Study On The Application Of Xylanase In Fast-growing Poplar Pulping

The fiber morphology observation and chemical composition analysis of both fast-growing poplar trunk wood and brushwood were investigated in this thesis as well as the fibers characteristics comparison of these two kinds of wood. Different xylanases were used and the effectiveness was tested and studied during the fast-growing poplar pulping process including enzyme-boosting refining, enzyme-boosting beating, enzymatic modification and enzymatic assistant bleaching. This thesis optimized the treatment conditions, and analyzed the changes of fiber morphology, chemical structure and lignin molecular weight during the xylanase treatment as well.The results showed that the fiber length and fiber coarseness of poplar trunk wood are better than that of branch wood as well as the ratio of fiber length to width (L/W). The holocellulose content of poplar trunk wood is higher than that of brushwood, however, the ash contenct, 1% NaOH extracted substances content, benzene-alcohol extracted substances extent and acid-insoluble lignin content are lower than that of brushwood. The fiber characteristics of brushwood have reached the pulping requirement, which indicates that this fast-growing poplar brushwood can be developed as material for pulp and paper making industry.Xylanase pretreatment before the second refining stage during Italian black poplar branch wood P-RC APMP process can reduce the refining energy consumption by 3 % ~ 8 % as well as 5%~9% for following beating energy consumption. In addition, the resulting pulp brightness can be improved by 1.0%ISO~2.3%ISO with minor change of pulp strength properties. Xylanase pretreatment before the third refining stage can reduce the refining energy consumption by 4 % ~ 8 % as well as 5%~13% for following beating energy consumption. The resulting pulp brightness is improved by 1.0%ISO~2.3%ISO with minor improvement of pulp strength properties. The application of xylanase before the second refining stage is better than that before the third refining stage. Among different xylanases used in this study, the AU-PE89 is best, and the optimal dosage of AU-PE89 is 20IU·g-1 and 30IU·g-1 respectively.The AU-PE89 xylanase pretreatment before the third refining stage during the mixed fast-growing poplar P-RC APMP process can reduce the refining energy consumption by 6%~10% and lower the following beating energy consumption by 4%~8% with the improvement of 1.7%ISO~1.9%ISO resulting brightness, 2%~18% breaking length, 8%~11% tear index, 3%~27% burst index and 50% folding endurance. The optimal dosage of AU-PE89 is 20IU·g-1 and 30IU·g-1.During the beating of mixed fast-growing poplar APMP and P-RC APMP, the 51024 xylanase pretreatment for APMP pulp can reduce 10%~18% beating energy consumption with the improvement of resulting brightness by 2.2% ISO, breaking length by 10%, tear index by 10%, burst index by 21% and folding endurance by 50%. The xylanase pretreatment for P-RC APMP pulp can decrease 10%~17% beating energy consumption with the improvement of 1.9%ISO brightness, 9% breaking length, 12% bursting index and 75% folding endurance.During the beating of Italian black poplar branch wood P-RC APMP pulp, the 51024 xylanase pretreatment can obviously improve the pulp beatability, and the beating energy consumption can be reduced by 22%~40% as well. The brightness, breaking length, tear index, burst index and folding endurance of pretreated pulp can be improved by 1.7%ISO, 6%, 7%, 4%, and 67% respectively. In addition, the treated pulp has lower PC number.The AU-PE89 xylanase pretreatment for fast-growing poplar APMP pulp can obviously improve the drainage capability. The beating degree of pretreated pulp decreased by 3.0°SR~12.0°SR and 0.5°SR~6°SR respectively for the beated pulp and non-beated pulp. The tear index of pretreated pulp is improved by 3%~16%, and the PC number decreased by 4%~15% with minor change of brightness, breaking length and burst index. Compared with the unpretreated pulp, the pretreated pulp has 1.64% lower pentosan content, and higher reversely cellulose content and Klason lignin content.The xylanase AU-PE89 treatment for fast-growing poplar APMP pulp can obviously improve the drainage capability and retention capability. The optimal treatment conditions for improving the pulp drainage are pH 7.0, AU-PE89 1.5IU·g-1, 45℃, 30min and 0.5% pulp consistency. Based on the optimal treatment conditions, the beating degree of treated pulp decreased by 4.5oSR, the dynamic drainage time reduced by 2.8s, Zeta potential (minus) reduced by 0.6mV, the retention of fines increased by 61.3%, the effect of AKD sizing promoted a little, and the breaking length, bursting index, and the tearing index increased by 6.2%, 9.5%, and 4.0% respectively.The bleach boosting effects of AU-PE89 xylanases are better than that of AU xylanases. For DP and HP bleaching sequences,the AU-PE89 xylanase pretreated pulp can has 1.1%ISO~3.1%ISO higher brightness than that of unpretreated pulp. At the condition of reaching similar brightness, the AU-PE89 xylanase pretreated pulp can can save 14%~33% bleaching chemicals.For fast-growing poplar Kraft pulp, the AU-PE89 can give better assistant bleaching effectiveness. The resulting pulp brightness can be improved by 1.8%ISO~2.7%ISO at the optimal treatment condition of pH 7.0. For fast-growing poplar NaOH-AQ pulp, both (OO)DXP and (OO)XDED, xylanase-including bleaching sequences, can make the resulting pulp brightness higher than 85% ISO. Especially the (OO)XDED bleaching sequence can give the resulting pulp 88.8% ISO brightness. For the TCF bleaching, the (OO)QXP can give the pulp higher than 80%ISO brightness.The fiber quality analysis showed that, compared with the untreated pulp, both the xylanase-treated poplar APMP and P-RC APMP have longer fiber length, higher fiber kink index, smaller fiber width, and lower fines content, which benefits the improvement of pulp strength properties.The SEM observation indicated that the xylanase-untreated pulp has stiffer fiber, less fines and lower fibrillation extent. In addition, the xylanase-treated fibers are softer and has higher fibrillation extent compared with the untreated fiber, and there are many hollows on the fiber surface. The xylanase treatment looses the fiber cell wall and softens the fiber so as to be easy for refining and beating.AFM observation showed that xylanase treatment made the microfibril texture on fiber surface more clearly, the surface of fiber became more coarse, and the smoothness of fiber surface decreased. Part of xylan on fiber surface was degraded by xylanase during the treatment and more microfibrils were exposed. The fibers treated by xylanase are easier to be splitted and broomed, which was responsible for the decrease of refining/beating energy consumption and the increase of pulp brightness and physical strength.XRD analysis indicated that degree of crystallinity of fiber increased after the xylanase treatment. In addition, the IR spectrum showed that the hydroxyl group content increased, however, the xylan content decreased. Part of lignin dissolved out from the fibers so that the pulp brightness increased and the pulp treated by xylanase is easier to be swollen.