• Energy Research

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG Biomass is commonly used for heat production in small-scale and domestic facilities. Particulate matter (PM) emissions from the combustion of solid biomass are a concern because of hazards to human health. Electrostatic precipitators (ESPs) have been supported as the most promising mitigation technology, although there is a lack of consolidation to provide and promulgate a solution. The currently published studies on small-scale biomass do not offer a deep performance analysis of an electrostatic precipitator; instead, different designs have been presented and validated through efficiency measurements. In this work, an easily attachable ESP design is tested to more deeply analyse its performance with emissions from a pellet boiler, measuring its retention efficiency as a function of the PM sizes generated by the boiler, and under the influence of the supplied voltage and the geometry considering different discharge electrode diameters. Increasing voltage give higher separation efficiencies until 90% are reached, and further improvements are not remarkable. This efficiency value is influenced by the limitations of the particle analyser. A thinner discharge electrode achieves both better efficiencies and stable operation at lower voltage than a thicker electrode. Regarding the PM size, the retention capacity of the ESP is lower for particles between approximately 0.17 μm and 0.7 μm due to the reduced ability of the charging mechanisms. The formation of agglomerates during re-entrainment is also observed. Xunta de Galicia | Ref. ED481A-2019/225 Ministerio de Ciencia, Innovación y Universidades | Ref. RTI-2018-100765-B100

Abstract This study presents a new modelling methodology for three-dimensional simulation of a reheating furnace of steel billets with natural gas burners. The movement of the billets inside the furnace is a periodically transient phenomenon. A modification in the energy transport equation is introduced through source terms in the billet region to convert the transient movements of the billets into a steady-state simulation, which significantly reduces the computational time without any loss of information. The combustion simulation inside the burners is performed using a global combustion mechanism that takes into account the combustion of the main gaseous species present in natural gas following a kinetic and turbulent control of the reaction through the Eddy Dissipation Concept (EDC). The results are contrasted and validated with the data obtained in a real facility though the SCADA of the plant. Some parameters, such as the surface temperatures of billets at the outlet and the distribution of power supplied by the fuel inside the furnace are reasonably close to the real case data obtained in temperature fields of both billets and the gas phase as well as the energy balance, leading to the conclusion that the proposed methodology is adequate for the purpose of simulating this system.

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.

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.

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 This work studied the importance of wood pellets, chips and wood logs for small- and medium-scale heat production. Pellet use can contribute substantially to reaching the renewable heat and electricity goals set by the European Union (EU) Renewable Energy Directive. Consequently, to integrate into European energy markets, pellet use must be a key piece of the EU energy policy. This study provides a wide perspective on the state-of-the-art small-scale biomass combustion units, particularly those that use pellets for fuel. Small-scale combustion units include stoves and boilers with capacities less than 200 kW. A wide market review has been conducted, including a review of the literature and information from manufacturers and test institutes. A database has been created, which includes 186 companies and offers 995 different models, providing an extensive view of the European market. The large number of new companies shows that the solid-fuel boiler market is continuously increasing across Europe. The technologies that are currently the most widely used are described and compared.

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.

In this research paper a study of the effectiveness of preheating in small pellet fixed bed stoves is presented. Two different set of experiences, with and without preheated air, were carried out and compared in a pilot 24 kWt pellet plant at the University of Vigo. The influence of preheating specially considering efficiency and emissions has been investigated through the application of statistical analysis on the results. Results show that preheating enhances the operation of the pellet stove specially form the energetic point of view