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Improving the three-stone fire
[top] [end]AimsThe open, wood-burning fire built on the ground with a pot supported above it - the 'three-stone fire' - is still an extremely common rural cooking device. The raising of such a fire on to a grate could lead to reduced heat losses to the ground and to increased completeness of combustion. This would favour both increased efficiency and reduced emissions. If these gains were significant the 'improved' open fire (IOF) might well constitute a good, essentially no-cost, stove.The aims of the present study therefore are:
See Figure 1. The open fire was built on a 50mm thick sand base with the pot supported on a metal tripod 10mm above the base. The side lengths of the triangular frame were 250mm as was the diameter of the floor of the pot. The improved open fire was of the same dimensions as the open fire, but was fitted with a wire grate of 10mm square pitch positioned 10mm above the ground. The grate area (maximum fuel bed area) was approximately half the pot floor area. The one-pot chimneyless metal stove was a typical example of current trends in stove design. It had light-gauge steel inner and outer walls, ceramic insulation and an adjustable fuel door to control the inlet air. The firebox diameter was 200mm. The stove was top-fed (the pot had to be removed to add fuel). [top] [end]EfficiencyThe percentage heat utilized (PHU) was determined by carrying out the standard water boiling test (VITA 1985).[top] [end]Emission measurementsThe stoves being tested were placed beneath an extraction hood fitted with a fan, a butterfly damper and an orifice plate flow meter. The extraction rate was set slightly above the minimum required to capture all smoke. The resulting air velocities at the stove were lower than typical air currents in a closed room.The pollutants measured were particulates (smoke), carbon monoxide (CO) and sulphur dioxide (SO2). Since the particulates emitted by wood fires are all in the respirable size range it is appropriate to measure total suspended particulates (TSP). TSP was measured by means of a light obscuration meter mounted in the exit duct of the extraction hood.
[top] [end]Fuel and fuellingThe exotic hardwood Eucalyptus grandis was used to simulate the hardwood types preferred in rural areas. Its moisture content was 11(±1) per cent. The open fires were fed radially and semi-continuously, following traditional practice. The stove was fed intermittently, once during the heating up phase and one to three times during the simmering.[top] [end]Results and discussionIt was found that smoke, CO, and SO2 follow similar emission patterns, see Table 1.Hydrocarbon emissions were not separately measured in our study. However, the most hazardous group of these, the polyaromatic hydrocarbons (PAHC) are emitted as particulates (Calle & Zeighami 1984) and are thus included in TSP. In summary: A wood-burning stove or fire having high emissions of CO has high emissions of smoke (which includes PAHC), SO2 and non-methane hydrocarbons. The emission of CO is thus used in the present study as an approximate health hazard index of wood-burning cooking devices. Figure 2 shows efficiency (averaged over the heating-up and the simmering phases) versus fire power for six tests on each of the cooking devices. Clearly there is a general trend to reduced efficiencies with increasing fire power. This would seem to be attributable largely to 'heat transfer efficiency' effects. The averages of the efficiencies shown in Figure 2 are given in Table 1. The fire built on a grid with an efficiency of 21 per cent is a substantial improvement on the open fire at 14 per cent (the corresponding reduction in fuel consumption is 32 per cent), and competes successfully with the metal stove at 20 per cent. The metal stove, however, reaches boiling in 16 minutes, as opposed to 22 minutes for the fires, a feature that is likely to be favoured by users. In presenting results on emissions we prefer to use the total mass of pollutant (CO) produced per test (cooking task). This mass is directly related to human exposure. Emission factors, because of their denominator term (per mass of fuel burned) include the effects of efficiency. They are tabulated for comparison. Figure 3 shows efficiency on the x axis and total CO emissions on the y axis for the 18 tests under discussion. The figure is not intended as a 'plot' in the functional relationship sense, but as a 'performance map' on which the top left-hand corner is the region of good performance and the bottom right-hand corner the region of poor performance.
Table 1: Performance characteristics of the cooking devices
[top] [end]Summary and conclusionsEfficiencies and emissions of an open fire, an improved open fire built on a grate, and a commercial, low-cost, one-pot, chimneyless metal stove were compared. Preliminary tests together with published data showed that CO emissions can be used as an approximate health hazard index of wood-burning cooking devices.On the basis of total CO emitted per test (cooking task) the improved open fire was the lowest-emitting device, followed closely but without statistically significant difference by the open fire. The stove was higher-emitting than the fires by a factor of three. With respect to SO2 emissions this factor was 10. The average efficiency was 21 per cent for the improved open fire which is substantially better than the open fire at 14 per cent (these figures correspond to a 32 per cent reduction in fuel consumption). The mean efficiency of the stove was 20 per cent, which is no improvement on the fire built on a grate. Clearly the improved open fire is a good overall performer, with efficiencies equal to that of the metal stove and emissions marginally (but not statistically significantly) lower than those of the open fire. [top] [end]Editorial noteThe laboratory tests carried out at Witwatersrand University show results which are in line with our field experience of three stone fires and unimproved metal stoves. We would be interested to see similar, comparative tests using a more popular and efficient metal stove, such as the improved KCJ (or a wood burning version of a similar stove) or the 'UMIME' shielded fire or a modern ceramic stove, such as the Sri Lankan Anagi.As the authors suggest, fire power (and consequent speed of cooking) along with smoke reductions are often the two most important considerations for stove users. If the research continues with field tests, the authors may find their claims for 32 per cent reduction in fuel consumption is not very meaningful at household level. [top] [end]Contents: Boiling Point 36: Solar Energy in the Home
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16 August 2007; Last edited:
20 August 2007; Version: 0 | |||||||||||||||||||||||||||||||||||||
Pagename: ImprovingTheThree-stoneFire @HEDON: RQGA | |||||||||||||||||||||||||||||||||||||
