Table 610

(continued)

Polyester (Terytex®)

Polyester fabrics offer good resistance to most acids, oxidizing agents, and organic solvents. Concentrated sulfuric and nitric acids are the exception. Polyesters are dissolved by alkalis at high concentrations. Maximum operating temperature is 270°F.

Nomex® fabrics resist attack by mild acids, mild alkalis, and most hydrocarbons. Resistance to sulfur oxides above the acid dew point at temperatures above 150° F is better than polyester. Flex resistance of Nomex® is excellent. Maximum continuous operating temperature is 400°F.

Ryton®

Ryton® fabrics offer exceptional chemical resistance through the pH range. They resist thermal oxidation and are affected by concentrated nitric, sulfuric, and chromic acids. Maximum continuous operating temperature is 375°F.

Glass

Glass fabrics offer outstanding performance in high-heat applications. In general, by using a proprietary finish they become resistant to acids, except by hydrofluoric and hot phosphoric acid in their most concentrated forms. They are attacked by strong alkalis at room temperature and weak alkalis at higher temperatures. Glass is vulnerable to damage caused by abrasion and flex; however, the proprietary finishes can lubricate the fibers and reduce the internal abrasion caused by flexing. Maximum operating temperature is 500°F.

P-84® fabrics resist common organic solvents and avoid high pH levels. They provide good acid resistance. P-84® offers superior collection efficiency due to irregular fiber structure. Maximum continuous operating temperature is 500°F.

Teflon®

Teflon® has excellent chemical resistance throughout the pH range, high particulate collection efficiency, and excellent abrasion resistance. Maximum continuous operating temperature is 500°F.

Tefair®

Tefair® has excellent chemical resistance throughout the pH range, excellent abrasion resistance, and high degree of efficiency. It is affected by concentrations of hydrofluoric acid and high concentrations of salts. Maximum continuous operating temperature is 500°F.

Source: Soud, H. N. and S. C. Mitchell, Particulate Control Handbook for Coal-Fired Plants, IEA Coal Research, London, 1997. With permission.

needle felts or polytetrafluorethylene (PTFE) membranes on woven glass, due to their ability to withstand higher temperatures (during system upsets which result in temperature excursions) and improved bag performance [57]. To protect bags against chemical attack, the fabrics are usually coated with other materials such as Teflon®, silicone, graphite, and Gore-Tex® [67].

Bags generally fall into two size categories: 30 to 36 ft in length by 1 ft in diameter or 20 to 22 ft in length by 8 in. in diameter [67]. Bag fabrics are constructed using combinations of texturized and untexturized yarns. Texturized yarns contain many broken filaments and are used to create fabric surfaces with properties suitable for retaining residual dust cakes to yield high collection efficiencies without excessive pressure drops. Smooth yarns are made of continuous, unbroken filaments and are stronger than texturized yarns.

Factors that Affect Baghouse Performance Key factors in proper baghouse design and operation are flue gas flow and properties, fly ash characteristics, and coal composition [8]. The baghouse must minimize pressure drop, maintain appropriate temperature and velocity profiles, and distribute the ash-laden flue gas evenly to the individual compartments and bags.

Particle size distribution of the fly ash and loading of the flue gas varies with type of combustion system [57]. Stoker-fired units produce ash with high carbon content, moderate loading, and large particle size distribution (compared to other combustion systems). Pulverized coal-fired systems produce ash with low carbon content, high loading, and fine particle size distribution. Cyclone-fired units produce ash with low carbon content, moderate loadings, and very fine particle size distribution. Fluidized-bed systems generally produce ash with high carbon content, high loading, and fine particle size distribution [8].

The sulfur content of the coal has been correlated to fabric filter operation. The cohesiveness of ash produced from high-sulfur coals is greater than from Western low-sulfur coals [8]. Also, maintaining the baghouse above the acid dew point is critical in high-sulfur coal applications. The fly ash properties are important because they affect the adhesion and cohesion characteristics of the dust cake which, in turn, affect the properties of the residual dust cake, collection efficiency, and cleanability of the bags.

Methods to Enhance Filter Performance The most recognized method to enhance fabric filter performance is the application of sonic energy, which was discussed previously. Virtually all reverse-gas baghouses have included sonic horns [8]. Gas conditioning has been explored for improving filter performance, although this technique is not performed commercially [8]. Low concentrations of ammonia and/or sulfur trioxide have been added in test programs to control fine particulate emissions and reduce pressure drop when firing low-rank fuels.

Hybrid Systems

Although the discussions of technologies in this chapter have mainly focused on commercial systems, this section briefly discusses two concepts that are under development for improving particulate capture. They are considered here because these technologies are expected to become commercial in the very near future, especially as particulate emissions become more stringent. Hybrid systems have been under development for over 10 years, as utilities are required to meet increasingly tighter emissions regulations for particu-late matter as well as sulfur dioxide. Fly ash resistivity and dust loadings are affected by switching to low sulfur coals or injecting sorbents for sulfur dioxide control, which in turn can reduce ESP efficiency. The desire to reduce fine particulate emissions is also leading to innovative technologies. Two such systems, the compact hybrid particulate collector (COHPAC) and the advanced hybrid particulate collector (AHPC) have been developed to address these issues.

Compact Hybrid Particulate Collector The COHPAC, developed by the EPRI, involves the installation of a pulse-jet baghouse downstream of the ESP or retrofitted into the last field of an ESP [69,70]. Because the pulse-jet collector is operating as a polisher for achieving lower particulate emissions, the low dust loading to the baghouse allows the filter to be operated at higher A/C ratios (8 to 20 ft/min) without increasing the pressure drop. This system allows for the ability to retrofit existing units and achieve high efficiencies at relatively low cost. The COHPAC technology has been demonstrated at the utility scale, including full-scale operation at Alabama Power's E.C. Gaston Station (272 MW) and TU Electric's Big Brown Plants (two units, each 575 MW) [69,71]. Results from COHPAC operation have been positive. For example, at E.C. Gaston Station, the COHPAC has been operated with both on-line cleaning and long filter bags (i.e., 23 feet) at filtration rates of 8.5 ft/min while providing low outlet emissions levels (<0.01 lb/MM Btu) and reduced pressure drops, even with occasional high inlet dust loadings. COHPAC is a promising technology for polishing particulate emissions and is expected to help utilities meet the more stringent particulate emissions standards.

Advanced Hybrid Particulate Collector Another hybrid system under development is the advanced hybrid particulate collector (AHPC). This technology, developed by the University of North Dakota Energy and Environmental Research Center (EERC) and being demonstrated by EERC, DOE, W. L. Gore & Associates, and Otter Tail Power Company, is unique because, instead of placing the ESP and fabric filter in series, the filter bags are placed directly between ESP collection plates [72]. A schematic diagram of the AHPC is shown in Figure 6-23 [73]. The collection plates are perforated with 45% open area to allow dust to reach the bags; however, because the particles become charged before they pass through the plates, over 90% of the particulate mass is collected on the plates before it ever reaches the bags [74]. The low dust loading to the bags allows them to be operated at a high filtration velocity (i.e., smaller device as 65 to 75% fewer bags are needed)

FIGURE 6-23. Schematic diagram of the Advanced Hybrid Particulate Collector. (From DOE, Advanced Hybrid Particulate Collector Fact Sheet, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., 2001.)

and to be cleaned without the normal concern for dust re-entrainment [75]. When pulses of air are used to clean the filter bag, the dislodged particles are injected into the ESP fields where they have another opportunity to be collected on the plates. Because these bags will not need to be cleaned as often as in typical baghouse operation, they are expected to have excellent performance over a long operating life, thereby leading to lower operating costs.

Particulate capture efficiencies of greater than 99.99% have been achieved in a 2.5 MW slipstream demonstration [76]. The AHPC technology is expected to increase fine particulate (PM2.5) collection efficiency by one or two orders of magnitude (i.e., 99.99 to 99.999%) [76]. A 450 MW demonstration is currently being conducted in the Big Stone cyclone-fired power plant, operated by Otter Tail Power Company and co-owned by Montana-Dakota Utilities, Northwestern Public Service, and Otter Tail Power Company, and burning coal from Wyoming's Powder River Basin.

Economics of Particulate Matter Control

As with other pollution control technology costs, the costs for particulate control systems are site specific and vary from country to country. They are influenced by the required emission limit and type of coal. This section summarizes the costs for ESPs, fabric filters, and hybrid systems using published data.

Electrostatic Precipitators The capital costs for a new ESP are between $40 and $60 kW, with the higher costs being associated with higher collection efficiencies [16]. Because most coal-fired power plants are already fitted with

ESPs, much of the published data relate to costs for upgrading existing ESPs. ESP rebuilds are less costly today due to greater market competition, the emergence of new construction techniques, and the use of wide plate spacing requiring less collecting plates. Wide plate spacing is one of the most economic and effective approaches to replacing internals. The cost benefits result from the need for fewer internal elements and materials, erection savings due to reuse of part of the original casing, and weight-savings effects on the existing support structures and foundations [16]. Costs for upgrading ESPs have been estimated at about $12/kW per field for a 500 MW unit, with the increased operating costs estimated to be $100,000 per year [16].

Flue gas conditioning has proved to be more cost effective than adding new fields. With difficult-to-collect fly ashes, conditioning allows operation without adding new fields. The reduction in ESP size with conditioning also lowers the operating costs because fewer fields and hoppers are used, thus decreasing the number of heaters and consequently the power consumption required [16]. A native SO3 conditioning system for a 500 MW power plant requires a capital cost of $4.50/kW. Adding an anhydrous ammonia conditioning system to an existing SO3 system would cost about $1/kW for a 50 MW unit, with the operating costs increasing by $50,000/year [16].

Staehle et al. [64] performed an economic analysis for using WESPs for SO3 control at three different levels of control: 50, 80, and 95%. The capital costs for the three levels of control were $10, $15, and $20/kW, respectively. The total operating costs, based on 8000 hours of operation per year, were $120,000, $160,000, and $200,000 per year, respectively.

Fabric Filters Fabric filters are reported to cost between $50 and $70/kW [16]. Reverse-gas baghouses have higher capital and operating costs than pulse-jet baghouses because reverse-gas baghouses operate at a lower A/C ratio. Fabric filters are generally more expensive than ESPs for collection efficiencies up to 99.5%; however, baghouses become more cost effective for higher collection efficiencies. In addition, high resistivity fly ashes need to be upgraded to achieve high collection efficiencies, and baghouses have economic advantages over ESPs for fly ash resistivity greater than 1013 to 1014 ohm-cm. Operating costs for baghouses are also higher than ESPs due to bag replacement and auxiliary power requirements.

Hybrid Systems A cost analysis that was performed for a COHPAC that uses a pulse-jet bag filter following an ESP estimated that the capital costs have varied from $57 to $70/kW and operating costs from $320,000 to $570,000 per year, both depending on the unit size [16]. The analysis was performed for upgrading ESPs at a few coal-fired units ranging in size from 150 to 300 MW. Less information is available for the AHPC because it is in the early stages of commercialization. According to Gebert et al. [72], retrofit or ESP conversion jobs have been quoted in North America and Europe comparing the AHPC to a COHPAC design; in those cases where the ESP was old and required significant upgrades for the hybrid filter system to function well, the AHPC had the economic advantage. For example, the 450 MW Big Stone power plant conversion has a project cost for the overall filter system of $25/kW. Gebert et al. [72] anticipate that these costs will decline further as more systems are built and the design is further refined and optimized.

Pollutants with Pending Compliance Regulation

As discussed in Chapter 4 (Coal-Fired Emissions and Legislative Action in the United States), controls for mercury emissions will be in place by the end of 2007. A rule was proposed in December 2003 that will be finalized by December 2004, with initial compliance required by the end of 2007. In anticipation of the regulation, which has been expected by the industry since 1999 when the EPA initiated the mercury Information Collection Request (ICR; see Chapter 4 for a discussion of the ICR), a number of companies, government agencies, and institutions have been working on developing mercury control technologies while others have been working on developing monitoring instruments. This section discusses some of the leading options for mercury control. This section is not inclusive, as many technologies are being investigated at the bench- and pilot-scale; however, it does discuss several of the options closest to commercialization.

0 0

Post a comment