Microaerobic removal of hydrogen sulphide from biogas

Resumen

Biogas production through anaerobic digestion represents one of the most important routes in order to fulfil the national and international regulations aiming for environment preservation and efficient utilisation of the natural resources. For profitable and safe use of its energetic potential, the biogas must satisfy the quality standards of the appliance. Biogas quality is crucial in both its methane content and purity. Hydrogen sulphide is one of the most common pollutants in biogas (Rasi et al., 2007). Several process-level (implemented in the reactor) and end-of-pipe strategies (applied in another unit) exist for its control, based on physical-chemical and biological phenomena (Cirne et al., 2008). In contrast to physical-chemical methods, the biological technologies are environmentally friendly and economical (Abatzoglou and Boivin, 2009). The removal of hydrogen sulphide directly in the reactor by imposing oxygen-limiting/microaerobic conditions is the most attractive biological solution due to its simplicity of implementation and operation, and the fact that the biogas is produced and desulphurised in a single unit (Fdz-Polanco et al., 2009; Díaz et al., 2010a, 2010b). By contrast, the operation of the alternative biological methods presents several difficulties (Mudliar et al., 2010). However, the possible costs arising from elemental sulphur accumulation in the gas space during microaerobic digestion could hinder the widespread application of this method of hydrogen sulphide control (Díaz, 2011). A potentially ideal external process for biogas desulphurisation would integrate the simplicity of the process of hydrogen sulphide removal in microaerobic digesters. The general objective of this thesis is to control the hydrogen sulphide content in the biogas produced during digestion by microaerobic processes. For this purpose, two different strategies are investigated: a process-level approach, which involves imposing microaerobic conditions directly in the reactor, and an end-of-pipe solution, implying the usage of an additional unit where the conditions present in the headspace of microaerobic reactors are reproduced. The thesis is structured as a compendium of eight research articles, which are organised into eight chapters. In order to achieve the aims of this thesis, two lab-pilot reactors and one industrial-pilot reactor are operated. At both scales, mixed sludge from a municipal wastewater treatment plant is treated under mesophilic conditions. The experiments aimed to develop a new end-of-pipe biotechnology for biogas desulphurisation based on the findings obtained from operation of lab-pilot microaerobic reactors are carried out at lab-pilot scale. Digestate from lab-pilot reactors treating municipal sewage sludge is used as the reaction media. Benefits of oxygen on the digestion process and the biogas quality can be reached simultaneously, for which the micro-oxygenation level must be precisely adjusted in order to achieve and maintain minimum concentration of both hydrogen sulphide and oxygen in the biogas. For this purpose, the production and the sulphide content of biogas can be used. Under variable organic loading rate and steady sulphur loading rate, biogas production is an efficient regulating parameter of the oxygen supply. However, under variable sulphur loading rate, hydrogen sulphide concentration must be the basis for the development of a reliable and precise control strategy. Elemental sulphur is the main by-product from sulphide oxidation in microaerobic reactors. When the moisture availability on the different surfaces of the gas space is sufficient, high amounts of this compound are deposited there, which can lead to increasing oxygen demand over time. As a result, the intervals of time at which the reactor must be cleaned can be reduced. Sulphide-oxidising bacteria grow all over the headspace. The composition, species richness and the size of this microbial community depended on the location (more specifically, on the moisture level) and the operation time. Although the hydrogen sulphide removal from biogas predominantly occurs in the gas space, the efficiency of the process is rapidly recovered after cleaning. Conversely, when the surfaces of the gas space suffer from dryness, elemental sulphur hardly accumulates there. The desulphurisation performance and the oxygen demand of the reactor are low relatively high (respectively) at the early stage of the microaerobic operation. Nevertheless, hydrogen sulphide can eventually be efficiently removed from the biogas under different configurations. The biogas recirculation raises the oxygen transfer rate to the liquid phase, which can increase the microbial richness and evenness and, in the log-term, cause an important shift in the biodiversity and structure of the bacterial and the archaeal communities. The conditions of biogas desulphurisation present in microaerobic reactors are successfully reproduced inside an external chamber called a ¿microaerobic desulphurisation unit¿. Microaerobic digestate is an efficient and durable reaction media. This system is robust against fluctuations in operating parameters, such as biogas residence time and mass loading rate and inlet concentration of hydrogen sulphide. Although neither nutrients nor water are added, it presents a high bacterial diversity. The microaerobic desulphurisation unit can be operated at 20ºC, and achieve almost the same removal efficiencies than microaerobic reactors. Nonetheless, relatively high temperatures at the start-up could be the key to achieving successful operation. Elemental sulphur is the main by-product, since the system performs successfully at oxygen/hydrogen sulphide (v/v) supplied ratios of up 1.0. Inside the MDU, hydrogen sulphide is oxidised through biological mechanisms. Abatzoglou, N., Boivin, S., 2009. A review of gas purification processes. Biofuels Bioprod. Biorefin. 3, 42-71. Cirne, D. G., van der Zee, F. P., Fdz-Polanco, M., Fdz-Polanco, F., 2008. Control of sulphide during anaerobic treatment of S-containing wastewaters by adding limited amounts of oxygen or nitrate. Rev. Environ. Sci. Biotechnol. 7, 93-105. Díaz, I., Lopes, A. C., Pérez, S. I., Fdz-Polanco, M., 2010a. Performance evaluation of oxygen, air and nitrate for the microaerobic removal of hydrogen sulphide in biogas from sludge digestion. Bioresour. Technol. 101, 7724-7730. Díaz, I., Pérez, S. I., Ferrero, E. M., Fdz-Polanco, M., 2010b. 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