Solid fuel cooking appliances such as charcoal ovens, tandoori ovens and wood-fired pizza ovens are becoming increasingly popular in restaurants these days, but it is very important to deal adequately with the fumes they emit.
These fumes emanate both from the solid fuel which is burned and also from the foodstuffs which are cooked, with the pressures to reduce these coming from two different directions:
- The need to make the kitchen a safe place to work
- The requirement to comply with air quality regulations in the local neighbourhood
Cooking is a complex process involving several different types of reaction including oxidation, pyrolysis and the Maillard reaction which is associated with browning. Cooking also leads to phase-changes, where solids become liquids and liquids become gases. Phase-changes differ from reactions in that they are reversible e.g. if animal fat turns from the solid phase to the liquid phase due to an increase in temperature, it will revert back to the solid phase when it cools down. The type of fumes released during cooking also depends on what is being cooked, with fats, proteins and carbohydrates emitting different chemical compounds.
The fumes produced from combustion also vary depending on what type of fuel is used. Wood or wood pellets largely consist of carbon and hydrogen, and therefore can theoretically be burned to produce only carbon dioxide and water. However, in reality they also produce toxic substances which are the result of incomplete combustion such as carbon monoxide and wood-tar compounds.
Charcoal differs from wood in that the wood-tar has been driven off during manufacture, however there is still a significant problem with emissions of carbon monoxide. Charcoal ovens generally vent their fumes directly into the kitchen, and even with an efficient extraction system the carbon monoxide levels can easily exceed the legal (and safe) limit. Since the effects of exposure to carbon monoxide at low levels include drowsiness and loss of concentration, giving carbon monoxide poisoning to staff who may be using sharp implements is best avoided.
Utilising catalytic converters in solid fuel cooking appliances to reduce fumes
Catalytic converters used in solid fuel cooking appliances work by oxidizing toxic compounds such as carbon monoxide and volatile organic compounds into harmless compounds like carbon dioxide and water vapour. The job of the catalytic converter is to facilitate the oxidation reaction at a lower temperature than it would otherwise occur. For example, carbon monoxide oxidises to carbon dioxide naturally at about 800oC, however with a catalytic converter the same reaction can occur at 200oC. Since this temperature is usually present in the flue gases of solid-fuel cooking appliances it is normally feasible to install the catalytic converter in the flue.
The oxidation reactions which the catalytic converter promotes are what is known as ‘exothermic’ i.e. they release heat. This means that the temperature of the flue gas usually increases as it passes through the catalytic converter, often by more than 100oC. One benefit of this is that the reactions tend to be self-sustaining, once they have got started. Another benefit is that heavier molecules such as soot and tar may be burned by the higher temperature.
Catalytic converters can only oxidise toxic substances if there is oxygen present in the flue gas, and sometimes this is in short supply, especially in the case of charcoal ovens where the aim is often to restrict the air supply to the charcoal so that it does not burn too vigorously. If there is an oxygen-deficiency in the flue-gas, the catalytic converter will not work properly. The best way to overcome this is by using ‘secondary-air’, which means introducing fresh air into the flue upstream of the catalytic converter. This secondary air can either be delivered by a pump, or drawn in by using a feature such as a venturi.
Whitebeam are happy to advise customers on which is the most suitable solution for their specific application based on our many years of experience in this field.