Control System Design for Effective Energy Utilization...A Case Study

Abstract

This paper describes the activities and results of a typical plant powerhouse energy management survey. The powerhouse provides electrical energy, steam supply at two different pressure levels, and compressed air for the process plant. The owners had already performed energy loss surveys and implemented maintenance and energy management programs. The activities described include a follow-up survey and a concentration on the improvement of the boiler operations and efficiencies. The Input-Output and Heat Loss methods of determining boiler efficiency, in accordance with ANSI & ASME PTC4.1, are explained. A comparison of the methods, including a sensitivity analysis, shows the effects of measurement inaccuracies on each.

Each boiler is studied and the required measurements identified and described. The importance of flue gas oxygen measurements is described along with the variations in readings obtained using dry or wet basis measurements. The existing operating efficiency of each boiler is determined and the effects of improved controls identified. The considerations for Security, Reliability, Maintainability, and Efficiency Improvement required by any control system are described. A control strategy is selected and hardware chosen to implement the improved boiler and overall steam generation performance.

Focus is shifted from the single boiler problem to the requirements for optimal loading of each boiler to meet total system load requirements. The economics of multiple boiler loading is discussed with the introduction of the Cost Function. This permits direct comparison of boilers even when using different fuels of varying costs. Provisions are included for operator limits of each boiler. These limits are required to permit identification of maximum boiler steaming rates or to provide hot standby (spinning) reserves. Minimum limitations on boiler loading such as burner turn-down are also recognized. In addition, the amount of boiler bias or offset can be limited on a per boiler basis. Lastly. a comparison is made between the powerhouse operation initially, following installation of improved controls. and finally when optimal boiler loading is implemented.