1.1 Principles and Characteristics of Oxy-fuel/Oxygen-enriched Combustion

The core principle of oxy-fuel/oxygen-enriched combustion is to replace the nitrogen (N₂) in air with high-purity oxygen (oxygen-enriched) or pure oxygen (oxy-fuel) as the oxidant, thereby significantly altering the characteristics of the combustion process.

The following are its detailed principles and characteristics:

1. Removal/Reduction of Nitrogen (N₂):

• In traditional air combustion, approximately 78% of the nitrogen in the air does not participate in the combustion reaction.
• This nitrogen absorbs a large amount of heat during the combustion process (being heated to the flame temperature) and is ultimately discharged into the atmosphere as high-temperature flue gas, carrying away a large amount of heat energy and causing energy loss (flue gas heat loss is one of the largest heat losses in equipment such as boilers).
• At high temperatures, nitrogen also reacts with oxygen to produce harmful nitrogen oxides (NOx).

2. Increasing Oxygen Concentration:

• Oxygen-enriched combustion refers to a situation where the volume concentration of oxygen in the oxidant is higher than 21% (air concentration), typically between 24% and close to 100%.
• Oxy-fuel combustion refers to a situation where the oxygen concentration in the oxidant is close to 100% (usually >95%).
• By increasing the oxygen concentration, a greater amount of fuel can be supported per unit volume of oxidant, resulting in more intense combustion.

1.2 Main Effects of Oxy-fuel/Oxygen-enriched Combustion

1. Significantly Increased Flame Temperature:

• This is the most direct and significant effect. Because there is no large amount of heat-absorbing nitrogen, the heat released by combustion is mainly used to heat the products (mainly CO₂ and H₂O) and a small amount of residual nitrogen (if any) or inert gases. This leads to a significant increase in adiabatic flame temperature (far higher than air combustion).
• Underlying Principle: Reduction of a large amount of "inert" gas (N₂) that does not participate in the reaction and only absorbs heat.

2. Increased Combustion Speed:

• Oxygen concentration is one of the key factors determining the rate of combustion reaction. The higher the oxygen concentration, the greater the probability of fuel molecules colliding with oxygen molecules to undergo a reaction, the faster the combustion reaction speed, and the more intense and complete the combustion.
• Underlying Principle: Increased concentration of reactants (O₂), accelerating chemical reaction kinetics.

3. Significantly Reduced Flue Gas Volume:

• Due to the removal of nitrogen, which accounts for approximately 78% of the volume of air, the volume of flue gas produced by combustion is significantly reduced (typically reduced to 1/4 to 1/3 of the flue gas volume of air combustion).
• Underlying Principle: Removal of the main "filler" gas (N₂).

• Advantages:
  1. Reduced flue gas heat loss: The heat carried away by high-temperature flue gas is greatly reduced, improving thermal efficiency (typically by 5%-25% or more).
  2. Reduced equipment size: Burners, flue ducts, induced draft fans, and dust removal and desulfurization equipment for flue gas treatment can be designed to be smaller.
  3. Reduced flue gas treatment costs: The total amount of pollutants to be treated is reduced.

4. Increased CO₂ Concentration in Flue Gas:

• Flue gas is mainly composed of combustion products CO₂ and H₂O vapor, plus a small amount of unburned gas, ash, and possibly residual O₂/N₂. After removing N₂, the CO₂ concentration can reach 70%-90% (dry basis), far higher than 15%-20% in air combustion.
• Underlying Principle: The main component of flue gas is CO₂, and the diluent N₂ is removed.
• Key Application: This is one of the most important applications of oxygen-enriched combustion in fields such as coal-fired power plants. The high concentration of CO₂ makes subsequent carbon capture, utilization, and storage relatively easy and economical, and is an important carbon reduction technology.

5. Reduced Nitrogen Oxide (NOx) Formation:

• The formation of thermal NOx (formed by the reaction of N₂ and O₂ at high temperatures) is closely related to temperature and directly related to nitrogen concentration. Although the increase in flame temperature theoretically favors NOx formation, the removal of most nitrogen fundamentally greatly reduces the precursor of NOx formation (N₂). The overall effect is usually a significant reduction in NOx emissions (can be reduced by 50%-90%).
• Underlying Principle: Removal of the main nitrogen source (N₂) for NOx formation.

6. Enhanced Radiative Heat Transfer:

• High-temperature flames and high-temperature flue gases mainly composed of triatomic gases (CO₂, H₂O) have stronger thermal radiation capabilities. This helps to improve the radiative heat transfer efficiency to the heating surface.
• Underlying Principle: Higher flame temperature and increased proportion of highly radiative gases (CO₂, H₂O) in the flue gas.

1.3 Overview of Main Application Industries of Oxy-fuel/Oxygen-enriched Combustion

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