Renewable aqueous ammonia from biogas slurry for carbon capture: Chemical composition and CO2 absorption rate
Graphical abstract
Introduction
Climate change is driving global concerns due to its profound impacts on our environment (Xie et al., 2015; Yang et al., 2012). Carbon dioxide (CO2) is considered as the primary greenhouse gas that contributes considerably to climate change. A wide range of technologies currently exist for CO2 separation/capture from gas streams, such as chemical absorption (Li et al., 2013a; Yan et al., 2015a), adsorption, membrane processes (He et al., 2018; Yan et al., 2014; Zhao et al., 2016). One of the leading technologies for CO2 capture in the near future may be amine-/ammonia-based CO2 absorption process because it can be easily retrofitted to the existing and new fossil-fuel based power plants, or other industry processes like natural gas engineering, bio-methane production (Abdeen et al., 2016; Ryckebosch et al., 2011; Sun et al., 2015), hydrogen and ammonia manufacturing processes (Darde et al., 2010a; Moser et al., 2011). Additionally, CO2 chemical absorption has been commercialized for many decades (Yu et al., 2012a), and the researches on CO2 chemical absorption have achieved great interests worldwide (Choi et al., 2009; D’Alessandro et al., 2010; Zhang et al., 2018).
However, chemical absorption has its drawbacks in carbon capture. First, CO2 chemical absorption process is always energy intensive due to the huge energy inputs required for absorbent regeneration, which may reduce the power production of a power plant by ∼ 30% (Yan et al., 2015b). Second, chemical absorbents may be degraded or have performance lost by the trace gases (e.g. SO2), metallic contaminants or exothermic reactions (Gao et al., 2011; Jeon et al., 2013). Last, most chemical absorbents are typically expensive and not environmentally friendly (Eide-Haugmo et al., 2012). CO2 capture in a once-through manner where absorbent regeneration is not required may reduce the regeneration energy (Bhown and Freeman, 2011; He et al., 2017b; Luis, 2016).
Derived from renewable resources like agricultural wastes, some renewable CO2 absorbents with easy availability and high CO2 removal efficiency (McLeod et al., 2014), may be promising to relieve the tension of chemical absorbent supply. For example, N-ethylethanolamine and N,N-diethylethanolamine are typical renewable CO2 absorbents because ethanol as the major raw material for synthesizing these alkanolamines can be prepared by renewable resources from agricultural products and/or residues (Vaidya and Kenig, 2007; Wang et al., 2017). Renewable ammonia should be worth noting since ammonia has a higher CO2 absorption rate and lower regeneration energy consumption compared with other amine absorbents (Darde et al., 2011, Darde et al., 2010b; Jiang et al., 2017; Puxty et al., 2010; Qin et al., 2010). Additionally, ammonia can be directly recovered and enriched through gas and/or thermal stripping methods from the ammonium-rich wastewater (Behrendt et al., 2002; McLeod et al., 2014), or biogas slurry which is the discharged effluent from anaerobic biogas digestion using biomass as the substrates (He et al., 2017a; Werber et al., 2017; Yan et al., 2013). Nitrogen-rich organic wastes are sufficient and renewable, and they can be converted into ammonium nitrogen in the treatment process (e.g. anaerobic digestion) (Drosg et al., 2015). So, recovery of ammonia from waste steams is not only helpful for CO2 separation and storage, but also beneficial for the regional environment protection (Bonmatı́ and Flotats, 2003; Carretier et al., 2015).
In our previous study, the renewable aqueous ammonia solution (RAA) was successfully achieved from biogas slurry, and its CO2 absorption performance was investigated as well (He et al., 2017a; Werber et al., 2017). RAA has comparable merits with aqueous ammonia solution like higher CO2 absorption capacity and less energy consumption for regeneration. RAA derived from renewable resources, its acquiring or generation is a fully green chemical process with little energy consumption. The application of RAA for CO2 capture in the once-through process without regeneration stage can combine CO2 capture and utilization and fertilizer production together (Strube et al., 2011). Thus, RAA has more advantages than the common absorbents e.g. amines. However compared to the conventional aqueous ammonia solution, some impurities like volatile fatty acids (VFAs) exist in RAA (He et al., 2017a; Werber et al., 2017), which may affect the CO2 absorption performance. Moreover, the concentration of RAA is lower than the conventional aqueous ammonia solution adopted in CO2 capture (Darde et al., 2010a; Diao et al., 2004; Liu et al., 2009; Yeh et al., 2005). Therefore, the mass transfer characteristic and rate of CO2 absorption into RAA should be intensively explored.
In this study, RAA was firstly obtained from biogas slurry by vacuum membrane distillation (VMD) to investigate the effect of initial total ammonia nitrogen (TAN) concentration in biogas slurry on the concentrations of free ammonia and the impurities in RAA. Then kinetics of CO2 absorption into RAA was studied. Finally, the effects of TAN concentration and different impurities on CO2 absorption into RAA were analyzed in a wetted-wall column.
Section snippets
Recovery of renewable aqueous ammonia solution (RAA) from biogas slurry
RAA was recovered by VMD with a hollow fiber membrane module containing hydrophobic microporous polypropylene membranes, supplied by Ningbo Moersen Membrane Technology Co., Ltd. Specifications of the membrane module are listed in Table 1. Experimental setup for ammonia recovery is shown in Fig. 1.
The properties of the raw biogas slurry are shown in Table 2. After 20 min centrifugal pretreatment of the raw biogas slurry to get rid of the plugging risk of membrane pores caused by the suspended
Overall kinetic constant of CO2 absorption into RAA solution
As we illustrated before, the primary component beneficial for CO2 absorption in RAA is ammonia (He et al., 2017a). Thus, the reactions between CO2 and ammonia can be treated as the main process when CO2 absorption into RAA solution and can be described by a zwitterion mechanism as shown in reactions (1) and (2) (Darde et al., 2011; Yu et al., 2012b):where ki is the kinetic rate constant for reaction i (L/mol s); B represents
Characteristics of RAA recovered from biogas slurry
During ammonia recovery from biogas slurry by VMD, all volatile components including water, CO2, ammonia and VFAs transport from the biogas slurry through membrane pores into gas phase, and then most of them can be condensed and collected on the permeate side (Chiam and Sarbatly, 2013; Pangarkar et al., 2014). Nitrogen can only transfer from the feed into the permeate in the form of ammonia, because nitrogen in biogas slurry exists primarily in the form of ammonia, ammonium, amino acid and
Conclusion
Renewable aqueous ammonia (RAA) for CO2 absorption was investigated in this study. Higher TAN concentration of RAA () up to 2.4 mol-N/L can be achieved through VMD, and increases with the increase of the initial TAN concentration of biogas slurry. Due to higher pH value of RAA (10.7–11.5), higher than 99% of the N element is formed as free ammonia, meaning a better affinity towards CO2. Despite much lower concentration compared to free ammonia in RAA, CO2 and some volatile
Acknowledgments
The authors thank the financial supports fromthe National Key R&D Program of China (No. 2017YFB0603300), the National Natural Science Foundation of China (No. 51676080, 51376078), and the Fundamental Research Funds for the Central Universities (No. 2662018PY046). Mr. Qingyao He acknowledges the support from China Scholarship Council (CSC) for studying at Macquarie University in Sydney (201606760032). Special thanks go to Dr. Hai Yu and Dr. Kangkang Li in CSIRO Energy for their significant help
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