Wood is an attractive alternative fuel for greenhouse heating – it is renewable, locally produced, and usually less expensive than other fuels. While wood is not for everyone, an increasing number of growers are turning to it as an economical heating alternative for their facilities. Fuel wood, waste wood and biomass are all potential sources of heat for greenhouses. An adequate supply at a low cost is needed to justify the cost of installing a wood-fired boiler.
Combustion of wood has three requirements: fuel, air, and heat. When all three are available in the correct proportion, combustion is self-sustaining, because the wood releases more than enough heat to initiate further burning.
Some equipment controls the amount of heat by controlling the amount of air. However, this is not a good approach because it leads to “smoldering” and the release of pollutants. Instead, it is better to utilize equipment that controls the rate of heat production by controlling the rate that fuel is fed into the fire.
Another important aspect of combustion is the energy content of the fuel. This is normally expressed in British thermal units (Btus) or Kilojoules (kJ) – one Btu is equal to 1.05 kJ. The energy content in wood is greatly affected by the moisture content and density of the wood, because the moisture does not contain stored energy – only the dry portion.
For example, hardwood and softwood at 50% moisture content (m.c.) will have about 4,700 Btu/lb (10,900 kJ/kg) whereas the same wood at 20% moisture will contain about 6,200 Btu/lb (14,400 kJ/kg). The same is true with wood chips - 4,000 Btu/lb green (50% m.c.) and 7,400 Btu/lb dry (10% m.c.). Hardwood has about twice the density of softwood and therefore twice the heat content. The same is true with wood chips – 4,000 Btu/lb (9,300 kJ.kg) green (50% m.c.) and 7,400 Btu/lb (17,200 kJ.kg) dry (10% m.c.).
Density of the wood is important if you are buying wood on a volume basis, such as by the cord (128 cubic feet). Hardwood has about twice the density of softwood and therefore twice the heat content per unit volume.
In the burning process, wood goes through three stages. In the first stage, the wood is heated to evaporate and drive off the moisture. In the second stage, starting at about 400 degrees F., the wood starts to break down chemically, and volatile matter is vaporized. The vapors contain between 50 and 60% of the heat value of the wood. These vapors have to be heated to about 1,100 degrees F. (600 degrees C.) to burn. If not, smoke is generated which can coat heat exchange surfaces and chimneys with creosote. In the third stage, once the volatile gases are released the remaining material (charcoal) burns as glowing embers at temperatures above 1,500 degrees F. All three stages can be present at the same time in a fire.
There is growing concern about the impact of wood combustion on air quality, partially due to the recent popularity of “outdoor wood boilers.” These devices can emit large quantities of smoke if they are operated with insufficient air or low burning temperatures. Growers who are concerned about pollution or are located in sensitive areas should think twice before using these devices for their facility. At very least, you should run the furnace at “high heat” as much as possible, to reduce the amount of smoke released to the air.
Automated-feed wood boilers, such as those designed to burn wood chips, are usually much cleaner burning as they control heat production by controlling the rate of fuel use, rather than by choking down the supply of air. They also can be fitted with pollution control devices such as cyclonic separators and/or bag houses, in order to meet even the most stringent air quality needs – such as at hospitals or schools. However, automated-feed boilers tend to be more expensive to install.
State and local regulations may put restrictions on the use of wood boilers in a commercial operation. Check with your local Extension agent or government representative before starting your project to avoid trouble down the road.
The amount of fuel needed depends on many factors including the heat required by the greenhouse, furnace efficiency, fuel type and moisture content. If you know your present consumption, you can estimate the firewood or chips you will need from the following table:
|#2 fuel oil (gal)||propane (gal)||natural gas (therm)||log wood (cord-128 cu.ft.)||wood chips (ton-2000 lb)|
(The values above are based on 75% heating system efficiency for fossil fuels and 70% for solid fuels. Cord wood at 20% moisture, wood chips at 45% moisture.)
A wood-fired heating system is a major investment that should be selected to give efficient and reliable operation for many years. It pays to spend a little more on the initial investment to get a unit that will reduce handling, increase efficiency and provide a safer operation. Consider the following:
Solid fuels offer a heat alternative for many growers throughout the United States. Their availability, low cost, and high heat value can replace expensive fossil fuels. Care in selection and installation is important.