Temperature control of an industrial furnace (Combustion control)

To control the temperature of an industrial furnace that uses hydrocarbon, it is necessary to do a combustion control to regulate the ratio between air and fuel. This air/fuel ratio improve the efficiency of the combustion on the required temperature, meaning reduction of gas consumption.

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Automatic temperature control of a Industrial LPG furnace

Every year, more and more industries improve their combustion control because it represents reduction of production cost and improve the quality of their products.

Let me give you an example of this, we have a food plant that uses LP for the combustion, the average LP consumed by the furnace is 72,80 Lt/hr, and the cost of the LP is $0.46 per liter, so in one day the cost of LPG consumed by the furnace is approximately $803. The temperature of the furnace is regulated by the operators manually without any combustion control. It was noticed that sometimes the color of the flame was yellow, indicating a bad air/fuel ratio, deficiency of air (more gas in the mix). With a good combustion control we could expect reduce the gas consumption. When the operators were able to get a blue flame on the furnace, meaning a good combustion, the LP consumed by the furnace was 64.41 Lt/hr, that means a daily cost of $711, almost $100 less per day, so in one month they could save about $3000 in gas consumption.

Another thing that makes companies improve the combustion are the government environmental regulations, this forces industries to stay in rule with the gas emission limits of the SO₂, NO_x and CO_2. These limits vary according with the country, so the recommendation is to check what are the permissible limits in your place.

The characteristics of the furnace in discussion are the following:

1. Fuel: LPG.
2. 3 passes furnace.
3. Gas exhaust fan of 1 HP.
4. Air fan 5 HP.

To implement the temperature control based on the combustion control, it is required to know what is the air/fuel ratio for the specific fuel that the combustion is using. Depending of the composition of the hydrocarbon, the ratio varies. I will show you the way to know this magic number: 

Calculating the Air/Gas ratio:

The combustion of the hydrocarbons in the air, involves hundreds of chemical reactions, but for this case, we are going to consider the general reaction of a hydrocarbon type C_mH_m. Its stoichiometric reaction is:

Stoichiometric reaction of the hydrocarbons

In this case, the hydrocarbon is the LPG, that is mainly composed by 60% propane (C₃H₈) and 40% butane (C₄H₁₀). Each if these hydrocarbons have the following atomic mass:

Atomic mass of the propane:

Atomic mass of the propane

Atomic mass of the butane:

Atomic mass of the butane

The atomic mass of the propane 44 Kg/mol and the butane 58 Kg/mol. Adding both atomic masses we can get the atomic mass of the LPG, 102 Kg/mol. So, the proportions of propane and butane in the LPG are:

Propane and butane proportions of LPG

Knowing this proportions, we can proceed to calculate the LPG atomicity:

Atomicity of LPG

Replacing the LPG atomicity in the stoichiometric reaction:

Stoichiometric reaction of LPG

Form the left 2 terms of the reactions (hydrocarbon and air) we can calculate the Air/LPG ratio:

Air/Fuel ratio calculation of LPG

To ensure a complete fuel combustion, normally 10% air excess is considered.

Calculating the average temperature of the passes of the furnace according to the flow on each pass.

Industrial furnace with 3 passes, shut off valve and pressure regulators

In this case, the furnace has 3 passes. Each pass has thermocouples to measure the temperature. Since the passes are not equal, their contribution to the temperature of the furnace is different, so it is important to consider the fuel flow on each pass to calculate the average temperature:

Average temperature according to the flow

The following picture shows the configuration of the passes of the furnace:

Now, that we know the air/fuel ratio of the LPG and the basic calculation of the average temperature, we are ready to implement the temperature control.

Implementing the temperature control based on the combustion control:

To visualize the controller that we are going to implement, is good to have a sketch of the system:

As we can see on the sketch, the control has the following devices for the temperature control: Temperature transmitter for each pass, flow transmitters for each pass, the air and fuel and DCS or PLC to configure the control.

In the following sections, we are going to describe the configuration of each control logic.

Temperature control TIC-101

This controller calculates the average temperature taking as consideration the flow on each pass:

Average temp of 3 passes considering the flow

This calculated temperature is used as PV (process variable) of the controller TIC-101. The output of the PID block is scaled to fuel flow range and send it to the combustion controller.

Function block configuration of the temperature controller

Oxygen control AIC-101

This controller receives the PV from the oxygen transmitter and it is controlled by a PID where the operator entries the desire SP. The output is scaled and represent the oxygen trim or air excess to be used by the combustion control. In this case, we can set the output scale 0.8 – 1.2 (±20%)

Function block configuration of the oxygen controller

Combustion Control

This controller has 3 parts:

1. Calculation of the fuel SP using the fuel equivalent air.
2. Calculation of the air SP using the air equivalent fuel or the air equivalent temperature.
3. Interlock to ensure the air controller FIC-105 and the fuel controller FIC-106 are in the correct mode.

Calculating the fuel equivalent according to the quantity of air supplied:

Calculation of the fuel required for a complete combustion according to the air supplied.

Taking on considerations possible changes of the process and disturbances, the fuel equivalent air could be:

 Calculation of the fuel required considering disturbances in the system.  

To determine the SP that is going to be send to the fuel controller FIC-106, it is selected the minimum value between the output of the temperature controller TIC-101 and the Fuel equivalent air:

Fuel gas set point

Calculating the air equivalent according to the quantity if fuel supplied:

Calculation of the air required for a complete combustion according to the fuel supplied

Taking on considerations possible changes of the process and disturbances, the air equivalent fuel could be:

Calculation of the airl required considering disturbances in the system. 

Calculating the air equivalent according to the temperature used:

Calculation of the temperature required according to the furnace temperature. 

To determine the SP that is going to be send to the air controller FIC-106, it is selected the maximum value between the air equivalent fuel and the air equivalent temp:

Air set point

Interlocks

It is required that the fuel gas controller and the air controller be in cascade mode at the same time in order to work properly with the combustion controller.

If the air flow controller FIC-105 is not in cascade, it is no receiving the SP form the combustion controller. Then, the fuel gas flow controller FIC-106 will match the mode of the FIC-105.

If the air flow controller FIC-105 is not in cascade, it is no receiving the SP form the combustion controller. Then, the fuel gas flow controller FIC-106 will match the mode of the FIC-105.

Air flow controller FIC-105

This controller receives the PV from the flow transmitter FI-105. In cascade mode, it receives the SP from the combustion control. The PID output controls the flow valve FV-105.

Fuel gas flow controller FIC-106

This controller receives the PV from the flow transmitter FI-106. In cascade mode, it receives the SP from the combustion control. The PID output controls the flow valve FV-106.