Burners Excess Air

What fuel are you using? Are you using a temp. controller? Do you have valves with actuators on both fuel and air? Do you have orifice plates with transducers to measure flow? Do you have a combustion analyzer to come up with that 125% figure?

I really have to know your setup to give you an answer. If I tried to cover all the possibilities, this would end up as a book.

I am responsible for the operation of large forge furnaces,(natural gas). We use zone temp. control, with combustion control. There are many ways to do this.

Temperature control and furnace operation is my life.

Thanks Bricktop.
We use gas.
Actually, I know the temperature of the combustion air (10 degrees celsius). I know the temperature of the gas going to the oven to cure our glass wool (250 degrees celsius).

I got the excess of air by using the dP air and gas max (measurements done by the contractor 2 years ago to adjust the ratio), meaning that we have a 125% excess of air when the burner runs at its full capacity. Thanks to the dP values and the burners' chart, I can deduce the excess of air.

We have a butterfly valve managing the air and gas flow, according to the contractor and also to the adjustments made by him at the lowest firing rate (dP air gas min), we operate with a much higher excess of air the lower the firing rate is...

125% become 300% at a 10% output. It is not the ideal situation I guess, since our process allow us to have a 15% excess of air without any risk of explosion.

I did not know anything before about combustion and it seems nobody know a lot about that at the plant, our contractor seems confused! weird

So I think you are adjusting air and gas valves with a linkage to one actuator. This is not ideal, as it will be non-linear. Using charts to compute ratios is not ideal ether, you don't have real time feedback. However, once you get it set up, this type of system works just fine.

To go to a system with concise ratio control is expensive, and it doesn't sound like that for your application. At low firing rates your xs-air will be very high because burners are set up to have some bypass air even when the valve is full closed. This protects the burner from getting to hot externally.

15% xs-air is virtually stoichiometric anyway. And having 40% xs-air is not a bad thing. You are assured of complete combustion, and you are creating more flow into your furnace, so your product heats up faster.

You don't have to worry about an explosion, as long as there is some fire in the burner. We often run -7.5% ratio when heating steel to minimize intermolecular participation, (scale from oxidation).

I suggest you purchase a flue gas analyzer, and you could periodically check your ratios.

http://www.einstrumentsgroup.com/

This link is for an American company of course, I sure you can find dealers in Europe also. I think a simple one, would cost about 2000 euro's, not cheap, but they can pay for themselves in a short time

Feel free to contact me.

By the way, where is Breda, I spend 2 weeks in Amsterdam every year. And another 3 weeks elsewhere in Europe, ever summer.

Mr. Brickman,

I for one agree 100%. The gentleman needs an exhaust analyzer. I do combustion 80% of my time and I believe he is close to a real problem. In addition he is wasting money.

I suggest "Testo" as an exhaust analyzer because they offer a wide range of equipment and they will have one at a good price to meet his needs. Testo is a German company and is shows in the repeatability and time between repairs. All of our trucks have them on board.Check out the new 330 model.

Mike

Breda,

I would be on a plane in a day to come and give you a hand if you picked up the airfare. The time between the contractor starting the equipment and a good stack analyisis has been to long. Seimens make a very good combustion managment system that if your equipment is big enough may have a good ROI. Their O2 trim system is the best for the money and it can be added to the LM52 system anytime after installation to spread out your investment. I have done many retrofit and they work well.

Respectfully,

Mike

Hey, I want to go too!

Hi, William Robyn!

What occurs to me is that the higher the burning temp, the greater the flow of air would be required to not only feed the flame, but to preserve the burner tips from exposure to the heat by forcing the flame to a greater distance from them. So it makes complete sense to me that more excess air would be required for an increase in capacity.

Mark

Well, no. You want to keep the flame in the burner. These burners really don't have a tip. There is a silicon carbide, or mullite cone in the center, and they rarely have to be replaced. You can go with XS-air to a point, but if you have to much air you get what is called "quenching", that is to much turbulence in the burner, and you rapidly loose efficiency. Burner setup is more of an art than science. So many variables.

In most cases the higher the input the better control you have and the lower the excess air. Regarding burner damage think of a cutting torch that is set up properly and burning. You lay it down on a steel table, the table gets very hot but may not burn a hole. If after the table top is glowing red you reach down and squeeze the trigger and bring in the needed excess air you will cut a hole (or kerf) very fast and continually as long as your speed is correct. There are a couple of burner manufactures in America that excess air above 5.5-6.5 % will cause damage to the burner head. In these cases the natural gas actually at lower excess air cools the burner head.

Boiler Guy

Thanks for your comments.

First Breda is in the southernpart of Netherlands, close to the Belgian border. Pretty nice place although I don't speak their language!

Well, I'm going to plan a meeting with the contractor and we will adjust the burners based upon the O2 remaining rate. Our research center advise sth around 3% after the combustion to operate in safe conditions. Do you agree?

I'm still confused regarding the excess of air at different output.
The burners firing rate is most of the time at 40%, then should we set up the excess of air (15% or a bit more to be sure we have a complete combustion) at that firing rate? We run it at 100% when we start up our curing oven, it means after each maintenance stop.

Thx

I have been tuning combustion systems for over 12 years and have never heard of having so much excess air in a furnace at a low fire or a high fire situation. I will be the first to admit I dont know everything there is about every process but will give you a little input from my experiences. First, if you are not an experienced combustions technician it is very unsafe to just start tweeking on any furnace especially if you dont know how all the fail safes work. Second, your burners you described sound pretty standard and should be considered to setup at 4-5% oxygen at low fire (Minimum heat and firing point) and 2-4% at high fire (Max heat). These numbers should be consistant with any furnace to keep efficiencies and reduce carbon monoxide output. In order to adjust the furnace correctly you will need a combustion analyzer as indicated in earlier responses capable of reading carbon monoxide (CO) and oxygen percentages. If the oxygen is to high then you will need to add gas to the burner(s) and monitor the CO if the CO increases you will need to remove gas. Keep in mind that most analyzers will have a delayed response on the readings of a minimum of 30 seconds, so be patient for your readings to stabelize before adjusting again.

Again, I do not encourage you to start adjusting any furnace without any proper taining. I recommend you to take any information supplied to you as intelect to oversee any contractor you are using.