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Integrated hydrogen production and power generation from microalgae
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e9
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Integrated hydrogen production and power generation from microalgae Muhammad Aziz* Solutions Research Laboratory, Tokyo Institute of Technology, 2-12-1-I6-25 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
article info
abstract
Article history:
An integrated system for hydrogen production, storage, and power generation from
Received 30 June 2015
microalgae using enhanced process integration technology is proposed. Enhanced process
Received in revised form
integration has two core technologies: exergy recovery and process integration. Exergy
20 October 2015
recovery is performed through exergy elevation and heat coupling to minimize exergy
Accepted 27 October 2015
destruction. The unrecoverable energy/heat in a single process is recovered and used in
Available online xxx
other processes through process integration. The proposed integrated system includes supercritical water gasification, hydrogen separation, hydrogenation, and a combined
Keywords:
cycle. The microalga Chlorella vulgaris is used for modeling and evaluation. Microalgae are
Microalgae
converted to syngas, then separated to produce highly pure hydrogen. To store the pro-
Hydrogen
duced hydrogen, the tolueneemethylcyclohexane cycle as a liquid organic hydrogen car-
Exergy recovery
rier is adopted. The remaining gas is used as fuel for combustion in the combined cycle to
Process integration
generate electricity. The effects of the fluidization velocity and gasification pressure on
Hydrogenation
energy efficiency are evaluated. From process modeling and calculation, high total energy
Energy efficiency
efficiency (higher than 60%), including electricity generation efficiency of about 40%, can be realized. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microalgae are a high-potential source of biomass for the production of food, industrial materials, pharmaceuticals, and energy [1]. Microalgae have very high photosynthetic fixation capability for CO2 that can be utilized in generating various algal cell components, energy, and molecular oxygen [2]. Furthermore, microalgae have characteristics superior to those of terrestrial biomasses including a higher growth rate, more efficient solar energy conversion, higher nutrient acquisition, and the ability to grow under severe conditions
[3]. Microalgae are rich in lipids and can thus be potentially used as an energy source and converted to different biofuels including bio-hydrogen, bio-diesel, biogas, and bio-oil [4]. The heating value of microalgae that are grown under optimum conditions usually ranges from 17 to 23 GJ per ton of dried microalgae [5]. As microalgae grow in an aqueous environment, they are generally cultivated remotely, possibly a far distance from their demand sites. Hence, the fuel produced from microalgae needs to be stored and transported. Among the secondary energy resources, hydrogen is highly versatile and efficient, has a wide variety of production and utilization technologies,
* Corresponding author. Tel.: þ81 3 5734 3809; fax: þ81 3 5734 3559. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2015.10.115 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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and is clean [6]. Hydrogen is widely used and its use is increasing. It is used in reciprocating combustion engines, combustion in a combined cycle, and fuel cells. It is also mixed with other fuels. Furthermore, water is produced during hydrogen oxidation, thus leading to clean energy utilization. Under ambient conditions, hydrogen has a relatively high energy density by weight of 33 kWh kg$H1 2 . Unfortunately, the energy density of hydrogen by volume, 3 Wh per liter of gaseous H2, is very low compared with that of other hydrocarbons. This leads to difficulty in storing and transporting the produced hydrogen. Recently, storage and transportation methods for hydrogen, including compression, liquefaction, and chemical and physical storage have been developed and applied. Among them, compressed and liquid hydrogen are considered established forms for storage and transport. However, these methods require high energy consumption and have a relatively low safety level. Hydrogen storage utilizing a liquid organic hydrogen carrier (LOHC) is considered promising because of its high safety level, high storage capacity, excellent reversibility, longer storage time, and lower CO2 emission [7e9]. Ammonia is another promising liquid hydrogen carrier, but unfortunately, it is toxic and corrosive, has a strong odor, and must be carried by a specially designed tanker. In the LOHC, the hydrogen is covalently bonded through hydrogenation. When the hydrogen is needed, it can be released from the LOHC through dehydrogenation. Available LOHC cycles include cyclohexaneebenzene, decalinenaphthalene, and tolueneemethylcyclohexane (MCH) cycles. In this study, a toluene (C7H8)eMCH (C7H14) cycle is adopted to store and transport hydrogen because toluene and MCH are cheap, stable, and easy to transport. In addition, both toluene and MCH exist as a liquid over a wide temperature range, which is favorable for long-term storage. Additionally, their applicability has been demonstrated in a large-scale test conducted by Chiyoda Corporation in Japan [10]. Microalgae can be biochemically and thermochemically converted to hydrogen. Unfortunately, biochemical conversion, including fermentation, has a slower conversion rate and lower conversion efficiency than thermochemical conversion. Hence, for large-scale energy conversion, thermochemical conversion, including gasification and pyrolysis, is generally adopted. Among forms of thermochemical conversion, gasification is considered to have the highest conversion efficiency [11]. Two gasification methods are currently available: conventional thermal gasification and supercritical water gasification (SCWG). In the former, the harvested microalgae need to be dried to quite a low moisture content to achieve a stable conversion with high efficiency. Unfortunately, the moisture content of microalgae is very high, ranging from 70 to 90 wt% on a wet basis (wb), leading to huge energy consumption for drying [12]. In contrast, supercritical water gasification can be performed without drying because gasification is performed under an aqueous state. However, the energy demand required to bring the microalgae to a supercritical condition is high, resulting in lower total energy efficiency. Hence, a novel system that resolves this problem is urgently required. To the author's knowledge, there has been no study focusing on the effort to effectively integrate the conversion of
microalgae to hydrogen and storage of the produced hydrogen. Many studies focusing on the application of SCWG to microalgae did not pay further attention to system development [13e15]. Furthermore, some researchers studied the production of hydrogen through SCWG using biomass including microalgae [16,17]. Fiori et al. [18] and Haiduc et al. [19] proposed a design for hydrogen and methane production from biomass including microalgae with SCWG. Unfortunately, no notable effort was made to realize energy/heat recirculation in their proposed systems; hence, what was still large exergy destruction was generated. In addition, the systems included no storage process for the produced hydrogen. In the present study, a novel integrated system consisting of gasification, hydrogen separation, hydrogenation, and a combined cycle is proposed on the basis of enhanced process integration (EPI). EPI is a technology that combines exergy recovery and process integration with the purpose of minimizing the exergy destruction throughout the system. Therefore, high energy efficiency can be achieved.
Integrated system for hydrogen production, storage, and power generation To minimize the exergy destruction throughout the system, the concept of EPI has been developed and applied to several materials including coal and biomasses [20e23]. EPI consists of two core technologies: exergy recovery and process integration. The former relates to the idea of heat circulation throughout a single process. Fig. 1 shows the concept of exergy recovery and two possible methods of exergy elevation. The dotted and solid lines represent streams with a high and low exergy rate, respectively. To realize the proposed exergy recovery, exergy elevation of the stream and heat coupling among the streams are performed (Fig. 1(a)). In exergy elevation, the exergy rate of the cold stream is raised, creating a hot stream by means of compression and heat combination (Fig. 1(b)). The stream is then used as the heat source and is paired with the cold stream through heat coupling (self-heat exchange). In heat coupling, the heat of the hot stream and the heat of the cold stream exchanged. Additionally, considerations are made of the heat type, heat amount and exergy rate to achieve an optimum balanced heat exchange. It is thus important to note that the idea of exergy recovery is different from that of conventional pinch or heat recovery technologies, which are essentially based on heat cascade utilization. Although heat recovery throughout a single process has been optimized through exergy recovery, there is still unrecoverable energy/heat relating to the minimum temperature approach during heat exchange, the imbalance of heat following a change in stream properties, and heat loss. To minimize the amount of unrecoverable energy/heat, a process integration is introduced where the unrecoverable energy/ heat from any process is used in other processes. As a result, exergy throughout the integrated systems can be further minimized, leading to high total energy efficiency. Fig. 2 shows a diagram of the basic schematic material and energy flows of the proposed integrated system. Solid and dotted lines represent material and energy flows, respectively.
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Fig. 1 e Concept of exergy recovery adopted in this study.
The system consists of continuous processes, namely SCWG, hydrogen separation, hydrogenation, and the combined cycle. The harvested microalgal biomass is pretreated initially to create an algae slurry that then flows to the SCWG reactor for conversion. Furthermore, in the SCWG reactor, microalgae are converted to syngas that mainly contains H2, CO, CH4, and CO2. The produced syngas then undergoes hydrogen separation to produce hydrogen with high purity. The separated hydrogen is then covalently bonded to toluene in hydrogenation to produce MCH, which is ready to be stored and transported. Meanwhile, the gas remaining from separation is used as fuel for combustion in the combined cycle to generate electricity. Part of the heat from the combined cycle is used to elevate the exergy rate in the SCWG module through heat combination. In addition, the flue gas from the combined cycle is recycled back to the cultivation as nutrients for algae to grow, leading to CO2 bio-sequestration. In this study, the syngas produced from SCWG, which is rich in hydrogen, is directly separated without being shifted for hydrogen enrichment. A shift reaction that converts CO to hydrogen is an endothermic reaction, hence heat must be supplied to the shift reactor, reducing the overall energy efficiency of the system. In addition, dual production of hydrogen (hydrogenated in MCH) and electricity is considered to maximize the total energy conversion efficiency. In-situ power generation utilizing the remaining syngas can supply electricity that is consumed within the system by pumps, compressors, and other auxiliaries. While a small part of the generated electricity is consumed inside the system, the larger part can be delivered and sold to the grid. A detailed schematic process flow diagram of the proposed integrated system is shown in Fig. 3. Algae slurry is pumped by pump P1 to a supercritical condition and then flows to a
Fig. 2 e Basic material and energy flows of the proposed integrated system consisting of SCWG, hydrogen separation, hydrogenation, and a combined cycle.
gasifier (GAS) after being preheated in preheaters HX1 and HX2 by condensate from a steam turbine (ST) and a condensed mixture of syngas and steam, respectively. Meanwhile, water is pumped and preheated in parallel with the algae slurry. This water is used as fluidizing gas and is blown from the bottom of the gasifier. Algae are converted to syngas in the gasifier and a mixture of syngas and steam is exhausted from the overhead of the gasifier. This mixture is then superheated in a superheater (SH) using the flue gas from a gas turbine (GT) to elevate its exergy rate and thus facilitate self-heat exchange. The superheated mixture of syngas and steam is then recirculated back to the gasifier through heating tubes immersed inside the gasifier and then flows to the preheaters. The syngas and steam are separated in condenser (CON) to produce relatively pure syngas. In addition, the compression energy of the syngas is recovered by the expander (EXP). The expanded syngas then flows to a separator (SEP) for hydrogen separation. The separated hydrogen (permeate) is mixed and preheated together with toluene before undergoing a reaction in a hydrogenator (HYD) that produces MCH. Meanwhile, the remaining syngas (raffinate) flows to a combustor (COMB) as fuel for combustion. The hightemperature and high-pressure gas flows to the gas turbine GT for expansion. The flue gas from the gas turbine is used to superheat the mixture of syngas and steam before it flows to the heat recovery steam generator (HRSG) for heat recovery. In addition, the condensate from the steam turbine is recirculated to the preheaters, hydrogenation module, and HRSG before it expands in the steam turbine.
Gasification of microalgae employing SCWG SCWG utilizes the advantages of supercritical water characteristics to convert thermochemically materials, especially biomass, to syngas. Generally, rich hydrogen and low carbon monoxide contents of syngas can be produced [24]. Under a supercritical regime, the water density decreases appreciably, leading to a lower static relative dielectric constant, and water thus behaves as a non-polar solvent [25]. In addition, as the hydrogen bonds weaken, a complete miscibility among gases and better transport properties can be achieved, facilitating the generation of a single homogeneous phase of fluid during the reaction. As a result, heat and mass transfers improve and faster conversion and a higher conversion efficiency can be achieved [26,27]. The syngas produced from SCWG is very clean with no formation of NOx or SOx; hence, syngas cleaning
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Syngas EXP
Toluene P4
P3
MCH
CON SEP Condensate
Microalgae slurry
HX7
HX6
H2 HYD
SH Flue gas (to cultivation)
Remaining syngas (raffinate) P1
HX1
HX2
GAS
Air
HRSG
COMB
HX5
P2 Water
HX3
HX4
GT
COMP
Electricity
ST GEN
Fig. 3 e Schematic diagram of the process flow of the proposed integrated system.
can be bypassed. The main reactions that occur during SCWG are steam reforming, a water gas shift, and methanation. In this study, a fluidized-bed-type SCWG reactor (gasifier) is adopted owing to its advantageous characteristics including an ability to avoid plugging, better particle mixing, a uniform temperature distribution across the bed, high rates of heat and mass transfers, and a high conversion rate. Researchers have also reported the successful application of the fluidized bed gasifier in SCWG [16,28]. Heating tubes are immersed inside the gasifier to facilitate a self-heat exchange between the superheated mixture of syngas and steam (hot stream) and algae slurry (cold stream). Hence, the heat required for gasification is basically supplied through this self-heat exchange. To improve the gasification efficiency in terms of carbon conversion, a catalyst of Ru/TiO2 is loaded inside the gasifier. The application of this catalyst leads to complete carbon conversion and syngas production with a rich hydrogen content [13]. In addition, to increase the dynamics of the particles inside the gasifier, fluidizing particles such as alumina can be adopted. Furthermore, the fluidizing particles prevent the formation of an ash layer and char on the gasifier wall and support the catalyst. As a result, high gasification performance can be achieved.
Hydrogen separation As the produced syngas is relatively clean, it can flow directly to hydrogen separation while bypassing gas cleaning. Technologies available for hydrogen separation include membrane, adsorption, absorption, and condensation technologies. Among them, membrane-based separation is considered a relatively mature technology that is ready for application. Membrane-based separation has the advantages of low energy consumption, an absence of additives, easy
control, easy scaling up and down, the possibility for continuous operation, and mild processing conditionings [29]. During separation, all impurities are rejected and hydrogen is preferentially passed into the product stream. There are basically four types of membrane separation: polymeric, nanoporous, dense metal, and ion conduction separations. Among them, polymeric membrane separation has the broadest commercial application. A polymeric membrane is a microporous film acting as a semi-permeable barrier that separates different materials according to their differences in physical properties, especially particle size. The advantages of the polymeric membrane include permselectivity, reasonable cost, and low operating temperature [30]. Hence, in the present study, a polymeric membrane is adopted for hydrogen separation. However, as the pressure gradient becomes the main driving force, compression of the feed stream to cover the pressure drop during separation is required. To address this issue, the pressure outlet of the expander EXP is set to the pressure required for separation, which is about 800 kPa. Hence, installation of a new compressor can be avoided.
Hydrogenation The separated highly pure hydrogen undergoes hydrogenation in which it reacts with toluene to produce MCH. Hydrogenation is an exothermic reaction, and the heat can thus be recovered for the preheating of steam for the steam turbine. The theoretical volumetric and gravimetric hydrogen contents of MCH are 47% and 6.2%, respectively. In addition, the structure of the LOHC is unsaturated, allowing recyclable hydrogen storage [8]. Hydrogenation including the catalyst used is considered a well-established technology. The reaction is C7 H8 þ 3H2
/ C7 H14
DH ¼ 205 kJ=mol:
(1)
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Combined cycle The remaining syngas that is rejected by the membrane separator is used as fuel for combustion in the combined cycle. Through combustion under high pressure in COMB, syngas is converted to thermal energy in the form of high-temperature and high-pressure gas that subsequently expands in gas turbine GT, producing a torque to rotate the generator (GEN). In addition, as the flue gas from the gas turbine is still at high temperature, it is used to superheat the mixture of syngas and steam exhausted from the SCWG reactor. Furthermore, the remaining heat of the flue gas is recovered in the HRSG to generate high-pressure steam that expands in steam turbine ST and generates additional electricity.
System modeling and analysis In this study, Chlorella vulgaris is adopted as a sample for system evaluation. It is a species of microscopic unicellular green alga that is rich in protein (about 50% protein). It can grow and multiple rapidly through highly efficient photosynthesis in fresh water with abundant sunlight and at relatively mild temperatures [31]. In addition, C. vulgaris has high potential for CO2 bio-sequestration owing to its ability to absorb CO2 efficiently and live under a high concentration of CO2 [32]. The properties of C. vulgaris including the results of proximate and ultimate analyses are listed in Table 1. In this study, the flow rate and initial moisture content of wet microalgae are fixed at 1000 t h1 and 90 wt% wb, respectively. Table 2 gives the gasification conditions and produced syngas composition. Alumina particles are used as fluidizing particles to enhance the fluidization performance of microalgae and to improve the heat and mass transfers across the gasifier. In addition, complete carbon conversion is assumed by adopting Ru/TiO2 as the gasification catalyst following the work of Chakinala et al. [13]. Two gasification pressures are evaluated: 25 and 30 MPa. Notably, as the gasification pressure increases, the properties of the water change accordingly. Hence, the flow rate of the fluidizing steam is different for each corresponding fluidization velocity and gasification pressure.
Table 2 e SCWG conditions and syngas composition used during calculation. Property SCWG condition Temperature ( C) Pressure (MPa) Fluidization velocity U/Umf () Fluidizing material () Density (kg m3) Particle diameter (mm) Sphericity () Voidage at minimum fluidization () Gasification catalyst () Weight ration of catalyst to wet algae () Syngas composition Carbon conversion efficiency (%) H2 (dry mol%) CO (dry mol%) CH4 (dry mol%) C2H6 and C3H8 (dry mol%) CO2 (dry mol%)
Property Moisture content (wt% wb) Dry solid content (wt% wb) Chemical composition Protein (wt% db) Fat (wt% db) Fiber (wt% db) Ash (wt% db) Carbohydrates (wt% db) Ultimate analysis Carbon (wt% db) Hydrogen (wt% db) Nitrogen (wt% db) Oxygen (wt% db) Calorific value (dried, MJ kg1)
Value 90 10 64.1 13 21.1 7 15 45.8 7.9 7.5 38.7 18.49
600 25, 30 1, 2, 3, 4 Alumina 3400 0.3 0.67 0.45 Ru/TiO2 2 100 46.1 3.1 18.1 4.9 27.8
The pressure drop required for fluidization inside the SCWG reactor is calculated as the pressure drop required to fluidize the alumina particles. This pressure drop across the gasifier, Dp, is approximated using the extended Ergun equation: 2 mg U g Dp ð1 εÞ2 1 ε rg Ug ¼ 150 ; 2 þ 1:75 3 3 H ε ε 4p dp 4p dp
(2)
where H, ε, m, U, r, 4, and d are the fluidization bed height, void fraction, dynamic viscosity, superficial velocity, density, sphericity, and diameter, respectively. In addition, subscripts g and p represent the fluidizing gas and particle, respectively. The pressure drop during fluidization is strongly affected by various factors, especially the friction factors under both laminar and turbulent conditions. The minimum fluidization velocity, Umf, is calculated using its relationship with the Reynolds number at the minimum fluidization velocity, Remf. This relationship can be expressed as Umf ¼
Table 1 e Proximate and ultimate analyses of Chlorella vulgaris.
Value
Remf mg ; rg dp
(3)
0:5 Remf ¼ 27:32 þ 0:0434Ar 27:3;
Ar ¼
d3p rg rp rg g m2g
;
(4)
(5)
where Ar and g are the Archimedes number and acceleration due to gravity, respectively. In addition, the pressure drop owing to the distributor at the bottom of the bed is considered to be 40% of the pressure drop across the bed during fluidization. In this study, to measure the effect of the fluidization velocity on the total energy efficiency, four fluidization velocities are evaluated: 1, 2, 3, and 4 Umf. The assumed conditions during hydrogen separation and hydrogenation are given in Table 3. A polymeric membrane is
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Table 3 e Hydrogen separation and hydrogenation conditions. Property
Value
Separation Type () Hydrogen recovery (%) Operating temperature ( C) Feed inlet pressure (kPa) Product outlet pressure (kPa) Product H2 concentration (mol%) Product CO concentration (mol%) Product other gas concentration (mol%) Hydrogenation Pressure (kPa) Temperature ( C) Catalyst () Catalyst particle size (mm) Sphericity ()
Polymeric membrane 70 100 800 110.0 99.5 0.0498 0.448 130 200 NieMo/Al2O3 0.3 0.5
adopted as the hydrogen separator and hydrogen of high purity can be produced as permeate. The hydrogen recovery is assumed to be 70%, which means hydrogen still exists in the remaining syngas, although its concentration is low. In addition, in hydrogenation, a fixed bed reactor is adopted as the hydrogenator. The catalyst is loaded inside the bed. The mixture of hydrogen and toluene that is preheated initially is fed from the bottom of the bed. The produced MCH is exhausted from the top of the bed. The pressure drop in the fixed bed of hydrogenation is approximated using the KozenyeCarman equation: 2
Dp 180mg ð1 εÞ ¼ Ug : 2 H 4p dp ε3
(6)
The conditions of the combined-cycle modules including combustor, gas and steam turbines are presented in Table 4. The gas turbine inlet temperature is fixed at 1300 C and the amount of air for combustion is controlled to achieve this controlled temperature. In addition, the steam turbine inlet temperature is fixed at 600 C, with the flow rate of steam controlled accordingly.
The modeling and system calculation are conducted using the steady-state process simulator SimSci Pro/II (Schneider Electric Software, LLC). Additional assumptions are made: (1) the SCWG reactor and hydrogenator are considered to consist of a mixer, conversion reactor, and heat exchanger, (2) the minimum temperature approached in all heat exchangers is 10 C, (3) heat exchange is conducted in counter-current mode, (4) the ambient temperature and pressure are 25 C and 101.33 kPa, respectively, (5) the adiabatic efficiency of the pump and compressor is 87%, (6) the adiabatic efficiency of the expander is 90%, (7) there is no heat loss from the system, (8) there is no air contamination inside the SCWG reactor and hydrogenator, and (9) in the SCWG reactor, heat is completely transferred from the superheated mixture of syngas and steam to microalgae. The total energy efficiency, htot, of the proposed integrated system is approximated as the ratio of the sum of the produced hydrogen calorific value and generated electricity to the calorific value, CV, of microalgae: htot ¼
Wout Win þ mH2 CVH2 ; malgae CValgae
(7)
Wout ¼ WGT þ WST þ WEXP ;
(8)
Win ¼ WCOMP þ WP1 þ WP2 þ WP3 þ WP4 ;
(9)
where WGT, WST and WEXP are actual work earned from the gas turbine, steam turbine and expander, respectively. WCOMP and WP are duties performed by compressor and pumps, respectively. The electricity generation efficiency, helec, is defined as the ratio of generated electricity to the calorific value of microalgae: helec ¼
Wout Win : malgae CValgae
(10)
Results and discussion Total energy efficiency Fig. 4 shows the relationship between total energy efficiency, htot, and fluidization velocity for different gasification
Table 4 e Assumed conditions for the combined cycle including combustion and gas and steam turbines.
Combustor and gas turbine Compressor outlet pressure (MPa) Compressor polytrophic efficiency (%) Combustor pressure drop (%) Gas turbine inlet temperature ( C) Gas turbine adiabatic efficiency (%) Air to fuel ratio () HRSG and steam turbine HRSG outlet pressure (MPa) Heat exchanger temperature difference ( C) HRSG pressure drop (%) Steam turbine inlet temperature ( C) Steam turbine polytrophic efficiency (%) Minimum outlet vapor quality (%)
Value 2 87 2 1300 90 10 10 10 1 600 90 90
Total energy efficiency (%)
Property
70 65 60 25 MPa
55
30 MPa
50 0
1
2 3 4 Fluidization velocity (xUmf)
5
Fig. 4 e Relationship between total energy efficiency and fluidization velocity under different gasification pressures.
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Table 5 e Total energy efficiency (%) for each fluidization velocity and gasification pressure.
Table 6 e Detailed simulation results for gasification pressure of 25 MPa.
Gasification pressure
Component
2
3
4
61.6 60.6
61.1 60.0
60.5 59.1
1
100
200
80
150
60
100
40 Net generated electricity Electricity generation efficiency
50
20 0
0 0
1
2 3 4 Fluidization velocity (xUmf)
Electricity generation efficiency (%)
Net generated electricity (MW)
pressures of 25 and 30 MPa. Generally, the average total energy efficiency of the proposed integrated system is very high; higher than 60%. The detailed total energy efficiency for each fluidization velocity and gasification pressure is given in Table 5. Regarding hydrogen production, about 3.5 t$H2 h1 can be produced and hydrogenated with toluene to produce MCH. The total energy efficiency decreases following an increase in fluidization velocity. In addition, gasification performed under a gasification pressure of 25 MPa has a higher total energy efficiency than that under 30 MPa. As the fluidization velocity increases, the amount of water for fluidization (fluidizing steam) increases accordingly. Therefore, the pump must work harder to bring the water to the designated pressure. As the amount of fluidizing steam increases, the flow rate of steam exhausted from the SCWG reactor increases accordingly. This has the effect of increasing the exchanged heat amount in the superheater. As a result, the heat available as hot stream in the HRSG decreases, leading to a decrease in actual work obtained from the steam turbine. Numerically, the exchanged heat amount in the superheater and actual work obtained from the steam turbine under a fluidization velocity of 1 Umf and gasification pressure of 25 MPa are 9.8 and 57.14 MW, respectively. When the fluidization velocity is increased to 4 Umf under the same gasification pressure, these values become 15.7 and 54.5 MW, respectively. Regarding the effect of the gasification pressure, as the pressure increases, the density of steam increases accordingly. Therefore, the flow rate of the fluidizing steam slightly increases although under the same fluidization velocity. As a result, similar to the phenomenon of the fluidization velocity increase, the increase in gasification pressure finally leads to lower actual work that can be obtained from the steam turbine.
250
Fluidization velocity U/Umf
5
Fig. 5 e Relationship among the net generated electricity, fluidization velocity, and electricity generation efficiency for when gasification pressure is 25 MPa.
Exchanged heat amount (MW) SCWG reactor GAS 347.9 Superheater SH 9.8 HRSG 168.1 Obtained works (MW) Gas turbine GT 282.3 Steam turbine ST 57.1 Expander EXP 13.8 Consumed works (MW) Compressors COMP 142.0 Pumps P 6.1
2
3
4
655.7 11.8 165.1
772.9 13.8 162
889.2 15.7 159
282.3 56.3 12.4
282.3 55.4 12.9
282.3 54.5 12.8
142.0 8.2
142.0 10.3
142.0 12.4
Electricity generation and detailed simulation results Fig. 5 shows the relationship among the net generated electricity, fluidization velocity, and electricity generation efficiency when the gasification pressure is 25 MPa. In addition, Table 6 gives detailed simulation results for when the gasification pressure is 25 MPa. There is no appreciable change in either the net generated electricity or electricity generation efficiency for the evaluated fluidization velocities. For gasification of 1000 t$microalgae h1, about 200 MW of electricity can be generated using the remaining syngas (raffinate). The efficiency electricity generation is relatively high, at about 40%. A large amount of heat is exchanged inside the SCWG reactor (gasifier). The amount of exchanged heat increases following the increase in fluidization velocity due to the increase in flow rate of the fluidizing steam. This large amount of exchanged heat demonstrates that self-heat exchange can be realized employing the proposed EPI technology, especially the exergy recovery. In addition, as the heat exchange in the superheater increases following the increase in fluidization velocity, the amount of exchanged heat in the HRSG decreases accordingly, leading to lower actual work that can be obtained from the steam turbine. There is almost no change in the work
250
100
200
80
150
60
100
40
50
20
Net generated electricity Electricity generation efficiency
0
0 0
1
2 3 Fluidization velocity (xUmf)
4
Electricity generation efficiency (%)
1 62.4 61.5
Net generated electricity (MW)
25 MPa 30 MPa
Fluidization velocity U/Umf
5
Fig. 6 e Relationship among the net generated electricity, fluidization velocity and electricity generation efficiency in the case that gasification pressure is fixed at 30 MPa.
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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fluidization velocity are 25 MPa and 1 Umf, respectively. The dotted and solid lines represent hot and cold streams, respectively. Generally, the streams are almost parallel to each other, leading to effective heat exchange between them (self-heat exchange). The largest heat exchange occurs in HX2, followed by HX5 (gasifier) and the HRSG. High energy efficiency can be achieved through the application of EPI. In this integrated system, exergy is recovered mainly through heat combination, especially in the superheater. The exergy rate of the cold stream (mixture of produced syngas and steam) is elevated using the hot flue gas from the gas turbine, which has a higher exergy rate. As a result, self-heat exchange can be realized. The compression work conducted by pumps for microalgae and water for fluidization can be recovered by the expander. Furthermore, through process integration, the available heat from one process can be used in other processes, leading to a further reduction of exergy destruction. Hence, high total energy efficiency can be achieved.
Table 7 e Detailed simulation results for gasification pressure of 30 MPa. Component
Fluidization velocity U/Umf 1
Exchanged heat amount (MW) SCWG reactor GAS 584.3 Superheater SH 10.7 HRSG 166.7 Obtained works (MW) Gas turbine GT 282.3 Steam turbine ST 56.7 Expander EXP 11.5 Consumed works (MW) Compressors COMP 142.0 Pumps P 8.1
2
3
4
990.9 13.2 162.9
997.9 15.6 159.1
1171.8 18.1 155.3
282.3 55.6 10.5
282.3 54.5 11.7
282.3 53.4 11.17
142.0 11.0
142.0 13.9
142.0 16.8
obtained from the gas turbine and expander because the flow rate and composition of the produced syngas are basically the same under all fluidization velocities. Fig. 6 shows the relationship among the net generated electricity, fluidization velocity and electricity generation efficiency under gasification pressure of 30 MPa. In addition, Table 7 gives detailed simulation results for gasification pressure of 30 MPa. Although both the net generated electricity and electricity generation efficiency are slightly lower than those under gasification pressure of 25 MPa, a similar tendency is observed for the two evaluated gasification pressures. As the density of steam increases following the increase in pressure, the exchanged heat amounts in the SCWG reactor (gasifier) and superheater are higher in the case of gasification pressure of 30 MPa. This is attributable to a higher flow rate of fluidizing steam. As a result, the heat exchange in the HRSG and the work from the steam turbine decrease. In addition, more pump work is consumed when gasification pressure is 30 MPa. Fig. 7 is a temperatureeenthalpy diagram of the proposed integrated system for when the gasification pressure and
1000
HX1
HX2
Conclusion A state-of-the-art integrated system for harvesting energy from microalgae including hydrogen production and power generation was proposed and evaluated. The integrated system is based on EPI technology that combines exergy recovery and process integration. The proposed system consists of a SCWG, hydrogen separation, hydrogenation, and a combined cycle. The effects of the fluidization velocity and gasification pressure on the total energy and electricity generation efficiencies were evaluated. The proposed integrated system provides high total energy efficiency higher than 60%. Furthermore, about electricity generation efficiency of 40% can be achieved. Although insignificant, lower fluidization velocity and gasification pressure lead to higher total energy efficiency and electricity generation efficiency.
HX3 HX4
HX5
SH
HRSG
HX6 HX7
Temperature (oC)
800 600
400 200 0
Enthalpy
Fig. 7 e Temperatureeenthalpy diagram of the proposed integrated system (gasification pressure and fluidization velocity are 25 MPa and 1 Umf, respectively). Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Acknowledgments This research is partly supported by the research development program of Solutions Research Laboratory, Tokyo Institute of Technology, Japan.
references
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[16] Lu YJ, Jin H, Guo LJ, Zhang XM, Cao CQ, Guo X. Hydrogen production by biomass gasification in supercritical water with a fluidized bed reactor. Int J Hydrogen Energy 2008;33:6066e75. [17] Wei L, Lu Y, Wei J. Hydrogen production by supercritical water gasification of biomass: particle and residence time distribution in fluidized bed reactor. Int J Hydrogen Energy 2013;38:13117e24. [18] Fiori L, Valbusa M, Castello D. Supercritical water gasification of biomass for H2 production: process design. Bioresour Technol 2012;121:139e47. [19] Haiduc AG, Brandenberger M, Suquet S, Vogel F, BernierLatmani R, Ludwig C. SumChem: an integrated process for the hydrothermal production of methane from microalgae and CO2 mitigation. J Appl Phsycol 2009;21:529e41. [20] Liu Y, Aziz M, Fushimi C, Kansha Y, Mochidzuki K, Kaneko S, et al. Exergy analysis of biomass drying based on self-heat recuperation technology and its application to industry: a simulation and experimental study. Ind Eng Chem Res 2012;51:9997e10007. [21] Liu Y, Aziz M, Kansha Y, Tsutsumi A. Application of the selfheat recuperation technology for energy saving in biomass drying system. Fuel Process Technol 2014;117:66e74. [22] Kotani Y, Aziz M, Kansha Y, Fushimi C, Tsutsumi A. Magnetocaloric heat circulator based on self-heat recuperation technology. Chem Eng Sci 2013;101:5e12. [23] Kansha Y, Aziz M, Kishimoto A, Tsutsumi A. Evaluation of a self-heat recuperative thermal process based on thermodynamic irreversibility and exergy. J Chem Eng Jpn 2013;46:87e91. [24] Lange S, Pellegrini LA. Study of hydrogen potentiality from supercritical water gasification of different biomasses: thermodynamic analysis and comparison with experimental data. Chem Eng Trans 2014;37:175e80. [25] Aziz M. Integrated supercritical water gasification and a combined cycle for microalgal utilization. Energy Convers Manage 2015;91:140e8. [26] Serani AL, Aymonier C, Cansell F. Current and foreseeable application of supercritical water for energy and the environment. Chem Sus Chem 2008;1:486e503. [27] Kersten SRA, Potic B, Prins W, VanSwaaij WPP. Gasification of model compounds and wood in hot compressed water. Ind Eng Chem Res 2006;45:4169e77. [28] Matsumura Y, Minowa T. Fundamental design of continuous biomass gasification process using a supercritical water fluidized bed. Int J Hydrogen Energy 2004;29:701e7. [29] Liu K, Song C, Subramani V. Hydrogen and syngas production and purification technologies. Hoboken, USA: Wiley; 2010. [30] Cersosimo M, Brunetti A, Drioli E, Firorino F, Dong G, Woo KT, et al. Separation of CO2 from humidified ternary gas mixtures using thermally rearranged polymeric membranes. J Membr Sci 2015;492:257e62. [31] Dejoye C, Vian MA, Lumia G, Bouscarle C, Charton F, Chemat F. Combined extraction processes of lipid from Chlorella vulgaris microalgae: microwave prior to supercritical carbon dioxide extraction. Int J Mol Sci 2011;12:9332e41. [32] Chinnasamy S, Ramakrishnan B, Bhatnagar A, Das KC. Biomass production potential of a wastewater alga Chlorella vulgaris ARC 1 under elevated levels of CO2 and temperature. Int J Mol Sci 2009;10:518e32.
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
Available online at www.sciencedirect.com
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Integrated hydrogen production and power generation from microalgae Muhammad Aziz* Solutions Research Laboratory, Tokyo Institute of Technology, 2-12-1-I6-25 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
article info
abstract
Article history:
An integrated system for hydrogen production, storage, and power generation from
Received 30 June 2015
microalgae using enhanced process integration technology is proposed. Enhanced process
Received in revised form
integration has two core technologies: exergy recovery and process integration. Exergy
20 October 2015
recovery is performed through exergy elevation and heat coupling to minimize exergy
Accepted 27 October 2015
destruction. The unrecoverable energy/heat in a single process is recovered and used in
Available online xxx
other processes through process integration. The proposed integrated system includes supercritical water gasification, hydrogen separation, hydrogenation, and a combined
Keywords:
cycle. The microalga Chlorella vulgaris is used for modeling and evaluation. Microalgae are
Microalgae
converted to syngas, then separated to produce highly pure hydrogen. To store the pro-
Hydrogen
duced hydrogen, the tolueneemethylcyclohexane cycle as a liquid organic hydrogen car-
Exergy recovery
rier is adopted. The remaining gas is used as fuel for combustion in the combined cycle to
Process integration
generate electricity. The effects of the fluidization velocity and gasification pressure on
Hydrogenation
energy efficiency are evaluated. From process modeling and calculation, high total energy
Energy efficiency
efficiency (higher than 60%), including electricity generation efficiency of about 40%, can be realized. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microalgae are a high-potential source of biomass for the production of food, industrial materials, pharmaceuticals, and energy [1]. Microalgae have very high photosynthetic fixation capability for CO2 that can be utilized in generating various algal cell components, energy, and molecular oxygen [2]. Furthermore, microalgae have characteristics superior to those of terrestrial biomasses including a higher growth rate, more efficient solar energy conversion, higher nutrient acquisition, and the ability to grow under severe conditions
[3]. Microalgae are rich in lipids and can thus be potentially used as an energy source and converted to different biofuels including bio-hydrogen, bio-diesel, biogas, and bio-oil [4]. The heating value of microalgae that are grown under optimum conditions usually ranges from 17 to 23 GJ per ton of dried microalgae [5]. As microalgae grow in an aqueous environment, they are generally cultivated remotely, possibly a far distance from their demand sites. Hence, the fuel produced from microalgae needs to be stored and transported. Among the secondary energy resources, hydrogen is highly versatile and efficient, has a wide variety of production and utilization technologies,
* Corresponding author. Tel.: þ81 3 5734 3809; fax: þ81 3 5734 3559. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2015.10.115 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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and is clean [6]. Hydrogen is widely used and its use is increasing. It is used in reciprocating combustion engines, combustion in a combined cycle, and fuel cells. It is also mixed with other fuels. Furthermore, water is produced during hydrogen oxidation, thus leading to clean energy utilization. Under ambient conditions, hydrogen has a relatively high energy density by weight of 33 kWh kg$H1 2 . Unfortunately, the energy density of hydrogen by volume, 3 Wh per liter of gaseous H2, is very low compared with that of other hydrocarbons. This leads to difficulty in storing and transporting the produced hydrogen. Recently, storage and transportation methods for hydrogen, including compression, liquefaction, and chemical and physical storage have been developed and applied. Among them, compressed and liquid hydrogen are considered established forms for storage and transport. However, these methods require high energy consumption and have a relatively low safety level. Hydrogen storage utilizing a liquid organic hydrogen carrier (LOHC) is considered promising because of its high safety level, high storage capacity, excellent reversibility, longer storage time, and lower CO2 emission [7e9]. Ammonia is another promising liquid hydrogen carrier, but unfortunately, it is toxic and corrosive, has a strong odor, and must be carried by a specially designed tanker. In the LOHC, the hydrogen is covalently bonded through hydrogenation. When the hydrogen is needed, it can be released from the LOHC through dehydrogenation. Available LOHC cycles include cyclohexaneebenzene, decalinenaphthalene, and tolueneemethylcyclohexane (MCH) cycles. In this study, a toluene (C7H8)eMCH (C7H14) cycle is adopted to store and transport hydrogen because toluene and MCH are cheap, stable, and easy to transport. In addition, both toluene and MCH exist as a liquid over a wide temperature range, which is favorable for long-term storage. Additionally, their applicability has been demonstrated in a large-scale test conducted by Chiyoda Corporation in Japan [10]. Microalgae can be biochemically and thermochemically converted to hydrogen. Unfortunately, biochemical conversion, including fermentation, has a slower conversion rate and lower conversion efficiency than thermochemical conversion. Hence, for large-scale energy conversion, thermochemical conversion, including gasification and pyrolysis, is generally adopted. Among forms of thermochemical conversion, gasification is considered to have the highest conversion efficiency [11]. Two gasification methods are currently available: conventional thermal gasification and supercritical water gasification (SCWG). In the former, the harvested microalgae need to be dried to quite a low moisture content to achieve a stable conversion with high efficiency. Unfortunately, the moisture content of microalgae is very high, ranging from 70 to 90 wt% on a wet basis (wb), leading to huge energy consumption for drying [12]. In contrast, supercritical water gasification can be performed without drying because gasification is performed under an aqueous state. However, the energy demand required to bring the microalgae to a supercritical condition is high, resulting in lower total energy efficiency. Hence, a novel system that resolves this problem is urgently required. To the author's knowledge, there has been no study focusing on the effort to effectively integrate the conversion of
microalgae to hydrogen and storage of the produced hydrogen. Many studies focusing on the application of SCWG to microalgae did not pay further attention to system development [13e15]. Furthermore, some researchers studied the production of hydrogen through SCWG using biomass including microalgae [16,17]. Fiori et al. [18] and Haiduc et al. [19] proposed a design for hydrogen and methane production from biomass including microalgae with SCWG. Unfortunately, no notable effort was made to realize energy/heat recirculation in their proposed systems; hence, what was still large exergy destruction was generated. In addition, the systems included no storage process for the produced hydrogen. In the present study, a novel integrated system consisting of gasification, hydrogen separation, hydrogenation, and a combined cycle is proposed on the basis of enhanced process integration (EPI). EPI is a technology that combines exergy recovery and process integration with the purpose of minimizing the exergy destruction throughout the system. Therefore, high energy efficiency can be achieved.
Integrated system for hydrogen production, storage, and power generation To minimize the exergy destruction throughout the system, the concept of EPI has been developed and applied to several materials including coal and biomasses [20e23]. EPI consists of two core technologies: exergy recovery and process integration. The former relates to the idea of heat circulation throughout a single process. Fig. 1 shows the concept of exergy recovery and two possible methods of exergy elevation. The dotted and solid lines represent streams with a high and low exergy rate, respectively. To realize the proposed exergy recovery, exergy elevation of the stream and heat coupling among the streams are performed (Fig. 1(a)). In exergy elevation, the exergy rate of the cold stream is raised, creating a hot stream by means of compression and heat combination (Fig. 1(b)). The stream is then used as the heat source and is paired with the cold stream through heat coupling (self-heat exchange). In heat coupling, the heat of the hot stream and the heat of the cold stream exchanged. Additionally, considerations are made of the heat type, heat amount and exergy rate to achieve an optimum balanced heat exchange. It is thus important to note that the idea of exergy recovery is different from that of conventional pinch or heat recovery technologies, which are essentially based on heat cascade utilization. Although heat recovery throughout a single process has been optimized through exergy recovery, there is still unrecoverable energy/heat relating to the minimum temperature approach during heat exchange, the imbalance of heat following a change in stream properties, and heat loss. To minimize the amount of unrecoverable energy/heat, a process integration is introduced where the unrecoverable energy/ heat from any process is used in other processes. As a result, exergy throughout the integrated systems can be further minimized, leading to high total energy efficiency. Fig. 2 shows a diagram of the basic schematic material and energy flows of the proposed integrated system. Solid and dotted lines represent material and energy flows, respectively.
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Fig. 1 e Concept of exergy recovery adopted in this study.
The system consists of continuous processes, namely SCWG, hydrogen separation, hydrogenation, and the combined cycle. The harvested microalgal biomass is pretreated initially to create an algae slurry that then flows to the SCWG reactor for conversion. Furthermore, in the SCWG reactor, microalgae are converted to syngas that mainly contains H2, CO, CH4, and CO2. The produced syngas then undergoes hydrogen separation to produce hydrogen with high purity. The separated hydrogen is then covalently bonded to toluene in hydrogenation to produce MCH, which is ready to be stored and transported. Meanwhile, the gas remaining from separation is used as fuel for combustion in the combined cycle to generate electricity. Part of the heat from the combined cycle is used to elevate the exergy rate in the SCWG module through heat combination. In addition, the flue gas from the combined cycle is recycled back to the cultivation as nutrients for algae to grow, leading to CO2 bio-sequestration. In this study, the syngas produced from SCWG, which is rich in hydrogen, is directly separated without being shifted for hydrogen enrichment. A shift reaction that converts CO to hydrogen is an endothermic reaction, hence heat must be supplied to the shift reactor, reducing the overall energy efficiency of the system. In addition, dual production of hydrogen (hydrogenated in MCH) and electricity is considered to maximize the total energy conversion efficiency. In-situ power generation utilizing the remaining syngas can supply electricity that is consumed within the system by pumps, compressors, and other auxiliaries. While a small part of the generated electricity is consumed inside the system, the larger part can be delivered and sold to the grid. A detailed schematic process flow diagram of the proposed integrated system is shown in Fig. 3. Algae slurry is pumped by pump P1 to a supercritical condition and then flows to a
Fig. 2 e Basic material and energy flows of the proposed integrated system consisting of SCWG, hydrogen separation, hydrogenation, and a combined cycle.
gasifier (GAS) after being preheated in preheaters HX1 and HX2 by condensate from a steam turbine (ST) and a condensed mixture of syngas and steam, respectively. Meanwhile, water is pumped and preheated in parallel with the algae slurry. This water is used as fluidizing gas and is blown from the bottom of the gasifier. Algae are converted to syngas in the gasifier and a mixture of syngas and steam is exhausted from the overhead of the gasifier. This mixture is then superheated in a superheater (SH) using the flue gas from a gas turbine (GT) to elevate its exergy rate and thus facilitate self-heat exchange. The superheated mixture of syngas and steam is then recirculated back to the gasifier through heating tubes immersed inside the gasifier and then flows to the preheaters. The syngas and steam are separated in condenser (CON) to produce relatively pure syngas. In addition, the compression energy of the syngas is recovered by the expander (EXP). The expanded syngas then flows to a separator (SEP) for hydrogen separation. The separated hydrogen (permeate) is mixed and preheated together with toluene before undergoing a reaction in a hydrogenator (HYD) that produces MCH. Meanwhile, the remaining syngas (raffinate) flows to a combustor (COMB) as fuel for combustion. The hightemperature and high-pressure gas flows to the gas turbine GT for expansion. The flue gas from the gas turbine is used to superheat the mixture of syngas and steam before it flows to the heat recovery steam generator (HRSG) for heat recovery. In addition, the condensate from the steam turbine is recirculated to the preheaters, hydrogenation module, and HRSG before it expands in the steam turbine.
Gasification of microalgae employing SCWG SCWG utilizes the advantages of supercritical water characteristics to convert thermochemically materials, especially biomass, to syngas. Generally, rich hydrogen and low carbon monoxide contents of syngas can be produced [24]. Under a supercritical regime, the water density decreases appreciably, leading to a lower static relative dielectric constant, and water thus behaves as a non-polar solvent [25]. In addition, as the hydrogen bonds weaken, a complete miscibility among gases and better transport properties can be achieved, facilitating the generation of a single homogeneous phase of fluid during the reaction. As a result, heat and mass transfers improve and faster conversion and a higher conversion efficiency can be achieved [26,27]. The syngas produced from SCWG is very clean with no formation of NOx or SOx; hence, syngas cleaning
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Syngas EXP
Toluene P4
P3
MCH
CON SEP Condensate
Microalgae slurry
HX7
HX6
H2 HYD
SH Flue gas (to cultivation)
Remaining syngas (raffinate) P1
HX1
HX2
GAS
Air
HRSG
COMB
HX5
P2 Water
HX3
HX4
GT
COMP
Electricity
ST GEN
Fig. 3 e Schematic diagram of the process flow of the proposed integrated system.
can be bypassed. The main reactions that occur during SCWG are steam reforming, a water gas shift, and methanation. In this study, a fluidized-bed-type SCWG reactor (gasifier) is adopted owing to its advantageous characteristics including an ability to avoid plugging, better particle mixing, a uniform temperature distribution across the bed, high rates of heat and mass transfers, and a high conversion rate. Researchers have also reported the successful application of the fluidized bed gasifier in SCWG [16,28]. Heating tubes are immersed inside the gasifier to facilitate a self-heat exchange between the superheated mixture of syngas and steam (hot stream) and algae slurry (cold stream). Hence, the heat required for gasification is basically supplied through this self-heat exchange. To improve the gasification efficiency in terms of carbon conversion, a catalyst of Ru/TiO2 is loaded inside the gasifier. The application of this catalyst leads to complete carbon conversion and syngas production with a rich hydrogen content [13]. In addition, to increase the dynamics of the particles inside the gasifier, fluidizing particles such as alumina can be adopted. Furthermore, the fluidizing particles prevent the formation of an ash layer and char on the gasifier wall and support the catalyst. As a result, high gasification performance can be achieved.
Hydrogen separation As the produced syngas is relatively clean, it can flow directly to hydrogen separation while bypassing gas cleaning. Technologies available for hydrogen separation include membrane, adsorption, absorption, and condensation technologies. Among them, membrane-based separation is considered a relatively mature technology that is ready for application. Membrane-based separation has the advantages of low energy consumption, an absence of additives, easy
control, easy scaling up and down, the possibility for continuous operation, and mild processing conditionings [29]. During separation, all impurities are rejected and hydrogen is preferentially passed into the product stream. There are basically four types of membrane separation: polymeric, nanoporous, dense metal, and ion conduction separations. Among them, polymeric membrane separation has the broadest commercial application. A polymeric membrane is a microporous film acting as a semi-permeable barrier that separates different materials according to their differences in physical properties, especially particle size. The advantages of the polymeric membrane include permselectivity, reasonable cost, and low operating temperature [30]. Hence, in the present study, a polymeric membrane is adopted for hydrogen separation. However, as the pressure gradient becomes the main driving force, compression of the feed stream to cover the pressure drop during separation is required. To address this issue, the pressure outlet of the expander EXP is set to the pressure required for separation, which is about 800 kPa. Hence, installation of a new compressor can be avoided.
Hydrogenation The separated highly pure hydrogen undergoes hydrogenation in which it reacts with toluene to produce MCH. Hydrogenation is an exothermic reaction, and the heat can thus be recovered for the preheating of steam for the steam turbine. The theoretical volumetric and gravimetric hydrogen contents of MCH are 47% and 6.2%, respectively. In addition, the structure of the LOHC is unsaturated, allowing recyclable hydrogen storage [8]. Hydrogenation including the catalyst used is considered a well-established technology. The reaction is C7 H8 þ 3H2
/ C7 H14
DH ¼ 205 kJ=mol:
(1)
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Combined cycle The remaining syngas that is rejected by the membrane separator is used as fuel for combustion in the combined cycle. Through combustion under high pressure in COMB, syngas is converted to thermal energy in the form of high-temperature and high-pressure gas that subsequently expands in gas turbine GT, producing a torque to rotate the generator (GEN). In addition, as the flue gas from the gas turbine is still at high temperature, it is used to superheat the mixture of syngas and steam exhausted from the SCWG reactor. Furthermore, the remaining heat of the flue gas is recovered in the HRSG to generate high-pressure steam that expands in steam turbine ST and generates additional electricity.
System modeling and analysis In this study, Chlorella vulgaris is adopted as a sample for system evaluation. It is a species of microscopic unicellular green alga that is rich in protein (about 50% protein). It can grow and multiple rapidly through highly efficient photosynthesis in fresh water with abundant sunlight and at relatively mild temperatures [31]. In addition, C. vulgaris has high potential for CO2 bio-sequestration owing to its ability to absorb CO2 efficiently and live under a high concentration of CO2 [32]. The properties of C. vulgaris including the results of proximate and ultimate analyses are listed in Table 1. In this study, the flow rate and initial moisture content of wet microalgae are fixed at 1000 t h1 and 90 wt% wb, respectively. Table 2 gives the gasification conditions and produced syngas composition. Alumina particles are used as fluidizing particles to enhance the fluidization performance of microalgae and to improve the heat and mass transfers across the gasifier. In addition, complete carbon conversion is assumed by adopting Ru/TiO2 as the gasification catalyst following the work of Chakinala et al. [13]. Two gasification pressures are evaluated: 25 and 30 MPa. Notably, as the gasification pressure increases, the properties of the water change accordingly. Hence, the flow rate of the fluidizing steam is different for each corresponding fluidization velocity and gasification pressure.
Table 2 e SCWG conditions and syngas composition used during calculation. Property SCWG condition Temperature ( C) Pressure (MPa) Fluidization velocity U/Umf () Fluidizing material () Density (kg m3) Particle diameter (mm) Sphericity () Voidage at minimum fluidization () Gasification catalyst () Weight ration of catalyst to wet algae () Syngas composition Carbon conversion efficiency (%) H2 (dry mol%) CO (dry mol%) CH4 (dry mol%) C2H6 and C3H8 (dry mol%) CO2 (dry mol%)
Property Moisture content (wt% wb) Dry solid content (wt% wb) Chemical composition Protein (wt% db) Fat (wt% db) Fiber (wt% db) Ash (wt% db) Carbohydrates (wt% db) Ultimate analysis Carbon (wt% db) Hydrogen (wt% db) Nitrogen (wt% db) Oxygen (wt% db) Calorific value (dried, MJ kg1)
Value 90 10 64.1 13 21.1 7 15 45.8 7.9 7.5 38.7 18.49
600 25, 30 1, 2, 3, 4 Alumina 3400 0.3 0.67 0.45 Ru/TiO2 2 100 46.1 3.1 18.1 4.9 27.8
The pressure drop required for fluidization inside the SCWG reactor is calculated as the pressure drop required to fluidize the alumina particles. This pressure drop across the gasifier, Dp, is approximated using the extended Ergun equation: 2 mg U g Dp ð1 εÞ2 1 ε rg Ug ¼ 150 ; 2 þ 1:75 3 3 H ε ε 4p dp 4p dp
(2)
where H, ε, m, U, r, 4, and d are the fluidization bed height, void fraction, dynamic viscosity, superficial velocity, density, sphericity, and diameter, respectively. In addition, subscripts g and p represent the fluidizing gas and particle, respectively. The pressure drop during fluidization is strongly affected by various factors, especially the friction factors under both laminar and turbulent conditions. The minimum fluidization velocity, Umf, is calculated using its relationship with the Reynolds number at the minimum fluidization velocity, Remf. This relationship can be expressed as Umf ¼
Table 1 e Proximate and ultimate analyses of Chlorella vulgaris.
Value
Remf mg ; rg dp
(3)
0:5 Remf ¼ 27:32 þ 0:0434Ar 27:3;
Ar ¼
d3p rg rp rg g m2g
;
(4)
(5)
where Ar and g are the Archimedes number and acceleration due to gravity, respectively. In addition, the pressure drop owing to the distributor at the bottom of the bed is considered to be 40% of the pressure drop across the bed during fluidization. In this study, to measure the effect of the fluidization velocity on the total energy efficiency, four fluidization velocities are evaluated: 1, 2, 3, and 4 Umf. The assumed conditions during hydrogen separation and hydrogenation are given in Table 3. A polymeric membrane is
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Table 3 e Hydrogen separation and hydrogenation conditions. Property
Value
Separation Type () Hydrogen recovery (%) Operating temperature ( C) Feed inlet pressure (kPa) Product outlet pressure (kPa) Product H2 concentration (mol%) Product CO concentration (mol%) Product other gas concentration (mol%) Hydrogenation Pressure (kPa) Temperature ( C) Catalyst () Catalyst particle size (mm) Sphericity ()
Polymeric membrane 70 100 800 110.0 99.5 0.0498 0.448 130 200 NieMo/Al2O3 0.3 0.5
adopted as the hydrogen separator and hydrogen of high purity can be produced as permeate. The hydrogen recovery is assumed to be 70%, which means hydrogen still exists in the remaining syngas, although its concentration is low. In addition, in hydrogenation, a fixed bed reactor is adopted as the hydrogenator. The catalyst is loaded inside the bed. The mixture of hydrogen and toluene that is preheated initially is fed from the bottom of the bed. The produced MCH is exhausted from the top of the bed. The pressure drop in the fixed bed of hydrogenation is approximated using the KozenyeCarman equation: 2
Dp 180mg ð1 εÞ ¼ Ug : 2 H 4p dp ε3
(6)
The conditions of the combined-cycle modules including combustor, gas and steam turbines are presented in Table 4. The gas turbine inlet temperature is fixed at 1300 C and the amount of air for combustion is controlled to achieve this controlled temperature. In addition, the steam turbine inlet temperature is fixed at 600 C, with the flow rate of steam controlled accordingly.
The modeling and system calculation are conducted using the steady-state process simulator SimSci Pro/II (Schneider Electric Software, LLC). Additional assumptions are made: (1) the SCWG reactor and hydrogenator are considered to consist of a mixer, conversion reactor, and heat exchanger, (2) the minimum temperature approached in all heat exchangers is 10 C, (3) heat exchange is conducted in counter-current mode, (4) the ambient temperature and pressure are 25 C and 101.33 kPa, respectively, (5) the adiabatic efficiency of the pump and compressor is 87%, (6) the adiabatic efficiency of the expander is 90%, (7) there is no heat loss from the system, (8) there is no air contamination inside the SCWG reactor and hydrogenator, and (9) in the SCWG reactor, heat is completely transferred from the superheated mixture of syngas and steam to microalgae. The total energy efficiency, htot, of the proposed integrated system is approximated as the ratio of the sum of the produced hydrogen calorific value and generated electricity to the calorific value, CV, of microalgae: htot ¼
Wout Win þ mH2 CVH2 ; malgae CValgae
(7)
Wout ¼ WGT þ WST þ WEXP ;
(8)
Win ¼ WCOMP þ WP1 þ WP2 þ WP3 þ WP4 ;
(9)
where WGT, WST and WEXP are actual work earned from the gas turbine, steam turbine and expander, respectively. WCOMP and WP are duties performed by compressor and pumps, respectively. The electricity generation efficiency, helec, is defined as the ratio of generated electricity to the calorific value of microalgae: helec ¼
Wout Win : malgae CValgae
(10)
Results and discussion Total energy efficiency Fig. 4 shows the relationship between total energy efficiency, htot, and fluidization velocity for different gasification
Table 4 e Assumed conditions for the combined cycle including combustion and gas and steam turbines.
Combustor and gas turbine Compressor outlet pressure (MPa) Compressor polytrophic efficiency (%) Combustor pressure drop (%) Gas turbine inlet temperature ( C) Gas turbine adiabatic efficiency (%) Air to fuel ratio () HRSG and steam turbine HRSG outlet pressure (MPa) Heat exchanger temperature difference ( C) HRSG pressure drop (%) Steam turbine inlet temperature ( C) Steam turbine polytrophic efficiency (%) Minimum outlet vapor quality (%)
Value 2 87 2 1300 90 10 10 10 1 600 90 90
Total energy efficiency (%)
Property
70 65 60 25 MPa
55
30 MPa
50 0
1
2 3 4 Fluidization velocity (xUmf)
5
Fig. 4 e Relationship between total energy efficiency and fluidization velocity under different gasification pressures.
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Table 5 e Total energy efficiency (%) for each fluidization velocity and gasification pressure.
Table 6 e Detailed simulation results for gasification pressure of 25 MPa.
Gasification pressure
Component
2
3
4
61.6 60.6
61.1 60.0
60.5 59.1
1
100
200
80
150
60
100
40 Net generated electricity Electricity generation efficiency
50
20 0
0 0
1
2 3 4 Fluidization velocity (xUmf)
Electricity generation efficiency (%)
Net generated electricity (MW)
pressures of 25 and 30 MPa. Generally, the average total energy efficiency of the proposed integrated system is very high; higher than 60%. The detailed total energy efficiency for each fluidization velocity and gasification pressure is given in Table 5. Regarding hydrogen production, about 3.5 t$H2 h1 can be produced and hydrogenated with toluene to produce MCH. The total energy efficiency decreases following an increase in fluidization velocity. In addition, gasification performed under a gasification pressure of 25 MPa has a higher total energy efficiency than that under 30 MPa. As the fluidization velocity increases, the amount of water for fluidization (fluidizing steam) increases accordingly. Therefore, the pump must work harder to bring the water to the designated pressure. As the amount of fluidizing steam increases, the flow rate of steam exhausted from the SCWG reactor increases accordingly. This has the effect of increasing the exchanged heat amount in the superheater. As a result, the heat available as hot stream in the HRSG decreases, leading to a decrease in actual work obtained from the steam turbine. Numerically, the exchanged heat amount in the superheater and actual work obtained from the steam turbine under a fluidization velocity of 1 Umf and gasification pressure of 25 MPa are 9.8 and 57.14 MW, respectively. When the fluidization velocity is increased to 4 Umf under the same gasification pressure, these values become 15.7 and 54.5 MW, respectively. Regarding the effect of the gasification pressure, as the pressure increases, the density of steam increases accordingly. Therefore, the flow rate of the fluidizing steam slightly increases although under the same fluidization velocity. As a result, similar to the phenomenon of the fluidization velocity increase, the increase in gasification pressure finally leads to lower actual work that can be obtained from the steam turbine.
250
Fluidization velocity U/Umf
5
Fig. 5 e Relationship among the net generated electricity, fluidization velocity, and electricity generation efficiency for when gasification pressure is 25 MPa.
Exchanged heat amount (MW) SCWG reactor GAS 347.9 Superheater SH 9.8 HRSG 168.1 Obtained works (MW) Gas turbine GT 282.3 Steam turbine ST 57.1 Expander EXP 13.8 Consumed works (MW) Compressors COMP 142.0 Pumps P 6.1
2
3
4
655.7 11.8 165.1
772.9 13.8 162
889.2 15.7 159
282.3 56.3 12.4
282.3 55.4 12.9
282.3 54.5 12.8
142.0 8.2
142.0 10.3
142.0 12.4
Electricity generation and detailed simulation results Fig. 5 shows the relationship among the net generated electricity, fluidization velocity, and electricity generation efficiency when the gasification pressure is 25 MPa. In addition, Table 6 gives detailed simulation results for when the gasification pressure is 25 MPa. There is no appreciable change in either the net generated electricity or electricity generation efficiency for the evaluated fluidization velocities. For gasification of 1000 t$microalgae h1, about 200 MW of electricity can be generated using the remaining syngas (raffinate). The efficiency electricity generation is relatively high, at about 40%. A large amount of heat is exchanged inside the SCWG reactor (gasifier). The amount of exchanged heat increases following the increase in fluidization velocity due to the increase in flow rate of the fluidizing steam. This large amount of exchanged heat demonstrates that self-heat exchange can be realized employing the proposed EPI technology, especially the exergy recovery. In addition, as the heat exchange in the superheater increases following the increase in fluidization velocity, the amount of exchanged heat in the HRSG decreases accordingly, leading to lower actual work that can be obtained from the steam turbine. There is almost no change in the work
250
100
200
80
150
60
100
40
50
20
Net generated electricity Electricity generation efficiency
0
0 0
1
2 3 Fluidization velocity (xUmf)
4
Electricity generation efficiency (%)
1 62.4 61.5
Net generated electricity (MW)
25 MPa 30 MPa
Fluidization velocity U/Umf
5
Fig. 6 e Relationship among the net generated electricity, fluidization velocity and electricity generation efficiency in the case that gasification pressure is fixed at 30 MPa.
Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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fluidization velocity are 25 MPa and 1 Umf, respectively. The dotted and solid lines represent hot and cold streams, respectively. Generally, the streams are almost parallel to each other, leading to effective heat exchange between them (self-heat exchange). The largest heat exchange occurs in HX2, followed by HX5 (gasifier) and the HRSG. High energy efficiency can be achieved through the application of EPI. In this integrated system, exergy is recovered mainly through heat combination, especially in the superheater. The exergy rate of the cold stream (mixture of produced syngas and steam) is elevated using the hot flue gas from the gas turbine, which has a higher exergy rate. As a result, self-heat exchange can be realized. The compression work conducted by pumps for microalgae and water for fluidization can be recovered by the expander. Furthermore, through process integration, the available heat from one process can be used in other processes, leading to a further reduction of exergy destruction. Hence, high total energy efficiency can be achieved.
Table 7 e Detailed simulation results for gasification pressure of 30 MPa. Component
Fluidization velocity U/Umf 1
Exchanged heat amount (MW) SCWG reactor GAS 584.3 Superheater SH 10.7 HRSG 166.7 Obtained works (MW) Gas turbine GT 282.3 Steam turbine ST 56.7 Expander EXP 11.5 Consumed works (MW) Compressors COMP 142.0 Pumps P 8.1
2
3
4
990.9 13.2 162.9
997.9 15.6 159.1
1171.8 18.1 155.3
282.3 55.6 10.5
282.3 54.5 11.7
282.3 53.4 11.17
142.0 11.0
142.0 13.9
142.0 16.8
obtained from the gas turbine and expander because the flow rate and composition of the produced syngas are basically the same under all fluidization velocities. Fig. 6 shows the relationship among the net generated electricity, fluidization velocity and electricity generation efficiency under gasification pressure of 30 MPa. In addition, Table 7 gives detailed simulation results for gasification pressure of 30 MPa. Although both the net generated electricity and electricity generation efficiency are slightly lower than those under gasification pressure of 25 MPa, a similar tendency is observed for the two evaluated gasification pressures. As the density of steam increases following the increase in pressure, the exchanged heat amounts in the SCWG reactor (gasifier) and superheater are higher in the case of gasification pressure of 30 MPa. This is attributable to a higher flow rate of fluidizing steam. As a result, the heat exchange in the HRSG and the work from the steam turbine decrease. In addition, more pump work is consumed when gasification pressure is 30 MPa. Fig. 7 is a temperatureeenthalpy diagram of the proposed integrated system for when the gasification pressure and
1000
HX1
HX2
Conclusion A state-of-the-art integrated system for harvesting energy from microalgae including hydrogen production and power generation was proposed and evaluated. The integrated system is based on EPI technology that combines exergy recovery and process integration. The proposed system consists of a SCWG, hydrogen separation, hydrogenation, and a combined cycle. The effects of the fluidization velocity and gasification pressure on the total energy and electricity generation efficiencies were evaluated. The proposed integrated system provides high total energy efficiency higher than 60%. Furthermore, about electricity generation efficiency of 40% can be achieved. Although insignificant, lower fluidization velocity and gasification pressure lead to higher total energy efficiency and electricity generation efficiency.
HX3 HX4
HX5
SH
HRSG
HX6 HX7
Temperature (oC)
800 600
400 200 0
Enthalpy
Fig. 7 e Temperatureeenthalpy diagram of the proposed integrated system (gasification pressure and fluidization velocity are 25 MPa and 1 Umf, respectively). Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115
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Acknowledgments This research is partly supported by the research development program of Solutions Research Laboratory, Tokyo Institute of Technology, Japan.
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Please cite this article in press as: Aziz M, Integrated hydrogen production and power generation from microalgae, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.115