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Outdoor Wood Boilers

Outdoor Wood Boilers & Chip/Pellet-fired Boilers

This note deals with boilers intended for installation either outdoors or in a dedicated building which is separate from the structures to be heated. They normally supply space heating and/or domestic hot water heating, and may be used for a range of residential and small commercial/industrial applications.

Heated water from the boiler is pumped through underground pipes to the building(s) where it either passes through a heat exchanger, releasing it; heat to a forced air stream, or is distributed to various areas supplying heat to baseboard or wall registers or through floor-heating coils before being sent back to the boiler through a second bank of underground pipes for reheating.

Outdoor boilers using firewood typically have the potential for very high levels of pollutant emissions and low efficiencies.

On the other hand, those boilers with automated chip or pellet systems may have much lower emission levels while efficiencies are improved.

Current status:

Outdoor boilers are typically a package including some form of weather protection and a relatively short venting system. Insulated piping moves hot water to buildings in the vicinity of the boiler, where the heat is supplied directly for heating domestic water or through a water to air heat exchanger for space heating. The need to go outdoors to refuel the boiler places an emphasis on long burn times, which leads to comparatively large fireboxes and, often, poor combustion. Manufacturers run the gamut of size and combustion expertise.

Outdoor boilers using normal firewood are widely available and common in rural areas. However, they generally have high emissions which have seriously impaired their acceptability, to the point where they are severely restricted or banned in parts of North America. Many manufacturers are working to develop improved systems as well as the necessary testing protocols to allow them to systematically improve their product and demonstrate this improvement.

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, 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 is reported using a wide range of test protocols, 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.

Outdoor wood boilers typically have a large firebox to maximize refuelling intervals. When combined with the large variation in heat demand over the heating season, many units end up operating at low output for much of the time. Manufacturers face the challenge of developing units which can burn cleanly, with minimal smoke and emissions, under these load conditions.

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 fence line maintenance or bushlot management activities, which may result in a decreased fuel costs in comparison to conventional heating sources. Equipment required for firewood processing is already present on most farms or commonly available at reasonable cost and harvesting and fuel preparation techniques are straightforward.

Firewood can be purchased in stores, or from firewood dealers listed in the local Yellow Pages. 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 gain e more knowledge and experience 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, traditionally 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 between $4.00 to $6.00. Corn pellets and kernel- 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. 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:

Manual outdoor boiler systems are sold in the 50 to 280 kW range. The great majority of these are sold for rural residential use although they have found application in commercial and warehouse buildings. They are delivered pre-assembled with shelter and water distribution systems. The unit is installed apart from the building to be heated, which allows convenient adjacent fuel storage and removes any fire hazard from the building. The firebox, which is relatively airtight, is fed manually with solid wood. An integral hot water jacket transfers heat for distribution. Heat output is controlled through air intake management. Ash removal is manual, and draft is natural. Estimates of installed costs are low and are around $100 / kW, which is competitive with conventional fuel systems.

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. Some outdoor boilers produce so much smoke and air pollution that numerous rural municipalities in Canada have either considered banning them or are proceeding to do so. In particular emissions are usually uncontrolled and at times can be at unacceptable limits because of factors such as:

the possibility of high moisture content in the chunk wood fuel;
the batch nature of the fuel supply combined with manual control of intake air can result in a wide range of combustion conditions
a cooling water jacket may keep fuel from attaining complete combustion.

Most outdoor boilers create a lot of smoke during operation for the following reasons:

They lack internal features that promote complete combustion
The relatively cool boiler surfaces quench the flame
Large firebox size promotes the use large pieces of unsplit, unseasoned firewood that does not burn efficiently
The units selected are often too large for the heat load, which results in smouldering which produce high level of smoke emissions
Their on/off operating cycle means excessive combustion which can result in high creosote pollution
They can smoulder for long periods when used to produce domestic hot water during the summer

Although wood smoke can be a potential annoyance to neighbours and a major health concern to the owners of a wood fired system, it can be controlled though 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 appliances except in British Colombia. Many manufacturers are well 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 determination of emissions and efficiency of all types of wood-burning 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.

There are no technical reasons preventing the development of very low emission outdoor firewood boilers. Meaningful emission and efficiency test methods and regulations should aid consumers in identifying high performance units, and manufacturers in improving their product. When this happens, the current bad image these products have is likely to change dramatically.

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. As fuel supplies continue to improve and conventional fuel costs rise, they are likely to enjoy increasing acceptance.