Waste heat recovery has long been the base standard for setting up cement manufacturing units. In China, a cement kiln will not get an official sign-off unless it has waste heat recovery systems. In Europe, any industry requiring large quantities of heat input for manufacturing must have waste heat recovery as a natural corollary, and sometimes its use could be extended to providing heat input to the neighbourhood and community for simple needs such as room and water heating systems.

The economics of Waste Heat Recovery Systems (WHRS) is preceded by sustainability concerns, as cement manufacturing in kilns continues to produce 8 per cent of the global greenhouse gas (GHG) emissions. The conversion of limestone to clinker is where the bulk of the heat energy is expended. However, this bulk of heat energy that is generated in six-stage preheater kilns by burning of any fossil fuel input actually gets wasted as shown in the pie-chart Fig 1, and only 58 per cent is used in the conversion process yielding clinker.

Theoretically 710 Kcal needs to be generated to convert 1 kg of clinker, out of which actual conversion process would need 410 Kcal, the rest ending up as losses, which can be recovered. In reality, the Indian cement industry average is 744 Kcal of heat input for producing 1 kg of clinker, which means the actual losses are even more than the theoretical possibility.

Out of the total generated heat there are some unavoidable losses, that include radiation loss, loss for evaporation of residual moisture in fine coal and raw meal and some part of heat going with clinker from cooler. The balance loss in pre-heater exhaust gases and the cooler exhaust gases are completely recoverable through WHRS.

Let us look at some statistics from the Indian cement industry. On an average, Indian cement plants require electrical power of 20 billion kWh per year. The coal needed for generating this much power accounts to 32 million tonnes per year. A significant portion of this power could be replaced by the WHRS that will use waste heat from the kilns to generate electricity.

In sustainability terms, this is equivalent to replacing a large component of the 32 million tonnes of coal to be otherwise burnt for producing electricity. WHRS are also very stable systems that operate on the Rankine cycle and it provides an avenue for utilising waste water from the process as well.

Balancing the investments

Let us now see the economics of putting up a captive power generating unit versus putting up a WHRS. The capital investment for WHRS is high at Rs 8 cr per MW going by the current costs, whereas the CPP units can come at Rs 4.5 cr per MW. However, the project Internal Rate of Return (IRR) would be very different as the cost of generation would be as low as Rs 0.40 per unit for the former while Rs 4.5 per unit for the latter, which given the current trajectory of fossil fuel prices is already under severe stress of upward correction. It is only the initial cost that continues to act as a deterrent for putting up a waste heat recovery unit.

The Indian cement industry must act responsibly and move quickly to put in investments that could raise the waste heat recovery installed capacity to cross the minimum threshold of 25 per cent of electricity consumption. That will still be far from the 20 billion KWhr of total electricity consumption by the industry.
The other area of concern is the price trajectory of fossil fuels, which would continue to move northwards. WHRS is one of the simpler ways of insulating the industry from the vagaries of future price increases.

Thus WHRS could be the natural hedge to fossil fuel price increases for a substantial portion of the electrical consumption. As matters stand today most WHRS would be the highest IRR projects that the industry as an ensemble can think of.

-Procyon Mukherjee