The heat energy that fuels fired heaters and boilers in a refinery or petrochemical plant is supplied by combustion, usually by burning natural gas or fuel gas. The optimal method to control heater firing has been debated over the years by automation and control engineers and fired equipment subject matter experts. The two most common methods of control include fuel gas supply flow or pressure control. Under steady operating conditions these two methods of control give adequate response and control of the heater. When there is a disturbance in the system though, especially caused by a change in the composition of the fuel supply, these control methods may be inadequate for the desired level of risk, fuel efficiency or environmental compliance.
Enabling small reductions – even as little as 1 percent – of excess O2 in the flue gas of fired heaters at an average-sized refinery or petrochemical plant can result in significant operational savings. On the other hand, operating with too low a level of O2 in the flue gas creates the risk of sub-stoichiometric (insufficient oxygen) combustion, possibly tripping the heater, or in the extreme case, causing damage to the heater. Sub-stoichiometric conditions can result when the composition of the fuel feeding the combustion suddenly changes to a richer fuel that is higher in heating value, requiring more oxygen. If this could be anticipated (i.e. feed-forward control), much of this challenge could be eliminated.
This article evaluates various methods of fuel gas control for natural draft fired heaters and describes the calculations needed to determine which method is best.