Abstract: modular flare stack (100) for combustion of combustible fluids comprises a gas feed pipe (101) and at least two burner elements (120) for combustion of the combustible fluids. This modular flare stack comprises an automated control system (190) for fixed stoichiometric combustion applied to air/gas venturi mixing system, based on feedback loop from flue gas temperature. Mixing ratio's are obtained either using a fan for gas flows at lower pressure (less than 0.5 barg), either using a venturi for gas flows at higher pressure (more than 1 barg). The control system also determines the number of operational burner elements and which burner elements are operational. The flare stack of the invention provides premixed surface burners for waste gas streams, guaranteeing extremely low emissions and high destruction efficiency by complete combustion with a high turndown ratio.