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Advances in Flue Gas Denitrification Technologies for Industrial Kilns and Boilers

1. Classification of Flue Gas Denitrification Methods

1.2 Wet Methods

(1) Alkali Absorption Method

(2) Oxidation-Reduction Method

(3) Chelation Absorption Method

(4) Microbial Method

2. SCR Flue Gas Denitrification Technology (Dry Method)

2.1 Selective Catalytic Reduction (SCR)

(1) SCR Method: Selective Catalytic Reduction (SCR) is currently the most researched and widely applied denitrification technology in industrial settings.

(2) SCR Invention: SCR was first developed by the Japanese and established as a demonstration project at the Shimoneski Power Plant in 1975. It has since been gradually promoted and applied in various countries in Europe and America.

(3) SCR Denitrification Principle: The primary principle involves using NH3 as a reducing agent to reduce NOx in the waste flue gas to N2 under certain reaction conditions. The reaction equations are as follows:

  • 4NH3 + 4NO + O2 → 4N2 + 6H2O
  • 8NH3 + 6NO2 →7N2+ 12H2O


2.2 SCR Process Flow Diagram

Figure 1: Schematic Diagram of SCR Denitrification Process for Coal-fired Boiler Flue Gas

2.3 SCR Denitrification Catalyst

The main active components of the catalyst are TiO, WO, V O, and MoO.

Figure 3: Schematic Diagram of SCR Denitrification Catalyst Reactor

2.3 SCR Denitrification Catalyst

Figure 4: Structural Diagram of Catalyst in SCR Denitrification Reactor

2.4 Analysis of Advantages and Disadvantages of SCR Denitrification

Advantages:

  • Medium-temperature reaction (320-420°C), high catalytic selectivity, and high denitrification efficiency. Generally, the denitrification rate can exceed 80%, reaching 90% or even higher under favorable conditions.

Disadvantages:

  1. Large investment; consumes reducing agents and requires periodic replacement of the catalyst.
  2. Safety hazards: NH3 is highly irritating and corrosive to pipelines, leading to secondary pollution in case of leaks.
  3. High operating costs.
  4. Not suitable for many industrial kilns and furnaces, such as ceramics, glass, coking, etc.
3. Selective Non-Catalytic Reduction (SNCR)

3.1 Selective Non-Catalytic Reduction (SNCR)

The SNCR method involves directly spraying a reducing agent, such as ammonia water, into high-temperature (850-1050 degrees) flue gas. It does not require a catalyst, and the reducing agent directly converts NOx into N2 and H2O. In the SNCR process, when using ammonia water or urea as a reducing agent, there may be issues with NH3 leakage.

SNCR Invention: Invented by the U.S. Exxon company and successfully applied in industrial settings in Japan in 1974.

3.2 SNCR Denitrification System Flow Diagram

Figure 5: Flow Diagram of SNCR Denitrification System

3.3 SNCR Denitrification Reducing Agents and Principles

(1) Liquid ammonia as a reducing agent:

4NH3 + 4NO + O2 → 4N2 + 6H2O 2 + 2NO2 + 2O2 + 6NO2 → 3N2 + 6H2O 2 → 7N2 + 12H2O

(2) Urea as a reducing agent:

(NH2)2CO → 2NH3 + CO2 2NH2 + NO → N2 + H2O CO + NO → N2 + CO2

3.4 Analysis of Advantages and Disadvantages of SNCR

Advantages:

  • Cost-effective equipment investment, no need for a catalyst, and convenient operation and management.

Disadvantages:

  1. Strong irritant properties, ammonia or ammonia water is prone to leakage, posing safety hazards, and ammonia escape can lead to secondary pollution.
  2. Strong alkalinity of ammonia can corrode kilns, products, and equipment, affecting product quality.
  3. Urea has poor activity, unstable denitrification effects, and is relatively expensive.
  4. Ammonia water and urea are easily oxidized at high temperatures, leading to ineffective consumption, increased chemical usage, and operating costs.
  5. Not suitable for use in industrial kilns and furnaces.
4. Industrial Kiln Flue Gas HSR Denitrification

4.1 High Selective Reduction (HSR) Denitrification

HSR Denitrification Method: It is a method that utilizes composite denitrification agents for in-situ selective reduction of nitrogen oxides (NOx) in high-temperature flue gas from industrial kilns/boilers. It was jointly developed by Jinan University and Guangzhou Lvhua Environmental Protection Technology Co., Ltd.

4.2 Applications of HSR Denitrification Technology

(1) Ceramic Industry: Flue gas denitrification in spray drying tower hot air furnaces, roller kilns, tunnel kilns, etc. (2) Glass Industry: Flue gas denitrification in industrial glass kilns and domestic glass kilns. (3) Cement Industry: Flue gas denitrification in cement kilns. (4) Industrial Boilers: Flue gas denitrification in large, medium, and small-sized coal, gas, oil, and biomass boilers. (5) Coking Industry: Flue gas denitrification in high-temperature flue gas from coke ovens. (6) Refractory Materials: Denitrification of high-temperature flue gas from tunnel kilns used for firing refractory materials. (7) Waste Incineration: Denitrification of high-temperature flue gas from chain grate incinerators. (8) Red Brick Industry: Denitrification of high-temperature flue gas from red brick tunnel kilns.

4.3 HSR Denitrification Principle

HSR Denitrification Principle: Under medium to high temperatures (650-1400℃), the HSR composite denitrification agent is used to selectively reduce and remove nitrogen oxides (NOx) in the flue gas of kilns. The reduction denitrification reaction is as follows: NO + NO2 + {HSR} → N + M2 In the equation, {HSR} represents the composite denitrification agent, and M represents other gaseous reduction products formed during the oxidation-reduction reaction process, such as H2O(g) and CO2, which are non-toxic and harmless substances.

4.4 HSR Composite Denitrification Agent

(1) HSR Composite Denitrification Agent: It utilizes various inorganic and organic denitrification materials to react under certain conditions, producing a multi-component composite HSR denitrification agent. (2) Classification of HSR Composite Denitrification Agent Applications:

  • HSR-1 Type Composite Denitrification Agent: Used in spray tower hot air furnaces, small and medium boilers, and cement kilns.
  • HSR-2 Type Composite Denitrification Agent: Used in roller kilns, tunnel kilns, and incinerator flue gas denitrification.
  • HSR-3 Type Composite Denitrification Agent: Used in flue gas denitrification in the glass industry.

(3) Characteristics of HSR Composite Denitrification Agent: Non-toxic, non-irritating, non-corrosive, non-flammable, environmentally friendly materials with safe storage, transport, and usage.

4.5 HSR Denitrification Process – Ceramic Hot Air Furnace Flue Gas Denitrification

4.6 HSR Denitrification Process – Glass Kiln (Horsehoe Flame Kiln)

4.7 HSR Denitrification Process – Glass Kiln (Cross Flame Kiln)

4.8 HSR Denitrification Process – Coking Industry Coke Oven

4.9 HSR Denitrification Process – Waste Incineration Furnace

4.10 HSR Denitrification Technology Features

(1) Advancement: No need to change the existing roller kiln equipment settings; only the addition of HSR denitrification devices and corresponding control systems on the existing facilities is required.
(2) Convenience: The HSR denitrification device control system is easy to operate and does not adversely affect the normal operation of the kiln.
(3) High Efficiency: The HSR denitrification rate can be adjusted between 30% and 80% according to denitrification requirements.
(4) Low Investment: The investment cost of HSR is slightly lower than SNCR, approximately 10% of the investment in SCR.
(5) High Safety: The HSR composite denitrification agent has stable storage, safe use, non-toxicity, and non-corrosiveness.
(6) High Efficiency and Energy Saving: The HSR composite denitrification agent has high efficiency, requires a small injection volume, does not need water dilution, has low heat loss, and can save 2.5 tons of raw coal per day for an 80,000 m3/h spray tower. This is equivalent to saving 5 tons of water coal slurry per day, resulting in a daily savings of 2500 yuan. Limitations: This method is only suitable for denitrification inside kilns and is not suitable for end-of-pipe denitrification in exhaust gas.

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