Investigations into the Thermo-chemical Conversion of Simulated Refuse Derived Fuels with Different Plastic Contents

Abstract

The burgeoning problem of municipal waste generation along with its disposal problem has urged the development of sustainable waste management practices such as waste-to-energy conversion. Refuse-derived fuel (RDF) represents the combustible fraction of municipal solid waste such as plastics, paper, textiles, and organics and is used for renewable energy generation. RDF utilization through gasification is considered a cleaner, promising, and efficient route, and is the focus of this work. In India, the RDF gasification technology is not well-established due to the various unaddressed challenges such as low fuel quality (fluffy form/lower density), high ash (~ 20 %), and high plastic content (~ 35 %) of the Indian commercial RDF. Besides, the lack of fundamental understanding of the effect of the plastic fraction on the RDF conversion process has further slowed down the development of RDF gasification technology. Plastic fraction intrinsically influences the RDF's physicochemical, and mechanical properties, which in turn can govern the conversion process of RDF. The present work has attempted to address these challenges of RDF gasification and finally demonstrated a proof of concept for the feasible plastic content in RDF for downdraft gasification applications. In this dissertation: the first objective reports the fundamental combustion characteristics of high plastic (~35 %) and high ash (~18 %) content RDF pellets produced commercially in India through single particle and packed bed studies. Single particle studies reported that RDF exhibited a unique behavior of swelling (5-10 %) due to plastic melting, resulting in a fragile char structure with reduced structural integrity. Packed bed experiments concluded that the high plastic content triggered the clinker formation of RDF ash even at a lower air mass flux. It was recommended that optimizing the existing plastic fraction in RDF while minimizing its ash content is important for successful RDF gasification. The second objective of the study was to simulate RDF compositions having different plastic fractions. Two RDF mixes having 35 % plastic fraction (RDF-1), and 5 % plastic fraction (RDF-2) were prepared based on Indian commercial RDF composition. The effect of the variation in the plastic fraction was investigated on the physicochemical, mechanical, and thermal properties of RDF. The thermogravimetric-based kinetic studies showed that the effect of plastic variation was a function of the reacting atmosphere. In the N2 atmosphere, RDF-1 reported higher activation energy (E), whereas, in the O2 atmosphere, RDF-2 reported higher E. The physicochemical characterization of the simulated pellets showed that increasing the plastic fraction from 5 % to 35 % reduced its pellet density, and mechanical strength by 25 %, and 60 %, respectively. The RDF-1 having lower pellet density showed lower conversion times (36%), and higher flame speed (11%), which are desirable conditions for faster conversion. However, packed bed studies showed limitations regarding the utilization of a high plastic content RDF-1. The RDF-1 had a narrow range of operating air mass flux due to the early advent of convective cooling of the bed. The third objective investigated the effect of the plastic fraction on the gasification behavior of these pellets in a downdraft gasifier. It was interesting to note that, in gasification experiments, as the plastic fraction increased, the gasifier bed temperatures reduced. Experiments showed that RDF-1 char was crushed inside the gasifier because of its lower initial strength and continued loss of strength during gasification. The crushed RDF-1 char decreased the bed porosity and finally leads to an increased pressure drop inside the reactor. Consequently, the producer gas calorific value, cold gas efficiency, and carbon conversion were reduced, and the tar content increased for RDF-1. So, it was advised to have a plastic fraction of < 35 % in RDF for gasification applications. Finally, in the fourth objective to maximize the effective utilization of plastic in RDF with suitable pellet properties, gasification experiments with a plastic content of 15 % (RDF-3) were conducted. The gasification operations for RDF-3 were stable with no crushing of char. The downdraft gasification of RDF-3 pellets reported an average gas calorific value of ~ 3 MJ kg-1 with a tar content of 1.4 g Nm-3. This work presented a systematic approach to investigate the effect of the different plastic fractions on the RDF thermo-chemical conversion process. The detailed investigation and analysis have helped in defining a workable plastic content in RDF for downdraft gasification applications, which marks an important step in addressing the ever-increasing municipal waste handling challenges.