WHR in the Cement Process

Worldwide, the cement industry is one of the most energy intensive sector in which energy represents 20% to 40% of the total production costs. The industry is also a significant source of greenhouse gases, accounting for about 5% of the annual global anthropogenic carbon dioxide emissions, about 1,800 million tons of CO2 emissions in 2005, emissions result from the use of fossil fuels and the chemical reactions during clinker processing. The average CO2 intensity ranges from 0.65 tons to 0.92 tons of CO2 per ton of cement across countries with a weighted average of about 0.83 tons of CO2/ton.

State-of-the-art New Suspension Process kilns (NSP) include multi-stage pre-heaters and pre-calciners to pre-process raw materials before they enter the kiln and an air-quench system to cool the clinker product.

Kiln exhaust streams from the clinker cooler and the kiln pre-heater system contain useful thermal energy that can be converted into power. Typically, the clinker coolers release large amounts of heated air at 250°C to 340°C (480°F to 645°F) directly into the atmosphere.

At the kiln charging side the 300°C to 400°C (570°F to 750°F) kiln gas coming off the preheaters is typically used to dry material in the raw mill and/or the coal mill and then sent to electrostatic precipitators or bag filter houses to remove dust before finally being vented to the atmosphere.

If the raw mill is down, the exhaust gas would be cooled with a water spray or cold air before it enters the dust collectors. Maximizing overall kiln process efficiency is paramount for efficient plant operation. Waste heat from the pre-heater exhausts and clinker coolers can however be recovered and used to provide low temperature heating needs in the plant or used to generate power to offset a portion of power purchased from the grid, or captive power generated by fuel consumption at the site.

Typically, cement plants do not have significant low ­ temperature heating requirements, so most waste heat recovery projects have been for power generation.

The amount of waste heat available for recovery depends on kiln system design and production, the moisture content of the raw materials, and the amount of heat required for drying in the raw mill.

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