Why do nitrogen oxides form in boiler flue gas?

Boilers are widely used in various industries and households to provide heat or power. However, the combustion of fuel in the boiler chamber generates not only heat and steam, but also various pollutants, including nitrogen oxides (NOx). NOx is a group of harmful gases that can cause acid rain, smog, and respiratory diseases. Therefore, controlling the emission of NOx from boilers is crucial for both environmental protection and human health.

The formation of NOx in boiler flue gas is a complex process involving both thermal and chemical reactions among the fuel, air, and nitrogen from the combustion air or the fuel itself. In brief, as the temperature in the flame zone exceeds 1300°C, some of the nitrogen and oxygen molecules in the air or fuel get dissociated and recombined into nitric oxide (NO) and nitrogen dioxide (NO2), which are collectively called NOx. The amount and ratio of NOx formation depend on various factors, such as the fuel type, combustion mode, excess air ratio, flame shape, and pressure. For example, natural gas has a lower nitrogen content than oil or coal, so it tends to produce less NOx. However, the use of lean premixed burners or staged combustion can reduce the flame temperature and the residence time of nitrogen and oxygen, thus minimizing NOx production.

To meet the increasingly stringent emission standards and customer demands, ZOZEN has developed various advanced flue gas systems for gas-fired boilers. The flue gas system consists of several components, such as the flue gas recirculation (FGR) system, the selective catalytic reduction (SCR) system, the selective non-catalytic reduction (SNCR) system, and the low-NOx burners. These systems can work together or separately to achieve different levels of NOx reduction, such as less than 30 mg/Nm3, 50 ppm, or 100 ppm.

The FGR system recirculates a portion of the cooled flue gas back to the combustion air inlet, which dilutes the oxygen concentration and reduces the peak flame temperature. This helps to inhibit the thermal NOx formation and increase the flue gas volume, which can enhance the heat transfer and reduce the fouling and corrosion of the boiler tubes. The SCR system injects a reductant agent, such as ammonia or urea, into the flue gas stream before it enters the catalyst bed, where the NOx molecules are chemically reduced into harmless nitrogen and water vapor. The SNCR system uses a similar principle but without the catalyst, by injecting the reductant agent into the furnace or the convective pass, where the temperature is between 900 and 1100°C. The low-NOx burners have a specially designed structure and fuel-air distribution, which can minimize the flame temperature and the nitrogen residence time.

ZOZEN's gas-fired boiler is a type of boiler that uses natural gas or liquefied petroleum gas (LPG) as the fuel, and has various models and sizes, from 0.7MW to 14MW, with a thermal efficiency of up to 98%. The gas-fired boiler is easy to install, operate, and maintain, and has a low emission level of NOx, CO, and SOx. The gas-fired boiler is widely used in many fields, such as food processing, chemical industry, textile industry, paper industry, and so on.

With the help of ZOZEN's flue gas system, the gas-fired boiler can achieve a low emission level of NOx, which meets or exceeds the national and international standards. The flue gas system can be customized according to the specific requirements of the customers, and can be retrofitted to the existing boilers. ZOZEN's flue gas system not only reduces the environmental impact of the boiler, but also improves the energy efficiency and the reliability of the boiler. ZOZEN's gas-fired boiler is your best choice for a cleaner and more efficient source of heat and power.