THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 292

Improved Construction of the Combustion Application : In order to obtain optimal
combustion with minimal emissions from incomplete combustion, one has to
achieve:

￿ Sufficiently high combustion temperatures

￿ Sufficiently long residence times

￿ Optimal mixing of fuel gases and air, at varying loads

These factors are partly determined by the combustion technology and design of
the furnace and partly by the operation of the combustion process. Process
variables that can directly be adjusted typically are the amount of fuel fed into
the furnace and the amount of primary and secondary combustion air supplied.
Staged-Air Combustion : It is widely applied in biomass combustion applications.
It reduces both the emissions caused by incomplete combustion and NO emissions
by separating devolatilization and gas-phase combustion, which leads to an
improved mixing of volatiles and combustion air. In the first stage, primary air
is added for devolatilization and gasification of the fuel. The oxygen-to-fuel molar
ratio in this stage is smaller than one, resulting in a gas that consists of mainly CO,
H 2 , hydrocarbons, CO 2 , and water. In the second stage, sufficient secondary air is
provided to ensure complete combustion of this gas with low emission levels. The
improved mixing of gas and air in the second stage reduces the amount of air
needed, which leads to higher flame temperatures and better burnout. Air staging
can also be used to reduce NO emissions from the fuel NO formation mechanism.
The fuel gas contains NH 3 and HCN, which are converted to NO if sufficient O 2 is
available. However, in fuel-rich conditions, NH 3 and HCN will react with NO,
forming N 2 . Therefore, the emissions of NO can be reduced by optimizing the
excess air ratio in the first stage.
Staged-Fuel Combustion and Reburning : These are other possible methods to reduce
NO x emissions in biomass combustion applications. In the first stage of staged-fuel
combustion, primary fuel is combusted at an air-to-fuel ratio larger than 1, leading to
a flue gas with a relatively high NO concentration. In the second stage, secondary
fuel is introduced into the flue gas without additional air supply. A fuel-rich condi-
tion is created inwhich theNOfromthe first stage reactswithNH 3 andHCNfromthe
secondary fuel, reducing the NO levels in the same way as in staged-air combustion.
An additional effect is that NO is converted back to HCN by reactions with HCCO
(ketenyl) andCH x radicals. This effect is called reburning. In the last stage, sufficient
secondary air is supplied to achieve complete burnout.

9.5.3.2 Secondary Measures for Emission Reduction Secondary measures are
applied to remove emissions from the flue gas once it has left the combustion cham-
ber. This concerns mainly emissions from complete combustion, in particular particle,
NO x , and SO x emissions. Emissions of other components (HCl, heavy metals, PCDD/
F) can be reduced by secondary measures as well, but they are not discussed here
in detail.