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Modelling of fixed bed combustion
Fuel Processing Technology, 36 (1993) 55-63 Elsevier Science Publishers B.V., Amsterdam
55
Modelling of fixed bed combustion N W J Ford, M J Cooke and P W Sage Coal Research Establishment, British Coal Corporation, Stoke Orchard, Cheltenham, Gloucestershire, GL52 4RZ, United Kingdom
Abstract As part of an investigation of methods for the control of sulphur dioxide emissions from stoker-fired plant, a computer-based model of combustion within the fuel bed was developed. The model was based on an existing model of coal combustion and developed to enable consideration of the fuel bed conditions in a travelling grate stoker. The model considers the fuel from the point of entry on the grate through devolatilisation, combustion and ash cooling. Heat and material balances are carried out throughout the fuel bed at selected time intervals. This enables prediction of combustion gas composition and temperature at any point within the fuel bed. As input the model requires details of coal composition, along with basic plant operational parameters such as grate speed, excess air and fuel burning rate. The model was used to predict fuel bed combustion gas composition and temperature profiles. Comparison with data obtained from operational plant and a novel fixed grate simulator showed good agreement with model predictions giving confidence in its representativeness.
1.
INTRODUCTION
As part of an investigation of methods for the control of sulphur dioxide (SO2) emissions from stoker-fired plant, the need for a means of predicting fuel bed conditions was identified. Practical studies, using a fixed grate simulator, had identified fuel bed conditions which were conducive to the absorption of SO2 by calcium-based sorbent. The need was to identify the operational arrangement which enhanced such conditions and hence enhanced SO2 removal. It was not considered that further practical studies on the fixed grate simulator would be particularly helpful, rather it was decided that a more efficient and appropriate approach would be the development of a predictive mathematical fuel bed model. With such a model a range of operating conditions could be studied and the resulting fuel bed conditions assessed to identify those which would be likely to favour sulphur capture. The fixed grate simulator could then be operated under the conditions identified to determine whether there was potential for an improvement in sulphur capture. It was considered that the model should be able to provide detailed predictions of fuel bed temperature and gas and solid composition throughout the bed. A literature survey of available models showed that most were not sufficiently detailed to enable this degree of analysis. A model by Hougen and Watson I was identified as providing an appropriate treatment of the major combustion reactions, albeit in a somewhat simplified manner. This model was selected for subsequent modification to enable a more detailed treatment of the fuel bed and to include other minor reactions which were of interest in the overall study.
56
j.~//
i~ ~/
f
~ dr
x~r~
x ./I
L J\
.-
]GHT
o)
sP
j/ I
i lED
•
/ /
,t ~
.-IL) J /
.
.- j ...i()> /.. ...-
f~
..-
~ J \ J ~_j~,~j I
•
TIME
(t)
COMBUSTION AIR
Figure i.
General Arrangement of Fuel Bed Structure
\/ l+i
,
,~/ /
i,t-I arlJcle
i,
t
l~oss
=
t-I
MASS
._/
i,t
I
,o~,
x i+I . %+I
i+l , t
t-I
i-I
, t
I-I , t+l
__/ Figure 2.
General Procedure for Model Calculations
...
57 2.
M O D E L DEVELOPMENT
2.1 Fuel Bed Structure For the purposes of this model the fuel bed is represented as a series of boxes (see Figure 1). At the front end of the grate where fresh coal enters, the coal is assumed to consist of spherical lumps of uniform size, stacked on top of each other. Each lump is considered to reside within a cube or box. Combustion air entering through the grate contacts the bottom box first and then flows up through the bed to the top box. For modelling purposes one stack of lumps is considered to be typical of all the stacks constituting the bed. The modelling calculations are therefore conducted on a bed area eqtiivalent to the base area of the initial box. Reactions within the box are considered over a time period of one minute, although this can be varied depending on the accuracy of modelling required. Thus for the first minute, material and heat balances are conducted on the first stack of boxes and solid and gas compositions along with temperature, are calculated for each box in the stack. At the beginning of the second minute the stack is considered to have moved down the grate (to a position equivalent to the specified grate speed and residence time). The lumps having reacted may have decreased in size and the boxes are therefore redrawn. The base dimensions remain the same as the original box (ie equivalent to the diameter of the initial coal lumps). However the height of the box changes to reflect the reduction in diameter of the particle after reaction. The calculation process is then repeated. Generally the time taken for coal entering an industrial stoker-fired boiler to pass through the furnace is around 40 minutes. In modelling this situation calculations might be carried out at minute intervals and thus 40 stacks would be considered The solid composition of the stack remaining after the specified combustion period corresponds to the ash remaining at the end of the grate of a 'real' stoker fired boiler. 2.2 Combustion and Associated Reactions As in the Hougen and Watson 1 model, the following three main stages are considered in the combustion of lump coal: a.
Oxygen in the main stream diffuses to the surface of the coal lump where it reacts to form carbon monoxide (CO) or carbon dioxide CO2. C
+ 02 ~
co 2
c + '/2 o 2 --, c o
(i)
(2)
Hougen and Watson I assume that due to the relatively high temperature involved the chemical reactions on the surface of the coal lump are so fast that chemical equilibrium between carbon (C), CO, CO2 and oxygen (02) is maintained. This indicates a diffusion controlled reaction. For completeness the chemical reaction control rate is also considered since, in some instances, bed temperatures are lower than the 1000°C suggested by Hougen and Watson I as giving a zero equilibrium concentration of both
co2 and 02.
58 b.
Carbon monoxide produced during combustion diffuses from the solid/gas interface into the gas stream where it may react with oxygen, if available, to produce CO~: 2C0
C.
+ 02 ~ 2 C02
(3)
As the carbon dioxide content of the gas stream increases (via 3), it diffuses to the solid/gas interface where it is reduced to CO: CO z + C ~ CO
(4)
Again due to the relatively high temperature involved at the solid/gas interface Hougen and Watson ~ consider that the final products of the above reaction approach their equilibrium composition containing virtually no CO2 and the reaction is therefore assumed to be diffusion controlled. The volatile matter associated with the coal is assumed to be released at various temperatures. The water associated with the coal is released once the temperature reaches 200°C with the remaining volatile species being released at 700°C 2. These species are assumed to pass through the fuel bed and bum above it. During combustion, under oxidising conditions, hydrogen and oxygen are assumed to combine to form water, whilst nitrogen and sulphur are released as N2 and SO2 respectively. Once the oxygen becomes limited, the hydrogen is assumed to combine with sulphur to produce H2S and any remaining is released as H2. Since hydrogen is present in the original coal in a larger quantity than sulphur there is always a surplus. Hydrogen, oxygen, nitrogen and sulphur are assumed to be released from the coal at a rate equivalent to the release of carbon. These along with CO and CO2, are the only gaseous species considered in the model. After release from the coal the resulting gases are assumed to join the bulk gas which is passing through the bed. Carbon bearing gases are assumed to react as in 3 and 4. Of the remaining gases nitrogen and water, once formed, are assumed to pass through the bed without further reaction. H2, H2S and SO2 are considered to be present at their equilibrium concentration. Thus as the bulk gas stream passes into a box the above concentrations are recalculated to account for the change composition of the bulk gas due to the addition of gases from the reaction of the coal and reaction within the bulk gas due to changes in composition and temperature. 2.3 Heat Balances In the Hougen and Watson I model a constant bed temperature is assumed and as such no heat balances are required. Since the rate of the reactions considered in the current model are dependent on temperature it was considered that the model would be more representative of a 'real' fuel bed if the temperature with each box were allowed to vary based on a heat balance.
59 Each box is considered to be subject to several sources of heat loss and heat gain: Heat transfer due to reaction Heat transfer from boxes above and below The purpose of the heat balance is to determine the temperature within the box, which will in turn determine the rates of reaction. This entails an iterative procedure where temperatures are selected, heat generation and. loss by subsequent reaction calculated and balanced with the heat gain and loss due to heat transfer. This balance is carded out for each box in the stack and for each selected time interval. Figure 2 illustrates the general arrangement of coal particles and gas flows within the model.
3.
M O D E L VALIDATION
Available literature does not provide sufficient information for the validation of this model. This is mainly due to the inherent difficulty in measuring fuel bed conditions on operational plant. In the absence of suitable existing data with which to validate the model a series of tests on a fixed grate simulator were conducted. The objective was to obtain detailed information on the composition of a stoker fuel bed to determine the representativeness of the model. Fixed grate simulators enable the progress of combustion to be reproduced by subjecting a mass of coal to the range of operating conditions that it would experience in a 'real' plant. The simulator used is considered to provide a reasonable representation of a 'real' plant3. The test programme featured the simulator operating under conditions typical of stokerfired plant. Combustion was quenched, using nitrogen, after a specified time. The bed was then removed in sections and each section submitted for chemical analysis. In total eight tests were conducted with combustion being quenched after 5, I0, 15, 20, 25, 30, 35 and 40 minutes. (In a 'real' stoker this would correspond to distance along the grate.) The chemical analyses of the various sections provided data on the mechanism and progress of combustion on the grate which could be compared with the predictions of the model. The model was run for the same overall operating conditions as for the testwork above. It was then possible to compare the results of the model with those obtained experimentally. Figure 3 compares the measured release of carbon from the coal with that predicted by the model. It may be seen that the release rate predicted by the model are in good agreement with that obtained in the testwork. Figure 4 compares the measured fuel bed temperature (two thermocouples at 40 mm and 100 mm above the grate) with that predicted by the model at these locations. Again reasonable agreement is shown between the measured and predicted values. Although it has not been possible to compare measured gas compositions within the fuel bed with predicted values, it is considered that the agreement shown between measured and predicted solid composition and bed temperature profiles suggests that the predicted gas composition profiles are also likely to be in agreement. Figure 5 shows the predicted concentration profile of 02 within the fuel bed. It may be seen that the general trends exhibited agree well with the generally accepted mode of combustion in a stoker fuel bed.
60
RELEASE (% of input)
1 O0
80
"-*- MEASURED -~ PREDICTED
40
2O
0
5
10
15
20
25
30
35
40
TIME (rains)
FIGURE 3 PREDICTED RELEASE OF CARBON FROM COAL
TEMPERATURE
(*C)
1,600
1,400
~*l.u~PLi ,*gmo.. *J,ovl **,TII
1,000
100mm -e- 4 0 r a m
800
-w" 4 0 m r n 600
........
400
......
'. . . . . . . . . . . . . . . .
-i..
" -lOOmm
MODEL MODEL
..,"
,oo ._.:.:,,::,j. :j .................. 0
6
1O
15
20 TIME
25
30
35
40
(rain)
FIGURE 4 COMPARISON
OF F U E L BED T E M P E R A T U R E S
61
1 60
140 1 20 ~100 I--i[]
~
80
6O 40 20 0
0
5
10
15
20
25
30
35
TIME (mln) OXYGEN CONCENTRATION (% by volumo) - - B E D HT ~ > 2 0 %
FIGURE
EE16-20% ~ 1 0 - 1 6 %
5 PREDICTED
FUEL
~6-10%
BED
~1-6%
OXYGEN
~<:1%
PROFILE
40
62
The general good agreement demonstrated between the model predictions and measured data provides confidence in the representativeness of the model.
4.
CONCLUSIONS
A computer model intended to enable the prediction of the fuel bed conditions in stokerfired plant has been developed. Comparison with experimental data derived from a fixed grate simulator indicates that the model predictions are representative of the fuel bed conditions during typical stoker firing.
5.
ACKNOWLEDGEMENTS
The work upon which this paper is based was funded jointly by British Coal and the European Coal and Steel Community. The views expressed are those of the authors, and do not necessarily reflect those of British Coal.
6.
REFERENCES
1.
O A Hougen and K M Watson, Chemical Process Principles - Combined Volume, John Wiley and Sons, London, 1950. R T Haslam and R P Russel, Fuels and their Combustion, McGraw-Hill Book Company, New York, 1926. N Ford, M J Cooke and M D Pettit, The Use of a Laboratory Furnace to Simulate Industrial Stoker-Fired Plant, Jnl Inst Energy, Vol LXV No 464, September 1992.
2. 3.
63
Discussion Modeling of fixed bed combustion N.W.J. Ford, M.J. Cooke and P.W. Sage
Question: w. Prins The agreement between model predictions and experimental results is amazing, considering the uncertainties in flow conditions (chanelling may cause hot spots and non-uniform combustion). Are there any fit parameters used in the model? What type of particle model is involved? Is it the socalled double-film model in which the CO, produced by CO2 gasification of the coal, is oxidized by the O 2 diffusing to the particle? Or is it a single-film model in which oxygen is allowed to reach the coal particle surface? How is (in the latter case) the ratio of CO to CO2, produced by the oxidation reaction, derived? Answer (1) There are no 'fit" parameters used in the model. (2) It is assumed that 02 reacts at the surface of the coal to form CO. The CO produced is then oxidised to CO 2 by the 02 diffusing to the particle (i.e., the double-fdm model). Question: P.D. Clark How are dust amounts quantified in various parts of the combustion system? Answer In the future gas ductwork dust burden and concentration is measured by isokinetic sampling using British Coal Utilisation Research Association (BCURA) equipment in accordance with requirements of British Standard BS 3405: 1983. We are developing a laser sheet illumination technique for determining particle velocities and directions inside coal fired equipment. So far, we have demonstrated it using cold models; soon we will be testing it in a coal-fired boiler.
55
Modelling of fixed bed combustion N W J Ford, M J Cooke and P W Sage Coal Research Establishment, British Coal Corporation, Stoke Orchard, Cheltenham, Gloucestershire, GL52 4RZ, United Kingdom
Abstract As part of an investigation of methods for the control of sulphur dioxide emissions from stoker-fired plant, a computer-based model of combustion within the fuel bed was developed. The model was based on an existing model of coal combustion and developed to enable consideration of the fuel bed conditions in a travelling grate stoker. The model considers the fuel from the point of entry on the grate through devolatilisation, combustion and ash cooling. Heat and material balances are carried out throughout the fuel bed at selected time intervals. This enables prediction of combustion gas composition and temperature at any point within the fuel bed. As input the model requires details of coal composition, along with basic plant operational parameters such as grate speed, excess air and fuel burning rate. The model was used to predict fuel bed combustion gas composition and temperature profiles. Comparison with data obtained from operational plant and a novel fixed grate simulator showed good agreement with model predictions giving confidence in its representativeness.
1.
INTRODUCTION
As part of an investigation of methods for the control of sulphur dioxide (SO2) emissions from stoker-fired plant, the need for a means of predicting fuel bed conditions was identified. Practical studies, using a fixed grate simulator, had identified fuel bed conditions which were conducive to the absorption of SO2 by calcium-based sorbent. The need was to identify the operational arrangement which enhanced such conditions and hence enhanced SO2 removal. It was not considered that further practical studies on the fixed grate simulator would be particularly helpful, rather it was decided that a more efficient and appropriate approach would be the development of a predictive mathematical fuel bed model. With such a model a range of operating conditions could be studied and the resulting fuel bed conditions assessed to identify those which would be likely to favour sulphur capture. The fixed grate simulator could then be operated under the conditions identified to determine whether there was potential for an improvement in sulphur capture. It was considered that the model should be able to provide detailed predictions of fuel bed temperature and gas and solid composition throughout the bed. A literature survey of available models showed that most were not sufficiently detailed to enable this degree of analysis. A model by Hougen and Watson I was identified as providing an appropriate treatment of the major combustion reactions, albeit in a somewhat simplified manner. This model was selected for subsequent modification to enable a more detailed treatment of the fuel bed and to include other minor reactions which were of interest in the overall study.
56
j.~//
i~ ~/
f
~ dr
x~r~
x ./I
L J\
.-
]GHT
o)
sP
j/ I
i lED
•
/ /
,t ~
.-IL) J /
.
.- j ...i()> /.. ...-
f~
..-
~ J \ J ~_j~,~j I
•
TIME
(t)
COMBUSTION AIR
Figure i.
General Arrangement of Fuel Bed Structure
\/ l+i
,
,~/ /
i,t-I arlJcle
i,
t
l~oss
=
t-I
MASS
._/
i,t
I
,o~,
x i+I . %+I
i+l , t
t-I
i-I
, t
I-I , t+l
__/ Figure 2.
General Procedure for Model Calculations
...
57 2.
M O D E L DEVELOPMENT
2.1 Fuel Bed Structure For the purposes of this model the fuel bed is represented as a series of boxes (see Figure 1). At the front end of the grate where fresh coal enters, the coal is assumed to consist of spherical lumps of uniform size, stacked on top of each other. Each lump is considered to reside within a cube or box. Combustion air entering through the grate contacts the bottom box first and then flows up through the bed to the top box. For modelling purposes one stack of lumps is considered to be typical of all the stacks constituting the bed. The modelling calculations are therefore conducted on a bed area eqtiivalent to the base area of the initial box. Reactions within the box are considered over a time period of one minute, although this can be varied depending on the accuracy of modelling required. Thus for the first minute, material and heat balances are conducted on the first stack of boxes and solid and gas compositions along with temperature, are calculated for each box in the stack. At the beginning of the second minute the stack is considered to have moved down the grate (to a position equivalent to the specified grate speed and residence time). The lumps having reacted may have decreased in size and the boxes are therefore redrawn. The base dimensions remain the same as the original box (ie equivalent to the diameter of the initial coal lumps). However the height of the box changes to reflect the reduction in diameter of the particle after reaction. The calculation process is then repeated. Generally the time taken for coal entering an industrial stoker-fired boiler to pass through the furnace is around 40 minutes. In modelling this situation calculations might be carried out at minute intervals and thus 40 stacks would be considered The solid composition of the stack remaining after the specified combustion period corresponds to the ash remaining at the end of the grate of a 'real' stoker fired boiler. 2.2 Combustion and Associated Reactions As in the Hougen and Watson 1 model, the following three main stages are considered in the combustion of lump coal: a.
Oxygen in the main stream diffuses to the surface of the coal lump where it reacts to form carbon monoxide (CO) or carbon dioxide CO2. C
+ 02 ~
co 2
c + '/2 o 2 --, c o
(i)
(2)
Hougen and Watson I assume that due to the relatively high temperature involved the chemical reactions on the surface of the coal lump are so fast that chemical equilibrium between carbon (C), CO, CO2 and oxygen (02) is maintained. This indicates a diffusion controlled reaction. For completeness the chemical reaction control rate is also considered since, in some instances, bed temperatures are lower than the 1000°C suggested by Hougen and Watson I as giving a zero equilibrium concentration of both
co2 and 02.
58 b.
Carbon monoxide produced during combustion diffuses from the solid/gas interface into the gas stream where it may react with oxygen, if available, to produce CO~: 2C0
C.
+ 02 ~ 2 C02
(3)
As the carbon dioxide content of the gas stream increases (via 3), it diffuses to the solid/gas interface where it is reduced to CO: CO z + C ~ CO
(4)
Again due to the relatively high temperature involved at the solid/gas interface Hougen and Watson ~ consider that the final products of the above reaction approach their equilibrium composition containing virtually no CO2 and the reaction is therefore assumed to be diffusion controlled. The volatile matter associated with the coal is assumed to be released at various temperatures. The water associated with the coal is released once the temperature reaches 200°C with the remaining volatile species being released at 700°C 2. These species are assumed to pass through the fuel bed and bum above it. During combustion, under oxidising conditions, hydrogen and oxygen are assumed to combine to form water, whilst nitrogen and sulphur are released as N2 and SO2 respectively. Once the oxygen becomes limited, the hydrogen is assumed to combine with sulphur to produce H2S and any remaining is released as H2. Since hydrogen is present in the original coal in a larger quantity than sulphur there is always a surplus. Hydrogen, oxygen, nitrogen and sulphur are assumed to be released from the coal at a rate equivalent to the release of carbon. These along with CO and CO2, are the only gaseous species considered in the model. After release from the coal the resulting gases are assumed to join the bulk gas which is passing through the bed. Carbon bearing gases are assumed to react as in 3 and 4. Of the remaining gases nitrogen and water, once formed, are assumed to pass through the bed without further reaction. H2, H2S and SO2 are considered to be present at their equilibrium concentration. Thus as the bulk gas stream passes into a box the above concentrations are recalculated to account for the change composition of the bulk gas due to the addition of gases from the reaction of the coal and reaction within the bulk gas due to changes in composition and temperature. 2.3 Heat Balances In the Hougen and Watson I model a constant bed temperature is assumed and as such no heat balances are required. Since the rate of the reactions considered in the current model are dependent on temperature it was considered that the model would be more representative of a 'real' fuel bed if the temperature with each box were allowed to vary based on a heat balance.
59 Each box is considered to be subject to several sources of heat loss and heat gain: Heat transfer due to reaction Heat transfer from boxes above and below The purpose of the heat balance is to determine the temperature within the box, which will in turn determine the rates of reaction. This entails an iterative procedure where temperatures are selected, heat generation and. loss by subsequent reaction calculated and balanced with the heat gain and loss due to heat transfer. This balance is carded out for each box in the stack and for each selected time interval. Figure 2 illustrates the general arrangement of coal particles and gas flows within the model.
3.
M O D E L VALIDATION
Available literature does not provide sufficient information for the validation of this model. This is mainly due to the inherent difficulty in measuring fuel bed conditions on operational plant. In the absence of suitable existing data with which to validate the model a series of tests on a fixed grate simulator were conducted. The objective was to obtain detailed information on the composition of a stoker fuel bed to determine the representativeness of the model. Fixed grate simulators enable the progress of combustion to be reproduced by subjecting a mass of coal to the range of operating conditions that it would experience in a 'real' plant. The simulator used is considered to provide a reasonable representation of a 'real' plant3. The test programme featured the simulator operating under conditions typical of stokerfired plant. Combustion was quenched, using nitrogen, after a specified time. The bed was then removed in sections and each section submitted for chemical analysis. In total eight tests were conducted with combustion being quenched after 5, I0, 15, 20, 25, 30, 35 and 40 minutes. (In a 'real' stoker this would correspond to distance along the grate.) The chemical analyses of the various sections provided data on the mechanism and progress of combustion on the grate which could be compared with the predictions of the model. The model was run for the same overall operating conditions as for the testwork above. It was then possible to compare the results of the model with those obtained experimentally. Figure 3 compares the measured release of carbon from the coal with that predicted by the model. It may be seen that the release rate predicted by the model are in good agreement with that obtained in the testwork. Figure 4 compares the measured fuel bed temperature (two thermocouples at 40 mm and 100 mm above the grate) with that predicted by the model at these locations. Again reasonable agreement is shown between the measured and predicted values. Although it has not been possible to compare measured gas compositions within the fuel bed with predicted values, it is considered that the agreement shown between measured and predicted solid composition and bed temperature profiles suggests that the predicted gas composition profiles are also likely to be in agreement. Figure 5 shows the predicted concentration profile of 02 within the fuel bed. It may be seen that the general trends exhibited agree well with the generally accepted mode of combustion in a stoker fuel bed.
60
RELEASE (% of input)
1 O0
80
"-*- MEASURED -~ PREDICTED
40
2O
0
5
10
15
20
25
30
35
40
TIME (rains)
FIGURE 3 PREDICTED RELEASE OF CARBON FROM COAL
TEMPERATURE
(*C)
1,600
1,400
~*l.u~PLi ,*gmo.. *J,ovl **,TII
1,000
100mm -e- 4 0 r a m
800
-w" 4 0 m r n 600
........
400
......
'. . . . . . . . . . . . . . . .
-i..
" -lOOmm
MODEL MODEL
..,"
,oo ._.:.:,,::,j. :j .................. 0
6
1O
15
20 TIME
25
30
35
40
(rain)
FIGURE 4 COMPARISON
OF F U E L BED T E M P E R A T U R E S
61
1 60
140 1 20 ~100 I--i[]
~
80
6O 40 20 0
0
5
10
15
20
25
30
35
TIME (mln) OXYGEN CONCENTRATION (% by volumo) - - B E D HT ~ > 2 0 %
FIGURE
EE16-20% ~ 1 0 - 1 6 %
5 PREDICTED
FUEL
~6-10%
BED
~1-6%
OXYGEN
~<:1%
PROFILE
40
62
The general good agreement demonstrated between the model predictions and measured data provides confidence in the representativeness of the model.
4.
CONCLUSIONS
A computer model intended to enable the prediction of the fuel bed conditions in stokerfired plant has been developed. Comparison with experimental data derived from a fixed grate simulator indicates that the model predictions are representative of the fuel bed conditions during typical stoker firing.
5.
ACKNOWLEDGEMENTS
The work upon which this paper is based was funded jointly by British Coal and the European Coal and Steel Community. The views expressed are those of the authors, and do not necessarily reflect those of British Coal.
6.
REFERENCES
1.
O A Hougen and K M Watson, Chemical Process Principles - Combined Volume, John Wiley and Sons, London, 1950. R T Haslam and R P Russel, Fuels and their Combustion, McGraw-Hill Book Company, New York, 1926. N Ford, M J Cooke and M D Pettit, The Use of a Laboratory Furnace to Simulate Industrial Stoker-Fired Plant, Jnl Inst Energy, Vol LXV No 464, September 1992.
2. 3.
63
Discussion Modeling of fixed bed combustion N.W.J. Ford, M.J. Cooke and P.W. Sage
Question: w. Prins The agreement between model predictions and experimental results is amazing, considering the uncertainties in flow conditions (chanelling may cause hot spots and non-uniform combustion). Are there any fit parameters used in the model? What type of particle model is involved? Is it the socalled double-film model in which the CO, produced by CO2 gasification of the coal, is oxidized by the O 2 diffusing to the particle? Or is it a single-film model in which oxygen is allowed to reach the coal particle surface? How is (in the latter case) the ratio of CO to CO2, produced by the oxidation reaction, derived? Answer (1) There are no 'fit" parameters used in the model. (2) It is assumed that 02 reacts at the surface of the coal to form CO. The CO produced is then oxidised to CO 2 by the 02 diffusing to the particle (i.e., the double-fdm model). Question: P.D. Clark How are dust amounts quantified in various parts of the combustion system? Answer In the future gas ductwork dust burden and concentration is measured by isokinetic sampling using British Coal Utilisation Research Association (BCURA) equipment in accordance with requirements of British Standard BS 3405: 1983. We are developing a laser sheet illumination technique for determining particle velocities and directions inside coal fired equipment. So far, we have demonstrated it using cold models; soon we will be testing it in a coal-fired boiler.