(219d) Three Concepts for Intensification of Nordic Kraft Pulp Processes

Background

The global change of paper consumption is moving the pulp production from northern countries, such as Scandinavia, US and Canada, towards southern hemisphere, to Uruguay, Brazil, Indonesia and China. Thus there is increasing interest for developing competitive future pulp mill concepts applicable in Nordic countries by intensification the current production processes. Proposed concepts of this study are (i) producing sulphur less or lean products, (ii) maximising energy production in pulp mill, and (iii) developing high yield pulps. Typical modern Nordic kraft mill is used as reference for these concepts.

Evaluation of concepts is conducted by literature review, by validating the pulp quality in fibre line with experimental trials (both cooking and bleaching), and by evaluating techno-economic feasibility of concepts by process simulation. In addition, rough investment estimates are given for new concepts.

Sulphur lean concept

The aim of the Sulphur lean concept is to produce birch pulp with low (sulphidity 10 %) or minimal (sulphidity close to 0 %) sulphur content. The main benefit of sulphur lean or free chemical pulping is the possibility to isolate sulphur-free side-products, such as lignin, to be used directly as a fuel or processed further for other uses. Strength is that the pulping process itself is virtually odourless

Pulp quality of sulphur lean concepts is partly better (WRV, bulk, stiffness) and partly worse (wet zero-span tensile strength, fracture toughness and tear strength) than reference pulp. Pulp itself is more difficult to bleach than kraft pulp which leads to higher bleaching chemical consumption and cost. The main disadvantage with concept is the significantly higher demand for causticizing capacity as all hydroxide must “pass through the lime cycle”. The attractiveness of proposed concept would be increased further by a breakthrough in auto-causticizing technologies.

Soda black liquors have been known to be challenging to evaporate and the smelt properties are somewhat different than in the kraft process. Black liquor gasification of soda liquors could increase the energy production in chemical pulping or open new possibilities for by-product generation. The gasification of black liquor is evaluated for this sulphur lean black liquor concept.

Energy mill concept

In the energy mill concept a scenario is evaluated where the energy production of the pulp mill is given a greater importance compared to today’s situation. As the energy price compared to pulp price is getting higher, there is increased interest in low kappa (<10) pulping. The heating value of carbohydrates (cellulose) is lower compared to lignin, and thus low kappa pulping with yield preserving methods are preferable. AQ and polysulphide are utilized as cooking aids. Cooking to low kappa enables omitting the oxygen stage and possible also chorine stages. (Possible savings in investment and energy costs). Both elemental (ECF) and total (TCF) chlorine free bleaching options were evaluated.

This study also showed that the pulp produced from the Energy mill ECF concept is of reasonable quality. In fact some properties are equally good or even better than the properties of the reference pulp. For example the tensile properties of the ECF pulps were better than those of the reference kraft. Also the very low extractive content in the pulp makes it an interesting pulp for special purposes.

This study also showed that the production costs of the Energy mill TCF concept are too high. Regarding the Energy mill ECF concept the results showed that cooking to kappa 10 and keeping the yield higher than 40 % is possible. If the electricity price would be the same as for electricity produced by wind, the energy mill concept would be economically very close to the reference kraft mill. Including the higher thermal energy output from the Energy mill concept, which in winter months would give increased revenue as well if CHP is installed.

High yield pulp concept

As wood is the major cost component of pulp in Nordic countries, the high yield pulp concepts is aiming to a significant increase in pulp yield. Two-stage pulping concept includes (i) high yield and high kappa cooking for stabilising hemicelluloses in pulp and (ii) novel oxygen delignification aiming to remove the lignin and reduce kappa while obtaining high yield. The proposed concept might include additional processes for recovering dissolved hemicelluloses from black liquor or for mechanically fibrillating pulp. Bleaching properties are evaluated as well.

Summary

Process intensification of Nordic kraft pulp mill can be achieved by applying proposed concepts. Sulphur lean concept enables production of value-added sulphurless by-products from pulp mill and efficiency of pulp mill if gasification of black liquor is applied. An energy mill concept simplifies process by removing oxygen delignification step and by increasing the energy utilisation of pulp mill, particularly if combined heat and power production (CHP) is applied. High yield pulp concepts utilise the most valuable component wood more efficiently towards pulp products.