September 1, 2009

Combined-Cycle Carbon Capture: Options and Costs, Part I

Justin Zachary, PhD, Bechtel Power Corp.

Upgrade the Steam Turbine

The main impact on the steam turbine for either amine- or ammonia-based CCS technology derives from the large steam extractions needed for solvent regeneration. A range of solutions, stand-alone or in combination, are available to cope with various amounts of extracted steam. The options presented here can be implemented without a significant reduction in steam turbine efficiencies.

A considerable amount of steam is required for solvent regeneration. The steam consumption for a representative amine-based postcombustion capture system is shown in Table 1. Typical supply steam conditions are 3 bar (45 psi) and 270C (518F). The amount of steam for 90% CO₂ recovery from the flue gas may be as high as 1.4 kg for 1 kg of CO₂ , equivalent to more than 40% of the LP steam turbine flow. Therefore, in all plant operational scenarios, it is imperative to consider the possibility that the CO₂ capture plant might not be able to receive part or all of the extraction steam from the LP section of the steam turbine.

It is also possible that the steam turbine intermediate pressure (IP) and LP turbines are located in a single casing, compounding the difficulty of extracting the necessary steam at the required pressures and temperatures. Also, venting such large quantities of steam is not an option in cases when the CO₂ plant is out of service. Any design must accommodate rapid configuration changes that allow the IP and LP modules to operate under zero-extraction conditions.

There are three practical options for extracting the necessary steam for the carbon capture process from a typical combined-cycle steam turbine system:

• Throttle cold reheat IP. This approach uses IP steam but with the addition of a throttle valve designed to keep the IP pressure constant even when large quantities of steam are extracted. This approach would be selected when the IP and LP turbines are located in the same casing and no IP-LP crossover or LP extraction is possible. Although significant throttling losses occur, this setup offers flexibility to extract any amount of steam needed (that is, for less than 90% CO₂ capture scenarios) and the capability to restore full power generation rapidly when the CO₂ capture system is not in operation (Figure 3).
  
  
  
  
  3. Steam extraction from IP/LP turbines. Source: Bechtel

• Backpressure turbine. If the steam extraction for the postcombustion capture plant is taken from an IP/LP crossover pipe, the pressure and temperature are too high for direct use in the sorbent regeneration process. The best solution is to recover much of the available energy to generate additional power through a noncondensing turbine to supply the required steam pressure and temperature to the carbon capture process while reducing the additional auxiliary load (Figure 4). The less-desirable alternative is to spray water in an oversize attemperator.
  
  
  
  4. Steam extraction from HP cold reheat steam line. Source: Bechtel

• Floating pressure LP. In the case of new plants, a better solution is to have the LP turbine operate at a pressure matching that of the reboiler. However, for retrofits, adding a noncondensing turbine may be the only way to recover some of the parasitic losses, despite the added complexity.

Table 1 also provides an example of the impact of a CO₂ capture plant, starting with the standard overall performance of a nominal 50-Hz, 450-MW net power plant and showing the CO₂ capture system performance and the incremental increase in auxiliary power expected with postcombustion capture. The amount of auxiliary electricity required is a function of how much CO₂ is removed. It should be emphasized that each project must conduct its own evaluation based on specific site conditions, selected capture technology, and type of sorbent used. Because each steam turbine vendor has a different cycle design with dissimilar IP module exhaust pressures, the output power of the noncondensing turbine varies accordingly.

In the given case, steam extraction for the postcombustion capture plant reduces steam turbine output by almost 23%. In addition to the reduction in plant gross output due to the steam extraction, net power output is significantly reduced by the increase in the auxiliary (parasitic) load.

When accounting for the power needed for CO₂ compression, the auxiliary loads for a plant with CCS are more than double than for the same plant without CCS. The use of the additional noncondensing turbine improves the plant heat rate. In this particular example the noncondensing turbine produces 25 MW. Without this turbine, auxiliary loads would be even higher.

Beyond the steam cycle losses, the net effect of CCS on the entire combined cycle includes poorer turbine section efficiency and capacity loss resulting from the gas turbine exhaust gas recirculation modifications.

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