Use Of Ozone In Chemical And High Yield Pulping Processes Jc Hostachy Full Paper Appita 2010
USE OF OZONE IN CHEMICAL AND HIGH YIELD PULPING PROCESSES
“Latest innovations maximizing efficiency and environmental performance”
Director – Key Account Pulp & Paper
ITT Water and Wastewater
As one of the strongest oxidant available in the pulp and paper industry, ozone can be used in both chemical and
By choosing ozone in their bleaching process, many pulp mills in various part of the world, producing hardwood
and softwood bleach pulps, have already improved product quality, environmental and process performance.
Over the three last years, significant progress has been made in the use of ozone for the emergence of Green
bleaching practice favoring on-site chemicals production including complete reuse of by-products, minimizing
the ecological footprint and reducing operating costs. Concerning optimization for on-site chemical production,
the overall concept is based today on 100 % oxygen recycling since each oxygen molecule is used twice: First to
produce ozone and in its second lifetime for other applications such oxygen delignification, white liquor
oxidation and effluent treatment. Data collected on industrial scales shows and explains why the results are
finally better than everybody initially expected. But Green bleaching is not only based on finding as many
internal synergies as possible, it also concerns the assessment and possibility to reduce the ecological footprint of
the whole process. By taking into account carbon emission for bleaching chemicals production, supply chain
(transportation, storage,) and local energy requirement, it can be demonstrated that adopting Green bleaching
with ozone reduces carbon emission between 40 to 60% when compared with conventional chemistry mainly
based on chlorine dioxide. Finally, it is shown that adopting Green bleaching practice doesn’t sacrifice pulp
quality but, on the opposite, opens opportunities to develop new paper grades in a changing environment.
The second field of innovation concerns the use of ozone in high yield pulping processes to significantly reduce
energy requirement of TMP/CTMP processes. Based on results obtained on a large pilot scale, it can be shown
that the net saving in energy requirement using ozone is about 25% whereas pulp quality (strength properties) is
generally improved. This is an important benefit since energy remains the limiting factor for the long term
viability of such processes. Concerning (Pressure) Groundwood pulp production, fibre availability has become a
critical issue, and consequently an important focus of many pulp and paper companies. The use of ozone on an
industrial scale shows that a PGW process can be switched from Spruce as raw material to 100% Pine. Ozone
eliminates the pitch content of the Pine pulp, which can be sent to the paper machine without facing the
conventional deposits problem using such a material. Moreover, printability tests have given positive feed-back
since no build-up has been observed.
The high oxidizing potential of ozone is commonly valorised in water treatment applications. From the chemical
point of view, ozone has the ability to react with most of the organic structures present in the wood matrix like as
ligno-celluloses and extractives. These properties are used for chemical pulp bleaching where its delignifying
and brightening abilities are remarkable. Based on the same chemistry on the wood components, the reaction of
ozone on high yield pulps is another promising field of new applications. Depending on the type of high yield
pulping processes, ozone reacts on the surface of the fibre facilitating their separation, or selectively destroys
extractives in wood species that normally cannot be used for papermaking.
GREEN BLEACHING PRACTICE
The first fibrelines producing ozone bleached pulp are mainly coming from the emergence of Total Chlorine
Free bleaching (TCF) during the 90s. Today, 28 pulp mills have chosen ozone to produce bleached pulp at high
brightness level. Since the start-up of the first ozone bleaching installations, a lot of alterations have been carried
out to improve all the components of the ozone bleaching systems (pulp mixing, ozone generation). Today,
ozone bleaching is fully adapted to the bleaching of both hardwood and softwood pulps . In most cases, ozone
is used to reduce the chemical cost and to improve the environmental impact. It is generally applied just after
oxygen delignification to finalize lignin removal before the final bleaching phases . Keeping a low bleaching
chemical cost, extended ozone delignification offers the possibility to reduce the effluent to be treated since the
filtrate from the ozone (Z) stage and further alkaline stages can be circulated back to the recovery boiler .
Finally, whether it concerns greenfield mills, capacity expansion project or retrofit options, ozone is frequently
investigated at the initial phase of the project giving the mill the opportunity to adopt an efficient bleaching
process like those, as latest references, in operation in India, Portugal or Australia.
For chemical pulp bleaching, the ozone charge is generally in the range of 4-7 kg per ton of pulp. Depending on
the mill expectations and equipment supplier, the ozone stage can be supplied at high (HC) or medium (MC)
pulp consistency. In addition to the hardwood and softwood pulps, ozone is also a promising option to bleach
non-wood pulps coming from annual plants or agriculture wastes (Bagasse from sugarcane, wheat straw …) .
Considering fibre availability for the long-term, the use of non-wood fibres, as renewable raw materials,
decreases pressure on the natural forest in emerging countries. There is not yet any reference where ozone is
used on non-wood pulps but the last results obtained on laboratory scale have shown that this approach is
promising. But what does “Green bleaching practice” concretely mean?
Green bleaching is based on the implementation of technology to:
Minimize as much as possible water usage with closure of water loops reducing environmental impact
(low or no AOX, limited COD and color emission) using ECF-light or TCF bleaching.
Decrease carbon footprint
Favor on-site chemicals production including partial or complete reuse of by-products (Avoid
transportation, delivery and storage of dangerous chemical precursors)
Reduce variable costs
Guarantee pulp quality at least equivalent to common standard (ECF) keeping opportunities to develop
new pulp and paper grades (Food contact, …)
A concrete example of the advantages of a Green Bleaching practice including reuse of by-product is given by a
pulp mill producing Pine/Eucalyptus bleached pulp in Portugal at Celtejo. In this plant, association of oxygen
delignification and ozone bleaching was decided to limit environmental impact and to reduce the operating cost.
The flexible delivered bleach plant allows operating three bleaching sequences and all of them have now been
• Light-ECF Pine/Eucalytus : OO-Ze-D1-D2
• Light ECF Pine/Eucalyptus : OO-Ze-D-P
• TCF Eucalyptus : OO-Ze-P1-P2
TCF sequence OO-Ze-P1-P2 to bleach Eucalyptus pulps has shown that no scaling problem has been observed.
The pulp can easily reach brightness level up to 89% ISO (Brightness after the Ze stage already reaches 70%
ISO). Additionally, the presence of an acidic ozone stage eliminates the need for chelating agent when using
peroxide. The low kappa obtained after the Z-stage together with the recycling of the Z-filtrate makes it possible
to minimize the bleach plant impact. Total bleach plant effluent volume is maintained approximately 6-8 m3/adt
in ECF and 4-6 m3/adt in TCF. Pulp viscosity with TCF has not been a problem so far. Final values reported by
the plant are 600-650 ml/g for SW, 850-950 ml/g for HW. Pulp strength properties are not affected whatever the
sequence. For ECF, an adequate balance between ozone, chlorine dioxide and peroxide charges was found to
optimize variable cost. The economy is even boosted by the recycling of the off gases.
After the reaction of the oxygen-ozone gas mixture in the pulp, the off-gas from the Z-Stage contains oxygen as
the main by-product. Recycling that oxygen should be viewed as a means to "save" oxygen and to reduce the
cost of ozone, but this possibility has to be balanced with additional investment (compressor unit, piping …)
especially when high pressure is required. Defining the most optimized solution is always done in connection
with the local conditions and constraints of the pulp mill answering to a specific situation.
A complete new designed oxygen recycling system was installed at Celtejo. Due to this new design, the off-gas
from the Z-stage contains of more than 85 % oxygen. The gas is recycled through a two stage liquid ring
compressor system. Several control and bypass systems warranty an independent optimisation of each stage.
Liquid oxygen is used as a back-up. Figure 1 presents a simplified diagram about the oxygen reuse.
34 t/day O2 30 t/day O2 + Applications
4 t/day O3
17.5 t/day 02
scrubber 30 t/day 02
Oxygen 12.5 t/day 02
tank Pulp out Ozone Destruction
Figure 1 – Ozone and oxygen production including total oxygen reuse from the Z-stage
As shown in figure 1, the recycled oxygen is supplied to two independent operating consumers. Approximately
12.5 tons per day is supplied to the Total Oxidized White Liquor (TOWL) process whereas up to 17.5 tons per
day can be consumed in the oxygen delignification stage. The philosophy is based on 100 % oxygen recycling
where each oxygen molecule is used twice. The “not consumed” part is handled in a catalytic ozone destruction
system which is finally considered as a back-up safety system. TOWL replaces about 25 kg NaOH per ton of
pulp normally used for oxygen delignification. Investment in liquid ring compressor and specific reacting device
for TOWL is balanced with the saving in operating cost. A pay-back calculation indicates that such investment is
beneficial for pulp mills having a production capacity higher than 700 tons per day. Each partners involved in
this project has done a good job and now this plant is very proud to be the first pulp mill world-wide which is
operating this new process.
Impact on carbon footprint
In connection with by-products reuse, a detailed assessment of different bleaching processes is performed on a
pulp mill producing 500, 000 tons per year of bleached hardwood kraft pulp. The two following bleaching
sequences, ODEopDnD and OZeDP, are investigated to point out the differences in carbon emission. In this
simulation, residual oxygen from the Z-stage is reused for oxygen delignification and white liquor oxidation, and
chlorine dioxide is produced from an HPA process as a modern production technology. Chemicals required to
bleach the pulp and energy for the operation of the fibreline and at full brightness (90+ISO) are summarized
respectively in Table 1 and 2.
Chemicals, kg/odt (Ze)DP D(Eop)(DnD)
ClO2 8 18
O3 6 0
NaOH 12 9
O2 0 3
H2O2 5 3
MgSO4 2 0
H2SO4 18 12
SO2 1 2
Table 1 – Bleaching chemicals consumption
Stage Pumps, kW Mixers, kW HC ozone, kW Presses, kW
Z 400 75 200
D 400 300 100
P 400 300
Total for (Ze)(DP), kW 2475
Total per ton of pulp bleached with (Ze)DP, kWh 33
Stade Pumps, kW Mixers, kW Presses, kW
D 400 300 100
Eop 400 300 100
D 400 300
D 400 300 100
Total for DEop (DnD), kW 3100
Total per ton of pulp bleached with D(Eop)(DnD), kWh 41
Table 2 – Energy requirement for the fibreline operation
Assessing the carbon footprint is not an easy task since many parameters have to be considered including the
perimeter of the study including countries where the pulp mill could be installed, origin of bleaching chemicals
and type of energy used. Carbon emission from energy production in different countries is integrated in the
simulation and is presented in table 3. The differences are directly connected to the type of infrastructures for
energy production (nuclear, hydraulic, fossil fuels …).
Country US Australia India Spain France Brazil
Kg Eq C/KWh 0.152 0.251 0.257 0.095 0.023 0.022
Kg Eq. CO2/KWh 0.558 0.921 0.943 0.349 0.084 0.081
Table 3 - Kg equivalent Carbon and Carbon dioxide emission per KWh produced (Source ADEME)
In this work, it has been decided to analyse the carbon emission regarding the influence of the following criteria:
- Type of energy required by the pulp mill for the operation of the bleaching process (pumping, mixing)
and for on-site production of bleaching chemicals (Oxygen, Ozone, Chlorine dioxide…). Energy can be
supplied internally (biomass from recovery cycle) or externally from the electrical network.
- Production, transportation and delivery of other bleaching chemicals or precursors (Sodium chlorate,
sodium hydroxide, Peroxide, Sulphuric acids, …)
- 27% 50426
- 48% 45804 - 51%
Carbon emission, tons per year
Energy from Electrical Netw ork
Pulp mill in France
NaClO3 from France
Pulp mill in France
Pulp mill in Brazil
NaClO3 from Spain Pulp mill in Spain
NaClO3 from US
NaClO3 from Spain
Figure 2 – Carbon emission versus type of bleaching sequence, location of the plant, energy used (on-site,
external chemical production, logistics)
The results obtained are summarized in figure 2 and the first point to be observed is that carbon emission related
to internal energy requirement (in blue) for bleaching chemical production on-site increases by about 50% with
ozone generation (energy is completely supplied by electrical network in that case). When the perimeter is
widened and includes influence of external chemical production and transportation, figure 2 shows that the main
contributor of carbon emission is the production of chemical precursors coming from “outside” the plant (mainly
sodium chlorate for chlorine dioxide generation). The value is directly connected to the country where the
sodium chlorate is produced. Significant differences can be observed from one country to the other. For example,
if the pulp mill is using sodium chlorate from a US supplier, carbon emission is more than twice as high as from
France. This highlights the importance of the origin of bleaching chemical precursors entering in the bleaching
process in the balance of global carbon emission. Depending on the location of the pulp mill and the origin of
chemicals, the carbon footprint is decreased between 39 to 51% with ZeDP bleaching sequence.
Another analysis is performed with a pulp mill using biomass from the recovery cycle as internal source of
energy. As showed in figure 3, the reduction of the carbon footprint with ozone-based pulp bleaching (ZeDP) is
in that case always higher than 50%.
- 56% - 56%
Carbon emission, tons per year
40000 - 55%
20000 8155 15489
Bleaching chem icals
Local Energy from biom ass
Pulp mill in France
NaClO3 from France
Pulp mill in France
NaClO3 from Spain Pulp mill in Brazil
Pulp mill in Spain
NaClO3 from US
NaClO3 from Spain
Figure 3 – Carbon emission versus type of bleaching sequence, location of the plant, energy used (on-site,
external chemical production, logistics)
The results confirm that significantly lower carbon footprint can be achieved using technology favouring on-site
production including by-products reuse. For sure, ozone increases internal energy requirement but this energy is
largely compensated by saving in chemical costs and reduction of carbon emission.
It is proven that ozone generation requires limited space and is considered as a “real” on-site chemical
production since the variable cost is local energy when oxygen is also produced on-site. Sustainability of
chemicals supply is a major focus of many pulp and paper companies and, the predictability of bleaching
chemical cost in the short run is always a challenge since the negotiation is based on volume and price for certain
duration. This work shows that, by investing in its internal resource, the pulp mill improves its independence
from the chemical market, better controls his variable cost and finally significantly reduces the carbon footprint.
OZONE IN HIGH YIELD PULPING
Contrary to chemical pulp bleaching where ozone dose is limited to 7 kg per ton of pulp (to avoid cellulose
degradation), the ozone charge on high yield pulps is between 10 to 30 kg ozone per ton of pulp to be
economically viable. As explained before, ozone reacts primarily with extractives and can be valorised in high
yield pulp production as follows:
• Ease fibre separation through pulping processes using refining stages (RMP, TMP, CTMP,…)
• Destroy “in situ” the extractives content (pitch) dissolved in the water phase or still attached on the
surface of the fibre.
Find the optimum between chemical and mechanical treatment
Fundamental and interesting practical studies have been carried out at the end of the 70s about the use of ozone
during high yield pulping. Ozone was initially investigated on thermo-mechanical pulp (TMP) collected from the
main or reject line. The objective was to lower energy requirement and to improve pulp strength properties 
. At that time, the main bottlenecks limiting industrial scale-up were the high ozone cost and availability of
mixing equipment on pilot and full scale to ensure satisfactory ozone application. In any case, these preliminary
works led to a better understanding of the fundamental aspects of the application. With the implementation of
new bleaching practices (ECF and TCF) in the 90s, ozone technology was improved and, the idea of the use of
ozone on high yield pulps reborn.
During pulp refining (RMP, TMP, CTMP), ozone plays the role of a “chemical” refining agent complementing
mechanical energy normally provided by refiners. The softening action of ozone facilitates fibre separation.
Among the factors affecting performance, it can be noticed:
Pulp consistency to limit interferences with carry-over remaining into the water phase
Fibre separation to guarantee homogeneity in the ozone action
pH management resulting from acidification of the pulp during oxidation
Retention time and pulp mixing
Modification of the bleaching chemistry to maintain bleachability (Peroxide or Dithionite based)
During the last decade, efforts were made to clarify the remaining limitations to achieve the full-scale
implementation. The main focal points were the identification of the best introduction point to maximize energy
reduction and, the build-up of the full scale mixing equipment regarding practical aspects (mixing, safety, ..).
Economically, reduction in energy consumption remains a priority, but is associated now, with the
diversification in raw material supply. The objective is to develop the use of wood species normally known to be
problematic for papermaking.
To illustrate the efficiency of ozone in a thermo-mechanical process, figure 4 shows an example of some results
obtained on a large pilot scale using 36’ diameter refiner unit. Pine and Spruce have been treated with ozone and
compared to the same pulps without treatment.
Specific Energy Consumption, MWh/t
0.65 MWh/t saved !
0 100 200 300 400 500 600 700
Canadian Standard Freeness, ml
Figure 4 – Effect of ozone treatment (20 kg ozone applied per ton of pulp) on specific energy consumption.
When energy requirement is compared to achieve the same final freeness, about 0.65 MWh per ton of pulp can
be saved using 20 kg/t ozone on both Spruce or Pine TMP pulps. Considering energy for oxygen and ozone
production, the net saving is about 0.4 MWh per ton of pulp. The value is not yet completely optimized and
higher saving is expected on full scale.
Concerning fibre length and strength properties, a short summary is given in table 4.
Spruce Ozonated Pine Ozonated
Reference Spruce Reference Pine
Energy consumption at 100 CSF 100 76 125 103
Tensile Index 100 130 65 100
Tear Index 100 110 62 85
Fiber Length 100 105 75 90
Table 4 – Relative changes in strength properties with pulps treated or not with 2% ozone
At the fibre level, fibre length increases because ozone softens fibre agglomerates and decreases aggressiveness
of mechanical refining and fibre cutting between the refining plates. From the chemical point of view, the
increase in strength properties can be explained by modifications such as pitch removal increasing fibre
flexibility and creation of carboxyl groups inducing better fibrillation and fibre bonding.
Among the drawbacks described in literature in the use of ozone on thermo-mechanical pulps, it can be
Significant brightness drop after ozone treatment. Partially induced by the formation of new
chromophores (light absorption), the main reason is a drastic change in the scattering coefficient of the
pulp. However, if the ozone dose is limited, intensive work has demonstrated that bleachability can be
totally restored when bleaching conditions are correctly controlled.
Decrease in pulp yield correlated to the ozone dosage. A part of the organic materials (mainly
extractives) are detached from the fibre and transferred to the water phase. COD but also
biodegradability of the effluent increases. If the plant plans to reuse the off-gas (oxygen) for wastewater
treatment, COD removal will be improved and sludge volume should decrease.
Direct pitch elimination
As early described, the second possibility is to directly use ozone to eliminate the pitch content. Raw materials
like Pine can not be fed into a (Pressure) Ground Wood process for paper manufacture. The high extractives
content induces major problems of deposits on the paper machine. All the known techniques (separation,
concentration, dissolution) to get rid of the pitch fraction have not given yet satisfactory results. Ozone treatment
represents a very simple way to eliminate these contaminants and has been directly performed at industrial scale
on low consistency pulp just after grinding. The effect of the ozone treatment has been investigated on the
process water recirculation, pulp bleaching (reductive in that case) and the pitch removal efficiency related to the
nature of chemical compounds present in the pitch fraction. It has been shown that one kg ozone can eliminated
about 0.8-0.9 kg extractives with a higher selectivity for resin acids and triglycerides.
Full scale application has validated the technical and economical feasibility of this process showing that good
paper quality can be produced from 100% Pine as raw material.
Risk and cost remain the main criteria of the decision making process in every project. When these factors are
balanced with long-term vision and guided by principles of environmental responsibility, Green Bleaching
practice represents the best alternative for chemical pulp bleaching. Of course, conventional ECF is still
considered today as the Best Available Technology but this statement is valid to guarantee maximum safety
regarding pulp quality only for softwood pulp production. For the rest, when the discussion is based on
arguments against arguments regarding cost, pulp quality and environment, and is not driven by other interests, it
can be easily demonstrated that ECF is more expensive and has already reached its “ecological terminus”.
The time to implement Green Bleaching practice has come, and our effort to develop the processes that help
minimize water, chemicals usage, energy requirement and carbon emission will continue to finally break the
Concerning the use of ozone in high pulping processes, a promising field of new applications is just opening.
Sustainability in raw material supply and better control of energy requirement are of major interest for many
pulp and paper companies. Improving management of the wood resource and reducing carbon footprint of such
processes, ozone will finally provide its own “limited” contribution to protect biodiversity.
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December 7-10, 2007.
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Conference, Stockholm, Sweden, June 14-16, 2005.
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practices”, 9th Int. Technical Conf. on Pulp Paper and Allied Industry, Paperex 2009, Proceedings, , pp
111-121, Delhi, India, December 4-6, 2009.
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5. Allison R.W., “Effect of ozone on high-temperature thermo-mechanical pulp” Appita Journal, Vol. 32,
No. 4, pp. 279-284 (1979).
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