Co-Firing wood and Coal

[JEB66]

I am part of a team studying the feasibility of co-firing wood into our coal fired fleet for the expected CO2 regulations. When we can, I plan on squeezing in dedicated burners. But some of our units do not have the space for dedicated burners. So I wish to find some details/pifalls of how wood (dry sawdust) can be injected into a pulverized coal transport line.

I know this is being done in Europe, but as far as I can tell it is not done in the US. I see issues with the fuel/ air ratio of the coal and of the wood messing with the burner. I also think that all of the burners on a mill group should get the same injection, so to keep the balance of fuel and air.

But mostly I am concerned with the possibility of fires resulting of coal layout near the injection point. How do you prevent layout/pluggage and keep the coal from going into the wood line.

Is there anyone out there with real life expierences?

 

[SlideRuleEra]

Contact Santee Cooper

http://www.santeecooper.org/greenpower/newsroom/releases/news_2005_0901.html

http://www.SlideRuleEra.net

 

[rmw]

Burning wood chips and burning dry sawdust are like burning apples and oranges.

What kind of a boiler are you trying to do this with? Or that is to say what kind of burners, wall fired, or corner fired?

rmw

 

[JEB66]

The plan is to study the feasibility of burning 10% heat input of sawdust on a 1300 MW, a 800 MW and a 600 MW boiler. We will take in all kinds of wood and process it all to sawdust. so it is sawdust to be burned. These are all wall fired dry bottom supercritical units.

This will not be pretty, to feed 1 800 MW unit 10% wood will need 13 trucks an hour, 10 hours a day for 6 days a week.

 

[rmw]

Since sawdust is by definition created by sawing wood and I doubt that you are going to process the wood you take in by sawing it, please describe just how you intend to "process" it. And, define what you mean by "dry" sawdust whatever the 'sawdust' ends up being.

In other words, what will the wood consistency and moisture content be as fired?

I am not being picky. Depending upon what type of wood you are bringing in, as grown/chipped/sawn vs. dry waste, and how you 'process' it down to be able to fire it, the firing mechanisms are vastly different and will bear on the "tips" that I might offer in this thread.

rmw

 

[JEB66]

Sorry, I took so long getting back to you, RMW.

the woody products delivered to the sight will be dryed down to 11% moisture and 0.75 and then ground down to the 0.03inch to 0.040inch range. I do not have a distribution, like I would see on a traditional coal pulverizer.

 

[mizzoueng]

What kind of burners are you currently using? I would assume this is at one plant, 3 boilers, so you may have three different types of burners here.

Also, what type of pulverizers/mills are you currently operating?

Can you post a semi-ultimate analysis of this sawdust?

 

[JEB66]

I cannot figure out how to copy and paste in this forum. I am really not looking forward to typing all that junk in here. Any body know how to past in pictures?

Let me ask you Mizzoueng, what does the type of mills have to do with this question?

 

[mizzoueng]

Primary air flow rates, air to fuel ratio

does it go through a cyclone, do you feed the air through the air heater first?

Do you just have a primary air fan (b4 pulverizer) or is there a pulverizer fan (b4 pulverizer) and a primary air fan (after pulverizer)?

To paste pictures, you need to load the pics on a hosting serivce like photobucket or something and then you can post pics. The pics need to be online before you can post them on the board.

 

[JEB66]

My understanding is that in Europe, they use a rotary lock valve and a blower to move the powered sawdust to the burner line. No preheating of the air. depending upon the distance and other losses, you would only have the heat of compression for warmiong the air.

So I have been wondering about the effect of the coal air on the velocity. I do not want fuel to lay out in the burner.

This injection scheme that is being studied does not send the wood through the mills, but after the mills and before the burners. We did not want to put the wood through the mills, because wood does not fracture like coal. We are choosing the primary option as a separate dedicated wood burner. But there is not a lot of free wall space. So to get to our atrgeted % heat input, this European scheme keeps popping up.

Our Low NOx Burners are designed around 2 to 2.2 lb air to lb coal, so I would expect the overall mix of wood/coal and air to be around that value. Changing that ratio will cause a change in emissions. That is something that will just have to be delt with, like the corrosive effects of burning wood and plants.

I really have not gotten comfortable with the idea of mixing these two streams together in the burner line. The folks doing the study say they will show me some examples of how they do this in europe. I am just hoping to think through this idea well enough so that we do not blow the place up. I am worried about lay up and interlocks. I was also hoping for a few other concerns.

I am not tring to design the system yet, The economics are not there to drive this. I just want to make sure the big problems are identified.


Typical analysis shows me 3% ash, 47% Carbon, 6% H2, 0.3 N2, 0.02% Cl, 0.04% S, and about 43% O2.

 

[rmw]

The type of wood burning that your are describing with the type of wood you say you will have that I have seen was where pulverized bone dry waste (even dryer than what you will have) was injected into wood burning furnaces where the primary solid fuel was 50% water (or more). So while the boiler struggled to keep the wet wood burning steadily, the dry waste burners just sat there and ran and looked like gas burners (blue flame in a sea of yellow.)

It actually had to be treated like burning gas with respect to its explosiveness.

As to injecting the wood and burning it, I don't see any problem. As to where to recommend to you to inject it, I haven't made up my mind. I am not comfortable with injecting it into the pulverized coal burners or the supply piping between the burner and the mills. Not saying that it can't be done, I just haven't put my mind around it yet. Balancing the pressures to prevent back flow in either direction would be a nightmare.

And, I'm not sure if the transport velocities would give good mixing of the coal with the wood and about issues regarding the ignition temperatures (and hence the ignition points) of the respective fuels.

Sawdust, if it is truly sawdust and not just nominal "sawdust" because it is hogged up wood waste contains silica from dirt from dragging logs through woods and from wind blowing dirt into the bark as it grows.

Silica and other such "attached" components can change the ash fusion temperature of the coal and cause or exacerbate clinkering. Sand and dirt in the ash can erode boiler and furnace wall tubing. Check that the velocities in your tube banks aren't so high that erosion would be a problem. Something as simple as erosion shields which are often used in coal fired boilers might be indicated.

I keep going back to a dedicated line(s). They wouldn't be very large diameter, so I don't see how you would have a problem fitting them in around the boiler. Have you considered putting them in a wall adjacent to the wall(s) with the coal burners?

Also, what is your back end package? Will the constituents of the wood ash have any detrimental effects on the precipitator if you have one? What would they do to the ash resistivity? Could be that it would have beneficial effects, but it has to be looked at.

What about ash sales? Would those constituents in either the bottom ash or the fly ash give you any problems with selling either one of them assuming that you could be.

I am following this thread and will only contribute when I think I can make a contribution. But I'm still out here.

rmw

 

[JEB66]

rmw
Thanks for your comments. I am right with you on all this and I have brought up those same issues (for the most part).

In Europe, they supposedly inject in the line without issue. So I must be open to the idea. But I struggle with how they do it. So I keep asking how.

The units which are getting dedicated burners are going into the sidewalls. They need to go in the middle elevation of burners because of me. I felt the bottom would allow the large peices of wood to drop out be unburnt in the ash hopper. and our founding father had this magic ratio of MW to ft3 furnace, so we have tiny boxes. That made me feel that there was not enough time for complete burn if they were on top.

I was expecting a injector, if you will, for the dedicated wood burners. But all of the designs I have been shown and found from overseas is a large complicated burner. Not unlike a coal burner. So there just is not much space available on the F and R wall. I have some units that have a wraparound windbox and secondary air ducting (in layers) on the sidewalls. Surely we could make some holes and go through, but that would be a maintenance nightmare for casing leaks.

The study focuses on using waste from cabinet making kind of processes. and then ground on site. So it should be the woody part of the tree (no bark and alkalies...) But the purchasing spec will address this too.

I thank you for your thoughts.

 

[rmw]

You must have a heck of a lot of cabinet makers in the area to be able to contemplate using that as a source. Wood is a solid and has to be hauled, so unless you have a large producer of good quality waste or even poor quality waste if you have the capability to burn it, forget the idea.

Now, regarding burners, I have seen a very successful design for bone dry waste (from inside the process where I saw it-strand board) where the burners resembled the cyclone burners that B&W used to be so famous for. They even slagged the ash and spit it out like the coal cyclone burners did.

It will be a while before I can remember who made them if I even can, but they worked well. You might want to seek out some wood products industry trade publications or go to some of their trade conventions. They all burn wood in its various forms.

rmw

 

[JEB66]

it is not just cabinets, it sawmills, pallets and other stuff. But we did go out 100 miles from the unit just to get the 10 % heat input. All of those trucks bringing in the wood will save the planet. If this comes to be reality, I do not know what I am gonna do. I have 40,000MW of coal fired generation to deal with.

Our buddies in Europe are not helping. In the Netherlands they get their wood for co fire generation from Western Canada. I am sure thos trains and boats go a long way to reduce CO2 in the atmosphere.

An now that G. W. mentioned switchgrsass...if I we to get 10 % heat input into one of my 1300 MW units, I will need 15 acres/hour. I am going to need to annex Canada. And all the labor to harvest the grass will solve the illeagle problem.

 

[ccfowler]

JEB66,

I agree with rmw. The fuel source that you are considering is going to be very expensive. Your worst problems are probably going to be the logistics of transportation, storage, processing, and finally, ash disposal. It seems as though you will need to contract with most of the cabinet shops in the country to maintain your required feed rate.

Dedicated trucks and rail cars will be a necessity. You can expect dreadful problems with contamination from "junk" in the source material alone without adding in more unrelated "junk" picked up in transportation. Some truck manufacturer will probably be very happy with the size of the fleet that you will be needing.

You may need to consider a broader range of wood sources and more processing of the chips, sawdust, etc. to maintain a more consistent fuel quality while drawing on a smaller geographic region. If you currently have a market for your coal ash, it will almost certainly go away as soon as the wood sourced fuel is introduced. Ash disposal is likely to be another ugly issue. Existing disposal permits for coal ash will almost certainly have to be re-evaluated by regulatory agencies.

My guess is that staying with all coal will look progressively more and more attractive as your study continues, but the study should be very interesting. Perhaps you will get lucky and will come up with a really great system.

 

[JEB66]

RMW,
Can you give me a manufacture of the injetors you mentioned? The burners I have been shown are bigger than the coal coal burners, for the same BTUs. They are nice burners with spin vanes, dampers and diffusers, but big.

As far as the back end. I have not seen any constituents that will be any worse then burning PRB. I think the biggest problem is the heat rate effect whcih will raise all my rates for a load. drying and grinding all that wood is a big aux power. Another way to save the planet.

 

[rmw]

JEB66,

I have been racking my mind to try to remember the name of the manufacturer of those burners and it has almost come to me several times. I can only remember that they are from Kansas, I think. I did the combustion controls for the units and I remember wondering where they got wood burning experience from in Kansas. Kansas doesn't have a single tree does it? (No flames from Kansas, now.)

You said something else in an earlier post about limited furnace volume as I understood it, and that gives me pause. Wood firing has established requirements for furnace volumes with respect to heat release rates. I'd have to open up some books to see if there is any significant difference, but I'll leave that to you. Steam by B&W is what I would look at, but it is at the office. I'd check it out.

I have seen wood and coal co-fired, but normally the coal is the incremental fuel with wood being the primary fuel. Most wood burning systems are designed for specific types and categories. Example, wet wood such as fresh bark or fresh "as sawed" sawdust is stoker or spreader fired over grates. Bone dry sander dust is fired like and burns like natural gas. The cyclone burners I mentioned were for particle board and strand board waste and trim, which is dry, but which has a lot of the phenolic resins in it which tend to slag up real bad.

(I'll never forget the picture I have in my mind of the boiler operator in a plywood plant who had a Melroe Bobcat on one end of a choker and the other end around a clinker inside a boiler furnace through an access door and he was trying to break up the clinker into small enough pieces to get out through the 18" wide door.)

Your thoughts about burning wood from a variety of sources gives me real pause.

I don't know what part of the country you are located in, but I suggest that you look up some industries in your area that are burning wood and go visit them; both to see how they fire the wood, and how they handle the material up to the burner. Paper mills are a real good place to start, as are saw mills, plywood plants, particle board plants, or OSB plants. Each of the ones mentioned do it a little bit differently.

Now, based on your comments about saving the planet, A) burning the wood returns the carbon to the atmosphere, while land-filling sequesters the carbon IMHO. In addition, B)the contribution to the atmosphere of all the truck and train exhaust that is going to be required to get the material to your boilers, not to mention the aux power that you bring up also cancel out a lot of any benefit gained by firing the wood instead of coal.

I'll keep trying to remember the name of the burner and if I get it, I'll post it. It will come to me, question is, when.

rmw

 

[rmw]

JEB66,

Danged if I ain't proud of myself. It wasn't due to my memory, because I don't think I would have gotten there via that route, but a little googling around got me there. This is who I think it is;

http://www.m-e-c.com/

Check their burner page.

And this is where I got there from;

http://www.thomasnet.com/products/rotary-dryers-24783201-1.html

There are more there any of which might have the burner technology you are looking for.

rmw

 

[tommy75]

Jeb66,

Wow, you've kinda opened a can of worms on this one buts I believe it could be a good can! So, I've been burning wood for roughly 10 years now and I can tell you it's alot of work. So let me throw some things at you and we can move from there...

1. How exactly do you plan to turn waste wood into injectible sawdust? All of our woodchips are either processed through a tub grinder or horizontal mill on-site of the wood job then trucked to our plant. The next step is to season the wood for roughly 30 to 60 days in a pile that gets an internal temperature of 180F very prone to fires. Once your fuel is dried a hammermill is an expensive piece to run with tramp metal, concrete, etc. tearing up hammers and grates left and right.

2. Once you have processed into sawdust (from now on only to be referred to as "ROCKET FUEL") I suppose that an appropriatly sized blower and pulsation damper would do you just fine (Detroit-Stoker) but wall tube box's and associated piping changes are costly.

3. Why, wood is hard to get and not to mention EXPENSIVE. When your paying $25.00 to $40.00 a bone dry ton for spec chips it makes the reality of it seem not so fun, not to mention with all the wood fired boilers in Canadas green houses they are really whoring up the wood market per-say.

4. If this still sounds like a good idea I would be more than happy to share my experience with you!!


Happy Holidays All