Publisher: The University of Edinburgh
Types: Doctoral thesis
Subjects: PCC, carbon capture, power plant, gas turbine, natural gas combined cycle, NGCC, Post-combustion CO2 capture, selective exhaust gas recirculation, S-EGR
Selective Exhaust Gas Recirculation (S-EGR) consists of selectively transferring CO2 from
the exhaust gas stream of a gas-fired power plant into the air stream entering the gas turbine
compressor. Unlike in “non-selective” Exhaust Gas Recirculation (EGR) technology,
recirculation of, principally, nitrogen does not occur, and the gas turbine still operates with a
large excess of air.
Two configurations are proposed: one with the CO2 transfer system operating in parallel to
the post-combustion carbon capture (PCC) unit; the other with the CO2 transfer system
operating downstream of, and in series to, the PCC unit. S-EGR allows for higher CO2
concentrations in the flue gas of approximately 13-14 vol%, compared to 6.6 vol% with
EGR at 35% recirculation ratio. The oxygen levels in the combustor are approximately 19
vol%, well above the minimum limit of 16 vol% with 35% EGR reported in literature.
At these operating conditions, process model simulations show that the current class of gas
turbine engines can operate without a significant deviation in the compressor and the turbine
performance from the design conditions. Compressor inlet temperature and CO2
concentration in the working fluid are critical parameters in the assessment of the effect on
the gas turbine net power output and efficiency. A higher turbine exhaust temperature allows
the generation of additional steam which results in a marginal increase in the combined cycle
net power output of 5% and 2% in the investigated configurations with S-EGR in parallel
and S-EGR in series, respectively. With aqueous monoethanolamine scrubbing technology,
S-EGR leads to operation and cost benefits. S-EGR in parallel operating at 70%
recirculation, 97% selective CO2 transfer efficiency and 96% PCC efficiency results in a
reduction of 46% in packing volume and 5% in specific reboiler duty, compared to air-based
combustion CCGT with PCC, and of 10% in packing volume and 2% in specific reboiler
duty, compared to 35% EGR. S-EGR in series operating at 95% selective CO2 transfer
efficiency and 32% PCC efficiency results in a reduction of 64% in packing volume and 7%
in specific reboiler duty, compared to air-based, and of 40% in packing volume and 4% in
specific reboiler duty, compared to 35% EGR.
An analysis of key performance indicators for selective CO2 transfer proposes physical
adsorption in rotary wheel systems as an alternative to selective CO2 membrane systems. A
conceptual design assessment with two commercially available adsorbent materials,
activated carbon and Zeolite X13, shows that it is possible to regenerate the adsorbent with
air at near ambient temperature and pressure. Yet, a significant step change in adsorbent
materials is necessary to design rotary adsorption systems with dimensions comparable to
the largest rotary gas/gas heat exchanger used in coal-fired power plants, i.e. approximately
24 m diameter and 2 m height. An optimisation study provides guidelines on the equilibrium
parameters for the development of materials.
Finally, a technical feasibility study of configuration options with rotary gas/gas heat
exchangers shows that cooling water demand around the post-combustion CO2 capture
system can be drastically reduced using dry cooling systems where gas/gas heat exchangers
use ambient air as the cooling fluid. Hybrid cooling configurations reduce cooling and
process water demand in the direct contact cooler of a wet cooling system by 67% and 35%
respectively, and dry cooling configurations eliminate the use of process and cooling water
and achieve adequate gas temperature entering the absorber.
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