ReviewRecent developments in biohythane production from household food wastes: A review
Graphical abstract
Introduction
Hydrogen and methane are widely used in chemical and process industries (Ellaban et al., 2014) because of their high calorific value of 143 kJ/g and 55 kJ/g for hydrogen and methane, respectively (Roy and Das, 2016, Sharma and Ghoshal, 2015). Hydrogen is recognised as a clean energetic fuel since it does not release CO2 in the atmosphere during combustion (Roy and Das, 2016). Unlike other fuels, methane and hydrogen combustion does not release any NOx (nitrous oxide) and SOx (sulphur dioxide), the major contributors to air pollution (Gaffney and Marley, 2009). Methane combustion, on the other hand, still generates the greenhouse gas CO2.
The term hythane has been coined in the early 90s by the Hydrogen Component Inc. (HCI), a company which was conducting several studies concerning the feasibility of the use of a blend of Compressed Natural Gas (CNG) and hydrogen as a fuel for internal combustion engines. They showed that the lean burn of mixture of hydrogen (7% by energy or 20% by volume) and CNG can reduce the emission of pollutants (mainly NOx) into the atmosphere, while maintaining the energy efficiency of CNG (Mishra et al., 2017). The use of this mixture does not require storage system neither particular changes both in the CNG engines and infrastructures. As a result HCI patented this mixture and the commercial name of this fuel was Hythane®.
Hythane displays remarkable advantages over CNG: it is a better vehicular fuel thanks to the presence of hydrogen, which improves the performance as far as the flammability range is concerned: hydrogen, in fact, it is characterized by a flame speed which is 8-fold that of methane (Moreno et al., 2012). Hydrogen stimulates methane combustion in the engine and, being an excellent reducing agent, contributes to a better catalysis also at lower exhaust temperatures, (Roy and Das, 2016). From the environmental point of view, hythane has the great advantage to reduce the greenhouse emissions into atmosphere because of the hydrogen presence which reduces the carbon content of this gaseous blend. To accentuate the environmental friendly nature of hythane, the investigation of renewable sources for hydrogen and methane production has been encouraged in the last decade. In fact, hythane is currently produced in intensive way from no sustainable processes: for example hydrogen can be obtained as main gaseous output from syngas production and methane reforming (Liu et al., 2018). The term “biohythane” has started to be used to indicate hythane produced from organic substrates, such as food wastes and agriculture residues (Mishra et al., 2017, Liu et al., 2018) by Anaerobic Digestion (AD) technology conducted in two separated phase. In this way, the production of biohythane, in comparison of the methane production by AD in a single stage, allows the reduction of the overall required fermentation time and, consequently, of the working volume of the reactors (Si et al., 2016). In addition, the attractiveness of producing hythane through a two-stage AD process, rather than the production of hydrogen alone, stems from the fact that the latter is not economically sustainable due to the low production rates and yields of dark fermentation (Valdez-Vazquez and Poggi-Varaldo, 2009, Abreu et al., 2016). Theoretically, a hydrogen yield of 4 mol/mol glucose can be achieved through dark fermentation. However, the hydrogen yield is seldom above 2 mol/mol glucose due to the limited metabolic fluxes and the generation of higher fatty acids (such as propionate and butyrate) and alcohols (Zhang et al., 2011), which means that only about 7.5–15% of the energy contained in organic wastes is converted to H2 (Si et al., 2016a, Mamimin et al., 2017, Luo et al., 2017).
Biohythane production advantages are well described by Life Cycle Assessment (LCA) models, which emphasise that single and two-stage AD processes allow for the reduction of the wastes led to landfills and the release of greenhouse gases (methane, carbon dioxide) to the atmosphere. AD, in fact, leads to a reduction of CO2 eq emissions by 90%, compared to the scenario where organic substrates are released in urban landfill or simply disposed on soil (Franchetti, 2013). This is attributed to the fact that, when the biogas is burned to produce electricity, all the CH4 is converted to CO2. On the contrary, with conventional uncontrolled disposal systems, the methane from wastes degradation, is all emitted unaltered into the atmosphere, increasing significantly the equivalent CO2 emissions, as methane is >20-fold effective as a greenhouse gas than CO2.
Recent LCA researches on biohythane production demonstrates that a two-stage AD process, such as that used for the production of hythane, produces a methane amount similar to the single-stage configuration. However, the simultaneous production of hydrogen, which is a carbon free fuel, allows to reduce by an additional 10% the overall CO2 equivalent emissions of the process (Franchetti, 2013). Lastly, the solid-liquid output from biohythane production is represented by digestate, which is more stable because of the lower amount of acid, nitrogen and carbon content, since the major part of the organic matter used for the AD has been already converted into biogas. In this way, apart from global warming potential (GWP), additional important LCA parameters show a better performance than single-stage AD. For example, Acidification and Eutrophication, which measure respectively the soil and air acidification, which leads to acid rain and a vegetation growth reduction (Ecoinvent database 2013) , as well as a negative accumulation of nutrients (mainly N and P compounds) in the environment (Stranddorf et al., 2005).
Section snippets
Main aspects of the biohythane production process
Anaerobic Digestion for the production of biogas, can be conducted in a single reactor (single-stage AD), or in two separate tanks (two-stage AD). Biohythane production may be carried out readily by a two-stage Anaerobic Digestion process. Over 17,000 CE full scale plants are present in Europe: 10,000 of which only in Germany, for a total generation of about 8,293 MWel (European Biogas Report, 2015). However, it is estimated that <1% of these plants are represented by full scale two-stage AD
Biohythane from household food waste
In the last decade, the increasing interest in the good performances of biohythane has attracted the attention on this technology with a consequent increase of laboratory and pilot-scale experiences. In particular, two types of wastes have been chosen by scientific community for the experimentation: food wastes and sewage sludge. The reason is due to their large worldwide availability: it has been estimated that the 72% of the single stage AD plants treating waste, actually treat food wastes,
Recent and innovative strategies for biohythane production
With the increasing interest in biohythane, innovative techniques to improve the overall yield of the two-stage AD have been investigated. As previously commented, the hydrogen molar yield is lower than the theoretical one. An alternative which is receiving great interest by the scientific community is represented by the combination of dark fermentation with a photo fermentation process. Contrary to dark fermentation, where hydrogen production occurs under anoxic or anaerobic conditions, during
Applications of biohythane
As previously described, hydrogen in combination with methane from clean organic biomasses, has several environmental benefits contributing to reduced CO2 equivalent and NOx emissions to the atmosphere, being a carbon free fuel. In addition, hydrogen is able to improve the performance of internal combustion engines, usually fed by methane from fossil sources, to reduce the methane number, which is expressed as the percentage of methane in the biohythane and is related to the knock resistance.
Conclusions
Biohythane is a gaseous blend, composed of 10–30% v/v hydrogen and 70–90% v/v methane generated by two-stage AD. Good performances have been achieved with HFW-sewage sludge codigestion. Biohythane is currently used to replace methane in the automotive sector, because hydrogen presence improves the combustion yield and reduces the CO2 equivalent and NOx emissions in the atmosphere. Anyway, to favour the adoption of cars and buses biohythane fuelled, bigger investments in the optimization of the
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