Company Unveils Major Breakthrough in Vertical Continuous Digester for Straw Pulping

By Hou-min Chang

 

Shandong Tralin Paper Co. of China recently invited nine experts to witness the unveiling of novel, up-right continuous digesters for the production of 100% wheat straw pulp. The digesters are fully automated from raw material preparation to blow tank. This is, without any doubt, a major breakthrough for the Chinese company, as previously, the continuous digester of a type similar to the Kamyr digester was regarded as infeasible for straw pulping. 

 

The innovation and persistence in research efforts of the mill allowed them to develop the first fully automated continuous digester for non-wood pulp production in the world. There are three breakthrough innovations that distinguish the Tralin continuous digester from the Kamyr digester.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The first innovation is the raw material preparation system. The mill uses a combination of the traditional straw cutting machine and Tralin’s designed rotary drum hammer mill to prepare cleaned straw for pulping. The system is fully automated and allows the mill to raise the straw acceptance rate from 75% to 92%. The second innovation is the feeding system to the digester. Unlike with wood chips, high pressure feeders cannot be used for wheat straw because of the difficulty in separating the liquor and straw at the top separator. The mill designed a dual screw feeding system at the top of the digester to overcome the problem. The third innovation is the screen system in the digester that allows liquor to be withdrawn from the digester for indirect heating and for delivery to the flash tank. The withdrawal of liquor from the digester is relatively easy for wood chips but is considered infeasible for straw that is prone to plugging the screen.

 

For several years, Tralin has been operating indirect heating batch digesters with the Rapid Displacement Heating (RDH) system for pulping wheat straw, which is the only one of its kind in the world. The core of this operating system is its innovative screen design, which allows Tralin to apply the screen to the continuous digester.

 

Other than the three innovations, the Tralin digester operates like a Kamyr digester. The straw is sent from the raw material preparation area by a belt conveyer to the top of the digester 30 meters above the ground, where the screw feeder feeds the straw to the digester. The straw then goes through the impregnation zone, the heating zone, and cooking zone, at the end of which liquor is withdrawn to the hot liquor tank and replaced by the washing liquor. The pulp continues to travel down the digester in the high heat washing zone and is cold blown to the blow tank at the bottom of the digester.

 

The Tralin continuous digesters started up in June, 2014 and exceeded the designed production capacity in September. The mill is now operating two continuous digesters, each producing more than 100,000 metric tons annually. The mill is in the process of installing four more lines.

 

The mill also is operating multistage oxygen delignification, the only mill in the world doing so for non-wood pulping. This technology allows the mill to produce both unbleached and chlorine-free bleached pulps. In addition, The Tralin Paper Co. is building two mills in Northeast China: one in Heilongjinag Province based on rice straw and one in Jilin Province based on corn stover. 

 

The nine invited experts unanimously agreed that this is a major breakthrough in non-wood pulping technology and could greatly impact not only China but other parts of the world where forest resources are limited.   

Hou-min Chang, Prof. Emeritus, North Carolina State University, Raleigh, N.C., USA, can be reached by email. Professor Chang was one of the nine experts (and the only one from a foreign country) invited to the unveiling event on Oct. 22-23, 2014.