Co-fire Sawdust in Boiler 1

[TCPhoenix]

I am working at a facility that wants to co-fire sawdust along with pulverized coal in the boiler. The boilers maximum capacity is 250,000 lb/hr. The boiler is front wall fired with three compartments and under normal operating conditions only operates two of them. We have an initial financial goal of obtaining 90% energy from coal and 10% energy from sawdust.

I have read a few technical documents on this subject and would like to discuss some of the details with an expert. I am looking for comments/advise/or books to read to advise me on the following points:

-Where should we inject the sawdust? (in the coal yard along with the coal, through a separate independent compartment, in the same compartment as the coal but through a different air stream, through soot blowers, through over fire air connections, or other)

-Do you recommend a fuel ratio? (sawdust/coal) As stated earlier our goal is 90-10 from an energy standpoint. Is 90-10 realistic?

-If injected separately what air/fuel ratio do you recommend for the sawdust?

-We are working with a few different sawdust providers. Are there certain moisture limits and particle size limits that our providers need to meet?

-Is glassing/slagging from the sand, a by-product of the sawdust, a concern?

-Has co-firing sawdust been done successfully in a front wall fired boiler? (the reports I have read are from tangentially fired boilers)

-Will most of my questions need to be answered via trial and error?


TCPhoenix

http://www.phoenix-engineer.com

 

[davefitz]

refer to the IEA-bioenergy website for a large number of tech papers that describe co-firing biomass in european boilers. <

http://www.ieabioenergy.com>

Placing the sawdust in the coal pile or in the coal feeder belt runs a risk of mill fires- the volatile content is too high- mill puffs are a distinct possibility above 5% heat input. Might then need to follow same rules as with PRB coal and provide a mill inerting system etc. Fine sawdust will be swept out of the mill table prior to pulverization and may exit the classifier as coarse dust due to its low density.

separate fuel nozzles to feed air-blown sawdust can be used to over 10% heat input, but if there is not good mixing in the furnace, one can get unbalanced O2 levels and high CO- if high CO levels persist to the upper furnace superheater tubes, than accelerated corrosion of the superheater can occur.

Some ash constituents of wood ash may contaminate the flyash such that it can no longer be used as a cement admixture

Some of these issues can be solved by burning chipped torrefied wood instead of raw sawdust.

 

[rmw]

Is it pure sawdust (as opposed to sawdust/bark mixed?) Is it sawdust sawdust (as opposed to other mill waste generically termed sawdust-planer shavings, etc?)

A lot of the answers to your question depend on the actual constituency of what you are going to fire. If it is wet sawdust right off of the saws, then it is too wet to put on the coal pile. The moisture will give you problems in your material handling equipment.

I would like to know more about what it is that you intend to fire before trying to get my mind around an answer. Paper Mills typically fire bark and other hogged wood fuel(similar to sawdust with respect to most of the problem areas) in combination, so you aren't inventing any wheels here.

rmw

 

[TCPhoenix]

rmw,
One of the products is described as green wood sawdust from saw mills. As received it feels dry, but there is moisture in the product. I performed a very crude test by vibrating it for 5minutes with a set of sieves. The only sieves I had were #4, #24 and #50 mesh. 100% went through #4 and 50% went through #24 mesh and none went through #50 mesh.

Our other products are much larger in size and look like mulch, we will probably shy away from these as they are much larger and will require processing.

TCPhoenix

http://www.phoenix-engineer.com

 

[rmw]

Depending on whether or not the logs being sawed were stored under water or not (sprinklers) your moisture could be as high as 55%-I've verified it to be that high. But probably never much less than about 35%. The mulch looking stuff is the bark-might be mixed somewhat with sawdust. Stay away from planer shavings as they are bone dry and have to be fired just like natural gas. I doubt that they are being offered as a waste fuel as they have value in other wood product manufacturing like particle board, etc.

If it were me, I'd inject this stuff right into the coal/primary air stream after the pulverizer as it leaves the compartment entering the furnace. I would not put it in before the pulverizers as I believe this would gum up the pulverizers.

Regarding the sand content, it is there. It gets onto the logs as they are handled through the woods and dust is blown onto the trees as they grow and it is inherent in the wood. Plan for it and its effects on the boiler with respect to erosion, presence in the ash (as davefiz has noted). The question is the wood's ash fusion temperature and where this may show up as ash build up in the boiler. I don't know what that is but I suspect that your temperatures in your coal fired furnace are such that you should be more worried about the ash fusion temperature of the coal ash rather than the wood ash.

Caution, if any of this is plywood or particle board plant waste or sawdust, beware as the phenolics in the glue can combine with the silica in the dirt in the wood to give an ash similar to molten glass. Nasty stuff.

Check your wood and wood ash constituents for their interactions with the products of combustion of the coal. Some may be beneficial and some might not. Check what the wood ash will do the precipitators.

All in all, I think what you want to do is workable based on the firing parameters alone. Now, that said, the material handling aspects of getting it conveyed up to the firing aisle and getting it injected into the coal/air stream will be a real engineering feat; sawdust wants to plug up the works at every chance it gets. But I guess that is what it is that your firm does, so good luck.

rmw

 

[macmet]

As rmw said, if the temperatures are high enough ash fusion could be a problem. This is something that is easily tested for low cost. The temperature received from testing is not going to be 100% accurate but it should give you a temperature range to stay below.

Also, testing can give you heat contents and moisture contents which could also be useful for you.

 

[dougarthur42]

just some info. Though I operate a coal fired boiler it is not pulverized, and may have a few different characteristics but may also be the same. We Co-fire bituminous coal, biomass, sawdust, petroleum coke, Tire chips, peat moss, and pulverized limestone ( as bed stabilizer). We have seen no ill effects on boiler ash, from running all coal or any mix. at 10 % fuel mix the saw dust should not effect flyash deposit in super heater lanes. because the sawdust should burn up first and hotter and more complete. I would suggest you not mix sawdust on to your coal piles. Maybe after pulverizers "Y " in the line and add sawdust. Either with a small eductor or venturi to create a small vacuum. This will be the best way to get fuel mix into boiler also.

 

[bamacoal]

The point of injection we use on our co-fired boilers is just above the coal feeders. It has its own fan and uses rotary dampers to blow bark/sawdust in. Most is burned in suspension and if we loose bark we drop the header almost immediately.

our moisture averages 35% from woodyard.
Ours is also a 250,000 lb/hr boiler and most load is from bark/sawdust.

 

[MJCronin]

Why not pay for an engineering study ?

I am sure that this has been done before....do you really want to rely on the opinions and recollections of a group of strangers ?

B&W (and others) offer boiler designs that are specifically configured for high moisture bio-fuels (bagasse, wood, bark etc.) You may be able to get away with a small percentage of sawdust ( 5-10% ??) but I would be sure to get an official evaluation done before I decided to burn large quantities...

-My opinion only

-MJC

 

[TCPhoenix]

We are in the process of gathering some quotes to perform an engineering study. Our issues is that the boiler mfg B&W is usually very expensive.

Does anyone know of any other companies that will due this type of study?

TCPhoenix

http://www.phoenix-engineer.com

 

[davefitz]

I understand KEMA of the Netherlands offers engineering studies of co-fining biomass in boilers- they are very active in european demonstrations , and have contributed tothe experience base at <

http://www.ieabioenergy.com>,

and had offered to study such applications in the USA.

 

[hacksaw]

Better do the engineering now or you'll be involved in a forensic investigation later, putting fuel in a boiler that is 5-6 stories tall is not a simple piece of work

 

[MachinaMan]

TCPhoenix,

Alstom is a direct competitor to B&W for boiler work.
They have always been good for us.

They have the type of experts that I would go to for this type of project.

Good Luck.

 

[MechBRR]

Another resource may be Energy Products of Idaho. They have a decent library of their papers on the subject, but they may be more focused on the BFB gasifier add-on approach to using biomass with coal.

 

[IFRs]

Middlebury College in VT has just commissioned a wood chip fired boiler / steam power plant. They may have some information useful to your study.