Role of primary freeboard on staged combustion of hardwood pellets in a fixed bed combustor

In staged fixed bed biomass combustion, primary air is supplied beneath the fuel bed with secondary air then provided above in the freeboard region. For fixed bed configurations, the freeboard is further divided into a primary freeboard length (LI), which is upstream of the secondary air and a secondary freeboard length (LII), measured from the secondary air all the way to the exhaust port. Despite extensive research into fixed bed configurations, no work has been successfully completed that resolves the effects of changing LI on fuel conversion, both in the fuel bed and within the freeboard of batch-type biomass combustors. In this study, experiments on a 202 mm diameter and 1500 mm long batch-type combustor have been conducted to determine the effects of changing primary freeboard length over three secondary to total air ratios (Qs/Qt) and two total air flow rates (Qt). The impact of these conditions has been studied on (i) intra-bed fuel conversion, measured through burning rate (kg/m−2 s−1), fuel bed temperature (°C) and ignition front velocity (mm-s−1), as well as (ii) post-bed fuel conversion in the freeboard, expressed through freeboard temperatures and emissions (NOx ppm, CO2%, CO ppm, O2%). The fuel used throughout the above experiments was Australian hardwood pelletised biomass. Results show that changes to primary freeboard length over LI = 200 mm, 300 mm and 550 mm, or LI/D = 1.00, 1.48 and 2.72, respectively, affect both intra-bed and freeboard (post-bed) performance indicators. The highest values of burning rate, ignition front velocity and fuel bed temperature were observed for interim values of LI/D = 1.48 at Qs/Qt = 0.25 and Qt = 0.358 kg/m−2 s−1. Primary freeboard lengths of LI/D = 1.00 and 1.48 were found to have higher freeboard temperatures, NOx and CO2 as well as lower CO and O2 values as compared to LI/D = 2.72 at Qs/Qt = 0.50 and 0.75. Increasing Qs/Qt from 0.25 to 0.50 for LI/D = 1.00 and 1.48 initially increased freeboard temperatures, with an accompanying increase in NOx and CO2 as well as decrease in CO values. However, further increase in Qs/Qt to 0.75 lead to lower freeboard temperatures for all primary freeboard lengths

Role of air staging in a batch-type fixed bed biomass combustor under constant primary air

Staged combustion of biomass is the most suitable thermo-chemical conversion for achieving lower gaseous emissions and higher fuel conversion rates. In a staged fixed bed combustion of biomass, combustion air is supplied in two stages. In the first stage, primary air is provided below the fuel, whereas in the later stage, secondary air is supplied in the freeboard region. The available literature on the effects of air staging (secondary air location) at a constant primary air flow rate on combustion characteristics in a batch-type fixed bed combustor is limited and hence warrants further investigations. This study resolves the effect of air staging, by varying the location of secondary air in the freeboard at five secondary to total air ratios in a batch-type fixed bed combustor. Results are reported for the effects of these controlled parameters on fuel conversion rate, overall gaseous emissions (CO2, CO and NOx) and temperature distributions. The fuel used throughout was densified hardwood pellets. Results show that a primary freeboard length (distance between fuel bed top and secondary air injection) of 200 mm has higher fuel conversion rates and temperatures as well as lower CO emissions, at a secondary to total air ratio of 0.75 as compared to primary freeboard length of 300 mm. However, NOx emissions were found to be lower for a primary freeboard length of 300 mm as compared to 200 mm. An increase in secondary to total air ratio from 0.33 to 0.75 resulted in higher freeboard temperatures and lower CO as well as NOx emissions. The outcomes of this study will be helpful in the effective design of commercial scale biomass combustors for more efficient and environmentally friendly combustion

Effect of air staging and porous inert material on the emission of volatile organic compounds in solid biomass combustion

The present paper delves into experimental data to assess the effects of the inclusion of a zirconium porous medium in the emissions of a laboratory-scale biomass combustor, with a focus on particulate matter and volatile organic compounds. While other studies have focused on the effects of the material on solid particulate matter or gaseous emissions, this research is focused on volatiles, their capture and storage. A novel sampling train for the capture of volatiles has been designed based on active carbon adsorption on a refrigerated environment, and its performance was evaluated through thermogravimetric analysis, showing the affinity of this porous medium towards lighter organic compounds. The retention time of the sample was also studied, and the data revealed that after three to six hours the sample had degraded significantly when stored in airtight plastic bags inside a glass desiccator at 22 °C. An analysis of the particulate matter emitted was also carried out. Volatile organic compounds were also found to follow the behavior of particulate matter, with the scenarios where low solid particles were emitted being also those in which volatiles release was minimized.Agencia Estatal de Investigación | Ref. PID2021-126569OB-I0

Characterization of biomass PM emissions using thermophoretic sampling: Composition and morphological description of the carbonaceous residues

In this paper, a complete characterization of particulate matter (PM) emitted by a low-scale biomass combustor was performed. Samples were collected in three different zones of the installation (in-flame, boiler outlet and chimney outlet) using the thermophoretic sampling (TPS) method, and the different particle structures present in the biomass were studied from a chemical and a morphological point of view. Five types of particles were found: soot agglomerates, organic films, organic particles with imbibed fibrous structures, condensed tars and condensed inorganic salts. Transmission electron microscopy (TEM) was used for shape description, and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) was used for determining elementary composition. Specifically, soot agglomerates were submitted to a deeper characterization in terms of fractal descriptors, obtaining values for primary particle diameter, dpo, of 21.7 ± 8.4 nm, 21.4 ± 6.6 nm and 26.0 ± 5.9 nm for the three sampled zones, as well as values for fractal dimension, Df, between 1.40 and 2.60 and fractal prefactor, kf, between 2.20 and 2.40. The results were in agreement with other soot formation sources and showed an increasing tendency to cluster growth and compaction with the residence time inside the facility, being higher as they went further from the combustion zone. Fractal descriptors were not fairly influenced by the applied thermal treatment.Ministerio de Economía y Competitividad | Ref. ENE2015-67439-

Experimental study on the stability and transient behavior of a closed-loop two-phase thermosyphon (CLTPT) charged with NOVEC 649

Currently, the growing need for efficient refrigeration resources in the industrial sector has led to an increasing interest in finding technologies with a higher heat removal potential and better cooling performance. Along these lines, two-phase liquid cooling appears to be a very interesting solution, with the CLTPT (closed-loop two-phase thermosyphon) being one of the leading alternatives. Most works in the scientific literature study loop thermosyphons that work in flow boiling conditions in steady state. The present paper analyzes the transient thermal behavior of a pool boiling CLTPT gravitational channel as a passive cooling system using NOVEC 649 as working fluid. The evaporator works with two submerged cylindrical heaters that represent different heat sources located in different positions. The initial transient behavior and consequent instabilities of a laboratory-scale facility were studied, followed by a stability analysis for various power inputs. Parameters such as temperature and pressure along the experimental setup were monitored, and the effects of internal pressure and room conditions were also tested. The results show some instabilities in the process to start the flow circulation and a relative stability and quick adaptation to changes when circulation is reached. The temperature in the evaporator chamber was highly homogeneous during the whole process; however, the temperature changes in the riser and the loop top were delayed with respect to the evaporator zone. The analysis shows several pressure and temperature raises before the vapor flux reaches the condenser. When the flow circulation is established, the system becomes highly stable and thermally homogeneous, decreasing the thermal resistance when increasing the power input. The stability analysis also showed that, when the system reaches the steady state, the changes in the power input produce a transient increase in the pressure and temperature of the fluid, followed by a quick decrease of the previous steady state values. The heat transfer analysis in the evaporator shows a higher heat flux on the upper heater caused by the buoyancy flow that rises from the lower heater. It was also observed that the lower heater reaches the CHF point with a lower heat flux.Ministerio de Ciencia, Innovación y Universidades | Ref. RTI-2018-100765-B10

A critical review on the numerical simulation related to Physical Vapour Deposition

Physical Vapour Deposition (PVD) is a process usually used for the production of advanced coatings regarding its application in several industrial and current products, such as optical lens, moulds and dies, decorative parts or tools. This process has several variants due to its strong evolution along the last decades. The process is commonly assisted by plasma, creating a particular low pressure and medium temperature atmosphere, which is responsible for the transition of atomic particles between the target and the parts to be coated into a vacuum reactor. Several parameters are directly affecting the deposition, namely the substrate temperature, pressure inside the reactor, assisting gases used, type of current, power supply, bias, substrate and target materials, samples holder and corresponding rotation, deposition time, among others. Many mathematical models have been developed in order to allow the generation of numerical simulation applications, trying to combine parameters and expect the corresponding results. Numerical simulation applications were created around the mathematical models previously developed, which can play an important role in the prediction of the coating properties and structure. This paper intends to describe the numerical simulation evolution in the last years, namely the use of Finite Elements Method (FEM) and Computational Fluid Dynamics (CFD).LAETA/CETRIB/INEGI Research Center- FLAD – Fundação Luso-Americana para o Desenvolvimento | Ref. 116/2018Fundação para a Ciência e a Tecnologia | Ref. UID/EMS/0615/201

Simulación y validación de captadores geotérmicos mediante modelos implementados en el software TRNSYS para una biblioteca situada en el campus de la Universidad de Vigo

de simulación del terreno y de los intercambiadores geotérmicos en un edificio del Campus de la Universidad de Vigo (Biblioteca de Ciencias del Mar). Para la ejecución de las simulaciones transitorias se ha empleado el motor de cálculo de TRNSYS. Asimismo, para el cálculo del perfil de temperaturas del terreno, se emplea un modelo basado en el estudio de T. Kusuda y P.R. Achembach, donde la temperatura del terreno es función del momento del año y de la profundidad, así como de la difusividad térmica del suelo, la temperatura media anual de la superficie, la amplitud de la temperatura superficial anual y el momento de menor temperatura superficial del año. Se utiliza como modelo del intercambiador de calor con el terreno el desarrollado por Göram Hellström. Los resultados obtenidos mediante simulación transitoria con TRNSYS son validados con los datos recopilados durante el año 2014 para el edificio de la biblioteca, el cual se encuentra calefactado mediante un sistema de bomba de calor geotérmica y suelo radiante. El sistema de producción térmica y el de captación geotérmica se encuentran monitorizados, proporcionando un registro de datos minutal. Los resultados muestran un alto grado de concordancia, lo cual indica que TRNSYS es una herramienta eficaz para la simulación de este tipo de sistemas.Está investigación ha sido parcialmente financiada a través del proyecto ITC- 20133033 TERESE3 subvencionado por el CDTI y Fondo Tecnológico -FEDER 2007-2013 Innterconecta apoyado por el Ministerio de Economía y Competitividad y Consejería de Economía e Industria a través Axencia Galega de Innovación (GAIN ) de la Xunta de Galicia