Cornerstone - Summer 2015 - (Page 67)
L E TTERS
VOLUME 3, ISSUE 1
UPGRADING THE EFFICIEINCY OF THE WORLD'S
COAL FLEET TO REDUCE CO2 EMISSIONS
hile I found a recent Cornerstone cover story interesting 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
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
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
In the long term USC/AUSC designs will improve the coal fired
fleet's Carnot cycle efficiency and reduce CO2 per megawatt
In the short term there are significant potential efficiency
improvements available in day-to-day operation and
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
Table of Contents for the Digital Edition of Cornerstone - Summer 2015
From the Editor: Urban Centers as Vehicles for Societal Development
Cover Story: Urbanization, City Growth, and the New United Nations Development Agenda
The High Cost of Divestment
South Africa’s Road to Growth is Paved with Coal
Driving India’s Next Wave of Urbanization
Transitioning Urbanization, Energy, and Economic Growth in China
ASEAN Urbanization and the Growing Role of Coal
Urbanization, Steel Demand, and Raw Materials
The Rise and Potential Peak of Cement Demand in the Urbanized World
Cogeneration Plants Close to Town Get the Most Out of Coal in Germany
Shenhua Guohua’s Application of Near-zero Emissions Technologies for Coal-fired Power Plants
Ashworth Gasifier-Combustor for Emissions Control From Coal-Fired Power Plants
Underground Coal Gasification: An Overview of an Emerging Coal Conversion Technology
Carbon Energy Delivers Innovations in Underground Coal Gasification
Cornerstone - Summer 2015