Optimising energy efficiency – part one

Introduction

The production of cement is one of the most energy-intensive production processes in the manufacturing industry. For this reason, cement companies have always recognised energy efficiency as crucial and a subject of major interest, long before the term sustainability became a worldwide issue. As a consequence, energy efficiency in cement plants is already high and potential for further improvements is limited.

The energy required to produce one t of cement is closely connected with technological and market issues, as well as with environmental regulations and emission level constraints.

In 2014, the fuel energy consumption of all cement works in Germany amounted to more than 90 million GJ, with the firing of alternative fuels representing a share of 63.4% of total fuel energy consumption. In the same period, cement works in Germany had a specific power consumption of 110 kW-hours per t cement, which corresponds to approximately 3.5 tW-hours, or in other words, an annual electricity cost of around f250 million for the entire sector.These figures reveal that small improvements with regard to energy efficiency can not only provide significant economic and competitive gains, but also represent small but nevertheless meaningful steps with regard to the sustainable manufacture of cement. However, only by conducting a simultaneous examination of the interactions between the energy performance of individual process steps (e.g. clinker cooling or cement milling) and the entire process chain, including the product portfolio, is it possible to further optimise energy efficiency in cement production in the long term under certain given constraints. For this reason VDZ has developed a structured and scientifically substantiated approach to analysing the energy performance of cement plants to define improvement potentials and measures.

Present and future trends concerning energy demand

Over the past years, the German cement industry has been continually challenged with regard to decreasing energy consumption. The changes that the German cement industry has experienced in the past decades, such as the increasing demand for cements of higher fineness, the rise in firing of alternative fuels (considered crucial in order to reduce production costs and CO2 emissions) and compliance with new environmental regulations, have more than counterbalanced the energy efficiency gains achieved in the same period. For this reason, the specific energy demand per t of cement in Germany has barely changed over the past decades.

A VDZ expert group has compiled a study with the main objective of looking at the various developments that can be expected up to 2030 in the German cement industry and to assess their potential impact on energy demand. Four main variables were identified as major players concerning energy efficiency:

• Optimisation of process/plant technology.
• Influence of the cement portfolio (fineness and clinker to cement ratio).
• Use of alternative fuels and increase of thermal substitution rate.
• Implementation of environmental protection measures due to new regulations.

The study concluded that the maximum theoretical potential reduction of energy demand in 2030 in the German cement industry, based on the absolutely unrealistic scenario that all kiln and grinding lines would be replaced by new ‘greenfield’ BAT installations, would only amount to approximately 14% relative to the year 2011. Usual technical developments (here called ‘BAU’ = business as usual) – under certain market and emission regulation constraints – would only offer a reduction potential of 6.8%.

Technology changes in a cement plant, such as the replacement of singular outdated major equipment (e.g. clinker cooler, cement mills, etc.) by others with higher efficiency, or even the construction of new BAT kiln lines, can provide considerable gains in terms of energy efficiency. However, both strategies require a considerably high investment and profitability must be carefully assessed on a case-by-case basis. The energy saving potential following this strategy has already been described in detail in the CSI/ECRA Technology Papers. Process optimisation without major technology changes is, for this reason, the most common strategy followed by plant managers with regard to the increase of energy efficiency. Strategies that may lead to a stepwise optimisation of the process, and to an increase in energy efficiency without significant investment, are described below.

The cement portfolio can affect both the thermal energy demands (indirectly) and the electrical energy demands (directly) of cement production. The reduction of the clinker to cement ratio, through the replacement of clinker by other constituents, generates a reduction in the cement-related thermal energy demand. On the other hand, some substitution raw materials, such as granulated blastfurnace slag (the primary clinker substitute material in Germany), are harder to grind than clinker and increase the electrical energy demand of the cement mills. Among all the variables identified as major contributors to energy efficiency, the lowering of the clinker to cement ratio is the one which presents the highest energy reduction potential. Potential losses in electric energy efficiency, e.g. due to lower grindability of mineral components like granulated blast furnace slag, are largely superseded by thermal energy gains from replacing clinker with unburnt materials. However, the reduction of the clinker to cement ratio is linked to the availability of substitute materials, which may limit the potential efficiency gains in the future. The availability of substitute materials becomes even more relevant in markets with increasing cement demand. Moreover, the market trend towards higher fineness cements, which can be observed in many European countries, tend to counterbalance energy efficiency gains.

Germany is a leading country in the use of alternative fuels in the cement industry. The whole industry recognises the importance of firing alternative fuels, not only due to its economic, but also its environmental, benefits. However, the rise of firing alternative fuels has contributed to a certain increase in energy demand, mainly due to the moisture content of the fuels and other fuel constituents that can destabilise the clinker burning process (e.g. chlorine and sulphur). The cement industry therefore still prefers to purchase alternative fuels of high quality, characterised, for example, by a high calorific value and/or a low chlorine content, in an attempt to reduce CO2 emissions and costs. It is not expected that alternative fuels of high quality, and with sufficient availability, will emerge on the market at competitive prices in the coming years. On the contrary, a falling calorific value and a rising moisture content of the alternative fuels are expected, which may consequently increase energy demand. Research is being carried out to counteract this foreseeable development by, for example, pre-treating the materials further compared to the current situation.

More stringent emission level regulations have led to the decrease of emissions from the cement industry over the past years. This decrease was only possible due to the investment in new environmental protection measures. The German ‘TALuft’ technical guidelines and the ‘17. BImSchV’ incineration of waste regulation define a new dust emission limit of 10 mg/m3 (10% O2) from 2016 onwards. The new NOX limit in Germany from 2019 onwards will be 200 mg/m3; 10% O2, and will particularly challenge the German cement industry due to the high investment costs entailed. New environmental protection measures will be necessary in order to comply with the new regulations and emission levels. The installation of SCR technology, as well as the replacement or retrofitting of process filters, may be required in several cement plants, which will lead to an increase in electric energy demand.

This is part one of a three-part article written for World Cement’s April issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the April 2016 issue here.

Read the article online at: https://www.worldcement.com/special-reports/25032016/optimising-energy-efficiency-1/