Cement Kilns: A Ready Made Waste to Energy Solution?

Due to the nature of cement production, cement kilns require temperatures of 2000C in their main burner.

The historic Coplay Cement kilns in Lehigh County, Pennsylvania Credit: Nicholas A. Tonelli

Due to the nature of cement production, cement kilns require temperatures of 2000°C in their main burner. Excellent conditions for waste derived fuels, they have become a standard fuel for many cement kilns around the world. But there is potential for this to grow fivefold.

by Jan Theulen

The initial driver for using waste based fuels in cement kilns was fuel costs - and it still is. Without the use of waste derived fuels, cement plants in most European countries are simply no longer competitive.

However, over the last decades reducing the environmental footprint of cement kilns has also become a target in itself to help ensure a sustainable future both for societies, and for the cement industry. The need to move towards a circular economy has pushed both waste generators and the cement industry to optimise the whole value chain from waste to a resource.

In a typical cement kiln some 200 tonnes per hour of sintering material passes the burner(s). The ashes of the fuel used to heat the kiln drops into the glowing material and reacts with the hot minerals to become an integral part of the end-product - clinker.

This clinker is then ground into cement. For such a kiln the fuel mix (both coal and waste derived fuel) might be around 30 tonnes per hour (tph) and produce some 20% ashes. This means that 6 tph of ashes are integrated into the 200 tph sintering material. Through this process, both the thermal and mineral value of waste is to 100% utilised by the cement kiln.

As the cement operator knows upfront the quantity and quality of this 3% addition by the fuel ashes to the sintering material, they will adapt the raw material mix accordingly.

The emissions of cement kilns co-processing waste derived fuels have the same limits as waste to energy plants. Due to the high temperature and long residence time, as well as to the fact that the lime in the standard suspension preheater is acting as a natural "scrubber", the emissions of heavy metals and/or dioxins is not increased when using waste derived fuels.

An intensive study from the German Cement Association (VDZ) illustrates this in figure 1.

Economics

In countries such as Japan, Western Europe and Canada the cost of landfilling waste is so high that waste to energy plants become commercially viable. In those countries both cement kilns and waste to energy plants work side by side in the waste management system.

However, in countries where GDP doesn't allow drastically increased landfill taxes, cement kilns offer the only viable alternative. One such example is Poland, which has reached a significant landfill diversion ratio, while utilising the thermal capacity of the existing cement kilns, as is illustrated in table 1. In order to reach EU-targets, Poland has started to build Waste to Energy (WtE) plants while growing the use of waste derived fuels in the cement industry.

Table 1. % of waste recovered by cement kilns as part of total thermal recovery / thermal treatment of MSW and C&I waste of various countries.

A similar development is arising in Indonesia, where landfilling is under pressure, especially in the metropolis of Jakarta. Due to the high organic content in the municipal solid waste the energy value is very low and waste to energy solutions as a result will have a limited power output. Waste generated power is supported by a surplus feed-inn tariff to encourage the market to take off. The cement industry however (having the thermal capacity already in place) can offer solutions to the local industries and municipalities at equal cost levels as a sanitary landfill and doesn't need subsiding.

Table 2. Emission limit comparison Incinerators and Co-incineration in Cement Kilns in EU.

However, cement kilns do require a major maintenance period of some three to four weeks annually. As the waste market needs to be serviced around the year, the cement industry has found the answer by operating as a network of kilns. Either a waste generator has multiple outlet contracts to various end-users, or the cement company (offering a full year service), diverts waste to other kilns of within its group.

In some cases agreements are made with waste to energy plants to take excess quantities in kiln overhaul periods against predefined rates.

Sewage sludge arising in China Long Term Commitments

One of the characteristics of cement kilns is that once they are built they will typically have an operational lifespan of some 40 to 80 years. Across the world concessions for quarrying are difficult to secure and the capital investment is high. Any company that finally accomplished a permit and made the significant capital investment will only get a return by operating the plant for several decades.

Against that background, cement kiln operators are aiming for long-term commitments on key resources - fuels and raw materials. Waste generators or waste management companies that are aiming for long term contracts (maybe due to large upfront investments) are very well matched to the principles of the cement operators.

The Polish Example

In the early years of this century the combined waste to energy and cement kiln capacity in Germany was not sufficient to absorb all the waste that was no longer allowed to be landfilled.

As such, the Polish cement industry capacity was utilised to recover excess flows of so-called Residue Derived Fuel (RDF) or Solid Recovered Fuel (SRF).

Nearly all cement plants have received permits from their local regulators to co-process these waste derived materials. The infrastructure built at the cement plants to handle these imports from Germany, and the experience of operating the cement kilns with these alternative materials, were accelerating factors in Poland's recent developments.

Gradually the imports from Germany were substituted by the locally produced RDF. A steady growth in the substitution of coal was achieved, and in parallel the start of landfill diversion of MSW and Commercial & Industrial (C&I) began. Now, waste management companies in Poland have started to convert C&I waste as well as MSW into RDF for cement kilns.

In 2011 the Polish Ministry of Environment launched a target to divert 50% of MSW from landfills in 2013 and reduce further landfilling to only 35% by 2020. Currently close to 25% of the MSW is converted into RDF, which is allowing the cement industry in Poland to provide with the highest contribution to the country's waste reduction targets.

Chinese Sewage Sludge

Chinese cities are extremely active in improving the waste management structure. A lot of wastewater treatment systems have been built over recent years. As a result the amount of sewage sludge being treated has increased rapidly, as illustrated in figure 2.

The Heidelberg Cement plant in Schelklingen, Germany

The cement industry is recognised by most institutes in China that are guiding/regulating the development of the wastewater system. Several demonstration facilities for the recovery of dewatered sewage sludge have been realised by Chinese cement plants.

The Cement Sustainability Initiative (CSI) from the World Business Council of Sustainable Development (WBCSD) has highlighted the HeidelbergCement-CRC joint venture plant in Guangzhou in the south of China as a role model for the cement industy. The plant was visited by the CSI-delegation in last Chinese gathering. The plant utilises waste heat from the kiln process to dry the sludge from 80% moisture down to 40%. In a next step the dried sludge is 100% recovered in the cement kiln.

The organic part is a fuel replacement for the coal, where-as the mineral part of the sludge (on dry basis around 40%) is replacing virgin raw materials. The silica-, calcium-, and aluminium components are valuable constituents of the raw material mix of a clinker process. There is no residual waste left.

Servicing three million inhabitants in the region, the authorities and the cement works are jointly developing further extension of this successful approach.

Hazardous Waste Treatment in Belgium

With the presence of some of Europe's biggest docks in the area - including Antwerp, Rotterdam and Amsterdam - the petroleum and chemical industries in Benelux are large. These sectors, and their downstream manufacturing industries, are unavoidably generating a wide range of hazardous waste materials that need to be treated carefully.

Some 25 years ago private investors decided to start a recovery plant for solid hazardous waste to produce fuel suitable for the cement industry.

Today in the Belgium market two very advanced hazardous waste treatment platforms exist, one in Seneffe, operated by Holcim's Geocycle, the other named Recyfuel (close to Liege) operated by a Joint Venture of Sita and HeidelbergCement. Together they take care of 50% of all hazardous waste generated in Belgium and on top of that are the recovery destination for a lot of specific hazardous flows of neighbouring countries.

Recyfuel is a pre-treatment centre for industrial and special household waste with good calorific value, such as: paint, resin, glue, sludge, tar, filtration cake, or active coal. It can handle bulky waste, as well as hazardous waste packed in drums, cans, plastic bottles, small containers etc.

An advanced treatment systems ensures that all steel is recycled, while the organic are mixed with sawdust to produce an 'impregnated saw dust'. This material is sent to cement kilns where it is completely burned out and the ashes used as a raw material replacement.

The core of this operation is the laboratory. When a waste is offered for treatment by a waste generator, samples are analysed, checked against the strictly monitored acceptance criteria, and weighted against the recipe of the alternative fuels to be delivered.

The fee that the generator will be charged is depends significantly on the waste's chemical and physical composition. When a contract is made and the truck arrives at the gate, each delivery is sampled and checked against the predefined criteria before unloading.

Conclusion

With its 5670 cement plants operating worldwide, the cement industry has a considerable capacity to give a local and economically viable solution to a considerable part of the world's challenges in waste management. Currently it is estimated that the total cement industry worldwide is valorising over 100 million tonnes of waste derived materials in cement kilns. This reflects only 20% of the potential it currently has.

The presence of the cement industry means that some countries can refrain from incinerators completely, as is the case for Norway for example, which has no need for any hazardous waste incinerator. However, in general a whole portfolio of waste treatment solutions will exist (or need to be developed), including landfilling and material and energy recovery solutions.

The major advantage of cement kilns is that they already exist, are quite tolerant to waste composition and offer environmentally safe and economical solutions in countries where tipping fees at landfills hardly surpasses €10 per tonne.

Jan Theulen is group director of alternative resources at HeidelbergCement.

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