June 15, 2008

Options for reducing a coal-fired plant’s carbon footprint: Part I

Dr. Justin Zachary, Bechtel Power Corp.

Impacts of postcombustion capture on plant design

Incorporating CCS technologies into a coal-fired plant significantly affects the plant’s design, thermal efficiency, and turbomachinery. For example, the need to sequester the captured CO₂ dictates considering the plant’s location. If it is located far from an adequate geological storage site or a site suitable for enhanced oil recovery, the cost of constructing a pipeline and the additional loads for pumping CO₂ must be accounted for. Space requirements and plant layout should also be considered.

By itself, CO₂ capture hardware has a large footprint. For amine scrubbing, CCS plant components (the absorber, stripper, compression stations, and various cooling and storage tanks) occupy a large area. In addition, the plant layout must provide large ducts for flue gas, which needs to be routed from the outlet of the block of air quality control systems to the amine scrubber without interfering with roads and buildings. As discussed in detail below, large low-pressure (LP) pipes in racks with adequate support also are needed to bring steam from the plant’s steam turbine to the amine scrubber.

Balance-of-plant equipment also must be augmented to meet CCS requirements. For example, the electrical design of transformers, transmission cables, and motor control centers may need to be enhanced. Particularly when an existing plant is being retrofitted for CCS, the ripple effect of adding a CO₂ removal system requires detailed and careful review. A final consideration is the plant’s heat sink. It should be sized to allow the condenser and cooling tower to accommodate the extra, unneeded steam that’s routed through the plant when the postcombustion capture system is not in operation.

A significant amount of steam is required to regenerate the solvent of an amine-based postcombustion capture system. At 44 psi and 518F, it may take between 2.9 and 3.5 pounds of steam to recover the amine needed to remove 2.2 lb of CO₂ from the flue gas. For a 90% CO₂ capture level, that level would represent more than 50% of the LP steam turbine’s flow.

Accordingly, it is imperative to consider that the CO₂ capture system might not be operational and, if so, would be unable to process all or part of the extraction steam it receives. This is especially important for plants whose steam turbine is permanently configured to operate with a reduced LP steam flow. Venting such large quantities of steam is not an option, so any design must enable rapid configuration changes that allow the LP modules to operate under zero-extraction conditions. There are four available options for extracting steam from the system: throttle LP, floating-pressure LP, LP spool with clutched LP turbine, and backpressure turbine.

Throttle LP. This configuration keeps the crossover pressure constant despite the fact that a large amount of steam is extracted. As Figure 4 shows, the arrangement requires the presence of a throttling valve downstream of the solvent steam extraction point. Although it would incur significant throttling losses, this setup is flexible enough to allow for the extraction of any amount of steam needed (to achieve less than 90% CO₂ capture, for example), as well as for returning to full-power operation rapidly when the CO₂ capture system is not in service. The type of throttling valve needed is commercially available for current LP crossover pipe sizes.

4. A throttle low-pressure arrangement for extracting the steam needed by a CO₂ capture system. Source: Bechtel Power Corp.

Floating-pressure LP. In this arrangement (Figure 5), the turbine’s intermediate pressure (IP) module must be able to operate with a variable backpressure. When the CO₂ capture system is operating, the crossover pressure is lower because the LP module’s last-stage loading and exit losses are higher. For retrofits, the IP section’s last stages can be replaced to match the desired operating conditions at both high and low steam flows, depending on the CO₂ capture system’s steam demand. Obviously, additional valves in the extraction line and downstream of the extraction point in the crossover pipe facilitate operational control of the different steam flows required by variable CO₂ capture rates.

5. A floating-pressure LP arrangement for providing extraction steam. Source: Bechtel Power Corp.

LP spool with clutched LP turbine. In this scheme, depicted in Figure 6, one of the steam turbine’s LP modules is connected via a clutch to the generator in an arrangement similar to that used by single-shaft combined-cycle plants. In this case, when the CO₂ capture system is operating, only one LP module is in use and the other is disconnected. The inlet flow and pressure of the operating LP module must be designed to accommodate the steam conditions at the anticipated CO₂ capture level(s).

6. The LP spool with clutched LP turbine extraction-steam configuration. Source: Bechtel Power Corp.

This option is costly (it requires additional structural pedestals and a longer turbine hall) and offers little flexibility for achieving various CO₂ capture rates. However, restoring the full capacity of the module when the CO₂ plant is not functioning is easy. In addition, the LP module that remains in operation performs at the design conditions, giving this option a higher efficiency than the other three.

One variant of this arrangement could even operate without a clutch. In this setup, the second LP module must rotate, so a minimum amount of steam (between 5% and 10% of its flow) must pass through the second module to prevent overheating or mechanical vibrations. The additional steam that is extracted without producing real power is an added loss for the system. A more permanent solution for the second LP module is to replace the bladed rotor with a dummy. In this scenario, when the CO₂ capture system is not operational, the steam cannot be returned to the cycle to produce power; it must either be vented or condensed.

Backpressure turbine. If the extraction steam for the CO₂ capture system is taken from the turbine’s IP/LP crossover pipe, its pressure and the temperature are too high for direct use by the sorbent regeneration process. One solution that would exploit the available energy is to add a noncondensing turbine to produce power (Figure 7). The power generated by the noncondensing turbine could then be used to reduce the auxiliary load of the carbon capture process.

7. Using a noncondensing, backpressure turbine to lower the temperature and pressure of extraction steam (and generate power). Source: Bechtel Power Corp.

Performance comparisons

Table 4 provides an example of the negative impacts of a CO₂ capture system on the overall performance of an 800-MW (net) power plant. It should be emphasized that each project must conduct its own evaluation based on specific site conditions, the selected carbon capture technology, and the type of sorbent used. Because each steam turbine vendor uses a different cycle design with dissimilar IP module exhaust pressures, the output power of the noncondensing turbine varies accordingly.

Table 4. Comparing the performance of plants with and without carbon capture and sequestration (CCS) capabilities. Source: Bechtel Power Corp.

In the example given, it can be seen that the steam extracted for the CCS system reduces the steam turbine’s output by more than 23%. Because the system also contains a CO₂ compressor, auxiliary loads increase by 95 MW. In this particular case, the noncondensing turbine produces 40 MW of power. Without this benefit, the increase in auxiliary loads would be even higher.

Figure 8 shows the thermal performance penalties of postcombustion CO₂ capture by comparing the relative efficiency losses of the four extraction steam options. This comparison of plant output does not account for auxiliary power losses incurred by CO₂ compression. As expected, the setup involving the addition of a noncondensing turbine has the lowest power loss (7%). Next is the clutch arrangement, which features the least steam throttling and the lowest LP turbine losses. However, both options require adding or making significant modifications to plant hardware. For a retrofit, these alternatives require substantial pre-investment and site preparation.

8. Comparing the power losses of the four extraction steam arrangements. Source: Bechtel Power Corp.

Finally, it is important to realize that operating a CO₂ capture system at less than 95% capture rate could have a significant impact on the system’s parasitic loads, either in terms of steam flow or electric power. Figures 9 and 10 show how changing the target CO₂ removal percentage affects the levels of steam and electricity required for carbon capture and compression. For example, reducing the CO₂ capture level from 95% to 80% reduces steam consumption by 20% and electricity consumption by 5%.

9. Steam consumption vs. CO₂ capture level. Source: Bechtel Power Corp.

10. Electricity consumption vs. CO₂ capture level. Source: Bechtel Power Corp.

—Dr. Justin Zachary (jzachary@bechtel.com) is senior principal engineer for Bechtel Power Corp., an ASME fellow, and a contributing editor to POWER.