Energy-efficient Conversion of Microalgae to ... - Science Direct

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ScienceDirect Energy Procedia 105 (2017) 453 – 458

The 8th International Conference on Applied Energy – ICAE2016

Energy-efficient conversion of microalgae to hydrogen and power Muhammad Aziza,*, Ilman Nuran Zainib a Institute of Innovative Research, Tokyo Institute of Technology, Tokyo 152-8550, Japan Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan

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Abstract An integrated system for H2 production from microalgae and its storage is proposed employing enhanced process integration technology (EPI). EPI consists of two core technologies, i.e. exergy recovery and process integration. The proposed system includes a supercritical water gasification, H2 separation, hydrogenation, and combined cycle. Microalga Chlorella vulgaris is used as a material for evaluation. The produced syngas is separated to produce highly pure H2. Furthermore, to store the produced H2, liquid organic H2 carrier of toluene-and-methylcyclohexane cycle is adopted. The remaining gas is used as fuel for combustion in combined cycle to generate electricity. The effects of fluidization velocity and gasification pressure to energy efficiency are evaluated. From process modeling and calculation, it is shown that high total energy efficiency, about 60 %, can be achieved. In addition, about 40 % of electricity generation efficiency can be realized. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). responsibility of of ICAE Selection peer-reviewofunder Peer-review and/or under responsibility the scientific committee the 8th International Conference on Applied Energy.

Keywords: algae; supercritical gasification; chemical looping; energy efficiency; hydrogen; energy efficiency

1. Introduction Microalgae are one of the highly potential sources of biomasses for production of food, industrial materials, pharmaceuticals, and energy. Microalgae have very high photosynthetic fixation capability of CO2 to be utilized in generating various algal cell components, energy and molecular oxygen [1]. Microalgae are rich in lipid content leading to their potential to be utilized as energy source and converted to different biofuels including bio-H2, bio-diesel, biogas, and bio-oil [2]. The heating value of microalgae which are grown under optimum conditions generally ranges from 19 to 25 GJ/t-dried-microalgae [3].

* Corresponding author. Tel.: +81-3-5734-3809; fax: +81-3-5734-3559. E-mail address: aziz.m.aa@m. titech.ac.jp.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.340

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Muhammad Aziz and Ilman Nuran Zaini / Energy Procedia 105 (2017) 453 – 458

As microalgae grow in aqueous environment, their cultivation is generally located remotely which might be far from their demand sites. Hence, the produced fuel from microalgae needs to be stored and transported. Among the secondary energy resources, H2 has some advantages of high versatility and efficiency, high variety of production and utilization technologies, cleanliness, etc. [4]. The utilization of H2 is very wide and increasing, including reciprocating engine, combustion in combined cycle, and fuel cell. Furthermore, water is produced during H2 oxidation leading to clean energy utilization. H2 has relatively high energy density by weight very low by volume. This leads to a difficulty in its storage and transportation. Among the H2 storage method, liquid organic H2 carrier (LOHC) is considered promising because of its high safety, high storage capacity, excellent reversibility, longer storage time, and lower CO2 emission [5]. In LOHC, H2 is covalently bond through hydrogenation. Furthermore, H2 can be released from LOHC through dehydrogenation. In this study, a cycle of toluene (C7H8) – methylcyclohexane (C7H14, MCH) is employed to store and transport the H2 because they are cheap, stable, easy to transport and have wide temperature as liquid which is favorable for long storage. The conversion of microalgae to H2 can be performed biochemically and thermochemically. Unfortunately, biochemical conversion has a slower conversion rate and lower conversion efficiency than thermochemical one. Therefore, for large scale energy conversion, thermochemical conversion is generally adopted including gasification and pyrolysis. Among thermochemical conversions, gasification is considered to have highest conversion efficiency [1]. Furthermore, 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 relatively very low moisture content to achieve a stable conversion with high efficiency. Unfortunately, the moisture content of microalgae is very high, about 70-90 wt% wb, leading to huge energy consumption for drying [2]. On the other hand, supercritical water gasification can be performed without drying because gasification is performed under aqueous state. However, the energy to bring the microalgae to supercritical condition is also large resulting in lower total energy efficiency. Hence, a novel system to answer this problem is urgently required. To the best knowledge of the author, there is no study focusing on the effort to integrate effectively the conversion of microalgae to H2 and storage of produced H2. Many studies focusing on the application of SCWG to microalgae with paying no further attention to system development. Fiori et al. [6] have proposed a design for H2 and methane production from biomass including microalgae with SCWG. Unfortunately, no significant effort for energy/heat recirculation in their proposed systems was performed, hence still large exergy destruction was generated. They also include no storage process for the produced H2. In this study, a novel integrated system consisting of gasification, H2 separation, hydrogenation, and combined cycle is proposed based on enhanced process integration (EPI). EPI is a technology combining exergy recovery and process integration with a purpose of minimizing the exergy destruction throughout the system [7]. Therefore, exergy loss is minimized and high energy efficiency can be achieved [8]. 2. System Modelling Fig 1 shows the basic schematic material and energy flows diagram of the proposed integrated-system. Solid and dotted lines represent material and energy flows, respectively. The system consists of some continuous processes: SCWG, H2 separation, hydrogenation, and combined cycle. The harvested microalgae from cultivation is pretreated initially to form an algae slurry before being fed to the SCWG reactor for conversion. Furthermore, in SCWG reactor, microalgae are converted to syngas which is mainly containing H2, CO, CH4, and CO2. The produced syngas is then going to H2 separation to produce H2 with high purity. The separated H2 is then covalently bond to toluene in hydrogenation producing MCH which is ready to be stored and transported. On the other hand, the remaining gas from separation is used as fuel for combustion in combined cycle for power generation.


Fig. 1. Conceptual diagram of the proposed integrated-system

A detailed schematic process flow diagram of the proposed integrated-system is shown in Fig 2. Algae slurry is pumped in P1 to supercritical condition and then flowing to gasifier GAS after being preheated in HX1 and HX2. On the other hand, water is also pumped and utilized as fluidizing gas, which is blown from the bottom of gasifier. Algae are converted to syngas in gasifier GAS. It is then superheated in SH utilizing the flue gas from gas turbine GT to elevate its exergy rate to facilitate self-heat exchange. The superheated mixture of syngas and steam is then recirculated back to gasifier GAS through heating tubes immersed inside the gasifier and then flowing to preheaters. Syngas and steam are separated in condenser CON producing relatively pure syngas. In addition, the compression energy owned by syngas is recovered by the expander EXP. The expanded syngas in then flowing to separator SEP for H2 separation. The separated H2 is mixed and preheated together with toluene before having reaction in hydrogenator HYD producing MCH. On the other hand, the remaining syngas is flowing to combustor COMB as fuel for combustion. As the result, high temperature and pressure of gas is flowing to gas turbine GT for expansion. The flue gas from gas turbine is utilized to superheat the mixture of syngas and steam before flowing to heat recovery steam generator (HRSG) for heat recovery. In addition, the condensate from steam turbine ST is recirculated to preheaters, hydrogenation module, and HRSG before it expands in steam turbine ST.

Fig. 2. Process flow diagram of integrated system to convert microalgae to H2 and power

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3. System analysis In this study, Chlorella vulgaris is adopted as a sample for system evaluation. In this study, the flow rate and initial moisture content of wet microalgae are fixed at 1,000 t/h and 90 wt% wb, respectively. Alumina particles are used as fluidizing particles enhancing the fluidization performance of microalgae as well as improving the heat and mass transfers across the gasifier. In addition, complete carbon conversion is assumed by adopting Ru/TiO2 as gasification catalyst following the work of Chakinala et al. [9]. The composition of produced syngas is shown in Table 1. Regarding the gasification pressure, two gasification pressures are evaluated: 25 and 30 MPa. Table 1. Compositions of produced syngas from SCWG Property

Value

Temperature (C)

600

Carbon conversion efficiency (%)

100

H2 (dry mol%)

46.1

CO (dry mol%)

3.1

CH4 (dry mol%)

18.1

C2H6 and C3H8 (dry mol%)

4.9

CO2 (dry mol%)

27.8

The modeling and system calculation are conducted using a steady-state process simulator SimSci Pro/II (Schneider Electric). Some additional assumptions are made: (1) SCWG reactor and hydrogenator are considered to consist of mixer, conversion reactor, and heat exchanger, (2) the minimum temperature approach in all heat exchangers is 10 °C, (3) heat exchange is conducted in counter-current mode, (4) the adiabatic efficiency of pump and compressor is 87 %, and (5) the adiabatic efficiency of expander is 90 %. The total energy efficiency, ηtot, of the proposed integrated-system is approximated as the ratio of the sum of produced H2 calorific value and generated electricity to the calorific value, CV, of microalgae. It can be presented as follows: K tot

Wout Win mH2 CVH2 malgae CValgae

(1)

4. Results and discussion Fig 3 shows the relationship between total energy efficiency, ηtot, and fluidization velocity for different gasification pressures. Generally, the average total energy efficiency of the proposed integrated-system is very high, higher than 60 %. Regarding H2 production, about 3.5 t-H2 can be produced and hydrogenated with toluene producing MCH. The total energy efficiency decreases following the increase of fluidization velocity. In addition, gasification which is performed under gasification pressure of 25 MPa shows a higher total energy efficiency than one of 30 MPa. As the fluidization velocity increases, the amount of water for fluidization (fluidizing steam) increases accordingly. Therefore, higher pump work is required to pump the water to the designated pressure. Furthermore, as the amount of fluidizing steam increases, the flow rate of steam exhausted from SCWG reactor also increases accordingly. This gives an effect to the increase of exchanged heat amount in the


superheater. As the result, the available heat as hot stream in HRSG decreases leading to the decrease of actual work obtained from steam turbine. Numerically, the exchanged heat amount in superheater and actual work obtained from steam turbine under 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 same gasification pressure, these values become 15.7 and 54.5 MW, respectively.

Fig. 3. Relationship between total energy efficiency and fluidization velocity in SCWG

Regarding the effect of gasification pressure, as the pressure increases, the density of steam increases accordingly. Hence, the flow rate of fluidizing steam slightly increases although under same fluidization velocity. As the result, similar to the phenomenon in the increase of fluidization velocity, the increase of gasification pressure finally leads to lower actual work which can be obtained from steam turbine. Fig 4 shows the relationship among net generated electricity, fluidization velocity and electricity generation efficiency under different gasification pressures. There is no significant change in both net generated electricity and electricity generation efficiency in the evaluated fluidization velocities. The largest amount of exchanged heat occurs inside the SCWG reactor (gasifier). The amount of exchanged heat increases following the increase of fluidization velocity due to the increase of flow rate of the fluidizing steam. This large amount of exchanged heat proves that self-heat exchange can be realized through the proposed EPI technology, especially the exergy recovery. In addition, as the heat exchange in superheater increases following the increase of fluidization velocity, the amount of exchanged heat in HRSG decreases accordingly leading to lower actual work which can be earned from steam turbine. (a)

(b)

Fig. 4. Relationship among net generated electricity, generation efficiency and fluidization velocity: (a) SCWG pressure of 25 MPa, (b) SCWG pressure of 30 MPa

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As the density of steam increases following the increase of pressure, the exchanged heat amounts in SCWG reactor (gasifier) and superheater are higher in case of gasification pressure of 30 MPa. This is due to larger flow rate of fluidizing steam. As the result, the heat exchange is HRSG and the work from steam turbine decrease accordingly. In addition, larger pump works are consumed in case of gasification pressure of 30 MPa. 5. Conclusion A-state-of-the-art integrated-system for energy harvesting from microalgae including H2 production and power generation has been proposed and evaluated employing EPI technology. The proposed system consists of SCWG, H2 separation, hydrogenation, and combined cycle. The effects of fluidization velocity and gasification pressure to the total energy and electricity generation efficiencies are evaluated. The proposed integrated system can result in high total energy efficiency of greater than 60 %. Acknowledgements This research is partly supported by JSPS KAKENHI Grant Number JP16K18355. References [1] Aziz M, Oda T, Kashiwagi T. Advanced energy harvesting from macroalgae-Innovative integration of drying, gasification and combined cycle. Energies 2014;7:8217-8235. [2] Aziz M, Oda T, Kashiwagi T. Integration of energy-efficient drying in microalgae utilization based on enhanced process integration. Energy 2014;70:307-316. [3] Aziz, M. Integrated supercritical water gasification and a combined cycle for microalgal utilization. Energy Convers Manage 2015;91:140-148. [4] Aziz M. Integrated hydrogen production and power generation from microalgae. Int J Hydrogen Energy 2016;41:104-112. [5] Jiang Z, Pan Q, Xu J, Fang T. Current situation and prospect of hydrogen storage technology with new organic liquid. Int J Hydrogen Energy 2014;30:17442-17451. [6] Fiori L, Valbusa M, Castello D. Supercritical water gasification of biomass for H2 production: Process design. Bioresour Technol 2012;121:139-147. [7] Aziz M. Power generation from algae employing enhanced process integration technology. Chem Eng Res Des 2016;109:297-306. [8] Kansha Y, Kotani 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;46:87-91. [9] Chakinala AG, Brilman DWF, van Swaaij WPM, Kersten SRA. Catalytic and non-catalytic supercritical water gasification of microalgae and glycerol. Ind Eng Chem Res 2010;49:1113-1122.

Biography Muhammad Aziz received B. Eng., M. Eng., and D. Eng. degrees from Kyushu University, Japan, in 2004, 2006 and 2008, respectively. He is an associate professor at Tokyo Institute of Technology, Japan. He has authored more than 100 peer-reviewed journals, book chapters, and proceedings focusing in energy systems, process design, and heat transfer.