Integrated drying and gasification of wet microalgae biomass to produce H2 rich syngas – A thermodynamic approach by considering in-situ energy supply

https://doi.org/10.1016/j.ijhydene.2019.02.165Get rights and content

Highlights

  • Present process configuration with a CLC provides energy for wet biomass drying.

  • The in-situ steam generation helps to increase the energy efficiency.

  • The CLC also reduces CO2-absorber duty.

  • Proper adjustment of CLC and gasification can enhance H2 concentration.

Abstract

A novel integrated drying and gasification of microalgae wet biomass process, involving a chemical-looping combustion (CLC) option to supply energy, is developed using Aspen Plus. The integrated gasification system consists of four primary units, including (i) a wet biomass drying unit, (ii) the gasification system, (iii) the CLC section, and (iv) the gas purification process. The model shows a good accuracy (relative error < 10%) in predicting the product compositions as compared to the experimental results under consistent operating conditions. The performance of the integrated gasification system is evaluated using Spirulina microalgae at various moisture contents (0–45 wt%). The effect of gasifying agents O2/steam and the fraction of the produced char used in the CLC section on the gasification performance is also evaluated. The tar is successfully reformed into syngas in the pyrolysis stage by adjusting the O2 flow rate. The C (char) to CLC provides to a positive effect on the syngas composition, particularly for gasification of wet biomass, but brings an adverse impact on the yield of the syngas product. The integration of the CLC process and CO2 absorber in the gasification system provides high-quality syngas by removing CO2. The separated pure CO2 can be used as a feedstock for other chemical industries.

Graphical abstract

Integrated drying and gasification of wet microalgae biomass, involving a CLC loop.

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Introduction

Recently, the microalgae biomass has received growing interest as a source of renewable energy. The cultivation and use of microalgae can address the CO2 emission issues as it recycles CO2 by photosynthesis [1]. The commercial scale production of microalgae biomass is more beneficial over terrestrial plants since it requires smaller land area per unit mass of biomass production [2]. The produced microalgae biomass can be directly combusted (after drying) to produce energy and/or converted into various useful products [3]. In this regard, the gasification is a promising approach for conversion of microalgae biomass into syngas (H2, CO). The flexibility of CO/H2 compositions in the produced syngas allows in a wide range of applications including in power generation and as a feedstock for commercial production of chemicals such as acetic acid, methanol, and ammonia [4].

Beside these attractive advantages, the large scale application of microalgae biomass gasification also encounters several challenges, including tar formation, which is responsible for reactor fouling, pipe blocking and production losses [5]. One way to control the fouling and blocking is physical separation of tar from the gasified products. However, the formation and separation of tar undermine the overall efficiency of the gasification process [6]. Consequently, the conversion of tar into syngas is consider to be preferable over the separation processes [7]. In biomass gasification, tar is mainly formed during pyrolysis stage, particularly at low operating temperature due to incomplete conversion of lignocelullosic biomass. Tar is a mixture of complex hydrocarbon oxi-compounds. The concentration of these compounds depend on the feedstock, the gasification process and the operating conditions [8]. A number of experimental studies reported in the open literature investigating various techniques to minimize tar formation. According to the literature, the efficient method for tar reduction includes: (i) efficient gasifier, (ii) use of efficient catalysts, and (iii) tuning of the parameters. The thermal cracking/gasification process is not economically preferable as it consumes high heat energy (high cost) to convert both the biomass and the tar into the syngas [7]. Typically, the operating temperature above 800 °C is required for tar reforming to ensure low tar concentration [9]. El Rub et al. [10] showed that the tar conversion is minimal at temperature below 700 °C. On the other hand, almost complete tar conversion can be achieved at over 800 °C [11]. The implementation of efficient gasifier design is another way to preserve overall efficiency by converting tar into syngas. Susanto and Beenackers [12] showed that the a continuous downdraft moving bed gasifier with internal recycle can effectively minimize tar formation. On the other hand, Brandt et al. [13] proposed a two-stage gasifier to minimize tar concentration in the gasification product. Application of a suitable catalyst also provides higher tar reforming relatively at lower temperature [7]. However, the catalytic process also faces some challenges, including the loss of textural properties, blocking pores and deactivation by coke formation [14], [15].

Apart from the after mentioned experimental investigations, there are studies reported in the open literature based on thermodynamic model evaluating the gasification performance at various process configurations. The thermodynamic modeling approach is faster and considerably cost-effective as compared the experimental investigations [16], [17]. The proper thermodynamic model can give an accurate prediction of the gasification performance under wide range of operating conditions. Surprisingly, only limited number of published articles considered tar formation and its subsequent gasification. One of the possible reasons is the complexity of tar composition and uncertainty of the model prediction due to improper selection of tar compounds [18]. Mostavi et al. [19] investigated the steam gasification performance of woody biomass with dry sorption CO2-capture. They used the empirical correlation by Fagbemi et al. [20] to predict tar yield as a function of operating temperature. The present research group also studied alternative process configurations using Aspen Plus demonstrating the gasification performance with C4H4O and mixture of CH2O, C4H6O3, C2H4O2 and C3H6O2 as tar compounds [21], [22].

In addition to tar minimization, handling of high water content in the microalgae is another important challenge in the gasification of microalgae biomass. The water content of the cultivated microalgae usually varies between 75 and 90 wt% on the wet basis [23], [24]. After separation, significant amount of water still remains in the produced biomass. Thus, the drying of microalgae is an energy-intensive stage prior to their gasification. In addition, the combination of high-water content feedstock and low temperature operation may leads to off specification product and even failure of gasification process. In order to negotiate the large amount of water in the microalgae, a number of researchers investigated the supercritical water gasification (SCWG) [25]. However, the main product of SCWG is CH4 (up to 50%) with minimum concentrations of H2 (∼10%) and CO (∼1%) [26], [27]. The high quality syngas can only be obtained at high temperature (∼650 °C) and very high pressure (∼250 bar) SCWG [28]. This severe operating conditions make the SCWG a cost intensive-process.

Recently, many attempts have focused on Spirulina microalgae due to its high-lipid content. However, few works reported the investigation on thermal conversion of Spirulina [29]. Therefore, it is critical to develop a cost-effective gasifier for wet microalgae gasification to produce high quality syngas. In this regard, the incorporation of chemical looping combustion (CLC) with the gasification process is considered as a promising approach for developing high-efficiency gasification of wet biomass [30]. The hot flue gas from both the reduction and the oxidation reactors of the CLC process can be used to evaporate the moisture in the biomass. In addition, the absence of N2 in the reduction reactor provides high concentration of CO2 in the flue gas [30]. Iron oxide-based metals (Fe2O3/Fe3O4) exhibited good performance as the oxygen carrier for CLC process. For instance, Chen et al. [31] reported almost complete conversion of spent isopropanol fuel using Fe2O3/Al2O3 with high concentration of CO2 in the combustion product. Serrano et al. [32] demonstrated 90% conversion of lubricating oil using Fe2O3 due to its catalytic activity for hydrocarbon cracking. The present group also reported a good performance of Fe2O3/Al2O3 on a heavy oil-fueled integrated CLC process for power generation and integrated CO2 capture [30].

The present study is focused on drying and gasification of wet microalgae biomass including the formation of tar during the pyrolysis stage. Indeed, the superior performance of drying can enhance the gasification performance due to minimum energy for drying. However, the vaporized water, which also contains significant amount of energy, is just disposed without further utilization. The novelty of the present work lays on the enhancement of the gasification performance by recapturing the energy for drying by sending the water vapor into the gasification system. In addition, vaporized water donates H atoms to form H2 at the given gasification condition. The introduction of the CLC unit also contribute to the efficient heat generation for the process with minimum CO2 emission. In this regard, a new integrated gasification process has been developed using Aspen Plus. As depicted in Fig. 1, the model consists of four major processing blocks: (i) drying of wet microalgae biomass and producing steam for gasification, (ii) gasification of the microalgae biomass, (iii) chemical-looping combustion (CLC) unit to supply energy for drying and gasification steps, and (iv) gas treatment process. The present model offers a new configuration for combining the advantages of the CLC flue gas (which is available at high temperature) for drying of wet biomass for gasification process. In addition, the inclusion of CLC process minimizes the required energy for CO2 separation (reduces CO2-absorber duty) due to the absence of N2 during syngas oxidation. The fuel for CLC process is a fraction of the produced gas from the gasification of char, which is obtained from the pyrolysis stage. Indeed, a critical modification on the char stream after pyrolysis stage is made in order to enhance H2 concentration and suppress CO and CO2 concentrations in the syngas. Thus, the suitable adjustment on the char split fraction (to the gasifier-1 and the gasifier-2) can provide the desired H2/CO ratio. One should note that the syngas composition is a major parameter when the syngas is used as the feedstock of other chemical industry. For example, the methanol synthesis requires syngas with H2/CO of 2 (two) [33]. One should note that high concentration of tar in the syngas is a serious problem. Thus, the configuration of the previous study by our group [21] is applied to reform tar from the pyrolysis stage into syngas. In this regard, tar is directed to the high-temperature combustion zone in order to facilitate steam reforming of tar. This method is adapted from the experiments by Susanto and Beenackers [12]. With this strategy, Susanto and Beenackers With this strategy, Susanto and Beenackers able to achieve the equilibrium condition successfully, which was indicated by minimum tar concentration using the modified down-draft gasifier. Therefore, it is suitable to model the present gasifier configuration using RGibbs.

The Spirulina microalgae species is selected as the biomass feedstock due to its high-lipid content. The system performance is determined by energy conversion efficiency and syngas quality. The energy conversion efficiency is determined by cold gas efficiency and gasification system efficiency while the syngas quality is decided by the dry-gas composition of the main species including H2, CO, CO2 and CH4. The performance of the integrated gasification system is examined using both oxygen and steam as the gasifying agents at various char split fraction and water content in the feedstock. The isothermal reaction condition is assumed in all simulations, except the CLC unit, which operates under adiabatic condition.

Section snippets

Process description

The details stream flow of the integrated drying (involving a CLC loop) and gasification process are presented in Fig. 2. The ultimate and proximate analysis and the heating value of the feedstock Spirulina microalgae biomass are presented Table 1. The simulations are carried out using a flow rate of 100 kg/hr Spirulina into the dryer (DY). High-purity O2 and steam are used as the biomass gasifying agents, while air is used in oxidation of the reduced oxygen carrier (Fe3O4/Al2O3 + O2 (N2) → Fe2O

Model development

The model of the main equipment (detail discussion is presented in the end of this section) is developed based on the Gibbs free energy minimization technique, which is a commonly used method to investigate the performance of the thermal conversion processes [34], [35]. The Peng-Robinson equation of state is applied in the simulation given the model can provide good accuracy for performance prediction of the thermal conversion processes [21], [36], [37]. Additional assumptions taken includes:

Performance evaluation

The performance of the integrated biomass drying and gasification system is determined according to (i) the composition of the syngas product, (ii) gasification system efficiency (GSE), and (iii) cold gas efficiency (CGE). The syngas composition is evaluated in dry gas basis, particularly for the concentration of H2 and CO. The GSE expresses the overall energy efficiency of the integrated gasification process, as shown in Eq. (15).GSE=msg×LHVsg+Qco1+Qco2+Qco3+Qco4mms×LHVms+Qpy+Qcm+Qgs2+Qab+

Model validation

The model validation is performed on the primary conversion units: (i) the gasification process and (ii) the CLC process. The accuracy of the gasification process is carried out by comparing the predicted composition of the pyrolysis product with the experimental composition of the pyrolysis product as reported by Hong et al. [29]. The simulation is carried out under isothermal conditions, which is consistent to the experimental works conducted by Hong et al. [29]. Table 4 depicts the predicted

Results and discussion

In this study, the parametric evaluation is performed by investigating (i) the influence of O2 and steam as the gasifying agents, (ii) the variation of the fraction of produced char used in the CLC loop and (iii) the variation of moisture content in the Spirulina microalgae biomass. The discussion of the parametric study is presented in the following sections.

Conclusions

A new configuration of the integrated gasification system, combining a wet biomass drying unit, two gasification units, a chemical-looping combustion (CLC) section for energy supply and the syngas purification system is developed using Aspen Plus Simulator. The Spirulina microalga is selected as the biomass feedstock. Following are the conclusion of this study:

  • i.

    A suitable feed rate of oxygen (O2 ER ratio) in the combustion zone allows complete conversion of tar into CO and H2 by reforming

Acknowledgements

The author(s) would like to acknowledge the financial support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. IN161022. The author (s) also would like to thanks the Department of Chemical Engineering at Universitas Islam Indonesia for the support and facilities.

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