Sodium Sulfite Denitrification (DeNOx)

Introduction

The core principle of sodium sulfite denitrification technology, commonly known as sodium sulfite absorption method or wet denitrification, is to use the reducing properties of sodium sulfite solution to reduce insoluble and chemically stable high valent nitrogen oxides (mainly NO) in exhaust gas into harmless nitrogen (N₂), so as to achieve nitrogen oxide removal.

Core chemical reaction principles

This technique primarily targets NOx in flue gases, which are typically dominated by NO (90-95%) and a small amount of NO₂. NO is insoluble in water and has stable chemical properties, which is the difficulty of denitrification. The sodium sulfite method solves this problem by following these steps:

1. Main Reduction Reaction (for NO)

Sodium sulfite (Na₂SO₃) itself reacts slowly with NO, so in actual engineering, catalysts (such as transition metal ion chelates) need to be added to facilitate the reaction. Its total chemical reaction equation can be expressed as:

4Na₂SO₃ + 2NO → 2N₂↑ + 4Na₂SO₄

4Na₂SO₃ + 2NO → 2N₂↑ + 4Na₂SO₄

The essence of this reaction is that sulfite ions (SO₃²⁻) reduce nitric oxide (NO) to nitrogen (N₂), and they are oxidized to stable sulfate ions (SO₄²⁻), and the resulting sodium sulfate (Na₂SO₄) is dissolved in water and can be discharged with wastewater or further treated.

2. Side reactions (against NO₂ and SO₂)

Reaction with Nitrogen Dioxide (NO₂):

If NO₂ is present in the flue gas, it will react rapidly with sodium sulfite to form N₂ and sodium sulfate.

2NO₂ + 4Na₂SO₃ → N₂↑ + 4Na₂SO₄

2NO₂ + 4Na₂SO₃ → N₂↑ + 4Na₂SO₄

Reaction with sulfur dioxide (SO₂):

This technique is commonly used for flue gas treatment containing both SO₂ and NOx (i.e., simultaneous desulfurization and denitrification). Sodium sulfite reacts very quickly with SO₂ to form sodium bisulfite (NaHSO₃).

Na₂SO₃ + SO₂ + H₂O → 2NaHSO₃

Na₂SO₃ + SO₂ + H₂O → 2NaHSO₃

The generated NaHSO₃ can be regenerated into Na₂SO₃ for recycling under certain conditions (such as heating), but the regeneration system is complex and costly.

Technical process

A typical sodium sulfite wet denitrification system mainly consists of the following units:

1. Flue gas pretreatment unit: The high-temperature flue gas is first pretreated with dust removal (such as electrostatic precipitator, baghouse) and cooling (such as spray cooling tower) to meet the operating conditions of the absorption tower.

2. Absorption Tower (Core Unit):

The pretreated flue gas enters from the bottom of the absorption tower.

The prepared sodium sulfite absorbent (usually containing catalyst) is sprayed down from the top of the tower and is in full contact with the flue gas flowing in the opposite direction.

In the tower, the absorption liquid undergoes the above reduction reaction with the NOx in the flue gas to form N₂ and Na₂SO₄.

At the same time, the SO₂ in the flue gas will also be absorbed by the absorbent solution.

3. Oxidation and by-product treatment unit:

The main component of the absorbent solution after reaction is sodium sulfate (Na₂SO₄).

This part of the waste liquid can be collected and evaporated and crystallized, and the by-product is industrial-grade anhydrous sodium sulfate (Yuanming powder) to achieve resource utilization.

If the system is designed with a regeneration link, the absorbent solution enriched with NaHSO₃ will enter the regeneration system, release SO₂ (acid production) by heating and other means, and reduce it to Na₂SO₃ for reuse.

4. Flue gas post-treatment unit: After the purified flue gas is removed by the mist eliminator to remove the entrained droplets, it is sent to the chimney by the induced fan for discharge.

Technical characteristics

Advantage:

1. High denitrification efficiency: Under suitable conditions, the removal efficiency of NOx can reach more than 85% or even higher.

2. Simultaneous desulfurization and denitrification: It can efficiently remove SO₂ and NOx simultaneously in one system, and the system is highly integrated.

3. The reaction product is harmless: the main product N₂ is directly discharged into the atmosphere without secondary pollution; The by-product sodium sulfate can be sold as a chemical raw material to achieve "turning waste into treasure".

4. Mature and reliable technology: As a type of wet process, the technology is relatively mature.

 Application scenarios

This technique is more suitable for:

• Flue gas treatment of small and medium-sized industrial boilers and kilns (such as glass kilns, ceramic kilns, and waste incinerators).

• When there is an urgent need for simultaneous desulfurization and denitrification.

• Limited space and need to be integrated with facilities.

• Enterprises with by-product sodium sulfate resource utilization channels.

For ultra-large-scale flue gas treatment such as large coal-fired power plants, the more mainstream processes are currently selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) technologies, as they have relatively more advantageous operating costs in large-scale applications.

Summary

Sodium sulfite denitrification technology is an efficient wet reduction technology, which essentially uses the reducing properties of Na₂SO₃ to convert NOx into harmless N₂ under the action of catalyst. It has the advantages of high removal efficiency and collaborative desulfurization, but it also faces challenges such as high operating costs, catalyst management, and by-product treatment. When choosing this technology, it needs to be comprehensively evaluated based on specific flue gas conditions, environmental requirements, economic costs and other factors.