EPA-R2-73-253
June 1973
Environmental Protection Technology Series
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EPA-R2-73-253
REDUCTION OF ATMOSPHERIC
POLLUTION BY THE APPLICATION
OF FLUIDIZED-BED
COMBUSTION AND REGENERATION
OF SULFUR-CONTAINING ADDITIVES
by
G.J. Vogel, E.L. Carls, J. Ackerman, M. Haas,
J. Riha, C.B. Schoffstoll, J. Hepperly, and A.A. Jonke
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439
Argonne No. ANL/ES-CEN-1005
Interagency Agreement EPA-IAG-0020
Program Element No. LA.2013
EPA Project Officer: D.B. Henschel
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, B.C. 20460
June 1973
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
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TABLE OF CONTENTS
Page
ABSTRACT 7
I. SUMMARY 7
II. INTRODUCTION 14
III. BENCH-SCALE, ATMOSPHERIC COMBUSTION EXPERIMENTS 16
A. Materials 16
1. Coal 16
2. Oil 16
3. Additives 16
4. Starting Bed Material 16
B. Equipment and Procedure 16
C. Results and Discussion 19
1. Air-Deficient Coal Combustion Experiments 19
2. Oil Combustion Experiments with an Excess of Oxygen. 27
3. Cyclone Collection Efficiencies during Coal
Combustion Experiments 32
4. Combustion Efficiencies for. Coal, Oil, and Natural
Gas Combustion with Excess Air 32
IV. REGENERATION OF SULFUR-CONTAINING ADDITIVES 34
A. Thermodynamic Analysis of Some Schemes for Regenerating
Partially Spent Additive from the Fluidized-Bed
Combustion of Coal 34
1. Thermal Decomposition of Calcium Sulfate 35
2. Reductive Decomposition of Calcium Sulfate 38
3. Roasting of Calcium Sulfide 45
4. Pressure Effects in the C0/C02 and H2/H20 Systems. . 48
5. Acid-Base Reaction of Calcium Sulfide with H20 and
C02 48
B. Experimental Studies 51
1. Reductive Decomposition of CaSO^ 51
2. Two-Step Process 52
V. PRESSURIZED COMBUSTION AND REGENERATION PILOT PLANT 64
A. Description 64
B. Petrocarb Solids Feeder Tests 69
VI. ACKNOWLEDGMENTS 72
VII. REFERENCES 72
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LIST OF FIGURES
No. Title Page
1 Simplified Equipment Flowsheet of Atmospheric Bench-Scale
Fluidized-Bed Combustor and Associated Equipment 18
2 Equipment for Feeding Oil to the 6-in.-dia Combustor 20
3 Effect of Air Feed Rate on Percent of Sulfur in Off-Gas as H2S 22,
4 Sulfur Retention in Oxygen-Excess and Oxygen-Deficient
Experiments 24
5 Effect on Sulfur Retention of Air Feed Rate to First Stage . . 24
6 Effect of Air Feed Rate and Fluidized-Bed Temperature on NO
Concentration in Off-Gas from the First Stage During Combus-
tion of Coal 24
7 Effect of Temperature on Sulfur Retention during Combustion
of Residual Fuel Oil in Excess Air 29
8 Effect of Ca/S Mole Ratio on Sulfur Retention during Combus-
tion of Residual Fuel Oil in Excess Air 30
9 Pressure of S02 in Equilibrium with C0/C02 Mixtures as a
Function of Temperature 39
10 Temperature and CO/C02 Conditions for Formation of CaSO^,
CaS03, and CaS 40
11 Equilibrium Dissociation Pressure of CaCO^ as a Function of
Temperature 41
12 82 and COS Concentrations at Equilibrium as a Function of
Temperature and CO/C02 Ratio 44
13 Pressure of S02 in the CaSO^-CaS-I^-I^O System 46
14 S2 and H2S Pressures in the System 52-1128-112-1120-0350^-035 . . 47
15 Equilibrium Constant as a Function of Temperature
C02 + H20 + CaS ->• CaC03 + H2S 49
16 Pressure of H2S in Undried Gas Stream as a Function of
Temperature at 10-atm P , 49
total
17 Schematic of Apparatus for Equilibrium Measurements on
CaSOit-CO/C02 System 51
18 Experimental and Calculated Partial Pressures of S02 Over a
Range of Prn/Prn Ratios 53
CO CO 2
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No. Title Page
19 Two-Inch-Diameter Fluidized-Bed Reactor 54
20 Sulfide Content of Bed During Reduction of Partially
Sulfated Dolomite with Hydrogen at Various Temperatures. ... 55
21 Effect of Temperature on Reduction of CaSO^ 55
22 Typical I^S Levels in the Outlet Gas for C02/H20 Regeneration. 56
23 H2S Concentration in Effluent Gas Stream in Regeneration Step. 59
24 Photomicrograph Showing Cross Sections of Particles from
Cyclic Experiment X100 62
25 Photomicrograph Showing Cross Sections of Particles from
Cyclic Experiment X100 63
26 Simplified Schematic of Pressurized Combustion-Regeneration
Equipment 64
27 Six-inch Fluidizing Gas Preheater 65
28 Six-inch Dia Pressurized Fluidized-Bed Combustor 66
29 Three-inch Dia Fluidized-Bed Regenerator 68
30 Model 16 ABC Petrocarb Injector 70
31 Cross Section of Mixing Valve Assembly 71
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LIST OF TABLES
No.
1
2
3
4
5
6
7
8
9
10
11
Title
Some Chemical and Physical Characteristics of Coal from
Properties of Esso Residual Fuel Oil Used in Combustion
Carbon Balances and Carbon Contents of Solids in Coal
Operating Data and Results, Combustion of Residual Fuel Oil
Particle Size Distributions and Bulk Densities for Solids
Pressure of 803 and S02 in Equilibrium with CaSO
HaS Concentration in Dried and Undried Product Gas Stream
Sieve Size Analysis of Starting Bed and Final Bed After
Page
17
17
26
28
32
36
38
50
57
60
63
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REDUCTION OF ATMOSPHERIC POLLUTION BY THE
APPLICATION OF FLUIDIZED-BED COMBUSTION
AND REGENERATION OF SULFUR-CONTAINING ADDITIVES
Annual Report
July 1971—June 1972
by
G. J. Vogel, E. L. Carls, J. Ackerman, M. Haas,
J. Riha, C. B. Schoffstoll, J. Hepperly, and A. A. Jonke
ABSTRACT
Fluldized-bed combustion is being studied as a means of
removing from the gas phase nearly all of the atmospheric
pollutants (sulfur and nitrogen compounds) generated during
the combustion of fossil fuels. Particulate lime solids
(additives) are introduced into the fluidized bed and react
with the sulfur compound formed during combustion. Information
has been obtained on (1) pollution control by fluidized-bed
combustion of oil with an excess of air and by the combustion
of coal with a deficiency of air, (2) the thermodynamics of
several proposed processes for regenerating additives, and
(3) experimental regeneration of sulfur-containing additive
by the two most promising processes—a one-step reductive
decomposition of CaSO^ and a two-step (reduction-C02/H20
regeneration) procedure.
I. SUMMARY
Argonne National Laboratory is investigating pollution control aspects
of fluidized-bed combustion in a program funded by the Control Systems
Division of the Environmental Protection Agency. The program consists of
investigating (1) the effects of operating variables in reducing pollutant
emissions when fossil fuels are combusted in a fluidized bed of a sulfur-
acceptor additive and (2) the regeneration of this additive for reuse and
the recovery of the sulfur.
Fossil fuels have been combusted in fluidized beds of either limestone
or dolomite and in either an excess or a deficiency of oxygen. Combustion
with an excess of oxygen is typical of utility and process steam plant
operations. Oxygen-deficient combustion might have potential for producing
a low-Btu gas that can be burned in gas turbines in a combined cycle
plant.
When sulfur contained in coal is released during combustion in an
oxygen-excess environment, CaSO^ is formed; in an oxygen-deficient environ-
ment, CaS is formed. For regenerating CaSO^, two favored regeneration
methods are being studied. One is reductive decomposition of CaSO^ to CaO
(or to CaC03 in 10-atm operation) and S02 at ^2000°F. A relatively concen-
trated stream of S02 is produced which can be processed in a sulfur recovery
plant. The other method is a two-step process in which CaSOit is reduced to
CaS at ^1600°F and then the CaS is reacted with C02/H20 at vL200°F to
release H2S for sulfur recovery.
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Equipment
The bench-scale, atmospheric-pressure fluidized-bed combustion equip-
ment consisted of a 6-in.-dia, 6-ft-long combustor with ancillary equip-
ment including gas supplies, control valves and metering systems, a gas
preheater, mechanical powder feeders for introducing coal and additive, and
an off-gas particulate removal system consisting of two cyclones and a
final filter.
During the year, the atmospheric unit was replaced with a combustion
system designed for 10-atm operation, and testing of this equipment has
started. The pressurized combustor is a 6-in.-dia unit with a pressurized
shell surrounding the heated zone. Additionally, a 3-in.-dia pressurized
regenerator has been constructed that will be operated separately from the
combustor. The same gas-feed and same off-gas equipment will be used for
the new combustor and the regenerator, though not simultaneously.
Equilibria of chemical reactions have been studied in a laboratory-
scale static system, using a horizontal, 3-in.-dia reactor. The two-step
regeneration reactions have been studied in a 2-in.-dia, batch fluidized-
bed reactor equipped with the necessary gas supplies, particulate handling
equipment, and analytical systems.
Bench-Scale Combustion Experiments at One Atmosphere Pressure
Combustion of coal in a deficiency of air has been studied to obtain
data on sulfur species formed in the gas phase and bed, on NO levels in
the flue gas, and carbon losses from the combustor. Oil combustion studies
with excess air present have explored the effects of the main operating
variables on sulfur retention in the bed and NO levels in the flue gas;
combustion efficiencies for oil combustion and coal combustion experiments
have been compared.
Coal Combustion Experiments with a Deficiency of Oxygen. Coal com-
bustion experiments were made at fluidized-bed temperatures of 1450-1650°F
and a range of air feed rates. When the air flow rate was decreased
(starting from a slightly oxygen-excess condition), the following were
observed:
1. The ratio, H2S/(H2S+S02), increased in the effluent gas.
2. Sulfur retention apparently decreased initially. With
further decreases in air feed rate and thus a more oxygen-
deficient condition, H2S formation increased and sulfur
retention rapidly increased. It is postulated that since
oxygen is required for form CaSOi,, decreasing the amount
of oxygen fed decreased the chance for CaSOit to form.
As a result, more of the S02 released during combustion
left in the flue gas, lowering sulfur retention. As the
quantity of oxygen fed decreased further, H2S was mainly
formed. H2S reacts rapidly with additive.
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3. The CO and hydrocarbon concentrations in the flue gas
increased.
4. The sulfide content of the bed increased. Sulfite in
low concentrations was also found in the bed.
5. The carbon content of the bed increased as did the
quantity of carbon elutriated from the bed.
6. The NO level in the flue gas (<200 ppm) was lower than
in the oxygen-excess experiments (>300 ppm). With an
oxygen deficiency, less NO appeared to be formed at
higher combustion temperatures and less at greater
oxygen deficiencies.
A two-stage combustion concept appears to have good prospects for low
sulfur emissions. In this concept, the first-stage fluidized bed is operated
under conditions that favor H2S formation (i.e., feeding of air at rates
constituting large oxygen deficiencies). In the second stage, combustible
gases from the first stage are burned with excess air. Prospects that NO
emission would be decreased by the two-stage combustion concept seem good,
but it has not yet been determined whether the gas leaving the first stage
contains nitrogen compounds other than NO and whether such compounds would
be oxidized to NO in the second stage.
Oil Combustion Experiments with an Excess of Oxygen. Residual fuel
oil (containing 1.97 wt % S) was burned in an excess of oxygen. Operating con-
ditions were bed temperatures of 1450 to 1650°F, Ca/S mole ratios up to
11.9, a gas velocity of ^3 ft/sec (except for one experiment at 5.5 ft/sec),
and 3 vol % oxygen in the flue gas (except in one experiment with 1 vol %
oxygen in the flue gas).
The following observations were made on the basis of experimental
results:
1. The effect of temperature on sulfur retention was similar to that
observed in coal combustion experiments—i.e., there is a temperature
yielding maximum sulfur retention. In the oil-combustion experiments,
maximum sulfur retention was at 1500-1550°F.
2. The shape of the curve for sulfur retention as a function of Ca/S
mole ratio resembles that obtained in coal combustion experiments. In the
oil combustion runs, sulfur retention increased as Ca/S mole ratio was
increased to about 5, then leveled off at 90% as the Ca/S ratio was increased
further. The slope of the curve for sulfur retention as a function of Ca/S
ratio in the oil combustion runs is less steep than the slope for the
combustion of Illinois coal (3.7 wt % sulfur) at similar operating conditions.
3. When oil was combusted (in the experiments with 3% 0 in the flue
gas), the NO levels in the flue gas ranged from 110 to 150 ppm. This may
be compared with the 400-800 ppm range observed when coal was burned.
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Cyclone Collection Efficiencies During Coal Combustion Experiments.
Data on par tide-removal efficiency of the ANL cyclone separators were
obtained as a basis for estimating the dust loading and the needed filter
area of a cartridge filter for the recently constructed pressurized combus-
tion bench-scale plant. In the atmospheric-pressure system, flue gas passed
through two cyclones in series, which have been modified for operation with
the pressurized equipment, and a final filter.
Collection efficiencies (defined as ratio of the weight of particles
removed to the weight of particles entering the cyclones) were compiled for
26 earlier ANL experiments in which the dust loadings at the combustor exit
were 0.16 to 1.78 g/ft3.* The combined efficiency of the two cyclones was
above 80% in 24 of the 26 experiments and above 90% in 21 of the experiments.
The dust loading in the flue gas leaving the second cyclone averaged 0.06
g/ft3 for the 26 runs; the maximum loading was 0.22 g/ft3.
Combustion Efficiencies for Coal and Oil Combustion with Excess Air.
The combustion efficiency for experiments performed in the combustor
was determined as the ratio of carbon burned to carbon fed, multiplied
by 100. Carbon loss was calculated by determining unburnt carbon leaving
the system by three routes: (1) carbon associated with the elutriated
solids, (2) incompletely burned gases, i.e., carbon monoxide and hydro-
carbons, and (3) carbon associated with fluidized-bed material taken from
the system. In no experiment was there recycle of fines.
Combustion efficiencies were 93 to 96% in ten coal-combustion experi-
ments with oxygen concentrations in the flue gas of about 3% and a
fluidized-bed temperature of 1600°F. In all experiments, carbon losses in
the bed material were negligible. Only about 10-20% of the carbon loss
was due to the formation of carbon monoxide and hydrocarbons. The major
carbon loss (80-90%) occurred as a result of elutriation of incompletely
combusted fine particles in the exhaust gases.
Combustion efficiency in oil-combustion experiments (bed temperatures
ranged from 1450-1650°F) was similar to that observed for coal combustion
experiments, ranging from 94 to 96% for experiments with 3% 02 in the flue
gas. Combustion efficiencies were lower when there was less excess oxygen
in the flue gas and when gas velocities were higher. In oil combustion,
the major carbon loss occurred as a result of incomplete burning of the
CO and hydrocarbons formed during combustion.
Regeneration of Sulfur-Containing Additives
When coal is burned in a fluidized bed containing limestone or
dolomite additive, the product of the reaction between sulfur-containing
gases and additive is calcium sulfate if combustion is carried out under
oxidizing conditions or calcium sulfide if combustion is carried out with
a deficiency of air.
Several regeneration processes are under consideration. These are
All volume measurements, unless indicated otherwise, are at 70°F and
one atmosphere absolute pressure.
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1. Reductive decomposition of calcium sulfate to form CaC03
and S02 (under the proposed operating conditions).
2. Roasting of calcium sulfide in air or oxygen to form S02
and CaO (or CaC03).
3. Reaction of calcium sulfide with steam and carbon dioxide
to form H2S and CaC03.
Processes 2 and 3 can be used to regenerate not only calcium sulfide,
but also material containing calcium sulfate if the sulfate is first reduced
to the sulfide.
Another alternative is to thermally decompose the CaSOu, but too high
a temperature would be required for this process if a high-S02-content gas
were to be obtained. For example, at 2500°F, the S02 equilibrium pressure
is only 0.06 atm.
The thermodynamics of the above processes were considered, and the
yields of gaseous sulfur-containing products, the compositions of solid
phases, and the variations of these yields and compositions with temperature,
pressure, and gas composition for a system at equilibrium were calculated.
Reductive Decomposition of CaSOu. To test the accuracy of the equilib-
rium compositions calculated for the reduction of CaS04 with C0/C02 mixtures,
experiments have been performed in a static system. The apparatus con-
sists of a horizontal tube reactor fabricated from recrystallized alumina
and equipped with the required gas supplies and sampling points. The tube
is 36 in. long and has an ID of 3 in. with 1/4-in. walls. One end of the
alumina reactor is closed, and the opposite end is capped with a stainless
steel 0-ring flange. Except for the flanged end, the tube is enclosed
within a furnace for temperature control.
Experimental results show good agreement, in most cases, between
experimental and calculated values of S02 levels when CaSO^ was reacted
with C0/C02 over a range of ratios. The data also indicate that CO and S02
were being slowly removed by a secondary process. Analyses of the gas
samples indicate that reaction was occurring in the ratio of one mole of
S02 to one mole of CO, forming one mole of C02 during the period of equi-
libration at 1900°F (the other reaction product was probably elemental
sulfur, which was found at the cooler end of the alumina tube).
Two-Step Process. A second regeneration procedure of interest is to
first reduce the CaSOi, to CaS at 1600-1700°F and then to react the CaS at
1000-1300°F with C02-H20 to form CaC03 and H2S. Experimental data on these
process steps were obtained in a 2-in.-dia batch fluidized-bed reactor.
Conversion of CaSO^ to CaS. The effect of temperature on the rate of
reduction of CaSOij to CaS with H2 and CO was studied. Results showed that
with partially sulfated dolomite and approximately three stoichiometric
equivalents of hydrogen added over the 5-hr period, less of the bed material
was converted to sulfide at 1350°F than at either 1450°F or 1600°F at
equivalent times. Percent conversions after 4.5 hr at 1350°F, 1450°F, and
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1600°F were 18%, 38%, and 86%, respectively. An additional experiment was
performed with CO, and conversion using CO agreed with the expected conver-
sion using H2 at the temperature employed (1500°F).
Reaction of CaS with C0?/H?0. The product of each of the reduction
experiments was carbonated (i.e., the unreacted CaO was reacted with C02
to form CaC03) at 10 atm to simulate a product that would be obtained in
an actual 10-atm combustion-reduction experiment. This material was then
reacted batchwise with an equimolar mixture of C02/H20 at 900 to 1100°F,
a gas velocity of approximately 1 ft/sec, and 10-atm pressure, and the
H2S concentration in the outlet gas was monitored.
The results showed that:
1. The reaction producing H2S was initially rapid, but the rate
decreased after a short time. Typically, the reaction rate dropped nearly
to zero after several minutes.
2. The peak concentration of H2S in the outlet gas was high, near
the expected equilibrium value.
3. Typically, half or less of the CaS reacted.
In continuing work, the effects of process variables are being studied
in an attempt to increase the quantity of CaS reacted.
Sulfation-Regeneration Cyclic Experiments. Since it will be desirable
to reuse additive material a number of times in commercial applications, a
cyclic experiment has been performed to obtain data on the pickup and
removal of sulfur from additive particles and to measure decrepitation and
attrition of additive particles during sulfat ion-regeneration cycles. Six
cycles of simulated combustion and two-stage regeneration were performed
with a single bed of additive. The starting material (1.2 kg) was part of
the final bed from a coal combustion experiment in which dolomite No. 1337
had been used as additive. The initial sulfur content of this material
was 15.4 wt %. The experiment was performed batchwise in the 2-in.-dia
fluidized-bed reactor and a cycle consisted of sulfation (in all except
the first cycle), reduction of the CaSOif to CaS with H2 or CO reductant,
and reaction with C02/H20 gas mixture to convert the CaS to CaCO^. A
sample of the bed material was taken after each step in the cycle and
analyzed for sulfur and sulfide content.
The data for the regeneration step showed that the peak concentration
of H2S in the effluent gas was 13 vol % (dry basis) in cycle 1 and decreased
to 0.5 vol % (dry basis) for cycles 5 and 6. The percentage of the calcium
sulfide converted to CaCO decreased to a very low indeterminant value after
several cycles. It appears from these data that a layer of material of low
permeability is built up on or within pores of the additive particles, in-
hibiting the removal of sulfur. The high sulfur loading of the bed particles
in these experiments may be a factor in poor conversion.
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Sulfiding-Regeneration Experiments. Because the presence of
might have contributed to the poor sulfur removals noted in the sulfation-
regeneration experiment discussed above, several cyclic experiments were
performed with dolomite or limestone starting beds in which the CaC03 con-
stituent was reacted directly with H2S, and then the product CaS was reacted
with C02/H20. The MgC03 in the dolomite bed was calcined before the H2S
reaction. Preliminary experimental results showed that:
1. The regeneration reaction was initially rapid, but slowed after
a short time.
2. The peak concentration of H2S in the outlet gas during regenera-
tion was near the expected equilibrium value in the first cycle.
3. The shapes of the curves for sulfur removal from the gas phase
by limestone and dolomite during H2S sorption tests appeared to be similar—
sulfur was initially removed at a high rate with each type of sorbent, and
not all sulfur was removed.
Pressurized Combustion and Regeneration Pilot Plant
Description. Equipment has been installed for combusting coal at
pressures up to 10 atm and for regenerating sulfated lime for reuse. The
equipment consists of a regenerator and fluidized-bed combustor which
have a common off-gas system (cyclones, filters, gas-sampling equipment,
pressure let-down valve, and scrubber). The two vessels will not be
operated simultaneously.
The combustion unit consists of a 6-in. schedule 40 pipe (Type 316
stainless steel), approximately 11 ft long, with an outer shell consisting
of 12-in. schedule 10 pipe (Type 304 stainless steel) over nearly the entire
length. A bellows expansion joint is incorporated into the outer shell to
accommodate differential thermal expansion of the inner and outer vessels.
The regenerator has a 3-in. ID. It consists of a 2 1/2-in. layer of
Plibrico castable refractory encased in an 8-in. schedule 40 pipe (316
stainless steel). This entire assembly is enclosed in a pressure shell
made of 12-in. schedule 20 carbon steel pipe.
Petrocarb Solids-Feeder Tests. The suitability of a modified-standard
Model 16 ABC Petrocarb solids feeder for feeding coal or limestone to a
pressurized combustor was tested. Uniform feed rates in the range of 20
to 100 Ib/hr are required in the high-pressure combustion system.
Tests were made at atmospheric and at ^80 psig pressure. Higher feed
rates, in general, gave more uniform solids flow than did lower ones; sized,
dried coal (1.2 wt % moisture, 20-200 mesh) flowed more evenly than as-
received coal (4.0 wt % moisture, -14 mesh). A 5/16-in. orifice in the
mixing valve was the smallest that could be used with -14 mesh coal without
sporadic plugging. When the length of the transport line was increased,
lower feed rates were achieved. However, results of these tests showed
that the feeder was not suitable for feeding at the lower feed rates.
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II. INTRODUCTION
Methods are being developed for lowering the concentrations of noxious
pollutants emitted from power- and steam-producing plants to meet standards
set by state and federal governmental agencies. Progress is reported here
on a continuing study of the removal of pollutant (S02, NO, particulate)
from the gas phase in the combustion of fossil fuels (such as coal, oil,
and natural gas) . The concept investigated involves introducing fuel and
a sulfur-reacting additive material such as crushed limestone or dolomite
into a hot fluidized bed of solids.
Two different combustion modes are possible, one with complete and the
other with partial combustion of the fuel in the fluidized bed. In the
complete-combustion mode (also called one-stage or oxygen-excess combustion) ,
oxygen in excess of the stoichiometric amount required to burn the fuel to
CC>2 and H20 is fed to the fluidized bed. In the second mode (called two-
stage or oxygen-deficient combustion) , a deficiency of air is added to the
fluidized bed, and the resulting H2 , CO and gaseous hydrocarbons are com-
busted to C02 and H20 by providing additional oxygen (air) either in the
region above the bed or in a separate combustor.
In the oxygen-excess combustion mode, the sulfur forms S02 , which reacts
with the calcium in limestone or dolomite to form
CaC03 + S02 + 1/2 02 •* CaSOi, + C02
In the oxygen-deficient combustion mode, the sulfur forms H2S, which reacts
to produce CaS
CaC03 + H2S -> CaS + C02 + H20
Since relatively large quantities of limestone (compared to the quantity
of coal ash) are required to efficiently remove the sulfur, regeneration
of the sulfated or sulfided lime or dolomite will probably be desirable.
Various regeneration schemes have been proposed, but the thrust of the
program is toward selecting one of two processes. The first is one-step
reductive decomposition of CaSO^ at ro2000°F.
CaSOit + CO -> CaC03 + S02
The second is a two-step scheme — reduction of CaSO^ to CaS at 1600-1700°F,
followed by reaction of the CaS with C02/H20 at 1000 to 1300°F to release
H2S
CaS04 + 4 CO -> CaS + 4 C02
CaS + C02 + H20 -»• CaC03 + H2S
Operation of the combustor system at 10-atm pressure is desirable in order
to take advantage of advanced power cycle concepts. Here, pressurized gas
from the combustor would operate a gas turbine to recover energy. Operation
of the regeneration reactor at 10-atm pressure is desirable in order to
match the pressure in the 10-atm combustor.
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15
The Argonne National Laboratory (ANL) work on the control of pollutant
emissions during fluidized-bed combustion of coal in excess oxygen at 1 atm
pressure has been reported in previous annual reports in this series. The
ANL work reported here has consisted of three major parts—(1) combustion
studies, (2) regeneration studies, and (3) construction of a bench-scale,
pressurized combustion and regeneration system. Coal was burned in one set
of experiments in a deficiency of oxygen, and in another set of experiments
oil was burned in an excess of oxygen (all experiments at atmospheric pressure).
The oil combustion experiments complemented previous coal combustion
tests made in excess oxygen. Regeneration studies have consisted of exam-
ination of the equilibria of different processes, experimental studies to
determine equilibrium 862 levels in the reductive decomposition reaction,
and experiments on the two-step process. Construction of the 10-atm regen-
erator and combustor pilot plants has been completed, and testing of the
units has started.
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16
III. BENCH-SCALE, ATMOSPHERIC COMBUSTION EXPERIMENTS
Objectives of the coal combustion and oil combustion experiments re-
ported below were to determine the effects of different operating conditions
on sulfur retention in the bed and NO levels in the flue gas, as well as
combustion efficiencies. An additional objective of the coal combustion
experiments was to determine the sulfur species formed in the gas phase and
bed.
A. Materials
1. Coal
Two shipments of Illinois coal were used in coal combustion ex-
periments, both from Peabody Coal Co. Mine 10, Seam 6, Christian County,
Illinois (furnished by Commonwealth Edison).
Data on the chemical and physical characteristics of coal samples
from the fourth shipment are presented in Table 1. The coal contained
4.1 wt % S (Table 1). Data on samples from the third shipment (see
ANL/ES-CEN-10041) showed that chemical characteristics were similar to
the fourth shipment. The average particle diameter was smaller than in
the fourth shipment, 350 ym and 429 urn.
2. Oil
The residual fuel oil used in the oil combustion experiments con-
tained 1.97 wt % sulfur and had an SSF (Saybolt Seconds Furol) viscosity
of 162.5 at 122°F and a flash point of 178°F. Other properties of this
oil are listed in Table 2. It was obtained from Esso.i"
3. Additives
Limestone No. 1359, used in the experiments reported here, was
obtained from M. J. Grove Lime Co., Stephens City, Va. and contained ^95
wt % CaCOs and vL wt % MgC03. The average particle size was 609 urn.
4. Starting Bed Material
In most of the bench-scale experiments performed during the report
period, the starting bed was calcined and partially sulfated limestone.
B. Equipment and Procedure
The major equipment item of the bench-scale combustor system (shown
in Fig. 1 and described in detail in pp. 21-23 and Appendix B, ANL/ES-CEN-1004)
is a vertical 6-in.-dia, 6-ft-high stainless steel combustor. To permit
control of the combustor temperature during an experiment, three 230-V
resistance heaters and four annular air-cooling chambers are mounted on
the wall at the combustion zone (the lower 24 in. of the combustor).
We are indebted to Mr. A. Skopp of Esso Research and Engineering Company
for obtaining the oil for us.
-------
17
TABLE 1. Some Chemical and Physical Characteristics of Coal
from Fourth Shipment from Commonwealth Edison Co.
Source of Coal:
Mine 10 (Peabody Coal Co.), Seam 6,
Christian County, Illinois
Proximate Analysis, wt %
Ultimate Analysis, wt %
Moisture
Ash
Volatile Matter
Fixed Carbon
Heating value 11
6.00
12.70
40.53
40.76
,304 Btu/lb
Carbon
Hydrogen
Nitrogen
Chlorine
Sulfur
Oxygen
Ash
62.94
4.50
1.25
0.0
4.09
8.48
12.70
Sieve Analysis
U.S. Sieve No.
-14 +25
-25 +35
-35 +45
-45 +80
-80 +170
-170 +325
-325
wt %
22.9
11.1
11.0
15.8
15.5
9.5
14.3
Average of two samples.
TABLE 2. Properties of Esso Residual Fuel Oil
Used in Combustion Experiments
API Gravity
Viscosity, SSF3 at 122°F
Bottoms, Sediment and Water
Flash Point
Pour Point
Flow Point
Sulfur
14.6
162.5
0.5%
178°F
+15 °F
+25°F
1.97%
Saybolt Seconds Furol
-------
18
TO
GAS-ANALYSIS
SYSTEM
TO GLASS FIBER
FINAL FILTER AND
VENTILATION EXHAUST
SECONDARY CYCLONE
PRIMARY CYCLONE
PREHEATER
Fig. 1. Simplified Equipment Flowsheet of Atmospheric Bench-Scale
Fluidized-Bed Combustor and Associated Equipment. ANL
Neg. No. 308-2870.
Fluidizing air for the combustor that has been preheated in a 3-in.-dia,
7-ft-long electrically heated, packed pipe is fed into the combustor through
a bubble cap gas distributor mounted on the bottom flange. Coal and additive
are continuously fed into 3/8-in. stainless steel lines by variable-drive
volumetric screw feeders mounted on scales. These solids are fed pneumat-
ically (entrained in a transport air stream) into the fluidized bed at a
point just above the gas distributor plate. Coal and fresh limestone are
metered as they are fed. Flue gas leaving the combustor passes through &
particle-removal system consisting of two in-series cyclones and a glass
fiber final filter. Approximately one-twentieth of the flue gas that has
passed through the second cyclone is diverted to the gas analysis system.
The water content of this flue gas stream is reduced to 3000 ppm (by conden-
sation and refrigeration) to prevent moisture interfering with gas analysis.
Continuous analyses of the dried gas for NO, S02, CO, CHi^, and Q£ are
carried out with infrared analyzers and a paramagnetic oxygen analyzer.
Gas chromatography provides intermittent analyses for C02. All instrument
signals, pneumatic and electrical, are routed to a data logger that produces
a paper tape record for further data processing as well as a typed output
of the signal values.
-------
19
Solids leave the combustor (1) as bed samples, (2) as excess bed
material, and (3) as elutriated flyash-limestone fines. Periodically
during a run, the bed and the elutriated_ solids are sampled for chemical
analysis (generally for C, Ca, S, and CO^) and to obtain material balances.
Bed material is removed intermittently (generally once per hour) during a
run to maintain a constant bed height, the quantity removed depending on
the quantities of limestone and coal fed and the quantities of solids
elutriated as fly ash and removed as bed samples.
In startup of all oil and coal combustion runs, the fluidized bed
of particulate solids is preheated to ^1000°F by passing heated air
through the bed and heating with the heaters mounted on the reactor
wall. Coal, or oil, is introduced into the heated bed, ignites, and
increases the bed temperature to the desired operating temperature
(e.g., ^1600°F). The bed is maintained within 10°F of the selected
temperature by passing air or an air-t^O mixture through annular chambers
on the exterior of the combustor wall. Additive is introduced continuously
into the bed. In the air-excess oil combustion experiments, the 62 con-
centration in the flue gas was
For coal combustion experiments made with a deficiency of combustion
air, the coal feed rate is adjusted until the oxygen concentration in the
flue gas is approximately 2%; the flow rate of air fed to the fluid bed
is next reduced until the oxygen concentration in the flue gas reaches
zero, then reduced an additional amount so that a deficiency of air is
fed to the fluid bed. An oxygen concentration of zero in the flue gas
was obtained at 1550°F with Peabody Mine 10 coal by lowering the air
flow from an above-zero oxygen condition to about 80 ft3 of air per
pound of coal fed. Additional air for the two-stage experiments was
injected above the bed region through a line that entered the top of the
combustor.
For the oil combustion experiments, which were performed at atmospheric
pressure, an oil feeding system (shown schematically in Fig. 2) was installed.
The oil in the primary oil vessel was heated, and the tank was then pressur-
ized to transfer the oil to the combustor. Near the combustor, air was
injected into the transport line to impel oil into the bed. The in-line
analytical instruments for measuring SC>2, NO, CHI+, CO, C02 and 02 concen-
trations in the flue gas were the same as those used for the coal combustion
runs.
C. Results and Discussion
1. Air-Deficient Coal Combustion Experiments
The concept of two-stage combustion provides for introduction
into the fluidized bed (the first stage) of a substoichiometric quantity
of air (that is, less air than is, required to burn the coal completely
to C02 and H20). Additional air may be injected into the disengaging
section above the fluidized bed (the second stage) to burn gaseous hydro-
carbons, H2, and CO in the gas stream from the first stage.
-------
HEATING ELEMENT
THERMOCOUPLE
BALL VALVEST
6"COMBUSTOR
N2 SUPPLY CXJ
THERMOCOUPLE
HEATING
ELEMENT
OIL OUTLET
DISTRBUTOR
PLATE
PRIMARY OIL VESSEL
PRESSURE
GAGE
COMBUSTION AIR
TRANSPORTER CALIBRATION TAP
Fig. 2. Equipment for Feeding Oil to the 6-in.-dia Combustor
-------
21
Two-stage combustion experiments of an exploratory nature were
conducted to determine if this combustion mode might have benefits, as com-
pared with single-stage fluidized-bed combustion. To simulate the condition
of combustion in the first stage only, experiments were performed in which
a substoichiometric quantity of air was introduced into the bottom of the
fluidized bed but no secondary air was fed. In other experiments, secondary
air was introduced above the fluidized bed. The bed consisted of coarse
lime particles in most of these experiments. In a few experiments, coal
was combusted in an Alundum bed to determine the H2S/(S02 + H2S) ratios
in the flue gas in the absence of limestone. In the lime bed experiments,
limestone was fed continuously into the bed during the experiment; no
limestone was fed during the alundum bed experiments.
a. Effect of Decreasing Air Input on Ratio of H?S to (H?S-fS02)
in Flue Gas. The concentration of H S in the off-gases was measured to
determine which operating conditions affect the formation of this sulfur
compound. In experiments in which air was introduced into the first stage
only, the amounts of I^S and SC>2 in the off-gas were compared. The percent-
age of sulfur in the off-gas as H2S was sensitive to the amount of air
introduced into the fluidized bed, increasing drastically when the air feed
rate was reduced below a value corresponding to 70% of the stoichiometric
quantity of air necessary to react with the coal fed* (see Fig. 3). At an
air feed rate equivalent to 50% of the stoichiometric quantity, the concen-
tration of H2S (611 ppm) was nearly equivalent to the concentration of S02
(660 ppm). At air inputs of 70-80% of the stoichiometric quantity, the
relative amount of sulfur as I^S fell to about 2% of the total sulfur in
the gas.
In those experiments in which secondary air was introduced
above the fluidized bed (Fig. 3), the H2S level in the off-gas was low—
corresponding to less than 1% of the total sulfur in the gas. This suggests
that any H2S in the gas leaving the first stage is oxidized to S02 by air
fed to the second stage.
No consistent relationship was apparent between H2S level and
either the temperature of the fluidized bed (1450-1650°F) or the temperature
of the off-gas in the freeboard above the bed (1100-1800°F).
The ratio of H2S to total sulfur (H2S+S02) in the flue gas
when coal was combusted in an alundum bed in the absence of limestone can
be compared to the limestone experiments in Fig. 3. When oxygen could be
detected in the flue gas, the relative H2S concentration in the flue gas
was low, less than 10% of the total sulfur in the flue gas. When oxygen
is no longer detected in the flue gas, the concentration of H2S increased
rapidly.
Not all of the sulfur was released to the flue gas, i.e.,
H2S+S02 levels in the flue gas were less than expected based on the coal
Although the parameter, air feed rate as a percent of stoichiometric, was
based on feed rates of coal and air, it is recognized that the quantity
of coal actually oxidized varies with other parameters (i.e., temperature,
etc.). For certain correlations, it may well be more suitable to use the
parameter, stoichiometric air feed rate based on the coal actually oxidized.
-------
22
70
60
o
o
so
(M
O
CO
-------
23
fed. Approximately 15% of the sulfur in the coal fed to the combustor
was retained in the alumina bed. The bed at the end of the experiments
contained ^5.5 wt % sulfur (1.9% from the coal present in the bed at the
end of the experiment and ^3. 6% from pickup of sulfur). At low stoichio-
metric air rates, the sum of the H2S and S02 contents of the fuel gas was
substantially less than the equivalent to the sulfur fed in the coal (less
than 50%). This suggests that a large part of the sulfur might have been
converted to sulfur vapor by a reaction between I^S and SC>2 (Glaus reaction)
at some point in the system upstream from the analyzers.
b. Sulfur Retention. Sulfur retention is defined as the percent-
age of the sulfur associated with the coal feed that is not contained in the
off-gas as S02 or H2S.* Experiments were performed with no introduction
of secondary air to determine sulfur retention as a function of the Ca/S
mole ratio in the feed at 1450, 1550, and 1650°F (Fig. 4). Also shown in
Fig. 4 (to allow comparison) is a curve representing data for experiments
carried out earlier under single-stage oxygen-excess conditions at 1450-
1470°F.
The data presented for the substoichiometric air experiments
show a large amount of scatter—principally due to variation in the quantity
of air fed to the fluidized bed, which was not the same in all experiments.
The percent of stoichiometric air added in these experiments ranged from
51 to 91. The best sulfur retention was observed at air additions less
than 60% of stoichiometric.
For experiments carried out at a Ca/S ratio of about 2 and
temperatures of 1450-1650°F (Fig. 5), no simple relationship between the
amount of air introduced into the bed and sulfur retention was evident;
however, a line has been fitted to the points as shown. At an air feed
rate of 100% of stoichiometric, sulfur retention was about 65%. As the
air feed rate was decreased, sulfur retention first decreased to about 45%
as the air rate decreased to 75% of the calculated stoichiometric require-
ment and then increased rapidly as the air rate was decreased further. This
suggests that in an oxygen-deficient region (75-95% of calculated stoichi-
ometry), removal of sulfur by lime in the form of SC>2 is poor, but that at
lower air flow rates sulfur is in the form of H2S and is efficiently removed.
This would be expected because oxidizing conditions are required for the
retention of 862 by lime (to convert a CaSQ^ intermediate to CaSO^), whereas
reducing conditions are required for the retention of H2S by lime.
The introduction of secondary air above the bed resulted in
erratic but generally lower sulfur retentions. The increased sulfur
content of the off-gas after secondary air was introduced was probably
caused by burning of entrained coal particles in the second stage to
produce additional S02-
c. NO Levels in the Flue Gas. When coal was burned with a defi-
ciency of air fed to the first stage, concentrations of NO in the off-gas
from the first stage as a function of the amount of air introduced into the
bottom of the fluidized bed were as shown in Fig. 6. To obtain these data
*
Because a small fraction of the carbon was not burned in these experiments
and some sulfur is retained with the coal, the sulfur retention values given
are probably higher than would be realized if all of the carbon were burned.
-------
24
100
90
80
70
Z 60
o
i- 50
UJ
cr
u. 40
O
30
20
10
091
061
69D Q6I
61A
O57
O80
THE NUMBERS ADJACENT
TO POINTS ARE THE PERCENT
OF STOICHIOMETRIC AIR ADDED
SINGLE-STAGE OXIDIZING
EXPERIMENTS (1450-I470°F)
BED TEMPERATURE
SUBSTOICHOMETRIC
AIR EXPERIMENTS
O I450°F
O I550°F
A I650°F
100
90
80
** 70
I 60
Z
UJ
L 50
or
i 40
u.
3 30
20
10
0
BED TEMPERATURE
D I450°F
O I550°F
A I650°F
I
I
_L
J_
_L
234
Ca /S mol ratio
Fig. 4. Sulfur Retention in Oxygen-
Excess and Oxygen-Deficient Experiments
0 50 60 70 80 90 100
AIR, % of sfoichiometric
Fig. 5. Effect on Sulfur Retention
of Air Feed Rate to First Stage
Ca/S mole ratio = 2
ANL Neg. No. 308-2875
300
250
200
150
100
50
I550°F
40
50
60
70
80
90
100
AIR, % of stoichiometric (based on feed rates)
Fig. 6. Effect of Air Feed Rate and Fluidized-Bed Temperature
on NO Concentration in Off-Gas from the First Stage
during Combustion of Coal.
ANL Neg. No. 308-2871.
-------
25
only the first stage was operated. The NO concentrations ranged from 50 to
250 ppm and apparently were affected by both the amount of air introduced
and the temperature of the fluidized bed. At the lower air feed rates,
the NO levels were generally lower. At a given air feed rate, NO levels
were higher at lower bed temperatures. Since earlier work at ANL showed
that nitrogenous compounds in coal are oxidized to NO during fluidized-bed
combustion, it can be postulated that the lower levels of NO observed at
higher temperature are due to more rapid decomposition of NO. This decom-
position may be.promoted by the presence of CO; another possibility is that
nitrogenous compounds other than NO may be formed in the highly reducing
atmosphere of the bed. The NO levels observed when coal was combusted with
an excess of air were greater than 250 ppm.
The data presented in Fig. 6 are not corrected to an equivalent
off-gas volume basis. However, if this correction were made, the dependence
of NO emission on air feed rate would be even more pronounced, assuming that
feed rates of coal were equivalent.
Upon the introduction of secondary air above the fluidized bed,
NO levels in the off-gas varied erratically—usually increasing. Possible
explanations for this behavior are (1) any reduction of NO by CO in the
zone above the bed would be suppressed by introducing secondary air or
(2) if a nitrogen compound such as ammonia was present in the gas, it might
be oxidized to NO by the secondary air.
d. Sulfur Species in the Bed. Results show that the sulfide
content of bed material decreased as air flow was increased. At air inputs
corresponding to 50% of stoichiometric, as much as 100% of the sulfur in the
bed was sulfide. In most experiments in which the air input exceeded 65%
of stoichiometric, sulfide content dropped rapidly to less than 1%. No
relationship was found between sulfide content and bed temperature.
The sulfite content of bed samples was erratic, ranging between
6.2 and <0.1 wt %. No correlation of sulfite content with either bed temper-
ature or air feed rate could be found.
e. Carbon Balance. Carbon balances were made for three experiments;
for three other experiments, all data for making the balances except the C02
level in the flue gas are available (see Table .3). The small quantity of
carbon not accounted for is represented (1) by hydrocarbons (other than CHi+)
for which no analyses were.made and (2) by a small loss of fine carbon
particulate from the combustion system. The data show that as the volume
of air added to the bed decreases (experiments 14-1A, -IB, -2) at the same
temperature, the CO content of the flue gas increases markedly, the CEi+
content increases slightly, and the quantity of carbon elutriated to the
first and.second cyclone separators from the fluidized bed increases.
The carbon content of the bed under low stoichiometric air
additions (55%) was as high as 31% (expt. 14-3B). Under these conditions
the amount of carbon elutriated was about 15% of that fed.
-------
26
TABLE 3. Carbon Balances and Carbon Contents of Solids
in Coal Combustion Experiments with a Deficiency of Air
EXPERIMENT
Carbon in coal, g/hr
Carbon Out, g/hr
First Cyclone
Second Cyclone
Flue Gas
CH4
CO
CO 2
Total Carbon Out, g/hr
Carbon Concentration in
Solids Streams, wt %
Bed at End of Experiment
First Cyclone
Second Cyclone
Run Conditions
Temp, °F
Coal Feed, Ib/hr
Air, % of Stoichiometric
14-1A
1428
71
22
>24
42
1230
>1470
29
52
1450
5.0
90
14-1B
1428
136
11
32
350
N.D.*
N.D.
39
53
1450
5.0
86
14-2
1684
174
24
39
441
840
1632
6
46
55
1450
6.3
71
14-1C
1799
167
7
42
521
N.D.
N.D.
39
56
1550
5.9
54
14-3A
1485
235
7
28
330
N.D.
N.D.
20
47
35
1600
5.2
64
14-3B
1713
233
16
32
455
800
1682
31
62
53
1600
6.0
55
N.D. - No Data
Value appears to be low. Sufficient sample was not available for making
another analysis.
-------
27
f. Preliminary Evaluation of the Concept. Although the work con-
ducted on two-stage combustion was exploratory in nature, a preliminary
evaluation of the concept can be made. The principal advantages of the
two-stage combustion concept over one-stage combustion are: (1) lower NO
emissions, (2) retention of sulfur in the form of calcium sulfide (rather
than sulfate) allowing for potentially easier regeneration of the additive,
(3) production of a combustible gas that could be used in conjunction with
a gas turbine.
The principal disadvantages are: (1) greater elutriation of
carbon, (2) possible complications in additive regeneration owing to the
high carbon content of the bed, (3) necessity for removing heat from the
bed under conditions that might be corrosive to immersed steam tubes.
Sulfur retention appears to be roughly equal for the two
concepts. It is notable, also, that no problems of coal caking were
encountered even at high bed carbon contents. Further work might be
warranted at lower air addition rates and higher bed temperatures to
avoid the need for heat removal from the bed.
2. Oil Combustion Experiments with an Excess of Oxygen
To assess the removal of S02 from combustion gases when residual
fuel oil is burned in a fluidized bed of sulfated lime with continuous
feeding of limestone additive, experiments were performed in the 6-in.-dia
fluidized-bed combustor at a variety of operating conditions. Residual
fuel oil (containing 1.9% S) was burned in an excess of oxygen at bed
temperatures ranging from 1450 to 1650°F, Ca/S mole ratios up to 11.9, a
gas velocity of ^3 ft/sec (except for one experiment at 5.5 ft/sec), and
with 3 vol % oxygen in the flue gas (except in one experiment with 1 vol %
oxygen in the flue gas).
Table 4 lists operating data, concentrations of some components
of the flue gas, and calculated sulfur retention and combustion efficiency
data for the experiments in which limestone was fed to the combustor.
Effect of Temperature on SO? Retention. The effect of temperature
on S0£ retention is similar to that observed in coal combustion experiments
(ANL/ES/CEN-1004, p. 251),for which there is a temperature at which sulfur
retention is at a maximum. In the oil-combustion experiments, maximum S02
retention was observed in the 1450-1650°F temperature range, possibly at
a temperature below 1550°F (Fig. 7).
Effect of Ca/S Mole Ratio on Sulfur Retention. As in the coal
combustion experiments, increasing the limestone feed rate at a fixed oil
feed rate increased the sulfur retention (Fig. 8). The slope of the curve
for sulfur retention as a function of Ca/S ratio is not as steep as in
experiments made with Illinois coal containing 3.7 wt % sulfur. Also,
the maximum observed sulfur retention of 90% is slightly lower than in
the coal experiments at equivalent operating conditions.
-------
1X5
00
Table 4
Operating Data and Results, Combustion of Residual Fuel Oil in Excess Air
Expt. OIL-
2
3A
3B
3C
4A
4B
4A-1
Fluidized-
Bed
Temp
1650
1550
1550
1550
1450
1650
1450
Oil
Feed Rate
(Ib/hr)
5.8
3.3
3.0
3.3
3.2
3.2
4.8
Equipment: ANL 6-in.-dia fluidized-bed combustor
Oil: Esso residual fuel oil, 1.97 wt % sulfur
Additive: Limestone No. 1359, as received (94.8 wt % CaCO , 0.9 wt % MgCO
609 urn average particle size)
Starting Fluidized Bed: 17.3 Ib partially sulfated and calcined limestone No. 1359
(%24-in. fluidized-bed depth) Note: The final bed from a run was
used as the starting bed for the following run.
0 Concentration in Flue Gas: ^3% (except 4A-1, 1%)
Limestone
Feed Rate
(Ib/hr)
0.6
0.8
2.2
1.1
1.2
1.2
1.2
Ca/S
Mole Ratio
1.6
4.1
11.9
5.4
6.0
6.0
4.0
Gas
Velocity
(ft/sec)
5.5
3.2
3.2
3.0
3.1
3.1
3.3
Flue Gas Composition
so2
(ppm)
1350
630
170
200
350
550
520
NO
135
110
140
150
140
130
190
CO
(ppm)
8500
6000
6000
5000
5000
5000
> 12000
(ppm) "
>3400
1500
800
1200
1100
1000
>3400
C02
No
Data
14.4
15.0
14.2
15.0
14.0
No
Data
Sulfur
Retention
16
60
90
88
78
66
68
Combustion
Efficiency
<92
95
94
No Data
95
96
No Data
-------
100
80
60
UJ
UJ
(T
o: 40
ID
20
0
I
OIL-3C
OIL-4A-I (~ l%02 IN FLUE GAS)
OIL-4B
OIL FEEDRATE : 3.2-3.3lb/hr
Cd/S : 5.4-6.0
GAS VELOCITY : 3.0-3.1 ft/sec
0 CONC : ~3%INFLUEGAS(EXCEPTINOIL-4A-I)
1
I
I
1400
1450 1500 1550 1600
FLUIDIZED-BED TEMPERATURE, °F
1650
Fig. 7. Effect of Temperature on Sulfur Retention During
Combustion of Residual Fuel Oil in Excess Air
K)
VO
-------
u>
o
100
80
2 60
z
LJ
UJ
* 40
D
20
OIL-3C
OIL-3B
OIL-3A
OIL FEED RATE : 3.0-3.3lb/hr
TEMPERATURE s I550°F (EXCEPT IN OIL-2)
GAS VELOCITY : 3.0-3.2 ft/sec (EXCEPT IN OIL-2)
02 CONC IN FLUE GAS : ~ 3 %
OIL-2 (I650°F, GAS VEL = 5.5 ft/sec,
5.8lb/hr OIL FEED RATE)
I I I I
6 8
Co/S MOLE RATIO
10
12
14
Fig. 8. Effect of Ca/S Mole Ratio on Sulfur Retention During
Combustion of Residual Fuel Oil in Excess Air
-------
31
NO Levels. Measured NO levels in the flue gas ranged from 110 to
150 ppm for experiments with 3 vol % oxygen in the flue gas. No trend was
apparent related to an effect of fluidized-bed temperature or gas velocity
on NO level.
Combustion Efficiency. Combustion efficiency, in percent, is
defined as 100 (1 - carbon out/carbon in). "Carbon out" is the sum of
carbon contents of elutriated solids, solids drained from the bed, and CO
and hydrocarbons in the flue gas. "Carbon in" is the carbon content of
the oil feed.
Combustion efficiency for the runs with 3 ft/sec gas velocity
ranged from 94 to 96% (Table 4). In the single experiment made at a
higher gas velocity of 5.5 ft/sec, the combustion efficiency dropped below
92%. An exact value could not be calculated because the CH^ concentration
(known to be above 3400 ppm) was not measured exactly. In the experiment
with a 5.5 ft/sec gas velocity, considerable gaseous combustion was
apparently taking place above the bed. The flame front was about two feet
above the bed. The CH^ and CO levels in the flue gas were high. Additionally,,
voluminous quantities of carbon floe were collected on the final filter.
For these reasons, in subsequent experiments the oil feed rate and com-
bustion air flowrate were lowered and the gas velocity was 3 ft/sec.
Bed temperatures in the 1450-1650°F range had little effect on
combustion efficiency in the experiments.
The maximum carbon content of the fluidized-bed solids in the
oil-combustion experiments was 1.1%. Most bed samples contained <1.0%
carbon, the lower limit of detectability by the LECO combustion method.
For the experiments with 3% excess oxygen and a gas velocity of 3 ft/sec,
the carbon content of solids removed from the first cyclone ranged from
3.4 to 5.1% (a carbon loss of 3.7 to 8.5 g/hr from this source). For
solids from the second cyclone, the carbon contents ranged from 6 to 20%
(a loss of 0.2 to 0.8 g/hr of carbon from this source).
Particle Size Distribution and Bulk Density of Solids. Particle
size distributions and bulk density data for the limestone additive, fluid
bed, and solids from the primary and secondary cyclones are presented in
Table 5 for experiment OIL-3C. For other runs, the bulk densities of the
first and second cyclone solids ranged from 0.57 to 1.20 g/cc and from
0.23 to 0.68 g/cc, respectively.
Hydrogen Concentration in Flue Gas. The hydrogen concentration
in the flue gas it not routinely measured with our available instruments.
To obtain an estimate, five flue gas samples from the OIL-series experiments
were analyzed with a gas chromatograph. The chromatographic column consisted
of 3 ft of l/4-in.-dia Molecular Sieve. Hydrogen concentration in the argon
carrier gas was measured, using a thermal conductivity detector. The hydrogen
concentrations in four samples from experiments made with 3% 02 in the flue
gas ranged from 46 to 340 ppm; for the experiment with 1% 02 in the flue gas
(OIL-4A-1), the hydrogen concentration was 2200 ppm.
-------
32
TABLE 5. Particle Size Distributions and Bulk Densities
for Solids Samples from Experiment OIL-3C
Percent in Screen Size Fraction, Bulk Density
U.S.
Bulk
S. Screen Size
+40
-40+60
-60+100
-100+170
-170+270
-270
Density, g/cc
Limestone
Additive
42.0
21.0
12.9
16.6
6.7
0.8
1.7
Bed
81.1
18.9
—
—
—
—
2.4
Primary
Cyclone
3.1
31.3
24.1
15.4
8.4
17.7
1.1
Secondary
Cyclone
1.3
1.4
1.7
4.1
75.2
16.3
0.4
3. Cyclone Collection Efficiencies During Coal Combustion Experiments
Data on the particle removal efficiency of the cyclone separators
were obtained as a basis for estimating the dust loading and filter area
of a cartridge filter for the recently constructed pressurized combustion
bench-scale plant. In the atmospheric-pressure system, flue gas passed
through two cyclones in series, which have been modified for operation with
the pressurized equipment, and a final filter. (The diameter of the first
in-line cyclone is 6 5/8 in. and the diameter of the second is 4 1/2 in.)
To determine the adequacy of the glass fiber, mat filters used in
the atmospheric plant, collection efficiencies (defined as ratio of the
weight of particles removed to the weight of particles entering the cyclone)
were compiled for 26 earlier ANL experiments. The flue-gas flowrates ranged
from 8 to 14 cfm, the coal feed rates from 4 to 7.3 Ib/hr, and the additive
feed rates from 1.1 to 2.3 Ib/hr; the dust loadings at the combustor exit
were 0.16 to 1.78 g/ft3. The combined efficiency of the two cyclones was
above 80% in 24 of the 26 experiments and above 90% in 21 of the experiments.
The dust loading in the flue gas leaving the second cyclone averaged 0.06
g/ft3 for the 26 runs; the maximum loading was 0.22 g/ft3.
4. Combustion Efficiencies for Coal, Oil, and Natural Gas Combustion
with Excess Air
The combustion efficiency for excess air experiments performed in
the combustor was determined as the ratio of carbon burned to carbon fed,
multiplied by 100. The carbon loss was calculated by determining unburnt
carbon leaving the system by three routes: (1) carbon associated with the
elutriated solids, (2) incompletely burned gases, i.e., carbon monoxide
and hydrocarbons, and (3) carbon associated with fluidized-bed material
taken from the system. All experiments were conducted without recycle
of fines. Oxygen concentration in the flue gas in these experiments was
approximately 3%.
-------
33
Combustion efficiencies in ten coal-combustion experiments (made
at a fluidized-bed temperature of ^1600°F) ranged from 93 to 96%.2 In all
experiments, carbon losses in the bed material were negligible. Only about
10-20% of the carbon loss was due to the formation of carbon monoxide arid
hydrocarbons. The major carbon loss (80-90%) occurred as a result of
elutriation of incompletely combusted fine particles in the exhaust gases.
Combustion efficiency can be increased by recycling the elutriated ash-
carbon mixture to the fluidized bed or to a carbon burnup cell.
Combustion efficiency in oil combustion experiments (fluidized
bed temperatures of 1450-1650°F) was similar to that observed for coal
combustion experiments under similar conditions, ranging from 94 to 96% for
experiments with 3% 02 in the flue gas. Combustion efficiencies were lower
when there was less excess oxygen in the flue gas and when gas velocities
were higher. In oil combustion, the major carbon loss occurred as a result
of incomplete burning of the CO and hydrocarbons formed during combustion.
Combustion efficiency can probably be improved by operating the combustor
with a deeper bed or by increasing the freeboard temperature.
In the combustion of natural gas,the elutriation of carbon-bearing
fine particles is negligible and the major loss of unburnt carbon is in the
carbon monoxide and hydrocarbons in the flue gas. Combustion efficiencies,
calculated from analyses of samples of the flue gas, ranged from 94.1 to
99.2% at 1600°F in runs with 3% oxygen in the flue gas.2' At 1800°F, com-
bustion efficiencies were 94.1-98.8%. These results are similar to data
reported by the USSR on combustion of gas in a fluidized bed.3 Although
the USSR data indicate that combustion efficiency is principally affected
by bed temperature, combustion efficiency is also likely to be a function
of bed depth, gas velocity, and excess oxygen concentration. For example,
in an experiment at 1800°F, combustion efficiency was decreased to 91% when
combustion conditions were intentionally made more reducing, i.e., 1.5
vol % 02 in the flue gas rather than 1.8-4.5 vol % 02.
-------
IV. REGENERATION OF SULFUR-CONTAINING ADDITIVES
When coal is burned in a fluidized bed containing limestone or dolomite
additive, the product of the reaction between sulfur-containing gases and
additive is calcium sulfate if combustion is carried out under oxidizing
conditions or calcium sulfide if combustion is carried out with a deficiency
of air. As the capacity of the bed material to retain sulfur is approached,
bed material must be removed from the combustor and replaced with fresh
material. Simply discarding the sulfur-bearing material would be unattractive
because of (1) the cost of replacement material, (2) the cost of transporting
fresh material to the furnace and used materials from the furnace, (3) dis-
posal costs, and (4) pollution problems associated with the disposal of
used material (especially if it contained calcium sulfide, which would
slowly release hydrogen sulfide if exposed to air) . A process for regener-
ating used bed material so that the lime may be recycled through the com-
bustor is needed, both to circumvent these problems and to recover the
sulfur value as sulfuric acid or elemental sulfur.
Several regeneration processes have been considered. These are
1. thermal decomposition of calcium sulfate to form S02 and CaO,
2. reductive decomposition of calcium sulfate to form CaO and S02 ,
3. roasting of calcium sulfide in air or oxygen to form S02 and CaO,
4. reaction of calcium sulfide with water and carbon dioxide to
form CaC03 and H2S.
S02 or H2S would be converted to sulfur in a Glaus plant or to
in an acid plant. Processes 3 and 4 can be used to regenerate not only
calcium sulfide, but also material containing calcium sulfate if the sulfate
is first reduced to the sulfide. Information is needed so that the most
economical and effective process may be identified.
A. Thermodynamic Analysis of Some Schemes for Regenerating Partially Spent
Additive from the Fluidized-Bed Combustion of Coal
Valuable information can be gained by considering the thermodynamics
of the regeneration processes. The yields of gaseous sulfur-containing
products, the compositions of solid phases, and the variations of these
yields and compositions with temperature, pressure, and gas composition
for a system at equilibrium can all be calculated. Optimum reactant feed
ratios and gas compositions necessary to reduce a given feed of spent
additive can also be calculated easily when product concentrations, compo-
sitions, and pressures are specified.
The equilibrium predicted product yield is, in effect, the upper limit
(i.e., the maximum amount of gaseous sulfur species) for a reaction system
operated in accord with the stated equilibrium system. Although equilibrium
considerations are of utility in indicating the limits of the system, they
cannot foretell the mechanism or kinetics of the reaction system, which can
be elucidated only by experiment. Because a large variety of reactions
can occur, a mixture of products at equilibrium is quite probable.
-------
35
The following predictions and conclusions are based on the assumption
that chemical equilibrium is achieved among the various phases. This implies
that the rates of all relevant chemical reactions are large for the time
scales being used, which scales are determined by mass transport rates
within the system. The maximum rates of all reactions at which this suppo-
sition is valid vary with temperature and must be determined in the labora-
tory and in the pilot plant. It is further assumed that the system is not
stoichiometrically limited. There must always be at least small amounts of
the appropriate solid phases present for the results of these calculations
to be valid. In actual processes, it is not expected that all of these
solid phases will be present, however.
The assumption is also made that solid solutions do not form to any
great extent. Exploratory experiments to date support this assumption. For
the calculation of equilibrium data, values of the free energy of formation
of the relevant compounds for the temperature range 1000-2500°F were com-
piled (Table 6). They are standard free energies of reaction (all gases at
one atmosphere); the actual free energies of reaction will depend on the
conditions existing in the regenerator during reduction.
The S02 pressures predicted from the free energies agree with the
pressures measured in exploratory experiments (see Section B below), indi-
cating that no serious inaccuracies exist in the free energy data.
1. Thermal Decomposition of Calcium Sulfate
Calcium sulfate decomposes by the reaction in Eq. 1 to give sulfur
trioxide.
CaS04 J CaO + S03 K = P (atm) (1)
The sulfur trioxide decomposes by the reaction in Eq. 2 to give sulfur
dioxide and oxygen.
S03 J S02 + 1/2 02 K = - - - (2)
P S03
The pressure of sulfur dioxide calculated from Eq . 2 is exactly equivalent
to that calculated from Eq . 3.
CaSOi, J CaO + S02 + 1/2 02 K = Pcn Pn 1/2 (3)
p , bU2 U2
Equation 3 represents the decomposition of calcium sulfate to sulfur dioxide
and oxygen. As can be seen from Table 7, the pressures of both sulfur dioxide
and sulfur trioxide at any temperature likely to be of interest are too
small to make thermal decomposition attractive. In this calculation of S02
pressure, it was assumed that no oxygen was present other than that from
the decomposition of CaSO^. The presence of additional oxygen would, of
course, decrease the S02 pressure.
-------
CO
ON
TABLE 6. AGC
f, Free Energy of Formation (kcal/g-mole)
RT(kcal)
°K
coa
CO a
CH,a
ti a
cosa
H2°a
S°2a
CaCO b
CaO<:3
CaSb
CaSO
NO3
MgO3
MgS3
MgSO a
1.6113
810.94°K
1000°F
-43.843
-94.560
-0.253
-50.415
-48.504
-11.972
-72.382
-77.756
-237.340
-130.637
-109.249
-224.478
-268.204
19.159
19.994
-122.788
-78.862
-227.334
1.7217
866.49°K
1100 °F
-45.096
-94.583
1.270
-50.528
-47.786
-11.329
-71.409
-75.581
-234.055
-129.304
-107.971
-219.680
-262.300
18.891
20.837
-121.374
-77.512
-221.320
1.8321
922.05°K
1200°F
-46.240
-94.603
2.601
-50.668
-47.063
-10.685
-70.436
-73.410
-230.776
-127.960
-106.682
-214.739
-256.414
18.823
21.689
-119.919
-76.121
-215.281
1.9425
977.60°K
1300°F
-47.385
-94.621
4.043
-50.802
-46.334
-10.036
-69.463
-71.250
-227.488
-126.599
-105.377
-209.936
-250.538
18.655
22.532
-118.401
-74.669
-209.206
(contd.)
2.0528
1033. 16°K
1400°F
-48.556
-94.638
5.494
-50.939
-45.600
-9.386
-68.492
-69.099
-224.216
-125.238
-104.038
-205.139
-244.707
18.359
23.382
-116.879
-73.197
-203.139
2.1633
1088. 72°K
1500°F
-49.725
-94.655
.6.951
-51.073
-44.862
-8.735
-67.523
-66.954
-220.954
-123.876
-102.676
-200.335
-238.895
18.319
24.225
-115.338
-71.712
-197.078
2.2737
1144. 27°K
1600°F
-50.886
-94.668
8.416
-51.211
-44.118
-8.081
-66.554
-64.821
-217.662
-122.471
-101.315
-196.314
-233.098
18.151
25.072
-113.797
-70.224
-191.048
2.3841
1199. 83°K
1700°F
-52.045
-94.681
9.883
-51.345
-43.373
-7.425
-65.585
-62.692
-214.368
-121.054
-99.954
-191.449
-227.297
17.983
25.914
-112.257
-68.735
-185.024
-------
TABLE 6.
AG°, Free Energy of Formation (kcal/g-mole) (contd.)
RT (kcal)
°K
°F
coa
CO/
CH?a
cos13
H2Sa
a
so2a
CaCO b
C J
CaSb
CaS03 c
N0a
NO/
MgOa
nfsoa
2.4944
1255. 38°K
1800°F
-53.199
-94.692
11.353
-51.478
-42.624
-6.771
-64.617
-60.577
-211.058
-119.610
-98.510
-186.584
-221.538
17.813
26.761
-110.716
-67.243
-179.042
o
JANAF Thermochemical
Fundamental Study of
2.6049
1310. 96°K
1900°F
-54.353
-94.703
12.828
-51.613
-41.873
-6.114
-63.649
-58.463
-207.759
-118.161
-97.071
-181.718
-215.795
17.645
27.605
-109.121
-65.698
-173.008
2.7152
1366. 5°K
2000°F
-55.498
-94.711
14.304
-51.745
-41.118
-5.439
-62.684
-56.366
-204.475
-116.721
-95.688
-176.857
-210.116
17.477
28.447
-107.321
-63.948
-166.805
2.8257
1422. 1°K
2100°F
-56.642
-94.719
15.784
-51.878
-40.361
-4.803
-61.719
-54.272
-201.237
-115.286.
-94.264
-171.987
-204.466
17.306
28.951
-105.136
-61.813
-160.228
Data, issued by Dow Chemical Company,
Sulfur Fixation by Lime and Magnesia,
2.9360
1477. 6°K
2200°F
-57.781
-94.725
17.262
-52.012
-39.603
-4.148
-60.757
-52.192
-197.999
-113.871
-92.840
-167.140
-199.024
17.138
30.128
-102.375
-59.102
-153.115
Midland, Mich.
Final Report,
3.0463
1533. 2°K
2300°F
-58.919
-94.732
18.742
-52.146
-38.842
-3.492
-59.793
-50.116
-194.761
-112.437
-91.416
-162.271
-193.478
16.968
30.968
-99.617
-56.393
-146.019
Contract PH
3.1568
1588. 8°K
2400°F
-60.055
-94.738
20.222
-52.280
-38.080
-2.836
-58.829
-48.044
-191.523 "
-110.991
-89.992
-157.392
-187.932
16.800
31.811
-96.864
-53.690
-138.944
86-66-108,
3.2673
1644. 3°K
2500°F
-61.184
-94.742
21.702
-52.411
-37.319
-2.181
-57.869
-45.989
-188.285
-109.570
-88.570
-152.543
-182.388
16.631
32.647
-94.125
-51.001
-131.922
Battelle Memorial Institute (June 30, 1966).
'J. P. Coughlin, Contributions to the Data on Theoretical Metallurgy, U.S. Bureau of Mines Bulletin NO. 542 (1954)
u>
-------
38
TABLE 7. Pressure of 803 and S02 in Equilibrium with
(assuming no C>2 pressure other than that from decomposition)
Temp (°F)
1700
1900
2100
2300
2500
P-n (atm)
SO 3
1.16 x 10~_.
_7
2.95 x 10 '
4.31 x'lO
3.90 x 10 ,
— u
2.72 x 10
Pgo (atm)
1.45 x 10~J
— u
1.78 x 10 ^
1.93 x 10
1.20 x 10,
— 7
5.97 x 10
2 . Reductive Decomposition of Calcium Sulfate
a. Reductive Decomposition of CaSOu with CO/CO? Mixtures. The
relative amounts of the species in an equilibrium mixture from the reduction
of calcium sulfate with carbon monoxide-carbon dioxide mixtures can be
calculated after the solid phases present at the various conditions of
temperature and carbon monoxide/carbon dioxide ratio have been determined.
The possible sulfur-containing solids are considered to be calcium sulfate,
calcium sulfite, and calcium sulfide.
(1) Conditions for the Presence of CaSO^ and CaS
The solid phases present at equilibrium with a PcO/^CO?
of 0.005-0.055 and temperatures of 1600 to 2400°F are shown in Fig. 9 (in
which temperature is the ordinate and Pco/^GOa *-he abscissa) . Examination
of the expression for K in the following reaction
CO 2
1/4 CaS04 + CO J 1/4 CaS + C02 K = - - (4)
P CO
shows that for any temperature, there is but one ratio of carbon monoxide
to carbon dioxide at which calcium sulfate and calcium sulfide can coexist
at equilibrium. The coexistence conditions appear as the line running from
the lower left to the upper right of Fig. 9 and represent the CO/C02 ratios
at which CaS and CaSO^ can both be present at equilibrium. In the area to
the right of this line, the gas mixture is so rich in carbon monoxide that
calcium sulfate is completely reduced to calcium sulfide. To the left of
the line, the gas mixture is so rich in carbon dioxide that calcium sulfide
is completely oxidized to calcium sulfate. This line is called the coexist-
ence line for calcium sulfate and calcium sulfide.
(2) Conditions for the Presence of Calcium Sulfite
Calcium sulfite is not stable in the presence of C0/C02
mixtures at any temperature from 1500 to 2400°F. This has been established
by plotting "coexistence" lines for calcium sulfite with calcium sulfate
and for calcium sulfite with calcium sulfide (Fig. 10). These are analogous
to the calcium sulfate-calcium sulfide coexistence line described above
and are determined in the same way from the equilibrium constants for
reactions 5 and 6.
-------
2400
2320
2240
2160
2080
a:
< 2000
tr
S02 PRESSURES (atm)
-0.001
1600
en
u
ir
a.
CJ
O
(ft
0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055
PCO/PC02
Fig. 9. Pressure of S02 in Equilibrium with C0/C02 Mixtures as a Function of Temperature
(10-atm total pressure)
-------
40
'CO;
+ CO £ CaS03 +
Kp-p
CO
1/3 CaS + C02 £ 1/3 CaS03 + CO K =
CO
5co2
(5)
(6)
u.
o
2400
2300
2200
2100
LU
cr 2000
£j 1900
Q.
UJ
1800
1700
1600
1500
-CaS04 + 4 CO -T CaS + 4 C02
CO-="CaS03
0.05
0.10
P /P
rco rcoz
0.15
Fig. 10. Temperature and CO/C02 Conditions for Formation
of CaSOit, CaSOs, and CaS
-------
41
(3) Conditions for the Presence of CaCO^ and CaO
When calcium sulfate is reduced or calcium sulfide is
oxidized by a mixture of CO and C02 , calcium oxide is formed. However, in
the presence of carbon dioxide at sufficient pressure, calcium oxide is
converted to calcium carbonate (Fig. 11).
2100
2000
UJ
o:
ID
< 1900
o:
LJ
1800
1700
I
8
10 II 12
13
14 15 16
KD,atm
Fig. 11. Equilibrium Dissociation Pressure of
as a Function of Temperature
(CaC03 J CaO + C02)
A coexistence line for the carbonate and the oxide is
determined by the equilibrium dissociation pressure of calcium carbonate
(reaction 7)
CaC03 ~t CaO + C02
KP = Pco2
(7)
It also appears as a nearly horizontal line at about 1950°F in Fig. 9. This
line represents the temperature at which the partial pressure of C02 in the
equilibrium mixture just equals the equilibrium pressure of C02 over calcium
carbonate.
The partial pressure of C0£ in the equilibrium mixture is
obtained by assuming a total pressure of 10 atm and subtracting the pressures
of S02 and CO. Clearly, if the total pressure is lowered or if an inert gas
-------
42
is added, the pressure of C02 will be lower and the horizontal line will be
at a lower temperature. It is also clear that calcium carbonate does not
exist above the horizontal line and that calcium oxide does not exist below
it.
(4) Sulfur Dioxide Pressure
The pressure of sulfur dioxide in the equilibrium mixture
can be calculated from the CO/C02 ratio and the equilibrium constant of the
reaction appropriate to the part of Fig. 9 under consideration; however, in
the areas labeled C and D, one must generate independent information about
the C02 pressure by making assumptions exactly analogous to those made in
the above discussion of calcium carbonate.
In area A of Fig. 9, S02 is generated by the following
reaction: p
S02 C02
CaS04 + CO ~t CaO + S02 + C02 K = (8)
p CO
The pressure of S02 is shown as a family of isobars slanted down toward
the right. In area B, S02 is generated by the oxidation of calcium sulfide
in accordance with the following reaction: , ,
[P 1 ' P
1 S02I CO
1/3 CaS + C02 J 1/3 CaO + CO + 1/3 S02 K = — (9)
P C02
The isobars of constant S02 pressure in area B curve down to the left,
meeting those of area A at the calcium sulfide-calcium sulfate coexistence
line. At any temperature, the S02 pressure is at a maximum at this junction.
For example, at 2000°F, the maximum attainable equilibrium pressure of S02
is 0.46 atm at a C0/C02 ratio of 0.020. This maximum in the S02 pressure
may be understood by examination of the appropriate equilibrium constants.
For example, the expression for the equilibrium constant for reaction 8
predicts that the pressure of S02 is directly proportional to the C0/C02
ratio. Thus, the pressure of S02 must increase as the C0/C02 ratio increases
as long as reaction 8 obtains. From the expression for the equilibrium
constant in reaction 9, it may be seen that the S02 pressure is inversely
proportional to the cube of the C0/C02 ratio. Thus, the S02 pressure
increases with decreasing C0/C02 ratio as long as reaction 9 obtains.
Reactions 8 and 9 occur simultaneously only along the coexistence line.
Thus, as one moves away from the coexistence line, the S02 pressure must
decrease.
In area C, reaction 10 applies.
PS02
CaSOit + CO £ CaC03 + S02 K = (10)
P CO
In this area, the pressure of S02 is dependent only on K and carbon monoxide
pressure, and the S02 isobars are nearly vertical. The main effect results
from the variation of Kp with the temperature. In area D, the S02 pressure
is once again a strong function of the C0/C02 ratio as may be seen from
reaction 11.
-------
43
Pso2 (pco)
CaS + 4 C02 ~t CaC03 + S02 + 3 CO K = -. —— (11)
P Iv \ ^
CO 2
The slope of the isobars in area D differs only slightly
from the slope in area B as a result of increased dependence on C02 pressure
in area D.
(5) Sulfur Pressure
The pressure of sulfur vapor is quite low (<10~2 atm) over
the temperature range 1700 to 2300°F (Fig. 12). The pressure of sulfur in
area B from reaction 12 was calculated.
P
CaS + C02 J 1/2 S2 + CO + CaO K =(?,,} ^ (12)
P \ S2/ PC02
Since the formation of sulfur is entirely analogous to the formation of S02,
sulfur concentration may be.expected to exhibit the same sort of maximum
at the calcium sulfide-calcium sulfate coexistence line.
(6) Carbonyl Sulfide Pressure
The carbonyl sulfide pressure from reaction 13 was
calculated.
COS
CaS + C02 J CaO + COS K = ^— (13)
P C02
Since PCQS -*-s dependent on the pressure of C02, assumptions made in calcu-
lating COS pressure are similar to those made in the discussion of calcium
carbonate. PCOS -"-s -'-ow (^10~3 atm) in this system.
(7) Solid-Solid Reaction of CaS with CaSO^
S02 is generated by the reaction of calcium sulfide with
calcium sulfate, as is shown in reaction 14.
1/3 CaS + CaSOit J 4/3 S02 + 4/3 CaO K = (P0^ W3 (14)
P V "^2/
\ /
Since reaction 14 is exactly equivalent to the sum of reactions 8 and 9, the
S02 pressure calculated from reaction 14 must be just that calculated from
reaction 8 or reaction 9, using the C0/C02 ratio at the coexistence line.
Another way of saying this is that the presence of both calcium sulfide and
calcium sulfate determines an oxidizing potential for the atmosphere with
which it is in equilibrium, and this oxidizing potential determines the
C0/C02 ratio of the atmosphere. If carbon monoxide and carbon dioxide are
present in the gas phase over a mixture of calcium sulfide and calcium
sulfate, they serve as a facile route to the production of S02 so that
rapid reaction rates for mixtures of the two solids are possible.
-------
2300
2200
i-
- 2100
UJ
cr
QL
UJ
Q.
2
UJ
2000
1900
1800
1700
1/4 CoS04-fCO
AREA A
atmatlOatmPf-
\
I LINE CALCULATED FOR 10 atm TOTAL PRESSURE
AREA C
\L
I
AREA D
I
I
I
0.01 0.02
0.03 0.04
P
rco
0.05 0.06 0.07
Fig. 12. 82 and COS Concentrations at Equilibrium as a Function
of Temperature and CO/C02 Ratio
-------
45
b. Reduction of CaSO^ with H2/H?0 Mixtures. The system calcium
sulfate-calcium sulfide-H2~H20 is exactly analogous to the system calcium
sul fate-calcium sulfide-CO-C02. This means that all the features of the
CO-C02 system are present in the H2-H20 system. The sulfur-containing
solid phases once again are calcium sulfate and calcium sulfide, but
calcium oxide and calcium hydroxide are the non-sulfur-containing solid
phases. The pressure of SC>2 at a given temperature has a maximum value
where the calcium sulfate and calcium sulfide are in equilibrium with the
H2-H20 mixture (Fig. 13). The instability of calcium sulfite can be shown
in the same way as in the CO-C02 system. In fact, the major differences
between the I^-t^O system and the CO-C02 system are that carbonyl sulfide
is replaced with H2S (see Fig. 12 for the pressure of H2S and sulfur vapor
as a function of temperature and H2/H20 ratio) and that at any given
temperature, the numerical value for the H2/H20 ratio differs from that of
the C0/C02 ratio.
An additional difference between the systems is that in the
H2-H20 system, calcium hydroxide can form at lower temperatures and higher
pressures of H20 (just as CaCOs can form in the CO-C02 system). However,
at 10-atm H20 pressure, Ca(OH)2 is not stable above 1200°F.
For any temperature and 862 pressure, the H2/H20 ratio can be
calculated from the equivalent CO/C02 ratio via reaction 15.
P /P
H2 + C02 £ H20 + CO K = pCO/pCO? (15)
P H2 H20
This is the familiar water-gas shift reaction. The principle involved here
is that equal S02 pressures are obtained in the H2-H20 system and the CO-C02
system when the oxidizing potentials of the atmospheres are the same, i.e.,
when the two atmospheres would be in equilibrium with each other.
An important conclusion is that the maximum pressure of S02
from any system in which calcium sulfate is reduced ,or calcium sulfide is
oxidized is the pressure of S02 observed along either the H2-H20 coexistence
line (Fig. 13) or the C0-C02 coexistence line (Fig. 9). The same is true
for S2 pressures. The basis for these rather far-reaching conclusions is
that in any process involving a reduction of calcium sulfate, the S02 and
82 pressures will increase with increasing reducing ability of the atmosphere
until calcium sulfide is formed. At that point, increasing the reducing
ability of the atmosphere no longer increases the amount of S02 or S2 formed,
but rather causes the calcium sulfate to be transformed into calcium sulfide.
Similarly, in a process in which calcium sulfide is oxidized, the S02 and
S2 pressures increase with increasing oxidizing ability of the atmosphere
until calcium sulfate is formed. This point is again a limit, and S02 and
82 pressures cannot be increased further.
3. Roasting of Calcium Sulfide
In the roasting process, calcium sulfide is oxidized with oxygen
or air according to Eq. 16.
CaS + 3/2 02 J S02 + CaO K = — - (16)
P
-------
PQft =0.01 atm
OUo
Ps = IO-3 atm
P.. =0.0001 atm
LJ
1100
0.003
0.005
0.007
0.009
0.011
P /P
rH2/fH20
0.013
Fig. 13. Pressure of SC>2 in the
System
-------
o
LJ
^
5
CL
2
UJ
1800
1700
1600
1500
1400
1300
1200
1100
= lO^atm
'=IO~4atm
— — -~ ""CaS
PH s= 2X I0"2atm
= IO~2atm
P,. =IO"7atm
Pu c= 7XlO"3atm_
rirtO
Pu c=3 IO"3 atm
n«o
I
0.003
0.005
0.007 0.009
P / P
^Hg H20
0.011
0.013
Fig. 14. 82 and H2S Pressures in the System S2-H2S-H2-H20-CaSOit-CaS
-------
48
As may be seen by examining the equilibrium constant for Eq. 16, the pressure
of SC>2 at any temperature increases with increasing pressure of oxygen.
However, it follows from the arguments presented above that above some
definite oxygen pressure (given at any temperature by K_ of Eq. 17),
1/2 CaS + 02 -" 1/2 CaSOi, K = ~- (17)
P
calcium sulfide is no longer stable, but is converted to calcium sulfate.
At this particular oxygen pressure, the S02 pressure is that observed along
the coexistence line in the CO-C02 system or in the ^-H^O system. Thus,
what appear to be two very different processes, the reductive decomposition
of calcium sulfate and the roasting of calcium sulfide, are in fact very
similar. Both processes give rise to identical maximum SC>2 pressures at a
given temperature.
4. Pressure Effects in the CO/C02 and H2/H2Q Systems
In all regeneration processes discussed above, the pressure of SC>2
is a function of the temperature or of the oxidizing ability of the atmosphere
(in the case of the roasting and reductive decomposition processes). The
pressure of S02 as a function of the total system pressure has not been
discussed because SC>2 pressure is independent of the total system pressure
in these processes. However, percent of S02 in the gas mixture is an inverse
function of the total system pressure since the pressure of SC>2 is fixed at
any temperature. The pressure of SC>2 is also independent of the presence
of inert gaseous diluents if sufficient oxidizing or reducing gas is present.
As stated above, in the case of reductive decomposition with CO-C02 mixtures,
the presence of inert gases may affect the C02 pressure enough to change
the reaction product from calcium carbonate to calcium oxide.
5. Acid-Base Reaction of Calcium Sulfide with H?Q and CO?
Reaction 18 has been proposed as a regeneration reaction for calcium
sulfide.
H2S
H20 + C02 + CaS t CaC03 + H2S K = (18)
P C02 H20
Calcium sulfide is formed in the additive by (1) burning coal in a fluidized
bed of limestone or dolomite with a deficiency of air or by (2) reducing the
CaSOi+ in additive from a run in which combustion was with an excess of air.
Unlike all other regeneration reactions discussed here, this reaction is
pressure-sensitive. The percentage and the pressure of H2S increase with
increasing total system pressure. The pressure of H2S is also sensitive
to the presence of inert-gas diluents, in contrast to the previously mentioned
regeneration schemes. The equilibrium constant for this exothermic reaction
becomes smaller as the temperature is increased (Fig. 15). This is in
direct contrast to the other (endothermic) regeneration schemes mentioned.
Maximum H2S yield is obtained when the H20/C02 ratio in the feed
gas is 1 to 1, as may be seen by an examination of the equilibrium constant
-------
49
0.151—
0.10 —
0.05 —
1000 1100 1200 1300 1400 1500
TEMPERATURE, °F
Fig. 15. Equilibrium Constant
as a Function of Temperature,
C02 + H20 + CaS -> CaC03 + H2S
2:0
H,0 + CO, + CaS — CaCO,+ H.S
Fig. 16. Pressure of H2S in
Undried Gas Stream as a Function
of Temperature at 10-atm P
r total
1000 1100 1200 1300 1400
TEMPERATURE, °F
-------
50
expression. Figure 16 shews the pressure of H2S as a function of temperature,
assuming 10-atm total pressure and an inlet gas stream composed only of H20
and C02 at various ratios. However, H20 may be readily removed from the
product gas stream by condensation. Thus, higher values of H2S concentration
in a dried gas stream may be obtained by operating with higher ratios of
H20/C02 in the inlet gas stream. For an inlet gas composition of 50% water
and 50% C02 and temperatures of 1000-1400°F, Table 8 gives the percentage
of H2S in the gas effluent from the reactor and the percentage of H2S in the
same effluent after it has been dried.
TABLE 8. H2S Concentration3 in Dried and Undried
Product Gas Stream at Equilibrium
Temp (°F) % H2S in Undried Gas % H2S in Dried Gas
1000
1100
1200
1300
1400
23.0
11.4
4.7
2.7
1.4
37.4
20.5
9.9
5.1
2.8
o
Assumptions are 10-atm total pressure and an inlet gas
of 50% H20 and 50% C02.
-------
51
B. Experimental Studies
1. Reductive Decomposition of
To test the accuracy of the equilibrium compositions calculated
for the reduction of CaSO^ with CO/C02 mixtures, experiments have been, per-
formed in a static system. The apparatus (shown in Fig. 17) consists of
a horizontal tube reactor fabricated from recrystallized alumina. The tube
is 36 in. long and has an ID of 3 in. with 1/4-in. walls. One end of the
alumina reactor is closed', and the opposite end is capped with a stainless
steel 0-ring flange. There are openings in the flanged end (which is out-
side the furnace) for gas lines and the thermocouple well.
AI203 TUBE 3"IDX3I/2"OD
iv.v v: »..-»-
^y^ry;-;^ '•. "::• '•::. ;•'.:-;.•:,•.:
\
C02 CO N2
iii
m
TC
WELL
FURNACE
VENT
VACUUM
PUMP
Fig. 17. Schematic of Apparatus for Equilibrium Measurements
on CaSO^-CO/CC^ System
The experiments were performed in the following manner. A 3-g
sample of CaSOit (Drierite) was placed in an alumina boat and loaded into
the reactor. The system was closed and leak-checked, and the CaSO^ was
dried at 500°F under vacuum for 15-20 hr. While the system was still at
500°F and isolated from the vacuum pump, a predetermined pressure (<1 atm)
of CO was added. The total pressure of the system was then increased to
1 atm by adding C02. The system was allowed to stand for 30 min; then a
gas sample was taken through a septum in the flange using a hypodermic
syringe. The gas sample was analyzed for CO by gas chromatographic techniques.
Next, the pressure of the system was reduced to 400 mm Hg and the
temperature was increased to 1900°F, increasing the pressure to ^1 atm. With
the system at 1900°F, samples of the gas mixture were obtained at selected
-------
52
intervals and analyzed for CO and 862. After the final gas sample was
obtained, the system was flushed with nitrogen to remove reactant and
product gases. The temperature was lowered to room temperature, and the
residue in the alumina boat was removed and analyzed for Ca, total S, and
S= by wet chemical analytical techniques.
Experimental results presented in Fig. 18 show good agreement, in
most cases, between experimental and calculated values of SC>2 levels when
CaSOi^ was reacted with CO/CC>2 over a range of ratios. Other data indicate
that CO and S02 were being slowly removed by a secondary process. Analyses
of the gas samples indicate that reaction was occurring in the ratio of one
mole of S02 to one mole of CO, forming one mole of C02 during the period of
equilibration at 1900°F (the other reaction product was probably elemental
sulfur, which was found at the cooler end of the alumina tube). This
secondary reaction, evident in those runs in which the equilibration time
was very long (^20 hrs) and/or the CO/C02 ratio was high, caused the points
to fall off the equilibrium line. In an experiment in which S02 was diluted
with N2 only, it was observed that S02 reacted with materials of construction
to a small extent; S02 concentration decreased from 15% initially to 14% in
2 hr and to 13% in 20 1/2 hr at 1900°F.
2. Two-Step Process
A second regeneration procedure of interest is to first reduce the
CaSOit to CaS and then to make t^S by the reaction of CaS with H20-C02. Ex-
perimental data on these process steps were obtained in a 2-in.-dia batch
fluidized-bed reactor. The regenerator and associated equipment are shown
in Fig. 19. The reactor system consisted of the reactor, a gas preheater,
and an off-gas filter.
The reactor was fabricated from Type 316 SS. Solids can be removed
from the fluidized bed in the reactor during an experiment by evacuating a
solids receiver tube and then transporting the solids to the tube by opening
a valve between the higher pressure reactor and the tube. Heating of the
reactor and preheater was done electrically. The pressure of the reactor
system was controlled by a pressure transmitter, pneumatic controller, and
a pneumatic valve. Analysis of the effluent gas was performed with infrared
analyzers, a gas chromatograph, and a quadrupole mass spectrometer.
The bed material consisted of either CaSO^ (Drierite) or partially
sulfated dolomite or limestone obtained from the coal combustion experiments.
Nitrogen and carbon dioxide were obtained from the evaporation of the liquids
or from cylinder sources. The other reactant gases were obtained from the
cylinder sources.
a. Conversion of CaSOu to CaS (Reduction Step). The effect of
temperature on the rate of reduction of CaSO^ to CaS with hydrogen was
studied and is shown in Figs. 20 and 21. The additive used was partially
sulfated dolomite (CaSO^/CaO/MgO, 15.4 wt % Sulfur) obtained in an earlier
experiment in which the dolomite (obtained from Charles Pfizer Co.,
Gibsonburg, Ohio) took up S02 during combustion of coal at 1550°F. In the
H2~reduction runs, the gas velocity was ^6 ft/sec and the system was at
atmospheric pressure. Approximately three stoichiometric equivalents of
-------
53:
0.15
E 0.10
•t-
o
N
O
0)
0.05
TEMP. I900°F
PRES. I dtm
" »A-9, Ihr
»A-l2,2hr -
A-l5,lhr
CO/CO, = 0.034
_A-7,l.5hr«
_A-l2,l9hr
A-15, IS^hr
CALCULATED EQUILIBRIUM PARTIAL
PRESSURE OF S02 FOR
CaS04 + CO— CdO-HCOg H
0.001
0.005
0.10
0.015
P /P
CO CO,
Fig. 18. Experimental and Calculated Partial Pressures
of S02 Over a Range of P_»/P__ Ratios
CO LU2
(Temp: 190.0"F1; total pressure: 1 atm)
-------
CONDENSER
SOLIDS
REMOVAL
LINE
T.C.WELL
T
RECEIVER
TO SAMPLING
INSTRUMENTS
PURGE ROTAMETER
N2,C02
Q
SOLIDS BALLAST VACUUM PUMP
RECEIVER
TAYLOR PRESSURE
TRANSMITTER
REACTOR
FILTER
CHAMBER
PREI
PNEUMATIC
CONTROLLER
PNEUMATIC VALVE
•— N2,C02lH2,CO
Fig. 19. TVo-Inch-Diameter Fluidized-Bed Reactor
-------
55
20
UJ
_i
Q.
5
<
V)
Q
UJ
CD
15
10-
UJ
t-
z
o
UJ
Q
U.
_l
Z>
Fig. 20. Sulfide Content of
Bed during Reduction of Partially
Sulfated Dolomite with Hydrogen
at Various Temperatures
100
200
TIME , min
300
100
Fig. 21- Effect of Temperature
on Reduction of CaSOtt (Dolomite)
(4.5-hr reduction time; 1-atm
pressure)
1200
1300 1400 • 1500 1600
TEMPERATURE, °F
1700
-------
56
pure hydrogen were added over the 5-hr period of the runs. As would be
expected, the sulfide concentration in the bed material was lower at
1350°F than at either 1450°F or 1600°F after equivalent reaction times.
Percent conversions after 4.5 hr at 1350°F, 1450°F, and 1600°F were 18%,
38%, and 86%, respectively. If all the sulfate had been converted to
sulfide, the sulfur content of the sample would be
Since in practice, CO may be the reducing gas, an additional
experiment was performed with CO. The percent reductions of CaSO^ with CO
and the expected reduction using H2 (Fig. 20) agreed at the temperature
employed (1500°F); however, this may not be true at all temperatures.
b. Reaction of CaS with C0?/H?0 (Regeneration Step). The product
of each of the reduction experiments was carbonated at 10 atm with C02 to
simulate a product that would be obtained in an actual 10-atm combustion-
reduction experiment. This material was then reacted batchwise with an equi-
molar mixture of C02/H20 at temperatures ranging from 900 to 1100°F, a
gas velocity of approximately 1 ft/sec, and 10-atm pressure in the 2-in.-
dia fluidized-bed reactor. The H2S concentration in the outlet gas was
monitored with a quadrupole mass spectrometer. Conditions and results
of the experiments are shown in Table 9.
The results to date have shown that:
1. The reaction producing H S was initially rapid, but the
rate decreased after a short time. Typically, the reaction rate dropped
nearly to zero after several minutes (Fig. 22).
2000
tr
<
CL
1000
02 4 6 8 10 12 14 16 16
TIME, min
Fig. 22. Typical H2S Levels in the Outlet Gas
for C02/H20 Regeneration
-------
TABLE 9. Regeneration Experiments, Conditions and Results
Reactor: 2-in.-dia batch reactor
Reactor Pressure: 10 atm
Conditions
and Reductant for Preparation of Material
for Regeneration Step ^.^
Stoichiometric Gas
Experiment
No.
CATS- 17
-18
-19
-20
-21
Reductant
100% H2
100% H2
100% H2
100% CO
85% CO/
15% C02
Equivalents
Temp. , °F Added
1600
1450
1350
1500 1.7
1600 5.0
Velocity,
ft/sec
0.7
0.6
0.16
0.40
1.0
Time,
hr
4.7
4.7
2.7
2.0
5.0
Regeneration Step
Starting Bed Temp. ,
S= v/ C °/ ® T
>/o o , /o r
17.7 20.4 1000
8.7 900
4.1 1100
12.2 19.1 1000
6.1 1000
H2S
Ending Bed Concentration, %
S-, % ST, % Peak
9.5 37
43.6
2.3 15.0 11.2
5.3 35.5
2.8 37.2
Equil.
37.4
59.0
20.5
37.4
37.4
**
Starting material was dolomite from Charles Pfizer Co. which had been sulfated in the 6-in.-dia fluidized bed combustor
at 1550°F. The sulfated dolomite contained 15.4 wt % sulfur. Reactor pressure during reduction step was 1 atm.
k
Reactor pressure was 10 atm. Reactant gas velocity was 1.3 ft/sec.
-------
58
2. The peak concentration of H2S in the outlet gas was high,
near the expected equilibrium value.
3. Typically, half or less of the CaS reacted.
In continuing work, the effects of process variables are being
studied in an attempt to increase the quantity of CaS reacted.
c. Sulfation-Regeneration Cyclic Experiments. Since it will be
desirable to reuse the additive material several times in commercial appli-
cations, a cyclic experiment has been performed to obtain data on the pick-
up and removal of sulfur from additive particles and to determine decrep-
itation and attrition of additive particles during sulfation-regeneration
cycles. Six cycles of simulated combustion and two-stage regeneration
were performed with a single bed of additive. The starting material (1.2 kg)
was the final bed from a coal combustion experiment in which dolomite No.
1337 had been used as additive. The initial sulfur content of this material
was 15.4 wt %. The experiment was performed batchwise in the 2-in.-dia
fluidized-bed reactor. Data on operating conditions and sulfur analysis
of the products of each step are shown in Table 10.
The sulfation portion of cycle 1 was omitted since the additive
already contained sulfur. For the remaining cycles, the constituents of
the synthetic combustion gas were N2, CC>2, H20, 62, CO, and SC>2. The sulfa-
tion reaction was allowed to proceed until the bed material had essentially
ceased further pickup of S02. After the bed had been sulfated, the CaSO^
was converted to CaS, using H2 or CO as reductant at 1550 to 1600°F and
10 atm. The bed was then reacted with a C02/H20 gas mixture at 1000°F
and 10 atm to convert the CaS to CaC03. A sample of the bed material was
taken after each step in the cycle and analyzed for sulfur and sulfide
content.
The effluent gas stream was analyzed for H2S concentration,
using the quadrupole mass spectrometer. A plot of I^S concentration vs
reaction time for each of the six cycles is presented in Fig. 23.
The results (see Table 10) for the sulfation cycles indicate
an increase in sulfur content with each succeeding cycle except between the
fourth and fifth cycles. It is not possible, however, to determine if this
means an actual increase in total sulfur content since the weight of the
bed after each cycle is unknown.
The results indicate that the conversion of CaSO^ to CaS in
the reduction step was ineffective. Only in cycles 1 and 4 was the con-
version to CaS greater than 50%. A possible cause is the interaction of
CaSOit and CaS to form nonporous surfaces; the formation of easily sinterable
cakes has been reported when CaS-CaSO^ are present.^
The data for the regeneration step indicate that the peak con-
centrations of H2S in the effluent gas decreased from 13 vol % (dry basis)
for cycle 1 to 0.5 vol % (dry basis) for cycles 5 and 6. Equilibrium con-
centrations of H2S would have been -25%. The percent sulfide converted
to CaO or CaCOs decreased from 9.7% to a very low indeterminant value.
-------
59
15
10 20
TIME, min
Fig. 23. H2S Concentration in Effluent Gas Stream in Regeneration Step
30
-------
TABLE 10. Conditions for Cyclic Experiment
Sulfation Cycle
1
2
3
4
5
6
Starting Bed: 1200 g of Sulfated Dolomite
Final Bed: 561 g
Analytical
Results for
T,°F
1500
1500
.1500
1500
1500
P,
atm
10
10
10
10
10
Time,
hr
^2
2
1.5
1
2
N
24
24
24
24
24
Gas
CO
L
12
12
12
12
12
Flowrate
HO
23
23
23
23
23
, cfh
°->
1.5
1.5
1.5
1.5
1.5
(10 atm,
CO
0.1
9.1
0.1
0.1
0.1
, 70°F)
SO
4.
0.5
0.5
0.5
0.5
0.5
Total
61.0
61.0
61.0
61.0
61.0
Gas Velv
ft/sec,
at T.I- .
2.9
2.9
2.9
2.9
2.9
Bed Sample
^Total'
wt %
15.4
16.4
17.8
18.0
16.7
N.S.
s',
wt %
-
-
-
-
-
Reduction Cycle
1
2
3
4
5
6
1550
1550
1550
1600
1600
1600.
10
10
10
10
10
10
4.5
4.5
4.5
5.5
5.0
5.0
^2
5.7
5.7
-
-
-
5.7
C£
_
-
4.1
5.1
5.1
-
co2
5.0
1.2
1.8
0.7
0.7
0.7
^2
6.0
3.0
0.5
3.0
3.0
-
Total
16.7
9.9
6.4
8.8
8.8
6.4
0.8
0.47
0.31
0.44
0.44
0.32
20.8
20.8
16.6
18. /
16.8
N.S.
15.5
9.2
7.6
13.9
5.9
N.S
Regeneration Cycle
1 1000 10 20
2 1000 10 21
3 1000 10 21
4 1000 10 21
5 1000 10 30
6 1000 10 26
co2
18
18
18
18
18
18
Total
18
18
18
18
18
18
36
36
36
36
36
36
1.37
1.37
1.37
1.37
1.37
1.37
18.9
18.8
17.7
19.6
18.2
15.5
14.0
8.4
6.8
14.8
6.0
2.0
N.S. - No sample.
-------
61
The sulfur contents in the initial and final beds were 15.4
and 15.5 wt %, respectively. The weights of sulfur in the initial and
final bed were 185 g and 87 g, respectively. A portion of the 98 g of
sulfur not accounted for in the bed was in bed samples. The total weight
of bed samples was about 200 g, possibly containing ^34 g of sulfur.
To obtain information on physical changes that occurred in
additive material during sulfation-regeneration cycling, a microscopic
examination was performed on additive material from the initial bed, bed
material sampled after the third cycle regeneration, and final bed
material. Photographs of particles are presented in Figs. 24 and 25.
It.is apparent from the photographs that with additional
recycling, the layer of material on the surface of the particles becomes
thicker, probably by diffusion into the particle rather than by buildup
of material on the surface. The mechanism for layer formation is not
known at present but may be solid solution formation. A more detailed
microscopic examination of this material will be performed, and hopefully
the composition of the layer will be determined.
The photographs also indicate that the porosity of the particles
may increase slightly as recycling proceeds.
To obtain data on the amount of attrition and agglomeration
of additive material during several cycles of sulfation-regeneration, a
particle size analysis was performed on starting bed material and final
bed material from the cyclic experiment. The data (presented in Table 11)
indicate that there was only slight agglomeration or attrition in six
cycles of sulfation-regeneration.
d. Sulfiding-Regeneration Experiments. Because the presence of
CaSOtt might have contributed to the poor sulfur removals noted in the
sulfation-regeneration experiments, several sets of cyclic experiments
with dolomite or limestone starting beds were performed in which CaCO^
was reacted directly with H2S and then the product CaS was reacted with
C02/H20. The MgO in the dolomite bed was calcined before the H2S reaction.
Preliminary experimental results from data available show that:
1. The regeneration reaction was initially rapid, but the
reaction rate decreased after a short time, the same as when CaSO^ was
present.
2. The peak concentration of H2S in the outlet gas during
regeneration was near the expected equilibrium value in the first cycle.
Experimental work is continuing, data are being evaluated,
and particles are being examined by different techniques to gain an insight
into the reaction mechanism.
-------
62
•\
~£*
•" w.v -? , . - *»*v»"*
:
Fig. 24. Photomicrographs Showing Cross Sections of
Particles from Cyclic Experiment (X100)
-------
63
mmjilj^'
&3&rA
• >• •*.
/' iC '-'•- .
-,-—«*_,
Fig. 25. Photomicrograph Showing Cross Sections of
Particles from Cyclic Experiment X100
TABLE 11. Sieve Size Analysis of Starting Bed and
Final Bed After Six Cycles
U.S. Mesh Size
+25
-25+40
-40+60
-60+80
-80+100
-100+230
-230
Starting Bed,
wt %
2.0
54.0
36.4
6.5
0.6
0.3
0.1
Final Bed,
wt 7,
11.0
45.6
34.6
6.2
1.0
0.8
0.8
TOTAL
100.0
100.0
-------
64
V. PRESSURIZED COMBUSTION AND REGENERATION PILOT PLANT
A. Description
Equipment has been installed for combusting coal at pressures up to
10 atm and for regenerating sulfated lime for reuse. A simplified equip-
ment schematic is shown in Fig. 26.
The solids feeders are of the rotary pocket type, equipped with
hoppers which are loaded batchwise.
The gas preheater (see Fig. 27) was designed in accordance with ASME
code requirements, and its design rating is 150 psig at 1500°F. Air (or
gas) passes through an annulus, reverses direction, and passes through a
central section containing electrical heaters.
The regenerator and the fluidized-bed combustor, which have a common
preheater and off-gas system (cyclones, filters, gas-sampling equipment,
pressure let-down valve, and scrubber), will not be operated simultaneously.
When the combustor or the regenerator is in operation, the other unit will
be disconnected from the off-gas line and flanged off.
The combustion unit (see Fig. 28) consists of a 6-in. schedule 40 pipe
(Type 316 stainless steel) approximately 11 ft long, with an outer shell
consisting of 12-in. schedule 10 pipe (Type 304 stainless steel) over
nearly the entire length. A bellows expansion joint is incorporated into
the outer shell to accommodate the differential thermal expansion of the
inner and outer vessels.
HIGH
REHEATER
PRESSURE
STEAM
COMPRESSOR
SURGE_
TANK
HOUSE
AIR
02 —
C02—
CO
H,
-PREHEATER CYCLONES
'COMBUSTOR
Lit
FILTERS
ANALYSIS
REGENERATOR
PRESSURE
LET -DOWN
VALVE
TO
STACK
SCRUBBERS
U|U
ROOM
VENTILATION
AIR
Fig. 26. Simplified Schematic of Pressurized
Combustion-Regeneration Equipment
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65
HEATER LEADS
BYPASS
HEATER CONTROL
THERMOCOUPLES
CLAMSHELL
HEATERS
TO REGENERATOR
TO
.^COMBUSTOR
GAS FEED
Fig. 27. Six-inch Fluidizing Gas Preheater.
ANL Neg. No. 308-3368.
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66
SIGHT GLASS
PURGE GAS OUTLET
HEATER CONTROL
THERMOCOUPLES
SHELL PURGE
GAS INLET
RUPTURE DISK
FLUE GAS TO
CYCLONE AND FILTERS
EXPANSION BELLOWS
RUPTURE DISK
12-in JACKET
24,36 OR 48 in
SOLIDS OVERFLOW
BED THERMOCOUPLE WELL
EXTERNAL COOLING COIL
(WRAPPED ON 6-in DIA WALL)
CALROD TYPE HEATER
INTERNAL COOLING COILS
BUBBLE CAP DISTRIBUTOR
ELECTRICAL HEATER
LEADS
PLIBRICO FILLED VOLUME
SOLIDS FEED LINES
FLUIDIZING AIR
Fig. 28. Six-inch Dia Pressurized Fluidized-Bed Combustor.
ANL Neg. No. 308-3366.
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67
The unit is of a balanced-pressure design, i.e., the annular chamber
between the two pipes is maintained under pressure so that there is no
differential pressure across the hot inner-pipe wall. The balancing
pressure for the shell is supplied by a bank of nitrogen cylinders.
A bubble-cap-type gas distributor is flanged to the bottom end of the
combustor inner vessel; thermocouples, solids feed lines (coal and additive
are injected upward into the bed through a 45° angle tube) and solids take-
off lines extend through the gas distributor. The outer wall of the 6-in.
pipe is wrapped alternately with sixteen 3000-W tubular resistance heaters
and 3/8-in.-OD cooling coils that are spray-metal-bonded. Internal cooling
coils of 3/8-in. pipe extend down into the interior of the 6-in. vessel
from the flanged top to provide additional heat transfer area. Water flow
to the cooling coils is regulated with flow indicators and is adjusted to
obtain the selected temperatures of the fluidized bed and reactor wall.
Both the annular pressure chamber and the reactor itself are equipped
with rupture disc assemblies and pressure relief valves vented to the room-
ventilation exhaust ducts.
The regenerator (see Fig. 29) has a 3-in. ID. It consists of a 2 1/2-in.
Plibrico castable refractory encased in an 8-in. schedule 40 pipe (Type
316 stainless steel). This entire assembly is enclosed in a pressure
shell made of 12 in., schedule 20 carbon steel pipe. Differential thermal
expansion between the inner and outer pipes is accommodated by using
packing glands on the lines entering the bottom flange of the unit. The
unit is of a balanced-pressure design. The balancing gas is nitrogen.
Since the annular space is not gas-tight with respect to the regenerator
inner vessel, the pressure in the annular space will be maintained slightly
higher than the regenerator pressure to prevent process gases from entering
the annulus. A pressure alarm gauge will monitor the pressure in the
annular space and will be set to warn of both high and low pressures.
A bubble-cap type gas distributor is connected to the bottom of the
inner regenerator vessel via a slip fit and held in place with retaining
screws. Thermocouples, solid feed lines, and solids take-off lines pass
through the gas distributor and through packing glands on the bottom
flange of the outer pressure vessel. The wall of the inner vessel is
wrapped alternately with 3000-W tubular resistance heaters and 3/8-in.-OD
cooling coils. Both the annular chamber and the regenerator itself are
equipped with rupture disc assemblies and pressure relief valves vented
to the room ventilation exhaust ducts.
The primary-filter cartridges (epoxy-impregnated cellulose-base
material with glass fiber substrate) are suitable for gas temperatures up
to 350°F. The secondary-filter cartridges are Rigimesh (woven metal wire).
Testing of the combustor and regenerator has started.
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68
SOLIDS FEED
SPARK-PLUG IGNITER
PRESSURE TAP
THERMOWELL
HEATER
CONTROL
THERMOCOUPLES
ELECTRICAL HEATERS
LEADS
KEROSENE
INLET LINE
OFFGAS TO CYCLONE
SEPARATIONS AND
FILTERS
RUPTURE DISKS
12-in JACKET
ELECTRICAL HEATERS
COOLING COILS
PLIBRICO CERAMIC LINER
THERMOWELL
BUBBLE CAP DISTRIBUTOR
THERMOCOUPLES
SOLIDS OVERFLOW LINE
24 inches
FLUIDIZING GAS
Fig. 29. Three-inch Dia Fluidized-Bed Regenerator.
ANL Neg. No. 308-3367.
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69
B. Petrocarb Solids Feeder Tests
A modified Petrocarb solids feeder was tested for its suitability
in feeding coal, limestone, or mixtures of coal and limestone to a pressur-
ized bench-scale combustor-regenerator. Uniform feed rates in the range
of 20 to 100 Ib/hr are required in the high-pressure combustion system.
A facility consisting of a feeder, a transport line, and a solids
collection system was constructed for testing the evenness and reproduc-
ibility of feeding the solids. The main features of the feeder (see Figs.
30 and 31) are a conical-bottom tank (that contains solids) and a mixing
valve assembly to mix the solids with the carrier gas. The solids in the
tank are aerated by a controlled stream of air at a selected pressure.
In a test, the aerated solids flowed down from the conical bottom through
an orifice in the mixing valve assembly into a controlled stream of air
and were pneumatically conveyed through a transport line to a solids
receiving vessel that contained a solids filter. For our use, a 1/4-in.
orifice mixing-valve assembly was selected that was projected to give
solids flows as low as 20 Ib/hr.
The solids feed rate was controlled by two factors—air pressure
in the tank and the total air flow rate. The tank pressure was controlled
by adjusting the tank pressure regulator; the gas flow rate was regulated
by adjusting a control valve until the prescribed flow was indicated on
the flowrator.
Eighteen exploratory runs were made with -8 mesh limestone at feed
rates of 11 to 300 Ib/hr. Tank pressure, except for one run at 70 psig,
was less than 20 psig. At feed rates much lower than 60 Ib/hr, limestone
flow was neither reproducible nor continuous. This was unsatisfactory
since limestone feed rates of 20 to 40 Ib/hr are required in bench-scale
development studies.
Nine exploratory runs were made with -14 mesh coal—five at atmospheric
pressure and four at above-atmospheric pressure (^80 psig). After feeding
of the coal was started, it was observed in each of the runs that during
an initial period of short duration (<10 min), the coal feed rates either
increased gradually or fluctuated widely before reaching a steady-state
period of long duration. The average coal feed rates during the steady-
state periods for the five runs at atmospheric pressure in the receiving
vessel ranged from 31.6 to 45.7 Ib/hr. The air flow rates ranged from
1.29 to 2.67 scfm. The maximum deviations of feed rates from the average
values were approximately ±15%, similar to those observed previously with
limestone. In the four above-atmospheric experiments, the average coal
feed rates ranged from 97.8 to 179.3 Ib/hr and the air flowrates from 2.21
to 3.27 scfm.
Another set of tests was made at 76-85 psig to determine if uniform
feed rates lower than the minimum previously obtained in high-pressure
operation could be achieved by modifying the equipment and/or using coal
with different properties. The equipment modifications investigated in
this work were orifice diameter and length of the transport line. The
coal properties studied were particle size distribution and moisture content.
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70
FILLING VALVE
TANK PRESSURE
GAUGE
TANK PRESSURE
REGULATOR
TANK PRESSURIZING —
VALVE
GAS SUPPLY CONNECTION
FLOWRATOR
MIXING ASSEMBLY
INJECTION HOSE
EXHAUST VALVE
PRESSURE RELIEF
VALVE
LINE PRESSURE
GAUGE
DILUTER PRESSURE
GAUGE
DILUTER PRESSURE
REGULATOR
DILUTER CONTROL
VALVE
DILUTER SHUTOFF
VALVE
DILUTER HOSE
BUSHING
FEED VALVE
QUICK DISCONNECT
COUPLING
(FOR 3/4"aSMALLER
SIZES ONLY)
Fig. 30. Model 16 ABC Petrocarb Injector
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71
SOLIDS FLOW FROM
INJECTION TANK
I
BUSHING
BALL
VALVE
ORIFICE
INSERT
QUICK
DISCONNECT
FROW
DILUTER
GAS LINE
SLEEVE HOLDER
SLEEVE-
Fig. 31. Cross Section of Mixing Valve Assembly
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72
Runs were made with as-received -14 mesh coal (moisture content of
4.0 wt %), and orifice diameters of 3/16, 1/4, and 5/16 in. In the run
with a 3/16-in. orifice, the coal did not flow under various operating
conditions. With a 1/4-in. orifice, coal flowed only for a short time
(8.3 min) before plugging. With a 5/16-in. orifice, the average feed rate
of coal was 136.9 Ib/hr at an air flowrate of 1.4 scfm. Thus, for flow
of as-received -14 mesh coal in the modified-standard Model 16 ABC Petrocarb
Injector, an orifice having a diameter of 5/16 in. or larger must be used.
In runs with a longer transport line, lower feed rates could be
achieved at a more uniform rate. The average feed rate for one run was
as low as 22.2 Ib/hr. One test was made with -14 mesh coal (containing
1. 2 wt % moisture) and a 50-ft long tansport line. In this run, feeding
of coal was initially irregular and eventually stopped.
Results of these exploratory tests showed that the feeder was not
suitable for feeding at the rates required for the pressurized combustion
unit tests and instead a rotary pocket feeder will be used.
VI. ACKNOWLEDGMENTS
We gratefully acknowledge the help given by Dr. R. C. Vogel,
Mr. D. S. Webster, Dr. S. Lawroski, and Mr. L. Link in directing and
reviewing the program, by Dr. S. Wood for his advice and assistance in
equilibrium calculations, by the analysis team of Mr. M. Homa, Mrs. C.
Blogg, Miss F. Ferry, Miss J. Williams, and Mr. Z. Tomczuk directed by
Dr. R. Larsen and Mr. E. Kucera, and by our secretary, Miss P. Wood.
VII. REFERENCES
1. A. A. Jonke et al., Reduction of Atmospheric Pollution by the
Application of Fluidized-bed Combustion, Annual Report, July 1970-
June 197L, ANL/ES-CEN-1004.
2. A. A. Jonke et^ al_., Reduction of Atmospheric Pollution by the
Application of Fluidized-bed Combustion, Annual Report, July 1969-
June 1970. ANL/ES-CEN-1002.
3. G. K. Rubtsov and N. I. Syromyatnikov, Investigation of Gas Combustion
in Fluidized Beds As Applied to Heating Furnaces, Russian Metallurgy
and Mining, No. 2, 50-57 (1964).
4. G. P. Curran e_t al., Coal-Based Sulfur Recovery Cycle in Fluidized
Lime Bed Combustion, Second International Hueston Woods Conference,
College Corner, Ohio (October 1970).
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73
BIBLIOGRAPHIC DATA 1- Report No. 2.
SHEET EPA-R2-73-253
4. Title and Subtitle
Reduction of Atmospheric Pollution by the Application of
Fluidized-Bed Combustion and Regeneration of
Sulfur -Containing Additives
7. Author(s) G. J. Vogel, E. L. Carls, J. Ackerman, M. Haas,
J. Riha, C. B. Schoffstoll, J. Hepperly, and A. A. Jonke
9. Performing Organization Name and Address
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439
12. Sponsoring Organization Name and Address
EPA, Office of Research and Monitoring
NERC/RTP, Control Systems Laboratory
Research Triangle Park, North Carolina 27711
3. Recipient's Accession N". ;
5- Report Date
June 1973
6.
8- Performing Organization Kepi.
N°ANL/ES-CEN-1005
10. Project/Task/Work Unit No.
11. Contract/Grant No.
EPA-IAG-0020
13. Type of Report & Period
covered Annual
July 1971-June 1972 :
14. :
1
15. Supplementary Notes
16. Abstracts
The report discusses fluidized-bed combustion (FBC) as a means of
removing from the gas phase nearly all of the atmospheric pollutants (sulfur and
nitrogen compounds) generated during the combustion of fossil fuels. Particulate
lime solids (additives) are introduced into the fluidized bed and react with the sulfur
compound formed during combustion. It discusses: pollution control by FBC of oil
with an excess of air and by the combustion of coal with a deficiency of air; the
thermodynamics of several proposed processes for regenerating additives; and
regeneration of sulfur-containing additive by the two most promising processes--
a one-step reductive decomposition of CaSO4 and a two-step (reduction-CO2/H2O
regeneration) procedure.
17. Key Words and Document Analysis. 17o.
Air Pollution
Fluidized-Bed Processing
Sulfur Oxides
Nitrogen Oxides
Limestone
Dolomite (Rock)
Combustion
Oils
Coal
17b. Identifiers/Open-Ended Terms
Air Pollution Control
Stationary Sources
Fluidized-Bed Combustion
Fluidized-Bed Oil Combustion
Two-Stage Coal Combustion
17e. COSAT1 Field/Group 13B
V' scr iptors
Calcium Sulfates
Sulfur
Additives
Fossil Fuels
Calcium Oxides
Stoichiometry
Thermodynamics
18. Availability Statement
Unlimited
19. Security Class (This
Report)
UNCLASSIFIED
20. Security Class (This
Page
UNCLASSIFIKD
21. No. of Pages
73
22. Price
$5.45
FORM NT1S-35 (REV. 3-72) USCOMIM-OC I4BE2-P72
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