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Ammonia production from algae via integrated hydrothermal gasification, chemical looping, N2 production, and NH3 synthesis
Energy 174 (2019) 331e338
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Ammonia production from algae via integrated hydrothermal gasification, chemical looping, N2 production, and NH3 synthesis Agung Tri Wijayanta a, Muhammad Aziz b, * a b
Department of Mechanical Engineering, Universitas Sebelas Maret, Kampus UNS Kentingan, Jl. Ir. Sutami 36A Kentingan, Surakarta 57126, Indonesia Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550 Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2018 Received in revised form 22 February 2019 Accepted 27 February 2019 Available online 28 February 2019
Novel integrated system to convert algae to NH3 is proposed with the objective of effective and thorough energy/heat circulation to achieve high total energy efficiency. The integrated system mainly consists of hydrothermal gasification (HTG), chemical looping, N2 production, NH3 synthesis, and power generation. Algae are converted initially to syngas through HTG, which is further converted to CO2 and H2 in chemical looping module. The produced H2 from chemical looping module is reacted with the produced highly-pure N2 from N2 production module to form NH3 in NH3 synthesis module. To realize high energyefficiency, an enhanced process integration, which simultaneously integrates both exergy recovery and process integration technologies, is applied. Therefore, the energy/heat involved in the integrated system is recirculated thoroughly and used partly for power generation. Macro alga of Cladophora glomerata (Chlorophyta) is used as the sample in the study. The effects of temperature and algae-to-water mass ratio during HTG are evaluated in terms of their influence to the total energy efficiency. From process modeling and calculation using SimSci Pro/II, the proposed integrated-system shows relatively high total energy efficiency of about 38%, including both NH3 and power production, achieved at HTG temperature of 380 C and mass ratio of 0.01. © 2019 Published by Elsevier Ltd.
Keywords: Algae Ammonia Hydrothermal gasification Chemical looping Energy efficiency Integrated system
1. Introduction Algae have received an intensive attention in today's industry covering energy source, chemicals, food and pharmacies. They have very large number of varieties and are distributed widely in aqueous environment including ocean, freshwater and marshland [1,2]. Compared to other biomasses, algae have higher growth rate, higher lipid content, and higher capability to live in wide range of environmental conditions [3]. Algae are able to convert sunlight to chemical energy with efficiency of near to 10% [4] and absorb CO2 at average rate of 36.7 t ha1 y1 [5]. Therefore, cultivation of algae is a very potential way for CO2 mitigation (bio-sequestration) [6]. The cultivation of algae near to CO2 producing sources, such as power plants, is considered as attractive strategy. Unfortunately, algae utilization still faces various obstacles during their cultivation, processing and utilization. In addition, algae have high moisture content ranging from 60 to 90 wt% on wet basis (wb) demanding extraordinary treatments during their utilization [7].
* Corresponding author. E-mail address: [email protected] (M. Aziz). https://doi.org/10.1016/j.energy.2019.02.190 0360-5442/© 2019 Published by Elsevier Ltd.
Focusing on the energy production from algae, in general, two routes of biochemical and thermochemical mechanisms are possible. Comparing both of them, thermochemical conversion seems more advantageous, due to faster conversion and higher carbon-conversion efficiency [8]. Dou et al. [9] have summarized well several issues related to thermochemical conversion of biomass to H2. As energy source, algae must be sufficiently competitive with other energy sources, even with fossil fuel. Therefore, an efficient energy production system from algae is considered urgent to be developed. Recently, H2 production from algae gains significant attention over last decades as it potentially combines the beneficial characteristics of both algae and H2 [10]. Compared to other energy carriers or fuels, H2 has characteristics of high energy efficiency, various production and utilization technologies, and clean and environmental-friendly [11]. Furthermore, an in-situ conversion of algae to H2 is believed more feasible due to better transportability of the energy harvested from algae, possibility of material recycling, and better material and fuel handling. However, H2 faces storage problems due to very low volumetric energy and physical density, which are 3 Wh L-H1 and 2 0.084 kg m3, respectively, under STP [12]. It results in the necessity for H2 storage with higher volumetric energy density. Some storage
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technologies are available today including compression, liquefaction, chemical and physical storages [13]. Compressed and liquid H2 are currently considered as established technologies. Unfortunately, compressed H2 has limitation on its storage capacity which is unsuitable for large scale storage [14]. In addition, although liquid H2 has some advantages of energy density, it faces some problems including intensive energy consumption and boil-off losses during liquefaction and storage, respectively. Boil-off is mainly affected by structure conversion of ortho- and para-H2, thermal leaks, sloshing and flashing [15]. Recently, among the available chemical H2 storages, NH3 received an intensive attention. It has characteristics of high energy density (about 22.5 MJ kg1), wide range of application, no generation of CO2, and can be liquefied under relatively low pressure [16]. In addition, utilization of NH3 as fuel for fuel cell leads to very clean energy production due to free CO2, SOx and soot [17] leading to the minimization of emitted greenhouse gases. Unfortunately, NH3 production leaves a big problem as its production consumes energy intensively [18]. Therefore, its reduction becomes very indispensable. Reviewing the literature related to H2 and/or NH3 production from algae, to the best of authors' knowledge by using journal database of Scopus and Web of Science, almost of them are focusing on the algal conversion technology. No comprehensive studies discussing on the integrated system proposal and intensification can be found, specifically on the effort to maximize the total energy efficiency. Therefore, it becomes very difficult to measure the feasibility of energy production, especially H2/NH3, from algae. Xu et al. [19] investigated H2-rich gas production from algal residue through low temperature gasification using Ni-based alumina catalysts. In addition, some researchers are focusing on hydrothermal gasification (HTG) of algae [20e22] and other biomasses [23] to produce H2-rich syngas with and without catalyst. Furthermore, Durak and Aysu [24] investigated a supercritical liquefaction of algae using several organic solvents, including acetone, ethanol, and isopropanol. Unfortunately, these studies are only focusing on the experiment-based conversion, without proposing and modeling any possible industrial process. Fiori et al. [25] have proposed an integrated system consisting of hydrothermal gasification (HTG), membrane-based H2 separation,
and combustion to supply the required heat for gasification. Although they performed simple heat recovery, their proposed system shows very low total energy efficiency. In addition, Aziz [26] has proposed H2 production from algae integrating HTG, membrane-based H2 separation, hydrogenation, and combined cycle for power generation. The proposed integrated system shows relatively high total energy efficiency, about 60%. However, as the electricity is the main output, the H2 production efficiency is very low which is only about 20%. This study focuses on the design proposal and evaluation of the novel system to produce H2 and further to store it as NH3 efficiently from algae. Process modeling employing the developed enhanced process integration (EPI) and intensification in terms of energy efficiency is performed in this study. The developed system consists of HTG, chemical looping, NH3 synthesis, N2 production, and power generation. 2. System proposal and analysis The novel system proposed in this study combines five continuous process modules: HTG, chemical looping, NH3 synthesis, N2 production, and power generation. Fig. 1 shows the flow diagram outlining the integrated system, including material, heat and electricity flows. In addition, the products of the proposed system are NH3 and electricity. Electricity is mainly used to cover the internal electricity consumption, and in case there is any surplus electricity, it can be sold to the grid for additional revenue. To intensify the heat circulation throughout the integrated system, the principles EPI is adopted in each module, as well as the whole integrated system. EPI is the combination of exergy recovery and process integration technologies [27]. The former focus on the optimum heat circulation in the same module in order to minimize the exergy destruction in the corresponding module. The focus on a single module relates to the possibility of effective exergy elevation and heat coupling due to better material and energy balance. However, as a complete energy recovery cannot be achieved in each module, the unrecoverable energy/heat is utilized and recovered in other modules, leading to a thorough heat recovery throughout the integrated system. EPI has been adopted in several systems and their energy efficiency has been improved significantly, including
Fig. 1. Outline flows of the integrated system for NH3 production from algae.
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2.1. HTG, chemical looping and power generation modules Conversion of algae to gaseous products can be conducted through gasification. In addition, as algae have very high moisture content (up to about 90 wt%), drying becomes a very energy intensive reducing the performance of their energy harvesting [31]. Hence, conventional thermal gasification is considered unfeasible. To resolve this issue, in the last decade, HTG technology has been proposed and evaluated for the materials having high moisture content, including algae [32]. In general, HTG has advantages of high gasification efficiency, high gas yield, and minimum char/tar yields [33,34]. Fig. 2 shows the process flow diagram of integrated HTG, chemical looping and power generation modules. Initially, algae are treated and stirred uniformly creating algal slurry, hence, it can be pumped and fed to the HTG. Then, it is preheated in heat exchanger HX1, especially for its sensible heat, using the remaining heat of syngas and water mixture. In HTG, a fluidized bed reactor is adopted in order to facilitate an effective heat transfer and rapid and uniform conversion [35]. The fluidizing particles (alumina) are inserted inside the reactor in order to improve the heat transfer and temperature distribution across the reactor. On the other hand, the algal slurry is used as the fluidization medium which is fed from the bottom of the bed. In addition, the gasification is conducted under bubbling fluidization regimes to achieve more uniform heat
transfer and conversion [35]. The pressure drop across the fluidized bed is approximated with the following Ergun equation as it shows relatively small deviation with the experimental values [36].
Dp L
¼ 150
2 m f Uf ð1 εÞ2 1 ε rf U f þ 1:75 2 4p dp ε3 ε3 4p dp
(1)
where L, ε, m, and U are fluidization height, voidage, viscosity, and superficial fluid velocity. In addition, 4, d and r represent sphericity, average diameter, and density of the used fluidizing particles, respectively. Furthermore, subscripts of f and p represents fluidization medium and particle, correspondingly. The minimum bubbling velocity, Umb, is calculated based on its correlation with the Reynolds number, Remb. In addition, Remb is calculated based on the work of Wei and Lu [36] for fluidization of supercritical water.
Umb ¼
Remb mf rf dp
(2)
Remb ¼ 2 108 Ar 2 9 108 Ar þ 1:4608
Ar ¼
d3p rg rp rf g
(3)
(4)
m2f
where Ar and g are Archimedes number and acceleration due to gravity, respectively. As the fluidization is conducted in bubbling fluidization regimes, the fluidization velocity (U) must be lower than the terminal fluidization velocity (Ut), which can be calculated as [37]:
8 > > g rp rf d2p 18mf Ret < 2 > > > > > < 2 1 Ut ¼ 4g2 rp rf d3p 225mf rf 3 2 < Ret < 500 > > > > > > > : 3:03g rp rf dp rf 500 < Ret =
coal drying [28], algae-based H2 production [29], and power generation from industrial wastes [30]. In general, the harvested algae are treated initially and converted to syngas through HTG. The produced syngas is further fed to the chemical looping module for conversion to H2, by-producing CO2. Produced H2 is then reacted in NH3 synthesis module with the N2 produced from N2 production module. The produced NH3 is expanded and condensed, before being transported to the demand side. As chemical looping also produces heat, it is recovered by combined cycle module for power generation. In addition, the purged gas from NH3 synthesis module is also utilized as fuel, which is further mixed and utilized in chemical looping module. O2-rich gas, which is the by-product during N2 production, is fed to the chemical looping module for combustion reactant.
333
Fig. 2. Schematic process flow diagram of HTG, chemical looping and power generation modules.
(5)
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HTG is a very complex process consisting of many chemical reactions. However, there are basically two main reactions during gas production [38] as follows: CHxOy þ (2 - y) H2O / CO2 þ (2- y þ 0.5 x) H2
(6)
CHxOy þ (1 - y) H2O / CO þ (2- y þ 0.5 x) H2
(7)
where x and y are H/C and O/C molar ratios of algae, respectively. In chemical looping module, three circulated reactors are employed: reducer (RDC), oxidizer (OXD), and combustor (CMB1). The heat is basically circulated among the reactors [39], while the rest of heat is recovered for power generation or utilized in other modules. Moving bed reactor is adopted for both RDC and OXD, while entrained bed is applied for CMB. In addition, to facilitate indirect contact between syngas and air, iron oxides (Fe2O3, FeO, Fe3O4) are employed as oxygen carrier circulated among the reactors. Initially, produced syngas from HTG module flows to the RDC from the bottom of reactor and reacts with Fe2O3 producing mainly CO2 and steam. On the other hand, Fe2O3 is then reduced to Fe and FeO. The reaction occurs in RDC can be simplified as follow based on the molar fraction of each component. aFe2O3 þ bCO þ cH2 þ dCH4 þ eC2H4 þ fC2H6 /gFeO þ hFe þ iCO2 þ jH2O
(8)
The produced CO2 and steam flow out from the top of RDC, which further go to the condensation (CD2) for separation. On the other hand, the reduced Fe and FeO particles are flowing down from the bottom of RDC due to gravitation and going to OXD. In OXD, steam is injected from the bottom as reactant to oxidize both Fe and FeO particles. As the production of oxidation, H2 is generated
and exhausted from the top of OXD together with the excess of steam. Like the mixture of CO2 and steam exhausted from RDC, this H2 and steam mixture also finally goes to condenser (CD2) for separation producing highly pure H2. On the other hand, both Fe and FeO are oxidized producing Fe3O4, which is going to CMB. The reaction in OXD can be approximated as follow:
mFe þ nFeO þ oH2 O/pFe3 O4 þqH2
(9)
In CMB, the fed Fe3O4 and heat carrier are immediately blown and entrained up to the top of the CMB reactor. Combustion occurs in a very short of time and Fe3O4 is oxidized to Fe2O3, which is further circulated to RDC. As a fluidizing medium and reactant, O2rich air which is by-produced in N2 production module is used.
xFe3 O4 þyO2 /zFe2 O3
(10)
As the reaction is exothermic, the exhausted gas from both OXD and CMB is in high temperature, which is possible to be recovered for superheating the produced syngas and water exhausted from HTG. In addition, as chemical looping is operated in relatively high pressure, the compression work of the stream exhausted from each reactor is recovered as much as possible using the expanders (EX2 and EX3). Therefore, higher energy recovery can be achieved. 2.2. N2 production and NH3 synthesis modules Fig. 3 shows the process flow diagram of N2 production and NH3 synthesis modules. Air is compressed through a multistage compression and cooled down in between for lower energy consumption. The compressed air is split into two streams, which are fed to the cryogenic separation column from different stages (middle and bottom). A single cryogenic separation column is
Fig. 3. Schematic process flow diagram of N2 production and NH3 synthesis modules.
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employed. To realize high energy efficiency, as well as minimize the exergy destruction in the system, the cryogenic system is basically conducted based on the principle of self-heat exchange. It is important to note that N2 has a lower boiling temperature than O2 under the same temperature and pressure. Therefore, N2 production is considered easier and energy-efficient than O2 production. During NH3 synthesis, the following reaction occurs:
N2 þ 3H2 /2NH3
(11)
The produced N2 is then fed to the Haber-Bosch (HB) module, together with the produced H2 from chemical looping module, for NH3 production. They are compressed to relatively high pressure of about 15 MPa. As the conversion per pass during NH3 synthesis is only about 15e25%, HB module is operated in a loop mode. The produced NH3 is separated via condensation (CD5), while the remaining gas and unreacted H2 and N2 are basically recycled. However, to avoid any build-up of impurities, including Ar, a part of the recycled gas is purged, which is further utilized as the additional fuel for the combustion in CMB (chemical looping module).
Table 2 Selected HTG parameters and the produced syngas composition (adapted from Ref. [40]). Temp. ( C) Mass ratio Pressure (MPa) Gas composition (mol kg1)
380 420 460 460 460
HHV ¼ 0:01 ð33:5 C þ 142:3 H 15:4 o 14:5 NÞ (12) Furthermore, lower heating value (LHV, MJ kg1) is obtained using the above calculated HHV (Eq. (12)) subtracted by the latent heat of water in the algae. HTG is conducted by following the experimental work performed by Safari et al. [40]. In addition, the compositions of the produced syngas during in HTG are listed in Table 2. In HTG, the effects of gasification temperature and mass ratio to the total energy efficiency are evaluated. The gasification is assumed to be performed under middle range of gasification temperatures of 380e460 C, as there is no significant effect of temperature during gasification above 460 C [38]. In addition, higher temperature also leads to higher energy input during gasification which potentially results in lower total energy efficiency. The mass ratio refers to the weight ratio of dried algae to the total water, including the inherent moisture inside the algae and additional water fed to HTG reactor. The algae are initially mixed and stirred to create an algal slurry
Table 1 Properties of Cladophora glomerata (adapted from Ref. [41]). Properties Proximate analysis Moisture (wt% wb) Volatile matter (wt% wb) Fixed carbon (wt% wb) Ash (wt% wb) Ultimate analysis Carbon (wt% db) Hydrogen (wt% db) Nitrogen (wt% db) Sulfur (wt% db) Oxygen (wt% db)
Value
Ref./Note
0.01 0.01 0.01 0.02 0.03
23.2 25.3 26.8 27.8 28.4
hNH3 ¼
43.5 9.2 0.8 1.6 38.5
CO2
CH4
H2
C2H4 C2H6
0.88 1.12 1.47 1.2 1.01
6.2 7.23 8.22 5.53 4.08
3.17 2.61 2.15 1.82 1.65
3.2 4.23 5.25 4.28 2.07
2.2 1.65 1.05 0.86 0.44
1.76 1.3 0.82 0.64 0.32
mNH3 LHVNH3 malgae LHValgae
hpower ¼
(13)
Wnet 3600 malgae LHValgae
(14)
htot ¼ hNH3 þ hpower
(15)
4. Results and discussion 4.1. Effect of HTG temperature Temperature significantly effects the decomposition and conversion of the algae during HTG. Fig. 4 shows the effect of HTG temperature on the produced H2 and NH3, total energy efficiency, and produced surplus electricity. As shown in Fig. 4(a) and (b), the produced H2 and NH3 decrease following the increase of HTG Table 3 Assumed conditions of chemical looping and power generation. Properties HTG module Temperature ( C) Fluidization velocity U/Umf Fluidizing particle Average particle diameter (mm) Density (kg m3) Sphericity Voidage Chemical looping module Operating pressure (MPa)
Normalized to 90% By difference 90 7.6 1.8 0.7
CO
before being pumped and fed to the gasifier. Table 3 lists the conditions in HTG and chemical looping modules. In addition, Table 4 represents the operating conditions during N2 production and NH3 synthesis. Wustite (Fe1-xO) is adopted as the catalyst inserted in the NH3 synthesis reactor. To measure the performance of the proposed system, the energy efficiency is categorized into NH3 conversion efficiency (hNH3), power generation efficiency (hpower), and total energy efficiency (htot).
3. Analysis Modeling of the system and its evaluation are conducted using a steady-state process simulator of SimSci Pro/II Process Engineering (Schneider Electric). Macroalga of Cladophora glomerata (Chlorophyta) is used as the sample with a flowrate of 100 t h1 and has moisture content of about 90 wt% wb. Table 1 shows the detailed properties of Cladophora glomerata used in this study. The higher heating value (HHV, MJ kg1) of raw algae is calculated using the following Dulong-based equation [38]:
335
Reducer temperature ( C) Estimated oxidation temperature ( C) Max. combustion temperature ( C) Circulated oxygen carrier entering RDC Average particle diameter (mm) Expander polytropic efficiency (%) Turbine max. inlet pressure (MPa) Turbine max. inlet temperature ( C)
Value
Ref./ Note [34,40]
380, 420, 460 2 Alumina 0.3 3400 0.67 0.5 [41e45] 3 (for RDC and OXD), 3.1 (for CMB) 800 1000 1100 Fe2O3 (75 wt%); Al2O3 (25 wt%) 0.3 90 3.1 1300
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Table 4 Conditions of N2 production, NH3 synthesis, and other general conditions. Properties N2 production Column stages Bottom pressure (kPa) Column pressure drop (kPa) Bottom feed ratio (%) Refed stream ratio (%) Feeding stages NH3 synthesis Operating pressure (MPa) Operating temperature ( C) Catalyst Conversion rate per pass (%) Purged stream ratio (%) General conditions HX min. temperature approach ( C) HX pressure drop (%) Pump and compressor polytropic efficiency (%) Ambient temperature ( C) Ambient pressure (kPa) Air composition (dry mol%)
Value
Ref./ Note [46,47]
48 560 20 70 70 20 and 47 [48] 15 450 Fe1-xO 20 20 15 1 87 20 101.33 N2: 58.11; O2: 20.96%; Ar: 0.93
temperature. Numerically, at HTG temperature of 380 C, the amounts of produced H2 and NH3 are about 758 and 2117 kg h1, respectively. They decrease to about 490 and 1368 kg h1, respectively. Fig. 4(c) shows the total energy efficiency, including both NH3 production and power generation. Similar to the produced H2 and NH3, the total energy efficiency also decreases following the increase of HTG temperature. The highest total energy efficiency can
be achieved in case of HTG temperature of 380 C, which is about 38%. However, as shown in Fig. 4(d), the power generation efficiency is very low (almost no surplus power) in case of lower HTG temperature (380 C). The amount of surplus electricity increases following the increase of HTG temperature. According to Table 2, the amount of H2 in the produced syngas from HTG is increasing following the increase of HTG temperature. However, the total calorific value of syngas is decreasing as the HTG temperature increases (under the same mass ratio of 0.01). In chemical looping, especially in RDC, all the components of syngas are basically oxidized with the oxygen brought by the oxygen carrier (reduction of oxygen carrier). Therefore, higher calorific value of syngas basically leads to the larger amount of reduced oxygen carriers (Fe and FeO) during reduction. The amount of Fe and FeO influence strongly the reactions occur in the OXD. Higher amount of Fe and FeO reacts with larger amount of steam, leading to larger amount of H2. Finally, the produced NH3 also increases accordingly.
4.2. Effect of HTG mass ratio Fig. 5 shows the effect of mass ratio of the fed algae on the produced H2 and NH3, total energy efficiency, and produced surplus electricity. As mass ratio refers to the amount of water which is mixed together with the algae, it also gives significant impacts to the system, especially related to the syngas composition and several duties, including compressor and pumps. As shown in Fig. 5(a) and (b), the amounts of produced H2 and NH3 decrease following the increase of mass ratio. Mass ratio of 0.01 is able to produce the highest amount of H2 and NH3, as well as total energy efficiency and surplus electricity. In case of mass ratio of 0.03, the generated electricity cannot cover the internallyconsumed electricity in the system, therefore, in this case, the
Fig. 4. Effect of HTG temperature (mass ratio of 0.01).
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337
Fig. 5. Effect of mass ratio (HTG temperature of 460 ).
system must be additionally supplied by any power generation from outside (cannot independently operate). Similar to the effect of HTG temperature, high mass ratio leads to lower calorific value of produced syngas from HTG. Therefore, the produced H2 and NH3 decrease accordingly. Although higher mass ratio means lower power consumed for pumping of the algal slurry, as the calorific value of the produced syngas decreases significantly, the total energy system drops significantly. Based on the above results and analysis, HTG temperature of 380 C and algae-to-water mass ratio of 0.01 are considered as the optimum condition leading to relatively high total energy efficiency, which is about 38%. This energy efficiency is considered high because the proposed system needs no further dewatering which is very energy intensive as algae have very high moisture content. To further improve the efficiency, the utilization of catalyst, including hydrochar [40], can be adopted.
mass ratio of 0.01 (considered as the optimum conditions). From process modeling and calculation, the proposed integrated-system shows relatively high total energy efficiency of about 38%, including both NH3 and power production. Higher HTG temperature leads to the decrease of total energy efficiency as well as H2 and NH3 production, although the surplus power increases slightly. The proposed system is considered feasible, especially in terms of energy efficiency, and no dewatering technology is demanded.
5. Conclusion
This work was supported by JSPS KAKENHI Grant Number 16K18355.
A state-of-art integrated system to harvest the energy of algae and convert it to NH3 has been modeled and evaluated. The proposed system consists of HTG for algae, chemical looping for H2 production and CO2 separation, N2 production, NH3 synthesis, and power generation. To realize high energy-efficiency, an enhanced process integration, consisting of exergy recovery and process integration. Hence, the energy/heat involved in each module and the whole system can be efficiently recirculated and recovered. Macro alga of Cladophora glomerata (Chlorophyta) is used as the sample in the study. The effects of temperature and algae-to-water mass ratio during HTG are evaluated in terms of their influence to the total energy efficiency. From process modeling and calculation using SimSci Pro/II, the proposed integrated-system shows relatively high total energy efficiency of about 38%, including both NH3 and power production, achieved at HTG temperature of 380 C and
Author contribution M.A. developed the idea, performed modeling and simulation, and prepared the manuscript. A.T.W. verified the data and checked the manuscript. Acknowledgments
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Ammonia production from algae via integrated hydrothermal gasification, chemical looping, N2 production, and NH3 synthesis Agung Tri Wijayanta a, Muhammad Aziz b, * a b
Department of Mechanical Engineering, Universitas Sebelas Maret, Kampus UNS Kentingan, Jl. Ir. Sutami 36A Kentingan, Surakarta 57126, Indonesia Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550 Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2018 Received in revised form 22 February 2019 Accepted 27 February 2019 Available online 28 February 2019
Novel integrated system to convert algae to NH3 is proposed with the objective of effective and thorough energy/heat circulation to achieve high total energy efficiency. The integrated system mainly consists of hydrothermal gasification (HTG), chemical looping, N2 production, NH3 synthesis, and power generation. Algae are converted initially to syngas through HTG, which is further converted to CO2 and H2 in chemical looping module. The produced H2 from chemical looping module is reacted with the produced highly-pure N2 from N2 production module to form NH3 in NH3 synthesis module. To realize high energyefficiency, an enhanced process integration, which simultaneously integrates both exergy recovery and process integration technologies, is applied. Therefore, the energy/heat involved in the integrated system is recirculated thoroughly and used partly for power generation. Macro alga of Cladophora glomerata (Chlorophyta) is used as the sample in the study. The effects of temperature and algae-to-water mass ratio during HTG are evaluated in terms of their influence to the total energy efficiency. From process modeling and calculation using SimSci Pro/II, the proposed integrated-system shows relatively high total energy efficiency of about 38%, including both NH3 and power production, achieved at HTG temperature of 380 C and mass ratio of 0.01. © 2019 Published by Elsevier Ltd.
Keywords: Algae Ammonia Hydrothermal gasification Chemical looping Energy efficiency Integrated system
1. Introduction Algae have received an intensive attention in today's industry covering energy source, chemicals, food and pharmacies. They have very large number of varieties and are distributed widely in aqueous environment including ocean, freshwater and marshland [1,2]. Compared to other biomasses, algae have higher growth rate, higher lipid content, and higher capability to live in wide range of environmental conditions [3]. Algae are able to convert sunlight to chemical energy with efficiency of near to 10% [4] and absorb CO2 at average rate of 36.7 t ha1 y1 [5]. Therefore, cultivation of algae is a very potential way for CO2 mitigation (bio-sequestration) [6]. The cultivation of algae near to CO2 producing sources, such as power plants, is considered as attractive strategy. Unfortunately, algae utilization still faces various obstacles during their cultivation, processing and utilization. In addition, algae have high moisture content ranging from 60 to 90 wt% on wet basis (wb) demanding extraordinary treatments during their utilization [7].
* Corresponding author. E-mail address: [email protected] (M. Aziz). https://doi.org/10.1016/j.energy.2019.02.190 0360-5442/© 2019 Published by Elsevier Ltd.
Focusing on the energy production from algae, in general, two routes of biochemical and thermochemical mechanisms are possible. Comparing both of them, thermochemical conversion seems more advantageous, due to faster conversion and higher carbon-conversion efficiency [8]. Dou et al. [9] have summarized well several issues related to thermochemical conversion of biomass to H2. As energy source, algae must be sufficiently competitive with other energy sources, even with fossil fuel. Therefore, an efficient energy production system from algae is considered urgent to be developed. Recently, H2 production from algae gains significant attention over last decades as it potentially combines the beneficial characteristics of both algae and H2 [10]. Compared to other energy carriers or fuels, H2 has characteristics of high energy efficiency, various production and utilization technologies, and clean and environmental-friendly [11]. Furthermore, an in-situ conversion of algae to H2 is believed more feasible due to better transportability of the energy harvested from algae, possibility of material recycling, and better material and fuel handling. However, H2 faces storage problems due to very low volumetric energy and physical density, which are 3 Wh L-H1 and 2 0.084 kg m3, respectively, under STP [12]. It results in the necessity for H2 storage with higher volumetric energy density. Some storage
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technologies are available today including compression, liquefaction, chemical and physical storages [13]. Compressed and liquid H2 are currently considered as established technologies. Unfortunately, compressed H2 has limitation on its storage capacity which is unsuitable for large scale storage [14]. In addition, although liquid H2 has some advantages of energy density, it faces some problems including intensive energy consumption and boil-off losses during liquefaction and storage, respectively. Boil-off is mainly affected by structure conversion of ortho- and para-H2, thermal leaks, sloshing and flashing [15]. Recently, among the available chemical H2 storages, NH3 received an intensive attention. It has characteristics of high energy density (about 22.5 MJ kg1), wide range of application, no generation of CO2, and can be liquefied under relatively low pressure [16]. In addition, utilization of NH3 as fuel for fuel cell leads to very clean energy production due to free CO2, SOx and soot [17] leading to the minimization of emitted greenhouse gases. Unfortunately, NH3 production leaves a big problem as its production consumes energy intensively [18]. Therefore, its reduction becomes very indispensable. Reviewing the literature related to H2 and/or NH3 production from algae, to the best of authors' knowledge by using journal database of Scopus and Web of Science, almost of them are focusing on the algal conversion technology. No comprehensive studies discussing on the integrated system proposal and intensification can be found, specifically on the effort to maximize the total energy efficiency. Therefore, it becomes very difficult to measure the feasibility of energy production, especially H2/NH3, from algae. Xu et al. [19] investigated H2-rich gas production from algal residue through low temperature gasification using Ni-based alumina catalysts. In addition, some researchers are focusing on hydrothermal gasification (HTG) of algae [20e22] and other biomasses [23] to produce H2-rich syngas with and without catalyst. Furthermore, Durak and Aysu [24] investigated a supercritical liquefaction of algae using several organic solvents, including acetone, ethanol, and isopropanol. Unfortunately, these studies are only focusing on the experiment-based conversion, without proposing and modeling any possible industrial process. Fiori et al. [25] have proposed an integrated system consisting of hydrothermal gasification (HTG), membrane-based H2 separation,
and combustion to supply the required heat for gasification. Although they performed simple heat recovery, their proposed system shows very low total energy efficiency. In addition, Aziz [26] has proposed H2 production from algae integrating HTG, membrane-based H2 separation, hydrogenation, and combined cycle for power generation. The proposed integrated system shows relatively high total energy efficiency, about 60%. However, as the electricity is the main output, the H2 production efficiency is very low which is only about 20%. This study focuses on the design proposal and evaluation of the novel system to produce H2 and further to store it as NH3 efficiently from algae. Process modeling employing the developed enhanced process integration (EPI) and intensification in terms of energy efficiency is performed in this study. The developed system consists of HTG, chemical looping, NH3 synthesis, N2 production, and power generation. 2. System proposal and analysis The novel system proposed in this study combines five continuous process modules: HTG, chemical looping, NH3 synthesis, N2 production, and power generation. Fig. 1 shows the flow diagram outlining the integrated system, including material, heat and electricity flows. In addition, the products of the proposed system are NH3 and electricity. Electricity is mainly used to cover the internal electricity consumption, and in case there is any surplus electricity, it can be sold to the grid for additional revenue. To intensify the heat circulation throughout the integrated system, the principles EPI is adopted in each module, as well as the whole integrated system. EPI is the combination of exergy recovery and process integration technologies [27]. The former focus on the optimum heat circulation in the same module in order to minimize the exergy destruction in the corresponding module. The focus on a single module relates to the possibility of effective exergy elevation and heat coupling due to better material and energy balance. However, as a complete energy recovery cannot be achieved in each module, the unrecoverable energy/heat is utilized and recovered in other modules, leading to a thorough heat recovery throughout the integrated system. EPI has been adopted in several systems and their energy efficiency has been improved significantly, including
Fig. 1. Outline flows of the integrated system for NH3 production from algae.
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2.1. HTG, chemical looping and power generation modules Conversion of algae to gaseous products can be conducted through gasification. In addition, as algae have very high moisture content (up to about 90 wt%), drying becomes a very energy intensive reducing the performance of their energy harvesting [31]. Hence, conventional thermal gasification is considered unfeasible. To resolve this issue, in the last decade, HTG technology has been proposed and evaluated for the materials having high moisture content, including algae [32]. In general, HTG has advantages of high gasification efficiency, high gas yield, and minimum char/tar yields [33,34]. Fig. 2 shows the process flow diagram of integrated HTG, chemical looping and power generation modules. Initially, algae are treated and stirred uniformly creating algal slurry, hence, it can be pumped and fed to the HTG. Then, it is preheated in heat exchanger HX1, especially for its sensible heat, using the remaining heat of syngas and water mixture. In HTG, a fluidized bed reactor is adopted in order to facilitate an effective heat transfer and rapid and uniform conversion [35]. The fluidizing particles (alumina) are inserted inside the reactor in order to improve the heat transfer and temperature distribution across the reactor. On the other hand, the algal slurry is used as the fluidization medium which is fed from the bottom of the bed. In addition, the gasification is conducted under bubbling fluidization regimes to achieve more uniform heat
transfer and conversion [35]. The pressure drop across the fluidized bed is approximated with the following Ergun equation as it shows relatively small deviation with the experimental values [36].
Dp L
¼ 150
2 m f Uf ð1 εÞ2 1 ε rf U f þ 1:75 2 4p dp ε3 ε3 4p dp
(1)
where L, ε, m, and U are fluidization height, voidage, viscosity, and superficial fluid velocity. In addition, 4, d and r represent sphericity, average diameter, and density of the used fluidizing particles, respectively. Furthermore, subscripts of f and p represents fluidization medium and particle, correspondingly. The minimum bubbling velocity, Umb, is calculated based on its correlation with the Reynolds number, Remb. In addition, Remb is calculated based on the work of Wei and Lu [36] for fluidization of supercritical water.
Umb ¼
Remb mf rf dp
(2)
Remb ¼ 2 108 Ar 2 9 108 Ar þ 1:4608
Ar ¼
d3p rg rp rf g
(3)
(4)
m2f
where Ar and g are Archimedes number and acceleration due to gravity, respectively. As the fluidization is conducted in bubbling fluidization regimes, the fluidization velocity (U) must be lower than the terminal fluidization velocity (Ut), which can be calculated as [37]:
8 > > g rp rf d2p 18mf Ret < 2 > > > > > < 2 1 Ut ¼ 4g2 rp rf d3p 225mf rf 3 2 < Ret < 500 > > > > > > > : 3:03g rp rf dp rf 500 < Ret =
coal drying [28], algae-based H2 production [29], and power generation from industrial wastes [30]. In general, the harvested algae are treated initially and converted to syngas through HTG. The produced syngas is further fed to the chemical looping module for conversion to H2, by-producing CO2. Produced H2 is then reacted in NH3 synthesis module with the N2 produced from N2 production module. The produced NH3 is expanded and condensed, before being transported to the demand side. As chemical looping also produces heat, it is recovered by combined cycle module for power generation. In addition, the purged gas from NH3 synthesis module is also utilized as fuel, which is further mixed and utilized in chemical looping module. O2-rich gas, which is the by-product during N2 production, is fed to the chemical looping module for combustion reactant.
333
Fig. 2. Schematic process flow diagram of HTG, chemical looping and power generation modules.
(5)
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HTG is a very complex process consisting of many chemical reactions. However, there are basically two main reactions during gas production [38] as follows: CHxOy þ (2 - y) H2O / CO2 þ (2- y þ 0.5 x) H2
(6)
CHxOy þ (1 - y) H2O / CO þ (2- y þ 0.5 x) H2
(7)
where x and y are H/C and O/C molar ratios of algae, respectively. In chemical looping module, three circulated reactors are employed: reducer (RDC), oxidizer (OXD), and combustor (CMB1). The heat is basically circulated among the reactors [39], while the rest of heat is recovered for power generation or utilized in other modules. Moving bed reactor is adopted for both RDC and OXD, while entrained bed is applied for CMB. In addition, to facilitate indirect contact between syngas and air, iron oxides (Fe2O3, FeO, Fe3O4) are employed as oxygen carrier circulated among the reactors. Initially, produced syngas from HTG module flows to the RDC from the bottom of reactor and reacts with Fe2O3 producing mainly CO2 and steam. On the other hand, Fe2O3 is then reduced to Fe and FeO. The reaction occurs in RDC can be simplified as follow based on the molar fraction of each component. aFe2O3 þ bCO þ cH2 þ dCH4 þ eC2H4 þ fC2H6 /gFeO þ hFe þ iCO2 þ jH2O
(8)
The produced CO2 and steam flow out from the top of RDC, which further go to the condensation (CD2) for separation. On the other hand, the reduced Fe and FeO particles are flowing down from the bottom of RDC due to gravitation and going to OXD. In OXD, steam is injected from the bottom as reactant to oxidize both Fe and FeO particles. As the production of oxidation, H2 is generated
and exhausted from the top of OXD together with the excess of steam. Like the mixture of CO2 and steam exhausted from RDC, this H2 and steam mixture also finally goes to condenser (CD2) for separation producing highly pure H2. On the other hand, both Fe and FeO are oxidized producing Fe3O4, which is going to CMB. The reaction in OXD can be approximated as follow:
mFe þ nFeO þ oH2 O/pFe3 O4 þqH2
(9)
In CMB, the fed Fe3O4 and heat carrier are immediately blown and entrained up to the top of the CMB reactor. Combustion occurs in a very short of time and Fe3O4 is oxidized to Fe2O3, which is further circulated to RDC. As a fluidizing medium and reactant, O2rich air which is by-produced in N2 production module is used.
xFe3 O4 þyO2 /zFe2 O3
(10)
As the reaction is exothermic, the exhausted gas from both OXD and CMB is in high temperature, which is possible to be recovered for superheating the produced syngas and water exhausted from HTG. In addition, as chemical looping is operated in relatively high pressure, the compression work of the stream exhausted from each reactor is recovered as much as possible using the expanders (EX2 and EX3). Therefore, higher energy recovery can be achieved. 2.2. N2 production and NH3 synthesis modules Fig. 3 shows the process flow diagram of N2 production and NH3 synthesis modules. Air is compressed through a multistage compression and cooled down in between for lower energy consumption. The compressed air is split into two streams, which are fed to the cryogenic separation column from different stages (middle and bottom). A single cryogenic separation column is
Fig. 3. Schematic process flow diagram of N2 production and NH3 synthesis modules.
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employed. To realize high energy efficiency, as well as minimize the exergy destruction in the system, the cryogenic system is basically conducted based on the principle of self-heat exchange. It is important to note that N2 has a lower boiling temperature than O2 under the same temperature and pressure. Therefore, N2 production is considered easier and energy-efficient than O2 production. During NH3 synthesis, the following reaction occurs:
N2 þ 3H2 /2NH3
(11)
The produced N2 is then fed to the Haber-Bosch (HB) module, together with the produced H2 from chemical looping module, for NH3 production. They are compressed to relatively high pressure of about 15 MPa. As the conversion per pass during NH3 synthesis is only about 15e25%, HB module is operated in a loop mode. The produced NH3 is separated via condensation (CD5), while the remaining gas and unreacted H2 and N2 are basically recycled. However, to avoid any build-up of impurities, including Ar, a part of the recycled gas is purged, which is further utilized as the additional fuel for the combustion in CMB (chemical looping module).
Table 2 Selected HTG parameters and the produced syngas composition (adapted from Ref. [40]). Temp. ( C) Mass ratio Pressure (MPa) Gas composition (mol kg1)
380 420 460 460 460
HHV ¼ 0:01 ð33:5 C þ 142:3 H 15:4 o 14:5 NÞ (12) Furthermore, lower heating value (LHV, MJ kg1) is obtained using the above calculated HHV (Eq. (12)) subtracted by the latent heat of water in the algae. HTG is conducted by following the experimental work performed by Safari et al. [40]. In addition, the compositions of the produced syngas during in HTG are listed in Table 2. In HTG, the effects of gasification temperature and mass ratio to the total energy efficiency are evaluated. The gasification is assumed to be performed under middle range of gasification temperatures of 380e460 C, as there is no significant effect of temperature during gasification above 460 C [38]. In addition, higher temperature also leads to higher energy input during gasification which potentially results in lower total energy efficiency. The mass ratio refers to the weight ratio of dried algae to the total water, including the inherent moisture inside the algae and additional water fed to HTG reactor. The algae are initially mixed and stirred to create an algal slurry
Table 1 Properties of Cladophora glomerata (adapted from Ref. [41]). Properties Proximate analysis Moisture (wt% wb) Volatile matter (wt% wb) Fixed carbon (wt% wb) Ash (wt% wb) Ultimate analysis Carbon (wt% db) Hydrogen (wt% db) Nitrogen (wt% db) Sulfur (wt% db) Oxygen (wt% db)
Value
Ref./Note
0.01 0.01 0.01 0.02 0.03
23.2 25.3 26.8 27.8 28.4
hNH3 ¼
43.5 9.2 0.8 1.6 38.5
CO2
CH4
H2
C2H4 C2H6
0.88 1.12 1.47 1.2 1.01
6.2 7.23 8.22 5.53 4.08
3.17 2.61 2.15 1.82 1.65
3.2 4.23 5.25 4.28 2.07
2.2 1.65 1.05 0.86 0.44
1.76 1.3 0.82 0.64 0.32
mNH3 LHVNH3 malgae LHValgae
hpower ¼
(13)
Wnet 3600 malgae LHValgae
(14)
htot ¼ hNH3 þ hpower
(15)
4. Results and discussion 4.1. Effect of HTG temperature Temperature significantly effects the decomposition and conversion of the algae during HTG. Fig. 4 shows the effect of HTG temperature on the produced H2 and NH3, total energy efficiency, and produced surplus electricity. As shown in Fig. 4(a) and (b), the produced H2 and NH3 decrease following the increase of HTG Table 3 Assumed conditions of chemical looping and power generation. Properties HTG module Temperature ( C) Fluidization velocity U/Umf Fluidizing particle Average particle diameter (mm) Density (kg m3) Sphericity Voidage Chemical looping module Operating pressure (MPa)
Normalized to 90% By difference 90 7.6 1.8 0.7
CO
before being pumped and fed to the gasifier. Table 3 lists the conditions in HTG and chemical looping modules. In addition, Table 4 represents the operating conditions during N2 production and NH3 synthesis. Wustite (Fe1-xO) is adopted as the catalyst inserted in the NH3 synthesis reactor. To measure the performance of the proposed system, the energy efficiency is categorized into NH3 conversion efficiency (hNH3), power generation efficiency (hpower), and total energy efficiency (htot).
3. Analysis Modeling of the system and its evaluation are conducted using a steady-state process simulator of SimSci Pro/II Process Engineering (Schneider Electric). Macroalga of Cladophora glomerata (Chlorophyta) is used as the sample with a flowrate of 100 t h1 and has moisture content of about 90 wt% wb. Table 1 shows the detailed properties of Cladophora glomerata used in this study. The higher heating value (HHV, MJ kg1) of raw algae is calculated using the following Dulong-based equation [38]:
335
Reducer temperature ( C) Estimated oxidation temperature ( C) Max. combustion temperature ( C) Circulated oxygen carrier entering RDC Average particle diameter (mm) Expander polytropic efficiency (%) Turbine max. inlet pressure (MPa) Turbine max. inlet temperature ( C)
Value
Ref./ Note [34,40]
380, 420, 460 2 Alumina 0.3 3400 0.67 0.5 [41e45] 3 (for RDC and OXD), 3.1 (for CMB) 800 1000 1100 Fe2O3 (75 wt%); Al2O3 (25 wt%) 0.3 90 3.1 1300
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Table 4 Conditions of N2 production, NH3 synthesis, and other general conditions. Properties N2 production Column stages Bottom pressure (kPa) Column pressure drop (kPa) Bottom feed ratio (%) Refed stream ratio (%) Feeding stages NH3 synthesis Operating pressure (MPa) Operating temperature ( C) Catalyst Conversion rate per pass (%) Purged stream ratio (%) General conditions HX min. temperature approach ( C) HX pressure drop (%) Pump and compressor polytropic efficiency (%) Ambient temperature ( C) Ambient pressure (kPa) Air composition (dry mol%)
Value
Ref./ Note [46,47]
48 560 20 70 70 20 and 47 [48] 15 450 Fe1-xO 20 20 15 1 87 20 101.33 N2: 58.11; O2: 20.96%; Ar: 0.93
temperature. Numerically, at HTG temperature of 380 C, the amounts of produced H2 and NH3 are about 758 and 2117 kg h1, respectively. They decrease to about 490 and 1368 kg h1, respectively. Fig. 4(c) shows the total energy efficiency, including both NH3 production and power generation. Similar to the produced H2 and NH3, the total energy efficiency also decreases following the increase of HTG temperature. The highest total energy efficiency can
be achieved in case of HTG temperature of 380 C, which is about 38%. However, as shown in Fig. 4(d), the power generation efficiency is very low (almost no surplus power) in case of lower HTG temperature (380 C). The amount of surplus electricity increases following the increase of HTG temperature. According to Table 2, the amount of H2 in the produced syngas from HTG is increasing following the increase of HTG temperature. However, the total calorific value of syngas is decreasing as the HTG temperature increases (under the same mass ratio of 0.01). In chemical looping, especially in RDC, all the components of syngas are basically oxidized with the oxygen brought by the oxygen carrier (reduction of oxygen carrier). Therefore, higher calorific value of syngas basically leads to the larger amount of reduced oxygen carriers (Fe and FeO) during reduction. The amount of Fe and FeO influence strongly the reactions occur in the OXD. Higher amount of Fe and FeO reacts with larger amount of steam, leading to larger amount of H2. Finally, the produced NH3 also increases accordingly.
4.2. Effect of HTG mass ratio Fig. 5 shows the effect of mass ratio of the fed algae on the produced H2 and NH3, total energy efficiency, and produced surplus electricity. As mass ratio refers to the amount of water which is mixed together with the algae, it also gives significant impacts to the system, especially related to the syngas composition and several duties, including compressor and pumps. As shown in Fig. 5(a) and (b), the amounts of produced H2 and NH3 decrease following the increase of mass ratio. Mass ratio of 0.01 is able to produce the highest amount of H2 and NH3, as well as total energy efficiency and surplus electricity. In case of mass ratio of 0.03, the generated electricity cannot cover the internallyconsumed electricity in the system, therefore, in this case, the
Fig. 4. Effect of HTG temperature (mass ratio of 0.01).
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Fig. 5. Effect of mass ratio (HTG temperature of 460 ).
system must be additionally supplied by any power generation from outside (cannot independently operate). Similar to the effect of HTG temperature, high mass ratio leads to lower calorific value of produced syngas from HTG. Therefore, the produced H2 and NH3 decrease accordingly. Although higher mass ratio means lower power consumed for pumping of the algal slurry, as the calorific value of the produced syngas decreases significantly, the total energy system drops significantly. Based on the above results and analysis, HTG temperature of 380 C and algae-to-water mass ratio of 0.01 are considered as the optimum condition leading to relatively high total energy efficiency, which is about 38%. This energy efficiency is considered high because the proposed system needs no further dewatering which is very energy intensive as algae have very high moisture content. To further improve the efficiency, the utilization of catalyst, including hydrochar [40], can be adopted.
mass ratio of 0.01 (considered as the optimum conditions). From process modeling and calculation, the proposed integrated-system shows relatively high total energy efficiency of about 38%, including both NH3 and power production. Higher HTG temperature leads to the decrease of total energy efficiency as well as H2 and NH3 production, although the surplus power increases slightly. The proposed system is considered feasible, especially in terms of energy efficiency, and no dewatering technology is demanded.
5. Conclusion
This work was supported by JSPS KAKENHI Grant Number 16K18355.
A state-of-art integrated system to harvest the energy of algae and convert it to NH3 has been modeled and evaluated. The proposed system consists of HTG for algae, chemical looping for H2 production and CO2 separation, N2 production, NH3 synthesis, and power generation. To realize high energy-efficiency, an enhanced process integration, consisting of exergy recovery and process integration. Hence, the energy/heat involved in each module and the whole system can be efficiently recirculated and recovered. Macro alga of Cladophora glomerata (Chlorophyta) is used as the sample in the study. The effects of temperature and algae-to-water mass ratio during HTG are evaluated in terms of their influence to the total energy efficiency. From process modeling and calculation using SimSci Pro/II, the proposed integrated-system shows relatively high total energy efficiency of about 38%, including both NH3 and power production, achieved at HTG temperature of 380 C and
Author contribution M.A. developed the idea, performed modeling and simulation, and prepared the manuscript. A.T.W. verified the data and checked the manuscript. Acknowledgments
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