Material Insights · Understanding the Material World

Bagasse and black-liquor cogeneration: Science and thermodynamics

Why boiler pressure decides how much power a mill can sell

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

Sugar and pulp mills burn their own residues (bagasse, black liquor) in boilers, pass the steam through back-pressure turbines for power and use the exhaust steam in the process. Overall efficiency is high, but electricity output depends heavily on boiler pressure and plant size.

Grade of energy

Favourable

Unlike most recovery tiles, the energy here is a residue fuel rather than waste heat: IRENA lists both bagasse and black liquor as process residues costing USD 0-4 per GJ. The boiler raises high-pressure steam; a back-pressure turbine then expands it only to the pressure the mill needs, typically 50, 150 or 250 psig (about 3.4, 10.3 or 17.2 bar gauge), and all exhaust steam goes to evaporators, dryers or juice heaters. The higher the boiler pressure relative to process pressure, the more power each tonne of steam yields. In India, Gujarat's regulator assumes bagasse has a gross calorific value of 2,250 kcal/kg and a plant heat rate of 3,600 kcal per kWh (GERC 2025). A peer-reviewed survey shows the payoff of pressure: old 22 bar back-pressure mills export almost nothing (0-10 kWh per tonne of cane, Brazilian data), while Indian mills with 67 bar, 495 C boilers and extraction-condensing turbines export about 90-120 kWh per tonne.121122

Bagasse and black liquor fuel cost USD 0-4/GJ2 Typical back-pressure exhaust 50, 150 or 250 psig1 Bagasse gross calorific value (GERC norm) 2,250 kcal/kg11 Station heat rate (GERC norm) 3,600 kcal/kWh11 Surplus power, 22 bar back-pressure vs 67 bar condensing 0-10 vs 90-120 kWh per tonne of cane22

Exergy versus energy efficiency

Mixed

Overall CHP efficiency for biomass is 70-90% (IRENA) or 80-90% (ETIP), because nearly all exhaust steam is used. The electricity share, which carries most of the exergy, varies widely with scale: IEA Bioenergy gives about 10-12% net electrical efficiency for back-pressure plants under 1 MWe, about 25% for 5-10 MWe and over 30% in cogeneration for plants around 50 MWe or more. Turbine isentropic efficiency rises from 60-70% at 250-500 kW to 80-85% at 25 MW. This is why upgrading old low-pressure sugar mill boilers to high-pressure units raises exportable power sharply.1234

Overall biomass CHP efficiency 70-90%2 Electrical efficiency, under 1 MWe back-pressure about 10-12%3 Electrical efficiency, 50 MWe and above (CHP) over 30%3 Turbine isentropic efficiency, 250 kW to 25 MW 60-70% to 80-85%1

Media compatibility and corrosiveness

Mixed

Black liquor is the spent pulping chemical solution: burning it in a recovery boiler both raises steam and recovers sodium chemicals as molten smelt. Smelt contacting water can cause violent explosions, the defining safety hazard of recovery boilers. Bagasse is fibrous, wet and ash-bearing, which limits combustion efficiency and erodes boiler tubes; boilers must tolerate seasonal stockpiled fuel. Fireside corrosion and fouling limit steam temperature in both cases.6

Variants compared1210913142324

VariantFuel or roleScaleNotes
Bagasse cogenerationCane fibre burned in mill boilersIndia 9,821 MW installed (Oct 2025, MNRE data as reported); plants 0.5-50 MWeMill self-sufficiency plus grid export; existing state tariffs about Rs 3.8-6.1/kWh, CERC proposes about Rs 7.5-9.4 for new 2026-27 projects
Black liquor recovery boilerSpent pulping liquor; also recovers chemicals1,950-7,200 t dry solids/day in recent Valmet unitsAanekoski mill: 260 MW electricity generation alongside 1.3 Mt/yr pulp (2017 start-up figures); 240% electricity self-sufficiency (company figure)
Back-pressure steam turbinePower from steam on its way to processunder 100 kW to over 250 MWUSD 650-2,700/kW; 0.5-1.0 US cents/kWh O&M

Sources

  1. Steam Turbines, CHP Technology Fact Sheet Series (DOE/EE-2976) US Department of Energy, 2025
  2. Biomass for Heat and Power, Technology Brief E05 IRENA and IEA-ETSAP, 2015
  3. Power generation and cogeneration (small-scale biomass technologies) IEA Bioenergy Task 32, 2026
  4. Biomass CHP facilities (factsheet) ETIP Bioenergy, 2018
  5. New process will recover more energy from black liquor and improve environmental performance and safety of paper making (1999, dated) US DOE Office of Industrial Technologies, 1999
  6. High efficiency levels: tapping bagasse-based cogeneration plants to add to India's renewable power (MNRE data) (trade press) Power Line, 2025
  7. Green frontiers: increasing scope of bioenergy in the energy transition (MNRE data, October 2025) (trade press) Renewable Watch, 2025
  8. Order No. 02 of 2025: determination of tariff for procurement of power from biomass and bagasse based cogeneration projects (control period 2023-27) Gujarat Electricity Regulatory Commission, 2025
  9. Valmet to deliver a new recovery boiler and an ash crystallization plant to Klabin's Monte Alegre mill (press release) Valmet, 2024
  10. Valmet's technology plays key role in Metsa Group's Aanekoski bioproduct mill (2017, dated) Bioenergy International, 2017
  11. Challenges in bioenergy production from sugarcane mills in developing countries: a case study (Colombo, Ocampo-Duque and Rinaldi), Energies 7(9), 5874-5898 MDPI Energies (peer-reviewed), 2014
  12. Pricing update: CERC proposes generic tariffs for select renewable technologies (CERC 2026-27 proposal) Power Line (trade press), 2026
  13. Aanekoski bioproduct mill (company figures) Metsa Fibre (Metsa Group), 2026