Biomass combustion and CHP: Efficiency
Why an open fire wastes up to 90% of its fuel
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Burning biomass turns anywhere from under 10% (an open fire) to 90% (a well-matched combined heat and power plant) of the fuel's heat into useful energy. Electricity alone rarely exceeds 40%, and plants below 1 MWe manage only 10-12%.
Chart: Efficiency ranges
In service CommercialEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Open fires and traditional stoves | 10 to 20%31 | In service | Heat in fuel to useful cooking heat (thermal efficiency); ISO Tier 0 (below 10%) is typical of open fires |
| Improved biomass cookstoves (ISO Tiers 2-5) | 20 to 50%1 | Commercial | Thermal efficiency thresholds of ISO/TR 19867-3:2018: Tier 2 at least 20%, Tier 5 at least 50% (laboratory tests) |
| Small back-pressure CHP plants (below 1 MWe) | 10 to 12%2 | Commercial | Heat in fuel to net electricity |
| Dedicated biomass power plants | 16 to 40%32 | In service | Heat in fuel to net electricity: 16% at 2 MWe, 30-32% at 25-44 MWe, 36% at 350 MWe; over 40% for large condensing plants |
| Co-firing biomass in large coal plants | 36 to 44%3 | Commercial | Heat in fuel to net electricity (biomass burned alongside coal in a big, efficient boiler) |
| Biomass combined heat and power | 70 to 90%34 | Commercial | Heat in fuel to electricity plus useful heat, when heat demand is well matched |
| Charcoal from earth mound kilns | 12 to 16% by mass5 | In service | Mass of charcoal per mass of wood loaded (Mozambique field measurements) |
| Slow pyrolysis (carbonisation) under controlled conditions | 35% by mass6 | Record | Char yield from dry wood at about 400 C over hours to days |
Where the energy is lost
| Step | Loss |
|---|---|
| Open fire: heat escaping around the pot, unburnt smoke and wet fuel | 80-90% of the fuel's heat (calculated from 10-20% efficiency)3 |
| Power plant: heat rejected in the steam condenser and flue gas | 60-84% of fuel heat in electricity-only plants (calculated from 16-40% efficiency)32 |
Bone-dry wood holds 18-20 MJ per kg, but wet wood far less (about 8.5 MJ/kg at 50% moisture) because water must be boiled off. An open fire heats the air around the pot, an enclosed stove channels hot gas against it, and a power station loses most of its heat at the steam condenser.
Size matters for power: a 2 MWe plant converts about a sixth of its fuel to electricity, a 350 MWe plant or a co-fired coal boiler 36-44%. Using the leftover heat (CHP) lifts total efficiency to 70-90%. Traditional charcoal kilns keep only 12-16% of the wood's mass, burning much of the rest to drive the process.
Sources
- Voluntary Performance Targets for cookstoves (ISO/TR 19867-3:2018 tiers for thermal efficiency, CO and fine particulate emissions) Clean Cooking Alliance, 2018
- Power generation and cogeneration IEA Bioenergy Task 32 (Biomass Combustion)
- Biomass for Heat and Power, Technology Brief E05 IEA-ETSAP and IRENA, 2015
- Biomass CHP facilities (factsheet) ETIP Bioenergy (European Technology and Innovation Platform)
- Review of the existing studies related to fuelwood and/or charcoal in Mozambique, section 3.2 Charcoal production FAO
- Fast pyrolysis: key points (table of product yields by pyrolysis mode) IEA Bioenergy Task 34 (Direct Thermochemical Liquefaction)