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With the development of the gas industry, there have been significant changes in China’s fuel structure: coal use has given way to gas, and natural gas has replaced artificial gas. This not only meets the needs resulting from the improvement in people’s living standards but also leads to a substantial improvement in environmental quality. Liquefied petroleum gas has always been an essential component of gas supply, especially in areas where urban gas pipelines are not available, as well as in rural and urban regions where the development of urban gas systems cannot meet demand in a timely manner; large amounts of liquefied petroleum gas are needed in such places. Any gas combustion device, while supplying heat energy, generates large amounts of smoke. The harmful components in this smoke can directly pollute the atmosphere, or they may first contaminate indoor air before affecting the atmosphere as well. The main pollutants in the flue gases produced by gas combustion are CO, SO2, and NOx. Among these, the environmental damage and health hazards caused by CO and SO2 are well known, and various measures have been taken to effectively reduce their generation. It was not until the 1940s that scientists began to pay attention to the environmental pollution caused by NOx and its harm to human health. NOx includes NO, NO2, N2O, N2O3, N2O4, N2O5, etc.; in flue gas, the main components of NOx are NO and NO2. NO is highly toxic; it binds very easily to hemoglobin Hb in the blood, causing hypoxia in the blood and leading to central nervous system paralysis. The affinity of NO for hemoglobin is several hundred to a thousand times greater than that of CO. NO2 is a yellow-brown gas with an irritating odor; its toxicity is 4–5 times higher than that of NO. It can irritate the respiratory system and cause emphysema. Exposure to a NO2 concentration of 16.9 ppm for 10 minutes can cause breathing difficulties and bronchospasm, while at concentrations of 90–100 ppm, exposure for three hours can be fatal. NOx not only causes primary pollution but also leads to secondary pollution of the environment. When NOx is released into the atmosphere and comes into contact with hydrocarbons, it undergoes photochemical reactions under the action of ultraviolet rays from sunlight, resulting in the formation of irritating light-blue smog and causing severe photochemical smog pollution. Furthermore, nitric acid produced from nitrogen oxides, together with sulfuric acid produced from sulfur oxides, will form acid rain. Photochemical smog pollution and acid rain not only pose serious threats to humans, but also cause severe pollution and damage to plants, buildings, water sources, and more. It is evident that NOx poses extremely serious threats to environmental pollution and human health. NOx in the atmosphere primarily comes from fuel combustion; therefore, controlling the generation and emission of NOx during the combustion process is a fundamental approach to protecting the environment. Reducing the generation and emission of NOx from gas appliances to protect environmental quality is an issue that needs to be addressed urgently. Some developed countries began establishing emission standards for NOx from gas equipment as early as the 1970s; for example, in the early 1980s, the United States and Japan set emission standards of 100 ppm and 150 ppm respectively for small gas boilers. Although our country established standards for atmospheric environmental quality in 1982, no standards have yet been set for the NOx emissions from gas equipment. However, an increasing number of industry professionals are calling for such standards, and the Beijing Environmental Protection Bureau has already set requirements regarding the NOx emissions from gas boilers. There are many factors that affect NOx generation, and the properties of the fuel gas play an important role in this process. Among coke oven gas, natural gas, and liquefied petroleum gas, burning liquefied petroleum gas results in the highest levels of NOx, with values that are significantly higher than those for the other two types of gas. Therefore, reducing NOx emissions from LPG burners is of even greater importance. 1 Mechanism of NOx formation: The NOx in flue gases is mainly NO, accounting for about 90%, and some of it is reoxidized to NO2 after being released into the atmosphere. Therefore, studying the mechanism of NOx formation essentially means studying the mechanism of NO formation. The generation forms of NO include fuel-type, temperature-type, and rapid temperature-type. The NO generated during the combustion process is mainly temperature-dependent NO (T-NO), with a portion being rapidly temperature-dependent NO (P-NO), also known as instantaneous NO. 1.1 Mechanism of T–NO formation T–NO is formed from nitrogen and oxygen in the air at high temperatures; its formation mechanism was proposed by the Soviet scientist Zeldvich in 1964. When the mixture of gas and air burns, the main reaction processes that lead to the formation of NO are as follows: N2 + O = NO + N ⑴ N + O2 = NO + O ⑵ According to the equations of chemical reaction kinetics and Zeldvich’s experimental results, the rate of NO formation can be expressed as: ⑶ Where: [NO], [N2], [O2] represent the concentrations in g/mol/cm2; t represents time in seconds; T represents the absolute temperature of the reaction in K; R represents the universal gas constant in J/gmol·K. For premixed flames with high oxygen concentration and low fuel content, the amount of NO calculated using equation (3) yields results that are quite consistent with actual values. However, at ratios lower than the chemical equivalence ratio, that is, when the fuel is too rich, the following reaction also occurs: N + OH = NO + H. As can be seen from equation (3), the rate of NO formation is related to T and other factors. Since the nitrogen concentration changes very little when the gas burns in air, its impact on the rate of NO formation is minimal. In equation (3), the value depends on the equivalence ratio between the gas and air during combustion; therefore, both the temperature and the equivalence ratio during combustion have a significant influence on NO formation. As shown in Figures 1 and 2, when the combustion temperature is below 1500 degrees Celsius, the amount of NO produced is extremely small, while when the combustion temperature is above 1500 degrees Celsius, the amount of NO produced increases significantly. As can be seen from Figures 1 and 2, for every increase of 100 K in temperature, the NO generation rate increases by about a factor of 5; moreover, the amount of NO generated increases proportionally with the oxygen concentration when there is an excess of fuel. The combustion temperature reaches its maximum near an equivalence ratio of 1, and the corresponding generation rate of NO also reaches its maximum at this point. When the excess air coefficient is far from 1, the formation rate of NO will decrease sharply. At the same time, the amount of NO generated increases as the residence time of the flue gas in the high-temperature zone increases. Furthermore, since equation (1) involves atomic oxygen O and nitrogen molecules N, the activation energy for this reaction is higher than that of the reaction between atomic oxygen and the combustible components in the fuel; as a result, the formation rate of NO is slower than that of the combustion reaction. Therefore, large amounts of NO are not generated within the flame. The formation of NO occurs at the rear end of the flame zone, that is, it is produced in large quantities downstream of the flame. In summary, the main factors affecting T—N0 formation are temperature, oxygen concentration, and residence time. 1.2. Mechanism of P–NO formation: Rapid-temperature NO is generated when hydrocarbon fuels are combusted in a premixed manner with an excess air coefficient of 0.7–0.8; it is formed not downstream of the flame front, but within the flame itself. The mechanism of its formation has not yet been clearly established. Bowman believes that the formation of P—NO is due to the oxygen atom concentration being much higher than the equilibrium concentration required for the dissociation of oxygen molecules. Fenimore thinks that P—NO is produced when hydrocarbon fuels are burned in excess; in this process, CH groups generated from the fuel collide with N2 molecules, resulting in the formation of CN-type compounds. The intermediate products such as N, CN, and NCH are then further oxidized to form NO. Generally, the amount of P—NO produced is not greatly affected by temperature, and it is one order of magnitude smaller than that of T—NO. 1.3 Formation of F—NO F—NO is formed when nitrogen atoms present in the fuel as compounds are oxidized during combustion. Nitrogen in fuel generates NO more easily than nitrogen in air, with a generation temperature of 600°C–700°C. In the combustion of gaseous fuels, due to their very low nitrogen content, little fuel-derived NO is produced during the combustion process, and it can be considered negligible. 1.4 The formation of NO2: NO2 is produced by the oxidation of NO, and this process occurs through the following reaction: NO + HO2 = NO2 + OH (5). Generally, NO2 can be detected in the reaction zone of premixed flames and diffusion flames, as well as in the low-temperature areas downstream of the flame front; very little NO2 is produced in the high-temperature areas downstream of the flame front. A large amount of NO is converted to NO2 after the flue gases are released into the atmosphere. ⑹ The reaction rate in the above equation is highly dependent on the concentration of NO in the air; a higher concentration leads to faster conversion of NO2, while a lower concentration results in slower conversion. 2 Methods for suppressing NOx during gas combustion. The nitrogen content in gas is extremely low, so almost no fuel-type NOx is generated during combustion. The amount of rapidly formed NOx is an order of magnitude lower than that of temperature-induced NOx; therefore, reducing NOx emissions in flue gases primarily involves suppressing the formation of T-NOx. Based on the formation mechanism of T–NOx, the corresponding suppression methods include: (1) reducing the combustion temperature, and paying attention to minimizing local high-temperature areas during combustion ； (2) Reduce oxygen concentration ； (3) To carry out the combustion process under conditions far from the theoretical air ratio ； (4) Reduce the residence time of flue gas in the high-temperature zone. 3 Experimental and theoretical analysis of factors affecting NOx generation. There are many factors that influence NOx generation; this paper investigates the relationship between the primary air coefficient and the shape of the combustion holes, and establishes an experimental system as shown in the figure below. The gas source used in this experiment is liquefied petroleum gas, and the burner is of atmospheric type (pure propane gas was selected for ease in design calculations). The pressure is 3 KPa, and the heat load is 11 KW. The combustion gas flows in two layers, inner and outer; the flame holes are vertically rectangular in shape with openings on the inner side, which prevents clogging of the holes and helps improve thermal efficiency. 3.1 Influence of mixing characteristics on NOx generation The NOx generation characteristics differ between premixed combustion and diffusion combustion of gaseous fuels; from the perspective of reducing generation levels, premixed combustion is superior to diffusion combustion. The amount of NOx generated in a premixed flame is influenced by the combined effects of temperature and O2 concentration changes resulting from variations in the air-to-fuel mixture ratio. The amounts of NOx and CO generated under different opening degrees of the air control vanes in the inner and outer rings during the tests were measured, with results shown in the table below: (All measured values have been adjusted to an excess air coefficient of 1.0 and are based on dry flue gas. Samples of the mixture at different vanes opening degrees were taken, and chromatographic analysis was used to determine the composition of the mixture; thereby, the primary air coefficient could be calculated.) Amounts of NOx and CO generated at different inner and outer ring opening degrees: Primary air coefficient for the inner ring’s vane opening degrees: a) Primary air coefficient for the outer ring’s vane opening degrees (a1): 1/3 (0.6987), 1/2 (0.7006), 2/3 (0.7275), 1 (0.8219). NOx, CO: 1/3 – 0.4469, 73.5, 58.2; 74.1, 59.3; 72.6, 56.6; 76.4, 71.5. 1/2 – 0.5147, 72.7, 52.9; 69.9, 58.2; 68.3, 46.7; 75.8, 53.2. 2/3 – 0.5583, 81.9, 65.5; 78.9, 62.5; 78.9, 63.2; 76.6, 67.6. 1 – 0.7174, 79.4, 56.5; 77.7, 63.8; 77.7, 55.9; 85.7, 62.8. As can be seen from the data in the table, when the primary air coefficient for the outer ring varies between 0.56 and 0.72, the amount of NOx generated remains relatively constant. As the primary air coefficient for the outer ring decreases from 0.55 to 0.45, the amount of NOx generated first decreases and then increases, with the lowest value occurring when the primary air coefficient is 0.51. When the inner ring air control valve is increased from 1/3 opening to 1/2 opening, the primary air coefficient increases only slightly; when it is increased from 1/2 opening to full opening, the primary air coefficient rises from 0.70 to 0.82, and the amount of NOx generated tends to increase as the primary air coefficient increases. It can be seen that the primary air coefficient has a significant impact on NOx formation, and it must be selected appropriately. According to the data in the table, the CO generation level is in the range of several dozen ppm, which is far below the requirements set by national standards. 3.2 The effect of rectangular fire holes on reducing NOx generation: Theoretical analysis and experimental observations show that vertical rectangular fire holes are effective in reducing NOx formation. When a vertical rectangular fire hole is burning, there is a temperature field between the outer fire hole and the outside of the burner head in the inner circle, as well as between the inner fire hole and the central axis. The greater the distance between these elements, the smaller the temperature gradient; the smaller the distance, the larger the temperature gradient. However, near the flame surface, the temperature gradient is very high regardless of the distance. Driven by the upward lift force, the smoke moves upwards; at the same time, due to the adsorption effect and the principle of concentration diffusion, the smoke moves upward along the walls of the fire holes. Therefore, due to the perturbing effect of this smoke, the flame temperature decreases, thereby suppressing NOx formation. As the distance between the circumferential seams of the inner and outer rings, as well as the fire holes in the inner ring and the central axis increases, the perturbation effect decreases; conversely, when this distance is smaller, the perturbation effect increases. However, if the distance is too small, insufficient secondary air will lead to the production of large amounts of CO. Therefore, it is crucial to select appropriately the length of the vertical rectangular fire holes and the diameter of the burners at the inner and outer perimeters in order to control excessive NOx formation and CO production. Through experiments on three types of gases—artificial gas, natural gas, and liquefied petroleum gas—it was found that vertical rectangular flame holes are particularly effective in reducing the NOx emissions from liquefied petroleum gas appliances. 4 Conclusions and Recommendations 4.1 Selecting an appropriate primary air coefficient will reduce the NOx generation in premixed flames ； 4.2 Vertical rectangular fire holes help reduce NOx generation, and are particularly effective in lowering NOx emissions from liquefied petroleum gas appliances. At the same time, attention should be paid to the selection of the length of the vertical rectangular fire hole and the diameters of the inner and outer ring heads. 4.3 There are many factors that affect NOx formation. Some of these factors not only influence NOx production but also have an impact on CO levels and thermal efficiency. Therefore, when designing LPG burners with low NOx emissions, it is necessary to take into account the relationships among NOx, CO, and thermal efficiency in order to achieve the best overall results. References: 1. Zhuang Yongmao, Shi Huibang. Combustion and Pollution Control. Shanghai: Tongji University Press, 1998. 2. Bai Liping, Fu Zhongcheng. Research on the use of flame coolers to reduce NOx emissions from gas water heaters. Gas & Heat, Issue 6, 1999. 3. Jiang Zhenghou. Gas Engineering Technology Manual. Shanghai: Tongji University Press, 1993