Tyres as an alternative fuel

The use of alternative fuels and raw material is common practice in the European cement industry. Dirk Lechtenberg gives an overview of scrap tyres as an alternative fuel in this excerpt from MVW Lechtenberg & Partner's Alternative Fuels & Raw Materials Handbook. Due to be published in the summer of 2011, the handbook will give an insight into over 80 different types of alternative fuels and raw materials with detailed descriptions of the availability, common use and practice in the cement industry. This includes processing considerations, the influence on the environment, clinker production and the economics of the various alternative fuels.

Scrap tyres are tyres that are no longer suitable or legal for their intended use due to damage, brittleness or low tread depth. By the time it is scrapped, a tyre generally loses around 20% of its weight. Tyres comprise natural rubber, synthetic rubber, carbon black, oil and various reinforcers.

Quantity and availability

Over 1Bn tyres are sold worldwide each year, which causes around 1Bn tyres to be scrapped. Despite an increase in the service life of tyres, volumes are growing constantly because of the increased vehicle use worldwide. The amount of tyres scrapped in various European countries in 2006 are shown in Table 1.

Composition of tyres

Tyres have a complex composition. Depending on their size and intended use they may vary in design, construction and total weight. The weight of a used passenger tyre in Europe is about 6.5kg but that of a truck tyre is about 53kg. These values and others are presented in Table 2.

Chemical composition

The main component of a tyre is rubber compound and the compositions of the tyres produced by different manufacturers are very similar. Table 3 shows the material composition of tyres.

Apart from differences in design and sizes, tyres are generally homogeneous. The average chemical composition is given in Table 4.

Tyres are 100% recyclable and are a valuable resource. Many countries with environmental legislation refer in their waste management practice to tyres in the so-called waste management hierarchy. This sets the priorities of what to do with scrap tyres. In priority order these are recycle, re-use, production of crumbs and strips, depolymerisation, energy recovery and disposal.

In Germany in 1990 about 44% of used tyres were recycled, 41% were converted to energy and 15% were disposed on landfill sites or disappeared unknown [H.Vest]. Since 2005, landfilling of tyres has been outlawed in Germany.

Economic value of tyres to cement plants

In order to calculate the benefits from substitution of primary fuels the following considerations have to be taken into account.

Calorific value - The calorific content value of tyres is between some 6450kCal/kg and 8000kCal/kg (S. Sprung). To substitute 1t of coal (some 6100kCal/kg) around 0.76 - 0.95t of scrap tyres is needed. Compared to pet coke (some 8200kCal/kg) the substitution factor is slightly more. Around 1.03 - 1.27t of scrap tyres is needed to substitute 1t of pet coke.

Biomass / CO2-value - Investigations show that on average 18.3% of the carbon in a passenger car tyre and 29.1% of that from a truck tyre is derived from biomass from natural caoutchouc. Taking this into account, the net CO2 emission factor is reduced from the default value of 85t of CO2/TJ (from IPCC) to approximately 59t of CO2/TJ for car tyres and 43t of CO2/TJ for truck tyres [C. Clauzade].

Hence the energetic utilisation of tyres has clearly a better CO2 balance than the incineration of fossil fuels as the emission factor for coal is 96t CO2/TJ.

Influences on clinker quality

The use of tyres as a fuel in cement production influences kiln operation and clinker composition.

Sulphur - Due to the high sulphur content the circulation of volatile salts increases - depending on the sulphur content of the substituted coal - and therefore the coating tendencies in the area of the kiln inlet, riser and lower cyclones also increase.

Iron - Normally the retention time for complete tyre burning in the kiln is long enough to convert the steel carcass into iron components that are incorporated into the clinker minerals. Therefore the alumina modulus decreases. This benefits the substitution of any corrective material, for example natural iron ore.

Zinc - The high concentration of zinc (approximately 1 – 2%) in tyres is crucial. Levels up to 500ppm zinc in clinker do not impact the cement properties (H Braun).

The usual maximum substitution rate by using tyres is approximately 25% of the whole thermal energy consumption due to process engineering restrictions. Even then this high level of zinc in clinker is not achieved.

However, investigations show that zinc concentration above 500ppm in clinker impact the workability of cement. Small amounts of zinc, between 0.01 and 0.2% have been reported to increase the reactivity of the C3A and in consequence lead to possible setting time problems (LEA et al).

Zinc levels above 0.4 % increase setting time [Scrap Tire Management Council] and experiments on laboratory clinker and cement show a clear retarding effect at 2.5% of zinc in clinker (D. Stephan).

Table 1: Tonnage of scrap tyres from different European countries. Source: Kurt Reschner, Altreifen-Recycling, (www.EnTire-engineering.de)

Table 2: Weights of different tyre types. Source: BASEL Convention, 'Technical guidelines on the identification and management of used tyres,' December 1999.

Table 3: Typical material composition of passenger car and truck tyres. Source: BASEL Convention - Technical guidelines on the identification and management of used tyres, December 1999.

Table 4: The chemical composition of tyres.Source 1: 'Bundesamt für Umwelt, Wald und Landschaft: Vollzugshilfe für die Lagerung, Behandlung und Verwertung von Altreifen' (Altpneus) – Entwurf, Stand. 10 December 2003.Source 2: H. Vest, 'Recycling of used car tyres,' Gate Information Service / gtz, 2000. (www.gtz.de/gate/gateid.afp)

Recommendations and considerations

1. Operators should ensure continuous mass flow into the feeding point of the kiln or calciner. This benefits the homogeneous combustion and hence the kiln operation. Continuous mass flow benefits the standard deviation of the iron oxide content of the clinker. The total iron oxide content consists of the iron oxide from raw meal plus the input from tyres. Normally the standard deviation of the raw meal is relatively low, and it increases while feeding whole tyres. The fluctuations of iron oxide in clinker become tremendous when tyre feeding is accidentally halted.

2. Winter operation with stored tyres in the open can lead to large amounts of water or ice trapped in the tyres. This changes the apparent feeding weight and impacts the calorific values and should be taken into account.

3. Operators should be careful about the total input of volatile elements (sulphur, sodium, potassium, chloride) from other alternative fuels and raw materials while using scrap tyres. The volatility will increase and hence cause coatings and cloggings.

4. Operators should be aware of the resulting zinc content in the clinker, especially when phosphate levels are high. Setting time and early strength could be greatly affected.

5. Operators should be careful about the feeding point and particle size of sliced tyres. Experience shows that shredded tyres fed into the calciner can result in deposits of unreacted wires in the lower-most cyclone. This results in clogging of the meal outlet. The gas flow in the riser duct sweeps the tyre pieces away. The tyre pieces burn and become lighter, forming deposits. A more reliable feeding point could be the meal chute of the second-lowest cyclone to the calciner or riser. This feeding channels the pieces directly into the kiln inlet so that the exhaust flow cannot sweep the pieces along into the cyclones.

6. Although the wires and carcasses seem to be destroyed completely in the kiln, some cement plants experience small metal iron pieces that remain in the clinker. To combat this, operators can install magnetic separators positioned before the clinker is fed into the cement mill.

References

Vest, H. 'Recycling of used car tyres,' Gate Information Service, 2000 (www.gtz.de)

Clauzade, C. 'Using used tyres as an alternative source of fuel - Reference values and characterisation protocols,' Reference document, R&D department ALIAPUR, July 2009.

Braun, H.: 'Sekundärbrennstoffeinsatz in der Zementindustrie - vom Altreifen bis zum Tiermehl,' (http://institute.unileoben.ac.at/ghiwww/braun.pdf).

Hewlett, P. C. (Ed.) 'LEA's Chemistry of Cement and Concrete, 4th Edition,' Arnold, London, 1998.

Stephan, D: 'Chrom, Nickel und Zink in Klinker und Zement - Einbau, Eigenschaftsänderung und Auslaugung,' Thesis - University Siegen, 1999.

Scrap Tire management Council: 'The Use of Scrap Tires in Cement Rotary Kilns,' 1992.

Sprung, S. 'Technologische Probleme beim Brennen des Zementklinkers,' Ursache und Lösung. Schriftenreihe der Zementindustrie, Verein Deutscher Zementwerke e.V.; Beton-Verlag GmbH, Düsseldorf, 43, 1982.