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j.energy.2018.07.199

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Energy 162 (2018) 35e44
Contents lists available at ScienceDirect
Energy
journal homepage: www.elsevier.com/locate/energy
The impact of bed material cycle rate on in-situ CO2 removal for
sorption enhanced reforming of different fuel types
Josef Fuchs a, *, Johannes Christian Schmid a, Florian Benedikt a, Stefan Müller a,
Hermann Hofbauer a, Hugo Stocker b, Nina Kieberger c, Thomas Bürgler c
a
b
c
TU Wien, Institute of Chemical, Environmental and Bioscience Engineering (ICEBE), Getreidemarkt 9/166, 1060 Wien, Austria
Voestalpine Stahl Donawitz GmbH, Kerpelystraße 199, 8700 Leoben, Austria
Voestalpine Stahl GmbH, Voestalpine-Straße 3, 4020 Linz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 December 2017
Received in revised form
12 June 2018
Accepted 29 July 2018
Available online 30 July 2018
A dual fluidized bed reactor system produces a nitrogen-free product gas by using steam as gasification
agent. Additionally, the usage of limestone as bed material allows for the in-situ removal of carbon dioxide out of the product gas. Hence, a hydrogen-rich product gas with a high reduction potential for the
steel industry can be generated. This so-called “sorption enhanced reforming” process has already been
proven applicable for wood as fuel, but since the costs for biomass like wood have increased significantly
during the last years, cheaper fuels are of interest. The experimental results of three different biogenic
fuel types (soft wood, rice husk and bark) and one fossil fuel type (lignite) are discussed in detail. In the
past, research mainly focused on temperature dependency of the process since it seemed to be the main
factor for carbon dioxide sorption in the gasification reactor and therefore product gas composition.
Within this work, it is shown that the bed material cycle rate should not be disregarded and is a key
factor within the process. The presented findings allow a detailed understanding of “sorption enhanced
reforming”, the influence of bed material cycle rate, and the influence of volatile matter in the fuel.
© 2018 Elsevier Ltd. All rights reserved.
Keywords:
Dual fluidized bed
Sorption enhanced reforming
Bed material cycle rate
Volatile matter
Limestone/calcium carbonate
Biogenic fuels/biomass
1. Introduction
The substitution of fossil fuels by renewables is one of the major
challenges for reducing greenhouse gas emissions. Biomass as a
renewable source releases the same amount of carbon dioxide as it
aggregates during its growth. Therefore, the gasification of biomass
is a reliable technology for the reduction of greenhouse gas emissions and for gaining a valuable product gas (also called producer
gas) for the production of heat, electric power, syngas and also for
the usage as a reducing agent in the iron and steel industry. The
utilization of cheap alternative biomass sources becomes important, since the increasing use of high-grade biomass (like wood
chips) leads to increasing fuel costs as well. Due to challenging
characteristics of alternative fuels (accompanying undesired
* Corresponding author.
E-mail addresses: josef.fuchs@tuwien.ac.at (J. Fuchs), johannes.schmid@tuwien.
ac.at (J.C. Schmid), florian.benedikt@tuwien.ac.at (F. Benedikt), stefan.mueller@
tuwien.ac.at (S. Müller), hermann.hofbauer@tuwien.ac.at (H. Hofbauer), hugo.
stocker@voestalpine.com
(H.
Stocker),
Nina.Kieberger@voestalpine.com
(N. Kieberger), thomas.buergler@voestalpine.com (T. Bürgler).
https://doi.org/10.1016/j.energy.2018.07.199
0360-5442/© 2018 Elsevier Ltd. All rights reserved.
chemical substances, high ash content, low ash deformation temperature, high tar and dust contents in the product gas, etc.), further
development of existing technologies towards biogenic residues is
of great relevance. Further, for the utilization of the product gas in
steel industry, a high reduction potential of the product gas is
required. To meet the mentioned requirements, the dual fluidized
bed (DFB) steam gasification is a suitable process for the thermochemical conversion of biomass into a nitrogen-free product gas,
which mainly consists of hydrogen (H2), carbon monoxide (CO),
methane (CH4), and carbon dioxide (CO2) [1e4]. A steam-blown
gasification reactor and an air-blown combustion reactor are the
main parts. The combustion reactor provides the necessary heat for
the overall endothermic steam gasification via combustion of residual char from gasification. If the residual char is not sufficient for
the heat balance, additional fuel can be fed to the combustion
reactor as well. The produced heat is transferred into the gasification reactor via the bed material, which is typical for fluidized beds.
Usually silica sand or olivine [5e7] are used for conventional
gasification applications as bed material. Besides these well-known
bed materials, beneficial effects like enhanced tar reforming [1] and
the selective in-situ removal of CO2 in the gasification reactor can
36
J. Fuchs et al. / Energy 162 (2018) 35e44
List of abbreviations
Symbols
m_ CO2;captured massflow of CO2 captured by the bed material in
the gasification reactor
m_ CaO;CR/GR massflow of CaO transported from combustion
reactor to the gasification reactor via the upper loop
seal
m_ char;GR/CR massflow of char which leaves the gasification
reactor via the lower loop seal
massflow of fuel fed into the gasification reactor
m_ fuel
m_ steam
massflow of steam fed to the gasification reactor
mCaO;initial initial bed material inventory as CaO
x
mass fraction
Abbreviations
add
additional
BeM
bed material
C
carbon
char
if not designated differently: char from gasification
reactor to combustion reactor via lower loop seal
CR
combustion reactor
be achieved by a calcium based bed material (calcium oxide (CaO)/
calcium carbonate (CaCO3)) [8e15]. The basic principle of the
gasification process with selective in-situ removal of CO2, the socalled sorption enhanced reforming (SER) process, is displayed in
Fig. 1.
The circulating bed material plays a crucial role for the basic
operation of the DFB gasification system as transport medium of
char from the gasification reactor into the combustion reactor and
as transport medium for heat from the combustion reactor into the
gasification reactor. Furthermore, the bed material can also act as a
catalyst and, therefore, contributes to improvement of the product
gas quality. Another possible function of the bed material is the
capture of gaseous components from the product gas: The sorption
enhanced reforming (SER) process uses limestone/calcium carbonate (CaCO3) as bed material. By operating both reactors in a
suitable temperature range, in-situ CO2 capture in the gasification
reactor according to Equation (1) and its release in the combustion
reactor are possible (Equation (2)). The decreased CO2 concentration leads to a more intensive reaction of steam (H2O) with CO.
Bed material
cooling
Fluegas
CO2 - enriched
Product gas
H2 - enriched
Heat
CaO
Fuel
Add. fuel
CaCO3/CaO
Char
Steam
Air
Fig. 1. Principle of gasification with CO2 capture (SER).
CxHy
DFB
gas
GCMS
GR
max
n.m.
Nm3
PG
PGY
S/C
S/F
SER
temp.
TGA
fuel
add. fuel
sum of ethene, ethane and propane
dual fluidized bed
gasification
gas chromatograph mass spectroscopy
gasification reactor
maximum
not measured
cubic meter at 0 C and 101.325 kPa
product gas
product gas yield
steam to carbon ratio
steam to fuel(daf) ratio
sorption enhanced reforming
temperature
thermogravimetric analyzer
fuel into gasification reactor
additional fuel into combustion reactor
Subscripts
th
daf
db
fuel
thermal
dry and ash-free
dry basis
fuel into gasification reactor
Thus, the water-gas shift reaction (Equation (3)) enhances the
production of H2 in the gasification reactor. Müller et al. [12]
showed that the SER process produces a hydrogen-based reducing
agent with a higher reduction potential than the gas produced from
the conventional gasification process with olivine as bed material.
CaO þ CO2 /CaCO3
DHR650 ¼ 170kJ=mol
(1)
CaCO3 /CaO þ CO2
DHR900 ¼ 166kJ=mol
(2)
CO þ H2 O 4CO2 þ H2
DHR650 ¼ 36kJ=mol
(3)
The suitable temperature range for gasification and combustion
during SER depends on the equilibrium partial pressure of CO2 in
Equation (1). The equilibrium curve according to [16] is reproduced
in Fig. 2, where typical operational conditions for both reactors are
outlined as well. Typical temperatures in the gasification reactor for
SER are between 600 and 700 C. In this temperature range, CO2
capture is possible, whereas in the combustion reactor, the bed
material is heated up above 810 C. There, a suitable residence time
ensures calcination to calcium oxide (CaO).
Since the correct temperature assures the driving force for CO2
capture in the gasification reactor, it has been identified as main
factor to influence the product gas composition of the process.
Several authors published product gas compositions depending on
gasification temperature [8,9,17e22]. The data from Ref. [8] was
obtained from SER of soft wood with the advanced 100 kWth test
plant at TU Wien. This test plant was used as well for the generation
of all data presented within this work.
The aim of this work is to point out that other influencing factors
besides the gasification temperature have been neglected in past
research activities. Therefore, SER experiments of four different fuel
types are investigated in detail. Especially the bed material cycle
rate is suspected to play a major role for the product gas
composition.
J. Fuchs et al. / Energy 162 (2018) 35e44
37
Partial pressure CO2 [bar]
0.4
0.35
0.3
CaCO
CaCO3 3
CR
0.25
0.2
0.15
0.1
CaO + CO2
GR
0.05
CaO + CO2
0
500
600
700
800
900
1000
1100
Temperature
[°C]
Temperature
[°C]
Fig. 2. Equilibrium partial pressure of CO2 for the System CaCO3/CaO.
2. Materials and methods
2.1. Advanced 100 kWth DFB test plant
TU Wien has designed and constructed an advanced DFB test
plant for the gasification of various fuels. A sketch of the plant is
shown in Fig. 3.
The advanced reactor design enhances the gas-solid contact by a
new countercurrent-column, which is placed subsequent to the
lower bubbling bed of the gasification reactor. The geometrical
modifications in this upper part of the gasification reactor lead to an
improved bed material hold-up [23] and enlarge the range of
applicable fuels because of higher tar and char conversion rates
compared to other DFB systems. Further, gravity separators with
gentle separation characteristics instead of cyclones support the
usage of soft bed materials such as limestone. The new system
prohibits high velocities of gas and particles and minimizes attrition effects. Additionally, a bed material cooling in the upper loop
seal enables the defined setting of temperature differences between the gasification and combustion reactor for SER. Further, a
staged air input into the combustion reactor (primary, secondary,
tertiary air in Fig. 3) allows an effective control of the bed material
cycle rate without changing the total amount of air.
2.2. Experimental
Three of the investigated fuels originate from renewable organic
sources - soft wood, rice husk, and bark. Lignite, the fourth presented fuel, is used for reasons of comparison and to point out the
fuel flexibility of the system. The detailed chemical analysis of the
mentioned fuels is given in Table 1 and was performed by the
accredited Test Laboratory for Combustion Systems at TU Wien. All
fuels were gasified with the same type of bed material (limestone,
Table 2), which mainly forms CaO during the calcination process
with high temperature in the combustion reactor. During the test
runs, the contents of the main product gas components like H2, CO,
CO2 and CH4 were analyzed: Therefore, a small amount of the
produced product gas was cooled down to condense the H2O.
Fig. 3. Advanced 100 kWth test plant at TU Wien.
Afterwards the gas passed four impinger bottles filled with rapeseed methyl ester to wash out the tar of the product gas and protect
the measurement devices. Online measurement of the main
product gas components is done by Rosemount NGA2000 measurement equipment. Higher gaseous hydrocarbons (CxHy) and N2
were analyzed by a gas chromatograph (Perkin Elmer ARNEL e
Clarus 500). The main product gas composition in this work is
published on N2-free basis to allow a better comparison between
the results. Typically, N2 in the product gas can be traced back to the
flushing of the measurement equipment and the fuel feeding system and is in the range of about 2 vol.-%db
A large number of temperature and pressure sensors and an
extensive measurement equipment for mass and volume flows of
the input and output streams guarantee an effective process control. A standardized arrangement of sampling equipment according
to [24] was used to analyze the content of dust, water, char and tar
in the product gas stream. Instead of using Isopropyl alcohol as a
38
J. Fuchs et al. / Energy 162 (2018) 35e44
Table 1
Ultimate analysis and fuel characteristics of the investigated fuels.
Parameter
Ultimate analysis
Water content
Ash content
Carbon (C)
Hydrogen (H)
Nitrogen (N)
Oxygen (O)
Sulphur (S)
Chlorine (Cl)
Additional fuel characteristics
Volatiles
LHV (wet basis)
Ash deformation temp. (A)
Ash flow temp. (D)
Standard/Equipment
Unit
Soft wood
Rice husk
Bark
Lignite
DIN 51701
DIN 14775
Perkin Elmer 2400 CHN Elemental Analyzer
wt.-%
wt.-%db
wt.-%daf
wt.-%daf
wt.-%daf
wt.-%daf
wt.-%daf
wt.-%daf
7.2
0.2
50.8
5.9
0.21
43.1
0.005
0.005
5.8
15.2
51.2
6.1
0.55
42
0.071
0.106
7.7
7.0
52.3
6.0
0.34
41.3
0.053
0.053
13.0
4.2
68.4
3.9
0.88
26.3
0.397
0.052
wt.-%daf
kJ/kg
C
C
86
17060
1330
1440
81
14740
1350
>1500
78
16480
1150
1210
54
20320
1340
>1500
Sum to 100 wt.-%daf
CEN/TS 15289
CEN/TS 15289
DIN 51720
DIN 51900 T2
DIN 51730/DIN 51719
Relative measurement uncertainty: Water content 4.3%; Ash content 9.15e12.65%; Sulphur 7.5%; Chlorine 7.1%; Volatiles 0.45%; LHV 1.0%; C,H,N not available, typically
around 5%.
Table 2
Chemical composition of bed material particles (limestone).
Parameter/name
Unit
Value
CaCO3
MgCO3
SiO2
Al2O3
Fe2O3
Loss of mass after calcination
Sauter mean diameter (dsv)
wt.-%
wt.-%
wt.-%
wt.-%
wt.-%
wt.-%
mm
95e97
1.5e4.0
0.4e0.6
0.2e0.4
0.1e0.3
~44
480/290
solvent for tar (as proposed in Ref. [24]), Toluene is used. This
procedure allows the determination of the water content in the
product gas via phase separation and volumetric determination. In
general all online measured values like temperatures, gas composition, fuel composition and input, or steam and air input were
taken over a period of about 30 min steady state operation for every
operation point presented in this work. Furthermore, all measured
and averaged values are validated with IPSEpro, which is a software
system for calculating energy and mass balances also for overdetermined processes. This approach guarantees accuracy and
reliability of the presented values, because the errors in measurement are balanced. Since presented values are validated by the
balanced and closed mass and energy balance of the whole test
plant, no raw measurement data is presented in the results section.
Therefore, standard deviation of the raw measurement data is not
provided in this work.
2.3. Performance indicating key figures
The calculation of mass and energy balances for different
operation points enables a validation of measurement data and the
calculation of performance indicating key figures. Equations
(4)e(10) show the applied definitions for the discussion of the results. Equation (4) shows the product gas yield, which indicates
how much product gas can be produced per fuel fed to the gasification reactor. The steam to fuel ratio and the steam to carbon ratio
(Equations (5) and (6)) are a measure of how much steam is used
for the gasification of the fuel and respectively of the carbon fed to
the gasification reactor. Further, the bed material cycle rate in
Equation (7) describes the turnover of the total bed material mass
based on CaO inventory in times per hour. The CO2 load of the bed
material (Equation (8)) indicates how much of the sorbent CaO in
the gasification reactor has reacted with CO2. The maximum value
would be 1, which means that the bed material is fully carbonated
when it leaves the gasification reactor. Equation (9) shows the
amount of carbon captured via the bed material in relation to the
total amount of carbon fed into the gasification reactor as fuel.
Equation (10) is similar to the previous key figure, but provides
information about the carbon which leaves the gasification reactor
as char to the combustion reactor in relation to the total amount of
carbon fed into the gasification reactor as fuel.
Product gas yield
.
PGY ¼ V_ PG;db m_ fuel;daf
Steamto fuel ratio
.
4SF ¼ m_ steam þ m_ fuel xH2O;fuel
m_ fuel;daf
. i
½Nm3 kg
(4)
(5)
½kg=kg
Steam to carbon ratio
.
4SC ¼ m_ steam þ m_ fuel xH2O;fuel
m_ fuel xC;fuel
½kg=kg
(6)
Bed material cycle rate
dCaO ¼ m_ CaO;CR/GR mCaO;initial ½h1
i
(7)
CO2 load of bed material
. .
XCO2 ¼ m_ CO2;captured 44
m_ CaO;CR/GR 56
½mol=mol
(8)
Carbon transport via bed material
. .
. tCO2 ¼ m_ CO2;captured 44
m_ fuel xC;fuel 12
½mol=mol
(9)
Carbon transport via char
. .
tchar ¼ m_ char;GR/CR xC;char 44
m_ fuel xC;fuel 12 ½mol=mol
(10)
3. Results and discussion
3.1. Overview of experimental test runs
The results of all test runs confirm that the necessary conditions
for the SER process in both reactors (gasification and combustion)
J. Fuchs et al. / Energy 162 (2018) 35e44
39
Table 3
Operational parameter and results of gasification experiments.
Initial bed material as CaCO3 (CaO)
Sauter mean diameter
Gasification temp. (lower GR)
Max. temperature in CR
Fuel to GR
Additional fuel to CR
Chem. energy in PG
H2
CO
CO2
CH4
CxHy
PG yield
CO2 in flue gas
Gravimetric tar
GCMS tar
S/F
S/C
Bed material cycle rateb
Pressure gradient upper CR
kg
mm
C
C
kW
kW
kW
vol.-%dba
vol.-%dba
vol.-%dba
vol.-%dba
vol.-%dba
Nm3db/kgfuel,daf
vol.-%db
g/Nm3db
g/Nm3db
kgH2O/kgfuel,daf
kgH2O/kgC,fuel (molH2O/molC,fuel)
h1
mbar/m
Soft wood I
Soft wood II
Soft wood III
Rice husk
Bark
Lignite I
Lignite II
Lignite III
70 (39)
480
630
840
110
4
81
69.5
8.6
5.6
14.0
2.3
0.9
28.9
n.m
n.m
0.8
1.6 (1.1)
4.3
0.25
80 (45)
290
680
820
100
0
59
58.5
7.2
18.8
12.8
2.7
0.8
22.4
1.1
6.0
0.9
1.8 (1.2)
14.6
1.03
80 (45)
290
690
820
102
0
58
56.4
8.0
21.5
11.7
2.4
0.8
21.9
0.9
4.8
0.9
1.7 (1.1)
20.4
1.52
80 (45)
480
640
810
105
29
76
55.5
13.5
11.6
15.8
3.6
0.9
23.7
17.8
29.4
1.2
2.3 (1.5)
12.5
0.8
80 (45)
480
630
840
102
6
77
67.5
6.4
8.8
14.3
3.0
1.0
27.8
n.m.
n.m.
1.1
2.0 (1.3)
4.0
0.31
72 (40)
480
660
920
100
0
45
75.7
4.4
9.8
8.4
1.7
0.9
25.6
n.m.
n.m.
1.1
1.6 (1.1)
8.8
0.39
72 (40)
480
660
880
100
0
42
72.0
4.8
14.3
7.0
1.9
0.9
23.1
n.m.
n.m.
1.6
2.3 (1.5)
12.9
0.67
72 (40)
480
650
920
100
0
42
76.1
4.3
11.1
7.0
1.5
0.9
23.7
n.m.
n.m.
1.5
2.2 (1.5)
9.5
0.37
n. m.: not measured.
a
PG composition on N2-free basis and without minor impurities; N2 content from flushing of measurement instruments for all presented test runs between 1.7 and 2.8 vol.%db.
b
Pure CaO in both reactors assumed
were fulfilled (see also Table 3). In general, the gasification reactor
has to work on the left side of the equilibrium curve of the system
CaCO3/CaO to capture CO2 and form CaCO3. Vice versa, the combustion reactor must be operated on the right side of the equilibrium curve to release CO2 and calcine CaCO3 to CaO. However, the
most important operating parameter of the process is the gasification temperature to guarantee an optimized CO2-sorption parallel to the thermochemical conversion of the fuel. It has been
shown in previous works that these two requirements are fulfilled
at a temperature around 650 C [18,20]. Therefore, the most
important control strategy is to keep this temperature in the lower
part of the gasification reactor (close to the fuel input). Further, the
temperature level in the combustion reactor is strongly dependent
on the chemical composition of the fuel. For SER of lignite a
significantly higher maximum combustion temperature was achieved. The reason for this behavior is the low content of volatile
matter in the fuel lignite (Table 1). This means that a high amount of
char (residual from gasification of the fuel) is transported to the
combustion reactor. This assumption is supported by the temperature profiles of the reactors in Fig. 4. During the gasification of
lignite, the temperature profile in the combustion reactor increased
with its height. This results from the high amount of char, which
burns along the reactor height with the fluidizing air. The temperature of the bed material at the exit of the combustion reactor
leads to higher temperatures in the countercurrent-column for the
gasification of lignite, which is advantageous for the reduction of
the tar content in the product gas [25]. A comparably smaller
amount of char reaches the combustion reactor in case of the
gasification of bark - the char is already combusted in the lower part
of the combustion reactor. The temperature decreases slightly with
the height of the reactor. Additionally, a slight fuel input directly
into the combustion reactor supports the calcination of bed material during the SER experiments with wood, rice husk and bark. The
described differences for lignite and bark are illustrative results for
fuels with low volatile matter and high volatile matter in general.
Comparing the product gas composition for the SER process of the
investigated fuels in Table 3 shows that for soft wood, bark and
lignite, H2 contents of about 70 vol.-%db were reached. Only the
gasification of rice husk led to a significantly lower H2 content in
the product gas. Several reasons can be identified for this behavior:
The comparably low maximum temperature in the combustion
reactor leads to an operation close to the equilibrium curve of the
chemical system CaCO3/CaO. It is assumed that complete calcination of the bed material was hindered, which may have impeded
the effective CO2 sorption in the gasification reactor. And last but
not least, the bed material cycle rate was comparably high, which is
unfavorable for a high CO2 sorption (this circumstance will be
discussed later on in this work). Consequently, a comparatively low
H2 content in the product gas was observed for SER with rice husk.
The reason for the lower system temperatures is the behavior of the
rice husk ash: Fluidization problems were encountered during
conventional gasification with gasification temperatures about
750 Ce800 C and high combustion temperatures of 1050 C [26].
To avoid limitations resulting from agglomeration effects of bed
material particles, SER system temperatures were kept as low as
possible. Thus, a more smooth operation regarding fluid dynamics
of the dual fluidized bed was found for SER of rice husk.
Especially, the gasification of wood, bark and lignite led to
preferable operational conditions without any limitations. Thus,
only these three fuels will be considered for further investigation in
this work.
3.2. Influence of bed material cycle rate
To demonstrate the influence of the bed material cycle rate on
the product gas composition, the two experiments Lignite II and
Lignite III were carried out and the results were compared. The
operating parameters like S/F ratio are very similar (see Table 3).
Further, the temperature profiles of the reactors in Fig. 5 are very
similar as well. This is not straightforward since the bed material
cycle rate influences the temperature profile heavily [9]. However, to
separate the influence of temperature from the influence of bed
material cycle rate, a similar temperature profile is indispensable.
Due to the bed material cooling in the upper loop seal, which is
installed at the test plant and can also be seen in the scheme of
Fig. 3, an operation with similar temperature profiles was possible.
Especially the temperature profiles of the gasification reactor, which
are most important, are nearly congruent. Higher temperatures in
Height [m]
J. Fuchs et al. / Energy 162 (2018) 35e44
Height [m]
40
Lignite I
Bark
Gasification temperature [°C]
Lignite I
Bark
Combustion temperature [°C]
Height [m]
Height [m]
Fig. 4. Temperature profile of the gasification reactor (left) and the combustion reactor (right) for SER of lignite and bark.
Gasification temperature [°C]
Combustion temperature [°C]
Fig. 5. Temperature profile of the gasification reactor (left) and the combustion reactor (right) for SER of Lignite II and Lignite III.
the upper part of the combustion reactor (Lignite III) are obvious.
However, since the deviation to the equilibrium concentration of the
system CaO/CaCO3 is high enough for both experiments favorable
conditions for calcination can be assumed in both cases.
By analyzing the fundamental figures of CO2 capture (Table 4)
calculated by mass and energy balances with IPSEpro, it turns out
that with a low cycle rate more CO2 is captured in total (8 kgCO2/h
vs. 6.9 kgCO2/h). Hence, a lower CO2 content can be found in the
product gas. Due to the water-gas shift reaction (Equation (3)) and
Le Chatelier’s principle, a higher H2 content can be obtained.
To get a more detailed insight into the process and its dependency on bed material cycle rate additional experiments
needed to be analyzed. All additional data were produced with soft
wood as fuel.
J. Fuchs et al. / Energy 162 (2018) 35e44
25
Table 4
Comparison of CO2 capture performance of experiments Lignite II and Lignite III.
Unit
Lignite II
Lignite III
Bed material cycle rate
h1
high (12.9)
low (9.5)
H2 in PG
CO2 in PG
Total CO2 capture
CO2 load of BeM.
vol.-%db
vol.-%db
kgCO2/h
molCO2/molCaO
72.0
14.3
6.9
0.017
76.1
11.1
8
0.027
Since the gasification temperature is strongly dependent on the
bed material cycle rate, a temperature increase with the bed material cycle rate could not be avoided (Fig. 6), even though the
installed bed material cooling was used. However, to demonstrate
the scale of influence of gasification temperature versus bed material cycle rate on the product gas composition, the Kendall rank
correlation is used. The Kendall rank t is equal to 1 (or 1) if
datasets are correlated and is 0 for non-correlated datasets. The
correlation between gasification temperature and CO2 content in
product gas versus bed material cycle rate and CO2 content in the
product gas is investigated in Fig. 7. The evaluation indicates that
the correlation with bed material cycle rate is higher than with
gasification temperature. Moreover, a quite high p-value for the
correlation with the gasification temperature is found, which
means that a significance of the result is not given. Therefore, from
the statistical point of view, a correlation between bed material
cycle rate and product gas composition is more probable. However,
the number of cases for this analysis is low and therefore must not
be seen as proof, but as additional indication for the significant
influence of the bed material cycle rate on the SER process.
3.3. General findings regarding bed material cycle rate
The general findings presented in this work deal with the carbon
balance of the gasification reactor. Carbon introduced by the fuel
can either leave the gasification reactor (i) in the product gas as CO,
CO2, higher hydrocarbons, tar, fly char or it is transported to the
combustion reactor as (ii) CO2 in the bed material (CaO þ CO2 /
CaCO3) or (iii) char (solid residual of gasification process). The
Soft wood I
Soft wood II
Soft wood III
Lignite
Add. soft III
wood
20
CO2 in product gas [vol.-%db]
Parameter/name
41
15
Low correlation
Kendall’s = 0.52
p-value = 0.14
10
5
0
620
640
660
680
700
720
Gasification temperature [°C]
25
Initial bed material as
CaCO3: 70 – 80 kg
Fuel Input: 88 – 110 kW
20
15
High correlation
Kendall’s = 0.90
p-value = 0.003
10
5
0
0
5
10
15
20
25
Bed material cycle rate [1/h]
Fig. 7. Kendall rank correlation between CO2 in product gas and gasification temperature/bed material cycle rate.
presented investigation focuses on the carbon which is transported
to the combustion reactor ((ii) and (iii)).
First, the CO2 load of the bed material is displayed as a logarithmic function of the bed material cycle rate in Fig. 8. It can be
learned that the CO2 load increases with a decreasing cycle rate.
This behavior is expectable, since it has already been shown that a
lower cycle rate increases the total amount of CO2 which is
captured in the gasification reactor and released again in the
Gasification temperature [°C]
700
680
660
Soft wood I
Soft wood II
Soft wood III
Rice
husk
Add. soft
wood
Bark
Lignite I
Lignite II
Lignite III
640
620
600
0
5
10
15
20
25
Bed material cycle rate [1/h]
Fig. 6. Correlation of bed material cycle rate and gasification temperature.
Fig. 8. Logarithmic CO2 load of bed material as a function of bed material cycle rate.
J. Fuchs et al. / Energy 162 (2018) 35e44
combustion reactor. If the cycle rate decreases while the total
amount of captured CO2 increases, the CO2 load must increase.
Considering the scale of the CO2 load for very low bed material
cycle rates (about 0.15 molCO2/molCaO), limitations regarding CO2
sorption for even lower bed material cycle rates are expected: It is a
well-known fact that the sorption capacity of limestones decreases
with calcination/carbonation cycles [10,27e31] due to changes of
the porous structure on its surface. Grasa & Abanades [27] proposed a residual sorption capacity of about 0.08 molCO2/molCaO for
calcination and carbonation in dry atmosphere (CO2/air mixture).
However, it is also well-known that steam increases the sorption
capacity significantly [32e34]. Thermogravimetric analyzer (TGA)
experiments of the limestone used for the test runs in the 100 kWth
test plant at TU Wien showed a remaining capacity of about 0.2
molCO2/molCaO after 40 cycles of calcination/carbonation in a
30 vol.-% steam atmosphere. Interestingly, the CO2 load seems to be
independent from the used fuel type. This means that for a certain
bed material cycle rate, the same CO2 load of the bed material is
obtained for every investigated fuel.
A similar behavior can be found for the carbon transport via
carbonation of the bed material (Equation (1)). Fig. 9 shows the
carbon transport via bed material with regard to the total carbon
introduced into the gasification reactor as a function of bed material cycle rate. For low cycle rates it can be shown that nearly 40% of
the carbon in the fuel (0.4 molCO2/molC, fuel) are removed from the
gasification reactor and transported to the combustion reactor sole
by CO2 sorption. The carbon transport via bed material decreases to
about 15% for a cycle rate of about 20 h1. The decrease can be
explained by a shorter residence time in the gasification reactor.
Again, no significant influence of the fuel type can be found.
Considering the carbon transported in the char from the gasification reactor to the combustion reactor, a different behavior is
obvious (Fig. 10): In case of lignite, about 60% of the carbon in the
fuel (0.6 molC, char/molC, fuel) was transported to the combustion
reactor as char, whereas for soft wood and bark only between 20 and
50% could be fed to the combustion reactor as char, depending on
the cycle rate. The most reasonable explanation for this behavior is
Initial bed material as CaCO3: 70 – 80 kg
Fuel Input: 88 – 110 kW
Gasification temperature: 620 – 700 °C
0.7
0.6
C transport via char
[molC,char/molC,fuel]
42
54 wt.-%daf
volatiles in fuel
0.5
0.4
0.3
0.2
78 – 86 wt.-%daf
volatiles in fuel
0.1
0
0
0.3
0.2
0.1
Initial bed material as CaCO3: 70 – 80 kg
Fuel Input: 88 – 110 kW
Gasification temperature: 620 – 700 °C
0
0
5
10
15
20
25
Bed material cycle rate [1/h]
Fig. 9. Carbon transport from GR to CR via bed material (CaO þ CO2 / CaCO3) as a
function of bed material cycle rate.
10
15
20
25
Fig. 10. Carbon transport from GR to CR via char as a function of bed material cycle
rate.
the volatile matter of the different fuel types. Lignite contains about
54 wt.-%daf volatiles only. Soft wood and bark contain between 78
and 86 wt.-%daf volatiles. Vice versa, the high amount of char in the
fuel lignite was not as easily devolatilized and gasified in the gasification reactor. Therefore, more char was transported to the combustion reactor. This behavior was found in earlier publications for
conventional DFB gasification without CO2 sorption [1,35] as well.
However, the increased residence time of the fuel in the gasification
Sum of C transport (bed material + char)
[molC/molC,fuel]
C transport via bed material
[molCO2/molC,fuel]
0.4
5
Bed material cycle rate [1/h]
Initial bed material as CaCO3: 70 – 80 kg
Fuel Input: 88 – 110 kW
Gasification temperature: 620 – 700 °C
0.5
Soft wood I
Soft wood II
Soft wood III
Rice
husk
Add. soft
wood
Bark
Lignite I
Lignite II
Lignite III
Soft wood I
Soft wood II
Soft wood III
Rice
husk
Add. soft
wood
Bark
Lignite I
Lignite II
Lignite III
1
54 wt.-%daf
volatiles in fuel
0.8
0.6
Soft wood I
Soft wood II
Soft wood III
Rice
husk
Add. soft
wood
Bark
Lignite I
Lignite II
Lignite III
78 – 86 wt.-%daf
volatiles in fuel
0.4
0.2
0
0
5
10
15
20
25
Bed material cycle rate [1/h]
Fig. 11. Sum of carbon transport from GR to CR as a function of bed material cycle rate.
Main product gas component [vol.-%db]
J. Fuchs et al. / Energy 162 (2018) 35e44
CO2 sorption in the gasification reactor and therefore in the product
gas composition. In this work it is shown that the bed material cycle
rate should not be disregarded. Subject of investigation is the carbon transport from the gasification reactor to the combustion
reactor, which is possible in two ways: CO2 in the bed material
(CaO þ CO2 / CaCO3) or char (solid residual of gasification
process).
The following conclusions can be drawn:
75
H2
65
55
Fuel: Soft wood
Fuel input: 88 – 110 kW
Gas. Temp.: 620 – 700 °C
25
5
0
CO2
CH4
15
CO
C xH y
0
5
10
43
15
20
Bed material cycle rate [1/h]
Fig. 12. Main product gas composition for soft wood as a function of bed material cycle
rate.
reactor due to low bed material cycle rates leads to an exceptionally
low carbon transport of about 20% (0.2 molC, char/molC, fuel) via char
for fuels with high volatile matter (soft wood and bark). It is
assumed that a similar behavior could be found for fuels with low
volatile matter (lignite in this case), but during the experiments such
low bed material cycle rates have not been investigated for lignite.
By considering the sum of both (Fig. 11) carbon transport via bed
material and via char, a nearly constant amount of carbon was
shifted to the combustion reactor e independent from the bed
material cycle rate, but dependent on the used fuel type: For lignite
about 80% of the total carbon in the fuel was removed from the
gasification reactor, whereas for the fuels with high volatile matter,
only about 63% on average were removed from the gasification
reactor. The independence from the cycle rate is surprising, since
both individual functions vary in a wide range.
Finally, the main product gas composition is displayed as a
function of the bed material cycle rate in Fig. 12. It is obvious that
the lowest CO2 and highest H2 contents in the product gas can be
obtained by low bed material cycle rates. This can be explained
easily by the findings of Fig. 9, which shows that a low bed material
cycle rate leads to a high CO2 removal in the gasification reactor.
Therefore, the water-gas shift reaction (Equation (3)) is stimulated
to produce H2.
4. Conclusion
Dual fluidized bed steam gasification of four different fuels,
operating the so-called sorption enhanced reforming (SER) process
for the production of a H2-rich reduction gas for steel industry, is
investigated in this paper. The SER process aims for the production
of a H2-rich product gas by in-situ CO2 removal from the gasification reactor via the bed material system CaO/CaCO3. Problem-free
operation conditions for the SER process with the four different
fuel types are demonstrated and it is shown that the amount of
volatiles in a fuel exerts a great influence on the operation conditions of the two interconnected reactors of a dual fluidized bed
gasification system.
In the past, research mainly focused on temperature dependency of the process since it seemed to be the main factor for
(i) The carbon transport via bed material is independent from
the investigated fuel type. In the experiments, up to 40% of
the carbon in the fuel is captured by the bed material at low
bed material cycle rate.
(ii) On the contrary, the carbon transport via char is dependent
on the volatile matter in the fuel. A significantly higher share
of carbon is transported as char to the combustion reactor for
lignite (low volatile matter). Again, the carbon transport via
char is highly dependent on the bed material cycle rate. Low
cycle rates lead to a high residence time of char in the gasification reactor and therefore a low amount of char can be
observed.
(iii) The sum of carbon transport to the combustion reactor
(carbon in bed material and char) is independent from the
bed material cycle rate. For soft wood and bark a total carbon
transport of 63% on average is observed, whereas for lignite
80% of the carbon is removed from the gasification reactor.
(iv) According to the findings (i) to (iii), higher H2 contents in the
product gas can be reached by applying low bed material
cycle rates.
Acknowledgements
The present work is part of the research project ERBA II in
cooperation with voestalpine Stahl GmbH and voestalpine Stahl
Donawitz GmbH. ERBA II receives financial support from the
research program “Energieforschung” funded by the “Austrian
Climate and Energy Fund”.
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