Advances in H2S-removal technologies for biogas upgrading are essential for mitigating environmental issues like air pollution and corrosion
Biogas, a renewable energy source derived from organic waste and typically comprised mainly of methane, holds great promise for sustainable energy production. However, the presence of hydrogen sulfide (H2S) in biogas poses challenges to its utilization due to its corrosive and toxic nature. Effective removal of H2S is crucial for upgrading biogas to meet quality standards for various applications, including power generation and injection into natural gas pipelines.
This article explores the different types of H2S removal technologies for biogas upgrading, with a focus on the advantages of regenerative technologies in addressing the unique challenges of H2S removal.
Biogas produced from anaerobic digestion processes often contains impurities, with H2S being a predominant contaminant. While H2S is naturally occurring, its presence in biogas streams can lead to equipment corrosion, air pollution and health hazards. Therefore, upgrading biogas to remove H2S is essential for ensuring safety, protecting infrastructure integrity and complying with regulatory requirements. Moreover, purified biogas can be utilized efficiently in various applications, contributing to more sustainable energy practices and reducing reliance on fossil fuels.
H2S removal technologies
Several technologies are available for removing H2S from biogas streams, each with its own unique mechanisms and advantages.
Chemical scrubbing. Chemical scrubbing involves removing H2S by absorption and neutralization. A base, such as sodium hydroxide or potassium hydroxide, is metered into the scrubbing liquid and reacts with the H2S to form salts. Often, an oxidizing agent, such as sodium hypochlorite, is added to convert unstable sulfite salts to stable sulfate salts. Chemical scrubbing is not selective to H2S, and CO2 interference can result in increased chemical consumption. While effective, chemical scrubbing requires significant chemical consumption, especially if CO2 is present. It also generates large volumes of wastewater, which increases operating costs.
Biological desulfurization. Biological desulfurization utilizes sulfur-oxidizing microorganisms to convert H2S into elemental sulfur or sulfuric acid. This regenerative process occurs in bioreactors or biofilters, offering a sustainable and environmentally friendly approach to H2S removal. However, biological desulfurization may be limited by process kinetics and sensitivity to operating conditions.
Adsorption. Adsorption technologies involve the physical adsorption of H2S onto porous media, such as activated carbon or zeolites. Adsorption offers versatility and can achieve high removal efficiencies, but the spent adsorbents require regeneration or disposal, adding to operational costs.
Scavengers. Scavengers are chemicals, such as metal oxides or triazines, that irreversibly react with H2S. For both adsorption and scavengers, a standby unit is often installed in order to replace the spent media without interrupting the process. While effective, scavengers generate waste products that require disposal, contributing to environmental concerns and operational costs.
Liquid redox. Liquid redox systems (Figure 1) utilize a catalyst, such as chelated iron, to convert H2S into solid sulfur. The chelated iron is not consumed in the process and the only losses are typically due to degradation over time. Liquid redox offers a sustainable solution for H2S removal by minimizing chemical consumption and waste generation. This technology regenerates and recycles the active catalyst used in the removal process, reducing operational costs and environmental impact.
FIGURE 1. In a liquid redox system, H2S is converted to elemental sulfur using a chelated iron catalyst, which is continually regenerated and reused in the system
Regenerative technologies
While all H2S removal methods aim to mitigate environmental risks and enhance operational efficiency, regenerative liquid redox technologies offer distinct advantages that make them increasingly attractive for biogas producers and industrial applications (Table 1). The following sections cover the multifaceted benefits of regenerative technologies, emphasizing their pivotal role in advancing process sustainability, cost-effectiveness and operational flexibility.
Environmental sustainability. One of the paramount advantages of liquid redox technologies lies in environmental sustainability. Unlike conventional chemical-scrubbing methods, which often entail significant chemical consumption and waste generation, liquid-redox technologies minimize environmental impact by optimizing reagent usage and promoting circular-economy principles.
Chemical consumption reduction. Liquid-redox desulfurization systems operate on regenerative principles, where reagents are continuously regenerated during the H2S removal process. For instance, chelated iron catalysts facilitate the oxidation of H2S to elemental sulfur, which can be separated from the process liquid through filtration, and recycled back into the process, minimizing the need for continual reagent makeup. Minimal chemical makeup is required to maintain the system pH.
Waste minimization. By significantly reducing chemical consumption and promoting reagent recycling, liquid-redox inherently minimizes waste generation. The continuous regeneration and reuse of reagents results in fewer waste byproducts compared to traditional chemical scrubbing methods. Consequently, biogas upgrading facilities employing regenerative technologies contribute to a more sustainable waste-management ecosystem and reduce their overall environmental footprint.
Cost-effectiveness. While upfront capital costs may be a consideration, liquid-redox technologies offer compelling long-term cost advantages that position them as economically viable solutions for biogas upgrading. Recent advancements in the technology have brought down capital costs, making it a feasible option for a wider range of applications. Furthermore, a modular design philosophy and skid-packaging can reduce installation time and costs (Figure 2). The inherent efficiencies of regenerative processes translate into substantial savings over the operational lifespan of H2S removal systems.
FIGURE 2. A skid-mounted pump package and modular vessels can minimize field work and installation costs for liquid-redox systems
Reduced operational costs. One of the primary cost advantages of liquid redox technologies stems from their ability to minimize operational expenses associated with chemical procurement, handling and disposal. By recycling or regenerating reagents, liquid redox desulfurization systems significantly lower ongoing chemical consumption, leading to substantial cost savings in the form of reduced chemical procurement and waste-disposal expenses and ultimately lower payback period depending on the application.
Enhanced lifecycle economics. Despite potentially higher initial capital investments compared to conventional methods, regenerative technologies offer superior lifecycle economics driven by their reduced operational costs and longer-term sustainability, especially for applications with a high inlet loading, because the cost per unit of H2S treated is lower than non-regenerative technologies. The ability to reuse or regenerate reagents extends the lifespan of H2S removal systems, maximizing return on investment (ROI) and delivering cost-effective solutions for biogas producers over the system’s operational lifetime.
Reduced water requirements. Since minimal waste is generated from liquid-redox technologies, there is a minimal water-makeup requirement compared to other technologies. The quality of water is an important consideration. Water that has low hardness, low dissolved solids and is free of solids and bacteria is beneficial for the process.
Operational flexibility. Liquid-redox technologies offer a high turndown and operational flexibility, enabling biogas producers to adapt to fluctuating feedstock compositions, process conditions, and operational requirements while still maintaining a high H2S-removal efficiency. This inherent flexibility enhances system resilience, reliability and adaptability in dynamic operating environments (Figure 3).
FIGURE 3. H2S removal systems should be designed for a high degree of process flexibility
Robust performance. Liquid-redox desulfurization systems demonstrate robust performance across a wide range of operating conditions, including variations in biogas composition, flowrates and H2S concentrations. The regenerative nature of the technologies ensures consistent and reliable H2S removal efficiencies, even in challenging operating environments, thereby minimizing disruptions and downtime.
Adaptability to process variability. Biogas production processes are inherently dynamic, with variations in feedstock composition and operating parameters. Liquid-redox technologies offer inherent adaptability to process variability by leveraging automated controller and instantaneous chemical reactions. This adaptability enables seamless integration with anaerobic digestion processes and enhances system stability and performance in diverse operating conditions.
Compliance and safety. By efficiently removing H2S from biogas streams, regenerative technologies ensure compliance with regulatory standards and safety requirements. Purified biogas is free from corrosive and toxic H2S, safeguarding equipment integrity and protecting human health and the environment.
Innovations in H2S-removal technologies for biogas upgrading are essential for advancing sustainable energy practices and mitigating environmental impacts. While various methods are available, regenerative technologies, such as liquid-redox, offer superior sustainability, cost-effectiveness and operational flexibility. ■
Edited by Mary Page Bailey
All images provided by the authors
Authors
Christopher Ristevski (421 Bentley Street, Unit 1, Markham, Ontario, L3R 9T2; Email: [email protected]) currently leads Macrotek Inc.’s Process Engineering team. He began working with Macrotek in 2009 and has a wide range of expertise in system design, process modeling and equipment and technology development. Ristevski has led the development of the Sulfcat H2S removal system from its inception in 2014, and is now responsible for the implementation of Sulfcat systems worldwide. He holds a B.S.Ch.E. from the University of Toronto.
Rosanna Kronfli is a process engineer at Macrotek Inc. (Same address as above; Email: [email protected]). She joined Macrotek in 2015 and has a wide range of experience in air-pollution-control equipment and process design. She holds B.S. and M.S. degrees in chemical engineering, both from the University of Toronto, and is a licensed professional engineer with Professional Engineers Ontario.