Although EPA states there is a direct relationship between coal-fired EGU efficiency and CO2 emissions, EPA recognized that other factors must be considered when comparing the effectiveness of GHG control technologies to improve the efficiency of a given coal-fired EGU. The study states that the actual overall efficiency that a given coal-fired EGU achieves is determined by the interaction of a combination of site-specific factors that impact efficiency to varying degrees, including:
EGU thermodynamic cycle – EGU efficiency can be significantly improved by using a supercritical or ultra-supercritical steam cycle.
EGU coal rank and quality – EGUs burning higher quality coals (e.g., bituminous) tend to be more efficient than EGUs burning lower quality coals (e.g., lignite).
EGU plant size – The electric-generating capacity of EGUs ranges from approximately 25 to
1,300 MW. Assuming an EGU efficiency of 33% (a typical efficiency for existing coal-fired
EGUs), this corresponds to a heat input range of 250 to 13,400 MMBtu/hr.
EGU efficiency generally increases with size because the boiler and steam turbine losses are lower for larger equipment. However, as equipment size increases [beyond a certain point] the differences in these losses start to taper off.
EGU pollution control systems – The electric power consumed by air pollution control equipment reduces the overall efficiency of the EGU.
EGU operating and maintenance practices – The specific practices used by an individual electric utility company [including] combustion optimization, equipment maintenance, can affect EGU efficiency.
EGU cooling system – The temperature of the cooling water entering the condenser can have impacts on steam turbine performance. Once-through cooling systems can have an efficiency advantage over recirculating cooling systems (e.g., cooling towers). However, once-though cooling systems typically have larger water related ecological concerns than recirculating cooling systems.
EGU geographic location – The elevation and seasonal ambient temperatures at the facility site ... may have a measureable impact on EGU efficiency. At higher elevations, air pressure is lower and less oxygen is available for combustion per unit volume of ambient air than at lower elevations. Cooler ambient temperatures theoretically could increase the overall EGU efficiency by increasing the draft pressure of the boiler flue gases and the condenser vacuum, and by increasing the efficiency of a condenser recirculating cooling system.
EGU load generation flexibility requirements – Operating an EGU as a base load unit is more efficient than operating an EGU as a load cycling unit to respond to fluctuations in customer electricity demand.
Based on the factors above, EPA concludes that coal-fired power plants “identical in design but operated by different utility companies in different locations may have different efficiencies. Thus, the level of effectiveness of a given GHG control technology used to improve the
efficiency at one coal-fired EGU facility may not necessarily directly transfer to a coal-fired EGU
facility at a different location.”40
EPA recognized in the study that a number of technologies to improve power plant efficiency are available for application to existing coal-fired EGU projects which can incrementally improve thermal overall efficiency, and expanded on the efficiency improvements reported by NETL in its
2008 report on efficiency improvement projects (shown in Table 1).
International Energy Agency Study
The International Energy Agency released a study41 in 2013 looking at opportunities to reduce CO2 emissions using upgrades and efficiency improvements at CFPPs. IEA concluded that substantial improvements (i.e., retrofits) may be seen as cost-effective if these economically restore the efficiency of a power plant.
Despite involving substantial outlay (typically US$100–200 million), retrofits will provide a payback in restored generation, fuel saving, extended plant life, and, in some countries, CO2 emissions cost savings. There are also benefits of reduced specific emissions of other pollutants.42
Retrofits include turbine upgrades, condenser optimization, increasing the capacity and efficiency of air-cooled condensers, boiler system improvements, and improvements to other systems where energy losses can occur (as shown in Figure 6).
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. IEA used the example of a conversion of a subcritical 500 MW unit in the United Kingdom to a supercritical pressure, which was projected to raise net generation efficiency from 38% to 44% (on a lower heating value basis). The upgrade was projected to reduce CO2 emissions by 500,000 tonnes per year.
Using Renewables to Improve Coal Plant Efficiency
Heat rate improvement could potentially be achieved using renewable technologies to either provide heat to reduce heat losses at various points in the steam cycle, or to provide power to the
equipment used to curb these heat losses, thus curbing on-site equipment electricity use. One such hybrid coal-solar power plant is already in operation in the United States, at the Xcel Cameo Generating Station in Colorado.
The demonstration project is expected to cut the use of coal at the power plant by around two or three percent, and could be scaled up to cut it by 10 percent. The system works through a series of parabolic trough solar collectors made of glass mirrors. On sunny days the mirrors concentrate the solar radiation onto a line of receiver tubes filled with a heat transfer fluid (mineral oil). The solar energy heats the circulating oil to about 300°C (575°F). The heated oil is then fed to a heat exchanger where the heat is transferred to water to heat it to around
200°C (407°F) before it enters the boiler. Having hotter water entering the boiler means less coal is needed to heat it and produce the steam that turns the turbine to generate electricity.43
Alternatively, using biomass has been suggested to co-fire with coal in a CFPP, or to replace coal altogether.
Combining the use of biomass with coal can be beneficial, particularly from an environmental standpoint although any such process may have its limitations or drawbacks. Each coal type and biomass feedstock has different characteristics although by combining the two, it may be possible to capitalize on the advantages of each, and minimize their individual disadvantages. An effective way is via [gasification and production of syngas, a mixture of hydrogen and carbon monoxide], and useful operating experience has been achieved in a number of large-scale coal-fuelled gasification and IGCC plants ... It also has the potential to form the basis of systems that combine coal and biomass use with other renewable energy technologies to create clean, efficient energy-production systems. Thus, various hybrid energy concepts, some based on coal/biomass [gasification], have been proposed or are in the process of being developed or trialed. Some propose to add yet another element of renewable energy to the system, generally by incorporating electricity generated by intermittent renewables such as wind or solar power. A number also aim to incorporate some form of carbon capture and storage.44
As biomass is generally considered carbon neutral,45 co-firing coal with biomass can provide advantages for electric power generation. However, using biomass on a large, commercial scale has a number of potential issues. Since the heating value and bulk density of biomass is lower than coal, the necessary volumes to be harvested and handled can be substantial, and the type and availability of different biomass materials tends to vary considerably with location. A potential source of biomass in the United States could be wastes from the forest products industry.46
Potential Barriers to Implementing Efficiency
Upgrades
Government regulations, regulatory regimes (i.e., competitive markets or traditional cost of service regulation), and industry factors (such as growth or a lack of growth in demand) all affect the motivation for implementing improvement projects at coal-fired power plants. These external forces add to the internal evaluation of the costs vs. benefits of improvements for a particular unit. This section will look at a few of these external forces which may hamper the implementation efficiency projects.
New Source Review
The New Source Review program was designed to prevent the degradation of air quality from the construction of new facilities or modification of existing facilities which have potentially harmful emissions. NSR was established by Congress as part of the 1977 Clean Air Act Amendments (P.L.
95-95).
The NSR process requires power plant operators to undergo a review for environmental controls if they build a new power generating unit, and to impose the Best Available Control Technology, as defined by the state permitting authority (or in some cases EPA). Efficiency improvements to power plants that reduce regulated pollutants should not theoretically trigger NSR requirements, unless the improvements result in an increase in emissions (e.g., because the modified plant operates for more hours). Establishment of a pre-improvement emissions baseline before and a post-improvement emissions report after efficiency upgrades seems like a logical step, but may not be easily achievable on a consistent basis.47 There are also ambiguities in the law which may serve to hamper efficiency projects from going forward.
Power plants built prior to 1971 are exempted from the limits on criteria pollutant emissions contained in the Clean Air Act, but may lose that exemption and be forced to undergo an NSR if the EPA determines that the plant has undergone non-routine maintenance which incre