USE OF OZONE IN CHEMICAL AND HIGH YIELD PULPING PROCESSES“Latest innovations maximizing efficiency and environmental performance”

Author

Jean-Christophe HostachyDirector – Key Account Pulp & Paper

ITT Water and WastewaterBoschstr. 4

D-32051 HerfordGermany

jean-christophe.hostachy@itt.com

ABSTRACT

As one of the strongest oxidant available in the pulp and paper industry, ozone can be used in both chemical and mechanical pulping.

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.

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INTRODUCTION

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 [1]. 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 [2]. 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 [2]. 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 …) [3]. 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 tested:

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

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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.

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 18O3 6 0NaOH 12 9O2 0 3H2O2 5 3MgSO4 2 0H2SO4 18 12SO2 1 2

3

ambient air

Pulp out

Pulp in

Z-Stage

TOWL

Oxygen Delignif.

OxygenOxygenApplicationsApplications

Gas scrubber

Ozone DestructionSystem

Compressor

OzoneGenerator

OxygenPlant

LOX

tank

OxygenBack-up

34 t/day O2 30 t/day O2 +4 t/day O3

30 t/day 02available

17.5 t/day 02

12.5 t/day 02

ambient air

Pulp out

Pulp in

Z-Stage

TOWL

Oxygen Delignif.

OxygenOxygenApplicationsApplications

Gas scrubber

Ozone DestructionSystem

Compressor

OzoneGenerator

OxygenPlant

LOX

tank

LOX

tank

OxygenBack-up

34 t/day O2 30 t/day O2 +4 t/day O3

30 t/day 02available

17.5 t/day 02

12.5 t/day 02

Table 1 – Bleaching chemicals consumption

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 BrazilKg Eq C/KWh 0.152 0.251 0.257 0.095 0.023 0.022Kg 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, …)

Figure 2 – Carbon emission versus type of bleaching sequence, location of the plant, energy used (on-site, external chemical production, logistics)

Stage Pumps, kW Mixers, kW HC ozone, kW Presses, kWZ 400   75 200e   300    D 400 300   100P 400 300    

Total for (Ze)(DP), kW 2475Total per ton of pulp bleached with (Ze)DP, kWh 33Stade Pumps, kW Mixers, kW Presses, kW

D 400 300   100Eop 400 300   100D 400 300  D 400 300   100

Total for DEop (DnD), kW  3100Total per ton of pulp bleached with D(Eop)(DnD), kWh 41

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DEopDnD

ZeDP

DEopDnD

ZeDP

DEopDnD

ZeDP

DEopDnD

ZeDP

Energy from Electrical Network

Transportation

Bleaching chemicals

Total

20516

12470

37959

19804

45804

33257

50426

24910

0

10000

20000

30000

40000

50000

60000

Car

bo

n e

mis

sio

n, t

on

s p

er y

ear

Pulp mill in FranceNaClO3 from France

Pulp mill in FranceNaClO3 from Spain Pulp mill in Spain

NaClO3 from Spain

Pulp mill in BrazilNaClO3 from US

- 27%- 51%- 48%

- 39%

DEopDnD

ZeDP

DEopDnD

ZeDP

DEopDnD

ZeDP

DEopDnD

ZeDP

Energy from Electrical Network

Transportation

Bleaching chemicals

Total

20516

12470

37959

19804

45804

33257

50426

24910

0

10000

20000

30000

40000

50000

60000

Car

bo

n e

mis

sio

n, t

on

s p

er y

ear

Pulp mill in FranceNaClO3 from France

Pulp mill in FranceNaClO3 from Spain Pulp mill in Spain

NaClO3 from Spain

Pulp mill in BrazilNaClO3 from US

- 27%- 51%- 48%

- 39%

- 27%- 51%- 48%

- 39%

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%.

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.

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DEopDnDZeD

PDEopDnD

ZeDP

DEopDnD

ZeDP

DEopDnD

ZeDP

Local Energy from biomass

Transportation

Bleaching chemicalsTotal

18000

8155

35443

15489

35443

15489

48028

20798

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

Car

bo

n e

mis

sio

n, t

on

s p

er y

ear

Pulp mill in FranceNaClO3 from France

Pulp mill in FranceNaClO3 from Spain Pulp mill in Spain

NaClO3 from Spain

Pulp mill in BrazilNaClO3 from US

- 56%

- 57%

- 56%

- 55%

DEopDnDZeD

PDEopDnD

ZeDP

DEopDnD

ZeDP

DEopDnD

ZeDP

Local Energy from biomass

Transportation

Bleaching chemicalsTotal

18000

8155

35443

15489

35443

15489

48028

20798

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

Car

bo

n e

mis

sio

n, t

on

s p

er y

ear

Pulp mill in FranceNaClO3 from France

Pulp mill in FranceNaClO3 from Spain Pulp mill in Spain

NaClO3 from Spain

Pulp mill in BrazilNaClO3 from US

- 56%

- 57%

- 56%

- 55%

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 [4][5][6][7]. 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:

Fibre coarseness Pulp consistency to limit interferences with carry-over remaining into the water phase Fibre separation to guarantee homogeneity in the ozone action Ozone dosage 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.

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.

6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 100 200 300 400 500 600 700

Canadian Standard Freeness, ml

Spec

ific

En

ergy

Con

sum

pti

on, M

Wh

/t

Pine referenceOzonated PineSpruce referenceOzonated Spruce

0.65 MWh/t saved !

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 100 200 300 400 500 600 700

Canadian Standard Freeness, ml

Spec

ific

En

ergy

Con

sum

pti

on, M

Wh

/t

Pine referenceOzonated PineSpruce referenceOzonated Spruce

Pine referenceOzonated PineSpruce referenceOzonated Spruce

0.65 MWh/t saved !

Spruce Reference

Ozonated Spruce

Pine Reference

Ozonated Pine

Energy consumption at 100 CSF 100 76 125 103Tensile Index 100 130 65 100Tear Index 100 110 62 85Fiber 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 mentioned:

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.

CONCLUSION

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 existing “status-quo”.

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

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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.

REFERENCES

1. Vehmaa J, Pikka O. ”Bleaching of hardwood kraft pulps with ozone” Paperex Conference, India, Delhi December 7-10, 2007.

2. Winnerstrom M., Carre G. “Ozone bleaching: An established technology” Int. Pulp Bleaching Conference, Stockholm, Sweden, June 14-16, 2005.

3. Hostachy J.C, “Bagasse pulp bleaching with ozone – It’s time to implement Green Bleaching practices”, 9th Int. Technical Conf. on Pulp Paper and Allied Industry, Paperex 2009, Proceedings, , pp 111-121, Delhi, India, December 4-6, 2009.

4. Allison R.W., “Low energy pulping through ozone modification” Appita Journal, Vol. 34, No. 3, pp. 197-204 (1980).

5. Allison R.W., “Effect of ozone on high-temperature thermo-mechanical pulp” Appita Journal, Vol. 32, No. 4, pp. 279-284 (1979).

6. Soteland N., Loras V., “The effect of ozone on mechanical pulps”, Norsk Skogind. Vol. 28, No. 6, pp. 165-169 (1974).

7. Lindholm C.-A, “Ozone treatment of mechanical pulps”, Paperi ja Puu, Special No. 4a, pp. 217-231 (1977).

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