Waste Heat Recovery Systems and Power Projects for Cement Plants.

TECHNOLOGY

As shown in Figure 1, the hot, dust-laden exhaust gases from the cement plant can be tapped from two sources — namely, clinker cooler and preheater cyclones. The hot gases are passed through the waste heat recovery boiler (WHRB) to generate medium-pressure superheated steam to drive the steam Rankine cycle-based power plant. The WHRB on preheater exhaust gases is generally installed parallel to the downcomer before the preheater fan. In select cases, it could be installed after the preheater fan. WHRB on clinker cooler is installed either on mid-tap or vent gases.

In cases where preheater exhaust gas temperature is typically above 360ºC, the clinker cooler WHRB is generally installed on cooler vent gases. If the preheater gas temperature is lower, the cooler WHRB is installed on mid-tap gases to maximize temperature of superheated steam, resulting in enhanced power output. Gases with high-grade heat are available if they are tapped at the middle of the clinker cooler. The high temperature and low quantity of gasses offer thermodynamic advantages for the generation of steam at higher pressure and help achieve a smaller sized boiler. Depending on the gas temperature levels, typical steam pressure is in the range of 10 to 30 bar, and the superheat temperature is in range of 300ºC to 400ºC.

The waste heat recovery based steam Rankine cycle power plant can be engineered with the option of condensation by either air cooler condenser or water cooled condenser. The option of an air cooled condenser is preferred in cases of scarcity of water. In case of abundant water availability, the option of a water cooled condenser is attractive since it offers higher power output.

As shown in the Figure 2, the hot gases can also be tapped for heating thermal oil or generating pressurized hot water to drive an organic Rankine cycle based power plant.

Hot gases from modern cement plants have lower temperatures than those from old cement plants. ORC based power generation, which operates on organic fl uid instead of steam, offers better outputs over steam cycle. Also, it has lower O&M costs because critical water treatment facilities are not required. ORC systems are gaining increasing popularity and are expected to take larger market share over steam Rankine cycle based systems.

The waste heat recovery heat exchanger (HE) explained above has a vertical construction, which occupies very little fl oor space. Usually, the preheater cyclone assembly of the cement plant is around 75 to 100m in height; therefore, the vertical design saves huge costs of supporting structure and ducting for conveying gases to the inlet of the HE. Also the vertical HE requires only a single dust collection point. It allows natural circulation of water through boiler tubes, eliminating the requirement of an additional circulation pump. With these features, it easily retrofi ts in the running cement plant without affecting the production capacities and quality of cement produced. The reliable high-temperature resisting guillotine dampers installed in the inlet and bypass ducts make it possible to isolate the HE for maintenance without requiring production shutdown.