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In Some refineries ( not in all ) There is provision Secondary air supply to Burner in Fired Heater to keep flame pattern in shaped. but as Secondary air is not taking part in Combustion process will it not effect to Excess O2 % calculation ?? Is value shown by Digital Device ( Excess 02 %) will be reliable in thus case ????
 
Answers
26/04/2016 A: Bruce Carr, Suncor Inc, matukaze1@hotmail.com
It doesn't matter how often maintenance is performed on an O2 analyzer. If there is more than one burner in a heater one can be starved while the other can have too much. The analyzer only measures the excess O2 in the breaching area.... Unless your heater is in pristine condition with no tramp air, fully sealed observation/inspection ports and explosion doors the O2 analyzer will not give a true representation of the combustion conditions. They should be used as a guide, understanding what a good flame looks like is your best control.
Another tip....only about 80% of the flame is visible to the naked eye if you want to see what is really going on with the flame, add a teaspoon of sodium bicarbonate (baking soda) into the air register. The Sodium will ionize and turn the total flame bright yellow. I've used this trick numerous times to show operators how a flame that looks normal is actually impinging on the tubes.
26/04/2016 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
The Flue Gas O2 analyzer readings can be faulty unless the soot deposits in the filters built in the sampling line of Flue Gas (inlet to O2 analyser) is cleaned up regularly. In other words the analyzer must adopt a regular Periodic Maintenance schedule for reliable results.
22/04/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Same reaction as primary air.
21/04/2016 A: Bruce Carr, Suncor Inc, matukaze1@hotmail.com
First off the terms primary air and secondary air are not just arbitrary terms. When burning Hydrocarbon fuel there are two reactions taking place within the flame. The first Primary reaction is the conversion of the Hydrogen and Carbon to Carbon Monoxide and water. The Secondary reaction is the conversion of the Carbon monoxide to Carbon dioxide, both are exothermic reactions. The primary air supports the primary reaction that is why it is normally introduced at the center of the flame. The secondary air supports the secondary reaction. When burning oil it is very important to separate the two reactions to liberate the maximum amount of heat from the fuel. The secondary reaction accounts 75% of the heat liberation from the fuel so it important to get it right....When burning gas the reactions take place so fast that normally only one source of air is required. (With exception being Pre-mix burners... a lot of heaters have been converted from burning oil to burning gas but kept the old air registers).
You are right that secondary air will control the flame pattern but too much primary air will create a smaller brighter flame that will not give off as much heat causing you to burn more fuel. This is because the flame will become hotter reacting with the Nitrogen in the air and forming NOx. This is an endothermic reaction so the energy of the fuel is not being used efficiently.
As for Excess O2 analyzers..... Never operate a heater based on them... too many things can give you false readings... Tramp air.. uneven distribution of the combustion air.... operate the heater based on flame pattern... try to minimize the excess air but don't set targets... energy conservation has caused more heater issues because of setting targets based on theoretical values. True efficiency if achieved by understanding the flame... save more money by properly training the operators on what a good flame looks like....
21/04/2016 A: L K Maheshwari, Essar Oil, laju1984@yahoo.co.in
But how Secondary Air will be consumed in firebox ??? As it is only for Shape stability of burner flame .
21/04/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
In any furnace, all oxygen entering the firebox is consumed in the burning except for the so-called excess O2. In other words, none of the secondary oxygen bypasses the burning area only because it is secondary. The O2 in the stack sample does not distinguish between the two types of O2 input.