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Simulation of hybrid biomass gasification using Aspen plus:A comparative performance analysis for food, municipal solidand poultry waste

Naveed Ramzan a,*, Asma Ashraf a, Shahid Naveed a, Abdullah Malik b

aDepartment of Chemical Engineering, University of Engineering and Technology, G.T. Road, Lahore 54890, Pakistanb Senior Principle Engineer, JACOBS, UK

a r t i c l e i n f o

Article history:

Received 18 March 2011

Received in revised form

30 May 2011

Accepted 2 June 2011

Available online 8 July 2011

Keywords:

Hybrid biomass Gasifier

Steady state simulation

Cold gas efficiency

Gibbs free energy

Gasifier performance

* Corresponding author. Tel.: þ92 342 500255E-mail address: ramzan50@hotmail.com

0961-9534/$ e see front matter ª 2011 Elsevdoi:10.1016/j.biombioe.2011.06.005

a b s t r a c t

Steady state simulation model for gasification has been developed using Aspen Plus. The

model can be used as a predictive tool for optimization of the gasifier performance.The

Gasifier has been modeled in three stages. In first stage moisture content of biomass feed is

reduced. In second stage biomass is decomposed into its elements by specifying yield

distribution. In third stage gasification reactions have been modeled using Gibbs free

energy minimization approach. The simulation results are compared with the experi-

mental results obtained through hybrid biomass gasifier. In the simulation study, the

operating parameters like Temperature, Equivalence Ratio (ER), Biomass Moisture Content

and Steam Injection have been varied over wide range and the effect of these parameters

on syngas composition, High Heating Value (HHV) and Cold Gas Efficiency (CGE) has been

investigated. Temperature increases the production of CO and H2. Increasing ER decreases

the production of CO and H2 which decreases the CGE. Biomass moisture content is an

important parameter affecting the heating value of the gas. Steam injection favors

hydrogen production. The performance of the simulated gasifier has been compared using

experimental data for Municipal Solid Waste (MSW), Food Waste (FW) and Poultry

Waste (PW).

ª 2011 Elsevier Ltd. All rights reserved.

1. Introduction powered with hydrogen fuel cells, which are three timesmore

Biomass gasification has emerged as a promising technology

to fulfill the increasing energy demands of theworld as well as

to reduce significantly the volume of biomasswaste generated

in developing societies [1]. Also the dramatic increase in the

price of petroleum, the finite nature of fossil fuels, increasing

concerns regarding environmental impact, especially related

to greenhouse gas emissions, and health and safety consid-

erations are forcing the search for new energy sources and

alternative ways to produce power [2e5]. Vehicles can be

5.(N. Ramzan).ier Ltd. All rights reserve

efficient than a gasoline-powered engine [6,7]. Gasification is

a thermo chemical conversion of solid fuel into syngas having

useable heating value. The gases produced by this process can

be used to generate electricity by integrated gasification

combined cycle (IGCC).

Gasification is carried out by reacting carbonaceous fuel

with a restricted amount of oxygen and often in combination

with steam. Heat evolved from the exothermic reactions of

oxygen with fuel serves to maintain the gasifier operating

temperature and drives certain endothermic reactions taking

d.

Table 1 e Gasification Reactions.

Reaction Reaction name Reaction number

Cþ O2/CO2 Carbon combustion R-1

H2 þ 0:5O2/H2O Hydrogen combustion R-2

CþH2O/ H2 þ CO Water gas R-3

CO2 þ C/2CO Boudouard R-4

COþH2O/CO2 þH2 CO shift R-5

Cþ 2H2/CH4 Methanation R-6

0:5N2 þ 1:5H2/NH3 NH3 formation R-7

H2 þ S/H2S H2S formation R-8

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place inside it. Steam can be the sole gasificationmedium if an

external source can provide the heat necessary for the endo-

thermic gasification reactions [8]. Air gasification produces

syngas with HHV in the range of 4e7 MJ/m3 and if oxygen is

used instead of air the syngas produced has a HHV in the

range of 10e18 MJ/m3 [9]. The gasification process can be split

into four physico-chemical processes as shown in Fig. 1.

First is the drying (>150 �C) in which the moisture content

of the fuel evaporates. Pyrolysis or devolatilization involves

a series of complex physical and chemical processes. Pyrolysis

is initiated at about 230 �C when thermally unstable compo-

nents and volatiles in feed are broken down and evaporate

with other volatile components. Pyrolysis yields char, tar and

light gases like H2, CO, CH4 etc. The tar being sticky represents

a great challenge to downstream equipments and reduces the

efficiency. However if the temperature is sufficiently high

some tars are cracked to form light hydrocarbons and thus

reduce the damage to downstream equipments. The compo-

sition of the products evolved is a function of the temperature,

pressure and gas composition during devolatilization. After

pyrolysis char is combusted with oxygen. Combustion

(700e1500 �C) is the most important reaction taking place

inside a gasifier, providing practically all the thermal energy

needed for the endothermic reactions. Oxygen supplied to the

gasifier reacts with the combustible substances present in the

feed; resulting in the formation of CO2 and H2O. Finally the

products of combustion react with remaining char in the

gasification zone. Gasification (800-100 �C) involves a series of

endothermic reactions supported by the heat produced from

the combustion reactions. Gasification yields combustible

gases such as H2, CO and CH4 [8,10e18]. Major gasification

reactions are water gas, boudouard, shift conversion and

methanation (see Table 1) [9].

The primary objective of this research was to develop

a steady state computer model for hybrid biomass gasifier

using commercial simulation software ASPEN Plus. Then this

Fig. 1 e Operational Zones of Gasifier.

model is used to describe the gasification of three different

biomass feed stocks. These are food waste (FW), municipal

solid waste (MSW) and poultry waste (PW). The effect of

operating parameters like temperature, equivalence ratio (ER),

biomass moisture content and steam injection on syngas

composition, high heating value (HHV), cold gas efficiency

(CGE) and hydrogen production has been investigated.

2. ASPEN Plus Simulation Model

A kinetic free equilibrium model has been developed for the

gasification process using ASPEN Plus. The gasification

process has been modeled in three stages. In the first stage

moisture content of the fuel is reduced before feeding to the

reactor. In second stage biomass is decomposed into volatile

components and char. The yield distribution for this stage has

been specified by using a FORTRAN statement in calculator

block. The third stage models the partial oxidation and gasi-

fication reactions by minimizing Gibbs free energy.

2.1. Assumptions

The basic assumptions in the model are:

1. Steady state kinetic free model

2. Isothermal system

3. All sulfur goes to H2S

4. Only NH3 forms, no oxides of nitrogen are produced

[19e21].

Table 2 e Description of ASPEN Plus unit operationmodels.

ASPENPLUSID

Block ID Description

RStoic DRIER Reduces the moisture content of the fuel

RYield DECOMP Yield reactor- converts non-conventional

biomass into conventional components

by using FORTRAN statement

RGibbs GASIF Gibbs free energy reactor- handles three phase

equilibrium and calculates syngas composition

by minimizing Gibbs free energy.

Sep SEPRATOR Separates gases from ash by specifying

split fractions

Fig. 2 e (a) ASPEN Plus Simulation model of hybrid biomass gasifier, (b) ASPEN Plus Simulation Calculation procedure.

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b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 6 2e3 9 6 9 3965

5. Tars and other heavy products are assumed as non-

equilibrium products to reduce hydrodynamics

complexity [22].

2.2. Physical property method

PengeRobinson equation of state with Boston-Mathias alpha

function (PR-BM) has been used to estimate all physical

properties of the conventional components in the gasification

process. The parameter alpha in this property package is

a temperature dependent variable. This parameter improves

the correlation of the pure component vapor pressure when

temperature is very high. That is why; this property package is

suitable for gasification process where temperature is fairly

high. The enthalpy and density model selected for both

biomass and ashwhich are non-conventional components are

HCOALGEN and DCOALIGT.

2.3. Model Description

Four ASPEN Plus blocks have been used to simulate the

gasifier (see Table 2). Fig. 2 (a, b) describes the ASPEN Plus

simulationmodel and its calculation procedure used. Biomass

is specified as a non-conventional component in ASPEN Plus

and defined in simulation model by using the ultimate and

proximate analysis. The information used to describe the

feedstock is given in Table 3. The ‘RStoic’ block has been used

to model the drying of the biomass where as the drying

operation is controlled by writing the FORTRAN statement in

the calculator block.

The RGibbs model is used to simulate gasification of

biomass. RGibbs models chemical equilibrium by minimizing

Gibbs free energy. However the Gibbs free energy of the

biomass cannot be calculated because it is a non-conventional

component. Therefore before feeding the biomass into the

RGibbs block it is decomposed into its elements (C, H, O, N, S,

etc). This is done by using RYield model with calculations that

Table 3 e Feedstock Conditions and their Ultimate andProximate analysis.

Food waste Municipal solidwaste

Poultry waste

Source Super market

in UK

Municipalities

in UK

Local chicken

shops

Proximate analysis (Mass percent)

FC 14.6 7.6 8.4

VM 51.1 18.8 40.3

MC 29.3 50.9 7.5

ASH 4.9 22.7 43.9

Ultimate analysis ( Mass percent)

ASH 6.9 46.3 43.4

C 56.65 36.35 22.4

H 8.76 4.96 3.8

O 23.54 10.13 27.1

N 3.95 1.43 2.6

S 0.19 0.83 0.7

are based on the component yield specification. It is assumed

that the total yield of volatiles is equal to the volatile content

of the parent fuel, determined by the proximate analysis of

the fuel. The yield distribution of biomass into its components

has been specified by FORTRAN statement in calculator block.

This statement specifies the mass flow rates of the

components in the outlet stream. The heat of reaction asso-

ciated with the decomposition of the biomass is carried by

a heat stream into the RGibbs reactor where gasification

reactions have been modeled. This reactor can be used to

model the gasification reactions because it handles three

phase chemical equilibrium. The reactor calculates the syngas

composition by minimizing the Gibbs free energy and

assumes complete chemical equilibrium. The decomposed

biomass and air enter into the RGibbs reactor where, partial

oxidation and gasification reactions occur. All the sulfur in the

biomass reacts with H2 to form H2S.Due to low contents of

sulfur in the fuel; inaccuracies of this simplification are

negligible. The assumption that only NH3 forms and no oxides

of nitrogen are produced has been used and this assumption

has already been used by others researchers [23]. Ash sepa-

ration from the syngas has been simulated by unit operation

model Sep. The outlet stream of the RGibbs reactor enters into

the Sep block which separates gases from ash based on

specified split fractions.

3. Validation of model

The simulation model has been validated using experimental

data from gasification of three wastes in a lab-scale hybrid

gasifier published by us somewhere else [1]. It may be

observed that the model results are in good agreement with

the experimental results for food waste and municipal solid

waste (see Table 4). However, Poultry waste presents prob-

lems for gasification because of its specific composition.

Therefore, there is considerable difference between the

experimental and simulation results.

4. Results and discussion

The model developed for the hybrid gasifier has been used to

perform sensitivity analysis. The effect of gasifier tempera-

ture, ER, biomass moisture content and steam injection on

syngas mole fraction, cold gas efficiency (CGE) and hydrogen

production has been investigated for the three wastes.

Cold gas efficiency (CGE) is calculated by using following

formula [9].

CGE ¼ _mgas � HHVgas

_mgas � HHVfuel

First the general trends of the parameters have been dis-

cussed and then the three wastes have been compared.

4.1. Effect of gasifier temperature

The gasifier temperature was varied from 400 �C to 1400 �C.The effect of gasifier temperature on syngas mole fraction for

Table 4 e Experimental results versus model predictions.

Food waste Municipal solid waste Poultry waste

Gascomposition[mole %]

Experiment Model diff Experiment Model diff Experiment Model diff

N2 67.01 69.25 2.24 67.34 68.67 1.33 60.5 69.23 8.73

O2 1.67 1.05 �0.62 1.2 1.09 �0.11 e e e

CO 11.29 11.17 �0.12 14.89 14.74 �0.15 28.1 18.7 �9.4

CO2 10.13 10.71 0.58 8.4 7.86 �0.54 3.7 4.2 0.5

H2 5.13 5.19 0.06 4.58 5.19 0.61 6.1 7.3 1.2

CH4 2.56 0.42 �2.14 1.54 0.39 �1.15 0.9 0.5 �0.4

Ar 2.21 2.21 0 2.07 2.07 0 e

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three wastes is shown in Fig. 3 (a, b, c). At very low tempera-

ture (400 �C) the carbon present in the biomass is not utilized

completely so the production of syngas is not good but with

increasing temperature more carbon is oxidized and the rate

of conversion increases. At low temperatures both unburnt

carbon and methane are present in the syngas but as the

temperature increases carbon is converted into carbon

monoxide in accordance with Boudouard reaction. Methane is

converted into hydrogen by reverse methanantion reaction.

This results in increasing the operating temperature of the

gasifier that favors the production of hydrogen and carbon

monoxide, consequently the heating value of gas improves.

This is in accordance with gasifier chemistry. According to

Boudouard reaction (R4) as the gasifier temperature increases

the mole fraction of carbon monoxide increases and that of

carbon dioxide decreases. Water gas reaction (R3) suggests

that high temperature increases the production of both

carbon monoxide and hydrogen. According to Methanation

reaction (R6) the mole fraction of methane in syngas

decreases and that of hydrogen increases with the increase in

temperature. At higher temperatures yield of H2 and CO starts

reducing. This is also attributed to theWater gas reaction (R3).

In case of food waste the increase in carbon monoxide and

hydrogen production is slightly at a greater rate as compared

to municipal solid waste and poultry waste but general

behavior is same. Methane production decreases sharply at

temperatures above 500 �C.

Fig. 3 e (a) Effect of Gasifier Temperature on Syngas Mole

Fraction (FW), (b) Effect of Gasifier Temperature on Syngas

Mole Fraction (MSW), (c) Effect of Gasifier Temperature on

Syngas Mole Fraction (PW).

4.2. Effect of equivalence ratio (ER)

Equivalence Ratio is defined as the ratio of the amount of air

actually supplied to the gasifier and the stoichiometric

amount of air. As ER is increased, the amount of oxygen

supplied to the gasifier increases due to which conversion of

carbon present in the fuel increases. But excess amount of

oxygen oxidizes the fuel completely and the production of

syngas declines. The ER in the gasifier has been varied from

0.1e0.9. Fig. 4(a, b, c) shows the effect of ER on syngas mole

fraction. Initially the amount of carbon monoxide and

hydrogen increases due to increased conversion of fuel but

after a certain limit (w0.3) the production of syngas decreases

due to complete combustion of feed. So the heating value of

the syngas decreases and the CGE also drops after this limit.

Foodwaste has highestmole fraction of carbonmonoxide and

hydrogen in syngas (0.38 and0.29 respectively) while poultry

waste has lowestmole fraction of carbonmonoxide (0.3) .ER at

maximum production of syngas lies between 0.26-0.35 in case

of food waste and municipal solid waste and is slightly low in

case of poultry waste.

Fig. 4 e (a) Effect of ER on Syngas Mole Fraction (FW), (b)

Effect of ER on Syngas Mole Fraction (MSW), (c) Effect of ER

on Syngas Mole Fraction (PW).

Fig. 5 e Effect of MC on Syngas HHV.

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4.3. Effect of moisture content (MC)

The moisture content of the biomass has been varied from 5

to 40% to investigate its effect on the performance of the

gasifier. Fig. 5 shows the effect of biomass moisture content

on HHV. Moisture content has a remarkable effect on

gasifier performance. Increasing moisture content strongly

degrades the gasifier performance. The production of syngas

decreases due to which heating value and cold gas efficiency

(CGE) also decrease. Food waste shows highest heating value

among the three wastes due to higher mole fraction of

carbon monoxide and hydrogen in the syngas. The produc-

tion of hydrogen and carbon monoxide is dependent on the

biomass composition and it is clear from the composition of

feed provided in Table 3 that food waste has highest

Fig. 6 e (a) Effect of Steam/Biomass on Syngas Mole

Fraction (FW), (b) Effect of Steam/Biomass on Syngas Mole

Fraction (MSW), (c) Effect of Steam/Biomass on Syngas

Mole Fraction (PW).

Fig. 7 e (a) Effect of Steam Injection on Hydrogen

Production (FW), (b) Effect of Steam Injection on Hydrogen

Production (MSW), (c) Effect of Steam Injection on

Hydrogen Production (PW).

Fig. 8 e (a) ER Vs CGE (FW), (b) ER Vs CGE (MSW), (c) ER Vs

CGE (PW).

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 6 2e3 9 6 93968

percentage of carbon in it and poultry waste has the lowest

among the three wastes.

4.4. Effect of steam to biomass ratio

The effect of steam injection on syngas production has been

studied. Saturated steam 200 kPa has been injected. The

steam/biomass ratio has been varied from 0.05-0.4 and the

result is shown in Fig. 6 (a, b, c). According to water gas

reaction (R-3) steam increases the mole fraction of hydrogen

and carbon monoxide in the syngas. According to CO Shift

reaction (R-5) the amount of hydrogen increases by steam

injection and that of carbon monoxide decreases. The same

result is predicted by model. As food waste has higher

production of combustible gases thus it may be concluded

that steam injection has a more pronounced effect on it. The

overall behavior is same for all the three wastes i.e., hydrogen

and carbon dioxide mole fraction increases with steam

injection and that of carbon monoxide decreases.

4.5. Effect of steam injection on hydrogen production

Steam injection has a remarkable effect on hydrogen

production. The effect of steam injection is compared in Fig. 7

(a, b, c). Saturated steam 200 kPa has been injected. In case of

foodwastemole fraction of hydrogen is enhanced from 0.23 to

0.26 by steam injection at an ER of 0.34. The same kind of

behavior is shown by MSW and PW but their capability to

produce hydrogen is low.

4.6. Effect of ER on CGE

Cold gas efficiency is the most important parameter which

practically demonstrates the economic efficiency of the

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 6 2e3 9 6 9 3969

gasifier. It depends on a large number of parameters but is

ultimately dependent on the amount of carbon monoxide,

methane and hydrogen in syngas. The composition of syngas

is controlled by ER hence it is the key parameter for predicting

CGE. In the model ER has been varied from 0.2e0.8 and the

corresponding CGE is calculated. The results have been

compared in Fig. 8 (a, b, c). Food waste shows highest CGE

(71%) at an ER of 0.3 while for municipal solid waste CGE is

maximum (54%) at an ER of 0.28. Poultry waste shows lowest

CGE (45%) among the three wastes.

5. Conclusion

A steady state equilibrium model was developed for hybrid

biomass gasifier using ASPEN Plus simulator. The model was

validated using experimental results from hybrid biomass

gasifier. The model is capable of predicting gasifier perfor-

mance under various operating conditions and the results

predicted are in good agreement with actual values. Sensi-

tivity analysis was performed and the effect of varying gasifier

temperature, ER, moisture content and steam/biomass on

syngas composition, HHV of the syngas, CGE and hydrogen

productionwas studied. Higher temperature improves gasifier

performance. It increases production of carbonmonoxide and

hydrogen in syngas which ultimately results in higher HHV

and CGE. ER controls the production of syngas by controlling

carbon conversion of fuel and extent of gasification reactions.

Moisture content affects the HHV of syngas. High moisture

content degrades gasifier performance and results in low CGE.

Steam injection favors hydrogen production but increases

heat input to gasifier so an optimum value of steam to

biomass ratio (0.15e0.3) should be used. Among the three

wastes considered food waste shows highest CGE (71%), HHV

and highest percentage of carbon monoxide and hydrogen in

syngas.Municipal solidwaste and poultrywaste have lowCGE

(53% and 45% respectively) and HHV as compared to food

waste but their performance can be further enhanced by

mixing with other feed stocks.

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