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MEMORANDUM 

Date: June 24, 2015 

Subject: Development of Background Information on the Final Area Source Emissions 

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

 

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

 

To: EPA Air Docket EPA-HQ-OAR-2010-1042 

I. Introduction 

Section 112(c)(3) and (k) of the Clean Air Act (CAA) require the U.S. Environmental 

Protection Agency (EPA) to identify and list the area source categories that represent 90 percent 

of the emissions of the 33 urban air toxics (urban HAP) associated with areas sources and to 

subject them to standards under section 112(d). Section 112(k)(3)(B)(ii) also requires EPA to 

assure that area sources accounting for 90 percent or more of the 33 identified urban HAP are 

subject to standards. The 33 urban HAP are listed in EPA’s Integrated Urban Air Toxics 

Strategy.1 EPA may elect to promulgate standards for area sources under section 112(d)(5) that 

provide for use of generally achievable control technology (GACT) or management practices by 

affected sources to reduce emissions of these HAP. For the development of area source 

standards, GACT includes 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 gas-fired 

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. All wool fiberglass furnaces are located at sources which, prior to development of the 

MACT standard in 1999, were major sources. Since that time, two-thirds of the facilities have 

become area sources due to the phaseout of phenol/formaldehyde binders on the bonded lines. 

No change is made to the furnace when facilities undergo this change, and we therefore 

considered all furnaces in the source category (regardless of whether they were located at major 

or area sources) in the GACT determination. 

During development of the residual risk and technology review for the major source 

NESHAP for wool fiberglass manufacturing (Subpart NNN), both the industry trade association 

(NAIMA) and EPA sent information requests to all major and area source wool fiberglass 

facilities in 2010 and 2012, respectively. Site visits were conducted and several meetings were 

held with industry regarding current emissions control practices at wool fiberglass plants. Two 

information requests were sent to industry: the first, a voluntary ICR, collected source test data 

and other information on representative processes across industry; the second, a CAA section114 

 

1

 See Urban Area Toxics information at http://www2.epa.gov/urban-air-toxics. 
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information collection request to conduct source testing at each wool fiberglass furnace in 

addition to providing information regarding furnace life, rebuild history, and refractory 

composition. The information requests included submission of any previous emissions tests for 

chromium and particulate matter (PM), in addition to new PM and chromium testing. Because of 

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

documents were submitted as confidential business information (CBI), the submitted test data is 

not included in this memorandum. 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 docket No. EPA-HQ-OAR-2010-1042. 

II. Glass-Melting Furnaces 

Glass-melting furnaces are heated by two types of energy sources: natural gas and 

electricity. Most furnaces are heated by only one energy source but a few are heated by a 

combination of both. The two general types of gas-fired glass-melting furnaces are air-gas and 

oxy-fuel. The general types of electric glass-melting furnaces currently used in the industry are 

steel shell and cold-top electric. Furnace diagrams were provided by NAIMA at an industry 

meeting in December 2012 (see Appendix). 

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

materials. Electric furnaces are started empty, heated with natural gas (during startup only), and 

when the furnace is sufficiently hot, clean cullet is added. When the furnace is full of molten 

glass, electricity is allowed to flow to the electrodes and a ‘cap’ of raw material is added to the 

furnace to contain the heat and maintain a molten furnace interior. 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. Molten glass flows from the electric furnace to the spinning process. 

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. 

Air-gas furnaces use ambient air as the combustion oxygen source, oxy-fuel furnaces mix 

natural gas with industrial-grade (99 percent 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, oxy-fuel fired furnaces operate with higher temperatures above the melted glass line 

(5,000 0F inside the furnace) than air gas furnaces (3,400 0F), whose temperatures are reduced by 

the dilution of oxygen in the outside air. According to industry data, only one air-gas furnace is 

still in operation in the US; all others have been replaced with oxyfuel furnaces. This is because 

permitting of an air-gas furnace involves accounting for emissions of PM and nitrogen oxides 

(NOx), which (because air-gas furnaces use outside air) are much higher for air-gas furnaces 

than for oxyfuel furnaces. Oxy-fuel furnaces are expected to replace all air-gas furnaces and 

many electric furnaces as the number of sources of industrial oxygen increases. 

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, recuperative 

air gas, or oxy-fuel). An average gas-fired furnace produces roughly three to four times the wool 

fiberglass of an average electric furnace. At least one gas-fired furnace also uses electric boost to 

provide additional heat at the start of the melt. 
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III. Glass-Melting Furnace Emissions 

Glass-melting furnaces emit particulate matter (PM), a fraction of which occurs as metals 

entrained in the particulate. 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. The gas-fired furnace 

refractory materials above the glass melt is a second and more significant source of chromium 

from these types of furnaces. Due to the high temperatures and corrosive nature of the gases at 

and above the melt (i.e., the glass-metal line), the furnace refractory wears with age; the 

refractory erodes in a process that releases refractory particles into the furnace exhaust as PM. 

Depending on the location in the furnace, chromium compounds make up 50-100 percent of the 

refractory products used to construct gas-fired furnaces at and above the glass-metal line.2,3 This 

results in much higher chromium emissions from gas-fired furnaces than from electric furnaces. 

Source tests show chromium constitutes an average of 1 percent of the PM emissions from gasfired furnaces, a 14 fold increase above the average for electric furnaces, which is 0.07 percent. 

IV. Glass-Melting Furnace PM Emission Controls 

The types of control devices used to control PM emissions (which include chromium) 

vary by furnace type. Steel shell electric furnaces are totally enclosed above the melt line and are 

equipped with a baghouse and/or fabric filter. Cold-top electric furnaces, which are not totally 

enclosed above the melt line, use a water spray to form a crust on the surface of the new material 

as the method of PM control. Air-gas furnaces currently use either electrostatic precipitators 

(ESP) or baghouses. Oxy-fuel furnaces generally use dry ESP only (with one using a scrubber in 

series with an ESP). Figure 1 shows PM emission rates by control device for all reporting 

furnaces in the wool fiberglass manufacturing source category. 

V. Glass-Melting Furnace Chromium Emissions 

The EPA evaluated the chromium emissions data for major and area source glass-melting 

furnaces and analyzed the operational differences. The EPA found that electric furnaces do not 

emit chromium at the levels of gas-fired furnaces, and chromium emissions from electric 

furnaces are multiple orders of magnitude lower than the chromium emissions from gas-fired 

furnaces as shown in Figure 2. As noted earlier, source tests show chromium constitutes an 

average of 1 percent of the PM emissions from gas-fired furnaces and less than 0.1 percent from 

electric furnaces. 

 

2

 See Information on SEFPRO high chromium oxide products in docket EPA-HQ-OAR_2010-1042 

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ICR responses on furnace chromium content from wool fiberglass facilities (submitted as CBI). 
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Figure 1. Furnace PM Emission Rate by Control Device 

Figure 2. Chromium Compounds Emission Rates by Furnace Type 

If a source of reasonably priced oxygen is available, the oxy-fuel glass-melting furnace is 

the design favored for use by glass manufacturers due to the glass-melting furnace’s low NOx 

Oxy Fuel Air Gas ESS Electric Melt 
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emissions, low energy demands per volume output of glass, increased production rate 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. (US DOE, 2006) The oxy-fuel glass-melting furnace design mixes the 

natural gas fuel with pure (99 percent industrial-grade) oxygen for combustion, thus reducing 

NOx emissions. (Oxygen-Enhanced Combustion, Baukal 1998) 

Although all glass-melting furnaces are constructed using chromium refractories at and 

below the glass/metal line, oxy-fuel and some air gas glass-melting furnaces have other glassmelting furnace parts or areas constructed using chromium refractories, such as the crown, 

forehearth, sidewalls, and appendages. These differences in design and operation of gas-fired 

glass-melting furnaces cause higher potential chromium emission relative to electric glassmelting furnaces. 

The temperatures above the melt in gas-fired glass-melting furnaces range from 3,400 to 

5,000 0F, while the temperatures above the melt in electric glass-melting furnaces are a few 

hundred degrees. Since chromium refractories provide the greatest resistance to heat and wear 

of any refractory in use today, gas-fired furnaces are constructed using chromium refractories at 

various parts or areas of glass-melting furnaces. The presence of chromium above the glass melt 

line, high temperatures, and corrosive environment cause gas-fired furnaces to have a higher 

chromium emission potential. Additionally, studies indicate that under normal industrial 

temperatures and oxidizing conditions, trivalent chromium present in the refractory oxidizes to 

hexavalent chromium (Nassaralla 1998). 

The degradation of the glass-melting furnace refractory indicates increasing chromium 

emissions with furnace age. This process, however, 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 

steady-state deformation rates are not appropriate (DOE and Oak Ridge National Laboratory, 

June 2006 p. 63). The following photographs show examples of the refractory degradation below 

the melt line at two different Owens Corning furnaces. 

For more information on this topic, see “Mechanisms of Chromium Emissions From 

Wool Fiberglass Glass-Melting Furnaces”, June 2015, in the docket to the rule. 
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Provided by Owens Corning during December 2012 site visits. 
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Provided by Owens Corning during December 2012 site visits. 

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 (EPA/Industry Meeting Minutes 2011), and that much of the loss occurs below the 

glass-metal line, which is believed to remain in the glass (Brosnan 2012). 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 glass-metal line is emitted as particulate to the outside air. Since 

chromium compounds are a significant component of the refractory used above the glass-metal 

line, there is a greater potential increase in chromium emissions which are 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 oxy-fuel 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., Cr6+). Because the increase in 

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

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

limit for gas-fired furnaces.4

 

4

 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 
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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 

VI. Determination of GACT for Glass-Melting Furnaces 

Wool fiberglass manufacturing was listed as an area source category based on emissions 

of chromium compounds. In developing area source rules, the CAA allows EPA the flexibility to 

specifically regulate the pollutants of concern (e.g., chromium compounds). After analyzing the 

data provided in Section V of this memorandum, we are finalizing GACT limits on chromium 

emissions from gas-fired glass-melting furnaces at area sources. 

We initially proposed GACT standards for area sources in the Wool Fiberglass 

Manufacturing source category on April 15, 2013 (78 FR 22377). To develop the proposed 

GACT standards, the EPA evaluated the methods, practices and techniques that are 

commercially available for gas-fired glass-melting furnaces. Because the design, operating, and 

emissions characteristics of glass-melting furnaces are the same at major and area sources, the 

EPA used data from both types of sources to determine GACT. As shown in Figure 1, most gasfired furnaces use an ESP and/or baghouse to control PM emissions, a fraction of which are 

chromium compounds. The EPA also evaluated the available data on controlled emissions shown 

in Figure 2 from gas-fired glass-melting furnaces and determined the control level that appeared 

to be generally achievable for most sources. Based on the emissions data in shown Figure 2, the 

EPA initially determined that an emissions level below 0.0001 lb/ton of melt would represent 

GACT. The EPA also compared the initial assessment of the GACT emission level to the 

chromium emissions level (6.0E-5 pounds of chromium compounds per ton of glass pulled) that 

was proposed on November 25, 2011 for gas-fired furnaces located at major sources as part of 

the technology review required by section 112(d)(6) of the CAA. Because the basis for a GACT 

determination and a technology review under CAA section 112(d)(6) are similar (i.e., the EPA 

determines what levels of control are generally achieved by the industry), and because furnaces 

located at major and area sources are identical from the standpoint of chromium emissions, the 

EPA believed it to be reasonable to set GACT for area sources equal to the emissions limit based 

on technology review for major sources (6.0E-5 pounds of chromium compounds per ton of 

glass pulled). 

 

Based on comments we received in response to the proposed chromium compounds limits for 

gas-fired glass-melting furnaces the November 2014 notice, we reviewed the cost and control 

options and found using new cost information that the limit as proposed was not as cost effective 

 

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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as we initially believed. Consistent with the legislative history on section 112(d)(5), in 

determining GACT, we may consider costs and economic impacts, in addition to considering 

technical capabilities of the facilities and the availability of control measures, which is 

particularly important when developing regulations for source categories that may have few 

establishments and many small businesses, or when determining whether additional control is 

necessary for sources with emissions that are already well controlled as a result of other existing 

or applicable standards. (Senate Report Number 101-228, December 20, 1989).

Here, wWe determined that it was appropriate to modify the proposed limit of 6.0E-5 pounds per 

ton of glass pulled because the cost effectiveness for the emission reduction options ($660,000 

per pound of chromium reduced for the raw material substitution option, and $620,000 per pound 

chromium reduced for the furnace rebuild) were not reasonable compared to other cases where 

the EPA has regulated highly toxic pollutants, such as hexavalent chromium. We therefore 

reviewed the data to determine whether a higher limit than previously proposed would be more 

cost effective while still significantly reducing chromium emissions from wool fiberglass gasfired glass-melting furnaces. The EPA found that most wool fiberglass gas-fired glass-melting 

furnaces at major sources currently emit chromium compounds at rates below 2.5E-04 pounds 

chromium per ton of glass pulled. We compared the chromium emission reductions that would 

have resulted under the previously proposed emission limit of 6.0E-5 pounds chromium per ton 

of glass pulled to the reductions that result from the final limit of 2.5E-04 pounds chromium per 

ton of glass pulled. We found that the proposed limit would have reduced chromium from major 

sources by 567 pounds per year, and that the final limit reduces chromium by 524 pounds per 

year. Because these emissions reductions are comparable, we finalized the emission limit for 

2.5E-04 pounds chromium per ton of glass pulled for gas-fired glass-melting furnaces located at 

wool fiberglass facilities that are major sources. 

Under CAA section 112(d)(5), we may set the GACT emission limit for area sources that 

provides for the use of control technologies that are generally available to reduce HAP, and we 

are not precluded from setting the limits for area sources equivalent to the limits for major 

sources. Because there are no differences between gas-fired glass-melting furnaces in use at area 

and major sources, we believe that the control measure for reducing chromium emissions (i.e., 

furnace rebuild) used by major sources is generally available for area sources, and we are 

finalizing the same emission limit of 2.5E-04 pounds total chromium per ton of glass pulled for 

gas-fired glass-melting furnaces at area sources, under CAA section 112(d)(5). Although all gasfired glass-melting furnaces located at area sources emit below the final GACT standards, the 

final limit sets a backstop so that gas-fired glass-melting furnaces that have high chromium 

emissions cannot be operated at an area source in this industry. 
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References: 

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

Refractory. Metallurgical and Materials Transactions, 1998. 

2. U. S. EPA, 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. U.S. DOE Energy Efficiency and Renewable Energy, Industrial Technologies Program, Final 

Technical Report. Compressive Creep and Thermophysical Performance of Refractory 

Materials”. Oak Ridge National Laboratories. June 2006. 

5. Baukal, Charles E., Jr., Oxygen-Enhanced Combustion, 1998.

6. Brosnan, D.A., Chromium Emissions from Wool Fiberglass Melting Furnaces. February 

2012. 

 
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Appendix
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