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INTRODUCTION
1.1 ABOUT TNPL
• Tamil Nadu Newsprint and Papers Limited (TNPL) Founded in the year 1979.
• It was established by the Government Of Tamil Nadu to produce newsprint and writing paper using bagasse, a sugarcane residue.
• Uses 1000000 tones of bagasse’s per annum and avoid deforestation in 30000 acres every year.
• Treated effluent water is pumped to TNPL Effluent Water Lift Irrigation Society (TEWLIS) consisting 396 farmer with 1500 acres amount spent sofar Rs.920 crore.
• Installation ozone treatment plant to reduce the colour of effilent water first in India.
• TNPL has obtained forest stewardship council certificate.
• Monitoring of land irrigated with TNPL treated effulent water trough TAMIL NADU AGRICULTURE UNIVERSITY (TNAU).
• Pulp bleaching sequence being changed from conventional chloride bleaching to elemental chlorine free bleaching.
• Bio-methanation plant producing methane gas of about 20,000m/day from bagasse wash water.
• Obtained ISO14001 certification for successfully stabilizing and applying environment management system for development, manufacture and supply of paper.
PRODUCTS of TNPL
Some of the major end-use segments of Printing & writing Paper (PWP) are printing applications, note books, computer stationery, office stationary, etc.
• TNPL Ultra White Map litho
• Radiant Printing
• TNPL Offset Printing
• Copy Crown
• TNPL Copier, etc.
TNPL is also exporting about 18% of the PWP production to 20 countries around the world. (Australia, Egypt, Indonesia, Malaysia, South Africa, U.A.E, U.K, U.S.A, etc.). Symbolic of TNPL’s commitment to the environment, the “world wide Fund for nature (WWF)” has entered into a pact with TNPL to use the “Panda" logo in TNPL’s branched products.
1.3 DEPARTMENTS IN TNPL
• Paper Machine.
• Pulp Mill.
• Energy Department.
• Water Treatment Plant.
• Effluent Treatment Plant.
• Soda Recovery Plant (SRP).
PAPER MACHINE:
The paper making machines are based on the principles of the four drainer machine, which uses a specially woven plastic fabric mesh conveyor belt in the forming section, where a slurry of fiber is drained to create a continuous paper web. After the forming section the wet web passes through a press section to squeeze out excess water, then the pressed web passes through a heated drying section.
The original fourdrinier forming section used a horizontal drainage area referred to as the drainage table.
Paper machines have four distinct operational sections:
• Forming section
• Press section
• Drying section
• Calender section
PULP MILL:
A pulp mill is a manufacturing facility that converts wood chips or other plant fiber source into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods (kraft and sulfite processes). The finished product may be either bleached, depending on the customer requirements.
Wood and other plant materials used to make pulp contain three main component (apart from water) cellulose fibers (desired for papermaking), lignin (a three-dimensional polymer that binds the cellulose fibers together) and hemicelluloses, (shorter branched carbohydrates polymer).
The aim of pulping is to break down the bulk structures of the fiber source, be it chips, stems or other plant parts, into the constituent fibers.
Chemical pulping achieves this by degrading the lignin and hemicelluloses into small, water-soluble molecules which can be washed away from the cellulose fibers without depolymerizing the cellulose fibers. The various mechanical pulping methods, such as ground wood (GW) and refiner mechanical (RMP) pulping, physically tear the cellulose fibers one from another. Much of the lignin remains adhering to the fibers.
The various pulping process:
• Mechanical pulp mills
• Chemical pulp mills
• Chemi-mechanical pulp mills
1.3.3 ENERGY DEPARTMENT:
TNPL has installed its own power generating facility to make it 100% self-sufficient through the installation of 61.18MW power generating equipment (TG sets) installed at the paper mill site.
1.3.4 WATER TREATMENT PLANT:
TNPL will set up a drinking plant with a capacity of 300 TPD at an investment of Rs.175 crore.
The water used in nearby every step of manufacturing processes, TNPL produces large volumes of wastewater and residual sludge waste, presenting a number of issues in relation to waste treatment, discharge, and sludge disposal.
Contaminants inherent to TNPL waste streams include effluent solids, sediments absorbable organic halides (AOX), chlorinated organic compounds and chemical oxygen demand (COD) and biological oxygen demand (BOD) contaminants. According to industry experts, approximately 85% of the water used in the TNPL industry is used as process water, resulting in relatively large quantities solutions.
Treatment option includes primary treatment such as clarification to remove solids and particulate matter, and secondary biological treatment processes for removing biodegradable organic matter and decreasing the effluent toxicity.
1.3.5 EFFLUENT TREATMENT PLANT (ETP):
Our effluent treatment plants provided effective solutions to effluent odor control, BOD reduction, aeration, clarification, phosphorous and nitrogen removal and more. Our process experts work with the design team to learn the various factors that contribute to a plant’s design, including effluent requirements, land availability, energy, labor and disposal cost.
The treatment technologies such as membrane filtration, UV disinfection, ion exchange, and granular activated carbon can also be employed to further treat effluent water to higher qualities.
LITERATURE REVIEW
2.1 GENERAL
Duke low, S.G. in the conference at Research Triangle Park: Instrument Society of America explain about In order to keep the limits and to remove any sludge ,loose scales and corrosion products, a certain quantity of boilers is to be regularly drained. This process is called a Blow down. This is due to the fact that water inside the boiler gets continuously evaporated due to steam generation. Concentration of dissolve solid, inside the drum increases and reach beyond the limit, so carryover of solids along with steam can occur. To prevent boiler tube chocking and overheating of the boiler tubes the blow down is necessary.
Stultz, S.C. in the year 1992 explain about boiler blow down designed to remove suspended solids, including any sludge formed in the boiler water. The manual blow down take-off is usually located in the bottom of the lowest boiler drum, where any sludge formed would tend to settle. Properly controlled intermittent manual blow down removes suspended solids, allowing satisfactory boiler operation.
J.B.Kitto 40th edition in 1993 suggested that these TDS can accumulate within boiler when water is evaporated to generate steam. To reduce the TDS of feed water the boiler blow down is performed regularly in boilers. The rate of water blow down can range from less than 1% when extremely high-quality feed water is available to greater than 20% in a system with poor-quality feed water. The quantity of water to be blow down will depend on the dissolved solids entering the boiler through the feed water. Thus parameters that are most often monitored to ensure this quality of steam are TDS and conductivity, pH, silicates and phosphates concentration. The boiler is blow down to reduce these levels and keep controlled to a point where the steam quality is not likely to be affected. A substantial amount of heat energy is lost in this process.
Franklin Kreith and Mark S. Bohn (1996), prescribed that the experimental studies it is shown that nearly 1.3% of fuel is wasted through blow down by installing the heat recovery system, recover nearly an 85% of total wasted fuel. This recovered energy is supplied to feed water to raise the water temperature. It will also increases the boiler efficiency. An average of 6degC rise in feed water temperature reduces the 10% of blow down water.
Everett B. Woodruff, Herbert B. Lammers, Thomas F. Lammers (1998), revealed that the heat recovery is done by two mechanisms, one by recovering energy in flash steam and the second one by latent heat in the waste water by heat exchanger. In flash recovery system, the blow down water is directly supplied to a flash vessel which is at atmospheric pressure. Due to the change in enthalpies of water at boiler pressure and atmospheric pressure nearly 13% of blow down water is converted into flash steam. This steam is directly supplied to the feed water tank hence the temperature of feed water is raised due to high enthalpy value of steam. The flash steam contains nearly 49% of energy contained in the blow down water. The latter one is using the latent heat in the water coming out of the flash vessel. Using this latent heat the temperature of makeup water coming to the deaerator is rose which will reduce the amount of coal required to produce that amount of steam.
CHAPTER 3
SODA RECOVERY PLANT
3.1 DESCRIPTION
The primary function of the soda recovery plant in pulp and paper mill is to recover and reuse the chemicals used for chemical pulping of various raw material viz. softwood, hard wood bagasse, etc. The main objective of unit is to minimize as efficiently as possible, the loss and subsequent make-up of the chemical used in the preparation of cooking liquor, commonly called as “WHITE LIQUOR”, which comprises mainly of sodium hydroxide, sodium sulphide and sodium carbonate.
The sodium and sulphur loss in kraft recovery cycle is made up with sodium sulphate (salt cake) and sodium hydroxide.
THE VARIOUS STEPS KRAFT RECOVERY IS AS FOLLOWS:
Concentration of weak black liquor (WBL) in evaporation plant. Incineration of concentrated black liquor (CBL) in chemical recovery boiler to remove organics.
Conversion of sodium carbonate to sodium hydroxide in recausticizing plant.
Calcination of lime mud (calcium carbonate) to lime (calcium oxide) in lime kiln for use in recausticizing plant.
3.2 EVAPORATION
The water content in the black liquor is the first concentrated by evaporation process before removing the organic matter present. This operation is carried out in the evaporator plant. The bagasse liquor and hardwood liquor at different concentration level are blented and processed in the evaporators. The blent liquor concentration is around 8-9%. The recovery plant has two streets of six-stage multiple effects and another street of seven effect falling flim evaporator. The concentration level of the evaporator output is 45% and it is called “semi concentrated black liquor (SCBL)”. Under MDP, the mill installed an additional evaporator to concentrate the black liquor up to 70% solids, which will enable firing of liquor directly in the recovery boiler. The plant has capacity to evaporator 500M^3 of water per hour.
3.3 INCINERATION
The concentrated black liquor is fired in the recovery boiler. Sodium Sulphate (salt cake) is added to concentrated black liquor to make up for the losses in the system. The concentrated black liquor contain the inorganic component are incinerated in the boiler. The primary function of the recovery boiler is to incinerate the organic matter in the controlled manner ad to recover the spent chemicals of the cooking liquor. In this controlled combustion, certain chemical reactions take place and the sodium sulphide (content of the cooking chemical) is enriched. This reaction is called as “Reduction”. The sodium salt melt in the furnace and flows out the molten form and it is called as smelt. The smelt contains mostly sodium carbonate and sodium sulphate. It is dissolved and is called as “green liquor”. The recovery boiler generates super-heated steam and is used to generate power.
3.4 CAUSTICIZING PLANT
The green liquor obtained on the smelt contains mostly sodium carbonate apart from sodium sulphide. To convert sodium carbonate to sodium hydroxide, which is the main chemical of the cooking liquor, the green liquor is treated with burnt lime and thereby white liquor is generated. The reaction is as follows:-
Sodium carbonate + Calcium hydroxide + Water = Sodium hydroxide + calcium carbonate
(Green liquor) (Burnt lime) (White liquor) (Lime sludge)
The reaction is carried out in the causticizing section. Burnt lime generated from the kiln (after crushing to required size) and hot green liquor (after clarification) is processed in the equipment called as slaker. In this reaction, white liquor and sludge are formed. It is fed to white liquor claridisc filter where the lime sludge and white liquor separated in pressure vessel. The clear white liquor flows from the white liquor separator and it is stored and supplied to pulp mill for cooking. White liquor claridisc has been installed which occupy lesser space and can generate high quality with clarity of white liquor.
The lime sludge scrapped from white liquor claridisc filter is pumped to mud washer and washed in the lime mud washer and filtered in mud filter. A lime kiln has been installed in lime recycle this sludge along with limestone and regenerated burnt lime required for the causticizing process. In this cycle of operation, there will be loss of sodium components. These losses are made up with sodium hydroxide (caustic soda) and sodium sulphate (salt cake).The caustic soda is directly mixed in the white liquor at required concentration, before pumping to pulp mill. The causticzing plant has capacity to generate 3600M^3/day of white liquor.
3.5 LIME KILN
Two lime kiln with the capacity of 170 Mt/day of the lime have been installed to recycle the lime sludge. Generated in the process of causticizing.
Raw lime stone is added with lime sludge and fed into the kiln. The kiln is fired with the furnace oil and bio gas. At the required temperature and burnt lime with purity of 78% Cao is generated which will be used in the causticizing as well as in bio methanation plant.
RECOVERY BOILER
4.1 DESCRIPTION
Recovery boiler is the part of pulping where chemicals for white liquor, which contains lignin from previously processed wood. The black liquor is burnt, generated heat which is usually in the process of making electricity, much as in air conventional steam power plant. The invention of the recovery boiler by G.H. Tomlinson in the early 1930’s was a milestone in the advancement of the kraft process. Recovery boiler is also used in the (less common) sulfite process of wood pulping.
4.2 FUNCTION OF RECOVERY BOILER
Concentrated black liquor contains organic dissolved wood residue in the addition to sodium sulphate from cooking chemical added at the digester. Combustion of organics portion of chemicals produces heat. In the recovery boiler is used to produce high pressure steam, which is used to generate electricity in a turbine. The turbine exhaust, low pressure steam is used for process heating. Combustion of black liquor in the recovery boiler furnace needs to be control carefully. High concentrated of sulphur requires optimum process condition to avoid production of sulphur dioxide and reduced sulphur gas emission in addition to the environmentally clean combustion in the reduction of inorganic sulphur must be achieved in the bed.
Function of the chemical recovery boiler is to fire the black liquor solids. The heat generated by the combustion of organic in the black liquor generates steam which in turn is utilized for cogeneration of power.
The recovery boiler has several unit process combustion of organics material in the black liquor to generate steam. Reduction of inorganic sulphur components to sodium sulphide, which exist at the bottom as smelt. The production of molten inorganics flow of mainly sodium carbonate and sodium sulphide, dust from flue gas to same chemicals. Production of sodium few to capture combustion residue sulphur compounds.
CONSTRUCTION
The boiler is an outdoor installation (with roof and side cladding) and of a screen less natural circulation, single drum suspended type with a membrane wall, water cooled furnace. The installation includes all mountings, fitting, and refectory. Casings, cladding, structural and support steel work and integral pipe work.
The boiler has both radiant and convective heating surface arranged suitably in order to cool the combustion gases and to facilitate effective cleaning of the heating surface. The boiler operates with natural circulation. The water walls of the boiler furnace are all welded construction with 63.5mm diameter tube and 12.7mm fins in between. Forming a gas tight construction with tubes on 76.2mm centers. The furnace has decanting bottom. In the bottom area there are suitable member of inlet opening on two levels in order to control the bad shape. A third level of air inlet of air openings higher up to finalizes the combustion. The three stages super heaters.
The super heaters are arranged in desuperheater in between the stages for the steam temperature control the flue gases leaving the super-heater pass through a real wall exit screen and then enter boiler tank.
The arrangement of axial flow panel type boiler tank give superior performance with regard to plugging behavior, case of erection and operational safety. The flue gases are leaving the boiler tank enter the three stage vertical panel type economizer.
The economizer’s panel are axial flow type which the optimum arrangement is resulting in the minimum plug gage of the economizer heating surfaces. The economizer has been sized for maximum flue gas outlet temperature of 180c. The super heaters, boiler tank and economizer heating surfaces will be cleaned by long retractable soot blowers. The soot blowers are operated through a remote/ auto sequence. Blankets will be provided as insulation in order to heat loss by radiation to surroundings the mineral wool will be converted with aluminum/pre-coated sheet for production against mechanical damage. The boiler will be supported from a steel ceiling structure above the boiler roof thus allowing free expansion downwards when the boiler is warmed up. Flue oil system for starting up the boiler and for stabilizing the furnace condition during low load operation or firing liquor one low heating vakue, 4 nos. Stat-up oil burner is provided. The starting burners consists of oil burner located in separate operating above the furnace hearth.
4.4 HEAVY BLACK LIQUOR DETAILS
• Density-1.4
• Temperature- 135°C
• Organic compound- 32% by weight
• Inorganic compound (C, Na, S, O2, H2, K, others)- 68% by weight
• Calorific value- 3200 Kcal/kg
4.5 SPECIFICATIONS OF RECOVERY BOILER
• Boiler firing capacity- 1300TPD
• Steam generation- 197T/hr
• Steam pressure-65 Kg/cm^2
• Feed water temperature- 135°C