Plasma gasifier modelling

5/19/2018 Plasma gasifier modelling

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ADVANCED PROCESS MODELS FOR BIOMASS GASIFIERS

E. Biagini1, L. Masoni1, M. Simone2, E. Bargagna2, G. Pannocchia2, C. Nicolella2, L. Tognotti2

1 Consorzio Pisa RicercheDivisione Energia Ambiente, Lungarno Mediceo, 40, 56125 PisaITALY2 Dipartimento di Ingegneria ChimicaUniversit di Pisa, Via Diotisalvi, 2, 56127ITALY

ABSTRACT: The optimization of biomass gasification should be studied with advanced models to evaluate the effect of the

operating conditions, quantify the by-products (CO2, CH4, tar) and compare different reactor configurations. In this work

different gasifiers (fixed beds, fluidized beds, entrained flow reactors) are modeled with Aspen Plus according to an

originally developed procedure. The innovative feature of the modeling procedure for all cases consists in the separation of

the steps of solid fuel gasification (devolatilization, oxidation, gasification of the char, homogeneous reactions and tar

cracking) and the development of dedicated sub-models (by adapting conventional blocks of the software or implementing

dedicated sub-models). All steps are connected to respect material and heat balances according to the gasifier configuration.

In addition a detailed description of the downdraft gasifier is realized as a distributed domain of several CSTRs and solving

energy and mass equations for gas and solid phases with gPROMS software. The entrained flow reactor is also developed

with Aspen HYSYS to give a more comprehensive and automatic solution. All gasifier models developed in this work are

powerful tools to be integrated in process study and optimization analysis.

Keywords: gasification, pyrolysis, fixed bed, dual fluidized bed, fluidized bed

1 INTRODUCTION

Biomass gasification is an attractive process to

convert a solid fuel into a gaseous product. Although

gasification is a relatively old process, the versatility of

the process (with production of syngas, electricity,

hydrogen or chemicals) and the multiplicity of

technological solutions (fixed beds, moving beds,

fluidized beds and entrained flow reactors) make it a

current topic of investigation. Process studies should be

performed for defining the best process configurations

and optimizing the operating conditions [1-2]. The

gasification reactor can be designed under very different

solutions [3]. The heat needed can be provided by partial

oxidation of biomass with air or pure oxygen, or by sand

recirculation. Steam may be added to promote

gasification. Temperatures, pressures and residence times

vary in wide ranges depending on the technological

configuration. In spite of all these differences, most

process studies in the literature modeled the gasifier as an

equilibrium reactor. This approach is indeed fundamental

for a preliminary study but hardly suitable for process

analysis and optimization procedures. Some issues arise

when introducing the equilibrium hypothesis in

optimization studies, as detailed below:

1. the relation between gasification temperature andthe operating conditions is limited, as essentially two

conditions can be set: isothermal or adiabatic conditions.

In all cases a realistic thermal profile can not be

introduced, nor a heat recovery (for instance in the

reactor jacket). In the real reactor, the reactor temperature

should arise from a global heat balance dependent on the

conditions (above all the oxygen-to-fuel ratio), and this

will be a goal of this work;

2. by-products in the syngas (e.g. CH4 and CO2) are

generally underestimated even though their value is

fundamental for the realistic evaluation of the process

efficiency. Residual char is not predicted in equilibrium

calculations, while the conditions for complete

conversion should be determined to assure highefficiencies and avoid problems in downstream units.

Similarly, tar is not predicted in most studies: its

quantification is actually fundamental to estimate the

process efficiency and the quality of the syngas

produced;

3. gasification is a complex ensemble of chemical

and physical phenomena. Each step can be operated

under different conditions and the configurations of

gasifiers can be compared only by developing a detailed

model. This is also the case of reactors that can be hardly

represented with an equilibrium reactor (e.g., due to the

low temperatures and residence times achieved).

For all the above points a gasifier model should be

developed instead of a gasification model. So, the aimof this work is the development of a procedure for

modeling different gasifiers and show some examples of

gasifier models.

2 MODELLING METHODOLOGY

A general procedure is developed to represent

different gasifiers as multizonal models. The main points

are summarized here and discussed in the next sections

along with some examples for different reactors:

- definition of the functional scheme of the

gasifier;

- separation of the characteristic steps of solid fuel

gasification (devolatilization, oxidation,

gasification of the char, homogeneous reactionsand tar cracking);

- development of sub-models of each step (by

adapting conventional blocks of the software or

implementing specific models);

- connection of all steps to respect the material

and heat balances according to the gasifier

configuration.

A pyramidal approach is developed dividing the

phenomena occurring in the gasifier on different levels.

(Figure 1). On the first one the evaluation of the heat and

mass transfer phenomena at the molecular level should be

based on the operating conditions and allows the initial

reactions to be described. The pyrolysis model is thebasic step of all gasification models and is known to

depend strongly on the operating conditions (temperature,

residence time and heating rate) besides the fuel

characteristics. The homogeneous reactions are fast and


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connected to the previous evaluations.

Heterogeneous reactions (involving the solid char

particle) are the controlling step of the entire system and

are studied on a second level as the consequent

transformations involve the particle (via size variation,

fragmentation, ash distribution and porosity evolutionphenomena). Diffusion of gasifying agents (O2, H2O and

CO2 above all), kinetics of char reactions, diffusion of

gaseous products should be represented in a realistic

model (considering intra-particle phenomena) to take into

account the variation of the conditions during the

gasification.

Gas-particle interactions, gas and solid fluid-

dynamic, solid-solid interaction (for instance in co-

gasification applications where different fuels can

interact) should be studied on a reactor level by

considering the reactor configuration. Also the heat

transfer on a macro-scale (e.g., presence of cooling

jackets or heat transfer surfaces) can be described only

once the geometry of the gasifier is defined.Finally the gasifier model should be validated with

experimental data. Lab-scale reactors can be used to

validate decoupled sub-models on molecular and particle

levels. Pilot-scale and large-scale gasifiers can be used to

validate the entire models.

Heat and MassTransfer Phenomena

PyrolysisDescription

Gas-PhaseReactions

Intra-ParticlePhenomena

ParticleEvolution

HeterogeneousReactions

Gas-SolidInteractions

GasifierGeometry

GASIFIERMODEL

ValidationLEVEL 1

LAB-SCALE

GASIFIER

LEVEL 3

LARGE-SCALE

GASIFIER

LEVEL 2

PILOT-SCALE GASIFIER

MOLECULAR

LEVEL 1

PARTICLE

LEVEL 2

REACTOR

LEVEL 3

Figure 1: Pyramidal approach for the development of a

gasifier model

The main sub-models will be described in the

following section for the reacting steps. They are adaptedfor the actual configuration of the gasifier once the

particle and reactor levels are defined. Some examples of

gasifier models will be discussed in section 4. All models

are developed using Aspen Plus and the model

approach will be compared.

Fixed bed (downdraft), fluidized beds (circulating

and dual beds) and entrained flow gasifiers are studied

here. Only in the first case the small scale justifies the

feed of biomass alone, while in the other cases a co-

gasification of biomass with coal can be studied. All

gasifier models are developed in view to be linked to

other unit models for studying different processes (for

example hydrogen production) in a global Aspen Plus

model.Also other codes are used for modeling some

gasifiers (gPROMS for a downdraft reactor and Aspen

HYSYS for an entrained flow gasifier). The

methodological approach is similar to the Aspen Plus

based models. The peculiarities and the capabilities of

these codes will be discussed in this work.

3 DESCRIPTION OF MAIN SUB-MODELS

All reactive sub-models are represented as Kinetic

Reactors (Plug Flow Reactor or Continuous Stirred Tank

Reactor depending on the reactor configuration).

Different thermal options (adiabatic, constant

temperature, thermal profile, constant coolant

temperature) can be set for the heat transfer according to

the reactor configuration. The list of all reactions is given

in Table 1.

3.1 Devolatilization sub-model

The first reacting step of the biomass is the

devolatilization. It is a thermal decomposition that

produces a solid residue (char, that will be the reactant in

the following gasification reactions), a condensableorganic product (tar) and the main gaseous species (CO,

CO2, CH4, H2O, H2, C2H4, N2, NH3, HCN, H2S, COS).

No conventional block can represent this step in any

commercial codes. Here, a structural model (ABCD

Advanced Biomass and Coal Devolatilization model [4])

is used for the biomass devolatilization. As a matter of

fact this model can simulate also the devolatilization of

different rank coals.

The ABCD model gives the yield of macro-products

and the speciation of gases once the fuel composition and

the operating conditions are given. The ABCD code can

be hardly implemented in Aspen Plus because of the

expensive computational cost. Therefore, aUser Routine

is developed for the devolatilization step (scheme of

Figure 2). Basically, it consists of a database and a

calculation function. The former is created with the

results of off-line simulations of the ABCD model

applied to the devolatilization of a specific biomass in a

wide range of pressures and temperatures. The

calculation function dialogues with the main model by

receiving the actual values of temperature and pressure,

interpolating the results of the database and returning the

speciation of the devolatilization product stream. In all

cases the material balance is verified.

MODEL in themain code

ABCD model

off-line simulations:

biomass composition

T = 600 1600 C

P = 1 40 bar

(T,P)

PYROLYSIS DATABASE

#########%H2

#########%CH4

#########%CO

#########%CO2

#########%H2O

#########%tar

#########%char

1000800600P 30

PYROLYSIS DATABASE

#########%H2

#########%CH4

#########%CO

#########%CO2

#########%H2O

#########%tar

#########%char

1000800600P 30Calculation

function(interpolation of

data and

normalization)devo

products

DEVO USER ROUTINE

Figure 2: Scheme of the devolatilization block

3.2 Homogeneous reactions sub-model

The homogeneous reactions are modeled assuming

parameters from literature. The general rate expression is

the following:

=

i

n

i

jm

jj cRT

ETAr exp

where A and E are the kinetic parameters of j-th

reactions, m is the exponent of the temperature, c is the

concentration of reactants and n is the order of the

reaction with respect to that reactant. For reversible


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reactions (e.g., the water gas shift reaction) the last term

involves also the concentration of the products.

3.3 Tar reactions sub-model

The quantification of the tar is a fundamental issue in

gasification models to define the quality of the producedsyngas, the downstream cleaning units and the end-use of

the process. Therefore it is important to develop a global

model that allows one to study the optimal conditions to

limit the tar formation or enhance the tar destruction. The

tar is formed during the pyrolysis step, while it is

destroyed in the subsequent steps: tar cracking (bond

scission caused by heat transfer to give light gases),

oxidation (if oxygen is available in the reactor) or

reforming (with reactions with H2O or CO2 to give

partially oxidized products).

The tar from biomass is here represented as

levoglucosane (C6H10O5) that is the monomer of cellulose

(while anthracene C14H10 represents the tar from coal). In

all cases, a power law expression is used for the reactionrate as that reported in the previous sub-section. As for

the kinetic parameters, they are adapted from [5,6] for the

tar cracking. Kinetics of heavy hydrocarbons (with

similar molecular weight) are adopted for both tar

oxidation and reforming.

3.4 Heterogeneous reactions sub-model

Heterogeneous reactions (those involving the char

from the previous devolatilization step) are modeled

assuming the unreacted core-shrinking model. Both the

diffusion and kinetic transfer coefficients are considered

for all heterogeneous reactions (of combustion as well as

gasification). The general expression is:

S

RD

j

j A

kk

cr

+=

11

where c is the concentration of the j-th species (O2,

H2O, CO2, H2), kD is the diffusion coefficients that takes

into account the mass transfer to the particle, kR is the

reaction coefficient (that has the Arrhenius form) and ASis a surface factor that depends on the external surface

area of the particle. This latter parameter varies during

the reaction. The shrinking of the particle is modeled by

considering the conversion and the ash content of the

biomass [7]. In most cases a density constant model isadopted, while only the particle diameter reduces from

the initial value to a critical value depending on the ash

content of the original fuel.

Different sets of kinetic parameters can be found in

literature for combustion/gasification systems. Every set

was validated and thus can be applied in a specific range

of operating conditions. In this work, kinetics of

combustion and gasification are divided in two ranges:

- range 1, for room pressure and relatively low

temperatures (800-1200 C);

- range 2, for high pressure (20-40 bar) and high

temperature (1200-1600C).

Kinetics for the heterogeneous reactions are adapted from[6,8] for range 1 and from [7] for range 2.

Table I: Reaction sets used in the sub-model

development

Devolatilization reactionsfuel char + tar + ligh gas

Combustion reactionsvolatile combustion H2 + O2 H2O

CO + O2 CO

CH4 + 2O2 CO2 + 2H2OC2H4 + 3O2 2CO2 + 2H2O

char oxidation C (char) + O2 CO

tar oxidation C6H10O5(tar) + 17/2 O2 6CO2 + 5H2O

Gasification reactionschar gasification C (char) + H2 O CO + H2

C (char) + CO2 2CO

C (char) + 2H2 CH4

tar reforming C6H10O5(tar) + H2O 6CO + 6H2

methane reforming CH4 + H2 O CO + 3H2

water gas shift CO + H2 O CO2 + H2

4 DEVELOPMENT OF GASIFIER MODELS

The development of different gasifiers are discussed

in this section. The first examples (fixed bed, fluidized

beds, entrained flow reactor) are reported for Aspen

Plus applications and compared among them as

uniform hypotheses are adopted. The last two examples

concerns the downdraft gasifier model developed with

gPROMS and the entrained flow reactor model with

Aspen HYSYS.

4.1 Example 1: downdraft gasifier

A scheme of the downdraft gasifier is shown in

Figure 3a. It is a versatile and proven solution in the

range 50-500 kWth. Most reactors are operated with sub-

stoichiometric air in commercial units, but also some

scientific investigations with steam/oxygen mixtures can

be found [9,10]. The internal peak temperature is around

1200 C, the syngas leaves the reactor at 700-1000C. In

this work we studied a 250 kg/h atmospheric gasifier.

The gasifier is modelled with Aspen Plus according

to the scheme of Figure 3a. After the heating of the

biomass, a first reactor block (the Yield Reactor

DECOMP) represents the devolatilization step, that gives

the pyrolysis macro-products (with the speciation of light

gases) balancing the moisture and ash content of the

biomass.The pyrolysis products are then mixed with the

gasifying agents to feed the reactor block (H-REACT) in

which all the above equations of combustion, gasification

and tar-cracking are modelled in a PFR configuration.

The temperature is calculated from the heat balance of

the system. The MIXHEAT block collects all heat

streams from heater and reactor blocks. A design

specification (a calculator tool in Aspen Plus) is used to

iteratively define the reaction temperature in the main

reactor, so that the value of the dispersion (the heat

stream DISP exiting the MIXHEAT block) converges to

the assumed value (5% of the generated heat).

Finally, a separator (SEPASH) separates the solid

residue from the syngas (according to an efficiency valueof 0.9) and acooler(H-REC) simulates the heat transfer

between the syngas and the reactor before the exit.


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4.2 Example 2: circulating fluidized bed gasifier

The circulating fluidized bed is formed by a riser

(where oxidation/gasification reactions occur) and a

downcomer (for the recirculation of the sand used as a

heat carrier). Fuel, oxygen and steam are fed in the upper

sections of the riser. Some syngas (from the downstreamunits) is also used as a recirculation gas to assure the

hydrodynamics of the system.

Some modeling aspects of the fast fluidized bed are

discussed in this section. The approach is used for

simulating both the circulating fluidized bed and the beds

of the dual bed gasifier.

The high temperature and heating rate allow one to

consider the devolatilization step completely separated

from the combustion and gasification reactions.

The hydrodynamics of the fluidized beds was

modeled by following the considerations on a fluidized

bed combustor model exposed in [11]. A similar

approach was considered also by Corella The reactor bed

was divided in two regions (see example in Figure 3b):- a dense lower region with a constant

suspension density (turbulent fluidized

bed);

- a more dilute upper region with a

decreasing suspension density.

The upper region is assumed to be axially composed

of two zones:

- the acceleration zone is at the bottom part

of the upper region where the solids are

accelerated to a constant upward velocity;

- the fully developed zone is located above

the acceleration zone, where the flow

characteristics are invariant with height.

In the acceleration zone, the axial voidage decreases

with the height of the riser. The average value of the

voidage between two height values of the riser can be

calculated using the expression proposed by Kunii and

Levenspiel [13]. The lower region is represented by a

single CSTR, while a series of CSTR with decreasing

voidage is used to take into account the solid fraction

variation in the upper region.

In the lower CSTRs both combustion and gasification

reactions are considered, while after a certain height only

gasification reactions are considered because of the

completely oxygen consumption. As discussed before,

heterogeneous reactions of combustion and gasification

are modelled with the unreacted core-shrinking model, soa mean particle diameter is assumed in each CSTR in

order to evaluate the reaction rates.

Another important topic is to determine the

conditions for the fast fluidization of the beds. The

superficial velocity of the gas should be at least higher

than the transport velocity [14]. An iterative procedure is

implemented to assure the proper superficial velocity by

varying the recirculation gas flowrate. Iterations are

necessary because variations in the fluidization gas

recirculated causes variations in the syngas produced.

4.3 Example 3: dual beds gasifier

The dual bed gasifier is formed of two fluidized beds(see Figure 3c):

- bed1 is fed with the fuel and the hot sand

and works as a gasifier;

- bed2 is fed with air and the cold sand from

bed1 (that contains also some residual char)

and work as a combustor to heat the sand.

The main pro of this configuration is the use of air

(instead of pure oxygen) to get a syngas with no nitrogen.

The sand recirculation is used as a heat carrier andvariations in the fuel-to-sand ratio can be studied only

with a detailed approach.

Both beds are modeled with the same approach

described in the previous sub-section.

4.4 Example 4: entrained flow gasifier

The reactor studied is an entrained flow reactor with

quench water (technology of Future Energy, scheme in

Figure 3d). The pressure is between 20 and 40 bar. A

burner in the top section feeds the fuel and sub-

stoichiometric oxygen. Very high peak temperatures are

achieved (1500-1600C), so steam can be added to

promote endothermic gasification reactions. A coolingscreen shields this part of the gasifier and recovers heat

for the production of low pressure steam. A cooling

jacket for the production of pressurized hot water

envelops the entire reactor. The high gasification

temperatures limit the tar in the syngas and make the ash

to melt, thus forming a slag on the reactor walls. This

slag is cooled and removed by the quench water in the

bottom part of the gasifier. In this work we studied a

5000 kg/h gasifier under a fixed pressure of 30 bar.

The gasifier is modelled with Aspen Plus according

to the functional scheme of Figure 3d. As in the previous

cases, the first step is the devolatilization of the fuel,

followed by a separated combustion chamber (modelled

as a PFR reactor with the oxidation reactions of table 1)

that represents the burner. The high heating rate expected

in this reactor allows to justify this hypothesis. The

subsequent gasification step includes the other reactions

of table 2.

The heat produced in the combustion step is used for

the other endothermic reactions (pyrolysis and

gasification), the pre-heating of the reactants and the heat

recovery in the cooling screen. The same iterative method

described in 4.1 is applied.

4.5 Example 5: downdraft gasifier with gPROMS

An accurate description of the mechanisms and

geometry of the fixed bed gasifier described in section

3.1 was done with gPROMS. A distributed model wasdeveloped to give an interpretative and diagnostic tool,

capable to aid the experimental activity with pilot scale

gasifiers, as well as provide indications about the effect

of the operating conditions and geometry.

The gPROMS model schematizes the gasifier as a 1-

D domain (only the axial variable distributions are

represented) meshed with 300-1000 cells. Figure 3a

reports a scheme of the gPROMS model. The simulation

of the system requires operational input (gas and solid

flow rates, wall temperature), gas and solid compositions

and some details about the pyrolysis behaviour of the

biomass (devolatilization kinetics, macroproducts

distribution and gas species).

The model is based on dynamic equations of heat andmass balance. Therefore the ignition behaviour can be

simulated as well as variations in the operating

conditions. The main outputs of the simulation are the

temperatures distribution along the gasifier axis and the


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gas species distribution.

4.6 Example 6: entrained flow gasifier with Aspen

HYSYS

The entrained flow gasifier described in the previous

sub-section 4.4 was modeled also with Aspen HYSYS.The functional scheme is the same of figure 3d. Aspen

HYSYS has a user-friendly interface, a good

thermodynamic package and the possibility of

implementing dedicated extensions that can enrich the

block library of the software and allow the simulation of

unconventional operations (as the devolatilization and

gasification steps). The extensions can be written with

programming codes (e.g., Visual Basic 6.0) that support

the automation, and create Unit Operations that can be

integrated in Aspen HYSYS in an easy, robust and

versatile manner.

The first extension was created for the

devolatilization step. Theoretically the basic approach is

the same ABCD model mentioned in the sub-section 3.1.This model is originally written in FORTRAN and is

available as an executable file: it requires an input text

file and writes the results to an output text file. The

Aspen HYSYS Devolatilization extension developed in

this work acquires the data (on fuel characteristics and

operating conditions) to write the input file, runs the

executable file and transfers the results from the output

file to the main code of Aspen HYSYS by defining the

stream of the Devolatilization Products-1 (see scheme in

Figure 4). This is a big step forward that avoids off-line

simulations of the ABCD model (with the generation of aDataBase as described in the sub-section 3.1), removes

the errors due to interpolation procedures and allows

quicker simulations with different fuels (in the previous

case we had to build a DataBase for all fuels tested).

The second peculiar extension developed in the

Aspen HYSYS model was for the gasification step.

Also in this case the unreacted-core shrinking model was

implemented (according to the approach described in the

sub-section 3.4) in an adiabatic PFR block. It is worth

noting that kinetic reactions of solid compounds is not a

conventional operation in Aspen HYSYS [16].

Finally the heat balance is set by algebraically adding

the heat streams of all blocks (heaters and reactors) and

the heat recovery (cooling screen and cooling jacket) tothe block BALANCE-1 (see figure 4). The Adjustment

tool (ADJ-1) allows the combustion temperature to be

iteratively defined to solve the heat balance.

(a)

air inlet

periodically removed solid

syngas

biomass

inlet

fixed bed over grate

GASIFICATION

ZONE

OXIDATION

ZONE

PYROLYSIS

ZONE

DRYING

ZONE

Solid Residues

t=0 IGNITION

Gasifying

agent

Biomass

SynGas

SOLID PHASEGAS PHASE

Heat streams

Mass streams

DispersionDispersion

P

FR

P

FR

P

FR

P

FR

gPROMS model

(b)

Sand

recirculation

AshBiomass

GASIFICATION

REACTOR

Oxygen

Steam

CYCLONE

Syngas

Recirculation gas

Figure 3: Sketch of the gasifiers studied and relative scheme for modeling (cont.)


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(c)

Hot Sand

20C

Ash

Syngas

COMBUSTOR

(BED 2)

PYROLYSIS/

GASIFICATION

REACTOR

(BED 1)

Exhaust

Char

+Sand Air

Fluidization Gas (from clean gas section)

Biomass

DEVOsub-model

GASIFICATIONsub-model

(n CSTR in series)

separation

COMBUSTIONsub-model

(CSTR)

pre-heating

DEDUST

hot sand

sand/char/ash

air

exhaustgases

excess solid

syngas

gas

recircula

tion

biomass

BED

1

BED

2

(d)

fuel pre-

heating

gasifying

agents pre-

heating

fuel oxygen

steam

DEVO

sub-model

(CPD Db)

COMBUSTIONsub-model (PFR)

GASIFICATIONsub-model (PFR)

i-th QUENCH

(MIXER)

i-th HEATRECOVERY

SEPARATIONsyngas

waste water

slag

quench water

hot water

LP steam

pressurizedwater

hot water

material

streams

heat

streams

HOMOGENEOUSREACTIONS (PFR)

COOLING JACKET

COOLING SCREEN

Figure 3: Sketch of the gasifiers studied and relative scheme for modeling

Figure 4: Scheme of the entrained flow gasifier model with Aspen HYSYS

Table II: Composition of fuels

Fuel Ultimate Analysis (wt% dry and ash free) Proximate Analysis (wt% dry) Moisture

C H N S Cl O VM FC ash (wt%)

Poplar wood 51.7 6.47 0.25 0.05 0.01 41.52 85.1 13 1.9 20

SA coal 81.6 4.84 1.75 1.27 - 10.54 26.7 58.6 14.7 7


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5 RESULTS AND DISCUSSION

Some examples are commented here remarking the

capability of each approach and the peculiar results with

respect to equilibrium or simplified models. It is worth

reminding that all models were developed to comparedifferent gasifier configurations and evaluate the effects

of the operating conditions on the syngas conversion to

optimize the gasifier performance or, in case, the global

process.

In the first example, the downdraft model developed

in Aspen Plus was used for the gasification of poplar

wood (properties in Table 2) with mixtures of oxygen and

steam. In all cases the temperature achieved in the

gasifier is limited to 1200C. The results are shown in

Figure 6 as functions of the Equivalent Ratio (ER), that is

the ratio between the actual oxygen present in the

gaseous feed and that needed for the complete oxidation

of the fuel. The higher the value of ER, the lower the

heating value of the syngas produced in the gasifier (dueto the higher oxidation level). So, the conditions for the

maximum LHV could be considered for a direct

combustion of the syngas (e.g., for power production).

The syngas composition should be taken into account

for hydrogen production processes. The hydrogen

produced in the gasifier shows a maximum for a value of

ER near 0.5 (see Figure 5). Above this value the excess

oxygen oxidizes the gaseous products and thus a decrease

in the production of hydrogen is observed. The syngas

contains also CO, CO2 and CH4 (not shown in the figure),

so the complete composition should be considered for

assessing the performance of the entire process (e.g., the

CO can be converted to give additional hydrogen in a

downstream water gas shift unit).

The tar at the exit of the gasifier is also shown in

Figure 5. The high tar content in the syngas for values of

ER between 0.35-0.40 makes prohibitive the direct

utilization of the syngas in engines or turbines for power

production. In this case onerous gas-cleaning units should

be installed and a loss in efficiency should be expected.

Values of HR slightly higher (0.45-0.50) can be

programmed for obtaining a syngas with a lower heating

value but with limited tar content. It is worth remarking

that this consideration can not be done with an

equilibrium model of the gasifier because tar can not be

predicted.

The gPROMS model of the same gasifier allows one

to obtain more detailed results. For instance, the thermal

profiles inside the reactor under various conditions areshown in Figure 6. The gas and solid temperatures in the

gasifier are reported for two values of the equivalence

ratio. The higher the ER, the higher the maximum

temperature of both phases. Three different zones can be

observed in the thermal profile:

1. the solid phase is heated up by the radiation

from the lower hot zone; consequently the gas

phase is heated up by convective heat transfer

from the solid phase;

2. the temperature peak is caused by the oxidation

reactions that mainly rise the gas temperature;

3. after the oxidation reactions the temperature

decreases due to gasification reactions and

thermal dispersion; in this zone the gas andsolid phases reach a thermal equilibrium.

Further results can be obtained with this detailed

approach. For instance the rate of char conversion due to

char oxidation and gasification which is an useful

information to estimate the reaction front position and

plan a sampling position in the reaction bed. Also some

dynamic simulation can give interesting evaluations on

conditions variations. Pressure drops due to the particle

size of the biomass can be estimated.A mixture of biomass and coal (properties in Table 2)

is studied for the circulating fluidized bed. A sensitivity

study can be performed by varying the biomass-to-coal

ratio, the steam-to-fuel ratio or the oxygen-to-fuel ratio

(also expressed as equivalence ratio). The example of

Figure 7 shows the syngas composition at the exit of the

gasifier for fixed values of steam and oxygen (ER = 0.4,

steam-to-fuel = 0.2) for the co-gasification of poplar

wood and coal. The higher the biomass sharing, the lower

the temperature of the gasifier; also the lower the syngas

quality (in terms of hydrogen and CO content). Also

some more tar can be observed and this can be a

consequence of the lower temperature achieved.

A mixture of biomass and coal is studied also for thedual beds gasifier. In this case the co-gasification is

dictated by the need of residual char that has to be passed

from bed1 to bed2 to assure the autothermal feasibility of

the system. Some biomass indeed produces a too low

quantity of char. This can be observed in Figure 8 where

the temperature of both beds are reported as function of

the biomass-to-coal ratio. The higher this ratio, the lower

the temperature of both beds. Also the residual char from

bed1 to bed2 decreases significantly as shown in the

figure. Finally, the hydrogen production decreases.

Additional analysis can be carried out with this tool

in order optimize the system. For instance, the sand-to-

fuel ratio and the gas recirculation flowrate can be varied

to define the optimal conditions.

0

2

4

6

8

10

12

0.3 0.35 0.4 0.45 0.5 0.55 0.6

ER

Specificunit

0

10

20

30

40

50

60 Hydrogenproduction

(gH2/kgbiomassdry)

H2 productionLHV syngas (MJ/Nm3)

ar (gTAR/kg

biomass dry)

Figure 5: Results of the downdraft gasifier model with

Aspen Plus for the gasification of poplar wood withoxygen/steam mixtures.

Figure 6: Results of the downdraft gasifier model with

gPROMS for the gasification of poplar wood (effect of

Equivalence Ratio on the temperature of solid and gas

phases).


8/9

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

Biomass-to-Coal ratio

Syngasm

olefrac

800

900

1000

1100

1200Peaktemperature(C)TAR

CO2

CO

H2

T

Figure 7: Results of the circulating fluidized bed gasifier

model with Aspen Plus for the gasification of

coal/poplar wood blends with oxygen/steam mixtures.

0

0.1

0.2

0.3

0.4

0 0.1 0.2 0.3 0.4 0.5

Biomass-to-Coal ratio

H2molefracinthesyngas,c

harfrom

bed1tobed2(kg/kgfee

d)

800

850

900

950

1000

1050

1100

Temperature(C)

Tbed1

Tbed2

H2

CHAR

Figure 8: Results of the dual bed system model with

Aspen Plus for the gasification of coal/poplar wood

blends (bed1: gasifier, bed2: combustor).

Also in the case of the entrained flow gasifier a

biomass/coal blend is considered. The effect of theoperating conditions on the performance of the gasifier

can be studied. The equivalence ratio, the biomass-to-

coal sharing, the ratio between the steam and the fuel

blend can be varied to study the syngas composition, the

temperature inside all sub-units of the model and the by-

products yields (CH4 and tar) [16].

For example, the syngas produced for the gasification

of a 10%wt biomass-to-coal blend in a mixture of oxygen

and steam (steam-to-fuel 0.24 kg/kg) is shown in Figure

9. The higher the equivalence ratio, the higher the

maximum temperature achieved in the reactor

(corresponding to the peak temperature of the combustion

unit in the scheme of Figure 3d). The singular point for

ER 0.3 can be explained once the residual char is alsoconsidered. Some residual char is present for low values

of ER (causing loss in efficiency). For high values of ER,

no residual char is observed so the excess oxygen

consumes the gaseous products. The maximum of the

hydrogen production (and also of CO) is just for values

of ER around 0.3. Correspondingly the tar production is

very low due to the high temperatures achieved. It is

worth noting that temperatures higher than 1200C are

crucial for this kind of gasifiers to melt and remove the

ash in the slag and minimize the formation of tar.

As remarked above, residual char and tar production

can be hardly quantified with equilibrium or simplified

model, as this is possible only with the implementation of

a detailed devolatilization sub-model able to predict the

pyrolysis products as functions of the operating

conditions. Therefore this gasifier model is a powerful

tool to carry out a global analysis by varying all the

operating variables and optimizing the performance

parameters (e.g., efficiency, hydrogen production)

considering the critical restrictions (e.g., maximum

temperature or tar content).

Finally, some example of the results from the

entrained flow gasifier developed with Aspen HYSYS

are reported in Figure 10. In this case the optimizationresults are shown. Starting from the reference values of

the operating conditions (peak temperature 1600C,

oxygen-to-fuel ratio 0.8, steam-to-fuel ratio 0.25), they

are varied in wide range to maximize the Cold Gas

Efficiency:

fuelfuel

syngassyngas

FLHV

FLHVCGE=

where the Low Heating Value and the mass Flowrate

of syngas and fuel are compared. For instance, increasing

the oxygen-to-fuel ratio between 0.80 to 0.90 and

maintaining constant the other operating conditions a

maximum in CGE can be found for values around 0.88.

Below this value the gasification is not complete, above

that excess oxygen oxidized the gaseous products.

However, combining all the operating parameters the

automatic procedure (implemented by the Hyprotech

SQP Optimizer) gave the optimized values reported in

Figure 10.

The automatic procedures included in the model and

powered by the software allow one to define also other

targets (for instance the hydrogen production of the entire

process) to obtain the optimal conditions. The developed

model is a powerful tool for process analysis and

optimization.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.2 0.25 0.3 0.35 0.4

ER

molefrac

1000

1200

1400

1600

1800

2000 Peaktemperature(C)

CO

H2

CO2

Char

TAR

T

Figure 9: Results of the entrained flow gasifier model

with Aspen Plus for the gasification of a coal/poplar

wood blends (10%wt of biomass) with oxygen/steam

mixtures.

6 CONCLUSIONS

A modeling procedure has been developed to provide

powerful tools for process analysis and optimization on

solid fuels gasification. The basic steps of pyrolysis,

combustion, gasification are described in detailed sub-

models that are combined to represent the reactor

configuration. Also heat streams are connected to respect

the heat balance of the system. This approach allows one

to optimize the operating conditions and compare

different configurations (fixed beds, fluidized beds and

entrained flow reactors) for a subsequent integration in

several processes of current interest (e.g., combinedpower production, hydrogen production). Different

commercial codes were used (Aspen Plus, Aspen

HYSYS, gPROMS) and peculiar aspects and

capabilities were commented.


9/9

0.65

0.7

0.75

0.8

0 0.2 0.4 0.6 0.8 1

steam-to-fuel,oxygen-to-fuelratios (kg/kg)

ColdGasEfficiency

0.65

0.7

0.75

0.8

1350 1400 1450 1500 1550 1600 1650 1700 1750

Peak Temperature(C)

steam-to-fuel

oxygen-to-fuelT

optimized values

0.50 0.81

T

1700

Figure 10: Results of the optimization of the entrained

flow gasifier modeled with Aspen HYSYS for the

gasification of a coal/poplar wood blends (10%wt ofbiomass) with oxygen/steam mixtures.

7 REFERENCES

[1] Zheng, L.; Furinsky, E. Comparison of Shell,

Texaco, BGL and KRW gasifiers as part of IGCC

plant computer simulations. Energy Conversion and

Management 2005, 46, 17671779.[2] Chiesa, P.; Consonni, S.; Kreutz, T.; Williams, R.

Co-production of hydrogen, electricity andCO2

from coal with commercially ready technology.

PartA: Performance and emissions.Int. J. Hydrogen

Energy, 30:747 (2005)[3] Collot, A.G.Matching gasifiers to coal. IEA Clean

Coal Centre 2002,CCC/65.

[4] Falcitelli M., Biagini E., Tognotti L. Development

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(ABCD) model. Proceedings of the 10th Conference

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July 2009

[5] Rath J., Staudinger G. Cracking reactions of tar from

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[6] Di Blasi C. Modeling Wood Gasification in a

Countercurrent Fixed-Bed Reactor, AIChE J. 50

(2004) pag. 2306.

[7] Wen, C.Y.; Chaung, T.Z. Entrainment coal

gasification modeling. Ind. Eng. Chem. Process

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[8] Hobbs, Michael L., Predrag T. Radulovic, and L.

Douglas Smoot, Modeling Fixed-Bed Coal

Gasifiers AIChE Journal, Vol. 38, No. 5, May

1992, pp. 681

[9] Dogru M., Howarth C.R., Akay G., Keskinler B.,

Malik A.A. 2002. Gasification of hazelnut shells in a

downdraft gasifier. Energy 27, 415427[10] Lv P., Yuana Z., Maa L., Wua C., Chena Y., Zhu J.

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air and oxygen/steam gasification in a downdraft

gasifier. Renewable Energy 32, 2173

[11] Sotudeh-Gharebaagh R., Legros R., Chaouki, Paris

J. Simulation of circulating fluidized bed reactorsusing Aspen Plus. Fuel 77 (1998) pag. 327.

[12] Corella J., Sanz A. Modeling circulating fluidized

bed biomass gasifiers. A pseudo-rigorous model for

stationary state. Fuel Processing Technology 86

(2005) pag. 1021

[13] Kunii D., Levenspiel O. High velocity fluidization.

In: Fluidization Engineering, 2nd ed. (1991) pag.

193. Butterworth-Heinemann, Boston, England.

[14] Basu P. Combustion and gasification in fluidized

beds, Taylor & Francis Group, LLC 2006[15] Aspen Technology Inc. Aspen HYSYS V7.1

Simulation Basis. Ten Canal Park, Cambridge (MA)

(2009)

[16] Biagini E., Bardi A., Pannocchia G., Tognotti L.

Development of an entrained flow gasifier model for

process optimization study. Ind.Eng.Chem.Res. 48

(2009), pag. 9028.

Universitdi Pisa

Chemical EngineeringDepartment

Via Diotisalvi, 256126 Pisa Italy

for this activity please contact:[email protected]: +390502217840fax: +390502217866

C. PisaRicerche

Energy & Environment Division

Lno Mediceo, 4056127 Pisa Italy

for this activity please contact:[email protected]: +390500988447fax: +390503869660
mailto:[email protected]:[email protected]:[email protected]:[email protected]

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