POWER February 2016 - 24

COMBINED HEAT AND POWER
Table 2. Technologies considered in EPRI's analysis, relative to the
EPA's standard. Source: EPRI
Technology
USC
USC
USC
USC
USC
USC
USC
USC
USC+CHP
USC+CHP
USC+CHP
USC+CHP
IGCC
IGCC
IGCC
IGCC
IGCC+HTCT
IG+SCO2
IGFC
IGFC
IGFC
IGFC
IGTC
1,500C combustion turbine
Notes: CHP = combined heat and power, HTCT = high-temperature combustion turbine, IG = integrated gasification,
IGCC = integrated gasification combined cycle, IGFC = integrated gasification fuel cell, IGTC = integrated
gasification triple cycle, SCO2 = supercritical CO2, USC = ultrasupercritical.
This increase in efficiency would not be
sufficient to meet the EPA's standard, but
deploying advanced Rankine cycles would
significantly decrease the amount of CO2
that would have to be captured and stored.
If materials could be developed, that would
allow Rankine cycle coal power plants to
reach steam temperatures greater than 760C
(1,400F). However, EPRI has calculated that
a plant capable of meeting the EPA standard
of 636 kg/MWh gross (1,400 lb/MWh) would
require steam temperatures around 1,125C
24
(2,050F). This is well beyond current materials
technology.
Coal Gasification Integrated with
Combined Cycles. Gasifying coal to produce
a synthesis gas (syngas) opens up additional
potential pathways for meeting CO2
emission limits without CCS. The syngas
consists predominantly of carbon monoxide
(CO) and gaseous hydrogen (H2) fuel. Contaminants,
specifically sulfur species and ash
mineral content, are removed. Clean syngas
is then fired in the open Brayton cycle comwww.powermag.com
1,700C
combustion turbine
Oxy + CO2 venting
Atmospheric pressure SOFC
Catalytic gasifier +
Atmospheric pressure SOFC
Pressurized SOFC
Catalytic gasifier +
Pressurized SOFC
Pressurized SOFC +
527
51.3
Details
290BAR/593C/621C
276BAR/593C/616C
276BAR/649C/671C
276BAR/704C/727C
290BAR/593C/621C +
High-quality coal
352BAR/680C/700C
276BAR/760C/760C
352BAR/680C/700C/700C
290BAR/593C/621C +
14% steam extraction
276BAR/760C/760C +
12.5% steam extraction
276BAR/760C/760C +
25% steam extraction
276BAR/760C/760C +
50% steam extraction
Siemens/2XGE7FB
E-Gas/GE7FB
Shell/3XGE7FB
Shell/3XGE7FB +
High-quality coal
Shell +
567
603
603
501
498
430
47.6
42.0
43.7
54.0
51.5
61.4
CO2 emissions intensity
(kg/MWh, gross)
789
777
754
735
734
723
721
715
636
636
568
465
743
702
676
627
Efficiency (%)
38.4
39.5
40.8
42.0
41.2
42.7
42.8
43.4
46.5
49.0
55.2
68.0
38.2
38.7
40.0
41.9
bustion turbine. The turbine exhaust is fed to
the Rankine cycle heat recovery steam generator
(HRSG), which also receives steam
from the syngas cooler.
State-of-the-art integrated gasification
combined cycle (IGCC) units (as is expected
for Mississippi Power's Kemper County
IGCC project, shown under construction in
May 2015 in the opening image) have thermal
efficiencies based on net power output
that are similar to those of state-of-the-art,
stand-alone Rankine cycle coal power plants,
offering significantly lower gross CO2 emission
intensities. However, the auxiliary power
consumption of IGCCs is greater-an important
consideration when gross CO2 emission
intensity is the parameter of merit.
IGCC efficiency gains (and resulting reductions
in CO2 emissions intensity) can be
realized by increasing the firing temperature
of the combustion turbine. EPRI's 2011
IGCC R&D roadmap concluded that going
from today's range of 1,370C-1,430C
(2,500F-2,600F) to 1,700C (3,100F) would
decrease heat rate by 16%. The technical feasibility
of a firing temperature approaching
1,700C (3,100F) has been established; the
timeline for commercial deployment of hightemperature
combustion turbines optimized
for IGCC operation is uncertain but appears
to be about a decade away.
Coal Gasification Integrated with
Supercritical CO2 Brayton Cycles.
Closed Brayton cycles using supercritical
CO2 (SCO2) as the working fluid are being
investigated by several organizations. (See
" Ten Advanced Combustion Systems That
Are Getting the Government's Backing "
at powermag.com.) SCO2 is introduced to
the burner to dilute the mixture because
firing syngas in oxygen would result in
temperatures above 2,750C (5,000F), far
exceeding the level current turbine technology
can tolerate. Recycling a large
flow of CO2 moderates the firing temperature
to 1,150C (2,100F).
The stream exiting the burner includes
a small amount of water vapor but consists
mostly of CO2. The ratio of recycled CO2 to
that produced by combustion of the syngas is
about 10:1. The flow is expanded in a turbine
to drive a generator. The turbine exhaust, still
at high temperature (>700C or 1,300F), is
directed to a large heat exchanger (recuperator),
then cooled further to condense out the
water vapor. The remaining flow, essentially
all CO2, is raised up to high pressure (165 bar
or 2,400 psi) and then preheated in the recuperator
prior to reintroduction to the burner.
A 2014 EPRI report (3002003734) summarizes
findings from an analysis of various
syngas-fed oxy-fired SCO2 Brayton plant
designs with 100% carbon capture. Various
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POWER February 2016

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