Most refining and petrochemical sites employ combined heat and power (CHP) plants to meet their demands of electricity, shaft power and steam. In this context, CHP systems typically comprise many energy-conversion units (e.g. boilers, steam and gas turbines) that entail substantial investment and large operating costs. Consequently, these plants offer an excellent opportunity to achieve significant savings, but, at the same time, minimising such expenditure also represents a challenging task due to the large number of design choices and complex operating interactions among the equipment.

Shell Global Solutions (GS) through its Utilities Group has provided consultancy to many sites worldwide in order to implement operating policies and/or infrastructure projects that effectively reduce costs and improve the reliability of their CHP plants. Throughout its long experience, Shell GS has adopted and developed several simulation and optimisation tools to understand the degrees of freedom offered by a CHP system in a comprehensive approach.

This paper examines a case study representing a refinery CHP plant subject to variable requirements and constraints (e.g. demands, fuel availability, electricity prices). This system needs to be revamped to reduce operational costs, increase efficiency, improve its reliability, and meet tougher emissions limits. Such objectives, together with the varying parameters, are simultaneously taken into account in order to arrive at a solution that includes, not only the new design, but also a suggested operational policy for the scenarios considered. Finally, a sensitivity analysis evaluates the impact of different situations and constraints on the conclusions for the project.

Keywords: Utilities, energy, optimisation, cogeneration, flexibility.