Mobile Wood-Burning Kiln

Is a wood-fired, movable lumber drying kiln feasible? Here are some ideas. July 13, 2006

Who knows the name of the company who specializes in kiln designs and hardware that one applies to an old reefer trailer? My dream kiln would be able to be moved occasionally to be close to the processing and storage of my client's projects. It should be single phase 110 or 220 volt, wood, fuel oil, and propane compatible. I've started to talk to Koetter because they have the wood fuel burning unit, but the insulated trailer compartment is not standard to their designs. We need to set up a processing site for 1-2 years and then move our kilns home or to another site. Is anyone out there working with a similar setup?

Forum Responses
(Sawing and Drying Forum)
From contributor L:
We have built a couple at Nyle and sold units to people who intended to be portable. But they almost never move once they are setup. It is more practical to bring the lumber to the kiln instead of bringing the kiln to the lumber.

From the original questioner:
Nyle can meet my fuel requirements? I want a high efficiency wood burning option and such a system might require a large thermal storage. The Koetter hot water heat exchange systems make sense to me... Would it not be a great deal more complicated to store heat in the form of steam?

It's true that we might only move this kiln once, but our client has equipment on their site that we could use (loaders and storage buildings). So it could really help us to start up our kilning operation on their site.

From contributor L:
There are different ways of burning wood. The most common is a hand fired wood boiler such as are made by a number of good companies. They are not too friendly to the environment and if you are in a neighborhood where the year-round smoke bothers people, it can be a big problem. A fully automatic wood fired system improves on this, but is very expensive - six figures at least. An efficient wood system that rapidly burns very cleanly is the most efficient and the best for the environment, but not much seen anymore. Those stored the heat in water but this is not practical for a kiln because the high temperature in a kiln makes the amount of stored heat way too small to do any good. For example, a 1000 gallon tank of water will only store about the amount of heat in 60 pounds of wood fuel when it is used with a kiln. All the portable units we have done have either been gas fired conventional or dehumidification

From the original questioner:
Well, that is exactly what I am talking about. Most of the wood boiler systems I have seen have terrible reputations for being dirty burners, which means they are wasting my energy and creating pollution to boot. I know the technology for clean burning has only been around a few hundred years. I'm surprised that there is no demand among wood fuel users for this. I want to manage my waste by burning it as cleanly as possible. I realize that wood will be a secondary fuel, so the system would have to have some way to overlap use of more than one fuel. Am I missing something?

From contributor U:
If you do use an old reefer trailer, be sure to allow sufficient space for airflow, and if drying from green you will need extra fans. Blowing air through a single 4 ft wide stack of lumber is less efficient of fan capacity than if you could have 2 or 3 stacks deep. I would put some thought into distribution of the air leaving the dehumidifier or heater as a long narrow kiln load can easily result in variation of relative humidity and temperature throughout the load. If you use a dehumidifier in a well insulated enclosure, you will only need auxiliary heat during the initial warm-up of the charge. Years ago I used a portable kerosene heater for this purpose (think fire safety).

If you are hoping to burn waste wood, consider the payback on the cost of the wood burner needed to burn the “free” fuel. We burn wood for heat, but not for the kilns - simply a matter of economics.

Think about how you will handle the lumber. Are you planning a cart loaded kiln? Drying oak from green will cause tremendous corrosion in a kiln, so choose components accordingly. I’ve done a lot of experimenting with this kind of thing in the past, and experimenting can be rather expensive in finance and time. Ask advice.

From contributor R:
Have you thought about using a genset, duel fueled with diesel or gas, and a gasifier (woodgas), to operate a dehumidification kiln?

From contributor L:
There are only a few places in the country where making electricity with a gen set is not quite a lot more expensive than using public power. Burning wood for warm-up with a dehumidification kiln might save as much as a penny a board foot. That is a lot of hassle for that little return. But if you have another use for the wood burner, then it might make good sense.

Back in the 70's energy crisis, Prof Dick Hill developed a downdraft burner that burned wood cleanly and you only needed a dry vent because there was no smoke and it was very clean. The wood burned quickly and the heat was stored in a large tank that was usually made of insulation board and swimming pool liner. Then the hot water would be pumped to heat the house. In a house you can get heat from the water as it cools to the house temp, say from 180 to 70 degrees, but in a kiln, you can only use the heat until it gets down to about 10 degrees above the kiln temp. That means a huge storage. Several manufacturers licensed the Hill boiler, but it petered out when oil became cheap again.

From the original questioner:
Contributor L, you are right on the mark. I am (still) looking for a company that specializes in this reefer container design so I don't have to reinvent the wheel with this kiln.

The only gasification designs I have seen are for after the end of the world; I can't imagine those engines running very long on smoke. If you know a company that has a small scale gasification unit and engine that runs on it, please let me know.

As for interior corrosion, since much of our stock will be on the thick side, we will be drying from green often, so before we start we should reseal the interior with paint.

Contributor L, such a burner as you described could be direct vented into the kiln; heat storage for later use seems to be the problem. Thanks for the help.

From Professor Gene Wengert, forum technical advisor:
There have been several designs using a two stage burner, which is very clean burning indeed. The emissions are only carbon dioxide and water... no smoke (creosote). Such burners require excess air, as typically wood is burned without enough air, so there is incomplete combustion, which leads to the formation of creosote, which we call smoke. The chemicals in smoke (the creosote) are so heavy that they fall to the ground rapidly, and do not result in pollution, as they are more or less naturally occurring chemicals (often found in high concentrations after a forest fire). Note that commercial wood burning systems (certainly boilers with 50 hp and larger burners) are very clean burning (unless they are malfunctioning or need maintenance). Smaller burners can also be efficient and clean if there is enough air added or if a two stage burner is used. As contributor L says, the price of wood burning is not low, and then making power from it (unless there are government tax credits) is not economical today, especially on the small scale.

Virginia Tech had a small 2-stage gasifier. It ran a diesel engine that could then be connected to a generator or other equipment. The only requirement is that the gasifier be close to the engine to prevent condensation of the gas and that when it was shut down, regular diesel fuel had to be used for a few minutes to clean the fuel system. If shut down with wood gas, the gas would condense and cause slug buildup. It is a neat system, but not economical with the low price of fuel here in the USA, even today. Remember that such a system has to carry its fuel. Also, remember that with any wood burner, you generate ash that must be safely disposed of. The ash will have high concentrations of certain metals that would be a pollutant if the ash is put in one spot.