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Wood Burning and the Enviroment

Wood Burning and the Enviroment

Regardless of the fuel you choose to heat your home, its use will have impacts on the environment. When it is not done properly, the burning of wood can have negativeimpacts on both indoor and outdoor air quality. Smoldering, smoky fires that produce a plume of blue-grey smoke from the chimney are the main cause of wood heat-related air pollution. Wood smoke can be harmful when it is breathed in by humans.

You can Control the amount of pollution from your wood heating activities in a number of ways.

  • Select an advanced technology stove, fireplace, furnace or boiler that is certified clean burning by the U.S. Environmental Protection Agency (EPA). These certified, clean-burning appliances reduce smoke emissions by as much as 90 per cent compared to conventional appliances.
  • Select an appliance of the correct size and, if it is a stove or fireplace, locate it in the main living area to make the most effective use of the heat it produces.
  •  Use a suitable, correctly installed chimney, sized to match to your appliance.
  •  Avoid smoldering fires by learning to use the burn techniques covered later in this book. By using these techniques, you can probably reduce the amount of smoke produced by as much as half.
  • Burn only seasoned firewood that is split to the right size for your appliance. Never burn garbage, plastics, painted or treated wood, plywood, particle board, cardboard or salt water driftwood.
  • Make your house more energy efficient so you will use less fuel to heat it. Less fuel consumption means less environmental impact.

 

Low-Emission Wood Burning Technology

Over the past 25+ years researchers and appliance designers have developed technologies that can reduce the amount of smoke and other pollutants produced by wood-burning appliances. These technologies are aimed at burning off the smoke before it leaves the firebox. This is not an easy task because to burn, the smoke must be extremely hot, have adequate oxygen available and have enough time to burn before being cooled.

Advanced technology wood-burning products have been highly successful, not just in reducing pollution and increasing the efficiency of wood heating, but also among Canadian householders who have accepted the technologies with enthusiasm. Already 40 per cent of wood stoves in use in Canada are advanced technology models. There are three different combustion technologies that successfully reduce smoke pollution: catalytic, non-catalytic and densified pellet combustion technology.

 

Catalytic Combustion

A catalyst is a substance that affects a chemical reaction without being consumed in the process. The catalysts used in wood-burning appliances are coated ceramic honeycombs through which the exhaust gas is routed. The catalytic coating lowers the ignition temperature of the gases as they pass through. This allows catalytic appliances to operate at low firing rates while still burning cleanly. Because the catalyst causes a restriction to gas flow through the appliance, these units always include a bypass damper into the flue. The damper is opened when the appliance is loaded and until a hot fire is established, then it is closed, forcing the gases through the combustor for an extended clean burn. The catalytic element degrades as it is used so it must be replaced after between two and six years of use. Catalytic combustion is used in fewer appliances now than in the past.

 

Non-Catalytic Combustion

Non-Catalytic combustion systems create the conditions necessary to burn the combustible gases without the use of catalysts. The technology has three main characteristics:

  • Firebox insulation to keep temperatures high;
  • Baffle plates to reflect heat back into the firebox, to create the gas turbulenceneeded for complete combustion and to give the gases a long and hot enough route so they will burn before being cooled.
  • A heated secondary air supply that is usually fed to the fire above the fuel bed through ducts with small holes.

When a non-catalytic stove is burning, you will often see little jets of flame coming from these small air inlet holes. This is because the combustion air is hot enough when it enters the firebox to mix with the gases and produce flames. Non-catalytic combustion has become the dominant advanced technology used in firewood burning appliances.

 

Densified Pellet Technology

Pellets are dried ground wood or other biomass waste compressed into small cylinders and larger blocks. The pressure and heat created during their production binds the pellets together without the need for additives. Pellet-burning appliances include a hopper to hold at least 20 kg (44lb.) of fuel and a screw auger to move the pellets from the hopper into the combustion chamber. Pellets usually burn cleanly because they are fed to the combustion chamber at a controlled rate and are matched with the right amount of combustion air. The larger compressed blocks can be place in the firebox and burned the same way as firewood. Pellet burning appliances are generally able to operate at lower emission levels than natural firewood appliances. The larger compressed blocks can be place in the firebox and burned the same way as firewood.

Excerpts taken from  A Guide to Residential Wood Heating handbook provided by the Canada Mortgage and Housing Corporation... Woodheat.org