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WHR in the cement industry – Part 2: Organic Rankine Cycle

World Cement,


ORC is a variant of the steam cycle, but in this case an organic compound (e.g. pentane or butane) serves as the working fluid. An important feature of this type of system is the incorporation of an intermediate loop so that heat is transferred initially from the energy source to a heat transfer fluid – usually a thermal oil or water – that then transfers heat to the organic compound, which vaporises. The intermediate loop provides greater operational flexibility and improved process control. Subsequently, the vapour is expanded through a turbine and the organic fluid is condensed in a cooling circuit. Consequently, an ORC unit is conceptually very similar to a Rankine Cycle, if somewhat more complex. It is claimed that, for a typical cement plant, 0.5 – 1 MW of power can be generated per kilotonne of clinker produced per day.

This type of system has several advantages relative to the steam cycle. Since the temperature of evaporation is lower, heat to boil the working fluid can be extracted from gases at a lower temperature (i.e. 350 °C) than would be feasible with a steam cycle. Furthermore, since the system is designed with the condensing pressure just above atmospheric, the need for a deaerator is eliminated since inward leakage of air is avoided. As a result of the density differences of the fluids, the turbine has fewer stages than in a steam cycle and the piping to the turbine from the heat exchanger can be smaller in diameter. These features can contribute to reduced costs. Significantly, the working fluid remains dry throughout the process so that the erosion risk noted in the case of the steam cycle is avoided; this is an important feature of the ORC.

There are also some disadvantages; an ORC unit will generate less power than a steam cycle operating with similar conditions and costs can be higher for specific applications. Also, as might be expected, the fluids used in an ORC cycle are combustible and if leakages occurred an environmental hazard could result.

The first unit of this type was installed by Ormat Technologies in the Lengfurt plant of HeidelbergCement in 1999 with a generating capacity of 1.2 MW.

Several ORC units have been installed in cement plants, including systems provided by Turboden, a Pratt and Whitney company, and by ABB, so that the Organic Rankine Cycle has matured to the stage where it is now considered to be a well-established technology for WHR.

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.

Read the article online at: https://www.worldcement.com/europe-cis/01082013/whr_cement_industry_organic_rankine_cycle_77/


 

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