Extended EDC local extinction model accounting finite-rate chemistry for MILD combustion

An extended Eddy Dissipation Concept (EDC) local extinction model is proposed to take into account the effects of finite-rate chemistry, normally occurring in Moderate to Intense Low oxygen Dilution (MILD) combustion, on the extinction limits. Local extinction is predicted when the local fine structure residence time is below a local critical value that is determined theoretically in the present study. The proposed model has been evaluated against experimental data reported for CH4/H2 jet-in-hot and diluted coflow flames. Comparison with the standard EDC extinction model is also presented. Results show that prediction of extinction threshold in MILD combustion conditions is attainable only through the application of the extended EDC extinction model on a well-resolved turbulence-chemistry interaction field. The effect of penetrating surrounding air into the reaction zone with subsequent flame cooling at downstream is also captured by the proposed extinction model. Despite its simplicity, the extended EDC extinction model is able to describe many features of localized extinction under MILD combustion as well as conventional combustion condition

Analysis of oxy-coal combustion through measurements in a pilot-scale entrained flow reactor

Coal combustion is investigated in both air and oxy-fuel conditions in a pilot-scale entrained flow reactor able to provide high temperatures, heating rates and residence times. Measurements are carried out with different levels of complexity and are aimed at: assessing the thermal field inside the reactor; evaluating conversions of devolatilization or char combustion tests; identifying phenomena such as volatiles ignition and measuring the ignition delay time. Computational Fluid Dynamics was also used in order to provide a better understanding of the experimental evidences. Among the results, the ignition delay time was found to be larger in oxy-fuel conditions than in air, mainly because of the larger specific heat of the oxy-fuel environment. The proposed investigation may help the qualification of advanced experimental apparatus as entrained flow reactors, with the purpose to make them suitable for heterogeneous kinetics studies in oxy-fuel conditions

Flame characteristics of pulverized torrefied-biomass combusted with high-temperature air

In this work, the flame characteristics of torrefied biomass were studied numerically under high-temperature air conditions to further understand the combustion performances of biomass. Three torrefied biomasses were prepared with different torrefaction degrees after by releasing 10%, 20%, and 30% of volatile matter on a dry basis and characterized in laboratory with standard and high heating rate analyses. The effects of the torrefaction degree, oxygen concentration, transport air velocity, and particle size on the flame position, flame shape, and peak temperature are discussed based on both direct measurements in a laboratory-scale furnace and CFD simulations. The results primarily showed that the enhanced drag force on the biomass particles caused a late release of volatile matter and resulted in a delay in the ignition of the fuel-air mixture, and the maximum flame diameter was mainly affected by the volatile content of the biomass materials. Furthermore, oxidizers with lower oxygen concentrations always resulted in a larger flame volume, a lower peak flame temperature and a lower NO emission. Finally, a longer flame was found when the transport air velocity was lower, and the flame front gradually moved to the furnace exit as the particle size increased. The results could be used as references for designing a new biomass combustion chamber or switching an existing coal-fired boiler to the combustion of biomass

High-temperature rapid devolatilization of biomasses with varying degrees of torrefaction

Torrefied biomass is a coal-like fuel that can be burned in biomass boilers or co-fired with coal in co-firing furnaces. To make quantitative predictions regarding combustion behavior, devolatilization should be accurately described. In this work, the devolatilization of three torrefied biomasses and their parent material were tested in an isothermal plug flow reactor, which is able to rapidly heat the biomass particles to a maximum temperature of 1400 ºC at a rate of 104 ºC/s, similar to the conditions in actual power plant furnaces. During every devolatilization test, the devolatilized biomass particles were collected and analyzed to determine the weight loss based on the ash tracer method. According to the experimental results, it can be concluded that biomass decreases its reactivity after torrefaction, and the deeper of torrefaction conducted, the lower the biomass reactivity. Furthermore, based on a two-competing-step model, the kinetic parameters were determined by minimizing the difference between the modeled and experimental results based on the least-squares objective function, and the predicted weight losses exhibited a good agreement with experimental data from biomass devolatilization, especially at high temperatures. It was also detected that CO and H2 are the primary components of the released volatile matters from the devolatilization of the three torrefied biomasses, in which CO accounts for approximately 45-60%, and H2 accounts for 20-30% of the total volatile species

Biomass furnace for externally fired gas turbine: Development and validation of the numerical model

Externally-fired gas turbines (EFGT) are currently being investigated for co-generation from biomass, because of their ability to deal with low-grade fuels without the complexity of gasification. Main drawbacks of the technology are related to the high thermal stresses experienced by the heat exchanger. The present work proposes a computational fluid dynamics (CFD) analysis of a grate-fired furnace installed in a EFGT cycle, with the purpose to provide a tool for detecting the most critical regions in the furnace. The model is complemented with a process simulation of the entire EFGT cycle. Different approaches for treating the fuel bed and their impact on the CFD analysis are discussed and validated through the availability of in-flame measurements of temperature and chemical species. Predictions indicate the need for a detailed fluid dynamic characterization of the grate region, which was found to largely impact the furnace flow and thermo-chemical fields