Natural-gas-based pot and muffle furnaces for glass industry

Natural-gas-based pot and muffle furnaces for glass industry

Widening a bottleneck

A polluting glass baking unit in Firozabad

Taj Mahal, the architectural wonder that brings the world to Agra's courtyard, has been robbed of its grandeur. Polluted air from various sources, including the teeming industrial units in its vicinity was believed to cause the damage.

But, beyond the fear of a medieval glory being hidden behind fumes, the looming reality is that the numerous polluting industrial units operating in the entire Taj Trapezium Zone – an area of 10 400 square kilometres around the Taj Mahal – were affecting the health of millions of inhabitants of the area. One of the reasons for this is the pollution caused by the smallscale glass industry cluster in the city of Firozabad, which was the nerve centre of this trade in India (Figure 1). T E R I technological innovation that substitutes coal with natural gas for these units provides some respite to the local industry.

An overdue intervention

While the bigger glass melting units in Firozabad employ tank furnaces, the smaller ones use pot furnaces. Also, a large number of muffle furnaces are being used for baking bangles. The technology in all these cases, however, had remained the same over the past many decades, and very little effort had been made to improve the working conditions and the technology. The traditional designs of coal-fired pot and muffle furnaces are very poor in terms of energy efficiency and environmental performance. As per the Supreme Court judgement, it was mandatory for polluting furnaces to shift to the use of natural gas within a definite time frame. But there was no off-the-shelf technology available for the pot and muffle furnaces to facilitate the shift.

To these industrial units looking for a way out of their predicament, TERI's intervention to provide them with energy-efficient and natural-gas-based technological solutions was much-needed indeed (Figures 3 and 4).

A conventional coal-fired pot furnace
The new natural-gas-fired pot furnance

Working within the comfort zone of the traditional glassmakers, TERI researchers developed a model for environmental performance – natural-gas-based pot and muffle furnaces that are energy-efficient, which also make good economic sense. By agreement with the glass industry association and the district authority of Firozabad, TERI set up two demonstration plants, one each for pot and muffle furnaces. Aiming at helping these units switch over to the new technology, the pilot programme was a joint initiative involving TERI and the Swiss Agency for Development and Cooperation.

The innovation

Pot furnace
A state-of-the-art natural-gas-fired pot furnace was set-up at Firozabad by TERI with support from British Glass. Keeping the traditional design and mode of operation unchanged, the new system minimized energy wastage by using pre-heated air for combustion, and utilizing the heat of the outgoing flue gases. Also, the installation of efficient combustion systems and use of better refractory and insulation materials reduced functional and structural losses. The refurbished furnace was centrally fired by a single burner mounted on the furnace crown for better heat distribution. Improved structural design and use of better quality material for the furnace crown extended its life to three yearsdegradation.

Similarly, better quality refractory materials like silimanite and zirmul were used for the floor in place of conventionally used refractories. To check the heat from escaping through the stack, a heat recovery system in the form of a metallic recuperator was installed to pre-heat incoming combustion air to 600 °C.

Muffle furnace
A natural-gas-fired muffle furnace was designed and demonstrated by adopting a participatory technology development approach (Figures below).

Natural-gas-based demonstration mufle furnance
Schematic diagram of the TERI model of gas-fired muffle furnace   

The gas-fired design is very similar to the existing coal-fired units, both in terms of construction and operation. The gas-fired furnace has the provision to use better-quality muffles made of silicon carbide that can enhance the muffle life to more than three years. The design was optimized to achieve complete combustion of gas and better distribution of heat. The technological innovation has made it possible to increase production by 10%.

A city in the line of fire

Of the total glass produced by small-scale units in India, 70% comes from Firozabad. The cluster of glass furnaces based in this little town on the outskirts of Agra meets the country's requirement of glass bracelets, showpiece items, tableware, headlight covers, laboratory ware, vacuum flasks, and the like.

However, the going got disrupted because of the landmark Supreme Court judgement in December 1996. In response to a public-interest litigation seeking to protect the Taj Mahal from industrial pollution, the apex court directed all polluting units in the Taj Trapezium Zone to stop the use of coal and switch over to natural gas, which the GAIL (India) Ltd was directed to supply.


The refurbished natural-gas-based furnaces are excellent environment performers in that there was a substantial reduction in energy consumption as well as emissions of pollutants. The glass units could benefit immensely as the gas-fired furnaces demonstrate energy savings up to 50% and 30% in the case of pot and muffle furnaces, respectively. TERI's initiative has motivated 45 muffle furnace units to switch over to the TERI model despite paucity of piped gas supply in the area. In the case of pot furnace, besides the demonstration plant, another unit based on the TERI model has been set up while two more are under construction.


Though both the conventional pot furnaces (that are in use in Firozabad) and the TERI-designed pot furnaces run on natural gas, the latter has an improved design. An incremental investment of 2.4 million rupees for TERI's pot furnace offers payback in 16 months. This is due to the reduction in gas requirement by nearly 35% as compared to the conventional gas-based furnaces. The capital cost required for setting up a gas-based muffle furnace is in the range of 15 000–30 000 rupees, depending on the construction material used. The investment is at par with the existing coal-fired systems.

While the new technologies definitely make economic sense, they also boast of an envious combination of being environmentally sound and, most important, much less harmful to the workforce that would now sustainably continue to manufacture glassware for the nation.