Variation of feedstock in a dual fluidized bed steam gasifier—influence on product gas, Sci ... [ Pobierz całość w formacie PDF ]
Variation of Feedstock in a Dual Fluidized Bed
Steam Gasifier—Influence on Product Gas, Tar
Content, and Composition
Johannes C. Schmid, Ute Wolfesberger, Stefan Koppatz, Christoph Pfeifer, and Hermann Hofbauer
Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9, A-1060 Vienna,
Austria; johannes.schmid@tuwien.ac.at (for correspondence)
Published online 27 February 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11607
industry in contrast to fossil fuels. Air-blown fluidized bed
concepts were proposed for the gasification of biomass.
However, conventional gasification with air yields a product
gas which is highly diluted with nitrogen, to the disadvantage
of the heating values of the product gas (4–6 MJ/m
3
stp
). In
contrast, a dual fluidized bed (DFB) steam gasifier system
enables the generation of a nitrogen-free product gas [2, 3].
The successful and prominent classical DUAL FLUID gasi-
fication technology for biomass was developed at the Vienna
University of Technology in the 1990s [4, 5]. ‘‘Classical’’
because the reactor concept combines a steam-blown bub-
bling fluidized bed (BFB) and an air-blown fast fluidized bed
(FFB), in contrast it is also possible to use two FFBs in DFB
systems. The basic principle of the entire DFB gasification
technology, nowadays named DUAL FLUID technology, is
shown in Figure 1. The product gas (syngas) yielded has a
high heating value of 11–15 MJ/m
3
stp
. Loop seals enable a
continuous circulation of solids between the separate reactor
parts. A circulation of solids matter, which mainly involves
bed material, serves to transport heat to the gasification
zone. Heat transfer to the fuel particles and the conversion of
biomass into a hydrogen rich product gas with high specific
energy content take place in contact with bed material par-
ticles inside the BFB. The heat demand for the allothermal
steam gasification is generated in a FFB by combustion of re-
sidual char with air. This part of the system is necessary for
the required circulation rate of bed material particles. There-
fore, the fluidized bed in the combustion zone has an effi-
cient transport characteristic.
The DFB concept is highly qualified for scale up. The
technical feasibility of the DUAL FLUID technology has been
proven in the early 2000s with the combined heat and power
plant (CHP) G¨ssing in Austria [6, 7]. In particular, a wide
range of experimental data was gathered at several but
almost similar generations of 100 kW DFB pilot plants at the
Vienna University of Technology [8]. These data highly sup-
ported the scale up of the CHP G¨ssing with a fuel input of
8 MW [9]. A sketch of the reactor system and a basic energy
flow sheet of the CHP in G¨ssing are displayed in Figures 2
and 3. With regard to efficiency consideration it has to be
mentioned that a process of drying the wet wood chips is
not realized in G¨ssing. The cold gas efficiency would be
significantly higher with drying the feedstock by usable low
temperature heat.
Further CHP based on the DUAL FLUID concept went
into operation in Oberwart/Austria [10], or are currently
A steam blown dual fluidized bed gasification plant was
used to yield a nitrogen (N
2
) free product gas (synthesis gas)
from various biomass fuels. In addition to the variation of
process parameters like temperature, steam to carbon ratio,
fluidization rate, and the influence of different bed materi-
als, various feedstock inputs affected the generation of the
product gas. This study focuses on the gasification of different
biomass feedstock. The variation of biomass implies wood
chips, wood pellets, sewage sludge pellets, and straw pellets.
The chosen evaluated experimental results are all gained
from the uniformly operated ‘‘classical’’ 100 kW ‘‘DUAL
FLUID’’ gasifier at Vienna University of Technology at con-
stant gasification temperatures between 800
C. In
the ‘‘classical’’ design, the gasification reactor is a bubbling
fluidized bed. The composition and ash melting behavior of
each feedstock is displayed, as well as the ranges of the prod-
uct gas compositions generated. Beside the main gaseous
product gas components, typical content ranges of dust and
char are highlighted. The content and composition of tar in
the product gas is discussed. Further it is possible to present
gravimetrical and gas chromatography coupled with mass
spectrometry measured tar values. Not less than five signifi-
cant component-groups of tar will also be outlined for each
feedstock.
2012 American Institute of Chemical Engineers Environ
Prog, 31: 205–215, 2012
Keywords: biomass, gasification, reforming, circulating
fluidized bed, synthesis gas
C and 810
8
8
INTRODUCTION
The thermo-chemical conversion of biogenous feedstock
is a promising option to advance the eco-friendly and effi-
cient production of heat and power, as well as the generation
of valuable products for the chemical industry based on
renewable sources. Biomass is particularly relevant as this
feedstock constitutes the only carbon source available within
the range of renewables [1].
Fluidized bed processing is applied by preference for the
gasification of various carbonaceous fuels, and therefore also
for biomass. This technology intensely promotes the conver-
sion of the solid feedstock into a valuable gas by an excel-
lent gas–solid contact and heat transfer. The application of
biomass derived product gas as precursor for various synthe-
ses might increase the share of renewables in the chemical
2012 American Institute of Chemical Engineers
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
July 2012
205
Figure 1. Basic principle of the classical DUAL FLUID gasifi-
cation technology at the Vienna University of Technology.
[Color figure can be viewed in the online issue, which is
available at
wileyonlinelibrary.com
.]
Figure 3. Basic energy flow sheet of the CHP in G¨ssing/
Austria. [Color figure can be viewed in the online issue,
which is available at
wileyonlinelibrary.com
.]
EXPERIMENTAL
Figure 4 presents the 100 kW gasifier at the Vienna Uni-
versity of Technology. All important elements like loop seals,
process media inputs, solids separator, and various feedstock
hopper arrangements are visible in this sketch. The dashed
line indicates the global solids circulation rate of the bed ma-
terial in the reactor system. Olivine sand with a mean particle
diameter of about 0.5mm is used as the bed material. Only
feedstock Hopper 1 was used for all experimental results
presented in this publication. Thus, the fuel input was always
realized directly into the BFB of the gasification reactor,
which favors a prompt intermixing of fuel particles into the
fluidized bed in contrast to top–down charging of the fuel
particles. There are specific requirements of the feeding sys-
tem in order to ensure the transport of fuels with various cal-
orific values and size distribution. As the feeding system of
solid fuels is a very important part of a gasification plant,
especially at industrial scale [20], a few words are required to
explain the other hopper installations. Hopper 2 and 3 can
be used to blend different fuels (like coal and wood chips)
in varying mixing ratios. The feeding arrangement of Hopper
3 is equivalent to a spreader feeding. It enables a comparison
of different locations of fuel input. The partially water cooled
screw feeding equipment of Hopper 4 allows the possibility
of feeding materials with low melting temperatures. In order
to guarantee the highest safety demands, all hoppers of the
100 kW gasification plant are locked gas tight and flushed
with nitrogen.
For comparison of the experimental results, all experi-
mental runs were operated within a gasification temperature
range of 800–810
8
C, which corresponds to the temperature
in the BFB. The range of several operating parameters for
the experimental test runs including characteristic values of
the fluid dynamics are listed in Table 1. Note that the real
Figure 2. Reactor system of the 8 MW CHP in G¨ssing/Austria.
being erected and undergoing commissioning [11, 12]. How-
ever, the technology has to meet new challenges. The system
flexibility with regard to the application of different feedstock
is a major issue of current experimental research [13–18].
Beside the application of conventional wood as feedstock, it
is aimed to enlarge the range of applicable fuels. Thus, the
extension of the feedstock basis for the DFB system pro-
motes the flexibility in terms of the economy of the system
[19]. The current work outlines the technical feasibility of dif-
ferent feedstock. The chosen evaluated data are all gained
from the uniformly operated 100 kW gasification plant at
Vienna University of Technology. Due to the up scaling
potential of DFB gasification processes, the clearly arranged
results in this publication are highly valuable and representa-
tive for larger gasification plants using the same steam blown
DUAL FLUID technology.
206
July 2012
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
Table 1. Typical ranges of operating conditions of the 100
kW
th
gasifier plant at the Vienna University of Technology.
Operating Conditions
General
Bed material particles
Olivine
(kg/m
3
)
Bed material particle density
2850
Mean particle size, d
p50
(
l
m)
520
Size distribution, d
p10
– d
p90
(
l
m)
450 – 630
Minimum fludization velocity, U
mf
(m/s)
0.1 – 0.2
Terminal velocity, U
t
(m/s)
4.5 – 6.9
Gasification Reactor (GR)
Typical fluidization regime
Bubbling
bed
Steam to fuel ratio
(kg
H2O
/
kg
fuel,dry
)
0.8 – 1.1
Thermal power, feed gasifier
(kW)
66 – 97
Temperature, gasification zone
(
C)
800 – 810
8
Superficial gas velocity, U
(m/s)
0.41 – 0.56
Fluidization ratio, U/U
mf
-
2.1 – 5.6
Fluidization ratio, U/U
t
-
0.06 – 0.12
Mean gas residence time,
bubbling bed
(s)
0.27 – 0.37
Mean gas residence time,
freeboard
(s)
3 – 4
Combustion Reactor (CR)
Typical fluidization regime
Fast bed
Thermal power, fuel to combustion
(kW)
22 – 31
Temperature, combustion zone
(
8
C)
840 – 900
Superficial gas velocity, U
(m/s)
8.8 – 10.1
Fluidization ratio, U/U
mf
-
44 – 100
Fluidization ratio, U/U
t
-
1.3 – 2.3
Mean gas residence time
(incl. separator)
(s)
0.73 – 0.85
Figure 4.
100 kW gasifier at the Vienna University of Tech-
nology.
distribution of contact time of gas and solids in a fluidized
bed is considered to be highly complex, due to the influence
of various factors on the overall contact behavior. Thus, a
simplified approach is used to assess a mean residence time
of the gas flow inside the bubbling bed. The approach for
calculation of the residence time involves:
an ideal stirred vessel for all solids (fuel and bed material
particles),
an ideal plug flow of the gaseous phase,
a typical fluidized bed porosity,
a linear release of volatiles from the fuel particles over the
bed height.
Hence, a mean residence time for the gas phase is calcu-
lated for the gasification zone inside the bubbling bed. Fur-
thermore, the gas residence time in the freeboard of the gasi-
fication reactor is displayed. The calculation of the mean resi-
dence time for the gas flow in the combustion reactor
includes the riser part and the solids separator downstream
zone where still a gas–solids contact is present.
Main product gas components like H
2
, CO, CO
2
, and CH
4
are analyzed by Rosemount NGA2000 measurement equip-
ment. C
2
H
4
,C
2
H
6
,C
3
H
8
, and N
2
values are measured with a
Syntech Spectras GC 955 gas chromatograph. A large number
of temperature (thermocouples) and pressure sensors guar-
antee an effective process control and a smooth and continu-
ous operation of the whole gasification facility. All process
media inputs are measured with high quality flowmeters
(rotameter) from the company Krohne. A standardized
arrangement of sampling equipment is used to analyze the
content of dust, char, and tar in the product gas stream [21].
For entrained dust and char contents as well as for the
amount and composition of tars a minimum of three samples
are generally taken. Therefore, the experimental results pre-
sented are average values of each series of samples. The
arrangement of sampling equipment for dust, char, and tars
is shown in Figure 5. The high molecular weight (heavy) tar
compounds are quantified as the mass of tars left after vac-
uum evaporation of the solvent (toluene). This is referred to
as ‘‘gravimetric’’ tar. The medium molecular weight tar com-
pounds such as naphthalene are detected by gas chromatog-
raphy coupled with mass spectrometry (GCMS). The tar sam-
pling and analysis procedure, as well as the comprehensible
chosen classification of tars, are described in detail elsewhere
[17, 22–24]. An overview of detectable GCMS tar components
and the classification of them with regard to the presented
experimental results is also described in Table 9. Since tolu-
ene is used as solvent, benzene, toluene, and xylene (BTX)
values are not detectable GCMS tar components.
The main elemental composition, volatiles, water, and ash
content, as well as the ash melting behavior of the feedstock
fuels were analyzed according to international standards at
the test laboratory for combustion systems at the Vienna Uni-
versity of Technology. Additionally, the X-ray fluorescence
spectroscopy (XRF) offers a rough overview of the main ele-
mental ash components. All value ranges are given in relative
percentage weight of elements. The ash melting behavior is
determined by the standardized characteristic temperature
method. Two temperatures are specified: The deformation
temperature, where the first rounding of the edges of a cubic
sample occurs; and the flow temperature, where the sample
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
July 2012
207
is molten to a flat disk with defined geometrical conditions
(specific height). The ash melting temperature is an impor-
tant and critical issue for fluidized bed operation. Ash melt-
ing has to be prevented inside fluidized beds, since solids
agglomeration and plugging may occur.
The software package IPSEpro is used for evaluation and
validation of the process data which were gathered during
the experiments. Further, the mass and energy balance for
the experimental runs is computed with this tool. IPSEpro is
a software package originating from the power plant sector,
which offers stationary process simulations based on flow
sheet handling. The software uses an equation-oriented
solver. Various validated models are also used to safeguard
up scaling results of future industrial plants and achieve reli-
able data for efficiencies and costs. The software package
IPSEpro is described in detail elsewhere [25, 26].
EXPERIMENTAL RESULTS
In the first part of this section, the results from the basic
analysis of softwood pellets, hardwood chips, straw/wood
blended pellets, straw pellets, and sewage sludge pellets
are presented in Tables 2–4. In Table 4 the typical carbon
to hydrogen ratio of biogenous fuels is evident. Thus it
appears that the ratios of the various fuels are all very simi-
lar. The ash melting behavior is displayed in Table 5. The
ash content of sewage sludge and straw is significantly
high. Therefore, the elemental ash composition for these
ashes is given in Table 6. Ash analyses were made directly
from the residual ash of the fresh fuel pellets. The pre-
sented values cannot be compared directly to downstream
ash probes from combustion or gasification plants. It
makes a difference if ash is taken after conversion in a flu-
idized bed as bed ash, cyclone ash, or bag filter ash. Fines
caused by abrasion and attrition effects of the fluidized
bed material are added to the discharged ash. The next im-
portant point is that all values are dependent on the source
of the fuel. Sewage sludge pellets of different sources have
a particularly wide range of composition. So the results of
the basic elemental analysis and the XRF analysis in Tables
2–6 only represent the presented and used fuels as basic
materials.
Considering the ash melting temperature of 100% straw in
Table 5, we are able to determine that this type of feedstock
is not suitable for gasification at around 800
8
C. Especially
because the combustion part of the system will have temper-
atures of up to 850
8
C. Therefore 40% straw shavings and
60% wood dust (by weight) were mixed together and then
pelletized. These blended pellets are suitable to reach the
minimum requirements for ash melting behavior. Further-
more, it is mentioned that the straw for the blended pellets
was not from the same source as the 100% straw pellets. All
fuel pellets were pelletized without the usage of a binder.
Despite the high ash content of sewage sludge there was no
problem in gasifying this type of pellets at 810
C. It was
observed that wood pellets and also blended pellets of straw
and wood have a very good cohesion behavior. Even in the
8
Figure 5. Dust, char, and tar sampling scheme.
Table 2. Physical properties, lower heating values, volatiles, ash contents, and water contents of various fuels.
Feedstock / Fuel
40% straw/60%
wood blended
pellets
Softwood
pellets
Hardwood
chips
100% straw
pellets
Sewage sludge
pellets
Mean cross section
(mm)
ø6
4
3
13
ø8
ø8
ø7.5
Range of length
(mm)
12 – 22
20 – 30
8 – 20
8 – 18
8 – 15
(kg/m
3
)
Bulk density
640
260
570
560
760
Lower heating value
(kJ/kg
dry
)
18,750
18,180
18,470
17,680
12,420
Lower heating value
(kJ/kg
moist
)
17,460
17,010
16,270
16,100
10,800
Water content
(wt %)
6.1
5.7
10.5
7.9
11.0
Ash content
(wt %
dry
)
0.3
1.0
1.6
6.7
41.5
Volatile matter
(wt %
dry
)
86.5
84.0
82.4
77.3
55.4
wt %
percentage by weight.
5
208
July 2012
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
Table 3. Main elements of various fuels, free from water, relative weight percentage.
Feedstock / Fuel
40% straw/60%
wood blended
pallets
Softwood
pellets
Hardwood
chips
100% straw
pellets
Sewage sludge
pellets
Ash content
(wt %
dry
)
0.3
1.0
1.6
6.7
41.5
C, carbon
(wt %
dry
)
50.2
48.8
49.9
46.9
29.7
H, hydrogen
(wt %
dry
)
6.0
5.9
5.7
5.4
3.7
O, oxygen
(wt %
dry
)
43.4
44.1
42.6
39.5
20.2
N, nitrogen
(wt %
dry
)
0.05
0.15
0.25
0.55
3.9
S, sulfur
(wt %
dry
)
0.005
0.02
0.02
0.52
1.01
Cl, chlorine
(wt %
dry
)
0.003
0.003
0.01
0.41
0.05
Table 4. Calorific elements and oxygen of various fuels, free from water, ash-free, relative weight percentage.
Feedstock/Fuel
40% straw/60%
wood blended
pellets
Softwood
pellets
Hardwood
chips
100% straw
pellets
Sewage sludge
pellets
C, carbon
(wt %
dry, ash-free
)
50.4
49.3
50.7
50.3
50.7
H, hydrogen
(wt %
dry, ash-free
)
6.1
5.9
5.7
5.8
6.3
O, oxygen
(wt %
dry, ash-free
)
43.5
44.6
43.3
42.3
34.5
N, nitrogen
(wt %
dry, ash-free
)
0.05
0.15
0.25
0.59
6.67
S, sulfur
(wt %
dry, ash-free
)
0.005
0.015
0.02
0.56
1.73
Cl, chlorine
(wt %
dry, ash-free
)
0.003
0.003
0.01
0.44
0.09
Table 5. Ash melting behavior of various fuels.
Feedstock / Fuel
40% straw/60%
wood blended
pellets
Softwood
pellets
Hardwood
chips
100% straw
pellets
Sewage sludge
pellets
Deformation temperature
(
8
C)
1400
1420
900
720
1140
Flow temperature
(
C)
1450
1460
1240
1080
1240
8
Table 6. Elemental ash characteristics of straw pellets and
sewage sludge pellets (XRF analysis), main elements, relative
values, dry, free of carbon and oxygen.
fluidized bed of the gasifier this type of pellets were not
pulverized immediately. Bed material samples from the lower
loop seal showed that char particles coming from the gasifi-
cation zone have almost the same shape as the initial pellets.
The attrition behavior of wood pellets and wood chips dur-
ing the gasification process in a fluidized bed is presented in
detail elsewhere [27]. The authors show that the common
pelletization procedure is able to give sufficient mechanical
strength to the pellets.
In the second part of this section, the main process
parameters due to the gasified fuels are shown in Table 7.
Figure 6 proposes the main gaseous product gas components
followed by an additional data set as shown in Table 8. Fur-
thermore it can be expected that the product gas from sew-
age sludge also comprises a significant content of ammonia
(NH
3
: 3.4 vol %), similar to investigations with other nitrogen
rich fuels [15]. The listed components given at dry basis com-
plete the product gas composition to 100 percent in volume.
It has to be taken into account that the value of nitrogen
(N
2
) is not a leakage of gas flow between the combustion
and gasification reactor. As already mentioned, the amount
of N
2
in the product gas is a result of flushing the feedstock
hoppers due to safety requirements. Gas leakage through
the two steam fluidized loop seals is almost zero. Rather
high differences in product gas output (volume flow),
Fuel
100% straw
pellets
Sewage sludge
pellts
Element
Na (wt %)
0.5
1
<
<
Mg (wt %)
3
3
<
<
Al (wt %)
1
6 – 10
<
Si (wt %)
45 – 50
13 – 16
P (wt %)
2
8 – 12
<
S (wt %)
<
2
2–4
Cl (wt %)
3 – 7
<
0.1
K (wt %)
30 – 35
2 – 5
Ca (wt %)
6 – 10
25 – 30
Ti (wt %)
<
0.5
<
3
<
1
3
10
2
4
V (wt %)
<
0.5
<
1
3
10
2
4
Mn (wt %)
<
0.5
Fe (wt %)
<
0.5
24 – 30
<
1
3
10
2
4
Cu (wt %)
<
0.5
<
1
3
10
2
4
Zn (wt %)
<
1
<
5
3
10
2
5
(per)
<
5
3
10
2
2
(per)
Cr, Co, Ni, As (wt %)
in
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
July 2012
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