Boiler combustion theory and analysis of factors affecting boiler combustion and thermal efficiency

Thread Content

Boiler, as the name suggests, is composed of "pot" and "furnace". It is a special equipment, a pressure vessel, and an energy conversion equipment. It is a working process in which fuel is burned in the boiler furnace, and the heat released is heated through the pipes to heat the water in the boiler pipes, causing the water to heat up, and finally producing hot water, boiling water or steam. At present, the mainstream products of boilers are gas boilers, oil boilers and electric boilers. With the strengthening of environmental policies and the advancement of boiler technology, there will be a pattern in which electric boilers are the main product and other boilers are supplementary. This is because electric boilers are the cleanest and most environmentally friendly boiler products, with thermal efficiency up to over 98%, high safety factor, and good product quality. As the operating cost of electric boilers decreases, electric boilers will inevitably become the dominant boiler product like traditional coal-fired boilers. Below, the editor of Gongcai.com will learn about boiler combustion theory and the factors that affect boiler thermal efficiency. Boiler combustion theory The furnace, as a combustion chamber, is one of the prerequisites to ensure the normal operation of the furnace. When burning pulverized coal, the requirements for the furnace are: 1. Create good ignition and stable combustion conditions, and ensure that the fuel is completely burned out in the furnace ； 2. There is no slagging on the heating surface of the furnace. ； 3. Arrange enough evaporation heating surfaces to prevent heat transfer from deteriorating. ； 4. Reduce the generation of pollutants as much as possible ； 5. It has wide adaptability to the combination of coal quality and load, as well as the reliability of continuous operation. Factors affecting combustion The combustion rate reflects the amount of combustibles burned per unit time. Since combustion is a complex physical and chemical process, the speed of combustion depends on the chemical reaction speed of combustibles and oxygen and the contact and mixing speed of oxygen and combustibles. The former is called chemical reaction speed, also called chemical conditions ； The latter is called physical mixing speed, also known as physical conditions. The speed of chemical reaction is related to the pressure, temperature and concentration of reacting substances in the reaction space, and is proportional to it. For the actual combustion of the boiler, the main factor affecting the chemical reaction speed is the temperature in the furnace. The higher the furnace temperature, the faster the chemical reaction speed. In addition to the chemical reaction speed, the combustion speed also depends on the speed of the air flow transporting oxygen to the surface of the carbon particles, that is, the physical mixing speed. The physical mixing speed depends on the relative speed of air and fuel, air flow disturbance, diffusion speed, etc. Chemical reaction speed and physical mixing speed are interrelated and both restrict the combustion speed. For example, the chemical reaction rate should be higher under high temperature conditions, but if the physical mixing rate is low and the oxygen concentration decreases, the combustibles will not receive sufficient oxygen supply, and the combustion rate will inevitably decrease as a result. Therefore, only when the chemical conditions and physical conditions are relatively suitable, a faster combustion speed can be obtained. Analysis of factors affecting boiler thermal efficiency 1. The total air volume of oxygen entering the furnace is closely related to the level of boiler thermal efficiency. If the total air volume is too large, the heat loss of the exhaust smoke will increase. If the total air volume is too small, the pulverized coal will not be fully burned. The CO content, fly ash combustible content and slag combustible content in the flue gas The increase in content will lead to an increase in chemical and mechanical incomplete combustion losses. The total air volume also affects the main air temperature and reheat air temperature. Therefore, selecting a reasonable total air volume entering the furnace can minimize the total heat loss, achieve the highest thermal efficiency of the boiler, and maintain a high temperature at low loads. It can be seen from the above that the oxygen content of boiler flue gas is the main indicator to measure whether the combustion process is economical. Flue gas oxygen content control controls the oxygen content within the optimal range by changing the air-fuel ratio. The main control method is the coordinated control of fuel volume and air supply volume. The key to controlling the oxygen content of flue gas is to have an oxygen analyzer with good dynamic characteristics and stable and reliable operation. A British SST high-temperature oxygen analyzer provided by Gongcai.com - OXY-Flex is a high-temperature zirconia oxygen analyzer with a power supply voltage of 24V and a variety of outputs.: 4~20mA, 0~10V, RS232, suitable for measurement situations or closed systems that are not easily exposed to gas, such as ventilation ducts, flues and containers. High Temperature Oxygen Analyzer OXY-FLEX Features: High precision linear output configurable output: 4-20mA and 0-10VDC or RS232 COM port optional output range: Standard ranges 0-25% and 0-100% or fully adjustable via RS232 in 0-100% mode. Externally triggered automatic or manual calibration can calibrate the period 3.3VDC in ordinary atmosphere or known concentration gas. Logic output can diagnose the quality of sensor pump circulation. Optional filtering settings allow suitable fast and dynamic output or slow and stable output. High temperature oxygen analyzer OXY-Flex specification 2. Furnace-windbox pressure difference. Under the condition that the boiler load and furnace outlet oxygen remain unchanged, the furnace - The air box pressure difference is related to the air volume ratio between auxiliary air, combustion air and combustion air. The greater the ratio, the greater the ratio will have a great impact on the stability of pulverized coal combustion, combustibility and NOx emissions. Therefore, choosing a reasonable furnace-air box pressure difference will improve the safety and economy of the boiler. 3. Burnout air volume The uppermost layer of the burner is the combustion air nozzle. The size of the combustion air volume affects the emission of NOx and the burning degree of carbon particles. This test only considers the impact of combustion air volume on boiler combustion. 4. The burner swing angle burner nozzle is designed to swing up and down, mainly by changing the height of the furnace flame center to adjust the reheat temperature and superheat temperature. However, the change in the flame center height will have a certain impact on the pulverized coal combustion. The burner swings upward, and the fly ash combustibles increase the efficiency of the boiler and decrease the amount of desuperheating water. 5. The primary air speed unit has a load of 600mv, and other operating parameters of the boiler remain unchanged. The primary air nozzle wind speed is changed by changing the air volume at the coal mill inlet. Due to the limitations of the pulverizing system, the primary air speed is difficult to change in a large range, so the boiler thermal efficiency has almost no change. This shows that changes in the primary air speed in a small range have little impact on the thermal efficiency of the boiler. 6. Fineness of pulverized coal: The fineness of pulverized coal becomes smaller, the combustible content of fly ash and slag decreases, and the thermal efficiency of the boiler increases. 7. Throwing coal mills are operated separately. The difference in boiler thermal efficiency is small, but it has a greater impact on the temperature.