Steady State Simulation Studies of Gasification System using Palm ...

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ScienceDirect Procedia Engineering 148 (2016) 1015 – 1021

4th International Conference on Process Engineering and Advanced Materials

Steady State Simulation Studies of Gasification System using Palm Kernel Shell Maham Hussain, Lemma Dendena Tufa* , Raja Nur Ariana Bt Raja Azlan, Suzana Yusup, Haslinda Zabiri Chemical Engineering Department, Universiti Teknologi Petronas, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia

Abstract In biomass gasification, the appropriate prediction of gasification products is one of the most important issues. In this work, a steady state equilibrium model for the pilot -scale circulating fluidized bed by using Aspen Plus is developed. The model is capable of predicting the composition of synthesis gas. The process is simulated in accordance with the fluidized bed gasifier assembly. The simulation represents entire unit operations involve, to process palm kernel shell followed by a series of equilibrium air and steam gasification reactions to obtain adequate Synthesis gas yield. The temperature is varied from 600°C to 700°C while for steam to biomass ratio, it is manipulated from 1.5wt/wt. to 2.5wt/wt. The optimum temperature and steam to biomass ratio is determined in this study.

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under under responsibility responsibility of Peer-review ofthe theorganizing organizingcommittee committeeofofICPEAM ICPEAM2016. 2016 Keywords:Biomass gasification system, Palm kernel shell, Steady state simulation, Aspen Plus software.

1. Introduction With global population growth, development and industrialization energy demands have been intensifying progressively. This trend would tend a challenge to energy generation industry owing to depleting fossil fuels reserves as the main source of energy. Consequently, alternative technologies such as gasification, which is considered as environmental friendly utilizing biomass have reviewed and discussed by many researchers. Among the biomass processing technologies, gasification is the most important process route [1]. Gasification can be achieved with a wide range of feedstock's such as coal, heavy oils, petroleum coke, heavy refinery residuals, refinery wastes, hydrocarbon contaminated soils, biomass and agricultural wastes can be used [2]. Gasification is a partial oxidation of biomass at a higher temperature i.e. above 700°C gives gaseous mixture known as Synthesis gas. Depending upon the process requirement air, steam or mixture of both in various ratios can be used as gasifying medium in gasification process [2]. There are many types of gasifiers available to facilitate gasification process [3-5]. These are classified on the basis of their operation and fluid circulation patterns. In comparison to other reactors fluidized bed gasifiers provides greater advantages like uniform temperature distribution and elevated operating temperature [6]. Recent developments have led to a renewed interest in the study of simulation models for the gasification process. With regard to design an optimum and sustainable gasification

*

Corresponding author. Tel.:+605- 368 7624 E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016

doi:10.1016/j.proeng.2016.06.523

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Maham Hussain et al. / Procedia Engineering 148 (2016) 1015 – 1021

biomass -based process. Mathematical modelling and simulation studies are essential [6]. Several different types of models have been developed for gasification systems [7-10].This can be used to investigate the applicability as well as limitation of gasifier design for a given feedstock. Furthermore, it is important to analyze and optimize the gasification process to enhance the overall performance of biomass gasification system. A process simulator Aspen Plus has been used by several investigators to simulate various processes includes methanol synthesis[18, 19], indirect coal liquefaction processes [20], integrated coal gasification combined cycle power plants[21], atmospheric fluidized bed combustor processes [22], fluidized bed coal gasifier [23],Coal gasification simulation[24]. A simulation is developed to simulate rice husk gasification process by incorporating aspen plus. The simulation follows Gibbs equilibrium, and it is based on material and energy balance along with chemical equilibrium relations. The temperature and steam to biomass ratio on syngas composition are demonstrated by this simulation [25].A mathematical model of gasification process using fluidized bed reactor is established. It is reported that 42.34 mole% hydrogen is expected to produce by it [26]. An Aspen Plus simulation model of gasification using pine saw dust is demonstrated. The tar formation is neglected and considered the system hydrodynamics. The temperature and equivalence ratio, steam to biomass ratio effects on Synthesis gas composition is reported. An amount of 44 mole% of hydrogen is predicted by this study [8]. In the present study, gasifier based on Gibbs free energy minimization approach is used to predict the composition of the produced gas. The influence of operating conditions like gasifier temperature, steam to biomass ratio on product gas composition is investigated. The overall process performance of is predicted by the simulation model which will further help to optimize the process. 2. Biomass

Palm kernel shell is used as a feedstock for gasification. According to Abdullah et al., for biomass gasification, Palm kernel shell is preferred feedstock owing to its extensive properties [2]. Its calorific value is high, which is 20.40 MJ kg-1. Calorific value is close related to theamount of gas produce where the high calorific value will give thehigher calorific value of producer gas. For palm kernel shell, the moisture content in it is 17.50 wt. % which is low compare to other biomass such as coconut shell (18.5 wt. %), empty fruit bunch (66.26 wt. %) and sugarcane residue (52.20 wt. %).Moreover, palm kernel shell contains 14.87% of fixed carbon and 81.03% of volatile matter. A high content of these matters can result in a more efficient gasification process. Ng et al. has also has stated that palm kernel shell gives the highest hydrogen production (28.48 g H2/ kg palm kernel shell) for gasification compared to bagasse, rice husk and coconut shell [3].

Fig. 1. Aspen Plus Simulation of Biomass Gasification System

Table 1: Properties of Palm Oil Kernal Shell Palm Kernel Shell Molecular Equation CH1.283O0.594N0.031 C (wt. % dry basis) 49.74 H (wt. % dry basis) 5.68 N(wt. % dry basis)

1.02

O (by difference)

43.36

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Higher heating value Calorific Value (MJ/kg)

18.46 20.40

3. Specification of Biomass Gasification System The different stages considered in Aspen Plus simulation is shown in figure 01.palm kernel shell as a feedstock is fed at the flow rate of 1.0kg/hr . At the feeding section, nitrogen gas is injected with the pressure of 3 barg at 7.2m3/hr to prevent the backflow of biomass. To avoid any decomposition of biomass it is passed through cooling water jacket before biomass is fed in the fluidized bed gasifier .In fluidized bed gasifier, biomass, super-heated steam, and nitrogen gas is fed in. The superheated steam is fed at a rate of 1.96 kg/hr. and a temperature of 350 °C. Steam and biomass will react to produce Synthesis gas such as H2, CO, CO2 and CH4 and also char .Some of the gaseous product are sent to a gas analyser for further analysis. The analyser is used to analyse the composition of the following gas components; H2, CO, CO2, N2 and CH4. The other products will pass through themicro filter with a porosity of 50μm to separate solids with gases. Guard bed gasifier is used to crack down 'heavy' gases such as gas tar while polishing bed reactor is used to further polish the product gases. Next, product gas is passed through steam the superheater (water spray tower) where it will cool down the gas and remove steam. After the temperature of product gas is reduced, it will pass through water separator to separate water and later will go through adsorption column system. From here, hydrogen is released. For water system management, some of the service water will be used for quenching and other will be used for steam generation. The service water is pump to reverse osmosis equipment where impurities are removed from thewater. The clean water is then stored in boiler water feed tank. The water is heated up to 150°C in the boiler, with the flow rate of 8.2 kg/hr, the generated steam is heated further in a superheater up to 250°C. This superheated steam is then fed to the bottom of fluidized bed gasifier and react with the biomass. Another water stream is used for quenched water system. The water is pump to RO system and is stored in the water tank. Later, it is pump at 2m3/hr with the pressure of 6.5barg to the steam de-superheater. This water is used to cool down the product gas. 3.1. Operating Condition of Equipment The operating conditions are based on the actual plant at Block P, UTP. The information is summarized in the Table. 2.These are the assumptions that considered in the simulation: x The gasification process is at steady state conditions and atmospheric pressure. x Biomass de-volatilization takes place instantaneously and volatile products mainly consist of H2, CO, CO2,CH4 and H2O. x The reaction occurs isothermally and at constant volume. x There will be perfect mixing and uniform temperature distribution in the gasifier. x Tar and char formation is negligible and ignored in the simulation. Table 2. List of Equipment in Biomass Gasification System Parameters

Parameters

Equipment

Temp (°C)

Pressure (barg)

Flow rate (m3/hr)

Cooling Water Pump

25

1

0.600

-

-

0.1

Boiler Water Feed Tank

-

Pump

-

8

0.015

Boiler

150 5060 350400

6

0.0082

Feeding Hopper Superheater Fluidized Bed Gasifier Micro Filter Guard Bed Gasifier

700 700 700

6.9

0.00196

6

-

6 6

-

Capacity (m3)

-

0.03

0.044 0.044

Equipment

Gas Polisher Reactor RO Feed Pump Quenched RO System Water Tank Quenched Water Pump Steam Desuperheater Water Separator Air Booster Pump Adsorption Column

Flow rate (m3/hr)

Temp (°C)

Pressure

700

6

-

6.5

2

-

7

0.5

-

-

0.5

-

Capacity (m3) 0.044 -

-

6.5

2

-

300

6

-

-

40

6

40

5.5

8.9

-

40

12

5

0.008

-

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3.2. Thermodynamic Property Package The PENG-ROBINSON equation of state has been used to estimate all physical properties of conventional components in Aspen PLUS steady-state Simulation. PENG–ROBINSON equation itself is the particular version of the general cubic equation of state. This package improves the correlation of pure component vapour pressure when the temperature is high which makes it suitable for gasification process where the temperature is fairly high [18]. The reaction kinetics that is used in gasification process is as follows [12].

1) 2) 3) 4)

 ൅ ଶ  ՜  ൅ ଶ ൅ ͳ͵ͳǤͷ Ȁ‘Že  ൅ ʹଶ ՜ ସ െ ͹ͶǤͺ Ȁ‘Ž‡ ସ ൅ ଶ  ՜  ൅ ͵ଶ ൅ ʹͲ͸ Ȁ‘Ž‡  ൅ ଶ  ՜ ଶ ൅ ଶ െ Ͷͳ Ȁ‘Ž‡ Table 3. Blocks used in Aspen Plus

Aspen Plus Block

R-stoic

Description

Block ID Reactor 1 Reactor 2

The model is a reactor which allow user to use reactions with molar extent conversion

Reactor 3 Sep. Heater Splitter Mixer

Separator Quench RO RO System Boiler Super heater Splitter 1 Splitter 2 Splitter 3 Mixer 1

The model is a separator which allow user to split fraction component The model is used for heating purpose The model is used to split one stream into several streams The model is used to mix several streams into one

*.The Table. 3 shows the description of Aspen Plus unit operation model. 4. Result and Discussion 4.1. Model Validation The product composition after the simulation is as reported in table 4.To validate the model, the gas composition from Aspen Plus is compared with the value from Khan et al, 2014. This paper is also using the exact plant at Block P, UTP. However, this result is based on experiments that they had done. From the results, there is not much difference in the percentage of hydrogen percentage. The percentage error of hydrogen production is 3.18%. Table 4. Simulation Result (Dry, N2 Free) Gas Composition C O2 CO CO2 CH4 H2 N2 H2 O Total

Kmole/hr 0.0000000 0.0243492 0.0343996 0.0267899 0.0098244 0.2189691 0.1005572 0.0038684 0.4187577

% 0.00 5.81 8.21 6.40 2.35 52.29 24.01 0.92 100.0

Dry N2 Free (%) 11.86 9.24 3.39 75.51 100.0

4.1.1. Effect of Temperature In this paper, the temperature of the gasifier is varied from the range of 600 to 750°C. From Figure 2 and 3(a-d), thehydrogen content in gas composition increase as the temperature increase from 600 to 675°C.However, for carbon monoxide production decrease at this temperature range and started to increase after 675°C. This occurs due to high reaction occur during the water gas shift reaction (Eq. 4). Less production of carbon dioxide occurred at 600 to 675°C and the amount started to rise at the temperature after 675°C as shown in Fig 3 and 4.

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For methane, low composition at the range of 600 to 675°C is because of low activity of methane steam reforming reaction (Eq. 3) as shown in figure 5. There is some error in prediction the composition of product gas because of ignoring tar and char production in the simulation.

Table 5. Experiment Result (Dry, N2 Free) Gas Composition CO CO2 CH4 H2 Total

% (Dry N2 Free) 9% 0% 13% 78% 100%

Fig. 2. (a) Effect of Temperature ion Hydrogen.(b) Effect of Temperature ion Hydrogen

Fig.3. (c) Effect of Temperature on Carbon Dioxide (d). Effect of Temperature on Methane

4.1.2. Effect of Steam to Biomass Ratio (S/B ratio) The influence of steam to biomass ratio on hydrogen production has also been studied. The flow rate of biomass is adjusted from 1.8kg/hr. to 1.35kg/hr. and 1.1kg/hr. while the flow rate of steam is maintained at 2.7kg/hr. to each reactor. This will change the steam to biomass ratio from 1.5 to 2 and 2.5. By increasing the S/B ratio, the production of hydrogen is increased from 79.92% to 82.4%. The increment is up to 2.5% of hydrogen production shown in figure 4(a). Besides that, it is clear from figure 4(b) production of methane is also increasing. However, it is shown in figure 5(c) carbon monoxide decrease, when S/B ratio is increased. It istherefore, likely that at a lower value of steam/biomass ratio, there was less steam present to react with the biomass, subsequently, it terminates water-gas shift reaction, steam reforming reaction.Therefore H2 yield improves with an increase in steam/biomass ratio. Reducing in biomass flow rate helps in increasing amount of steam in the process, thus, enhance the water gas shift and methane reforming reactions. This evidence proved that increasing the S/B can increase composition of hydrogen.

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Figure 4. (a) Effect of Steam to Biomass ratio on Hydrogen (b). Effect of Steam to Biomass ratio on Methane.

Fig.5. Effect of Steam to Biomass ratio on Carbon Monoxide. (d). Effect of Steam to Biomass ratio on Carbon Dioxide.

5. Conclusion This study set out with the aim to investigate the steam/biomass ratio and gasifier temperature effects on product gas composition. The simulation model was developed by using the experimental data of PKS gasification for lab-scale fluidized bed gasifier at block-P UTP. In the present paper, the aspen plus process simulation software package was employed to develop a simulation model of PKS gasification system. The proposed simulation model has the ability to predict gasifier performance over a range of operating conditions. The effect of steam to biomass ratio, gasifier temperature is being performed and reported.. The temperature is varied from 600°C to 700°C and steam to biomass ratio is manipulated from 1.5wt/wt. to 2.5wt/wt.

Acknowledgments The authors gratefully acknowledge financial grant FRGS: 0153AB-K40 by Universiti Teknologi PETRONAS.

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