Utilization of waste products as alternative fuels for cement industry

Cement production is an energy intensive industry and is mostly dependent on fossil fuels like coal and natural gas to fulfil the energy demand. Excessive usage of fossil fuels leads to higher CO2 emissions. About 5%–6% of anthropogenic CO2 is released from the cement industry which is a significant concern for the environment. Besides CO2, other greenhouse gases like NOX and SO2 and some heavy metal discharges from cement industry place it under continuous scrutiny of local government and environmental protection agencies. In contrast to fossil fuel, waste derived alternative fuels offer cheaper energy sources which are capable of reducing the pollutant emissions and are environmentally sustainable. Still there are concerns on using alternative fuel regarding the quality of the cement and some emission issues as not all waste derived fuel reduces the greenhouse gas emissions. Previous studies show that no single alternative fuel could replace the entire energy requirement for cement manufacturing. In this context there is a need to identify a perfect blend of alternative fuels which could replace the fossil fuel for cement production. The main aim of this study is to investigate the utilisation of waste products as alternative fuels in cement manufacturing and maximise their usage. This study explored the impact of using waste-derived alternative fuels on pollutant emissions and on the quality of clinker. This thesis determined the maximum substitution rate of major alternative fuels in the cement industry. The study was divided into four major sections with specific research goals. Firstly, a feasibility study was undertaken to identify the potentialities of alternative fuel sources in Australia, in particular solid alternative fuels. The second part of the study was to develop a novel computational process model capable of predicting the outcomes of using different solid alternative fuels in terms of emissions and clinker quality. The process model was developed in three stages: the preheater tower model, the kiln model and the integrated model. The third part of the study consisted of using the computational model to optimise the usage of solid alternative fuels in cement production. This part also determined a perfect blend of alternative fuels which could replace all or a major portion of the fossil fuel that was being used for energy supply. Finally, the simulation model predicted the potential opportunity of energy saving by reducing the energy requirement for clinker production without altering the process parameters and clinker quality. Alternative fuel substitution rates in Australia are way below best practice all over the world. This study is the first of its kind in the Australian context to investigate and determine the maximum substitution rates of potential alternative fuels for cement manufacturing. Many alternative fuels were considered throughout the study, but only five were examined using the integrated model, namely waste tyres, MSW, MBM, plastic waste and bagasse. Based on the simulation results of the investigated model, it was found that the maximum substitution rates of selected alternative fuels for the operating conditions of local plants are: waste tyres 18%, MSW 15%, MBM 20%, plastic waste 12% and bagasse 5%. Emission results indicate that, in optimised operating conditions, each alternative fuel can reduce CO2 emissions and about a 5% reduction in CO2 was achieved for MSW and MBM. It is to be noted that the selected alternative fuels have very minimal influences on clinker quality. In the process of determining maximum substitution rates, a baseline emission standard was set as no such standard is available at the Australian national level. The baseline standard outlined in this study could be the starting point for policy makers to develop a national emissions standard for Australia. The integrated model identified the potential energy saving opportunity through employing alternative fuels and a maximum of 6.4% energy saving can be achieved by using MBM in optimised conditions. This study provided a clear understanding and guidelines to Australian cement manufacturers and stakeholders on using different alternative fuels in optimal proportion. The computational model presented in this thesis could be one of the important tools for the cement industry to identify new alternative fuels before being tested and implemented. This study also promoted alternative fuels as a climate friendly sustainable energy source for the cement industry which may help the community to develop a green future.