Low-pressure hydrothermal processing of mixed polyolefin wastes into clean fuels

Highlights

• A new LP-HTP method was developed for converting mixed polyolefins to clean fuels.

• Reaction pathways for conversion of PE/PP mixtures in LP-HTP were found.

• LP-HTP saves 90% of capital costs and 80% of energy compared to SWL.

• Energy efficiency and GHG emissions are better than producing fuels from crude oil.

• LP-HTP can produce 190 Mt of fuels annually and save 1.5 billion BOE energy.

Abstract

The amount of polyolefin waste, which is 63% of the total plastic waste, has grown exponentially over the past six decades. Its degradation products pose a serious threat to ecosystems and human health. Polyolefins can be converted to clean oils or fuels by using the supercritical water liquefaction (SWL) method at high pressures (≥23 MPa), which require high capital and energy costs. A new low-pressure (~2 MPa) hydrothermal processing (LP-HTP) method is developed to convert polyolefin waste with 50% or more polypropylene into oils with 87% yield at 450 °C and 45 min. The oils, which are C₄ to C₂₅ hydrocarbons, can be distilled into clean gasoline and ultra-low sulfur diesel. With this method, up to 190 million tons of fuels can be produced annually from polyolefin waste, resulting in savings of 92% of the energy and 71% of the GHG emissions compared to conventional methods for producing fuels.

Introduction

The amount of plastic waste has grown exponentially over the past 60 years and accelerated as COVID-19 spread [1], [2], [3], [4]. Only 9% of the total plastic waste is recycled, and 12% is incinerated [1], [5]. The rest, about 6 billion tons, accumulates in landfills and oceans, where it degrades over decades into microplastics, releasing toxic chemicals into the environment, Fig. 1 [1], [6], [7], [8], [9], [10]. Current technologies for removing plastic pollutants from water cost about $0.003 per gallon [11], [12]. Cleaning up the oceans, containing 3.5 × 10¹⁸ gallons of water, could cost thousands of times the global GDP. Microplastics have been found in drinking water, plant roots, animals, and human organs [13], [14], [15], [16]. Their impact on ecosystems and human health is potentially devastating [15], [17], [18], [19]. This plastic pollution could be a more urgent threat to all life on land or below water than climate change.

Conventional methods, including incineration, mechanical recycling, and pyrolysis, are ineffective in reducing the plastic waste. Incineration releases greenhouse and toxic gases, has low energy recovery, and requires tipping fees ($20/ton) to be profitable [20], [21], [22]. Mechanical recycling of mixed waste typically results in dark-colored, lower-value products. After several (<10) cycles, polymer properties degrade, and the wastes must be landfilled or incinerated [23], [24], [25]. Pyrolysis can convert mixed plastic waste to oils with yields from 50 to 90%, but the oils have a wide carbon number distribution [26], [27], [28], [29], [30], [31], [32]. Fast catalytic pyrolysis generates significant amounts of polycyclic aromatic hydrocarbons and up to 40% char, resulting in catalyst deactivation [33], [34]. The oils from pyrolysis need extensive upgrading and separation to produce transportation fuels or other chemicals. Gasification converts mixed plastic waste into gases such as CH₄, H₂, CO and CO₂ [35], [36], [37], [38]. Gasification has a high energy consumption as it requires a temperature between 500 and 800 °C. Embodied energy in plastic waste is lost because no polymer structure or carbon chain is preserved in gasification. The use of supercritical water in gasification can help reduce char formation, but the combination of high temperature and high pressure (≥23 MPa) could result in a high capital cost [39]. Globally, more than 350 million tons of plastic waste is generated annually [1]; 63% are polyolefins, polyethylene (PE) and polypropylene (PP), which have short lifetimes (<6 months) and low recycling rates (~5%) in the United States [40]. Thus, almost all new polyolefin products are made from virgin feedstocks.

Supercritical water liquefaction (SWL) was shown to convert plastic waste into lower molecular-weight chemicals and the water serves as a solvent, reactant, or catalyst [41], [42], [43], [44]. Polyolefin waste was converted into oils with high yields (~90%) and no char using SWL. PP waste was converted mainly into naphtha; PE waste was converted into wax, clean gasoline blendstock, or diesel [41], [45]. Previous studies were limited to sorted polyolefin wastes, using high operating pressures (≥23 MPa) and requiring high capital and energy costs. Here, we developed efficient and economical low-pressure (~2 MPa) hydrothermal processing (LP-HTP) methods for converting polyolefin mixtures into clean fuels. The conversion pathways of PE, PP, and their mixtures at various pressures were found from comprehensive two-dimensional gas chromatography analysis of the products at different reaction times. Our new optimal LP-HTP methods produced oils with high yields (87%) and little char (<0.5%). The oils produced from the mixed wastes with 50% or more PP were distilled to produce qualified clean gasoline and ultra-low sulfur diesel fuels. With LP-HTP, 220 million tons of polyolefin wastes can be converted to 190 million tons of fuels annually, while saving 1.5 billion barrels of crude-oil-equivalent energy and associated GHG emissions compared to conventional fuel production methods. The oils can also be used for producing other monomers to help achieve a circular economy, Fig. 1.