European Biomass Conference and Exhibition 2017 Proceedings

25th European Biomass Conference and Exhibition, 12-15 June 2017, Stockholm, Sweden

ALGAE CONVERSION TO HYDROGEN AND POWER BY INTEGRATION OF DRYING, GASIFICATION, AND CHEMICAL LOOPING COMBUSTION Ilman Nuran Zaini, Muhammad Aziz Institute of Innovative Research, Tokyo Institute of Technology, Tokyo, Japan 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan

ABSTRACT: A process simulation-based study was done to evaluate the integration of steam gasification and chemical looping combustion to produce hydrogen while generating power from algae (Fucus serratus). The process simulation was done using ASPEN Plus software package. The integrated system consists of drying, steam gasification, chemical looping combustion, and power generation. Enhanced process integration technology was adopted to maximize the heat circulation throughout the system. Chemical looping combustion used in this study consists of fuel, steam, and air reactors. Iron oxide was utilized as the circulated oxygen carrier in the chemical looping system which was reduced and oxidized through the reactors. The performance of the integrated system was evaluated under different steam-to-biomass ratios during gasification and operational pressures of the chemical looping combustion. The simulation result shows that the integrated system had a relatively high total efficiency (about 72%) which consisted of hydrogen production and power generation efficiencies of about 57% and 15%, respectively. Keywords: algae, hydrogen, electricity, integration, efficiency

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Firstly, wet algae were dried using a superheated steam to improve their characteristics. A rotary steam tube dryer (Figure 2(a)) was applied in this study as its characteristics are suitable for drying of algae which includes low emissions, low energy consumption, large heat transfer area, high thermal efficiency, and flexibility in handling various sizes and shapes of feedstocks [8,9]. After drying, the process was then followed by steam gasification by using a dual circulating fluidized bed gasifier.

INTRODUCTION

Algae are promising biomass fuel due to their higher growth rate than the terrestrial biomass and their characteristics as a non-direct competitor of food crops [1,2]. Currently, studies regarding utilization of algae as a fuel are mainly focused on the algae conversion into biooil through biochemical based methods. This method includes transesterification and fermentation, which produce biodiesel and ethanol, respectively. Although biochemical methods commonly require less energy and are more environmentally friendly than thermochemical methods, their production rate and energy yield are low [3]. In contrast, the conversion of algae by using thermochemical methods is believed to give faster reactions and a higher carbon conversion rate than the biochemical methods, thus making it favorable for the conversion of algae [4]. Recently, chemical looping combustion (CLC) technology has been gaining attention as an environmentally friendly option for fuel combustion. The combustion process in CLC system only involves oxygen. Therefore emissions from the fuel reactor contain only CO2 and water [5]. More importantly, CLC system has an ability to produce hydrogen (H2) by adding another oxidation reactor in the system [6], thus making it a more desirable method as H2 is considered as a promising clean source of energy. Therefore, a CLC based conversion system is proposed in this study to utilize algae for power generation and H2 production by means of process simulation.

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Figure 1: Schematic diagram of the cogeneration system. Figure 2(b) shows the process flow diagram of the algae gasification process coupled with the CLC and power generation systems. The dual circulating fluidizedbed gasifier that consisted of a gasifier and a combustor operated at near-atmospheric pressure was used for the conversion of algae. Steam, which acted as a gasification reactant and fluidizing agent, flowed from the bottom of the gasifier. In the dual fluidized-bed gasification system, the required heat was supplied by the char combustion process and was transferred to the gasifier by the circulation of inert material (such as sand). The syngas produced from gasification was then fed into the CLC module. The CLC module used in this cogeneration system consisted of three reactors: a fuel reactor (reduction), a steam reactor (oxidation), and an air reactor (combustion). The reactors were modeled using RGIBBS and RSTOIC blocks in Aspen Plus. The iron

METHODOLOGY

2.1 Proposed cogeneration system Figure 1 shows a schematic diagram of the proposed cogeneration system. The system consists of drying, steam gasification, CLC, and power generation. To realize an optimum heat circulation throughout the integrated system, enhance process integration (EPI) technology is adopted. It consists of two main technologies: exergy recovery and process integration [7].

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(a)

(b)

Figure 2: Process flow diagram of the (a) drying process and (b) gasification and chemical looping systems. gases were treated as ideal gases; air was assumed to consist of 79 mol% N2 and 21 mol% O2; and the ambient pressure was 101.325 kPa. Simulations were carried out with the operating condition of the drying, gasification, and CLC system as explained in Table II.

oxide that acted as an oxygen carrier was circulated throughout the reactors. In the fuel reactor, iron oxide was reduced by reactions with syngas producing CO2 and steam. The process was then followed by the oxidationby steam in the steam reactor, in which H2 is produced and exhausted together with the excess steam. In the air reactor, finally, iron oxide was recovered by combustion using air. To carry and provide effectively the heat for the circulating reactors, inert materials of Al2O3 and SiC were added to the system as an additional heat carrier. The mass fraction of the solids circulated in the SCL system was assumed to be 70% Fe2O3, 15% SiC, and 15% Al2O3 [6]. Expanders were then used in CLC to generate power by utilizing the heat generated from each reactor.

Table I. Fuel properties of Fucus serratus [4] Property Ultimate composition (dry basis, wt.%) C H N O S Proximate composition (dry basis, wt.%) Volatile matter Fixed carbon Ash

2.2 Algae composition Fucus serratus, a brown macroalgal species, was used in this study as the biomass feedstock. Table I shows the fuel composition of Fucus serratus that was obtained from literature [4]. In addition, the moisture content of algae entering the drying process was assumed to be 80 wt.%.

Higher heating value, HHV (dry basis, MJ/kg)

Value 33.5 4.78 2.39 34.4 1.31

51.5 27.4 21.1 17.5

The performance of the cogeneration system was evaluated based on the power generation and H2 production efficiency. The power generation efficiency was defined as the ratio of net power generation to HHV value of algae. While the H2 production efficiency was defined as the ratio of the calorific value of produced H2 from the steam reactor to the HHV value of dried algae.

2.3 Process simulation The Aspen Plus version 8.8 software package (Aspen Technology, Inc.) was used in this study to perform the simulation of the cogeneration system. The following general assumptions were used during the simulation: the process was operated under steady-state conditions;

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The total sum of those efficiency values was defined as the total efficiency of the cogeneration system. The system was evaluated on different target moisture contents (MCdry) for drying process and the steam-tobiomass ratios (S/B) for gasification. Table II. Assumptions used in the process simulation Process

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Operating condition

Algae Flowrate

100 t h-1

Drying Minimum temperature approach ΔTmin

20 °C Figure 3: Energy consumption for each process in the cogeneration system (at MCdry = 5% and S/B = 0.7).

Gasification Gasifier temperature Combustor temperature

700 °C 850 °C

Chemical looping combustion System pressure Fuel reactor temperature Steam reactor Air reactor

3.0 MPa 900 °C 815 °C 1000 °C

Power generation Expanders discharge pressure

100 kPa

Heat exchangers Minimum temperature approach ΔTmin

20 ºC

In the proposed cogeneration system, power was generated by recovering the heat from each CLC reactor’s product stream using an expander. The amount of the power generated from each expander was shown in Figure 4. It is clearly shown that the expander in steam reactor module generated the highest power. The reason was due to the high amount of steam in the steam reactor product stream which carried a high amount of latent heat.

RESULTS AND DISCUSSION

3.1 Energy consumption and generation Figure 3 presents the amount of the consumed energy for drying, gasification, and CLC process at MCdry and S/B equal to 5% and 0.7 respectively. It can be seen in the figure that CLC consumed the highest energy in the cogeneration system. The high amount of energy was mainly due to the duty of compressors in fuel and steam reactors as CLC was performed at a high operating pressure (3.0 MPa). Moreover, a significant amount of energy was also needed to dry the wet algae feedstock. The energy required to reduce the moisture content of algae from 80 wt.% to 5 wt.% was 2.3 MW. On the other hand, gasification consumed much less energy than other processes. In general, the energy consumed in the integrated system is considered low compared to the conventional system. For the example, in conventional algae drying, the energy which is generally consumed can reach about 3.6 MJ kg-water-1 [10], which is equivalent to about 40-60% of the total calorific value of algae.

Figure 4: Power generation by expanders in CLC process (at MCdry = 5% and S/B = 0.7). 3.2 Effect of target moisture content, MCdry, during drying Figure 5 shows the duty and outlet pressure of the compressor for drying brown algae with MCdry values in the range between 2.5% and 40%. In general, the simulation results show that at MCdry equal to 10%, the lowest compressor duty was obtained as a duty of 2.320 MW was required to dry the algae. Furthermore, MCdry values of less than 10% required a slightly higher compressor duty. On the other hand, at MCdry values of greater than 10%, the drying duty was found to increase for higher MCdry values.

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4 3 2 1 0

400 300 200 100 0

Pressure (kPa)

Duty (MW)


0 5 10 15 20 25 30 35 40 45 Target moisture content, MCdry(%)

Compressor duty for drying Outlet pressure

Figure 7: Effect of S/B values during gasification on the yield and composition of the produced syngas (at MCdry = 5%).

Figure 5: Effect of the target moisture content MCdry on the duty and outlet pressure of the dryer compressor (at values of S/B of 0.7).

Figure 8 shows the efficiency of the cogeneration system and the net power generated at different S/B values during gasification. The chemical looping system using syngas produced by gasification with an S/B value of 0.7 had the highest total efficiency of nearly 72%. Above an S/B value of 0.7, rising S/B resulted in a reduction in total efficiency, following a significant decrease in the production of H2 in the steam reactor. This phenomenon was caused by a similar reason to the case of gasification of algae with a high value of MCdry. At a high S/B, the production of Fe in the fuel reactor decreased due to a lower concentration of CO and CH4 in the syngas feed which reduced the H2 production efficiency.

Figure 6: Effect of the target moisture content MCdry on total efficiency, H2 production efficiency, and power generation efficiency (at S/B = 0.7). Figure 6 presents the performance of the cogeneration system at different MCdry values for the drying process. In general, the results show that a higher moisture content in the dried algae reduced the total efficiency of the system. the highest total efficiency (71.87%) was achieved when MCdry value was 2.5%. The efficiency of the system then gradually decreased corresponding to the increase in the value of MCdry.The reason of the phenomenon was the reduction of CO and CH4 production during gasification at a higher MCdry. It caused the reduction reactions of Fe2O3 in the fuel reactor produced less Fe and more FeO which was followed by the decrease of H2 production in the steam reactor. Therefore, the total efficiency decreased as the H2 production efficiency decreased.

Figure 8: Effect of S/B values during gasification on the total efficiency, power generation efficiency, H2 production efficiency, and net power generated (at MCdry = 5%).

3.3 Effect of steam-to-biomass ratio, S/B, during gasification Figure 7 shows the composition and yield of syngas produced by gasification for values of S/B from 0.6 to 1.0. It is clearly shown that the yield of syngas on a volume basis increased in proportion to the increase in S/B values, which was attributed to the production of more H2 and CO2. An increase in the steam feed during gasification boosted the rates of the water-gas shift and steam-methane reforming reactions. Thus, the gasification process produced more H2 and CO2 and less CO and CH4.

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CONCLUSION

A cogeneration system has been proposed to produce H2 and generate power based on the integration of drying, steam gasification, CLC, and power generation. The simulation showed promising results, as the total efficiency, H2 production efficiency, and power generation efficiency were approximately 72%, 57%, and 15%, respectively, when the cogeneration system was performed at values of MCdry of 5% and S/B of 0.7.

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REFERENCES

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ACKNOWLEDGEMENTS

This work was supported by JSPS KAKENHI Grant Number 16K18355.

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