Full Length ArticleEffects of air swirl on the combustion and emissions characteristics of a cylindrical furnace fueled with diesel-biodiesel-n-butanol and diesel-biodiesel-methanol blends
Introduction
Today, the importance of energy and its role in economics and politics is clear. This is important not only for the advanced industrialized countries that are major consumers of the world's energy resources but also for oil-rich countries [1], [2]. Since these countries also have to find out the fact that oil resources are limited, the lack of understanding of this issue in countries like Iran as a developing country can seriously cause some problems with the living and economy of future generations [3], [4]. Therefore, the production of fuels from renewable sources is not only a choice but also is necessary and important [5]. On the other hand, gaseous emissions from internal combustion engines, diesel generators, and industrial furnaces fuelled with fossil fuels, have inevitably necessitated using alternative fuels [6], [7]. In recent years, biofuels mostly biodiesel, ethanol, methanol, and n-butanol have been widely used in gasoline and diesel engines [8]. So, one of the most discussed issues in the world is the use of biofuels as a mixture with diesel fuel [9], [10]. In addition, utilizing biofuels in boilers and industrial furnaces is very important [11].
Air swirling causes an intensified turbulent recirculation zone, which makes the flame stable and improves the mixing of fuel and air and produces a homogeneous mixture in a short time [12].
As a result of air swirling during the combustion, the adverse axial pressure gradient causes an internal recirculation region through the furnace axis and eliminates the outer recirculation region near the walls simultaneously [3], [13].
According to high combustion temperatures of some burners and their combustion conditions above stoichiometric air to fuel ratios resulting in the production of nitrogen oxides, carbon monoxide, and unburned hydrocarbon, it is necessary to investigate the effect of new mechanism such as air swirling and using biofuels on the pollutant emissions in burners [14].
Motamedifar and Shirneshan [3] investigated the effects of different vane angles of air swirlers on the pollutant emissions in a cylindrical combustion chamber fuelled with diesel–biodiesel-ethanol blends. The results of this research showed that the minimum CO and HC emissions belong to the fuel blend included 75% diesel, 25% biodiesel and 5% Ethanol and air swirler with the angle of 15° and the maximum CO and HC emission occurs for net diesel and swirler air vane angle of 90°. The results also showed that ethanol can more effective to increase CO2 values than that of biodiesel. Besides, the minimum CO2 emission obtained for fuel blend D100. In addition, air swirling condition with higher vane angles of 45° leads to higher premixed combustion and temperature and causes an increase in NOx emission.
Based on the literature review presented in the study conducted by Motamedifar and Shirneshan [3], Ti et al. [15] showed that NOx emissions decreased with OFA swirl air vane angles of 25°, 35°, 45°, and 90°, and flue gas temperature increased as the OFA swirl air vane angle increased from 25° to 45°.
Wang et al. [16] reported that the effect of the high-temperature recirculating region under the arch upon the combustion and NOx emission characteristics of the boiler is greater than that of the high-temperature flue gas entrained by the swirl burner itself.
The results of Li et al. research [17] revealed that the lower swirl air angles cause an increase in CO and a reduction in NOx emission generally.
Sung and Choi [18] concluded that the control of swirl vane angle was more effective than that in the control of the primary zone stoichiometric ratio.
There are also some new researchers that investigated the effect of air swirl on the combustion and emission characteristics in the burners and boilers:
Wang et al. [19] investigated two cases of the main swirler vane angle (20° and 30°) of the TeLESS-II combustor. High-speed camera, planar laser-induced fluorescence, and computational fluids dynamics were used to better understand the ignition results. The ignition results at room inlet temperature and pressure demonstrated that the ignition performance of the 30° vane angle was better than that of the 20° vane angle. The results indicated that the radial dimensions of the primary recirculation zone of the two cases were very similar, and the dominant cause of the different ignition results was the vapor distribution of the fuel. The concentration of the kerosene vapor of the 30° vane angle was much higher than that of the 20° vane angle case. The authors suggested the swirler vane angle of 30° for better ignition performance.
Pourhoseini and Asadi [20] investigated the effects of the angle of an air swirler vane on the combustion characteristics of liquid fuels. They performed the tests according to three vanes with angles of 0°, 40°, and 80°. The results indicated that the angle of the swirler vane had significant effects on the temperature, combustion efficiency, and NO and CO emissions. In addition, the optimum combination of the contact area and time maximizes the mixing rate of the inlet air and fuel jet at the optimum angle. Moreover, the combustion efficiency is higher at small and large swirl angles, and soot, CO and NOx emissions have their minimum values at the optimum vane angle.
Sung et al. [21] experimentally investigated the swirl intensity's influence on the internal recirculation zone and exhaust tube vortex of pulverized coal-methane flames. They used various inner swirl numbers (0.43, 0.63, 0.9, and 1.3) and an outer swirl number of one. The results indicated that after the sudden gas expansion by pulverized coal combustion in zone two, the axial velocity gradient was increased by the temperature increase caused by the combustion reaction near the stagnation point, accelerating the forward flow downstream. This driving source of axial momentum by gas expansion with the remaining part of the swirl component from the internal recirculation zone caused to create an exhaust tube vortex with a hollow-tube structure.
Wang et al. [22] proposed a novel high-efficiency low-NOx technology for a down-fired boiler with swirl burners. They performed experiments under three combustion technologies and different loads. The results indicated that the ignition, furnace temperature and flame fullness in the primary combustion zone were improved, the air-staged level was modified further, and coal combustion in the lower furnace was well distributed. Moreover, NOx emissions and carbon in fly ash were reduced at different loads.
Yang et al. [2] applied numerical simulation and experiments to study a 600 MWe boiler under original and new combustion systems. The novelty of the research involves changing the operation mode of swirl burner, rearranging secondary air, and applying separated over-fire air. The results indicated that the NOx emissions and the unburned combustible in flue dust decreased with the arrangement of the swirl burner changing from co-rotating mode to counter-rotating one. This study provided a feasible and economic approach for down-fired boilers to perform high efficiency and low emissions.
Zhou et al. [23] evaluated the flame stability mechanism of the swirl burner in the annular recirculation zone. The results demonstrated that the specially designed swirl burner structures, including the pulverized coal concentrator, flame stabilizing ring and baffle plate caused an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. Furthermore, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as a fuel-rich environment to contribute to in-flame NOX reduction.
Ti et al. [24] presented an experimental research on cold flow, combustion characteristics, and NOx emissions of a swirl burner with air staging. The investigation was performed in a 0.5 MW laboratory furnace in order to optimize the primary-air cone length of a centrally fuel-rich swirl burner. The results showed that the flame stability, performance of ignition, and burnout were significantly improved by decreasing the primary-air cone length. Moreover, the NOx emissions with a cone length Lp/Li = 0 decreased by 50.3% compared to those measured with a cone length Lp/Li = 1.0. The results showed that the optimum value of primary-air cone length was observed to be Lp/Li = 0.
Yang et al. [25] simulated the turbulence radiation interaction in a swirling gas-fired furnace to understand the thermal radiation behavior under oxy-combustion conditions. The results showed that the contribution of radiation to total heat flux on the wall was more than 45% and a local increase of 18% in radiative heat flux was caused by turbulence radiation interaction. Moreover, increasing swirl number weakened the turbulence radiation interaction effect.
Using biofuels in industrial furnaces is a strategy that can improve the combustion and emission characteristics. Literature reviews showed that there are so far no researches focused on the investigation the effects of air swirl and methanol or n-butanol as a mixture with diesel–biodiesel blends on the combustion and emissions characteristics of a cylindrical furnace; therefore, this research investigates the pollutant emission from a cylindrical furnace with the mixtures of diesel–biodiesel-n-butanol and diesel–biodiesel-methanol under air swirling condition. In this study, the CO, CO2, and NOx emissions and the temperature of combustion gas for the fuel blends D70B25Bu5, D75B20Bu5, D80B10Bu10, D85B10Bu5 as well as D70B25M5, D75B20M5, D80B10M10, D85B10M5 and D100 were measured and analyzed.
Section snippets
Fuel preparation
In the present research, biodiesel from waste cooking oil was produced in TMU biofuel laboratory by transesterification reaction by using methanol and potassium hydroxide tablets as the alcohol and catalyst respectively. Then, the produced biodiesel was analyzed according to ASTM D6751 standard [26], [27]. Table 1 shows the physicochemical properties and fatty acid profile of waste cooking oil (WCO). Moreover, methanol and n-butanol were supplied from local bio-alcohol suppliers. Fig. 1 shows a
Effect of air swirling on the CO emission
Fig. 6, Fig. 7 show the CO emission amounts at different vane angles of the air swirler for various diesel–biodiesel-n-butanol and diesel–biodiesel-methanol blends.
As the results show, the amount of CO emissions changes with the vane angle of the air swirler. According to the results, for all fuel blends, CO emissions were firstly reduced when the angle changed from 15° to 25°. The vane angle of 45° had the worst air swirling and allowed the maximization of the CO emissions at this angle for
Conclusion
This study investigated the effects of air swirling on the emission and combustion characteristics of a cylindrical furnace fueled with various diesel–biodiesel-n-butanol and diesel–biodiesel-methanol blends. According to the results, air swirler with vane angles of 35° and 60° causes an improvement in CO and HC emission; in addition, CO2 and NOx emissions improved at the angles of 45° and 60°. The results indicated that using n-butanol and methanol as the mixture with diesel–biodiesel blends
CRediT authorship contribution statement
Mohsen Amiri: Data curation, Methodology. Alireza Shirneshan: Formal analysis, Supervision, Writing - original draft, Writing - review & 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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