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| GLASS &
CERAMICS INDUSTRY |
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Glass industry
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The major areas where energy conservation
can be thought of in a glass industry are
- Raw material section
- Melting furnace
- Auxiliaries
Raw Material
Section
Beneficiation
Silica sand is the basic raw material for the glass
industry. The quality of silica sand available in India
is poor which especially contains high iron oxide and
needs beneficiation. The beneficiation is basically
washing with water which reduces contaminants to acceptable
limits. The optimum iron oxide (Fe2O3) content is 0.03
to 0.08%. We do not have organized sectors in our country
beneficiating the sand at source and supplying to the
industry. Many of the glass manufacturing units have
sand washing plants in the factory itself. They transport
about 10 to 20% of unwanted material which is rejected
in processing at the manufacturing facility. A captive
sand washing plant should be designed for installation
at the sand mine either individually or collectively
by a number of companies. This would result in conservation
of energy in transporting.
Hence the high capacity glass factories over 200 TPD
can individually or collectively consider installing
captive sand washing plants at or close to sand mines.
Drying Typically, natural drying (solar drying) is
carried out after washing to minimise the moisture.
The allowable moisture in sand is 3 to 4%.
Melting Aids Calumite which is processed blast furnace
slag accelerates melting. Hence calumite should be introduced
in the raw batch composition.
Grain size Both coarse grains and very fine grains
should be eliminated for better thermal performance.
Normally all sand should be between 30 and 80 mesh i.e.,
the grain size being between 500 and 180 microns.
There should be no grain coarser than 20 mesh or finer
than 100 mesh. Hence the acceptable grain size is between
+ 100 to - 20 mesh size.
Raw material mix
Generally a mix of fresh raw material (batch) and cullets
(recycled glass) is used as raw material input to the
furnace. Higher the percentage of cullets, better will
be the fuel efficiency. Another advantage is that the
cullets are cheaper. Fuel efficiency improvement to
the tune of 10 to 20% is possible by increasing the
cullet percentage to 90%. However there is a practical
limitation on cullet usage beyond 90%, due to retardation
of movement of raw material at filling point.
Pre-heating of raw material
Pre-heating of raw material is in initial stages of
development in developed countries. The pre-heating
is carried out by any of the following methods.
- Direct heating in case of cullets
- In-direct heating in case of fresh raw material
The pre-heating is feasible where large volumes of cullets
are handled.
Melting furnace
Melting furnace is the major consumer of energy in glass
industry. The energy conservation aspects to be considered
in design stage in melting furnace are highlighted in
the following sections.
Size of the furnace
Size of the furnace determines the operating efficiency
and the overall profitability of the company. The specific
fuel consumption varies with the size of the furnace.
Typical data is shown below
| Size Fuel |
Glass ratio |
| Upto 300 TPD |
1:10 |
| Upto 100 TPD |
1:5 |
The suggested minimum size of furnaces are to have economy
of scale as follows
| Type of Glass |
Size of the plant |
| Container Glass |
200 - 250 TPD |
| Float Glass |
500 TPD |
Campaign life
Campaign life of a melting furnace in glass industry
decides the overall economics of the plant. Higher the
campaign life, better will be the economics. The campaign
life in early 1950's was only 2 to 3 years and has been
increased to 6 to 8 years due to development in the
refractory material and technology. Few new plants have
incorporated good design features to achieve the campaign
life of 10 years. The range varies from 2 years in high
value optical glass electric melters and from 10 to
12 years in large float glass furnaces. Hence it is
prudent to incorporate appropriate design features to
improve the campaign life in all types of glass melting
furnaces.
Refractory material
Selection of type of refractory material and type of
joints play a major role in the life of the furnace
and energy consumption.
The suggested refractory materials are as follows
| Identification |
Type of refractory material |
| Glass contact area
|
AZS Electrocast with
Zirconia |
| Super structure |
AZS Zircon and Silica
|
| Crown |
Super Silica |
The joints have to be perfect and the gaps should be
minimum while constructing the furnace.
Oxy fuel burners
In the present context of environmental regulations,
oxy-fuel burner have an advantage of reduced Nox emissions.
In some developed countries oxy-fuel burners are established
and yielding good results from environmental point of
view. In our country, use of oxy-fuel burners are not
economical at the present costs. However from environmental
point of view, the incorporation of oxy fuel firing
may have to be resorted to confirm to the strict environmental
regulations in near future. Energy conservation by oxyfuel
is still a grey area.
Sealed in burner blocks
The underport oil firing system also plays a role in
fuel consumption. Conventionally an opening around the
burner is provided to allow induced cold air to cool
the burner tip. This cold air entry consumes higher
heat energy than hot air through regenerator. Firing
systems should be designed to seal the burner in the
burner block preventing any induced cold air. Now-a-days
"Sealed in burner blocks" are available in
the market and can save 3 to 5% of energy
Hence installation of a sealed in burner blocks for
burners should be considered for maximum energy saving
at design stage.
Insulation
Selection of right type of insulation reduces fuel consumption
at furnace enormously. In 1935, the overall efficiency
of furnace was about 19% and the present efficiency
is more than 50%. The major reason for their improvement
is the development of better refractory and insulating
materials. The insulating material has to be compatible
with refractory material.
Different areas of the furnace should be insulated with
specific insulating materials compatible with refractory
materials.
Pre-heating of furnace during start
up
Present trend during furnace start up is to pre-heat
the air up to 8000C and inject directly into the furnace.
After 800°C, direct heating with burners is practiced.
The rate of heat has also been increased from 1°C/hr
to 5 to 10oC/hr. With this approach the pre-heating
time would come down from 20 days to 6 days. This reduces
the over-all fuel consumption in pre-heating. All joints
have to be thoroughly sealed after pre-heating of furnace.
Waste heat recovery system
The typical waste heat system at the furnace is
Furnace exhaust Regenerator or Recuperator
Raw material
Chimney pre-heating
Regenerators and recuperators are used to recover waste
heat in the melting furnaces. Regenerators are relatively
more efficient when compared to recuperators.
Regenerator The checkered
work in modern regenerator is of Magnesite brick on
top and High alumina at the bottom.
Recuperator For low capacity
furnaces (10T/day) the recuperator can be of metallic/refractory
material. The metallic recuperators have a temperature
limit (Max. 900 to 1000°C). The refractory recuperators
can with stand high temperatures but the leakages should
be taken care of.
Instrumentation system
The flue gas analysis is essential to sustain efficient
combustion in furnaces. An on-line oxygen analyser can
be installed to check oxygen and fine tune the air fuel
ratio to achieve optimum combustion efficiency. Hence
installation of on-line oxygen analyser for melting
furnace is suggested.
Fans and blowers
The flux line cooling and throat cooling fans can be
fitted with variable speed drives (VSDs), preferably
variable frequency drives (VFDs). The advantage of these
drives is, depending on the capacity requirement at
the furnace, the capacity of fans can be varied automatically
and achieve optimum efficiency.
Hence install correct size fans for flux line cooling
and throat cooling fitted with variable frequency drives.
Lehrs
Generally, electrically heated annealing lehrs are used
to carry out annealing of glass products.
The following points are to be considered while selecting/operating
lehrs
- Explore the possibility of thermic fluid heated
lehrs instead of electrically heated lehr.
- Install atleast two standard size lehrs matching
the different product size. Operate the lehr, which
can allow little gap between the components and
the lehr body to minimise energy consumption.
- Recirculate hot air from central zone to side
zones should be practised.
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