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Waste heat to electricity
Waste heat generated in cement manufacture has proven to be amenable to conversion into electrical energy, provided it is tapped in adequate measure and the temperature is sufficiently high to make the project viable. In a typical Indian cement plant, the potential generation of power from waste heat is estimated at roughly 20 – 25 kWh/t of clinker. The process goes through four basic stages:
- Heat tapping and extraction.
- Heat conversion.
- Heat dissipation.
- Electricity feed and control.
Three technologies are recognised as being well developed and effective in the conversion of heat into electricity – using a steam Rankine cycle, an organic Rankine cycle, or the Kalina process. All these technologies involve a pressurised working fluid (water in the case of the steam cycle or an organic compound for the organic Rankine cycle) to be vaporised by the hot exhaust gases channelled through a heat recovery boiler, or heater, and then passed into a turbine that drives a generator.
Steam Rankine cycle
This is a thermodynamic cycle that converts heat into work (power in this case). Hot exhaust gases are directed into a waste heat recovery boiler where they exchange heat with the working fluid (water) that is converted into high pressure steam, which then expands in the turbine causing it to rotate and produce electricity. The expanded vapour is condensed into a low pressure liquid in the water-cooled condenser and then is recycled back into the boiler to continue the cycle. The system consists of a suspension preheater boiler, air quenching cooler boiler, steam turbine generator, distributed control system, water-circulation system and dust removal system. This is the most common type of WHR system in cement plants and was chosen for the Gagal plant.
Organic Rankine cycle
This process uses organic fluids. Their inherent ability to evaporate at low temperature and yield good levels of condensation allows these fluids to deliver considerable energy during their expansion in the turbine.
Rankine Kalina cycle
This is a relatively new concept in heat recovery and power generation, which is a thermodynamic process for converting thermal energy into usable mechanical power. It uses a working fluid mixture, made up of 70% ammonia with 30% water. This process offers the potential of significant efficiency gains as compared to the conventional Rankine cycle. It is usually more suitable for medium to low gas temperature heat recovery systems.
Sustainability and other issues
WHR units score highly in environmental terms and simultaneously offer several advantages. The primary environmental benefit of the WHR power plant is to produce electric power without burning any additional fossil fuel or contributing any additional greenhouse gas (GHG) emissions. These systems play a vital role in energy conservation as they utilise waste heat and do not need any additional fuels to generate electricity. They help conserve fuels and reduce overall carbon emissions. Where they substitute power from an external grid or a captive power plant, there is an additional advantage of reduced fuel consumption and lower CO2 emissions. Since it is based on waste heat, the energy produced is green energy that is equivalent to renewable energy.
The WHR at Gagal is expected to lead directly to a reduction of over 44 000 tpa of CO2 emissions. By a rough rule of thumb, it can be said that such units can help reduce up to 25 kg of CO2 per tonne of clinker produced. The investment involved in setting up a WHR plant is reasonable. On average, the cost of a waste heat based power generation plant would fall in the range of US$2 – 2.5 million per MW.
As experienced by ACC at Gagal, the cost of electricity generated by WHR units is cheaper than both captive power and power purchased from an external grid. Encouraged by the Gagal WHR project, ACC has plans to set up similar systems at its other major cement plants in the country in a move towards enhancing energy security.
India’s cement sector already has several working WHR plants and undoubtedly such plants will become a feature in this fast expanding market. Waste heat recovery can comprise an economical and reliable supplement to captive power generation in an energy-intensive industry like cement, particularly in an energy-deficient country like India. In addition, this practice comprises energy conservation and efficiency that helps the cement industry meet its low carbon technology roadmap for the future.
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.
The authors acknowledge the input of Navneet Chauhan, Sr General Manager Operations, ACC Gagal and Dominic Fernandes, General Manager Energy & Environment, ACC Limited.
Read the article online at: https://www.worldcement.com/asia-pacific-rim/16072014/waste_heat_recovery_in_india_part_2_86/