| The PGW and PGW-S Processes for Lower Electrical Energy Demands | Finland | 1991 | Full scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 35
Background
This case study was submitted on the part of the Working Group on cleaner Production in Pulp and Paper Industries in the framework of the UNEP IE/PAC Cleaner Production Program with the support of the Technical Research Center of Finland's Non-Waste Technology Research Unit.
The pressurised groundwood (PGW) and super pressurised groundwood (PGW-S) processes produce pulps of almost same strength properties as TMP (thermomechanical) pulps, but significantly lower electrical energy demands are required.
There are almost twenty mills all over the world using the PGW technology in grinding and the two given below in Number 8, are using the PGW-S technology.
Cleaner Production Principle
Process modification
Cleaner Production Application
The groundwood pulp flows out of the pressurised grinder through a shredder, discharged through a blow valve into a blow cyclone, and then into a filter. From the filter the hot filtrate returns to a shower water tank and the discharge pulp is re-diluted to a controlled consistency ahead of screening.
In the PGW and PGW-S processes the grinder has been built to be pressure proof. The grinder can be pressurised by using air, thus the grinding temperature can be raised. Due to the higher temperature and pressure the lignin is softened in the grinding, resulting in stronger and more fibrillated pulp.
The effect of the shower water temperature on groundwood pulp properties was compared in the tests. Shower water temperatures ranged up to 140oC and grinder overpressure up to 450 kPa. From the results it can be confirmed that mechanical pulp strength properties can be developed without more energy. The use of superheated shower waters and higher overpressures improves the strength properties of super pressure groundwood by 15-20% compared to conventional PGW.
The first commercial PGW plant was started in the summer of 1980, and the first PGW-S plant in 1991. The method has been patented in the countries most important from the point of view of paper production . There are pressure grinders operating all over the world.
Since 1976 Tampella Ltd (Finland) has done research into the possibilities of improving the properties of groundwood. Preliminary trials were made in 1977 and a full-sized prototype construction was started in 1979. The first commercial PGW plant started in 1980 and the first PGW-S plant in 1991.
Environmental and Economic Benefits
The main goals of the research and development work have been to produce fiber that better meets the market need, to lower the consumption of energy, and to keep the consumption of more and more limited wood material as low as possible
Due to better properties of pressure groundwood pulps, the share of chemical pulp in paper production can be decreased from 20% to 10%.
| Material Category | QTY Before TMP | QTY After PGW-S |
| Waste Generation : BOD7 | 15-25kg/t paper | 12-17kg/t paper |
| Feedstock Use : | N/A | N/A |
| Water Use: | N/A | N/A |
| Energy Use:electricity | 1,7-1,9 Mwh/t | 1,1-1,3 MWh/t |
| Steam | 4,2 GJ/t | |
| Pulp yield : | 95% | 97% |
Constraints
Spruce, different pine species and populus are the best raw materials for PGW and PGW-S processes. Hardwood can't be used. Some exotic hardwoods can be tested in the pilot plant.
Contacts
Review Status
This case study was submitted by the UNEP Working Group on Cleaner Production in the Pulp and Paper Industries, based at the Technical Research Center of Finland (address above) in 1992, as part of a contract for UNEP IE. Before submission, the case studies were reviewed at the Center. They were edited for the ICPIC diskette in June 1997.
Subsequently the case study has undergone another technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.