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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: [email protected]
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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