Flame Stability Analysis of Natural Gas Runners in Heat Recovery Steam Generators

A Heat Recovery Steam Generator or HRSG is an energy recovery heat exchanger that recovers heat from a hot gas stream. The HRSGs are generally fitted in the downstream of a Gas Turbine to recover the Turbine Exhaust Gas (TEG) heat. The HRSG is typically used to produce steam that can be used in a cogeneration process or is used to drive a steam turbine in a combined cycle system. The temperatures in TEG will vary from 350°C to 650oC depending on the class of the upstream Gas turbine. The HRSG is designed with a supplementary firing system to augment its steam output. A supplementary firing system comprises of a log type duct burner arrangement mounted on the inlet duct of HRSG. Duct burner system in the HRSGs is capable of raising the exhaust temperatures from 350°C - 650oC to 600°C - 900oC or even up to 1000°C. In a duct burner arrangement, fuels like oil and gas can be fired in separate dedicated oil burners and gas runners stacked at multiple elevations similar to a log. Fuel gases like Natural gas fired runners are most commonly used in duct burners and the oxygen required for combustion of natural gas is utilized from the Gas turbine Exhaust. Typically the oxygen concentration in TEG will vary from 12-15% depending on the class of Gas turbine. This oxygen content will serve as the stoichiometric and excess air for complete combustion of the hydrocarbons in Natural Gas. Maximum supplementary firing refers to the maximum Natural Gas that can be fired with the oxygen available in the TEG. An advanced class Fr 6FA Gas Turbine is characterized by low oxygen concentration, low GT Exhaust flow and temperatures at lower GT loads. So in order to enhance the HRSG steam generation, very high supplementary firing up to a capacity of 70 Mkcal/h of Natural Gas will be fired in the Gas runners of the duct burners. These gas runners that are stacked in multiple elevations will each have number of fuel holes along its axis and will have flame holding stabilizers to obtain a sustained Natural Gas flame. As Natural Gas is let in through these fuel holes, initial ignition energy is provided to obtain flame and with the flame stabilizers directing the TEG oxygen towards the flame, a sustained combustion is obtained. However in case of Fr 6FA GT at lower gas turbine loads, due to lesser TEG oxygen percentage and low exhaust temperatures, instability happens in natural gas combustion and the flame tends to get put off. To understand the flame stability and oxygen concentration, different stabilizer arrangements of the gas runner were analyzed using a Computational Fluid Dynamics study using ANSYS Fluent v 14.5 tool. The results of the study are presented in this paper.