Sawdust as fuel

      Building a sawdust-fired boiler to heat kiln and shop. November 18, 2002

Question
We are considering burning our sawdust using radiant heat for a dry kiln. The kiln will hold approximately 12,000 bd. ft. Does anyone else use this type of heat as an alternative to expensive electric or gas? If so, are you burning sawdust pellets or just sawdust? Is it difficult to regulate sawdust as a fuel? Would this form of heat be acceptable to the EPA?

Forum Responses
From contributor R:
I have an outside wood burner for heating my house. One day I thought I would gather up a garbage bag of sawdust and throw it in the furnace. It ignited like I threw in a gallon of gas. Just a thought.



From contributor D:
It is often done in large operations where the feeding can be done automatically. However, by the time you get it installed, an automatic feeding sawdust boiler will cost in the six figures, way too much for a 12,000 BF kiln. I am not sure what you mean by radiant heating, but if you are talking about a radiant floor, it is not a good choice in a kiln. Pellets would be more expensive to run than an electric dehumidification kiln.

As for the EPA, unless you are in a sensitive area, it is not likely you would have any problem with this size system unless you did have some crude system that blew sparks into the air or produced excessive smoke. If you buy a commercial system such as a Messersmith (sp?), you should be fine.



From contributor S:
My buddy used to use workshop waste (primarily sawdust and planer shavings) in his outdoor wood-fired boiler to heat his shop and house. Worked great. Only one warning: if your stove has a fan to force air into the stove to increase the quality of the burn, don't turn it on until the door is shut.


How about boxing the sawdust up to toss into the boiler? Would be way safer than shoveling it in.


From contributor R:
I've thought about it more than a few times. There doesn't seem to be anything practical (cost effective) at the small scale end. I had thought of a small auger feed, somewhat along the style of a pellet stove feed, but the dust issue would be hard to deal with. Someone I know was messing around with sawdust and wax, but I think he was just trying to make an entry into the firelighter biz - during one of the wood burning stove comeback eras. Also, just trying to *dry* the stuff to the right level would probably be hazardous enough.


From contributor S:
My buddy just used an aluminum shovel and kept the dust in barrels. Intensive, but I think it increased his burn time by a couple of hours when used in conjunction with solid wood.


From the original questioner:
What I meant by radiant heat is that I am building a wood-fired boiler. I purchased a tank that is 7 feet in diameter and 20 feet long. I also have a smaller stainless steel tank that will be welded into the larger tank with a door similar to a Taylor woodstove, only on a much larger scale. The tank is actually much larger than what I would need to heat a 12,000 bd ft kiln, but I'm building it with the idea that I can always add another kiln without constructing another boiler. The sawdust will be automatically fed with a chain (variable speed motor for burn control). The chain will drop the shavings into a pipe, which will then be blown into the firebox. The chain and blower will be controlled by a thermostat. Radiator type heaters will be placed near the ceiling of the dry kiln with fans blowing air over the hot coils.

I haven't checked the pricing on commercial models, but frankly I hope they do cost six figures, as I have spent less than a thousand dollars for the two tanks I mentioned earlier. Saving six figures will be the equivalent of making six figures. I'm estimating that I will have less than 2500.00 in our heating system, which will also heat our workshop. We spent in the neighborhood of 400.00 per month last winter buying kerosene and it was still cold! Anyone see any holes in my idea?



From contributor B:
I used to burn coal in a proper coal burning stove and each night would add a shovel-full or two of sawdust on top of the already burning bed of coals. This worked really well in terms of extending the life of the fire through the night. However, I needed a strong fire with air flow delivered from below for this to work (many coal stoves can deliver air through the shaker grates). Too much sawdust would smother the coal fire without that good bottom draft.

When I tried sawdust alone, I couldn't keep it burning. But with a coal or log fire below, it worked well. In a large scale burning situation like yours, I'd be curious to learn if an already strongly burning bed of sawdust would keep the fire going as new dust is added. In my situation, the sawdust actually burned from the bottom of the pile upward rather than from the top down.



From contributor D:
Just a reminder... don't buy the unit heaters like they use in garages, etc. The fan motors won't stand up in a kiln and the steel cases rust fast in that environment. The coil ratings are greatly reduced because of the vastly different delta T. Use finned pipe and regular kiln fans.


From contributor R:
The "holes" are likely to be in the implementation more than the ideas themselves. I fear the risk of burn-back along the feed path, and as dust accumulates (and it will) this becomes an explosive risk. I might look into the fire log processors again - the ones that compress sawdust, paper and other wood refuse into three inch or so diameter fake logs - they may mix in something else as a binder, perhaps wax.


One concern about wood waste is keeping it working 24 hours a day, 7 days a week. This means either a lot of your labor or an automatic system. Also, you will need to store enough fuel for a 4 day weekend, plus you will need fuel when business is slow. In other words, fuel storage is important. Also, the fuel must be a form that you can use - dust, small hunks, etc - so some sort of fuel grinder may be necessary. (Dust is very explosive, as mentioned.) The MC must also not be too high or the fuel will freeze. Also, you need a good feeder that will stop the fire from going up the feed system. The feeder needs to control the quantity.

The EPA is not really a problem; rather the EPA clean air laws are enforced by the individual states. Often the fines will be levied from the time the infraction occurred, and not just from the day they find the problem. Just because you are small does not mean they will not look for you. Of course, with a wood burning system, you will also have to get different insurance. A "business" fire will not be covered by home owners, etc.

And the list goes on. In short, small wood burning is not really a good idea without a substantial investment.

Gene Wengert, forum technical advisor



I've built a few controllers for sawdust augers. You input water temperature or steam pressure into a loop controller. Take the output of the controller to input a freq drive. Run the auger motor proportionally with the freq drive. These systems can work very well.

Systems burning wet dust need induced draft. You can mix dry and wet in a hopper and burn without an induced draft. If you burn dry dust, you can add sprays to prevent burn-back. Wet or dry, you can add gaps to the feed with drops from auger to auger.



From the original questioner:
Gene made a very good point about the fuel storage. We have been paying a guy to leave a semi-trailer in which our sawdust was blown into. He would take the sawdust to another milling operation where they would burn it in their boiler system. This is another expense that I will eliminate using this system. We added a 30 x 12 room with a 14 ft ceiling onto our warehouse to accommodate the dust.

Someone once told me that the EPA doesn't frown on burning sawdust as long as you're using it for heat. I would tend to think it wouldn't be as hard on the environment as burning used oil or other petroleum products that I've seen in mechanics' garages and other shops. I'm purely guessing that this will be enough to keep it running through the 4 day weekends.



From contributor B:
Beware of spontaneous combustion! I don't see why, if it can happen with hay, it can't happen with wood dust.


It sounds like you are creating a pressure vessel. State and local laws may govern the periodic inspection of this.


I have two Taylor hot water furnaces. I use one to heat my 3,000 sq ft house and the other to heat my kiln. During the day in the cold of the winter I fuel each stove with sawdust from my Mobile Dimension Sawmills, which are circle saws. Of course, the material is much coarser than one gets from band mills, therefore I would think that the explosiveness of the dust may be far less likely. I use the sawdust during the day when necessary and during the dark hours, I use solid wood. The openings for each are 2' x 2'x 3'. In each unit, if I was to not fill them with solid wood for the evening and just use the dust, I would have more than enough residue to ignite the fire for the following day. You can also get these units so that they have a supplemental fuel, such as natural gas, propane, or a used oil burner system. I have a used oil furnace in a building that is 32 x 120 and it does real fine for heating. I also can acquire all of the used oil I want for nothing, as most places have to pay to have it removed.


From the original questioner:
I don't think this unit could be considered a pressure vessel since the water capsule will be vented. No pressure involved. The stove simply heats the water, which is circulated through various radiator type heaters. My moulder used electric motors instead of steam engines!

All the sawdust we have is from kiln-dried lumber. It's a mixture of planer shavings and shavings from the moulder. We built a similar heating system a few years ago. Looked a lot like a Taylor. We experimented with sawdust shavings, which were basically put into the stove in a bulk manner. The shavings burned completely but we noticed that we didn't generate as much heat as we needed. I think the sawdust would burn much better if it were blown into an already hot fire in a small quantity but in a consistent manner. I do know of one local furniture company that burns their sawdust just to get rid of it. Seems like a waste of energy to me, considering they don't use the heat from it. I would also be interested in a method of making sawdust pellets. Does anyone know where I can get information on how the pellets are processed?



I too looked at this for quite some time. What stopped me from implementing something was:
1. Without pressure, I wouldn't be able to get the water hot enough to set pitch, thus expensive system required.
2. A system without insurance is too risky and non-certified systems will be uninsurable, thus expensive system required.
3. In BC you can burn dirty, filthy fires of brush and muddy stumps with gallons of smoky diesel accelerant, but if you want to use the energy for something useful, you must have the cleanest fire around, thus expensive system required.


Hot water for kilns can and has been used. However, with the lower temperatures (under 212 F) without pressure, you will need substantially more radiating surfaces (fin pipe) to achieve hot kiln temperatures.

Gene Wengert, forum technical advisor



The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor H:
Sure, raising the pressure of water will allow you to attain a hotter temperature by raising the boiling point, but though oil is a little messier to handle and slower to attain its temperature, it has a couple benefits that water doesn't. I think it has a higher boiling point and shouldn't corrode. I might be wrong, but in my biodiesel attempt, it seemed the oil remained hot longer than a water-antifreeze mix would have. To heat oil is a little less efficient than water from my reading. I think the number was 18% less efficient, but if you have access to excess, then the price is negligible.



Comment from contributor Y:
Dried sawdust is used as fuel in a great number of furniture factories, and most often, for their drying kilns. Steam is most often the heat delivery medium because of its heat energy transfer capabilities. The dry sawdust is blown into the combustion firebox, similar to a large coal burning powerhouse. Most also have provisions to burn their small end cuts and such and most have a fuel gas burner also for backup and starting. You will have to comply with EPA and Boiler Code standards, though. With as many furniture factories that have closed, a boiler system such as described should be available somewhere. Also you mentioned the Taylor-style wood burner. Taylor and the other manufacturers will openly tell you that their unit is not made to combust any sawdust. Their combustion air delivery systems cannot force air into a pile of the stuff for good combustion. But you sure can briquette the stuff and burn it in 'em, and no, you do not need a bonding agent if you use enough pressure to release the lignin in the wood particles.


Comment from contributor Z:
My grandfather heated a very large three-story home with coarse sawdust from a circular saw. The sawdust was green coniferous, and was fed into the furnace at a steep angle in a hopper that held 24 five gallon pails of sawdust. With all air flow shut down this would last 24 hours. Wide open, which it most often was (this helped prevent chimney clogging), it would last about 7 or 8 hours.

Coarse green sawdust simply won't backfire, and poses no threat of fire creeping back up the feed. Clean and stick free, it won't hang up, but even a small stick will cause problems. Empty hoppers will sometimes smoke if left empty with a closed chimney damper.

They also used a sawdust hopper on the cook stove, which used about 4 five gallon pails per 8 or 10 hours. That stove was considered the best for sawdust, and was called a "Leader". I believe they all had home built hoppers, as the factory ones held less and hung up all the time. The most successful hoppers were steep sided, and built by a local tin smith.



Comment from contributor F:
I have a 9,000 b/f kiln. While I do not burn the sawdust, I do utilize all the slab wood that comes off the mill. Heat from burning wood is as dry as popcorn; no de-humidifier is needed. The kiln is a well-sealed building with a hatch behind the stove for fresh air and one at the opposite corner for exhaust. (This one has a small fan mounted in it.) The stove makes it simple to maintain about 110 degrees F in the kiln except for a week of the drying process in which I crank it to 140 degrees F to kill the bugs. There is a humidistat to turn on the exhaust fan when the humidity hits 85%. On the rare occaision I have pine in the kiln, I have to supplement the heat with a torpedo heater in the last week of drying to get the temp to 185 degrees F to set the pitch. This heater draws outside air, so it must be done in the last week when the wood is already nearly dry so as not to check all the lumber. The sawdust (from my circular mill) is sold to people with horses as bedding in stalls. (Just don't include black walnut - it rots their hooves.) If you have a bandsaw mill, there are pellet manufacturers who will pay for hardwood sawdust. If you are a small mill and are sawing pine, I have to ask why...


Comment from contributor M:
I work at a mill that uses an energex for the dry-kilns. Our sawdust and wood shavings are collected from the planer mill and collected into a large hopper. The sawdust goes from the hopper to a hammer mill - this pulvurizes the sawdust into a flour-like powder. When the energex is first started, a jet of natural gas is used inside the burn chamber. Once the chamber gets to a certain temperature, air is blown into the burn chamber and the gas is turned off. Then I guess air is blown past the burn chamber and this is how the air is heated to dry the wood - saving the company a load of cash!


Comment from contributor E:
The area where I live has lots of private mills that are finding themselves at odds with the government because of the mounds of sawdust they produce. I too was wondering how this could be used as fuel. My research found that the only units designed specifically for this are known as "fluid bed" combustors or "cyclonic" burners. These units tend to be both large and expensive. However, as the technology
improves, these units may become more compact. Unfortunately, as with pellet style burners, mechanical draft and feed systems are needed. Perhaps you may be able to learn of and adapt this technology to your home made boiler.


Comment from contributor V:
Sawdust is regularly used as a fuel in many biomass plants, from small heating plants to utilities. The design of the combustion system is dependant upon the qualities of sawdust to be burned. What is the moisture content of the sawdust? What is the maximum and minimum size? I am currently working on a design for a new biomass heating plant with three hybrid boilers using green sawdust (40-60% moisture). Green sawdust has half the heating value of dry sawdust (4000 BTU/lb versus 8000 BTU/lb). The fuel is cheap, but the capital cost of the boiler and material handling equipment is very expensive.

The big hole in the idea of a homemade boiler built for only a few thousand is that I expect it will violate numerous design codes and be a very large safety risk. I doubt your insurance company will cover you in the event of a mishap. The original question notes that a "boiler" is to be built, and not a furnace. Boilers are pressure vessels and must be designed to the ASME Boiler and Pressure Vessel Code. This is done in the interest of safety, as many people have been injured and killed due to shortcuts in design, construction, and operation.



Comment from contributor A:
I used to be a field engineer for Energex, who made waste product fired burners for direct firing of process applications like dry kilns and rotary dryers. The burners I worked with were as small as 15 million BTU/hr up to 60 Million BTU/hr. I know one of my fellow field engineers made one at home that was about the size of a coffee can.

Most of our customers were people like Weyerhaeuser, L&P, Georgia Pacific, Broyhill Furniture and others.

These burners burned clean and were the answer to the old teepee burners the EPA banned back in the 70's. They had about a 5:1 turn down ratio and were very responsive. Key components were a burn chamber that had combustion air wipe around the outer brick before entering at a tangent to the inner burner surface. This kept the outside burner surface cool and preheated the combustion air.

To give some rough scaling, the 27 million BTU/hr burner was about 6 feet high and about 3 feet inside diameter with about an 18 inch choked exit. The air would enter at several holes up and down the burner. I believe three rows with 4 holes 90 degrees apart. Inside there would be two vortices - one wiping the inner wall moving from the bottom up to the head and a very tight vortex exiting down, counter to the incoming air, and out the choke. The spinning vortex would keep the heavy particles to the outside, where they would gasify with the combustion taking place in the inner vortex.

Fuel was fed via a rotary airlock into a two inch line feed by a roots blower. A differential pressure transmitter would measure the pressure drop across this mixing tee. The output was proportional to the bulk density of the fuel being fed and was used to adjust the combustion air so the air/fuel ratio could be maintained to sustain combustion. I believe that the combustion blower was around 50 or 70 HP for this size burner, but I'm not sure.

The last time I worked with one was back in the early 80's. Used to get $300/day plus all expenses paid. Seemed like a lot back then, even though the companies were saving about $100,000/month in natural gas expenses.

Funny thing is that most mills didn't put them in to save money, but because they couldn't find an easy way to get rid of their scrap sander dust and planer shavings and trimmings.

I have no idea what happened to the company, but at the time I was one of three field experts in the US and Canada. We didn't have PLC's, but used timer relays and math modules. Would be a whole lot easier to control the process today.



Comment from contributor P:
At a milk processing plant that I worked at, we had a Foster Wheeler trash grate boiler which burnt sawdust both wet (sodden) and dry. The boiler was made in 1917 and is still in good repair.

Trash grates are sometimes called "step grates" as they are primarily set up like stair treads at about 30 degrees of slope. At the bottom of the step grate is a smaller vertically barred grate. Other than a good natural draft created by a 60 - 70 foot chimney, nothing else is needed.

The sawdust burns on both the upper and underside of the grate. The volatile gasses given off by the sawdust above are burmed under a brick arch which acts as a reverbatory furnace, further toasting the sawdust. By the time it has collapsed down into the bottom of the grate, it is just a mass of glowing coals which then hit the air flowing up through the vertically barred grate and erupt into a suspended mass of flame burning with a white flame that is hard to look at. The technology is old, but it works like a dream.



Comment from contributor T:
My dad has a cylindrical wood burning stove from Turkey which can also burn sawdust. There is a removable steel bucket in which the fuel burns. You have to place a drain pipe or something similar down the middle, pack sawdust around it and then carefully withdraw the pipe. The sawdust is then lit with a paper taper and burns steadily from the middle outwards. The heat output is not as good as wood, but is fine for background heat. The idea is you have several buckets and just switch them over when it runs out. Cool or what?


Comment from contributor G:
It would seem that sawdust of consistent size and fuel grade (species) would allow for a simple control system to dry a consistent volume of wood from consistent moisture. Notice the word consistency, as you deviate from your design, you will introduce variables, which require a more sophisticated control system. The wood species (grain tightness and how fast it gives off moisture) would ideally be the only variable, as it would be more easily adjusted for by varying the dry times.

If you are pursuing a clean burn efficient fire, you need to design the system for certain volumes. You will need a lot of air and fuel, while also paying attention to the firebox size. There are ratios to be met.

The concept is good though, a high surface area fuel with high BTU rating being air injected into a good fire box. All electrical plants that are coal based grind the coal to a fine dust and air inject it into the boiler space. The injection ports look like flamethrowers.



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