Renewable aqueous ammonia from biogas slurry for carbon capture: Chemical composition and CO₂ absorption rate

Highlights

• Renewable aqueous ammonia up to 2.4 mol-N/L can be achieved from biogas slurry.

• Higher than 99% of N element in RAA is formed as free ammonia.

• CO₂ and VFAs can be extracted from biogas slurry and transferred into RAA.

• k₂ value of RAA with 2 mol-N/L is about 4306 L/mol s at 298 K.

• Butyric acid in RAA has the greatest negative impact on CO₂ absorption rate.

Abstract

Renewable aqueous ammonia (RAA) can be recovered from biogas slurry by vacuum membrane distillation (VMD). Bio-natural gas can be produced from biogas after removing its CO₂ using RAA as the CO₂ absorbent. However, RAA contains some impurities, e.g. organic carbon (mainly the volatile fatty acids, VFAs) and inorganic carbon (mainly the dissolved CO₂) that may influence its CO₂ absorption performance. In this study, RAA was firstly recovered by VMD to investigate the concentrations of free ammonia and impurities in RAA changed with the initial total ammonia nitrogen (TAN) concentration. Then CO₂ absorption performance of RAA was investigated in a wetted-wall column, and compared with the unloaded aqueous ammonia (AA). Through VMD, higher TAN concentration of RAA up to 2.4 mol-N/L can be achieved. Due to higher pH value (10.7–11.5), higher than 99% of N element is formed as free ammonia. Additionally, CO₂ and some VFAs mainly including ethanol, acetic acid, propionic acid and butyric acid can be extracted from biogas slurry and transferred into RAA. However, their concentrations are much lower than free ammonia in RAA. Furthermore, at 2 mol-N/L, the second-order reaction constant (k₂) of RAA is about 4306 L/mol s at 298 K, which is slightly lower than that of AA (4772 L/mol s) due to the negative effect of VFAs. The average overall mass transfer coefficients of RAA are lower by 4.67%–17.31% than that of unloaded aqueous ammonia. Furthermore, increasing the concentration of VFAs and CO₂ loading in RAA leads to a rapid decline of the overall mass transfer coefficient of CO₂. Butyric acid has the greatest negative impact on CO₂ absorption, followed by acetic acid, ethanol and then CO₂. However, the increase of TAN concentration in RAA can minimize the negative influence of impurities on CO₂ absorption performance.

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.