Combustion in fired heaters and boilers is typically based on either volumetric flow control or pressure control of the fuel gas feeding the burner. Using mass flow control could help lower energy costs and emissions.
The heat energy supplied by fired heaters and boilers in a refinery or petrochemical plant is created by combustion, usually by burning natural gas or fuel gas made up of various refinery off-gases.
Automation and control engineers and fired-equipment subject matter experts have debated the optimal method to control heater firing over the years. The most common approach controls the fuel gas volumetric flowrate or pressure. In this control scheme, the outlet temperature of the heater cascades and resets a volumetric-flow controller or pressure controller. Under steady operating conditions, this technique provides adequate response and control of the heater. However, any disturbance caused by a change in the fuel supply composition can render this control method inadequate for the desired level of risk, fuel efficiency, or environmental compliance.
This article describes some of the challenges of controlling combustion in a fired heater or boiler, and suggests a better method of control that measures the mass flow or the actual energy of the fuel gas being directed to the burner. The article evaluates the economic benefits of several methods of fuel gas control for natural-draft fired heaters.
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