Gasifying black liquor with recycling of generated hydrogen sulphide gas to the gasifier

A method of preparing a digesting liquor having high sulphidity from a spent liquor obtained from digesting cellulosic fiber material, optionally without a conventional causticizing step is provided. Thermal decomposition of the spent liquor is conducted under reducing conditions in a reactor at a pressure of from atmospheric pressure up to about 150 bar and at a temperature of about 500.degree. to 1600.degree. C. so that a combustible gas phase containing hydrogen sulphide is formed in the reactor and extracted therefrom, and a phase of solid or molten material of substantially sodium sulphide or potassium sulphide, or mixtures thereof. The solid or molten material is dissolved in an aqueous liquid to produce said digesting liquor. The invention hydrogen sulphide is recovered from the gas phase and returned to the reactor to be present during the thermal decomposition of the spent liquor.

FIELD OF THE INVENTION 
The present invention relates to a method of preparing a digesting liquor 
having high sulphidity from a spent liquor obtained from digesting 
cellulosic fiber material. 
BACKGROUND OF RELATED ARTS 
During combustion of spent liquor from the sulphate pulp industry, for 
instance, two partial goals are aimed at, the first being that the organic 
wood substance released is combusted so that its chemical energy is 
converted to useful thermal and electric energy and the second being that 
the inorganic chemicals used shall be recovered and converted to active 
form. This means, among other things, that the sulphur shall be recovered 
in sulphide form, which requires under-stoichiometric combustion, while 
the energy recovery requires over-stoichiometric conditions. This means 
that two opposing processes must be performed simultaneously in one and 
the same combustion chamber which, with conventional soda recovery unit 
technology, results in optimizing problems. 
The inorganic part recovered from the soda recovery unit, the melt, is 
dissolved to green liquor and consists of a plurality of chemical 
compounds. Most of these compounds consist of sodium carbonate and also 
sodium sulphide, the latter constituting the first one of the recovered 
active digesting chemicals in the white liquor. However, the cycle is not 
a hundred per cent efficient and some of the digesting chemicals are 
obtained in inactive form, thereby constituting ballast in the white 
liquor. 
Preparation of the white liquor is the last step in the recovery of 
chemicals. Here the sodium carbonate in the green liquor is converted to 
sodium hydroxide by the reaction with calcium oxide (CaO), i.e. caustic 
lime. The second one of the two active digesting chemicals in the white 
liquor has thus been recovered. 
The process is generally termed causticizing and an insoluble sludge 
consisting of calcium carbonate lime sludge is obtained as a by-product. 
Calcium oxide is recovered from the lime sludge by combusting it in a 
long, rotating furnace. The chemical cycle requires considerable energy 
and is not particularly harmless to the environment. The chemical 
composition of a normal melt from a soda recovery unit is typically as 
follows: 
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Compound % of melt (approx.) 
______________________________________ 
Sodium carbonate Na.sub.2 Co.sub.3 
70 
Sodium sulphide Na.sub.2 S 
27 
Sodium sulphate Na.sub.2 SO.sub.4 
3 
Sodium sulphite Na.sub.2 SO.sub.3 
Sodium thiosulphate Na.sub.2 S.sub.2 O.sub.3 
small quantities 
Sodium chloride NaCl 
______________________________________ 
After causticizing, the chemical composition of a normal white liquor is 
typically as follows: 
______________________________________ 
Compound Content in g/l (as NaOH) 
______________________________________ 
NaOH 100 
Na.sub.2 S 40 
Na.sub.2 CO.sub.3 
20 
Na.sub.2 SO.sub.4 
2.8 
______________________________________ 
The white liquor recovered through a conventional soda recovery unit 
process has a sulphidity of 25-30%. 
##EQU1## 
Swedish patent SE-C-8502731-6 describes an alternative process to the soda 
recovery unit technique, which is based on a gasification technique in 
which the organic substances in the black liquor are gasified in a first 
process in a pressurized reactor by means of "flash-pyrolysis" to provide 
CO, CO2, H2 and H2S, the residue obtained consisting primarily of the 
inorganic components of the spent liquor in solid or molten form having a 
composition corresponding to the melt from the soda recovery unit, i.e. 
mainly consisting of sodium carbonate and a small proportion of sodium 
sulphide. As with the soda recovery unit technique, this melt is dissolved 
and a green liquor is obtained which is treated in the same known manner 
as when the soda recovery unit technique is used. 
The chemical composition of spent liquor from the sulphate pulp industry is 
typically as follows: 
______________________________________ 
Sodium + Potassium 
8.7 mol/kg DS black liquor 
Sulphur 1.8 mol/kg DS black liquor 
Chlorine 0.05 mol/kg DS black liquor 
Carbon 29.6 mol/kg DS black liquor 
Hydrogen 39.2 mol/kg DS black liquor 
Oxygen 21.0 mol/kg DS black liquor 
______________________________________ 
The mole ratio Na:S is 4.88:1. 
Under certain operating conditions, such as at a pressure of 1.5 bar abs. 
and a temperature of 950.degree. C. in the gasification reactor, a melt is 
obtained consisting of 59 per cent by weight of Na.sub.2 CO.sub.3 and 31 
per cent by weight of Na.sub.2 S. 
Through Swedish patent SE-C-465 039 it is known to continuously supply 
sulphur compounds to the black liquor in order to alter the composition of 
the melt. By altering the mole ratio Na:S to 1.5:1 by the addition of 4 
mol sulphur/kg DS thick liquor a melt having high sulphidity consisting 
substantially of 14 per cent by weight of Na.sub.2 CO.sub.3 and 80 per 
cent by weight of Na.sub.2 S is obtained under otherwise identical 
operating conditions. 
Due to its large content of sodium sulphide the melt thus consists for the 
most part of active digesting chemicals, i.e. white liquor. 
However, the difficulty is to maintain the low mole ratio Na:S, and if 
possible to reduce it even further, in order to obtain a white liquor of 
nearly 100% sulphidity. The amount of available sulphur compounds in a 
sulphate factory, for instance, is limited and only a small portion of the 
total amount of black liquor can therefore be treated in a gasification 
process of this type, unless sulphur is supplied externally. In practice 
this is impossible to maintain continuously for sulphur balance reasons. 
It is therefore difficult to maintain a low mole ratio under certain 
operating conditions--as is accentuated by the following operating 
example. 
Increased pressure displaces the reaction towards the formation of 
carbonate in the following equilibrium reaction: 
EQU Na.sub.2 CO.sub.3 +H.sub.2 S.revreaction.Na.sub.2 S+CO.sub.2 +H.sub.2 O. 
When the gas produced is supplied to a gas turbine a higher pressure is 
desirable in the gasification reactor. 
In this operating example, therefore, the melt obtained contains an even 
larger proportion of sodium carbonate and an even larger addition of 
sulphur compounds is necessary. If, for instance, the previously specified 
chemical composition of the black liquor and the amount of sulphur added 
remain the same but the pressure is increased from 1.5 bar abs. to 25 bar 
abs., i.e. the mole ratio Na:S of 1.5:1 is maintained, a melt is obtained 
consisting substantially of 52 per cent by weight of Na.sub.2 CO.sub.3 and 
39 per cent by weight of Na.sub.2 S, i.e. a melt in which the carbonate 
content has increased from 14% to 52% by weight. This means that 
causticizing is still required and also that the sulphidity in the melt 
decreases, which can be a disadvantage. 
It is clear from the above that if increased pressures in the gasification 
reactor are used, mole ratios are required which are lower the higher the 
pressure is, and increasing amounts of sulphur compounds must be added in 
order to displace the above reaction to the right. It will be recognized 
that the quantities of sulphur required would be unmanageably large, in 
particular in the upper pressure levels. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of preparing 
digesting liquor having high sulphidity from spent liquor which enables an 
essential part of, or even all the spent liquor produced in a digesting 
process to be dealt with so that conventional recovery technique such as 
causticizing and lime sludge burning are reduced and eliminated, 
respectively. 
The method according to the invention is characterized in that hydrogen 
sulphide is recovered from the gas phase extracted and is returned to the 
reactor to be present during the thermal decomposition of the spent liquor 
.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
An embodiment relates to a method of preparing a digesting liquor having 
high sulphidity from a spent liquor obtained from digesting cellulosic 
fiber material, the method comprising the steps of thermal decomposition 
of the spent liquor in at least one reactor at a pressure of from 
atmospheric pressure up to about 150 bar and at a temperature of about 
500.degree. to 1600.degree. C. The thermal decomposition is carried out 
under reducing conditions without addition of oxygen-containing gas or by 
the controlled addition of an oxygen-containing gas in a quantity of from 
nearly 0 up to 80% of the quantity of oxygen required stoichiometrically 
for complete oxidation of substances produced during the thermal 
decomposition. A combustible gas phase is formed in the reactor and 
extracted therefrom. A phase of solid or molten material comprising 
substantially sodium sulphide, potassium sulphide or a mixture thereon is 
also formed in the reactor. The solid or molten phase is dissolved in 
aqueous liquid to produce the digesting liquor. The gas phase contains 
hydrogen sulphide which is recovered and returned to the at least one 
reactor. 
Preferably, the quantity of hydrogen sulphide returned is adjusted so that 
the mole ratio between sodium and/or potassium supplied by the spent 
liquor and sulphur supplied partly by the spent liquor and partly by the 
return of hydrogen sulphide amounts to 4:1 and below, preferably 2.8:1 and 
below, most preferably 1.5:1 and below. Recovery and return of hydrogen 
sulphide preferably take place continuously. 
The invention enables gasification of black liquor under the required low 
mole ratio of (Na and/or K):S in order to produce a melt in the 
gasification step consisting substantially of sodium sulphide (Na.sub.2 S) 
and/or potassium sulphide (K.sub.2 S) which, after being dissolved in 
liquid (in a quenching step, for instance, or in a liquid film cyclone), 
immediately produces a white liquor having very high sulphidity, 
preferably 100% sulphidity, for example, if sodium sulphide is present: 
Na.sub.2 S+H.sub.2 O.fwdarw.NaHS+NaOH. A similar reaction would occur for 
potassium. 
The invention enables the complicated and expensive causticizing step, also 
including the lime kiln, to be eliminated from the recovery cycle. White 
liquor having high sulphidity also enables alteration of the digesting 
process. High sulphidity is particularly favorable in the initial stages 
of pulp digestion. 
Recirculation of hydrogen sulphide establishes such a high partial pressure 
of hydrogen sulphide in the gasification step that the equilibrium 
reaction Na.sub.2 CO.sub.3 +H.sub.2 S.revreaction.Na.sub.2 S+CO.sub.2 
+H.sub.2 O is displaced so far to the right that the formation of Na.sub.2 
CO.sub.3 is suppressed. A similar displacement of the equilibrium reaction 
for potassium carbonate and potassium sulfide would occur. 
According to thermo-chemical equilibrium calculations performed, this is 
achieved at a gasification pressure of 1.5 bar abs., for instance, and 
when the mole ratio of (Na and/or K):S in the black liquor supplied to the 
gasification reactor, including the sulphur in the recirculated hydrogen 
sulphide gas, is 1.0 or lower. At the higher gasification of 25 bar abs. 
mentioned earlier, the above mole ratio must be reduced to 0.5 or lower. 
In the two operating cases exemplified the actual partial pressure of H2S 
can be seen in the following table: 
______________________________________ 
Operating pressure 
Mole ratio H.sub.2 S partial 
reactor bar abs. 
(Na and/or K):S 
pressure bar abs. 
______________________________________ 
1.5 .ltoreq.1.0 0.07 
25.0 .ltoreq.0.5 2.0 
______________________________________ 
The partial pressure 0.7 bar abs. (0.0492.times.1.5=0.0738) in the first 
case corresponds to 4.7% of the total pressure 1.5 bar abs. and the 
partial pressure 2.0 bar abs. in the second case corresponds to 8% of the 
total pressure 25.0 bar abs. This means, therefore, that the concentration 
of hydrogen sulphide must be increased from 4.7% to 8% when the total 
pressure in the reactor is increased from 1.5 bar abs. to 25.0 bar abs. 
To maintain the higher partial pressure of H.sub.2 S required in order to 
obtain a white liquor having high sulphidity, preferably 100% sulphidity, 
in a quenching step, for instance, according to the invention the H.sub.2 
S content in the process gas is returned to the gasification reactor. 
This return is achieved by allowing the gas to pass through a gas washing 
apparatus for selective and regenerative absorption of the H.sub.2 S 
content. Examples of such absorption processes are the Purisol process 
which utilizes N-methylpyrrolidone as an absorption liquid, and the Dow 
Gas/Spec-process which utilizes methyldiethylamine (MDEA) as an absorption 
agent. Using these processes, for example, more than 99% of the H.sub.2 S 
content of the gas can be washed out, driven off from the absorption 
liquid via a regeneration step, and returned to the gasification step. 
Some form of sulphur or sulphur compound is added to the gasification 
reactor when it is started up in order to build up the required high 
H.sub.2 S level in the system/cycle, and thus the necessary (Na and/or 
K):S mole ratio at the actual reactor pressure. Such external additions of 
sulphur may also be carried out during the process in order to compensate 
any sulphur losses in the circulation system or when the partial pressure 
for H.sub.2 S is to be increased due to increased pressure in the reactor. 
Such supplementary sources of sulphur may consist of a sulphurous 
secondary fuel, sulphurous gas from the digester house or evaporation 
plant, which can be supplied via the secondary air for the reactor or via 
the above-mentioned hydrogen sulphide regeneration step, elementary 
sulphur, sodium sulphate and/or potassium sulphate or sulphurous residue 
acid from the production of chlorine dioxide. However, if the entire flow 
of black liquor from the factory is gasified--i.e. no soda recovery unit 
is used--and without chlorine dioxide bleaching, said sodium sulphate 
and/or potassium sulphate, and residue acid are not available. 
The regenerated H.sub.2 S gas is thus returned to the gasification reactor 
to maintain the required H.sub.2 S partial pressure. According to a 
suitable embodiment the H.sub.2 S gas may be utilized entirely or 
partially as atomization gas for the black liquor addition in the burner 
nozzle. 
The H.sub.2 S gas recovered from the process gas from a reactor can be 
distributed so that one part is returned to this first reactor and the 
remainder to a second reactor which is supplied with the same black 
liquor. Thus, according to this embodiment of the invention, two white 
liquors can be produced one having high sulphidity and the other somewhat 
lower sulphidity. 
It is preferred that all the H.sub.2 S gas recovered is recirculated to the 
reactor. If for some reason the entire quantity of the H2S gas recovered 
is not to be recirculated, the surplus is subjected to a suitable process 
such as the one generally known as the Claus process, for the recovery of 
elementary sulphur. 
A measuring device may be disposed in the pipe for the process gas leading 
from the reactor, to measure the content of H.sub.2 S or S in the gas. 
This measuring device is connected to control means that, via a valve, 
controls the supply of supplementary sulphurous material when the process 
is started up and during continuous operation if a sulphur loss occurs in 
the circulation or, alternatively, if not all the H.sub.2 S recovered is 
to be circulated and instead elementary sulphur is to be recovered for 
some reason either temporarily or continuously. 
The invention is applicable to spent liquors from both sulphate and 
sulphite pulp industry processes. 
The thermal decomposition occurs under reducing conditions without the 
supply of oxygen-containing gas or with controlled supply of an 
oxygen-containing gas in a quantity corresponding to from nearly 0 up to 
80%, preferably up to 60%, of the quantity of oxygen required 
stoichiometrically for complete oxidation of substances produced during 
the thermal decomposition. 
The invention is further illustrated by means of the following examples. 
Black liquor having an Na:S mole ratio of partly 4.88:1 (normal sulphidity) 
and partly 2.0:1 (100% sulphidity) was fed continuously into a reactor in 
which the temperature was maintained at 950.degree. C. and the pressure at 
1.5 bar abs. The dry solids content of the black liquor was 65%. Air 
pre-heated to a temperature of 500.degree. C. was supplied in a controlled 
manner so that the quantity of oxygen reached 45% of quantity of oxygen 
required stoichiometrically for complete oxidation. The results from 
various tests are shown in the following Table 1. 
Test 1 relates to conventional run, that is with normal sulphidity and 
without recirculation of H.sub.2 S. An addition of sulphur was made in the 
subsequent tests (2-6) to provide the reactor with the amount of 
supplementary sulphur necessary so that the stated Na:S mole ratio was 
set. Furthermore, in tests 3-6 H.sub.2 S gas was recirculated to 
participate in this increase in added sulphur and subsequently to be 
responsible for maintaining the stated mole ratio. Increased recirculation 
of H.sub.2 S gas resulted in a corresponding increase in its partial 
pressure so that the amount of sodium carbonate formed was reduced and the 
amount of sodium sulphide correspondingly increased. The gas extracted 
continuously from the reactor was caused to pass a gas washing apparatus 
for the H.sub.2 S content in the gas to be absorbed. 
TABLE 1 
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Mole Re- Mole Con- 
ratio circ. ratio tent 
Na:S H.sub.2 S 
Na:S H.sub.2 S 
in DS kmol/ in in dry 
Melt composition, 
Test Dry ton reac- gas weight-% 
No. Solids DS tor vol-% Na.sub.2 CO.sub.3 
NaOH Na.sub.2 S 
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1 4.88:1 0.0 4.88:1 
0.15 59.1 8.7 31.5 
2 2.0:1 0.0 2.0:1 1.01 23.8 5.9 69.3 
3 2.0:1 0.5 1.8:1 1.32 19.6 5.5 74.0 
4 2.0:1 1.35 1.5:1 2.18 14.2 4.7 80.1 
5 2.0:1 2.70 1.2:1 3.55 4.8 3.1 91.2 
6 2.0:1 4.05 1.0:1 4.92 2.5 1.5 95.5 
______________________________________ 
The results show that a forced increase in the H.sub.2 S partial pressure 
obtained through recirculation of H.sub.2 S gas provides a controlled 
decreased Na:S mole ratio in the reactor and accompanying conversion of 
the black liquor to white liquor having high sulphidity, without 
causticizing and lime sludge burning. If potassium is present, the (Na 
and/or K):S mole ratio will be similarly decreased. 
While the invention has been described in detail and with reference to the 
specific embodiments thereof, it will be apparent to one of ordinary skill 
in the art that various changes and modifications can be made therein 
without departing from the spirit and scope thereof.