Automatic Combustion Control (ACC)
When you have a large firebox, with slowly moving, steady heat release, you should consider using Chevron’s patented Automatic Combustion Control (ACC) technique. It is now in use on many of Chevron’s oil and gas fired process heaters to reduce fuel consumption and NOx emissions. All heaters with fired duties greater than 50 million BTU/HR are candidates for ACC. Typical payback periods are 1 year or less.
Mainly applied to natural draft heaters, ACC is also used on heaters with forced and induced draft (FD/ID) controls and on multibox heaters (three to four combustion zones with common convection sections and multiple stacks).
ACC is a computer program using a constraint control technique to reduce excess air and to maintain the combustion process at the best possible efficiency over a wide range of process conditions. ACC is not a draft control, O2 control, or CO control technique, but rather controls the entire combustion process. See Figure 600-3 for a schematic representation.
A set of “constraint conditions” is determined by testing the candidate heater while monitoring certain process variables. To test, excess air is slowly reduced until flue gas CO starts to rise above 150 ppm. This establishes firing and heater conditions for the highest attainable efficiency and sets the constraint values for CO, O2, and draft. It also reveals potential limits to efficient heater operation, e.g., draft, flame impingements, instrumentation problems, air leaks.