### Wet Electrostatic Precipitator (WESP) Control for Meeting Metals Emission

Standards

Paper # 585

Andrew C. Bartocci

Envitech, Inc.

2924 Emerson Street- Suite 320 San Diego, CA 92106 Ph: 619-223-9925, ext. 203



### ABSTRACT

Several industrial processes face the challenge of meeting increasingly aggressive metals emission standards. In many cases these standa rds exceed the capability of existing air pollution control equipment including bag-houses, pack ed bed absorbers, and Venturi Scrubbers.

The metals of concern can include mercury, ar senic, lead, cadmium, nickel and others, depending on the process. A control strategy us ing a wet electrostatic precipitator (WESP) in conjunction with sub-cooling was used on a seco ndary lead smelter to meet more stringent emission standards. This approach achieved > 98% removal of arsenic and > 92% removal of

lead and other condensed metals downstream of a bag-house. This substantially reduced the plant’s cancer risk index and helped to meet reduced fence line lead emission limits. This control strategy will be discussed and how it can be applied to other industrial processes that face similar challenges, including lead refiners, refinery sludge incinerators, hazardous waste incinerators, and geothermal energy plants.



### INTRODUCTION

A secondary lead smelter recovers lead from us ed automobile batteries by using a combination of combustion processes including kilns and fu rnaces. Emissions from the combustion sources are highly controlled for particul ate and acid gases. Despite thes e controls, stack tests show low amounts of various heavy metals including mercury, arsenic, lea d, cadmium, nickel, and several

other metals. Concentrations for some of the compounds are non- detectable, while others may be in the range of 0.002 to 0.2 mg/dscm. These lo w concentrations result in a cancer risk index that is higher than new state st andards allow. The pollution c ontrol challenge is therefore a “polishing step” to significantly reduce toxic air em issions from very low in let concentrations.

This challenge was met with a wet electrostatic precipitator (WESP) with an integrated packed bed condenser/absorber (C/A). St ack tests indicate greater than 98% removal of arsenic and greater than 92% removal of lead was achieved. This substantially reduced the plant’s cancer risk index and helped to meet more stringent fence line lead emission standards. There are many other industrial process that face similar challeng es including lead refiners , refinery sludge and hazardous waste incinerators, and geothermal energy plants. The results achieved on the secondary lead smelter suggest the same appro ach can be applied to these other industrial

processes.ADDITIONAL CONTROL REQUIREMENTS FOR HEAVY METLS

There are several industrial processes that emit hazardous air pollutants (HAPs) which can include lead, arsenic, cadmium, and/or mercury. These processes are hi ghly controlled for particulate and/or acid gases. Some facilities, ho wever, are being subject to emission limits that exceed the capability of existing control equipment.

Figure 1 provides a summary of some of th ese processes including the additional removal efficiencies that can be required downstream of existing air pollution controls. Most of the processes are from combustion sources and use a range of air pollution co ntrols including baghouses, packed bed absorbers, and Venturi scrubber s. In some cases a combination of controls are used. Despite existing controls, very low con centrations of heavy metals can be emitted. In the case of bag-houses for instance, the operating temperature may be in a range that some of the metals are in a gas phase. In such case they will not be collected by part iculate control devices. In other cases, the concentrations of submicr on, condensed phase heavy metals may exceed the removal capability of controls like pack ed bed absorbers or Venturi scrubbers.

The performances achieved on a secondary lead smelter using a WESP, suggest the approach can be used on these other processes to remove residua l concentrations of conde nsed metals. In the case of mercury, the ability to remove it with a WESP depends on whether it is in a condensed form. This requires reliable speciation data to ma ke that determination. More specifically, the mercury must be in a particulate or oxid ized form for it to be removed by a WESP.

Figure 1: Additional control requirements for heavy metals by process type.

PROCESS Upstream Controls Compounds

Requiring Polishing Add

‐ on

Removal

Efficiency 1

Secondary Lead Smelters Bag

‐ houses,
packed bed absorbers Lead (Pd)

Arsenic (As) 92% ‐ 98%

Primary Lead Smelters Bag

‐ houses Arsenic (As) > 85%

Hazardous Waste Incinerators Venturi scrubbers,
packed bed absorbers Lead (Pd)

Cadmium (Cd)

Mercury (Hg) 80% ‐ 90%

Refinery Sludge Incinerators Venturi scrubbers,
packed bed absorbers Cadmium (Cd) 98%
‐ 99%

Geothermal Plants Packed bed absorbers Arsenic (As)

Mercury (Hg) > 90%

1 Refers to additional removal effici ency after the upstream controls.

### WESP SYSTEM DESIGN

A WESP is primarily a particulate removal device. Its main advantage is the ability to collect submicron particulate with high removal efficien cy. Figure 2 illustrates this capability which compares the particle removal efficiency of a WE SP with a 40 inch W.C. pressure drop Venturi.

The curves show that the Venturi removal efficien cy drops off dramatically for particles below 1 micron in size. The WESP, however, has removal effi ciencies greater than 90% for most particle sizes below 1 micron. The dip in collection effi ciency that occurs around 0.4 micron is due to a transition in particle collection from inertial to diffusion forces. Because the par ticle sizes of condensed metals are predominately submicron, th e WESP is ideally suited for collecting these compounds as a polishing step.

Figure 2: WESP Particle removal efficiency

Figure 3 shows a schematic for an upflow WE SP arrangement. The inlet gas enters the conditioning section at the botto m of the WESP. The conditioni ng section ensures the gas is saturated and evenly distributes the flow to the collection sec tion above it. A packed bed condenser/absorber (C/A) can be integrated into the conditioning section. This enables the WESP to be used as a multi-pollutant device fo r particulate and acid gas removal. Acid gases are removed by a process of absorption and neutra lization by metering a caustic solution into the packed bed recirculation line. A liquid cooling circuit consisting of a c ooling tower and a plateand-frame heat exchanger sub-cools the gas to condense the water vapo r in the gas stream.Figure 3: Upflow WESP arrangement.

Sub-cooling provides several advantages. Th e gas volume is reduced which allows the collection section to be smaller and saves on capit al and operating cost, and space. Additionally, sub-cooling maximizes condensation of gas phase heavy metal compounds prior to entering the collection section. This is important because co ndensed heavy metals can be collected in the WESP. This is illustrated by the vapor pressure curve for arsenic trioxide in Figure 4. At a subcooled temperature of 100°F, the vapor phase As 2 O 3 concentration is negligible at 0.0001 ppmv. This means any As 2 O 3 present in the gas will be in a condensed form that can be collected by the WESP. At a higher temperature of 300°F, there can be as much as 29.8 ppmv in the gas phase which will pass through a bag-house. Lastly, su b-cooling eliminates the water vapor plume under favorable climatic conditions. This helps to lower the profile of the plant within the surrounding community.Figure 4: Arsenic trioxide vapor pressure curve.

0.0001

0.001

0.01

0.1

1 10 100

0 50 100 150 200 250 300 350VaporConcentration,ppmv

Temperature, °F 

## As 2 

## O 3 

## Vapor Pressure Curve

As2O3

After passing through the conditioning section, the gas enters the collection section for particulate collection. The collect ion section is an array of he xagonal shaped tubes with an electrode protruding in the center of each t ube. Figure 5 shows a picture of a hexagonal collection tube array and a WE SP collection tube illustration.

Particulate collection entails three steps which ar e depicted in the collection tube illustration. First, particles are given a strong negative ch arge by an ionizing corona produced by a high voltage electrode. Second, the el ectric field between the electrode and the tube wall causes the charged particles to migrate to the grounded tube wall where they accumulate. Finally, a falling water film inside the tube removes the collected material to a discharge drain. The falling water film is created by a periodic flush that occurs above the collection section.Figure 5: WESP Collection Tube Section

Picture of a hexagonal, cross-secitonal array looking from bottom to top. Collection Tube Illustration

The final section of the WESP is the outlet section. The outlet section supports the high voltage grid and houses the wash header to provide the we t film for particulate removal. It includes an air purge system to keep the high voltage insula tors dry. As the gas ex its the outlet section, it passes through an entrainment sepa rator for water droplet removal.



### WESP PERFORMANCE

The size of the WESP is a function of the desire d removal efficiency and the inlet volumetric flow rate. The specific collecti on area or SCA is the active colle ction tube surf ace area per 1,000 cubic feet per minute of gas. It is a metric used to size a WESP to meet a given performance requirement. Figure 6 shows the WESP removal efficiency for vari ous specific collection areas (SCAs) represented by the letters A through E. Th e curve illustrates that more collection area (higher SCA) is required to achieve greater removal efficiency.Figure 6: Predicted WESP Efficiency



### STACK TEST RESULTS

Table 2 compares the stack test results to the performance guarantee for a secondary lead smelter WESP system. The removal efficiencies are calc ulated from WESP inlet and outlet testing. The results confirm the system is above the perf ormance guarantee for both compounds and achieved greater than 92% removal.

Table 2: System Performance

Compound Approximate Inlet, mg/dscm 1 Guarantee Efficiency, % Performance Test Efficiency, %

Arsenic a x 10 -02 92% 98.7%

Lead b x 10 -02 92% 92.4%

1 Indicates order of magnitude for actual value.



### CONCLUSION

Several industrial processes incl uding lead refiners, secondary lead smelters, refinery sludge incinerators, hazardous waste incinerators, and geot hermal energy plants face challenges to meet more stringent emission standards for heavy me tals. In many cases, new standards exceed the capability of existing air pollution control equipment including bag-houses, packed bed absorbers, and Venturi scrubbers.

A control strategy using a wet elec trostatic precipitator (WESP) in conjunction with sub-cooling was used on a secondary lead smelter to reduce the plant’s cancer risk index. This approach successfully removed greater than 92% of vari ous condensed metals from very low inlet concentrations in the range of 0.002 to 0.2 mg/dscm. Stack test results in dicate greater than 98% removal of arsenic and greater than 92 % removal of lead was achieved.The results confirm that a WESP can be an eff ective control device to remove condensed heavy

metals with low inlet concentra tions. This approach can be us ed on similar industrial processes facing the same challenge.



### REFERENCES

1. Crawford, M. Air Pollu tion Control Theory, 1976.

2. Calvert, S.; Englund, M. Handbook of Air Pollution Technology, 1984



### KEY WORDS

Wet electrostatic precipitator, electrostatic pr ecipitator, WESP, hazardous air pollutants, HAPs, heavy metals, geothermal plants, secondary lead smelters, refinery sludge incinerators, hazardous waste incinerators, primary lead smelters.