Horse manure derived biofuel production via microwave induced pyrolysis

Abstract

Uncontrollable amount and unsystematic management of generated wastes due to the ever-increasing global population of human and animal have led to the alarming environmental pollution and health issues. Thus, waste-to-product transformation process is gaining popularity in recent years as this allows for the conversion of zero-value waste into beneficial end products. Horse manure (HM) is deemed to be a feasible feedstock for waste-to-energy transformation through thermochemical conversion. Conventional HM management methods, such as natural composting, have led to undesirable environmental pollution. The aim of this study was to determine the feasibility of HM to be converted into bio-products through microwave pyrolysis. The decomposition and kinetics behaviour of HM were investigated through thermogravimetric analyser (TGA) at temperatures of 27-900 °C and heating rates of 1-10 °Cmin-1. The feedstock experienced rapid decomposition at temperatures between 190 °C and 400 °C under oxygen-free environment. Such a decomposition process was found to be endothermic and endergonic in nature, giving an activation energy of approximately 148.5-300.1 kJ/mol. The finding suggested that pyrolysis is the preferred thermal conversion pathway. A lab-scale microwave-heated pyrolysis reactor was then setup for the decomposition process of HM whilst collecting end products in the form of solid (bio-char), liquid (bio-oil) and gas (bio-gas). The produced bio-gas consists up to 70.2 vol% of syngas (H2 and CO) along with CH4 (12.6 to 23.4 vol%), making it a gaseous fuel candidate for heat/power generation. Although the bio-oil is made up of oxygenated compounds that resulted in low heating value (6.2-15.8 MJ/kg), its high phenolic content (up to 79 wt.%) remains useful for application as bio-chemical product. The bio-char derived from the microwave pyrolysis of HM is highly porous (surface area of up to 698.4 m2g-1), having potential as bio-adsorbent products. The bio-char produced also gained in heating value (up to 111.1%), indicating solid fuel-like properties. The optimum pyrolysis process parameter in yielding desired amount of end products was measured statistically through a full factorial design (FFD). The results indicated AC/HM ratio as the most influential factor, followed by temperature and carrier gas flow rate. Optimization of the end products suggested that a process condition with AC/HM ratio of 2.0, N2 flow rate of 0.5 Lmin-1 and temperature of 550 °C, giving end product yields of 13.5 wt.% of bio-char, 32.5 wt.% of bio-oil and 39.2 wt.% of bio-gas. Lastly, a life-cycle-analysis was also conducted and the results show that HM feedstock is more environmentally friendly as compared to swine manure, when both are processed through pyrolysis. The management of HM through pyrolysis is also found to be the preferred pathway as compared to incineration, anaerobic digestion and natural composting. Overall, the study demonstrated that microwave-induced pyrolysis has the potential to thermochemically-convert HM into beneficial end products. Coupling this with the positive outcome from the life-cycle-analysis, it could be further summarised that HM is indeed a viable feedstock to be considered for valorisation purposes through microwave-induced pyrolysis.