Sawdust-Fueled Boilers

      Practical considerations around burning sawmill sawdust for heat. October 4, 2007

Question
I'm wondering about a sawdust boiler for my Wood-Mizer sawdust. Wood-Mizer is expected to come out with a boiler this year, but knowing them, it will be 20k. I'm not worried about drying the sawdust - I can figure that out. I'm more interested in the boiler issue. Any ideas on research?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Burning green sawdust is commonly done throughout the industry. I am not sure what questions or research you have in mind. Can you please clarify?



From contributor J:
Burning anything has become a hot politically charged issue in some states, like California. Some of the burners available for wood waste are gasifier units; they burn and re-burn the waste gases, and they are really efficient, but need permits and such regulation to operate legally. It's a headache to use them unless you determine that the regulations are worthwhile to your operation.

I have seen some home built units, and wouldn't sleep at night without being next to them at all times. Sawdust - any powder - is explosive. So with the static electricity issues and the fact that there is fire and blowers, you can see the safety issues. As with anything, if you're going to do it do it right, cutting corners isn't worth it.



From contributor C:
It is my understanding that the stove would cost in the neighborhood of 6 to 8 grand.

Sawdust is not explosive unless it is blown in the air at a certain low moisture content and a spark ignites it. Moisture content of sawdust for these stoves can be in the 30 plus percent range also. No need for drying. I have looked at the 2 prototype burners and the system looked a lot safer than any outdoor wood furnace I have seen. Also they burn smokeless after a few seconds of start up. Will burn wood flour all the way up to circle sawdust. Also, depending on the hopper size, you could go 3 days or more before refilling. A big enough hopper and you could go 1 week.

With such a wide range of particle size and moisture content of feedstock, these stoves look to be well worth looking into. One should never assume what a company will charge for something. It can lead to bad decisions.



From contributor J:
I might have misunderstood... You want to use a wood fired boiler to heat a kiln? Or to produce electricity?

One unit has the capacity to consume the gases that are produced by burning above the 1200 degree range... Higher temp equals better, cleaner burning. Some of the units don't last long at these temperatures. They burn through the firebox. Shorter lifespan, etc.

Getting a unit that can handle the higher temps for sustained times - and you can manage the feedstock - that is the best. For electricity production, I didn't know of a smaller unit produced in the USA, only higher capacity electric generation co-gen units. Very spendy.

As for dusty environments being explosive, it only takes a spark. Sounds like a camp song. I have seen coco powder explode. Not pretty. Sawdust and flour will do the same. Static and temp, moisture, etc. You're bound to run into one or all the ignitable environments in your operation. Just be careful.



From contributor G:
I too would be interested in a sawdust burner. Dry sawdust, in my case. I just returned from Germany where I toured two shops. One was a 3 man shop and the other a 16 man shop. Both burned their sawdust for heat. From the less than perfect translation I got, it seems the burners are very common and very expensive, but well worth it considering the high price of fossil fuel there. Their burners were German or European made and seemed to be very safe and efficient.


From Professor Gene Wengert, forum technical advisor:
There are a tremendous number of wood burners in the industry and I have yet to hear of one exploding. The fuel does catch fire from time to time before it enters the burner, but huge explosions, as suggested previously, are rare, if they do occur at all.

As suggested, it is important to use proven technology. A boiler does require state licenses and may even require a licensed operator 24/7. Do your homework to find out what your state requires. Also, check with your insurance company to find out what requirements they may have.

Also, you will have to have a storage facility that is heated in cold weather to prevent freezing. Further, green waste bridges, so feed systems will have to be fairly complex.
In short, small scale wood powered boilers are very expensive. Larger boilers will require an engineer to design the total system. It is not a DIY project.



From Professor Gene Wengert, forum technical advisor:
Particleboard plants and OSB plants dry small wood particles. I was unable to find any reports of explosions from the dust in these operations. Fires were reported from time to time.


From contributor J:
Explosions? Wood flour and static electricity. I have seen the systems that convey wood chips, dust and flour and an issue that is always brought up in the safety meetings is the proper handling of the material. One broken 90 degree elbow in a plant (it leaked out wood flour) into a storage room, exploded when a motor turned on. It happens...

Some boiler operations use pneumatic piping for carrying the fuel. I have also seen the Rube Goldburg setup with a big pile of ground up wood waste no farther than three feet from a burner, complete with ash spilled on the floor.

Now in most plants, we use T's instead of 90degree elbows, where the other end of the T is packed with the material being blown, the product hits the packed material and blows into the 90 degree. It handles the wear and greatly increases the functional life of the bend.

In a shop situation, I can see using the blower system to blow all the wood waste into the cyclone and then into an auger to feed the boiler, or co-gen plant.



From contributor C:
Wood-Mizer is not coming out with a boiler. It is simply a stove to burn sawdust that heats water similar to other outdoor wood furnaces. This furnace is to heat homes and buildings.


From the original questioner:
Thanks everybody - I'm just looking for a way to burn the sawdust. How about this idea? Run the sawdust through a hammermill to pulverize it. I have an old farm feed grindermill. Run it over a screen and then press the powder into log size pellets maybe 4'' diameter by 24'' long. I have an outdoor furnace already (Aquatherm).


From Professor Gene Wengert, forum technical advisor:
I have used a regular feed mill to pelletize wood. It works best with a softer (low density) species.


From contributor J:
Why pulverize a product that is already sawdust? Knife grinders or particulators can make wood flour. Contact your burner manufacturer and see what the requirements for your burner are. Auger systems to feed the boiler, limiting switches to keep from overfilling. These are readily available and if you're sort of handy, easy to install. (Cost for maintenance of equipment increases greatly when you make a smaller product out of a larger one.) I operate wood waste grinders, and we contract grind plastic for a decking company. Full boards down to 1/4" minus. If your burner can handle the material right off the saw, you're better off.


From contributor R:
I was at the Wood-Mizer 25th anniversary celebration in Wausau last weekend and got the chance to see the "Bio-Mizer". I also got the chance to talk with Jeff Laskowski. As I walked up to the Bio-Mizer on a brisk Saturday morning, the main thing I noticed coming out of its short stack was... well, nothing. There was no smoke being emitted from the stack - it burns the sawdust quite efficiently, it would seem. This is quite different from a pellet burner or wood stove. It is an air to air heat exchanger and will utilize the same type of nozzle as an ordinary household LP or natural gas furnace to heat up the chamber where the sawdust is ignited. Once the chamber has reached temperature, the alternate fuel is added and it uses that fuel continuously until it is gone. Once it is gone, the LP or natural gas is then again used. Alternate fuels will include virtually any biomass, i.e. chicken manure, wood shavings, etc. Apparently, there is virtually no dust or ash, as the combustion chamber is so hot. I am not an engineer by trade, so I don't know all of the logistics. The wood dust they were using on display was dried. It will use wet sawdust, but the downside is it just uses more of it, which may or may not matter since sawdust is very plentiful. To dry theirs, they put their dust in a narrow type gunny-sack, and hung it up to air dry. The one million BTU model will sell for the approximate $20k price tag.


From the original questioner:
Thanks for the info. It looks like the residential model will be $6-8k. That's a fair price, I think.


From contributor J:
To dry the sawdust, if you're already making heat, wrap water jacket piping around an enclosed auger (then insulate it). That boiling water will heat the jacket to a sawdust drying temperature pretty quick. Depending on the slow low gearing of your auger, to get the sawdust to be dry, you might need a longer auger. Depending on the volume that you are consuming.

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