3Mansfield Unit 2 was forced off-line Sunday, , as a result of a large slag fall that caused multiple leaks in the north and south bottom ash hopper slope tubes.
4SEQUENCE OF EVENTSStarted the long term dolomite test on Unit 2.Operations noted unusually heavy slag build-up on the pendant slope area.Approximate start of high sulfur fuel deliveries.Slag observation guidelines were provided to Operations and the dolomite feeders were shut off daily to improve slag observations.Initiated water blasting of pendant slag to evaluate its effectiveness.
5SEQUENCE OF EVENTS (CONT) G mill was taken off for an overhaul increasing top mill operation at high loads.Boiler average furnace exit gas temperature (FEGT) started to trend upward on Unit 2.Boiler slope blowers 25 and 26 became obstructed with slag that is believed to have slid down from the upper pendants.Unit 2 tripped off-line.
6THEORETICAL CONTRIBUTORS TO SLAGGING CHARACTERISTICS OF THE COALIron and ash contentASH FUSIBILITYFurnace Exit Gas Temperature (FEGT)
7PRACTICAL CONTRIBUTORS TO SLAGGING HIGH SULFUR MCELROY FUELDOLOMITE INJECTION2G MILL OUT-OF-SERVICE FOR REBUILDHIGH UNIT CAPACITY FACTORSBOILER COMBUSTION DEFICIENCIES
8FUEL QUALITY(Max levels based on individual barge analysis)
9DOLOMITE INJECTIONBLOCKS SLAG OBSERVATIONS VISUALLY AND BY THE CONTROL ROOM CAMERASPOTENTIALLY INCREASES SLAGGING TENDENCIES OF SOME FUELS BY LOWERING THE ASH FUSION TEMPERATURERESULTS IN HIGHER ASH LOADINGCHANGES CHARACTERISTICS OF THE SLAG THAT IS FORMED
102G MILL UNAVAILABILITY REQUIRES THE USE OF UPPER MILL 2D FOR LOAD ADVERSLY EFFECTS COMBUSTION DUE TO SINGLE TOP MILL OPERATIONREQUIRES HEAVIER MILL LOADING DUE TO THE UNAVAILABILITY OF A 7th MILLINCREASES FURNACE EXIT GAS TEMPERATURE (FEGT)
12BOILER COMBUSTION DEFICIENCIES INCREASES FURNACE EXIT GAS TEMPERATURESCREATES A REDUCING ATMOSPHERE HIGHER IN THE FURNACE LOWERING THE ASH FUSION TEMPERATURE OF THE COALCAUSES COMBUSTION INBALANCES THAT RESULT IN LOCALIZED SLAGGINGAGGRIVATED BY HIGH MILL OUTPUT
13Coal Contribution to U2 Slag Incident Jim Mooney Slagging ReportCoal Contribution to U2 Slag IncidentJim Mooney
14Comments from 1997 Testing Documentation (Dr.Simon Hansen, CONSOL) The one parameter in coal that can be used to predict slagging is sulfur.“For every 1/10th lb. Of Sulfur per mmbtu you will increase peak FEGT by 20 degrees.”Primary Slag Controls are:FEGT control, Coal, Burner Arrangement, Firing Rate and Sootblowing
21December Consol Supplied #S/MMBTU December Average #S/MMBTU 3.4089,664Through 12-20
22Short Prox Results Recent McElroy Bunker Samples Post Trip Yard
23Jake Davis GTSD-FSS January 9th, 2001 Affect of Operating Parameters on Slag Accumulation in Mansfield Unit 2 Upper FurnaceJake DavisGTSD-FSSJanuary 9th, 2001
24Background“The characteristics of slag deposits… are a function of deposit temperature and deposit composition… which is a function of the local atmosphere, particularly for ash with significant iron content.”B & W Steam Book40th Edition
25Factors Affecting Slag Formation and Accumulation in the Furnace Coal - Jim MooneyDolomite - Mark LamisonFurnace Exit Gas TemperatureCombustion - O2, CO, LOI, Balance
26Affects of Combustion on Slagging Local reducing atmospheres in the furnace negatively affect the ash fusion temperatures of most Mansfield coals by up to 250 oF.Local reducing atmospheres are caused by:Inadequate excess air in the furnaceUnbalanced fuel and air flows at the burnersUnstable combustion, or “swings”Combustion also affects FEGT.
27Affect of Combustion on Ash Fusion Temperatures We are at about 10%Mineral analysis doesn’t necessarily mean Fe as Fe2O3...
28Desired Conditions for Boiler Optimization Maintain 2.6% minimum oxidizing environment at furnace exitMaintain bulk exit gas temperature of 2200 oF with no areas above 2350 oF.MN2 = 1.7% in centerMN2 = 2182 oF Peak = oFMN2 = 98.5%Maintain mill fineness 99.5% passing 50 meshBalance pulverizer fuel flow to + 10%MN2 = 17.7%
31Mansfield Unit 2 Furnace CO Maximum CO in center of furnace of 35,000 ppm
32Combustion StabilityUnstable combustion results in localized reducing atmospheres in the upper furnaceUnstable combustion results from changes in unit conditionsChanging mills / burnersChanging loadOFA control problems
33OFA Port Swings Resulting in Unstable Combustion
34Reasons for OFA Flow Swings High OFA port temperature due to low OFA port flow (high NOx curve)Controls open 100% on high port temperature and then close back to control NOx to setpoint once temperature is OK
35Combustion Conclusions Mansfield Unit 1 and 2 combustion profiles result in localized reducing areas of the furnace, which lower ash fusion temperatures by 250 oF.OFA Port control issues contributed to combustion instability on the unit.
36Affects of FEGT on Slag Accumulation “When temperatures in the furnace are below the measured initial deformation temperature, the majority of the ash particles… impacting on heating surface will bounce off and be re-entrained in the gas stream. At temperatures above the IT ash… particles have a greater potential to stick to heating surface.”B & W Steam Book40th Edition
37Factors Affecting FEGT SootblowingMill CombinationsAvailable Heat Transfer SurfaceLoadExcess OxygenCoalCombustionMill Fineness
38Sootblowers on Mansfield Unit 2 11 of 58 furnace blowers running at time of tripUsually ran 3 to 4 times per day
39Affect of Furnace Wall Blowers on FEGT 1993 Diamond Power reportFEGT decreases about 60 deg. when running the C and D orfurnace wall blowers just one time.
40Affect of Mill Combinations on FEGT Running with one or both upper mills in service generally raises FEGT by 80 to 120 oF.During the period leading up to the significant slag accumulation on Mansfield 2, a top mill was in service almost without interruption.
41Top Mill Operation Affect on FEGT 1993 Diamond Power ReportPutting just one top mill in service raises FEGT by 80 to 120degrees. This example shows 80 to 90 degree of change.
42Affect of Increase in Load on FEGT Capacity factor for December was 87% compared to normal values of 65 to 75%.
43Affect of Furnace Surface Area on FEGT More SA to absorb heat yields lower FEGTCorners of Mansfield furnaces have more SA and lower temperaturesSlag generally worse in middle of boiler
45High FEGT on Mansfield 2 Prior to Slagging Incident 11 of 58 wall blowers operated in furnace.Average run cycle of 3.4 times per day.High load demand on the unit in December87% capacity factor MTD leading up to tripRan top mill non-stop from 12/3 to 12/17Temperature stratified in center of boilerApproximately 150 oF hotter than baseline FEGT
49FEGT ConclusionsFEGT took an approximately 150 oF step change higher during the two weeks prior to the unit trip.Load was significantly higher than typical for long periods of timeTop mills were run more than during low NOx firing conditionsCurrent wall blower operation is not effective in maintaining low FEGT and reducing slag
50RecommendationsAchieve mill performance of + 10% Fuel Balance, 75% passing 200 mesh fineness and 99.5% passing 50 mesh fineness.Maintain balanced oxidizing atmosphere in upper furnace through combustion improvements.Lower the FEGT
51Mill Performance Balance the burners by clean air testing to + 2%. Maintain mill outlet temperatures above 170 oF.Maintain mill ball charges to ensure 135 to 140 mill motor amps.Continue investigation of adjustable classifier modification on 2F mill.Reinvestigate using smaller balls with larger lift bars to improve fineness.Test all auxiliary air dampers for leakage. This could be affecting mill balance.Model classifiers to troubleshoot fineness and distribution problems.
52Combustion Improvements Balance furnace excess oxygen profile using HVT probe to achieve an average 2.6% O2 with no points < 2%.Pay close attention to combustion characteristics in noted high slag areas.Adjust secondary air registers to balance combustion.More accurately profile the O2 distribution along the side-walls.Experiment with biasing of mills to troubleshoot burner problem and optimize combustion.Experiment with upper mill burner configurations to optimize combustion.
53Combustion Improvements (contd) Correct the OFA port “swings” to aid combustion stability.Increase utilization of the Unit 2 MK Engineering LOI and Temperature monitoring device to assist with combustion balancing.Maintain stack CO indications below 100 ppm at high loads. Low load CO should be minimal at all times.Inspect all burner swirlers.Inspect all burner and damper tolerances during the outage.
54Lowering FEGT Return to service all available furnace wall blowers. Investigate use of water lances in the furnace area.Optimize combustion in the upper furnace.Compare furnace HVT profile to current FEGT measurement to better understand its range.Model the Mansfield 2 boiler to assist in evaluating operational changes on the FEGT.
55FEGT ConclusionsFEGT took a 100 to 200 oF step change higher during the two weeks prior to the unit trip.Load was significantly higher than typical for long periods of timeTop mills were run more than standard practiceSootblowing on Mansfield 2 is not as effective as desirable
60Low Temperature Fouling Microbeam study indicated a higher propensity for low temperature foulingSignificant fouling has occurred in the convection passes.Fouling has not blocked gas path.Economizer outlet temp up 60 to 80 °FFiring boiler harder to achieve temperatures
62Slagging Slagging indices indicate varied affects from dolomite Lab blending tests% Basic Vs Ash Fusion CurveDolomite increases ash loadingMicrobeam Study indicated no increase in propensity to slagging, but possibly higher strength9 day test in July 1999 showed no increased slag even possible improvementDOE Dolomite testing indicated some slag accumulation, but also with MacElroy coal above 3.8% Sulfur.
63Slagging Indicators Conflicting information from indicators The value are indicators and have a wide scatterAssuming lignitic ashDolomite is not mixed directly with coalAsh fusion temperature biased down by CaSO4 & MgSO4
682A Mill Dolomite Injection Difficult controlling rate of injectionCan not minimize injection at low loadsHighly stratified injectionAuxiliary air high to maintain velocities at low injection rates
69Mill Dolomite Flow Pipe to Pipe (Deviation from Average) EASTWEST
70RecommendationsFurther evaluation of slagging potential (actual viscosity measurements)Inject dolomite with lower slagging coalTry injecting dolomite through outside burner pair - (lower FEGT area)Variable speed drives on mill feedersDiscuss mixing directly with coalExplore the use of convection pass sonic horns
71Deposition Analysis FirstEnergy Mansfield Station Review MeetingMansfield StationN. S. HardingJanuary 9, 2001
73DefinitionsSlagging – deposition where radiation is the predominant form of heat transferFouling – deposition where convection is the predominant from of heat transfer
74Deposition Mechanisms Condensation of inorganic vaporsInertial impaction and sticking of particlesChemical reactionsThermophoresis
75Principal Effects of Deposition Retard heat transfer and eventually reduce boiler efficiencyGrow until they restrict flow through the boilerCan be associated with corrosion
76Selective Species Role in Deposition Alkali (Na and K)Mostly volatilized and react with sulfur to form low melting sulfates; very dense and reflectiveCan react also with iron and sulfur to form corrosive iron trisulfatesAlkaline Earth (Ca and Mg)More refractory and probably not completely volatilizedIf intimately mixed with ash, reduces melting temperature
77Selective Species Role in Deposition SulfurCompletely vaporized and forms very low melting solidsUsually found as “glue” which holds deposits togetherIronReacts with alkalis and sulfur to produce low melting materialsHas relatively low melting temperature and causes ashes to melt at lower temperatures
78Deposition Parameters Coal Slagging and Fouling ParametersASME Publication, Research Committee on Corrosion and Deposits from Combustion GasesSlagging and Fouling in Pulverized-Coal-Fired Utility BoilersEPRI Publication, CS-5523 (Work performed by Battelle)
79Ash Definitions Ash Type Base-to-Acid Ratio If CaO + MgO < Fe2O3 then Bituminous ashIf CaO + MgO > Fe2O3 then Lignitic ashBase-to-Acid RatioSum of bases (Na2O+K2O+Fe2O3+MgO+CaO) divided bySum of acids (Al2O3+SiO2+TiO2)
89Summary McElroy coal may be problematic Especially if higher sulfurFurnace Exit Gas Temperature is above ash melting temperatureDolomite addition enhances problems
90Recommendations Monitor FEGT, maintain below ~2300 F Limit sulfur content to maximum in contractMaintain operation of wall blowersAdd dolomite to side burners-avoid hot spotsOperate without top row of burners as much as possible
91CONCLUSIONS SHORT / LONG TERM CORRECTIVE ACTION C. Swanson
92High sulfur McElroy fuel PRIMARY CONTRIBUTORHigh sulfur McElroy fuelSECONDARY CONTRIBUTORSHigh furnace exit gas temperatures2G mill unavailabilityCombustion stratificationInsufficient wall blower availabilityUnavailability of mid sulfur fuel to replace McElroyDolomite injection
93SHORT TERM CORRECTIVE ACTION Control tuning is progressing to minimize OFA flow swings.Combustion O2 curves have been modified to increase minimum full load O2 from 3.2 to 3.4% to improve combustion.Initiated increased slag monitoring and observations.Draft flow charts have been developed to provide operator guidance on required corrective action for light-heavy slagging conditions.More aggressive load reductions have been taken based on observed slag conditions.
94SHORT TERM CORRECTIVE ACTION (CONT) High pressure water blasting has been used with some success to remove slag build-up from pendant leading edge tubes.Precision Blasting, Inc. is looking at methods to reach center slag build-up for explosive removal of slag.GTSD is providing full time support to identify and correct combustion issues related to slagging.Storm Engineering will be brought in to support GTSD with combustion testing and analysis.Unit operation has been limited to 2 PA fan operation to optimize mill performance.
95SHORT TERM CORRECTIVE ACTION (CONT) A detailed sootblower monitoring program has been developed by operations for review at the daily plant status meeting.A cross-functional team of plant personnel has been set-up and has begun addressing sootblower unavailability.
96LONG TERM CORRECTIVE ACTION Installation of 6 leading edge sootblowers will occur on Unit 2 during the Spring scheduled outage.An additional FEGT monitor will be installed to give improved indication of exit gas temperatures.Training for plant operations personnel is being looked into to provide general awareness of combustion and slagging issues.Restoring the Digital Fuel Tracking System (DFTS) to monitor fuel quality is in progress.GTSD will investigate the use of Diamond Power’s water lances for select wall blower replacement.