Energy & Fuels 1993, 7, 835-841
835
Measurement of Radiant Heat Flux and Local Particle and Gas Temperatures in a Pulverized Coal-Fired Utility-Scale Boiler B. W. Butler+ and B. W. Webb’ Department of Mechanical Engineering, Brigham Young University, Provo, Utah 84602 Received April 8, 1993. Revised Manuscript Received September 8, 1999
This paper reports experimental measurements of local particle and gas temperatures in a fullscale utility boiler. The boiler is a nominally 80 MW, tangentially-fired unit operated by New York State Electric and Gas. Gas temperatures were measured using a 4-m-long triply-shielded suction pyrometer. Particulate temperature data were collected using a two-color pyrometer. Temperature data were acquired at all levels in the boiler for two different coals. Results show reacting coal particle temperatures above those of the local gases only at selected positions in the burner region. At the top of the radiant section and just above the boiler nose, the particle temperatures were below the local gas temperatures by about 150 K. The results illustrate the strong three-dimensionality of the temperature field primarily in the near-burner region. Above the flame zone both particle and gas temperatures were nearly invariant across the boiler cross-section. Differences in particle and gas temperatures resulting from different fuels and firing conditions were revealed. Radiant heat flux profiles from the two tests illustrate significant dependence of the boiler radiant energy field on the coal type and mass mean particle size. Wall incident fluxes of over 500 kW/m2 were measured in the near-burner region.
Introduction Demands on world energy resources to meet the basic human needs of food, shelter, and clothing have increased significantly in the last century. These increases in energy use have resulted in severe reductions in the worlds fossil fuel reserves and increased demands on the earth’s ecologicalsystems to compensate for increased production of fossil fuel combustion byproducts. The reduction in cost of coal-derived energy and the environmentally safe burning of low-rank coals require that the combustion process be better understood from a fundamental mechanistic viewpoint. Predictive models must be accurate enough to reflect the physics of the complex process, yet simple enough for economicaluse. Comprehensive models include submodels for the prediction of the turbulent flow structure, particle dispersion, detailed chemical reaction kinetics, radiation transport, etc. Prediction methods for the many physical processes involved are constantly being refined. However, for the predictions to be accepted as physically accurate, advances in mathematical models must be accompanied by corresponding increases in the accuracy and availability of experimental data from the many physical processes occurring during combustion. Radiative energy transport has been shown to be of significant importance in coal combustion models, particularly in large-scale industrial furnaces. The radiative properties of the constituents in the energy field can play as large a role in submodel predictive uncertainty as inaccuracies in the models. Heat flux predictions exhibit a significant dependence on accurate specification of the gas and particle temperature field.’ Advances in radiation t Preaently with the USDA Forest Service Intermountain Research Station’s Fire Sciences Laboratory, Missoula, MT. Abstract published in Aduance ACS Abstracts, October 15, 1993. (1)Mengijq, M. P.; Viskanta, R. Combust. Sci. Technol. 1987,51,51-
74.
model predictive accuracy have outpaced the supply of well-documented comprehensive data from full-scale systems. For this reason a critical need exists for spatially resolved gas and particle temperature and radiation heat transfer data. This study focuses particularly on spatially resolved gas and particle temperature and heat flux data.
Prior Work Several areas of need for detailed experimental data in pulverized coal combustion systems have been identified in previous s t u d i e ~ In . ~particular, ~~ local gas and particle temperatures, particle size and number densities, particle velocities, and wall radiant and total heat flux have been identified as lacking adequate research attention.&’ These data are vital to the evaluation of present and future thermal radiation submodels. Others have conducted pulverized coal-fired tests in laboratory-scale furnaces;a11 however, none reported (2)Butler, B. W. An Experimental Evaluation of Radiant Energy Transport in Particle-Laden Flames. Ph.D. Dissertation,Brigham Young University, Provo, UT, 1992. (3)Phillips, S. D. Data Book for Evaluation of Three-Dimensional Combustion Models. M.S. Thesis, Brigham Young University, Provo, UT, 1989. (4) Jamaluddin, A. S.; Smith, P. J. Combust. Sci. Technol. 1988,59, 321-330. (5)Lockwood, F. C.; Mahmud, T. Twenty-Second Symposium (Int’l.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 165-173. (6)Lockwood, F.C.; Rizvi, S. M. A.; Lee, G. K.; Whaley, H. Twentieth Symposium (Int’Z.)on Combustion; The Combustion Institute: Pittsburgh, PA, 19W;pp 513-522. (7) Truelove, J. S. Twentieth Symposium (Int’l.)on Combustion;The Combustion Institute: Pittsburgh, PA, 1 9 W pp 523-530. (8)Thurgood, J. R. Mixing and CombustionofPulverized Coal. Ph.D. Dissertation, Brigham Young University, Provo, UT, 1979. (9) Harding, N. S. Effects of Secondary Swirl and Other Burner Parameters on Nitrogen Pollution Formation in a Pulverized Coal Combustor. Ph.D. Dissertation, Brigham Young University, Provo, UT, 1980.
0S87-0624/93/2507-0S35$04.oo/o 0 1993 American Chemical Society
836 Energy & Fuels, Vol. 7, No. 6,1993
complete spatially resolved gas and particle temperature or heat flux data. Lockwood and Mahmud5 report gas temperature data. However, no heat transfer or particle temperature data are reported. Lockwood et a1.6 and Truelove and Williamsl2present detailed studies with gas temperature measurements, but no particle temperatures are provided. Costa and co-workers13present gas species concentration, gas temperatures, and char burnout data over a range of burner swirl numbers. A few studies report combustion measurements made in pilot-scale systems, one of the most widely referenced being that published by Be6r.14 This well-documented study consists of relatively complete profiles of gas temperatures, wall radiant heat fluxes, particulate concentration and chemical composition at several different seta of operating conditions. Other reported works include those by T r u e l o ~ e , ~Hein J ~ and Leukel,16 Michel and Pyne,17Hein,ls and Visser et a1.19 However, none of these sets include particle temperature measurements. Only a few relevant studies report measurements from industrial-scale boilers. Because the bulk of energy transfer in these systems occurs by radiation, and the radiation is dominanted by flame emission rather than wall effects, such data are critical for the evaluation of radiation transport models as well as the other submodels comprising the comprehensive models.20 Wall and Stewart21present gas temperature data from the near-burner region of a 20 MWe tangentially-fired boiler; Lowe et aL22 present gas and heat flux data from a 900MWt pulverized coal tangentially-fired boiler. Ku and Rajaram23present some wall heat flux and temperature data from a fullscale boiler operating at two different firing rates. Boyd and Kentz4report comparisons between predictions and gas temperature and heat flux data from a 500 MWe tangentially coal-fired boiler. Cetegen and Richter25 present some gas temperature and wall radiant heat flux measurements from a 420 MW, boiler. Blokh26 reports gas temperature and heat flux data from a range of utility(IO) Rees, D. P. Pollutant Formation During Pulverized Coal Combustion. Ph.D. Dissertation, Brigham Young University, Provo, UT, 1980. (11)Assay, B. W. Effects of Coal Type and Moisture Content on Burnout and Nitrogeneous Pollutant Formation. Ph.D. Dissertation, Brigham Young University, Provo, UT, 1982. (12)Truelove, J. S.;Williams,R. G. Twenty-SecondSymposium (Znt’l.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988;pp 155-164. (13)Costa, M.; Costen, P.; Lockwood, F. C.; Mahmud, T. TwentyThird Symposium (Znt’l.)on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 973-980. (14)Be&, J. M. J. Inst. Fuel 1964,37, 286-313. (15)Truelove, J. S.Twenty-First Symposium (Znt’l.) on Combustion; the Combustion Institute: Pittsburgh, 1986;pp 275-283. (16)Hein, K.;Leuckel, W. Trials C-14.Further Studies on the Effect of Swirl on Pulverized Coal Flames. IFRF Report No. F32/a/40;IFRF: Ijmuiden, The Netherlands, 1969. (17)Michel; Payne. Detailed Measurement of Long Pulverized-Coal Flamesfor the Characterization of Pollutant Formation. IFRF Document No. F 09/a/23;IFRF: Ijmuiden, The Netherlands, 1980. (18)Hein, K.Preliminary Results ofC.15 Trials, P. F. Panel Meetings, August and December, 1970;I F R F Ijmuiden, The Netherlands. (19)Visser, B. M.; Smart, J. P.; Van De Kampf, W. L.; Weber, R. Twenty-Third Symposium (Znt’l.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 949-955. (20)Smoot, L. D.;Hill, S. C. Prog. Energy Combust. Sci. 1983,9,77103. (21)Wall, T. F.; Stewart, I. McC. Fourteenth Symposium (Znt’l,)on Combustion; The Combustion Institute: Pittsburgh, PA, 1973;pp 689697. (22)Lowe, A.; Wall, T. F.; Stewart, I. McC. Fifteenth Symposium (Int’l.)on Combustion; The Combustion Institute: Pittsburgh, PA, 1974; pp 1261-1270. (23)Ku, A.; Rajaram, S. J. Inst. Energy 1983, Dec, 217-228. (24)Boyd, R. K.;Kent, J. H. Twenty-First Symposium (Znt’l.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 265274.
Butler and Webb
scale pulverized coal-fired units. Holve and co-workers27 report particle size distribution, speed, and gas temperatures made near the nose and inlet and exit of the electrostatic precipitator of a 250 MWe wall-fired boiler. More recently, Butler and Webb28have reported spatially resolved gas temperature and heat flux data measured in a tangentially-fired 80 MW, boiler. These measurements were part of a large effort which included the measurement of gas species concentration, gas velocities,B and particle size distribution and number density measurement^.^^ Despite the efforts of many researchers to characterize energy transport in full-scalepulverized coal-firedsystems, few if any particle temperature data are reported. Additionally, none of the studies provide a complete set of the data needed for effective model evaluation studies. This paper reports spatially resolved gas and particle temperature data which were acquired concurrently with measurements of heat flux, gas species, gas velocities, particle size distribution and number density, particle velocity, and char burnout data reported e l s e ~ h e r e . ~ . ~ ~ ~ ~ ~
Experimental Apparatus A schematic of the utility boiler is shown in Figure 1. The boiler is approximately 25 m in height and of rectangular crosssection with average inside dimensions of 7.5 m on the sides and 7.7 m across the front and back. The boiler was divided into seven levels, six of which were available for data acquisition. Ports near the corners were used on levels 1-3 and 5-7 for measurement of incident wall radiant flux. Where structurally possible, corner porta and some wall-centered ports on levels 2, 3,5,and 7 were used for particle and gas temperature measurement. Gas temperature data were also collected in the economizer. For reference, the four burner nozzles at each corner of the boiler (all operating during the testa) were located nearest levels 2 and 3. Boiler operating conditions are listed in Table I. An attempt was made to maintain these variables at near-constant levels throughoutthe test period. Although it was necessary to operate soot blowers intermittently, repeatability testa on several measured variables revealed that boiler operation was quite steady. Ultimate and proximate analyses of the coal burned during the testa are found in Table 11. Of particular note is the difference in ash, volatiles, and carbon in the two coals. Coal 1has nearly twice the volatile content as coal 2,but significantly lower fixed carbon. The ash content of coal 1 is 30% less than that of coal 2. The mass mean particle sizesof the two coals are also radically different; while the mill settings were identical for the two testa, the considerably higher grindability of coal 2 resulted in a mean particle size nearly half that of coal 1. The mass mean coal particle size was approximately 30 pm duringtest 1and 18pm during test 2. The coal size distribution was measured using a Malvern 2600 sizer. Figure 2 presents the coal particle size distribution data. (25)Cetegen, B.M.; Richter, W. Heat Transfer Modeling ofa Large Coal-Fired UtilityBoiler and Comparisons with Field Data,Proceedings of the 2nd ASME/JSME Thermal Engineering Joint Conference, Honolulu, HI, 1987. (26)Blokh, A. G.Heat Transfer in Steam Boiler Furnaces; Hemisphere: Washington, DC, 1988. (27)Holve D. J.; Meyer, P. J.; Muzio, L. J.; Shiomoto, G. H. Western States Section of the Combustion Institute. Salt Lake City, - . UT,. March 21,1988,paper No. 8840. (28)Butler, B. W.; Webb, B. W. Fuel 1991, 70,1457-1464. (29)M o b , J. Gas Phase VelocityMeaeurementsinan Industrial Scale, Coal-Fired Boiler. M.S. Thesis, Brigham Young University, Provo, UT, 1993. (30)Bonin, M. P. Optical Analysis of Particle Dynamics in Pulverized Coal Flames. Ph.D. Dissertation,BrighamYoungUniversity,Provo,UT, 1992. (31)Oettli, M. Analysis of Gas and Particle Composition in a FullScale Utility Power Station. M.S. Thesis, Brigham Young University, Provo, UT, 1993.
Energy & Fuels, Vol. 7, No. 6,1993 837
Measurement of Local Particle and Gas Temperatures test/ coal type
gross
generation (MW)
1
85 82
2 0
Table I. Furnace Operating Parameters net coal flow rate air flow rate generation (MW) ((kg/h) x 10-3) ((kg/h) X 10-3) 80 76.5
29.1 28.35
burner tilt? (deg)
excess oxygen (%)
0 0
4.5 4.4
487.4 463.7
Positive burner tilts indicate firing above the horizontal toward the boiler nose; negative burner tilts indicate firing below the horizontal.
3
.
High TemperatureSuper Heater Pendeots
Table 11. Proximate and Ultimate Coal Analyses coal type --
Reheater Pendents
d
k
1
2
Average Proximate Analysis (mass % ,as received) 4.9 6.15 moisture 7.4 10.4 ash 34.95 18.75 volatile fixed carbon 52.30 64.75 2.01 1.41 sulfur heating value (kJ/kg) 31,249 30,470 Average Ultimate Analysis (mass % ,as received) 77.00 73.5 carbon hydrogen 3.69 4.20 nitrogen 1.22 1.10 oxygen 3.85 3.00 Average Coal Particle Size Information -30 pm -18pm mass mean grindability -59 --9o 3.45 3.29 coal density (g/cm3) Average Ash Composition (mass % ,as received) Si02 43.6 42.7
21.5m
23.4 22.0 1.20 4.00 0.70 0.40 1.40 2.40 3.44
A1203
Fee03 Ti02 CaO
Front Wall
MgO
Na2O K20
so3 wall
Figure 1. Schematic of utility boiler. These data are required input to both comprehensive and radiative transport models.
36.3 11.8 1.7 3.7 1.0 0.1 1.4
1.2
ash density (g/cmS) 3.30 Average Ash Fusion Temperatures (K,reducing/oxidizing) 141111540 1795/ 1810 init deform 1530/1644 1810/1810 fluid 1464/ 1590 1810/1810 softening 1500/1622 1810/1810 hemispherical 6 1
I
Instrumentation Gastemperatures were measured using a triply-shielded,watercooled suction pyrometer 4 m long and 63 mm in diameter with a 27-mm outside diameter and l&cm-long triple-walled ceramic radiation shield affixed to the end. The pyrometer is of conventionaldesign. All measurements were made with the probe axis oriented normal to the furnace wall through which it was inserted. Repeated measurements made a t the same conditions suggest an uncertainty estimate of 1 4 0 K. Butler2 and Butler and Webb28 provide further information on the instrument, measurement uncertainties, and sampling procedures. Particle temperatures were measured using a two-color pyrometer. Figure 3 is a schematic of the instrument. The optical portion of the instrument is enclosed in a 4-m-long, 83-mmdiameter water-cooled sight tube and was configured to observe particle clouds in the flow channel located approximately 4.1 m from the pyrometer detectors. Cooling water was circulated through the cold target, which never exceeded 310 K. The flow channel formed between the cold target and end of the sight tube isolated small pockets of particles,permitting approximately local measurements. The cooled target was blackened to prevent contamination of the collected signal by reflection of radiation from the flame external to the flow channel. The image diagnostic volume was a 10-cm-longtruncated cone 2 and 2.5 cm in diameter on the small and large cone ends, respectively. Infrared radiation is transmitted through interference filters with 30-nm bandwidths centered at 1.27 and 1.60
9
0.1
1.o
10.0
100.0
Particle
Diameter
(mlcrons)
1000.0
Figure 2. Comparison of particle size distribution for the two coals investigated. pm. These wavelengths were chosen to avoid the absorption bands of C02 and water vapor. Voltage data were acquired from the two channels using a 16-bit A/Ddata acquisition system. Measurements consisted of the average of 1500 blocks of data, with each block comprising the average of eight sequentially acquired samples collected a t
Butler and Webb
838 Energy & f i e l s , Vol. 7, No. 6, 1993 Water
:
Water
o
o
o
2 1800
1
Water
Optlcal Train Detail
Tpa rt I c I
-t=JwW
Relay Lenses
Level 2 0.5
0.0
1.0
1.5
2.0
2.5
3.0
3.5
Distance From Boiler Wall (m) Figure 4. Measured particle and gas temperatures a t burner levels (levels 2 and 3). Convection
Figure 3. Schematic of two-color pyrometer for particle temperature measurement. a frequency of 22 kHz. The calibration procedure and potential sources of uncertainty are discussed in detail elsewhere.2J2 The objective in the design of the water-cooled probe was to make the sample volume small enough relative to the dimensions of the furnace that local measurements could be approximated. It should be emphasized that the measurement technique is unable to resolve particle-to-particle temperature fluctuations; measurements reported are of particle clouds rather than individual particles. Previous analytical work suggests that for the conditions present in industrial-type pulverized coal-fired flames, the pyrometric measurement of reacting coal/char particles in the devolatilization zone is always high due to the presence of soot.sa This difference increases with increasing soot concentrations and temperaturedifference between the soot and particles (Le., assuming that the soot temperatures are greater than the particle temperatures). Rather than a bipolar uncertainty error estimate, the uniformly high temperaturespredicted using two-color theory suggest that an uncertainty estimate perhaps as high as 200 K in the near-burner region is possible (Le., the reported temperature is less than 200 K higher than the actual particle cloud temperature). The estimate of uncertainty for measurements made outside the reaction zone is considerably lower due to the absence of soot; 80 K is appropriate for measurements made in these regions. Heat flux measurements were acquired using a water-cooled ellipsoidal radiometer. This instrument consists of a 43-mmdiameter, 0.75-m-long water-cooled jacket encasing an inner sensing head and electronics. The instrument operation and calibration are discussed more fully e1sewhere.m
Discussion of Results Particle and gas temperature profiles from these tests are shown in Figures 4 and 5. Test 1was fired with coal 1, at a mass mean particle size of 30 pm, and coal 1was (32) Butler,B.W.; Wileon,T.;Webb,B.W. lbenty-Fourth Symposium (Znt'l.)on Combustion; The CombustionInstitute: Pittabrugh,PA, 1992; pp 1333-1339. (33) Grosehandler, W. L. Combust. Flame 1984,55,59-67. (34)Lafollette, R. M.; Hedman, P. 0.;Smith, P. J. Combust. Sci. Technol. 1989,66,93-101.
Sect ion
o 2140 m
-----""I
,001
1
7001 n
Y
500 nnn i
Y
2 7
c
2-12.5 m o
'Oo0
0
0.0
p i Test T 2, {
right side port
o left side port
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Distance From Boiler Wall (m)
Figure 5. Measured particle and gas temperatures at levels 5 and 7 and in the convective section.
significantly higher in volatiles and lower in ash and carbon than coal 2 fired during test 2. Coal 2 was pulverized to a mass mean of 18pm. Burner tilt and the nominal boiler firing rate were unchanged between these two tests. Burner inlets were located at the boiler corners on levels 2 and 3 with one bank of burners at each comer. Burners were directed 4O off the diagonal to induce a counterclockwise rotating flow as viewed from above.28 Burner region temperature data are shown in Figure 4. The gas and particle temperatures measured at level 2 (z = 4.8 m) were initially 100-120 K greater for test 1than those of test 2. However, as the distance into the furnace increased, the gas temperatures from test 1approached those of test 2. The initially higher gas temperatures of test 1 are likely a result of the increased heat release
Measurement of Local Particle and Gas Temperatures
associated with devolatilization of coal 1. However, as one moves toward the center of the boiler the gas and particle temperatures of test 1exhibit a slight drop while those of test 2 remain constant, suggesting that most of the devolatilization is occurring near the burner inlets. At level 3 (z = 7.4 m) the differences between the nearwall temperatures of the two tests were much greater than seen at level 2 (Le., >200 K). Test 1particle temperatures nearest the wall were greater than the associated gas temperatures; however, within 0.5 m of the wall the gas temperatures had increased to levels near those of the particles. The temperatures then remained nearly constant into the center of the furnace volume. The data of test 2 show a very different profile over the same space. Initially the gas and particle temperatures were nearly the same magnitude; however, a gradually increasing trend of more than 100 K/m is shown in the test 2 data. In fact, the gas temperatures increased at a faster rate than the particle temperatures for distances greater than 1m from the wall. The gas temperatures of test 2 increased to levels 200 K greater than those of test 1. One explanation for this temperature rise may be that the gases of test 2 were experiencing energy input due to char oxidation. The greater carbon content of the coal burner during test 2 may result in greater heat release during this later stage of the combustion process than observed during test 1. Differences in the temperature profiles between the two tests suggest that the majority of the furnace volume in this section was comprised of three-dimensional temperature fields which were highly dependent on fuel parameters such as rank and pulverization. It was not possible to acquire temperature data at level 4 due to a lack of access ports. Temperature profiles acquired above the burner levels are shown in Figure 5. Level 5 is just below the boiler nose. At this level the gas and particle temperature fields from both tests were uniform across the furnace volume. There was very little difference between the particle and gas temperatures, and in fact, differences were within the individual instrument measurement uncertainties. Gas temperature profiles measured on opposite sides of the front face of the boiler were similar in magnitude and structure, indicating a uniform temperature field across the boiler volume. Oxygen concentration and particle burnout measurements indicated that combustion reactions were essentially complete.31 As was the case with level 4, structural barriers prevented temperature measurements on level 6, which is adjacent to the boiler nose. Gas temperature measurements made on level 7 (z = 13 m) remain relatively constant at approximately 1200 K across the measurement volume over both tests. Whereas the particle temperatures were essentially identical to the gas temperatures on level 5, particle temperatures were from 120 to 200 K lower than the gas temperatures on level 7. Some structure is shown in the test 2 particle temperature profile. This may be attributed to the differences in flow patterns due to the vortex flow in the lower portion of the boiler. This is supported by B~nin,~O who reported measurements of particle number densities and particle velocities made from the same locations. It was suggested that differences in particle densities were due to preferential flow streamlines induced by the vortex flow of the burner region which directed the bulk of the particle field toward specific locations in the boiler.
Energy & Fuels, Vol. 7, No. 6, 1993 839 Front of Boiler (Generator Side)
Level 1
Level 5
mers
m 23
277
Level 6
Back of Boiler (Convective Section Side) access port location for temperature measurements Incident radiant fluxes shown in kW/m2
Figure 6. Local incident radiation flux variation over the periphery and height of the boiler for test 1.
The increasing difference between the particle and gas temperature profiles between levels 5 and 7 suggests that the particles were cooling more rapidly than the gases. Previous work has suggested that the heat transfer mechanism in this region of the boiler is accomplished by convective energy transport between the gases and particles followed by radiant energy exchange between the particles and furnace walls.32 Because the particle temperatures decreased more rapidly than the gas temperatures, it appears that radiant energy transfer from the particulates to the boiler walls occurs much more readily than convective energy exchange between the gas and particles. This conclusion is further reinforced by the observation that the particle temperature drop between level 5 and level 7 is greater for test 2, which had a significantly smaller mean coal size and higher ash content. This would suggest a higher concentration of fly-ash particles in the upper region of the boiler for test 2, with correspondingly higher radiative transfer to the walls. Gas temperature measurements made further downstream in the convective section are also shown (z = 23 and 40 m). Particle temperature measurements in these locations were deemed unreliable due to the low signalto-noise ratio in the sampled data. However, a generally decreasing trend in gas temperatures due to energy exchange with the boiler heat transfer surfacesis indicated. As expected, there is very little variation in temperature across the boiler volume. Local incident radiant heat fluxes at the walls were measured at various locations over seven elevations in the boiler. The results are shown graphically in Figures 6 and 7. The data are depicted as hemispheres sized in proportion to the measured heat fluxes; the greater the hemisphere diameter, the larger the measured incident
840 Energy & Fuels, Vol. 7, No.6, 1993
Butler and Webb
Front of Boiler (Generator Side)
500
'ners
-
400 -
300 200 -
U
100-
Level 3
Burners
rl Level 6
t
Level 5
1
Axlal Dlstance (m)
Figure 8. Circumferentially-averaged incident wall radiant fluxes as a function of boiler elevation for both testa.
1 3 5 Level
7
U
Back of Boiler (Convective Section Side)
access port location for temperature measurements Incident radiant fluxes shown in kW/m2
Figure 7. Local incident radiation flux variation over the periphery and height of the boiler for test 2.
radiation at the corresponding port. The numerical value of the measured flux is shown adjacent to each hemisphere. Port locations relative to the boiler walls are shown, but the figures are not drawn to any specific scale. The large arrows at each corner in the schematics corresponding to levels 2 and 3 indicate the burner nozzle orientation with respect to the boiler cross-section. As was observed in a previous study,% there are significant variations in wall radiant flux around the boiler periphery, and from level to level. The wall-centered ports show generally higher incident fluxes than those near the corners, due to the proximity.of the corner ports to the relatively cool walls. Magnitudes of the incident flux are higher at the burner levels due to the emission from the flame. The flux magnitudes show a generally decreasing trend as one proceeds up the boiler; the incident radiant fluxes measured above the nose on level 7 are only 2025% of those in the near-burner region. Generally speaking, the peak local flux occurs at higher elevations in the boiler for test 2 than for test 1, although the magnitude of the peak flux is lower for the test 2 case. This is illustrated more clearly in the periphery-averaged incident flux variation with boiler elevation explored in the next section. Average wall radiant fluxes at each boiler level were calculated from the data of Figures 6 and 7 by computing the mean of the local data at each elevation. Those for levels 2 and 3 were computed by averaging the local measurements taken at each of the four corners. Averages for levels 1,5,and 6 were determined by taking a simple mathematical average of the measured heat fluxes at a specific level from the side of the boiler with the most complete data set. Because there were more corner port data than wall-centered data, and the data were weighted
equally, this averaging procedure may be slightly inaccurate. Nevertheless, the trends indicated by the averages thus determined can provide useful information about the radiation transport in the boiler. The variation of the average wall incident radiant flux is shown in Figure 8. The heat fluxes of test 1were greater than those of test 2 in the lower half (near-burner region) of the boiler, with the peak energy flux occurring at level 3 (adjacent to the top row of burners). The test 1profile then shows a steady decrease in wall incident radiant flux over the remainder of the boiler volume. This suggests that more of the energy release occurs during devolatilization of coal 1. Coal 2, whose proximate analysis showed lower volatiles but greater fixed carbon, produced a longer (temporally) reaction zone wherein later heat release due to char oxidation downstream from the burners resulted in nearly constant heat fluxes in the upper portion of the reaction zone (e.g., the relatively flat heat flux profile between levels 3 and 5). These trends suggest that the majority of energy release due to devolatilization occurs at or very near the burners while char oxidation occurs over a longer time period. The relative heat flux levels were reversed in the upper portions of the boiler. This reversal was likely due to differences in the volatiles and ash contents of the two coals. The more rapid decrease in the associated particle temperature profiles of test 2 over test 1 from level 5 to level 7 suggests that radiant energy transfer between the particles and boiler walls was greatest during test 2. This observation is matched in the heat flux data. The increased energy release observed in the test 2 data in the upper portion of the boiler may be due to increased fly-ash number densities. This is supported by others who have associated finer coal grind with enhanced radiant energy exchange between the particles and boiler walls, probably due to the higher number of fly-ash particles.lS6sa This conclusion presumes that a greater ash content in the parent coal results in more ash particles for the same load and heating value. This is corroborated by a recent study on ash fragmentation,%which concluded that bituminous coals may form as many as 100 fly-ash particles from each parent coal particle larger than 80pm, and approximately 10 fly-ash particles from parent coal smaller than 20 pm. (35) Steward, J. R.; Guruz, H.K.In Heat Transferin Flames;Afgan, N H. Beer, J. M., Eds.; John Wiley and Sone: New York, 1974. (36) Baxter, L. L. Combust. Flame 1992,90, 174-184.
Measurement of Local Particle and Gas Temperatures
It is interesting to note that despite the differences in average wall incident radiative flux with elevation in the boiler shown in Figure 8, the integrated radiative flux for the two tests is within 5 % . This integrated flux is indicative of the total radiant heat transfer to the boiler walls, and was determined simply by integrating the average flux over the local boiler wall surface area. This agreement in total radiative heat transfer to the boiler walls concurs with the identical nominal load for the two tests.
Energy Balance An overall combustion side energy balance was calculated as a check of the measured wall radiant heat flux data. This was performed assuming radiation as the dominant mode of heat transfer to the walls. The heat flux profiles in Figure 8 were integrated over the total boiler wall surface area of the radiant section and yielded values of 186 MW for test 1 and 177 MW for test 2. Using a mean wall emissivity value of 0.77, which is reported as repre~entative,~~ and assuming grey walls with uniform wall temperatures2 of 1350 K, the total radiant energy leaving the boiler walls (emitted plus reflected) was estimated to be approximately 130 MW. The net radiant energy absorbed by the water walls is defined as the difference between these values, vielding values of 56 MW for test 1 and 47 MW for test 2.. Total energy release due to reaction was estimated at 6o MW for these tests. The net energy absorbed by the boiler walls must equal that lost from the combustion products over the same area* Such an balance is subject to the assumptions made in the however, the level Of (37) Singer, J. G., Ed. Combustion, Fossil Power Systems, 3rd ed.; Combustion Engineering, Inc.: Windeor, CT, 1981.
Energy & Fuels, Vol. 7, No. 6, 1993 841
agreement lends some confidence to the heat flux measurements reported herein.
Conclusions Experimental measurements of gas and particle temperatures and wall incident radiant heat fluxes have been made in an 80 MW, pulverized-coal corner-fired utilityscale boiler. The radiative energy transport was observed to vary significantly with distance from the burner inlet. Both the temperature and heat flux data suggest that energy transport between the combustion products and boiler walls is limited by convective energy transport between the gases and particles rather than radiant energy exchange between the particulates and boiler heat transfer surfaces. The gas and particle temperature and heat flux profiles reflect the influence of the tangential firing pattern, coal characteristics, and pulverization. As would be expected considering the relative heating values of the two coals, total energy release for the two tests in the radiant section of the boiler was within 5 % . However, there is some indication that the overall heat release pattern in the boiler may be tailored through the selection of coal properties and pulverization. These data will provide combustion modelers with new information regarding particle and gas temperature histories and radiant heat flux profiles in pulverized-coal-fired boilers.
Acknowledgment. The financial support for this project was provided by the Empire State Electric Energy Research Gorp. (ESEERCO), New York State Electric and Gas (NYSEG), and the Advanced Combustion Engineering Research Center (ACERC). ACERC funds are received from the National Science Foundation, the State Of Ut*, 26 participants, and the Department of Energy.
