Bagasse Pulp Mill Machinery: Converting Sugarcane Waste into Paper-Grade Pulp

Why Bagasse Is India's Most Important Agro Pulp Raw Material

India produces approximately 350 million tonnes of sugarcane annually, generating 100–120 million tonnes of bagasse — the fibrous residue remaining after juice extraction. Historically burned for boiler fuel in sugar mills, bagasse is increasingly recognised as a high-value pulping raw material that can substitute for imported wood pulp in India's writing, printing, and tissue paper industries.

Bagasse fibre is a short hardwood-equivalent fibre with good formation properties for writing and printing paper. Bleached bagasse pulp at 78–84 ISO brightness is suitable for maplitho, offset printing, and tissue grades — often blended with 20–30% bleached softwood pulp for strength. Proximity to sugar mills (most major Indian sugar-growing regions) means raw material transport costs are low for mills located in UP, Maharashtra, Karnataka, Tamil Nadu, Andhra Pradesh, and Gujarat.

The economics are compelling: bagasse costs INR 1,500–2,500/tonne ex-sugar mill (or often supplied free in exchange for disposing the sugar mill's bagasse surplus), versus INR 40,000–55,000/tonne for imported BHKP. A bagasse pulp mill producing bleached pulp at INR 12,000–18,000/tonne for INR 2,000/tonne raw material cost achieves raw material margins that imported pulp users simply cannot match.

• India bagasse generation: 100–120 million TPY; major surplus available
• Proximity: sugar mills in UP, Maharashtra, Karnataka, Tamil Nadu, Andhra Pradesh, Gujarat
• Bagasse cost: INR 1,500–2,500/tonne wet; often available free for disposal arrangement
• Bagasse fibre type: short fibre (0.8–1.5 mm) — similar to hardwood kraft pulp
• Applications: writing paper (50–70% blend), maplitho, tissue, newsprint base

Depithing: The Critical First Step

Fresh bagasse contains 25–35% pith — the spongy, non-fibrous cellular tissue from the inner core of the sugarcane stalk. Pith contributes no useful fibre to the pulp; it simply consumes cooking chemicals and reduces pulp yield while generating extra black liquor. Depithing removes 75–85% of pith before the bagasse enters the digester, dramatically reducing chemical consumption and improving pulp quality.

Dry depithing, using a rotating drum screen (wet-type depither), is the most common depithing system for Indian bagasse pulp mills. Wet bagasse from the sugar mill is fed into the rotating drum; the pith particles, being smaller and lighter than fibre bundles, pass through the screen perforations while the cleaned bast fibre advances to the digester feed conveyor. A well-designed depither achieves 8% or lower residual pith content in the accepted fibre stream.

Depithing is not optional — it is the first essential process step. Mills that skip depithing to reduce capex find that cooking chemical consumption increases 30–40%, pulp yield drops 4–6%, and black liquor handling becomes disproportionately difficult. The depither capital cost (INR 40–80 lakh for a 50 ODT/day system) is recovered within 3–4 months of operation through chemical savings alone.

• Pith content in raw bagasse: 25–35% of dry weight
• Target pith after depithing: ≤ 8% for good pulp quality
• Dry depither: rotating drum screen; simplest and most common in India
• Wet depither: hydraulic separation — better pith removal; higher water use
• Bagasse moisture: 48–52% as received; must be stored covered to prevent fire and degradation

Soda-AQ Cooking: The Right Chemistry for Bagasse

Bagasse is pulped using the soda process (NaOH cooking) or soda-AQ process (NaOH + anthraquinone catalyst) in a batch or continuous digester. Soda-AQ is strongly preferred — the anthraquinone addition at 0.05–0.1% on dry bagasse increases screened pulp yield by 5–8% and improves pulp strength, with AQ cost recovered many times over in yield improvement.

Cooking conditions for bleachable-grade bagasse pulp: active alkali 14–18% (as NaOH on OD bagasse), maximum temperature 165–170°C, H-factor 400–700. The resulting unbleached pulp has kappa number 14–20, suitable for ECF bleaching to 78–84 ISO brightness. For kraft liner grades, higher kappa (25–35) at lower alkali charge gives a stronger but lower-brightness pulp.

Batch digesters — spherical rotary digesters of 25–50 m³ volume — are the standard cooking equipment for 30–100 ODT/day bagasse pulp mills. Their simple mechanical design, ease of maintenance, and suitability for seasonal batch-mode operation (matching sugar mill bagasse supply) make them well-suited to Indian operating conditions. Continuous digesters (Kamyr-type) are used for larger mills above 150 ODT/day where the higher capital cost is justified by operating labour savings.

• Active alkali: 14–18% as NaOH on OD bagasse
• Cooking temperature: 165–170°C; heating time: 60–90 min
• H-factor target: 400–700 for bleachable grade (kappa 14–20)
• AQ dosage: 0.05–0.1% on OD bagasse; yield improvement 5–8%
• Batch digester size: 25–50 m³ for 30–100 ODT/day; spherical or vertical cylinder

Pulp Washing, Screening, and Bleaching

After cooking, the pulp must be washed to remove spent liquor (black liquor containing dissolved lignin and cooking chemicals), screened to remove uncooked fibre bundles (shives), and bleached if the target paper grade requires brightness above 65 ISO.

Washing is performed on vacuum drum washers (3 stages in series) or pressure diffuser washers. Target: washing efficiency above 95% (black liquor carry-over below 5 kg Na₂O/ADT pulp). Poor washing means black liquor carryover into the bleach plant — increasing bleaching chemical consumption and creating downstream quality problems.

ECF (Elemental Chlorine Free) bleaching using a D0-Eop-D1 sequence (chlorine dioxide — alkaline extraction with oxygen/peroxide — chlorine dioxide) achieves 78–84 ISO brightness for bleached bagasse pulp. ECF bleaching is the current industry standard; elemental chlorine bleaching is no longer used in modern mills due to environmental concerns. Rajshree designs ECF bleach plants as standard; totally chlorine-free (TCF) bleaching is available for specialty markets.

• Brown stock washer: 3-stage vacuum drum washer OR pressure diffuser
• Washing efficiency target: ≥ 95% (black liquor carry-over below 5 kg Na₂O/ADT)
• Pressure screen: 0.15–0.2 mm slot for shive removal before bleaching
• ECF bleaching sequence: D0-Eop-D1 or D0-Ep-D1 for 78–84 ISO target
• ClO₂ generation: on-site SVP Mathieson or R8 process for medium/large mills

Business Case: Integrated Bagasse Pulp + Paper Mill

The most compelling investment model is an integrated bagasse pulp + writing paper mill located adjacent to a sugar mill. The sugar mill supplies bagasse at low or zero cost; the integrated mill converts it to bleached pulp and then to maplitho or offset printing paper, capturing the full value chain from agricultural waste to finished paper.

At 50 ODT/day pulp production from a 150 ODT/day bagasse input, the integrated mill produces approximately 45 TPD of finished writing paper (85–90% pulp-to-paper conversion). With bagasse at INR 2,000/tonne and finished paper at INR 62/kg, the gross margin structure is dramatically better than a mill buying imported BHKP at INR 48/kg: the raw material advantage alone adds INR 18,000–20,000/tonne of pulp to EBITDA versus an imported pulp user.

Total capex for a 50 TPD integrated bagasse pulp + writing paper mill: INR 90–130 crore (pulp mill + paper machine + ETP + utilities). EBITDA of INR 25–35 crore/year at design capacity gives a payback of 3–5 years — among the best returns available in India's paper sector for the 50 TPD scale.

• Bagasse input for 50 ODT/day pulp: ~150–160 ODT/day dry bagasse
• Screened pulp yield: 42–46% on OD bagasse (soda-AQ cooking)
• Bleached pulp production cost: INR 12,000–18,000/tonne all-in
• Comparison: imported BHKP at INR 45,000–55,000/tonne
• Integrated mill EBITDA payback: 3–5 years at design capacity

Frequently Asked Questions