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Syngas production from steam gasification of Palm kernel shell with subsequent CO2 Capturing using CaO sorbent: An Aspen plus modelling Muhammad Shahbaz, Suzana Yusup, Dr. Abrar Inayat, Muhammad Ammar, David Onoja Patrick, Angga Pratama, and Salman Raza Naqvi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02670 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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Syngas production from steam gasification of Palm kernel shell with subsequent CO2 Capturing using CaO sorbent: An Aspen plus modelling. Muhammad Shahbaz1,3, Suzana Yusup1*, Abrar Inayat2, Muhammad Ammar1, David Onoja Patrick1, Angga Pratama1, Salman Raza Naqvi1 1. Biomass Processing Lab, Centre of Biofuel and Biochemical Research , Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. 2. Department of Sustainable and Renewable Energy Engineering, University of Sharjah, 27272 Sharjah, United Arab Emirates 3. Department of Chemical Engineering, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan *[email protected] Abstract: The current work is based on the simulation modelling of steam gasification of PKS with CO2 capturing through sorbent (CaO) using Aspen plus®. The simulation model is developed using Gibbs free energy minimization method. The objective of this work is to investigate the effect of key parameters like temperature, steam/biomass ration and CaO/biomass ratio on syngas yield. The system performance was also evaluated through carbon conversion efficiency, cold gas efficiency and gasification efficiency, lower and higher heating values by varying the gasification temperature, steam/biomass ratio and CaO/biomass ratio. The H2 concentration increased from 65 vol% to 79.32 vol% with the increase of temperature from 650-700°C. The CO2 content was reduced from 20 vol% to 5.32 vol% by increase in CaO/biomass ration from 0.5-1.42. The maximum hydrogen content predicted is 79.32 vol% and minimum CO2 content is 5.42vol% found at operating parameter including temperature of 700°C, steam/biomass ratio of 1.5 and CaO/biomass ratio of 1.42. In addition the simulation model predicted results were compared with experimental data obtained from the experimental set up used in the simulation. Key words: PKS, CaO/biomass ratio, Syngas, CO2 capturing, Aspen Plus Nomenclature: Carbon conversion efficiency: CCE 1 ACS Paragon Plus Environment

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Cold gas efficiency: CGE Gasification efficiency: GE Palm kernel shell: PKS lower heating value of gases: LHVgas Higher heating value of gases: HHVgas Empty Fruit bunches: EFB 1. Introduction: The world’s commercial and domestic energy demand comprises of fuels from coal, petroleum, natural gas which makes up about 81.1% of the world energy mix 1. This ample dependence on fossil fuel for energy has not only increased the average world temperature but also rattled weather cycle in most part of the world. This is evident

by its 84%

contribution to total greenhouse gas (GHG) emissions 2. In addition, finite and unequal distribution of reserves, imbalance energy trade and brusque changes in oil prices have raised concerns on energy security 3. The above mentioned issues forced the world to make the Doha amendment to Kyoto Protocol in 2012 and Paris agreement in 2016, aimed at to reducing the use of fossil fuel and acclimate efforts to find new and sustainable resources 4. Biomass can be a favourable option to replace the fossil in the near future. It has advantages such as; sewing up the gap between supply and demand of energy due to its abundant availability about 200-700 EJ/yr 5, no depletion threat due to sustainable life cycle

6

and

environmentally friendly due to CO2 neutrality 7. Biomass is only renewable source that can be converted into both liquid and gaseous fuels 8. The extraction of energy through biomass conversion is well documented 9. In comparison to the biochemical conversion of biomass, thermo-chemical gasification process is more propitious for fuels production like syngas and methane. These fuels can be used as substitutes for petroleum products without any major modification in existing infrastructure and machinery8, 10. Syngas is very importance in the current and future energy mix due to its numerous applications in fuels like methane and hydrogen, and production of chemicals like ammonia, urea, methanol and ethanol

11, 12

. In gasification process, biomass is converted into the

gaseous products (syngas) in a controlled amount of gasifying agent (oxygen, air and steam). Syngas produced is a mixture of H2, CO2, CH4, CO, N2 and water vapours along with other 2 ACS Paragon Plus Environment

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impurities like H2S, NH3 and tar 13. Steam gasification is a preferable option over the other gasifying agents as it enhances the quality of syngas by increasing the H2 content through acceleration of water gas shift reaction

14

. Basically the three types of gasifiers used in

gasification are the fixed bed gasifier, fluidized bed gasifier and entrained flow gasifier 5. Fluidized bed gasifier is often preferred to others due to advantages such as tar reduction, good solid–gas contact which enhances heat and mass transfer and good control of temperature 15. The utilization of biomass for syngas production has been the prime focus of research from the last decade. Like every country, Malaysia is also focusing on exploring its biomass potential for value added product. Malaysia has 41% share of palm oil export in the world that generated about 198 million tonnes of palm oil waste every year 1. Several research works in Malaysia concentrated on syngas production using biomass, especially palm oil residue

16, 17

. Khan et al.

18

used palm kernel shell for steam gasification in fluidized bed

gasifier using Ni catalyst and CaO, and reported 82 vol% of hydrogen. The simulation modelling is very useful since it provides precise analysis of a process without performing experimental study. Suitable selection of simulation model helped to find the optimized process that can reduce time and expenses involved in experimental studies

19

. Aspen plus

simulator is a software tool used for the simulation of gasification process, combustion and coal gasification along with the integrated coal/biomass gasification to predict the syngas, synthetic natural gas (SNG) and power production

19, 20

. Puig et al.

21

summarized the

simulation work on gasification. An equilibrium model based on Gibbs energy minimization for steam gasification of biomass is developed to predict the optimum parameters for gasification by Sreejith et al.

22

. They ignored the tar conversion and assumed 100%

conversion of char to products. The model predicted a maximum H2 content of 59.3% at an optimum temperature of 973K and steam/biomass ratio of 1. Nikoo and Mahinpy

20

used

Aspen plus simulator to develop a model of fluidized bed gasification along with hydrodynamics and compared it to experimental results. They used the reaction kinetics in the simulation and studied the effect of equivalence ratio (ER), temperature, particle size and steam/biomass ratio on syngas production. Naveed et al. 23 also used reaction kinetics for the development of a simulation model for gasification in three stages, drying, decomposition of biomass and gasification on the basis of Gibbs free minimization approach. They varied the operating conditions like temperature, ER, steam/biomass ratio, moisture content and

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analysed the effect on the predicted syngas from the gasification of municipal poultry and food wastes. Hydrogen is an important component of syngas that has received a lot of attention in the last a few decades due to its clean and high energy density 19. Hydrogen content of syngas can be increased by using steam gasification as well as removal or capturing of CO2. CO2 is normally removed in the downstream process by using Slexol® and amine solution. CaO was used to capture CO2 within the process and it was found to be very effective for CO2 sorption 24

. CaO captured the CO2 through carbonation reaction and increase the H2 content in syngas

25

. Very few experimental and simulation studies have reported the use of CaO for CO2

capture. Acharya et al.

26

developed a thermodynamic model on the basis of Gibbs free

energy and used sorbent for CO2 capturing. The simulation result was compared with experimental result in which predicted hydrogen content was very compared to the experimental result. The simulation model was not in thermodynamic equilibrium hence it resulted in overproduction of H2. The deviation was overcome by using appropriate coefficients to ensure thermodynamic equilibrium. The use of CaO sorbent for CO2 was deeply studied by Khan et al. 27 and introduced integrated catalytic gasification system. They utilized palm kernel shell (PKS) for the steam gasification by using Ni catalyst and CaO as a sorbent bed material for CO2. They varied the temperature, particle size, steam/biomass ratio, fluidization velocity and sorbent to biomass ratio and obtained about 82.01% H2 content with the negligible amount of CO2 at a temperature of 675°C and steam to biomass ratio of 2. In case of modelling with Matlab, Inayat et al.

28

used CaO sorbent in kinetic modelling for the

EFB gasification and predicted that 76% of hydrogen alongside 12% of CO2. From above discussion, it could be concluded that very few work has been reported for modelling of sorption based gasification process. Some of the reported works were based on kinetic modelling. Gasification process using CaO sorbent for CO2 in equilibrium model has not been discussed adequately in literature. According to author’s best knowledge, no work has been reported on Aspen plus simulation for steam gasification by enabling the CO2 capturing with CaO sorbent. In addition, very few modelling studies are available for PKS steam gasification for syngas production. The objective of this study has two parts, First to develop a simulation sorption-enabled model for steam gasification of PKS while investiging the effect of four process parameters temperature (650-750°C), steam/biomass ratio (1-1.5) and CaO/biomass ratio (0.5-1.5) on syngas composition, lower heating value (LHVgas), higher heating value (HHVgas), carbon conversion efficiency (CCE), gasification efficiency 4 ACS Paragon Plus Environment

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(GE) and cold gas efficiency (CGE). In the second part, the result of the simulation model is compared with the experimental results obtained from the pilot scale setup that was simulated in the model. 2. Methodology 2.1 Experimental setup: The pilot-scale gasification setup used for the validation of predicted results obtained from the model is shown in Figure 1. The system consists of two reactors, steam generating system, water treatment system, water scrubbing and cooling, cyclone and gas analysing system. In this process, the biomass is fed into the fluidized bed gasifier at a flow rate of 1 kg/hr using screw feeder. Silica sand is used as a bed material to maintain fluidization. Water is supplied in the boiler to generate steam at 150°C and heated up to 350°C in the superheater. The high-temperature steam is introduced to the fluidized bed gasifier. The gasification reaction takes place in the gasifier and the product gases passed through the second reactor where they come in contact with the CaO bed to enable the sorption of CO2. Both reactors are heated to the desired temperature with the aid of external jacketed electrical heater. The ash is removed at the bottom of the reactor. After the sorption action, the gases pass through the cyclone separator to remove the tiny solid particle from the gases. The cleaned gases are passed to the water scrubbing system at a temperature above 600°C where the gases are cool down to 25°C. Water is separated in a separator in the form of condensate. The cool and clean gases are directed to the on-line gas analysing system to measure the composition of syngas.

Figure.1 Process flow diagram of Pilot scale plant for sorption enhanced steam gasification. 2.2 Model development 5 ACS Paragon Plus Environment

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The Gibbs energy based model for steam gasification of biomass is developed using Aspen plus v8®. The model is incorporated with CaO to analyse the effect of sorbent to capture the CO2 and its consequence on syngas composition. The process flow sheet developed shows the different step involved in this sorption enhance steam gasification as shown in Figure. 2. Some assumptions made for the formulating of smooth simulation are given below. •

Steady state process on Gibbs free energy minimization

•

Uniform temperature and pressure inside the gasifier

•

The system operates at atmospheric pressure

•

All gases are considered ideal

•

Char is considered as C-graphite

•

Biomass is considered as non-conventional

•

Tar and higher hydrocarbon are not considered

•

The catalytic effect of CaO is not considered

•

All the N2 assumed to be converted into NH3.

2.3 Physical property and method In order to use solid biomass gasification in the simulation process, the “solid with metric unit” flow sheet type was chosen that enable the use of the solid component in the simulation. The stream class MIXNCPSD was picked for simulation. This stream class deal with aqueous stream (MIXED) nonconventional solid biomass with particle size distribution (NCPSD) and ash with NCPSD stream. The method is used commonly to describe the thermodynamic properties and calculation of conventional components. Peng-Robinson with Boston Modification (PR-BM) property package is selected. PM-RM is a recommended property package for refinery, petrochemical units, crude oil conversion and gas processing units. This is a very useful method for mixture of hydrocarbon and light gases (H2, CO, CO2 and H2) and result generated is good for all temperatures and pressures 7. Both biomass and ash are considered to be non-conventional. The density and enthalpy of the biomass were taken to be similar to that of coal by using HCOALGEN and DCOALIGT method provided by Aspen Plus. The ash properties were also considered by using HCOALGEN and DCOALIGT. Thermophysical data based on conventional component in Aspen Plus were used for the fluid streams. The solid component like CaO, CaCO3 and C-graphite were modelled using


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thermophysical data stored in Aspen plus data bank. In addition, no input data was required for some components namely: CO, CO2, H2, CH4, N2 and CH4. Table 1 Proximate and ultimate analysis of PKS 1 Moisture

9.70

Proximate analysis (dry mass fraction basis) Volatile matter (%) Fixed carbon (%) Ash content (%)

80.81 14.25 4.94

Ultimate Analysis (dry mass fraction basis) C (%) H (%) N (%) S (%) O (%) (by difference) HHV(MJ/kg)

48.78 5.70 1.01 0.21 44.3 18.82

2.4 Model Description: The Aspen Plus flow sheet for sorption enhanced gasification process is shown in Figure 2. The description of the blocks and their used are summarised in Table 2. The feed material stream, BIOMASS was considered to be a non-conventional material fed into a yield reactor, DECOMP which converted non-conventional biomass into conventional component C, H, O, S, H2O, N2 and ash according to yield distribution shown in Table 1. Ash was considered to be a non-conventional material and 100% ash was selected during component attributes for proximate and ultimate analysis. The stream ELEMENTS containing decomposed elements entered into Gibbs free reactor GASFIR1. The stream WATER entered into the BOILER where it converted into steam at 150°C. The stream, STEAM was heated into SPRHTR up to 350°C and directed to GASIFIR1. The GASIFIR1 is an equilibrium reactor working on the Gibbs free energy minimization method that converted biomass into product gases through gasification reactions as mentioned in Table 2. The PROG1 went through another Gibbs free energy reactor named GASFIR2 where CO2 is captured by CaO through carbonation reaction. The ash and CaCO3 were separated in FILTER and SOLIDSEP by using SSplit block. The resulting gases were cooled to 25°C in a COOLER and water is separated with the aid of a separator unit named SEPARATOR to obtain clean gas.


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Table 2. Description of blocks used in the simulation. Block ID Aspen plus Description name DECOMB RYield Yield reactor: use to convert the biomass into conventional constituent elements according to elemental analysis. GASFIR1 RGibbs Equilibrium reactor in which gasification reaction occurs and useful when Temperature and pressure are known. Stoichiometric and reaction kinetics are not known. GASFIR2 RGibbs Equilibrium reactor used for Gasification reaction along Carbonation and gasification reaction. BOILER Heater Used to convert the water into steam at 15O °C. SPRHTR Heater It is used to heat the steam at a temperature of 350 °C. FILTER SSplit It is used to remove ash from syngas. SOLIDSEP SSplit It is used to remove CaCO3. COOLER Separator It is used to cool down the product gas temperature to 50 °C. SEPRTOR Separator It is used to remove the water from product gas.

CL EAN GAS

COOL GAS SEPRATOR WATER1 GAS

COOLER

SOLIDSEP WATER2

WATER3

SYNGAS DECOMP

GASIFIR2

GASIFIR1 PROG1

BIOMASS

PROG2

FILTER

ELEMENTS CACO3

CAO STEAMG1 BOILER WATER

ASH

SPRHTR STEAM

Figure 2. Process flow diagram developed in Aspen Plus simulation for steam gasification of PKS with enabled CO2 capturing 2.5 Model Validation and performance: The experimental results obtained from the steam gasification of PKS in a pilot-scale gasification system were used to validate the simulation model. The detail of set up has been given above. After, the validation of predicted gas composition of model with the experimental gas production. The model performance was further evaluated in terms of


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Lower heating value (LHV), Higher heating value (HHV), Carbon conversion efficiency (C.C.E), gasification efficiency (G.E) and cold gas efficiency (C.G.E). The LHVgas and HHVgas have been defined as 27, 29. LHVgas= (30 X CO + 25.7 X H2 + 85.4 X CH4) X 0.0042

(1)

HHVgas =(H2 X 30.52 + CO X 30.18 +CH4 X 95) X 0.0041868

(2)

The cold gas efficiency is based on the LHV of syngas and LHV of biomass feed is defined as. C.G.E = (LHVgas /LHVbio)%

(3)

The carbon conversion efficiency is defined as 30, 31. CCE= (Moles of carbon in syngas/moles of carbon in feed)%

(4)

On the other hand Gasification efficiency is based on total mole of syngas produced GE = (total mass of syngas/ total biomass feed)%

(5)

3. Results and Discussions: 3.1 Effect of Temperature: Temperature is one of the most important factors with a significant effect on biomass gasification process. The effect of temperature from 650-750°C on product gas composition at steam/biomass ratio of 1 and CaO/biomass ratio of 1.42 is shown in Figure 3. It is observed that H2 concentration increased with an increase in temperature from 650 to 700°C but dropped at a higher temperature of 750°C. A similar effect on CO concentration is observed with smaller extent. The increase in H2 and CO content is due to water gas reaction, water gas shift reaction and steam methane reforming reaction at a temperature range of 650-700°C. Acharya et al.

32

also reported similar effect of temperature on H2 concentration in the

presence of CaO. The decrease in H2 and CO content at higher temperature is due to the presence of CaO 27. Whereas, the CO2 decreased with an increase in temperature from 650 to 700°C and then a reverse effect is observed beyond 700°C. The reduction in CO2 content is due to the activation of carbonation reaction enabled by the presence of CaO as shown in equation 6.


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Carbonation reaction 19

CaO (s) + CO2 (g) → CaCO3 (s) (Exothermic)

(6)

At higher temperature, the enrichment of CO2 occurred due to reverse exothermic carbonation reaction (Calcination) on the basis of Le, Chatelier’s principle highlighted by equation 7. Calcination reaction 19

CaCO3 (s) → CaO(s) + CO2 (g) (Endothermic) Rupesh et al.

19

(7)

also showed an evidence of the deactivation of CaO at a higher temperature

(727°C) in sorption enabled gasification of biomass. This reversal effect of carbonation reaction is also reported at a higher temperature of 727°C by Xu et al. 33 and 675°C by Khan et al. 27. The methane formation decreased with an increase in temperature. The suppression of CH4 formation is due to the acceleration of methane reforming reaction at a higher temperature in the presence of steam. Methane reduction with the increase in steam is well documented 28, 29. H2 CO CO2 CH4

80 70 60 50

Vol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 640

660

680

700

720

740

760

Temperature (C)

Figure 3. Effect of temperature on gas composition The heating values of product gases determined on the basis of the molar content of CO, CH4 and H2 are described in Eq. (1) and (2). It was observed that both LHVgas and HHVgas decreased with an increased in temperature as shown in Table 3. The maximum LHVgas and HHVgas of the gas (11.60 and 13.46 MJ/Nm3) were obtained at 650°C. There is a decreased in heating values due to a decrease in CH4 and CO content of the product gas as temperature increased. This fact was also observed and reported by other researchers 27, 34. Table 3 shows


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the effect of temperature on carbon conversion efficiency (CCE), gasification efficiency (GE) and cold gas efficiency (CGE). The lower CCE at 670°C is due to lower content of CO2. The higher CCE at higher temperature of 750°C is as a result of an increase in CO2 due to the reverse carbonation reaction. Zakir et al.

27

also observed an increase in CCE with an

increase in temperature that due to carbonation reaction. Gasification efficiency is defined as the total mass of syngas produced from total biomass. The syngas has high content of hydrogen (more that 60%) in each case thus resulting in a higher GE value compared to CCE. The GE followed the same trend as the CCE in which it increased with increase in temperature from 670 to 750°C. Many researchers have reported similar observation for GE in the temperature range of 700-900°C

35

. The cold gas efficiency depends on the Lower

heating value of the gas. So it also decreased in the same way as LHVgas with increase in temperature. The reduction in CGE has the same reason as LHVgas reduction. Table 3. Effect of temperature on LHV,HHV, C.C.E, G.E, and C.G.E at steam/biomass ratio of 1.5 and adsorbent/biomass ratio of 1.42 Temperature (°C) LHVgas (MJ/Nm3) HHVgas(MJ/Nm3) Carbon conversion efficiency (%) Gasification efficiency (%) Cold gas efficiency

650 11.7009 13.46253 44.41913352

700 10.9623 12.66127 42.05617

750 9.55582 11.11366 61.94718

59.41993

61.02957

97.91532

66.56978

60.72241

52.93163

3.1.1 Validation of model. In order to validate the sorption-enhance based model, the experiment was performed using same parametric condition in a pilot-scale fluidization set up as discussed above. For this comparison, steam gasification of PKS was performed at 650, 700 and 750°C, steam to biomass ratio of 1.5 and CaO/ biomass ratio of 1.42. The comparison between predicted and experimental gas composition is shown in Figure 4. It is clear from Figure 4 that experimental results are in good agreement with results predicted by the model. The H2 content is higher at 650°C as predicted by the model. The model has not considered the formation of tar and higher hydrocarbon. Laxmi et al.7 is also made the same argument for higher hydrogen content. At higher temperature both experimental and predicted H2 concentrations have no much difference. Tar reduction is higher at elevated temperature. It is 11 ACS Paragon Plus Environment

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observed that the comparison of CO2 shows a good agreement between predicted and experimental result. At higher temperature, the lower formation of CO2 predicted by model compared to experimental data shows the activity of reverse carbonation reaction. This might be because catalytic activity of CaO was ignored in the model. Secondly, it is also important to note that the model has shown lower production of methane compared to the experimental data. This shows the higher activation of methane reforming reaction. Similar phenomenon is observed by other researchers 7, 19. H2(Exp)

H2(Mod)

CO(EXp)

CO(Mod)

CO2(Exp)

CO2(Mod)

CH4(Exp)

CH4 (Mod)

90 80 70 60 Vol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 40 30 20 10 0 650

700

750

Effect of temperature (ºC)

Figure 4. Comparison between experimental and model predicted result by varying temperture 3.2 Effect of steam/biomass ratio: The gasifying agent is key and contributions significantly to syngas composition and heating values. In this study, steam/biomass ratio is varied from 1 to 1.5 and its effect is observed on syngas composition, heating values, and CCE, GE and CGE. Figure 5 shows the effect of steam/biomass ratio at a temperature of 700°C and CaO/biomass ratio of 1.42. It is observed that H2 content increased with increased in steam/biomass ratio from 1-1.5. The enrichment of H2 content in product gas composition comes from the acceleration of water gas shift reaction, char gasification and steam reforming of methane. The activation of these reactions toward the enrichment of H2 is observed by many researchers

19, 27

. The use of steam

enhances the hydrogen yield by shifting the equilibrium of water gas shift reaction in the forward direction as reported in the literature

36

. The use of steam as a gasifying agent 12

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increases the H2 content in the syngas compared to air gasification

16

. Methane content is

found to reduce in product gas with increase in steam/biomass ratio which shows the activation of steam methane reforming reaction in the presence of steam. It is observed that CO2 content increased slightly from 5 to 8% with increase in steam to biomass ratio. The reason for this slight increase in CO2 could be decrease in CO and CH4 content due to the enhancement of water gas shift and steam methane reforming reaction. Khan et al.

27

also

give the same justification for the increase in CO2 at higher steam/biomass ratio in the steam gasification of PKS in the presence of CaO. Acharya et al.

32

also noticed a similar trend in

CO2 content for the steam gasification of sawdust. Rupesh et al.

19

also observed the same

phenomenon in his air-steam sorption enabled gasification model.

H2 CO CO2 CH4

80 70 60 50

Vol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 1.0

1.1

1.2

1.3

1.4

1.5

steam/biomass ratio

Figure 5. Effect of steam/biomass ratio on gas composition The effect of steam/biomass ratio on the heating values of product gas in terms of LHVgas and HHVgas is shown in Table 4. It is observed that increase in steam to biomass ratio has an inverse effect on heating values of product gas. This might be due to the reduction of CO and CH4 content with the increase of steam. This argument is supported by the Eq. (1) and (2) that shows the fractional part of CO and CH4 is higher than H2. A similar observation was also reported by khan et al.

27

for steam gasification of PKS in the presence of CaO. Table 4

shows that the steam/biomass ratio has an inverse effect on CGE, GE and CCE. The decrease in CCE with the addition of steam is due to the lower content of CO, CO2 and CH4 in product gas as CCE is the ratio of mole of C in product gas and in feed. Zakir et al. 27 also explained the decrease in CCE at higher steam/biomass ratio of 2-2.5. Gasification efficiency 13 ACS Paragon Plus Environment

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calculation is based on the total syngas production on the mass of feed. It first decreased and then increased when steam to biomass ratio increased. The first decrease might have resulted from activation of some reaction like char gasification and then enhance due to activation of water gas shift reaction. The CGE is calculated on the basis of heating values of product gas and it decreased with the decreased in methane content at higher temperature. Table 4. Effect of steam/biomass ratio on LHV,HHV, C.C.E, G.E, and C.G.E at steam/biomass ratio of 1.5 and adsorbent/biomass ratio of 1.42 Steam/biomass ratio 1 1.3 1.5 LHVgas (MJ/Nm3) 12.17667 11.13628 10.9623 HHVgas(MJ/Nm3) 13.85902 12.8116 12.66127 Carbon conversion 53.41872 45.3037 37.21735 efficiency (%) Gasification 65.24754 62.95305 61.02957 efficiency (%) Cold gas efficiency 67.56183 61.68613 60.72241

3.2.1 Validation of model: In order to validate the model predicted results, the experiments are performed in pilot-scale gasification setup by varying the steam/biomass ratio from 1 to 1.5 at a temperature of 700°C and CaO/biomass ratio of 1.42. Figure 6 shows a good agreement between gas composition of model predicted and experimental data. At steam/biomass ratio of 1, the H2 and CO concentration predicted by the model are higher than the experimental result. It might be because tar was ignored in the model. In the case of 1.5 steam/biomass ratio, the model predicted H2 is similar to experimental result. Whereas the methane formation is suppressed in simulation, that is due to the high performance of steam methane reforming reaction. It normally happens in simulation and also reported in literature7.


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H2(Exp)

H2(Mod)

CO(EXp)

CO(Mod)

CO2(Exp)

CO2(Mod)

CH4(Exp)

CH4(mod)

90 80 70 60 Vol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

50 40 30 20 10 0 1

1.5 Effect of Steam/biomass ratio

Figure 6. Comparison between experimental and model predicted result by varying steam/biomass ratio 3.3 Effect of CaO/biomass ratio The main feature of this simulation study is to understand the effect of CaO sorbent on syngas production in terms of CO2 reduction and enhancement of H2. For this purpose the effect of CaO/biomass ratio on syngas composition, heating values and CCE, CGE and GE has been investigated. Figure 7 shows the significance of CaO/biomass ratio on syngas composition. There is a direct relationship between CaO/biomass ratio and H2 content. H2 content increased from 63.72 to 79.77 vol% with the addition of CaO from 0.5 to 1.42. CaO successfully adsorbed the CO2 from the product gas through carbonation reaction and its reduction can be noticed in Figure10. The enrichment of H2 content in the product gas is due to the shifting of water gas shift reaction in the forward direction due to a drop in partial pressure of the system as CO2 is adsorption. The presence of CaO also increased the activation of steam methane reforming reaction and enriches the H2 content 37, 38. Zakir et al. 29

also reported the increase in H2 content with the increase of A/B ratio from 0.5-1.5 in

steam gasification of PKS. The methane formation is very low and decreased with the increase in CaO/Biomass ratio from 0.5-1.42 due to the high activity of methane reforming reaction

29

. The slight increase in CH4 content and decrease in H2 content at CaO/biomass

ratio of 1.5 shows the slower propagation of methane reforming reaction.


Energy & Fuels

H2 CO CO2 CH4

80 70 60 50

Vol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 0.4

0.6

0.8

1.0

1.2

1.4

1.6

CaO/biomass ratio

Figure 7. Effect of CaO/biomass ratio on gas composition There is a direct relationship between CaO/biomass ratio and heating values of product gas as shown in Table 5. The HHVgas increased from 11.2 to 12.6 MJ/Nm3 with an increase in CaO/biomass ratio from 0.5-1.42. LHVgas followed a similar pattern but with lower range of value (9.6-10.2 MJ/Nm3). The higher heating values of product gases at CaO/biomass ratio of 1.42 is due to the higher content of CO and CH4 as methane contributed more in heating values in comparison to H2 and CO 34. A similar observation was reported by Khan et al. 29 in steam gasification of PKS. Table 5 shows the effect of CaO/biomass ratio on CCE, GE and CGE. The CGE is calculated on the basis of heating values of product gas and follow a similar pattern like LHVgas. It increased from 53.2% to 60.72% with increase in CaO/biomass ratio. On the other hand, CCE dropped at higher CaO/biomass ratio. This drop is due to decrease the in the moles of carbon containing gases in the product and increase in the mole of H2. The decreased in CCE with the increase of CaO is well documented efficiency also decreased with increase in CaO/biomass ratio. Khan et al.

29

29

. Gasification

also noticed the

decrease in GE with the increase in CaO/biomass ratio from 0.5-1.5. Table 5. Effect of CaO/biomass ratio on LHV,HHV, C.C.E, G.E, and C.G.E at steam/biomass ratio of 1.5 and temperature 700 °C CaO/biomass ratio 0.5 1 1.42 LHVgas (MJ/Nm3) 9.68745 10.49557 10.9623 3 HHVgas(MJ/Nm ) 11.20833 12.12221 12.66127 Carbon conversion 65.15186 51.1673 37.21734 efficiency (%) Gasification 99.60567 75.69615 61.02956 efficiency (%) Cold gas efficiency 53.75048 58.13713 60.72241 16 ACS Paragon Plus Environment

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3.3.1 Validation of Model: Figure 8 shows that the predicted gas composition has good agreement with experimental result obtained. H2 content is higher for predicted model at CaO/biomass ratio of 0.5 because tar reduction was ignored in the model. It can be observed from Figure 8 that both experimental and simulation show that the production of CO2 decreased with an increase in CaO/biomass ratio. The reduction of CO2 shows that the activity of CaO reduced the CO2 production through carbonation reaction. The methane formation is suppressed in simulation due to the high performance of steam methane reforming reaction. This normally happens in simulation also reported in literature 7. H2(Exp) CO2(Exp)

H2(Mod) CO2(Mod)

CO(EXp) CH4(Exp)

CO(Mod) CH4(mod)

90 80 70 60 Vol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

50 40 30 20 10 0 0.5

Effect of CaO/biomass ratio

1.42

Figure.8 Comparison between experimental and model predicted result by varying CaO/biomass ratio Conclusion: A steady state Aspen plus simulation model developed for the steam gasification of PKS which enabled the use of CaO for CO2 was used to study the effect of operating parameters on syngas yield. The model is made by using Gibbs free energy minimization method. The model predicted result is validated with the result obtained from the same experimental setup. It was found that H2 content increased from 650-700°C and CO2 decreased in the same range of temperature. The heating values of the product gases decreased with increase in temperature. The H2 concentration increased with an increase of steam/biomass ratio from 1-


Energy & Fuels

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1.5 wheras, methane and CO decrease by increasing steam/biomass ratio. The heating values of the product gas were also shows to have an inverse trend for increase in steam/biomass ratio. In case of CaO/biomass ratio the CO2 decreased with the increase of CaO/biomass ratio up to 1.42. H2 content was also increased with an increase in temperate. Heating values increased with increase of sorbent. The maximum hydrogen content (79 vol%) and minimum CO2 (5.42 vol%) were obtained at 700°C, steam/biomass ratio of 1.5 and CaO/biomass ratio of 1.42. Acknowledgement: This research project is funded by the Ministry of Higher Education Malaysia under the Long-term Research Grant Scheme (LRGS). The authors would like to thank Universiti Teknologi PETRONAS for providing facilities to conduct this research work.

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