1 This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). INFUB-14 Algarve, Portugal, 2 - 5 April 2024 Fluidized bed gasification of plastic waste and residual biomass – an overview of HTW® and Chemical looping technology development Fabiola Panitz*, Jens Kaltenmorgen*, Marc Siodlaczek*, Jochen Ströhle*, Bernd Epple* fabiola.panitz@est.tu-darmstadt.de *University of Darmstadt, Department of Mechanical Engineering, Institute for Energy Systems and Technology Abstract Mechanical recycling of plastic waste to second generation products is the currently mostly applied technology to reuse the valuable elements contained, particularly carbon, but often leads to a loss in product quality. Furthermore, its economic potential is limited due to heterogeneous waste streams, and the remainder - non-recyclable composites - is disposed in landfills or thermally utilized with the challenge of carbon capture. In contrast, chemical recycling preserves the carbon and other elements such as hydrogen by transforming them into valuable base chemicals or synthetic fuels. This enables a circular economy, which is a key factor in achieving a net-zero emission society. Thus, the Institute for Energy Systems and Technology at the Technical University of Darmstadt is investigating the promising path of fluidized bed gasification processes with downstream gas cleaning and synthesis at pilot scale. Its 1 MWth modular pilot plant allows for different process configurations as bubbling bed, circulating bed and dual-fluidized bed gasification under autothermal conditions. The whole process chain from the waste to a valuable product such as methanol or Fischer-Tropsch has been demonstrated and optimized in test campaigns of several weeks. Based on these pilot tests, an upscaling to industrial scale is conducted in combination with modelling and simulation. Expertise has been built up from 2015 in the High-Temperature Winkler (HTW®) gasification of lignite, where meanwhile residual feedstock as biomass and plastic waste (solid recovered fuel = SRF) have been proven to be suitable. Chemical looping gasification (CLG) was demonstrated successfully and investigated for the first time at pilot scale in 2022 at the institute using biogenic residues as feedstock. 1. Introduction It is evident that fossil fuels must be replaced as carbon sources to meet the 1.5 °C target, set in the Paris Agreement of 2015 [55], and only an interaction of multiple approaches can lead to net zero emissions. While electrification is assumed to be the solution for road transportation - aviation, shipping and heavy-duty land transport will most likely rely on hydrogen and other alternative fuels [30, 31]. The latter take advantage of the liquid fuel being easy to handle and having a high energy density by mass [29]. The chemical production cannot be decarbonized as most products are made of carbon [11]. Residual biomasses and plastic waste can replace fossils as carbon sources to produce base chemicals, that are further processed into green and synthetic fuels as well as chemical products. The conversion of plastics to these base chemicals - referred to as chemical recycling - additionally contributes to a valorization of non-recyclable waste streams and a closed carbon cycle [23]. Gasification of the solid waste streams opens one path to transfer the carbon – and other valuable elements as hydrogen – to the gas phase, that is purified and used for synthesis such as Fischer- Tropsch or methanol synthesis. Other processes such as low to medium temperature pyrolysis or liquefaction require homogenous feedstock since the molecules are only partially re-arranged and impurities stay in the product. Gasification provides virgin base chemicals able to replace fossil fuels for many applications. [4] The process of gasification includes a sequence of interdependent chemical reactions to convert the carbon bound in the feedstock to the syngas at temperatures between 650 °C and 1800 °C. The feedstock devolatilizes releasing gases such as CO2, CO, CH4, H2, H2S, NH3, primary hydrocarbons such as tars and phenols and steam, with the composition being highly dependent on the temperature of the devolatilization and the heating rate. Subsequently, the char and pyrolysis products react primarily with free oxygen (O2), or oxygen bound in gasification agents such as steam or CO2 forming the valuable http://creativecommons.org/licenses/by/4.0/ 2 syngas components CO and H2, as well as CH4 and byproducts. The gasification reactions are mostly endothermic. Thus in autothermal gasification processes, energy/heat must be supplied by exothermal combustion of parts of the char and volatiles with oxygen, that is either supplied pure or in air, with the disadvantage of N2-dilution. [10, 29] The syngas quality is influenced by the feedstock, gasification agents, and operating conditions [29]. Hence, the gasification of waste challenges due to the heterogeneity of the feedstock, the little fixed carbon and correspondingly high volatile amounts and the high purity of the syngas required for subsequent synthesis of chemical intermediates [6]. Recent developments in research [13, 51, 52] and industry [50] show, that fluidized bed gasification is a promising technology for gasification for a broad variety of biomasses, plastics, and other residuals. It was developed by Fritz Winkler in 1926 for coal and commercialized later. Fluidized bed technologies are characterized by rather easy scale-up capabilities and high flexibility for feedstock and load change, as gasification agent to feedstock ratios, and discharge can easily be adjusted [10, 33]. Feedstock preparation is not necessary or moderate in comparison to entrained flow gasification, where the feedstock must be ground to small particle sizes, which is challenging for some kinds of plastics and biomasses without additional processing. Fluidized beds allow for enhanced heat exchange and reactions between the gas phase and the particles and a high heating rate of the feedstock. Thus, uniform and stable temperatures in the bed can be achieved. To prevent slagging, fluidized bed gasifiers must be operated at rather low temperatures below the ash softening temperature [10]. This leads to an enhanced formation and reduced decomposition of long-chain hydrocarbons and tars, even more for biomass and plastics due to their complex molecule structure and long-chain hydrocarbons. Different fluidizing bed technologies counteract with catalytic active bed materials, elevated temperatures in a free board, or additional downstream gas cleaning steps such as partial oxidation (POX) or catalytic tar removal. [10, 29] The High-Temperature Winkler gasification technology (HTW®) applies a bubbling bed for the devolatilization and gasification of the char. It counteracts the enhanced release of long-chain hydrocarbons through a post-gasification zone (= PGZ). The bubbling bed and the PGZ are combined in a single reactor geometry, producing a syngas of a quality suited for synthesis of liquid chemicals. The bed temperature ranges from 600 °C to 950 °C. In the PGZ additional oxygen is injected, consequently increasing the temperature up to 1000 °C, which is aiming to crack hydrocarbons as well as to gasify entrained char, resulting in additional syngas, that enhances the yield of hydrogen and carbon monoxide. The gasification agents oxygen and steam are distributed via dual flow nozzles on different reactor levels in the bed and PGZ. [32] Chemical looping gasification (CLG) is a novel technology and a further development of Dual fluidized bed gasification (DFBG) and Chemical looping combustion (CLC). Through the separation of the heat- supplying combustion reactions from the gasification step in a separate reactor (= air reactor, AR), a nitrogen-free syngas is produced without the need of an air separation unit. An oxygen carrier, circulating between the two coupled reactors, acts as bed material, providing both heat for the endothermic steam gasification reaction as well as oxygen for the (partial) oxidation of light hydrocarbons and tars in the gasification reactor (= fuel reactor, FR). The reduced oxygen carrier is circulated back to the air reactor and exothermically oxidized with air. Thus, in comparison to DFBG, carbon does not need to be combusted inside the air reactor for heat generation. Hence, CO2 emissions are avoided, and the process is carbon neutral or even possesses a negative carbon footprint (in case a sustainably sourced biogenic feedstock is used and captured CO2 is stored). [15] 2. Previous work Pilot testing at the TU Darmstadt aims to demonstrate and investigate the above-mentioned gasification processes for different kinds of feedstocks as residual biomasses and plastic waste at a scale that allows reliable scale-up. While for HTW® gasification various lab, pilot and commercial plants were already operated with coal and also short-term tested with residuals, the first autothermal pilot testing for CLG was conducted at the TU Darmstadt in 2022 with biomass. An overview of the work, that the pilot testing at TU Darmstadt is based on, is given in the following. 2.1. High-Temperature Winkler® gasification The High-Temperature Winkler® gasification is a further development of the autothermal, bubbling bed Winkler gasification by Rheinische Braunkohle Werke (later RWE Power AG) as a licensor in cooperation with UHDE GmbH (later thyssenkrupp Industrial Solutions AG). Pilot plant testing with coal 3 at the RTWH Aachen University from 1974 on (0.5 MWth / 1.7 t/day, atmospheric pressure), and with lignite at the Rheinbraun Wachtberg/Frechen facility (1978 to 1985, 2 MWth / 34 t/day, pressurized to 10 bar) led to several improvements. An elevated pressure up to 30 bar allows for a higher size-related gas throughput but leads to a higher CH4 concentration and significant amounts of unreacted water vapor and CO₂ due to the “Le Chatelier” principle. Thus, it needs to be counteracted with a higher temperature in the fluidized bed. A PGZ for cracking of hydrocarbons/tars and a recycling of fine and therefore entrained particles via a cyclone and return leg enhances the carbon conversion. [20, 34] Based on this successful pilot testing of HTW® gasification, several demonstration and commercial plants were under operation: − From 1986 to 1997 the demonstration plant Berrenrath (Germany, Rheinbraun) was operated at a pressure of 10 bar and a load of 600 t/day of lignite producing 300 t/day of methanol. [1, 48] − The HTW® plant in Oulu (Finland, Uhde) produced 240 t/day of Ammonia from gasification of 650 t/day of peat and saw dust in the years 1988 to 1994. − For investigation of pressurized HTW® gasification up to 25 bar, the plant in Wessling/Cologne (Germany, Rheinbraun) was run from 1989 to 1992 using brown and bituminous coal for energy production. At the end of the 1990s environmental concerns regarding landfilling of residuals led to several test campaigns of co-gasification of municipal solid waste (MSW), auto shredder residue and sewage sludge with lignite in Berrenrath. Co-gasification of up to 50% of MSW with lignite could be demonstrated in three test campaigns of 3 days each at commercial scale in Berrenrath in 1997. [1] − These findings were applied in the Sumitomo Heavy Industries Ltd’s Niihama facility “Kemira Oy” in Sikuku (Japan) from 1999 until 2002, where MSW was gasified with air for generation of electricity. The facility was using the Krupp Uhde PreCon®process, that combines the HTW® gasification technology and a combustion furnace. [1] From 1994 the RWE Power AG engineered a pressurized HTW® gasifier with a scale-up factor of 5 compared to the Berrenrath plant for the KoBra IGCC in Hürth (Germany) but decided in 1998 to not continue this project in favour for the realisation of the lignite-fired power plant BoA in Niederaußem (Germany) [2, 19, 58]. As part of the IGCC power plant in Vreŝova (Tschechien) two HTW® gasifiers had been planned but never installed [42, 59]. The HTW® technology was taken over by ThyssenKrupp Uhde as licensor in 2010 anticipating a high potential for gasification for MSW in the future. [53] ThyssenKrupp Uhde was planning an HTW® plant for bio-methanol production for the company VärmlandsMetanol AB with a thermal input of 111 MWth wood. Even though the engineering, licensing and environmental and risk assessment had been conducted, the project is on hold since 2013 due to uncertainty concerning the future taxation of biofuels. [5] The gasification of coal to more valuable products gained again attention with upcoming concerns on the remaining coal reserves in Germany and the carbon footprint of coal combustion for electricity generation. Nevertheless, only energy-efficient technologies could compete with the price and efficiency of chemicals production from fossil crude oil. [3] Consequently, further investigation on the HTW® technology was needed to optimize the process in terms of efficiency and regarding the feasibility of using the syngas for synthesis. Till then, alternative feedstock such as biomass, MSW or solid recovered fuel (SRF) had only been tested in co-gasification or for electricity generation. Therefore, an existing pilot plant for carbon capture technologies at the Technical University of Darmstadt (TU Darmstadt) was modified for HTW® gasification in 2 years of construction. After commissioning in 2015, parameter studies for the gasification of lignite and hard coal were conducted, e.g. in the EU-funded L2L project. In 2021, a gas cleaning test rig and a synthesis test rig for Methanol and Fischer-Tropsch synthesis were commissioned in the EU-funded FABIENE project allowing to demonstrate the feasibility of the syngas produced for synthesis. With the climate crises rising more awareness in politics, and the coal phase-out decision in 2020, pilot testing started focussing on replacing the lignite by SRF and biomass first in co-gasification and later as 100 % feedstock. Just recently, mono-gasification SRF, wood and sewage sludge could be successfully demonstrated within the VERENA project, funded by the Federal Ministry for Economic Affairs and Climate Action of Germany. In 2019 GIDARA Energy B.V. acquired the technology HTW® from ThyssenKrupp [54] and is currently planning the two industrial HTW® gasification plants Advanced Methanol Amsterdam (AMA) and Advanced Methanol Rotterdam (AMR) in the Netherlands for the production of methanol. The plants 4 will have a capacity of around 180,000 tonnes of non-recyclable waste input to produce approximately 87,500 tonnes of methanol for fuel blending per year. [24, 25] 2.2. Chemical looping gasification The development of the Chemical looping gasification process is not as linear as of the HTW® technology and research is conducted in different countries and projects in parallel. CLG has emerged from Chemical looping combustion (CLC) and Dual fluidized bed gasification (DFBG) and was first mentioned in the literature in 2014, according to Mendiara et al. 2018 [41]. Several recent reviews [9, 14, 26, 37, 41, 45] give an overview of the status of investigation regarding process configuration, operating parameters, oxygen carrier material, different feedstocks, and economical potential for Chemical looping gasification. However, pilot testing at TU Darmstadt in 2022 is not included in these reviews, yet. While existing experimental investigation primarily focuses on the conversion of different kinds of biomasses and fossil fuels, plastic waste has not been tested in continuous CLG. Apart from multiple studies focussing on oxygen carrier selection and reaction kinetic, lab scale testing has been and is conducted in the following externally heated units. Here, focus is placed on the oxygen carrier performance, the influence the of oxygen/biomass ratio, as well as the effect of steam/biomass ratio and the gasification temperature on the process efficiency: − At the Spanish National Research Council CSIC (Spain), two CLG units with a scale of 1.5 kWth and 50 kWth are available for continuous testing. While the smaller unit applies two bubbling bed reactors and a bed inventory of approximately 2.5 kg, the 50 kWth unit is composed of a circulating fluidized bed fuel reactor, an air reactor in bubbling mode at the bottom and a fast fluidized bed on top and a carbon stripper to recycle entrained char from the fuel reactor. [7, 8, 49] − The Chalmers University of Technology (Sweden) currently operates three different CLG units for solid feedstock with a thermal capacity of 10 kWth, 100 kWth and 2 - 4 MWth. Feedstock enters the 10 kWth unit via a chute into a bubbling bed with recirculation. The bed inventory comprises approximately 15 kg. The 100 kWth reactor system consists of two coupled circulating fluidized bed reactors and a carbon stripper to enhance the carbon conversion. [43, 51] The 2 - 4 MWth bubbling bed gasifier is coupled with a circulating fluidized bed air reactor and works without external heating of the reactor. However, the unit cannot be considered fully autothermal, as the air reactor is highly over-dimensioned and most of the heat generated within it is not coming from the oxidation of the reduced oxygen carrier recycled from the fuel reactor. Instead, heat is provided from the combustion of wood chips directly fed to the air reactor, serving the purpose of heating the university campus. Besides the conversion of wood pellets with different kinds of oxygen carriers, automotive shredder residue has been investigated as feedstock, where its own ash functioned as bed material. [46, 47] During a total of 650 hours of continuous CLG operation different feedstocks such as industrial wood pellets, wheat straw pellets, pine forest residue and black pellets as feedstock in combination with oxygen carriers such as ground steel converter slag, ilmenite, iron-titanium ore, iron ore and LD slag were investigated in cooperation in the 1.5 kWth unit at CSIC and the 10 kWth unit at the Chalmers University. Evolving from this testing, ilmenite was chosen for experiments in 50 kWth and 100 kWth scale. [43, 51] − The Chinese Academy of Sciences (China) uses a 10 kWth chemical looping fluidized bed reactor consisting of a bubbling bed fuel reactor with internal circulation and an air reactor in bubbling mode at the bottom and fast fluidized bed on top. Sawdust of pine and torrefied eucalyptus were gasified with 5.8 kg of oxygen carrier e.g. iron ore, Fe2O3/Al2O3, and NiFe2O4. [56, 57, 60] − At the Southeast University Nanjing (China) a 25 kWth unit with a high-velocity fluidized bed as air reactor and a bubbling fluidized bed as fuel reactor was set up for CLG of rice husk with 4 kg of hematite oxygen carrier in a mixture with silica as bed material. [21, 22] Furthermore, with smaller changes in the reactor configuration, other chemical looping processes, such as CL pyrolysis gasification [60] were investigated. Moreover, the system can be utilized as a system of three reactors with an additional steam reactor for adjustment of H2/CO ratio [37]. The pilot plant at the TU Darmstadt was modified for autothermal Chemical looping gasification in 2021/2022, which is discussed in detail by Marx et al. 2021 [38]. A pilot plant design was developed based on reactor hydrodynamics and heat and mass balances for six operational cases, derived using a validated chemical looping gasification model. This design allows for reasonable operation up to 1.8 MWth in autothermal region and serves the purpose of being a reference for scale-up. Both 5 reactors are operated as circulating fluidized beds with an internal circulation in the air reactor and require approximately 1,000 kg as bed inventory. Thus, only naturally occurring or waste materials can be used as oxygen carriers. Therefore and as recent studies at CSIC had shown promising performance [8], ilmenite was chosen for the pilot testing at TU Darmstadt. Marx et al. 2021 [38] posited, that autothermal operation of CLG limits the freely selectable operation parameters to the thermal load, the oxygen carrier to fuel equivalence ratio, and the global solid circulation, which can only be adjusted dependently, whereby the global solid circulation is set indirectly via fluidization velocities. In contrast to lab-scale externally heated CLG units, for the autothermal pilot plant at TU Darmstadt, a process control strategy to de-couple heat and oxygen transport must be employed. Such a concept was developed by Dieringer et al. 2020 [15] based on equilibrium-based process simulations. The process control strategy proposes to restrict the oxygen transport to the fuel reactor to prevent full oxidation of the syngas by limited oxygen availability in the air reactor and thus incomplete oxidation of the oxygen carrier. With this approach, the oxygen carrier circulation can be kept high to provide heat in the fuel reactor. These assumptions and process control strategies were investigated in pilot testing in 2022, utilizing three different kinds of biomass as feedstock. 3. Experimental The 1 MWth thermal modular pilot plant at the Technical University of Darmstadt allows for the investigation of different fluidized bed processes at ambient pressure. A fluidized bed reactor with an inner diameter of 400 mm and a height of 11 m can be operated in stand-alone mode as bubbling fluidized bed (HTW® gasification [27, 28, 32, 36]) or as circulating fluidized bed (CFB gasification). A second coupled fluidized bed reactor (= air reactor, AR) of 9 m height and an inner diameter of 600 mm allows to decouple the heat providing reactions - either being feedstock combustion (DFBG) or oxidation of an oxygen carrier material (CLG) - from the gasification reactions [17, 18, 38]. Different reactor configurations are shown in Figure 1. The refractory lining, consisting of an inner layer of fireproof cement and two layers of insulation bricks, allows for autothermal operation. Thermal loads range from 0.35 MWth to 1.8 MWth depending on the process. Thermocouples and differential pressure transmitters provide online information on the process conditions in both reactors. (a) HTW® configuration (b) DFBG / CLG configuration [17] Figure 1: HTW® configuration (a) and CLG configuration (b) at the pilot plant of TU Darmstadt A fully automated feedstock dosing system transports pelletized or granulated feedstock from big bags, containers, or a bulk silo via an oscillating and pneumatic system to a lock hopper, that supplies a weighted dosing hopper. The dosing screw conveyer ensures a constant and quantified feedstock flow that enters the gasification reactor via a cooled screw conveyor. Superheated steam and either individually preheated O2 or air serve as gasification and fluidization agents. CO2 is used for purging of instrumentation. Both reactors have a cooled discharging screw conveyor. The product gas leaves the gasification reactor at the top, with particles being separated from the gas stream by a cyclone and transported back to the reactor system. The raw gas cooler, in the form of a fuel bottom product gasification agents syngas quench (water) 11 m MexOy Feedstock Depleted air Syngas Steam A ir R e acto r – 9 5 0 °C LS Fu e l R e acto r – 8 5 0 °C LS CO2 MexOy Feedstock Depleted air Syngas Steam A ir R e acto r – 9 5 0 °C LS Fu e l R e acto r – 8 5 0 °C LS CO2 6 horizontal double tubular exchanger, cools down the product gas to an outlet temperature of around 300 °C. The cooling agent is pressurized water to avoid evaporation and thus simplify utility infrastructure. In addition, a water quench can be activated at the top part of the reactor. Particles (>35 µm) that remain in the cooled gas are removed in a hot gas filter. The raw syngas produced is piped to a gas cleaning test rig, that is capable of purifying up to 250 Nm³/h of syngas including aromatic components. This cost-efficient downstream concept was developed by RWE, ThyssenKrupp Uhde and the TU Darmstadt within the EU-funded FABIENE project. After gas cleaning, the syngas meets the required purity for methanol or Fischer-Tropsch synthesis, which can be conducted for small gas quantities in a modular tube synthesis reactor. This allows for a demonstration and investigation of the full process chain starting from the feedstock to the liquid fuel. Alternatively, the syngas can be combusted in a post oxidation chamber. To evaluate the process performance, the product gas is analysed for the permanent gases CO2, CO, CH4 O2 and H2, for the moisture content, for alkanes, alkenes, and alkynes (C3 – C6) via gas sample bags and gas chromatography as well as for hydrocarbons / tars according to the tar protocol [44]. Furthermore, solid samples of the reactor inventory and the dust are taken and analysed for proximate and ultimate composition, and particle size distribution. All online data is transmitted to the process control system (Simatic PCS 7) and to a trend system (KRIS³) for continuous control and optimization of the process. 4. Results and Discussion 4.1. High-Temperature Winkler® gasification Starting with the commissioning of the HTW® reactor in 2015, till now 16 successful test campaigns with a total gasification time of more than 2040 hours (85 days) have been conducted. The first test campaigns focussed on the gasification of various lignite, optimizing the reactor setup, gaining operational experience and the commissioning of the gas cleaning and synthesis test rig. From 2020, fossil fuels were replaced by biomass and SRF, as well as sewage sludge. The demonstration of the feasibility of new feedstock – especially in full-chain operation - already represents a major aim. Operating ranges were determined and optimized to produce a syngas suitable for fuel synthesis by variation of the reactor settings such as oxygen and steam total input, their ratio and distribution over the reactor height, thermal load, fluidization, bottom product discharge and many more. Key performance indicators are the carbon conversion, the cold gas efficiency, the syngas yield and the content of methane, hydrocarbons, and minority species. The test campaigns have been conducted at first in cooperation with ThyssenKrupp Uhde and later in the scope of German or European Union funded projects as well as on behalf of GIDARA Energy B.V.. Table 1 gives an overview of all HTW® test campaigns conducted at the TU Darmstadt pilot plant, which will be discussed in the following. Experiences from gasification of lignite In the first test campaign in summer 2015 (C1) the functionality of the PGZ could be proven for the gasification of lignite with a stable methane content below 2 %. Higher temperatures in the PGZ enhance the highly endothermic and beneficial reforming of methane to H2 and CO but consequently result in increased amounts of CO2 originating from the necessary combustion reactions. Nevertheless, the bed temperature needs to be above approximately 730 °C to reach this syngas quality. Besides the oxygen input, the bed temperature is influenced by the moisture content of the feedstock and the pre-heating temperature of the gasification agents. It could also be found, that the methanation reaction can be neglected under the operating pressure of 1 bar, when devolatilized hearth furnace coke was gasified. [28] In a following test campaign (C2) the feasibility of a low reactive high volatile bituminous coal (HVBC) could be demonstrated, showing that higher temperatures are needed due to slower kinetics in comparison to lignite. [32] The influence of CO2 as gasification agents (with dried lignite = WTA) and high loads of LEG on the syngas quality was investigated with a focus on minority species (C3, C4). Cold gas efficiencies of up to 65 % were found and are limited by the constant reactor heat loss around of 50 kWth. [27] 7 Table 1: HTW® test campaigns in the TU Darmstadt pilot plant No. Year project feedstock thermal load [kW] duration [h] product temp. bed [°C] temp. PGZ * [°C] research topic publi- cation C0 May. 2015 Thyssen Krupp Rhenish lignite 24 h syngas commis- sioning Herdel et al. 2017 [28] C1 Jun. 2015 Thyssen Krupp Rhenish lignite 390 - 410 170 h syngas 735 - 770 845 - 977 C2 Jul. 2015 Shandong Shengxing Group HVBC 415 - 565 120 h syngas 875 - 920 905 - 955 less reactive feedstock Krause et al. 2019 [32] C3 Jan. 2019 FABIENE WTA 477 - 520 36 h syngas 740 - 790 830 - 860 feedstock + CO2 gasification Heinze et al. 2023 [27] C4 Apr. 2019 FABIENE LEG 610 - 690 80 h syngas 750 790 - 840 high loads, minority species C5 Mar. 2020 Lig2Liq LEG + 20 % SRF 400 - 420 58 h syngas 730 760 800 feedstock Langner et al. 2023 [36] C6 May. 2020 GIDARA (on behalf) waste wood + 25 to 100 % SRF 205 - 235 125 h (incl. 13 h 100% SRF) syngas 700 - 780 850 - 880 industrial testing report not available to public C7 Sep. 2020 Lig2Liq LEG + 20 to 100 % SRF 350 - 430 140 h (incl.18 h 100 % SRF) syngas 700 – 750 720 - 815 feedstock Langner et al. 2023 [36] C8 Feb. 2021 FABIENE LEG 460 155 h cleaned syngas 670 770 commis- sioning gas cleaning Heinze et al. 2023 [27] C9 Apr. 2021 FABIENE LEG 365 230 h methanol 675 760 Lig2Liq LEG + 25 % SRF 367 65 h 675 - 730 720 - 800 feedstock + methanol Langner et al. 2023 [36] C10 May. 2021 GIDARA (on behalf) feedstock blend waste wood + SRF 368 110 h syngas 650 - 790 > 860 industrial testing Report not available to public C11 Jun. 2021 Lig2Liq LEG + 25 - 50 % SRF 315 - 360 240 h Fischer- Tropsch 650 - 730 700 - 865 Fischer- Tropsch Langner et al. 2023 [36] C12 Nov. 2022 GIDARA (on behalf) fresh/waste wood + 20 % SRF 355 - 430 120 h syngas 720 - 745 790 - 870 pre-testing AMR + AMA plant report not available to the public C13 May. 2023 VERENA pine forest residue 340 - 460 100 h methanol 720 - 740 840 - 880 feedstock + operating range not published yet C14 Aug. 2023 GIDARA (on behalf) waste wood + 20 % SRF 430 65 h syngas 720 - 745 < 1000 pre-testing AMR + AMA plant report not available to public C15 Aug. 2023 VERENA sewage sludge 430 48 h syngas 690 - 750 800 - 870 feedstock not published yet C16 Sep. 2023 VERENA SRF 440 165 h syngas 670 - 730 < 1000 feedstock + operating range not published yet LEG = lignite energy grained (dried lignite with a grain size of < 4 mm) WTA = fine grain (150 µm) Renish lignite dried in fluidized bed drying with integrated heat recovery (Wirbelschicht Trocknung mit integrierter Abwärmenutzung) HVBC = high volatile bituminous coal * = temperature measured, but actual temperature potentially higher due to slagging on thermocouples over time and thus malfunction Gasification of residual feedstock (SRF, wooden biomass, sewage sludge) Wooden biomass and SRF contain significantly more volatile matter (68 – 81 % / 74 - 82 %) than lignite (45 %). While wooden biomass still comprises around 15 % of fixed carbon, SRF consists of only up to approximately 9 % fixed carbon and consequently with up to 15 % more ash (4 % for lignite). The spread in the feedstock composition given Table 2 shows the inhomogeneity of SRF already in a small number of batches and could be a challenge in SRF gasification. Sewage sludge also is composed of only a small fraction of fixed carbon (4.5 – 6.5 w-%), but of more than 30 % ash and thus a medium volatile content of 50 – 55 w-%. The lower heating value of SRF is comparable to lignite at around 22 MJ/kg whereas significantly lower for wooden biomass (15 - 19 MJ/kg) and sewage sludge (13 MJ/kg). Additionally, the ash is comprised differently for every feedstock and softening tends to start at lower temperatures in reducing atmosphere in comparison to lignite. 8 Table 2: proximate analysis and lower heating value for different feedstock used for gasification experiments in pilot scale at the TU Darmstadt feedstock (a.r.) lignite SRF pellets fresh wood pellets waste wood pellets pine forest residue pellets sewage sludge dried LHV [MJ/kg] 21.5 - 22.7 20.8 - 23.5 15.8 - 16.8 16.6 - 17.1 18.3 - 18.4 12.3 - 13.6 Moisture [%] 10.5 - 13.0 2.6 - 5.7 9.9 9.9 3 - 4.7 3.5 - 5.8 volatiles [%] 44.0 - 45.0 74.0 - 82.0 72.2 68.7 76.7 - 80.7 51.4 - 54.2 fixed carbon [%] 39.0 - 41.0 1.8 - 8.6 14.8 15.3 12.3 - 16.6 4.5 - 6.3 ash [%] 3.5 - 4.0 12.4 - 14.8 3.1 6.1 1.9 - 2.8 36.4 - 38.3 The effects of these feedstock characteristics on the reactor conditions and the operability were investigated at the TU Darmstadt from 2020. Starting with co-gasification of increasing amounts of SRF at first with lignite (C5, C7) and later with waste wood (C6), 13 hours (C6 [35]) and 18 hours (C7 [36])of SRF mono-gasification could be achieved. To prevent agglomerations and sintering in the first experiments, a high CO2 or steam fluidization was provided and the temperatures were limited to moderate 700 - 750 °C in the bed and 720 - 815 °C in the PGZ. With rising SRF amount, the inventory level in the gasifier decreased due to more volatiles being released, and thus lower amount of fixed carbon remaining. To counteract, the oxygen input into the bed was reduced to decrease the carbon conversion rate in the bed. Consequently, the bed temperature decreased, which affects the devolatilization of the feedstock. On the other hand, the higher fraction of volatiles led to lower temperatures in the PGZ. This was reacted to with increased O2 input in the PGZ, temporarily increasing the temperature to > 1,000 °C to decompose tars. The total oxygen supply ranged from a lambda of 0.36 to 0.44. Under this reactor conditions a methane content of 6.9 % (dry) for 100 % SRF could be achieved in comparison to 2.5 % for 100 % lignite. [36] However, such high temperatures in the PGZ are critical for operation in the TU Darmstadt pilot plant, as the steam enveloped oxygen jet can hit the opposite reactor wall due to the small reactor diameter and cause slagging at refractory lining and subsequently blockage. This risk is not anticipated for commercial scale HTW gasifiers with larger geometry and an optimized hydrodynamic of the oxygen jet. During the commissioning of the gas cleaning unit in 2021 (C8, C9), the first production of methanol in the 1 MWth pliot plant was achieved with lignite and 25 w-% SRF as feedstock [27]. Synthesis of Fischer- Tropsch products from co-gasification of 50% SRF with lignite was demonstrated for the first time in June 2021 (C11) [36]. Parameter studies in these test campaigns have proven, that the methane content and the amount of tars decrease and the yield of the valuable syngas components H2 and CO increases with rising gasification temperatures - already in the fluidized bed but additionally with higher temperatures in the PGZ. By replacing lignite with SRF the yield of H2 and CO does not change significantly, which underlines the promising opportunities for residual feedstock for HTW® gasification. On the other side, the methane, Naphthalene and other hydrocarbons content increases. Aromatic compounds (C6 - C9) comprise the biggest fraction of hydrocarbons, even increasing with higher temperatures as aromatics are very stable and thus a decomposition product of aliphatic hydrocarbons. This leads to the assumption, that the syngas quality is highly influenced by the tar cracking reactions and methane reforming in the PGZ due to the high volatiles content and that the heterogeneous water gas reaction plays a minor role. For mono-gasification of 100 % SRF a cold gas efficiency of 53.7 % was achieved, showing potential for optimizing the reactor conditions, especially regarding the methane content and tar conversion. [27, 36] Furthermore, different qualities of wooden biomass (fresh wood, pine forest residue and plastic- containing waste wood) were investigated in co-gasification with SRF but also as single feedstock in several test campaigns (C6, C10, C12, C13, C14), underlining the assumptions taken. In May 2023 the synthesis of methanol from pine forest residue via HTW® could be demonstrated for the first time (C13). In September 2023 a test campaign using 100% SRF was successfully operated for 165 hours (C13), where operating ranges were exploited to optimize the syngas quality and reduce the amount of methane and tars. At a thermal load of 440 kWth, the bed temperature was varied between 660 °C and 760 °C at PGZ temperatures < 1000 °C. A bottom product recycle was established to increase the carbon conversion rate. Additionally, approximately 50 % of the fluidized bed was replaced by olivine for parts of the test campaign to investigate the potential catalytic effect on tar cracking. A total of 7 complete sets of samples could be taken and is currently analysed as well as online data is post- processed. 9 A test campaign of 48 h was conducted successfully for sewage sludge (C15), which is a challenging feedstock due to the large component of ash and requires a permanent bottom ash discharge. The pilot testing of (co)-gasification of lignite, SRF, biomass and sewage sludge has provided an extensive operational experience regarding operational ranges, potential for process optimization and handling of changes in feedstock composition and load, as well as of unforeseen issues. Furthermore, the experiments have proven the reproducibility of the pilot testing. 4.2. Chemical looping gasification The Chemical looping gasification reactor configuration at the TU Darmstadt was successfully commissioned in a test campaign of >100 hours with industrial wood pellets and ilmenite as oxygen carrier in March 2022. Pine forest residue and pretreated wheat were investigated in CLG using two different granulations in two further test campaigns. Thus, the feasibility of autothermal CLG for biomass could be demonstrated during more than 400 hours of gasification operation. The thermal input was set to 1.1 - 1.8 MWth, which corresponds to a feed rate of 230 - 360 kg/h. Reactor temperatures reached 805 - 950 °C in the air reactor and consequently lower temperatures of 715 - 840 °C in the fuel reactor. The latter are mainly adjustable via the oxygen carrier circulation and restricted by the plant scale and autothermal operation (i.e. lack of external heating). CLG achieves cold gas efficiencies up to 50 % (up to 80 % expected for industrial plants [12]) and a high calorific syngas with decent methane content, but lower amounts of higher hydrocarbons compared to other biomass gasification technologies. [39] The suggested process control strategy (see section 2.2) could be evaluated and proven with an AR flue gas recirculation, that limits the availability of oxygen in the air reactor. This allows for independent control of hydrodynamics and heat as well as oxygen transport between air and fuel reactor and is discussed in Dieringer et al. 2023 [17]. For further understanding and scale-up of the process, particular focus was placed on the measurement of the solid flux measurement as the main influencing parameter [40], the system hydrodynamics [18] (complemented by > 50 hours of cold flow model operation), the fate of ilmenite in long-term operation as the core of the process [16] and the formation of tars [39]. Based on the findings of the three CLG test campaigns, as well as the operational experience gained, it is expected that process efficiency can be further optimized to achieve improved KPIs. Moreover, testing of different feedstocks will answer whether the CLG technology is suitable for the conversion of other waste streams. Therefore, a CLG test campaign with SRF is planned for mid-2024 to open up the process of CLG for non-recyclable plastic waste. At commercial scale co-feeding of biomass and waste is conceivable in plants of 50 to 300 MWth, depending on the regional feedstock availability. 5. Conclusion and outlook The HTW® technology as well as the Chemical looping gasification technology were demonstrated to be promising technologies for the industrial production of syngas suitable for liquid fuel synthesis. Gasification of biomass and waste (only for HTW®) has achieved competitive yields, syngas qualities, carbon conversions and cold gas efficiencies in comparison to other gasification technologies. With > 2040 hours of total operating time for HTW® and > 400 hours for CLG reliable sets of data were gained, giving insight to the highly complex processes. An overview of the findings is given in section 4 with reference to papers for more detailed information. The results of the latest test campaigns for HTW® with SRF, pine forest residue and sewage sludge are still under evaluation and will be published. Furthermore, the researchers at the TU Darmstadt built up an extensive operational experience through numerous test campaigns in 24/7 shifts dealing with the commissioning of new reactor configurations, unforeseen process behaviour and new feedstock. This expertise is also important and valuable for planning of industrial setups. However, further investigation is beneficial – especially for CLG operation and the gasification of non- recyclable plastic waste as feedstock – with the purpose of: − demonstrating the feasibility and performance of different feedstocks with a particular focus on validating business cases and for dedicated applications in industrial plants. − validating the base-case used for scale-up. − reviewing identified findings and verifying whether they can be transferred to other feedstocks. − evaluating the performance of the downstream gas cleaning and synthesis depending on the gasification process. 10 − analysing minority species that may harm downstream processing or are subject to regulation. − characterizing residual streams as bottom product and ash to meet regulatory requirements and potentially valorise. − assessing the effects of catalytic active bed material on the tar formation and cracking especially for HTW®. − observing the performance of long-term operation. Acknowledgements: The authors gratefully acknowledge the funding of the German Federal Ministry for Economic Affairs and Climate Action under grant agreement No. 03EE5044A (VERENA: Gasification processes with integrated excess electricity integration for flexible power generation and production of synthetic fuels from residuals). References [1] Adlhoch, W., Sato, H., Wolff, J., and Radtke, K. 2000. High-Temperature Winkler Gasification of Municipal Solid Waste. 2000 Gasification Technologies Conference, San Francisco, California, USA. [2] Adlhoch W, Meyer B, Schumacher H-J. 1992. Stand der HTW-Vergasungstechnologie. Chem Ing Tech 64, 5, 476. [3] Ahrens, R. H. 2016. Braunkohle wird zu Synthesegas. VDI Verlag GmbH (Jul. 2016). [4] Biessey, P., Vogel, J., Seitz, M., and Quicker, P. 2023. Plastic Waste Utilization via Chemical Recycling: Approaches, Limitations, and the Challenges Ahead. Chemie Ingenieur Technik 95, 8, 1199–1214. DOI: https://doi.org/10.1002/cite.202300042. [5] Björn Gillberg. 2016. VärmlandsMetanol - a Pioneer Project: A Gasification Project temporarily on hold. 9th International Seminar on Gasification in Malmö, Malmö. [6] Ciuta, S., Tsiamis, D., and Castaldi, M. J. 2018. Gasification of Waste Materials: Technologies for Generating Energy, Gas, and Chemicals from Municipal Solid Waste, Biomass, Nonrecycled Plastics, Sludges, and Wet Solid Wastes. Elsevier. [7] Condori, O., Diego, L. F. de, Garcia-Labiano, F., Izquierdo, M. T., Abad, A., and Adánez, J. 2021. Syngas Production in a 1.5 kWth Biomass Chemical Looping Gasification Unit Using Fe and Mn Ores as the Oxygen Carrier. Energy & fuels : an American Chemical Society journal 35, 21, 17182–17196. DOI: https://doi.org/10.1021/acs.energyfuels.1c01878. [8] Condori, O., García-Labiano, F., Diego, L. F. de, Izquierdo, M. T., Abad, A., and Adánez, J. 2021. Biomass chemical looping gasification for syngas production using ilmenite as oxygen carrier in a 1.5 kWth unit. Chemical Engineering Journal 405, 126679. DOI: https://doi.org/10.1016/j.cej.2020.126679. [9] Dai, J. and Whitty, K. J. 2022. Chemical looping gasification and sorption enhanced gasification of biomass: A perspective. Chemical Engineering and Processing - Process Intensification 174. DOI: https://doi.org/10.1016/j.cep.2022.108902. [10] De, S., Agarwal, A. K., Moholkar, V. S., and Thallada, B. 2018. Coal and Biomass Gasification. Springer Singapore, Singapore. [11] Dechema Gesellschaft für Chemische Technik und Biotechnologie e.V. 2017. Low carbon energy and feedstock for the European chemical industry. [12] Detsios, N., Atsonios, K., Grammelis, P., Dieringer, P., Ströhle, J., Nikkanen, V., and Orfanoudakis, N. G. 2023. A Comparative Analysis and Assessment of Dual Fluidized Bed and Chemical Looping Gasification: Design Considerations for Commercial Use and Applicability in BTL Schemes. European Biomass Conference and Exhibition Proceedings 31st EUBCE - Bologna 2023, 588–593. DOI: https://doi.org/10.5071/31stEUBCE2023-4AO.5.2. [13] Di Carlo, A., Savuto, E., Foscolo, P. U., Papa, A. A., Tacconi, A., Del Zotto, L., Aydin, B., and Bocci, E. 2022. Preliminary Results of Biomass Gasification Obtained at Pilot Scale with an Innovative 100 kWth Dual Bubbling Fluidized Bed Gasifier. Energies 15, 12, 4369. DOI: https://doi.org/10.3390/en15124369. [14] Di Giuliano, A., Capone, S., Anatone, M., and Gallucci, K. 2022. Chemical Looping Combustion and Gasification: A Review and a Focus on European Research Projects. Ind. Eng. Chem. Res. 61, 39. DOI: https://doi.org/10.1021/acs.iecr.2c02677. [15] Dieringer, P., Marx, F., Alobaid, F., Ströhle, J., and Epple, B. 2020. Process Control Strategies in Chemical Looping Gasification—A Novel Process for the Production of Biofuels Allowing for Net Negative CO2 Emissions. Applied Sciences 10, 12. DOI: https://doi.org/10.3390/app10124271. [16] Dieringer, P., Marx, F., Lebendig, F., Müller, M., Di Guiliano, A., Galucci, K., Ströhle, J., and Epple, B. 2023. Fate of Ilmenite as Oxygen Carrier during 1 MWth Chemical Looping Gasification of Biogenic Residues. Applications in Energy and Combustion Science. DOI: https://doi.org/10.1016/j.jaecs.2023.100227. [17] Dieringer, P., Marx, F., Michel, B., Ströhle, J., and Epple, B. 2023. Design and control concept of a 1 MWth chemical looping gasifier allowing for efficient autothermal syngas production. International Journal of Greenhouse Gas Control 127. DOI: https://doi.org/10.1016/j.ijggc.2023.103929. 11 [18] Dieringer, P., Marx, F., Ströhle, J., and Epple, B. 2023. System Hydrodynamics of a 1 MWth Dual Circulating Fluidized Bed Chemical Looping Gasifier. Energies 16, 15. DOI: https://doi.org/10.3390/en16155630. [19] FAZ. 1998. "BoA-Plus" soll Wirkungsgrad von 50 Prozent bei Braunkohleverstromung ermöglichen. [20] Franken, G., Adlhoch, W., and Koch, W. 1980. Herstellung von Synthesegas aus Braunkohle. Chemie Ingenieur Technik 52, 4, 324–327. DOI: https://doi.org/10.1002/cite.330520407. [21] Ge, H., Guo, W., Shen, L., Song, T., and Xiao, J. 2016. Biomass gasification using chemical looping in a 25 kW th reactor with natural hematite as oxygen carrier. Chemical Engineering Journal 286, 174–183. DOI: https://doi.org/10.1016/j.cej.2015.10.092. [22] Ge, H., Guo, W., Shen, L., Song, T., and Xiao, J. 2016. Experimental investigation on biomass gasification using chemical looping in a batch reactor and a continuous dual reactor. Chemical Engineering Journal 286, 689–700. DOI: https://doi.org/10.1016/j.cej.2015.11.008. [23] German Environment Agency. 2020. Chemical recycling. German Environment Agency. [24] GIDARA Energy. 2021. Gidara Energy annouces a new facility converting non-recyclabe waste into advanced biofuels (June 2021). Retrieved January 16, 2024 from www.gidara-energy.com/advanced- methanol-amsterdam. [25] GIDARA Energy. 2022. GIDARA Energy announces next advanced methanol facility in the Port of Rotterdam (April 2022). Retrieved January 16, 2024 from www.gidara-energy.com/news-articles/gidara- energy-announces-next-advanced-methanol-facility-in-the-port-of-rotterdam. [26] Goel, A., Moghaddam, E. M., Liu, W., He, C., and Konttinen, J. 2022. Biomass chemical looping gasification for high-quality syngas: A critical review and technological outlooks. Energy Conversion and Management 268. DOI: https://doi.org/10.1016/j.enconman.2022.116020. [27] Heinze, C., May, J., Langner, E., Ströhle, J., and Epple, B. 2023. High Temperature Winkler gasification of Rhenish lignite in an optimized 500 kWth pilot plant. Fuel 333, 126289. DOI: https://doi.org/10.1016/j.fuel.2022.126289. [28] Herdel, P., Krause, D., Peters, J., Kolmorgen, B., Ströhle, J., and Epple, B. 2017. Experimental investigations in a demonstration plant for fluidized bed gasification of multiple feedstock’s in 0.5 MW th scale. Fuel 205, 286–296. DOI: https://doi.org/10.1016/j.fuel.2017.05.058. [29] Higman, C. and van der Burgt, M. 2008. Gasification. (2nd ed.). Gulf Professional Pub./Elsevier Science, Amsterdam, Boston. [30] Intergovernmental Panel on Climate Change. 2023. Climate Change 2023 Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Geneva, Switzerland. DOI: https://doi.org/10.59327/IPCC/AR6-9789291691647. [31] International Energy Agency. 2023. Net Zero Roadmap: A Global Pathway to Keep the 1.5 °C Goal in Reach: 2023 Update. [32] Krause, D., Herdel, P., Ströhle, J., and Epple, B. 2019. HTW™-gasification of high volatile bituminous coal in a 500 kWth pilot plant. Fuel 250, 306–314. DOI: https://doi.org/10.1016/j.fuel.2019.04.014. [33] Kunii, D. and Levenspiel, O. 1991. Fluidization engineering. Butterworth-Heinemann Series in Chemical Engineering. Butterworth-Heinemann, Stoneham, Massachusetts. [34] Lambertz, J., Schrader, L., and Teggers, H. 1986. Neuere Ergebnisse des Betriebes der Pilotanlage zum Hochtemperatur‐Winkler‐Verfahren. Chemie Ingenieur Technik 58, 8, 637–643. DOI: https://doi.org/10.1002/cite.330580804. [35] Langner, E. 2020. Gasification Test at HTW Pilot Plant TU Darmstadt for GI Dynamics. Institute for Energy Systems and Technology. [36] Langner, E., Kaltenmorgen, J., Heinze, C., Ströhle, J., and Epple, B. 2023. Fluidized bed gasification of solid recovered fuels in a 500 kWth pilot plant. Fuel 344, 127901. DOI: https://doi.org/10.1016/j.fuel.2023.127901. [37] Lin, Y., Wang, H., Wang, Y., Huo, R., Huang, Z., Liu, M., Wei, G., Zhao, Z., Li, H., and Fang, Y. 2020. Review of Biomass Chemical Looping Gasification in China. Energy Fuels 34, 7, 7847–7862. DOI: https://doi.org/10.1021/acs.energyfuels.0c01022. [38] Marx, F., Dieringer, P., Ströhle, J., and Epple, B. 2021. Design of a 1 MWth Pilot Plant for Chemical Looping Gasification of Biogenic Residues. Energies 14, 9. DOI: https://doi.org/10.3390/en14092581. [39] Marx, F., Dieringer, P., Ströhle, J., and Epple, B. 2023. Process efficiency and syngas quality from autothermal operation of a 1 MWth chemical looping gasifier with biogenic residues. Applications in Energy and Combustion Science 16. DOI: https://doi.org/10.1016/j.jaecs.2023.100217. [40] Marx, F., Dieringer, P., Ströhle, J., and Epple, B. 2023. Solid flux measurement in dual fluidized bed processes based on solid samples. Fuel 341. DOI: https://doi.org/10.1016/j.fuel.2023.127589. [41] Mendiara, T., García-Labiano, F., Abad, A., Gayán, P., Diego, L. F. de, Izquierdo, M. T., and Adánez, J. 2018. Negative CO2 emissions through the use of biofuels in chemical looping technology: A review. Applied Energy 232, 657–684. DOI: https://doi.org/10.1016/j.apenergy.2018.09.201. [42] Modern Power Systems. 2008. Report from Vřesová: 12 years of operating experience with the world’s largest coal-fuelled IGCC - Modern Power Systems. [43] Moldenhauer, P., Linderholm, C., Rydén, and M. et al. Experimental investigation of chemical-looping combustion and chemical-looping gasification of biomass-based fuels using steel converter slag as oxygen carrier. 12 [44] Neeft, J., Knoef, H., Zielke, U., Sjöström, K., Hasler, P., Simell, P., Dorrington, M., and Greil, C. 2001. Tar Protocol: Development of a standard method for the measurement of organic contaminants (tar) in biomass producer gases. Proceedings of the First World Conference on Biomass for Energy and Industry, 582–585. [45] Nguyen, N. M., Alobaid, F., Dieringer, P., and Epple, B. 2021. Biomass-Based Chemical Looping Gasification: Overview and Recent Developments. Applied Sciences 11, 15. DOI: https://doi.org/10.3390/app11157069. [46] Pissot, S., Berdugo Vilches, T., Maric, J., Cañete Vela, I., Thunman, H., and Seemann, M. 2019. Thermochemical Recycling of Automotive Shredder Residue by Chemical-Looping Gasification Using the Generated Ash as Oxygen Carrier. Energy Fuels 33, 11, 11552–11566. DOI: https://doi.org/10.1021/acs.energyfuels.9b02607. [47] Pissot, S., Berdugo Vilches, T., Maric, J., and Seemann, M. 2018. Chemical looping gasification in a 2-4 MWth dual fluidized bed gasifier. 23rd International Conference on Fluidized Bed Conversion, Seoul, Korea. [48] Renzenbrink W, Wischnewski R, Engelhard J, Mittelstädt A. 1998. High temperature Winkler (HTW) coal gasification: a fully developed process for methanol and electricity production: Rheinbraun AG. Gasification Technology Conference. [49] Samprón, I., Diego, L. F. de, García-Labiano, F., Izquierdo, M. T., Abad, A., and Adánez, J. 2020. Biomass Chemical Looping Gasification of pine wood using a synthetic Fe2O3/Al2O3 oxygen carrier in a continuous unit. Bioresource technology 316, 123908. DOI: https://doi.org/10.1016/j.biortech.2020.123908. [50] Shahabuddin, M., Alam, M. T., Krishna, B. B., Bhaskar, T., and Perkins, G. 2020. A review on the production of renewable aviation fuels from the gasification of biomass and residual wastes. Bioresource technology 312, 123596. DOI: https://doi.org/10.1016/j.biortech.2020.123596. [51] Soleimani Salim, A. H., Linderholm, C. J., Condori, O., Samprón, I., de Diego, L. F., García-Labiano, F., Abad, A., and Mattison, T. 2022. Investigating the chemical looping gasification of biomass for syngas production during continuous operation in 1.5 to 100 kW units: 2nd International Conference on Negative CO2 Emissions. 2nd International Conference on Negative CO2 Emissions, Göteborg, Sweden. [52] Szul, M., Głód, K., and Iluk, T. 2021. Influence of pressure and CO2 in fluidized bed gasification of waste biomasses. Biomass Conv. Bioref. 11, 1, 69–81. DOI: https://doi.org/10.1007/s13399-020-00840-9. [53] thyssenkrupp Industrial Solutions AG. 2011. Uhde erweitert sein Technologie-Portfolio: Uhde übernimmt HTW-Verfahren zur Kohlevergasung von RWE Power. [54] Ullrich, U., Abraham, R., and Toporov, D. Verfahren zum Vergasen von kohlenstoffhaltigen Feststoffen mit gasförmigen, sauerstoffhaltigen Vergasungsmitteln in einem Wirbelschichtgasprozess in einem Vergaser, insbesondere in einem Hochtemperatur-Winkler-Vergaser, Filed Nov. 9th, 2023. [55] United Nations Framework Convention on Climate Change (UNFCCC). 2015. The Paris Agreement. [56] Wei, G., He, F., Huang, Z., Zheng, A., Zhao, K., and Li, H. 2015. Continuous Operation of a 10 kW th Chemical Looping Integrated Fluidized Bed Reactor for Gasifying Biomass Using an Iron-Based Oxygen Carrier. Energy Fuels 29, 1, 233–241. DOI: https://doi.org/10.1021/ef5021457. [57] Wei, G., He, F., Zhao, Z., Huang, Z., Zheng, A., Zhao, K., and Li, H. 2015. Performance of Fe–Ni bimetallic oxygen carriers for chemical looping gasification of biomass in a 10 kWth interconnected circulating fluidized bed reactor. International Journal of Hydrogen Energy 40, 46, 16021–16032. DOI: https://doi.org/10.1016/j.ijhydene.2015.09.128. [58] Wischnewski, R. and Engelhard, J. Two years` successful operation of a hot gas filter in the Rheinbraun High Temperature Winkler coal gasification demonstration plant. Pittsburgh Coal Conference, Pittsburgh, PA (United States). [59] Z. Bucko, J. Engelhard, J. Wolff, H. Vierrath. 400 MWe IGCC Power Plant with HTW Gasification in the Czech Republic. [60] Zheng, A., Fan, Y., Wei, G., Zhao, K., Huang, Z., Zhao, Z., and Li, H. 2020. Chemical Looping Gasification of Torrefied Biomass Using NiFe 2 O 4 as an Oxygen Carrier for Syngas Production and Tar Removal. Energy Fuels 34, 5, 6008–6019. DOI: https://doi.org/10.1021/acs.energyfuels.0c00584.