POWER February 2016 - 23

COMBINED HEAT AND POWER
Table 1. Recent CO2 emissions
standards for new coal plants.
Source: EPRI
Country or state
New York
Canada
Standard (CO2/MWh)
420 kg (925 lb) net
420 kg (925 lb) net
United Kingdom 450 kg (992 lb) net
California
Maine
Washington
United States
China
500 kg (1,100 lb) net
500 kg (1,100 lb) net
500 kg (1,100 lb) net
636 kg (1,400 lb) gross
763 kg (1,679 lb) net
Based on high-level assessments described
in a recent white paper published by
the Electric Power Research Institute (EPRI),
the answer is a qualified " yes. " The paper,
" Can Future Coal Power Plants Meet CO2
Emission Standards Without Carbon Capture
& Storage? " (EPRI report 3002006770)
is available at www.epri.com. This article
offers highlights of that report, with an emphasis
on the only currently commercially
available and proven technology route, CHP
(Figure 1).
In Search of Alternatives to CCS
Some industry executives are beginning
to wonder if an easier path to regulatory
compliance might be found through advanced
coal technology. The answer is
clear: Without CCS, meeting or beating
the U.S. standard can only be achieved
by increasing the thermal efficiency of
the energy conversion processes involved
in generating electricity from coal-a
challenge more easily stated than accomplished
(Table 2). EPRI found:
■ Even with steam temperatures exceeding
800C (1,500F)-some 200C (360F)
higher than those currently achievable-
USC coal plants based on the conventional
Rankine steam-electric cycle alone are not
capable of meeting the standard.
■ USC plants used in high-efficiency CHP
applications are capable of meeting the
standard-but only at sites with " thermal
hosts " capable of using large volumes of
steam.
■ Gasifying coal and then firing the synthesis
gas in a conventional combined cycle
configuration can meet the standard-but
only for certain types of gasifiers, and
only when the integrated plant is fueled by
high-quality coal.
■ Assuming further technological progress,
coal gasification provides multiple pathways
for achieving the standard, including
gasifiers integrated with solid oxide fuel
February 2016 | POWER
1. How coal technologies stack up. Courtesy: EPRI
cells; with combined cycle plants having
firing temperatures for the combustion
turbine approaching 1,700C (3,100F); or
with novel cycle designs.
■ Greater public-private investment in research
and development (R&D) is needed
to accelerate commercialization of gasification-based
cycles and component technologies.
■
Power producers interested in new coal
plants could explore potential thermal
hosts for CHP projects or the economics
of gasifying and firing high-quality coal.
Coal Generation Technologies
That Could Meet CO2 Emissions
Standards
High-quality bituminous fuel makes up
only about half of the world's coal resource.
EPRI's report focuses on possible technology
options for meeting the EPA's standard
while firing subbituminous Powder River
Basin coal, representative of widely available
lower-rank fuels.
www.powermag.com
Rankine Cycle Plants with Higher
Steam Temperatures. The overwhelming
majority of coal-fired power plants are based
on the Rankine cycle, in which high-pressure
steam is raised from the heat released while
burning pulverized coal. Efficiency can be
improved by increasing the temperature ratio
of the hottest and coldest points in the cycle.
For the Rankine cycle, this means increasing
the temperature of the steam entering the turbine
and/or decreasing the temperature in the
condenser at the turbine exit.
Since 2001, an R&D consortium principally
funded by the U.S. Department of Energy
and the Ohio Coal Development Office has
been pursuing advanced materials for coalfired
boilers and steam turbines, with EPRI
serving as the technical lead. Nickel alloys
in particular show promise of allowing steam
temperatures to rise to 760C (1,400F). That
would allow an increase in gross thermal efficiency
of at least 10%, from the current ~41%
for USC plants to ~45% for next-generation
advanced USC plants.
23
http://www.epri.com http://www.powermag.com
POWER February 2016

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