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Woodburners: Time to Clear the Air?

Is your woodburner efficient and environmentally friendly?
Probably not, and it might be your fault.

by J.C.F. Walker

On cold calm winter evenings, temperature inversions over Christchurch can see smoke pollution build up during the evening until it is ten or more times that of the daytime level. Anti-pollution legislation provides some control over the burning of coal and wood, new open fires are prohibited and today only "approved" woodburners can be installed. However, the designs and efficiencies of such woodburners still leave a great deal to be desired.

The effects on public health of various forms of air pollution have been debated at length. Open fires and woodburners may present risk factors for some respiratory problems and upper digestive and respiratory tract cancers. The main constituents of woodburner emissions are suspended particulate matter (soot), polycyclic organic hydrocarbons and carbon monoxide. The amounts present increase enormously as the efficiency of combustion decreases.

It has been some 15 years since the resurgence of woodburners. The first generation were better than open fires, but claims of "clean burning" should be judged by the proportion of incompletely burnt gases and wasted energy.

Of the total energy that could be delivered by the complete combustion of wood, the open fire provides a derisory 10-20%. In a simple woodburner, this rises to 40-50%; sophisticated designs can raise efficiencies as high as 85% or more.

"Approved" woodburners are not necessarily intrinsically efficient and clean burning. The approval simply means that the appliance met the compliance requirements of the Clean Air Council when tested under carefully circumscribed conditions. These restricted emissions to 35 grams of particulates in smoke per hour and imposed smoke opacity limitations of less than 20%. Today the Clean Air Act has been superseded by the Resource Management Act, and local authorities are still pondering their new strategies.

Most existing woodburners sold in New Zealand are not as efficient as they might be. In addition, current woodburners are easy to misuse, thus creating pollution far in excess of that indicated by the manufacturer. New standards introduced in Australia and New Zealand last year aim to address both concerns. They have yet to be cited by any regional council.

Geoff White of the Victorian Environmental Protection Agency says that the new standard "will result in a five to ten-fold reduction in flue gas emissions from wood heaters." It would be hard to agree with this conclusion without making two assumptions -- too many people damp the fire down and leave it to smoulder overnight, and too much poorly dried wood is being burnt. To understand the consequences of such actions it is necessary to know a little about the combustion of wood.

Wood Burning

When wood is burnt, heat is needed initially to dry the wood and raise the temperature to about 275oC, at which point the wood constituents begin to break down spontaneously (pyrolysis), releasing volatile tars and gases and leaving a small residue of charcoal. Oxygen is not necessary at this stage.

At even higher temperatures the tars and oxygenated hydrocarbons crack to simple gases. Only then do these gases need to be mixed with oxygen and burnt. Once the volatile fraction has been driven off and the flames diminish, the charcoal itself comes in contact with the in-coming air and becomes incandescent.

In an open fire, there is little burning of the volatiles, as it takes very high temperatures and good mixing with oxygen to get these gases to burn efficiently. The unregulated supply of air passing through the hot char on the grate is depleted in oxygen and lifts the unburnt volatiles away from the heat and up the chimney -- the volatiles do not get a good chance to burn. This is unfortunate as the volatiles constitute around 80% of the wood.

By enclosing the fire and turning it into a simple woodburner, the air supply can be regulated and the efficiency increased by a factor of two to three. However, if the burner is simply a metal box, a significant amount of heat is lost by conduction, convection and radiation, and the temperature of the flames is still too low for effective combustion. Further, the temptation to damp down the fire overnight by building it up and turning down the air-supply is a recipe for poor combustion and excessive emissions of unburnt flue gases. Good design overcomes these problems.

In most woodburners an adjustable supply of preheated air sweeps over the burning wood, releasing unburnt volatile gases. Although this air is quickly depleted of its oxygen, it is hot enough to break-down or pyrolyse the wood. One effective scheme is to burn the gases arising from pyrolysis in a separate insulated combustion chamber using a second source of preheated air, so that burning takes place at much higher temperatures of 1,000oC or more. Downdraft designs in which the volatile gases must pass through the hot char before being burnt are particularly clean burning. Only when the hot burnt gases escape from the combustion chamber should one extract heat to cook, to warm the house and provide hot water.

As a bonus, the heavy refractory lining of the combustion chamber stores a large amount of heat, so that even when the fuel has burnt out long ago the burner will keep the house warm overnight without any temptation to bank up the fire. To date manufacturers have shown little inclination to adopt such basic ideas, which were demonstrated in the mid-1970s.


The Consumers Institute recently tested a number of woodburners and concluded "that all the burners performed well enough for you to make a choice based on the appearance and/or price and not lose out too badly", but this assertion is questionable. Their own performance ratings of eight models showed values of efficient performance ranging from around 50% to 80% when operating at low burn, with the best model some 60% more efficient than the poorest. The differences are less dramatic at medium and high burn rates.

Quite apart from the distinction in efficiency, one needs to consider the amount of smoke produced. On a crude rule of thumb, the worst burner would have a stink factor at least two and a half times as great as the best burner. Woodburners may well operate under conditions less favourable than manufacturers and legislators intend, and pollution may be much greater than expected.

The term "efficiency" needs to be defined. It is the product of both combustion and heat transfer, and can be equated with the ratio of useful heat divided by the wood energy input. There is a general trend that woodburners operating under slow burn conditions have lower combustion efficiencies and higher heat transfer efficiencies, whereas under high burn conditions these relative efficiencies are reversed.

A woodburner inset into an existing fireplace produces only 70-80% of the heat given by an identical free-standing model. The fire will burn as efficiently, but there is a smaller surface for heat transfer, so less heat gets into the room and the benefit of an exposed flue pipe is lost.

A further complication is that heat may be given off when it is not needed, possibly as much as 30%. Finally, the heat output of the burner needs to be matched to the size of the room or house. Few homes need burners with outputs of 15 kW/hr unless they are being relit every evening when the family returns to a cold house.

A properly informed public needs details of the certified performance tests, ideally indicating the range of efficiency, particulate emission (g/hr) and burn rate (kg/hr), especially when operating correctly at the lowest burn rate. Compliance requirements could be written to ensure that a banked-up fire is unable to smoulder away all night, by ensuring that woodburners are designed with adequate secondary air to maintain good combustion.

One should note that the new "rigorous" Standard still permits smoky burning under slow combustion conditions, provided the average value meets the regulations. Further, under some operating conditions efficiencies of less than 50% are still possible, hence the need for a full disclosure of test data. The response of the industry to new standards has been largely cosmetic -- a bit of insulating firebrick and more secondary air.

Even an efficient woodburner will perform badly if the wood has not been properly dried. Some 60% of the weight of freshly felled pine is water. So much energy is needed to evaporate this moisture that the temperature in the burner is too low for efficient combustion and it burns with very little heat and copious smoke. Regulations require the moisture content of wood to not exceed 25%, usually requiring at least three months drying after cutting.

Of course, people may buy woodburners for reasons of other than efficiency. Wood fires have a primitive appeal -- the warm glow on the hearth, the security of supply against power crises; they make a simple statement that even yuppies have souls.

Modern burners are more attractive with brightly coloured baked-enamel finishes, but internally little has changed. Unless the improvement in performance is a sufficient incentive, existing relatively inefficient woodburners will remain in place for the next 20-30 years as they have few moving parts and will not wear out quickly.

In discussing winter warmth it is self-evident that the first prerequisite is to have a well-insulated house. But if people are to burn wood, then they should be aware that many existing woodburners perform quite poorly, that they are a source of major pollution -- though they pall before the open fire in that respect -- and that they have a responsibility to operate them correctly. Finally, wood suppliers, manufacturers and legislators all have an incentive and opportunity to do something about it.

Dr J.C.F. Walker is a lecturer at the University of Canterbury's School of Forestry.