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Operational investigation of a bubbling fluidized bed biomass gasification system
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Operational investigation of a bubbling fluidized bed biomass gasification system Lim Mook Tzeng and Z.A. Zainal School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia 14300 Seberang Prai Selatan, Pulau Pinang, Malaysia E-mail (Zainal): [email protected]
The operational characteristics of a bubbling fluidized bed biomass gasifier (BFBG) are discussed. A gas cleaning and cooling (GCC) system that consists of cyclones and natural and forced convection air-to-gas heat exchangers is connected to the BFBG to facilitate removal of particulates, moisture and tar. The performance of the GCC with and without a second condenser and its effect is presented. The secondary condenser doubles the condensate removal rate. Temperatures across the components of the GCC remain constant after a period of time. The fluidization regime of the BFBG with varying air-flow rate was investigated by measuring the pressure drop (∆P) with a manometer. A change in the slope of the ∆P curve indicated a change in fluidization regime from homogeneous fluidization to bubbling fluidization. The fluidization regime changes to bubbling fluidization once the flow rate exceeds 110 kg/hr regardless of the bed height, which causes intense solid particle mixing. The varying air-flow rate gives different equivalence ratio (ER) values. The upper limit of ER is constrained by the lower calorific value (LCV) of producer gas. The bed temperature and fluidization regime of the BFBG is determined by ER and has to be regulated so as to obtain maximum cold gas efficiency from the BFBG. Low ER corresponds to homogeneous fluidization and low bed temperature, while higher ER gives bubbling fluidization and higher bed temperatures. Further investigation on the relationship between these two parameters is still going on. 1. Introduction Biomass energy is becoming more prominent in the effort to increase sustainable energy utilization. It is a renewable resource, whereby, for instance, such materials as agricultural waste can be used to generate electricity. Thermal conversion of biomass for power generation through the integrated gasification combined cycle (IGCC) is far more efficient than direct combustion, and the utilization of fluidized bed gasifiers for this purpose is gaining momentum as well, due to its versatility and high reaction and heat transfer rate within the reactor [Warnecke, 2000]. Currently large-scale applications of fluidized bed gasifiers are already available, many in Finland. These applications have the fluidized bed gasifiers providing gas to fire up a boiler [Wilén et al., 2004]. In this project a bubbling fluidized bed biomass gasifier was constructed for demonstration and development purposes. The gasifier is designed to provide producer gas for electricity generation via a petrol and diesel engine, each producing 100 kWe. In this paper the results cover an investigation of the gas cleaning and cooling system (GCC) and the fluidization regime of the gasifier. 1.1. Process issues Tar content in gas from fluidized bed gasifiers has proven to be a major obstacle towards widespread implementation of the technology. Tar arises from the decomposition of biomass and its constituents are mainly condensable 88
hydrocarbons or aromatic compounds, with high nitrogen and oxygen content. As the dew point of tar is approximately 500º C, it gives rise to operational issues downstream of the system by adhering to surfaces, sometimes forming liquid deposits inside pipes. It is reported that tar will polymerize once it reaches ambient temperature, causing fouling and plugging of instruments or equipment [Van Paasen and Kiel, 2004]. The primary removal method is to increase the operational temperature of the fluidized bed gasifier, as tar is decomposed into gaseous form at temperatures higher than 900º C [Van Paasen and Kiel, 2004]. However, due to possibilities of clinkering inside the bed, the control of bed temperature is essential to prevent excessive temperature, causing alkali metals and silica from biomass and sand respectively to form clinker [Yan et al., 2005]. This factor raises the issue of consistent fuel feeding into the gasifier, as interruption of biomass feed will cause the bed temperature to increase undesirably. Another option for tar removal would be to clean the gas downstream through coolers and condensers. The condensed liquid mixture of moisture and tar can then be processed in a waste-water treatment system. 2. Experimental set-up A bubbling fluidized bed biomass gasifier (BFBG) with an inner diameter of 400 mm and 130 mm thick refractory
Energy for Sustainable Development • Volume XI No. 1 • March 2007
Articles
Figure 1. Schematic of BFBG system with GCC
lining was constructed. The BFBG consisted of a freeboard section and a disengaging zone as well. A distribution plate was mounted above the plenum zone to allow air to flow into the bed. A liquified petroleum gas (LPG) injection point was fabricated for start-up purposes. The bed was heated up to 350º C before biomass feeding commenced. In this case, the fluidizing agent is air, provided by a blower at a pressure of 1.3 bar. The blower is controlled by a variable speed drive (VSD) and therefore the air-flow rate can be regulated. Air flows through the air distribution plate, then into a bed of sand. The amount of sand used is 96 kg, corresponding to a bed height of approximately 500 mm. The fluidization regime for the BFBG was determined by obtaining the pressure drop curve. The mean particle size of sand used as inert bed material was in the range of 425-600 µm and it had a density of 1520 kg/m3 The fuel feeding system consisted of an intermediate hopper, a screw feeder, a rotary valve and then a screw conveyor, which fed the biomass into the BFBG. The screw conveyor’s center line was 400 mm above the air distribution plate. The screw feeder was regulated by a VSD to vary the biomass feed rate. Once biomass was fed in, the reaction started and gas was produced. It flowed out into the gasifier cyclone where solid particles were separated before it flowed into the downstream GCC. The GCC is meant to remove moisture and tar condensates from the gas stream. In the GCC there is an air-to-gas natural convection heat exchanger, followed by another cyclone. A condenser was installed to remove moisture by passing the condensates out though a 1-inch (1 inch = 2.54 cm) drain pipe. The condenser is a forced convection air-to-gas heat exchanger, where gas flows into the steel tubes while air at ambient temperature flows on the outside of the pipes to bring down the temperature.
An expansion tank was installed to collect additional moisture. The schematic of the system is shown in Figure 1. The air flow rate was measured by using an orifice meter, coupled to a water manometer. The pressure drop measured from the manometer was used to calculate the volume flow rate of air by means of Bernoulli’s equation. The biomass feed rate was measured by collecting the amount fed from an outlet chute in a period of 1 minute. The collected amount was then weighed on a weighing scale, which would then give an estimate of the feed rate in terms of kg/hr. An automated weighing feeder was not purchased in this project due to cost constraints. The airfuel ratio (AFR) can be calculated from the air-flow rate and biomass feed rate, and it governs the temperature and efficiency of the BFBG. The ratio of AFR for gasification to AFR for stoichiometric combustion of biomass is regarded as the equivalence ratio (ER). ER is used as an indicating parameter of the BFBG with respect to its bed temperature, calorific value and efficiency. In this paper the effect of ER, temperature and fluidization regime is discussed. A type-K thermocouple[1] was placed 400 mm above the air distribution plate to measure the bed temperature. Four PT-type thermocouples were installed along the downstream GCC components to measure the heat loss as the gas flowed through. These four temperature sensors were located at the gasifier outlet, at the condenser inlet, after the expansion tank and at the stack. After every test run the collecting bin of the gasifier cyclone (see Figure 1) was checked for the amount of residue collected. It was weighed on a weighing scale and the value was recorded. The condensation rate from the condenser was estimated by collecting the liquid condensates in a container for a period of 1 minute. The mass of the container was deducted from the total mass, giving
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Figure 2. Bed temperature profile of ER = 0.15 and ER = 0.17
only the mass of liquid, which would then give the condensation rate in terms of g/min. The pressure drop across the BFBG was measured by using a water manometer as well. Two locations were used for pressure tapping, one just before the plenum zone while the other was at the disengaging zone. The two columns of water were connected to the pressure tapping. It was ensured that the level of water was equal in the manometer columns when the gasifier was not operational. Static bed heights in various fluidized bed gasifiers were found to vary from 370 to 800 mm, with aspect ratios (ratio of static bed height to the reactor inner diameter) ranging from 0.95 to 3.00 [Sanchez and Silva, 1994; Gomez et al., 1995; Guo et al., 2003; Formisani et al., 2002]. As the biomass feeding-point is 400 mm above the air distribution plate, the bed material should at least be at the same height, so that biomass comes in contact with the hot bed material once it enters the interior of the BFBG. A static bed height of 400 mm was chosen. A second test with 500 mm static bed height was chosen for comparison. These two values correspond to aspect ratios of 1.00 and 1.25 respectively. The air-flow rate was varied in a descending order first, followed by ascending flow rate. 3. Results and discussions 3.1. Gasifier temperature With continuous biomass-feeding, the BFBG was able to obtain a stable operating temperature range within 30 min. Presently all tests are done at a biomass feed rate of 160 kg/hr. With varying AFR the temperature profile differs as well. Figure 2 shows the temperature trend for ER at 0.15 and 0.17. At ER = 0.15 the air-flow rate was 144 90
kg/hr while at ER = 0.17 it was 163 kg/hr. At 144 kg/hr the start-up took a longer time to reach 350º C. Stable operating temperature was achieved at almost the same time for both cases. For ER = 0.15 the temperature maintained was 400-450º C, while for ER = 0.17 the temperature maintained was 600-650º C. The temperature was found to be lower for lower ER as the amount of air flow into the BFBG was reduced, decreasing the amount of oxygen reacting with the biomass, thus lowering the amount of heat released and consequently the temperature. With higher air-flow rates, there was more oxidant that could be used for reaction, thus increasing the bed temperature. 3.2. Gas cleaning and cooling (GCC) system From the condenser the flow rate of condensates from the outlet drain, mfirst, was at an average rate of approximately 147 g/min. The expansion tank was able to collect 2.76 kg of condensates in a two-hour test run and the average condensation rate, mexp.tank, was 23 g/min. Overall the condensation removal rate, mremoval, was 6.38 % of the biomass feed rate. A decision was made to install a secondary condenser to increase the condensation rate. Due to space constraints it had to be connected after the expansion tank rather than after the first condenser. A test run was then repeated and the condensate flow rates from it were estimated. The second condenser condensate flow rate, msec, was at an average of approximately 170 g/min, while mfirst was 165 g/min. The expansion tank collected 2.45 kg of condensates in the two-hour test run. This test run gave a condensation removal of 13.25 % of the biomass feed rate. The incorporation of the second condenser doubled the removal of moisture. Table 1 shows the data for the
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Table 1. Average condensate flow rate from test runs with and without second condenser mfirst (g/min)
msec (g/min)
mexp.tank (g/min)
mremoval (%)
Std. dev. of condensate mremoval
Test run without condenser
147
-
23
6.38
0.32
Test run with condenser
165
170
20
13.25
0.37
10.01
42.27
2.50
1.79
-
SE mean at 95 % confidence interval
Table 2. Average temperature of components across GCC for different bed temperatures Equivalence ratio, ER
Average bed temperature (ºC)
Tsec (ºC)
Standard deviation of Tsec
Tfinal (ºC)
Standard deviation of Tfinal
0.135
448
55
8.64
37
4.68
0.156
644
59
1.07
41
1.57
0.188
686
57
1.00
37
1.10
654
[1]
3.84
59
2.16
0.646
-
0.889
-
0.172 P-value
-
0.001
Note 1. Test run did not incorporate secondary condenser into system.
Table 3. Characteristics of collected char in gasifier cyclone from four test runs Mass of char (kg)
Percentage of feed rate (%)
22
10.57
Standard deviation
1.91
2.22
SE mean at 95 % confidence interval
0.89
0.86
P-value
0.20
0.22
Average
average condensation collection rate for test runs with and without the second condenser. The standard deviation for mremoval was low, 0.32 and 0.37 respectively. The means of standard error (SE) for the condensate flow rates at 95 % confidence interval are shown in Table 1 as well. Table 2 shows the average temperature across each of the components in the GCC with average bed temperatures from separate test runs. When the second condenser was not incorporated, the final gas temperature, Tfinal, was considerably higher at 59º C. When the second condenser was fabricated and used in the system, Tfinal dropped significantly to approximately 40º C. A lower temperature would indicate loss of sensible heat, causing the temperature to drop and eventually reach the dew point of water. Increased condensation would then occur in the second condenser. Table 2 showed that the temperature in the components of the GCC was maintained as well, regardless of the bed temperature (Tbed). The second condenser inlet temperature, Tsec, tends to be maintained at approximately 60º C, while the final gas temperature tends to be approximately 40º C or lower. P-values from analysis of
variance were 0.646 and 0.889 respectively, meaning that there is no significant difference between the data when compared at different bed temperatures. Characterstics of the collected char from the gasifier cyclone from several separate test runs are shown in Table 3. The amount of char collected is not consistent and varies with every test run but averages 21.5 kg and 10.57 % of the biomass feed rate. Sieve analysis shows that the particle size of char is 100 µm (i.e., 0.1 mm). The particle size is essential for the char to be utilized in solid fuel combustors. In cement plants coal needs to be pulverized into particles to be fired in kilns. Char has the potential of being used as solid fuel in the same manner as well. Char has higher energy content than biomass, and has higher volatile content than coal. Even on wet basis char was determined to have a calorific value of 18.10 MJ/kg, which is roughly the value for biomass on dry basis. It therefore presents a viable option to be developed into pulverized solid fuels. However, there are alternative applications whereby it is used as an adsorbent due to the change in its properties during pyrolysis [Haykiti-Acma et al., 2006]. The analysis of variance of the mass of char collected showed that the P-value is higher than 0.05, indicating that there is no significant difference between the test runs. The means of standard error (SE) for the average collected char at 95 % confidence interval are shown in Table 3 as well. 3.3. Pressure drop, ∆P The bed material used for fluidization can be classified into four different groups of particles. These are called Geldart Group A, B, C and D particles. Each has its own fluidization behavior which is fundamentally related to its particle diameter and density. Group C particles are very fine with particle size of less than 30 µm and are difficult
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Figure 3. Pressure drop versus air-flow rate
to fluidize due to their strong cohesive force. Fluidization for Group A and B particles is easier, with the latter having larger particle diameters [Gibilaro, 2001]. In this study the bed material corresponds to Geldart Group B particles, which have a mean particle size in the range of 100 µm (0.1 mm) to 1.0 mm and a density of 1520 kg/m3. Figure 3 shows the curve of the pressure drop, ∆P, against the air-flow rate for two different bed heights. The test for descending flow rate is denoted (a) while for ascending flow rate it is denoted (b) [Fan and Zhu, 1998; Gupta and Sathiyamoorthy, 1999]. ∆P for ascending and descending flow rate for both bed heights follows the same trend, though descending flow rates are preferred [Formisani et al., 2002] to ensure all particles have been “loosened” from each other. ∆P for 400 mm bed height was lower than for 500 mm bed height as expected. However, both showed a change in the slope of the curve. This point of change in slope is almost at the same air-flow rate, which is at 110 kg/hr. The curve then continues its increase in ∆P with increasing flow rate. The point where the curve changes slope shows that the bed has undergone a transition from a homogeneous fluidization regime. For Geldart Group B particles, it shows instantaneous bubbling once minimum fluidization is reached, unlike for Geldart Group A particles. Group A particles have mean particle sizes in the range of 30-100 µm [Gibilaro, 2001]. They have a transition period when 92
minimum fluidization is reached before any bubbles start to form within the bed, unlike Group B particles which exhibit bubble motion once minimum fluidization is reached. For gasification, bubbling fluidization is desirable as the turbulent mixing of bed solid particles is required to ensure uniform reaction. From the ∆P experiment bubbling fluidization would only occur for flow rates above 110 kg/hr. The extent of the increase in air-flow rate however is limited to the elutriation rate of bed particles, which from visual observation occurs at 190 kg/hr. Sand was observed to be collected at the gasifier cyclone when this flow rate was tested. Also if ER were to be too high it would lead to a decrease in lower calorific value (LCV) of the producer gas [Yan et al., 2005]. The ER could not be lowered by increasing the biomass feed rate due to the limited capacity of the rotary valve. The rotary valve tends to malfunction when the biomass feed rate is increased to above 180 kg/hr. There needs to be a compromise between the selection of fluidization regime and the required ER for maximum cold efficiency. Comparing with the bed temperature profile of Figure 1, at an air flow rate of 110 kg/hr, with homogeneous fluidization the temperature is maintained at 400-450º C. For ER = 0.17 the air-flow rate is more than 110 kg/hr, giving bubbling fluidization, and the bed temperature is 600-650º C. This shows that the fluidization regime and
Energy for Sustainable Development • Volume XI No. 1 • March 2007
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the temperature are related to the air-flow rate. The impact of this on the LCV producer gas is still under study.
Formisani, B., Girimonte, R., and Pataro, G., 2002. “The influence of operating temperature on the dense phase properties of bubbling fluidized beds of solids”, Powder Technology, 125(1), pp. 28-38.
4. Conclusion
Gibilaro, L.G., 2001. “Fluidization-dynamics”, Chapter 10 in Fluidization Quality, ButterworthHeinemann, Oxford, pp. 108-110.
With lower air-flow rates, the ER and temperature would decrease. Homogeneous fluidization corresponds to lower bed temperatures while bubbling fluidization corresponds to higher bed temperatures with regard to their respective ER values. The varying bed temperatures of the BFBG did not influence the steady state temperature of the downstream GCC components as they were shown to be stable. Incorporation of the secondary condenser lowered the temperature to increase the condensation of moisture and tar, giving clean producer gas. Char collected from the gasifier cyclone showed that on average char amounting to 10.57 % of the biomass feed rate was produced, having a mean particle size of 100 µm, which can be used in the development of solid fuel combustors. Note 1. Type-K thermocouples use nickel-chromium or nickel-aluminum alloys as the base metals for measuring the temperature. They can be used to measure high temperatures. References Fan, L.S., and Zhu, C., 1998. “Principles of gas-solid flows”, Chapter 9 in Dense Phase Fluidized Beds, Cambridge University Press, Cambridge, UK, pp. 378.
Gomez, E.O., Silva, E.L., and Cortez, L.A.B., 1995. “Constructive features, operation and sizing of fluidized-bed gasifiers for biomass”, Energy for Sustainable Development, 2(4), pp. 52-57. Guo, Q.J., Yue, G.X., Suda, T., and Sato, J., 2003. “Flow characteristics in a bubbling fluidized bed at elevated temperature”, Chemical Engineering and Processing, 42(6), pp. 439-447. Gupta, C.K., and Sathiyamoorthy, D., 1999. “Fluid bed technology in materials processing”, Chapter 1 in Hydrodynamics of Two-Phase Fluidization, CRC Press, Boca Raton, Florida, USA, pp. 23-24. Haykiti-Acma, H., Yaman, S., and Kucukbayrak S., 2006. “Gasification of biomass char in steam-nitrogen mixture”, Energy Conversion and Management, 47(7-8), pp. 1004-1013. Sanchez, C.G., and Silva, E.L., 1994. “Biomass fluidized bed gasification research in the State University of Campinas”, Energy for Sustainable Development, I(4), pp. 31-34. Van Paasen, S.V.B., and Kiel, J.H.A., 2004. “Tar formation in fluidized-bed gasification – impact of gasifier operating conditions”, Paper presented at the 2nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection, Rome, Italy. Warnecke, R., 2000. “Gasification of biomass: a comparison of fixed bed and fluidized bed gasifier”, Biomass and Bioenergy, 18(6), pp. 489-197. Wilén, C., Salokoski, P., Kurkela, E., and Sipilä, K., 2004. Finnish Environment Institute Finnish Expert Report on Best Available Techniques in Energy Production from Solid Recovered Fuels, Edita Publishing Ltd. Helsinki, Finland. Yan, R., Liang, D.T., and Tsen, L., 2005. “Case studies-problem solving in fluidized bed waste fuel incineration”, Energy Conversion and Management, 46(7-8), pp. 1165-1178.
Energy for Sustainable Development and its role in IEI’s mission IEI’s journal Energy for Sustainable Development (ESD) is meant to be a vehicle of communication between Northern and Southern energy actors on the energy goals and policies as well as energy plans and projects in developing countries. It also aims to promote South-South interactions to tackle these problems. ESD attempts to provide a balanced treatment of: renewables and end-use efficiency (in the industrial, transport, domestic, commercial and agricultural sectors); generation (including development) of energy technologies and their dissemination; hardware (for the conversion of primary energy as found in nature into convenient secondary energy, for transmission/transport and distribution and for utilisation in end-use devices); and software (relating to energy policies, institutions, management and financing). Articles and short articles published are subject to a formal process of anonymous peer review. Find out more about ESD on the Internet at www.ieiglobal.org/esd.html.
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Operational investigation of a bubbling fluidized bed biomass gasification system Lim Mook Tzeng and Z.A. Zainal School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia 14300 Seberang Prai Selatan, Pulau Pinang, Malaysia E-mail (Zainal): [email protected]
The operational characteristics of a bubbling fluidized bed biomass gasifier (BFBG) are discussed. A gas cleaning and cooling (GCC) system that consists of cyclones and natural and forced convection air-to-gas heat exchangers is connected to the BFBG to facilitate removal of particulates, moisture and tar. The performance of the GCC with and without a second condenser and its effect is presented. The secondary condenser doubles the condensate removal rate. Temperatures across the components of the GCC remain constant after a period of time. The fluidization regime of the BFBG with varying air-flow rate was investigated by measuring the pressure drop (∆P) with a manometer. A change in the slope of the ∆P curve indicated a change in fluidization regime from homogeneous fluidization to bubbling fluidization. The fluidization regime changes to bubbling fluidization once the flow rate exceeds 110 kg/hr regardless of the bed height, which causes intense solid particle mixing. The varying air-flow rate gives different equivalence ratio (ER) values. The upper limit of ER is constrained by the lower calorific value (LCV) of producer gas. The bed temperature and fluidization regime of the BFBG is determined by ER and has to be regulated so as to obtain maximum cold gas efficiency from the BFBG. Low ER corresponds to homogeneous fluidization and low bed temperature, while higher ER gives bubbling fluidization and higher bed temperatures. Further investigation on the relationship between these two parameters is still going on. 1. Introduction Biomass energy is becoming more prominent in the effort to increase sustainable energy utilization. It is a renewable resource, whereby, for instance, such materials as agricultural waste can be used to generate electricity. Thermal conversion of biomass for power generation through the integrated gasification combined cycle (IGCC) is far more efficient than direct combustion, and the utilization of fluidized bed gasifiers for this purpose is gaining momentum as well, due to its versatility and high reaction and heat transfer rate within the reactor [Warnecke, 2000]. Currently large-scale applications of fluidized bed gasifiers are already available, many in Finland. These applications have the fluidized bed gasifiers providing gas to fire up a boiler [Wilén et al., 2004]. In this project a bubbling fluidized bed biomass gasifier was constructed for demonstration and development purposes. The gasifier is designed to provide producer gas for electricity generation via a petrol and diesel engine, each producing 100 kWe. In this paper the results cover an investigation of the gas cleaning and cooling system (GCC) and the fluidization regime of the gasifier. 1.1. Process issues Tar content in gas from fluidized bed gasifiers has proven to be a major obstacle towards widespread implementation of the technology. Tar arises from the decomposition of biomass and its constituents are mainly condensable 88
hydrocarbons or aromatic compounds, with high nitrogen and oxygen content. As the dew point of tar is approximately 500º C, it gives rise to operational issues downstream of the system by adhering to surfaces, sometimes forming liquid deposits inside pipes. It is reported that tar will polymerize once it reaches ambient temperature, causing fouling and plugging of instruments or equipment [Van Paasen and Kiel, 2004]. The primary removal method is to increase the operational temperature of the fluidized bed gasifier, as tar is decomposed into gaseous form at temperatures higher than 900º C [Van Paasen and Kiel, 2004]. However, due to possibilities of clinkering inside the bed, the control of bed temperature is essential to prevent excessive temperature, causing alkali metals and silica from biomass and sand respectively to form clinker [Yan et al., 2005]. This factor raises the issue of consistent fuel feeding into the gasifier, as interruption of biomass feed will cause the bed temperature to increase undesirably. Another option for tar removal would be to clean the gas downstream through coolers and condensers. The condensed liquid mixture of moisture and tar can then be processed in a waste-water treatment system. 2. Experimental set-up A bubbling fluidized bed biomass gasifier (BFBG) with an inner diameter of 400 mm and 130 mm thick refractory
Energy for Sustainable Development • Volume XI No. 1 • March 2007
Articles
Figure 1. Schematic of BFBG system with GCC
lining was constructed. The BFBG consisted of a freeboard section and a disengaging zone as well. A distribution plate was mounted above the plenum zone to allow air to flow into the bed. A liquified petroleum gas (LPG) injection point was fabricated for start-up purposes. The bed was heated up to 350º C before biomass feeding commenced. In this case, the fluidizing agent is air, provided by a blower at a pressure of 1.3 bar. The blower is controlled by a variable speed drive (VSD) and therefore the air-flow rate can be regulated. Air flows through the air distribution plate, then into a bed of sand. The amount of sand used is 96 kg, corresponding to a bed height of approximately 500 mm. The fluidization regime for the BFBG was determined by obtaining the pressure drop curve. The mean particle size of sand used as inert bed material was in the range of 425-600 µm and it had a density of 1520 kg/m3 The fuel feeding system consisted of an intermediate hopper, a screw feeder, a rotary valve and then a screw conveyor, which fed the biomass into the BFBG. The screw conveyor’s center line was 400 mm above the air distribution plate. The screw feeder was regulated by a VSD to vary the biomass feed rate. Once biomass was fed in, the reaction started and gas was produced. It flowed out into the gasifier cyclone where solid particles were separated before it flowed into the downstream GCC. The GCC is meant to remove moisture and tar condensates from the gas stream. In the GCC there is an air-to-gas natural convection heat exchanger, followed by another cyclone. A condenser was installed to remove moisture by passing the condensates out though a 1-inch (1 inch = 2.54 cm) drain pipe. The condenser is a forced convection air-to-gas heat exchanger, where gas flows into the steel tubes while air at ambient temperature flows on the outside of the pipes to bring down the temperature.
An expansion tank was installed to collect additional moisture. The schematic of the system is shown in Figure 1. The air flow rate was measured by using an orifice meter, coupled to a water manometer. The pressure drop measured from the manometer was used to calculate the volume flow rate of air by means of Bernoulli’s equation. The biomass feed rate was measured by collecting the amount fed from an outlet chute in a period of 1 minute. The collected amount was then weighed on a weighing scale, which would then give an estimate of the feed rate in terms of kg/hr. An automated weighing feeder was not purchased in this project due to cost constraints. The airfuel ratio (AFR) can be calculated from the air-flow rate and biomass feed rate, and it governs the temperature and efficiency of the BFBG. The ratio of AFR for gasification to AFR for stoichiometric combustion of biomass is regarded as the equivalence ratio (ER). ER is used as an indicating parameter of the BFBG with respect to its bed temperature, calorific value and efficiency. In this paper the effect of ER, temperature and fluidization regime is discussed. A type-K thermocouple[1] was placed 400 mm above the air distribution plate to measure the bed temperature. Four PT-type thermocouples were installed along the downstream GCC components to measure the heat loss as the gas flowed through. These four temperature sensors were located at the gasifier outlet, at the condenser inlet, after the expansion tank and at the stack. After every test run the collecting bin of the gasifier cyclone (see Figure 1) was checked for the amount of residue collected. It was weighed on a weighing scale and the value was recorded. The condensation rate from the condenser was estimated by collecting the liquid condensates in a container for a period of 1 minute. The mass of the container was deducted from the total mass, giving
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Figure 2. Bed temperature profile of ER = 0.15 and ER = 0.17
only the mass of liquid, which would then give the condensation rate in terms of g/min. The pressure drop across the BFBG was measured by using a water manometer as well. Two locations were used for pressure tapping, one just before the plenum zone while the other was at the disengaging zone. The two columns of water were connected to the pressure tapping. It was ensured that the level of water was equal in the manometer columns when the gasifier was not operational. Static bed heights in various fluidized bed gasifiers were found to vary from 370 to 800 mm, with aspect ratios (ratio of static bed height to the reactor inner diameter) ranging from 0.95 to 3.00 [Sanchez and Silva, 1994; Gomez et al., 1995; Guo et al., 2003; Formisani et al., 2002]. As the biomass feeding-point is 400 mm above the air distribution plate, the bed material should at least be at the same height, so that biomass comes in contact with the hot bed material once it enters the interior of the BFBG. A static bed height of 400 mm was chosen. A second test with 500 mm static bed height was chosen for comparison. These two values correspond to aspect ratios of 1.00 and 1.25 respectively. The air-flow rate was varied in a descending order first, followed by ascending flow rate. 3. Results and discussions 3.1. Gasifier temperature With continuous biomass-feeding, the BFBG was able to obtain a stable operating temperature range within 30 min. Presently all tests are done at a biomass feed rate of 160 kg/hr. With varying AFR the temperature profile differs as well. Figure 2 shows the temperature trend for ER at 0.15 and 0.17. At ER = 0.15 the air-flow rate was 144 90
kg/hr while at ER = 0.17 it was 163 kg/hr. At 144 kg/hr the start-up took a longer time to reach 350º C. Stable operating temperature was achieved at almost the same time for both cases. For ER = 0.15 the temperature maintained was 400-450º C, while for ER = 0.17 the temperature maintained was 600-650º C. The temperature was found to be lower for lower ER as the amount of air flow into the BFBG was reduced, decreasing the amount of oxygen reacting with the biomass, thus lowering the amount of heat released and consequently the temperature. With higher air-flow rates, there was more oxidant that could be used for reaction, thus increasing the bed temperature. 3.2. Gas cleaning and cooling (GCC) system From the condenser the flow rate of condensates from the outlet drain, mfirst, was at an average rate of approximately 147 g/min. The expansion tank was able to collect 2.76 kg of condensates in a two-hour test run and the average condensation rate, mexp.tank, was 23 g/min. Overall the condensation removal rate, mremoval, was 6.38 % of the biomass feed rate. A decision was made to install a secondary condenser to increase the condensation rate. Due to space constraints it had to be connected after the expansion tank rather than after the first condenser. A test run was then repeated and the condensate flow rates from it were estimated. The second condenser condensate flow rate, msec, was at an average of approximately 170 g/min, while mfirst was 165 g/min. The expansion tank collected 2.45 kg of condensates in the two-hour test run. This test run gave a condensation removal of 13.25 % of the biomass feed rate. The incorporation of the second condenser doubled the removal of moisture. Table 1 shows the data for the
Energy for Sustainable Development • Volume XI No. 1 • March 2007
Articles
Table 1. Average condensate flow rate from test runs with and without second condenser mfirst (g/min)
msec (g/min)
mexp.tank (g/min)
mremoval (%)
Std. dev. of condensate mremoval
Test run without condenser
147
-
23
6.38
0.32
Test run with condenser
165
170
20
13.25
0.37
10.01
42.27
2.50
1.79
-
SE mean at 95 % confidence interval
Table 2. Average temperature of components across GCC for different bed temperatures Equivalence ratio, ER
Average bed temperature (ºC)
Tsec (ºC)
Standard deviation of Tsec
Tfinal (ºC)
Standard deviation of Tfinal
0.135
448
55
8.64
37
4.68
0.156
644
59
1.07
41
1.57
0.188
686
57
1.00
37
1.10
654
[1]
3.84
59
2.16
0.646
-
0.889
-
0.172 P-value
-
0.001
Note 1. Test run did not incorporate secondary condenser into system.
Table 3. Characteristics of collected char in gasifier cyclone from four test runs Mass of char (kg)
Percentage of feed rate (%)
22
10.57
Standard deviation
1.91
2.22
SE mean at 95 % confidence interval
0.89
0.86
P-value
0.20
0.22
Average
average condensation collection rate for test runs with and without the second condenser. The standard deviation for mremoval was low, 0.32 and 0.37 respectively. The means of standard error (SE) for the condensate flow rates at 95 % confidence interval are shown in Table 1 as well. Table 2 shows the average temperature across each of the components in the GCC with average bed temperatures from separate test runs. When the second condenser was not incorporated, the final gas temperature, Tfinal, was considerably higher at 59º C. When the second condenser was fabricated and used in the system, Tfinal dropped significantly to approximately 40º C. A lower temperature would indicate loss of sensible heat, causing the temperature to drop and eventually reach the dew point of water. Increased condensation would then occur in the second condenser. Table 2 showed that the temperature in the components of the GCC was maintained as well, regardless of the bed temperature (Tbed). The second condenser inlet temperature, Tsec, tends to be maintained at approximately 60º C, while the final gas temperature tends to be approximately 40º C or lower. P-values from analysis of
variance were 0.646 and 0.889 respectively, meaning that there is no significant difference between the data when compared at different bed temperatures. Characterstics of the collected char from the gasifier cyclone from several separate test runs are shown in Table 3. The amount of char collected is not consistent and varies with every test run but averages 21.5 kg and 10.57 % of the biomass feed rate. Sieve analysis shows that the particle size of char is 100 µm (i.e., 0.1 mm). The particle size is essential for the char to be utilized in solid fuel combustors. In cement plants coal needs to be pulverized into particles to be fired in kilns. Char has the potential of being used as solid fuel in the same manner as well. Char has higher energy content than biomass, and has higher volatile content than coal. Even on wet basis char was determined to have a calorific value of 18.10 MJ/kg, which is roughly the value for biomass on dry basis. It therefore presents a viable option to be developed into pulverized solid fuels. However, there are alternative applications whereby it is used as an adsorbent due to the change in its properties during pyrolysis [Haykiti-Acma et al., 2006]. The analysis of variance of the mass of char collected showed that the P-value is higher than 0.05, indicating that there is no significant difference between the test runs. The means of standard error (SE) for the average collected char at 95 % confidence interval are shown in Table 3 as well. 3.3. Pressure drop, ∆P The bed material used for fluidization can be classified into four different groups of particles. These are called Geldart Group A, B, C and D particles. Each has its own fluidization behavior which is fundamentally related to its particle diameter and density. Group C particles are very fine with particle size of less than 30 µm and are difficult
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Figure 3. Pressure drop versus air-flow rate
to fluidize due to their strong cohesive force. Fluidization for Group A and B particles is easier, with the latter having larger particle diameters [Gibilaro, 2001]. In this study the bed material corresponds to Geldart Group B particles, which have a mean particle size in the range of 100 µm (0.1 mm) to 1.0 mm and a density of 1520 kg/m3. Figure 3 shows the curve of the pressure drop, ∆P, against the air-flow rate for two different bed heights. The test for descending flow rate is denoted (a) while for ascending flow rate it is denoted (b) [Fan and Zhu, 1998; Gupta and Sathiyamoorthy, 1999]. ∆P for ascending and descending flow rate for both bed heights follows the same trend, though descending flow rates are preferred [Formisani et al., 2002] to ensure all particles have been “loosened” from each other. ∆P for 400 mm bed height was lower than for 500 mm bed height as expected. However, both showed a change in the slope of the curve. This point of change in slope is almost at the same air-flow rate, which is at 110 kg/hr. The curve then continues its increase in ∆P with increasing flow rate. The point where the curve changes slope shows that the bed has undergone a transition from a homogeneous fluidization regime. For Geldart Group B particles, it shows instantaneous bubbling once minimum fluidization is reached, unlike for Geldart Group A particles. Group A particles have mean particle sizes in the range of 30-100 µm [Gibilaro, 2001]. They have a transition period when 92
minimum fluidization is reached before any bubbles start to form within the bed, unlike Group B particles which exhibit bubble motion once minimum fluidization is reached. For gasification, bubbling fluidization is desirable as the turbulent mixing of bed solid particles is required to ensure uniform reaction. From the ∆P experiment bubbling fluidization would only occur for flow rates above 110 kg/hr. The extent of the increase in air-flow rate however is limited to the elutriation rate of bed particles, which from visual observation occurs at 190 kg/hr. Sand was observed to be collected at the gasifier cyclone when this flow rate was tested. Also if ER were to be too high it would lead to a decrease in lower calorific value (LCV) of the producer gas [Yan et al., 2005]. The ER could not be lowered by increasing the biomass feed rate due to the limited capacity of the rotary valve. The rotary valve tends to malfunction when the biomass feed rate is increased to above 180 kg/hr. There needs to be a compromise between the selection of fluidization regime and the required ER for maximum cold efficiency. Comparing with the bed temperature profile of Figure 1, at an air flow rate of 110 kg/hr, with homogeneous fluidization the temperature is maintained at 400-450º C. For ER = 0.17 the air-flow rate is more than 110 kg/hr, giving bubbling fluidization, and the bed temperature is 600-650º C. This shows that the fluidization regime and
Energy for Sustainable Development • Volume XI No. 1 • March 2007
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the temperature are related to the air-flow rate. The impact of this on the LCV producer gas is still under study.
Formisani, B., Girimonte, R., and Pataro, G., 2002. “The influence of operating temperature on the dense phase properties of bubbling fluidized beds of solids”, Powder Technology, 125(1), pp. 28-38.
4. Conclusion
Gibilaro, L.G., 2001. “Fluidization-dynamics”, Chapter 10 in Fluidization Quality, ButterworthHeinemann, Oxford, pp. 108-110.
With lower air-flow rates, the ER and temperature would decrease. Homogeneous fluidization corresponds to lower bed temperatures while bubbling fluidization corresponds to higher bed temperatures with regard to their respective ER values. The varying bed temperatures of the BFBG did not influence the steady state temperature of the downstream GCC components as they were shown to be stable. Incorporation of the secondary condenser lowered the temperature to increase the condensation of moisture and tar, giving clean producer gas. Char collected from the gasifier cyclone showed that on average char amounting to 10.57 % of the biomass feed rate was produced, having a mean particle size of 100 µm, which can be used in the development of solid fuel combustors. Note 1. Type-K thermocouples use nickel-chromium or nickel-aluminum alloys as the base metals for measuring the temperature. They can be used to measure high temperatures. References Fan, L.S., and Zhu, C., 1998. “Principles of gas-solid flows”, Chapter 9 in Dense Phase Fluidized Beds, Cambridge University Press, Cambridge, UK, pp. 378.
Gomez, E.O., Silva, E.L., and Cortez, L.A.B., 1995. “Constructive features, operation and sizing of fluidized-bed gasifiers for biomass”, Energy for Sustainable Development, 2(4), pp. 52-57. Guo, Q.J., Yue, G.X., Suda, T., and Sato, J., 2003. “Flow characteristics in a bubbling fluidized bed at elevated temperature”, Chemical Engineering and Processing, 42(6), pp. 439-447. Gupta, C.K., and Sathiyamoorthy, D., 1999. “Fluid bed technology in materials processing”, Chapter 1 in Hydrodynamics of Two-Phase Fluidization, CRC Press, Boca Raton, Florida, USA, pp. 23-24. Haykiti-Acma, H., Yaman, S., and Kucukbayrak S., 2006. “Gasification of biomass char in steam-nitrogen mixture”, Energy Conversion and Management, 47(7-8), pp. 1004-1013. Sanchez, C.G., and Silva, E.L., 1994. “Biomass fluidized bed gasification research in the State University of Campinas”, Energy for Sustainable Development, I(4), pp. 31-34. Van Paasen, S.V.B., and Kiel, J.H.A., 2004. “Tar formation in fluidized-bed gasification – impact of gasifier operating conditions”, Paper presented at the 2nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection, Rome, Italy. Warnecke, R., 2000. “Gasification of biomass: a comparison of fixed bed and fluidized bed gasifier”, Biomass and Bioenergy, 18(6), pp. 489-197. Wilén, C., Salokoski, P., Kurkela, E., and Sipilä, K., 2004. Finnish Environment Institute Finnish Expert Report on Best Available Techniques in Energy Production from Solid Recovered Fuels, Edita Publishing Ltd. Helsinki, Finland. Yan, R., Liang, D.T., and Tsen, L., 2005. “Case studies-problem solving in fluidized bed waste fuel incineration”, Energy Conversion and Management, 46(7-8), pp. 1165-1178.
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