Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1992-09-30
1994-12-06
Jones, W. Gary
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 301, 162252, 60 3905, 60 3912, 60 39182, D21C 1104
Patent
active
053707729
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for generating heat and electricity by means of pressurized gasification of black liquor by using combined power plant technology in a sulphate pulp mill. Black liquor is gasified in a pressurized state in a gasifier by means of air pressurized in a compressor. The gases thereby produced are purified and combusted in a gas turbine combustor by means of pressurized air from the compressor. The exhaust gases formed in the combustor are expanded in the gas turbine for generation of electricity. From the gas turbine, the gases are further conducted into a waste heat boiler, where superheated high-pressure steam is generated, which is then expanded in a steam turbine for generation of electricity. Exhaust steam from the steam turbine is used to cover the heat demand of the pulp mill.
In a pulp mill, steam is usually also generated in an auxiliary boiler, where bark, wood waste or other fuel is combusted.
Production of sulphate pulp is the most important pulp making method in the world. Its share was about 58% of the pulp produced in 1987. Heat and electricity demands of sulphate pulp mills have continuously decreased during the past years as a result of development work and, today, sulphate pulp making is over self-sufficient with respect to energy or at least with respect to process heat.
In Finland, the average annual heat demand of a sulphate pulp process is about 10000 to 13000 MJ/tm and the average annual electricity demand about 1900 to 2900 MJ/tm. Both electricity and heat demands vary in accordance with seasons, i.e., outside temperatures. The difference between average nominal heat consumptions in winter and summer may be nearly 20% and between nominal electricity consumptions about 6%.
Energy is produced in a pulp mill mainly by combusting black liquor in a recovery boiler and wood wastes and bark in an auxiliary boiler. The bark of the wood raw material used in the process and the organic substance of generated black liquor together normally cover the whole energy demand. If more energy is needed, additional electricity or fuel may be purchased. The additional fuel is combusted with the bark in the auxiliary boiler.
Electricity is consumed in the cooking, washing, bleaching and evaporation plants primarily for pumping of black liquor, water and pulp and for transporting of chips. Furthermore, the dryer section consumes electricity for the drives and the ventilation in the section. Electricity is also consumed in lighting and in treatment of raw water and waste waters.
In a sulphate pulp process, heat is needed for inducing and increasing the velocity of desired process reactions. In the cooking plant, for example, heat is needed for heating the black liquor and the chip circulation systems. During the cooking process, the chips and the cooking liquor are heated to the cooking temperature indirectly with high pressure steam. The dryer section stands for approximately 29% of the heat consumption.
The evaporation plant is the greatest heat consumer of all separate processes. It stands for 31% of the entire energy consumption of a sulphate pulp mill. The secondary heat obtained from the evaporation plant is sufficient to produce warm water (40.degree. C.) required by the sulphate pulp mill. The power plant in the pulp mill stands for about 18% of the electricity consumption and about 10% of the heat consumption.
Today, energy is produced in the following way: a recovery boiler and an auxiliary boiler, where waste bark from the mill is combusted, generate superheated high pressure steam. The steam hereby generated is forced through a back-pressure steam turbine/turbines and the steam released is used to cover the heat demand of the mill. The turbine and the generator connected thereto produce the electricity needed by the mill.
Electricity is usually generated by means of a back-pressure turbine provided with one or more bleed lines. The back-pressure is usually 3 to 4 bar and the bleeding pressure 10 to 13 bar. Electricity may also be generated by a
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Ron McDonald, The Pulping of Wood, vol. 1, 1969, pp. 308-309.
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Arpalahti Olli E.
Pekkinen Jukka
A. Ahlstrom Corporation
Jones W. Gary
Nguyen Dean T.
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