SCOPE
Furnace 80 %Other 5 %
3.7 Special glass
45 % Melting energy
22 % Forming and downstream processes 28 % Annealing/
tempering lehr
5 % Other
(includes building heating)
Source:[140, Domestic Glass 2008]
Figure 3.8: Energy usage in soda-lime-silica glass tableware production
Some other processes within the sector, particularly lead crystal production, are carried out on a much smaller scale and pot furnaces may be used. The energy usage distribution for lead crystal glass production differs significantly from one plant to another, with a variation in the energy required for the melting process from 16 to 85 % of the total energy consumption.
The overall energy consumption for lead crystal manufacture can be even higher (up to 28 GJ/tonne of finished product), when the calculated theoretical energy requirement for melting from normal raw materials is only around 2.5 GJ/tonne. The difference can be due to many factors, but the main ones are given below.
High-quality requirements may lead to high reject levels. The pot is slowly dissolved by the glass, leading to cords and stones in the product.
The glass is frequently hand worked and the yield from forming may be below 50 %, and the articles may need reheating during forming.
The pots have to be ‘founded’ or fired up to a high temperature before use, and they have a very limited lifetime compared to continuous furnaces.
Electric melting of lead crystal allows for the use of high-quality refractories, which give a much higher glass quality and therefore lower reject rate and better yield. The continuous nature of electric melting and the fact that there are not hot flue-gases from combustion often result in a more efficient automated forming. However, the overall energy demand including the downstream activities can lead to energy consumption close to the figure of 25 GJ/tonne of product.
3.7.1 Process inputs [26, Special 1998]
The chemical composition of the special glass varies depending on the glass type and the end use, and is generally expressed in terms of the oxides of the elements it contains. It is difficult to identify ‘typical’ batch compositions for such a diverse sector. The basic raw materials are selected and blended to give the final desired glass compositions following melting. The typical glass types and composition ranges are shown in Section 2.8. Table 3.33 shows the main raw materials used to achieve these compositions.
More detailed information is given for the inputs of glass ceramics, borosilicate glass tubes and soda-lime glass bulbs in Table 3.34, where data concerning four specific example processes are reported.
Table 3.33: Materials utilised in the special glass sector
Description Materials
Glass-forming material Silica sand and high-purity quartz sand, process cullet Glass intermediate and
modifying materials
Sodium carbonate, potassium carbonate, limestone, dolomite, alumina, aluminium hydroxide, zirconium oxide, borax, boric acid (pure for some applications), carbon, lead oxide, titanium oxide, tin oxide, strontium carbonate, lithium carbonate, barium carbonate, spodumene, fluorspar, nepheline syenite, feldspars, sodium chloride, phosphates
Glass oxidants and fining agents
Sodium sulphate, sodium nitrate, potassium nitrate, arsenic (As2O3), antimony (Sb2O3), carbon
Glass colouring agents Iron chromite, iron oxide, cobalt oxide, selenium or zinc selenite, cerium Fuels Fuel oil, natural gas, electricity, butane, propane, acetylene
Water Mains supply and local natural sources (wells, rivers, lakes, etc.)
Ancillary materials
Packaging materials including plastics, paper, cardboard, and wood Mould lubricants, generally high-temperature graphite-based release agents Machine lubricants, predominantly mineral oils
Process gases including nitrogen, oxygen, hydrogen and sulphur dioxide Water treatment chemicals for cooling water and waste water
Table 3.34: Overview of inputs and outputs for example glass ceramic, borosilicate glass tubes and soda-lime glass lamp bulbs processes
Glass ceramic
Glass tubes (borosilicate)
Glass lamp bulbs (soda-lime)
Type of furnace Oxy-fuel Oxy-fuel Cross-fired
regenerative
Cross-fired regenerative
Furnace capacity 30 – 65 t/d 10 – 55 t/d 10 – 55 t/d 50 – 150 t/d
Inputs Units/tonne
melted glass
Energy, gas GJ 5.5 – 11 10 – 15 14 – 17 5 – 14
Energy, electricity GJ 1 – 8
SiO2 (calculated) kg 660 – 685 740 – 760 740 – 760 400 – 700
Al(OH)3
(calculated) kg 310 – 340 22 – 26 22 – 26
CaO, CaCO3 kg 18 – 22 18 – 22 100 – 400
K2O, K2CO3 kg 20 – 100
Na2CO3, Na2O kg 22 – 28 22 – 28 100 – 300
CaF2 kg 3 – 7 3 – 7
TiO2 kg 12 – 45
Li2CO3
(calculated) kg 85 – 110
B2O3 kg 220 – 240 220 – 240 10 – 100
NaNO3, KNO3 kg 9.5 – 15 20 – 25 20 – 25 50 – 250
ZrO2 kg 12 – 45
ZnO kg 12 – 45
Minor mineral
ingredients kg 3.5 – 10 1 – 2 1 – 2 0.5 – 20
Internal cullet kg 250 – 550 200 – 400 150 – 350 100 – 500
Water m³ 1.5 – 2.5 1.7 – 2.8 1.7 – 2.8 Closed water circuit
Outputs Emissions to air Waste gas abatement
system Bag filter Bag filter/ESP Bag filter/ESP ESP
CO2 kg 410 – 500 900 – 1150 950 – 1300 400 – 600
NOX (as NO2) kg 3.6 – 6.5 5 – 8 7 – 12 0.1 – 6
SOX (as SO2) kg 0.02 – 0.07 0.02 – 0.07 0.01 – 0.05
HCl kg 0.02 – 0.08 0.02 – 0.08 0.02 – 0.08
HF kg 0.002 – 0.004 0.002 – 0.004
Dust kg 0.001 – 0.08 0.001 – 0.08 0.001 – 0.08 0.001 – 0.08
Heavy metals kg 0.003 – 0.02 0.001 – 0.02 0.001 – 0.02
Waste water m³ 0.8 – 1.5 1 – 1.6 1 – 1.6 closed water circuit
Source: [141, Special glass 2008]
3.7.2 Emissions to air
3.7.2.1 Raw materials
In most special 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.
However, a characteristic of this sector is that some batch plants are relatively small and due to the specialised nature and lower volumes of some of the products, there is a higher level of manual (and semi-manual) handling and transfer. Emissions from these activities will depend on how well systems are controlled. Clearly where materials containing potentially more toxic compounds (e.g. lead oxide, arsenic, etc.) are handled, there is the potential for emission of these substances.
3.7.2.2 Melting
In the special glass sector, the greatest potential environmental emissions are emissions to air from melting activities. The main substances emitted and the associated sources are identified in Section 3.2.2.1. The wide range and specialised nature of the products of the special glass sector lead to the use of a wider range of raw materials than encountered in most other sectors. For example: CRT funnels and some optical glasses contain high levels of lead of over 20 % and up to 70 %; certain glass compositions may involve the use of specialised refining agents such as oxides of arsenic and antimony; and some optical glasses can contain up to 35 % fluoride and 10 % arsenic oxide. Emissions of fluorides, lead, arsenic and other metals are directly related to the use of compounds which contain these substances in the batch.
Due to the diverse nature of the sector, most of the melting techniques described in Chapter 2 can be found. However, the low volumes of production mean that most furnaces are quite small, and the most common techniques are the use of recuperative furnaces, oxy-gas furnaces, electric melters and day tanks. Regenerative furnaces are also used; for example, they were applied for the production of CRT glass and now, more rarely, in the production of borosilicate glass tubes or other glass types (e.g. soda-lime silica glass bulbs). The melting temperatures of special glasses can be higher than for more conventional mass-produced compositions. CRTs, borosilicate glass and glass ceramics, in particular, require melting temperatures of more than 1650 °C.
These high temperatures and complex formulations can lead to higher emissions per tonne than for example, soda-lime products. The higher temperatures favour higher rates of volatilisation and NOX formation, and the greater use of nitrate-oxidation agent or sulphate fining agents can result in higher NOX, SO2, and metal emissions. The lower scale of production coupled with higher temperatures also means that energy efficiency is generally lower.
Emission levels for a particular furnace can depend on many factors, but principally batch composition, furnace type, abatement techniques utilised, the operation of the furnace and the age of the furnace. Emission levels expressed in kg/tonne of melted glass product are given in Table 3.34 for four different example processes.
3.7.2.3 Downstream activities
Emissions from activities downstream of the furnace are very case specific and must be considered for each site. However, there are some general issues.
Several types of products may require cutting, grinding and polishing, which could lead to emissions of dust and for some products (e.g. optical glass and CRT funnels and panels), lead may be present in the emissions. These operations are usually carried out under liquid or have air extraction and dust filtration. Thus emission levels are generally very low.
3.7.2.4 Diffuse/fugitive emissions
The main sources of diffuse/fugitive emissions specific to the special glass sector may vary with the type of glass article produced. They usually concern the doghouse area of the furnace, forehearth channels, forming area and fire-finishing operations.
Emissions from the batch-charging area (doghouse) are related to carryover of batch composition (dust emissions) and combustion gases from the furnace, and are in common with the container and domestic glass sectors.
When discontinuous furnaces are used for the production of glasses with batch formulations which contain potentially harmful raw materials (e.g. compounds of As, Sb, Pb, F), an extraction system may be present over the charging area of the pot furnace or day tank, conveying the diffusing waste gases to a treatment system.
Combustion gases and evaporation products may be released from the forehearth channels.
In the forming area, mists of mineral oil and other lubricating products may be released.
Combustion gases may arise from the thermal treatment of the moulds and from the annealing lehr.
Fire-finishing operations produce combustion gases which are normally released in the ambient atmosphere.
Measures to avoid any leakage, spilling and fugitive emissions, together with the control of ammonia usage, are normally applied when SCR and SNCR techniques for NOX abatement are operated in special glass installations.
In general, these sources do not give rise to significant emissions to air and most issues are managed according to health and safety regulations.
3.7.3 Emissions to water
As with other sectors of the industry, the major water uses include cooling and cleaning, and aqueous emissions will contain the cooling water system purges, cleaning waters and surface water run-off. In general, the cleaning waters do not present any particular issues that would not be common with any industrial facility, i.e. inert solids and potentially oil. Cooling system purges will contain dissolved salts and water treatment chemicals. Surface water quality will depend on the degree of drainage segregation and site cleanliness.
However, the diversity of the sector means it is not possible to identify all of the potential emissions, and each case must be assessed specifically. The raw materials used for each product and the processing undertaken must be considered. Any potentially harmful raw materials used on site will have the potential to enter waste water streams, particularly where materials are handled and products are cut or ground. For example, the grinding and polishing of articles, such as CRT funnels and some optical glasses, may generate an aqueous stream which contains the grinding and polishing aids and fine glass containing lead. In general, solids will be removed and the liquid will be recycled as far as practicable, but there will be a certain level of discharge and a potential for spillage. Some quantitative data concerning the specific water consumption and discharges per tonne of melted glass are provided in Table 3.34 above for four example processes.
3.7.4 Other wastes
In general, most internally-generated glass waste (cullet) is recycled back to the furnace and waste levels are generally quite low. General wastes from packaging and furnace repairs are the same as with other sectors. Waste from dust control systems and dry scrubbing are recycled to the furnace where practicable. In processes involving grinding and cutting, the sludges separated from the water circuits must be disposed of if they cannot be recycled or reused. Some quantitative data concerning the use of internal cullet back to the melting process is provided in Table 3.34 above, for four example processes.
3.7.5 Energy
For such a diverse sector, it is very difficult to give general information on energy consumption.
In Table 3.34 specific energy consumption data for the melting furnaces are indicated for three different types of products, ranging from a minimum of 5 GJ/tonne up to 17 GJ/tonne of melted glass, depending on the type of product, furnace size and melting technique. A wide variation of energy consumption data may be observed depending on the batch formulation, the melting technique, and how the plant is designed and operated. Data in the range of 12 – 16 GJ/tonne of finished product have been reported in particular for soda-lime silica glasses [tm29 Infomil][30, Infomil 1998]. [75, Germany-HVG Glass Industry report 2007] [111, Austrian Special glass plant 2006].
The general description in Section 3.2.3 is applicable to this sector and the discussion of energy efficient techniques in Chapter 4 provides further information. Considerations specific to special glass are that the melting temperatures for special glasses are generally higher than those for mass produced glasses, and that special glass furnaces are, in general, smaller than in other sectors of the glass industry. Both of these factors result in higher CO2 emissions and higher specific energy consumption.