A macroscopic determination of the impact of Lethabo coal quality upon the optimum combustion air quantity

Abstract

Lethabo Power Station burns a unique low grade coal with low volatile content. This merits the optimisation of the total combustion air quantity with efficiency as main criterion to produce an air flowsteam flow curve as a function of load as well as coal quality. This project demonstrates the optimisation exercise of the unit as a whole, including the boiler as well as the turbine (a 600 MW turbo-alternator set) , therefore producing optimum air flow quantities different to the conventional wisdom. Ninety tests were performed with varying total air flow, at five loads ranging from full load to lowest load with three different coal qualities. The application of the Station Thermal Efficiency Program (STEP) as optimising tool employs a simultaneous direct and indirect calculation method (opposed to the traditional indirect only methods and programs) . The test results were used to customise the targets of the STEP program for Lethabo as well as to introduce new philosophies in the formulae. The expected trend of higher unit efficiencies with resulting improved total emissions were confirmed. In the process operational enhancements such as new milling plant settings and sootblowing philosophies were also confirmed to contribute to the improved efficiencies. The findings were operationally implemented which included statutory procedurisation.