FINAL REPORT

SLAG CHARACTERIZATION AND REMOVAL USINGPULSE DETONATION TECHNOLOGY DURING COAL GASIFICATION

DE-FG22-95MT95010--10

Submitted to:

Federal Energy Technology CenterAAD Document Control

U. S. Department of EnergyP. O. Box 10940, MS 921-143

Pittsburgh, PA 15236

Submitted by:

Dr. Ziaul Huque, Dr. Daniel Mei,Dr. Paul O. Biney and Dr. Jianren Zhou

Department of Mechanical EngineeringCollege of Engineering

Prairie View A&M UniversityP. O. Box 397

Prairie View, TX 77446

Tel: (409) 857-4023 Fax: (409) 857-4395E-mail: zhuque@melab1.me.pvamu.edu

Date: July 30, 1998

TABLE OF CONTENTS

Disclaimer 1

Abstract 2

Acknowledgements 3

EXECUTIVE SUMMARY 4

1.0 INTRODUCTION 5

1.1 Ash Formation and Deposition Processes 51.2 Deposit Removability 71.3 Contract Objectives 91.4 Background on Pulse Detonation 9

2.0 RESULTS AND DISCUSSION 11

2.1 Slag Characterization Study 112.1.1 Morphological Analysis and Porosity Calculations 112.1.2 SEMPC Analysis - Deposit Phase and Compositional 17

Analysis

2.2 Computational Fluid Dynamics Analysis 192.2.1 CFD Codes Used 202.2.2 Initial Conditions 202.2.3 Grid 202.2.4 CFD Results 20

2.3 Experimental Results 212.3.1 Experimental Setup and Procedure 212.3.2 Input Conditions 232.3.3 Pressure and Velocity Distribution 23

2.3.3.1 Single Pulse Testing 232.3.3.2 Multi Pulse Testing 25

2.4 Economic Feasibility Study 262.4.1 Preliminary Economic Analysis 26

3.0 CONCLUSIONS 28

4.0 REFERENCES 29

LIST OF FIGURES

No. Description Page No.

Figure 1 Distribution of Liquid Phases as a Function of Deposit 7Temperature

Figure 2 Basic Schematic of Pulse Detonation Generator 9

Figure 3a Backscattered Electron Image of Polished Cross-Section 13Of Sample MTI 96-55 (Riverside #8 Reheater) at 100X

Figure 3b Backscattered Electron Image of Polished Cross-Section 13Of Sample MTI 96-55 (Riverside #8 Reheater) at 500X

Figure 4a Backscattered Electron Image of Polished Cross-Section 14Of Sample MTI 96-56 (Riverside #8 Economizer) at 100X

Figure 4b Backscattered Electron Image of Polished Cross-Section 14Of Sample MTI 96-55 (Riverside #8 Reheater) at 500X

Figure 5a Backscattered Electron Image of Polished Cross-Section 15Of Sample MTI 96-54 (Riverside #8 Airheater Inlet) at 100X

Figure 5b Backscattered Electron Image of Polished Cross-Section 16Of Sample MTI 96-54 (Riverside #8 Airheater Inlet) at 100X

Figure 6 UTA Detonation Tube Apparatus Used in CFD Modelling 30

Figure 7 Initial Condition Zones 31

Figure 8 Grid Used to Model Heat Exchanger Tubes -- 232X100 32

Figure 9 Time History as Detonation Wave Passes Over Lead Tube 33

Figure 10 Time History as Detonation Wave Passes Over Rear Tubes 34

Figure 11 Time History As Interactions Produce High Pressure on Rear 35Stagnation Point Of Lead Tube

Figure 12 Time History As Interactions Produce High Pressure on Rear 36Stagnation Point Of Lead Tube

Figure 13 Shock Function Based on Pressure Gradient 37

Figure 14 Shock Function Based on Pressure Gradient--Showing The 38Shock Setting Up In Front Of Front and Rear Cylinders

Figure 15 Velocity Vectors Colored By Mach Number Showing Reverse 39Flow On Rear Stagnation Area Of Lead Tube

Figure 16 Temperature Contours Normalized To 520 Degre R, 1 Atm 40Pre-Ignitiond Condition

Figure 17 Test Chamber Schematic 41

Figure 18 Test Chamber Setup 42

Figure 19 Picture of Injection Line 42

Figure 20 Pressure Plot of Single Pulse Strong Detonation Wave 43Inside the Chamber

Figure 21 Velocity Plot of Single Pulse Strong Detonation Wave 44Inside Chamber

Figure 22 Pressure Plot of Multi Pulse Weak Detonation Wave 45Inside the Chamber

Figure 23. Picture of Soft Slag Sample Before and After the Test with Weak 46Detonation Wave at a Distance of 5 cm Away From the Exit of theDetonation Tube

Figure 24. Picture of Soft Slag Sample Before and After the Test with Weak 47Detonation Wave at a Distance of 20 cm Away From the Exit of theDetonation Tube

Figure 25. Picture of Medium Soft Slag Sample Before and After the Test 48with Weak Detonation Wave at a Distance of 20 cm Away Fromthe Exit of the Detonation Tube

Figure 26. Picture of Hard Slag Sample Attached at the Side of the Tube 49Before and After the Test with Weak Detonation Wave at aDistance of 5 and 10 cm Away From the Exit of the Detonation Tube

Figure 27. Picture of Soft Slag Sample Attached at the Side of the Tube 50Before and After the Test with Weak Detonation Wave at aDistance of 8.5 cm Away From the Exit of the Detonation Tube

Figure 28. Picture of Soft Slag Sample Attached at the Side of the Tube 51Before and After the Test with Weak Detonation Wave at aDistance of 20 cm Away From the Exit of the Detonation Tube

Figure 29. Picture of Medium Soft Slag Sample Before and After the Test 52with Weak Detonation Wave at a Distance of 8.5 cm Away Fromthe Exit of the Detonation Tube

Figure 30. Picture of Hard Slag Sample Before and After the Test with Strong 53Detonation Wave at a Distance of 8.5 cm Away From the Exit of theDetonation Tube

Figure 31. Picture of Hard Slag Sample Before and After the Test with Strong 54 Detonation Wave at a Distance of 20 cm Away From the Exit of the Detonation Tube

Figure 32. Picture of Medium Soft Slag Sample Before and After the Test with 55Weak Detonation Wave at a Distance of 15 cm Away From the Exitof the Detonation Tube

Figure 33. Picture of Medium Soft Slag Sample Before and After the Test 56with Weak Detonation Wave at a Distance of up to 8.5 cm AwayFrom the Exit of the Detonation Tube

Figure 34. Picture of Hard Slag Sample Before and After the Test with Weak 57 Detonation Wave at a Distance of up to 8.5 cm Away From theExit of the Detonation Tube

Figure 35. Picture of Soft Slag Sample Before and After the Test 58With Tube Banks

Figure 36. Picture of Hard Slag Sample Before and After the Test 59With Tube Banks

LIST OF TABLES

No. Description Page No.

Table 1 Area Percent Epoxy in the Deposit 16

Table 2. Riverside #8 Deposits - SEMPC Analyses - Phases by Region 18

Table 3. Riverside #8 Deposits - SEMPC Analyses - Chemical Composition 18

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Disclaimer

This report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, or assumesany legal liability or responsibility for the accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, or represents that its use would notinfringe privately owned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, or otherwise does notnecessarily continue or imply its endorsement, recommendation, or favoring by theUnited States Government or any agency thereof. The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United States Governmentor any agency thereof.

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Abstract

Pulse detonation technology for the purpose of removing slag and foulingdeposits in coal-fired utility power plant boilers offers great potential. Conventional slagremoval methods including soot blowers and water lances have great difficulties inremoving slags especially from the down stream areas of utility power plant boilers. Thedetonation wave technique, based on high impact velocity with sufficient energy andthermal shock on the slag deposited on gas contact surfaces offers a convenient,inexpensive, yet efficient and effective way to supplement existing slag removalmethods. A slight increase in the boiler efficiency, due to more effective ash/depositremoval and corresponding reduction in plant maintenance downtime and increased heattransfer efficiency, will save millions of dollars in operational costs. Reductions in toxicemissions will also be accomplished due to reduction in coal usage.

Detonation waves have been demonstrated experimentally to have exceptionallyhigh shearing capability, important to the task of removing slag and fouling deposits. Theexperimental results describe the parametric study of the input parameters in removingthe different types of slag and operating condition. The experimental results show thatboth the single and multi shot detonation waves have high potential in effectivelyremoving slag deposit from boiler heat transfer surfaces.

The results obtained are encouraging and satisfactory. A good indication has alsobeen obtained from the agreement with the preliminary computational fluid dynamicsanalysis that the wave impacts are more effective in removing slag deposits from tubebundles rather than single tube. This report presents results obtained in effectivelyremoving three different types of slag (economizer, reheater, and air-heater) t a distanceof up to 20 cm from the exit of the detonation tube. The experimental results show thatthe softer slags can be removed more easily. Also closer the slag to the exit of thedetonation tube, the better are their removals. Side facing slags are found to shear offwithout breaking. Wave strength and slag orientation also has different effects on thechipping off of the slag. One of the most important results from this study is theobservation that the pressure of the waves plays a vital role in removing slag. The wavefrequency is also important after a threshold pressure level is attained.

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Acknowledgements

The authors would like to thank the following individuals for project direction andguidance on this work: our Contracting Officer's Representatives, and Mr. AnthonyMayne as well as the management personnel at DOE/FETC. Special thanks also to Dr.Steve Benson of Microbeam Corporation and Dr. Luis Hunter at Lockheed-Martin for allthe assistance provided in finishing the projects. Technical consultation and supportprovided by FETC staff are gratefully acknowledged.

Research sponsored by the U.S. Department of Energy's Morgantown Energy Technology Center, undercontract DE-FG22-95MT95010 with Mechanical Engineering/Prairie View A&M University, College ofEngineering and Architecture, P.O. Box 397, Prairie View, TX 77446-0397; Tel: (409) 857-4023 Fax:(409) 857-4395

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EXECUTIVE SUMMARY

Pulse detonation technology for the purpose of removing slag and foulingdeposits in coal-fired utility power plant boilers offers great potential. Conventional slagremoval methods including soot blowers and water lance cleaning devices are being onlypartially successful in removing deposits specially from the downstream side of heattransfer tube bundles in the convective pass sections. They also require considerablemaintenance and reduce boiler efficiency when in use. The detonation wave technique,based on high pressure and velocity impact with sufficient energy and thermal shock onthe slag deposited on gas contact surfaces, offers a potentially convenient, inexpensive,yet efficient and effective way to supplement existing on-line slag removal method. Aslight increase in the boiler efficiency, due to more effective ash/deposit removal andcorresponding reduction in plant maintenance downtime and increased heat transferefficiency, will save millions of dollars in operational costs. Reductions in toxicemissions will also be accomplished due to reduction in coal usage resulting clean andhealthy environment.

The final report presents experimental results on the study of slag removal usingboth single pulse and multiple pulse detonation waves. The experiments were carried outat the University of Texas at Arlington Detonation Testing Laboratory. Three differenttypes of slag (obtained from Northern States Power) deposited on economizer, reheaterand air-heater were used. The slag were placed at several positions up to a distance of 20cm from the exit of the detonation tube. The experiments show that softer slag are removed more easily. Multiple wavereverberation established in soft slag cavities contribute additional removal of slag.Results also show that slag sample at the front of the tubes facing the detonation wavebreaks up into several small pieces, whereas, the slag on the side of the tube shear offwithout breaking into smaller pieces. The wave is also capable of removing slag from theback of the tube. The effectiveness of this removal depends on the pressure intensity andthe tube configuration. Tube bundle configuration provide wave reflection and bettercleaning at the back. As expected, closer the slag from the exit of the detonation wavemore effective is the slag removal. The strength of the detonation wave also has directimpact on cleaning effectiveness. A good indication has also been obtained from theagreement with the preliminary computational fluid dynamics analysis that the waveimpacts are more effective in removing slag deposits from tube bundles rather than singletube.

These experimental results show tremendous potential of using pulse detonationwave as an alternative technique in removing slag/fouling deposit from heat transfersurfaces in coal-fired utility power plants.

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1.0 INTRODUCTIONHistorically, boiler slagging and fouling as a result of inorganic impurities in

combustion gases being deposited on heat transfer tubes have caused severe problems incoal-fired power plant operation. These problems are fuel, system design, and operatingcondition dependent. Ash in conventional power system is known to be a major problemthat results in the loss of millions of dollars annually as a result of decrease in efficiency,unscheduled outages, equipment failures, and cleaning requirements. The ash depositionproblems persist in both the high temperature environment in the main boiler, where iron-rich phases and/or silicate based slagging dominates, and in the lower temperatureconvective pass areas, where both silicate-based and sulfate based fouling dominate onthe superheater, reheater, and the economizer tube bundles. These problems are severelynoticed in cases of boilers fired with coal of low calorific value and high content ofmineral constituents, especially those that tend to accumulate on heat transfer solidsurfaces. Utility companies very often choose western coals in their boilers. Westerncoals are low in sulfur content, but have high ash deposit, which is the most importantfactor limiting boiler design and operating conditions. Using western coals, require somestrategy to remove fouling ash/slag deposits.

1.1 Ash Formation and Deposition ProcessesDuring combustion or gasification the inorganic materials are transformed into

ash species that are in the form of gases, liquids, and solids. These intermediates arecomplex and variable; their characteristics reflect the interactions of the inorganiccomponents in the coal during combustion. Research on ash (fly ash) indicate a bimodalsize distribution [1]. The submicron size particles form as a result of condensation offlame-volatilized species upon gas cooling. Flame volatilized species may also condenseon the surfaces of larger particles or deposits. The larger particles are sometimes referredto as residual ash, which is largely derived from mineral grains. The composition andsize distribution of the larger particles result from the transformations and interactionsbetween discrete mineral grains in higher rank coals. In lower rank coals the interactionof the organically-associated inorganic elements with mineral grains occurs as well asmineral-mineral interactions. In addition, the low-rank coals that contain high levels oforganically-associated calcium and other alkali and alkaline earth elements produce highlevels of very small, < 5µm, calcium-rich particles. The chemical composition,mineralogy, and size of the ash particles ultimately determine the likelihood of the ashdepositing on heat transfer surfaces or causing other operational problems.

The size of the intermediate ash species in the flue-gas stream, boiler design, gasvelocity, and temperature determine how the ash particles are transported to heat-transfersurfaces. The ash species are transported to the heat transfer surfaces by severalmechanisms based on their size and state. Submicron particles are transported to thesurfaces by diffusion and thermophoresis. Larger particles are transported to the surfaceby inertial impaction. Ultimately, one has to understand both the size and chemistry ofthe particles entrained in the bulk gas flow through the boiler in order to understand howash deposits form.

Although there are many types of ash deposits, the most common deposits can becategorized into two major groups: high-temperature silicate-based deposits, and low-

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temperature calcium-based deposits. These deposit types form under very differenttemperatures and from very different ash materials.

Figure 1 illustrates the type of liquid components present in deposits as a functionof temperature. At lower temperatures, sulfates dominate, while at higher temperatures,silicates are more prone to cause deposits. In high-temperature fouling, the bonding ofparticles is due to silicate liquid phases and in low-temperature fouling, the bonding is aresult of the formation of sulfates. Condensed sulfur species, principally in the form ofCaSO4, are stable and form the matrix or bonding material in the low-temperaturedeposits.

High temperature fouling occurs in regions of the utility boiler wheretemperatures exceed the stability of the sulfate-bearing phases (>1700 oF). In combustorsburning coals that contain high levels of alkali and alkaline earth elements, high-temperature fouling can be a significant problem. Much work has been done on hightemperature fouling due to sodium. In most cases, the innermost layers consist primarilyof small particles, rich in flame-volatilized species such as sodium and sulfur, which aretransported to the surface by vapor phase diffusion and thermophoresis. Larger particlesalso impact the surface. The initial deposit layers may provide a sticky surface fortrapping inertially impacting particles which are not sticky. In addition, the initial layersmay provide fluxing materials that will cause larger particles to melt. These particlesprovide sites for continued deposition to form islands of particles. These initial islandsare the precursors of the more massive upstream deposits that form in the secondarysuperheater and reheater sections of a utility boiler. Coatings also form on the surfaces ofentrained ash particles as a result of the condensation and reaction of flame-volatilizedspecies to form a molten or plastic surface. Condensation on surfaces of deposited ashparticles can also occur. As a result of the insulating effect of the deposit layer on thetube, the outer layers of the deposit are formed at higher temperatures. The highertemperature causes melting and interaction of the particles to form a liquid phase. Once aliquid phase has formed on the outside of the deposit, it becomes an efficient collector ofash particles, regardless of the individual melting characteristics of the particles.

Low-temperature ash deposition occurs at temperatures in the range of 1000 -1650 oF. In systems which exhibit low temperature fouling, the sulfate phases dominatethe matrix or bonding mechanism between particles. Detailed examination of low-temperature deposits shows high levels of calcium in the deposits. Formation of low-temperature deposits is dependent upon the availability of small calcium oxide particlesand the process of sulfation. Low-temperature deposits form when small calcium oxideparticles in a deposit undergo sulfation through reaction with sulfur dioxide in the gasstream. This reaction produces calcium sulfate which causes particle-to-particle bondingand fills in the available pore space in the deposits. This pore filling produces verystrong, brick-like deposits which are difficult to remove.

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Figure 1. Distribution of liquid phases as a function of deposit temperature.

1.2 Deposit RemovabilityThe primary factors that influence the ability to be removed are the strength of the

deposit and the adhesive bond between the ash deposit and the heat transfer surface. Theremoval of the deposit involves breaking the deposit matrix and/or breaking the bondbetween the deposit and heat transfer surface. Methods typically used to remove depositsinclude load reduction and on-line cleaning. Load reduction results in cooling of thedeposits to cause the breaking of bonds between the deposit and heat transfer surfacebecause of a differential in the thermal expansion coefficients between the deposit andheat transfer surface. On-line ash deposit cleaning devices are called sootblowers. Theblowing medium is either high pressure steam or compressed air. The blowing mediumis directed at the deposit through a nozzle. The impacting fluid causes the deposit tocrack and be eroded away. The design of the blowers vary depending upon the locationin the boiler. The wall blowers are short blowers that extend a short distance into theradiant section of the boiler and are rotated to remove the deposit in a circular patternaround the point of entry into the boiler. Long retractable blowers are required to cleanthe surface of the tube banks in the convective passes of utility boilers. For extremelydifficult to remove deposits water-jet sootblowers have been used. These work well toremove large accumulations of deposits in the furnace. The impact of water is muchgreater than that of air or steam so it is effective in removing deposits. In addition, the

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water can quench or cool the deposit resulting in fracture of the deposit and breaking theadhesive bond with the heat transfer surface. The use of sootblowers varies widely due tofuel quality and ranges of operating conditions. Many operators will operate eachsootblower every eight hours or once a shift. Some have the blower operation linked toboiler cleanliness monitors so cleaning can be made on demand. Some use visualobservations to assist in determining sootblowing cycles. Sootblowing is a significantexpense in terms of operation and maintenance. In addition, the use of sootblowers cancause tube wastage due to the impaction of deposits blown from heat transfer surfaces.

The removability of a deposit depends upon its physical structure. Wain andothers (1992) initiated work to develop relationships between the nature of slag and therelevant thermal and mechanical properties that effectiveness of sootblowers. Theyexamined ten different slag samples to determine the thermal conductivity, coefficient ofthermal expansion, compressive strength and elastic modulus, porosity, and crystallinity.The ability of a deposit to be removed is dependent upon a thermal shock parameter. Thethermal shock parameter is the change in temperature needed to develop crackpropagation. When a sootblowing medium encounters a deposit, a change in temperatureneeds to occur to cause thermal stress leading to crack initiation and propagation. Theoperators of a boiler must be able to remove a deposit when it is still largely glassy andhas a relatively high porosity. Deposits that have had sufficient time at temperatures thatallow for significant strength development through viscous flow sintering will likelybecome very dense, highly crystalline and very hard to remove. Typically depositshaving porosities of less than 25% are extremely difficult to remove.

Thus conventional ash and fouling deposit removal methods are being onlypartially successful in removing deposits, they also require considerable maintenance,and they reduce boiler efficiency when in use. Alternative technologies are thereforeneeded to remove ash deposits, more efficiently and effectively, from heat transfersurfaces of utility boilers during full power plant operation. New alternative methods areespecially needed to remove deposits from the downstream side of heat transfer tubebundles in the convective pass sections.

Pulse detonation wave technique, based on the action of mechanical and thermalshock on material, deposited on heat transfer surfaces, offers potential solutions to manyof the slag deposit problems by providing simple, inexpensive, yet efficient and effectiveways to supplement existing ash removal methods, without expensive plant shut down.

1.3 Contract ObjectivesThe objectives of the program was to evaluate the technical feasibility issues of

applying pulse detonation technology to remove ash/slag deposits in coal-fired utilitypower plant boilers. The key objectives of this program are summarized as follows:

• Study of slag characterization• Pulse detonation technology study and its development• Feasibility study through preliminary CFD analysis• Perform parametric technology demonstration experiments• Perform economic analysis• Recommendation for future development

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was established more than a century ago, but has been modified considerably over thepast decades on the basis of extensive laboratory research exploiting laser generatedextremely short light pulses: photons, fast transducers, and other modern instruments.

According to the classic detonation theory [4], a detonation wave can easily begenerated in a sufficient long tube, closed at one end, by igniting a homogenized and pre-mixed combustible fuel-oxidant mixture by a spark. The combustion products, boundedby the tube closed end, side walls, and the flame front, will travel toward the open end ofthe tube into unburned mixture, initially at a relatively slow propagation speed of a fewmeters per second for about ten tube diameters, but accelerating quickly toward the flamefront until a steep detonation front is established. The detonation wave propagatessubsequently at its own propagation, which may reach several thousand meters persecond.

The first ideas for employing detonation waves for cleaning boiler slag depositscould be traced in the literature back in 1965, though the first experiments were notreported until 1974 (Schelokov, et. al.). The extensive paper of Podimov et. al. [5] on theso called "Impulse Technique", published in Russian, revealed that this topic wasresearched extensively in the USSR in connection with cleaning of waste heat boilers thatutilize hot gases from iron and steel-works, nonferrous smelters, and other industries. In1980 the Institute of Thermal Equipment (VUEZ) in Tlmac, Czechoslovakia, and theThermal Power Plant "Kakanj" near Sarajevo, Bosnia, decided to test jointly the basicidea by installing a simple detonation wave generator at an old coal-fired boiler in the"Kakanj" Power Plant. In the papers (K. Hanjalic and I. Smajevic, 1994), [6,7], theauthors have described the principles and reported on several years of experience inapplying the detonation wave technique in full scale operation in two large coal-firedboilers. But information are scarce on ash/slag formation and deposition, their physicalstructure and characteristics, shape and bonding in the literature. Limitations in applyingdifferent types of detonation wave have also been found from above literature cited.Besides, during slag removal operation, no extensive research on parametric study havebeen performed.

This program represents extensive research and experiments on slagcharacterization, pulse detonation technology studies and parametric study in removingdifferent types of utility boiler slags. The experiments were performed at The AerospaceResearch Laboratory at the University of Texas at Arlington (UTA). Pulse detonationcombustion experiments have been investigated in UTA laboratory for past several years.UTA detonation engine have been used to perform both single and multi pulse testing.

2.0 RESULTS AND DISCUSSION

2.1 Slag Characterization StudyMicrobeam Technologies Incorporated (MTI) worked with Prairie View A&M

University to develop and demonstrate the new method to remove deposits from coal-fired utility boilers. MTI provided background information on fuel properties, ashformation, ash deposition, and ash removal. In addition, MTI also provided depositscollected from a full scale utility boiler. MTI performed analysis of deposits obtainedfrom Basin Electric and from Northern States Power (NSP). The analyses wereconducted using scanning electron microscopy/microprobe techniques. The chemical and

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physical properties of the deposits were determined. The results are presented in thefollowing sections.

2.1.1 Morphological Analysis and Porosity CalculationsThe deposit samples used for testing were collected from a Babcock and Wilcox

cyclone fired boiler of Northern States Power Company's Riverside Unit #8. The depositsare:

• MTI 96-54 is an air heater inlet deposit• MTI 96-55 is a reheater deposit• MTI 96-56 is an economizer deposit

The deposits were mounted in epoxy resin, allowed to harden, cross-sectioned andpolished for analysis in the scanning electron microscope. The SEM was used to obtainimages at two magnifications for each of the samples.

The reheater deposit (MTI 96-55) cross-section is illustrated in Figure 3a and 3b.Figure 3a is a low magnification (100 X) backscattered electron image that illustrates theoverall microstructure. Figure 3b is a higher magnification image (500 X) that showsbonding between individual ash particles. The larger particles are bonded together bysurface coatings. Detailed examination of the coating layers indicates that the matrixmaterial that bonds the larger 10 to 20 micrometer particles together is rich in calciumsulfur likely in the form of calcium sulfate.

The economizer deposit (MTI96-56) is shown in Figures 4a and 4b. Figure 4a islow magnification image showing a very dense region of the deposit and a porous region.The very dense region is likely a deposit fragment from another part of the boiler that wastrapped in the porous deposit. The more porous appearing material is much morerepresentative of an economizer deposit. A higher magnification image, Figure 4b,shows that the deposited particles are very small (less that 10 micrometers). Thesedeposits, if not removed, can develop very high strengths because of the high surface areaof the deposited particles. Particle sintering (densification) due to heat treatment andreaction with gas phase SO2 and SO3 will be rapid.

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Figure 3a. Backscattered Electron Image of polished cross-section of sample MTI 96-55(Riverside #8 Reheater) at 100X.

Figure 3b. Backscattered Electron Image of polished cross-section of sample MTI 96-55(Riverside #8 Reheater) at 500X.

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Figure 4a. Backscattered Electron Image of polished cross-section of sample MTI 96-56(Riverside #8 Economizer) at 110X.

Figure 4b. Backscattered Electron Image of polished cross-section of sample MTI 96-56(Riverside #8 Economizer) at 500X.

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The airheater inlet deposit (MTI96-54) is shown in Figures 5a and 5b. Figure 5ais a low magnification image illustrating the overall morphology of the deposit. Thedeposit is made up primarily of particles greater in size than 5 micrometers. The particleappear to be bonded by surface coatings rich in sulfates. A high magnification image ofthe deposit cross-section is shown in Figure 5b.

Figure 5a. Backscattered Electron Image of polished cross-section of sample MTI 96-54(Riverside #8 Airheater Inlet) at 100X.

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Figure 5b. Backscattered Electron Image of polished cross-section of sample MTI 96-54(Riverside #8 Airheater Inlet) at 500X.

Measurements were conducted to determine the area of the epoxy and deposit inthe cross-section. The numbers are not an actual porosity they represent the percent ofthe plug surface that is epoxy. The percent epoxy provides an indication of the voidspace between particles. The percent epoxy was determined from the gray scalehistogram of the backscattered electron image. The results obtained for the NSPRiverside deposits are shown in Table 1. The highest area percent epoxy was found forthe air heater followed by the economizer. The lowest percent epoxy was found for thereheater deposit. This deposit was subjected to higher temperatures and resulting in moresintering and in lower porosity.

Table 1. Area percent epoxy in the deposit.

MTI # Description Area % EpoxyMTI 96-55 Reheater 29.4MTI 96-56 Economizer 38.6MTI 96-54 Airheater 44.4

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2.1.2 SEMPC Analysis - Deposit Phase and Compositional Analysis

The distribution of phases were determined in the deposits by the scanningelectron microscopy point count routine. The composition of the deposit as well as theformation of various phases in the deposit significantly effect the ability of the deposit todevelop strength and be resistant to soot blowing. For example, in some cases thedeposits highly enriched in crystalline phases can be susceptible to crack propagation andweakening of the deposit allowing them to be easily removed by sootblowing. While inother deposits the formation of a highly sulfated deposit may cause significant bonding tooccur in the deposit causing it to be difficult to remove. The SEMPC analysis of thedeposits are summarized in Tables 2 and 3. Table 2 shows the major phases present inthe samples. The most abundant phase is the unclassified phase which is largely made upof glassy fly ash particles or partially sulfated materials. The compositions of theunclassified phases are known however, the compositions do not fit into classificationcriteria designated for the phases. The next most abundant phase in the reheater andeconomizer samples is anhydrite (CaSO4). This material is the primary bonding materialin the deposits. The air heater samples contains some partially sulfated materials that isdesignated unclassified that is contributing to the bonding. The bulk and amorphousphase composition are summarized in Table 3. (The compositional information in Table3 is the elemental composition of the material expressed as equivalent oxide. Thisconvention is used by the coal research and industrial community when reporting thecomposition of coal ash and related ash-derived materials.) All of the samples haverelatively high levels of SO3 indicating a sulfate based deposit. The amorphous phase isthe composition of the silicate based material that makes up the amorphous material. Thesulfates are not considered. The physical properties of deposits at high temperature ishighly dependent upon the composition and abundance of the amorphous phase in thedeposit.

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Table 2. Riverside #8 Deposits - SEMPC Analyses - Phases by RegionPhase, wt. % Airheater Inlet

MTI 96-54Reheater

MTI 96-55EconomizerMTI 96-56

Gehlenite 2.7 .0 .0Anorthite .7 .7 .7Albite .0 .3 .7Pyroxene .3 .0 .3Hauyne .0 .0 1.0Calcium Aluminate .3 .0 .0Quartz 1.0 .3 1.0Anhydrite 1.0 12.7 17.7Sulfated Dolomite 2.0 3.0 .0Sodium Calcium Sulfate .0 3.0 1.7Unclassified 89.0 76.9 74.6Pure Kaolinite (amorp) .3 .0 .0Kaolinite Derived 2.0 2.7 .7Illite .3 .0 .3Montmorrilonite (amorp) .0 .0 .7Calcium Derived .0 .0 .3

Table 3. Riverside #8 Deposits - SEMPC Analyses - Chemical CompositionAirheater Inlet

MTI 96-54Reheater

MTI 96-55EconomizerMTI 96-56

Bulk Amorp Bulk Amorp Bulk AmorpSiO2 16.5 24.5 14.3 24.2 18.6 27.7Al2O3 13.0 20.6 10.9 19.1 13.0 21.2Fe2O3 4.8 7.4 3.8 6.8 4.8 8.0TiO2 .9 1.4 .7 1.2 .9 1.5P2O5 1.1 1.8 1.3 2.2 1.1 1.8CaO 21.7 31.4 21.2 30.8 20.9 23.9MgO 5.6 7.5 4.2 6.5 5.1 8.2Na2O 2.6 3.6 4.4 7.0 3.6 5.2K2O .3 .6 .8 1.3 .9 1.3SO3 32.0 .0 37.8 .0 30.1 .0BaO .8 1.2 .5 .7 .8 1.2MnO .1 .1 .0 .1 .1 .1ClO .6 .0 .1 .0 .2 .0

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In summary, the deposits obtained from Northern States Power for testing atPrairie View were characterized to determine some of the chemical and physicalproperties that effect the strength of the deposit as well as the resistance to removal. Thedeposits range in porosity (area percent epoxy) from 44.4 for the weakest to 29.4 % forthe strongest. The critical porosity value for removal full scale utility boilers by a sootblower is about 25% porosity (Wain and others (1992)). The deposits are primarilysulfate based with varying levels of sulfation. The microstructure indicated coating onthe particles that are likely contributing to the strength development. These chemical andphysical properties of the deposits influence the ability of conventional sootblowers toremove the deposits.

2.2 Computational Fluid Dynamics AnalysisPulse detonation technology for the purpose of removing fouling slag/ash deposits

in heat exchangers was evaluated using 2-D time dependent methods to determine theeffect of a detonation wave passing through a 3-tube heat exchanger. Pressure, pressuregradients, temperature and velocities were recorded as a function of time. Wavereverberations were recorded on the backside of the heat exchanger lead tube.Conventional methods clean the front side of the tubes, but have great difficulty cleaningthe back side. This analysis shows large negative velocities impinging on the back side ofthe lead tube as a result of a shroud being placed over and under the tube bank and thewave interaction between the tubes.

Three cases investigated were as follows:(1) The detonation tube before ignition was at 1 atm, with stoichiometric

propane/oxygen reactants.(2) The detonation tube before ignition was at 2 atm, with stoichiometric

propane/oxygen reactants.(3) The detonation tube before ignition was at 3 atm, with stoichiometric propane/oxygen reactants.

These single shot detonation studies indicate peak pressures of several hundredatmospheres which act over a very short time and which fall of very rapidly. Pressuregradients move around the tubes and large velocities alternate with pressure which serveto scrub all surfaces circumferentially to remove the slag deposit. The three cases notedabove correspond the experimental testing conditions at UTA.

Figure 6 indicates the UTA detonation tube which is used in CFD modeling. Thetube is 18 inches long and is enclosed by diaphram at one end and end plate at the otherend. Tubing for fuel, oxidizer, venting and vacuum are shown on end wall ( end plate).On the diaphram side a fixture is used to mount the heat exchanger tubes in variousarrangements for the experiments. In CFD analysis, the detonable mixture were ignited atthe closed end of the tube as were done in the experiments.

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2.2.1 CFD Codes UsedThe CFD codes used in this analysis are a pulse detonation code, which is a 1-D

code [MOZART, NASA], which processes the detonation chemical reactions in regionsjust downstream of the detonation wave, the expansion wave region and the end wallregion, where the velocity is zero. After the detonation wave have traversed the length ofthe tube, those properties are frozen and become the initial conditions for the other code,a full 2-D Navier-Stokes code [FALCON, Lockheed commercial].

2.2.2 Initial ConditionsThe initial conditions are derived from the 1-D pulse detonation code. These

conditions are shown in Table 4 for the four regions shown below:Region I: The region next to the end wall. This region occupies approximately

the first 10 inches of the detonation tube.Region II: The expansion region. This region occupies approximately 4 inches

and is modeled as a single entity.Region III: The jump condition region. This is the Chapman-Jouguet solution and

represents the conditions across the detonation wave and combustion zone.

Region IV: This region is the expansion region which surrounds the heat exchanger tubes. Initially the conditions are ambient.

All the four regions are defined and shown in Figure 7.

Table 4. Initial Conditions for CFD Analysis

I II III IV1 ATM

P (PSF) 50128 95950 141772 2116T (R) 13520 15096 16692 520U (FT/SEC) 0 2500 5200 0

2 ATMP (PSF) 100256 191906 283566 2116T (R) 13520 15096 16693 520U (FT/SEC) 0 2500 5200 0

3 ATMP (PSF) 150447 287000 425350 2116T (R) 13520 15096 16692 520U (FT/SEC) 0 2500 5200 0

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2.2.3 GridFigure 8 shows the grid which is used to model heat exchanger tubes. The

constant area detonation region, the expansion region and the 3-tube heat exchanger isused as grid models. The grid was generated using the commercial grid generation codeGRIDGEN. The boundary conditions in the 2-D full Navier Stokes code allow blankingin the interior of the flowfield, which is a combination of rectangular objects. In this case,the three rectangular objects in transformed space to represent the three cylinder heatexchanger tubes. Each side of the interior rectangle must have a specified boundarycondition. The boundary conditions for the side walls, end wall, nozzle walls and heatexchanger cylinders will be modeled with slip wall boundary conditions.

2.2.4 CFD ResultsA time dependent normalized pressure history (normalized to 1 atm) is shown in

Figures 9-12. As the detonation wave passes through the tube bundle , reflecting from thevarious surfaces, the pressure starts to build up on the back side of the lead tube, negativevelocities impinge on the back side and it appears that cleaning the back sides of thetubes may be accomplished using detonation waves. While the large pressures are shownon the front side of the tubes and subside very quickly, the pressure stays elevated on theback side of the lead tube. Pressure gradient plots are shown in Figures 13-14. Theseplots show shock formations around the heat exchanger tubes, where the tubes block theflow sufficiently to form almost steady formations before the lead cylinder, where theflow goes subsonic, accelerates around the lead cylinder, shocks down before the reartubes goes supersonic around the rear tubes. This activity also serves to cause enoughdisturbances in the flow field to scrub the entire surfaces of the heat exchanger tubes.Figure 15 shows the velocity vectors colored by mach no. It shows large negativevelocity developed on the back side of the rear tubes which contributes in removingslags. This same CFD exercise was accomplished on a single cylinder, where the resultsshow a much less active flow field, where there was no scrubbing on the rear of thecylinder. The 2 and 1 atmosphere cases were essentially the same as the 3 atm case. Thetime history of the normalized temperature plot for I atm case is shown in Figure 16.

2.3 Experimental Results

2.3.1 Experimental Setup And ProcedureA pulse detonation facility specially designed to study detonation waves at the

University of Texas at Arlington (UTA) was utilized to perform experiments onremoving boiler slag. The main component of the pulse detonation facility are the testchamber, the injection system, the ignition system and the instrumentation. To hold theslag samples at the exit of the detonation tube at different axial positions and orientations,three different fixtures were fabricated. The test chamber consists of steel tubes ofvarying length connected end to end in different combinations but with the same crosssectional area. The schematic of the test chamber is shown in Figure 17. The threesegment lengths are 7.62 cm (3 in.), 15.24 cm (6 in.), and 30.38 cm (12 in.). Each sectionhas an inner diameter of 7.62 cm (3 in.) and an outer diameter of 13.97 cm (5.5 in.). Aflange of 1.905 cm (0.75 in.) thickness is welded to each end of the test sections. Eachsection of the chamber has provisions for mounting pressure transducers, thermocouples,

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thin film gauges, and heat flux gauges every 7.62 cm (3 in.). The ignition plug is mountedin a 7.62 cm (3 in.) section and can be inserted any where along the length of the tubebetween other sections. One end of the chamber is sealed with a plate. The fuel andoxidizer is injected through this plate. The various sections of the chamber are flangedand bolted together at each joint. The open end of the chamber is bolted to a thrust standto hold the chamber in place. The chamber is sealed by fastening a Mylar diaphragm,0.254 - 0.381 mm (.01 in - .015 in.) thick, to the open end of the chamber. The closed endof the chamber is sealed by bolting a 1.905 cm (0.75 in.) thick steel plate to the chamber.This has seven holes in it in addition to the eight bolt holes used to fasten it to the testchamber. One of these holes is used as a sensor port and the other six are used for theinjection system. Figure 18 shows a picture of the test chamber and its setup.

The fuels and oxidizers are injected separately into the test chamber through theend plate. All of the lines are controlled remotely. There are six stainless tubing injectionlines attached to the end plate of the chamber. Figure 19 shows the injection lines. Thefirst line is used for fuels. This line connects to a fuel cylinder located outside thebuilding. The cylinder can be changed to use desired fuel: propane, hydrogen or methane.The second line is connected to an oxygen cylinder while the third line is connected to ahigh pressure air source. The high pressure air is used as an oxidizer, to dilute thechamber contents if the test is aborted, or to flush out the products after a test run. One ofthe remaining line is for a Baraton pressure transducer. This allows the partial pressuresfor the fuel and oxidizer inside the test chamber to be recorded before the test isconducted. The fifth line is vacuum line used to evacuate the chamber so that a nearstoichiometric mixture of fuel and oxidizer can be injected into the test chamber. Finally,a vent, open to the air is connected to the last line. This line is opened whenever thediaphragm does not break during a test or the test run is aborted. This allows the exhaustproducts or reactants to be safely flushed from the test chamber. The injection system hasbeen designed for hydrogen, propane, or methane as the fuel and oxygen or air as theoxidizer. Other oxidizers could be used if they are compatible with the valve materialsand seals. The fuel and oxidizer are injected perpendicular to the axis of the detonationchamber and in such a way as to impinge upon each other during the injection process butnot into the supply line of the other. This is an effort to mix the fuel with oxidizer. Thefuel and oxidizer is measured by setting the valve supply pressure according to regulatorflow rate charts and injected using rotary valves.

The tests are started by igniting the fuel and oxidizer mixture in the chamber. Thisis done by an arc plug developed specially for this program. An arc welding supplysource has been modified so that it can be used to create an ionized path through the gasbetween the two electrodes of the arc plug. This reduces the resistance of the gas. Afterthe gas path is ionized, a capacitor bank discharges a high voltage to the arc plug. Ahigher amount of energy can be dissipated into the mixture in this way using a normalspark plug. The arc plug is inserted into the 7.62 cm (3 in.) test section. When the gaspath is ionized sufficiently, the discharge capacitors discharge through the path in theform of a high current arc.

Once the mixture is ignited, the process of the experiment is monitored usingthree types of sensors. One of each type of sensors is located at each axial station. Thefirst set of sensors used is a set of seven PCB dynamic pressure transducers model111A24. These transducers measure a change in pressure from initial value of zero to

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6894.8 kPa (1000 psia) with a response time of 1 microsecond. The second type ofsensors used is Type E thermocouples model TCB-031-E, from Medtherm, with aresponse time of 1 micro second and a temperature range of up to 1000oC. Finallyplatinum thin film gauges, model PTF-100-20292 from Medtherm, are used to measureheat transfer rates. These gauges have a response time of 2 micro seconds and a full scalerange of 16.34 MW/(m2.oC). Six of these transducers may be used along the wall of thechamber and the seventh one in the end plate. The instrumentation used to obtain theexperimental data are the seven pressure transducers. The instrumentation sensors aremounted in the side wall at 7.62 cm (3 in.) increments with the capability for all types ofsensors to be mounted at the same axial locations. The instrumentation are connected to aDSP technology data acquisition system which has the capability of 100 kHz samplingrate, 12 bits of accuracy, and 48 channels, each with its own amplifier and analog todigital converter to allow for simultaneous sampling for all channels. The system has 512kilobytes of memory available for distribution to the channels being utilized. The dataacquisition system is controlled by a PC which retrieves the data, stores it on a hard drive,and analyzes the data.

2.3.2 Input ConditionsInput parameters for the experiments were as follows:

Types of slag: MTI 96-54 (soft), MTI 96-55 (hard), MTI 96-56 (medium soft)

Wave pattern: Weak detonation and C-J detonation

Wave frequency: 1 Hz (single pulse testing), 10-20 Hz (multi pulse testing).

Types of Fixture: (a) axial configuration (four axial positions)(b) triangular configurations (three triangular positions)(c) matrix configurations (nine positions, 3X3 form)

Slag orientation: Front back and side attached.

2.3.3 Pressure and Velocity DistributionVoltage readings from the pressure transducers were converted into pressure

readings and plotted against time. The pressure plots were used to obtain experimentalwave diagram. The time interval between the observed abrupt rise in pressure fromadjacent transducers was used to calculate wave propagation speed. All the data were tomeasure pressure and velocity of detonation waves inside the detonation tube. Nomeasurements were taken at the sample locations due to complexity of instrumentation atthe present time.

The pressure developed within the test chamber varied from 1718-2404 kPa (250-350 psi) for weak and 3093-4466 kPa (450-650 psi) for strong detonation waves. On theother hand, velocity varied from 549-914 m/sec (1800-3000 ft/sec) for weak and 671-2438 m/sec (2200-8000 ft/sec) for strong detonation waves. Figures 20 and 21 show

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typical pressure and velocity plots for strong single pulse detonation waves. The pressureand velocity are expected to decrease drastically as the wave leaves the exit of thedetonation tube. Figure 22 shows a pressure plot of multi pulse weak detonation waveinside the chamber at a location 3.81 cm (1.5 in.) from the exit.

The results on slag removal by detonation waves are discussed below:

2.3.3.1 Single Pulse TestingEach sample consist of a solid stainless steel bar, 1.62 cm (.64 in) dia. And 16.5

cm (6.5 in.) long. Solid slags were cut into pieces with approximate dimensions of 2.5 cmx 1.25 cm x 2.5 cm (1 in. x 0.5 in. x 1 in.). Each piece is attached to a bar with the help ofepoxy resign J-B weld. Use of this epoxy assures that the bonding between the slag andthe solid surface is stronger than the bonding within the slags. Each sample is then placedin desired location and orientation with the help of two fixtures attached with the exit ofthe detonation tube.

Effect of axial locationTo study the effect of distance of the slag from the exit of the detonation tube,

weak detonation waves were used. Both soft slag from air-heater inlet deposit (MTI 96-54) and medium soft slag from economizer deposit (MTI 96-56) were used. The waveswere found to chip off the whole soft slag at 5 cm and almost 95% of the slag at 20 cm.Observations at 10 cm and 15 cm locations also showed complete removal. Anotherobservation noted was that the chipped off samples were broken into several pieces. Thenumber of pieces increased for the samples closer to the exit of the detonation tube.Figure 23 shows the pictures of the sample before and after the test, with soft slagattached at the front, at a location of 5 cm from the exit of the detonation tube. Figure 24shows for sample at a distance of 20 cm with other conditions remaining same. The wavewas also found to remove a substantial amount of the medium soft slag positioned at 20cm from the detonation tube as shown in Figure 25.

Effect of Slag OrientationBoth the strong and weak detonation wave are capable of chipping off soft slag

samples attached at the side of tube up to a distance of 20 cm from the exit of thedetonation tube. The shearing of slag samples took place during removal. No brokensample pieces have been observed after test runs. As expected the weak detonation waveswere less effective in removing slag from the back sides of the tubes. The waves havebeen found effective in removing slags from the back side of a tube upto a distance of 10cm from the exit of detonation tube. The waves have been found to be more effective asthe slags are put directly on the path of the traveling waves. The tube obstacles reduce thewave strength and velocity before striking the samples at the back side of the tube. Withweak detonation wave and soft slag attached on the side of the tube, the experimentsshow that the shearing off of the entire slag takes place without any breaking. Figure 26shows pictures of hard slag sample (attached at the side of the tube) before and after thetest at a distance of 5 and 10 cm from the exit of the detonation tube. The picture showscomplete removal of slag from the closest position, i.e., 5 cm from the exit. Figure 27shows complete removal of soft slag attached at the side of the tube at a distance of 8.5cm from the exit. Similar results were also observed at a distance of 5 cm from the exit.

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Figure 28 shows for location 20 cm from the exit of the detonation tube with otherconditions remaining same. Thus the wave was found to be effective in shearing off ofthe soft slag attached to the side of the tube even at a distance of 20 cm from the exit ofthe detonation tube. Similar tests were performed with slag attached to the back of thetube. Results at 5 cm and 15 cm positions show complete removal, but the number ofbroken pieces are less than those from slag attached in front position of the tube.

Thus the effect of slag orientation results support the computational fluiddynamics (CFD) analysis. The analysis indicated that the detonation wave is capable ofremoving slag circumferrentially from the tube.

Effects of Wave PatternA strong detonation (C-J) wave develops more pressure and velocity at the exit of

the detonation tube than a weak detonation wave and therefore is expected to remove slagmore effectively. Figure 29 shows the pictures of medium soft slag samples before andafter the test with weak detonation wave at a distance of 8.5 cm from the exit of thedetonation tube. Figure 30 shows the pictures of hard slag samples before and after thetest with C-J detonation wave at a distance of 8.5 cm from the exit with the same slagorientation. The hard slag were completely removed with the strong wave but the softerslag were only partially removed with the weak wave. Figure 31 shows hard slag beforeand after the test at a distance of 20 cm from the exit of the detonation tube. The waveused was C-J detonation wave. The test shows complete removal of hard slag at thefurthest position. Whereas at the same position a weak wave could not completelyremove a medium soft slag. Tests at three other positions, namely, 5 cm, 10 cm, and 15cm also showed complete removal of slag with strong wave. Another observation notedwas that the number of broken pieces of the chipped off slag samples increased with C-Jdetonation waves. Hence, stronger the wave more effective will be the slag removal.Limitations on the wave strength will be dictated by the ability of boiler tubes inwithstanding the waves.

Effects of Slag Types In addition to the wave strength (pressure and velocity), slag type is also an

important parameter. The detonation wave can remove soft slag more easily than the hardones. Tests show that the weak detonation waves were capable of removing almost theentire slag of soft and medium soft samples and about 50% of the hard sample at adistance of 5 cm and almost the entire soft slag, about 70% of medium soft and about30% of hard sample at a distance of 15 cm from the exit of the detonation tube. Figure 32shows the pictures of medium soft slag attached at front of tube before and after test at adistance of 15 cm. Pictures show almost 70% removal of slag. Figures 33 and 34 showthe pictures of medium soft and hard slag samples before and after the tests at a distanceof 8.5 cm from the exit of detonation tube with other conditions remaining same. Resultsshow more removal of medium soft slag than the hard one.

With same slag types, the wave have been found to be more effective in triangulartube orientation than axial orientation. The reasons are attributed to wave reflections andcollision of chunks of slag from front position, which facilitated the removal of additionalslag in triangular orientation. Thus with single tube position, without reverberation and

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wave reflection, the effectiveness of the wave decreases with distance and strength of theslag.

2.3.3.2 Multi Pulse Testing

Several experiments were performed with multi-cycle detonation wave at theAerospace Research Center at University of Texas at Arlington. The matrix configurationfixture was used to investigate the wave reflection effects among the sample tubes andreverberation effects within the slag. In the multi-cycle tests the pressure build up insidethe detonation wave chamber varied from 35-50 psi, which was almost one-eighth of thepressure range developed in the single shot test runs. Test results indicated that thepressure is an important factor in removing slag, in addition to the other factors such aswave reflection, thermal shock, and reverberation. During the first test a sizeable portionof the soft slag were removed from the first two rows closest to the exit of the detonationtube. Repeating the experiments four more times showed additional removal of slag fromthe central column. Figure 35 shows the picture of soft slag before and after the test runs.Figure 36 shows pictures of hard slag before and after the test. The pictures show moreremoval of hard slag from all positions. The reason most likely are the collision of theremoved slag from front position with slag in rear position facilitating additionalremoval. One very important observation from the tests (both single and multi-pulse)were that to remove slag a certain pressure level of the wave is needed. Below thethreshold pressure, the waves are not effective in removing slags. At the same time theboiler tube banks must be able to with stand the pressure.

2.4 Economic Feasibility StudyA preliminary economic analysis of Pulse Detonation Engine soot blower

installation was carried out for the Sherburne Co. plant of Northern States Power (NSP).This analysis is made with the cooperation of Mr. Joe Brojberg (senior analysis engineerof NSP) and Dr. Steve Benson (EERC of North Dakota, slag and ash specialist) and Mr.Paul Johnston of Diamond Power Specialty. According to Paul Johnston, "Effectivecontrol of ash accumulation throughout the boiler tubes costs big money by limiting theoutput of steam to the turbine and by requiring more fuel to make a pound of steam.Typical savings on a mid to large size boiler can be as much as $3,000,000 a year fromsteam output and $500,000 for fuel". If we assume the size of the plant to be 600 MW,then 5833 $/MW are lost due to heat exchanger blockage.

The Sherburne Co. plant consists of 3 units, a 800 MW and two 750 MW units.The 800 MW unit was recently taken off line for excessive ash build up in the heatexcchanger unit in the convective pass. Evidently, the lead heat exchanger in theconvective pass takes the brunt of the ash accumulation. In addition, the King unit ofNSP, but not a part of Sherburne Plant, had to be taken off line to clean the fouled heatexchangers which had "one foot accumulations of ash", according to Mr. Brojberg.

The 800 MW unit was taken down for 4 days at $100,000 per day, plus the cost ofcleaning the unit which we estimate at $80,000 for total of $480,000. We will assume thatthe King unit down time cost the same amount so that the cost incurred at NSP due tofouling before the annual cleaning cost approximately one million dollars per year. Forthis amount of money, 33 pulse detonation unit could be purchased where $25,000 is the

26

estimated cost of each Pulse Detonation Engine (PDE) soot blower, which resembles thesteam units currently in use at NSP, and $5,000 for installation. The PDE units will beretractable and will work about the same frequency as the current steam blowers now inuse.

Cost:The primary costs of the ash build up are as follows:

(1) down time to clean unit before annual maintenance

(2) with detonation equipment down time cycles may be extended to 15 months instead of 12 months

(3) loss of output of steam

(4) additional fuel to make a pound of steam

These costs will be estimated for the Sherburne Plant of NSP only and will bebased on experience where possible.

For the Sherburne units, we will assume annual costs for taking down units forannual maintenance

1500 MW X 600 $/MW = $900,000

(cost for 2 750MW units)

cost per month = $75,000

Cost per month of the 800 MW unit with maintenance required every 7 months$69,000/month. Therefore annual maintenance for the 3 plants is $1,728,000/year. If thiscycle can be extended to 15 months for all units due to PDE soot blowers, the followingsavings occur

Without PDE for 15 months $1,125,000 + $1,035,000

With PDE for 15 months 230MW X 600/MW = $1,380,000

Or a savings of $780,000 per 15 months ($52,000/month). This saving will allow 21 PDEsoot blowers to be purchased per year.

If we book keep the lost efficiency to slag and ash fouling, for the Sherburne plantat $5,800/MW x 2300 MW = $13,340,000. The Sherburne plant consumes 7,500,00 tonsof coal per year at a cost of $130,000,000 in its present status. On an environmental basis,if 1-2% of the coal was saved to produce the same amount of power, 150,000 tons coalproduces approximately 500,000 tons of carbon dioxide, which could be prevented fromentering the atmosphere. Taken world wide, the environmental savings are enormous.

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3.0 CONCLUSIONS

The goal of this program was to investigate the technical and economicalfeasibility in applying detonation waves for removing coal fired utility boiler slags. Thepreliminary CFD analysis results are very encouraging. The analysis shows largenegative velocities impinging on the back side of tubes. These negative velocities helpedto clean the back side of the tubes. The CFD solutions also shows that more the tubes,better will be the removal of slags by means of wave reflection.

Several conclusions can be drawn from the results obtained during single shotdetonation wave testing. They are as follows:

• Softer slag samples can be removed more easily.• Sample with forward attachment breaks into several small pieces.• Side facing slag shear off without breaking.• Closer the slag is to the exit of the detonation tube, the better are their

removals.• Stronger waves can remove slag more effectively.• More effective removal if the slag is directly on the path of the wave.

The single shot detonation waves have been found more effective in removingslags over multi-cycle waves because of higher pressure. The tube bank arrangementshave been found to be more effective due to wave reflections.

There are several areas where studies are needed to successfully develop thealternative slag removing technique. More experiments with pressure variations, alongwith pressure and velocity measurements near the slag. A more comprehensive CFDanalysis with 3-D fluid flow simulations with combustion will provide a more accuratepressure and velocity distributions around the tubes. In the slag characterization areamore work to characterize fragments of deposits removed to determine where the depositbonds were broken will definitely provide more insight. Test should also be performed tomake relationships between deposit physical and chemical properties to removability.

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4.0 REFERENCES

1. Benson, S.A., Jones, M.L. and Harb, J.N. Ash Formation and Deposition--Chapter 4. In: Fundamentals of Coal Combustion for Clean and Efficient Use, edited by Smoot, L.D. Amsterdam, London, New York, Tokyo: Elsevier, 1993, p. 299-373.

2. Wain, S.E., Livingston, W.R., Sanyal, A., and Williamson, J. “Thermal and Mechanical Properties of Boiler Slags of Relevance to Sootblowing,” In: Inorganic Transformations and Ash Deposition During Combustion, Ed. S.A. Benson, ASME, 1992.

3. Hurley, J.P. and Benson, S.A., “Ash Deposition at Low Temperatures in Boilers Burning High-Calcium Coals. 1. Problem Definition,” Energy and Fuels, 9, 1995, 775-781.

4. Courant, R., and Friedricks, K. O., 1956, Supersonic Flow and Shock Waves, Interscience Publ., 2nd edition.

5. Podimov, V. N., 1979, On the Mechanism of Impulse Cleaning, Kazan University.

6. Hanjalic, K. and Smajevic, I., "Detonation - Wave Technique for On Load Deposit Removal From Surface Exposed to Fouling: Part I - Experimental Investigation and Development of the Method", Journal of Engineering for Gas Turbine and Power, Vol. , pp. 116-223, 1994.

7. Hanjalic, K. and Smajevic, I., "Detonation - Wave Technique for On LoadDeposit Removal From Surface Exposed to Fouling: Part II - Full-Scale Application", Journal of Engineering for Gas Turbine and Power, Vol. , pp. 231-236, 1994.