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 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 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: firstname.lastname@example.org E-Mail address: email@example.com (J. C. Schmid), firstname.lastname@example.org (J. Fuchs), email@example.com (A.M. Mauerhofer), firstname.lastname@example.org (S. Müller), email@example.com (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) , (ii) Oberwart, Austria (8.5 MWth) , (iii) Senden, Germany (15 MWth)  and (iv) Gothenburg, Sweden (32 MWth) . 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  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 ACCEPTED MANUSCRIPT 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 . 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. . Detailed experimental studies on various fuels in the DFB steam gasification process were reported at TU Wien [15–18] and by Schweitzer et al. . 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 . 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 . This advanced pilot plant went in operation in 2014 and is documented in detail by Schmid . 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. ACCEPTED MANUSCRIPT 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] ACCEPTED MANUSCRIPT 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. . 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 . 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 . 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 . 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 . 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. . 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. ACCEPTED MANUSCRIPT 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 . 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.  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. ACCEPTED MANUSCRIPT 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]. ACCEPTED MANUSCRIPT Figure 3: Basic flow sheet of the advanced pilot facility at TU Wien Figure 3  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 ACCEPTED MANUSCRIPT 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 ACCEPTED MANUSCRIPT 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.” . 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. . 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) . More detailed descriptions of the measurement setup of the pilot plant can be found in literature . 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. , is displayed. GCMS tar compounds, which are considered in this study, are presented. ACCEPTED MANUSCRIPT 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. ACCEPTED MANUSCRIPT 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. . A comprehensive model library for biomass gasification has been developed by Pröll and Hofbauer . 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. . 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. and Müller et al . 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. . Boström et al.  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.  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. . 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 References                            Codina Gironès V, Moret S, Peduzzi E, Nasato M, Maréchal F. Optimal use of biomass in large-scale energy systems: Insights for energy policy. Energy 2017;137:789–97. doi:10.1016/j.energy.2017.05.027. International Energy Agency (IEA), http://www.iea.org/etp/etp2014/ [accessed August 3rd 2018] 2018. Patterson MG. What is energy efficiency? 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ACCEPTED MANUSCRIPT 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.