Waste incineration and waste-to-energy (WtE) plants play a key role in waste management worldwide. To avoid the high amounts of CO2 emissions associated with waste incineration, cost-effective capture solutions are required. Nevertheless, most capture processes entail high economic penalties, making them unprofitable under current carbon taxes. In this work, we compare two concepts for capturing CO2 emissions using carbonate looping (also known as calcium looping, CaL) technology. One concept involves retrofitting the capture facility at the back end. The other is a novel integration concept that uses pretreated waste to fire the calciner, replacing one incineration line. The CaL concepts are analyzed for retrofitting a German WtE plant, which treats 200 kt of waste per year. We performed a techno-economic assessment that includes process modeling using the software Aspen Plus. The process simulations were supported using reactor models validated with pilot plant data. The calculated mass and energy balances were used to dimension components and calculate economic indicators, including a sensitivity analysis. We obtained CO2 avoidance costs (CAC) of ca. 140 €/tCO2,av for the tail-end concept, in agreement with previous studies. On the other hand, the integrated concept has CAC of 27 €/tCO2,av, including compression but excluding transport and storage, making it competitive with the current price of CO2 certificates in the European Union. To the best of our knowledge, this is the lowest value reported in the scientific literature for CO2 capture from waste incineration plants to date.

The chemical looping gasification process is an efficient technology for chemical recycling and bioenergy. While the process has been tested in lab and pilot scale, a scale-up to demonstration or industrial scale has not yet been attempted. A potential tool to assist the scale-up is coupled computational fluid dynamics (CFD) and discrete element method (DEM) simulation. However, with conventional simulation techniques the required simulation time is unfeasibly long. In this work the simulation was accelerated by a hybrid approach using both, graphics processing unit (GPU) and central processing unit (CPU) achieving a speed-up of 80 times. An analysis of the main influences on the simulation time was conducted to improve the simulation efficiency. With these results an upscaled simulation of a 200 MWth plant was performed achieving plausible results in a reasonable time. Thus, this work provides a proof of concept for CFD-DEM simulation of fluidized bed gasification at industrial scale.

Corrosion monitoring in industrial boilers is typically performed using offline techniques, such as coupons or ultrasonic testing, to assess component lifetime. These techniques lack temporal resolution, hindering their ability to detect dynamic influences on corrosion attack, such as fuel inhomogeneity, load changes, or unfavourable operational parameters. Electrochemical online monitoring has been successfully implemented in the chemical industry, yet studies in power plants remain limited to short tests or laboratory-scale parameter investigations. This work aims to highlight the added value of online monitoring and evaluate the ability to record reliable data in industrial environments. Additionally, the study presents an approach to quantify sensor data and enable operators to derive actionable recommendations. Twelve sensors were strategically positioned around burners in the membrane wall and monitored for three years to investigate spatial distribution and temperature influence on corrosion attack. Sampling of near-wall gas atmosphere during partial and full-load conditions facilitated research into gas phase influence. Multiple analytical approaches validated sensor data: Temperature measurements demonstrated correlation between deposit accumulation and corrosion signals. Event-based analysis revealed intense corrosion during plant shutdowns across all sensor positions. Statistical evaluation established correlation between mill utilization and corrosion intensity. Gravimetric quantification methods, expressing results in mm/1000 h, enhanced interpretability and chemical analysis of deposits provided insight into dominant corrosion mechanisms. This study offers significant insight to operators as well as researchers, enabling them to evaluate the benefits of online monitoring systems and to compare laboratory experiments with industrial results.

This study presents experimental results on co-combustion of hard coal and solid recovered fuel (SRF) conducted in a circulating fluidized bed (CFB) reactor with an internal diameter of 0.59 m, a height of 8.6 m, and a thermal load of 1 MWth. The high flexibility of the test facility enables testing of a variety of input materials under changing test conditions. The combustion of various fuel mixes ranging from 100 % hard coal to 100 % waste was tested to assess the impact of varying fuel ratios on the overall combustion process. As the proportion of SRF increased, a reduction in reactor differential pressure and particle density was observed. Moreover, combustion shifts to higher parts in the reactor when the share of SRF is increased. To further investigate the combustion conditions inside the reactor, three in-bed gas measurements were conducted, capturing the horizontal gas profiles of CO2, O2, and CO. An increase in SRF content led to elevated CO and O2 levels, alongside a reduction in CO2, indicating an upward shift in the combustion zone. The behavior is attributed to the higher volatile content of SRF, which is in this experiment roughly 2.5 times the share of volatiles in hard coal and the lower bulk density of SRF. The study concludes by proposing potential strategies for reducing the impact of high SRF content on combustion conditions.

Carbonate looping (CaL) is a promising technology for CO2 capture from carbon-intensive processes. The performance of CaL strongly depends on sorbent kinetics and carrying capacity. In this work, we present a novel particle sub-model to predict the carbonation behavior of CaO for CO2 capture in a CaL process. The sub-model deals with sorbent deactivation and reaction kinetics. We follow an original approach to non-ideal calcination and introduce a new calculation methodology for sorbent aging that considers fluctuations in sorbent circulation and make-up rates. The sub-model presented in this article is used in conjunction with the reactor model of Part I [1] to predict the CO2 capture efficiency of a CaL pilot plant. The results of the particle sub-model were validated with experimental data from thermogravimetric analysis (TGA) and pilot tests at the 300-kWth scale. Our study provides new insights for more accurately selecting assumptions and better modeling sorbent behavior in CaL systems.

The transition from coal-based power generation to carbon-neutral alternatives remains a critical challenge in mitigating climate change. Circulating Fluidized Bed (CFB) boilers offer fuel flexibility, enabling the integration of more environmentally friendly biogenic or waste-derived fuels such as Solid Recovered Fuel (SRF). However, replacing conventional fuels with high-volatile alternatives poses challenges related to combustion stability and efficiency. Oxygen Carrier Aided Combustion (OCAC) with ilmenite as a bed material enhances combustion efficiency and reduces emissions by facilitating oxygen transport within the fluidized bed. Additionally, oxyfuel combustion offers a promising pathway for carbon capture but is hindered by high oxygen demand. This study combines OCAC and oxyfuel combustion, presenting the first autothermal Oxyfuel-OCAC (Oxy-OCAC) experiments conducted at the 1 MWth scale, utilizing 100 % SRF as feedstock. The pilot plant enables oxyfuel operation with wet flue gas recirculation and pure oxygen supply, allowing a controlled transition from air-fired to oxyfuel conditions in 16 min. Differential pressure profiles revealed increasing particle loads in the free board zone with increasing inlet oxygen concentration, leading to a more uniform temperature distribution throughout the CFB reactor. Flue gas analysis confirmed that Oxy-OCAC improves combustion stability compared to oxyfuel combustion with sand as bed material, enhancing oxygen distribution within the reactor. These findings demonstrate that Oxy-OCAC is a promising approach to increasing the efficiency and economic viability of oxyfuel combustion in CFB systems. The combination of ilmenite with SRF in an oxyfuel environment enhances CO₂ capture potential while ensuring stable reactor operation, supporting sustainable energy production.

Oxy-fuel combustion is a promising way to avoid process-based CO2 emissions. In this paper, the operational range of a new semi-industrial oxy-fuel combustion chamber for pulverized biomass is analyzed. This approach is used to gain a deeper understanding of the combustion setup and to examine the differences between air and oxy-fuel combustion on an industrial scale. Both analyzed parameters—flame spread and temperature distribution—have a significant influence on heat transfer in commercial boilers. The stability of various operating conditions is assessed by monitoring the CO content in the flue gas via a gas analyzer unit. For stable operation using walnut shells as fuel in an air atmosphere, an overall air-to-fuel ratio of 1.57–1.75 and a local air-to-fuel ratio of 0.75–0.95 provide the most stable conditions. A high swirl number of 0.9 is found to be critical for stability, as the increased fuel momentum entering the combustion chamber promotes a fuel jet-dominated swirl flame. For the corresponding oxy-fuel combustion with the same volume flows and three different oxygen concentrations between 27% and 33%, stable combustion behavior is also observed. Using a camera setup to analyze flame shape and spread, it is observed that the flame formed with an oxygen content of 33% most closely resembles the flame shape achieved under air combustion conditions. However, the combustion temperatures most closely match those of the air operating condition when the oxygen content is 27%. Overall, it is shown that the approach for corresponding oxy-fuel conditions features similar flame shapes to oxy-fuel combustion with flue gas recirculation in a semi-industrial combustion chamber.

Carbonate looping (CaL) is a CO2 capture technology with the potential to efficiently decarbonize power plants and carbon-intensive industries, such as cement and lime production. Many pilot tests have demonstrated the feasibility of operating CaL in oxy-fuel and indirect-heating (IHCaL) modes. Still, there is no commercial facility in operation or planning. To support the scale-up of the technology, reliable reactor models are required. However, little progress has been made in calciner modeling in recent years. The available models are either too demanding in terms of computation complexity or lack support from empirical data. In this work, we develop a novel calciner model by combining a particle sub-model with a one-dimensional reactor sub-model. The model is validated with results from experiments in two different pilot plants in the 300-kWth and 1-MWth scales. The predictions of the model are interpreted using stochastic methods and dimensionless numbers. Furthermore, we introduce a three-step approach to designing calciners for CO2 capture. Calciners with oxy-fuel combustion should be operated at 930-965 degrees C to achieve sufficient sorbent regeneration. For indirectly heated calciners, an operating temperature of 950 degrees C is necessary for high performance, but lower temperatures (e.g., 900 degrees C) are also possible using steam for fluidization. Considerations regarding particle residence time are also discussed. Our guidelines are straightforward and enable the design of a calciner with simple calculations.

The study investigates biomass chemical looping gasification (BCLG) using nickel smelter slag as an oxygen carrier (OC). Key operating parameters, including reactor temperatures (800-900 °C), OC-to-biomass ratio (OCBR, 4:1-15:1), and steam as a gasification medium, were evaluated in a 5kWth fluidized bed reactor using pine forest residue. Performance metrics including gas composition and process efficiencies were assessed. OCs were characterized using XRD, BET and SEM-EDS analyses. Optimal performance was achieved at 850 °C, an OCBR (10:1) and a steam-to-biomass ratio (1.4). The gas composition was 38.87 vol% H2, 19.65 vol% CO, 34.48 vol% CO2, and 6.61 vol% CH4, with a product gas yield of 1.24Nm3/kg-biomass. Carbon conversion efficiency was 77.85 %, cold gas efficiency 58.70 %, and levelized cost of fuel was 0.15 €/Nm3 for product gas and 4.55 €/kg for H2.The results suggest that steam addition significantly enhanced char conversion, improving overall BCLG efficiency. Moreover, nickel smelter slag demonstrated stability, consistent reactivity, and limited sintering behavior.

Oxygen Carrier-Aided Combustion (OCAC) has been extensively studied for biomass and coal combustion, yet its application to Solid Recovered Fuel (SRF) remains underexplored. This study investigates the use of ilmenite as an oxygen carrier in a 1 MWth circulating fluidized bed (CFB) pilot plant, systematically analyzing SRF-Coal cofiring at 60 %, 70 %, and 100 % SRF (by heating value). The stepwise increase in SRF share is particularly relevant for retrofitting coal-fired CFB boilers. A direct comparison between ilmenite and quartz sand as bed materials evaluates ilmenite's influence on exhaust gas composition. Additionally, four in-bed gas measurements (IBGMs) were conducted at a height of 4.2 m for 70 % SRF and 100 % SRF, providing unique insights into combustion characteristics within the reactor - critical for operational optimization but rarely reported in the literature. The study highlights key SRF-specific challenges - such as heterogeneity, high volatile content, and contaminant presence - contrasting them with biomass to emphasize the study's importance. The results demonstrate that ilmenite enhances combustion efficiency by transporting oxygen to fuel-rich zones, particularly in areas near the fuel feed, mitigating oxygen deficiencies and improving the oxidation of volatile components. Compared to quartz sand, ilmenite led to significant reductions in CO (up to 69 %), CH4 (up to 95 %), and NO (up to 7 %). Furthermore, findings suggest that OCAC with ilmenite enables more uniform combustion, offering promising pathways for retrofitting coal-fired plants with high SRF shares while maintaining low emissions and stable operation.

The indirectly heated carbonate looping process (IHCaL) is a promising technology for decarbonizing the lime and cement industry. Advantages of the IHCaL are the synergy with these industries using same solid materials and the avoidance of an air separation unit (ASU), since no technical pure oxygen is necessary in this capture process. Former pilot tests showed the feasibility of the IHCaL for applications in the power plant sector. However, the integration of the IHCaL into cement and lime plants, as well as the usability of spent sorbents as educts in these processes, has not yet been proven in industrially relevant conditions. In this study, the modification of an existing 300 kWth pilot plant for demonstrating the IHCaL process are described, aiming in accelerating the demonstration in an industrially relevant environment for cement and lime industries. A flue gas circulation system and a solid fueling system into the combustor, allowing the heat generation for the calcination with e.g. waste derived fuels, have been designed and installed. Results gained during the operation with the modified reactor configuration, are presented in order to assess the performance of the new components in the framework of the IHCaL configuration. The external combustor was operated while co-firing propane with either dried lignite or waste derived fuels. The carbonator was fluidized with a real flue gas from the external combustor, having CO2 concentrations between 11 and 18 vol-%dry. The results, obtained within more than 200 h of continuous testing with modified reactor configuration, show a good hydrodynamic behavior and adequate reactor performance, while providing a sound basis for further improvement and up scaling of the technology.

Carbonate looping (CaL) technology has the potential to efficiently capture CO2 from power plants and carbon-intensive industries such as cement production. Its feasibility has been demonstrated in numerous pilot campaigns, including tests validating advanced features such as indirectly heating the sorbent regenerator. However, some issues, such as the role of reactor hydrodynamics, need to be discussed more thoroughly to reliably scale up CaL plants for commercial operation. In this work, we present a novel carbonator model based on a systematic literature review and experimental data from pilot tests, including a rigorous analysis of carbonator fluidization regimes. We highlight that modeling assumptions commonly found in the literature can result in a significant overestimation of carbonator performance. Additionally, we offer guidelines for the appropriate selection of these assumptions. Our carbonator model provides an effective tool for the next scale-up step of CaL technology.

The release behavior of sulfur (S), chlorine (Cl), and nitrogen (N) compounds contained in torrefied poplar (TP) is investigated using an entrained flow reactor. Due to the binding forms of S, Cl, and N in biomass and the high volatile content, pollutants are released during the first stage of combustion, devolatilization. Detailed knowledge of this process is essential for understanding the release and formation of pollutants. In this study, the release is investigated in a nitrogen (N-2) and carbon dioxide (CO2) atmosphere to investigate the influence of a CO2-rich environment on pollutant release. The experiments are carried out at three different temperatures (T = 900, 1000 and 1100 degree celsius). The results show that the atmosphere and temperature have a significant influence on the formation of pollutants. Corrosive gases can be expected at low temperatures and corrosive deposits can be expected at high temperatures in both atmospheres. An increasing temperature reduces the formation of sulfur dioxide (SO2) and carbonyl sulfide (COS) at a residence time of 0.5 s in both atmospheres. In N-2 atmosphere, no SO2 and COS are detected at the temperature of 1100 degree celsius. However, both corrosive species can be expected at 1100 degree celsius in CO2 atmosphere. A decreasing formation of hydrogen chloride (HCl) with increasing temperature is observed in both atmospheres and residence times (0.22 and 0.5 s). In CO2 atmosphere, less HCl is formed at temperatures of 900 and 1000 degree celsius as in N-2 atmosphere. The nitrogen contained in the fuel is predominantly released as hydrogen cyanide (HCN) and nitric oxide (NO). A high temperature favors the formation of HCN in a N-2 atmosphere. The NO formation decreases with increasing temperature. Comparatively less HCN and NO are formed in a CO2 atmosphere. The nitrous oxide (N2O) concentration decreases with increasing temperature at a residence time of 0.5 s in a CO2 atmosphere, while the N2O concentration increases in a N-2 atmosphere. In the CO2 atmosphere, a lower overall formation of nitrogen species is observed.

Oxyfuel is a combustion technology where the oxidant consists mainly of oxygen and carbon dioxide instead of oxygen and nitrogen. Since carbon dioxide has strongly absorbing bands in the thermal spectrum, the radiation properties of the flame change in an oxyfuel atmosphere compared to conventional combustion. When retrofitting an existing air-fired combustion system to an oxyfuel process, the oxygen content in the oxidant must be adjusted so that similar values for heat transfer by radiation are achieved. This measure allows the system to be operated with otherwise unchanged parameters. In this work, the thermal radiation of natural gas, pulverised walnut shells and lignite under an air and oxyfuel atmosphere is investigated in a semi-industrial combustion chamber with water-cooled membrane walls, at different oxygen concentrations and combustion parameters. While the radiative heat fluxes for natural gas with an oxygen content of 28 vol% in the oxidant are significantly higher than those for firing with air, the values for lignite are still below the air-firing, even with an oxygen content of 30 vol%. For walnut shells, the oxyfuel results are close to the air case for all oxygen concentrations between 27 and 33 vol%. The walnut shells show higher radiative emissions than the lignite at the same thermal output. For non-swirled flames, the radiative heat flux is lower than for swirled flames.

Chemical looping gasification (CLG) is a novel dual fluidized bed gasification process that enables the conversion of solid feedstocks to a nitrogen-free syngas through in situ air separation, avoiding a costly air separation unit. While there have been recent advances in experimental studies, modeling of CLG is almost exclusively restricted to lab-scale units or 1D models. In this study, a 3D CFD-DEM model of a 1 MWth fuel reactor for the conversion of solid biomass was developed. Due to the high computational demand of the DEM method, a coarse-grained approach was used in combination with a simplified reaction network. The hydrodynamics were modeled with an EMMS drag model. Simulations were conducted for two woody biomasses and wheat straw based on experimental data of a 1 MWth CLG reactor. The model was able to predict the pressure profile over the reactor accurately, with a mean error below 10%. Carbon conversion and oxygen carrier oxidation were in good agreement with the experimental data with mean deviations below 5%, while reasonable values below 8 mol % mean error were achieved for the gas composition. Discrepancies in the gas composition as well as temperature profile indicate that further work is needed in the pyrolysis step of the model.

The index of refraction (IOR) is required to model thermal radiation interaction with pulverized solid fuels. In this work, the complex index of refraction of biomass (walnut shell) is therefore determined using pulverized particles. Single particles are irradiated, and the scattered radiation is measured in different directions. To avoid falsification of the scattering pattern (phase function), the particles are kept contactless in an acoustic levitator. Here, over 1000 different phase functions are measured. The measured scattering patterns are evaluated using an inverse evaluation procedure to determine the IOR. Mie theory serves as the basis for the mathematical modeling of the radiation properties of the particles. The measured IOR is then compared to data from the literature on coal. For the wavelength range lambda=2000-4000nm no distinct differences are noticed between the coal and biomass IOR. For lambda>4000nm the real part of the biomass IOR is larger and the differences increase with increasing wavelength. However, the order of magnitude still matches that of coal IOR, and thus, only minor differences in the radiative properties of coal and biomass are expected.

Power-to-Ammonia (P2A) is a promising technology that can provide a low-emission energy carrier for long-term storage. This study presents an optimization approach to a novel small-scale containerized P2A concept commissioned in 2024. A dynamic nonlinear optimization problem of the P2A concept is set up, employing the non-commercial MOSAIC® software V3.0.1 in combination with the NEOS® server. In total, seven optimization solvers, ANTIGONE®, CONOPT®, IPOPT®, KNITRO®, MINOS®, PATHNLP®, and SNOPT®, are used. The first and main part of this work optimizes several disturbance scenarios of the concept and aims to determine the optimal reactor temperature profile to counter the disturbances. The optimization results suggest, for example, lowering the reactor temperature profile if the hydrogen and nitrogen inlet streams into the system decrease. The second part of this work presents a crude dynamic optimal scheduling model. This part aims to determine the amount of ammonia to be produced and sold given a randomized price of electricity for three consecutive points in time. The optimization results recommend decreasing production when the price of electricity is high and vice versa. However, the dynamic model must be improved to include fluctuations in the price of ammonia. Then, it can be used as a real-time optimization tool.

On the way to climate-neutrality with low resource consumption, the economics worldwide needs ways to obtain high-quality products from existing material flows. If waste is used as a feedstock for gasification, high-quality syngas is obtained from which new products for the chemical industry (e.g. methanol) or the transport sector (Fischer-Tropsch products) can be obtained. The fluidized bed-based High-Temperature-Winkler process is particularly suitable for the gasification of solid recovered fuel (SRF) and biomass that are difficult to grind since the feedstock can be processed in lump form. The gasification characteristics of SRF pellets were investigated in a 500 kWth pilot gasifier at the Technical University of Darmstadt. After a co-gasification with pre-dried lignite, a mono SRF gasification could be realized. This paper presents the effects of operating temperature and the SRF content in the feedstock on the syngas quality, which is characterized by the target components CO and H2, the undesired fraction of CH4, and some representative higher hydrocarbons. It could be shown, that the syngas gas components CO and H2 depend mainly on the gasification temperature and not on the used feedstock. On the other hand, the higher amount of volatiles in the SRF fraction rises the methane concentration and the amount of higher hydrocarbons in the syngas. It was observed that these components are decomposed at higher gasification temperatures and an almost tar-free syngas is produced.

Lime is an essential raw material for iron and steel production, in construction and agriculture, in civil engineering, in environmental protection, and in manifold chemical manufacturing processes. To address the problem of unavoidable process CO 2 emissions associated with the production of lime, efficient capture technologies need to be developed and implemented. The indirectly heated carbonate looping (IHCaL) process is an efficient candidate for this application because it utilizes lime as the sorbent for the CO 2 capture. In this work, a retrofit configuration of this process is presented and analyzed for net negative CO 2 emissions. This is done considering different fuels that provide the heat required for the regeneration of the sorbent. The different scenarios were simulated with an AspenPlus® model, key performance indicators were calculated, and the process was compared with other post-combustion capture methods. The results show that net negative CO 2 emissions as high as −1805 kg CO2 /t CaO , calculated with a state-of-the-art coal power plant energy scenario ( η e = 44.2 %; e ref,el = 770 kg CO2 /MWh el ), can be obtained. This represents an equivalent CO 2 avoidance of more than 230% with respect to the reference plant without capture (1368 kg CO2 /t CaO ). A specific primary energy consumption for CO 2 avoided ( SPECCA ) lower than 1.5 MJ LHV /kg CO2,av was achieved for the same energy scenario. Particularly promising results can be accomplished when applying fuels with high biogenic fraction and low specific CO 2 emissions, such as solid recovered fuels (SRFs) with a high calorific value.