• Energy Research

Abstract The primary objective of this work is to study the blending of natural gas in equimolar proportions with three high hydrogen content syngases in a radiant porous media burner. We examined the effects of the composition of the syngases, the fuel-to-air ratio and the thermal input on the flame stability, the radiation efficiency and the pollutant emissions (CO and NOx). In this study, we emulated the syngases with H2–CO mixtures, in which the H2 to CO ratio was varied between 1.5 and 3. Additionally, pure natural gas was also used as a base fuel for comparison. The thermal inputs evaluated in this study correspond to two values (300 and 500 kW/m2) found in practical applications. The results indicate that the thermal input and the fuel-to-air ratio significantly influenced the temperature profile in the radiant porous media burner, the radiation efficiency, and the pollutant emissions. On the other hand, contrary to what was observed in other studies for lower hydrogen concentrations, we found that substituting natural gas with high hydrogen content syngases (up to 50%) affected the flame stability limits. Significant differences were also observed for the radiation efficiencies and pollutant emissions.

Abstract The effect of the injection of externally sourced carbon dioxide (CO2) on the stability of the flameless combustion regime was evaluated numerically and experimentally, taking temperature uniformity and pollution emissions (NO and CO) as criteria. The flameless combustion regime was studied in a lab-scale furnace fueled with natural gas (NG) at a thermal power of 20 kW based on the low heating value (LHV). The CO2 was injected into the lower part of the furnace to directly affect the reaction zone. Computational fluid dynamics (CFD) simulations were performed using the ansys-fluent software. The models used to describe the turbulence, the radiation heat transfer, and the turbulence–chemistry interaction were the standard k–ɛ model, discrete ordinate model (DOM), and eddy dissipation concept (EDC) model, respectively. The NG oxidation was described with a seven-step global reaction mechanism with the EDC model. Three excess air conditions were analyzed, 20%, 25%, and 30%, combined with various CO2 injection flows. At 30% excess air, the flame exhibited destabilization without any CO2 injection. Adding CO2 attenuates the destabilization because of the dilution effect. Increasing either the CO2 or excess air flow resulted in a considerable decrease in the global temperature of the process, consequently producing an increase in CO emissions and a decrease in NO emissions. Finally, for the conditions studied, increasing the mass flow of externally sourced CO2 did not destabilize the flameless combustion regimen. This result shows the potential of the implementation of flameless combustion in industrial processes where CO2 is releasing as a result of a reaction external to the combustion process, such as cement, ammonia, or lime production among others.

Abstract An analysis of the effect of burner location on the performance of a walking-beam type reheating furnace for an austenitizing process is presented in this work. Four configurations were evaluated, where the main difference was the position of four high-speed self-recuperative burners. The analysis was done through computational fluid dynamics (CFD) simulations, using a set of models suitable, and previously validated, to consider combustion, heat transfer, and billet heating, all in a 3D steady-state calculation. The self-recuperative burners were modeled by programming a custom user-defined-function (UDF) for the specific burner. This UDF calculates air preheating temperature in each burner as a function of air mass flow rate and the flue gas temperature entering the burner recuperator. The efficiency of the heating process, the billet heating characteristics, and the heat transfer rate to the billets for the different configurations were analyzed and compared. The results show that position and type of burners have a great effect on the furnace performance. The entrance of cold air through the furnace openings was responsible for the lower efficiencies and some billet heating problems observed. The configuration with the burners staggered on the sidewalls presented the best results in terms of energy efficiency and the billet heating characteristics required for an austenitizing process (heating rate, austenitizing temperature, holding time, and temperature uniformity).

Abstract This work presents the computational fluid dynamic (CFD) simulation of a single-ended non-recirculating radiant tube burner (RTB). In the RTB evaluated, the mixing and main combustion reactions take place inside a combustion chamber, which differs from most of the RTB configurations found in the literature. The eddy-dissipation-concept (EDC) model and the Steady-Diffusion-Flamelet (SDF) model were compared to contrast their performance in the studied burner. Five chemical kinetic mechanisms were evaluated with the two combustion models. The chemical equilibrium approach with a PDF tabulation was also included. The CFD simulations were made using an axisymmetric two-dimensional (2-D) computational domain. The performance of the CFD simulation was evaluated by comparing its predicted outer radiant tube (RT) temperature with experimental measurements. Temperature and mole fraction of CO and OH were also compared between models and kinetic mechanisms. A third model, the Flamelet Generated Manifold (FGM), was subsequently included in the analysis, due to its capability to describe partially premixed combustion. Finally, the results of the 2-D simulation were contrasted with a three-dimensional (3-D) simulation, determining the effect of geometry simplification and confirming the suitability of 2-D CFD models in the studied case.

Abstract The present study presents a numerical simulation of the effects of using self-recuperative burners on the performance of a walking-beam reheating furnace. The study was done using CFD (Computational Fluid Dynamics) simulations where a low computational cost method was implemented to simulate the billet heating as a steady state system. The preheating temperature of the air was defined as a function of the air mass flow and the flue gas temperature in each burner, using a UDF (User-Defined Function). The results of the billet heating profile were validated with experimental measurements in a furnace not utilizing heat recovery and showed good agreement with a maximum deviation of 54 K. Efficiency was found to increase from 32.7% to 48.5% with the use of self-recuperative burners. This result was reflected in a fuel consumption decrease of 31.3%, or an increase in furnace production of 51.3%. The heat transfer in the furnace and the billet heating characteristic were analyzed, which were observed to have changed with the different burners. This effect was lesser when the thermal input was decreased, and greater when the production was increased or when both remain constant. With and without heat recovery, radiation represents about 90% of the total heat flux on the billets. Due to the importance of radiation, solid and gas radiation were also analyzed. An improvement upon a methodology found in the literature was proposed. Using this new methodology it was found that, for the furnace evaluated, around 25% of the radiation absorbed by the billets comes from the flue gases. The effect of defined a constant billet emissivity on the results of the simulation was also discussed.