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Sample preparation for the analysis of secondary fuels - part one

World Cement,


A cement plant typically consumes 3 to 6 GJ of fuel per t of clinker produced, depending on the raw materials and the process used. Most cement kilns today use coal and petcoke as primary fuels, and to a lesser extent natural gas and fuel oil. Selected waste and byproducts with recoverable calorific value can be used as secondary fuels in a cement kiln, replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications.

Inhomogeneous samples

Secondary fuel can be inhomogeneous, containing all types of polymers, paper, wood and low fractions of metal. Recycled wood or parts of automobile tyres might also be used as secondary fuel. Before its use as fuel in the production of cement, several chemical analyses are necessary for quality control. One is the determination of the calorific value.

Determination of the fuel requirements

To determine the fuel requirement during clinker production, the gross and net calorific values of the fuels utilised must be identified. The determination of the calorific values is of particular significance for characterising different secondary fuels, the compositions of which frequently vary. The gross calorific value is determined in a bomb calorimeter, according to DIN 51900, in an oxygenated atmosphere and at a pressure of 30 bar: a so-called ‘bomb’ is inserted into a metal tank that can be assumed to be adiabatic. The tank is filled with water and brought to a suitable temperature. The fuel to be examined is put into the bomb, ignited by arcing, and combusted. The gross calorific value can be determined by measuring the warming of the bomb calorimeter.

The gross calorific value (GCV) of a fuel is defined as the quantity of heat generated by the combustion and subsequent cooling of the exhaust gases to 25°C. Both the energy required to heat the combustion air and the exhaust gases, and the heat generated by the evaporation or condensation of liquid – particularly water – are taken into account in this parameter.

The net calorific value (NCV) indicates the heat quantity of a fuel that can actually be utilised. It is calculated on the basis of the gross calorific value, taking into account the exhaust gases released during the combustion process, which discharge part of the fuel’s energy. As a consequence, the NCV is lower than the GCV. The moisture contained in the fuel is particularly significant in this context. The NCV of a dry fuel is higher than that of a moist fuel of the same kind.

This is part one of a two-part article written for World Cement’s March 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 December 2015 issue here.

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