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Waste Heat Recovery

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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.

ACC’s Gagal cement plant commissioned in 1985 is the major cement plant in the state, representing a total cement capacity of approximately 4.4 million tpa. The plant utilizes power from the state’s grid and from a standby captive DG power plant. Most of the cement that Gagal manufactures comprises flyash-based Portland pozzolana cement, which reduces carbon emissions. 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.

The WHR system introduction at the Gagal cement plant marks an important step in energy conservation for the company, as it is also the first such project in the state of Himachal Pradesh. The system harnesses waste heat discharged in the manufacturing process as exhaust gases, channeling these gases into a waste heat boiler that runs a steam turbine and converts it into useful electrical energy. The unit can generate about 7.5 MW of electricity and 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.

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

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 recognized 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 pressurized working fluid (water in the case of the steam cycle or an organic compound for the organic Rankine cycle) to be vaporized by the hot exhaust gases channeled through a heat recovery boiler, or heater, and then passed into a turbine that drives a generator.

Steam Rankine cycle 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 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. Alternatively, Rankine Kalina cycle 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.

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.

Sustainability

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. The cost of electricity generated by WHR units is cheaper than both captive power and power purchased from an external grid and ACC has plans to set up similar systems at its other major cement plants in the country in a move towards enhancing energy security.

 

K. N. Rao, Director

Environment and Energy

Conservation. ACC Limited,

India

 

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