Cogeneration in biomass-based power plants

Cogeneration (combined heat and power) is the production of electricity and useful heat from the same fuel at the same time. Heat that would be rejected in a power-only plant is used for a process.

Why it is useful in biomass-based plants

1. Low efficiency of biomass power alone – small biomass steam plants have electrical efficiency of only about 15–25% due to low steam pressure and small size. Using the exhaust/back-pressure steam for heat raises total efficiency to about 60–80%.
2. Heat is needed on site – biomass plants are normally attached to agro-industries that need steam or hot air:
   • Sugar mills burn bagasse; steam is used for juice heating and evaporation, and power for crushers.
   • Rice mills use rice husk; heat for parboiling and drying paddy.
   • Paper, timber and tea industries use residues for drying.
3. Fuel available at the site – the residue is produced at the same factory, so no transport cost; the plant is a captive plant.
4. Better economics – selling surplus electricity to the grid plus saving fuel for heat shortens payback.
5. Environment – waste disposal problem is solved and carbon-neutral fuel replaces diesel or coal.

 Bagasse -> Boiler -> HP steam -> BP turbine -> Gen
                                     |
                             LP steam to process
                            (juice heating, drying)

Example: sugar mills in the Terai of Nepal use bagasse cogeneration to meet their own demand during the crushing season.