As a fundamental process, combustion plays a crucial role in supplying the energy needed for everyday life and industrial applications. Understanding its basic principles is not only essential but also significant for optimizing and controlling its processes. Chemistry serves as the backbone of this occurrence, providing the underlying framework for combustion concepts. Hence, reviewing and emphasizing the core chemical principles outlined in the following discussion will be instrumental in studying the combustion process.
Process of Combustion
This process is fundamentally an oxidation process in which the oxidizable components of fuel react, and this can be represented by a chemical equation. Throughout this process, the mass of each element remains unchanged, making it essential to adhere to the law of conservation of mass in all reactions. According to Equation 1, the simplest type of reaction is the one that occurs between carbon and oxygen.
Equation 1
C + O2 → CO2
This equation states that 12 kilograms (equivalent to one kilomole) of carbon react with 32 kilograms (equivalent to one kilomole) of oxygen to produce 44 kilograms (equivalent to one kilomole) of carbon dioxide. The substances participating in the reaction are known as reactants, while the resulting compounds are called products. The general form of a combustion reaction is expressed by the next equation.
Equation 2
Fuel + Oxidizer → Water Vapor + Carbon Oxides + Nitrogen + Energy
Combustion reactions primarily produce water vapor and carbon dioxide as their main products. The physical state of water—whether liquid or vapor—depends on the temperature and pressure of the products. In most practical cases, instead of using pure oxygen as the oxidizer, a specific amount of air is used.
The molar composition of air consists of 21% oxygen, 78% nitrogen, and 1% argon and other gases. In combustion calculations, argon and other trace gases are typically neglected, and it is assumed that air is composed of 21% oxygen and 79% nitrogen on a molar basis. Under this assumption, for each mole of oxygen in the air, 3.76 moles of nitrogen are present.
The article “Clean Combustion” offers an in-depth discussion on clean fuels. It is suggested to read this article for further details.
Theoretical (Stoichiometric) Air and Excess Air
The minimum required air for the complete process is referred to as theoretical or stoichiometric air. When combustion occurs with the exact stoichiometric air, no excess oxygen is found in the products. This reaction is expressed in Equation 3.
Equation 3
The stoichiometric coefficients in Equation 3 can be obtained using the principles of balancing. These coefficients are provided in Table 1.
![\[ \Large % تغییر سایز به بزرگتر \renewcommand{\arraystretch}{2} % افزایش فاصلهی عمودی برای وسطچین شدن متن \begin{array}{|c|c|} \hline \multicolumn{1}{|c|}{x + \frac{y}{4}} & \multicolumn{1}{c|}{v_{O_2}} \\ % وسطچین \hline \multicolumn{1}{|c|}{x} & \multicolumn{1}{c|}{v_{CO_2}} \\ % وسطچین \hline \multicolumn{1}{|c|}{\frac{y}{2}} & \multicolumn{1}{c|}{v_{H_2O}} \\ % وسطچین \hline \multicolumn{1}{|c|}{3.76\left(x + \frac{y}{4}\right)} & \multicolumn{1}{c|}{v_{N_2}} \\ % وسطچین \hline \end{array} \]](https://raadmanburner.com/wp-content/ql-cache/quicklatex.com-264d5417fde2ab626eb820683851cbba_l3.png "Rendered by QuickLaTeX.com")
Table 1- Stoichiometric coefficients of combustion reaction
Therefore, to completely combust one mole of a hydrocarbon fuel with a specified chemical formula, (x + y/4) × 4.76 moles of air are required. This corresponds to 100% stoichiometric air. However, in practice, complete combustion occurs when the amount of air supplied is slightly higher than the theoretical air requirement. To calculate the excess air required for combustion, two parameters, AF and λ, are introduced. The AF parameter refers to the air-to-fuel ratio and is defined in Equations 4 and 5, in terms of mass and moles, respectively.
Equation 4
![\[ AF_{mass} = \frac{m_{air}}{m_{fuel}} \]](https://raadmanburner.com/wp-content/ql-cache/quicklatex.com-02b9280f62162f5e9d74c875fdac945e_l3.png "Rendered by QuickLaTeX.com")
Equation 5
![\[ AF_{mole} = \frac{n_{air}}{n_{fuel}} \]](https://raadmanburner.com/wp-content/ql-cache/quicklatex.com-776421eb0ff00ab9773bbfa8ed9423c4_l3.png "Rendered by QuickLaTeX.com")
The equivalence ratio of ϕ is defined by Equation 6 as the ratio of the stoichiometric air-to-fuel ratio divided by the actual air-to-fuel ratio.
Equation 6
![\[ \phi = \frac{AF_s}{AF_a} \]](https://raadmanburner.com/wp-content/ql-cache/quicklatex.com-1fa614365a75044112625201947c5f9e_l3.png "Rendered by QuickLaTeX.com")
If ϕ < 1, the fuel mixture is considered lean in terms of fuel and the combustion process has excess air. If ϕ > 1, the mixture is considered rich in fuel. The actual amount of air used can be expressed by the excess air percentage λ, as given in Equation 7.
Equation 7
The subscript “s” refers to the value of the parameter in the stoichiometric state. The application of 20% excess air (%λ = 20) is commonly used in industrial combustion.
Pollutants Resulting from Combustion
As mentioned, complete combustion in industry occurs when the amount of air supplied is slightly greater than the stoichiometric air requirement. Incomplete process results in by-products that act as combustion pollutants. The most important and notable pollutants of this process are carbon monoxide and nitrogen oxides (NOx). Adjusting the level of excess air allows for the control of pollutants. The following figure illustrates the production of carbon monoxide and NOx pollutants based on the air-to-fuel ratio in a combustion process.
NOx emission is highest near the stoichiometric air-to-fuel ratio, and as the air-to-fuel ratio decreases, NOx production decreases while carbon monoxide emissions rise. By increasing the excess air relative to the stoichiometric air, both carbon monoxide and NOx emissions will decrease simultaneously. However, excessive increase in this ratio leads to a decrease in flame temperature and an increase in stack losses, both of which lower efficiency. Hence, an optimal amount of excess air should always be considered in every combustion process.
Emissions Caused by Nitrogen
The presence of nitrogen oxides in the atmosphere leads to the formation of smog, acid rain, global warming, and damage to the ozone layer. The key nitrogen oxide compounds are nitrogen monoxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O).
There are three primary mechanisms for the formation of nitrogen oxides in combustion reactions which lead to: thermal NOx, prompt NOx, and fuel NOx.
Thermal NOx is mainly formed at high temperatures and in the presence of large amounts of nitrogen and oxygen. Under such conditions, nitrogen molecules decompose due to the high temperatures and combine with oxygen.
This type of NOx is the most prevalent in most combustion processes, especially in industrial equipment such as boilers, gas turbines, and internal combustion engines. Prompt NOx is generated by the reactions of free radicals produced during combustion. These reactions usually take place in the initial stages and result in the formation of unstable nitrogen compounds, which rapidly transform into nitrogen oxides.
Fuel NOx forms when nitrogen compounds in the fuel chemical structure break down during combustion, and the released nitrogen atoms react with oxygen. This type of NOx is notably produced in substantial amounts in liquid and solid fuels like fuel oil and coal. Its quantity depends on the chemical composition of the fuel, conditions, and flame temperature.
Carbon Monoxide (CO) Emission
Carbon monoxide (CO) is generally produced as an incomplete combustion product in a few combustion processes. It is a flammable, colorless, odorless, tasteless, and generally non-corrosive gas. Carbon monoxide is found in all combustion products of carbon-based fuels. In equilibrium, carbon monoxide is formed according to Equation 8.
Equation 8
CO2 → CO + 0.5O2
Carbon monoxide concentration is influenced by temperature and excess air. In fuel-rich combustion areas, the carbon monoxide level rises due to insufficient oxygen. Only when enough air is mixed with the fuel at high temperatures does the carbon monoxide level decrease. In combustion with low fuel levels, carbon monoxide will combine with air to form carbon dioxide molecules.
Carbon monoxide is primarily produced from the incomplete combustion of a hydrocarbon fuel. In a fuel-lean system, excess oxygen is used to ensure complete process and minimize carbon monoxide emissions. The figure below shows the air-to-fuel ratio in the combustion process and the amount of carbon monoxide emissions. In this graph, the horizontal axis represents the air-to-fuel ratio in the combustion process, with a value of one considered as the stoichiometric air-to-fuel ratio.
Raadman burners, with their precise engineering design, are capable of operating with low excess air, resulting in increased combustion efficiency, reduced emissions, and optimized fuel use. To precisely control the excess air amount, CO/O₂ Trim sensors like those from AutoFlame can be employed. These systems continuously monitor the levels of oxygen and carbon monoxide in the exhaust gases and use a smart controller to adjust the air-to-fuel ratio.
Adiabatic Temperature of Flame
When there are no heat losses, the temperature of the products reaches its highest value, known as the adiabatic flame temperature. The figure below illustrates the adiabatic flame temperature for hydrogen, methane, and propane fuels in terms of the equivalence ratio.
The peak temperature (maximum temperature) for fuels in the figure above occurs under conditions close to stoichiometric. In many real flames, the maximum temperature occurs under lean fuel conditions. This is carried out to avoid incomplete process, which requires additional oxygen to complete the process. Almost all industrial applications operate in lean fuel conditions to ensure that carbon monoxide levels remain low. Thus, with proper burner design, the flame temperature can approach the peak temperature (the condition that optimizes heat transfer).
The highest temperature that a combustion chamber can withstand depends on the material of the chamber. As a result, the adiabatic flame temperature plays a critical role in the design of combustion chambers for gas turbines, boilers, etc. In Table 2, the adiabatic flame temperature for several fuels is provided.
One of the key challenges in high-temperature flames is the maximization of NOx formation. The production of thermal NOx increases exponentially with temperature, which is why many modern designs focus on reducing flame temperature to minimize NOx emissions. In this regard, Raadman burners with their advanced design achieve optimal fuel and air distribution, preventing the formation of high-temperature zones and, as a result, minimizing NOx production.
Different Types of Combustion
There are two types of combustion, which will be addressed in the following:
Premixed Combustion
In a homogeneous premixed flame, the fuel and air are initially completely mixed together, and then ignition occurs using an ignition source such as a spark. The flame then forms at the spark location and begins to propagate. The boundary between the burned and unburned regions is referred to as the flame front. The flame front moves at a fixed speed toward the unreacted regions, where the combustion intensity is very high.
Premixed type is applied in premixed burners, such as the advanced raadman burners. These industrial burners are designed in such a way that fuel and air are mixed before entering the combustion series, and combustion takes place at the point where the flame is formed. This technology leads to higher efficiency and a significant reduction in pollutants such as CO and NOx, which are key advantages of Raadman premixed burners.
Non-premixed Combustion
In a non-premixed flame, the combustion reactions take place before the fuel and oxidizer combine. Once the fuel and air enter the combustion chamber and mix, ignition occurs simultaneously as the fuel molecules diffuse into the oxidizer. The intensity in such flames is influenced by the extent of mixing between fuel and oxidizer. Ignition in diesel engines, gas turbines, match flames, spark plugs, furnace burners, etc., is of the non-premixed type.
Non-premixed type is employed in nozzle mix burners. In this type of burner, fuel and air remain apart until they exit the nozzle. When the fresh air and fuel reach the nozzle opening or beyond, they combine, and combustion takes place. Packman, with its advanced technical knowledge, designs and manufactures nozzle mix burners according to national and international standards. Raadman burners, focused on reducing pollutants and optimizing energy consumption, are designed to enhance combustion processes across various industries.
Concluding Remarks: Efficient Combustion with Advanced Industrial Burners
Industrial burners are vital in optimizing the combustion process and minimizing environmental pollutants. Understanding the principles of this process, including stoichiometric ratios, adiabatic flame temperature, and the mechanisms of pollutant formation such as NOx and CO, is essential for designing efficient combustion systems. Accurate control of excess air and the implementation of modern burner technologies can significantly enhance combustion efficiency and reduce emissions.
In this context, raadman burners, with their advanced design, facilitate optimal fuel and air mixing, which not only enhances combustion efficiency but also contributes to the reduction of environmental pollutants. This technology improves flame temperature control and prevents excessive heat zones, reducing the formation of pollutants like NOx. As a result, applying engineering expertise and advanced burner design technologies is a highly effective strategy for minimizing environmental impact and achieving optimal, sustainable combustion.
