THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 293

Particle Control Technology : In most biomass combustion applications, particle
emissions are significant, and secondary measures are needed to comply with emis-
sion limits. Since there are many different types of particles, there is also a wide
range of particle control technologies. These include settling chambers, cyclones,
electrostatic precipitators (ESPs), fabric filters, and scrubbers. Settling chambers
are large compartments in which the gas velocity is reduced leading to sedimen-
tation of the particles by gravity. Settling chambers have low collection efficiency
and require much space, but they have simple designs and low investment and
maintenance costs. In cyclones, the particles are separated based on the principle
of centrifugal forces. The particle-laden gas is brought into a swirling motion by
injecting it tangentially into a cyclone. Due to the centrifugal forces, the particles
drift to the wall of the cyclone and then slide down into a container. The gas leaves
the cyclone at the top of the cyclone. Cyclones have higher collection efficiency
than settling chambers. To improve the collection efficiency, the centrifugal forces
can be increased by reducing the diameter of the cyclone. To keep the same capac-
ity, many small cyclones can be put in parallel, then called a multicyclone.
The disadvantages of a multicyclone are the more expensive construction and
the higher pressure drop, which leads to a higher energy consumption. At tolerable
pressure loss, multicyclones can only remove particles larger than 10 m. For

the removal of finer particles, ESPs or fabric filters need to be used. In an ESP,
the particles are first electrically charged and then exposed to an electric field,
in which they are attracted to the collecting electrode. The collecting surface is
cleaned periodically by vibration. The collected particles then drop into a con-
tainer. ESPs have problems removing particles with a high electrical resistance,
such as fly ash from dry straw combustion. Fabric filters are then preferred. In
fabric filters, the flue gas has to pass through a tightly woven cloth consisting
of special fibers. The filters are periodically cleaned by vibration or pressurized
air. Bag filters can remove fine particles at high efficiency, but the cloths are often
sensitive to high temperatures and fouling by condensing tars. For woody biomass,
it is common practice to use cyclones for coarse separation and ESPs or fabric
filters for fine separation. Another technology to remove particles is a scrubber.
In scrubbers, a mist of small droplets is created, through which the flue gas flows.
The particles collide with the droplets and are carried away by them. The droplets
are collected at the bottom of the chamber resulting in a stream of wastewater that
needs to be cleaned. The advantage of a scrubber is its ability to remove SO x ,NO x ,
and HCl as well by gas absorption. Disadvantages are corrosion and erosion
problems and the added cost of wastewater treatment.
NO x and SO x Control Technology :NO x emission levels of biomass combustion
applications are usually very low because of the small amount of fuel nitrogen
in biomass (Spliethoff, 2010). For biomass fuels with relatively high nitrogen
content (e.g., straw), primary measures are usually sufficient to meet the emission
limits. Secondary measures for NO x emission reduction are selective catalytic
reduction (SCR) and selective noncatalytic reduction (SNCR). In both methods,
a reducing agent

μ

is injected in the flue gas to reduce
NO to N 2 . In SCR, platinum, titanium, or vanadium oxide catalysts are used in a

— usually ammonia or urea —