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Exergy analyses of biogas production from microalgae biomass via anaerobic digestion
Bioresource Technology 289 (2019) 121709
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Exergy analyses of biogas production from microalgae biomass via anaerobic digestion
T
⁎
Chao Xiaoa,b, Qiang Liaoa,b, Qian Fua,b, , Yun Huanga,b, Ao Xiaa,b, Weifeng Shenc, Hao Chena,b, Xun Zhua,b a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, China Institute of Engineering Thermophysics, School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China c School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Exergy analyses Biogas production Microalgae biomass Anaerobic digestion Hydrothermal pretreatment
Biogas production from microalgae biomass without pretreatment and with hydrothermal pretreatment involve the energy with different quality and quantity, which makes it complex to evaluate thermodynamic performance. In this paper, exergy analyses were conducted in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment. The results showed that the materials and energy flow affected exergy efficiency in biogas production from microalgae biomass. The biogas production from microalgae biomass with solar-driven hydrothermal pretreatment achieved the highest exergy efficiency (40.85%), compared with that without pretreatment (26.2%) and with hydrothermal pretreatment (35.98%). In addition, the maximum exergy loss was caused by biogas residue, which accounted for 60.58%, 38.54%, and 35.13% of overall exergy input in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. Exergy analyses provide important theoretical guidance to improve the performance of biogas production from microalgae biomass.
1. Introduction The survival and development of human beings are threatened by environmental pollution and resources exhaustion. Microalgae technology is a promising technology for alleviating stress from environmental pollution and energy exhaustion (Sun et al., 2016). Specifically, microalgae cell can capture CO2 and purify sewage while producing biomass for biofuels production (Chang et al., 2018). In addition, microalgae have several advantages over other energy crops, such as a high photosynthetic efficiency, no competition with food crops in lands, and well tolerance of high CO2 concentration (Li et al., 2008). Microalgae biomass has been used as a feedstock for biofuels production, for example, biodiesel production by hydrothermal liquefaction (Li et al., 2014) and pyrolysis (Miao et al., 2004), and biogas production by anaerobic digestion (Bennion et al., 2015; Collet et al., 2011). Generally, microalgae biomass still has a low concentration in microalgae slurry after harvesting, and dewatering of microalgae slurry is required for biodiesel production through hydrothermal liquefaction
and pyrolysis. However, dewatering is a process of energy intensity, which is the bottleneck of biodiesel production from microalgae biomass (Uduman et al., 2010). Notably, although hydrothermal liquefaction and pyrolysis can converted microalgae biomass into biodiesel at high pressure and temperature, both of them achieve a negative energy gain (Bennion et al., 2015). In comparison, anaerobic digestion can convert microalgae biomass with high moisture content into biogas without dewatering, which remarkably achieves a positive energy gain (Sun et al., 2019). Therefore, anaerobic digestion is considered as a promising approach for biofuel production from microalgae biomass. However, biogas production from microalgae biomass via anaerobic digestion achieves a poor biogas yield, due to the compact cell wall of microalgae biomass. For instance, Sialve et al. (2009) showed that all of organic matters in microalgae biomass can be converted into biogas, and methane yields from proteins, lipids and carbohydrates were theoretically 0.851, 1.014 and 0.415 L/g volatile solid (VS), respectively. However, the methane yield from microalgae biomass was merely 0.28 L/g VS in practical (Murphy et al., 2015). For this issues, hydrothermal
Abbreviation:HTP, hydrothermal pretreatment ⁎ Corresponding author at: Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, China; Institute of Engineering Thermophysics, School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (Q. Fu). https://doi.org/10.1016/j.biortech.2019.121709 Received 29 May 2019; Received in revised form 19 June 2019; Accepted 26 June 2019 Available online 27 June 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Composition of microalgae biomass (A), and system boundary for biogas production from microalgae biomass via anaerobic digestion (B).
performance is required in biogas production from microalgae biomass in industry. The improvement of thermodynamic efficiency relies on the breakthrough of bottleneck in biogas production from microalgae biomass. The thermodynamic analysis can be utilized to find out the bottleneck for the improvement of thermodynamic performance of biofuel production from microalgae biomass. Thus, it is necessary to analyze the thermodynamic performance of biogas production from microalgae biomass. On the other hand, biogas productions from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment involve different kinds of energy, e.g., electric energy, biomass energy, chemical energy, thermal energy, and solar energy. Notably, all of the energy have different both quantity and quality, which belong to different categories and should be taken into consideration in the assessment of thermodynamic performance in biogas production from microalgae biomass. Thus, it is imperative to analyze the thermodynamic performance of biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment in a unified standard. Energy analysis, which based on the first law of thermodynamics, merely takes the quantity of energy into consideration. In contrast, the exergy covers both the quantity and quality of energy, and exergy analysis is usually utilized to evaluate the thermodynamic performance of processes (Sorguven and Ozilgen, 2010), where different kinds of energy are involved. Li et al. (2019) conducted an exergy analysis of biomass staged-gasification for hydrogen-rich syngas, and the main exergy losses lay in the internal exergy loss. Kumar et al. (2018) conducted an exergy analysis of hydrogen production from acetone-butanol-ethanol-water mixture via steam reforming. Cohce et al. (2011) conducted an exergy analysis of hydrogen production from palm oil waste by thermochemical biomass gasification. However, the exergy analysis of biogas production from microalgae biomass via anaerobic digestion has never been reported.
pretreatment was proposed to destroy the compact cell wall of microalgae cell and further improve the performance of biogas production from microalgae biomass via anaerobic digestion (Passos et al., 2015). Generally, the hydrothermal pretreatment of microalgae biomass was investigated at a temperature blow 180 °C and a pressure blow 2 MPa (Mendez et al., 2014). Alzate et al. (2014) showed that the methane yield from microalgae biomass with hydrothermal pretreatment in anaerobic digestion was 1.39 times as great as that without pretreatment. Mendez et al. (2013) reported that there was 1.93-fold increase in the methane production from microalgae biomass with hydrothermal pretreatment via anaerobic digestion, compared with that using microalgae biomass without pretreatment as a substrate. Cho et al. (2013) reported that the methane yield from microalgae biomass with hydrothermal pretreatment via anaerobic digestion increased by 20.5%, in comparison to that without pretreatment. Although hydrothermal pretreatment is a promising method for the improvement of methane yield from microalgae biomass in anaerobic digestion, the process of hydrothermal pretreatment has amounts of energy requirement. Cho et al. (2013) demonstrated that the net energy gain was 1.47 times more in biogas production from microalgae biomass without pretreatment via anaerobic digestion than that using raw microalgae biomass as a substrate. For energy saving in hydrothermal pretreatment, Xiao et al. (2019) proposed a solar-driven hydrothermal pretreatment system, using solar energy as a heat resource for hydrothermal pretreatment of microalgae biomass. In the following anaerobic digestion, the methane yield of biogas production from microalgae biomass with the solardriven hydrothermal pretreatment raised 1.57 times, in contrast to that using raw microalgae biomass as a substrate for anaerobic digestion. Biogas productions from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment go through different processes, and acquire different thermodynamics efficiencies. In addition, an excellent thermodynamic 2
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was 1.2 in the gas boiler and the flue gas temperature was 120 °C. The anaerobic digestion was conducted at the temperature of 35 °C and pressure of atmospheric pressure.
In this paper, exergy analyses were conducted for biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment via anaerobic digestion. The material and energy flow, exergy efficiency and exergy destruction were investigated in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment via anaerobic digestion.
2.4. Anaerobic digestion with solar-driven hydrothermal pretreatment The microalgae slurry firstly absorbed the waste heat from the pretreated microalgae slurry in a heat exchanger, and then absorbed solar energy in a solar-driven hydrothermal pretreatment system, and eventually the microalgae slurry was heated up to 160 °C. In the solardriven hydrothermal pretreatment, the carbohydrates and the proteins were hydrolyzed, and the hydrolysis rate of carbohydrates and proteins were 0.431 and 0.474, respectively (Xiao et al., 2019). The solar-driven hydrothermal pretreatment of microalgae slurry was conducted at the temperature of 160 °C and pressure of 2 MPa. After solar-driven hydrothermal pretreatment, the microalgae slurry was cooled down to 35 °C through the heat exchanger and then flowed into a fermentation reactor. The methane yield from microalgae biomass with solar-driven hydrothermal pretreatment via anaerobic digestion was 348 L/kg VS. The anaerobic digestion was conducted at the temperature of 35 °C and pressure of atmospheric pressure.
2. Material and methods 2.1. Process description and system boundary Microalgae biomass was used as a feedstock for biogas production. The composition of microalgae biomass is shown in Fig. 1A (Xiao et al., 2019). It was reported that microalgae biomass had a growth rate of 25 g/(m2·d), obtaining a daily productivity of 25000 kg/d in raceway pond of 100 ha (Collet et al., 2011). Additionally, the mass fraction of microalgae biomass increased and obtained 5% after flotation and settling (Zamalloa et al., 2011). Therefore, it was assumed that the mass fraction of microalgae biomass was 5% in microalgae slurry in this paper, and further the microalgae slurry of 500 m3/d with a mass fraction of 5% was utilized. It was assumed that the ambient temperature was 25 °C. Fig. 1B shows the system boundary for exergy analyses on biogas production from microalgae biomass. The methane yield from microalgae biomass without pretreatment was 222 L/kg VS, while the methane yield from microalgae biomass with solar-driven hydrothermal pretreatment increased by 57% and obtained 348 L/kg VS (Xiao et al., 2019). Additionally, it was assumed that the biogas yield from microalgae biomass with hydrothermal pretreatment was the same as that with solar-driven hydrothermal pretreatment.
2.5. Mathematical modeling The NRTL-RK model was selected as property methods, and the reactions were simulated by stoichiometric reactor model. It was simplified that carbohydrates and proteins were hydrolyzed into glucose and amino acid, respectively, in hydrothermal pretreatment and solardriven hydrothermal pretreatment as shown in Eqs. (1),(2). In anaerobic digestion, the conversion of non-nitrogenous organic matters (such as glucose and lipid) and nitrogenous organic matters (such as amino acid) can be described as Eq. (3) and Eq. (4) (Manchala et al., 2017), respectively. The enthalpy of materials can be acquired from Aspen plus except for carbohydrates and proteins. The enthalpy of carbohydrates and proteins are shown in Table 1.
2.2. Anaerobic digestion without pretreatment The microalgae slurry flowed into a fermentation reactor directly without any pretreatment, under the drive of a pump. The methane yield from microalgae biomass without pretreatment in anaerobic digestion was 222 L/kg VS (Xiao et al., 2019). The biogas produced in anaerobic digestion was divided into two parts, one of which was transported to burn in a gas boiler for internal requirement of energy. Microalgae slurry was heated from 25 to 35 °C in anaerobic digestion, and the energy requirement was provided by gas boiler. The excess air coefficient was 1.2 in the gas boiler for sufficient burning of biogas, and the flue gas temperature was 120 °C. The anaerobic digestion was conducted at the temperature of 35 °C and pressure of atmospheric pressure.
(C6 H10 O5 )x + H2 O → C6 H12 O6
(1)
(C2 H3 NO)x + H2 O → C2 H5 NO2
(2)
b c a b c a b c Ca Hb Oc + ⎛a − − ⎞ H2 O → ⎛ + − ⎞ CH4 + ⎛ − + ⎞ CO2 4 2 2 8 4 2 8 4 ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ (3)
(
b 4
) CH + (
a 2
Ca Hb Oc Nd Se + a −
(
a 2
+
b 8
−
c 4
−
3d 8
−
e 4
4
−
c 2
−
b 8
+
3d 4
+
c 4
+ +
e 2 3d 8
) H O→ + ) CO 2
e 4
2
+ dNH3
+ eH2 S
2.3. Anaerobic digestion with hydrothermal pretreatment
(4)
2.6. Exergy analysis
The microalgae slurry firstly absorbed the waste heat from the pretreated microalgae slurry in a heat exchanger, and then absorbed the heat from biogas combustion in a gas boiler, and finally the microalgae slurry was heated up to 160 °C. In hydrothermal pretreatment, the carbohydrates and proteins were hydrolyzed, and the hydrolysis rate of carbohydrates and proteins were 0.431 and 0.474, respectively (Xiao et al., 2019). The hydrothermal pretreatment of microalgae biomass was conducted at the temperature of 160 °C and pressure of 2 MPa. The pretreated microalgae slurry was cooled down to 35 °C through a heat exchanger, and then flowed into a fermentation reactor. The methane yield from microalgae biomass with hydrothermal pretreatment via anaerobic digestion was 348 L/kg VS. The produced biogas from microalgae biomass via anaerobic digestion was divided into two parts, one of which was transported to burn in the gas boiler for internal requirement of energy. The microalgae slurry was heated up to 160 °C during hydrothermal pretreatment, and the energy requirement came from the biogas combustion in the gas boiler. The excess air coefficient
The exergy of materials consists of kinetic, potential and internal exergy (Rahbari et al., 2018). It was assumed that the variation of kinetic and potential exergy was negligible throughout the system. Therefore, the exergy of materials was the sum of physical and chemical exergy. The physical exergy (Exhp) is defined by Eq. (5) (Ofori-Boateng and Lee, 2013).
Exhp = (H − H0) − T0 (S − S0)
(5)
Table 1 The enthalpy of carbohydrates and proteins (Wooley and Putsche, 1996).
3
Components
Molecular formula
Solid enthalpy of formation (kJ/kmol)
Carbohydrates Proteins
C6H10O5 C2H3NO
−976,362 −187,196
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Table 2 Standard chemical exergy of material. Components
Chemical formula
Chemical exergy (kJ/kg)
Carbohydrates Glucose Proteins Glycine Lipids Glycerol Oleic acid Water Ammonia gas Carbon dioxide Methane
C6H10O5 C6H12O6 C2H3NO C2H5NO2 C57H104O6 C3H8O3 C18H34O2 H2O NH3 CO2 CH4
18,808 (Boesch et al., 2012) 15,518.89 (Ojeda et al., 2011) 24,488 (Boesch et al., 2012) 13,869.71 a 45,861.76 (Sorguven and Ozilgen, 2010) 18,539.13 (Ojeda et al., 2011) 42,067.65 (Ozilgen and Sorguven, 2011) 50 (Ojeda et al., 2011) 19,818.29 (Fishman, 2017) 433.86 (Ojeda et al., 2011) 51,981.25 (Barati et al., 2017)
a
The chemical exergy of glycine is calculated by Eq. (6).
where, H and S are the enthalpy and entropy of material, respectively, H0 and S0 are the enthalpy and entropy of material at the temperature of T0 (25 °C) and the pressure of p0 (1 atm), respectively. The specific chemical exergy (Exch) of complex substances are calculated by Eq. (6) (Hepbasli, 2008), when the elemental compositions of substrates are known. The standard chemical exergy of material used in this paper are shown in Table 2.
Fig. 2. Total exergy efficiency of biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment (HTP), and with solardriven hydrothermal pretreatment via anaerobic digestion (AD).
microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment by anaerobic digestion. The total exergy efficiencies were 26.2%, 35.98%, and 40.85% in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The difference in total exergy efficiency was ascribed to the different material and energy flow in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment. Fig. 3 shows the material and exergy flow in biogas production from microgalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment via anaerobic digestion. The net methane yields were 8.06, 11.55, and 14.39 kmol/h in biogas production from microalgae biomass without any pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The energy requirement for heating microalgae slurry in anaerobic digestion was 243.06 kW in biogas production from microalgae biomass without pretreatment. The energy requirements for hydrothermal pretreatment and solar-driven hydrothermal pretreatment were 608.51, and 780.14 kW, respectively. The pump powers were 0.75, 14.2, and 14.2 kW in biogas production from microalgae biomass without any pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The exergy efficiency (35.98%) of biogas production from microalgae biomass with hydrothermal pretreatment increased compared with that (26.2%) using raw microalgae biomass as a substrate. The reason was that the net output methane and exergy output increased in biogas production from microalgae biomass with hydrothermal pretreatment, compared with that without pretreatment. Hydrothermal pretreatment was useful for the enhancement of biogas production from microalgae biomass, which was different with Cho et al. (2013). The reason was that the methane yield got a great increase and recovery of waste heat reduced the energy requirement for hydrothermal pretreatment in this paper. The exergy efficiency (40.85%) of biogas production from microalgae biomass with solar-driven hydrothermal pretreatment was higher than that without pretreatment (26.2%). The reason was that biogas production from microalgae biomass was enhanced by solar-driven hydrothermal pretreatment, and further more exergy output was obtained than that without pretreatment. The exergy efficiency (40.85%) of biogas production from microalgae biomass with solar-driven hydrothermal pretreatment was higher than that with hydrothermal pretreatment (35.98%). The reason was that the exergy requirement for solar-driven hydrothermal pretreatment was substituted by renewable solar energy, eliminating the internal exergy loss
8,177.79[C ] + 5.25[N ] + 27,892.63[H ] − 3,173.66[O]⎞ Ex ch = 4.1868 × ⎛ ⎝ + 0.15[O](7,837.78[C ] + 33,888.89[H ] − 4,236.1[O]) ⎠ (6) ⎜
⎟
where C, H, O and N represent the mass fraction of carbon, hydrogen, oxygen and nitrogen in the biomass, respectively. Exergy efficiency (ηtotal ) was defined as the ratio of biogas exergy (Exbiogas ) to the total exergy input (∑ Ex input,t ) in biogas production from microalgae biomass, as shown in Eq. (7). Exergy efficiency of operational unit (ηunit ), except for the exergy efficiency of solar collector, was defined as the ratio of exergy output (Ex output,u ) to exergy input (∑ Ex input,u ) of operational unit, as shown in Eq. (8). Generally, the exergy efficiency of solar collector was defined as the ratio of the increased exergy of heat transfer fluid to the input exergy of solar radiation. The exergy efficiency of solar radiation was defined as Eq. (9) (Chafie et al., 2018; Hepbasli, 2008; Mehrpooya et al., 2018). The percentage of exergy destruction (ηdes ) in operational unit was defined as the ratio of exergy destruction (Ex des ) in operational unit to total input exergy (∑ Ex input,t ) in biogas production, as shown in Eq. (10). Notably, the target product was just biogas in biogas production from microalgae biomass via anaerobic digestion, thus the exergy associated with biogas slurry and biogas residue were considered as exergy loss in this paper.
ηtotal =
ηunit =
Exbiogas ∑ Ex input , t
(7)
∑ Ex output , u ∑ Ex input , u
(8) 4
ηsolar = 1 +
ηdes =
1 ⎛ T0 ⎞ 1 T − ⎛ 0⎞ 3 ⎝ Tr ⎠ 3 ⎝ Tr ⎠ ⎜
⎟
⎜
⎟
Ex des ∑ Ex input , t
(9)
(10)
where, Tr is the solar radiation temperature (6,000 K), T0 is the reference temperature (298 K). 3. Result and discussion 3.1. Exergy efficiency of biogas production from microalgae biomass Fig. 2 shows the total exergy efficiency in biogas production from 4
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Fig. 3. General diagram of biogas production from microalgae biomass without pretreatment (A), with hydrothermal pretreatment (B), and with solar-driven hydrothermal pretreatment (C) via anaerobic digestion.
microalgae biomass with solar-driven hydrothermal pretreatment was different with syngas production and biodiesel production from microalgae biomass. For example, Rahbari et al. (2018) reported that the exergy efficiency of syngas production by concentrated solar supercritical water gasification from algal biomass with a concentration of 25 wt.% was 72%. Ofori-Boateng et al. (2012) reported that the exergy efficiency of the biodiesel production through oil extraction and transesterification from microalgae biomass was 36%. The exergy efficiency (72%) of syngas production from algal biomass by concentrated solar supercritical water gasification was higher than that (40.85%) of biogas production from microalgae biomass with solardriven hydrothermal pretreatment. The reason was that the utilization efficiency of organic matters raised in concentrated solar supercritical
and resulting in the increasing output methane and exergy, compared with that with hydrothermal pretreatment. The pump power (0.75 kW) in biogas production from microalgae biomass without pretreatment was lower than that with hydrothermal pretreatment (14.2 kW) and with solar-driven hydrothermal pretreatment (14.2 kW). The reason was that the pump was just used for overcoming the flow resistance of microalgae slurry in biogas production from microalgae biomass without pretreatment. Comparatively, the pumps in biogas production from microalgae biomass with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment were used for not only overcoming the flow resistance of microalgae slurry, but also providing extra pressure for preventing microalgae slurry boiling in pretreatment. In addition, the exergy efficiency of biogas production from 5
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solar-driven hydrothermal pretreatment, respectively. The exergy efficiency of anaerobic digestion increased 57.2% in biogas production from microalgae biomass with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment, compared with that without any pretreatment. The reason was that the methane yield from microalgae biomass with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment via anaerobic digestion increased, as against that without pretreatment. Therefore, the exergy output of anaerobic digestion increased in biogas production from microalgae biomass with hydrothermal pretreatment and with solar driven hydrothermal pretreatment, in comparison to that without pretreatment.
water gasification, as against that in biogas production from microalgae biomass with solar driven hydrothermal pretreatment. In syngas production from microalgae biomass, the organic matters of microalgae biomass was all converted into hydrogen, CO and CO2 in supercritical water gasification. However, the biogas residue contained amounts of organic matters, which were not converted into biogas in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment. Additionally, the supercritical water gasification contained a process of steam methane reforming (Rahbari et al., 2018), in which the water was converted into hydrogen, and further the exergy output increased in syngas production from microalgae biomass. However, the syngas production from algal biomass by concentrated solar supercritical water gasification had a demand on the concentration of algal biomass (25 wt.%). Therefore, the dewatering during the preparation of algal biomass was imperative for syngas production by concentrated solar supercritical water gasification and had amount of energy requirement, which was not included in the system boundary of exergy analysis. On the other hand, the exergy efficiency (36%) of biodiesel production by oil extraction and transesterification from microalgae biomass was lower than that (40.85%) in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment. The reason was that the oil extraction and transesterification merely utilized the lipid in biodiesel production. The exergy efficiency had a strong dependence on the lipid content of microalgae cell in biodiesel production from microalgae biomass by extraction and transesterification. Therefore, biodiesel production through oil extraction and transesterification not only made a higher requirement on the composition of microalgae biomass, but also led to the waste of carbohydrates and proteins, compared with biogas production from microalgae biomass with solar-driven hydrothermal pretreatment.
3.3. Exergy destruction of biogas production from microalgae biomass Fig. 5 shows exergy distribution of biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment. As shown in Fig. 5, the biogas residue caused the maximum exergy loss, and followed by the irreversibility of fermentation reaction in biogas production with three different pathways. The exergy loss associated with biogas residue accounted for 60.58%, 38.54%, and 35.13% of overall exergy input in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. Obviously, after hydrothermal pretreatment and solar-driven hydrothermal pretreatment, the exergy loss associated with biogas residue decreased, compared with that without pretreatment. The reason was that hydrothermal pretreatment and solar-driven hydrothermal pretreatment enhanced the degradability of microalgae biomass, causing the decrease of organic matters and exergy loss associated with biogas residue. After hydrothermal pretreatment and solar-driven hydrothermal pretreatment, amount of organic matters were still not utilized fully, which was probably because of the ammonia–nitrogen toxicity (Ward et al., 2014). Thus, elimination of ammonia inhibition was necessary for the reduction of exergy destruction in biogas production from microalgae biomass. In addition, the internal exergy loss, which was caused by exergy requirement for anaerobic digestion, accounted for 3.67% of overall exergy input in biogas production from microalgae biomass without pretreatment. The internal exergy loss for hydrothermal pretreatment accounted for a small proportion (6.31%) of overall exergy input, which was ascribed to the recovery of waste heat, as mentioned above. Furthermore, the irreversible processes (throttling, heat transfer, hydrolysis reaction and solar-thermal conversion) caused exergy destruction, which accounted for a small proportion of overall exergy input.
3.2. Exergy efficiency of operational unit in biogas production from microalgae biomass Fig. 4 shows the exergy efficiency of operational unit in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment in anaerobic digestion. The exergy efficiency were almost 100% in the unit of pump, exchanger, pretreatment and valve. The exergy efficiency of gas boiler were 96.49% and 94.35% in biogas production from microalgae biomass without pretreatment and with hydrothermal pretreatment, respectively. Correspondingly, the exergy efficiency of solar collector was 25.91% in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment. Additionally, the exergy efficiency of anaerobic digestion were 29.88%, 46.97%, and 46.97% in biogas production from microalgae biomass without any pretreatment, with hydrothermal pretreatment, and with
4. Conclusion The maximum exergy efficiency (40.85%) was acquired in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment, compared with that without pretreatment (26.2%) and with hydrothermal pretreatment (35.98%). Biogas residue caused the maximum exergy destruction, which accounted for 60.58%, 38.54%, and 35.13% of overall exergy input in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The further enhancement of degradability of microalgae biomass was beneficial for the reduction of exergy destruction in biogas production. The exergy analyses provide important guidance for the improvement of thermodynamic performance for biogas production from microalgae biomass. Acknowledgment The authors are grateful for the financial support provided by the State Key Program of National Natural Science of China (Grant number 51836001), National Natural Science Foundation of China (No.
Fig. 4. Exergy efficiency of operational unit in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment (HTP) and with solar-driven hydrothermal pretreatment via anaerobic digestion (AD). 6
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Fig. 5. Exergy distribution of biogas production from microalgae biomass without pretreatment (A), with hydrothermal pretreatment (B) and with solar-driven hydrothermal pretreatment (C) via anaerobic digestion.
51776025), the Venture & Innovation Support Program for Chongqing Overseas and Returnees (No. cx2017017), and the Fundamental Research Funds for the Central Universities (2018CDQYDL0049). No conflicts of interest, informed consent, human or animal rights are applicable to this investigation.
methane potential of microalgae biomass after lipid extraction. Chem. Eng. J. 243, 405–410. Barati, M.R., Aghbashlo, M., Ghanavati, H., Tabatabaei, M., Sharifi, M., Javadirad, G., Dadak, A., Soufiyan, M.M., 2017. Comprehensive exergy analysis of a gas engineequipped anaerobic digestion plant producing electricity and biofertilizer from organic fraction of municipal solid waste. Energ. Convers. Manage 151, 753–763. Bennion, E.P., Ginosar, D.M., Moses, J., Agblevor, F., Quinn, J.C., 2015. Lifecycle assessment of microalgae to biofuel: Comparison of thermochemical processing pathways. Appl. Energ. 154, 1062–1071. Boesch, P., Modarresi, A., Friedl, A., 2012. Comparison of combined ethanol and biogas polygeneration facilities using exergy analysis. Appl. Therm. Eng. 37, 19–29.
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Exergy analyses of biogas production from microalgae biomass via anaerobic digestion
T
⁎
Chao Xiaoa,b, Qiang Liaoa,b, Qian Fua,b, , Yun Huanga,b, Ao Xiaa,b, Weifeng Shenc, Hao Chena,b, Xun Zhua,b a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, China Institute of Engineering Thermophysics, School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China c School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Exergy analyses Biogas production Microalgae biomass Anaerobic digestion Hydrothermal pretreatment
Biogas production from microalgae biomass without pretreatment and with hydrothermal pretreatment involve the energy with different quality and quantity, which makes it complex to evaluate thermodynamic performance. In this paper, exergy analyses were conducted in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment. The results showed that the materials and energy flow affected exergy efficiency in biogas production from microalgae biomass. The biogas production from microalgae biomass with solar-driven hydrothermal pretreatment achieved the highest exergy efficiency (40.85%), compared with that without pretreatment (26.2%) and with hydrothermal pretreatment (35.98%). In addition, the maximum exergy loss was caused by biogas residue, which accounted for 60.58%, 38.54%, and 35.13% of overall exergy input in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. Exergy analyses provide important theoretical guidance to improve the performance of biogas production from microalgae biomass.
1. Introduction The survival and development of human beings are threatened by environmental pollution and resources exhaustion. Microalgae technology is a promising technology for alleviating stress from environmental pollution and energy exhaustion (Sun et al., 2016). Specifically, microalgae cell can capture CO2 and purify sewage while producing biomass for biofuels production (Chang et al., 2018). In addition, microalgae have several advantages over other energy crops, such as a high photosynthetic efficiency, no competition with food crops in lands, and well tolerance of high CO2 concentration (Li et al., 2008). Microalgae biomass has been used as a feedstock for biofuels production, for example, biodiesel production by hydrothermal liquefaction (Li et al., 2014) and pyrolysis (Miao et al., 2004), and biogas production by anaerobic digestion (Bennion et al., 2015; Collet et al., 2011). Generally, microalgae biomass still has a low concentration in microalgae slurry after harvesting, and dewatering of microalgae slurry is required for biodiesel production through hydrothermal liquefaction
and pyrolysis. However, dewatering is a process of energy intensity, which is the bottleneck of biodiesel production from microalgae biomass (Uduman et al., 2010). Notably, although hydrothermal liquefaction and pyrolysis can converted microalgae biomass into biodiesel at high pressure and temperature, both of them achieve a negative energy gain (Bennion et al., 2015). In comparison, anaerobic digestion can convert microalgae biomass with high moisture content into biogas without dewatering, which remarkably achieves a positive energy gain (Sun et al., 2019). Therefore, anaerobic digestion is considered as a promising approach for biofuel production from microalgae biomass. However, biogas production from microalgae biomass via anaerobic digestion achieves a poor biogas yield, due to the compact cell wall of microalgae biomass. For instance, Sialve et al. (2009) showed that all of organic matters in microalgae biomass can be converted into biogas, and methane yields from proteins, lipids and carbohydrates were theoretically 0.851, 1.014 and 0.415 L/g volatile solid (VS), respectively. However, the methane yield from microalgae biomass was merely 0.28 L/g VS in practical (Murphy et al., 2015). For this issues, hydrothermal
Abbreviation:HTP, hydrothermal pretreatment ⁎ Corresponding author at: Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, China; Institute of Engineering Thermophysics, School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (Q. Fu). https://doi.org/10.1016/j.biortech.2019.121709 Received 29 May 2019; Received in revised form 19 June 2019; Accepted 26 June 2019 Available online 27 June 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Composition of microalgae biomass (A), and system boundary for biogas production from microalgae biomass via anaerobic digestion (B).
performance is required in biogas production from microalgae biomass in industry. The improvement of thermodynamic efficiency relies on the breakthrough of bottleneck in biogas production from microalgae biomass. The thermodynamic analysis can be utilized to find out the bottleneck for the improvement of thermodynamic performance of biofuel production from microalgae biomass. Thus, it is necessary to analyze the thermodynamic performance of biogas production from microalgae biomass. On the other hand, biogas productions from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment involve different kinds of energy, e.g., electric energy, biomass energy, chemical energy, thermal energy, and solar energy. Notably, all of the energy have different both quantity and quality, which belong to different categories and should be taken into consideration in the assessment of thermodynamic performance in biogas production from microalgae biomass. Thus, it is imperative to analyze the thermodynamic performance of biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment in a unified standard. Energy analysis, which based on the first law of thermodynamics, merely takes the quantity of energy into consideration. In contrast, the exergy covers both the quantity and quality of energy, and exergy analysis is usually utilized to evaluate the thermodynamic performance of processes (Sorguven and Ozilgen, 2010), where different kinds of energy are involved. Li et al. (2019) conducted an exergy analysis of biomass staged-gasification for hydrogen-rich syngas, and the main exergy losses lay in the internal exergy loss. Kumar et al. (2018) conducted an exergy analysis of hydrogen production from acetone-butanol-ethanol-water mixture via steam reforming. Cohce et al. (2011) conducted an exergy analysis of hydrogen production from palm oil waste by thermochemical biomass gasification. However, the exergy analysis of biogas production from microalgae biomass via anaerobic digestion has never been reported.
pretreatment was proposed to destroy the compact cell wall of microalgae cell and further improve the performance of biogas production from microalgae biomass via anaerobic digestion (Passos et al., 2015). Generally, the hydrothermal pretreatment of microalgae biomass was investigated at a temperature blow 180 °C and a pressure blow 2 MPa (Mendez et al., 2014). Alzate et al. (2014) showed that the methane yield from microalgae biomass with hydrothermal pretreatment in anaerobic digestion was 1.39 times as great as that without pretreatment. Mendez et al. (2013) reported that there was 1.93-fold increase in the methane production from microalgae biomass with hydrothermal pretreatment via anaerobic digestion, compared with that using microalgae biomass without pretreatment as a substrate. Cho et al. (2013) reported that the methane yield from microalgae biomass with hydrothermal pretreatment via anaerobic digestion increased by 20.5%, in comparison to that without pretreatment. Although hydrothermal pretreatment is a promising method for the improvement of methane yield from microalgae biomass in anaerobic digestion, the process of hydrothermal pretreatment has amounts of energy requirement. Cho et al. (2013) demonstrated that the net energy gain was 1.47 times more in biogas production from microalgae biomass without pretreatment via anaerobic digestion than that using raw microalgae biomass as a substrate. For energy saving in hydrothermal pretreatment, Xiao et al. (2019) proposed a solar-driven hydrothermal pretreatment system, using solar energy as a heat resource for hydrothermal pretreatment of microalgae biomass. In the following anaerobic digestion, the methane yield of biogas production from microalgae biomass with the solardriven hydrothermal pretreatment raised 1.57 times, in contrast to that using raw microalgae biomass as a substrate for anaerobic digestion. Biogas productions from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment go through different processes, and acquire different thermodynamics efficiencies. In addition, an excellent thermodynamic 2
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was 1.2 in the gas boiler and the flue gas temperature was 120 °C. The anaerobic digestion was conducted at the temperature of 35 °C and pressure of atmospheric pressure.
In this paper, exergy analyses were conducted for biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment via anaerobic digestion. The material and energy flow, exergy efficiency and exergy destruction were investigated in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment via anaerobic digestion.
2.4. Anaerobic digestion with solar-driven hydrothermal pretreatment The microalgae slurry firstly absorbed the waste heat from the pretreated microalgae slurry in a heat exchanger, and then absorbed solar energy in a solar-driven hydrothermal pretreatment system, and eventually the microalgae slurry was heated up to 160 °C. In the solardriven hydrothermal pretreatment, the carbohydrates and the proteins were hydrolyzed, and the hydrolysis rate of carbohydrates and proteins were 0.431 and 0.474, respectively (Xiao et al., 2019). The solar-driven hydrothermal pretreatment of microalgae slurry was conducted at the temperature of 160 °C and pressure of 2 MPa. After solar-driven hydrothermal pretreatment, the microalgae slurry was cooled down to 35 °C through the heat exchanger and then flowed into a fermentation reactor. The methane yield from microalgae biomass with solar-driven hydrothermal pretreatment via anaerobic digestion was 348 L/kg VS. The anaerobic digestion was conducted at the temperature of 35 °C and pressure of atmospheric pressure.
2. Material and methods 2.1. Process description and system boundary Microalgae biomass was used as a feedstock for biogas production. The composition of microalgae biomass is shown in Fig. 1A (Xiao et al., 2019). It was reported that microalgae biomass had a growth rate of 25 g/(m2·d), obtaining a daily productivity of 25000 kg/d in raceway pond of 100 ha (Collet et al., 2011). Additionally, the mass fraction of microalgae biomass increased and obtained 5% after flotation and settling (Zamalloa et al., 2011). Therefore, it was assumed that the mass fraction of microalgae biomass was 5% in microalgae slurry in this paper, and further the microalgae slurry of 500 m3/d with a mass fraction of 5% was utilized. It was assumed that the ambient temperature was 25 °C. Fig. 1B shows the system boundary for exergy analyses on biogas production from microalgae biomass. The methane yield from microalgae biomass without pretreatment was 222 L/kg VS, while the methane yield from microalgae biomass with solar-driven hydrothermal pretreatment increased by 57% and obtained 348 L/kg VS (Xiao et al., 2019). Additionally, it was assumed that the biogas yield from microalgae biomass with hydrothermal pretreatment was the same as that with solar-driven hydrothermal pretreatment.
2.5. Mathematical modeling The NRTL-RK model was selected as property methods, and the reactions were simulated by stoichiometric reactor model. It was simplified that carbohydrates and proteins were hydrolyzed into glucose and amino acid, respectively, in hydrothermal pretreatment and solardriven hydrothermal pretreatment as shown in Eqs. (1),(2). In anaerobic digestion, the conversion of non-nitrogenous organic matters (such as glucose and lipid) and nitrogenous organic matters (such as amino acid) can be described as Eq. (3) and Eq. (4) (Manchala et al., 2017), respectively. The enthalpy of materials can be acquired from Aspen plus except for carbohydrates and proteins. The enthalpy of carbohydrates and proteins are shown in Table 1.
2.2. Anaerobic digestion without pretreatment The microalgae slurry flowed into a fermentation reactor directly without any pretreatment, under the drive of a pump. The methane yield from microalgae biomass without pretreatment in anaerobic digestion was 222 L/kg VS (Xiao et al., 2019). The biogas produced in anaerobic digestion was divided into two parts, one of which was transported to burn in a gas boiler for internal requirement of energy. Microalgae slurry was heated from 25 to 35 °C in anaerobic digestion, and the energy requirement was provided by gas boiler. The excess air coefficient was 1.2 in the gas boiler for sufficient burning of biogas, and the flue gas temperature was 120 °C. The anaerobic digestion was conducted at the temperature of 35 °C and pressure of atmospheric pressure.
(C6 H10 O5 )x + H2 O → C6 H12 O6
(1)
(C2 H3 NO)x + H2 O → C2 H5 NO2
(2)
b c a b c a b c Ca Hb Oc + ⎛a − − ⎞ H2 O → ⎛ + − ⎞ CH4 + ⎛ − + ⎞ CO2 4 2 2 8 4 2 8 4 ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ (3)
(
b 4
) CH + (
a 2
Ca Hb Oc Nd Se + a −
(
a 2
+
b 8
−
c 4
−
3d 8
−
e 4
4
−
c 2
−
b 8
+
3d 4
+
c 4
+ +
e 2 3d 8
) H O→ + ) CO 2
e 4
2
+ dNH3
+ eH2 S
2.3. Anaerobic digestion with hydrothermal pretreatment
(4)
2.6. Exergy analysis
The microalgae slurry firstly absorbed the waste heat from the pretreated microalgae slurry in a heat exchanger, and then absorbed the heat from biogas combustion in a gas boiler, and finally the microalgae slurry was heated up to 160 °C. In hydrothermal pretreatment, the carbohydrates and proteins were hydrolyzed, and the hydrolysis rate of carbohydrates and proteins were 0.431 and 0.474, respectively (Xiao et al., 2019). The hydrothermal pretreatment of microalgae biomass was conducted at the temperature of 160 °C and pressure of 2 MPa. The pretreated microalgae slurry was cooled down to 35 °C through a heat exchanger, and then flowed into a fermentation reactor. The methane yield from microalgae biomass with hydrothermal pretreatment via anaerobic digestion was 348 L/kg VS. The produced biogas from microalgae biomass via anaerobic digestion was divided into two parts, one of which was transported to burn in the gas boiler for internal requirement of energy. The microalgae slurry was heated up to 160 °C during hydrothermal pretreatment, and the energy requirement came from the biogas combustion in the gas boiler. The excess air coefficient
The exergy of materials consists of kinetic, potential and internal exergy (Rahbari et al., 2018). It was assumed that the variation of kinetic and potential exergy was negligible throughout the system. Therefore, the exergy of materials was the sum of physical and chemical exergy. The physical exergy (Exhp) is defined by Eq. (5) (Ofori-Boateng and Lee, 2013).
Exhp = (H − H0) − T0 (S − S0)
(5)
Table 1 The enthalpy of carbohydrates and proteins (Wooley and Putsche, 1996).
3
Components
Molecular formula
Solid enthalpy of formation (kJ/kmol)
Carbohydrates Proteins
C6H10O5 C2H3NO
−976,362 −187,196
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Table 2 Standard chemical exergy of material. Components
Chemical formula
Chemical exergy (kJ/kg)
Carbohydrates Glucose Proteins Glycine Lipids Glycerol Oleic acid Water Ammonia gas Carbon dioxide Methane
C6H10O5 C6H12O6 C2H3NO C2H5NO2 C57H104O6 C3H8O3 C18H34O2 H2O NH3 CO2 CH4
18,808 (Boesch et al., 2012) 15,518.89 (Ojeda et al., 2011) 24,488 (Boesch et al., 2012) 13,869.71 a 45,861.76 (Sorguven and Ozilgen, 2010) 18,539.13 (Ojeda et al., 2011) 42,067.65 (Ozilgen and Sorguven, 2011) 50 (Ojeda et al., 2011) 19,818.29 (Fishman, 2017) 433.86 (Ojeda et al., 2011) 51,981.25 (Barati et al., 2017)
a
The chemical exergy of glycine is calculated by Eq. (6).
where, H and S are the enthalpy and entropy of material, respectively, H0 and S0 are the enthalpy and entropy of material at the temperature of T0 (25 °C) and the pressure of p0 (1 atm), respectively. The specific chemical exergy (Exch) of complex substances are calculated by Eq. (6) (Hepbasli, 2008), when the elemental compositions of substrates are known. The standard chemical exergy of material used in this paper are shown in Table 2.
Fig. 2. Total exergy efficiency of biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment (HTP), and with solardriven hydrothermal pretreatment via anaerobic digestion (AD).
microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment by anaerobic digestion. The total exergy efficiencies were 26.2%, 35.98%, and 40.85% in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The difference in total exergy efficiency was ascribed to the different material and energy flow in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment. Fig. 3 shows the material and exergy flow in biogas production from microgalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment via anaerobic digestion. The net methane yields were 8.06, 11.55, and 14.39 kmol/h in biogas production from microalgae biomass without any pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The energy requirement for heating microalgae slurry in anaerobic digestion was 243.06 kW in biogas production from microalgae biomass without pretreatment. The energy requirements for hydrothermal pretreatment and solar-driven hydrothermal pretreatment were 608.51, and 780.14 kW, respectively. The pump powers were 0.75, 14.2, and 14.2 kW in biogas production from microalgae biomass without any pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The exergy efficiency (35.98%) of biogas production from microalgae biomass with hydrothermal pretreatment increased compared with that (26.2%) using raw microalgae biomass as a substrate. The reason was that the net output methane and exergy output increased in biogas production from microalgae biomass with hydrothermal pretreatment, compared with that without pretreatment. Hydrothermal pretreatment was useful for the enhancement of biogas production from microalgae biomass, which was different with Cho et al. (2013). The reason was that the methane yield got a great increase and recovery of waste heat reduced the energy requirement for hydrothermal pretreatment in this paper. The exergy efficiency (40.85%) of biogas production from microalgae biomass with solar-driven hydrothermal pretreatment was higher than that without pretreatment (26.2%). The reason was that biogas production from microalgae biomass was enhanced by solar-driven hydrothermal pretreatment, and further more exergy output was obtained than that without pretreatment. The exergy efficiency (40.85%) of biogas production from microalgae biomass with solar-driven hydrothermal pretreatment was higher than that with hydrothermal pretreatment (35.98%). The reason was that the exergy requirement for solar-driven hydrothermal pretreatment was substituted by renewable solar energy, eliminating the internal exergy loss
8,177.79[C ] + 5.25[N ] + 27,892.63[H ] − 3,173.66[O]⎞ Ex ch = 4.1868 × ⎛ ⎝ + 0.15[O](7,837.78[C ] + 33,888.89[H ] − 4,236.1[O]) ⎠ (6) ⎜
⎟
where C, H, O and N represent the mass fraction of carbon, hydrogen, oxygen and nitrogen in the biomass, respectively. Exergy efficiency (ηtotal ) was defined as the ratio of biogas exergy (Exbiogas ) to the total exergy input (∑ Ex input,t ) in biogas production from microalgae biomass, as shown in Eq. (7). Exergy efficiency of operational unit (ηunit ), except for the exergy efficiency of solar collector, was defined as the ratio of exergy output (Ex output,u ) to exergy input (∑ Ex input,u ) of operational unit, as shown in Eq. (8). Generally, the exergy efficiency of solar collector was defined as the ratio of the increased exergy of heat transfer fluid to the input exergy of solar radiation. The exergy efficiency of solar radiation was defined as Eq. (9) (Chafie et al., 2018; Hepbasli, 2008; Mehrpooya et al., 2018). The percentage of exergy destruction (ηdes ) in operational unit was defined as the ratio of exergy destruction (Ex des ) in operational unit to total input exergy (∑ Ex input,t ) in biogas production, as shown in Eq. (10). Notably, the target product was just biogas in biogas production from microalgae biomass via anaerobic digestion, thus the exergy associated with biogas slurry and biogas residue were considered as exergy loss in this paper.
ηtotal =
ηunit =
Exbiogas ∑ Ex input , t
(7)
∑ Ex output , u ∑ Ex input , u
(8) 4
ηsolar = 1 +
ηdes =
1 ⎛ T0 ⎞ 1 T − ⎛ 0⎞ 3 ⎝ Tr ⎠ 3 ⎝ Tr ⎠ ⎜
⎟
⎜
⎟
Ex des ∑ Ex input , t
(9)
(10)
where, Tr is the solar radiation temperature (6,000 K), T0 is the reference temperature (298 K). 3. Result and discussion 3.1. Exergy efficiency of biogas production from microalgae biomass Fig. 2 shows the total exergy efficiency in biogas production from 4
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Fig. 3. General diagram of biogas production from microalgae biomass without pretreatment (A), with hydrothermal pretreatment (B), and with solar-driven hydrothermal pretreatment (C) via anaerobic digestion.
microalgae biomass with solar-driven hydrothermal pretreatment was different with syngas production and biodiesel production from microalgae biomass. For example, Rahbari et al. (2018) reported that the exergy efficiency of syngas production by concentrated solar supercritical water gasification from algal biomass with a concentration of 25 wt.% was 72%. Ofori-Boateng et al. (2012) reported that the exergy efficiency of the biodiesel production through oil extraction and transesterification from microalgae biomass was 36%. The exergy efficiency (72%) of syngas production from algal biomass by concentrated solar supercritical water gasification was higher than that (40.85%) of biogas production from microalgae biomass with solardriven hydrothermal pretreatment. The reason was that the utilization efficiency of organic matters raised in concentrated solar supercritical
and resulting in the increasing output methane and exergy, compared with that with hydrothermal pretreatment. The pump power (0.75 kW) in biogas production from microalgae biomass without pretreatment was lower than that with hydrothermal pretreatment (14.2 kW) and with solar-driven hydrothermal pretreatment (14.2 kW). The reason was that the pump was just used for overcoming the flow resistance of microalgae slurry in biogas production from microalgae biomass without pretreatment. Comparatively, the pumps in biogas production from microalgae biomass with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment were used for not only overcoming the flow resistance of microalgae slurry, but also providing extra pressure for preventing microalgae slurry boiling in pretreatment. In addition, the exergy efficiency of biogas production from 5
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solar-driven hydrothermal pretreatment, respectively. The exergy efficiency of anaerobic digestion increased 57.2% in biogas production from microalgae biomass with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment, compared with that without any pretreatment. The reason was that the methane yield from microalgae biomass with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment via anaerobic digestion increased, as against that without pretreatment. Therefore, the exergy output of anaerobic digestion increased in biogas production from microalgae biomass with hydrothermal pretreatment and with solar driven hydrothermal pretreatment, in comparison to that without pretreatment.
water gasification, as against that in biogas production from microalgae biomass with solar driven hydrothermal pretreatment. In syngas production from microalgae biomass, the organic matters of microalgae biomass was all converted into hydrogen, CO and CO2 in supercritical water gasification. However, the biogas residue contained amounts of organic matters, which were not converted into biogas in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment. Additionally, the supercritical water gasification contained a process of steam methane reforming (Rahbari et al., 2018), in which the water was converted into hydrogen, and further the exergy output increased in syngas production from microalgae biomass. However, the syngas production from algal biomass by concentrated solar supercritical water gasification had a demand on the concentration of algal biomass (25 wt.%). Therefore, the dewatering during the preparation of algal biomass was imperative for syngas production by concentrated solar supercritical water gasification and had amount of energy requirement, which was not included in the system boundary of exergy analysis. On the other hand, the exergy efficiency (36%) of biodiesel production by oil extraction and transesterification from microalgae biomass was lower than that (40.85%) in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment. The reason was that the oil extraction and transesterification merely utilized the lipid in biodiesel production. The exergy efficiency had a strong dependence on the lipid content of microalgae cell in biodiesel production from microalgae biomass by extraction and transesterification. Therefore, biodiesel production through oil extraction and transesterification not only made a higher requirement on the composition of microalgae biomass, but also led to the waste of carbohydrates and proteins, compared with biogas production from microalgae biomass with solar-driven hydrothermal pretreatment.
3.3. Exergy destruction of biogas production from microalgae biomass Fig. 5 shows exergy distribution of biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment. As shown in Fig. 5, the biogas residue caused the maximum exergy loss, and followed by the irreversibility of fermentation reaction in biogas production with three different pathways. The exergy loss associated with biogas residue accounted for 60.58%, 38.54%, and 35.13% of overall exergy input in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. Obviously, after hydrothermal pretreatment and solar-driven hydrothermal pretreatment, the exergy loss associated with biogas residue decreased, compared with that without pretreatment. The reason was that hydrothermal pretreatment and solar-driven hydrothermal pretreatment enhanced the degradability of microalgae biomass, causing the decrease of organic matters and exergy loss associated with biogas residue. After hydrothermal pretreatment and solar-driven hydrothermal pretreatment, amount of organic matters were still not utilized fully, which was probably because of the ammonia–nitrogen toxicity (Ward et al., 2014). Thus, elimination of ammonia inhibition was necessary for the reduction of exergy destruction in biogas production from microalgae biomass. In addition, the internal exergy loss, which was caused by exergy requirement for anaerobic digestion, accounted for 3.67% of overall exergy input in biogas production from microalgae biomass without pretreatment. The internal exergy loss for hydrothermal pretreatment accounted for a small proportion (6.31%) of overall exergy input, which was ascribed to the recovery of waste heat, as mentioned above. Furthermore, the irreversible processes (throttling, heat transfer, hydrolysis reaction and solar-thermal conversion) caused exergy destruction, which accounted for a small proportion of overall exergy input.
3.2. Exergy efficiency of operational unit in biogas production from microalgae biomass Fig. 4 shows the exergy efficiency of operational unit in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment and with solar-driven hydrothermal pretreatment in anaerobic digestion. The exergy efficiency were almost 100% in the unit of pump, exchanger, pretreatment and valve. The exergy efficiency of gas boiler were 96.49% and 94.35% in biogas production from microalgae biomass without pretreatment and with hydrothermal pretreatment, respectively. Correspondingly, the exergy efficiency of solar collector was 25.91% in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment. Additionally, the exergy efficiency of anaerobic digestion were 29.88%, 46.97%, and 46.97% in biogas production from microalgae biomass without any pretreatment, with hydrothermal pretreatment, and with
4. Conclusion The maximum exergy efficiency (40.85%) was acquired in biogas production from microalgae biomass with solar-driven hydrothermal pretreatment, compared with that without pretreatment (26.2%) and with hydrothermal pretreatment (35.98%). Biogas residue caused the maximum exergy destruction, which accounted for 60.58%, 38.54%, and 35.13% of overall exergy input in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. The further enhancement of degradability of microalgae biomass was beneficial for the reduction of exergy destruction in biogas production. The exergy analyses provide important guidance for the improvement of thermodynamic performance for biogas production from microalgae biomass. Acknowledgment The authors are grateful for the financial support provided by the State Key Program of National Natural Science of China (Grant number 51836001), National Natural Science Foundation of China (No.
Fig. 4. Exergy efficiency of operational unit in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment (HTP) and with solar-driven hydrothermal pretreatment via anaerobic digestion (AD). 6
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Fig. 5. Exergy distribution of biogas production from microalgae biomass without pretreatment (A), with hydrothermal pretreatment (B) and with solar-driven hydrothermal pretreatment (C) via anaerobic digestion.
51776025), the Venture & Innovation Support Program for Chongqing Overseas and Returnees (No. cx2017017), and the Fundamental Research Funds for the Central Universities (2018CDQYDL0049). No conflicts of interest, informed consent, human or animal rights are applicable to this investigation.
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