Wood Heat for Greenhouses

Farm Energy March 23, 2012|Print
Outdoor wood boilers are increasingly subject to state and local and regulations aimed at minimizing air pollution. Photo courtesy of Vern Grubinger

 

Introduction

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 basics

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.

Emissions and air quality

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.

Fuel quantity

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:

Amount of Wood Fuel Needed to Replace Fossil Fuel
#2 fuel oil (gal) propane (gal) natural gas (therm) log wood (cord-128 cu.ft.) wood chips (ton-2000 lb)
10,000 15,000 13,800 74 194
20,000 30,000 27,600 148 389
30,000 45,000 41,400 222 584
40,000 60,000 55,200 296 778
50,000 75,000 69,000 370 973

(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.)

Equipment selection

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:

  • Size of system. The unit should be sized to offset the heat losses. Too large a unit may create inefficiencies in fuel usage and excess smoke and pollution. The installation of modular units will allow for expansion of the growing area and greater fuel efficiency during mild weather. Because of this, it is often better to have two small units than one large one.
  • Furnace (hot air) or boiler (hot water). Most units are boilers as it is easier to get the heat where you need it with a heated water system. Water temperature can be modulated for root zone heating.
  • Firewood or chips. There is little savings to be gained if you have to pay the homeowner rate. Solid wood or chips may be available from landscapers or arborists at little or no cost but requires time to get it sized to fit the firebox. Larger firewood units require handling several hundred pounds of wood a day. Chips and sawdust are delivered in bulk and are automatically fed to the firebox, saving you the trouble of feeding the fire manually. A high quality, sustainable, and affordable supply source is needed.
  • Indoor or outdoor location. Placing the combustion unit inside the greenhouse or headhouse results in shorter supply piping. An indoor installation also provides the benefit of heating the house with heat lost through the appliance's jacket that would otherwise be lost to the outside air. An outdoor installation can be located close to the wood storage. It also keeps smoke away from the greenhouse. When installing indoors, one needs to pay special attention to changes in draft conditions based on other operations in the house (e.g. ventilation fans.) A sealed combustion unit in which combustion air is ducted from outside to the burner helps relieve this situation.
  • Lined or unlined firebox. A firebrick lined firebox will usually burn hotter, create less smoke and be more efficient than an unlined one, especially if it has a water jacket.
  • Gasification. In these units, the volatiles are driven off in an oxygen-deprived chamber and then moved through a burner nozzle where they are superheated and mixed with air for complete and even combustion.
  • Natural or forced draft. The chimney on a natural draft unit needs to be tall to entrain combustion air and promote adequate draw on the combustion exhaust. A forced draft maintains a hotter, more efficient fire and decreases smoke, creosote and ashes. This reduces the need for a water jacket with a large capacity because temperature recovery time is reduced.
  • Primary and secondary air. Choose a unit that provides both primary and secondary air to the fire. Many new designs have electronic controls that regulate the rate of firing, draft inducers that provide the right amount of air, heat storage that absorbs extra heat and heat exchangers to capture the heat of combustion before it escapes up the chimney.
  • Dual fuel capability. Some units are available with fossil fuel burners for starting the wood and also providing backup if the solid fuel fire goes out.
  • Will the unit meet local and state codes? Larger units usually have to meet emission codes for particulate matter, carbon dioxide and other pollutants. In some states, outdoor wood furnace installation and operation are regulated, especially when their capacity exceeds a certain size.

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.

Additional Resources for Greenhouse Energy Conservation and Efficiency

  • Virtual Grower, from USDA, is a decision support tool for greenhouse growers. Using this downloaded software, you can "virtually" build a greenhouse with a variety of materials for roofs and sidewalls, design the greenhouse style, schedule temperature set points throughout the year, and predict heating costs for over 230 sites within the US. Different heating and scheduling scenarios can be predicted with few inputs.

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