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Gaseous emissions from circulating fluidized bed combustion of wood
0961-9534/93$6.00+ 0.00 ‘C 1993PergamonPressLtd
BiomassandBioenergyVol. 4, No. 5, PP. 379-389,1993 Printedin GreatBritain.All rightsreserved
GASEOUS
EMISSIONS FROM CIRCULATING BED COMBUSTION OF WOOD
FLUIDIZED
B. LECKNERand M. KARLSSON Chalmers University of Technology, Department of Energy Conversion, S-412 96 Goteberg, Sweden (Received I7 February 1993; revised received 23 March 1993; accepted 6 April 1993)
Abetract-The emissions of NO, N,O, CO and SO, were investigated during combustion of wood in a circulating fluidii bed boiler as a function of various parameters, including bed temperature, air supply and load. Emissions from mixtures of wood and coal were also investigated. The results show that the nitrogen oxide emission is directly related to the nitrogen content of the wood. The small amount of char in the bed during combustion of wood results in a much smaller reduction in the NO formed in the case of wood compared to that during combustion of coal and reduces the impact of bed temperature and air supply on the NO emissions during combustion of wood as compared to coal. Emission of N,O during combustion of wood was negligible. Keywords--Gaseous
emissions, biofuels, circulating fluidized bed combustor, nitrogen oxides.
1. INTRODUCTION
Fluidized bed combustion is an environmentally advantageous method of burning solid fuels of different types, including bio-fuels. Wood is sometimes claimed to be a comparatively clean fuel which is simple to burn. However, few systematic studies of gaseous emissions from wood combustion in fluidized beds have been carried out. Information is available from single tests where operating conditions have not been specified and from studies where operating conditions have been varied in an unsystematic way. Figure 1 shows an example of emissions of NO recorded for various bed temperatures from a 165 MW (thermal power) circulating fluidized bed (CFB) boiler fired with wood waste. The results are quite scattered. However, this behaviour could not be explained, since the boiler operation was normal in all respects and similar tests with coal and peat had been carried out during the same test program’ without finding such irregularities in the emission values. In order to determine the reason for the scatter an extended test program focusing on wood was carried out. The purpose of this work was to determine the influence of various combustor operating parameters on the emissions of nitrogen oxides, gaseous monoxide, and other carbon components of the flue gases (e.g. unburned hydrocarbons) from combustion of wood in an atmospheric CFB which was large enough to be representative of commercial boilers and where
the conditions could be carefully controlled. A direct comparison with combustion of coal was also made. 2. EXPERIMENTAL
CONDITIONS
2.1. The boiler The tests were carried out using a 12 MW (thermal power) CFB boiler located in the power centre of Chalmers University of Technology. Figure 2 shows the boiler. The chamber dimensions of the combustion (number 1 in Fig. 2) are 1.7 x 1.7 x 13.5 m. The bottom part, below a height of 2 m, as well as two of the walls, is refractory-lined. The remaining sides of the combustion chamber are membrane-tube walls having a temperature of around 220°C. The cyclone is water-cooled but protected by refractory lining.
.d
z
~,‘770 .,.,,“ .,,,.,t’ ‘,(,(,‘ ,,,’ ,‘, %o 810 850 890 Bed temperature,‘C
Fig. I. Nitrogen oxide emissions from combustion of wood waste in a 165 MW (thermal power) CFB boiler as a function of bed temperature and under otherwise constant conditions. 379
B.LECKNERand M.KARLSON
380 Air inlet
5
Air inlet 4 + Air inlet 3 -w Air inlet l/2 3 <
.-. t Fig. 2. The boiler. (1) Combustion chamber. (2) Air plenum and start-up combustion chamber. (3) Fuel feed chute. (4) Cyclone. (5) Exit duct. (6) Convection cooling section. (7) Particle seal. (8) External heat exchanger (particle cooler). 0 Measurement holes. x Pressure taps. ?? Thermocouples + Secondary air nozzles (air inlets l-5).
The fuel is introduced through the inclined fuel chute (3) on the front of the boiler from two fuel hoppers. The boiler can be fed simultaneously from both of these, if required, allowing mixtures of fuels to be used. In addition to these hoppers from which woodchips and coal were fed, in some tests sawdust was also supplied to the fuel feed conveyor just upstream of the fuel chute by means of a specially installed and calibrated screw feeder. The maximum capacity of the screw feeder corresponded to a fuel power of 3 MW. Primary air is injected through nozzles in the bottom of the combustion chamber and secondary air can be supplied through several inlets in the form of groups of horizontally arranged nozzles, numbered 1 to 5 in Fig. 2. The secondary air inlet l/2 was normally used during the present tests, inlets 3 and 4 are located at higher levels of the combustion chamber, and inlet 5 consists of six holes in the gas exit duct of the cyclone. On some occasions air was also added at the entrance to the cyclone. The
location denoted
of this secondary “C” in Fig. 2.
air addition
has been
The temperature of the combustion chamber during changes in the thermal conditions, such as those caused by a change of fuel or by a variation in moisture content of the fuel, was controlled by adjusting the external heat exchanger (8). Fine tuning of the control of the bed temperature was accomplished by flue-gas recirculation. In this way the most important boiler parameters could be maintained constant and changes in the conditions, for instance of the moisture content of the fuels, could be compensated for without affecting the emissions. The small changes of fluidization velocity caused by adjustment of flue-gas recirculation have little impact on the emissions. The boiler is fitted with instruments for continuous recording of temperatures, pressures, flows of gases and solids, etc. A number of thermocouples are installed in the bottom bed, in the top of the combustion chamber, and upstream (Ti”) and downstream (Tl, T2, T3 and T4) of the cyclone, as illustrated in Fig. 3. 2.2. Gas analyses Flue gas analyses were carried out using two independent systems (A and B) as shown in Fig. 4. An 0, analyzer [O,(KC)] downstream of the cyclone measures the oxygen concentration used in calculation of the boiler excess-air ratio. (The excess-air ratio is denoted m in some of the following figures.) Gas samples were normally taken downstream of the baghouse filter,
T4
Tin
Fig. 3. The top of the combustion chamber, the cyclone and the cyclone exit duct. Temperature measurement locations (T) and holes of the secondary air registers 5 and C are also shown.
Gaseous emissions from CFB combustion
381
Flue gas duct From cyclone
System B
System .I
I
Pump
Pump /
Fig. 4. The two systems for gas analysis.
upstream of the flue-gas fan (System A), and also in the inlet of the cyclone (System B). The CO analyzers were corrected for interference from N,O. The analyzers are listed in Table 1. The analyzers were calibrated at regular intervals. The results of the emission measurements were converted to a standard flue gas with an oxygen concentration of 6% 0,. For this conversion, the oxygen concentration at the location of the gas analyzer was used.
2.3. Fuels and bed material The fuels were specially cut wood-chips of trunks from birch or fir, a Colombian bituminous coal and sawdust from fir. The properties of the fuels are given in Table 2 in the form of average values from several tests. In general, the composition of the wood from various sources is similar, although the form and the moisture content were different. There was also a variation in the nitrogen and moisture contents
Table I. The gas analyzers used. For System B, the symbols within brackets are used to name the different analyzers Gas
Range
Type
System A
02 co
NO
N,O SO*
o-10% o-loft0 DDID O-250 pb;n 0-lOO, O-500 ppm O-500 ppm
Magnos ST, paramagnetic Binos. NDIR Beckman 955, chemiluminescence Spcctran 647, NDIR Binos, NDVIS/UV
o-10% &250 ppm o-0.5% &lo% o-10% o-10%
Magnos 5T, paramagnetic Beckman 955, chemiluminescence Uras 7N, IR Scott IIIC, IR Binos, NDIR Rosemount, model 132, Zirconia
System B 02
NO CO(CTH) COW) CO(MKS) O,(KC)
382
B. LECKNERand M. 100 E?
0 0
4J L G 75 s c
0”50 2 J m 25 ‘B E Birch chips
T
Fir chips
Day of test Fig. 5. Variation of moisture and nitrogen content during the test period.
of a particular fuel, as shown in Fig. 5 by the recordings taken for wood-chips over the test period of one month. The heating values (lower) were 31.3 MJ kg-’ for the coal and about 19 MJ kg-’ for the wood, expressed on an ash-and-moisture-free basis. The bed material consisted of 0.35 mm average diameter silica sand mixed with a minor quantity of the fuel ash remaining in the bed after combustion. 2.4. Test conditions Each test was run for one to two hours under constant boiler operation conditions. The results presented are mean values over this period of time. 3. TEST STRATEGY
The tests consist of two parts. First, the wood-chips were used in a parameter study to investigate the influence on the emissions of changes in bed temperature, air supply and load. Then mixes of wood and coal were used. In the parameter study, one parameter was varied at a time, while the others were kept constant and equal to the values for standard boiler operation. The study covered a time-span
KARLSON
of one month and was divided into several test series (one for each parameter). Each test series included a period of standard operation in order to verify that nothing was changed unintentionally during the parameter study. As a consequence, runs under standard conditions were repeated 18 times. The average values recorded during these runs are shown in Table 3. Variations between the standard runs were very small. The largest variation was observed in flue-gas recirculation which was used to control the bed temperature. The amount of flue-gases recirculated was small, however, compared to the rate of air supply, and its influence on the emissions was negligible. Other variations were caused by the contents of nitrogen and moisture of the fuel (Fig. 5). These variations were not deliberate, but reflected differences in deliveries of wood from different locations. However, the chips were always cut in the same way. The moisture variations could be handled by the boiler control system as explained above, but the variations in the content of nitrogen affected the results and several test series were repeated in order to determine the effects of differing nitrogen content of the fuel. The temperature of the combustion chamber, the bed temperature, was fairly even. There was only a small rise in temperature from bottom to top, 10°C for coal and 20-30°C for wood. The temperature in the bottom of the combustion chamber T,, is used to characterize the bed temperature in the following presentation. The walls of the cyclone cause some cooling of the gas-particle suspension which is to some extent counterbalanced by heat release of combustion in the cyclone. In the case of coal and regular-sized wood-chips the extent of combustion in the cyclone was small and the temperature drop between the inlet (T,,) and the outlet of the cyclone (Tl) was about 50°C. Fine
Table 2. The fuel properties (average values),%. amf = ash and moisture free Coal Combustibles (as delivered) Moisture (as delivered) Ash (as delivered) Carbon (amf) Hydrogen (amf) Oxygen (amf) Nitrogen (amf) Sulphur (amf) Volatile matter (amf) Size, mm thickness or diameter length
19.9 13.7 6.4 78.7 5.3 13.8 1.50 0.8 39 < 10
Sawdust
Birch chips
Fir chips
86.5 13.1 0.4 50.7 6.4 42.8 0.10 0.02 82
62.6 37.0 0.4 50.2 6.2 43.4 0.22 0.05 82
61.1 38.6 0.3 50.7 6.3 43. I 0.15 0.02 82
0.8
1-3 545
1-3 545
Gaseous emissions from CFB combustion
383
Table 3. Average values during standard boiler operation
Number of cases Total air, kg s-l Secondary/primary air ratio Flue-gas recirculation, kg s-r Temperatures, “C bed cyclone inlet cyclone outlet (TI) cyclone outlet duct Fluidization velocity, m s-l 0, (KC)% (wet) Combustion chamber pressure drop, kPa
fuels, like the sawdust, on the other hand, show a considerable afterburning in the cyclone. The fine fuel particles tend to rise in a plume in the combustion chamber, partly without access to oxygen, mix with the oxygen in the cyclone, burn and raise the temperature. The amount of sawdust added in the present tests was not very large and hence the corresponding temperature rise in the cyclone was less than 50°C. The difference between the gas temperature at the cyclone outlet (Tl) and the cyclone outlet duct [(T2 + T3 + T4)/3] presented in Table 3 is probably explained by radiative heating of Tl from the cyclone and radiative cooling of the unshielded thermocouples from the cold wall of the convection pass. 4. RESULTS
4.1. Nitrogen content of the fuel The fuel nitrogen content vs. the emission of nitric oxide (NO) is shown in Fig. 6 for operation under standard conditions. There is a strong influence of the nitrogen content of the fuel on the emission of NO. All other possible infiuencing parameters were equal to those of the standard case.
Coal
Birch chips
2 3.6 1.0 1.0
10 3.7 1.0 0.34
854 857 817 791 6.9 3.5 5.7
The effects on the NO emission of bed temperature, excess air, primary-air stoichiometry, level of secondary air supply and load (fluidization velocity) are shown in Figs 7, 8, 9, 10 and 11. Usually, data for both birch and fir chips are plotted, since these differ reflecting the nitrogen content of the fuels. Except for this difference, each figure is based on deliveries of fuel mostly from the same origin and the data are only affected by small variations in nitrogen content. The NO emission is only weakly dependent on bed temperature and total excess-air ratio as seen in Figs 7 and 8. This conclusion is supported by the data shown in Table 4, where a comparison is made with boilers burning coal under otherwise similar conditions. This table shows the change of emission per degree of bed temperature or percent excess air. The temperature dependence of NO emission is almost five times as great for coal as for wood and the dependence of the emission on the excess air is almost twice that for coal as for wood. The results with fir chips illustrated in Fig. 7 show almost no dependence on temperature. This is probably caused by a slight variation of the fuel nitrogen content between the measurement
$
0.0
Fuel
%rogen
y/bfJ
Fig. 6. The variation of NO emission as a function of nitrogen supplied with the fuel during the standard case runs.
850 880 822 778 6.7 3.5 5.5
4.2. Parameter variations
.I
ou
8 3.6 1.0 0.18
851 871 834 782 6.7 3.4 5.6
4
2
Fir chips
50
1
I2 075: lm=lup Bed temperature,“C BOO
850
900
Fig. 7. The influence of bed temperature on the NO emission. (m is excess-air ratio.)
B. LECKNERand M. KARLWIN
384
2 * t
.
Birch
chipa
0 Fir 0 ““” chips 1.1 1.2
‘,,“‘I
1.3
1.4
Total excess air ratio Fig. 8. The influence of excess air on the NO emission. (Tb is bed temperature.)
points. If these points were considered as they stand together with the birch chips’ points, the dependence of the NO emission on temperature shown in Table 4 becomes even smaller. Primary-air stoichiometry is defined as the ratio of air supplied in the form of primary air and the stoichiometric air-demand for complete combustion of the fuel. A variation of the primary-air stoichiometry has a very small effect on the NO emission as shown in Fig. 9. This is similar to results with coal3 In the coal tests3 a reduction in NO emission occurred, however, when the supply of secondary air was moved from inlets l/2 (see Fig. 2) to higher levels in the combustion chamber. No such effect was observed in the present tests with wood (Fig. 10). Only in the extreme case when air was supplied from inlets 4 and 5 (50% of the secondary air in each inlet) was there a slight decrease in the NO emission. The load of the boiler is proportional to the fluidization velocity at a constant excess-air ratio. Figure 11 therefore shows the influence on the NO emission of both load and fluidization velocity. At low fluidization velocity, most of the bed material remains in the bottom of the combustion chamber and the gases are cooled as they pass the heat-transfer surfaces of the
f
oj,, ,,, /, 0.0
Primary
0.8
0.4
air
l/2
3
Secondary
3jc
4
air
4/c
4/5
register
Fig. 10. The influence of the level of secondary air supply. The numbers on the horizontal axis indicate the secondary air register(s) where the secondary air was supplied. The slash indicates that the secondary air is equally split between the two registers.
combustion chamber. As a result, the temperature at the top of the combustion chamber (and in the cyclone and flue gas duct) falls as shown in Fig. 11. The NO emission also decreases. The reason for this smaller NO emission at low fluidization velocities is that more bed material and also more fuel remain in the bottom bed below the secondary air nozzles where now a truly sub-stoichiometric combustion takes place, assisted by the secondary air which accomplishes final burn-out of the gases leaving the sub-stoichiometric primary combustion zone. This is in contrast to the situation at full load (8 MW) when the solid fuel is partly carried away with the flue gases to regions of the combustion chamber above the secondary air nozzles where over-stoichiometric conditions prevail. The nominally sub-stoichiometric bottom zone does not play a great role in NO reduction at full load, since at full load the fuel does not burn there and the bottom zone does not actually become oxygen-deficient. The extension of combustion to the entire combustion
,I 1.2
stoichiometry
Fig. 9. The influence on NO of the primary air stoichiometry.
Fig. 11. The influence of fluidization velocity (load). The corresponding temperatures in the top part of the combustion chamber are also shown.
385
Gaseous emissions from CFB combustion Table 4. Influence of bed temperature and excess air on the emission of NO from wood and coal
Parameter Bed temperature, ppm/“C Excess air, ppm/% excess air
Coal references
Wood present work
I
2
3
4
0.2 1.1
0.6 I.91
I.0 -
0.7 2.0
0.9 1.8
*Uncertain.
chamber also explains the small effect of changes in the bottom zone below the secondary air inlet, as shown in Figs 9 and 10. The emissions of N,O from combustion of wood are negligible for all parameters investigated. An example is shown in Fig. 12. This is different from observations with CFB combustion of coal from which emissions of NzO in the order of 50 to 200ppm have been measured. ‘~~3~ The emissions of CO were low (around 50 ppm) in all cases investigated and no unburned hydrocarbons were recorded in the flue gases leaving the boiler. 4.3. Mixtures of fuels
‘R 0
z
E .t:
50
I_
z ;50Bed
NO+C-+;N,+CO
(Rl)
NO + CO+ ;N, + CO, (catalyzed by char).
Coal and wood combustion in fluidized bed differ due to the different volatile matter contents of the fuels (cf. Table 2). During coal combustion a considerable part of the fuel burns as char in the bed, whereas in the case of wood the combustion of volatiles dominates. Moreover, the reactivity of wood char is higher than that of coal, and the char concentration in the bed becomes very low in the case of wood combustion. Measurements made during the present tests with 100% coal or 100% wood combustion show char concentrations of 2.0% or 0.2% in the bottom part of the combustion chamber and 0.5% or 0.04% respectively in samples taken in the particle return duct after the cyclone. The difference between bottom and top of the combustion chamber, the latter
z
represented by the samples taken after the cyclone, shows that char concentration varies with height of the combustion chamber. The measured char concentrations also show that the char concentration for wood is about a tenth of that of bituminous coal during the present combustion conditions. The concentration of char in the combustion chamber is important for the reduction of nitrogen oxides, since two of the most significant reduction reactions are6
a
800
850
900
temperature,‘C
Fig. 12. The influence of bed temperature on the emission of N,O.
(R2)
Hydrogen, H,, may play the same role as CO in (R2). The combustion of various mixtures of fuel, from 100% wood-chips to 100% coal, was compared under standard conditions of boiler operation. In some tests sawdust was used instead of wood-chips to simulate a fine-particle fuel. The results for emission of NO, CO, SOI and N,O are shown in Fig. 13. The extreme cases with 100% wood or 100% coal illustrated in Fig. 13a show that the emission of NO from combustion of wood is normally higher than that from combustion of coal. It can also be seen that a small addition of coal to wood gives a higher emission of NO than that from wood only. This can be explained (see Appendix) in terms of the higher nitrogen content of coal, which leads to a greater formation of NO than during combustion of wood. When the fraction of coal added is small, the amount of char in the bed is also small, and the NO formed is only reduced to nitrogen to a small degree. When the fraction of coal fed to the boiler is further increased, the amount of char in the bed increases, and hence a greater proportion of the NO formed is reduced. As a result, the NO emission gradually decreases from its maximum value as the fraction of coal increases, and only a few percent of the fuel nitrogen is converted to NO in a 100% coal-
B. LECKNERand M.
386
c
100
Energy
J
e
100
Energy
crml rood
+
100 0
fraction,%
wood
0
fractions
(4
100
e
Energy
&
"0 100
e
Energy
rood
0
fraction,%
coal rood
+
100 0
fraction,%
Fig. 13. (a) The emission of NO from mixtures of coal and wood. Other conditions constant. (b) The emission of CO from mixtures of coal and wood. Other conditions constant. (c) The emission of SO2 from mixtures of coal and wood. Other conditions constant. (d) The emission of N,O from mixtures of coal and wood. Other conditions constant.
KARLSON
fired bed, whereas the conversion of the fuel nitrogen of wood to NO is almost 20%. Figure 13b shows that the emission of CO increases steadily with an increasing fraction of coal despite the fact that the CO concentration in the combustion chamber upstream of the cyclone is higher with wood than with coal as a fuel. The CO concentration in the entrance to the cyclone was about 0.5% during combustion of wood-chips and locally as high as 1 to 2% with sawdust. After the cyclone, however, the CO concentration was low, but it was higher with coal than with wood. Generally, the large decrease of the CO concentration in the cyclone with wood as well as with coal is caused by the mixing of the gases in the cyclone, which readily oxidizes the CO. In the case of coal, the higher concentration of char particles contained in the particle flow in the cyclone burns and additional CO is produced. In the case of wood-chips there was practically no char in the cyclone, almost all had been consumed in the combustion chamber. Thus, the CO level becomes higher with coal than with wood. Sawdust is seen to decrease the CO emission even more than wood-chips. This is most likely a consequence of the increased cyclone temperature during sawdust addition (see below). As shown in Fig. 13c, the emission of SO2 also increases in proportion to the amount of coal burned, since the sulphur content of the wood is negligible and all the sulphur is contained in the coal. The maximum value, at 100% coal, corresponds to the sulphur content of the coal minus a self-absorption in the coal ash amounting to 10% to 15% of the sulphur. The small amount of ash in the wood had no measurable influence in binding sulphur. As shown in Fig. 13d, the emission of N,O originates from the coal, since wood alone emits negligible amounts of N,O. It is seen, however, that the relationship is not entirely linear; the addition of wood-chips and especially that of sawdust decreased the N,O emission. This effect could have been caused by a slightly higher bed temperature (about 1OC) in the top of the combustor when wood was added, and during addition of sawdust also by a temperature rise in the cyclone due to afterburning. The cyclone temperature rise was about 50°C at maximum sawdust addition of 27%. In fact, one of the reasons for adding the sawdust was to investigate the possibility of raising the cyclone temperature. This effect was not very great, however, with the quantities of sawdust used,
387
Gaseous emissions from CFB combustion
and hence the decrease in NzO caused by the temperature rise was moderate. The addition of wood could also have had an effect on the N20 formation and decomposition through the combustion of wood and its involving volatiles, resulting in reactions hydrogen or hydrogen radicals. However, no measurements were made to support this assumption. 5. DISCUSSION
It was shown in Fig. 6 that the NO emission from wood combustion in a fluidized bed is closely related to the fuel-nitrogen content. A similar result has been obtained for flame combustion of oil’ and for grate combustion of wood.* For fluidized-bed combustion of coal in commercial-size combustors, however, no clear relationship has been reported between fuelnitrogen content and NO emission.’ The reason for this difference between coal and wood lies in the significant contribution of the char contained in the coal-fired fluidized bed to the reduction of the NO formed, reactions (Rl) and(R2). The NO emission from a coalfired fluidized bed corresponds to only a few percent of the coal nitrogen. Various parameters such as bed temperature and air supply affect the char concentrations of the bed, and thus the NO emission, and have a stronger impact on the emission than the coal-nitrogen content. Therefore, it has been difficult to distinguish the influence of the nitrogen content of the coal from the effect of other parameters. In the wood-fired fluidized bed, on the other hand, the concentration of char is small, the conversion of the fuel nitrogen is high and the relationship between the emission of NO and the fuelnitrogen content is clear. For the same reason, the influence of boiler parameters, bed temperature and excess air ratio, such as shown in Table 4, is small. The fuel used to produce the results shown in Fig. 1 was wood waste from the forest, containing branches and even green needles. The composition of the fuel could vary from one
5o[NO increase,
ppm
Fig. 14. Influences of cyclone combustion on temperature and NO increase in the cyclone. (NO reduction is negative increase).
delivery to another. This has a significant impact on the nitrogen content of the fuel, as shown in Table 5, which clearly demonstrates that the nitrogen content of wood-waste greatly depends on the parts of the tree included in the wood-waste. Most likely the scattered results shown in Fig. 1 can be explained in terms of varying nitrogen content of the fuel. The influence of cyclone combustion and of changes in the cyclone temperature have not had a very great importance in the present range However, some of parameter variations. changes in the character of the cyclone activities are as shown in Fig. 14, which indicates the relationship between cyclone combustion (temperature increase from the entrance to the exit of the cyclone) and NO release in the cyclone (NO increase; the difference between the concentrations of NO in the entrance of the cyclone and the stack). In the standard case of boiler operation (coal or wood-chips) there is some cooling in the cyclone and some NO reduction. When combustion in the furnace is retarded by moving the secondary air to higher levels (inlets 3 and 4) some NO is produced in the cyclone instead, and when sawdust is added to wood-chips there is both a rise in temperature due to cyclone combustion and an evident increase in NO. The increase in the NO emission in these cases is clearly caused by formation of NO in the cyclone and insufficient time for reduction. 6. CONCLUSIONS
Table 5. Nitrogen content of various parts of tree. (Average values from different types of tree”) Part of tree Trunk Bark Branches Needles
Nitrogen content % 0.05 0.4 0.6 1.0
The following conclusions can be drawn concerning fluidized-bed combustion of wood: (i) the fuel-nitrogen content is the most important parameter affecting the NO emission; (ii) the emission of NO is generally higher than from coal combustion; (iii) the influence of bed temperature and air supply is less than in coal
388
B. LECKNERand M. KARLSWN
combustion; and (iv) practically no N,O is emitted. Furthermore, it is concluded that: (i) emissions from mixtures of coal and wood are approximately proportional to the mixing ratio of the fuels and to the emission properties of the respective fuels; (ii) an influence of the NO reduction of char is clearly demonstrated in the tests with mixtures of coal and wood; (iii) the cyclone has a beneficial effect for CO combustion, making the CO emissions from wood combustion very low; and, finally, (iv) extremely fine fuel powder tends to rise in a plume in the combustion chamber and burns to a large extent in the cyclone. Acknowledgements-This work was financed by the following organizations: Swedish National Board for Industrial and Technical Development (NUTEK), Swedish Energy Development Corporation (SEU), Kvaerner Generator AB and &ebro Energi AB.
APPENDIX
Estimation
of the NO-char
relationship
The explanation of the NO emission from combustion of mixes of wood and coal under otherwise similar conditions (Fig. 13a) can be verified by a simple model assuming a plug flow of gas through a matrix of char particles in the bed. The essential assumptions are: (1) Nitric oxide is formed from combustion, proportional to the nitrogen content of the coal n,, the wood n, and the fuel flow, cow n,p + n,(l - P)K/K
where p is the energy fraction of coal, and H,, H, are heating values of coal and wood. (2) The rate of reduction of NO depends mostly on reduction on the surface of char A or on reduction of CO or H, catalyzed by char6 k N Ac”[CO]~
REFERENCES
1. B. Leckner, M. Karlsson, M. Mjornell and U. Hagman, Emissions from a 165 MW,,... circulating fluid&d bed boiler. J. Inst. Energy 65, 122-l 30 (1992). 2 M. Mjdmell, B. Leckner, M. Karlsson and A. Lyngfelt, Emission control with additives in CFB coal combustion. In 11th Int. Conf: on Fluidized Bed Combustion (E. J. Anthony, Ed.), pp. 655-664. ASME Book No 10312B (1991). 3 L.-E. Amand and B. Leckner, Influence of air supply on the emissions of NO and N,O from a circulating fluid&d bed boiler. In 24th Symposium (Int.) on Combustion, pp. 1407-1414. The Combustion Institute, Pittsburgh (1992). 4. B. Leckner and L.-E. Amand, Emissions from a circulating and a stationary fluidized bed boiler-a comparison. In 1987 Int. Conf on Fluidized Bed Combustion (J. P. Mustonen, Ed.), pp. 891-897. ASME Book No 10232A (1987). 5 L.-E. Amand and B. Leckner. Influence of fuel on the emissions of nitrogen oxides (NO and N,O) from an 8 MW fluidized bed boiler. Cornbust. Flame 84, 181-196 (1991). 6. J.-E. Johnsson and K. Dam-Johansen, Formation and reduction of NO, in a fluidized bed combustor. In I 1th Int. Co& on Fluidized Bed Combustion (E. J. Anthony, Ed.), pp. 1389-1396. ASME Book No 10312B (1991). I. D. W. Pershing, J. E. Cichanowicz, G. C. England, M. P. Heap and G. B. Martin, The influence of fuel composition and flame temperature on the formation of thermal and fuel NO, in residual oil flames. In 17th Symposium (Int.) on Combustion, pp. 715-726. The Combustion Institute, Pittsburgh (1978). 8. T. Nussbaumer, Stickoxide bei der Holzverbrennung. Heiz. Klima 15, 51-62 (1988). 9. T. Shimizu, J. Tatebayashi, H. Terada, T. Furusawa and M. Horio, The combustion characteristics of different types of coal in the 20 t/h fluidized-bed boiler. In Proc. of the Eighth Int. Con/ on Fluidized-Bed Combustion, pp. 231-239. US Dept. of Energy, Morgantown (1985). 10 J. Johansson, Characterisation of fuels-nitrogen compounds (in Swedish with English summary). U(B)l991/39, Swedish Power Board, Vattenfall (1991).
(1)
(2)
where c is the concentration of NO and [CO] is the concentration of CO symbolizing also the influence of hydrogen, Hz. (3) The surface of char in the bed A is proportional to the concentration of char in the bed and to the combustible part of the fuel flow added. A N GP + C,(l -
PVW,/W,~~,)
(3)
where C is the concentration of char in the bed originating from coal and wood, and d is the combustible fraction of the fuel. (4) The CO concentration is assumed to be 1% in the case of wood and p %(p < 1) in the case of coal, and [CO] = 1 -pp.
(4)
For a simple plug flow reactor dc/dt = -
+mr)‘-” - L(P + m&l
-PP)~~“(‘-‘) (5)
Gaseous emissions from CFB combustion
0 100
+_
Energy
cod wood
+
100 0
fraction,%
Fig. Al. Case (1) Standard data. The same CO concentration in the case coal or wood-chips. m, = 0.2; a = 0.4; WI,= 0.15; p = 0. (2) Same as above but less nitrogen in the wood-chips. m, = 0.15; a = 0.4; m2 = 0.15; p = 0. (3) Same as Case 1, but different CO concentrations in the case of coal and wood. m, = 0.2; a = 0.4; m2 = 0.15; b = 0.4; p = 0.4.
The constants 5, and & are determined from the two extreme cases in Fig. 13a. P = 1 (coal only) c(p = 1) = 52 ppm i
p = 0 (wood only) c(p = 0) = 78 ppm. i
The values of m, and m2 are given by the constants in eqns (1) and (3). The measured char mass concentrations in the bottom bed (2 and 0.2% for coal and wood respectively) and in the top bed (0.5 and 0.04%) give C, = 0.1 C,. The other values, d, H and n, are obtained from Table 2 or from the text. The
389
reaction orders a = 0.4 and b = 0.4 are chosen as average values from data of Ref. 6. p is the only uncertain quantity. The gas concentrations behave differently in wood-fired and coal-fired fluidized beds, but the order of magnitude of the concentration is the same. Therefore the approximation p = 0 is not unreasonable for the purposes of the present calculation, and other values of p are only used for the sensitivity analysis. The result of calculations with eqn. (5) is plotted in Fig. Al together with the measured emissions from Fig. 13a. The model describes the maximum of the measured emission data which supports the explanation given. The good agreement between model and measured data is probably a coincidence. Minor adjustments of the model inputs and assumptions could displace the result somewhat. For instance, a 5% higher nitrogen content of the coal (or lower nitrogen content of the wood) might displace the result to the curve representing Case 2, Fig. Al. It is also seen that a higher CO concentration in the wood case than in that of coal would lower the results and make the maximum values smaller (Case 3). In general, however, the existence of a maximum in the model results supports the assumption of the char as a major reducing agent for NO.
BiomassandBioenergyVol. 4, No. 5, PP. 379-389,1993 Printedin GreatBritain.All rightsreserved
GASEOUS
EMISSIONS FROM CIRCULATING BED COMBUSTION OF WOOD
FLUIDIZED
B. LECKNERand M. KARLSSON Chalmers University of Technology, Department of Energy Conversion, S-412 96 Goteberg, Sweden (Received I7 February 1993; revised received 23 March 1993; accepted 6 April 1993)
Abetract-The emissions of NO, N,O, CO and SO, were investigated during combustion of wood in a circulating fluidii bed boiler as a function of various parameters, including bed temperature, air supply and load. Emissions from mixtures of wood and coal were also investigated. The results show that the nitrogen oxide emission is directly related to the nitrogen content of the wood. The small amount of char in the bed during combustion of wood results in a much smaller reduction in the NO formed in the case of wood compared to that during combustion of coal and reduces the impact of bed temperature and air supply on the NO emissions during combustion of wood as compared to coal. Emission of N,O during combustion of wood was negligible. Keywords--Gaseous
emissions, biofuels, circulating fluidized bed combustor, nitrogen oxides.
1. INTRODUCTION
Fluidized bed combustion is an environmentally advantageous method of burning solid fuels of different types, including bio-fuels. Wood is sometimes claimed to be a comparatively clean fuel which is simple to burn. However, few systematic studies of gaseous emissions from wood combustion in fluidized beds have been carried out. Information is available from single tests where operating conditions have not been specified and from studies where operating conditions have been varied in an unsystematic way. Figure 1 shows an example of emissions of NO recorded for various bed temperatures from a 165 MW (thermal power) circulating fluidized bed (CFB) boiler fired with wood waste. The results are quite scattered. However, this behaviour could not be explained, since the boiler operation was normal in all respects and similar tests with coal and peat had been carried out during the same test program’ without finding such irregularities in the emission values. In order to determine the reason for the scatter an extended test program focusing on wood was carried out. The purpose of this work was to determine the influence of various combustor operating parameters on the emissions of nitrogen oxides, gaseous monoxide, and other carbon components of the flue gases (e.g. unburned hydrocarbons) from combustion of wood in an atmospheric CFB which was large enough to be representative of commercial boilers and where
the conditions could be carefully controlled. A direct comparison with combustion of coal was also made. 2. EXPERIMENTAL
CONDITIONS
2.1. The boiler The tests were carried out using a 12 MW (thermal power) CFB boiler located in the power centre of Chalmers University of Technology. Figure 2 shows the boiler. The chamber dimensions of the combustion (number 1 in Fig. 2) are 1.7 x 1.7 x 13.5 m. The bottom part, below a height of 2 m, as well as two of the walls, is refractory-lined. The remaining sides of the combustion chamber are membrane-tube walls having a temperature of around 220°C. The cyclone is water-cooled but protected by refractory lining.
.d
z
~,‘770 .,.,,“ .,,,.,t’ ‘,(,(,‘ ,,,’ ,‘, %o 810 850 890 Bed temperature,‘C
Fig. I. Nitrogen oxide emissions from combustion of wood waste in a 165 MW (thermal power) CFB boiler as a function of bed temperature and under otherwise constant conditions. 379
B.LECKNERand M.KARLSON
380 Air inlet
5
Air inlet 4 + Air inlet 3 -w Air inlet l/2 3 <
.-. t Fig. 2. The boiler. (1) Combustion chamber. (2) Air plenum and start-up combustion chamber. (3) Fuel feed chute. (4) Cyclone. (5) Exit duct. (6) Convection cooling section. (7) Particle seal. (8) External heat exchanger (particle cooler). 0 Measurement holes. x Pressure taps. ?? Thermocouples + Secondary air nozzles (air inlets l-5).
The fuel is introduced through the inclined fuel chute (3) on the front of the boiler from two fuel hoppers. The boiler can be fed simultaneously from both of these, if required, allowing mixtures of fuels to be used. In addition to these hoppers from which woodchips and coal were fed, in some tests sawdust was also supplied to the fuel feed conveyor just upstream of the fuel chute by means of a specially installed and calibrated screw feeder. The maximum capacity of the screw feeder corresponded to a fuel power of 3 MW. Primary air is injected through nozzles in the bottom of the combustion chamber and secondary air can be supplied through several inlets in the form of groups of horizontally arranged nozzles, numbered 1 to 5 in Fig. 2. The secondary air inlet l/2 was normally used during the present tests, inlets 3 and 4 are located at higher levels of the combustion chamber, and inlet 5 consists of six holes in the gas exit duct of the cyclone. On some occasions air was also added at the entrance to the cyclone. The
location denoted
of this secondary “C” in Fig. 2.
air addition
has been
The temperature of the combustion chamber during changes in the thermal conditions, such as those caused by a change of fuel or by a variation in moisture content of the fuel, was controlled by adjusting the external heat exchanger (8). Fine tuning of the control of the bed temperature was accomplished by flue-gas recirculation. In this way the most important boiler parameters could be maintained constant and changes in the conditions, for instance of the moisture content of the fuels, could be compensated for without affecting the emissions. The small changes of fluidization velocity caused by adjustment of flue-gas recirculation have little impact on the emissions. The boiler is fitted with instruments for continuous recording of temperatures, pressures, flows of gases and solids, etc. A number of thermocouples are installed in the bottom bed, in the top of the combustion chamber, and upstream (Ti”) and downstream (Tl, T2, T3 and T4) of the cyclone, as illustrated in Fig. 3. 2.2. Gas analyses Flue gas analyses were carried out using two independent systems (A and B) as shown in Fig. 4. An 0, analyzer [O,(KC)] downstream of the cyclone measures the oxygen concentration used in calculation of the boiler excess-air ratio. (The excess-air ratio is denoted m in some of the following figures.) Gas samples were normally taken downstream of the baghouse filter,
T4
Tin
Fig. 3. The top of the combustion chamber, the cyclone and the cyclone exit duct. Temperature measurement locations (T) and holes of the secondary air registers 5 and C are also shown.
Gaseous emissions from CFB combustion
381
Flue gas duct From cyclone
System B
System .I
I
Pump
Pump /
Fig. 4. The two systems for gas analysis.
upstream of the flue-gas fan (System A), and also in the inlet of the cyclone (System B). The CO analyzers were corrected for interference from N,O. The analyzers are listed in Table 1. The analyzers were calibrated at regular intervals. The results of the emission measurements were converted to a standard flue gas with an oxygen concentration of 6% 0,. For this conversion, the oxygen concentration at the location of the gas analyzer was used.
2.3. Fuels and bed material The fuels were specially cut wood-chips of trunks from birch or fir, a Colombian bituminous coal and sawdust from fir. The properties of the fuels are given in Table 2 in the form of average values from several tests. In general, the composition of the wood from various sources is similar, although the form and the moisture content were different. There was also a variation in the nitrogen and moisture contents
Table I. The gas analyzers used. For System B, the symbols within brackets are used to name the different analyzers Gas
Range
Type
System A
02 co
NO
N,O SO*
o-10% o-loft0 DDID O-250 pb;n 0-lOO, O-500 ppm O-500 ppm
Magnos ST, paramagnetic Binos. NDIR Beckman 955, chemiluminescence Spcctran 647, NDIR Binos, NDVIS/UV
o-10% &250 ppm o-0.5% &lo% o-10% o-10%
Magnos 5T, paramagnetic Beckman 955, chemiluminescence Uras 7N, IR Scott IIIC, IR Binos, NDIR Rosemount, model 132, Zirconia
System B 02
NO CO(CTH) COW) CO(MKS) O,(KC)
382
B. LECKNERand M. 100 E?
0 0
4J L G 75 s c
0”50 2 J m 25 ‘B E Birch chips
T
Fir chips
Day of test Fig. 5. Variation of moisture and nitrogen content during the test period.
of a particular fuel, as shown in Fig. 5 by the recordings taken for wood-chips over the test period of one month. The heating values (lower) were 31.3 MJ kg-’ for the coal and about 19 MJ kg-’ for the wood, expressed on an ash-and-moisture-free basis. The bed material consisted of 0.35 mm average diameter silica sand mixed with a minor quantity of the fuel ash remaining in the bed after combustion. 2.4. Test conditions Each test was run for one to two hours under constant boiler operation conditions. The results presented are mean values over this period of time. 3. TEST STRATEGY
The tests consist of two parts. First, the wood-chips were used in a parameter study to investigate the influence on the emissions of changes in bed temperature, air supply and load. Then mixes of wood and coal were used. In the parameter study, one parameter was varied at a time, while the others were kept constant and equal to the values for standard boiler operation. The study covered a time-span
KARLSON
of one month and was divided into several test series (one for each parameter). Each test series included a period of standard operation in order to verify that nothing was changed unintentionally during the parameter study. As a consequence, runs under standard conditions were repeated 18 times. The average values recorded during these runs are shown in Table 3. Variations between the standard runs were very small. The largest variation was observed in flue-gas recirculation which was used to control the bed temperature. The amount of flue-gases recirculated was small, however, compared to the rate of air supply, and its influence on the emissions was negligible. Other variations were caused by the contents of nitrogen and moisture of the fuel (Fig. 5). These variations were not deliberate, but reflected differences in deliveries of wood from different locations. However, the chips were always cut in the same way. The moisture variations could be handled by the boiler control system as explained above, but the variations in the content of nitrogen affected the results and several test series were repeated in order to determine the effects of differing nitrogen content of the fuel. The temperature of the combustion chamber, the bed temperature, was fairly even. There was only a small rise in temperature from bottom to top, 10°C for coal and 20-30°C for wood. The temperature in the bottom of the combustion chamber T,, is used to characterize the bed temperature in the following presentation. The walls of the cyclone cause some cooling of the gas-particle suspension which is to some extent counterbalanced by heat release of combustion in the cyclone. In the case of coal and regular-sized wood-chips the extent of combustion in the cyclone was small and the temperature drop between the inlet (T,,) and the outlet of the cyclone (Tl) was about 50°C. Fine
Table 2. The fuel properties (average values),%. amf = ash and moisture free Coal Combustibles (as delivered) Moisture (as delivered) Ash (as delivered) Carbon (amf) Hydrogen (amf) Oxygen (amf) Nitrogen (amf) Sulphur (amf) Volatile matter (amf) Size, mm thickness or diameter length
19.9 13.7 6.4 78.7 5.3 13.8 1.50 0.8 39 < 10
Sawdust
Birch chips
Fir chips
86.5 13.1 0.4 50.7 6.4 42.8 0.10 0.02 82
62.6 37.0 0.4 50.2 6.2 43.4 0.22 0.05 82
61.1 38.6 0.3 50.7 6.3 43. I 0.15 0.02 82
0.8
1-3 545
1-3 545
Gaseous emissions from CFB combustion
383
Table 3. Average values during standard boiler operation
Number of cases Total air, kg s-l Secondary/primary air ratio Flue-gas recirculation, kg s-r Temperatures, “C bed cyclone inlet cyclone outlet (TI) cyclone outlet duct Fluidization velocity, m s-l 0, (KC)% (wet) Combustion chamber pressure drop, kPa
fuels, like the sawdust, on the other hand, show a considerable afterburning in the cyclone. The fine fuel particles tend to rise in a plume in the combustion chamber, partly without access to oxygen, mix with the oxygen in the cyclone, burn and raise the temperature. The amount of sawdust added in the present tests was not very large and hence the corresponding temperature rise in the cyclone was less than 50°C. The difference between the gas temperature at the cyclone outlet (Tl) and the cyclone outlet duct [(T2 + T3 + T4)/3] presented in Table 3 is probably explained by radiative heating of Tl from the cyclone and radiative cooling of the unshielded thermocouples from the cold wall of the convection pass. 4. RESULTS
4.1. Nitrogen content of the fuel The fuel nitrogen content vs. the emission of nitric oxide (NO) is shown in Fig. 6 for operation under standard conditions. There is a strong influence of the nitrogen content of the fuel on the emission of NO. All other possible infiuencing parameters were equal to those of the standard case.
Coal
Birch chips
2 3.6 1.0 1.0
10 3.7 1.0 0.34
854 857 817 791 6.9 3.5 5.7
The effects on the NO emission of bed temperature, excess air, primary-air stoichiometry, level of secondary air supply and load (fluidization velocity) are shown in Figs 7, 8, 9, 10 and 11. Usually, data for both birch and fir chips are plotted, since these differ reflecting the nitrogen content of the fuels. Except for this difference, each figure is based on deliveries of fuel mostly from the same origin and the data are only affected by small variations in nitrogen content. The NO emission is only weakly dependent on bed temperature and total excess-air ratio as seen in Figs 7 and 8. This conclusion is supported by the data shown in Table 4, where a comparison is made with boilers burning coal under otherwise similar conditions. This table shows the change of emission per degree of bed temperature or percent excess air. The temperature dependence of NO emission is almost five times as great for coal as for wood and the dependence of the emission on the excess air is almost twice that for coal as for wood. The results with fir chips illustrated in Fig. 7 show almost no dependence on temperature. This is probably caused by a slight variation of the fuel nitrogen content between the measurement
$
0.0
Fuel
%rogen
y/bfJ
Fig. 6. The variation of NO emission as a function of nitrogen supplied with the fuel during the standard case runs.
850 880 822 778 6.7 3.5 5.5
4.2. Parameter variations
.I
ou
8 3.6 1.0 0.18
851 871 834 782 6.7 3.4 5.6
4
2
Fir chips
50
1
I2 075: lm=lup Bed temperature,“C BOO
850
900
Fig. 7. The influence of bed temperature on the NO emission. (m is excess-air ratio.)
B. LECKNERand M. KARLWIN
384
2 * t
.
Birch
chipa
0 Fir 0 ““” chips 1.1 1.2
‘,,“‘I
1.3
1.4
Total excess air ratio Fig. 8. The influence of excess air on the NO emission. (Tb is bed temperature.)
points. If these points were considered as they stand together with the birch chips’ points, the dependence of the NO emission on temperature shown in Table 4 becomes even smaller. Primary-air stoichiometry is defined as the ratio of air supplied in the form of primary air and the stoichiometric air-demand for complete combustion of the fuel. A variation of the primary-air stoichiometry has a very small effect on the NO emission as shown in Fig. 9. This is similar to results with coal3 In the coal tests3 a reduction in NO emission occurred, however, when the supply of secondary air was moved from inlets l/2 (see Fig. 2) to higher levels in the combustion chamber. No such effect was observed in the present tests with wood (Fig. 10). Only in the extreme case when air was supplied from inlets 4 and 5 (50% of the secondary air in each inlet) was there a slight decrease in the NO emission. The load of the boiler is proportional to the fluidization velocity at a constant excess-air ratio. Figure 11 therefore shows the influence on the NO emission of both load and fluidization velocity. At low fluidization velocity, most of the bed material remains in the bottom of the combustion chamber and the gases are cooled as they pass the heat-transfer surfaces of the
f
oj,, ,,, /, 0.0
Primary
0.8
0.4
air
l/2
3
Secondary
3jc
4
air
4/c
4/5
register
Fig. 10. The influence of the level of secondary air supply. The numbers on the horizontal axis indicate the secondary air register(s) where the secondary air was supplied. The slash indicates that the secondary air is equally split between the two registers.
combustion chamber. As a result, the temperature at the top of the combustion chamber (and in the cyclone and flue gas duct) falls as shown in Fig. 11. The NO emission also decreases. The reason for this smaller NO emission at low fluidization velocities is that more bed material and also more fuel remain in the bottom bed below the secondary air nozzles where now a truly sub-stoichiometric combustion takes place, assisted by the secondary air which accomplishes final burn-out of the gases leaving the sub-stoichiometric primary combustion zone. This is in contrast to the situation at full load (8 MW) when the solid fuel is partly carried away with the flue gases to regions of the combustion chamber above the secondary air nozzles where over-stoichiometric conditions prevail. The nominally sub-stoichiometric bottom zone does not play a great role in NO reduction at full load, since at full load the fuel does not burn there and the bottom zone does not actually become oxygen-deficient. The extension of combustion to the entire combustion
,I 1.2
stoichiometry
Fig. 9. The influence on NO of the primary air stoichiometry.
Fig. 11. The influence of fluidization velocity (load). The corresponding temperatures in the top part of the combustion chamber are also shown.
385
Gaseous emissions from CFB combustion Table 4. Influence of bed temperature and excess air on the emission of NO from wood and coal
Parameter Bed temperature, ppm/“C Excess air, ppm/% excess air
Coal references
Wood present work
I
2
3
4
0.2 1.1
0.6 I.91
I.0 -
0.7 2.0
0.9 1.8
*Uncertain.
chamber also explains the small effect of changes in the bottom zone below the secondary air inlet, as shown in Figs 9 and 10. The emissions of N,O from combustion of wood are negligible for all parameters investigated. An example is shown in Fig. 12. This is different from observations with CFB combustion of coal from which emissions of NzO in the order of 50 to 200ppm have been measured. ‘~~3~ The emissions of CO were low (around 50 ppm) in all cases investigated and no unburned hydrocarbons were recorded in the flue gases leaving the boiler. 4.3. Mixtures of fuels
‘R 0
z
E .t:
50
I_
z ;50Bed
NO+C-+;N,+CO
(Rl)
NO + CO+ ;N, + CO, (catalyzed by char).
Coal and wood combustion in fluidized bed differ due to the different volatile matter contents of the fuels (cf. Table 2). During coal combustion a considerable part of the fuel burns as char in the bed, whereas in the case of wood the combustion of volatiles dominates. Moreover, the reactivity of wood char is higher than that of coal, and the char concentration in the bed becomes very low in the case of wood combustion. Measurements made during the present tests with 100% coal or 100% wood combustion show char concentrations of 2.0% or 0.2% in the bottom part of the combustion chamber and 0.5% or 0.04% respectively in samples taken in the particle return duct after the cyclone. The difference between bottom and top of the combustion chamber, the latter
z
represented by the samples taken after the cyclone, shows that char concentration varies with height of the combustion chamber. The measured char concentrations also show that the char concentration for wood is about a tenth of that of bituminous coal during the present combustion conditions. The concentration of char in the combustion chamber is important for the reduction of nitrogen oxides, since two of the most significant reduction reactions are6
a
800
850
900
temperature,‘C
Fig. 12. The influence of bed temperature on the emission of N,O.
(R2)
Hydrogen, H,, may play the same role as CO in (R2). The combustion of various mixtures of fuel, from 100% wood-chips to 100% coal, was compared under standard conditions of boiler operation. In some tests sawdust was used instead of wood-chips to simulate a fine-particle fuel. The results for emission of NO, CO, SOI and N,O are shown in Fig. 13. The extreme cases with 100% wood or 100% coal illustrated in Fig. 13a show that the emission of NO from combustion of wood is normally higher than that from combustion of coal. It can also be seen that a small addition of coal to wood gives a higher emission of NO than that from wood only. This can be explained (see Appendix) in terms of the higher nitrogen content of coal, which leads to a greater formation of NO than during combustion of wood. When the fraction of coal added is small, the amount of char in the bed is also small, and the NO formed is only reduced to nitrogen to a small degree. When the fraction of coal fed to the boiler is further increased, the amount of char in the bed increases, and hence a greater proportion of the NO formed is reduced. As a result, the NO emission gradually decreases from its maximum value as the fraction of coal increases, and only a few percent of the fuel nitrogen is converted to NO in a 100% coal-
B. LECKNERand M.
386
c
100
Energy
J
e
100
Energy
crml rood
+
100 0
fraction,%
wood
0
fractions
(4
100
e
Energy
&
"0 100
e
Energy
rood
0
fraction,%
coal rood
+
100 0
fraction,%
Fig. 13. (a) The emission of NO from mixtures of coal and wood. Other conditions constant. (b) The emission of CO from mixtures of coal and wood. Other conditions constant. (c) The emission of SO2 from mixtures of coal and wood. Other conditions constant. (d) The emission of N,O from mixtures of coal and wood. Other conditions constant.
KARLSON
fired bed, whereas the conversion of the fuel nitrogen of wood to NO is almost 20%. Figure 13b shows that the emission of CO increases steadily with an increasing fraction of coal despite the fact that the CO concentration in the combustion chamber upstream of the cyclone is higher with wood than with coal as a fuel. The CO concentration in the entrance to the cyclone was about 0.5% during combustion of wood-chips and locally as high as 1 to 2% with sawdust. After the cyclone, however, the CO concentration was low, but it was higher with coal than with wood. Generally, the large decrease of the CO concentration in the cyclone with wood as well as with coal is caused by the mixing of the gases in the cyclone, which readily oxidizes the CO. In the case of coal, the higher concentration of char particles contained in the particle flow in the cyclone burns and additional CO is produced. In the case of wood-chips there was practically no char in the cyclone, almost all had been consumed in the combustion chamber. Thus, the CO level becomes higher with coal than with wood. Sawdust is seen to decrease the CO emission even more than wood-chips. This is most likely a consequence of the increased cyclone temperature during sawdust addition (see below). As shown in Fig. 13c, the emission of SO2 also increases in proportion to the amount of coal burned, since the sulphur content of the wood is negligible and all the sulphur is contained in the coal. The maximum value, at 100% coal, corresponds to the sulphur content of the coal minus a self-absorption in the coal ash amounting to 10% to 15% of the sulphur. The small amount of ash in the wood had no measurable influence in binding sulphur. As shown in Fig. 13d, the emission of N,O originates from the coal, since wood alone emits negligible amounts of N,O. It is seen, however, that the relationship is not entirely linear; the addition of wood-chips and especially that of sawdust decreased the N,O emission. This effect could have been caused by a slightly higher bed temperature (about 1OC) in the top of the combustor when wood was added, and during addition of sawdust also by a temperature rise in the cyclone due to afterburning. The cyclone temperature rise was about 50°C at maximum sawdust addition of 27%. In fact, one of the reasons for adding the sawdust was to investigate the possibility of raising the cyclone temperature. This effect was not very great, however, with the quantities of sawdust used,
387
Gaseous emissions from CFB combustion
and hence the decrease in NzO caused by the temperature rise was moderate. The addition of wood could also have had an effect on the N20 formation and decomposition through the combustion of wood and its involving volatiles, resulting in reactions hydrogen or hydrogen radicals. However, no measurements were made to support this assumption. 5. DISCUSSION
It was shown in Fig. 6 that the NO emission from wood combustion in a fluidized bed is closely related to the fuel-nitrogen content. A similar result has been obtained for flame combustion of oil’ and for grate combustion of wood.* For fluidized-bed combustion of coal in commercial-size combustors, however, no clear relationship has been reported between fuelnitrogen content and NO emission.’ The reason for this difference between coal and wood lies in the significant contribution of the char contained in the coal-fired fluidized bed to the reduction of the NO formed, reactions (Rl) and(R2). The NO emission from a coalfired fluidized bed corresponds to only a few percent of the coal nitrogen. Various parameters such as bed temperature and air supply affect the char concentrations of the bed, and thus the NO emission, and have a stronger impact on the emission than the coal-nitrogen content. Therefore, it has been difficult to distinguish the influence of the nitrogen content of the coal from the effect of other parameters. In the wood-fired fluidized bed, on the other hand, the concentration of char is small, the conversion of the fuel nitrogen is high and the relationship between the emission of NO and the fuelnitrogen content is clear. For the same reason, the influence of boiler parameters, bed temperature and excess air ratio, such as shown in Table 4, is small. The fuel used to produce the results shown in Fig. 1 was wood waste from the forest, containing branches and even green needles. The composition of the fuel could vary from one
5o[NO increase,
ppm
Fig. 14. Influences of cyclone combustion on temperature and NO increase in the cyclone. (NO reduction is negative increase).
delivery to another. This has a significant impact on the nitrogen content of the fuel, as shown in Table 5, which clearly demonstrates that the nitrogen content of wood-waste greatly depends on the parts of the tree included in the wood-waste. Most likely the scattered results shown in Fig. 1 can be explained in terms of varying nitrogen content of the fuel. The influence of cyclone combustion and of changes in the cyclone temperature have not had a very great importance in the present range However, some of parameter variations. changes in the character of the cyclone activities are as shown in Fig. 14, which indicates the relationship between cyclone combustion (temperature increase from the entrance to the exit of the cyclone) and NO release in the cyclone (NO increase; the difference between the concentrations of NO in the entrance of the cyclone and the stack). In the standard case of boiler operation (coal or wood-chips) there is some cooling in the cyclone and some NO reduction. When combustion in the furnace is retarded by moving the secondary air to higher levels (inlets 3 and 4) some NO is produced in the cyclone instead, and when sawdust is added to wood-chips there is both a rise in temperature due to cyclone combustion and an evident increase in NO. The increase in the NO emission in these cases is clearly caused by formation of NO in the cyclone and insufficient time for reduction. 6. CONCLUSIONS
Table 5. Nitrogen content of various parts of tree. (Average values from different types of tree”) Part of tree Trunk Bark Branches Needles
Nitrogen content % 0.05 0.4 0.6 1.0
The following conclusions can be drawn concerning fluidized-bed combustion of wood: (i) the fuel-nitrogen content is the most important parameter affecting the NO emission; (ii) the emission of NO is generally higher than from coal combustion; (iii) the influence of bed temperature and air supply is less than in coal
388
B. LECKNERand M. KARLSWN
combustion; and (iv) practically no N,O is emitted. Furthermore, it is concluded that: (i) emissions from mixtures of coal and wood are approximately proportional to the mixing ratio of the fuels and to the emission properties of the respective fuels; (ii) an influence of the NO reduction of char is clearly demonstrated in the tests with mixtures of coal and wood; (iii) the cyclone has a beneficial effect for CO combustion, making the CO emissions from wood combustion very low; and, finally, (iv) extremely fine fuel powder tends to rise in a plume in the combustion chamber and burns to a large extent in the cyclone. Acknowledgements-This work was financed by the following organizations: Swedish National Board for Industrial and Technical Development (NUTEK), Swedish Energy Development Corporation (SEU), Kvaerner Generator AB and &ebro Energi AB.
APPENDIX
Estimation
of the NO-char
relationship
The explanation of the NO emission from combustion of mixes of wood and coal under otherwise similar conditions (Fig. 13a) can be verified by a simple model assuming a plug flow of gas through a matrix of char particles in the bed. The essential assumptions are: (1) Nitric oxide is formed from combustion, proportional to the nitrogen content of the coal n,, the wood n, and the fuel flow, cow n,p + n,(l - P)K/K
where p is the energy fraction of coal, and H,, H, are heating values of coal and wood. (2) The rate of reduction of NO depends mostly on reduction on the surface of char A or on reduction of CO or H, catalyzed by char6 k N Ac”[CO]~
REFERENCES
1. B. Leckner, M. Karlsson, M. Mjornell and U. Hagman, Emissions from a 165 MW,,... circulating fluid&d bed boiler. J. Inst. Energy 65, 122-l 30 (1992). 2 M. Mjdmell, B. Leckner, M. Karlsson and A. Lyngfelt, Emission control with additives in CFB coal combustion. In 11th Int. Conf: on Fluidized Bed Combustion (E. J. Anthony, Ed.), pp. 655-664. ASME Book No 10312B (1991). 3 L.-E. Amand and B. Leckner, Influence of air supply on the emissions of NO and N,O from a circulating fluid&d bed boiler. In 24th Symposium (Int.) on Combustion, pp. 1407-1414. The Combustion Institute, Pittsburgh (1992). 4. B. Leckner and L.-E. Amand, Emissions from a circulating and a stationary fluidized bed boiler-a comparison. In 1987 Int. Conf on Fluidized Bed Combustion (J. P. Mustonen, Ed.), pp. 891-897. ASME Book No 10232A (1987). 5 L.-E. Amand and B. Leckner. Influence of fuel on the emissions of nitrogen oxides (NO and N,O) from an 8 MW fluidized bed boiler. Cornbust. Flame 84, 181-196 (1991). 6. J.-E. Johnsson and K. Dam-Johansen, Formation and reduction of NO, in a fluidized bed combustor. In I 1th Int. Co& on Fluidized Bed Combustion (E. J. Anthony, Ed.), pp. 1389-1396. ASME Book No 10312B (1991). I. D. W. Pershing, J. E. Cichanowicz, G. C. England, M. P. Heap and G. B. Martin, The influence of fuel composition and flame temperature on the formation of thermal and fuel NO, in residual oil flames. In 17th Symposium (Int.) on Combustion, pp. 715-726. The Combustion Institute, Pittsburgh (1978). 8. T. Nussbaumer, Stickoxide bei der Holzverbrennung. Heiz. Klima 15, 51-62 (1988). 9. T. Shimizu, J. Tatebayashi, H. Terada, T. Furusawa and M. Horio, The combustion characteristics of different types of coal in the 20 t/h fluidized-bed boiler. In Proc. of the Eighth Int. Con/ on Fluidized-Bed Combustion, pp. 231-239. US Dept. of Energy, Morgantown (1985). 10 J. Johansson, Characterisation of fuels-nitrogen compounds (in Swedish with English summary). U(B)l991/39, Swedish Power Board, Vattenfall (1991).
(1)
(2)
where c is the concentration of NO and [CO] is the concentration of CO symbolizing also the influence of hydrogen, Hz. (3) The surface of char in the bed A is proportional to the concentration of char in the bed and to the combustible part of the fuel flow added. A N GP + C,(l -
PVW,/W,~~,)
(3)
where C is the concentration of char in the bed originating from coal and wood, and d is the combustible fraction of the fuel. (4) The CO concentration is assumed to be 1% in the case of wood and p %(p < 1) in the case of coal, and [CO] = 1 -pp.
(4)
For a simple plug flow reactor dc/dt = -
+mr)‘-” - L(P + m&l
-PP)~~“(‘-‘) (5)
Gaseous emissions from CFB combustion
0 100
+_
Energy
cod wood
+
100 0
fraction,%
Fig. Al. Case (1) Standard data. The same CO concentration in the case coal or wood-chips. m, = 0.2; a = 0.4; WI,= 0.15; p = 0. (2) Same as above but less nitrogen in the wood-chips. m, = 0.15; a = 0.4; m2 = 0.15; p = 0. (3) Same as Case 1, but different CO concentrations in the case of coal and wood. m, = 0.2; a = 0.4; m2 = 0.15; b = 0.4; p = 0.4.
The constants 5, and & are determined from the two extreme cases in Fig. 13a. P = 1 (coal only) c(p = 1) = 52 ppm i
p = 0 (wood only) c(p = 0) = 78 ppm. i
The values of m, and m2 are given by the constants in eqns (1) and (3). The measured char mass concentrations in the bottom bed (2 and 0.2% for coal and wood respectively) and in the top bed (0.5 and 0.04%) give C, = 0.1 C,. The other values, d, H and n, are obtained from Table 2 or from the text. The
389
reaction orders a = 0.4 and b = 0.4 are chosen as average values from data of Ref. 6. p is the only uncertain quantity. The gas concentrations behave differently in wood-fired and coal-fired fluidized beds, but the order of magnitude of the concentration is the same. Therefore the approximation p = 0 is not unreasonable for the purposes of the present calculation, and other values of p are only used for the sensitivity analysis. The result of calculations with eqn. (5) is plotted in Fig. Al together with the measured emissions from Fig. 13a. The model describes the maximum of the measured emission data which supports the explanation given. The good agreement between model and measured data is probably a coincidence. Minor adjustments of the model inputs and assumptions could displace the result somewhat. For instance, a 5% higher nitrogen content of the coal (or lower nitrogen content of the wood) might displace the result to the curve representing Case 2, Fig. Al. It is also seen that a higher CO concentration in the wood case than in that of coal would lower the results and make the maximum values smaller (Case 3). In general, however, the existence of a maximum in the model results supports the assumption of the char as a major reducing agent for NO.