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WHR in the cement industry – Part 3: Kalina Cycle

World Cement,

The Kalina Cycle resembles the Rankine Cycle in that a fluid is used to transfer heat from the source and the hot fluid is then expanded through a turbine to drive the generator. The critical difference is that a mixture of ammonia water is used and the great advantage of a binary fluid of this type is that both evaporation and condensation occur over a range of temperatures and the fluid will continue to increase in temperature as evaporation occurs. As a result, there is a much better match with the heat source than can be achieved in a Rankine Cycle where the temperature remains constant during boiling until all the water has been converted to steam. Thus the Kalina Cycle will show an improvement in efficiency compared with a conventional steam cycle.

A WHR system based on the Kalina Cycle has only recently been applied to a cement plant and this initial unit is being commissioned (spring 2013) at the DG Khan cement company in Khaipur, Pakistan. The WHR unit is being supplied by FLSmidth, a Kalina licensee in partnership with Recurrent Engineering, and will generate 8.6 MW of power using waste heat from both the preheater and the clinker cooler with a Kalina Cycle especially developed for the cement industry. A second plant will be supplied to the Star Cement Company in the United Arab Emirates. In this case, waste heat will be recovered only from the clinker cooler and a power output of 4.75 MW is estimated.

While the application of the Kalina Cycle to the cement industry is a recent development, power plants using this technology have been in operation since 1992 when the Canoga Park demonstration unit was commissioned. Units have been built and are operating successfully using waste heat from steelmaking processes, in the petrochemical industry, and in geothermal and solar applications. A demonstration unit recovering waste heat from waste incineration operated for a two-year trial period and demonstrated a 20% greater efficiency compared to comparable units using other WHR technologies. The unit at the Kashima works of Sumitomo Metals provides 3.5 MW of electric power and has operated since 1999 with 97 – 99% availability. These and other Kalina installations provide a solid pedigree and strong technical basis for further exploration of this innovative technology.

The two component working fluid and the opportunity to vary ammonia concentration throughout the cycle, coupled with recuperative heat exchangers, provides the basis for a high efficiency system. If waste heat from both the preheater and the clinker cooler is used, two Heat Recovery Vapour Generators (HRVGs) are required for the preheater and the clinker cooler exhausts, respectively, and both HRVGs are used for evaporation, but only the higher temperature (preheater) source for superheating. The superheated vapour is then expanded through a back pressure turbine and the exhaust diluted with fluid having a low concentration of ammonia from the separator/demister to allow condensation to take place in the low pressure condenser. The pressure is then increased and most of the fluid is partially boiled and sent to the separator/demister through recuperative heat exchangers while the remainder is mixed with the high ammonia concentration stream from the separator/demister. This process optimises the ammonia-water concentration for the heat acquisition stage of the cycle and the working fluid is then condensed in the high pressure condenser and passed to the HRVGs.

Written by Thomas B. Gibbons. This is an abridged version of the full article, which appeared in the August 2013 issue of World Cement. Subscribers can view the full article by logging in.

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