NOX EMISSIONS FROM KRAFT PULP MILLS
KEIJO SALMENOJA
Team Manager, Oy Metsä-Botnia Ab, Technical Development
P.O. Box 165, FI-26101 Rauma, Finland
Email: keijo.salmenoja@botnia.com,
Mobile: +358 50 598 5011
ABSTRACT
Total production of the Finnish pulp mills was 12.9 million tons in 2007. Production has
risen by 60% over the last 15 years. Over the same period, emissions from the mills per
ton produced have decreased by 70–80%. The main reason for the reduction is the
implementation of new processes and emission reduction techniques in the industry.
Sulfur oxide (SOx) emissions have mainly been solved by introducing high dry solids
firing. An effort is also being made to reduce nitrogen oxide emissions, but the work
seems to be more challenging.
Nitrogen oxides (NOx) are one of the biggest gaseous emission sources of kraft pulp
mills. Generally, specific emissions from black liquor recovery boilers are low
compared to e.g. solid fuel boilers, but due to the large flue gas quantity, NOx emissions
from recovery boilers can be locally relevant. Reported average NOx emissions from
Finnish pulp mills in 2007 were 1.49 kgNO2/ADt. This includes emissions from the
recovery boiler, lime kiln, and dedicated odorous gas incinerator.
All new pulp mills have both source specific (mg/m3n) and mill specific (kgNO2/ADt)
NOx emission limits. Typically, mill specific emission limits are set according to the
BAT BREF guidelines, which state that specific NOx emissions below 1.5 kgNO2/ADt
should be accessible. However, the total NOx emission from a pulp mill is dependent on
several things; wood raw material nitrogen content, recovery boiler furnace load, degree
of odorous gas collection and place of combustion, selection of lime kiln fuels, and total
pulp yield of the mill. Therefore, large differences in the reported specific NOx
emissions from the pulp mills can be found.
NOx emissions from the recovery boiler correspond to around 2/3 of the total emissions.
Therefore, biggest efforts have been put on reducing NOx emissions from the recovery
boilers and all efficient and best available measures are already taken into account in the
design and operation. On the other hand, strong odorous gases (CNCGs) are a
significant source of NOx emissions if they are burnt in the lime kiln or in a dedicated
boiler. Therefore, special attention should be paid on where and how CNCGs are burnt
in a pulp mill.
This paper presents results from theoretical studies, dedicated field tests and measuring
campaigns. Nitrogen routes in kraft pulp mills and nitrogen balances in the recovery
area are also discussed, as well as the measures and possibilities to reduce NOx
emissions from pulp mills.
Keywords: Pulp mill, NOx, recovery boiler, lime kiln, odorous gas, BAT, emissions.
INTRODUCTION
The control of emissions from European pulp mills and the development of emissions
regulations are guided by the IPPC BREF document, which specifies, among others, the
best available techniques (BAT) guidelines, for limiting nitrogen oxide emissions from
pulp mills (European Commission, 2001). The document sets the range for nitrogen
oxide emissions at 1.0-1.5 kgNO2/ADt, when a pulp mill is using a suitable combination
of best available techniques. The figures include emissions from recovery boilers, lime
kilns and odorous gas boilers, but not emissions from auxiliary or bark boilers.
The total production of pulp in Finland was 12.9 million tons in 2007, from which
Metsä-Botnia's share was ca. 2.5 million tons. The share of chemical pulp of the total
production was 7.7 million tons. Production of chemical pulp has risen by 35% over the
last 15 years. Over the same period, emissions from the mills have decreased by 70–
80% (Finnish Forest Industries, 2008). The most significant reduction in chemical pulp
production has been obtained in sulfur dioxide (SOx) emissions, mainly due to the
introduction of high dry solids firing in the 1980s. An effort is also being made to
reduce nitrogen oxide (NOx) emissions, but the work seems to be extremely
challenging. The forest industry sector contributes some 10% of Finland’s total nitrogen
oxide emissions.
All new pulp mills have both source specific (mg/m3n) and mill specific (kgNO2/ADt)
NOx emission limits. Typically, mill specific emission limits are set according to the
BAT guidelines. Figure 1 shows the annual average NOx emissions from Finnish pulp
mills in 2007. As can be seen from Figure 1, reported NOx emissions varied from 0.5 up
to 2.2 kgNO2/ADt. Only 40% of the Finnish mills reported emissions lower than 1.5
kgNO2/ADt. Reported average NOx emissions from Finnish pulp mills in 2007 were
1.49 kgNO2/ADt. Specifying and comparing emissions from pulp mills as kgNO2/ADt
contains, therefore, a strong effect of the pulping yield.
2.50
kg NO2/ADt
2.00
1.50
1.00
0.50
0.00
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
Cumulative Production (1000 ADt/a)
Figure 1: Annual average NOx emissions from Finnish pulp mills in 2007 (Finnish Forest Industries,
2008.
NOx emissions from the recovery boiler correspond to around 2/3 of the total emissions
from the mill (Salmenoja et al., 2007). Typical NOx emission from a recovery boiler
furnace is around 200 mgNO2/m3 n (Vakkilainen, 2005), which corresponds to ca. 1.5
kgNO2/ADt. Therefore, biggest efforts have been put on reducing NOx emissions from
recovery boilers. However, the total NOx emission from a pulp mill is dependent on
several things; wood raw material nitrogen content, recovery boiler furnace load, degree
of odorous gas collection and place of combustion, and the selection of lime kiln fuels.
If the measures to reduce NOx emissions are only aimed at the recovery boiler, the
results may be slender. Therefore, all the relevant factors should be taken into account.
The large variation (1.7 kgNO2/ADt) in reported NOx emission numbers also implies
that there are differences in the treatment of nitrogen containing streams in the mill, as
well as in the measuring and reporting procedures.
The purpose of this paper is to give an overview of the formation of NOx emissions at
kraft pulp mills and possibilities to reduce the emissions. The review is based on
theoretical studies, dedicated field tests, and numerous mill-scale follow-up studies.
FORMATION OF NOx EMISSIONS FROM KRAFT PULP MILLS
The formation of nitrogen oxide emissions from pulp mills is significantly more
complicated than was perceived before. Detailed studies on the formation of nitrogen
oxides in the burning of black liquor began in the 1990s. However, a complete picture
of the cycle of nitrogen compounds at pulp mills has only been clarified relatively
recently (Kymäläinen, 2001). In particular, the sodium cyanate (NaOCN) forming in the
recovery boiler smelt and the ammonia (NH3) generated through it in the chemical
recovery cycle are new key factors, the importance of which has only been understood
in the last few years.
Nitrogen is mainly introduced into the mill with wood chips, but other nitrogen sources
are also available, including defoamers, anti-scaling agents, chelating agents, etc. Their
contribution has been considered to be negligible, but they may, however, have an
impact on the black liquor nitrogen content in the future. The recycling of fiberline
filtrates back to the recovery cycle in a mill with a high degree of closure may increase
the nitrogen content of black liquor significantly (Telkkinen, 1997). Raw wood material
contains organic nitrogen compounds as natural constituents, typically 0.05-0.50% of
dry matter (Martius, 1992, Nichols et al., 1993, Verveka et al., 1993, Kymäläinen,
2001). In normal cooking, nitrogen compounds in the wood dissolve more or less
completely in the alkaline cooking liquor and are transferred as part of the black liquor
to the evaporation plant and further to the recovery boiler.
Black liquors from wood species typically contain 0.05-0.50% of nitrogen in dry solids.
This suggests that the nitrogen in wood could account for all the nitrogen present in
black liquors. Hardwood black liquors typically contain more nitrogen than softwood
liquors (Kymäläinen, 2001), which was also confirmed in this study. Measured average
black liquor nitrogen contents for softwood and hardwood black liquors were 0.06%
and 0.09%, respectively.
NOx formation in the black liquor recovery boiler furnace
NOx emissions from the black liquor recovery boiler furnace can originate either from
fuel-NO or thermal-NO formation. However, thermal-NO formation i.e. the nitrogen
oxide (NO) originating in nitrogen from the air, have shown not to play any role in
normal black liquor combustion (Vakkilainen et al., 2005). On the other hand, black
liquor nitrogen or fuel nitrogen is the dominant source for NOx emissions from a
recovery boiler furnace (Nichols, et al., 1993, Iisa et al., 1998).
Nitrogen is released either as ammonia during the pyrolysis of black liquor droplets or
as NO during the char oxidation. Some nitrogen can also be released in molecular form
(N2) during the oxidation stage in oxygen-lean conditions. According to a recent study
(Brink et al., 2008) droplet size has a considerable effect on the NOx emissions from the
recovery boiler furnace. Smaller droplet sizes promote higher NOx emissions. Black
liquor dry solids content, however, seems not to have significant effect on the NOx
emissions from the recovery boiler furnace (Vakkilainen, 2005).
The conversion of black liquor nitrogen into NOx in the furnace varies from boiler to
boiler, but on the average around a third of the nitrogen in the black liquor is oxidized
into NOx (Kymäläinen, 2001). However, conversions lower than 15% have been
reported (Saviharju et al., 2007), but these extremely low NOx conversions were
achieved in special conditions.
Figure 2 presents measured and calculated conversions to NOx during normal operation.
Measured conversions are based on nitrogen balance measurement campaigns at five
kraft pulp mills (Salmenoja et al., 2007). Calculated conversions are based on year 2007
mill statistics, which correspond to the normal operation at the mills.
Conversion to NOx (%)
50.0
Measured
45.0
Calculated
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
Mill A
Mill B
Mill C
Mill D
Mill E
hardwood
Mill E
softwood
Figure 2: Measured and calculated conversions to NOx in recovery boiler furnaces. Measured values
represent one day averages during the campaigns. Calculated values are based on 2007 mill statistics.
As can be seen from Figure 2, measured and calculated conversions are close to each
other. Average measured and calculated conversions are 35% and 37 %, respectively.
The average conversions agree very well with the 33% rule of thumb. This data
suggests that it seems hard to achieve lower than 30% conversion to NOx in the
recovery boiler furnace in normal operation conditions. However, high carbon
monoxide (CO) concentration seems to favor low NOx emissions from the furnace
(Saviharju et al., 2007). Thus, allowing higher CO levels in the flue gases, lower
conversions to NOx and consequently lower NOx emissions can be obtained.
Effect of recovery boiler furnace load
Furnace temperature affects the combustion chemistry, reaction kinetics, and retention
time in the lower furnace. Although, the formation of fuel-NO is relatively insensitive to
temperature in the range 800-1000 °C (Nichols et al., 1993), decreasing the lower
furnace temperature seems to decrease NOx emissions (Vakkilainen, 2005). A low load
means lower furnace temperature, which in turn enables better combustion control in
the furnace. Thus, furnace temperature has also an effect on the recovery boiler NO x
emissions. However, reducing NOx by lowering the furnace temperature is problematic,
since SO2 emissions will increase and reduction degree will decrease.
Several factors contribute to the temperature in the lower furnace including primary and
secondary air temperatures, black liquor dry solids content, heating value of black
liquor, and dry solids load to the furnace. Generally, recovery boilers are operated at
constant air temperatures, black liquor dry solids content and heating value. Thus, the
only factor that affects the lower furnace temperature in normal operation is the dry
solids load, in other words the heat load into the furnace. The higher the dry solids load,
the higher the lower furnace temperature.
Figure 3 shows furnace load (tds/d) and relative NOx emissions from a recovery boiler
during a two-year period. The load figures are collected from the boiler control (DCS)
system and NOx emission values are based on on-line analyzers. According to Figure 3,
NOx emissions from this recovery boiler appear to follow rather nicely the changes in
the dry solids load to the boiler. The average dry solids load during the two-year period
was close to 3000 tds/d, which is around 90% of the design load. Average NOx emission
from the recovery boiler during 2006-2007 was equivalent to ca. 1.4 kgNO2/ADt.
1.20
4 500
1.00
3 500
0.80
3 000
2 500
0.60
2 000
0.40
1 500
1 000
0.20
Relative NO x Emission
Furnace Load (tds/d)
4 000
500
0
0.00
2006-2007
Figure 3: Relative NOx emissions (line) and furnace load (bar) during 2006-2007 from a recovery boiler
in normal operation. NOx emissions calculated according to the measurements by on-line analyzers.
Effect of NCGs
A significant fraction of the organic nitrogen in the black liquor is transferred into the
recovery boiler smelt as NaOCN. The cyanate nitrogen in the smelt passes into the
green liquor through the dissolving tank along with the smelt’s other salts. In the green
liquor, alkaline hydrolysis gradually transforms cyanate nitrogen into NH3 (DeMartini
et al., 2004). The ammonia formed in the green liquor and in the white liquor partially
evaporates into dilute gases (DNCGs). However, most of the ammonia continues as part
of the white liquor back to the cooking. From the cooking, the ammonia passes into the
black liquor together with the organic nitrogen compounds of the wood. The ammonia
is separated from the liquor in the evaporation plant, passing into concentrated gases
(CNCGs) and methanol. The ammonia in CNCGs ends up – depending on the mill’s
processing operations – in the recovery boiler, lime kiln, or in a dedicated incinerator.
Total emissions of nitrogen compounds from a pulp mill are, therefore, dependent on
how the ammonia containing flows, generated in the chemical recovery cycle, are
treated.
At old mills where the treatment of DNCGs is deficient, most of the ammonia vapors
generated in the production of white liquor escapes into the atmosphere as ammonia
emissions with other DNCGs. In the latest processes, DNCGs are collected efficiently
and combusted either in the recovery boiler, lime kiln or dedicated incinerator.
Depending on the particular case, these ammonia containing streams may increase the
total NOx emissions from the pulp mill. Practical studies have shown that burning
NCGs in a recovery boiler does not increase the NOx emissions (Janka and Tamminen,
2003). The increase in NOx emissions can more or less be avoided if the malodorous
gases are injected in the correct way and in the right location in the recovery boiler
furnace.
Effect of lime kiln fuels
Experiences has shown that burning oil in a lime kiln produces lower NOx emissions
than natural gas, even though oil contains significant quantities of organic nitrogen that
is not present in natural gas (Salmenoja et al., 2007). This is because of the higher
temperatures with the natural gas flame. This will cause a higher thermal-NO x
formation than the burning of oil. Low-NOx burner technology can be applied to lime
kilns, but it is still under development and has not been widely utilized in the pulp mills.
In most cases burning the NCGs in the lime kiln results in a significant increase in the
NOx emissions. Developing low-NOx technology for lime kiln burning, when both
NCGs and the main fuel are present, will require additional investments, and currently
there are no reliable technical solutions available. The main reason for burning NCGs in
the lime kiln are the high heating value and the high sulfur content of the gases. The use
of NCGs as lime kiln fuel can replace around 15% of the heavy oil, which means
considerable annual fuel savings. Typically, CNCGs contain 3-5 kgS/ADt and those
mills that suffer from high sulfidity levels can not burn concentrated gases in the
recovery boiler and recycle the sulfur. Therefore, CNCGs are burnt in the lime kiln or in
a dedicated incinerator and the sulfur recovered as sodium bisulfite (NaHSO3) with an
alkali scrubber.
Liquid methanol has a high heating value and therefore some mills use methanol as a
lime kiln fuel. Due to the high nitrogen content in methanol, NOx emissions from the
lime kiln will also increase during the combustion of methanol. Lime kiln emissions
represent around 15% of the total emissions from the mill (Salmenoja et al., 2007) and
therefore methanol combustion in lime kiln may have a considerable impact to total
NOx emissions.
A practical test was carried out in a mill where methanol is used as a lime kiln fuel.
Measured nitrogen content in methanol was 2.5%. This is five times higher than the
nitrogen content in heavy oil (0.5%) used in the lime kiln. Thus, methanol increased the
nitrogen flow into the lime kiln three-fold. When methanol flow was turned on, a rapid
increase was also seen in the NOx emissions from the lime kiln. Since most of the NO x
in flue gases is in NO form, the alkali scrubber after the lime kiln is not able to handle
increased emissions. Therefore, total emissions from the mill will increase.
METHODS FOR REDUCING NOx EMISSIONS
There are a number of methods under development or undergoing testing that could
potentially reduce NO emissions at pulp mills. Some of these are also mentioned in the
IPPC document. Existing best available techniques include only combustion or socalled primary measures. Additional air levels above the conventional ones can provide
more freedom for managing NOx emissions with combustion technology, and according
to the experiences could result in a 10-25 % reduction depending on the original NO
emission level (Janka et al., 1998).
It has become apparent that NOx reduction techniques based on optimizing air
distributions are not efficient if the boiler load is very high, or if boilers are operating
overloaded, as is the case at several pulp mills. In addition to this, optimizing the air
system will require special furnace wall tube materials that can withstand corrosion in a
reducing atmosphere. These so-called compound tubes would have to be extended from
the lower part of the boiler to the highest air level, which would be a significant and
expensive alteration in a recovery boiler.
Selective non-catalytic reduction (SNCR)
Injecting ammonia into hot (ca. 900-1000 °C) flue gases causes a selective non-catalytic
reaction, in which nitrogen oxide is converted into N2 by reacting with ammonia or urea
((NH2)2CO). The effectiveness and applicability of SNCR for recovery boilers is still
unclear. One problem is changing boiler load which moves the temperature window
optimal for the SNCR technique to a different location in the furnace. As a result, the
efficiency of the NOx reduction varies. The potential reduction in recovery boilers (low
initial NOx level, varying temperature, effect of other components in the gases) is likely
to be below 50 %.
Selective catalytic reduction (SCR)
This process, in which ammonia is injected in a special catalytic reactor into cooled flue
gases after the electrostatic precipitator (ESP), is another well-known technology that is
commercially in use in power boilers, but has only reached the trial stage in recovery
boilers. The problem with this technique is ensuring sufficient flue-gas dust removal
before the process in the catalytic reactor and the durability of the catalyst. Optimal
temperature window for this process is between 250-350 °C, which means that either
hot ESP or extra heat transfer surfaces after the ESP must be used. This means heavy
investments in existing boilers. Reductions over 70% can be easily achieved with SCR.
In both SNCR and SCR techniques, unreacted ammonia remaining in the flue gases as
ammonia emissions is also a factor about which there is still insufficient practical
empirical data and which may restrict the effectiveness of these techniques in reducing
NO emissions.
CONCLUSIONS
Nitrogen oxide (NOx) emissions from kraft pulp mills were reviewed. The purpose of
the paper is to give an overview of the formation of NOx emissions at kraft pulp mills
and possibilities to reduce the emissions. The review is based on theoretical studies,
dedicated field tests, and numerous mill-scale follow-up studies. The mills represent
Finnish state-of-the-art kraft pulping technology and use the best available techniques
(BAT) in pulp production. Different pulping methods and raw materials are used in the
mills and several different fuel streams are fed into the recovery boiler and lime kiln.
Reported NOx emissions from Finnish pulp mills varied from 0.5 up to 2.2 kgNO2/ADt
in 2007. Only 40% of the Finnish mills reported emissions lower than 1.5 kgNO2/ADt,
which is the upper limit according to BAT BREF recommendations. Reported average
NOx emissions from Finnish pulp mills in 2007 were 1.49 kgNO2/ADt. The large
variation (1.7 kgNO2/ADt) in reported NOx emission numbers also implies that there are
differences in the treatment of nitrogen containing streams in the mill, as well as in the
measuring and reporting procedures.
The results in this study show that:
• The nitrogen content of the wood raw material largely determines the NO x level of
the recovery boiler. The nitrogen content of hardwood is usually significantly higher
than that of softwood.
• It is hard to achieve lower than 30% conversion to NOx in the recovery boiler furnace
in normal operation conditions. Lower conversions can be obtained only in special
conditions.
• Recovery boiler load has a considerable effect on NOx emissions. Lower furnace load
means lower furnace temperature and enables better combustion control.
• The way how and where DNCGs and CNCGs are treated is crucial. The high
ammonia content in CNCGs must be taken into account when considering the place
of combustion of these streams.
The effort to reduce NOx emissions from Finnish pulp mills is continuing, but the work
seems to be extremely challenging. The formation of nitrogen oxide emissions from
pulp mills is significantly more complicated than was perceived before. However,
recent studies have shed light to the possibilities to reduce NOx emissions especially
from the recovery boiler. Latest measurements have also revealed that at softwood kraft
pulp mills, which are operated at full load and have a complete odorous gas collection
system, reduction of total NOx emissions below 1.5 kg/ADt is impossible even by using
the BAT processes.
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