-------
                      a.   8x  enlargment
  b.  Rolled structures
c.  Rolled and stacked structures
Figure 4-9.   Examples of Thermacomb corrugated ceramics, produced by
             American Lava Corporation.
                          4-18

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                     HC HONEYCOMB
         B ) SC  SPLIT  CELL  (Note  Separator)
         XFSC
     CROSS-FLOW,
      SPLIT CELL
     Note separators
     and corrugations
         at 90°
    XXSC
 CRISS-CROSS,
  SPLIT CELL
Note separators
and corrugations
    at 45°
    XXHC
 CRISS-CROSS,
 HONEYCOMB
with corrugations
   at 45°
Note there is no
   separator.
Figure 4-10,  Structural  types  of  Thermacomb
                corrugated  ceramic monoliths.
                     4-19

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Figure 4-11,  Examples of Celcor cordierite  monoliths,
             produced by Corning Glass Works.
                        4-20

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                      a.   Straight honeycomb
b.  Slant cell  honeycomb             c.   Crossflow honeycomb



     Figure 4-12.  Examples of DuPont's TORVEX ceramic  honeycomb.
                            4-21

-------
    ••••••••••••••••••••••••••••••••••••••••I
    ••••••••••••••••••••••••••••••••••••••••I
    ••••••••••••••••••••••••••••••••••••••••I
    ••••••••••••••••••••••••••••••••••••••••I
    ••••••••••••••••••••••••••••••••••••••••I
    ••••••*••••••••••«•••••••••••••••••••••»'

    ••••••••••••••••••••••IB
Figure 4-13.   Examples of General  Refractories Company's

               Versagrid ceramic  honeycomb.
                        4-22

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Figure 4-14.  Norton Company's SPECTRAMIC  honeycomb  material
                           4-23

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Upon ignition, the Cerafelt supports the combustion on the surface, but has no tendency
to flash back.  Porous heat-insulating fibers such as asbestos, silica, or basaltic fi-
ber are proposed for use in catalytic heaters in Reference 33, and Reference 34 describes
the use of Johns-Manville's Fibrechrome refractory felt as a burner face material.
       While asbestos is suitable for low-temperature applications, high-temperature
requirements call for fibers of silica or alumina.  For example, Johns-Manville's
Fibrechrome is formed from an alumina-silica-chromia composition and has a maximum
use temperature of 2700°F.  This material is shown in Figure 4-15.  Imperial Chemi-
cal Industries United States Incorporated markets both alumina and zirconia fiber
materials, under the trade name SAFFIL, with Figure 4-16 showing their alumina fiber.
Table 4-2 lists some important properties of these fiber materials; one advantage of
all fiber pads is their low thermal conductivity, thus preventing flashback since
the plenum side of the pad remains well below ignition temperature.

4.2.1.3  Surface Impregnated Pellets
       One advantage of monolithic supports is the presence of a large geometric
surface area and the possibility of depositing the catalytic materials on this sur-
face.  This increases the availability of the active catalyst to the main stream of
the combustion mixture, a property that increases performance in these fast reaction
systems.  The presence of the catalyst as close to the geometric surface of the sup-
port as possible is important to minimize diffusion limitations of the reactants into
pores.  This property can be achieved with highly porous pelleted catalysts as well.
It requires the application of a special technique of catalyst preparation called
surface impregnation.  The pellets, or catalyst carriers, come in a variety of mate-
rial composition, including alumina and silica.  Formed by a variety of methods, such
as extrusion, the catalyst carriers come in different shapes and forms.  Both Norton,
Chemical Process Products Division and Houdry Division, Air Products and Chemicals
offer catalyst carriers that have a wide range of surface areas and pore size dis-
tributions.  Typical shapes and forms of Norton catalyst carriers are shown in
Figure 4-17.

4.2.2  Substrate Wash Coat Materials
       As mentioned earlier, the low  surface area of the monolith structure requires
the  application  of a thin coat of oxide such as Al^O,.  This wash coat, which strongly
adheres to  the ceramic or refractory  support, provides a uniform high surface area
while  still insuring that the catalytic material which is subsequently impregnated on
the  wash  coat is  close  to the main  flow of reactants.  The thickness of the wash coat
is usually  between 10 x 10~6 and 20 x  10"6 m (10-20 microns).
                                       4-24

-------
! i
I
:- ,
CJ1
                                 Figure4-15.  Johns-Manvilie's Fibrechrome - a fiber pad.

-------
ro
CPi
                               Figure 4-16.  ICI United States'  SAFFIL -an  alumnia fiber.

-------
                                                           TABLE 4-2.   FIBER  PAD  DATA
Manufacturer
ICI United
States Inc.
Johns-Manville
Product
SAFFIL
(Standard
Alumina)
SAFFIL
(High Temperature
Alumina)
SAFFIL
(Standard Zirconia)
SAFFIL
(High Temperature
Zirconia)
Cerafelt
Fibrechrome
Description
Inorganic alumina
fiber
Inorganic alumina
fiber
Inorganic zirconia
fiber
Inorganic zirconia
fiber
Silica-alumina
fiber
Silica-alumina-
chromia fiber
Temperature
Limit (°F)
1850
2750
2000
2900
2300
2700
Specific Heat
(Btu/lb-°F)
 1000°F
0.95
N/A
0.90
N/A
0.7a
0.75a
4=-
ro
            Density  10  lb/ft:

-------
4»

oo
                                                                                                   *Z£^
                                                                                                   ***.. .».^4.-ii-^:.»v.. if -•- '. - • ' • . — •».• S
                                                                                               Stars
                                                     Fluidizable
                                                     -20 mesh)
                                                     Aggregate
                                                     ( + 20 mesh)
                                                                  Spheres
                                                                                                Saddles
                                             Figure  4-17.   Examples of  Norton  Company's catalyst
                                                              carriers.

-------
       Many different materials can be used as wash coats.   ZrCL.  for example,  has
been used in test runs for the catalytic combustion of hydrocarbons  in automotive
exhaust.  Variation in the type of wash coat can have important consequences  in the
stability and life of the catalyst system.

4.2.3  Catalyst Coatings
       Although the field of catalysis has  progressed substantially  over the  past
decade, its theoretical aspects are not yet at the degree of sophistication which
would enable one, a priori, to choose or design an active oxidation  catalyst  with a
given set of catalytic properties.  However, a vast amount of data exists which per-
mits the development of correlations useful for choosing promising candidates based
on excellent catalytic activity and catalyst stability.   Over the  last decade there
has been a rapid development of the art, science and technology necessary to  synthe-
size, test and manufacture ultrastable oxidation catalysts for the abatement  of auto-
motive emissions.  These catalysts operate  under the most stringent  conditions, and
sometimes at temperatures close to 2000°F.   The expertise developed  in this area will
be useful in the development of a stable catalytic combustion system.
       Two broad classes of catalytic coating materials  are available:  metals  and
oxides.  These are discussed separately in  the following sections.

4.2.3.1  Metals
       The metals of catalytic interest, which are readily synthesized in the zero
valent state at reasonable conditions, are  listed in Table 4-3.  Of  these metals,
the only ones which have a possibility of remaining in the metallic  state in  a  high-
temperature, oxidizing environment are the  noble metals.  The others readily  form
oxides, and will be discussed in Section 4.2.3.2.  Of the noble metals, a large vol-
ume of data and correlations are available  for Pt and Pd because of  their use as
automotive oxidation catalysts.  They are among the most active catalysts for the
oxidation of a number of fuels, e.g., methane (Reference 35), methanol (Reference
36) and hydrogen (Reference 37).  The high  activity of these metals  is related to
their ability to activate H^, O^, and carbon-hydrogen and oxygen-hydrogen bonds.
Palladium and platinum are readily prepared in a highly dispersed  form on a  number
of support materials.  Because of the high  activity of these metals  per unit  area of
metal surface (specific activity) and the ability to attain high dispersions, only
small amounts are necessary for catalytic combustion (0.1 - 0.5 wt%).  Ruthenium
metal is another possible candidate for catalytic combustion; however, under oxidi-
zing conditions it forms a volatile oxide (RuO.), which is rapidly removed from con-
ventional catalyst supports.  This problem has been recently solved  by researchers
interested in the NO  reduction properties  of Ru.  One approach has  been to anchor
                    A
Ru to a support by forming a relatively stable perovskite structure with certain
                                       4-29

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TABLE 4-3.  METALS OF CATALYTIC INTEREST FOR CATALYTIC COMBUSTION
GROUP
Fe Co
Ru Rh
Os Ir
VIII* GROUP IB*
Ni
Pd
Pt

Cu
Ag
Au

              Enclosed metals are considered noble.
                              4-30

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oxides such as La_0- (Reference 38).   Osmium is even more volatile and poisonous
than ruthenium in an oxidizing environment.   Also, as with iridium and rhodium,  it
is very costly, and available in limited supply.   The use of the latter two metals,
if at all, would be restricted to small quantities in multimetallic systems.   Silver,
while quite active at low temperatures for the activation of (L, melts at low temper-
atures and therefore sinters to a significant degree at the high temperatures of
catalytic combustion.  However, it could find use as an additive in a multimetallic
catalyst.  Gold is very inactive for oxidation, and would therefore only be consid-
ered as a structural or electronic modifier for multimetallic catalysts.
       Based on the above considerations, those metals which show the greatest pro-
mise for use in a single-active-element catalyst system are:
       •   Pt
       •   Pd
       •   Ru (stabilized)
Highly active and stable catalyst systems can be produced through the use of multi-
metallic systems and the addition of structural and electronic promoters.  For exam-
ple, Pd/Pt bimetallic clusters, which have been extensively studied as automotive
oxidation catalysts, combine the high catalytic activity of platinum with the rela-
tively low cost of palladium.  Similarly, extensive studies with metals have shown
that the rate of catalytic oxidation of hydrocarbons parallels the catalytic activity
for CL isotope exchange:

                              0216 + 0218 -»• 2018016

Thus, exchange is thought to be sensitive to the electronic properties of the metal.
These properties are readily changed by alloying or forming supported multimetallic
clusters.  Therefore, Groups VIII and VIII-IB combinations appear promising.  Bime-
tallic combinations can have high structural and thermal stability, especially if
one of the metals is high melting.

4.2.3.2  Metal Oxides
       The catalytic properties of metal oxides have been extensively studies by a
number of research groups (Reference 39).  Some of the simple oxides, e.g., Co-0.,
have oxidation activity comparable to the very active noble metals like Pt and Pd.
A primary difference between these metals and the oxides'is the so-called "light-
off" temperature.  This relates the ability of a catalyst to reach a significant
conversion level at low temperatures and in short periods of time.  This is more of
a problem for automotive oxidation systems where a significant amount of emissions
occurs after a cold start.  Therefore, it is imperative that the oxidation catalyst
                                       4-31

-------
bed reach operation temperature as quickly as possible.  This could also be a prob-
lem for catalytic combustion installations, although for some units a precombustor
might be built into the system for startup.  Other alternatives include doping a
high light-off temperature catalyst with small amounts of noble metals to initiate
low temperature combustion, or using a multi-bed catalyst where the first small bed
is a noble metal catalyst.
       There has been a significant amount of work with oxides in the area of cataly-
tic oxidation.  The specific catalytic activity, i.e., per unit surface area, can be
varied over very wide limits.  It has been found, as for the case of metals, that
the activity for hydrocarbon oxidation parallels the ability of the catalyst to cata-
lyze the homomolecular exchange of oxygen.  Figure 4-18 illustrates the extensive
range of activities one can achieve for this exchange reaction.  It also shows the
parallel behavior of various oxidation reactions.  One will  note that the oxides of
transition metals containing ions with partially filled d-orbitals have the greatest
activity.  It also has been found that the rate of hetero-exchange,

                        0018 + O16   - -»• 018016 + O18   .
                         2        surf               surf

over oxides which contain an equilibrium amount of oxygen in the surface layer is
equal to that for homomolecular exchange (Reference 40).  The coincidence of rates
of homomolecular and hetero-exchange suggests that on oxides with an equilibrium
oxygen content, simple isotopic exchange of molecular oxygen takes place with parti-
cipation of oxidic lattice oxygen.  Since this exchange parallels oxidation rate
(Figure 4-18) one can deduce that the strength of the oxygen bond in the oxide plays
a significant role in determining activity.  This point is further amplified by the
data of Figure 4-19, which shows the correlation between activation energies for
methane oxidation and oxygen bond strength for a number of oxide catalysts.  An anal-
ysis of these data suggests that the rate limiting step for methane oxidation invol-
ves the breaking of an oxygen-catalyst bond (Reference 40).   The oxygen lattice en-
ergy in many cases can be controlled by the addition of certain dopants.  Thus,
Figure 4-20 shows that one can control this parameter and therefore the rate of ex-
change and the catalyst's oxidation activity.
       The above considerations have indicated that the most active simple oxides
are:

                    Co304, Mn02, NiO, CuO, Co^, Fe^, V^.

Other simple oxides such  as Ti02 and ZnO have very low activity, and would therefore
be  of interest  only in the consideration of complex oxides and multicomponent systems.
                                       4-32

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C7>
O
     10
           TiO
ZnO
     Figure  4-18  ,   The  catalytic  activity of  oxides  of  the  elements
                    of the  fourth  period  in  the  homomolecular exchange
                    of oxygen  (1), oxidation of  hydrogen  (2), oxida-
                    tion of methane  (3) and  nitrogen  oxide decomposi-
                    tion (4) at  300°C.
                                4-33

-------
        Co304
NiO

  I
              Fe2°3
               V2°5
                              I
                         I
     10
20
30          40

 q,  kcal/mole
ZnO   TiO,
                         I
                                                     50
         60
Figure 4-19.  Activation energies of homomolecular oxygen exchange
              reaction (I), methane oxidation (II), as a function
              of oxygen bond energy, for oxides of elements of the
              fourth period (Reference 40).
                                   4-34

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 Logy
                                     K2S04
Rb2S04
Cs2S04
Figure 4-20.  Catalytic activity of vanadium pentoxide promoted  with  sulfates
              of various alkali  metals.   Curve 1  - reaction of exchange  of
              molecular  oxygen; curve 2 - reaction of oxidation of sulfur
              dioxide (Reference 40)
                                     4-35

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       As with the metal, multi-component catalysts may provide increased activity
and stability.  Materials of interest include stable spinel structures such as Co304/
CuO, Cr203/CuO and La20/Co304-  These materials are likely to be very active and show
good thermal stability.  Doping, using alkali, alkaline-earth metals and rare-earth
metals, may also show increased activity and thermal stabilization.

4.2.4  Temperature Capability of Catalyst/Support System
       The choice of an optimum catalyst/support combination requires a consideration
of the temperature capability of a given combination.  The low temperature limit is
dictated by the so-called "light-off" temperature.  This limitation  has been discus-
sed in Section 4.2.3.2.  Solutions to this problem are not complex for most catalytic
combustion systems, and some of the possible solutions have been discussed in that
section.  The major thermal limitation for catalytic combustion is the high tempera-
ture of operation.  The phenomena which lead to catalyst deactivation at high temper-
atures are:
       •   Sintering of the catalytic species
       t   Changes in catalyst stoichiometry
       •   Sintering of the wash coat
       t   Thermal degradation of the ceramic support material
       0   Mechanical failures
       Sintering of the catalytic species will lead to a concomitant loss in surface
area and therefore a loss in catalyst activity per unit mass of catalyst.  The tem-
perature at which sintering occurs is a function of the catalytic material.  Thus,
Ag will sinter at much lower temperatues than Pt or Rh.  In an oversimplified man-
ner this sintering or crystallite growth is a function of the melting point for
metals.  Thus, Ag melts at much lower temperatures than Pt or Rh, and therefore Ag
atoms will  have greater mobility at a given temperature than Pt or Rh atoms.  Such
mobility is required for crystallite growth.  A number of techniques have been de-
veloped to minimize or prevent sintering.  For example, multimetallic systems can be
prepared which are more stable than the constituent metals.  A number of structural
promoters are known.  For example, A1203 is used to stabilize high surface area Fe,
which  is active for NH3 synthesis, and Th02 is similarly used to stabilize Fe for
Fischer-Tropsch syntheses.
        Stoichiometry changes for oxides can lead to a catalyst with  dramatically dif-
ferent  catalytic  properties.  As previously discussed, loss of lattice oxygen can
cause  an increase in the energy required to activate 02 and thereby  decrease combus-
tion activity.  As indicated  in Section 4.2.3.2, one method for overcoming this
                                       4-36

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problem is to dope the lattice with a material  which causes  a decrease  in  the  energy
necessary to activate oxygen and also stabilizes the stoichiometry of the  lattice.
       A third mechanism of thermal deactivation is wash coat sintering.   At temper-
atures above 1650°F high surface area n- or Y-Al^O., undergoes a phase change to  a-
A^O-j with concomitant sintering.  The change may be from a  surface area of 300  m2/g
to about 5 m2/g.  This sintering results in pore closure and a "burying" of active
catalytic sites in the Al_0,.  To minimize this problem, it  is possible to work  with
catalysts for which the catalytic phase has been deposited on a presintered AUO.,.
Alternatively, other materials which are thermally more resistant may be investiga-
ted as wash coats.  These might include ZHL, Th02 and UOp.
       At very high temperatures, the ceramic support, be it spheres or monolithic
honeycomb structures, begins to degrade.  Typical substrate  fail  temperatures  have
been given in Tables 4-1 and 4-2.  Although catalytic combustion  units  are not ex-
pected to operate continuously at the temperatures listed, the possibility of  local
hot spots occurring due to uneven air/fuel mixing and local  catalyst bed nonuniform-
ities must be considered.
       Thermal expansion and contraction may also occur at high temperatures.  Both
pellet and monolithic substrates have thermal expansion coefficients different than
their mounting hardware.  Considerable catalyst mechanical attrition can occur if
the catalyst becomes loose, thus causing a decrease in catalytic  performance.

4.2.5  Poisoning Effects
       Two types of adsorptive poisoning can be distinguished; (a) homogeneous ad-
sorption of the poisoning molecule, and (b) selective adsorption.  In homogeneous
adsorption, the poison is uniformly distributed over the catalyst surface  and  through-
out the pores.  This usually occurs when the poison is weakly adsorbed  on  the  surface
and usually with a strength comparable to that of the reactants.   An example of  this
type of poisoning is the effect of small amounts of sulfur in a dehydrogenation  re-
action over a Pt catalyst.  In this case the sulfur will be  removed in  the presence
of hydrogen at reaction conditions and activity can be recovered.  Selective poison-
ing occurs when the poison is so strongly adsorbed that the  pore mouth  in  the  cata-
lyst becomes almost completely poisoned before the pore interior is affected.  For
this type of poisoning, one can measure the progressive poisoning across the cross-
section of the catalyst pellet, provided the concentration of poison is low in the
reactant stream.  This type of poisoning is usually irreversible and leads to  a
stoichiometric reaction with the catalytic phase.  A typical example is the sulfur
poisoning of a nickel catalyst, where the interaction results in a compound forma-
tion of nickel sulfide.
                                       4-37

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       Participate poisoning is also possible and occurs when the catalyst bed acts
as a filter and removes participate material such as carbonaceous particles from the
combustion products.  This coats the catalyst surface and acts as a physical barrier
to the diffusion of reactant species to the catalyst.  Deposits of poisons such as
lead and inorganic materials are very harmful, as shown by previous work with auto-
motive catalysts.
       In general, lead, sulfur and phosphorous are considered strong poisons for
metal catalysts because of the possibility of particulate poisoning (lead) or selec-
tive adsorption (sulfur and phosphorous).  This is confirmed by studies performed in
the automotive industry.  Other potential poisons are zinc, barium, halides and other
trace elements.  Restricting consideration to natural gas and light distillate oils,
many of these poisons are not expected.  Possibly phosphorous, sulfur and certain
halides would be the major poisons encountered.

4.2.6  Characterization Techniques
       A common basis of comparison is crucial for the effective evaluation of a
series of different catalysts.  Since catalytic action occurs on the surface of the
solid, surface area is the basis that has to be used.  Therefore, the only meaning-
ful activity comparison for catalysts is to express the rate on a surface area basis,
e.g., moles/sec/cm2.  If this is not done, a catalyst coating of normally very high
activity may appear to be less active than one which is normally of lower activity
because the former is poorly dispersed and the latter is highly dispersed.  A know-
ledge of the surface characterization of the catalyst is needed to determine the
light-off characteristics of the catalyst and the area available for the diffusion
of  reactant species to active catalyst sites.  Ideally, only the surface area of the
active catalyst component should be included on this basis.  By selective adsorption
of  either H2 or CO, one can determine the amount of catalyst exposed at the surface.
The titration technique pioneered by Benson and Boudart (Reference 41) can be used to
determine the surface area of the noble metal catalysts.  No such technique is avail-
able for oxides.  Total surface area, e.g., by the BET technique (which includes the
support), is used for the evaluation of these catalysts.
       The correlation of catalytic behavior with the physical properties of a cata-
lyst is crucial in understanding the reasons for catalyst performance.  Measurement
of  the physical properties before and after reaction is particularly important in
assessing the thermal stability of a catalyst.  Sintering of the catalyst will lead
to  a loss in surface area and thus a loss in catalyst activity per unit mass of cata-
lyst.  Wash coat sintering results in pore closure and the burying of active catalyst
sites, again leading to a loss in surface area and a lessening of catalyst activity.
The properties that should be determined for the catalysts tested would be:
       t   Total surface area by the Brunauer, Emmett, Teller method (BET)
       •   Selective surface area of metallic catalysts by titration, H2 or CO ad-
           sorpti on
                                        4-38

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       t   Pore volume and pore size distribution by physical  adsorption or Hg
           porosimetry
       •   Structure and particle size by x-ray crystallography
       •   Metal or oxide distribution within the catalyst pellets by scanning micro-
           probe analysis.
       In cases where catalyst agglomeration cannot be measured by selective chemi-
sorption, x-ray diffraction can be used to determine an average particle size.  This
technique, however, is limited to particles greater than 40 A in size and crystal-
line in structure.  In the case of oxides, x-ray crystallography is also helpful to
determine the phase of the oxide and any changes in stoichiometry that may occur due
to the high temperatures of the reaction.

4.3    EXISTING APPLICATIONS OF CATALYTIC OXIDATION CONCEPTS*
       The existing applications of catalytic oxidation concepts in industry are
largely in the low-temperature area at this time.  Several current research programs
are concerned with high-temperature catalytic combustion applications, and these
will be discussed in Section 4.4.  In this section the use of catalytic combustion
for the following applications will be discussed:
       •   Nitric acid plant tail gas cleanup
       •   Industrial  odor control
       •   Small catalytic heaters
       •   Automotive oxidation catalyst systems.

4.3.1  Nitric Acid Plant Tail Gas Cleanup
       Nitric acid is produced commercially by reacting ammonia with air to produce
nitrogen oxides, which are then absorbed in water to yield nitric acid.  The basic
steps in the production of nitric acid from ammonia are:
       •   Reaction of ammonia with air over a catalyst at high temperature to pro-
           duce nitric oxide
       •   Oxidation of the nitric oxide by oxygen remaining in the gas stream to
           produce nitrogen dioxide
       0   Absorption of the nitrogen dioxide in water to produce nitric acid, with
           additional  nitric oxide being released which must be reoxidized.
 This discussion will deal only with catalytic combustion, as defined in Section 2,
 rather than with high-temperature refractory surface combustion; Reference 7 re-
 viewed applications in this second category in detail.
                                       4-39

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Large amounts of unabsorbed nitrogen oxides in the tail gas from the absorption
tower pose a pollution problem, which has been solved in some cases by the catalytic
processing of this tail gas over platinum-group metals to yield useful energy in the
form of steam and/or power (References 42-26).  The basic reactions for the cataly-
tic treatment of nitric acid tail gas over platinum, palladium, or rhodium for me-
thane reducing fuel are:
       •   CH4 + 4N02 -»• 4NO + C02 + 2H20 (decolorization)
       •   CH4 + 02 -»• C02 + 2H20 (combustion)
       •   CH4 + 4NO -»• C02 + 2H20 + 2N2 (abatement)
The tail gas reactor outlet temperature is usually between 1250°F and 1380°F.  Sup-
port materials for the catalyst have included nichrome wire, alumina pellets, and
alumina honeycomb materials

4.3.2  Industrial Odor Control
       The elimination of organic fumes is desirable from both air pollution and
fire safety points of view.  The platinum metals have been used for several years as
catalysts for the oxidation of carbon monoxide and a wide range of organic molecules
in the presence of air or oxygen.  Most odors are caused by the emission of low con-
centrations of organic molecules to the air, with these organic molecules always con-
taining carbon and hydrogen and generally sulphur, nitrogen, and oxygen as well.
Since most industrial odor problems are caused by organic compound concentrations
well below the level required for spontaneous combustion in air, it is necessary to
raise the contaminated air stream temperature to a level at which combustion can oc-
cur   The use of catalysts lowers the temperature needed to achieve odor removal,
and also lowers the  necessary residence time at the combustion temperature.  A typi-
cal catalytic incinerator using a platinized ceramic honeycomb catalyst is shown in
Figure 4-21.
       Various catalyst  systems presently in use for industrial odor control are dis-
cussed in References 48-54.  In Reference 49, heat removal from the catalyst bed is
achieved through the use of pipe coils embedded in the catalyst layers.  This tech-
nique  is very effective  in limiting the catalyst temperatures.

4.3.3  Low-Temperature Catalytic Heaters
       Low-temperature catalytic heaters are characterized by the small portable
 radiant  heaters that operate at temperatures below 800°F.  Operating characteristics
 include  heat  release rates of approximately 75 Btu/in2 and lifetimes of several
 thousand  hours.  A typical low-temperature catalytic heater is shown in Figure 4-22.
 Gaseous  fuel  enters  the  metal disk  through an orifice.  The disk is designed to
                                        4-40

-------
                                      Catalytic combustion unit
       Process
       sir inlet
      Clean air
       outlet
  *f
  3 5
 2 o "S
•SaT»
 •5 2-2.
inised
Figure  4-21.  Flow diagram of a typical catalytic incinerator
               using a honeycomb catalyst   (Reference  11).
                              4-41

-------
   Metal  dish
Orifice
   Catalytic pad
                                                                   Protective screen
   Figure 4-22.  Schematic diagram - low temperature catalytic heater.
                                   4-42

-------
evenly distribute the gas to all parts of the catalytic pad, and combustion takes
place on the surface of the fibers of the pad.  A metal screen protects the front
face of the pad.  A variety of applications for low temperature catalytic heaters
is given in Reference 55.
       The Control Systems Laboratory of the EPA has conducted an extensive emissions
testing program on a number of catalytic heaters, as reported in Reference 56.   No
correlation between emissions and specific heat release rates was found, and CO and
UHC levels were generally quite high.

4.3.4  Automotive Exhaust Catalysts
       Since platinum metal catalysts were selected for emission control on conven-
tional automotive internal combustion engines for the 1975 model year, a great deal
of development work went into these emission control systems.  In general, NO  emis-
sions are controlled by a reduction mechanism, and CO and UHC emissions are removed
by catalytic oxidation with secondary air added to the exhaust stream to ensure com-
plete oxidation.
       A reactor for catalytic exhaust emission control has three essential design
features:
       •   A ceramic support on which the catalyst is deposited in a manner that
           prevents attrition
       •   Rapid warm-up of the catalyst unit, and hence rapid light-off
       •   Nearly complete conversion efficiency.
       The essential features of a satisfactory CO/UHC oxidation catalyst for auto-
motive emission control are (Reference 57):
       •   A light-off temperature in the region of 250°C
       •   Conversion efficiencies in excess of 90 percent for both CO and UHC at
           space velocities up to 150,000/hr
       •   High temperature stability of the support and platinic crystallite system
           at temperatures to 950°C
       •   Poison resistance to compounds containing lead, phosphorous, and sulfur.
       Platinum-palladium catalysts have been extensively tested in full-scale vehi-
cle tests (Reference 58).  Two different locations were tested:  at the exhaust mani-
fold outlet flange (up-front), and at the exhaust system "Y", where the exhaust
streams merge.  Figure 4-23 shows the effect of catalyst composition on CO and UHC
emissions.  As seen here, the differences between Pt and Pd catalyst performances
were quite small.
                                       4-43

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 Figure 4-23.   Effect of catalyst composition  on 1975 FTP emis-
                 sions   (Reference 58).
                             4-44

-------
       Reference 59 discusses possible substitute catalysts  for platinum in  automo-
bile emission control.  In this study it was concluded that  base metal  catalysts
generally are not as effective as platinum/palladium catalysts  for  automobile  emis-
sion control, especially for hydrocarbon and carbon monoxide oxidation.   For NO
                                                                               /\
reduction, certain base metal catalysts initially are more active and selective
than are noble metal catalysts; however, most suffer from rapid deactivation.

4.4    CURRENT RESEARCH PROGRAMS IN CATALYTIC COMBUSTION
       Since catalytic combustors have excellent potential for  low  NO  emissions,  a
                                                                     A
number of research programs investigating their use in automotive,  gas  turbine, do-
mestic appliance, and home furnace applications are currently going on.   Much  of
this work is being conducted under proprietary secrecy agreements,  making it impos-
sible to gather information on the catalyst material and method of  preparation.
       This section describes on-going research programs in  catalytic combustion.
It cannot provide an all-inclusive compilation of catalytic  combustion  research,
due to the proprietary nature of the subject.

4.4.1  Air Force Aero Propulsion Laboratory
       The AFAPL has conducted an in-house program to study  the feasibility  of using
solid catalytic beds in the reaction zone of aircraft gas turbine combustors,  and
reported this work in Reference 60.  The catalytic unit used in this program was
supplied by Engelhard Industries at no charge under a secrecy agreement as to  the
nature of the combustor.  The bed was of the monolith type,  with pressure drops of
approximately 1 percent at 40 fps inlet velocity and 4 percent  at 80 fps inlet ve-
locity.  No igniter was needed as preheated air at 800°F was used to get the bed
to operating temperature.
       All tests were run at low equivalence ratios (fuel-lean  operation) on JP-4
fuel.  Flashback and pre-ignition were observed during the tests, causing the  Air
Force to do considerable work in developing an acceptable fuels introduction system.
       At all operating conditions tested, NO  concentration was below 2 ppm.   Vir-
                                             A
tually 100 percent combustion efficiency was obtained at combustor  inlet temperatures
above 700°F and fuel-air ratios above 0.02, and specific heat release rates  of 3.5 x
106 Btu/atm-hr-ft3 were achieved.  A diagram of the AFAPL catalytic combustor  system
assembly is shown in Figure 4-24, with CO and hydrocarbon emission  data shown  in
Figure 4-25, and combustion efficiency in Figure 4-26.  Figure  4-25 indicates  that
emissions were sharply reduced when the fuel-air ratio was above 0.0212.  These data
were then translated into the combustion efficiency curve of Figure 4-26, where com-
bustion efficiencies of nearly 100 percent are noted for fuel-air ratios above 0.0195.
                                       4-45

-------
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en
                                                                                                                         DMT
                                                           sown* ourr/f DUCT
W7AUT77C UHlT OUTOJ DUCT
                                             Figure 4-24.  AFAPL's catalytic  combustor system assembly.

-------
                 CALCULATED EXHAUST TEMPERATURE (°F)
                1800
                               2000
2200
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 1000

  600
  400

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      .4

      .2
                                 COMBUSTOR INLET
                                   TEMPERATURE
                                 COMBUSTOR PRESSURE - 7 atm
                                 INLET VELOCITY     - 44 Ft/Se<
                                                 = 710°F
        .015
                .0175        .02          .0225
                         FUEL-AIR  RATIO
    .025
 Figure 4-25.  CO and C H  emission index variation with  fuel air
               ratio (Reference 60).
                              4-47

-------
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    90
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CO
CO
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    60
               1800
CALCULATED EXHAUST TEMPERATURE (°F)

       2000            2200
                                                                  2400
                                           INLET TEMPERATURE = 710°F
                                                    PRESSURE = 7 atm
                                          REFERENCE VELOCITY = 44 Ft/Sec
       .015
                         ,020
                           .025
.030
                                    FUEL-AIR RATIO
   Figure 4-26.  Combustion efficiency variation with fuel-air ratio (Reference  60)

-------
       The combustor tested in this program was designed for high-power application,
and the AFAPL is presently involved in selecting a contractor to begin  work on  a  low-
power catalytic combustor.  Work on the low-power phase should begin  by June 1, 1976.

4.4.2  Bratko Corporation
       Rudolph Bratko, owner of the Bratko Corporation, is engaged  in industrial  pro-
cess heating work, and is also active in the use of fiber pads as combustors in
radiant-furnace applications.  Mr. Bratko holds patents for the application of  Johns-
Manville fiber pads to surface or catalytic combustion applications (References 32,
34), and is presently working on a home furnace concept in which the  fiber pad  is
built in the shape of a top hat.  Using natural gas or propane as the fuel, Bratko
passes a premixed air-fuel stream through the fiber pad and ignites the mixture on
the pad surface.  The extremely low thermal conductivity of the pad prevents flash-
back, and Bratko claims 100,000 Btu/ft2 heat release,  with combustor  surface temper-
atures of approximately 1700°F.  The top hat combustor has been placed  in a conven-
tional home furnace by Bratko, with excellent results.

4.4.3  Detroit Diesel Allison Division, General Motors Corporation
       Detroit Diesel Allison has conducted a test program in which three different
catalysts, consisting of Pt-catalyst coated ceramic monoliths having  different  cata-
lyst surface concentrations, were used to investigate  the combustion  of propane-air
mixtures at inlet temperatures of 700°K as a function  of fuel-air ratio.   No details
are available on the composition of the catalysts.  These tests, reported in Refer-
ence '61, are intended as screening tests for gas turbine applications.   A typical set
of data for one of the catalysts is shown in Table 4-4, and shows that at a fuel-air
ratio of 0.0152, excellent NO , HC, and CO emission levels are attained.
                             /\

4.4.4  Engelhard Industries
       Engelhard has done a great deal of work on gas  turbine catalytic combustors,
both in-house and in liaison arrangements with other investigators.  Because of
Engelhard's proprietary concerns, however, they are able to make very little of
their information public.  Their efforts in catalytic  combustion include the
following areas:
       •   Catalyst screening
       t   Catalyst evaluation/performance studies (fuel-air ratio, inlet tempera-
           ture, etc.)
       •   Liaison area-working with others (AFAPL, NASA-Lewis, industries)
       •   Interfacing of catalyst with product.
                                       4-49

-------
                                 TABLE 4-4.  COMBUSTION PARAMETERS FOR CATALYST C,  DETROIT DIESEL ALLISON TESTS,

                                             7.6 CM LENGTH x 8.9 CM DIAMETER9
en
o
Run
No.
B.C.
1
2
3
4
5
6
Air
(kg/hr)
41.8
41.8
41.8
41.8
41.8
41.8
41.8
(Mass/Mass)
C3H8/A1r
0
0.00586
0.00895
0.0106
0.0121
0.0143
0.0152
°K
Inlet
708
711
711
714
714
719
719
Outlet
653
686
739
789
869
1000
1144
% C02
0.08
0.30
0.70
1.17
1.77
2.83
4.95
9/kg C3H8
NOX
--
0
0
0
0.013
0.022
0.042
HC
--
712
542
465
376
244
0
- CO
—
0.164
1.08
1.91
54.2
82.3
0.222
«%
0
26.3761
41.6383
53.8397
65.2915
78.9164
99.9998
                          B.G.  denotes background readings obtained prior to run.

-------
From data generated internally on small test rigs, Engelhard's present operating
range for the catalyst bed is:
       •   Inlet temperature:  400°F   1000°F
       •   Operating temperature:  2000°F   2600°F
       0   Pressure:  1-10 atm
       0   Heat release rate:  100,000   20,000,000 Btu/hr-ft3atm
       0   Fuel —Gaseous, lead-free distillate (almost all  fuels have been tested)
       0   Combustion efficiency:  99.99 percent
       0   NO  emissions:  1 ppm.
       Engelhard has done some studies on catalyst life, and found good performance
after 1000 hours of operation for some combinations.   They have also tested both pel-
let bed and honeycomb monoliths, but prefer the honeycomb because of its low pres-
sure drop.
       Engelhard is now actively engaged in conducting full-scale tests of catalytic
combustors for gas turbine application.  Some of their test results to date include:
       0   Size:  5- to 7-inch diameter, 0.5- to 2-foot length
       0   Fuels:  Gasoline, JP-4, diesel, propane
       0   Operating range:  Catalyst inlet temperature 600°F   1000°F
                             Catalyst exit temperature:  2000°F   2800°F
                             Pressure:  3-10 atm
       0   Emissions:  HC = 0.01 g/kg fuel
                       CO = 0.1 g/kg fuel
                       NOV   0.1 g/kg fuel.
                         A
The goal now is to interface the catalyst with the engine and develop a burner that
is not simply a laboratory device.  Their in-house work is actively pursuing this
goal.

4.4.5  Institute of Gas Technology
       IGT has been involved in the development of catalytic combustors since the
early 1950's.  Their main investigations have involved the use of methane and pro-
pane for fuels in catalytic combustion devices.  Currently, IGT has three programs
going on in the catalytic combustion area:
       0   Subcontractor to Engelhard on an EPA contract on cracked ammonia
       0   Contractor on a Southern California Gas Company contract for vent!ess
           home appliances
       0   Contractor on a joint  EPA-SCG range-top burner development for hydrogen
           catalytic combustion

                                         4-51

-------
       •   Contractor on methane and propane catalytic combustion units for various
           commercial sponsors.
       Since there are different temperature requirements for various appliances, IGT
feels that more than one burner design will be necessary for the variety of home ap-
pliances.   They have investigated the complete combustion and emissions aspects of
ventless home appliances, using reformed natural gas as the fuel.  The gas produced
by steam reforming of methane is approximately 80 percent hydrogen, 20 percent carbon
dioxide, and a trace of carbon monoxide.  Using this fuel, a variety of appliances
have been developed (Reference 62).  These burners use a noble metal catalyst placed
on a permeable support plate, and are self-igniting at room temperature (Reference
63).  Emissions data have been reported for these ventless appliances (Reference 64),
and show very small amounts of NO  and CO (typical values are 1.1 x 10~" lb/106 Btu
for NOV, 4 x 10~2 lb/106 Btu for CO).
      A
       In summary, IGT's work on ventless home appliances shows the following advan-
tages and disadvantages:
       t   Advantages
             Produce minimal quantities of pollutants
           - Ventless appliances reduce building construction costs
             Can humidify homes concurrently with heating
             High efficiency from catalytic combustion
             Lower operating temperatures reduce fire hazard
             Self-igniting catalytic appliances eliminate pilots or ignition systems
       t   Disadvantages
             Hydrogen not currently available as fuel
             Catalyst life may be limited
             Odorant required for safety which must not poison catalyst
             Safety hazard from extreme combustibility of hydrogen
             Excessive humidification of homes may result

4.4.6  Jet Propulsion Laboratory
       JPL has been conducting tests over the past 18 months on a compact onboard
hydrogen generator for use with a hydrogen-enriched gasoline internal combustion en-
gine.  This modified fuel system, called partial hydrogen injection, substantially
lowers NOX emissions and also increases engine efficiency considerably.  Details of
the  system are discussed in References 65 and 66.
                                       4-52

-------
       The hydrogen generator uses gasoline and air in a partial  oxidation reactor
to produce a gaseous product consisting of hydrogen, carbon monoxide, minor amounts
of methane, carbon dioxide and water, and nitrogen.  It was found that a nickel
catalyst speeds up the partial oxidation reaction, and commercially available nickel
catalysts known as steam reforming catalysts were used.  A sketch of JPL's catalytic
reactor is shown in Figure 4-27, and an exploded view of the reactor is seen in  Fig-
ure 4-28.  Alumina pellets have usually been used as the support, but monolith struc-
tures are also being tested.
       Typical catalytic generator operating conditions and output are given in
Table 4-5.  Air preheat of 450°F is utilized to prevent any condensation of the
vaporized gasoline.  The air-fuel ratio of 5.15 is the theoretical optimum operating
point, where a hydrogen/fuel mass ratio of 0.136 can be obtained  under soot-free con-
ditions.  Note that the hydrogen/fuel mass ratio actually obtained in this test  was
0.12.

4.4.7  NASA-Lewis Research Center
       The NASA-Lewis program in gas turbine catalytic combustor  development began
when Engelhard provided a test unit to Lewis on a no-cost basis and under a secrecy
agreement.  This unit had very low CO, UHC, and NO  emissions, and encouraged Lewis
                                                  A
to continue working with Engelhard on catalytic combustor development for gas tur-
bines.
       In addition to contractual work with Engelhard, NASA-Lewis is currently pur-
suing two in-house programs on catalytic combustion.  Both programs are just being
initiated at this time.  One program will consider the screening  of a large number
of catalyst-support combinations, determining such parameters as  ignition temperature,
maximum temperature, and poisons.  The goal is to select several  good candidate
catalyst-support combinations for further development work.
       The second in-house program is concerned with the fuel-air preparation sys-
tems for introduction of the premixed fluid to the catalyst bed.   Fuel distribution
and start-up properties of the catalyst system will be studied, with an effective
carburetion system as the goal.

4.4.8  Oxy-Catalyst, Incorporated
       Oxy-Catalyst, long active in fume abatement by catalytic oxidation, has been
assigned a patent on a dual-catalytic bed device for the oxidation of internal com-
bustion engine exhaust gases (Reference 67).  In this device, shown in Figure 4-29,
a catalytic hot resistance element of small mass and surface area is placed upstream
of a catalyst bed of large mass and surface area.  The hot resistance element quickly
reaches catalytically effective temperature, and its heat plus the heat of catalytic
                                       4-53

-------
                                              .Air InUt
                                                                                                                    Fuel Preheat Co))
                 Pressure tap
                                                                                                                                            Friitur* Tap
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                                                       :. i • II'III:: ii .;•.; / / / /'-: -Jt!  :..'..''/.'/.._;. //
                 (Us SanpHng Tap
                                     Catalyst Bed
                                              Air Preheat &
                                                Catalyst Bed-
                                                Cooling Channel
                                                                                     Needle Valve
                                                                                                                              Fuel Line
                                             Figure  4-27.   JPL Small  Scale Catalytic  Combustor  System.

-------
in
un
              Figure  4-28.   Exploded  view  of  catalytic  compact  reactor  containing cold-start  burner, air

                            pre-heat  heat  exchanger,  and  catalyst  bed containing 1.5 pounds of catalyst

                            pellets.

-------
TABLE 4-5.  TYPICAL JPL CATALYTIC GENERATOR OPERATING  CONDITIONS
Input Condition
Airflow rate, Ib/h
Fuel flow, Ib/h
A/F
Equivalence ratio
Generator pressure, psig
Catalyst temperature, °F
Mole
Output Condition Fraction
H2 0.2160
CO 0.2360
CH4 0.0103
C2H4 0.0009
C0£ 0.0123
H20 0.0120
N2 0.5125
1.0000
Mean molecular weight
H2/ hydrocarbon mass ratio
H/C atomic ratio
Exit pressure, psig
Exit temperature, °F
Generator efficiency
Value
45.6
8.9
5.15
2.83
1.4
1774
Mass Mass
Fraction Output, Ib/h
0.0194 1.06
0.296 16.19
0.0074 0.404
0.0011 0.062
0.024 1.326
0.0097 0.529
0.6420 35.15
1.000 54.72
22.33
0.12
1.925
1.0
1527.0
0.785
                              4-56

-------
•f*

U1
            Catalytic
            Hot
            Resistance
            Element
Large
Catalytic
Bed
                            Figure 4-29.  Oxy-catalyst's dual-catalytic  bed  oxidation apparatus  (Reference 67)

-------
oxidation serve to accelerate the heating of the second, large catalyst bed.   The
device thus serves as an aid in overcoming the problems of cold starting, wherein
the catalytic bed is ineffective until considerable time has passed.

4.4.9  Tokyo Gas Company. LTD
       In order to reduce NO  emissions, the Tokyo Gas Company has developed  a
                            A
technique for two-stage combustion wherein partial combustion at high fuel-air ratios
is carried out in the first stage, secondary air is added, and the combustion is com-
pleted in the second stage.  Nitrogen compounds such as HCN, NH3 and  NO are formed in
the first stage and are converted to NO  in the second stage.  These  nitrogen com-
                                       A
pounds can be reduced by carrying out the primary combustion with the aid of a cata-
lytic bed.  Reference 68 discusses the work done in detail; a schematic diagram of
the experimental facility is shown in Figure 4-30.
       The amount of nitrogen compounds NO, HCN, and NH, formed in the fuel-rich
conditions of the first stage is closely related to the type of primary combustion.
Figure 4-31 shows the variation in nitrogen compounds as a function of primary com-
bustion chamber equivalence ratio.  The use of catalysts to promote the partial com-
bustion and thus suppress the formation of these nitrogen compounds is shown in Fig-
ure 4-32.  As the nitrogen compounds decrease in the primary combustion, NO  in the
                                                                           J\
exhaust gas is also reduced.

4.5    CONCLUSIONS
       Based upon the review of the catalytic combustion process, the available
catalyst materials and their properties, the existing applications of catalytic
combustion, and the current research programs in catalytic combustion, the following
conclusions have been made:
       •   The overall success of a catalytic combustion system in reducing CO and
           UHC to low levels is a function of both heterogeneous and  gas-phase re-
           actions; surface reactions alone cannot achieve the desired low values.
       •   New high-temperature catalyst support materials that are capable of main-
           taining their structural and thermal integrity at temperatures up to
           3000°F are necessary for high system efficiency unless methods for cooling
           the catalyst bed are used.  These materials come in pellet, monolith, and
           fiber pad geometries.  However, the catalyst coating is limited to much
           lower temperatures (i.e., 1800°F) than the support.
       •   The most promising catalyst materials are the noble metals, such as plat-
           num and palladium, because of their high activity and relatively low ac-
           tivation temperature.
                                       4-58

-------
                                          Primary combusion chamber
en
vo
                                                                                                     Cooling  water    Sampling

                                                                                                        outlet         probe
                                                         Cooling water  inlet
                                                                                       Secondary combustion chamber
                    Figure 4-30.  Schematic diagram —Tokyo Gas Company's two-stage combustion method for NO  reduction (Reference 68),

-------
-o
c
3
o
Q.

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-------
      40
      30
o.
a.
•o
c
o
a.
o
o
 C71
 o
      20
      10
NO  in the exhaust gas
                                            HCN at the exit of the primary

                                                combustion chamber
                                            NO
                Nothing
                                           Catalyst
        Figure  4-32.   Effect of various catalysts on the formation of nitrogen compounds

                      (Reference 68).
                                            4-61
-------
Metal oxides have activities which approach those of the noble metals,
but have significantly higher light-off temperatures, probably limiting
their use to spinel structures.
The principal catalyst poisons for natural gas and light distillate oils
will probably be phosphorous, sulfur, and certain halides.   Their effect
on a variety of catalysts is not known.
The existing applications of catalytic oxidation concepts in industry,
such as in nitric acid plant tail gas cleanup, industrial odor control,
small catalytic heaters, and automotive oxidation catalyst systems, are
generally in the low-temperature range of applicability.  The applicabil-
ity of this technology to high-temperature catalytic combustion is there-
fore quite limited, since the problems of catalyst sintering and thermal
degradation of the support are avoided at low temperatures (i.e.,  <1400°F).
Research is presently being conducted in the area of high-temperature
catalytic combustion for application to gas turbine engines.  This re-
search is done with extremely fuel-lean conditions, making it difficult
to relate directly to domestic, commercial, and industrial  space heaters,
where near stoichiometric conditions are desirable.
Two-stage combustion, wherein the first stage is run fuel-rich and secon-
dary air is added in a catalytic second stage to complete the combustion,
is effective in controlling emissions of NO .   CO and UHC emissions can
                                           A
be controlled at the same time by proper design, keeping the system fuel-
air ratio near stoichiometric.  Staging should be examined carefully for
application to catalytic combustor technology.
Because of the necessity of avoiding local "hot spots" in the catalyst
bed, premixed fuel-air systems are needed for catalytic combustors.  To
prevent blockage of the passages in a catalyst bed, liquid fuels must be
pre-vaporized.  The problems associated with fuels introduction to the
catalyst bed are formidable ones that require careful attention.
At the present time, it is assumed the catalyst bed temperature must be
held below 2800°F to minimize the formation of NO .  Further research
                                                 /\
work should be undertaken to determine this temperature.
The current research programs have all been conducted at catalyst bed
temperatures well below 2800°F (i.e., 2400°F), and have all demonstrated
a successful minimization of NO  formation.
Because much of the research concerning catalytic oxidation is proprietary
information and therefore not available to the general research community,
an experimental program which examines catalyst-support performance and
addresses the application of these catalysts in various combustor-heat ex-
changer systems is needed.  Information on the proper methodology to fol-
low in selecting a catalyst is also unavailable from catalyst manufacturers.
                             4-62
-------
                                     SECTION  5
                    APPLICABILITY OF CATALYTIC FUELS OXIDATION
                             CONCEPTS TO AREA SOURCES

       The usefulness of the information presented in the previous sections will be
proven through the development of a complete combustion system.  This study has been
limited to assessing the applicability of the information generated to three area
sources:  home heating systems, commercial boilers and industrial boilers.  The fol-
lowing section includes a discussion of the options and problems associated with the
applicability of catalytic combustion to these areas.  The ultimate test of the con-
cept through actual hardware development must await a further study.
       This section attempts to discuss the factors which must be considered when
assessing the applicability of catalytic combustion to this equipment.  Following
this, some conceptual designs are presented and the advantages and disadvantages
discussed.
       This section begins with a discussion on the potential matrix of possibilities
that confronts the designer.

5.1    POTENTIAL DESIGN MATRIX
       The potential design matrix may be considered by reference to the "design
tree" illustrated in Figure 5-1.  Six levels of design choices, consisting of New/
Retrofit, Applications, Catalyst Support Arrangement, Cooling, Staging, and Flue Gas
Recirculation (FGR), are shown on the tree.  The complete tree, of course, is not
shown.  Whenever there is a branch ending with "same" and an arrow pointing to the
next level, this indicates that the identical set of branches is below each of those
boxes.  Even this extensive tree does not represent the full choice of options avail-
able to the designer, since within the catalyst support structure category there are
different monolith styles (materials, open area, shape, wall thickness), packed beds
(shape, size, material), or fiber pads that could be considered.  The objective is
to prune this tree; to either eliminate whole application areas (if only because
another area appears more promising) or to narrow the choices for a given area.  It
                                         5-1
-------
                          CXjn.icA.Te
01
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                                                          [ COAMBRCIAI- UNITS
                             [ml (Vfruit cetA»oTT. An |
                                                               Figure 5-1.   Potential design matrix.
-------
may be necessary to make some arbitrary choices when there is no rational  reason
for one option over another, but clearly these options must be reduced in  some logi-
cal manner to limit the choices to a practical number.  A brief review and discus-
sion of each of these areas is given below.

5.1.1  New Versus Retrofit
       A new design requires a totally different design approach than a retrofit de-
sign.  In a new design one can arrange the heat transfer area as required  to take
advantage of the unique characteristics of catalytic combustion particularly where
radiation is required.  The design will also not necessarily be restricted to design
for minimum pressure drop.  It also may be easier in a new design to reduce the over-
all volume and thus cost using catalytic combustion.  Finally, in a new design there
are many more options available to the designer.  This is good in the sense that he
is not as restricted but unfortunately it makes it more difficult to select the best
approach.
       In a retrofit design there will always be certain dimensional as well as pro-
cess constraints.  Pressure drop and particularly heat transfer problems have to be
overcome using existing equipment.  But, perhaps most importantly, cost will be a
much more significant factor in a retrofit design.
       At this point it seems possible to come up with designs for both the new and
retrofit application areas.

5.1.2,  Applications
       Section 3 discussed the important features of the potential application
areas in some detail, and indicated that the design requirements for each  of the
three application areas vary considerably.  Here the relationship of each  of the ap-
plications to the other design variables as shown in Figure 5-1 is discussed.

5.1.2.1  Residential Furnaces
       As can be seen in Figure 5-1 there are basically two furnace types  that are
of interest:  warm air furnaces and hot water boilers (either gas or oil fired).
Gas fired warm air furnaces will be discussed first, followed by oil fired warm air
furnaces, gas fired hot water and oil fired hot water boilers.
       Warm air furnaces, under present design practices, are quite pressure-drop
limited on both the flue gas and process side (when the process fluid is air).  There
is no inherent reason, however, other than cost of blowers and perhaps noise, why the
pressure drop should be so limited in new designs.  To retrofit, however,  care must
be given to these pressure levels and if higher pressure drops are required it may
                                         5-3
-------
necessitate replacing or adding a combustion air blower or induced draft fan.   In
addition, higher velocities may be required on the process side to increase heat
transfer rates.   Whatever is done that might influence cost, it should be remembered
that home owners and developers operate on a lowest first cost basis rather than
total operating cost; this will significantly influence the maximum permissible sys-
tem cost.  In addition, no flue gases may be allowed to leak to the process side and
the ratio of process stream/flue gas is on the order of 100 to 1.   Home furnaces have
a heat release rate of 10,000 to 300,000 Btu/hr and operate at nominally atmospheric
pressure.
       Also of prime concern for the home heating application is the cyclic nature
of the equipment as discussed in Section 3.2.   Where thermal masses may be signifi-
cantly higher in a catalytic system it may be advisable to try to  modulate the firing
rate to match the load.  Either way presents problems with flame stability and off-
design heat transfer and special  problems for liquid fuels, as will be discussed in
a later section.  Also of importance here is the ratio of warm-up  time to cycle-on
time because most any catalytic system will require a thermal combustion warm-up
period.  If the thermal mass of the unit can be kept low then the  period of higher
pollutant formation will be kept low.
       For the gas fired warm air furnace, then, the felt pad may  be a good choice
for a catalyst support material.   However, both the monolith and packed bed approach
are also feasible.  The monolith is a better choice from a pressure drop view
point.  Gas fired units typically operate with uncontrolled secondary air.   Thus,
some form of closure will be required if the fuel/air ratio is to  be controlled.
Staging, cooling and flue gas recirculation will be discussed in a later section.
       Most any of the design options look feasible for new designs, although with
warm air furnaces an auxiliary fluid may be required to achieve significant radia-
tion heat transfer from a bed.
       The oil fired warm air furnace typically uses a relatively  large refractory
or refractory felt-lined combustion chamber with a separate heat exchanger.  This
volume should allow sufficient room to incorporate most any design configuration
which involves staging, internal  bed cooling,  various catalyst supports and an
oil vaporizer.  One drawback is that little heat transfer surface  or local  heat sink
will be available to assist in cooling the bed.  Problems associated with all  oil
systems will be discussed in a section on fuels.
       The oil fired hot water furnace (or cast iron boiler) also  utilizes a similar
large combustion volume.  One advantage in this case is that oftentimes the combustion
                                         5-4
-------
volume is backed by a flooded water surface.  This surface may be able to be utilized
as a heat sink to aid in cooling the bed.
       Gas fired hot water furnaces come in a variety of designs.  Some look almost
identical to the oil fired cast iron boilers except the combustion volume is fore-
shortened.  Usually the same "gas logs" are used as with the warm air furnace and
the combustion volume is open for secondary air addition.  Therefore, the same com-
ments apply as with the warm air furnace except that the combustion volume is con-
siderably larger which may allow greater latitude in design concepts.

5.1.2.2  Commercial Units
       Commercial heating systems can be broken down into basically three categories -
warm air roof top, cast iron boilers and firetube boilers.  Any of these may be gas
fired, oil fired and dual fuel fired.  The gas fired warm air system utilizes a for-
ced combustion system with a Maxon type burner even though the heat release rates
are not much different than residential furnaces.  Considerable excess air is still
used, however, to keep the hot flue gas temperature down.  Units employ a variety of
flue gas to air heat exchange surfaces but most have higher heat transfer rates than
the home furnace.  This may allow sufficient radiation cooling of the catalyst bed.
The cast iron commercial boilers are not unlike the residential units and offer con-
siderable combustion volume to work with.  Dual fuel units present the problem of
trying to design a complete oil fired system with all the associated ramifications
and cost penalties as well.  Firetube designs also offer considerable volume to work
with and have the added advantage of a water wall for heat transfer purposes.
       With regard to a preference for any of the design parameters at this time no
one design branch seems to have an advantage over another.  These systems may allow
greater pressure drop and a more sophisticated design due to greater capital cost of
this size of equipment.  This may allow use of the packed bed approach with its
greater pressure drop.  Since most commercial heating systems utilize water it
will easily be available as a heat transfer medium.  If two stage catalytic combus-
tion is eventually shown to be one of the optimum routes to pursue there surely will
be ample volume to incorporate that approach.  It may even be possible to introduce
the second stage air at the first return manifold in a multipass firetube configura-
tion and to use the smaller tubes as a catalyst holder in the second stage.
       Cycling will also be a factor but will not be as severe as in the residential
application.  Many manufacturers are now advertising using several small units
to achieve modulation by bringing them on line only as needed.  The same approach
could be taken using catalytic combustion.
                                        5-5
-------
5.1.2.3  Industrial Boilers
       In the lower heat release range the units are predominately firetube whereas
in the larger sizes they are almost exclusively watertube, either package or field
erected.  Essentially the same comments apply with the firetube boilers in the indus-
trial size range as with the commerical boilers.  The only additional criteria gov-
erning the design will be that again greater capital costs will be involved and the
loads will be more uniform.
       Watertube boilers are generally of the water-wall  design with single or multi-
ple burners in front wall, horizontally opposed or tangentially fired configurations.
Although there certainly will be sufficient volume to accommodate most any catalytic
replacement burner design by extending into the combustion volume it may be rather
difficult to accomplish for a retrofit design.  The water walls will usually provide
only a minimum opening or access for the burner into the combustion space.  If the
catalytic burner cannot be accommodated into this open area it will either require
extending the burner into the firebox or to the backside.  If it extends into
the firebox this will present many materials and structural problems.  If it extends
to the backside some auxiliary internal cooling will be required.  Again at this
point any of the catalyst support materials are candidates as well  as any of the
other NO  control schemes.  Flue gas recirculation may be a strong candidate due to
        A
the simplicity, especially for the retrofit application.   Testing will be required
to determine the performance of catalytic combustion with FGR.  New designs for in-
dustrial boilers will probably arrange to facilitate radiation heat transfer from
the bed.  To do this in the very large heat release rates of large industrial boilers
may require a very different design approach.   For example, the burner may become a
large radiative slab in the center of the boiler.   Water walls could be incorporated
into the structure of this burner to help support it and perhaps provide some inter-
nal bed cooling as well.
       A very large packed bed with internal cooling may be an alternate approach
for the industrial boiler.  The bed itself would provide the initial heating of the
water-steam circuit.  The bed would be followed by conventional convective sections.
Perhaps the chief problem with this approach is accessibility for maintenance to the
water or steam tubes buried within the bed.  The dual fuel capability must also be
built into this equipment.

5.1.3  Catalysts
       Section 4.2 indicates many possible candidate catalysts.  Taking the above
applications into account as well as start-up problems, catalyst support, cooling,
staging, and fuel, different catalysts may be appropriate for different applications.
From an applications viewpoint, cost will also be a significant factor.
                                        5-6
-------
5.1.4  Catalyst Support Arrangement
       The choice of the catalyst support arrangement will  be a strong function  of
the proposed design.  Different supports can be formed into a variety of shapes  to
give different pressure drop characteristics and activity area per unit volume.
Table 5-1 lists some of the characteristics of the various  support arrangements  from
an application point of view, along with the respective advantages and disadvantages.
As we shall see later in this section, a significant feature of the monolith (honey-
comb) structure is the ability for this material to be fabricated into a crossflow
heat exchanger (either metal or ceramic).  From an applications standpoint,  at this
time we feel the monolith, pellet bed, and felt pad to be the primary catalyst sup-
port materials.  A wire mesh is limited by temperature but  is suitable for lower tem-
perature application.  The fluidized bed will probably suffer from attrition of  the
catalyst surface although this problem should be further explored.  Combinations of
various support arrangements may also be feasible.  For example, monolith slabs  may
be used to containerize a packed bed.

5.1.5  Low NO  Techniques
             A
       There are a great variety of potential arrangements  involving catalytic com-
bustion which could be used to aid in lowering N0x emissions, while maintaining  load
and keeping CO and UHC emissions low.  These possibilities  include:
       •   Catalytic combustion with cooling
       •   Staged combustion
       •   Flue gas recirculation
       •   Catalytic combustion with variable catalyst loading to control heat
           release
       0   Combinations of any or all of the above.
       Table 5-2 delineates the unique features of each of these schemes, together
with the advantages and disadvantages.  Table 5-2 also shows the various combinations
within each NO  reduction scheme.  For example, nine different techniques of cooling
the different types of beds are illustrated.  An attempt has been made to rank these
schemes from an applications point of view.  Further analysis and testing will be re-
quired to verify this ranking.  Some addition general comments appropriate to these
various low NO  techniques are in order.
              A

5.1.5.1  Cooling Schemes
       Some form of cooling the bed will be  required either through active cooling,
by staging with intercooling or by flue gas  recirculation, to keep the catalytic
bed within the materials structural temperature limits and to keep the thermal N0x
levels low.
                                       5-7
-------
                                            TABLE 5-1.  CATALYST BED TYPES FROM AN APPLICATION POINT OF VIEW
                     Bed Types
          Description
          Advantages
         Disadvantages
           A.  Monolith
•  Honeycomb ceramic
•  Square or round

•  Can be made into cross flow
   heat exchanger

t  Most any size, thickness,
   and pore size
   Can be formed in cross flow
   heat exchanger

   Not as subject to abrasion
   as others

   High temperature capability
   (2200°F - 2800°F)
•  Subject to cracking and
   breakage; thermal  shock

t  Heat exchanger configura-
   tion may be expensive to
   manifold
           B.   Packed Bed
tn
oo
t  Any size or shape

0  Particle size any size or
   shape

0  Must containerize incom-
   patible material

0  Coated pellets, solid cata-
   lyst, sintered balls
0  Can be "filled" to any size
   and shape

0  High temperature capability
0  Can imbed cooling coils in
   any position or orientation

0  Easy Replacement
0  Subject to abrasion

0  Must containerize (tem-
   perature considerations)
           C.  Coated Tubes
0  Coat the inside surfaces of
   conventional firetube
   boiler
0  No special "catalyst bas-
   ket" required
0  Low surface area

0  Low temperature capability
           D.  Felt Pad of Catalyst
               Fibers
0  Felt blanket material de-
   rived from insulating
   material
0  May be all catalytic fibers
   or impregnated with a few
   fibers
•  Can be surface combustion
   or reactor bed provided
   temperature low enough
0  Simple to form in most any
   size or shape
0  Cheap
0  Life unknown

0  Limited to 2600°F

0  Unsupported life questionable
-------
                                                                 TABLE 5-1.   CONCLUDED
                      Bed Types
          Description
          Advantages
         Disadvantages
            E.   Wire  Mesh
0  Stainless, high alloy steel
   or other metal coated with
   catalyst or mesh made of
   catalytic material
•  Can be formed into almost
   any size or shape
•  Temperature limited

•  May be difficult to coat
   surfaces with catalyst
            F.   Fluidized  Bed
CJI

-------
                                                           TABLE 5-2.  POTENTIAL LOW NOX CATALYTIC CONCEPTS

                                                              (a)  Thermal Combustion/Catalytic Afterburning
                         Combustor Concept
                          Description
                                           Advantages
                                           Disadvantages
                    Air
—   To  load
t  High temperature thermal
   combustion followed by re-
   duction of NOX by high tem-
   perature catalysis
                     Fuel
•  Can achieve good flame
   stability by firing with
   minimum of excess air

•  Good cyclic operation

•  Does not require prevapor-
   ized fuels
                                                                                                                      •  May not work

                                                                                                                      •  High temperature catalysis
                                                                                                                         support materials required
in

o
                                                (b)  Thermal Combustion With Cooling Followed by Catalytic Afterburning
                         Combustor Concept
                          Description
                                           Advantages
                                          Disadvantages
                   Air
                                           To load
                •   Lower temperature thermal
                   combustion  followed  by
                   catalytic oxidation  of  CO
                   and  H/C.
                     Fuel
                                 •  Lower catalyst bed tempera-
                                    tures

                                 •  Achieve low NO

                                 »  Possibly good cyclic opera-
                                    tion

                                 •  Does not require prevapor-
                                    ized fuels
                                                                                                                      •  May have flame stability
                                                                                                                         problems if cooled too much
-------
                                                     TABLE 5-2.  CONTINUED

                                             (c)  Catalytic Combustion With Cooling
      Combustor Concept
          Description
           Advantages
         Disadvantages
1.  Monolith Sandwich
            JL	L
—
- —

•  Monolith could be heat
   exchanger

•  Sandwich material could be
   same support structure as
   catalytic surface or other

•  Cooling could be in tubes

   -  Process fluid; gas or
      liquid

   —  Combustion air

   -  Flue gas

   -  Intermediary fluid
•  Compact

•  Amenable to many design
   configurations

•  Any cooling medium may be
   utilized
•  Cooling rates have to be
   carefully controlled

•  May not be practical to use
   water cooling

•  Manifolding cold side may
   not be simple

•  Thermal stress problems
2.  Packed Bed-Cooling Coil
•  Cooling coil  imbedded in
   bed

•  Cooling fluid
   —  Process fluid gas or
      liquid

   —  Combustion air
   -  Flue gas
   -  Intermediary fluid
•  Cooling can be distributed
   anywhere in bed

•  Cooling can be crossflowi
   counterflow, or parallel
   flow
•  Must watch temperature of
   metal cooling tubes
•  Cooling uniformly across the
   bed could be difficult
3.  Fluidized Bed-Cooling Coil
•  Same as above
•  Same as above
•  Excellent heat transfer
   characteristics
•  Metal cooling coils tempera-
   ture limited
             t  »t
4.  Fluidized Bed-Moving Bed
                    Cold
•  Fluidized catalytic bed is
   continuously or on a batch
   basis moved from the hot bed
   to a cold bed where it ex-
   changes heat

•  Bed will achieve some steady
   state lower temperature than
   otherwise
•  Can replenish catalyst

•  Can clean catalyst

•  Good heat transfer character-
   istics

•  May be solution to poison
   (S)
t  Probably expensive

t  Carryover of residues and
   other unburned hydrocarbons,
   etc., to cold side

•  Transfer of catalyst back to
   hot bed not simple
-------
                                                                         TABLE 5-2.  CONTINUED

                                                                           (c)  (Continued)
                          Combustor Concept
                    5.   Rotary  Bed
                                       hot
                                                ^
                                               cold
in

ro
                                               Hot
                   7.  Heat Pipe Immersed in Bed

                             *
                   8.  Radiation to Cold Surface
                                         ICOL D
         Description
•  Monolith, packed bed, wire
   mesh, tube bundle or other
   can make up the bed

•  Alternately exchanges heat
   with cold fluid in transient
   mode
•  Hot and cold streams alter-
   nately flow through two beds
   by appropriately valving
•  Bed may be of any type de-
   scribed in Table 2-11

•  Beds always operate in the
   transient mode
•  Multiple heat pipes immersed
   in bed or between monolith
   sandwich and transferring
   heat to a cold stream
•  Any type catalytic bed
   transferring heat prin-
   cipally by radiation (IR)
          Advantages
•  Uniform average temperature
   of catalyst
•  No metal cooling tubes to
   worry about
                                                                                     •  Thermal stress and high
                                                                                        temperature material prob-
                                                                                        lems minimized
   May be attractive from a
   construction standpoint for
   same designs
   Manifolding and sealing prob
   lems between hot and cold
   streams may be minimized

   Some thermal stress problems
   may be minimized
   Amenable to many design
   configurations

   May be combined with convec-
   tive cooling schemes (sand-
   wich concept above #1)

   Can achieve large ratios of
   hot to cold side heat flux
         Disadvantages
   Carryover of exhaust gases
   and even unburned hydrocar-
   bons
   Heat transfer fluid must be
   combustion air
   Sealing problems
   May be expensive to drive
   the wheel
                                    Carryover of partial  products
                                    of combustion

                                    Thermal  cycling could cause
                                    problems
                                    Valving  may cause pulsations
                                    in combustion
                                    Cold fluid must be combustion
                                    air
•  Nonuniform bed and sandwich
   cooling

•  Heat pipe compatible with
   temperature may be excessive
   (liquid Na)

t  Bed design must be compatible
   with heat pipe configuration;
   i.e., the vapor "end" is
   relatively short
•  Bed must be kept relatively
   thin

t  If cooling fluid is air,
   metal temperatures must be
   carefully designed
-------
                                                     TABLE 5-2.   CONTINUED

                                                        (c)   (Concluded)
      Combustor Concept
          Description
          Advantages
         Disadvantages
9.  Direct Injection of Water
•  Introduce water in the
   middle of a packed bed

•  Could be "slow leak" in
   combustion with cooling
t  Becomes steam/methane
   reformer
•  Thermal shock

•  Generation of steam

•  Consumes water
-------
                                                                          TABLE 5-2.   CONTINUED


                                                                         (d)  Staged Combustion
in
i
Combustor Concept
1 . 1st Stage:
2nd Stage:
isr
A(f\ ^
T
FUEL
2. 1st Stage:
2nd Stage:
i"
Fuel rich/
catalytic
Remaining excess
air/conventional
combustion
, 0 «<>
— i x'
	 )- *-
~- l
I




Fuel rich/
catalytic
Remainder excess
air/catalytic
combustion
2nd

Fue L
3. 1st Stage:
2nd Stage:
F
/"" •> r
'| li
1
ft/£t
4. 1st Stage:
2nd Stage:
/1/Rv =
T i

1
FUEL
Fuel lean/
catalytic
Remainder fuel/
conventional
ii .n.
-1



Fuel lean/
catalytic
Remainder fuel/
catalytic
1
il i
A
— i

i

— 1
Description
• First stage could be pre-
mixed or mixed in the
chamber rapidly or slowly
t Any of the catalyst con-
figurations
• Second stage air mixed
rapidly or slowly
• Air preheat or not
• Same as above
• First stage could be pre-
mixed or mixed in the
chamber
• Any of the catalyst con-
figurations could be used
• Additional fuel in second
stage mixed rapidly or
slowly
• Air preheated or not
• Same as (3)
Advantages
• Relatively simple to design
such a concept for many of
the applications
• Controlled heat release
between stages
• Same as (1)
• Same as ( 1 )
• Same as (1)
Disadvantages
• Depending on fuel (e.g., HL)
even first stage may produce
excessive peak temperatures
• Flame stability problems
(this may be helped by cata-
lyst)
• Second stage temperatures
may become too high with-
out intercooling
• Same as (1)
• Flame stability problems
(this may be helped by
catalyst)
• Second stage temperatures
may become too high without
intercooling
• Same as (3)
-------
                                                                       TABLE 5-2.   CONTINUED

                                                                         (d)  (Concluded)
in
I
                         Combustor Concept
5.   1st Stage: Thermal; fuel
              lean or rich
    2nd Stage: Catalytic, re-
              mainder fuel or
          r-, Pir
r^jpr
                      fue.L
                   6.   Multiple Stage/All
                       Catalytic/Air
                       Fu'tL
                   7.  Multiple Stage/All
                      Catalytic/Fuel
                    AIR
                                      Description
                                                   •  Normal thermal combustion
                                                      operated  in fuel lean or
                                                      rich  to achieve low NOX

                                                   •  Remaining air or fuel
                                                      added assisted by catalytic
                                                      oxidation of unburned H/C
                                                      and CO.
                            •  Catalytic bed could be mono-
                               lith  sandwich or packed bed
                            •  Air injection in controlled
                               fashion  (perhaps transpira-
                               tion  type surface)
                            •  Air injection could be
                               coolant  as well
                            •  Catalytic bed could be mono-
                               lith  sandwich or packed bed

                            •  Fuel  injection in controlled
                               fashion  (perhaps transpira-
                               tion  type surface)
                            •  Fuel  injection could be
                               coolant  as well
          Advantages
                                                            •  Good cyclic operation

                                                            •  Possible  retrofit
•  Controlled  rate of reaction
   and hopefully  temperature
•  Controlled  rate of reaction
   and hopefully  temperature

•  May be good way to vaporize
   oil or other liquid fuel
                                                                                                         Disadvantages
                                t  Possible flame  stability and
                                   light-off problem  in first
                                   stage
                                t  Second stage temperatures
                                   may become too  high without
                                   interceding
                                                                                                •  Lots of manifolds and meter-
                                                                                                   ing

                                                                                                •  Lots of experimenting to
                                                                                                   achieve proper configuration
                                                                                                •  Manifolding  and metering

                                                                                                •  Design  to  avoid hot spots and
                                                                                                   injection  tube failure
-------
                                                                         TABLE 5-2.  CONTINUED
                                                                      (e)  Flue Gas Recirculation
01
I
                         Combustor Concept
                   1.  Single Stage Catalytic
s ^
1
't

-~—
=



                   2.  Two Stage/Catalytic
full
                   3.  Multiple Stage Catalytic
                    AIR
                                     Description
                           •  FGR introduction into
                              —  combustion air
                              —  combustion chamber prior
                                 to catalyst
                              -  the catalyst bed
                              —  gaseous fuel
FGR introduction into
—  combustion air
—  combustion chamber prior
   to catalyst
-  second stage air
-  second stage combustion
   chamber
-  either catalyst bed
—  gaseous fuels
                           •  FGR introduction into
                              -  combustion air
                              -  gaseous fuel
                              —  bed at any location
                                        Advantages
                              •  Applicable to many systems
                              •  Easy method to lower tem-
                                                                                        perature
                                                               Same as (1)
                              •  Same as (1)
         Disadvantages
•  May require auxiliary high
   temperature blower although
   natural draft feasible in
   some systems
•  Care must be taken to avoid
   condensation of 1^0 in flue
   gas
•  Same as above
•  Same as (1)
-------
                                                       TABLE 5-2.  CONCLUDED

                        (f)  Catalytic Combustion with Variable Catalyst Loading to Control  Heat Release
      Combustor Concept
          Description
          Advantages
         Disadvantages
Variable Catalyst Loading
t  Would probably require
   multiple thin beds
t  Simple system
•  May not work; after a minimum
   threshold of catalyst you
   may get full activity
                                               (g)  Combinations-of A,  B,  C,  D,  E,  and  F
      Combustor Concept
          Description
          Advantages
         Disadvantages
For example, see C-3
   A unique combination of any
   of the above allowing cool-
   ing of the bed, staged com-
   bustion and flue gas recir-
   culation may prove useful
-------
       The choice of a cooling fluid is dictated by the type of catalyst support.
For example, water cooling should not be considered using a monolith heat exchanger
due to the high thermal  stress that could occur.  Conversely, if metal  cooling tubes
are buried in a packed bed, air cooling may not be sufficient to keep the tubes cool.
It will probably be difficult to cool the felt pad arrangement except by radiation.
Retrofit designs must carefully consider if there is sufficient cooling to accommo-
date a radiation cooled  catalyst bed.  Very likely those heat transfer surfaces that
are air cooled will have insufficient heat transfer coefficients.   Water backed units
can probably accommodate the load.

5.1.5.2  Staging
       Staging is an alternate method of maintaining low thermal  NO  emissions. As
                                                                   A
previously shown in Figure 5-1, the first stage can be either lean  or rich.   Some
interceding between stages will always be required to keep the peak flame tempera-
ture low in the second stage.  Either stage could be catalytic or thermal.   A thermal
first stage has the advantage with  oil  that the fuel would not require vaporization
in the first stage.  However, a stable fuel rich condition might be difficult to
achieve  without sooting and possibly plugging the catalyst.   Thus  the first stage
using oil (if the first  stage is thermal) would most likely be fuel lean with a pre-
vaporized fuel oil  addition in the  second stage.  At this time the  consideration of
other fuels or two catalytic stages gives no evidence of a preferred first stage sto-
ichiometry.  In fact, for gaseous fuels there is no preference for  which stages should
be catalytic or thermal.   In a retrofit application one might want  to operate with the
existing first thermal stage and add a  catalytic second stage.   Some retrofit appli-
cations might not lend themselves to staging.   For example, it would probably be
difficult to incorporate a staged concept into the existing gas fired home heating
furnace.  However,  there is more than enough room for such a concept in the cast iron
and firetube boilers.  In particular, the turning point in the tube of a Scotch ma-
rine boiler is an excellent point to inject secondary air for a two-stage catalytic
system.
       At this point, too, it is difficult to judge whether a staged approach with
internal cooling may offer some advantage.   Some interstage cooling will certainly be
required.
       The choice of a catalyst support will  not be influenced greatly if staging  is
used, although the packed bed may offer an advantage if the first stage is sooty.
It seems likely that the monolith would plug more easily under those conditions and
be more unlikely to cure itself.

5.1.5.3  Flue Gas Recirculation
       Flue gas recirculation is another known method for NO  reduction which might
be very effective in lowering the peak flame temperature in a catalytic combustion
                                        5-18
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process.  It would most likely be applied in a single stage system by introducing
it with the combustion air.  However, it can be applied in a two-stage concept as
well.  There are other locations where the flue gas might be introduced other than
the combustion air to achieve different types of mixing, depending on the final
physical configuration of the system.  Combustion instabilities can be caused in
conventional combustion systems by introducing the flue gas into the combustion
air.  However, with a premixed catalytic system this is not anticipated to be a pro-
blem.  In fact, it may be desirable to use the combustion air blower to draw in flue
gases as well.  This can be done by appropriately sizing the ducts and/or orifices.
This requires that the fan be able to take the higher exhaust gas temperatures and
that the combined temperature of the combustion air and flue gas not be below the
dew point of the mixture.  Alternatively a separate fan could be employed.  In the
case of oil fired units the flue gas could be introduced upstream or downstream of
the preheated air, again as along as condensation is avoided.  From an applications
point of view flue gas recirculation could be applied to any of the proposed appli-
cations; either new design or retrofit.  The primary problem will be economic; can
flue gas recirculation (fans, valves, ducting) be accomplished less expensively than
other options.
       Initially flue gas recirculation coupled with a single stage catalytic combus-
tion system looks like one of the most promising schemes.  The simple ducting coupled
with a premixed arrangement may result in one of the least expensive systems, provided
that significant benefits result.  This is especially true for a retrofit applica-
tion.  However, flue gas recirculation may not work as well for fuel oil applications
where fuel nitrogen could contribute to the total NO  picture.
       In the final analysis, the highest ranking scheme will probably be a combina-
tion of several of these schemes.  For example, two-stage combustion combined with
cooling the bed may prove to be the most promising technique.  Of course, it should
be kept in mind that what may be an optimum scheme for a new design may not be suit-
able for a retrofit application.

5.1.6  Fuels
       The fuel to be considered can greatly influence the potential design, particu-
larly if it is a liquid fuel.  Several factors which must be kept in mind are noted
below:
       •   Lightoff characteristics
       •   Flame speed
       0   Flammability limits
       t   Adiabatic flame temperature as a function of equivalence ratio
                                        5-19
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       t   Sooting characteristics
       •   Vaporization characteristics
       •   Sulfur content
       •   Nitrogen content
       Most of the parameters were discussed in Section 4.2.   From an applications
point of view they can influence size,  shape,  equivalence ratio of a stage,  residence
times, degree of cooling, and startup procedures.
       Problems associated with liquid  fuels (No.  2 fuel  oil)  as applied to  home
heating can be significant.   Conventional  designs  operate in  a highly cyclic fashion
to meet the load.   Strieker (Reference  15) reported typical cyclic firing times on
the order of 5 to 6 minutes (see Figure 3-15 of Section 3.2).   (This will vary de-
pending on the load and heater size per unit floor area.)  Typical firing times in
some West Coast furnace installations indicate a much shorter firing time, on the
order of 2 to 3 minutes.   Off cycles between firings can  be on the order of  10 min-
utes to an hour depending on the load.
       Typical catalyst warmup periods  have recently been reduced to about 1 minute
in an automotive application (Reference 65).  However, until  the bed is hot  the com-
bustor will require thermal  combustion  which will  probably be a higher polluting
regime.  Thus, it can be seen that this polluting  regime  may  be a significant frac-
tion of the firing cycle.  In addition, the cooldown cycles can be sufficiently long
to allow the bed to cool, requiring a warmup period each  time the furnace is cycled.
This problem is most severe with No. 2  oil which requires either fuel atomization or
an electric prevaporizer during warmup.  (It is assumed that  the liquid fuel will re-
quire vaporization during normal running in catalytic combustion.)  An important task
in designing an oil fired catalytic furnace will therefore be the development of a
fast response oil  vaporizer.  Factors which must be considered in the vaporizer de-
sign are:
       •   Coking and cracking of the oil
       t   Flow instabilities due to boiling and two-phase flow (particularly during
           initial startup and shutdown)
       •   Turndown
       •   Start-up and shutdown.
The problem is to volatilize a liquid which has a  range of boiling points from 325°F
to 750°F and contains some ash and residue.  Several approaches are listed in Table
5-3 with their advantages and disadvantages.
                                        5-20
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                                                            TABLE 5-3.  OIL VAPORIZER DESIGNS
en
ro
                                          Approach
                                 1.   Use heat of combustion
                                 2.   Clamshell electric
                                     heater-tube with
                                     small  quartz balls
                                      V.'S.'.'S* t.V.V. «.•> J.
                                      • ?• .•„•••»» "v» •l_f-;
3—
                                 3.   Hot pot of Dowtherm G
                                     with oil coil
        Advantages
•  Does not require auxil-
   iary heat source

t  Can be built into the
   apparatus
•  Smoothes out two phase
   flow problem

0  Good heat transfer

•  Can electrically main-
   tain precise tempera-
   ture control

•  Good capacitance
•  Good capacitance

•  Good temperature control
                                             Disadvantages
                                         Must start up on alter-
                                         nate method

                                         Must be carefully
                                         designed to avoid coking

                                         Shut down/soak down
                                         may be a problem
                                      •  High pressure drop

                                      •  May clog up with time

                                      •  Requires auxiliary
                                         power
                                      •  Flow instabilities es-
                                         pecially on start up may
                                         still be there.  (May
                                         want to fill oil tube
                                         with quartz balls)

                                      •  Requires auxiliary
                                         power
-------
       Cycling problems can be reduced if the furnace is modulated to meet the load.
Modulation will probably require a turndown ratio of at least 10 to 1 and will also
necessitate a corresponding turndown on the process side.  Under the low fire condi-
tion care would have to be taken that the temperature in the catalyst bed did not
fall below a critical  level.  It may be that the lowest firing level to sustain this
critical  level would be too high for sound energy conservation practices.  That is,
even though an energy saving (as well as emissions) could be realized by using a mod-
ulated approach, the required "pilot" for a catalytic combustor could negate those
positive savings.   It should also be noted that the combustion air will require pre-
heating as well to avoid any condensation of the oil  vapor.   Thus, at this point,
and without detailed analysis, there is no clear direction on which approach to take
when designing an  oil  fired catalytic system.   Perhaps the best approach would be to
go with a two-stage approach, the first stage thermal followed by a catalytic second
stage to overcome  these startup problems.  Other possibilities include dual catalyst
systems involving  noble metal startup or pilot catalysts (lower ignition temperature),
and metal oxide operating catalysts (slower deactivation rate at high temperature).

5.2    OTHER DESIGN CONSIDERATIONS
       As we have  seen in the previous sections a great number of parameters must
be considered when formulating a new design.  There are in addition some practical
design problems which are discussed briefly below:
       •   Flashback — Flashback to a stable thermal  combustion zone as well as
           flashback in an unstable manner must be prevented for safety, for equip-
           ment lifetime, and to minimize pollutant emissions.  This can be accom-
           plished through quenching, maintaining velocities of a premixed mixture
           above the flame speed or remaining outside the flammability limits.
       •   Partial Plugging — This may be a serious problem.  If for some reason
           local combustion produces soot and plugs the catalyst pores, the reac-
           tivity will fall off, the temperature will drop,  thus producing a local
           instability.
       •   Nonuniformities Across the Bed — Likewise, nonuniform cooling might pro-
           duce similar results.  Local conduction will offset both the above prob-
           lems and help maintain a uniform bed temperature.
       •   Mixing of Staged Air -As in any combustion process, proper mixing is a
           must to prevent bypassing of CO and UHC.
       •   Cooling the Bed - Methods of cooling the bed were discussed at some length
           in Section 5.1.4.  However, the bed must not be cooled too much or the re-
           actions will not be sustained.
                                       5-22
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       t   Sulfur Content of Liquid Fuels —The sulfur content is of concern for
           two reasons:  (1) Sf^ and S0.j emissions and (2) poisoning of the catalyst.
           It may be possible to combine catalytic combustion with a sulfate removal
           process such as insertion of a bed of ZnO to collect the S as ZnS.
       •   Pressure Drop Available — This will always be coupled to the overall cost
           allowed and also possibly to noise.
       •   Intermediary Cooling Fluid - Required heat absorption rates for cooling
           the catalyst bed by radiation may exceed the capability or pressure drop
           limitations using air.  Consequently, an intermediary fluid could be used
           to remove the heat and then transport it to the process fluid.
       •   Thermal Shock — Thermal shock to refractory materials must be evaluated
           and avoided if possible.
       •   Thermal Stresses and Low Cycle Fatigue — One of the most serious problems
           in conventional furnace design is thermal fatigue of the heat exchanger.
           In any heat exchanger design great care must be taken to avoid severe
           thermal stresses.
       t   Size — Overall size is always a factor.  It is conceivable that a new
           catalytic combustion system would be made more compact than conventional
           designs.
       •   Cost -Overall cost of the system will probably be the single most impor-
           tant factor.  Unfortunately, most customers look only at the capital cost
           of the equipment as opposed to the overall operating cost.
       •   Maintenance — Ease of maintenance, and periods between maintenance must
           be considered.
       •   Retrofit Compatibility - Designs for retrofit should be considered as well
           as new designs.

5.3    OVERALL CONCLUSIONS
       In general, it has been shown that the matrix of possible design choices for
each of the application areas is quite large.  The initial survey to this point does
not indicate a clear direction to follow, except for a few isolated examples, either
for a retrofit or new design application.  Flue gas recirculation with a single stage
catalytic combustor may be one of the better and least expensive variations for many
of the applications.  Gas fired home furnaces present special problems for retrofit
due to limited volume, low existing heat transfer coefficient and their cyclic nature.
Any oil fired unit also presents special problems including oil vaporization, startup
and air preheating.
                                        5-23
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       Section 6 following will  examine several specific design problems and present
suggested solutions for the following cases:
       •   Home heater - retrofit
       t   Home heater - new design
       •   Commercial  boiler - retrofit
       •   Commercial  boiler — new design
       •   Industrial  boiler — retrofit
       •   Industrial  boiler — new design.
                                        5-24
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                                     SECTION  6
                   COMMERCIAL APPLICATION OF CATALYTIC CONCEPTS
                                TO  SELECTED SYSTEMS

       This section presents a number of possible design concepts for several  of the
application areas.  For three of these concepts detailed sizing calculations have
been made.  It should be emphasized that the concepts presented here are by no means
the optimum for any candidate application, but only represent a possible design.
Which of the many possible designs has the greatest potential for success will
ultimately depend on the cost of the hardware.   Many possible good design options
(branches) could not be considered within the scope of this program.
       Application areas where designs have been developed include
       §   Residential heating — retrofit
       •   Residential heating — new design
       •   Commercial boiler — retrofit
       •   Commercial boiler - new design
       •   Industrial boiler — retrofit
       •   Industrial boiler — new design
Summaries of the work performed on each of these areas is presented below.

6.1    HOME HEATER RETROFIT
       Due to Aerotherm's prior experience with home heater design studies, a pro-
bably disproportionate number of design possibilities come to mind.  However, it is
also felt that this may be one of the most feasible applications for catalytic com-
bustion.  Also, although some of the concepts offered here are biased towards a home
heater application, many of the same principles or concepts could be applied to the
other areas.  They must be evaluated in light of the system limitations (cost, pres-
sure drop, etc.) of alternate applications.
       There are basically four retrofit home heating applications which must be
considered.  These include
       •   Gas fired - warm air
                                        6-1
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       •   Gas fired — hot water
       •   Oil fired — warm air
       •   Oil fired — hot water
       Although it is conceivable to design a room or floor heater using catalytic
combustion, these do not significantly contribute to the population of furnaces,
especially for new units being sold.  Thus only the units listed above will be dis-
cussed in the following text.
       As was shown in Section 3.2, nearly all  gas fired burners, whether in a warm
air or hot water application,  consist of the typical naturally aspirated "stick" or
"log" type burner.  These burners are generally of the premixed type and rely on am-
ple quantities of secondary excess air to keep  the "combustion chamber" cool.  In
reality, there usually is not  a combustion chamber as such; firing is direct to the
heat exchange surface.  The units have no control over the secondary air opening.
Thus, in a retrofit application, if the overall excess air is to be controlled, the
combustion chamber or volume will require modification to seal it around the burner.
Furthermore, there will usually be little volume to work with and radiation heat
transfer and maximum metal  temperature must be  considered in the design.  There is
generally no refractory lined  or refractory fiber lined combustion chamber with gas
fired units.
       Figures 6-1 to 6-3 show several potential designs.  All these units replace
the conventional log burners.   In the first case (Figure 6-1) heat is transferred from
a monolithic or felt pad bed by radiation only.  Preliminary calculations show that
heat transfer rates are sufficient to keep the  bed below 2800°F without exceeding a
maximum metal temperature of 900°F.  A summary  of the calculations for Figure 6-1 is
shown in Table 6-1.  These calculations are based on some rather optimistic assump-
tions.  For example, radiation heat transfer is calculated on the basis of a slab
radiating to two perpendicular walls.  The convective heat transfer from the metal
heat exchange surface and the  thermal entry length heat transfer relationships were
used in combination with free  convection coefficients.  This combined coefficient is
three to four times higher than the usual average heat transfer coefficient given
for warm air furnaces.  Thus it may be necessary to increase the heat transfer coef-
ficient on the warm air side.   This can be accomplished by increasing the local ve-
locity around the heat exchanger, inducing turbulence, or adding heat exchange area
by the use of fins.  The first two options will increase pressure drop and may re-
quire a change in the air circulation fan.  Testing of the concept will be required
to verify these calculations.   Figures 6-2 and  6-3 show convective cooling schemes.
A summary data sheet on each design is included as Tables 6-2 and 6-3.  The first
scheme utilizes a monolith structure in the form of a crossflow heat exchanger.
This ceramic heat exchanger, discussed in Section 4.2.1.1, is cooled using air
                                        6-2
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Premixed
air-fuel
                                                                   Catalyst bed
                    Figure  6-1.   Home  heater  retrofit  design  #1
                                         6-3
-------
              TABLE 6-1.   DATA ON DESIGN #1
Length:  15 inches
Width:  2 inches
Height:  0.5 inch
Volume:  15 in3
Surface Area per Unit Volume:  50 ft2/ft3
Height Required to be Coated with Catalyst:   0.25 inch
Heat Release in Bed:  25,000 Btu/hr
Heat Transferred by Radiation:  11,400 Btu/hr
Heat Transfer Coefficient on Air Side:  8.9  But/hr-ft2-°F
Pressure Drop through Bed:  < 0.25"H20
Bed Temperature:  2200°F
Wall Temperature:  900°F
Type Bed:  Celcor Honeycomb Monolith
Number of Units:  4
                             6-4
-------
Coolant
out
      Purified
      fuel  air
                  Coolant
                  in
                   Figure 6-2.   Design #2 cross flow ceramic heat exchanger.
                                           6-5
-------
               TABLE 6-2.  DATA ON DESIGN #2
Length:  15.00 inches
Width:  1.0 inch
Height:  0.50 inch
Heat Released in Bed:  25,000 Btu/hr
Heat Transferred by Radiation:  4612 Btu/hr
Heat Transferred by Convection:  6788 Btu/hr
Cooling Air Temperature Rise:  621°F
Cooling Air Flow Rate:  105 scfm
Cooling Air Pressure Drop Across Bed:  ~ 3.0"H2
Effectiveness of Heat Exchanger:  0.46
Number of Transfer Units (NTU):  0.825
Surface Area Per Unit Volume:  600 ft2/ft3
                          6-6
-------
en
            Coolant Out
                     Cooling Tubes
                     Insulating Pad
                                                                                                              1    	  Conventional  Heat
                                                                                                                        Exchanger  Surface
                                                                                                                        Cooling  Air
                                                                                                                       Premixed  Fuel
                                                                                                                       and Air
                                                      Figure 6-3. Home  heater  retrofit design
-------
               TABLE 6-3.   DATA ON DESIGN #3
Width:  2 inches
Length:  15 inches
Height:  1.0 inch
Heat Release Per Burner:  25,000 Btu/hr
Total Number of Burners:  4
Number of Cooling U-Tubes:  6
OD of Cooling Tubes:  1/16"
Heat Transferred from Bed by Radiation:  4612 Btu/hr
Heat Absorbed by Tubes:  6788 Btu/hr
Coolant Flow Rate Per Burner:  21.5 gal/hr
Exit Gas Temperature:  2200°F
Coolant Inlet Temperature:  70°F
Coolant Exit Temperature:  120°F
Pressure Drop through Cooling Tubes:  11.16 ft H20
                            6-8
-------
(water would cause excessive thermal shock).  The cooling air is supplied using a
separate small fan and the inlet and exit manifold are at the same end of the burner.
The heat exchanger manifolding represents one of the most difficult technological
problems.  Representatives of the American Lava Corporation claim they have solved
this problem, although the design is proprietary at this point and has only been
tried on an experimental basis.  In addition, in its present configuration the cera-
mic material from which the exchanger is made is permeable and will allow some leak-
age depending on the pressure differential.  This situation cannot be tolerated if
the leakage is to the coolant side (and perhaps even to the hot side).  American
Lava is developing a nonpermeable material which will soon be available in this
cross flow heat exchanger form.  In that case, this cooling air can then be fed back
to the main cooling air or routed to a separate room or basement.  Radiation heat
transfer in this case was calculated using the assumption that the convective coef-
ficient from the heat exchanger was the more conventional 2.7 Btu/hr-ft2-°F.  The
heat exchanger surface is an American Lava Thermocomb material with approximately
8 corrugations per inch.
       The last design, shown in Figure 6-3 (followed by a data sheet in Table 6-3),
shows a pebble bed approach for the retrofit application to a gas fired furnace uti-
lizing small U-tubes of cooling water.
       The heat absorbed by the cooling water is then transferred to the circulating
air just prior to entering the furnace.  This arrangement requires an auxiliary pump
and finned tube heat exchanger.  The water cooling circuit is arranged to come on
after the catalytic bed has reached a specified temperature.  It may also be possi-
ble to run cooling coils between a sandwiched monolithic bed or just above the bed.
In the latter case the cooling water must be flowing during the initial light-off,
where thermal combustion will occur.  The entire transient problem must be studied
in detail in any of these designs to insure that an unsafe condition does not occur
For example, in conventional furnaces the burner comes on for a minute prior to the
circulating air fan.  Depending on the length of time required for the water system
to heat, it may be necessary that the circulating air fan come on sooner than current
practice to insure that heat is extracted from the water and boiling does not occur.
Similarly, in the pebble bed case (Figure 6-3) timing of the cooling water flow and
recirculating air flow must be carefully controlled.
       The three designs just discussed cover the gas fired warm air and hot water
retrofit applications.  There remain the oil fired applications.
       Oil fired warm air heaters utilize a refractory or refractory felt combustion
chamber connected to a heat exchanger of varying design  (unlike the gas fired units
which are all very similar in design).  All the heat exchanger designs are  low pres-
sure drop devices, resulting in low velocities, low heat transfer coefficients, and
thus large heat transfer areas.  Generally a relatively  small fraction of the heat is
                                         6-9
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transferred from the combustion chamber.   This fact presents a difficult design
problem for a cooled retrofit catalytic combustor.   However, it may lend itself to
a multistage configuration where a larger volume may be required.
       There are a number of problems that will  be  common to any potential  design
using oil.   These are:
       •   The oil will  require prevaporization  when operating in  the catalytic
           mode
       •   The combustion air must be preheated  above 600°F to avoid condensation
           of vaporized  oil
       •   The oil may be vaporized using the heat  of combustion or electrically
       0   Sooting of a  thermal combustion phase must be avoided
       •   Light-off and heat-up of the catalyst system will either require a thermal
           combustion phase or air electrical preheating scheme
       0   Thermal combustion of the vaporized fuel must be avoided prior to the
           catalyst bed  during the normal operating mode.
The designs previously discussed and shown in Figures 6-1 through 6-3 could be applied
to oil combustion by adding an air preheater and oil vaporizer.  However, it may be
more appropriate to develop a design more suited to the combustion volume of the oil
fired furnace.
       A design that initially seems ideally suited to this volume is the concept
developed by JPL and shown in Figure 4-27 of Section 4.4.6.  This design would fit
into the combustion volume and solves the six common problems mentioned earlier.
The heat up period for this design has been reduced to about 1 minute, which may be
acceptable in a home heater application.   It is  important to note  that the  JPL de-
sign operates only in the fuel-rich mode at this time.   One drawback is that little
heat is extracted from the bed.  As with the gas fired  design, in  order to  achieve
low NOX the maximum combustion temperature must  be  reduced.  This  can be achieved
by cooling the bed or going to a two-stage or flue  gas  recirculation concept.   Cool-
ing the bed is easily adaptable to the JPL design.   Water cooling  coils can  be embed-
ded in the packed bed.   This heated water then exchanges heat with the incoming
house red railating air  using a finned tube heat exchanger.  Alternatively,  a ceramic
heat exchanger (monolith) using a separate cooling  air  supply can  be incorporated.
The principal problem is in ducting and manifolding the air.  Flue gas recirculation
in conjunction with the  JPL approach is also feasible.
       Another potential problem with the JPL design is assuring that sooting does
not occur with the relatively cold combustion chamber walls when firing #2  fuel oil.
JPL has achieved a no-sooting condition on kerosene, and it is conceivable  that the
design would work on #2  oil.
                                       6-10
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       It would be convenient to utilize an existing burner in a retrofit applica-
tion to provide combustion air as well as to provide the light-off and heat-up  capa-
bility.  Several factors discourage this approach:
       •   During normal catalytic running, the combustion air must be preheated;
           this would necessitate diverting the flow downstream of the burner blower
           a difficult task.
       •   Conventional burners soot or smoke during light off when the walls are
           still cool.
       0   The blowers are pressure drop limited.
       •   The burners are not built to handle preheated air.
Another approach, shown in Figure 6-4, utilizes a  catalyst impregnated felt pad.
The chief design questions with this approach for  a retrofit oil system are as
follows:
       •   Lightoff — What type of lightoff system should be used with this design?
           Unless the combustion air and all structural  parts are above the boiling
           point of the oil it is likely that condensation of the oil  will  occur
           and perhaps plug the pad.
           In order to achieve preheating either a thermal combustion  system or elec-
           trical preheating of both the combustion air  and oil vaporizer may be em-
           ployed.  Electric preheating offers the advantage of simplicity.  Since
           electrical power will be required in any case to operate fans, etc., it
           seems to be the preferred method with this design.
       •   Heat Transfer
           -   In a cylindrical or conical arrangement,  as shown in Figure 6-4,
               there has to be sufficient heat transfer  by radiation alone to get
               the flame temperature below the acceptable limit for low NO ; espec-
               ially when minimum excess air is used. Heat must be transferred to
               three places:  (1) to the combustion air, (2) to the room recircula-
               ting air, and (3) to the oil to be  vaporized.  In the first case
               this heat transfer does not aid in  lowering the combustion tempera-
               ture since the air is part of the combustion process.  In the second
               case current heat transfer coefficients on the heat exchanger may not
               be sufficiently high to achieve low enough combustion temperatures.
               Therefore, it may require some auxiliary  heat transfer  surface, pos-
               sibly water cooled.
       t   Oil Vaporization -The best arrangement in this case would  be to wrap
           coils around the combustor in the proper temperature regime.  Care must
           be taken to avoid overheating.
                                       6-11
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Combustion air
          Cooling Air
                                                                     Heat  exchanger
                                                                     Electric  air  preheater
                                                                     Oil  vaporization  tube
                                                                     Cooling Fluid
 Catalytic Impregnated
 pad
Cooling air
                                                                 Combustion products
                         Figure 6-4.  Catalytic pad oil burner.
                                             6-12
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       •   Shutdown and Standby
           Since the nature of a home heating system is currently a cyclic one,  it
           may be desirable to keep the oil  vaporizer hot during standby periods.
           This is accomplished through auxiliary electrical  heating or burying  the
           tubes in a high heat capacitance material.  (This  requires a longer ini-
           tial warm up period.)
Although this design is feasible it requires considerable modification and new hard-
ware.  It may be more appropriate to consider it for a new design concept.
       In summary, it appears that the most reasonable approach for an oil fired re-
trofit application is a single catalytic system using a thermal lightoff and warm
up.  The oil would be vaporized during the normal running and the catalyst bed would
be actively water or air cooled.

6.2    HOME HEATER - NEW DESIGNS
       The freedom of a completely new design presents fewer  constraints on physical
size and heat transfer arrangement.  There will still be an overall size limit (simi-
lar to current practice) but that is easily met.  Two other basic criteria which
will not be as easy to meet are cost and noise, the latter because of the higher
pressure fans that may be required to achieve higher heat transfer rates.  Within
these limits literally thousands of arrangements are potential  candidates, as seen
in Section 5.1.  Gas and oil fired warm air and hot water heating systems are con-
sidered.  For oil fired units the requirements of preheated combustion air and va-
porized oil still exist.  For either fuel it is necessary to  take into consideration
either cyclic operation or modulation to match the variation  in load.  Coupled with
those aspects is the transient response of the furnace.  At this point it appears
quite feasible to design a gas warm air or hot water furnace.  Oil fired units pre-
sent considerably more problems, none of which are insolvable.   Following are des-
criptions of several designs which warrant further study.

Home Heater Design #1
       This design, pictured in Figure 6-5, involves radiant  cooling of the catalyst
bed, lean catalytic first stage, addition of fuel before the  second stage, and cata-
lytic combustion of the second stage.  The unit is compact, involving a concentric
cylinder arrangement of the catalyst bed, fuel addition units, and outer shell.   The
outer shell can either be water or air cooled, with additional  fins placed in the
outer shell if it were air cooled.  The catalyst bed could either be a packed bed,
a fiber mat, or possibly a monolith bed.  Fuels for this application would most
likely be the gaseous fuels, but a prevaporized oil could also be used.  Thermal or
electrical start up similar to the retrofit design would be required for oil to
                                       6-13
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Flue Out
                                                              Connective Section
                                                                    .Premixed Fuel Lean
                                                                    1st Stage
Coolant Out
                                                                 2nd Stage Catalyst Bed
                                                                 1st Stage Catalyst Bed
                                                                           Coolant In
                          Figure 6-5.  Home heater design #1.
                                          6-14
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vaporize the fuel and preheat the combustion air.  Oil vaporizing tubes would be
placed between the cold shell and the catalytic surface.  Preliminary calculations
have been made on this design to size the first and second stage catalyst beds and
the heat transfer area.  Table 6-4 summarizes  the important  design  parameters  cal-
culated.  Figure 6-6 shows this design to scale.  This unit is air-cooled and could
utilize an existing house air recirculation fan; it could also be designed as a water-
cooled unit.  Note that the overall dimensions are well within current practices,
although the room air fan, combustion blower,  and fuel controls must be packaged in-
to the unit as well.
       It must also be noted that sufficient heat must be extracted from the first
stage to achieve two goals:
       •   To insure the maximum flame temperature in the second stage does not
           exceed 2200°F ->- 2500°F to achieve low N0x and be within the limits of
           the catalyst
       0   To lower the temperature sufficiently to prevent thermal combustion prior
           to the second stage catalyst bed after introduction of the second stage
           fuel.
The latter criterion can be circumvented if the velocities are greater than the
flame speed.  This can certainly be achieved by appropriately sizing the passages.
       The catalyst volume calculation was based on ~ 11,000 Btu/hr-ft2 of catalyst
surface area, based on experimental JPL criteria.  Within this constraint, the sizes
shown in Figure 6-6 and Table 6-4 depend on radiative and convective heat transfer
calculations.

Home Heater Design #2
       Figure 6-7 shows a variation of design  #1.  Premixed fuel and air (fuel lean
or rich) passes through a cylindrical catalytic bed which transfers heat by radia-
tion and convection to water cooled tubes connected with sintered copper balls.  The
exhaust products are passed through a second stage with fuel or air addition and
finally out through additional heat exchange area.  In this case the first stage is
catalytic, whereas the second stage is thermal.  The heat transfer surface could be
tailored to achieve the proper temperature distribution or be designed with more sur-
face on the process side if air cooling were incorporated.  No detailed calculations
were performed on this design.

Home Heater Design #3
       This concept,  shown in Figure 6-8, is patterned after the Bratko furnace de-
sign with some improvements.  A conical fibrous catalyst surface radiates to an outer
                                       6-15
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TABLE 6-4.   DATA SUMMARY HOME HEATER DESIGN #1,  100,000 BTU/HR
                                      1st Stage  2nd Stage
  Heat  Release  (Btu/hr)                  68,750     31,250
  Stoichiometry                           1.60       1.10
  Gas Inlet Temperature  (°F)               70        HOI
  Gas Exit Temperature (°F)               2200       2591
  Catalytic Surface Requirement  (ft2)     6.12       2.78
  Surface Area/Unit Volume (ft2/ft3)      200        200
  OD  (inches)                             2.86       2.43
  ID  (inches)                             2.00       2.00
  L  (inches)                               4          4
  Pressure Drop  ("H20)                  -0.04     > 0.1
  Heat  Transfer  Area Downstream:  12.26 ft2
  Average Overall U = ~  5 Btu/hr-ft2-°F
  Exit  Gas Temperature:  400°F
  Use 28 Fins 1.3 inches long on either side
  Cooling Air Flowrate:  1000 scfm
  Pressure Drop  Air Side:  0.122"H20
                              6-16
-------
Catalytic surface
        2nd stage
        Room air
          Catalytic  surface
                  1st  stage
                                                                                    12 x 8
                                                                          Room air
Fuel plus combustion
air
                                        Additional fuel
                         Figure 6-6.  Scaled home heater design #1.
                                            6-17
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     Flue Out
                                                 Coolant Out
                                          Cooling Tubes and
                                          Porous Fins
                                          Thermal  2nd Stage
                                          1st  Stage
                                          Catalyst Bed
                                              Coolant  In
                             Premixed  1st  Stage
                             Fuel  and  Air
Premixed 2nd Stage
Fuel and Air
     Figure 6-7.  Home heater design  #2.
                    6-18
-------
                                      Flue Out
  Coolant Out
Fibrous Catalytic
Mat
                                                         Convective
                                                         Heat Exchange
                                                         Section
                                                              Radiant Section
                                                                            Cooling
                                                                            Fluid
                                                Premixed Fuel and Air
                         Figure 6-8.   Home heater design #3.
                                        6-19
-------
air or water cooled shell.   A premixed mixture of air and fuel would be routed behind
the felt pad and ignited at the outer surface.  After exchanging heat radiatively with
the outer shell the flue gases are passed through the tubes of a convective section.
The process stream is passed through in a multipass crossflow arrangement outside the
tubes.  This design is single stage, operating with minimum excess air but relying on
radiation cooling to keep the catalyst surface cool.   It could be designed to operate
on oil.

Home Heater Design #4
       Figure 6-9 shows a concept utilizing heat pipes between sections of catalysts.
The heat is picked up by the room air stream flowing  adjacent to either side of the
combustion chamber.  This design could easily use oil  or gaseous fuel.   The first
combustion chamber would be modified to utilize a startup oil spray nozzle firing
thermally and later switched to vaporized oil.  The oil  vaporization tubes could also
be located between catalyst beds.   The remaining heat is finally extracted through a
convective section at the right hand side of the figure.

Home Heater Design #5a, b,  c
       Figure 6-10 shows three variations of a common design.  This unit could be
built as a single test rig  and modified for the alternate approaches.   The concepts
include staged combustion,  first-stage thermal or catalytic,  second-stage thermal or
catalytic, integrally cooled catalyst bed or radiatively cooled catalyst bed.  This
concept is probably the most versatile as far as testing numerous concepts.  Flue
gas recirculation could be  added to any of these schemes.

6.3    COMMERCIAL BOILER - RETROFIT
       As indicated in Section 3.3, the principal existing commercial  boiler styles
in use are the cast iron and firetube designs.  Within the firetube design, packaged
scotch and firebox are prevalent.   All of these designs  have  relatively large initial
combustion volumes, whether gas or oil fired.  The firebox designs, including some of
the cast iron boilers, employ a refractory or refractory felt-lined combustion cham-
ber.  The others (packaged  scotch) utilize a large water backed volume for the ini-
tial combustion chamber.  The on-off cycles again are highly  dependent on the indi-
vidual load and can conceivably be as short as the home heater application.
       The principal difficulty is to extract sufficient heat from the bed to achieve
low N0x.  All the problems  associated with oil fired home heater applications also
apply  in this instance.  The most significant difference is that the capital cost of
these  units is an order of magnitude higher than the home heating systems.  This
will allow considerably more latitude in potential designs.
                                       6-20
-------
at
ro
                                        Catalyst Beds   —i
                     Premixed Fuel  v
                     and Air        V,
                                                                          Convective Heat
                                                                          Exchange Section
Fuel Out
                                                                                                       Out
                                                                                          Heat Pipes
                                                    Figure 6-9.  Home heater design #4.
-------
               Coolant  In
                                                                       Flue Out
                                                                    Convective
                                                                    Heat  Exchanger
                                                     Catalyst  Bed
                   Catalyst  Bed
 Prenrixed
Fuel & A1r
                    Coolant  Out
2nd Stage Fuel
or Air
                                       (0
                                      Heat  Exchanger        Coolant  In
                                                                      Connectlve
                                                                      Heat Exchanger
                                                              -j— Catalyst  Bed
   „  T   *—Catalyst
   Premixed    Be()
  Fuel &  Air
                                       2nd Stage Fuel  or  Air
                                (b)
                                       Catalyst Bed  Heat  Exchanger
                                                                        Flue Out
                                                                          Convective
                                                                          Heat Exchanger
                                                             •- Coolant In
   Premlxed
  Fuel S Air
 2nd Stage
 Fuel or Air
                                  (a)
                          Figure 6-10,  Home heater design #5.
                                        6-22
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       First consider a design with a refractory lined firebox.   In  this  case  a  lar-
ger version of the home heater retrofit design shown in Figure 6-3 could  equally
well be applied.  In the case of internal cooling of the catalyst bed a portion  of
the feed water or circulating water could be used as the coolant.  If it  is  desir-
able to use air as the internal catalyst coolant, as with a monolith heat exchanger,
it should not be the combustion air but rather an auxiliary air  supply which would
than transfer heat to the recirculating water.  This does not seem to be  a very  logi-
cal approach and therefore a monolithic heat exchange catalyst would not  be  chosen
for this application.
       The felt pad in a conical or cylindrical arrangement is also  a possibility for
a retrofit combustor.  Much commercial equipment, being oil fired, presents  the  prob-
lem of incorporating oil firing with the felt pad mentioned in previous sections.

6.4    COMMERCIAL BOILER - NEW DESIGN
       Figure 6-11 shows a potential new design concept for a firetube boiler.  The
firetubes are filled with pellets of catalyst or coated with a catalytic  material.
A combustion chamber is added of any desired length for first-stage  thermal  combus-
tion, if required.  In addition, a second-stage can be added as desired,  followed by
additional cooling.  FGR can be added to this design as well.
       A catalytic water wall design is shown in Figure 6-12.  Actually,  this  is very
similar to the Home Heater Design #1 in that it employs a cylindrical catalytic  com-
bustor radiating to the water wall.  This radiant section is followed by  a convective
section.  This design is a single stage and relies on the high radiant cooling.

6.5    INDUSTRIAL BOILERS - RETROFIT
       The retrofit designs for the firetube commercial boilers could equally well apply
to the small industrial boiler.  The problems associated with a watertube design were
briefly discussed in Section 5.  Figure 6-13 shows one approach, incorporating a sin-
gle stage catalyst and flue gas recirculation.  This unit would be built as a replace-
ment combustor and would be required to fit into an existing burner port.  For oil
firing preheated air and an oil vaporizer are required.

6.6    INDUSTRIAL BOILERS - NEW DESIGN
       As mentioned in Section 5, it may be advantageous to  incorporate radiation
cooling of the catalyst bed into a new design for an industrial waterwall boiler.
This approach is shown in Figure 6-12 under new commercial boiler designs.  Figure
6-14 shows slight modification of that approach.  This design looks more  like the
luminous wall furnace as built by Hoi den (Reference 69).
                                       6-23
-------
                                              .Conventional
                                              Burner
                                                                         Catalyst Plug
                        Fuel
ro
                               Air
                                                                                                         Water Cooled
                                                                                                         Shells
                                                                                                                  2nd Pass Heat
                                                                                                                  Exchanger
                                                                   Optional  Staging  Injection  Section
                                        Figure 6-11.   Commercial  boiler  design  II  Firetube boiler.
-------
Coolant Out
      Coolant  In
                                                    Air
                                                                     Convective Section
                                                                       Connective Cooling Tubes
                                                                 Radiant Section
                                                              Catalytic Bed
                                        t
                                      Fuel
                 Figure 6-12.  Comnercial boiler design #2 water wall configuration.
                                            6-25
-------
                                 Burner mounting flange-
A1r
              Fuel
                    FGR
                                                                                Refractory lined
                                                                                burner throat
                                                                              Water wall of boiler
                                                                 A-IOS77
                  Figure 6-13.  Industrial  boiler replacement  catalyst burner.
-------
                   Luminous wall
                catalytic combustion
Water wal1
                Premixed air/fuel  mixture
                        Figure 6-14.   Luminous wall  catalytic furnace.
                                   6-27
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6.7    CONCLUSIONS
       Data which would be useful  to the design for sizing the catalyst bed and cal-
culating heat release rates are almost nonexistent.   Nevertheless, several  possible
designs have been generated.
       •   Gas fired warm air and  hot water retrofit designs appear to be feasible,
           and although problems exist, are the easiest retrofit methods to accom-
           plish.
       t   Oil fired warm air and  hot water retrofit designs present special prob-
           lems concerning oil  vaporization prior to combustion, and oil preheating
           without thermal combustion occurring.
       •   The necessity for preheated air for some catalyst-fuel  combinations to
           activate the catalyst bed must be taken into consideration in any final
           design.
       t   New designs based on oil- or gas-fired warm air and hot water furnaces
           offer fewer constraints than retrofit designs.   Based on limited exist-
           ing information, these  designs appear feasible.
       •   Designs of both retrofit and redesign systems for commercial and indus-
           trial boilers have been made.  These new designs stress radiative heat
           transfer, while retrofit designs stress catalyst bed cooling and flue
           gas recirculation.  All appear feasible.
       •   At the present time it  is not possible to make  a reasonable cost esti-
           mate of catalytic combustors, due to a general  lack of knowledge con-
           cerning bed depth required.  This information must be obtained to allow
           cost analyses to be made.
                                         6-28
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                                     SECTION 7
                                  RECOMMENDATIONS

       Based upon extensive reviews of stationary combustion source characterizations
and existing catalytic combustion knowledge,  a complete design matrix has  been  de-
fined, and several conceptual designs from this matrix have been completed.   Because
of the proprietary nature of most of the existing data on catalytic oxidation,  the
designer is severely hampered in his ability  to do realistic calculations.   It  is
therefore recommended that a comprehensive testing program on high-temperature  cata-
lytic oxidation be conducted, in order to provide the designer with firm data for use
in sizing calculations.  This program should  include:
       •   Screening tests of various catalyst-support combinations with a  variety  of
           fuels, in order to determine activation energies and kinetic rate constants,
           and to study heat release limits,  catalyst life, poisoning effects,  ignition
           and light-off for various air/fuel  ratios, preheat temperatures,  and surface
           temperatures.
       •   Development of a mathematical model for combined homogeneous/heterogeneous
           combustion which will be capable of predicting the performance  of a  cata-
           lytic bed.  This model will make use of the data obtained in the screening
           tests.
       •   Tests which demonstrate the feasibility of catalytic combustion  in flue
           gas recirculation, catalyst bed cooling, and two-stage combustion concepts.
           Since the goal of low NO  emissions is dependent on holding the  bed  tern-
                                   A
           perature down, the mechanisms of active bed cooling must be studied  in
           detail.
       •   Prototype design and development of catalytic furnaces using flue gas re-
           circulation, catalyst bed cooling,  and/or two-stage combustion.   Since
           little data of any value presently exists to aid the designer in sizing
           a catalytic combustor, this data must be obtained experimentally before
           further prototype design is attempted.
       The results of such a test and analysis program would provide a base of  design
information, currently lacking, sufficient to allow an accurate assessment of the
prospects for NO  reduction by catalytic combustion concepts and of the commercial
                A
prospects of catalytic combustion in area-source applications.
                                        7-1
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                                         R-2
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                                         R-4
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                                 TECHNICAL REPORT DATA
                          (Please read ImLnictiuns on the reverse before completing)
 1. REPORT NO.
  EPA-600/2-76-037
      2.
            3. RECIPIENT'S ACCESSION-NO.
 4. TITLE ANDSUBTITLE
   Catalytic Oxidation of Fuels for NOx Control from
    Area Sources
                                 5. REPORT DATE
                                  February 1976
                                 6. PERFORMING ORGANIZATION CODE
 7. AUTHOR(S)
          J. P.Kesselring, R. A. Brown,
  R. J.Schreiber, and C. B. Moyer
                                 8. PERFORMING ORGANIZATION REPORT NO
 9. PERFORMING ORGANIZATION NAME AND ADDRESS
  Aerotherm Division, Acurex Corporation
  485 Clyde Avenue
  Mountain View, California 94040
                                 10. PROGRAM ELEMENT NO.

                                 1AB015:  ROAP 21AUZ-019
                                 11. CONTRACT/GRANT NO.
                                 68-02-1318, Task 12
 12. SPONSORING AGENCY NAME AND ADDRESS
 EPA,  Office of Research and Development
 Industrial Environmental Research Laboratory
 Research Triangle Park, NC  27711
                                 13. TYPE OF REPORT AND PERIOD COVERED
                                 Task Final:  10/74-4/75
                                 14. SPONSORING AGENCY CODE

                                 EPA-ORD
 15. SUPPLEMENTARY NOTES
  EPA Project Officer for this report is B.Martin, 919/549-8411, Ext 2235.
 16. ABSTRACT  Tne repOrt gives results of a review of the state-of-the-art of catalytic
  combustion concepts, and of an assessment of the applicability of catalytic com-
  bustion to gas-  and oil-fired home heaters and commercial and industrial boilers.
  Newly developed high-temperature support materials will greatly enhance the field
  of high-temperature  catalytic combustion, but current catalyst systems are  limited
  by the calatyst coating to much lower temperatures than the supports.  To keep
  combustor temperatures below those that would  cause catalyst degradation,  to
  achieve high system  efficiency, and  to prevent NOx formation, combustion system
  concepts  such as two-stage combustion,  flue gas re circulation,  and bed heat
  removal appear necessary.  Application of these concepts  to home furnaces  appears
  feasible,  but their application to larger size units may be more attractive because
  of their greater initial cost, generally more sophisticated  controls, better super-
  vision of  equipment,  and heat transfer characteristics.
 7.
                             KEY WORDS AND DOCUMENT ANALYSIS
                DESCRIPTORS
                                          b.lDENTIFIERS/OPEN ENDED TERMS
                                             c. COSATI Field/Group
 Air Pollution
 Combustion
 Nitrogen Oxides
 Catalysis
 Oxidation
 Fuels
Hydrocarbons
Air Pollution Control
Stationary Sources
Area Sources
Staged Combustion
Flue Gas Recirculation
Bed Heat Removal
13 B
21B
07B
07D
07C
21D
 Z. DISTRIBUTION STATEMENT
 Unlimited
                                          19. SECURITY CLASS (This Report)
                                          Unclassified
                                             21. NO. OF PAGES
                                                 194
                                          20. SECURITY CLASS (Thispage)
                                          Unclassified
                                                                   22. PRICE
EPA Form 2220-1 (9-73)
                                        F-l
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