The maximum temperature can be reduced and fuel combustion with the smallest possible amount of air can be achieved by optimizing the configuration of the fuel and the air streams and by adapting these streams to the firebox geometry. In this work, powerful secondary gas stream systems were modified in three ways in a rectangular prolonged firebox. The measurements established that the flow rate and geometrical distribution of these streams can determine the CO production rate, the excess air ratio must be finely tuned for complete combustion, and all these factors determine the production of NOx. If CO burn out is not achieved, it is reasonable to separate the gas into finer streams. The calculations demonstrate that long gas streams with higher flow rates along the firebox produce more CO, and a longer firebox is needed for the complete burn off of CO. In a second case, a very fine gas injection with streams grouped in packages near the burner was utilized in a firebox containing curved flame torches. Low CO yields were achieved, and the fuel combustion at nearly stoichiometric conditions resulted in lower NOx emissions. (C) 2012 Elsevier Ltd. All rights reserved.