(524f) CO2 Post Combustion Capture Operational Flexibility and Scale-Up For Power Plants

CO₂ Post Combustion
Capture Operational Flexibility and Scale-up for Power Plants

Prachi Singh, John Davison

International Energy Agency Green House Gas R&D
Programme (IEAGHG), The Orchard Business Centre, Stoke
Orchard, Cheltenham, GLOS UK, GL52 7RZ, UK

1.Introduction

Several governments
and international organizations have set 2020 as the target for broad
commercial deployment of Carbon Capture and Storage (CCS). With this target in
mind, it is clear that the initial commercial and full scale capture plants
will have to be based on currently available technologies. These commitments
and agreed targets create an important role for the solvent‐based
post‐combustion capture technology, which is
considered to be the most mature of the capture technologies available today. CO₂
Post combustion capture technology provides a retrofit possibility and is
already available on relatively small industrial scale. Moreover now a days
existing and new power plants must face the challenges of the liberalized
electricity market and the requirement to cover intermediate and peak load
constraints, in order to respond to the daily and seasonal variation of the
electricity demand. Most studies undertook CO₂ post combustion
capture at power plant base load operation, but it is now clear that CO₂
post combustion capture technology will also need to be able to respond to the
requirements of the new liberalized electricity market. Therefore, in this
work, current capabilities of conventional Ultra Supercritical Pulverised Coal
(USCPC) and Natural Gas Combined Cycle (NGCC) fired power plants, without and
with conventional solvent‐based CO₂ post combustion capture
technology to operate flexibly in response to the demand of the electricity
market and factors that may constrain the operation flexibility and cost
implications were evaluated by process modelling (IEAGHG 2012).

Scale-up issues
related to conventional solvent‐based CO₂
post combustion capture technology for Supercritical Pulverized Coal (SCPC)
900MWe and NGCC 809MWe power plant are also evaluated in this work (IEAGHG
2013). However due to the different scope requirement for operational
flexibility and scale-up study, the design of solvent based CO₂ post
combustion capture process may not be identical for both studies. The
difference in the CO₂ capture process may differ mainly in the heat
integration, absorber design and solvent characteristics. Although this
difference will not change the main outcome on operation flexibility and scale
up issues for post combustion capture technology.

  2.CO₂
Post Combustion Capture Technology Operational Flexibility

The introduction of
post combustion CO₂ capture may impose additional constraints on the
start-up and fast load changing of a power plant but techniques are available
to overcome these constraints. In an NGCC plant the gas turbine starts up more
rapidly than the heat recovery steam generator (HRSG) and the steam turbine.
The regenerator in the CO₂ capture plant requires steam from the
HRSG or steam turbine and the regenerator needs to be heated to its operating
temperature. To avoid constraints on start-up time and to avoid CO₂
emissions during start up, the CO₂ absorber could be operated using
lean solvent from a storage tank and the CO₂ rich solvent from the
absorber would be stored and fed to the regenerator later. This would enable an
NGCC or USCPC plant with CO₂ capture to start up and change load as
quickly as a plant without capture. This technique is evaluated in this work is
discussed later in this paper.

2.1.Turn off or turn down of CO₂ capture

The net power output
of a plant could be increased by turning down or turning off the CO₂
capture and compression units and emitting more CO₂ to the
atmosphere. The ability of a plant with CO₂ capture to ramp up power
output could in principle be better than that of a plant without capture if the
load of the CO₂ capture unit was reduced at the same time as the
load of the power generation unit was increased. This work assessed the option
of turning off CO₂ capture but various intermediate options
involving turning off or turning down parts of the CO₂ capture plant
may also be attractive. Turning down or turning off capture would increase
emissions of CO₂ to the atmosphere so regulations would have to
permit CCS plants to emit more CO₂ during times of peak power demand. Turning down or turning off CO₂ post
combustion capture would reduce the plant's internal consumption of electricity
and the low pressure steam that would otherwise be consumed by the CO₂
capture unit could be used to further increase the net power output, provided
the plant was built with the necessary extra low pressure turbine capacity.

Table 1, Turning off
CO₂ capture plant

It can be noticed
from Table 1 that having the capability to turn off capture increases the
capital cost of the plant (per kW of normal power output), mainly because of
the need for greater steam turbine capacity, but the cost per kW of peak power
output is lower. The net capital cost per kW of extra peak power generation
capacity is relatively low, probably less than the cost of other types of peak
generation capacity.

2.2.Solvent storage

Solvent from post
combustion capture can be stored during times of peak power demand for
regeneration during times of lower power demand. This reduces the requirement
for other peak generation capacity. The extra generation during peak times
would have low CO₂ emissions, unlike the alternatives of by-passing
CO₂ capture or using peaking plants such as simple cycle gas
turbines without CCS.

Table 2, Storage of
post combustion CO₂ capture solvent

Two operating
scenarios described below were assessed in this study as an illustration but it
is recognised that in reality power plant operations will depend on many
external factors which may change during the operating life of a plant. The
?weekly' and ?daily' scenarios involve different amounts of solvent storage and
peak load operation. It can be noticed from Table 2, that the solvent storage
has very little effect of the thermal efficiency except for the NGCC weekly
scenario, in which one of the gas turbines has to operate at minimum
environmental load at off-peak times to regenerate solvent. The solvent storage
tanks are conventional sized tanks as used at oil refineries but they are
nevertheless large, particularly in the weekly scenario. In the weekly scenario
the capital cost per kW of additional peak generation capacity is greater than
the cost of the reference power plant, which indicates that this scenario is
unlikely to be attractive. In the daily scenario the capital cost per kW of
additional peak generation capacity is less than the cost of the reference
plant but it is probably higher than the cost of the leading alternative
technology for peak load generation, namely simple cycle gas turbines. Solvent
storage may be attractive in this scenario, depending on fuel prices, carbon
emission costs and the electricity price profile.

  3.CO₂
Post Combustion Capture Technology Scale-up Issues

CO₂ post
combustion capture design cases for SCPC and NGCC were reviewed to identify the
major issues that will likely be faced when moving from the current
demonstration scale to constructing and operating large commercial scale units.
In general, there appear not to be any major risks which have not been
addressed either in power generation or elsewhere in the heavy industrial
sector. Integration of the each component of the CO₂ capture
facility at large scale and incorporation into existing power plant designs may
represent the largest challenge. The main issues related to scale up for post
combustion capture process which emerged in the study are construction of
absorber and regenerator, stiffing of the boiler, ductwork and flue gas equipment,
bypass of flue gas, increase in gas turbine back pressure, amine emission
issues etc.

  4.References

IEAGHG, Operating Flexibility of Power Plants
with CCS, 2012/6, June, 2012.

IEAGHG, Post Combustion CO₂
Capture Scale-up Study, 2013/05, February 2013.