Thread Content
1. Where should the flue gas sampling points for heating furnaces be located? (1) Sampling points in the radiation chambers: For heating furnaces with two or more radiation chambers, flue gas sampling points should be installed at corresponding locations in each radiation chamber, so as to check whether the air supply to each chamber is the same; such points are not necessary for furnaces with only one radiation chamber. (2) A flue gas sampling point is installed before entering the convection chamber; this point is used to check the actual amount of air supplied for combustion, thereby controlling the air flow rate of the burner. (3) Install flue gas sampling points in the convection chamber; use the excess air coefficient at these points to calculate the furnace’s thermal efficiency and to check for air leaks in the convection chamber. (4) When a waste heat recovery system is used, flue gas sampling points should be installed before and after the air preheater. 2. What issues need to be considered when installing smoke sampling points? (1) There should be no air leaks at the location of the sampling point and in its vicinity. (2) The sampling point should preferably extend to the middle of the flue gas flow cross-section; if this is not possible, the extension length should be at least 500 mm. (3) The sampling point for controlling the combustion air supply must be placed before entering the convection chamber. (4) The sampling point for measuring thermal efficiency should be located in the non-turbulent area of the chimney. 3. What should be the acceptable range for the carbon monoxide content in the flue gases of a heating furnace? In the flue gases of a heating furnace, the carbon monoxide (CO) content should be kept between 0.015% and 0.025% as a reference standard; if the carbon monoxide level in the flue gases is too low or even absent, it indicates that there is an excess of air inside the furnace ； Conversely, if the carbon monoxide level is higher than this value, it indicates incomplete combustion inside the furnace. Therefore, the heating furnace can be adjusted based on flue gas analysis to bring the fuel combustion close to its optimal state. 4. When analyzing flue gas, is it better to use an oxygen analyzer or a carbon dioxide analyzer? By analyzing the O2 or CO2 content in the flue gas, it is possible to determine the excess air coefficient. At a normal excess air coefficient, the total volume percentage of O2 and CO2 in the fuel flue gas is approximately 17%. The greater the excess air volume, the higher the volume percentage of O2. The oxygen in the flue gas mainly comes from the excess air; the oxygen content in the fuel is very low. Therefore, when the amount of excess air remains constant, the oxygen content in the flue gas varies little among different fuels. Due to the significant differences in their hydrocarbon ratios, different fuels produce greatly varying amounts of CO2 as a result of the combination of carbon and oxygen. In the heating furnaces of petrochemical plants, since combustion can be done using oil alone, gas alone, or a mixture of both, it is convenient to use an O2 analysis meter to control the excess air coefficient. 5. What are the factors that affect the thermal efficiency of a heating furnace? The main factors affecting the thermal efficiency of a heating furnace are as follows: (1) The higher the flue gas temperature of the furnace, the lower its thermal efficiency. (2) The higher the excess air coefficient, the lower the thermal efficiency. (3) The greater the loss due to incomplete chemical combustion, that is, the more CO and H2 in the exhaust gases, the lower the thermal efficiency. (4) The greater the mechanical incomplete combustion loss, that is, the higher the content of unburned carbon particles in the exhaust gases, the lower the thermal efficiency. (5) The greater the heat loss from the furnace wall, the lower the thermal efficiency. 6. What are the measures to improve the thermal efficiency of heating furnaces? The main measures to improve the thermal efficiency of heating furnaces are as follows: (1) Reduce heat loss ① Strengthen management and establish reasonable operating procedures. ②Control the \"three doors and one panel\" to reduce the furnace’s excess air coefficient. ③A control system and computer operation are used. ④Zirconia sensors are used to control the oxygen content in the radiation chamber. ⑤Reduce heat loss from the furnace wall. (2) Absorb more heat using the convection chamber ① The convection chamber uses pin tubes or finned tubes. ②Set up the soot blower. ③Increase the convective tube or appropriately lengthen it. ④The walls in the convection chamber are made of baffle bricks. ⑤Pure radiation furnace with convection chamber. (3) Add a waste heat recovery system: ① Use a rotary air preheater. ②The convection chamber uses cold feed. ③Add a fixed air preheater～steel pipe, cast iron pipe, or glass tube. ④A heat-tube air preheater is used. ⑤Air is preheated using a circulating heat carrier. ⑥A waste heat boiler is used. (4) Other methods: ① Install a heater to preheat the combustion air. ②Use forced air supply or high-energy, high-intensity burners. ③Reduce flue gas from refineries and eliminate torches. 7. What is meant by “three valves and one plate”? “Three valves and one plate” refers to the throttle valve, valve, air valve, and chimney damper. ““Three doors and one plate” is an abbreviation for a heating furnace operation method; by appropriately adjusting the valves for the fuel oil and atomizing steam, it is possible to ensure that the flame lengths of each burner are roughly uniform and that the atomization is optimal. The adjustment of the burner damper or the butterfly valve in the burner’s air duct must be coordinated with that of the chimney baffle. If the chimney dampers are opened too wide, or if the burner air valves or duct butterfly valves are closed too tightly, it will result in excessive negative pressure inside the furnace, allowing too much air to leak in ； If the baffle is set too low and the damper or butterfly valve is opened too wide, it may create a positive pressure in certain areas of the furnace, allowing hot flue gases to leak out. The general control parameter should set the negative pressure at the inlet of the flow chamber at -2 to -4 mmHg (-20 to -40 Pa). 8. What is the excess air coefficient α? Fuel requires oxygen for combustion; in air, oxygen accounts for about 21% while nitrogen accounts for about 79%, so air must be supplied for the fuel to burn. The theoretical amount of air required for the combustion of 1 kg of fuel oil (α = 1) is approximately 14.2 kg (1 l Nm³). In actual heating furnaces, not all of the air that enters from the combustion gases can be used in the combustion process. Additionally, air leaks in from other unsealed areas of the furnace, so the actual amount of air that enters the furnace is always greater than the theoretical amount. The ratio of the former to the latter is called the excess air coefficient. Excess air amount = (α – 1); for example, if α = 1.35, then the excess air amount is 0.35, which corresponds to 35%. 9. What are the disadvantages of a too high excess air coefficient α? When the exhaust temperature of the heating furnace remains constant, a higher α leads to an increased amount of exhaust gas, and as a result, more heat is released into the atmosphere through the chimney. This **reduces the thermal efficiency of the furnace. For example, at a flue gas temperature of 400°C, α ranges from 1.3 to 1.7, and the thermal efficiency of the furnace decreases by about 6%. High α values will also exacerbate the oxidative corrosion of the furnace tubes ； Increase the dew point temperature of the flue gases, thereby expanding the range of low-temperature corrosion ； Furthermore, it also promotes the formation of NOx, thereby exacerbating environmental pollution. 10. What considerations should be taken when reducing the excess air coefficient α? Although there are many advantages to reducing the excess air coefficient, an important prerequisite is that complete combustion of the fuel must be ensured. Otherwise, it will increase the losses due to chemical incomplete combustion and mechanical incomplete combustion; seemingly this reduces α and improves thermal efficiency, but in reality it increases the heat losses from incomplete combustion, so the overall thermal efficiency of the furnace does not improve, and it is likely that more fuel is burned.