A modular flare stack used for the combustion of combustible fluids comprises: a gas feed pipe, at least two burner elements, and an automated control system for applying a fixed stoichiometric combustion to an air/gas venturi mixing system based on a feedback loop from the flue gas temperature. Mixing ratios are obtained either using a fan for gas flows at a lower pressure (less than 0.5 barg) or a venturi for gas flows at a higher pressure (more than 1 barg). The control system also determines the number of operational burner elements and which of the burner elements are to be operational. The flare stack provides premixed surface burners for waste gas streams, thus guaranteeing extremely low emissions and high destruction efficiency by complete combustion with a high turndown ratio.