Is it Worth Kiln-Drying Firewood?
Considering the cost of kiln construction and energy inputs versus the value of extra-dry firewood, it's hard to see how the dollars make sense. January 27, 2008
Question
What is the most economical/quickest way to build a kiln to dry firewood?
Forum Responses
(Sawing and Drying Forum)
From contributor D:
Don't burn oil or gas. Don't do it with a hot water boiler unless you are okay with long (1-2 weeks) drying times. Wood fired steam boilers are best as a heat source with steam coils and fans in the kiln. But that is expensive even if you buy used and build it yourself. A lot depends on your planned sales amount and location and how you sell.
From Professor Gene Wengert, forum technical advisor:
Following up on contributor D's comments, in simple terms, it takes energy to remove water from wood... about 1100 BTU per pound of water and a cord of wood that weighs 6000 pounds green will have about 2500 pounds of water to be removed to achieve 20% MC or below. The only way to avoid energy costs, which can be quite expensive if you use gas, oil, or electricity, would be to use wood (but that is also not cheap as you are burning some of your raw material that you would use to make a profit). So, we are left with air-drying as the major way to remove water (and maybe a day in the kiln to heat the wood to sterilize it). Air drying is free except for the time involved.
From contributor L:
What about a solar kiln?
From contributor D:
Depends on where you are selling. If you are bagging it and selling in stores, it has to be heated enough to be sure all the bugs and molds, etc. are killed. It is also a selling point to homeowners that you are not delivering them firewood with bugs in it.
From Professor Gene Wengert, forum technical advisor:
A solar kiln similar to the lumber kilns would cost you about $1500 and would dry one cord. How many loads would you have to dry to recover this cost? Each load is going to require about 30 days of warm sunny weather... maybe 4 to 6 loads a year. Then there would also be the labor cost of loading and unloading and the cost of electricity for the fans. I would think that you would want to be a profit making organization and with a solar kiln, it might be tough.
From contributor L:
Well, it was a thought. I figured that the fans would run on solar voltaic power and they would be electric charge free. Considering it takes 7 months to season wood by air drying I would figure that 7 times this amount would be better. I'm sure you could build them cheaper than $1500, if you have materials around or don't use new. If you are thinking of drying hundreds of cords, that's a different story. You didn't mention quantity.
From the original questioner:
Has anyone ever fitted a used 20 or 40 ft container? These are 8ft wide and 7ft high. Does anyone have an idea of time to dry it out? I live in Albany NY. I want to dry 75-100 cords a year.
From Professor Gene Wengert, forum technical advisor:
Drying time depends on the amount of energy you can supply. Solar powered fans with enough power for drying operations are quite expensive.
From contributor S:
Check out firewoodkiln.com.
From the original questioner:
I looked at this site. I can buy a used container locally for $2600ish. Isn't $18,500 steep to fit this with heat and fans? Couldn't it be done more reasonably?
From Professor Gene Wengert, forum technical advisor:
The US Forest Products Lab has a report on the kiln drying time for oak firewood. Unfortunately, they kiln-dried wood that started at about 50% MC, while green red oak firewood would have been at 75% to 80% MC. They also dried the wood to 20% MC, but of course, there must have been some variation in final MC. They report a kiln drying time of 260 hours at 140 F, 90 hours at 180 F, and 30 hours at 220 F. Their kiln had a large heating system and high speed fans. If the wood had been green, times would probably be doubled, as they removed 30% MC in kiln drying, but for green wood, 60% MC would have to be removed. They used steam for heat.
I believe that it would be hard to dry firewood at less than $50 per cord, counting in capital costs, labor and energy, and also taxes and insurance. Wood that has been kiln-dried to 20% MC in a week or less will be free of insects, and so can demand a good price, compared to air-dried material. Will the price gain for kiln drying offset the expense? What about firewood that is kiln-dried in April or May or June? Will there be an immediate market or will an inventory of stock be carried until sales pick up in the fall? Cash flow will be erratic and that might cause some problems.
From contributor D:
It has to be a refrigerated container. Never try to use a standard container as a kiln or wood dryer. There is almost no way to do it that doesn't cost a lot more than starting out with a used reefer.
From the original questioner:
Thanks. Why a reefer versus a standard container? Has anyone ever used a standard container to dry wood in? What are the pros and cons?
From contributor B:
Not for sure, but I'd think it is because a reefer trailer is insulated and a standard one is not.
From contributor M:
That's why I cut today and sell next year.
From contributor J:
If you're trying to kill bugs, couldn't you just use a small propane heater, like a pilot light, to heat the chamber, get the wood to 140 degrees or higher? Vent the chamber to take off any moisture... So what if it cracks the firewood.
I believe that a refer container has a suitable floor that wouldn't corrode with the acids in the wood, dripping and eating the wood floors in an uninsulated container. I think insulated containers are made of aluminum... and the other ones are steel.
From contributor T:
If you're not dead-set on a kiln, there is a cheap way to increase air drying times. I was told about this method many years ago but haven't tried it myself. Hot sunny days will set up a chimney effect and, or so I was told, and dry the wood in as little as 3 months.
Start with a good supply of free pallets. Arrange a row in a north-south direction so it gets even sun to both sides. Stack two ricks side by side, leaving an air space between them. Reduce the air space between them as the ricks get higher until they touch at the top. Make an effort to have the top sloped away from the center. This should form a corbelled arch and be fairly stable. Finally, attach clear plastic starting just above the pallets and ending near the middle of the top. The top center should remain open to allow air flow but not so wide as to allow excess rain into the stack.
Keep the base of the pallets clear of weeds and grass to allow for good air flow. Also, you should put down a vapor barrier below the pallets to keep ground moisture out of the pile. Maybe some old 4' wide metal sheeting from your local scrapyard is in order.
