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Figure 7. Minute variations of biomethane purity: concentration (% V/V) of methane, carbon dioxide, oxygen, and others. the daily variations in methane concentration downstream the VSA. Effect of Single VSA Phases To assess the impact of each VSA phase on biomethane quality, a minute resolved analysis was performed for 1 h. Figure 7 reports the obtained results. During the switching from one column to another, a strong CH4 concentration decrease was observed, together with an increase of second- ary components. Since CO2 do not significantly contribute to these changes in biomethane composition, it is possible to state that the reduction in biomethane purity during the col- umn switches is mainly due to the residual atmospheric air used for the column purge. This observation is a further con- firmation that a higher biomethane purity can be achieved using a biomethane recirculation for column regeneration. Hydrogen Sulfide Monitoring Hydrogen sulfide ranged from 142 to 210 mg Nm23 (95– 140 ppm) in the raw biogas, and from 6 to 12 mg Nm23 (4– 8 ppm) after the desulfurization unit. Hence, during all the experimentations, the VSA unit was used for the upgrading of a biogas with a low H2S concentration. This has enhanced the lifetime of natural zeolites, since CO2 breakthrough and adsorption capacity can dramatically change in the presence of H2S [18]. However, a light desulfurization effect was observed even in these conditions, since the H2S concentra- tion in biomethane downstream the VSA was in the range 3– 10.5 mg Nm23 (2–7 ppm): this light desulfurization effect is in good agreement with related literature, even if most of these studies were performed at higher H2S concentartions [18,21]. Emissions from VSA Off-Gas The composition of the VSA off-gas was monitored to assess the environmental impact of the proposed technology. During the regeneration step, atmospheric air flows (6.67 3 1025 Nm3 s21) through the column and reach the vacuum pump, which dilutes the flow with auxiliary air (50.0 3 1025 Nm3 s21), as described in the paragraph “Prototype description.” This dilution led to an enhancement in the detection and quantification limits of the gas analyses, and only CO2 was quantifiable. Results are shown in Table 5. DISCUSSION Comparing requirements listed in Table 1 and the results of Table 4, it is important to highlight that CH4 concentration was never lower than 95% during all the experimentation time: hence, natural zeolites were demonstrated to be able to perform an efficient biogas upgrading, and to maintain their properties for up to three months. Table 5. VSA off-gas emission. Parameter Methane Carbon dioxide Hydrogen sulfide Emission <1.5 g s21 56.4–112.2 g s21 <4.2 g s21 6 Month 2017 Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep The most critical parameter is O2, which can be higher than 1.5%, affecting the possibility to use the produced bio- methane for the intended scope. The high concentration of O2 in biomethane could be attributed both to the high oxy- gen content of the considered biogas and to the atmospheric air used during the regeneration step. Further tests are need- ed in order to validate this hypothesis. This study focused its attention also on another critical disadvantage of PSA and VSA upgrading techniques: the emission of greenhouse gases in the atmosphere, including CO2 from off-gas and methane losses [25,26]. In the per- formed experiments CO2 concentration was always lower than 0.3%, in compliance with requirements of all countries as reported in Table 1. Hence, CO2 adsorption efficiency is higher than 99%, as expected from previous investigations on these materials [21]. However, the most significative envi- ronmental disadvantage of the VSA technique is the emission of CO2 in the off-gas, after its adsorption. However, CH4 con- centration was below than the detection limit (Table 5), and this is an important result since the global warming potential of this compound is estimated to be 28–36 times higher than CO2 (over 100 years) [27,28]. Compared with synthetic zeolites, natural zeolites are characterized by a lower adsorption capacity 45 mg of car- bon dioxide per g of adsorbent, while this values reaches 298.5 for 133 [18]. On the other hand, the performances of standard adsorbents are strongly affected by interfering sub- stances [5], especially moisture [19]. Results obtained in this study show that the impact of moisture and other biogas interferents on the efficiency of natural zeolites is lower. Indeed, Figure 5 shows that biomethane purity do not decrease during all the experimental time (determination coefficient R2 5 0,0003). Therefore, in the experimental con- ditions, natural zeolites are able to perform a biogas upgrad- ing to biomethane without losing their properties, for more than 3 months. Finally, during all the experimental time, a biomethane with a very stable composition was produced (relative standard deviation 5 0.013% for methane concentra- tion). The same stability was observed for the other biogas components: this is a very important property for an industri- al scale application of the proposed technology, deriving from the efficiency of the adsorbing material and the stabilityPDF Image | Vacuum swing adsorption on natural zeolites
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