Utilization of waste heat from a flue gases up-stream gas scrubbing system

A thermal seawater desalination plant which utilizes waste heat from flue gases upstream, flue gas desulphurization (FGD), is discussed. This type of “low-grade” heat is suitable for low-temperature desalination technologies such as multi-effect distillation (MED) technology. Two different alternatives for using the heat is analyzed and compared. In these schemes, about 8,500-10,000 m3/d of high-quality water (~10 mg/l TDS) may be produced from one 575 MW generating unit. Moreover, a great amount of water is saved due to reduction in flue gas temperature and therefore reduction in water evaporation in the flue gas scrubber. An economics evaluation of the alternatives is presented. The water cost obtained is in the range of 53–62 US cent/m3 and depends on the MED and gas cooler capital costs, unit capacity factor and required rate of return. The obtained costs are comparable to water cost obtained from state-of-theart, large-scale sea-water reverse osmosis (SWRO) plants even without taking into account the higher water quality.

1. Two alternatives for utilizing the waste heat of the flue gases in a typical coal-fired power plant as a heat source in a MED plant were assessed. 2. The suggested schemes utilize only wellproven technology components; therefore, the technological risk is very low. 3. The MED plant may adopt to operate in thermal vapor compression mode. Small quantities of steam (30–40 t/h for an 8,500– 10,000 m3/d plant) from the power plant cycle are the motive steam. In this mode the availability of the desalination plant is kept while the flue gases are by-passing the FGD system for maintenance reasons. 4. The cost of water for the base case is 53–62 cent/m3. A major cost reduction may be achieved in the future due to reduction in MED plant cost. 5. The energy utilization in Alternative B is more efficient. As a result, the water cost in Alternative B is lower than the expected cost of water in Alternative A. However, the complexness involved in Alternative B may overrule the difference in production costs. 6. The lower production rate in Alternative B dictates a better rate of return. Nevertheless, there is no big difference between the two alternatives in this factor. 7. The expected cost of water is very attractive even as compared to a state-of-the-art large-scale desalination plant where the acceptable water price in these projects is 60– 80 cent/m3. 8. The suggested schemes present a very promising medium-size desalination option where it possible to integrate it with a power station or any other source of boiler that generates waste gases. 9. Another promising option is to integrate this plant with an existing/new seawater reverse osmosis plant, which is not designed to produce high-quality water. Mixing the two sources of water can produce potable water at a very attractive price.