A STUDY ON IMPROVED INSTITUTIONAL BIOMASS STOVES S.C. Bhattacharya, A.H. Md. M. R. Siddique, M. Augustus Leon, H-L. Pham and C.P. Mahandari Energy Program, Asian Institute of Technology, P. O. Box 4, Klong Luang, Pathumthani, Thailand Fax No. (662) 524 5439, e-mail: bhatta@ait.ac.th Abstract - Realisation that improved cooking stoves (ICS) can relieve pressure on biomass resources led to ICS programs in most developing countries of the world. Most of the ICS programs are directed towards development of improved household cooking stoves, while relatively less work has been done on development of bigger stoves that could be used in institutional kitchens or certain traditional rural cottage industries. Three different designs of such stoves, using biomass briquettes as fuel, have been studied: 1. INTRODUCTION The experimental study was within the framework of a regional research and dissemination programme, 'Renewable Energy Technologies in Asia', funded by the Swedish International Development Cooperation Agency (Sida). One of the main objectives of the project was to design, fabricate and test improved biomass briquette burning stoves suitable for institutional kitchens or traditional cottage industries. Several institutional stoves were designed, fabricated and tested to evaluate their performances. While designing, main attention was focused on utilization of briquettes as fuel and to get clean combustion. Both ricehusk and sawdust briquettes were used as fuel. Testing on the stoves were carried out to determine overall efficiency using water boiling test and to observe general operational features, e.g. smoking, ease of start-up etc. This paper presents descriptions of the experimental stoves and test procedures as well as results of the study. 2. EXPERIMENTAL SET-UP AND PROCEDURE The efficiency of a stove is usually defined as the ratio of heat transferred to the cooking medium to heat supplied by fuel. The stove efficiency could be evaluated by a number of standard methods such as Constant Heat Output Method, Constant Temperature Rise Method, Constant Time Method, and Water Boiling Test (Prasad and Verhaart, 1983). Of these, the Water Boiling Test appears to be most commonly used; this test method is used in the present study as well. 2.1 Water boiling test In a Water Boiling Test, a known quantity of water is heated on the cookstove. No lid is used to cover the vessel so that evaporated water freely escapes from the vessel. The quantity of water evaporated after complete burning of the fuel is determined to calculate the efficiency by using the following formula: η = m w,i c p,w m w,i x c p,w x (T e - T i ) + m w,evap x H l m f x H f = initial mass of water in the cooking vessel, kg = specific heat of the water, kj/kg C = mass of water evaporated, kg = mass of fuel burned T e = temperature of boiling water, C T i = initial temperature of water, C H l = latent heat of evaporation at 100 C and 10 5 Pa, kj/kg = Calorific value of fuel, kj/kg m w,evap m f H f 2.2 Apparatus for water boiling test i) A pan without lid. ii) Thermocouples for measuring the ambient and boiling water temperature. iii) A digital balance for measuring the weight of fuel, water and pan. 2.3 Procedure The fuel and pot to be used in the test were separately weighed. The pot was partially filled up with water and weighed again. The initial temperature of water was recorded. The stove was ignited to initiate heating of the pot. Boiling temperature of water was recorded. After burning of the fuel was complete, weight of water left on the pot was recorded. 3. BRIQUETTE STOVES Ricehusk and sawdust briquettes were the main fuels used in the study. The briquettes were available in cylindrical shape of about 55-60 mm diameter, and in 20-30 cm length. These were used in whole lengths or cut into small pieces of thickness in the range of 20-25 mm, as required. The measured properties of the briquettes are given in Table 1. Table 1: Properties of ricehusk and sawdust briquettes Property Ricehusk briquette Sawdust briquette Bulk density, (kg/m 3 ) 522 483 Moisture content, % 6.0 7.9 Volatile matter, % 57.5 75.1 Fixed carbon, % 12.7 15.7 Ash, % 20.6 1.3 Lower Calorific Value (MJ/kg) 11.7 18.8 Three designs of biomass briquette-fired stoves suitable for institutional kitchens or cottage industries are studied in this
paper. They are: i) a gasifier stove, ii) a two-stage top burning stove, and iii) a charcoal making stove. 3.1 Gasifier stove For preliminary studies, a cross flow gasifier stove was designed and tested. Based on the test results, the design was improved and a final design arrived at (Figures 1,2). The most important part of the stove, the base (lower part) was constructed of brick and a few pieces of steel plates. The hopper allowed extended operation without frequent loading of fuel. The stove could be ignited from the bottom by opening the ash pit door and using a flame torch. A steel net was provided at one side of the combustion zone to let primary air in. A curved channel section was fixed to the stove base for primary airflow into the stove. The chimney consisted of 2-3 segments. The bottom segment attached to the stove was 30 cm tall. Among the rest two segments, one was 30 and the other one was 20 cm tall, both being detachable. Inside surface of all the segments were insulated by fire clay. On the top of the chimney, three small bars were welded to allow a pot to rest on it. The stove could accommodate flat or spherical bottomed vessels of 15-25 cm diameter. Small pieces of briquettes/wood could be loaded into the metallic hopper from the top. The top cover of the hopper was closed tightly so that there was no air leakage. For start-up, the fuel had to be ignited from the bottom of the stove. A flame torch was held below the grate by opening the ash pit door. A few minutes later briquettes were ignited, the bottom layer first. During start-up, considerable amount of smoke generated from the stove. Flame appeared inside the chimney after it became heated enough to establish a steady draft of air through the stove. Figure 2: Gas burner details 3.2 Charcoal making stove This was a single pot, portable metallic stove as shown in Figures 3,4 and 5. It consisted of two concentric cylinders, with annular space in between; the annular space was covered at the top permanently, with a mild steel sheet, by welding. Briquettes were loaded vertically in the annular space from the bottom of the stove, and the bottom of the annular space was covered with a removable metal lid. The inner cylinder had a grate at the bottom through which air entered the stove. It also had some small holes at its surface to allow pyrolysis gas to enter into the combustion zone. The pyrolysis gas would be burned inside the stove for cooking. The stove was insulated externally by ceramic fibre insulation. The stove could accommodate spherical or flatbottomed vessels of 20-30 cm diameter. The entire stove stands on three legs that hold it about 15 cm above the ground to allow free primary air access. Figure 1: The Gasifier Stove Figure 3: Loading briquettes in Charcoal Making Stove
For startup, the inner cylinder of the stove was loaded from the top with some charcoal, and was ignited (For consequent use of the stove, the charcoal produced during the previous run of the stove was used). As briquettes placed in the annular space start to pyrolyse, the pyrolysis gas enters the combustion zone through the small holes in the wall of the inner cylinder, and burns. When pyrolysis is complete, i.e., when all the briquettes get pyrolysed, the flame dies, and charcoal remains in the annular space. This charcoal may be used for the next run of stove use. 3.3 Two-stage top-down burning stove Based on the experience with the preliminary design, a simple durable design was developed as shown in Figures 6-9. The stove consisted of two identical stages. Two empty paint cans of five US gallon capacity were used to make this stove. There was a grate at the bottom of each stage to hold fuel above it. Primary air could enter through the bottom of each stage. Secondary air enters through several 1 cm holes located at the middle of the stage. To use as a two-stage stove, one stage is placed above the other one. Four supports were provided at the top of each stage for this purpose. Inside surface of each stage was insulated by fire clay to reduce heat loss to the surrounding. The stove could accommodate spherical or flat-bottomed vessels of 20-30 cm diameter. The external diameter of the stove was 29 cm and height of the twostage stove 34 cm. Both small and large pieces of briquette could be used as fuel in this type of stove. Option 1(briquette pieces): Biomass briquettes were cut into small pieces to burn in this option of stove operation. Figure 4: Fixing the bottom cover Single stage operation: The stove can be loaded completely or partially depending upon the desired duration of operation. On top of the fuel bed, some kindling materials, e.g. small pieces of dry wood soaked with kerosene (Figure 7) were placed. Double stage operation: In this case, the lower stage was loaded completely such that the fuel touches the grate of the upper stage. For combustion to propagate from the upper stage to the lower stage, this is very important. The upper stage can be loaded half or full, depending on the desired duration of stove operation. After loading the upper stage, some small pieces of dry wood were placed at the top for kindling. Continuous operation: By fully loading both the two stages, up to about three hours of operation is possible. If the stove is to be run for more than three hours, the two stages of the stove should be fully loaded as in double stage operation. About one and a quarter hour later, the top stage should be removed and refilled with pieces of briquette; it is next placed below the other stage. For continuous operation the upper stage should be removed, refilled and reused as bottom stage in this way. Normally after one round of refilling, the stove could be operated for about 1 hour and 15 minutes. Option 2 (whole briquette): Large pieces of briquette (20-25 cm) can also be burned in this stove. Briquettes could be placed vertically inside the stove. The top 4-5 cm vacant space should be filled up with small pieces of briquette. For starting combustion, kindling should be placed as mentioned in option 1. Figure 5: Combustion in the Charcoal Making Stove Combustion could be started by igniting the kindling by a match stick or a cigarette lighter. Briquettes get ignited within a few minutes. Combustion of the fuel propagates from the top downwards. It was observed that the combustion was very clean,
steady and strong. The flame reached a height of about 30 cm above the stovetop. In case of large briquettes loaded vertically, the combustion flame was stronger and more intense than with small pieces. High combustion flames, which escape through the surrounding area of the cooking pot also take away heat and impairs the efficiency of the stove. Flame height could be reduced and duration of burning increased by closing primary air supply through the grate of the bottom stage. Figure 6: Loading large briquettes in Two-stage Stove Figure 8: Single-stage Stove burning. Figure 7: Initial ignition. 4. RESULTS AND DISCUSSION 4.1 Gasifier Stove Sawdust briquettes showed very good results in terms of smokeless combustion after start-up. The flame reached up to the top of the chimney while burning sawdust briquette. It was observed that some smoke occasionally emerged from the air inlet passage during steady stove operation. Efficiency of the gasifier stove for several experiments are shown in Table 2. Flame-to-pot contact is essential to achieve higher efficiency. It was observed that when the chimney height was 60 cm, flame-to-pot contact was better and higher efficiency was obtained compared to a chimney height of 80 cm. The stove could not operate satisfactorily with rice husk. Since it Figure 9: Two-stage Stove burning.
had no provision for forced ash removal, it could not be operated steadily with rice husk because of ash accumulation. Table 2: Efficiency of Gasifier Stove Expt Efficiency, % Fuel no. Chimney height 80 cm. Chimney height 60 cm. 1 9.60 15.76 Sawdust 2 10.21 11.80 3 10.15 13.63 4.2 Charcoal Making Stove Combustion was very clean due to the perfect burning of the volatiles produced from the briquettes. The flame height was reasonable for both ricehusk and sawdust briquettes. Occasionally some smoke was seen to escape from the stove. The burning time of the stove was 1-1.5 hours with a batch of fuel. The efficiency values of the stove are given in Table 3. Higher ash content of ricehusk could be the reason for lower efficiency observed in case of ricehusk compared with sawdust. Amount of charcoal produced per batch was about 0.7 kg, which was about 15% of fuel loaded. Table 3: Efficiency of Charcoal making Stove Fuel Exp. no. Efficiency, % 1. 15.38 Sawdust 2. 12.96 3. 12.47 4. 11.39 Ricehusk 5. 13.20 6. 15.47 Level of smoking was found to be much less with top-down burning compared to conventional burning from the bottom. 5. CONCLUDING REMARKS In the Gasifier Stove, combustion was almost smokeless under certain conditions. Highest efficiency observed was about 16% for sawdust briquettes. Two-stage top-down combustion stove could be used for longer duration with whole or small pieces of briquettes. Combustion was almost smokeless with efficiency of about 27% for ricehusk. The Charcoal making stove was found compact with a little occasional smoking. Charcoal produced in one operation was enough to start the stove for the following operation. Highest efficiency was found about 15% both for ricehusk and sawdust. All stoves designed during the present study showed low levels of smoke emission. These stoves could be disseminated to rural areas of developing countries. The gasifier stove which showed cleanest combustion in terms of smoke could be considered for food drying in a cabinet dryer. REFERENCES Prasad K. Krishna, and Verhaart, P., 1983, Wood Heat for Cooking, Indian Academy of Sciences, Bangalore, Macmillan India Press. Overall, the stove was found compact with very little occasional smoking. Charcoal produced in one test run could be used to start the stove for the following run. 4.3 Two-stage top-down burning stove The efficiency of the stove was measured with half of a single stage loaded with briquettes. Time needed to boil water was around 13 minutes. The efficiency found from several experiments are given in Table 4. Relatively low efficiency observed for sawdust compared with rice husk appears to be due to higher losses caused by stronger flame. Table 4: Efficiency of single stage stove Fuel Exp. no. Efficiency, % 1 21.64 Sawdust 2 19.97 3 20.31 4 27.17 Ricehusk 5 27.27 6 26.20