Closed-loop control of fuel/air mixture equivalence ratio in gas turbine becomes today more important for energy industries. The mass flow of fuel is well metered but there is no easy way to establish the accurate equivalence ratio without fast on-line measurement of gas composition and Wobbe index. Reduction of emissions is another indisputable requirement, which can be only achieved by closed-loop control of combustor in a range of equivalence ratio avoiding increase of CO and NOx.