Chemical Technology June 2015

WASTE MANAGEMENT

Conventional Power Station Natural gas (grammes per gigajoule) Oil (grammes per gigajoule)

Coal (grammes per gigajoule)

0

590 140

940 270

65

Table 1.3: Emissions from Blue Circle Cauldon cement kiln during tire burning trial

Percentage Change

Usual Fuel ( Coal & Coke ) milligrams/ m 3 Usual Fuel with 15% tyres milligrams/ m 3

Pollutant particulates

60

60

No change

oxides of nitrogen sulphur dioxide carbon monoxide chlorine and fluorine

1180 500 985 1.13 129

800 500 948

-32

No change

-4

1.0

-12 -47 -75

volatile organic compounds

68

dioxins

0.12 nanogrammes per metre cubed

0.03 nanogrammes per metre cubed

Source: Blue Circle Industries plc

either to the precalciner or into the kiln at the same end as the feed. If cement kiln operators want to use substitute fuels they must apply to the Agency for permission to undertake a trial. A full assessment is made of the possible environmental impacts of burning substitute fuels on a case-by-case ba- sis. Following a trial, the operator must then apply to the Agency for permission to carry out continuous burning of a substitute fuel. The Agency is committed to full public consultation be- fore and after trials. In January 1998 the Agency published a draft protocol on the use of substitute fuels in kilns for public consultation (Environment Agency, 1998a). The Substitute Fuels Protocol updates the Bedford Protocol which was issued by the former Her Majesty’s Inspectorate of Pollution in 1994. In most cases, the trials have shown either no change in the concentration of pollutants or a decrease when burn- ing tyres (Table 1.3). The emissions of nitrogen oxides are reduced by up to 40 % due to staged combustion effects in which less thermal nitrogen oxides are generated. Because tyres contain iron, using them as a fuel reduces the amount of iron oxide added to the process. An additional benefit is that no residual ash is produced from the cement process. Tyres are already used as a fuel in cement kilns in many countries. The British Cement Association estimates that the UK cement industry can potentially recover up to 190 000 tonnes of used tyres, which is equivalent to 50 % of the annual arising of used tyres in the UK .

clay or shale, mixed with other materials to form clinker. Temperatures in excess of 1 400 ºC are required to produce the cement clinker. The following characteristics make ce- ment kilns suitable for burning tyres: • high temperature;

• long residence time; • oxidising atmosphere; • high thermal inertia; • alkaline environment; • no ash residue; • continuous fuel requirement.

Organic constituents are destroyed due to the high tem- peratures, long residence time and oxidising conditions in the cement kiln, and produce carbon dioxide and water. The majority of the inorganic constituents combine with the raw materials in the kiln and leave the process as part of the cement clinker. Heavy metals remain bound in the cement and in its subsequent use. The remaining inorganic con- stituents are expelled to the chimney where 99,9 % should be captured through the use of electrostatic precipitators or filters (Parker, 1987). Tyres are used, either chipped or whole, to replace part of the conventional fuel. They are treated in a different way depending on the type of cement kiln. At ‘wet’ kilns, whole tyres are dropped into the kiln about halfway along its length, and shredded tyres are added at the fuel end of the kiln. The tyres are subject to the very high temperatures inside the kiln and any residues left after burning combine with the final product. In ‘dry’ kilns the tyres are added

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Chemical Technology • June 2015