Paper manufacturing is a very energy intensive process. About one-half of the total energy used is in the pulping process, which is the cooking of the wood chips to open the fibers that create the paper materials. Therefore, a mill that is 'integrated' (it includes pulping and paper manufacturing), will use at least twice as much energy as the same size mill that uses 'market pulp' (pulp produced at another location and sold as a raw material). The largest single energy user at a paper mill is boiler fuel. The graphic below shows the major thermal energy flow through the conventional integrated mill.
Paper Mill Boilers
Packaged Boilers are typically natural gas or oil. Power House boilers are typically coal, wood waste, black liquor, oil, natural gas, or multi-fueled, depending on the size of the mill and if it includes a pulping operation. It is common for several different kinds of boilers to be used at the same mill.
There are three basic methods to produce pulp:
Mechanical Pulp (yield 90%)
Mechanical pulping uses mechanical abrasion to separate cellulose fibers which are held together by a material called lignin. In the process called "Groundwood" wet wood is ground by large stones. In Thermo Mechanical Pulping (TMP), metallic plates rub steam heated chips at high speeds, separating fibers
Chemical Pulp (yield 50%)
Uses chemicals to dissolve lignin. Kraft pulp is most common pulp.
Uses chemicals to soften lignin, and mechanical abrasion in refiners - Chemi Thermo Mechanical Pulping
The kind of paper that can be made is determined by the wood fiber and other raw or recycled materials used and the pulping method.
If the pulp will not be used on-site it becomes "Market Pulp", and shipped to another location. Due to many factors but mostly environmental and local wood supply, many mills no longer produce their own pulp.
Before the pulp is shipped, it is dried on a machine called a Flakt Dryer. The Flakt Dryer has multi-level decks or webs with steam-heated air jetted through the pulp and rollers that squeeze it to remove the water.
Most mills also include a steam turbine(s) to generate electricity. This may be because the mill is older than the public electric system in the area, but continues to make operational sense. The boilers produce high pressure steam (in excess of 500 psi) that first passes through the steam turbine. The steam turbine takes energy out of the steam by reducing its pressure to about 100 psi, which then runs the rest of the mill. Mills may also have direct steam drives in areas that would otherwise have electric motors. Steam drives resemble mini-steam turbines, and are much smaller than an electric motor of the same horse power.
The main sources of fire in paper machines are generation of static electricity due to rollers, mechanical sparks due to poor lubrication, electrical sparks due to poor insulation and short circuits. The combustion possibility and spread of combustion is quite fast due to the movement of rollers and depends on the extent of moisture content at various stages in the process. Modern paper mills have complicated rolls running at high speeds which accentuates the loss. Damage is often caused by the rubber covers coming of the suction rolls due to wear, foreign matter getting on to the wire screen belt and stress corrosion cracking in the cylinder shells of the suction rolls and couching presses. Other exposures are due to steam heater drying cylinders bursting due to crack formation, overheating of roller bearings, fracturing of journal bearings and cylinder end discs cracking.
Risk Management Recommendations
The following recommendation are being made in the light of the above exposures
Segregation of raw material preparation areas like de-barking, chipping etc away from the other blocks with special electrical installation of armoured cables, dust proof electrical fittings, double earthing.
High standard of house keeping and maintenance including removal of dust and waste, lubrication of moving equipment etc is required.
Wall Thickness of critical equipment like digester, black liquor boiler etc at regular intervals and is recommend. Non Desctructive Testing (NDT) like ultrasonic testing, magnetic particle testing etc is recommended. For non magnetic components, penetrant testing is recommended. For lime kilns thermographic tests are recommended which helps not only to identify cold or hot spots but also for monitoring damage in the refractory lining. Vibration measurement of rotating equipment like rolls, cylinders, bearings, etc., is recommended.
Process parameters like black liquor concentration, pressure, temperature, machine speeds etc should be constantly monitored.
Storage areas should be well regulated to provide proper aisle space and height while stacking. and.
Installation of fire hydrant and sprinkler systems is also strongly suggested considering the high fire load.
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