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Date: September 29, 2014

Subject: Development of Background Information on 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. 2-09

RTI Project No. 0213199.002.009

From: Cindy Hancy

Dave Reeves

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

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 HAPs. 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. 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 114 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

 

1 See Urban Area Toxics information at http://www2.epa.gov/urban-air-toxics. 
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information requests included submission of any previous emissions tests for chromium and 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 main types of gas-fired glass-melting furnaces are air-gas and oxyfuel. The main 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 

(3,5000 F. inside the furnace) than air gas furnaces (2,4000 F), whose temperatures are reduced 

by the dilution of oxygen in the outside air. 

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. Oxy-fuel furnaces are expected to replace all airgas furnaces and many electric furnaces as the number of sources of industrial oxygen increases.

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 
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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. 

Chromium compounds make up 95-100% 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 gas-fired 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.

Figure 1. Furnace PM Emission Rate by Control Device

V. Glass-Melting Furnace Chromium Emissions

 

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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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.

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 

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 

Oxy Fuel Air Gas ESS Electric Melt
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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 2,5000 to 

4,5000F, while the temperatures 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.
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Provided by Owens Corning during December 2012 site visits.

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., Cr+6). 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

 

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Information obtained during site visits in December 2012 indicated that differences in furnace designs, placement 
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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 proposing to set GACT limits on 

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

In order to determine GACT for chromium emissions 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 emissions, 

a fraction of which are chromium compounds. The EPA then evaluated the available data on 

controlled emissions shown in Figure 2 from gas-fired glass-melting furnaces and then 

determined the control level that appears to be generally achievable for most sources. The design 

and operating of a glass-melting furnace would be exactly the same regardless of whether or not 

the furnace is located at a major source or an area source. Therefore, the emission characteristics 

of a glass-melting furnace located at an area source versus a major source are identical. In this 

case it is appropriate to use the data from both types of sources to determine GACT. The data in 

Figure 2 show that generally chromium emissions are well below 0.0001 lb/ton of melt for most 

facilities, and the facilities above that level emit above 0.00015 lb/ton. Therefore EPA initially

determined that an emissions level below 0.0001 lb/ton of melt would represent GACT.

As a second step in determining GACT, EPA compared the emission level above to the 

chromium emissions level which was proposed for gas-fired furnaces located at major sources as 

part of the technology review required by section 112(d)(6) of the CAA. In the technology 

review analysis, which included furnaces located at major sources only, the EPA had determined 

that a chromium emissions level of 6.0E-5 pounds of chromium compounds per ton of glass 

pulled reflects a level achievable by most furnaces that are not considered high emitters. The 

basis for a GACT determination and a technology review are similar, in that the EPA determines 

what levels of control are generally achieved by the control technology. As noted above, 

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

 

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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emissions. Therefore, it is reasonable to set GACT for area sources equal to the emissions limit 

based on technology review for major sources, namely 6.0E-5 pounds of chromium compounds 

per ton of glass pulled. We note that reducing GACT from 0.0001 to 6.0E-5 only results in one 

additional furnace being above the proposed limit, and that most furnaces are still generally 

below this level.

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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