T262 emissions
Scrubbing NCG with white liquor
to remove reduced sulfur gases
By Z. Zhang, C.Q. Jia, H. Tran, P. Rouillard and B. Adams
Z. ZHANG
University of Toronto
Toronto, ON
Abstract: Depending on the gas sources, non-condensible gases (NCG) contain 3 to 18% total
reduced sulphur gases (TRS) on a dry basis. Of the TRS, methyl mercaptan (CH3SH) accounted
for more than 50%, while hydrogen sulphide (H2S) and dimethyl sulphide (DMS) accounted for
less than 25% each, and dimethyl disulphide (DMDS) less than 2%. H2S and CH3SH were effectively scrubbed by white liquor. DMS and DMDS were not, and in some cases, they appear to have
been formed in the scrubber as a result of reactions between white liquor and NCGs.
N KRAFT PULP MILLS, non-condensible
gases (NCG) are emitted mostly from
digesters, evaporators, turpentine systems, strippers, brown stock washers,
and liquor storage tanks. These gases
contain large amounts of methane (CH4) and
reduced sulphur gases (known as TRS), which are
mainly hydrogen sulphide (H2S), methyl mercaptan (CH3SH), dimethyl sulphide or DMS
(CH3SCH3) and dimethyl disulphide or DMDS
(CH3SSCH3).
Due to the malodorous, hazardous natures of
these gases, NCGs must be collected and treated
properly before they can be released into the
atmosphere. The most common method of NCG
treatment is to burn the gases in a combustion
system available at the mill, such as a lime kiln, a
power boiler, a recovery boiler, or a dedicated
incinerator. Combustion eliminates CH4 and oxidizes TRS into less odorous SO2. The practice,
however, often results in high SO2 emissions from
the boiler and kiln stacks, high sulphur losses
from the mill, and low lime availability. In some
mills, excessive ring formation in lime kilns and
accelerated corrosion in bark boilers has also
been attributed to the burning of NCG [1].
In attempts to minimize the problems and to
comply with increasingly stringent environmental
regulations on TRS and SO2 emissions, many
mills have installed NCG scrubbers to remove the
TRS from the NCG before burning. The TRS
removal efficiency of scrubbers has been found to
vary widely from mill to mill depending on many
factors, including: the design and operating conditions of the scrubbers, the sources of NCG
streams, the concentration and composition of
the TRS, and the strength and flow rate of the
white liquor used for scrubbing. Understanding
these factors and their effects on TRS removal is
of vital importance for kraft mills in order to maximize the scrubber performance, and to minimize sulphur losses and problems associated with
high TRS and SO2 emissions.
This paper discusses results of a systematic
study on the TRS removal efficiency of a newly
installed NCG scrubber at the Domtar Espanola
mill. The study involves field trials to determine
the compositions and concentrations of TRS in
I
H. TRAN
University of Toronto
Toronto, ON
P. ROUILLARD
Domtar Inc.
Espanola Mill
Espanola, ON
C.Q. JIA
University of Toronto
Toronto, ON
B. ADAMS
Domtar Inc.
Espanola, ON
74 • 107:12 (2006) • PULP & PAPER CANADA
the NCG stream before and after the scrubber,
and laboratory experiments at the University of
Toronto to obtain fundamental data on the possible chemical reactions between white liquor and
the individual components of the TRS, and how
they may affect the TRS removal efficiency of
NCG scrubbers.
EXPERIENCE AT ESPANOLA
The Domtar Espanola mill produces 450
ADMT/day of fully bleached softwood kraft
pulp and 550 ADMT/day of hardwood kraft
pulp. The softwood is pulped in a Kamyr single
vessel hydraulic continuous digester and the
hardwood, in five 5800 ft3 batch digesters. Both
pulping lines are supported by a common chemical recovery cycle.
A single concentrated NCG (CNCG) system
collects TRS from continuous and batch sources
and sends them to one of the two identical lime
kilns for incineration. Continuous TRS sources
include chip bin exhaust, turpentine relief gases, excess flash steam, and evaporator condensate off-gases. Batch sources include a semi-continuous volume of batch digester cooking relief
gases and the concentrated NCG generated with
each blow.
The incineration of TRS gases in both NCG
and stripper-off-gases (SOG) appears to have
been the main contributing factor to the excessive ringing problem in the kiln. A ringing outage
occurred every one to three months while incinerating TRS gases. On one occasion, the kiln was
forced to shut down for ring removal after only 13
days in operation. The second kiln, which is used
for back-up incineration of TRS gases, typically
operates without interruption between major
shutdowns, while processing the same materials.
A white liquor scrubber was commissioned in
March 2003 to reduce the sulphur load to the
kiln, in an attempt to minimize the ringing problem. The white liquor scrubber was designed to
treat a gas stream of 1050 ACFM at 50°C with 150
gal/min of white liquor at 90°C. It has a 3’ OD by
20’ column of 1” 316SS Koch IMTP #50 random
packing. As shown in Fig. 1, NCGs are directed
through, or bypassed around, the scrubber by
three automatic block valves. Clarified white
emissions
liquor is used for a single pass, and is then
sent to the suction side of the continuous
digester feed pumps to minimize TRS
emissions from the atmospheric clarifier.
Steam is directly injected into the gas inlet
to control the white liquor discharge temperature, to minimize scaling and to prevent overloading of the digester heaters.
The NCG flow is typically 400-500 ACFM.
It operates under a vacuum of 15-17”
water column, as provided by a downstream steam ejector. A vent gas cooler
removes moisture from the NCG stream
and cools it from 90°C to 45°C before it is
burned in the kiln. Condensate from the
cooler is sent to the stripper.
Sulphur dioxide (SO2) from the kiln
stack dropped from 400 ppm to 10 ppm
immediately after the scrubber was put
on-line. The stability of the kiln controls
also improved significantly. However, the
SO2 emissions increased again to 200
ppm over a period of three weeks until
the kiln was shut down for ring removal.
Although other factors may have contributed to the accelerated ring formation
in the kiln, the elevated SO2 emissions
from the kiln stack indicated that the
scrubber may not have been performing
as expected.
It became apparent at this point that
direct measurement of the NCG composition and the determination of the scrubber efficiency would be needed in order
to understand and to resolve the problem.
These measurements were carried out in
three phases during the period from June
to October 2003.
Phase I was a two-week baseline study
performed in June, with the scrubber offline to fine tune the sampling and analysis procedures, to calibrate the gas chromatograph (GC) used for gas analysis,
and to determine the natural variability of
the NCG. Phase II was carried out from
the second half of July to mid-August to
determine the NCG composition before
and after the scrubber, as well as the efficiency of the scrubber. During this period,
the study was interrupted several times
due to unrelated problems at the mill.
Phase III was carried out in October to
determine the scrubber efficiency at
increased NCG flow rates.
NCG SAMPLING AND
ANALYSIS PROCEDURES
The sampling and analysis of NCG are not
easy tasks, due to safety concerns over
high TRS concentrations. In this study,
much effort has been put into developing
a safe sampling procedure, and into constructing a reliable, odour-free sampling
system so that the collection, handling,
purging and disposal of samples and gas
lines were possible without releasing
NCGs into the atmosphere. Other precautions included using a “buddy system”
for continuous gas monitoring, using supplied air during sampling, and minimiz-
FIG. 1. NCG scrubber at Domatar Espanola Mill.
ing the dehydration hazard. The combination of warm (45-50°C) ambient temperatures in the vicinity of the scrubber
with the dry supply air increased the risk
of dehydration significantly. Respiratory
protection was required while sampling
the TRS-enriched, spent white liquor at
the scrubber discharge.
The gas was collected from the NCG
gas line at two locations, as shown in Fig.
1. Location A is the scrubber inlet, and
location B is the scrubber outlet after the
condenser. The gas sample was drawn
through an ice-chilled impinger filled
with 85% phosphoric acid to remove
moisture, and then into a standard 1 L
Tedlar sample bag. The bag was stored in
a dark cooler with ice to prevent possible
decomposition of the gases in the sample.
Sample dilution and injection into the GC
were carried out manually using gas-tight
syringes. The sampling process required
20 to 60 minutes to complete.
The samples were collected under two
conditions: “Blow” and “No-Blow”. The
No-Blow samples were collected between
batch digester blows. They represented
the NCG from continuous sources together with the relief gases from batch
digesters. The Blow samples were collected during the batch digester blow, and
thus they included both the No-Blow
NCG and the NCG from batch digesters.
For each sampling, one No-Blow sample
and one Blow sample were collected. The
No-Blow sample was collected first so that
it would not be contaminated by the residual TRS from the previous blow gases.
The Blow sample was collected during the
middle of a blow to allow sufficient time
for the blow gases to reach the scrubber.
The collected samples were transported
immediately to the main lab at the mill for
analysis. A Varian 3800 gas chromatograph with a PFPD detector and a Restek
Rt-sulphur micropack column was used in
this study, with temperature programmed
from 60 to 200°C.
NCG COMPOSITION
In the Phase I-baseline study, a total of 6
No-Blow and 4 Blow NCG samples were
collected at the scrubber outlet, location
B in Fig. 1, and analyzed. Since the scrubber was off-line and bypassed during this
phase, this sampling location was essentially the same as the scrubber inlet (location A). The samples were analyzed for
H2S, CH3SH, DMS, DMDS and COS; however, COS was not detected in any sample.
The results are summarized in Table I,
along with the concentration of TRS
which was assumed to be the total of the
four main reduced sulphur gases.
Although the concentrations of these gases in both No-Blow and Blow NCGs varied
widely from day to day, the data consistently show that the concentration of
CH3SH was the highest, followed by DMS
and H2S, and then by DMDS. The concentrations of these gases in the Blow
NCGs were 4 - 6 times higher than those
in the No-Blow NCGs. The concentrations of TRS (assumed to be the sum of
the concentrations of the four main
reduced sulphur gases) varied from
20000 to 50000 ppm (or 2 to 5%) in the
No-Blow NCGs, and from 13 to 17% in
the Blow NCGs.
The difference in composition
between the No-Blow and Blow NCGs can
be seen more clearly in Fig. 2, which
shows the average concentrations of H2S,
CH3SH, DMS and DMDS during the baseline study. The No-Blow NCG, on a dry
basis, contained 0.8% H2S, 2.0% CH3SH,
0.9% DMS and less than 0.1% DMDS,
while the Blow NCG contained up to
about 4 times more of these gases (3.2%
H2S, 7.5% CH3SH, 4.2% DMS and 0.4%
PULP & PAPER CANADA • 107:12 (2006) •
75
T263
T264 emissions
FIG. 2. Concentrations (on dry basis) of reduced sulphur
gases in NCGs during Phase I study.
DMDS). This is presumably due to the following reasons: i) the Blow NCG contained blow gases which had been “accumulated” in the batch digesters over a
longer period of time compared to those
in the continuous digester, ii) the release
of batch digester blow gases occurred in a
short blow duration, and iii) the batch
digesters and the continuous digester
were operated under different cooking
conditions due to different wood species.
Although the concentrations of the individual reduced sulphur gases were significantly higher in the Blow NCG than in the
No-Blow NCG, their distributions in the
TRS were essentially the same for both
Blow NCG and No-Blow NCG. As shown in
Fig. 3, CH3SH was the most dominant
species, accounting for more than half,
while H2S and DMS accounted for less than
a quarter each, and DMDS less than 2%.
SCRUBBING EFFICIENCY
Table II summarizes the daily average concentrations of reduced sulphur gases for
both No-Blow NCGs and Blow NCGs at
the scrubber inlet and outlet during
Phase II of this study. The concentrations
at the scrubber inlet varied widely from
day to day, but collectively were much
higher than the concentrations obtained
in Phase I. CH3SH and DMS were the
dominant components in the inlet gases
(7 to 8% of each in the No-Blow NCGs
and 18 to 20% of each in the Blow
NCGs), followed by H2S (about 2% in the
No-Blow NCGs and 4% in the Blow
NCGs), and then by DMDS (less than 2%
in both No-Blow and Blow NCGs).
As in the baseline study in Phase I,
while the concentrations of the individual
reduced sulphur gases at the scrubber
inlet were higher in the Blow NCGs than
in the No-Blow NCGs, their distributions
in the TRS were similar, as shown in Fig. 4.
CH3SH and DMS account for about 40%
each, while H2S accounted for about 10%,
and DMDS, for less than 6%. Compared
to the baseline study, Fig. 3, the concentrations of DMS and DMDS in Phase II
were much higher, implying that the recy-
FIG. 3. Distribution of reduced sulphur gases in TRS during
Phase I study.
TABLE I. Daily average concentrations of reduced sulphur gases in NCGs during
Phase I - Baseline Study.
No-Blow NCGs
Sampling Date H2S (ppm)
06/25/2003
06/26/2003
07/02/2003
07/03/2003
07/04/2003
07/07/2003
Average
Blow NCGs
Sampling Date
06/26/2003
07/02/2003
07/03/2003
07/07/2003
Average
CH3SH (ppm)
6800
5100
11400
8600
12900
1000
7630
23200
16500
30100
19200
28300
5200
20400
H2S (ppm)
CH3SH (ppm)
15200
34200
23500
53300
31600
63700
70700
64900
102300
75400
DMS (ppm) DMDS (ppm)
900
<10**
11900
11900
8300
5200
8640
<10*
100
400
400
300
700
380
DMS (ppm) DMDS (ppm)
56800
59600
47700
2000
41500
1300
2000
1400
9800
3630
TRS*
30900
21700
48800
40100
49800
22100
35600
TRS*
137000
166500
137500
167400
152100
Note: *: Assummed to be the sum of the four reduced sulphur gases analyzed
**: Below the 10 ppm detection limit
cled white liquor might have changed the
equilibrium and/or kinetics of TRS formation in the continuous digester.
The concentrations of H2S and CH3SH
at the scrubber outlet were very low,
either below the detection limit of the GC
(10 ppm), or less than 2800 ppm (0.3%).
The results suggest that these gases were
effectively removed by the scrubber. DMS
was also removed by the scrubber, but the
removal was not as effective as that of H2S
and CH3SH. The concentration of DMDS
at the outlet varied widely, and was lower
than the concentration at the inlet. In a
Blow NCG sample, however, the outlet
concentration was significantly higher
than the inlet concentration, while in
another sample, it was the same as the
inlet concentration. The results suggest
that the removal of DMDS was not effective as compared to the other gases, and
that DMDS may have been generated in
the scrubber as a result of reactions
76 • 107:12 (2006) • PULP & PAPER CANADA
between NCGs and white liquor.
Based on the concentrations of the
individual and the total reduced gases
before and after the scrubber, the efficiencies of the scrubber in removing these
reduced gases and TRS from the NCGs
can be estimated, as shown in Table III.
On average, the scrubbing efficiency was
over 99% for H2S and CH3SH, about 93%
for DMS and about 50% for DMDS. For
the TRS, the scrubbing efficiency averaged at 95%. No significant difference in
scrubbing efficiency was found between
No Blow NCGs and Blow NCGs, except
for DMDS.
EFFECT OF NCG FLOW RATE
OF SCRUBBING EFFICIENCY
Table IV summarizes the concentrations
of reduced sulphur gases for Blow NCGs
at the scrubber inlet and outlet during
Phase III study, in which the NCG flow
rate was increased from the normal oper-
emissions
TABLE II. Concentrations of reduced sulphur gases in the “No-Blow” NCG during Phawe II study.
No-Blow NCGs
Sampling Date
7/23/2003
7/24/2003
7/29/2003
7/30/2003
7/31/2003
8/05/2003
8/06/2003
8/07/2003
Average
26800
10200
18600
48100
21900
27300
15600
16600
23100
Blow NCGs
Sampling Date
7/23/2003
7/24/2003
7/29/2003
7/31/2003
8/05/2003
8/06/2003
8/07/2003
Average
H2S (ppm)
Inlet
Outlet
CH3SH (ppm)
Inlet
Outlet
10
10
*<10
570
260
40
350
10
180
94700
82400
133200
107700
69400
60800
32600
29800
76300
H2S (ppm)
Inlet
Outlet
6700
16000
67200
75400
37700
60700
28600
41800
70
630
50
1700
1350
2800
250
*<10
980
CH3SH (ppm)
Inlet
Outlet
60
10
760
20
20
10
20
130
69700
172000
216000
184000
280000
261000
188000
196000
490
360
1900
550
20
10
220
500
DMS (ppm)
Inlet
Outlet
112700
95900
88200
64600
21800
118000
65900
9500
72100
600
1040
1170
7300
11000
17300
10500
4900
6700
DMS (ppm)
Inlet
Outlet
95400
233000
264000
40400
268000
163000
170000
176000
1400
1300
1800
16800
21600
16000
15500
10600
DMDS (ppm)
Inlet
Outlet
1000
8350
2000
54000
7800
4100
5000
11300
11700
350
690
530
2500
5000
6100
2800
2300
2500
DMDS (ppm)
Inlet
Outlet
4200
12500
22700
12400
24800
13100
39500
18500
1500
1300
1400
**23900
***24800
20800
12100
12300
Note: * Below detection limit of 10 ppm, ** higher than inlet value, *** same as inlet value.
TABLE III. Scrubbing efficiency (%) dring Phase II Study.
No Blow NCGs
H2S
CH3SH
DMS
DMDS
TRS*
Average
99
99
91
79
94
Min.
98
95
48
–49
85
Max.
100
100
99
95
100
Blow NCGs
Average
100
100
94
34
95
Min.
99
99
58
–93
87
Max.
100
100
99
94
99
Note: *Assumed to be the sum of the four reduced sulphur gases analyzed.
ating value of 450 ACFM to 600 ACFM.
The concentrations were consistent with
those obtained during Phase II.
Due to the wide variation in the inlet
gas concentration and composition, the
efficiency of the scrubber did not appear
to change appreciably with an increase
NCG flow rate, Table V. The DMS removal
efficiency dropped significantly. It was
essentially 0%, varying from -18% to 16 %.
The cause of this low DMS removal efficiency is not known.
EFFECT OF NCG SCRUBBING
ON SULPHUR BALANCE
Despite the low DMS and DMDS removal
efficiencies, the scrubber helped stabilize
the kiln operation by removing most of
the H2S and CH3SH during the batch
digester blows. The blow cycle represents
a major process disturbance when the
scrubber is off-line.
FIG. 4. Distribution of reduced sulphur gases in TRS at the
scrubber inlet during Phase II study.
Direct measurement of NCG composition and scrubber efficiency shows that
the increased SO2 emissions from the
kiln stack were not caused by closed-loop
accumulation of TRS gases which overload the scrubber. While the data was too
scattered to confirm or disprove the
hypothesized long-term TRS enrichment
phenomena, there is evidence to support
the transfer of TRS compounds to stripper off-gas (SOG).
LABORATORY STUDIES
The ability of white liquor to capture
reduced sulphur gases in the scrubber
depends greatly on the rate of mass transfer of the gases to the white liquor, and on
chemical reactions between the gases and
the white liquor. In order to understand
how effectively reduced sulphur gases are
scrubbed by the white liquor, the absorption and chemical reactions between the
gases and the white liquor were studied in
the laboratory using a gas-liquid reactor,
as schematically shown in Fig. 5.
The reactor consists of a Pyrex glass
container with openings for introducing
and collecting gases and liquids. The temperature is controlled using a water bath.
After a known amount of a reduced sulphur gas has been introduced into the
head space above the liquid surface in the
reactor, gas samples are withdrawn from
the head space at different times for analysis. As the gas diffuses and dissolves in the
solution, its concentration in the head
space decreases and eventually reaches
equilibrium with the liquid phase. Since
the rate of absorption is controlled by mass
transfer to, and chemical reactions at, the
liquid surface, the departure of the gas
concentration from the equilibrium value
is the driving force for the gas to dissolve.
By determining the change in concentra-
PULP & PAPER CANADA • 107:12 (2006) •
77
T265
T266 emissions
TABLE IV. Concentrations (in ppm) of reduced sulphur gases in blow NCGs at different NCG flow rates during Phase III
study.
Sampling Date
9/30/2003
10/01/2003
10/02/2003
Flow Rate
(ACFM)
450
500
600
450
600
450
H2S (ppm)
Inlet
Outlet
20900
27100
23700
14100
8100
10000
360
960
1500
240
*<10
350
CH3SH (ppm)
Inlet
Outlet
148900
189000
153000
259000
172000
258000
1600
930
3400
1300
350
230
DMS (ppm)
Inlet
Outlet
140000
170000
106000
93000
100100
120000
135000
143000
99200
**224000
**118000
120000
DMDS (ppm)
Inlet
Outlet
8100
10000
6500
12000
7400
10200
**20000
5700
7500
5400
2100
1800
Note: *Below detection limit of 10 ppm, **outlet value higher than inlet value.
TABLE V. Effect of NCG flow rate on scrubbing efficiency (%) during Phase III
study.
Sampling Date
9/30/2003
10/01/2003
10/02/2003
FIG. 5. Laboratory Apparatus.
tion of the gas in the head space with time,
it is possible to evaluate the extent of the
gas-liquid absorption process.
Since CH3SH was found to be the
dominant reduced sulphur gas, Figs. 3
and 4, its absorption in an aqueous solution of NaOH was examined first in this
study. Figure 6 shows the change in concentration of CH3SH in the head space
with time, as the gas was exposed to various caustic solutions ranging from 0
(water) to 4 M NaOH at 40°C. The
CH3SH concentration decreased (absorption rate increased) rapidly as the NaOH
concentration increased. The results
clearly indicate the involvement of a
chemical reaction between CH3SH and
NaOH in the absorption process.
CH3SH + NaOH
៬ CH3SNa + H2O
Similar results were obtained when
white liquor was used instead of the
NaOH solution. Although the NaOH concentration in the white liquor used was at
least 2M NaOH, the absorption occurred
slowly in the beginning, but was complete
in 30 minutes. The apparent slower
absorption of CH3SH in white liquor compared to pure NaOH solutions is probably
due to the fact that white liquor contains
sulphide (S2–) which hinders dissociation
of CH3SH in the solution.
Figure 7 shows typical chromatograms
of No Blow NCGs collected at the inlet
and outlet of the scrubber during Phase II
study. Not only did the ratio of DMDS
peak to DMS peak increase during the
scrubbing process, but the DMS peak
Flow Rate
H2S
CH3SH
DMS
DMDS
TRS*
450
500
600
450
600
450
98
96
94
98
100
97
99
100
98
99
100
100
4
16
6
–141
–18
0
–148
42
–16
55
72
82
51
62
61
39
58
69
Note: * Assumed to be the sum of the four reduced sulphur gases analyzed.
seems to have split into two peaks, and at
least two other unknown peaks are visible.
Similar results were also obtained for all
Blow NCGs at the scrubber outlet. No
explanation can be given for the peak
split and the presence of unknown peaks
at this time.
However, the split of the DMS peak
and the presence of the unknown peaks
were not observed in scrubber inlet samples that had been stored for a day before
the analysis. These findings imply that
DMS may be unstable in the scrubber
environment, and that DMDS may have
been generated in the scrubber as a result
of chemical reactions between white
liquor and other reduced sulphur gases.
CH3SH, for instance, is known to react
with O2 to generate DMDS [3,4]:
2CH3SH + 1⁄2 O2
៬ CH3SSCH3 + H2O
SUMMARY
A study was conducted to determine the
concentration of various reduced sulphur
gases in the NCGs and the sulphur
removal efficiency of an NCG scrubber
using white liquor at the Domtar Espanola
mill. The results show that:
• Depending on the gas sources, NCG
contained 3 to 18% total reduced sulphur
gases (TRS) on a dry basis. Of the TRS,
methyl mercaptan (CH3SH) accounted
for more than a half, while hydrogen sulphide (H2S) and dimethyl sulphide
(DMS) accounted for less than a quarter
each, and dimethyl disulphide (DMDS)
less than 2%.
• The TRS concentration in the NCGs
during a batch digester blow was significantly higher than that during the time
between blows. This confirms that batch
Résumé: Selon leur source, les gaz non condensables contiennent de 3 à 18 % de soufre réduit
total (SRT) à l’état sec. Le SRT compte plus de 50 % de méthylmercaptan (CH3SH), un peu moins
de 25 % chacun de sulfure d’hydrogène et de sulfure de diméthyle (DMS), et moins de 2 % de
disulfure de diméthyle (DMDS). La liqueur blanche a permis d’épurer efficacement le H2S et le
CH3SH. Le DMS et le DMDS n’ont pas été épurés et, en certains cas, ils semblent s’être formés
dans l’épurateur à la suite de réactions entre la liqueur blanche et les gaz non condensables.
Reference: ZHANG, Z., JIA, C.Q., TRAN, H., ROUILLARD, P., MCDONALD, R., ADAMS, B.
Scrubbing NCG with white liquor to remove reduced sulfur gases. Pulp & Paper Canada 107(12):
T262-267 (December, 2006). Paper presented at the 2004 International Chemical Recovery Conference in Charleston, SC, June 6-10, 2004. Not to be reproduced without permission of PAPTAC.
Manuscript received December 21, 2005. Revised manuscript approved for publication by the
Review Panel April 7, 2006.
Keywords: NON-CONDENSIBLE GASES, REDUCED SULPHUR GASES, NCG, TRS, SCRUBBER, EMISSIONS, DIGESTER, KRAFT MILL
78 • 107:12 (2006) • PULP & PAPER CANADA
emissions
FIG. 6. Time dependence of head-space CH3SH concentrations when dissolving in water, white liquor and aqueous
NaOH solutions at 40°C.
digesters are a major source of TRS.
• H2S and CH3SH were effectively
removed by the scrubber. DMS and
DMDS were more difficult to remove. In
some cases, they appear to have been
formed in the scrubber as a result of reactions between white liquor and NCG.
• DMS and DMDS removal efficiencies
vary widely, probably due to the complexity of the chemistry of DMS and DMDS in
the scrubber and the relatively low solubility of these gases in water.
• Results of laboratory studies show that
the absorption of CH3SH in caustic solutions is a chemical process enhanced by
the solution alkalinity. The results also
confirm that white liquor can be used as
effectively as NaOH solutions of similar
alkalinity to scrub CH3SH.
FIG. 7. Typical chromatograms of no-blow NCGs at scrubber
inlet and outlet.
Granger, C. Rollins, G. Rose, T. Reid and
K. Simpson of the Domtar Espanola mill
and L. Allen of PAPRICAN for their assistance in gas sampling.
LITERATURE
1. BURGESS, L.T. Collecting and Burning Noncondensible Gases. Tappi Kraft Recovery Operations Short
Course. Tappi Press (2002).
2. SEGALL, J., WEN, Y., SINGER, R., DULLIGAN, M.,
WITTIG, C. Vibrationally Resolved Translational
Energy-Release Spectra from the Ultraviolet Photodissociation of Methyl Mercaptan. Journal of Chemical
Physics 99 (9): 6600-6606 (1993).
3. DALAI, A.K., TOLLEFSON, E.L., YANG, A.,
SASAOKA, E. Oxidation of Methyl Mercaptan over an
Activated Carbon in a Fixed-Bed Reactor. Industrial &
Engineering Chemistry Research 36 (11): 4726-4733
(1997).
4. BAGREEV A, BASHKOVA S BANDOSZ TJ. Dual
role of Water in the Process of Methyl Mercaptan
Adsorption on Activated Carbons. Langmuir 18 (22):
8553-8559 (2002).
ACKNOWLEDGMENTS
This work was part of the research program on “Increasing the Throughput and
Reliability of Recovery Boilers and Lime
Kilns” jointly supported by Alstom Power
Inc., Andritz Corporation, Aracruz Celulose S.A., Babcock & Wilcox Company,
Boise Paper Solutions, Bowater Canadian
Forest Products Inc., Canfor Inc., ClydeBergemann Inc., Daishowa-Marubeni
International Ltd., Domtar Inc., Georgia
Pacific Corporation, International Paper
Company, Irving Pulp & Paper Limited,
Kvaerner Power, MeadWestvaco, Stora
Enso Research AB, Tembec, Votorantim
Celulose e Papel, and Weyerhaeuser Company, and by the Natural Sciences and
Engineering Research Council of Canada.
The authors also wish to acknowledge T.
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