- Email: [email protected]
Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part II — Model validation and parameter variation
F U E L P RO CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8) 6 60 –6 6 6
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part II — Model validation and parameter variation Priyanka Kaushal⁎, Tobias Pröll, Hermann Hofbauer Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, A-1060, Vienna, Austria
AR TIC LE I N FO
ABS TR ACT
Article history:
The two-phase combustion model for biomass char combustion in a riser of a dual fluidized
Received 25 September 2007
bed gasification unit that has been presented in part I is validated using the data obtained
Received in revised form
from the 8 MWth dual fluidized bed reactor at Guessing/Austria. The model is capable of
6 December 2007
calculating the average temperatures in all zones, the gas phase composition, solid hold up,
Accepted 10 December 2007
char feed rates and air ratio. The model predictions for the temperature profile along the riser and for the exiting gas composition are in good agreement with the measured values. The
Keywords:
simulation results show that the residual char from the gasifier is only partly converted in
Fluidized bed
the riser for char particles larger than 0.6 mm. Un-combusted char is circulated back into the
Biomass
gasification reactor. Parameter variations show that the exact location where additional
Char
liquid fuels are introduced in the middle zone of the riser does not affect the global behaviour
Combustion
of the combustion reactor. Based on the simulation results it is proposed that external supply of char (additional) may be a very effective method for reducing producer gas recycling to the riser, which is currently necessary to obtain the desired gasification temperatures. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
The development of a one dimensional two-phase steady state char combustion model for a fast fluidized bed reactor is reported in part I [1,2]. The model is capable of predicting the bed temperature, gas composition profile and the air ratio under a wide range of operating conditions. It is therefore essential to validate the model with a set of data from a running plant. One operating industrial plant is the Guessing gasifier which uses a dual fluidized bed gasification reactor. For the industrial plant only few detailed measurements are available such as temperature and flue gas composition at the exit of the riser. Nevertheless, these data can be used for model validation. After model validation the behaviour of the plant is also studied during simulated variations of the key parameters. Such a parameter variation is neither possible nor economical at the plant itself. The mathematical model turns
out to represent an important tool to understand and optimize the performance of the dual fluidized bed gasifier.
2.
Process description
The fundamental idea of the dual fluidized bed gasification system is to physically separate the gasification reaction and
⁎ Corresponding author. Tel.: +43 1 58801 15965; fax: +43 1 58801 15999. E-mail address: [email protected] (P. Kaushal). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.12.009
Fig. 1 – Scheme of dual fluidized bed gasifier.
F U E L P R O CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8 ) 6 6 0–6 6 6
661
of the gasification reactor. Therefore, additional fuels have to be introduced into the combustion reactor. These fuels can be a fraction of recycled producer gas, residues from the process (e.g. fly ash separated from the producer gas, scrubber liquid saturated with tar, etc.) or even an additional external fuel. Furthermore, several air inlet points (bottom, primary and secondary air) are realized at the industrial plant not only to get a good controllability for the combustion process but also to control bed material circulation. The high complexity of this combustion reactor makes it difficult to predict the behaviour. Therefore, a mathematical model for the combustion reactor including a number of aspects has been established to facilitate the understanding and optimization of the combustion reactor. The aim of this study was to validate the model by comparing the model prediction with literature data and with the measured values from the 8 MW (fuel power) CHP plant [3].
Fig. 2 – The mean gas phase profile along the height of riser.
the combustion reaction in order to gain a largely nitrogen-free product gas (Fig. 1). The endothermic gasification of the fuel takes place in a stationary fluidized bed (gasification reactor) and the exothermic combustion is carried out in a fast fluidized bed riser (combustion reactor). The heat transfer from the combustion reactor to the gasification reactor is obtained via the circulating bed material by closing the circulation loop between these two reactors with a cyclone followed by a loop seal and an inclined chute. Char which is available in the gasification reactor after devolatilization of the gasified biomass together with the circulating bed material is transported into the combustion reactor. Char is in most cases the main fuel but normally not enough to satisfy the heat demand
Fig. 3 – Gas profile in dense zone. B: bubble; E: emulsion.
3.
Model evaluation
3.1.
Gas composition
In Fig. 2 the overall gas composition profiles along the height of the riser are shown. The concentrations are represented in mole fractions on the y axis. The concentration of CO2 and H2O
Fig. 4 – Comparing measured and predicted gas composition.
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Fig. 5 – Char conversion profile along the height of riser.
Fig. 7 – Effect of char composition on the temperature of riser.
increases while that of CO and O2 decreases with the height of the riser. At a height of 2 m and 4 m the local concentration of O2 is increased while that of other gas components dips down. This is due to the primary and secondary air addition at the respective levels. In the middle zone (2–4 m) the concentration of CO increases steeply before subsidizing. This increase is due to the quick combustion of the additional fuel (producer gas) introduced in the middle zone. The oxygen in the emulsion phase goes down swiftly until it is completely consumed. There is some oxygen always present due to the mass transfer formulation between the phases. Though there is abundance of oxygen in the bubble phase, inefficient gas mixing between zones is one of the major weaknesses of one dimensional modelling. Nevertheless, the increased hydrogen concentration in emulsion phase shows that the gasification reactions are significant in
the emulsion phase once oxygen is depleted. Such behaviour has been reported by Adanez et al. [4], Hannes and Renz [5] and Yan et al. [6] and is also observed in Fig. 3 as a result of the mathematical model in the dense zone. Fig. 4 is drawn for three different operating conditions (C1, C2 and C3). The operating condition (i.e. the flow rates, temperatures and composition of incoming streams) varied widely between the three sets of data and therefore it is not logical to draw Fig. 4 with respect to any one operating or model parameter. Hence only the measured and predicted values are compared. The gases analysed were CO, CO2 and O2. It can be seen that the model predictions are in good agreement with the measured values. Yet the model is flexible enough with respect to fuel composition, cross-section and fluidization regime to be used for studying other combustion process as well [1,2,7].
Fig. 6 – Average temperature in the dense, middle and upper zones of riser.
Fig. 8 – Effect of char composition on the char circulation and CO concentration.
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663
the mathematical model. The model complexity with respect to the consecutive zones has been chosen just as detailed as necessary for adequate model validation by the available measurements. The average temperature of each zone predicted by the model is in excellent agreement with the measured temperature as shown in Fig. 6.
4.
Parameter variation
In the sections below, the most critical, uncertain and sensitive parameters are varied individually to observe their influence on the performance of the riser.
4.1.
Fig. 9 – Effect of char size on the char circulation rates.
3.2.
Char conversion profile
Fig. 5 shows the reaction profile of the char along the height of the riser. A major portion of char conversion is contributed by the combustion reaction. Apart from that there is always a constant amount of char that is gasified. If Figs. 2 and 5 are compared it can be seen that at the exit of the riser where the oxygen concentration is low and the steam fraction has increased, the ratio between amount of char gasified and combusted is ~1:1. Fig. 5 also shows that the amount of char that is combusted varies along the height of the riser and is a strong function of the oxygen concentration.
3.3.
Temperature profile
Actual temperature profiles are available from measurements done at the industrial plant and are compared to results from
Char composition
In the overall process wood chips are introduced into the gasifier. Inside the gasifier the wood chips undergo the thermal conversion steps of drying, devolatilization and partial gasification. However the solid residence time in the gasifier does not allow for complete conversion and residual char is transported to the riser with the circulating bed material. The exact composition of this char is not known. Therefore, it is important to study the effect of variations in char composition. The raw biomass contains ~50% carbon and therefore in this simulation the carbon concentration (weight carbon/weight char) has been varied from 50% to 100% carbon. There is no prominent effect of char composition on the average temperature of the three zones. However, it is interesting to see the temperature effect in the dense zone when char is 100% carbon. At this point, the average temperature of the dense zone goes down. This is a model limitation and it can be explained as: In the lack of hydrogen and oxygen (char is 100% carbon) there is no steam present. Hence, once the oxygen in emulsion is consumed (Fig. 7) almost all reactions stop and the dense zone begins to cool while the remaining char is transported to the middle zone. In this case more char is pushed into the middle zone where apart from primary air and secondary fuels, steam (in form of emulsion) is also added and
Fig. 10 – Effect of operating parameter on CO concentration in flue gas.
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Fig. 11 – Relation between producer gas and char for constant air ratio (1.02).
reaction rates are accelerated. The higher char input to the middle zone leads to a higher mean temperature of the middle zone though the final exit temperature remains almost the same and is not much affected by the char composition. The CO concentration in the flue gas is a strong function of char composition and it increases with increasing carbon concentration in the char (Fig. 8). This simulation result also shows that with increasing carbon percentage the necessary mass flow of char is decreased.
4.2.
Char size
Among all the char properties the most effective and sensitive parameter is the mean char size. The effect of char size has been studied for a size range of 0.5 mm to 30 mm. Assuming
constant carbon conversion (constant air ratio), the feed rate of char increases with increasing diameter of char particles (Fig. 9) and except for very small diameters the char particles are not completely combusted in the riser and are, therefore, partly recycled into the gasifier. During the shut down operation of the industrial plant pieces of solid char can be actually found in the return leg of the cyclone. For char sizes below 0.5 mm, the incoming char is completely combusted inside the riser and there is no char recycled back to the gasifier in this case. In Fig. 10 each curve is drawn at constant char input with increasing char diameter. The calculated CO content is a function of the air ratio and drops as soon as complete char conversion in the riser is obtained, i.e. if no solid carbon is present in the riser exit cross-section. The point of inflection, where CO concentration begins to drop shows the maximum size for a given char feed rate for complete combustion (i.e. no recirculation of char). Fig. 10 shows that for small sized char particles CO concentration is a strong function of both size and air ratio, whereas for big char particles it is almost independent of char size and depends on the air ratio only.
4.3.
External char supply and recycled producer gas
To maintain and to control the temperature of the riser a part of the clean producer gas is recycled into the riser whereas the economy and efficiency of the plant suggest that a maximum amount of producer gas should be used to produce electricity. The effect of the recycled producer gas feed rate on the char circulation is shown in Fig. 11. For a constant air ratio λ (i.e. constant char conversion) the amount of char circulation increases as the producer gas feed rate decreases. The relation between char circulation and producer gas feed rate is almost linear. Theoretically, if the producer gas supply is reduced to zero, nearly 1000 kg/h of char must be supplied (externally and explicitly) in the riser with the circulating bed material in order to maintain the gasification temperature.
Fig. 12 – Effect of average temperature of gasifier on temperature and char rates of riser.
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Table 1 – Model prediction for the equal distribution profile defined in Fig. 14 Parameters
Char hold up [kg] Temperature [°C] Char rate [kg/h] Flue gas: yCO yCO2 yO2 yH2O
Fig. 13 – Effect of average temperature of gasifier on CO concentration of flue gas.
Dense zone
Middle zone
2.54 859 1070
1.7 896
Upper zone 3.4 932 675 0.00241 0.178 0.005 0.0958
temperature dependency of the char conversion reaction rates. There is also an acute effect of the average temperature of the gasifier on the CO composition in the flue gas. It can be seen in Fig. 13 that the CO concentration decreases with increasing temperature. A similar result is also reported by Ducarne et al. [8].
4.5. 4.4.
Value
Mixing pattern of spent scrubber liquid
Temperature of the gasifier
The temperature of the gasifier determines the temperature of the bed material and char feed stream to the riser. The solid feed stream temperature is assumed to be equal to the measured bed temperature in the gasifier. The simulation results show a significant effect of the average temperature of the gasifier on the performance of the riser (Fig. 12). For a constant air ratio of λ = 1.02 it has been found that, when the average temperature of the gasifier is increased the necessary char circulation decreases while the average bed temperature in the three zones increases along with the increasing gasifier temperature. The reason for the decrease in necessary char circulation with increasing operating temperature is the
The spent, tar loaded scrubber liquid, which consists of rape seed oil methyl ester (RME), biomass tars and water, is cocombusted in the fluidized bed combustion reactor (i.e. in the riser). The location in the riser where this emulsion is introduced falls in the height range of the middle zone of the model. The mixing pattern of the evaporating liquid in the fast fluidized bed is not known exactly. Therefore, different mixing patterns are compared in simulation. The terms increasing, decreasing and parabolic in Fig. 14 mean that the emulsion added in the cells of the middle zone (part I) increases, decreases or first increases then decreases (parabolic) along the height of the middle zone. As shown in Fig. 14, the steam profile is only affected in the middle zone of the riser. At the boundary of the middle zone the gas composition is similar in all cases. Table 1 lists the key model parameter for the default case (i.e. for equal distribution). It is safe to say that the values reported in Table 1 are also true for other mixing patterns, since the relative error in the values reported, for different mixing patterns is always below 0.1%. Therefore it can be said that evaporation and mixing patterns in the middle zones have no effect on the overall performance of the riser. Hence it does not matter where exactly the liquid is introduced and evaporated within the middle zone.
5.
Fig. 14 – Mixing profile of emulsion, 1: equal; 2: increasing; 3: parabolic; 4: decreasing.
Conclusions
Model validation was performed using the measured values of 8 MWth dual fluidized bed biomass gasification plant at Guessing/Austria and qualitatively from literature. Following the few measurements available from the industrial plant (e.g. temperature profile, exit flue gas composition) a good agreement between the model results and the measurement can be found. Hence it can be concluded that the reported results are preliminary and further validation studies are needed. Concerning the char properties only the net char combusted was
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known from the plant balancing while the actual particle size of char and the char flow rate are the unknown parameters. It was observed for the typical operating conditions at Guessing that for mean char particle sizes N0.6 mm, the incoming char is only partly converted in the riser and is partly circulated back to the gasification reactor. Model results show that within the operating range, CO is always below 1 vol.% in the primary flue gas from the riser. The mathematical model is flexible enough with respect to fuel composition, bed geometry and fluidization regime and can be used for studying another combustion process as well.
Acknowledgement The authors gratefully acknowledge the financial support from RENET Austria (Knet/Kind public funds program, Austria).
REFERENCES [1] P. Kaushal, T. Proell, H. Hofbauer, Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: part I — model development and sensitivity analysis, Fuel Processing Technology (2007).
[2] P. Kaushal, T. Proell, H. Hofbauer, Model development and validation: co-combustion of residual char, gases and volatile fuels in the fast fluidized combustion chamber of a dual fluidized bed biomass gasifier, Fuel 86 (17–18) (2007) 2687–2695. [3] H. Hofbauer, Scale up of fluidized bed gasifiers from laboratory scale to commercial plants: steam gasification of solid biomass in a dual fluidized bed system, 19th FBC Conference, 21–24 May, Vienna, Austria, 2006. [4] J. Adanez, P. Gayan, L.F. de Diego, F. Garcia-Labiano, A. Abad, Combustion of wood chips in a CFBC. Modelling and validation, Industrial and Engineering Chemistry Research 42 (5) (2003) 987–999. [5] J. Hannes, U. Renz, The IEA Model for Circulating Fluidized Bed Combustion. Fluidized Bed Combustion-1, ASME, 1995. [6] H. Yan, C. Heidenreich, D. Zhang, Mathematical modelling of a bubbling fluidised-bed coal gasifier and the significance of ‘net flow’, Fuel 77 (9–10) (1998) 1067–1079. [7] L. Amand, A. Lyngfelt, M. Karlsson, B. Leckner, Fuel Loading of a Fluidized Bed Combustor Burning Bituminous Coal, Peat or Wood Chips. Report A97-221, Departrment of Energy Conversion, Chalmers University of Technology, Goeteborg, Sweden, 1999 http://www.entek.chalmers.se/~leam/ Input_model/Fuel_loading1/Charload_rev1.pdf. [8] D.E. Ducarne, J.C. Dolignier, E. Marty, G. Martin, L. Delfosse, Modelling of gaseous pollutants emissions in circulating fluidized bed combustion of municipal refuse, Fuel 77 (13) (1998) 1399–1410.
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part II — Model validation and parameter variation Priyanka Kaushal⁎, Tobias Pröll, Hermann Hofbauer Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, A-1060, Vienna, Austria
AR TIC LE I N FO
ABS TR ACT
Article history:
The two-phase combustion model for biomass char combustion in a riser of a dual fluidized
Received 25 September 2007
bed gasification unit that has been presented in part I is validated using the data obtained
Received in revised form
from the 8 MWth dual fluidized bed reactor at Guessing/Austria. The model is capable of
6 December 2007
calculating the average temperatures in all zones, the gas phase composition, solid hold up,
Accepted 10 December 2007
char feed rates and air ratio. The model predictions for the temperature profile along the riser and for the exiting gas composition are in good agreement with the measured values. The
Keywords:
simulation results show that the residual char from the gasifier is only partly converted in
Fluidized bed
the riser for char particles larger than 0.6 mm. Un-combusted char is circulated back into the
Biomass
gasification reactor. Parameter variations show that the exact location where additional
Char
liquid fuels are introduced in the middle zone of the riser does not affect the global behaviour
Combustion
of the combustion reactor. Based on the simulation results it is proposed that external supply of char (additional) may be a very effective method for reducing producer gas recycling to the riser, which is currently necessary to obtain the desired gasification temperatures. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
The development of a one dimensional two-phase steady state char combustion model for a fast fluidized bed reactor is reported in part I [1,2]. The model is capable of predicting the bed temperature, gas composition profile and the air ratio under a wide range of operating conditions. It is therefore essential to validate the model with a set of data from a running plant. One operating industrial plant is the Guessing gasifier which uses a dual fluidized bed gasification reactor. For the industrial plant only few detailed measurements are available such as temperature and flue gas composition at the exit of the riser. Nevertheless, these data can be used for model validation. After model validation the behaviour of the plant is also studied during simulated variations of the key parameters. Such a parameter variation is neither possible nor economical at the plant itself. The mathematical model turns
out to represent an important tool to understand and optimize the performance of the dual fluidized bed gasifier.
2.
Process description
The fundamental idea of the dual fluidized bed gasification system is to physically separate the gasification reaction and
⁎ Corresponding author. Tel.: +43 1 58801 15965; fax: +43 1 58801 15999. E-mail address: [email protected] (P. Kaushal). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.12.009
Fig. 1 – Scheme of dual fluidized bed gasifier.
F U E L P R O CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8 ) 6 6 0–6 6 6
661
of the gasification reactor. Therefore, additional fuels have to be introduced into the combustion reactor. These fuels can be a fraction of recycled producer gas, residues from the process (e.g. fly ash separated from the producer gas, scrubber liquid saturated with tar, etc.) or even an additional external fuel. Furthermore, several air inlet points (bottom, primary and secondary air) are realized at the industrial plant not only to get a good controllability for the combustion process but also to control bed material circulation. The high complexity of this combustion reactor makes it difficult to predict the behaviour. Therefore, a mathematical model for the combustion reactor including a number of aspects has been established to facilitate the understanding and optimization of the combustion reactor. The aim of this study was to validate the model by comparing the model prediction with literature data and with the measured values from the 8 MW (fuel power) CHP plant [3].
Fig. 2 – The mean gas phase profile along the height of riser.
the combustion reaction in order to gain a largely nitrogen-free product gas (Fig. 1). The endothermic gasification of the fuel takes place in a stationary fluidized bed (gasification reactor) and the exothermic combustion is carried out in a fast fluidized bed riser (combustion reactor). The heat transfer from the combustion reactor to the gasification reactor is obtained via the circulating bed material by closing the circulation loop between these two reactors with a cyclone followed by a loop seal and an inclined chute. Char which is available in the gasification reactor after devolatilization of the gasified biomass together with the circulating bed material is transported into the combustion reactor. Char is in most cases the main fuel but normally not enough to satisfy the heat demand
Fig. 3 – Gas profile in dense zone. B: bubble; E: emulsion.
3.
Model evaluation
3.1.
Gas composition
In Fig. 2 the overall gas composition profiles along the height of the riser are shown. The concentrations are represented in mole fractions on the y axis. The concentration of CO2 and H2O
Fig. 4 – Comparing measured and predicted gas composition.
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Fig. 5 – Char conversion profile along the height of riser.
Fig. 7 – Effect of char composition on the temperature of riser.
increases while that of CO and O2 decreases with the height of the riser. At a height of 2 m and 4 m the local concentration of O2 is increased while that of other gas components dips down. This is due to the primary and secondary air addition at the respective levels. In the middle zone (2–4 m) the concentration of CO increases steeply before subsidizing. This increase is due to the quick combustion of the additional fuel (producer gas) introduced in the middle zone. The oxygen in the emulsion phase goes down swiftly until it is completely consumed. There is some oxygen always present due to the mass transfer formulation between the phases. Though there is abundance of oxygen in the bubble phase, inefficient gas mixing between zones is one of the major weaknesses of one dimensional modelling. Nevertheless, the increased hydrogen concentration in emulsion phase shows that the gasification reactions are significant in
the emulsion phase once oxygen is depleted. Such behaviour has been reported by Adanez et al. [4], Hannes and Renz [5] and Yan et al. [6] and is also observed in Fig. 3 as a result of the mathematical model in the dense zone. Fig. 4 is drawn for three different operating conditions (C1, C2 and C3). The operating condition (i.e. the flow rates, temperatures and composition of incoming streams) varied widely between the three sets of data and therefore it is not logical to draw Fig. 4 with respect to any one operating or model parameter. Hence only the measured and predicted values are compared. The gases analysed were CO, CO2 and O2. It can be seen that the model predictions are in good agreement with the measured values. Yet the model is flexible enough with respect to fuel composition, cross-section and fluidization regime to be used for studying other combustion process as well [1,2,7].
Fig. 6 – Average temperature in the dense, middle and upper zones of riser.
Fig. 8 – Effect of char composition on the char circulation and CO concentration.
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663
the mathematical model. The model complexity with respect to the consecutive zones has been chosen just as detailed as necessary for adequate model validation by the available measurements. The average temperature of each zone predicted by the model is in excellent agreement with the measured temperature as shown in Fig. 6.
4.
Parameter variation
In the sections below, the most critical, uncertain and sensitive parameters are varied individually to observe their influence on the performance of the riser.
4.1.
Fig. 9 – Effect of char size on the char circulation rates.
3.2.
Char conversion profile
Fig. 5 shows the reaction profile of the char along the height of the riser. A major portion of char conversion is contributed by the combustion reaction. Apart from that there is always a constant amount of char that is gasified. If Figs. 2 and 5 are compared it can be seen that at the exit of the riser where the oxygen concentration is low and the steam fraction has increased, the ratio between amount of char gasified and combusted is ~1:1. Fig. 5 also shows that the amount of char that is combusted varies along the height of the riser and is a strong function of the oxygen concentration.
3.3.
Temperature profile
Actual temperature profiles are available from measurements done at the industrial plant and are compared to results from
Char composition
In the overall process wood chips are introduced into the gasifier. Inside the gasifier the wood chips undergo the thermal conversion steps of drying, devolatilization and partial gasification. However the solid residence time in the gasifier does not allow for complete conversion and residual char is transported to the riser with the circulating bed material. The exact composition of this char is not known. Therefore, it is important to study the effect of variations in char composition. The raw biomass contains ~50% carbon and therefore in this simulation the carbon concentration (weight carbon/weight char) has been varied from 50% to 100% carbon. There is no prominent effect of char composition on the average temperature of the three zones. However, it is interesting to see the temperature effect in the dense zone when char is 100% carbon. At this point, the average temperature of the dense zone goes down. This is a model limitation and it can be explained as: In the lack of hydrogen and oxygen (char is 100% carbon) there is no steam present. Hence, once the oxygen in emulsion is consumed (Fig. 7) almost all reactions stop and the dense zone begins to cool while the remaining char is transported to the middle zone. In this case more char is pushed into the middle zone where apart from primary air and secondary fuels, steam (in form of emulsion) is also added and
Fig. 10 – Effect of operating parameter on CO concentration in flue gas.
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Fig. 11 – Relation between producer gas and char for constant air ratio (1.02).
reaction rates are accelerated. The higher char input to the middle zone leads to a higher mean temperature of the middle zone though the final exit temperature remains almost the same and is not much affected by the char composition. The CO concentration in the flue gas is a strong function of char composition and it increases with increasing carbon concentration in the char (Fig. 8). This simulation result also shows that with increasing carbon percentage the necessary mass flow of char is decreased.
4.2.
Char size
Among all the char properties the most effective and sensitive parameter is the mean char size. The effect of char size has been studied for a size range of 0.5 mm to 30 mm. Assuming
constant carbon conversion (constant air ratio), the feed rate of char increases with increasing diameter of char particles (Fig. 9) and except for very small diameters the char particles are not completely combusted in the riser and are, therefore, partly recycled into the gasifier. During the shut down operation of the industrial plant pieces of solid char can be actually found in the return leg of the cyclone. For char sizes below 0.5 mm, the incoming char is completely combusted inside the riser and there is no char recycled back to the gasifier in this case. In Fig. 10 each curve is drawn at constant char input with increasing char diameter. The calculated CO content is a function of the air ratio and drops as soon as complete char conversion in the riser is obtained, i.e. if no solid carbon is present in the riser exit cross-section. The point of inflection, where CO concentration begins to drop shows the maximum size for a given char feed rate for complete combustion (i.e. no recirculation of char). Fig. 10 shows that for small sized char particles CO concentration is a strong function of both size and air ratio, whereas for big char particles it is almost independent of char size and depends on the air ratio only.
4.3.
External char supply and recycled producer gas
To maintain and to control the temperature of the riser a part of the clean producer gas is recycled into the riser whereas the economy and efficiency of the plant suggest that a maximum amount of producer gas should be used to produce electricity. The effect of the recycled producer gas feed rate on the char circulation is shown in Fig. 11. For a constant air ratio λ (i.e. constant char conversion) the amount of char circulation increases as the producer gas feed rate decreases. The relation between char circulation and producer gas feed rate is almost linear. Theoretically, if the producer gas supply is reduced to zero, nearly 1000 kg/h of char must be supplied (externally and explicitly) in the riser with the circulating bed material in order to maintain the gasification temperature.
Fig. 12 – Effect of average temperature of gasifier on temperature and char rates of riser.
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Table 1 – Model prediction for the equal distribution profile defined in Fig. 14 Parameters
Char hold up [kg] Temperature [°C] Char rate [kg/h] Flue gas: yCO yCO2 yO2 yH2O
Fig. 13 – Effect of average temperature of gasifier on CO concentration of flue gas.
Dense zone
Middle zone
2.54 859 1070
1.7 896
Upper zone 3.4 932 675 0.00241 0.178 0.005 0.0958
temperature dependency of the char conversion reaction rates. There is also an acute effect of the average temperature of the gasifier on the CO composition in the flue gas. It can be seen in Fig. 13 that the CO concentration decreases with increasing temperature. A similar result is also reported by Ducarne et al. [8].
4.5. 4.4.
Value
Mixing pattern of spent scrubber liquid
Temperature of the gasifier
The temperature of the gasifier determines the temperature of the bed material and char feed stream to the riser. The solid feed stream temperature is assumed to be equal to the measured bed temperature in the gasifier. The simulation results show a significant effect of the average temperature of the gasifier on the performance of the riser (Fig. 12). For a constant air ratio of λ = 1.02 it has been found that, when the average temperature of the gasifier is increased the necessary char circulation decreases while the average bed temperature in the three zones increases along with the increasing gasifier temperature. The reason for the decrease in necessary char circulation with increasing operating temperature is the
The spent, tar loaded scrubber liquid, which consists of rape seed oil methyl ester (RME), biomass tars and water, is cocombusted in the fluidized bed combustion reactor (i.e. in the riser). The location in the riser where this emulsion is introduced falls in the height range of the middle zone of the model. The mixing pattern of the evaporating liquid in the fast fluidized bed is not known exactly. Therefore, different mixing patterns are compared in simulation. The terms increasing, decreasing and parabolic in Fig. 14 mean that the emulsion added in the cells of the middle zone (part I) increases, decreases or first increases then decreases (parabolic) along the height of the middle zone. As shown in Fig. 14, the steam profile is only affected in the middle zone of the riser. At the boundary of the middle zone the gas composition is similar in all cases. Table 1 lists the key model parameter for the default case (i.e. for equal distribution). It is safe to say that the values reported in Table 1 are also true for other mixing patterns, since the relative error in the values reported, for different mixing patterns is always below 0.1%. Therefore it can be said that evaporation and mixing patterns in the middle zones have no effect on the overall performance of the riser. Hence it does not matter where exactly the liquid is introduced and evaporated within the middle zone.
5.
Fig. 14 – Mixing profile of emulsion, 1: equal; 2: increasing; 3: parabolic; 4: decreasing.
Conclusions
Model validation was performed using the measured values of 8 MWth dual fluidized bed biomass gasification plant at Guessing/Austria and qualitatively from literature. Following the few measurements available from the industrial plant (e.g. temperature profile, exit flue gas composition) a good agreement between the model results and the measurement can be found. Hence it can be concluded that the reported results are preliminary and further validation studies are needed. Concerning the char properties only the net char combusted was
666
F U E L P RO CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8) 6 60 –6 6 6
known from the plant balancing while the actual particle size of char and the char flow rate are the unknown parameters. It was observed for the typical operating conditions at Guessing that for mean char particle sizes N0.6 mm, the incoming char is only partly converted in the riser and is partly circulated back to the gasification reactor. Model results show that within the operating range, CO is always below 1 vol.% in the primary flue gas from the riser. The mathematical model is flexible enough with respect to fuel composition, bed geometry and fluidization regime and can be used for studying another combustion process as well.
Acknowledgement The authors gratefully acknowledge the financial support from RENET Austria (Knet/Kind public funds program, Austria).
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