Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR THE PARTIAL COMBUSTION OF CELLULOSE SPENT LIQUOR
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Technical field
The present invention relates to a process for partial combus-
tion of cellulose spent liquors from the cellulose industry in
a burner connected to a reactor, which burner comprises a cen-
trally arranged burner gun or liquor Lance equipped with a noz-
zle at its front end which adds liquor and, a coaxially ar-
ranged tubular channel around the liquor lance, in which chan-
nel an oxygen containing gas is added to support partial com-
bustion, whereby the oxygen containing gas, prior to entry in
the coaxially arranged channel, has been given a vortex move-
ment.
The object of the present invention is to facilitate partial
combustion of the cellulose spent liquor through use of a
burner creating a stable, self-igniting flame at low air/fuel
ratios.
Background of the invention
The cellulose industry generates spent liquors differing in
composition according to the delignification process used.
Within the sulphate pulping industry, spent Liquor, commonly
referred to as black liquor, contains valuable chemicals and
energy in the form of combustible carbonaceous compounds. At
the present time these chemicals and energy are normally recov-
ered in a re-covery boiler in which the black Liquor is com-
pletely burned.
Partial combustion of black Liquor in a gasification reactor as
in the present invention generates a combustible gas comprising
H2, C0, CO2, and droplets of molten inorganic chemicals.
In conjunction with pulp bleaching, a diluted liquor comprising
organic matter and sodium salts is obtained. Mechanical and
semi-chemical pulping processes also generate diluted liquors
of different compositions. These as well as other waste and
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spent liquors generated in the cellulose industry can, after
concentration be used as a feedstock in the process of the pre-
sent invention.
Although the following description describes the present inven-
tion as it applies to black liquor it is not restricted only to
this particular liquor in its application.
The mechanisms related to partial combustion of black liquor
are fairly well understood and are applied inter alia in the
lower part of the soda recovery boiler. The difference between
the present burner and a liquor burner in a soda recovery
boiler is, however, great inter alia due to the low degree of
Liquor atomization in recovery boiler burners and the absence
of a well-defined liquor flame.
A major difference between the burner of the present invention
and conventional oil burners is that a stable flame has to be
formed with the use of a considerably lower amount of air or
oxygen carrier.
As the exemplification below show black Liquor as a fuel is
characterized by a relatively low calorific value and high wa-
ter and ash contents.
Calorific value of
the dry substance 13 GJ/ton dry substance (DS)
Elementary composition C29H34~20Na9S2
Dry solids content 65
viscosity at 100°C 100 cSt.
The presence of sodium compounds in the black liquor and its
inherently high oxygen content make it a very reactive fuel,
which means, provided an adequate burner design is at hand that
the carbon conversion already in the flame zone becomes high,
in spite of the fact that the combustion is substoichiometric.
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The vortex burner described in the present invention provides
high combustion efficiency and flame stability using black liq-
uor as fuel in a relatively small reactor volume. The tempera'
lure in the reactor is above 700°C, preferably around 900°C.
The molten inorganic chemicals, substantially sodium carbonate
and sodium sulphide, are separated from the process gas in a
quench dissolver connected to the reactor. The process gas is
substantially composed of carbon monoxide, carbon dioxide, and
hydrogen. The volume ratios of carbon monoxide, and carbon di-
oxide, in the process gas is allowed to vary between 0.8:1, and
1.8:1, and is controlled by i.a. the amount of oxygen added.
The flow pattern near the burner is influenced to a great ex-
tent by the level of vortex which can be controlled by e.g. ad-
jesting the vortex btading. The radial flow rate of the oxygen
containing gas is thereby markedly affected with a maintained
axial flow rate. The main principle of the vortex burner is to
recircutate a portion of the gases through an internal
recirculation zone towards the Liquor lance. This internal
recirculation zone facilitates combustion and stabilizes the
flame and the recirculated hot gases add energy for ignition of
the Liquor spray. The internal recirculation zone also serves
as a depot for heat and reactive gas components.
The mixing of the liquor spray and the combustion air is sup-
ported by the turbulent shear surface between the recircutation
zone and the discharged gas and liquor droplets.
An outer recirculation zone, however, of less importance for
the stability of the flame is also developed. Its shape is in-
fluenced more by the geometry of the reactor than by the geome-
try of the burner.
The degree of atomization of the liquor is of great importance
for obtaining a stable black liquor (tame, the extension of the
flame and the high carbon conversion. The Theological proper-
ties of the black liquor are of significant importance to the
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degree of atomization which can be achieved in a given nozzle.
The viscosity of the black liquor can be influenced by e.g.
heating and/or the addition of additives and normally the black
liquor is being heated to above 100°C for use in the present
invention. The viscosity of the black liquor at the moment of
atomization should preferably be below 200 cSt, more preferably
less than 100 cSt.
Atomization of the black liquor can be further enhanced
by flashing the liquor into the reactor in which case the liquor
is preheated to a temperature above its boiling point at the
operating pressure of the reactor.
Several types of atomizing nozzles are available but only
a few varieties are suitable for atomizing cellulose spent
liquors, such as black liquor, in the present invention.
"Twin-fluid" nozzles are most suitable for use in the
present burner. A common feature of '°twin-fluid" nozzles is that
a relatively high gas flow rate is necessary for the supply of
energy for the atomization. Another important feature of these
nozzles is that the resulting size of th.e droplets decrease with
increasing density of the atomizing gas. Depending on how the
two fluid phases are brought together several mechanisms for
forming droplets, such as shearing between ligaments, combination
and formation of spheres of liquor droplets and high turbulence
decomposition of the liquor spray can be anticipated.
Description of the Present Tn~ention
The present invention describes a process for efficient
substoichiometric combustion of cellulose spent liquors, using a
burner connected to a reactor, which invention is characterized
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in that at least half of the non-fuel related amount of oxygen
which shall be added to the reactor to partially burn the black
liquor added through the burner, is added to the reactor in the
form of a hot oxygen containing gas which gas is added through a
tubular channel arranged coaxially around a liquor lance arranged
for the addition of said cellulose spent liquor, the weight ratio
between the amount of oxygen of the oxygen containing gas added
through the burner and of the added cellulose spent liquor solids
being in the range of 0.1°0.7:1, preferably 0.15-0.5:1.
In preferred embodiments: the stream of liquor in the
liquor lance is brought into contact with a gas having a high
velocity and a higher pressure than said cellulose spent liquor,
increasing the velocity of the stream of liquor which forms a
finely dispersed divergent spray of cellulose spent liquor
exiting from 'the lower part of said liquor lance; the cellulose
spent liquor is discharged from the lower part of said liquor
lance through at least three symmetrically arranged orifices or
through a circular gap; the oxygen containing gas is passed
through a diverging nozzle arranged coaxially around the liquor
lance, which nozzle ends in the reaction zone of said reactor;
and the vortex blading is arranged substantially behind the
atomizing nozzle of the liquor lance.
The attached drawing shows a vortex burner and two
different "twin-fluid" nozzles, whereby
FIG. 1 schematically shows a vortex burner with its
recirculation zone;
FIG. 2 shows an embodiment of a "twin-fluid'° nozzle in an
axial cross-section;
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FIG. 3 shows a front view of the nozzle according to
FIG. 2 seen along the line III-III of FIG. 2;
FIG. 4 shows a second embodiment of a "twin-fluid"
nozzle in axial cross-section; and
FIG. 5 shows a front view of the nozzle according to
FIG. 4 seen along the line V-V of FIG. 4.
FIG. 1 schematically shows a vortex burner 1 placed in
a combustion chamber 22, and a vortex generator 24 arranged in a
channel 23 for the purpose of adding air. The unbroken line in
FIG. 1 shows the spatial distribution of the internal recircula-
tion, the dotted line,the internal recirculation zone, and the
dashed line shows the limit of the return flow, i.e., the limit
along which the recirculation turns (the axial zero-velocity
line). The dashed line in the lower part of FIG. 1 also shows
the limits of the outer recirculation. The vortex generator 24
is placed substantially behind the lower part of the liquor lance
which means that the combustion air added outside the nozzle will
circulate around the liquor lance before it meets and carries the
atomized black liquor. By arranging the vortex generator
adequately a flame having a toraidal vortex is developed, an
important advantage for the stability of the flame and the course
of the partial combustion.
FIGS. 2 and 3 show a twin-fluid nozzle where the liquor
and gas are mixed and then forced under high pressure through
several symmetrically arranged circular openings 3. These openings
are the ends of so-called Y-jet atomizing nozzles comprised of two
tubes 4 and 5, the former in contact with an outer tube 6 for the
purpose of adding black liquor and the latter in contact with an
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27243-55
inner concentric annular tube 7 for the purpose of adding atomiz-
ing gas, such as air or steam. The openings 3 diverge producing
divergent atomized jets from the lower part 2 of the liquor lance
1. A hood 9 fitted to the body 10 of the liquor lance, holds the
Y-jet atomizer 8 in place. The body 10 encloses the concentric
annular tubes 6 and 7. The black liquor is introduced into the
liquor lance 1 through an inlet tube 20 and the air through
another inlet tube 21.
FIGS. 4 and 5 show an embodiment of the burner gun having
three concentric annular tubes 11, 12, and 13. Air is fed
through the outer and the inner tubes ll, and 13, while black
liquor is fed through the intermediate tube 12. The air is
divided through the 18 symmetrically distributed holes 14, and 15
shown in the figures, while black liquor is forced through an
annular gap 16. The holes 14 are hereby obliquely directed in one
direction and the holes 15 obliquely directed in the opposite
direction. The black liquor is fed through the gap 16 and meets
a lip 17 forcing it in an inward direction. Now in the form of
a film the black liquor is met by the air coming through the
holes 16 and is atomized. This initial air-black liquor mixture
is met by additional air outside the lip 17, creating a diverging
jet of finely dispersed black liquor. The black liquor is added
to the burner through an inlet tube 20 and the air through two
inlet tubes 21.
Although air has been used in the description above the
invention is not restricted to air but other gases, such as
steam, nitrogen or oxygen enriched air can be used as atomizing
gas.
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When designing burners great attention has to be paid
to the weight relationship between the air and fuel added.
The black liquor described herein is a fuel possessing
unusual properties and thus a burner which shall provide a stable
flame
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must be designed accordingly.
Different fuels contain different amounts of chemically bound
oxygen. Bitumenous coal usually contains between 4-10 % of
bound oxygen. Fuel oils contain less than 1 % of bound oxygen.
Black liquor dry solids contains about 35 % by weight of bound
oxygen calculated on dry matter. This affects the design of
burners for combustion of black liquor since only a small
amount of oxygen, air or oxygen enriched air can be added to
the burner to obtain the desired level of combustion.
The air/fuel ratio (by weight) for some fuels at stoichiometric
combustion are exemplified below:
Antracite Air/fuel 10-12:1
Ethyl alcohol "- 9:1
Black liquor "- 4-5:1
Diesel oil/heavy oil "- 13-15:1
A burner for partial combustion of black liquor in accordance
with the present invention is designed for an air/fuel solids
ratio in the order of 0.5-3:1 which thus is considerably lower
than ratios applied at stoichiometric as well as
substoichiometric combustion of most other fuels. Since air
consists of about 23 % by weight of oxygen the present black
Liquor burner is thus designed for an oxygen added/black liquor
solids added ratio in the range of 0.1-0.7:1. To compensate for
the low air/fuel ratios and to achieve reasonable gas veloci-
ties the air should be preheated to at least 100°C, preferably
to 300oC and it should further be given vortex movement.
Preheating of the air adds energy in close proximity of the
burner, which further supports the stability of the flame. The
larger part of the oxygen required for the partial combustion
is added through a channel arranged coaxially around the liquor
lance which channel in turn ends in the reactor in a divergent
burner tile. Part of the oxygen required for the partial com-
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bustion can be added to the flame zone through the atomizing
nozzle and optionally, another part can be added through secon-
dary air gates in the upper part of the reactor.
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