Coke oven gas desulfurization, denitrification, and waste heat recovery integration

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Sulfur dioxide and nitrogen oxide removal from coke oven flue gas, along with integrated waste heat recovery, is increasingly becoming mature in China. Our company primarily promotes the following technology: SCR denitration + pure dry or semi-dry desulfurization + flue gas waste heat recovery

PrimaryProcess flowProcess:

   Flue gas from the coking furnace first enters the flue gas denitrification system, after which the gas temperature is reduced.The flue gas, around 15 degrees, enters the desulfurization system for desulfurization treatment. Afterward, the treated flue gas is directed to the flue gas waste heat recovery unit, where it is heat-exchanged before being returned to the underground flue. It is then emitted through the original chimney.

Engineering Applications

This technology has been successfully applied at the Inner Mongolia American Coal & Coke Co., Ltd., which operates four coke ovens, each with a production capacity of600,000 tons/year, each pair of coke ovens shares one chimney, with each chimney emitting 250,000 Nm3/h of flue gas; the flue gas contains 250 mg/Nm3 of SO2, and NOx concentration is 1000 mg/Nm3.

Each chimney to be equipped with an integrated flue gas desulfurization and denitrification waste heat recovery unit, respectively installed.Completed and successfully tested in June and August 2016, the facility has been running smoothly for over half a year. The treated flue gas meets the following standards: SO2 concentration less than 30 mg/Nm³, NOx concentration less than 150 mg/Nm³. Not only does it comply with the environmental protection requirements of the Wuhai area, but it also meets the requirements of Table 6 for special areas in GB16171-2012 "Emission Standards for Pollutants from Coking Chemical Industry," and produces 12 tons of 0.6 MPa low-pressure saturated steam per hour.

Technical Features

1) Pre-desulfurization followed by denitrification, then waste heat recovery is conducted. The烟气 post-desulfurization contains a lower SO2 content, which is beneficial in reducing the amount of denitrification catalyst required and extending the catalyst's lifespan. The desulfurization by-products are classified as conventional solid waste, easy to handle, and can be further utilized for resource recovery.

2) The first application of dry desulfurization in actual engineering projects, with minimal temperature drop, significantly lower than that of semi-dry and wet desulfurization methods. The entire unit, aside from heat dissipation, has no temperature drop, ensuring the temperature of flue gas during denitrification and facilitating subsequent waste heat recovery.

3) Utilizes mobile bed desulfurization technology and has developed a dedicated desulfurizing agent, achieving high desulfurization efficiency and reduced pressure.

4) The mobile bed dry desulfurization system features high-efficiency dust removal, significantly reducing dust content in flue gas to meet national standards, eliminating the need for an additional dust removal unit.

5) The flue gas entering the waste heat boiler has been purified, significantly reducing boiler corrosion and extending the boiler's lifespan.

6) Utilizing low-temperature denitrification with imported porous high-efficiency low-temperature denitrification catalysts, the denitrification efficiency is approximately twice that of conventional denitrification catalysts with the same filling volume, and the catalyst filling volume is reduced. The service life can reach 3-4 years or more. Designed using CFD simulation for the ammonia injection system, it maximizes the catalyst's effectiveness and minimizes ammonia escape.

7) Post-waste heat recovery, the exhaust gas temperature is high, and the treated flue gas is returned to the original chimney, keeping the original chimney in a hot standby state, and also preventing the emission of a large amount of white smoke at low altitudes.

8) The underground original flue damper door is equipped with an automatic opening system upon power failure, ensuring the safety of the coke oven under special operating conditions.

9) The complete unit consists of only three main equipment: the desulfurization tower, the denitrification reactor, and the waste heat boiler. The reduced number of equipment allows for flexible arrangement, minimizing land use and investment costs, making it feasible to either build a new integrated unit or expand desulfurization and denitrification in an existing waste heat recovery system.

Integrated Solution Technology

In response to customer requirements and the existing coke oven gas treatment process, researchers at Zhonggang Heat Energy have developed and integrated a unified technology for desulfurization and denitrification of coke oven flue gas as well as the recovery and utilization of waste heat.

1. Proposal Description

Coke oven gas treatment process (as shown in Figure 1): Coke oven→Coke oven flue gas → De-nitration reactor → Heat pipe flue gas heat exchanger → Pressure booster fan → Desulfurization tower → Tower top chimney emissions.

Firstly, the gas from the coke oven's underground flue is extracted to the ground before entering the existing gate valve. An electric regulating valve is installed on the pipeline, and the flue gas is then piped to the denitrification reactor. After denitrification, the flue gas enters the flue gas waste heat recovery unit, which primarily recovers the sensible heat of the flue gas to produce 0.6 MPa saturated steam. The heated flue gas, after being cooled in the waste heat recovery unit, is pressurized by an air blower and then enters the wet flue gas desulfurization unit. The desulfurized flue gas is then emitted through the stack at the top of the tower. A bypass pipeline is installed before the denitrification unit, which uses a small standby fan and a diesel generator to send the flue gas back into the underground flue. This is to provide a short-term emergency start in case of a fault in the desulfurization or denitrification system or during a power outage, to push the high-temperature flue gas into the underground flue, increasing the flue and stack temperatures. Once the stack draft is sufficient, the standby fan is turned off and the underground flue gate is opened.

2. Denitrification Reactor

Currently, commonly used denitrification methods include Selective Non-Catalytic Reduction (SNCR), Oxidation Absorption, and Selective Catalytic Reduction (SCR). Selective Catalytic Reduction (SCR) is a method for denitrifying exhaust gases.

The principle of Selective Catalytic Reduction (SCR) for NOx removal is: a certain amount of ammonia is added to the exhaust gas..Gas, using ammonia as a reducing agent, is reduced to N2 on the catalyst surface, the reaction

Formula: NOx + NH3 + O2→Ｎ２＋Ｈ２Ｏ。

Liquid ammonia source is used in the denitrification reaction..A 20% concentrated ammonia solution is produced in the ammonia or ammonia steam section and introduced through pipes into the denitrification reaction system. After flow control via a regulating valve, it is mixed evenly with flue gas in a mixer. NOx sensors are set at both the inlet and outlet of the denitrification reactor to monitor the NOx concentrations in real-time and control the amount of ammonia added based on feedback signals.

In the denitrification process, the denitrification catalyst is crucial. The catalyst is selected with an integrated coating structure based on ceramic honeycomb, consisting of ceramic honeycomb, metal oxide coating, and active components. The oxide coating is uniformly and firmly attached to the outer surface of the ceramic honeycomb, with the active components dispersed on the oxide coating. This catalyst boasts high denitrification rates, high operating velocity, low resistance, good selectivity, low ammonia escape rate, broad temperature range, and small coefficient of thermal expansion, making it an ideal overall catalyst for flue gas NOx treatment.

Even with high NOx concentrations at the inlet (2,000 to 3,000 mg/m³), this type of catalyst can achieve a high NOx removal accuracy, with the exhaust gas NOx concentration after reaction below 150 mg/m³. Should the emission standards be further upgraded, no modifications to the catalyst or reaction unit are necessary; merely a slight increase in ammonia dosage is needed to provide the required ammonia for the reaction, ensuring the exhaust gas NOx concentration is below the specified emission limits.

Continuously monitor the concentration of various substances in the flue gas at the butterfly valve outlet, with NOx, NO, SO2, O2, and NO2 concentrations around 1100, 720, 450, 190, and 10 mg/m³, respectively. Every 15 minutes, the composition of the coke oven flue gas undergoes periodic fluctuations, which are caused by the coke oven operating schedule.

Figure 2 shows the nitrogen oxide concentrations at the reactor's inlet and outlet during the denitrification experiment, along with the denitrification efficiency. It can be observed that during the experimental run, the nitrogen oxide concentration in the flue gas fluctuated between 800 to 1200 mg/m³. After denitrification treatment, the nitrogen oxide concentration can be reduced to below 20 mg/m³, with a denitrification efficiency of over 98%.

Conclusion

(1) Denitrification efficiency consistently remains above 98% (detectable NO content is in the order of several 10^-6, which can be considered as system error), nearing 100%, reflecting the ultra-high denitrification efficiency of the catalyst.

(2) The operating air speed of the experiment is approximately 16,000 h^-1, which is about four times that of the traditional denitrification catalyst.

(3) Catalyst bed pressure drop is around 300 Pa, significantly reducing fan energy consumption.

(4) Catalysts are designed with a modular approach.Sure, here is the translation: VeryMaximized avoidance of future engineering scaling issues, conducive to achieving engineering scale-up.