Common problems with heating furnaces

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1. High excess oxygen content: The fuel gas in the heating furnace cannot burn completely at the chemical equilibrium air volume (i.e., the theoretical air volume); it requires a certain amount of excess air to burn completely. In actual operation, if the excess air volume increases, a large amount of excess air will be carried away with the exhaust gases, taking heat with it and releasing it into the atmosphere. This results in increased heat losses from the flue gases and a lower thermal efficiency of the heating furnace. When the excess air volume is too high, it also accelerates the oxidation of the furnace tubes and internal components, increases the heat absorption in the convection section, promotes the conversion of SO2 to SO3, and exacerbates corrosion due to low-temperature dew points in the flue gases. The aging and failure of the heat pipes in the air preheater can lead to air leaking into the flue gases. Large fluctuations in the negative pressure within the furnace, untimely adjustment of the dampers, and poor sealing of the viewing ports, manholes, and dampers in the direct exhaust ducts can all result in a high level of excess oxygen in the furnace, thereby increasing the oxygen content in the exhaust gases. 2. High NOx content in flue gas. The \"Special Emission Limits for Air Pollutants\" specified in GB 31570-2015 set limits on the emission concentrations of pollutants from process heating furnaces: the limit for SO2 is 50 mg/m3, the limit for NOx is 100 mg/m3, and the limit for particulate matter is 20 mg/m3. At present, the levels of SO2 and particulate matter in the flue gas emitted by the facility are well below the specified emission limits. However, NOx levels are affected by the composition of the fuel gas; specifically, when high-calorific value gases such as catalytic dry gas or hydroprocessed residue gas are used as fuel, the NOx levels in the flue gas exceed the allowed limits. The fuel gas composition of the heater and the actual NOx emission concentrations measured by the CEMS were compared under three operating conditions: C2 recovered dry gas fed into the system (Condition 1), catalyzed dry gas fed into the system (Condition 2), and light fractions from residue hydroprocessing fed into the system (Condition 3). Under Condition 1, the NOx emission concentration measured by the CEMS was 80.43 mg/m3, which meets the requirements of GB 31570-2015. However, when operating under Conditions 2 or 3, the flue gas NOx emission concentrations exceeded the existing standard values, representing severe excess emissions. To meet the standard requirements, it is urgent to upgrade the heating furnace to ensure compliant emissions. Condition 1 is the main operating condition of the device, while the other two conditions are used for shorter periods of time; therefore, all data presented in this text are based on Condition 1. 3. The thermal efficiency of the heating furnace is low. According to statistics, when the thermal efficiency of the heating furnace is above 90%, the heat loss through exhaust gases accounts for 70%~80% of the total heat loss ； When the thermal efficiency is around 70%, the heat loss due to exhaust gases accounts for over 90% of the total heat loss. It is evident, therefore, that reducing the exhaust gas temperature and thereby minimizing heat loss from exhaust gases has a significant impact on improving the thermal efficiency of the heating furnace. This unit uses a heat-tube type air preheater, and the flue gas temperature is greatly influenced by the ambient temperature. The average flue gas temperature for F-1101/1201/1801 is 127°C; in summer, this temperature rises above 130°C, which has a significant impact on the heating furnace’s thermal efficiency. The thermal efficiency is significantly higher in winter than in summer. Under the same conditions, the average thermal efficiency of the heating furnace tends to decline. The main reason for this is the rapid failure of the heat tubes in the air preheater, with a failure rate of about 5% per year. As a result, the flue gas temperature increases over time, failing to meet energy-saving requirements.