Is the waste a gas, liquid, solid or a combination?
Downfired thermal oxidizers should be considered when burning waste streams that contain salts or solids. Waste gas and liquid streams can be destroyed in horizontal or vertical TOs.
What is the waste gas composition and heating value?
Gases with high heating values or fuel like waste gases can be introduced in the burner. Low heating value gases like tail gases should be introduced away from the main burner flame.
Guidelines for introducing waste liquids into a TO are similar to those of gases but must be atomized quickly for efficient destruction.
Small particulate suspended in the stream can be burned in a stationary downfired thermal oxidizer while larger solids should be fed to a rotary kiln.
What is the required destruction efficiency? What is the design flow rate? What is the normal flow rate?
These parameters are used to size the combustion chamber. Combustion engineers talk about the three “Ts”. Time, temperature and turbulence. For a given destruction efficiency, a waste gas must be properly mixed (TURBULENCE). It must reach the proper TEMPERATURE for the proper amount of residence TIME. Mixing is generally accomplished by how the waste gas is introduced into the combustor. Time is dictated by the velocity of the waste stream and the size of the combustor. Reducing the size of the combustor reduces the residence time and increases the required temperature for the specified destruction rate. This can reduce capital costs but increases operating costs. Increasing the combustor size increases residence time and reduces the required temperature. This reduces the operating costs but increases capital costs. EnviroPro can help find the best compromise.
What is the required turndown? How quickly will the unit cycle?
A system that runs in steady state for long periods of time will have a simpler control system than one that requires turndown and will cycle from maximum rate to minimum fire quickly.
What are the NOx and other emission requirements?
This will impact how the thermal oxidizer burner is designed. If the waste has bound nitrogen in the waste stream, designs like Low NOx burners should be considered to reduce emissions. NOx limitations could also require an SCR be installed on the back end.
Other emission requirements like particulate emissions may require post combustion treatment of the flue gas. Treatment methods often include baghouses, quench systems, venturis, dry or wet electrostatic precipitators.
Is heat recovery desired?
In many instances it becomes economical to recovery the heat generated. This is typically done by generating steam in a waste heat boiler or hot oil in a waste heat recovery unit.