User-centric approach for the design and sizing of natural convection biomass cookstoves for lower emissions

Energy 115 (2016) 1202e1215

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User-centric approach for the design and sizing of natural convection biomass cookstoves for lower emissions Milind P. Kshirsagar a, Vilas R. Kalamkar b, * a b

Department of Mechanical Engineering, St Vincent Pallotti College of Engineering and Technology, Nagpur, India Department of Mechanical Engineering, Visveswaraya National Institute of Technology, Nagpur, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 February 2016 Received in revised form 8 June 2016 Accepted 6 September 2016

Biomass cookstove is a cuisine and user specific device. Every cuisine represents a unique cooking situation and the requirements such as; the range of cooking power required and the shape and size of utensils are almost rigid at the user-end. Hence, a single cookstove model cannot replace all the traditional stoves in the field, and user must be free to choose a stove size depending upon his/her own requirements. Moreover, the paradigm of cookstove design is shifting from energy-efficient stoves to the emissions efficient stoves. Hence, we have to ensure that, a stove design must fulfill user's requirements as well as new energy and environmental norms. The paper presents a user-centric design and sizing procedure, based upon findings from the parametric analysis of experimental data and simulations of validated heat and mass transfer model; subjected to different constraints. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Biomass cookstoves User centric approach Design and sizing Emissions Parametric analysis

1. Introduction The cookstove is a cuisine specific device, as the nature of cooking task varies from place to place. Every cuisine represents a unique cooking situation and hence requires a cookstove designed for the purpose. The local cooking requirements such as the range of cooking power and the shape and size of utensils are uncompromising at the user-end. Hence, a single cookstove design is not sufficient to solve the problem of traditional stove-use in developing countries, and we must come up with a number of better stove designs and size, suitable to different cuisines. At the same time, we have to ensure that, these designs fulfill new energy and environmental norms. 2. Need of the standard design and sizing for biomass cookstoves Historically, there were some efforts to lay down design procedure for biomass cookstoves. Verhaart [1] in 1982 gave first design guideline; discussing various considerations influencing stove design and different heat transfer modes and combustion

* Corresponding author. E-mail addresses: [email protected] (M.P. Kshirsagar), vilas. [email protected] (V.R. Kalamkar). http://dx.doi.org/10.1016/j.energy.2016.09.048 0360-5442/© 2016 Elsevier Ltd. All rights reserved.

models. Baldwin [2] in 1987 came up with rough guidelines for designing a biomass cookstove; discussing different processes, heat losses, and heat transfer correlations. In 1993, Sharma [3] presented the basic principles of improved cookstove design and discussed the design criteria based on different factors related to the consumers needs. In 1990, Bhatt [4,5] presented a rating and design systems for woodstoves. Recently, Kshirsagar and Kalamkar [6,7] have felt the need of a standard design and sizing procedure as “a prerequisite to construction and testing; to avoid failure at the latter stages of stove development”. However, the design guidelines from last century [1e5] were insufficient to cope with new energy and emissions norms; set by IWA 11 tiers of performance [8,9]. A good variety of 'improved' and 'advanced' cookstove designs exist in the fields and the labs, and the most famous among these is the 'rocket stove' [9e13]. However, the design of the rocket stove, as any other biomass-cookstove is based upon old guidelines [13] and hence does not guarantee fulfillment of new norms. Moreover, any such design may be suitable for a specific purpose; but not for all, and hence, cannot be accepted universally. Hence, a need was strongly felt for a standard design and sizing procedure capable of dealing with the local requirements and new performance norms. 3. Need of emission based biomass cookstove design According to an estimate, about 2.6 billion people across

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

different nations do not have access to clean cooking devices, and the numbers will not change even up to 2030 [14]. About 75% of rural households in developing nations like India still fulfill their cooking energy demand by using fuel wood in the traditional stoves [15]. Indoor air pollution from solid-fuel cooking is responsible for up to 4% of the total disease-related deaths in the poorest nations, killing about 2 million people prematurely per year; the majority being women and children [16,17]. Kumar et al. gave a good review of the impact of cookstoves on environment and health [18]. Increasingly, the cookstove experts are demanding more attention to the reduction in emissions than the increase in efficiency of a cookstove [18e22]. In fact, it is because of emission-related health issues that the entire world is now interested in this mundane device. Some of the advanced biomass cookstoves are already found to decrease exposure to indoor air pollution by a significant amount [22e24]. Hence, emissions based design and sizing procedure was much required to reduce the intensity of this severe issue.

4. Scope of the work The paper presents a novel user-centric standard design and sizing procedure, for a natural convection biomass cookstove; based upon the range of cooking power required and the vessel diameter used. The results of the parametric analysis of experimental data were used to define the ‘technical’ constraints related to the emissions and efficiency norms as per the IWA 11 tiers of performance (limiting excess air ratio and thermal efficiency). The design space was further refined using the ‘functional’ or ‘social constraints (the range of cooking power required and the range of utensil diameter), and the ‘safety and comfort’ constraints (the maximum allowable surface temperature and the height of the stove) [7]. The validated heat and mass transfer model [6] was then used along with a VBA search code, to search all feasible designs within the space bounded by these constraints. The results are presented in the form of ready to use design charts. With the help of this chart, an artisan/user depending upon his/her requirements can decide the design specifications of a Tier 2 level natural convection rocket stove.

5. International Workshop Agreement, IWA 11 A modern cookstove is expected to cut down on CO and PM emissions while ensuring enhanced thermal efficiency and comfortable cooking experience. This requires quantification of the effect of design parameters on the emissions and efficiency of a cookstove. Hence, standard protocols are essential in the process of technical decision-making. In order to provide a fair comparison of different cookstove models, the testing protocols of cookstoves have gone through an evolution process from WBT to the most recent 'the International Organization for Standardization (ISO) International Workshop Agreement, IWA 110, agreed upon in 2012 [8,9]. A good review and comparison of different cookstove testing protocol are available in the literature [7,25]. The IWA 11 aims at substantial improvement in cookstove performance, by introducing the system of 'Tier of performance’, “to stretch goals for targeting ambitious health and environmental outcomes”. These Tiers of performance are associated with the ‘VITA Water Boiling Test (WBT)’ and the ‘stove safety protocol developed at Iowa state university’ [8]. Tier performance levels associated with the ‘water-boiling test 4.2.2’ for high-power thermal efficiency, and indoor emissions are given in Table 1[8,9].

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Table 1 Tier performance levels associated with the water-boiling test 4.2.2. IWA tiers of performance 0 1 2 3 4

No improvement over the open fire/baseline Measurable improvement over baseline Substantial improvement over baseline Currently achievable technology for biomass stoves Stretch goals for targeting ambitious health and environmental outcomes

Tier performance levels Tier High power thermal efficiency (%) 0 1 2 3 4

<15 15 25 35 45

Indoor emissions CO Indoor emissions PM 25 (g/min) (mg/min) >0.97 0.97 0.62 0.49 0.42

>40 40 17 8 2

6. Parametric analysis in search of decisive parameter for emission control If we want to control the effect, we must control the cause. Hence, the search for the decisive parameter affecting combustion quality (CO and PM emissions) is mandatory. For the purpose, parametric analysis of a consistent and cohesive data set identified from literature was carried out. Any such decisive parameter must be of the dimensionless form, if we want to use these results for the design of different geometries. Hence, the experimental data set available for different geometries was chosen to identify any trustworthy dimensionless behavior. The data set as shown in Table 2 is taken from the series of experiments performed by J Agenbroad et al. [26e28] for three different configurations i. e. rocket elbow tested with pot (single elbow size) and without pot (two different elbow sizes). By rearranging the data from Table 2 (irrespective of geometries), it was observed that the l, not only directly affects the flame temperature and combustion efficiency, but also limits the CO and PM emissions, and hence, is the decisive parameter for the overall performance of the biomass cookstove. The residential time of burning volatiles in the combustion chamber, availability of oxygen and the temperature of flue gas affect the combustion quality in a natural convection biomass cookstove. For a natural convection cookstoves, the velocity of flow does not vary much and is close to 1 m/s; resulting in enough residential time for the complete burning of volatiles [6,29]. However, flame temperature and the availability of oxygen for combustion are the real variables affecting the combustion quality and interestingly; both the parameters are controlled by l. The availability of oxygen is judged by % O2 in flue gas and is directly related to l by the formula:

l ¼

% O2 ð21  % O2 Þ

(1)

Fig. 1 shows the plot of flame temperature (Tg) versus l for the readings from Table 2. It can be observed from Fig. 1, that, l furthermore, controls the flame temperature as well a very low value of l leads to unavailability of oxygen required for complete combustion and a very high range of flame temperatures. However, high temperatures are favorable for good combustion efficiency, the insufficient oxygen levels dominate the situation and the combustion quality deteriorates. On the contrary, for a very high value of l, though the oxygen is available in abundance, the low flame temperatures resulting from the dilution of hot flue by associated high mass flow rate of combustion-air, leads to a poor combustion efficiency. Hence, for good combustion efficiency and

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Table 2 Experimental data used for parametric analysis. Type of geometry

la

Tg (K)

h c (%)

PM mg/min

CO gm/min

Chimney height: H ¼ 200 mm Chimney diameter: D ¼ 100 mm With pot

1.04 1.06 1.80 2.01 2.38 3.02 3.14 3.65 3.84 5.77 1.85 1.99 2.75 3.45 4.42 4.75 5.80 6.91 12.00 29.48 1.96 2.43 3.33 3.75 4.55 4.76 5.23 7.35 7.71 12.06 15.99

1082 1017 1018 929 895 799 806 727 697 589 940 943 821 762 681 670 612 587 466 402 991 922 785 773 711 696 669 575 564 472 453

90.82 92.76 99.52 99.42 99.47 98.97 99.49 98.68 97.61 97.10 99.63 99.61 99.19 98.49 98.76 97.52 95.51 95.41 93.08 90.56 99.82 99.71 99.25 99.18 98.88 97.70 98.71 95.50 96.95 95.10 91.99

103 91 24 7 37 8 4 4 9 4 6 6 9 11 14 15 18 22 37 88 6 8 11 12 15 15 17 23 25 38 48

1.54 1.09 0.07 0.10 0.08 0.06 0.12 0.18 0.06 0.16 0.07 0.07 0.13 0.21 0.15 0.27 0.41 0.38 0.36 0.19 0.07 0.10 0.18 0.22 0.28 0.44 0.25 0.56 0.41 0.40 0.41

Chimney height: H ¼ 255 mm Chimney diameter: D ¼ 130 mm Without pot

Chimney height: H ¼ 200 mm Chimney diameter: D ¼ 100 mm Without pot

a

- calculated from % O2 values as: l ¼ [%O2/(21-% O2)].

optimum l between 1.8 and 2 [30e38] for different small-scale biomass combustion chambers, all have confirmed the basic trend as obtained in Fig. 2. In Fig. 2, the nature of variation of combustion efficiency with the l in both the regions is a polynomial type with good values of coefficient of determination (R2); however, the correlations are not the same. The combustion efficiency is correlated to the l as:

1200 1100

Flame Temperature (K)

1000 900 800

y = 1157.x R² = 0.967

700 600 500

hc ¼ 0:014l2  0:785l þ 101:2

400




 R2 ¼ 0:91

(2)

ðPre  optimum l region; l  1:95Þ

300 0

5

10

15 Excess Air Ratio

20

25

30

Fig. 1. Variation of flame temperature with the excess air ratio.

100

Combustion efficiency (%)

99

lower emissions (CO and PM), the l must be maintained inbetween these two extremes. The dependence of combustion efficiency on l is already a wellestablished fact [30e36] and the results obtained from the parametric analysis agreed well with the typical behavior. All types of the combustion chambers have an optimum value of l corresponding to the best combustion point [30e36]. Hence, the combustion behavior can be divided into two regions; the 'preoptimum l region' and the 'post-optimum l region'. The parametric analysis predicted the optimum value of l as 1.95 and the best combustion efficiency as 99.8% for the natural convection biomass cookstove. Nussbaumer [30] in his work on different combustion devices also agreed, on the optimum value of l for a cookstove to be about 2. Though different researchers predict a range of values for

98 97 96 95 94 93 92 91 90 0

5

10

15 20 Excesss Air Ratio

25

Fig. 2. Variation of combustion efficiency with the excess air ratio.

30

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

hc ¼ 10:26l2 þ 39:66l þ 61:43




 R2 ¼ 0:98

ðPost  optimum l region; l < 1:95Þ

(3)

1205

well-designed Tier 2 stove which represents 'Substantial improvement over baseline' is a much better than an ill-designed cookstove.

8. Limiting inlet area ratio for ensuring good combustion 7. Limiting excess air ratio for ensuring good combustion In biomass cookstoves, CO and PM are the pollutants responsible for a majority of diseases, and hence; a good stove design must limit these pollutants by using applicable techniques. To establish the relationship between these pollutants and the l, variations of CO and PM emissions from Table 2 are plotted against the corresponding l as shown in Fig. 3. As expected, there exists a good combustion region surrounded by a poor combustion region on either side as discussed in Section 6. The region of good combustion is bounded by limiting values of the excess air ratio, l ¼ 1.95e5.8. If somehow, l is held in the region of good combustion, the stove performance of ‘Tier 2’ level can be assured. As pointed out by different researchers, it is easier to fulfill CO emissions level for Tire 3 & 4 than the PM emissions level [8,9]. Only forced draft stoves are found to reach PM emissions corresponding to Tier 4 [9e11]. Hence, it is concluded that the natural convection stoves can reach up to a collective performance corresponding to Tier 2 only. Nonetheless, a

Kshirsagar and Kalamkar [6] while developing a validated heat and mass transfer model identified an important operational parameter named as 'Inlet area ratio'. The parameter was found to have considerable effect on the combustion quality; and the other related stove performance parameters. The 'Inlet area ratio' was defined as [6]:

Ar ¼

Ain Area unoccupied by fuel at the feed door ¼ Cross sectional area of the combustion chamber A (4)

The Ar in essence is the fraction of stove cross-sectional area, which is available for the entry of combustion air at the stove inlet. The Ar controls the entry of air in the stove; hence the l, and the combustion quality. The Ar was found to affect the quality of combustion drastically for values of Ar lower than 0.7 [6]. Hence, a minimum ‘Inlet area ratio’ of 0.7 (Primary air area factor) is exclusively made available for primary air entrance. To accommodate

Fig. 3. Variation of CO and PM emissions with excess air ratio.

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this new feature, a modified rocket stove design as shown in Fig. 4 was proposed. The modification ensures that even in worst conditions (highest possible firepower) a minimum ‘breathing space’ is available with the stove for air entry. 9. Simulation results and parametric analysis The validated mathematical tool for natural convection biomass cookstoves [6] was then used for simulating the heat and mass transfer behavior of the modified rocket stove design. The results were used to study the effect of variations in different geometrical and operational parameters on the overall performance of a biomass cookstove. The effect of input parameters like inlet area ratio, firepower, insulation thickness, height of the combustion chamber, pot gap width, and pot diameter; on different output parameters like excess air ratio, overall efficiency, surface temperature, and cooking power, were studied. 9.1. Effect of the inlet area ratio Fig. 5. Variations of excess air ratio with the inlet area ratio.

The model was applied to study the effect of Ar on the l, for different firepowers. Fig. 5 shows the simulation results and the values of other parameters, which were held constant. As expected, at the higher Ar, the l is almost constant for a given firepower. The findings are in line with the previous work [6]. The important thing to learn is that there exists a safe combustion region bounded by l ¼ 1.95e5.8 and Ar  0.7. The firepowers corresponding to the l ¼ 1.95 (2.15 kW) and the l ¼ 5.8 (4.15 kW) are the permissible limits of firepowers for the given stove geometry, and for any firepower in between, the emissions performance of the stove will be within the limits of Tier 2. For any given geometry of natural convection biomass cookstove, the maximum firepower (hence, the maximum fuel-burning rate) occur at l y1 [27]. Hence, every stove-pot combination has a unique value of maximum firepower occurring at l ¼ 1, which is 6.15 kW; for the given geometry. In actual working conditions, no stove should work at its maximum firepower as it leads to higher emissions and lower efficiency. The best combustion point (l ¼ 1.95) results at the firepower of 4.15 kW which is about 2/3rd of the highest firepower possible. Hence, to

avoid unsafe combustion zone, one has to design an overrated stove; with about 50%, more firepower than the maximum required by the user. If the user required firepower is out of the safe range (2.15e4.15 kW for the present geometry), the emissions performance of the stove will deteriorate. It further means that designing a stove is only a half of ‘ensuring safe performance’; the latter half rests with the user. The possible solution is an overrated stove design and an optimal match between the stove performance and the user requirements. 9.2. Effect of the chimney height Fig. 6 shows the effect of chimney height on the overall efficiency of the stove, for different firepowers. Decreasing chimney height increases overall efficiency, because of increase in view factor and hence, the radiative heat transfer. Also with decreasing chimney height the total stove height decreases, which is highly

6

dp

w

t h-Hf

t 5

D

4

4 Wf

Hf

Hp

h

Ht

1

1

3

Wp

1 cm air gap

2

2 Front view

1 – Inlet area for Fuel +Air (Variable) 3 – Fuel holding mesh 5 – Combustion chamber or chimney

Cross sectional i

2 – Fixed inlet area for primary air (Constant) 4 – Insulation 6 – Cooking pot

Fig. 4. The modified rocket stove design.

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215 d

120

mm

h

180-320

mm

37%

t w dp

70 10 240

mm

35%

37%

mm mm

d h t w

120 200 10-100 10

mm mm mm mm

dp

240

mm

100 mm 70 mm

33%

32%

31%

Overall efficiency (%)

Overall efficiency (%)

39%

1207

6.0 kW

29%

5.6 kW

27%

5.2 kW

25%

4.8 kW

23%

4.4 kW 4.0 kW 3.6 kW 3.2 kW 2.8 kW 2.4 kW

21% 19%

27%

22%

330

320

310

300

290

280

260

270

250

240

230

220

210

200

190

170

180

17%

Chimney height (mm)

17% 1.8

2.3

2.8

3.3

Fig. 6. Variations of overall efficiency with the chimney height.

hf ¼ Cf 

P2/5 f

(5)

where Cf varies from 75 to 110 mm/kW0.4 depending upon the situation [3,39]. To be on the safer side the value of Cf ¼ 110 mm/ kW0.4 is adopted for flame height calculations. The general household purpose fuel-stick diameter is in the range of 50e70 mm [40,41]; hence, a feed door height (Hf) of 80 mm (on a higher side) is provided in the proposed modified rocket stove design. In addition, a minimum of 100 mm height (h-Hf) is provided to serve as the chimney to create draught and facilitate combustion. Hence, from the practical point of view a total chimney height (h), less than 180 mm is not entertained. In addition, from Equation (5), a stove with chimney height less than 180 mm will have a maximum safe firepower less than 3.5 kW, which will lead to inadequate cooking power.

9.3. Effect of the insulation thickness Figs. 7 and 8 shows the effect of insulation thickness on the overall efficiency and surface temperature of the stove, respectively. Increasing insulation thickness increases efficiency and decreases the surface temperature of the stove, due to the reduction in heat losses. Fig. 7 shows that after certain thickness; insulation loses its effectivity. According to the new stove safety-protocol, the surface temperature of the stove should not be greater than the surrounding temperature by 50  C for ‘fair ‘rating [42]. However, as per Fig. 8 it is not possible to meet this expectation with a reasonable insulation thickness, especially at higher firepowers. Hence, it is proposed that a wire-mesh cover will be provided to the

5.3

5.8

6.3

Fig. 7. Variations of overall efficiency with the insulation thickness.

630 610 590 570 550

Surface temperature (K)

desirable from ergonomic point of view. However, the ‘flame height’, which is the distance between the fuel bed and the visible flame tip, imposes restriction on the minimum allowable chimney height for the given stove. If the height of the chimney is less than the flame height, the flame will meet the cold pot, resulting in quenching of the flame and deterioration of the combustion quality. Prasad et al. [39] conducted a series of experiments on biomass cookstove to determine actual flame heights and compare those values; with the model predicted values to conclude that, the flame height is practically independent of the other parameters and varies with the firepower only. Prasad et al. and several other researchers [3,39] agree that the flame height varies according to the law:

3.8 4.3 4.8 Firepower (kW)

5.6 kW 5.2 kW 4.8 kW

200 10-100 10 240

mm mm mm mm mm

4.4 kW

530

4.0 kW

510

3.6 kW

490

3.2 kW

470

120

d h t w dp

6.0 kW

2.8 kW

450 430 410

2.4 kW 2.0 kW

390 370 350 330 310 0

10

20

30

40

50

60

70

80

90

100

Insulation thickness (mm)

Fig. 8. Variations of the surface temperature with the insulation thickness.

stove outer surface maintaining a gap of 10 mm to avoid any physical contact with the stove. Increasing thickness is not practical due to increasing size of stove and cost of insulation with decreasing effectivity, hence, the insulation thickness of 70 mm is taken for final analysis. 9.4. Effect of the pot gap width Fig. 9 shows that decreasing pot gap width increases overall efficiency, due to increase in view factor, and increase in flame temperature due to the reduction in the excess air ratio. However, a very small pot gap width is not recommendable practically. A pot gap width less than 10 mm will be prone to blockage by soot deposition, and will either need frequent cleaning or will lead to very small ‘actual pot gap’ forcing the stove behavior into the

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M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

42%

41% 39% 37%

120

mm

h

200

mm

t

70

mm

w dp

10 120-360

mm mm

6.0 kW 5.4 kW 5.0 kw 4.6 kW 4.2 kW

Overall efficiency (%)

37%

35%

Overall efficiency (%)

d

33% 31% 29%

5 mm

27% 25%

3.8 kW 3.4 kW

32%

3.0 KW 2.6 kW

27%

2.2 kw

22%

23% 21% 10 mm 19%

20 mm 30 mm 40 mm 50 mm

17% 15%

1.8

2.2

2.6

3.0

3.4

3.8

4.2

4.6

d

120

mm

h

200

mm

t

80

mm

w dp

5-50 240

mm mm

5.0

5.4

5.8

6.2

17% 100 120 140 160 180 200 220 240 260 280 300 320 340 360

Pot diameter (mm)

Fig. 11. Variation of overall efficiency with the pot diameter.

10. System centric versus user centric design approach Fig. 9. Variations of overall efficiency with the pot gap width.

unsafe region. Fig. 10 shows the variation of the excess air ratio with the firepower for the given pot gap. The same stove geometry with 10 mm pot gap width is capable of producing a safe firepower of about 4.25 kW, whereas with 5 mm pot gap width the safe firepower is only 3.2 kW. It means the reducing pot gap width, also reduces the ability of the same stove configuration to produce maximum safe firepower. This will lead to the design of a larger stove for generating same maximum safe firepower. Hence, a pot gap width of 10 mm is recommended for further analysis. 9.5. Effect of the pot diameter Fig. 11 shows the effect of pot diameter on the overall efficiency of the stove. Increasing pot diameter leads to almost linear increase in the overall efficiency, because of increasing heat transfer area and partly because of the increase in flame temperature due to the decrease in the excess air ratio. Similar trends were observed by Varun Kumar [43] while experimenting with gasifier stove.

7.0 50 mm 40 mm 6.5 30 mm 20 mm 6.0

d

120

mm

h

200

mm

t

80

mm

w dp

5-50 240

mm mm

5.5 10 mm

Excess air ratio

5.0 4.5 4.0

5 mm

3.5 3.0 2.5 2.0 1.5 1.0 0.5 1.8

2.2

2.6

3.0

3.4

3.8

4.2

4.6

5.0

Firepower (kW)

Fig. 10. Effect of pot gap width on stove size.

5.4

5.8

As discussed in Section 2, no standard design or sizing approach for biomass cookstoves, capable of coping up with new energy and emissions-norms is available. The only modern design guideline in existence is by the Aprovecho research center, which presents two different design strategies given by Baldwin and Winiarski [13]. Winiarski recommends designing stoves by maintaining a constant cross-sectional area throughout the stove-pot system, which decides different gap sizes. Baldwin's method requires picking a maximum high power for the stove design, which then determines the size of the channel gaps. The guidebook recommends ‘respect for local knowledge’, however; do not consider variation in the user's need depending upon the locality, the type of the food item and the size of the family. Both the approaches suggest stove geometry suitable for a single pot diameter only. In reality, no household uses a single type and size of pot, hence; a stove must perform well and safe with ‘a range’ of pot diameter, which is generally 150e360 mm; worldwide [44e49]. Moreover, the socalled ‘optimized’ stove designs are nothing but the oversimplified ‘lab fantasies’. For example, a stove with optimized skirt gap is suitable only for a single pot diameter, and not for the rest. For a fixed skirt, the skirt-gap will change with the pot diameter, whereas, a flexible skirt is too tedious to use with different pot shapes and sizes. None of the existing design principles [1e5,13] considered this variety in demand at the user's end and hence; all these design principles can be termed as ‘system (stove) centric’. The proposed ‘User-centric‘ design approach takes into account, the use of different pot diameters by the same user, and more importantly, the range of cooking power required at the user end; instead of firepower supplied by the stove. Frequently throughout the stove literature, one comes across the term; ‘Firepower’, which is the product of fuel burning rate and calorific value of the fuel. However, useful ‘Cooking power’, which is the product of firepower and the overall efficiency, is the real need of the cooking process. LPG stoves with 1e2 kW firepower and 60% efficiency serve the same purpose, as that of, an ‘Improved’ biomass stove with the firepower of 2e4 kW and efficiency 30%. Every cuisine has a preferable range of required cooking power. The maximum and minimum cooking powers can be obtained by conducting surveys in the ‘targeted community’ or from the related literature sources, for example, the preferable cooking power range in India is 1.2e0.6 kW [45e47]. For the ‘user centric’ stove-design, cooking power is the

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

Firepower

Cooking power 1.55

4.7

1.53 1.51

4.6

1.49

Fire Power (kW)

1.47 4.5

1.45 1.43

4.4

1.41 1.39

4.3

1.37 1.35

4.2

Cooking Power (kW)

1.33 1.31

4.1

1.29 1.27

4.0 380

360

340

320

300

280

240

260

220

180

200

160

120

140

100

1.25

Pot Diameter (mm) Fig. 12. Effect of pot diameter on maximum safe firepower and cooking power.

d

120

2.5

h

200

mm mm

t

70

mm

2.3

w dp

10 120-360

mm mm

Pc_max

2.1 Cooking power (kW)

deciding parameter, as it directly links with the user's requirement and is given the due to consideration in the proposed stove design and sizing procedure. However, both the ‘user defined’ parameters i. e. cooking power and pot diameter are interlinked as shown in Fig. 12. It shows the variation of maximum safe firepower and the corresponding cooking power of the pot diameter. For a given stove configuration, the maximum safe firepower decreases with increasing pot diameter, whereas the cooking power increases. It means that if a stove is capable of supplying a certain value of cooking power for a given pot diameter, then, for any larger pot diameter, it will safely supply the same or even higher cooking power. Hence, the minimum allowable pot diameter, which is equal to that of the chimney, is chosen as the base pot diameter for rating its maximum cooking power. Similarly, the maximum pot diameter of 360 mm is chosen as the base pot diameter for rating its minimum cooking power. Fig. 13 shows the crucial values of cooking power corresponding to the different firepowers. Maximum possible cooking power (2.5 kW) is corresponding to maximum possible firepower (5.8 kW) and the maximum pot diameter (360 mm). This is the maximum power output of the stove, but is not safe from the emissions point of view and hence, of no practical use. Maximum safe cooking power (about 1.3 kW) is the cooking power corresponding to maximum safe firepower (4.6 kW) for the minimum pot diameter (120 mm). This is the maximum useful cooking power supplied by the stove for a minimum pot diameter within safe limits of emissions. The reason behind using minimum pot diameter is to make sure that, the maximum safe cooking power is available even with the smallest pot. Minimum safe cooking power (about 0.5 kW) is the cooking power value corresponding to minimum safe firepower (2 kW) for the maximum pot diameter (360 mm). This is the minimum useful cooking power supplied by the stove for a maximum pot diameter within safe limits of emissions. The reason behind defining this value for maximum pot diameter is to make sure that, the minimum safe cooking power is available even with the largest pot.

1209

1.9 1.7 1.5 1.3

Pc_max safe

1.1 0.9 0.7

11. User-centric sizing of biomass cookstoves subjected to different constraints

Pc_min safe

0.5 0.3

A cookstove designer must consider social (or functional), technical and economic aspects of the design, for ensuring long-run adoption of a cookstove by the customer [7]. Important functional considerations include; local food habits, locally available fuels, the range of required cooking power, combustion chamber diameter, the prevalent pot shapes and sizes, user's safety, and cooking comfort. Important technical considerations are the targeted energy and environmental norms; arising from international protocols like IWA 11, and choice of the diameter and height of the combustion chamber. Table 3 presents the different constraints imposed on a biomass cookstove design by these considerations. The design space as defined by Table 3 was searched for the feasible sizing combinations using the earlier developed performance prediction tool for natural convection biomass cookstoves [6]. For all the simulations, some input parameters to the model are assumed constant and assigned values as given in Table 4. The mathematical tool consists of a set of algebraic equations and uses the iterative procedure based upon the SOLVER add-in in the Microsoft Excel® program to establish energy and mass balance within 0.1%

mflue ¼

LHV  hc  Qchar

radiation

 Qflame

radiation

Q  Qdoor  heat loss  hg  ha

120 140 160 180 200 220 240 260 280 300 320 340 360 Pot diameter (mm)

Fig. 13. Relation between the cooking power and the pot diameter.

accuracy. The most important equations from Kshirsagar and Kalamkar [6] that are underlying this mathematical tool are repeated here; along with a new set of required equations:

Firepower : Pf ¼ m_ f  NCV

(6)

The mass balance equation for the combustion chamber:

mflue ¼ Cd 

353 1  A _f Tg m

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi Tg 2gh 1 Ta

(7)

The heat balance equation for the combustion chamber:

loss

 QH2

moisture

 Qfuel

moisture

(8)

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M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

Table 3 Different constraints. Sr No

Constraint type

Parameter

Targeted range/Value

Reference

1 2

Functional

Cooking power required Utensil diameter Combustion chamber diameter Total height of the stove Height of the feed door Surface temperature of the stove High power thermal efficiency Combustion chamber height Inlet area ratio Excess air ratio

0.5e2.4 kW 150- 360 mm 100-200 mm <330 mm 50-70 mm <¼ 50  C above the ambient >25% hf ¼ 110  Q2/5 (mm) 0.7e1.72 1.95e5.8

[9,10,44e47] [45,48,49] [40,45] [40] [40,41] [42] [8,9] [3,39] e e

3 4 5

Technical

6

Table 4 Operating parameters used during parametric study and search process. Parameter

Value

Reference

Net calorific value of wood fuel (General value for most of the wood species on dry basis) Moisture content of the fuel (Wet basis) Surface temperature of pot (Average) Ambient temperature (Standard) Char emissivity Char temperature Amount of water in pot (Standard) Insulation thermal conductivity (Average) Bed voidage

18000 kJ/kg 15% 336 K 300 K 0.85 1100 K 5L 0.1 W/m K 0.69

[50e53] [1,2,10] e e [6] [6] [9e11] [59] [6]

Cooking power : Pc ¼ Pf  ho

Solving Equations (7) and (8) simultaneously, the flame temperature (Tg) inside the combustion chamber was obtained. The excess air ratio was estimated using equation:

l¼

ma ¼ ms

ma

 1 C  þ H  8  O þ S  23

(12)

Height of primary air entrance : Hp ¼

(9)

A  Af ¼ 0:55  d WP (13)

8 3

Where, Generally, C, H, O and S are obtained from ultimate analysis of fuel; however, for a general situation, the following average values can be used for most of the wood fuel types: C ¼ 50%, H ¼ 6.5%, O ¼ 42% and S ¼ 0.2% [2,3,50e58]. For the different Pot zones [6] such as the ‘pot bottom zone’, the ‘pot gap zone’, and the ‘pot side zone’, the following general heat balance equation was used to determine temperatures at the different important locations:

Q_ supplied

by flue gas

¼ Q_ transfer

to the pot

þ Q_ side

(10)

heat loss

WP ¼ Wf ¼ d

(14)

The height of feed door : Hf ¼ 0:8  d

(15)

Nominal diameter of fuel sticks ¼ 0:5  d

(16)

Total height of the stove : Ht ¼ 10 þ Hp þ h þ w

(17)

As defined in Section 9.1, the minimum allowable Ar:

Finally, the overall efficiency of a stove was calculated as:

ho ¼

Q_ char

radiation

þ Q_ flame

radiation

þ Q_ conv

2

þ Q_ rad

3

þ Q_ conv

3

þ Q_ rad

4

þ Q_ conv

Pf

The complete set of equations and the property correlations used by the mathematical tool is described in detail by Kshirsagar and Kalamkar [6]. The tool was validated against the experimental data from literature for a rocket type natural draft cookstove prototype [27,28] for three most important parameters i. e. flame temperature, % O2 and flue gas mass flow rate; simultaneously [6]. However, to cope up with new search task, a fresh set of equations was formed and integrated with the earlier mathematical tool [6]:

4

 100

Ar_min ¼ Af ¼ 0:7




Pf ¼ Pf_max

(11)



(18)

whereas, the maximum possible value of Ar:

Ar

max

¼ Af þ


 Wf  Hf ¼ 1:7 Pf ¼ 0 A

(19)

As the Ar varies in between these extremes, the firepower will also vary. Two intermediate values of Ar are important from the design point of view: Ar_max safe FP and Ar_min safe FP. A correlation

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

1211

Specify fixed input parameters d, w = 10 mm, t = 80 mm Initiate Pf and assign Ar =1, dp = d+1 Calculate hf = Cf *Pf 2/5 & assign h = Max (180, hf) Generate solution by establishing heat & mass balances If >= 1.95

Decrement Pf

No

Yes Pf_max safe = Pf, Pc_max safe = Pc and the related o/p parameters Initiate Pf and assign Ar=1.36, dp =360 Generate solution by establishing heat & mass balances If

<= 5.8

Increment Pf

No

Yes Pf_min safe = Pf, Pc_min safe = Pc and the related o/p parameters Initiate Pf and assign Ar=0.7, dp = d+1 Generate solution by establishing heat & mass balances If

No

>= 1

Decrement Pf

Yes Pf_max = Pf, Pc_max = Pc and the related o/p parameters Initiate Pf = Pf_max safe and assign dp = 240

Decrement Pf

Initiate Ar=1.05 Generate solution by establishing heat & mass balances

If Pc <= Pc_max safe & >= 1.95

No

Increment Ar

Yes Pf_rated = Pf, Pc_rated = Pc and the related o/p parameters End Fig. 14. Scheme of operation for the VBA search code.

Yes

If Ar <1.3

No

For Pf _dimensionless ¼ 0/Ar ¼ 1:7

326 325 324 323 322 321 319 318 317 315 312 99.7 99.7 99.7 99.6 99.6 99.5 99.4 99.4 99.3 99.1 98.6 23.4 24.2 25.1 26.0 27.1 28.3 30.0 31.5 33.2 35.4 36.5 3.4 3.0 2.7 2.4 2.1 1.8 1.6 1.3 1.1 0.9 0.7





Pf ¼ Pf_max Pf ¼ 0





(21)

(22)

Assuming linear variation the following correlation between Ar and Pf_dimensionless is formed:

17.6 18.2 18.9 19.8 20.7 21.7 22.9 24.3 25.9 27.7 28.9

For Pf ¼ Pf_max

Pf

safe /Pf_dimensionless

(23)

max

¼ 0:68/Ar ¼ 1:02y1

7.39 6.54 5.73 4.97 4.29 3.64 3.05 2.52 2.04 1.63 1.33

(24)

3.4 3.1 2.7 2.4 2.1 1.8 1.6 1.3 1.1 0.9 0.7

22.0 22.4 22.8 23.2 23.8 24.3 24.9 25.7 26.5 27.1 26.5 2.05 2.04 2.06 2.09 2.09 2.15 2.17 2.20 2.25 2.35 2.66 15.5 13.8 12.0 10.3 8.9 7.5 6.3 5.2 4.2 3.3 2.5 27.1 27.6 28.2 28.9 29.7 30.6 31.6 32.8 34.0 35.0 35.0 6.48 5.83 5.19 4.63 4.09 3.56 3.08 2.68 2.26 1.86 1.45 1.02 1.01 1.02 1.01 1.01 1.02 1.02 1.01 1.01 1.03 1.12

h0

200 190 180 170 160 150 140 130 120 110 100 200 190 180 170 160 150 140 130 120 110 100

329 314 297 280 264 246 229 212 195 180 180

10 10 10 10 10 10 10 10 10 10 10

70 70 70 70 70 70 70 70 70 70 70

459 438 416 393 372 348 326 304 281 260 255

Hp Ht t w

All dimensions in mm

h

Abed A

(26)


For Ar  1/Afr ¼ 1 Pf  Pf _max

 safe


 For Ar ¼ 1:7/Afr ¼ 0 Pf ¼ 0

(27)

(28)

Assuming linear variation the following correlation between Ar and Afr is obtained:

Afr ¼ 2:43  1:43  Ar

(29)

The revised equations for radiative heat transfer from charcoal bed and the flame are given as:




Q_ char

radiation

¼

110 104 99 93 88 82 77 71 66 60 55

160 152 144 136 128 120 112 104 96 88 80

23.9 21.1 18.4 16.0 13.8 11.6 9.8 8.2 6.6 5.3 4.1

kW

Pc

hc h0

% kW

Pc

l

e kW

Wf Hf

Pf

%

Afr ¼

91.1 91.1 91.1 91.0 91.0 91.2 91.3 91.0 91.1 91.4 92.9

(25)

l

¼ 0:34/Ar ¼ 1:36

e

safe /Pf_dimensionless

Pf

For Pf ¼ Pf_min

kW

99.7 99.7 99.7 99.7 99.7 99.6 99.6 99.6 99.6 99.5 99.3

%

dimensionless

¼ 1:7 

Pf

1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.4

14.4 12.5 10.8 9.1 7.7 6.4 5.2 4.2 3.3 2.5 1.8

For Pf _dimensionless ¼ 1/Ar ¼ 0:7

(20)

5.77 5.77 5.77 5.78 5.75 5.76 5.78 5.77 5.80 5.84 5.88

97.2 97.2 97.2 97.2 97.2 97.2 97.2 97.2 97.2 97.2 97.1

2.02 2.04 2.08 2.14 2.19 2.26 2.41 2.50 2.63 2.89 3.55

Touter

K

hc h0

%

Pc

kW

l

e

Pf

kW

hc

%

; 1  Pf _dimensionless  0

kW

h0

max

e

Pc

Pf Pf

From Equation (23), value of Ar for any intermediate firepower can be predicted as follows:

kW

l

¼

dimensionless

Ar ¼ 1:7  Pf

%

Pf

Pf

%

hc

between Pf and Ar can be used to determine these values. However, to form a useful correlation both the quantities must be dimensionless in nature. Hence, a new term ‘dimensionless firepower’ is defined as:

In earlier model [6], it was assumed that the charcoal bed area remains constant, irrespective of firepower variations. However, from practical point of view the area occupied by charcoal bed will vary with the firepower and with the Ar. In general, the charcoal bed area Ab will decrease with increase in Ar. To facilitate the comparison; another dimensionless factor ‘charcoal bed radiation factor’ was defined as:

d

Performance corresponding to the minimum safe firepower (dp ¼ 360 mm) Performance corresponding to the maximum safe firepower (dp ¼ d mm) Performance corresponding to the maximum possible firepower (dp ¼ d mm) Geometrical configurations of the stove

Table 5 Results from search code run subjected to various constraints.

%

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

Performance corresponding to the standard pot diameter (dp ¼ 240 mm)

1212

Q_ flame

radiation

4 s  A  Tchar  Tp4 ð1εc Þ εc

þ

2

  Afr

(30)

ð1þFcharpot Þ


¼ s  A  εg Tg4  ag Tp4  Afr

(31)

This modified mathematical tool requires 19 different inputs in the form of 9 operational and 10 physical parameters; for the given stove geometry and operating conditions. In addition to the 31 output parameters and overall thermal efficiency; predicted by earlier mathematical tool [6], the modified mathematical tool predicts 3 new important output parameters namely, cooking power, height of primary air entrance, and the total height of the stove as given by Equations (12)e(17). The user-defined VBA search code

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215

1213

Table 6 Ready to use decision table for artisans/designers. Safe cooking power range (kW)

Suitable pot diameter range

Max diameter of fuel sticks that can be used

Geometrical configuration

Max

Min

mm

mm

All dimensions in mm

3.4 3.1 2.7 2.4 2.1 1.8 1.6 1.3 1.1 0.9 0.7

1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.4

200e360 190e360 180e360 170e360 160e360 150e360 140e360 130e360 120e360 110e360 100e360

160 152 144 136 128 120 112 104 96 88 80

200 190 180 170 160 150 140 130 120 110 100

d

h

w

329 314 297 280 264 246 229 212 195 180 180

10 10 10 10 10 10 10 10 10 10 10

t

Ht

Hp

Hf

Wf

70 70 70 70 70 70 70 70 70 70 70

459 438 416 393 372 348 326 304 281 260 255

110 104 99 93 88 82 77 71 66 60 55

160 152 144 136 128 120 112 104 96 88 80

200 190 180 170 160 150 140 130 120 110 100

Rated overall efficiency % 23.4 24.2 25.1 26.0 27.1 28.3 30.0 31.5 33.2 35.4 36.5

Table 7 Diameter dependent sizing correlations for a natural draft biomass cookstove. S. No.

Input parameters

Symbol

Correlation/Fixed value

Unit

1 2 3 4 5 6

Insulation thickness Pot gap width Width of the feed door and primary air entrance Height of the feed door Height of the primary air entrance Chimney height

t w Wf ¼ Wp Hf Hp h

mm mm mm mm mm mm

7 8 9 10

Suitable pot diameter range Maximum diameter of fuel sticks that can be used Maximum safe cooking power Minimum safe cooking power

e dfuel_max Pc_max safe Pc_min safe

70 10 d 0.8  d 0.55  d 1.687  d -7 (Not less than 180 mm) d-360 0.8  d 1.778  105*d2.3 1.1  105 *d1.771

in Microsoft Excel® was written to determine the suitability of a given stove geometry for a cooking power range. Fig. 14 summarizes the scheme of different steps followed by the code for obtaining the feasible designs. Table 5 shows preliminary results from the search code run for a single chimney diameter. Table 6 shows ready to use results for artisan/designers. Only the range of cooking power and pot diameter are required for choosing a natural draft, Tier 2 biomass cookstove design. There exists a very good correlation between the chimney diameter and other parameters shown in Table 6. Table 7 shows different chimney diameter (in mm) dependent correlations, for sizing a natural draft biomass cookstoves. One can readily calculate all required dimensions for a given stove diameter from these equations.

mm mm kW kW

there must be an optimal match between the stove performance and the user requirements. Unlike the existing ‘system-centric’ stove design principles the proposed ‘User-centric‘ design approach takes into account, use of different pot diameters by the same user, and the range of cooking power required at the user end; instead of the firepower. A number of stove configurations, each suitable for a cooking power range and general pot diameter range are possible. With the help of these results, an artisan/user depending upon his/ her requirements can decide the design specifications of a Tier 2 level natural convection rocket stove. However, it is not possible to obtain a performance better than Tier 2, with a natural draft biomass cookstove, and an ‘advanced’ type of biomass cookstove, such as, a forced draft or a gasifier type stove may deliver a better performance.

12. Limitations of the present stove sizing approach References The mathematical tool used for design and sizing procedure is applicable only for chimney type of wood burning natural draft biomass stoves. It is not applicable for charcoal stoves, gasifier type stoves or fan assisted forced draft stoves. Furthermore, the results are not applicable for pot diameters less than the chimney diameter. The model is not applicable for stoves with shielded pot and is applicable only for flat-bottom cooking utensils. 13. Conclusion In this paper, a validated mathematical tool and the results of the parametric analysis were used for laying down a novel emissions based design and sizing procedure for the natural draft biomass cookstove. It is concluded that, to avoid unsafe combustion zone, one has to design an overrated stove; with about 50%, more firepower than the maximum required by the user. In addition,

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[37]

[38]

[39]

[40]

[41]

[42] [43]

[44]

[45] [46] [47] [48]

[49]

[50]

[51] [52] [53]

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Nomenclature A: Cross sectional area of the chimney or the combustion chamber (m2) Abed: Actual area of the charcoal bed inside combustion chamber (m2) Af: Primary air area factor Afr: Charcoal bed radiation factor Ar: Inlet area ratio Ar_min: Minimum allowable Inlet area ratio Ar_max safe FP: Inlet area ratio corresponding to the maximum safe firepower Ar_max: Maximum possible Inlet area ratio Ar_min safe FP: Inlet area ratio corresponding to the minimum safe firepower Cf: Flame constant (mm/kW0.4) C: Carbon content of the fuel as percentage of dry weight (%)

M.P. Kshirsagar, V.R. Kalamkar / Energy 115 (2016) 1202e1215 Cd: Coefficient of discharge CO: Carbon monoxide (g/min) D: Chimney diameter (mm) dfuel_max: Maximum diameter of fuel sticks that can be used with given stove dp: Pot diameter (mm) hC: Combustion efficiency (%) hO: Overall efficiency (%) h: Chimney height (mm) H: Hydrogen content of the fuel as percentage of dry weight (%) ha: Enthalpy of outside ambient air (kJ/kg of air) hf: Flame height (mm) Hf: Height of the feed door (mm) hg: Enthalpy of hot flue gases inside the combustion chamber (kJ/kg of flue) Hp: Height of the primary air inlet (mm) Ht: Total height of the stove (mm) L: Excess air ratio ma: Actual air supplied for combustion (kg of air/kg of dry fuel burnt) mflue: kg of flue produced per kg of dry fuel burnt ms: Stoichiometric air required (kg of air/per kg of dry fuel burnt) NCV: Net calorific value on dry basis (kJ/kg) O: Oxygen content of the fuel as percentage of dry weight (%) Pc: Cooking power (kW) Pc_max: Maximum possible cooking power (kW) Pc_max safe: Maximum safe cooking power (kW) Pc_min safe: Minimum safe cooking power (kW) Pf: Firepower (kW) Pf_dimensionless: Dimensionless firepower

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Pf_max: Maximum possible firepower (kW) Pf_max safe: Maximum safe firepower (kW) Pf_min safe: Minimum safe firepower (kW) PM: Particulate matter (mg/min) Qchar radiation: Radiative heat transfer from char bed to the pot bottom (kJ/kg of dry fuel) Qdoor loss: Radiative heat lost through the feed door opening (kJ/kg of dry fuel) Qflame radiation: Radiative heat transfer from flame to the pot bottom (kJ/kg of dry fuel) Qfuel moisture: Heat loss due to evaporation and sensible heating of fuel moisture (kJ/kg of dry fuel) Qheat loss: Heat lost from the stove through insulation (kJ/kg of dry fuel) QH2 moisture: Heat lost due to sensible heating of H2 related moisture (kJ/kg of dry fuel) Q_ char radiation : Radiative heat transfer from char bed to the pot bottom (W) Q_ flame radiation : Heat transfer from flame to inner wall (W) Q_ conv 2 : Convective heat transfer to the pot from pot bottom zone (W) Q_ rad 3 : Radiative heat transfer to the pot from pot gap zone (W) Q_ conv 3 : Convective heat transfer to the pot from pot gap zone (W) Q_ rad 4 : Radiative heat transfer to the pot from pot side zone (W) Q_ conv 4 : Convective heat transfer to the pot from pot side zone (W) S: Sulphur content of the fuel as percentage of dry weight (%) T: Insulation thickness (mm) Ta: Ambient temperature (K) Tg: Flue gas temperature in the combustion chamber (K) Touter: Surface temperature (K) W: Pot gap width (mm) Wf: Width of the feed door (mm) Wp: Width of the primary air entrance (mm)