Development of an Innovative and Sustainable Technological Process for Biogas Purification Through the Reuse of Autoclaved Aerated Concrete Waste

Abstract

This study demonstrated the effectiveness of using autoclaved aerated concrete AAC waste as a low-cost filtering material for removing hydrogen sulfide (H₂S) from gas streams. A long-term experiment (89 days) was conducted in a packed bed reactor to purify synthetic biogas composed of N₂, CO₂, H₂S, and O₂. Optimal H₂S removal efficiencies, reaching up to 100%, were achieved under highly acidic conditions (pH ≈ 1–3) and low oxygen concentrations (<1%). In the presence of oxygen, calcium oxides in the AAC waste react with H₂S to form gypsum (CaSO₄ 2H₂O). The simultaneous removal of both oxygen and H₂S by AAC waste, following an approximate 2:1 molar ratio, may be particularly beneficial for biogas streams containing unwanted traces of oxygen. The transformation and lifespan of AAC waste were monitored through sulfur accumulation in the material and pressure drop measurements, which indicated structural changes in the AAC waste. At the end of its lifespan, the AAC waste exhibited an H₂S removal capacity of 185 g_H2S kg_AAC⁻¹. This innovative and sustainable process not only provides a cost-effective and environmentally sound solution for the simultaneous removal of H₂S and O₂ from biogas, but also promotes waste valorization and aligns with circular economy principles.

1. Introduction

On a global scale, it is estimated that more than 4.8 billion tons of concrete waste from construction and demolition will be produced annually by 2030 [1]. Therefore, efforts must be made to continue developing innovative sustainable technologies for recovering this waste to reduce the carbon footprint of concrete [2,3]. Among the wide variety of concrete waste, this study focuses on autoclaved aerated concrete (AAC), also known as cellular concrete. It is a lightweight building material with excellent thermal insulation and fire-resistant properties. It is composed of silica sand, cement, calcium oxides, water, and aluminum powder, which acts as an expansion agent. The main recycling options for AAC waste, aimed at avoiding landfilling, are related to the building industry. These include the production of new AAC, light mortar, lightweight aggregate concrete, and shuttering blocks made from concrete without fine fractions [4]. But recycling alternatives do exist, particularly in the field of gas purification, notably for gases laden with hydrogen sulfide (H₂S) [5]. Hydrogen sulfide is a toxic and corrosive gas commonly encountered in various industrial and natural environments, including gas processing facilities, wastewater treatment plants, landfills, and anaerobic digesters used in biogas production. Given the increasing role of biogas as a renewable energy source (biogas is a mixture composed primarily of methane (CH₄) and carbon dioxide (CO₂), along with trace components such as nitrogen (N₂), oxygen (O₂), ammonia (NH₃), and H₂S [6]), ensuring its clean utilization is critical to supporting environmental sustainability and climate change mitigation efforts [7]. Current technologies for H₂S removal from biogas include physicochemical techniques (membrane separation, absorption, cryogenic distillation, advanced oxidation processes, adsorption) as well as biological techniques (biofilters, biotrickling filters, and bioscrubbers) [8,9]. The predominant techniques are absorption, which involves the use of a scrubbing solution (alkanolamines, ionic liquids, deep eutectic solvents, or hybrid blends), and adsorption, which involves packed columns containing sorbents such as activated carbons, zeolites, and/or metal oxides. In terms of H₂S separation capacity, some activated carbons outperform other materials such as biochars, zeolites, metal-organic frameworks (MOFs), and metal-based adsorbents [9]. However, to be efficient, these activated carbons must be functionalized by impregnation or metal loading, which significantly increases cost. It is also important to consider the effects of water and oxygen (O₂), both of which can have an impact on H₂S separation. Moreover, powdered activated carbon can cause substantial pressure drops, which increases energy consumption due to additional gas compression. Regeneration of these sorbents can pose difficulties and reduce their adsorption capacity. Furthermore, the elimination of non-regenerable sorbents poses an environmental risk due to the formation of sulfuric acid (H₂SO₄), an oxidation product of the H₂S separation process. These limitations underscore the need for innovative, cost-effective, and sustainable materials that align with circular economy principles. Interestingly, the natural degradation of concrete observed in sewer systems and wastewater treatment infrastructure, caused by sulfuric acid generated from microbial activity [10,11,12], can inspire new pollutant removal technologies. Preliminary studies in our laboratory have demonstrated that humidified fixed beds of AAC waste can act as an efficient, low-cost, and robust purification system for H₂S-laden gases [13,14,15]. The removal mechanism involves complex physico-chemical interactions with AAC constituents, particularly calcium and ferric oxides. Given the abundant availability of AAC waste, this simple technology could offer a sustainable pathway for biogas purification. The use of AAC waste for H₂S removal in biogas is a novel area of research. To our knowledge, no data currently exist in the literature, aside from our results, that allow for direct comparison with other adsorbent materials. But a recent biogas biofiltration study using cellular concrete waste as a packing material enriched with sulfur-oxidizing bacteria (Thiobacillus) reported an elimination capacity (EC) of 100 g_H2S m⁻³ h⁻¹ and a moderate removal efficiency (RE) of 75% [16]. However, the authors did not address the influence of oxygen, which is a critical factor in both the biofiltration process and the physical H₂S removal by AAC waste presented in the present study.

Our initial laboratory studies were carried out using a synthetic gas mixture composed of nitrogen (in place of methane for safety reasons), H₂S, and uncontrolled traces of oxygen. The presence of O₂ appeared to influence the removal performance, although it could not be precisely quantified [14]. Other parameters, such as CO₂ concentration and pH effects, were not considered. Therefore, the present experimental work was undertaken using synthetic gas mixtures more representative of actual biogas, with controlled levels of N₂, CO₂, O₂, and H₂S. The objective was to investigate the influence of O₂ and CO₂ on H₂S removal efficiency, and to determine the reaction stoichiometry, including the ratios of H₂S removed to O₂ consumed and to CO₂ consumed or produced. The impact of pH on these reactions was also examined.

2. Materials and Methods

2.1. Material

The autoclaved aerated concrete used in this study was provided by Ecominero, a French eco-organization dedicated to managing waste from construction, demolition, and excavation activities (https://www.ecominero.fr/, accessed on 29 May 2025). Before being introduced as a packing material in the PBR, the AAC waste was sieved to isolate pieces approximately 10–15 mm in diameter (Figure 1). The elemental composition of AAC was determined using an Energy Dispersive X-ray Fluorescence Spectrometer (EDX-800HS, Shimadzu Company, Kyoto, Japan). Prior to contact with H₂S-laden gas, the AAC waste was primarily composed (by weight) of SiO₂ (50.5%), CaO (24.6%), SO₃ (19.7%), Al₂O₃ (2.2%), P₂O₅ (1.4%), and Fe₂O₃ (1.3%). These data agree with those reported by [13]. A complementary analysis was carried out using X-ray Diffraction (XRD) (Siemens Brüker D5000, Karlsruhe, Germany). The following phases were identified: quartz (SiO₂), calcium carbonate (CaCO₃), gypsum (CaSO₄, 2H₂O), aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), and calcium silicate hydroxide hydrate (Ca₄₅Si₆O₁₅(OH)₃, 2H₂O). Specific surface area was determined using a Micromeritics ASAP^® 2020 gas adsorption analyzer (Micromeritics Company, Norcross, GA, USA). The specific surface area (44 ± 0.8 m² g⁻¹) was calculated by the Brunauer-Emmett-Teller (BET) method. Internal porosity (64%) and apparent density (547 kg m⁻³) were measured using a mercury porosimeter, Micrometrics autopore IV 9500 (Micromeritics Company, Norcross, GA, USA). The water retention capacity of AAC waste, defined as the maximum mass of water retained per gram of dry material, was measured by immersing the material in water for 1 h, followed by draining for 24 h. The increase in mass was used to calculate the water retention, which was found to be 56 ± 2%. The pH of AAC waste was measured at 9 using a pH electrode connected to a Consort C561 multi-parameter analyzer (Consort, Turnhout, Belgium). A CHNS elemental analyzer (Carbon, Hydrogen, Nitrogen, Sulfur, Flash EA Series 1112, Thermo Scientific, Waltham, MA, USA) was also used to quantitatively determine the variation in sulfur content in the material between the beginning and the end of the experiment. The composition of AAC waste at the start of the experiment was (mass percentage): calcium Ca: 59.4%, silicon Si: 29.0%, sulfur S: 5.5%, iron Fe: 2.8%, aluminum Al: 1.2%, potassium K: 0.8%, and phosphorus P: 0.6%.

2.2. Experimental Setup

The experimental setup (Figure 1) consisted of a PVC cylindrical column (with internal diameter 100 mm), referred to as the “packed bed reactor” (PBR) in the text, filled with 7.8 L of AAC over a height of 1 m, along with a water tank. The PBR was equipped with six sampling ports located at 0, 20, 40, 60, 80, and 100 cm from the bottom for sampling, concentration determination, and pressure drop measurements. The synthetic gas in contact with the AAC was a mixture of N₂-CO₂-H₂S-O₂ simulating raw biogas. A BrezzaNiGen nitrogen generator (LCMS 40-1, with up to 99.9% purity) from Gengaz Company (Wasquehal, France) provided a continuous nitrogen supply to the PBR. The nitrogen flow rate was controlled and measured with a mass flowmeter (Model 58500, Brooks Instruments, Hatfield, PA, USA). Despite the generator’s capability to purify nitrogen from air, a low fraction of uncontrolled oxygen was consistently measured in the nitrogen gas. Consequently, an air flow rate regulated by a mass flowmeter (Brooks Instruments, Hatfield, PA, USA) was added in order to adjust the O₂ concentration to the desired level in the gas entering the PBR (around 0.3%, i.e., 3000 ppm). A CO₂ flow (99.5% purity) and a H₂S flow (99.7% purity), both controlled by dedicated mass flow meters (Brooks Instruments, Hatfield, PA, USA), were also mixed into the main N₂ stream before entering the PBR. The concentrations of N₂, CO₂, H₂S, and O₂ were simultaneously measured using a gas chromatography analyzer (micro GS Fusion^® gas analyser, Inficon, Bad Ragaz, Switzerland), which was alternately connected to the six sampling ports. A sodium hydroxide (NaOH) trap, positioned after the PBR, was used to remove residual H₂S at the end of the treatment. The treated gas was then vented outside.

All experiments were carried out under humid conditions, monitored by a humidity sensor (Model EE08, E+E Electronik GmbH, Engerwitzdorf, Austria) located at the top of the PBR. To maintain optimal humidity conditions, the AAC was continuously moistened with tap water using a drip system. The water circulated in a closed loop between the top and bottom of the PBR. The relative humidity measurements of the gas at the PBR outlet consistently exceeded 97%. The gradual accumulation of sulphate (SO₄²⁻) in the water due to H₂S oxidation was measured using a High Pressure Ion Chromatography (940 Professional IC Vario, Metrohm, Switzerland). Thermocouples (K type) were installed to measure temperatures at various levels within the PBR, and pressure drops were measured with a Setra pressure sensor (Setra Systems, Inc., Boxborough, MA, USA; 0–700 Pa).

Figure 1. Sketch of the experimental setup used for the removal of H₂S, and photos of AAC waste (right: raw material at the beginning of experiment; Left: piece retrieved from the packed bed reactor (PBR) at the end of experiment).

Figure 1. Sketch of the experimental setup used for the removal of H₂S, and photos of AAC waste (right: raw material at the beginning of experiment; Left: piece retrieved from the packed bed reactor (PBR) at the end of experiment).

2.3. Operating Conditions

The experiment focused on monitoring the behavior of AAC waste in contact with the loaded gas throughout the lifespan of the material, as well as determining the optimal operating conditions, particularly in terms of pH. The experiment was carried out in two distinct phases (

Table 1. Operating conditions.

Transient Phase	Steady State
From days 1 to 33 Residence time 1: 60 ± 1 ↘ 22 ± 1 s H2S concentration: 105 ↗ 400 ↘ 300 ± 6 ppm Loading Rate 2: 9.0 ± 0.5 ↗ 72 ± 4 gH2S m−3 h−1 O2 concentration: 2–3% (20,000–30,000 ± 600 ppm)	From days 34 to 89 Residence time = 22 ± 1 s H2S concentration = 300 ± 6 ppm Loading Rate = 72 ± 4 gH2S m−3 h−1 O2 concentration: 3 ↘ 0.3% (20,000–3000 ± 60 ppm)

¹ Residence time = volume_AAC (m³)/gas flowrate (m³ s⁻¹). ² Loading rate LR = H₂S concentration (g m⁻³) × gas flowrate (m³ h⁻¹)/volume_AAC (m³). Note that 1 ppm_H2S = 1.4 mg m⁻³ at 25 °C and 101 kPa.

): a transient phase (one month), during which the loading rate (LR) was progressively increased from 9.0 ± 0.5 to 72 ± 4 g_H2S m⁻³ h⁻¹ by raising the applied H₂S concentration and reducing the residence time, and a steady-state phase (two months) at the maximum loading rate. The loading rate refers to the mass of H₂S applied to the treatment system per unit volume of AAC in the PBR and per unit of time. At the start of the experiment, the pH of the tap water used to moisten the AAC was close to neutral.

3. Results and Discussion

3.1. H₂S Removal Efficiency

The evolution of the capacity of AAC waste to remove H₂S in the experiment is shown in Figure 2 through a bar chart illustrating the removal efficiency RE = 100 × (C_inlet − C_outlet)/C_inlet. During the transient phase, between days 1 and 25, RE decreased from 80 ± 4 % to 20±1% in relation to the increase in the loading rate from 9.0 ± 0.5 to 65 ± 4 g m⁻³ h⁻¹. The pH of the recirculating water was roughly constant around neutral, and the O₂ concentration was very high, around 2–3%, due to a malfunction in the nitrogen generator. On day 24, 6 L of acidic water (pH = 1.2 ± 0.1) was added to the water tank to improve the efficiency of the AAC waste. Consequently, the pH dropped from 7.3 ± 0.1 to 5.4 ± 0.1 before increasing to 6.6 ± 0.1, due to the alkaline nature of the material. The alkalinity of AAC waste helps neutralize the acidic byproducts generated during reactions with H₂S, in the same way that alkaline materials, such as lime, are used to enhance the efficiency of industrial gas scrubbers treating H₂S [17]. This operation proved beneficial, as the pH then steadily decreased due to the production of sulfuric acid, confirmed by the accumulation of sulphate (SO₄²⁻) in the water. By the end of the transient phase, the pH was around 4.0 ± 0.1, and RE increased to 60 ± 3%. This result highlights the importance of performing an acid attack on the material to achieve significant treatment performance. During the steady-state phase, two different situations can be observed in Figure 2. Between days 26 and 50, RE ranged from 50 ± 3% to 65 ± 3%, whereas between days 54 and 78, the removal efficiency exceeded 95 ± 4% and occasionally reached 100%. These two situations differ in terms of oxygen concentration and pH. Between days 26 and 50, pH values were around 3.0, and the O₂ concentration was approximately 3%. In contrast, between days 54 and 78, the O₂ concentration was 10 times lower, around 0.3%, and pH values decreased from 3.0 to 1.0. Although it is difficult to draw definitive conclusions about the respective influence of these two parameters on the efficiency of H₂S elimination, it seems necessary to operate at low O₂ concentration and very acidic pH values, around 2–3 (as will be confirmed later in Section 3.5). In practical applications, pH values can be easily regulated, whereas O₂ concentration, which is typically lower than 1% in biogas, cannot be controlled. After day 81, it was observed that RE decreased significantly to 30 ± 2%, despite the fact that the pH was close to 1.0 and the O₂ concentration remained around 0.3%. This drop in RE corresponds to the end of the AAC’s lifespan, meaning that the main components of the AAC capable of reacting with H₂S, i.e., calcium oxides [14], were almost entirely consumed.

3.2. Pressure Drops

The physico-chemical reactions that occur between H₂S and the components of AAC waste lead to a significant change in the pressure drops (ΔP) recorded over time (Figure 3). Initially, the pressure drops across the PBR were very low (around 5.0 ± 0.3 Pa·m⁻¹), which is very favorable in terms of fan power consumption. Then, the progressive transformation of the AAC waste resulted in a moderate and linear increase in ΔP over time (up to 23 ± 2 Pa·m⁻¹ on day 71), followed by a dramatic rise on day 81 (up to 60 ± 3 Pa·m⁻¹, i.e., a three-times increase). This sharp increase in ΔP, concomitant with the drop in efficiency observed in Figure 2, indicates that the nature of the AAC waste has been profoundly altered. According to Poser et al. [14], the primary reaction between H₂S and AAC waste involves the disappearance of calcium oxides (CaO SiO₂·nH₂O and CaCO₃) and the formation of calcium sulfate (CaSO₄·nH₂O, i.e., gypsum), which represents a significant transformation of the material. After transformation, the highly moistened material obtained can be easily crushed, whereas the original AAC waste could not. The transformation of AAC waste into gypsum leads to a significant change in the pressure drops recorded over time. Consequently, this rapid increase in ΔP over a short period serves as a reliable indicator that the material’s capacity to treat H₂S is nearly exhausted and that it is necessary to replace it. From an industrial perspective, monitoring this indicator, which is reliable, easy to implement, and cost-effective, represents a major advantage for controlling the H₂S purification process and the replacement of used AAC waste. Considering that the main reactive capacities with H₂S are significantly altered from day 81 onward, it can be deduced that the removal capacity of material was 185 ± 8 g_H2S kg_AAC⁻¹ (Figure 3). However, this value does not represent the maximum, as the AAC was still capable of removing part of the H₂S with efficiencies ranging from 20 ± 1% to 30 ± 2% (Figure 2). This value is slightly higher than that reported in a previous study (169 g_H2S kg_AAC⁻¹ by [14]) and is of the same order of magnitude as that of commercial adsorbents used for H₂S removal, generally between 50 and 200 g_H2S kg⁻¹ [9,18,19,20]. Consequently, the use of AAC waste could represent a very promising, low-cost alternative to the use of activated carbon for H₂S removal.

3.3. Change in Composition of AAC Waste over Time and Chemical Reactions

The accumulation of sulfur in AAC waste is clearly demonstrated by the increase in the sulfur content of the material, measured using the energy dispersive X-ray Fluorescence spectrometer (Figure 4). The transformation of the material is due to chemical reactions between AAC waste and H₂S under humid conditions, resulting in a change in the mechanical structure of the material [21], observable through pressure drops in the PBR. From XRD analysis, Poser et al. [14] demonstrated that the primary reaction between H₂SO₄ (resulting from the oxidation of H₂S as indicated hereafter) and AAC waste leads to the consumption of calcium oxides and the formation of calcium sulfate (CaSO₄·2H₂O, i.e., gypsum), as shown in Equations (1)–(4).

Tobermorite: Ca₅Si₆O₁₆(OH)₂ 4H₂O + 5H₂SO₄ → 5CaSO₄ 2H₂O + 6SiO₂ + 4H₂O,

(1)

Calcium carbonate: CaCO₃ + H₂SO₄ + 2H₂O → CaSO₄ 2H₂O + CO₂,

(2)

Calcium hydroxide: Ca(OH)₂ + H₂SO₄ → CaSO₄ 2H₂O,

(3)

Calcium-Silicate-Hydrate: 3CaO 2SiO₂ nH₂O + 3H₂SO₄ → 3CaSO₄ 2H₂O + 2SiO₂ + mH₂O,

(4)

The formation of gypsum is confirmed through Scanning Electron Microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy, as illustrated in Figure 5. As indicated by Equations (1)–(4), gypsum formation results from the acid attack of calcium oxides by H₂SO₄ [11]. Sulfuric acid may form via the oxidation of H₂S in an aqueous environment (Equation (5)) or through the reaction of SO₃, which is present in substantial amounts in AAC waste, with water (Equation (6)). The direct oxidation of H₂S with O₂ in the gas is also possible (Equation (7)), and elemental sulfur (S⁰) may further convert into sulfate (SO₄²⁻) in the presence of excess oxygen. Additionally, sulfur removal may occur through the reaction between iron(III) oxide in AAC waste and H₂S, forming iron(II) sulfide (Equation (8)). In the presence of oxygen, iron(II) sulfide can further react, regenerating iron(III) oxide (Equation (9)), as detailed by [22].

H₂S + 2O₂ → H₂SO₄,

(5)

SO₃ + H₂O → H₂SO₄,

(6)

H₂S + 0.5O₂ → S⁰ + H₂O,

(7)

H₂S + 2Fe³⁺ + 2OH⁻ → 2Fe²⁺ + 2H₂O + S⁰,

(8)

2Fe²⁺ + H₂O + 0.5O₂ → 2Fe³⁺ + 2OH⁻,

(9)

According to the stoichiometry of Equation (5), 2 moles of oxygen are required to remove 1 mole of H₂S, whereas Equation (7) requires only 0.5 moles of oxygen per mole of H₂S. Similarly, the reaction of H₂S with iron(III) oxide (Equations (8) and (9)) also consumes 0.5 moles of oxygen per mole of H₂S to regenerate Fe₂O₃. Consequently, H₂S removal is accompanied by oxygen consumption. Oxygen consumption was monitored between days 50 and 89 of the experiment under steady-state conditions, with constant H₂S and O₂ concentrations in the gas entering the PBR (Figure 6). Between days 50 and 70, the O₂/H₂S ratio fluctuated between 1.5 ± 0.2 and 2.5 ± 0.2. After day 70, a clear downward trend was observed, approaching 0.5 ± 0.2 by the end of the experiment. This evolution correlates with a change in H₂S removal efficiency (Figure 3). Between days 50 and 70, when H₂S removal efficiencies were close to 100%, it is likely that all reactions occurred simultaneously, including gypsum formation, direct oxidation of H₂S to elemental sulfur (S⁰), and FeS formation. The presence of elemental sulfur (S⁰) and iron sulfide (FeS), visible on the material surface as a yellow deposit and a black coloration (as shown in the top left of Figure 1, i.e., AAC used material), was confirmed by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy. However, after day 70, gypsum formation reactions became less frequent, leading to a simultaneous decrease in H₂S removal efficiency and O₂ consumption. By the end of the experiment, removal efficiencies of 20–30% can be attributed to mechanisms described by Equations (7) and (8), which require 0.5 moles of O₂ per mole of H₂S. In other words, the acid attack of calcium oxides by H₂SO₄ is the dominant mechanism in H₂S removal, achieving near 100% efficiency, whereas reactions involving Fe₂O₃ play a secondary role with limited effectiveness.

The influence of CO₂ was also monitored between days 50 and 89 of the experiment (Figure 6). Unlike oxygen, no clear conclusion can be drawn regarding CO₂ production or consumption, as the CO₂/H₂S ratio fluctuated between −0.4 ± 0.2 and +0.6 ± 0.2 during this phase of the experiment. Thus, CO₂ is unlikely to participate in reactions between AAC waste and H₂S. In other words, AAC waste does not seem suitable for reducing CO₂ levels in biogas.

3.4. Lifespan of AAC Waste

During the period of near 100% efficiency (between days 54 and 78), the daily H₂S removal capacity of the material remained constant at 4.0 ± 0.2 g_H2S kg_AAC⁻¹ day⁻¹ (Figure 3), which provides a good indication of the potential lifespan of the material under optimal pH and O₂ concentration conditions. For the applied load of 72 ± 4 g_H2S m⁻³ h⁻¹, this lifespan would be 46 days for RE→100% (i.e., 185 g_H2S kg_AAC⁻¹ divided by 4.0 g_H2S kg_AAC⁻¹ day⁻¹). This value can be compared with the estimated lifetime determined by [14] the following:

$$\mathrm{t}={\displaystyle \frac{{\mathrm{\rho }}_{\mathrm{A}\mathrm{A}\mathrm{C}}}{\mathrm{R}\mathrm{C}}}\mathrm{\%}\mathrm{C}\mathrm{a}\mathrm{O}{\displaystyle \frac{{\mathrm{M}}_{{\mathrm{H}}_2\mathrm{S}}}{{\mathrm{M}}_{\mathrm{C}\mathrm{a}\mathrm{O}}}},$$

(10)

With t representing the lifespan of the AAC waste (in h), ρ_AAC the AAC bed density (Mass_AAC/V_AAC, in kg m⁻³), the removal capacity (RC) is defined as the mass of H₂S removed per unit volume of the PBR and per unit of time (in kg_H2S m⁻³ h⁻¹). The removal capacity equals the loading rate when RE = 100%. %CaO is the fraction of calcium oxides constituting the material, and M_H2S and M_CaO are the molar masses of H₂S and calcium oxides, respectively (in kg mol⁻¹). The time calculated according to Equation (10), with ρ_AAC = 410 kg m⁻³ (derived from the mass of dry AAC waste introduced in the PBR, i.e., 3.2 kg), CaO = 0.246 and RC = 0.072 kg_H2S m⁻³ h⁻¹, underestimates the material’s lifespan (36 days). However, Poser et al. [14] noted that this equation considers only the reaction of calcium oxides with H₂S, whereas other elements present in the material may also react, as indicated above. Furthermore, these authors highlighted significant uncertainty in determining the parameter ρ_AAC, as the mass of the dry AAC waste introduced at the start of the experiment will have to be reconsidered upwards due to the addition of water and the progressive accumulation of sulfur in the material over time. Consequently, without precise knowledge of the density of the wetted material bed, Equation (10) can only provide a rough estimate of the material’s lifetime.

3.5. Packed Bed Reactor Design and Operation Strategies for a Large Scale Biogas Purification

Measurements of H₂S removal efficiency and pressure drops showed that AAC waste can be efficiently and cost-effectively used to remove H₂S from gases, provided that the material is sprayed with highly acidic water and that gases contain some oxygen. However, the relatively short lifetime of the material, due to its high H₂S removal capacity, means that a system with two parallel reactors filled with AAC waste should be designed for industrial-scale treatment of large biogas flows. While one reactor operates, the other undergoes material replacement. Once replaced, the material could be repurposed either for plasterboard production or as a soil amendment [17,23]. The recyclability of AAC waste, transformed into gypsum through reaction with H₂S for potential use as a soil amendment, appears to be the most promising avenue. However, this possibility must be investigated through dedicated studies, including toxicological assessments, to confirm its safety and feasibility. The switching of operation from one reactor to the other will be controlled by monitoring the pressure drops in the operating reactor. However, this operating strategy requires controlling the switch from one reactor to the other to maintain effective H₂S treatment.

A short additional experiment was therefore carried out to verify whether AAC waste are capable of reacting very quickly and effectively with H₂S under optimum operating conditions. The PBR was emptied, and new raw AAC pieces were introduced. These pieces were then sprayed with acidic water at pH = 3.0 ± 0.1 and subjected to the same gas flow used at the end of the previous experiment (i.e., residence time = 22 ± 1 s, H₂S concentration = 420 ± 20 mg m^−3, and oxygen concentration around 0.30 ± 0.01%). The results showed that H₂S removal efficiency reached 100% within a few days (less than one week), demonstrating that the transient period was considerably reduced compared to that observed in the long-term experiment (Figure 1). Then, RE remained constant at 100% during the remainder of the experiment (10 days), with a removal capacity of 70 ± 4 g m⁻³ h⁻¹. The pH decreased slightly to 2.2 ± 0.1, while the oxygen concentration varied between 0.3 and 0.7% (3000–7000 ± 100 ppm). Consequently, the continuous irrigation of raw AAC waste with highly acidic water ensures high efficiency in H₂S removal, demonstrating the feasibility of large-scale biogas purification using AAC waste.

3.6. Advantages and Drawbacks of Using AAC Waste for H₂S Removal in Biogas

Compared to conventional adsorbent materials, the treatment performance of AAC waste is superior to that of zeolites (40 g_H2S kg⁻¹) and metal-organic frameworks (MOFs) (130 g_H2S kg⁻¹), and comparable to that of activated carbons and biochars (50–200 g_H2S kg⁻¹) [9,24]. However, certain impregnated or doped activated carbons can reach adsorption capacities as high as 850 g_H2S kg⁻¹ when treating synthetic biogas in which nitrogen replaces methane [9]. Nevertheless, these functionalized activated carbons are highly expensive, whereas the low cost of AAC waste (on the order of a few tens of euros per ton) clearly positions such waste as a viable alternative to activated carbons, even though some activated carbons can be partially regenerated at high cost. It is also important to note that studies conducted on real biogas streams containing CH₄, CO₂, and H₂S have shown that the adsorption capacities of biochars, zeolites, and activated carbons are significantly lower than those obtained using N₂ as the carrier gas (<70 g_H2S kg⁻¹). However, adsorption capacities of 383 g_H2S kg⁻¹ and up to 1266 g_H2S kg⁻¹ have been reported in the literature. It is therefore essential to assess the performance of AAC waste in the treatment of real biogas in order to enable accurate comparisons.

Regarding end-of-life considerations for materials, activated carbons and AAC waste can be compared in terms of their respective disposal routes. On the one hand, the incineration of activated carbons requires specialized facilities capable of treating emitted gases, while landfilling presents notable environmental constraints. On the other hand, AAC waste offers the advantage of transforming upon contact with H₂S into a new material that can be potentially valorized as a soil amendment. The main limitation in the use of the resulting gypsum lies in the origin of the AAC waste, specifically its initial composition, and in particular the presence of potentially toxic metallic compounds. The composition of AAC waste varies depending on the waste source and its degree of association with other demolition materials. With respect to available waste streams, in Germany, for example, AAC waste generation was estimated at 1.2 million tons in 2020, with projections exceeding 4 million tons by 2050 [25]. These volumes would be more than sufficient to meet the country’s biogas treatment needs. Assuming a treatment capacity of 185 g_H2S kg⁻¹, the amount of AAC waste required annually to treat the biogas output of a digester producing 150 Nm³ h⁻¹ with an H₂S concentration of 1000 ppmv (1400 mg m⁻³) is approximately 10 tons per year. The available AAC waste quantities are thus more than adequate to support large-scale H₂S removal. Nonetheless, attention should be given to sourcing AAC waste locally, in order to minimize transportation needs, both for the supply of fresh material and for the removal of the AAC waste transformed into gypsum byproduct.

The pressure drops generated during filtration using randomly AAC waste also highlight the advantages of this material over activated carbons. The centimeter-scale particle size of AAC waste results in pressure drops of only a few pascals per meter (Figure 3), which are significantly lower than those typically observed in packed beds of powdered or granular activated carbon [26].

Moreover, the removal of H₂S from biogas using AAC waste offers the advantage of being compatible with humid conditions and the presence of oxygen, factors that can negatively affect the performance of activated carbons. The simultaneous removal of both oxygen and H₂S by AAC waste, following a 2:1 stoichiometric ratio, can be particularly beneficial in anaerobic digesters employing micro-oxygenation to reduce H₂S concentrations in the digester headspace [27,28]. By controlling the amount of air introduced during micro-oxygenation, it may be possible to regulate the O₂/H₂S ratio in order to optimize H₂S removal by AAC waste, ultimately yielding a biogas stream that is free of both H₂S and residual oxygen after treatment.

4. Conclusions

This study demonstrated the effectiveness of AAC waste as a material for removing H₂S from gas streams under specific operating conditions. By maintaining a highly acidic environment and ensuring the presence of oxygen at low concentrations (<1%), AAC waste exhibited H₂S removal efficiencies close to 100% for moderated H₂S concentrations (300 ppm, i.e., 420 ± 20 mg m⁻³). The transformation of AAC waste, mainly into gypsum, indicated a progressive consumption of the calcium oxides of the material, thereby determining its operational lifespan. This study also highlighted the importance of monitoring pressure drops (ΔP) as a reliable and cost-effective indicator for determining the end of AAC’s functional lifespan.

Based on these results, an industrial H₂S treatment system can be designed using two parallel reactors filled with AAC waste. While one reactor is in operation, the other undergoes material replacement. Although the use of AAC waste has not yet been considered by the industry for H₂S treatment in gases such as biogas, the significant recyclability potential of this waste as gas filtration material is evident, offering a promising alternative to the use of expensive adsorbents in terms of H₂S separation capacity. Thus, the use of AAC waste for H₂S removal in biogas could fit into a dual circular economy approach, contributing to energy transition goals by reducing greenhouse gas emissions and establishing a value chain for AAC waste.

The next steps of this work will involve confirming the obtained performance, as well as the durability of AAC waste, for the treatment of real biogas produced under industrial conditions. Furthermore, the recyclability of AAC waste transformed into gypsum, notably as a soil amendment, will also be studied.