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The banded approach or box model is a method to include the non-grayness of combustion gases in radiation heat transfer calculations. However, the determination of the correct limits for the bands and the effective band absorption coefficients is still something of a black art. In this study, the line by line (LBL) spectral absorption coefficient profile has been implemented to obtain the effective number of bands, and bands’ limits for pure H2O, pure CO2 and a H2O-CO2 gas mixture. A mathematical technique has been used to smooth the LBL profiles of pure gases in atmospheric pressure in order to be used for identifying the gray bands. The optimization for selecting the bands is done by analyzing the radiative heat transfer in several one-dimensional benchmarks. After obtaining the optimal band dividing scheme, a set of correlations for the pressure based gray band absorption coefficient of pure gases is found by integrating the line by line spectral absorption coefficient weighted by the corresponding black body intensity along the bands. In contrary to the previous similar works, by using the LBL data for pressure based gray absorption coefficient of the bands, the current correlations are independent of gas concentration and path length. The present approach could successfully support the non-gray walls. The method has been validated using several benchmarks and exhibited comparable accuracy with other available models.

Sorption enhanced gasification (SEG) is a promising technology for producing gas derived from renewable feedstock to be used in biofuel synthesis processes. As a response to the growing need for renewable fuels, an SEG reactor design was developed for industrial-scale dimethyl ether (DME) production. A 100 MW th scale SEG reactor concept for wood pellets as a feedstock was created by a model-based approach. Thus, a 1D modeling tool for the coupled circulating fluidized beds was developed. The model was used to investigate the dual fluidized bed system’s operation in the gasifier temperature range of 730–790°C. In this range, the optimal producer gas composition without external hydrogen for the downstream DME synthesis was achieved at gasifier temperature 730°C: 63 % v o l , d b H 2 , 11 % v o l , d b CO , 13 % v o l , d b CO 2 . The model prediction was successfully compared against experimental data and modeling results from the literature. The developed 1D model enables the investigation of the composition and yield of the producer gas with different operating parameters, such as the part-load operation. This advanced capability can be used to develop new control strategies for the SEG system and investigate the impact of various operating parameters on the producer gas composition and yield.

Sewage sludge, a by-product of the wastewater treatment process, should not be considered as a waste, but as a potential resource for renewable energy and nutrient recovery. However, sewage sludge contains harmful contaminants (e.g. pathogens, pharmaceutical residuals, and microplastics), and if the sludge – composted or digested – is utilized directly in agriculture, these contaminants may leak into the environment. To tackle this issue, an ad-vanced combustion scheme is proposed. In this process, mechanically dewatered sludge (20–25 wt% dry solids con-tent) from a nearby wastewater treatment plant is first dried at 110 °C in a circulating mass dryer and then combusted at 850 °C in a circulating mass reactor. Thereby, all the unwanted organic compounds are eliminated, and the ash – relatively high in nutrients and sufficiently low in heavy metals – can be further processed into fertilizers and used in forestry or farming. The process is self-sufficient in terms of energy; the excess heat extracted from the process can be utilized, e.g., as district heat. This paper describes the process in detail and reviews initial experiences from the first commercial plant in Rovaniemi, Finland. Also, the potential of the technology for efficient and sustainable han-dling of municipal sewage sludge is discussed., Proceedings of the 29th European Biomass Conference and Exhibition, 26-29 April 2021, Online, pp. 113-118

Sorption-enhanced gasification (SEG) is a promising technology for producing renewable feedstock gas to be used in biofuel synthesis processes, especially in dimethyl ether (DME) synthesis. To adopt the technology on a commercial scale, it is necessary to acquire knowledge about the related operational characteristics. The SEG process is carried out at lower temperatures than those employed in conventional gasifiers. A typical operating range is from 600 ◦C to 800 ◦C. Fuel decomposition experiments have shown distribution of the decomposition products to vary by the process temperature in this operating range, and thus, it is important to adapt this phenomenon for modelling the SEG process. To model the temperature dependence of the decomposition products, a fuel model was developed. Fuel decomposition experiments were conducted to obtain the boundary conditions for the fuel model. The developed fuel model was implemented to an SEG model frame, and the model prediction was compared against data from a 200 kWth dual fluidised bed facility. The model gave satisfactory predictions for producer composition and temperature trends. Furthermore, the main balances of the model were in agreement with typical trends of the SEG process. The conducted simulations improved our understanding of material balances in SEG reactors. Knowledge from physical operations governing the process is of value in further development of the technology.

Gas-solid fluidized beds have heterogeneous flow structures, which affect the reactions due to uneven distribution and limited mixing of reactants. Some model approaches neglect these heterogeneous structures and use a homogeneous assumption instead. This work demonstrates the effects of neglecting this heterogeneity. Eulerian multiphase modeling is utilized to model fuel conversion in a fluidized bed riser. The effects of the chemical reactivity of the fuel, the amount of fuel in the reactor and the fluidization regime are investigated. The model data are time- and space-averaged into 1D results and compared with a 1D modelling which assumes homogeneous distributions in radial direction. Based on this study, the negligence of the heterogeneous flow structures leads to significantly higher local and overall reaction rates and conversion in the system. © 2020 Elsevier Ltd

Bioenergy with CO2 capture and storage (BECCS) is a promising negative emission technology (NET). When using sustainably produced biomass as fuel, BECCS allows the production of power and heat with negative CO2 emissions. The main technical challenges hindering the deployment of BECCS technologies include energy penalties associated with the capture process. This work evaluates the performance of an advanced CO2 capture technology, chemical looping with oxygen uncoupling (CLOU), in conjunction with biomass-fired combined heat and power (CHP) generation. Results from a MATLAB/Simulink reactor model were incorporated in a plant and integration model developed in a commercial process simulation software to quantify the key performance indicators of the CLOU-integrated CHP plant. Both energy and exergy analysis were conducted. The results show a remarkably low efficiency penalty of 0.7% compared to a conventional reference plant, and a high carbon capture efficiency of 97%. The low efficiency penalty is due to the high moisture and hydrogen contents of the biomass, and the separation of combustion products and excess air streams in the CLOU process, these together provide an opportunity to recover a significant amount of heat by flue gas condensation at a higher temperature level than what is possible in a conventional boiler. The condensing heat recovery yields an 18 MW generator power increase (3 MW loss in net power output) for the CLOU plant, in the reference plant with conventional boiler, the same scheme could achieve an increase of 9 MW (generator) and a decrease of 8 MW (net).

The hydrodynamic characteristics of gas and solid flows in circulating fluidized beds (CFB) can also influence other processes. To investigate various features of flows and mass circulation in CFB, a computational study was performed with the support of experiments measured in the riser section of a lab-scale apparatus of CFB. The fluidization velocity was slightly below the terminal velocity to avoid a major solid entrainment. The Gidaspow's model was used to calculate the gas-solid drag term in computational cells, which led to a good agreement with the experimental data of pressure profile. The gas inlet conditions were shown to be uninfluential in the hydrodynamics of major part of the upper riser, and the solid inlet (return) imposed non-uniformity in radial and tangential directions. The time-averaged profiles presented a good indication of the wall layer formation. The rate of circulating mass in the riser from simulations was obtained 25% lower than the experiments. The coefficient of restitution was a parameter with notable influence on the hydrodynamics and the present study suggests that a variable coefficient of restitution in Eulerian models can describe the behavior of CFB system more accurately.

The study analyzes a potential to improve the local waste and wastewater management in a Finnish community. The Lappeenranta wastewater treatment plant (WWTP) is used as a case study. Two different technological setups are considered for improving the wastewater treatment plant. These are used to construct four alternative wastewater reclamation scenarios. The mass and energy balances for the considered scenarios are developed and used as input for the profitability evaluation. The utilization of the sewage sludge from the WWTP and municipal solid waste fraction that cannot be recycled for the generation of heat and electricity at the CHP plant is investigated with the aim to improve the economic performance of the wastewater treatment facility. The studied scenarios are initially compared based on their investment and operational costs. The cost of water treatment increases significantly in the case of the investigated tertiary treatment systems: higher amounts of chemicals and electricity are needed to improve the water quality. At the same time, the study indicates that the profitability of a WWTP integrated with a CHP plant can be reasonably high in a wide range of likely price scenarios for alternative wastewater purification systems. The results of the analysis showed a significant potential for the investigated wastewater reclamation systems to improve the efficiency of solid waste and wastewater management in the community.

The primary goal when designing a CFB boiler is to ensure its high energy efficiency. Additional constraints come from environmental regulations, which are constantly becoming more difficult to assess, and require improvements in the boiler design process. Local on-site measurements are often restricted to short distances inward from the furnace wall, leaving most of the core unmeasured. Computer simulations are practically the only feasible tools to investigate the combustion processes and to support the design of CFB units. Three-dimensional steady-state semi-empirical CFB furnace models predict adequate results within reasonable times but these models require tuning to existing objects, which obviously excludes their application for new object design. The present study represents a step toward using the hybrid Euler–Lagrange (HEL) technique to model flow variables in order to replace the measured data. The information collected using the HEL model for an arbitrary furnace can be used to tune a semi-empirical model. To validate the HEL model, measurements of a 235 MWe CFB were used. The simulations show good agreement between the numerical and experimental results, indicating that the presented idea is worth investigating in the future.

Power-to-gas technology can facilitate the transition toward a renewables-based energy system by converting excess electricity to hydrogen and then into methane via methanation. Unlike traditional chemical methanation, biological methanation uses an aqueous solution of biomass (archaea), which consumes H 2 and CO 2 to produce CH 4 . The process is limited primarily by the gas–liquid mass transfer step. In addition to experimental research, modeling is often used to guide and expedite the development and scale-up of bioreactors from the laboratory to the pilot and commercial scales. Modeling has been used to optimize and test various operation conditions outside the range of experimentation. Estimations of gas–liquid mass transfer and the related stirring power are important for optimization and feasibility studies in the application of biological methanation to power-to-gas systems. Related published literature, however, is limited. In this study, a dynamic model for a continuously stirred biomethanation reactor was developed with novel approach that combines semi-fundamental modeling of gas–liquid mass transfer, hydrodynamics, and biological reactions. The model was validated against existing experimental data and used in a sensitivity analysis of critical parameters, a scale-up study of a biomethanation reactor, and process dynamics studies. In each of the varying operational conditions, the model reproduced the trends observed in the experimental studies. The sensitivity analysis showed that biological parameters have a minimal effect on methane production. Conversely, the model is very sensitive to the gas–liquid mass transfer properties, such as the geometry of the impeller and reactor. The scaled-up study of biomethanation reactors with a CH 4 production capacity of 56–508 Nm 3 /h revealed that the required stirring power is 0.7–1.1% from the electrolyzer power and decreases as the size of the reactor increases. High output quality (∼98%) of the methane could be reached in each of the studied cases, and the overall efficiency of the power-to-methane process was roughly 50%. Dynamic simulations showed that the modeled process is tolerant to large gradients in the input parameters. After correctly setting the reactor- and process-specific parameters, the model can be used to perform scaled-up and dynamic studies of various reactor designs and different biomass solutions.

Fluidized beds are common in the energy and chemical industries because they provide a good environment for heterogeneous reactions owing to the combination of mixing and multiphase flow hydrodynamics. The hydrodynamics of gas–solid suspensions in fluidized beds is extremely complicated and requires accurate study. A wide range of simulation approaches have been used in fluidized bed design and operational studies, but it is still not clear how the phase interaction should be treated. In this study, a practical characterization method and modeling tool for gas–solid flows are presented, along with a definition of the phase interaction. In this method, the phase interaction is derived for a real solid material with specific particle size, shape and density distributions. In addition, a macroscopic 1D modeling tool for gas–solid flow that is capable of practical stationary and transient simulations is presented. This tool solves the governing equation for the solid-phase momentum and provides physical predictions for transient gas–solid flow investigations. The proposed characterization method and modeling tool are validated against experimental data. Post-print / Final draft

Synthetic hydrocarbons can be produced sustainably with power-to-gas processes, resulting in a net reduction of greenhouse gas emissions due to the substitution of conventional natural gas and other fossil fuels with carbon-neutral alternatives. Acquisition of the CO 2 for the synthetic fuel production can be implemented in multiple ways. This work introduces a node-based model to assess different implementation strategies of CO 2 utilization systems, taking into account temporal effects, regional variation, and economies of scale for CO 2 capture. Intermediate storage volumes, capture costs, transport quantities, and other relevant infrastructural aspects of the CCU system can be estimated with the model. Finland is used as a case study, focusing specifically on the national and regional scale. CO 2 capture costs are significant, being nearly four times larger than the cost of storage in the baseline scenario (354 M€, 85 M€). CO 2 sources with smaller annual emissions increases capture costs by 14% compared to baseline. This increase in cost is comparable to the cost of transporting over a quarter of all captured CO 2 to off-site processing (varying distance, 100–400 km). Seasonal storage of CO 2 is found to be beneficial for the cost-efficient production of synthetic fuels, owing to the temporal disparity between CO 2 emissions and utilization, as well as the overall cost structure of the components. Five key decision categories are proposed for a carbon utilization system: scale, type, units, location, and technological decisions. These may be applied to describe any carbon utilization system, helping to form a more comprehensive picture of a future energy system, where carbon is widely used as a raw material.

Oxygen-fired combustion in circulating fluidized bed has been extensively studied during the last decade in different experimental scales from small laboratory devices to large pilot units. The development and scaling up of a new combustion technology require both experimental and modelling work. This paper summarizes the tests carried out at CIUDEN's 30 MWth oxy-CFB facility. The test data has been applied to validate a semi-empirical three-dimensional process model of the furnace. The 3D-model has been applied for predicting the performance of full scale oxy-CFB units: first a 300 MWe Flexi-Burn® CFB operated in air-fired and oxygen-fired modes and then a 600 MWe oxy-CFB with 40%-vol inlet oxygen concentration. Based on the experience of oxy-CFB tests at CIUDEN and model results of commercial scale units, there are no technological barriers which would prevent application of oxy-CFB based carbon capture and storage.

This work is concerned with the research field of ash deposition phenomena. An unsteady CFD model of the convective section of a 100-kW PC pilot-scale combustor is implemented aiming to simulate and study the ash deposition trends. Emphasis is placed at analyzing how particles of different sizes may contribute differently to the deposit buildup. A discrete particle tracking and a rebound–stick submodel are used. Four simulation cases are implemented to investigate the effect of the inlet flow velocity and the fouling conditions (clean vs. fouled tubes). The sticking efficiencies decreased with the particle diameter d p after being constant and higher than 95% for d p 1 μ m. It also seemed to decrease with the flue gas inlet velocity. The normalized particle arrival rates to the tubes were rather constant up to particle sizes of d p = 8 μ m. The studied cases of faster flow inlet velocity showed higher values for both the deposition and arrival trends as a consequence of a higher overall particle Stokes’ number and the total ash mass input to the model. The cases with fouled tubes presented higher deposition rates (approximately, 7.91% more for the fast velocity case) than what is observed for clean tubes.

Limestone is used widely in fluidized bed energy applications for sulphur capture. The conditions of the novel fluidized bed energy processes can differ from the conditions (i.e., temperature and gas concentrations) in conventional fluidized bed applications for energy production. The influence of H 2 O(g) on calcination and indirect sulphation was examined with one limestone type in a bench-scale reactor. A time-dependent multilayer particle model was used for analysing the experimental results. The studied atmosphere included 0%, 10% or 20% H 2 O(g) and two different CO 2 concentration levels (15% and 50%). The temperature level was the same in all tests (∼1188 K). The added H 2 O(g) increased the conversion degree compared to conditions without H 2 O(g) in all test conditions. The model was used to explicate the observed differences between test results with and without H 2 O(g) and determine the conversion curve, conversion profile and magnitude of reactions and diffusion as a function of radius and time. The results show that different sulphation patterns and conversion degrees can be explained with different limitations inside the particles in terms of time and in different conditions.

The buildup of ash deposits represents a major challenge in the operation of industrial boilers since it entails significant losses in heat transfer performance. Therefore, the research of predictive tools is of great value in boiler operation. A CFD model for ash deposition and fouling growth prediction of industrial furnaces is presented. Considerations are taken regarding grid resolution and accuracy requirements available in literature. A 2D transversally-periodic bundle of four in-line tubes of a kraft recovery boiler bank is modeled with the gas laden with discrete solid fume ash particles. A sticking-rebound submodel determines whether to account the mass of a hitting particle for the deposit growth or to make it rebound, re-entraining it back into the flow. A dynamic mesh modifies periodically the model grid, simulating the expansion of deposits. 0.7 μm diameter particles showed round and uniform deposits caused by thermophoresis. The deposit thickness grew up to about 3 mm after 100 min of fouling. 3.62 μm particle showed more irregular distributions with some high local peaks, but lower deposition rates in average. For this particular case modeled, the constant particle sticking probability approach showed no major differences with respect to the particle sticking submodel.

Sorption enhanced gasification (SEG) is a promising technology for production of a renewable feedstock gas for biofuel synthesis processes. The technology has been previously demonstrated at pilot scale. Scaling of the technology to an industrial size requires knowledge from governing phenomena. One-dimensional bubbling fluidized bed (BFB) reactor model was developed and validated against experimental data from 200kWth dual fluidized bed facility. Sub-models for biomass gasification, reactive bed material and fluidized bed hydrodynamics were incorporated into the model frame. The model gave satisfactory predictions for bed material conversion, temperature profiles and gas composition of the producer gas. The conducted validation improved understanding of bed material conversion, water–gas shift reaction and hydrodynamics and their role in SEG reactors. Further refinement and comprehensive validation of the model with additional data from the pilot is required. The knowledge from the comprehensive validation can be utilized in simulation of an industrial size reactor. Post-print / Final draft

In modeling of large-scale circulating fluidized bed (CFB) furnaces, semi-empirical 3D process models are used in furnace analysis, design and optimization as their accuracy versus computational times make them more favorable than high resolution computational fluid dynamics (CFD) in many engineering type of studies. One of the key aspects in determining the furnace performance and efficiency is the correct modeling of the fuel flow as it determines where the thermochemical reactions take place in the furnace, affecting the temperature and gaseous species distributions. In modeling, this requires the suitable approximation of the momentum exchange between the fuel and the gas phase as well as the fuel and the bed material. To model the momentum exchange with current models presented in the literature, information on the material properties, namely the fuel particle size has to be given. The determination of particle size is affected by the particle shape and for irregular, non-spherical material, such as biomass, currently there are no readily applicable methods to determine the effective momentum exchange based on the particle geometry. In this work, a new gas–fuel momentum exchange model is developed for the fuel flow, which acknowledges the effect of physical properties of the fuel, namely the shape and size. Due to impracticality of determining a representative fuel particle size, characterization data is utilized to directly obtain information on the gas–fuel momentum exchange and the particles are classified based on their elutriation behavior, rather than particle size. The new momentum exchange model is utilized with an existing semi-empirical 3D process model and applied in simulation of a utility-scale CFB furnace firing biomass. The simulation results are compared with measurements and are found to be in good agreement, which demonstrates the applicability of the modeling approach. The presented method improves the modeling of fuel flow with CFB process models, especially with irregular, non-spherical fuels such as biomass. The improved modeling of fuel flow within large-scale CFB furnaces enables faster and more reliable design and analysis of CFB furnaces.

This work presents a CFD model of a transversally-periodic bundle of four in-line tubes of a kraft recovery boiler bank for ash deposition calculations. The flue gas was laden with discrete solid ash particles. Particle contact and sticking–rebound mechanics were used. The ash deposition parameters (arrival rate, sticking efficiency, deposit rates) were computed locally, studying the dependency on the particle diameter. Emphasis was put regarding grid resolution, unsteady flow solving, and the differences observed among different tubes and locations. Thermophoresis was responsible for 94.1% of the total deposition rates for submicron (0.7 μ m ) particles, becoming markedly less significant for coarser particles. These small particles showed a much higher sticking efficiency (above 95%) than other studied particles. The particles with the largest diameter studied (18.7 μ m ) had much higher arrival rates to the cold surfaces since they did not follow the flue gas flow paths (inertial impaction). Due to their low sticking efficiency (about 15%), they formed more irregular deposition distributions. Particles with an intermediate diameter (3.62 μ m ) showed the smallest arrival rates among the three particle sizes considered.

Ca-Looping technology has experienced a substantial development in the technical readiness level in the last years, especially in its application as post-combustion CO 2 capture technology in power plants. Experimental results from MW-scale power plants worldwide have confirmed post-combustion Ca-Looping process using interconnected circulating fluidised bed reactors as a promising technology for CO 2 capture. Among the different fields of research having contributed to this breakthrough, modelling activity aiming at assessing sorbent properties, interpreting results from experimental reactors or assessing technology scale-up through large-scale reactors and process integration have played a crucial role. This paper aims at reviewing and discussing findings obtained by different research groups worldwide about post-combustion Ca-Looping process modelling. Assumptions made with respect to sorbent performance, reactor operating conditions and process integration between different components are crucial when evaluating the performance of the Ca-Looping process as a post-combustion technology for CO 2 capture. With the aim of understanding the importance of these assumptions, this paper covers particle reaction and reactor models for carbonation and calcination steps, assessing the impact of the conditions used for their determination into their reactivity predictions, as well as process modelling works that assess performance obtained when integrating a Ca-Looping process into a power plant. Indications on the research needs detected among the reviewed works have also been highlighted in this work to contribute to the advancement of the knowledge on the Ca-Looping technology.

Bioenergy with CO2 capture and storage (BECCS) has been introduced as a promising negative emission technology (NET) that opens up the possibility of producing power and heat with negative CO2 emissions. By combining 1.5D reactor modelling with flowsheet simulation of a complete full-scale cogeneration plant, this study assesses the applicability and potential of an advanced CO2 capture technology, namely chemical looping with oxygen uncoupling (CLOU), for CO2 capture from a biomass-fired combined heat and power (CHP) plant generating electricity, district heat (DH) at 75–90 °C supply and 45 °C return temperatures, and process steam at 10 and 4.5 bar(a) pressures. Nordic wood (50% wet-basis moisture) is used as fuel. The key performance indicators of the CLOU-integrated CHP plant were quantified and compared with those of a non-CCS reference plant. Part-load operation at reduced DH loads was considered. At 100% fuel load, the CLOU plant captured 99.0% of the CO2 from the combustion of biomass and still achieved a net efficiency of 80.1%LHV, a value very close to that of the reference plant without CO2 capture or flue gas condensation (81.1%LHV). Depending on the fuel load, the specific negative CO2 emissions from the CLOU plant ranged from 439 to 504 kgCO2/MWh.

To develop an efficient and practical spectral radiation model for CFD simulations, a banded approach is proposed for a mixture of carbon dioxide and water vapor in varying thermodynamic states. Using a previously reported band dividing scheme, a statistical narrow band model is implemented to provide gray band absorption coefficient databases. The databases were then approximated by certain simple correlations, which can be readily used in CFD calculations of RTE solvers. The correlations were validated in several 1D and 3D benchmarks representing various combustion conditions. The accuracy and CPU cost of the proposed banded approach were studied and compared with those of other similar methods. The results demonstrated that the new approach is an efficient and accurate method that can be conveniently applied in a commercial CFD code for spectral radiation; moreover, it can handle non-gray walls. As a practical case study, the proposed approach was used to simulate radiative heat transfer within a back pass channel of a CFB boiler. The effect of combustion scenarios, i.e., air- and oxygen-fired, boiler load, inlet flow conditions, and wall material, was analyzed by the CFD model. The predictions of two different RTE solvers, i.e., P1 and DO, and the required CPU time were compared for gray and non-gray models. Post-print / Final draft

This article discusses the market potential estimation results at different stages of second-generation oxy-fuel technology for carbon capture and storage, where oxygen content is increased in order to increase efficiency and product life cycle. The article highlights the challenges to commercializing an energy technology with long development lead times. Challenges related to external market environment, regulations and market price for carbon dioxide (CO₂) also need to be resolved. The study is based on a second-generation oxy-CFB technology evaluation, SWOT analysis and product life-cycle estimation. The commercialization of second-generation oxy-fuel technology, estimated as a basis of oxy-CFB technology, is largely found to be dependent on technical, sociological and economical aspects related to the energy market environment. The CO₂ price in European Union (EU) emissions trading is currently below 10 €/ton, which does not favour investment in CCS. CO₂ storage also faces public opposition and EU directive limitations. The CO₂ price needs to be in the range of 30 €/ton by 2030, the estimated time of commercialization. Technological development or subsidies would also be required for the second-generation oxy-CFB power production concept to become viable. This is one of the critical factors in market deployment as the price development is influenced by several external factors, including the EU’s environmental policy, politics and public acceptance.

CFD tools are of great value in the design and operation of boilers. One particular aspect that can be modeled by CFD is the ash deposition and plugging of heat transfer surfaces of boilers. Fouling and slagging are the most typical causes of unscheduled boiler shutdowns. Consequently, appropriate predictions of deposit geometries and rates are of considerable interest. CFD multiphase flow simulations are capable of modeling particle-laden streams and constitute a suitable tool for study of material deposition. Fouling phenomena have a complicated and multidisciplinary nature involving thermo-fluid mechanics, sticking/rebounding of particles, sintering, etc. If the deposit growth rate has been accurately calculated by a multiphase model appropriate for particle-laden flows, CFD dynamic mesh techniques are able to move the interphase fluid-deposit according to the growth rate. This work develops and presents a CFD model for prediction of deposition shapes in a classical boiler bank of a Kraft Recovery Boiler by combining a multiphase and a dynamic mesh model. The effect of tube transversal spacing is also analyzed. The paper highlights the features and effects of the dynamic mesh model.

Oxyfuel combustion is one of the leading technologies considered for capturing CO2 from power plants with CCS. This involves the process of burning the fuel with nearly pure oxygen instead of air. In order to control the flame temperature, some part of the flue gas are recycled back into the furnace/boiler. Since the publication of the Special Report on CO2 Capture and Storage by the International Panel for Climate Change (IPCC, 2005), the development of oxyfuel combustion technology has progressed significantly and could be considered at par in terms of technology maturity as compared to other leading CO2 capture technologies. This paper presents an overview to the current state-of-the-art technology on the development of oxyfuel combustion applied to (a) PC and CFB coal fired power plants and (b) gas turbine based power plant. It should be noted that it is not the intention of this paper to provide a comprehensive review but to present what have been achieved in the past 10 years of RD&D efforts. For coal fired power plant using oxyfuel combustion, this paper primarily presents the different development aspects of the burners and boilers (combustion and heat transfer), emissions, operation of the plant (i.e. start-up and turndown) and its integration to the ASU and CPU. For gas turbine based power plant using oxyfuel combustion, the different GT cycles are described, looking at the different aspects in combustion, emissions, cycle efficiency and development of the turbomachineries. Also presented in this paper is a snapshot to what we have learned from the operation of the different large-scale pilot plants and development of large scale demonstration projects worldwide. The paper concludes by presenting the potential of this technology and highlighting the importance of realizing large scale demonstration plant as a necessary step to achieve its ultimate goal of technology commercialization.

Solid fuel combustion via chemical looping with oxygen uncoupling (CLOU) represents a novel route for CO 2 capture with minimal energy penalty. An essential part of the viability of a CLOU system is based on the behaviour of the fuel reactor. This paper presents a one-dimensional fluidized bed model capable of describing the operation of the fuel reactor involved in CLOU. The model considers physical phenomena relevant to CLOU, such as oxygen carrier decomposition, combustion of coal with molecular oxygen, fluidized bed hydrodynamics, and transfer of heat within the reactor. The performance of a CLOU fuel reactor fed with bituminous coal and a TiO 2 -supported CuO oxygen carrier was evaluated by means of modelling. A reference case was first defined and simulated, after which the effect of various process parameters on the results was assessed by parameter variation. For the reference case conditions with a temperature of 960 °C and solids inventory of 400 kg/MWf in the reactor and without utilization of a carbon separation system, a CO 2 capture efficiency of approximately 90% was predicted. A CO 2 capture efficiency of 95% could be reached by optimizing the operating conditions. It was found that when using low-reactive coals, a carbon stripper is required to obtain a feasible CO 2 capture rate. Overall, the model predictions appeared to be in agreement with results presented in the literature.

A Eulerian multiphase model was used to study the hydrodynamics of the gas–solids flow in a circulating fluidized bed (CFB) furnace. Alternative modeling approaches were tried to evaluate their effects on the predicted solids distribution in the furnace. Due to high computational costs, large scale calculations are commonly performed by using a coarse mesh, which leads to too uniform solids flow field and an overestimated gas–solid drag force. Thus, a suitable form of correction of the gas–solid drag is needed for coarse mesh simulations. This study presents fluid dynamics simulations of a 235 MWe CFB furnace by two sets of interphase momentum exchange coefficients: the model based on the homogeneous flow condition and the model based on the energy-minimization multi-scale (EMMS) approach. The calculations with one solid phase included studies with two mesh resolutions and different particle sizes. Additional simulations were performed using two solid phases and four solid phases to analyze the effect of polydispersity of particles. The model results were compared with the measured vertical pressure profile of the furnace and the locally measured solid concentration and velocity profiles. The results show that with a homogeneous model, the model matches better with measurements if the mesh resolution is finer. The EMMS approach can be used to improve the calculations in a coarse mesh if the average particle size can be determined correctly for the model. With multiple solid phases, the vertical and horizontal segregation could be simulated. However, increasing the number of solid phases did not change the total solid concentration and pressure profiles if the Sauter mean diameter of the total system was constant. The results indicate that with a coarse mesh, similar drag corrections would be necessary between the solid phases that have been developed for gas–solid drag.

A set of correlations for direct exchange area (DEA) between zones are presented. The correlations are simpler and much faster than the classical method used for DEA calculations in zone method. Additionally a unique form of correlation supports both singular and non-singular DEA calculation and no extra effort for non-singular cases is needed. Using the new correlations, the correlation based zone method (CBZM) is introduced and validated by several benchmarks. The CBZM results were in excellent agreement with the benchmark solutions. As an application case, by using the CZBM the gray and non-gray radiative heat transfer has been analyzed in a large back pass channel of a CFB boiler for the case of air and oxygen-fired combustion scenarios. The effect of the spectral radiative behavior of combustion gases on the predicted radiative heat fluxes on the walls is addressed. The effect of combustion scenario on the operation of the unit is also discussed.

Two-fluid Eulerian method is widely used in computer simulations of gas–solid systems such as fluidized beds. The models for describing drag forces play key roles in the reliability of simulation results. The solid–solid drag model is calculated by the formulas such as Syamlal-O'Brian's model [Syamlal, M. The particle–particle drag term in a multiparticle model of fluidization. Topical report, DOE/MC/21353-2373, NTIS/DE87006500, National Technical Information Service, Springfield, VA. 1987]. However, the performance of the model has not been examined in the mixtures of solid phases with a large size ratio between the particles of the phases, which is found in fluidized beds containing biomass particles (big particles) and the neutral phase particles (small particles) such as sand. The present study compares the results of Eulerian simulations using the Syamlal-O'Brian's formula with the results of Lagrangian simulations performed by the discrete element method (DEM) in the bidisperse mixtures of the size ratio of 10. Results suggest a significant difference between the Eulerian model and the DEM, which is basically due to the inhomogeneities triggered by the existence of large particles. The current methodology can be used to present correction factors for the solid–solid drag forces in Eulerian simulations for the bidisperse mixtures of large size ratios.

A 3D semi-empirical model for reactive two-phase flow in a circulating fluidized bed furnace (CFB3D) is modified by implementing the radiative zone method to solve the radiation heat transfer. The radiative properties of the gas and particle phase have been calculated using detailed information of gas and particle distribution obtained from the CFB3D model. A recently published WSGGM for oxygen-fired combustion has been used to calculate the absorption coefficient of gaseous combustion products. The results of implementing the radiative zonal approach have been compared with those obtained using empirical radiative correlations. The temperature field obtained by using the radiative zone method is more uniform than the one obtained by empirical correlation, and the total heat flux to the wall is slightly higher. The long distance effect of radiation has been found more important in the upper furnace where the gas is the dominant phase. Detailed discussion concerning the obtained results is presented.

Mesoscale flow structures such as clusters and streamers of particles are characteristic features of gas–solid flow in fluidized beds. Numerical simulations of gas–solid flows for industrial-scale fluidized beds are often performed using the Eulerian description of phases. An accurate prediction of this type of flow structure using the Eulerian modeling approach requires a sufficiently fine mesh resolution. Because of the long computational time required when using fine meshes, simulations of industrial-sized units are usually conducted using coarse meshes, which cannot resolve the mesoscale flow structures. This leads to an overestimation of the gas–solid drag force and a false prediction of the flow field. For these cases, a correction must be formulated for the gas–solid drag. We have simulated a large-scale circulating fluidized bed furnace using different gas–solid drag models and compared the model results with measurements.

The indirect steam gasification in circulating fluidized bed reactors was studied by modelling. The object of study was a coupled 12 MWth gasifier-combustor system, which was fired by woody biomass. The heat for the steam-blown gasifier was produced in the air-blown combustor and transported by circulating solids between the interconnected reactors. The system was modelled by a semi-empirical three-dimensional model, which simulated the fluid dynamics, reactions, and heat transfer in the coupled process. The studied cases included different temperature levels, which were controlled by the amount of additional fuel feed to the combustor. The model concept can be later applied to study sorption enhanced gasification, which is a promising method for sustainable production of transport fuels to substitute fossil based fuels. Post-print / Final draft

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland, A calcium looping (CaL) process is a carbon capture technology which utilizes calcium oxide to remove carbon dioxide from the flue gas of a power plant. Like most capture technologies, CaL process has a high energy demand, which reduces power plant efficiency. The energy penalty and the operating and capital costs of the unit can be reduced by increasing the concentration of O2 in the oxidant flow to calciner. In this study, a calciner has been studied with a three dimensional, steady-state, CFB process model. First, the model was validated by test data of the calciner in la Pereda CaL pilot. Next, a 3D model was created for a 200 MWth commercial scale calciner, in which the inlet oxygen concentration was increased up to 75% to map the potentials of improving the heat balance of the system and to investigate how the calciner operates in these conditions. Based on the simulations, the CaL process is feasible even at very high inlet oxygen concentration., The work presented in this paper is being partially funded by the European Commission under the RFCS “CaO2” project.

Chemical looping with oxygen uncoupling (CLOU) is a promising concept for efficient combustion of solid fuels with an inherent capture of the greenhouse gas CO 2 . This paper presents a CLOU process scheme with stoichiometric mass, energy, and exergy balances. A CLOU reactor system using medium volatile bituminous coal as fuel and silica-supported CuO as an oxygen carrier is analyzed. The analysis includes the estimation of various design and operational parameters, thermal considerations, and evaluation of the overall performance. The operation of a reactor system of two interacting circulating fluidized beds (CFBs) is greatly influenced by the hydrodynamics. For the CuO oxygen carrier, the hydrodynamic operating range appeared feasible considering the maximum solid circulation rates in current CFB boilers. Depending upon the reactor temperatures, oxygen carrier inventories of 400–680 kg/MW in the system were found necessary for stoichiometric combustion of the fuel. The temperature difference between the reactors should not exceed 50 °C, as otherwise, problems may arise with the heat balance. Exergetic efficiencies in the range of 63–70% were obtained for different combinations of relevant design parameters. It is evident that the possible operating conditions in the system are closely related to the properties of the chosen oxygen carrier. However, the calculation procedure and design criteria presented here are applicable to any oxygen carrier to be used in the process.

To study the mixing and flow of fuel inside commercial scale circulating fluidized bed (CFB) furnaces, a new model was introduced to solve the flow of fuel in an existing semi-empirical, steady state, three-dimensional Eulerian model frame. The fuel flow model is a simplified momentum equation which is based on the momentum balance considering the fuel inertia, gravity, and drag force from gaseous and solid phase. The model improves the prediction capabilities of fuel flow patterns in the existing model frame for conventional and especially renewable fuels, such as biomass. The fuel flow model improves the analysis of CFBs and can be utilized in the design and development of commercial CFB units.

The HITEMP 2010 spectral emissivity database has been employed in line by line (LBL) calculation to produce an accurate total emissivity database for H2O–CO2 mixtures of the composition characteristics for the oxy-fired combustion. A wide range of temperatures, pressure-path length products and molar fraction ratios have been covered in the database. By using the LBL based emissivity database, an accurate set of coefficients has been obtained for the weighted sum of gray gases model (WSGGM). Compared to the standard WSGGM, the present model includes the molar ratio of H2O–CO2 mixtures in its formulation which leads to one set of coefficients required to represent the entire range of molar ratio. This simplifies the implementation of the model in the simulation of radiative heat transfer in high inhomogeneous media providing higher accuracy. The model is validated against the benchmark solutions for oxy-fired conditions. Obtained results have been compared with some of the previously reported WSGGMs.

The limestone behavior in an oxy-fuel CFB process was studied by means of modeling. Different modeling methods were combined: the calculation of particle trajectories in a three-dimensional steady state CFB furnace model was utilized to investigate the real process environment which the particles experience and a particle model was used to study the effects of the transient environment on limestone reactions. The studied cases included the air-fired and oxygen-fired combustion and conditions on both sides of the equilibrium curve for calcination, which enabled both indirect and direct sulfation mechanisms. Also the effect of the return leg system was studied; it was shown that depending on the conditions in the return leg, a complex behavior consisting of simultaneous reactions (calcination–carbonation–sulfation–direct sulfation) is conceivable. Moreover, the modeling approach was used to evaluate whether the transient environment has an effect on the limestone behavior compared to the averaged conditions and in which conditions the effect is the largest. The difference between the averaged and transient conditions was notable only in the conditions which were close to the equilibrium curve.

Eulerian gas–solid CFD simulations are applied to derive interphase mass transfer coefficients for the Geldart B and D solids in a bench and a pilot size fluidized bed combustion reactor. The interphase mass transfer coefficients obtained from the bubbling and turbulent churning fluidized bed CFD simulations are compared to the empirical data and correlations, which are primarily available only for the Geldart A solids. Particularly, the effect of fluidization velocity, particle diameter and bubble size on the interphase mass transfer coefficients is evaluated.

The shape and size of particles have a major effect on the fluidization behavior of the particles through the drag force from the fluid phase. Several methods to estimate the particle size exist, but without the consideration of the particle shape of irregular particles these methods can produce varying results. The particle shape can be described in numerous ways, but currently no method can successfully describe irregular particles without ambiguity. Many computational models consider all particles as monodisperse perfect spheres, although the real particles often have size and shape distribution. Especially for biomass materials these distributions are usually wide. Disregarding this in modeling might lead to inaccurate results. Another way to describe material drag force is through experimental testing and without detailed knowledge about the material particle size and shape distribution. This paper describes a characterization method of an average drag force between gas and different materials. Material sizes and shapes are defined with optical measurements and the averaged values are combined with the characterization method for comparison with standard drag curves of single particles.

A 1-D dynamic calcium looping model was applied to a large scale calcium looping concept capturing CO 2 from a 250 MWt power plant. Several new features were added to the existing model frame in order to successfully simulate the large scale unit. Models were needed for new material fractions, such as ash and CaSO 4 , sulfur capture and heat transfer in the solid return system. The plant was dimensioned based on the experience from large CFB units and the heat transfer design was evaluated based on initial simulations of the design case. The unit was then simulated in five load scenarios ranging from full load to zero load, no flue gas flow to the carbonator. The scale-up of the process is feasible from the model’s point of view keeping in mind the assumptions and simplifications made in the modeling. The results from the simulations confirmed that successful operation of a large scale calcium looping unit requires good heat transfer design including cooling of the hot solids coming the calciner. Also the recirculation of flue gas in both reactors is necessary to ensure the sufficient fluidization for different flue gas flows from the source combustor. Solid circulation control is also critical because it affects heavily the thermal balance of the system as well as the capture efficiency and the CO 2 balance. Results show that utilizing the experience gained from the large CFB-units coupled with new innovations, the scale-up of this process could be feasible in the near future. The ability to operate flexibly in different modes could give this technology an advantage needed for wide industrial utilization of the process.

Many industrial processes in circulating fluidized beds (CFB) include different solid phases which can have different properties and sizes. For instance, the combustion of biomass particles fluidized by air in a CFB containing sand particles as the inert fluidized particles is one of the processes absorbed a great deal of attention in computer simulations. However, the large size ratios between the sand particles (~0.3 mm) and the biomass particles (1–10 mm) can critically question the applicability of continuum equations such as Syamlal’s formula to evaluate the solid–solid drag forces used in Eulerian simulations. In this paper, we employ DEM simulations to find out how the two solid phases with large size ratios between their particles interact. We quantify the volumetric drag force and compare it to the Syamlal’s formula. The main problem with the latter is associated with the inhomogeneity that the size difference imposes locally, which is against the assumption of homogeneous phases in the derivation of the formula. This can create a way to modify the existing drag models based on DEM simulations.

In this study, the influence of CO2 on sulfur capture efficiency was studied during fluidized-bed desulfurization by experiments and modeling. During calcination–sulfation and direct sulfation, the effect was examined with one limestone type. A time-dependent multilayer particle model was used for analyzing the experimental results. The model determines the magnitude of the reactions and the diffusion as a function of the radius and time. In high temperatures (∼1200 K), CO2 increased the conversion degree during calcination–sulfation. In direct sulfation, the effect of CO2 was opposite; lower conversion was obtained when the CO2 concentration was increased. When the CO2 concentration was increased in low temperatures (∼1100 K) (close to the calcination curve), CO2 retarded the conversion strongly. The detected differences between the results are explained with the development of the Thiele number, conversion curve, and conversion profile during the reactions.

Chemical looping combustion (CLC) is an emerging combustion technology with an inherent separation of the greenhouse gas CO2. The feasibility of CLC has been proven in various small-scale units worldwide, but the large-scale realization of theoretical or small-scale units is still lacking due to many technical challenges. Most of the existing CLC installations use a configuration of two interacting fluidized bed reactors, and even though the fluidized bed technology is mature and well-established, a high level of uncertainty is included in the attempts to up-scale the reactor system involved in CLC. For progressive scale-up of the new technology, a preliminary design of a 100 MWth pre-commercial CLC unit for gaseous fuels is presented. A reactor-level model was used to predict the performance of such a system, and as a result, the operation of the system was characterized and valuable information of the parameters affecting the process was received. For cost-effective energy generation with efficient CO2 capture, the power plant-level integration of CLC must be conducted carefully, and different plant configurations need to be investigated to find the most optimal solution. Hence, the integration of the reactor system and steam turbine cycle for power production was studied resulting in a suggested plant layout including a CLC boiler system, a simple heat recovery setup, and an integrated steam cycle with a three pressure level steam turbine. A plant-level model was used to evaluate the viability of the plant, and without the purification and compression of CO2, the net cycle efficiency of 42.8% was obtained. It was also found that a drop of 2% points in the degree of fuel conversion decreases the net cycle efficiency by about 1% point.

The calcium looping process is a fast-developing post-combustion CO 2 capture technology in which combustion flue gases are treated in two interconnected fluidized beds. CO 2 is absorbed from the flue gases with calcium oxide in the carbonator operating at 650 °C. The resulting CaCO 3 product is regenerated into CaO and CO 2 in the calciner producing a pure stream of CO 2 . In order to produce a suitable gas stream for CO 2 compression, oxy-combustion of a fuel, such as coal, is required to keep the temperature of the calciner within the optimal operation range of 880–920 °C. Studies have shown that the calcium looping process CO 2 capture efficiencies are between 70% and 97%. The calciner reactor is a critical component in the calcium looping process. The operation of the calciner determines the purity of gases entering the CO 2 compression. The optimal design of the calciner will lower the expenses of the calcium looping process significantly. Achieving full calcination at the lowest possible temperature reduces the cost of oxygen and fuel consumption. In this work, a 1.7 MW pilot plant calciner was studied with two modeling approaches: 3-D calciner model and 1-D process model. The 3-D model solves fundamental balance equations for a fluidized bed reactor operating under steady-state condition by applying the control volume method. In addition to the balance equations, semi-empirical models are used to describe chemical reactions, solid entrainment and heat transfer to reduce computation effort. The input values of the 3-D-model were adjusted based on the 1-D-model results, in order to model the behavior of the carbonator reactor realistically. Both models indicated that the calcination is very fast in oxy-fuel conditions when the appropriate temperature conditions are met. The 3-D model was used to study the sulfur capture mechanisms in the oxy-fired calciner. As expected, very high sulfur capture efficiency was achieved. After confirming that the 1-D model with simplified descriptions for the sorbent reactions produces similar results to the more detailed 3-D model, the 1-D model was used to simulate calcium looping process with different recirculation ratios to find an optimal area where the fuel consumption is low and the capture efficiency is sufficiently high. It was confirmed that a large fraction of the solids can be recirculated to both reactors to achieve savings in fuel and oxygen consumption before the capture efficiency is affected in the pilot unit. With low recirculation ratios the temperature difference between the reactors becomes too low for the cyclic carbonation and calcination. As a general observation, the small particle size creates high solid fluxes in the calcium looping process that should be taken into account in the design of the system.

Chemical looping combustion (CLC) is an emerging combustion technology with an inherent separation of the greenhouse gas CO 2 . The technique typically employs a dual fluidized bed reactor system where a metal oxide is used as a solid oxygen carrier that transfers the necessary oxygen from air to the fuel combustion. In this work, a comprehensive simulation tool for the investigation of a CLC system consisting of two interconnected fluidized bed reactors is introduced. One-dimensional, dynamic fluidized bed reactor model is implemented into the Matlab/Simulink environment. The model is based on the conservation of mass and energy, and semi-empirical correlations are used for the calculation of reaction kinetics, hydrodynamics, and heat transfer. The main outputs of the model are the global solids circulation rate, the conversion of the carrier and the gas composition at the reactor exit, the vertical profiles of temperature, reaction rates and gas concentrations, and the distribution of solids in the reactors. For validating and evaluating the capabilities of the model, a reference case based on the operation of a 120 kW th CLC test rig was defined and simulated. Good agreement was observed between the experiments and simulations, and the model structure and submodel forms turned out to be appropriate to describe the studied process.