat rates across coal-fired generation units can partly be explained by technical
characteristics determined at the time of plant construction that cannot be changed without a
major overhaul. This category includes size, age, firing type, and the technology employed.
Higher efficiency is generally associated with plants that are used more heavily because
efficient units are less costly to operate.