Low temperature recovery of kraft black liquor

A kraft black liquor recovery system utilizing three separate reactors for liquor pyrolysis, sulfate reduction and carbon plus organics combustion, respectively. Oxidized black liquor is pyrolyzed in a fluid bed reactor. The temperature in the fluid bed reactor is 600.degree. C. or lower. The resulting char, containing Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4 and a significant amount of carbon, is separated from the pyrolysis gases and introduced in an indirect heated reactor where reduction of Na.sub.2 SO.sub.4 to Na.sub.2 S takes place in the solid state under an atmosphere generated by the reduction. The reduced char is cooled and leached to produce green liquor. The leached char and gases from the pyrolysis and reduction reactors are burned in a fluid bed combustion unit operating below the melting point of mixtures of Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4. The fluid bed particles, consisting mainly of Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4, serve to remove the volatile oxidized sulfur species formed by combustion of the pyrolysis gas. The overflow of pellets are ground and dissolved in the incoming heavy black liquor feed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a pulp mill recovery system. More 
specifically, the present invention relates to a low temperature kraft 
spent liquor recovery system utilizing separate reactors for pyrolysis, 
combustion and sulfate reduction. 
2. Description of the Prior Art 
The central piece of equipment for recovery of cooking chemicals and energy 
from kraft black liquor is the so-called Tomlinson furnace. Black liquor 
at about 65% dry solids content is sprayed into the furnace. During their 
descent, the black liquor droplets lose the remaining water by evaporation 
and the solids pyrolyze to form a char bed at the bottom of the furnace. 
The char bed burns under reducing conditions at a temperature of about 
750.degree.-1050.degree. C. and the recovered chemicals, mainly Na.sub.2 
CO.sub.3 and Na.sub.2 S, are drained from the furnace as a smelt. The 
smelt is dissolved in water to produce so-called green liquor, the 
precursor of the cooking liquor called white liquor. The gases generated 
during pyrolysis and burning of the char are fully combusted at a higher 
location in the furnace. The furnace is provided with suitable heat 
exchange means to recover heat from the hot combustion gases for steam and 
electricity generation. 
Although the objective of the recovery of chemicals and energy is 
adequately achieved in present commercial operations, the use of the 
Tomlinson furnace presents a number of problems. For example, inadvertant 
contact between water and the inorganic smelt has resulted in serious 
explosions. Also, high char bed temperatures lead to increasing emission 
of sodium salts and excessive fouling of the steam pipes in the upper part 
of the furnace. 
To solve these problems, and also to reduce capital investment and increase 
the energy efficiency of the recovery operation, a number of kraft 
recovery alternatives have been described. In some of these alternatives 
the smelt-water explosion hazard is eliminated and the emission of sodium 
salts reduced by keeping the inorganic chemicals in solid rather than 
molten form. This principle was used by Copeland et al., U.S. Pat. No. 
3,309,262, where spent liquor is concentrated and introduced by 
atomization into a fluidized bed reactor. The resulting waste liquor spray 
encounters residual inorganic chemicals derived from the combustion of 
previous spent liquors. Additionally, the fluidized bed reactor may 
contain inert materials such as silica grains in admixture with the 
inorganic chemicals. In the fluidized bed reactor, operated with excess 
air, all the organic material is combusted below the fusion point of the 
inorganic salt mixture. The sodium sulfate in the inorganic pellets are 
reduced with hydrogen in a second fluidized bed (Arnold, Can. Pat. 
828,654). Alternatively, the first fluid bed can be used as a means to 
provide incremental recovery capacity, while the reduction of sodium 
sulfate is achieved by injecting the pellets into the conventional 
recovery furnace (Tomlinson II, U.S. Pat. No. 4,011,129). 
Flood, U.S. Pat. No. 3,322,492, describes a two-stage fluid bed process 
where weak black liquor at about 20% solids content is dried to solid 
granules in the first bed at a temperature of about 175.degree. C. The 
sodium sulfate in the granules is reduced to sodium sulfide by virtue of 
carbon monoxide derived from decomposition of the organic matter in the 
second bed. The operating temperature of the second fluid bed is about 
800.degree. C. 
Osterman, U.S. Pat. No. 3,523,864, presents a three-zone fluid bed reactor 
which would replace the conventional chemical recovery furnace and lime 
kiln. Black liquor is dried and burned under reducing conditions at about 
650.degree.-700.degree. C. in the intermediate zone. The reducing gas from 
the intermediate zone is burned and serves as fluidizing medium for the 
top fluidized bed. Here predried CaCO.sub.3 is introduced to be calcined 
to CaO pellets. These CaO pellets overflow first to the intermediate zone 
and then subsequently to the lower bed with a coating of mainly char, 
Na.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 from the burned black liquor. The 
reduction of Na.sub.2 SO.sub.4 is said to take place in the lower 
fluidized bed at about 700.degree.-760.degree. C. with air and/or 
combustion gases as a fluidizing medium. 
In the process of Shah, U.S. Pat. No. 3,574,051, kraft black liquor is 
concentrated by contact with a stream of heated air. The resulting 
concentrated black liquor is then burned with excess air in a fluidized 
bed reactor while the bed temperature is maintained at about 
250.degree.-600.degree. C. The solid salts are then passed through another 
reactor and subjected to a reducing gas stream containing mainly carbon 
monoxide. It is claimed that in the range of 250.degree.-500.degree. C. 
the sodium sulfate is reduced to sodium sulfide. Green liquor is produced 
by dissolution of the salts in water. 
Lange, Can. patent 1,089,162, presents a low temperature process where the 
organic portion of black liquor is gasified in a fluidized bed, operating 
not in excess of 760.degree. C. so as to keep the inorganic portion of 
black liquor in the solid state. The solid particles leaving the bed will 
typically contain 90% Na.sub.2 CO.sub.3, 9% Na.sub.2 S, less than 1% 
Na.sub.2 SO.sub.4, and less than 1% carbon. After dissolving the solids in 
water, and separation of the carbon, the liquor will be used to remove 
H.sub.2 S from the gas produced in the fluidized bed reactor. The spent 
absorbing medium can then be treated to form the cooking liquor which is 
returned to the digestion process. 
In all the above alternatives to the conventional kraft recovery process 
(except for the process of Tomlinson II, U.S. Pat. No. 4,011,129), 
Na.sub.2 S and Na.sub.2 CO.sub.3 are produced from black liquor in 
reactors operating below the fusion point of the inorganic salt mixture. 
As far as is known, only the Copeland process is used on a commercial 
scale. However, in this process the end products are pellets consisting of 
mainly Na.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 rather than mainly Na.sub.2 
S and Na.sub.2 CO.sub.3. There are two main reasons for the absence of 
commercial utilization of these low temperature processes. First, the 
relatively high temperature required for fast and complete conversion of 
Na.sub.2 SO.sub.4 to Na.sub.2 S and, secondly, the ease of formation of 
H.sub.2 S when Na.sub.2 S is contacted with combustion gases below the 
melting point of the inorganic salts. So, while the reduction is favored 
by a high temperature, the above alternative processes require a 
relatively low temperature just below the melting point of the inorganic 
salt mixture. The consequence is that in fluid bed processes operating in 
the reducing mode, most of the formed Na.sub.2 S is rapidly converted to 
H.sub.2 S (and some COS) according to the overall reaction 
EQU Na.sub.2 S+CO.sub.2 +H.sub.2 O.fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 S 
resulting in a low yield of solid Na.sub.2 S. 
It is an object of this invention to provide a kraft recovery process 
whereby Na.sub.2 CO.sub.3 and Na.sub.2 S are formed below the melting 
point of the inorganic pulping chemicals with a minimum production of 
sulfurous gases. 
It is a further object of this invention to provide an assembly for 
carrying out the process, more especially an assembly of reactors. 
SUMMARY OF THE INVENTION 
The process of the invention provides for the recovery of energy and kraft 
pulping chemicals in a system of multiple reactors, all operating below 
the melting point of the mixture of inorganic pulping chemicals. 
In accordance with one aspect of the invention there is provided a process 
for the treatment of kraft black liquor which comprises i) pyrolyzing 
black liquor which contains inorganic salts, including an oxysulphur 
component and a carbonate component, at a temperature of not more than 
600.degree. C. to produce a char; ii) subjecting the char to reducing 
conditions effective to reduce the oxysulphur component to a sulphide salt 
component inside the char; the reduction is carried out at a temperature 
above 600.degree. C. and below the melting temperature of the salts in the 
char in an atmosphere generated by the reduction itself; iii) cooling the 
resulting char; iv) leaching the cooled resulting char from iii) with an 
aqueous leaching liquid to leach inorganic salts from the char; and v) 
recovering the aqueous liquid bearing the salts from iv) as a green 
liquor. 
In a particular embodiment of the process volatile components from the 
pyrolysis and reduction stages, for example pyrolysis gases, are combusted 
in a fluid bed reactor and the heat energy of combustion is recovered. The 
leached char may also be passed to the fluid bed reactor. 
In another aspect of the invention there is provided an apparatus for the 
treatment of kraft black liquor which comprises a pyrolyzer, a reduction 
reactor, a char leacher and a fluid bed combustor for carrying out the 
several stages of the process of the invention. Flow lines are provided 
between the several parts of the apparatus, in particular a first line 
between the pyrolyzer and the reduction reactor, a second line between the 
reduction reactor and the char leacher, a third line for green liquor from 
the char leacher, a fourth line from the pyrolyzer to the fluid bed 
combustor, and a fifth line from the reduction reactor to the fluid bed 
combustor. 
The inorganic salts are in particular sodium salts, especially sodium 
carbonate and sodium salts of oxysulphur acids, for example sodium 
sulphate, sulphite and thiosulphate. 
Thus in a particular embodiment the present invention employs a fluidized 
bed pyrolyzer where black liquor at 30-100% dry solids, but preferably 
60-100% dry solids, is pyrolized with hot combustion gases and some air. 
It is preferred that the black liquor is previously oxidized. Air is 
premixed with the combustion gases and used for temperature control. The 
temperature of the solids in the reactor is 600.degree. C. or lower. This 
minimizes the formation of Na.sub.2 S and subsequent formation of 
sulfurous gases from the decomposition of Na.sub.2 S. The resulting char, 
containing Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4 but mostly free of 
Na.sub.2 S, is separated from the pyrolysis gases and introduced in a 
reactor where reduction of Na.sub.2 SO.sub.4 to Na.sub.2 S takes place 
under an atmosphere generated by the reduction itself. The low partial 
pressures of H.sub.2 O and CO.sub.2, the presence of carbon, and a 
temperature above 600.degree. C. but preferably slightly below the onset 
of smelt formation, favor conversion of Na.sub.2 SO.sub.4 to Na.sub.2 S 
with minimum production of H.sub.2 S or other sulfur containing gases. The 
char leaving this reduction reactor is cooled and contacted with water to 
produce green liquor and leached char. The leached char and gases from the 
pyrolysis and reduction reactors are burned in a fluid bed combustion unit 
operating below the melting point of the mixture of Na.sub.2 CO.sub.3 and 
Na.sub.2 SO.sub.4. The fluid bed pellets, consisting mainly of Na.sub.2 
CO.sub.3 and Na.sub.2 SO.sub.4, serve to remove the gaseous oxidized 
sulfur species formed by combustion of the sulfurous components produced 
in the reduction and pyrolysis reactor. The overflow of pellets is ground 
and mixed with the black liquor feed. Alternatively, the leached char 
could be combusted in a typical coal fired furnace. In this case, flue gas 
cleaning equipment must be added to minimize sulfur emission.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a schematic illustration of one form of the present invention. As 
shown in FIG. 1, the present invention includes as main pieces of 
equipment the fluid bed pyrolyzer 5, the indirect heated reducer 10, the 
char leacher 14, and the fluid bed combustor 25. Strongly oxidized black 
liquor is fed via line 1 to the fluid bed pyrolyzer and sprayed onto the 
fluid bed particles. The fluid bed particles are either black liquor char 
pellets or inert particles like sand or Al.sub.2 O.sub.3 coated with black 
liquor char. The black liquor may contain 30-100% solids and, in the case 
of high dry solids content, the black liquor solids are injected under the 
surface of the fluidized bed with a carrier gas. The carrier gas can be 
air and/or cooled combustion gas. Air in line 2, mixed with combustion gas 
in line 3 from the fluid bed combustor 25 is used as a fluidizing medium 
in the fluid bed pyrolyzer 5. The temperature in pyrolyzer 5 is controlled 
by air flow rate in line 2 and the temperature of the combustion gases in 
line 3. Additionally, the pyrolyzer can be indirectly cooled or heated to 
obtain the required fluid bed temperature. The temperature of the fluid 
bed pyrolyzer is kept below about 600.degree. C. to minimize formation of 
Na.sub.2 S and subsequent formation of sulfurous gases from the 
decomposition of Na.sub.2 S. The flue gases leaving the pyrolyzer 5 via 
line 4 also contain high boiling point organic compounds and elutriated 
black liquor char particles. The particles are separated from the gas in 
cyclone 6 operating at essentially the same temperature as the fluid bed 
pyrolyzer 5. The char is transported by gravity or mechanical means via 
line 7 to reduction reactor 10. Alternatively, the char pellets may be 
removed directly from the fluid bed and transported to the reduction 
reactor. Reactor 10 is indirectly heated by the flue gases in line 26 from 
the fluid bed combustor 25 or heated by other means. The temperature in 
the reduction reactor is about 750.degree. C., i.e. slightly below the 
value where the onset of smelt formation occurs. A relative motion between 
the char and internal surface of reactor 10 is maintained by either 
internal mechanical agitation or rotation/oscillation of the reactor 10 
itself. The gases produced in reactor 10 are vented via line 9 to the 
fluid bed combustor 25. The admission of gases which contain CO.sub.2 or 
H.sub.2 O to reactor 10 should be minimized to reduce the formation of 
sulfurous gases from Na.sub.2 S. The addition of CO to reactor 10 on the 
other hand is favorable for suppression of sodium emission from reactor 
10. Thus the gas in reactor 10 is, preferably, high in CO content and low 
in H.sub.2 O and CO.sub.2 content. The char leaving the reduction reactor 
10 contains mainly Na.sub.2 CO.sub.3 and Na.sub.2 S as the inorganic 
salts. The char is fed via line 11 to a steam producing heat exchanger 31, 
and subsequently to the char leacher 14 via line 12. Water is added via 
line 15 to remove, to a large extent, the inorganic salts from the char. 
The extracted char is separated from the resulting green liquor and enters 
a filter press 19 via line 17. In the filter press additional green liquor 
is removed from the char and combined with main green liquor streams in 
line 16. The leached and dewatered char is transported via line 39 to the 
fluid bed combustor 25. The particles in the fluid bed combustor consist 
mainly of Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4 originating from 
Na.sub.2 CO.sub.3 and Na.sub.2 S remaining in the char after the filter 
press 19. Air enters reactor 25 and is mixed with the gas streams 8 and 9. 
The energy, generated by combustion of carbon, volatile organics, CO and 
H.sub.2 in the fluid bed reactor 25 is used to generate steam leaving via 
line 20. The combustion products of sulfurous gases combine with Na.sub.2 
CO.sub.3 to form Na.sub.2 SO.sub.4. The overflow of particles from the 
fluid bed combustor 25 are ground and mixed with heavy black liquor to be 
reintroduced in the present process. Part of the combustion gases from 
reactor 25 are recycled to reactor 5 and a part is vented to atmosphere 
after particulate removal in cyclone 32 and heat exchange in reactor 10 
and heat exchanger 30. Alternatively, the leached and dewatered char in 
line 39 could be combusted in a typical coal fired furnace. In this case, 
flue gas cleaning might be added to minimize the emission of sulfur and 
sodium containing species. Finally, in order to increase the throughput 
through the reactors 5, 10 and 25, the gas pressure in the reactors can be 
increased to levels considerably above atmosperic. 
EXAMPLE 1 
Black liquor was obtained by cooking black spruce chips at 170.degree. C. 
with white liquor at a liquor-to-wood ratio of 4 L/kg o.d. chips. The 
heat-up time from 80.degree. to 170.degree. C. was 90 minutes and the time 
at 170.degree. C. was 45 minutes. The white liquor had a sulfidity of 
29.82% and an effective alkali concentration of 30.07 g/L. After 
completion of the cook, the cooking liquor was blown from the digester and 
separated from the chips. The kappa number of the chips was 104. The black 
liquor was subsequently strongly oxidized in a continuously stirred batch 
pressurized reactor operating at 130.degree. C., by bubbling air through 
the liquor for 180 minutes. Some of the liquor was then transferred to an 
Al.sub.2 O.sub.3 dish and dried under I.R. lamps for 7 hours. The dried 
black liquor solids were put in an Al.sub.2 O.sub.3 boat which was 
subsequently inserted in the quartz tube of a tube furnace preheated to 
600.degree. C. The volatiles produced during pyrolysis of black liquor 
solids were removed by a flow of 0.55 L/min (at room temperature) of 90% 
helium and 10% CO. The boat was removed from the furnace after 30 minutes 
at 600.degree. C. Samples were taken for analysis and the boat was 
reintroduced in the tube furnace which was now increased in temperature to 
750.degree. C. The flow of 90% helium and 10% CO was maintained at 0.55 
L/min. After 45 or 60 minutes at 750.degree. C., the boat was again 
removed from the furnace and the black liquor char was analyzed for total 
sulfur, sulfide, oxy-sulfur and carbonate ion content. The analysis of the 
black liquor solids, the 600.degree. C. pyrolyzed char and the char 
treated at 750.degree. C. are shown for the two samples in Tables 1 and 2 
respectively. The difference between the treatment conditions of the 
samples is the reduction time at 750.degree. C. Also included are the 
yield and the sulfur loss for each treatment as well as the reduction 
efficiency after treatment at 600.degree. C. and 750.degree. C. The 
reduction efficiency is defined as 
##EQU1## 
The different ion contents were determined by ion chromatography of the 
solution obtained by leaching the solids or char. The total sulfur content 
was determined by the Schdniger combustion method and subsequent ion 
chromatographic analysis of the produced SO.sup.-2.sub.4. The percentages 
of total sulfur and all the anions are based on the original weight of the 
black liquor solids. 
The results in Tables 1 and 2 show that the reduction efficiencies after 
pyrolysis at 600.degree. C. are low, 8.6 and 8.3% for samples 1 and 2 
respectively. However after treatment at 750.degree. C. the reduction 
efficiencies increase to 87 and 83.8% respectively. It should be noted 
that the sulfur in the form of S.sup.2- and SO.sup.2-.sub.4 after 
pyrolysis at 600.degree. C. accounts for 90.7 and 98.5% of the total 
sulfur in samples 1 and 2 respectively. Also after further treatment at 
750.degree. C., the amount of sulfur as S.sup.2- and SO.sup.2-.sub.4 is 
relatively unchanged at 88.9 and 97.6% respectively of the total sulfur. 
Finally the total sulfur loss during pyrolysis and reduction are 24.3 and 
6.8% for samples 1 and 2 respectively. 
TABLE 1 
______________________________________ 
Pyrolysis and reduction of oxidized black liquor solids. (Sample 1) 
Black liquor 
Black Black liquor 
char treated 
liquor solids pyrolyzed 
at 750.degree. C. 
solids at 600.degree. C.* 
for 60 minutes* 
______________________________________ 
Initial weight (g) 
-- 0.1817 0.2004 
Total S (%) 
2.80 2.12 2.13 
SO.sub.4.sup.2- (%) 
4.96 4.93 0.43 
SO.sub.3.sup.2- (%) 
0.37 &lt;0.1 &lt;0.1 
S.sub.2 O.sub.3.sup.2- (%) 
&lt;0.05 &lt;0.05 &lt;0.05 
S.sup.2- (%) 
&lt;0.1 0.28 1.75 
CO.sub.3.sup.2- (%) 
15.4 23.3 21.0 
yield (%) -- 74.1 89.6 
Sulfur loss (%) 
-- 24.3 0.0 
Reduction &lt;3.2 8.6 87.0 
efficiency (%) 
______________________________________ 
*Total sulfur and anion percentages are based on the weight of the 
original black liquor solids. 
TABLE 2 
______________________________________ 
Pyrolysis and reduction of oxidized black liquor solids. (Sample 2) 
Black liquor 
Black Black liquor 
char treated 
liquor solids pyrolyzed 
at 750.degree. C. 
solids at 600.degree. C.* 
for 45 minutes* 
______________________________________ 
Initial weight (g) 
-- 0.2241 0.1261 
Total S (%) 
2.76 2.02 1.96 
SO.sub.4.sup.2- (%) 
5.30 5.13 0.55 
SO.sub.3.sup.2- (%) 
0.1 0.1 &lt;0.1 
S.sub.2 O.sub.3.sup.2- (%) 
&lt;0.05 &lt;0.05 &lt;0.05 
S.sup.2- (%) 
&lt;0.1 0.28 1.73 
yield (%) -- 74.9 87.0 
Sulfur loss (%) 
-- 26.8 3.0 
Reduction &lt;3.0 8.3 83.8 
efficiency (%) 
______________________________________ 
*Total sulfur and anion percentages are based on the weight of the 
original black liquor solids. 
TABLE 3 
______________________________________ 
Pyrolysis and reduction of non-oxidized black liquor solids. 
Black liquor 
Black Black liquor 
char treated 
liquor solids pyrolyzed 
at 750.degree. C. 
solids at 600.degree. C.* 
for 60 minutes* 
______________________________________ 
Initial weight (g) 
-- 0.2971 0.1356 
Total S (%) 
2.37 1.30 1.16 
SO.sub.4.sup.2- (%) 
0.27 0.47 0.56 
SO.sub.3.sup.2- (%) 
2.78 &lt;0.1 &lt;0.1 
S.sub.2 O.sub.3.sup.2- (%) 
&lt;0.1 &lt;0.16 &lt;0.1 
S.sup.2- (%) 
&lt;0.1 0.40 0.46 
CO.sub.3.sup.2- (%) 
12.8 -- 8.6 
yield (%) -- 74.6 91.3 
Sulfur loss (%) 
-- 45.0 11.0 
Reduction -- 58.0 58.0 
efficiency (%) 
______________________________________ 
*Total sulfur and anion percentages are based on the weight of the 
original black liquor solids. 
EXAMPLE 2 
In this example the same black liquor as described in Example 1 was used 
except that the oxidation in the continuously stirred reactor was deleted. 
Again the dried black liquor solids were pyrolyzed at 600.degree. C. under 
helium and 10% carbon monoxide and subsequently exposed at 750.degree. C. 
to the same gas mixture. The analysis of the black liquor solids, the 
600.degree. C. pyrolyzed char and the char treated at 750.degree. C. are 
shown in Table 3. The analysis shows that the main inorganic sulfur 
containing species in black liquor solids is SO.sup.2-.sub.3, contrary to 
Example 1 where SO.sup.2-.sub.4 is the dominant ion. Subsequent pyrolysis 
at 600.degree. C. gives a slightly higher sulfide content for the 
non-oxidized sample compared to the oxidized samples in Example 1. However 
the 45% sulfur loss is considerably larger than in Example 1. Further 
treatment of the non-oxidized sample at 750.degree. C. increases the total 
sulfur-loss to 56%, while the reduction efficiency is unchanged at 58%. 
Thus from comparison of Examples 1 and 2 it is clear that a strongly 
oxidized black liquor is preferred in order to minimize the sulfur-loss 
and maximize the reduction efficiency. 
EXAMPLE 3 
About 10 mg of oxidized black liquor solids were pyrolyzed in a 
thermobalance by linearly increasing the temperature from 20.degree. to 
750.degree. C. at a rate of 20.degree. C./minute. The gas atmosphere was 
pure nitrogen up to 550.degree. C. and 88% N.sub.2 plus 12% CO above 
550.degree. C. After stabilization of the temperature at 750.degree. C., 
CO.sub.2 is added to a concentration of 20%, with the remaining gas being 
10% CO and 70% N.sub.2. The addition of CO.sub.2 leads to gasification of 
the carbon in black liquor char as indicated by the recorded weight-loss 
and CO production. The composition of black liquor char during 
gasification is shown in Table 4. The results in Table 4 show a continuous 
decrease in inorganic sulfur content, while the reduction efficiency is 
maintained at 80-90%. COS was measured gas chromatographically as the only 
sulfur gas produced during gasification. The reaction responsible for the 
sulfur-loss is 
EQU Na.sub.2 S+2CO.sub.2 .fwdarw.COS+Na.sub.2 CO.sub.3 
The high S.sub.2 O.sup.2-.sub.3 content is due to rapid oxidation of 
S.sup.2- in aqueous solution before analysis of the water leachate of 
black liquor char by ion chromatography. The small sample size and the 
presence of carbon makes it extremely difficult to prevent the oxidation. 
It should also be noted that Na.sub.2 S.sub.2 O.sub.3 cannot exist at 
750.degree. C. Combining this result with the preceding examples, it can 
be concluded that gasification leads to gaseous sulfur emission due to 
reaction between Na.sub.2 S and CO.sub.2 (and/or H.sub.2 O vapor). 
TABLE 4 
______________________________________ 
Composition of sulfur species 
in black liquor char during CO.sub.2 gasification. 
Gasification 
Carbon Reduction 
time burn- S.sup.2- SO.sub.4.sup.2- 
S.sub.2 O.sub.3.sup.2- 
efficiency 
(min) off (%) (% wt)* (% wt)* 
(% wt)* 
(%) 
______________________________________ 
0 0 0.96 0.17 0.7 90 
4 25 0.5 0.13 0.5 86 
9.5 50 0.7 0.13 0.4 90 
16 75 0.3 0.13 0.6 80 
36 100 0.4 0.10 0.4 87 
______________________________________ 
Conditions: 
1) Temperature 750.degree. C. 
2) CO concentration 10% 
3) CO.sub.2 concentration 20% 
*Based on the weight of dry black liquor solids. 
EXAMPLE 4 
About 10 mg of oxidized black liquor char solids were pyrolyzed in a 
thermobalance under an atmosphere of pure helium by linearly increasing 
the temperature from 20.degree. C. at a rate of 20.degree. C./minute. The 
sample was kept at a final pyrolysis temperature until no further 
weight-loss occurred. The composition of the pyrolysis residue for 
different final pyrolysis temperatures is listed in Table 5. The table 
shows that no sulfur is lost under an inert atmosphere, and that high 
reduction efficiencies are achieved. It should also be noticed that a 
considerable loss of Na.sub.2 CO.sub.3 occurs at higher pyrolysis 
temperatures in an inert atmosphere. 
TABLE 5 
______________________________________ 
Composition of char after pyrolysis in helium. 
T S.sub.total 
S.sup.2- 
S.sub.4.sup.2- 
S.sub.3.sup.2- 
S.sub.2 O.sub.3.sup.2- 
Na.sup.+ 
CO.sub.3.sup.2- 
(.degree.C.) 
(%) (%) (%) (%) (%) (%) (%) 
______________________________________ 
b.l. 3.1 -- 1.2 3.6 -- 19.5 10.5 
solids 
675 2.3 1.8 0.9 &lt;0.1 &lt;0.05 18.1 17.9 
775 2.3 2.0 0.3 &lt;0.1 &lt;0.05 5.73 2.96 
800 2.4 2.2 0.2 0.2 &lt;0.05 -- -- 
______________________________________ 
1) Pyrolysis in helium until negligible weightloss. 
2) Percentages given are based on original weight of black liquor solids. 
TABLE 6 
______________________________________ 
Composition of char after pyrolysis in 88% He 
and 12% CO for 30 minutes at T.sub.final. 
T.sub.final 
S.sub.total 
S.sup.2- 
S.sub.4.sup.2- 
S.sub.3.sup.2- 
S.sub.2 O.sub.3.sup.2- 
Na.sup.+ 
CO.sub.3.sup.2- 
(.degree.C.) 
(%) (%) (%) (%) (%) (%) (%) 
______________________________________ 
b.l. 3.1 -- 1.2 3.6 -- 19.5 10.5 
solids 
750 2.4 1.7 1.1 0.1 0.2 17.6 15.5 
800 2.4 2.2 0.1 &lt;0.1 0.1 17.7 10.6 
______________________________________ 
EXAMPLE 5 
About 10 mg of oxidized black liquor solids were pyrolyzed in a 
thermobalance under an atmosphere of 88% helium and 12% carbon monoxide. 
The temperature of the oven was linearly increased from 20.degree. C. to a 
final temperature at a rate of 20.degree. C./minute. The composition of 
the pyrolysis residue after being kept at the final pyrolysis temperature 
for 30 minutes is seen in Table 6. The results listed in Table 6 show that 
contrary to Table 5, no significant amount of sodium is lost at the higher 
pyrolysis temperatures when CO is present besides helium. Again no sulfur 
is lost at the higher pyrolysis temperatures. This shows that sodium 
emission can be suppressed by the presence of CO in the pyrolysis 
atmosphere.