Thermoeconomic analysis of alternative fuels on rotary kiln performance of Obajana cement plant

Abstract

A process model for the operation of rotary kiln in the cement production process was developed using Aspen Plus to study the effect of alternative fuels such as used tyre, municipal waste (MSW) and bagasse on clinker quality, emissions and thermal stability by introducing 5%, 10%, 15%, 20%, 25% and 30% of the alternative fuel mix with coal. The three fuels increased the Lime Saturated Factor (LSF) compared to 100% coal, but the observed values were within acceptable limits. Bagasse had the best silica ratio (SR between 2.49458 to 2.49405), with Used Tyre and MSW having a low silica ratio below 1.6. However, the low SRs were complemented by their alumina ratios (ARs). Used Tyre and Bagasse can maintain the kiln temperature (1640 0C to 1690 0C) up to the reference case. Tyre and MSW can reduce CO2 emission, but Bagasse does the opposite with a capacity of up to 9.67 kg/tonne clinker increment in emission at 30% fuel mix. The three alternative fuels considered were found to increase NOx emissions. Used Tyre was the most preferred alternative fuel due to its ability to reduce CO2 emission while ensuring thermal stability in exergy efficiency and kiln outlet temperature, coupled with its worldwide availability. However, caution must be employed to monitor its SOx emissions; hence it should not exceed 20% in the fuel mix to keep SOx emissions below 310 mg/Nm3. The alternative fuels were more cost-effective (as higher configurations resulted in lesser costs, particularly in MSW with a cost-saving up to 613.92 $/hr in 30% fuel mix) but less efficient than coal in terms of exergy, with bagasse as the potential best (difference of less than 3%). It can be inferred from the results obtained that the development of alternative fuel from blending bagasse and coal for a rotary kiln is economical and feasible.