Coal occurs naturally with water present (3-60 weight %), and the combustion process releases additional water as hydrogen in the coal is reacted with oxygen. The amounts of water that can be recovered from flue gas are sufficient to substantially reduce and in some cases eliminate the need for off-plant sources of water. At the present time, there is no practiced method of extracting the water found in the power plant stack gas. Some work has been done on using mechanical heat rejection to condense water vapor. Such systems would require massive and expensive heat rejection equipment, would be severely limited by high ambient temperatures, and would result in decreased gas turbine performance as a result of higher back pressure due to closed heat exchangers in the flow path. This research uses liquid desiccant-based dehumidification technology to efficiently extract water from the power plant flue gas, requires minimal heat rejection equipment, can function across the entire ambient range, and results in only a small increase in exhaust pressure.

This project employs a liquid desiccant-based dehumidification system placed at the exhaust end of coal-fired power plants and utilizes low-grade heating and cooling sources available in the power plant. A conceptual schematic of the proposed prototype system is shown in Figure 1. The flue gas is cooled and then subjected to a liquid desiccant absorption process, which removes water from the flue gas. By stripping off the absorbed water, the weak desiccant solution is regenerated back to the strong desiccant solution. The water vapor that is produced during the regeneration process is condensed and subsequently made available for plant reuse or for clean water export. The water recovered from the exhaust is expected to be of demineralized quality and would be used in water-consuming plant processes, such as steam drum blowdown, IGCC inlet evaporative cooling, and fuel processing (syngas production).

Figure 1.  Conceptual Design of Liquid Desiccant Process

The development of a liquid desiccant-based absorption system to reduce power plant water consumption through flue gas recovery is founded on commercial process technology currently used for industrial dehumidification applications, such as natural gas dehumidification. This new application for this technology represents significant scientific/technological advancements in several areas:

• The desiccant absorber-contacting process must be scaled up from its relatively small commercial basis to the very large volumetric flow rates of power plant flue gases to result in economically acceptable equipment.
• The dehumidification process must be properly integrated with available low-grade heat sources and cooling sources in the power plant to maintain acceptable power plant thermal efficiency and to result in economic feasibility.
• The development of the dehumidification desiccant and its process steps must consider the exposure to power plant flue gas contaminants that might interact with and influence the functions of the process.
• The dehumidification process might be utilized to capitalize on interactions with power plant emission species (e.g., SOx, NOx, halides, volatile organics, and trace metals such as mercury) to provide an additional stage of flue gas cleaning that acts as a polishing process.

The scientific and technical basis for the proposed liquid desiccant-based dehumidification process is founded on three areas of commercial technology experience: 1) the smaller-scale commercial design and operating experience with industrial gas dehumidification systems, 2) the electric utility-scale commercial design and operating experience with large flue gas–slurry absorption contacting in flue gas desulfurization (FGD) processes, and 3) power plant thermal/mechanical energy management optimization.

This project seeks to develop economical and environmentally effective technology with the ability to substantially reduce the water consumption of fossil fuel-fired power plants by recovering a large fraction of the water present in the plant flue gas and to perform an engineering evaluation to determine how such technology can be integrated into various power-generating systems (e.g., steam turbine or combined cycle systems), both to recover water and to improve efficiency and reduce emissions of acid gases and carbon dioxide. Depending on plant configuration and location, this recovered water could be used within the power plant—for example, for flue gas desulfurization makeup, for integrated gasification combined cycle (IGCC) fuel gas humidification, or in an evaporative cooler or fogger system to substantially augment plant output—or it could be exported to external customers.

The Energy & Environmental Research Center (EERC) has teamed with Siemens Westinghouse Power Corporation (SWPC) to conduct small-scale, coal-fired, dehumidification process testing and power plant evaluations as the initial stage of the commercialization of this process.

The overall objective of this research is to develop a liquid, desiccant-based flue gas dehydration process technology to reduce water consumption in coal-fired power plants. The specific objective is to generate sufficient subscale test data and conceptual commercial power plant evaluations to assess the process feasibility and merits for commercialization.

The scope of this program consists of bench and pilot-scale testing activities and conceptual engineering activities:

• A desiccant selection evaluation and characterization is conducted by ranking the merits of potential desiccants based on available physical–chemical property data and desiccant performance data and by subjecting candidate desiccants to laboratory screening tests.
• A comprehensive, subscale test program is performed at an EERC coal-fired test facility to produce key performance and operating data for the flue gas dehydration process.
• Sufficient test data is collected to develop an understanding of the potential emissions control benefits and issues of the desiccant process.
• Conceptual commercial evaluations and a market study of the flue gas dehydration process for utility coal-fired power plants are generated.
• An engineering evaluation is performed to determine how the desiccant technology can be integrated into various power-generating systems (e.g., steam turbine or combined cycle systems), both to recover water and to improve efficiency.