Note: Descriptions are shown in the official language in which they were submitted.
'7'7~
The presen-t invention rela-tes to a method for wet
combustion of organic material.
More particularly, the invention relates to a method
o~ directly wet combusting an aqueous solution con-tain-
ing dissolved or finely dispersed organic material by molecu-
lar oxygen at elevatecl pressure and at high temperature.
It is known to totally or partially combust organic
material dissolved or finely suspended in water by molecular
oxygen or gases containing molecular oxygen, such as air,
under pressure and at elevated temperature, which temperature
depending on the degree of combustion and the type of the
organic substance, should be in the range of 180 to 340C.
The process is suitably carried out continuously, and the
combustion can be effected using both concurrent flow and
counter-current flow with an almost complete use of the mole-
cular oxygen. When using air in the combustion of e.y. ligno-
cellulose-containing biological substances, such as wood,
peat, and bagasse, or waste liquors obtained by the acid
or alkaline pulp digestion of biological substances, the
exhaust combustion gases seldom contain more tharl 0.2% of
molecular oxygen. If, nevertheless, an almost complete com-
bustion of the organic material is to be obtained, -the combus-
tion temperature usually must exceed 300C, e.g. be between
300 and 340C.
Due to the continuously reclining content of oxyyen
during the combustion process, -there occurs a corresponding
reduction of the complete oxidation of the organic material
into carbon dixoide and water. This contributes during the
combustion process, especially in the combustion of lignocel-
lulosic ma-terial, to the formation of difficult oxidizable
compounds which are mainly low molecular weiyht acids, such
as acetic acid, proprionic acid, or salts thereof. This
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is independent of whether the combustion takes place in con-
current or counter-curren-t flow.
In the wet combustion process, the incoming liquid
may lose volatile combustible material by expulsion with
the exhaust mixture of steam and gas formed by the combustion,
irrespective whether the combustion takes place in counter-
current and concurrent flow. Volatile products can be present
in the incoming liquid and also be formed during the combus-
tion. Due to the low concentration of molecular oxygen in
exhaust flue gas in -the final stage of the combustion, there
is the risk that the volatile produc-ts remain unoxidized
and either remain in the solution as e.g. salts, or are en-
trained in the s-team generated.
However, experiments have shown that excess molecu~
lar oxygen at high concentrations, such as in air, of from
20 to 50%, facilitates the combustion of the difficult oxi-
dizable compounds which, when they result from combustion
of biological substances or products thereof, normally consist
of low molecular weight fatty acids~ primarily acetic acid.
Where the incoming liquid is alkaline and -the formed
acids are bound in the form of salts, generally the same
problem exists, viz. the decomposition of the acids into
carbon dioxide and water, as in the case of free acids.
A great advantage with the combustion in alkaline solution
is, however, tha-t the generated steam is free from acidity,
which facilitates its use for heating and power generating
purposes and simplifies -the selec-tion of suitable construc-tion
material for these purposes.
It is further known from experience tha-t the combus-
tion of lignocellulosic biological substances or products
thereof can be effected under relatively moderate ternperature
conditions, between 180 and 300C when the generated heat
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is restricted to between 75 and 90~ of the calorific value
of the organic material, but that higher te~peratures are
required to release the last 5 to 10% of the calorific value,
and where this organic material is cons-ti.tuted by low molecu-
lar weight acids, the combustion -temperature must substantial-
ly exceed 300C. From experiments with wet combustion of
alkaline waste liquor from digestion of wood using pure
sodium hydroxide solution, and from the resul.ts to be referred
to below, it becomes evident that use of a surplus and high
concentration of oxygen gas in the final stage of the combus-
tion process facilitates the decomposition of the difficultly
oxidizable compounds into carbon dioxide and water.
There was combusted, for example, a waste liquor
obtained by digestion of pine-wood using 220 g NaOH and 2
g of anthraquinone per kilogram wood calculated as bone dry
substance at a temperature of 170C into a pulp yield of
47.8%. The waste liquor has a dry solids content of 14.7o
with a calorific value of 3,762 Cal per kg and contained
24.6% of Na2O calculated as bone dry substanceO In -the com-
bustion of this waste liquor in an autoclave while using
air with an initial pressure of 3,800 kPa at 20C, resulting
in a partial pressure of the oxygen gas of 800 kPa at 20 C,
83% of the calorific value of the waste liquor were released
at a temperature of 275C, the partial pressure of the oxygen
gas thereunder falling to 400 kPa. Thereupon, the temperature
was raised to 300C, whereby additional heat was released
so that, calculated on the original waste liquor, 90% or
the calorific value was released~ Then the partial pressure
of the oxygen gas dropp^ed to 250 kPa.
In a similar experiment, the combustion was started
with air having the same par-tial pressure of the oxygen gas
of 800 kPa as in the preceding experimen-t~ Then, 8~% of
the calorific value of the waste liquor was released when
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a temperature of 275C had been reached. Pure oxygen gas
was then supplied and the temperature was raised to 300C,
when altogether 96% of the calorific value of the waste liquor
was released. Then the partial pressure of the oxygen gas
was 500 kPa calculated at 20C and represents double surplus
of oxygen gas in the final stage as in the previous experi-
ment, in which the total combustion process until a final
temperature of 300C was carried out with that quantity of
molecular oxygen which was present in the air initially sup-
plied.
To reach a high degree of combustion in the combus-
tion of the lignocellulosic biological substance withou-t
resorting to extraordinary conditions of temperature subs~an-
tially exceeding 300C, such as e.g. to 340C, the combustion
in the final stage must be effected with a great surplus
of molecular oxygen, and so as at the same time to limit
the consumption of oxygen for all the organic material present
in the waste liquor~ the combustion must be effected in two
separate steps. In the first step the combustion of the
incoming liquid containing organic substance is carried to
such degree that between 75 and 95% of the calorific value
is released, which can be effected with small excesses or
molecular oxygen. In the second step the remaining organic
substance is combusted with a great surplus of molecular
oxygen in such a manner than steam and gas effluent from
this second step can be fed to the first step with a content
of oxygen gas adjusted so that the combustion in this step
can be effected to the aforesaid degree of 75 to 95%. If
desired, an extra addition of molecular oxygen may be supplied
to the effluent steam and gas so that the stated combustion
degree is reached. The yas containing the molecular oxygen
incoming into the second step must be satura-ted with steam
at 300C in order to avoid cooling of the liquid in the second
step and thereby staying at a too low combustion temperature.
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Assuming that 10%, for example, of the combustion
heat of the earlier mentioned wasteliquor is preserved after
the first step and that air is used for the combustion, the
surplus of molecular oxygen in the second step becomes about
times greater than the theoretical requirement, if the
whole quantity of air necessary for the combustion of the
incoming organic substance is supplied to the second step.
Instead of air, it is, to advantage, possible to
use air enriched with oxygen gas, e.g. with between 20 and
50% of 2~ or other non-reactive gases having a higher content
of molecular oxygen than air. Considering solely the reaction
mechanism, it is advantageous to use pure oxygen gas. However
for safety reasons, it appears to be inappropriate to operate
with higher oxygen contents than 30-50% of the gas entering
into the combustion zone.
The compressed air enriched with oxygen can be
prepared depending on local conditions eithex by mixing toge-
ther air and oxygen gas at atmospheric pressure and there-
upon compressing the gas mixture, or by mixing compressed
air with oxygen under pressure, e.g. by vaporization of liquid
molecular oxygen.
The combustion gases leaving the wet combustion
plant can also be recirculated under almost the same pressure
that prevails in the combustion apparatus after it has been
cleared, for instance under pressure, of generated carbon
dioxide and possible excess of non-reactive gases, e.g. nitro-
gen, and thereupon having been supplied with an adequate
quantity of oxygen gas under pressure.
Usually, the wet combustion process is carried
out in concurrent flow, but it may in many cases be more
suitable to carry out the combustion in the second step in
a counter-current flow, which then also takes into account
that the liquid fed into the second step is of a relatively
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small quantity due to -the evaporation of incoming aqueous
solution which occurred due to escape of vapour during the
combustion.
When e.g. waste liquor containing 18~ of dry sub-
stance, of which 81~ is organic material, from a pure soda
process is combusted, the waste liquor must be diluted with
water so that the generated heat can be totally converted
into vapour. Furthermore, water must be added for removal
of the soda formed in the combustion. In this case, only
10-12% of the quantity of water entering the first step will
be supplied to the second step and finally oxidized. This
is done most effectively in a counter-current flow in, e.g.
a tower filled with annular elements, which increase the
surface of contact between the liquid and the molecular oxygen
; containing gas, which facilitates the diffusion of the gas
~; into the liquid and thereby accelerates the combustion reac-
tion. The filling material of the tower may be a material
which in a catalytic manner stimulate oxidation, such as,
for example, nickel, or chromium, vanadium and titanium con-
taining alloyed steel, or the tower filling material may
be coated with active material, e.g. platinum or nickel,
precipitated on ceramic materials. It is also possible to
utilize heterogeneous catalysts in the form of powder, e.g.
copper chromite, finely divided platinum which is added to
the incoming liquid and after completion of the oxidation
is separated or precipitated and, if desired, reactivated
and recirculated. The types of catalysts depend mainly whe-
ther the combustion of the incoming organic material is to
be performed in an acid, neutral or alkaline environment.
According to the present invention, therefore,
there is provided in the wet combustion method of combusting
an aqueous solution containing organic material in which
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the organic material is oxidized by introducing into the
solution molecular oxygen material at a temperature ranging
between 180C and 340C and a correspondingly elevated pres-
sure, the improvement comprising: a) a first oxidizing step,
in which the organic material in said aqueous solution is
oxidized so as to release an amount of the heat of combustion
ranging between 75% and 9S% thereof; b) a second oxidizing
step, in which the residual organic material is finally oxidi-
zed by introducing surplus molecular oxygen in an amount
sufficient to impart to the gaseous effluents generated by
said second step a content of molecular oxygen sufficient
to achieve the oxidation of the first step; and c) a feeding
step, in which said gaseous effluents are supplied to said
first step. Suitably, the organic material comprises cellu-
lose-containing biologic material and in which low molecular
acids formed in the first step and resistant to oxidation
therein are oxidized in said second step. Desirably, the
molecular oxygen comprises a stream of compressed air. Pre-
ferably, the oxidation of the organic material in the first
step is carried out in concurrent flow with the stream of
compressed air and in which the oxidation in step two is carried out
in counter-current flow to -the flow of said stream of com-
pressed air.
In one embodiment of the present invention, the
flue gases cooled by said gaseous effluents are liberated
from carbon dioxide and excess of non-condensible gases formed
during the combustion process, and in which the residual non~condensi-
ble gases are recirculated together with molecular oxygen
to the second oxidation step.
In order to exemplify how the process may be carried
out, reference is made to the following example and the accom-
panying drawing, in which the single figure is a flow sheet
indicating the essential equipment parts of a plant for carry-
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7~7~
ing out combustion oE black liquor from the production of
kra~t pulp by means of sulphur-free sodium hydroxide solu-
tion and recovery of soda. Since the waste liquor is alka-
line, no problems arise regarding purification of the water
vapour leaving the process~ Otherwise, when non-combusted
volatile compounds are formed and pass with the water vapour,
e.g. free acetic acid, such acid must be removed either
directly from the water vapour or from the condensate formed
thereform.
Referring now to the Figure, black liquor from
a pulp production of 20 t/h consisting of e.g. 127,560 kg
of water and 2B,000 kg of dissolved solids, of which 81%
is organic substance, is fed into vessel 1 through pipe 2.
Simultaneously, 47,823 kg of steam condensate at 40C and
6,092 kg of warm water at 151C are fed into a storage vessel
1 through pipes 3 and 4, respectively and furthermore, 13,425
kg of steam at 100C through pipe 24, so that altogether
194,900 kg of diluted black liquor at 80C is present in
the vessel 1. The black liquor solution at a temperature
of 80C is pumped by a pump 5 through pipe 6 into preheater
7, into which at the same time 29,026 kg of steam at 5 atmos-
pheres absolute pressure is introduced through pipe 8, from
steam generator 9, which steam passesthe solution from
the preheater 7 to a high-pressure pump 10 at a temperature
of 151C for further transport through pipe 11 to reactor
vessel 12, which is under a pressure of s-team and gas of
149 atmospheres above atmospheric and which consti-
tutes the first combustion stage in which 90% of the combus-
tion heat of the liquor is considered to be set free. Simul-
taneously, 113,000 m3 of air compressed to 150 atmospheres
absolute pressure is supplied to the reactor from compressor
13. Of this quantity, o~ air, 50,000 m3 is fed through pipe
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14 to scrubber 15, within which the air in counter-current
flow meets an aqueous solution at 310C coming from cyclone
16 and supplied through pipe 17 to the top of the scrubber
and recycled from the bottom thereof by means of pump 29
into the reactor vessel 12. In the scrubber the air is
saturated with steam and preheated to about 300C and suppli-
ed via pipe 18 to the bottom section of reactor 19 for final
oxidation. At the same time from the cyclone 16, about 50,000
kg of solution containing soda and Na-salts and having a
temperature of 310C is fed to the top of the reactor 19
through pipe 20. In the final oxidation step 6,400,000 Cals
are produced which generate about 20,000 kg of steam at 310C,
which passes from the top of the reactor 19 together with
50,000 m3 of air containing about 2~5% CO2, and the escaping
gas, since it is saturated with steam of 310C, carries along,
in addition to the 20,000 kg of steam generated in the reactor
19, also about 29,000 kg of steam which the air has taken
up in the scrubber 15, when being preheated by direct contact
with the water having the temperature of 310C. The mi~ture
of steam and gas from the reactor 19 is introduced through
the pipe 11 together with 63,000 m3 of air coming from the
compressor 13 via pipe 21 into the reactor vessel 12, enough
of molecular oxygen thus being supplied to this reactor vessel
12 for combustion of 90% of the organic substance contained
in the black liquor. At the same time, there escape from
the top of the reactor vessel 12 via the cyclone~ 16,170,000 kg of steam at
310C and 156,250 kg of gas under a stearn-gas pressure of 149 atmospheres
above atmospheric, i.e. 0.92 kg of gas per kg of stearn. Theoretically, a
wor3cing pressure of 124 atmospheres above atrnospheric should be sufficient,
but in order to ensure reliability in operation, sorne predetermined over-
pressure rnust exist, and 149 atmospheres above atmospheric should guarantee
that difficulties due to a fall of te}nperature in the reactor will not
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arise as a consequence of escape of a steam-gas mix-ture too
rich in steam. The residual burn-out portion of the black
liquor is derived from the reactor 19 through pipe 22. This
residual portion amounting to 20,000 kg of water and 4,770
kg of soda, of which about 10~15% may consis~ o~ sodium
acetate, are recycled to the pulp cooking equipment with
the causticized liquor. The withdrawn soda solu-tion is
expanded to atmospheric pressure in a cyclone 23, where 13,425
kg of steam escape through pipe 24 to the vessel 1. From
the cyclone the soda solution of 100C leaves through pipe
28 and is diluted at the same time with 13,425 kg of water
at 40C through pipe 25, whereby the soda solution regains
a volume of 30,000 kg of water containing 4,770 kg of soda,
and which while having a temperature of about 25C is fed
to tank 26. From the tank 26, the warm soda solution is
conveyed through plpe 27 to become causticized for further
feed to a digester.
The steam and gas escaping from the reactor 12
via the cyclone 16 and consisting of 170,000 kg of steam
and 156,250 kg of combustion gases are introduced into a
heat exchanger 30 and cooled down under a full pressure of 149 atmospheres
above atmospheric for generation of steam at 34 atmospheres above atmospheric
from feed water of 151&. The steam and gas of 310C are thus cooled down
to 249&, while at the same time 134,355 kq of saturated steam under
a pressure of 34 atmospheres above atmospheric leave~ steam
boiler 31 through pipe 32. Steam, condensate and gas of
249C from the heat exchanger 30 are fed to heat exchanger
33 through the pipe line 34 for generation of steam of 4
atmospheres above atmospheric from water at 151C. The steam
` 30 is obtained from the steam generator 9 through pipe 36 in
a quantity of 41,690 kg, of which 29,026 kg are supplied
to the preheater 7 via the line 8, and in this way the quan-
-- 10 --
11 1~3'~"7~
tity of steam of 4 atmospheres above atmospheric wil] amount
to 12,624 kg.
From residual condensate, steam and gas are
still under the pressure of 149 atmospheres above atmospheric,
warm water at 151C is produced by causing steam condensate
at 20C to exchange heat with condensate and gas from the
heat exchanger 33 and conveyed through pipe 37 to a second
heat exchanger 38 for production of warm feed water. Conden-
sate and gas leaving t~e heat exchanger 38 are collected
in a pressure vessel 40, within which they have a temperature
of 40 C and are under a gas pressureof 149 atmospheres above
atmospheric. 182,137 kg of condensate of 20C are conveyed
from vessel 41 by pump 42 through pipe 39 to the heat exchan-
yer 38, where the condensate is heated to 151C and conveyed
further to a column 45 via pipe 44 and relieved from dissolved
carbon dioxide and other gases before the feed water of
150C is supplied to the steam generator 9 and steam boiler
31 by pump 46 via pipe 49. A surplus of warm water of 151 C
amounting to 6,092 kg is conveyed through pipe 4 to the vessel
1 for dilution of the black liquor. 134~355 kg of steam
subjected to a pressure of 34 atmospheres above atmospheric
is fed from the boiler 31 via pipe 32 to superheater 50,
where the steam is superheated to 420C and conveyed further
to a reaction turbine 51, which de]ivers 15,700 kW at a back
pressure of 11 atmospheres above atmospheric. Back pressure
steam is drawn off from a steam accumulator 52. From the
gas and condensate streaming to the pressure vessel 40 the
latter is conducted through pipe 55 to a water turbine 56
driving an electric generator which delivers 480 kW. The
gas still under pressure is passed through pipe 57 via a
superheater 58 to an expansion machine 59 which drives an
electric generator producing 27,000 kW. The superheaters
~ 11 --
50 and 58 are heated by hot flue gases from the furnace 60,
the quantity of heat consumed thereby corresponding to 2.7
tons of oil per hour.
In the wet combustion of black liyuor according
to the preceding example for production of steam and recovery
of the chemicals, the heat content of the s-team represents
92% of the calorific value of the dry substance conten-t of
the liquor. For comparison may be mentioned that according
to a corresponding manner of calculation for a plant with
soda furnace the result is about 56%~
When considering the additional heat required
for superheating the steam and non-condensable gas producing
a minor surplus of power, the calories in the steam represent
74.3% of the calories in the liquor and the additional fuel,
and then all power needed for operation of pumps, auxiliary
machines and compressors has been produced also. The addi-
tional heat which is supplied, corresponds to 0.095 Swedish
Crowns per kWh, if the price for heavy oil is assumed to
be Swedish Crowns l,000 per ton.
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