	Introduction

Coal has long been the major fossil fuel used to produce electricity. However, the Environmental Protection Agency (EPA) lists coal-fired electric power plants as one of the largest sources of air pollution in the United States, with greenhouse gas (GHG) emissions from burning fossil fuels believed to be the largest contributor to global climate change. Regulations under development at EPA would impose new requirements on power plants to control GHG emissions. First, in September 2013 EPA proposed standards for the control of carbon dioxide (CO 2 ) emissions from new electric generating units burning fossil fuels. EPA's proposals for control of GHG emissions from existing power plants are expected by June 2014, with many options for reducing GHGs under consideration. EPA may target emissions on a state or plant-by-plant basis, with companies likely given choices for compliance, and increasing coal-fired power plant (CFPP) efficiency may be one of those choices. In its simplest form, a plant's heat rate (for a particular period) can be defined as follows:
HR = F / E 
where,
HR = heat rate (Btu/kWh)
F = heat energy input supplied by fuel to the power plant for a period (BTU)
E = energy output from the power plant in a period (kWh)
Since the equivalent BTU content of a single kWh of electricity is 3,412 BTU, thermal efficiency can be calculated as:
TE = (100) ( 3412 ) / HR 
where ,
TE = thermal efficiency (%)
As an example, using the average heat rate in 2011 of 10,444 BTU/kWh for coal-fired power plants (i.e., all coal types), the average efficiency for coal-fired plants was 33%. Efficiency Improvements to Reduce GHG Emissions

The overall efficiency of a power plant encompasses the efficiency of the various components of a particular generating unit. Lower power plant efficiency results in more CO 2 being emitted per unit of electricity generated. The options most often considered for increasing the efficiency of CFPPs include equipment refurbishment, plant upgrades, and improved O&M schedules. Cost of the improvements is often compared to the expected return in increased efficiency as a primary determinant of whether to go forward with a program. According to an Asia-Pacific Economic Cooperation Working Group (APWG) study in 2001, projects to improve combustion, steam cycle, and O&M required low to medium costs, and these expenditures were predicted to produce as much as a 3.5% net overall efficiency improvement. However, the study found that replacing the older CFPPs with new power plants was not practical because the expenditure for a new plant could not be justified by the improved performance. Instead, efficiency and operational improvements were seen as a possible alternative considering a range of equipment upgrades and refurbishment options to various CFPP systems. NETL then undertook an analysis of the efficiency of U.S. CFPPs, concluding that while the average efficiency was 32% in 2007, the efficiency of the top 10% was five points higher at 37.4%. NETL suggested that if GHG emissions reduction was a goal, then heat rate efficiency improvements could enable a power plant to generate the same amount of electricity with lower CO 2 emissions. In 2010, NETL completed a new study of U.S. CFPP efficiency, dividing the results into 10 deciles of equal capacity (see Table 2 ). According to NETL's analysis, retirements of lower efficiency units combined with increased generation from higher efficiency refurbished units, and advanced refurbishments with improved operation and maintenance, would be the key to potentially increasing average fleet efficiency beyond the best-in-class units. However, IEA affirmed that major plant retrofits and upgrades (i.e., conversion of subcritical PC units to super- or ultra-supercritical PC units) would raise efficiencies more substantially. Within such a system, efficiency improvements can be an important contributor. NETL observed in its 2010 report that based on a scenario where CFPP generation was constant at the 2008 level, increasing the average efficiency from 32.5% to 36% could reduce U.S. GHG emissions by 175 MMmt per year or 2.5% of total U.S. emissions in 2008. One possible approach might be to follow NETL's suggestion of using the top decile of CFPP efficiency as a benchmark for U.S. fleet efficiency, used with an efficiency frontier. Using statistical methods, benchmarks could be used to improve efficiency of the CFPP fleet. The efficiency standards could increase over time, and require CFPPs not meeting these standards to retire.
Coal has long been the major fossil fuel used to produce electricity. However, coal-fired electric power plants are one of the largest sources of air pollution in the United States, with greenhouse gas (GHG) emissions from burning of fossil fuels believed to be the major contributor to global climate change. Regulations under development at the Environmental Protection Agency (EPA) would impose new requirements on fossil-fueled (mostly coal-fired) power plants (CFPPs) to control GHG emissions. The first of these requirements was issued in September 2013 with proposed standards for the control of carbon dioxide (CO2) emissions from new electric generating units burning fossil fuels. EPA's proposals for control of GHG emissions from existing power plants are expected by June 2014, with many options under consideration. EPA may target emissions on a state or plant-by-plant basis, with companies likely given choices for compliance. Within such a system, efficiency improvements can be an important contributor.
The overall efficiency of a power plant encompasses the efficiency of the various components of a generating unit. Minimizing heat losses is the greatest factor affecting the loss of CFPP efficiency, and there are many areas of potential heat losses in a power plant. Efficiency of older CFPPs becomes degraded over time, and lower power plant efficiency results in more CO2 being emitted per unit of electricity generated. The options most often considered for increasing the efficiency of CFPPs include equipment refurbishment, plant upgrades, and improved operations and maintenance schedules.
Cost of the improvements is often compared to the expected return in increased efficiency as a primary determinant of whether to go forward with a program. A study by the Asia-Pacific Working Group (APWG) found that at the low to medium end of cost expenditures are combustion, steam cycle, and operations and maintenance improvements. Replacing the older CFPPs with new power plants was not generally seen as being practical because the expenditure for a new plant could not be justified by the improved performance. Instead, efficiency and operational improvements were seen as a possible alternative considering a range of equipment upgrades and refurbishment options to various CFPP systems.
The National Energy Technology Laboratory (NETL) took APWG's analysis a step further, finding that while the average efficiency of U.S. plants was 32% in 2007, the efficiency of the top 10% was five points higher at 37.4%. NETL suggested that if GHG emissions reduction was a goal, then heat rate efficiency improvements could enable a power plant to generate the same amount of electricity from less fuel and decrease CO2 emissions.
In 2010, NETL completed a new study of U.S. CFPP efficiency, concluding that if generation levels were held constant at 2008 levels, overall fleet efficiency could be raised from 32% to 36%, resulting in an overall reduction in U.S. GHG emissions of 175 million metric tonnes per year, or 2.5% of total U.S. GHG emissions in 2008.
According to subsequent analyses, NETL concluded that retirements of lower efficiency units combined with increased generation from higher efficiency refurbished units, and advanced refurbishments with improved operation and maintenance, would be necessary to achieve this goal. These improvements would generally be considered low to medium cost upgrades. However, at the higher cost end are major plant retrofits and upgrades (i.e., conversion of subcritical CFPP units to super- or ultra-supercritical CFPP units), which would raise efficiencies more substantially.
One possible approach to achieve fleet-wide efficiency improvement might be to follow NETL's suggestion of using the top decile of CFPP efficiency as a benchmark for the U.S. fleet, and establish an "efficiency frontier" that would be revisited periodically to reset the benchmark. This could be combined with possible incentives to improve efficiency or retire less efficient power plants. Other federal approaches could use tax incentives to encourage greater efficiency, or employ energy efficiency standards focused on improving efficiency of CFPPs. The overall cost of these or other programs to increase CFPP efficiency has yet to be determined.