Biofuels
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A kinetic-based simulation model of palm kernel shell steam gasification in a circulating fluidized bed using Aspen Plus®: A case study Maham Hussain, Lemma Dendena Tufa, Suzana Yusup & Haslinda Zabiri To cite this article: Maham Hussain, Lemma Dendena Tufa, Suzana Yusup & Haslinda Zabiri (2018): A kinetic-based simulation model of palm kernel shell steam gasification in a circulating fluidized bed using Aspen Plus®: A case study, Biofuels, DOI: 10.1080/17597269.2018.1461510 To link to this article: https://doi.org/10.1080/17597269.2018.1461510
Published online: 02 May 2018.
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BIOFUELS, 2018 https://doi.org/10.1080/17597269.2018.1461510
A kinetic-based simulation model of palm kernel shell steam gasification in a circulating fluidized bed using Aspen Plus®: A case study Maham Hussain, Lemma Dendena Tufa, Suzana Yusup and Haslinda Zabiri Department of Chemical Engineering, Universiti Teknologi Petronas, Malaysia
ABSTRACT
ARTICLE HISTORY
A detailed simulation model for hydrogen production using catalytic steam gasification of palm kernel shell in an atmospheric dual fluidized bed gasifier using an Aspen Plus® simulator is developed. The catalytic adsorbent-based steam gasification of palm kernel shell is studied in a pilot scale dual fluidized bed reactor using coal bottom ash as a catalyst for hydrogen and syngas production. The use of a catalyst along with the adsorbent improved tar cracking and enhanced the hydrogen content of syngas. The effect of temperature and the steam–biomass ratio on hydrogen yield, syngas composition and lower and higher heating values was studied. An increase in steam–biomass ratio enhanced the hydrogen content from 60 to 72 mol%%. The maximum value of hydrogen production, i.e. 72 vol% was achieved at a steam–biomass ratio of 1.7. The use of adsorbent and coal bottom ash had a significant effect on hydrogen and syngas yield. A maximum of 80.1 vol% hydrogen was achieved at a temperature of 650 C with a 1.25 steam–biomass ratio and 0.07 wt% coal bottom ash.
Received 29 September 2017 Accepted 22 March 2018
Introduction Gasification is a thermochemical process in which solid fuels (such as coal, biomass and municipal solid waste, etc.) or liquids fuels (such as waste lubricant oil, olive oil, etc.) devolatilize and are converted into high calorific value hydrogen and syngas [1]. Synthesis gas, or syngas, is a promising alternative energy due to its clean fuel properties. It is composed of, mainly, hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), nitrogen (N2), water vapor and other hydrocarbons. The gasification product gas usually contains impurities like tar particulate matter, sulfur oxide, nitrogen oxide and ammonia. Tar is a sticky material that deposits in the downstream equipment and blocks the supply lines [2]. Catalytic hot gas cleaning is a promising method by which tar can be completely removed and converted into product gas. It is an advantageous approach, but it may cause catalyst deactivation and adsorption of poisonous gas build up [2]. Syngas is used in various applications, such as transport fuel production and energy generation [3]. Biomass is the fourth most important source of energy after coal, petroleum and natural gas and provides about 10% of global energy [3,4]. The conversion of biomass into bio-energy products is an economical and environmentally-friendly process among the existing industrial processes [5]. In Malaysia, 12% of gross national income (GNI) is contributed by the agricultural sector and 8% comes from palm oil plantation. [3]. A significant amount of biomass is produced from palm
CONTACT Lemma Dendena Tufa
[email protected]
© 2018 Informa UK Limited, trading as Taylor & Francis Group
KEYWORDS
Palm kernel shell; fluidized bed reactor; gasification; coal bottom ash
plantation that is either landfilled as waste or left for mulching as organic fertilizer [6]. This palm biomass can be used as a feedstock for syngas production [6]. The biomass waste produced is in the form of, mainly, fibers and shells, which are typically burnt as fuel in boilers to produce steam or hot gas for drying [6]. In the past decade, the utilization of palm oil waste for fuel generation has increased substantially for air gasification. Hydrogen production of about 12–38 vol% was reported for empty fruit bunch (EFB) gasification at a high temperature of 1000 C [7]. Inayat et al. [8] studied gasification of EFB and reported that 76.5 vol.% of hydrogen at a steam/biomass of 2.5 was obtained. The syngas production via gasification of palm oil waste provides a viable solution to palm oil waste management and clean fuel production. Hydrogen plays a significant role as a fuel and chemical reactant due to its high energy content and low emission problems [9]. The hydrogen produced from biomass gasification can be used directly as a feedstock in proton exchange membrane (PEM) fuel cells or as a medium to produce numerous chemicals, such as methanol [10]. Among all the hydrogen production methods, gasification is the most promising method that can produce hydrogen along with syngas [11]. Several gasification agents, such as steam, pure oxygen, CO2, air or a mixture of O2 and steam, can be used in the gasification process. Schuster et al. [12] reported that gasification using air produces a synthesis gas of
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heating value of 4–7 MJ/N m3, where gasification with a mixture of oxygen and steam produces syngas possessing a heating value ranging between10–18 MJ/N m3. Steam is a preferred gasification agent due to its higher yield of N2-free syngas with a high H2 content, which makes it economical at both laboratory and commercial scales [13]. Steam gasification has advantages such as residence time characteristics, effective tar reduction and higher heating value [14]. The use of a catalyst improves the gas quality by converting higher hydrocarbons to lighter gases [15]. Also, use of a catalyst improves the reaction rate thereby making it possible to achieve gasification at a low temperature [16]. The conventional catalysts used in gasification processes are Ni, Fe, dolomite, olivine and alkaline earth metals [17,18]. These catalysts have their own advantages and disadvantages. Dolomite is cheap and good for tar reduction but its activity is low. Ni and Fe catalysts are more reactive but are expensive; they also pose problems such as deactivation and regeneration. Many authors have reported efficient and costeffective catalysts for biomass gasification systems [19– 22]. Coal bottom ash is a waste produced from the combustion of coal in power plants. It is mostly used in construction industries, and its dumping process is expensive because it contains toxic elements that pollute water and the atmosphere [23]. Recently, many researchers have reported a reasonable amount of alkaline metals, mainly Ca and Al in coal bottom ash [24]. Alkaline and CaO are conventional catalysts used in biomass gasification [25]. Xiong et al. [24] used coal bottom ash as a bed material for pyrolysis of coal and reported a good effect on tar reduction compared to sand. Umeki et al. [26] observed that coal bottom ash could maintain its catalyst activity in biomass gasification. It could be concluded that very limited work has been done on coal bottom ash utilization in biomass gasification. The net carbon dioxide equivalent (CO2e) and greenhouse gas (GHG) emissions from biomass combustion can be determined using the life cycle assessment (LCA) analytical method. LCA is a tool used to determine the sustainability of biofuels. The main steps in LCA include goal and scope, life cycle inventory, life cycle impact assessment, and interpretation [27]. This method provides an assessment of environmental impacts of the considered products and technologies using detailed input and output parameters that operate within the system boundaries. Bioenergy resources may be considered to be carbon neutral (no net effect on GHGs), carbon negative (net reduction in GHGs) or carbon positive (net increase in GHGs) depending on the net CO2e GHGs produced by the considered processes [28]. Akhil Kadiyala et al. [29] reported on gasification-based biofuel production systems considering the effect of different integration options producing
synthetic natural gas, methanol and Fischer–Tropsch (FT) fuels on the net annual profit (NAP), fuel production cost (FPC) and GHG emission reduction potential. An economic evaluation model can evaluate a bioenergy plant profitability. It can be developed by estimating the effects of varying parameter values on plant costs and revenues [30]. Biomass gasification simulation models are useful tools providing rigorous qualitative as well as quantitative information about the biomass gasification process. Some of the advantages of simulation studies over experimental studies are safety, time and cost. Simulation models can be broadly classified into steady-state models and dynamic (transient-state) models. The steady-state models are classified into kinetic rate models and kinetic-free equilibrium models [31–33]. Aspen Plus® is a process simulation software that includes the complete chemical unit operation modeling [34] necessary for developing both steady-state and dynamic models. Aspen Plus® is a problem-oriented software tool commonly used for biomass gasification modeling, to predict syngas and hydrogen composition for a specific feedstock, gasification conditions and gasification agents [14]. The Aspen Plus® process simulator, has been used by researchers to simulate coal conversion, including methanol synthesis [35,36], indirect coal liquefaction processes [37], integrated coal gasification combined cycle (IGCC) power plants [38] and fluidized bed gasification processes [39–42]. Aspen Plus® can be used for understanding the gasification process. Simulation models can be performed in a Gibbs reactor where no kinetics are applied [43,44]. The Gibbs reactor works on Gibbs free energy minimization to calculate the equilibrium. It does not require specified reaction stoichiometry [44]. Doherty et al. [44] developed the Aspen Plus® model for a circulating fluidized bed gasifier and validated it against actual plant data. The model was based on Gibbs free energy minimization using the equilibrium method. The effect of operating parameters such as gasification temperature, biomass moisture content, steam– biomass ratio, air–fuel ratio, and temperatures of air and steam were reported. The results of syngas composition, heating value and cold gas efficiency were reported to be in good agreement with published literature. Fernandez et al. [45] developed a manure dual gasifier equilibrium model based on Gibbs free energy minimization using Aspen Plus®. The effect of gasification temperature, steam, CO2 as a gasifying agent, gasifying agent–biomass ratio on syngas composition and LHV of syngas were studied. The results showed increases in H2 and CO content and decreases in CH4 and CO2 content. Although there are some equilibrium-based Aspen Plus® models for gasification [46–49], there are limited
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studies on the development of the kinetic-based Aspen Plus® model in the literature [50–52]. Gartner et al. [50] developed a semi-empirical kinetic-based entrained flow gasifier model in Aspen Plus®. Ternary fuel blends (dried lignite, extraction residue and char) in an air-blown pressurized entrained flow reactor (PEFR) using the pyrolysis product distribution data of Mill was used. Lee et al. [52] examined the different types of burner and the gasifier temperature of benchscale entrained flow gasifier in Aspen Plus®. The model is validated with the experimental data obtained from the 1-ton-per-day oxygen-blown Korea Electronic Power Corporation (KEPCO) research institute gasifier. In Aspen Plus®, a library model of the fluidized bed is added to simulate the fluidized bed unit operation. In this study, a simulation model of a dual fluidized bed reactor unit capable of predicting the steady state performance of an atmospheric PKS steam gasification using coal bottom ash as a catalyst is developed. The reaction rate kinetics were used for hydrogen and syngas production in the simulation model. The pre-mixed biomass and coal bottom ash with steam as a gasifying agent was fed inside the fluidized bed gasifier. The effects of temperature and steam/biomass were studied. A range of parameters was selected by experimental studies: (500¡700) C temperature and (0.5¡2) wt/wt steam/biomass. For the pilot-scale biomass gasification plant under study, 0.07% of coal bottom ash to biomass and 1.4% adsorbent to biomass were reported to be optimal amounts [53]. The effects of temperature and the steam–biomass ratio on hydrogen and syngas composition, as well as lower and higher heating values of product gases, were evaluated.
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Table 1. Proximate and ultimate analysis, the calorific value in the dry basis of palm kernel shell Proximate Analysis (wt. %) Ultimate analysis (wt. %) Moisture content 4.6 § 0.32 Carbon (C) 45.66 § 0.44 Volatile matter 72.4 § 0.50 Hydrogen (H) 6.82 § 0.38 Ash content 1.4 § 0.41 Nitrogen (N) 0.66 § 0.07 Fixed carbon (by 21.6 § 0.35 Sulphur (S) 0.46 § 0.05 difference) 46.4 § 0.94 Calorific value 17 § 0.45 MJ/Nm3 Oxygen (O) (By Difference) (dry basis)
the elemental analyzer. The furnace temperature is maintained at a constant temperature of 1000 C. Calorific value was determined by the IKA C-5000 model bomb oxygen calorimeter based on the ASTM E711-87 procedure. Approximately 5 mg of the sample is placed in a crucible and then put inside the decomposition vessel. The sample is ignited using oxygen gas of 99.98% inside the decomposition vessel. The calorific value obtained represents the higher heating value (HHV) of the sample. The determinations were repeated several times to check the accuracy of experimental results. The data showed that deviation higher than § 5% was discarded to assure the repeatability of experimental data. The results for proximate, ultimate and calorific values of the PKS are presented in Table 1. Sand was used as the bed material in the first reactor while calcium oxide was used in the second. The nitrogen flow rate varied from 6–8 m3 /h. Dolomite (CaO) was used as an adsorbent, which also acts as a catalyst and facilitates in tar cracking. It was ground and sieved to a particle size of 0.150–0.250 mm and obtained from the kinetic chemical producer, Sdn Bhd, Malaysia. The dolomite contains 88.5% of CaO, which is suitable for adsorption application [54].
Material and method Materials
Coal bottom ash
The palm kernel shell (PKS) used in the experiment was supplied by Kilang Sawit Nasaruddin Sdn Bhd located in Perak, Malaysia. It was sun dried for 4–5 days and then dried in an oven to ensure removal of free and bounded moisture. It was ground using a Fritsch 19/25 analytical mill and sieved to a particle size ranging from 0.5–2.0 mm. The proximate analysis, fixed carbon, volatile matter and ash content were determined using the thermogravimetric analyzer EXSTAR TG/DTA 6300 (Seiko Instrument Inc.) on a dry basis. In this analysis, an approximate 5 mg of the sample was heated from a temperature of 25 to 1000 C at a constant nitrogen flow rate of 100 C min¡1. The composition was determined based on the ASTM E-872 standards. In the ultimate analysis, elemental compositions such as carbon, hydrogen, nitrogen, sulfur and oxygen on dry wt% basis were determined using the LECO CHNS 932 elemental analyzer. An approximate 2 mg of sample is placed in a silver capsule before being transferred into
Coal bottom ash was used as a catalyst and is obtained from the boiler of the TNB Janamanjung Sdn Bhd power plant, Selangor, Malaysia. Figure 1 shows the surface structure and particle shape determined using Zeiss Supra 55 VP in a magnification range between 1000–5000 £. It was observed that the surface was made up of coarse and irregular-shaped particles. The coal bottom ash is composed of alkaline oxides as revealed in X-ray fluorescence (XRF) analysis (see Table 2). The presence of Fe2O3, Al2O3 and MgO content reveals its potential as a catalyst. In the literature, the catalytic effect of alkaline metal oxides in the gasification process is widely reported [24,55,56].
Experimental setup The pilot-scale gasification system for PKS steam gasification is as shown in Figure 2. The gasification process setup consists of dual fluidized bed reactors with
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Figure 1. (a) FESEM image of the bottom ash at 1000£ magnification; (b) at 5000£ magnification.
external heaters, a biomass feeding system, cyclone separators, a steam generation system with superheater, cyclone, a wet scrubber, a water treatment system, a steam generation system, a water separator and an online gas analysis system. During fluidized bed conditions, a perforated plate was used for air and steam flow. The length and diameter of the reactor were 250 and 15 cm, respectively. Silica sand was used as bed material in the fluidized bed. The biomass was pre-mixed with coal bottom ash, and approximately 1 kg/h was fed continuously using N2 gas to ensure forward flow. A cooling jacket with a feeder was installed to avoid thermal degradation of the biomass before entering it into the gasifier. Steam was generated in the boiler and heated in a superheater up to 350 C. The steam was supplied at 0.5 to 1.5 kg/h to the gasifiers. The gases (H2, CO2, CO and CH4) produced in the first reactor were passed through the dolomite bed of the second reactor. The height and diameter of the second reactor were 250 and 15 cm, respectively. Both reactors are made of Inconel 625 material, allowing its use up to 850 C. Uniform heating was ensured using eight electrical heaters, and thermocouples were used to determine the temperature profile within the gasifier. The entire piping system was encased with insulation tape to avoid clogging caused by tar condensation. A cyclone separator was used to remove tiny solid particles from the product gas. A water scrubbing system was used to reduce the temperature of gases up to 40 C. A separator was used to remove residual water from the product gas. Finally, product gas was fed into the sampling unit,
where its composition was measured using an online gas analyzer (Teledyne 7500). The gas analyzing system measures gas composition every minute for all experimental runs. The system was purged with nitrogen before the start of each run.
Aspen Plus® simulation model Model assumption The following assumptions were considered in the modeling of gasification process: 1. The de-volatilization occurs rapidly and yields volatile products comprising of H2, CO, CO2, CH4 and H2O. 2. The particles are of spherical and uniform diameter. 3. The char is comprised of carbon and ash. 4. The char gasification starts in bed and continues in the freeboard. 5. The tar and higher hydrocarbons are not considered. 6. The catalytic effect of CaO is not considered.
Aspen Plus® model description The schematic flowsheet of the PKS steam gasification process developed in Aspen Plus ®is as shown in Figure 3. The flowsheet represents the experimental setup used for validation of the simulation model. Overall, the process is composed of four main sections:
Table 2. XRF, BET surface area and average pore diameter analysis of coal bottom ash Chemical composition %
SiO2 Al2O3 MgO 38 24.8 13.7
CaO 6.3
SO3 Na2O 6.2 5.98
Fe2O3 2.16
P2O5 MnO 1.1 0.47
K2O 0.3
Cr2O3 Rh2O3 0.08 0.05
Surface properties Adsorption average pore width (4 V/A by BET): (m2/g)
47.64
Adsorption average pore width (m2/g)
0.014 Average pore size nm
4
Particle density (kg/ m3)
1400
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Figure 2. Process flow diagram of the pilot-scale gasification system.
steam generation, biomass feeding system, biomass gasification, solid separation unit, moisture and impurities separation unit. The description of Aspen Plus® blocks involved is summarised in Table 3. The Aspen Plus® yield reactor block was used to decompose biomass material. In Aspen Plus®, biomass is defined as a heterogeneous solid and regarded as a non-conventional stream because of non-defined molecular weight. In this reactor, biomass is converted into carbon, hydrogen, oxygen, nitrogen, sulfur and ash. The yield distribution is specified according to biomass ultimate analysis. The constituent stream from the R-yield reactor was fed into the fluidized bed reactor block. Perfect mixing inside the fluidized bed reactor was ensured by continuous and vigorous movement of bed material. The reaction kinetics are defined in each reactor block. The fluidized bed reactor
block is composed of two main sections – the dense bed and freeboard – that are defined through hydrodynamic parameters. The gas phase reactions were well-defined as equilibrium and kinetic rate reactions and were used to estimate the char gasification products. The super-heated steam at 350 C as a gasifying medium was supplied to the fluidized bed reactors. The physical properties of solids, the non-conventional components, were calculated using the Peng– Robinson-Boston—Mathias (PR-BM) model. This property package defined both the pure and mixed component properties. The HCOALGEN and DCOALIGHT property models were used to calculate the enthalpy and density of the biomass, char and ashlike components. The chemical reactions involved in the biomass gasification process are listed in Table 4 [43].
Figure 3. Aspen Plus® simulation schematic flowsheet for PKS gasification.
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Table 3. Process blocks description used in the simulation Unit Yield reactor Super-heater Fluidized bed reactor Cyclone Flash
Aspen Plus Block R-yield Sup-HTR CFB Cyc-Sep
Description To perform elemental decomposition of feed by specifying reaction yields of each component. It increased the temperature of water to make it superheated steam. This model is used when the reaction kinetics are known. It simulates the process efficiently involving solids such as char in the reactions. It separates solids from the product gas. It is known as two-outlet flash. This model works on the principle of rigorous vapor–liquid or vapor–liquid–liquid equilibrium.
Table 4. Chemical reactions involved in PKS gasification Reaction # 1 2 3 4 5 6 7 8 9
Reaction name
Reaction equation
Combustion reaction Combustion reaction Bourdouard reaction Methanation reaction Methanation reaction Water gas shift reaction Water gas shift reaction Carbonation reaction Steam reforming
C þ O ! CO C þ O2 ! CO2 C þ CO2 $ 2CO C þ 2H2 $ CH4 2C þ 2H2 O ! CH4 þ CO2 C þ H2 O $ CO þ H2 CO þ H2 O $ CO2 þ H2 CO þ CaO $ CaCO3 CH4 þ H2 O ! CO þ 3H2
Results and discussion
can be considered valid in terms of representing the gasification process.
Model validation To validate the simulation results, five experimental data from gasification of PKS in a pilot-scale fluidized bed gasifier at temperatures of 500, 550, 600, 650 and 700 C and steam–biomass ratios of 0.5, 1, 1.5 and 2 were used, as explained in the previous section. Simulation results were compared with all sets of experimental data. The sum-squared deviation method was used to calculate the accuracy of simulation results: RSS ¼
2 N X yie yip i¼1
yie
(1)
e = Experimental P = Predicted RSS N pffiffiffiffiffiffiffiffiffiffiffi Mean Error ¼ MRSS MRSS ¼
Heat of reaction ΔH(KJ/mol) ¡111 ¡283 +172 ¡75 +103 +131 ¡41 ¡170.5 +206
(2) (3)
The steam gasification of PKS was carried out by varying the temperature from 500 to 700 C and the steam–biomass ratio (0.5–1) at an adsorbent– biomass ratio of 1.42 and 0.07% coal bottom ash as a catalyst at atmospheric pressure. In this simulation, maximum H2 content was found at a reactor temperature of 680 C and a steam–biomass ratio of 2. It was also observed that a lower temperature is needed when the steam–biomass ratio is higher. The comparison of experimental results with model results is shown in Table 5. The variations are somewhat due to the presence of impurities like tar and char particles, which were not considered in the simulation model and may be a possible cause of the discrepancies. Consequently, a simulation model
Effect of gasifier temperature on synthesis gas composition In this study, PKS steam gasification was carried out at a temperature range of (500–700) C. The effect of temperature on product gas composition and lower and higher heating values were discussed. For the pilot-scale biomass gasification plant under study, 0.07% of coal bottom ash to biomass and 1.4% adsorbent to biomass were reported to be optimal amounts [53]. The hydrogen and syngas composition vary significantly with temperature. Hydrogen production increased from 45.5 to 80.1 vol% as temperature increased from 600 to 705 C at a steam–biomass ratio of 1.5, as shown in Figure 4. The percentage composition of CO decreased from 20.1 to 8.0 vol% when temperature increased. The endothermic and exothermic reactions (1) to (7) were responsible for the product gas composition. The hydrogen and syngas composition was due to the water gas shift reaction (6) in the temperature range from 500 to 700 C. A similar trend for catalytic steam gasification of palm oil waste for a temperature range from 650 to 692 C has been investigated [15]. The CaO as an adsorbent is significant for syngas composition in the gasification process. The CO content ranges from 20.1 to 8.0 vol%, as shown in Figure 4. The use of adsorbent increased syngas and hydrogen yield, giving the lowest value of CO at 700 C. The effect of CaO on product gas composition in steam gasification was also reported between the temperature range 600–670 C in the literature [57]. The composition of methane decreased from 23.4 to 10.1 vol % when the temperature was increased from
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Table 5. Synthesis gas composition (dry N2 vol%) obtained with the simulation and plant experimentation data for temperature and steam–biomass ratios (a) Effect of temperature (500–700) C on syngas composition at an adsorbent–biomass ratio of 1.42 and a steam–biomass ratio of 1.25 (w/w) used with 0.07% coal bottom ash as a catalyst Temperature 500 C 550 C 600 C 650 C 700 C Components Sim. Exp. Sim. Exp. Sim. Exp. Sim. Exp. Sim. Exp. Overall mean error H2 45.1 40.1 58.4 57.3 70.4 67.2 74.3 72.3 80.1 71 0.0077 CO 20.1 17 24.3 27.4 13 12.3 12 14 8 7 0.0180 23.4 22 14.1 12.5 14.4 12.2 11.1 10.2 10.1 10 0.0121 CH4 11.3 10 9.6 8.3 6.5 7 5.3 5 4.6 2.3 0.2100 CO2 (b) Effect of steam–biomass ratios of 0.5–2 on syngas composition at an adsorbent–biomass ratio of 1.42 and a gasifier temperature of 650 C with 0.07% coal bottom ash as a catalyst Steam–biomass 0.5
1
1.5
2
Components
Sim.
Exp.
Sim.
Exp.
Sim.
Exp.
Sim.
Exp.
Overall mean error
H2 CO CH4 CO2
60.1 16.1 12.5 10.5
58.6 14.3 9.5 8.9
67.9 15 12.1 10
64.8 12.7 11.5 9
73.5 10.2 11 9.4
70.9 9.2 8.1 7.6
75.5 11.5 8 5.8
72.4 9.3 7.1 4
0.0391 0.1705 0.2483 0.2753
500 to 700 C, as shown in Figure 4. This result was due to an endothermic methane reforming reaction that enhanced the H2 and CO content in the syngas. It is reported that a temperature between 650–800 C is important for H2 production with CaO sorption in the fluidized bed gasifier [58]. In the literature, a decrease in methane content was observed with an increase in temperature from 650 to 700 C in biomass steam gasification using a fluidized bed gasifier [59]. The endothermic tar-cracking reaction supported at higher temperatures, which increased the hydrogen composition in the product gas [60]. In the present study, it was observed that H2 content increased with temperatures up to 700 C while the CO, CH4 and CO2 content decreased. Inayat et al. [8] investigated an increase in hydrogen yield for
in-situ steam gasification of empty fruit bunches (EFBs). The increase in hydrogen yield in relation to temperature was also observed for pine sawdust feedstock in steam catalytic gasification with CaO sorbent [61]. The lower and higher heating values using the molar content of CO, H2, and CH4 were determined by Equations (1) and (2), respectively as shown in Figure 5 [15,62]. LHVgas ¼ ð30 CO þ 25:7 H2 þ 85:4 CH4 Þ 0:0042
(4)
HHVgas ¼ ðH2 30:52 þ CO 30:18 þ CH4 95Þ 0:0041868
(5)
From Equations (10) and (11) it is seen that the product gas heating value is dependent on the H2, CO and CH4 content. The LHVgas decreased from 17.5 to 11.01 MJ/Nm3 with an increase in temperature from 500 to 700 C as shown in Figure 5. Gupta et al. [63] determined a similar trend for the heating value of palm residue steam gasification in a semi-batch reactor.
Effect of the steam–biomass ratio on synthesis gas composition
Figure 4. Effect of gasifier temperature (500–700) C on syngas composition at an adsorbent–biomass ratio of 1.42 and a steam–biomass ratio of 1.5 (w/w) used with 0.07% coal bottom ash as a catalyst.
Steam is one of the most promising gasifying agents for hydrogen and syngas production in the gasification system. In the present work, steam gasification of PKS catalytic gasification using CaO as a sorbent has been studied. The effect of steam–biomass ratios ranging from 0.5 to 2 (w/w) on product gas composition, lower and higher heating values were studied. The following parameters were used in this study: temperature
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Figure 5. Effect of gasifier temperature (500–700) C with a steam–biomass ratio of 1.5 (w/w) on (a) LHV and (b) HHV of syngas composition at an adsorbent–biomass ratio of 1.42 and a steam–biomass ratio of 1.25 (w/w) used with 0.07% coal bottom ash as a catalyst.
650 C, biomass flowrates 1 kg/h, sorbent–biomass ratio 1.42 and coal bottom ash 0.07% of biomass weight. Hydrogen content increased with an increase in the steam–biomass ratio, as shown in Figure 6. The hydrogen content produced was 60.1 vol% at a 0.5 steam–biomass ratio and it increased to 67.9% at a steam–biomass ratio of 1. The maximum hydrogen content produced was 75.5 vol% at a steam–biomass ratio of 2. The increase in hydrogen yield with steam resulted from the methane reforming reaction (9), water gas shift reaction (7) and char gasification reaction (6). Syngas production increased as the steam–biomass ratio increased. Figure 6 shows an H2 increase with an increase in the steam–biomass ratio. In contrast, the concentration of CO, CO2 and CH4 decreased with an increase in the steam–biomass ratio. The increase in steam supply shifts the equilibrium of the water gas
shift reaction (7) in a forward direction. A similar trend was observed in the literature [51]. At a steam–biomass ratio of 2, CO content was high at approximately 16.1 vol%. The decrease in CO composition results from the water gas shift reaction (7) that consumed CO produced in the steam methane reforming reaction (9). These reactions were also reported previously for steam biomass gasification [59]. The methane content produced decreased from 12.1 to 8 vol% in steam gasification of PKS. The increase in steam promoted a steam methane reforming reaction and decreased methane content in the product gas. The optimum steam–biomass ratio is significant for syngas production whereas excessive steam reduced the gasification temperature [64]. The lower content of CO2 shows sorption activity of CaO in the process. The higher syngas composition is due to the presence of Al2O3, Fe2O3 and MgO in the coal bottom ash for catalytic steam gasification. The Fe2O3 and Al2O3 content improves the char gasification [65]. The LHVgas and HHVgas can be calculated using the following equations: [13,49]. LHVgas ¼ ð30 CO þ 25:7 H2 þ 85:4 CH4 Þ 0:0042
(6) The LHVgas of gas is mainly contributed by methane, hydrogen and carbon dioxide content. The decrease in LHVgas was because of a decrease in CH4 and CO content as shown in Figure 7. Khan et al. [15] also reported a decrease in heating value of gas for steam gasification of PKS. The higher heating value (HHVgas) decreased from 17.7 to 14.38 MJ/Nm3 with an increase in the steam–biomass ratio as calculated using Equation (2). Figure 6. Effect of steam–biomass ratios of 0.5–2 on syngas composition at an adsorbent–biomass ratio of 1.42 and a gasifier temperature of 650 C with 0.07% coal bottom ash as a catalyst.
HHVgas ¼ ðH2 30:52 þ CO 30:18 þ CH4 95Þ 0:0041868
(7)
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Figure 7. Effect of steam–biomass ratios of 0.5–2 on (a) LHV and (b) HHV on syngas composition at an adsorbent–biomass ratio of 1.42 and a gasifier temperature of 650 C with 0.07% coal bottom ash as a catalyst.
The decrease in HHVgas is due to the low content of CH4 and CO at a steam–biomass ratio of 2 as shown in Figure 7. It was noticed that the decrease in CO and CH4 content was higher than the increase in H2 content. Li et al. [66] reported similar trend results for palm waste gasification. The performance of gasifier regarding cold gas efficiency is determined using a lower heating value and product gas composition. Xiong et al. [24] observed the presence of Fe2O3, Al2O3, CaO and MgO in coal bottom ash, which is important in the determination of product gas composition, tar reduction and higher gasification efficiency [17]. The hydrogen content increased while the CO, CO2 and CH4 content decreased. The increase in hydrogen content is due to the water gas shift reaction (7) and steam reforming reaction (9). The use of coal bottom ash on tar reduction was also observed in pyrolysis of coal [24]. The catalyst improved the solid carbon conversion into the gaseous product and also enhanced tar cracking, which results in enhanced hydrogen content in the syngas. Chin et al. [67] reported a lower amount of residue left after catalytic gasification in comparison to noncatalytic gasification. The CaO used as an adsorbent for CO2 sorption in PKS steam gasification promotes the water gas shift reaction and steam methane reforming reaction in the gasification [68]. The CaO adsorbed CO2 via the water gas shift reaction (7) in which it reacts with CO2 and results in CaCO3 formation. This CaO effect occurs in two stages: in the first stage, production of hydrogen increased due to the catalytic effect of methane reforming and tar-cracking reactions; in the second stage, hydrogen production is enhanced due to CO2 capture via CaO [15,69]. The CO2 adsorption shifted the equilibrium of the water gas shift reaction and steam methane reforming reaction in the forward direction, which consequently
increased the hydrogen content in the syngas composition. Khan et al. [15] identified the vital effect of CaO in the biomass steam gasification process. The syngas and hydrogen sensitivity to CaO was also reported in the literature [70].
Conclusion Catalytic-sorption-based steam gasification of PKS using coal bottom ash as a catalyst in a pilot-scale plant was investigated. Hydrogen production increased from 45.5 to 80.1 vol% as temperature increased from 500 to 700 C. The percentage composition of CO decreased from 20.1 to 8 vol% as the temperature increased. The use of an adsorbent increased the syngas and hydrogen yield, producing the lowest value of CO2 at 700 C. The LHVgas decreased from 17.5 to 11.1 MJ/Nm3 with an increase in temperature from 500 to 700 C. The steam has a significant effect on hydrogen content as a gasifying agent but at a specific range. The steam–biomass ratio increased the hydrogen from 60 to 70.5 vol%. Hydrogen production was 60.1 vol% at a 0.5 steam– biomass ratio and increased to 70.5% at a steam– biomass ratio of 1. The maximum hydrogen production achieved was 67.9 vol% at a steam–biomass ratio of 1.5. The higher heating value (HHVgas) decreased from 17.7 to 14.38 MJ/Nm3 with an increase in the steam– biomass ratio.
Nomenclature BET EFB HHV LHV MRSS
Brunauer–Emmett–Teller Empty fruit bunch Higher heating value Lower heating value Mean residual sum of squares
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PKS RSS S–B XRF
Palm kernel shell Residual sum of squares Steam–biomass ratio X-ray fluorescence
[15]
[16]
Acknowledgements This research project is funded by the Ministry of Higher Education, Malaysia under the Research Grant Scheme (FRGS 0153AB- 40) and the University of Teknologi PETRONAS, Malaysia.
Disclosure statement
[17]
[18]
No potential conflict of interest was reported by the authors. [19]
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