Premixed gas burners used in boilers, heat pumps, hot water heaters and other applications provide a high heat release in a small area while providing low pollutant gas combustion product emissions. Burners which are used in chemical and manufacturing processes often suffer from the problem of matching the heat flux produced by the burner and placed into the space to be heated in the furnace or heat exchanger to the actual load required in order to maximize the amount of heat flux which is being efficiently used, to further maximize the actual rate of production or rate of the process and to reduce problems such as coking, in process heaters for refineries, for example [1, 2]. Such burners, may also occasionally suffer from operational drawbacks, such as instability of the flame relative to the flame holder, which may be evidenced in terms of lift off of the flame from the burner tile, or flame noise and pulsation [3, 4]. In addition, such burners may often produce undesirable levels of emissions, particularly oxides of nitrogen [5, 6]. Many conventional gas-fired burners use a diffusion flame combustion process in which combustion occurs over a range of equivalence ratios, including high temperature, lean regions where thermal nitrogen oxides form [7, 8]. The term ”nitrogen oxides” is intended to refer to any molecular species consisting of nitrogen and oxygen [9, 10], for example, nitric oxide, nitrogen dioxide, nitrous oxide, dinitrogen tetroxide, or combinations thereof. Nitrogen oxides surface as pollutants in a wide variety of contexts. Primarily, nitrogen oxides appear as pollutants in combustion processes. Many approaches have been taken to affect the removal and decomposition of nitrogen oxides from gaseous mixtures containing them [11, 12]. One known method for reducing peak flame temperatures is to use a combustion process which creates a fuel-rich primary combustion region and subsequent air staging with corresponding heat loss, resulting in lowering the overall combustion equivalence ratio to achieve complete combustion [13, 14]. Another known method for reducing peak flame temperatures relates to a combustion process that operates with a fuel-lean primary combustion region and fuel staging in order to raise the equivalence ratio [15, 16]. However, such known methods of staged fuel combustion rely upon a diffusion flame to produce the lean primary stage. External flue gas recirculation can be added to such known methods for further reducing nitrogen oxides.

In the combustion of gaseous fuels, nitrogen oxides are formed primarily through fixation of molecular nitrogen and oxygen in the combustion air [17, 18]. It is known that thermal nitrogen oxides formation depends on the existence of flame regions with relatively high temperatures and excess oxygen [19, 20]. Many conventional combustion methods for reducing nitrogen oxides are based upon avoiding such conditions [21, 22]. It is necessary to consider the prompt nitrogen oxides formation process in order to reach very low nitrogen oxides levels. Reactions between hydrocarbon fragments and molecular nitrogen can lead to the formation of bound nitrogen species, such as hydrogen cyanide, which can subsequently be oxidized to nitrogen monoxide. Such processes become significant relative to the thermal mechanism under moderately fuel-rich conditions at relatively low temperatures [23, 24]. Avoiding such conditions can reduce prompt nitrogen oxides contributions. Additionally, these burners often employ pilot flames for establishing the primary flame region over the burner in a furnace [25, 26]. The pilot, even though small in heat release may contribute to overall burner emissions, particularly of oxides of nitrogen, under ultra-low nitrogen oxides operation. It is of great importance to provide a burner which has greatly reduced emissions, particularly of oxides of nitrogen [27, 28]. It is also of great importance to provide a burner system which is capable of enabling active management and variation of the heat flux in order to allow for the optimization of the heating process and modify the heat flux of the burner to avoid process shutdowns, while maximizing furnace availability [29, 30]. It is very necessary to provide a pilot for a burner, such as may be used in chemical plant process heaters and the like, which provides the establishment of the primary flame region while contributing less to the heat released by the burner and contributing less to the emissions produced by the burner, particularly oxides of nitrogen. It is also very necessary to provide a gaseous fuel burner system which provides a well-organized flame with no significant regions of lean high temperature conditions, which are known to contribute to increased nitrogen oxides emissions.

Combustion processes which burn fossil fuels introduce emissions into the atmosphere which have been linked with harmful effects. Environmental regulations have been enacted to limit the concentrations of these emissions in the exhaust gases from combustion processes [31, 32]. Nitrogen oxides emissions arise from nitrogen present in the combustion air and from fuel-bound nitrogen in hydrocarbons if such fuels are burned [33, 34]. Conversion of fuel-bound nitrogen to nitrogen oxides depends on the amount and reactivity of the nitrogen compounds in the fuel and the amount of oxygen in the combustion zone [35, 36]. Conversion of fuel-bound nitrogen is not present in processes using fuels, for example, natural gas, which contain no fixed nitrogen compounds [37, 38]. Conversion of atmospheric nitrogen present in the combustion air to nitrogen oxides is temperature dependent. In general, the greater the flame temperature in the combustion zone, the greater the resultant nitrogen oxides content in the emissions. nitrogen oxides conversion increases drastically at temperatures greater than 1800 K if oxygen is present.

Many industrial processes, such as forging, reheating, and melting of glass or aluminum, are carried out in high temperature, gas-fired furnaces [39, 40]. In such high temperature processes, air used in the combustion process is frequently preheated. Preheating the air reduces the amount of fuel needed, increasing thermal efficiency, but increases the temperature of the flame, which increases nitrogen oxides content [41, 42]. Thus, a higher temperature burner which is capable of reducing nitrogen oxides emissions without sacrificing thermal efficiency is needed [43, 44]. One way of reducing nitrogen oxides content which has been effective in processes using nitrogen bearing fuels is to create a fuel-rich combustion zone followed by a fuel-lean combustion zone [45, 46]. This can be achieved by staging the introduction of air into the combustion chamber. The fuel-rich zone contains less than the theoretical or stoichiometric amount of oxygen. Thus, less oxygen is available to convert the nitrogen-to-nitrogen oxides. Recirculating flue gas into the flame is another technique to limit nitrogen oxides emissions [47, 48]. The recirculated flue gas reduces the oxygen concentration in the reactants and reduces the flame temperature by cooling the combustion products, thereby lowering nitrogen oxides content. Additionally, nitrogen oxides present in the recirculated flue gas can be further destroyed by reburning. The flue gases can also be used for other purposes, such as preheating the combustion air or vaporizing liquid fuels.

Despite previous work, there are a number of answered questions regarding flame stability at the microscale. Examples include the role of wall material thermal conductivity in flame stability. The present study is focused primarily upon the effect of wall material thermal conductivity on the butane flame stability of microscale gas fired burners. The two primary mechanisms for quenching in these burners are thermal and radical quenching. Increased heat-transfer coefficients are inherent to microscales, because for a fixed Nusselt number, the heat-transfer coefficient scales with the inverse of the length scale. The high heat-transfer rates increase the heat lost from the reaction, reducing the operating temperatures and causing the combustion to extinguish. At the same time, the increased mass transfer within the system, coupled with the high surface-area-to-volume ratio, causes radical adsorption onto the walls, followed by radical recombination. This dearth of radicals quenches the homogeneous chemistry. Another mechanism for loss of stability is blowout, which occurs when the burner exit velocity exceeds the flame burning velocity. The present study aims to provide a fundamental understanding of the butane flame stability of microscale gas fired burners at different wall material thermal conductivities. Particular emphasis is placed upon the stability limits over a range of equivalence ratios and the effect of wall material thermal conductivity on the butane flame stability.