POWER February 2016 - 25

COMBINED HEAT AND POWER
2. CO2 emission intensities of CHP plants based on USC technology
with various levels of steam extraction to a thermal host. Courtesy: EPRI
mal host's location, the EPA's standard offers
CHP plants full credit for any heat that is exported
and put to good use, according to the
following formula:
Pgross= [(PeST + PeCT + PeIE - PeFW)/T]
+ (PtHR + PtIE + PtPS)
Where:
Pgross = Gross energy output of affected
facility in MWh.
PeST = Electric energy output plus mechanical
energy output (if any) of steam
turbine(s) (ST) in MWh.
PeCT = Electric energy output plus mechanical
energy output (if any) of combustion
turbine(s) (CT) in MWh.
PeIE = Electric energy output plus meconfigurations
offered increases ranging
from >25% to >40% in net thermal efficiency
relative to state-of-the-art IGCC plants with
90% CCS, providing CO2 emission intensities
below the EPA target.
Coal Gasification Integrated with
Solid Oxide Fuel Cells. Fuel cells have long
held the promise of higher-efficiency power
generation based on natural gas, hydrogen
fuel produced via the electrolysis of water,
and other gaseous fuels. Combining a coal
gasifier with a fuel cell offers a similar opportunity.
In
integrated gasification fuel cell (IGFC)
power plants, solid oxide fuel cell (SOFC)
technology essentially replaces the combustion
turbine in IGCC designs. Syngas is fed
to the anode side of the cell, and oxygen is
supplied by blowing air into the cathode side
of the cell. This enables an electrochemical
reaction that produces heat and induces direct
current, which is converted to grid-compliant
alternating current via an inverter.
The chemical reaction in the SOFC does
not fully consume the syngas. Any remaining
CO and H2 are mixed with hot air leaving
the cathode and then are combusted in a
burner. The HRSG applies the combustion
exhaust from the burner to raise steam, which
is mixed with steam produced in the syngas
cooler, to drive a steam turbine.
An IGFC plant design operating on
CH4-enriched syngas and at elevated pressure
would have a CO2 emission intensity
of 430 kg/MWh (946 lb/MWh). SOFCs
are still at an early stage in the development
cycle; according to EPRI's analysis,
megawatt-scale modules are not expected
until after 2020.
Coal Gasification Integrated with Triple
Cycles. University of Tokyo professor Dr.
Shozo Kaneko, among others, has promoted
February 2016 | POWER
the concept of integrating SOFCs with conventional
combined cycle plant configurations
to create " triple-cycle " plants that could be
fueled by natural gas, coal-derived syngas, or
other sources. The potential thermal efficiency
advantage of integrated gasification triple cycle
(IGTC) over IGFC designs is that hotter turbine
inlet temperatures can be achieved by having
the SOFC feed a combustion turbine.
EPRI developed a simplified IGTC model
and simulated performance based on a conventional
coal gasifier, a pressurized SOFC,
and a G-class turbine with a 1,500C (2,732F)
firing temperature. Results indicate a possible
net thermal efficiency of greater than
51% (HHV basis) and CO2 emission intensity
of approximately 527 kg/MWh gross
(1,159 lb/MWh), sufficient to achieve the
EPA standard. Room for improvement in
IGTC performance-to achieve even lower
limits-appears to exist through design optimization
and perhaps eventually through the
achievement of a 1,700C (3,100F) turbine firing
temperature. SOFC technology remains
the limiting factor.
Combined Heat and Power Applications.
Another way to increase the thermal
efficiency of pulverized coal power plants
is to utilize the input fuel's energy to produce
both electric power and useful heat.
Combined heat and power or cogeneration
(cogen) plants can approach very high utilization
rates. The technology is commercially
mature. Examples of typical " thermal hosts "
for the exported heat from CHP plants are
oil refineries, food processing facilities, and
central heating (and/or cooling) districts for
commercial buildings, large hotels, hospitals,
and university campuses.
Because exported heat can displace the
burning of fossil fuel that would otherwise
have been used to generate heat at the therwww.powermag.com
chanical
energy output (if any) of affected
facility's integrated equipment (IE) that provides
electricity or mechanical energy to the
affected facility or auxiliary equipment in
MWh.
PeFW = Electric energy used to power
boiler feedwater (FW) pumps at steam-generating
units in MWh.
T = Electric transmission and distribution
factor, set to 1.0 for most affected facilities
and to 0.95 only for a facility where at least
20% of Pgross consists of electric or direct mechanical
output on an annual basis and 20%
of Pgross
output, measured relative to standard ISO
conditions, from heat recovery (HR) for applications
other than steam generation or
performance enhancement of the affected
facility in MWh.
PtIE = Useful thermal energy output relative
to ISO conditions from any integrated
equipment that provides thermal energy to
the affected facility or auxiliary equipment
in MWh.
PtPS = Useful thermal energy output of
steam, measured relative to ISO conditions,
that is used for applications that do not generate
additional electricity, produce mechanical
energy output, or enhance the performance of
the affected facility.
The term PtPS is calculated in MWh as:
PtPS =
Qm x H
3.6 x 109
Where:
Qm = Measured steam flow in kg (lb) for
the operating hour.
H = Enthalpy of the steam at measured
temperature and pressure relative to ISO conditions
in J/kg (Btu/lb).
3.6 x 109
(3.413 x 106
= Conversion factor in J/MWh
Btu/MWh).
Assuming that only steam is exported to a
25
consists of useful thermal energy
output on a rolling three-year basis.
PtHR = Hourly useful thermal energy
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POWER February 2016

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