A problem arises when instead of the standard fuel-gas for which the apparatus is set, a fuel-gas from the same family as said standard fuel-gas but prone on flame blow-off or prone to flash back are fed. Specifically, the burner flame openings surface may attain critical temperature value, and in some other occasion, the flame may become unstable, resulting in poor combustion of fuel-gas.
A method, applicable to highly premixed but only atmospheric burners, to maintain stable the temperature of the flame openings and reduce the harmful emissions by the variation of --.lambda.-- in the mixture according to the temperature of the flame openings itself is described in the European patent of the author EP0606527A1 deposited on Aug. 16, 1993, but don't take into consideration the flame, its position, shape or density, particularly is not considering a lamellar flame detached from the area of the flame openings.
It is also known a method described in patent DE3630177 dated Sep. 4, 1986 where the signal of the ionisation current, inside the large volume of a turbulent flame, is used for the variation of --.lambda.-- nevertheless it is a signal relating to the combustion conditions inside the big volume of the turbulent flame itself, not of the ionisation conditions of the unburned mixture just upstream the flame front, so at conditions prior to the combustion. This signal is typical of the combustion conditions, specifically as to the limit of stability for the turbulent combustion, don't give any information about the position of the flame front or the flame distance. Furthermore the considered emission of UV, without description of how it is and how it can be used, would seem to determine the combustion condition as to the limit of the stability for turbulent combustion.
Control systems which vary the total quantity of air or of primary air basing itself on a temperature in combustion chamber, on the excess of air in fuels, either combined or not with air variation according to the flow rate of fuel-gas fed, are known. However none of these take into account the influence of gases different from the standard one which can be distributed in sequence without notice and therefore feed the combustion system, nor can they maintain the stability of the flame in large ranges of capacity modulation, nor take into account the combustion of a hyperstoichiometric mixture in substantially laminar flow, particularly with lamellar flames.
These latter control systems and similar ones are complex, in particular for the type and positioning of the sensors, consequently, too expensive for gas appliances of flow limited to even few KW.
None of the previously considered control systems take into account the temperature as well the outflow velocity of the mixture.