Flue Gas Analysis questions

[jyro]

I am trying to understand the relationship of O2, NO, H20 and CO to combustion efficency in a 125 ton per hour nat gas reheat furnace. O2 is excess air, NO is nitrous oxide, h2o is water vapor, CO is carbon monoxide. I have a onsite gas monitor that gives a constant readout. I am having trouble getting a handle on the CO while trying to keep the NO low. I have a 4% excess 02 but still have CO. I need to get the 4% O2 down but when I do the CO goes up over 100ppm.
what does the H2O number tell me? When tuning ratios( this furnace has 7 zones with from 5 to 14 burners in each zone) I can't go any lower than 10.2 ratio or the CO goes high.
the furnace presure is maintained at .01 wc so I have no air influx.
Can anyone help me understand how to tune this thing?
thanks in advance

 

[LOUDOG]

I recomend phoning a professional. You have to understand where these chemicals come from to try to troubleshoot. NOx comes from high temperature combustion. CO comes from low excess air levels. If your O2 is still 4% it does not seem as though you should have high CO levels. Maybe your burners are not getting their proper distribution of air, or they are not mixing fuel and air properly.

 

[rmw]

You CAN have CO with excess O2 although it is unusual. CO is an indication of unburned (partially combusted) fuel. If you have a poorly tuned burner and pass fuel through the combustion zone without it going to complete combustion you will get CO no matter how much O2 there is nearby. As stated above, NOx is a function of flame temperature. So you could literally have zones in your flame envelope that are starving for air while others are getting all that is needed and producing high flame temperatures in that zone. The secret to complete combustion is to get good mixing. The secret to reducing NOx is to reduce flame temperature.

Sorry, but it sounds to me like tuning that many burners in that many zones would be a tinkers nightmare.

Your H2O is a product of combustion (and whatever there is in the atmospheric air) so your combustion calculations should predict pretty accurately what you should expect.

rmw

 

[jyro]

RMW wrote--- "it sounds to me like tuning that many burners in that many zones would be a tinkers nightmare."

Luck for me, I don't have to tune individual burners.
They all are connected to their respective zone controls. Something I discovered today, this furnace has VF drives on the entering rolls and the exit rolls, when starting and stoping the pneumatic air and gas controllers in 2 zones react to the VF drives turning on and off. The I/P converters recieve a 4-20ma signal from a plc and send out a 3-15 psi control presure to the valve operators, the 3-15 presure takes a jump when either conveyor starts and stops. The flow is measured by an orifice plate and the ratio gets off when this happens. I'll change those 4 I/p converters and see what happens. I know about the effects of RF from the VF drives, the 4-20 line is in it's own conduit and the varible speed motors are run with the correct VF wire. The only space they share is under the floor in the drive room.

 

[rmw]

Also check your pneumatic valves for "stiction" which is a term for the fact that a pneumatic valve at rest takes a certain pressure change in order to break it away and start it moving. If that "stiction" is too high, then your response from the valve is a step function as well. In a valve with packing, I can change the control system characteristic by cranking down good and tight on the packing. If yours is a dirty process and the valve stems have drug a lot of airborne debris into the packing, the stems could be sticking when called on to respond.

rmw