REMOVAL OF NAPHTHALENE AS THE MODEL TAR COMPOUND ON CALCINED DOLOMITES, OLIVINE
AND COMMERCIAL NICKEL CATALYST IN A FIXED BED TUBULAR REACTOR, APPLIED FOR BIOMASS
GASIFICATION
a
Z. Abu El-Rub*a , E. A. Bramera, G. Brema,b
University of Twente, Laboratory of Thermal Engineering, P. O. Box 217, 7500 AE Enschede, The Netherlands
*
Fax: +31534893663, Tel: +31534891091, Email: [email protected]
Fax: +3153 4893663, Tel : +31534892530, Email: [email protected]
b
TNO, P. O. Box 342, 7300 AH Apeldoorn, The Netherlands
Fax: +31555493287, Tel: +31555493290, E-mail: [email protected]
ABSTRACT: A project is carried out to develop a so called Entrained Flow Cracker (EFC) for tar cracking downstream
of a gasifier making use of cheap catalysts. Calcined dolomites and olivine are considered as such active and inexpensive
catalysts for hot gas cleaning of a gas produced by biomass gasifiers. The efficiency of tar removal for these materials is
compared with that of an inert material (silica sand) and with that of high activity material (commercial nickel catalyst).
Tar is modeled using naphthalene with a concentration of about 40 g/Nm3 and tar decomposition over the catalyst
materials is modeled by naphthalene reaction with CO2 and steam. Tests are carried out in a fixed bed tubular reactor at a
temperature of 900 oC under atmospheric pressure and a gas residence time in the catalyst bed of 0.3 s. For the selected
catalyst, kinetic studies will be carried out in the temperature range of 750 to 925 oC and atmospheric pressure in a plug
flow conditions without external or internal mass transfer limitations. A simple first order kinetic model will be used to
describe the tar conversion. The gas composition and tar content in the gas are analyzed downstream the reactor using
on-line and off-line gas analysis.
1 INTRODUCTION
The Dutch government has formulated the policy that
by the year 2020, 10% of the energy generation has to
come from sustainable energy. Amongst the sustainable
energy sources, biomass has the highest potential on a
worldwide scale.
Biomass can be converted to energy carriers by
biological or thermochemical processes. Among the
various thermochemical processes gasification has
attracted significant interest due to the higher efficiencies
produced by this technology either for small or largescale systems.
The fuel gas from most gasification reactors
(producer gas) contains particulates and heavy
hydrocarbons. At a joint meeting of experts and
authorities in Brussels in March 1998, it was decided to
define tars as hydrocarbons of molecular weight higher
than benzene [1]. If tar content in the exit gas is high,
then it will condense in the gas transfer lines and cause
severe plugging problems resulting in serious operational
interruptions. For engines, high concentration of tar can
damage them or lead to unacceptable level of
maintenance. The efficiency of the gas-cleaning step is
therefore fundamental to the successful operation of the
integrated biomass gasification system. Moreover, the tar
problem is considered as one of the most important
technical barriers for the penetration of the technology in
the power markets [2].
For most people working on gasification, catalytic tar
removal is the best method for producer gas cleanup.
With a catalyst placed downstream of the gasifier, the
producer gas can be not only cleaned but also upgraded
[3].
Tar removal over calcined dolomites (MgO.CaO) is
mostly due to steam and dry (CO2) reforming reactions
[4]. The reacting network for all the overall tar
elimination over calcined dolomite is not known yet, but
at least includes: reactions of steam and CO2, thermal
cracking and hydrocracking of tars [5]. Simell et. al., [6]
related the activity of dolomite to the large pore size,
CaO surface area of the corresponding calcinates,
presence of active metal such as iron and may be due to
its relatively high alkaline (K, Na) content. The main
problem with dolomite is its fragility [7]. It has lower
mechanical strength than olivine [8,9]. Factors that
increase the life time of dolomite are [9]: space time or
space velocity, (high space velocity > 0.17 kg
dolomite.h/m3), Particle diameter, (particle diameter <
1.9 mm) and temperature (T > 840 0C). Simell et al
(1999), concluded that thermal reactions at a temperature
of 900 oC did not have a measurable effect on the
decomposition rates of tar model component [10].
Olivine consists mainly of a silicate mineral in which
magnesium and iron cations are embedded in the silicate
tetrahedral. Natural olivine is ((Mg,Fe)2SiO4) [8]. Its
mechanical strength is comparable to that of silica sand,
even at high temperatures, it presents higher attrition
resistance than dolomite [7,8]. Olivine is available in
market at about the same price as that for dolomite (120
Euro per metric ton) [7].
Model tar compounds are simple molecules chosen
as substitutes for real tar mixture. Since tar is a complex
mixture of hydrocarbons, it is difficult to prepare and to
study its conversion. Researchers used several model tar
components to represent the real tar in their studies for
tar reduction. The commonly used model tar compounds
in literature are naphthalene [11], benzene [10], toluene
[12], cyclohexane and n-heptane [13].
Naphthalene represents a light poly aromatic
hydrocarbon or a tertiary tar and at a temperature of 900
0
C it is the major component in tars. [14]. The order of
thermal reactivity of tar model compounds as found by
Jess [11] is toluene >> naphthalene > benzene. Based on
information from literature, it seems that it is more
accurate to use naphthalene as a model tar compound
than other model tar compounds.
This paper presents a comparative study of the
catalytic activity of dolomite and olivine compared with
that of an inert material (silica sand) and with that of high
activity material (commercial nickel catalyst). Other
cheap catalysts like char, ashes and FCC catalyst will be
investigated.
2 EXPERIMENTAL
This is the first stage of developing a so called
Entrained Flow Cracker (EFC) for hot gas cleaning
downstream of a biomass gasifier. In this stage, catalysts
screening tests were done using a fixed bed tubular
reactor at a mean temperature of 900 oC, an operating
pressure of 1 atm. and a residence time of 0.3 s.
2.1 Apparatus
The experiments were performed in a tubular reactor
at atmospheric pressure and a temperature of 900 oC. The
feed gas is typically 70 L.h-1 (NC). The gas (N2) is
metered from a cylinder controlled by critical nozzles.
The nitrogen is used as the carrier gas, which flows
through naphthalene and water saturation units made
from glass. The desired mole fractions are obtained by
controlling the temperatures of the saturators. CO2 is
added to the feed gas and the mixture is passed through
the reactor. The reactor is an empty quartz tube with an
internal diameter of 4 cm and a length of 91 cm, and
heated by a tubular electric furnace of 62.5 cm height and
5 cm heating space inside which the reactor is placed.
The reactor has two thermocouples, the first is in the
center (considered as the reference temperature) and the
second is close to the wall. These two thermocouples can
be moved along the bed height in small quartz pipes of 6
mm outside diameter to measure longitudinal
temperature profiles. Temperatures are measured by Ktype thermocouples. The bed material is supported by a
porous quartz disc, which allows the gas to pass through
its bottom. The catalyst is placed above a bed of an inert
silica sand of equal height to the catalyst bed. Fig. 1
shows the scheme of the setup.
2.2 Test procedure
The experimental runs were started by pouring a
weighed sample of the bed material on the reactor grid.
The feed gases flow rates were regulated to give the
desired space time, which is 0.3 s. dolomite calcination
was carried out in situ at 900 oC and atmospheric
pressure for 1 h at constant nitrogen flow. After
calcination, all the gaseous reagents were fed and the
reactor is heated to the required temperature. The
catalysts were then stabilized for at least 15 minutes [12]
before tests were started.
2.3 Tar analysis
The product gas composition and tar content in the
gas are analyzed downstream the reactor using on-line
and off-line gas analysis: the on-line gas analysis is
performed by a GC with flame ionization (FID) and
thermal conductivity (TCD) detectors and the off-line gas
analysis is performed with a GC/MS.
2.4 Catalysts and reagents
Dolomite, olivine and nickel catalyst were obtained
from commercial suppliers. The particles of dolomite and
olivine were sieved to a particle size range of 1.40-1.70
mm. The particles of nickel were crushed and sieved to
the particle size range of
1.40-1.70 mm. Table 1
shows some chemical characteristics of the catalysts.
Ventilation
Ventilation
TC
TI
TI
9
8
PI
GC / TCD
& FID
SPA
7
10
11
4
GC/MS
6
5
FIC
PI
FIC
2
TIC
3
FIC
2
TIC
1
Figure 1: Experimental setup
1. water saturator; 2. oven; 3. naphthalene saturator; 4. tubular
furnace; 5. quartz tubular reactor; 6. inert bed; 7. catalyst bed; 8.
quartz tubes for thermocouples; 9. water condenser; 10. filter;
11. heated pump; GC: gas chromatography; FID: flame
ionization detector; TCD: thermal conductivity detector; MS:
mass spectrometry; SPA: solid phase adsorption.
Table 1: Chemical composition of the catalysts (wt%)
Element
MgO
CaO
SiO2
Fe2O3
Al2O3
NiO
MnO
Cr2O3
NiCO3
Olivin
48.5-50.0
0.05-0.10
41.5-42.5
6.8-7.3
0.4-0.5
0.3-0.35
0.05-0.10
0.2-0.3
-
Dolomite
21.5
30.5
0.15
0.20
0.061
-
Ni-catalyst
7
12
70
5
3 EXPERIMENTAL DATA EVALUATION
3.1 Tar conversion
The conversion of the tar model compound
(naphthalene) was calculated from the inlet and outlet
naphthalene concentrations of the gas. This tar
elimination includes catalytic conversion as well as
thermal cracking. Tar conversion reactions are related to
steam and dry (CO2) reforming reactions with the model
tar component. The concentrations of steam and CO2 in
the feed gas are maintained at 10 vol.% for each
component. The conversion is defined as:
(Cn ,in − Cn ,out )
Cn ,in
100%
100%
(1)
3.2 Residence time
The residence time (τ) in the catalyst bed with
respect to the empty reactor is defined as:
τ=
VR ,cat
Q(T , Ptot )
61%
40%
(2)
(−rn ) = k app ,n Cn
(3)
Under plug flow conditions, the apparent kinetic constant
can be integrated as:
[− ln(1 − X n )]
2%
0%
Sand
Sand+Olivine
Sand+Dolomite
Sand+Nickel
Bed material
Figure 3: Effect of bed material on conversion of the
model tar component. Bed temperature 900 oC, 1 atm and
0.3 s residence time.
The relatively low activity of the used dolomite and
olivine could be related to the low active iron content,
especially in dolomite and the large particle size used in
the experiments, which caused internal mass transfer
limitations. Commercial nickel based catalyst is, as
expected, much more active than dolomite and olivine,
but they are more expensive and can be also deactivated.
Other types of cheap catalysts like char, ashes and FCC
catalysts will be investigated in the near future in
addition to catalyst life time studies.
CONCLUSIONS
τ
(4)
4 RESULTS AND DISCUSSION
Experiments were performed at such high
temperature in order to get high tar conversion or low tar
content in the exit gas. Fig. 2 illustrates the temperature
profile inside the reactor.
As a first stage of developing a so called entrained
flow cracker downstream a biomass gasifier, the catalytic
conversion of tarry fuel gas was studied with different
bed materials. The experiments showed that the used
commercial nickel catalyst is the most active one to
convert tarry fuel gas into a clean gas.
ACKNOWLEDGEMENT
This work is carried out within the framework of the
Center of Separation Technology of University of
Twente. This center is financed by the Netherlands
organization for Applied Scientific Research (TNO) and
the Process Technology Institute Twente (PiT).
70
60
50
Height (cm)
55%
60%
20%
3.2 Kinetic model
Anzar et. al., indicates that some European
Institutions participated in some EU financed projects on
hot gas cleaning few years ago to use the same, easy to
handle kinetic model for comparison of results obtained
in tar removal [14]. This model is a first order kinetic
equation with respect to the tar (naphthalene). This
model will be used when making kinetic study for the
selected catalysts.
k app ,n =
80%
x(%)
Xn =
conversion of the model tar component is illustrated in
Fig. 3. The order of reactivity obtained is: commercial
nickel > dolomite > olivine > silica sand.
40
30
NOMENCLATURE
20
10
Cn,in
0
0
200
400
600
800
10 0 0
T e m p e r a t u r e ( oC )
Cn,out
kapp,n
Figure 2: Reactor temperature profile
The effect of the tested four bed materials on
Ptot
Q
rin
T
VR,cat
Inlet naphthalene concentration to the reactor,
g.m-3
outlet naphthalene concentration to the
reactor, g.m-3
Apparent kinetic constant for naphthalene
removal, s-1
Total pressure, Pa
Volume flow rate, m3.s-1
Reaction rate with respect to the volume of
empty reactor, m3
Reaction temperature, oK
Volume of catalyst bed with respect to the
Xn
τ
volume of empty reactor, m3
Conversion of naphthalene, g.g-1
Residence time with respect to the empty
reactor, s
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