Tangential Flue Gas Recirculation (TFGR) technique for enhancement of radiation characteristics and reduction of NOx emission in natural gas burners
Introduction
In the recent years, there has been an increasing use of natural gas as a clean fuel that can replace liquid and solid fossil fuels in industrial burners and that does not suffer from the economic and environmental problems of solid and liquid fossil fuels (Martins et al., 2019; Semin, 2008; Kumar, 2013; Liao et al., 2005). However, from the point of view of heat transfer, natural gas flame has low heat transfer efficiency, which is a limiting factor for natural gas burners (Pourhoseini and Asadi, 2017a). Also, from the environmental point of view, natural gas burners emit the dangerous NOx, whose emissions should be reduced (Dupont et al., 1993; Chen and Liu, 2015). Convection and radiation heat transfer are the main mechanisms of heat transfer from flame. Based on Stefan-Boltzmann law, in the high temperatures of flame, radiation is the dominant method of heat transfer (Andersson and Johnsson, 2007; Yasuhisa et al., 2011; Mehta et al., 2009). Therefore, to enhance the low heat transfer efficiency of natural gas flame, we focus on radiation heat transfer. Flame temperature, flame emissivity coefficient, flame lateral surface area and the residence time of radiative species within the flame are the parameters which can affect radiation from natural gas flame (Draper et al., 2012; Andersson et al., 2008). The flame temperature is dependent on the mixing process. In the case of natural gas flame, unlike liquid and solid fossil fuels, since natural gas and air create a homogenous mixture, the mixing rate is appropriate (Pourhoseini and Asadi, 2017b). Flame emissivity coefficient is dependent on the types of flame species and their concentrations. To enhance the emissivity of flame, the concentration of the most radiative species of flame must be increased. These species include the products of complete combustion-CO2 and H2O gaseous species-and soot particles, which have high absorptivity and emissivity coefficient (Gorog et al., 1981; Centeno et al., 2014; Paul and Paul, 2014). Natural gas flame does not produce the same amount of soot and does not have the same rate of soot formation as heavy solid and liquid fossil fuels do (Pourhoseini, 2017; Koylu and Faeth, 1991; Williams et al., 2007). Soot particles, however, act as highly emissive gray bodies in flame structure and have high emissivity coefficient (Song and Jacobs, 2014; Viskanta and Mengüç, 1987; Johansson et al., 2013; Bäckström et al., 2014). This has motivated many researchers to try to increase the concentration of intermediate soot particles in natural gas flame, which consequently enhances its radiation. They fulfill this goal by methods such as preheating of natural gas and combustion air (Yang and Blasiak, 2005; Atreya, 2006) and synchronous combustion with liquid and solid fuels that produce large amounts of soot (Pourhoseini and Moghiman, 2015a, 2015b). These methods noticeably enhance the radiative characteristics of flame. However, they have some important limitations. First of all, from the environmental point of view, the rate of soot emission, as a pollutant, must be controlled and kept in the standard level. Second, from the economic point of view, they need high temperature preheating devices. Finally, in some applications, it is impossible to use supplementary solid and liquid fuels with high sooting tendency.
Flue Gas Recirculation (FGR) is a method reported to effectively reduce NOx pollutant emissions by recirculating part of the flue gas (Tesfa et al., 2012; Sahin et al., 2014; Subramanian, 2011). Sidorkin et al. (2016) investigated the effect of FGR on the environmental performance of an oil-shale burner. They observed that the recirculation of flue gas significantly decreased the NOx emission. In another work, Chen et al. (2016) studied the effects of FGR on emissions from a small-scale wood chip fired boiler. The mathematical modeling showed that the flue gas recirculation reduced the temperature level in the furnace. Furthermore, the results showed that FGR reduced the NOx emissions from the wood chip fired boiler while it led to higher CO emissions. Similarly, Qian et al. (2011) investigated the effect of FGR on the NOx emission from a pilot-scale vortexing fluidized-bed combustor (VFBC) with coal as the fuel. The experimental results showed that using flue gas recirculation in the VFBC effectively decreased NOx emission. They mentioned the following reasons for the lower NOx level. FGR returns NOx to the reactor, which provides the opportunity to decrease even more NOx. The greater amount of CO2 that FGR supplies provides the next cause of more NOx reduction since its reaction with char produces CO, which reacts with NOx.
An overview of the studies on the FGR technique reveals that it was used for the purpose of increasing the energy recovery and decreasing the NOx emitted from solid and liquid fossil fuels (Liuzzo et al., 2007; Michel and Belles, 1993; Jiang and Zhang, 2004; Sander et al., 2011; Yu et al., 2016). However, we expect this method to have the following advantages in natural gas burners. First, since the radiation characteristics of natural gas flame are poor, thermal energy cannot be transferred from flame and consequently the exhaust gas temperature is high. This method, by recirculating the flue gas into the flame, raises the residence time of high temperature combustion products in the furnace and prevents the energy loss in the chimney. Second, since CO2 and H2O, as the main products of complete combustion of natural gas, are the most important radiative species of flame, the recirculating of these gaseous species into the flame increases their concentrations in the flame reaction zone and consequently enhances the emissivity and radiation heat transfer of flame. Third, natural gas is the lightest of hydrocarbon fossil fuels and has a low carbon number in its chemical structure. Besides, natural gas and air create a homogenous mixture. These two factors make the fuel-oxidant mixing rate in natural gas flame an appropriate one. Consequently, compared with liquid and solid fossil fuels, natural gas flame makes a more complete combustion and produces little CO emission and a small flame with hot-spot regions in it, which strengthens the rate of NOx formation (Pourhoseini and Asadi, 2017a; Pourhoseini, 2017, 2020; Zhang et al., 2014; Pourhoseini et al., 2021). Therefore, increased CO emission, as the negative consequence of FGR in burners that use liquid and solid fossil fuels, is not a problem in natural gas flames. It seems that FGR methods decrease NOx emission in natural gas burners, but they do not increase CO emission to a degree above the standard level. Based on the above arguments, in the present work, we investigate the effect of Tangential Flue Gas Recirculation (TFGR) on thermal, radiative and pollutant emission characteristics of natural gas flame.
Section snippets
Experiments
The experiments were done on a laboratory cylindrical furnace. Five measuring holes, each with an ID of 2.5 cm, were embedded on the furnace wall to measure the combustion parameters such as temperature, luminosity and radiation heat flux. The holes were 0.07, 0.13, 0.19, 0.24 and 0.30 m distant from the inlet of the burner. For the sake of durability at high temperatures of flame, the furnace body was made of high temperature resistant steel AISI316. A chimney, which was 3000 mm long and
Results and discussion
Fig. 2 shows pictures of the flame at different Tangential Flue Gas Recirculation (TFGR) ratios. It can be seen that the TFGR technique changes the nonluminous natural gas flame (at TFGR ratio = 0) to luminous flame. The blue color of flame is attributed to CO2 and H2O gaseous species. Soot and CO, when burning in the flame, create a yellow colored flame (Pourhoseini and Yaghoobi, 2018). As mentioned above in the Introduction section, previous studies on solid and liquid fossil fuel burners
Conclusion
The Tangential Flue Gas Recirculation (TFGR) technique was proposed and investigated for the enhancement of flame radiation characteristics and for reduction of NOx emission in natural gas burners. The main findings are as follows:
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TFGR enhances the luminosity of natural gas flame by the dissociation of the recirculated CO2 to CO.
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TFGR enhances the tangential momentum of flame and generates a voluminous flame, increases the heat transfer area of flame and eliminates the hot-spot region in the
Author statement
S.H. Pourhoseini: Investigation, Methodology, Conceptualization, Data curation. I. Taghvaei: Data curation, Investigation, Editing. M. Moghiman: Supervision, Writing- Reviewing and Editing. M. Baghban: Data curation, Writing- Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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