COMPARISON OF A

CONTINUOUS OPACITY MONITOR and METHOD 9 OBSERVATIONS to
BROKEN BAG DETECTOR SIGNALS
on an EAF BAGHOUSE© 2001
authors:
Joseph C. Wesselman, Corporate Director of Environmental Affairs IPSCO Enterprises Inc.
Charles W. Askins, P.E., President AG Environmental Services, Inc.

Introduction
This report presents the results of a test program comparing the output of a triboelectric broken bag detection system to a Continuous Opacity Monitor (COM), both of which are installed on the Electric Arc Furnace/Ladle Metallurgy (EAF/LMF) Baghouse at the IPSCO Steel Inc. (IPSCO), Montpelier Works located in Muscatine, Iowa. During the comparison period, elevated opacity was created artificially, using surrogate particulate introduction, to better identify the comparison of opacity to triboelectric signals at opacities higher than 5%. USEPA Method 9 visible emission observations were made concurrently during the surrogate particulate introduction periods. The simulated opacity generated during the comparisons was tracked and recorded by the TribolinkTM system, the COM and Certified Visible Emission (VE) observations. The comparison program began on October 12, 2000 and was completed on November 25, 2000.

Background
IPSCO was interested in replacing the COM which was installed on the stack as a result of the NSPS requirements. The COM has been a high maintenance device because of the high vibration within the stack of the negative pressure baghouse. Further, the COM has a documented problem measuring the 3% opacity standard applied to the EAF emissions. Bag penetration by sparks in the negative pressure, pulse jet cleaned baghouse has been a continuing maintenance problem, and the COM has shown itself to be ineffective as a tool for identifying the compartment location of failed bags. The COM opacity identified in the stack, when actually present, provides the operator with no useful information as to the source of emissions, and when more than one compartment is involved, successive isolation of individual compartments provides no compartment identification information when using the COM as the emission indicator.

To provide operators with a more proactive and useful tool, IPSCO investigated the feasibility of using a broken bag detection system. After a review of broken bag detection system technologies, a triboelectric base system was selected.

Because of the lower levels of particulate change detected by the triboelectric technology, and the problems associated with using a COM in this application, IPSCO applied to USEPA for the use of alternative monitoring technology on the EAF/LMF baghouse. The use of a triboelectric broken bag detection system was proposed for replacement of the COM presently installed on the stack. The proposed technology change was conditionally approved by USEPA, with the condition that IPSCO run a forty five (45) day comparison to establish the relative relationship of the triboelectric signals to opacity as measured by the COM. Since compliance with the New Source Performance Standard (NSPS) 3% opacity standard for the EAF baghouse is established by VE observations using USEPA Method 9, VE observations were also included by IPSCO in the comparison.

Discussion of Instrument Measurement Mechanisms
Fundamental compliance with the particulate standard for EAF emissions is based upon the particulate mass emissions from the baghouse. Since continuous monitoring of mass emissions is a practical impossibility for fume control systems, the use of surrogate measurements has been adopted to characterize the particulate emissions from the baghouse. The surrogate measurement adopted by USEPA in the NSPS for the EAF baghouse is relative opacity. Opacity is the measurement of the amount of light lost when passing through an emission plume, and is characterized by percent (%) loss of light. The opacity measurement range starts with 0%, being no loss, and 100%, being complete loss of light, or fully opaque.

USEPA established a standard method of visual evaluation for particulate emissions that uses a trained human observer to establish opacity of exhaust plumes. This procedure is found in 40 CFR 60 as Reference Method 9 (USEPA Method 9). The requirement for visual opacity measurements using USEPA Method 9 has been established in the EAF - NSPS to allow for operators and regulators to monitor the EAF baghouse compliance with the particulate mass emission standard.

The measurement of opacity (by either Method 9 or a COM) and the measurement of triboelectric change are surrogate measurements for the mass emissions from the baghouse, and measure the amount of particulate present in an exhaust gas stream. Neither system measures particulate mass emissions.

COM System
The measurement of opacity can be done using an instrument such as a COM. The COM uses the same basic principle as VE observations, measuring the loss of light associated with passage of a light though an exhaust plume. In the case of a COM, the source of light is a beam that is directed across a duct or stack striking a mirror on the opposite side of the duct. The mirror is aligned to reflect the light back to the light source. The instrument then measures the reduction in light intensity after this double path length, and calculates the loss as relative opacity.

The COM measurement of opacity is based upon the principle that particles present in the gas stream will effect the amount of light returning. This is based upon the fact that particles have the inherent and variable characteristics of light absorbence, refraction or reflection. These light effecting characteristics are associated with the size, shape and structure of the respective particle, as well as the quantity of particles present in the gas stream. These factors have nothing to do with the density or mass of the respective particle. The COM presumes that all light lost is associated with particles in the gas stream.

Since there is neither a correlation to the gas flow volume nor the concentration of particulate present, the mass emission associated with the gas flow cannot be measured by the COM.

Opacity, whether measured by Method 9 or a COM, is a qualitative surrogate for the quantity of particulate matter being emitted. The standard for opacity in the EAF - NSPS is < 3% opacity averaged over 6 minutes of continuous readings. The human eye can distinguish opacity to the level of 5% increments, and the minimum detectable change recorded by the human eye is 5%. Therefore the EAF-NSPS of < 3% opacity represents a no visible emission standard. The COM is able to measure opacity in fractions of a percent, however it has been identified by ASTM to be accurate only to the level of 10% opacity. ( ASTM Standard: D 6216-98, Standard Practice for Opacity Monitor Manufacturers to Certify Conformance with Design and Performance Specifications.)

Triboelectric Broken Bag Detector System
The measurement principle of the triboelectric broken bag detection system is based upon measuring the small changes in electrical charge of an energized probe placed within the exhaust gas stream. The probe is a stainless steel or other metallic material rod that is energized with a DC electrical voltage. The particulate present in the gas stream strike the probe, and the strikes act to change the electric field of the probe. This mechanism is similar to the release of static electricity that has been accumulated in a person’s clothing or skin surface. The small changes in the electric field associated with the particle impacts are measured in pico-amps. These pico-amp changes are the measurements that quantify the triboelectric signal.

The triboelectric signal is an analog output that is displayed as a percent of scale. The impact of no particles is measured as 0%, with the relative increase of particle strikes measured up to 100% of the scale.

Similar to the COM, the particle characteristics of size, shape and structure of the respective particle, as well as the quantity of particles present in the gas stream effect the relative change in triboelectric signal. Once again, these factors have nothing to do with the density or mass of the respective particle.

Equipment Specifications & Descriptions
The system specifications for the instruments used in the comparison are as follows:

COM System
The COM system installed by IPSCO for use on the EAF/LMF baghouse is manufactured by SICK, and reads out its measurements to a monitor that displays the instantaneous opacity and the most recent six (6) minute average opacity. The COM instrument is installed on the baghouse stack, approximately seventy five (75) feet above grade, at the sample testing port level on the stack. The total stack height is one hundred fifty (150) feet above grade. The monitor is located in the baghouse control building located at grade immediately East of the baghouse structure.

The signal from the COM reads out as a digital display on the console face of the controller. For the purpose of tracking COM data during this comparison, the COM 4-20 mA signals (instantaneous and 6-minute average opacities) were connected to the Auburn computer. A real time readout was available on the PC monitor and a record of these signals was tracked and stored on the PC hard drive. The COM system specification is as follows:

Manufacturer: SICK, Inc. 6900 West 110th Street Bloomington, MN 55438
Model: OMD 41
Number of COM Locations: 1
COM Material of Construction: Carbon Steel
COM Ambient Temperature Range: -4 °F to 131 °F
Relative Moisture: 95%
Signal Output: 4 - 20 mA
Operating System: Dedicated COM Software


Broken Bag Detector System
The broken bag detection system installed by IPSCO for use on the EAF/LMF baghouse is manufactured by Auburn Systems, Inc. (Auburn), and uses the triboelectric principle of particle detection. The triboelectric system specification is as follows:

Manufacturer: Auburn Systems, LLC 8 Electronics Avenue Danvers, MA 01923
Model: TribolinkTM
Number of Probe Locations: 4
Number of sensors/location: 2
Detector Material of Construction: 316 Stainless Steel
Probe Temperature Range: -40 °F to 450 °F
Input/Output Interface: PC
Operating System Platform: Windows 98


The TribolinkTM detectors are located in four (4) locations in the exhaust plenum of the baghouse. Two (2) TribolinkTM detectors (A-2, B-2) are located in the east compartment group plenum, and two (2) detectors (A-1, B-1) are located in the west compartment group plenum. Each detector group includes 2 probes. The detector configuration design places the probes such that there are eight (8) compartments upstream of the B group probes, and fourteen (14) compartments upstream of the A group of probes. Figure 1-1 illustrates the general orientation of the baghouse compartments and the respective location of the TribolinkTM detectors and the COM.

EAF/LMF Baghouse
The EAF/LMF Baghouse is a negative pressure, pulse jet cleaning baghouse that was installed for fume control of an EAF and LMF melt shop subject to the New Performance Standards (NSPS) for EAF’s. The negative pressure baghouse has a stack exhaust, and as such, is obligated to install and operate a COM on the exhaust stack.

The EAF is a twin shell DC electrode furnace, and is initially designed to produce 164 tons/hour (tph) of steel. At this base design rating, the EAF baghouse is sized and permitted to operate using twenty four (24) compartments, with one compartment off line for service and/or cleaning during operations. The baghouse (under a phased PSD Permit) has been permitted and built to allow for the EAF to operate at a production rate of 200 tph. At the 200 tph production rate the baghouse system is designed to operate with twenty eight (28) compartments, with one compartment off line for service and/or cleaning during operations.

During the surrogate particulate introduction tests and the 45 day comparison period, the EAF was operating within the 164 tph permitted production range.

Figure 1-1
EAF Baghouse Compartment Configuration
& Broken Bag Detector Locations



QA for Equipment Prior to Comparison
Prior to beginning the comparison, a manufacturer’s calibration was conducted on the respective instrument and monitoring system. Each of the certified VE observers used during the comparison had a current certification.

COM System Calibration
A manufacture’s calibration to insure that the COM conformed to the manufacturer’s specifications was conducted by SICK during the period of September 25, through September 28, 2000. The output signal from the COM to the TRIBOLINKTM PC was connected and verified as to its equivalence with the COM console display on September 26, 2000. The signal interface to the computer continued to operate without problems throughout the comparison period.

Problems with COM Performance
As noted previously, the COM was initially calibrated by the manufacturer prior to beginning the USEPA requested comparison on October 12, 2000. The manufacturer’s technician was on site for two (2) days during the pretest calibration period. However, the COM continued to perform erratically throughout the test period, and was re-calibrated a total of three (3) times before and during the comparison. Manufacturer’s calibrations were conducted on the following dates:

September 25 - 28, 2000
October 18, 2000
November 6, 2000

IPSCO was concerned that the COM should operate as close as possible to the manufacturer’s performance specification and PS-1, and therefore elected to have all of the calibration work done by the manufacturer’s trained technicians, presuming that a superior level of calibration would be obtained. After the completion of the comparison study, the COM was evaluated by a qualified engineer for conformance with the USEPA Performance Specification 1, Appendix B of 40 CFR 60. This conformance certification was requested by USEPA, and the instrument was certified as conforming to PS-1.

The circumstances associated with the repeated calibrations during the comparison study are discussed by event date.

September 25 - 28, 2000
As previously noted, this calibration was performed with the intent that the COM would be at its best operating performance before beginning the comparison. Upon completion of the calibration, the COM appeared to operate in conformance with the VE observations made of the stack exhaust plume.

October 12, 2000
Prior to beginning the surrogate particulate comparison program on October 12, 2000, it was observed that the COM was reading an average of 4.9% opacity, despite the fact that the Melt Shop was not operating and there were no emissions. The weather was clear, temperature of 65 °F, with a 5 to 10 mph wind from the southeast. Surrogate particulate (iron oxide) was introduced into the baghouse exhaust plenum to verify the relative performance of the COM. The COM did respond to increased levels of opacity in the exhaust, however the unit never returned to 0%, even after dust introduction had ceased, and VE observations using Method 9 showed an opacity of 0%.

Given the observed poor performance of the COM at low level opacities, a second calibration of the instrument was requested. Routine daily and additional Method 9 observations were made during the time period of October 12 to October 18, when the COM was re-calibrated, to confirm that there was 0% opacity in the exhaust plume, even though the COM indicated opacities ranging from 3% to 10% during this time period.

October 18, 2000
The COM was inspected on October 18, 2000 by a technician from Spectrum Systems, the manufacturer’s representative, and some reprogramming was done. The complete instrument package was re-calibrated. At the time, the technician left the site the COM was reading 0% opacity, the same level of opacity recorded by Method 9 observations of the stack exhaust plume.

November 1, 2000
Surrogate particulate (iron oxide) comparison was scheduled for November 1, 2000, and the COM was observed at 0% opacity prior to beginning the introduction of particulate. The weather was clear, temperature 70 °F, with the wind at 10 to 15 mph from the southeast. The COM registered no change in relative opacity when the surrogate particulate was introduced into the exhaust plenum of the baghouse, even though VE observations indicated opacities ranging from 5% to 25%. It appeared that the COM was in a flat line signal mode, even though no adjustments had been made to the unit since the technician made his calibration on October 18, 2000. The COM continued to read 0% opacity or nearly 0% even though surrogate particulate was introduced on November 1, 2000.

A third re-calibration of the COM was requested from the manufacturer, with the expressed requirement that SICK provide the best technician available to correct the problems experienced to date with this COM unit. The opacity indicated by the COM remained at 0% until November 6, 2000.

November 6, 2000
The COM was again inspected on this date by a SICK technician from the German home office. The technician made programming changes to correct the flat line signal induced by portions of the programming done by the Spectrum Systems technician. The complete instrument package was again re-calibrated. After re-calibration by the SICK technician, the COM erroneously read false-positive opacities between 5% and 6%, even when there were no Melt Shop operations. The SICK technician instructed IPSCO engineering personnel on the procedure to zero the COM during non-operations, and this procedure was performed. After the zero procedure was accomplished the COM read 0% opacity during non-operating periods, as well as during operating periods, as verified by Method 9 observations. It became evident after a few days that the COM read a true 0% opacity only on dry days, and whenever there was moisture in the air, the COM read a baseline of 3% to 4% opacity, even during non-operating periods. This elevated, false-positive baseline opacity reading was evident during the two days of surrogate particulate comparison done on November 15, and 16, 2000.

Broken Bag Detector System
At the present time, there are no Performance Specifications for triboelectric based system published in Appendix B of 40 CFR 60, however, USEPA published a guidance document, "Fabric Filter Bag Leak Detection Guidance Document" for such systems. A broken bag detector system certified calibration was completed by Auburn on the broken bag detector system during the period of September 20 through September 21, 2000. During this calibration, all of the detector probes were removed, inspected and cleaned. The system operated throughout the comparison period without hardware or software problems.

Instrument Comparison General Objectives
USEPA did not establish any specific objectives for the comparison, other than the comparison must be for a period of 45 days. To better qualify the expectations for the comparison, several objectives were identified by IPSCO for the 45-day comparison. These objectives were accepted by USEPA prior to beginning the comparison. The numbering of the objectives does not imply any hierarchy.

- Identify a base line normal range for triboelectric signals for each of the four (4) detector locations.
- Compare the relative detection ranges for the COM and Triboelectric System.
- Compare the real time reliability between the COM and the Triboelectric System.
- Perform introduction of iron oxide into each of the two (2) clean side plenums upstream of both detector groups in the respective plenum to compare system performance at opacities above 5%.
- Establish the relative relationship between the COM and the Triboelectric System.
- Establish the relative relationship between the Triboelectric System and the VE Observations.

Comparison Study Discussion
As noted previously, the comparison study program was begun on 10/12/00. This section of the report presents the findings of the forty five (45) day comparison, as well as specific review of the two (2) surrogate particulate introduction tests performed to evaluate the tracking of higher opacity levels in the exhaust plume.

General Discussion of Comparison Test Program
The comparison test program was generally divided into the two programs by IPSCO. These programs were:

1. Comparison during surrogate particulate introduction
2. Comparison during scheduled operations

Since it was anticipated that opacity levels above 5% would not occur during the normal operations of the baghouse, IPSCO determined that a program for artificially generating elevated opacity levels would be needed during the comparison program. The test program for the surrogate particulate introduction was initiated late in the comparison period because of the difficulties encountered with the COM. The surrogate particulate introduction was done on November 15 & 16, 2000. During the surrogate particulate tests the follow objectives were established by IPSCO:

1. Identify a baseline normal range for triboelectric signals for each of the four (4) detector locations.
2. Perform introduction of iron oxide into each of the two (2) clean side plenums upstream of both detector groups in the respective plenum.
3. Introduce the iron oxide for a sufficient time period to allow for at least two (2) successive 6-minute observations by a certified VE observer.
4. A total of two (2) tests would be conducted in each of the plenums.
5. At least a 3% opacity should be achieved for the tests, as identified by the COM and the VE observer.
6. If possible, opacities of at least 20% should be generated during the comparison.

November 15 & 16, 2000 Surrogate Particulate Introduction
Since the Melt Shop was not operating on November 15, the procedures for introduction of particulate and correlation of COM and VE readings were coordinated. Data collection procedures were established. The Melt Shop operated on November 16, and the base load of particulate associated with fume collection was present for the comparison study.

Discussion of Fe2O3 Introduction Method
Creation of opacity by making bag penetrations to allow for leakage of EAF dust was considered for generating the opacity. However, this method of opacity creation was not used for several reasons:

- The penetration of bags destroyed good equipment.
- The amount of dust could not be regulated.
- It was not possible to generate data for a single probe group, since once the bags had been cut, they would continue to leak particulate until replaced.
- Use of penetrated bags does not produce a range of data for opacities.

The surrogate particulate was introduced using a compressor, sandblasting pot and a single introduction point in the respective exhaust plenum. The dust introduction point was located up stream of the "B" probe locations in the respective plenum.

Compressed air for conveying the particulate was supplied from a compressed air line located at the penthouse level of the baghouse. The sandblasting pot and hoses were moved from the introduction location in the east compartment line to the west compartment line for the successive comparison runs. The introduction points were under negative pressure, and dispersion of the dust with in the plenum was accomplished by the natural draft and turbulence in the respective plenum.

Particulate Size Distribution
A surrogate particulate was used to create opacity in the emissions from the respective plenums of the baghouse. A commercial grade of Iron Oxide (Fe2O3) was purchased for use as the surrogate particulate, and samples of the Fe2O3 and EAF dust collected by the baghouse were submitted for particle size analysis at an independent laboratory. The mean particle size of the Fe2O3 is 1.857 µm, and the mean particle size of the EAF dust is 1.818 µm. The particle size distribution is summarized in Table 1.1.

Particulate 10% 25% 50% 75% 90%
EAF dust, µm <1.235 <1.581 <1.874 <2.107 <2.276
Fe2O3, µm <1.022 <1.328 <1.780 <2.346 <2.819


To accomplish the higher level opacity comparisons (> 5% opacity) it was necessary to introduce iron oxide particulate into the off gas stream to generate various levels of opacity in the exhaust from the stack. The iron oxide used as a surrogate particulate for the EAF dust had particle sizes distribution similar to the EAF dust. This particle size range is representative of the size range of particles in the exhaust from an EAF baghouse that had penetrations in the filter bag fabric.

Comparison of VE Opacity Observations and Triboelectric Signals
During the surrogate particulate introduction comparison conducted on November16, a comparison was made between the VE opacity observations and the Triboelectric signal output. The surrogate particulate was introduced into the east side and west side exhaust plenums of the baghouse, and VE observations and COM opacity were made on the stack and compared to the Triboelectric data recorded on the Auburn PC. The COM comparison to Triboelectric signals is discussed separately in the following section. The comparison of VE observation data correlates very well for the four particulate introduction periods on November 16. Chart 1.1 illustrates the linear regression curves of the VE opacities to the Triboelectric signals on a real time basis.

Figure 1.1
Linear Regression of VE Opacity to Triboelectric Signals, 11/16/00


The VE opacity points are for one (1) minute averages of fifteen (15) second readings according to USEPA Method 9. Triboelectric signals are also based upon the one (1) average of signal measurements made for the corresponding minute. The regression of the B-1 and B-2 probes with the VE opacity observations show similar correlations. The linear regression of the VE opacity to the Triboelectric signals produced the following linear equations with associated R2 values for the regressions.

Test #1, Probe A-1 to VE: y = 1.5634x - 0.1031 R2 = 0.9646
Test #2, Probe A-1 to VE: y = 1.155x + 7.0787 R2 = 0.8210
Test #3, Probe A-2 to VE: y = 1.1969x + 0.4331 R2 = 0.9734
Test #4, Probe A-2 to VE: y = 1.358x + 2.2674 R2 = 0.9791


The relatively high R2 values (R2 of 1.0 is a perfect correlation) for these equations indicate a very good correlation.

During the surrogate particulate comparison conducted on November 16, the weather changed. The sky became overcast, the temperature was 38° F at the start of the comparison, and it began to snow flurry by the end of the fourth test. Throughout the test period the COM indicated a base level of opacity at 3 to 4% while, according to simultaneous Method 9 readings, there was no observable opacity in the exhaust plume. The false-positive base line opacity of the COM continued for the remainder of the day.

For relative comparison purposes with the previously plotted VE to Triboelectric data, the same probe data for the triboelectric signals is plotted against the COM data. The COM opacity to the triboelectric signals (A-1, A-2 probes) for the four tests on November 16 are plotted as both logarithmic and linear regression curves in Chart 1.2.

Chart 1.2
Regression of COM Opacity to Triboelectric Signals, 11/16/00


The COM data points are one (1) minute averages of the COM continuous opacity output signal. The triboelectric signals are also one (1) minute average data from that system output.

The respective regression curve formulae for the Triboelectric signal comparison to the COM signals (Chart 1.2) with the associated R2 values are as follows:

Test #1 Probe A-1 to COM: y = 1.878x + 22.485 R2 = 0.4054
y = 24.205Ln(x) - 7.1213 R2 = 0.5782
Test #2 Probe A-1 to COM: y = 0.7932x + 31.156 R2 = 0.3311
y = 22.549Ln(x) - 13.203 R2 = 0.7056
Test #3 Probe A-2 to COM: y = 1.4231x + 8.8582 R2 = 0.8854
y = 21.934Ln(x) - 17.881 R2 = 0.9282
Test #4 Probe A-2 to COM: y = 1.8411x + 13.226 R2 = 0.7561
y = 24.875Ln(x) - 17.229 R2 = 0.9070