Gás de combustão

• author: Gabriel Chaves Cathoud Pinheiro, Giulia Clare de Carvalho Andrade Santos
• date: April 04th, 2022

Note from authors

In order to make our assignment more accessible, we decided to write it in English. It is supposed to be included at the page “Industrial Utilities” from DEQWiki as a new section, called “Flue Gas” (in Portuguese, “Gás de Combustão”). It can be considered an industrial utility, and it is intrinsically connected to the topic of heat integration and has not been explored yet in detail at the DEQWiki.

We also suggest that, for the following years, students are encouraged to write their contributions in English as it would make the DEQWiki more accessible and give more visibility to the Department of Chemical Engineering of the FCTUC.

Flue Gas

Flue gas is the gaseous effluent obtained from the combustion of a fuel. The gas produced is released to the atmosphere via a flue, which is a pipe or a channel. Combustion is an exothermic redox chemical reaction between a fuel and an oxidant. The products formed are, normally, carbon dioxide and steam. It is generally used in industrial plants to generate thermal energy. This can be achieved in boilers, furnaces, and other equipment. In this case, the oxidant is oxygen, which is present in atmospheric air. There is a variety of fuels, which can be divided into solid fuels (e.g., biomass), liquid fuels (e.g., fuel oil), and gaseous fuels (e.g., biogas).

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:

Figura 1- Combustion reaction

Whereas \(\sigma F^in\)\ ( 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.

Since the combustion is a very exothermic reaction, it will release heat. The heat generated will provide the thermal energy needed in the plant. Additionally, the gases produced will be in a very high temperature. These hot gases can be used to provide further thermal energy in other parts of the industrial plant. The temperature of the product gases will depend on the fuel used, and the operating conditions and purpose of the combustion unit. In boilers, for instance, the temperature of the forming gases will reach between 150 °C – 260 °C. In an incinerator, this temperature can reach values as high as 1000 °C. Table 1 obtained from [3] illustrates some temperature ranges depending on the combustion unit.

Table 1. Temperature ranges of the flue gas in different combustion units [3]

Installation and Heat Recovery

At industrial plants, the flue gas produced is usually seen as a process waste, that only needs to be treated so it can be discharged according to regulations. However, with a properly approach, it can bring great benefits such as heat recovery and even reduction of pollutants emissions⁴.

As mentioned, in boilers, fuel gas is normally expelled at temperatures in the range of 150 – 260 °C. A heat exchanger can then be installed at the exit of the boiler, so that a cold stream heats as the flue gas temperature decreases. Therefore, the heat is recovered before the gas is treated or released to the atmosphere.

Aiming heat integration, the heat recovered from the flue gas can be used in other parts of the process. Normally it is used to preheat the cold water that will be fed to the boiler, reducing the amount of fuel required to achieve the desired steam conditions. It can also be used to preheat the combustion air that will be fed to the boiler, also leading to fuel economy. In both cases the thermal efficiency of the boiler is improved. Since less fuel is consumed, this strategy also reduces CO₂ and other pollutants emissions⁴.

The heat exchanger used for this purpose is called an economizer, since it economizes energy. They can be used in different processes, but the goal is usually to recover energy or preheat a fluid. In this section we are focusing on economizers for boilers. The first successful economizer was invented by Edward Green in 1845, aiming efficiency increase⁵. Usually in economizers the cold water passes through tubes surrounded by hot gas, as can be seen in Figure 1⁶. The heated water is then fed to the boiler. Economizers are usually specifically designed for the target installation, so that its size is in accordance with the process characteristics and the performance is optimized.

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.

In contrast, in wet economizers (WE) the products of the combustion are condensed. Compared to DE, in WE the heat recovered is much bigger since latent heat is involved in the phase transition. However, a problem that comes with this solution is corrosion. As mentioned before, CO₂ is a significant component of flue gas. Other acid gases such as SO₂ can also be present, depending on the type of fuel. The condensate formed at the economizer is then very acid and can corrode its interior. Therefore, wet economizers are made of corrosion-resistant materials, such as stainless steel⁷. Another aspect is that the condensate formed must be captured and properly treated before discharge. As expected, the use of resistant materials and the need for additional treatment of the condensate increase the utility costs. Therefore, a detailed study should be conducted to assess if the heat recovery increased with condensation compensates the extra costs that emerge.

Other option is to recirculate flue gas into the boiler’s burner, in a strategy called “flue gas recirculation (FGR)”. This reduces the flame temperature since it dilutes the combustion air fed to the boiler. The main advantage of this strategy is reducing the NOₓ emissions, but it can also reduce the burner capacity⁸.

Treatment

One main problem associated with flue gas is the presence of pollutants, such as particulates, NOₓ, CO₂, SO₂ and mercury. If released straight to the atmosphere, it would affect the quality of the air and damage the environment. Therefore, emissions of pollutants are regulated by legislations, that vary from country to country. In order to achieve regulations limits, the flue gas should undergo treatment before being released to the atmosphere. However, a main aspect here is the cost since the technologies needed are very expensive⁹.

At industrial plants, these technologies are usually disposed in series to remove the different pollutants in the flue gas, depending on its expected composition. The technologies used also differ according to the type of process that originated the flue gas, for example boilers or incineration. The range of temperatures used can lead to different pollutants composition in the flue gas.

Usually, the first step is to remove ash and particulates. The equipment used for this purpose are filters, for example bag filters, or electrostatic precipitators. The choice of the equipment and its design should take in count the size distribution of the particulates, so that they can be efficiently removed⁹.

The next treatment aims to remove nitrogen oxides. Selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) are the well-known and commercially available strategies for this purpose. The goal is to react nitrogen oxides with ammonia or urea and form nitrogen gas (N₂), which can be released into the atmosphere with no further concern¹⁰. Since SNRC is not catalyzed, the reaction needs very high temperatures to occur, around 1000 °C. That way SNRC strategy is mainly used in incineration process, feeding ammonia or urea directly in the combustion chamber. For flue gas from boilers, the strategy recommended is SCR, which will implicate in the acquisition of a catalyst.

After that, adsorption-based process is used to remove SO₂. Usually the technologies involve scrubbers, which traditionally designates devices that use a liquid to remove pollutants from a gaseous stream. Scrubbers can be based on chemical or physical adsorption. For the removal of SO₂, the strategy is usually called Flue Gas Desulfurization (FGD) and the scrubbers can be called wet or dry, depending on the phase of the final by-product – liquid or solid.¹¹ The removal efficiency can reach up to 90% and the costs vary depending on the chosen strategy.⁹

An interesting approach is to implement a SNOX process, developed by Haldor Topsoe. It includes technologies to remove SO₂, NOₓ and particulates from the flue gas in an integrated process. The steps are as follows: removal of dust, SCR of NO₂ with NH₃, catalytic oxidation of SO₂ to SO₃, and cooling of the gas, so that sulfuric acid condenses and can be easily removed as a concentrated liquid, which can even be commercialized. This process is, therefore, recommended when the flue gas is rich in sulfur. The diagram in Figure 2 explains in detail the SNOX process.¹²

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

References

1. Wikipedia. Combustion. https://en.wikipedia.org/wiki/Combustion.
2. U.S. Department of Energy’s Industrial Technologies Program. Technical advice on boiler combustion control. https://www.reliableplant.com/Read/22138/technical-advice-on-boiler-combustion-control.
3. The Engineering ToolBox. Fuels - Exhaust Temperatures. https://www.engineeringtoolbox.com/fuels-exhaust-temperatures-d_168.html.
4. exodraft. Flue Gas Heat Recovery. https://www.exodraft-heatrecovery.com/flue-gas-heat-recovery/.
5. Wikipedia. Economizer. https://en.wikipedia.org/wiki/Economizer.
6. alwepo. What is Function of Economizer Boiler..? https://alwepo.com/what-is-function-of-economizer-boiler/ (2021).
7. Waldron, R. BOILER ECONOMIZER : WHAT YOU NEED TO KNOW. https://www.rasmech.com/blog/boiler-economizer/ (2021).
8. Pett, P. THE PROS AND CONS OF FLUE GAS RECIRCULATION. https://www.hvpmag.co.uk/The-pros-and-cons-of-flue-gas-recirculation-/10382 (2017).
9. Hosansky, D. Flue gas treatment technology. https://www.britannica.com/technology/flue-gas-treatment.
10. G Miller, B. CHAPTER 6 - Emissions Control Strategies for Power Plants. in Coal Energy Systems 283–392 (2005).
11. ICAC. Acid Gas/SO2 Controls. https://www.icac.com/page/Acid_Gas_SO2_Control.
12. Wikipedia. SNOX process. https://en.wikipedia.org/wiki/SNOX_process.
13. Lin, F. et al. Flue gas treatment with ozone oxidation: An overview on NOx, organic pollutants, and mercury. Chemical Engineering Journal vol. 382 (2020).