How to Optimize the Fuel / Air Ratio on a Reheat Furnace

Reheat furnaces are used to bring a metal billet to the correct temperature for rolling or extrusion processes, typically in the range 1000 °C to 1250 °C (1830 °F to 2280 °F). 

Design details vary depending on the size and the application with walking beam, rotary hearth and roller hearth designs being commonly used. Fuels also vary but the most widely used fuel is natural gas. The amount of fuel burned in a reheat furnace means that efficiency improvements give large benefits. A gas-fired furnace typically consumes 25 m3 of fuel per metric ton of steel processed. Efficiency improvements can reduce fuel consumption by up to 10%, saving money and reducing emissions. For a large furnace, the savings can total in the tens of thousands of pounds per month.

Derek Stuart - Blog
Determining the optimum fuel/air ratio is key to minimizing both fuel costs and pollution emissions. Insufficient air results in incomplete combustion and wasted fuel, with a high concentration of carbon monoxide (CO) in the exhaust gases. This condition is shown in red. An excess air condition ensures all the fuel is burned but the losses, in this case, are dominated by the large volume of hot air leaving the furnace. The energy used to heat all this air is not available to heat the billet within the furnace. Excess air losses are shown in yellow.

Minimizing these losses requires an understanding of the process conditions, measurements of the oxygen concentration in the furnace exhaust are a good starting point. However, the oxygen concentration does not tell the full story – it does not tell you whether one or more burners are poorly-regulated and producing a high concentration of CO. Adding carbon monoxide measurements permits  more precise regulation of combustion conditions by operating closer to the theoretically-ideal setpoint without risking high CO emissions or a dangerous flameout condition.

The AMETEK Land FGA930 oxygen and CO analyser is ideal for such measurements. It uses sensitive and highly specific electrochemical sensors and includes sample handling and signal processing components in a compact, weatherproof enclosure. It has proven its worth in reheat furnaces, boilers and other combustion applications worldwide. A large billet reheating furnace operation in Spain recently experienced abnormal CO emissions and installed an FGA System to continuously monitor the flue gas. By installing the analyser, they were able to continuously monitor CO emissions, detect unburnt gas and prevent burner failure improving the overall efficiency of its furnace combustion which has improved asset management and reduced operating costs.
 

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