Air/fuel ratio strategies
Five air/fuel ratio control strategies on the same load step: when the flame goes rich and what safety costs.
Simulator developed by José Geraldo da Silva Moreira · Energy-efficiency lab
Air/fuel ratio: five strategies on the same load step
Demand rises from 40% to 85% and then drops back. The air loop (fan, damper) is slower than the fuel loop.
Strategies on the λ chart
Result by strategy
Minimum λ below 1: flame short of air (CO, soot, risk). High maximum λ: excess cold air. Response: time for fuel to reach 95% of the new load.
Actual air/fuel ratio (λ)
Flows (% of maximum)
DemandFuelAir ÷ design λ
What to watch
- Parallel and fuel-leads-air: fast response, but the load increase passes through λ < 1. That is the puff of smoke on every ramp-up.
- Air leads fuel: safe, slow ramp-up; on the way down air drops first and the flame can turn slightly rich.
- Single cross-limiting: never rich; the price is transient excess air on the way down.
- Double cross-limiting: keeps λ inside a band in both directions, at the cost of a slower response. Too narrow a band leaves the furnace stuck.
Simplified teaching models with order-of-magnitude illustrative values. Do not use them for design or operating decisions without validation against plant data.