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Biomass Boiler Efficiency

We suggest that the following formulae can be used to estimate the thermal efficiency of modern biomass boilers. The estimate required for the standing losses should be made after inspection where possible. If the outside of the boiler and combustion chamber are mildly warm to the touch then the standing losses would be around 1.5% MCR(boiler rated output). If they are hot but would not burn you, then we suggest that you allow 3% MCR, otherwise 6% would be a reasonable figure. 

          fgl = 0.74 * CO2 / (Tf - Ta)                          eff = 100 - fgl - sl

where :-
fgl = flue gas loss
CO2 = %CO2 in the dry flue gas
Tf = flue gas temperature at the boiler outlet in deg C
Ta = ambient air temperature in the boiler house in deg C
eff = estimated thermal efficiency of the boiler
sl = estimated standing losses

The calculations give values based on the net calorific value of the fuel and not the gross CV as normally given for gas.
The flue gas loss formula is based on average carbon, hydrogen, and oxygen contents published in the trade literature and on the Internet for dry ash free samples. However, as the content of the elements can vary from one species to another the answers must be considered as estimates and not facts. We have not carried out any quantitative tests to confirm the factor of 0.74.

This table gives approximate net calorific values for softwoods.

Calorific values of softwood

Thank you for your interest,
George Fletcher

updated 18/04/2009

George Fletcher