The trend towards increasing costs of primary energy sources such as fossil fuels, and the need to reduce harmful emissions – particularly of carbon dioxide – is driving a worldwide effort to improve the efficiency of energy usage in industrial processes. Estimates suggest that roughly 66% of all the energy input is lost as waste heat and in the cement industry specifically the loss is estimated as 40%. With the tendency towards ever more stringent environmental constraints and the need to reduce the carbon footprint for individual organisations, the potential to generate electrical energy from waste heat will continue to be an attractive option.
The first major waste heat recovery (WHR) system in a cement plant was the 15 MW unit installed by Kawasaki Heavy Industries for Taiheiyo Cement in 1982. This was a conventional Rankine Cycle using heat from both the kiln and the clinker cooler. As the benefits became generally recognised within the industry, WHR units, the vast majority of which involved the conventional Rankine Cycle, were installed to provide up to about 30% of the power requirements of the plant. The main sources of waste heat were the exhaust from both the preheater and the clinker cooler and, in some of the developing countries where power outages are not unusual, the WHR system may be the only source of reliable power available to the plant operator.
Improvement in the overall efficiency of cement manufacture has resulted in lower exhaust gas temperatures and this development has provided opportunities for alternative technologies, notably the Organic Rankine Cycle (ORC) and the Kalina Cycle, which are more effective in recovering waste heat from lower temperature gases.
Conventional Rankine Cycle
By far the most widely used technology for the generation of power from waste heat involves the Rankine Cycle. As the name suggests, the conventional Rankine Cycle involves the transfer of heat from the waste heat source to convert water from the condenser to steam, which is then superheated – i.e. heated to a temperature above the saturation temperature at the given plant operating pressure. Subsequently, the steam is expanded through a steam turbine driving the generator to produce power. This is the process used in conventional fossil fuel-fired power plants, but in the case of WHR in cement plants the maximum temperatures are much lower – usually in the range of 300 to 350 °C – and consequently the plant operating pressure is also significantly lower, i.e. around 2.5 MPa (25 bara). In order to maximise the efficiency of the cycle, the pressure in the condenser is minimised, which will affect the steam quality, thereby increasing the content of water droplets that can cause erosion of the turbine blades. In a cement plant there can also be significant variations in the temperature of the hot air emerging from the clinker cooler, resulting in variability in the steam temperatures in the system, potentially exacerbating the erosion problem.
A key point is that if waste heat from both the preheater and the clinker cooler is used to generate power it is necessary to provide two boilers, i.e. one for each heat source, and this adds to the cost and complexity of the system.
A good example of the current trend towards larger WHR plants is provided by Najran Cement Company of Saudi Arabia, which commissioned the Chinese company, Sinoma Energy Conservation Limited, to install a turnkey unit at the Sultana cement plant. The WHR unit is designed to provide 27 MW of power at a projected cost of just less than US$45 million and will be the largest WHR unit in the global cement industry in terms of generating capacity. The system consists of a total of eight boilers plus associated equipment using waste heat from the cement plant along with ten boilers taking waste heat from diesel generators. It is estimated that CO2 emissions will be reduced by 145 000 tpa when the plant is fully operational.
Written by Thomas B. Gibbons. This is an abridged version of the full article, which appeared in the August 2013 issue of World Cement. Subscribers can view the full article by logging in.
Read the article online at: https://www.worldcement.com/europe-cis/31072013/whr_cement_conventional_rankine_cycle_76/