Exergy analysis of a steam production and distribution system including alternatives to throttling and the single pressure steam production

The operative steam production and distribution system of Kårstø natural gas processing plant at four operating conditions was studied by exergy analysis. The eight boilers, of which two are direct fired produce steam at a single pressure. The steam (at 59 bar, 420 °C) is then distributed by extensive use of throttling. Most of the steam is utilized at low pressure (7 bar, 200 °C). The effect of implementing steam turbines in steam distribution system, increase of steam production pressure (from 59 to 120 bar) or two-stage pressure steam production (120/59 bar) were examined. The exergy efficiency of the existing system was 44.3%. Implementing steam turbines or elevation of production pressure (single/dual) resulted in marginal exergy efficiencies of 92%, 89.8% and 98.7%, respectively. Combinations of steam turbines and elevated pressure gave a ratio of 90.9%. In terms of lower heating value, the marginal electric efficiencies ranged from 89% to 104%. The single most fuel exergy demanding alternative was the elevated pressure, with 3.6% increase, which resulted in 18.5 MW of extra electric power. The corresponding figures for steam turbines and two-stage pressure were 3.0%/16.2 MW and 2.6%/15.6 MW. Thus, the study provided an example from an existing, industrial steam system that illustrates both the losses in throttling and low-pressure steam production, and the practical potentials for improvement.