The major unit operations involved in wood-based mills are pulping, washing, screening, bleaching, chemical recovery and paper making. The details of these processes are elucidated :

The major process involved in RCF-based mills is elucidated :

New and Emerging and Energy Efficient Technologies
1. Desilication of Silica Rich Black Liquor
2. Thermal Treatment for Reduced Viscosity of Black Liquors
3. Bio Refinery Concept for Pulp and Paper Mills
4. Black Liquor Gasification
5. Higher Dryness of Paper Entering at Dryers
6. Impulse Drying
7. Magnetically-Coupled Adjustable-Speed Drives
8. Advanced Fibrous Fillers
9. Stickies and Efficient Contaminant Removal
10. Bio drying Technology for the Drying of Pulp and Paper Mixed Sludges

1. Desilication of Silica Rich Black Liquor

Higher quantity of silica, a non-process element has always been a matter a matter of great concern for agro residue black liquors. Silica entering the process cycle with the raw material gets dissolved in the black liquor and poses severe problem in the evaporator, recovery boiler and causticization operations. To overcome the problem, CPPRI developed a desilication technology for selective precipitation of silica adopting stepwise gentle carbonation technique employing flue gas from the recovery stack. CPPRI desilication process removes more than 90% silica with a residual silica content around 0.3%.

Benefits
Removal of silica from black liquor will give following advantages:
1. Reduced scaling in evaporator leading to lower down time in evaporator
2. Improved thermal properties facilitate burning of black liquor in recover furnace and generation of more steam and power
3. Improvement in rheological properties helps in achieving higher dry solids during evaporation. This will further help in generation of more steam and power
4. Better fluidity of smelt
5. Fast settling of mud in white liquor clarifiers therefore improved white liquor clarity
6. Improved calcinations of lime sludge and lower fuel requirement in lime kiln

2. Thermal Treatment for Reduced Viscosity of Black Liquors

The black liquor viscosity characteristics, specifically from non-wood based pulp and paper mills are such that even a normal operating temperatures of 100-115oC, the viscosity at 55% w/w total solids concentrations exceeds 500 cps. Thus, the higher viscosity problems lead to firing of black liquor at low solids concentration and thereby lower steam generation.

Studies at CPPRI indicated that high molecular weight organic residues like polysaccharides linked with lignin-carbohydrates complex (LCCs) and the high molar mass lignin molecules are mainly responsible for abnormally high viscosities of agro residues black liquors.

Benefits
Studies conducted at CPPRI have shown that viscosity of bagasse and straw black liquor can be reduced substantially by adoption of thermal treatments. Major benefit of liquor thermal treatment include more than 60% reduction in viscosity leading to better combustion of black liquor, higher steam generation, reduced emissions and lower steam demand in evaporation.

3. Bio Refinery Concept for Pulp and Paper Mills

In case of the integrated mills, there is emphasis on development of theBio-refinery Concept. Bio-refineries would typically integrate several biomass raw materials to produce different biomaterials and bio-products – i.e. integrating forestry wastes, agricultural residues and other sources of biomass, separating the fibres for paper production and converting the wastes into value-added chemicals and clean energy. Even in the non-integrated mills, the bio-refinery concept could be applied to process wastes, potentially augmented by other biomass (agricultural residues).

4. Black Liquor Gasification

Black liquor accounts for a significant fraction of the fuel consumed by the pulp and paper industry. Kraft mills combust black liquor in recovery boilers to recover chemicals and generate process steam and on-site electricity (via a steam turbine). The efficiency of such boilers is typically low, around 60-65%. Black liquor gasification may also be used to create a clean synthesis gas (syngas) from black liquor by converting its biomass content into a gaseous energy carrier. The syngas can be used in boilers or in combined cycle processes to generate on-site electricity and process steam.

Benefits The potential advantages of black liquor gasification are the greater end use flexibility offered by a gaseous fuel, reduced air pollutant content, and higher electricity-to-heat ratios in combined cycle systems than standard recovery boiler steamturbine systems. Research is being pursued in black liquor gasification technologies and applications.

5. Higher Dryness of Paper Entering at Dryers

The dryer is the key energy consuming step of the paper making process using 68% of the heat and an estimated 13% of the total electricity. To reduce the heat demand, the dryers content of the web at thedryer entry needs to be higher. A 5% reduction of the water content entering the dryer would lead to a 10% reduction in drying energy requirement.

Benefits
Innovation in wire and press design together with improvements to the vacuum systems helps in achieving higher dryness of paper prior to dryer section.

6. Impulse Drying

Impulse drying may lower the moisture content of the paper web entering the drying section by up to 38%, thereby significantly lowering the energy required in the paper machine’s drying stage. Impulse drying involves pressing the paper between one very hot rotating roll (150-500oC) and a static concave press with a very short contact time. The pressure is about 10 times higher than that in press and CondeBelt drying.

Benefits
Potentially, energy savings can be significant. Reports from literature show an estimated potential savings of 50-75% in drying steam consumption. Other reported benefits of this technology includes reduced capital costs, increased machine productivity, improved strength, reduced fibre use, and increased recycled fibre content allowed for any given paper strength.

However, current results from pilot operations show limited energy efficient improvements when compared to state-of-the-art efficient paper machines. Hence, further research is needed to realize the promises of impulse drying.

7. Magnetically-Coupled Adjustable-Speed Drives

Magnetically-coupled Adjustable-Speed Drives (MC-ASDs) are a new type of ASD, in which the physical connection between themotor and the driven load is replaced with a gap of air. Torque is generated by the interaction of rare-earth magnets on one side of the drive with induced magnetic on the other side. The amount of torque transferred is controlled by varying the air gap distance between the rotating plates in the assembly. Compared to existing ASDs, various advantages of MC-ASDs have been reported. These are:

• A greater tolerance for motor misalignment
• Little impact on power quality
• The ability to be used with regular duty motors (instead of inverters)
• Expected lower long term maintenance costs
• Extended motor and equipment lives, due to elimination of vibration and wear on equipment

The results of the trials in mills in US have shown promise to adopt this technology. However, introduction of this technologymay need techno-economic evaluation in Indian mills.

Advanced Fibrous Fillers
Mineral fillers are commonly used to replace fibres in the production of paper products. In Indian mills, the filler loading reported is up to 30% in some agro-based mills. However, it still needs to be validated with respect to the product quality requirements. Therefore, major paper mills limit the filler loading to roughly 15-20% due to paper strength and quality requirements.

It is reported that new inorganic fibrous fillers have been developed that could raise the filler loading limit to up to 50%, while maintaining paper strength and quality in many products.

Benefits
Reportedly, the use of fillers could reduce energy consumption by 25% and costs by Rs 500 to 2000 per ton of paper produced, attributable to saving in pulp and reduced drying energy due to an increase in the percentage of press solids in the sheet.

8. Stickies and Efficient Contaminant Removal

Adhesive materials (often called ‘stikies’) on secondary fibre feedstock can significantly degrade the quality of recycled paper products. The existing method used to remove stickies by screening, dispersion and kneading are not very effective. In US, a demonstration on the principle of electrohydraulic discharge indicated that it can remove these contaminants effectively and in an energy efficient manner. The technology uses the discharge of sparks in cleaning and screening processes to enhance the removal efficiency of stckies in screening and cleaning and to increase the efficiency of flotation of deinking.

Benefits
The technology is at trial stage. However, it is reported that apart from improved stickies removal, it could lead to direct energy use reductions of 10-15% in contaminant removal and cleaning equipment. The technology may be very useful for Indian mills, as the major capacities expansion on plan are on waste paper.

9. Bio drying Technology for the Drying of Pulp and Paper Mixed Sludges

Effective sludge management is increasing critical for pulp and paper mills due to high landfill costs and complex regulatory frameworks. Sludge dewatering challenges are exacerbated at many mills due to improved in-plant fibre recovery coupled with increased production of secondary sludge, leading to a mixed sludge with a high proportion of biological matter that is difficult to dewater. Various drying technologies have emerged to address this challenge of sludge management, whose objective is to increase the dryness of mixed sludge to above critical levels (~ 42% dryness) for efficient and economic combustion in the boiler for steam generation. A bio-drying process, enhanced by biological heat generation under forced aeration, has been developed that hassignificant promise.

Source: Compendium of Census Survey of Indian Paper Industry. Central Pulp & Paper Research Institute, 2015