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.
A second factor is how the boiler is used. The relationship between the heat rate and
utilization is nonlinear, as efficiency tends to be lower at very low and very high levels of
utilization ... Units with lower utilization may be ramped up and down more frequently,
which requires additional fuel input as temperature in the boiler fluctuates. The result could
involve efficiency losses at least partly outside the control of plant decision makers. Plant
managers control several other factors that affect heat rates. Techniques, management, or
technology may improve the efficiency of the plant by targeting the major components of the
coal combustion process: oxygen, temperature, and pressure. Excessive deviations in any of
these areas may decrease efficiency through waste or shortfalls ...Maintenance and
performance testing are also critical for identifying and preventing losses.20