A FIBRELINE DESIGNED FOR BAMBOO PULPING

Stig Andtbacka, Kvaerner Pulping AB

In April 2005 Kvaerner Pulping AB signed the contract to provide a cooking plant to Chitianhua in China. The fibreline will operate on bamboo, and the capacity for the continuous digester is 850 ADT/d. The cooking process is COMPACT COOKING™, which is most suitable for bamboo.

Bamboo as raw material for pulping

Bamboo is botanically classified as a grass and for pulping purposes it is considered as an annual plant. Bamboo pulp is from fibre length and strength point of view very similar to softwood (figures 1 and 2).

Figure 1

Figure 2

The fibre length for bamboo is much longer than for hardwood and this also results in a stronger pulp. However, pulping conditions are very similar to eucalyptus (figure 3). This means that bamboo can be pulped together with hardwood. This mixed cooking also gives a stronger pulp than if the hardwood is pulped separately. Bamboo can also be cooked together with acacia.

Figure 3

An important characteristic of bamboo in comparison with other annual plants is that proper chips can be produced, which makes it possible to pulp bamboo in a continuous digester (figure 4). Length and thickness distribution is, for this sample anyhow, very similar to normal wood chips. The chip weight corresponds to the chip weight for eucalyptus, which also is important if mixed cooking is done.

Figure 4

Bamboo is a grass and one consequence of this is that the silica content is much higher than for wood plants  (figure 5). The presence of silica is the way that the plant is protecting itself from the environment. Trees use bark for protection while annual plants have high silica content, which acts as a "skin" for the plant.

Figure 5

When a bamboo mill is designed the high silica content must be taken into consideration. The high silica content gives scaling problems in the cooking plant and in black liquor evaporation and also difficulties in lime reburning. The silica balance in figure 6 shows that if the silica content in bamboo is 2 %, approximately 65 % of the lime must be purged to maintain an equilibrium SiO2 content of 650 mg/l in the white liquor.

Figure 6

For minimizing scaling problems in the cooking plant a continuous digester with few or no circulations for heating is recommended. COMPACT COOKING™ is in this case a very suitable solution. In figure 7 some characteristics for bamboo pulping are summarized.

Figure 7

A reference list for continuous digesters operating on bamboo is shown in figure 8. The Chitianhua digester shown in the list is designed for COMPACT COOKING™.

Figure 8

Compact Cooking™

The first Kamyr digester was started in 1949. However it took eight years before a real breakthrough for continuous cooking was made. This came when the so called "cold blow" system was introduced. Different development steps are shown in figure 9.

Figure 9

Today Kvaerner Pulping is promoting COMPACT COOKING™, for which the process significants, mill results and benefits have been shown and can be summarized as follows:

• Liquor/wood ratio completely flexible between impregnation and cooking
• Optimal chemical profile
• Low cooking temperature and low steam consumption
• Strong pulp
• Good bleachability
• Easy to operate and high availability
• Reduced investment and operational costs
• Possibilities to tailor-make different pulp qualities

The total world production of chemical pulp is today approx. 140 million tons/year of which 65-70 % is produced in continuous digesters (figure 10).

Figure 10

Results from lab study on bamboo

Kvaerner Pulping has done several comprehensive lab studies on bamboo. Figure 11 shows some cooking results. Bamboo pulp is rather easy to bleach. It will also be noticed that the hexaneuronic acid content is on the same level as for softwood pulp. The HexA concentration is 0.01 - 0.015 mm ol HexA/g BD pulp. In an ECF sequence a conventional D0 stage can be used and neither an A nor a DualD™ stage is necessary.

Figure 11

Bamboo pulp with an ingoing kappa number of 8 after oxygen delignification can be bleached to full brightness, 90 % ISO, with the three-stage sequence D0(EOP)D. The total chlorine consumption is around 30 kg/ADT. Peroxide charge in the pressurized EOP stage is 4 kg/ADT, figure 12. If a lower brightness, 85-87 % ISO, can be accepted the two-stage sequence (DQ)(PO) can be used, figure 13. The peroxide consumption is 20-25 kg/ADT and active chlorine charge 12 kg/ADT, figure 14.

Figure 12

Figure 13

Figure 14

Results for the four-stage ECF sequence D0(EOP)-D-PO is shown in figure 15. For bleaching to brightness 90 % ISO, total active chlorine charge is 24 kg/ADT and H₂O₂ consumption
10-12 kg/ADT. Bleaching to brightness 89-90 % ISO is also possible with the four-stage
TCF sequence Q(EOP)Q(PO). Total peroxide consumption for bleaching to 89 % ISO is
30-35 kg/ADT, figure 16. With the short TCF sequence Q(PO) brightness 80 % ISO can be obtained, figure 17. The PO stage is in all shown sequences pressurized (PREPOX™) with a retention time of two hours and a temperature of 105°C. Peroxide consumption for bleaching to brightness 80 % ISO is approx. 30 kg/ADT. These were only some examples of modern bleaching sequences for bamboo. Other alternatives are of course also possible.

Figure 15

Figure 16

Figure 17

Kvaerner Pulping proposal for a bamboo fibreline

For cooking we recommend COMPACT COOKING™ of the so called second generation (G2). This digester has very few liquor circulations for heating and is therefore less
sensitive for scaling than other continuous digesters. Cooking is performed at a low cooking temperature, around 140-142°C. The pulp produced has good strength properties, figure 18, and good bleachability. The whole fibreline is shown in figure 19. In the digester blow line we recommend installing a pressure diffuser. With COMPACT COOKING™ the total reject content will be very low, 0.5 - 1 %. We therefore recommend taking out both knots and fine rejects from the screen room. After screening we install a COMPACT PRESS™ as final wash stage before oxygen delignification.

Figure 18

Figure 19

For oxygen delignification we recommend the two-stage DUALOX™ system. We have here a short first stage, approx. 6 minutes, and a second stage with 60 minutes retention. Between the stages a second MC-pump is installed. With this we can increase the pressure in stage two to up to 6 bar on the reactor top. This is important from a kappa reduction point of view . For post-washing after oxygen delignification we recommend two Compact Press™ in series. Kappa after cooking is 16-18 and after oxygen delignification around 8.

For bleaching there are several possible sequences as earlier discussed. This is depending on brightness demand, environmental issues, etc. The sequence Q(PO)D(PO) is an alternative if we go for light ECF. In this case the storage tower between the two post-oxygen wash presses also acts as a Q stage. We can for this bleaching sequence expect the following consumption figures for bleaching to brightness 89 % ISO:

• ClO₂ as active chlorine 12 kg/ADT
• H₂O₂ (charge) 18 kg/ADT
• NaOH 32 kg/ADT
• H₂SO₄ 8 kg/ADT
• Water including white water 15-20 m3/ADT
• Steam ~ 500 kg/ADT
• Total effluent volume ~ 10 m3/ADT
• COD in effluent 24-26 kg/ADT

Another possible bleaching sequence is D0(EOP)D1(PO), figure 20, which is earlier discussed. With this sequence the peroxide charge can be reduced to approx. 12 kg/ADT, but the ClO2 charge has to be increased accordingly to approx. 24 kg act. Cl/ADT. For this sequence we also need totally 7 wash presses for the fibreline instead of 6 for the light ECF sequence.

Figure 20

Finally:

The continuously increasing demand for virgin pulp has made less conventional materials for pulping interesting.

As shown in this presentation bamboo fibre is such a material.

Bamboo is in many aspects an excellent raw material for pulp. Some special aspects must however be taken into consideration when designing a bamboo pulp mill due to the high SiO₂ content in the raw material.

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