Efficiency and energetic analysis of the production of gaseous green fuels from the compressed steam and supercritical water gasification of waste lube oils

Highlights

• H₂ and CH₄ are obtained from steam reforming and supercritical water gasification of waste lube oils.

• More than 70% of the energy contained in the waste lube oil can be recovered as H₂ and CH_4.

• The valorization is energetically more efficient when H₂ and CH₄ are the main products obtained.

• H₂ and CH₄ production are promoted at high temperatures and long reaction times.

• Diverse influence of pressure on kinetics, H₂ and CH₄ production, and energy efficiency are analyzed.

Abstract

The gasification of a waste lube oil (WLO), using water under vapor and supercritical states leads to its valorization into green fuels with high energetic power like H₂ and CH₄. This work investigates how the most important variables of a flow continuous gasification process, temperature, pressure and reaction time, influence the efficiency of gasification and the amounts and characteristics of the gases produced. The study of the influence of these variables on the energetic efficiency of the process was also completed. The highest production yields, 2.4 10⁻² mol_H2 g_oil⁻¹ and 3.0 10⁻² mol_CH4 g_oil⁻¹, were registered at supercritical water conditions (750 °C, 250 bar and 1.87 min). The energetic study revealed that the energetic efficiency increased as the reaction time was lengthened. Gasification kinetics were slower as pressure increased but the reaction times inside the flow reactor were longer. A pressure range from 150 bar, steam region, to 250 bar, supercritical region, was identified as optimal since allowed the most suitable balance between kinetics and the reaction times achieved.

Introduction

Automotive engine oils are consumed for the internal protection of engines. Usually, 90% of their composition is made of petroleum fractions while the remaining 10% are additives like antioxidants and anti-wear agents [1]. They must be regularly replaced since the severe temperature and friction they are subjected to degrade and pollute the oil with heavy metals, soot and aromatic hydrocarbons [2]. The inappropriate use and removal of the generated waste lube oils (WLOs) can cause huge damages to the environment and human health. The decision 2014/955/EU of the European Commission indicates clearly that WLOs should be classified as hazardous waste since the substances that they contain are classified within categories HP5 (specific organ toxicity), HP 7 (carcinogenic) and HP4 (ecotoxic) [3].

For several years, the energy contained in WLOs has been recovered by combustion and incineration. These techniques remove the residue but are not considered as the best option for the environment due to the emission of different toxic and greenhouse gases [2], [4]. The investigations are currently exploring other alternatives like regeneration and valorization. Oladimeji et al. discuss about some of the current regeneration methods of WLOs that allow the reuse of the lubricant, like vacuum distillation, hydrofinishing process or solvent extraction [5]. The valorization alternative makes the most of the energetic power of the WLOs generating valuable fuels. In this field, the technology of pyrolysis and its variants are the most investigated [6], [7], [8], [9], [10]. These processes are usually focused on turning the WLOs into secondary fuels similar to diesel. Their use as second-generation fuels reduce the consumption of primary fuels [2], although the products obtained by pyrolysis also contribute to global warming and greenhouse effect because of the types of gases generated in their combustion. Obtaining clean fuels like CH₄ and above all, H₂, is then becoming necessary to face this problem since the atmospheric emissions produced in their combustion are less harmful that those produced by other fuels.

Hydrogen can be produced through different methods like water electrolysis, solar light photoelectrolysis, natural gas steam reforming, coal or biomass gasification [11], [12], [13], [14], [15]. Steam catalytic reforming of natural gas/methane is one of the most wide-spread methods [13], [16], [17]. Large reactors must be employed due to the slow kinetics of the process and when catalysts are used to accelerate it, they are easily deactivated [18]. Methane is the main component of natural gas, one of the most globally demanded fuels. It provides a relative clean combustion thanks to its low ratio C/H [19], [20], [21]. Methane is also obtained through different technologies like power to gas (electrolysis of water plus methanation) [22], a combination of coal or biomass gasification and methanation [23], [24] anaerobic digestion of biomass [25] or microbial methanogenesis of coal [26].

In spite of the existence of several methods that produce H₂ and CH₄, the search for new environmentally friendly alternatives that support a sustainable energetic development keeps being necessary. Regarding this situation, the current work studies an attractive approach, the valorization of WLOs into green fuels through their gasification with compressed water steam and supercritical water (SCW). These processes can offer important advantages regarding the production of H₂. This species is produced in greater amounts when reforming reactions take place so the use of water, either in steam or supercritical state, seems an interesting option. Specifically, it has been stated that when some typical reforming reactions such as the water-gas shift reaction are carried out under supercritical conditions [27], their kinetics are faster and the achieved conversions are greater thanks to the especial nature and properties of SCW. Namely, the changes that the properties of water experience beyond the critical point (P_c = 221 bar and T_c = 374 °C) turn SCW into a favorable medium for the valorization of organic waste. Complex organic compounds, hydrocarbons and gases are dissolved in SCW, what results in homogeneous reaction and low mass-transfer resistance [28]. The gas-like viscosity of water at supercritical conditions increases its diffusion coefficient and reaction rates. Furthermore, supercritical pressures enhance particle collision and the possibility of chemical reaction to occur within the reactants [29]. Then, despite the difficultness and the high investment of the scale-up of this technology, it may result in high H₂ production yields in comparison with other gasification alternatives. This technology is also considered as environmentally friendly since it does not use organic solvents, catalysts, or any other reagents apart from water. This friendly character is even more marked in this case since the green fuels are produced from the abundant and toxic residue WLO as raw material.

This valorization method has received scarce attention in spite of its potential. As far as we know, only Ramasamy et al. reformed a WLO using SCW at 450 °C and 221 bar, and atmospheric-pressure steam from 715 to 880 °C [30]. They mainly studied how the use of different catalysts influenced the gasification, no other temperatures or pressures were explored within the supercritical and compressed steam regions. The present work shows new insights about the potential of this technology to turn WLOs into valuable gases such as H₂ and CH₄. It first explores how this residue is gasified under steam and supercritical conditions (50–500 bar) at different temperatures (500–750 °C) and reaction times (0.21–1.87 min). This initial approach is then used as basis to face the two main objectives and novel contributions of the work. Firstly, the calculation of the produced amounts of the objective gases H₂ and CH₄ (and others) and their concentrations in the gaseous streams produced, and the influence of the investigated variables on both parameters. Secondly, the compiled information is used for developing an energetic analysis of the process that supports the search for the optimal conditions of H₂ and CH₄ production from WLO steam and supercritical water gasification. As far as we know, this is the first time that an energetic analysis of this WLO valorization method is reported.