Biogas industry: Novel acid gas removal technology using a superacid solvent. Process design, unit specification and feasibility study compared with other existing technologies
Graphical abstract
Introduction
Nowadays, biogas use has risen as an alternative energy source counterbalancing the dependence on fossil fuels. This gas is produced by fermentation of organic matter, such as primary and secondary agricultural residues, industrial and household waste, and sewage. The significant methane (CH4) content in raw biogas streams, usually above 40 vol.%, allows either producing biomethane (upgraded biogas above 95 vol.% in CH4) through an intensive purification treatment, or producing partially purified combustion gas for turbines, engines and boilers (depending on the equipment specifications).
Overall biogas composition depends on the source of the organic matter, as summarized in Table 1. Other components different to methane in biogas streams have rather a negative effect on industrial equipment, combustion processes, emissions quality and the environment, as shown in the same table.
In terms of the combustion performance, the low Wobbe Index values for raw biogas (Table 1), as compared to natural gas, confirms the operational need for purification before its further use. The Wobbe index for natural gas sales is approximately (Deublein and Steinhauser, 2010; Eimer, 2014). In addition, environmental regulations establish maximum impurities content in purified combustion gases between for sulfur hydroxide (H2S) and between 2 and 8 vol.%, for carbon dioxide (CO2), depending on every country sales gas specifications (Kohl and Nielsen, 1997; Nexant-Chem systems, 2006). Regulations also apply to sulfur dioxide (SO2) gas emissions, produced by the H2S combustion, to a maximum between 4.0 and 133.0 ppmv, depending on the combustion facility location (European Commission, 2015; Kohl and Nielsen, 1997).
Specific content of siloxanes and the non-methane volatile organic compounds (NMVOCs) in biogas streams, resulting from its anthropogenic sources, renders necessary a pretreatment for its separation. Siloxanes are produced by industrial matter degradation, such as hygienic and cosmetic products, food additives and other man-made products. Their chemical structure is consisting of silicon, oxygen and alkyl groups in lineal (Li) or cyclic (Di) arrangements (Table 1). The siloxanes noted as D4 and D8 are normally reported as the most frequently found in the biogas (Soreanu et al., 2011; Tansel and Surita, 2014). Siloxanes oxidation, during the gas combustion, leads to silica particles () precipitation, with a size distribution ranging between 4 and 500 nm, depending on the flame temperature of the combustion chamber. Part of the aggregates precipitate in the combustion chambers and chimneys (below 1200 K), whereas a small fraction of solids remains dispersed in the gas emissions (Tansel and Surita, 2014). Negative effects of precipitated are described in the Table 1.
In wake of the mentioned process specifications and environmental regulations, biogas impurities need to be removed seeking a final energy use. Acid gases (H2S and CO2) removal is well known and established in decades, based on natural gas processing. Few of these technologies are however used for an overall biogas upgrading through adsorption and absorption-based separation processes. Commonly used adsorption-based activated carbons normally exhibit capacity around 7 times higher for siloxanes and 4 times higher for NMVOCs as compared to H2S (Bansal and Goyal, 2005; Ford, 2007; Largitte and Pasquier, 2016), whereas carbon molecular sieves, usually Zeolite 5A – lead to estimated separation capacity around 1.5 times higher for H2S than for CO2 and a specific selectivity to siloxanes and NMVOCs (Awe et al., 2017; Maddox, 1982). Separation by absorption-regeneration scrubbing systems using water, organic solvents (e.g., polyethylene glycol, methanol, N-metyl-2-pyrrolidone and carbonate propylene) or inorganic salts (e.g., NaHCO3, K2CO3) in aqueous solutions rely on the gas solubility and therefore separate the heavy compounds, siloxanes, NMVOCs, CO2 and H2S as compared to the CH4; however large recycling volumes usually limit its implementation (Ford, 2007; Kohl and Nielsen, 1997). Recent studies propose the use of alkanolamines aqueous solutions but discarding the idea of a regeneration step while used for biogas upgrading (Abdeen et al., 2016; Tippayawong and Thanompongchart, 2013).
Alternatively, other technologies are rather used to eliminate specific impurities from raw biogas streams, such as membrane separation for CO2 removal, biological or chemical H2S desulfurization units and low-temperature siloxanes condensation units, among others. Polymeric membranes are currently implemented for CO2 removal from pretreated biogas streams, usually achieving between 92 and 97% pure biomethane by means of either multiple separation steps with pressure gradients, or membrane contactors with concentration gradient as driving force. Among others, these membranes are usually made of polyimide, polysulfone and cellulose acetate polymeric materials (Angelidaki et al., 2018; Ballaguet et al., 2018; Ford, 2007; Kerber and Repke, 2016). Specific H2S chemical desulfurization may be similarly feasible by using strong oxidation agents, such as H2O2, KMnO4, Cl2 and O3 as currently done for wastewater treatment. However, possible methane oxidation frequently limits its application for biogas valorization (Bernard, 2013; Cadena and Peters, 1988; Lewkiewicz-Malysa et al., 2008). Biological H2S removal has become increasingly popular with a wide range of techniques, e.g., continuous stirred-tank reactors, biofilters, and membrane contactors, which use alkaline solvents to produce sulfide ions in liquid phase to thereafter biologically oxidize it to elemental sulfur. Frequent problems related to biomass accumulation and clogging, internal pH gradients, unwanted pressure drops and backmixing lead to decreased performance on these processes (Roman, 2016).
The use of a complete treatment unit, as well as integration of complementary units, is reported to be expensive and energy demanding for biogas treatment, due to its permanent changes of acid gases content, the singular presence of siloxanes, the low pressure (around 202.6 kPa) and the low flow rate (lower than ) of raw biogas streams (Clément, 2016; Krischan et al., 2012; Vienna University of Technology, 2012).
Based on the described problem, a novel purification system has been developed for biogas applications. It allows eliminating the acid gases, siloxanes and NMVOCs up to the required emissions specifications, leading to a reduction of the total load factor of further separation equipment and thereafter energy savings. This system is based on the use of a superacid solvent mixture capable of performing a selective chemical absorption of the gas contaminants, by means of a bubble column reactor. The use of a superacid solvent has not been reported before for this type of applications.
This paper presents the process design considerations of the novel system: details on the solvent formulation, gas–liquid reaction mechanisms, transport properties specifications, equipment-dimensioning criteria and process simulation specifications. The newly proposed system was designed based on average composition values and flow rate of 500 Nm3/h of a real biogas facility. The impurities content was set in dry basis equal to 400–1500 ppmv H2S composition range, 30% CO2, 5 ppmv siloxanes, and 340 ppmv NMVOCs. A technical and energy-based feasibility study was also performed to the new process as compared to other commercialized technologies, i.e., the physical absorption with methanol, based on the Rectisol® process, and the chemical absorption with the monoethanolamine, MEA, aqueous solutions.
Section snippets
Superacid solvent formulation
The proposed superacid solvent is a mixture of sulfuric acid (H2SO4(aq), commercial grade, 95.0 wt.%) and acetic acid glacial (CH3COOH(l), commercial grade, 99.8 vol.%). Scientific reports have been published describing the ability of this mixture to react with weak acids for which the H2SO4 was not capable by itself (Kotov et al., 1969; Olah et al., 2009).
Based on the acids mixture reaction properties, the solvent formulation for the biogas treatment has been established to a molar ratio of
Process modeling and base case scenario
Fig. 1 presents the process simulation diagram of a biogas purification unit for biomethane production, without considering any desulfurization unit. This type of process lay-out is particularly used in French biogas upgrading units, usually employing either activated carbon or NaOH scrubbing units, coupled to a PSA or a membrane separation unit for biomethane production (European Biogas Association (EBA), 2017). Biogas properties, compositions and overall operation conditions, considered on
Novel purification system design
Table 5 presents the process related results of the novel biogas purification system using of the Superacid 9/1 in a bubble column reactor. Process simulations were performed considering three different conditions of H2S content in the raw gas: 400, 1000 and 1500 ppmv (dry basis). The novel purification unit was optimized as part of the commonly used biogas process aimed to produce biomethane, establishing a maximum SO2 content of 100 ppmv in the gas emissions while burning the off-gases
Conclusions
A novel purification system is proposed for raw biogas streams. This system consists on a bubble column reactor equipped with a solvent mixture of sulfuric acid (H2SO4(aq), ) and acetic acid glacial (CH3COOH, 99.8 vol.%), both commercial grade. A mixture of these acids in a molar ratio of 9/1 for H2SO4(aq)/ CH3COOH respectively, offers physico-chemical properties within the realm of superacids. Sufficient physical absorption capabilities from each acid solvent as well as its specific
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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