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Comparison of five rural, wood-burning cooking devices: Efficiencies and emissions
Pergamon
Biomassand &energy Vol. 11,No. 5, pp. 419-430, 1996 Conviiaht 0 1996ElsevierScience Ltd Printed’k drea~gritain. All rights reserved PII: so961-%34(%)ooo4o-2 0961-9534/96$15.00+ 0.00
COMPARISON OF FIVE RURAL, WOOD-BURNING COOKING DEVICES: EFFICIENCIES AND EMISSIONS G. BALLARD-TREMEERand H. H. JAWUREK* School of Mechanical Engineering, University of the Witwatersrand, P. 0. WITS 2050, Johannesburg, South Africa (Received 11 July 1995; revised 28 May 1996; accepted 3 June 1996) Ahstract-The
following cooking devices were compared: an open tire built on the ground, an “improved” open fire built on a raised grate, a one-pot metal stove, a two-pot ceramic stove and a two-pot metal stove. Efficiencies (ratios of energy entering the pot to the energy content of the fuel consumed) were determined by carrying out a computer-controlled version of the standard Water Boiling Test. Emission concentrations of smoke, carbon monoxide and sulphur dioxide were measured by means of a fume extraction hood, an optical smoke-density meter and an electrochemical flue-gas analyzer. Average emissions of smoke were lowest for the improved open fire and the two-pot ceramic stove, with the remaining devices higher emitting by factors from 1.5 to 3. Emissions of carbon monoxide and sulphur dioxide were lowest for the two open fires; the stoves were higher emitting by factors ranging from 2 to 3 for carbon monoxide and 3 to 10 for sulphur dioxide. Average efficiencies were 14% for the open fire, 21% for the improved open fire, and (with no statistically significant difference) 20 to 24% for the stoves. Copyright 0 1996 Elsevier Science Ltd. Keywords--Stoves;
woodstoves; cooking fires; efficiency; emissions; biomass.
1. BACKGROUND AND AIMS
The cooking devices to be compared in this study are: an open fire, an “improved” open fire built on a raised grate, a commercial one-pot metal stove, a prototype two-pot ceramic stove and a prototype two-pot metal stove. The open, wood-burning fire built on the ground with a pot supported above it-the “three stone fire”-is the most basic of cooking devices, and in many rural areas of the world the most common. It is thus a natural point of reference in a comparison of stove types. Open fires are widely held to have low heat transfer efficiencies (ratios of energy entering the pot to that liberated by combustion) and high levels of pollutant emission. It is to be expected that raising an open fire onto a grate would lead to reduced heat losses to the ground and, by supplying primary air *Author to whom all correspondence should be addressed. E-mail: [email protected]. tVITA publications are obtainable from: Volunteers in Technical Assistance, 1815 North Lynn Street, Suite 200, Arlington, Viiginia 22209-2079, U.S.A. WHO publications are obtainable from: PEP, World Health Organization, 1211 Geneva 27, Switzerland. Boiling Point is an international journal for stove workers published by the Intermediate Technology Development Group (ITDG), Myson House, Railway Terrace, Rugby, CV21 3HT, U.K.
from beneath the fuel bed, would lead to increased completeness of combustion. This would favour both increased efficiency and reduced emissions. Krishna Prasad’ further mentions that a grate “helps in defining the combustion space better, thereby promoting the use of compact fires”-as opposed to wastefully large ones. The only quantitative study of open fires with and without grates appears to be that of Visser and Verhaard of the Eindhoven Woodburning Stove Group, which is reported in summary form by Gopalakrishnan.z For wood (white fir) of 10% moisture content and with the pot elevated 110 mm above the fuel bed (both reasonable values in practice) the efficiencies were 20.5% for the open fire and 22% for the fire on a grate. The corresponding figures for oven-dried wood were 21% and 24%. These are not large improvements. However, the test methods that were employed differed from the VITA? (Volunteers in Technical Assistance) tests that are currently accepted as standard’,’ and this may have biased the results; certain test methods are known to favour certain stove types5 Baldwin,6 on the other hand, in dealing with West African uninsulated metal (“malgache”) stoves, reports that the fitting of
419
420
G. BALLARD-TREMEER and H. H. JAWUREK
a grate increased their efficiency from about 18% to nearly 25%. Emissions were not measured in either study. Early stove work was aimed largely at increasing fuel efficiency. The last decade, however, has seen an increasing awareness of the problem of emissions. In 1992, the World Health Organization’ stated that emissions from rural, biomass-burning cooking sites were a health hazard affecting half the world’s population (approximately 2500 million people). Emissions are now considered at least as important as efficiencies. The commercial one-pot metal stove used in this study is a typical example of current trends in stove design. It has light-gauge steel inner and outer walls, ceramic insulation and an adjustable damper to control the inlet air flow. The stove is chimneyless and top-fed (the pot has to be removed to add fuel). The stove was advocated as being fuel-efficient, but there was no information on its emission characteristics. Presumably many buyers-perhaps even the manufacturers-share the widely held view that any stove must be lower polluting than an open fire. In fact, the opposite is more likely. Increased efficiency (“Percentage Heat Utilized”) is frequently achieved at the cost of reduced completeness of combustion and thus of increased emissions.* Nangale in comparing a three stone fire with an “improved” metal stove obtained an increase in mean efficiency from 22.8% to 24.2% and increases in emissions by factors of about 5 for both carbon monoxide and total suspended particulates, and 2.2 for hydrocarbon acidity. The two-pot ceramic stove of this study is similar in design to the TERI (TATA Energy Research Institute, Bombay) Improved Chimneyless Woodfuel Cookstove. It consists of two pot holders; the first contains the firebox and is the main cooker; a tunnel connects the first to the second pot holder; combustion gases escape from the latter around the second pot which is intended for simmering and for maintaining food at temperature. The design respects the two-pot cooking that is practised in many parts of the world where meals consist of a starch-based main dish and a smaller dish of vegetables or meat. Additionally, the second pot recuperates energy from the flue gases, thus improving efficiency. Emissions were not considered at the time of design (1990). The two-pot metal stove represents an attempt to provide, at low cost, the features of
a “proper” stove-the widely desired cast iron range. The features are: durability-the result of construction in appropriate metals; the ability to accommodate two (or more) pots; a flat cooking surface; operation at waist height; refuelling without removal of the pot(s); and a chimney. The two-pot metal stove of this study has galvanised steel inner and outer walls enclosing a layer of ceramic fibre insulation, a removable heavy-gauge steel cooking plate (hob), a stainless steel fire box and a chimney. We are informed (by an associate who has worked on this stove) that a stove of strikingly similar design is widely used in Lapland. The two-pot ceramic stove and the two-pot metal stove were developed in the School of Mechanical Engineering in co-operation with the Wits Rural Facility (WRF), an interdisciplinary unit of our University concerned with rural upliftment and operating in Mpumalanga (previously Eastern Transvaal) province. The ceramic stove was designed to be manufactured by rural potters using locally available materials and traditional techniques, and several units were successfully manufactured in this manner. Field tests were conducted in the vicinity of the WRF, our study area. Despite initial community enthusiasm for the stove, it failed to achieve long-term acceptance.” The stove remains of interest, however, because it represents a recognised type of cooking device that is in use elsewhere. The two-pot metal stove was designed after extensive consultation with the inhabitants of our study area. The design was modified a number of times (for improved operation and greater ease of local manufacture) and appears to have community acceptance. For the purposes of the present comparison the stove is an example of a distinct class of rural cooking device-the range-type stove. The aims of the present study are to compare the efficiencies (determined by standard VITA tests) and the emissions of the five rural cooking devices of markedly different designs as discussed above. 2. APPARATUS AND PROCEDURE
2.1. The cooking devices (“stoves”) These are shown in Fig. 1. The open fire was built on a sand base 50 mm thick and beneath the tripod shown. The tripod supported an aluminium pot of 250 mm diameter and of 5 1 capacity, filled at the beginning of each test with
Comparison of five rural, wood-burning cooking devices
2 1 of water. The same pot size and charge was used throughout, also as the second pot in the case of two-pot stoves. The improved open fire was of the same dimensions as the open fire, but was fitted with a wire grate of 10 mm square pitch on which the fire was built. The grate area (maximum fuel bed area) was approximately half the area of the bottom of the pot. The main features of the three stoves have already been discussed. Note that the two-pot metal stove is drawn to a smaller scale than the other cooking devices. 2.2. Eficiency Efficiency was determined by carrying out the Water Boiling Test (WBT) recommended by VITA.3 Stewart4 describes this as a “generally accepted...test procedure that can be applied universally”. The WBT involves operating the stove under conditions of heating up and simmering, using water to simulate food. Water is brought to the boil as rapidly as possible during the heating-up phase (commonly called the “high-power phase”) and is then maintained within 5°C of boiling for 30 min during the simmering phase (called the “low-power phase”). In this study the efficiency of each phase was determined (though only the overall efficiency over the entire test is reported here). In order to
Improved
Open fire
2 pot ceramic stove
421
determine the separate efficiencies it is necessary to know the energy content of the combustibles remaining at the end of each phase. At the end of the simmering phase (end of test) this presented no problem. The wood and the char (each of known calorific value) were separated and weighed. At the end of the heating-up phase (and with open fires) wood and char were separated and weighed rapidly, the fire was reassembled and the test was continued into the simmering phase. With enclosed stoves, however, the removal of combustibles at the end of the heating-up phase proved too problematic. The stoves were thus fuelled in such a way that char only remained at the end of the heating-up phase. The required control of the fire was difficult to achieve with both procedures, and repeatability was inherently low, as also noted by Nangale.’ Efficiency is defined as the ratio of energy entering the pot to the energy content of the fuel consumed. The energy entering the pot produces two measurable effects: raising the temperature of the water to the boiling point and evaporating water. The former was measured by a thermocouple (others measured fire temperature), the latter by a digital weighing platform on which the stove rested. The initial mass of wood, added wood and char (and where applicable, wood) remaining at the end of each
1 pot metal stove
open fire
2 pot metal stove
Fig. 1. The cooking devices.
G. BALLARD-TREMLZR and H. H. JAWUREK
422
phase were similarly determined by weighing. Thus the energy content of the fuel consumed could be calculated. For the one-pot cooking devices, efficiency was given by: rl=
@AT m&f-
+ h,m, mchc
’
where rl is efficiency (fractional); c,, is the specific thermal capacity of water (kJ kg-’ K-l); E is the average mass of water in the pot during the heating-up phase (kg); AT is the rise in water temperature for the heating-up phase (K); h, is the enthalpy of vaporisation of water (kJ kg- ‘); mfe is the mass of water evaporated (kg); m, is the mass of fuel (wood) used during the test (or phase) (kg); hf is the enthalpy of combustion (lower calorific value) of the fuel (kJ kg-‘); m, is the mass of char remaining at the end of the test (or phase) (kg); and h, is the enthalpy of combustion of the char (kJ kg - ‘). The resulting efficiencies are, for all practical purposes, identical with the widely used “Percentage Heat Utilised” of Baldwin.6 (He uses initial rather than average mass of water in the first term of the numerator.) For the two-pot stoves, the numerator term-the energy absorbed-were summed over both pots. Baldwin6 recommends that the second pot be ignored since “ ...the additional heat recuperated...is ineffective in actually cooking food because it is too low in temperature and because it cannot be easily controlled”. This has not been our experience during fieldwork: we have seen the second pot used very effectively on our stoves. The efficiencies of cooking devices are frequently reported as a function of “fire power”. Fire power is defined as the ratio of energy content of the fuel consumed during a test (or phase) to the duration of the test (or phase). 2.3. Direct emissions
versus
indirect
measurement
of
The direct measurement of emissions involves measurement at the source-the stove-and uses an extraction hood to capture the emissions.” Indirect methods measure the influence of the stove on a dilution chamber
(simulating a dwelling) and calculate the “source strength” (emission rate) on the assumption that it is constant.‘* The “hood method” was used in the present study. It requires fewer assumptions than the “chamber method” and it has the potential of providing more information. The question of “whether the mechanically induced air flow [changes] the combustion characteristics of the stove”‘* was addressed in preliminary tests discussed below. 2.4. Extraction rate The stove and the weighing platform were placed beneath an extraction hood fitted with a butterfly damper and an orifice plate flow meter. The volumetric extraction rate was set at three different levels, 0.049, 0.056 or 0.065 m3 s-‘, in order to study the effect of extraction on stove performance. The lowest of these rates was one that was sufficient to capture all smoke, but that had no visible effect on the flame of a lit match held in the position normally occupied by the stove. The highest rate was one that had a small, but clearly discernible effect on the flame. Statistically designed experiments (analysis of variance) showed that extraction at the above levels had no effect (at 95% confidence) on stove efficiency, fire power, fire temperature and the emission of all pollutants measured with the exception of carbon monoxide (CO). There was a small decrease in CO emissions with increasing extraction, but there was no interaction (at 95% confidence) between this and stove type. This means that the CO emissions of all stoves were equally influenced by extraction. An empirical hood design equation was used to calculate the air (“capture”) velocities at the stoveI corresponding to the above extraction rates. The velocities ranged from 0.10 to 0.12 m s-l. Typical air currents in a closed room are 0.25 m s-‘.‘~ The hood method can therefore be used with confidence, provided that the extraction rate is appropriately low and is not changed between tests. 2.5. Pollutants measured The pollutants measured were particulates (smoke), carbon monoxide (CO) sulphur dioxide (SO3, oxides of nitrogen (NO,) as well as carbon dioxide (CO*). Among these, CO constitutes both a short- and long-term health hazard, while particulates, SO, and NO, have long term health effects. Hydrocarbon emissions were not measured. However, the most hazardous group of these, the polyaromatic
Comparison of five rural, wood-burning cooking devices
hydrocarbons-many of which are known carcinogens-are emitted as particulates.14 Since the particulates emitted by wood fires are predominantly in the respirable size range,” I4 it is appropriate to measure total suspended particulates (TSPs). TSPs were measured by means of a light obscuration (attenuation) meter consisting of a light-emitting diode and a light-dependent resistor operating in the linear range. The meter was mounted in the exit duct of the extraction hood. Obscuration was expressed in terms of specific optical smoke density (OSD, m-l) which is defined as: OSD = 1og’,(100/100 - &)x- ‘, where S, is the light obscuration (Oh) over the path length x (m). The correlation between OSD and mass concentration of TSPs was established by use of a standard filter sampling system as employed in air pollution measurements. Flue samples were drawn through two Nucleopore acid etchedmembrane filters in series, the first of 8 pm pore size, the second of 0.4 urn. These filters had 50% collection efficiencies for particles of 2.5 and 0.01 pm aerodynamic diameters respectively.15 Sample volume (over a sampling period of a few minutes) was measured by means of an air volume (dry gas) meter. The collected particulates were measured by weighing the filters. (The masses collected on the fine filters were 10 times those collected on the coarse filters; this confirms the largely respirable nature of the particulates.) Eleven such calibration data points were established at different naturally occurring stages of smokiness of a stove. The correlation between mass concentration and OSD, time-averaged over each sampling period, was linear (correlation coefficient 0.98), as also found in other studies.‘6.‘7 Gas was sampled from the exit duct of the extraction hood and concentrations (above background) of CO, SO?, NO, and CO, were measured by means of an electrochemical flue gas analyser manufactured by Industrielle Messund Regelsysteme fur Umwelttechnologie, Heilbronn, Germany, the IMR 3000P. The emissions of NO, encountered in this study were frequently close to or below the detection limit of the unit and are thus not reported. 2.6. Data acquisition, water evaporated, fuel burned and burn rate The concentrations of the gaseous pollutants, light obscuration,
orifice plate pressure drop
423
and mass (of stove, fuel, pot and water, combined) were recorded digitally at 10 s intervals throughout the test. Additionally, at the start of the test and at the end of each phase the pot was briefly lifted off the stove. From these mass records, and with wood and char treated as discussed in Section 2.2, the separate masses of wood supplied, char (and where applicable, wood) remaining, and water evaporated could be determined for each phase. In a number of tests the pot was briefly lifted off the stove every 2-3 min throughout both phases. This provided records versus time of water mass, and hence fuel mass, and thus the burn rate. It was found that the mass of water decreased essentially linearly with time during the simmering phase (as expected) and nearly linearly during the heating-up phase. Furthermore, the mass of water evaporated during the heating-up phase was a small fraction of the mass of fuel burned. Thus, for those tests in which only the total mass evaporated was measured, a linear change in mass was assumed for both phases. Thus, the instantaneous burn rate could be estimated for these cases as well. 2.7. Fuel and fuelling The wood used for testing was Eucalyptus plantation wood. The wood is not used in rural areas, but is considered to be a good barbecue fuel by urban dwellers. It is a hardwood, as are the types of wood preferred by rural users. Its moisture content was 10.7( + 1.3)%. The effect of type of wood on stove performance was studied in a separate set of experiments. The improved open fire and the one-pot metal stove were used to compare Eucalyptus grandis and Pinus patula, a softwood considered too fast-burning for satisfactory cooking. There was no difference (at the 95% confidence level) in the efficiency and emissions results for the two types of wood. This would seem to indicate that wood type is not a critical factor in this study. (The calorific values of the woods were obtained from a database of the Forestek Division of the South African Council for Scientific and Industrial Research; the values were 19.69 MJ kg - ’ for E. grandis and 20.90 MJ kg-’ for P. patula. The calorific value of char was taken as 28.0 MJ kg-‘, the value given by Baldwin.‘) Pieces of wood chopped to a “diameter” of less than 15 mm (capable of passing through a
grandis, a non-indigenous
424
G. BALLARD-TREMEER and H. H. JAWUREK 600
- - - - - - Sulphur dialid= - - - Total suapudod pwticuhos
0
500
1000
1500
2000
2500
3000
Time (seconds)
Fig. 2. Instantaneous concentrations
of CO, TSPs and SO*, one-pot metal stove.
ring of 15 mm internal diameter) were used as kindling. Wood with a diameter of 30-60 mm was used as main fuel. All pieces were 100 mm in length. The initial fuel charge consisted of 0.03 kg of kindling and approximately 0.12 kg of main fuel for the four smaller devices, and 0.75 kg for the larger two-pot metal stove. The open fires were fed radially and semi-continuously, following traditional practice. The stoves were fed intermittently, generally once during the heating-up phase and l-3 times during the simmering phase. 3. RESULTS AND DISCUSSION
It was found that for the one-pot metal stove, TSPs, CO, and SOz follow similar emission patterns during a test (see Fig. 2). Thus, if the ratios of the instantaneous concentrations of TSP and of SO, to that of CO were to be calculated they would approximate to constants. In the terminology of atmospheric chemistry,‘* the emission ratios of TSPs and SO, to CO are essentially constant over a test. Such behaviour has been noted by Lobert et aI.18 and Helas (personal communication, see also Ref. 19) for TSP, CO and various hydrocarbon products of incomplete combustion emitted from real and simulated Savannah fires. This finding has important implications for the taking of “grab samples” of fire plumes and in the estimation of the emissions of trace gases affecting atmospheric chemistry and climate.lg It should therefore be noted that considerably
more variable emission ratios than those for the one-pot metal stove are exhibited by the two other stoves of this study, and that this variability is even more marked in the case of the open fires. Figure 3 shows the emission patterns of the improved open fire; the SO,/CO emission ratio, for example, varies with time from 0.17 to 1.2; if the emission ratio were to be based on mean concentrations, or on cumulative masses emitted, its value would depend on the duration of the test. Care must therefore be exercised when using emission ratios in the study of stoves and cooking fires. The TSP data shown in Fig. 3 have the appearance of including a superimposed drift to higher values with time. It was initially thought that this was due to condensation of volatiles on the optical surfaces during a test. Subsequent checks, however, showed that such “window contamination” amounted at most to 1% obscuration over a test. Further, the drift-like effect was absent in numerous tests, see for example Fig. 2. The TSP emission patterns for the traditional open fire were very similar to those of the improved open fire. These fires therefore become increasingly smoky with time. (In absolute terms, however, they are low-smokeemitting devices--compare the TSP concentration scales in Figs 2 and 3.) A possible explanation for this might be the following: the fires were fed semi-continuously; they thus always contained “new” wood which means that there was always some smoke; the wood
Comparison of five rural, wood-burning cooking devices
425
04
0
500
1000
1500
2m
2500
co
3000
Time (saconds)
Fig. 3. Instantaneous
concentrations
of CO, TSPs and S02, improved open fire.
was fed radially inwards; this caused accumulations of ash in the combustion space in the middle of the fires, leading to the progressive reduction of air supply and thus to increased smoking. By the same argument, CO, also a product of incomplete combustion, should display a trend to higher concentrations with time. This trend is clearly discernible in Fig. 3, superimposed over emission peaks somewhat sharper than those for TSPs. It may well be that the dying-away of SO, emissions in the later stages of this test is attributable to the same reduction in combustion air that caused the increased emissions of TSPs and of CO. The
traditional open tire displayed similar CO and SO, emission patterns. The above behaviour is very different from that exhibited by the one-pot metal stove (Fig. 2) for which the products of incomplete and complete combustion-CO and TSPs, and SO,-respectively, varied “in phase”, as already noted. (In-phase emissions of CO and SO, have also been reported for a residential coal stove.“) Figure 4 shows the burn rate of the one-pot metal stove for the same test as that of Fig. 2. The burn rate varied very sharply with time (for example by a factor of 15 between 1000 and 1500 s) and it seems that this masked any
L)E-4 T 7E-4 -6E-4 -5i $
IE-4.-
d t
4E.4
--
E 3
38-4
--
2E-4
--
1 E-4 OE+O
t 0
I/
, 500
1000
1500
2ao0
2500
Time (suonds)
Fig. 4. Instantaneous
bum rate, one-pot metal stove.
A
v\
, 3000
426
G. BALLARD-TREIUEER and H. H. JAWUREK 20-
26 --
20 -s 6 .f I6 .Y fi
?? Improved open fire
00
10 --
5 --
O-l 0
4
1
2
3
4
5
6
Power (kw) Fig. 5. Efficiency and fire power.
changes in emission factor (mass pollutant emitted per mass fuel burned) that occurred during the test. (The three main peaks relate to the addition of fuel, one during the heating-up phase and two during the simmering phase.) The open fires on the other hand were operated more closely to steady state; they were thus less burn-rate-dominated and this allowed the changes in emission factor discussed above to be detected. The burn-rate effect was, however, not absent; thus the simultaneous peaks in CO and SO, at 1000 and 2000 s in Fig. 3 correspond to peaks in burn rate. The presentation of the main results starts with efficiencies. Figure 5 shows overall efficiency (over the heating-up and the simmering phases) versus fire power for the 29 tests that were conducted. Focusing attention initially on the one-pot devices (open data points) we note a general trend to reduced efficiencies with increasing fire power. This was also reported by Joshi et al.z1 in their study of four metal stoves fuelled with wood, crop residue or dung. It would appear that for our devices and those of Joshi et al. and for the ranges of power that were studied, the higher the power of a fire, the more of its energy is lost to the surroundings, rather than transferred to the pot. Further, some cooking devices are inherently more prone to losses than others. The open fire is thus doubly penalised: it has high losses to the ground and so needs to be built quite large if it is to achieve boiling, and it therefore suffers the additional size-related losses. (We should like to
stress that our open fires were sensibly operated; considerably lower efficiencies could have been achieved by building larger fires.) The improved open fire has reduced losses to the ground and can therefore be built economically small. Additionally, it is self-limiting in power, the grate being able to accommodate no more than a finite amount of wood at a time. Compared with the improved open fire the one-pot metal stove, being insulated, has reduced losses to the surroundings, but it incurs the added loss (as do all stoves) of raising its own mass to operating temperature. The stove does not, as might perhaps have been expected, outperform the improved open fire. Turning now to the two-pot stoves (solid data points) we note that they also obey the “economy of smallness” rule noted above. The data lie roughly parallel to those for the one-pot devices, but displaced to slightly higher efficiency. This would appear to be due to the energy recuperation by the second pot. The efficiencies of the improved open fire and of our stoves are slightly higher than those of Joshi et a1.2’ at the same power and with wood-fuelling. However, Joshi et al. extended their measurements to powers as low as 2 kW and achieved efficiencies of 30 and 37% with their “TARA” and “CP” stoves respectively. Our devices were difficult or impossible to operate at such low powers. The averages of the efficiencies shown in Fig. 5 are given in the first row of data in Table 1. The standard deviation is given in
Comparison
of five rural, wood-burning
brackets alongside each entry, and beneath this the range of data for the (generally) six tests per cooking device. The bold number by each cell is a rating on a scale of l-3 where 1 rates highest; where two (or more) devices have the same rating, their differences in efficiency (or other performance data) are not statistically significant at the 90% confidence level. The fire built on a grate with an efficiency of 21% is a substantial improvement on the open fire at 14% (the corresponding reduction in fuel consumption is 32%) and it competes successfully with the one-pot metal stove at 20%. The two-pot stoves had slightly higher efficiencies, but the differences between these figures and that for the improved open fire were not significant at the 90% confidence level. In presenting results on emissions we prefer to use the total mass of pollutant emitted per test (cooking task). This mass is directly related to human exposure. Emission factors, because of their denominator term (mass of fuel burned), include the effects of efficiency. (Emission factors for CO are shown in Table 1 for purposes of comparison.) Joshi et ul.*‘***also prefer the use of mass emitted per task, but generally report both it and emission factor. However, they define the task not as the simulated cooking cycle of the WBT, but as the transfer of a fixed, and much smaller, amount of energy-879 kJ-to the pot. Figure 6 shows efficiency as the abscissa and total CO emissions as the ordinate for the 29 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 comer is the region of good performance and the bottom right-hand comer the region of poor performance. Clearly the improved open fire is a good overall performer, with efficiencies not significantly different from that of the stoves and emissions significantly lower, see also Table 1. Its CO emissions are also marginally lower than those of the open fire, but not statistically so at 90% confidence. (The use of emission factors eliminates the difference between the two fires, the less efficient device, the traditional open fire, being unrealistically favoured.) Table 1 shows the stoves to have had CO emissions (mass per task) higher than those of the fires by factors of 2 to 3, with the one-pot metal stove the highest emitter. CO emission factors ranged from 19 to 66 g kg-‘; this is in close agreement with Joshi et al.*’ whose mean values were from 17 to JBB
II/S-E
cooking
devices
427
62 g kg-‘. Also shown in Table 1 is the CO to CO, molar ratio (ratio of total moles emitted during test). It is included partly because it is of interest to atmospheric chemistry,lg and partly as a further indicator of incompleteness of combustion. The ratio is by far the highest for the one-pot metal stove. It would appear that the incompleteness of combustion was due not to a shortage of primary air, but rather due to quenching associated with the very narrow gap between the top of the stove and the bottom of the pot-there was a visible reduction in smoke emissions when the pot was briefly lifted off the stove. The SO2 emissions were lowest for the open fire and the improved open fire; the stoves were higher emitting than the (averaged) fires by factors of 3 to 10, with the one-pot metal stove again the worst emitter. Emissions of TSPs were lowest for the improved open fire and the two-pot ceramic stove; the other devices were higher emitting by factors of 1.5 to 3. It should be noted that the health hazard of the two-pot metal stove is reduced by the presence of the chimney which removes the emissions from the immediate vicinity of the cooking site. Joshi et al.*’ observe increasing emission factors for CO and TSPs with increasing efficiency, but observe no such correlation for emission mass per task. The correlation is also absent in the present study (see Fig. 6). The positive correlation between emission factor and efficiency would thus seem to be due to the fact that the two quantities have denominators that are proportional to each other-mass of fuel burned, and energy content of fuel burned. This, together with the (real) negative correlation between efficiency and power, explains the negative correlation between emission factors and power observed by Joshi et al.“, and the absence of correlation between emission mass and power observed in the present study. Figure 6 shows the performance points of the open fires to be considerably less scattered than those of the stoves. Data of the Aprovecho Institute23 similarly shows open fires to behave more consistently than enclosed (“Louga”) stoves. We believe that the reason for this is that open fires are tended near-continuously, they are visible and allow immediate intervention in the event of any irregular or undesired behaviour. Stoves are fed intermittently and operate in a highly transient manner. This favours irregularities of combustion which,
CO to CO2 mol ratio,
%
1.30 (0.16) 1.08-1.56
22 (3) 18-25
0.058 (0.025) 0.03AM9
19 (3.7) 13.323.4
Time to reach boil, min
kg-’
0.891 (0.160) 0.65-l .08
SO, per test, g
factor,
15 (2.1) 12.9-18.0
14 (2.1) 11.6-16.8
Open fire (6)
Total TSPs per test, g
Total
CO emission
g
% (St. dev.)
Total CO per test, g
Efficiency,
Stove type (no. of tests)
1
2 1.02 (0.12) 0.85-1.17
22 (3) 17-27
0.523 (0.198) 0.28-0.78
0.082 (0.034) 0.05-0.13
1 2
19 (3.7) 15.4250
12 (2.2) lO.Sl5.5
1
1
21 (1.3) 19.7-23.1
2
metal
16 (2) 13-19 5.25 (1.39) 3.64-7.60
1
0.976 (0.278) 0.4w.12
1 2
0.694 (0.114) 0.5M.80
66 (15.2) 43.680.5
43 (9.8) 32.7-54.4
1
1
1
1
l-pot
devices ceramic
3
1
2
0.218 (0.062)
3
1.48-2.42
’1.83 (0.32)
22 (2) 19-25
0.492 (0.122) 0.34-0.61
0.14-0.33
38 (9.9) 23.4-52.8
28 (6.1) 20.1-37.5
24 (3.1) 18.1-26.2
2-pot
3
3
1
stove (6)
of the cooking
20 (3.1) 16.CL24.7
characteristics
open fire (6)
1. Performance
Improved
Table
2
2
1
2
2
2
1
stove (6)
metal
1.00 (0.62) 0.31-1.88
32 (5) 26-39
1.595 (0.306) 1.22-1.94
0.17-0.42
0.303 (0.113)
22 (8.0) 12.tL31.6
30 (11) 17.5Al.l
22 (3.0) 18.2-25.9
2-pot
1
3
3
2
1
2
1
stove (5)
$ 7e
5: r
&
&
I w
g A
Y F
n
Comparison of five rural, wood-burning cooking devices 20
26
I
20 -'
0 0 00
00
?? A A
8 0
?n ?
429
%I,
??
Q e
A
??
A
3 3 8 w
'I
A
0 Open tire
00
0 Improved open fire
10 -
A 1 pot metal 0 2 pot ceramic 6-
??2 pot metal
04 0
10
20
30
40
so
I 60
Total Emissions (g) Fig. 6. Efficiency and total CO emissions.
because the devices are closed, go uncorrected for extended periods. Taking a final and broad look at the ratings shown in Table 1 we note the following: the improved open fire rates first, or jointly first, in all categories except time to reach boiling. The one-pot metal stove rates first in time to reach boiling (this would endear it to users), jointly first in efficiency, and last with respect to emissions (which, smoke excepted, are invisible). Clearly, efficiencies and emissions need to be determined before a stove design is disseminated. The traditional open fire is not the disaster it is generally held to be; while less efficient than the other devices, it rates second in overall emissions. The remaining two stoves rate joint first in efficiency, but are overall higher emitting than the traditional open fire.
4. SUMMARY AND CONCLUSIONS
For the one-pot metal stove the emission ratios TSPs/CO and SO,/CO were approximately constant over the duration of a test. The behaviour of the other two stoves was more variable, and for the open fires emission ratios changed with time by factors of up to 7. Care must therefore be taken when using emission ratios in the context of biomass-burning stoves and fires. Care is also required in dealing with grab samples (for subsequent analysis) of the flue gases of such devices.
The one-pot metal stove exhibited very sharp variations in burn rate. It seems that these overshadowed any changes in emission factor with time, thus leading to essentially constant emission ratios. The open fires were semi-continuously fed and operated more closely to steady state; this permitted additional factors influencing emissions to be detected. The average emissions of TSPs were lowest for the improved open fire and the two-pot ceramic stove; the other devices were higher emitting by factors of up to 3. The emissions of CO and SO, were lowest for the two open fires; the stoves were higher emitting by factors of up to 3 for CO and up to 10 for SO*. Overall, the improved open fire was the lowest emitting device and the commercial one-pot metal stove the highest emitting. It is thus important that new stove designs be tested for emissions before being disseminated. Mean efficiencies were 14% for the traditional open fire, 2 1% for the improved open fire and 20%-24% for the stoves, with no statistically significant difference (at 90% confidence) between the latter four devices. Acknowledgements-The authors offer their thanks to the following: the Director and staff, particularly Mr E.-J. P. Harvey, of the Wits Rural Facility (WRF) for their support and for facilitating field work related to this study; Dr D. I. Banks, previously of the WRF, for design input, student supervision, friendship, advice and criticism; Dr H. Annegarn and Mrs M. Kneen of Annegam Environmental Research for help with the calibration of the smoke meter; Dr. G. Helas of the Max Planck Institute for Chemistry, Ma&, Germany, for advice and helpful discussions; and the Foundation for Research Development (FRD) and the University for financial assistance.
G. BALLARD-TREMFER and H. H. JAWUREK
430 REFERENCES
1. K. Krishna Prasad, Analysis of some stove designs. In A Woodstove Compendium, ed. G. de Lepeleire. The Woodburning Stove Group, Eindhoven Technical University, 1981. 2. N. K. Gopalakrishnan, Experiments on cooking stoves. In Cookstove Handbook, Pilot Edition. TATA Energy Research Institute (TERI), Bombay, 1982. 3. VITA, Testing the Efficiency of Wood-burning Cookstoves: International Standards. Volunteers in Technical Assistance, Arlington, VA, 1985. 4. B. Stewart, Improved Wood, Waste and Charcoal Burning Stoves-A Practitioners’ Manual. Intermediate Technology Publications, London, 1987. 5. S. Baldwin, New directions in woodstove development, VITA News, January issue, pp. 3-23. Volunteers in Technical Assistance, Arlington, VA, 1984. 6. S. Baldwin, Biomass Stoves: Engineering Design, Development and Dissemination. Volunteers in Technical Assistance, Arlington, VA, 1987. 7. WHO, Epidemiological, Social and Technical Aspects of Indoor Air Pollution from Biomass Fuel. World Health Organization, Geneva, 1992. 8. Editorial comment, Boiling Point 28, 18, 1982. 9. G. F. Nangale, Stove emission monitoring. Boiling Point 28, 15-18 (1992).
10. G. Ballard-Tremeer, From clay and wood to cast iron and coal in South Africa. Boiling Point 29, 15-16 (1992).
11. S. S. Butcher, U. Rao, K. R. Smith, P. Azuma and H. Fields, Emission factors and efficiencies for small-scale standard open biomass combustion: towards measurement techniques. Paper presented at the Annual Meeting of the American Chemical Society, 1984. 12. D. R. Ahuja, V. Joshi, K. R. Smith and C. Venkataraman, Thermal performance and emission characteristics of unvented biomass-burning cookstoves: a proposed standard method for evaluation. Biomass 12, 247-270 (1987).
13. Committee on Industrial Ventilation, Industrial Ventilation, 17th edn. CIV, MI, 1982. 14. E. E. Calle and E. A. Zeighami, Health risk assessment of residential wood combustion. In Indoor Air Quality, ed. P. J. Walsh, C. S. Dudney and E. D. Cop&aver. CRC Press, Boca Raton, FL, 1984. 15. M. Lippman and J. N. Harman, Sampling aerosols by filtration. In Methods of Air Sampling and Analysis, 3rd edn, ed. J. P. Lodge. Lewis Publishers, MI, 1989. 16. BS 2811, Specifications for Smoke Density Indicators and Recorders. British Standards Institution, London, 1969. 17. J. S. Nader, Source monitoring. In Air Pollution, 3rd edn, Vol. 3, ed. A. C. Stern. Academic Press, New York, 1976. 18. J. M. Lobert, D. H. Scharffe, Wei-Min Hao, T. A. Kohlbusch, R. Seuwen, P. Wameck and P. J. Crutzen, Experimental evaluation of biomass burning emissions: nitrogen and carbon containing compounds. In Global Biomass Burning: Atmospheric, Climatic, and Biospheric Implications, ed. J. S. Levine. MIT Press, Cambridge,
MA, 1991. 19. G. Helas, Emissions of atmospheric trace gases from vegetation burning. Philosophical Transactions of the Royal Society, London 351, 297-312 (1995). 20. D. W. Macumber and D. R. Jaasma, Efficiency and
emissions of a hand-fired residential coalstove. In Proceedings: 1981 International Conference on Residential Solid Fuels, Environmental Impacts and Solutions,
ed. J. A. Cooper and D. Malek, Oregon Graduate Centre, OR, pp. 873-880, 1982. 21. V. Joshi, C. Venkataraman and D. R. Ahuja, Emissions from burning biofuels in metal cookstoves. Environmental Management 13, 763-772 (1989). 22. V. Joshi, C. Venkataraman and D. R. Ahuja, Thermal
performance and emission characteristics of biomassburning heavy stoves with flues. Pacific and Asian Journal of Energy 1, l-19 (1991). 23. Aprovecho Institute/German Appropriate Technology Exchange (GATE), Fuel Saving Cookstoves. Friedrich
Vierweg und Sohn, Braunschweig/Wiesbaden,
1984.
Biomassand &energy Vol. 11,No. 5, pp. 419-430, 1996 Conviiaht 0 1996ElsevierScience Ltd Printed’k drea~gritain. All rights reserved PII: so961-%34(%)ooo4o-2 0961-9534/96$15.00+ 0.00
COMPARISON OF FIVE RURAL, WOOD-BURNING COOKING DEVICES: EFFICIENCIES AND EMISSIONS G. BALLARD-TREMEERand H. H. JAWUREK* School of Mechanical Engineering, University of the Witwatersrand, P. 0. WITS 2050, Johannesburg, South Africa (Received 11 July 1995; revised 28 May 1996; accepted 3 June 1996) Ahstract-The
following cooking devices were compared: an open tire built on the ground, an “improved” open fire built on a raised grate, a one-pot metal stove, a two-pot ceramic stove and a two-pot metal stove. Efficiencies (ratios of energy entering the pot to the energy content of the fuel consumed) were determined by carrying out a computer-controlled version of the standard Water Boiling Test. Emission concentrations of smoke, carbon monoxide and sulphur dioxide were measured by means of a fume extraction hood, an optical smoke-density meter and an electrochemical flue-gas analyzer. Average emissions of smoke were lowest for the improved open fire and the two-pot ceramic stove, with the remaining devices higher emitting by factors from 1.5 to 3. Emissions of carbon monoxide and sulphur dioxide were lowest for the two open fires; the stoves were higher emitting by factors ranging from 2 to 3 for carbon monoxide and 3 to 10 for sulphur dioxide. Average efficiencies were 14% for the open fire, 21% for the improved open fire, and (with no statistically significant difference) 20 to 24% for the stoves. Copyright 0 1996 Elsevier Science Ltd. Keywords--Stoves;
woodstoves; cooking fires; efficiency; emissions; biomass.
1. BACKGROUND AND AIMS
The cooking devices to be compared in this study are: an open fire, an “improved” open fire built on a raised grate, a commercial one-pot metal stove, a prototype two-pot ceramic stove and a prototype two-pot metal stove. The open, wood-burning fire built on the ground with a pot supported above it-the “three stone fire”-is the most basic of cooking devices, and in many rural areas of the world the most common. It is thus a natural point of reference in a comparison of stove types. Open fires are widely held to have low heat transfer efficiencies (ratios of energy entering the pot to that liberated by combustion) and high levels of pollutant emission. It is to be expected that raising an open fire onto a grate would lead to reduced heat losses to the ground and, by supplying primary air *Author to whom all correspondence should be addressed. E-mail: [email protected]. tVITA publications are obtainable from: Volunteers in Technical Assistance, 1815 North Lynn Street, Suite 200, Arlington, Viiginia 22209-2079, U.S.A. WHO publications are obtainable from: PEP, World Health Organization, 1211 Geneva 27, Switzerland. Boiling Point is an international journal for stove workers published by the Intermediate Technology Development Group (ITDG), Myson House, Railway Terrace, Rugby, CV21 3HT, U.K.
from beneath the fuel bed, would lead to increased completeness of combustion. This would favour both increased efficiency and reduced emissions. Krishna Prasad’ further mentions that a grate “helps in defining the combustion space better, thereby promoting the use of compact fires”-as opposed to wastefully large ones. The only quantitative study of open fires with and without grates appears to be that of Visser and Verhaard of the Eindhoven Woodburning Stove Group, which is reported in summary form by Gopalakrishnan.z For wood (white fir) of 10% moisture content and with the pot elevated 110 mm above the fuel bed (both reasonable values in practice) the efficiencies were 20.5% for the open fire and 22% for the fire on a grate. The corresponding figures for oven-dried wood were 21% and 24%. These are not large improvements. However, the test methods that were employed differed from the VITA? (Volunteers in Technical Assistance) tests that are currently accepted as standard’,’ and this may have biased the results; certain test methods are known to favour certain stove types5 Baldwin,6 on the other hand, in dealing with West African uninsulated metal (“malgache”) stoves, reports that the fitting of
419
420
G. BALLARD-TREMEER and H. H. JAWUREK
a grate increased their efficiency from about 18% to nearly 25%. Emissions were not measured in either study. Early stove work was aimed largely at increasing fuel efficiency. The last decade, however, has seen an increasing awareness of the problem of emissions. In 1992, the World Health Organization’ stated that emissions from rural, biomass-burning cooking sites were a health hazard affecting half the world’s population (approximately 2500 million people). Emissions are now considered at least as important as efficiencies. The commercial one-pot metal stove used in this study is a typical example of current trends in stove design. It has light-gauge steel inner and outer walls, ceramic insulation and an adjustable damper to control the inlet air flow. The stove is chimneyless and top-fed (the pot has to be removed to add fuel). The stove was advocated as being fuel-efficient, but there was no information on its emission characteristics. Presumably many buyers-perhaps even the manufacturers-share the widely held view that any stove must be lower polluting than an open fire. In fact, the opposite is more likely. Increased efficiency (“Percentage Heat Utilized”) is frequently achieved at the cost of reduced completeness of combustion and thus of increased emissions.* Nangale in comparing a three stone fire with an “improved” metal stove obtained an increase in mean efficiency from 22.8% to 24.2% and increases in emissions by factors of about 5 for both carbon monoxide and total suspended particulates, and 2.2 for hydrocarbon acidity. The two-pot ceramic stove of this study is similar in design to the TERI (TATA Energy Research Institute, Bombay) Improved Chimneyless Woodfuel Cookstove. It consists of two pot holders; the first contains the firebox and is the main cooker; a tunnel connects the first to the second pot holder; combustion gases escape from the latter around the second pot which is intended for simmering and for maintaining food at temperature. The design respects the two-pot cooking that is practised in many parts of the world where meals consist of a starch-based main dish and a smaller dish of vegetables or meat. Additionally, the second pot recuperates energy from the flue gases, thus improving efficiency. Emissions were not considered at the time of design (1990). The two-pot metal stove represents an attempt to provide, at low cost, the features of
a “proper” stove-the widely desired cast iron range. The features are: durability-the result of construction in appropriate metals; the ability to accommodate two (or more) pots; a flat cooking surface; operation at waist height; refuelling without removal of the pot(s); and a chimney. The two-pot metal stove of this study has galvanised steel inner and outer walls enclosing a layer of ceramic fibre insulation, a removable heavy-gauge steel cooking plate (hob), a stainless steel fire box and a chimney. We are informed (by an associate who has worked on this stove) that a stove of strikingly similar design is widely used in Lapland. The two-pot ceramic stove and the two-pot metal stove were developed in the School of Mechanical Engineering in co-operation with the Wits Rural Facility (WRF), an interdisciplinary unit of our University concerned with rural upliftment and operating in Mpumalanga (previously Eastern Transvaal) province. The ceramic stove was designed to be manufactured by rural potters using locally available materials and traditional techniques, and several units were successfully manufactured in this manner. Field tests were conducted in the vicinity of the WRF, our study area. Despite initial community enthusiasm for the stove, it failed to achieve long-term acceptance.” The stove remains of interest, however, because it represents a recognised type of cooking device that is in use elsewhere. The two-pot metal stove was designed after extensive consultation with the inhabitants of our study area. The design was modified a number of times (for improved operation and greater ease of local manufacture) and appears to have community acceptance. For the purposes of the present comparison the stove is an example of a distinct class of rural cooking device-the range-type stove. The aims of the present study are to compare the efficiencies (determined by standard VITA tests) and the emissions of the five rural cooking devices of markedly different designs as discussed above. 2. APPARATUS AND PROCEDURE
2.1. The cooking devices (“stoves”) These are shown in Fig. 1. The open fire was built on a sand base 50 mm thick and beneath the tripod shown. The tripod supported an aluminium pot of 250 mm diameter and of 5 1 capacity, filled at the beginning of each test with
Comparison of five rural, wood-burning cooking devices
2 1 of water. The same pot size and charge was used throughout, also as the second pot in the case of two-pot stoves. The improved open fire was of the same dimensions as the open fire, but was fitted with a wire grate of 10 mm square pitch on which the fire was built. The grate area (maximum fuel bed area) was approximately half the area of the bottom of the pot. The main features of the three stoves have already been discussed. Note that the two-pot metal stove is drawn to a smaller scale than the other cooking devices. 2.2. Eficiency Efficiency was determined by carrying out the Water Boiling Test (WBT) recommended by VITA.3 Stewart4 describes this as a “generally accepted...test procedure that can be applied universally”. The WBT involves operating the stove under conditions of heating up and simmering, using water to simulate food. Water is brought to the boil as rapidly as possible during the heating-up phase (commonly called the “high-power phase”) and is then maintained within 5°C of boiling for 30 min during the simmering phase (called the “low-power phase”). In this study the efficiency of each phase was determined (though only the overall efficiency over the entire test is reported here). In order to
Improved
Open fire
2 pot ceramic stove
421
determine the separate efficiencies it is necessary to know the energy content of the combustibles remaining at the end of each phase. At the end of the simmering phase (end of test) this presented no problem. The wood and the char (each of known calorific value) were separated and weighed. At the end of the heating-up phase (and with open fires) wood and char were separated and weighed rapidly, the fire was reassembled and the test was continued into the simmering phase. With enclosed stoves, however, the removal of combustibles at the end of the heating-up phase proved too problematic. The stoves were thus fuelled in such a way that char only remained at the end of the heating-up phase. The required control of the fire was difficult to achieve with both procedures, and repeatability was inherently low, as also noted by Nangale.’ Efficiency is defined as the ratio of energy entering the pot to the energy content of the fuel consumed. The energy entering the pot produces two measurable effects: raising the temperature of the water to the boiling point and evaporating water. The former was measured by a thermocouple (others measured fire temperature), the latter by a digital weighing platform on which the stove rested. The initial mass of wood, added wood and char (and where applicable, wood) remaining at the end of each
1 pot metal stove
open fire
2 pot metal stove
Fig. 1. The cooking devices.
G. BALLARD-TREMLZR and H. H. JAWUREK
422
phase were similarly determined by weighing. Thus the energy content of the fuel consumed could be calculated. For the one-pot cooking devices, efficiency was given by: rl=
@AT m&f-
+ h,m, mchc
’
where rl is efficiency (fractional); c,, is the specific thermal capacity of water (kJ kg-’ K-l); E is the average mass of water in the pot during the heating-up phase (kg); AT is the rise in water temperature for the heating-up phase (K); h, is the enthalpy of vaporisation of water (kJ kg- ‘); mfe is the mass of water evaporated (kg); m, is the mass of fuel (wood) used during the test (or phase) (kg); hf is the enthalpy of combustion (lower calorific value) of the fuel (kJ kg-‘); m, is the mass of char remaining at the end of the test (or phase) (kg); and h, is the enthalpy of combustion of the char (kJ kg - ‘). The resulting efficiencies are, for all practical purposes, identical with the widely used “Percentage Heat Utilised” of Baldwin.6 (He uses initial rather than average mass of water in the first term of the numerator.) For the two-pot stoves, the numerator term-the energy absorbed-were summed over both pots. Baldwin6 recommends that the second pot be ignored since “ ...the additional heat recuperated...is ineffective in actually cooking food because it is too low in temperature and because it cannot be easily controlled”. This has not been our experience during fieldwork: we have seen the second pot used very effectively on our stoves. The efficiencies of cooking devices are frequently reported as a function of “fire power”. Fire power is defined as the ratio of energy content of the fuel consumed during a test (or phase) to the duration of the test (or phase). 2.3. Direct emissions
versus
indirect
measurement
of
The direct measurement of emissions involves measurement at the source-the stove-and uses an extraction hood to capture the emissions.” Indirect methods measure the influence of the stove on a dilution chamber
(simulating a dwelling) and calculate the “source strength” (emission rate) on the assumption that it is constant.‘* The “hood method” was used in the present study. It requires fewer assumptions than the “chamber method” and it has the potential of providing more information. The question of “whether the mechanically induced air flow [changes] the combustion characteristics of the stove”‘* was addressed in preliminary tests discussed below. 2.4. Extraction rate The stove and the weighing platform were placed beneath an extraction hood fitted with a butterfly damper and an orifice plate flow meter. The volumetric extraction rate was set at three different levels, 0.049, 0.056 or 0.065 m3 s-‘, in order to study the effect of extraction on stove performance. The lowest of these rates was one that was sufficient to capture all smoke, but that had no visible effect on the flame of a lit match held in the position normally occupied by the stove. The highest rate was one that had a small, but clearly discernible effect on the flame. Statistically designed experiments (analysis of variance) showed that extraction at the above levels had no effect (at 95% confidence) on stove efficiency, fire power, fire temperature and the emission of all pollutants measured with the exception of carbon monoxide (CO). There was a small decrease in CO emissions with increasing extraction, but there was no interaction (at 95% confidence) between this and stove type. This means that the CO emissions of all stoves were equally influenced by extraction. An empirical hood design equation was used to calculate the air (“capture”) velocities at the stoveI corresponding to the above extraction rates. The velocities ranged from 0.10 to 0.12 m s-l. Typical air currents in a closed room are 0.25 m s-‘.‘~ The hood method can therefore be used with confidence, provided that the extraction rate is appropriately low and is not changed between tests. 2.5. Pollutants measured The pollutants measured were particulates (smoke), carbon monoxide (CO) sulphur dioxide (SO3, oxides of nitrogen (NO,) as well as carbon dioxide (CO*). Among these, CO constitutes both a short- and long-term health hazard, while particulates, SO, and NO, have long term health effects. Hydrocarbon emissions were not measured. However, the most hazardous group of these, the polyaromatic
Comparison of five rural, wood-burning cooking devices
hydrocarbons-many of which are known carcinogens-are emitted as particulates.14 Since the particulates emitted by wood fires are predominantly in the respirable size range,” I4 it is appropriate to measure total suspended particulates (TSPs). TSPs were measured by means of a light obscuration (attenuation) meter consisting of a light-emitting diode and a light-dependent resistor operating in the linear range. The meter was mounted in the exit duct of the extraction hood. Obscuration was expressed in terms of specific optical smoke density (OSD, m-l) which is defined as: OSD = 1og’,(100/100 - &)x- ‘, where S, is the light obscuration (Oh) over the path length x (m). The correlation between OSD and mass concentration of TSPs was established by use of a standard filter sampling system as employed in air pollution measurements. Flue samples were drawn through two Nucleopore acid etchedmembrane filters in series, the first of 8 pm pore size, the second of 0.4 urn. These filters had 50% collection efficiencies for particles of 2.5 and 0.01 pm aerodynamic diameters respectively.15 Sample volume (over a sampling period of a few minutes) was measured by means of an air volume (dry gas) meter. The collected particulates were measured by weighing the filters. (The masses collected on the fine filters were 10 times those collected on the coarse filters; this confirms the largely respirable nature of the particulates.) Eleven such calibration data points were established at different naturally occurring stages of smokiness of a stove. The correlation between mass concentration and OSD, time-averaged over each sampling period, was linear (correlation coefficient 0.98), as also found in other studies.‘6.‘7 Gas was sampled from the exit duct of the extraction hood and concentrations (above background) of CO, SO?, NO, and CO, were measured by means of an electrochemical flue gas analyser manufactured by Industrielle Messund Regelsysteme fur Umwelttechnologie, Heilbronn, Germany, the IMR 3000P. The emissions of NO, encountered in this study were frequently close to or below the detection limit of the unit and are thus not reported. 2.6. Data acquisition, water evaporated, fuel burned and burn rate The concentrations of the gaseous pollutants, light obscuration,
orifice plate pressure drop
423
and mass (of stove, fuel, pot and water, combined) were recorded digitally at 10 s intervals throughout the test. Additionally, at the start of the test and at the end of each phase the pot was briefly lifted off the stove. From these mass records, and with wood and char treated as discussed in Section 2.2, the separate masses of wood supplied, char (and where applicable, wood) remaining, and water evaporated could be determined for each phase. In a number of tests the pot was briefly lifted off the stove every 2-3 min throughout both phases. This provided records versus time of water mass, and hence fuel mass, and thus the burn rate. It was found that the mass of water decreased essentially linearly with time during the simmering phase (as expected) and nearly linearly during the heating-up phase. Furthermore, the mass of water evaporated during the heating-up phase was a small fraction of the mass of fuel burned. Thus, for those tests in which only the total mass evaporated was measured, a linear change in mass was assumed for both phases. Thus, the instantaneous burn rate could be estimated for these cases as well. 2.7. Fuel and fuelling The wood used for testing was Eucalyptus plantation wood. The wood is not used in rural areas, but is considered to be a good barbecue fuel by urban dwellers. It is a hardwood, as are the types of wood preferred by rural users. Its moisture content was 10.7( + 1.3)%. The effect of type of wood on stove performance was studied in a separate set of experiments. The improved open fire and the one-pot metal stove were used to compare Eucalyptus grandis and Pinus patula, a softwood considered too fast-burning for satisfactory cooking. There was no difference (at the 95% confidence level) in the efficiency and emissions results for the two types of wood. This would seem to indicate that wood type is not a critical factor in this study. (The calorific values of the woods were obtained from a database of the Forestek Division of the South African Council for Scientific and Industrial Research; the values were 19.69 MJ kg - ’ for E. grandis and 20.90 MJ kg-’ for P. patula. The calorific value of char was taken as 28.0 MJ kg-‘, the value given by Baldwin.‘) Pieces of wood chopped to a “diameter” of less than 15 mm (capable of passing through a
grandis, a non-indigenous
424
G. BALLARD-TREMEER and H. H. JAWUREK 600
- - - - - - Sulphur dialid= - - - Total suapudod pwticuhos
0
500
1000
1500
2000
2500
3000
Time (seconds)
Fig. 2. Instantaneous concentrations
of CO, TSPs and SO*, one-pot metal stove.
ring of 15 mm internal diameter) were used as kindling. Wood with a diameter of 30-60 mm was used as main fuel. All pieces were 100 mm in length. The initial fuel charge consisted of 0.03 kg of kindling and approximately 0.12 kg of main fuel for the four smaller devices, and 0.75 kg for the larger two-pot metal stove. The open fires were fed radially and semi-continuously, following traditional practice. The stoves were fed intermittently, generally once during the heating-up phase and l-3 times during the simmering phase. 3. RESULTS AND DISCUSSION
It was found that for the one-pot metal stove, TSPs, CO, and SOz follow similar emission patterns during a test (see Fig. 2). Thus, if the ratios of the instantaneous concentrations of TSP and of SO, to that of CO were to be calculated they would approximate to constants. In the terminology of atmospheric chemistry,‘* the emission ratios of TSPs and SO, to CO are essentially constant over a test. Such behaviour has been noted by Lobert et aI.18 and Helas (personal communication, see also Ref. 19) for TSP, CO and various hydrocarbon products of incomplete combustion emitted from real and simulated Savannah fires. This finding has important implications for the taking of “grab samples” of fire plumes and in the estimation of the emissions of trace gases affecting atmospheric chemistry and climate.lg It should therefore be noted that considerably
more variable emission ratios than those for the one-pot metal stove are exhibited by the two other stoves of this study, and that this variability is even more marked in the case of the open fires. Figure 3 shows the emission patterns of the improved open fire; the SO,/CO emission ratio, for example, varies with time from 0.17 to 1.2; if the emission ratio were to be based on mean concentrations, or on cumulative masses emitted, its value would depend on the duration of the test. Care must therefore be exercised when using emission ratios in the study of stoves and cooking fires. The TSP data shown in Fig. 3 have the appearance of including a superimposed drift to higher values with time. It was initially thought that this was due to condensation of volatiles on the optical surfaces during a test. Subsequent checks, however, showed that such “window contamination” amounted at most to 1% obscuration over a test. Further, the drift-like effect was absent in numerous tests, see for example Fig. 2. The TSP emission patterns for the traditional open fire were very similar to those of the improved open fire. These fires therefore become increasingly smoky with time. (In absolute terms, however, they are low-smokeemitting devices--compare the TSP concentration scales in Figs 2 and 3.) A possible explanation for this might be the following: the fires were fed semi-continuously; they thus always contained “new” wood which means that there was always some smoke; the wood
Comparison of five rural, wood-burning cooking devices
425
04
0
500
1000
1500
2m
2500
co
3000
Time (saconds)
Fig. 3. Instantaneous
concentrations
of CO, TSPs and S02, improved open fire.
was fed radially inwards; this caused accumulations of ash in the combustion space in the middle of the fires, leading to the progressive reduction of air supply and thus to increased smoking. By the same argument, CO, also a product of incomplete combustion, should display a trend to higher concentrations with time. This trend is clearly discernible in Fig. 3, superimposed over emission peaks somewhat sharper than those for TSPs. It may well be that the dying-away of SO, emissions in the later stages of this test is attributable to the same reduction in combustion air that caused the increased emissions of TSPs and of CO. The
traditional open tire displayed similar CO and SO, emission patterns. The above behaviour is very different from that exhibited by the one-pot metal stove (Fig. 2) for which the products of incomplete and complete combustion-CO and TSPs, and SO,-respectively, varied “in phase”, as already noted. (In-phase emissions of CO and SO, have also been reported for a residential coal stove.“) Figure 4 shows the burn rate of the one-pot metal stove for the same test as that of Fig. 2. The burn rate varied very sharply with time (for example by a factor of 15 between 1000 and 1500 s) and it seems that this masked any
L)E-4 T 7E-4 -6E-4 -5i $
IE-4.-
d t
4E.4
--
E 3
38-4
--
2E-4
--
1 E-4 OE+O
t 0
I/
, 500
1000
1500
2ao0
2500
Time (suonds)
Fig. 4. Instantaneous
bum rate, one-pot metal stove.
A
v\
, 3000
426
G. BALLARD-TREIUEER and H. H. JAWUREK 20-
26 --
20 -s 6 .f I6 .Y fi
?? Improved open fire
00
10 --
5 --
O-l 0
4
1
2
3
4
5
6
Power (kw) Fig. 5. Efficiency and fire power.
changes in emission factor (mass pollutant emitted per mass fuel burned) that occurred during the test. (The three main peaks relate to the addition of fuel, one during the heating-up phase and two during the simmering phase.) The open fires on the other hand were operated more closely to steady state; they were thus less burn-rate-dominated and this allowed the changes in emission factor discussed above to be detected. The burn-rate effect was, however, not absent; thus the simultaneous peaks in CO and SO, at 1000 and 2000 s in Fig. 3 correspond to peaks in burn rate. The presentation of the main results starts with efficiencies. Figure 5 shows overall efficiency (over the heating-up and the simmering phases) versus fire power for the 29 tests that were conducted. Focusing attention initially on the one-pot devices (open data points) we note a general trend to reduced efficiencies with increasing fire power. This was also reported by Joshi et al.z1 in their study of four metal stoves fuelled with wood, crop residue or dung. It would appear that for our devices and those of Joshi et al. and for the ranges of power that were studied, the higher the power of a fire, the more of its energy is lost to the surroundings, rather than transferred to the pot. Further, some cooking devices are inherently more prone to losses than others. The open fire is thus doubly penalised: it has high losses to the ground and so needs to be built quite large if it is to achieve boiling, and it therefore suffers the additional size-related losses. (We should like to
stress that our open fires were sensibly operated; considerably lower efficiencies could have been achieved by building larger fires.) The improved open fire has reduced losses to the ground and can therefore be built economically small. Additionally, it is self-limiting in power, the grate being able to accommodate no more than a finite amount of wood at a time. Compared with the improved open fire the one-pot metal stove, being insulated, has reduced losses to the surroundings, but it incurs the added loss (as do all stoves) of raising its own mass to operating temperature. The stove does not, as might perhaps have been expected, outperform the improved open fire. Turning now to the two-pot stoves (solid data points) we note that they also obey the “economy of smallness” rule noted above. The data lie roughly parallel to those for the one-pot devices, but displaced to slightly higher efficiency. This would appear to be due to the energy recuperation by the second pot. The efficiencies of the improved open fire and of our stoves are slightly higher than those of Joshi et a1.2’ at the same power and with wood-fuelling. However, Joshi et al. extended their measurements to powers as low as 2 kW and achieved efficiencies of 30 and 37% with their “TARA” and “CP” stoves respectively. Our devices were difficult or impossible to operate at such low powers. The averages of the efficiencies shown in Fig. 5 are given in the first row of data in Table 1. The standard deviation is given in
Comparison
of five rural, wood-burning
brackets alongside each entry, and beneath this the range of data for the (generally) six tests per cooking device. The bold number by each cell is a rating on a scale of l-3 where 1 rates highest; where two (or more) devices have the same rating, their differences in efficiency (or other performance data) are not statistically significant at the 90% confidence level. The fire built on a grate with an efficiency of 21% is a substantial improvement on the open fire at 14% (the corresponding reduction in fuel consumption is 32%) and it competes successfully with the one-pot metal stove at 20%. The two-pot stoves had slightly higher efficiencies, but the differences between these figures and that for the improved open fire were not significant at the 90% confidence level. In presenting results on emissions we prefer to use the total mass of pollutant emitted per test (cooking task). This mass is directly related to human exposure. Emission factors, because of their denominator term (mass of fuel burned), include the effects of efficiency. (Emission factors for CO are shown in Table 1 for purposes of comparison.) Joshi et ul.*‘***also prefer the use of mass emitted per task, but generally report both it and emission factor. However, they define the task not as the simulated cooking cycle of the WBT, but as the transfer of a fixed, and much smaller, amount of energy-879 kJ-to the pot. Figure 6 shows efficiency as the abscissa and total CO emissions as the ordinate for the 29 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 comer is the region of good performance and the bottom right-hand comer the region of poor performance. Clearly the improved open fire is a good overall performer, with efficiencies not significantly different from that of the stoves and emissions significantly lower, see also Table 1. Its CO emissions are also marginally lower than those of the open fire, but not statistically so at 90% confidence. (The use of emission factors eliminates the difference between the two fires, the less efficient device, the traditional open fire, being unrealistically favoured.) Table 1 shows the stoves to have had CO emissions (mass per task) higher than those of the fires by factors of 2 to 3, with the one-pot metal stove the highest emitter. CO emission factors ranged from 19 to 66 g kg-‘; this is in close agreement with Joshi et al.*’ whose mean values were from 17 to JBB
II/S-E
cooking
devices
427
62 g kg-‘. Also shown in Table 1 is the CO to CO, molar ratio (ratio of total moles emitted during test). It is included partly because it is of interest to atmospheric chemistry,lg and partly as a further indicator of incompleteness of combustion. The ratio is by far the highest for the one-pot metal stove. It would appear that the incompleteness of combustion was due not to a shortage of primary air, but rather due to quenching associated with the very narrow gap between the top of the stove and the bottom of the pot-there was a visible reduction in smoke emissions when the pot was briefly lifted off the stove. The SO2 emissions were lowest for the open fire and the improved open fire; the stoves were higher emitting than the (averaged) fires by factors of 3 to 10, with the one-pot metal stove again the worst emitter. Emissions of TSPs were lowest for the improved open fire and the two-pot ceramic stove; the other devices were higher emitting by factors of 1.5 to 3. It should be noted that the health hazard of the two-pot metal stove is reduced by the presence of the chimney which removes the emissions from the immediate vicinity of the cooking site. Joshi et al.*’ observe increasing emission factors for CO and TSPs with increasing efficiency, but observe no such correlation for emission mass per task. The correlation is also absent in the present study (see Fig. 6). The positive correlation between emission factor and efficiency would thus seem to be due to the fact that the two quantities have denominators that are proportional to each other-mass of fuel burned, and energy content of fuel burned. This, together with the (real) negative correlation between efficiency and power, explains the negative correlation between emission factors and power observed by Joshi et al.“, and the absence of correlation between emission mass and power observed in the present study. Figure 6 shows the performance points of the open fires to be considerably less scattered than those of the stoves. Data of the Aprovecho Institute23 similarly shows open fires to behave more consistently than enclosed (“Louga”) stoves. We believe that the reason for this is that open fires are tended near-continuously, they are visible and allow immediate intervention in the event of any irregular or undesired behaviour. Stoves are fed intermittently and operate in a highly transient manner. This favours irregularities of combustion which,
CO to CO2 mol ratio,
%
1.30 (0.16) 1.08-1.56
22 (3) 18-25
0.058 (0.025) 0.03AM9
19 (3.7) 13.323.4
Time to reach boil, min
kg-’
0.891 (0.160) 0.65-l .08
SO, per test, g
factor,
15 (2.1) 12.9-18.0
14 (2.1) 11.6-16.8
Open fire (6)
Total TSPs per test, g
Total
CO emission
g
% (St. dev.)
Total CO per test, g
Efficiency,
Stove type (no. of tests)
1
2 1.02 (0.12) 0.85-1.17
22 (3) 17-27
0.523 (0.198) 0.28-0.78
0.082 (0.034) 0.05-0.13
1 2
19 (3.7) 15.4250
12 (2.2) lO.Sl5.5
1
1
21 (1.3) 19.7-23.1
2
metal
16 (2) 13-19 5.25 (1.39) 3.64-7.60
1
0.976 (0.278) 0.4w.12
1 2
0.694 (0.114) 0.5M.80
66 (15.2) 43.680.5
43 (9.8) 32.7-54.4
1
1
1
1
l-pot
devices ceramic
3
1
2
0.218 (0.062)
3
1.48-2.42
’1.83 (0.32)
22 (2) 19-25
0.492 (0.122) 0.34-0.61
0.14-0.33
38 (9.9) 23.4-52.8
28 (6.1) 20.1-37.5
24 (3.1) 18.1-26.2
2-pot
3
3
1
stove (6)
of the cooking
20 (3.1) 16.CL24.7
characteristics
open fire (6)
1. Performance
Improved
Table
2
2
1
2
2
2
1
stove (6)
metal
1.00 (0.62) 0.31-1.88
32 (5) 26-39
1.595 (0.306) 1.22-1.94
0.17-0.42
0.303 (0.113)
22 (8.0) 12.tL31.6
30 (11) 17.5Al.l
22 (3.0) 18.2-25.9
2-pot
1
3
3
2
1
2
1
stove (5)
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Comparison of five rural, wood-burning cooking devices 20
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10
20
30
40
so
I 60
Total Emissions (g) Fig. 6. Efficiency and total CO emissions.
because the devices are closed, go uncorrected for extended periods. Taking a final and broad look at the ratings shown in Table 1 we note the following: the improved open fire rates first, or jointly first, in all categories except time to reach boiling. The one-pot metal stove rates first in time to reach boiling (this would endear it to users), jointly first in efficiency, and last with respect to emissions (which, smoke excepted, are invisible). Clearly, efficiencies and emissions need to be determined before a stove design is disseminated. The traditional open fire is not the disaster it is generally held to be; while less efficient than the other devices, it rates second in overall emissions. The remaining two stoves rate joint first in efficiency, but are overall higher emitting than the traditional open fire.
4. SUMMARY AND CONCLUSIONS
For the one-pot metal stove the emission ratios TSPs/CO and SO,/CO were approximately constant over the duration of a test. The behaviour of the other two stoves was more variable, and for the open fires emission ratios changed with time by factors of up to 7. Care must therefore be taken when using emission ratios in the context of biomass-burning stoves and fires. Care is also required in dealing with grab samples (for subsequent analysis) of the flue gases of such devices.
The one-pot metal stove exhibited very sharp variations in burn rate. It seems that these overshadowed any changes in emission factor with time, thus leading to essentially constant emission ratios. The open fires were semi-continuously fed and operated more closely to steady state; this permitted additional factors influencing emissions to be detected. The average emissions of TSPs were lowest for the improved open fire and the two-pot ceramic stove; the other devices were higher emitting by factors of up to 3. The emissions of CO and SO, were lowest for the two open fires; the stoves were higher emitting by factors of up to 3 for CO and up to 10 for SO*. Overall, the improved open fire was the lowest emitting device and the commercial one-pot metal stove the highest emitting. It is thus important that new stove designs be tested for emissions before being disseminated. Mean efficiencies were 14% for the traditional open fire, 2 1% for the improved open fire and 20%-24% for the stoves, with no statistically significant difference (at 90% confidence) between the latter four devices. Acknowledgements-The authors offer their thanks to the following: the Director and staff, particularly Mr E.-J. P. Harvey, of the Wits Rural Facility (WRF) for their support and for facilitating field work related to this study; Dr D. I. Banks, previously of the WRF, for design input, student supervision, friendship, advice and criticism; Dr H. Annegarn and Mrs M. Kneen of Annegam Environmental Research for help with the calibration of the smoke meter; Dr. G. Helas of the Max Planck Institute for Chemistry, Ma&, Germany, for advice and helpful discussions; and the Foundation for Research Development (FRD) and the University for financial assistance.
G. BALLARD-TREMFER and H. H. JAWUREK
430 REFERENCES
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