Research articleOnce-through CO2 absorption for simultaneous biogas upgrading and fertilizer production
Graphical abstract
Introduction
Bioenergy as a renewable energy source has attracted growing interest recently, due to its significant roles in improving energy security, reusing wastes and reducing greenhouse gas emissions [1], [2], [3], [4]. Combining bioenergy production with carbon capture and storage (Bio-CCS) is a promising way to mitigate climate change and generate renewable energy [5], [6], [7]. Globally, Bio-CCS could remove approximate 10 billion tons of CO2 from the atmosphere every year, which is equal to 1/3 of the current global energy-related CO2 emissions [8].
Among biofuels, biogas production through anaerobic digestion of agricultural residues and crops has several advantages compared with other biological processes, such as its simplicity and capacity to process a wide range of substrates [1]. In anaerobic digestion, anaerobic microorganisms convert waste organic matters into two main products: biogas and nutrient-rich digestate. Biogas (mainly CH4: ~ 60% and CO2: ~ 40%) can be used to produce heat, electricity, or compressed natural gas and liquefied natural gas after upgrading [9], [10].
Various technologies have used for biogas upgrading, such as water scrubbing, pressure swing absorption, chemical absorption and membrane separation [9], [11], [12], [13], [14], [15]. The main drawback of these commonly used methods (e.g. water scrubbing and pressure swing absorption) is the high CH4 loss (which may range from 2% to 20%). It is important to minimize the CH4 loss in biogas upgrading, as the greenhouse effect of CH4 is much (~ 23-fold) higher than that of CO2 [14]. Chemical absorption has negligible CH4 loss (< 0.1%) and high CH4 purity in biogas upgrading because of the significant solubility difference between CO2 and CH4 [16]. However, chemical absorption consumes huge energy mainly due to solvent regeneration at high temperature [10]. In addition, captured CO2 is unlikely to be completely and safely stored [17]. Thus, the “once-through” process without chemical solvent regeneration, combining CO2 capture and storage with production of commodity chemicals, is a sustainable carbon reduction process because it can obtain good repayment to compensate the cost of CO2 capture and maximize the net balance between captured CO2 and emitted CO2 [17], [18]. Using ammonia to capture CO2 and simultaneously produce the NH4HCO3 fertilizer is a typical once-through CO2 capture method [17].
Generally, alkaline wastes are excellent absorbents for once-through carbon capture, which does not exhaust the global supply of chemicals, making a meaningful reduction in CO2 emissions [19], [20]. As a byproduct of anaerobic digestion, the renewable biogas slurry (BS, the liquid phase nutrient-rich digestate) can be used as a once-through CO2 absorbent due to its weak alkalinity [14], [21]. BS contains relatively high concentration of nutrients and organic carbon that can be beneficially used as a liquid fertilizer and soil amendment [22]. The huge volume of BS in an anaerobic digestion plant suggests a large CO2 absorption capacity, and main reactions include: NH4+ → NH3 → NH4HCO3CO3 [1], [22], [23], [24].
However, direct application of BS to soil may cause severe environmental risks [23], [25] due to the high concentration of total ammonium nitrogen (TAN) in BS (0.5–5.0 g N/L) [22], [25], [26]. Various methods have been employed to remove or recover TAN from BS to minimize the risks and produce valuable products [23], [27], [28], [29], [30]. These technologies include reverse osmosis [29], air-stripping by stripping towers and acid absorption [27], zeolite adsorption by ion exchange [31], co-precipitation with phosphate and magnesium to form struvite [28], and low pressure processes with gas permeable membranes [32]. However, few studies on the application of TAN to capture CO2 to realize production of fertilizers (e.g. NH4HCO3) and reduction in CO2 emissions. Previously, we studied the CO2 absorption capability of BS by adding chemical absorbents and vacuum regeneration [17], [21].
The objective of this study is to develop a simple and efficient process to achieve high CO2 absorption performance for biogas upgrading with BS, minimized BS phytotoxicity and valuable products. The process contains a renewable ammonia recovery step by vacuum membrane distillation and a two-stage CO2 absorption using treated BS and recovered aqueous ammonia (Fig. 1). The products of the process are bio-methane with high CH4 content, treated BS with low phytotoxicity and high concentration NH4HCO3 as fertilizers. CO2 absorption performance of the treated BS and recovered aqueous ammonia, and phytotoxicities of the treated BS and untreated BS are evaluated.
Section snippets
Biogas slurry and its properties
Raw BS was collected from a large-scale mesophilic anaerobic biogas digestion plant (digestion substrate: pig manure; digestion temperature: ~ 35 °C), located at Caoda Village of Yingcheng City, Hubei province, China. The collected raw BS was stored anaerobically at ambient temperature prior to experiments until no biogas was produced. Undissolved solids and partial suspended solids were separated by centrifuging (4000 rpm) for 20 min. Characteristics of the centrifuged BS measured at 15 ± 2 °C are
Ammonia separation by VMD
When BS solutions at different pH values are used in VMD, ammonia removal performance is shown in Fig. 3. As anticipated, the TAN concentration in the feed solution decreases and the TAN removal ratio increases with operating time. The measured TAN concentrations in the feed (treated biogas slurry) agree well with an exponential decay curve (i.e. the 1st order kinetics) as shown in Fig. 3(a). At higher pH values, the TAN concentration in the feed reduces dramatically with time, particularly in
Conclusion
A simple and efficient process is developed for biogas upgrading using the TAN in biogas slurry. Increasing the pH of BS to 10 can ensure that > 90% TAN transfers into free ammonia, leading to high TAN removal ratios. However, further increasing the pH of BS beyond 10 has limited effects. VMD has higher kinetics constants and thus is a more effective way to recover and enrich ammonia from BS compared with thermal or air stripping. After VMD treatment, the recovered aqueous ammonia with high
Acknowledgment
We thank the financial support from the National Natural Science Foundation of China (No. 51376078) and Open Research Fund Program of Collaborative Innovation Center of Membrane Separation and Water Treatment (2016YB01). Qingyao He acknowledges the financial support from China Scholarship Council (CSC) for studying at Macquarie University in Australia (201606760032).
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