Efficiency and energetic analysis of the production of gaseous green fuels from the compressed steam and supercritical water gasification of waste lube oils
Graphical Abstract
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 (Pc = 221 bar and Tc = 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.
Section snippets
Materials
Most of assays in this work used the synthetic automotive engine WLO Repsol 5W40 as feeding oil. The oil was used in a diesel car engine with a runtime of 15,000 km. Its density was 0.85 g cm−3 and its elemental composition was 84.32 wt% carbon, 13.28 wt% hydrogen, 0.33 wt% nitrogen and 0.19 wt% sulfur (determined by combustion at 1000 °C in a LECO CHNS-932 equipment). Inorganic components in the WLO were analyzed by inductively coupled plasma optical emission spectroscopy. The used ICP-OES
Carbon gasification efficiency
CGE is a parameter associated with the conversion of the carbon contained in the oil to carbon-containing gases, so that it allows evaluating the efficiency of the gasification. Gas Yield allows evaluating the total production of gases, including the non carbon-containing too.
The effect of temperature on CGE and Gas Yield for treatments at supercritical conditions 250 bar, 0.3 min and 0.85 wt% is shown in Fig. 2a. The increase in temperature caused an increase in the conversion of carbon from
Conclusions
This work explored the gasification of WLO with water under steam and supercritical states. The influence of important variables like temperature, pressure and reaction time on the gasification efficiency, the production yields of valuable gases and the energetic performance of the process were investigated. Gasification assays with a few samples of automotive engine WLOs were carried out to analyze the reproducibility of the obtained results.
2.4 10−2 molH2 goil−1 and 3.0 10−2 molCH4 goil−1 were
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial support from the Spanish Ministerio de Economía y Competitividad (Project CTQ2015-64339-R) and Anticipos Fondos Feder is acknowledged.
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2023, Renewable and Sustainable Energy ReviewsCitation Excerpt :After the reaction, the gaseous, liquid, and solid products are collected for analysis. Among these, the gaseous products can be analyzed ex situ as usual (gas bag in Figs. 2 and 3) or quantified in situ using a mass flow meter and analyzed further by gas chromatography (GC) (Fig. 4) [74,79,84,86]. The residence time for the semi-batch and continuous flow tube reactors is calculated using Eq. (1).