Innovative technologies for biogas upgrading

Abstract

The biogas from the anaerobic digestion of wastewaters or organic solid waste represents a key renewable energy vector in order to mitigate the use of fossil fuels. Biogas upgrading is required prior use as a vehicle fuel or injection into natural gas networks. In this context, several international regulations exist setting the maximum and minimum allowed concentrations of each biomethane component depending on its final application. Multiple physical-chemical and biological technologies are nowadays commercially available in order to remove CO2 and H2S from biogas. However, most of these technologies must be sequentially implemented to remove both H2S, CO2 and trace contaminants such as siloxanes or volatile organic contaminants. In this sense, this thesis focuses on the study of new technologies supporting the simultaneously removal of H2S and CO2 from biogas in a single step process, in a sustainable manner and with low operating costs. The state-of-the-art of biogas upgrading technologies is presented in the Introduction section. The objectives, approach and strategies followed in this thesis are summarized in the Aims and Scope section. In Chapter 1, the bioconversion of biogas to biomethane coupled to centrate treatment was evaluated during summer time in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external CO2-H2S absorption column (AC) via settled broth recirculation. CO2-removal efficiencies ranged from 50 to 95% depending on the alkalinity of the cultivation broth and environmental conditions, while a complete H2S removal was achieved regardless of the operational conditions. A maximum CH4 concentration of 94%, along with a limited O2 and N2 stripping, were recorded in the upgraded biogas at recycling liquid-to-biogas (L/G) ratios in the AC of 1 and 2. Process operation at a constant biomass productivity of 15 g m-2 d-1 (controlled via settler waste) and the minimization of effluent generation supported high carbon and nutrient recoveries in the harvested biomass (C = 66±8%, N = 54±18%, P ≈ 100% and S = 16±3%). Finally, the low diversity in the structure of the microalgae population was likely due to harsh environmental and operational conditions imposed. In Chapter 2, the influence of the diffuser type and L/G ratio on biogas upgrading performance in an outdoor pilot scale HRAP was evaluated. Four different types of biogas diffusers (metallic of 2 μm, porous stone, and two ceramic tubular membranes of 0.2 and 0.4 μm) were evaluated to improve the quality of biomethane. Each type of diffuser was tested independently using three different L/G ratios (0.5, 1 and 2). No significant difference was recorded in the CH4 concentrations of biomethane (i.e. > 93.0%) working with the different types of diffusers at L/G ratios > 1. Only the metallic biogas diffuser supported CH4 concentrations higher than 94.0% at a L/G ratio of 0.5. The increase in L/G ratio induced the stripping of the dissolved N2 and O2 into the biogas, which compensated the decrease in CO2 concentration mediated by the higher pH value of the scrubbing solution in the absorption column. An analysis of variance (ANOVA) of the results here obtained confirmed that both the type of biogas diffuser and the L/G ratio significantly determined the quality of the upgraded biogas. Chapters 3 and 4 evaluated for the first time the influence of seasonal variation on biogas upgrading coupled with digestate treatment in an outdoors pilot scale algal-bacterial photobioreactor. In Chapter 3, the yearly variations of the quality of the upgraded biogas and the efficiency of digestate treatment were evaluated in an outdoors pilot scale HRAP interconnected to an external AC via a conical settler. CO2 concentrations in the upgraded biogas ranged from 0.7% in August to 11.9% in December, while a complete H2S removal was achieved regardless of the operational month. CH4 concentrations ranged from 85.2% in December to 97.9% in June, with a limited O2 and N2 stripping in the upgraded biogas mediated by the low recycling L/G ratio in the AC. Finally, microalgae diversity was severely reduced throughout the year likely due to the increasing salinity in the cultivation broth of the HRAP induced by process operation in the absence of effluent. On the other hand, in Chapter 4, the influence of the daily and seasonal variations of environmental conditions on biomethane quality was evaluated. The high alkalinity in the cultivation broth resulted in a constant biomethane composition during the day regardless of the monitored month, while the high algal-bacterial activity during spring and summer boosted a superior biomethane quality. CO2 concentrations in the upgraded biogas ranged from 0.1% in May to 11.6% in December, while a complete H2S removal was always achieved regardless of the month. A limited N2 and O2 stripping from the scrubbing cultivation broth was recorded in the upgraded biogas at a recycling L/G ratio in the AC of 1. Finally, CH4 concentration in the upgraded biogas ranged from 85.6% in December to 99.6% in August. In Chapter 5, three innovative operational strategies to improve the quality of biomethane under unfavorable environmental conditions and without external alkalinity supplementation were evaluated in an outdoors pilot scale HRAP interconnected to an external AC: i) the use of a greenhouse during winter conditions, ii) a direct CO2 stripping in the HRAP via air stripping during winter conditions and iii) the use of digestate as make-up water during summer conditions. CO2 concentrations in the biomethane ranged from 0.4% to 6.1% using the greenhouse, from 0.3% to 2.6% when air was injected in the HRAP and from 0.4% to 0.9% using digestate as make up water. H2S was completely removed under all strategies tested. On the other hand, CH4 concentrations in biomethane ranged from 89.5% to 98.2%, from 93.0% to 98.2% and from 96.3% to 97.9%, when implementing strategies i), ii) and iii), respectively. The greenhouse was capable of maintaining microalgae productivities of 7.5 g m-2 d-1 during winter under continental weather conditions, while mechanical CO2 stripping increased the pH in order to support an effective CO2 and H2S removal. Finally, the high evaporation rates during summer conditions allowed maintaining high inorganic carbon concentrations in the cultivation broth using centrate as make-up water, which provided a cost effective biogas upgrading. In Chapter 6, the influence of L/G ratios and alkalinity on the biogas upgrading performance was evaluated in a 11.7 m3 outdoors horizontal semi-closed tubular photobioreactor interconnected to a 45 L AC. CO2 concentrations in the upgraded biomethane ranged from <0.1 to 9.6% at L/G of 2.0 and 0.5, respectively, with maximum CH4 concentrations of 89.7% at a L/G of 1.0. Moreover, an enhanced CO2 removal (mediating a decrease in CO2 concentration from 9.6 to 1.2%), and therefore higher CH4 contents (increasing from 88.0 to 93.2%), were observed when increasing the alkalinity of the AC cultivation broth from 42±1 mg L-1 to 996±42 mg IC L-1. H2S was completely removed regardless of the L/G or the alkalinity in AC. The continuous operation of the photobioreactor with optimized operating parameters resulted in contents of CO2 (<0.1%-1.4%), H2S (<0.7 mg m-3) and CH4 (94.1%-98.8%) complying with international regulations for biomethane injection into natural gas grids. In Chapter 7, the potential of purple phototrophic bacteria (PPB) for the simultaneous treatment of piggery wastewater (PWW) and biogas upgrading was evaluated batchwise in gas-tight photobioreactors. PWW dilution was identified as a key parameter determining the efficiency of wastewater treatment and biomethane quality in PPB photobioreactors illuminated with infrared radiation. Four times diluted PWW supported the most efficient total organic carbon (TOC) and total nitrogen removals (78% and 13%, respectively), with CH4 concentrations of 90.8%. The influence of phosphorous concentration (supplementation of 50 mg L-1 of P-PO43-) on PPB-based PWW treatment coupled to biogas upgrading was investigated. TOC removals of ≈ 60% and CH4 concentrations of ≈ 90.0% were obtained regardless of phosphorus supplementation. Finally, the use of PPB and algal-bacterial consortia supported CH4 concentrations in the upgraded biogas of 93.3% and 73.6%, respectively, which confirmed the potential PPB for biogas upgrading coupled to PWW treatment. Finally, Chapter 8 evaluated the potential of a novel Fe/EDTA/carbonate-based scrubbing process for the simultaneous removal of H2S and CO2 from biogas in a 1.8 L absorption column interconnected to a 2.0 L air-aided regeneration column. This work evaluated the influence of Fe/EDTA molarity (M), carbonate concentration (IC), and biogas (B), air (A) and liquid (L) flow rates on biogas upgrading performance using a Taguchi L16(45) experimental design. The ANOVA demonstrated that the molarity of the Fe/EDTA solution was a significant factor influencing H2S concentration. The inorganic carbon concentration impacted on the concentrations of CO2, N2 and CH4, and the biogas and recycling liquid flow rates affected CO2, O2, N2 and CH4 content. Finally, the air flow rate in the regeneration column impacted on CO2, H2S, N2 and CH4 concentrations. Process optimization via analysis of the effect of the interaction between M and IC provided the optimal conditions for each control factor. Continuous biogas upgrading operation at M of 0.05 M, IC of 10000 mg L-1, and B, A and L flow rates of 10 mL min-1, 1000 mL min-1 and 30 mL min-1, respectively, provided a CH4 concentration of 97.4% in the upgraded biogas with very low levels of CO2, O2, N2 and H2S (1.4, 0.29, 0.97 and 0%, respectively), which fulfilled with most international biomethane regulations. The results obtained in the present thesis confirmed the potential of photosynthetic biogas upgrading under outdoors conditions as a cost-effective and sustainable tool for the upgrading of biogas. In addition, this thesis provided two proofs of concept of new physical/chemical and biological technologies for biogas upgrading.