Thermal performance and emission characteristics of unvented biomass-burning cookstoves: A proposed standard method for evaluation

Biomass 12 (1987) 247-270

Thermal Performance and Emission Characteristics of Unvented Biomass-burning Cookstoves: A Proposed Standard Method for Evaluation Dilip R. Ahuja, Veena Joshi, Kirk R. Smith* and Chandra Venkataraman Tata Energy Research Institute, 7, Jor Bagh, New Delhi 110 003, India (Received 3 December 1986; accepted 12 March 1987)

A B S T R A CT A method is proposed to measure emissions of air pollutants from unrented biomass-burning cookstoves and to incorporate a measure of these emissions in the existing way of rating cookstoves by thermal efficiency. Emission factors for the three metal stoves tested burning Acacia nilotica were found to range between 13 and 68 g kg-I for carbon monoxide and between 1.1 and 3.9 g kg- i for total suspended particulates and to increase with increasing thermal efficiency both within a stove and across stoves. Emissions for a uniform standard task -- the proposed performance index -- were, however, lower for total suspended particulates for the more efficient stoves but higher for CO, indicating that the increases in efficiency were not able to offset the greatly increased CO emission factors. Key words: C o o k s t o v e performance evaluation, indoor air polution, thermal efficiency, emission factors.

INTRODUCTION

Biomass fuels will continue to meet the cooking energy needs of a majority of people in poorer countries in the coming decades. Even though small-scale combustion of biomass often takes place under conditions that degrade air quality and are thermally inefficient, the increase in the prices of cleaner substitutes and their unavailability in many locations make remote the possibilities of a rapid shift away from the use of * Present address: Environment and Policy Institute, East West Centre, Honolulu. 247 Biomass 0 1 4 4 - 4 5 6 5 / 8 7 / S 0 3 . 5 0 - © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

248

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

traditional fuels. Worries about the effects of deforestation around human settlements and increased time and effort expended by rural families in the collection of combustible materials have caused a number of governments to attempt both to increase fuelwood supply by social forestry plantations and to decrease demand by promoting the use of more efficient cookstoves.l In addition to fuel economy, many improved stove programmes have claimed reduction in human smoke exposures. Indeed, before the 1970s, smoke was often the primary concern of such efforts in developing countries. But in the years immediately after the first energy crisis, reduction in smoke exposures seemed to be relatively less important in most programmes. Recently, a number of studies have documented that unvented biomass-burning stoves emit large amounts of health-damaging air pollutants, often leading to extremely high human exposures. 2-4 As a result, there has been increased interest in relative smokiness as an important attribute by which to judge stoves. Pollutant emissions and fuel consumption, of course, are only two of the attributes upon which to rank stoves. Other criteria for stove evaluation include time to perform a task, ease of cooking particular types of food, the ability to see and monitor the fire, the ease of manufacture and maintenance, multifuel capability and interaction of the stove with the structural integrity of the dwelling. In addition, there may be important lessons to be learned by comparing the determinations of such characteristics achieved by measurement to those achieved by seeking the perceptions of the users. Although not wishing to downplay the roles of these other attributes and of human perceptions our intention in this study is to help provide the information needed to make numerical comparisons about the two factors that are of principal concern in most programmes designed to enhance human well-being through dissemination of improved stoves: fuel economy and smokiness. We do not intend to propose modifications to the methods recently developed by others to measure fuel economy, but will propose a new method to enable quantitative comparisons of stoves using a measure that incorporates both thermal efficiency and emissions. In addition, as will become clear, the two attributes are not independent and trade-offs exist between efficiency and emissions. 1.1 Present status of standard methods

Dissemination programmes for improved cookstoves have sometimes been marked by claims of large fuel savings based on laboratory tests. In the past, these claims were difficult to verify partly due to a lack of stand-

Environmental and thermal performance of cookstoves

249

ard methods for testing fuel economy both in the laboratory and under field conditions. A range of often incompatible methods were developed and used by different investigators. The resulting difficulty of comparison led to the development and publication of a set of proposed standard methods through the efforts of a workshop with representation from a number of the principal research groups 5 and, after a period of review and revision, to a final set of standards. 6 Although there are still some remaining controversies pertaining to these standards, 7 it is now possible for agencies wishing to promote improved cookstoves systematically to test the thermal efficiency of different stove-fuel-pot combinations prior and subsequent to large-scale dissemination activities. Widespread application of these methods, however, has yet to occur in many countries. Unfortunately, there are as yet no standard methods for measuring the 'smokiness' of unvented biomass-buming cookstoves in the laboratory or the field, although there have been some initial efforts. 8,9 There are, however, methods available for unvented gas-fired stoves ~° and woodfired heating stoves with flues, ~! as well as many other less closely related combustion devices with flues, chimneys or exhaust pipes. This gap may seem surprising in that the open biomass-burning stove is still the most common combustion device in the world. There are special characteristics of such stoves that make emissions monitoring difficult: Because there is no venting, it is not possible to measure pollutant concentration in a flue along with flue air velocity to determine total emissions as is the strategy for most emissions measurement methods; -- Because biomass fuels have substantially different emissions at different times during the burn, it is not appropriate to measure short-term steady-state emissions as is possible with liquid and gaseous fuels. Because cooking is not a single continuous process, measurements need to be made over some sort of cooking cycle in a manner analogous to the driving cycle used for monitoring the fuel and emissions characteristics of motor vehicles.

-

-

-

-

There have been basically two approaches to handle these problems. The first is to place the stove under a hood into which all the flue gases are drawn mechanically. The hood method (sometimes called the 'direct' method) has been used in studies of unvented gas cookstoves ~2and kerosene space heaters ~3 in developed countries. It can be used for stoves with flues, as in the proposed standard method for wood heating

250

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

stoves. 1~ Preliminary tests have been done also with the unvented biomass-burning stove types of interest here. By either directly measuring the air flow in the hood s or by estimating the air flow through mass balance calculations using nitrogen and carbon] 4 the dilution of outside air can be estimated and the pollutant emissions per unit fuel burned can be determined. A variant of this approach has been used in which ratios of pollutants are monitored without any determination of air flow for stove-fuel combinations, such as gas cookstoves, where steady state combustion conditions are achievedJ ° Several difficulties are apparent with the hood method, In general, the determination of air flow requires a significant increase in cost for the facility and substantial care in operation. Even so, its accuracy is often a limiting factor in the final result. Another problem is the potential for the mechanically induced air flow to change the combustion characteristics of the stove. A third problem is the potential for the hood to physically interfere with the tending of the cookstoves during measurements. Finally, the hood system, while not expensive by laboratory standards, costs somewhat more to build and operate than the alternative described below and is limited to the laboratory setting. The second approach, the chamber (or 'indirect') method, requires no ductwork and air flow calibrations. In principle, it can be done in any chamber or even in a remote village house where the ventilation conditions are relatively constant over the period of measurement. The stove is simply put through a cooking cycle in a room and the pollutant concentrations are monitored within the same room. Thus, although sharing with the hood method the same need for pollution monitoring equipment, this method requires little other expense. In addition, airflow conditions around the stove can be simulated much more closely with this method. There are disadvantages as well. One is the need for the operators to subject themselves to the smoke while tending the stove and instruments inside the chamber. This exposure can be avoided, of course, by remotely operating these systems as is usually done with such measurements in developed countries for unvented gas cookstoves ~2 and kerosene space heatersJ 5 It is doubtful, however, that biomass-fired cookstoves could be successfully operated by remote means at reasonable cost because of the relative complexity of the cooking and fueling cycle compared to those for other stoves. Another problem shown in previous chamber studies, 9'16 is that such stoves produce a significant degree of smoke stratification in the room, making difficult the determination of mean concentrations.

Environmental and thermal performance of cookstoves

251

1.2 Approach in this study In this paper, we discuss development of an improved chamber method for use with unvented biomas-burning cookstoves. In particular, we have systematized the means of determining ventilation rate and have tested the effect of introducing fans to eliminate stratification of pollutants within the chamber. We have also been able to utilize relatively inexpensive and portable equipment that need not be dedicated to this system but be used for other types of research as well. Our approach has been to design the emissions monitoring system such that it can be operated simultaneously with determinations of thermal performance. In this way, trade-offs between thermal and emissions performance can be investigated. Two pollutants are monitored: carbon monoxide (CO) and total suspended particulates (TSP). Both these pollutants are known to be health-damaging and to occur at high levels in biomass smoke and inside houses burning biomass in unvented appliances. 3'~7 CO is perhaps most important as a potential short-term (acute) hazard, while TSP is the best indicator of long-term (chronic) toxicity. Indeed, TSP, under the name of 'tar', has been used as the single best indicator of the hazard of the most well studied biomass smoke, that from tobacco. TM Furthermore, having a physical particulate sample enables later laboratory determination of individual hazardous compounds such as benzo(a)pyrene. Both are indicators of poor combustion for, unlike solid fossil fuels, most biomass fuels contain few toxic contaminants that remain after complete combustion. To illustrate our proposed method, the emission factors and other parameters for three unvented cookstoves are reported and compared with values obtained through other methods.

EXPERIMENTAL PROCEDURE Both thermal and emissions performance during biomass combustion are affected by a bewilderingly large constellation of factors. 7,17 These include the fuel species and conditions, stove designs, pot sizes, shapes and materials, the manner in which the stove is tended, the types and amount of food being cooked and, of course, the ventilation conditions. Thus, in order to obtain a reliable ranking of stoves, as many factors as possible need to be held constant during the tests.

252

D. R. Ahufa, It. Joshi, K. R. Smith, C. Venkataraman

2.1 Test site

All experiments were conducted in a structure constructed in the courtyard of a two-storeyed house in a predominantly residential section of New Delhi. It was sheltered from busy roads about 100 m to its west and north by two separate rows of houses. To simulate conditions in a village kitchen, a 16 m 3 hut with mud walls and a thatched roof was constructed in the centre of the courtyard (Fig. 1 ). ~

Enclosure

1 9

13

N

4

6

12

5 I0

II I

Im

I

Fig. 1. Plan of the test-site showing the hut and the enclosure. 1, Stove; 2, monitors; 3, door; 4, 5, 6, windows; 8, door; 7, 9, 10 and 1 1, windows; 12, fan (oscillating); 13, fan (stationary).

2.2 Ventilation conditions

The mud structure was surrounded on all sides by an asbestos sheet enclosure (volume 52 m 3) to provide an extra layer of shelter in addition to that provided by the building and the courtyard. This outer enclosure reduced the wind effects and helped to keep ventilation conditions uniform throughout an experiment and between experiments. The location of doors and windows are shown in Fig. 1. The windows were more numerous and larger than found in a typical village setting but were sized to enable ventilation conditions to be varied, if desired. The volumes of the two buildings were similar to those in many Indian village houses. 2 During the initial experiments on smoke stratification, only the door of the hut and the southern window of the outer enclosure were kept closed. During subsequent experiments all the openings of the inner

Environmental and thermal performance of cookstoves

253

enclosure were left open along with the same southern window of the outer enclosure. Also, in the second set of experiments, two low-speed pedestal fans (1.25 m high) were used in the space between the two enclosures to reduce vertical stratification of pollutants. The one at the north window of the hut was oscillating and facing inwards, the other at the south window was stationary and facing east. Both were tilted slightly towards the roof so as not to interfere with the combustion. 2.3 Fuel characteristics The wood used for the experiments was Acacia nilotica (local names -keekar, babul) obtained in one lot from a wholesale dealer, split and airdried. Three or four similar semi-cylindrical pieces of wood (2-4 cm in diameter and 20-25 cm in length) were used during each experiment. The moisture content of the fuelwood was determined periodically. At the end of the entire series of experiments, the calorific value of the fuelwood and the charcoal left over were determined using a bomb calorimeterJ 9 2.4 Description of stoves Three metal cookstoves were used in the experiments. One was a conventional U-shaped design procured from a retailer whereas the other two were more efficient designs obtained from their respective developers: T A R A from Development Alternatives in New Delhi and the other from SMVM Community Polytechnic in Tanuku, Southern India. The stove design parameters are summarized in Table 1. As shown in Fig. 1, stoves were placed in the corner of the hut on a digital platform balance (accuracy 10 g) which monitored the instantaneous weight loss (Model 8040-D, Libra Industries).

TABLE 1 Stove Design Parameters

Stove

Weight Wall Combustion Height from Heat (kg) thickness" volume grate to pot transfer (mm)

(litres)

bottom (cm)

area

#'m:) TARA Community polytechnic stove Conventional metal stove

1"6 2-0 2.3

0'9 0-9 1-0

3"5 4"0 6-8

12"3 8"5 22"0

900 450 450

254

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

2.5 Placement and use of pollutant monitors

The x and y distances of the CO and the TSP monitors were 1 m and 0.5 m respectively relative to the centroid of the stove combustion zone (Fig. 1 ). There was in all experiments a pair of monitors with a z coordinate of 1 m; other monitors, were placed at various heights along the same axis. Particulates were collected on a 37 nun teflon-coated glass fibre filter (Paliflex Corporation) with a low-flow personal air sampling pump (Model HFS 113, Gilian Corporation). The flow rate of the pump was adjusted to about 3 litres min-~ and calibrated before each experiment with a bubble tube. 2° Experimental filters were weighed immediately before and after each experiment on an analytical balance accurate to 0.01 mg (Model SB-4, Scientific Instrument Co.) The difference between the post-test and pre-test weights was recorded as the collected particle mass. 20 In some of the experiments, we also monitored instantaneous particulate concentrations with a handheld light-scattering monitor (MINIRAM, Model PDM-3, G C A Corporation). This optical monitor was calibrated using the gravimetric personal air sampler. All TSP readings include background concentrations which, in separate measurements in and around the hut, were found to be of the order of 140/~g m-3. Carbon monoxide concentrations were measured with a GasTech Corporation portable monitor (Model CO-82). The gain of the CO monitor was adjusted before each experiment by using a span gas of known concentration (108 ppm). Since the CO monitors are 'zeroed' before each experiment, the readings give the contribution from the cookstove and do not include background levels. 2.6 Experimental sequence

The method proposed here attempts to minimize variations in emission characteristics by using a single charge of fuel and by minimal tending of fire so that a single test could be used to estimate both thermal and environmental performance. Three and a half kilograms of water were boiled in a covered, flatbottomed, aluminium vessel of 5 litres capacity with a thermometer (0.5°C accuracy) inserted through a rubber stopper in the lid. Just enough wood was used in each experiment to bring this water to the boil and keep it boiling for about 1 5 minutes. The quantity was different for each stove and was determined on the basis of trial runs and rated stove efficiencies. The vessel, lid and thermometer, water, and wood were all weighed on a pan balance ( 1 g accuracy).

Environmental and thermal performance of cookstoves

255

Ends of all the pieces of wood were dipped in a measured amount of kerosene before lighting. T h e fire was tended only to ensure a steady flame. Both types of pollutant monitors were turned on as the fire was being lit. CO concentrations were recorded every minute and water temperature every four minutes. The time at which water came to a boil was also noted. Ambient temperature, pressure and relative humidity and wind speed in the courtyard were recorded. The water boiling test was terminated and the TSP pump was stopped when the water temperature dropped by 0.5°C. The vessel was reweighed to determine the mass of water evaporated. T h e remaining pieces of charcoal were weighed and then removed from the proximity of the test site. Subsequently, the decay of CO concentration in the room, now without any CO sources, was recorded for 20 minutes. At the time our experiments were done, an international standard method 5 called the 'water-boiling test' was available for evaluating the thermal performance of stoves. The water boiling test method proposed herein simulates a cooking cycle only in so far as it includes a relatively high power burning phase followed by a low power phase, albeit of varying durations. It differs from the VITA standards in that we limited ourselves to a single charge of fuel and minimal fire tending to minimize variations in emissions characteristics.

DATA ANALYSIS Since there can be confusion about the definition of seemingly simple terms, the exact methods used for calculations are listed here and the symbols are defined in the Appendix.

3.1 Moisture (M) and energy ( H ) content The moisture content (dry basis) is calculated on the basis of the weight loss of a sample that was oven-dried at 100°C for about 36 h until the weight of the sample stabilized. M(°/o) =

100( Ww- wd) W~

(1)

About 6 g of fuel chips from different parts of the fuel were used for moisture content measurements. T h e bomb calorimeter tests yielded gross calorific values of wood and wood charcoal. The net calorific

256

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

values determined 19 and used in subsequent calculations were 17.6, 30.4 and 41.9 MJ kg- ~for dry wood, charcoal and kerosene, respectively.

3.2 Burn rate (F) The burn rate is corrected for the amount of kerosene used, the charcoal remaining and the moisture content of the fuelwood: F(kgh-l)=l[t[

lOOWw W k H k ilO0+M) +-Hw

W~H~Hw

(2)

3.3 Thermal efficiency (n) The amount of heat used to evaporate water is considered as useful heat input to the vessel since our primary interest is to compare stoves rather than derive an absolute number for cooking efficiency for any given stove. The burn rate and net corrected calorific value of wood are used in the calculation of thermal efficiency:

n(O/o)=(Wwia(Tf-Ti)+(WW,-FtHw Wwf)L) x l 0 0

(3)

This expression gives the overall thermal efficiency and is the product of the combustion efficiency, n c, which measures the extent to which the chemical energy in the wood is converted to heat, and the heat transfer efficiency, nt which indicates the fraction of the heat transferred to the pot and its contents. The heat absorbed in the pot is considered as a loss in our calculations. If this were to be considered as useful heat, it would typically raise our calculated overall thermal efficiencies by about 3%, i.e., an efficiency of 33% would become 34%.

3.4 Air exchange rate (S) The air exchange rate (S) of the test chamber was determined by the slope of a least square fit to the natural logarithm of the decay of CO concentration with time after the charcoal had been removed from the hut, using the following relationship: S(h-') = (In Co - In C~)/t~

(4)

Thus, a plot of ln(C~) against time will have a slope of - S . Figure 2 shows this graph for one of the experiments.

Environmental and thermal performance of cookstoves

257

6O 5O

-o

40

% 30

° .

E

v

~ 20 o

Q)

© (D

I0

I

0

Fig. 2.

I

OI

t

I

0.2 Time (h)

I

0.3

Air exchange rate (3.8 h-~) for a typical experiment as determined by the decay of CO concentration with time.

3.5 Measured mean pollutant concentration (Ca) The change in the mass of the filters, the flow rate of the sampler and the test duration are used in the calculation of the mean TSP concentration as follows: Ca(mg m - 3) = ( Wff - Wfi)/(Qt)

(5)

The mean CO concentrations are calculated by averaging the instantaneous concentration values.

3.6 Theoretical pollutant concentrations (Ca, Ce, Cm) The tests were always started with the chamber free of smoke. In addition, we have assumed that outdoor concentrations of these pollutants are negligible and that little smoke reinfiltrates from the outside after exiting the chamber. Finally, we assume that little deposition, resuspension or chemical transformation of these pollutants occurs in the time flame of concern. With these assumptions, the instantaneous indoor concentration can be estimated by: 21

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

258

Ci(mgm-3)=(FE/VS)(1

- exp[-

tiS])

(6)

When the monitoring time is large (t ~> S - ~), the equilibrium concentration becomes:

Ce(mgm -3) = FE/VS

(7)

Many instruments, such as the sampling pump used here -- for particulates -- monitor mean concentrations over extended time periods. The mean concentration during a period, t, after the start of the fire is:

Cm(mgm-3)=(FE/VS)(1

+(exp[- tS]-

1)/tS)

(8)

Since the tests were always started with the chamber free of smoke, the measured concentration during a test period is always less than the concentration that would eventually be reached under equilibrium conditions. It is the latter, however, that is of most interest in this study because it reveals the emission factor separately from the time constant imposed by the chamber. The equilibrium concentration can be calculated by two methods. The first uses the measured mean concentration: C~(mg m-3) = Cm/(a + ( e x p [ -

tS]- 1)/tS)

(9)

The second utilizes the measured instantaneous concentration: Ce(mg m - 3) =

Ci/(1 -

exp[ - tiS])

(10)

There are important trade-offs among measurement duration, air exchange rate and measured mean and instantaneous concentrations. Shown in Fig. 3, for example, is the effect of air exchange rate on the extent to which measured mean and instantaneous concentrations repre-

~-- [ Instont(Ih) /

"~60 L / / X ~

O 0

Fig.

3.

2

4

"~MeOn(O'Sh)

~ 6 8 10 12 14 16 Air Exchange Rate (h-I)

18 20

M e a n and instantaneous i n d o o r pollutant concentrations as a function of air exchange rate. Curves for m e a s u r e m e n t periods of 0-5 and 1 h are shown.

Environmental and thermal performance of cookstoves F-

"~" C~C~ ~--

~-

~

259

Instant(6h -I )

..->"- ..----"~I3h-,~

60

~)

0

0

0.4

08 12 16 Measurement Durofion(h)

2

Fig. 4. Mean and instantaneous indoor pollutant concentrations as a function of measurement duration. Curves for air exchange rates of 3 and 6 h - ~are shown.

sent equilibrium concentrations. Note that for a measurement duration of 30 min, the measured mean concentration is less than 90% of the steady state value for all air exchange rates less than about 20 h- 1. Fig. 4 shows the effect of duration where, at an air exchange rate of 6 h- 1, for example, the measurement duration would need to be at least 1.5 h in order for the measured mean concentration to approach 80% of the steady state value. These figures illustrate that for most conditions of practical interest, i.e. measurement durations of less than 1.5 h and air exchange rates less than 10 h-1, corrections to the measured mean values will be needed to accurately estimate equilibrium or steady-state values. In reporting equilibrium concentrations, instead of using eqn (10), we have utilized a least-squares optimization procedure (curvilinear regression) to reduce the effect of short-term variations in measured concentrations. In those tests where we measured only mean particulate concentrations, the equilibrium particulate concentrations were obtained by multiplying these by the ratio of Ce and Ca for CO. 3.7 Emission factor ( E )

The emission factor for a particular pollutant is derived (from eqn 7) by assuming equilibrium conditions, uniform mixing and constant air exchange and burn rates during the test, i.e. E(g k g - ' ) = C~SV 1000 F

(11)

260

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

3.8 Emissions per standard task

(Etask)

In order to compare stoves using a composite index that incorporates both efficiency and emissions, we defined a standard task as one that would raise the temperature of 3.5 kg of water by 60°C, i.e., transfer 879 kJ of energy to the contents of the pot. We can then calculate the total emissions of a pollutant during the performance of this task. Etask ----"E

x Fx/'task

(12)

If the task were defined as that required to transfer a unit of energy (kWh), then it is a trivial matter to calculate emission per unit of useful heat delivered.

RESULTS A N D DISCUSSIONS The stratification experiments yield information about the distribution of concentrations and consequently might be used to estimate the human exposures. It has been found, however, that it is most often necessary to utilize personal monitors worn by householders to obtain accurate representations of true exposure because of the many spatial and temporal variations of indoor concentrations and human behaviour in village houses. 22 One of the advantages of our method is that it utilizes such monitors, which then can serve double duty in a research group interested in conducting exposures studies as well as emissions measurements. In tests without mixing fans, strong vertical stratification was verified, as shown for CO in Fig. 5 with the concentrations at 3 m above the combustion zone (near the roof) being thrice those at 1 m. The use of the two fans was shown to be effective in eliminating stratification (Fig. 5). Table 2 summarizes the measurements and the calculations for the stratification tests on the T A R A and the conventional metal stove (CM) whereas Table 3 conveys the same information and the derived emission factors for the three stoves with mixing. As would be expected, mixing increased the mean air exchange rates, from 2.6 h - ~to 5.2 h - ~for the conventional stove and from 2-7 to 4.1 h -~ for TARA. Increased air exchange rates, however, do not have a consistent effect on burning rates. The higher mean efficiencies for T A R A during stratification experiments and for the conventional stove during mixing experiments can be explained by changes in their mean burn rates (Fig. 6). In spite of lower air exchange rates for the stratification experiments, the average values of measured mean concentrations for these experi-

Environmental and thermal performance of cookstoves '~280 ~

•

8,

uJ 0

261

°

40 0

+ ............ L

0

I 0.4

l 08

+

I

I I J _I L 1 I I I 12 16 2 24 28 Monitor Heights (m)

Fig. 5. Equilibrium concentratmn of CO at different heights within the test site. Broken lines represent experiments in which the mixing fans were used to remove stratification of smoke that resulted in experiments where fans were not u s e d (solid lines).

TABLE 2 Stratification Experiments: Thermal and Environmental Parameters

Stove

S (h- i)

F (kg h- ~)

n (%)

track

[CO L

[CO]e

[TSP]a

[TSP]c

(h)

(mg m-3)

(mg m- 0

(mg m- 0

(mg re-c)

TARA

1'8 2"5 3"6 1"5 3"1 1"3 3"7 3'7

0"41 0'47 0'53 0"57 0"59 0"59 0"62 0"65

30 29 37 43 38 34 36 39

0"32 0"35 0'30 0'25 0'20 0"35 0'23 0"27

44 44 35 21 37 35 30 26

82 86 55 48 58 108 44 38

---1"8 2"1 -1'6 --

---4"0 3"2 -2-3 --

Mean

2"7

0-55

36

0"28

34

65

1.8

3'2

CM

2.4 3'6 1.2 3'2

0.61 0"73 0.91 0.65

12 15 17 15

0'65 0.40 0.30 0.50

38 28 33 .

60 47 110 .

1.9 3.0 6.7

3'1 5'1 22.2

2.6

0.73

15

0.46

33

3.9

10" 1

Mean

.

72

.

ments for both CO and TSP are lower than in mixing experiments. This is a reflection of the particular vertical location chosen for samplers in the stratification experiments. Whereas for the mixing experiments the position of monitors is not critical, it is possible to get lower, the same or

S

3"8 5'0 3"5

4'1

5"1 4"9 4"7 6'4

5"3

3"3 6'7 5'7

5'2

Mean

CP

Mean

CM

Mean

~h-')

TARA

Stove

0"94

0"75 1"02 1"05

0"44

0"37 0"38 0"43 0"59

0"42

0"38 0"42 0"46

F (kg h -~)

15

13 17 16

37

40 37 36 34

31

0"35

0"52 0"23 0"30

0"33

0"35 0"38 0"35 0"23

0"39

0"42 0"37 0"38

(h)

28 31 34

/task

n

(%)

40

39 45 35

72

58 77 84 69

46

35 39 65

(mg m - 3)

[CO]a

57

56 65 51

98

90 102 117 84

67

47 56 97

(mg m - -~)

[CO]e

16

13 22 14

62

65 68 66 48

33

24 35 39

(g kg - ')

E (CO)

4"9

4"9 5"3 4'5

8"7

8"4 10"0 10"0 6"5

5"3

3"9 5"3 6'8

(g task - i)

Etask(CO)

4"5

3"9 5"5 4"1

3"1

3"4 3"6 2"9 2"5

2"9

3"0 1'3 4"5

(rag m - 3)

[YSP]a

TABLE 3 Mixing Experiments: Thermal and Environmental Parameters

[YSP]e

6'5

5"5 8"0 6"0

4"3

5"3 4"8 4"0 3"1

4"2

4"0 1"8 6"8

(mg m - 3)

1'9

1"3 2"7 1"7

2"8

3'8 3"2 2"3 1"8

2"0

0"55

0"50 0"62 0"52

0"38

0"48 0"46 0"33 0"24

0"32

0"33 0"17 0"47

(g task- 9 Cg k g - 9

2"1 1"1 2"7

Eta~k (TSP)

E (TSP)

tO

tO

Environmental and thermal performance of cookstoves 42

38

--

o ~.

® ~..-~..% /

34-

263

/ /

.~

o

O

O x TARA

cP

@ / Y~-~

30

~,~

26-

"

22i-

18 i

+

I(3 ~ 0.3

Fig. 6.

~)~r

*_t~ . . . . . . . . . .

L4

[

I 0.5

1+~

I

1

t

0.7 0.9 Burn Rate (kg h-I)

] I.I

Thermal efficiency as a function of burn rate. Circled points indicate experiments with mixing fans.

higher concentrations when the pollutants are allowed to stratify depending upon the height of the monitors. This uncertainty in the measurement of concentrations and emission factors is avoided by the provision of mixing fans. For both stoves, however, the (TSP)/(CO) ratios are similar for the two experimental conditions, indicating that particulates from biomass combustion (mostly respirable) diffuse like gases. This is also expected from their size distribution of less than 0.5/.tm mass median diameter.9 The emission factors derived from the mixing experiments should be closer to the true values because the assumption that the pollutants are well mixed is more closely satisfied. The emission factors for the three stoves are, as shown in Table 3, between 13 and 68 g kg -~ for CO and between 1.1 and 3.8 g kg-~ for TSP. The reported values in the literature ~7 for CO are 11-180 g kg -1 for fireplaces, 43 g kg -~ for cigarette sidestream smoke and about 40-100 g kg-~ for typical cooking fires and 50-300 g kg- ~for space heating woodstoves.23 Our values for CO conform to this range. Similar values quoted ~7 for TSP are 2-29 g kg- 1 for fireplaces, 24 g kg-~ for sidestream cigarette smoke, 0-3-15.0 g kg-1 for cooking fires and 3.0-50.0 g kg-~ for space heating woodstoves}3 Our values lie at the lower end of the range quoted. Though not mimicking indoor conditions that would obtain in practice, the use of mixing fans allows us to arrive more accurately at an estimate of true emission factors. Although this was not their focus, Smith et al. 9 w e r e able to estimate emission factors in a chamber study designed to measure particle size distribution of smoke from open clay cookstoves using different

264

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

fuelwood and burn cycles. They reported values of 2-6 g kg-~ for CO and 0.1-0.8 g kg -1 for TSP with somewhat higher levels for cowdung combustion. These values are substantially lower than ours but also do not reflect corrections for stratification and the difference between mean and equilibrium concentrations. Butcher e t al. 8 proposed and tested a hood method for making such measurement. They used a different wood and a different burn cycle in several open cookstoves to obtain emission factors of 39-110 g kg- =for CO and 0.3-15.0 g kg -1 for TSE We have reported here somewhat lower levels for each. More work will be needed to discover whether the differences are due to the variations in fuels, stoves or the measurement technique. Among three stoves tested, the conventional metal stove had the highest burn rate and the lowest emission factors during mixing experiments. Both TARA and the Community Polytechnic (CP) stove had mean burn rates that cannot be distinguished from one another and emission factors and efficiencies that were higher than those for the conventional stove. A plot of mean emission factors against mean efficiencies (as shown in Fig. 7) reveals that the emission factors for CO increase sharply with increasing efficiency. This relationship is not only displayed values when mean values for individual stoves are compared but also within the experimental set for a given stove (Fig. 8). Even though the relationship of mean emission factors vs efficiency is not as striking for particulates as it is for CO (Fig. 7), within a stove TSP emission factors also seem to increase with increasing efficiency (Fig. 9). This figure also illustrates the relatively large variability of TSP emissions. This increase

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Environmental and thermal performance o f c o o k s t o v e s

265

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in emission factors with increasing overall thermal efficiency (n) indicates that the improvements in n come from improvements in the heat transfer efficiency with the simultaneous deterioration in the combustion efficiency. Though a higher efficiency stove might have higher emission factors, the total emissions for the performance of a task, however, could be less because a smaller quantity of wood would be required. Emissions per

266

D. R. Ahuja, V. Joshi, K. R. Smith, C. Venkataraman

task can be used as a performance index to compare between stoves. Emissions per task for CO and TSP are also listed in Table 3 for the three stoves. It is seen from Table 3 that the increased efficiency is able to compensate for the increased emission factors for both T A R A and CP over CM in the case of particulates but is not enough to offset the greatly increased carbon monoxide emission factors. A n obvious dilemma is produced by the choice between two stoves where one is more efficient but produces more emissions per task as well. Since exposures are also related to housing conditions and human behaviour, the best procedure would be to compare actual thermal performance in the field with measured personal exposures. A n advantage of this method over the hood method is that such studies can be carried out in the field in developing country conditions as they have in developed countries for gas cookstoves. 24 As long as the ventilation conditions of the household can be determined using the CO decay procedure (Section 3.4), stoves can be tested in situ. The flexibility of doing such field work is another advantage of using the handheld instruments. It might be expected, however, that stove performance on both parameters (fuel and smoke) will be both poorer and more variable under field conditions. Redefinition of the standard task might help pin down the relative performance of stoves for food types or cooking patterns of particular interest. Our purpose here, however, is to suggest ways of evaluating stoves for these factors in laboratory studies. A stove is likely to be preferred over an alternative if it has a higher mean thermal efficiency and lower emissions per task. By this token, both T A R A and CP stoves would be preferred over the CM if TSP emissions per task is the deciding performance index. However, this does not help us choose between stoves where one has a higher efficiency but also higher emissions per task. This is the case with the three stoves if the emissions of CO per task is chosen as the performance index or for that matter between T A R A and CP even for TSP emissions per task. To determine how much of an improvement in efficiency is worth a deterioration in air quality, some sort of analysis including the costs and benefits of each would need to be performed. 25

CONCLUSIONS We believe that the physical system described here offers a means through which standard methods can be developed for simultaneous monitoring of stoves for efficiency and emissions. We have applied it to

Environmental and thermal performance of cookstoves

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three unvented stoves and, though our conclusions are based on few data points, have concluded that: 1. The assumption that pollutants are well mixed, often made in indoor air quality studies, is untenable. We found significant vertical stratification but little horizontal stratification within a room with the stove. Depending upon the choice of placement of monitors, emission factors calculated from 'stratified' conditions will underestimate true values by factors between 1.6 and 3. 2. For different stoves, emission factors seem to increase markedly with improvements in efficiency because these are often accompanied by reductions in burn rates. Therefore, it is important to monitor both efficiency and emissions on the basis of a standard task to determine the potential effect on human air pollution exposures. 3. Finally, unlike the hood method, we believe that our approach can be easily adapted for testing the indoor smoke leakage of improved stoves with chimneys as well as a range of unvented combustion appliances burning solid fuels other than biomass.

ACKNOWLEDGEMENTS This research was funded in part by a grant (No. 323) from the Environmental Research Committee of the Department of Environment, Government of India. At the time this research was done, Kirk R. Smith was a Visiting Senior Fellow at the Tata Energy Research Institute, New Delhi. KRS is grateful for support received from the Indo-US Fellowship Program of the Council for International Exchange of Scholars and the US National Science Foundation. We thank Ms Sharmila Sengupta for help with the conduct of the experiments and Dr Ashok Gadgil for his comments on an earlier draft of this paper.

REFERENCES 1. Foley, G. & Moss, P. (1983). Improved cooking stoves in developing countries, Earthscan Energy Information Programme Technical Report No. 2, London. 2. Smith,K. R., Aggarwal,A. L. & Dave, R. M. (1983). Air pollution and rural biomass fuels in developing countries: A pilot village study in India and implications for research and policy. Atmospheric Environment, 17, 2343-62.

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3. DeKoning, H., Smith, K. R. & Last, J. M. (1985). Biomass fuel combustion and health. Bulletin of the World Health Organization, 63 (1), 11-26. 4. Ahuja, D. R. (1985). Domestic air pollution from biomass combustion, Energy Environment Monitor, 1 (2), 2-20. 5. Volunteers in Technical Assistance, Inc. (1982). Testing the efficiency of wood burning cookstoves: provisional international standards, Arlington, VA. 6. Volunteers in Technical Assistance, Inc. (1985). Testing the efficiency of wood burning cookstoves: international standards, Arlington, VA. 7. Baldwin, S. (1986). Biomass stove technologies: engineering design, development and dissemination, Volunteers in Technical Assistance and Princeton University, Arlington, VA. 8. Butcher, S. S., Rao, U., Smith, K. R., Osborn, J. E, Azuma, P. & Fields, H. (1984). Emission factors and efficiencies for small scale open biomass combustion: toward standard measurement techniques. Meeting of the American Chemical Society, Washington, DC. 9. Smith, K. R., Apte, M., Menon, P. & Shrestha, M. (1984). Carbon monoxide and particulates from cooking stoves: results from a simulated village kitchen. Third International Conference on Indoor Air Quality and Climate, Swedish Council for Building Research, Stockholm, 4, 389-95. 10. Indian Standards Institute (1984). Specification for domestic gas stoves for use with liquefied petroleum gas, IS: 4246, New Delhi. 11. American Society for Testing and Materials ( 1985). Proposed test methods for heating performance and emissions of residential wood-fired closed combustion chamber heating appliances. ASTM, Philadelphia. 24 pp. 12. Davidson, C., Borrazzo, J. E. & Hendrickson, C. T. (1986). Pollutant emission factors for gas stoves: a literature survey. US EPA, Report CR-81254301-0, Research Triangle Park, NC, USA. 13. Lionel, T., Martin, R. J. & Brown, N. J. (1986). A comparative study of combustion in kerosene heaters. Environment Science and Technology, 20, 78-85. 14. Islam, N. (1985). Experimental study of small scale downdraft combustion for combustion efficiency and combustion generated air pollution. Unpublished masters thesis in Mech. Engg, University of Hawaii. 15. Ragland, K. W., Andren, A. W. & Manchester, J. B. (1985). Emissions from unvented kerosene heaters. The Science of the Total Environment, 46, 171-9. 16. Dollar, A. M., Menon, P. & Smith, K. R. (1982). Air pollution from biomass cooking fuels: the simulated village house. Conference on Womens Studies in Different Cultural Contexts (November), University of Hawaii/East West Centre, Honolulu, HI. 17. Smith, K. R. (1987). Biomass, air pollution and health: a global review. Plenum Publishing Co., New York City (forthcoming). 18. US Surgeon General (1981). The health consequences of smoking -- the changing cigarette: a report of the Surgeon General US Dept of Health and Human Services, Rockville, MD. 19. Venkataraman, C., Raman, P. & Kohli, S. (1987). Calorific value measurements for some biomass fuels (manuscript under preparation), The Tata Energy Research Institute, New Delhi.

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20. World Health Organization (1984). Evaluation of exposure to airborne particles in the work environment, WHO, Geneva, Offset publication No. 80. 21. World Health Organization (1982). Estimating human exposure to air pollutants, WHO, Geneva, EFP]82.31, 58 pp. 22. Reid, H., Smith, K. R. & Sherchand, B. (1986). Indoor smoke exposures from traditional and improved cookstoves: comparisons among rural Nepali women. Mountain Research and Development, 6 (4) 293-304. 23. Burnet, Paul, G., Edmisten, N. G., Tiegs, P. E., Houck, J. E. & Yoder, R. A. (1986). Particulate, carbon monoxide, and acid emission factors for residential wood burning stoves. J. Air Pollution ControlAssoc., 36, 1012-18. 24. Borrazzo, J. E., Osborn, J. E, Fortmann, R. C., Keefer, R. L. & Davidson, C. I. (1986). Modelling and monitoring of CO, NO, and N O 2 in a modern towrthouse. Atmospheric Environment, 21 (2) 299. 25. Smith, K. R. and Ramakrishna, J. (1986). Traditional fuels and health: social economic and technical links, ERG Monograph No. 98, International Development Research Centre and United Nations University. Wiley Eastern Ltd, New Delhi.

APPENDIX: GLOSSARY OF SYMBOLS

G G,m,i,

G, [CO]e,a E

Etask F

Hc,k,w L M

n

Q S S

t ti /task

Ti,f,

Measured average concentration (mg m-3) of a pollutant. Equilibrium, mean and instantaneous indoor pollutant concentrations (mg m - 3). Concentration in chamber at time when fire is extinguished (mg m-3). Calculated equilibrium and measured mean concentrations of carbon monoxide (mg m-3). Emission factor (g kg- 1). Emissions during performance of standard task (g task- ~). Burn rate (kg h - 1). Low heating values for charcoal, kerosene and wood (MJ kg-t). Latent heat of vaporization of water at 100°C (MJ kg- ~). Moisture content of wood on a dry basis (%). Thermal efficiency of stove (%). Airflow rate of sampling pump (m 3 h - ~). Specific heat of water (MJ kg- i oC - 1). Air exchange rate (h- ~). Total test duration (h). Instantaneous time (h). Time required to perform a standard task (h). Initial and final temperature of water (°C).

270

[rSP]e,a V

Wc,k,w

Wd Wfi, Wff

Wwi,wf

D. R. Ahu]a, V. Joshi, K. R. Smith, C. Venkataraman

Calculated equilibrium and measured mean concentrations of Total Suspended Particulates (mg m-3). Volume of chamber (m3). Weight of charcoal, kerosene, and wood (kg). Weight of dry wood (kg). Initial and final weights of test filters (mg). Initial and final weights of water in pot (kg).