2 Post-combustion capture

Post-combustion capture is the technology normally applied for the CO2 capture in conventional PC (or CFB) boiler and natural gas combined cycle power plants. Published information on the flexibility of these plant types mainly focuses the attention on the flexible operation of coal-fired boiler power plants, but their contents are also valid for the natural gas combined cycles, though there are potentially important differences in the design of the two plants.

In general, in order to improve the flexibility of fossil-fired power plants with CO2 capture, changes in operating procedures shall be identified, in response to the daily electricity grid demand and prices variation. In most works, the following alternatives are considered and investigated for these plant types:

  • Varying the CO2 capture rate, depending on electricity prices and CO2 costs;
  • Turning on and off the CO2 capture plant;
  • Providing solvent storage to decouple plant operation (boiler or GT) from the CO2 capture, allowing the power plant to increase/decrease load, following its own ramp up/down rates.

All the possibilities require extra investment costs, related to the over-sized capacity of some units in the power plant or to the additional equipment necessary for a specific operating mode.

These solutions allow to generate extra power, when required, or to store solvent and decouple the plant operation from the CO2 capture unit, thus meeting the same objective.

To estimate if the increased plant revenues associated with the improved plant flexibility and the capability to offer ancillary services are economically convenient, i.e. if the benefits recompense the additional investment cost, is not an easy task, as the analysis is strongly dependent on the future market conditions, which are unpredictable.

2.1 Impact of post-combustion capture on power plant capabilities

2.1.1 Start-up time and cycling capability

In general, it is expected that post-combustion capture facilities do not limit start-up times of the power plant, since flue gas can be released to the atmosphere. However, in electricity markets where there is a cost related to the CO2 emissions, releasing carbon dioxide during start-up is an important additional cost that should be as much as possible reduced, compatibly with the plant start-up requirements. This issue could be avoided with moderate amounts of solvent storage, in order to allow the decoupling of the boiler (or GT) from the CO2 capture unit during start-up or when fast overall plant load changes are required.

With this configuration, the CO2 capture column can be put in “stand-by” operation, with full amine circulation, waiting feed gas from the boiler (or the GT). Therefore, the amine is initially circulated to the absorption column by-passing the regeneration section, without any flue gases entering the column. When boiler (or GT) is put in operation with its own ramp-up rate, the exhaust gases pass through the absorption column where the CO2 capture is made. During the first phases, when the ratio between gas and liquid is lower than the design conditions, the CO2 capture will likely be lower than the nominal. Until the power island is not able to provide a stable amount of steam for the regeneration, the rich amine can be stored in a dedicated storage tank while, simultaneously, the lean amine is taken from an equally-sized lean amine tank.

Once the steam cycle is started-up and LP steam is available, the regeneration section can be put in operation in accordance to its own ramp rate. The storage tanks shall be sized properly, taking into account the duration of these transients.

However, since the steam cycle and the CO2 capture plant are thermally integrated, the power plant output and the overall plant efficiency is influenced by the required steam extracted for solvent regeneration. Therefore, some constraints to the power plant start-up could occur, depending on the ability of the plant to handle variable steam flows in the Steam Turbine, mainly in relation to its minimum stable load.

The same configuration with rich and lean storage tank can promote the cycling operation of power plants, because the regeneration and compressions can be completely decoupled from the absorption column.

Regarding the CO2 compressors, it can be stated that these machines do not add specific constraints on plant capabilities to change loads, or, more in general, to the plant flexibility. Ramp up and down rates depend on compressor type, e.g. “in-line” or “integral-gear” centrifugal, but they are typically very short, in the order of a few seconds.

It seems that other aspects on flexibility have not been investigated yet. For example, a relatively narrow band of temperatures of the steam used for solvent regeneration is acceptable, without affecting the characteristics and properties of the solvent itself. However, it is expected that steam supply pressures and temperatures can be appropriately regulated, also when a boiler is operated under sliding pressure conditions.

2.1.2 Partial load operation

Power plant efficiency is reduced in power plants with CCS when operating at partial load, mainly due to the compressor power consumptions and the steam required for the solvent regeneration that is extracted from the steam turbine.

In fact, as explained in Section C, the minimum load for a stable and efficient operation of a compressor is around 70-75%. Below this value, a recirculation of compressed stream is necessary to keep the machine in operation, thus impacting its efficiency. Moreover, the lower the load of the Steam Turbine, the higher the penalty related to the LP steam extraction at constant pressure for solvent regeneration.

As a consequence, it is essential for the economics of the plant to identify those operating conditions, in terms of solvent circulation and lean/rich loading, that correspond to the lowest heat requirement at the regenerator reboiler.

When the boiler is required to operate at partial load, then flue gas mass flow and composition vary with respect to the base load operation. At lower load, the flue gas mass flow decreases and, as a consequence of the increased air ratio in the boiler, CO2 content decreases while oxygen content increases.

These changes in the flue gas conditions influence the liquid to gas (L/G) ratio in the absorber. In fact, the optimum L to G ratio, corresponding to the minimum heat demand of the reboiler for solvent regeneration, while maintaining a constant CO2 capture rate, tends to decrease when decreasing unit load. Therefore, when CO2 capture unit is operating at partial load, lower specific steam consumption is required in the reboiler.

Moreover, the higher oxygen content in the flue gases entering the CO2 capture section has a negative impact on the amine degradation rate and unit operation. In fact, one of the main concerns with the amine-based solvents is the high-level of corrosion and degradation in the presence of oxygen, as well as of other impurities (e.g. SOx, NO2, etc). This characteristic leads to the need of addition of inhibitors in the solvent, to counteract the oxygen activity. These inhibitors also protect the equipment against corrosion and allow for use of conventional materials of construction, mostly carbon steel. Therefore, the design of CO2 capture section and specification of such inhibitors shall be properly made, taking into account the operation at partial load where O2 content in flue gases increase.

With respect to the plant without CCS, the energy penalty associated with the steam extraction from the steam cycle increases at partial load, mainly due to the increasing of throttling losses in the steam turbine extraction. In fact, in order to have a constant extraction pressure for the LP steam used in the regeneration section, the steam extraction from the turbine shall be properly controlled. It can be done throttling the steam at LP section inlet, with the effect of decoupling LP steam inlet pressure from LP section bowl pressure. The lower the plant load, the heavier the steam throttling for having a constant pressure and the higher the efficiency penalty of the steam turbine. Therefore, the Steam Turbine and in particular the LP module shall be optimized, taking into account this operating mode.

This efficiency penalty is more evident for retrofitted power plants, because constraints on the steam pressure for solvent regeneration have not been considered in the steam turbine design, so heavy throttling is required when plant operates at partial load. However, retrofitting the power cycle of an existing plant with let-down back pressure turbines would lead the steam cycle to achieve performances close to the new-build power cycle with CCS. These subjects have been more deeply investigated in IEA GHG report 2011/02.

Another aspect that partially affects the overall plant efficiency at partial load is the compressor behaviour. Typical efficient turndown of CO2 compressor with electric drivers, operating at constant discharge pressure, is approximately 70-75% of full load. If throughput is reduced below this limiting load, the CO2 capture plant can continue operating, but it is associated to an extra power required per unit of CO2 captured, as the stable operation of the CO2 compression system requires flow recycle.

No significant issues, with the exception of efficiency penalties, are expected to maintain discharge pressure at part load, as long as recycling CO2 is used to ensure that compressor throughput remains in the manufacturer’s allowable operating range.

It is to be noted that, in most power plants applications, often multiple train of CO2 compressions are required. In this case, when the plant operates at partial load, it would be possible to turn off one or more compression trains, so that any remaining operating compressor has a throughput higher than 70-75% of full load.

Although for economical reasons the compressors suppliers are trying to increase as much as possible the maximum size of the single machine, the selection of a 2×50% compression trains configuration may be driven not only by turndown reasons, but also by the maximum size of the compressors available in the market.

Further investigations are required for other potential changes in plant efficiency at variable loads. For example, waste heat rejected in the CO2 compression process is supposed to be used to provide heat, where possible, within the power cycle. However, as long as CO2 capture plant load is varied, the potential for heat transfer between the compression section and the steam cycle could also vary, with associated impacts on power plant efficiency.

2.2 Tuning capture level

Flexible CO2 capture operation is particularly suited for post-combustion CO2 capture systems, which generally offers the ability for flexible or on/off operation.

The on/off operation is based on the possibility to totally by-pass the CO2 capture unit, when required or economically convenient. It allows the plant to have the possibility of saving the energy required for CO2 capture and compression when it is preferred to increase the plant output in response to the electric grid demand, though releasing CO2 to atmosphere.

Nevertheless, this operating option requires that the power plant is properly sized to handle the increased steam flow in the low pressure steam turbine module and condenser. Alternatively, if no margins on LP steam section are considered, the boiler operating load has to be reduced in line with the steam cycle capacity constraints, but in this configuration the plant is not exploited at its maximum capacity.

On the other hand, for retrofitted plants, sufficient capacity in critical items like the low pressure (LP) steam turbine and generator is available and increased net power can be produced, when the capture plant is bypassed.

For new plants, designed for CCS, some areas of plant including the low pressure turbine section, condenser and generator will require appropriate design to accommodate the large variation in flows associated with tuning the CO2 capture rate. Therefore, investors have to decide whether any expected increase in revenues associated with the additional power exported when the CO2 capture is bypassed is sufficient to justify the related extra capital cost.

As a matter of fact, the relevant profitability of these operating options depends on the selling price of both the electricity and the CO2. If electricity prices are high and/or CO2 prices are low, it might be economically attractive to bypass the post-combustion capture unit. When CO2 prices increase, the breakeven point in terms of electricity selling price required for the plant to switch from CO2 capture to no capture mode is also increased.

For low CO2 prices, the plant would not capture the CO2, regardless of the electricity selling price, unless other constraints (e.g. environmental law) require this operation. Alternatively, the CO2 capture unit can be kept in warm stand-by, with amine continuously circulating, without feeding steam to the reboiler. In this case, the stresses related to the on/off unit operation are avoided, but some of the O&M costs shall be taken into account. On the other hand, this operating option allows a quicker re-start up of the unit, when the CO2 capture is required, by feeding the steam to the reboiler.

In addition, it is noted that by turning down or off the post combustion capture unit it is possible to ramp-up the steam turbine more quickly than the ramp rate of a conventional coal fired boiler, which generally limits the capacity of the steam turbine for these plant types. From this point of view, the load-tuning of the capture unit could increase the rate at which the power output is ramped-up, resulting in an operating flexibility of the boiler plants with CCS higher than the plants without CCS.

2.3 Rich-solvent storage

Providing solvent storage tanks for rich solvent from the CO2 absorber allows continuously capturing the CO2 from the flue gas flow, delaying most of the energy penalty requirements associated with the CO2 capture and compression units.

During peak demand periods, when electricity selling prices are high, power plants could operate removing the CO2 from the flue gas in the absorber column as during base load operation, but with the solvent regeneration and CO2 compression processes halted.

In this way, the rich solvent containing CO2 leaves the absorber column and is temporarily stored in solvent storage tanks, avoiding the majority of the energy penalty for the amine capture process, which is related to the steam extracted from the steam cycle and to the CO2 compression. Typically, when lower electricity selling prices reduce the revenues of the plant output, the rich stored solvent can be regenerated.

To allow the delayed regeneration, while maintaining the power plant in operation at full load, over-sizing of the regenerator section of the CO2 capture plant, i.e. stripper and reboiler, and of the compression train is required, implying an additional investment cost.

If no over-sizing were provided, when the stored solvent has to be regenerated, the power plant should be in operation at partial load, while the compression train and the reboiler are in operation at base load. The selection of this solution shall be based on careful market evaluation, to assess if expected cycling operation of the plant is in line with such behaviour of the capture plant.

Accordingly to the information provided by the main technology Licensors, the dynamic modelling of the post combustion capture unit has been performed, even if not to deeply investigate the decoupled operation of the regenerator and absorber sections. However, no particular critical aspects are foreseen by main technology Licensor to operate independently both the absorber and the regenerator between their minimum and maximum load.

Available information on chemical stability of solvent for CO2 capture highlights that degradation of amine solution increases when increasing temperature and CO2 loading. As a consequence, rich solvent degradation is possible, when stored, so further investigation with referenced Licensors of this technology is recommended.