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Solid Fuel Cooking

Overview of Solid Fuel Cooking

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.

The pressures to reduce fumes come 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

Chemistry of solid fuel cooking fumes

Discover the problems and dangers caused by emissions from solid fuel ovens and grills.

The nature of emissions from wood and charcoal fired ovens

There are two different sources of fumes from solid-fuel cooking appliances – the cooking of the food and the combustion of the fuel.

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.

Minimising emissions from solid fuel cooking appliances

Discover how catalytic converters reduce fumes from wood and charcoal fired ovens.

Abatement of solid fuel cooking 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 800°C, however with a catalytic converter the same reaction can occur at 200°C. 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 100°C. 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 so the charcoal does not burn too quickly. 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.

Measuring the fumes from solid fuel cooking appliances

Quantifying emissions from wood fired and charcoal ovens

Probably the most important factor to test on a solid-fuel cooking appliance is the emissions of carbon monoxide from the burning of the fuel. This can be done on site using a flue-gas analyser, however it is necessary to use one which can work with high concentrations. We use a Testo 330 2LL, which has a range up to 30000 ppm (3%) which is adequate most of the time.

Measuring the emissions from the cooking processes is more complicated and is best done using a flame-ionisation detector (FID). When using the FID it is also necessary to quantify the mass flow rate of the gases in the flue, therefore it is best to do this sort of testing in a facility which was designed for the purpose. Whitebeam has such a facility and can offer a testing service to its customers.

Regulated emissions from solid fuel cooking appliances

More about the rules which apply to fumes from ovens fired by wood and charcoal.

Air quality in commercial kitchens

There are two categories of regulations which are concerned with the emissions from solid-fuel cooking appliances: the first deals with health and safety regulations inside the building whilst the second focuses on the environmental impact in the neighbourhood.

From a health and safety standpoint the main concern is carbon monoxide gas. This can lead to tiredness, headaches and nausea at low concentrations, followed by unconsciousness and death at higher concentrations. Since even mild carbon monoxide poisoning can impair people’s judgment, the likelihood of accidents also increases in busy kitchens, and they are already hazardous places to work. Carbon monoxide concentrations above 30 parts per million (ppm) are potentially dangerous in the workplace, however we have measured over 1,000 ppm in close proximity to a charcoal oven and over 30,000 ppm (3%) in the flue itself!

The Health and Safety Executive in the UK has recently published a useful information sheet entitled ‘Preventing exposure to carbon monoxide from use of solid fuel appliances in commercial kitchens’ and this can be downloaded from this link:

Health & Safety Executive guide (PDF)

Regulations which focus on the environmental impact of solid-fuel burning usually vary depending on the type of neighbourhood where it takes place. In the UK many cities are designated as ‘Smoke Control Areas’ which means that appliances which burn solid fuel need to comply with strict limits for their particulate emissions. Appliances are tested by DEFRA to determine whether they meet these standards. Restaurants also need to avoid creating a nuisance, for example offensive smells, and if they fail to do so they may attract complaints from neighbours followed by enforcement action by the local government.

Products to reduce emissions from solid fuel cooking appliances

Catalytic solutions for fumes in commercial kitchens

The choice of catalytic converter for solid fuel cooking applications is driven by two factors: the concentration of carbon monoxide and the flue gas temperature.

Charcoal ovens tend to emit very high levels of carbon monoxide, and therefore a catalytic converter with a large surface area is required. Since this normally needs to fit inside the flue, compactness is also important, which means that catalytic converters based on metal honeycombs are usually the best choice.

Wood-fired pizza ovens tend to produce much lower concentrations of carbon monoxide and also have lower flue-gas temperatures. The priority in these cases is normally to find a catalytic converter which can store heat effectively so that it can keep functioning temporarily if the flue gas temperature drops too low. This means that catalytic converters based on ceramic honeycombs are most suitable.

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