Fully aerobic bioscrubber for the desulfurization of H₂S-rich biogas

Highlights

• A fully aerobic process for purification of H₂S-rich biogas was developed.

• H₂S removal efficiencies between 85% and 94% were achieved.

• High and stable sulfur oxidation performance was obtained.

• The bioscrubber was robust and resilient towards operational shutdowns.

• Thioalkalimicrobium cyclicum was the most abundant bacteria.

Abstract

A fully aerobic bioscrubber for the desulfurization of H₂S-rich biogas was developed in the present study by coupling an absorption column and a bubble column bioreactor. The bioscrubber treated H₂S loading rates of 37, 59, and 100 g S m_liquid⁻³ h⁻¹ at gas residence times of 6.6, 4.1 and 2.4 min in the absorption column, respectively. Stable H₂S removal efficiencies above 80% were recorded at all the conditions tested. The bioscrubber was robust towards short- and long-term operation shutdowns (5 and 18 days), the H₂S removal performance being recovered after few hours. The aerated bubble column bioreactor was operated at slightly alkaline conditions (pH 8 ± 0.5), which prevented H₂S stripping and allowed sulfur oxidation performances higher than 75% regardless of the operating conditions tested. Bacterial community characterization by 16S rRNA pyrosequencing revealed that the operating conditions (slightly alkaline pH, sulfate concentrations of up to 25,000 g m⁻³ and dilution rates of 5.7 × 10⁻³–8.6 × 10⁻³ h⁻¹) shaped a specialized community highly efficient for H₂S oxidation. Three Proteobacteria were predominant in the bubble column bioreactor, namely Thioalkalimicrobium cyclicum, Stappia sp. and Ochrobactrum sp. with relative abundances of 68.18, 13.49 and 7.61%, respectively.

Introduction

Biogas, mainly composed of ∼60–65% CH₄ and ∼35–40% CO₂ is a renewable fuel obtained from the anaerobic degradation of organic matter [1]. Depending on the characteristics of the organic matter, biogas may contain a sort of pollutants that must be removed prior its use in engines or turbines to produce heat and power [2]. H₂S is one of the most common biogas pollutants, which causes corrosion to internal combustion devices and industrial pipes [3]. H₂S is also toxic for humans, being lethal at a concentration of 300 ppm_v (0.03% v/v) [4]. The biogas generated from sulfur-rich effluents (e.g. wastewaters from pulp and paper mills, citric acid production, sugar/ethanol production from molasses, edible oil industry refineries, wine production and intensive pig farms) typically contains H₂S concentrations between 2000 and 20,000 ppm_v (0.2–2.0% v/v) [5], [6], [7]. However, the maximum H₂S concentration acceptable in combined heat and power engines is in the range of 100–500 ppm_v [4]. Therefore, H₂S-rich biogas must be desulfurized before its use for heat and power generation.

Physical-chemical technologies for biogas desulfurization such as adsorption on activated carbon, chemical scrubbing or thermal decomposition are costly and generate secondary pollution [8], [9]. Chemical scrubbing, a common purification technology for H₂S-rich biogas, requires large volumes of alkaline water that must be subsequently treated [10]. Alternative technologies based on the activity of sulfur-oxidizing bacteria are also available even at industrial scale. This is the case of the commercial THIOPAQ^® process, which is a reference technology for biogas desulfurization at industrial scale (as well as its variation SHELL-PAQUES^® developed for purification of natural gas under pressure) [11], [12]. In this technology, H₂S is absorbed in a scrubber under alkaline conditions (H₂S + OH⁻ → HS⁻ + H₂O). Then, the absorbed HS⁻ is partially oxidized to elemental sulfur (S⁰) by chemolithoautotrophic bacteria in an aerated bioreactor (HS⁻ + 0.5 O₂ → S⁰ + OH⁻). There are also commercial desulfurization technologies based on biotrickling filtration that target the full H₂S oxidation to sulfate [13], [14], [15]. This is the case of the Sulphus^® and SOX^® technologies developed by Pure Air Solutions and Aeris, respectively [16], [17]. Fertilizer production is a main application of the sulfur recovered during biogas desulfurization [18], [19]. Since SO₄²⁻ is the chemical form taken up by plants, the liquid stream leaving the desulfurization system might be valorized as liquid fertilizer after adding adequate proportions of other nutrients (e.g. N, P or K) [11], [20]. Hence, the production of sulfate rather than S⁰ could be in some instances more convenient [21].

In biotrickling filters the H₂S absorption and biological oxidation occur in the same unit. The absorbed HS⁻ is oxidized by chemolithoautotrophic bacteria adhered on a packed bed under aerobic conditions (HS⁻ + 2O₂ → SO₄²⁻ + H⁺) [6], [13], [14], [15] However, a critical operating issue in biotrickling filters treating H₂S-rich biogas is the frequent clogging of the packed bed due to accumulation of elemental sulfur [9], [14], [21], [22]. Bed clogging comes from supplying less O₂ than that required to achieve the full oxidation of H₂S to SO₄²⁻ (2 mol of O₂ per mole of H₂S). On the contrary, biogas dilution is likely to occur in biotrickling filters operated with excess of O₂ supply [9], [14], [22], [23]. Therefore, fully aerobic biological technologies must still be improved for the efficient and robust desulfurization of H₂S-rich biogas.

The performance of a fully aerobic two-stage bioscrubber for the aerobic desulfurization of H₂S-rich biogas was investigated in the present study. In this configuration air and biogas do not mix, which avoids biogas dilution, while allowing the full oxygenation of the liquid phase to prevent elemental sulfur accumulation. The scrubber was operated under slightly alkaline pH conditions to prevent H₂S stripping from the aerated liquid phase. The desulfurization performance was evaluated in terms of S-H₂S removal efficiency, S-H₂S elimination capacity and the percentage of S-H₂S consumed that was fully oxidized to S-SO₄²⁻. The bacterial communities established after 80 days of operation were also characterized by 16S rRNA pyrosequencing.