Co-processing of agricultural plastic waste and switchgrass via tail gas reactive pyrolysis

Authors: Dorado, Christina; Mullen, Charles; Boateng, Akwasi

Submitted to: Industrial and Engineering Chemistry Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2015
Publication Date: 9/21/2015
Publication URL: http://handle.nal.usda.gov/10113/61636
Citation: Dorado, C., Mullen, C.A., Boateng, A.A. 2015. Co-processing of agricultural plastic waste and switchgrass via tail gas reactive pyrolysis. Industrial and Engineering Chemistry Research. 54:9887-9893.

Interpretive Summary: We have previously demonstrated that biomass from agricultural and forestry sources is a sustainable and abundant feedstock for the production of fuels and commodity chemicals via a process called pyrolysis. The liquid produced from the pyrolysis of biomass, called pyrolysis oil, has low value as a fuel due to its high oxygen content, high acidity and low energy content. Pyrolysis oils with reduced oxygen content and acidity and increased energy content can be obtained from the catalytic pyrolysis of biomass in the presence of a catalyst such as HZSM-5. Recently, a promising method for the production of pyrolysis oils with properties similar to that produced from catalytic pyrolysis has been developed by the US Department of Agriculture called tail gas reactive pyrolysis or TGRP. The TGRP process uses the non-condensable gases from the pyrolysis of biomass diluted with nitrogen to produce pyrolysis oil that is deoxygenated and has lower acidity and an increased energy content compared to the pyrolysis of the feedstock under nitrogen. The benefit being that pyrolysis oils with improved properties can be obtained without the need for catalyst. The TGRP process is similar to catalytic pyrolysis in that deoxygenation leads to a loss of carbon in the subsequent oil. This could be alleviated by the addition of a carbon rich co-feedstock for the pyrolysis of biomass such as plastic. Plastics are often used in agriculture for temperature regulation, reducing evaporation, pest control and preservation and transportation of crops. This agricultural plastic waste eventually ends up in landfills, is incinerated or buried. Using agricultural plastic waste as a source of carbon for the co-pyrolysis of biomass can be beneficial as a method for waste utilization and also for the production of pyrolysis oil with improved properties. In this study we set out to find the conditions necessary to produce pyrolysis oils with improved properties from the TGRP of switchgrass and hay bale covers used in agriculture. Even at low concentrations of hay bale cover plastic mixed with switchgrass, pyrolysis oils with improved properties compared to the regular pyrolysis of switchgrass alone were produced via TGRP. The results of this work can be beneficial to those looking to develop processes for the production of pyrolysis oils with improved properties.

Technical Abstract: Mixtures of agricultural plastic waste in the form of polyethylene hay bale covers (PE) (4-37%) and switchgrass were investigated using the US Department of Agriculture’s tail gas reactive pyrolysis (TGRP) at different temperatures (400-570 deg C). TGRP of switchgrass and plastic mixtures significantly reduced the formation of waxy solids that are produced during regular pyrolysis. Under an atmosphere of approximately 70% recycled tail gas, mostly non-condensable gases were produced along with highly deoxygenated and aromaticized pyrolysis oil. When the atmosphere was diluted further to a recycled tail gas concentration of about 55%, higher yields of liquid product were achieved, but with less deoxygenation. TGRP of low plastic mixtures (4-8%) produced oils with increased carbon and reduced oxygen content compared to the fast pyrolysis of switchgrass alone. Non-condensable gas fractions containing high concentrations of H2, CO, ethylene and other light hydrocarbons remained a significant portion of the product mixture at temperatures above 500 deg C.