Manufacturing cement is highly energy-intensive. First, energy is used in the form of fuel to fire the rotary kilns to produce the cement clinker. Second, electrical energy is used to operate various units – in particular raw material and cement grinding systems. Today, electrical energy consumption in cement works makes up over 10 per cent of the total energy consumption, with the energy costs being split almost equally between fuel and electricity. In total, the German cement industry used 92.5 million gigajoules of fuel in 2014, whilst electricity consumption was 3.57 terrawatt hours (TWh).
Back in the 1950s and 1960s, considerable improvements were made to the energy efficiency of rotary kiln plants in German cement works. After the reunification of Germany, construction and modernisation of works in eastern Germany led to a further reduction in specific energy consumption. Throughout this time, several kiln lines in the old federal states were also replaced by three new plants, and existing plants were optimised. Overall, this has led to specific fuel-based energy consumption for cement manufacture now being close to the optimum in terms of process technology – based on 2,800 kilojoules for one kilogram of cement. What is key here is the fact that the energy efficiency level in the manufacture of the intermediate product of clinker is currently over 80 per cent.
The high requirement for electrical energy is due to the raw material preparation (approx. 35 per cent of the total requirement), the burning and cooling of the clinker (approx. 22 per cent), and cement grinding (approx. 38 per cent). Whilst the specific fuel use has been reduced more or less continually over several decades, the specific electricity consumption was on the increase until the mid-1980s. In Germany, the maximum value was achieved in 1987, at 111.2 kilowatt hours (kWh) for the manufacture of one tonne of cement; this corresponded to absolute electricity consumption of 3.8 TWh. Following a considerable reduction to a level of around 99 kWh in 2008, the specific electricity consumption has increased again: in 2014, an average of 110 kWh was required per tonne of cement; this corresponded to absolute consumption of around 3.57 TWh.
The increase in electricity consumption in the years following 2008 has various reasons. These include increasing demand from the building materials industry for high-performance, and often finely ground, cements and increasing production of cements with other main constituents, which contain significant proportions of substances such as granulated blast furnace slag or limestone in addition to clinker. On the one hand, the manufacture of such composite cements leads to lower specific carbon dioxide emissions; on the other hand, the use of these materials increases the grinding effort required to achieve the desired product quality. The electrical energy requirement in the cement industry is also increasingly being influenced by stricter requirements regarding exhaust gas cleaning, particularly in terms of reducing nitrogen oxide emissions. The use of denitrification plants or additional filter technology, for example, slows down the flow of exhaust gas, meaning that higher drive power, and thus an increased specific electricity input, are required to comply with the same flow velocities.
Despite this trend, in 2012 German industry entered into an agreement with the government to increase energy efficiency, committing to continually reducing its specific energy consumption between now and 2020. This took account of the fact that an increasing specific energy input is to be expected in certain branches of industry in future, due to the outlined contrary effects.