Co-pyrolysis of rice husk and plastic wastes: liquid optimization through response surface methodology

In the near future it is predictable that fossil fuels will continue to have the greatest contribution to energy production. Today’s energy needs account for almost 85% from fossil fuels, including coal, oil and natural gas. As a result, the increasing need to find alternative fuels, together with aiming to reduce the negative environmental impact of wastes accumulation has led to the idea of studying the thermochemical conversion of biomass in the presence of plastic wastes. The increasing amount of these wastes found in Portugal as well as in EU countries encourages scientific community to develop and implement new projects on this subject. For this purpose, co-pyrolysis of rice husk (RH) and polyethylene (PE) waste was performed aiming at liquid fuel production as well as the production of valuable chemicals raw materials. The overall process of rice production generates rice husk and straw wastes and also plastic bags used to transport seeds, fertilizers and plastics bags used for rice packaging. Plastic wastes usually end-up in landfills, because their amount of dirt and dust do not allow them to be recycled. Thus, the main objective of this work is to study the energetic and organic valorisation of rice production main wastes by co-pyrolysis to produce bio-fuels to substitute fossil fuels and electricity consumption during rice milling processes. Thus, the use of fossil fuels will decrease, together with CO2 emissions, due to the use of biomass wastes. Plastic and biomass pyrolysis has been studied by several authors, however the information about co-pyrolysis of biomass and plastic wastes blends is limited. Co-pyrolysis of rice husk and PE bring new challenges, especially due to the high silica contents of rice husks. On the other hand the presence of PE may favour biomass conversion, as PE is easily transformed into liquids by pyrolysis, which increases heat and mass transfer in the reaction medium. The work done aimed to study the effect of experimental conditions to optimize liquid products yields and properties to allow its use as fuels. A set of experimental runs was conducted to define the waste blend to be used in the process. Response surface methodology (RSM) was employed in the definition of the operational conditions (reaction temperature, initial pressure and reaction time) that maximize liquid production. Regression analyses of experimental data were performed according to RSM. In addition, this methodology not only allowed defining which operation condition affected more liquid yield but also reduced the experimental effort to be undertaken. Experimental apparatus used was composed by 1 liter capacity batch reactor of Hastelloy C276 built by Parr Instruments. According to accumulated knowledge and experimental constrains of reactor, the range of operational conditions studied were: 350-430 ºC for reaction temperature, initial pressure from 2 to10 bar and reaction time varied between 10 and 60 minutes. This study also discusses the viability of using pyrolysis technology applied to these wastes, the effect of operation conditions on products yields and composition as well as the possible synergetic effects that may occur during the pyrolysis of these wastes.