Ultra High Pressure (10-12 bar) TMP and Its Energy Recovery Options, 2007 International Mechanical Pulping Conference

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Traditionally TMP for graphic papers is produced at low chip preheating pressures of 1-2 bar(g) (or even at atmospheric pressure in the case of PRMP) for 1,5 to 3 minutes and at refining pressures of about 3,0 to 4,0 bar(g) in order to obtain adequate pulp properties at reasonable energy requirements and a recovered steam pressure which is high enough to be used in the dryer section of the paper machine without further compression. Many investigations have shown that the residence time of the chips at higher pressures/temperatures during preheating and refining has to be short and well controlled in order to avoid pulp discoloration and to keep the energy consumption at acceptable levels. Short preheating times, in the order of about 13 seconds and pressures/temperatures of about 5,5 bar(g)/162°C are used in the first refining stage of the RTS process.

Several limitations of existing commercial and pilot installations to operate at minimal retention times in preheating and refining at even high pressures/ temperatures have hindered research work in this area. A recently modified pilot plant allows to produce TMP at pressures up to 12 bar(g) (appr. 192°C ) and at very short retention times. RTS-TMP was produced and ultra high pressure TMP at the practically shortest possible retention time. The results confirm the importance of short and well controlled exposure of chips respectively fibers to high temperatures in order to obtain proper fracture zones in the wood matrix and sufficient fines generation at comparable refining energy levels and to avoid excessive pulp discoloration. Options are presented how the recovered high pressure steam could be used for power generation, for efficient drying applications, for e.g. effluent evaporation prior to paper drying or as motive steam to boost low pressure steam to usable levels in order to improve the overall energy balance of TMP production.