Characteristics of combustion in fuel and gas boilers

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Characteristics of combustion in fuel and gas boilers 　　　I. Characteristics of fuel combustion 　Fuel is a liquid fuel, and its boiling point is always lower than its ignition point; therefore, the combustion of fuel always occurs in a gaseous state. After the fuel is atomized into fine droplets that are injected into the furnace, they are heated by the high-temperature flue gases inside the furnace, resulting in vaporization. The vaporized fuel gas then reacts with oxygen in the surrounding air to form a flame. Some of the heat generated by combustion is transferred to the oil droplets, causing them to vaporize and burn continuously until they are completely burned out. The smaller the diameter of the oil droplets, the faster they burn. Similarly, the oxygen required for the combustion of oil droplets is supplied in a timely manner, resulting in faster combustion of those droplets. Therefore, to enhance oil combustion, the following steps must be taken: 1. Improve the atomization quality and reduce the diameter of oil droplets ； 　　2. Increase the relative velocity between air and oil particles ； 　　3. Proper air distribution. 　　II. Combustion characteristics of gas 　Gas with a high calorific value requires a large amount of air; in order for it to burn fully, a significant quantity of air must be mixed with it. The combustion process of gas does not involve the atomization and vaporization processes of fuel. The way in which gas and air are mixed has a significant impact on the intensity of combustion, flame length, and flame temperature. Depending on the mixing method, gas combustion can be divided into three types: 1. Diffusion combustion. In this type of combustion, there is no prior mixing; instead, the gases mix together at the nozzle tip and then burn. Its advantages include stable burning and a simple burner structure, but the flame is long, which can lead to incomplete combustion and carbonization of the heated surface. 　　2. Premixed air combustion: In this combustion method, a portion of air is mixed with the fuel in advance before combustion (with the primary air excess coefficient ranging from 0.2 to 0.8), and then combustion takes place. Its advantages are a clear combustion flame, enhanced combustion, and high thermal efficiency. However, the combustion is unstable, and high requirements are placed on the control of primary air as well as the combustion components. This combustion method is commonly used in gas burners. 　　3. Flameless combustion: In this type of combustion, the air required for the gas is mixed evenly with the gas prior to combustion. The primary air excess factor is equal to the air excess factor when the fuel burns completely, and no oxygen needs to be drawn from the surrounding air during the combustion process. When the gas-air mixture reaches the combustion zone, it can burn up instantly. 　　III. Burners The burners in fuel-gas boilers are key components of these boilers; the selection of burners is crucial for ensuring compatibility with the boiler, as well as for controlling and regulating combustion. This topic will be discussed in detail in future articles. 　　Structural Features and Types of Fuel and Gas Boilers I. Structural Features of Fuel and Gas Boilers Fuel and gas boilers differ from coal-fired boilers in that they require burners to inject fuel into the boiler’s furnace; combustion takes place within a combustion chamber, without the need for grates. Since fuel and gas boilers do not produce any fuel ash after combustion, they do not require ash discharge facilities. Oil and gas injected into the furnace can explode easily if they mix with air to a certain extent or go out. Therefore, both fuel and gas boilers require automated combustion and control systems. Fuel and gas boilers have a compact design; the small boiler unit, along with its ventilation, water supply, control, and auxiliary equipment, are all mounted on one chassis. Larger and medium-sized boilers can also be assembled before being shipped out. The main pressure-bearing components and connection methods of fuel and gas boilers are described as follows: 1. The main pressure-bearing components of fire-tube boilers: boiler shell, tube sheet, furnace chamber, flue tubes ； 　　2. The main pressure-bearing components of a water-tube boiler: drum, water wall, boiler tube bank, coil tubes, header ； 　　3. Connection method of the main pressure-bearing components of fuel and gas boilers: mainly welding, with expansion joining also used in some cases. 　　II. Types of Fuel and Gas Boilers 1. Horizontal Internal Combustion Fire-Tube Boilers (1) Definition A fire-tube boiler in which the longitudinal axis of the boiler shell is parallel to the ground and the combustion chamber is located within the boiler body is referred to as a horizontal internal combustion fire-tube boiler. 　　(2) Characteristics of horizontal internal combustion fire-tube boilers The furnace chamber is the combustion area of this type of boiler. The burner nozzle is located at the front of the furnace chamber; the high-temperature flue gases generated by combustion extend to the back. After leaving the furnace chamber, they turn back into a space known as the flashback chamber, and then enter the second passage (flue tube). If they turn back once, it is called a two-pass boiler; if they turn back twice, it is called a three-pass boiler, and so on. Generally, the number of turns back does not exceed four, with three-pass boilers being the most common. Boilers of this type can be classified into dry-back boilers and fully wet-back boilers based on the structure of the flue gas return space at the rear of the furnace chamber (schematic diagrams of dry-back boilers and fully wet-back boilers are shown in Figure 4-1 on page 17 of the \"Regulations for Safety Supervision of Steam Boilers\"). The flue gas return space in a dry-back boiler is enclosed by refractory materials ； The flue gas return space of a fully wet-back boiler is composed of a flashback chamber submerged in water. There is also another type of backfire boiler: in this case, the rear wall of the furnace chamber is sealed inside the boiler shell, and when the flue gases encounter it, they are redirected backward toward the front along the inner walls surrounding the furnace chamber. Such boilers can also be considered full wet-back boilers. Although dry-back boilers have a simple structure, the refractory material at the back of the furnace chamber tends to get damaged, and the rear tube sheet is frequently exposed to direct exposure from high-temperature smoke gases, resulting in large temperature differences. As a result, dry-back boilers are considered obsolete, and few manufacturers produce them these days ； Although full wet-back boilers have a more complex structure, they avoid the problem of smoke leakage in the return space, making them more suitable for combustion at slightly positive pressure; therefore, most manufacturers currently produce this type of boiler as full wet-back boilers. 　　(3) Technical limitations of horizontal internal combustion fire-tube boilers The boiler shell and furnace chamber of horizontal internal combustion fire-tube boilers are both cylindrical components; to ensure safe operation, they must possess sufficient strength and stiffness. Since the wall thickness of a cylindrical element is proportional to the product of the cylinder’s diameter and pressure, whether it is the boiler shell or the furnace chamber, an increase in diameter and an increase in pressure both lead to an increase in wall thickness. Through technical and economic comparison, if the diameter of the boiler shell is too large, its manufacturing cost will exceed that of water-tube boilers with the same capacity and pressure, making it uneconomical ； For the furnace chamber, if the wall thickness exceeds 21–22 mm, it will result in excessive thermal stress, posing a threat to the safe operation of the boiler. Therefore, the operating pressure of fuel and gas horizontal internal-combustion fire-tube boilers generally does not exceed 2.0 MPa ； The output of a boiler: for boilers with a single furnace, it generally does not exceed 15 t/h ； Double-chamber boilers generally do not exceed 30 t/h. 2. Water-tube boilers When the boiler capacity is ≥30 t/h, the performance parameters of water-tube boilers are significantly better than those of fire-tube boilers. The only thing to note is that the combustion characteristics of fuel and gas require a slight positive pressure for ventilation; therefore, high requirements are placed on the strength and sealing of the boiler walls. The main types of water-tube boilers are Type D, Type A, and Type O. The common features of the three types are: a horizontal layout with burners installed horizontally, easy operation and maintenance, relatively small width and height dimensions, and considerable length flexibility, making them suitable for the serialized production of boiler drums. Among them, the Type D boiler is of the double-drum vertical type; the water wall is located on the right (left) side, while the convective tube bank is situated between the upper drums on the left (right) side. A superheater or economizer can be installed as needed ； Type O is also of the double-drum vertical configuration, with the upper drum being longer and the lower drum shorter. The water wall tubes on both sides of the front furnace chamber bend toward each other at the lower part and are connected to the lower header; the burner is arranged in front of this area ； It is of Type A, single-drum vertical configuration; both the furnace and the convection tube bank are composed of tubes located between the upper drum and the lower collectors on both sides. Burners are arranged at the front part, and a filter can be installed at the rear of this boiler. 　　3. Small vertical boilers: Boilers whose longitudinal axis of the boiler shell is perpendicular to the ground are considered vertical boilers. Its burner is generally located on the top of the furnace. The core is a furnace chamber; flue gas flows upward in a loop at the bottom of the furnace, passing through the jacket formed by the corrugated tubes and being heated there, with steam being separated out in the upper part ； The burner can also be arranged on the lower side; after burning inside the furnace chamber, it forces smoke upward through the smoke pipes, and the water is heated inside the outer shell surrounding these smoke pipes. 　　III. Re-adjustment of combustion after the boiler is delivered to the customer’s site Conventional boiler products of various specifications and models are, provided that combustion conditions permit, generally subjected to combustion testing in the factory. Why is it necessary to readjust the boiler before it is delivered to the user? There are mainly the following factors at play, so adjustments are needed. 　　1. Environmental factors 　After the boiler is delivered to the user, it is usually placed in a boiler room, and the building is equipped with a tall smoke exhaust chimney. In this way, as the external environment changes, it becomes necessary to readjust the oil-air ratio of the burner. Secondly, due to the altitude of the geographical location where the user is situated, adjustments to the combustion process are also necessary to ensure that the boiler can meet the designed load requirements. 　　2. Fuel factors: After purchasing a boiler, users must adhere to strict fuel requirements. Furthermore, depending on the use of the boiler and the external ambient temperature, select fuel that meets **the relevant standards. Fuel that does not meet the requirements often fails to burn properly, and may even damage the lifespan of the combustion equipment ； With different grades of fuel, the combustion process also needs to be adjusted. 　　3. Adjustment of control and protection devices Before a boiler is put into operation, there are often discrepancies between the settings of the control and protection devices and the actual requirements of the user. Moreover, after transportation, the boiler itself and its accessories may require readjustment. 　　IV. Selection Principles for Fuel and Gas Boilers When selecting fuel and gas boilers, in addition to considering the principles of combining technology and economics, it is also necessary to take into account the owner’s intentions, as well as the opinions of environmental protection, fire safety, and labor authorities, with an emphasis on safety and environmental protection. The following are several selection principles to keep in mind: 1. It should operate automatically, with guaranteed safety, as well as reliable automatic control and protection devices. 　　2. Choose boiler manufacturers with a good reputation and excellent after-sales service. 　　3. The boiler’s performance must be consistent with the user’s requirements for heat and steam, offering good adaptability. When there are significant changes in user load, sensitivity should be high, tracking speed should be fast, and stability under pressure should be maintained. 　　4. Choose between a vertical or horizontal boiler depending on the boiler’s installation location and requirements. 　　5. Depending on the user’s requirements regarding steam supply time, a fast boiler can be chosen, which is generally able to supply steam within 3–5 minutes. 　　6. Users are required to provide a load curve in order to verify the output and performance of the selected boiler. 　　V. Development Trends of Fuel and Gas Boilers With the continuous deepening of reform and opening up in China, the rapid development of economic construction across the country, and the swift growth of high-rise residential buildings in cities, there are higher demands for environmental protection. Along with intensive exploitation of oil and gas resources, the use of fuel and gas boilers is increasing year by year. Looking at the development of fuel and gas boilers in recent years, we can summarize the future trends as follows: 1. High efficiency of boilers. 　　2. Simple structure. 　　3. Use simple auxiliary equipment. 　　4. Fully intelligent automatic control with a multi-level protection system. 　　5. Equipped with a burner (blower) and flue noise reduction system to reduce the noise generated during boiler operation. 　　6. An automatic dosing device and a water treatment system should be installed. 　　7. Other monitoring and control devices shall be installed to ensure the safe operation of the boiler 24 hours a day without supervision.