Tel: +1 908 500 7500 Stephen R. Parker, Sales Engineer
Ringoes, New Jersey
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Process Furnace System Design Considerations

1. When is a dangerous situation created by mixing of waste gases with combustion air?
A fuel/air mixture of a small quantity of fuel and a lot of air is a lean mixture. As the amount of fuel in the ratio increases, the mixture approaches the Lower Explosive Limit (100% LEL). 100% LEL is the minimum amount of fuel needed to support combustion. As the ratio increases, the flame will be able to achieve higher temperatures; the ratio will eventually reach the stoiciometric ratio. This is the theoretical ratio that means that all of the fuel will burn with all of the air. As the ratio increases, all of the air is used, but the exhaust will contain unburned fuel.

More fuel and the ratio will eventually reach the point where there is so much fuel, and so little air, that the flame will be extinguished. This is the Upper Explosive Limit (UEL). Under these conditions, gas stream will contain virtually no oxygen and so this ratio is above the UEL. To burn this gas stream, we must mix in air, in a controlled environment at high temperature, such as in our combustion chamber so that combustion takes place continuously and not all at once. Whether the gas stream is injected through the orifice/venturi in the burner or through an orifice/venturi in the chamber will not effect the destruction efficiency or the margin of safety of the equipment.

The burners that are typically used by HTT are medium velocity packaged burners that fire down a stainless steel tube assembly. We can introduce the customers waste gas stream directly into the combustion air blower or can add a second stainless tube around this firing tube for injection of your gas stream directly into the chamber. Under these conditions the gases are both heated and mixed with the products of combustion and the excess air. As the mixing takes place continuously in the chamber, there is no possibility of an explosion or rough light-off. As a further safety feature the package could include the optional spark arrester in the waste gas duct to the oxidizer.

2. What is the source of primary and secondary air to the premix burner?
The main burner combustion blower, if properly sized, will supply both the primary and secondary air. The oxidizer must control the flow of combustion air and auxiliary fuel regardless of the amount of the makeup of the waste gas being feed to the chamber. We have designed the HTT Systems with an oversized burner, which has a very high turndown ratio. For the system to operate properly, the combustion air must be adjusted until the exhaust has a minimum O2 content of 8-10% (100% excess air). Below 8%, the system may not properly destroy the waste gases and may even smoke. The burner is sized larger to provide all of the combustion air necessary for the auxiliary fuel and for the waste gas stream combined, with a good safety margin. Two temperature controllers will adjust the fuel and air separately.

3. How does the HTT system control the secondary air in high VOC concentrations streams?

We have designed a system that will control the secondary air and fuel separately. By adding only as much air as the process requires, the system will lower overall operating costs and keep the chamber at temperature in standby condition. The systems will startup with minimal secondary air. The burners will bring the unit up to operating temperature. Once temperature is reached, the waste gas dampers will open to direct the flow to the unit. The waste gases will enter the chamber, mix with the secondary air and will pass through the burner flame.

During startup, the blower butterfly valve will be set at minimum airflow as required for output and the fuel butterfly will be set at maximum output. This is adjusted to bring the oxidizer up to the set point as required. Once the unit approaches set point, the air will remain constant and the fuel input will decrease until the temperature set point is maintained.

Upon reaching this set point, the duct valve will open and the injection of the waste gases into the oxidizer will begin. Since the gases will contain a heat value, the temperature will begin to rise. When this happens, the fuel output will drop to a preset minimum low-fire. If the heat value does not maintain temperature, the burner will control the auxiliary fuel to maintain the set temperature. If the burner drops to low fire and the temperature begins to rise above the first set point, a second set point on the combustion air controller will increase the secondary air until the second set point is maintained.

Because the gases will contain heat value, the temperature will rise in the chamber. This will cause the burner controls to decrease burner output. If the heat content is high enough, the burners will drop to low fire and the temperature will continue to rise. The secondary air dampers will be operated from the second controller. Once this set-point is reached, the damper opens and the secondary air is increased. The secondary air volume controls the temperature. Typically, this set point is 50 F above the burner set-point. As the heat value of the gas changes the secondary air damper will adjust.

The system shall include a 6" ceramic fiber refractory lined horizontal/vertical chamber. The waste gas connection will be on the side of the unit or at the blower inlet and the exhaust stack connection will be on the top. The installation will require setting unit, utility connections, and bolting on the stack and customer provided supply duct.

I have also designed the systems using Nema 4 components and can include an optional air/N2 pressure regulator for implementation of a Class X or Z purge. This will satisfy your requirements for XP area classification.