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.

Thank
you for your interest,

George
Fletcher

updated 18/04/2009