Elsevier

Energy Conversion and Management

Volume 99, 15 July 2015, Pages 151-160
Energy Conversion and Management

Optimization of a premixed cylindrical burner for low pollutant emission

https://doi.org/10.1016/j.enconman.2015.04.039Get rights and content

Highlights

  • The mixing uniformity of methane closely relates to low burning emissions.

  • Optimal exit position and diameter of nozzle are obtained with high methane mixing.

  • Low emissions of optimal burner are experimentally validated.

Abstract

A premixed cylindrical burner is numerically and experimentally investigated to realize low pollutant emission. The geometrical parameters of nozzle exit position and nozzle diameter are optimized by using a validated Computational Fluid Dynamics model. The natural gas-air mixing in the mix chamber indicates that the uniformity of methane concentration increases with the increase of distance from ejector outlet. It is found that the nozzle exit position at −3.0 mm improves the overall performance of premixed cylindrical burner, when nozzle diameter is not less than 1.6 mm. The emission characteristics of nitrogen oxides and carbon monoxide are also examined by experimental approach. It is found that load factor has a great influence on nitrogen oxides and carbon monoxide emissions, but the effect is gradually disappeared when air coefficient is not less than 1.4. When nozzle exit position is −3.0 mm, nozzle diameter is not less than 1.6 mm and air coefficient is not less than 1.4, the emissions of nitrogen oxides and carbon monoxide are less than 20 ppm and 50 ppm, respectively.

Introduction

In recent years, environmental problems are becoming more and more attention. Air pollution is one of the major problems of modern world. The low-pollutant emission technology has received increasing attentions. One of the main pollutants is NOx formed by the oxidization of nitrogen during combustion process. Wei et al. [1] investigated the effects of operating parameters on the NOx emission and found out the optimal operating conditions to reduce the NOx emission for high-temperature air combustion furnaces. The concentration of NOx emission reduces by 25.45% in their optimal operating condition. Gonca [2] conducted a steam injected method to supply ethanol–diesel blend fuel for a diesel engine. The results showed that the NO emission is reduced to 34% compared with conventional diesel engine (D) and steam injected diesel engine (D + S20). Kesgin [3] researched the relationship between design and operational parameters and the NOx emission of a turbocharged natural gas engine. By reducing the charge temperature, it was found that there is an increase in the excess air ratio, which causes a significant decrease in NO emission and the concentration of emissions is decreased to a level to meet the international standards. Coelho described a numerical simulation of Eulerian particle flamelet mode in mild combustors to calculate NO emission in a post-processing stage. The combustor was characterized by relatively lower NOx emission [4]. The further investigation [5] pointed out that N,N′-diphenyl-1, 4-phenylenediamine (DPPD) antioxidant additive can reduce NOx emission significantly. In this case of the addition of 0.15% (m) DPPD additive in JB5, JB10, JB15 and JB20 (Jatropha methyl ester is blended with diesel at 5%, 10%, 15% and 20% by volume), the reduction in the NOx emission is 8.03%, 3.503%, 13.56% and 16.54%, respectively, compared to biodiesel blends without the additive under the full throttle condition. Although an increase in CO emission with the addition of DPPD antioxidant to all Jatropha biodiesel blends was observed, the value is low in comparison to diesel emission. However, the low-NOx emission approaches are based on non-premixed combustion in the above investigations, which focus mainly on engine and biodiesel fields.

Premixed combustion can notably reduce NOx emission because it does not produce Fuel NO (F-NO) and Prompt NO (P-NO), which decrease the chance of Themal NO (T-NO) generation. Authors’ research group have designed and developed a plate-type premixed burner, which have the advantages of high efficiency and low emissions. After increasing the mixing effect in mixing chamber, NOx and CO emissions was found to be less than the conventional burners in present market [6], [7]. It is characterized by high temperature, relatively short flame and excellent ductility. To improve premixed combustion behavior, some of research devoted to study the geometry parameters of burning system. Zhang et al. [8] optimized the geometry parameters of diversion plate to improve the uniformity at the outlet of gas mixing system. The hole diameters and the arrangement of a cylindrical multi-hole premixed burner were investigated by Lee et al. [9]. The results showed that the NOx and CO emissions are less than 40 ppm and 30 ppm for a 0% O2 basis, respectively. Panwar et al. [10] evaluated the performance of premixed type industrial burner with swirl vane for mixing the air and gas. However, the premixed cylindrical burner (PCB) with ejectors of mixing the air and natural gas has not been studied.

In China, it has experienced a process to establish NOx and CO emissions standards for gas appliance. In the early 1990s, China formulated her own standards to the European standards gradually. As environmental awareness increasing, the national standards of domestic gas instantaneous water heater are established step-by-step. This makes it necessary to investigate low NOx emission gas combustors.

Traditional optimal ways of combustors are based on empirical models, and a large number of experiments are required. It costs not only a lot of time, but also numerous resources. With the development of computer, the numerical simulation is becoming an attractive method for its high flexibility and low cost application. The computational fluid dynamics (CFD) approach is capable of visualizing the detailed information of flow field to optimize its performance. Sharfi and Boroomand [11] validated the deviation between the numerical model and experiment data. The maximum difference between the numerical and experimental results is about 9.7% and 10.6% for the two models, respectively. Zhu et al. [12] employed the CFD technique to research the ejector geometry parameters, and showed the optimum primary nozzle position and converging angle in particular operating condition. In order to study a supersonic air ejector, Hemidi et al. [13] dealt with the comparisons between CFD and experiments. Good validation results were obtained for a wide range of operating conditions. Arghode et al. [14] numerical investigated the combustion characteristics for application to gas turbine combustors by commercial software FLUENT. It was revealed that the numerical method was able to capture the overall flow field behavior and provide insights to achieve reactions in volume distributed combustion regime to reduce NOx and CO emissions. Sukumaran and Kong [15] used the CFD modeling to study the combustion process inside the burner, and the reduced mechanism was able to predict NOx emission for different feedstock and operating conditions. Chui et al. [16] investigated burner design concepts of reducing NOx formation via improved staging by CFD. It indicated that a new burner design approach can potentially reduce the NOx compared to the existing design methods.

The present developed PCB adopts a new design principle of premixed combustor. It has the advantages on simple manufacture, convenient operation, low NOx and CO emissions and highly combustion efficiency compared to the existing burner. The stainless steel radial heat exchanger and compact aluminum heat exchanger are used. The purpose of the present study is to optimize geometry parameters of PCB by CFD. An innovative approach of random sampling method is introduced to analyze the mixing effect in mixing chamber and the experiments are conducted to study the combustion characteristics.

Section snippets

Description of the premixed cylindrical burner and experimental rig

Fig. 1 is the schematic diagram of our designed PCB, which consists mainly of a mixer and a cylindrical burner. The mixer has 6 major components: gas distribution chamber, nozzles, air chamber, ejectors, insulating chamber and mixing chamber. For reducing gas static pressure before entering the ejectors, equal number nozzles are located at the outlet of gas distribution chamber. The ejectors described by Liu et al. [6] are arranged in circle. The distance between nozzle and ejector is defined

CFD model

In order to improve the accuracy and reduce time consumption of solution, several assumptions are made before numerical simulation. The air and natural gas are treated as Newtonian fluid by satisfying the law of Newton inner friction. They are assumed to be incompressible for present low Mach number flow. The natural gas is simplified as methane, and the effects of radiation are neglected in the process of mixing.

The commercial software FLUENT 14.0 and Gambit 2.4 are used as the CFD solver and

Grid independent test

To reduce the total number of grid cells, the computational region is divided into ten sub-regions. The grid independence test is conducted between two grid systems. The cell numbers of the two systems are 2.13 × 106 and 4.50 × 106, respectively. In this case, the mass flow rate of gas and air inlets is 1.89 × 10−4 kg/s and 3.96 × 10−3 kg/s, respectively. On the central axis of the mixing chamber, eight sample points are extracted from the numerical results. Fig. 3a shows the location of the sample

Results and discussions

Liu et al. [6] studied NXP and nozzle diameter for optimal design of household appliance burner of fuel-gas mixing in a single ejector. It was found that the pressure of the gas inlet increases with the distance between nozzle outlet and ejector inlet when nozzle diameter keeps constant. And the nozzle diameter had significant effects on the pressure of gas inlet. The results of Zhu et al. [12] indicated that the parameter NXP is importance to the ejector entrainment ratio. However, there is no

Conclusions

The numerical and experimental study is used to investigate the mixing and combustion performance of PCB. The effects of mixing on burning characteristic are discussed and the optimal geometric parameters and the operation conditions of PCB are obtained. The main results are summarized as follows:

  • (1)

    The effect of NXP and d on the static pressure of air inlet is negligible. However, with respect to the natural gas inlet, the ejector performance is improved when nozzle outlet moves away from the

Acknowledgements

The authors gratefully acknowledge the support of scientific and technological cooperation project of Wuqing District and Tianjin University; project (NR2013K04) supported by Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering, China; project (20130909) supported by the Higher School Science and Technology Development Fund of Tianjin, China.

Cited by (25)

  • Feeding H<inf>2</inf>-admixtures to domestic condensing boilers: Numerical simulations of combustion and pollutant formation in multi-hole burners

    2022, Applied Energy
    Citation Excerpt :

    So far CFD has been applied to condensing boilers mainly to address the problem of gas distribution and mixing upstream of the burner and only a few works have dealt with the simulation of the reactive flow. Zhao et al. [14] investigated numerically, though non-reactive CFD simulations, the influence of geometrical parameters, i.e. nozzle exit position and nozzle diameter, on the gas distribution from a perforated cylindrical burner. Zhang et al. [15] used CFD to optimize the gas mixing system of a premixed burner.

  • Mathematical modelling and experimental study of an ejector burner

    2022, Experimental Thermal and Fluid Science
    Citation Excerpt :

    Therefore, besides emphasizing specific techniques that one can resort to get a stable flame, the authors presented flameless combustion as a promising method, as it promotes the uniformity of the thermal field — specially for combustors with a low thermal load. The non-negligible impact of the thermal load on the pollutant emissions were furthermore analysed by Zhao et al. [4], who presented solutions to optimize a premixed cylindrical low-pollutant burner. Moreover, Zhang et al. [5] concluded that the uniformity of the mixture to be burnt in a premixed burner has a determinant impact on emissions.

  • Prediction of the equilibrium compositions in the combustion products of a domestic boiler

    2021, Energy
    Citation Excerpt :

    These studies have been conducted with numerical, experimental, and theoretical methods. Several numerical and experimental studies were performed focusing on pollutant emissions and environmental impacts [5–12], effects of various gaseous fuels on combustion [13–16], combustion stability [17,18], system optimization [19–21], combustion performance and energy efficiency [22–24], emission predictions [25–27]. Emerging emissions resulting from combustion are predicted by kinetic/dynamic models and chemical equilibrium methods, mainly known as equilibrium constants or minimization of Gibbs energy [28].

  • Emissions and thermal efficiency for premixed burners in a condensing gas boiler

    2020, Energy
    Citation Excerpt :

    Liu [10] studied the flow and mixing process of low nitrogen oxide town gas ejector by experimental method. Zhao [11] reduced NOx and CO emissions by optimizing geometric parameters of the cylindrical burner. Minutolo [12] studied the performance of a new type of household burner and obtained the appropriate excess air rate.

View all citing articles on Scopus
View full text