Hi everyone. This is Doug Simmers talking again about improving your combustion processes. The primary goal of combustion flue gas analysis is optimizing the fuel/air ratio, which minimizes fuel cost, lowers NOx emissions, and also minimizes the amount of carbon dioxide greenhouse gases emitted into the atmosphere. One overlooked use of flue gas analyzers is for balancing the combustion across large multi-burner furnaces.
Historically, a single flue gas analyzer placed downstream or in the smokestack was considered satisfactory for setting fuel/air ratios. Many furnace operators have moved the measurement location upstream, closer to the furnace, in order to get a faster speed of response, and added more analyzers for redundancy. This has proven to be a mixed blessing, since the analyzers almost never read the same O2 levels, which lowers the operators’ confidence in the measurement, and subsequently their willingness to operate at the lowest possible O2 setpoint. After many calibrations, it becomes clear that all the analyzers are all reading correctly, and the problem is that the combustion inside the furnace is unbalanced. When you think about it, this makes sense.
The combustion process is the burner. This is where the fuel and air are mixed and combusted. The furnace is mostly just a heat exchange envelope. If you have 50 burners in a large furnace, you have 50 discrete combustion processes, and some variability can be expected. This is the case for all large multi-burner furnaces, be they a 500 MW coal-fired boiler, a large crude process heater furnace in a refinery, or a steel reheat furnace. Strategically placing an array of analyzers in the flue gas ductwork can help operators keep the furnace balanced, and prevent many problems.
Flue Gas Stratification Tells a Story
Forward-thinking operators will use these varying O2 indications as a diagnostic tool to look for problems in the furnace, such as:
- Fuel Gas BTU variation – will be indicated by both O2 and CO variation.
- Fouled burners – depending on the consistency and quality of fuel burned in a furnace, a burner will change over time.
- Flame carryover – combustion sometimes progresses well into and beyond the convective zone, and is indicated by lower O2 readings, higher CO readings, or both, along with rising outlet temperatures.
- Tube leaks – an unbalanced furnace can cause flame impingement and associated tube wastage.
- Furnace casing leaks – will result in elevated O2 readings, since 21% of the air leaking in is oxygen.
- Sticking dampers – stack outlet dampers are sometimes poorly maintained, and sluggish control of O2, COe, and furnace draft are indicative of problems.
Combustion flue gas analyzers have become true analytical tools for determination of all kinds of furnace problems. We’ll talk about others in future blogs.
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