Utilidades industriais/Gases industriais/G\303\241s de combust\303\243o.md ..
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The composition of the flue gas will vary depending on the fuel used in the combustion. The main combustion products are carbon dioxide (CO₂) and steam. Nitrogen is also present, since it is a component of the atmospheric air used in the combustion and is normally inert, so it will exit in the flue gas along with the other gases. Oxygen may also be present, due to excess used to enforce complete combustion. Other components can also be present: if the combustion is incomplete, carbon monoxide (CO) and carbon particles can also be formed; if the fuel has nitrogen or sulfur in its composition, the combustion can form nitrogen oxides (NOₓ) and/or sulfur oxides (SOₓ). Furthermore, if the temperature of the combustion is high enough, nitrogen will no longer be inert, and it will oxidize, forming nitrogen oxides. The composition of the flue gas can be estimated doing the molar balance to the combustion unit. For example, if the combustion of biogas, which is mainly methane (CH₄, ~60%, CO₂ ~40%), is considered, the molar balance can be done as follows:
Whereas \( F_i^{in} \) and \( F_i^{out} \) are the inlet and outlet molar flows of the component \( i \), respectively; \( \alpha \) is the excess coefficient (e.g., if the air excess is 20%, \( \alpha \) is equal to 1.2); and \( y_i \) is the molar fraction of component \( i \) in the flue gas. It is considered that the molar composition of the air is 79% nitrogen and 21% oxygen.
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*Figura 1-* Design of an economizer used for preheating the water fed to the boiler⁶
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*Figura 2-* Design of an economizer used for preheating the water fed to the boiler⁶
There are two main types of economizers for boilers, depending on the amount of heat transferred. In dry economizers (DE) the fuel gas temperature is only reduced, without phase transition. Since the flue gas remains as a gas at the exit of the economizer, this equipment is usually made of carbon steel⁷.
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*Figura 2-* SNOX process diagram.¹²
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*Figura 3-* SNOX process diagram.¹²
There are also other pollutants that should be removed from flue gas, such as mercury and CO₂, but the technologies are not as established as the previously mentioned. In general, flue gas treatment is widely researched nowadays, and recent studies have been made trying to improve and develop new strategies, with an effort to remove more than one pollutant in a single equipment.¹³
