Emission Controls for Small Wood-Fired Boilers - Western Forestry ...

9.0 OVERVIEW OF EMISSION CONTROLS FOR OTHER RELEVANT
POLLUTANTS
9.1 Nitrogen Oxides (NOx)
There are two major sources of NOx emissions. The first, “fuel NOx” is NOx produced by the oxidation of
fuel bound nitrogen during combustion. The second, “thermal NOx” is NOx produced by the oxidation of
nitrogen in the combustion air. The latter is produced at temperatures typically much higher than those
occurring during biomass combustion. Therefore, the total NOx is most influenced by the fuel nitrogen
content.
Combustion controls are the only way NOx emissions are controlled apart from add‐on controls such as
selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR).
Increasing excess air can help control thermal NOx emissions by reducing flame temperature. Oxygen
concentration is an indicator of the amount of excess air; therefore, monitoring oxygen concentration and
linking oxygen measurements with automated controls establish the appropriate quantity of excess air
on a continual basis and prevent excess thermal NOx emissions.
Staged combustion is another means for controlling thermal NOx emissions.
9.2 Sulfur Dioxide (SO 2 )
SO 2 emissions from wood combustion are negligible given very low sulfur content in biomass.
9.3 Carbon Monoxide (CO)
Carbon monoxide emissions are minimized by good combustion conditions, specifically, maintaining the
proper air to fuel ratio. Plume opacity observation, proper operation and maintenance, periodic
combustion efficiency testing, and in‐situ oxygen concentration monitoring are ways to ensure ongoing
good combustion conditions. A combustion chamber designed with staged combustion increases the
degree of combustion completeness and is therefore useful for minimizing CO emissions.
9.4 Volatile Organic Compounds (VOCs)
The same measures for minimizing CO emissions will minimize VOC emissions.
9.5 Hazardous Air Pollutants (HAPs)
Hazardous air pollutants include both gaseous and particulate based pollutants. The aforementioned
controls for CO and are also effective at controlling HAPs, but most effective at controlling gaseous HAPs.
Particulate HAPs are also controlled with an add‐on control device.
Resource Systems Group, Inc.
Emission Control Technologies for Small Wood‐Fired Boilers
6 May 2010 Page 24