The red-hot air furnace for an externally fired gas turbine power unit – Designing aspects and performance testing

Boilers commonly used in industry, for the most part, operate on the principle of transferring heat from the flue gas to the liquid medium (with or without evaporation). This paper focuses on red-hot air furnaces, intended to supply externally fired gas turbines. The general design issues, related to permissible limits of thermal and flow parameters, and to the contamination of biomass-combustion flue gas, are discussed. The preliminary test results for the pilot-scale red-hot air furnace with steel-tube heat exchange sections, powered by the gas burner are presented and analyzed. The device consists of a high-temperature chamber located above the firebox and a medium-temperature (convection) exchanger. The tests were carried out for three burner powers - 140, 195, and 250 kW, and for various volumetric airflows within the range between 500 and 800 kg/h. The effectiveness of heat exchange in this system ranged from 79 to 87%, depending on the case. During the tests, the furnace was operated with an underestimated amount of exhaust gases, as the target system provided for the dosing of the outlet turbine air into the combustion chamber and baffle. This led to the reduced outlet temperature of exhaust gas (from 54 to 82°C). The study outcomes reveal that the proposed design type for the red-hot air furnace is promising in terms of efficiency, reliability, and total costs. However, considering today's need for maximizing the utilization of biomass, further design development should also involve low-dust combustion technologies.