Analysis of hot spots in boilers of organic Rankine cycle units during transient operation

A. Benato, Martin Ryhl Kærn, Leonardo Pierobon, A. Stoppato, Fredrik Haglind

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Abstract

This paper is devoted to the investigation of critical dynamic events causing thermochemical decompositionof the working fluid in organic Rankine cycle power systems. The case study is the plant of an oiland gas platform where one of the three gas turbines is combined with an organic Rankine cycle unit toincrease the overall energy conversion efficiency.The dynamic model of the plant is coupled with a one-dimensional model of the once-through boilerfed by the exhaust thermal power of the gas turbine. The heat exchanger model uses a distributedcross-flow physical topology and local correlations for single- and two-phase heat transfer coefficients.The results indicate that severe load changes (0.4–1.0 MWs-1) can lead to exceedance of thetemperature limit of fluid decomposition for a period of 10 min. Ramp rates lower than 0.3MWs-1 areacceptable considering the stability of the electric grid and fluid decomposition. It is demonstrated thatthe use of a spray attemperator can mitigate the problems of local overheating of the organic compound.As a practical consequence, this paper provides guidelines for safe and reliable operation of organicRankine cycle power modules on offshore installations.
Original languageEnglish
JournalApplied Energy
Volume151
Pages (from-to)119-131
ISSN0306-2619
DOIs
Publication statusPublished - 2015

Keywords

  • Hot spot
  • Organic Rankine cycles
  • Fluid thermochemical decomposition
  • Waste heat recovery unit

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