September 07, 2010

Solid Biofuels - Combustion

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Small-scale Combustion of Solid Biofuels (Solid Wood)

Introduction

This note deals with wood and pellet stoves, furnaces, and boilers intended for installation within the space they are to heat. This type of unit may be fuelled with firewood, with pellets made from wood or agricultural wastes, or from grains such as corn. Wood stoves, including cookstoves, have been in use in Canada and on Canadian farms since the land was settled. This long usage can lull one into a false sense that this is an old, stagnant technology. In fact, major advances in the 1990's, particularly with wood and pellet stoves, have made wood burning significantly cleaner, more efficient and convenient. Wood burning continues to be an effective and economical way to heat spaces as either the primary heating source or as a secondary heating source to complement existing oil, gas or electric heat sources.

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. 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 by a number of sensible factors, which include:

• Buying an EPA-approved woodstove, fireplace, or fireplace insert
• 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 appliance
• Never burn garbage, plastic, particle board, plywood or any painted or treated wood products as this will release the toxic chemicals into the space

 

Certainly, as far as firewood is concerned, by far the best starting point is to have a wood stove, fireplace or fireplace insert that meets the EPA 1990 standard for emissions. The pollution from such an appliance is typically 80-90% lower than that from a non-certified appliance. Well-designed pellet stoves have the potential to be even cleaner burning, and more efficient.

The reward for making this effort is an environmentally friendly, sustainable, reliable, and often low-cost form of heating unlike any other.

Equipment availability:

Firewood fuelled equipment is readily available and requires operating skills within reach of most homeowners, as are the venting systems required for such equipment. Suppliers range from specialty retailers to "big-box" type stores. Choosing the supplier is a major decision, not only for warranty and parts reasons, but also to obtain good advice on selection, installation and operation.

Pellet-fired equipment is also readily available, and requires relatively low operating skills. As with stoves, there are a wide range of potential suppliers, and product support should, to an even greater extent, be a major consideration in choosing where to obtain a unit. Also look for one that has been tested for efficiency. A good unit should be above 70%.

Technical Background:

There are significant technical challenges which may impact the future desirability of heating with wood, as well as the market for firewood. Relative to current oil and gas/propane heating systems, firewood combustion produces significantly higher particulate, carbon monoxide (CO), and hydrocarbon (HC) emissions which can have serious human health effects. However, because wood-burning allows CO2 to be recycled, if the wood is harvested sustainably, (trees capture CO2 as they grow, then release it when burnt) wood heating can make a significant contribution to reducing greenhouse gas emissions. The challenge facing firewood-based heating systems is to reduce the harmful emissions while taking advantage of the effective zero CO2 emissions.

A great deal of effort has gone into improving the emissions characteristics of wood stoves, fireplaces and fireplace inserts. Performance standards, specifically EPA 1990 and CSA B415.1, identify the newer, cleaner burning units and have resulted in a labelling system (Fig 1) that provides information for customer comparison. It should be noted that the EPA test method does not cover central systems and most pellet stoves.

For wood stoves, wood-burning fireplaces and inserts, EPA-certified systems should always be purchased in preference to unapproved units, as they will give you a much cleaner, more efficient burn. As well, some states in the US have adopted more stringent requirements than the federal ones. Installing a stove which meets these emission levels can result in a unit with further reduced emission levels. Because so many factors determine actual emissions for individual appliances, small differences (less than 0.5 g/h) in certified emission rates are not likely to be significant. If rated emissions are close, selection can on other considerations such as operator convenience, after-sales support, etc.

An additional Canadian CSA standard covering central furnaces and boilers has been developed. At the time of preparation of this note has not yet been adopted by Canadian regulatory authorities, and few manufacturers have rated their equipment using it. When it is adopted, it should provide the consumer with an increased level of information, including a means of comparing appliance efficiencies, and cover a much broader selection of appliance types.

Using a pelletized fuel allows control of heat output based on fuel input rate rather than on restricting air supply which can result in more efficient clean-burning systems. Operator convenience is also typically a major benefit of these systems, as a fuel hopper can be periodically refuelled using relatively clean pellets. As with cord wood based systems, there is a wide variation in the emissions and efficiency of current units and systems certified to CSA B415.1 or EPA 1990 standards should be purchased in preference to unapproved units.

Fuel:

Fuel costs can be lower than conventional heating as many farms produce firewood as a consequence of fence line improvement or bush lot maintenance or have grain corn that could be used in some pellet systems. It should be noted that both firewood and pellets will require an area for storage that ensures that the wood or pellets used remain dry.

Firewood can be purchased in many stores, from large home improvement stores to grocery stores. Large quantities can be purchased from firewood dealers listed in the local Yellow Pages, or in local newspaper ads. 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!, a booklet available from the government of Canada either through its outlets or online at http://publications.gc.ca, it provides useful information to help operators become more familiar about ordering firewood. In fact, most wood is sold by the "face" cord (1.2 x 2.4 x 0.4m), or about 1/3 the size of a full "cord".

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 in the range of $4.00 to $6.00. Corn pellets and kernel-corn may be available through agricultural co-ops, or similar suppliers.

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.

Table 1: Energy Content of Wood and Pellets

Well placed wood or pellet stoves can achieve heating efficiencies ranging from 55% to 80% which is comparable to some conventional systems. Table 2 lists the typical efficiencies of many heating systems.

Table 2: Heating Efficiencies of Various Systems

In most cases, operating costs are typically higher for pellet based systems. Offsetting this is the relative ease of handling of pellet fuels, and potentially longer interval between refuelling.

Unit Selection:

There are two main types of systems currently available; "room" heaters (stoves) and central systems (furnaces or boilers). Both these types are available fuelled by firewood or pellets. A number of considerations are involved in selecting and installing a stove or central system. A Guide to Residential Wood Heating, available from the government of Canada either through its outlets , online at http://publications.gc.ca, or from many woodstove retailers offers guidance on types and sizes of systems, stove location, installation and operation. The cost of either type of system will depend on many factors such as size and heat load requirements.

The future:

The introduction of federal emission limits in the United States has resulted in a significant reduction in wood stove particulate emissions. However, technology advances have slowed since emission requirements have yet to evolve into stricter standards or be made applicable to a wider range of wood-burning equipment.

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 appliances.
• 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.

 

Additional Sources of Information:

Industries, Trade Groups and Governments

Regency Fireplace Products
http://www.regency-fire.com/

Pacific Energy
http://www.pacificenergy.net/index.php

RSF Woodburning Fireplaces
http://www.icc-rsf.com/en/fireplaces/accueil_foyer.asp

Drolet
http://www.drolet.ca/

Napoleon Quality Fireplaces (formerly Wolf Steel)
http://www.napoleonfireplaces.com/

Natural Resources Canada
http://www.canren.gc.ca/default_en.asp

Woodheat.org
http://www.woodheat.org/

Hearth Patio Barbecue Association
www.hpba.org.

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