(173r) Conversion of Waste Plastic to Oils for Feed to High-Pressure Gasifiers

The focus of this work is to understand the properties of oils produced from waste plastics and make plastic oils specifically tailored for gasification. The plastic oil will be one of the components of a slurry that also consists of biomass pyrolysis liquids and ground coal waste; this blend will be gasified for the production of hydrogen or other high-value products.

Polystyrene (PS), Polypropylene (PP), Low-Density Polyethylene (LDPE), High-Density Polyethylene (HDPE), and mixtures of these plastic pellets are subjected to thermal pyrolysis in a two-stage fixed-bed reactor (batch system) to create improved plastic oil that flows easily at room temperature for gasifier feed. The reactor is operated over a range of conditions (including various temperatures and residence times) to understand relative yields and compositions of liquid, gas, and char, as well as the viscosity characteristics of the resulting liquid. The goal is to identify conditions that will produce suitable plastic oil for gasification with minimal processing intensity.

For each test, 100 g of plastic pellets were loaded inside a container that was placed into the first section of the reactor. As pyrolysis vapors evolve from the plastic bed, they are carried into the second section of the reactor by the nitrogen carrier gas with a flow rate of 200 ml/min. Reactor sections 1 and 2 can be heated at different temperatures (400 to 600⁰C), then, the condensable plastic pyrolysis volatiles are collected as plastic oil in a series of chillers, which are chilled to

-15⁰C. Two acrylic tubes in series filled with activated carbon and silica gel, respectively, are used to clean the gas of any remaining oil and moisture before passing through a micro-GC, which allows the identification of resulting gaseous species such as H₂, CO, and light hydrocarbons (C₁ through C₃).

Figures 1 and 2 summarize the preliminary experimental results obtained from the pyrolysis process in the two-stage fixed bed reactor.

1. a) Liquid Yield: Represents the mass % of the material loaded that was collected as liquid in the chilled jars.
2. b) Char Yield: Represents the mass % of the material loaded in the reactor that stayed as carbonaceous/char-type material.
3. c) Gaseous Yield: represents the expected mass % of the material loaded that should be converted to gaseous species, these numbers are obtained by difference. % of gaseous products yield = 100% minus % reported in a+b.

Liquid products: PS produces large amounts of liquid products (more than 95% of the mass loaded) at both 500⁰C and 600⁰C (a slight yield reduction is observed at 600⁰C vs 500⁰C). The liquid yield for PP is considerably larger at 500⁰C than at 600⁰C, but the resulting liquid product at 500⁰C is very thick and flows with difficulty at room temperature. About 70% mass of the loaded plastics (LDPE and HDPE) is collected as liquid material when pyrolyzed at 600⁰C, but they are thick fluids that flow with difficulty at room temperature. These two plastics (LDPE and HDPE) were not tested at 500⁰C because they would have created even thicker materials that could have clogged some components of the experimental system.

The plastic oils produced from PS, PP, and the mixture of PS-PP-HDPE (33.3% each) are liquids and flow easily. The presence of PS and PP in the mixture helps dissolve the thick product created from HDPE.

Solid products: Small amounts of char-type material are created from all plastics (around 1% or less of the mass loaded). LDPE and HDPE produce the higher amounts, followed by PS and PP.

Gaseous products: The expected yield % of gaseous species needed to close the mass balance of the test are shown in figures 1 and 2 (gaseous products, yield %), but the gaseous species detected by the micro-GC (hydrogen, methane, ethylene, ethane, propylene, and propane), are only a portion of the total gaseous species.

Other mixtures/plastic ratios will be tested to better understand the influence of PS and PP on polyethylene plastic oils and rheology measurements will be conducted to determine the viscosities, cloud point, and pour point of the plastic oils.

Other parameters to be evaluated include changing the residence time of the gases in the pyrolysis reaction zone (by changing the N₂ flow rate), the presence of contaminants such as oil, paper, soil, and bio-solids in the plastic pellets and see their effect on plastic oil characteristics.