Integration of Renewable Energy on Farms - Solid Biofuels

You are here: Home » Technologies » Bio-Energy - Combustion » Solid Biofuels - Central Heating

Solid Biofuels - Central Heating

Central Wood Furnaces and Boilers & Chip/Pellet-fired Boilers

This note deals with solid biomass-fired central heating furnaces and boilers intended for installation within the heated space. They normally supply space heating through a central heat distribution system - warm air ducts and registers if a furnace, and pipes and radiators/convectors if a boiler. They can be used for a range of residential and small commercial applications.

Most cordwood-fired furnaces and boilers have the potential for very high levels of pollutant emissions, creosote deposition and low efficiencies.

On the other hand, automated chip- or pellet-fired boilers may have much lower emission levels.

Current status:

Central heating with wood-fired furnaces and boilers is much less common than it was 25 years ago. There are 3 main reasons:

The advent of higher efficiency airtight woodstove,
The fact that the furnace and boiler combustion technology has generally not kept pace with the new advanced combustion generations of woodstoves, with the result of high pollutant emissions, potential for creosote deposition and chimney fires, and generally low efficiencies.
with houses becoming more and more energy efficient and being easier to heat with efficient woodstoves and fireplaces, which also have the aesthetic benefit of fire viewing

The need to be able to supply the heat demand for the whole house means the need to refuel the appliance places an emphasis on long burn times. This leads to generally large fireboxes and, most often, poor combustion.

Automated systems burning wood chips, sawdust, or wood pellets are also available. Their relative sophistication, combined with lower demand, has resulted in fewer manufacturers, who tend to be both experienced and knowledgeable. Although these systems can offer efficiency and emissions performance rivalling conventional oil and gas systems, as well as extended operation without refuelling, they also have shortcomings which include increased cost and complexity, potential difficulties procuring a fuel supply, requirement for a fairly large fuel storage facility, potential feeding problems, and the need for operator involvement.

Technical challenges:

No standard test method for determining the emissions, or the efficiency of these appliances, or regulation of performance currently exists which makes it very difficult both for manufacturers to develop a high performance product and for consumers to identify the best units on the market. Currently performance may be reported using a wide range of test protocols, if any, making it challenging to perform meaningful comparisons between units. Presently, in the United States, a test protocol is under development to standardize performance reporting and may be adopted by the US EPA.

Wood furnaces and boilers typically have a large firebox to maximize refuelling intervals. When combined with the large variation in heat demand over the heating season, most unit cycle very often, "stewing" at very low fire for much of the time. Every time the thermostat calls for heat, there is a further burst of emissions while the combustion stabilizes; similarly, each time the thermostat is satisfied, there is another burst as the air is closed down. Manufacturers face the challenge of developing units which can burn cleanly, with minimal smoke and emissions, under cyclic load conditions. This issue is particularly difficult for furnaces. Some boilers attack this issue by burning at a higher firing rate for an extended period of time, with the excess heat generated after the thermostat having been satisfied being storing in a large, well-insulated storage tank, for use the next time the thermostat asks for heat, rather than "short-cycling" the unit.

The major operational problem experienced by chip-fired systems result from the jamming of the fuel delivery system with odd size chips. This problem is minimized with good quality chips but they cost more to produce, hence to purchase. The smaller the system, the more critical this problem can become, which has tended to limit the minimum size of chip-fired systems to units able to heat two or more dwelling-size structures. Although jamming of the fuel delivery system is less of a problem with pellet fired systems, there is an external technical challenge to producing fuel at a sufficiently low cost to make overall operating costs price-competitive.

Fuel:

Firewood based systems offer the prospect of using fuel produced on the farm, sometimes waste wood from fenceline maintenance or bushlot management activities, which may result in a decreased fuel costs in comparison to conventional heating sources. Equipment required for firewood processing (chainsaw, axe/maul, mechanical splitter, hauler, etc) is already present on most farms or commonly available at reasonable cost and harvesting and fuel preparation techniques are straightforward.

Firewood can be purchased from many vendors. Sometimes large home improvement stores may be a source. More generally it can be purchased from firewood dealers listed in the local Yellow Pages or advertising in the local papers. The official unit of measurement is a cord, which measures 1.2 x 2.4 x 1.2 m. Buying Firewood? Don't Get Burned!, available from the government of Canada either through its outlets or online at http://publications.gc.ca, provides useful information to help operators become more knowledgeable about ordering firewood.

Chip-fired systems can take advantage of smaller wood stems and branches often rejected for firewood use, but require a chipper for fuel preparation. Basic chippers tend to produce a chip of non-uniform size which can cause frequent jamming of fuel feed systems, so the cost of a chipper suitable for the purpose is often significant.

Many feedstocks, including woodwaste, are potentially suitable for pellet production. While pellet stoves normally require premium pellets made from high quality wastes, larger systems are often able to operate on industrial or high ash wood pellets available at lower cost. Crop residues show promise as feedstocks for pellet production, and are worth investigating in areas where they are available. In-house pelletizing is not a realistic alternative for almost all potential users.

Bags of wood pellets can be purchased in the seasonal section of home improvement stores or through agricultural co-ops. The bags come in a range of sizes with the most common being 18.1 kg (40 lbs), which may cost approximately $4.00 - $6.00. You may also be able to buy these bags in bulk, on a pallet. Corn pellets and kernel-corn may be available through agricultural co-ops.

The amount of fuel required as well as the cost will be determined by the size of the system, its efficiency and the heating load.

When harvested, wood can have a high water content (as much as 60% or more). It should be air-dried for at least one summer season, before burning. Otherwise, most of the energy in the wood goes to boiling off the water, the combustion is poor, and emissions and creosote are very high.
There is a significant difference in the energy content of various species of firewood, with the densest woods (oak, sugar maple, beech) having ~ 30 GJ/full cord, to medium density woods (e.g. birch, ash, tamarack) having ~25 GJ/full cord, to the softwoods (poplar, spruce, pine) having ~ 18 GJ/full cord. Table 1 provides the energy content of various types of wood.

It should be noted that dedicated storage area is required to keep firewood, chips and pellets from becoming contaminated with dirt and to control moisture content.

Fuel	Energy Content
Hardwood (air dried)	30 600 MJ/cord
Softwood (air dried)	18 700 MJ/cord
Mixed hardwood (air dried)	25 000 MJ/cord
Wood Pellets	19 800 MJ/tonne

Table 1: Energy Content of Wood and Pellets

Unit Selection:

Most cordwood-fired central furnace and boiler systems are sold in the 30 to 100 kW range. While wood-burning furnaces can be stand-alone units, Combination wood-oil, wood-electric and certified add-on wood furnaces to existing conventional furnaces are the most typical. For combined units, the controls and duct systems are linked, and the heat distribution system is shared by both energy sources. A certified add-on furnace can share a chimney with its oil furnace partner, provided the chimney is certified for use with wood.
All wood-burning furnaces and boilers must be certified to meet the CSA B366 safety test standard. Installation specifics are determined during certification, and are listed in the manufacturer's instructions for the product.

In particular, for wood furnaces, the certification includes a requirement for plenum and duct clearances to combustible materials which is typically greater than for oil or gas furnaces.
When visiting units during the selection process, it is very important to ensure that the fuel being used and heat load are similar to that envisioned for the new installation, as these will have a great impact on operation. Although there is currently no standardized performance test methods, the majority of the units on the market are considered to be less efficient than similar output conventional heating sources due to inefficient combustion and distribution losses.

Wood can be a difficult fuel to burn cleanly and efficiently, and users need to be prepared to devote time and effort to proper selection and operation of this type of appliance to ensure safe and low-pollution operation. Many central heating furnaces and boilers create significant amounts of smoke, pollutant emissions and creosote during operation for the following reasons:

They lack internal advanced features that promote complete combustion now found on well-designed wood stoves
Large firebox size promotes the use large pieces of unsplit, unseasoned firewood that does not burn efficiently
The user does not use air-dried, well-split wood
The units selected are often too large for the heat load, which results in smouldering a major portion of the heating season which, in turn, produces high level of smoke emissions
Their on/off operating cycle means excessive combustion which can result in high creosote pollution
If a wood boiler is used to produce domestic hot water in the non-heating season, (not recommended as a rule) it can smoulder for long periods when used to produce domestic hot water during the summer.
Furthermore if the house is well-sealed (i.e. tight), and there are other significant air exhausting devices in the home, such as dryers, and kitchen and bathroom fans, the weak draft during the smouldering can be overcome, and the pollutants (including carbon monoxide), may spill into the house, creating a potentially serious health concern for the occupants.

To a fair degree, the above problems can be reduced through sensible operating practices which include:

Keeping the fire small and hot
Feeding it regularly with dry, well seasoned wood that has been split properly
Ensuring that the system is not overloaded and that there is air movement within the firebox
Never burn garbage, plastic, particle board, plywood or any painted or treated wood products as this will release the toxic chemicals into the space

The future:

In Canada there are currently no national emission standards for wood-burning central heating appliances. Some manufacturers are aware of the concerns surrounding their products, and have devoted considerable resources to addressing the issues. Critical to increased acceptance of wood burning are:

standards for determining emissions and efficiency of all types wood-burning furnaces and boilers.
identification of maximum acceptable emissions of critical pollutants.
a prohibition on the sale of appliances with unacceptable emission levels.
removal of existing appliances with unacceptable emission levels.

Chip-fired systems currently available offer very high performance levels. A lack of publicity, lack of standards allowing manufacturers to demonstrate the performance of their product, and failure to recognize and promote this technology has been the main impediment to growth in this area.

Pellet fired systems have all the advantages of chip fired systems plus less fuel feed system challenges. However they also face many of the same challenges mentioned above. If fuel supplies continue to improve and remain at a relatively stable price, while conventional fuel costs rise, they are likely to enjoy increasing acceptance.