CO2/CH4 separation by hot potassium carbonate absorption for biogas upgrading

https://doi.org/10.1016/j.ijggc.2019.02.011Get rights and content

Highlights

  • CH4 emissions in off-gas affect biomethane sustainability.

  • Biomethane was produced by absorption in hot K2CO3 solution in a pilot plant.

  • Methane slip with HPC is <0.1%.

  • Capital expenditure and electricity consumption are lower.

Abstract

In biogas upgrading to biomethane, the release of CO2 off-gas into the atmosphere is generally regarded as a carbon-neutral emission, but a significant loss of CH4 can occur in this step: considering the global warming potential of this latter compound, methane slip can worsen or even nullify the CO2 savings associated to biomethane. This study investigates a novel approach for biogas upgrading to biomethane, aimed at reducing the methane loss. A plant based on hot potassium carbonate was fed with 150–200 Nm3 h−1 of biogas from municipal waste. CO2 is removed in a K2CO3 absorption column, with negligible CH4 absorption. An assessment of biomethane quality was performed to check its compliance with recent National and European standard specifications. Results show that a methane slip below 0.1% can be achieved with this technology, thus significantly reducing the greenhouse gas emissions associated to biomethane industry. This leads to a lower capital expenditure because no off-gas post-treatment is required. Heat and electricity consumption were monitored, and operational expense resulted to be lower than membrane separation in the specific case study, by applying life cycle cost (LCC) methodology.

Introduction

Biomass has gained a key role in climate change mitigation, in light of its large energetic potential. Several strategies are available for converting biomass to energy, including direct combustion, pyrolysis and gasification, bioethanol and biodiesel production and anaerobic digestion. In the last decade, biogas production through anaerobic digestion has become a consolidated approach for the energetic exploitation of several kinds of biomass, including agricultural and zootechnical by-products, sewage sludge, food waste and energy crops. Biogas is a gas mixture whose main components are CH4 and CO2 (85-40% and 15–60%, respectively) (Andriani et al., 2014).

Biogas is generally used for producing heat and/or electricity (Batstone et al., 2015; Demirbas et al., 2016; Bachmann, 2015). On the other hand, biogas conversion into biomethane is being regarded as an interesting application. Biogas upgrading to biomethane aims at obtaining a density and a calorific value comparable to natural gas, by removing CO2 (Andriani et al., 2014; Munoz et al., 2015; Wesley Awe et al., 2017). The development of biomethane technology allows to increase the share of biomethane end-users: indeed, biomethane can be injected into natural gas grids (Semple et al., 2014; Bekkering et al., 2015) and can be used as vehicle fuel (Yang et al., 2014; Patterson et al., 2011). Biomethane use as gasoil substitute can significantly reduce the contribution of vehicular traffic to urban air quality, due to its lower emission factors for nitrogen oxides, particulate matter and polycyclic aromatic hydrocarbons (EEA, 2016).

The CO2/CH4 separation can be achieved through several approaches, including absorption, adsorption (pressure swing adsorption and vacuum swing adsorption, PSA/VSA), membranes and cryogenic separations (Bauer et al., 2013; Hoyer et al., 2016). Regardless of which separation technique is used, biogas upgrading consists in the production of two gas streams: biomethane and off-gas. While biomethane mostly consists in CH4 and is used as a fuel, the off-gas is mainly composed of CO2 and is generally released into the atmosphere. This emission is regarded as “carbon-neutral”, since it corresponds to CO2 absorbed in previous decades during biomass production. Furthermore, the amount of CO2 released as off-gas would be similarly emitted in the atmosphere during conventional biogas burning. Nevertheless, high purity off-gas streams might be used in industrial and food applications, as well as in carbon capture and storage processes including microalgae growth (Colling Klein et al., 2017).

The choice of the optimum upgrading technology for a specific anaerobic digestion plant depends on a number of factors: in rough approximation, the three main parameters are biomethane purity, energy consumption and methane slip. Biomethane purity is generally defined as the volumetric concentration of CH4 in biomethane outlet: it affects several parameters which must comply with regulatory requirements, namely density, heating value, methane number, CO2 concentration etc., depending on the specific Country. Energy consumption (kWh m−3) is defined as the energy used, in terms of heat and electricity, for a given volume of processed biogas or produced biomethane. This parameter strongly affects the total operational cost of the plant and is also related to the global balance of fossil fuel saving.

Methane slip, also known as methane loss, is defined by Eq. 1, where Qi and Qf are inlet and outlet methane mass flow rate, respectively.Methaneslip%=100Qi-QfQi

This latter parameter is a fundamental issue on both an economic and an environmental level. Methane slip are obviously associated to a waste of a fraction of the available biofuel, resulting in a reduced revenue from the unexploited renewable energy. Along this trivial consideration, CH4 concentration in off-gas can dramatically affect the environmental sustainability of a biogas upgrading plant, since the global warming potential of CH4 is 28 times higher than that of CO2 (IPCC, 2013; Paolini et al., 2018a). As a consequence, the release in the atmosphere of low quality off-gas streams would result in the emission of a significant amount of CH4. This greenhouse gas would not be emitted if biomass is used in applications different from anaerobic digestion; similarly, low methane emission factors are observed in conventional biogas burning. Methane slip has been identified as a major contribution to total methane emissions of biogas industry (Liebetrau et al., 2017). The equivalent CO2 saving of biomethane production considerably raises if methane slip is limited to 0.05%, while the process results no longer sustainable when methane slip reach 4% (Ravina and Genon, 2015).

Table 1 reports a general overview on biogas upgrading technologies available at a commercial scale.

As shown in Table 1, most of commercially available techniques have a methane slip far above 0.05%. Since biomethane production is undergoing a fast development, it is foreseen that a large number of biogas upgrading plants will be installed in the next decade. As a consequence, it is expected that off-gas emissions will have a growing role in the mitigation of climate change. In this framework, biogas upgrading technologies should be thoroughly revised in order to reduce the methane slip.

Currently, the lowest methane slip can be archived by amine absorption, but this technology involves the use of chemicals such as monoethanolamine or diethanolamine. The acute inhalation toxicity of a mixture containing ethanolamines has been investigated in rats and the lethal concentration (LC50) was estimated to be 2.48 mg L−1 (Anon., 2010a). The repeated dose toxicity has also been assessed: the dominant effects of a continuous exposure to 5–6 ppm were skin irritation and lethargy; dogs and rodents exposed to 66–102 ppm ethanolamine vapor had behavioural changes, pulmonary and hepatic inflammation, hepatic and renal damage, and hematological changes; mortality was reported in dogs exposed to 102 ppm ethanolamine vapor for 2 days, and in rodents exposed to 66–75 ppm ethanolamine vapor for 24–28 days (Elder, 1983). The inhalation of ethanolamine can also cause bronchoconstriction (Kamijo et al., 2009). Weak mutagenic effects were reported in one study on genotoxicity in vitro, and a weak positive response was reported in human lymphocytes (Arutyunyan et al., 1987). Ethanolamine inhalation by humans has been reported to cause immediate allergic responses of dyspnea and asthma and clinical symptoms of acute liver damage and chronic hepatitis (Elder, 1983). A case of occupational asthma in an industrial worker exposed to ethanolamine was reported (Savonius et al., 1994). Exposure to vapours from ethanolamine can irritate the nose, throat, and lungs (Bello et al., 2009). As a consequence, the Occupational Safety and Health Administration (OSHA) permissible exposure level for ethanolamine is 6 mg m-3 as an 8 h time-weighted average concentration (Anon., 1996). Similarly, the National Institute for Occupational Health and Safety (OSHA) has a recommended exposure limit of 8 mg m-3 for a 10 h workday and 40 h work week; the short-term exposure limit is 15 mg m-3, for periods not to exceed 15 min (Anon., 2010b). Diethanolamine is classified as “possibly carcinogenic” by the International Association of Research on Cancer (IARC) (Anon., 2013).

Aim of this study is to assess a novel approach for CO2/CH4 separation, with the final goal of reducing methane slip while keeping biomethane purity and energy consumption at a competitive level. The investigated approach consists in the application of hot potassium carbonate (HPC) technology to biogas upgrading. It is basically an absorption on K2CO3 solutions, through the reaction described in Eq. 2 (Sanyal et al., 1988):K2CO3 + CO2 + H2O ⇄ 2 KHCO3

The obtained KHCO3 solution is continuously regenerated by reducing the pressure and increasing the temperature (Kamps et al., 2007). HPC is widely used at industrial scale for CO2/N2 separation in ammonia synthesis (Mahmoodi and Darvishi, 2017). The advantages of K2CO3 absorption include good CO2 capacity (e.g. 0.7–3.6 CO2/K2CO3 ratio, in the temperature range 313–393 K) (Kamps et al., 2007; Imle et al., 2013) and moderate heat consumption (e.g. 51.64 kJ mol−1 for a K2CO3 solution 15% in weight, with a loading ratio of 0.86 mol CO2/mol K2CO3 at 303 K) (Kim et al., 2016). In light of these advantages, HPC is being successfully applied to other gas streams, including post-combustion (Hetland and Christensen, 2008; Thee et al., 2012) and gasification (Urech et al., 2014; Li et al., 2014): since these latter applications have significant amounts of dust, tar, sulfur compounds and acidic components, the HPC confirmed to have a good resistance to impurities. Being a consolidated approach for CO2 capture, HPC could quickly become a valid and reliable technology for biogas upgrading to biomethane. Despite this potential, to the best of our knowledge no experimental data are available with regard to this topic, even if HPC was recently suggested as a suitable biogas upgrading method in a couple of simulation studies (Arshad et al., 2014; Zhang et al., 2017). Hence, aim of this study is to investigate the technical feasibility of a HPC plant for biomethane production, in terms of long term performances, energy consumption at industrial scale, resistance to biogas impurities, compliance with fuel standard requirements, methane retention and emission in the off-gas. For this purpose, biogas upgrading to biomethane by absorption on K2CO3 is investigated for the first time in a pilot HPC plant.

Section snippets

Biogas production and cleaning

Tests were performed in a waste management facility located in Este, North-Eastern Italy. The facility includes an anaerobic digestion plant for the treatment of the organic fraction of municipal solid waste. Fig. 1 shows the layout of the plant. The plant is divided into a cleaning module and an upgrading module. The cleaning module aims at removing secondary components such as non-methane volatile organic compounds (VOC) and sulfur compounds. It is basically constituted by a series of

Biomethane purity

Fig. 3 shows the CH4 and CO2 concentration in biomethane produced by the prototype during four operational days, representative of the average performance of the plant. The average CO2 concentration is 1.2 ± 0.4%v/v, following absorption in the potassium carbonate solution. This result shows that, in terms of biomethane purity, the considered methodology is competitive with the currently used alternative approaches (Andriani et al., 2014; Ryckebosch et al., 2011).

A detailed characterisation of

Conclusions

In this study, a demonstrative plant based on HPC absorption was validated for biogas upgrading to biomethane. Results obtained showed that the investigated methodology can be successfully applied for the production of biomethane for grid injection or for use as automotive fuel, in compliance with current standard requirements.

The main advantage is related to methane slip, which is significantly lower compared to currently used technologies. Specifically, an average methane slip below 0.1% was

Acknowledgements

This study was funded by GM Green methane in the framework of the scientific consulting activity CNR IIA protocol 0001517 of April 26th 2017. Activities are included in the cooperation agreement between the Institute of Atmospheric Pollution Research of the National Research Council of Italy (CNR IIA) and the “Unità di ricerca per l'ingegneria agraria” of the “Consiglio per la Ricerca in agricolutura e l’analisi dell’Economia Agraria” (CREA ING), protocol CNR IIA 0000369/2015 and CRA

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