CO reduction in biomass suspension burner 1

[paburner]

Here is the problem. I have a biomass suspension burner that is producing too much carbon monoxide (CO). It runs on clean, dry, hardwood wood that has been dried and size reduced in a hammermill through 1/4" screens to a moisture content of about 9%. That fuel is blown into the suspension burner and kept in suspension by two tangental overfired air fans. It's a typical webb burner design. I can adjust the burner temperature by dampering the overfired air fans. I can also adjust the volume of air introduced by the fuel feed blower. More than 100% of the air needed for combusion is introduced with the fuel feed blower. I'm seeing CO levels of 500+ ppm. Everyone tells me that I should have very little CO, but I do. It's a 45MM BTU burner that I'm running at 20MM BTUs. Any thoughts on how I can reduce CO to meet my DEP permit levels would be desperately appreciated.

 

[rmw]

I can't offer a comprehensive answer because I have no idea what your furnace looks like but two thoughts come to mind.

One is that dry fuel burns very rapidly and sometimes can burn faster than the oxygen needed to sustain complete combustion can be replenished into the flame zone, quenching the flame before combustion is complete.

Second is that with dry fuel, the fuel is light to start with and before combustion can be completed, the fuel burns off the volatiles and leaves behind the unburned carbon making a very light cinder. Those light cinders are then picked up (swept up) and carried out of the furnace to a cooler zone of the (whatever this furnace is attached to) and the combustion is quenched before it can be completed. It is very difficult to keep these light particles from being swept up into the air flow and carried out of the furnace.

Do you have any type of cinder collection device that can collect them and reinject them? Sometimes that is good and sometimes that is bad because by the time they get collected and reinjected they are too cool to start burning again, but light enough to be picked up and swept out all over again.

My suggestion would be to try to get some overfire air into the flame zone high right about the end of the flame envelope to provide O2 to complete the combustion of the particles trying to leave the furnace.

If you have that much CO, I would suspect that you have some smoke and that is where you will see smoke coming off of your flame. I have seen that work real well for dry fuel before. If done right, that air blast can burn as blue as if you were using a gas burner.

rmw

 

[paburner]

Thanks RMW. I've included a picture of the burner for your review. I appreciate any additional thoughts you might have.

My thoughts too have been that the excess air and the overfired air are the place to focus. We recently reduced the CFM of air that was being used to carry the fuel to the burner and thus injected into the burner with positive results. We reduced the diameter of the pipe from 10" to 6" to keep the velocity high enough to transport the fuel.

I believe the air carrying the fuel has sufficient O2 for combustion and too much was inhibiting proper combustion, although I don't know why. More usually seems better, but it wasn't. Any ideas why?

Perhaps too much overfired air is also bad if it sweeps the cinder away? I thought it would have been helpful to improve the cyclonic action and keep the particles in the burner longer. The balance is hard to judge.

Interestingly, the opacity doesn't seem to be tied to the CO levels. We have perfectly clear exhaust when the CO is running wild.

Any other thoughts on what is necessary for proper combustion of wood will be appreciated. You never know when the light-bulb idea will pop up.

Thanks in advance.

 

[macmet]

Paburner,

The reason that too much excess air has a negative effect is because of what RMW said, quenching of the flame. Essentially the excess air cools the flame and you lose the temperature needed for combustion.

What is the refractory like in this system? Can you install baffles or something inside to direct flow and then inject the air to spots to encourage mixing? It's also possible that you need to extend the refractory area to maintain temperature and allow for further combustion.

I'm not familar with the Webb Burner design, but in the systems we design we use refractory to direct flow and increase mixing. I'm not sure if you can do this with a cyclonic action though.