• Energy Research

This paper presents a transient model for the simulation of biomass combustion in a fixed bed boiler fed through particle drop. The method combines classical CFD techniques, which are used to simulate the gas phase, with a set of Eulerian variables defined to model the solid phase and Lagrangian trajectories that model the particle drop. Several submodels are implemented to simulate the numerous processes that occur during the combustion of the solid phase. These submodels include the thermal conversion of biomass, heat and mass transfer, particle shrinkage and bed compaction as well as the interaction of the DPM (Discrete Phase Model) trajectories and the bed variables. The predictions are contrasted with various experimental tests, which provide reasonably good results and demonstrate the overall good behavior of the models. The simulation facilitates an understanding of the heat transfer inside the boiler and the instability of the emission measurements.

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.

Abstract In this work, we study the operation of a water heater storage tank using CFD modelling and analyse its limitations in order to propose a redesign that improves its characteristics. The study method is based on calculating the main variables that affect the thermal performance using the finite volume method. With this method, the Navier-Stokes equations are applied to calculate the internal movements of the fluid and its heat exchange with the coil. A determining factor for the operation of a water heater tank is natural convection, which is solved by means of the Boussinesq approximation. Different reservoir simulations are carried out in continuous operation mode (stationary) and in heating mode (transient). In both cases, the results show very homogeneous heating, which means poor temperature stratification. As an alternative for improvement, a redesign of the system is proposed by means of an auxiliary tank where the heating coil is located, which injects the hot water, at different heights, into the main tank. Several simulations of this redesign are carried out with different configurations in order to achieve better temperature stratification, reduce the heating time and maintain the outlet temperature of the domestic hot water (DHW) between 45 and 60 °C. As a result, the redesign significantly improves the stratification of temperatures and reduces the heating time necessary to obtain a DHW temperature of 45 °C from 45 to 8 min.

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

Abstract This work proposes a method based on CFD techniques for the study of thermal water heater tanks. The method is based on using the Navier-Stokes equations to calculate the internal movements of the fluid and the transport of energy through the flow. The Boussinesq approximation is applied to solve the fluid movements that occur by natural convection, which represents a great computational savings compared to other methods based on variable densities with very short time steps. Because the tank has a corrugated coil whose mesh poses major computational challenges, a fitting method is used to calculate the heat transfer of the corrugated surface in a smooth coil. The model is applied to a tank with 150 L of water, which is heated by a heat exchanger in the form of an outer jacket. The heat accumulated in the tank is exchanged with the drinking water by means of a corrugated coil located inside the tank. The model is tested by simulating several steady-state continuous consumption experiments and by conducting a transient test of heating and subsequent cooling. For the comparison of results, the data used include the flow rates through the water jacket and the inner coil, the inlet and outlet temperatures of these flows and various temperatures recorded by thermocouples located inside the tank at different heights. The results obtained from the simulations are reasonably close to the experimental observations. The experimental data show an aggressive temperature increase in all thermocouples, except the highest, at the beginning of heating and a softer increase at the end. A similar behavior occurs during cooling, aggressive at the beginning and softer as time progresses. This behavior is also obtained in the model predictions, especially in the lower thermocouples. The greatest discrepancy is found for the highest thermocouple during cooling. The experimental tests show a noticeably slower cooling than the model predictions.

Abstract CFD codes are well equipped in the resolution of the gas phase combustion, however the solid phase modelling still needs to be developed. The aim of this work is to implement several submodels of thermal conversion of solid fuels and the interaction with the gas phase in a commercial CFD code. For this, a set of submodels taken from literature is proposed for simulating the combustion of solid biomass in packed beds. The modelling method implements several variables that represent the main parameters of the solid mass in the framework of a commercial CFD code. The transport equations of the solid phase are used to predict the transient evolution of the bed and the interaction with the gas phase within the bed and the area surrounding it. The radiative heat transfer is modelled by modifying the standard Discrete Ordinates model to consider the temperature difference between the solid and gas phases and the high absorptivity of the medium. A compaction model is introduced to account for the local shrinkage of the bed due to the collapse of regions weakened by their combustion. The results of the model are presented and discussed by the contrast of the model through the simulation of an experimental burner whose ignition rates, maximum temperatures, and drying, devolatilisation, and char thicknesses are known. The model shows a reasonably accuracy in the predictions of some variables as ignition rates, maximum temperatures and the bed height transient evolution. The comparison of the drying, devolatilisation, and char thicknesses for different air mass fluxes shows reasonably good tendencies even thought the values are excessively high. Compared with previous works done by the authors and by others, it can be seen that the introduction of a bed compaction submodel, as the one presented here, can help in the realistic estimation of the processes involved in packed be combustion of biomass and point to particle shrinkage and bed’s mechanics as an important process to be considered.

Abstract The thermally thick treatment of fuel particles during the thermal conversion of solid biomass is required to consider the internal gradients of temperature and composition and the overlapping of the existing biomass combustion stages. Due to the implied mixture of scales, the balance between model resolution and computational efficiency is an important limitation in the simulation of beds with large numbers of particles. In this study, a subgrid-scale model is applied to consider the intraparticle gradients, the interactions with other particles and the gas phase using a Euler–Euler CFD framework. Numerical heat transfer and mass conservation equations are formulated on a subparticle scale to obtain a system of linear equations that can be used to resolve the temperature and position of the reacting front inside the characteristic particle of each cell. To simulate the entire system, this modelling is combined with other submodels of the gas phase, the bed reaction and the interactions. The performance of the new model is tested using published experimental results for the particle and the bed. Similar temperatures are obtained in the particle-alone tests. Although the mass consumption rates tend to be underpredicted during the drying stage, they are subsequently compensated. In addition, an experimental batch-loaded pellet burner was simulated and tested with different air mass fluxes, in which the experimental ignition rates and temperatures are employed to compare the thermally thick model with the thermally thin model that was previously developed by the authors. The results show a better approximation of the model in this study, which addresses some of the main limitations of a thermally thin model regarding the bed reaction stability in overstoichiometric regimes.

Abstract The use of biomass is growing as it is considered as a renewable energy by a society with increasing ecological awareness. Biomass has the advantage of being easily used in existing installations with non-renewable solid fuels, either as a final fuel or as a transition fuel. However, biomass use presents serious operational problems, such as slagging and fouling, which have slowed its development. This work continues the development of an advanced and flexible fouling model that is embedded in a fixed-bed biomass combustion model developed by the University of Vigo for the commercial code, ANSYS-Fluent. The modifications to the algorithm enable the analysis of commercial water-tube and fire-tube combustion systems, with diverse types of fuel-feeding systems. To validate the model, a numerical study of the combustion and fouling phenomena of two commercial systems that have already been analysed experimentally and published was carried out. The results obtained are in good agreement with the experimental results and demonstrate the accuracy and flexibility of the proposed model. After this validation, future research lines can be opened that consider more complex phenomena.

Abstract A ThermoGravimetric Analyser with Differential Scanning Calorimeter (TGA-DSC) has been simulated to evaluate the influence of different parameters on thermal lag. Two virtual materials with selected properties were created and combined with two heating rates. A three-dimensional mesh was created based on measurements in the actual TGA-DSC. Specific models have been applied and boundary conditions have been configured, including a programmed Proportional Integral Derivative (PID) controller to regulate temperature. The results showed that a sample with a lower thermal diffusivity had greater temperature differences when compared to the furnace temperature than a sample with a higher thermal diffusivity. Biot number has also been computed to analyse temperature differences within the sample. Thermal diffusivity and Biot number effects were miniscule compared to the heating rate, which increased sample temperature instability and temperature differences within the sample.