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A comparative study of charcoal gasification in two types of spouted bed reactors
Energy 31 (2006) 228–243 www.elsevier.com/locate/energy
A comparative study of charcoal gasification in two types of spouted bed reactors P. Abdul Salam, S.C. Bhattacharya* Energy Field of Study, School of Environment, Resources and Development, Asian Institute of Technology, P.O. Box 4, Klongluang, Pathumthani 12120, Thailand Received 8 April 2004
Abstract Gasification is considered to be a favourable method for converting a solid fuel into a more versatile gaseous fuel. Performance of a gasifier depends on the design of the gasifier, type of fuel used and air flow rate, etc. The applications of spouted bed for a variety of processes such as drying, coating, pyrolysis, gasification and combustion have been reported. Gasification of solid fuels in a spouted bed, which has certain potential advantages over other fluid bed configurations, appears to be an under-exploited technique so far. Central jet distributors are the most commonly used in the experimental studies that has been reported in the literature. Circular slit distributor is a new concept. This paper presents results of a comparative experimental study on air gasification of charcoal in central jet and circular slit inert sand spouted beds. The experiments were carried for an equivalence ratio of 0.25. The effect of spouting velocity and type of the distributor on the gasification performance were discussed. The steady state dense bed temperature varied between 979 and 1183 8C for central jet spouted bed and between 964 and 1235 8C for circular slit spouted bed. At higher spouting velocities, the gasification efficiency of the circular slit spouted bed was slightly more compared with that of central jet spouted bed. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction Gasification is the process of converting a solid fuel into a gaseous fuel by supplying a restricted amount of oxygen, either pure or from air. In case of gasification using air, its nitrogen content does not participate in the gasification process but remains as an inert component in the final gas. A gaseous fuel * Corresponding author. Tel.: C66 02 524 5403; fax: C66 02 524 5439. E-mail address: [email protected] (S.C. Bhattacharya). 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.01.004
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can be distributed easily; its combustion can be controlled to give high efficiency with low emissions. The gas can be burned directly for cooking and heating uses, or used in internal combustion engines or gas turbines for producing electricity or shaft power. The major types of biomass gasifiers are fixed bed gasifiers, i.e. updraft, downdraft and crossdraft and fluidized bed gasifiers, i.e. bubbling fluidized bed and circulating fluidized bed. Although not common, entrained bed reactors, e.g. cyclone reactors have also been used for biomass gasification. Performance of a gasifier depends on the design of the gasifier, type of fuel used and air flow rate, etc. The difference between Fluidized Bed (FB) and Spouted Bed (SB) lies in the dynamic behavior of the solid particles. In a fluidized bed, air is passed through a uniform distributor plate to float the particles which move up and down in groups. In a spouted bed, fluid enters the bed through a small orifice at the center of a conical base. The high fluid velocity causes a stream of solid particles to rise rapidly in a hollow central core or spout within the bed. The particles, having reached somewhat above the bed level, fall back forming a fountain onto the annular space between the spout and the container wall, where they slowly travel downward and, to some extent, inward as a loosely packed bed. As the fluid travels upward, it flares out into the annulus. The overall bed thereby becomes a composite of a dilute phase central core with upward moving solids entrained by a concurrent flow of fluid and a dense phase annular region with counter-current percolation of the fluid. A systematic cyclic pattern of solid movement is established with effective contact between the gas and the solids. The hydrodynamic features of a spouted bed are significantly different from those of a fluidized bed. A fluidized bed can be considered as consisting of two phases, the bubble and emulsion. A spouted bed on the other hand, has three well-defined regions, the annulus, the spout, and the fountain. The applications of spouted bed for a variety of processes such as drying, coating, pyrolysis, gasification and combustion have been reported. The modification of standard spouted bed to include the characteristics of fluidized bed, which is called spout-fluid bed, has also received much attention due to its better solids mixing and annular solid-fluid contact. The spout-fluid bed involves a substantial fluid flow through a single central inlet orifice, as in spouted bed, and an auxiliary fluid flow through a series of holes in the surrounding distributor, as in fluidized bed. The distributor can be incorporated into either a flat or conical base of the spout-fluid bed column. Central jet distributors are commonly used in the spouted bed experimental studies reported in the literature. Circular slit (modified) distributor, on the other hand, is a new concept. In this distributor, originally proposed by Rasul and Bhattacharya [1], air is supplied to the bed of particles through a circular slit. Rasul and Bhattacharya [1] reported that the contact efficiency of a spouted bed having a circular slit distributor was higher compared with a central jet spouted bed. It implies that an improvement in the design of the spouted bed reactors may be achieved by using circular slit distributors. Gasification of solid fuels in a spouted bed, which has certain potential advantages over other fluid bed configurations, appears to be an under-exploited technique so far. In particular, theoretical models for predicting the performance of spouted bed gasifiers under different operating conditions are scanty. Gasification in a fixed bed can be regarded as consisting of four different reaction zones, e.g. drying, pyrolysis, reduction and combustion. In fluidized or spouted bed gasifiers, because of good mixing, separate reaction zones do not exist. All the processes of drying, pyrolysis, reduction and combustion can be regarded as taking place simultaneously throughout the reactor volume although the intensity of
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any particular process may vary depending on the location. In case of charcoal, the pyrolysis is less significant in comparison with wood. The reactions which describe the gasification process are [2]: (a) Heterogeneous (gas–solid) reactions Oxidation of carbon: C C 1=2O2 Z CO
(1)
C C O2 Z CO2
(2)
Boudouard reaction: C C CO2 Z 2CO
(3)
Water gas reaction: C C H2 O Z CO C H2
(4)
Methane formation: C C 2H2 Z CH4
(5)
(b) Homogeneous (gas-phase) reactions CO C H2 O Z CO2 C H2
(6)
CH4 C H2 O Z CO C 3H2
(7)
Although, in practice, there may be additional reactions taking place, they can be considered as combinations of the above reactions [3]. This paper presents the results of a comparative experimental study on air gasification of charcoal in central jet (conventional) and circular slit (modified) inert sand spouted beds.
2. Apparatus and materials 2.1. Experimental set-up for gasification Schematic diagram of the experimental set-up used in this study is shown in Fig. 1. The reactor column consisted of a stainless steel pipe of inner diameter 13.3 cm, thickness 3 mm and height 134 cm. The cone angle of the conical section was 608. The air distributors were 15 mm thick and made of mild steel with 458 champering at the air entrance face to reduce head loss of supplied air. Air entrance face of the central jet distributor contained a stainless steel screen of aperture 1 mm which was fastened with the distributor plate by four screws. The details of the distributors are shown in Fig. 2. The air inlet areas of the chosen distributors were 353.4, 530.1 and 706.8 mm2. Thus, the opening areas are two, three and four times of 176.7 mm2. The outer diameter of the slit was 67.7 mm for all the circular slit distributors. The diameter of the narrowest end of the cone of this set-up was fixed at 72 mm to have a more stable spouting. By changing the inner
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Flare
Filter
P5
Gas cooling
T5 Cyclone Reactor Column
P4
T6
Online gas analysers
Cyclone
Viewing port
Temperature recorder T4
Gas sampling
P3
Fuel Feeder
Heater
Ash collector Water Manometer T3
Water Jacket
T2
P2 Distributor P1
Air Box
Control panel Motor
T1 Air preheater Blower
Rotameters
Fig. 1. Schematic diagram of experimental setup.
Fig. 2. Designs of spouted bed distributors.
T1 to T6 - Temperature Probes P1 to P5 - Pressure Probes
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diameter of the slit, the required air inlet areas of the slits were produced. Air was supplied from a 15-hp rotary air blower. A screw feeder with hopper was used to feed the charcoal continuously to the bed. Screw diameter of the feeder was 60 mm. A variable speed motor was used to drive the feeder through a speed reduction pulley arrangement. A simple conical spout deflector is used to restrain the height of the spout, to prevent blowout of the inert material to the cyclone, during start-up. A cyclone was provided between the exit of the bed and the ceramic fiber filter to separate entrained fly ash and sand particles from the producer gas. The lower end of the cyclone contained a gate valve and a container to collect the separated particles. At the outlet of the cyclone separator, a ceramic fiber filter was placed as shown in Fig. 1 to separate the fine ash particles from the gas. The filter was constructed of a steel pipe of diameter 18 cm and height 8 cm with one end closed. Ceramic fiber of thickness 1.5 cm was used as the filter material. It was placed at the middle of the cylinder by using two stainless steel screens of aperture 2.5 mm and two iron wire stands. It was important to avoid leakage of gas around the edges of the ceramic fiber and at the same time reduce pressure drop of flowing gas. The ceramic fiber layer was changed after each run. The outside of the filter as well as the inlet and outlet pipes were thermally insulated to avoid condensation of tar and moisture inside the filter. To prevent excessive heat loss from the reactor and the air preheater, ceramic wool insulation of 2.5 cm thickness (CARBORUNDUM-FIBERFRAX) was wrapped around the air preheater, the reactor, ceramic fiber filter, cyclone and different connecting pipes. 2.2. Measuring instruments Fig. 3 shows the details of the gas sample collection and measurement system. It consisted of a ceramic fiber filter, counter flow gas cooler, condensate/tar collector, on-line gas analyzer, cotton filter, gas meter, vacuum pump, etc. The gas cooler was made of glass and ice water was used in the cooler to condense tar and water vapour present in the gas. Airflow was measured by using three rotameters of range 0–45 free m3/h air at 27 8C and 760 mm Hg abs. These rotameters were connected in parallel and a convenient air flow circuit was constructed. In case of the central jet air distributor, the gate valves were controlled in such a way that air passed through the rotameters and then mixed in a single pipe before entering the air preheater. In case of circular slit air distributor, initially air flow path was same as central jet air distributor but entered the air box at three chambers. The air box is divided in to three equal chambers by three walls each at 1208 apart. When the gasification started, in order to ensure uniform spouting, gate valves were so controlled that equal amounts of air flew in the three chambers of the air box, through the rotameters, bypassing the air preheater. Temperatures at different locations of the bed and of the producer gas were measured by using Chromel–Alumel (K-type) thermocouples inserted at six fixed positions as shown in Fig. 1. They were connected to a digital temperature indicator. The pressure drop across the bed and the distributor were measured by means of a U-tube water manometer. Two on-line gas analyzers were used to measure directly and continuously gas composition of the producer gas. One of them was Fisher–Rosemount NGA 2000 model used to measure the volume percentage of CO (0–30%), CO2 (0–20%), H2 (0–20%), CH4 (0–5%) and the other was Rosemount
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Product gas in
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Filter
Water out
Gas cooling column Online Gas Analyzers Cotton Filter
Cool water in Cotton Filter
Gas meter
Vacuum pump
Gas sampler
Fig. 3. Gas sample collection and measurement system.
BINOS 100M used to measure volume percentage of O2. Also gas samples were collected and the composition was determined, on a random basis, in gas chromatography. 2.3. Materials used and their properties In this study, Mangrove wood charcoal (5–7 mm size) was used as gasifying fuel and silica sand (1.18–1.70 mm size) was used as the inert bed material. The ultimate and proximate analyses of the charcoal used in this study are presented in Table 1. Particle and bulk density of the sand was 2469 and 1360 kg/m3, respectively. Table 1 Proximate and ultimate analysis of charcoal Proximate analysis
Ultimate analysis
As received basis
Ash and moisture free basis
Ash and moisture free basis
Material
Weight (%)
Material
Weight (%)
Element
Weight (%)
Moisture Ash Combustibles
6.2 2.7 91.1
Volatiles Fixed carbon
25.2 74.8
Carbon Hydrogen Oxygen Nitrogen Sulphur
77.53 4.30 17.88 0.22 0.05
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3. Experimental procedures 3.1. Calibration of screw feeders Maintaining the feed rate of fuel is important for carrying out gasification at the desired operating conditions, i.e. gasification temperature, air–fuel ratio, etc. The charcoal screw feeder was calibrated by measuring feed rate against motor speed (rpm). During gasification tests, charcoal feed rates were determined indirectly from the motor speed and corresponding calibration curve. 3.2. Experimental procedure for minimum spouting condition For a particular distributor, and a bed height, a predetermined weight of sand of specific particle size was loaded into the bed column. The gate valve was then gradually opened until over-developed spout was formed. Then the gate valve was slowly closed until the minimum spouting condition was attained. A slight reduction of air velocity at this condition causes the spout to collapse and the pressure drop in the bed to rise suddenly. For this condition, air flow rate was recorded from rotameter, pressure drop across the bed and pressure drop across the distributor were measured from the water manometer. 3.3. Gasification tests The bed was first charged with the sand to provide a desired static bed depth. Janarthanan and Clements [4] found that spout is better defined and highly stable when the ratio of operating spouting velocity (Us) to minimum spouting velocity (Ums) is in the range of 1.1–1.3. Hoque and Bhattacharya [5] carried out gasification experiments in a conventional spouted bed at spouting velocity (Us) equivalent to 1.2–1.5 Ums. In this study, the gasification experiments were carried out at a operating superficial spouting velocity of 1.2 Ums. Sufficient air flow rate to form a stable spout was maintained as indicated by the spouting pressure drop which remained constant irrespective of the spouting air flow rate. The section of the bed, which is connected to fuel feeder, was externally heated by two 950 W (FIBROTHAL) half cylinder heaters of length 25 cm. Also, two 950 W (FIBROTHAL) half cylinder heaters of length 25 cm were used as the air preheater. During heating of the inert bed material, a small part of the hot air was passed through the ceramic fiber filter to avoid blocking of filter material by condensation of tar or steam of produced gas at the beginning of gasification. When the inert material of primary bed was preheated by the supplied hot air to a temperature of approximately 350 8C, the fuel feeder was turned on; gasification then sustained by itself and bed temperature increased rapidly. Once the gasification started, the air preheater and the heater around the bed were turned off. The gate valve of the water jacket located around the casing of screw feeder was opened at the same time to avoid blocking of screw by partial pyrolysis of fuel in the feeding path. For a particular air flow rate, the desired air fuel ratio required for a particular equivalence ratio was maintained by controlling the speed of the feeder screw. During measurement of gas composition by online gas analyzers and collection of gas sample, positive pressure inside the circuit was created by partially closing the valve located at the gas outlet end of the cyclone. A small part of the producer gas was passed through the ceramic filter and the condenser where tar or water incoming with the gas separated; the clean gas was then passed through the on-line gas analyzers. It took about 30 min, after
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starting feeding of the fuel, to reach steady state gasification. The gas compositions were measured after a steady state was attained. The average dense bed and free board temperatures were estimated from six readings of the thermocouples placed at the dense bed (T2, T3 and T4) and the free board (T5), taken at 5 min interval each, after the steady state was reached. When all the data recording was complete the charcoal feeding was stopped and the air flow rate allowed to burn the char still left in the reactor. Dry ash accumulated in the container located at the bottom of the cyclone separator was collected at the end of each run. The ash was analyzed for the presence of sand particles and carbon/ash ratio was determined. 3.4. Layout of experimental design for gasification tests Table 2 shows the experimental design layout for the gasification tests. The effects of the three air inlet areas of the distributors, three different level of bed heights (i.e. 10, 14 and 18 cm), and superficial spouting velocities on the performance of the spouted bed charcoal gasifiers with central jet and circular slit distributors were studied. The study was carried out for an equivalence ratio (i.e. the air–fuel ratio divided by the air–fuel ratio of stoichiometric combustion) of 0.25. As per the elemental analysis of the charcoal, it is estimated that, one kg of the charcoal requires 8.78 kg of air for stoichiometric combustion.
Table 2 Experimental design layout for gasification tests Distributor type
Air inlet area of the distributor (mm2)
Abbreviation used
Bed height (cm)
Air supply (m3/h)
Superficial spouting velocity (m/s)
Charcoal feeding rate (kg/h)
Equivalence ratio
Central jet distributor
353.4
A2C
530.1
A3C
706.8
A4C
353.4
A2S
530.1
A3S
706.8
A4S
10 14 18 10 14 18 10 14 18 10 14 18 10 14 18 10 14 18
18 22 25 21 24 26 22 25 29 24 27 29 24 27 29 27 29 31
0.36 0.44 0.50 0.42 0.48 0.52 0.44 0.50 0.58 0.48 0.54 0.58 0.48 0.54 0.58 0.54 0.58 0.62
9.46 11.73 12.87 10.60 11.73 12.87 11.73 12.87 15.14 12.87 14.00 15.14 12.87 14.00 15.14 14.00 15.14 16.28
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
Circular slit distributor
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4. Results and discussions 4.1. Superficial spouting velocity The superficial spouting velocity was estimated based on the air flow rate at the stable spouting condition. Since the cross sectional area of the reactor column remained same for the central jet and circular slit spouted beds, the superficial spouting velocity is same for a given air supply irrespective of the air feed arrangement. In order to maintain similar bed hydrodynamics the ratio of the operating spouting velocity to the minimum spouting velocity, i.e. Us/Ums, was held constant at 1.2 in this study. As shown in the Fig. 4, in order to make the fuel (charcoal) mix with the inert bed material (sand) and to sustain the spouting, the air flow rate was increased with the increase of bed height, which resulted in the increase of the superficial spouting velocity. For a particular bed height, the superficial spouting velocity of the circular slit spouted bed was higher than that of the central jet spouted bed. In a central jet spouted bed the gas from the spout diffuses out through cylindrical area of spout-annulus interface whereas in case of circular slit spouted bed the gas from the spout diffuses out through cylindrical areas of spoutinner core interface and spout-annulus interface. Higher area of gas diffusion from the spout requires more air supply to counter the diffusion out of the spout to the packed bed, so that the spout does not collapse. 4.2. Gasifier dense bed temperature Fig. 5 shows the overall effect of variation in superficial spouting velocity on average dense bed temperatures of the spouted beds. The bed temperature varies from 979 to 1183 8C and from 964 to 1235 8C in the central jet and circular slit spouted beds, respectively. The corresponding freeboard temperatures are between 767 and 1051 8C and between 920 and 1090 8C, respectively. In every A2C
A3C
A4C
A2S
A3S
A4S
Superficial Spouting Velocity (m/s)
0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 10 cm
14 cm
18 cm
Bed Height (cm)
Fig. 4. Variation of superficial spouting velocity with bed height.
P. Abdul Salam, S.C. Bhattacharya / Energy 31 (2006) 228–243 Central Jet Distributors
237
Circular Slit Distributors
1300
Bed Temperature (°C)
1250 1200 1150 1100 1050 1000 950 900 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 5. Effect of variation in superficial spouting velocity on bed temperature.
experiment the freeboard temperature was less than the bed temperature. The average temperature difference observed between the bed and the freeboard was 130 8C. The bed temperature was found to increase with the increase of the superficial spouting velocity in all the beds. This is due to the fact that increasing the amount of air increased the rate of the exothermic oxidation reactions. This is similar to the observation of Mansaray et al. [6] who found that the bed temperature increased with the increase of fluidization velocity in a fludized bed. For a particular superficial spouting velocity, the bed temperature of central jet spouted bed was higher than that of circular slit spouted bed. 4.3. Gas composition The average compositions of the produced gas from the spouted beds at different bed heights and different superficial spouting velocities are given in Table 3. Fig. 6 shows the overall effect of variation in superficial spouting velocity on CO and H2 concentration for different central jet and circular slit spouted bed designs. In general, the concentration of CO was higher for the circular slit spouted beds than the central jet spouted beds at all superficial spouting velocities. As explained earlier, in the circular slit spouted bed, because of more interface area, more air diffuses from the spout into the packed solid bed; this improves the gas–solid contact and enhances formation of CO. With the increase of superficial spouting velocity, in the case of central jet spouted beds, the concentration of CO remained almost constant, whereas, in the case of circular slit spouted beds, the concentration of CO decreased. The concentration of H2 was slightly higher for the central jet spouted beds than the circular slit spouted beds. Fig. 7 shows the overall effect of variation in superficial spouting velocity on CO2 and CH4 concentration for different central jet and circular slit spouted bed designs. The concentration of CO2
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Table 3 Average gas compositions Distributor type
Bed height (cm)
Superficial spouting velocity (m/s)
Temperature (8C)
Gas composition (vol%)
Dense bed
Free board
CO2
O2
N2
CO
CH4
H2
A2C
10 14 18 10 14 18 10 14 18 10 14 18 10 14 18 10 14 18
0.36 0.44 0.50 0.42 0.48 0.52 0.44 0.50 0.58 0.48 0.54 0.58 0.48 0.54 0.58 0.54 0.58 0.62
979 1083 1167 1011 1055 1109 1086 1122 1183 1103 1169 1208 964 1037 1081 1089 1168 1235
767 886 968 889 904 950 914 946 1051 962 1054 1038 920 1012 1016 1031 1090 1079
14.08 13.66 15.42 13.90 13.62 13.90 13.86 14.75 15.08 13.58 14.22 13.41 12.39 14.41 13.14 12.87 14.19 14.05
2.65 2.53 3.03 3.00 1.98 2.68 2.22 1.93 2.65 0.18 0.36 0.58 0.20 0.15 0.17 0.89 0.51 0.26
56.36 55.67 57.31 56.63 55.71 56.51 54.75 54.48 55.64 56.47 56.80 56.55 55.10 56.70 56.00 55.68 58.14 58.59
13.96 14.44 12.31 12.79 16.02 13.79 14.77 15.64 14.49 17.65 16.58 17.19 19.76 16.00 17.68 18.42 15.50 15.32
0.56 0.69 0.57 0.94 0.77 0.68 0.93 0.61 0.55 0.68 0.51 0.62 0.93 0.84 0.75 0.85 0.90 0.88
12.39 13.01 11.36 12.74 11.89 12.43 13.48 12.60 11.61 11.44 11.55 11.64 11.62 11.90 12.27 11.29 10.75 10.90
A4C
A2S
A3S
A4S
CO - Central jet
H2 - Central jet
CO - Circularslit
H2 - Circular slit
20
CO, H2 (Vol. %)
A3C
15
10
5 0.30
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 6. Effect of variation in superficial spouting velocity on concentration of CO and H2.
P. Abdul Salam, S.C. Bhattacharya / Energy 31 (2006) 228–243 CO2 - Central jet CO2 - Circular slit
239
CH4 - Central Jet CH4 - Circular slit
20
CO2, CH4 (Vol. %)
15
10
5
0 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 7. Effect of variation in superficial spouting velocity on concentration of CO2 and CH4.
was in the range of 12.4–15.4 vol%. The concentration of CO2 is found to show a increasing trend with increase in spouting velocity in all spouted beds; higher CO2 concentration is obviously due to higher availability of oxygen in the spouting fluid at higher velocities. The concentration of CH4 was low in the range of 0.51–0.94 vol% and remained almost constant with the increase of superficial spouting velocity. The concentration of O2 was lower for the circular slit spouted beds than the central jet spouted beds for all superficial spouting velocities; this was due to higher gas–solid contact efficiency in case of the circular slit distributor as pointed out above. The percentage of N2 varied in the range of 54.5–58.6 vol%. 4.4. Higher heating value of produced gas The gasification performance parameters obtained in this study are summarized in Table 4. The higher heating value (HHV) of the produced gas was calculated based on the measured gas composition and the heating value of the individual gas components (13.1, 13.2 and 41.2 MJ/Nm3 for CO, H2, and CH4, respectively [7]). Fig. 8 shows the overall effect of variation in superficial spouting velocity on HHV. The HHV varies in the range of 3.35–4.10 MJ/Nm3 for central jet spouted beds and 3.81–4.51 MJ/Nm3 for circular slit spouted beds. The HHV decreased slightly with higher values of superficial spouting velocity in all the spouted beds. This was due to the decrease in the concentration of CO and H2 at higher superficial spouting velocity. For a given superficial spouting velocity, HHV of the produced gas from circular slit spouted beds was higher than that of central jet spouted beds; this was due to higher CO concentration of produced gas from the circular slit spouted beds.
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Table 4 Gasification performance parameters Distributor type
Bed height (cm)
Superficial spouting velocity (m/s)
Gas yield (Nm3/kg of fuel)
Carbon conversion percentage
Carbon conversion rate (kg/h)
Cold gas gasification efficiency
Carbon closure (%)
Calorific value (MJ/Nm3)
A2C
10 14 18 10 14 18 10 14 18 10 14 18 10 14 18 10 14 18
0.36 0.44 0.50 0.42 0.48 0.52 0.44 0.50 0.58 0.48 0.54 0.58 0.48 0.54 0.58 0.54 0.58 0.62
2.59 2.57 2.59 2.69 2.82 2.74 2.63 2.73 2.64 2.58 2.65 2.60 2.59 2.61 2.60 2.68 2.52 2.48
56.12 56.17 55.54 56.36 65.10 58.92 58.83 64.25 60.25 62.46 62.99 61.42 65.05 61.74 62.31 65.30 58.54 56.75
3.75 4.65 5.05 4.22 5.39 5.36 4.87 5.84 6.44 5.68 6.23 6.57 5.91 6.11 6.66 6.46 6.26 6.52
44.23 46.34 40.07 46.60 52.04 47.22 49.78 50.11 44.63 48.98 47.95 48.53 54.05 48.36 51.11 52.72 44.60 43.58
70.4 75.2 83.2 71.6 86.9 84.8 78.0 89.0 89.3 97.9 99.7 97.2 90.2 96.6 95.9 96.7 91.0 94.1
3.69 3.89 3.35 3.74 3.99 3.73 4.10 3.96 3.66 4.10 3.91 4.04 4.51 4.01 4.24 4.25 3.82 3.81
A4C
A2S
A3S
A4S
Central Jet Distributors
Circular Slit Distributors
5.0
Higher Heating Value (MJ/Nm3)
A3C
4.5
4.0
3.5
3.0 0.3
0.35
0.4
0.45
0.5
0.55
0.6
Superficial Spouting Velocity (m/s)
Fig. 8. Effect of variation in superficial spouting velocity on HHV.
0.65
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4.5. Gas yield The gas production rate was estimated by an overall nitrogen balance, i.e. by equating amount of N2 in the air supplied plus in the charcoal fed to the gasifier to the amount of N2 in the produced gas plus in the unburned char collected in the cyclone. The product gas yield, which is defined here as the total dry gas flow rate at 27 8C and 101.3 kPa per kg of charcoal fed. The difference of gas yield between the central jet and circular slit spouted beds was very marginal, i.e. in the range of 0.1–0.2 Nm3/kg of fuel. The highest gas yield was in the central jet spouted bed with medium air inlet area (A3C) at 14 cm bed height. The lowest gas yield was in the case of circular slit spouted bed with larger air inlet area (A4S) at 18 cm bed height. 4.6. Carbon conversion The carbon conversion, defined as the degree to which the carbon in the fuel has been converted into gaseous products, is an important parameter in deciding the performance of a gasifier. The carbon conversion percentage for central jet spouted bed was in the range of 55.5–65.1%, for circular slit spouted bed was in the range of 56.8–65.3%. The carbon conversion percentage found to decrease with the increase of the superficial spouting velocity; the decrease in carbon conversion can be attributed to the higher attrition and elutriation at higher superficial spouting velocity as observed by Wongvicha [8] for spouted bed combustion. Increasing the superficial spouting velocity resulted in reducing the residence time of gases and enhancing the carry over of fine char particles from the bed. Since the escaped fines are not completely converted, this resulted in decrease in carbon conversion. For a given superficial spouting velocity, the carbon conversion percentage for the circular slit spouted bed was higher than that of central jet spouted bed; this suggests that gas–solid contact in the circular slit spouted bed is higher compared with the central jet spouted bed. In the present study, with the increase of the superficial spouting velocity, the fuel feeding rate was increased to achieve a constant equivalence ratio, i.e. 0.25. This leads to an increase in the bed temperature, which in turn tends to improve reaction rates and carbon conversion. The increase in superficial spouting velocity also increases the carry over rate; this tends to reduce the carbon conversion rate. In this study, the carbon conversion rate was found to increase with the increase of the superficial spouting velocity. Fig. 9 shows the variation of char carry over with the superficial spouting velocity. For a given superficial spouting velocity, the carry over in circular slit spouted bed is higher than that of central jet spouted bed. As observed by Wongvicha [8], shear in the spout-annulus interface contributes to the attrition of bed particles. A bed with circular slit distributor has higher spout-packed bed interface area than a bed with central jet distributor. Higher carry over rate in the circular slit spouted bed could be contributed to the higher attrition rate due to higher shear in the interface. The carbon balance closure was in the range of 70.4–89.3% for central jet spouted bed, and 90.2–99.7% for circular slit spouted bed. This is in the range of carbon balance closure reported in the literature [3,4,9]. 4.7. Gasification efficiency The cold gas thermal efficiency was estimated as the ratio of energy out in the produced gas to the energy into the gasifier from the charcoal. The efficiency varied in the range of 40–52% for central jet
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Circular Slit Distributors
5.0
Char Carry Over (kg/hr)
4.0
3.0
2.0
1.0
0.0 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 9. Variation of char carry over with superficial spouting velocity.
and 43.6–54.1% for circular slit spouted beds. The overall effect of variation in superficial spouting velocity on the gasification efficiency is shown in Fig. 10. Highest gasification efficiency was achieved for circular slit spouted bed with medium sized inlet air area (A3S) and at 10 cm bed height. The lowest efficiency was achieved for central jet spouted bed with large sized air inlet (A4C) area and at 18 cm bed Central Jet Distributors
Circular SlitDistributors
Gasification Efficiency (%)
55
50
45
40
35 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 10. Effect of variation in superficial spouting velocity on gasification efficiency.
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height. At higher superficial spouting velocity the efficiency decreased in both central jet and circular slit spouted beds. At higher spouting velocities, the gasification efficiency of the circular slit spouted bed was slightly more compared with central jet spouted bed. This was due to the higher heating value of the gas produced in the circular slit spouted beds.
5. Conclusion This paper presents the results of a comparative experimental study on air gasification of charcoal in spouted bed reactors having two types of distributor: central jet and circular slit. The bed temperature was found to increase with increase of the superficial spouting velocity in all the beds. For a particular superficial spouting velocity, the bed temperature of central jet spouted bed was higher than that of circular slit spouted bed. Overall, the circular slit distributor design is associated with higher CO concentration in the gas, and higher carbon conversion as well as heating value in comparison with the central jet distributor. Char carry over rate in case of circular slit spouted bed is higher than that of central jet spouted bed. The gasification efficiency observed for both the types of spouted bed designs was low (i.e. 40–52% for central jet and 43.6–54.1% for circular slit) because of higher energy loss associated with the char carry over (i.e. 14–28% for central jet and 24–36% for circular slit); in practical systems the carry over loss could be reduced by recycling the char particles. Further study is recommended on the scale-up of the circular slit spouted bed with different bed diameters, and for different equivalence ratios to evaluate the usefulness of circular slit spouted bed as a gasifier.
References [1] Rasul MG, Bhattacharya SC. Performance comparison of three spouted bed reactor designs. J Inst Eng India 1992;(72): 73–6. [2] Buekens AG, Schoeters JG. Modeling of biomass gasification. In: Overend RP, Milne TA, Mudge LK, editors. Fundamentals of thermochemical biomass conversion. London: Elsevier; 1985. p. 619–89. [3] Sue-A-Quan TA, Cheng G, Watkinson AP. Coal gasification in a pressurised spouted bed. Fuel 1995;74:159–64. [4] Janarthanan AK, Clements LD. Gasification of wood in a pilot scale spouted bed gasifier. In: Bridgwater AV, Boocock DGB, editors. Developments in thermochemical biomass conversion, vol. 2. London: Blakie Academic & Professional; 1997. p. 945–59. [5] Hoque MM, Bhattacharya SC. Fuel characteristics of gasified coconut shell in a fluidized and a spouted bed reactor. Energy 2001;26:101–10. [6] Mansaray KG, Ghaly AE, Al-Taweel AM, Hamdullahpur F, Ugursal VI. Air gasification of rice husk in a dual distributor type fluidized bed gasifier. Biomass Bioenergy 1999;17:315–32. [7] Reed TB, Das A. Handbook of biomass downdraft gasifier- engine systems. SERI/SP-271-3022. Golden: Solar Energy Research Institute; 1988. [8] Wongvicha P, Bhattacharya SC. Attrition of lignite char in a spouted bed combustor. Int J Energy Res 1994;18:9–20. [9] Mansary KG. Gasification of rice husk in a fluidized bed gasifier. A PhD dissertation submitted to Dalhousie University— Daltech. Halifax, Nova scotia. Daltech Libarary; 1998. p. 1–460.
A comparative study of charcoal gasification in two types of spouted bed reactors P. Abdul Salam, S.C. Bhattacharya* Energy Field of Study, School of Environment, Resources and Development, Asian Institute of Technology, P.O. Box 4, Klongluang, Pathumthani 12120, Thailand Received 8 April 2004
Abstract Gasification is considered to be a favourable method for converting a solid fuel into a more versatile gaseous fuel. Performance of a gasifier depends on the design of the gasifier, type of fuel used and air flow rate, etc. The applications of spouted bed for a variety of processes such as drying, coating, pyrolysis, gasification and combustion have been reported. Gasification of solid fuels in a spouted bed, which has certain potential advantages over other fluid bed configurations, appears to be an under-exploited technique so far. Central jet distributors are the most commonly used in the experimental studies that has been reported in the literature. Circular slit distributor is a new concept. This paper presents results of a comparative experimental study on air gasification of charcoal in central jet and circular slit inert sand spouted beds. The experiments were carried for an equivalence ratio of 0.25. The effect of spouting velocity and type of the distributor on the gasification performance were discussed. The steady state dense bed temperature varied between 979 and 1183 8C for central jet spouted bed and between 964 and 1235 8C for circular slit spouted bed. At higher spouting velocities, the gasification efficiency of the circular slit spouted bed was slightly more compared with that of central jet spouted bed. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction Gasification is the process of converting a solid fuel into a gaseous fuel by supplying a restricted amount of oxygen, either pure or from air. In case of gasification using air, its nitrogen content does not participate in the gasification process but remains as an inert component in the final gas. A gaseous fuel * Corresponding author. Tel.: C66 02 524 5403; fax: C66 02 524 5439. E-mail address: [email protected] (S.C. Bhattacharya). 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.01.004
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can be distributed easily; its combustion can be controlled to give high efficiency with low emissions. The gas can be burned directly for cooking and heating uses, or used in internal combustion engines or gas turbines for producing electricity or shaft power. The major types of biomass gasifiers are fixed bed gasifiers, i.e. updraft, downdraft and crossdraft and fluidized bed gasifiers, i.e. bubbling fluidized bed and circulating fluidized bed. Although not common, entrained bed reactors, e.g. cyclone reactors have also been used for biomass gasification. Performance of a gasifier depends on the design of the gasifier, type of fuel used and air flow rate, etc. The difference between Fluidized Bed (FB) and Spouted Bed (SB) lies in the dynamic behavior of the solid particles. In a fluidized bed, air is passed through a uniform distributor plate to float the particles which move up and down in groups. In a spouted bed, fluid enters the bed through a small orifice at the center of a conical base. The high fluid velocity causes a stream of solid particles to rise rapidly in a hollow central core or spout within the bed. The particles, having reached somewhat above the bed level, fall back forming a fountain onto the annular space between the spout and the container wall, where they slowly travel downward and, to some extent, inward as a loosely packed bed. As the fluid travels upward, it flares out into the annulus. The overall bed thereby becomes a composite of a dilute phase central core with upward moving solids entrained by a concurrent flow of fluid and a dense phase annular region with counter-current percolation of the fluid. A systematic cyclic pattern of solid movement is established with effective contact between the gas and the solids. The hydrodynamic features of a spouted bed are significantly different from those of a fluidized bed. A fluidized bed can be considered as consisting of two phases, the bubble and emulsion. A spouted bed on the other hand, has three well-defined regions, the annulus, the spout, and the fountain. The applications of spouted bed for a variety of processes such as drying, coating, pyrolysis, gasification and combustion have been reported. The modification of standard spouted bed to include the characteristics of fluidized bed, which is called spout-fluid bed, has also received much attention due to its better solids mixing and annular solid-fluid contact. The spout-fluid bed involves a substantial fluid flow through a single central inlet orifice, as in spouted bed, and an auxiliary fluid flow through a series of holes in the surrounding distributor, as in fluidized bed. The distributor can be incorporated into either a flat or conical base of the spout-fluid bed column. Central jet distributors are commonly used in the spouted bed experimental studies reported in the literature. Circular slit (modified) distributor, on the other hand, is a new concept. In this distributor, originally proposed by Rasul and Bhattacharya [1], air is supplied to the bed of particles through a circular slit. Rasul and Bhattacharya [1] reported that the contact efficiency of a spouted bed having a circular slit distributor was higher compared with a central jet spouted bed. It implies that an improvement in the design of the spouted bed reactors may be achieved by using circular slit distributors. Gasification of solid fuels in a spouted bed, which has certain potential advantages over other fluid bed configurations, appears to be an under-exploited technique so far. In particular, theoretical models for predicting the performance of spouted bed gasifiers under different operating conditions are scanty. Gasification in a fixed bed can be regarded as consisting of four different reaction zones, e.g. drying, pyrolysis, reduction and combustion. In fluidized or spouted bed gasifiers, because of good mixing, separate reaction zones do not exist. All the processes of drying, pyrolysis, reduction and combustion can be regarded as taking place simultaneously throughout the reactor volume although the intensity of
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any particular process may vary depending on the location. In case of charcoal, the pyrolysis is less significant in comparison with wood. The reactions which describe the gasification process are [2]: (a) Heterogeneous (gas–solid) reactions Oxidation of carbon: C C 1=2O2 Z CO
(1)
C C O2 Z CO2
(2)
Boudouard reaction: C C CO2 Z 2CO
(3)
Water gas reaction: C C H2 O Z CO C H2
(4)
Methane formation: C C 2H2 Z CH4
(5)
(b) Homogeneous (gas-phase) reactions CO C H2 O Z CO2 C H2
(6)
CH4 C H2 O Z CO C 3H2
(7)
Although, in practice, there may be additional reactions taking place, they can be considered as combinations of the above reactions [3]. This paper presents the results of a comparative experimental study on air gasification of charcoal in central jet (conventional) and circular slit (modified) inert sand spouted beds.
2. Apparatus and materials 2.1. Experimental set-up for gasification Schematic diagram of the experimental set-up used in this study is shown in Fig. 1. The reactor column consisted of a stainless steel pipe of inner diameter 13.3 cm, thickness 3 mm and height 134 cm. The cone angle of the conical section was 608. The air distributors were 15 mm thick and made of mild steel with 458 champering at the air entrance face to reduce head loss of supplied air. Air entrance face of the central jet distributor contained a stainless steel screen of aperture 1 mm which was fastened with the distributor plate by four screws. The details of the distributors are shown in Fig. 2. The air inlet areas of the chosen distributors were 353.4, 530.1 and 706.8 mm2. Thus, the opening areas are two, three and four times of 176.7 mm2. The outer diameter of the slit was 67.7 mm for all the circular slit distributors. The diameter of the narrowest end of the cone of this set-up was fixed at 72 mm to have a more stable spouting. By changing the inner
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Flare
Filter
P5
Gas cooling
T5 Cyclone Reactor Column
P4
T6
Online gas analysers
Cyclone
Viewing port
Temperature recorder T4
Gas sampling
P3
Fuel Feeder
Heater
Ash collector Water Manometer T3
Water Jacket
T2
P2 Distributor P1
Air Box
Control panel Motor
T1 Air preheater Blower
Rotameters
Fig. 1. Schematic diagram of experimental setup.
Fig. 2. Designs of spouted bed distributors.
T1 to T6 - Temperature Probes P1 to P5 - Pressure Probes
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diameter of the slit, the required air inlet areas of the slits were produced. Air was supplied from a 15-hp rotary air blower. A screw feeder with hopper was used to feed the charcoal continuously to the bed. Screw diameter of the feeder was 60 mm. A variable speed motor was used to drive the feeder through a speed reduction pulley arrangement. A simple conical spout deflector is used to restrain the height of the spout, to prevent blowout of the inert material to the cyclone, during start-up. A cyclone was provided between the exit of the bed and the ceramic fiber filter to separate entrained fly ash and sand particles from the producer gas. The lower end of the cyclone contained a gate valve and a container to collect the separated particles. At the outlet of the cyclone separator, a ceramic fiber filter was placed as shown in Fig. 1 to separate the fine ash particles from the gas. The filter was constructed of a steel pipe of diameter 18 cm and height 8 cm with one end closed. Ceramic fiber of thickness 1.5 cm was used as the filter material. It was placed at the middle of the cylinder by using two stainless steel screens of aperture 2.5 mm and two iron wire stands. It was important to avoid leakage of gas around the edges of the ceramic fiber and at the same time reduce pressure drop of flowing gas. The ceramic fiber layer was changed after each run. The outside of the filter as well as the inlet and outlet pipes were thermally insulated to avoid condensation of tar and moisture inside the filter. To prevent excessive heat loss from the reactor and the air preheater, ceramic wool insulation of 2.5 cm thickness (CARBORUNDUM-FIBERFRAX) was wrapped around the air preheater, the reactor, ceramic fiber filter, cyclone and different connecting pipes. 2.2. Measuring instruments Fig. 3 shows the details of the gas sample collection and measurement system. It consisted of a ceramic fiber filter, counter flow gas cooler, condensate/tar collector, on-line gas analyzer, cotton filter, gas meter, vacuum pump, etc. The gas cooler was made of glass and ice water was used in the cooler to condense tar and water vapour present in the gas. Airflow was measured by using three rotameters of range 0–45 free m3/h air at 27 8C and 760 mm Hg abs. These rotameters were connected in parallel and a convenient air flow circuit was constructed. In case of the central jet air distributor, the gate valves were controlled in such a way that air passed through the rotameters and then mixed in a single pipe before entering the air preheater. In case of circular slit air distributor, initially air flow path was same as central jet air distributor but entered the air box at three chambers. The air box is divided in to three equal chambers by three walls each at 1208 apart. When the gasification started, in order to ensure uniform spouting, gate valves were so controlled that equal amounts of air flew in the three chambers of the air box, through the rotameters, bypassing the air preheater. Temperatures at different locations of the bed and of the producer gas were measured by using Chromel–Alumel (K-type) thermocouples inserted at six fixed positions as shown in Fig. 1. They were connected to a digital temperature indicator. The pressure drop across the bed and the distributor were measured by means of a U-tube water manometer. Two on-line gas analyzers were used to measure directly and continuously gas composition of the producer gas. One of them was Fisher–Rosemount NGA 2000 model used to measure the volume percentage of CO (0–30%), CO2 (0–20%), H2 (0–20%), CH4 (0–5%) and the other was Rosemount
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Product gas in
233
Filter
Water out
Gas cooling column Online Gas Analyzers Cotton Filter
Cool water in Cotton Filter
Gas meter
Vacuum pump
Gas sampler
Fig. 3. Gas sample collection and measurement system.
BINOS 100M used to measure volume percentage of O2. Also gas samples were collected and the composition was determined, on a random basis, in gas chromatography. 2.3. Materials used and their properties In this study, Mangrove wood charcoal (5–7 mm size) was used as gasifying fuel and silica sand (1.18–1.70 mm size) was used as the inert bed material. The ultimate and proximate analyses of the charcoal used in this study are presented in Table 1. Particle and bulk density of the sand was 2469 and 1360 kg/m3, respectively. Table 1 Proximate and ultimate analysis of charcoal Proximate analysis
Ultimate analysis
As received basis
Ash and moisture free basis
Ash and moisture free basis
Material
Weight (%)
Material
Weight (%)
Element
Weight (%)
Moisture Ash Combustibles
6.2 2.7 91.1
Volatiles Fixed carbon
25.2 74.8
Carbon Hydrogen Oxygen Nitrogen Sulphur
77.53 4.30 17.88 0.22 0.05
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3. Experimental procedures 3.1. Calibration of screw feeders Maintaining the feed rate of fuel is important for carrying out gasification at the desired operating conditions, i.e. gasification temperature, air–fuel ratio, etc. The charcoal screw feeder was calibrated by measuring feed rate against motor speed (rpm). During gasification tests, charcoal feed rates were determined indirectly from the motor speed and corresponding calibration curve. 3.2. Experimental procedure for minimum spouting condition For a particular distributor, and a bed height, a predetermined weight of sand of specific particle size was loaded into the bed column. The gate valve was then gradually opened until over-developed spout was formed. Then the gate valve was slowly closed until the minimum spouting condition was attained. A slight reduction of air velocity at this condition causes the spout to collapse and the pressure drop in the bed to rise suddenly. For this condition, air flow rate was recorded from rotameter, pressure drop across the bed and pressure drop across the distributor were measured from the water manometer. 3.3. Gasification tests The bed was first charged with the sand to provide a desired static bed depth. Janarthanan and Clements [4] found that spout is better defined and highly stable when the ratio of operating spouting velocity (Us) to minimum spouting velocity (Ums) is in the range of 1.1–1.3. Hoque and Bhattacharya [5] carried out gasification experiments in a conventional spouted bed at spouting velocity (Us) equivalent to 1.2–1.5 Ums. In this study, the gasification experiments were carried out at a operating superficial spouting velocity of 1.2 Ums. Sufficient air flow rate to form a stable spout was maintained as indicated by the spouting pressure drop which remained constant irrespective of the spouting air flow rate. The section of the bed, which is connected to fuel feeder, was externally heated by two 950 W (FIBROTHAL) half cylinder heaters of length 25 cm. Also, two 950 W (FIBROTHAL) half cylinder heaters of length 25 cm were used as the air preheater. During heating of the inert bed material, a small part of the hot air was passed through the ceramic fiber filter to avoid blocking of filter material by condensation of tar or steam of produced gas at the beginning of gasification. When the inert material of primary bed was preheated by the supplied hot air to a temperature of approximately 350 8C, the fuel feeder was turned on; gasification then sustained by itself and bed temperature increased rapidly. Once the gasification started, the air preheater and the heater around the bed were turned off. The gate valve of the water jacket located around the casing of screw feeder was opened at the same time to avoid blocking of screw by partial pyrolysis of fuel in the feeding path. For a particular air flow rate, the desired air fuel ratio required for a particular equivalence ratio was maintained by controlling the speed of the feeder screw. During measurement of gas composition by online gas analyzers and collection of gas sample, positive pressure inside the circuit was created by partially closing the valve located at the gas outlet end of the cyclone. A small part of the producer gas was passed through the ceramic filter and the condenser where tar or water incoming with the gas separated; the clean gas was then passed through the on-line gas analyzers. It took about 30 min, after
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starting feeding of the fuel, to reach steady state gasification. The gas compositions were measured after a steady state was attained. The average dense bed and free board temperatures were estimated from six readings of the thermocouples placed at the dense bed (T2, T3 and T4) and the free board (T5), taken at 5 min interval each, after the steady state was reached. When all the data recording was complete the charcoal feeding was stopped and the air flow rate allowed to burn the char still left in the reactor. Dry ash accumulated in the container located at the bottom of the cyclone separator was collected at the end of each run. The ash was analyzed for the presence of sand particles and carbon/ash ratio was determined. 3.4. Layout of experimental design for gasification tests Table 2 shows the experimental design layout for the gasification tests. The effects of the three air inlet areas of the distributors, three different level of bed heights (i.e. 10, 14 and 18 cm), and superficial spouting velocities on the performance of the spouted bed charcoal gasifiers with central jet and circular slit distributors were studied. The study was carried out for an equivalence ratio (i.e. the air–fuel ratio divided by the air–fuel ratio of stoichiometric combustion) of 0.25. As per the elemental analysis of the charcoal, it is estimated that, one kg of the charcoal requires 8.78 kg of air for stoichiometric combustion.
Table 2 Experimental design layout for gasification tests Distributor type
Air inlet area of the distributor (mm2)
Abbreviation used
Bed height (cm)
Air supply (m3/h)
Superficial spouting velocity (m/s)
Charcoal feeding rate (kg/h)
Equivalence ratio
Central jet distributor
353.4
A2C
530.1
A3C
706.8
A4C
353.4
A2S
530.1
A3S
706.8
A4S
10 14 18 10 14 18 10 14 18 10 14 18 10 14 18 10 14 18
18 22 25 21 24 26 22 25 29 24 27 29 24 27 29 27 29 31
0.36 0.44 0.50 0.42 0.48 0.52 0.44 0.50 0.58 0.48 0.54 0.58 0.48 0.54 0.58 0.54 0.58 0.62
9.46 11.73 12.87 10.60 11.73 12.87 11.73 12.87 15.14 12.87 14.00 15.14 12.87 14.00 15.14 14.00 15.14 16.28
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
Circular slit distributor
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4. Results and discussions 4.1. Superficial spouting velocity The superficial spouting velocity was estimated based on the air flow rate at the stable spouting condition. Since the cross sectional area of the reactor column remained same for the central jet and circular slit spouted beds, the superficial spouting velocity is same for a given air supply irrespective of the air feed arrangement. In order to maintain similar bed hydrodynamics the ratio of the operating spouting velocity to the minimum spouting velocity, i.e. Us/Ums, was held constant at 1.2 in this study. As shown in the Fig. 4, in order to make the fuel (charcoal) mix with the inert bed material (sand) and to sustain the spouting, the air flow rate was increased with the increase of bed height, which resulted in the increase of the superficial spouting velocity. For a particular bed height, the superficial spouting velocity of the circular slit spouted bed was higher than that of the central jet spouted bed. In a central jet spouted bed the gas from the spout diffuses out through cylindrical area of spout-annulus interface whereas in case of circular slit spouted bed the gas from the spout diffuses out through cylindrical areas of spoutinner core interface and spout-annulus interface. Higher area of gas diffusion from the spout requires more air supply to counter the diffusion out of the spout to the packed bed, so that the spout does not collapse. 4.2. Gasifier dense bed temperature Fig. 5 shows the overall effect of variation in superficial spouting velocity on average dense bed temperatures of the spouted beds. The bed temperature varies from 979 to 1183 8C and from 964 to 1235 8C in the central jet and circular slit spouted beds, respectively. The corresponding freeboard temperatures are between 767 and 1051 8C and between 920 and 1090 8C, respectively. In every A2C
A3C
A4C
A2S
A3S
A4S
Superficial Spouting Velocity (m/s)
0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 10 cm
14 cm
18 cm
Bed Height (cm)
Fig. 4. Variation of superficial spouting velocity with bed height.
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237
Circular Slit Distributors
1300
Bed Temperature (°C)
1250 1200 1150 1100 1050 1000 950 900 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 5. Effect of variation in superficial spouting velocity on bed temperature.
experiment the freeboard temperature was less than the bed temperature. The average temperature difference observed between the bed and the freeboard was 130 8C. The bed temperature was found to increase with the increase of the superficial spouting velocity in all the beds. This is due to the fact that increasing the amount of air increased the rate of the exothermic oxidation reactions. This is similar to the observation of Mansaray et al. [6] who found that the bed temperature increased with the increase of fluidization velocity in a fludized bed. For a particular superficial spouting velocity, the bed temperature of central jet spouted bed was higher than that of circular slit spouted bed. 4.3. Gas composition The average compositions of the produced gas from the spouted beds at different bed heights and different superficial spouting velocities are given in Table 3. Fig. 6 shows the overall effect of variation in superficial spouting velocity on CO and H2 concentration for different central jet and circular slit spouted bed designs. In general, the concentration of CO was higher for the circular slit spouted beds than the central jet spouted beds at all superficial spouting velocities. As explained earlier, in the circular slit spouted bed, because of more interface area, more air diffuses from the spout into the packed solid bed; this improves the gas–solid contact and enhances formation of CO. With the increase of superficial spouting velocity, in the case of central jet spouted beds, the concentration of CO remained almost constant, whereas, in the case of circular slit spouted beds, the concentration of CO decreased. The concentration of H2 was slightly higher for the central jet spouted beds than the circular slit spouted beds. Fig. 7 shows the overall effect of variation in superficial spouting velocity on CO2 and CH4 concentration for different central jet and circular slit spouted bed designs. The concentration of CO2
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Table 3 Average gas compositions Distributor type
Bed height (cm)
Superficial spouting velocity (m/s)
Temperature (8C)
Gas composition (vol%)
Dense bed
Free board
CO2
O2
N2
CO
CH4
H2
A2C
10 14 18 10 14 18 10 14 18 10 14 18 10 14 18 10 14 18
0.36 0.44 0.50 0.42 0.48 0.52 0.44 0.50 0.58 0.48 0.54 0.58 0.48 0.54 0.58 0.54 0.58 0.62
979 1083 1167 1011 1055 1109 1086 1122 1183 1103 1169 1208 964 1037 1081 1089 1168 1235
767 886 968 889 904 950 914 946 1051 962 1054 1038 920 1012 1016 1031 1090 1079
14.08 13.66 15.42 13.90 13.62 13.90 13.86 14.75 15.08 13.58 14.22 13.41 12.39 14.41 13.14 12.87 14.19 14.05
2.65 2.53 3.03 3.00 1.98 2.68 2.22 1.93 2.65 0.18 0.36 0.58 0.20 0.15 0.17 0.89 0.51 0.26
56.36 55.67 57.31 56.63 55.71 56.51 54.75 54.48 55.64 56.47 56.80 56.55 55.10 56.70 56.00 55.68 58.14 58.59
13.96 14.44 12.31 12.79 16.02 13.79 14.77 15.64 14.49 17.65 16.58 17.19 19.76 16.00 17.68 18.42 15.50 15.32
0.56 0.69 0.57 0.94 0.77 0.68 0.93 0.61 0.55 0.68 0.51 0.62 0.93 0.84 0.75 0.85 0.90 0.88
12.39 13.01 11.36 12.74 11.89 12.43 13.48 12.60 11.61 11.44 11.55 11.64 11.62 11.90 12.27 11.29 10.75 10.90
A4C
A2S
A3S
A4S
CO - Central jet
H2 - Central jet
CO - Circularslit
H2 - Circular slit
20
CO, H2 (Vol. %)
A3C
15
10
5 0.30
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 6. Effect of variation in superficial spouting velocity on concentration of CO and H2.
P. Abdul Salam, S.C. Bhattacharya / Energy 31 (2006) 228–243 CO2 - Central jet CO2 - Circular slit
239
CH4 - Central Jet CH4 - Circular slit
20
CO2, CH4 (Vol. %)
15
10
5
0 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 7. Effect of variation in superficial spouting velocity on concentration of CO2 and CH4.
was in the range of 12.4–15.4 vol%. The concentration of CO2 is found to show a increasing trend with increase in spouting velocity in all spouted beds; higher CO2 concentration is obviously due to higher availability of oxygen in the spouting fluid at higher velocities. The concentration of CH4 was low in the range of 0.51–0.94 vol% and remained almost constant with the increase of superficial spouting velocity. The concentration of O2 was lower for the circular slit spouted beds than the central jet spouted beds for all superficial spouting velocities; this was due to higher gas–solid contact efficiency in case of the circular slit distributor as pointed out above. The percentage of N2 varied in the range of 54.5–58.6 vol%. 4.4. Higher heating value of produced gas The gasification performance parameters obtained in this study are summarized in Table 4. The higher heating value (HHV) of the produced gas was calculated based on the measured gas composition and the heating value of the individual gas components (13.1, 13.2 and 41.2 MJ/Nm3 for CO, H2, and CH4, respectively [7]). Fig. 8 shows the overall effect of variation in superficial spouting velocity on HHV. The HHV varies in the range of 3.35–4.10 MJ/Nm3 for central jet spouted beds and 3.81–4.51 MJ/Nm3 for circular slit spouted beds. The HHV decreased slightly with higher values of superficial spouting velocity in all the spouted beds. This was due to the decrease in the concentration of CO and H2 at higher superficial spouting velocity. For a given superficial spouting velocity, HHV of the produced gas from circular slit spouted beds was higher than that of central jet spouted beds; this was due to higher CO concentration of produced gas from the circular slit spouted beds.
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Table 4 Gasification performance parameters Distributor type
Bed height (cm)
Superficial spouting velocity (m/s)
Gas yield (Nm3/kg of fuel)
Carbon conversion percentage
Carbon conversion rate (kg/h)
Cold gas gasification efficiency
Carbon closure (%)
Calorific value (MJ/Nm3)
A2C
10 14 18 10 14 18 10 14 18 10 14 18 10 14 18 10 14 18
0.36 0.44 0.50 0.42 0.48 0.52 0.44 0.50 0.58 0.48 0.54 0.58 0.48 0.54 0.58 0.54 0.58 0.62
2.59 2.57 2.59 2.69 2.82 2.74 2.63 2.73 2.64 2.58 2.65 2.60 2.59 2.61 2.60 2.68 2.52 2.48
56.12 56.17 55.54 56.36 65.10 58.92 58.83 64.25 60.25 62.46 62.99 61.42 65.05 61.74 62.31 65.30 58.54 56.75
3.75 4.65 5.05 4.22 5.39 5.36 4.87 5.84 6.44 5.68 6.23 6.57 5.91 6.11 6.66 6.46 6.26 6.52
44.23 46.34 40.07 46.60 52.04 47.22 49.78 50.11 44.63 48.98 47.95 48.53 54.05 48.36 51.11 52.72 44.60 43.58
70.4 75.2 83.2 71.6 86.9 84.8 78.0 89.0 89.3 97.9 99.7 97.2 90.2 96.6 95.9 96.7 91.0 94.1
3.69 3.89 3.35 3.74 3.99 3.73 4.10 3.96 3.66 4.10 3.91 4.04 4.51 4.01 4.24 4.25 3.82 3.81
A4C
A2S
A3S
A4S
Central Jet Distributors
Circular Slit Distributors
5.0
Higher Heating Value (MJ/Nm3)
A3C
4.5
4.0
3.5
3.0 0.3
0.35
0.4
0.45
0.5
0.55
0.6
Superficial Spouting Velocity (m/s)
Fig. 8. Effect of variation in superficial spouting velocity on HHV.
0.65
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4.5. Gas yield The gas production rate was estimated by an overall nitrogen balance, i.e. by equating amount of N2 in the air supplied plus in the charcoal fed to the gasifier to the amount of N2 in the produced gas plus in the unburned char collected in the cyclone. The product gas yield, which is defined here as the total dry gas flow rate at 27 8C and 101.3 kPa per kg of charcoal fed. The difference of gas yield between the central jet and circular slit spouted beds was very marginal, i.e. in the range of 0.1–0.2 Nm3/kg of fuel. The highest gas yield was in the central jet spouted bed with medium air inlet area (A3C) at 14 cm bed height. The lowest gas yield was in the case of circular slit spouted bed with larger air inlet area (A4S) at 18 cm bed height. 4.6. Carbon conversion The carbon conversion, defined as the degree to which the carbon in the fuel has been converted into gaseous products, is an important parameter in deciding the performance of a gasifier. The carbon conversion percentage for central jet spouted bed was in the range of 55.5–65.1%, for circular slit spouted bed was in the range of 56.8–65.3%. The carbon conversion percentage found to decrease with the increase of the superficial spouting velocity; the decrease in carbon conversion can be attributed to the higher attrition and elutriation at higher superficial spouting velocity as observed by Wongvicha [8] for spouted bed combustion. Increasing the superficial spouting velocity resulted in reducing the residence time of gases and enhancing the carry over of fine char particles from the bed. Since the escaped fines are not completely converted, this resulted in decrease in carbon conversion. For a given superficial spouting velocity, the carbon conversion percentage for the circular slit spouted bed was higher than that of central jet spouted bed; this suggests that gas–solid contact in the circular slit spouted bed is higher compared with the central jet spouted bed. In the present study, with the increase of the superficial spouting velocity, the fuel feeding rate was increased to achieve a constant equivalence ratio, i.e. 0.25. This leads to an increase in the bed temperature, which in turn tends to improve reaction rates and carbon conversion. The increase in superficial spouting velocity also increases the carry over rate; this tends to reduce the carbon conversion rate. In this study, the carbon conversion rate was found to increase with the increase of the superficial spouting velocity. Fig. 9 shows the variation of char carry over with the superficial spouting velocity. For a given superficial spouting velocity, the carry over in circular slit spouted bed is higher than that of central jet spouted bed. As observed by Wongvicha [8], shear in the spout-annulus interface contributes to the attrition of bed particles. A bed with circular slit distributor has higher spout-packed bed interface area than a bed with central jet distributor. Higher carry over rate in the circular slit spouted bed could be contributed to the higher attrition rate due to higher shear in the interface. The carbon balance closure was in the range of 70.4–89.3% for central jet spouted bed, and 90.2–99.7% for circular slit spouted bed. This is in the range of carbon balance closure reported in the literature [3,4,9]. 4.7. Gasification efficiency The cold gas thermal efficiency was estimated as the ratio of energy out in the produced gas to the energy into the gasifier from the charcoal. The efficiency varied in the range of 40–52% for central jet
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P. Abdul Salam, S.C. Bhattacharya / Energy 31 (2006) 228–243 Central Jet Distributors
Circular Slit Distributors
5.0
Char Carry Over (kg/hr)
4.0
3.0
2.0
1.0
0.0 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 9. Variation of char carry over with superficial spouting velocity.
and 43.6–54.1% for circular slit spouted beds. The overall effect of variation in superficial spouting velocity on the gasification efficiency is shown in Fig. 10. Highest gasification efficiency was achieved for circular slit spouted bed with medium sized inlet air area (A3S) and at 10 cm bed height. The lowest efficiency was achieved for central jet spouted bed with large sized air inlet (A4C) area and at 18 cm bed Central Jet Distributors
Circular SlitDistributors
Gasification Efficiency (%)
55
50
45
40
35 0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
Superficial Spouting Velocity (m/s)
Fig. 10. Effect of variation in superficial spouting velocity on gasification efficiency.
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height. At higher superficial spouting velocity the efficiency decreased in both central jet and circular slit spouted beds. At higher spouting velocities, the gasification efficiency of the circular slit spouted bed was slightly more compared with central jet spouted bed. This was due to the higher heating value of the gas produced in the circular slit spouted beds.
5. Conclusion This paper presents the results of a comparative experimental study on air gasification of charcoal in spouted bed reactors having two types of distributor: central jet and circular slit. The bed temperature was found to increase with increase of the superficial spouting velocity in all the beds. For a particular superficial spouting velocity, the bed temperature of central jet spouted bed was higher than that of circular slit spouted bed. Overall, the circular slit distributor design is associated with higher CO concentration in the gas, and higher carbon conversion as well as heating value in comparison with the central jet distributor. Char carry over rate in case of circular slit spouted bed is higher than that of central jet spouted bed. The gasification efficiency observed for both the types of spouted bed designs was low (i.e. 40–52% for central jet and 43.6–54.1% for circular slit) because of higher energy loss associated with the char carry over (i.e. 14–28% for central jet and 24–36% for circular slit); in practical systems the carry over loss could be reduced by recycling the char particles. Further study is recommended on the scale-up of the circular slit spouted bed with different bed diameters, and for different equivalence ratios to evaluate the usefulness of circular slit spouted bed as a gasifier.
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