Presentation on theme: "MANSFIELD UNIT 2 BOILER SLAG ANALYSIS JANUARY 9, 2001."— Presentation transcript:
MANSFIELD UNIT 2 BOILER SLAG ANALYSIS JANUARY 9, 2001
AGENDA Introduction Coal Characteristics Combustion Contributors to Slagging Dolomite Issues Slagging Analysis Conclusions / Corrective Actions C. Swanson J. Mooney J. Davis M. Lamison S. Harding C. Swanson
Mansfield Unit 2 was forced off-line Sunday, 12-17-00, as a result of a large slag fall that caused multiple leaks in the north and south bottom ash hopper slope tubes.
SEQUENCE OF EVENTS 10-02-00 Started the long term dolomite test on Unit 2. 10-30-00 Operations noted unusually heavy slag build-up on the pendant slope area. 11-01-00 Approximate start of high sulfur fuel deliveries. 11-08-00 Slag observation guidelines were provided to Operations and the dolomite feeders were shut off daily to improve slag observations. 11-22-00 Initiated water blasting of pendant slag to evaluate its effectiveness.
SEQUENCE OF EVENTS (CONT) 11-28-00 2G mill was taken off for an overhaul increasing top mill operation at high loads. 12-05-00 Boiler average furnace exit gas temperature (FEGT) started to trend upward on Unit 2. 12-16-00 Boiler slope blowers 25 and 26 became obstructed with slag that is believed to have slid down from the upper pendants. 12-17-00 Unit 2 tripped off-line.
THEORETICAL CONTRIBUTORS TO SLAGGING CHARACTERISTICS OF THE COAL – Iron and ash content ASH FUSIBILITY – Furnace Exit Gas Temperature (FEGT)
PRACTICAL CONTRIBUTORS TO SLAGGING HIGH SULFUR MCELROY FUEL DOLOMITE INJECTION 2G MILL OUT-OF-SERVICE FOR REBUILD HIGH UNIT CAPACITY FACTORS BOILER COMBUSTION DEFICIENCIES
FUEL QUALITY (Max levels based on individual barge analysis)
DOLOMITE INJECTION BLOCKS SLAG OBSERVATIONS VISUALLY AND BY THE CONTROL ROOM CAMERAS POTENTIALLY INCREASES SLAGGING TENDENCIES OF SOME FUELS BY LOWERING THE ASH FUSION TEMPERATURE RESULTS IN HIGHER ASH LOADING CHANGES CHARACTERISTICS OF THE SLAG THAT IS FORMED
2G MILL UNAVAILABILITY REQUIRES THE USE OF UPPER MILL 2D FOR LOAD ADVERSLY EFFECTS COMBUSTION DUE TO SINGLE TOP MILL OPERATION REQUIRES HEAVIER MILL LOADING DUE TO THE UNAVAILABILITY OF A 7th MILL INCREASES FURNACE EXIT GAS TEMPERATURE (FEGT)
HIGH CAPACITY OPERATION
BOILER COMBUSTION DEFICIENCIES INCREASES FURNACE EXIT GAS TEMPERATURES CREATES A REDUCING ATMOSPHERE HIGHER IN THE FURNACE LOWERING THE ASH FUSION TEMPERATURE OF THE COAL CAUSES COMBUSTION INBALANCES THAT RESULT IN LOCALIZED SLAGGING AGGRIVATED BY HIGH MILL OUTPUT
Slagging Report-1-9-00 Coal Contribution to U2 Slag Incident Jim Mooney
Comments 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
Monthly Avg 3.51 #S/mmbtu
October 2000 Consol Supplied S#/MMBTU 3.47# Sulfur/MMBTU on Monthly Average 298,453 Tons
October Cumberland #Sulfur 70,651 Tons
November 2000 Consol Quality Average Consol Supplied Coal 3.57 295,453 TONS
November Cumberland 63,435 Tons
December Consol Supplied #S/MMBTU December Average #S/MMBTU 3.40 89,664 Through 12-20
Short Prox Results Bunker Samples Post Trip Recent McElroy Yard Samples
Affect of Operating Parameters on Slag Accumulation in Mansfield Unit 2 Upper Furnace Jake Davis GTSD-FSS January 9 th, 2001
Background 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 Book 40th Edition - 1992
Factors Affecting Slag Formation and Accumulation in the Furnace Coal - Jim Mooney Dolomite - Mark Lamison Furnace Exit Gas Temperature Combustion - O 2, CO, LOI, Balance
Affects of Combustion on Slagging Local reducing atmospheres in the furnace negatively affect the ash fusion temperatures of most Mansfield coals by up to 250 o F. –Local reducing atmospheres are caused by: Inadequate excess air in the furnace Unbalanced fuel and air flows at the burners Unstable combustion, or swings Combustion also affects FEGT.
Affect of Combustion on Ash Fusion Temperatures
Desired Conditions for Boiler Optimization Maintain 2.6% minimum oxidizing environment at furnace exit Balance pulverizer fuel flow to + 10% Maintain mill fineness 99.5% passing 50 mesh Maintain bulk exit gas temperature of 2200 o F with no areas above 2350 o F. MN2 = 1.7% in center MN2 = 98.5% MN2 = 2182 o F Peak = 2475 o F MN2 = 17.7%
Typical Mansfield 2 Combustion Profile
Mansfield Unit 2 Furnace O 2
Mansfield Unit 2 Furnace CO Maximum CO in center of furnace of 35,000 ppm
Combustion Stability Unstable combustion results in localized reducing atmospheres in the upper furnace Unstable combustion results from changes in unit conditions –Changing mills / burners –Changing load –OFA control problems
OFA Port Swings Resulting in Unstable Combustion
Reasons 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
Combustion Conclusions Mansfield Unit 1 and 2 combustion profiles result in localized reducing areas of the furnace, which lower ash fusion temperatures by 250 o F. OFA Port control issues contributed to combustion instability on the unit.
Affects 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 Book 40th Edition - 1992
Factors Affecting FEGT Sootblowing Mill Combinations Available Heat Transfer Surface Load Excess Oxygen Coal Combustion Mill Fineness
Sootblowers on Mansfield Unit 2 11 of 58 furnace blowers running at time of trip Usually ran 3 to 4 times per day
Affect of Furnace Wall Blowers on FEGT
Affect of Mill Combinations on FEGT Running with one or both upper mills in service generally raises FEGT by 80 to 120 o F. During the period leading up to the significant slag accumulation on Mansfield 2, a top mill was in service almost without interruption.
Top Mill Operation Affect on FEGT
Affect of Increase in Load on FEGT Capacity factor for December was 87% compared to normal values of 65 to 75%.
Affect of Furnace Surface Area on FEGT More SA to absorb heat yields lower FEGT Corners of Mansfield furnaces have more SA and lower temperatures Slag generally worse in middle of boiler
Mansfield Unit 2 Furnace Temperature
High 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 December –87% capacity factor MTD leading up to trip Ran top mill non-stop from 12/3 to 12/17 Temperature stratified in center of boiler Approximately 150 o F hotter than baseline FEGT
Baseline FEGT on Mansfield 2
FEGT on Mansfield 2 during December
FEGT Conclusions FEGT took an approximately 150 o F step change higher during the two weeks prior to the unit trip. –Load was significantly higher than typical for long periods of time –Top mills were run more than during low NOx firing conditions –Current wall blower operation is not effective in maintaining low FEGT and reducing slag
Recommendations Achieve 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
Mill Performance Balance the burners by clean air testing to + 2%. Maintain mill outlet temperatures above 170 o F. 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.
Combustion Improvements Balance furnace excess oxygen profile using HVT probe to achieve an average 2.6% O 2 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 O 2 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.
Combustion 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.
Lowering 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.
FEGT Conclusions FEGT 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 time –Top mills were run more than standard practice –Sootblowing on Mansfield 2 is not as effective as desirable
DOLOMITE January 9, 2001
Dolomite Visual Results
Potential Problems Increased Low Temperature Fouling Increased Slagging Increased Economizer Outlet Temperature Increased Scrubber Scaling Injection Method and Location SCR Catalyst Scrubber Operation Thickener Chemistry Unit #3 ESP
Low Temperature Fouling Microbeam study indicated a higher propensity for low temperature fouling Significant fouling has occurred in the convection passes. Fouling has not blocked gas path. Economizer outlet temp up 60 to 80 °F Firing boiler harder to achieve temperatures
Unit #1 Superheater Fouling 13th floor -11/24/99
Slagging Slagging indices indicate varied affects from dolomite Lab blending tests % Basic Vs Ash Fusion Curve Dolomite increases ash loading Microbeam Study indicated no increase in propensity to slagging, but possibly higher strength 9 day test in July 1999 showed no increased slag even possible improvement DOE Dolomite testing indicated some slag accumulation, but also with MacElroy coal above 3.8% Sulfur.
Slagging Indicators Conflicting information from indicators The value are indicators and have a wide scatter Assuming lignitic ash Dolomite is not mixed directly with coal Ash fusion temperature biased down by CaSO 4 & MgSO 4
2A Mill Dolomite Injection Difficult controlling rate of injection Can not minimize injection at low loads Highly stratified injection Auxiliary air high to maintain velocities at low injection rates
Mill Dolomite Flow Pipe to Pipe (Deviation from Average) EAST WEST
Recommendations Further evaluation of slagging potential (actual viscosity measurements) Inject dolomite with lower slagging coal Try injecting dolomite through outside burner pair - (lower FEGT area) Variable speed drives on mill feeders Discuss mixing directly with coal Explore the use of convection pass sonic horns
Deposition Analysis FirstEnergy Mansfield Station Review Meeting Mansfield Station N. S. Harding January 9, 2001
Slagging vs Fouling Slagging Fouling
Definitions Slagging – deposition where radiation is the predominant form of heat transfer Fouling – deposition where convection is the predominant from of heat transfer
Deposition Mechanisms –Condensation of inorganic vapors –Inertial impaction and sticking of particles –Chemical reactions –Thermophoresis
Principal Effects of Deposition –Retard heat transfer and eventually reduce boiler efficiency –Grow until they restrict flow through the boiler –Can be associated with corrosion
Selective Species Role in Deposition –Alkali (Na and K) Mostly volatilized and react with sulfur to form low melting sulfates; very dense and reflective Can react also with iron and sulfur to form corrosive iron trisulfates –Alkaline Earth (Ca and Mg) More refractory and probably not completely volatilized If intimately mixed with ash, reduces melting temperature
Selective Species Role in Deposition –Sulfur Completely vaporized and forms very low melting solids Usually found as glue which holds deposits together –Iron Reacts with alkalis and sulfur to produce low melting materials Has relatively low melting temperature and causes ashes to melt at lower temperatures
Deposition Parameters –Coal Slagging and Fouling Parameters ASME Publication, Research Committee on Corrosion and Deposits from Combustion Gases –Slagging and Fouling in Pulverized-Coal-Fired Utility Boilers EPRI Publication, CS-5523 (Work performed by Battelle)
Ash Definitions –Ash Type If CaO + MgO < Fe 2 O 3 then Bituminous ash If CaO + MgO > Fe 2 O 3 then Lignitic ash –Base-to-Acid Ratio Sum of bases (Na 2 O+K 2 O+Fe 2 O 3 +MgO+CaO) divided by Sum of acids (Al 2 O 3 +SiO 2 +TiO 2 )
Example: Slagging Indices (95% McElroy/5% Dolomite)
Example: Fouling Indices (95% McElroy/5% Dolomite)
Example: Hensel-Halfinger Plot
Estimated Ash Temperatures
Ternary Diagram (Si-CaO-Fe 2 O 3 ) 2200 o F 2800 o F 4800 o F
Quaternary Diagram (Si-Mg-Ca-Al) 2370 o F
Summary McElroy coal may be problematic Especially if higher sulfur Furnace Exit Gas Temperature is above ash melting temperature Dolomite addition enhances problems
Recommendations Monitor FEGT, maintain below ~2300 F Limit sulfur content to maximum in contract Maintain operation of wall blowers Add dolomite to side burners-avoid hot spots Operate without top row of burners as much as possible
CONCLUSIONS SHORT / LONG TERM CORRECTIVE ACTION C. Swanson
PRIMARY CONTRIBUTOR High sulfur McElroy fuel SECONDARY CONTRIBUTORS High furnace exit gas temperatures 2G mill unavailability Combustion stratification Insufficient wall blower availability Unavailability of mid sulfur fuel to replace McElroy Dolomite injection
SHORT 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.
SHORT 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.
SHORT 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.
LONG 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 Powers water lances for select wall blower replacement.