Doug Simmers here again. My last post (“Combustion Geek: Part Deux”) discussed different methods of measuring the oxygen remaining in the flue gases coming off of combustion processes. My next few posts will discuss the economic advantages of having a reliable O2 measurement, and of dynamically controlling combustion air to maintain the optimum firing condition.
Overview and Traditional Application
Combustion flue gas analysis has been used by power plant operators for decades as a method of optimizing fuel/air ratio. By measuring the amount of excess oxygen and/or CO in the flue gases resulting from combustion, plant operators can operate at the best heat rate efficiency, lowest NOx, and also generate the least amount of greenhouse gas. The theoretical ideal, or the stoichiometric point, is where all fuel is reacted with available oxygen in the combustion air, and no fuel or O2 is left over.
Operating furnaces never attain this ideal, however, and the best operating point usually will result in 1-3 percent excess air, and 0-200PPM of CO. This optimum operating point is different for every furnace and also varies for differing loads, or firing rates. A higher firing rate induces greater turbulence through the burner(s), providing better mixing of fuel and air, and enabling operation with a lower excess O2 before unburned fuel (represented by CO) appears, or “breaks through.”
This breakthrough point is usually established by cutting the combustion air back until a portable CO analyzer sees a CO increase. It happens quickly, since this is only a parts per million (PPM) measurement. Again, this ideal O2 operating point will vary with firing rate, so a curve is usually developed from test data to assign the ideal O2 control point based upon an index of firing rate, such as fuel flow or steam flow.
Customers with a permanently mounted CO analyzer can see this breakthrough point as part of their normal operations, but customers operating with only an O2 analyzer will redo the testing with a portable CO analyzer at different firing rates, resulting in a curve something like shown above. This curve will be entered into the control system, and the combustion air will be “trimmed” to maintain the optimum flue gas O2 for a given firing rate. Firing rate is usually determined by measuring the fuel flow to the burner(s), or steam flow, if no fuel flow measurement is available.
It should be noted that this curve should be reestablished from time to time as burners wear, and other furnace conditions change over time. The curve for burners using natural gas and light oil fuels will tend to remain valid for long periods of time (years). Burners firing solid fuels such as coal, petroleum coke, or pellitized biofuels will experience more frequent pluggage and other degradation in the burners and fuel delivery systems, and will benefit from more frequent reestablishment of this curve.
Operation with dynamic O2 trim control can save considerable fuel cost – see for yourself here.
Future posts will discuss some of the other advantages that good combustion analysis affords, including:
- Operating with lower NOx emissions
- Operating coal-fired units with less slag
- Using an array of flue gas analyzers as a diagnostic for detecting furnace leaks, imbalances, and other problems