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This paper presents a methodology to simulate the combustion of fixed beds of biomass particles using computational fluid dynamics (CFD) techniques. The models presented were used in the simulation of a domestic pellet boiler working under operating conditions and the model predictions were compared with measurements of heat transfer, temperature and species concentration. The same procedure was then used to simulate the same domestic boiler working with different values of water temperature and the influence of water temperature variations on the main variables was analyzed.

Ash-related issues such as fouling and slagging are likely the main operational problems of most commercial solid fuel burners. To study this kind of system, a full 3D-transient bed model embedded into the commercial CFD code ANSYS-Fluent was developed to describe the main processes that occur inside the bed. The model employs several sub-models that have been validated in previous studies (e.g., drying, devolatilisation, char reaction, radiation) and were combined with an ash evaporation model that functions in conjunction with a fine-particulate ejection model for predicting typical ash-related problems. In this work, the model is applied to simulate a pilot plant where the deposition of ash on refrigerated tubes is investigated. Several operational points were tested and simulated to assess the capability of the model to explain and predict the experimental fouling rates on the tubes, using which we show the relevance of the bed thickness variation and the primary air flow in the deposition profile. The ash evaporation and fine-particulate ejection models work symbiotically with the existing packed-bed biomass combustion model. The results obtained in this work show that this is a powerful tool for improving the operation of most existing appliances and contributes to the creation of a complex ash-layer deposition model. Ministerio de Economía y Competitividad | Ref. ENE2015-67439-R

A numerical model is proposed to perform CFD simulations of biomass boilers working in different operating conditions and analyse the results with low computational effort. The model is based on steady fluxes that represent the biomass thermal conversion stages through the conservation of mass, energy, and chemical species in the packed bed region. The conversion reactions are combined with heat and mass transfer submodels that release the combustion products to the gas flow. The gas flow is calculated through classical finite volume techniques to model the transport and reaction phenomena. The overall process is calculated in a steady state with a fast, efficient, and reasonably accurate method, which allows the results to converge without long computation times. The modelling is applied to the simulation of a 30 kW domestic boiler, and the results are compared with experimental tests with reasonably good results for such a simple model. The model is also applied to study the effect of air enrichment in boiler performance and gas emissions. The boiler operation is simulated using different oxygen concentrations that range from 21% to 90% in the feeding air, and parameters such as the heat transferred, fume temperatures, and emissions of CO, CO2, and NOx are analysed. The results show that with a moderated air enrichment of 40% oxygen, the energy performance can be increased by 8%, CO emissions are noticeably reduced, and NOx remains practically stable.

Two particle treatments, thermally thin and thick, are applied to Eulerian combustion modeling for biomass packed beds and tested through the simulation of an experimental plant. The paper shows the efficiency of the Eulerian approach for large packed beds and tests the behavior of both particle treatments, tested with in-bed and flame temperatures and released volatiles measurements at different locations, which is not common in the literature for a full size boiler. Both approaches are implemented in a model with a comprehensive framework that includes several submodels for the thermal conversion kinetics, bed motion, heat and mass transfer with the gas phase, and gas flow and reaction. Two experiments are performed with wood chips fuels with different moisture contents. The simulations of the two cases result in reasonably good predictions for both particle treatments. The results are similar for higher moisture content and, for the low-moisture test, the bed temperature distribution and reaction fronts are slightly different due to the different predictions of the drying and devolatilization fronts. The volatile measurements show that the T. Thin model results in slightly more accurate predictions than the T. Thick, possibly because the wood chips have a more thermally thin behavior. Acknowledgments. This research was financially supported by the project PID2021-126569OB-I00 of the Ministry of Science, Innovation and Universities (Spain). Funding for open access charge: Universidade de Vigo/CISUG.

This study analyzes a buffer tank simulated in both continuous operation mode and heating mode using CFD techniques. The analysis is focused in the thermal behavior of the tank, especially in parameters such as heat exchanged, heating time, and temperature distributions into the tank, in order to propose a better design. The results of the different simulations show that the tank heats water extremely slowly and extremely evenly when producing domestic hot water (DHW), which negatively affects the thermal stratification that is critical for rapidly reaching the DHW temperature. Therefore, the main problem of the tank is an inefficient heat exchange and a poor distribution of temperature. In order to overcome these operational limitations, a new design is proposed by installing a tube inside the tank that encloses the heating coil and sends hot water directly to the tank top region such that high-temperature DHW is rapidly provided, and thermal stratification is improved. Several simulations are performed with different open and closed configurations for the outlets of the inner tube. The different results show that the heating times significantly improve, and the time needed to reach the 45 °C set point temperature is reduced from 44 to 15 min. In addition, the simulations in which the opening and closing of the water outlets are regulated, the outlet DHW temperature is kept within 45–60 °C, which prevents overheating to unsafe use temperatures. Furthermore, the results of the simulation in continuous operation mode show a clear improvement of thermal stratification and an increase in the heat transmitted to the inside of the tank.

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG Entre as diferentes estratexias de modelización aplicadas para a simulación da combustión de biomasa durante as dúas últimas décadas, este traballo describe en detalle dous modelos baseados en dous dos enfoques máis utilizados e proba o seu rendemento mediante a simulación dunha caldeira de pellets de madeira de 60 kW. Isto contribúe a analizar o comportamento de ambos enfoques traballando en diferentes condicións de funcionamento e a determinar cales son as condicións favorables para a aplicación dos modelos. O primeiro modelo presentado é un método experimental 1D que introduce os produtos de conversión térmica da biomasa a través de varias seccións da superficie do leito. Os cálculos baséanse nos balances de masa e enerxía e na determinación experimental de fluxos reactivos. O segundo modelo é un método analítico 3D que calcula a conversión térmica da cama de biomasa dentro do dominio CFD. Isto aplica cálculos máis complexos cun custo computacional máis elevado. Para ambos os modelos, a reacción de oxidación do char calcúlase mediante catro correlacións que devolven diferentes relacións CO/CO2. Os modelos de conversión de leito e as correlacións de oxidación do char aplícanse a dúas probas diferentes coa caldeira funcionando a media carga e a plena carga con diferentes combustibles. Os resultados mostran que ambos os modelos de combustión teñen un comportamento xeral similar. O modelo 3D compórtase razoablemente ben en todos os casos e non se ve afectado significativamente polas diferentes correlacións de oxidación de char. Ambos modelos dan resultados similares cando as condicións de combustión son favorables (proba a plena carga). O modelo 1D é moi sensible ás correlacións de oxidación do char, especialmente na proba de media carga. Neste caso, funciona mellor con correlacións que producen relacións CO/CO2 máis baixas. A análise dos contornos no fogar da caldeira mostra que, no modelo de leito 3D, unha parte importante do proceso de combustión ocorre no volume do leito, que non está engranado no modelo 1D, e que o avance na combustión compensa as diferenzas de CO emitido pola cama. Among the different modelling strategies applied for the simulation of packed bed biomass combustion during the last two decades, the present paper detailed describes two models based on two of the most commonly used approaches and tests their performance through the simulation of a 60-kW wood pellet boiler. This contributes to analyze the behavior of both approaches working in different operating conditions and to determine what conditions are favorable for the models application. The first presented model is a 1D experimental method that introduces the products of the biomass thermal conversion through several sections of the bed top surface. The calculations are based on mass and energy balances and experimental determination of the reactive fluxes. The second model is an analytical 3D method that calculates the packed bed thermal conversion inside the CFD domain. This applies more complex calculations with a higher computational cost. For both models, the char oxidation reaction is calculated though four correlations that returns different CO/CO2 ratios. The bed conversion models and the char oxidation correlations are applied to two different tests with the boiler operating at half and full load with different fuels. The results show that both bed models have a similar overall behaviour. The 3D model has a reasonably good behaviour in all cases and is not significantly affected by the different char correlations. Both models give similar results when combustion conditions are favourable (full-load test). The 1D model is highly sensitive to the char oxidation correlations, especially in the half load test. In this case, it has a better behaviour with the correlations that produce lower CO/CO2 ratios. The analysis of the contours in the freeboard shows that, in the 3D bed model, an important part of the combustion process occurs in the bed volume, which is not meshed in the 1D model, and that advance in the combustion compensates the differences in the CO emitted by the bed. Agencia Estatal de Investigación | Ref. PID2021-126569OB-I00 Slovenian Research Agency | Ref. P2-0424

In this work, a CFD-based model is proposed to analyse the effect of phase change materials (PCMs) on the thermal behaviour of the walls of a cubicle exposed to the environment and on the resistance of the walls to climate changes. The effect of several days of exposure to the environment was simulated using the proposed method. The results of the simulation are compared with experimental data to contrast the models. The effects of exposure on the same days were simulated for several walls of a cubicle made of a mixture of concrete and PCM. The results show that the PCM stabilizes temperatures within the cubicle and decreases energy consumption of refrigeration systems.