Issues regarding the design and operation of SCR for flue gas desulfurization in catalytic cracking

This post was last edited by hean*n2007 on 2016-3-31 at 12:36. Our plant has not yet installed an SCR system; I would like to share some considerations regarding the design: 1. Liquid ammonia storage issue: We have eliminated the need for liquid ammonia storage tanks, and instead entrust the management of such tanks to specialized personnel in the logistics department. The process involves the logistics department transporting the liquid ammonia to our plant’s area. 2. Electric heaters: The dilution air needs to be heated to a certain temperature using electric heaters before being mixed with the vaporized ammonia and fed into the ammonia injection grid. If your company has the means, it is possible to install two electric heaters, one in use and one as a backup; if such conditions do not exist, it is advisable to specify this requirement during the initial bidding process. The thermocouples of the electric heaters should be arranged in separate zones to prevent local short circuits that could cause the SCR system to stop operating. Furthermore, higher requirements should be placed on the anti-Bao rating of electric heaters. 3. Ammonia evaporator: In the SCR process, the ammonia evaporator is a crucial device. It can be operated through various methods such as indirect heating using a steam-water bath or direct heating with hot water. It is advisable for your company to consider these aspects thoroughly from the initial design stage, ensuring that the load on the ammonia evaporator remains stable, that it is easy to operate and adjust, and that there is an adequate supply of heat source ； 4. For ammonia medium tube bundles, 316L is the preferred material ； 5. Ammonia-air mixer: Ammonia-air mixers are categorized into static mixers and Venturi-type mixers. There are various opinions regarding the mixing efficiency of these two types, and it is up to your company to make a judgment; we have chosen the Venturi-type mixer. 6. Ammonia spraying grid: When designing the ammonia spraying grid, it is advisable to use a flow meter for measurement, with the signal from the flow meter sent to the DCS; control valves should preferably have a self-locking function. 7. Precautions during operation: In my opinion, the key precautions for SCR operation include: 7.1 Site safety (implementing necessary interlocks to prevent the ammonia concentration from reaching dangerous levels). 7.2 Catalysts (catalysts have a sintering temperature; it is strictly forbidden to allow this temperature to be reached or exceeded during operation). 7.3 Ammonia leakage (ammonia leakage is an ongoing issue, and to prevent excessive amounts of ammonia from escaping, close attention must be paid during operation, as high levels of ammonia leakage can lead to the formation of ABS sulfuric acid on the outer walls of the economizer tubes, affecting their heat transfer efficiency and ultimately causing severe corrosion and perforation of those tubes over time). 7.4 Catalyst bed pressure drop (The catalyst bed pressure drop is a major issue that has long been a point of criticism in SCR-based denitration processes. Excessive pressure drop leads to an increase in the backpressure ahead of the boiler and in the flue gases, which is detrimental to energy savings; in severe cases, it can even cause the plant to stop operating. Therefore, great care must be taken when selecting soot blowers – important things should be said three times!) :lol:lol These are just my personal opinions; please correct me if I’m wrong.

Thank you so much; I’ve learned a lot~~~

2. Electric heaters: The dilution air needs to be heated to a certain temperature using electric heaters before being mixed with the vaporized ammonia and fed into the ammonia injection grid. If your company has the means, it is possible to install two electric heaters, one in use and one as a backup; if such conditions do not exist, it is advisable to specify this requirement during the initial bidding process. The thermocouples of the electric heaters should be arranged in separate zones to prevent local short circuits that could cause the SCR system to stop operating. Furthermore, higher requirements should be placed on the anti-Bao rating of electric heaters. Why use hot air? We usually use a dilution fan to supply air directly, which mixes with the ammonia gas coming from the evaporator and is then sent to the flue through an ammonia spraying grid.

It’s not that diluted air without heating is unacceptable; it depends on your company’s design. What I’m referring to is the distance between the ammonia injection grid and the SCR catalyst – whether this distance is sufficient to ensure uniform temperature after the mixture of ammonia-rich air and hot flue gas comes together. The electric heater is installed primarily to raise the temperature of the dilution air to around 300°C, thereby preventing the ammonia-air mixture emitted by the spray nozzles of the ammonia spraying grid from having too low a temperature and thus forming supercooling points on the surface of the SCR catalyst; the formation of such supercooling points can lead to problems in the production of ABS.

What is the abbreviation for ABS? We don’t have a heater installed at the moment, and due to space constraints related to the modifications, the mixing distance is very short

ABS refers to sulfuric acid an and ammonium bisulfate; installing a heater will be more beneficial for long-term operation