Wood Gasification

Winter 2016-2017
Diagram of a wood gasification boiler
A wood gasification boiler. Wood is burned in the firebox (top), and gases travel downward and are burned at 1,800 to 2,000 F in the ceramic chamber below. The hot gases then pass through a fire-tube heat exchanger to transfer heat to water stored in a large tank.

Flue temperatures are usually under 350 F, and there is no creosote. Wood must be dry (preferably two years old). From Eko-Vimar Orlanski Installation, Operation and Maintenance Manual, https://www.newhorizoncorp.com/PDF/ekomanual.pdf; used with permission.

Using a cutting crib enables quick, efficient bucking of many small-diameter logs and branches at a time. This method is recommended only for those trained in chainsaw safety and experienced with using a chainsaw.

Be extremely careful when cutting small branches, and put the largest ones on top, as small pieces can get thrown out, toward the sawyer. Keep the saw revved up, as a slow chain can catch on small wood and jerk it toward the sawyer. Photos courtesy of the author

Properly dried and burned wood is an excellent green fuel for rural heating. Wood gasification is a process by which wood is burned at extremely high temperature to create syngas. This fuel can power furnaces, stoves and even cars. In the winter months, it is a great alternative fuel source.

Why use wood?

Wood is the ideal fuel for rural heating in Maine, especially if you own a woodlot. Firewood harvests are an opportunity to improve the forest when you remove dead, dying, diseased and poorly formed trees. This enables residual trees to grow faster, produce more oxygen and use more CO2 greenhouse gases.

If you are a gardener, wood ash adds calcium, potassium, other nutrients and bio-char to the soil. BUT make sure to apply these only after and according to recommendations of a soil test, as wood ashes can raise the soil pH quickly and excessively).

Buying wood from a local supplier is also far better than buying pellets from afar and minimizes consumption of motor fuels. It also provides local employment and keeps money in the local economy.

Wood gasification

Wood preparation

Since freshly cut wood may be 60 percent water, the key to minimizing wood cutting, splitting and stacking is to let it dry for at least a year. If you don’t, you burn about 40 percent of your wood just to drive off water – no heat results. Most stoves burn with 40 to 60 percent efficiency, and outdoor wood boilers usually get 30 to 50 percent. Meanwhile a wood gasifier gets 80 to 92 percent – but the key is dry wood.

After a year, wood moisture content may be 20 to 35 percent; after two years, 10 to 20 percent. My gasifier calls for 15 to 25 percent moisture for maximum efficiency, so I dry wood for two years and recently completed a solar wood dryer to try to cut drying time.

How to gasify wood

With gasification, wood is burned initially in a conventional firebox, then gases are directed to a ceramic combustion chamber where temperatures reach 1,800 to 2,000 F. All gases and tars are burned, no smoke comes from the chimney and the chimney stays clean. Despite the high gasification chamber temperatures, by the time the gases pass through the boiler’s fire-tube heat exchanger, flue temperatures may hit 350 F.

My flue temperatures are usually under 250, indicating the efficiency of the fire tube heat exchanger. If wood is too wet, the fire is cooled and white steam exits the chimney. Since wood gasification boilers are mass produced in Europe, they are much cheaper than domestic models. Mine was made in Poland.

I now heat my 1,400-square-foot house, basement and domestic water from fall until late spring on about 3-1/2 cords. Before installing the boiler, my house was 1,000 square feet, and with a wood stove for heat, I burned 3-1/2 cords plus 150 to 200 gallons of oil for domestic hot water.

Heat production

In cold weather I build one fire a day and keep it going for about eight hours. The key to efficiency is dry wood and a hot, fast burn. My small, 85,000 BTU boiler heats water stored in a 500-gallon propane tank, and this water circulates, on demand, to heat living spaces and domestic water. With year-old wood, my tank reached a maximum temperature of 170 degrees, but with really dry wood, it gets to 180, which greatly increases the BTUs.

One fire is good for a day during Maine winters but has lasted for two to three days in the mild fall of 2015. With oil prices down, I burned oil rather than wood in the summer; one fire might supply domestic hot water for a week, but a lot of tank heat would be lost to the basement. A large family taking many showers and doing much laundry would likely benefit from a weekly burn.

By Ben Hoffman

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