full scale EP system design and performance on a BOF application

Vol. 6, pp. 227-232, 1981 Printed in the USA. All rights reserved.

0160-4120/81/010277-06502.00/0 Copyright©1982Pergamon Press Ltd.

Environmentlnternational,

PILOT PLANT/FULL SCALE EP SYSTEM DESIGN AND PERFORMANCE ON A BOF APPLICATION Douglas Ruth United McGill Corporation, P.O. Box 820, Columbus, Ohio 43216, USA

David Shilton CF & I Steel Corporation, Pueblo, Colorado 81002, USA

An ESP pilot plant study was done on emissions from a BOF process which is cyclicwith veryhigh and low gas volumes, temperature and grain loadings. Data collectedwere EP performance vs. gas velocity, and collection area. Also measured was particle size distribution, dust resistivity, and grain loading vs. opacity at the EP outlet. From this data, a full size EP system was designed and installed. Recentperformance tests indicate the data agrees well with the initial pilot plant study. Emissions have been reduced from 11 grs/scf (27.1 g/m 3) to less than 0.033 grs/scf (81 mg/m3) and opacity to 2007oor less.

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gr/scf (27.1 g/m3), the temperature goes from 175 °F (79 °C) to near 400 °F (200 °C), and the gas volume increases to it's maximum value of about 300,000 acfm (142 m3/s). The particulate emission is primarily Fe2 03, 90o7o o f which is below 10 micron in size, with 10o70 of these below 1 micron. The dust resistivity at flue gas conditions is about 10 ~0 ohm-cm. It was the desire of CF & I to reduce these emissions to 0.033 grs/scf (81 m g / m 3) and not to exceed a visual stack opacity of 20o70. For these requirements, United McGill designed and installed an electrostatic precipitator system.

The BOF shop where these tests were done, CF & I, Pueblo, Colorado, is operated with two 90 metric ton vessels, with one vessel active at any given time; the other vessel concurrently undergoing a reline. A precipitator system installed would accordingly be designed to handle the emission from one active vessel. Specifically, the volume to the precipitator would be 300,000 acfm (142 m3/s) at 350 °F (177 °C), with moistures between 10oT0 to 30°70. This shop utilizes an open hood concept with a spark box cooling chamber immediately after the vessel. This system removes the bulk o f the heavy solids and is directed with a separate clarifier-lagoon water treatment system. The emission from a BOF furnace is cyclic, with high grain loading for a period o f 20 minutes, then a quiescent period lasting 20 minutes. After the first 20 minute period, oxygen lance reblows may be necessary to satisfy metalurgical requirements on the steel being melted. The highest grain loadings are usually associated with reblows. Scrap iron is poured into the converter, which is filled with molten iron, then oxygen lanced. This process takes about 20 minutes, and it is during this time that the outlet flue volume, gas temperature and particulate emissions increase markedly. During this blow period, emissions go from essentially zero to around 11

Pilot Plant Study

One of the best possible ways to specify the design and size o f a full size electrostatic precipitator system is to do a pilot plant study on the site where it is to be used. This is so the actual stack emissions to be collected are specifically characterized and actual operating data are obtained with a unit exactly like the full scale system, only reduced in size. This was done at CF & I during April 1976 by United McGill Corporation, using one o f their mobile EPs and testing laboratories. This mobile is a completely self-contained EP system, which has three fields, each o f which has its own TR set and voltage control, rapper system, hopper, and rotary 227

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Douglas Ruth and David Shilton

Fig. 1. Pilot plant installationat BOF plant. valve. Also included are the fan, duct work, outlet stack, evaporative cooler, control panel, and inlet/outlet testing ports including opacity transmissometers. Each field uses the McGill needle/plate design, with needles on the leading and trailing edges of the discharge plates, which are held at a positive potential of about 25KV. The mobile EP cross-sectional area is 25 ft 2 (2.3 m2), and the fan is capable of pulling 8000 cfm (3.8 m3/s) at temperatures up to 800 °F (427 °C). The entire EP system is mounted on a trailer for ease of transportation. It can usually be set up and be operating in 8 hours. The accompanying laboratory trailer is fully equipped for performing the standard EPA stack tests, gas analysis by infared spectrophotometry, and some chemical analysis. At CF & I, the mobile EP was connected to a breech in the existing downcomer from the process by a 85 ft. (26 m) long by 20 in. (51 cm) diameter insulated duct. Fig. 1 shows the mobile EP at this installation. Fig. 2 is another view of the installation.

Fig. 2. Anotherviewof MobileEP at BOF site.

Fig. 3. MobileEP outletwithall fieldsoff duringblow cycle. Emission collection performance of the pilot EP was done only during the blow or reblow part of the BOF cycle, since this is the period of maximum emission. A crew from CF & I tested at a point before the EP inlet using the W.P.-50 apparatus, while a simultaneous EP outlet test was done by a United McGill crew, using EPA Method 5. An observer was located in the BOF shop who gave signals to the test crews at the beginning of a cycle. Within 1 minute, the equipment would be readied and a portion of a test completed during the 20 minutes blow cycle. The EP inlet test could only last 8 minutes due to the high grain loading. The outlet tests were done for two blow/reblow cycles, but with testing halted during the quiet cycle. Fig. 3 shows the EP outlet during a blow cycle with all fields de-energized. Fig. 4 shows the outlet with one field on, Fig. 5 shows 2 fields on and Fig. 6 with all three fields energized. The white plume at this point is merely water condensation. Fig. 7 and 8 show that indeed the EP collects the dust. The primary data to be obtained were that of optimum EP flow-through velocity, specific collection area, and rapping timing. Operation temperature is more or less fixed by the process, which was 360 °F (182 °C) during the periods of heavy emission, so no at-

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tempt was made to change this, except to insulate the duct. Also of interest, was the chemical make up of the dust, dust resistivity, particle size, and opacity measurements. The EP velocity was varied by changing the volume flow with a damper control on the fan section and was varied between 1.5 to 3.0 ft/sec (46-91 cm/s). The specific collection area was also varied in this manner, and also by de-energizing one or two fields from 400 to 800 ft 2 per 1000 acfm (78.7-157 m 2 per m3/s). From this data, graphs were generated showing the relation between outlet grain loading EP flow-through velocity, and specific collection area. From this, one could then determine the full size EP design. Fig. 9 shows the result of a dust resistivity measurement vs. temperature and moisture. Fig. 10 shows a typical particle size distribution, a correlation between grain loading and opacity was done, with results shown in Fig. 1l, for a 48 in. (1.2 m) diameter stack.

EP Design From a graph relating EP gas velocity and outlet grain loading, obtained from the pilot testing, the crosssectional area of the full size unit was chosen. Also

Fig. 5. Mobile EP outlet with two fields.

taken into account was the particle size, especially in regards to re-entralument. This influenced the choice in the number of fields required, as well as the velocity. As for plate area or total collection area, the following procedure was used. A curve of the form outlet = e-(a × sea + b)

Fig. 4. Mobile E P outlet with one field on.

was fit to the data by means of regression analysis, i.e., least square fit. This technique finds values for the fitting parameters a, b. Once these are determined, one can calculate the required SeA needed to achieve the desired outlet grain loading, providing the operating conditions for the EP, such as inlet loading, temperature, and moisture, are in the range that they were during the pilot testing. It was assumed that this would be so since the process would not be expected to change. The equation is only a fitting technique and no physical meaning is given to it, it just happens to fit this type of data. It might be argued that it is a modified Dentsch-Anderson equation, however, one could find more argument against this than for it. As mentioned, the basic EP size was determined by the EP velocity/gas volume relation, which in this case, dictated a 1800 ft 2 (167 m 2) chamber. The total collec-

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Douglas Ruth and David Shilton

Fig. 8. Mobile EP plates with BOF.

mg/m3). For these requirements, it was decided that a McGill Model 5-600 x 4 EP would be the best choice. This consists of four chambers, each 600 ft 2 (56 m 2) in cross-sectional area, each with five fields, and each with about 79,000 ft 2 (7340 m 2) of collection area. Four, rather than the necessary three units, were chosen so Fig. 6. Mobile EP outlet with three fields.

i0 II tion surface area required determined by the above technique, was found to be about 238,000 ft 2 (22100 m 2) to treat 300,000 acfm (142 m3/s) of gas. This would yield a predicted outlet grain loading of 0.033 grs/scf (81

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Fig. 9. BOF dust resistivity.

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EP system design and performance

231

that there would be three active units and one standby. Using three units for 300,000 cfm (142 m3/s) of gas at 360 °F (182 °C) and l l grs/scf inlet loading gives an average EP flow through velocity of 2.7 fps (82 cm/s), an SCA of 790 (156 m 2 per m3/s) for an expected outlet of 0.033 grs/scf (81 m g / m 3) and 20% or less visual opacity. The EPs are paired, and each pair share the same TR set on the first field. Fields 2 and 3 and fields 4 and 5 on each EP have their own TR set. These were designed to operate at 25 kV at 500 ma each. Since each of the four precipitators is the same, flow modeling was done on only one unit. A 1/16 scale model was constructed of plexiglas. The velocity pressure was set the same in the model as would be found in the full size system (Euler Modeling) and a velocity profile measured across the face of the model. The standard technique of fitting turning vanes and perforated plates to provide an even flow distribution was used. The final configuration installed was diagonal grid of channel pieces with a 6 in. (15 cm) opening, followed by a perforated plate and then a set of splitter vanes. Another design consideration was dust handling. Each row of EP hoppers are connected by a screw conveyor. These in turn empty into a pnuematic conveyor to a dust silo. Dust from the silo is pelletized and carried away by rail car. This hopper dust has a bulk density of around 60 lbs/ft a (960 kg/m3). One-third of the dust from the process is quite heavy, near 140 lbs/ft 3 (2240 kg/m3). For this, a single dropout chamber was built before the EP system inlet. This dust is also conveyed to the silo for pelletizing and subsequent re-use. One final concern was that of possible explosion due to the high CO levels possible. In light of this, a CO monitoring system was designed to de-energize the fields. This reduces the possibility of a spark induced explosion in the EP. Fabrication and Installation The unique McGill design allows for a modular construction; that is, each EP is made up of one or more of five standard modules. These individual modules are

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manufactured complete before shipping, including plates, side skins, and rapper boxes where required. The top structures, hoppers, and transitions are also shop produced and shipped ready for installation. In this particular case, the 600 model is made of four 150 ft 2 (13.9 m 2) modules, stacked 2 × 2, each field. Therefore, each of the four EPs is a 5 x 4 array of modules for a total of 80 modules to make up the system. Field erection then consists only of pouring the foundation, setting up the support structure and hoppers, and then lifting the modules in place. Top structures and transitions are then installed. Finally the wiring, piping, insulation, and siding is installed. Using this method, the quality of the final product is higher, because most of the EP is shop produced and assembled, under controlled supervision. This means that the quality of the installation is not totally in the hands of a contractor who may not have the necessary experience in areas where critical assembly is needed. This is especially true in the case of plate alignment. 750

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Fig. 12. Performance of pilot plant (o) vs. full size system (x).

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Full Scale System Performance Colorado regulations require that the full system emission be less than 40 lbs/hr (18 kg/hr), and less than 20°7o opacity. The three unit system operation easily meets the process weight requirement. Fig. 12 shows a comparison between the pilot plant data and the full system performance data. As can be seen, the agreement between the two sets of data is good. With three of the four units in operation, there are occasional excursions above 20°7o opacity. Most typically, the readings over 20°7o opacity are brief and associated with commencement of the blow or reblows. These puffs are indigenous to the BOF process and well controlled with all four units operating.

Operation and Maintenance All four EPs were installed and operating within 2 years after the pilot plant study. The biggest operating problem with the system has been associated with the high density dust transporting system; evidenced by plugging in the line or defective gate valves. Periodic cleaning

Douglas Ruth and David Shilton

of all the transporters is needed to maintain continuity of dust removal. It was also found necessary to attend to the "automatic" pelletizer full time, due to the required water feed adjustments. However, one 8 hour shift can pelletize dust from a 24 hour collection period. It is also necessary to blow off the ledges between modules with compressed air once every shift, due to the high buildup in this area. The biggest problem area with EP operation seems to be failure of the insulators in the rapper boxes, especially in colder weather. A re-piping of the purge air system has reduced this problem somewhat.

Summary The pilot plant testing and subsequent data has provided for a successful full size EP design and installation for this BOF application. The problems unique to this process were solved at the early stage of the pilot plant program when design changes were easily made. As a result, the required emission level and opacity is within that allowed by state regulation with no major problems in day-to-day operation.