PRINCIPLES OF FLUE GAS WATER RECOVERY SYSTEM (2005)

This paper discusses a commercially viable and novel technology that enables the recovery of the water vapor contained in power plant flue gas streams. Currently, fossil fuel-based power plants consume vast quantities of water for heat rejection, fuel preparation, power augmentation, emissions control, and cycle makeup purposes. At the same time, global water resources are becoming more difficult to procure as water consumption outstrips the pace of renewal. At present, the strategy used to reduce power plant water consumption in areas where water restrictions are stringent is to employ an air-cooled condenser as opposed to oncethrough cooling or a cooling tower. However, even plants with air-cooled condensers to minimize water consumption require a significant amount of water in several cases in order to allow for required steam drum blowdown, power augmentation systems, and gas turbine inlet evaporative cooling or fogging systems in addition to the aforementioned uses. To offset these observed escalating consumption rates, a new system was devised that can recover a substantial portion of the water vapor present in a flue gas stream. The water recovered by this system is of a quality similar to that of a reverse osmosis system outlet (RO Out). This water can be used for various plant needs as recovered or can receive minimal treatment so it can be used as direct cycle makeup. Siemens Power Generation, Inc. (SPG) and the Energy & Environmental Research Center (EERC) at the University of North Dakota (UND) have successfully completed the first phase of a U.S. Department of Energy (DOE) supported pilot-scale program that verified the technical viability of this system. SPG, with the cooperation of the EERC, is currently developing a commercial partnership with the goal of having a 1/10-scale slipstream commercial demonstration by the end of 2007. This paper provides a detailed description of the pilot-scale system and process conditions, presents the reportable results of the recent pilot-scale test program, and discusses the expected commercial characteristics. INTRODUCTION AND BACKGROUND The electricity industry is second only to agriculture as the largest domestic user of water, accounting for 39% of all freshwater withdrawals in the nation, of which 71% is used in fossil fuel-based electricity generation. Power plants have to compete with other industrial customers, agricultural interests, and households for this limited commodity. The availability of water for use in electric power generation is limited in many parts of the United States and is an important factor in obtaining site permits for new plant construction. Difficulty or the inability to obtain necessary water permits can lead to delayed or abandoned projects. Also, it is often the case that additional infrastructure is required to provide a suitable supply, adding cost and environmental impact. In areas that do not have an adequate water source, power plant construction is not even considered, even though these locations are ideal in other respects. The goals of this flue gas water recovery project are to develop technology for recovering water from combustion flue gases to reduce the net water requirements 1 © Siemens AG 2005. All rights reserved. of power plants burning fossil fuels and to perform an engineering evaluation to determine how such technology can be integrated into various power-generating systems, including steam turbine and combined-cycle plants. Power plants burning fossil fuels have in the past been designed to generate electricity at least cost under circumstances of abundant coal and natural gas resources and adequate supplies of water for plant cooling. Future plants will increasingly need to be designed and operated to conserve both fuel and water. Water is becoming scarce and expensive in many parts of the United States including California, where there is already a strong economic incentive to reduce the net cooling water requirements of power plant subsystems cooling steam turbine condensers and scrubbing stack gases. Currently, coal-fired power plants require access to water sources outside the power plant for several aspects of their operation in addition to steam cycle condensation and process cooling needs. In integrated gasification combined-cycle (IGCC) systems, significant water is used in the coal gasification process and for syngas saturation, which is lost through the power plant stack. In pulverized coal (PC) power plants, water inherent in the coal as well as water associated with flue gas scrubbing is lost through the stack. Currently, the strategy used to reduce water consumption in areas where water restrictions are stringent is to employ an air-cooled condenser as opposed to once-through cooling or a cooling tower. However, even plants with aircooled condensers to minimize water consumption require a significant amount of water in several cases in order to allow for required steam drum blowdown, power augmentation systems, and gas turbine inlet evaporative cooling or fogging systems. At the present time, there is no practical method of extracting the usually abundant 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. The process being investigated in this project 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. The advantage of using a liquid desiccant dehumidification system (LDDS) is to facilitate the recovery of useful amounts of water at flue gas temperatures that can be reasonably achieved during power plant operation. Direct contact cooling with a desiccant solution can be engineered to minimize pressure drop, and any water evaporating into the flue gas from an upstream scrubber would be recovered for reuse. The alternative of indirect cooling in an air/flue gas-condensing heat exchanger without a desiccant, which would be limited to applications involving low ambient temperatures, raises significant engineering and economic problems involved with the size and cost of the heat exchange equipment, exhaust pressure drop, corrosion, fouling, and discharge of non-buoyant stack gas. This project is a two year program to demonstrate the feasibility and merits of a liquid desiccant-based process that will efficiently and economically remove water vapor from the flue gas of coal-fired power plants (IGCC and PC steam plants) to be recycled for in-plant use or exported for clean water conservation. Reduction of water consumption by power plants is quickly becoming a significant issue when 2 © Siemens AG 2005. All rights reserved. 3 © Siemens AG 2005. All rights reserved. attempting to obtain permits for power plants and when required to meet new, more restrictive water consumption allowances currently being considered by the U.S. Environmental Protection Agency (EPA) under proposed Rule 316b.