Boiler flue gas desulfurization issues?

The sodium-calcium double-alkali desulfurization process is currently widely used. I. Process characteristics: The calcium-sodium double-alkali desulfurization process is also known as the double-alkali method. This method is primarily used to remove SO2 gas from gases. Suitable for desulfurization of boiler flue gas, coke oven gas, and waste gases generated in boiler operations. 　　The calcium-sodium double-alkali method involves first using a sodium-alkaline absorption solution for flue gas desulfurization, and then regenerating the desulfurization solution with lime powder. Since the entire reaction takes place between the liquid and gas phases, scaling in the system is avoided. Moreover, it features a high absorption rate, a low liquid-to-gas ratio, high utilization of the absorbent, lower investment costs, and reduced operating expenses. 　　1. Desulfurization is carried out using NaOH (Na2CO3); the desulfurization solution is primarily an aqueous solution of NaOH (Na2CO3). This helps to reduce corrosion, erosion, and blockages in water pumps, pipes, and other equipment during the circulation process, thereby facilitating the operation and maintenance of such equipment. 　　2. The sodium-based absorption solution reacts rapidly with SO2, resulting in a low liquid-to-gas ratio and thus a high desulfurization efficiency, generally ≥90%. 　　3. The regeneration of the desulfurization agent and the formation of desulfurization sludge both take place within the tower, which prevents blockages and wear inside the tower, improves operational reliability, and reduces operating costs. 　　4. Using empty-tower spraying as the structure for the desulfurization tower ensures high operational reliability, a low incidence of accidents, and low tower resistance, with △P≤600Pa. II. Process Principle 　　1. Reaction Principle 　　SO2 absorption reaction: Na2CO3 + SO2 → Na2SO3 + CO2↑ 　　Absorbent regeneration reaction: CaO + H2O → Ca(OH)2 　　Ca(OH)2 + Na2SO3 + H2O → 2NaOH + CaSO3·H2O 　　2. Process Flow 　　The flue gas from the steel furnace is cooled to ≤200°C through heat exchange, and then enters the desulfurization tower from the bottom via the flue duct. Several layers of dozens of nozzles are installed within the desulfurization tower; these nozzles emit fine liquid droplets that are evenly dispersed throughout the volume of the tower. The flue gas comes into full contact with the sprayed desulfurization liquid, allowing SO2 and dust in the flue gas to be fully absorbed and reacted by the desulfurization liquid, thereby achieving the purpose of removing dust and SO2. The cleaned flue gas after desulfurization and washing is dehydrated by the mist eliminator at the top of the tower, and then enters the chimney through the upper part of the desulfurization tower to be released into the atmosphere. After SO2 is removed through gas-liquid contact inside the tower, the desulfurization circulation fluid flows via pipes at the bottom of the tower into a sedimentation tank, where dust is settled. The clear liquid then overflows to the reaction and regeneration tank, where it undergoes a regeneration reaction with lime slurry brought from the lime slurry preparation tank. The regenerated fluid flows into the pump previous circulation tank to have Na2CO3 added to it, and from there it is pumped to the top of the desulfurization tower to remove SO2 for reuse. The regenerated CaSO3 and CaSO4 formed from the excess oxygen in the flue gas are precipitated and separated in a sedimentation tank. III. Process Advantages 　　1. Flue Gas System 　　The flue gas from steel forging enters the desulfurization tower directly through a flue gas exhaust fan. The desulfurization tower features a spray structure with an empty tower. The design air velocity is low (4.0 m/s), and the pressure drop in the tower is low (≤600 Pa). Desulfurization, dust removal, and flue gas emission are integrated into one process; the flue gas rises to the top of the tower and is discharged into the atmosphere through the chimney. After the desulfurization tower is manufactured and subjected to sandblasting treatment, it is coated with epoxy resin for anti-corrosion purposes six times. The internal components of the tower, namely the nozzles and mist eliminators, are all made of 304 stainless steel. When the desulfurization pump fails, the desulfurization process is suspended, and the flue gas can be released into the atmosphere through the chimney. 　　2. SO2 absorption system in the desulfurization tower: The flue gas enters the desulfurization tower and rises, where it comes into counter-current contact with the liquid sprayed downward, allowing for thorough gas-liquid interaction to absorb SO2. The desulfurization tower uses nozzle-type spray in the empty tower; due to the atomizing effect of the nozzles, the liquid is divided into countless small-diameter droplets, increasing its total surface area by thousands of times. This facilitates thorough contact between the gas and the liquid. The greater the contact area between the gas and the liquid, the more efficient the mass transfer and thermal reactions between the two phases. Therefore, spray tower structures are widely used in many unit operations in chemical production, offering advantages such as high efficiency, energy savings, and low cost. Inside the desulfurization tower, the alkaline solution atomizes to absorb SO2 and dust, producing Na2SO3; at the same time, NaOH and Na2SO3 are consumed. The desulfurization liquid is discharged from the tower into the regeneration tank where it reacts with Ca(OH)2 to regenerate sodium ions; Na2SO3 (or NaOH) is then added, and the mixture is pumped via a circulation desulfurization pump into the desulfurization cycle to absorb SO2. 　　A demister is installed at the top of the desulfurization tower; through the deflection action of the demister’s baffle plates, the dust and other water droplets and solid particles carried in the flue gas are captured and separated by the demister. The demister is equipped with a regular flushing mechanism to prevent it from becoming clogged. 　　3. Treatment of desulfurization products The desulfurization products are ultimately gypsum slurry, specifically CaSO3, CaSO4, as well as some oxidized Na2SO4 and dust. The potential cement slurry is pumped out of the sedimentation tank, treated, and then dried through natural evaporation. Since the gypsum slurry contains solid impurities that affect its quality, the disposal method is generally considered the best approach. The slurry from the sedimentation tank can be passed through a hydrocyclone and a thickener to increase its consistency and solid content, before being discharged to the slag dump for disposal. 　　4. Solutions to secondary pollution The sodium-calcium double-alkali flue gas desulfurization method can address the issue of secondary pollution associated with single-sodium-alkali desulfurization. The sodium-calcium double-alkali method uses sodium alkali to absorb SO2, and the resulting product is regenerated using lime milk to produce more sodium alkali for further use. Since this method reduces alkali consumption, it also eliminates the problem of secondary pollution. A small amount of Na2SO4 that cannot be regenerated is carried into the gypsum slurry; after solid-liquid separation, the resulting solid residue is recovered and stored for further use. The solution flows back to the regeneration tank for reuse, thus preventing secondary pollution. 　　5. Features of the scheme: Desulfurization is carried out using NaOH (Na2CO3); the desulfurization solution is primarily an aqueous solution of NaOH (Na2CO3). This helps to reduce corrosion, erosion, and blockages in water pumps, pipelines, and equipment during the circulation process, thereby facilitating the operation and maintenance of these devices. Sodium-based absorption solutions react rapidly with SO2, thus having a lower liquid-to-gas ratio and achieving a high desulfurization efficiency, generally ≥90%. 　　The regeneration of the desulfurization agent and the formation of desulfurization sludge both take place within the tower, which prevents blockages and wear inside the tower, improves operational reliability, and reduces operating costs. Using an empty tower for spraying as the structure of the desulfurization tower ensures high operational reliability, a low incidence of accidents, and low tower resistance, with △P≤600Pa. 　　6. Efficiency of SO2 absorption and main influencing factors pH value: A higher pH value results in a faster SO2 absorption rate and higher desulfurization efficiency; at the same time, it reduces the likelihood of scaling, thereby preventing purification on the surface of the absorbent. 　　Temperature: Lower temperatures facilitate gas-liquid mass transfer and the dissolution of SO2, but they slow down the reaction rate. Therefore, the temperature of the desulfurizer is not an independent constant factor; it depends on the temperature of the flue gas entering the system. 　　Lime particle size and purity: The purity of the lime is required to be ≥95%, with the particle size controlled within the range of Pc200–300 mesh. 　　Slurry concentration: maintain at 10–15%.

This post was last edited by qazxsw119 on 2011-8-19 09:07. What is the desulfurization efficiency of the sodium-calcium double-alkali desulfurization process during normal operation? If the sulfur content in coal is >2%, the flue gas can be treated to meet the standards after desulfurization (SO2

It’s impossible for such a thing to happen. The ammonia method itself belongs to the alkaline method. Furthermore, ammonia-based desulfurization is listed in my **Environmental Protection Technologies Catalogue**.

“The desulfurization product is ultimately a gypsum slurry, specifically consisting of CaSO3, CaSO4, as well as some oxidized Na2SO4 and dust. " Can these solids be reused? Is there any way to deal with it?

Reply to 3# qazxsw119: The double-alkali method has lower equipment investment and operating costs, is more stable, offers better desulfurization performance than the gypsum method, and ensures compliance with standards without any issues

Oh, the ammonia-based desulfurization method is widely used these days. We currently use quicklime for desulfurization, but the results are not very good; there are also many issues with the equipment, and the service life of the bag filters is short. We plan to adopt the ammonia-based desulfurization method, but after hearing the above information, we have doubts. As of now, things remain unclear; however, the sulfuric an product works fine for now

Additionally, I learned quite a lot yesterday about Tianyu’s efforts in this area; I’m wondering which companies in Hubei have adopted it. Regarding flue gas desulfurization, we would like to get some information on that as well

The double-alkali method is a bit misleading~~~ It’s not possible to achieve a high yield of the alkaline solution; moreover, it’s not clear whether what is produced is calcium sulfate or sodium sulfate... and it’s difficult to handle

Oh, looking forward to the debate; anyway, I believe different people will have different opinions! We believe our question can provide some food for thought for everyone – and that is the best satisfaction

As far as I know, Shandong Province no longer encourages the use of the ammonia-based desulfurization method; it is only permitted for enterprises that generate waste ammonia to install such facilities, while this method is not recommended for other enterprises. Furthermore, in theory, the double-alkali method can prevent scaling, but in most cases there is significant loss of alkali solution; if not operated properly, scaling can still occur severely. Therefore, based on my many years of experience in desulfurization projects, wet desulfurization using the magnesium and calcium methods is the most mature and reliable approach, with a desulfurization efficiency of over 95%.