August 15, 2008

Condensate polishers add operating reliability and flexibility

Colleen M. Layman and Lisa L. Bennett, Bechtel Power Corp.

Making the best choice

So far we have surveyed the technology and several technical factors that affect the decision to invest in a condensate polishing system. This discussion raises an interesting question: Is it possible to assess the need for a condensate polisher based simply on high-level plant design data?

We believe the answer is ”yes” and present a decision tree for selecting condensate polishing equipment that we find useful in our work. The decision tree uses a series of primary and secondary technical factors to arrive at a polisher solution (see sidebar). The decision tree is primarily a screening tool; final purchase decisions must be weighed against the specifics of a plant site, makeup water, and first-cost and life-cycle cost considerations.

A selection guide for condensate polishing equipment

Some plant designs or configurations, by their nature, require a condensate polisher; with others, condensate polisher may be optional. The decision tree in Figure 4 provides a simple approach to quickly determine when a polisher is needed and what type of polisher will work best on a power project. It is based on specific plant configurations and the risks related to the cycle configuration and steam turbine chemistry limits recommended by EPRI, regardless of the chemistry control program.

4. To polish or not to polish? These are the questions that will help you determine whether or not a condensate polisher is necessary. Source: Bechtel Power Corp.

In our opinion, there are a series of five key factors (presented as questions in Figure 4) that determine if a condensate polisher is required. An answer of “yes” to any of these questions suggests that a condensate polisher is absolutely required. In addition, a series of seven secondary factors influence the decision to include a condensate polisher.

Making the final decision to include a condensate polisher may require a complicated series of economic analyses and design trade-offs that include plant location, wastewater disposal issues, acceptability of risk, owner preferences, and start-up considerations. In any case, this selection guide will get you very close to the right answer, regardless of plant design.

Bechtel Power has been involved in many condensate polisher evaluations during the past few years. Below, we share five project case histories that illustrate how our condensate polisher selection guide (Figure 4, in sidebar) was used on particular projects. These case histories also illustrate how additional plant design criteria and owner preference will enter into the final selection decision.

Case history #1. Two new 855-MW base-loaded pulverized coal-fired units on a greenfield site in the midwestern U.S. use a once-through supercritical boilers with a 3,700 psia throttle pressure. The units were designed to be commissioned on AVT chemistry and transition to OT chemistry once the cation conductivity of the feedwater was consistently below 0.15 µS/cm and the units had reached a stable operating load. Cold-lime-softened river water was the makeup water source for the plant’s cooling tower. The plant was designed to internally recycle as much of its own wastewater as possible, but it has a permit that allows discharge of some wastewater from the plant to a nearby river.

The selection guide determined that a deep-bed condensate polisher was required. The plant water balance included significant internal wastewater recycling in the wet flue gas desulfurization (FGD) system and as ash-conditioning water. The remainder went to an outfall. External regeneration was selected due to the high-quality condensate required for OT operation. The final design used 3 x 50% service vessels for each of the two units with a common, centrally located regeneration station to minimize capital costs.

Case history #2. A new 450-MW base-loaded pulverized coal-fired unit on a site with two existing units in the southwestern U.S. uses a drum boiler design with a 2,400 psia throttle pressure. The unit was designed to operate on an AVT chemistry program with phosphate injection available for emergency use only. Cold-lime-softened well water is the makeup water source for the plant’s cooling tower. All nonrecyclable wastewater is treated in a zero-liquid-discharge (ZLD) system and disposed of in on-site evaporation ponds.

None of the five primary factors in the selection guide applied to this unit, but secondary factors did: the throttle pressure, a steam cation conductivity limit of 0.2 µS/cm, and a condensate system designed to operate on AVT chemistry. Based on the selection guide, a precoat condensate polisher was selected for this project. The final design provided for 2 x 100% precoat polisher service vessels.

Case history #3. A 2 x 1 combined-cycle power plant with a 1,880 psia throttle pressure located in the UK was designed for a phosphate chemical treatment program. The plant was expected to be routinely cycled and was designed for rapid start-up. The turbine exhaust steam was cooled with an ACC.

None of the selection guide’s primary selection factors came into play, and only two of the secondary factors were present, so no condensate polisher was required. However, based on the owner’s preference, a 1 x 100% precoat condenser polisher was included as part of the plant design. The precoat polisher was selected because of its ability to handle the high levels of iron transport associated with an ACC operating in cycling service. The potential for carbon dioxide in the condensate, due to air in-leakage in the ACC, was another key factor in its selection.

Case history #4. Two new 750-MW base-loaded pulverized coal-fired units on a site with one existing unit in the midwestern U.S. use a once-through 3,700 psia supercritical design. The units were designed to be commissioned on AVT and to transition to OT chemistry once cation conductivity of the feedwater was consistently below 0.15 µS/cm and the units had achieved a stable operating load. Cold-lime-softened well water is the cooling tower makeup water source. The plant uses a ZLD system with all nonrecyclable wastewater disposed of in on-site evaporation ponds. Minimal opportunities for wastewater recycling exist within the facility because a dry FGD system and production of saleable ash were part of the overall plant design.

The selection guide determined that a deep-bed condensate polisher was best for this plant. However, because the plant has a ZLD system and minimizing wastewater was a key project design goal, off-site regeneration of the polisher resin was selected. The final design used 3 x 50% vessels for each of the two units with a common, centrally located resin transfer and storage station to minimize capital costs. The resin storage facilities were designed to store three charges of fresh resin—one spare charge for each new unit plus one extra charge.

Case history #5. Two new 750-MW base-loaded pulverized coal-fired units on a greenfield site in the southwestern U.S. use once-through supercritical boilers operating at 3,690 psia. The units were designed to be commissioned on AVT, with the transition to OT chemistry occurring once cation conductivity of the feedwater was consistently below 0.15 µS/cm and the units had achieved a stable operating load. The plant recycles large amounts of internal wastewater to the wet FGD system and as ash-conditioning water. High-quality well water is available for plant cooling water on a limited basis. Therefore, a hybrid wet-dry cooling system was planned for this facility. The hybrid design includes an ACC as well as a wet mechanical draft cooling tower to minimize water usage while maximizing cooling capability and unit efficiency.

The selection guide pointed to the choice of a deep-bed condensate polisher. Internal water recycling capability indicated that on-site regeneration of the polisher resin would make economic sense. The final design used 3 x 50% service vessels for each of the two units with a common, centrally located regeneration station to minimize capital costs. Because the design included an ACC, pleated septa condensate filters were included upstream of the deep-bed polisher vessels to protect the deep-bed polishers from high levels of particulates during start-up. These filters could be bypassed during normal operation once condensate cleanliness is established.

—Colleen M. Layman (cmlayman@bechtel.com) is manager of water treatment engineering, and Lisa L. Bennett (llbennet@bechtel.com) is a water treatment engineer for Bechtel Power Corp.