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Reduction of Bed Agglomeration in CFB Combustion Biomass with Aluminium-Contain Bed Material
REDUCTION OF BED AGGLOMERATION IN CFB COMBUSTION BIOMASS WITH ALUMINIUM-CONTAIN BED MATERIAL R. Liu , B. Jin, Z. Zhong and J. Zhao Key Laboratory on Clean Coal Power Generation and Combustion Technology of Ministry of Education, Southeast University, Nanjing, PR China.
Abstract: Two kinds of bed materials were used as bed materials when a biomass of cotton stalk was burnt in a circulation fluidized bed (CFB) combustion pilot plant (0.2 MW). After a long time operation, silica bed materials were found sintered while aluminous bed materials slightly changed. The particles of both kinds of bed materials were sampled and three instrumental approaches were performed (XRF, SEM/EDS, XRD) to identify the bed materials in order to find out the mechanics of agglomeration. The result shows that both of the bed materials were enriched with alkali metals but the changing speed of enrichment of aluminous particles was much slower than that of silica particles. The CFB could go steadily and well at least 72 h under our experimental condition when aluminous particles were used as bed material. Agglomeration of aluminous bed materials was reduced due to the protective element aluminous. Keywords: CFB; bed materials; agglomeration; biomass; combustion.
INTRODUCTION
Correspondence to: Dr R. Liu, Key Laboratory on Clean Coal Power Generation and Combustion Technology of Ministry of Education, ThermoEnergy Engineering Research Institute, Southeast University, Nanjing 210096. E-mail: [email protected]
DOI: 10.1205/psep07010 0957–5820/07/ $30.00 þ 0.00 Process Safety and Environmental Protection Trans IChemE, Part B, September 2007 # 2007 Institution of Chemical Engineers
1996; Bryers, 1996; Michelsen et al., 1998; Nielsen et al., 2000; Werther et al., 2000). Two possible mechanisms for bed agglomeration in FBC are known (Skrifvars et al., 1994). One mechanism believed that partial melting of alkali compounds in biomass ash causes stickiness of the particles and the agglomeration of the particles. Another main sintering mechanism is chemical reaction. Chemical reaction sintering is related to gassolid reaction between gases of SO2 and CO2 and particles CaO forming dense and hard particles of CaSO4 and CaCO3. The main objective of present work was to experimentally determine the agglomeration tendencies of some common biomass in the combustion of the fluidized bed by using a new bed material based on the controlled fluidized bed agglomeration test and to compare the results with the traditional bed materials. In order to understand the formation mechanics of bed agglomeration in the FBC reactor, a full investigation of biomass and bed materials is conducted by several approaches, including X-ray fluorescence (XRF), scanning electron microscope (SEM) and X-ray diffraction (XRD). The research focused on the accumulation rate of ash alkali in the bed materials, and the mechanism by which the agglomeration takes place. This study can be utilized to predict and reduce ash sintering, the result would help
Fluidized bed combustion (FBC) is well known as an effective technology for burning both conventional and renewable solid fuel. The flexibility towards the fuel, the high combustion efficiency, and the low environmental impact are major advantages of FBC. However, during combustion of biomass fuels, a number of operation matters occur because of the different ash components. The major ash-related problem encountered in fluidized beds is bed agglomeration which is the worst case may result in operational failures with FBC. Therefore, preventing bed agglomeration is a relevant task to avoid unscheduled shutdowns and costly maintenance stops for boiler. Much attention was paid to agglomeration of bed materials in fluidized bed combustion/ gasification in the mid-seventies (Gluckman et al., 1976). For agglomeration occurring in FBC of biomass, the ash chemistry is of the most importance. It is generally believed that the elements which influence sintering and agglomeration the most are potassium, sodium, calcium, magnesium, silicon, sulphur and chlorine (Carty et al., 1988; Jenkins et al., 1998). Combustion processes with biomass which has a high content of alkaline elements are prone to experience bed agglomeration and sintering (Miles, 1994; Miles et al.,
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to improve the design of combustion plants and plan strategic changes in the combustion processes.
METHODS AND MATERIALS CFB Combustion Pilot Plant and Combustion Conditions CFB combustion pilot plant, where the biomass combustion tests were made, is shown in Figure 1. The biomass supply system was consisted of a conical bin and a screw conveyor with a variable speed. The biomass which was discharged, around 30 kg h21, was 30 cm above the distributor with a bed height in shutdown around 30 cm (35 kg of bed materials). The furnace was cubic in shape, with an inner length of foursquare side 0.23 m and a height of 4 m. The primary air (60% of the total air 150 Nm3 h21) was distributed through multiple nozzles on a plate. The bed temperature and the mean fluidization velocity were maintained to 8508C + 208C and 3 m s21 during the combustion tests. The duration of each combustion test under steady state condition was over 8 h, with a start up around 2 h. Solids circulation rate is about 5 according to the current experimental condition.
Table 1. Physical and chemical characterization of biomass tested in the CFB. Density (g cm23) LHV (kJ kg21) Proximate analysis (as received, wt%) Moisture Volatiles Fixed carbon Ash Ultimate analysis (air dry basis, wt%) Carbon Hydrogen Nitrogen Sulfur Oxygen (diff.) Ash composition (wt%) CaO MgO K2O Na2O Fe2O3 Al2O3 SiO2 P2O5 TiO2 MnO
Materials
0.6 12372 8.41 68.12 21.87 1.75 43.63 5.5 0.27 0.11 48.69 17.33 9.00 32.67 3.67 1.57 3.30 6.33 7.33 0.16 0.06
Samples
A biomass fuel of cotton stalk taken from Shandong province in China was used for all experimental tests. Two kinds of particles (silica and aluminous particles) size of 0.60 mm were used as bed materials. The physical and chemical characterization of the biomass fuel used is summarized in Table 1.
In order to obtain data on the inorganic elements with relevance for bed agglomeration, some bed materials and ashes have been sampled in the atmospheric CFB combustion pilot plant. XRF analyses of two fresh bed materials were showed in Table 2. Other bed materials samples were taken
Figure 1. CFB. Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441– 445
CFB COMBUSTION BIOMASS WITH ALUMINIUM-CONTAIN BED MATERIAL
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Table 2. XRF analyses of fresh bed materials (wt%). Sample
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
K2O
MnO
P2O5
Na2O
LOI
Silica Aluminous
98.9 17.92
0.09 75.93
0.09 1.28
0.04 0.17
0.00 0.01
0.01 2.80
0.02 1.18
0.15 0.01
0.03 0.11
0.08 0.03
0.24 0.28
LOI ¼ loss on ignition at 10008C.
from the bottom of the furnace and crushed smaller than 0.074 mm in diameter.
Analytical Methods Laboratory sample of biomass, initial and final bed material, ash were obtained by an adequate pre-treatment consisting of dividing, drying, homogenization and grinding. Analyses of calorific value, volatile matter, ash at 5508C, moisture, bulk density, distribution of particle size, chlorine and ultimate analysis (C, H, N, S) were carried out for biomass samples by internal procedures, which are mainly based on ASTM norms for wood, refuse derived fuels and coal. With scanning electron microscope (SEM), X-ray fluorescence spectrometer (XRF) and ionic chromatography and atomic emission spectrophotometry (AES), a lot of analyses were carried out for different samples including two types of fresh bed materials, degraded bed materials and ash. The SEM uses electrons to form a three-dimensional image of the outer surface of the test materials and indicates the elemental distribution in detail. SEM with energy dispersive X-ray spectroscopy (EDS) technique was used to form image which shows the bonding part of sand particles in the bed materials and indicates the chemical composition of inner of bed materials. XRF is a spectroscopic method that is widely used to measure the elemental composition of material; the bed material samples can be tested rapidly without acidic digestion. AES is used to determine the concentrations of trace elements presented in digested sample by acidic solution. The results and discussions obtained from the above-mentioned analyses are provided in the subsequent text.
bed material samples of aluminous particles were also taken separately when the CFB cumulate operating time was 3, 7, 14, 28, 38 h. After comparing the samples of the two kinds of bed materials, the result shows that the colour of the bed materials was darker after combustion, respectively. The shape of silica bed material particle was more approximate to sphere, while that of the aluminous particles bed materials were kept originally.
XRF Analysis The result from XRF analysis of the silica bed material samples is shown in Figure 2. In this figure, aluminum, calcium, iron, magnesium, potassium, phosphorus and sodium were enriched in the bed material particles. The longer the operation was continue, the more content of the elements mentioned were found in the bed material particles. Among those increased species, the content of potassium increased most. There was almost no potassium in the initial silica bed material samples, when cumulate operation time reach 21 h, the content of potassium turned out to be 6.85%. The same result without detail was mentioned by Chirone et al. (2006). The element titanium, iron, manganese existed in the bed material particles. Potassium took the primary responsibility for the significant agglomeration formation comparing with calcium. The result from XRF analysis of the aluminous particles bed material samples is shown in Figure 3. When choosing aluminous particles as bed material, the result showed that the content of the samples increased with the time. The speed of enrichment was much lower than it did in silica bed material samples. When cumulate operation time reach 38 h, K, Ca, Mg and Na were enriched in the bed materials.
RESULTS AND ANALYSES In this study, silica and aluminous particles were used as bed materials under the same experimental condition; the temperature of the region of dense phase in FBC was controlled around 8508C the whole experimental period; The pressure was normal atmosphere and solids circulation rate was around 5 as mentioned before. Several pressure probes were set in the furnace in order to observe the fluidization in the bed. In the experiment that silica is used as bed material, when operating time lasts 8 h, the fluidization in the bed became unsteady; the pressure between the gas chamber and the top part of the furnace changed from 8000 Pa to 10 000 Pa; when the operating time accumulates to 21 h, the circulation fluidized bed could not maintain running. The bed material samples were taken separately when the running time of CFB reaches 2, 8, 11, 14 and 21 h. In the experiment that aluminous particles were used as bed materials, when the operating time accumulates to 38 h, the circulation fluidized bed still remained stable. The
Figure 2. XRF analyses of silica bed materials (wt%).
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441 –445
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Figure 5. SEM image of the edge of aluminous bed material.
Figure 3. XRF analyses of aluminous bed material (wt%).
After comparing with silica bed material, the result shows that Ca did enrich much more than it did in silica particles. Because of the different component of the two kinds of bed materials, it is hard to form agglomeration for lack of low melting point compound although there was a lot of K species in the aluminous particles bed material.
SEM/EDS Analyses Representational bed material particles were selected to do some SEM/EDS analyses. Two kinds of particles were selected: silica particles that have been being burnt for 21 h and aluminous particles that have being burned for 38 h. Figures 4 and 5 were SEM images of those two kinds bed material particles. A part of cutting surface of those two kinds of particles was photographed in order to observe clearly. In Figure 4 there is a about 4 um reaction layer on the surface of the silica particles. A huge difference between the center and the surface of the particles was appeared. The region located in the center of the particles was higher
compacted than the 4 um sticky superficial layer which was probably composed by some adsorbed ashes. Figure 5 shows that the aluminous particles were microporous. The cutting surface of aluminous particle was full of tiny holes. No interface layer was found in the particles. It seemed that aluminous particles did not react with ashes inside. After a long time operation, this kind of bed material still worked well. Figure 4 and Table 3 were the results of SEM-EDS to silica particles. In Figure 4 and Table 3, there were enriched elements Ca, Mg and K on the surface of silica particles with little S, Cl. The outside 4 um layer might be some adsorbed cotton stalk ash. In which, SiO2 in the silica particles reacted with ash from combustion to form low melting point silicate. The melting point of some silicates is lower than 7508C (Miles, 1994) . The compact region below the above 4 um layer was possibly some coagulum of melted silicates. Figure 5 and Table 3 were results of SEM-EDS to aluminous particles. In Figure 5 and Table 3, there is no layer found in section of particles. Species in the section of particles were distributed out of order. After comparing with the initial aluminous particles, content of Ca and K increased rapidly and that of Mg, Na, S increased slowly. Probably there was not many low melting point compounds were produced on the surface of aluminous particles; cotton stalk ash
Table 3. SEM/EDS of two kinds of bed material. Silica 1 Na Mg Al Si S Cl K Ca Fe O
2
Aluminous 3
Ave
1
2
1.84 0.85 0.71 1.13 3.69 2.45 11.19 2.63 4.72 6.18 2.1 1.37 1.32 0.34 5.74 2.46 7.51 10.04 12.95 23.72 21.53 19.4 8.22 11.52 1.11 1.25 0.64 1.00 0.55 3.58 0.2 0.34 0.1 0.21 0.09 0.11 3.18 7.31 9.85 6.78 26.58 27.27 28.44 21.63 13.07 21.04 18.77 5.14 1.7 0.46 1.91 1.35 2.19 1.33 38.08 41.48 41.73 40.43 32.47 37.18
3
Ave
4.99 3.71 2.89 2.12 5.37 7.64 14.31 11.35 0.72 1.61 0.01 0.06 18.8 24.21 15.34 13.08 1.44 1.65 36.16 35.27
Figure 4. SEM image of the edge of silica bed material. Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441– 445
CFB COMBUSTION BIOMASS WITH ALUMINIUM-CONTAIN BED MATERIAL could not be adsorbed, so content of elements on the surface or inner seemed have no big difference.
XRD Analysis The only mineral detected by XRD in samples of the fresh silica bed materials and the degrader silica particles is quartz (SiO2), whereas XRD analysis of the clinker shows that the quartz is transformed to the high-temperature form: cristobalite and tridymite. Other mineral could not be detected for the low concentration. This result consistent with the reported conclusion (Rong Yan et al., 2003). Some aluminous particles were also detected by XRD, the result showed that there were no KCl, NaCl, K2Ca(CO3)2 in the particles, i.e., aluminous could stop the low melting point compound from sticking to particles. Finally, it is proved to be a good bed material for CFB to combust biomass which has high concentration alkali in ash.
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. When traditional silica particles were used as bed materials, agglomeration was not only associated with gas –solid reaction between gaseous alkaline with bed material particles, but also connected to the partially melting of eutectics or silicates. Firstly, gaseous alkali metal and alkali earth metal in the ash of biomass will stick on the surface of silica particles and react with SiO2 to form eutectics or silicate with low melting point. Secondly, partially melting silicates could adsorb ashes on the surface to form a viscous layer to from a layer where agglomeration occurs when this layer is viscid enough to catch up particles. . When aluminous particles was chosen as bed materials, for not much SiO2 exists in the particles and inside the particles was full of pores, ash could not be attached on the surface of the particles and little mount of silicate with low melting point could be formed. The CFB could go steadily and well at least 72 h under our experimental condition.
REFERENCES DISCUSSION AND CONCLUSIONS In this experiment, the chosen biomass has much more alkali metals and alkaline-earth metals than coal especially in ash. Different bed materials should be selected to adapt this kind of biomass fuel. In the first 8 h of combustion traditional silica bed material was used well and in the rest experiment time it began to form agglomeration. Through XRF, SEM/EDS on the particles after combustion, alkali metal and alkaline-earth metal were found enriched on the surface of particles. Under the condition of high temperature, lot of alkali metal was released after cotton stalk was combusted (Xiaolin et al., 2005). Alkali metal contained in the fly ash would melt and became viscous. When those viscous fly ash met with silica particles, they would attach themselves to the surface of particles and reacted with SiO2 to form eutectics of silica with low melting point near 7508C. Those partial melting compounds on the surface of the particles were viscid enough to catch up two near particles. A mount of sticking particles could form agglomeration. When aluminous particles were used as bed material, the CFB has been going well in 38 h without any problem. After XRF analysis to final aluminous particles, it showed that the components in the particles changed the same way as it did in silica particles, but the changing speed of enrichment was much slower than that of silica particles. Interior of aluminous particles was found full of pores through SEM/EDS analyses. When being combusted under high temperature, cotton stalk released alkali metal into gas phase. Because of the internal structure of particles, alkali metal in the gas phase would reach the center of the particles and react with SiO2 inside the aluminous particles to form low melting point silicate. That was why the components of aluminous particles changed. Since the mount of silicate was much less than it was in silica particles and most of them exist inside the particles, it is not viscous enough to form agglomeration. From this study, we can get our conclusions. . When biomass with alkali ash was burnt in CFB, alkali metal and alkali earth metal such as K, Ca, Na would enrich in the bed materials more and more along with the process time.
Bryers, R.W., 1996, Fireside slagging, fouling and high-temperature corrosion of heat transfer surface due to impurities in steam raising fuels, Prog Energy Combust Sci, 22(1): 29–120. Carty, R., Mason, D.M. and Babu, S.P., 1988, Reaction kinetics and physical mechanisms of ash agglomeration, Final report, Doe/ MC/21313-Doe/MC/2593. Chirone, R., Miccio, F. and Scala, F., 2006, Mechanism and prediction of bed agglomeration during fluidized bed combustion of a biomass fuel: effect of the reactor scale, Chem Eng J, 123(3): 71–80. Gluckman, M.J., Yerushalmi, J. and Squires, A.M., 1976, Defluidization characteristics of sticky materials on agglomerating bed, Fluidization Technology, 2: 395– 422. Jenkins, B.M., Baxter, L.L. and Miles, T.R., 1998, Combustion properties of biomass, Fuel Process Technol, 54(1): 17– 46. Michelsen, H.P., Frandsen, F.J., Dam-Johansen, K. and Larsen, O.H., 1998, Deposition and high temperature corrosion in a 10 MW straw fired boiler, Fuel Processing Technology, 54(1–3): 95–108. Miles Jr, T.R. and Miles, T.R., 1994, Alkali deposits found in biomass power plants. Summary report for National Renewable Energy Laboratory (NREL), NREL subcontract TZ-2-11226-1, 1 –72 (NREL, USA). Miles Jr, T.R., 1994, Alkali deposits found in biomass power plants. Summary report for National Renewable Energy Laboratory (NREL), NREL subcontract TZ-2-11226-1, 1– 72 (NREL, USA). Miles, T.R., Miles Jr, T.R., Baxter, L.L., Bryers, R.W., Jenkins, B.M. and Oden, L.L., 1996, Boiler deposits from firing biomass fuels, Biomass Bioenergy, 10(2): 125–138. Nielsen, H.P., Frandsen, F.J., Dam-Johansen, K. and Baxter, L.L., 2000, The implications of chlorine-associated corrosion on the operation of biomass-fired boilers, Prog Energy Combust Sci, 26(3): 283 –298. Rong Yan, David, T.L. and Karin Laursen, 2003, Formation of bed agglomeration in a fluidized multi-waste incinerator, Fuel, 82(7): 843– 851. Skrifvars, B.J., Hupa, M., Backman, R. and Hiltunen, M., 1994, Sintering mechanisms of FBC ashes, Fuel, 73: 171– 176. Werther, J., Saenger, M., Hartge, E.U., Ogada, T. and Siagi, Z., 2000, Combustion of agricultural residues, Progress in Energy and Combustion Science, 26(1): 1 –27. Xiaolin, W., Uwe, S. and Klaus, R.G.H., 2005, Behaviour of gaseous chlorine and alkali metals during biomass thermal utilization, Fuel, 84(7– 8): 841 –848.
ACKNOWLEDGEMENTS The authors would like to thank Xiaoli Zhong, Yiqiang Mao for analyses of the samples and Jing liu for helping in English writing. The manuscript was received 26 January 2007 and accepted for publication after revision 3 April 2007.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441 –445
Abstract: Two kinds of bed materials were used as bed materials when a biomass of cotton stalk was burnt in a circulation fluidized bed (CFB) combustion pilot plant (0.2 MW). After a long time operation, silica bed materials were found sintered while aluminous bed materials slightly changed. The particles of both kinds of bed materials were sampled and three instrumental approaches were performed (XRF, SEM/EDS, XRD) to identify the bed materials in order to find out the mechanics of agglomeration. The result shows that both of the bed materials were enriched with alkali metals but the changing speed of enrichment of aluminous particles was much slower than that of silica particles. The CFB could go steadily and well at least 72 h under our experimental condition when aluminous particles were used as bed material. Agglomeration of aluminous bed materials was reduced due to the protective element aluminous. Keywords: CFB; bed materials; agglomeration; biomass; combustion.
INTRODUCTION
Correspondence to: Dr R. Liu, Key Laboratory on Clean Coal Power Generation and Combustion Technology of Ministry of Education, ThermoEnergy Engineering Research Institute, Southeast University, Nanjing 210096. E-mail: [email protected]
DOI: 10.1205/psep07010 0957–5820/07/ $30.00 þ 0.00 Process Safety and Environmental Protection Trans IChemE, Part B, September 2007 # 2007 Institution of Chemical Engineers
1996; Bryers, 1996; Michelsen et al., 1998; Nielsen et al., 2000; Werther et al., 2000). Two possible mechanisms for bed agglomeration in FBC are known (Skrifvars et al., 1994). One mechanism believed that partial melting of alkali compounds in biomass ash causes stickiness of the particles and the agglomeration of the particles. Another main sintering mechanism is chemical reaction. Chemical reaction sintering is related to gassolid reaction between gases of SO2 and CO2 and particles CaO forming dense and hard particles of CaSO4 and CaCO3. The main objective of present work was to experimentally determine the agglomeration tendencies of some common biomass in the combustion of the fluidized bed by using a new bed material based on the controlled fluidized bed agglomeration test and to compare the results with the traditional bed materials. In order to understand the formation mechanics of bed agglomeration in the FBC reactor, a full investigation of biomass and bed materials is conducted by several approaches, including X-ray fluorescence (XRF), scanning electron microscope (SEM) and X-ray diffraction (XRD). The research focused on the accumulation rate of ash alkali in the bed materials, and the mechanism by which the agglomeration takes place. This study can be utilized to predict and reduce ash sintering, the result would help
Fluidized bed combustion (FBC) is well known as an effective technology for burning both conventional and renewable solid fuel. The flexibility towards the fuel, the high combustion efficiency, and the low environmental impact are major advantages of FBC. However, during combustion of biomass fuels, a number of operation matters occur because of the different ash components. The major ash-related problem encountered in fluidized beds is bed agglomeration which is the worst case may result in operational failures with FBC. Therefore, preventing bed agglomeration is a relevant task to avoid unscheduled shutdowns and costly maintenance stops for boiler. Much attention was paid to agglomeration of bed materials in fluidized bed combustion/ gasification in the mid-seventies (Gluckman et al., 1976). For agglomeration occurring in FBC of biomass, the ash chemistry is of the most importance. It is generally believed that the elements which influence sintering and agglomeration the most are potassium, sodium, calcium, magnesium, silicon, sulphur and chlorine (Carty et al., 1988; Jenkins et al., 1998). Combustion processes with biomass which has a high content of alkaline elements are prone to experience bed agglomeration and sintering (Miles, 1994; Miles et al.,
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Vol 85 (B5) 441–445
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LIU et al.
to improve the design of combustion plants and plan strategic changes in the combustion processes.
METHODS AND MATERIALS CFB Combustion Pilot Plant and Combustion Conditions CFB combustion pilot plant, where the biomass combustion tests were made, is shown in Figure 1. The biomass supply system was consisted of a conical bin and a screw conveyor with a variable speed. The biomass which was discharged, around 30 kg h21, was 30 cm above the distributor with a bed height in shutdown around 30 cm (35 kg of bed materials). The furnace was cubic in shape, with an inner length of foursquare side 0.23 m and a height of 4 m. The primary air (60% of the total air 150 Nm3 h21) was distributed through multiple nozzles on a plate. The bed temperature and the mean fluidization velocity were maintained to 8508C + 208C and 3 m s21 during the combustion tests. The duration of each combustion test under steady state condition was over 8 h, with a start up around 2 h. Solids circulation rate is about 5 according to the current experimental condition.
Table 1. Physical and chemical characterization of biomass tested in the CFB. Density (g cm23) LHV (kJ kg21) Proximate analysis (as received, wt%) Moisture Volatiles Fixed carbon Ash Ultimate analysis (air dry basis, wt%) Carbon Hydrogen Nitrogen Sulfur Oxygen (diff.) Ash composition (wt%) CaO MgO K2O Na2O Fe2O3 Al2O3 SiO2 P2O5 TiO2 MnO
Materials
0.6 12372 8.41 68.12 21.87 1.75 43.63 5.5 0.27 0.11 48.69 17.33 9.00 32.67 3.67 1.57 3.30 6.33 7.33 0.16 0.06
Samples
A biomass fuel of cotton stalk taken from Shandong province in China was used for all experimental tests. Two kinds of particles (silica and aluminous particles) size of 0.60 mm were used as bed materials. The physical and chemical characterization of the biomass fuel used is summarized in Table 1.
In order to obtain data on the inorganic elements with relevance for bed agglomeration, some bed materials and ashes have been sampled in the atmospheric CFB combustion pilot plant. XRF analyses of two fresh bed materials were showed in Table 2. Other bed materials samples were taken
Figure 1. CFB. Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441– 445
CFB COMBUSTION BIOMASS WITH ALUMINIUM-CONTAIN BED MATERIAL
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Table 2. XRF analyses of fresh bed materials (wt%). Sample
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
K2O
MnO
P2O5
Na2O
LOI
Silica Aluminous
98.9 17.92
0.09 75.93
0.09 1.28
0.04 0.17
0.00 0.01
0.01 2.80
0.02 1.18
0.15 0.01
0.03 0.11
0.08 0.03
0.24 0.28
LOI ¼ loss on ignition at 10008C.
from the bottom of the furnace and crushed smaller than 0.074 mm in diameter.
Analytical Methods Laboratory sample of biomass, initial and final bed material, ash were obtained by an adequate pre-treatment consisting of dividing, drying, homogenization and grinding. Analyses of calorific value, volatile matter, ash at 5508C, moisture, bulk density, distribution of particle size, chlorine and ultimate analysis (C, H, N, S) were carried out for biomass samples by internal procedures, which are mainly based on ASTM norms for wood, refuse derived fuels and coal. With scanning electron microscope (SEM), X-ray fluorescence spectrometer (XRF) and ionic chromatography and atomic emission spectrophotometry (AES), a lot of analyses were carried out for different samples including two types of fresh bed materials, degraded bed materials and ash. The SEM uses electrons to form a three-dimensional image of the outer surface of the test materials and indicates the elemental distribution in detail. SEM with energy dispersive X-ray spectroscopy (EDS) technique was used to form image which shows the bonding part of sand particles in the bed materials and indicates the chemical composition of inner of bed materials. XRF is a spectroscopic method that is widely used to measure the elemental composition of material; the bed material samples can be tested rapidly without acidic digestion. AES is used to determine the concentrations of trace elements presented in digested sample by acidic solution. The results and discussions obtained from the above-mentioned analyses are provided in the subsequent text.
bed material samples of aluminous particles were also taken separately when the CFB cumulate operating time was 3, 7, 14, 28, 38 h. After comparing the samples of the two kinds of bed materials, the result shows that the colour of the bed materials was darker after combustion, respectively. The shape of silica bed material particle was more approximate to sphere, while that of the aluminous particles bed materials were kept originally.
XRF Analysis The result from XRF analysis of the silica bed material samples is shown in Figure 2. In this figure, aluminum, calcium, iron, magnesium, potassium, phosphorus and sodium were enriched in the bed material particles. The longer the operation was continue, the more content of the elements mentioned were found in the bed material particles. Among those increased species, the content of potassium increased most. There was almost no potassium in the initial silica bed material samples, when cumulate operation time reach 21 h, the content of potassium turned out to be 6.85%. The same result without detail was mentioned by Chirone et al. (2006). The element titanium, iron, manganese existed in the bed material particles. Potassium took the primary responsibility for the significant agglomeration formation comparing with calcium. The result from XRF analysis of the aluminous particles bed material samples is shown in Figure 3. When choosing aluminous particles as bed material, the result showed that the content of the samples increased with the time. The speed of enrichment was much lower than it did in silica bed material samples. When cumulate operation time reach 38 h, K, Ca, Mg and Na were enriched in the bed materials.
RESULTS AND ANALYSES In this study, silica and aluminous particles were used as bed materials under the same experimental condition; the temperature of the region of dense phase in FBC was controlled around 8508C the whole experimental period; The pressure was normal atmosphere and solids circulation rate was around 5 as mentioned before. Several pressure probes were set in the furnace in order to observe the fluidization in the bed. In the experiment that silica is used as bed material, when operating time lasts 8 h, the fluidization in the bed became unsteady; the pressure between the gas chamber and the top part of the furnace changed from 8000 Pa to 10 000 Pa; when the operating time accumulates to 21 h, the circulation fluidized bed could not maintain running. The bed material samples were taken separately when the running time of CFB reaches 2, 8, 11, 14 and 21 h. In the experiment that aluminous particles were used as bed materials, when the operating time accumulates to 38 h, the circulation fluidized bed still remained stable. The
Figure 2. XRF analyses of silica bed materials (wt%).
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441 –445
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Figure 5. SEM image of the edge of aluminous bed material.
Figure 3. XRF analyses of aluminous bed material (wt%).
After comparing with silica bed material, the result shows that Ca did enrich much more than it did in silica particles. Because of the different component of the two kinds of bed materials, it is hard to form agglomeration for lack of low melting point compound although there was a lot of K species in the aluminous particles bed material.
SEM/EDS Analyses Representational bed material particles were selected to do some SEM/EDS analyses. Two kinds of particles were selected: silica particles that have been being burnt for 21 h and aluminous particles that have being burned for 38 h. Figures 4 and 5 were SEM images of those two kinds bed material particles. A part of cutting surface of those two kinds of particles was photographed in order to observe clearly. In Figure 4 there is a about 4 um reaction layer on the surface of the silica particles. A huge difference between the center and the surface of the particles was appeared. The region located in the center of the particles was higher
compacted than the 4 um sticky superficial layer which was probably composed by some adsorbed ashes. Figure 5 shows that the aluminous particles were microporous. The cutting surface of aluminous particle was full of tiny holes. No interface layer was found in the particles. It seemed that aluminous particles did not react with ashes inside. After a long time operation, this kind of bed material still worked well. Figure 4 and Table 3 were the results of SEM-EDS to silica particles. In Figure 4 and Table 3, there were enriched elements Ca, Mg and K on the surface of silica particles with little S, Cl. The outside 4 um layer might be some adsorbed cotton stalk ash. In which, SiO2 in the silica particles reacted with ash from combustion to form low melting point silicate. The melting point of some silicates is lower than 7508C (Miles, 1994) . The compact region below the above 4 um layer was possibly some coagulum of melted silicates. Figure 5 and Table 3 were results of SEM-EDS to aluminous particles. In Figure 5 and Table 3, there is no layer found in section of particles. Species in the section of particles were distributed out of order. After comparing with the initial aluminous particles, content of Ca and K increased rapidly and that of Mg, Na, S increased slowly. Probably there was not many low melting point compounds were produced on the surface of aluminous particles; cotton stalk ash
Table 3. SEM/EDS of two kinds of bed material. Silica 1 Na Mg Al Si S Cl K Ca Fe O
2
Aluminous 3
Ave
1
2
1.84 0.85 0.71 1.13 3.69 2.45 11.19 2.63 4.72 6.18 2.1 1.37 1.32 0.34 5.74 2.46 7.51 10.04 12.95 23.72 21.53 19.4 8.22 11.52 1.11 1.25 0.64 1.00 0.55 3.58 0.2 0.34 0.1 0.21 0.09 0.11 3.18 7.31 9.85 6.78 26.58 27.27 28.44 21.63 13.07 21.04 18.77 5.14 1.7 0.46 1.91 1.35 2.19 1.33 38.08 41.48 41.73 40.43 32.47 37.18
3
Ave
4.99 3.71 2.89 2.12 5.37 7.64 14.31 11.35 0.72 1.61 0.01 0.06 18.8 24.21 15.34 13.08 1.44 1.65 36.16 35.27
Figure 4. SEM image of the edge of silica bed material. Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441– 445
CFB COMBUSTION BIOMASS WITH ALUMINIUM-CONTAIN BED MATERIAL could not be adsorbed, so content of elements on the surface or inner seemed have no big difference.
XRD Analysis The only mineral detected by XRD in samples of the fresh silica bed materials and the degrader silica particles is quartz (SiO2), whereas XRD analysis of the clinker shows that the quartz is transformed to the high-temperature form: cristobalite and tridymite. Other mineral could not be detected for the low concentration. This result consistent with the reported conclusion (Rong Yan et al., 2003). Some aluminous particles were also detected by XRD, the result showed that there were no KCl, NaCl, K2Ca(CO3)2 in the particles, i.e., aluminous could stop the low melting point compound from sticking to particles. Finally, it is proved to be a good bed material for CFB to combust biomass which has high concentration alkali in ash.
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. When traditional silica particles were used as bed materials, agglomeration was not only associated with gas –solid reaction between gaseous alkaline with bed material particles, but also connected to the partially melting of eutectics or silicates. Firstly, gaseous alkali metal and alkali earth metal in the ash of biomass will stick on the surface of silica particles and react with SiO2 to form eutectics or silicate with low melting point. Secondly, partially melting silicates could adsorb ashes on the surface to form a viscous layer to from a layer where agglomeration occurs when this layer is viscid enough to catch up particles. . When aluminous particles was chosen as bed materials, for not much SiO2 exists in the particles and inside the particles was full of pores, ash could not be attached on the surface of the particles and little mount of silicate with low melting point could be formed. The CFB could go steadily and well at least 72 h under our experimental condition.
REFERENCES DISCUSSION AND CONCLUSIONS In this experiment, the chosen biomass has much more alkali metals and alkaline-earth metals than coal especially in ash. Different bed materials should be selected to adapt this kind of biomass fuel. In the first 8 h of combustion traditional silica bed material was used well and in the rest experiment time it began to form agglomeration. Through XRF, SEM/EDS on the particles after combustion, alkali metal and alkaline-earth metal were found enriched on the surface of particles. Under the condition of high temperature, lot of alkali metal was released after cotton stalk was combusted (Xiaolin et al., 2005). Alkali metal contained in the fly ash would melt and became viscous. When those viscous fly ash met with silica particles, they would attach themselves to the surface of particles and reacted with SiO2 to form eutectics of silica with low melting point near 7508C. Those partial melting compounds on the surface of the particles were viscid enough to catch up two near particles. A mount of sticking particles could form agglomeration. When aluminous particles were used as bed material, the CFB has been going well in 38 h without any problem. After XRF analysis to final aluminous particles, it showed that the components in the particles changed the same way as it did in silica particles, but the changing speed of enrichment was much slower than that of silica particles. Interior of aluminous particles was found full of pores through SEM/EDS analyses. When being combusted under high temperature, cotton stalk released alkali metal into gas phase. Because of the internal structure of particles, alkali metal in the gas phase would reach the center of the particles and react with SiO2 inside the aluminous particles to form low melting point silicate. That was why the components of aluminous particles changed. Since the mount of silicate was much less than it was in silica particles and most of them exist inside the particles, it is not viscous enough to form agglomeration. From this study, we can get our conclusions. . When biomass with alkali ash was burnt in CFB, alkali metal and alkali earth metal such as K, Ca, Na would enrich in the bed materials more and more along with the process time.
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ACKNOWLEDGEMENTS The authors would like to thank Xiaoli Zhong, Yiqiang Mao for analyses of the samples and Jing liu for helping in English writing. The manuscript was received 26 January 2007 and accepted for publication after revision 3 April 2007.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 441 –445