Note: Descriptions are shown in the official language in which they were submitted.
1 31 9055
The present invention relates to improvements in
steam generating boilers. ~ore par-ticularly, the invention
is directed toward improving combustion conditions in steam
generatillg boilers such as the recovery boilers which are
used in -the pulp and paper mills for the combus-tion of spen-t
liquor frorn sodium-based pulping process.
The main objec-tives in operating a recovery
boiler are to recover the pulping chemicals in the reduced
s~ate and to recover -the heat released by the combustion of
carbonaceous material to generate steam for the process. The
spent liquor from the pulping process is sprayed in small
drops over the cross section of the boiler furnace through
the upwardly flowing combustion gases so as to dry the
liquor droplets to a concentration where the heat value of
the char material with the residual moisture is sufficient
to keep a reasonably stable combustion going. The dry liquor
solids settle on the bottom of the boiler forming a carbona-
ceous char bed. The char bed has two functions: to reduce
-the spen-t chemicals for further recycle and to supply heat
by reacting with the oxygen in the air being blown horizon-
tally over the char bed.
ln such boilers, the air is introduced peripher-
ally through ports located in the boiler sidewalls and into
the lower section of the boiler. ~n most designs, the total
air supply is divided in two or more streams which are
introduced peripherally at different levels of the boiler.
These air streams are referred to as primary, secondary and
tertiary air, conventionally starting from the bottom of the
boiler
1 3 1 9055
Because of the influence of the induced draft Ean
and of the large size of the boiler, only a very small
fraction of the peripherally introduced air reaches the
central region of the boiler's cross-section.
Peripheral air is introduced either horizontally
or slightly downwardly at subsonic velocities ranging from
about 25 to 100 m/sec, which causes an upward deflection of
the air along the walls of the boiler.
In cases where the fuel which supplies heat for
the steam generation is concentrated towards the center of
the boiler's cross-section, such as in the case where a char
bed containing carbonaceous materials sits on the bottom of
the boiler, the peripherally introduced air will not readily
combine with the combustible species, either gaseous or
finely divided solids, resulting in poor combustion in the
lower section of the boiler.
A fundamental limitation to the burning capacity
of these boilers is due to such poor mixing between the
combustible species and the oxygen required as a comburant.
As the air is in-troduced peripherally through sidewall ports
and blown into the lower section of the boiler, due to the
relative low pressure and subsequent low velocity of the air
flow, there is a preferential upward flow along the side-
walls, leading to poor mixing with the combustible species.
The lack of intimate mixing of air with the
combustible species in the lower section of the boiler
limits its capacity not only because heat transfer to the
boiler tubes is poor since peripheral air behaves as a
coolant, but also because the lack of mixing lengthens the
combustion zone, resulting in a vertical temperature profile
_A 2
;,~,
1 3 1 9 055
which promotes carry over of unreacted inorganic ma-terial
and in unnecessarily higher -temperatures in the upper
section of -the boiler, where screen tubes and superhea-ter
-tu~es are located.
Layers of carried over deposits on the screen and
superheated tubes can be over 20 mm thick, thus drastically
obstructing heat -transfer and reducing the sectional area
for the passage of gases, eventually bottlenecking the
boiler when the high pressure drop through the upper sec-tion
limits the air blowing capacity of the boiler, forcing
scheduled or non-scheduled shut-downs for deposit removal.
Another problem associated with poor mixing of
the air with the combustible species is -the emission of
reduced sulfur species in the exit gas. In conventional
boilers, eventhough an overall 2 excess of over 2% may
exist in the exit gas, some reduced sulfur species mix with
the available oxygen only at the top of the boiler, where
no-t enough time is available for a complete oxidation to
occur and/or the gases are already at a lower temperature
than necessary for complete oxidation.
From a thermochemical equilibrium view point, as
long as there is more than 1~ vol. 2 in the flue gases
exiting the boiler furnace, there should be less than 20 ppm
total reduced sulfur ~TRS) speciesO However, because of
imperfect gas mixing, equilibrium is not at-tained and
therefore oxygen and combustible species coexist in the flue
gases.
A better mixing of oxygen and the combustible
species would modify -the ver-tical temperature profile of the
boiler resulting in a temperature increase in the lower
1 31 qO55
section oE the boiler and consequent shorter combustion zone
and lower exit gas tempera-cure in the upper section of the
boiler wi-th -the following advan-tages:
1. Reduc-tlon of carried over deposits.
2. Lowering of TRS emissions at similar excess 2
in flue gas.
3. Increased chemical recovery capacity.
4. Inereased steam generating capacity.
5. Reduced shut-down frequency for deposit re-
moval.
6. Srnoother boiler operation.
Enriching the combus-tion air wi-th oxygen would
-further allow burning capaeity increases without subsequen-t
inerease in earry over deposits due -to lower gas veloeities
relative to air eombustion.
It is -therefore an object of the presen-t inven-
tion to improve the air distribution in che lower section of
a s-team generating boiler so as to provide intimate mixing
of the air with the combustion speeies, thereby improving
combustion.
In accordance with one aspect of the invention,
-there is provided a method of improving combustion in a
steam genera-ting boiler having a bottom wall supporting a
char bed and sidewalls with por-ts -through which air is
admitted for combus-tion of combus-tible species in the char
bed and emanating therefrom, which comprises introducing an
oxygen-con-taining gas into a lower central zone of the
boiler, from at least one point remote from the sidewalls to
thereby cause intimate mixing of -the oxygen contained in the
gas with the combustible species.
1319055
According to a further aspect of the invention,
there is also provided in a steam generating boiler having a
bot-tom wall supporting a char bed and sidewalls with ports
through w'nich air is admitted for combustion of combustible
species in the char bed and emanating therefrom, the impro-
vement which comprises means for blowing an oxygen-contain-
ing gas into a lower central zone of the boiler, from at
least one point remote from the sidewalls to -thereby cause
intimate mixing of the oxygen contained in the gas with the
combustible species.
Applicant has found quite unexpectedly that by
introducing an oxygen-containing gas into the lower central
zone of the boiler, remotely from the sidewalls of the
boiler, a better mixing of oxygen and combus-tible species
could be achieved and that the oxygen deficient zone which
is charac~eristic of boilers where air is introduced only
peripherally through sidewall por-ts could be greatly reduced
as a result of the improved mixing of the oxygen with the
combustible species. This improvement can be attained
without disrupcing the char oed formation which is essential
to achieving chemical recovery. Due to the resulting tempe-
rature increase in che lower section of the boiler, oxida-
tion of Na2S, H2S or organic sulrides would occur in tha-t
section of the boiler, thereby lowering TRS emissions.
Examples of suitable oxygen-containing gases
which can be introduced non-peripherally include air,
oxygen-enriched air and mixtures of oxygen with other gases
2' 2' CO, CE~4, C3E18, natural gas, H2O vapour
N2O, flue gases, etc. It is of course also possible to use
commercial 2 having a molecular oxygen content generally
I ;~ 1 9055
between 90 and 99.5% by volume. On -the other hand, where -the
oxygen-containing gas comprises a mixture of 2 and CO2,
such a gaseous mix-ture is preferably nitrogen-free, -that is,
havi.ng a N2 content of less than about 4% by volume, so as
-to enable the CO2 to be recovered. Preferably, up to about
60~ of -the total oxygen re~uirement is introduced via the
non-peripheral blowing of the oxygen-containing gas, the
balance being supplied i.n the form of air introduced peri-
pherally ~hrough the sidewall ports.
The oxygen-containing gas can be introduced
non-peripherally by blowing the gas either downwardly from
an upper section of the boiler or upwardly from above the
char bed, or by a combined blowing of the gas both down-
wardly froin an upper section of the boiler and upwardly from
above the char bed. The oxygen-containing gas can be blown
at any pressure, from atmospheric ~when a negative pressure
exists in the boiler because of an induced draf-t fan) to
about 10 atm., -the preferred pressure range being between
about 1.2 and about 5 atm. (absolute). Thus, the gas veloci-
-ty can range from about 1 ft/sec to over sonic velocity,
preferably from about 10 to about 1000 ft/sec.
The oxygen-containing gas is conveniently blown
downwardly by means of at least one elongated lance arranged
in the upper section of the boiler and ex-tending downwardly
to discharge through at least one orifi.ce thereof at least
one stream of the oxygen-containing gas, remotely from -the
sidewalls of the boiler. For example, a single lance can be
suspended from the top of the boiler to ex-tend vertically
and centrally of the boiler, or can be mounted in the
so-called "bull nose cavity" of the boiler, in which case
1 3 1 qO55
the lance is angularly inclined. Such a lance is preferably
provided wi-th a plurality of di.scharge orifices spaced from
one anothe~ and each oriented at an angle no-t greater than
about 60 relative to the longi-tudinal axis of the lance. On
the other hand, where a plurality of lances are used, the
lances can be evenly distributed relative -to a central
vertical axis of the boiler or they can extend in a common
plane, in spaced-apart parallel relationship; in the latter
case, -the lances may extend either vertically or at an angle
relative -to the vertical.
Upward blowing of the oxygen-containing gas, on
-the other hand, is advan-tageously effected by means of a-t
least one injector arranged on the bottom wall oE the boiler
remotely from the sidewalls thereof and extending through
the char bed. Preferably, the injector protrudes from the
surface of the char bed immediately surrounding -the injector
a distance ranging from about 1 cm to about 30 cm, so as to
not in-terfere with the chemical reactions occurring in the
char bed and to prevent blockage of the gas discharge
orifice of the injector by the liquid smelt.
According -to a particularly preferred embodiment,
the injector comprises an elongated conduit of temperature
and corrosion resistant metal extending through the bottom
wall, and a protective refractory struc-ture surrounding the
conduit, the conduit and refractory structure coextending
from the bottom wall to above the char bed.
The pro-tective refractory structure should be
made of a refractory material which is chemically resistant
to the smel-t and capable of mechanically withstanding
impac-ts caused hy falling deposits from the upper section of
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1;~1'3055
the boiler. Examples of suitable refractory mate:rials
include alumina, silica, silicon carbide, magnesite and
chrome-magnesite.
In the case where a single injector is used, i-t
is preferably disposed centrally of the boiler. In -the case
of a plurali-ty of injectors, on the other hand, -these are
preferably arranged to impart a swirling motion to the
oxygen-containing gas.
It is also possible to pneumatically inject with
the lance and/or injector particulate solids which can act
as seeds to cause agglomeration of volatilized inorganic
mat-ter, or as a source of heat to control furnace or char
bed temperature, the oxygen-containing gas further serving
in this case as a carrier gas. Such injection of particulate
solids is also useful in removing accretion build-up from
the gas discharge orifices of the lance or injector. For
example, particles of sodium sulfate can be used as agglome-
ration seeds whereas particles of carbonaceous materials
such as coal or sawdust can be used as a source of heat.
However, where use is made of coal particles, the carrier
gas should not contain more than about 21~ vol. oxygen.
The present invention finds application not only
in recovery boilers used in pulp and paper mills, bu-t also
in other types of steam generating boiler such as those
opera-ted in coal fired power plants and boilers burning any
mixture of biomass, hydrocarbons, fossils or by-product
fuels for the purpose of generating steam and optionally
recovering chemicals.
1 31 qO55
Further features and advantages of the invention
will become more readil.y apparent from the following des-
cription of preferred embodiments as illustrated by way of
example in the accompanying drawings, in which:
Fig. 1 is a schematic vertical cross-section of a
kraft recovery boiler equipped with a top blowing lance
according to the invention;
Fig. 2 is a view similar to Fig. 1, illustrating
a different location of the lance;
Fig. 3 is a fragmentary section view of the lance
shown in Figs 1 and 2, illustrating -the discharge end
thereof;
Fig. 4 is a bottom view of a lance according to
another preferred embodiment;
Fig. 5 is a fragmentary section view taken along
line 5-5 of Fig. 4;
Fig. 6 is a schematic vertical c,oss-section of a
kraft recovery boiler equipped with a bottom blowing injec-
tor according to the invention;
Fig. 7 is a fragmentary vertical section view
illustrating the injector shown in Fig. 6i
Fig. 8 is a fragmentary top view of the injector
shown in Fig. 7;
Fig. 9 is a view similar to Fig. ~, illustrating
an injector according to ano-ther preferred embodiment; and
Figs 10, 11 and 12 which are on the same sheet as
Fig. 6 are schematic horizontal section views illustrating
different arrangements of injec-tors.
I 3 1 9055
Referring first to Fig. 1, there is illustrated a
kraft recovery boiler generally designated by reference
numeral 20 and seen having a slan-ted bottom wall 22 and
vertical sidewalls 24. The bottom wall 22 is formed of
closely spaced tubes 26 with welded fins therebetween
whereas the sidewalls 24 are lined with similar tubes 26
allowing circulation of water introduced through inlet 28
and fed to manifold 30 for distribution to the tubes 26.
slack liquor from the kraft pulping process is sprayed by
means of spray nozzles 32 in small drops to collect as black
liquor dry solids in a char bed 33 supported by the bottom
wall 22. As water rises through the tubes 26, it is gradual-
ly heated by the heat released by the combustion of the
black liquor solids and vaporizes into steam to be collected
in the upper drum 34 of the boiler tube bank 36 comprising a
plurality of boiler tubes 38. Saturated steam is then sent
from the upper drum 34 via line 40 to superheater tubes 42
for the generation of high-pressure dry steam which is
discharged at the outlet 44 and may be used at various
points in the pulp and paper mill.
Air for the combustion of the black liquor solids
is supplied at three different levels in ~he boiler, by
means of primary, secondary and tertiary windboxes 46,4~3 and
S0 which respectively blow primary, secondary and tertiary
air through ports 52,54 and 56 provided in the sidewalls 24.
The primary air is blown through por-ts 52 and which may
account for up to 60~ of the total air supply serves to
control the height and shape of the char bed 33. The char
bed is a mixture of inorganic salts and carbonaceous mate-
rials which provides a reducing environment to chemically
1, )
-- 10 --
'331qO5~
reduce sodium sulfate -to sodi.um sulfide and sodium hydroxide
-to sodium carbona-te, the active chemicals in the liquid
smel-c produced and discharged through spout 5~. These
cher,licals are subsequently recycled to the diges-tion stage
of -the pulp mill for -the -trea-tmen-t of incoming wood.
The secondary air which is blown through ports 54
may account for up to 50% of the total air supply and
provldes the oxidant which first meets the incoming black
liquor from the spray nozzles 32. Besides causing flash
dehydration of the black liquor salts, it supplies oxygen to
burn carbon monoxide formed at the char bed 33 and should
oxidize the reduced sulfur species either contained in the
black liquor or genera-ted during the combus-tion of dry
solids.
The tertiary air blown through ports 56 supplies
the balance of air needed to attain an excess 2 in the exit
gas represented by the arrow 60. The 2 concen-tration in the
exit gas varies in practice from about 0.1 to about 6% by
volume, but for the purpose of the present invention it is
preferably within the range of 1.0 to 2.5~ by volume. The
purpose of -the tertiary air is to take to completion the
oxidation of combustible species emanating from the lower
section of the boiler 20.
Hot gases and entrained volatilized matter are
carried -to the upper section of the boiler 20. ~s temperatu-
re decreases, the volatilized matter forms crusty deposits
on screen tubes 62 and the boiler must therefore be periodi-
cally shut down to remove such deposits. The screen tubes 62
1 31 qO55
form an independent hot water circui-t which -takes hot water
from the lower drum 64 via line 66 and discharges steam via
line 68 into -the upper drum 34 of the boiler -tube bank 36.
The hot gases containing mainly nitrogen, carbon
dioxide and water vapor from the combus-tion of organic
matter also carry ash and chemical fumes, which after the
superheater tubes 42, cross the boiler tubes 38 and enter an
economizer ~not shown). The economizer is a heat exchanger
which uses the sensible heat in the exit gas 60 to indirec-
tly preheat the feed water before it reaches the boilertubes 38 and subsequently the water introduced through the
inlet 28 at the bottom of the boiler 20.
A fundamental characteristic of traditional steam
generating boilers which limits efficient burning of combus-
tible species is the lack of intimate mixing of the seconda-
ry and tertiary air supplies with the intermediate products
of combustion. The low velocity air tends to flow upwards
peripherally along the sidewalls 24, resulting in a
relatively cold gas containing large 2 excess. In the
central zone, an 2 defficient plume 70 forms which may
reach as high as the screen tubes 62 before complete mixing
with the peripheral lean gas takes place.
The delayed mixing has important detrimental
effects for the boiler operation. Should intimate mixing
-take place at the tertiary air level, or not too high over
it, complete combustion would be attained, thus the longitu-
dinal temperature profile would change, resulting in a
shorter but hotter combustion zone, with a subsequent lower
temperature at the upper section of -the boiler.
- 12 -
i 3 1 9055
In order to overcome these drawbacks and to
reduce the oxygen-defEicien-t zone 70, an oxygen-con-taining
gas is blown downwardly into the lower central zone of the
boiler by means of a water-cooled lance 72 suspended from
-the top of the boiler by a retaining collar 74 and arranged
centrally of the boiler. An oxygen-containing gas such as
air or oxygen-enriched air is thus blown cen-trally into the
lower section of the boiler, thereby causing intimate mixing
of oxygen with the combustible species and resulting in a
much shorter O2-defficient plume 70'.
Instead of posi-tioning the lance 72 vertically
and centrally of the boiler, it is also possible to mount a
shorter lance 76 in the so-called bull nose cavity 78 of the
boiler, as shown in Fig. 2. In this case, the lance 76 is
angularly inclined and s~ill provides non-peripheral down-
ward blowing of oxygen-containing gas into the lower central
zone of the boiler 20'.
Fig. 3 illustrates the structure of the water-
cooled lance 72, which may also be the same for the lance 76
shown in Fig. 2. As shown, the lance 72 is formed with a
central conduit 80 for conveying the oxygen-con-taining gas,
which merges wi-th an outwardly diverging gas discharge
orifice 82. Two concentric tubular conduits 84 and 86 are
provided for circulating water -to cool the lance, the con-
duits 84 and 86 communicating with one another at their
lower ends by means of an annular elbow 88 formed in the tip
90 of the lance. The lance tip 90 can be made of a high
thermally conductive metal, such as copper or a copper
alloy. The ou-ter wall 92 of the lance, on -the other hand,
can be made of corrosion resistant metal such as a ferrous
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1 3 1 9055
alloy (e.g. stainless steel), whereas the inner walls 94 and
96 can be made oE thermally conductive metal such as carbon
steel, for adequate cooling.
Figs 4 and 5 illustrate the discharge end of a
similar water-cooled lance 72', but having a modified tip
90'. As shown, the tip 90' is formed with three gas dischar-
ge orifices 82' equidistantly spaced from one another and
each oriented at an angle of about 45 relative to the
longitudinal axis of the lance.
In the recovery boiler 20" illustrated in Fig. 6,
the non-peripheral blowing of oxygen-containing gas is
effected by blowing the gas upwardly from above the char bed
33 into a substantially gaseous phase by means of an injec-
tor 98 arranged on the bottom wall 22' and extending -through
and above the char bed 33. The injector 98 comprises an
elongated conduit 100 extending through -the bottom wall 22'
for conveying the oxygen-containing gas and a protective
refractory structure 102 surrounding the conduit 100, as
best shown in Fig. 7. The conduit 100 and refractory s-truc-
ture 102 coextend from the bottom wall 22' to above the char
bed 33. The refractory structure 102 has a conical configu-
ration, the gas discharge orifice 104 being located at the
apex of such a conical structure. The flow of oxygen-con-
taining gas can be regulated by means of the valve 106.
Where the oxygen-containing gas is air and it is desired to
enrich the air with oxygen, molecular oxygen can be admixed
via the conduit 108 connected to conduit 100 and provided
with a valve 110 for regulating the flow of molecular oxygen
admixed.
- 14 -
13190~i5
The bot-tom wall 22' is formecl of closely spaced
tubes 26 wiLh welded fins 112 therebetween, as is -the bo-ttom
wall 22 shown in Figs 1 and 2. However, in order to ins-tall
-the injector 98 and enable the conduit 100 -thereof to extend
between the bo-ttom wall tubes, -the two tubes 26' immediately
adjacent the conduit 100 are ben-t downwardly and ou-twardly
to provide sufficien-t spacing for accommodating the conduit
100; as best shown in Fig. 8. In order to also allow ther-
mally induced deformations, the fins connected -to the tubes
26' are made in two parts 112' and 112" which are movably
engaged with one another by means of a tongue and groove
arrangement 11~.
Fig. 9 illustLaLes a similar bot-tom injector 98'
with a protective refractory struc-ture 102' having a pyrami-
dal configuration. As shown, the injector 98' is provided
with four gas discharge orifices 104', one in each of the
four upwardly converging sidewalls o:E the pyramidal refrac-
tory struc~ure 102'.
As shown in Fig. 10, the injector 98 is arranged
centrally of the boiler 20" so as to blow the oxygen-con-
taining gas vertically upwardly in the center of -the boiler.
It is also possible to arrange the injector 98 off-center
and to install two pyramidal--type injector 98" each having a
single gas discharge orifice 104' in the refractory StLuc-
ture 102" thereof such as to blow two streams of oxygen-con-
taining gas angularly upwardly in a direction toward the
vertical stream of oxygen-containing gas blown by the
injector 98, as shown in Fig. 11. Four pyramidal-type
injectors 98" can also be arranged in a manner such that the
respective gas discharge orifices 104' thereof blow a stream
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~ 3 1 ~055
of oxygen-containing gas angularly upwa:rdly while imparting
to the oxygen-containing gas a swirling mo-tion, as shown in
Fig. 12.
It should be noted in connec-tion with -the embodi-
ments illustra-ted in Figs 1 and 2 that the lance 72 or 76
need not necessarily be wa-ter cooled as other types of
cooling means can be utilised. For instance, -the lance can
comprise a first tubular conduit for blowing the oxygen-con-
taining gas and a second tubular condui-t concentrically
arranyed with respect to the first conduit to defi.ne a
channel of annular cross-section surrounding the first
conduit for blowing a gas shrouding -the oxygen-containing
gas. The shrouding gas can be any gas or mixture of gases
which may serve as a coolant or as a gaseous shield to
protect the tip of the lance from 2 attac]~. Examples of
shrouding gas which may be used to this end include air,
argon, N2, CO2, CO, CH4, C3M8, H2O vapour and flue gases.
With respect to the embodiments shown in Figs
8-12, the refractory s-truc-ture 102, 102' or 102" is entirely
optional since when -the 2 concentration of the oxygen-con-
taining gas blown by the injector 98,98' or 98" is less than
about 35% by vol., a single s-teel pipe is adequate. For 2
concentrations of 35% by vol. and over, use can be made of
an injector comprising a first tubular conduit of temperatu-
re and corrosion resistant rnetal for blowing the oxygen-con-
taining gas and a second -tubular conduit of temperature and
corrosion resistant metal concentrically arranged with
respect ~o the first conduit to define a channel of annular
cross-section surrounding the firs-t conduit for blowing a
gas shrouding the oxygen-con-taining gas, the Eirst and
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~ 3~ qO55
second conduits coextending through the bottom wall and char
bed. A refractory structure such as that represented by
reference numeral 102 can optionally surround the second
conduit, the refractory structure and second conduit coex-
tending from the bottom wall through the char bed.
When solid carbonaceous or oxygen reactive
ma-terials are pneumatically injected into the boiler, a
concentric double condui-t type injector as described above
can be advantaqeously utilized, wherein a gas which is
non-reactive to the solid carbonaceous or oxygen reactive
materials is used as a carrier and blown together with the
solid carbonaceous or oxygen reactive materials through the
central conduit while the oxygen-con-taining gas is blown
through the annular channel defined between the condui-ts.
The carrier gas can consist of a hydrocarbon gas or of a
gaseous mixture of hydrogen, carbon monoxide and hydrocar-
bons. It is also possible to inject the solid carbonaceous
or oxygen reactive ma-terials through the central conduit by
means of a liquid hydrocarbon, the oxygen-containing gas
being blown through -the annular channel between -the con-
duits.
According to a further preferred embodiment, usecan be made of a concentric triple conduit injector, that
is, an injector comprising a first tubular condui-t of
temperature and corrosion resistan-t metal for pneumatically
injecting a solid oxygen-reactive material in particulate
form with a carrier gas which is non reactive to the oxygen
reactive material, a second tubular conduit of temperature
and corrosion resistant metal concentrically arranged with
respect to the first conduit to define a first channel of
1 3 ~ ~ ~ 5 5
annular cross-sec-tion surrounding -the first conduit for
blowing -the oxygen-containing gas, and a third tubular
condui-t of -temperature and corrosion resistant metal concen-
trically arranged wi-th respect -to -the second conduit to
define a second channel of annular cross-section surrounding
the second conduit for blowing a gas shrouding -the oxygen-
containing gas, -the first, second and third conduits coex-
tending through the bottom wall and the char bed.
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