Combustion And Related Subjects

This section covers the fundamental principles of combustion in fired heaters, including burner design, air-fuel ratios, combustion efficiency, and related topics essential for proper heater operation.

The Combustion Process

The combustion in fired heaters takes place in the burner. The types of burners and how they function are not covered in detail in this section. The amount of heat released can be easily calculated for a gas when we know the composition of the fuel and the heating values of the various components. For liquid fuels, the heating values are obtained by a calorimeter test.

From these values and using the standard combustion equation, we can determine the composition of the flue gas. As an example, the combustion of methane could be stated:

Of course for fuel gases containing many more components and burning in air rather than pure oxygen, the equation gets more complicated. Therefore, a task that in itself is quite simple, becomes a burden to do by hand, but can be easily accomplished by a simple computer program. The heating values normally used in fired heater design are the LHV, lower heating values.

Lower Heating Values (LHV) Reference Table

The following Lower Heating Values are commonly used in combustion calculations:

Component	Btu/lb	Component	Btu/lb	Component	Btu/lb
CH₄	21,520	C₂H₆	20,432	C₃H₈	19,944
N-C₄H₁₀	19,680	I-C₄H₁₀	19,629	N-C₅H₁₂	19,517
I-C₅H₁₂	19,478	C₆H₁₄	19,403	CO	4,347
H₂	51,623	N₂	0	CO₂	0
C	14,093	S	3,983	C₂H₄	20,295
C₃H₆	19,691	C₄H₈	19,496	C₆H₆	17,480
H₂O	0	O₂	0	H₂S	6,545

Burner Types & Selection

Burners for fired heaters can be generally divided into two categories, natural draft and forced draft. The natural draft type burner requires less pressure differential to provide the required air for combustion than the forced draft burner. Air pressure differential, or pressure drop, across a natural draft burner would normally fall in the 0.1 to 1.0 in H₂O range, where the forced draft burner would normally require 0.3 to 4.0 in H₂O. Burner combustion air can also be induced by the fuel gas flowing through a venturi section. Also burners have different air registers for primary and secondary air intake. The air may be delivered to the registers by an air plenum.

In addition to burners being classified by the draft requirements, they are also described by the fuel they burn such as oil or gas or combination. There are numerous fuels which may be burned including:

Refinery Gas	Propane or Heavier Gas
Natural Gas	High Hydrogen Gas
Waste Gas	No. 2 Fuel Oil
No. 6 Fuel Oil	Other Liquids

Burners may be of the low NOₓ which may incorporate staged air or fuel designs. These burner types have become almost standard in the developed world where environmental air standards demand the best combustion technology available.

Burners may be designed for mounting in the heater floor, side walls, or end walls. The burner tips can be designed for various shapes or flame patterns.

In conclusion, there is such a wide variety of types and configurations of burners available from the many manufacturers, that selecting a particular burner for a design requires that the designer work closely with the burner manufacturer to assure the correct selection. But, for many furnaces, the burner(s) can be selected and sized using the standard data provided in the manufacturer's catalog.

Burner Sizing Example

NOₓ and Other Considerations

The fired heater industry has concentrated on the two primary sources of Nitrogen Oxides, NOₓ. These are normally referred to as Thermal NOₓ and Fuel NOₓ. The technology for NOₓ reduction via the combustion focuses on one or more of the strategies:

• Reduce peak temperatures of the flame zone
• Reduce gas residence time in flame zone
• Reduce oxygen concentration in the flame zone

Both process modifications and new burner/furnace designs rely on the following concepts to implement the three main strategies for reducing NOₓ emissions:

1. Reduce Peak Temperatures

• Using a fuel-rich primary flame zone
• Increasing the rate of flame cooling
• Decreasing the adiabatic flame temperature by dilution

2. Reduce Gas Residence Time

• Changing the shape of the flame zone
• Using the steps listed in strategy 1

3. Reduce O₂ Content in Primary Flame Zone

• Decreasing the overall excess air rates
• Controlled mixing of fuel and air
• Using a fuel-rich primary flame zone

Burning with Low Excess Air

For many years, it was not uncommon to see furnaces operating with 50 to 100% excess air. It was simply easier for the operator to just make sure there was plenty of air. Of course this method of operation also resulted in reduced efficiency and more NOₓ generation. As fuel costs and environmental concerns have risen, these practices have changed. With the excess air at levels of 15 to 30% (lower for gas and higher for oil), the furnace could be operated with a minimum of monitoring. In recent years, development of advanced instrumentation has allowed continuous automatic furnace monitoring and control of excess air, and the percent excess air can now be reduced below the 10-30% limit. EPA tests concluded that a 19% average reduction of NOₓ can be achieved by reducing the percent excess air from an average of 20% to an average of 14%.

Air Staging

Burner designs for accomplishing air staging vary among the burner manufacturers, but they basically work the same. Only a portion of the air flows across the fuel injection zone and this forms a fuel rich primary combustion zone where the fuel is only partially burned. As a result, only a portion of the fuel nitrogen decomposes to form molecular nitrogen, thus reducing NOₓ formation. And since excess air is not available, Thermal NOₓ is also reduced. The remainder of the air is injected downstream to complete the combustion.

Fuel Staging

When firing gas fuels, thermal NOₓ can be controlled using fuel staging. The fuel is divided into two or more streams. Only a portion of the fuel is injected into the primary combustion zone. The NOₓ levels are very low because the flame temperature is low. The rest of the fuel requirement is introduced downstream. Flame lengths in staged fuel burners are generally shorter and more defined compared to staged air burners. A lower excess air is achievable with this type burner.

Selective Catalytic Reduction (SCR)

Postflame treatment processes add a reducing agent to the combustion gas stream to take oxygen away from NO. For large heaters ammonia is used as the reducing agent. The desired reaction is:

This requires 2/3 ammonia molecules for each molecule of NO. However, there is always some oxygen present, which leads to reactions like:

These reactions can be carried out over a zeolite catalyst at 700° to 800°F. This catalytic process has been employed on a large scale and gives up to 85% reduction in NOₓ.

Emissions Correction Formula

To correct emission readings to 3% oxygen (15% excess air), use the following equation:

Purpose Of Excess Air

Perfect combustion is achieved when all the fuel is burned using only the theoretical amount of air. Perfect combustion cannot be achieved in a fired heater. Complete combustion is achieved when all the fuel is burned using the minimal amount of air above the theoretical amount of air needed to burn the fuel. With complete combustion, the fuel is burned at the highest combustion efficiency. Incomplete combustion occurs when all the fuel is not burned, which results in the formation of soot and smoke.

Oxygen for combustion is obtained from the atmosphere, which is about 21% oxygen by volume or 23% by weight. About 2000 cubic feet of air is required to burn one gallon of fuel oil at 80% efficiency at sea level. About 15 cubic feet of air is required to burn one cubic foot of natural gas at 75% efficiency at sea level.

Air Classification

Air required in combustion is classified as:

Primary Air

Primary air controls the rate of combustion, which determines the amount of fuel that can be burned.

Secondary Air

Secondary air controls combustion efficiency by controlling how completely the fuel is burned.

Excess Air

Excess air is air supplied to the burner that exceeds the theoretical amount needed to burn the fuel.

Natural gas contains more hydrogen and less carbon per unit of heat content than oil and consequently its combustion produces a great deal more water vapor which withdraws a greater amount of heat from the flame. Therefore gas efficiency is always slightly less than oil efficiency.

The good news about excess air is that it provides a measure of safety. The bad news is that it wastes fuel. The less excess air used results in the least amount of "waste".

Preheated Combustion Air

"Technology is now available to increase efficiencies to 92 percent and better." These words were written in 1979 for a paper presented to the Mid-Year API meeting. The words are still true today. The single, most significant obstacle to overcome in this application is high flue gas dew points versus percent sulfur in both oil and gas fuels.

There are basically two ways to obtain preheated combustion air and reduce the amount of fuel required. One process would use heat from an external source to preheat the combustion air, such as waste steam or flue gases from other sources. The other method would be to utilize the flue gas from the heater to preheat the combustion air.

The amount of heat available in the flue gas can be calculated using the enthalpy of the flue gas at the entering and exiting temperatures:

Where:

• q_avail = Heat available in flue gas, Btu/hr
• h_in = Enthalpy of gas at inlet temperature, Btu/lb
• h_out = Enthalpy of gas at outlet temperature, Btu/lb
• W_g = Flue gas mass flow, lb/hr

Acid Dew Point Of Flue Gas

To improve the thermal efficiency of combustion equipment it is necessary to cool the flue gas to a low outlet temperature, to recover as much heat as possible. But the temperature must not be so low as to allow sulfur from the fuel to condense as sulfuric acid, resulting in a very corrosive flue gas.

The calculation of acid dew point is based on the method described by A. G. Okkes in Hydrocarbon Processing, dated July 1987, pp 53-55. The SO₃ conversion from SO₂, if calculated, is assumed to be set by equilibrium at 1000°C. The flue gas is assumed to be at atmospheric pressure.

Typical Flue Gas Composition

Component	Typical Mole %
Nitrogen	72.74
Oxygen	1.72
Carbon Dioxide	8.66
Water	16.71
Argon	0.00
Sulfur Dioxide	0.16
Sulfur Trioxide	Calculated

Flame Temperature

Based on article by Carl A. Vancini, in Chemical Engineering, March 22, 1982

Here a simple heat balance serves as the basis for calculating the flame temperature. The increase in enthalpy between the unburned and burned mixtures is set equal to the heat produced by combustion.

Up to a flame temperature of about 2,500°F, the burned mixture generally includes such ordinary gases as CO₂, N₂, SO₂, H₂O, and residual O₂ (from excess air). At higher temperatures, CO₂ appreciably dissociates to CO and O₂; H₂O to O₂ and OH⁻; O₂ to O⁻²; H₂ to H⁺; N₂ to N⁻³ and NO (produced by N₂ and O₂) to N⁻³ and O⁻². These dissociation reactions absorb an enormous amount of energy (heat), substantially lowering the flame temperature being calculated.

Heat Balance

The heat balance is calculated as follows: At constant pressure, the heat, Q, required to bring the temperature of one pound of gas from temperature 0 to temperature t is:

The variation of cₚ with temperature can be approximated by a polynomial:

where, a, b, c, and d are constants that depend on the nature of the gas.

For transferring heat at a constant pressure:

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