EPA-600/2-77-067
March 1977
Environmental Protection Technology Series
EVALUATION OF MOLTEN SCRUBBING FOR FINE
PARTICULATE CONTROL
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
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recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-067
March 1977
EVALUATION
OF MOLTEN SCRUBBING
FOR FINE PARTICULATE CONTROL
by
G.G. Poe, L.R. Water-land, andR.J. Schreiber
Aerotherm Division/Acurex Corporation
485 Clyde Avenue
Mt. View, California 94042
Contract No, 68-02-1318, Task 24
ROAPNo. 21ADL-029
Program Element No. 1AB012
EPA Task Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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FOREWORD
This document contains a review and evaluation of molten scrubbing as a novel concept for
the control of fine particulate.
The study was performed for the Environmental Protection Agency, Research Triangle Park,
North Carolina. Dr. D. C. Drehmel was the EPA Task Officer. The Aerotherm Program Manager was
Mr. Fred Moreno. Acting as Technical Advisor for the task was Dr. C. B. Moyer. The study was
performed during the period December 1975 through February 1976.
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TABLE OF CONTENTS
Section Page
1 SUMMARY 1
2 INTRODUCTION ......... ...... 3
3 LITERATURE SEARCH ..... . 5
4 PROCESS'DESCRIPTIONS 6
5 PERFORMANCE DATA 10
6 THEORETICAL DISCUSSION . 12
6.1 Particle Collection Mechanisms . 12
6.2 High Temperature/Pressure Effects .... 18
6.3 Pressure Drop and Paniculate Collection Efficiency ........ 22
7 PROCESS EVALUATION 24
7.1 Material Corrosion 24
7.2 Mist Elimination 25
7.3 Molten Salt Reactions 25
7.4 Particulate Build-Up in the Molten Salt 25
7.5 Process Applications and Economics 26
REFERENCES 29
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LIST OF ILLUSTRATIONS
Figure Page
1 Schematic of Battelle-Northwest Molten Scrubbing Process 7
2 Operation of a Venturi Scrubber 13
3 Fluid Streamlines and Particle Trajectories Around a Sphere 14
4 Experimental and Calculated Target Efficiencies for Spheres 16
5 T^ Versus K . for f = 0.25 19
6 Penetration Versus Aerodynamic Particle Diameter for Q./Q =
1.19 x 10'3 ? 20
7 Penetration Versus Aerodynamic Particle Diameter for u = 9310 cm/sec . . 21
8 Penetration Versus Temperature for Q0/(L = 1.19 x 10"3; u_ = 9310
cm/sec *. ? ? 23
9 Theoretical Power and Pressure Drop Versus Aerodynamic Cut Diameter ... 27
iv
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LIST OF SYMBOLS
C1 Cunningham correction factor (dimensionless)
CD drag coefficient (dimensionless)
d diameter (cm)
d aerodynamic diameter (cm /g/cm3)
"a
f empirical proportionality constant (dimensionless)
K inertia! impaction parameter (dimensionless)
AP pressure drop (cm hLO)
P. penetration (dimensionless)
Q flowrate (cm3/sec)
r radius (cm)
Re Reynolds number (dimensionless)
t time (sec)
u velocity (cm/sec)
y distance from collector axis to limiting streamline (cm)
Z axial distance (cm)
n target efficiency (dimensionless)
y viscosity (poise)
p density (g/cm3)
a surface tension (dyne/cm)
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Subscripts
d drop
9 gas
& liquid
p particle
r relative
t throat
0 reference
Superscripts
— average
dimensionless
VI
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SECTION 1
SUMMARY
The purpose of this study was to analyze and evaluate molten scrubbing as a new fine
(£ 3 microns) particulate control concept and to investigate its applicability to high tempera-
ture, high pressure processes.
The concept of molten scrubbing for particulate capture is indeed a novel one. Previous
experience in molten scrubbing was almost exclusively concerned with sulfur removal, and placed
no emphasis on particulate collection. The literature search for the study revealed the pau-
city of data on this subject. Only two projects were found to have any application to this sub-
ject at all. IGT's molten metal scrubbing process, however, is proprietary and their test rig
is currently shut down. The work of Battelle Northwest appears to be the only applicable study
with available data. Their process involves the scrubbing of both hydrogen sulfide and par-
ticulate with a molten liquid of alkali metal salts for application to hot fuel gas from a coal
gasifier.
Existing theories provide a reasonably detailed description of the particle collection
mechanisms involved in atomized liquid scrubbers. These theories should apply equally well at
the high temperatures found in molten scrubbers, so the effects of the high temperatures on
particle collection can be predicted. The collection efficiency for a given particle size de-
creases as the temperature of the gas increases, according to the best available design equa-
tions. These predictions, however, have not been confirmed experimentally for molten scrubbers.
Thus far, the experimental work at Battelle has revealed the following:
• Overall collection efficiency is not as great as expected (this may be attributed
to poor design and/or operational problems).
0 The high corrosiveness of the scrubbing medium presents a difficult materials hand-
ling problem.
• Carryover of molten liquid droplets in the scrubbed gas was much greater than the
allowable specs on gas turbine inlets.
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• Molten salt reactions with other gaseous pollutants can potentially degrade the
scrubbing liquid.
• Decomposition of the molten salts occurs at temperatures around 900°C; thus, limit
controls for very high temperatures are necessary.
• Insoluble particulate build-up necessitates the use of in-line filters in any re-
generable scrubbing process.
• Energy requirements for molten scrubbers are estimated at 7 to 10 kW/Mcfm, which
are typical for scrubbing processes at more conventional temperatures.
Even though it has not been adequately demonstrated as yet, the application of molten
scrubbing to high temperature/high pressure advanced energy processes for fine particulate con-
trol appears possible. However, before effective molten scrubbing systems for particulate re-
moval are available, the developmental problems described above need to be solved.
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SECTION 2
INTRODUCTION
One important proposed use of low Btu fuel gas derived from coal gasification processes
is the generation of electric power by combustion and expansion of the gas through a gas turbine.
Process temperatures associated with coal gasification are typically near 3000°F, which dictates
that the product gas be cooled to the 1800°F gas turbine inlet temperature. However, planned
improvements in turbine blade technology may allow inlet temperatures up to 3000°F. Combining
the high temperature gas turbine with a repowered boiler may result in an overall cycle efficiency
of greater than 40 percent.
Successful operation of such a system is predicated on the conservation of sensible heat
of the fuel gas. However, the raw gas from the gasification process contains impurities, notably
sulfur compounds and particulate, which are incompatible with turbine operation and must there-
fore be removed. Since the fuel gas must remain at elevated temperatures, a high temperature
clean-up process is required. Such technology is not immediately available, but many novel
concepts have been proposed. In addition to conserving the sensible heat of the hot gas stream,
many of these concepts have been proposed as fine (<_ 3 microns) particulate cleanup processes.
Fine particles have recently come to be recognized as being much more significant air pollutants
than larger particles.
One proposed concept, molten scrubbing, utilizes venturi or spray scrubbing with molten
fluids that remain in the liquid state at the cleaning temperature. This report presents a
review and evaluation of the molten scrubbing processes currently under development in the U.S.
The work presented in this report was performed by Acurex/Aerotherm for the EPA's Indus-
trial Environmental Research Center as Task 24 on Contract 68-02-1318. The scope of work was
defined by EPA as follows:
1. Conduct a limited literature search to determine if similar scrubbing devices have
been developed or proposed by others. Determine if the device is truly novel and
define the mechanisms which are responsible for particulate capture.
2. Determine the reliability and significance of any experimental data submitted.
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3. Assess the practicality of the device for collection of fine particulate. Estimate
the capital and operating costs for the device evaluated.
4. Determine which sources might be controlled by the system and estimate the probability
of successful application.
The evaluation of the molten scrubbing concept is presented in the following sections of
this report.
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SECTION 3
LITERATURE SEARCH
The search for literature concerned with molten scrubbing occurred in three steps:
• In-house literature review
• Telephone interview with molten scrubbing researchers for information on their
processes
• Computer-aided literature search of NTIS, COMPENDIX, and Physical and Chemical Ab-
stracts resources, performed by the Lockheed DIALOG service.
The first step produced very little in the way of useful information. The telephone
calls uncovered reports, papers, and the general status of the Battene, I6T, and Atomics In-
ternational processes. The final step using the computer search produced several papers of
general interest, all of which are listed at the end of the reference section of this report.
In all, the literature search clearly revealed that molten scrubbing is a very new and
developing process, and that there is a growing recognition of its potential as a possible
high temperature fuel gas purification process.
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SECTION 4
PROCESS DESCRIPTIONS
Although some pilot- and lab-scale experience with molten scrubbing has been obtained by
the Pacific Northwest Laboratory of the Battelle Memorial Institute (Battelle-Northwest), the
Atomics International Division of Rockwell International (AI), and the Institute of Gas Tech-
nology (IGT), in all cases the data reported have been, at best, incomplete, especially with
respect to particulate scrubbing.
The Institute of Gas Technology (IGT) in Chicago recently operated a small lab-scale mol-
ten metal droplet contactor designed for sulfur and particulate removal. The metal in this case
was presumably molten iron. The device is currently shut down, and since the process is pro-
prietary, no further information is available (Reference 2).
AI investigated a pilot scale molten scrubbing process aimed at removing S02 from
boiler stack gases (Reference 3). The AI scrubber used the ternary eutectic of (Li,K,Na)2CO,
(M.P. 396°C) as the working fluid. Gas scrubbing was effected with a spray contactor operating
at 455°C, and mist elimination was accomplished with a York wire mesh demister. However,
particulate collection was of secondary concern in the AI process; in fact, an electrostatic
precipitator was used before the scrubber to reduce the flyash load of the gas before scrubbing
the S02.
BatteHe-Northwest has developed a molten scrubbing process designed to remove H2S and
particulate from fuel gas produced by coal gasification (Reference 1). Because the Battelle-
Northwest data is the most comprehensive, this process will be singled out for more thorough
discussion below. Observations and conclusions based on Battelle-Northwest's experience should
be generally applicable to all molten scrubbing processes, as other proposed processes should
differ in, at most, choice of working fluid or gas-liquid contacting scheme. The underlying
principles of operation supporting the Battelle-Northwest process, and the problems encountered
should be universal to all molten scrubbing processes.
The Battelle-Northwest pilot-scale scrubbing process is shown schematically in Figure 1.
Fuel gas from a gasifier is heated to 850°C and flows through a venturi where it atomizes a stream
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Temperature
Flow
Gas sample
Pressure P & AP
Salt transfer
line
From
gasifier
-CxJ-r—
Purge
Gas
heater
Venturi
Salt
level
_Q and press
H2S
injection
Deimster and
de-entrainment
section
«• To burner
(Waste to drums
or drain)
Pump
Figure 1. Schematic of Battelle-Northwest molten scrubbing process (Reference 1).
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of molten salts entering from a reservoir. Good gas-liquid contact is obtained and both sulfur
compounds and particulates are entrained in the liquid droplets. Gas-liquid separation is ac-
complished in a de-entrainment, mist eliminator section, with the cleaned gas flowing to a bur-
ner, and the spent molten salt returning to the reservoir.
The working fluid used in the Battelle scrubber was a salt mixture of approximately 36 wtl.
Na2C03, 37 wt% K2C03, 13 wt% Li2C03, and 14 wt% CaC03- CaC03 was added to facilitate H2$ scrub-
bing; and the amount of Li2C03 was decreased from the 1:1:1 weight ratio of alkali metal carbo-
nates in a eutectic mixture, since the lithium salt was by far the most expensive of the three
carbonates used. The above mixture is a noneutectic and has a melting range of 393°C to 541°C.
Above 541°C the mixture consists of one liquid phase.
No provision was made in the Battelle pilot facility for either insoluble particulate re-
moval or the continuous regeneration of metal sulfides to metal carbonates in the spent salts,
although batch carbonate regeneration was accomplished by blowing CCL and steam through the spent
melt. In a continuous regeneration system, insoluble particulates presumably could be filtered
from used salts and the batch carbonate regeneration scheme could be adapted to a continuous sys-
tem.
Gas-liquid contacting in the Battelle pilot plant was accomplished by a venturi atomizer.
As discussed in Section 6, particulate removal in a venturi scrubber is effected by particle
inertia! impaction onto liquid droplets. The Battelle venturi was designed to remove 55 percent
of the particles 4.0 microns or larger.
A key consideration in any scrubbing scheme is the final gas-liquid phase separation after
the scrubbing is complete. Mist elimination in the Battelle pilot plant was accomplished in two
stages. A de-entrainment section consisting of a column packed with 1-inch diameter ceramic
(A1203) balls was designed to remove 50 percent of the droplets 3.5 microns in diamter and 95+
percent of the droplets greater than 10 microns. This section was followed by a demister device
consisting of packed rolls, 3/4 inch in diameter, made of 3-inch wide stainless steel screen.
Perhaps the most important consideration in the design of a scrubbing process utilizing
molten salts is materials choice. The combination of high temperature (850°C in the Battelle pro-
cess) and the high corrosive nature of molten salts makes the choice of materials of construction
quite difficult. Both AI and Battelle conducted short-term static corrosion screening studies on
a wide range of metals, metal alloys, and ceramics. In addition, AI conducted long-term corrosion
tests in rotating capsules. AI found that Type 347 stainless steel was suitable for service below
500°C, while Battelle found aluminized Type 304 stainless steel sufficiently corrosion resistant
8
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at 760°C. However, neither material was exceptionally resistant to attack by the hot molten
(Li.KjNaJgCaCOj; CaS mixture. Alumina ceramics proved quite resistant but these materials were
rejected as not being readily amenable to conventional fabrication and assembly techniques.
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SECTION 5
PERFORMANCE DATA
Performance data on particulate removal from the Battelle molten scrubber is presented
below. This scrubber is the only one for which particulate removal data are available, and
even these data are scanty and inconclusive because of the start-up and operating problems ex-
perienced.
The Battelle molten scrubber was operated for six performance runs during late 1974.
Of these, only three runs yielded H2S scrubbing data and only two of these yielded particulate
removal data with any degree of reliability.
Several shake-down problems were encountered, most of which resulted in salt crystalliza-
tion and subsequent line plugging throughout the process stream. This occurred in spite of the
care taken with trace heating in the pilot plant design.
A gas flowrate of 70 scfm at 750°C - 800°C and 1 to 3 psig was used in the two runs
yielding particulate removal data. The salt flow for these runs was estimated at 2 gpm. Par-
ticulate gas samples were isokinetically withdrawn using high temperature stainless steel probes
both prior to and after the molten salt scrubbing step. For the preventuri sample, the total
inlet particulate burden was determined by passing the sample through a heated glass cyclone
followed by a high-temperature glass wool filter capable of collecting particles 0.3 microns
in diameter. Post-venturi samples were collected and sized using an Anderson stack sampling
head capable of sizing particles in the 0.3 to 20 micron range. Data for the two productive
runs appear in Table 1. During Run No. 3, the wire mesh demister downstream of the alumina
balls de-entrainment section was not used.
Table 1 shows that the overall scrubber collection efficiency was 70 to 90 percent,
versus a design efficiency of 99 percent for particles > 4 microns. However, most significant
was the fact that approximately 30 wt% of the collected particles in the scrubber outlet gas
were entrained salt crystals. This observation has ominous implications if turbine quality
gas is the desired product. The Westinghouse Company has specified a maximum allowable limit
for alkali metals in gases to present-day turbines of 40 ppb. The large quantity of salt
10
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carryover experienced in the Battelle pilot plant suggests that closer attention be paid to mist
eliminator design.
TABLE 1. PARTICULATE REMOVAL WITH BATTELLE MOLTEN SALT SCRUBBER*
Inlet gas parti cul ate burden
Outlet gas parti cul ate burden
Overall collection efficiency
Run No. 3
0.1 gr/scf
0.0122 gr/scf
60 wt% < 4 pm
88%
Run No. 6
0.0674 gr/scf
0.0223 gr/scf
76 wt% < 4 pm
67%
Reference 1
The Battelle molten scrubber was in operation for a total of about 25 hours. After Run
No. 6 it was disassembled, inspected and cleaned. Visual inspection showed no evidence of
major corrosion, though some material erosion was evident immediately downstream of the venturi
throat. However, it is doubtful that significant material corrosion evidence would exist after
such a short period of operation.
11
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SECTION 6
THEORETICAL DISCUSSION
The Battelle-Northwest pilot plant employed a verturi scrubber for gas-liquid contacting.
The principles of operation and collection mechanisms for venturi scrubbers, the effects of high
temperature and pressure, and the effects of using a molten scrubbing medium are briefly reviewed
in this section.
6.1 PARTICLE COLLECTION MECHANISMS
Venturi scrubbers employ gradually converging, then diverging sections of pipe as shown
in Figure 2. The venturi throat is the intersection of the converging and diverging sections of
pipe. Generally, liquid is introduced into the gas stream at the throat, and is quickly atomized
into small droplets by the high velocity gas. Particles entrained in the gas stream are collected
by the liquid droplets primarily through particle inertial impaction onto the droplets. This col-
lection proceeds until the droplets are accelerated to the gas (and presumably the particle)
velocity (References 4 and 5).
Consider a single spherical collector (liquid droplet) in a uniform velocity field (gas)
containing a suspension of small particles as shown in Figure 3. As the fluid stream approaches
the collector the streamlines envelop it. However, due to inertial forces, the particles tend
to cross fluid streamlines, impact the collector, and stick to it or become entrained within it.
Thus a single collector target efficiency may be defined for the spherical collector,
(1)
where y is the distance from the sphere's axis to the limiting streamline for impaction and
r. is the collector (liquid droplet) radius.
12
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Clean gas
and liquid
droplets
0° 0.0 •
1
Liquid in
Dirty
gas
Figure 2. Operation of a venturi scrubber.
13
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Fluid streamlines
Figure 3. Fluid streamlines and particle trajectories around a
sphere (Reference 4).
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The dimensionless equations of motion for particles obeying Stoke' s law are:
(2)
where u,v = u/u , v/u ; u is a reference velocity,
0' ""0' 0
(3)
and
Kp - 18^rd • <«>
The quantities u and v are the x and y components of velocity, respectively, t is time, K is
termed the inertia parameter, and C1 is the Cunningham correction factor for the drag on a
sphere in steady motion. Equation (2) may be solved for a variety of collector shapes and im-
posed velocity fields allowing, eventually, a calculation of y , and hence r\. Walton and
Woolcock (Reference 6) experimentally determined n for K > 0.2 and found that
approximated the data very well. A comparison of Equation (5) with the theoretical prediction
of n vs. K for potential flow appears in Figure 4.
If it is assumed that:
1. Particles move only with the gas stream and are collected only by liquid droplets
2. The liquid drop diameter is given by the Sauter mean diameter from Nukiyama and
Tanasawa (Reference 7),
,
15
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Sphere, theoretical
potential flow
Sphere, experimental,
Walton & Wool cock
i, .c' "p V •«
T> " 18 Mn <•„
Figure 4. Experimental and calculated target efficiencies for spheres (Reference 4).
16
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where the relative velocity between gas and liquid is assumed to be the gas velocity
ug (cm/sec), a is the liquid surface tension (dyne/cm), y£ is the liquid viscosity
(poise), p£ the liquid density (g/cm3), and Q£ and Q are volume flowrates of liquid
and gas, respectively. Drop diameter, d., is in microns.
3. The liquid droplets' drag coefficient (Cp) is given by Ingebo's (Reference 8) data
in the drop Reynolds number (Re.) range of 6 to 400.
CD =
4. Particle collection is by inertial impaction only.
5. Particle concentration is uniform in any plane perpendicular to the gas flow.
6. Liquid is not atomized and distributed over a cross section until
I*,, = f Ug . (8)
where u is the relative velocity between gas and liquid and f is an empirical pro-
portionality constant.
then the single particle size penetration (one minus efficiency) may be derived for a venturi
scrubber in the following manner (References 4 and 5).
For venturi scrubbers the inertial impaction parameter is defined in terms of the rela-
tive velocity between gas and liquid droplets. Thus,
Kp= 9 Vd"= *v£ (9)
where the aerodynamic diameter of a particle is defined as
dD = (PC')1/2 d (10)
va v v
Taking into account the continuous change in ur from f u to zero, the average target effi-
ciency (n) is expressed in terms of K t = MO by integrating Equation (5) over Kp to obtain
fVt + .,.Tt^0.M.(^iI)
_ r ** ' txn4- \ /
17
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The single particle size penetration, P^., is obtained from a material balance over a differen-
tial volume and then integrating. Thus,
where Z is the length over which scrubbing occurs. Since particle impaction is operative only
when a relative velocity exists between particles (gas) and liquid droplets, Z will be given by
the length required to accelerate a liquid droplet to the gas stream velocity. Assuming a con-
stant acceleration given by C, then
(13)
where n is given by Equation (11). A plot of r\ vs. K t for f = 0.25 is shown in Figure 5.
Standard operating conditions for the Battelle molten scrubber were Q^ - 2 gpm and
Q = 70 scfm at 750°C - 800°C and 1 to 2 psig through a 1.5 inch venturi throat (Reference 1).
For these values, Q^/Q = 1.19 x 10~3, and throat gas velocity, u = 9310 cm/sec. Shown in
Figure 6 is a plot of predicted particle penetration for f = 0.25 as a function of aerodynamic
particle size with gas velocity as a parameter. Note that Pt is a strong function of particle
size in the 1 to 10 micron range, and that scrubber efficiency (1 - Pt) is low for subitvicron
particles. Figure 7 shows Pt as a function of dp with 0-/Qa as a parameter. Note here that
P. is a very strong function of the liquid-to-gas ratio. Again, scrubber efficiency is ob-
served to be quite low for submicron particles at all liquid/gas ratios.
6.2 HIGH TEMPERATURE/PRESSURE EFFECTS
The effects of temperature and pressure on predicted venturi performance are quite im-
portant (References 9 and 10). A wealth of experience has been gained with venturi operations
at temperatures < 100°C using water, however, molten scrubbing requires operation at tempera-
tures up to 900°C - 1000°C. Referring to Equations (5), (9), and (13), it is clear that the
effects of temperature and pressure on P are exhibited largely through changes in y and C1.
Strauss and Lancaster (Reference 9) have shown that changes in C1 with temperature ana pressure
are much less significant than changes in the gas viscosity, so only viscosity effects will be
considered here.
The viscosity of a gas increases with both temperature and pressure, so the effects of
increased temperature or pressure will be to decrease the inertial impaction parameter K , and
18
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0.4
0.001
10
50
Figure 5. n" versus K . for f = 0.25 (Reference 4).
pi
19
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0.01
0.3
1.0
10.0
30.0
d (pm
Figure 6. Penetration versus aerodynamic particle diameter
for Q£/Qg = 1.19 x lO'3.
20
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1.0
0.1
0.01
0.001
Q£/Qg = 0.25 x 10'3
I \ I I I I I I I
0.3
1.0
10.0 30.0
Figure 7. Penetration versus aerodynamic particle diameter
for ug = 9310 cm/sec.
21
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hence, decrease the target efficiency n- Consequently, n/u decreases and P increases with in-
y t
creasing pressure or temperature. The viscosity of air is roughly directly proportional to tem-
perature, however, pressure effects on viscosity are significant only at very high pressures
(> 100 atm) or at low temperatures (< 300°C). The effect of temperature on predicted single
particle penetration is illustrated in Figure 8 for the Battelle molten scrubber. This figure
shows P. for air as a function of temperature with the aerodynamic particle diameter as a param-
eter. The associated calculations considered only variations in gas viscosity with temperature;
changes in the Cunningham correction factor and in the liquid physical properties were neglected.
It is clear from this figure that penetration increases significantly with increasing tempera-
ture for all particle sizes.
6.3 PRESSURE DROP AND PARTICULATE COLLECTION EFFICIENCY
Assuming the pressure drop across a venturi scrubber is determined solely by the energy
required to accelerate liquid droplets to the gas velocity (wall frictional losses are negligible)
and assuming the gas velocity is constant, the pressure drop is given by
u_2 Q.
AP = 1.03 x ID'3 9n * (14)
W9
where AP is in cm H?0 and u is in cm/sec (Reference 4). Since pressure drop is the direct
^ 9
measure of the energy consumed in the scrubbing process, it is clear from Equation (14) that
additional energy would be required to increase the collection efficiency for a given particle
size and to significantly increase the efficiency of collecting fine particles (5 3 microns).
This becomes quite evident after examining the curves in Figures 6 and 7. This is discussed
further in Subsection 7.5.
22
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1.0
0.1
0.01
0.00
200
dn, = 0.3 ym (g/cm3
800 900
Figure 8. Penetration versus temperature for Q£/Qg = 1.19 x 10"3;
U = 9310 cm/sec.
23
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SECTION 7
PROCESS EVALUATION
Although no molten scrubbing schemes have yet been suggested with the primary goal of
particulate removal (Battelle's scrubber treated particulate removal as a secondary considera-
tion and the AI scheme considered particulates to be an unavoidable salt contaminant), this
technique may be advanced as such in the future, especially in instances where sulfur compound
removal is simultaneously desired. Venturi scrubbers (as well as all scrubbing methods which
rely on particle-droplet impaction for particulate collection) require considerable energy con-
sumption for efficient removal of submicron particles. For particles larger than 2 or 3 microns,
gas-liquid scrubbing is proven to be an efficient particulate collection method. On the whole,
this method remains effective even at the elevated temperatures required for fuel gas scrubbing
with molten salts. However, the use of molten salts at elevated temperatures presents a new
set of problems which warrants further attention. Some of these points of concern are discus-
sed below.
7.1 MATERIAL CORROSION
Perhaps the greatest drawback to the use of molten salts as a scrubbing liquid lies with
the high corrosiveness of the medium. Molten alkali metal carbonates at high temperatures pre-
sent quite a severe environment to virtually all materials commonly used in the construction of
processing equipment. Moreover, the presence of metal sulfides in the gas compounds this pro-
blem significantly. Based on corrosion tests, Battelle found aluminized stainless steels to be
an acceptable material and reported no problems with severe corrosion during pilot plant opera-
tions, however, the pilot plant was in operation for less than 30 hours. Based on longer term
dynamic corrosion tests, AI used an austenitic stainless steel as a balance between cost,
availability, and corrosion resistance in the construction of their stack gas scrubbing system.
However, during operation of their pilot plant AI experienced many problems with stress corro-
sion cracking of piping and transfer lines, even though the AI process operated at a relatively
low (455°C) temperature. AI also considered using aluminized stainless steel but rejected this
material because (a) the aluminum is difficult to apply uniformly thus yielding uncoated patches
and (b) the aluminizing technique is quite expensive, especially for parts with large exposed
24
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surface areas (Reference 12). At the present time there is no material that is highly resistant
to corrosion by hot molten salts and readily amenable to the common process equipment fabrica-
tion and assembly techniques at an acceptable cost.
7.2 MIST ELIMINATION
An integral part of any gas-liquid scrubbing scheme is the final separation of entrained
liquid from the cleaned gas. This is a special concern when cleaning fuel gas for eventual
turbine expansion using molten alkali metals since the allowable concentration of alkali metals
in turbine inlet gases is quite low. Both Battelle and AI experienced significant problems with
mist elimination (References 11 and 12). The Battelle data on salt carryover was presented and
discussed in Section 5. AI reported similar problems with salt crystallization and subsequent
salt carryover in wire mesh demisters.
An associated problem is that of salt vaporization and subsequent salt loss in the clean
gas. Although the vapor pressures of alkali carbonates are quite low, the vapor pressure of
alkali chlorides are significant (e.g., the vapor pressure of NaCl at 800°C is 395 ppm). The
use of alkali metal chlorides and other salts with significant vapor pressures must definitely
be avoided.
7.3 MOLTEN SALT REACTIONS
Alkali and alkaline earth metal carbonates begin to decompose to their oxides at temper-
atures around 500°C. Since the oxides of these metals have higher melting points than the car-
bonates, this decomposition increases the possibility of salt crystallization and line clogging.
Another concern is the possibility of gas reactions with the molten salt liquids. Even
though Battelle found no change in the CO, HZ, C02> 02> N2 or CH4 composition of fuel gas passing
through their molten carbonate scrubber, equilibrium thermodynamics suggest this could be a pro-
blem under other test conditions.
7.4 PARTICIPATE BUILD-UP IN THE MOLTEN SALT
For a molten particulate scrubbing process to be economically and environmentally feasi-
ble, the particulates must be removed from the spent salt melt. AI found flyash sparingly solu-
ble in molten carbonates at 450°C, however, at 800°C Battelle found that 45 to 50 wt% of carbon-
free flyash dissolved in the molten carbonates. Certainly the effects of a long-term build-up
of these soluble inorganics on the properties of the melt should be studied in greater detail
in any future process development work.
25
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Filtering of the used salt melt is required to remove the insoluble components. Other
than corrosion, AI reported no problems in filtering insolubles from the molten carbonates using
sintered stainless steel filters. However, they concluded that continuous removal of the filter
cake was necessary.
7.5 PROCESS APPLICATIONS AND ECONOMICS
Molten scrubbing has been applied to two types of emission problems over the past few
years. Atomics International developed the process for removing SOg from power plant stack
gases, and both IGT and Battelle-Northwest proposed the use of molten scrubbing for H^S and
particulate removal from hot fuel gas from a coal gasifier. We have concluded from the present
study that molten scrubbing seems most ideally suited and most appropriately applied to H?S and
particulate removal from hot fuel gases, since an additional step in the regeneration of the
molten liquid is required for SO^ removal.
At present there are no process cost estimates for a molten salt fuel gas scrubber. The
only cost estimates available were prepared by AI for their flue gas S0? scrubbing concept.
These, of course, would not apply to a proposed fuel gas particulate scrubber because of the
different gas volumes and salt regeneration requirements. However, for an order of magnitude
illustration AI estimated in 1971 (Reference 3) that, for a 800 MW power plant (3.6 x 106 cfm)
capital requirements would be $17/kW and annual operating costs would be 0.95 mills/kWh (3.53
mills/Mcf).
In the absence of other data, the relative operating costs for gas cleaning systems may
be estimated from the energy requirements of these systems. For atomized liquid scrubbers such
as venturi scrubbers the energy required for particle scrubbing is essentially proportional to
the gas pressure drop through the scrubber. Calvert (Reference 13) has developed a relation-
ship between scrubber pressure drop and the aerodynamic particle diameter at which penetration
is 50 percent (dp ) for a variety of gas scrubbing schemes. Figure 9 (Reference 14} shows
dp as a function of gas pressure drop for a sieve plate, impingement plate, packed column,
and venturi scrubber. Also shown are the theoretical power requirements for each of these tech-
niques. The plot shows that the energy efficiency of venturi scrubbing is comparable to that
of other scrubbing schemes, and is, in fact, more energy efficient for collecting smaller,
submicron, particles. The figure indicates that, for collecting particles in the 0.5 to 2.0
micron range, energy requirements for venturi scrubbers are 1 to 10 kW/Mcfm. It is indeed sig-
nificant that the amount of energy required for scrubbing increases quite strongly as the par-
ticle size decreases.
26
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Theoretical power (kW/Mcfm)
5.0
0.2
2.0
i.o
0.5
IO
a.
0.2
0.1
No.
1
2
3
4
0.5 1.0
2.0
5.0 10.0
Sieve
Venturi, f = 0.25
Impingement plate
Packed column
I
I
10 20 50
Pressure drop (cm
1
100 200
20.0
500
Figure 9. Theoretical power and pressure drop versus aerodynamic cut
diameter (Reference 14).
27
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Typical energy usages for other particulate control systems are (Reference 15):
• Cyclones — 1 to 7 kW/Mcfm
• Fabric filters - 3 to 4 kW/Mcfm
• Electrostatic precipitator - 1.5 to 2 kW/Mcfm
Indeed, the energy costs for venturi scrubbing are comparable to those of other gas cleaning
methods, and may be lower for smaller particles. Therefore, from an economic standpoint,
molten scrubbing appears promising as a high temperature fine particulate clean-up process.
28
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REFERENCES
1. Moore, R. H., Allen, C. H., Schiefelbaum, G. F., and Maness, R. F., "A Process for Cleaning
and Removal of Sulfur Compounds from Low Btu Gases," OCR/R&D 100/Int. 1, NTIS PB 236 522,
August 1974.
2. Telephone interview of Mr. Dennis Duncan, Institute of Gas Technology, Chicago, Illinois,
January 7, 1976.
3. Atomics International, "Development of a Molten Carbonate Process for Removal of Sulfur
Dioxide from Power Plant Stack Gases," APTD-0752, NTIS PB 203 466, July 1971.
4. Calvert, S., Goldschmid, J., Leith, D., and Mehter, D., "Wet Scrubber System Study, Vol. 1,
Scrubber Handbook," EPA-R2-72-118a, July 1972.
5. Calvert, S., "Venturi and Other Atomizing Scrubbers Efficiency and Pressure Drop," AIChE. J.,
Vol. 16, No. 3, pp. 392 - 396, May 1970.
6. Walton, W. H. and Woolcock, A., Intern. J. Air Pollution, Vol. 3, p. 129, 1960.
7. Nukiyama, S. and Tanasawa, Y., Trans. Soc. Mech. Engrs. (Japan), Vol. 4, p. 86, 1938.
8. Ingebo, R., NASA Tech. Note 3762, 1956.
9. Strauss, W. and Lancaster, B. W., "Prediction of Gas Cleaning Methods at High Temperatures
and Pressures," Atmospheric Environ., Vol. 2, pp. 135 - 144, 1968.
10. Thring, M. W. and Strauss, W., "The Effects of High Temperature on Particle Collection
Mechanisms," Trans. Inst. Chem. Eng., Vol. 41, p. 248, 1963.
11. Telephone interview of R. H. Moore, Battelle-Northwest Laboratories, Richland, Washington,
January 6, 1976.
12. Telephone interview of R. D. Oldenkamp, Atomics International Division, Rockwell Interna-
tional, Canoga Park, California, January 19, 1976.
13. Calvert, S., "Engineering Design of Fine Particle Scrubbers," J. APCA, Vol. 24, No. 10,
October 1974.
14. Calvert, S. and Yung, S., "Evaluation of Petersen Scrubber," APT, Inc. report prepared for
EPA Contract 68-02-1328, Task 12, June 1975.
15. Hegarty, R. and Shannon, L. J., "Evaluation of Sonics for Fine Particle Control," EPA-600/
2-76-001, January 1976.
Supplementary Bibliography
1. Plyler, E. L. and Maxwell, M. A., "Proceedings, Flue Gas Desulfurization Symposium 1973,
New Orleans, Louisiana," EPA-650/2-73-038 (NTIS PB 230 901), December 1973.
2. Singmaster and Breyer, New York, "An Evaluation of the Atomics International Molten Carbo-
nate Process," NTIS PB 207 190, November 1970.
3. Atomics International, Canoga Park, California, "Development of a Molten Carbonate Process
for Removal of Sulfur Dioxide from Power Plant Stack Gases. Part I. Process Chemistry-
Reduction," NTIS PB 191 957, October 1968.
4. ibid., " Part II. Process Chemistry-Regeneration," NTIS PB 191 958, October 1968.
5. ibid., " Part III. Materials Studies," NTIS PB 191 959, October 1968.
6. ibid., " Part IV. Contractor Development," NTIS PB 191 960, October 1968.
7. Mandelin, D. J., "Removing Sulfur Dioxide from Gas Streams using Molten Thiocyanates,"
Patent No. 3773893, Occidental Petroleum Corporation, February 1970.
29
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-067
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Evaluation of Molten Scrubbing for Fine
Particulate Control
5. REPORT DATE
March 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
G.G. Poe, L.R. Waterland, and
R.J. Schreiber
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Aerotherm Division/A cur ex Corp.
485 Clyde Avenue
Mt. View, California 94042
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-029
11. CONTRACT/GRANT NO.
68-02-1318, Task 24
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AJM
Task Final; 1
PERIpp_CpVERED
14. SPONSORING AGENCY CODE
EPA/600/13
is.SUPPLEMENTARY NOTES IERL.RTP task officer for this report is D.C. Drehmel, Mail
Drop 61, 919/549-8411 Ext 2925.
16. ABSTRACT
repOrj. gjves results of an evaluation of molten scrubbing for fine parti-
culate control, a concept that study results indicate as seeming to be feasible. Appli-
cation of the concept to fine particulate clean-up in advanced energy processes seems
possible. Molten scrubbing is especially well-suited to processes where simultan-
eous removal of sulfur compounds is desired. However, before effective molten
scrubbing systems can be developed for particulate removal, two important problems
need to be solved: (1) finding construction materials at an acceptable cost which can
adequately withstand the highly corrosive scrubbing medium presented by hot molten
liquids; and (2) improving gas/liquid separation and mist eliminator designs so that
liquid carryover satisfies emission standards or gas turbine inlet specifications.
Based on the report's observations and on the above conclusions , it appears that
considerable development work would be required to investigate the aforementioned
problems before a final assessment of the feasibility of this concept could be made.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Dust
Scrubbers
Fused Salts
Desulfurization
Gas Turbines
Air Pollution Control
Stationary Sources
Particulate
Molten Scrubbing
13B
11G
07A
07D
13G
3. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
36
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
30'
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