Cement production is energy-intensive and currently a mix of coal, petcoke, biomass and waste materials is used as a fuel source. Use of alternative fuels like biomass or waste materials has an immediate impact on the industry’s carbon profile, and while the industry already uses large quantities of such materials, this may well be increased in the future. The unique process and energy requirements of the cement industry enable use of fuel mixes that would not be suitable for many other industries. This ability to mix fossil fuels like coal or gas with waste materials, biomass and industrial by-products is beneficial both from a resource efficiency and security of supply point of view.
In Europe, the cement industry has replaced a large part of its traditional fuel sources with biomass or waste. From a technical point of view, much higher substitution rates are possible, with some plants using up to 80% alternative fuels1.
Apart from direct effects of replacing carbon-intensive fossil fuels with lower-carbon-intensity of alternative fuels, there can also be indirect benefits. Waste materials disposed of via landfill or by incineration give rise to their own greenhouse gas emissions, such as methane in landfill gas. By using these materials as alternative fuels in cement plants, harmful decomposition-related emissions are avoided. Use of waste as an alternative fuel in cement kilns contributes to lowering overall CO2 emissions by replacing fossil fuels and their related CO2 emissions with waste materials, which would otherwise have to be incinerated or landfilled with corresponding emissions of greenhouse gases. Typical alternative fuels classed as waste products include waste tyres, waste oil and solvents, pre-treated industrial and domestic waste, and plastic, textile and paper waste. There may also be transport-related benefits if local alternative fuels replace imported fossil fuels.
Pure carbon-neutral biomass fuels used in the cement industry today include animal meal, waste wood, sawdust and sewage sludge. Besides these fuels, many other organic waste materials are utilised as fuels in the cement industry globally, but to a lesser extent. Wastes containing biomass are mainly pre-treated industrial and domestic products (containing organic fibres, textiles and paper).
Co-processing waste offers a cheaper solution than investing in dedicated facilities, which require a huge capital investment and in which operating costs tend to be higher.
Sewage sludge and cement production Sewage sludge is a by-product of waste water treatment. Until recently, the main solutions for sewage sludge were open dumping, landfill disposal or use in agriculture. However, there is increasing concern about organic residues in sewage sludge, making its use in agriculture questionable. Sewage sludge can also be used as both an alternative fuel source and raw material in the high-temperature clinker manufacturing process, destroying organic residues. It is important to note that there is a surplus of sludge, so the need for alternative forms of processing is becoming all the more acute.
The Netherlands and Spain are just two examples of countries where the cement industry is providing a solution for sewage sludge. Since March 2000, the ENCI cement plant located in Maastricht (the Netherlands) has been working together with the Limburg Purification Board to develop ways of receiving pre-treated sewage sludge from their sewage water treatment plants (following treatment in the Board’s own thermal sludge dryers). Today, 80,000 tonnes of dried sewage sludge is co-processed annually in a kiln with a capacity of 865,000 tonnes of clinker per year.
In 2005, the cement sector in Catalonia (Spain) reached an agreement with the Catalan administration, trade unions and local councils to launch a trial to monitor the environmental behaviour of thermally dried sewage sludge from the Barcelona area as an alternative fuel in cement plants. The aim was to use more than 60,000 tonnes of dried sewage sludge every year as a petcoke substitute, providing a solution for the high volume of sewage sludge that cannot be used for agricultural purposes.
In addition, a cement plant in Spain is drying sewage sludge using exhaust gases from the clinker cooler, and then uses the dried sewage sludge as alternative fuel for the kiln. Other national governments should follow suit to maximise the potential of sewage waste by making it available for use in cement manufacture.
In order to use biomass as a fuel, it has to be affordable, secure and continually available. As industries try to reduce their CO2 emissions and demand for green power grows, there will be increased competition with other industry sectors (power plants, steel industry, biomass boilers) over access to biomass.
Higher replacement rates of fossil fuels by alternative fuels will be facilitated if waste legislation, in the EU and its Member States, restricts landfill and allows controlled waste collection, treatment and alternative fuel production. Better understanding and social acceptance of using waste as an alternative fuel in cement plants is also key.
It has been estimated that by 2050, 40% of kiln energy could potentially come from traditional sources, i.e. coal (30%) and petcoke (10%), While 60% of kiln energy could potentially be provided by alternative fuels of which 40% could be biomass. This fuel mix would lead to an overall decrease of 27% in fuel CO2 emissions. In order to achieve this level of reduction, a number of policy modifications are required, which are outlined below.