Limestone wet scrubber

Since both the direct and phased approaches offer, at least in principle, equal promise for ultimate success (i.e., comprehensiveness and complete characterization), it is worthwhile to examine their relative resource requirements. Several studies were conducted with the objective of comparing the costs of direct and phased (with elimination of low priority streams) sampling and analysis approaches. (2,3] A number of processes were evaluated during these studies and the results for two unit operations — a limestone wet scrubber and full-scale low-Btu coal gasifier — are taken as examples. The scrubber involved seven feed or waste stream sampling sites. The gasifier contained 70 identifiable stream sampling points. The total estimated costs for both processes by both approaches are shown in Table I. 

Calcium sulfite and calcium sulfate scaling in the system can be a problem for the lime/limestone wet scrubber systems. Scaling occurs when the solutions are supersaturated to a point where heterogeneous crystallization (crystallization on foreign surfaces such as the scrubber walls, overfiow pots, marbles) takes place, resulting from nucleation. The ratios of the products of the activities (A) of Ca and S04 " or to their solubility product constants Kgp) as a measure of the degree of supersaturation are ... 

The settling characteristics of the solid sulfur compounds produced in the lime/limestone scrubber systems vary considerably. Although the clarifier-thickeners are being designed by the vendors, C-E is developing a computer program that could be used to design the clarifier-thickeners for lime/limestone wet scrubber systems. 

The solids concentration of the underflow from the clarifier-thickeners is between 20 and 40 wt % and is not concentrated enough for transportation by either trucks or railroad cars to the disposal areas. For this reason vacuum filters are usually used to concentrate the underflow to 60-80 wt % solids. Ponds are also being used to store solids and to accomplish solid-liquid separation. C-E is doing developmental work to determine the leaching characteristics of the sulfur compounds present in the sludge and also to determine the most suitable filtration system for lime/limestone wet scrubber systems. 

Calcined limestone or lime reacts with sulfur oxides, which are then removed by wet scrubbers. 

The main purpose of magnesia addition to a limestone wet scrubbing system is to facilitate high S02 removal. For a wet scrubber that cleans flue gas from a utility coal-fired boiler, the scrubber inlet gas S02 concentration is typically about 700 ppm by volume per one weight percent sulfur in the fired coal. For sub-bituminous coal having only 0.7 weight percent sulfur content, the inlet S02 concentration is about 500 ppm, and, for example, the outlet S02 has to be less than 50 ppm to achieve 90 percent removal. In order to avoid serious inhibition of mass transfer because of S02 back-pressure, the equilibrium S02 partial pressure should be about four or more times lower than the actual S02 partial pressure in the gas. Thus, Figure 3 indicates that for this low-sulfur coal system, the scrubber inlet pH should be at least 5.5, and the outlet pH at least... 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

A preliminary-level economic evaluation (13) performed by EPRI (Electric Power Research Institute) and TVA (Tennessee Valley Authority) indicates that a combination of electrostatic precipitators (or bag house), ammonia-based SCR system, and wet lime/limestone FGD scrubber range between 20% to 185% cheaper than wet process for complete control of particulates, N0X and S02 The lower percentage is for the second type and higher percentage for the first type of process. Therefore, the second type of process appears to be more promising and will be the subject of further discussion in this paper. 

The Air Quality Control Systems (AQCS) using lime/limestone wet scrubbing have three basic types of chemical process equipment (1) scrubbers, (2) reaction tanks, and (3) solid-liquid separators, in addition to several auxiliary pieces of equipment such as pumps, demisters, and reheaters. The SO2 in the flue gas is transferred into the liquid in the scrubber, the sulfur in the liquid is converted to solid calcium sulfite, and calcium sulfate in the reaction tanks and solid calcium sulfite and sulfate are separated from the liquid and disposed from the solid-liquid separators such as clarifiers, vacuum filters, and ponds. 

Fig. 2 Basic schematic of a wet scrubber column. Absorbent slurry percolates down through the packing, while the flue gases flow upward. The most common absorbents for sulfur oxides are limestone (calcium carbonate), lime (calcium hydroxide), and magnesium-enhanced lime made from dolomite. The sulfur-bearing sludge for some scrubbers is market-grade gypsum, but for other scrubbers it is a waste product that must be landfilled.

In a limestone-based wet scrubber, the dissolution of calcium carbonate [Eq. (7)] is the primary rate-limiting reaction, because of the low solubility of calcium carbonate. The rate for the dissolution reaction can be expressed as ... 

Spray-dry scrubbers are an alternative to conventional wet scrubbers. In this type of scrubber, an alkaline slurry or solution is sprayed in fine droplets into a reaction vessel, along with the flue gas. The droplefs rapidly react with the sulfur dioxide while drying to a fine powder of sulfite salts. This powder is entrained in the gas stream, and is carried to a dust precipitator where it is collected, as shown in Fig. 7. Most of the sulfur dioxide is collected in liquid-phase reactions while the droplets are drying, but 10-15 /o additional sulfur dioxide can be absorbed in gas/solid reactions, as the absorbent powder is swept through the ductwork and particulate collector. These are cocurrent devices, and so the limestone utilization and sulfur removal efficiency are inherently lower than those of countercurrent devices such as wet scrubbers. Partial recycle of the sorbent is often used to improve the sorbent utilization. 

The addition of adipic acid to limestone-based FGD wet scrubbers results in improved limestone utilization and enhanced S02 sorption kinetics. The use of adipic acid was first proposed by Rochelle (1) and has been tested by the EPA in pilot systems at the Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina and at the TVA Shawnee Test Facility at Paducah, Kentucky. Adipic acid in the concentration range of 1,000-2,000 mg/1 has been found effective as a scrubber additive. During scrubber operation, however, adipic acid is lost from the system in the liquid and solid phase purge streams and by chemical degradation (2,3). 

This paper summarizes the results of tests conducted from July 1978 through March 1981 at the EPA, 10-MW equivalent, lime/limestone wet-scrubbing FGD test facility, during which adipic acid as an additive was tested and shown to be a powerful scrubber additive for improving SO2 removal. The optimum concentration of adipic acid is only 700 to 1500 ppm at a scrubber inlet pH of 5.2 or higher. SO2 removal efficiencies in excess of 90 percent and reliable operation were demonstrated in four long term, limestone/adipic acid runs. Factorial tests were also conducted to characterize SO2 removal as a function of gas and slurry flow rates, pH, and adipic acid concentration. Intermediate duration optimization runs and favorable economics are also reported. 

Henson, L.J., "TVA/EPRI Shawnee Cocurrent Scrubber Test Results," 1980, Proceedings Fifth Industry Briefing on IERL-RTP Lime/Limestone Wet Scrubbing Test Programs, December 1979, EPA-600/9-80-032 (NTIS PB 80199813), 116-165. 

The burning of pulverized coal in electric power plants produces sulfur dioxide (SO2) gas emissions. The 1990 Clean Air Act and its subsequent amendments mandated the reduction of power plant SOj emissions [66-70]. The Best Demonstrated Available Technology (BDAT) for reducing SOj emissions is wet scrubber flue gas desulfurization (FGD) systems. These systems are designed to introduce an aUcahne sorbent consisting of lime or limestone in a spray form into the exhaust gas system of a coal-fired boiler. The aUcaU reacts with the SOj gas and is collected in a liquid form as calcium sulfite or calcium sulfate slurry. The calcium sulfite or sulfate is allowed to settle out as most of the water is recycled [66-80]. 

Emission control systems in a power plant also require water. Hue gas desulfurization (FGD) systems in coal-fired power plants remove SO2 from the exhaust flue gas by reacting it with lime or limestone in a wet scrubber system generating wastewater that must be treated before discharge. FGD wastewater treatment involves removal of the gypsum precipitate, adjustment of the pH, and the removal of any toxic dissolved metals. This is typically accomplished using a combination of gravity-driven processes, chemical floc-culants, absorption, and biological processes. Membrane approaches are currently not economically competitive for this application [58],... 

Other systems control the coal burn to minimize emissions of sulfur dioxide, nitrogen oxides, and particulates (Darcovkic et al., 1997 Bhanarkar et al., 2008). Wet scrubbers, or flue gas desulfurization (FGD) systems (Table 22.3), remove sulfur dioxide, a major cause of acid rain, by spraying flue gas with limestone and water. The mixture reacts with the sulfur dioxide to form synthetic gypsum, a component of drywall. 

FGD materials Derived from a variety of processes used to control sulfur emissions from boiler stacks. These systems include wet scrubbers, spray dry scrubbers, sorbent injectors, and a combined sulfur oxide (SOx) and nitrogen oxide (NOx) process. Sorbents include lime, limestone, sodium-based compounds, and high-calcium coal fly ash. 

Flue gas desulfurization (FGD) Removal of the sulfur gases from the flue gases (stack gases) of a coal-fired boiler—typically using a high-calcium sorbent such as lime or limestone three primary types of flue gas desulfurization processes commonly used by utilities are wet scrubbers, dry scrubbers, and sorbent injection. 

Flue-gas desulfurization (FGD) is the best way to remove sulfur dioxide from fossil fuel power plants. FGD commonly uses either wet or dry scrubbers. In wet scrubbers, a slurry of limestone, CaC03, or hydrated lime Ca(OH)2 is sprayed... 

Saarberg-HoUer (S-H-V) Process. The S-H-U process is a limestone wet scrubbing process with formic acid enhancement The S-H-U absorber has both cocunent and counter-current sections. The flue gas enters the absorber at the top of the cocurrent flow section and flows downward past several levels of spray nozzles where the pH drops rapidly. The scrubber slurry collects in the sump. The flue gas then turns upward into the second scrubbing stage, the countercurrent flow section, where the final increment of SO2 removal occurs. The flue gas exits either through a combination of a vertical flow and a horizontal flow mist eliminator or vertical flow mist eliminators. 

In wet scrubbing of SOp from boiler flue gas by limestone slurry, the concentration of dissolved species in the scrubbing liquor that can react with incoming SOp gas is very low, about one to two m-mole/1. This is far below the SOp make-per-pass in the scrubber, typically about 10 m-mole of SOp absorbed per liter of liquor for one pass through the scrubber. Therefore, the SOp absorption rate is largely dependent upon the slow rate of limestone dissolution into the liquor passing through the scrubber. 

In the lime or limestone FGD process, SO2 is removed from the flue gas by wet scrubbing with a slurry of calcium oxide or calcium carbonate [3]. The waste solid product is disposed by ponding or landfill. The clear hquid product can be recycled. Many of the lime or limestone systems discharge scrubber waters to control dissolved solids levels. 

For the required coal feed rate, scale the results from Problem 14.6 to determine for each scrubber train the actual flow rates (kg/h) of wet solids and filtrate from the filter, the slurry flow rates entering and leaving the scrubber, the flow rates of gas (kg/h and m /min) entering and leaving the scrubber, and the flow rates (kg/h) of fresh water and limestone fed to each blending tank. 

---