Page 89 - Wastewater Solids Incineration Systems

58 Wastewater Solids Incineration Systems

For efficient operation, the high range of oxygen must be 5% based on wet gas.
Anything greater than 5% will have a quenching or cooling effect on the bed and will
reduce overall efficiency.

3.5 Turbulence
Combustible materials and combustion air must be mixed to optimize combustion or
to efficiently operate at low excess air. The combustible material must be provided
with adequate surface area to contact with oxygen and react. Highly agitated hot
sand quickly fragments feed material into small particles, which in turn are quickly
heated to volatilization temperature without depressing the bed temperature because
of the large heat inventory of the sand bed. With turbulence, feed is better distributed
to the bed and every particle of the fluid bed is exposed to the fluidizing combustion
air, providing almost infinite extended surface. On the other hand, with lack of tur-
bulence, feed material will be poorly distributed and a larger portion of the
volatilized organics will reach the freeboard before being oxidized in the bed. This
phenomenon can lead to excessive over-bed burning and subsequent higher emis-
sion of hydrocarbon volatiles and other products of incomplete combustion.
If the system is being operated in violation of one or more of the above five main
principles of combustion then incomplete or poor combustion will result.

4.0 FLUID BED DESIGN CONSIDERATIONS
To design a fluid bed incinerator that is as efficient as possible with minimum emis-
sions, the system must not operate with scale formation in the exhaust system nor
formation of agglomerates (clinkers) in the sand bed at minimum auxiliary fuel con-
sumption. Although scaling and formation of clinkers depends on the characteristics
of the feed material, emissions of pollutants and auxiliary fuel consumption are
directly linked to the fundamental parameters of design.

4.1 Fundamental Design Parameters

To determine equipment size and characteristics, it is necessary to determine
required airflow to the furnace, flue gas quantities, supplementary fuel requirements,
and cooling water requirements. First, a mass balance is determined. Then the heat
balance can be prepared. Finally, system exit characteristics can be found. Heat and
mass balances are discussed elsewhere in this chapter; this section emphasizes the
fundamental parameters of design commonly used in the sizing of the incinerator.