Cupola emissions controls: wet scrubber vs. dry baghouse.

By comparing the wet and dry emissions controls at Neenah Foundry, you can make an informed decision on whether or not a system change makes sense.

The decision to change foundry emission control systems can be an expensive endeavor, potentially exhausting both time and money. In an area important to both public perception and environmental compliance, it's vital to stay on top of the best choices for your foundry in terms of operating requirements and costs. Your best course of action, however, isn't necessarily to scrap your current emission control system in favor of the most advanced system on the market.

A baghouse, or dry, system is capable of extremely low emission rates and lower operating costs, but if experience isn't an integral part of the system design, you can have many costly problems, including failed bags, poor dust handling, bridging of dust in the hoppers and excessive corrosion. If regulations allow you to keep or update your wet system by making simple, less costly modifications, you might want to bite the bullet on operating costs.

Regulations undoubtedly will drive the need to upgrade emissions control, but, keeping in mind the high cost of completely changing systems, you may be wise to wait until change is absolutely required. Using Neenah Foundry Co.'s experience with both wet and dry emissions control, you can compare operating costs to decide what's best for your operation: updating or changing your emission systems or doing nothing at all.

This article highlights the differences between a high-energy venturi wet scrubber and a pulse jet baghouse, including a comparison of cupola particulate control operating costs and the incremental cost of upgrading a wet system to a higher efficiency baghouse.

Neenah's Rationale

Neenah operates two cupola melt foundries, Plant 2, a 600-ton/day gray iron municipal/industrial casting shop, and Plant 3, a 500-ton/day ductile iron industrial casting shop. Both cupolas are the same size and style and are capable of similar melt rates, but they operate with different emission control systems, Plant 2 uses a pulse jet dust collector, while Plant 3 uses a high-energy venturi wet scrubber.

Originally, Neenah updated existing wet scrubbers on both cupolas in 1989. In 1991, regulatory requirements necessitated that the efficiency of the scrubber in Plant 3 be increased. Due to numerous design flaws, including the lack of a variable throat venturi and the absence of a water treatment system, the scrubber was not capable of achieving acceptable emission rates. Neenah started fixing one problem at a time, but when all was said and done, the cost of "fixing" the problem was much higher than expected.

At the time, Plant 2's scrubber was operating acceptably, but it was of marginal capacity. Knowing what was spent to make Plant 3 efficient, combined with increasing melt rates ad tighter environmental regulations, Neehah decided that it would be more cost effective to install a new baghouse system.

Baghouse Cupola

To understand operating costs associated with the system, it's first necessary to understand the cupola and emissions control setup in both foundries. Plant 2 melts with an 84-in. acid-lined, front-slagging, above charge take-off cupola with 1000F (538C) hotblast and tuyere oxygen injection. The cupola typically melts 24-29 tons/hr. The upperstack combustion system utilizes a refractory gas-mixing orifice, two 6 million Btu/hr main afterburners and two 1.5 million Btu/hr pilot gas burners for carbon monoxide (CO) combustion. Two 10 gal/min-maximum stack sprays also are installed just above the burners for temperature control during upset and burn down conditions.

Hot off gases enter a large drop out chamber fitted with five 10 gal/min-maximum air/water sprays for fly ash removal and recuperator inlet temperature control. Hot gases pass through a long, refractory-lined duct to a vertical heat exchanger and exit the recuperator at approximately 900F (482C). Gases then enter an 8-ft-diameter, 40-ft-tall gas-cooling tower with 10 4 gal/min-maximum air/water spray nozzles that cool the gases to near baghouse inlet temperature (about 550F [288C]).

Cooled gases pass through a large, low-pressure drop spark arrestor prior to entering the baghouse at 450F (232c). Dust stabilizing reagent is added immediately prior to the baghouse, a pulse jet collector with 10 cells that are isolated one at a time for off-line cleaning. The filter medium is an acid-resistant woven 22-oz fiberglass with an expanded PTFE membrane. The baghouse exhausts through a high-efficiency airfoil fan powered by a variable-frequency drive for flue gas volume and cupola upper stack temperature control. The baghouse's tube sheet pressure differential operates in the range of 3-5 water column in. Using screw conveyors, dust is transported from the baghouse into a small silo, which feeds a high-speed pin mixer for wetting the dust prior to placement in a landfill. Upper stack temperatures are maintained at a constant 1550F (843C) [+ or -]50F during melting by continuously [TABULAR DATA FOR TABLE 1 OMITTED] controlling the exhaust volume using the fan's variable-frequency drive.

Wet Scrubber Cupola

The cupola at Plant 3 is virtually identical to the one in Plant 2, but a wet system is used to control emissions. The difference in the cupola setup is Plant 3's slightly lower melt rate, in the 22-27 tons/hr range, and slightly higher coke ratios for metallurgical reasons. Hot-off gases enter a water spray quencher in which the gas stream is boosted to saturation. The saturated gas enters a variable-throat venturi and passes through a flooded elbow into a chevron demister with city water face sprays before entering into a high-static pressure radial blade fan. A venturi is essentially a restriction in the ductwork that accelerates the gases, and the position of the venturi plug is continuously adjusted to control exhaust gas volume to maintain an upper stack temperature of 1550F (843C). The fan always is operated at its full load amperage (and static pressure). This way, the maximum pressure drop (and maximum scrubbing) is maintained across the venturi throat. Fan static pressure operates at about 60 in. of water column static pressure.

Dirty quencher water drains to a large drag chain tank for removal of fly ash and grit before being pumped back to the quencher. The dirty scrubber water from the flooded elbow and demister drains into an 8000-gal flocculation tank in which pH is controlled using magnesium hydroxide. Flocculation is the act of adding a cationic polymer to dirty water to gather together the suspended particulate, allowing it to settle out.

The flocculated effluent then is clarified in a 14,000-gal sludge contact/inclined plate clarifier; the solids are pumped to a sludge-thickening tank. Twice per day sludge is pumped into a 60-cu-ft frame-and-plate filter press where it is dewatered and discharged into a 6-ton hopper that empties into dump trucks headed for the landfill.

A sidestream of clean hot water from the clarifier is discharged at a rate of 45 gal/min to the sanitary sewer for control of dissolved solids in the water system. The remaining clean hot water is cooled by a 12 million Btu/hr cooling tower before being pumped back to the venturi for scrubbing. The venturi spray water typically operates at 90F (32C) with a suspended solids content of less than 50 ppm. The use of cool, clean water has improved scrubbing efficiency as well as general systems operation.

Cost of Emission System Operation

For both emission systems, the cost of operation was analyzed (Table 1), excluding capital depreciation, landfill disposal costs and major equipment replacements. Maintenance costs were tallied over 9 months and prorated for an annual total. For the purpose of calculating compressed air, electrical and water consumption, and chemical costs, both cupolas were assumed to be operating on blast at 20 hr/day, 260 days/year at a 25-ton/hr melt rate, with an equivalent output of 130,000 tons of iron. Operating costs for off-blast periods were excluded for the sake of obtaining a production rate comparison. However, the off-blast operating cost for the wet scrubber is substantially higher than that for the dry system due to the higher connected horsepower (hp) associated with the exhaust fan and water system pumps.

Operating Cost

The overall operating cost of the wet scrubber at Neenah is 1.5 times greater than that of the dry system, or $575.04/ton of particulate matter (PM) for the wet system and $373.34/ton PM for the dry system.

The electrical cost associated with the high fan hp of the wet system and associated water system pumps (that operate continuously) are the primary energy consumers. The lower design static pressure of the baghouse conserves energy. This, combined with a low tubesheet differential pressure, results in a low-hp system.

Electrical cost for the baghouse system, however, is offset greatly by the high cost of air-atomized cooling sprays that are needed in a system with only partial heat recuperation. Compressed air often is the most overlooked and one of the highest cost energy sources in the foundry. High-efficiency sprays are needed, however, to prevent water-related system problems. For this reason, a system should recuperate flue gas to the greatest extent possible. Recovering (or wasting) heat without the use of water in a dry system will conserve energy, coke, water and compressed air and prevent the condensation of acids associated with high moisture levels in the dry system.

Maintenance costs for the wet system appear to be only slightly higher than those of the baghouse, but represent a much larger percentage of the total operating cost for the baghouse due to the lower total annual operating cost.

Collector Efficiencies

Particulate grain loadings for the wet system are nearly 10 times higher than those for the baghouse, but the total emission rate is only 5 times higher. This is attributable, in part, to [TABULAR DATA FOR TABLE 2 OMITTED] the smaller volume of gas associated with the cooler saturated gas stream in the wet system.

The PM control efficiency of a properly engineered and operated wet scrubber nearly approaches that of a dry system (Table 2). If one looks at the incremental cost of control per ton of PM to replace a wet scrubber with a baghouse (at the cost of $2 million) it is apparent that to collect the extra 17.2 tons/year PM out of a potential 890 tons/year, the incremental cost of control can well exceed $100,000/ton PM.

Public perception is perhaps one of the most obvious and non-technical differences between the two systems. The dry system has no visible plume except during the coldest days, while the water vapor exhaust of the wet scrubber implies environmental degradation to the public. This may or may not be a good justification for replacement of a properly operating wet system.

Combustion and CO Recuperation

The cost to operate a recuperative hotblast at Neenah is much less than that of operating a natural gas-fired preheater. The related benefits of cooling cupola gases without the use of water for dry collectors are very substantial. Some of the benefits are: reduced flue gas volume (smaller fan, less hp), lower moisture content in the flue gas, less ductwork corrosion, smaller ductwork, lower air-to-cloth ratios, reduced baghouse size and reduced spray nozzle compressed air usage. While the control of CO continues to gain prominence, the effective recovery of heat from CO combustion for process air preheat and metallurgical control significantly improves overall system efficiency while reducing operating costs.

This article was adapted from a presentation at the 1998 AFS 2nd International Cupola Conference. Conference proceedings are available from AFS Publications at 800/537-4237.