SCOPE
3 PRESENT CONSUMPTION AND EMISSION LEVELS .1 Introduction
3.2 General overview of the glass industry
3.2.2 Process outputs
the products of fossil fuel combustion and the high-temperature oxidation of nitrogen in the combustion atmosphere (i.e. sulphur dioxide, carbon dioxide, and nitrogen oxides)
particulate matter arising mainly from the volatilisation and subsequent condensation of volatile batch materials
gases emitted from the raw materials and melt during the melting processes.
Where 100 % cold-top electrical heating is used, the emissions of combustion products and thermally-generated NOX are eliminated and particulate emissions arise principally from batch carryover. The partial substitution of fossil fuel firing with electrical heating will reduce direct emissions from the installation, depending on the level of substitution and the particular combustion conditions. Oxy-fuel firing greatly reduces the level of nitrogen in the furnace and so reduces the potential for NOX formation. There are usually off-site emissions associated with the generation of electricity and oxygen, which should be taken into consideration when assessing the overall environmental impact.
The furnaces encountered within the glass industry, and within each sector, vary considerably in size, throughput, melting technique, design, age, raw materials utilised, and the abatement techniques applied. Therefore, there is considerable variation in the emissions reported. There are also significant differences in the methodologies used for measuring emissions, and this can make direct comparisons of some actual data misleading. The minimum values are not always necessarily indicative of the best techniques and may only reflect more favourable operating conditions (e.g. high-volume stable production, or low-emission compositions) or plants with lower output. Clearly many of the lower releases represent those modern plants with advanced abatement measures, or ‘clean’ technologies. This issue has been taken into account in the determination of BAT-AELs which are discussed more fully in Chapters 4 and 4.9.
Air emissions are normally presented as concentrations (mg/Nm3) or mass emissions (kg/tonne of glass). All values given in concentrations refer to standard conditions: dry gas, temperature 273 K, pressure 1013 hPa. Unless stated otherwise, the standard conditions for the figures presented throughout the sections of Chapter 3 and the following chapters are given in Table 3.2.
Table 3.2: Reference conditions of emission data
Operating conditions Unit Reference conditions
Melting activities Conventional furnaces
(continuous melters) mg/Nm3 8 % oxygen by volume Conventional furnaces
(discontinuous melters) mg/Nm3 13 % oxygen by volume Oxy-fuel fired furnaces kg/tonne melted glass
The use of specific mass emissions (kg/tonne melted glass) is more appropriate. However, if emission concentrations are reported, the correction to a reference oxygen is not applicable Electric furnaces kg/tonne melted glass
or mg/Nm3
The correction of emission concentrations to a reference oxygen is not applicable
Frit melting furnaces (1) kg/tonne melted glass or mg/Nm3
Concentrations refer to 15 % oxygen by volume.
The specific mass emissions refer to one tonne of melted frit All types of furnaces kg/tonne glass The specific mass emissions refer to one tonne of melted glass Non-melting activities
All processes mg/Nm3 No correction for oxygen
All processes kg/tonne glass The specific mass emissions refer to one tonne of produced glass
(1) The use of concentrations (mg/Nm3) or mass emissions (kg/t glass) depends on the operating conditions (oxy-firing, oxygen-enriched air/gas firing-see Tabelle 5.1).
The main emissions arising from melting activities within the glass industry are summarised in Table 3.3.
Table 3.3: Summary of emissions to atmosphere arising from melting activities
Emission Source/Comments
Particulate matter
Volatilisation of batch components from the molten glass and subsequent condensation into submicron dust particles.
Carryover of fine material in the batch Product of combustion of some fossil fuels Nitrogen oxides
Thermal NOx due to high melting temperatures and prompt NOX formation Decomposition of nitrogen compounds in the batch materials
Oxidation of nitrogen contained in fuels Sulphur oxides
Sulphur in fuel
Decomposition of sulphur compounds in the batch materials in particular from the fining process with sulphates
Oxidation of hydrogen sulphide in hot blast cupola operations Chlorides/HCl
Present as an impurity in some raw materials, particularly synthetic sodium carbonate and external cullet
NaCl used as a raw material (fining agent) in some special glasses
Fluorides/HF
Present as a minor impurity in some raw materials, including external cullet
Added as a raw material in the production of enamel frit to add certain properties to the finished product
Added as a raw material in the continuous filament glass fibre sector to influence the forming process (surface tension) and in some glass batches to improve melting, or to produce certain properties in the glass, e.g. opalescence
Where fluorides are added to the batch, typically as fluorspar, uncontrolled releases can be very high
Heavy metals
(e.g. V, Ni, Cr, Se, Pb, Co, Sb, As, Cd)
Present as minor impurities in some raw materials, post-consumer cullet, and fuels Used in fluxes and colouring agents in the frits sector, in particular for enamel frits (predominantly lead and cadmium)
Used in some special glass formulations (e.g. lead crystal and some coloured glasses) Selenium is used as a colourant (bronze glass), or as a decolourising agent in some clear glasses and may generate both gaseous and solid emissions
Carbon dioxide
Combustion product
Emitted after decomposition of carbonates in the batch materials (e.g. soda ash, limestone)
Carbon monoxide Product of incomplete combustion, particularly in hot blast cupolas
Hydrogen sulphide Formed from raw material or fuel sulphur in hot blast cupolas due to the reducing conditions found in parts of the furnace
Heavy metal and trace element emission concentrations can be significant from some processes, and are generally present in the dust. Table 3.4 gives the classification groups generally used for metals emissions on the basis of their estimated relative potential environmental impact (see TA Luft 1986, French and Italian legislations).
Table 3.4: Classification of metals and their compounds Group 1 metals
and their compounds
Group 2 metals and their compounds
Arsenic Antimony
Cobalt Lead
Nickel Chromium III
Selenium Copper
Chromium VI Manganese
Cadmium Vanadium
Tin
Some actual examples of emission levels, taken from [42, VDI 1997][162, ICG-TC 13 2006], are shown in Table 3.5, which reports illustrative maximum figures for heavy metals not indicative of the use of BAT.
Table 3.5: Potential heavy metal emissions from glass processes without abatement
Metal Container glass Flat glass Lead crystal
glass Vanadium (when firing fuel oil) Up to 4 mg/Nm3 Up to 2 mg/Nm3
Nickel (when firing fuel oil) Up to 0.5 mg/Nm3 Up to 0.4 mg/Nm3 Chromium, total (green glass) Up to 3 mg/Nm3
Selenium, total (green container glass) Up to 0.8 mg/Nm3 Selenium, gaseous (flint hollow glass) Up to 14 mg/Nm3 Selenium, total (flint hollow glass) Up to 25 mg/Nm3
Selenium, total (float bronze glass) Up to 80 mg/Nm3
Lead Up to 4 mg/Nm3 Up to 1 mg/Nm3 Up to 700 mg/Nm3
Cadmium Up to 0.3 mg/Nm3 Up to 0.1 mg/Nm3
Antimony Up to 10 mg/Nm3
Arsenic Up to 20 mg/Nm3
Source: [42, VDI 1997] [162, ICG-TC 13 2006]
Downstream activities
This term is used to describe activities undertaken following melting, for example, forming, annealing, coating, processing, etc. The emissions from downstream activities can vary greatly between the different sectors and are discussed in the sector-specific sections. Although many of the sectors share some similar melting techniques, the downstream activities tend to be exclusive to each sector. In general, emissions to air can arise from:
the coating application and/or drying (e.g. mineral wool, continuous filament glass fibre, container glass, and some flat glass)
any activities performed on the materials produced such as cutting, polishing, or secondary processing (e.g. mineral wool, domestic glass, special glass, HTIW)
some product-forming operations (e.g. mineral wool, and HTIW).
Diffuse/fugitive emissions
Diffuse and fugitive emissions may be associated with different operations of the glass manufacturing process; however, in general, they do not represent a main concern for the sector.
The main sources of diffuse/fugitive emissions common to all the sectors of the glass industry are related to the following areas:
material storage and handling
the charging area of the furnace (doghouse)
the melting furnace.
Material storage and handling
Solid emissions may arise from sand and/or cullet deposited in open spaces and leakages from storage silos. Gaseous emissions may arise from the storage and handling of volatile liquids and/or gaseous chemicals, mainly related to downstream activities or flue-gas treatments (i.e.
ammonia storage). Information regarding the prevention and minimisation of diffuse/fugitive emissions from storage can be found in the Reference Document on Emissions from Storage (EFS BREF) [121, EC 2006]. In general, the impact of diffuse and fugitive emissions in the working area is managed by Health and Safety regulations at work, which include awareness and compliance. Occupational exposure limit values (OELs) have been set for a select number of substances at the European level, while many other OELs are based on national or international legislations and threshold limit value lists (e.g. European OSHA; ACGIH, US;
MAK, Germany, etc.). Diffuse emissions of respirable crystalline silica (silica sand, an essential component of the batch formulation for glass manufacturing, could give rise to respirable crystalline silica particles) are the subject of a European Social Dialogue Agreement:
‘Agreement on workers’ health protection through the good handling and use of crystalline silica and products containing it’, signed in 2006 [135, NEPSI 2006] [169, NEPSI-Good Practice Guide 2006].
Charging area of the furnace (doghouse)
Solid and gaseous emissions may arise from carryover, evaporation and decomposition phenomena from the charging of the batch formulation into the melting furnace. In general, the charging area (doghouse) is kept closed as much as possible in order to prevent both air infiltration and diffuse emissions. In some cases the doghouse area may be equipped with extraction systems that discharge outside or, less frequently, inside the building, close to the roof; in other cases, for specific types of furnaces, the doghouse is totally enclosed.
Melting furnace
Diffuse emissions may arise from combustion gases of the fossil fuel and from evaporation/condensation phenomena of the volatile components in the batch formulation. The melting furnace may not be totally sealed due to inspection holes, burner ports, and slits between the refractory bricks. An estimate of the volume of fugitive gases can be assessed through a mass balance of a significant pollutant, e.g. sulphur dioxide, proving that the amount of waste gases leaking from the furnace is quite low compared to the total waste gas volume produced during melting.
3.2.2.2 Emissions to water
In general, emissions to the water environment are relatively low and there are few major issues that are specific to the glass industry. In general, water is used mainly for cleaning and cooling and can be readily recycled or treated using standard techniques.
Most activities will use some liquids, often limited to water treatment chemicals, lubricants or fuel oil. All liquid raw materials pose a potential threat to the environment through spillage or containment failure. In many cases, basic good practice and design is sufficient to control any potential emissions. Specific issues relating to aqueous emissions are discussed in the sector-specific sections. As an example, a typical flow chart of water distribution in the container glass manufacturing industry is shown in Figure 3.1.
Preparation (unit operation, mixing) Fresh water stream
10 – 40 %
Separation
(purges, supernatant) Output wastewater 10 – 40 % Sludge,
residue vapour
Glass processing:
cooling 30 – 40 % forming, cleaning and
utilities 60 – 70 %
Recycled water stream 6 – 90 % Input water stream
Output water stream
Source:[101, Bruno D. BATwater 2007]
Figure 3.1: Typical water distribution in a container glass plant
3.2.2.3 Emissions of other wastes
A characteristic of most of the glass industry sectors is that the great majority of internally generated glass waste is recycled back to the furnace. The main exceptions to this are the continuous filament sector, the HTIW sector and producers of very quality-sensitive products in the special glass and domestic glass sectors. The mineral wool and frits sectors show a wide variation in the amount of waste recycled to the furnace ranging from nothing to almost 100 % for some stone wool plants. Other waste production includes waste from raw material preparation and handling, waste deposits (generally sulphates) in waste gas flues, and waste refractory materials at the end of the life of the furnace.
In some sectors of the glass industry, refractories which contain chromium are used for the construction of upper walls, crowns and regenerators. The chromium when combined with magnesia to form magnesium-chrome bricks is very resistant to batch carryover and combustion products at the high temperatures that exist in the regenerator chambers. The chromium used in the preparation of these materials, Cr3+, is essentially non-hazardous, has low solubility and presents little risk. However, at high temperatures under alkaline and oxidising conditions, small amounts of the chromium will convert to Cr6+ during the furnace campaign. Cr6+ compounds are highly soluble, toxic and carcinogenic.
As with all furnace waste, every effort is made at the end of a campaign to have the materials recycled. Where this is not possible, the Cr6+ content of the used mag-chrome refractories will be determined to ensure that they are correctly classified and disposed of appropriately. The industry is gradually reducing the amount of refractories which contain chromium by development and redesign.
Small tonnages of high-purity chromic oxide refractories may also be used. They are generally purchased on the basis that at the end of a campaign they will be taken back by the manufacturer for recycling. In most continuous glass filament furnaces, large amounts of this material are used.