UNIVERSITÀ DEGLI STUDI DI CATANIA
DEPARTMENT OF INDUSTRIAL ENGINEERING
MASTER-DEGREE IN CHEMICAL ENGINEERING FOR INDUSTRIAL SUSTAINABILITY
LUCA BARBAGALLO
MODELING GASIFICATION AND PYROLYSIS
PROCESSES OF RESIDUAL BIOMASSES WITH
CHEMCAD AND ASPEN PLUS
RELATORE :
PROF. FRANCESCO PATANIA
PROF. ANTONIO GAGLIANO
CORRELATORE: ING. MARIA BRUNO
ACADEMIC YEAR 2015 -2016
GOALS OF THE THESIS
The aim of the thesis is the Simulation of the Gasification and Pyrolysis processes of
Biomass through:
CHEMCAD and ASPEN PLUS
Softwares
The implemented models were validated and calibrated through experimental data
INTRODUCTION
“The crude oil age” is not very far from the end, because of:
the international political conflicts
the increase of the demand and the price of the
conventional fossil fuels
the need to minimize the environmental impacts
INTRODUCTION
In this context, the production of energy from renewable sources is a very
considerable resource for the future.
BIOMASSES and particularly RESIDUAL BIOMASSES from the agriculture and
food factory, can be a very important source of energy.
From the residual biomasses it is possible to produce energy reducing:
 the dependence by conventional fossil fuels;
 the emission of carbon dioxide (and other pollutants) in the atmosphere;
 the problem of the disposal of the residual biomasses.
BIOMASS IS ALL BIOLOGICALLY-PRODUCED MATTER BASED ON CARBON,
HYDROGEN, NITROGEN AND OXYGEN.
Several kinds of biomass have been identified in:
 Ligno-cellulosic biomass: fiber sorghum, eucalyptus and black locust etc.;
 Byproducts of tree crops: pruning of olive trees, citrus, fruit trees, almond and hazelnut;
 Byproducts of the forestry: high forests and coppice;
 Agro-industrial wastes: exhaust residues of olives from oil production, wood industry, etc.;
 Municipal solid wastes: biomass derived from wet urban wastes, compost etc.
THERMOCHEMICAL PROCESSES OF BIOMASS
O2
N2
 GASIFICATION
BIOMASS
Char
Tar
CO2
N2
H2O
CH4
 PYROLYSYS
BIOMASS
450-650 °C
CH4
Bio-oil
Biomass reacts with oxygen at
temperatures between (7501000) °C, to form mainly
hydrogen,
carbon
dioxide,
carbon monoxide, methane,
water and nitrogen.
Minor products are Char and
Tar.
Biomass is involved in a thermal
decomposition at temperatures
between (450-650) °C, in absence
of air. The pyrolysis products are
bio-oil, char and gas in
percentages which are function of
the operative conditions (slow,
intermediate, fast pyrolysis).
THERMOCHEMICAL PROCESSES OF BIOMASS
200-300 °C
 TORREFACTION
PELLETS
CO
CO2,
trace of H2
CxHy
H2O
The process occurs in the
absence of oxygen, in
atmospheric pressure and
temperatures of the order of
(200-300) °C. Torrefaction
products are pellets of the
same biomass, CO, CO2 and
traces
of
H2,
lights
hydrocarbons and water.
THERMOCHEMICAL PROCESSES OF BIOMASS
Biomass thermochemical conversion processes, pyrolysis/gasification, are very
complex and the products yields, as well as their quality for the specific use to which
they are addressed, are affected by a great number of parameters, especially process
temperature, residence time and heating rate, biomass characteristics.
The modeling of the processes has the aim to predict the yields and the quality of the
process products as a function of the operative conditions and the biomass physicchemical characteristics.
Consequently, it could be very useful to the design and the management of the
gasification and pyrolysis processes.
GASIFICATION THERMODYNAMIC EQUILIBRUM MODEL
The equilibrium model is based on the following global gasification reaction:
CHhOoNn + wH2O(liq) + a(O2 + 3,76N2) ↔ x1H2 + x2CO + x3CO2 + x4H2O(vap) + x5CH4 + x6N2
+ x6TAR+ x7Char
 CHhOoNn is chemical formula of biomass;
 w are moles of water in the biomass;
 a are moles of the input air in the gasifier;
 The reaction inside the gasifier takes place in conditions of thermodynamic equilibrium at pressure of 1 atm and the
reactions proceed in an isothermal system;
GASIFICATION THERMODYNAMIC EQUILIBRUM MODEL
The number of moles w of water, contained in the biomass, can be determined according to the humidity U:
w=
𝑴𝑾𝒃𝒊𝒐 ∗ 𝑼%
𝑴𝑾𝒘𝒂𝒕𝒆𝒓∗(𝟏−𝑼%)
Typically, in a gasifier, ERis (equivalent ratio) defined as:
ER =
𝑨𝒓𝒆𝒂𝒍/𝑭
𝑨𝒔𝒕𝒐𝒊𝒄/𝑭
Where Arealand Astoichare expressed in Kg/h of air and F is the feed in Kg/h of input biomass. In function of the ER we can
evaluate the moles equivalent of air a by the following expression:
𝒉
𝒏
𝒐
𝟒
𝟐
𝟐
a= 𝟏 + + −
∗ 𝑬𝑹
in which (1+h/4+n/2-o/2), corresponds to the stoichiometric amount of oxygen required for complete combustion of 1
mole of biomass. Finally the model is based on the choice of two intermediate reactions:
Methanetion
C + 2H2 ↔ CH4
Water gas shift
CO + H2O ↔ CO2 + H2
PYROLYSIS MODELS
Pyrolysis has been simulated both with a kinetic and an equilibrium model, based on the reactions of
methanation and water gas shift.
The general reaction of pyrolysis is:
CHhOoNn + ΔH(MJ) ↔ aH2 + bCO + cCO2 + dH2O(vap) + eCH4 + fCxHy + gChar
ΔH (MJ) is the heat provided to biomass;
C + 2H2 ↔ CH4
CO + H2O ↔ CO2 + H2
The Kinetic model is based on the following expression of the kinetic rates of the methanation and water gas shift
reactions
rm = km*CCH4 ;
rwgs = kwgs*CH2*CCO2
where k is the kinetic constant rate of the Arrhenius expression
ki = Ai*exp[-Eai/RT];
i= 2
PYROLYSIS (EQUILIBRIUM) MODEL
•
The equilibrium constants for the methanation and water-gas shift reactions in the equilibrium model are the following:
Ln (Km ) = Am + Bm/T
Ln (Kwgs) = Aw + Bw/T
SIMULATION OF THE GASIFICATION AND PYROLYSIS PROCESSES
Rubber wood
 GASIFICATION
Process parameters:
Biomass moisture content
and Equivalent Ratio
In Aspen
plus and
Chemcad
Wood pellets
Olive kernels
Olive pits
 PYROLYSIS
Process parameters:
Temperature
In Chemcad
Olive OilResidues
MODELING PELLETS GASIFICATION THROUGH ASPEN PLUS
Seven tests have been carried out considering different moisture content MC and ER
Input data:
Wood pellets (CH1,622O0,628 ) (Barrio et al, 2002)
MW = 23,67 [Kg/Kmol]
BIOMASS ULTANAL
ASH
CARBON
CHLORINE
HYDROGEN
NITROGEN
OXYGEN
SULFUR
BIOMASS SULFANAL
ORGANIC
PYRITIC
SULFATE
BIOMASS PROXANAL
ASH
FC
MOISTURE
ultimate analysis %w dry basis
0
50,7
0
6,9
0
42,4
0
sulfunate analysis %wt
0
0
0
proximate analysis %w dry basis
0,39
0
7,25
test
ER
MC%
8b
9
12
13a
13b
13c
14
0,232
0,229
0,230
0,225
0,238
0,235
0,227
7,25
6,9
7,02
6,38
8
7,58
6,67
Areal/F (Kga/Kgf) A real (Kg a)
1,44
1,447
1,45
1,42
1,50
1,48
1,44
144,13
144,25
145,46
142,23
150,42
148,29
143,69
T gasific.(K)
1073,70
1075,89
1075,14
1079,15
1069,03
1071,64
1077,33
ER values were calibrated by using following equation:
ER = 0,008*MC% + 0,174
MODELING PELLETS GASIFICATION THROUGH ASPEN PLUS
The Ryield block is fundamental
to simulate the devolatilization of
biomass in which the nonconventional components are
converted
into
conventional
components as H2, CO, CO2, CH4
and H2O. Temperature is set at
773 K.
The Mixer block allows
to add air at the gases
coming from Ryield
Pressure is set 1 atm.
The Rgibbs block allows to
simulate the final step of
gasification where the
processes of methanation
and WCS are developed.
Temperature
is
set
between (1069-1080) K
RESULTS FOR WOOD PELLETS GASIFICATION WITH ASPEN
The tests carried out through Aspen have been compared with experimental data (Barrio et al., 2002)
6,99%
14,9%
25,5%
27,5%
12,4%
11,7%
0,15%
16,9%
6,56%
1,33%
2,38%
7,34%
3,93%
6,93%
• The model predictions are quite satisfactory, even if the H2
percentage in the producer gas is always overestimated by the
model (16,55 %).
• The model predictions are very satisfactory in all the
cases analyzed (different moisture content e equivalent
ratio);
• Anyway the overestimation of the H2 percentage is a typical
behavior of the thermodynamic equilibrium models as
confirmed by other literature studies.
•
Indeed the average percentage error was (4,09 %)
RESULTS FOR WOOD PELLETS GASIFICATION WITH ASPEN
15,0%
33,5%
29,9%
29,3%
15,2%
10,5%
33,3%
• CH4 is underestimated if compared with experimental data.
0,73%
5,37%
4,51%
6,71%
1,61%
5,02%
6,21%
• Lower heating values (MJ/Nm3) are slightly overestimated, but
they show a very good agreement with experimental data.
• Indeed the average percentage error was (23,81 %)
• Indeed the average percentage error was (4,31 %)
MODELING OIL RESIDUES PYROLYSIS THROUGH CHEMCAD
Four tests have been conducted with different process temperatures
Input data:
Tests
T °C
Biomass [Kg/h]
1
2
3
4
400
500
550
700
24,44
24,44
24,44
24,44
Oil-residues (CH1,366O0,675N0,02 ) (Uzun Et al, 2012)
MW = 24,22 [Kg/Kmol]
BIOMASS PROXANAL
ASH
FC
MOISTURE
VM
BIOMASS ULTANAL
ASH
CARBON
CHLORINE
HYDROGEN
NITROGEN
OXYGEN
SULFUR
% weight
5,12
17,3
8,83
68,75
% weight
0
49,08
0
5,59
1,14
44,19
0
two models for pyrolysis were used :
•
Kinetic model
•
Equilibrium model.
MODELING OIL RESIDUES PYROLYSIS THROUGH CHEMCAD
COMBUSTION PRODUCT OF CH4
The Pyrolyzer decomposes biomass
at (400, 500, 550 and 700) °C in
absence of oxygen.
It is set as PFR (plug flow reactor) in
isothermal mode:
CH4
MIXER
COMBUSTOR
BIOMASS
3
4
5
AIR
• KREA (kinetic reactor)
DR YIER
• REQUIL (equilibrium reactor).
HEAT FOR DRYING BIOMASS
Combustion of CH4 is used for
generating the heat supplied for
drying the biomass
PYROLYZER
SYNGAS
8
6
7
BIO-OIL
9
SPLITTER
The Dryer vaporizes
the water content of biomass
at 120 °C
10
CHAR
Splitter separates:
• Syngas from the top
• Bio-oil (tar) from the central part
• Char from the bottom
RESULTS FOR OIL RESIDUES PYROLYSIS WITH CHEMCAD
Experimental data (Uzun et al., 2013), have been compared with those obtained from the simulations
with equilibrium and kinetic models.
• At the temperatures of 500 °C, that is the most
suitable temperature for a pyrolysis process, the
kinetic model is very reliable.
• At the temperatures of 500 and 550 °C, that are the
most suitable temperatures for a pyrolysis process,
the equilibrium model is very reliable.
• Average error, evaluated from kinetic and equilibrium
model at 500 °C is (12,16 %).
• Average error between 500 and 550 °C for
equilibrium model is (24,6 %)
RESULTS FOR OIL-RESIDUES PYROLYSIS WITH CHEMCAD
• At the temperatures of 500 and 550 °C, that are the
most suitable temperatures for a pyrolysis process,
equilibrium model is sufficiently reliable.
• Average error for equilibrium model at 500 and 550
°C is (3,5 %).
• At the temperatures of 500 and 550 °C, that are the
most suitable temperatures for a pyrolysis process,
both the models are sufficiently reliable.
RESULTS FOR OIL-RESIDUES PYROLYSIS WITH CHEMCAD
•
Char and Bio-oil have showed a very bad results respect to
literature references with kinetic model
RESULTS FOR OIL RESIDUES PYROLYSIS WITH CHEMCAD
• The results show that the char decreases when temperature increases from 400 up to 700 °C, in
accordance with literature data.
• This result is related with the char gasification when the temperature rises.
CONCLUSIONS
In this study we have implemented different models (equilibrium models or kinetic models) for modeling the
gasification and pyrolysis processes.
The models proposed allows to predict of the chemical composition (H2, CO, CO2, CH4, C2H4, C2H6) and the lower
heating value of the producer gas.
The validation and calibration of the processes were carried out through the comparison with experimental data that
have allowed the refinements of the chosen process variables.
The comparison between simulated and experimental data shows a good agreement, both for the CHEMCAD and the
ASPEN simulations.
- (16,55 % for H2, 4,09 % for CO and 23,81 % for CH4) (aspen-model).
- (12,16 % for H2, 24,6 % for CO and 3,5 % for CH4) (chemcad-model)
- Lower heating value (LHV) shows an average error of (4,31 %).
Considering the very high complexity of the pyrolysis and gasification processes, which are affected by a great
number of parameters, it is possible to assert that the developed model gives quite reliable results.
The models implemented, could be very useful for the prediction of the gas composition with the aim to produce a
syngas of high quality from a renewable resource.
I would like to thank my supervisor prof.: A. Gagliano,
the professor F. Patania
and
co-rapporteur engineer: M. Bruno
\