Gas Absorption

It has been in operation since 1983. The absorber was a tray tower with a single tray designed for 90% SIS removal and supplied by The Babcock & Wilcox Company (B&W). The boiler fires high sulfur eastern bituminous coal producing SIS loadings up to 7. 5 lbs/Numb. To increase the removal in the WIFE system, MASC. and B&W added a second tray in 2002 to take advantage of dual tray technology. SIS removal has been increased from 90% to 98% without the use of organic acids. Performance tests and parametric tests have been performed on the system over the last two years. The testing also included tests forSOB, HP, HCI, and PM. This paper discusses the absorber design, operating parameters, results of the testing and system chemistry. Introduction The need for maximum SIS removal is necessary with the upcoming Clean Air Interstate Rule (CARR). To meet these demands, high efficiency scrubbers are necessary. This paper provides an evaluation based on limestone, forced oxidized systems since t his is the most common type of system being used now and in the foreseeable future. High efficiency can be gained by changes to the chemistry of the slurry or by increasing the contact of gas and slurry in the absorber.The typical change to the system chemistry is the addition of dipodic acid or a composite adiabatic acid such as DAB. Contacting can be increased by increasing the liquid to gas ratio (L/ G), which also increases the alkalinity available per pass. It is difficult to add more L/ G, physically and financially, because this usually involves adding more spray headers and absorber recirculation pumps. There is usually not enough space available to accommodate this. Increased contacting can also be achieved by the addition of a contacting device such as an absorption tray or by reducing the open rear of an existing tray.In these cases the LEG remains the same, but the absorption of SIS per unit volume of slurry increases. In order to add another tray, the space must be avai lable to do so. Adding a tray or decreasing the tray open area increases the absorber pressure drop. B&W has 6 units operating with two trays and has 19 other dual tray units in the design, construction and startup phases. The purpose of the described project was to convert a conventional, 90% SIS removal scrubber into a high efficiency (>95% SIS removal) scrubber. B&W proposed to maximize SIS amoeba by adding another tray to this limestone forced oxidized scrubber.Michigan South Central Power Agency was an ideal location for the demonstration because their wet flue gas decentralization (WIFE) system is a limestone, forced oxidized system with a single existing absorption tray and because of the good working relationship between the two companies. Their absorber design also had provisions for an additional future tray. The baseline performance measured in 1998 is as follows (Table 1): After the modifications were made to the system, field performance tests were carried out. The last of the testing occurred during November 2005.In addition to SIS being tested, some multi-pollutants were also tested to gather baseline data of the plant. Those multi- pollutant tests included solid particulate matter, sulfuric acid mist, hydrogen chloride and hydrogen fluoride. These results are presented with the SIS data. Background Michigan South Central Power Agency's Endicott Generating Station, Unit 1, was supplied by The Babcock & Wilcox Company in 1981. See Fig. 1. The boiler is a Sterling design rated for 480,000 lb/hrs steam flow and a nominal 55 MM while burning bituminous coal.The air quality control system (SACS) consists of a cold Table 1 MASC. 1998 performance Parameter Units Values SIS removal Inlet SIS loading pH Geochemistry Absorber pressure drop lb/Numb -ca/S CPM/masc. in. Wag 6. 5 5. 6 1. 06 85 3. 6 electrostatic precipitated (ESP.) and wet FIG. The FIG system is a forced oxidation design consisting of a single absorber, a reagent preparation system, and prima ry and secondary dewatering systems. Prior to adding the new, second tray, the scrubber performance had deteriorated and the plant struggled to meet compliance t high sulfur loadings, > 6. Lb/Numb and high boiler loads, 60 MM. The primary reasons for this poor performance were sulfite blinding and inadequate flow of limestone slurry to the absorber. The limestone feed slurry density was about 13% which limited the limestone flow. The pH was limited to 4. 6 to 4. 8. SIS removal averaged about 83% with a single tray. The absorber system consists of one 22'6†³ diameter absorber tray tower. The original design was a single absorption tray. In October 2003, a second absorption tray was added. The material of construction above the inlet is LOLL.The absorber inlet has a CHIC awning and side shields to prevent absorber inlet plugging. The absorber has two slurry spray levels operating above the tray. There is no spare spray level. Three absorber recirculation pumps, two operating and one standby, are provided to feed the slurry spray headers. The absorption spray zone is lined with Stabbing tile to protect it from spray impingement. Two stages of mist eliminators are supplied in the absorber tower with automatic sprays above and below the 1st stage and below the 2nd stage of mist eliminators (ME).The 2nd stage ME overspread is provided with a wash header, manually operated. A sparse grid oxidation system is provided for insist oxidation of the tank. Additionally, four air lances have been installed at the bottom of the tank to provide additional air flow and to fully oxidize the tank at the higher than designed removal efficiency and inlet SIS loading. The scrubbing reagent used is limestone, which is ground by a single, 100% vertical tower mill. Dewatering consists of primary and secondary systems. The absorber blown slurry is sent to a single, 100% thickener for primary dewatering.The underflow from the thickener is sent to a thickener underflow tank and then batched to the rotary drum vacuum filters. Two vacuum filters are provided, one operating and one spare. A gypsum byproduct is produced from the cake of the vacuum filter and is currently being landfill. In 2003, a second absorption tray was added to the absorber tower. See Fig. 2. This new tray did not have the same pressure drop as the existing tray because of limitations on the ID fan. Improvement of SIS removal on the system was seen, but the full effect was not observed. The removal increased from 83% to bout 89%.The main reason was continued sulfite blinding. It was expected that the delivered oxidation air was adequate for the increase of SIS removal, but that was not the case. Also, the limitations on Fig. 2 A second absorption tray was added in 2003. The ID fan were observed to be an absorber inlet plugging related problem. At least twice per year, the absorber inlet needed to be cleaned before upsetting boiler operations because of high differential pressure drop. In 2005, a few more modifications were made to the system for system reliability and to enable the second tray to be more effective.During the plant outage, the CHIC inlet awning was cleaned to remove some hard solids, which developed over the years, at the top gap in the awning. After cleaning, it was discovered that the awning was severely corroded and needed replaced. This new awning resolved the inlet plugging issues. Next, some rubber plugs were added to the second absorption tray to increase the pressure drop so that it equaled the first tray. Lastly, oxidation air lances were added to the bottom of the absorber reaction tank to improve oxidation and stop sulfite blinding.The combination of these improvements allowed the FIG system to reach its full performance capacity. SIS removals can now reach as high as 98%. SIS absorption theory Scrubber design and SIS absorption The design of a wet scrubber can be reduced to Just two basic requirements. The first is to contact the gas and slurr y. The second is to provide alkalinity to neutralize the acid formed when SIS is absorbed. Contacting the gas and slurry can be accomplished using only the absorber slurry sprays (L/G) such as in an open spray tower design. The key to this is getting good gas striation and good slurry spray distribution.In an open spray tower the contact surface of the spray droplets and the contact of the gas with these droplets is the primary means of removing SIS. This leaves one primary parameter, pump flow, to be adjusted to achieve the desired performance. The contact surface can also be increased by providing more droplet surface area (higher nozzle pressure drop), but also at the expense of pump power, and increasing the droplet surface area quickly reaches the point of diminishing returns. The gas and slurry contacting and SIS amoeba can be greatly improved by using some type of contacting device(s).B&Ws choice of contacting device is a perforated tray(s). The absorber tray provides intimat e contacting between the gas and slurry. The contact surface provided by the tray is much more effective for SIS removal than that of the slurry droplets in a spray tower design. Fig. 1 Michigan South Central Power Agency's Endicott Station. 2 Gas distribution First the tray provides a resistance to distribute the gas flow uniformly over the tower cross-section. This resistance is provided at the start of the gas and slurry contact one, or the absorption zone, in the absorber.Therefore, the contact of the gas with slurry is optimized over the full height of the absorption zone. In an open spray tower, the pressure drop across each spray level will tend to distribute the gas. However, by the time the successive pressure drops have redistributed the gas, the gas has already traveled through much of the absorption zone. This is not making full use of the LEG being provided. Uneven gas distribution results in areas of high and low LEG within the absorber. In areas where the LEG is highe r than the design L/G, theSIS removal will be higher than design. However, areas of lower LEG will have less than the average removal. When designing to 98% removal, the area of lower LEG does not have to be too large to seriously limit the overall SIS removal efficiency. Contacting effectiveness The tray provides a much more efficient means of gas and slurry contact than slurry sprays. Contacting devices are well known to provide the optimum design in most gas-liquid absorption systems. In fact, most of the first utility scrubber designs were based on the use of some type of packing or trays.The use of a tray in an absorber is typically worth 25 to 30 L/G. That is, the absorber with a tray requires 25 to 30 LEG less than an open spray tower design. This is illustrated in field unit test data presented in Fig. 3. The curves indicate that 80% removal efficiency could be achieved operating without a tray at 60 LEG or with a tray at about 35 L/G. The data also indicates 95% removal at 60 LEG and using a single absorption tray. Fig. 3 SIS removal with and without tray. Table 2 Effect of Trays on SO 2 Removal Unit Whinny Pilot MASC. # Trays 70 Removal 82 93 82. 4 92. 6