New Technology Will Recover Heat &Water from Flue Gas

By Barbara Carney

Recovering heat and water from flue gas can reduce water usage and boost power plant performance. The National Energy Technology Laboratory has tested a promising technology that condenses water and recovers heat from flue gas.

The image of billowing clouds of condensing water flowing from the stack and cooling towers is how many Americans picture thermoelectric power plants, but they may not know that plume abatement towers can be used to capture the water from the cooling towers. Escaping flue gas contains heat and water that is generally considered a waste stream that is lost to the atmosphere, but if it could be recovered and reused, it could hold the potential to lower plant water usage and increase plant efficiency.

The U.S. Department of Energy’s (DOE) National Energy Technology Laboratory (NETL) supports several projects that are developing ways to lessen the amount and impact of water usage at thermoelectric power plants, including condensing water from cooling towers and flue gas. Three basic methods can be used to recover water: condensing heat exchangers, membranes, and desiccants. NETL helped commercialize a condensing heat exchanger for cooling towers, the SPX plume abatement cooling tower ClearSky, which captures an average of 18 percent of the water evaporated from a cooling tower. Flue gas loses less water overall to the atmosphere, but does have a high heat content that may be recovered.

Until now, there has been no practical commercial technology available for recovering both waste heat and water from the power plant flue gas. Heat is removed from the flue gas through the use of economizers and air preheaters, but condensing water (and acid gas) is a limitation in how low the temperature can be taken. Condensing flue gas moisture by simply removing heat in a heat exchanger is a problem because a large surface area is required and equipment corrosion often occurs because of the acidic condensate. The recovered water needs further treatment before it can be used for any other processes due to the high acidity and other contaminates that may present in the water.

TMC Design

NETL has partnered with Gas Technology Institute (GTI) and Media & Process Technology Inc. (MPT) who have developed a nanoporous ceramic membrane device that condenses water and recovers heat from flue gas. The innovation is called the Transport Membrane Condenser (TMC). The TMC has been tested in various boilers and configurations and has been shown to be effective at heat and water recovery. The technology can be particularly beneficial for coal‐fired power plants that use high‐moisture coals and/or a wet flue gas desulfurization (FGD) for flue gas cleanup.

The TMC is constructed from a ceramic membrane that is heat and chemical resistant. The unit consists of tubes of alumina that are partially fired to give high porosity. These tubes are coated with another layer of alumina to form an intermediate layer with smaller pore sizes, then a final very thin coating of Zirconium dioxide (ZrO2).

The intermediate layer is made of distinct particle sizes that can be controlled to give well-defined nano-size membrane pores, and the final coating makes the outside tube surface smooth and slippery to prevent particulate fouling. The tubes are bundled into a shell to form essentially a nanoporous heat exchanger. In the case of flue gas water recovery, the membrane tube spacing is optimized to fit into the flue gas duct with minimal flue gas side pressure drop and maximum membrane tube surface area for heat and mass transfer.

Flue gas flows through the duct and comes into contact with TMC tubes. Water vapor condenses in the pores of the membrane tubes and passes into the inside of the tubes, which have water flowing through them. Non-condensable gases such as CO₂, O2, NOX, and SO₂ are inhibited from passing through the membrane by the condensed water clogging the tube membrane pores. The low pressure difference across the membrane tube wall and its nanoporous membrane coating on the tubes inhibits particulates from clogging the pores. The recovered water is of high quality and mineral free, and therefore can be used directly as boiler makeup water, as well as for other processes.

TMC Function

For a coal‐fired power plant boiler equipped with a wet FGD unit, flue gas exits at up to about 180 °F, with nearly 100 percent relative humidity. It contains up to about 40 percent, in volume, of water vapor. For coal‐fired power plant boilers with a dry FGD, the flue gas moisture content is still comparable with the industrial gas‐fired boiler flue gases, with a dew point at 130 to 140 °F, or about 20 percent in volume of water vapor in the flue gas stream. If 40 to 60 percent of this water vapor and its latent heat could be recovered and reused, the plant thermal efficiency could be significantly improved while providing a water recovery benefit.

The current project for power plant application is aimed at optimizing heat and water recovery in a two‐stage TMC design. For the first stage, the TMC will use condensed steam from the steam turbine in the tubes. The condensed steam is typically at 90°F to 110°F, which is sufficiently cool to provide a driving force for water transport across the membrane. The recovered water and heat are entered back into the steam cycle downstream of the first low-pressure feedwater heater, thus requiring less steam to heat. Calculated efficiency improvement is about 0.7-0.8 percent.

The amount of water and heat collected this way is limited by the amount of make-up water required for power plant boilers, which is typically about 2 percent. Research is underway to increase the amount of heat recovered from the flue gas. For the second stage, the TMC will recover a larger part of water from the flue gas and add it into the plant cooling water stream. This portion of recovered water can replace part of the cooling tower water makeup, or it can be used in other processes of the power plant.

Past Project Successes

• Industrial Steam Boilers were outfitted with TMC modules to form a TMC unit with controls. Boiler fresh makeup water (typically 10 percent to 50 percent of the boiler feedwater flow rate) was used in the TMC unit to recover flue gas water vapor and heat. The preheated makeup water coming out of the TMC requires less fuel from the boiler, therefore, the boiler efficiency can be improved by 5-10 percent depending on the boiler makeup water requirement. At the same time, the boiler makeup water amount was also reduced due to water vapor recovery from the flue gas. Advanced TMC-based heat recovery systems for industrial, large commercial, and institutional boilers have been made commercially available by Cannon Boiler Works as the Ultramizer® product. Current sizes include 10-20 MMBtu/hr units operating at 92-95 percent efficiency with ongoing developments to scale up to larger sizes of over 20 MMBtu/hr.
• Laundry Steam Tunnels were equipped with TMC units on top of their stacks to recover the unused steam, which were used for preheating the hot water to be used in the washing machine. This saved water and steam requirement from a steam boiler leading to cost reductions for plant operations.
• A Coal Power Plant was equipped with a pilot TMC unit for slip stream testing with five-week continuous operation and has produced good heat and water recovery results.
• Home Heating Systems were outfitted with a similar waste heat and water recovery system, however, the water vapor condensed in the tube membrane pores is re-evaporated into the furnace circulating air stream. The preheated circulating air with added water vapor can not only improve the furnace efficiency but also raise the air humidity level, thus providing home occupants a comfortable home environment. In this configuration it is called the Transport Membrane Humidifier (TMH) and can raise the efficiency of a standard mid-efficiency furnace by about 15 percent. This version of TMH can be used for existing low to mid-efficiency furnace retrofit applications, which account for more than 50 percent of current installed furnace population. A TMH module can be also integrated into a high-efficiency furnace to provide air humidification with some efficiency improvement. Besides maintaining a lower pressure at the flue gas side inside the TMH, a pressure switch is also used to ensure that no combustion gases can enter into the circulating air stream through the TMH.

Building on these successes, another NETL-managed project with GTI and MPT will develop and test a high-pressure modular version of the TMC in GTI’s pilot-scale pressurized coal combustor. The project will evaluate its performance and analyze the results for future commercial-scale pressurized oxy-combustion power plants. Testing over the next year will culminate in a scale-up and integration evaluation for a commercial-scale power plant at the end of next summer.

Input from utility equipment suppliers and industry groups will be solicited to guide the commercialization efforts to meet the needs of utility customers.

Because cost is a significant challenge associated with the technology, ongoing work is focused on improving the performance of the TMC by increasing the water flux and lowering the manufacturing cost of the membrane and integrating it in the boiler at various stages of the thermoelectric power plant.

The most promising applications are downstream of a FGD where flue gas water content is high and inlet temperature relatively low. Another possible application is for a Natural Gas Combined Cycle (NGCC) plant with a fired heat recovery steam generator (HRSG).

A Different Approach to Water Usage and Treatment Challenges

Energy and water are both becoming more constrained and it is a challenge to use both more wisely as population and demand increases.

The interconnectedness of energy and water has been realized. Tremendous amounts of cooling water are needed for condensing steam in the thermoelectric Rankine cycle to produce electricity, and large amounts of energy are typically required to transport and treat water. The DOE initiative to meet this challenge is called the Water-Energy Nexus, and power plants represent a promising opportunity to exploit synergies among water and energy systems.

Challenges to more widespread use of this technology are the current low cost of energy and water. Even with these low prices, several applications of the TMC, such as industrial boilers and laundry operations, are already cost competitive with a relatively quick return on investment of less than two years.

However, it is difficult to achieve widespread adoption of this energy and water saving technology.

Power plants will have similar challenges, but if the TMC is commercialized, thick clouds of condensing water exiting a power plant stack will become a less common site at thermoelectric power plants because the majority of the water and heat that create them will no longer escape into the atmosphere. Instead, they will be used to reduce water consumption, increase plant efficiency, and potentially lower costs.

Author

Barbara Carney is project manager with the U.S. Department of Energy’s National Energy Technology Laboratory

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