SCOPE
3 PRESENT CONSUMPTION AND EMISSION LEVELS .1 Introduction
3.4 Flat glass
3.4.2 Emissions to air
[19, CPIV 1998] [65, GEPVP-Proposals for GLS revision 2007]
3.4.2.1 Raw materials
In most modern flat glass processes, silos and mixing vessels are fitted with filter systems which reduce dust emissions to below 5 mg/Nm3. Mass emissions from both filtered and unfiltered systems will clearly depend on the number of transfers and the amount of material handled.
3.4.2.2 Melting
In the flat glass sector, the greatest potential environmental emissions are emissions to air from the melting activities. The substances emitted and the associated sources are identified in Section 3.2.2.1. Almost all of the furnaces in this sector are fossil-fuel fired (both natural gas and fuel oil), cross-fired regenerative furnaces.
The overview of the furnaces equipped with systems for the control of air pollution in the flat glass sector is presented in the Table 3.23. Data presented in the table refer to a situation where the abatement of dust applied to the sector, generally coupled with a scrubbing system for acid gaseous pollutants (SOX, HF, HCl), consists of 34 electrostatic precipitators and one bag filter.
The control of NOX consists of SCR applications, Fenix technology, control of combustion parameters (primary measures) and the 3R technique.
Table 3.23: Overview of air pollution control (APC) systems installed in the flat glass sector in Europe
Year
Abatement of
dust, SOX, HCl, HF, metals Control/abatement of NOX Total number of furnaces APC(1) Equipped APC(1) Equipped
EU-15 2000 16 33.3 % 8 16.7 % 48
EU-25 2005 28 51.9 % 22 40.7 % 53
EU-27 2007 35 60.3 % 28 48.3 % 58
(1) APC= air pollution control systems.
Source: [65, GEPVP-Proposals for GLS revision 2007] [127, Glass for Europe 2008]
A summary of the range of emissions to air is given in Table 3.24 below. This table shows figures separately for furnaces without any abatement systems and furnaces with primary and/or secondary abatement techniques installed. The data cover both gas and oil-fired furnaces making clear float glass under normal operating conditions and show measurements from 2005.
Data reported are the result of a survey from members of the European flat glass trade association (Glass for Europe) and concern the EU-25. The statistical analyses of data might have produced results that show significant differences from the previous survey carried out within members of EU-15 for the elaboration of the first version of the BREF.
Note that sampling and measurement techniques used for the collection of data are not uniform and when standardised methods are used, the uncertainty of them is not taken into account in expressing the results.
Table 3.24: Emission levels from flat glass furnaces with and without abatement systems Substance Unabated furnaces in mg/Nm3
(kg/tonne glass melted)
Abated Furnaces
primary/secondary methods in mg/Nm3 (kg/tonne glass melted)
Oxides of Nitrogen
(as NO2) 1250 – 2870 (2.9 – 7.4) 495 – 1250 (1.1 – 2.9)
Oxides of sulphur
(as SO2) 365 – 3295 (1.0 – 10.6) 300 – 1600 (0.5 – 4.0)
Particulate matter 80 – 250 (0.2 – 0.6) 5.0 – 30 (0.02 – 0.08)
Fluorides (HF) <1.0 – 25 (<0.002 – 0.07) <1.0 – 4.0 (<0.002 – 0.01)
Chlorides (HCl) 7.0 – 85 (0.06 – 0.22) 4.0 – 40 (<0.01 – 0.1)
Metals other than Se
(Ni, V, Co, Fe, Cr) <1.0 – 5.0 (<0.001 – 0.015) <1.0 (<0.001) Selenium
(coloured glass) 30 – 80 (0.08 – 0.21) <5 (<0.015)
NB: Reference conditions are: dry, temperature 0 °C (273 K), pressure 101.3 kPa, 8 % oxygen by volume.
Source: [65, GEPVP-Proposals for GLS revision 2007]
The term ‘unabated furnaces’ refers to furnaces operating normally with no specific primary or secondary pollution control technology.
For unabated furnaces, the highest emissions of NOX were from highly loaded gas-fired plants, and the lowest are from oil-fired plants. The abated furnaces are equipped with primary measures like the Fenix process or by secondary measures like SCR (selective catalytic reduction) or 3R (addition of hydrocarbons fuel oil or natural gas, for the chemical reduction of NOX).
The highest emissions of SOX for unabated furnaces are from oil-fired plants and the lowest are from gas-fired plants.
The highest emissions of particulate matter for unabated furnaces are from oil-fired plants, and the lowest are from low loaded gas-fired plants with high cullet levels. The particulate matter emitted from an uncontrolled furnace is mainly derived from the condensation in the waste gases of soda and sulphate compounds volatilised from the melt. The main component of the particulate matter is sodium sulphate, a relatively harmless soluble compound. The other minor components are derived from the raw materials, the furnace structure, and fuel oil if it is used.
For unabated furnaces, the highest emissions of HCl and HF are from plants with relatively high levels of chlorides and fluorides in the raw materials.
The highest emissions of metals from unabated furnaces are from oil-fired plants (the nickel and vanadium content of the fuel oil) or those using colouring agents (Se, Co, Fe and Cr), and the lowest are from gas-fired plants producing clear glass.
The ranges of emissions of SOX, dust, HCl, HF and metals from abated furnaces are associated with installations operating particulate abatement systems (an electrostatic precipitator and, in one case, a bag filter), in combination with acid gas scrubbing, in order to meet local permit requirements. Under these conditions, emissions of metals are often beneath detection limits.
When tinted glasses containing selenium as the colourising agent are produced, the uncontrolled emissions of selenium are typically between 30 and 80 mg/Nm3. The emissions are normally less than 5 mg/Nm3 with values in the range of 1–3 mg/Nm3 when secondary measures are applied (filtration combined with acid gas scrubbing). Very few experiences exist today on the abatement of selenium emissions from float glass furnaces, especially in the case of fuel-fired furnaces.
The efficiency of the control equipment depends on the type of reagent and the presence of other gaseous pollutants (concurrent species to be absorbed) in the flue-gas, such as SOX, with the consequence of competitive parallel reactions.
3.4.2.3 Downstream activities
Because of the air-tight sealing of the tin bath section, the emissions of tin vapour from the float bath have been found to be very low and these are generally monitored only to ensure low workplace exposure levels. This issue is not considered further in this document.
Hot treatment of the flat glass surface at the exit of the float bath is normally carried out with the purpose of improving the chemical resistance of glass. The process requires the use of SO2
with subsequent gaseous emissions, typically in the range of 150 – 300 mg/Nm3 (0.02 – 0.04 kg/tonne glass melted) [84, Italy Report 2007]; however, in the case of special productions the emissions may be higher.
The on-line coating processes applied to flat glass are very case specific and the raw materials used and the pollutants emitted will vary. Among the coating technologies, one of the most important is on-line pyrolytic chemical vapour deposition (CVD) involving the use of a gaseous chemical mixture which reacts with the hot surface of the glass leading to the deposition of a coating which bonds to the glass. A variety of materials consisting in general of metals and oxides are deposited on the glass surface.
The coating application by the sputtering of metals from metal targets onto the glass surface is generally performed off-line at very low pressure in dedicated vacuum chambers. Emissions will typically contain acid gases (HF, HCl) and fine particulate matter (e.g. oxides of silicon and tin).
In general, downstream activities do not generate a significant source mass emission, although, they are usually subject to the general local environmental legislation and abatement systems are installed accordingly. Limited information is available concerning emissions levels from these activities. Typical emission limit values applied are, for example, HCl: 10 to 30 mg/Nm3, HF: 5 mg/Nm3 HF, particulate matter: 20 mg/Nm3, and tin compounds: 5 mg/Nm3.
The production of mirrors represents another important downstream activity for the flat glass sector. The process and related emission levels will not be discussed here since it is already covered in the Surface Treatment Using Organic Solvents (STS) BREF
[139, European Commission 2007].
3.4.2.4 Diffuse/fugitive emissions
The main source of diffuse/fugitive emissions in the flat glass sector is related to the batch charging area of the melting furnace.
Emissions of dust from batch carryover, combustion gases which contain volatile compounds present in the batch formulation are the main issues. Selenium used for colouring the glass may be present in the emissions from the charging area.
Extraction systems are often used to discharge emissions from the charging area into the atmosphere and bag filters are applied to remove dust.