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j.energy.2018.08.146

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Accepted Manuscript
Fuel Flexible Gasification with an Advanced 100 kW Dual Fluidized Bed Steam
Gasification Pilot Plant
F. Benedikt, J.C. Schmid, J. Fuchs, A.M. Mauerhofer, S. Müller, H. Hofbauer
PII:
S0360-5442(18)31682-7
DOI:
10.1016/j.energy.2018.08.146
Reference:
EGY 13622
To appear in:
Energy
Received Date:
18 December 2017
Accepted Date:
20 August 2018
Please cite this article as: F. Benedikt, J.C. Schmid, J. Fuchs, A.M. Mauerhofer, S. Müller, H.
Hofbauer, Fuel Flexible Gasification with an Advanced 100 kW Dual Fluidized Bed Steam
Gasification Pilot Plant, Energy (2018), doi: 10.1016/j.energy.2018.08.146
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ACCEPTED MANUSCRIPT
Fuel Flexible Gasification with an Advanced 100 kW
Dual Fluidized Bed Steam Gasification Pilot Plant
F. Benedikt1,*, J. C. Schmid1, J. Fuchs1, A. M. Mauerhofer1, S. Müller1, H. Hofbauer1
TU Wien, Institute of Chemical, Environmental and Bioscience Engineering, Vienna, 1060 Getreidemarkt 9/166
* Corresponding author: florian.benedikt@tuwien.ac.at
E-Mail address: johannes.schmid@tuwien.ac.at (J. C. Schmid), josef.fuchs@tuwien.ac.at (J. Fuchs), anna.mauerhofer@tuwien.ac.at (A.M.
Mauerhofer), stefan.mueller@tuwien.ac.at (S. Müller), hermann.hofbauer@tuwien.ac.at (H. Hofbauer)
1
Abstract
Steam gasification enables the conversion of heterogeneous solid fuels into homogeneous gaseous energy carriers.
The utilization of biogenic residues and waste fractions as fuel for this technology offers a sustainable waste
management solution to produce heat and power, secondary fuels and valuable chemicals after several cleaning
and upgrading steps of the product gas. However, residues and waste fuels show unfavorable properties for
gasification and, therefore, cause technical challenges. This paper presents experimental results carried out at an
advanced 100 kWth dual fluidized bed steam gasification pilot plant from nine single test runs. In the following the
fuels that were gasified will be listed: (i) Five biogenic fuels, mainly residues: softwood, sugar cane bagasse,
exhausted olive pomace, bark and rice husks; (ii) two different waste-derived fuels: a municipal solid waste
fraction and a shredder light fraction; and (iii) a mixture of municipal solid waste fraction with a 25% blending of
lignite based on lower heating value as well as pure lignite. Thereby, various product gas qualities were generated.
The presented results offer the basis for a sustainable and promising waste management solution for the tested
waste fuels.
Keywords: waste gasification, biogenic residues, municipal solid waste fraction, shredder light fraction, calcium
oxide
1. Introduction
Taking into account the forecast of the International Energy Agency (IEA), that the global energy demand
increases up to 70% and the CO2-emissions increase up to 60% in 2050 compared to 2011, the research on
renewable energy systems still is and becomes more and more important [1,2]. Especially, energy systems and
technologies with a high energy efficiency are relevant for the future, because energy efficiency refers to the use
of less energy to produce the same amount of services or useful output [3,4]. Only, if this is fulfilled by a
technology or an overall energy system, it is possible to contribute to the decrease of fossil fuel demand and the
decrease of CO2 emissions. Additionally, due to the fact, that fuel costs influence the economic behavior and
process efficiency in particular, great attention should be paid on the use of non-conventional sources of energy,
like biomass in form of residuals and waste materials [5,6]. With regard to this issue, the following work focuses
on a thermochemical energy conversion process, the dual fluidized bed (DFB) steam gasification, of alternative
fuels to produce a high-valuable product. The DFB steam gasification offers a well-proven technology to produce
heat, electricity, secondary liquid or gaseous energy carriers and valuable chemicals from solid fuels. Utilizing
residues and waste fractions provides a high potential to produce these commodities in a sustainable, eco-friendly
way. Energy or fuel production from biomass via gasification offers an opportunity for a continuous process in
contrast to the fluctuating wind or solar based renewable energy sources. Additionally, with a suitable processing
of the syngas, a direct storage of the produced commodities (e.g. methane, hydrogen, FT-Diesel) is possible to
balance the fluctuating supply by other renewable energy sources.
DFB steam gasification was demonstrated for the gasification of wood at industrial scale in (i) Güssing, Austria
(8 MWth fuel power) [7], (ii) Oberwart, Austria (8.5 MWth) [8], (iii) Senden, Germany (15 MWth) [9] and (iv)
Gothenburg, Sweden (32 MWth) [10]. However, some of these plants suffer from difficult economic conditions if
high-grade wood chips are used as solid fuel - especially because the biomass costs increased significantly in the
last decade. In all quoted cases, woody biomass (in form of pellets, chips or partially forest residues) was used as
feedstock. Several economic considerations on DFB steam gasification processes showed that the fuel costs have
a significant impact on the economic performance as mentioned above [6,11]. In an extensive review on dual
fluidized bed gasifiers by Corella [12] it is stated, that the key problem or weakness to this technology is its
economic and not the technical feasibility. Thus, fuel flexibility in order to use low-cost fuels is an important issue.
By the use of alternative, low-grade fuels, which arise anyway as by-product of different processes, the life cycle
of these materials can be increased and the overall CO2 emissions reduced. Thus, the focus of this paper lies on
the conversion of residuals and waste material into a high-valuable product. Additionally, high carbon and
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hydrogen contents of fuels, which is the case in the mentioned fuels above, contribute to a great extent to the lower
heating value of the product gas [13]. However, alternative fuels often show unfavorable properties for gasification
and, therefore, cause technical challenges and limitations. Thereby, low product gas qualities may occur in form
of tar rich gas and/or other unwanted impurities in the product gas. The formation of tar during biomass gasification
is also stated as key issue by Devi et al. [14]. Detailed experimental studies on various fuels in the DFB steam
gasification process were reported at TU Wien [15–18] and by Schweitzer et al. [19]. Literature shows, that an
efficient gas-solid contact between devolatilized gaseous components and the bed material is crucial for a high
quality product gas [20,21]. The admixture of catalytic active particles into conventional bed materials like quartz
and olivine sand can also increase the hydrogen content and decrease tar contents in the product gas [22]. Beside
catalytic effects of the bed material, the steam to carbon ratio and the gasification temperature play a major role
for sufficient steam gasification conditions [23,24].
An advanced 100 kWth gasification pilot plant was developed with the aim to increase the severity of reaction for
the introduced fuels. This was realized through a new design of the gasification reactor. For this reason, the upper
gasification reactor was divided into a sequence of sections, which ensure improved gas-particle interaction using
flow obstacles in defined height intervals [25]. This advanced pilot plant went in operation in 2014 and is
documented in detail by Schmid [26]. The objective of this study is to demonstrate the product gas qualities and
performance indicating key figures for gasification, which derive from gasification test runs with the advanced
100 kWth gasification plant at TU Wien with a wide range of fuels. The experimental results are meaningful
because the plant has a significant size and it is possible to gasify fuels under steady state conditions for hours to
get average measurement values. Thus, the study offers the technical basis for economic considerations in terms
of industrial sized DFB steam gasification of several fuels.
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LIST OF ABBREVIATIONS
BA
CR
DFB
ECN
EOP
FG
GCMS
GR
IEA
ILS
LHV
LIG
LLS
MWF
MWF & LIG
n.a.
PG
RH
SCB
SLF
SW
temp.
ULS
bark
combustion reactor
dual fluidized bed
Energy Research Centre of the Netherlands
exhausted olive pomace
flue gas
gas chromatography coupled with mass spectrometry
gasification reactor
International Energy Agency
internal loop seal
lower heating value
lignite
lower loop seal
municipal solid waste fraction
municipal solid waste fraction and lignite
not analysed
product gas
rice husk
sugar cane bagasse
shredder light fraction
softwood
temperature
upper loop seal
LIST OF SUBSCRIPTS
daf
db
th
stp
dry and ash-free
dry basis
thermal
standard temperature & pressure (273.15 K, 101.325 Pa)
LIST OF SYMBOLS
H/C
LHVCR,fuel
LHVGR,fuel
LHVPG
PGY
ṁC,GR,fuel
ṁCR,fuel
ṁGR,fuel
ṁGR,fuel,daf
ṁH2O,GR,fuel
ṁH2O,PG
ṁsteam,GR
Q̇loss
V̇PG
XH2O
ηCG
ηCG,o
φSC
φSF
elemental ratio of hydrogen (H) to carbon (C) in the fuel [mole/mole]
lower heating value of fuel into the CR [MJ/kg]
lower heating value of fuel into the GR [MJ/kg]
lower heating value of PG on dry basis [MJ/m³stp,db]
product gas yield [m³stp,db/kgfuel,daf]
massflow of carbon in the fuel into the GR [kg/h]
massflow of fuel into the CR [kg/h]
massflow of fuel into the GR [kg/h]
dry and ash-free massflow of fuel into the GR [kg/h]
massflow of water in the fuel into the GR [kg/h]
massflow of water in the product gas [kg/h]
massflow of fluidization steam into the GR [kg/h]
heat losses in the gasification and combustion reactor [kW]
volume flow of product gas on dry basis [m³stp,db/h]
steam-related water conversion [kgH2O/kgH2O]
cold gas efficiency [%]
overall cold gas efficiency [%]
steam to carbon ratio [kgH2O/kgC]
steam to fuel ratio [kgH2O/kgfuel]
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Economic assessments are a key issue regarding technology development. However, an economic assessment
based on the findings in this study was not performed so far and should be examined in subsequent investigations.
The basic principle of the DFB steam gasification process is shown in Figure 1. The process is based on two
interconnected fluidized bed reactors, the gasification reactor (GR) and the combustion reactor (CR). In the
gasification reactor, solid fuels are converted with steam as gasification agent into a nitrogen-free product gas,
which mainly consists of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethylene
(C2H4), and other hydrocarbons in smaller amounts. The circulating bed material is heated up in the combustion
reactor, transported to the gasification reactor, and there provides the necessary heat for the overall endothermic
steam gasification. The connections between the two reactors do not allow the exchange of gases, but the
transportation of solids between the reactors. This can be realized through e.g. a loop seal as described by Stollhof
et al. [27]. The unconverted fuel from the gasification reactor, the so-called char, supplies the fuel for the
combustion reactor, which is fluidized with air. A flue gas (FG) stream leaves the combustion reactor and the
valuable product gas (PG) stream leaves the gasification reactor. The gasification reactor is only operated with
steam as gasification agent, and therefore no nitrogen enters it in gaseous form in contrast to a gasification process
with substoichiometric air supply. Thereby, a high-calorific nitrogen-free product gas with a lower heating value
(LHV) in the range of 10 to 16 MJ/m³stp,db is generated. An overview for product gas applications is given in [28].
product gas
flue gas
H2,CO,CO2,CH4,C2H4
N2,CO2,H2O,O2
H2O
Gasification
heat
circulating
bed material
800-850°C
Combustion
900-950°C
char
fuel steam
air
Figure 1: Basic principle of the DFB steam
gasification process
Olivine sand is typically used as circulating bed material in DFB steam gasification plants. Olivine is a common
magnesium iron silicate based mineral and after some time of operation in a DFB gasification plant as a bed
material, it forms calcium-rich layers on its surface, caused by interactions with biomass ash [29]. At high
temperatures these outer layers form calcium oxide and lead to an increased catalytic activity with respect to an
enhanced water gas shift reaction and tar reduction by steam reforming [30]. Therefore, limestone consisting of
calcium carbonate (CaCO3) as bed material for DFB steam gasification is a focus of investigations at TU Wien in
the last years. CaCO3 will be calcined to calcium oxide (CaO) in the reactor system at temperatures higher than
750 °C, according to thermodynamic equilibrium [31]. Calcium oxide shows relative low abrasion resistance in
comparison to commercial applied bed materials for fluidized bed applications like olivine or quartz sand. An
extensive overview on attrition phenomena in fluidized bed systems is given by Scala et al. [32]. Gravity separators
for smooth separation of bed material particles from gases are installed downstream the gasification and the
combustion reactor in the advanced design of the 100 kWth pilot plant at TU Wien to prevent excessive attrition
of soft bed materials.
To sum up, this paper should provide an overview of DFB steam gasification of alternative, low-grade fuels with
an extensive discussion of the results. The generated results should serve as a basis for further research to shift the
conventional biomass gasification of wood to a sustainable gasification process, where low-grade fuels are
converted. By reaching this goal, biomass gasification has the possibility to overcome the economic weaknesses,
which occur in particular from fuel prices as mentioned above and contribute to a sustainable energy engineering.
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2. Experimental Setup and Applied Methods
2.1 The advanced 100 kWth DFB steam gasification pilot plant and measurement
equipment
The advanced design of the DFB steam gasification pilot plant was implemented at TU Wien with an inner height
of 4.7 m and a diameter of 125 mm for the cylindrical combustion reactor and an inner height of 4.3 m with an
rectangular cross sectional area of 68 times 490 mm² for the gasification reactor in the bubbling bed. The overall
pilot plant facility has a height of about 7 m. More detailed information on the dimensions of the reactor system
are found in Figure 7. The pilot plant facility including fuel supply equipment, a control room and equipment for
gas cooling, cleaning and measurement, covers two floors of around 35 m². Figure 2 shows the advanced design
of the 100 kWth DFB steam gasification pilot plant at TU Wien, which basically follows the principle shown in
Figure 1. Thereby, a product gas (PG) is generated in the reducing atmosphere of the gasification reactor (GR) and
a flue gas (FG) results from the oxidizing atmosphere in the combustion reactor (CR). The product gas is analyzed
at the indicated sample point. The inputs of air and steam in the pilot plant are indicated with blue (steam) and
light grey (air) arrows. The pilot plant at TU Wien is equipped with a cooled feeding screw of the on-bed feeding
system onto the bubbling bed of the lower gasification reactor (LOWER GR), which is indicated with a brown
arrow in Figure 2 (fuel to GR). This feeding system is favorable for the use of various fuel types, especially for
the utilization of waste fuels like plastic residues with low melting temperatures. The feeding screw is not directly
in contact with the hot bed material particles of the bubbling bed. Thus, blockage of the screw is prevented. At
least, in comparison to in-bed feeding, the inertization of the fuel feeding system is easier with on-bed feeding,
because of a much lower counter-pressure towards the fuel input [33]. The upper gasification reactor (UPPER GR)
is designed as a countercurrent column with hot bed material flowing down and product gas streaming upwards.
An increased bed material holdup is realized with restrictions in the upper part causing locally smaller diameters
and, therefore, higher superficial velocities to form turbulent fluidization regimes. An extensive discussion on the
fluid dynamics was presented by Schmid et al. [25] An enhanced gas-solid contact and a higher residence time in
these turbulent fluidized zones promote tar cracking and reforming reactions by the use of a catalytic bed material.
Thus, the conversion efficiency is increased.
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Figure 2: Advanced design of the 100 kWth DFB
steam gasification pilot plant
The gasification and combustion reactor are equipped with gravity separators for the coarser bed material, which
allow smooth separation in contrast to cyclones, where higher gas and particle velocities occur. Downstream
cyclones separate fine particles for each reactor to obtain a nearly particle-free product gas. The lower loop seal
(LLS) and the upper loop seal (ULS) connect the two reactors and close the global circulation of bed material
particles. The internal circulation in the gasification reactor is realized through the internal loop seal (ILS).
Additional fuel in the combustion reactor compensates for the relatively high specific heat losses in this small pilot
plant and enables control of the gasification temperature for the gasification experiments. The design of the
gasification reactor allows the gasification of various fuels. The volatile content of a fuel is a good indicator for
the gasification behavior, especially for the tar content in the product gas. If volatile matter is available in large
amounts, devolatilization and gasification occur more quickly than with fuels containing a low volatile content.
Therefore, fuels with high content of volatile matter tend to show high tar values. The fuel flexible pilot plant has
been in operation since 2014 and first experimental results from test runs with various fuels or bed materials can
already be found in literature [31,33–36].
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Figure 3: Basic flow sheet of the advanced pilot facility at TU Wien
Figure 3 [26] shows a basic flow sheet of the advanced pilot plant facility. The facility is divided into three main
categories: solid fuel supply, production of two gas streams (the pilot plant from Figure 2), and gas cooling,
cleaning and utilization. Additionally, process media supply systems, measurement and control technology, the
control station, and an encompassing safety technology are displayed. Starting from the fuel hoppers through to
the chimney, the arrows inside the scheme show flow paths of the solid streams and gas/fluid flows. The sample
point for product gas measurement is marked within the scheme, located downstream the gasification reactor at
the radiation cooler outlet. At this sample point, all presented product gas analyses were executed. As there is no
product gas application installed at the pilot plant site for the presented test campaigns, the product gas and flue
gas stream are led together into a secondary combustion chamber and are burned. In the following, the off gas is
led through a water boiler and a fabric baghouse filter and leaves the chimney. Figure 4 shows a picture of the
upper floor of the pilot plant facility at TU Wien. The two fuel hoppers painted in blue and red are located in the
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front of the picture. Prior to a test run, the hoppers are filled with fuel and the fuel capacity of each hoppers assures
constant fuel feeding for several hours with its separate dosing screw. The duration is depending on fuel density
and fuel feeding rate. For the standard fuel softwood pellets, typically a feeding rate of 20 kg/h results in a fuel
power of 100 kWth. The smaller blue hopper has a capacity of around 150 kg of softwood pellets, which enables
an operation time of around 7.5 hours with 100 kWth fuel power. The availability of two hoppers provides the
possibility to switch between them and to assure constant fuel feeding for a longer duration. Another advantage is
the possibility to blend two fuels, which can be fed over the screw feeder constantly in the gasification reactor (see
also Figure 3). In addition, some parts of the steel construction of the pilot plant can be seen as grey steel columns.
The control room is located in the rear left and the insulated pilot plant in the center of the picture, behind the
hoppers. On the far right, a part of the secondary combustion chamber is visible in Figure 4.
The lower floor of the pilot plant facility is shown in Figure 5. In the very top left, the water container filled with
softened water for electrical steam generation can be seen. At the bottom left picture edge, the valves and pipes
from the steam generator direct steam to the control and measurement equipment of the control station. Next to
the softened water tank, the electric motor of the screw feeder for the pilot plant is visible. From there, the pipe
with fuel containing the screw feeder is going horizontally into the lower part of the gasification reactor. The
insulated pilot plant is situated in the background of the picture. The whole reactor system is constructed with heat
resistant steal (type: 1.4841). For a quick heat up rate, no fireclay is used. Three sampling containers for solids
from the cyclone downcomers and for cleaning one of the radiation coolers are visible at the bottom of the picture.
The whole plant is mounted suspended. Well thought out expansion equipment compensate the stretching because
of the high temperatures (up to 1050 °C). The water boiler is on the right side of the picture. In front of the water
boiler the black cooling trough comprising the washing bottles for the online gas measurement can be seen in
Figure 5. On the rightmost pictures edge a part of the fabric filter is visible.
Figure 4: Pictures of the upper part of the advanced 100 kWth DFB steam
gasification pilot plant facility at TU Wien
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Figure 5: Pictures of the lower part of the advanced 100 kWth DFB steam
gasification pilot plant facility at TU Wien
Table 1 presents the applied online measurement devices of the pilot plant. The pilot plant is controlled with a
programmable logic controller (PLC). The PLC continuously measures and records data of all relevant flow rates,
temperatures, pressures as well as the main gas composition of the product gas (H2, CO, CO2, CH4). Additionally,
C2H4 was analyzed every 12 to 15 minutes by a gas chromatograph (Perkin Elmer ARNEL – Clarus 500). An
adapted standardized arrangement of sampling equipment was used to analyze the tar content in the product gas.
Single tar components are measured by gas chromatography coupled with mass spectrometry (GCMS). For tar
measurements at the advanced pilot plant, toluene is used as solvent instead of isopropanol, as it is proposed by
the tar guideline, because the solubility for tar in toluene is higher and the water content in the PG can be measured
simultaneously in a simple way. However, toluene itself cannot be detected with this setup as GCMS tar component
and also the detection of benzene and xylene is not easy. The IEA (International Energy Agency) Bioenergy
Gasification Task suggests not to include benzene in the tar definition, as they define tars being “hydrocarbons
with molecular weight higher than benzene.” [37]. However, as benzene was measured at the most test runs as
GCMS component, it is presented in the results section, but not included in the sum of total GCMS tar content in
the product gas. A more detailed description of the tar measurement procedure was reported by Wolfesberger et
al. [38]. The tar dew point is defined as an important value for fouling as well as for long-term operation of biomass
gasification systems regarding the impact on downstream equipment. The tar dew points of the detected GCMS
tar compounds were calculated with the online tool from the Energy Research Centre of the Netherlands (ECN)
[39]. More detailed descriptions of the measurement setup of the pilot plant can be found in literature [36].
Table 1: Setup of online measurement devices for the gasification test runs
Parameter
Device
Number
Temperature
thermocouple (type K)
100 +
Pressure
Kalinsky DS2 (pressure sensor)
70 +
Main air and steam flows
Krohne (float-type or vortex)
8
Fuel input
Scalesa and dosing equipment
3
Main gas composition
Rosemount NGA 2000
12b
Fluidized bed behavior
inspection glasses
3
a: additionally through calibration of the feeding system (via dosing screws)
b: number of online measured FG and PG components
c: one inspection glass each to look inside the gravity separator and one to look directly on top of the
bubbling bed
Generally, tars can be classified according to different parameters. In Table 2 the classification of tars by physical
properties, which was proposed by Rabou et al. [40], is displayed. GCMS tar compounds, which are considered in
this study, are presented.
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Table 2: Classification of tars by physical properties
Tar
class
1
Class name
Property
Compounds considered in this study
GCundetectable
None
2
Heterocyclic
aromatics
Very heavy tars,
cannot be detected by
GC
Tars containing
heteroatoms; highly
water soluble
3
Light aromatics
Light hydrocarbons
with single ring; not
problematic in terms
of condensation and
water solubility
4
Light PAHa
2- and 3-ring
compounds; condense
at intermediate
temperatures at
relatively high
temperatures
5
Heavy PAHa
Larger than 3-ring
compounds; condense
at high temperatures at
low concentrations
a: PAH = poly-aromatic hydrocarbon
Benzofuran, 2-methylbenzofuran, dibenzofuran, quinoline, isoquinoline, indole,
carbazole, 2-methylpyridine, 3&4-methylpyridine, 1-benzothiophene,
dibenzothiophene, phenol, 2-methylphenol, 4-methylphenol, 2,6dimethylphenol, 2,5&2,4-dimethylphenol, 3,5-dimethylphenol, 2,3dimethylphenol, 3,4-dimethylphenol, 2-methoxy-4-methylphenol , eugenol,
isoeugenol
Phenylacetylene, styrene, mesitylene, o-xylene, m&p-xylene, ethylbenzene
1H-indene, 1-indanone, naphthalene, 1-methylnaphthalene, 2methylnaphthalene, 1-vinylnaphthalene, 2-vinylnaphthalene, biphenyl,
acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene, 4&5methylphenanthrene, 9-methylanthracene
Fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene,
benzo[k]fluoranthene, benzo[a]pyrene, benzo[e]pyrene, benzo[g,h,i]perylene,
dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene, perylene, coronene
2.2 Bed materials and fuels
Within this paper, nine gasification test runs are presented. Table 3 shows the applied bed materials: Limestone,
quartz and olivine in various mixtures were used for the test runs. The investigation of limestone as bed material
has been a major research focus at TU Wien for the last years due to the catalytic activity of calcium oxide (present
at temperatures above 800 °C) derived from limestone at high temperatures. However, limestone has poor
properties in terms of abrasion resistance compared to olivine and quartz and the particle density of calcium oxide
is nearly half of that of quartz or olivine. Through the lower particle density of calcium oxide, the heat transport
between the combustion and the gasification reactor is lower. In general, by using mixtures of bed materials the
supplementing properties of two different bed materials can fulfill the desired requirements for the gasification
process.
Table 3: Bed materials for the gasification test runs
unit
Limestone Quartz Olivine
Al2O3
mass-%
0.1
CaCO3
mass-%
95-97
<0.1
CaO
mass-%
<0.4
Fe2O3
mass-%
0.1-0.3
0.04
8.0-10.5
MgCO3
mass-%
1.5-4.0
48-50
SiO2
mass-%
0.4-0.6
99.8
39-42
Hardness
Mohs
3
7
6-7
Particle density kg/m³
2650
2650
2850
(1500) a
a: particle density after full calcination
Figure 6 shows pictures of the fuels which were tested at the 100 kWth pilot plant and are presented within this
paper. The fuels can be divided into three categories: (i) lignocellulosic or biogenic fuels: softwood (SW), sugar
cane bagasse (SCB), exhausted olive pomace (EOP), bark (BA) and rice husks (RH), (ii) waste-derived fuels:
shredder light fraction (SLF) and a municipal solid waste fraction (MWF) and (iii) a fossil fuel: lignite. These fuels
were all tested in single gasification test runs. Additionally, a mixture of municipal waste fraction and lignite
(MWF &LIG), consisting of 25% MWF and 75% lignite based on the lower heating value, was investigated. It is
obvious that some of the fuels were used as pellets because constant fuel feeding is easier with this shape at pilot
plant scale. Fuel preparation apart from drying is not necessary for industrially applied DFB steam gasification
plants. Due to a very low ash deformation temperature for exhausted olive pomace, limestone was premixed to the
fuel during pelletization.
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Figure 6: Pictures of fuels for the gasification test runs at the 100 kWth pilot plant
Table 4 presents the proximate and ultimate analyses of the used fuels for the gasification test runs. In the
following, the main differences are emphasized:
 The ash and water contents of the fuels varied significantly. Therefore, the elemental analyses and volatile
matter are presented on a dry and ash-free basis.
 The carbon content for biogenic fuels was around 50 mass-%, for waste-derived fuels between 75 and
80 mass-% and for the tested lignite 68 mass-% on a dry and ash-free basis. A consideration of the
elemental H/C ratio of hydrogen (H) to carbon (C) in the fuel shows that the tested biogenic fuels had
values around 1.4, waste-derived fuels around 1.8 and lignite the lowest value of around 0.7 mole
hydrogen per mole carbon.
 The volatile matter for the tested biogenic fuels was between 77 and 86 mass-%, for waste-derived fuels
around 92 mass-% and for lignite around 54 mass-% on a dry and ash-free basis.
 The energy content was mainly dependent on water, ash and oxygen content of the fuel. The tested
biogenic fuels had a heating value of around 14-17 MJ/kg, waste-derived fuels of around 30 MJ/kg, and
lignite of around 21 MJ/kg.
Table 4: Proximate and ultimate analyses of fuels for the gasification test runs
Unit
Water content
Ash content
Carbon (C)
Hydrogen (H)
Nitrogen (N)
Sulfur (S)
Chlorine (Cl)
Oxygen (O)a
Volatile matter
LHV, dry
Ash deformation
temp. (A)
Ash flow temp.
(D)
mass.%
mass.%
mass.%daf
mass.%daf
mass.%daf
mass.%daf
mass.%daf
mass.%daf
mass.%daf
MJ/kg
db
SW
SCB
EOP
BA
RH
SLF
MWF
LIG
7.2 ± 0.3
7.7 ± 0.3
11.8 ± 0.8
7.6 ± 0.3
7.5 ± 0.3
7.1 ± 0.3
1.6 ± 0.07
13.0 ± 0.56
0.2 ± 0.018
2.3 ± 0.33
11.0 ± 1.4
7.0 ± 0.89
15.2 ± 1.9
12.2 ± 1.5
7.8 ± 1.0
4.2 ± 0.54
50.8 ± 0.5
48.9 ± 0.5
52.4 ± 0.5
52.3 ± 0.5
51.2 ± 0.5
80.5 ± 0.8
76.1 ± 0.76 68.4 ± 0.68
5.9 ± 0.3
5.9± 0.3
6.2 ± 0.3
6.0 ± 0.3
6.1 ± 0.3
12.1 ± 0.4
11.3 ± 0.6
3.9 ± 0.2
0.2 ±
0.01
0.005 ±
0.001
0.005 ±
0.001
0.409 ±
0.02
0.051 ±
0.004
0.061 ±
0.004
0.554 ±
0.03
0.071 ±
0.005
0.106 ±
0.008
0.490 ±
0.03
0.112 ±
0.008
0.146 ±
0.01
0.342 ±
0.02
0.053 ±
0.004
0.053 ±
0.004
0.25 ± 0.03
2.213 ±
0.28
0.575 ±
0.03
0.082 ±
0.002
1.185 ±
0.15
0.877 ±
0.04
0.397 ±
0.05
0.052 ±
0.007
43.1
44.7
40.1
41.3
42.0
4.4
10.8
26.3
85.6 ± 0.4
85.7 ± 0.4
85.2 ± 0.4
77.7 ± 0.3
80.7 ± 0.4
91.8 ± 0.4
93.3 ± 0.4
54.1 ± 0.3
18.9 ± 0.19 17.8 ± 0.18 17.6 ± 0.18 18.2 ± 0.18 15.9 ± 0.16 31.1 ± 0.17 31.5 ± 0.17
24.3 ± 0.3
1.12 ± 0.06
°C
1330
1180.0
750-850
n.a.c
>1350
1210
1180
n.a.c
°C
1440
1330
>1440
n.a.c
-d
1320
1290
n.a.c
a: calculated by difference to average 100 mass-%daf
b: consists of 4.4 mass-% initial ash and 6.6 mass-% CaCO3 addition before pelletization
c: not analyzed
d: did not occur
ACCEPTED MANUSCRIPT
2.3 Process simulation & performance indicating key figures
By using the process simulation software IPSEpro, it is possible to calculate mass and energy balances of the
process data, which are recorded during gasification test runs. IPSEpro is an equation-oriented process simulation
software, which originates from the power plant sector and offers the user stationary process simulation.
Furthermore, IPSEpro enables validating measured experimental data. The validation of raw data was performed
using the software module PSValidate of the IPSEpro program package. The process model is solved with a data
adjustment algorithm that minimizes the weighted sum of the squares of the differences between redundant
measured values. For the validation of each presented test run a system model with more than 8500 variables was
solved. Thereby, several redundant values were defined with a permitted deviation for each value according to its
expected measurement inaccuracy. A similar validation approach is explained in more detail by Weber et al. [41].
A comprehensive model library for biomass gasification has been developed by Pröll and Hofbauer [42]. It enables
the user to calculate important values via mass and energy balances, which cannot be measured directly. A process
flowsheet and further information were presented by Müller et al. [31]. By the use of process simulation, following
performance indicating key figures were calculated, which were also used in previous work on the advanced DFB
steam gasification pilot plant by Benedikt et al. [36]and Müller et al [31].
The steam to fuel ratio φSF gives the amount of total steam from fluidization and fuel water to dry and ash-free fuel
fed into the gasification reactor and is given in Eq. 1.
φSF =
Eq. 1
msteam,GR + mH2O,GR,fuel
mGR,fuel,daf
The steam to carbon ratio φSC is given in Eq. 2. It is used to enable a comparison between test runs with different
fuels.
φSC =
Eq. 2
msteam,GR + mH2O,GR,fuel
mC,GR,fuel
The product gas yield PGY is presented in Eq. 3 and gives the ratio between dry product gas to dry and ash-free
fuel introduced into the gasification reactor.
PGY =
Eq. 3
VPG
mGR,fuel,daf
The steam-related water conversion XH2O shows the relation of water consumed and water introduced into the
gasification reactor and is depicted in Eq. 4.
XH2O =
Eq. 4
msteam,GR + mH2O,GR,fuel ‒ mH2O,PG
msteam,GR + mH2O,GR,fuel
Eq. 5 gives the cold gas efficiency ηCG, which describes the ratio of chemical energy in the product gas to the
chemical energy in the fuel that is fed into the gasification reactor, based on the lower heating value.
ηCG =
VPG ∙ LHVPG
mGR,fuel ∙ LHVGR,fuel
Eq. 5
∙ 100
Eq. 6 describes the overall cold gas efficiency ηCG,o, which is an extension of the cold gas efficiency. ηCG,o also
takes the fuel which is fed into the combustion reactor as well as the relatively high heat losses of the 100 kWth
pilot plant into account.
ηCG,o =
VPG ∙ LHVPG
mGR,fuel ∙ LHVGR,fuel + mCR,fuel ∙ LHVCR,fuel ‒ Qloss
∙ 100
3. Results and Discussion
Eq. 6
ACCEPTED MANUSCRIPT
Table 5 presents the main operation parameters for the steady state operation of the presented gasification test
runs. Each column presents a single gasification test run with an experimental duration of approximately a day and
an experimental validation period of at least 30 minutes of steady state operation. Softwood, being the standard
fuel, represents the mean values from two test runs with equal operating parameters. For each fuel, operating
parameters for a steady state operation were found, but during the test runs with exhausted olive pomace and rice
husks, problematic ash behavior occurred. The ash composition of the exhausted olive pomace had a high share of
potassium and therefore, showed a low ash deformation temperature. Hence, a certain share of limestone was
premixed to the fuel during pelletization prior to the gasification test run (see Table 4). However, after 4 hours of
operation, temperature fluctuations in the lower part of the reactors occurred. This behavior indicated a poor
intermixing of the fluidized beds. Concurrently, the observation of the bed via the inspection glasses showed
untypical viscous behavior. The plant was shut down immediately to prevent it from suffering any damage caused
by the formation of solid phases. Another ash limiting behavior occurred during the gasification of rice husks. It
was not possible to maintain a steady state operation longer than for approximately 45 minutes. The reason for that
was a blockage of the bed material circulation in the upper loop seal. In this case, the ash had a high amount of
silicon, maybe in combination with a small amount of potassium. This prevented the ash from falling apart and
formed stable ash skeletons. The limitations of the ash behavior for these fuels were reported in more detail by
Fuchs et al. [43]. Boström et al. [44] proposed an extensive model for ash transformation chemistry during
combustion of various biomasses and suggest a conceptual model to describe prevailing phenomena. Kuba et al.
[45] described the deposit build-up and ash behavior in the DFB steam gasification plant in Senden using logging
residues as fuel.
Table 5: Main operating parameters of the 100 kWth DFB steam gasification pilot plant
Value
Bed material
Steam/fuel
Steam/carbon ratio
Temp. in lower GR
Unit
mass-%
kgH2O/
kgfuel,daf
kgH2O/
kgC
°C
MWF
&
LIGa
67%
quartz
& 33%
limestone
SW
SCB
EOP
BA
RH
SLF
MWF
LIG
100%
limestone
100%
limestone
78%
olivine
& 22%
limestone
100%
limestone
90%
limestone &
10%
olivine
100%
limestone
100%
limestone
0.7
0.7
1.1
1.0
0.9
1.5
1.4
1.6
1.0
1.4
1.5
2.0
1.6
1.7
2.1
1.9
2.2
1.5
100%
limestone
789
753
761
761
761
807
754
838
847
Temp. in upper GR °C
991
975
856
965
904
976
965
985
984
Mean temp. in CR
°C
1021
1021
874
998
946
995
998
1008
992
Fuel input GR
kW
101
102
84
102
105
113
110
123
100
Fuel input CR
kW
52
51
55
56
36
55
55
65
35
a: 75 % MWF and 25 % lignite based on the lower heating value
At first, the initial bed material composition for all test runs is given in mass percent. For the presented experiments,
it was tried to keep the steam to fuel and steam to carbon ratios in a comparable range. A certain steam input is
needed for uniform fluidization of the bubbling bed and to assure bed material circulation in the DFB reactor
system. Due to the relatively high energy density of the waste-derived fuels (see Table 4), but a similar need for
steam input for fluidization and circulation, the steam to fuel ratio was higher for the waste-derived fuels. The
lower gasification reactor is equipped with nine temperature sensors over the height. A bubbling fluidized bed
typically shows a very even temperature distribution. However, some temperatures show cold spots due to the
steam injection nearby or because they are located close to cold reactor walls (see Figure 7). Therefore, in the
lower gasification reactor, a mean temperature value at the fuel input is given. The reentry temperature of hot bed
material coming from the upper loop seal in the upper countercurrent gasification reactor is given as temperature
in the upper GR. The listed mean temperature in the combustion reactor is a mean value of the temperature sensors
of the upper combustion reactor. Fuel input in the gasification reactor was between 84 and 123 kW. Additional
fuel input was fed into the combustion reactor to compensate for the relative high heat losses of the pilot plant
caused by a large surface to volume ratio, flanges, instrument nozzles etc. Thus, temperatures higher than 750 °C
were reached in the lower gasification reactor.
Figure 7 shows the temperature profile of the gasification test runs with bark, shredder light fraction and lignite as
representative results for the respective group (biogenic, waste-derived and fossil). The measured temperatures at
the top of the combustion reactor, being approximately the temperature of hot bed material entering the upper loop
seal coming from the combustion reactor, were in a very close range of around 1000 °C. The temperatures stayed
ACCEPTED MANUSCRIPT
relatively similar in the upper gasification reactor and then dropped to around 760 °C for bark, 810 °C for the
shredder light fraction and 850 °C for lignite in the narrow part of the lower gasification reactor. For all three test
runs, limestone was used as bed material. It seems that the temperature difference in the lower gasification reactor
was caused by different volatile contents (see Table 4) and related gasification behavior of the fuels. Residual char
from lignite was mainly transported to the gasification reactor, which can also be seen at the low additional fuel
input into the CR from Table 5. Thereby, the influence of endothermic devolatilization and gasification reactions
did not lower the temperature in the lower gasification reactor in the same way as for the other fuels. The volatile
content of the SLF was very high (see Table 4), resulting in fast devolatilization of the fuel and a moderate drop
of the temperature in the lower gasification reactor. It is more likely that the devolatilization for SLF took place
primarily on top of the bubbling bed and a bed intermixing was achieved. The gasification test run with bark
showed the highest temperature drop in the lower gasification reactor caused by endothermic devolatilization and
steam gasification reactions in the bubbling bed.
5
5
Lignite
SLF
Bark
4.5
4.5
4
4
3.5
3
3
2.5
2.5
2
2
1.5
1.5
1
1
0.5
0
height [m]
height [m]
3.5
0.5
700
800
900
temperature GR [°C]
1000
800
900
1000
0
temperature CR [°C]
Figure 7: Temperature profile of the gasification reactor (left) and the combustion reactor (right) for the
gasification test runs with bark, shredder light fraction and lignite. Height indications for temperature
measurement points and fuel, steam and air inputs are depicted in the drawing (center)
Figure 8 shows the dry main product gas composition for the gasification test runs. The gasification of the biogenic
fuels, SW, SCB, EOP, BA and RH, led to similar product gas compositions. The use of 100 wt.-% limestone as
bed material enhanced the H2 formation as well as the tar reduction. Comparable results of the advanced DFB
steam gasification pilot plant using softwood and sugar cane bagasse as feedstock, where mixtures of limestone
and olivine or pure olivine were investigated are shown in literature [33,46]. The waste-derived fuels, SLF and
MWF, generated similar product gas compositions and differed considerably from the biogenic product gas
compositions with around two to three times higher CH4 contents and slightly lower CO and CO2 contents in the
dry product gas. The product gas composition for the mixture of MWF and LIG shows a different product gas
composition in comparison to merely MWF as fuel due to the increase of fixed carbon in the fuel.
Lignite showed the highest H2 and CO and the lowest CH4 content in the product gas. In case of softwood, bark
and lignite very low C2H4 contents in the product gas were observed, which is a good indicator for tar
decomposition reactions and therefore, led to a low tar content in the product gas (see Figure 9). The correlations
of fuel composition regarding contents of volatiles to attained product gas yields and lower heating values of the
product gases are presented in Figure 11 for these test runs. Figure 12 shows the correlation between CH4 content
in the product gas and molar H/C ratio in the fuel.
ACCEPTED MANUSCRIPT
Figure 8: Product gas composition of the test runs
Figure 9 presents different tar related properties of the gasification test runs. The benzene content, total GCMS
content (excluding benzene) and gravimetric tar contents in the dry product gas as well as the calculated tar dew
points are displayed. The benzene content was not measured for the test run with exhausted olive pomace. Biogenic
fuels and lignite showed low tar contents. Softwood, bark and lignite generated the lowest tar contents of around
1 g/m³stp,db total GCMS, or gravimetric tar content, in the product gas and around 5-8 g/m³stp,db benzene content.
For these fuels, also the lowest tar dew points were calculated. However, a direct correlation between the total
GCMS content and tar dew point was not found, because small quantities of heavier components have a greater
influence on the tar dew point than larger amounts of light components. The waste-derived fuels MWF and SLF
produced significantly higher tar contents in the product gas compared with the other fuels. In case of the mixture
of MWF with lignite, the sum of GCMS tar was reduced by around a quarter and the benzene concentration in the
product gas was reduced by around a third in comparison to pure MWF. However, the gravimetric tar concentration
showed a slight increase for the fuel blending of MWF and lignite. Figure 10 displays the share of tar classes
according to physical properties (see Table 2). The very small total GCMS tar content of the product gas for the
gasification test runs with softwood and bark was nearly completely comprised of class 4 tars (light PAHs), which
mainly contained naphthalene. It is notable that exhausted olive pomace and rice husk were the only fuels with
relevant amounts of class 2 tars (heterocyclic aromatics), which could originate from fluidization and circulation
problems due to the unfavorable ash properties for these two fuels. The class 2 tars from EOP were mainly
composed of nitrogen containing heterocyclic aromatics, which derived from a high nitrogen content in the fuel
(see Table 4) and for RH of phenols. Not only the amount of total GCMS was the highest for SLF and MWF but
also the composition was most problematic with the highest amounts of class 5 tars (heavy PAHs), which increased
the tar dew point significantly. In this study, a broad range of different fuels and bed materials were tested in a
DFB steam gasification pilot plant and several product gas qualities with a broad spectrum of tar concentrations
were generated. A detailed explanation for possible tar formation and reduction phenomena is given in more detail
by Milne et al. [47].
ACCEPTED MANUSCRIPT
Figure 9: Benzene and tar concentrations and calculated tar dew points of the test runs.
Figure 10: Share of tar classes of total GCMS tar content according to Table 2. Shares smaller than
1 mass-% are not displayed
Figure 11 shows the product gas yield and the lower heating value of the product gas in dependence on the content
of volatile matter in the fuel for the test runs. It becomes obvious that certain trends for these key figures appear.
Biogenic fuels show product gas yields from 1.2-1.7 m³stp,db/kgdaf, while waste-derived fuels show values above
2 m³stp,db/kgdaf mainly due to their high volatile content. Furthermore, due to the high volatile content and the
ACCEPTED MANUSCRIPT
resulting high methane content, the lower heating value of the product gas was on the top range with around
16 MJ/m³stp,db for the waste-derived fuels. Vice versa, the lower heating value of the product gas from lignite
gasification was the lowest.
Figure 11: Correlation of product gas yield (PGY)
and lower heating value (LHV) of the product gas
dependent on the content of volatile matter in the fuel
Figure 12 displays a correlation between the CH4 content in the dry product gas and the molar H/C ratio of
hydrogen (H) to carbon (C) in the fuel. The higher the hydrogen content in the fuel, the higher CH4 contents were
reached. This indicates that CH4 is a decomposition or devolatilization product from pyrolysis of the fuel and once
it is generated, it is very unlikely to react in further gas-gas reactions.
Figure 12: Correlation between CH4 content of the
product gas and the molar H/C ratio in the fuel
Table 6 shows the performance indicating key figures for DFB steam gasification. The steam related water
conversion XH2O was around 0.3 for the biogenic fuels except for RH. Maybe the value for RH was lower due to
the bed material circulation problems caused by the ash skeletons. MWF and SLF showed even higher water
conversion rates of around 0.4 and at the test run with lignite around 60% of the inserted water was converted into
product gas. The high water conversion for lignite can be explained by the high share of fixed carbon in the fuel
(see Table 4). Thereby, high CO concentrations were found in the product gas (Figure 8), which led to favorable
conditions for the water-gas shift reaction. The catalytic active CaO and high shares of CO in the product gas
promoted a very high water consumption.
The cold gas efficiencies varied a lot but if the additional fuel and heat losses are taken into account for all tested
fuels, overall cold gas efficiencies of around 70% were calculated. The H2/CO ratio is a vitally important value for
possible downstream syntheses of the product gas. Within the presented test runs, the H2/CO ratio was between
2.2 and 2.8 for biogenic fuels and lignite but significantly higher with values up to 3.9 for the waste-derived fuels
SLF and MWF. This could also originate from the higher H/C ratio in the waste-derived fuels (see Table 4).
ACCEPTED MANUSCRIPT
Table 6: Overview of performance indicating key figures for the gasification test runs
Value
Unit
SW
SCB
PGYa
m³stp,db/kgdaf
1.4
1.2
Product gas volume flowa m³stp,db/h
28.2
24.9
Product gas powera
kW
88.7
81.2
LHVPGa
MJ/m³stp,db
11.3
11.7
XH2O
kgH2O/kgH2O
0.36
0.29
ηCG
%
88
79
ηCG,O
%
73
68
H2/CO ratio
mole/mole
2.2
2.4
a: dry basis, char and tar free
b: 75 % MWF and 25 % lignite based on the lower heating value
EOP
1.7
25.9
83.2
11.6
0.34
99
74
2.7
BA
1.4
27.0
81.7
10.9
0.34
80
67
2.8
RH
1.2
23.8
82.5
12.5
0.18
79
71
2.4
SLF
2.3
23.4
100.5
15.3
0.43
89
68
3.9
MWF
&
MWF LIGb
2.0
2.1
21.9
29.0
97.6 115.2
16.1
14.3
0.41
0.43
89
94
72
69
3.6
2.8
LIG
1.6
23.9
70.5
10.6
0.58
71
67
2.2
4. Conclusions
Within this paper, the results of nine gasification test runs with eight different fuels carried out at an advanced
100 kWth dual fluidized bed steam gasification pilot plant are presented. Softwood, sugar cane bagasse, exhausted
olive pomace, bark and rice husks were tested as examples for biogenic fuels. Shredder light fraction and a
municipal solid waste fraction were gasified as examples for waste-derived fuels. Lignite and a mixture of 25%
lignite and 75% municipal solid waste fraction were also tested. In the following, the key findings are summarized:





The tested biogenic fuels showed very similar gasification behavior at the pilot plant. Thereby, product
gas with a lower heating value of around 11-12.5 MJ/Nm³db was generated. The lowest total GCMS and
gravimetric tar concentrations were around 1 g/m³stp,db. The measured steam-related water conversions
were around 0.3 for the gasification test runs of the biogenic fuels, except for rice husks. This means that
around a third of the utilized steam and fuel water were converted into valuable product gas components.
The gasification of two waste fractions and a mixture of lignite with municipal solid waste was
successfully demonstrated in pilot plant scale. The product gas composition showed high CH4 and slightly
lower CO and CO2 contents for the waste-derived fuels compared to biogenic fuels. Thereby, relatively
high lower heating values of the product gas of 14-16 MJ/Nm³db were reached. However, the gravimetric
tar contents of the product gas were high in a range of around 15-20 g/Nm³db for the waste gasification.
The total GCMS tar content including benzene was reduced by around 33% for the fuel blend of municipal
solid waste fraction with lignite compared with solid waste alone. For these fuels, the steam-related water
conversion was even larger than 0.4.
Exhausted olive pomace and rice husks were the only fuels with relevant amounts of class 2 tars
(heterocyclic aromatics), which could originate from fluidization and circulation problems due to the
unfavorable ash properties for these two fuels.
The product gas yields and lower heating values of the generated product gases were compared. A direct
correlation was revealed:
o The higher the content of volatile matter in the fuel, the higher the product gas yields and the
lower heating values.
o The higher the elemental H/C ratio of the fuel, the higher the CH4 contents in the product gas.
This indicates that CH4 is a decomposition or devolatilization product from pyrolysis of the fuel
and once it is generated, it is very unlikely to react in further gas-gas reactions.
Overall cold gas efficiencies of around 70% were calculated for all presented test runs.
The presented results offer the basis for a sustainable and promising waste management solution for the tested
biogenic residues, a municipal solid waste fraction and a shredder light fraction. The presentation and discussion
of an extensive product gas cleaning, which is necessary for the utilization of the product gas from these fuels, is
not part of this study. Economic assessments are a key issue regarding technology development. However, an
economic assessment based on the findings in this study was not performed so far and should be examined in
subsequent investigations.
ACCEPTED MANUSCRIPT
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nine different fuels are converted into valuable product gases,
certain tar species in some product gases indicate poorer gasification conditions,
correlations between fuel compositions and product gas quality are shown,
overall cold gas efficiencies of around 70% were calculated for all test runs.
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