Controls on Combination Bark/Gas Boilers

[mpullin]

I have a controls issue with a bark and gas fired boiler. I new at this so I need a little help. Our operators are running the boiler with bark air wide open at all times. I was given the assignment to redesign that system. I've trying to find control schemes on the web and have not had any luck. I don't believe this practice is good for the efficiency of the boiler. If anybody has any input please send me a link, I would appreciate it.

 

[rmw]

Define what you mean by "bark air". And describe what type of furnace/stoker that you have where you are trying to do this. Do you have traveling grates, fixed grates, (no grates), or what, and what type of gas burner/registers do you have?

I don't think you are going to find combination bark/anything control schemes on the web. You are going to have to look at specialized publications like TAPPI or in specific vendor literature for vendors who specialize in those types of controls.

You have two problems here. First, bark needs lots of excess air normally speaking, while the gas burners can be regulated to within a few O2 percentage of stochiometric.

Normally there is enough excess air associated with the bark side of the process so that you can run your gas burner registers tight on air and still have enough excess air in the funace to keep from smoking. And besides, with the bark smoke, who would notice a little incomplete gas combustion smoke, not that you would want any.

Give us a few more details about what it is that you are trying to do and maybe we can help you.

rmw

 

[mpullin]

Thanks rmw for respnding to this forum. What I mean by bark air wide open is that the dampener coming off the fd fan is operating at 100% open. The type of grates are traveling grates. It is a CE four corner burner system.

 

[rmw]

OK, do you have any of estinating how the air is divided between the burner registers and under the grate? It would seem to me that one or the other is getting too much air.

Too much air under the grates picks the fuel up off the grate before it can complete combustion causing excessive carry over of light weight thumbnail sized pieces of unburned bark.

Too much air in the gas burner registers pushes the gas flame out too far into the furnace.

One or the other, and maybe both need to be pinched back. Does your control system have O2 measurement/control? If so, I have a suggestion, but won't get into it until I know if you measure O2 (and how and where.)

When you say CE four corner burners, where are the bark distributors. Are they along one or more walls. I assume that your gas burners are the corner burners. Help me visualize this. Do you have overfire air (on the walls, or in the gas burner nozzles/buckets?) Are your corner burners tilting or fixed?

rmw