The objective of designing and developing boilers is to recover the required steam using a duct burner equipped with fresh air firing under conditions where the gas turbine is not functional. A stand-alone boiler is a type of heat recovery steam generator (HRSG) that is capable of producing the steam required by a combined cycle power plant via a gas turbine or independently (without the need for a gas turbine). The main use of this boiler is when the gas turbine is not functional and there is a need for the steam recovered by the HRSG – but no need for the electricity generated by the gas turbine. Under this condition, the energy required for generating the steam is supplied by a duct burner. In addition, the oxygen needed for the occurrence of combustion in the duct burner is supplied by fresh air firing with a fan. Compared with conventional heat recovery steam generators, stand-alone boilers can provide the steam required for power plants with higher reliability and therefore have a better performance in electricity and steam cogeneration projects. When a stand-alone boiler (equipped with fresh air firing) is utilized, the energy needed for generating steam is provided by a duct burner. Compared with duct burners used in heat recovery steam generators, duct burners used in stand-alone boilers have a relatively greater capacity because they must increase the temperature of air from ambient temperature to the gas turbine’s output temperature (about 500 ⁰C). Also, given that fresh air is mostly perpendicular to the boiler, designing air stream rectifiers including baffle and mesh filters is more difficult. The aim of this project is to achieve the knowledge of designing and constructing duct burners for stand-alone boilers.