Lars-André Tokheim

Mathematical Model for Alternative Fuel Combustion in a Rotary Cement Kiln Burner

Increased use of alternative fuels in cement kilns is a trend in the world. However, replacing fossil fuels like coal with different alternative fuels will give various impacts on the overall kiln process due to the inherent fuel characteristics. Hence, it is important to know to what extent the fossil fuels can be replaced by different alternative fuels without severely changing process conditions, product quality or emissions. In the present study, a mathematical model based on a mass and energy balance for the combustion of different alternative fuels in a cement rotary kiln was developed. First, the impact of different fuel characteristics on kiln gas temperature, kiln gas flow rate and air requirement were observed by using coal (reference case), meat and bone meal (MBM), two different wood types, refuse derived fuel (RDF) and a mixture of saw dust and solid hazardous waste as the primary fuel. It was found that the key process parameters depend largely on the chemical characteristics of the fuel. It appears that MBM shows quite different results from other alternative fuels investigated. Next, simulation of combustion of a mixture of coal and MBM in the rotary kiln burner was carried out in three steps in order to find the reduction in production capacity compared to the reference case. Around 9% of reduction in clinker production rate could be observed when replacing 59% of the coal energy input. Results from a full-scale test using the same mixture of coal and MBM verified the simulation results.

Substitution of coal by refuse derived fuels (RDF) in the precalciner of a cement kiln system

Cement manufacturing is an energy intensive process where coal is the main energy source. Replacement of coal with refuse derived fuels (RDF) will reduce costs and CO2 emissions as RDF is partly carbon neutral. In modern cement kilns, most of the calcination (CaCO3(s)  CaO(s) + CO2) is carried out in the precalciner, and the process conditions there make it a good reactor for combustion of alternative fuels like RDF. However, most waste-derived fuels have a poorer quality than coal, and this could negatively affect the process. In this study, a model of the calcination and fuel combustion in the precalciner has been developed, and different fuel input cases have been simulated. The results indicate that it is feasible to use RDF to fully replace coal as an energy source in the precalciner. However, to obtain the same degree of calcination, the specific energy consumption will increase by 10% compared to coal if the RDF moisture content is 25% (a quite typical value). This is due to the fuel characteristics. As a result of the increased energy consumption, the exhaust gas flow rate also increases. Using RDF with 25% moisture content increases the exhaust gas volume flow rate from the kiln system by 11%. This is equivalent to a reduction in the production capacity. Considering that waste fuels like RDF are usually less expensive than coal, operating a precalciner with RDF as a coal replacement becomes a trade-off between lower fuel costs and lower production capacity.