Energy Technology 2014: Carbon Dioxide Management and Other Technologies Edited by: Cong Wang, Jan de Bakker, Cynthia K. Belt, Animesh Jha, Neale R. Neelameggham, Soobhankar Pati, Leon H. Prentice, Gabriella Tranell, and Kyle S. Brinkman TMS (The Minerals, Metals & Materials Society), 2014
PERFORMANCE EVALUATION, TECHNICAL AND ENVIRONMENTAL ASPECTS OF BIOMASS COOKSTOVES: AN EXERGY APPROACH S. K. Tyagi 1, A. K. Pandey, Kunwar Pal
Sardar Swaran Singh National Institute of Renewable Energy, Kapurthala 144601 (Punjab) India Keywords: Improved biomass cookstove, emission reduction, energy and exergy analysis
Abstract This study deals with the performance evaluation, technical and environmental aspects of different cookstove models (NIRE-02, NIRE-03 & NIRE-04). These cookstove models were designed, fabricated and evaluated following Bureau of Indian Standards (BIS) and their performance was compared from technical and environmental points of views. The theoretical efficiencies of all the cookstove models are found to be in good agreement with the experimental one. Also, the overall performance of these models is found to be in the order of (NIRE-04 > NIRE-03 > NIRE-02) which is also observed to be much better than that of the traditional cookstoves being used by the people in most of the developing countries around the world. The CO 2 emission reduction from these models is found to be between 2.0-3.0 tons per household annually which is ultimately in good agreement with the experimental values. Introduction More than 2.5 billion people around the world depends on the biomass based resources for cooking and heating applications which includes wood, cattle dung, charcoal and agriculture residue [1,2]. Generally, people of rural area cook their food on unvented open fire, which burn biomass fuel inefficiently besides it creates many thermal and environment pollution hazardous causing health problems in the women and girl children due to more exposure of emissions. Also the consumption of biomass is very high as compared to the fossil fuels due to the fact that the large population of the developing countries lives in the rural area than that of in the urban area. It is estimated that about 3% of the diseases are caused due to incomplete combustion of biomass, which results in the around 1.8 million premature death every year including around 0.9 million death under five year of their age [3,4]. Ramanathan and Carmichael [5] recently found that the black carbon of unburned smoke is playing a major role in global warming. The emission factors for different combinations of fuel and stove tested in China for direct and indirect GHGs as well as other airborne pollutants such
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Corresponding Author: E-mail(s):
[email protected]
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as, CO 2 , CO, CH 4 , TNMHC, N 2 O, SOx, NOx, TSP etc. for a typical set of operating parameters [6]. The improved biomass cookstove technology came into assistance which is technically feasible, socially reliable and economically viable [7, 8]. The improved biomass cookstove not only reduces pressure on the biomass resource, but also reduces the environmental pollution and also minimize the time required for collection of fuel wood [9, 10]. Sharma et al. [11] emphasized on the adoption and large-scale propagation of efficient stoves to improve the health of rural women and to reduce the consumption of firewood and environmental pollution. Very recently Tyagi et al., [12] presented the experimental study of various improved biomass cookstove models based on energy and exergy analysis. They found that the exergy efficiency is generally, lower than that of the energy efficiency for all type of cookstove model studied by them. Kumar et al. (2013) [13] presented the thorough review on biomass cookstove based on various performance parameters. Improved biomass cookstoves also have the higher potential to get carbon credits [14], not only because of their contribution to climate-change mitigation but also they can yield major co-benefits in terms of energy access to the mass populations around the globe. Carbon trading offsets is growing source of interest comes from improved biomass cookstove program on carbon market for both voluntary and certified emission reduction, under clean development mechanism (CDM) of the UNFCCC following Kyoto Protocol [14,15]. Materials and Methods In the present article a modified traditional cookstove with different modifications was designed and fabricated using locally available materials. Apart from modifications in the traditional cookstove two improve biomass cookstove models (NIRE-03 and NIRE-04) were also design and fabricated. These improved cookstove work on the principle of down-draft gasifier where pyrolysis, gasification and combustion of biomass are taking place simultaneously. These cookstoves were fabricated using mild steel whereas for insulation clay and wheat straw mixture was used. The photographic view of NIRE-02, NIRE-03 and NIRE-04 are shown in the figure 1. The water boiling test was performed for all the cookstove models and the thermal efficiency of each cookstove was calculated according to BIS standard (IS 13152). The temperature of water, flame and cookstove body was measured with the help of digital temperature sensors, whereas, the temperature of pot, plate and ambient was measured with mercury-glass thermometers. As per the detailed reports available in the literature [16, 17], the thermal efficiency of traditional cookstove is less than 10% and can be verified by following the similar testing protocols as in the present case. Sheesham (Dalbergia sissoo) wood was used as the fuel and prepared as per BIS protocol for cookstove. The fuel wood was cut from the same log into pieces of 3x3 cm square cross-section and length of half the diameter/length of combustion chamber so as to be housed inside the combustion chamber easily. For experimental analysis, the wood samples were crushed into powder form. Calorific value of wood was estimated with Bomb calorimeter. Finally The performance of different types of cookstoves has been evaluated out using energy and exergy analysis following earlier authors [10, 12]. The modified traditional cookstove NIRE02 is a portable model made up on mud platform and suitable for indoor and outdoor according to the weather condition. This model was tested in three different modes such as, tested with grate (G), tested with grate and top spacing (S) and tested only with top space (S). However, the IBC model NIRE-03 was tested with and without insulation and with.
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253 260
25
260
Top view
150 Pot support
25
35
Wick Secondary air inlet
60 50 Space for Flame
190
15
35
Handle
Bottom view
410 23
40
Insulation
Grate
Air
Platform 200
Grate
200
Handle for primary Air adjustment
50
Fig.1(a): NIRE-02
50
Fig.1(b):NIRE-03
Primary air
253 TOP VIEW 25
125 Pot Support
25 60
Wick
50
Secondary Air Inlet û Handle
410
BOTTOM VIEW
Insulation
23
Air Gap Grate Handle for Primary Air Adjustment 50 50
Primary Air Inlet
Fig.1(c): NIRE-04 Fig.1: Schematic diagram of the different cookstove models evaluated in the present study Analysis of Cookstove Improved biomass cookstoves can reduce indoor air pollution, deforestation, climate change, and therefore, the quality of life can be improved on a universal scale. The better design of these cookstoves can significantly impact their performance and emissions. Although these improved biomass cookstoves have been studied for a long time however, a theoretical understanding of their operating behaviour and the development of engineering tools for an improved cookstove based on natural convection is still missing. Energy and Exergy Analysis Energy efficiency of a cookstove may be defined as the ratio of energy utilized to the energy produced by complete combustion of a given quantity of fuel based on the net calorific value of the fuel and this can be written as below [10,12]:
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»
Energy output Energy input
E o § m w C p (Tfw Tiw ) m pot C pA 1 (Tfp Tip ) · ¸¸ {¨ E in ¨© m wd c1 xu du c 2 ¹
(1)
However the exergy efficiency may be defined as the ratio of output exergy to the input exergy and given by [10,12]:
ý
Exergy output Exergy input
E xo E xin
T T · § ¨ m w C p (Tfw Tiw )(1 a ) m pot C pA 1 (Tfp Tip )(1 a ) ¸ Tfw Tfp ¸ ¨ ¸ ¨ T m wd c1 (1 a ) u » xu du c 2 ¸¸ ¨¨ Tfuel ¹ ©
(2)
where m w is the mass water in the pot, C p is the specific heat of water, T fw is the final temperature of water, T iw is the initial temperature of water, m pot is the mass of pot, C pA1 is the specific heat of Aluminium pot, T fp is the final temperature of pot, T ip is the initial temperature of pot, m wd is the mass of wood, c 1 is the calorific value of wood, T a is the ambient temperature, T fuel ! $ ! * ` » ! ` * ` density of kerosene and c 2 is the calorific value of kerosene. Emission reduction Calculation The emission reduction from each cookstove is based upon fraction of biomass that can be saved during the project year and the calorific value of the biomass used and this can be calculated according to the formula given below [14]:
ER y
B y ,savings u f NRB , y u NCVbiomass u EFprojected _ fossilfuel
(3)
where ERy is the emission reductions during the year y in tCO 2 , f NRB , y is the fraction of woody biomass saved by the project activity in year y, NCVbiomass is the net calorific value of the nonrenewable woody biomass that is substituted (IPCC default for wood fuel, 0.015 Tj/tonne), EFprojected _ fossilfuel is the emission factor for the substitution of non-renewable woody biomass by similar consumers and one can use of value of 81.6 tCO 2 /TJ. B y , savings is the quantity of woody biomass that is saved in tonnes which can be calculated by the following formula:
By , savings
§ K · Bold .¨¨1 old ¸¸ © K new ¹
(4)
where B old is the quantity of woody biomass used in the absence of the project activity in tonnes, » old is the efficiency of the baseline system, » new is the efficiency of the system being deployed as part of the project activity (fraction), as determined using the water boiling test (WBT) protocol.
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Results and Discussion Based on the biomass characteristics, the performance of different cookstoves has been carried out using energy, exergy and carbon emission reduction analysis at a typical location in India, while the discussion of results is given as below: Modified Traditional Cookstove
Energy() and Exergy() efficiency (%)
The energy and exergy efficiencies were evaluated for NIRE-02 and plotted against the quantity of wood as shown in the Fig. 2. From the figure 2, it is found that as the quantity of wood is increases from 1- 5 kgs, both the energy and exergy efficiencies also increase and found to be highest for 5 kg of fuel wood in the case of with top space. On the other hand, in case of grate only, the energy efficiency is found to be increasing in nature for 1-3 kgs of wood. However, for a given capacity, there should be an optimum time of operation and amount of fuel wood required to get the best possible performance of a cookstove. This due to the fact that the overall heat loss from the cookstove increases as the cooking time and the amount of fuel wood is increased. In other words, the increase in the energy efficiency from 1-3 kgs is found to be significant, while it is almost constant for 4-5 kg of wood consumption. However, the change in the energy efficiency is found to almost constant as the fuel wood supply is increased from 4-5 kgs. Again in the case with top spacing nature of variation in energy efficiency is found to be same as that of the case with grate and space while the average energy efficiency with grate is found to be higher than that of the other two cases mentioned above. The maximum energy and exergy efficiency is found to be 29.28% and 4.79% respectively in the case with top space and with 5 kg of wood. However, the optimum performance of the NIRE-02 model is found for the case with grate only. (NIRE-02) w G
30 24 18 12 6 0 »
ý 1
»
(NIRE-02) w G & S
ý
»
2
ý 3
NIRE-02 w S
»
ý 4
»
ý 5
Mass of wood (kg)
Fig-2: Energy and exergy efficiency of NIRE-02 with different modification parameter and with varying quantity of wood. (G= Grate, S= Spacing between upper edge of cookstove and pot) The variation of CO 2 emission reduction values of NIRE-02 cookstoves is shown in the figure 3. The value of emission reduction is continuously increases as the fuel wool increases from 1-5 kgs for the case when top space is used only. The emission reduction from NIRE-02 with grate only and with grate and top space is similar in nature. From these modification the value of emission reduction is first increasing as the fuel wood increases from 1-2 kgs and then almost constant with varying quantity of fuel wood from 3-5 kgs. It is clearly understood for the figure 3 that the NIRE-02 model performs good and gives a large amount of emission reduction value i.e. 2.26 tonnes of CO 2 reduced per household per year when it is use with top space only.
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(NIRE-02) w G
Emission reduction (tonneCO2 /household/year)
3
(NIRE-02) w G & S
NIRE-02 w S
2 1 0 1
2
3 Mass of wood (kg)
4
5
Fig-3: Emission reduction from NIRE-02 with different modification parameter and with varying quantity of wood. Improved Cookstove Models The variation in energy and exergy efficiency against the quantity of the wood of NIRE-03 cookstove model with different variations in the design is shown in Fig. 4. From Fig. 4, it is found that as the quantity of wood increases from 1-5 kgs, the energy efficiency of NIRE-03 cookstove model is almost constant without insulation. In the case with insulation and without wick, it was observed that the energy and exergy efficiency first increases with increasing the wood from 1-3 kgs but decreases with further increase in wood from 3-5 kg. Again, it was observed that the energy efficiency first decreases with the increase in the wood quantity from 12 kgs and then increases as the quantity of wood increases from 2-5 kgs in the case with wick. Thus it is concluded from this study that the use of wick should be used to regulate and control the fire for specified cooking requirement only. (NIRE-03) w/0 Ins.
Energy() and exergy() efficiency (%)
40 32 24 16 8 0 »
ý 1
(NIRE-03)w Ins. w/0 W
»
ý
»
2
ý 3
(NIRE-03) w Ins. & W
»
ý 4
»
ý 5
Mass of wood (kg)
Fig-4: Energy and exergy efficiencies of NIRE-03 with different modification parameter and with varying quantity of wood. (Ins= insulation, w/0 = without, W= wick) Figure 5 shows the variation of emission reduction potential (tonneCO 2 /household/year) of NIRE-03 cookstoves. It is clear from the Fig. 5 that there is good potential for annual CO 2 emission reduction with improved cookstove. However, the highest potential in emission reduction is found for the case where insulation without wick was applied, and the similar case was also found for the thermal efficiency as shown in the Fig. 4. The maximum value of emission reduction 2.46 tonne CO 2 /household/year was found with 3 kg of wood. It is clearly understood from the results that the NIRE-03 model performs best and gives a large amount of emission reduction value i.e. 2.46 tonnes of CO 2 reduced per household per year. This means if
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Emission reduction (tonneCO2 /household/year)
approximately 10,000 of NIRE-03 cookstoves are provided to the consumers it will save approximately 24,600 tonnes of CO 2 in one year. (NIRE-03) w/0 Ins. 2.5 2 1.5 1 0.5 0 1
(NIRE-03)w Ins. w/0 W
(NIRE-03) w Ins. & W
2 3 mass of wood (kg)
4
5
Fig-5: Emission reduction from NIRE-03 with different modification parameter and with varying quantity of wood.
Energy() and Exergy() Efficiency (%)
The NIRE-04 cookstove model is the modified version of NIRE-03 in which an air is provided to utilize the heat which is going to waste through the stove body.The combustion chamber as shown in the Fig.1. The performance of NIRE-04 model without insulation has been evaluated and shown in the Fig. 6, further modifications are underway and enhancement in the efficiency is also expected by 10-12% as compared with present case with similar trend for emission reduction potential. Energy »
30
Exergy þ
20 10 0 1
2 3 Mass of wood (kg)
4
5
Fig-6: Energy and exergy efficiencies of NIRE-04 without insulation and with varying quantity of wood. Conclusions In the present study comparative exergetic, energetic efficiency and CO 2 emission potential of different designs of modified traditional and improved cookstove models have been presented. Based on the experimental study and observations, the following conclusions are drawn: x Based on the experimental observations the energy and exergy efficiency and as well as the emission reduction potential for NIRE-02 model was found to be best when both grate and top space was provided. x The performance of NIRE-03 cookstove was found to be best with insulation for 3kg of fuel wood. Similar results were also observed for the emission reduction potential. x Again, the performance and the CO 2 emission reduction potential of NIRE-03 model is found to be higher than that of NIRE-02 for all set of operating parameters. In general the
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x
efficiencies and CO 2 emission reduction potential of improved cookstoves models to be higher than that of traditional cookstoves i.e., without grate and top space for all set of operating parameters. Also energy efficiency was found to be always higher than that of exergy efficiency which is due to the energy gained by the hot water at that particular temperature. References
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