United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-070 July 1984
SER& Project Summary
Kinetic Studies Related to the
LIMB Burner
A. Attar
Theoretical and experimental studies
were conducted on subjects related to
the limestone injection multistage
burner (LIMB). The main findings
include data on the rate of evolution of
HzS from different coals and on the
dependence of the rate of evolution on
the distribution of organic sulfur func-
tionalities in the coal.
A method was developed for deter-
mining the pore structure of solids at
high temperatures which also allows
estimates of the diffusion and adsorp-
tion coefficient of the gases onto the
solid surface using the pulse dispersion
method (PDM) and Fourier analysis.
The method was applied to the sintering
of lime. In addition, the PDM was used
to determine the influence of sulfation
on the diffusion of gases into lime.
Several other studies were conducted,
including analyzing the influence of ash
composition on ash fusion tempera-
ture and developing new correlations
for their estimate. The surface forces on
coal and how they affect the adherence
of minerals to the surface were deter-
mined. Also, the rates of reduction of
iron pyrite and doped iron pyrite by
hydrogen were measured. A compre-
hensive survey of the chemistry and
kinetics of the various reactions which
occur in LIMB is presented, along with
new findings.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory, Research Triangle
Park. NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
The project included theoretical and
experimental work related to the lime-
stone injection multistage burner (LIMB).
The theoretical part included: (1) a
literature survey of the chemistry,
kinetics, and thermodynamics of the
reactions of sulfur-containing gases with
limestone and lime during staged coal
combustion (Chapter 1); (2) development
of novel correlations between coal ash
fusion temperatures and ash composition
(Chapter 10); (3) development of compu-
ter programs which allow on-line mea-
surement of the diffusion coefficient, the
adsorption coefficient, and porosity of
solids at high temperatures (Chapters 2,
3, 4, and 11); and (4) development of
computer programs which predict the
rate of decomposition of limestone
(Chapter 1).
The main experimental work concen-
trated on: (1) building a pulse dispersion
analysis system and using it to study
both the rate of sintering of lime at
different temperatures (Chapter 3), and
the rate of poor plugging of lime during
sulfation (Chapter 4); (2) the composition
and porosity of limes and of doped lime
(Chapters 5 and 9); (3) the rate of
reduction of iron pyrite by hydrogen and
the effect of impurities on the rate of
reduction (Chapter 6); (4) the surface
forces on coal particles and their effect on
the adherence of minerals to the coal
(Chapter 7); (5) the rate of evolution of H2S
from different coals as a function of the
distribution of organic sulfur groups in
the sample (Chapter 8); and (6) trace
elements in different size fractions of
Fredonia Valley limestone.
Accomplishments
The main accomplishments and find-
ings are:
A. Developing a clear qualitative and
semi-quantitfitive understanding of
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the chemistry, kinetics, and thermo-
dynamics of the reactions occurring
in LIMB.
B. Devising a method for on-line
studies of the porosity, adsorption
coefficients, and diffusion coefficient
of gases in solids at high tempera-
tures, simultaneously with the
progress of chemical reactions.
C. Establishing the kinetics of sintering
of lime.
D. Establishing the roles of SO2 con-
centration and temperature on the
change in the porosity of lime and in
the diffusion coefficient of gases in it
during sulfation.
E. Obtaining qualitative and semi-
quantitative information relative to
the rate of evolution of H2S from coal
as a function of the distribution of
sulfur groups in it.
F. Developing new and simpler corre-
lations for ash fusion temperature
as a function of ash composition.
G. Determining the rate of reduction of
FeS2 by H2 and the influence of
temperature and impurities on its
magnitude.
H. Establishing the role of surface
functions on the adherence of
inorganic materials to coal surface.
On-Line Measurement of Solid
Properties at High
Temperatures
A method was developed for on-line
measurement of solid properties at high
temperatures. It consists of passing a
pulse of a tracer through a bed packed
with the solid to be studied. The porosity
of the solid and the interaction of its
surface with the tracer change the shape
of the pulse, depending on the magnitude
of the interaction and the properties of
the solid. The pulse is detected by a device
that produces a signal proportional to the
tracer concentration. The signal is
digitized and decomposed to its Fourier
coefficients. A mathematical model
which describes the various gas/solid
interactions was developed and solved in
the Fourier domain. The solid porosity
and the parameters of the gas/solid
interaction are selected by a multidimen-
sional search routine to obtain the best
match.
The pulse dispersion method was
applied to several systems, including:
lime sintering, pore plugging in lime due
to sulfation, and pore plugging in
catalysts due to metal deposition (this
was used to verify the usefulness of the
method).
Lime Sintering
Lime surface area sinters in two
stages: a very fast one (1 -15 seconds) and
a slow one (15 seconds-hours).
The kinetics of the slow sintering stage
can be described by:
reactions with Arrhenius temperature
dependence:
. d[sulfur functions] _
dt
/So\z=i+
\sJ
where S is the BET (Brunauer, Emmett,
and Teller) surface area, and S0 is the
initial specific surface area of the sample:
k(T) = k0e-E/RT= k0e-60'000/RT;
i.e., the sintering rate constant follows
approximately the Arrhenius correlation
with an activation energy of about 60
kcal/mole.
The initial specific surface area of the
sample influences the kinetics of the
sintering.
Calcite decomposes in parallel sheets
rather than according to a dense shrink-
ing-core.
Impact of Lime Sulfation on
Gas Diffusion into the Solid
Larger concentrations of SO2 and/or
higher reaction temperatures sulfate the
lime faster, but also result in a smaller
overall conversion.
The maximum conversion achievable
is never reached by sulfation because of
diffusion limitations in the sulfate layer
formed. A fixed limit on the achievable
conversion exists, however, and depends
on the conditions of sulfation: the limit is
lowered as the temperature and/or
concentrations are increased.
The Kinetics of Sulfur
Evolution from Coal as H2S
The distribution of organic and inorganic
sulfur in coals determines to a large
extent the rate of evolution of HgS from
coal particles in nonoxidizing atmospheres.
Since the distribution of sulfur groups in
different coals varies, the rate of evolu-
tion of H2S from them will vary. As a part
of a DOE contract, the distribution of
organic sulfur functionalities and inorganic
sulfur components in 19 coals and in 6
crude oils was determined. Started under
this EPA project was a determination of
the rate parameters for the evolution of
H2S from different sulfur functionalities.
In the three coals tested by the end of the
project, 2-18% of the organic sulfur in
them evolved as H2S in the first 200 msec
in the combustor. (More recent studies
with additional coals found sulfur volatili-
zation as H2S as high as 60%.) The
approximate rate parameters for H2S
formation from different groups can be
approximately correlated by first-order
dt
Function
functions]
k0 sec E kcal/mole
Aliphatic thiols
Aromatic thiols
Aliphatic sulfides
Aryl sulfides
13.8 9.9
836 12.6
2700 19.5
48000 26.0
The coal particle size exerts a tremendous
influence on the rate of heating of the
particles and therefore on the rate of
evolution of H2S from them. A computer
program was prepared which allows the
prediction of the rate of evolution of H2S
from coal particles with a known distribu-
tion of sulfur groups, particle sizes, etc.
Predicting Ash Fusion
Temperatures from Ash
Compositions
Traditionally, ash fusion temperatures
were correlated empirically with the ash
composition using a variety of equations.
A new semi-theoretical approach was
developed which allows equally as
accurate predictions of fusion tempera-
tures with fewer parameters than other
methods.
The method assumes that if a proper
"average" ash composition is selected,
then the concentrations of the different
components of any actual ash will deviate
from it by only a small magnitude. Thus,
relative to the "average" ash, the acutal
ash behaves like an ideal solution. If the
concentration closure condition is applied
together with Van't-Hoff type-T depend-
ence, the actual ash fusion temperature,
T, can be described by:
N
J_=JL+ I Jlln(1 -AX,)
T To i=1 A,
where T0 is the fusion temperature of the
"average" ash. A, is a constant that
depends on each ash component, and AX,
is the molar concentration deviation for
each ash component from the average
ash. Several hundred ash-composition/
fusion-temperature data points were
examined and were tabulated relative to
the "best" T0, A,, and average ash
compositions to be used for low and for
high calcium ashes for the Initial Deform-
ation Temperature (IDT), Half-Sight
Temperature (HST), and Melting Temper-
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ature (MT) in both reducing and oxidizing
atmospheres.
Rate of Reduction of FeSz
byHz
Iron-pyrite and doped iron-pyrite were
reduced with hydrogen in the tempera-
ture range of 300-800°C using the pulse
dispersion technique. The rate of reduc-
tion is initially limited by the rate of
nucleation of the new phase, FeS; but
after nucleation occurs, a layer of FeS
begins to form which partially reduces
the rate of diffusion of sulfur ions in the
coal matrix. Certain impurities, when
doped on the FeSa surface, enhance the
rate of nucleation and help form a more
porous FeS layer which allows a faster
rate of reduction of the FeS2.
Coal Surface Forces
The surface forces on the organic coal
matrix attract the coal mineral particles
and make them adhere to the organic
particle surface. The surface forces are
either ionic or Van Der Waals forces.
The ionic forces were determined in
several coals, and two methods were
developed for determining the Van Der
Waals force's density and intensity.
Chemical treatments which increase the
surface force's density and/or intensity
seem to result in a stronger adherence of
minerals to the organic surface and vice
versa. Data are presented for several
coals.
A. Attar is with North Carolina State University, Raleigh, NC 27650.
Robert H. Borgwardt is the EPA Project Officer (see below).
The complete report, entitled "Kinetic Studies Related to the LIMB Burner,"
fOrder No. PB 84-209 485; Cost: $25.0O, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
U S GOVERNMENT PRINTING OFFICE, 1984 — 759-015/7759
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Environmental Protection
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