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The effect of vitiated air on the combustion of pulverized coal for applications to coal fired combined cycle steam generators
Eighteenth Symposium (International) on Combustion
The Combustion Institute, 1981
T H E E F F E C T O F V I T I A T E D AIR ON T H E C O M B U S T I O N OF P U L V E R I Z E D COAL F O R A P P L I C A T I O N S TO COAL F I R E D C O M B I N E D CYCLE STEAM G E N E R A T O R S FRANCESCO FLORIS Sezione Macchine, Istituto di Meccanica della Facoltd di Ingegneria, Piazza D'Armi, 09100 Cagliari, Italy
A 15 Kg/h pulverized fuel two dimensional furnace was used to determine combustion efficiency, ignition stability, CO, CmH ., NO x profiles of experimental combustors under oxygen-lean and variable excess vitiated air conditions for applications to gas turbine-steam turbine combined engines. A domestic bituminous coal mined in Sardegna was investigated. Results show that the total vitiated air excess must be maintained above 30% and oxygen content in the primary stream cannot drop below 17% to avoid a carbon loss of over 3% wt. and unsteady flame formation. The secondary air oxygen content has a lesser influence on combustion efficiency: the tests performed show that an amount of 15% oxygen is sufficient to support a steady flame with over 0.97 combustion efficiency, if the proper swirl generator is used and the total excess of oxidizer is above the limit of 30%. The secondary vitiated air temperature and an inertial vortex generator offered a successful method of providing swirling turbulent coal flame and smoke, CO, and unburned hydrocarbons under environmental conditions.
Introduction The possibility of improving the conversion of primary to electrical energy by combining a gas turbine cycle with that of a steam turbine has suffered a set-back after the 1974 oil price crisis, due to the restrictive measures regarding the use of the imported energy sources, fuel oil and natural gas. Up to December 1973 in most cases to be found in practice"2~, an open cycle gas turbine preceded the waste heat steam generating plant with supplementary firing and both coupled plants burned gas or light fuels in the TG and fuel oil in the boiler. Despite this fact, there is no reason to reject the development of a combined cycle, if coal can be used in the steam generator in addition to oil or gas in the TG. The use of light fuels in the TG is necessary to keep the input temperature between 1200-1400 K and consequently an efficiency above 0.3, which, coupled with the 0.4 efficient steam cycle, results in a combined cycle with an efficiency of 0.43 to 0.47. In addition to the improved utilization of energy due to these improvements in efficiency, emission values also diminish considerably with respect to
power produced, because of reduced heat consumption; the drop in these values can contribute to cost saving, particularly on the flue gas side, if desulphurisation plants should be required. To obtain information on pulverizing, combustion and fouling characteristics of Sulcis bituminous coal, including the effect of vitiated air, a double combustion chamber with intercooling, designed to burn pulverized coal, was installed in the Istituto di Meccanica of Cagliari University. The paper describes the adjustments on the postcombustion burner to optimize flame shape to overcome fouling and emissions and the influence on combustion efficiency of the primary and secondary air temperatures, burner port velocity and oxygen content of the flue gas, closely simulating the operation of a gas turbine exhaust.
1313
ExperimentalApproach In order to obtain postcombustion data under closely controlled laboratory conditions, a double refractory lined furnace, pioneered by others, 3~ was adjusted to burn pulverized coal and fuel oil at a nominal rate of 10 K g / h on the first burner and
1314
COAL COMBUSTION
15 Kg/h of coal and/or fuel oil on the supplementary burner. An artist's view of the test rig is shown in Fig. 1. The recycle loop inserted in the primary air line to the second burner is necessary as a coal transport line via a Venturi mixing device. Sulcis as-received coal, for which the analysis is given in Table 1, is processed at a location apart from the furnace; it is crushed by feeding coal between the two toothed rolls of a double roll crusher. The end product is pulverized in a slow speed tube mill up to 90% through 200 mesh. The pulverized is discharged into a storage bin and later conveyed via a vibrating screen, whose frequency is given by an electric vibrator in a range from hundreds to thousands of vibrations per minute. The fine particles come down to the screen discharge end and are mixed with the primary air stream in a Venturi device, then burned through the burner in the front wall of the second rectangular furnace which is 450 mm wide, 650 mm high and 1670 mm deep. A cross section of one of the burners used for pulverized coal combustion is shown in
Fig. 2. The burner is designed to impart swirl to the primary vitiated air stream, whose velocity in the transport line ranges from 14 to 25 m/s. Both primary and secondary air can be preheated from 350 to 650 K. The fuel used in the first combustion chamber and as "start up" fuel in the second is of the fluid type (3-5 Engler at 325 K). In order to investigate the composition of flue gas from the flame without drastically changing the fluid dynamic conditions, a probe having a tip with rounding-off angles of ca. 30~ was designed, (Fig. 3), cooled by water circulation and able to sample in isokinetic conditions, as indicated in the literature. 4~The probe is mounted on a traversing mechanism which has the precision of location of 0.1 mm, in order to explore the gas concentration profiles in the axial and radial directions. CO, CO~, 02, C,~H, are analyzed by an automatic continuous control console, consisting of NDIR and polarographic analyzers. SO2, SO3, NO~, soot analyses are carried out by traditional methods of wet chemical analysis.
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FIG. 1. Artist's view of the test facility used at the "'Istituto di Meeeanica."
APPLICATIONS TO COAL FIRED COMBINED CYCLE STEAM GENERATORS
1315
TABLE 1 A n a l y s i s of f u e l s Seruci coal as received Proximate, %wt moisture 7.2 vol. matter 43.3 fixed carbon 39.4 ash 10.1
U l t i m a t e , %wt hydrogen 5.4 carbon 63.5 nitrogen 2.0 oxygen 12.5 sulfur 6.5 ash 10.1
H i g h e r H e a t i n g Value = 6 0 5 0 K c a l / K g
A s h a n a l y s i s , %wt SiOz AI203 TiOa
1.53 1.08 0.05
Acidic
2.66
Fe20 a CaO MgO NazO K~O
9.92 37.19 5.76 0.99 0.10
Basic S u l f u r as SO 2 -
53.96 43.38
F u e l oil a n a l y s i s D e n s i t y at 293 K = 0.917 K g / l Viscosity at 323 K = 37 cSt Moisture 0.72 %wt Carbon 84.00 %wt Hydrogen 12.00 %wt Sulfur 3.33 %wt H i g h e r H e a t i n g Value = 10324 K c a l / K g Nickel 14 p p m Vanadium 44 p p m Sodium 80 p p m
FIG. 2. O r i g i n a l fuel o i l / p u l v e r i z e d coal b u r n e r a s s e m b l y .
1316
COAL COMBUSTION
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Handling and Combustion Characteristics The coal received from the Seruci mines was under 50 mm maximum size and had a moisture content of 7.0%. Difficulties were encountered in pulverizing in tube mills because of the low grindability of Sulcis coal (about 50) and its tendency to form hard agglomerates. Difficulties, using indirect firing systems, were also experienced in hoppers, on the vibrating screen and in the transport line due to unusual build-up of packed pulverized coal. This was attributed to the moisture content and the problem was solved by heating the stock and increasing the temperature of the transport fluid. A cross section of the supplementary burner and combustion chamber is shown in Fig. 2. The windbox was designed to impart swirl to both the primary
and secondary air streams. A screw swirl inducer was adopted in combination with a conical shield in order to obtain the following effects: a--prevention of fuel from entering the core of the primary zone, which is full of burning products and lacks oxygen b - - i m p r o v e d chances of combining coal with available oxygen c--improved mixing of coal with the secondary air stream d--protection of the stream against radiation from the flame Initial tests, using the burner design of Fig. 2 resulted in fouling virtually with all oxygen and temperature conditions selected, as summarized in Table 2. A long bushy flame of the high combustion
TABLE 2 Summary of experimental results with burner of Fig 2 Inlet to 1st eombustor Air flow 124 k g / h Air temp. 293 K CO 2 0.0 %vol. 02 21.0 %vol. N2 79.0 %vol.
Inlet to 2nd eombustor
Outlet from 2nd eombustor
vitiated p.air flow 63 K g / h
flue gas 136 K g / h
Vit.s.air flow 96 K g / h p.air
s.air
Fuel oil burnt 5.0 K g / h COz HzO O2 N2
3.0 0.3 16.4 80.3
%vol. %vol. %vol. %vol.
coal burnt 7.0 K g / h excess oxy. 45% p.air velocity 23 m / s p.air temp. 390 K s.air temp. 500 K
7.2 1.0 11.0 80.8
COz 15.0 %vol. H20 1.5 %vol. 02 6.0 %vol. N2 77.5 %vol. CO 2.2 m g / N m 3 CmH" 0.5 m g / N m ~ carbon loss 2.5 m g / n m 3 comb. efficiency 95%
APPLICATIONS TO COAL F I R E D C O M B I N E D CYCLE STEAM GENERATORS
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FIG. 5. Effect of primary vitiated air oxygen content on fly-ash emissions FIG. 4. Modified pulverized coal burner assembly. intensity type was produced. The swirl inducer did not give symmetrical mixing of the primary and secondary streams, so that the flame shape was not uniform in length all around the burner mouth. This drawback, coupled with the high fouling which reduced efficient operation in a few hours, suggested modifying the design of the primary air s w i r l impeller. Figure 4 shows the new burner. It consists of a tube body vortex generator and outlet tube. Coal entering the tube is given a swirling motion by the vortex generator. This motion throws the particles radially outward toward the wall of the tube, by centrifugal force. The pitch and length of
the vortex generator determines the degree of mixing of the primary and secondary streams. Port velocity in the range 30-40 m / s was achieved with the face of the body turned toward the conical shield. The combined results of the above modifications produced a combustor with higher combustion efficiency and lower CO and soot levels, as shown in Table 3. Carbon conversions are lower than those obtained with above stoichiometric air combustion, practically for every vitiated air and oxygen value. The operating excess air levels cannot be reduced to any significant extent due to the properties of the coal being fired. Suleis coal has a high propensity to deposit ash which slags in the firing zone. As shown in Fig. 5, fly ash and soot formation seems to be a function of the excess air parameter. Consequently,
TABLE 3 Summary of experimental results with burner of Fig 4
Test
p.air temp. K
Excess a.
oxygen in p.air
Comb. efficiency
Flame temp. K
1 2 3 4 5 6 7 8 9 10 11 12
370 370 370 370 390 390 390 390 390 385 390 390
35 36 38 40 35 37 37 38 38 38 40 40
16 17 18 19 16 17 18 19 16 17 18 19
96.5 97.0 97.0 97.3 96.6 97.2 97.3 97.5 97.1 98.1 98.0 98.2
1420 1450 1450 1500 1500 1510 1550 1540 1505 1510 1550 1600
Secondary air temperature = 470 K
1318
COAL COMBUSTION
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Fzc. 6. Effect of primary air oxygen content on CO emissions in coal supplementary firing reduced exhaust gas excess operation is not a practical method of supplementary firing due to unacceptable levels of flyash emissions. In the tests carried out in our laboratory we had to operate at superabundant levels of excess air to reduce soot emission. Thus, we found an excess of oxidizer equal to 30 percent or more to be the dividing line between an acceptable combustion efficiency capable of being improved and an unacceptable production of pollutants. At higher levels, this dividing line excess air is practically ineffective in reducing unburnt products and NO, or improving
combustion efficiency; these depend on oxygen concentration, temperature of the primary and secondary streams and turbulence. It was found that vitiated air circulation to the primary flame zone is impossible only when oxygen levels in the primary oxidizing mixture drop below 17 percent because of loss of ignition. On the contrary, the oxygen content in the secondary air can reach 15% or less without excessive formation of soot, CO and hydrocarbons. In a few tests, combustion and ignition were maintained with a secondary air oxygen content equal to 14% and a primary air content of 20%; the total excess oxydant was 45%. This effect can be explained by the reactivity of Sulcis coal due to the high content of volatile matter and the porosity of the powder after escape of the volatile elements. The flyash, carbon monoxide, CmH, and NO x emissions measured are shown in Figs. 5, 6, 7, 8. The data plotted on the right hand of all figures correspond to conditions when non vitiated air is present, i.e., the air is passed through a fired preheater to bring it to 450 K and then to the second burner. These data form a base line for comparison with results obtained with vitiated air. The CO, soot and flyash values become asymptotic to the values pertaining to a simple direct combustion with preheated air. Although the actual concentration of CO and hydrocarbons was higher under vitiated air conditions, the contribution of CO and C , H, made by supplementary coal firing was relatively smaller at oxygen contents above 18%. The operation with low oxygen combustion air is slightly effective in reducing NO~ emissions as shown in Fig. 8. This can be attributed to the fact that flue gas circulation in the primary stream is effective in reducing thermal NO~ by flame temperature reduction, but ineffective in reducing fuel
350
tr~ess o~tydant
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secondary a i r 0 2
17.6
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~100
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50 . . . . . . . . . . . . . . . . . . . . . . . . .
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150
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excess oxydant
30
secondary air 0 2
16.5 %
%
0.2 15
16
17
18
19
2o
21
02%
dry volumetric basis
C/ 15
Fzc. 7. Effect ot primary air oxygen content on unburned hydrocarbons emissions in coal supplementary firing.
16
17
18
19
20
21
02%
Fro. 8. Effect of primary vitiated air oxygen content on NO, emissions from 2nd coal burner.
APPLICATIONS TO COAL F I R E D C O M B I N E D CYCLE STEAM GENERATORS
99-
98
o
~
r
n
a
r
Y
air
18,6 %
97
P 96
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FIG. 9. Effect of secondary air temperature on combustion efficiency of supplementary coal firing.
nitrogen conversion. Sulcis coal typically has nitrogen contents ranging from 1.0 to over 2.0 percent by weight, '5~ dry, mineral matter free basis, which can represent a significant source of NO x formation in coal-fired furnaces. To assure stability of ignition, the temperature of the primary air and coal leaving the mixer had to be at least 350 K. The maximum temperature of the primary air/coal mixture chosen in the laboratory tests was a compromise between the tendency of Sulcis coal to coke on the burner parts and the need to remove surface moisture; we found that 370390 K was a good compromise between these two needs, as shown in Table 3. Therefore the influence of temperature on combustion efficiency could be investigated only in the secondary vitiated air duct. The effect of the secondary air temperature on combustion efficiency is illustrated in Fig. 9. The figure shows that there is a penalty paid in terms of efficiency when lowering the temperature to 450 K. On the contrary, higher temperatures have been shown to be effective in increasing efficiency above 98%. The possibility of utilizing this tool to reduce unburnt combustible loss below 2-3 percent by weight will probably depend on the complexities of the design of windbox, vanes and registers of utility units, due to additional port velocity and coal erosion.
Summary and Conclusions
1--Bituminous Sulcis coal could be burned efficiently in furnace combustors when burner modifications were made to maintain the excess vitiated air above 30% and the oxygen content in the primary stream above 17%, in both cases paying a slight penalty in combustion efficiency.
1319
The oxygen level in the secondary air was considered less important for ignition and combustion stability. Its concentration could be maintained in the range of 15-18%. For excess air and oxygen contents in primary stream below 30% and 17%, respectively, a marked reduction in combustion efficiency is obtained and an unacceptable production of pollutants is noted. Also, ignition of pulverized coal was no longer stable, without the use of supporting fuel. 2--CO, CmH,, smoke emissions could be maintained substantially equal to air direct firing emissions, if the oxygen and excess air values in the primary stream were in the above ranges. NO,emissions were slightly reduced as expected for a vitiated air operation. 6'7's'. 3 - - T h e secondary air temperature and turbulence proved to be the most successful methods for improving combustion efficiency. A compact inertial vortex generator offered a practical solution for providing swirl to the coal/air mixture with little pressure loss, improving the primary air/coal distribution and mixing of coal with the secondary air stream.
Acknowledgment The author wishes to thank Prof. Mario Carta and the personnel of the Istituto di Arte Mineraria, Cagliari, for the preparation of pulverized coal. Ing. Enrico Mascia is to be commended for his excellent work in operating the combustion facility and developing the sampling handling. The author is also grateful for financial support provided by the Consiglio Nazionale delle Ricerche under contracts No. 77.01377.07, No. 78.00742.07 and No. 79.00383.07.
REFERENCES 1. L^nR G.: Gas-Wdrme Int., Bd. 16, No. 7, 378, 1967 2. FLADJ.: Operation of a Combined District Heating and Power Production Scheme with a Combined Cycle Gas Turbine, Paper presented at the Von Karman Lecture Series 6, Brussels, April 1978 3. ACTONO., SAa'rAA.: I1 Calore, 39, 329, 1968 4. DiLOBENZOA., MASl S.: Annali di Chimica, 68, 619, 1978 5. AGus M., CAaTAM.: Delle caratteristiche mineropetrografiche, chimiche e tecnologiche (]el carbone della miniera Seruci-Nuraxi Figus, paper presented at the 1st Convegno Minerario e Metallurgico Italo-Sovietico, Cagliari, October, 1976 6. HAZARDH. R.: ]. of Engineering for Power, 96, 235, 1974 7. KOLaACKR., ACETO L. D.: Recirculation Effects
1320
COAL C O M B U S T I O N
in Gas T u r b i n e Combustors, ASME paper No. 74-WA/GT-3, 1974
8. SULUVAND. A.: J. of Engineering for Power, 99, 145, 1977.
COMMENTS B. Vollerin, Battelle, Switzerland. I would like to just come back to the results y o u presented in Figure 8, i.e. NO~ reduction versus vitiation of primary c o m b u s t i o n air and also to your last conclusion w h e n you say "'NO, is reduced w h e n u s i n g vitiation as expected." t) I u n d e r s t a n d that the results presented here are mainly preliminary results a n d that you will be continuing this program in the future. At Battelle-Geneva we have some on-going research programs dealing with flue-gas recirculation/vitiation of c o m b u s t i o n air allowing 02 concentration to get clown to 15 or 16% a n d simultaneous combustion staging (or more precisely delayed aerod y n a m i c mixing). T h e fuels we b u r n are natural gas, distillate a n d heavy fuel-oil. Particularly, in heavy fuel-oil combustion, we noticed that an optim u m can be f o u n d between the relative m o m e n t u m s of the primary a n d the secondary flows and the "chemical quality" (i.e. the partial pressure of oxygen) of these two flows of oxydizers, in order to get the m i n i m u m NO~ level at the stack while keeping the level of particulate emission at an acceptable level. I would like to k n o w if you already carried out similar experiments a n d if so, what kind of results did y o u obtain? 2) It is generally not expected to get a dramatic reduction on NO~ level w h e n u s i n g vitiation of air for combustion of coal or heavy fuel-oil, apart some effect on the thermal NO Xwhich doesn't represent quite a significant part of the total NO~ w h e n b u r n i n g these fuels. T h e results y o u presented in Figure 8 are basically due to some aerodynamic staging in your experimental set up. Could you tell us what are the velocities and swirl n u m b e r of your primary and secondary flows in order to compare the NO~ reduction you mentioned with some other results presented in the literature? Author's Reply. T h e exercise with vitiated air is quite similar, for some extent, to the recireulation
of c o m b u s t i o n gases w h e n introduced into the primary flame zone t h r o u g h the windbox auxliary secondary air compartments. NO~ emissions on tangentially fired gas and oil boilers were reduced u p to 50% w h e n oxygen levels in the combustion air/recirculated flue gas mixture drop below 16 to 17% .cl> The same effect was relatively lower on our coal fired unit. T h e general opinion is that this can be attributed to the fact that vitiated air operation is effective in reducing thermal NOx, by flame temperature reduction, b u t ineffective in reducing fuel nitrogen conversion. Nevertheless, the relatively ineffective flue gas oxidant operation as N O , control in our apparatus was due to the following characteristics of the burner: - - h i g h primary air, equal or higher than 20% - - m a x i m u m mixing of the secondary air with the coal prior to ignition of the volatile fractions, because of quasi-radial coal injection. T h i s rapid mixing between coal and vitiated air with sufficient swirl to form an internal recireulation zone was a necessary design characteristic to provide heat for early ignition. U p to now, we were not able to support coal/vitiated air combustion with flames which give minim u m NO~ emissions, as simple jet flames, low fuel air mixing rate p r o d u c e d with zero or very weak swirlJ 2>
REFERENCES
(I)SELKEBA. P. "'Program for Reduction of NO x from Tangential Coal Fired Boilers Phase II" E.P.A. 650/2-73-005-a, June 1975. 12>HEAP M. P. et al. " B u r n e r Design Principles for M i n i m u m NO= E m i s s i o n s " Proc. of E.P.A. Coal C o m b u s t i o n Seminar, Research Triangle Park (1973) I.F.R.F. Doc, nr k 2 0 / a / 6 7 .
The Combustion Institute, 1981
T H E E F F E C T O F V I T I A T E D AIR ON T H E C O M B U S T I O N OF P U L V E R I Z E D COAL F O R A P P L I C A T I O N S TO COAL F I R E D C O M B I N E D CYCLE STEAM G E N E R A T O R S FRANCESCO FLORIS Sezione Macchine, Istituto di Meccanica della Facoltd di Ingegneria, Piazza D'Armi, 09100 Cagliari, Italy
A 15 Kg/h pulverized fuel two dimensional furnace was used to determine combustion efficiency, ignition stability, CO, CmH ., NO x profiles of experimental combustors under oxygen-lean and variable excess vitiated air conditions for applications to gas turbine-steam turbine combined engines. A domestic bituminous coal mined in Sardegna was investigated. Results show that the total vitiated air excess must be maintained above 30% and oxygen content in the primary stream cannot drop below 17% to avoid a carbon loss of over 3% wt. and unsteady flame formation. The secondary air oxygen content has a lesser influence on combustion efficiency: the tests performed show that an amount of 15% oxygen is sufficient to support a steady flame with over 0.97 combustion efficiency, if the proper swirl generator is used and the total excess of oxidizer is above the limit of 30%. The secondary vitiated air temperature and an inertial vortex generator offered a successful method of providing swirling turbulent coal flame and smoke, CO, and unburned hydrocarbons under environmental conditions.
Introduction The possibility of improving the conversion of primary to electrical energy by combining a gas turbine cycle with that of a steam turbine has suffered a set-back after the 1974 oil price crisis, due to the restrictive measures regarding the use of the imported energy sources, fuel oil and natural gas. Up to December 1973 in most cases to be found in practice"2~, an open cycle gas turbine preceded the waste heat steam generating plant with supplementary firing and both coupled plants burned gas or light fuels in the TG and fuel oil in the boiler. Despite this fact, there is no reason to reject the development of a combined cycle, if coal can be used in the steam generator in addition to oil or gas in the TG. The use of light fuels in the TG is necessary to keep the input temperature between 1200-1400 K and consequently an efficiency above 0.3, which, coupled with the 0.4 efficient steam cycle, results in a combined cycle with an efficiency of 0.43 to 0.47. In addition to the improved utilization of energy due to these improvements in efficiency, emission values also diminish considerably with respect to
power produced, because of reduced heat consumption; the drop in these values can contribute to cost saving, particularly on the flue gas side, if desulphurisation plants should be required. To obtain information on pulverizing, combustion and fouling characteristics of Sulcis bituminous coal, including the effect of vitiated air, a double combustion chamber with intercooling, designed to burn pulverized coal, was installed in the Istituto di Meccanica of Cagliari University. The paper describes the adjustments on the postcombustion burner to optimize flame shape to overcome fouling and emissions and the influence on combustion efficiency of the primary and secondary air temperatures, burner port velocity and oxygen content of the flue gas, closely simulating the operation of a gas turbine exhaust.
1313
ExperimentalApproach In order to obtain postcombustion data under closely controlled laboratory conditions, a double refractory lined furnace, pioneered by others, 3~ was adjusted to burn pulverized coal and fuel oil at a nominal rate of 10 K g / h on the first burner and
1314
COAL COMBUSTION
15 Kg/h of coal and/or fuel oil on the supplementary burner. An artist's view of the test rig is shown in Fig. 1. The recycle loop inserted in the primary air line to the second burner is necessary as a coal transport line via a Venturi mixing device. Sulcis as-received coal, for which the analysis is given in Table 1, is processed at a location apart from the furnace; it is crushed by feeding coal between the two toothed rolls of a double roll crusher. The end product is pulverized in a slow speed tube mill up to 90% through 200 mesh. The pulverized is discharged into a storage bin and later conveyed via a vibrating screen, whose frequency is given by an electric vibrator in a range from hundreds to thousands of vibrations per minute. The fine particles come down to the screen discharge end and are mixed with the primary air stream in a Venturi device, then burned through the burner in the front wall of the second rectangular furnace which is 450 mm wide, 650 mm high and 1670 mm deep. A cross section of one of the burners used for pulverized coal combustion is shown in
Fig. 2. The burner is designed to impart swirl to the primary vitiated air stream, whose velocity in the transport line ranges from 14 to 25 m/s. Both primary and secondary air can be preheated from 350 to 650 K. The fuel used in the first combustion chamber and as "start up" fuel in the second is of the fluid type (3-5 Engler at 325 K). In order to investigate the composition of flue gas from the flame without drastically changing the fluid dynamic conditions, a probe having a tip with rounding-off angles of ca. 30~ was designed, (Fig. 3), cooled by water circulation and able to sample in isokinetic conditions, as indicated in the literature. 4~The probe is mounted on a traversing mechanism which has the precision of location of 0.1 mm, in order to explore the gas concentration profiles in the axial and radial directions. CO, CO~, 02, C,~H, are analyzed by an automatic continuous control console, consisting of NDIR and polarographic analyzers. SO2, SO3, NO~, soot analyses are carried out by traditional methods of wet chemical analysis.
k k
./'/
i Ill
I
9 - r
9
9
C - Nta~ E~ 9 D -- ~ A I V A m FL 9 E -- U I ~ N ~VI 9 UI tm r
-- P U L V E 9
C~L
O -- l u m , t l m m v A ~ N - ctmiusvloh - ~Bota 9 tim - CONV 9
~. I'
prom* r op|lc*
PAmlt
9 -- I C 9 VO n t m M T t - *,O 9149 TAR
O -
F~
lUmat 9
Oit
9
9
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FIG. 1. Artist's view of the test facility used at the "'Istituto di Meeeanica."
APPLICATIONS TO COAL FIRED COMBINED CYCLE STEAM GENERATORS
1315
TABLE 1 A n a l y s i s of f u e l s Seruci coal as received Proximate, %wt moisture 7.2 vol. matter 43.3 fixed carbon 39.4 ash 10.1
U l t i m a t e , %wt hydrogen 5.4 carbon 63.5 nitrogen 2.0 oxygen 12.5 sulfur 6.5 ash 10.1
H i g h e r H e a t i n g Value = 6 0 5 0 K c a l / K g
A s h a n a l y s i s , %wt SiOz AI203 TiOa
1.53 1.08 0.05
Acidic
2.66
Fe20 a CaO MgO NazO K~O
9.92 37.19 5.76 0.99 0.10
Basic S u l f u r as SO 2 -
53.96 43.38
F u e l oil a n a l y s i s D e n s i t y at 293 K = 0.917 K g / l Viscosity at 323 K = 37 cSt Moisture 0.72 %wt Carbon 84.00 %wt Hydrogen 12.00 %wt Sulfur 3.33 %wt H i g h e r H e a t i n g Value = 10324 K c a l / K g Nickel 14 p p m Vanadium 44 p p m Sodium 80 p p m
FIG. 2. O r i g i n a l fuel o i l / p u l v e r i z e d coal b u r n e r a s s e m b l y .
1316
COAL COMBUSTION
H~O H#
A ,~---]
J~
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t
I
"1 I 0
---~SAMPLE
,Jl A
Fie. 3. Sampling probe schematic
Handling and Combustion Characteristics The coal received from the Seruci mines was under 50 mm maximum size and had a moisture content of 7.0%. Difficulties were encountered in pulverizing in tube mills because of the low grindability of Sulcis coal (about 50) and its tendency to form hard agglomerates. Difficulties, using indirect firing systems, were also experienced in hoppers, on the vibrating screen and in the transport line due to unusual build-up of packed pulverized coal. This was attributed to the moisture content and the problem was solved by heating the stock and increasing the temperature of the transport fluid. A cross section of the supplementary burner and combustion chamber is shown in Fig. 2. The windbox was designed to impart swirl to both the primary
and secondary air streams. A screw swirl inducer was adopted in combination with a conical shield in order to obtain the following effects: a--prevention of fuel from entering the core of the primary zone, which is full of burning products and lacks oxygen b - - i m p r o v e d chances of combining coal with available oxygen c--improved mixing of coal with the secondary air stream d--protection of the stream against radiation from the flame Initial tests, using the burner design of Fig. 2 resulted in fouling virtually with all oxygen and temperature conditions selected, as summarized in Table 2. A long bushy flame of the high combustion
TABLE 2 Summary of experimental results with burner of Fig 2 Inlet to 1st eombustor Air flow 124 k g / h Air temp. 293 K CO 2 0.0 %vol. 02 21.0 %vol. N2 79.0 %vol.
Inlet to 2nd eombustor
Outlet from 2nd eombustor
vitiated p.air flow 63 K g / h
flue gas 136 K g / h
Vit.s.air flow 96 K g / h p.air
s.air
Fuel oil burnt 5.0 K g / h COz HzO O2 N2
3.0 0.3 16.4 80.3
%vol. %vol. %vol. %vol.
coal burnt 7.0 K g / h excess oxy. 45% p.air velocity 23 m / s p.air temp. 390 K s.air temp. 500 K
7.2 1.0 11.0 80.8
COz 15.0 %vol. H20 1.5 %vol. 02 6.0 %vol. N2 77.5 %vol. CO 2.2 m g / N m 3 CmH" 0.5 m g / N m ~ carbon loss 2.5 m g / n m 3 comb. efficiency 95%
APPLICATIONS TO COAL F I R E D C O M B I N E D CYCLE STEAM GENERATORS
7.6 9
2S
30
, ,.o ~
1317
20
):,:
6.6 o
secondary l i r 0 2 16 %
\ o
9
o
,o
\#k,
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..................................
15
16
17
"_r 18
19
20
__ 21
02%
FIG. 5. Effect of primary vitiated air oxygen content on fly-ash emissions FIG. 4. Modified pulverized coal burner assembly. intensity type was produced. The swirl inducer did not give symmetrical mixing of the primary and secondary streams, so that the flame shape was not uniform in length all around the burner mouth. This drawback, coupled with the high fouling which reduced efficient operation in a few hours, suggested modifying the design of the primary air s w i r l impeller. Figure 4 shows the new burner. It consists of a tube body vortex generator and outlet tube. Coal entering the tube is given a swirling motion by the vortex generator. This motion throws the particles radially outward toward the wall of the tube, by centrifugal force. The pitch and length of
the vortex generator determines the degree of mixing of the primary and secondary streams. Port velocity in the range 30-40 m / s was achieved with the face of the body turned toward the conical shield. The combined results of the above modifications produced a combustor with higher combustion efficiency and lower CO and soot levels, as shown in Table 3. Carbon conversions are lower than those obtained with above stoichiometric air combustion, practically for every vitiated air and oxygen value. The operating excess air levels cannot be reduced to any significant extent due to the properties of the coal being fired. Suleis coal has a high propensity to deposit ash which slags in the firing zone. As shown in Fig. 5, fly ash and soot formation seems to be a function of the excess air parameter. Consequently,
TABLE 3 Summary of experimental results with burner of Fig 4
Test
p.air temp. K
Excess a.
oxygen in p.air
Comb. efficiency
Flame temp. K
1 2 3 4 5 6 7 8 9 10 11 12
370 370 370 370 390 390 390 390 390 385 390 390
35 36 38 40 35 37 37 38 38 38 40 40
16 17 18 19 16 17 18 19 16 17 18 19
96.5 97.0 97.0 97.3 96.6 97.2 97.3 97.5 97.1 98.1 98.0 98.2
1420 1450 1450 1500 1500 1510 1550 1540 1505 1510 1550 1600
Secondary air temperature = 470 K
1318
COAL COMBUSTION
6O0
excess oxydant
=
30%
see.me.cry a i r 0 2
=
16%
6O0
4OO
00
0 2OO O 100
~
2
/'r 15
16
17
18
19
i
i
20
21
02%
*dry volumetric basis
Fzc. 6. Effect of primary air oxygen content on CO emissions in coal supplementary firing reduced exhaust gas excess operation is not a practical method of supplementary firing due to unacceptable levels of flyash emissions. In the tests carried out in our laboratory we had to operate at superabundant levels of excess air to reduce soot emission. Thus, we found an excess of oxidizer equal to 30 percent or more to be the dividing line between an acceptable combustion efficiency capable of being improved and an unacceptable production of pollutants. At higher levels, this dividing line excess air is practically ineffective in reducing unburnt products and NO, or improving
combustion efficiency; these depend on oxygen concentration, temperature of the primary and secondary streams and turbulence. It was found that vitiated air circulation to the primary flame zone is impossible only when oxygen levels in the primary oxidizing mixture drop below 17 percent because of loss of ignition. On the contrary, the oxygen content in the secondary air can reach 15% or less without excessive formation of soot, CO and hydrocarbons. In a few tests, combustion and ignition were maintained with a secondary air oxygen content equal to 14% and a primary air content of 20%; the total excess oxydant was 45%. This effect can be explained by the reactivity of Sulcis coal due to the high content of volatile matter and the porosity of the powder after escape of the volatile elements. The flyash, carbon monoxide, CmH, and NO x emissions measured are shown in Figs. 5, 6, 7, 8. The data plotted on the right hand of all figures correspond to conditions when non vitiated air is present, i.e., the air is passed through a fired preheater to bring it to 450 K and then to the second burner. These data form a base line for comparison with results obtained with vitiated air. The CO, soot and flyash values become asymptotic to the values pertaining to a simple direct combustion with preheated air. Although the actual concentration of CO and hydrocarbons was higher under vitiated air conditions, the contribution of CO and C , H, made by supplementary coal firing was relatively smaller at oxygen contents above 18%. The operation with low oxygen combustion air is slightly effective in reducing NO~ emissions as shown in Fig. 8. This can be attributed to the fact that flue gas circulation in the primary stream is effective in reducing thermal NO~ by flame temperature reduction, but ineffective in reducing fuel
350
tr~ess o~tydant
35
secondary a i r 0 2
17.6
\
3OO 250
"
1A EPA standard ........................
_o . . . . .
1.2
Oo~
tO
0
O
0 0
~100
0
*o
0
0
0.6
0
-~_
Z
o .......
50 . . . . . . . . . . . . . . . . . . . . . . . . .
j
0
Q.a
150
~
/l"
0.4
excess oxydant
30
secondary air 0 2
16.5 %
%
0.2 15
16
17
18
19
2o
21
02%
dry volumetric basis
C/ 15
Fzc. 7. Effect ot primary air oxygen content on unburned hydrocarbons emissions in coal supplementary firing.
16
17
18
19
20
21
02%
Fro. 8. Effect of primary vitiated air oxygen content on NO, emissions from 2nd coal burner.
APPLICATIONS TO COAL F I R E D C O M B I N E D CYCLE STEAM GENERATORS
99-
98
o
~
r
n
a
r
Y
air
18,6 %
97
P 96
4'so
s'oo
sso
soo
K
FIG. 9. Effect of secondary air temperature on combustion efficiency of supplementary coal firing.
nitrogen conversion. Sulcis coal typically has nitrogen contents ranging from 1.0 to over 2.0 percent by weight, '5~ dry, mineral matter free basis, which can represent a significant source of NO x formation in coal-fired furnaces. To assure stability of ignition, the temperature of the primary air and coal leaving the mixer had to be at least 350 K. The maximum temperature of the primary air/coal mixture chosen in the laboratory tests was a compromise between the tendency of Sulcis coal to coke on the burner parts and the need to remove surface moisture; we found that 370390 K was a good compromise between these two needs, as shown in Table 3. Therefore the influence of temperature on combustion efficiency could be investigated only in the secondary vitiated air duct. The effect of the secondary air temperature on combustion efficiency is illustrated in Fig. 9. The figure shows that there is a penalty paid in terms of efficiency when lowering the temperature to 450 K. On the contrary, higher temperatures have been shown to be effective in increasing efficiency above 98%. The possibility of utilizing this tool to reduce unburnt combustible loss below 2-3 percent by weight will probably depend on the complexities of the design of windbox, vanes and registers of utility units, due to additional port velocity and coal erosion.
Summary and Conclusions
1--Bituminous Sulcis coal could be burned efficiently in furnace combustors when burner modifications were made to maintain the excess vitiated air above 30% and the oxygen content in the primary stream above 17%, in both cases paying a slight penalty in combustion efficiency.
1319
The oxygen level in the secondary air was considered less important for ignition and combustion stability. Its concentration could be maintained in the range of 15-18%. For excess air and oxygen contents in primary stream below 30% and 17%, respectively, a marked reduction in combustion efficiency is obtained and an unacceptable production of pollutants is noted. Also, ignition of pulverized coal was no longer stable, without the use of supporting fuel. 2--CO, CmH,, smoke emissions could be maintained substantially equal to air direct firing emissions, if the oxygen and excess air values in the primary stream were in the above ranges. NO,emissions were slightly reduced as expected for a vitiated air operation. 6'7's'. 3 - - T h e secondary air temperature and turbulence proved to be the most successful methods for improving combustion efficiency. A compact inertial vortex generator offered a practical solution for providing swirl to the coal/air mixture with little pressure loss, improving the primary air/coal distribution and mixing of coal with the secondary air stream.
Acknowledgment The author wishes to thank Prof. Mario Carta and the personnel of the Istituto di Arte Mineraria, Cagliari, for the preparation of pulverized coal. Ing. Enrico Mascia is to be commended for his excellent work in operating the combustion facility and developing the sampling handling. The author is also grateful for financial support provided by the Consiglio Nazionale delle Ricerche under contracts No. 77.01377.07, No. 78.00742.07 and No. 79.00383.07.
REFERENCES 1. L^nR G.: Gas-Wdrme Int., Bd. 16, No. 7, 378, 1967 2. FLADJ.: Operation of a Combined District Heating and Power Production Scheme with a Combined Cycle Gas Turbine, Paper presented at the Von Karman Lecture Series 6, Brussels, April 1978 3. ACTONO., SAa'rAA.: I1 Calore, 39, 329, 1968 4. DiLOBENZOA., MASl S.: Annali di Chimica, 68, 619, 1978 5. AGus M., CAaTAM.: Delle caratteristiche mineropetrografiche, chimiche e tecnologiche (]el carbone della miniera Seruci-Nuraxi Figus, paper presented at the 1st Convegno Minerario e Metallurgico Italo-Sovietico, Cagliari, October, 1976 6. HAZARDH. R.: ]. of Engineering for Power, 96, 235, 1974 7. KOLaACKR., ACETO L. D.: Recirculation Effects
1320
COAL C O M B U S T I O N
in Gas T u r b i n e Combustors, ASME paper No. 74-WA/GT-3, 1974
8. SULUVAND. A.: J. of Engineering for Power, 99, 145, 1977.
COMMENTS B. Vollerin, Battelle, Switzerland. I would like to just come back to the results y o u presented in Figure 8, i.e. NO~ reduction versus vitiation of primary c o m b u s t i o n air and also to your last conclusion w h e n you say "'NO, is reduced w h e n u s i n g vitiation as expected." t) I u n d e r s t a n d that the results presented here are mainly preliminary results a n d that you will be continuing this program in the future. At Battelle-Geneva we have some on-going research programs dealing with flue-gas recirculation/vitiation of c o m b u s t i o n air allowing 02 concentration to get clown to 15 or 16% a n d simultaneous combustion staging (or more precisely delayed aerod y n a m i c mixing). T h e fuels we b u r n are natural gas, distillate a n d heavy fuel-oil. Particularly, in heavy fuel-oil combustion, we noticed that an optim u m can be f o u n d between the relative m o m e n t u m s of the primary a n d the secondary flows and the "chemical quality" (i.e. the partial pressure of oxygen) of these two flows of oxydizers, in order to get the m i n i m u m NO~ level at the stack while keeping the level of particulate emission at an acceptable level. I would like to k n o w if you already carried out similar experiments a n d if so, what kind of results did y o u obtain? 2) It is generally not expected to get a dramatic reduction on NO~ level w h e n u s i n g vitiation of air for combustion of coal or heavy fuel-oil, apart some effect on the thermal NO Xwhich doesn't represent quite a significant part of the total NO~ w h e n b u r n i n g these fuels. T h e results y o u presented in Figure 8 are basically due to some aerodynamic staging in your experimental set up. Could you tell us what are the velocities and swirl n u m b e r of your primary and secondary flows in order to compare the NO~ reduction you mentioned with some other results presented in the literature? Author's Reply. T h e exercise with vitiated air is quite similar, for some extent, to the recireulation
of c o m b u s t i o n gases w h e n introduced into the primary flame zone t h r o u g h the windbox auxliary secondary air compartments. NO~ emissions on tangentially fired gas and oil boilers were reduced u p to 50% w h e n oxygen levels in the combustion air/recirculated flue gas mixture drop below 16 to 17% .cl> The same effect was relatively lower on our coal fired unit. T h e general opinion is that this can be attributed to the fact that vitiated air operation is effective in reducing thermal NOx, by flame temperature reduction, b u t ineffective in reducing fuel nitrogen conversion. Nevertheless, the relatively ineffective flue gas oxidant operation as N O , control in our apparatus was due to the following characteristics of the burner: - - h i g h primary air, equal or higher than 20% - - m a x i m u m mixing of the secondary air with the coal prior to ignition of the volatile fractions, because of quasi-radial coal injection. T h i s rapid mixing between coal and vitiated air with sufficient swirl to form an internal recireulation zone was a necessary design characteristic to provide heat for early ignition. U p to now, we were not able to support coal/vitiated air combustion with flames which give minim u m NO~ emissions, as simple jet flames, low fuel air mixing rate p r o d u c e d with zero or very weak swirlJ 2>
REFERENCES
(I)SELKEBA. P. "'Program for Reduction of NO x from Tangential Coal Fired Boilers Phase II" E.P.A. 650/2-73-005-a, June 1975. 12>HEAP M. P. et al. " B u r n e r Design Principles for M i n i m u m NO= E m i s s i o n s " Proc. of E.P.A. Coal C o m b u s t i o n Seminar, Research Triangle Park (1973) I.F.R.F. Doc, nr k 2 0 / a / 6 7 .