Emissions and efficiency of a domestic gas stove burning natural gases with various compositions
Introduction
Natural gas (NG) has been extensively used as a fuel in many heating processes. In particular, natural gas is recognized as one of the most important cooking fuels for domestic gas burners in developed countries. Because of the increasing emphasis on environmental protection, indoor air pollution has become of great concern [1], [2], [3], [4], [5], [6], [7], [8]. The ever increasing demand for higher efficiency and precise controllability coupled with more stringent pollution emission requirements, imposes considerable pressures on combustion research and development [9]. Therefore, much attention has been paid to higher thermal efficiencies and lower emissions of domestic gas burners for a number of years.
The prime objectives of the burner designer are to ensure that (1) the correct mixture of fuel and air is supplied, (2) ignition is controlled and reliable, (3) the resultant flame is of the required shape and its structure is stable and (4) the appliance is inherently safe [9]. In order to achieve these goals, an understanding of the design, operation and fuel factors of domestic gas burners is most crucial. Although many studies have been focused on the first two parameters, very little research has been done on the effects of variations in gas composition. Junus et al. [10] employed the factorial experimental design method to analyze the effects of design factors on emissions from natural gas cooktop burners. The design factors reported include cap material, cap shape, cap dimension, cap mass, cap age, location on cooktop, grate height, port shape and port spacing. In addition, operational factors are related to the conditions under which the burner operates, such as maximum firing rate, turndown ratio, primary aeration and peripheral secondary aeration. In their study, the thermal input, port perimeter to area ratio, port spacing, port size, port shape, central secondary aeration and flame insert had significant effects on the emission rates. Lower emission rates were obtained with slit ports, without central secondary aeration and without flame insert. Furthermore, decreasing the port perimeter to area ratio led to the reduction of emission rate.
Stubington et al. [11] investigated the emissions and efficiencies of production cooktop burners firing natural gas. It was found that the emissions and thermal efficiencies were markedly affected by (1) the thermal input, with or without a load on the burner, and (2) the load height to flame length ratio with a load on the burner. Raising the thermal input or the load height to flame length ratio affects the emission rate of each of the species differently, and it generally reduces the thermal efficiency. Both NOx and NO generally rise with either thermal input or load height to flame length ratio. The emission rate of CO and hydrocarbon fell with increasing thermal input. Ashman et al. [12] used a single production cooktop burner to determine the effects of loading height and thermal input on its efficiency and gaseous emissions. It was found that the thermal efficiency of the burner decreases with increasing loading height. At a constant (low) loading height the thermal efficiency is higher at lower thermal inputs. At low loading heights, NOx increases with increasing loading height, but CO increases with decreasing loading height.
At present, the domestic gas burner most widely used is the conventional Bunsen type, i.e. partially aerated. The typical partially aerated burner entrains primary air naturally by a momentum sharing process between the high velocity gas jet and the ambient air. The fuel commonly used in domestic gas appliances is natural gas. The composition of the world resources of natural gas varies greatly, such that domestic natural gas supplies are multi-component mixtures, liable to variation in composition. Table 1 illustrates the widely differing composition of natural gas supplied from different resources. The heating value of the fuel, which depends on its composition, strongly affects burner performance, including the combustion characteristics (such as flame length, flame stability, temperature distribution etc.), thermal efficiency and emissions. Clearly, gas composition is a particularly vital parameter affecting burner performance. Hence, the interchangeability of gaseous fuel is of great significance and should receive much more attention.
Using the same gas stove to burn natural gas with various heating values is inappropriate and hazardous due to the possible occurrence of incomplete combustion (i.e. a great increase of CO emissions and/or soot formation), liftoff and flashback. Although a considerable amount of published work exists on the development of low emission, energy efficient gas stove burners, the literature on the effect of changes in gas composition on burner performance is limited. In this study, we aim to assess the effects of changes in gas composition on burner performance and propose suitable design or operational factors for domestic gas stoves burning natural gas with various heating values. The results obtained in this work would help us to understand what parameter dominates the variation of thermal efficiency, complete combustion and emission pollution. Additionally, our results are of great importance to provide optimum design or operation concepts and real application for manufacturing domestic gas stoves burning natural gas with various heating values.
Section snippets
Experimental
Fig. 1 shows the schematic diagram of the experimental apparatus system. The domestic gas burner adopted in the present work is a so-called double ring burner, which is one of the most typical for burning natural gas (i.e. most popular models found in the market). The burner is made of copper. The burner head consists of two rings with round holes. The outer ring is the major part of the burner, which supplies the primary heat source (heat addition), or higher thermal input. The inner ring is
Results and discussion
In the experiment, θF is defined as the opening of the fuel regulator. At θF=90°, both the inner ring and outer ring burner heads have the maximum flow rate. In other words, θF=90° corresponds to the maximum thermal input. When θF=60°, the inner ring and outer ring burner heads have the middle flow rate of fuel and moderate thermal input. These two values of θF (60° and 90°) are commonly used in cooking. They have a double ring flame structure, and their corresponding thermal input is shown in
Conclusions
Using the same gas stove for burning natural gas with various heating values other than the intended fuel is inappropriate and hazardous due to the possible occurrence of incomplete combustion (i.e. a great increase of CO emissions and/or soot formation), liftoff, flashback and inadequate heat input. In the present study, a single gas burner, which was originally designed to burn a natural gas with low heating value, as adopted to investigate the effects of variations in gas composition on the
Acknowledgements
This work was supported by the National Science Council, Taiwan, ROC, under contract NSC89-CPC-7-168-001.
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