Once-through CO2 absorption for simultaneous biogas upgrading and fertilizer production

Fuel Processing Technology 166 (2017) 50–58

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Research article

Once-through CO2 absorption for simultaneous biogas upgrading and fertilizer production Qingyao He a,b,c, Ge Yu a,b, Wenchao Wang a,b, Shuiping Yan a,b,⁎, Yanlin Zhang a,b, Shuaifei Zhao c,⁎⁎ a b c

College of Engineering, Huazhong Agricultural University, No.1, Shizishan Street, Hongshan District, Wuhan 430070, PR China The Cooperative Innovation Center for Sustainable Pig Production, Wuhan 430070, PR China Department of Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia

a r t i c l e

i n f o

Article history: Received 28 March 2017 Received in revised form 23 May 2017 Accepted 24 May 2017 Available online xxxx Keywords: Carbon capture Biogas upgrading Renewable absorbent Vacuum membrane distillation Biogas slurry CO2 absorption

a b s t r a c t A new process is developed for biogas upgrading using the total ammonia nitrogen (TAN) in biogas slurry as a renewable absorbent. TAN in biogas slurry can be transferred into free ammonia by adding NaOH to increase the solution pH. Increasing the pH of biogas slurry to 10 causes that N90% TAN transfers into free ammonia, leading to high TAN removal ratios. However, further increasing the pH of biogas slurry has limited effects. Vacuum membrane distillation (VMD) has higher kinetics constants and thus is a more effective way to recover and enrich ammonia from biogas slurry compared with thermal or air stripping. After VMD, the recovered aqueous ammonia solution with high TAN concentrations and the enhanced biogas slurry can be used as “once-through” CO2 absorbents. With alkaline addition, VMD does not increase the CO2 absorption capacity, but significantly minimizes the phytotoxicity of biogas slurry. When NaOH dosage is below 0.25 M, superior ammonia separation performance with high kinetics constants and low phytotoxicity can be achieved. The recovered aqueous ammonia solution also has excellent CO2 absorption performance for biogas upgrading and can help obtain high content of methane. This study provides an effective process for biogas upgrading with low costs and generation of valuable products, including high purity bio-methane, low phytotoxicity biogas slurry for agricultural application and high concentration NH4HCO3 as a fertilizer. © 2017 Elsevier B.V. All rights reserved.

1. 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 [1–4]. Combining bioenergy production with carbon capture and storage (Bio-CCS) is a promising way to mitigate climate change and generate renewable energy [5–7]. 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%)

⁎ Correspondence to: S. Yan, College of Engineering, Huazhong Agricultural University, No.1, Shizishan Street, Hongshan District, Wuhan 430070, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (S. Yan), [email protected] (S. Zhao).

http://dx.doi.org/10.1016/j.fuproc.2017.05.027 0378-3820/© 2017 Elsevier B.V. All rights reserved.

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 [9,11–15]. 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 (b 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 “oncethrough” 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,

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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: NH+ 4 → NH3 → NH4HCO3CO3 [1,22–24]. 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 [23,27–30]. 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.

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 shown in Table 1. The supernatant liquid (i.e. BS) was used for further measurement and vacuum membrane distillation (VMD). Chemical oxygen demand (COD) and pH value of the BS were measured with a CM-03 COD meter (Beijing Shuanghui Jingcheng Electronics Co., Ltd.) and a pH meter (Metler Toledo, FE20K), respectively. Total ammonia nitrogen (TAN), was determined in a Smartchem 200 Discrete Auto Analyzer (Italy AMS-Westco) [33]. Total solid (TS) concentration was measured by the standard methods [34]. Volatile fatty acid (VFA) concentration was determined using GC-FID (SP-2100A) [22]. The turbidity was determined using a photoelectric turbidity meter (WZT-1, Shanghai Jingjia Scientific Instrument Co., Ltd.). Electric conductivity (EC) of the BS was determined with a conductivity meter (DDS307A, Shanghai INESA Scientific Instrument Co., Ltd.). The total carbon (TC) and total inorganic carbon (TIC) concentration were determined with a TC/TN Analyzer (multi N/C 2100, Analytik Jena AG, German). Each liquid sample was measured at least three times to determine the average values and standard deviations. The effects of uncertainties from the readings and device accuracies were also considered. The proportion of free ammonia in TAN as a function of pH and temperature can be calculated by [27]:

2. Materials and methods

A microporous hydrophobic polypropylene membrane module from Hangzhou Jiefu Membrane Technology Co., Ltd. was used for VMD. Specifications of the hollow fiber membrane module are listed in Table 2. The VMD experimental setup for ammonia recovery is schematically shown in Fig. 2a. Sodium hydroxide (NaOH) was added into BS to adjust its pH to different levels: 7.93 (0 M), 9.68 (0.1 M), 10.33 (0.25 M), 11.53 (0.33 M), 12.80 (0.50 M). 600 mL BS was circulated on the lumen side of the hollow fiber membrane by a peristaltic pump (Leifu YZ25, Baoding

2.1. 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

½NH3  ¼

½TAN −8

1 þ 10410

T3 þ910−5 T2 −0:0356Tþ10:072−pH

ð1Þ

where [TAN] and [NH3] are the concentrations of TAN and free ammonia (M), respectively. T is the temperature of the solution (°C). 2.2. Membrane module and vacuum membrane distillation

Fig. 1. Biogas upgrading by treated biogas slurry and recovered aqueous ammonia.

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Equilibrium partial pressure data of NH3 above pure aqueous solutions over a range of temperatures and concentrations can be calculated by the Henry's law [36]:

Table 1 Properties of the biogas slurry. Parameters

Values

Units

pH Electric conductivity (EC) Turbidity Chemical oxygen demand (COD) Total ammonia nitrogen (TAN) Total solid (TS) Volatile fatty acid (VFA)

7.93 ± 0.21 25.51 ± 0.32 347.17 ± 0.58 2725.47 ± 11.29 0.14 ± 0.07 5589 ± 56.98 0.011 ± 0.001

– mS/cm NTU mg/L M (mol/L) mg/L mg/L

Leifu Fluid Science and Technology Co., Ltd., China) under stirring and heating. The liquid flow rate was maintained at 60 mL/min by adjusting the rotation speed of the pump. The inlet temperature of the liquid was 55 °C and monitored by k-type thermocouples. A vacuum pump (Yvhua Instrument Co., Ltd., Gongyi, China) was used to generate vacuum on the shell side of the membrane (3 kPa). The recovered ammonia and water vapor were condensed and collected under 0 °C. Each experimental run lasted about 180 min until the volume of feed solution was reduced by half (i.e. concentration factor = 2). In each experiment, 5 mL feed solution and all the condensate were sampled every 10 min, and the same volume of pH adjusted raw BS was added into the feed tank to ensure a relatively stable experimental system. After each experiment, the weights of the feed solution and the condensate were also measured. The ammonia removal ratio Rmov and recovery ratio Rcov can be calculated by: Rmov ¼

C0 V0 −Ct Vt  100% C0 V0 0

Rcov ¼

ð2Þ

0

Ct Vt  100% C0 V0

ð3Þ

where V0 and C0 are the initial volume (L) and TAN concentration (M) of the BS, respectively; Vt and Ct are the volume and TAN concentration of the BS at time t (min); V't and C't are the volume and TAN concentration of the condensate at time t (min). To evaluate the stripping performance and compare with other results, the concept of time constant was used [35]. The experimental TAN concentration profile at each stripping condition was fitted to an exponential decay curve (1st order kinetic) as shown following: −k

Ct ¼ C0 e

ð4Þ

1 τ

ð5Þ

k¼

where k is the pseudo-first-order kinetics constant (min−1) of TAN removal. The time constant τ (min) represents the time required to reduce the TAN concentration by (1− 1e ) (~ 63% of the initial concentration), allowing quantitative comparison between experimental results at different initial and final conditions.

Table 2 Specifications of the hollow fiber membrane contactor. Parameters

Values

Units

Fiber inner diameter Fiber outer diameter Membrane pore size Membrane porosity Module inner diameter Module outer diameter Number of fibers Total module length Effective hollow fiber length Effective contact area

350–370 450 100–200 40–50 18 22 500 400 320 0.176

μm μm nm % mm mm – mm mm m2

pNH3 ¼

100γNH3 mNH3 KH

ð6Þ

where pNH3 (kPa) is the partial pressure of NH3, mNH3 is the free ammonia molality (mol/kg), γ denotes the activity coefficient (γ = 1 when the solution is diluted enough). The Henry's law constant (KH) can be calculated by: ln K H ¼ −8:09694 þ

3917:507 −0:00314ð273:15 þ TÞ 273:15 þ T

ð7Þ

2.3. Packed column absorber and biogas upgrading A stainless steel randomly packed column was used to absorb CO2 from simulated biogas (40 vol% CO2 and 60 vol% CH4). Characteristics of the absorber are listed in Table 3. Sodium hydroxide (NaOH), aqueous ammonia and monoethanolamine (MEA) solutions were used as absorbents in the packed column (Fig. 2b). The concentrations of the absorbents were based on the VMD results. The simulated biogas was fed into the bottom of the packed column with a constant gas flow rate of 750 L/h under standard conditions. The chemical absorbents countercurrently flowed from the top of the column at different flow rates (10– 60 L/h). In the column, the gas and liquid phases contacted, and CO2 was absorbed by the chemical absorbents. The treated gas left from the top of column, and CO2-rich solution flowed to the rich solution tank. Before each experimental run, the system was cleaned for at least 5 min by deionized water to eliminate the influence of the former run [37]. All data were obtained at a steady state when the CO2 concentration in the outlet gas stream was constant, determined by an infrared biogas analyzer (Gas-board 3200 L, Wuhan Cubic Optoelectronics Co., Ltd.) after drying. To minimize ammonia loss, biogas upgrading experiments were carried out at 8 ± 2 °C [38]. Selection of the operational parameters was based on previous relevant studies [37,39]. 2.4. Phytotoxicity test Phytotoxicity of the CO2-rich BS was tested through the germination of Chinese cabbage seeds [17]. The CO2-rich BS was diluted 10 times before the seed germination experiment. For raw BS, 0.5 mL CO2-rich BS was mixed with 4.5 mL distilled water; for VMD treated BS, 0.25 mL CO2-rich BS was mixed with 4.75 mL distilled water. Then, 5 mL diluted solution was put in a 9-cm petri dish where 20 Chinese cabbage seeds were placed on a piece of filter paper. Each treatment was replicated three times. These petri dishes covered with lids were placed into a lightless incubator (MLR-350, Versatile Environmental Test Chamber) for seed germination at 25 ± 0.5 °C and a relative humidity of 80%. Water loss in each dish was monitored everyday by weighing and distilled water was added if necessary. Adsorption and degradation of BS were negligible in the test. Seed germinated when the radicle was over 2 mm in length. After the radicle length of the control seed in distilled water was N20 mm, germination experiments could be terminated [40]. In this study, the average germination time was about 48 h. Finally, the percentage of seed germination and the length of roots were measured. The application phytotoxicity of the CO2-rich BS was evaluated by germination index (GI) [17]: GI ¼

MRB  MRLB MRC  MRLC

ð8Þ

where MRB and MRC are the mean seed germination rates in the CO2-rich BS and the control, respectively; MRLB and MRLC represent the mean root lengths in the CO2-rich BS and the control, respectively.

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Fig. 2. Experimental setup for (a) ammonia recovery from biogas slurry by vacuum membrane distillation, and (b) CO2 absorption in biogas upgrading.

3. Results and discussion Table 3 Characteristics of the packed column absorber.

3.1. Effect of pH value on ammonia recovery

Parameters

Values

Units

Column inner diameter Total height Packing material Packing size Packing height Specific area Void fraction

0.04 2 Stainless steel Pall rings 6 × 6 × 0.3 1.5 273 0.914

m m – mm m m2/m3 –

3.1.1. 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,

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higher pH values, there are higher free ammonia concentrations in the feed, which can be removed from BS by VMD [39,41,42]. The free ammonia content is 20.67% at a pH value of 7.9, while it increases to higher than 98% at a pH value of 10.3 [36]. Therefore, the pH value of the feed BS should be adjusted to be higher than 10.0 when the free ammonia content is higher than 95%, so that high TAN removal efficiency can be achieved. However, further increasing the pH value of the feed solution does not change much in terms of TAN removal efficiency (Fig. 3). VMD is an effective way to minimize the TAN concentration in BS. Compared with the vacuum thermal, air or biogas stripping for ammonia removal, VMD has much larger kinetics constants and smaller time constants (Table 4). This is mainly caused by the lower ammonia partial vapor pressure due to the vacuum, leading to a higher driving force for ammonia transfer [43,44]. Additionally, the larger kinetics constants in VMD result from the higher interfacial areas of the hollow fiber membrane [39]. Thus, VMD is preferable for ammonia removal from BS due to its high efficiency (large kinetics constant and small time constant).

Fig. 3. Effect of pH on total ammonium nitrogen (TAN) removal performance from biogas slurry: (a) TAN concentration profiles in treated biogas slurry (TBS), and (b) TAN removal ratio.

the TAN concentration in the feed reduces dramatically with time, particularly in the first 60 min. However, the TAN concentration in the feed and the TAN removal ratio change little with the pH variation from 10.3 to 12.8. This tendency is in agreement with the pseudo-first-order kinetics constant variation (Table 4), which slightly increases from 0.053 to 0.063 when the pH value rises from 10.3 to 12.8. The variation of TAN removal from BS is determined by free ammonia concentration that is a function of the pH value as shown in Eq. (1). At

3.1.2. Ammonia collection performance The removed ammonia and water vapor were condensed and collected at 0 °C and 3 kPa and the collection performance is shown in Fig. 4. It is expected that the ammonia concentration in the condensate declines with operating time, since the TAN concentration in the feed solution decreases with time. However, the TAN concentration in the condensate increases in the first 30 min, maintains stable for about 20 min, and then slightly declines when the pH of the BS is 7.9. This is because that at lower pH value (7.9) free ammonia content in the feed is relatively low (20.67%) and the main chemical balance is controlled by CO2 and NH3. In the first 30 min, CO2 comes out of the BS and the pH value increases to 9.06 that results in the free ammonia content increased to 77%. As a result, the TAN concentration in the condensate increases at the beginning. This phenomenon was also observed in air stripping of biogas slurry [47]. The highest TAN concentration collected in the condensate is 0.9 M when the pH values of the treated BS are 11.53 and 12.80 (Fig. 4a). All TAN transfers into free ammonia when the pH values of the treated BS are higher than 10.33 based on Eq. (1). Thus, the solid line and the dash line are very close (Fig. 4a). Obviously, a pH value higher than 10.33 is helpful to increase the free ammonia concentration in the condensate. After VMD, TAN and free ammonia concentration in the condensate is much higher than those of the initial concentrations (~0.03 M) in the biogas slurry, suggesting that the recovered ammonia has higher potential for CO2 absorption [48]. Table 5 shows the properties of the recovered aqueous ammonia solution. The main components that transfer into the condensate are CO2 and volatile organic acids (VFAs), such as ethanol and acetic acid. TC content indicates the quantity of total impurities. TC concentration in recovery aqueous ammonia solution is about 0.06 mol/L (703.5 mg/L), b 10% of TAN, suggesting a low concentration of impurities. TIC concentration was

Table 4 Effect of separation conditions on values of pseudo-first-order kinetics constant (k) and time constant (τ). Separation methods and conditions Vacuum membrane distillation (55 °C)

Vacuum thermal stripping

Air stripping (37 °C) Air stripping (50 °C) Biogas striping (70 °C) G/L means gas/liquid flow rate ratio.

pH = 7.93 pH = 9.68 pH = 10.33 pH = 11.53 pH = 12.80 50 °C, 16.6 kPa 70 °C, 33.6 kPa 100 °C, 101.3 kPa G/L = 1.0, pH = 11.0 G/L = 10, pH = 9.0 G/L = 2.0, pH = 11.0 G/L = 14.0, pH = 10.0 G/L = 0.25, pH = 11.43 G/L = 0.375, pH = 13.86

Kinetics constant k (min−1)

Time constant τ (min)

R2

Refs.

0.0006 0.019 0.053 0.060 0.063 0.017 0.025 0.026 0.00075 0.0027 0.0027 0.0144 0.0006 0.0019

1666.67 52.63 18.87 16.67 15.87 58.82 40 38.46 1333.33 370.37 370.37 69.44 1666.67 526.32

0.99 0.99 0.94 0.99 0.98 – – – 0.97 0.97 0.98 0.99 0.73 0.95

This study

[43]

[45] [46] [35]

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Ammonia recovery in the condensate is a typical gas-liquid equilibrium process that can be explained by the Henry's law [36]. The ammonia partial pressures at different free ammonia concentrations and temperatures can be calculated by Eq. (6) (Fig. 5). Ammonia will come out of the collecting system if the total pressure applied on the gas side is lower than its partial pressure. Thus, higher pressure, lower temperature and lower ammonia concentration are beneficial for minimizing ammonia loss. In VMD, the permeate can be condensed and collected before or after the vacuum pump [42]. Collecting condensate at ambient or higher pressure after the vacuum pump could be better at industrial scales. However, collecting condensate before the vacuum pump is more suitable for bench-scale test, because the vapor is likely to condense in the vacuum pump if collection is taken after the pump. In this study, ammonia was collected under vacuum condition (3 kPa) before the vacuum pump, and the ammonia recovery ratios are b 70% (Fig. 4b). The superior ammonia recovery ratio is obtained at a pH of 9.68 due to the lower ammonia partial pressure based on the Henry's law [36]. Overall, high aqueous ammonia with high concentrations and recovery ratios could be achieved when the operating conditions are optimized [39,42,49]. 3.2. CO2 absorption capacity and phytotoxicity CO2 absorption capacities of different types of BS are compared with the equilibrium CO2 loadings before and after CO2 saturation [17]. The CO2 absorption capacity of condensate with VMD is based on the free ammonia content with an assumption of 100% ammonia recovery [21]. Additives can be put into BS to improve the CO2 absorption capacity, but the phytotoxicity should be restricted to ensure the applicability [17].

Fig. 4. Effect of pH on ammonium recovery performance from biogas slurry: (a) variations of total ammonium nitrogen (TAN) concentration in the condensate, and (b) ammonia recovery ratio.

about 0.05 mol/L (616 mg/L), higher than 80% of the TC, which is caused by CO2 dissolution in the recovered aqueous ammonia solution. Ethanol is the main component in VFAs (Table 5) because ethanol is independent on pH, while other VFAs (e.g. acetic acid and propionic acid) greatly depends on pH. Therefore, VFAs is anticipated to have little effect on CO2 absorption performance due to its low concentration in the aqueous ammonia. CO2 dissolved in the aqueous ammonia solution is the main factor that needs to be restricted. Increasing pH of BS might be a good way to reduce the TIC concentration in the recovered aqueous ammonia solution.

3.2.1. VMD only Without VMD the CO2 absorption capacity of the BS is only 0.03 M, and it reaches 0.12 M with VMD (Fig. 6). The improvement of the absorption capacity is mainly caused by the higher total free ammonia content in both BS and condensate (Fig. 7) [21]. Free ammonia content in TBS first increases from 0.03 to 0.09 M in the first 10 min as VMD regenerates partial CO2 dissolved in BS [21], and then decreases with the TAN concentration in TBS. At higher pH values of the condensate (~ 9.7), ammonia transfers from the feed to the condensate mostly in the form of free ammonia. The total free ammonia content maintains steady at 0.12 mol after 30 min (Fig. 7), suggesting that VMD is helpful to improve the CO2 absorption capacity. However, because of the relatively low free ammonia concentrations in TBS (0.01 M) and condensate

Table 5 Properties of recovered aqueous ammonia solution in the first 10 min (pH of treated biogas slurry is 10.33). Property parameters

Values

Units

pH Total ammonia nitrogen (TAN) Total carbon (TC) Total inorganic carbon (TIC) Ethanol (CH3CH3OH) Acetic acid (CH3COOH) Propionic acid (CH3CH3COOH) Butyric acid (CH3CH3CH3COOH)

10.95 ± 0.20 0.69 ± 0.03 703.5 ± 22.24 616.0 ± 57.13 34.21 ± 29.16 1.46 ± 1.07 0.16 ± 0.06 2.75 ± 1.11

– M (mol/L) mg/L mg/L mg/L mg/L mg/L mg/L

Fig. 5. Ammonia partial pressures at various free ammonia concentrations and temperatures based on Eq. (6).

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Fig. 6. CO2 absorption capacity change with vacuum membrane (VMD) and NaOH addition.

Fig. 8. Phytotoxicity of biogas slurry with and without vacuum membrane distillation (VMD) at different NaOH dosages.

(0.2 M), their CO2 absorption performance may not be good enough for biogas upgrading [48]. Although ammonia is an important nutrient for plant growth, high concentration of ammonia is harmful for plants [50] and causes environmental risks [25,27]. VMD is an effective way to reduce the TAN concentration in BS and thus the phytotoxicity (Fig. 8). Without NaOH in BS, the difference in germination index between the VMD treated BS and untreated BS is not much, and the phytotoxicity is not severe. However, the phytotoxicity of biogas slurry becomes more and more serious with the rise in NaOH concentration. Particularly, the untreated BS seriously inhibits germination even at a low NaOH concentration. For instance, the germination index is 0.5 for untreated BS, while it is near 0.9 for VMD treated BS when the NaOH concentration is 0.1 M. Generally, when the germination index value is N 0.8, the phytotoxicity can be neglected; when the germination index value is below 0.6, the phytotoxicity is serious and seed breeding is inhibited severely [17].

economically available [19,51]. To minimize the phytotoxicity, a large quantity of water is required to dilute the CO2-rich biogas slurry, which may not be technically or economically feasible in practical operations.

3.2.2. NaOH addition only As expected, CO2 capacity of BS increases linearly with the dosage of NaOH (Fig. 6). However, the germination index value of the CO2-rich biogas slurry drops sharply to below 0.5 with NaOH addition, suggesting NaOH addition increases the phytotoxicity. Therefore, alkali, such as NaOH should be added carefully, although alkali wastes may be

3.2.3. VMD combining NaOH addition The total CO2 absorption capacity of BS (i.e. the absorption capacity from both recovered ammonia and treated biogas slurry) increases dramatically with NaOH addition and VMD (Fig. 6). The difference in total CO2 absorption capacity with and without VMD is insignificant (P b 0.01). CO2 absorption capacity with VMD mainly comes two parts: treated biogas slurry and recovered aqueous ammonia. CO2 absorption capacity of the treated biogas slurry is mainly caused by the added NaOH. In VMD, apart from ammonia, CO2 can also transfer from the feed solution into the condensate, leading to a rise in pH of the feed. However, adding a small amount of NaOH into the feed solution will inhibit the regeneration of CO2 from biogas slurry, which can lower the pH rise of the feed, leading to low free ammonia content. As a result, the CO2 absorption capacity of the recovered ammonia with NaOH dosage of 0.1 M is slightly lower than that without NaOH addition (Fig. 6). With the increase of NaOH dosage (N 0.25 M), CO2 absorption capacity of the recovered ammonia becomes stable because CO2 regeneration is inhibited and all ammonia transfers to the condensate. Fig. 8 shows that when the NaOH dosage is lower than 0.25 M, the germination index of the CO2-rich BS treated by VMD is higher than 0.8, suggesting VMD is effective to promote seed breeding. However, the untreated BS has low germination indexes (b 0.5) even with a small NaOH dosage. Considering the results in Figs. 6 and 8, we can conclude that adding NaOH into BS, the CO2 absorption capacity of BS does not improve by VMD, but VMD significantly minimizes the phytotoxicity and thus improves the applicability of the CO2-rich BS for agricultural application. In addition, VMD enriches the ammonia content in the condensate (i.e. high TAN concentration), which could improve biogas upgrading performance. 3.3. Biogas upgrading performance

Fig. 7. Variations of total ammonium nitrogen (TAN) and free ammonia in treated biogas slurry (TBS) and condensate with vacuum membrane distillation operating time.

Concentrations of aqueous ammonia (0.5–2.0 M) applied in this work are based on the ammonia recovery performance and previous studies [39,41,42], and they could be higher if the operating parameters and collection performance are optimized. Since the treated BS contains various concentrations of NaOH with good CO2 absorption capacities after VMD, NaOH solutions were selected for comparison in biogas upgrading (Fig. 9). As expected, biogas upgrading performance increases with the increase of the liquid flow rate. This is primarily caused by the improved

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using MEA solution. Thus, the CO2 absorption performance of NaOH or ammonia solutions is slightly lower than that of MEA solution under the same conditions [59]. The reaction of CO2 with typical absorbents, such as primary and secondary amines (RNH2) and ammonia, can be described by the Zwitterion mechanism [59,60]. When deprotonation is almost instantaneous, the Zwitterion formation is rate-determining compared with the reverse reaction. Therefore, the rate of CO2 absorption can be expressed as r CO2 ¼ k2 C CO2 C RNH2

ð9Þ

where rCO2 represents the CO2 absorption rate, k2 is the reaction kinetics constant, CCO2 and CRNH2 are the concentration of CO2 and the absorbent, respectively. 3.4. Advantages of this work

Fig. 9. Comparison of biogas upgrading performance with different solvents in a packed absorber (gas flow rate: 750 L/h, pressure: 110 kPa, temperature: 8 °C).

mass transfer coefficients due to the reduced boundary layer effect at high liquid flow rates [37,52]. Hence, high liquid flow rates lead to a decrease in the resistance of the liquid phase, and thus increase the mass transfer rates [37,48,53–55]. Such performance also agrees well with the modelling for biogas upgrading [56]. Water shows the lowest performance in biogas upgrading due to its low physical absorption rate and capacity [53]. At lower concentrations (b1 M), aqueous ammonia has better upgrading performance than NaOH solutions, particularly when the liquid flow rates are relatively high. MEA has the best biogas upgrading performance due to its high reaction rate and mass transfer coefficient as expected [10,57]. The methane contents after upgrading become similar for NaOH and ammonia when their concentrations increase to 2 M, and the upgraded methane content can be up to 97%. These results demonstrate that recovered ammonia by VMD can achieve a high efficiency for biogas upgrading. CO2 loading of aqueous ammonia is also compared with NaOH and MEA solutions (Fig. 10). CO2 loadings of the absorbents decrease with the rise in the liquid flow rate. Although the CO2 absorption capacities of NaOH and NH3 solutions could be up to 1.0 mol/mol, their CO2 absorption rates would decrease with the increase of CO2 loadings [58]. When CO2 is absorbed into NaOH and ammonia solutions the main product is − CO2– 3 or HCO3 , which results in a higher CO2 partial pressure than that

In this study, simultaneous biogas upgrading, ammonia recovery and fertilizer production can be achieved. Compared with typical amine absorption for CO2 capture, less energy is required since absorbent regeneration is not needed, and the absorbent comes from biogas slurry, which is renewable. Ammonia can be recovered from biogas slurry to reduce the phytotoxicity risk when biogas slurry is used as liquid fertilizer [9]. At the same time, two main products: aqueous ammonium bicarbonate (a typical fertilizer) and bio-methane (an energy resource) can be produced after CO2 absorption. The by-product, i.e. treated biogas slurry, can be directly employed for agricultural applications, minimizing the risk of eutrophication due to the reduced TAN concentration. 4. 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 N90% 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 concentrations and the enhanced BS can be used as “oncethrough” absorbents. Without alkaline addition, VMD can enhance the CO2 absorption capacity of BS by increasing the content of free ammonia. With alkaline addition, VMD does not increase the CO2 absorption capacity, but significantly minimizes the phytotoxicity of the BS. When NaOH dosage is below 0.25 M, superior ammonia separation performance with high kinetics constants and low phytotoxicity can be achieved. The recovered aqueous ammonia also has excellent CO2 performance for biogas upgrading and can help obtain high content of methane. The demonstrated process could be more promising when abundant cheap alkaline wastes are available in practical applications. 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). References

Fig. 10. Effect of liquid flow rate on CO2 loading of the chemical absorbent (measured at the bottom of absorber).

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