Process design for green hydrogen production
Introduction
There is a worldwide consensus that the increase of anthropogenic CO2 emissions, due to increase of population and increased energy use in developing countries, results in more pressing request for more sustainable energy systems. In this regard, hydrogen is often referred to as a perfect energy carrier that could bring us in the so-called “hydrogen economy” [1].
Hydrogen as energy carrier is very interesting as it can be efficiently converted into power without emissions of CO2. However, hydrogen (as gas) needs to be produced, and unfortunately, a large part of hydrogen is still produced worldwide via conversion of fossil fuels, which thus results in a large quantity of CO2 emissions.
To circumvent (or at least reduce to a large extent) CO2 emissions, more research is being devoted to efficiently produce hydrogen from bio-renewable feedstocks via thermo-chemical conversion processes such as pyrolysis, gasification, steam gasification, supercritical water gasification of biomass and steam reforming of bio-fuels (liquid or gas fuels produced from biomass). The large variety and availability of bio-based feedstock will also contribute to both energy security and socio-economic situation of developing countries [2].
Hydrogen can indeed be produced from virtually any bio-based feedstock, however, for the large availability (see Table 1), easy production, easy handling and high hydrogen capacity, ethanol is often referred to as the most promising bio-based feedstock for hydrogen.
Ethanol is also very reactive at relatively low temperatures; Ruocco et al. [5] have reported as ethanol is reformed at temperatures below 480 °C in a few milliseconds to a hydrogen containing gas mixture. The main drawbacks of this conversion is that also methane is generally produced along hydrogen, and this is why ethanol reforming is carried out at even higher temperatures up to 800 °C, to convert the methane to hydrogen [6], [7].
More in details the ethanol reforming is a system of equilibrium limited reactions, where the most important reaction (methane reforming) requires high temperatures for complete conversion. As hydrogen is the product of these equilibrium reactions, the equilibrium constrains of conventional reactors can be circumvented if hydrogen is separated in situ during the reaction. This can clearly be attained with integration of hydrogen selective membranes in a membrane reactor. It has already been shown that higher ethanol conversion per pass and pure hydrogen production can be achieved in membrane reactors with either thick self-supported membranes in packed beds [8], [9], [10], [11], [12], or with thin supported membranes [13], [14], [15], [16], [17], [18].
While all these studies have demonstrated that membrane reactors are beneficial for hydrogen production via ethanol reforming, it is still not clear if a full process based on ethanol reforming can be economically attractive for hydrogen production. The integration of Pd-membranes in a process for industrial hydrogen production poses in fact feasibility aspects, the examination of which must guide through a careful selection of those reaction stages more suited to such integration.
The scope of this paper is to carry out a full-scale plant design for hydrogen production through ethanol reforming by using membrane reactors. Different reactors and level of integration are considered for membranes and catalyst. Aspen simulations have been used for performing the heat and material balance of the process scheme, with an evaluation of the performance in terms of efficiency and hydrogen yield. An economic evaluation is also reported to check the economic feasibility of the proposed process scheme.
Section snippets
Process scheme development
Steam reforming of ethanol has been extensively studied and is widely reported in literature as one of the main routes for converting a bioderived renewable feedstock into hydrogen [19]. The ethanol molecule has in fact a high H/C ratio (namely 3) which can ensure a high hydrogen yield, and its conversion via steam reforming, among other possible conversion routes like partial oxidation and oxidative steam reforming, is reported as the most economical method [20]. Accordingly, the overall
Comparative performance and economic analysis
The comparative process performances, represented by means of the efficiencies, the hydrogen yields and the specific CO2 emissions are reported in Table 8 for an overall analysis and comparison of the different processes.
Regarding the efficiency indices and hydrogen yield, the following can be observed:
- a)
the reference SER process has a lower hydrogen efficiency than the benchmark case (ɳ H2 = 66,10% vs 68,12%). This is due to the intrinsic ethanol features, having a lower LHV per carbon atom and
Conclusions
Among possible bio-based feedstock, ethanol represents one of the most promising raw material for green hydrogen production, due to its large availability and distribution on the worldwide market as fuel additive. The introduction of at least a pre-treatment and pre-reforming step upstream a conventional Steam Methane Reforming process allows the use of a concentrated stream of ethanol, like ‘fuel grade’ ethanol requests for pure hydrogen production; the resulting modified process, here
Acknowledgement
This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 734561. The paper reflects only the author's view and that the Agency is not responsible for any use that may be made of the information it contains.
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