United States
                   Environmental Protection
                   Agency
Air and Energy
Engineering Research Laboratory
Research Triangle Park NC 27711
                   Research and Development
 EPA/600/S2-88/069  Mar. 1989
x°/EPA         Project  Summary
                   Fundamental Studies of Dry
                   Injection  of Calcium-Based
                   Sorbents  for  SCte Control  in
                   Utility  Boilers
                   G. H. Newton, D. K.Moyeda, G. Kindt, J. M. McCarthy, S. L Chen, J. A. Cole,
                   and J. C. Kramlich
                    A research  program  was con-
                   ducted to  determine the mecha-
                   nisms which  limit the  extent  of
                   reaction between sulfur  dioxide
                   (SOa)  and  calcium-based sorbents
                   [CaCOa and Ca(OH)2l by  measuring
                   the in situ physical  structure and
                   reactivity of sorbent injected into a
                   combustion environment for resi-
                   dence times as short as 35 ms. Four
                   models of the sulfatlon reaction were
                   used to guide the research and
                   interpret the data. The  extent  of
                   sorbent utilization was found to be
                   limited by porosity losses during the
                   sorbent activation  process. In-sltu
                   porosities a fraction  of  that theo-
                   retically possible were measured In
                   the absence of SO2- At temperatures
                   below 1000°C, this porosity loss was
                   determined to  be caused by COa*
                   actlvated sintering. The presence of
                   SO2 during calcination reduced the
                   extent of porosity loss and at optimal
                   temperature sulfation  conditions no
                   loss in porosity  was observed. At
                   temperatures above 1200°C, porosity
                   losses may result from an increased
                   rate  of  thermal sintering  or a
                   decrease in the rate of the sulfatlon
                   reaction. Calcines from CaCOa suf-
                   fered greater losses in porosity than
                   those  from Ca(OH>2  which, along
                   with the  larger CaCOa particle size,
                   accounts for the substantial differ-
                   ences in SO2 capture between these
                   two sorbents.
  This Project Summary was devel-
oped by  EPA's Air and  Energy
Engineering Research  Laboratory,
Research Triangle Park, NC,  to an-
nounce key findings of the research
project that is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).

Introduction
  Upper furnace  injection of calcium
based sorbents to adsorb SOa has been
studied  extensively in recent years. A
complete understanding of the funda-
mental physical and chemical processes
which occur at furnace temperatures has
not, however, been achieved. Previous
studies of the sulfation process and well
established  theories have allowed the
rates of external mass transfer,  pore
diffusion, and product layer diffusion to
be calculated. These processes coupled
with the chemical reaction, the unknown
physical structure of a calcined sorbent,
and the extremely high rate of the overall
process (the reaction  is primarily over
within a few hundred milliseconds) make
it difficult to determine  which of these
mechanisms controls  the extent  of
sorbent utilization. Other studies  have
determined the  physical  structure  of
calcined sorbent but only at times much
longer than those available in full scale
utility boilers. This program was designed
to determine the  physical structure of
calcined  sorbents at these early times so

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that  the mechanisms  that limit sorbent
utilization can be understood and higher
levels of S02 capture achieved.

Experimental Facilities
  Fundamental  experiments  were con-
ducted in three research  facilities:  the
Isothermal Reactor (ITR); the Short Time
Reactor  (SIR);  and  the Controlled
Temperature Tower (CTT). Firing rates in
these three facilities range from 9.4  kW
for the ITR to 26.4 kW for the CTT.
  The ITR is  an electrically heated
drop-tube  furnace down-fired by  a
premixed  flat-flame burner.  Sampling
residence times normally range from  100
to 500  ms with isothermal  operating
temperatures  of 800-1500°C. The STR
is a back-fired furnace which operates
isothermally  at temperatures of 930-
1370°C with  residence times of  35-400
ms.   The  STR  can  operate  either
isothermally or under  quenched condi-
tions and  can  fire coal  as well  as
gaseous fuels. The  CTT  is a back-fired
reactor designed to simulate the time-
temperature histories of a wide variety of
coal-fired boilers. The  CTT can also  fire
either gas or coal.
  The ITR  was used  to obtain data on
SC>2  capture  as a function  of time,
temperature,  and sorbent type. This
information was supplemented with data
obtained  from the CTT on the influence
of quench rate on  S02 capture.  To
determine the in-situ  physical structure
of calcined sorbent, solids were sampled
from  the STR  as a  function of time,
temperature,  SC>2  concentration   and
sorbent type. A  CO  flame  was used
during solid sampling to allow sorbent to
be obtained without the presence of H20
in the  sampling system  while   still
providing a sulfation environment similar
to that in  a  coal or  natural gas fired
furnace. Surface area,  porosity, and pore
size  distribution  were  determined  for
sorbent collected from the STR.

Modeling
  The complexity  of  the   sulfation
reaction  requires that models  of  the
sulfation process be used to define which
process controls the  extent of sorbent
utilization. Four sulfation  modeling
approaches  were considered  in  this
program: a grain model, a  pore  tree
model, and two distributed pore models.
Each  model  assumes  that sorbent
particles  are spherical and fully calcined
prior to the onset of sulfation. In addition,
an  activation model of the calcination
reaction based on the grain model  was
utilized.
  The activation model was developed to
simulate  simultaneous  calcination  and
surface  area  loss. It considers CaCOa
decomposition at  the CaO/CaCOs inter-
face, diffusion of CC>2 through the CaO to
the particle  surface, diffusion  of  CC>2
from the particle surface to the bulk  gas,
and continuous, finite rate surface area
loss for the calcined  material  (sintering).
The  calcination process  is represented
by a spherical, shrinking core model  with
the intrinsic calcination  rate  dependent
only  on the chemical rate. The sintering
rate  is  assumed  to depend on  CC>2
concentration and  surface area.
  All of the sulfation  models include the
following sequential mechanisms:  exter-
nal mass transfer; pore  diffusion;  solid
state diffusion; heterogeneous chemical
reaction; and product layer buildup on
the internal surfaces. Each  model,  how-
ever, view the physical  structure of the
calcine differently.
  The  grain  model  treats  the  CaO
particle as an agglomerate of CaO grains
whose distribution of  sizes is set to
match  the measured BET surface  area.
The  pore  tree model  describes  the
sorbent pore structure as a set of trees of
various sizes whose trunks are located at
the particle surface. The size  distribution
of the pores is  proportional to  1/rp3,
where rp is the pore radius. Two versions
of the  distributed pore model  were
considered:  one viewed  the  pore
structure as  being made  up of  an
interconnected network  of pores, and the
other viewed the pore structure as having
non-intersecting pores.  Both  distributed
pore models use  experimentally  deter-
mined pore size distributions.

Results

Influence of Temperature
  Capture of SOa  for sorbents sulfated in
the ITR and  for the  Linwood hydroxide
sulfated in the STR fired with a CO flame
shows that a maximum in  SO2 capture
occurs between 1100 and 12008C.  The
activation and grain models indicate that
the  surface area of   a sorbent  will
increase to  a  maximum  immediately
upon calcination  and then rapidly de-
crease to an equilibrium value within a
few tenths of a second. The surface  area
of the  Linwood hydroxide sampled from
the STR, both with and without  SO2
present, does not show this. The surface
area  of these  samples  did  not
significantly  vary with either  time or
temperature.  The  measured  porosity of
these solids, plotted in Figure 1, also did
not vary with time or temperature and are
much less than the porosity theoretical!
possible from Ca(OH)2 (0.49).
  The  porosity of the calcined  sorbei
sampled from the STR with S02 preset
has,  however,  been  reduced  from il
original value  by the  buildup  of  th
CaS04 product layer within  the por
structure. These original porosities, whic
can  be calculated when the extents <
sorbent utilization are  known, are  plotte
in  Figure  2.  A maximum  in calculate
original  porosity  is observed  betwee
1100 and  1200°C  which is nearly equ;
to  the theoretical porosity of Ca(OH)2. /
lower  and  higher temperatures   th
calculated original porosities were  les
than  the theoretical value. A compariso
of  these calculated original porosities t
the  measured  porosities  without  SO
present (Figure 1) reveals that when SO
is present the extent of porosity loss fror
the theoretical porosity is either reduce
or  prevented.
  This dependency  of  the calculate
original porosity on temperature and th
presence of S02 allows a new hypothesi
to  be proposed:

  The process which causes this porosit
  loss (sintering) occurs at approximatel
  the same temperature as the sulfatio
  reaction. Porosity  loss  and  sulfatio
  may therefore be viewed  as processe
  which compete  for  available sulfatio
  sites. When the mechanism  whic
  causes porosity  loss is fast compare
  to  sulfation, the loss  in porosity result
  in  low  sorbent utilization.  Whe
  sulfation is  fast, porosity  loss  does nc
  occur and  greater  levels of sorber
  utilization result.

The  dependence of   porosity  loss o
temperature could be caused by either c
two  scenarios. At  low temperature
(~970°C) the sulfation reaction is  sky
due  to kinetic and diffusional limitation;
and  a  large loss in porosity occurs. /
intermediate temperatures (~1180°C
the  sulfation  reaction  is fast enough t
prevent  porosity  losses. At   highe
temperatures (>1200°C) the increasin
rate  of CaS04  decomposition  reactio
results in a net decrease in the rate of th
overall sulfation reaction, and porositie
again drop.
  The  most  likely  mechanism respor
sible for  this  porosity  loss  is  eithe
thermal or C02 activated sintering. EPA'
B. K. Gullett conducted a  similar set c
experiments  in an electrically  heate
nitrogen  (CO2  free)  flow furnace c
1000°C.  The measured porosities (with-
out  SO2) and  the calculated  origin*
porosities  (with  S02)  showed  n

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 1*
 I!
 oi o
 !«•
   x
   5
0.25


0.2





 0.1





  0


 0.2





 0.1





  0


 0.2





 0.1
             •o-
                                                      970°C
                                                            J270°C
                                                          Linwood
                                                          CO Flame
                                                    Q   w/o SO2

                                                   _ I
                     0.1
                                  0.2           0.3

                                     Time (sec)
                   0.4
                                                                    0.5
Figure 1.    Measured porosities of sorbent sampled from the STR.
significant  drop in porosity from  the
theoretical porosity of a  calcined  hy-
droxide within the first 500 ms.  Porosity
losses therefore  occur  only  in  the
presence of COg,  indicating  that CCV
activated sintering is  responsible for
porosities less than the theoretical value
(at the low temperature range).
  The second scenario describing  the
dependence of porosity loss on temper-
ature  is that at the lower temperatures
(970 °C) the high  rate of  COa-activated
sintering  is responsible  for  porosity
losses. At the intermediate temperatures
(~11808C) the rate of  COa-activated
sintering  slows, and porosity losses no
'•>nger occur. At  higher  temperature
  inge, porosity loss  is caused  by   an
increasing rate of thermal sintering rather
than a decreasing rate of sulfation. Data
are not currently available  to determine
which hypotheses is correct.
  A  modeling approach  was developed
based on the data obtained in this study.
Pore size distributions from the earliest
time sampled and an  average calculated
porosity at each temperature were used
as  inputs  for the   interconnected
distributed  pore model. The  results,
plotted as solid lines  in Figure 3, match
the data extremely well. To predict SOa
capture  at the highest  temperature
(1270°C),  the equilibrium  concentration
of 802 above CaSC^  had to be included
in the model, as indicated by the dashed
line.
                                          Influence of Mixing Rate
                                            Mixing of sorbent is known to influence
                                          the  extent of 862  capture. A  venturi
                                          throat in the ITR provides a completely
                                          mixed stream within 7-10 ms,  while a
                                          straight throat doesn't provide complete
                                          mixing until 24-40 ms.  Use of the faster
                                          mixing throat  results  in  significantly
                                          increased levels of SOa capture. It  was
                                          concluded that  slower mixing in  an
                                          isothermal furnace  affects 862 capture
                                          by:

                                          (1) Delaying the onset of sulfation.

                                          (2) Exposing  the sorbent to a distribution
                                          of temperatures. When mixing  is slow,
                                          eddies  within the sorbent/air jets  may
                                          obtain temperatures  between  the  jets'
                                          initial temperature  and  the furnace
                                          temperature.  While at these intermediate
                                          temperatures the sorbent will calcine and
                                          obtain an original porosity (see Figure 2)
                                          different than sorbent which calcines at
                                          the furnace temperature. The distribution
                                          of original porosities will result in a lower
                                          average porosity and  lower average
                                          sorbent reactivity.

                                          (3) Allowing incomplete mixing to occur.
                                          Mixing which is  not complete at the  end
                                          of a  furnace may result in eddies  with
                                          high  levels of sorbent  (and  depleted of
                                          862) and eddies containing low levels of
                                          sorbent (and high levels of
The ITR fitted  with the venturi throat has
extremely rapid mixing and none of these
phenomena  are  likely  to influence  SOa
capture.  When fitted with  the straight
throat,  factors  (1)  and (2)  above affect
capture.

Influence  of Quench Rate
  SOa  capture at  higher quench  rates
result from two mechanisms:

(1) The time  available at temperatures
where sulfation occurs is reduced.

(2) A finite mixing  time exposes the
sorbent to a distribution of temperatures.
Calcination occurs  at  a  range  of
temperatures, and   a  distribution  of
original porosities  results.  The  lower
average sorbent porosity leads to lower
sorbent reactivity and lower 802 capture.
This  effect also  results in a shift in the
temperature  where  the maximum  802
capture occurs.

When  this  second  mechanism  was
included in the interconnected distributed
pore model, it is able to account for the

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     0.5
     0.4
     0.3
I
     0.2
     0.1
          ™  Theoretical Porosity
                                                            Calculated
                                                             Original
                                                              Porosity
                                                            w/SOz
Measured
Porosity
w/o SO2
                                                          -318-369 ms
                                                             I	
        900         1000         1100         1200

                                  Temperature (°C)


Figure 2.   Measured porosities and calculated original porosities.
                                            1300
1400
differences in SOg  capture due to
quench rate.

Influence of Sorbent Type
  Ca(OH>2 from different commercial
sources are known to vary in their ability
to capture 802- Capture by a Mississippi
hydroxide in the STR is significantly less
than capture by the Linwood hydroxide.
The calculated original porosities for the
Mississippi hydroxide, plotted  in  Figure
4, are less for those than for the Linwood
hydroxide. Interconnected distributed
pore model predictions indicate that the
differences in capture  by these two
hydroxides result from the slightly  larger
particle size of the Mississippi Ca(OH)2
and  from the greater  porosity loss it
experiences.
  The difference in S02 capture between
carbonates and hydroxides was investi-
gated in the STR with a CO flame.  It has
been hypothesized that lower  levels  of
                          capture  by carbonates  result from  their
                          large particle size. Calculation of  their
                          original  porosities,  plotted  in  Figure  5,
                          reveals  that carbonates are  subject  to
                          greater losses  in   porosity  than
                          hydroxides. Predictions by the  inter-
                          connected  distributed  pore model
                          indicate that lower  SC-2  capture by
                          carbonates results (approximately)
                          equally from their larger particle size and
                          their greater porosity loss.  Analysis  of
                          the porosity data also indicates that
                          those portions of the carbonates  which
                          calcine  most rapidly have  low original
                          porosities while  those fractions calcining
                          more slowly have the theoretical porosity
                          of a calcined calcium carbonate.

                          Influence of  Physical Structure
                            Average  pore  diameters  for the
                          Linwood hydroxide sampled from the
                          STR without S02  present,  varied  as  a
                          function of temperature but not  as  a
function of time.  Pore size  distributio
corrected to  a  constant porosity a
plotted  in  Figure 6  ofor  the  Linwoi
hydroxide. Above 80 A, the  distributio
are about the same while at  below 80
the  distributions are  scattered.
determine  the  extent  that  the:
differences make  in   SOa capture, tl
pore size distributions in Figure 6 we
entered in the interconnected distribut
pore  model.  The  results of  the;
calculations,  presented  in  Figure
indicate that the observed differences
pore size  distributions  have  litt
influence  on  S02 capture.  To furth
determine  the influence  of pore  si,
distribution  on  sulfation  pore si.
distributions from  the  Linwood sorbe
sampled at 970°C  and  1270°C, tl
Vicron 45-3 carbonate, the  Mississip
hydroxide,  and the Fredonia hydroxi*
sampled from a  nitrogen  reactor
1000°C  were  corrected to  a  consta
porosity and  used  as inputs for tl
model. The  predictions exhibit  no mo
than 6% variation in  SC>2 capture. Tl
variations in sorbent pore size distribute
resulting  from  different sulfatic
conditions therefore  have no significa
effect on SC>2 capture.

Conclusions
  The goal of  this  program was
determine the mechanism(s) which tin
calcium  utilization.  This  was accor
plished  by investigating the relationsh
between sorbent physical structure ai
various  sulfation parameters.  In-si
sorbent physical  structure  was  dete
mined by sampling from the Short Tin
Reactor (STR),  an  isothermal react
fired with a CO flame. The use of a C
flame allowed sorbent to be sampled
the absence of H2O which was  shown
degrade calcined  sorbent  durir
sampling.
  Investigation  of  the  influence
injection temperature on SO2 capture ai
sorbent physical  structure revealed th
calcination of sorbent in  a  combustic
environment  resulted in   porositi<
dramatically  less than  theoretical
possible -  0.49 for  Ca(OH)2.  Tt
presence of S02 was found to reduce tt
extent of or completely prevent porosi
loss,  depending on  the  injectic
temperature.  The  cause of  this porosi
loss  was  determined  to be   CO
activated sintering.  A  hypothesis wj
proposed based on this information:
   Porosity loss and  sulfation  may I
  viewed as  processes which  compe
  for available sulfation sites. When tl
  mechanism  which causes  porosity lo

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      60
      50
    3 40

    I
    Cj
      30
      20
      10
Linwood
CO Flame
Ca/S = 2
                                 t       t

                            -1400 ppmS02
                                                    1180°C
                                           Equilibrium Concentration
                                           of SOi Above CaSOt
        0.0     0.05
                        0.10    0.15    0.20     0.25     0.30

                              Sorbent Residence Time (sec)
                                                    0.35
0.40
          cining  at longer  times  obtained  the
          theoretical porosity of a carbonate (0.54).
            Modeling indicated that,  although
          average  pore diameters varied  as  a
          function of temperature  (but  not as  a
          function of time), the observed variations
          in pore  size distribution had little  effect
          on SO2  capture  when corrected to  a
          constant  porosity. The use of pore size
          distributions from  carbonates and from  a
          sorbent  sampled  from  a  nitrogen
          environment did  not  change the
          predicted levels of SOa capture.
  Figure 3.    Interconnected distributed pore model predictions based on calculated original
             porosities.
  is fast compared to sulfation (at  low
  temperatures,  ~970°C), the  loss in
  porosity  results  in  low  sorbent
  utilization.  When  sulfation  is  fast
  (~1180°C), porosity  loss  does  not
  occur and greater  levels of  sorbent
  utilization  result.  At higher tempera-
  tures (>1200°C) the increasing rate of
  CaS04 decomposition  reaction results
  in a net decrease  in the rate of  the
  overall sulfation reaction and porosities
  again drop.

It was also noted  that,  at the  higher
temperature range  porosity losses may
result from an increased  rate of thermal
sintering  rather than  a decrease in  the
rate  of  the  sulfation  reaction.  A
distributed  pore  model, which used  the
measured pore  size  distributions  and
these porosities, was  able to  adequately
predict the observed  SO2  capture
without the use of adjustable parameters.
  Mixing was  found  to  influence BO2
capture by  (1)  delaying the  sulfation
reaction,  and  (2) altering the extent of
porosity loss  by  changing the  sorbent
thermal  history.  In an  isothermal
environment with slow mixing,  transient
                             temperatures (between a sorbent's initial
                             temperature  and the ambient  temper-
                             ature) may occur resulting in a porosity
                             loss not directly related to the  injection
                             temperature.
                               In  a quenched  environment,  mixing
                             which occurs over  a period  of time will
                             expose the sorbent to  a distribution of
                             temperatures and a resulting distribution
                             of porosity losses. This results in (1) an
                             average  porosity  different from  the
                             isothermal case (for a given injection
                             temperature)  and  a  resulting different
                             capture and (2) a shift in the temperature
                             where maximum SOg capture occurs.
                               The extent  of porosity loss and sorbent
                             particle  size were found  to  be  the
                             primary factors which determine sorbent
                             reactivity. Different  hydroxides experi-
                             enced significantly different levels of
                             porosity  loss, and  carbonates suffered
                             dramatically  greater losses  in porosity
                             than  did hydroxides.  The  degree  of
                             porosity  loss in carbonates  varied with
                             time.  Those portions  of  a  carbonate
                             which calcined under 35  ms  had  a
                             porosity  equal  to  that  of  a  carbonate
                             calcined without SC>2 present (the lowest
                             porosity  possible),  while  portions cal-

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      0.5
      0.4
      0.3
      0.2
      0.1
       0:
Theoretical Porosity
                               Linwood
                                             ft 1BO°C)
                           •o—.
                                                <1160°C>
                                               1400ppmS02
                                               CO Flame
                      0.1
      0.2
   Time (sec)
                                                   0.3
0.4
Figure 4.   Calculated original porosities of two hydroxides.

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0.6
0.5
0.4
g 0.3
§.
0.2
0.1
0.0
0
III
Theoretical Porosity
Projected
fj Calculated
-f^ Original (w/SOz
A£ N r
|£ £ Measured (w/o SOii
Vicron 45-3 (1 1 urn)
III
0 0.1 0.2 0.3
Time (sec)
•
•
•
•
• • i
Theoretical Porosity
["] Projected
_ JJ*
• ^VO Calculated
Cj Original (w/SOz
' ^ t^N


Measured (w/o SOi)
7fjm Vicron
ill
0.4 0.1 0.2 0.3
Time (sec)
0.4
Figure 5.    Measured porosities  of solids sampled without  SO2  and calculated original
             porosities of solids sampled with SOi corrected for extent of calcination.  The
             projected original porosities were calculated by assuming that the uncalcined
             portion of the collected sorbent would have a porosity of 0.54 upon complete
             calcination.
       15
       W
           10
                      t= 35ms
                      t= 69ms
                      t=110ms
                      t=369ms
                                                                          I  I I I
                    Linwood
                    1180°C

                     I     I    I  I  I  I  I  II
                        i    I   I  I  I I I
      100

Pore Diameter (A)
1000
  Figure 6.    Pore size distributions corrected to a constant porosity.

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              Linwood, STR
              CO Flame.  1180°C
              1400 ppmSOz
              Ca/S = 2
                                           0.2
                                        Time (sec)
                                                             0.3
                                                                              0.4
Figure 7.     Predictions of  the interconnected distributed pore model based on pore  size
             distributions from Figure 6.

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G. H. Newton, D. K. Moyeda, G. Kindt, J. M. McCarthy, S. L. Chen, J. A. Cote, „„«
  J. C. Kramlich are with Energy and Environmental Research Corp., Irvine,  QA
  92718-2798.
Brian K. Gullett is the EPA Project Officer (see below).
The complete report, entitled "Fundamental Studies of Dry Injection of Calcium-
  Based Sorbents  for SO2 Control in Utility  Boilers," (Order No. PB 89-134
  1421 AS; Cost: $36.95 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:
    Air and Energy  Engineering Research Laboratory
    U.S. Environmental Protection Agency
    Research Triangle Park, NC 27711
United States                   Center for Environmental Research
Environmental Protection         Information
Agency                         Cincinnati OH 45268
Official Business
Penalty for Private Use $300

EPA/600/S2-88/069
QOOQ3Z9    PS

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                u  S
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