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Tralin unveils world's first fully automated continuous digester for non-wood pulp production

(Editor's note: The following description of the new continuous digester for straw pulping at China's Tralin Paper Company, was submitted by Hou-min Chang, Prof. Emeritus, North Carolina State University, who was one of nine experts (and the only one from a foreign country), invited to the unveiling event on October 22-23, 2014.)


By Hou-min Chang

Shandong Tralin Paper Company of China invited nine experts to witness the October unveiling of novel up-right continuous digesters for the production of 100 percent wheat straw pulp.

The digesters are fully automated, from raw material preparation to blow tank. This is, without a doubt, a major breakthrough for the Chinese company as previously the continuous digester, of a type similar to the Kamyr digester, was not regarded as feasible for straw pulping.

CDP Report

The major driver of global deforestation comes from the demand for four critical agricultural commodities: cattle products, palm oil, timber products, and soy. These commodities feature in the supply chains of countless companies across sectors, from food to fuel. The resulting deforestation is itself a key driver of climate change, accounting for approximately 10-15 percent of the world's greenhouse gas emissions—equivalent to the emissions from the entire transport sector.

The mill's innovation and persistent research efforts 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 prepares cleaned straw for pulping using a combination of a traditional straw cutting machine and Tralin-designed rotary drum hammer mill. The system is fully automated and allows the mill to raise the straw acceptance rate from 75 percent to 92 percent.

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. To overcome the problem, the mill designed a dual screw feeding system at the top of the digester.

CDP Report

The third innovation is the screen system in the digester that allows liquor to be withdrawn from the digester for indirect heating and for delivery to the flash tank.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 not considered feasible 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 these three innovations, the Tralin digester operates like a Kamyr digester. Straw is sent by belt conveyor from the raw material preparation area to the top of the digester, 30 meters high, where the screw feeder feeds it to the digester. The straw then goes through the impregnation, heating and cooking zones, 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 by September they had exceeded the designed production capacity. The mill is now operating two continuous digesters, each producing over 100,000 metric tons annually. The mill is in the process of installing four more lines.

The mill is also operating multistage oxygen delignification, the only mill in the world doing so for non-wood pulping. The technology allows the mill to produce both unbleached and chlorine-free bleached pulps. In addition, The Tralin Paper Company 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, can be reached at: [email protected]

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