One method of improving the overall efficiency of a power plant or increasing the power output is to re-power the existing boilers, using gas turbines in a combined cycle system. For the case of coal -fired plants, this option known as gas turbine topping is limited by the difficulties in coal burnout due to the low oxygen content (12 vol%) in the exhaust gas of modern gas turbines. A possible option to resolve the problem could be the introduction of an oxidation catalyst in order, first, to avoid the incomplete combustion of the coal and, second, to decrease the CO and CxHy emissions. This paper presents the results of coal combustion tests in an existing atmospheric fluidized bed with simulated gas turbine exhaust flue gas. The purposes of the trials were the examination of the possibility to achieve the combustion of low-grade lignite with about 1214 vol% O2 content in the turbine exhaust gas and the experimental investigation of the catalytic oxidation on the burnout behaviour as well as the resulting CO, SO2, N2O, CxHy and NOx emissions. Two series of measurements were carried out, with and without catalysts. The influence of the catalysts on the burnout of coal and the combustion temperature is examined and discussed. The main conclusion of this experimental investigation is that it is possible to achieve combustion of the low-grade lignite in combined cycle applications, using turbine exhaust gas as a fluidizing and oxidant medium with the support of the catalysts examined in this study, as is shown by the drastic reduction of CO and CxHy emissions.

Keywords : combined cycle, coal combustion, catalytic oxidation