AGRE-0044 Upgrading Straw into Pulp, Paper and Polymeric Materials

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Biocomposites/Boards : Biological Conversion : Drying/Pretreatment : ECLAIR Cluster II - Lignocellulose : Fibre : Paints/Coatings/Plastics : Paper/Pulp : Process Engineering : Pulping : Straw

SUMMARY

Wheat straw, surplus of which is available in many EU Member States, has been used in the past for papermaking. However, environmental and quality issues have generally prevented its use. As a fibre source, straw still has significant potential, if suitable methods can be developed. This was the objective of this project which investigated a novel pulping process, together with the equipment required to provide suitable chopped straw as feed, in order to produce unbleached straw pulp for use in packaging grade papers. The project also investigated the industrial use of straw for the manufacture of composite materials for which there is an increasing demand. In addition, it looked at the environmental impacts of straw processing by this new process and investigated effluent treatment. Overall, it provided the technical and economic information required in order to make a decision to build full-scale straw processing plants.

INTRODUCTION

Wheat straw is regarded by many as a waste product and as such is readily available in many EU countries. It has been used in the past for papermaking but environmental and quality issues has generally stopped its use. As a fibre source, straw still has significant potential, if suitable methods can be developed. This was the objective of this project which investigated a novel pulping process, together with the equipment required to provide suitable chopped straw as feed, in order to produce unbleached straw pulp for use in packaging grade papers. The project also investigated the industrial use of straw for the manufacture of composite materials for which there is an increasing demand. As the main drawback to exploitation of lignocellulosics is the multicomponent nature of these materials and their low compatibility with synthetic matrices, alternative process technologies were investigated. The project also looked at the environmental impacts of straw processing by this new process and investigated effluent treatment. Overall it provided the technical and economic information required in order to make a decision to build full-scale straw processing plants.

ACTIVITIES

The project had 5 inter-linked sub-groups which considered the following aspects:

1. straw varieties, preparation and storage;
2. straw pulping using the Bivis extruder;
3. straw pulping using the steam explosion process;
4. polymers and biocomposites from straw and
5. treatment of straw pulping effluents.

RESULTS

Straws: Some varieties appeared to be more suitable for papermaking, based on the assumption that the best straws should have a high content of cellulose and a low content of silica, water extractables, alcohol/benzene extractables and lignin. Among the wheat varieties investigated, Soissons appeared the best whilst triticale and barley straws also gave good results. The average silica content was around 3%; a few samples as low as 1% were found. The average lignin and cellulose contents were 16% and 35% respectively. Hemicellulose was between 24% and 28%.

Preparation: The straw preparation system included a bale breaking device which consisted of two horizontal shredder rotors into which the bales were fed. The loosened straw was then held in a moving floor reservoir before being fed to an inclined elevator and metering rake, followed by a rotary slicing device. Besides requiring less power, the slicing equipment was shown to be quieter, produce less dust and have lower maintenance requirements than conventional chopping machines. The median particle length of the slicer output was 30 mm. Aspiration of leaf and leaf sheath from sliced straw was found to remove 60% of these fractions and subsequent aspiration of the hammer milled residue gave 70% removal of nodes with a 5% loss of the internode fraction.

Pulping: Steam explosion was investigated as a means of manufacturing a high yield straw pulp. The operating conditions were defined at pilot scale in a batch system as
20 bar (214°C) for 3 min in the presence of 5% sodium sulphite on straw, followed by an instantaneous pressure release during which the cooked straw particles exploded. The ratio liquor to straw was 1:1. The resulting pulp had the following characteristics, measured at 66°SR on handsheets of 120 g/m2: breaking length 4250 m, burst index 2.3 kPa m2/g, tear index 4.7 mNm2/g, Concora Medium Test 192 N, Ring Crush Test 220 N. These values were slightly higher than those of the pulp from the reference mill. However the pulp yield was very low level (61%). Several attempts to improve the yield failed. No more work was done on this process and the more promising Bivis extrusion process was focused on. This produced a high yield of unbleached pulp. Following laboratory trials, optimum operating conditions were determined (use of a solution of 9% sodium sulphite and 3% sodium carbonate at about 140°C) giving a pulping yield around 86%. Under these conditions the mechanical properties of the pulp were as follows: breaking length 4000 m, burst index 3.0 kPa m²/g, tear index 4.0 mNm²/g, Concora Medium Test 190 N, Ring Crush Test 150 N. These values were higher than those of pulps produced from waste paper currently used in the manufacture of fluting medium papers. It was found that an increase from 100 kg/h to 300 kg/h in the feed rate improved pulp quality. The average fibre length (measured after Kraft cooking) was around 1 mm.

Improving Pulp: Treating the straw pulp after extruder pulping but before papermaking to optimise the stiffness characteristics was also necessary. This was because the pulp from the extruder contains a significant amount of shive. Fractionating the shive away from the pulped fibre is an attractive alternative, since specific refining could be directed to each of the pulp fractions using a defibering action to break up the shive.

Effluent Treatment: Pulp production form straw generates an effluent containing a significant pollution load. Several technologies of effluent treatment were investigated. This included ultrafiltration in a pilot plant which was operated for about 180 days without any major problem. Unfortunately, investment and operating costs are currently too high to justify the implementation of this technique. Biotreatments for decolorisation of straw-pulping effluents were designed after screening 84 fungal strains in several selection steps, 4 fungi (T. versicolor, G. australe, C. gallica and P. gigantea) were chosen for effluent decolorisation under agitated culture conditions and two ( Paecilomyces sp and P. chrysosporium) under stationary conditions. The best decolorisation, (close to 90% under stationary conditions) was attained at 28°C and were comparable with the best results reported during fungal treatment of other pulp and paper effluents.

EXPLOITATION

After a series of pilot plant paper machine trials the decision was made to design, build and commission an industrial scale pilot plant pulping unit capable of producing between 2 and 3 tonnes of straw pulp per hour based on a Clextral KRO 200 Bivis extruder. Straw pulp from this pilot plant was used for a series of trials on industrial paper machines.

PARTICIPANTS

Industrías Celulosa Aragonesa (Spain), Stazione Sperimentale per la Cellulosa Carta e Fibre Tessili Vegetali ed Artificiali (Italy), AFRC Institute of Engineering Research (UK), CNRS (France),CSIC (Spain), GIE Grainval (France), ASCAF (France), Universidad de Alcala de Henares (Spain), Università di Padova (Italy).