Techno-Economic Evaluation of Refuse Derived Fuel Torrefaction

Abstract

The last decade has seen an increase in the world wide energy demand, it was recorded as the highest spike encountered in the energy demand over a decade. On analyzing the main sources which were able to satisfy this overwhelming demand it was found that the majority was still from non-renewable sources or fossil fuels such as coal, oil and natural gas. The aftermath of the continued use of these fossil fuels are already well known, one can argue that there has still not been a serious attempt to phase out these non-renewable sources even though the concerns regarding the harmful emissions being released with their continued use are subject of numerous discussions and studies. It is paramount that the share of renewable energy technologies in the overall energy mix needs to be increased,various options must be explored such that it is possible to substitute these renewable energy sources in place of fossil fuels particularly in energy and emission intensive industries without a comprise in meeting their energy demand. Refuse derived fuel (RDF), which is the combustible fraction that has been separated from municipal solid waste (MSW) has gained attention due to it being seen as an alternative to convectional fossil fuels as well as a method of sustainable waste management and disposal. The reason for their use as an alternative fuel is mainly attributed to their physical and chemical characteristic such as their low moisture content, high grindability and calorific value. However, the challenges faced in their application as a substitute fuel or feedstock is attributed mainly to the high variability in the properties of the RDF material that is being supplied to the relevant industries. Therefore a standardization in its properties needs to be carried out particularly with regards to their moisture content and calorific value in order to promote their application. Torrefaction, also referred to as mild pyrolysis is a thermal treatment method that usually is carried out in temperature ranges of 200-300°C and residence times of 30-60 min in the absence of oxygen. The material that is being subject to this treatment method partly decomposes to release volatiles,and the left over solid product has been reported to undergone a modification its physical and chemical characteristics when compared to the initial solid material. From a chemical point of view the final solid product is reported to have an increase in the carbon content and decrease in the hydrogen and oxygen content which leads to a higher calorific value than the original material. Several experimental studies have been conducted for the torrefaction of RDF with aim of improving and standardizing the calorific value and have yielded promising results. However there has not yet been a work that has focused on the torrefaction of RDF on large scale, that takes into account the process design and economics associated with their production. The aim of this thesis was to carry out a techno-economic evaluation into the torrefaction of refuse derived fuel. An initial composition of raw RDF was selected from literature which has a high moisture content and low calorific value. The raw material is then subject to several process such as drying, torrefaction, grinding and pellitization with the aim producing torrefied RDF pellets. These process were simulated and optimized through Aspen Plus in order to obtain a better understanding of the influence of process parameters such as moisture content, drying temperature, torrefaction temperature and residence time on the final product as well as to select the optimum route for their production. Economic evaluation was also carried out to determine if the implementation of such a project is feasible. This was done on the basis of certain economic or profitability indicators. Results from simulations show that the optimum torrefaction conditions were 250°C and residence time of 30 min, which resulted in the final torrefied RDF having an increased calorific value. The economic analysis provides positive results particularly when moving towards a higher processing capacity of initial raw RDF. Finally, the effect of substitution of the torrefied RDF in a cement plant was also evaluated and has shown that significant savings can be achieved through the reduction in fossil fuel consumption and carbon dioxide emissions.