Sasol Technology Proprietary Limited

Rian Wessels

Franco Gasparini

Corné Welgemoed

A method of operating a combined heat and power plant () (CHP plant) includes generating hot flue gas and cooling the hot flue gas in a sequence of cooling steps to recover heat and to generate steam in a heat recovery steam generator () (HRSG). The HRSG () includes an LP steam evaporator () designed to generate steam at least over a pressure range of from 2 bar(g) to 18 bar(g) so that either LP steam or MP steam can selectively be generated by the LP steam generator (), thereby to cool the hot flue gas, and an MP steam superheater () upstream of the LP steam evaporator () to superheat MP steam in heat exchange with the hot flue gas thereby to cool the hot flue gas. The method further includes, when no or insufficient MP steam is being imported to the MP steam superheater () to ensure safe operation of the MP steam superheater, and/or when a demand exists for exporting MP steam which cannot be satisfied by imported MP steam superheated in the MP steam superheater (), and/or when a demand exists for MP steam in the CHP plant which cannot be satisfied by imported MP steam, operating the LP steam evaporator () at an operating pressure in the range of between 8 bar(g) and 18 bar(g) to generate MP steam to wet the MP steam superheater () and/or to satisfy at least to some extent said demand for MP steam, and thereafter, when sufficient MP steam is being imported to the MP steam superheater () from external of the CHP plant () to ensure safe operation of the MP steam superheater (), and/or when any demand for exported MP steam is satisfied by imported MP steam which is superheated in the MP steam superheater () and then exported, and/or when there is no more demand for exporting of MP steam, and/or when the demand for MP steam in the CHP plant () is being satisfied at least to some extent by imported MP steam, reducing the operating pressure of the LP steam evaporator () to a pressure in the range of between 2 bar(g) and 8 bar(g) thereby to generate LP steam.

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Pretoria, South Africa

Rian Wessels