Exploring Alternative Kiln Technologies

Sawmillers try out, wonder about, and report on the Green Dri desiccant-based drying kiln from Japan. June 8, 2011

I just watched a video here on WOODWEB by green-dri kilns. They say they use Nobel Prize winning technology from Japan. They also say they can dry mixed species and mixed thicknesses. They state that the specially designed walls remove the moisture (which appear to be wood boards) and show a fan in an upper corner and a good sized vent open in the top of the unit. Is anyone familiar with this kiln? I am curious as to how it actually works.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
I hope they install a kiln so we all can see how dry wood walls will absorb moisture from wet wood and achieve 7% MC. The Nobel Prize stuff is about living cells from Johnís Hopkins. I cannot see the application to drying, but maybe we will learn.

From contributor W:
I wonder how they dry the walls out so it is ready for the next load. Physics and common sense are often a barrier to genius.

From contributor X:
I can't remember if it was here or elsewhere but I was involved in a discussion about this kiln technology and the general consensus was the same. No one had any firsthand knowledge and the company seems unable or unwilling to provide any verifiable third party testing other than the very limited one on their site with Japanese cedar.

I am just using my healthy dose of skepticism regarding any "new drying technology" because they seem to come and go. I hope they are the real deal because the claims they're making make it an attractive alternative for many species. What makes me think it might actually work as advertised, is if you look at it, it works essentially like a solar kiln but instead of using the sun they use infrared, and instead of trapping the heat and moisture and slowly venting it out, their unit allows it to slowly migrate out through the wall membranes. I wonder how the wood walls will perform long term, when species that need high temps to kill bugs are given a good dose of 130ļ plus supplemental heat for four to six hours?

I don't know how significant it is, but they make the clear distinction that their product is not a wood kiln, but a wood drier. Maybe that's what they're saying - that their unit is not intended to kill bugs, just dry species that aren't prone to infestation? Because they also note in their third party testing that Japanese cedar is termite and bug resistant.

From contributor H:
I purchased one of the Green Dri biological wood driers and put it in operation a couple of months ago. I operate a Nyle dh 200 and dry about 2000 to 3000 bf per charge. I have had the Nyle for about eight years now and use the weight samples of four boards per charge and weigh samples every day. Enough about my DH kiln.

I was not having luck finding someone that owned one. I was interested in the Greenstone statement that some of the dry times could be faster and with less heat. They stated that the wood will dry stronger and with less stress.

I am a one man Woodmizer saw mill and drying operation and I sell my rescued lumber from Florida retail. I also have a Lucas slabbing mill. I normally mill 3" table top slabs from oaks and maples and a few others. My dry time is between two and three years per slab. I wanted to speed up the drying time as cost effective as I could so I was looking for another method to dry and Vacuum kiln was too costly for my small business.

Last year I came across Greenstone lumber drier. It took some time to buy in on the process but after much discussion with the owner and some isolated testing on my own I decided to buy one of their units. My unit is 8' deep x 8' high and about 16' in length. My first load was a mix of big yellow pine timbers (10) 6"x 12" and 10' in length and on top of the timbers I placed (2) 3" x 42" wide and 100" in length laurel oak slabs that had been air dried for several months. I started the unit and in about three weeks I was getting exterior shell readings on my Lignomat moisture at 3 percent on the pine and oak. The pine was showing surface cracks and the oak was drying fine. I was satisfied that this lumber slabs was in good enough shape to work with. The pine i cut off 12" off the end to check the interior. I found the interior to be from 18mc to 30mc. my average was around 18mc was ok as I was taking the wood to be pressure treated. I did not cut the slabs in half to check the interior.

I have just finished a small load of about 500 bf of 4/4 live and laurel oak. After 18 days in the drier my eight sample boards were indicating that they were at 8 mc. I am really happy about what seems like express drying but uneasy that all is evenly dried. I decided to cut the live oak in half and check the core. I found the core to be at 12-15 percent. The exterior was indicating generally 9 to 12 mc. I am still testing with meters and I am milling some of the lumber to see how it works. Sorry this is so long, but I felt most of the story was important enough to mention. I do feel like this drier really works but canít seem to get a hold on what is happening on the inside.

From contributor X:
Maybe 18 days is not long enough. If you go another few days or week maybe they would be thoroughly dry. If they dry too fast they'll case harden but maybe your wood drier cannot dry it too fast? I'd like to know more about what kind of RH and temps you noted during the cycle. Did you use any kind of schedule or did you just load the chamber and crank up the dials?

From Gene Wengert, forum technical advisor:
Can you explain what you mean by started? Is there an energy source? Do you have any idea where the water goes? If it goes into the walls, when does it come out? Can you reach a low MC? Are you able and willing to have visitors? Do you have to sign any sort of agreement?

From Gene Wengert, forum technical advisor:
Apparently this dryer is a basic desiccant dryer using several types of desiccants in the walls, including wood. As such, desiccant drying of wood has been studied before. It is not yet clear to me if there is other technology present. Indesicant dries, there are two basic requirements: a source of energy and a means of drying out the desiccant so it can be used again. If the desiccant is merely air dried, then it will not be able to dry under 12% MC after it has been used and then dried.

From contributor X:
Is 106ļ the max without adding more lamps, or just the temp you wanted to maintain? Gene, if the chamber is relatively hotter than the ambient air, and at the end of the cycle as the moisture drops into the 6% range, won't the desiccant in the walls and the interior wood veneer also come down to that same percentage or at least right above it provided the lamps are left on long enough? If that is so, I wonder how much longer the lamps would have to stay on. I also wonder if the lamps and fans would have to be turned back on after the charge is removed just to dry the desiccant and chamber faster than if the charge we left in. I guess I could what if all day but it is a curious way to dry wood.

From Gene Wengert, forum technical advisor:
Contributor X - what you say is true in general if the desiccant releases moisture fast enough. If it releases the moisture slowly, then we would begetter off to just bring in outside air and heat it and not use a desiccant.

Here is my question: If the kiln has a temperature and relative humidity cycle that dries wood more quickly than a standard schedule, why not use these new settings with a DH kiln? If this kiln dries using air a certain temperature, RH and air flow, we should be able to use the same settings in a more conventional kiln and get the same results. Is it possible that the faster drying reported is merely because a faster drying schedule is used? This modified schedule often is possible with many kilns, if you check the drying manuals, as the basic schedule is conservative as it must work for all sorts of variations in wood quality, equipment variations, and operator skill. Schedule acceleration is encouraged.

From contributor A:
If you have a shell at 3%MC and a core at 18 to 30% MC in SYP in three weeks you have not really done much. My Nyle will do that. The walls of my kiln are raw wood and the insulation is sawdust. It does pull moisture out of the chamber when the load first starts and then puts it back later on when the wood gets so dry. The thing that worries me the most is the very dry shell and wet core. It can be prone to case hardening with hardwoods.

From contributor A:
I have dried lots of big SYP in mine. Yes it will check but it has to for the most part. Timbers like that I try to get to the 12 to 16% range for frames and I want the shell and core to be within 2% of each other. 20% is fine if the whole piece is the same.

From Gene Wengert, forum technical advisor:
Years ago (maybe 50) the US Forest Products Lab had a wooden walled kiln for the DIY operation. They also had a heated room dryer. Note that if a certain temperature and humidity condition dries wood at a given rate and quality. You should be able to duplicate the same conditions and get the same results in your DH kiln. Have you tried the new conditions in your DH kiln? You need the same velocity too.

From contributor X:
Do those times fit within any oak schedule normally associated with a DH kiln? The wood doesn't know what medium is being used to remove the water from the kiln whether air movement and venting or absorption through the walls, so I would think like Doc said, that those times should be able to be duplicated in any kind of kiln.

If there's something special about the Green Dry kiln that allows faster drying with no more defects than any kind of conventional drying, I fail to see where it's explained anywhere. The drying time for those thick slabs seems to be very fast to me, faster than anything other than vacuum drying which is sort of its own animal in my mind