A combustion/deposition entrained reactor for hightemperature/pressure studies ofcoal and coal mineralsRodney J. Anderson, Ronald G. Logan, Charles T. Meyer, and Richard A. Dennis Citation: Review of Scientific Instruments 61, 1294 (1990); doi: 10.1063/1.1141176 View online: http://dx.doi.org/10.1063/1.1141176 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/61/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Experimental Study on Capture of PM10 Emitted from Coal Combustion with High Gradient Magnetic Field AIP Conf. Proc. 914, 587 (2007); 10.1063/1.2747485 Devices for maintaining optical access in hightemperature coal combustion environments. II. Sonic andsupersonic flow regions Rev. Sci. Instrum. 65, 466 (1994); 10.1063/1.1145158 Computerized thermogravimetric reactor with video microscopy imaging system for coal pyrolysis andcombustion studies Rev. Sci. Instrum. 64, 1541 (1993); 10.1063/1.1144024 Devices for maintaining optical access in hightemperature coal combustion environments Rev. Sci. Instrum. 62, 624 (1991); 10.1063/1.1142519 Pulsed differential reactors and their use for kinetic studies of gas–solid reactions—application to mechanisticstudies of the reactions of hydrogen sulfide and the alkaline minerals in coal Rev. Sci. Instrum. 50, 111 (1979); 10.1063/1.1135653

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A combustion/ deposition entrained reactor for highmtemperature/ pressure studies of coal and coal minerals

Rodney J. Anderson, Ronald G. Logan, Charles T. Meyer, and Richard A. Dennis

United States Department of Energy, J';1organtown Energy Technology Center, 1'11orgalltown, West Virginia 26507-0880

(Received 10 November 1989; accepted for publication 3 January 1990)

The combustion of coal and coal-derived fuels in heat engines poses significant technical challenges in terms of establishing high combustion rates and efficiencies, controlling emissions, and minimizing the impact offuel contaminants on engine components. An entrained reactor has been designed and constructed to study coal particle combustion, the tendency of coal ash to form deposits on heat engine components, and the effects of fuel additives on residual ash composition. The reactor is designed for high temperature/pressure conditions similar to those of a coal-fired gas turbine. Optical access ports and advanced instrumentation allow the in situ measurement of gas and particle temperatures, and vapor phase alkali concentrations. The reactor has been used to study the deposition potential of several coals as a function of process conditions, and to determine the effects of selected additives on the deposition rate.

INTRODUCTION

Direct coal-fired gas turbines are potentially attractive alter­natives to conventional steam cycle electric power genera­tion due to their higher conversion efficiencies. However, a possible barrier to their utilization is ash deposition. Dcpos­its are formed by the adherence of ash particles to cooled surfaces (e.g., blades and guide vanes) of the turbine. A suf­ficiently thick deposit layer can interfere with both the aero­dynamic and heat transfer performance of these components and result in increased maintenance and decreased equip­ment lifetime.2 A combustion/deposition entrained reactor (CDER) was designed and constructed to allow deposition phenomena to be studied under controlled conditions simu­lating the high temperatures and pressures found in a direct coal-fired turbine. The CDER's versatile design also allows for the study of several other related phenomena including coal combustion and the effect offuel additives on character­istics of both the gaseous combustion products and the resid­ual ash composition.

I. BACKGROUND

The CDER is an electrically heated, entrained flow reactor designed to operate at temperatures up to 1450°C and pressures up to 12 atm. Entrained coal and/or mineral matter is injected into a hot gas stream inside the reaction zone, where it is burned. The particle-laden gas is then accel­erated through a nozzle to velocities similar to those found in conventional turbines. Upon exiting the nozzle, the particles impinge on a deposition target where the ash buildup is mon­itored. Optical access ports in the reactor allow for visual observation and optical monitoring of these events.

Previous reactors used to study deposition are signifi­cantly different from the CDER 2

5 Rosner's apparatus, used for vapor deposition studies, consists of a combustion gas supply system, nebulizer-aerosol producer, fiat flame burner, platinum ribbon target with electrical power supply,

1294 Rev. Sci. !nstrum. 61 (4), April 1990

and an optical signal detector. The combustion gas (pro­pane, oxygen, and nitrogen) is supplied to the burner, oper­ated at atmospheric pressure, and seeded with a highly dis­persed aerosol of aqueous alkali salt solutions. The deposition is measured on the 5-mm-wide platinum rib­bon. 24 A device more similar to the CD ER has been used by Benson et ai., 5 for pulverized coal applications. This appara­tus consists of an electrically heated drop-tube reactor which has a maximum temperature of 1550 °C and a preheat injec­tor system that can heat the secondary air to 1100"C. The 50-em-long drop tube has a 6.35-cm i.d. ending in a 1.27-cm nozzle. Nozzle velocities are on the order of 3 m/s. The CDER offers several advantages over both of these reactors for turbine-related research including: testing at elevated pressures, greater flexibility with target aerodynamics, high­er gas velocities (by one to two orders of magnitude), and optical access for in situ laser diagnostics.

II. HARDWARE

A. Reactor

The CDER (Fig. 1 ) consists oftwo attached vessels: the heater and the test section. This arrangement is similar in operation to an entrained reactor described by Gullett et al.6

The heater uses Super Kanthal 33 heating elements that can attain temperatures of up to 1820"C. Its function is to heat the ambient temperature carrier gases to their preset final temperature before entering the test section. Heat losses betwecn the two vessels result in gas temperatures up to 1500 cC entering the test section. Guard heaters within the test section prevent further temperature loss and permit iso­thermal operation.

The test section has two flow zones: an upper laminar flow zone for combustion studies (reaction zone) and a low­er acceleration zone for deposition studies (Fig. 2). The re­action zone is 50.8 cm long with a 5.08 cm diameter. Pre­heated carrier gas enters the top of the reaction zone and

1294

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Test Section

G[~r. «=L_

\ \\ '.~

~~ e

°_6

Particle Injection Access Port

.----i- Optical Access Ports

Process __ (Qjl )

;1& ~1~ :~~r~ I I \ \ \ ~po,;Uo,

FIG. 1. Combustion/deposition entrained reactor (eDER).

Flow G\\ Inlet ) \

I \ \ Target I \ Access Port

D ' I \

encompasses a water-cooled injection probe extending about 15.2 em into the reaction zone. The injection probe is used to introduce particles and/or reactive gases. Cooling from the water-cooled surfaces of the probe as well as the mixing of ambient temperature carrier gas from the probe result in some decrease in temperature along the center line of the reaction zone. However, this temperature is rapidly recov­ered resulting in a near-isothermal environment. One exam­ple of the radial temperature gradient for a preset reactor temperature of 1100 °C is shown in Fig. 3. These experimen­tal data were collected at a vertical position about 15 cm below the tip of the injector nozzle.

Experiments in the CDER are designed to simulate de­position on the leading edge of a gas turbine airfoil where the primary mode of particle delivery to the surface of the airfoil is inertial impaction (Fig. 4). At the exit of the combustion zone the products of combustion are accelerated through a 3. 18-mm-diam nozzle, creating ajet which impinges on a fiat platinum disk at a velocity of approximately 300 m/s. This is within the range expected in the first stage of a gas turbine. The platinum target is positioned two nozzle diameters, or 6.35 mm below the nozzle aperture (Fig. 5). This nozzle­/target configuration was developed according to proce-

1295 Rev. SCi.lnstrum., Vol. 61, No.4, April 1990

~ dures recommended by Marple and Willeke7 to insure that all particles larger than O.Spm are forced by inertia to im­pact the target, as would occur on the leading edge of a gas turbine stator or blade. Since most particles tested are signif­icantly larger than 0.5 m, all particles exiting the nozzle ef­fectively strike the target. As a result, the particle mass flux exiting the nozzle is equivalent to that striking the target. Platinum is used as an inert target material to eliminate sur­face reactions peculiar to a specific blade material which could effect the experimental results. The target surface is cooled from the underside by an opposing jet of cooling air. Thus the targets can be cooled over a range of temperatures by varying the cooling air flow rate. The target temperature is measured throughout each test via a two-color optical py­rometer monitoring the back side of the platinum target.

Viewing ports are located in both the reaction and depo­sition zones of the test section. The two locations within the reaction zone are designed to accommodate laser and optical monitoring of combustion and associated phenomena. Two apertures with an/number of 4 (F4), 1800 apart, and one aperture with an/number of 15 (F15), perpendicular to the F4line of sight, are located at both levels. Each F15 port is designed as an access for high-intensity laser light and has a

Combustion reactor for coal 1295

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... ~

~ ~ ,... :;

~ j1

~ 0'1 ... z P i'> )00

~ ... CD

~

o o

f g: :;,

I ... ~

i

... N

:E

Item

PRESSURE, ABSOLUTE, atm

FLOW RATE, slpm

VESSEL LENGTH, em

VESSEL OUTSIDE DIAMETER, em

WALL THICKNESS, mm

PROCESS STREAM EXIT TEMPERATURE, °C

VESSEL SKIN TEMPERATURE, °C

ELEMENTAL TEMPERATURE (maximum), DC

HEATING ELEMENTS

HEATER OUTPUT, watts

POWER SUPPLY, VIA

EXHAUST FLOW COMPOSITION

T est Section

1-12

10-350

152

61

95 (seh 40-304LSS)

1,400

< 150

1,650

KANTHAL SUPER 33 9(18

7,060

35/350

DILUTE COAL POCI FILTERED

EXHAUST FLOW < 2 atm; < 200°C PRESSURBTEMPERATURE

FiG, 2, CDER specifications,

Injection of Coal/Mineral Matter

[%";,;~

,. l-e ~

'rTTI' Deposition Target

Access Port

TEST SECTION

I Access PorI

,. '.

-==--t.JOPlical Access Port

Optical Access Pori

Nozzle

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1150r

1100

U 105()

~ 1000

" E &. 950

II

'" ill III II II

.. II

II • II lIila IS! _ ill III! II iii "

~ :::l I 800 -r---.,--, ... --,-.-.---.-.--,--,-. .-....----.J

·30 ·25 ·20 -15 -10 -5 10 15 20 25 30

North-Soulh Cross Section (mm)

FrG. 3. Drop tube temperature profile.

beam dump positioned on the opposite wall of the reactor. While termed laser access ports, this setup also allows for the insertion of thermocouples directly into the drop tube and may be used for other measurements as the project evolves. There are also three access ports in the deposition zone of the test section having the same optical aperture ratings and ori­entation as those in the upper portion.

B. Particle feeder

The particle feeder was designed to feed pulverized coal and coal/mineral mixtures at a stable rate into the CDER. The feeder (Fig. 6) is a syringe-type feeder with a novel particle takeoff. The stainless-steel coal reservoir is 5.08 cm in diameter and 25 cm long. A 9.5-mm-diam feed tube pro­vides entrainment air as it extends down from the top of the reservoir. A smaller (4.75 mm diameter) concentric tube serves as the outlet for the particle-laden carrier gas exiting the feeder. In operation, the feed tube is driven down into the rotating coal reservoir by a syringe pump gear drive. Paddles on the feed tube and rotation of the reservoir combine to

-"--too

Gas/Particle Flow

Particle Capture Zone

Deposition

Pressure Surface

'-_. Deposition

.~ .. _ Suction Surface

FIG. 4. Ash deposition on turbine airfoils.

1297 Rev. SCi.lnstrum., Vol. 61, No.4, April 1990

\ \ t-NOZZlE

\ 8 ... __ JET EXIT

\ \~- STREAMLINE

\ \ \ r- TRAJECTORY OF

\\ ~ED PARTICLE

"­'\"

, , ------TARGET COOLING :

2-~ :ET;p:r- TARGET SUPPORT

..:...;:-.L..,,-" ..:....J ~...-_ FIBER ~PTIC CABLE

FHi. 5. CDER nozzle/target configuration.

keep the coal level uniform which prevents sudden bursts of particles with the feed. The entrainment air enters the reser­voir through holes in the annulus between the concentric tubes of the feed tube and entrains the coal as it exits the reservoir via the 4.75-mm tube. Just above the feeder is a makeup air line designed to increase the gas velocity and minimize particle loss in the transport line to the CDER. The higher velocity of the particle-laden gas also provides better mixing upon exiting the water-cooled injection probe into the reaction zone. The feeder is capable of coal feed rates of 1-10 g/h, with an accuracy of ± 10 %,

C. Sampling probes

Deposit samples are obtained by placing the target, a 12.7-mm-diam platinum disk, 011 the target holder which is raised pneumatically into the lower part of the test section and locked in place. Two target holders are currently in use. The first holder provides a jet of cooling air directed at the underside of the target (see Fig. 5), allowing the target sur­face temperature to be controlled independent of the gas temperature. The target surface temperature can be varied over a range of 200 0c. Tests with cooled targets should ac­curately simulate deposition on cooled engine components. The second holder places the target at a 30" angle to the incident flow. Tests with the angled target will be used to determine ifthe flow angle has an effect on the fraction of ash that sticks to the surface and should aid in the prediction of deposition on complex airfoil shapes.

The flux of particles entrained in the jet is determined by filter samples using quartz filters and an air-cooled filter holder. Airflow from orifices in the filter holder rapidly cools the hot jet to a temperature of about 300 DC before the gas encounters the quartz filter. The filter sample is weighed to determine the particle arrival rate.

Combustion reactor for coal 1297

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To Test Section

t

~I fn 3/8" TUbe-..\1 ill)l_

II !lll]-· '""',l::,,~' I~II) 1

111'

Linear Translating III Stage Iltl'

} 'lilli' T1~Tr-1 !IIIII I [:~ I 1~111. --11~lljl~

!i: I > III:I[ I !:I~ JL--III, . I II:' lJ I II ReCIprocating 8. ~:: : --=-~. ill I i R~ting O-Ring Seal

oo;,:rl: ~J fc==~~llltl ~'O,,,"g I ~,I I ~ 111111.ll-.J- a-Ring

I Ii I I Ilf' I~I ~ 9001

~ '°01 F" 1_ .~ r ,,,",' I J Re~~~~rs-1 \_I ___ ~ / j

Variable Speed Motors

FIG. 6. Solids feeder design.

D. Basic instrumentation

Gas flow rate, pressure, and temperature are measured and controlled at various points within the COER by stan­dard process instrumentation. These parameters are moni­tored by the operator from digital readouts on the control panel and by an automatic data acquisition and control sys­tem (ADACS) that archives large quantities of data.

Tylan model 260 mass flow controllers are used to con­trol the gas flow of entrainment air to the coal reservoir, the makeup air, and the target cooling air. A Tylan model 262 mass flow controller is used to monitor the gas flow through the filter sample. The carrier gas flow and back pressure regulator are controlled by Moore Micro 352 single loop digital controllers. Barber-Colman model 570 series pro­grammable temperature controllers are used to power the heating elements in the heater and the test section. The con-

1298 Rev. SCi.lnstrum., Vol. 61, No.4, April 1990

trollers ramp up the temperature in both vessels at a con­trolled rate to a preset operating temperature. Each con­troller also includes the sequences to power down the CDER. Both of these controllers are linked to AOACS and the test section controller is also linked to a high-tempera­ture alarm. Temperature and pressure alarms are also locat­ed at eight strategic points throughout the COER system.

Types Rand K thermocouples measure the gas and re­fractory temperatures at various locations within the heater and the test section and a single point in the exhaust stream. Optical pyrometers are available to measure the temperature of the heating elements.

E. Advanced instrumentation

Most of the advanced instrumentation in use and planned for the COER is nonintrusive and utilizes the opti­cal access ports in the body ofthe test section. These devices include a multimonitor, an on-line mass spectrometer, a Fourier transform infrared (FTIR) spectrophotometer, and a two-color pyrometer. The most significant advantage of­fered by the optical techniques is that by measuring at a precise time and location within the reaction zone, research­ers will have access to information which potentially can provide mechanistic understanding of the combustion/ de­position process.

The computer-controlled multimonitor consists of three instruments that can simultaneously provide real-time optical measurements ofpartic1e temperature, gas tempera­ture, and alkali vapor concentrations. The optical signals for all three instruments are collected by an elliptical mirror and carried by fiberoptic light guides to lock in detection systems (Fig. 7). The elliptical mirror has a focal volume of about 3 mm at the center of the reaction zone. Therefore, the multi­monitor is able to address an area small enough to be consid­ered a point source device capable of mapping the flow stream in up to three dimensions. The particle temperature is measured by a high-speed, four-wavelength optical pyrome­ter. It was designed to look at single particles (assumed to be greybody sources) in the hot reacting flow stream. The four wavelengths were selected to avoid emission interferences. The gas temperature is monitored by measuring the hy­droxyl radical (OH-) emission. A rapid scanning, low-reso­lution monochromator measures the emissions at 307 and 309 nm, A two-point fit to an empirical function is used to generate the temperature. The vapor concentrations of sodi­um, potassium, and calcium are measured by atomic absorp­tion using hollow cathode lamps as sources. The alkali and calcium concentrations are measured because they are be­lieved to aggravate ash deposition.

The on-line mass spectrometer (QuestaI' high-speed process analyzer) is used to sample combustion gases from two locations in the reaction zone. The sampling rate (15 cc/min) is sma}] enough that it will not cause any distur­bances of the flow stream in the reaction zone. The mass spectrometer provides data on the type and amount of the gases present. Sulfur species are of major interest to the de­position experiments and to studies of sulfur sorbents.

An FTIR is used in several ways to assist in coal reaction studies: measurement of the gas temperature by analyzing

Combustion reactor for coal 1298

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Lens

Lense

[f~~·,-D=== /<\ i)" ) Photo.

Optical Fiber

/,,",'-::-'~ 0 Diode

Transmitter

Optical Fiber

Hollow Cathode Chopper

FIG. 7. Nouintrusive analytical in­strumentation.

Lamp

wi ,~ ~,>

the rotational distribution in the carbon monoxide stretch­ing band, identification and quantitative analysis of gas phase species in the combustion zone and the nozzle exit, ane! infrared mapping of ash deposits (by specular reflec­tance). The major components of the system include the Mattson Instruments Cygnus 100 FTIR, a Spectratech IR­Flan infrared microscope accessory, and a set of Analect OPTIBUS components arranged into three-energy-transfer double-periscope assemblies.

The surface temperature of the platinum target is mea­sured by a two-color pyrometer using ftberoptic light guides. The pyrometer is capable of measuring target temperatures ranging from 600 to 1650°C. The two-color approach was chosen so that variations in the emissivity of the target would not affect the temperature readings. A sapphire rod, mount­ed in each of the target sample holders, transmits the light from the back of the platinum target to a bifurcated fiberop­tic cable. The cable divides the gathered light into two branches. At the exit of each branch, the light is collimated by double convex lenses and passed through dichroic filters (950 and 1064 nm) to be detected by photodiodes. A stan­dard blackbody is used to calibrate the signal from the py­rometer to an uncertainty of ± woe.

III. OPERATING PROCEDURE

After achieving stable operation of the CDER and parti­cle feeder (see Table I for normal operating conditions), several filter samples are taken to determine both the particle flux and the ash characteristics (ultimate analyses and parti-

'1299 Rev. Sci. Instrum., Vol. 61, No.4, April 1990

cle size) in the deposition zone. Subsequently, deposition targets are placed under the particle-laden jet for intervals ranging from 2 to 40 min. The deposition rate is determined by monitoring the weight gain of the platinum target as a function of time.

The primary objective of these experiments is to mea­sure the sticking coefficient of different coals as a function of the experimental conditions: reactor temperature, pressure, target surface temperature, residence time, fuel/air ratio, and with various additives mixed with the coal. The sticking coefficient, deiined as the fraction of impacting particles that adhere to the target, is the basis for an in-house theoretical

TABLE I. Normal CDER operating conditions.

Test parameter

Coal feed rate Entrainment air flo ... ; Makeup air Heater air Total air flow rate Pressure

Variables

Range

3.0 g/h 3.0 (pm 2.0 (pm

23.6 fpm 28.6/pm I atm

Reactor temperature 1100-1300 'C Target temperature 560 -1000 "C

Combustion reactor for coal 1299

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model of ash deposition. The combined experimental and theoretical modeling approach will be used to develop a framework to evaluate coals for their fouling tendencies, to determine the effectiveness of coal cleaning/additives tech­niques, and to determine optimum operating conditions.

IV. CHARACTERIZATION

A. Particle heating

A iow mass flow rate of carrier gas is used to entrain the particles for injection into the COER reaction zone. This sheath of ambient temperature carrier gas mixes with a much higher mass flow of preheated gas within the COER near the top of the reaction zone. Although mixing of the sheath and preheated gases may result in some particle slip velocity, the heat transfer by convection is expected to make a small contribution for heating the particles. As a result, particles in the reaction zone are assumed to be heated both by conduction from the hot gas stream and by thermal radi­ation from the reactor walls. The relative importance of these two heat transfer processes is assessed by calculating characteristic heating times of the particles in the reaction zone. The characteristic heating time of a particle is defined as the time taken to heat up the particle to 64% of the tem­perature of the heat source. The characteristic heating time by gas-phase conduction and radiation from hot walls to particles were calculated using the method outlined by Ko­bayashi.s The results are shown in Fig. 8. The characteristic heating time of a particle is presented for both gasphase con­duction and thermal radiation from the reactor walls. The calculations were performed for reactor temperatures of 730, 1130, and 1530 0c. For these calculations, the thermal properties of air were used, and it was assumed that the reac­tor is isothermal (i.e., both the reactor walls and carrier gas are at the same temperature). The plots show that the heat­ing time increases with particle size, as expected. The plots also show that characteristic heating times are much smaller for conduction than for radiation; hence, heat transfer to the

100,-------------,,-------------------, 730"C- __ /

Radlatio//;: -1130·C Radiation

" ~- ---'''' C R"'''''''

,., b"";, ' 1,---"",,-c ""O".ct;,"

'""'l '--- -~- - -1530"C Conduction 0.1

0.01 -t--------r------+---------------....J 10 100

Particle Size (Microns)

FIG. 8. Characteristic particle heating time.

1300 Rev. Sci.lnstrum., Vol. 61, No.4, April 1990

particles is dominated by gas-phase conduction. Gas-phase conduction is at least an order of magnitude larger than the contribution to particle heating by wall radiation. These re­sults are specific to the assumed conditions in the COER and could change if the range of conditions used in these calcula­tions is exceeded.

The characteristic heating times of the particles by gas­phase conduction varies little with temperature. This is at­tributed to the small change in thermal conductivity of air for the given temperatures. The result is that heating times for conduction for a given size particle are essentially pro­portional to the temperature gradient between the reactor and the particle. The assumption of uniform temperature within the particle is made to simplify the calculations. For more precise calculations of characteristic heating times, the exact solution for thc temperature distribution in a sphere heated by conduction is given by Carslaw and Jaeger.9 In order to utilize this exact solution. the thermal conductivity for coal particles at high temperatures must be known. The thermal conductivity of coal, found experimentally in the temperature range of 25-975 T, increases significantly with temperature. 2X Values of thermal conductivity beyond this range are not readily available, so the exact solution was not used in the calculations. The assumption of a uniform tem­perature distribution in the particles will result in a slightly smaller characteristic heating time.

The data also show that the radiation mechanism should have more relative inft.uence as the temperature in­creases. The data shown in Fig. 8 indicate that radiation will become the dominant heat transfer mechanism for particle sizes greater than 100 pm and temperatures above 1725 °C. The choice of the heat transfer mechanism governs the theo­retical and experimental considerations for obtaining more accurate heating times of the particles for deposition studies in the CDER.

Preheated gas in the CDER mixes with the ambient temperature carrier gas used to introduce particles into the CDER. The mixing of the small quantity of carrier gas with the preheated gas results in a time lag for the heating of the gas surrounding the particles. The rate of heating of the car­rier gas can be estimated from data for laminar jets. I I This time lag results in an increase in the characteristic heating time of the particle due to gas-phase conduction, potentially enhancing the significance of radiative heating.

B. Nozzle/target assembly

The nozzle/target assembly of the CDER is designed so that all particles exiting the nozzle will impinge on the tar­get. The target surface, which is made of a 12.7-mm-diam platinum disk, is perpendicular to the axis of the nozzle and is separated from the nozzle by a distance of two nozzle di­ameters, as shown in Fig. 5. It should be noted that particles with diameters greater than some predetermined value (i.e., the critical diameter) will strike the target while the relative­ly smaller particles will follow the gas streamlines and by­pass the target. Upon impingement, some of the particles will stick to the target. The value of this probability of stick­ing or adherence is labelled the sticking coefficient.

Combustion reactor for coal 1300

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One-dimensional compressible flow theory has been used to demonstrate what the nozzle does to the flow in the reactor. 12 The one-dimensional approximation is made to alford simplicity in calculations and, at the same time, pro­vide the preliminary information on the experimental setup. Since the nozzle is short, the frictional effects may be ne­glected and the flow may be considered to be reversible and, therefore, isentropic. The converging nozzle at the exit of the CDER test section (see Fig. 2) results in an acceleration of the carrier gas and entrained particles with a corresponding decrease in the pressure and temperature of the carrier gas.

As illustrated in Fig. 5, a jet of particle-laden gas emerges from the nozzle incident upon the target. The im­pingement of the particles entrained in the jet is governed by an inertial impaction mechanism. That is, all particles larger than a predetermined size will cross the gas streamlines and strike the target while the smaller particles with less inertia remain airborne and bypass the target. The critical particle diameter is primarily governed by particle density, gas/par­ticle velocity, and gas characteristics (i.e., temperature and pressure). Marple and Willeke showed that the collection efficiency of a nozzle/target configuration with the target perpendicular to the flow can be calculated very accurately when the Reynold's number of the jet is between 500 and 5000 and when the target is separated from the nozzle exit by a distance of 1-5 nozzle diameters. 7 The distance of two noz­zle diameters was chosen for the CDER nozzle/target as­sembly to provide a margin for uncertainiies in the position of the target with respect to the nozzle. That is, slight varia­tions in the position of the target will not affect the calculated critical particle diametcr for the nozzle/target assembly.

Results of calculations of the critical particle diameter, as a function of the CDERjet temperature and pressure, are prescnted in Fig. 9. The calculations are based on the 3.18-mm CDER nozzle with air as the carrier gas, a particle den­sity of 3.0 g/cm, and a jet velocity of 152.4 m/s. These re­sults, shown for both 1 and 10 atm pressure in the CDER, illustrate the increasing tendency of the critical particle di­ameter with increasing jet temperature. This effect is due to the positive influence of temperature on the viscosity of the carrier gas (air). The primary elfect of increasing pressure is increased viscosity which causes a decrease in the critical diameter.

The high-temperature, high-velocity jet of particle-lad-

Temperature ('G)

FIG. 9. Critical particle diameter.

1301 Rev. Scl.lnstrum., Vol. 61, No.4, April 1990

en gas that emerges from the test section through the expan­sion nozzle heats the target to near test section temperatures. The opposing jet of cooling air from a 6.35-mm-diam nozzle impinges on the back of the target holder with controlled high velocity and exits with the carrier gas from the CDER. By varying the flow rate of the cooling air, the target surface temperature can be controlled over a range similar to that expected in first-stage turbine blades of a coal-fired gas tur­bine. Figure 10 shows the effect of cooling air flow rate on the temperature of the target. While the radial variation in target temperature was not measured, calculations predict radial variations ofless than 60"C. The deposit should encompass minimal radial target temperature variations since the depo­sit is formed in a localized area near the center of the target.

v. RESULTS AND DISCUSSION

The deposition characteristics of two coals were tested in the CDER for temperatures and gas velocities similar to those which might be encountered in a coal-fired combus­tion gas turbine. The tests were performed at atmospheric pressure. The baseline coal used in this study was Arkwright Pittsburgh bituminous. In addition, a highly cleaned Ken­tucky Blue Gem bituminous coal was also studied. Ash anal­yses of the Arkwright and Blue Gem coals are shown in Table II. Figure 11 shows the effect of surface temperature on the sticking coefficient of Arkwright coal at several reac­tor temperatures. The data show that at the lowest reactor temperature (1100 "C), the sticking coefficient was indepen­dent of the target surface temperature. However, at the high­est reactor temperature (1300 °C), the sticking coefficient decreased by an order of magnitude as the temperature of the target surface was cooled from 960 to 780°C. This phenome­non could be the result of a number of physical processes occurring at the target surface. The lower surface tempera­ture could freeze molten ash particles as they traverse the boundary layer above the cooled target, as well as slow chemical reactions producing molten mixtures in the depos­its.

For a given surface temperature, the sticking coefficient was observed to decrease with increasing reactor tempera­ture. For example, at a target surface temperature of 780 °C, the sticking coei1icient was 100 times lower at a reactor tem­perature of 1300 °C compared to results at 1100 0C. This was

900 ~-:.-------.---

r--·-~

:~:=:==--=-~==== 600 I t---___________

500 L._L __ L I I ~......L-_-+...L...._--'

tl100'C

812QO·C

o 5 10 15 20 25 30 35 40 45

Coolanl Flow (!pm)

FIG. 10. Target temperature control.

Combustion reactor for coal 1301

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TABLE II. Ash characteristics of coals tested in CDER experiments.

Arkwright Blue Gem Pittsburgh ( cleaned)

bituminous bituminous

% ASTMash 6.93 0.56 Elemental analyses (wt. %)

Sia, 48.09 16.86 AI,O, 25.07 22.75 FecO, 10.95 29.57 TiO, 1.27 1.95 1'20 , 0.18 0.48 CaO 5.78 7.03 MgO 1.25 2.46 K 20 1.16 0.53 Na20 0.90 1.54 SO, 5.34 8.07

Ash Fusion Temperature (oxidizing. !:: 40°C)

Initial deformation 1321 1343 Softening 1421 1385 Hemispherical l432 1418 Fluid 1471 1454

unexpected since it was thought that the sticking coefficient would be lower at the lower temperature due to fewer molten species in the ash and a reduction in vapor phase alkali. The data suggest that a change in the mechanism of deposition occurs in this range of temperatures. 13 -14

Figure 12 shows a comparison of sticking coefficients for both Arkwright and Blue Gem at a reactor temperature of 1300 °C with cooling of the target surface. The data show a similar trend for both coals, although the decrease in stick­ing coefficient for Blue Gem with target cooling was less pronounced. The ash fusion temperatures of the coats are similar, however there are differences in the composition of the ashes. The Arkwright ash has a high silica content, while the major species in the Blue Gem ash is iron. Thus, while target cooling has a similar effect on the sticking coefficients of the two coals, the difference in the magnitude ofthe reduc­tion is probably due to differences in the ash chemistries.

In summary, a novel entrained reactor has been de­signed and constructed with special characteristics to study

450 550 650 750 850 950 105e

Target Temperalure iCC)

FIG. 11. Sticking coefficient measurements for the baseline coal.

1302 Rev. Sci, lostrum., Vol. 61, No.4, April 1990

1.0

§ I 0.10 ~ I " ~

f " ~ 0.010 ~ I " ~~ 0 u

'" I " :il '-~ • BlueGem ,!,!

co II. Arkwright

~ 1.0E·04 I I 700 750 aoo a5() 900 95() 1000

Target Temperature (0C)

FIG. 12. Comparison of sticking coeffcients for two coals.

the combustion of coal as wen as the properties of residual mineral matter contained in the products of combustion. Many of the capabilities of the reactor have been demon­strated in a study of the tendency of residual particles from two micronized coals to form deposits on a target surface. The ability to independently control target and reactor tem­peratures indicated a trend in decreased deposition with de­creasing target temperature for selected reactor conditions. This trend is potentially useful in the control of deposit for­mations. The utility of this reactor to quantitatively explore processes associated with particular components of coals will be useful in providing insights into effective methods of directly burning coal in combustion gas turbines.

ACKNOWLEDGMENTS

We would like to acknowledge,Eswar Josyula for his contributions in deriving critical particle diameters, Max Hooper for his help in designing the CDER, and Masood Ramezan for analyses of the nozzle/target configuration.

11. E. France, U. Grimm, R. J. Anderson, and J. 1. Kovach, U.S. Depart­ment of Energy, Report DOE/METC/84-17 (1984).

'D. E. Rosner, and K. Seshadri, 18th International Symposium on Com­bustion (Combustion Institute. Pittsburg, PAl, pp. 1385-1394 (1985). 'D. E. Rosner, and R. M. Atkins, in Fouling and Stagging Resultingfrom Impurities in Combustion Gases, edited by R. Dryers (Engineering Foun­dation, New York, 1981), pp. 469--492.

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'So A. Benson, M. Tangsathitkulchai, and L. G. Austin, in Proceedings of the 2nd Pittsburgh Coal Conference (U.S. DOE, Pittsburgh, PA, 1985), pp. 689-·695.

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lOS. Badzioch, D. R. Gregory. and M. A. Field, Fne143, 267 (1964). liS. J. Pai, Fluid Dynamics of Jets (Van Nostrand, New York 1954). 12 A. H. Shapiro, 111e Dynamics and Thermodynamics of Compressible Flow,

Vol. I (Wiley, New York, 1953). uG. A. Richards, R. G. Logan, C. T. Meyer, and R. A. Andersoll, to he

published in Trans. ASME. ;4R. G. Logan, G. A. Richards, C. T. Meyer, ami R. 1. Anderson, to be

published in Progress in Energy and Combustion Science.

Combustion reactor for coal 1302

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