An experimental study on bio-oil production from co-pyrolysis with potato skin and high-density polyethylene (HDPE)

Abstract

In order to investigate the synergistic effect of high dense polyethylene (HDPE) addition to potato skin (pS) during co-pyrolysis, pure HDPE was pyrolysed as a model compound for determination of the optimum process conditions. Thermal decomposition of pure HDPE was performed at constant heating rate of 5 °C min^− 1 under various pyrolysis temperatures (400, 420, 450, 470, 500 and 550 °C) and sweeping gas flow rates (100, 200, 400 and 800 cm³ min^− 1). The maximum bio-oil yield was 56% at 500 °C under sweeping flow rate of 400 cm³ min^− 1. Then, co-pyrolysis of pure HDPE-pS and waste HDPE-pS mixtures were pyrolysed with various proportions such as 1:0, 1:1, 1:2, 2:1 and 0:1. The yield of liquids produced during co-pyrolysis enhanced with increasing weight ratio of HDPE in mixtures. Obtained bio-oils were analyzed in detail with various spectroscopic and chromatographic methods. Co-pyrolysis oil had higher carbon and hydrogen contents, lower oxygen contents with higher heating value than those of pyrolysis. It was concluded that addition of HDPE improved the liquid yields in terms of both quality and quantity.

Introduction

The increasing consumption of polymeric materials in modern daily life causes loss of natural resources, the environmental pollution, and the depletion of landfill space [1]. Nevertheless, the non biodegradable polymeric waste materials such as plastics, tires require the main difficult disposal methods. Presently, the most common solution to deal with organic wastes, namely land filling and incineration do not appear to be the most suitable ones since they present various problems related with the environment. For instance; landfill areas are limited. Also, land filling could pollute environment such as phthalates and various dyes found in plastic additives leak into ground water. Incineration is an alternative to landfill. However, this process causes formation of unacceptable emissions of gases such as nitrous oxide, sulfur oxides, dusts, dioxins and toxins [1], [2].

The utilization of such waste materials is indeed of importance from economical and environmental aspects. Especially, packing materials make up the 50–70% of the total plastic disposal, containing mainly 89% polyolefins (polyethylene, polypropylene, polystyrene, polyvinyl chloride). Nevertheless, decomposition of polyethylene and polypropylene do not give high yields of ethylene and propylene with the ordinary recovery techniques [3], [4]. Pyrolytic processes are suitable to convert polyolefins and cellulose (or lignin) derived materials into valuable feedstock and the specific benefits of these methods potentially include: the reduction of the volume of waste, the recovery of chemicals, and the replacement of fossil fuels [5].

The aim of mixing polymers with biomass wastes is to improve the liquid fraction of the products, and to evaluate the H-donor effect of polymers. There are several studies about co-pyrolysis of biomass–polymers which has indicated the synergistic effect on liquid production [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. According to these studies, the composition and the nature of the biomass and synthetic polymer as well as the pyrolysis conditions have a great influence on the yield, the chemical structure and the physical properties of the products. The addition of polymer to biomass has a positive effect on the yield and basic physical properties of bio-oil.

Potato skin is a promising biomass candidate. According to our previous study, resultant oil from pyrolysis of potato skin has a high aliphatic content which is very beneficial for artificial fuel production. Potato is an essential nutrition in the world. In Turkey, potato is grown on 200 thousand ha, and 4.5 million tons of potato is produced every year. Revealed residue amount is 495,000–594,000 tons/yr. Converting the organic matters in residues such as polymers and food industry waste into more valuable, concentrated forms of energy by pyrolysis will be a sustainable way for waste management [16].

The thermal degradation of the organic materials, the product distribution and consequently the economics of the process are strongly influenced by the experimental conditions used. In the present work, co-pyrolysis of pS with pure and waste HDPE was investigated for the first time to optimize the experimental conditions for bio-oil production. Obtained bio-oils were analyzed in detail with various spectroscopic and chromatographic methods. As a result, the addition of HDPE caused an increase of yields and also improvement of fuel properties.