Energy & Fuels 1993, 7, 768-773
768
Fuel Switching: A Pilot-Scale Approach to Boiler Performance Predictions Benjamin R. Pease,* Armand A. Levasseur, and Oscar K. Chow ABB Power Plant Laboratories, Research and Technology, Combustion Engineering, Inc., Windsor, Connecticut 06095 Received May 21, 1993. Revised Manuscript Received September 7,1993@
Application of pilot-scale test results in conjunction with computational models offers an alternative approach to full-scale testing when considering fuel switching options. This paper discusses ABB Power Plant Laboratories' pilot-scale testing capabilities and its computer models used to generate full-scale predictions from pilot-scale data. The focus is on the measurement and prediction of coal slagging, fouling, and erosion potentials and subsequent boiler performance impacts.
Introduction
In today's environment of increasingly strict emissions regulationsand changing coal prices, electric power utilities are often compelled to investigate fuel switching, either as part of their strategy to comply with the 1990 Clean Air Act Amendment or as a fuel cost reduction strategy. For example, conversion of a plant burning high sulfur coal to low sulfur coal could enable a utility to avoid high capital and operating costs associated with scrubber additions. However, coal performance impacts can be critical to the overall feasibility of a fuel conversion. Changes in coal properties could significantly affect plant operation and may require major boiler modifications to achieve satisfactory performance. The methods used in assessing fuel impacts are often limited in their ability to accurately predict performance. Well-planned boiler field tests provide the greatest level of confidence on the performance of alternate coals. However, field testing is costly and time-consuming. Up front equipment modification costs may be incurred if the unit was designed to fire a coal with significantly different characteristics. Pilot-scale testing can provide an alternate means of reliably evaluating the suitability of replacement fuel at relatively low cost, without operational risk to the utility unit. This paper discusses impacts fuel switching can have upon utility boiler performance an3 operating capacity. It focuses on the application of pilot-scaletesting to reliably assess the behavior of an alternate fuel in a specific boiler and provides illustrations of the analysis. Impacts of Coal Switching on Power Plant Performance
Coal properties have a significant influenceon the design and operation of virtually every aspect of steam generation from fuel handling to the stack (Figure 1). Conversion of a plant to utilize a coal which has significantly different characteristics than the originaldesign coal requires careful evaluation in order to fully define overall impacts and accurately assess true costslsavings.'
* Abstract published in Advance ACS Abstracts, October 15, 1993.
(1)Llinares, V.; Levasseur, A. A. Fuel Switching and the State-ofthe-Art of Coal and Boiler Performance Analyses. International Joint Power Generation Conference & Exposition, Atlanta, GA,1992.
O887-062419312507-0168$04.O0/ 0
P
Raw Coal Bunker
i Stack
-
Figure 1. Coal utilization areas impacted by coal quality.
For example, firing a western low-sulfursubbituminous coal in a unit designed for an eastern bituminous coal often creates several concerns. The higher moisture content and lower calorific value of subbituminous coal require higher coal throughput (for an equivalent boiler generating capacity) which could result in pulverizer capacity limitations. In order to overcome pulverizer capacity limitations and minimize overall unit derating, several options could be pursued. These options range from increasing mill air temperature (through air heater modifications or duct burner addition) to complete mill replacement. Similarly, the greater coal flow and transport airlwater vapor flow associated with firing the subbituminous coal may require resizing of coal piping to reduce velocities and minimize erosive wear. Resizing of windbox compartments to maintain flame stability and boiler turndown performance would also be typically required. Due to the increasing regulatory pressure to reduce NO, emissions, often utilizes considering fuel switching for SO2 reduction would also use the opportunity to install a low NO, firing system tailored to the new fuel. Differences in fuel reactivity are another significant consideration when switching coals. The higher combustion reactivity of subbituminous coal compared to bituminous can be advantageous from a carbon burnout standpoint, particularly under low NO, firing applications. Further, the grinding requirements (fuel fineness) for a higher reactivity coal are not as stringent to obtain the same combustion performance, which can also help reduce pulverizer capacity limitations. Thermogravimetric anal1993 American Chemical Society
Fuel Switching
Energy & Fuels, Vol. 7, No. 6, 1993 769 Table 1. Boiler Performance a n d Economic Impacts of Ash Deposits
ash effecta reduced heat transfer impeded gas flow slag falls bridging of hopper
performance impacts excessive spray flow excessive tube temperature physical damage draft loss
ysis techniques as well as indices based upon fuel composition provide qualitative assessment of combustion reactivity. Methodologies utilizing combustion kinetic data generated through drop tube furnace testing, coupled with computational modeling to account for specificboiler design and operating conditions, have been developed and successfully a ~ p l i e d . ~ a Ash Performance Effects
The amount of ash generated by a coal and how it behaves during the combustion process are important factors impacting several aspects of boiler performance. Control of ash-related impacts is generally the most dominant factor in furnace sizing and convection pass design. Ash slagging,fouling, and erosion strongly impact boiler performance, operating capacity, and reliability (Table I). Several bench-scale indices have been developed and used to help predict coal ashslagging and fouling potential. However, these indices, generally based upon ASTM ash fusion temperatures and ash composition data, are of limited application. Their reliability is often poor because they cannot account for the variability of mineral composition and distribution within the coal particles. During pulverized-coal firing, the characteristics of individual coal particles largely dictate their behavior. Specific properties of the mineral matter in a given coal particle play a decisive role in fly ash formation and the behavior of that fly ash particle in a boiler. Ash deposition in a boiler is an extremely complex process involving numerous processes including coal devolatilization, char combustion, fly ash formation, transport, and deposition. Each of these processes is in turn influenced by many factors including heating rates, temperature regime, localized gaseous environment, bulk and local fluid dynamic conditions, etc. These phenomena are often interrelated and are strongly affected by equipment design and operating conditions. Considerable progress has been made in the fundamental understanding of these processes. There have been substantial advances over the last 5 years in utilizing analytical tools such as scanning electron microscopy to provide more detailed coal property data. Substantial advancements have also been made in applying these data and using computational techniques to model and predict ash deposition in commercial boilers.P6 However, gaps still remain in the fundamental understanding of certain mechanisms involved during ash formation/deposition and (2) Nsakala, N.; Patel, R. L.; Borio, R. W. An Advanced Methodology for Prediction of Carbon Loss in Commercial Pulverized Coal-Fired Boilers. Joint Power Generation Conference, 1986; Paper 86-JPGCFACT-L. (3) Hurt,R. H.; Mitchell, R. E. Combustion Kinetics of Heterogeneous Char particle Populations. Proceedings of the 24th International Symposiumon Combustion;The Combustion Institute: Pittsburgh, PA, 1992. (4) Wilenski, G.; Srinivasachar, S.; Sarofim. Modeling of Mineral Matter Redistribution and AahFormation In PulverizedCoal Combustion, Proceedings of the Engineering Conference-Inorganic Transformations and Ash Deposition During Combustion, Palm Coast, FL, 1991.
economic imDacta ~~
decreased boiler efficiency decreased boiler availability increased maintenance decreased unit capacity
in the ability to consider fully the multitude of processes simultaneously occuring in a large boiler. Prediction of ash deposit effects on boiler performance, such as deposit thermal impacts, deposit response to soot blowing, etc., is very difficult and typically expressed in a qualitative, comparative fashion. When a high level of reliability is essential, full-scale combustion testing is the approach presently used to predict ash performance effects. Evaluation of the overall performance of a coal in the actual unit under actual operating conditions provides the ultimate characterization. This approach can be very expensive and may pose a risk to existing equipment. If field testing is not well planned and properly controlled, results may not be conclusive. It is necessary to ensure that testing is conducted at the appropriate operating loads desired after conversion. Pilot-scale combustion testing coupled with computational boiler analysis techniques offer another option for evaluating boiler performance impacts. The application of pilot-scale testing to predict boiler performance and operation charateristics is discussed in depth in the following sections. Prediction of Ash Performance Impacts
Although ABB Power Plant Laboratories and others are enthusiastically pursuing the prediction of ash deposit behavior from fundamental fuel properties, it is believed that pilot-scale testing can be currently used to quantify deposit characteristics and provide direct measurement of key ash performance effects. However, it is important to note the significance of replicating key commercial operating parameters during pilot-scale testing since they play such adominant role in deposit behavior. Key furnace operating parameters must be measured and controlled to allow accurate application of test results to commercial situations. Time-temperature history plays a dominant role in the ash formation process. Heat transfer surface temperature and furnace heat flux dictate deposit temperatures which in turn control reactions within the deposit and ultimately the physical properties of the deposit. Each of these operating parameters may be measured and controlled in ABB’s Fireside Performance Test Facility (FPTF),developed to simulate important boiler operating parameters and allow testing over the typical range of conditions experienced in utility coal-firedboilers. Details of the PFTF, testing approach, and specific measurements employed have been described else~here.~v* Testing ( 5 ) Benson, S. A.; et al. Predicting Ash Behavior In Utility Boilers: Assessment of Current Status. EPRI Conference-The Effects of Coal Quality on Power Plants, La Jolla, CA, 1992. (6) Baxter, L. L.; Dorra, L. The Combustion Behavior of a Blend of Eastern and Western Coale: Comparison Between a Blend and ita Individual Components. International Joint Power Generation Conference & Exposition, Atlanta, GA, 1992. (7) Borio, R. W.; Levasseur, A.; Chow, 0.;Miemiec, L. S. Ash Deposition: A Boiler Manufacturer’s Perspective. Engineering Foundation Conference-Inorganic Transformation and Ash DepositionDuring Combustion, Palm Coast, FL, 1991.
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770 Energy & Fuels, Vol. 7, No. 6, 1993
75,000 Blu/hr/fl 80,000 Blulhrlfl
-
' 90,000 Bfulhrlfl
105.000 Blulhrlfl' 115,000 Bfulhrlffz
CoelB
'
107.000 Blulhrlf12 130,000 BlUlhrlflZ
2400
-
2300
-
FPTF
445 MWe UTILITY UN/T
Figure 2. Comparison of full-scale and pilot-scale furnace heat flux.
Table 11. Thermal Conductance of Deposits Generated at Various Elevations in The FPTF thermal conductance MAX,Btu/(h ft2 O F ) fuel coal A coal B elevation 1 elevation 3 elevation 4 average
CodA
38 32 37 35.7
49 48 48 48.3
focuses on defining the effects of ash deposition on heat transfer and determining physical properties of deposits. Of particular interest is deposit response to soot blowing and how deposits are affected by boiler thermal conditions. A key objective of testing is to establish the maximum or "critical" thermal conditions at which deposit removal through conventional soot blowing becomes marginally effective. These conditions represent a boiler capacity limit for continuous operation. Due to scale-up effects (on flame radiation, gas velocity, etc.), potential differences in pilot-scale furnace heat transfer rates from those in a commercial furnace must be considered. Figure 2 provides an example of heat flux measurements at various furnace elevations in a utility boiler and in the FPTF when firing the same coal. Recent studies at ABB have indicated that furnace heat flux may be the single most important operational parameter influencing wall deposit strength and response to soot blowing.9 Deposits generated in the FPTF under equivalent thermal conditions have shown excellent correlation with utility boiler deposits in terms of physical properties, . chemical composition,and thermal characteristics.1° The effects of deposits on boiler performance are of particular importance from an application standpoint. Table I1 provides the average wall deposit thermal conductance values determined in the FPTF for two fuels which were extensively tested in a utility boiler. Figure 3 shows the variation in measured furnace outlet temperature for the same two fuels fired under similar conditions in the field. The higher furnace outlet temperatures associated with firing Coal A are consistent with lower thermal conductances. With the exception of one test point which was obtained during thermal loading transition and fuel changes, average FPTF thermal conductance values have shown excellent correlation with back-calculated field values determined from field test data (Figure 4). (8)Levasseur, A. A.; et al. Combustion Characterization of EPRI Cleaned Coala. EngineeringFoundation Conference-Fine Coal Cleaning, Santa Barbara, CA, 1987. (9)Borio, R.W.;Levasseur,A. A.; Thomock, D. E. Use of Pilot-Scale Combustion Tests to Predict the Effects of Ash Deposita on Boiler Performance. International Joint Power Generation Conference, San Diego, CA, 1991. . (10)Benson,S.A.; et al. Examination of Ash DepositionIn Full-,Pilot-, and Bench-Scale Testing. EPRI Conference-The Effects of Coal Quality on Power Plants, 1992,La Jolla, CA, 1992.
0 2200
s
3 2100 d
2000
I
I 10
0
I 20
I 30
BOILER WIDTH (ff)
1 1 50
I 40
- LEFT TO RIGHT
Figure 3. Furnace outlet gas temperature profiles for similar boiler heat inputs for field-tested coals.
/ I H
/ 20 20
I 30
I 40
-
I 50
60
FIELD klU, B W h f i r O F
Figure 4. Comparison of average FPTF deposit thermal conductance versus utility boiler deposit thermal conductance for different coals over an equivalent temperature range. 100
h
40
d 2
3.3 MBtulhr 12.5% EA 2806 O F
3.2 MBtulhr 2Q% EA 2822 O F
3.3 MBtulhr 30% EA 2815 "F
Figure 5. A comparison of heat flux recoveries for coal A firing at various excess air levels in the FPTF.
Pilot-scale testing allows greater control of test conditions, facilitating systematic evaluation of the effects of various boiler operating conditions. It also allowsisolation of specific phenomena required to develop fundamental cause and effect relationships. Figure 5 illustrates changes in waterwall heat absorption rates and deposit response to soot blowing when firing at different excess air levels while thermal loading and furnace temperature profile are kept relatively constant in the FPTF. Results show increased slagging behavior with lower (12.5 % ) excess air firing. Soot blowing was less effective due to the formation of more tenaciouslybonded deposits. Apparently, reduced
Fuel Switching P
2600
2300
Energy & Fuels, Vol. 7,No. 6,1993 771
,
I
I
2.0
2.5
I 3.0
I 3.5
a,.
I 4.0
I
I 5.0
4.5
I
.E
% voi
Figure 6. Furnace outlet temperatures as a function of excess air for a 445-MWe unit firing coal A. m
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I
F
G
H
6.0
5.5
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L
M
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Figure 8. Fly ash erosion rates for various coals. FprF oep.wi.neimd lnpvn
Pwnm Moduks
I
m COALC
a 2000
2100
2200
2300
2400
2500
TEMPERATURE, "F
Figure 7. Effect of gas temperature on convective tube deposit
bonding strengths.
Figure 9. Boiler performance flow chart.
environments promote formation of low melting eutectic compounds. The FPTF results are consistent with field measurements. As shown in Figure 6, furnace outlet temperature increased as excess air levels are decreased. The increase in measured furnace outlet temperature is attributed to both increased wall slagging and reduced gas mass flow rate associated with lower excess air input. In parallel with the FPTF furnace wall slagging measurements, fouling evaluations are conducted to generate similar performance data with respect to upper furnace and convection pass sections. On-line deposit-to-tube bonding strength measurements are made to quantify relative force required to remove deposits, The technique has been applied in the field and calibrated to soot blower effectiveness. Bonding strength measurements are used to establish critical gas temperature which could limit full load capability. The effect of gas temperature on the deposit bonding strength of different coals is illustrated in Figure 7. Data on deposit buildup rates and changes in convection tube heat absorption rates are generated and used to establish surface effectiveness factors which define thermal performance. In addition to the slagging &d fouling evaluations, FPTF testing provides quantitative measurement of ash erosion rates. High gas velocity sections located downstream of the convective tube section in the FPTF accelerates the ash-laden gas flow up to 200 ft/s and direct it toward target tube specimens. The wear data generated for different coals are used to establish gas velocitv limitations for acceptable tube life (Figure
mance programs, represented by the flow chart in Figure 9. For a particular unit under study, the FPTF results are used to define key model inputs for furnace wall conditions, such as the deposit conductivities, emissivities, and absorptivities necessary to calculate lower furnace heat absorption. The computer-calculated furnace gas temperatures are compared with gas temperature limits determined by FPTF testing, to predict whether maximum continuous rating can be maintained. A similar process is used to evaluate the fouling characteristics of the coals being studied based on the gas temperature limits along with the interaction of fouling factors and surface effectiveness factors for radiant pendant panel and platen surfaces in the upper furnace and convection surfaces above the furnace arch and backpass.l3 The programs are run in an iterative mode enabling the boiler designer to establish load limits of upper furnace and backpass superheater, reheater, and economizer that are based on reaching or exceeding threshold values for gas temperatures, or excessive superheater and reheater desuperheating spray flow, or by surpassing tubing and header pressure part metal temperature limits. Since boiler thermal conditions and ash deposit properties are so strongly interdependent, it is essential that input variables defining deposit characteristics are appropriately adjusted to match predicted operating conditions during the iterative calculation process. The boiler performance program also generates other important information for
s).
Application of Test Results The FPTF furnace slagging, convection fouling, and erosion results are integrated with ABB's boiler perfor-
(11)b a s k , E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Co.: New York, 1986. (12) Singer, G.J., Ed. Combustion: Fossil Power; Combustion En-
gineering (13) Chow, In'*:0.K.; et al. Combustion CTplN1. Characterization of the Kentucky
No. 9 Cleaned coals, EPRI cs-4994 Project 2425-1 find report, 1987.
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772 Energy & Fuels, Vol. 7, No. 6, 1993
Table 111. Fuel Analyses for Study Unit Test C o d e parametersa baseline coal altemate coal proximate, wt 95 21.1 28.4 moisture 33.0 30.8 volatile matter fixed carbon 36.7 33.9 9.2 6.9 ash 100.0
100.0
HHV, Btu/lb
9060
8470
ultimate hydrogen carbon sulfur nitrogen oxygen ash total
4.6 67.2 0.8 1.2 14.6 11.6 100.0
4.5 69.9 1.o 1.1 14.0 9.5 100.0
Figure 10. Schematic of the 350-MWe Study Unit.
ash loadings, Ib/MBtu
10.1
8.0
plant heat rate analysis (boilerefficiency) and other boiler island and downstream equipment, such as desuperheater spray controls, electrostatic precipitators, etc.
ash fusion temp, "C I.T. S.T. H.T. F.T.
2090 2180 2200 2220
2210 2260 2270 2280
43.7 18.6 4.7 13.4 3.6 0.2 0.6 1.0 0.3 10.1 96.2
16.7 10.5 11.8 24.3 6.3 2.2 0.3 0.3 0.5 25.8 98.7
total
0 Refracfable Soof Blowers I Wall Blowers
Case Study
To illustrate how results from the PFTF are used with the boiler performance program, a fuel switchingcase will be examined in which a utility unit would switch from a subbituminous coal to an alternate coal due to potential fuel cost savings. A schematic of the 350-MWe boiler studied is shown in Figure 10. Typical fuel analysis data for the baseline coal presently burned in this unit and the alternate coal evaluated are provided in Table 111. Although these two coals vary in many characteristics, the most significant difference is in the ash chemical composition. The alternate coal ash has much higher sodium and calcium concentrationsthan the baseline coal ash. These constituents are known to contribute to high fouling propensity.11J2 Key performance data established during FPTF testing of each coal are summarized in Table IV. FPTF test results indicated slagging characteristicsof the two coals wold be controllable and fairly similar at furnace conditions equivalent to full load in the study unit. Additionally, FPTF results indicated that convection pass fouling with the alternate coal would become uncontrollable in spaced sectionsat gas temperatures above 2200 OF. Comparative deposit-to-tubebonding strength measurements obtained for the baseline coal from the field unit confirmed that the FPTF data could be directly applied. Therefore, an operational limitation was established at 2200 O C for the gas entering the convective reheater surfaces when firing the alternate coal. The convective gas temperature limit for the baseline coal was 2350 OF, which was well above those temperatures projected for the study unit. FPTF results also indicated that firing the alternate coal reduced convection tube deposit buildup rates by onethird compared to the baseline coal at equivalent conditions. However, the more tenaciously bonded deposits of the alternate coal and their resistance to smt blowing must be considered. Results indicated that the potential benefits on heat transfer anticipated due to lower deposit buildup rates would be negated by reduced soot blower effectiveness for the alternate coal. As previously discussed, the effects of these ash deposit characteristicson boiler performance are strongly influenced by boilerspecific factors. The study unit has a design capacity 350
ash composition,w t 95 Si02 &?Os Fez03 CaO MgO Na2O K20 Ti02 p206
so3
total
a Proximate analyses on as-receivedbasis, all others on moisturefree basis.
Table IV. Comparison of FPTF Performance Results between Baseline and Alternate Coals parameters baseline coal altemate coal Furnace Slagging critical flame temp for removal 3030 2990 deposita, OF deposit physical state molten sintered/molten 41.2 40.8 thermal conductance, Btu/(h ft2 O F ) ConvectivePass Fouling critical gas temp for removal 2350 2200 deposita, OF relative soot blowing interval, h 6 8 friable/sintered highly sintered deposit physical state Erosion normalized wear at 60 ft/s and 3.8 10 OOO h operation, mil fly ash particle size, Im 2.8 1.8 quartz, wt 5% of fly ash 16.8 5.0
MW, and was built to fire the baseline subbituminous coal. The boiler has a maximum continuous rating (MCR) of 2 472 OOO lb/h main steam flow and 2 173 OOO lb/h of reheat steam flow; main and reheat outlet steam conditions are 1005 OF/2619 psig and 1005 OF/548 psig, respectively. Superheatoutlet steam temperature is controlled by means of superheat attemperator spray. Reheat outlet steam temperature is controlled by means of fuel nozzle tilt and reheat attemperator spray. The boiler model was configured to represent the study unit. Performance runs utilizing baseline operating conditions (fuel fineness, excess air, etc.), fuel analysis, and FPTF data were conducted to assess data quality and
Fuel Switching
Energy & Fuels, Vol. 7, No. 6,1993 773
Table V. Comparison of Study Unit Performance Firing Baseline and Alternate Coals parameters
baseline coal
Steam Generator gross generator output, MW 346 percent MCR,% 94 boiler efficiency, % 87.81 main steam flow, 109 lb/h 2330 reheat steam flow, 103 lb/h 2235 superheater steam outlet temp, OF 1005 superheater steam outlet pressure, psig 2369 superheater spray flow, % 2.5 reheater steam outlet temp, O F 1005 reheater steam outlet pressure, psig 491 reheater steam spray flow, % 6.9 Furnace Performance net heat input, MBtu/h 3460 heat release rate, MBtu/(h ft2) 1.89 furnace outlet temp, O F 2233 burner tilt, deg -2OO Pulverizer Performance hardgrove grindability 52 fuel moisture, wt % 24 fuel fineness, % minus 200 mesh 85 mill power consumption, kW 1765 Air Heater Performance inlet air temp, O F 79 inlet gas temp, OF 777 outlet air temp, O F 700 outlet gas temp, OF 251 efficiency, 7% 75.4
alternate coal
280 80 90.12 1978 2140 1005 2369 0.5 1005 410 0.2 2883 1.57 2187 +30° 83 15 85 735 80 697 639 224 76.7
calibrate the model for boiler-specific factors. Modeling results are summarized in Table V. The results of the performance predictions indicate the study unit, when firing the alternate coal, would experience fouling problems at full load as gas temperatures would exceed the reheater foulinglimitation of 2200 O F . Although many input variables can be manipulated to offset fouling impacts, for the case presented in this model study, a simplifying assumption was made that the gas temperatures would be reduced through load reduction and that operating variables such as steam temperature would be maintained a t the design values. Under this scenario, the generating capacity would be reduced to approximately 80% of design capacity in order to meet the fouling temperature criterion. In addition to the fouling concern at this load, superheater and reheater steam temperatures would be only marginally achieved. With upward tilts already at the maximum position (+30°), even minor decreases in heat transfer to the superheater or reheater could result in a failure to achieve design steam temperatures. Normally superheat and reheat temperatures can be maintained with fuel/air nozzle tilts in the horizontal position. Firing with burner nozzle tilts in the maximum upward position, coupled with low attemperator spray flows indicates very limited control flexibility. Several other operational compromises would typically be considered when evaluating derating requirements, such as increased excess air, reduced steam temperatures, etc. Although the model may be applied to support evaluation of performance and economic trade-offs, these compromises are generallyunit specific. Equipment modifications including additional soot blower coverage in the finishing
reheater section of the unit could improve deposit cleaning capability and facilitate continuous higher load operation by improving heat transfer. Platenizing the reheater sections also could improve soot blower effectiveness, and furnace surface additions could reduce furnace outlet temperature while still providing required heat absorption. The primary purpose of the case study presented here is to illustrate the application of pilot-scale results to boiler performance predictions. As shown in the above analysis, depending on unit specifics, alternatives can be chosen to optimize boiler operations firing a new fuel. Use of FPTF inputs to the boiler model greatly reduced the number of assumptions and subjective approximations required to run the analysis. The ability of the model to utilize deposit property data in an interactive fashion (adjustment of ash performance effects as boiler operating conditions are changed) is very important to the reliability of the predictions.
Summary Accurate assessment of boiler performance and operational impacts associated with coal switchingare essential in determining the overall feasibility of a conversion. Different approaches can be used to evaluate these impacts which provide various levels of confidence. Significant advances have been made in analyzing coal properties and in applying computational modeling to predict behavior in commercial systems. These techniques can greatly improve the reliability of assessments as compared to assessments made solely with ASTM-based indices. However, full-scaleutility boiler combustion testing is still the primary approach used to determine coal performance impacts with a high degree of reliability. Pilot-scale combustion testing can provide an alternative means of reliability evaluating replacement coals at relatively low cost and no operational risk to the utility. A methodology has been developed for quantitative assessment of ash-related effects of boiler performance. Pilot-scale combustion testing is used to define key ash deposit physical and thermal properties and to establish boiler operational limits. This information is used in computational boiler models to quantitatively predict boiler capacity and unit performance characteristics. Results from pilot-scale tests have shown excellent correlation with utility boiler performance under comparative conditions. The strong interdependence of coal performance characteristics and boiler operating conditions makes it essential that coal ash behavior be evaluated under appropriate conditions. Similarly, predictive models need to effectively interact and adjust ash-related behavior of changing boiler conditions. The methodology discussed in this paper has been applied to accurately predict changes in the performance of utility boilers when firing different coals.
Acknowledgment. The authors acknowledge Messrs. D. E. Thornock, B. F. Griffith, and R. W. Borio and Ms. L. S. Miemiec of ABB Power Plant Laboratories for their contributions to work reported. The authors also acknowledge Dr. A. K. Mehta of EPRI for his technical and financial support.
