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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).
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.
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.
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.
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.
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.
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.
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.
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:
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).
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.
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.
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
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
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:
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.
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.