coal-water fuel droplet combustion and secondary
TRANSCRIPT
FUNDAMENTAL ASPECTS OFCOAL-WATER FUEL DROPLET COMBUSTION AND
SECONDARY ATOMIZATION OF COAL-WATER MIXTURES
FINAL REPORT, Volume II
by
T.U. Yu, S.W. Kang, J.M. BedrJ.D. Teare and A.F. Sarofim
MIT EL 87-003 February 1987
DOE/PC/70268-F2
FUNDAMENTAL ASPECTS OFCOAL-WATER FUEL DROPLET COMBUSTION
ANDSECONDARY ATOMIZATION OF COAL-WATER MIXTURES
FINAL REPORT
Professor J.M. BeerProfessor A.F. SarofimPrincipal Investigators
Volume II
The Energy Laboratoryand
Department of Chemical EngineeringMassachusetts Institute of Technology
Cambridge, Massachusetts 02139
Date Prepared - November 1986
Prepared for
United Stated Department of EnergyPittsburgh Energy Technology Center
Fossil Energy Program
Under Grant Number: DE-FG22-84PC70268
i
DISCLAIMERS
This report was prepared as an account of work sponsored by an agency
of the United States Government. Neither the United States nor any agency
thereof, nor any of their employees, makes any warranty, expressed or
implied, or assumes any legal liability or responsibility for any third
party's use of the results of such use of any information, apparatus,
product or process disclosed in this report, or represents that its use by
such third party would not infringe privately owned rights.
ii
FOREWORD
Grant Number DE-FG22-84PC70268 covered research carried out at the
Massachusetts Institute of Technology during the period October 1984-
September 1986. All of this research related to combustion of coal-water
fuels, but the workscope covered two entirely separate tasks which involved
experimental work in three different facilities at MIT.
The Task 1 investigations dealt with Fundamental Aspects of Coal-Water
Fuel Droplet Combustion, and the experiments were carried out in Laminar
Flow Reactors. The Technical Monitor at the DOE Pittsburgh Energy
Technology Center for this portion of the work was Dr. James D. Hickerson.
The Task 2 research was on Secondary Atomization of Coal-Water
Mixtures, and this involved measurements on a Spray Test Rig to characterize
spray atomization quality of fuels with various treatments, followed by
combustion tests in a 1-3 MWth Combustion Research Facility. The Spray Test
Rig was also used to characterize the nozzle used in the Task 1 work. The
Task 2 Technical Monitor was Mr. Charles McCann, who was also the Project
Manager for the Grant.
Because of the disparate nature of these separate-but-related Tasks,
this Final Report is issued in two volumes; Volume I covers the Fundamental
Aspects and Volume II deals with Secondary Atomization.
iii
RESEARCH TEAK
Principal Investigators
Professor J.M. Be6rProfessor A.F. Sarofim
Research Group Tasks
Prof. J.M. Be6r 1, 2
Dr. D. Froelich 1
Mr. S.9. Kang 1, 2
Mr. S.G. Kang 1
Prof. A.F. Sarofim 1, 2
Dr. S. Srinivasachar 1
Dr. J.D. Teare 1, 2
Mr. L.D. Timothy 1
Dr. M.A. Toqan 2
Dr. P.M. Walsh 2
Dr. T.U. Yu 2
Reports Prepared by:
Volume I Volume II
S.W. KangA.F. SarofimJ.D. TeareJ.M. Be6r
T.U. YuS.W. KangJ.M. Be6rJ.D. TeareA.F. Sarofim
ACKNOWLEDGEMENTS
The contributions of the members of MIT-CRF group to the experimental
program, and of Ms. Bonnie Caputo in the report preparation, are gratefully
acknowledged.
ABSTRACT
This Final Report is issued in two volumes, covering research into the
combustion of Coal Water Fuels (CWF). Two separate but related tasks are
discussed; Volume I contains results obtained under Task 1 - Fundamental
aspects of Coal-Water Fuel Droplet Combustion in which the experiments were
carried out in Laminar Flow Reactors. The present report, Volume II, covers
experiments under Task 2 - "Secondary Atomization of Coal-Water Mixtures".
Three methods of improving spray fineness by fuel treatment were
investigated - 1) the heating of the CWF under pressure to produce steam as
the pressure drops during passage of the CWF through the atomizer nozzle 2)
the absorption of CO02 gas in the CWF to produce a similar effect, and 3) the
addition of a chemical additive which will cause microexplosions in the
droplets upon heating. These treatments are expected to produce disruptive
atomization, i.e., the disintegration of slurry droplets subsequent to their
leaving the atomizing nozzle, and therefore to yield better burnout and
finer fly ash particle size distribution upon combustion. The effects of
disruptive atomization upon CWF spray size distribution were studied using a
spray test chamber equipped with a laser diffraction particle size analyzer;
the data were fitted to the Rosin-Rammler particle size distribution
function. The combustion characteristics of the treated CWFs were
investigated in the MIT Combustion Research Facility.
The spray chamber tests established that thermally-assisted atomization
produced reductions both in the mean droplet size and in the mass fraction
of large particles in the spray. For fuel delivery temperatures up to 100*C
this effect is attributable to lowered fuel viscosity, while further heat of
the CWF (to 1500C in these experiments) produces disruptive atomization
In-flame measurements and high speed cine pictures made during
combustion tests provided detailed information for comparisons of treated
and untreated CWF. Thermally-assisted atomization was the most effective of
the methods studied for improving carbon conversion efficiency and reducing
fly ash particle size. CO02 and picric acid addition techniques showed
substantial improvements but they were less effective.
v
Table of Contents
Page
Disclaimers i
Foreword ii
Research Team iii
Acknowledgements iii
Abstract iv
Table of Contents v
1. Introduction 1
2. Experimental Apparatus and Conditions 3
2.1 Spray Test Rig and Optical Spray Measurement 32.2 Experimental Equipment, Measurement Methods and 6
Experimental Variables2.3 Fuel Treatments 11
3. Experimental Results and Discussion 11
3.1 Spray Tests 113.1.1 Thermally-Assisted Atomization 113.1.2 C02 -Assisted Atomization 20
3.2 Combustion Tests 24
4. Summary and Conclusions 36
References 38
Appendix A A-IAppendix B B-1Appendix C C-l
1. Introduction
During coal water slurry combustion there is a tendency for the coal
particles to agglomerate within individual droplets while the water is
evaporating; the agglomerates are then dried out and undergo thermal
decomposition in the flame. Hence the resulting coal particle size
distribution (p.s.d.) will be determined more by the size distribution of
the atomized fuel spray than by the initial particle size of the coal.
Large particles formed through agglomeration have increased burnout times
and produce large fly ash particles which accelerate convective tube bank
erosion. Such erosion can be reduced if the fly ash particles are
sufficiently small that they follow the gas streamlines around tubes rather
than impact on them.
In pulverized coal combustion it is known that finer grinding of the
coal will yield reduced fly ash p.s.d. The relationship between coal
particle size and fly ash size, however, is less straight-forward for CWF.
If finer coal p.s.d. in the slurry could permit use of smaller atomizer
nozzle orifices, this might lead to finer fly ash p.s.d. via improved
atomization, with the fineness of atomization being related to the orifice
dimensions of the atomizer. Unfortunately, finer solids p.s.d. leads to
increased slurry viscosity for a given CWF solids loading, and this, in
turn, may lead to coarser atomization unless the viscosity is reduced by
means of an additive or by diluting the slurry with water.
An alternative route to finer p.s.d. of the spray droplets and of the
fly ash is the use of fuel treatment to produce disruptive atomization; the
atomizer would deliver as fine a spray as readily achievable, but fuel
treatment or additives would cause further disintegration of the atomized
droplets, yielded finer droplet p.s.d. for combustion.
Three treatments to produce microexplosions in atomized droplets have
been studied at MIT:
i) CWF heating, which was first studied in Germany (1) and
successfully tested (2) by Babcock & Wilcox; if a pressurized fuel
is heated to a value approaching its saturation temperature, the
water content of the slurry 'flash vaporizes' as its pressure is
rapidly reduced in the atomizing nozzle. The resulting volumetric
change from steam release causes the droplets initially formed by
the nozzle to shatter, yielding a substantial decrease in p.s.d.
ii) The absorption of CO2 in the slurry under pressure, as initially
proposed at MIT (3); CO2 is dissolved into the fuel by injection
into the line between the pump and fuel nozzle. During the
pressure release in the atomizing nozzle, the dissolved CO2 evolves
in gaseous form and disrupts CWF droplets.
iii) The use of picric acid as an additive as suggested by Dr. Kenneth
Olen (4); upon the injection of the CWF spray into the flame the
water-soluble and thermally unstable chemical produces
microexplosions on the surface of the coal particles in the hot
environment, resulting in separations of partially agglomerated
coal particles.
Some quantitative characteristics of these three treatment techniques
are presented in Appendix A.
In the following sections are reported the results of experimental and
analytical studies carried out at MIT to provide information on the effects
of disruptive atomization in reducing p.s.d. of droplet and fly ash
particles; the influence of the three types of fuel treatment is evaluated
in terms of the spray droplet p.s.d., the overall flame behavior including
flame stability and carbon burnout, the p.s.d. of particulates within the
flame and at exit from the combustor, and the fly ash deposition.
Experimental results showed that the fuel treatments were effective in
reducing p.s.d. of droplet and fly ash particles (5, 6, 7).
2. Experimental Apparatus and Conditions
2.1 Spray Test Rig and Optical Spray Measurement
A schematic diagram of a spray test rig is shown in Figure 1. CWF
droplet p.s.d. for a spray discharged into a 1.25 x 0.45 x 1.00 m chamber
was measured by means of a laser diffraction spray analyzer (8). Two sides
of the chamber have plexiglas walls for optical observation and measurement;
the other sides are fitted with honeycomb sections through which outside air
to be entrained by the spray can pass. This supply of outside air is
necessary to suppress any recirculation of small particles inside the
chamber. A description of the spray test rig and the laser diffraction
analyzer is given in Appendix B.
The measurement technique of the analyzer is based on the Fraunhofer
diffraction pattern superimposed on the geometrical image produced by
droplets in the path of a monochromatic light beam. The analyzer,
manufactured by Malvern Instruments Inc., comprises a laser light source
that passes light through the two plexiglas plates perpendicular to the fuel
spray flow, a 31-element photodetector that receives the light signal on the
other side of the chamber, and a minicomputer and a control terminal that
process output signals from the photodetector and print out droplet size
distribution parameters. In the processing of the signal a functional form
for the size distribution is fitted to the data; for example, the software
yields best-fit Rosin-Rammler (R-R) parameters from which other spray
characteristics, including mass median diameter (MMD), can be calculated.
EXHAUSTTOOUTSIDE AIR
EXHAUSTFAN
ROOM AIR
I I I I
FLOWMETER
ATOMIZINGAIR SUPPLY
AIR FLOWDAMPER
WATER
SPRAYCHAMBER
FILTER
P : PRESSURE GAUGE
T : TEMPERATURE GAUGE
V : VALVE
R : REGULATOR
TO STORAGE PUMPTANK
CO2
INJECTION
STIRRER
FUELMIXINGTANK
FILTER
STEAM OUT
STEAM IN
V WATER
FUEL PUMP
Schematic diagram of spray test facilityFigure i.
5
An assessment was made of the relative error due to our choice of this R-R
size distribution by comparison with fits based on a "model-independent"
size distribution. The index used to show the quality of the least-square
fit of the data was defined as
31Z Log [light intensity calculated - light intensity measured]2
1
had a value always less than 4.3, and its value did not improve (i.e.,
decrease) when the model-independent size distribution was used instead of
the R-R functional form.
The laser beam was aimed perpendicular to the spray axis, 30 cm
downstream from the nozzle tip. The transmissivity of the spray was
monitored, and multiple scattering effects were determined according to the
empirical calibration technique developed by Dodge (9). The error on MMD
due to multiple scattering was always less than 5%. A 300 mm focal length
lens was used for the laser-diffraction particle size measurements. This
gave an observable size range between 5 and 560 Mm.
During spray experiments the thermally-assisted and C02-assisted
atomization techniques were tested. The fuel flow rate was maintained at
163 kg/h, while the atomizing air/fuel mass flow ratio and the nozzle
diameter were varied. The atomizer nozzle used for spray tests was of the
OR-KVB dual fluid type developed by Occidental Research Corp. and KVB, Inc;
it is of internal mixing and has a single exit orifice.
Both the spray tests and the combustion studies were carried out with
Coal-Water Fuels Prepared by Atlantic Research Corporation to the same
specification (coal particle size, viscosity, and fuel additive). The coal
in the CWF used for the spray tests was "Montcoal", and that in the CWF
prepared for the combustion experiments was "Splashdam". The origins of
these coals were different, but in each case the coal type was bituminous;
the two coals were of the same rank and were similar in composition. There
is no reason to believe that differences in coal type used in the
formulation of the CWF caused any differences to exist in the qualitative
behavior of the CWF sprays used in the two types of experiments. Indeed,
experiments were subsequently carried out at MIT with a range of coal types
in CWFs, and these later spray tests have confirmed that the behavior of a
CWF under disruptive atomization is insensitive to the coal type (10).
2.2 Experimental Equipment, Measurement Methods and ExperimentalVariables
The MIT Combustion Research Facility (CRF) is a 1.2 m x 1.2 m cross-
section, 10 m long combustion tunnel equipped with a single burner capable
of up to 3 MW firing rate. The combustion tunnel, shown in Figure 2, is
comprised of a number of 0.3 m wide, watercooled, refractory lined or bare
metal interchangeable sections. The sections can be arranged to control
heat extraction along the length of the flame, and thus to simulate the
thermal environment of a wide range of industrial and utility flames. The
facility is extensively instrumented to permit accurate control of flame
conditions and detailed characterization of internal flame structures. The
experimental burner consists of an infinitely variable swirl generator (of
IFRF moveable block design). Combustion air from the swirl generator passes
through a 0.176 m diameter nozzle coaxial with a 0.06 m diameter fuel gun.
The fuel gun and atomizer are moveable along the axis of the burner,
permitting variation in the position of fuel injection relative to the
nozzle throat. Detailed descriptions of the burner and combustion chamber
have been given by Be6r et al. (11) and Farmayan et al. (12).
In-flame measurements were obtained with water-cooled probes which
could be inserted into the combustion chamber at various axial stations.
Solids sampling was carried out with probes in which the sampling lines were
Burner Experimental Cylindrical Cold-Wall ExhaustChamber Afterburner Chamber Section
Figure 2. Furnace assembly and air supply
either water-cooled or steam-heated. In the former case a water spray was
injected into the line at the sampling nozzle to quench reaction in the
collected sample, and in the latter case the probe was connected to a
cascade impactor where particulates could be classified aerodynamically into
several size ranges. The various techniques used for in-flame measurements
of temperature, gas composition and solids concentration have been reported
elsewhere (11, 12).
Fine and regular-grind CWFs supplied by ARC were used for the
combustion tests. Experiments were carried out to determine the effect of
addition of CO2 , picric acid, and the preheating of the fuel to 110*C prior
to atomization. Over this range of preheat, neither slurry caused any
plugging in the pipeline or nozzle. For each slurry a baseline study was
carried out using untreated fuel.
Specifications of the two ARC CWFs (Fine and regular-grind; Coal type:
- Splashdam) are included in Table 1. The experimental conditions
maintained constant during the combustion tests are given in Table 2. It
should be noted that the excess air in the flames studied was maintained at
a level unusually low for coal combustion (8-9%) to accentuate the
improvements due to the fuel treatments. However, the two types of CWF
(fine and regular-grind) were tested under different thermal inputs (1.0 and
1.3 MW, respectively). Thermal input was increased to raise the flame
temperatures in the regular-grind CWF experiments, but this would result in
improved combustion efficiency at a given axial location relative to the
corresponding fine-grind case. Thus quantitative comparisons between
equivalent fine and regular-grind cases are not available.
Table 1
Specifications on the ARC Coal-Water Fuels*
A. Montcoal (Used for Spray Tests)
- Solids p.s.d. in slurrySize (pm) 300 150 75 45 10 5 1% Passing 100 97 81 72 29 15 2
- Slurry Solids: 68.1%
-I- App. Viscosity (Haake): 800 ± 50 cp in interval 2000 - 8000 sec-
B. Splashdam (Used for Combustion Tests)
o ARC Regular (Runs 324 to 326)- Solids p.s.d. in slurry
Size (pm) 850 70 20 7.6% Passing 100 80 50 30
- Slurry solids: 70.3% -1- App. Viscosity (Haake): 616 cp at 70*F and 102 sec
o ARC Fine (Runs 320 to 323)- Solids p.s.d. in slurry
Size (pm) 600 75 30 9.9 4.6% Passing 100 96.9 80 50 30
- Slurry solids: 69.6%
-1- App. Viscosity (Haake): 416 cp at 70*F and 102 sec
o Characteristics of the Parent Coals- Proximate Analysis: As received Dry Basis% Moisture 1.07 -% Ash 5.50 5.56% Volatile 30.44 30.77% Fixed Carbon 62.99 63.67Btu/lb 14561 14718
- Ultimate Analysis (Dry)% Carbon 82.91% Hydrogen 5.06% Nitrogen 1.50% Chlorine 0.11% Sulfur 0.61% Ash 5.56% Oxygen (diff.) 4.25
*Analyses of experimental slurries provided by Atlantic Research Corp.
Table 2
Experimental Conditions of Combustion Tests (Runs 320 to 326)
Fixed Conditions
o Atomizer: Solid Cone 50* OR-KVB Nozzle, 3.175 mm Orifice Diameter
o Burner Type: 25* Half Angle Divergence, Refractoryo Combustion Air Swirl: S = 2.8o Burner Nozzle Diameter: 0.176 mo Atomizer Position: At the entrance of the divergent quarlo Combustion Chamber Configuration (from burner to outlet):
7 Water-cooled refractory lined sections2 Water-cooled bare metal sections5 Water-cooled refractory lined sections
ARC Fine (Runs 320 to 323)
o CWF Type: ARC Fine, Splashdam, 67.5%, Coal Loadingo Fuel Flow Rate: - 188 kg/h (-1.0 MW Firing Rate)o Fuel Pressure at Nozzle: -1.65 MPa (-1.20 MPa with Heating)o Fuel Temperature: -260C (-1100 C with Heating)o Atomizing Air Flow Rate: -35.9 kg/ho Atomizing Air Pressure: -1.20 Mpao Combustion Air Flow Rate: - 1119 kg/ho Combustion Air Preheat: -290*Co Excess 02: -2%
ARC Regular (Runs 324 to 326)
o CWF Type: ARC Regular, Splashdam, 69.5% Coal Loadingo Fuel Flow Rate: - 232 kg/h (-1.3 MW Firing Rate)o Fuel Pressure at Nozzle: -1.72 MPa (-1.40 MPa with Heating)o Fuel Temperature: -270C (-1100C with Heating)o Atomizing Air Flow Rate: -42.9 kg/ho Atomizing Air Pressure: -1.34 Mpao Combustion Air Flow Rate: - 1570 kg/ho Combustion Air Preheat: -310*Co Excess 02: -2%
2.3 Fuel Treatments
For the thermally-assisted atomization studies, steam-heated lines
connecting the fuel pump to the Spray Test Rig and to the CRF burner are
capable of raising the CWF temperature to about 150*C. Each line is about
12 m long and equipped with gauges for monitoring pressures and temperatures
of both steam and CWF.
For the study of fuel treatment by CO2 absorption, a 15 cm section
containing a CO2 injection assembly can be readily inserted into either main
fuel line at the high pressure side of the fuel pump. Figure 3 shows a
schematic diagram of the injection system. The maximum CO2 flow rate which
could be injected into the line without producing pulsating sprays or flames
was approximately 4 g/kg CWF. This is about 50% of the theoretical
saturation limit.
The chemical treatment of the CWF was effected by premixing picric acid
with CWF, with special care being taken to ensure uniform mixing. The
"nominal" picric acid concentration was chosen to be 0.35 g/kg CWF, which is
within a factor of two of that required to cover the CWF coal particles with
a monolayer of picric acid molecules. Several picric acid concentrations in
excess of the nominal were tested, but visual observation of the flames
showed no discernable improvement in atomization quality. Thus the in-flame
measurements were carried out with the nominal picric acid concentration.
3. Experimental Results and Discussion
3.1 Spray Tests
3.1.1 Thermally-Assisted Atomization
The mass median diameter (MMD) of the spray was calculated from the
laser diffraction measurement data and plotted in Figure 4 as a function of
P,T
ROTOMETER
ELECTRICALIIEATINGELEMENT
FUEL LINE SECTION
CO2 INJECTION ASSEMBLY
CO2 CYLINDER
Schematic diagram of CO2 injection systemFigure 3.
ARC CWF (68/32,OR-KVB NOZZLEFuel Flow Rate 2.72
standard)
kg/min
x X xA
400]
E
ZL
I-
U)U)2
xx *xxx
XX
o2 oo B
Nozzle OrificeDiameter (mm)
3.3023.1753.1753.1752.794
CoalMMD
0
Particles= 26.3km
I I I I I50
I I ~ ~ I
OC100 150
CWF TEMPERATURE
Effect of CWF temperature on mass median diameter of CWF spray(CWF - ARC Montcoal)
80
60 O- x0
Air/ FuelMass Ratio
0.3190.2730.2610.1330. 133
40
20
RunI
- 2345
Figure 4.
I I I I I I I I I I
CWF temperature. For both high (0.261 - 0.319) and low (-0.133) A/F ratios,
the MMD decreases monotonically with increasing CWF temperature. Up to
100*C the reduction in spray droplet size is due to the decrease of
effective viscosity of the CWF with increasing temperature. At higher
temperatures the continuing reduction in MMD may be partly due to further
decrease in viscosity, but it is also attributable to disruptive flash
vaporization of water in the droplets. It does not appear feasible to
quantify the relative roles of these two mechanisms without first obtaining
CWF viscosity data measured at high shear rate and high temperature (in the
range 100-150*C).
For the high A/F ratio it may be observed that the measured MMD
approaches the MMD of the parent coal particles in the CWF, indicating the
potential of thermally-assisted atomization for improvement of spray
quality. At the low A/F ratio of 0.133 there is a similar decrease in MMD
in the range 100*C to 150*C, but the smallest droplet MMD measured is still
much larger than that of the parent coal particles.
For Run 3 (0.261 A/F ratio and 3.175 mm orifice dia.), the experimental
data were further analyzed in the form of particle mass distribution.
Figure 5 shows the relative mass distribution of sprays for three different
temperatures (21*C, 100*C and 148*C) and of the parent coal particles used
in the formulation of the ARC CWF. The curves represent differential forms
of the cumulative Rosin-Rammler mass distribution.
The beneficial effect of heating CWF from room temperature to 100*C and
then to 148*C can be seen in Figure 5. As temperature increases not only
does the mean diameter decrease, but also the spray size distribution
becomes more uniform; i.e., the number of large droplets existing in the
spray diminishes. At room temperature (-21*C), for example, 13% of the
2.0
1.0
0
Size distribution of coal particles and CWF droplets
at various CWF temperature (CWF - ARC Montcoal)
0OO
I)O
100 200
PARTICLE DIAMETER (/.Lm)
Figure 5.
spray mass is contained in droplets greater than 100 microns, whereas at a
temperature of 148*C the corresponding percentage of the spray mass is 1.3.
Figure 5 also shows that the mass distribution of the spray at 148*C in the
large size range is close to that of the parent coal particles. Due to
forcing of the data to the Rosin-Rammler functional form, the size
distribution of 148*C CWF shows fewer large particles above 80 pm than the
parent coal. This error, however, is small ( < 1%) and does not change the
above conclusion.
The extent of disruptive atomization can be estimated using the area
enclosed by two mass distribution lines of 100*C and 148*C. These two lines
intersect each other at a particle diameter of approximately 43 microns.
The enclosed area to the right of this abscissa represents the total amount
of large particle mass lost due to disruptive atomization, and it is equal
to the area to the left of 43 microns, which is the total amount of fine
particle mass gained. The area is calculated to be 0.2, i.e., it can be
said that as much as 20% of the total mass of spray droplets is converted
into finer droplets.
The strength of flash vaporization from a sudden expansion of high
temperature pressurized water into atmospheric pressure can be related to a
"volume change". The volume change occurs because some of the water will
vaporize to steam as it undergoes a sudden pressure drop below its
saturation pressure, typically to 1 atm. The bulk liquid must cool down to
the saturation temperature corresponding to the reduced pressure, and the
heat released is available for the production of steam, which results in a
greater total volume. The values of enthalpy of saturated water between 373
K and 500 K, obtained from Steam Tables (13), can be reasonably well
represented by a linear function of temperature given by
h - 4.32 x 103 Ts - 1.193 x 106, (J/kg) for 373 K < Ts < 500 K,
where Ts is the saturated water temperature in K. Therefore, the change of
enthalpy of the water from its value at 373 K is
Ah - 4.32 x 103 Ts - 1.612 x 106, (J/kg), (1)
and this is available for steam production. If it is assumed that the
saturated water at high temperature can be passed through a rapid pressure
drop to atmospheric pressure without any transient process of vaporization,
the volume change compared to its original volume could be represented by
Ah
AV v vfh- fg 3.20 Ts - 1.194 x 103V- 1 1 3 1 ((2)
vfv + 1 1.043 + 1- ( )Y
with h = 2.255 x 106 J/kg (heat of vaporization of water,1 atm & 373 K).
vg = 1.672 m3/kg (specific volume of steam, 1 atm & 373 K)
vf- 1.043 x 10-3 m3/kg, (specific volume of water, 373 K)
Pc 1.3 x 103 kg/m3 (density of coal),
and where 7 is the mass fraction of water in the CWF.
In addition, the mass fraction of vaporized steam is
Am Ah -3m h 7 = (1.916 x 10 Ts - 0.715) 7 (3)
fg
These relationships are approximately linear with temperature and are
displayed in Figure 6 as a function of water mass fraction of the CWF. For
example, if the water heated to 150*C in a pressurized line undergoes sudden
expansion to atmospheric pressure through the atomizing nozzle, the water
temperature should decrease to -100*C, liberating an "excess" enthalpy of
2.15 x 105 J/kg. This is sufficient to vaporize 10% of the water and
produce a 150-fold increase in specific volume. If the water forms part of
a 70/30 coal-water slurry, the instantaneous flash vaporization produces
Absolute Pressure
500
(kpa)
1000 1500 2000
150°C 200
I I I , 1400
I I I I
450
Saturation Temperature
Volume and mass fraction of steam generated from CWF as a functionof saturation temperature. y: Water mass fraction in CWF
100300 r--
200
I00
>,
0
o
0.15
El
E
0.1
-0
0°-
0.05
100
I I I
Figure 6.
- 0.03 kg of steam per kilogram of CWF, representing a net 55-fold increase
of specific volume.
For the present case of the CWF atomization shown in Figure 4 the fuel
flow rate at 2.72 kg/min and at 150*C (corresponding to a fuel volume flow
rate of 4 x 10-5 m3/s) will result in a vaporized steam flow rate of -2.3 x
10-3 m3/s. This in turn corresponds to 36% of the atomizing air volume flow
rate (6.4x10-3 m3/s) at the A/F ratio of 0.133. Assuming that the vaporized
steam flow acts like atomizing air flow, the effective A/F mass ratio
changes from 0.133 to (0.133 + 0.03)/(1.0-0.03) - 0.168, which is a 26%
increase due to 0.03 kg of steam per CWF kilogram. The assumption is
conservative, because the specific volume of the steam produced is 58%
larger than that of the atomizing air, thus calling for higher flow
velocities to maintain a given volumetric flow rate.
One question which arises is whether the heating of CWF from 100*C to
150*C provides better atomization quality than would be obtained by directly
increasing the atomizing air mass flow by 26% at 100*C. As of this time,
there have been no direct experimental comparisons to clarify such a
question.
However, the data in Figure 4 show the extent of improvement achievable
with CWF at 100*C when the A/F ratio is doubled (from 0.133 to 0.261).
Since this A/F increase actually quadruples the momentum of the atomizing
air, it is reasonable to postulate that a 41% increase in A/F ratio, or a
doubling of momentum, would yield approximately half as much improvement
relative to the data at A/F - 0.133; i.e., would yield an improvement
roughly equivalent to that obtained by increasing fuel temperature to 150*C
while maintaining A/F ratio unchanged at 0.133.
From an operational viewpoint it can be argued that the cost of
achieving a given improvement in atomization quality should be lower when
the fuel heating technique is used than when the same improvement is
obtained by increase in flowrate of the atomizing medium. The compression
work for pressurized air is usually provided by electrically-driven motors,
while use of steam as the atomizing medium will significantly increase the
requirement for demineralized make-up water supplied to the boilers; these
factors make use of high atomizing-to-fuel mass flow ratios prohibitively
expensive.
The experimental tests on thermal atomization have provided new
information on the effectiveness of flash vaporization on CWF atomization.
However, better understanding of the phenomena associated with flash
vaporization, both upstream and downstream of the nozzle orifice, is
required to provide an improved way of controlling and optimizing thermally-
assisted atomization.
3.1.2 CO02 -Assisted Atomization
The role of dissolved CO2 in spray atomization has been studied by
measuring spray droplet size distributions obtained using CWF with a range
of CO2 concentrations. The experimental measurements of MMD as a function
of CO2 concentration indicate that CO2 by itself produces a more modest
improvement in atomization quality (Figure 7).
One possible explanation for these results can be seen from the
comparison of volume expansion between CO2 atomization and thermally-
assisted atomization. Since the solubility of CO2 in water at 31*C is
-3AV = 6.38 x 10 P scc/g H20, (4)
where P is the nozzle fuel pressure in kPa, the ratio of the volume of
maximum soluble CO02 to the CWF volume becomes
OR CWS (standard, 70/30)OR - KVB NOZZLE
NOZZLE ORFuel Flow RA/F Moss RFuel Nozzle
Run22
IFICEate =atio =
DIA. 3.175mm2.27 kg/min0.151
Pressure 720 KPa
E
C
a)
aL
Er
In0
C
0aa"3
U)U)
I I I I I I I
0.1 0.2
C0 2 /CWS Mass0.3
Ratio (%)
Effect of CO2 concentration on mass median diameterof CWF spray
120
I 10 -
00 --
o
Xo
90-
80
60 1-
5010.4
Figure 7.
)
-3AV 6.38 x 10 3Py (5)
V 10- + - (l-y)PC
where - is the mass fraction of water in CWF, and Pc is the coal density
which is approximately 1.3 x 103 kg/m3 . The mass fraction of CO2 is
Am -5-m - 1.15 x 105 PY (6)
m
Equations (5) and (6) are graphically displayed in Figure 8. At 1000 kPa,
the volume change achieved from an expansion of CO2 in water is 6.4.
However, the corresponding volume change when the CO2 is dissolved in
(70/30) CWF is only 2, which is about 25 times smaller than the volume
expansion achievable by heating to the saturation temperature of 150*C.
This relatively weak volume change is probably the primary reason why the
C02 -assisted atomization did not show significant improvement in spray
quality.
One problem noted during the CO2 experiments was that whenever the
injected mass of CO2 exceeded approximately 50% of its maximum soluble
amount, the resulting sprays from the nozzle became non-uniform and
pulsating. This phenomenon is probably due to CO2 gases left undissolved in
the fuel line. Since in our case the residence time for dissolution of CO2
is finite, it is to be expected that the calculated value of CO2 solubility
in H20 would be larger than the actual value.
In order to reach a final conclusion on the effectiveness of this CO2 -
assisted atomization technique, further studies are needed. How the
absorbed CO2 escapes from the water during a sudden expansion and/or after
primary atomization remains unanswered.
1000Absolute Pressure (kpa)
22
EE
O
1.5 u
oI0.5
0.5
0,
2000
Volume and mass fraction of CO2 dissolvable into CWF as afunction of absolute pressure-y: Water mass fraction in CWF
I;>1>>
00U
)E030
I0
5
Figure 8.
3.2 Combustion Tests
Experimental data of gas composition, solids concentration and carbon
conversion efficiency for the baseline case and for three fuel treatments
are compared in Tables 3 and 4 for the ARC fine and regular-grind CWFs,
respectively. The carbon conversion efficiency improvements due to fuel
treatments are accompanied by corresponding reductions in 02 concentration
at the furnace exit. The lower 02 concentrations are concomitant with
increased CO2 concentrations and lower final concentration of CO. The data
show that the thermally-assisted atomization is the most effective method in
improving the carbon conversion efficiency. CO2 injection is slightly more
effective than picric acid addition.
Photographs of the flames of ARC fine CWF taken during the four scoping
trials (Runs 322 -0 to -3) are shown in Figure 9. It can be seen from a
close observation of the still photos that the different treatments yield
varying improvements in flame stability and fuel-air mixing. A fuller
flame length was especially evident when the CWF was heated, consistent with
the measurements of more complete carbon burnout.
The effect of fuel treatments was studied further in terms of the
particle size distribution (p.s.d.) of flame solids, as determined by a
cascade impactor. Particles larger than 20 pm were captured by a cyclone
upstream of the impactor and sieved manually. Particle size distributions
of particulates taken in flames of fine and regular-grind CWFs at a distance
of X/D - 17.1 (D - 0.176 m is the combustion air nozzle diameter) are
plotted in Figures 10 and 11. In Figure 11 mass percentage of unburned
carbon as a function of particle size is also plotted for the thermally-
assisted and baseline cases, indicating substantial reduction in the amount
of unburned carbon in the larger particles for the former case. With the
Table 3
Summary of Experimental Data from Scoping Tests withVarious Secondary Atomization Treatments for ARC Fine CWF (Run 322)
322-0
Base
27°C
322-1
CO2
322-2
Picric Acid
3.9 g/kg CWF 0.35 g/kg CWF
322-3
Heating
1080C
Axial PositionX/D, D - 0.176 m
Temperature K
02 %
CO %
CO2 %
Solid Concentration*
(g/m3 , NTP)
3.3 17.1 3.3 17.1
- 1352
3.52
0.0202
- 14.57
- 1353
- 2.70
- 0.0067
15.57
63.9 20.1 58.8 8.7
3.3 17.1
- 1354
- 3.30
- 0.0076
14.98
57.0 8.8
3.3 17.1
1353
0.71
0.0035
- 17.21
22.7 2.6
Ash (%)
Carbon Conversion*Efficiency (%)
5.7
3.7
7.9
- 32.1
CWF Type: ARC Fine Splashdam
* Water-quench solids sampling probe (X/D - 3.3)Steam-heated solids sampling probe (X/D - 17.1)
t Carbon conversion efficiency at combustion exit (X/D - 28) for all cases was >99%.
Run No.
Treatment
7.4
26.3
11.0
52.9
Table 4
Summary of Experimental Data from Scoping Tests withVarious Secondary Atomization Treatments for ARC Regular CWF (Runs 325)
Run No.
Treatment
Axial PositionX/D, D = 0.176 m
Temperature K
02%
CO %
CO2 % 1
Solid Concen- 9tration*
(g/m3 , NTP)
Ash (%)
Carbon ConversiontEfficiency (%)
325-1
Base
270C
3.3 17.1
1727
0.8
0.8
6.0
9.0
325-1
CO2
3.9 g/kg CWF
3.9 17.1
1459 - 1486
2.56 - 2.46
0.0071 - 0.0071
16.5 - 16.6
2.3 79.01 1.9
5.8 39.0 7.4 47.6
325-2
Picric Acid
0.35 g/kg CWF
3.3 17.1
- 1490
- 2.22
- 0.0072
- 16.8
80.78 2.2
6.7 44.0
325-3
Heating
1100
3.3 17.1
1771
0.8
15.9
52.2
1496
1.92
0.0057
18.2
1.5
8.0 71.5
4.4 90.8 26.3 93.5 18.0 92.5 32.3 97.7
CWF Type: ARC Regular Splashdam
* Water-quench solids sampling probe (X/D - 3.3)Steam-heated solids sampling probe (X/D - 17.1)
t Carbon conversion efficiency at combustion exit (X/D = 28) for all cases was >99%.
BASE (Run 322-0)
CO2 (Run 322-1)
PICRIC ACID (Run 322-2)
HEATING (Run 322-3)
Photographs of CWF flames with various treatments(Run 322, CWF - ARC Fine Splashdam)
Figure 9.
4.5 - CO2 ASSISTED
- PICRIC ACID
o 1.5 a BASELINE
x
ARC-FINEzoH
1.0H
4
MI
0.5
0 100 200 300
PARTICLE DIAMETER ( / M )
Figure 10. Fly ash and residual char (unburned carbon) particle sizedistribution for the effect of different fuel treatments(CWF - ARC Fine Splashdam; Flame thermal input - 1.0 MW)
1002,5
2.0
E o
0
m
B
100 200 300
PARTICLE
Figure 11. Fly ash and residualdistribution for the
DIAMETER d(/Am)
char (unburned carbon) particle size
effect of different fuel treatments
(CWF: ARC Regular Splashdam; Flame thermal input - 1.3 MW)
0z0
5050wzmZ:3:Qz
0,5
fuel treatments, improvement in size distribution by reduction of the large
particle mass fraction and corresponding increase in the small particle mass
fraction can be seen for both the fine and the regular-grind CWFs. CO2 and
picric acid addition resulted in appreciable improvement in p.s.d. However,
the technique of CWF heating produced the best p.s.d. observed.
Trimodal distributions of particle size seen on Figures 10 and 11
indicate that several mechanisms may contribute to the production of fly ash
and residual char particles during CWF combustion. As small and
intermediate size particles are formed through the partial disintegration of
coal agglomerates, these may continue to burn, with each producing a single
fly ash particle; this is roughly equivalent to a yield of one fly ash
particle per original coal particle in the CWF, and it creates the middle
peak of the p.s.d. Alternatively several very small fly ash particles can
be produced through fragmentation of a single parent coal particle, which
corresponds to the left-most peak of p.s.d., while some of the large
particles formed initially through agglomerations may still exist at the
axial station at which sampling was carried out (X/D - 17.1). Disruptive
atomization causes effects of coal particle size on char burn-out and fly
ash particle size to become more important. This is evidenced by the number
of large particles surviving to the axial location of X/D - 17.1 (see Figure
10 and 11 - the right-most peaks); the number is higher for the flames of
the regular-grind CWF than for those of the fine-grind CWF. The middle
peaks of these trimodal distributions are centered at 45 pm and 20 Mm for
the fine and regular-grind cases, respectively. In the latter case the
unburned carbon content of the 20 pm particles was very small, but carbon
content was not measured at X/D - 17.1 for the fine-grind flames. No direct
comparison of the relative extents of burnout for these two fuels can be
made, because (as indicated earlier) the regular-grind CWF was burned in
flames with higher thermal input than the fine-grind.
Detailed measurements (see Appendix C) on the centerline of the flames
of both fine and regular-grind CWFs were made for the case of thermally-
assisted atomization for comparison with the baseline data; some radial
traverses were also carried out for one flame (Table C-2). The centerline
distributions of flame velocity and temperature are plotted in Figure 12.
Figure 13 shows that solids concentrations from each thermally-assisted
flame are lower than those of the corresponding baseline case and,
furthermore, the carbon burnout is better.
High speed cine pictures and photographs of the flames show a wider
spray angle for the thermally-assisted case relative to that of the
baseline, and corresponding improved flame stability is manifested by
reduced ignition distance and absence of low frequency fluctuations in flame
front position. The improvement in combustion conditions can be further
illustrated by SEMs (Scanning Electron Micrographs) taken for the solid
particles captured from the centerlines of the thermally-assisted and
baseline flames at X/D - 17.1. Comparison of the SEMs of 250 - 355 pm
particles in Figure 14 shows that the state of oxidation is more advanced in
the flame with thermally-assisted atomization.
For the baseline and thermally-assisted cases further comparisons were
made by determining deposition rates of fly ash on ceramic tubes inserted
perpendicular to the flame axis and thermally equilibrated with the flame
gases. The transverse distribution of deposition rate was determined from
the total amount of fly ash deposited per unit length of deposition probe
during a period of 20 minutes. Effects of CWF heating on the deposition
rates for tube diameters of 25.4 and 6.35 mm are shown in Figure 15. The
o BASELINE
• * THERMALATOMIZATION
ARC-REGULAR
I I I L I20 30
2000
1500
10000
I I10 20 3
Figure 12. Comparison of gas velocity and temperature at the centerlineof CWF flames for baseline and thermal atomization cases
Left:CWF - ARC Regular Splashdam
Right:CWF - ARC Fine Splashdam
Flame thermal input - 1.3 MW Flame thermal input - 1.0 MW
40
1000
I I I I i
o BASELINE
STHERMALATOMIZATION
ARC-FINE
4 V%,%
I00
I-
E
z0
I-
0aC3
,)
- TIIERMALAIOMIZATION -
I BASELINE
0 10 20
X/D, DISTANCE FROM BURNER
I 0°
0I I I I I
20
X/D, DISTANCE FROM BURNER
Figure 13. Solids concentrations and unburned carbon percentageon the centerline of CWF flames for baseline and thermalatomization cases
Left:CWF - ARC Regular Splashdam
Right:CWF - ARC Fine Splashdam
Flame thermal input - 1 3 MW Flame thermal input - 1.0 MW
SIO
'10E
z
I-g
zaUjzo 10
CJ
0(n
(A) BASELINE
(B) THERMALLY ASSISTED ATOMIZATION
Figure 14. SEM photographs of particles collected from the centerlineof the flame at X/D = 17.1, 250 - 355 pm size range(CWF - ARC Regular Splashdam)
101
X/D= 17.1 IBASELINE THERMAL
ATOMIZATION
_ O TUBE DIA. 6,35 mm
C U0 TUBE DIA. 25.40 mmE
t 100
O
-
W
0
100 20 40 60
DISTANCE FROM FLAME AXIS (cm)
Figure 15. Transverse distributions of fly ash deposition rate per unitarea of ceramic tubes at X/D - 17.1, for thermally-assistedand baseline cases (CWF - ARC Regular Splashdam)
deposition rate with CWF heating for a 25.4 mm tube is less than that
withoug heating (baseline) case at all transverse locations, by a factor of
about 0.5 - 0.6.
As would be expected, the smaller tube has higher capture efficiency by
particle impaction which results in higher normalized deposition rates.
However, for this tube the CWF heating produces increased deposition rates
relative to the baseline case in the region close to the flame axis. The
reason for this could be related to the reduction of the mass fraction
larger particles which are capable of eroding the deposited material upon
their impaction. Thus by their removal the risk of tube erosion is reduced
but the heat exchange surface might collect more of the fine particle
deposits.
4. Summary and Conclusions
Three methods of inducing disruptive atomization to improve the quality
of spray droplet p.s.d., and thereby yield finer fly ash p.s.d., have been
studied using the MIT Spray Rig and CRF. They include 1) thermally-
assisted, 2) C02 -assisted and 3) chemically-assisted atomization. The spray
rig studies involved measurements of droplet p.s.d. and mass median diameter
of sprays using a laser diffraction analyzer. In-flame measurements made
during combustion experiments in the CRF served to determine the influence
of these three methods of disruptive atomization on flame stability, carbon
burnout, and resultant fly ash p.s.d.
During the spray tests the thermally-assisted atomization of CWFs was
found to improve spray quality. This improvement was probably due to a
decrease in viscosity at temperatures lower than 100*C, and to disruptive
atomization at temperatures higher than 100*C. Compared to normal
atomization the CO02-assisted method had a more modest effect upon reduction
of CWF spray particle size. This can be explained by the differences in the
volume expansion due to flash vaporization and the evolution of gaseous CO2
in the CWF droplet.
During combustion experiments the characteristics of the three modes of
disruptive atomization studied to identify the effectiveness of each method
in reducing the fly ash p.s.d. The most effective method was the thermally-
assisted atomization, judged by reduction of solids concentration and p.s.d.
determined along the length of the flames. While not as effective as
thermal atomization, CO2 and picric acid additions to the slurry have also
given beneficial results. The improvement in atomization quality due to CO2
absorption was slightly greater in the flame than in sprays introduced into
a cold environment. The chemically-assisted atomization was in the third
place in ranking behind thermal and CO02 -assisted atomization.
38
References
1. Merten, M. and Homer, M., Section in Final Report on "Combustion of
Coal/Water Suspension Power Plants," Steinkohlen bergbauvereim (LigniteMining Association), Essen, Germany, January, 1972.
2. Daley, R.D., Farthing, G.A., Jr. and Vecci, S.J. Coal Water SlurryEvaluation, Vol. 2, Final Report CS-3413, Research Project 1895-3 EPRIPalo Alto, CA 1984.
3. Reid, R.C., Sarofim, A.F., and Be6r, J.M., MIT, Cambridge, MA., privatecommunication (1983).
4. Olen, K.R., "Chemically Enhanced Combustion of Water-Slurry Fuels,"U.S. Patent No. 4,445,150, June 19, 1984.
5. Yu, T.U., Kang, S.W., Toqan, M.A., Walsh, P.M., Be6r, J.M., andSarofim, A.F., "Secondary Atomization of Coal-Water Slurry Fuels,"Seventh International Symposium on Coal Slurry Combustion andTechnology, New Orleans, Louisiana, May 22-24, 1985.
6. Yu, T.U., Kang, S.W., Toqan, M.A., Walsh, P.M., Teare, J.D., Beer, J.M.and Sarofim, A.F., "Disruptive Atomization and Combustion of CWF," 8thInternational Symposium on Coal Slurry Preparation and Utilization,Orlando, Florida, May 27-30, 1986.
7. Yu, T.U., Kang, S.W., Toqan, M.A., Walsh, P.M., Teare, J.D., Be6r, J.M.and Sarofim, A.F., "Effect of Fuel Treatment on Coal-Water FuelCombustion," 21st Symposium (International) on Combustion, WestGermany, August, 1986.
8. Swithenbank, J., Bear, J.M., Taylor ,S.S., Abbot, D. and McCreath,G.C., "A Laser Diagnostic Technique for the Measurement of Droplet andParticle Size Distribution," AIAA 14th Aerospace Sciences Meeting, AIAApaper No. 76-69, 1976.
9. Dodge, L.G., "Change of Calibration of Diffraction-Based ParticleSizers in Dense Sprays," Optical Engineering, Vol. 23, No. 6, 1984.
10. Beer, J.M., Toqan, M.A., Teare, J.D., "Reduction of Fly Ash ParticleSize in Coal-Water Fuel Flames" Phase IV Final Report September, 1986.
11. Be6r, J.M., Jacques, M.T., Farmayan, W.F., and Teare, J.D., "DesignStrategy for the Combustion of Coal-Derived Liquid Fuels," EPRI InterimReport, (RP1412-6), Palo Alto, California, 1982.
12. Farmayan, W.F., Srinivasachar, S., Monroe, L., Ditaranto, F., Teare,J.D. and Beer, J.M., "NOx and Carbon Emission Control in Coal-WaterSlurry Combustion," Proceedings Sixth International Symposium on CoalSlurry Combustion and Technology, p. 165, U.S. DOE, Pittsburgh EnergyTechnology Center, 1984.
13. Keehan, J.H., Keyes, F.C., Mill, P.G. and Moor, J.G., "Steam Tables,"John Wiley & Sons, Inc., 1969.
A-i
Appendix A - Characteristics of Fuel Treatment Techniques
Al. Thermal Atomization
If water is heated to 200*C (saturated vapor pressure then being 1550
kPa, or - 15 atm) its enthalpy in the liquid state is 0.853 MJ/kg. On
sudden expansion to atmospheric pressure the water temperature must decrease
to -1000 C, which represents a drop on enthalpy of -0.43 MJ/kg H20. This is
sufficient to vaporize -0.2 kg/kg H20, since the heat of vaporization of
water at 100*C is 2.26 MJ/kg. Thus the net result of a sudden expansion of
water at 200*C is the conversion of -20% of the water into steam, with a
300-fold increase in specific volume.
If the water forms part of 70/30 coal/water slurry, the instantaneous
flash vaporization produces -0.06 kg of steam per kg of CWF, representing a
net specific volume change of the mixture of -100-fold increase. Transfer
of the heat stored in the coal particles (initially at -2000 C) to the
remaining water would add to the extent of flash vaporization and enhance
the specific volume increase.
A2. Carbon Dioxide Injection
At 310 C the solubility of CO2 in water is -0.65 P cc (NTP) per gram of
H20, with P in atmospheres. Thus at a nozzle pressure of -7atm the
saturation level would be -4.5 cc/g H20, or -0.009 g/g H20, corresponding to
-0.0027 g/g CWF (for a 70/30 slurry), or -0.0045 g/g CWF (for a 50/50
slurry). It was expected that pronounced secondary atomization effects
would be observed with lesser amounts of dissolved C02 , at levels on the
order of 0.1% by weight of the CWF.
Provision must be made to prevent freezing of the CWF as a result of
temperature drop in the CO2 as it is expanded from storage pressure. This
A-2
was accomplished by making the injection section a part of the heated fuel
line discussed in Al.
A3. Picric Acid
An upper limit estimate of the 'desirable' amount of picric acid to be
added can be made by assuming a monomolecular coating for the coal particles
in the slurry. For instance, for spherical coal particles of diameter d,
the picric acid mass per particle would be rd2WM/ANA kg, where WM - 0.229
kg/mole, A = 20 x 10- 2 0 m2 (molecular 'area'), and NA = 6.02 x 1023/mole.
The particle mass would be rpd 3 /6 kg, with p - 1200 kg/m3 , so that the
picric acid requirement would be 6WM/pdANA kg/kg of coal, or (for a 70/30
slurry) 4.2 WM/pdANA - 3.3 x 10.4 kg/kg CWF for d - 20 pm. This corresponds
-3to 1.1 x 10 kg/kg H20 in the slurry, which is in turn approximately 10% of
the room temperature saturated solution concentration.
Picric acid dissolves rapidly in water, but its concentration at
saturation is too low (- 1%) to permit addition to the CWF by metering a
saturated solution, since this would cause excessive dilution of the CWF.
For the CRF experiments the most practicable method of introducing the
additive to the fuel was by premixing.
B-I
Appendix B. - Spray Test Rig and Optical Analyzer
B-1. Spray Test Rig
A schematic of the spray test facility is shown in Figure 1. Figure B-
1 is a photograph of the spray test rig. The atomizing air supply and fuel
pump systems are shared with the 3 MW combustion facility. The atomizing
air is supplied at pressures up to 2200 psig. It passes through a 1/2 inch
diameter line equipped with a pressure regulator, flow meter and a flexible
stainless steel hose, to a SPC[y nozzle. Atomizing air pressure and
temperature are measured at the- entrance of the fuel gun. The CWF is
delivered by a Moyno pump which can provide pressures up to 550 psig. On
its path to the spray nozzle the fuel passes through a flow meter (Micro
Motion Model C 25). Fuel pressure and temperature are measured at the gun
entrance.
A spray gun transporting the CWF and the atomizing air can be adjusted
vertically and horizontally to permit the traversing of different segments
of the conical spray by the laser beam of the optical spray analyzer.
The spray rig test chamber is 49" x 18" x 40". Two sides of the
chamber have plexiglas walls for optical observation and measurement. About
half of the area of the other sides of the chamber is comprised of honeycomb
sections through which air to be entrained by the spray can pass. The
supply of outside air is necessary to suppress the recirculation of small
particles into the path of the laser beam. Air at room temperature enters
through the honeycomb sections when the exhaust fan is switched on. At exit
from the spray chamber this entrained air and the atomizing air stream are
separated from the CWF, and then flow through a filter and a flexible hose
en route to the exhaust system of the CRF. The used CWF is collected in a
storage tank through a pump.
I I --~~-iL-i--~--=rr=2=~f;-re=~-~-L-----L-- - ~-~-mm~-
B-2
JEL LINE
f"
1-ILASER LIGHTSOURCE
ATOMIZINGAIR LINE
I: r;*~u
f~IT~I2
HEATED FUELSUPPLY LINE
, _ = .~a~i
EXHAUSTHOSE
S (
Photograph of spray test facility
SPRAYCHAMBER
I
Figure B-1.
B-3
B-2. Optical Analyzer
Figure B-2 shows a schematic diagram of a Laser Diffraction Spray Analyzer.
This analyzer (Malvern Instrument) consists of a He-Ne laser light source
that passes light through the two plexiglas plates perpendicular to the fuel
spray flow, of a 31 multi-element photodetector that receives the light
signal from the other side of the chamber, and of a minicomputer and a
control terminal that process output signals from the photodetector.
The operational principle of the laser diffraction spray analyzer is
based on the Fraunhofer diffraction pattern superimposed on the geometrical
image, produced by the droplets in the path of the monochromatic coherent
light beam. The diffraction pattern is large compared with the image. The
resulting light energy distribution is collected through a lens by a multi-
element detector consisting of 31 semi-circular rings. The lens acts
effectively as a Fourier transform lens by bringing all the scattered light
from droplets at various locations in the beam into the focal plane of the
lens. For monosize particles the light distribution pattern at the focal
plane would consist of alternate bright and dark fringes, the position of
which would depend upon the size of the droplets. When droplets of many
different sizes are present an aggregate light energy distribution is
obtained from which the drop size distribution can be calculated. The light
energy falling on one ring of the photo-detector located between radii si
and sj can be expressed (8) according to
M 2 2 2 2 2E.i - C M Nkk [(J + J ) - (J 0 + J1 ) ] B-Ii k Z1 0 1 si 0 1 s
where C is a constant, N is the number of droplets of size X, J0 and J1 are
Bessel functions, and M is the number of drop size ranges. The total light
energy distribution is also the sum of the product of the energy
B-4
A TD
NOZZLE
COLLIMATED ,, DIFFRACTEDBEAM B EAM
.ET SPRAY , I.
LIQUID
He - NeLASERI
SIZE
DISTRIBUTION
PRINT OUT
Figure B-2. Schematic diagram of laser diffraction analyzer
MULTIELEMENT
LIGHT
DETECTOR
MINICOMPUTER
CONTROL
TERMINAL
DROPL
I
B-5
distribution for each size range and the weight or volume fraction in that
range. This can be expressed in the form of a matrix equation
E - TW B-2
where W is the weight fraction and T contains the coefficients which define
the light energy distribution curves for each droplet. Rewriting the above
equation as W = T-1E, then with the knowledge of the inverse matrix T-1 the
weight distribution can be calculated from the measured light energy E. An
approach to the solution of Equation B-2 is to assume a form for W and
adjust the parameters by iterative means until the sum of the squared errors
Z(E-TW)2 is a minimum. The Malvern Instrument software is capable of using
various weight distributions for W, including Rosin-Rammler, normal
distribution, or "model independent". To determine the diffraction pattern
the 31 semi-annular detectors are scanned sequentially by a solid state
switch controlled by a microprocessor, both with and without the droplets
present in the beam.
If, for example, the Rosin-Rammler distribution is postulated, then in
the processing of the signal the microprocessor assumes that the size
distribution is a good approximation to:
R = 1 - v - exp [-(X/X)n] B-3
where R is the weight fraction contained in particles of diameters greater
than X, X is the Rosin-Rammler mean diameter (for which R - 36.8%), and the
exponent n indicates the spread of diameters about the mean. For a fuel
spray typical values of n will be in the range 1.1 to 3, but n can increase
to 15-20 for near monosize droplets.
B-6
The microprocessor selects initial values of X and n and the light
energy distribution corresponding to the Rosin-Rammler distribution is
calculated through Equation B-2. A least squares error criterion is used to
determine the quality of fit between calculated and measured light energy
distribution. The parameters X and n are then iteratively adjusted to give
the best fit with minimum error. The Rosin-Rammler distribution in 15 size
ranges together with the calculated and measured light energy distribution
is printed by the microprocessor using the appropriate values of X and n.
Using X and n, the mass median diameter (MMD), which is the droplet
diameter below or above which lies 50 percent of the mass of the droplets
(i.e., R - 0.5), can be calculated by
MMD - X [In 2] 1/n B-4
The Sauter mean diameter, SMD, also can be related by
SMD - X/ r(l-1/n) B-5
where r is the gamma function. The SMD is the diameter of a droplet having
the same volume/surface ratio as the entire spray.
The mass distribution of a spray as the weight fraction in any size
increment is given by the derivative of Equation B-3, i.e.,
v n X ndv n exp[-(X/X)n] B-6dx
iXX
C-1
Appendix C - Experimental Data
The following four tables summarize the experimental data obtained by
in-flame measurements in the Combustion Research Facility. Centerline
distributions of flame temperature, velocity, gaseous species concentrations
and particle concentrations are tabulated. Some radial distributions are
included in Table C-2.
Table C-1
Experimental Data, Run 320 (ARC Fine, Base)*
Distance from
Air Nozzle
X(m) X/D
Gas
Temp.
T(K)
Gas
Velocity
u (m/s)z
Mole Fractions
(as measured, dry basis)
X2 XCO2 XCO XNO XSO2
(%) (%) (%) (ppm) (ppm)
Particle
Concentration
(g/m3 , NTP)
0.4 16.2 2.300 497 875
0.3 17.6 1.000 457 750
1.7
2.5
2.3
2.0
2.0
2.1
16.7
16.1
16.3
16.7
16.6
16.6
0.111
0.651
0.028
0.019
0.006
0.003
500
492
495
2.7 15.9 0.002 489
*Centerline Measurement
0.17
0.27
0.42
0.57
0.74
0.88
1.19
1.49
1.80
2.09
3.00
3.61
4.22
4.52
1.0
1.5
2.4
3.3
63.67
4.2
5.0
36.02
6.8
8.5
10.3
11.9
17.1
20.6
24.1
25.8
103.0
23.1
16.0
7.5
4.0
2.3
1.9
2.4
2.6
2.9
2.5
2.5
5.4
6.7
1515
1638
1681
1620
1580
1538
1450
1390
1309
1301
1297
1289
625
600
500
0
5.61
3.03
3.20
3.21
3.23
Table C-2
Experimental Data, Run 323 (ARC Fine, Heating)
Radial Distance from Gas Gas Mole Fractions ParticleDistance Air Nozzle Temp. Velocity (as measured, dry basis) Concentration
R(m) X(m) X/D T(K) u (nVs) X X XO 2 (g/m 3 , NTP)
(%) (%) (%) (ppm) (ppm)
0 0.17 1.0 - 106.00 0.27 1.5 1645 38.20 0.42 2.4 1719 33.0 - - - - -0 0.57 3.3 1697 14.3 1.6 14.9 1.150 700 525 26.210.1 1641 -4.1 6.4 12.5 0.056 580 513 9.47
CN0.2 1581 14.5 7.1 11.9 0.025 535 525 6.900.3 1570 11.8 6.1 12.9 0.012 550 538 4.360.4 1555 5.0 6.0 13.2 0.010 560 713 -0 0.88 5.0 1600 0.0 1.8 16.1 0.320 590 650 7.240.1 1602 -3.9 2.9 15.5 0.073 610 625 5.030.2 1619 -2.9 3.7 14.8 0.017 600 625 4.780.3 1618 2.0 4.4 14.4 0.014 610 563 4.780.4 1593 7.1 5.0 13.7 0.015 600 550 -0 1.19 6.2 1583 -3.4 3.0 15.5 0.040 590 650 -0 1.49 8.5 1566 -3.0 3.3 15.3 0.040 570 620 2.890.2 1584 2.1 3.2 15.3 0.011 590 638 -0.4 1589 2.8 3.6 14.9 0.008 600 650 2.530 1.80 10.3 1541 -1.8 3.4 15.1 0.015 - 625 -0 2.09 11.9 1500 1.5 3.2 15.3 0.016 - 625 3.550 3.00 17.1 1377 2.1 2.9 15.7 0.007 505 663 2.600 3.61 20.6 1356 2.1 2.6 16.3 0.007 - 575 -0 4.22 24.1 1326 2.8 2.6 15.9 0.008 - 675 3.33
C-3
Table C-3
Distance fromAir NozzleX(m) X/D
0.17
0.27
0.42
0.57
0.74
0.88
1.19
1.49
1.80
2.09
3.00
3.61
4.22
1.0
1.5
2.4
3.3
4.2
5.0
6.8
8.5
10.3
11.9
17.1
20.6
24.1
G
TerT(
Experimental Data, Run 324 (ARC Regular, Base)
as Gas Mole Fractions
np. Velocity (as measured, dry basis)K) uz(m/s) X0 XCO XCO XNO XSO
(%) (%) (%) (ppm) (ppm)
1393
1620
1771
1760
1757
1706
1667
1623
1576
1459
1450
1439
55.6
44.6
12.4
-6.9
-4.3
-4.2
-3.7
-3.7
-2.6
-0.9
0.7
0.8
0.5
0.4
1.8
3.6
3.1
2.8
2.4
2.5
2.2
15.7
17.0
17.7
18.1
17.2
15.5
16.1
16.3
16.9
16.7
17.1
3.300
1.600
0.800
0.900
0.093
0.027
0.016
0.011
0.007
0.006
0.005
740
750
690
630
710
740
740
720
700
710
680
1125
775
763
788
600
525
538
575
575
588
600
ParticleConcentration
(g/m3 , NTP)
423.73
99.00
28.66
6.92
4.30
3.04
2.36
2.34
*Centerline Measurements
C-4
Experimental Data,
Distance from
Air Nozzle
(m) X/D
Gas
Temp.
T(K)
Gas
Velocity
u (m/s)z
Table C-4
Run 326 (ARC Regular, Heating)*
Mole Fractions
(as
XO2
(%)
measured, dry basis)
XCO2 XCO XNO SO2
(%) (%) (ppm) (ppm)
43.6
14.0
10.4
-7.3
-8.3
-7.8
-7.0
-6.4
-5.7
-5.5
-4.0
-2.1
0
2.2
1.1 15.2
0.8 15.9
0.9
0.9
0.7
2.1
2.9
2.5
2.4
2.2
1.8
*Centerline Measurement
Particle
Concentration
(g/m3 , NTP)
166.48
0.17
0.27
0.34
0.42
0.57
0.63
0.74
0.88
0.94
1.19
1.49
1.80
2.09
3.00
4.22
1.0
1.5
1.9
2.4
3.3
3.6
4.2
5.0
5.4
6.8
8.5
10.3
11.9
17.1
24.1
1543
1664
1688
1727
1736
1748
1775
1739
1708
1681
1655
1610
1490
1460
4.000
3.500
1.300
0.900
0.700
0.175
0.037
0.025
0.013
0.006
0.007
700
670
700
690
670
710
710
710
720
710
670
16.9
17.3
17.8
17.4
15.9
16.8
16.9
16.8
17.4
700
1150
525
575
575
575
600
625
588
638
638
52.25
19.29
5.84
2.52
1.44
2.00
1.50