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

	NGCC	USCPC
Increase in power output with no capture, %	15.9	27.4
Thermal efficiency, %
Reference plant with capture	50.6	34.8
Plant with capability to turn off capture	50.2	34.2
Plant with capture turned off	58.6	44.3
Capital cost
Change in cost per kW of normal output, %	+5.8	+3.9
Change in cost per kW of peak output, %	-8.7	-18.5
Cost of extra peak power capacity, €/kW	354	322
CO2 emissions
Tonnes CO2 per MWh of extra peak power	2636	2944

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

Power plant type	NGCC	USCPC	NGCC	USCPC
Storage scenario	Weekly	Weekly	Daily peak	Daily peak
Hours per week of peak output	80	80	10	10
Increase in power output at peak times, %	6.2	4.8	12.1	22.2
Thermal efficiency, %
Reference plant efficiency, 100% load	50.6	34.8	50.6		34.8
Reference plant time weighted average efficiency	50.6	33.6	50.6		33.6
Storage plant time weighted average efficiency	45.3	33.5	50.5		33.6
Capital cost
Change in cost per kW of normal output, %	+19.6	+6.1	+9.3		+5.8
Change in cost per kW of peak output, %	+12.6	+1.2	-2.6		-13.5
Cost of extra peak generation, €/kW	3116	2891	752		589
Solvent storage
Quantity of solvent storage, 103m3	286	199	30		46

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.