CHAPTER 1 INTRODUCTION

Transcription

1 CHAPTER 1 INTRODUCTION 1.1 Significance of paper Paper is one of the essential commodities in daily life all over the world. It is considered as an index of a country s growth. Several grades of paper are manufactured to meet the demands for writing, printing, packaging, industrial needs, hygiene and toilet use. The per capita consumption in India is estimated around 9.3 kg (IPMA, 2014). Overall paper consumption in India is million metric tonnes (MT) (ICRA, 2013) and domestic production of paper & paper board production is estimated to be million tons. The paper consumption is estimated to touch MT (IPMA, 2014) by The Indian paper industry accounts for about 1.6% (IPMA, 2014) of the world s production of paper and paperboard. The Indian paper industry is expected to grow to meet the increasing paper demand. The industry has earned a notorious name as a highly polluting industry (Pokhrel et al., 2004) with high energy consumption (Szabo et al., 2009). There has been a growing awareness and effort to make the industry efficient in terms of usage of resources, use cleaner technologies to minimize the pollution load and close water cycles. These can be better appreciated when one looks at the paper making process. 1.2 Pulp & Paper manufacturing processes The pulp & paper manufacturing process essentially consists of pulp making with chemical recovery and paper making sections. The pulp making requires cellulosic raw materials and the pulp is converted into a sheet of paper. The cellulosic raw materials can be wood, agro residues or recycled paper. In India, over 60% of the paper is produced from wood and agro residue raw materials. Wood and agro residues contain primarily cellulose with lignin (Lycksam et al., 2012) as a binding material. In the pulping process, the fibers are separated by either a chemical process dissolving lignin (Naqvi et al., 2010) or by a mechanical/thermal process (Pokhrel et al., 2004) separating fibers but retaining the lignin. The chemical pulp gives stronger fibers with paper having longer shelf life. Mechanical or Thermal pulp on the other hand gives higher yields but have lesser shelf 1

2 life. In the chemical pulping process, the chemical dissolves the lignin where as cellulose is separated out. The pulping process can be alkaline or acidic (sulphite). Alkaline processes are preferred due to lower environmental impact. The alkaline process can be soda pulping (where only sodium hydroxide is used) or Kraft process (where a combination of sodium hydroxide and sodium sulphide) is used. As the name suggests, it gives stronger pulp due to selective dissolution of lignin (Cardosso et al., 2009a) and better preservation of cellulose. This happens due to the presence of sulphide ion. In a typical Kraft pulping process, the most favored pulping process worldwide (Alen et al., 1995), the raw material is cooked in a digester with the cooking liquor (called white liquor essentially containing NaOH and Na 2 S) at temperature around o C and around 6 to 7 kg f /cm 2 (g) pressure. After cooking, pulp (around 50% of the raw material) (Marklund, 2007) is separated from the liquid called black liquor due to its color (essentially containing all the dissolved organics and inorganics). The black liquor is separated in the washing process where clean pulp from the washers is sent for further processing. Calciner Lime Pulp Caustization White Liquor Make up chemicals Digester Wood Chips Blow Tank Washer Sludge Green Liquor Pulp Filtration Water Smelt Weak BL Storage Tank Smelt Dissolving Tank Recovery Furnace Strong BL Storage Evaporator High pressure steam Figure 1.1: Schematic of chemical pulping process Condensate 2

3 The black liquor on the other hand containing the dissolved solids is processed for recovering the inorganic chemicals on one hand and recovering heat from the dissolved organic compounds. This process is called chemical recovery process. All modern chemical pulp mills have recovery plant for the energy and chemicals recovery from black liquor otherwise this will be single most polluting streams. The chemical recovery process including the digester is shown in Figure Black liquor Black liquor contains all the valuable organic and inorganic chemicals (Saw et al., 2009). It is a viscous, thick and dark brown liquid. Every ton of pulp produced generates about 1.5 tons of dry black liquor solids or about tons of weak black liquor (10 to 18 % dissolved solids depending on the degree of dilution during pulp washing) with roughly two-thirds organics and one-third inorganics. This weak black liquor is concentrated to 60-85% solids (Karlsson et al., 2013) for further processing in a recovery boiler. The organic content present in the black liquor serves as a fuel in recovery boiler to produce high pressure steam which in turn is utilized for producing power. The heat content possessed by black liquor solids (13-15 MJ/kg) is less than that of fossil fuel. However, the efficiency of heat recovery depends on the concentration of black liquor and heat carried by the molten smelt which is drained out of recovery boiler. Typical composition of dry black liquor solids is given in Table 1.1 Table 1.1: Typical composition of dry BL solids (Marklund, 2011) Element Wt. (%) C 36.4 H 3.5 O 34.3 N 0.14 Na 18.6 K 2.02 Cl 0.24 S 4.8 Total

4 1.4 Recovery Boiler Figure 1.2: Schematic of recovery boiler furnace (Macek, 1999). The schematic of a typical recovery boiler is shown in Figure 1.2. The black liquor, which is concentrated in multiple effect evaporators (Johansson et al., 2009) is sprayed in the recovery boiler as droplets through the injection nozzles. These nozzles are provided about 5.5 m height from the bottom of the furnace. The droplet while moving down in the hot gas ambience, undergo three different phases (Miikulainen et al., 2004) before it hits the bottom of the furnace. Immediately after being sprayed inside the furnace, the droplet loses its moisture which is termed as drying phase (Whitty et al., 2008a) is the first phase. The dried black liquor droplet experiences a temperature rise as it goes down resulting in a devolatilization process (primarily a depolymerization reaction process) that is strongly temperature dependent and this process is called as second phase. The particle not only loses its mass but is likely to experience an increase in size due to swelling. These processes have a great influence on the operation of the recovery furnace. The particle 4

5 finally hits the bottom of the furnace, which is called char bed, where it experiences a combustion process. Combustion is the third phase in which the droplet residue starts burning and hits the char bed. The char bed consists of molten smelt covered with char residue. The char residue, while burning, supplements the heat energy for the reduction reaction of sodium sulfate with carbon and to keep the smelt in molten form. Smelt primarily consists of all the inorganic chemicals that were charged in the reactor during the cooking process. Air for combustion is supplied in the recovery boiler in a multi-tier port system to maximize the combustion of char and volatiles. Primary air supply is used to control the shape & position of the char bed. The secondary air controls the position of the tip of the bed and burns the hydrocarbon volatiles. The tertiary air supplies the air to complete the combustion process. The bed temperature increases while increasing the air until all the carbon present in the bed is burnt and then bed starts cooling down due to excess air. In a typical three tier air distributed recovery boilers, the air supply proposition is as follows (Table 1.2) and it depends on the proportions of volatiles in BL solids. The heat generated in the recovery boiler is received by feed water and in turn converts to high pressure steam while passing through economizer, boiler tube banks, water curtain walls and super heater to generate power & to meet other process heat requirements. Part of the heat generated as radiant heat is absorbed by water wall curtains. Table 1.2: Typical air flow distribution in a recovery boiler (Wessel, 2008) Percentage of total air Air supply supply Primary air flow Secondary air flow Tertiary air flow Char bed reactions are very complicated in nature involving at least four of the five major black liquor elements: carbon, oxygen, sodium and sulfur. All the inorganic pulping chemicals present in the black liquor droplet reach the char bed except for some losses in the carry over with hot gases, separate from the burning char in a molten, reduced state and flow out of the furnace through the smelt spouts. The principal reaction is the 5

6 reduction of sodium sulfate by carbon. The major gaseous products of this process, CO and CO 2, move upward to add to the boiler gas stream, while the reduced Na 2 S join other molten salts, primarily Na 2 CO 3, to form a smelt that is drained in to a water bath and this solution is called as green liquor. The oxidation reduction cycle is shown in Figure 1.3. The green liquor is further processed to recover the chemicals that were recycled to the digester. Na 2 SO4 C Na2S CO /CO2 Oxidation Na 2 S Reduction by Carbon CO /CO O 2 2 Na 2 SO 4 Figure 1.3: Oxidation-Reduction cycle 1.5 Operational criticalities of Recovery Boiler Black liquor droplets are generated when the black liquor is injected through nozzles and droplet size distribution is important in the operation of the boiler. The initial droplet diameter has a direct impact on the condition of the char bed, temperature profile of the boiler and recovery of inorganic chemicals. The droplets take some time to reach the char bed after they are sprayed in the recovery boiler and the time taken to travel through the hot gaseous environment of the boiler to reach the char bed is called residence time. Droplet residence time in the hot ambience is a critical factor for the droplet to complete drying and devolatilization processes before it reaches the char bed. Residence time depends on several factors and size of the droplet is one of the important factors. If the size of the droplet is big enough, it becomes difficult to complete the drying phase. When the droplet touches the char bed in a wet condition and process continues for longer 6

7 period of time, the char bed reactions may be killed which sometimes, may lead to smeltwater explosion or volatile cloud explosion due to accumulation of volatiles. If the size of the droplet is very small, the droplet is carried away with the hot gases and may stick to boiler tube banks. Due to this phenomenon, the gas path between the tubes is plugged resulting in less heat transfer (Tran and Vakkilainnen, 2008) as well as loss of chemicals. Thus it is very significant to understand the effect of droplet size, and its dynamics in the recovery boiler to sustain the longer operation cycles of recovery boiler and to contain the loss of makeup chemicals due to carry over. The black liquor recovery furnace operation gets influenced due to many variations occurring in the pulp mill operations. These include change in the raw material used in the pulping, presence of varying quantities of chemicals, concentration of black liquor injected in the furnace, size of the droplet and the temperature of the furnace. The operations can be unstable when suitable adjustments are not made. One of the primary variables which can be controlled is the black liquor droplet size entering the furnace which determines its fate as to whether: a. The particle is dried, devolatilized and reaches the char bed. b. The particle is dried, partly devolatilized and reaches the char bed. c. Particle is dried but not devolatilized and reaches the char bed. d. The particle is wet and reaches char bed. e. The particle is too small and entrains out of the bed. For a stable operation, conditions (d) and (e) must be avoided while condition (a) is the best option. These considerations lead to the present work. 1.6 Scope of the thesis The scope of the thesis is to develop a mathematical model of the various phenomena happening to a black liquor droplet that is sprayed in to a chemical recovery boiler before it reaches char bed. The results of the model will be helpful in identifying the desirable droplet size distribution that makes the boiler operation safe and economical. The model considers various important parameters such as initial black liquor concentration, initial droplet diameter, initial droplet velocity, travel distance, prevailing gas temperatures and swelling of the droplet during pyrolysis etc. 7

8 In order to determine the weight loss occurring to the droplet during the devolatilization stage of its flight in the recovery boiler, it is proposed to conduct experiments on dried black liquor solids in a muffle furnace for various temperatures and residence times. The experimental results can be integrated into a kinetic model that can be used to predict the weight loss occurring due to devolatilization in the furnace during the flight of the particle. This kinetic model may further be used in the mathematical model developed for the dynamics of the droplet. The results from the mathematical model such as drying time, devolatilization time, desirable particle size ranges as a function of various parameters will be very useful in identifying practical and simple operating maps that will aid the operator in safe and reliable operations of the recovery boiler. 1.7 Organization of the thesis The first chapter consists of a brief introduction to the pulp and paper manufacturing process, recovery boiler and briefly outlines the scope/organization of the thesis. The literature review pertaining to the problem is given in chapter 2. The mathematical model of the drying phenomena of droplet in recovery boiler is developed in chapter 3. Chapter 4 consists of the experimental work on weight loss of dried black liquor solids and development of model for estimating the kinetics of devolatilization. In chapter 5, the mathematical model on the devolatilization of black liquor droplets is presented. Chapter 6 consists of the simulation results of the mathematical models presented in chapter 3 and 5 with detailed discussions. Finally, chapter 7 summarizes the important conclusions and gives suggestions for further work in this interesting area. 8