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How to Treat Nitrogen-containing and Salt-prone Waste Gases in RTO system
2025-09-10
With the continuous expansion of industrialization, human quality of life has steadily improved, and the supply of daily necessities has continued to increase. However, at the same time, the increasingly deteriorating ecological environment has posed significant challenges to human survival and development.
In recent years, severe environmental issues such as smog, the greenhouse effect, ozone layer depletion, and photochemical smog—all of which are caused by air pollution—have garnered widespread public attention.
In China, sources of organic waste gas pollution are primarily concentrated in the petroleum and chemical industries. During production processes, these industries generate nitrogen-containing organic waste gases and inorganic waste gases as a result of energy consumption. The release of these diverse and high-concentration waste gases into the atmosphere inevitably impacts the surrounding environment and human health.
To address these waste gases, technical treatment is necessary to ensure they meet emission standards before being released, thereby reducing environmental pollution.
Treatment Processes for Nitrogen-containing Waste Gases
Process Selection
Among various VOC treatment technologies, incineration is currently the most reliable method for treating VOCs. The most widely used equipment for treating VOCs via incineration includes Thermal Oxidizers (TO), Regenerative Thermal Oxidizers (RTO), Catalytic Oxidizers (CO), and Regenerative Catalytic Oxidizers (RCO).
Among various VOC treatment technologies, incineration is currently the most reliable method for treating VOCs. The most widely used equipment for treating VOCs via incineration includes Thermal Oxidizers (TO), Regenerative Thermal Oxidizers (RTO), Catalytic Oxidizers (CO), and Regenerative Catalytic Oxidizers (RCO).
The working principle of an RTO involves heating organic waste gases to above 760°C, where the VOCs in the waste gas are oxidized into carbon dioxide and water. The high-temperature gas generated by oxidation flows through specially designed ceramic heat storage media, heating the ceramics and storing thermal energy. This stored heat is then used to preheat subsequent incoming organic waste gases, thereby reducing the fuel consumption required for heating the waste gases. Each heat storage chamber undergoes a cyclic process of heat storage, heat release, and purging, operating continuously. After a chamber completes the "heat release" phase, it must be purged with a portion of the treated, clean gas before it can re-enter the "heat storage" phase.
RTOs can be categorized into two types: valve-switching (fixed-bed) and rotary valve types.
Valve-switching RTOs include first-generation two-chamber and second-generation three-chamber or multi-chamber RTOs. These consist of two or more ceramic-filled heat storage chambers, where switching valves change the airflow direction to preheat the VOC waste gases.
Valve-switching RTOs include first-generation two-chamber and second-generation three-chamber or multi-chamber RTOs. These consist of two or more ceramic-filled heat storage chambers, where switching valves change the airflow direction to preheat the VOC waste gases.
Generally, the more heat storage chambers there are, the higher the purification efficiency. Rotary RTOs feature 12 chambers and a cylindrical design. They use a single rotary valve to replace multiple switching valves for waste gas diversion, representing third-generation technology and currently the most advanced RTO system in the world.
Structure of Rotary RTO
The Rotary RTO primarily consists of a combustion chamber, ceramic packing beds, and a rotary valve.
The furnace is divided into 12 chambers: 5 for intake of waste gas, 5 for output of purified gas, 1 for purging, and 1 for isolation.
The Rotary RTO primarily consists of a combustion chamber, ceramic packing beds, and a rotary valve.
The furnace is divided into 12 chambers: 5 for intake of waste gas, 5 for output of purified gas, 1 for purging, and 1 for isolation.
The waste gas distribution valve, driven by a motor, rotates continuously and steadily. Under the action of the distribution valve, the waste gas is gradually and continuously switched among the 12 chambers.
The sealing structure of the Rotary RTO employs contact sealing, characterized by wear resistance, high-temperature resistance, and corrosion resistance.
The sealing structure of the Rotary RTO employs contact sealing, characterized by wear resistance, high-temperature resistance, and corrosion resistance.
Analysis of Combustion Products of Nitrogen-containing Organic Compounds
Mechanism of Nitrogen Oxides Formation
Nitrogen oxides (NOx) are generated through the following three pathways:
Nitrogen oxides (NOx) are generated through the following three pathways:
- Thermal NOx: Formed by the oxidation of nitrogen in the air at high temperatures (above 1400°C), accounting for 15% to 25% of total NOx generation.
- Prompt NOx: Generated when CH radicals from the high-temperature decomposition of hydrocarbons in fuel volatiles react with nitrogen in the air to form HCN and N, which then rapidly further react with oxygen to form NOx. This typically accounts for less than 5% of total NOx generation.
- Fuel NOx: Generated when nitrogen compounds in the fuel are oxidized during combustion.
Factors Influencing NOx Generation
The combustion products of nitrogen-containing organic compounds are a mixture of N2 and NOx. The NOx content depends on various factors, including temperature, concentrations of nitrogen (N) and oxygen (O), fuel characteristics, and residence time. When the fuel gas contains nitrogen components such as HCN, pyridine, or quinoline, the nitrogen in these compounds is first converted into HCN during combustion (in the flame, unlike thermal NOx, which forms downstream of the flame), and then into NH or NH2.
The combustion products of nitrogen-containing organic compounds are a mixture of N2 and NOx. The NOx content depends on various factors, including temperature, concentrations of nitrogen (N) and oxygen (O), fuel characteristics, and residence time. When the fuel gas contains nitrogen components such as HCN, pyridine, or quinoline, the nitrogen in these compounds is first converted into HCN during combustion (in the flame, unlike thermal NOx, which forms downstream of the flame), and then into NH or NH2.
NH and NH2 can react with oxygen to form NO + H2O: 2NH2 + 2O2 → NO + 2H2O. Alternatively, they can react with NO to form N2 + H2O. In the flame, the proportion of fuel nitrogen converted to NO depends on the NO/O2 ratio. When α is less than 0.7, almost no fuel NO is generated.
Experiments show that 20% to 80% of the nitrogen components in fuel are converted into NO during combustion. If there is insufficient oxygen during combustion (α < 1), some of the formed NO can be reduced to N2, lowering the NO content in the waste gas. The following conclusions can be drawn:
- Higher combustion temperatures facilitate NOx formation. It is recommended to control the combustion temperature when burning nitrogen-containing organic compounds.
- A higher oxygen content during combustion facilitates the formation of NOx. If nitrogen-containing organic compounds contain oxygen (e.g., nitro compounds), this also increases NOx content.
- Nitrogen-containing organic compounds are first converted into HCN during combustion, then into NH or NH2, which react with oxygen to form NOx. Therefore, the NOx generation amount follows the order: (sub)amino compounds (-NH or -NH2) > cyano compounds (-CN).
Analysis of Combustion Products of Some Nitrogen-containing Organic Compounds
The following conclusions can be drawn from the calculations above:
- As the temperature increases, the slope of the conversion rate becomes steeper. Higher temperatures result in more thermal NOx.
- The addition of more substituents and longer chains results in lower conversion rates. More substituents and longer chains make the molecules less stable, more prone to react with oxygen, and reduce the likelihood of N combining with O.
- Nitrogen-containing organic compounds with oxygen exhibit higher conversion rates.
Introduction to Treatment Routes for Nitrogen-containing Process Waste Gases
Nitrogen-containing waste gases are collected and directed into the RTO system. The main fan carries the gases through a flame arrestor into the RTO, where they are distributed by the rotary valve into the heat storage chambers. During the upward flow, the gases absorb heat and are preheated to above 850°C before entering the reaction chamber. When the temperature in the top reaction chamber reaches 900°C, the nitrogen components are almost completely oxidized and decomposed.
The waste gas remains in the top chamber for a residence time of at least 1 second. After complete combustion in the high-temperature reaction chamber, the resulting high-temperature clean gas enters another five heat storage chambers to release heat. The gas then exits through low-temperature pipelines, mixes with high-temperature pipelines, and enters a plate heat exchanger for cooling. The cooled gas is treated by an SCR denitrification device to remove secondary pollutant NOx generated during combustion. The purified gas is then discharged through an exhaust stack via an induced draft fan.
Considering that ammonia-containing waste gases generate NOx after combustion, anti-corrosion measures are required for the low-temperature pipelines of the RTO.
Yurcent Environment's self-developed proprietary technologies, including "RTO + SCR Denitrification Technology" and "Ammonium Salt Anti-blocking Technology" for nitrogen-containing and salt-prone waste gases, provide clients with high-standard, low-emission Waste Gas Treatment Solutions. These technologies have demonstrated significant effectiveness and have gained recognition from numerous customer enterprises.