Design and construction of a gasifier using long stick wood as the feed

Transcription

1 CHAPTER 3 Design and construction of a gasifier using long stick wood as the feed In this chapter the design aspects and experimental set up for different modes in the gasifier are briefly discussed at first. 3.1 Categories of Gasifier Many different gasifier designs have been oped for wood chips and it is developed over the last century using both experience and intuition. We are in a new period, during which the principles of combustion science are being applied to develop a better understanding of gasification using long stick wood. In this new gasifier such as the stratified updraft gasifier and the high temperature gene gasifier one notices the promise to expand the range of usable fuels and to produce an even cleaner product gas. Consequently gasifier design has taken new dimensions. Only time may tell whether this long stick wood based gasifier concept will result in cleaner, more versions of versatile gasifiers at an acceptable cost. The common names of gasifiers (updraft, down draft, fluidized bed and entrained flow) reflect the way the fuel flows and are supported and simultaneously the way the air/oxygen flows to the fuels. In the gasification process heat can be supplied by direct combustion of the pyrolysis gases (flaming pyrolysis in down draft) 41

2 or by the combustion of the charcoal sepaly (updraft) or by a combination (fluidized beds). Every gasifier and combustion device employs some form of heat recycling to gene the 5 to 15% heat required for pyrolysis as shown in Table 1. Arranging to have this heat delivered to the incoming fuel is the principal problem in gasifier design and accounts for the wide variety of gasifiers. Table 1 - Sources of Heat for Gasification of Various Types of Gasifiers TYPE OF GASIFIER SOURCE OF HEAT FOR PROCESS Updraft Combustion of Charcoal Downdraft Partial combustion of Volatiles Cross draft Partial combustion of Volatiles and Charcoal Fluidized bed Partial combustion of Volatiles and Charcoal Updraft (Counter flow) The updraft gasifier is widely used for coal gasification and non volatile fuels such as charcoal. Counter flow refers to the fact that fuel is introduced at the top and flows down, while air/oxygen is introduced below a g on which the charcoal produced by pyrolysis sits. The gases flow up as shown in fig.4. Fig 4. Diagram of Updraft gasifier 42

3 From top to bottom the processes are 1. The down flowing fuel is dried by up flowing hot gases with good heat recovery. 2. Down flowing dry fuel is then pyrolysed by the up flowing gasification gases, producing prompt gas/vapor and charcoal and recovering their heat. 3. Down flowing charcoal at C reacts with up flowing CO 2 and H 2 O resulting from charcoal combustion to produce CO and H 2 4. Down flowing charcoal burns with entering air at the g at very high temperatures 5. Down flowing ash falls to ash disposal The updraft counter flow gasifier is occasionally used for biomass in situations involving high ash or where the tars don t need to be removed for subsequent combustion Downdraft In down draft gasifier air enters at the combustion zone and the gas produced leaves near the bottom of the gasifier as shown in fig 5. In this type of gasifier, the volatiles and tar produced from the descending fixed bed have to pass through the reduction zone, where they are cracked and gasified. Also a throat construction provided in the hearth ensures that the gaseous products pass through the hottest zone. The gas produced contains less of tar and more of ash. This gasification is suitable for fuels like wood and agriculture wastes. 43

4 3.1.3 Cross draft Fig 5. Diagram of downdraft gasifier The cross draft gasifier schematically illustd in fig 6 is an adaptation for the use of charcoal. It is the lightest and simplest gasifier. Charcoal gasification results in very high temperatures (1500 C and higher) in the oxidation zone, which can lead to material problems. In cross draft gasifiers insulation against these high temperatures is provided by the fuel (charcoal) itself. Kaupp and Goss (1984) observed that for the cross draft gasifier, high temperature reached requires a low ash fuel to prevent slagging. Fig 6. Diagram of cross draft gasifier Advantages of the system lie in the very small scale at which it can be oped. Installations below 10 kw (shaft power) can under certain conditions be 44

5 economically feasible. The reason is the very simple gas-cleaning train (only a cyclone and a hot filter), which can be employed when using this type of gasifier in conjunction with small engines. A disadvantage of cross-draft gasifiers is their minimal tar-converting capabilities and the consequent need for high quality (low volatile content) charcoal. It is because of the uncertainty of charcoal quality that a number of charcoal gasifiers employ the downdraft principle, in order to maintain at least a minimal tar-cracking capability Fluidized bed The operation of both up and downdraught gasifiers is influenced by the morphological, physical and chemical properties of the fuel. Problems commonly encountered are: lack of bunker flow, slagging and extreme pressure drop over the gasifier. A design approach aiming at the removal of the above difficulties resulted in the fluidized bed gasifier illustd schematically in fig. 7. Air is blown through a bed of solid particles at a sufficient velocity to keep these in a state of suspension. The bed is originally externally heated and the feedstock is introduced as soon as a sufficiently high temperature is reached. The fuel particles are introduced at the bottom of the reactor, very quickly mixed with the bed material and almost instantaneously heated up to the bed temperature. As a result of this treatment the fuel is pyrolysed very fast, resulting in a component mix with a relatively large amount of gaseous materials. 45

6 Fig 7. Diagram of Fluidized bed gasifier Further, gasification and tar-conversion reactions occur in the gas phase. Most systems are equipped with an internal cyclone in order to minimize char blowout as much as possible. Ash particles are also carried over the top of the reactor and have to be removed from the gas stream if the gas is used in engine applications. The major advantages of fluidized bed gasifiers, as reported by Van der Aarsen (1984) stem from their feedstock flexibility resulting from easy control of temperature, which can be kept below the melting or fusion point of the ash (rice husks), and their ability to deal with fluffy and fine grained materials (sawdust etc.) without the need of pre-processing. 3.2 Motivation for our preliminary study Improved processing of wood into suitable shape and size along with drying of freshly cut wood required for use in a gasifier is needed to popularizing gasifiers. Feed stock preparation is an important aspect of the complete systems planning and such preparation typically involves cutting wood into required size. Electrical power, manpower, and storing facility involved in such processes make the functioning 46

7 cumbersome especially in an industrial area where meeting out all these parameters are difficult. Saw cutting alone genes 10% sawdust on weight basis. Thus it is felt that, it is right time to develop a new model of gasifiers working on long stick wood as fuel. The perfect functioning of a gasifier is highly dependent on the wood properties such as energy content, moisture content, bulk density, reactivity of the fuel, size and size distribution. Wood having ash content less than 2% by weight and moisture content up to 30% by weight is generally suited for gasification. Wood fuel needs some form of processing before combustion, primarily size reduction and drying. Storage will also be required. These activities maybe in the forest or where biomass is produced or at the gasification user plant site. The gasifier used widely during World War II had specially prepared 1 X 2 X 2 cm 3 hardwood blocks. Wood chips for such gasifier reactor required manual cutting. Mechanical cutter, electrical cutter, briquettes and charcoaling also control such dimensions. However, the above methods in preparation of wood chip have the following disadvantages such as consumption of electricity, high manpower, dust production and unexpected accident during processing. Furthermore, in case of water pumping systems, electricity is typically not available at the site and belt driven system may need to be employed in order to reduce the manual labour input in feedstock preparation. Also the wood chip cutter seemed to be less convenient, more dangerous and consumed more power supply. Up and downdraft gasifiers are limited in the range of fuel size acceptable in the feed. Fine grained and/or fluffy feedstock may cause flow problems in the bunker section of the gasifier as well as an inadmissible pressure drop over the reduction zone and a high proportion of dust in the gas. Large pressure 47

8 drops will lead to reduction of the gas load of down draft equipment, resulting in low temperature and tar production. A large range in size distribution of the feedstock will generally aggravate the above phenomena. Too large particle sizes can cause gas-channelling problems, especially in updraft gasifiers. Acceptable fuel sizes in gasification systems depend to a certain extent on the design of the units. In general, wood gasifiers ope on wood blocks and woodchips ranging from 8 x 4 x 4 cm. to 1 x 0.5 x 0.5 cm. Charcoal gasifiers are generally fuelled by charcoal lumps ranging between 1 x 1 x 1 cm. and 3 x 3 x 3 cm. Table 2 shows different types of gasifiers using the relevant feedstock size and moisture content. Table 2 - Feed stock size and Moisture content for various types of gasifiers Types of Gasifiers Feedstock size Moisture content Updraft 2 50mm <50% Down draft mm <30% Fluid bed 0 25mm <50% Source: T.B.Reed (2001) Feedstock Preparation Size adjustment of biomass is often required. Coarse inert components can be removed by sieving or by magnetic and ballistic separators. Fines can be removed by sieving and or air classification. Size reduction to 100mm is done with low speed shredders. Fast speed chippers and hammer mills are suited for further size reduction 48

9 until 1-10mm. Hos and Groeneveld (1981) report that densification by pelletizing or briquetting operation is only a solution for the improvement of the flow, storage and transport of the biomass, but does not improve gasifier performance. Three types of equipment are generally suitable for breaking wood fuel into smaller particles: 1. Chippers are well-proven units with sharp blades, which cleave the wood against a stationary anvil. Intended primarily for green wood (drier timbers result in high wear s), there is dust production and chipping can be uneven. The machines are vulnerable to soil and stone contamination and, because they are noisy, may be subject to restricted working hours. 2. Hammer mills use fast-moving blocks of metal to shatter wood and break it further against a metal screen. The machines are easy to maintain and tole contaminated material, although they tend to produce a coarse, variably sized chip. Like chippers, hammer mills are noisy and may be subject to restricted operating hours. 3. Shredders are low speed machines, which shear the feedstock with rotors and offer the advantage of lower noise levels. Very variable size particles will be produced unless a screen is fitted. Where wood chips are being used in equipment using gasification, automatic combustion, the chip size required by the manufacturer of the gasifier feed system must be ascertained before obtaining wood chips from a supplier, or before acquiring any chipping or shredding equipment. The charcoal yield from a biomass feedstock is highly dependent on the of heating and size of the biomass particles. Industrial charcoal manufacturers use very slow heating s to achieve charcoal yields of more 49

10 than 30% of the initial dry weight of the biomass. The very rapid heating s encountered when small biomass particles are gasified and combusted realize charcoal yields of less than 15% of the initial dry weight of the biomass; large size feed stocks produce 15% to 25% charcoal. The various ranges of woodcutters manufactured by Chauhan and Jain (1991) include the following: Table 3 - Cutter and feed stock parameter Sl No Type of cutter 1 Belt driven cutters for pumping systems 2 Motorized circular saw based rotary cutters for small gasifiers systems 3 Motorized circular saw based rotary cutters for larger gasifiers systems 4 Eccentric / Reciprocation / Multi blade cutters for larger gasifier system 5 Chaff cutter type cutters for agricultural feedstock / twigs and branches etc. Desirable feed stock parameters Maximum Acceptable cross sectional area (equivalent dia in mm) Maximum length of cut piece (mm) Hourly output (kg/hr) to to to 60 Up to 100 Up to 150 Up to to to A belt-driven model with mounting attachment for integration with gasifier engine pump-sets. 50

11 2. Single blade, single phase and three-phase electric motor driven cutters using 200mm, 250mm and 300mmcircular saw blades, for sizing feedstock of different diameters ranging between40 to 85mm (1.1/2 3.1/2 ). 3. A three bladed electric motor driven high-speed circular cutter for sizing feedstock of upto90mm diameter and output of well above 100 kg/hour. 4. A two bladed motor driven high-speed circular cutter for economically sizing feedstock of 80mm to 100mm diameter and output of around 500 kg/hour. 5. A Chaff Cutter type device, capable of sizing agricultural residues, thorny bushes, twigs etc., and with outputs of up to 110 kg/hour Economics of wood sizing Table4 shows that the cost per kg of wood sized works out to be indeed very high. However, the drudgery and lack of reliability force one to look for alternatives. Table 4 - Cost of sizing woody biomass Sl.No. Parameter Single blade Two blade high electric cutter speed cutter 1 Rated output (kg/hr) Motor rating (kw) Assumed daily operation (hours) Estimated price of the cutter (Rs.) 9,500 25,000 5 Operating cost in Rs/100kg - Man Rs.25/day - Rs.1.5 kwhr - Interest & depreciation - (Capital 20% Total Cost / kg of woody biomass 59.45Rs/- 44.5Rs/- 51

12 3.2.3 Experimental setup Saravanakumar and Haridasan (2002) in their preliminary studies to understand the temperature and time required for cracking a piece of wood used an LPG burner. LPG was preferred due to the convenience of getting pointed flame for thermal cracking. Thermal cracking of wood means that heat energy is supplied to the wood for burning and then to get cracked into two small pieces. From this observation it is estimated at which temperature and what time the fuel wood was thermally cracked completely. We have taken a 15-inch (38 cm) length and 6 cm thickness firewood. Then the bottom of the firewood was heated with LPG burner. The distance between the bottom of the firewood and top of the LPG burner is 7 cm. From this experimentation, it is identified that the firewood started cracking at the temperature of 253 to 276 C. At temperature above 300 C, the firewood was broken into two pieces. During this experiment, the temperature reading was noted as in Table 5 for every fifteen minutes. Table 5 - Wood cracking temperatures readings S.No Time (sec) Temperature ( C) 1 Initial

13 The following fig 8 is drawn between time vs. temperature for wood cracking. The cracking start at about 253 C to 276 C and the splitting is materialized at about 300 C. At the temperature of 237 C a small crack takes place in the wood. The time taken is about 120 Seconds. In other flames, which are not as pointed as coming from LPG, this time interval may be more. Ratio of volume loss of wood per second to volume of the wood is found to be per second. So thermal cracking with pointed flame is possible with negligible loss of fuel wood. In other words 1 kg of fuel wood feed suffers 0.18 kg loss during the cracking process. 400 Temperature (Deg.C) Time (Sec) 338 Fig 8. Time Vs Temperature for wood Cracking The ratio of volume loss of wood per second to volume loss of the wood used is to found to be per second indicating the long-range feasibility of adopting this method. However challenges still exist in extending the presently suggested process for large-scale applications in real life situations. From this experiment, it is also learnt that controlled pyrolysis of wood is a desirable stgy. The loss of wood in the process is negligible. The time for fuel wood preparation is short. 53

14 3.3 Design of Bottom lit updraft Gasifier using Long stick wood From these observations of the preliminary work, it was decided to design and construct a 25 capacity updraft gasifier. Since the interest here is just in exploring and validating of this concept, the design was restricted to an updraft gasifier. This gasifier attains a high-energy release per unit area due to high inlet air velocity and activated reaction in combustion zone Old and New Updraft Gasifiers Experimental studies (Bamford et al. (1946), Tran and White (1992)) provide temperature as a function of time with in the wood under packed bed combustion conditions. Earlier studies on updraft gasifiers were confined to small wood pieces as feed. For example, Khummongkol and Arunlaksadamrong (1990) have studied an updraft gasifier with sun dried mangrove wood size 2-5cm long and about 5cm diameter. Bhattacharya et al. (1999) worked with two stage gasification using wood chips like cubic shape with sides in the range 10-15mm, dried by solar dryer. On the other hand, Bryden and Ragland (1996) have studied the whole tree by utilizing a deep, fixed bed combustor/gasifier. Wood logs 20cm in diameter were smouldered in a 3.7m deep fuel bed. Saravanakumar et al. (2005) used an updraft gasifier with packed bed for long stick wood of length 68cm and thickness of 6cm. A limited number of biomass pyrolysis experiments that have been performed using large woody particles. Some works include experiments for a 2-cm diameter moist cylinder (Lee, Chaiken, and Singer 1976), a 1-cm diameter cylinder (Chan, Kelbon, and Krieger 1985), 0.95-cm diameter cylinders (Tinney 1965), several spheres ranging 54

15 from 2 to 5.6-cm in diameter (Bilbao et al. 1993), 1-cm cubes (Bonnefoy et al. 1993), and 15-cm by 15-cm by 6.4-cm blocks (Tran and White 1992). Kayal et al. (1994) used an updraft gasifier with bundles of long jute sticks (15cm long and 1cm o.d). Conventional updraft gasification has the advantage that it can burn very wet wood and converts all of the biomass to combustible gas. It has the disadvantage that the gas can contain up to 20% volatiles from the wood, and so is unsuitable for operation of engines. A new form of updraft gasifier, the top lit updraft gasifier (also called the inverted downdraft gasifier ), has been developed starting in 1991 (Reed, 1991). If the reaction takes place at the top of the fuel charge, the volatiles are burned by the incoming air from the bottom forming a bed of charcoal on top of the fuel. These gases then pass through the charcoal resulting in tar levels from ppm, depending on superficial velocity of the gases (Reed et al. 1999, 2001). Because the reaction moves counter current to the air, the fuel bed is burned at the same as the reaction moves against the fuel. The top lit updraft gasifier is now being widely used for cooking and power generation. With dry wood it can also produce greater than 20% of a good grade of charcoal. Top and bottom lit updraft wood gasifiers using long sticks as feed are fundamentally very different. The bottom lit gasifier is a char burning, tar making gasifier, while the top lit gasifier is a tar burning, charcoal making gasifier. Both forms of gasifier have been tested and the results discussed in this chapter. 55

16 3.3.2 Fabrication of Bottom lit Updraft Long stick wood Gasifier The gasifier in Fig 9 was designed and constructed using mild steel sheet with 4.3mm thickness with a volume is 0.714m 3. It is rectangular in cross section. The total height of the gasifier is 1.34m. The internal length of the gasifier is 75cm. HOPPER Hopper is the primary storage area for the fuel wood. The Feedstock size for long stick wood of length 68cm and thickness of 6cm is placed into the top of the hopper through the inlet. Fig 9. Pilot model of bottom lit updraft long stick wood gasifier 56

17 The hopper is designed in such a way that it is able to hold wood, which can produce the gas continuously for 5 to 6 hours in a batch process. The wood sticks had a typical moisture content of 25%. The energy content of the wood was 20 MJ/m 3 on a dry basis. Initially, charcoal pieces are first loaded up to air nozzle height. Then long stick wood is packed up to the full capacity of the hopper. HEARTH Hearth is made up of castable cement with a mild steel exterior cladding to encounter higher temperature up to 1600 C. The air from the blower for partial combustion enters the hearth through air nozzle. Charcoal is filled inside the hearth up to the nozzle level. Just the region around the air nozzle is the combustion zone, where the oxidation of wood takes place. AIR NOZZLE The air nozzle tube is made up of S.S. metal 1.5-inch diameter. There are two air nozzles fixed with opposite side of the hearth in fig 9. GRATE The positioning of the g below the hearth zone in the gasifier is to help the reduction reactions. Also the g is used to control reactor pressure drop and hence to maintain the gas production. ASH COLLECTION UNIT It provides screw system for removing char and ash from the reactor. 57

18 3.3.3 Operation of Gasifier using Long stick wood Initially, charcoal pieces are first loaded up to air nozzle height. Then long stick wood is packed up to the full capacity of the hopper. The moisture content of the wood was typically 25%. The hearth is made up of castable cement to resist higher temperatures up to 1600ºC with a mild steel exterior cladding. The air from the blower for partial combustion enters the hearth through an air nozzle. The air nozzle tube is made up of SS material 1.5-inch diameter. There are two air nozzles fixed with opposite sides of the hearth. The positioning of the g below the hearth zone in the gasifier helps the reduction reactions. The g directly supports the combustion zone and must be capable of letting ash fall through, without excessive loss of fuel. In addition, the g is used to control reactor pressure drop and hence to maintain the gas production. The long stick wood is packed into the hopper in a horizontal position. Air is supplied to the gasifier using an electric blower with a control valve capable of supplying the necessary air at constant speed. Air enters below the combustion zone and the producer gas leaves near the top of the gasifier. Air and gas flows are measured with an orifice and differential manometer, as shown in fig Performance of Gasifiers The gasifier system was oped for 9 runs in three modes for bottom lit updraft, top lit updraft and cross draft. Totally this gasifier system was experimented with 27 runs for these modes. 58

19 3.4.1 Performance of Bottom lit (conventional) Updraft Operation The gasifier system was run nine times, each for a period of 5 hours and 15 minutes in the bottom lit updraft configuration. In the first three runs, the feed Prosophis Species was utilised and the time needed for the gas to support combustion from the initial flaring was 15 minutes in the 1 st, 2 nd and the 3 rd run. For the 4 th to 6 th runs, the feed Albesia Species was used and the time needed for the gas to support combustion from the initial flaring was 12 minutes in the 4 th, 5 th and the 6 th run. The feed used for the 7 th, 8th and 9 th runs was Tamarind Species and the time needed for the gas to support combustion from the initial flaring was 10 minutes. The total wood consumption in the first three runs was 42kg and in the second three runs was 45kg and in the final three runs was 40kg. The performance details from run one to run ten are given in Table 6 to 17 respectively. 59

20 Run 1 Gasifier starting time = 10.00am Gas flame at burner time = 10.15pm Duration of gas ignition = 20minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 42 kg Table 6. Run#1 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 60

21 Run 2 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 15minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 42 kg Table 7. Run#2 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 61

22 Run 3 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 15minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 42 kg Table 8. Run#3 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 62

23 In the first three runs, the feed Prosophis Species was utilised and the time needed for the gas to support combustion from the initial flaring was 15 minutes in the 1 st, 2 nd and the 3 rd runs. The air/fuel ratio increases from 1.04 to 6.87 as more and more of the charcoal are gasified and the process approaches complete combustion. A result of operation of gasifier runs 1, 2 and 3 were given below Table 9. Results of operation of Bottom lit updraft gasifier for first three runs Measured parameters Gasifier Runs Run 1 Run 2 Run 3 Moisture Content of Prosophis wood (%) Size of wood (cm) 68 x 6 68 x 6 68 x 6 Average Air Flow Rate () Average Gas Flow Rate () Average Gas Flame Temperature ( C) Initial Fuel (kg) Final Charcoal (g) Tar + Particulate (g) CO (%) O 2 (%) CO 2 (%) Calorific value of gas (kcal / Nm 3 ) Accordingly, with the calorific value of the producer gas at 3.5 MJ/m 3 and calorific value of the solid wood at 15.9 MJ/kg the gasifier efficiency was found to be around 65%. 63

24 Run 4 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 15minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 45 kg Table 10. Run#4 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Combustion Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 64

25 Run 5 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 15minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 45 kg Table 11. Run#5 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Combustion Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 65

26 Run 6 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 15minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 45 kg Table 12. Run#6 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 66

27 In the next three runs, the feed Albesia Species was utilised and the time needed for the gas to support combustion from the initial flaring was 12 minutes in the 4 th, 5 th and the 6 th run. The air/fuel ratio increases from 1.04 to 6.87 as more and more of the charcoal are gasified and the process approaches complete combustion. A result of operation of gasifier runs 4, 5 and 6 were given below Table 13. Results of operation of Bottom lit updraft gasifier for second three runs Measured parameters Gasifier Runs Run 4 Run 5 Run 6 Moisture Content of Albesia wood (%) Size of wood (cm) 68 x 5 68 x 5 68 x 5 Average Air Flow Rate () Average Gas Flow Rate () Average Gas Flame Temperature ( C) Initial Fuel (kg) Final Charcoal (g) Tar + Particulate (g) CO (%) O 2 (%) CO 2 (%) Calorific value of gas (kcal / Nm 3 ) Accordingly, with the calorific value of the producer gas at 3.5 MJ/m 3 and calorific value of the solid wood at 14.9 MJ/kg the gasifier efficiency was found to be around 65%. 67

28 Run 7 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 40 kg Table 14. Run#7 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 68

29 Run 8 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 40 kg Table 15. Run#8 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 69

30 Run 9 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 40 kg Table 16. Run#9 Performance of 5hours and 15minutes run by the bottom lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Combustion Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 70

31 The feed used for the 7 th, 8th and 9 th runs was Tamarind Species and the time needed for the gas to support combustion from the initial flaring was 10 minutes. The air/fuel ratio increases from 1.04 to 6.87 as more and more of the charcoal are gasified and the process approaches complete combustion. A result of operation of gasifier runs 7, 8 and 9 were given below Table 17. Results of operation of Bottom lit updraft gasifier for third three runs Measured parameters Gasifier Runs Run 7 Run 8 Run 9 Moisture Content of Tamarind wood (%) Size of wood (cm) 68 x 3 68 x 3 68 x 3 Average Air Flow Rate () Average Gas Flow Rate () Average Gas Flame Temperature ( C) Initial Fuel (kg) Final Charcoal (g) Tar + Particulate (g) CO (%) O 2 (%) CO 2 (%) Calorific value of gas (kcal / Nm 3 ) Accordingly, with the calorific value of the producer gas at 3.5 MJ/m 3 and calorific value of the solid wood at 15.9 MJ/kg the gasifier efficiency was found to be around 68%. 71

32 Flame Temperature C - Air/Fuel Ratio X Air / fuel X 100 Com bustion mode start up Pyrolysis Time Hours Flame temperature C Char gasification mode Fig 10. Bottom lit Updraft Time Vs Flame temperature-air/fuel ratio Figure 10 shows operations in a combustion mode at start up, a pyrolysis mode for the middle part of the run and a charcoal gasification mode at the end of the run. The heat release is unsteady in three phases for the drying phase; the pure pyrolysis phase and the charcoal burnout Performance of top lit (Inverted down draft) updraft operation The gasifier system was run nine times, each for a period of 5hours and 15minutes in the top lit updraft configuration. In the course of our present work, we also happened to come across the work of Reed and Ronal Larson (1996) on their development of a top lit updraft gasifier using the fuel wood size 1-3 cm softwood 72

33 chips and 1-2 X 10 cm hardwood sticks. They used the stick size of 2 X 1 X ½ cm placed vertically in the hopper. Accordingly we have also used our set up for such a top lit operation with our long stick feed stacked horizontally. Thus in this work we have examined the scope of an updraft gasifier employing longer stick wood as the feed (68cm long and 6cm dia). In this set up charcoal is filled above the long stick wood in the hopper. Air enters the bottom of the drying zone with a pyrolysis flame passing up through the fuel wood mass. The arrangement is shown in fig 11. Fig 11. Schematic diagram of a top lit updraft long stick wood gasifier When the wood gas is passing through char combustion zone, volatiles and tar compounds are cracked at the higher temperature. We can get fairly clean tar free gas as compared to the classical updraft mode using bottom lit setup. 73

34 The gasifier system was run ten times, each for a period of 5 hours and 15 minutes in the bottom lit updraft configuration. In the first three runs, the feed Casuarina Species was utilised and the time needed for the gas to support combustion from the initial flaring was 12 minutes in the 10 th, 11 th and the 12 th run. For the 13 th to 15 th runs, the feed Leucaena Species was used and the time needed for the gas to support combustion from the initial flaring was 10 minutes in the 13 th, 14 th and the 15 th run. The feed used for the 16 th, 17 th and 18 th runs Acacia Species and the time needed for the gas to support combustion from the initial flaring was 10 minutes. The total wood consumption in the first three runs was 45kg and in the second three runs was 40kg and in the final three runs was 42.5kg. The performance details from run one to run nine are given in Table 18 to 29 respectively. 74

35 Run 10 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 45 kg Table 18. Run#10 Performance of 5hours and 15minutes run by the top lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 75

36 Run 11 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 45 kg Table 19. Run#11 Performance of 5hours and 15minutes run by the top lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 76

37 Run 12 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 45kg Table 20. Run#12 Performance of 5hours and 15minutes run by the top lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 77

38 The feed used for the 10 th, 11 th and 12 th runs was Casuarina Species and the time needed for the gas to support combustion from the initial flaring was 10 minutes. The air/fuel ratio increases from 1.04 to 6.87 as more and more of the charcoal are gasified and the process approaches complete combustion. A result of operation of gasifier runs 10, 11 and 12 were given below Table 21. Results of operation of Top lit updraft gasifier for first three runs Measured parameters Gasifier Runs Run 10 Run 11 Run 12 Moisture Content of Casuarina wood (%) Size of wood (cm) 68 x 6 68 x 6 68 x 6 Average Air Flow Rate () Average Gas Flow Rate () Average Gas Flame Temperature ( C) Initial Fuel (kg) Final Charcoal (g) Tar + Particulate (g) CO (%) H 2 (%) CO 2 (%) N 2 (%) Calorific value of gas (kcal / Nm 3 ) Accordingly, with the calorific value of the producer gas at 4.2 MJ/m 3 and calorific value of the solid wood at 15.9 MJ/kg the gasifier efficiency was found to be around 73%. 78

39 Run 13 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 40 kg Table 22. Run#13 Performance of 5hours and 15minutes run by the top lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 79

40 Run 14 Gasifier starting time = 10.00am Gas flame at burner time = 10.10am Duration of gas ignition = 10minutes Gasifier stopping time = 15.00pm Total hours of gasifier operation = 5 hours and 15 minutes Total wood consumption = 40 kg Table 23. Run#14 Performance of 5hours and 15minutes run by the top lit updraft gasifier Time Flame Air flow Gas flow Fuel Air/Fuel MODE Hours Temperature C Flow kg/hr Combustion Combustion Combustion Combustion Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Flaming Pyrolysis Charcoal Gasification Charcoal Gasification Charcoal Gasification Charcoal Gasification 80