Firewood kiln
Methods and facilities used to dry firewood. November 26, 2001
Question
What's the most economical method of building a kiln for quick drying oak firewood? Preferably, it will be a wood-fired system.
Forum Responses
I have quite often thought about buying an old insulated tractor-trailer. I run a very large generator all the time, so I could put ducts inside the trailer and channel the air from the radiator. Kiln drying firewood wouldn't be anywhere as detailed as lumber, so a few big fans inside to move the air would work good. I have never done it, so I don't know how it would work exactly. The reason I aimed for a trailer is because I run a Lx985 New Holland skidsteer. It would fit right inside to push the wood. I assume a forced air furnace would work the same as my generator.
Regarding the insulated semi trailer, wouldn't you still need to take the moisture out of the environment somehow, while still keeping the temperature up and the air moving? I have been wondering the same thing as our pre-pay propane is going to be .90 cents a gallon this year, so I think it will be another good year for firewood selling. One big firewood operator here uses those cheap hoop house greenhouses and packs them full of firewood in the spring and early summer, and has good dry wood for the late fall.
You need to consider airflow. Air does not go through piles of firewood. It goes around them. The pieces on the surface of the pile will dry very quickly while the other material will take forever to dry.
Looks like a perfect application for a DH type kiln. I know it's not wood-fired, but there is a lot of surface area exposed in split firewood and the condenser sets right in the chamber where the moisture can be removed via a drain line. Sawmill and Woodlot Magazine did a story on these a few issues back. Shoot - with the payback they are showing for seasoned firewood, it shouldn't take long to recoup the investment.
I have seen many of these operations. The key is moving lots of air through the pile of wood. You'll need as much heat as you can get, if you want to process the wood quickly, and a venting system to remove the moisture from the air. Because of this, I don't think DH would be an economical method.
Mother Nature is your best economic solution. Moving firewood is not saving any money. We have been selling firewood for years in western PA and had no issues with it not being dried. The shortest time to dry is about 3 months. Cover the pile with a large tarp (solar kiln) and allow the bottom edge of the tarp to flap in the breeze (vents). Wood that is very dry (kilned) burns very fast, even oak. A little moisture in the wood controls the rate of burn. If you are very concerned about the dryness of your wood, let it sit in the pile for a year.
DH is not really a good choice. It is not designed for fast crude drying with lots of splits and checks. We have supplied equipment for a number of steam-heated firewood dryers and they are pretty simple. The best way is to have a wood fired boiler and run it at about 30 psig so the temperatures can be over 225F. Burn the culls and scraps and it makes sense economically.
This summer I have dried 8,000 bf of hardwood lumber down to 12% moisture content in a simple 12' x 20' plastic covered frame. The walls are 6 mil black plastic and the roof is the corrugated clear material, though one or two layers of clear 6 mil could be used. I also have two household dehumidifiers running that I found at the town transfer station. They each needed about 1 hour of work but they run just fine. I have a baffle that holds several small household fans and creates a good draft between the stickers.
Running cost per 4000 bf of hardwood is $150. Running time from 25% down to 12% is 6 weeks. The dehumidifiers shut off during 100F + temperatures, but then all through the night the water just pours out. My stack is 6' x 20' x 5' height. I think that if you stack your split wood with even air spacing throughout the load, it will work just fine.
That is a lot of money to spend on energy to dry 4000 BF from 25% to 12%. If you used that approach to dry firewood, you would be out of the market. If you are drying firewood to be packaged and sold in retail stores, you have to get it hot enough to kill any bugs. That is why air-drying is not adequate for this market. DH takes too long to get that hot (it will never with residential systems). If you intend to operate in this market, there really is no option but to install a direct fired, hot oil or steam kiln.
Every large-scale firewood kiln I know of is high temperature (200 degrees F) with no regard to what would be considered "degrade" in lumber. I would get an old reefer trailer and set it up with a sawdust burner and/or waste wood burner. The name of the game is high airflow speeds, and high temperatures.
