CENTRAL PULP AND PAPER RESEARCH INSTITUTE SAHARANPUR Innovative Approach for Improved Energy & Chemical Recovery Efficiency in Straw Based Paper Mill Dr. R.K. Jain, Dr. A.K. Dixit, Dr. Tarun CENTRAL PULP AND PAPER RESEARCH INSTITUTE SAHARANPUR

Structure of Presentation Problems Associated with Agro Residue Black Liquors Characterization Agro residue and Wood Black liquor Viscosity of Agro and Wood Black Liquor Impact of Higher Viscosity on Chemical Recovery System CPPRI Effort in Improving Thermal Efficiency of Agro Residue based Chemical Recovery Process Flow-sheet of Desilication followed by Liquor Heat Treatment Optimization of Desilication Process Conditions Studies on Liquor Heat Treatment of Desilicated Black Liquor Analysis of the Control, Desilicated and Heat Treated Black Liquor samples Observation Conclusion

Problems Associated with Agro Residue Black Liquors Chemical recovery operation in the agro based paper mills particularly the mill employing wheat straw as a major raw material suffers several operational problems leading to poor energy & chemical recovery efficiency Presence of high content of silica and LCC cause higher viscosity of black liquor restricting to achieve thermal efficiency of chemical recovery units in wheat straw / bagasse based mills

Characterization Agro residue and Wood Black liquor Parameters Agro residue Black Liquor Wood Black Liquor   Total Solids, %w/w. 11 - 13 16 pH 12 12-13 RAA, as Na2O, gpl 5 - 6 SVR, ml/g 8 - 9 20 - 22 Silica as SiO2, %w/w. 2.8 - 3.5 0.05 - 0.1 Inorganic as NaOH, %w/w 34 35 Organics, %w/w. 66 65 Lignin Carbohydrate Complex, % w/w 14-20% 3-5% Carbon as C, %w/w 35.05 34.3 Hydrogen as H, %w/w 5.47 4.5 Sulphur as S, %w/w 1.0 3.0

Viscosity of Agro and Wood Black Liquor Total solids, %w/w. Viscosity, cps at 1000C Bagasse Black Liquor Wood Black Liquor 45 50.2 - 50 138 20 55 384 79 60 1073 165 65 3016 369 High viscosity of Agro Residue black liquor shows poor burning and low thermal efficiency in chemical recovery boiler

Impact of Silica on Chemical Recovery Cycle Following are impact of silica on Chemical Recovery Cycle:- Scaling in Evaporator Poor burning behavior of Black Liquor Slow settling of White Liquor Make lime mud reburning difficult

Impact of Higher Viscosity on Chemical Recovery System Viscosity is an important parameter for black liquor handling. It has impact on following unit operations:- - Evaporator Capacity and energy requirement - Heat Transfer - Mixing operation - Droplet size

CPPRI Effort in Improving Thermal Efficiency of Agro Residue based Chemical Recovery To combat the problems of high silica content and higher viscosity of agro residue black liquor, a process consisting of Desilication followed by Liquor Heat Treatment was developed for efficient operation of chemical recovery system in the paper mills employing wheat straw and /or Sarkanda as major raw material. Desilication process is based on controlled carbonation of the black liquor using flue gas in a submerged bubble reactor resulting in selective precipitation of silica at optimized pH level. Liquor Heat Treatment Process involves heating of the black liquor at temperature more than pulping temperature (180 - 185° for 15-20 mins. maintaining optimum level of residual active alkali)

Process Flow-sheet of Desilication followed by LHT

Optimization of Desilication Process Conditions Desilication Efficiency at different pH Black Liquor Characteristics Desilication (%) Before Desilication After Desilication pH SiO2, %w/w PH 12.5 3.03 10.5 2.8 17.5 10.3 1.6 47.2 10.2 0.8 73.6 10.1 0.5 83.5 10.0 0.40 86.8 9.9 0.36 88.2

FLOW SHEET OF DESILICATION PROCESS TC

Characterization of original and desilicated black liquor Parameters Control BL Desilicated Re-alkalized BL Total Solids, %w/w. 11.2 11.3 pH 12.54 12.6 RAA, as NaOH gpl 3.84 4.35 % 3.44 4.92 SVR, ml/g 8 10 Silica as SiO2, %w/w. 3.03 0.36 Inorganic as NaOH, %w/w 31.59 33.82 Organics, %w/w. 68.41 66.18 Residual Silica – 0.04% SVR improved to 10 showing better burning behavior

Heat Treatment of Desilicated Black Liquor Process involves heating of black liquor at 180 -190° for 15-20 minutes, maintaining optimum residual active alkali level. Cleavage of the high molecular weight Lignin carbohydrates bonds (LCC) occurs due to depolymerization leading to generation of lower molecular weight entity.

SCHEMATIC OF LIQUOR HEAT TREATMENT

Variable affecting Liquor Heat Treatment Residual Active Alkali Retention time Temperature Studies were conducted on optimization

Optimization of RAA Optimization of Residual Active Alkali (RAA) at 1800C RAA, %w/w as NaOH 1800C (% reduction in Viscosity) 50% solids 55% solids 60% solids 3.5 44.72 45.10 48.24 4.5 50.91 52.83 56.41 5.5 61.72 63.08 66.53 6.5 66.58 68.28 70.07

Optimization of Retention Time Optimization of Retention Time at 1800C Time, minutes % reduction in viscosity (RAA 5.5% / 180°) 55% solids 60% solids 65% solids 15 62.8 66.1 66.6 20 65.7 68.5 68.9 25 69.9 70.2 72.4

Optimization of Temperature Optimization of Temperature (RAA 5.5%) Temperature, 0C % reduction in viscosity 50% solids 55% solids 60% solids 175 61.5 63.1 66.6 180 62.9 64.3 67.1 185 63.9 65.5 68.1

Studies on Liquor Heat Treatment of Desilicated Black Liquor Liquor Heat Treatment of desilicated re-alkalized black liquor was carried out at optimized process conditions. Control black liquor, desilicated black liquor and Heat treated black liquors were analyzed for various parameters to observe the impact of Desilication coupled with LHT on black liquor properties

Desilicated-Re-alkalized-Heat Treated BL Analysis of the Control, Desilicated and Heat Treated Black Liquor samples Parameters Control BL Desilicated-Re-alkalized-Heat Treated BL pH 12.54 12.55 RAA, as NaOH gpl 3.84 5.71 % 3.44 2.37 SVR, ml/g 8 15 Silica as SiO2, %w/w. 3.03 0.38 Inorganic as NaOH, %w/w 31.59 34.74 Organics, %w/w. 68.41 65.26

Viscosity of control and treated Black Liquor Total Solids, %w/w. Value, cps Control BL (Without LHT) Treated BL (After LHT) 1000C 1050C 50 350 295 111 93 55 885 718 195 155 60 2215 1735 342 259 65 5545 4200 600 432 70 - 1051 719

Observations The following observations are made from the study conducted: Liquor Heat Treatment coupled with desilication of wheat straw black liquor conducted at optimized conditions resulted in more than 75% reduction in viscosity. Treated liquor have shown better combustion behavior reflected by higher Swelling Volume Ratio (SVR) of treated black liquor (15 ml/g) as against untreated black liquor (8 ml/g). It is possible to achieve 5-6% higher concentration of treated black liquor due to lower viscosity and a concentration of more than 65% can be achieved as against 60% of untreated black liquor. Firing of black liquor at higher dry solids level will also reduces emissions from chemical recovery boiler. Reduction in silica will help the agro residue based mills in installing full-fledged chemical recovery system In addition to above benefits, reduction in silica content will make lime sludge suitable for reburning and will improve dryness of lime sludge. This will reduce fresh lime requirement and less fuel will be required for lime mud reburning.

Conclusions The development of Liquor heat treatment in combination with desilication of silica rich wheat straw black liquor and demonstration of the same should help the Indian paper Industry employing wheat straw as raw material in achieving higher chemical recovery and thermal efficiency of chemical recovery operation.

THANKS Central Pulp & Paper Research Institute