Reducing the risks of poisonous emissions from stoves

Grant Ballard - Tremeer and Harald H Jawurek, University of Witwatersrand, Johannesburg, South Africa.

To keep warm in cold climates, fires and stoves are needed for space heating as well as for cooking. These fires are often in rooms with little ventilation and they bum for long periods. People are thus exposed to high levels of combustion emissions for a long time - the health impact of these emissions on the users is therefore particularly severe.

The health effects from combustion emissions range from headaches and breathing difficulties to death. These effects may be immediate or occur after being exposed to the pollutants for a long time. Some symptoms may show up only many years after exposure. The effects depend upon the type and quantity of the pollutants, the duration of exposure to them, and on the age and health of the person exposed. There is increasing evidence that chronic exposure to carbon monoxide (CO) constitutes a long-term health risk.

At the University of the Wilwatersrand in Johannesburg, South Africa, the CO and smoke emission patterns have been recorded for a number of cooking devices including the traditional 'three stone' fire, as well as a number of improved stoves. It was found that enclosed stoves all have greater stove efficiencies than the open fire but also had greater emission levels.

In an attempt to improve efficiency, thermal contact between the fire and the base of the pot has been increased in improved stoves by enclosing the fire, but this results in the combustion gases being less completely burnt. In addition, in an enclosed fire, the flames are 'forced' on to the base of the much cooler pot, thus quenching them and causing 'freezing' of the volatiles and their emission in partially burnt states.

Emission rates were recorded for CO and smoke every ten seconds throughout a bum cycle; this involved heating water to boiling point rapidly and then simmering for 30 minutes. Figure 1 shows room concentration of CO for a one-pot metal stove with ceramic insulation which is top fed. Notice the high room concentrations reached for the metal stove. The line shown at 0.1g/m3 (equivalent to 87ppm) is the level of the 15 minute World Health Organisation air quality guideline for Europe. Notice that both the three-stone fire and the metal stove exceed this level for most of the bum cycle. After 40 minutes, room concentrations for the metal stove are twice as high as for the open fire. From the above discussion we offer the following recommendations:

Figure 1: CO concentration in a room with poor ventilation

· For space heating, improved stoves must have chimneys so that combustion gases are removed from the dwelling. The Indian Chulha with a chimney, although its efficiency is low (as can be expected for large mass mud stoves) is a good example. Mud stoves without chimneys are not recommended.

· Do not assume that improved stoves without chimneys are safer than the traditional open fires. Enclosed stoves in general emit higher levels of poisonous gasses than three-stone fires. Stoves which are 'fed' through the same opening as that which supplies air for combustion have the danger of being over-stoked (in an attempt to prolong burning). The more fuel in the combustion chamber, the less space there is for air, and emissions will therefore increase significantly. Stoves providing combustion air principally from beneath the fire through a metal or ceramic grate can clog with ash and gradually cause the fire to smother; again greatly increasing emissions.

· Particularly large quantities of poisonous fumes are emitted during stove lighting and refueling because cold pot sides and stove sides cool the flames and result in less complete combustion. Because of this, portable stoves without chimneys should be lit and operated out of doors for at least ten minutes before being brought indoors. This practice is frequently followed in South African informal settlements with coal-burning braziers, 'Mbaulas', made from 25 litre drums. Refuelling indoors is dangerous (although peak emissions after refuelling are usually lower than after initial ignition owing to reduced quenching on the sides of the stove). No fire (even a glowing one) should, however, be operated in a room with poor ventilation.

· Overall efficiency can easily be improved without reducing combustion efficiency by raising the fire off the ground by means of a grate. This improved three stone fire has an efficiency comparable with enclosed stoves (21 per cent) but with much lower emissions.