On-Line Monitoring of Gaseous Flames for Air-Fuel Ratio Control

This paper provides the initial steps in the overall design and implementation of a control system that utilizes optical sensors to monitor individual burners in a furnace system. The key component of such a system is the optical sensor, which produces a signal that corresponds to the air-to-fuel ratio for the given flame. A single propane or natural gas fueled flame was monitored with a single optical sensor responsive to emission wavelengths between approximately 350 to 1100 nm. Air or fuel flow was controlled in order to shift the air/fuel ratio from fuel-lean to fuel-rich and back to fuel-lean. Results show that when the optical sensor was correctly positioned its normalized output voltage had a pronounced peak value that occurred very close to the stoichiometric air/fuel ratio. The normalized response decreased by approximately 30% at equivalence ratios of 0.8 and 1.25. This detection scheme shows great promise for controlling the desired local and global flame stoichiometry in the burner, and may be easily adapted to any burner system without major alterations and expense.