Optimising the Steam and Power System

Roger Grundy, Breckland Ltd, UK, UK

Much of the energy consumed in a refinery/petrochemical asset is for the generation of steam and power.  For example, in a world-scale petrochemical asset this can represent a cost in the order of 200 $million/year or more, based on typical pricing of natural gas.

In our experience, even new industrial complexes continue to be designed with a large proportion of power generated on a "condensing" cycle, which is much less efficient than by a cogeneration cycle.  We term this "the conventional power station syndrome."

Using modern, high efficiency aero-derivative gas turbines in a cogeneration arrangement that recovers exhaust heat from the waste gas to generate steam, we were able to reduce total site energy consumption of the design for a new petrochemical facility by almost 10%, worth 30 $million/year in reduced natural gas consumption.  Our SAPSIM program was used extensively to develop and substantiate the scheme finally adopted by the client as the project moves into implementation.

We developed SAPSIM since we have not found any industry standard software that simulates a steam system conveniently and to the level of detail that we require for our work.  Key features of SAPSIM include the ability to model steam flows between "nodes" (i.e. points at which steam enters or leaves a header) and also to converge a solution with given steam input/output constraints: this problem turns out to be quite challenging mathematically.

We emphasise however that the process of optimisation needs both engineering experience and good judgement as well as good simulation tools.