Co-firing PRB and Rail Road Ties in a stoker boiler...

[Bunji51]

Anyone have any experience co-firing PRB coal and rail road ties with a traveling grate stoker? One of the main issues I'm having is Coking on the surface of the grates which prevents fuel from achieving proper air flow and causes the operators to have to rake off the grates quite often.

Is this a characteristic of the PRB or the combination of the two fuels together?

Thanks

 

[rmw]

Are you spreading both fuels, or pulverizing the coal and blowing it in over the grate, and what is the size of your fuel if you are spreading? Based on no more information than you gave, I have this mental picture of a huge stack of railroad ties all piled up in the middle of your grate burning like a huge bonfire. HA.

Seriously, the RR ties, depending on their age has some preservatives in it, and if older, maybe heavier tar. If you have a lot of tar, and don't have a really hot fire on the grate, you will get some build up from that.

On the other hand, if you have a hot enough fire to completely burn the tar out of the wood, you may be running into ash fusion temperature problems with the PRB coal.

What is the grate speed?

What temperature is your combustion air, and what percentage comes through the grate?

Do you have overfire air?

We really need a lot more information here in order to start to digging into this one.

rmw

 

[macmet]

Agreed with RMW, need more information.

I'd also like to know if you have any flue gas recirculation? What % excess air are you running?

Based on your limited info, it sounds like you're having clinker issues, which I've always seen with ash fusion issues caused by excessive temperatures.

 

[rmw]

I did a lot of wood fired, stoker type, fixed grate, and traveling grate boilers many years ago when I was just a pup, and I noticed from plant to plant that the plywood plants, burning the same basic bark, sawdust and wood trim waste that a sawmill down the road (sometimes same company, same boiler types, etc) was burning, that the sawmills, paper mills, etc had minimal problems with clinkers on the grate; not that there weren't any, but just that they weren't huge issues.

The plywood plants on the other hand were such that the operators would have to take Bobcat skid steer type loaders and run cables into the furnace and "lasso" or hook the clinkers and try to jerk them hard enough to break them into smaller pieces small enough to be able to drag through the furnace clean out doors (about 18" wide). I've seen clinkers that had to be broken in several pieces before they could be dragged out of the furnace.

I always attributed that (my opinion only) to a combination of the amount of dry waste - burns hotter than wet waste - and something in the glue (phenolic compounds???) that acted to fuse the ash.

Many of the sawmill and paper mill boilers burnt dry waste with the wet bark and sawdust, but none had the problems of the plywood plants. And it was pretty universal among the plywood plants that I dealt with and that measured in the several of dozens.

RR ties are going to be dry waste to start with, and there may be something in the preservative chemicals that are cooked into the RR ties that is acting like the (supposed) glue in the plywood dry trim waste.

rmw