Combustion of municipal solid waste (MSW) and refuse-derived fuel (RDF) with low rank coal; For waste to energy concepts

Abstract

With rapid increases in population and urbanization, uncontrolled municipal solid waste (MSW) is a threat to public health and environmental safety. In this study, we explore its generation, treatment, and characteristics of physical/chemical composition, and assess the potential of MSW and refuse-derived fuel (RDF) as an alternative renewable energy source in Lahore, the second largest city in Pakistan. Based on the average generation rate of MSW (i.e., 0.65 kg/capita/day), the daily production of MSW in this city would reach 7,150 tons/day. However, its disposal in a safely engineered way has been restricted due to the lack of: (a) pre-planning, (b) infrastructure, (c) political will, and (d) public awareness. Various samples of MSW considering socio-economic structure were collected. The physical components of MSW in Lahore were found to be in the descending order of biodegradable, nylon plastic bags, textile, diaper and paper. The inductively coupled plasma optical emission spectroscopy (ICP-OES) technique was used to determine the heavy metal content and leachability of the MSW components to check for the environmental contamination risk. The proximate and ultimate analysis of this MSW was also carried out along with its heating values. The average high heating value of MSW was measured as 14,490 kJ kg-1. Energy recovery potential of 48 MW was assessed further from 2000 tons of MSW/day. In addition to basic characterization, the thermal behavior of fossil fuel (coal) and solid wastes such as municipal solid waste (MSW) and refuse derived fuel (RDF) were investigated by thermogravimetric analysis (TGA) to compare their thermal decomposition behavior in combustion and co-combustion processes.The experiments were performed in a thermogravimetric analyzer by using non-isothermal condition with temperature range of 105 °C to 1000 °C, at four heating rates (10, 20, 30 and 40 °C /min). The TGA profiles of samples indicate low reactivity of coal, whereas solid wastes present greater reaction rate reflecting their low ash and high volatile content. The obtained thermal data of cobumstion process was used to calculate the kinetic parameters using model-fitting methods (i.e., Arrhenius and Coats-Redfern) and model-free (isoconversional) methods (i.e., Kissinger-Akahira-Sunose, Flylnn-Wall-Ozawa, and Friedman, and Vyazovkin). In case of model-fitting mehtods, the percentage difference between activation energy values found from Arrhenius and Coats-Redfern models was in range of 4.1-26.5%. However, the Coats-Redfern model exhibited consistency in activation energy values, followed by high value of R2 . Whereas in case of solid wastes, it is possible to say that all isoconversional methods presenting similar trend of activation energy for the conversion range of 0.1 - 0.6 and 0.7-0.9. In case of coal, Friedman model exhibits lower and inconsistent values of activation energy than others selected isoconversional methods. According to these models the fuels could be arranged in order of activation energy as MSW & RDF & Coal. The co-firing of these solid wastes (10%, 20%, 30% and 40%) with coal show lower temperature region in most of the thermal properties. Another existing challenge for incineration industries is the management of solid residue such as bottom ash and fly ash. This research work aslo aims to integrate information on fly ash derived from the combustion of municipal solid waste (FA1) and biomass (FA2) in fluidized bed incinerator facilities. Fly ash samples were comparatively analyzed by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM), and inductively coupled plasma optical emission spectroscopy (ICP-OES) to study the mineralogy, morphology, total heavy metal content, and leaching behavior, respectively. The analysis revealed that the two types of fly ash differ in their characteristics and leaching behavior. The concentration of most of the heavy metals in both is low compared to the literature values, but higher than the regulatory limits for use as a soil conditioner, whereas the high contents of Fe, Cu, and Al suggest good potential for metal recovery. The leaching ability of most elements is within the inert waste category, except for Hg, which is slightly above the non-hazardous waste limit. This study recommends that the combustion and co-combustion of solid wastes (MSW and RDF) with coal could be a promising alternative renewable energy source resulting in better waste management strategies, including a reduction in GHG emission. The results of this study should be helpful for policy makers to establish a MSW management strategy for its use as potential renewable energy alternative, resulting in high return with environmental benefits.