Kiln Drying Firewood

From time to time the question crops up "can I speed up the seasoning of my firewood by using a kiln?" A bit more digging usually reveals a stove owner who doesn't have enough properly seasoned wood put by and is looking for a bit of a short cut to help them get through the winter.

Well inevitably the answer is yes - putting your split logs in a hot kiln with plenty of air circulation will definitely speed up drying but is the cost and hassle worth the benefit of cutting a few months off your seasoning times?

Here was discuss the pros and cons of using a kiln, as well as some special cases where it might be essential.

General principals of kiln drying

When using a kiln they tend to follow some basic principals:

• They need heat, and plenty of it. This is often provided by burning waste wood, sometimes natural gas, but could also be waste heat from another industrial process.
• They need excellent air circulation and ventilation. Heat alone won't remove moisture, there needs to be plenty of fresh dry air entering,being heated, then passing over the wood.
• Some kilns use dehumidifiers to remove moisture from the air and then recirculate the dry air.

Benefits of kiln drying

While open air seasoning is usually perfectly adequate there are some benefits to using a kiln:

• The high temperatures in a kiln kill pathogens. This is a huge benefit at a time when various diseases are wiping out forested regions. Kiln drying makes firewood safe to harvest and sell from these areas.
• If you absolutely must burn wood that is not fully seasoned, perhaps because you are new to processing your firewood and haven't yet got three years ahead, you can get jump start on the seasoning process.
• If you are a commercial firewood processor then kiln drying gives significant brand and marketing advantages - consistency of product, "premium" product status, lower land footprint and inventory needs etc...

Note that while kiln dried wood is sometimes marketed as a premium product it is no better or worse than properly seasoned firewood that has been in the open air. Don't get sucked in by the marketing!

That said - if you are running short in the middle of winter, then spending a little extra to get quality kiln dried wood, rather than fresh cut unseasoned logs is probably good value for money.

The Solar Kiln

Solar kilns are simple to construct, usually out of timber and lightweight plastics. The basic idea is build a box with a large collector area for sunlight. During the day the heat from the sun warms the stacked firewood inside and drives out moisture.

Ventilation is usually either by a small fan or using a simple solar chimney.

Solar kilns tend to be slower than wood fired kilns due to the day/night cycle but have few moving parts and need no additional fuelling once they are setup.

This site has a lot more technical information on solar drying, although it is mostly aimed at drying lumber for construction.

Wood Fired Kilns

These kilns are generally designed for commercial operation as they are expensive to build and operate. Typically they are constructed from steel shipping containers to give them sufficient durability under the high heat loads they experience.

Loads of firewood are stacked in the container and hot air is supplied from outside. Usually the air is heated by a furnace burning wood scraps - timber of too low quality to be sold as part of a finished product.

A wood fired kiln cycle can be completed in as little as 7 days, compared to 3 months for solar kilns, but require much more consistent attention to maintain correct temperature and air flow.

Kits to convert a container start from around $18,000, excluding the cost of the container