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[Enginerdo]

Hi,

I was looking for some clarification on the desired flame temperatures in burners, and what effects it has on the flue gas stack. I'm dealing with an OTSG, that uses a mixture of natural gas (86% by mass) and produced gas (contains 10% mole CO2 and 0.83% mole H2S if SRU is down).

From my understanding (please correct me if I'm wrong), as your flame temp increases:

• SO3 composition will decrease, thus lowering your acid gas dew point (favorable for heat recovery in the stack)
• NOx emissions will increase
• Increased coking in the burner tip, resulting in plugging

Is there an ideal flame temperature?
Are there any other considerations that I've missed?

Any input will be appreciated.

Thanks,

Enginerdo

[kkala]

I was looking for some clarification on the desired flame temperatures in burners, and what effects it has on the flue gas stack. I'm dealing with an OTSG, that uses a mixture of natural gas (86% by mass) and produced gas (contains 10% mole CO2 and 0.83% mole H2S if SRU is down).
SG certainly means steam generator, OT means what? (outdoor?) It must be a boiler. Thus there are heated water tubes between radiant section and stack. Temperature in stack gases is limited by their acid dew point, not practically depending on flame temperature. Under same rate of burnt fuel, a short flame is hotter than a long flame, since the latter is diluted with more air.
Normal conditions are assumed below, then flame temperature increases by decreasing flame length, e.g. by increasing primary air while total combustion air remains as before.
From my understanding (please correct me if I'm wrong), as your flame temp increases:
1. SO3 composition will decrease, thus lowering your acid gas dew point (favorable for heat recovery in the stack)
Right, higher temperature will shift the SO3/SO2 equilibrium to less SO3, so acid dew point will be lower. http://www.jehar.com/dewpnt.htm.
But flame temperature will increase only locally (in and around the flame), so resulting acid dew point is not expected to have a high drop. Probably this drop is withing the errors of dew point estimate (usually not being quite precise).
2. NOx emissions will increase
Right, since high flame temperature creates NOx. Combustion in successive "stages" develops lower temperatures and lower NOx.
Burners and radial section should have low NOx configuration for a guaranteed result.
3. Increased coking in the burner tip, resulting in plugging
No experience in coking. However gas burners create less coke than diesel / fuel oil burners. CH4 is "resistant" to coking, probably to 600 oC (http://www.cheresour...ng-temperature/, http://www.cheresour...ture-of-methane), but flame temperature could approach 1800 oC. Domestic natural gas burners would not have plugging problems in the one case or the other.
4. Is there an ideal flame temperature?
For fuel oil boilers ordered in local refineries, conditions of low NOx burning (that is low temperatures in radiant section) were considered as preferable, to reduce NOx formation. Smoke / particulates were retained in Electrostatic precipitator, acid dew point was considered as dependent only on SO2 content of fuel (flame temperature neglected).
5. Are there any other considerations that I've missed?
Local trend is to establish continuous monitoring of stack gas emissions after few years.
Any input will be appreciated.
Hope it is useful, an expert could help (especially with coking issue).