VOLUME 3, ISSUE 1
UPGRADING THE EFFICIENCY OF THE WORLD’S COAL FLEET TO REDUCE CO2 EMISSIONS
While I found a recent Cornerstone cover story inter-esting in its discussion of reducing global CO2 emissions from the global coal-fired power plant fleet through USC and AUSC designs, I found myself wondering about the two primary options for coal plant efficiency improvements.
There are two types of efficiencies that apply to a typical Rankine cycle steam electric generating station: Carnot cycle efficiency and equipment efficiency. There is a mistaken impression in some quarters that the equipment, especially the steam generator or turbine, are the source of improved SC/USC/AUSC efficiencies.
According to Carnot a thermodynamic heat engine operates between a heat source and a heat sink where the heat available for conversion is determined by the difference in thermodynamic properties between the source and the sink.
A typical turbine steam path converts about 85% of the steam energy between the throttle valves and the used energy end point (UEEP). Consider two different plant options: 1) 2400 psig, 1000°F, 1460 Btu/lb and 2) 3100 psig, 1100°F, 1506 Btu/lb. Condenser back pressure is 4.0 inches Hgabs, 125 °F, 1064 Btu/lb. With a steam flow of 5,000,000 lb/h, ΔH, and 85% efficiency case 1) will produce 493 MW and case 2) 550 MW.
Efficiency savings must be balanced with the additional costs incurred for equipment and materials that must handle both short and long term stresses. T/P91 & 92 are popular materials for USC, but has enough time and experience accumulated to predict long-term reliability? There are several vintage SC plants, however USC operates at higher temperatures which have an exponential impact on metallurgical reliability.
Heat rate and fuel carbon content are the important factors. Coal gets most of its energy from carbon, rather than hydrogen. For a typical power plant the equipment energy losses are distributed in this general manner.
About 10% to 15% of the fuel input is lost up the stack. About 5% is dry gas losses which depend on the amount of flue gas and temperature. About 5% is latent heat of water vapor from the products of combustion. This water vapor heat counts as a loss if HHV is used for the fuel heat content and does not count if LHV is used for the fuel heat content. The third 5% is from boiler radiation losses, unburned fuel, bottom ash heat, etc.
The turbine’s steam path is actually fairly efficient, converting about 85% to 90% of the steam between the throttles and UEEP mechanical energy (i.e., driving the generator).
The circulating water system that carries away the heat from the turbine’s condensing exhaust steam is the most misunderstood and neglected of the plant systems. This system carries 50% of the energy that entered the turbine throttles.
The heat rates for typical utility power plants frequently run as high as 5% to 10 % above design. For decades utilities in the U.S. have been reluctant to pursue significant and expensive plant efficiency improvements because public utility commission mandated fuel/energy cost adjustment factors burden the utility with the costs while passing the savings on to the rate payer.
In the long term USC/AUSC designs will improve the coal fired fleet’s Carnot cycle efficiency and reduce CO2 per megawatt hour.
In the short term there are significant potential efficiency improvements available in day-to-day operation and maintenance.
Nicholas Schroeter, PE
President and Master Navigator
Heat Rate Navigation Services, Inc.
Response: It is true that some decent benefits in coal-fired power plant efficiency can be gained by close attention to operation and maintenance. This should be part of best practices for all utilities. In my experience, some utilities are very attentive to this approach, while others can be less so. However, once one has “wrung out” the last fractions of a percent by this route, equipment upgrades need to be considered to allow the higher steam temperatures and pressures needed for further efficiency improvements.
IEA Clean Coal Centre
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