Waste heat recovery plants offer a reliable supplement to captive power generation in an energy-intensive industry like cement, particularly in an energy-deficient country such as India. ACC Limited, part of the Holcim group, recently launched its first Waste Heat Recovery (WHR) system at the Gagal cement plant in the north Indian state of Himachal Pradesh. The WHR system harnesses waste heat discharged in the cement manufacturing process as exhaust gases, channelling them into a boiler that runs a steam turbine and converts it into useful electrical energy. The new WHR project generates electricity at a cost that is significantly lower than that of a captive power plant and only a fraction of the cost of grid power. ACC sees the project as an important step in energy conservation and is exploring the possibility of installing similar systems at a few of its other cement plants.
ACC’s Gagal cement plant is nestled in the picturesque mountainous state of Himachal Pradesh in northern India. Commissioned in 1985, it is the major cement plant in the state, with two production lines – Gagal I and Gagal II – representing a total cement capacity of some 4.4 million tpa. Gagal utilises power from the state’s grid and from a standby captive DG power plant. The plant pursues a wide-ranging agenda that aspires to achieve high standards of sustainable development in all aspects of its operations – beginning with meticulous hill mining techniques and extending to efficient manufacturing, sound environment management, afforestation and tree plantation, waste water treatment, as well as an array of community development and social volunteering activities. Most of the cement that Gagal manufactures comprises flyash-based Portland pozzolana cement, which reduces carbon emissions. Gagal also promotes the use of alternative fuels and raw materials.
WHR at ACC Gagal
A few months ago, ACC Limited launched its first WHR system at the Gagal cement plant. This marks an important step in energy conservation for the company, as it is the company’s first WHR plant and also the first such project in the state of Himachal Pradesh to deploy WHR technology. To put it briefly, a WHR system harnesses waste heat discharged in the manufacturing process as exhaust gases, channelling these gases into a waste heat boiler that runs a steam turbine and converts it into useful electrical energy. The newly commissioned WHR unit, set up at a cost of about US$16 million, can generate about 7.5 MW of electricity. This supplements the output of Gagal’s captive power plant. Nantong Wanda supplied the boilers for the project, while the turbine and generator were supplied by Qingdao Jieneng and Shangdong Jinan, respectively.
WHR energy at Gagal has proved to be remarkable in several ways – the first of which is that it requires no additional fuel. The cost of generating such energy has turned out to be significantly lower than that of the captive power plant and only a fraction of the cost of purchasing grid power.
Heat recovery for power generation
The cement manufacturing process is energy intensive, requiring very high temperatures in the order of 1400 °C in the kilns. Thermal energy is also used in other stages of the process, including the preheater, during grinding in the coal mill and raw mill and for drying additives such as flyash and slag. Significant amounts of heat energy are released as exhaust streams in different stages of the cement manufacturing process, chiefly from the kiln exhaust streams, clinker cooler, kiln preheater and precalciner. The manufacturing process in Gagal releases about 1000 tph of waste hot flue gases at temperatures well above 300 °C that are exhausted into the environment. Waste kiln gases exit at about 260 – 400 °C depending on the number of preheater stages in the plant. The cooler generates hot air of about 200 – 300 °C and 80 – 130 kcal/kg. Some of the hot air is used as combustion air in kiln furnaces and elsewhere; the rest of the hot gases are expelled as exhaust into the atmosphere. All these waste gases contain useful energy that can be gainfully exploited. This is the basis of the WHR system deployed at Gagal.
Apart from a cement plant’s capacity, the availability of waste heat is directly influenced by process efficiency parameters and other factors. The number of preheater stages in a cement plant has a significant bearing on the overall thermal energy consumption and waste heat recovery potential. The higher the number of stages, the better the thermal energy consumption and hence the lower the WHR potential. Similarly the moisture content in limestone, coal, flyash, slag and other materials used in a plant affects the potential for waste heat recovery as considerable heat would be required to dry raw materials. Again, improvements in plant and machinery efficiencies would offer lower potential for generation of waste heat.
Part 2 of this article can be read here.
Written by K. N. Rao, Director – Environment and Energy Conservation, and R. Nand Kumar, Vice President – Corporate Communications, ACC Limited, India. This is an abridged version of the full article, which appeared in the July 2014 issue of World Cement. Subscribers can view the full article by logging in.
Read the article online at: https://www.worldcement.com/asia-pacific-rim/14072014/waste_heat_recovery_in_india_part_1_85/