Energy efficient and environmentally friendly cupola furnace
Prosanto Pal and Abhishek Nath
TERI, Habitat Place, Lodhi Road, New Delhi 110003.
Tel: 011-4682100 or 4682111, Fax. 011-468 2144 or 468 2145
Email: mailbox@teri.res.in
Background
There are over 6000 small-scale foundries in India. Most of these foundries are concentrated in geographical
cluster. The foundry industry is characterized by its high energy consumption and particulate emissions.
Melting is the most energy intensive process in foundries and also results in release of major air
contaminants. Cupola furnaces, is by far, the most common type of melting furnace in Indian foundries.
Majority of the cupolas in operation today are based on conventional energy inefficient designs. Rising cost
of energy inputs, increasing competition and environmental pressures are serious threats to the survival of
foundry units today.
In order to popularize an energy efficient and environmentally friendly cupola among Indian foundries, Tata
Energy Research Institute (TERI) has set-up a demonstration cupola melting plant at a foundry unit,
nominated by Indian Foundry Association (IFA) in Howrah. After setting-up of the model unit, TERI has
been trying to disseminate the demonstrated technology to other foundries in India.
The paper outlines the improvement in energy and environmental performance possible through adoption of
a properly designed divided blast cupola (DBC) and pollution control device.
Initial diagnosis
Energy audits of a range of cupolas were conducted by TERI in Howrah and Agra foundry clusters. The
charge coke percentage, which is a measure of energy efficiency of a cupola, was found to vary over a wide
range. The most energy efficient cupola was found to be using 13.6 percentage charge coke (coke:metal ::
1:7.5) and the least energy efficient cupola was operating at a charge coke percentage of 26.5 (coke:metal ::
1:4).
Some of the factors that contributed to the poor energy performance of operating cupolas were found to be
the following:
§ Incorrect blast rate
§ Lower blast air pressure
§ Incorrect distribution of air between the top and lower tuyeres
§ Turbulent (non-uniform) entry of air into the cupola
§ Incorrect sizing of cupola parameters such as tuyere area, well depth, and stack height among others
§ Poor operating and maintenance practices
§ Poor control of feed materials (shape, size, weight, sequence)
Advantages of DBC
Divided blast cupola (DBC) is a well-proven technology for improving the energy performance at a modest
investment. As si evident from its name, a DBC supplies blast air to the cupola furnace at two levels
through a double row of tuyeres. The blast air is almost equally divided between the top and bottom row of
tuyeres, and the spacing between the tuyeres is about one metre apart, irrespective of the diameter of the
cupola. Some comparative advantages of a DBC, as found in studies conducted by BCIRA, are given
below:
§
a higher metal tapping temperature (approximately 45-50oC more) and higher carbon pick-up
(approximately 0.06%) are obtained for a given charge-coke consumption
§
charge-coke consumption is reduced by 20-32% and the melting rate is increased by 11-23%, while
maintaining the same metal tapping temperature.
However, in the initial survey conducted at Agra and Howrah foundry clusters, it was found that
conventional cupolas are commonly used by Indian foundry units and DBCs, where ever adopted, are of
sub-optimal designs. Hence the intervention aims to demonstrate and disseminate the benefits of a well designed DBC among Indian foundries.
TERI's DBC design
TERI's DBC design incorporates the specific melting requirements of the individual foundry unit. Salient
features of the cupola design include:
§
Optimum selection of blower specifications (quantity and pressure)
§
§
§
§
§
§
§
§
Optimum ratio of the air delivered to the top and bottom tuyers
Minimum pressure drop and turbulence of the combustion air
Separate wind-belts for top and bottom tuyeres
Correct tuyere area, tuyere number and distance between the two rows of tuyeres
Optimum well capacity
Higher stack height
Mechanical charging system
Stringent material specifications
Energy savings and other benefits
A demonstration plant was installed at Bharat Engineering Works, Howrah, a unit nominated by the Indian
Foundry Association (IFA). The foundry, manufacturing ingot moulds, had a charge coke percentage of
13.6 percent (coke:metal :: 1:7.5) which was brought down to 8 percent (coke:metal :: 1:12.5). Hence, the
energy saving achieved in the new plant was about forty percent co mpared to their earlier cupola. On an
average monthly melting of 430 tons, the yearly saving in coke is 270 tons which is equivalent to Rs. 8 lakh.
Additionally there was an increase in metal tapping temperature and reduction in silicon and manganese
losses.
Energy saving of about 40 percent was achieved in a replication unit set-up at a foundry unit in Nagpur
which make thin walled sanitary castings. The charge coke consumption reduced from 22 percent
(coke:metal :: 1:4.5) earlier to about 13 percent (coke:metal :: 1: 7.7) . This translates to a coke saving of
280 tpa worth about Rs. 11 lakh on a melting of 300 tons per month in the foundry. The total capital
investment of the cupola, inclusive of civil work, platforms, bucket charging system etc.) was about Rs. 12
lakh. Thus the payback on the investment is one year considering saving in coke only. Additional benefits
of the DBC were better analytical and temperature control of molten metal leading to substantial reduction
in rejection of finished castings. The payback is more attractive if the decrease in rejection rate of finished
casting on account of better analytical and temperature control is considered.
70
65
Energy saving (%)
60
50
40
30
20
35
40
25
10
0
Cupola at Howrah
Cupola at Nagpur
Range of saving expected
in other cupolas
Comparison of energy performance
Pollution control device
Environmental measurements were carried out by TERI at different locations to assess the emission level of
untreated cupola stack gases. The suspended particulate matter (SPM) emissions without any gas cleaning
device was found to vary in the range of 1300 mg/Nm3 to 3900 mg/Nm3 . The measu rements show that the
emissions from cupola are much higher than the statutory limits and hence a pollution control device needs
to be installed along with the cupola to comply with environmental regulations.
TERI's venturi scrubber system
The venturi scrubber is a highly efficient device for removing particulate matter and sulphur dioxide from
stack gases. Since cupola stack gases contain a significant percentage of fine particulates, it was found that a
venturi scrubber is the most effective device to bring down the emissions below the more stringent
particulate emission limit of 150 mg/Nm³.
Some of the salient features of the venturi scrubber design are:
§ Variable venturi throat to clean the gas by binding the particles to water droplets
§ Optimum gas velocity at the throat, liquid/gas ratio and throat geometry for maximum efficiency
§ Dewatering cyclone after venturi to retain water droplets in the gas stream
§ Induced draft fan to ensure sufficient pressure drop
§
§
§
§
Stainless steel construction to prevent corrosion
Closed circuit recirculation to minimize water requirement
Lime dosing to maintain the pH of the recirculating water
Explosion doors and gas tight construction to prevent explosion
SPM and sulphur dioxide of the outlet gas from the pollution control device installed at a foundry in
Howrah was measured. The SPM was found to be about 50 mg/Nm3 and sulphur -dioxide was measured to
be about 40 mg/Nm3 . The results have been validated by the monitoring done by the pollution control
authorities independently. Bharat Enginerring Works, Howrah, the foundry unit where the pollution control
device is installed, has recently won the prestigious Environmental Excellence Award 2000-2001 given by
West Bengal Cleaner Production Centre, set up jointly by the Indian Chamber of Commerce and the West
Bengal Pollution Control Board.
Comparison of suspended particulate matter (SPM) emissions
Conclusion
The results of the demonstration project show that there is a huge potential for energy saving and pollution
reduction among foundries in India. Technical assistance for the design and implementation of energy
efficient cupola melting furnace is available at subsidized cost to foundry units desirous of upgrading their
melting units. The following services are provided to t he foundry units by TERI:
• Energy and environment audit of their existing cupola
• Design of DBC and pollution control device
• Advisory support during vendor selection, fabrication and installation
• Assistance during test firing and fine-tuning of the plant
• Training of operators on best operating practices
• Post implementation energy and environment audit to quantify energy savings and environmental
performance.
Services of the best available international foundry technology expertise can be availed by the Indian small scale foundry units at subsidized cost, since the intervention is supported by the Swiss Agency for
Development and Cooperation (SDC).
It is hoped that the Indian foundry industry would avail of the services being offered to modernize their
melting operations.