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Date: March 6, 2013

Subject: Development of Background Information on Proposed Area Source Emissions 

 Limit - Determination of GACT for Wool Fiberglass Furnaces at Area Sources

EPA Contract No. EP-D-11-084; EPA Work Assignment No. 1-07

RTI Project No. 0213199.001.07

From: Cindy Hancy

Dave Reeves

To: Susan Fairchild, EPA/OAQPS/SPPD/MMG

I. Introduction 

Section 112(k) of the Clean Air Act requires the U.S. Environmental Protection Agency (EPA) 

to develop standards for area sources which account for 90 percent of the emissions in urban 

areas of the 33 urban hazardous air pollutants (HAP) listed in EPA’s Integrated Urban Air 

Toxics Strategy. These area source standards can require emission control levels that are 

equivalent to generally achievable control technology (GACT) for affected sources. For the 

development of area source standards, GACT is meant to include methods, practices and 

techniques which are commercially available and appropriate for application by the sources in 

the category considering economic impacts and the technical capabilities of the firms to operate 

and maintain the emissions control systems.

The purpose of this memorandum is to describe how GACT was determined for glass-melting 

furnaces, which is the affected source for the proposed National Emission Standards for 

Hazardous Air Pollutants (NESHAP) for Wool Fiberglass Manufacturing Area Sources. In the 

development of the residual risk and technology review of the major source NESHAP for wool 

fiberglass manufacturing (Subpart NNN), the industry trade association (NAIMA) and EPA sent 

information requests to all major and area source wool fiberglass facilities and had several 

meetings with industry regarding current practices at wool fiberglass plants. Because of the large 

number of emission test reports and other documents reviewed, and the fact that most documents 

included information claimed as confidential business information (CBI), the specific references 

for the test data are not listed in this memorandum. However, a summary of the test reports and 

all of the other documents that were used in the analysis of GACT, other than those that were 

claimed to be CBI, can be found in the docket.

II. Glass-melting Furnaces

Glass-melting furnaces vary according to size, design, fuel source, raw material mixtures, 

melting times and methods of heat application. Glass-melting furnaces can be electric steel shell, 
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cold top electric, air-gas, oxyfuel, or combinations of gas and electric. There are two types of 

electric furnaces currently used in the wool fiberglass industry: electric steel shell and cold top 

electric. There are two types of gas-fired glass-melting furnaces: air-gas, and oxy-fuel. There 

may also be combinations of gas fired and electric furnaces (known as ‘electric boost’) in 

operation. About 80 percent of the glass-melting furnaces used in the wool fiberglass industry

are electric (e.g., steel shell or cold-top) and about 20 percent are gas-fired (e.g., air gas or oxyfuel).

Electric furnaces operate using electrodes embedded beneath the surface of the raw materials. 

The electrodes melt the raw material mixture while new layers of raw material are constantly

added with a water spray on the surface of the molten layer. The electric steel shell furnace 

extrudes molten glass from the end of the furnace; the cold-top electric furnace is tapped from 

the bottom of the furnace. Temperatures above the raw material mixture are less than 3000 F.

Gas-fired furnaces (oxy-fuel and air gas) feed raw materials into one end, melt the raw materials 

using natural gas, and extrude the molten glass from the far end of the furnace. Air gas furnaces 

use outside ambient air to provide oxygen for combustion of natural gas in the furnace. Oxyfuel 

furnaces mix natural gas with industrial-grade (99% pure) oxygen to create a flammable mixture 

that is burned in the furnace. Because a gas/ oxygen mixture fires at a higher temperature than an 

air/gas mixture, oxyfuel fired furnaces operate with higher temperatures above the melted glass 

line (35000 F. inside the furnace) than air gas furnaces (24000 F), whose temperatures are 

reduced by the dilution of oxygen in the outside air.

III. Existing Glass-melting Furnace Emission Controls 

The type of control device used to control particulate matter (PM) emissions varies by furnace 

type. Electric steel shell furnaces use a baghouse and/or fabric filter; cold top electric furnaces 

use batch wetting as the method of PM control. Air-gas furnaces currently use a combination of 

electrostatic precipitators (ESP) and baghouses; oxyfuel furnaces use an ESP only (with one

oxyfuel furnace using a scrubber along with an ESP). Figure 1 shows PM emission rates by

control device for all furnaces in the wool fiberglass manufacturing source category that 

submitted PM data.

IV. Glass-Melting Furnace Emissions 

Chromium Emissions

Wool fiberglass Manufacturing is being listed as an area source category based on emissions of 

chromium compounds. In the development of the residual risk and technology review of Subpart

NNN (for major sources), the industry trade association (NAIMA) and EPA received emissions 

information from the majority of all major and area source wool fiberglass facilities. The EPA 

evaluated chromium emissions data for major and area source glass-melting furnaces and 

analyzed the operational differences of the different types of glass-melting furnaces. The EPA 

found that electric furnaces (electric steel shell and cold-top electric) do not have the potential to 

emit chromium at the levels of gas-fired furnaces, and that chromium emissions from electric 
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furnaces are about 2 orders of magnitude lower than the chromium emissions from gas-fired 

furnaces as shown in Figure 2. 

Figure 1. Furnace PM Emission Rate by Control Device

Figure 2. Chromium Compounds Emission Rates by Furnace Type
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This higher chromium emission potential for gas-fired glass-melting furnaces is due to 

differences in design and operation of gas-fired glass-melting furnaces compared to electric 

glass-melting furnaces. Although all glass-melting furnaces are constructed using chromium 

refractories at and below the line of contact defined by the refractory wall and the molten glass 

within the glass-melting furnace (the glass/metal line), oxy-fuel and some air gas glass-melting

furnaces have other glass-melting furnace parts constructed using chromium refractories, such as 

the crown, sidewalls, front wall, back wall, and forehearth. The use of chromium refractories 

above the melt line is necessary to obtain the desired furnace life and reduce the necessity for hot 

repairs of the furnace. When the hot, corrosive and reactive gases of a gas-fired glass-melting 

furnace come in contact with the high chromium refractories lining the area above the glass melt 

in high temperature glass-melting furnaces, the chromium is available to be oxidized and 

converted into its hexavalent form. 

A chromium refractory product has the greatest resistance to heat and wear of any refractory in 

use today. The temperatures above the melt in gas-fired glass-melting furnaces range from 2,500◦

to 4,500◦F, while the temperatures in electric glass-melting furnaces are less than 3000 F. Due to 

their higher operating temperatures, gas-fired glass-melting furnaces are constructed using 

chromium refractories at various parts of the glass-melting furnace that are above the molten 

glass, including the crown. Chromium refractories may contain over 90% chrome bearing 

compounds, specifically, chomite ore (as Cr2O3) and chromic oxide (as Cr3O4). The use of 

chromium refractories above the glass melt line and corrosivity due to thermal and chemical 

degradation of the furnace interior contribute to the greater potential for gas-fired furnaces to 

chromium compounds. 

Studies indicate that, under normal industrial temperatures and oxidizing conditions, trivalent 

chromium, which is present in the refractory, oxidizes readily to its hexavalent state.1

Additionally, while the degradation of the glass-melting furnace refractory indicates increasing 

chromium emissions, that process does not necessarily follow a normal and predictable pattern. 

The degradation of refractories within the glass-melting furnace is a function of numerous 

factors, including temperature, time, stress, and the composite effects of aging and creep 

response. These processes are highly nonlinear, so the traditional equations that assume steadystate deformation rates are not appropriate.

2 We have also found that as the refractories of the 

gas-fired glass-melting furnaces degrade, the chromium of those refractories at and above the 

metal/glass line is emitted as particulate to the outside air. Chromium from the refractories below 

the metal/glass line is absorbed into the molten glass and becomes vitrified with the other raw 

minerals. Industry commented that refractory loss from degradation of the refractory walls in use 

is approximately 20,000 pounds of refractory annually.

3 When that loss occurs below the glass 

melt line, it is believed to be incorporated into the molten glass. When that loss occurs above the 

glass melt line, it is emitted from the furnace. While existing air pollution controls achieve 

reductions in the total PM, a greater fraction of that PM consists of chromium in the gas fired 

furnaces than exists in the electric furnaces, and have been measured from gas-fired furnaces at 

over 500 pounds per year. Therefore, additional chromium reductions for gas-fired furnaces in 

this particular industry are needed.
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EPA has learned that if a source of reasonably priced industrial-grade oxygen is available, the 

oxyfuel glass-melting furnace is the design favored for use by glass manufacturers due to the 

glass-melting furnace’s low NOx emissions, low energy demands per volume output of glass, 

and extended furnace life span. The low NOx emissions of an oxy-fuel glass-melting furnace 

result from the fact that no air (which contains nitrogen) is introduced into the high temperature 

zone above the glass melt. Instead, the oxy-fuel glass-melting furnace design mixes the natural 

gas fuel with pure oxygen for combustion, thus reducing NOx emissions. 

In the 1999 Wool Fiberglass Manufacturing NESHAP (subpart NNN), EPA promulgated 

emission limits for PM (0.5 lb PM per ton of glass pulled) as a surrogate for metal HAP emitted 

from glass-melting furnaces. The rationale for this surrogacy is that the metal HAP are contained 

(in trace amounts) in the PM, and that the PM control devices remove metal HAP with the same 

efficiency as they remove total PM. Therefore, a limit on PM also creates equivalent control of 

the metal HAP.

IV. Technical Basis For Setting Emissions Limits For Both PM And Chromium for Gas 

Fired Furnaces

We continue to believe that PM is an appropriate surrogate for metal HAP for electric glassmelting furnaces at wool fiberglass manufacturing facilities. The EPA may use a surrogate to 

regulate HAP if there is reasonable basis to do so. We have used PM as a surrogate to regulate 

hazardous pollutants because: “(1) HAP metals are invariably present . . . in PM;” (2) “PM 

Control technology indiscriminately captures HAP metals along with other particulates;” and (3) 

“PM control is the only means by which facilities achieve reduction in HAP metal emissions.” 

National Lime v. EPA, 233 F.3d at 639. We have also always reasoned that “[i]f HAP metals are 

invariably present in ...PM, then even if the ratio of metals to PM is small and variable, or simply

unknown, PM is a reasonable surrogate for the metals-assuming ... that PM control technology

indiscriminately captures HAP metals along with other particulates.” Id. Additionally, we believe 

that PM controls are still effective for chromium emissions at the same efficiency as total PM 

and other metal HAP. However, we believe that in the case of gas-fired furnaces a separate 

chromium limit is necessary in order to insure control of chromium emissions.

In general, the PM emissions from glass-melting furnaces result from the entrainment of small 

particles of the feed in the exhaust gas. The feed materials going into the furnace contain trace 

amounts of chromium and other HAP metals. Therefore, the resulting PM also contains HAP 

metals (including chromium) in trace amounts. However, there is also a second particulate 

source which is the refractory above the glass melt line of the furnace. Due to the chemical and 

thermal properties of the gases, a corrosive environment exists within a gas-fired furnace. This 

causes erosion of the refractory lining of the furnace, which releases chromium-bearing 

refractory particles into the furnace exhaust gas as PM. 

All electric furnaces are lined with chromium refractories at and below the glass melt line. The 

chromium refractory in an electric furnace extends up to two inches above the melt. Gas-fired 

furnaces may construct the entire furnace (crown, sidewalls, front and back walls, forehearth, 

etc.) from chromium refractories. In the case of gas-fired furnaces, the refractory does not 

contain chromium in trace amounts, rather chromium compounds make up a significant portion 
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of the refractory composition above the glass melt line. This results in gas-fired furnaces 

emitting particulate that contains chromium in larger amounts than that of electric furnaces. 

Based on our dataset, chromium constitutes an average of 0.96 percent of PM emissions for gasfired furnaces, which is 13 times higher than the average for electric furnaces (0.07 percent of 

PM emissions are chromium). 

As an example, if a furnace is emitting 100 pounds per year (lb/yr) of uncontrolled PM and has a 

99.9 percent efficient control device; assume that total metal HAP are about 1 percent of the 

uncontrolled PM, and chromium makes up 10 percent of the metal HAP. Controlled PM 

emissions would be 0.10 lb/yr of PM, and controlled chromium emissions would be 0.0001 lb/yr. 

However, if the 100 lb/yr of PM came from the chrome refractory located above the glass melt 

line, the chromium content might be as high as 50 percent of the PM. This would result in the 

same level of controlled PM emissions, 0.10 lb/yr, but controlled chromium emissions of 0.05 

lb/yr. This is a 500 fold increase in chromium emissions but no increase in total PM emissions. 

Because chromium compounds are a significant component of the refractory used above the 

glass melt line, a greater potential for chromium emissions exists, which is not associated with a 

commensurate increase in emissions of other HAP metals. Historical test data show there can be 

a delay in the increase of chromium emissions due to the wear of the furnace refractory. Table 1 

shows measured total chromium emissions over one campaign of a single oxyfuel furnace. The 

emissions testing conducted in 2010 for the furnace summarized in Table 1 showed that 93 

percent of the chromium was in the hexavalent state (i.e., Cr+6). Because the increase in 

chromium emissions may occur as a result of numerous variables1, and may be closely related to 

specific furnace design and maintenance elements, we believe it is appropriate to set a chromium 

limit in addition to the PM limit.

Table 1. Summary of Chromium Emissions from 2004 – 2010

Year Chromium Emissions from Highest Emitting Glass-Melting Furnace 

at permitted production rate, pounds per year

2004 <5

2005 30

2008 114

2010 540

Additionally, emissions testing conducted at wool fiberglass furnaces raise pollutant-specific 

issues. Emissions test data for glass-melting furnaces at wool fiberglass manufacturing facilities 

indicated that the phenomena discussed above can result in emissions levels of chromium up to 

50 lb/year, and in one case as high as 540 lb/year from gas-fired furnaces, while emissions from 

electric furnaces at similar PM emissions levels are below 5 lb/year. Table 2 shows a comparison 

 

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Information obtained during site visits in December 2012 indicated that differences in furnace designs, placement 

of refractory products, composition and manufacturing specifications of refractory products, differences in 

installation of refractory products, and composition of refractory patches used in “hot” repairs all may influence 

chromium emissions from gas-fired furnaces. 
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of the average and maximum total chromium emissions between electric and gas-fired glassmelting furnaces. Chromium has been identified as one of the urban air toxics based on its 

prevalence and toxicity in urban areas.

Table 2. Average Annual Total Chromium Emissions for Glass-melting Furnaces1

Average Chromium 

Emissions per 

Furnace (lb/yr)

Maximum Chromium 

Emissions from a 

Furnace (lb/yr)

Electric 

Furnaces 0.38 1.7

Gas-fired 

Furnaces

42 540

1. Based on furnaces for which PM and chromium emissions testing was submitted

In lieu of setting both a PM and chromium limit, we considered setting a chromium limit alone. 

However, for all the reasons noted above (i.e. the fact that the PM controls are still effective, but 

the chromium content of the particulate is increased) we believe it would be difficult to improve 

the efficiency of the PM controls to meet a chromium limit based on GACT. 

We also note that the control technology that would be required to meet the proposed chromium 

emissions limit is a packed bed sodium hydroxide (NaOH) scrubber. However, this type of 

control is not effective in reducing PM emissions. Therefore, in establishing only a chromium 

limit, PM emissions (and thus emissions of other HAP metals) would be left essentially

uncontrolled.

V. Determination of GACT for Glass-Melting Furnaces

In order to set PM and chromium emission control levels that are equivalent to GACT, the EPA 

evaluated the methods, practices and techniques that are commercially available for gas-fired 

glass-melting furnaces. As shown in Figure 1, most gas-fired furnaces use an ESP and/or 

baghouse to control PM and chromium emissions. The basic approach to determining GACT 

was to evaluate the available data on controlled emissions from gas-fired glass- melting furnaces 

and then determine the control levels that appear to be generally available for reducing emissions 

of chromium and PM from gas-fired furnaces. Test data from facility emission test reports were 

compiled into a spreadsheet and sorted by pollutant and furnace type. Figure 2 shows the 

emission rate test data that were provided by facilities for chromium compounds. 

EPA reviewed these data and then used production data for each furnace to estimate the annual 

PM and chromium compound emissions. There are no differences between furnaces located at 

area sources and major sources. The reason is that the differences between major and area 

sources relate to emissions from the binding process, not to emissions or emissions controls on 

the furnaces. Therefore we looked to the entire data set to develop options for GACT.

In the case of PM we evaluated several options as potential GACT. The first option was 0.33 

pounds PM per ton of glass pulled This are the same PM limit proposed for major source 

furnaces as a result of technology review performed under 112(d)(6). Given the fact that major 

and area source furnaces are no different, this was a logical option to evaluate and we believe 
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represents the least stringent option we should consider. This level would result in no costs or 

emission reductions because all furnaces currently meet this limit. However, this limit would 

codify the current level of PM emissions and prevent any future PM emissions increase.

We also looked evaluated potentially lower limits as GACT. These were 0.2, 0.1, and 0.05 lb 

PM/ton, and at a level of control equal to MACT of 0.0093 lb/ton glass for existing sources and 

0.0018 lb/ton glass for new sources. At these levels 5 to 12 furnaces would have to invest in 

additional add-on PM emissions control. We estimate these costs would exceed $50,000 per ton

of PM reduction for the first three options. In the case of the MACT floor levels of control, the 

cost of PM reductions would be $480,000 per ton PM removal ($200,000 per furnace for a total 

of $2,400,000 for 12 furnaces that currently do not meet the limit to remove five tons of PM 

annually). These cost effectiveness values are significantly higher than any the Agency has 

accepted as reasonable for PM. 

For chromium the first GACT option evaluated was 0.00006 lb/ton glass pulled. As was the case 

with the PM limit, this is the same chromium limit proposed for major source furnaces as a result 

of technology review performed under 112(d)(6). There are three area source gas-fired furnaces 

that currently do not meet the proposed GACT for chromium. However, data are available for 

industries with similar control requirements that demonstrate that there are effective chromium 

control technologies available. We searched other industries for controls that would remove 

chromium and found that a sodium hydroxide (NaOH) scrubber is used in both high temperature 

metallurgical industries and in the chromium electroplating industry for removal of hexavalent 

chromium.2 Based on the effectiveness of this technology on to two different types of exhaust 

gas streams, we believe this control technology is transferable to wool fiberglass furnaces. 

Though there are currently no NaOH scrubbers applied in the wool fiberglass industry, there is 

currently one gas-fired furnace equipped with a PM control followed by a wet scrubber for SO2

control. This is directly analogous to using a NaOH wet scrubber downstream of the PM controls 

to achieve additional chromium removal. 

Assuming that the facilities not currently meeting the proposed chromium emission limit opted 

to use the NaOH scrubbers to achieve compliance, the cost of the proposed chromium emissions 

limit is $7,600 per pound of chromium. This is a reasonable cost given that chromium is an 

urban air toxic and that a significant portion of the chromium emitted from gas-fired glassmelting furnaces is hexavalent chromium, which is extremely toxic and carcinogenic even in low 

amounts. We note that we found $11,000 per pound chromium removed to be a reasonable cost 

in the final Chromium Electroplating RTR rulemaking, where we regulated chromium 

compounds (77 FR 59220, September 19, 2012). 

For chromium we evaluated a MACT floor level emissions limit of 0.000022 pounds (lb) of 

chromium compounds per ton of glass pulled (0.022 lb per thousand tons glass pulled) for new 

and existing sources. In this case of the MACT floor levels of control, the cost per pound of 

chromium removed would be $34,400 annually ($250,000 for 10 furnaces that currently do not 

meet the limit to remove 75 pounds of chromium compounds annually). Therefore, the costeffectiveness of achieving the chromium emissions levels would be about five times higher than 

 

2 NaOH Scrubber Information. Telephone discussion and emails between vendors, companies and EPA. Steffan 

Johnson, Measurement Policy Group. USEPA/OAQPS/SPPD. 
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the first option GACT standard which we are proposing. These cost effectiveness values are 

significantly higher than any the Agency has accepted as reasonable for chromium. We also 

considered some level between 0.00006 and 0.000022, but we noted that any level below 

0.00006 would incur similar costs and emissions reductions. 

In summary, we found the cost effectiveness of the proposed GACT limits to be 

reasonable. We evaluated more stringent emissions limits that those proposed, but the overall 

emission levels would not be significantly reduced (because most of the area sources are 

relatively low emitters), and the cost and cost-effectiveness of achieving these emissions would 

be significantly higher than the cost of proposed GACT standards without significant added 

environmental benefit. In addition, and more significantly, setting more stringent limit that those 

being proposed this would result in a situation where area source gas-fired glass-melting 

furnaces would be subject to stricter regulations than the major source gas-fired glass-melting 

furnaces, which would not be appropriate given that there are no discernible differences between 

area and major source furnaces.

References: 

1. Lee, Y., Nassaralla, CL., Minimization of Hexavalent Chromium in Magnesite-chrome 

Refractory. Metallurgical and Materials Transactions, 1998. 

2. Wool Fiberglass Insulation Manufacturing. Background Information for Proposed Standards. 

December 1983. 

3. Minutes of EPA Meeting with Representatives of the Wool Fiberglass Industry and NAIMA. 

August 31, 2011.

4. Notes from Steffan Johnson on Caustic Scrubbing of Hexavalent Chromium Emissions from 

Industrial Sources. October 26, 2011.