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After years of development of the dual fluidized bed gasification process with woody biomass, new challenges arise with a wider range of feedstocks, such as agricultural residues or waste fractions, gaining importance. This work presents the results from the operation of a 1 MW advanced dual fluidized bed (aDFB) gasifier at the Syngas Platform Vienna, with cashew shells (CS) as feedstock and pure olivine or olivine mixed with 33% limestone as the bed material. Two operation points, different in their main process parameters, with CS as feedstock were performed for a minimum of 16 h. Due to the high calorific value (22.7 MJ/kg dry) and high content of volatile components (81.7 wt % dry), CS are well suited for gasification. During the operation, a product gas with a H2/CO ratio of 1.93-2.29 was produced, which suits the use of the syngas as input for Fischer-Tropsch synthesis. Due to the low ash softening temperature of CS of about 840 °C, determined in ash fusion tests and predicted by thermodynamic equilibrium calculations, the operating temperature in the gasifier was kept below 820 °C in the first operation point with around 20 h of runtime. Bed material samples after the operation revealed the formation of agglomerates, which were analyzed by SEM and EDS for their morphology and composition. During the second operation point, temperatures in the gasifier were increased to 870 °C for 16 h, while no agglomeration tendencies were observed. Both operations did not show changes in fluidization behavior compared to operations with wood chips (WC). The evaluated key performance indicators were compared to aDFB steam gasification of WC as the reference.

The production of sewage sludge, a by-product of wastewater treatment plants, is steadily increasing. With its hazardous properties but valuable resources and materials, sewage sludge requires innovative management strategies to tackle environmental challenges and create opportunities for energy recovery and circular resource utilization. The most common applications currently include incineration, agricultural use and composting. The trend towards thermal treatment is steadily increasing, especially in Europe. State-of-the-art is to incinerate sewage sludge, which offers the benefits of volume reduction and the destruction of toxins, pathogens as well as the potential for recovery of the critical resource phosphorus from the generated ash. The latter is becoming increasingly important, particularly due to emerging regulations within the EU. Gasification as a thermal treatment method is also gaining traction as a suitable approach. The thermochemical process offers the same benefits as incineration but additionally produces gas (synthesis gas), a versatile fuel for energy generation and a precursor for the chemical industry. Sewage sludge gasification has yet to be adopted on a large scale due to gaps in research, technological development and systemic requirements as well as financial constraints. This review provides a comprehensive overview of legislation related to sewage sludge management and feedstock characteristics, including ash chemistry. Furthermore, it explores the fundamentals of gasification, focusing on optimizing parameters for sewage sludge by summarizing findings from studies on the effects of feedstock parameters, process designs and operating conditions on efficiency and output. It concludes with an outlook on large-scale implementation to maximize waste valorization through gasification.

The advanced dual fluidized bed steam gasification technology allows the generation of a medium-calorific product gas from various feedstocks. Thereby, biogenic residues, municipal and industrial wastes e.g., sewage sludge or rejects from pulp and paper industry can be utilised. This paper presents the development step of this technology from pilot to demonstration plant scale i.e. the step from technology readiness level 4 to 6. The newly erected demonstration plant at the Syngas Platform Vienna is designed for 1 MW_th fuel input power and incorporates a counter current column as the upper part of the gasification. This design choice already resulted in an improvement of product gas quality in pilot scale. By comparing the data gathered from multiple years of 100 kW_th pilot scale testing at TU Wien with first results from the new demonstration plant via mass and energy balance simulation, fluidisation regimes and temperature and pressure profiles, the success of the scale-up of the reactor design is proven. The first full load operation achieved a conversion of 1 MW fuel input power into 769 kW product gas power. This translates to 256 kg/h dry biomass being converted into 245 Nm³/h of dry product gas. Although first experiments with the reference feedstock high-grade wood chips showed good reproducibility of the results achieved in pilot scale, challenges still remain. The expected product gas composition was different compared to pilot scale results, as the volume share of hydrogen was lower and the relative content of carbon monoxide and carbon dioxide inverted. While the results show an important intermediate step in the process development, the challenges of improving product gas quality and increasing overall conversion efficiency remain to be tackled.

First experiments with biogenic residues and a plastic-rich rejects and woody biomass blend were conducted in an advanced 1 MW dual fluidized bed steam gasification demonstration plant at the Syngas Platform Vienna. Wood chips, bark, forest residues, and the plastic-rich rejects and woody biomass blend were tested and the tar composition was analyzed upstream and downstream of the upper gasification reactor, which is designed as a high-temperature column with countercurrent flow of catalytic material. Each feedstock was gasified with olivine as bed material in demonstration scale and is compared to the gasification of softwood pellets with olivine and limestone in pilot scale. A reduction in tar content was observed after countercurrent column for all feedstocks. However, a shift in tar species occurred. While styrene, phenol, and 1H-indene were predominant upstream, naphthalene and polycyclic aromatic hydrocarbons (PAHs) were the prevailing tar species downstream the countercurrent column. Hence, an increase of i.e. anthracene, fluoranthene, and pyrene from the upstream concentration was observed. For pyrene, up to twice the initial concentration was measured. This recombination to PAHs was observed for all feedstocks in demonstration- and pilot-scale. The only exception occurred with limestone as bed material, characterized by a higher catalytic activity in comparison to the typically used olivine. In the perspective of the integrated product gas cleaning, tar with higher temperature of condensation are separated more efficiently in the installed scrubbing unit. Hence, the recombination facilitates an overall decline of tar content after the gas cleaning.

K-feldspar was utilized as bed material for fluidized bed combustion of bark, chicken manure, and their mixture. Bed samples were extracted after 4 and 8 h and the samples were analyzed with scanning electron microscopy to study the impact of P-rich chicken manure on the bed material. The results were compared to fixed bed exposures with different orthophosphates to investigate their influence in detail. The fresh bed material used for this study exhibited an uneven surface with many cavities which facilitated the deposition and retention of the fuel ash. Utilizing pure chicken manure as fuel led to the formation of Ca- and P-rich particles which accumulated in these cavities. At the same time, larger ash particles were formed which consisted of the elements found in chicken manure ash. The co-combustion of bark and chicken manure led to the interaction of the two ash fractions and the formation of a thicker ash layer, which consisted of elements from both fuel ashes, namely Ca, P, Si, K and S. The layer appeared to be partially molten which could be favorable for the deposition of ash particles and thereby the formation of a mixed Ca/K-phosphate. Fixed bed exposures of the K-feldspar particles with Na3PO4 or K3PO4 caused particle agglomeration which means presence of alkali-phosphates should be limited. The co-combustion of bark with chicken manure showed promising results both regarding a shift from Ca-phosphates to more bioavailable Ca/K-phosphates and an acceleration in ash layer formation. The formation of an ash layer after only 4 h of exposure with the mixture of bark and chicken manure could be advantageous for catalytic activation of the bed material.

The use of biomass, whether wood or waste materials, represents an opportunity to reduce dependence on fossil fuels. Using dual fluidized bed (DFB) steam gasification technology for conversion of biomass, biogenic residues and waste is a possible approach to exploit this potential. A demonstration-scale 1 MW gasification plant with subsequent product gas and flue gas cleaning has been constructed at the Syngas Platform Vienna. The performance of this plant during its first continuous operation was evaluated in this study by analyzing temperature and pressure profiles recorded over a period of nearly 6 hours of continuous operation and by evaluating various performance indicators, such as steam-to-fuel ratio, gasification temperatures, and product gas composition. The resulting data sets were compared with existing results from the 100 kW pilot plant at TU Wien. The discrepancies found and their avoidance is discussed. In addition, this paper highlights the mechanical challenges encountered during commissioning, such as high temperatures at the fabric filter and additional condensate drain in steam supply pipes, and identifies potential challenges during the installation and operation of future gasification plants.

This study aims to determine the fate of P during fluidized bed co-combustion of chicken litter (CL) with K-rich fuels [e.g., wheat straw (WS)] and Ca-rich fuels (bark). The effect of fuel blending on phosphate speciation in ash was investigated. This was performed by chemical characterization of ash fractions to determine which phosphate compounds had formed and identify plausible ash transformation reactions for P. The ash fractions were produced in combustion experiments using CL and fuel blends with 30% CL and WS or bark (B) at 790–810 °C in a 5 kW laboratory-scale bubbling fluidized bed. Potassium feldspar was used as the bed material. Bed ash particles, cyclone ash, and particulate matter (PM) were collected and subjected to chemical analysis with scanning electron microscopy–energy-dispersive X-ray spectrometry (SEM–EDS) and X-ray diffraction. P was detected in coarse ash fractions only, that is, bed ash, cyclone ash, and coarse PM fraction (>1 μm); no P could be detected in the fine PM fraction (<1 μm). SEM–EDS analysis showed that P was mainly present in K–Ca–P-rich areas for pure CL as well as in the ashes from the fuel blends of CL with WS or B. In the WS blend, P was found together with Si in these areas. The crystalline compound containing P was hydroxyapatite in all cases as well as whitlockite in the cases of pure CL and WS blend, of which the latter compound has been previously identified as a promising plant nutrient. The ash fractions from CL and bark blend only contained P in hydroxyapatite. Co-combustion of CL together with WS appears to be promising for P recovery, and ashes with this composition could be further studied in plant growth experiments.

The use of phosphorus-rich fuels in fluidized bed combustion is one probable way to support both heat and power production and phosphorus recovery. Ash is accumulated in the bed during combustion and interacts with the bed material to form layers and/or agglomerates, possibly removing phosphorus from the bed ash fraction. To further deepen the knowledge about the difference in the mechanisms behind the ash chemistry of phosphorus -lean and phosphorus-rich fuels, experiments in a 5 kW bench-scale-fluidized bed test-rig with K-feldspar as the bed material were conducted with bark, wheat straw, chicken manure, and chicken manure admixtures to bark and straw. Bed material samples were collected and studied for layer formation and agglomeration phenomena by scanning electron microscopy combined with energy dispersive X-ray spectrometry. The admixture of phosphorus-rich chicken manure to bark changed the layer formation mechanism, shifting the chemistry to the formation of phosphates rather than silicates. The admixture of chicken manure to straw reduced the ash melting and agglomeration risk, making it possible to increase the time until defluidization of the fluidized bed occurred. The results also highlight that an increased ash content does not necessarily lead to more ash melting related problems if the ash melting temperature is high enough.

Understanding layer formation on bed materials used in fluidized beds is a key step for advances in the application of alternative fuels. Layers can be responsible for agglomeration-caused shut-downs but they can also improve the gas composition in fluidized bed gasification. Layers were observed on K-feldspar (KAlSi3O8) impurities originating from the combined heat and power plant Senden which applies the dual fluidized bed (DFB) steam gasification technology. Pure K-feldspar was therefore considered as alternative bed material in DFB steam gasification. Focusing on the interactions between fuel ash and bed material, K-feldspar was tested in combustion and DFB steam gasification atmospheres using different fuels, namely Ca-rich bark, Ca -and P-rich chicken manure, and an admixture of chicken manure to bark. The bed particle layers formed on the bed material surface were characterized using combined scanning electron microscopy and energy-dispersive X-ray spectroscopy; area mappings and line scans were carried out for all samples. The obtained data show no essential influence of operational mode on the layer-formation process. During the combustion and DFB steam gasification of Ca-rich bark, a layer rich in Ca formed while K was diffusing out of the layer. The use of Ca -and P-rich chicken manure inhibited the diffusion of K, and a layer rich in Ca and P formed. The addition of P to bark via chicken manure also changed the underlying layer-formation processes to reflect the same processes as observed for pure chicken manure.