Different options to upgrade engine oils by gasification with steam and supercritical water

Highlights

• Upgrading engine oils by gasification with steam and supercritical water up to 500 bar.

• Two different ways of upgrading are proposed depending on the temperature.

• This method achieves turn more than 85% of oil into valuable gases.

• The effect of pressure on gasification depends on the structure of feedstock.

Abstract

This work reports for the first time a treatment to manage engine oils in an environmentally friendly manner which, besides, upgrades them into valuable gases: gasification with steam and supercritical water (SCW). Pressure, temperature and wt% ranges from 50 to 500 bar, 500–800 °C and 0.42–2.0%, respectively, were investigated. The process can be faced in two different ways regarding the temperature. A low-temperature gasification at 550 °C to transform up to 60% of oil into carbon-containing gases such as CH₄ and ethane in less than 2 min; and a high-temperature gasification at 750 °C that produces H₂, CH₄ and CO₂ as main gaseous products. Namely, SCW gasification at 250 bar, 750 °C and 0.83 wt% for 1.9 min transformed more than 85% of the oil into a gaseous mixture containing 37% H₂ and 29% CH₄, two valuable green fuels. The treatment is herein used to upgrade fresh engine oils but it is potentially applicable to the upgrading of such an abundant and polluting residue as waste engine oils.

Introduction

Lube oils are complex blends of hydrocarbons generated in the refining process of crude oil. These oils must be periodically changed because the original hydrocarbons are progressively degraded, and water and charred residues are added to the blend during their lifetime; thus oils are unable to accomplish their original role. Currently, waste lube oils (WLOs) are the most abundant liquid pollutant in Europe [1]. Furthermore, WLOs are highly pollutant residues since they contain polycyclic aromatic hydrocarbons (PAHs) and high levels of heavy metals [2].

WLOs can be managed in different ways. The regeneration of WLOs is achieved through a series of treatments (removal of water and fuel added during its lifetime; removal of solid particles, such as heavy metals and soot; fractionating stage; and removal of chlorinated, oxygenated, and sulphurated species) that allows recovering and reusing a fraction of the original blend [3,4]. Regeneration leads to the greatest reduction of the environmental damage [5], but not all of the WLOs can be regenerated. It is estimated that only 60–65% of WLOs can be regenerated since they must fit specific polluting levels and viscosity indexes [3]. Furthermore, a highly polluting fraction is discarded during the regenerating process that demands further purification treatments.

Bioremediation is an alternative to regeneration, in which WLOs are degraded by the biological activity of certain microorganisms, usually bacteria [6,7]. The pollutants are destroyed, but the energetic potential contained in the blend is wasted. WLOs can also be reused without any treatment sometimes, e.g., as a lube in other applications [8] or as a binder in the manufacturing of asphalt and cements [9,10].

Different techniques exist that allow making the most of the energetic power contained in WLOs. Their use as fuels in cement kilns was the most employed valorization method until recent decades [2], but this method damages the environment because it releases significant amounts of polluting gases. Fractionating the hydrocarbons contained in the WLOs to transform them into valuable species is a trending alternative to direct burning [5]. Thermal cracking is the most classical method to crack hydrocarbons, and when followed by a distillation stage, allows obtaining short-chain hydrocarbons from WLOs [11]. Most of efforts concerning this technology are focused on the production of liquid diesel-like hydrocarbons [12,13]; however, depending on the process conditions, volatile hydrocarbons can be obtained [14,15]. Classical heating with hot inert gases is the starting point to other technologies, such as catalytic thermal cracking [16,17], pyrolytic distillation [18,19], electric-arc pyrolysis [20], or pyrolysis with microwave heating [21]. High-temperature oxidation is another alternative to upgrade WLOs. Guo et al. [22] made a partial oxidation of a mixture bio-oil/WLO with air at 1050−1100 °C to produce syngas; however, their ultimate goal was not gasifying the mixture but producing syngas as a reagent to synthesize lower alcohols (C₁–C₅ mixed alcohols).

On the other hand, gasification with steam is one of the most common methods to upgrade different types of biomass with an organic base. Gasification is likely to be an ideal and practical method for waste to energy application if the extremely high process temperature (up to 1600 °C), which is the main limitation of this technology, can be reduced to a lower process temperature [11]. When steam is compressed above the critical pressure of water (P_c = 221 bar), the resulting fluid shows different properties to steam. Supercritical water (SCW) dissolves organic compounds and gases, what reduces mass transfer limitations, and it also behaves as a reagent and a catalyst [23].

Steam and SCW have been used to gasify and upgrade different types of biomass and organic pollutants [[24], [25], [26], [27], [28]]. SCW gasifies the biomass more efficiently than steam given its above-commented properties; thus, supercritical gasification can be performed at lower temperatures than steam gasification. Although this method is widely spread to upgrade biomass, the gasification with steam and SCW of WLOs, a different type of organic residue, has not been explored to the best of our knowledge. For the first time, this study reports the gasification of engine lube oils with water under different states of the matter, steam and SCW, and their conversion into different valuable gases.

WLOs contain noticeable amounts of heavy metals and PAHs [[29], [30], [31]], highly harmful compounds for human health. The health risks that the performance of many gasification assays using WLOs may imply led us to perform the study using fresh synthetic lube oil, a blend with a similar composition but with low aromatic contents and without heavy metals. Linear and branched paraffins with high molecular weights are the main compounds contained in these blends, but they have also a meaningful cyclic paraffin content [32,33].