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Karlstrom O.,Abo Akademi University | Brink A.,Abo Akademi University | Hupa M.,Abo Akademi University | Tognotti L.,University of Pisa | Tognotti L.,International Flame Research Foundation
Combustion and Flame | Year: 2011

In industrial pulverized fuel combustion, char oxidation is generally limited by the combined effects of chemical reactions and pore diffusion. Under such conditions, char oxidation is frequently predicted by power law models, which despite their simplicity, are widely used in the comprehensive CFD modeling of pulverized coal boilers. However, there is no consensus on the apparent reaction order given by such models. This study developed a systematic approach which gives consistent values over a range of conditions. Apparent reaction orders for 10 bituminous coal chars were investigated with three different oxygen concentrations, ranging from 4 to 12vol.%, and a gas temperature of 1223K for each char. Experimental burnout profiles of the chars were obtained by means of an Isothermal Plug Flow Reactor operating at industrially realistic heating rates (104K/s). For various reaction orders between 0.05 and 2.00, kinetic parameters were independently determined, following numerical procedures recently suggested in the literature. The resulting values were incorporated into an empirical power law model and compared to experimental data for the 10 chars, over a burnout range of 0-75%. The best fit to the experiments occurs with apparent reaction orders of around one for all the chars. © 2011 The Combustion Institute. Source

Karlstrom O.,Abo Akademi University | Brink A.,Abo Akademi University | Biagini E.,Consorzio Pisa Ricerche | Hupa M.,Abo Akademi University | And 2 more authors.
Proceedings of the Combustion Institute | Year: 2013

The influence of concentration of oxygen on the oxidation rates of 5 anthracite chars is investigated at gas temperatures ranging from 1223 K to 1673 K. Reaction orders and kinetic parameters are determined with a multivariable optimization method in which modeled burnout profiles are fitted to experimental burnout profiles from a 4 m isothermal plug flow reactor operating at industrially realistic heating rates, i.e., 104-105 K/s. The determined reaction orders are compared to reaction orders of 10 bituminous coal chars investigated at similar conditions in a previous study. The results show that the optimized reaction order of the anthracite chars is zero, while the reaction order of the bituminous coal char is one. The difference in the reaction orders cannot be explained by using the two semi-global oxidation reactions: C(O) + O2 → CO/CO2 and C(O) → CO. However, by also considering 2C(O) → CO2 + C as a possible reaction step, the reaction order difference between the anthracite chars and the bituminous coal chars can be explained. In addition, a first attempt to apply the same multivariable optimization method to determine the reaction order for a biomass char is presented. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Source

Karlstrom O.,Abo Akademi University | Brink A.,Abo Akademi University | Hercog J.,Polish Institute of Power Engineering | Hupa M.,Abo Akademi University | And 2 more authors.
Energy and Fuels | Year: 2012

In this study, the oxidation of 22 bituminous coal chars is modeled with (i) an individual activation energy for each char and (ii) constant activation energy for all the chars. Modeled burnout profiles, from 0 to 75% of burnout, are compared to experimental measurements from a 4 m isothermal plug flow reactor operating at temperatures and heating rates typical of pulverized fuel industrial combustion. The fuel and the gas rates are chosen such that temperature gradients in the radial direction and along the centerline of the reactor are minimized. In this study, the objective is to predict the burnout profiles with a model suitable for the comprehensive computational fluid dynamics (CFD) modeling of pulverized fuel boilers. Therefore, a power law model that takes into account external diffusion and apparent kinetics is used. The kinetic parameters that are used in the model are determined with a suggested multivariable optimization method. The results show that the experimental burnout profiles of the 22 individual chars are not predicted with a significantly higher accuracy using separately determined activation energy for each char than they were using a constant activation energy for all the chars. Thus, only one fuel specific parameter (i.e. the pre-exponential factor) is needed to model the burnout profiles. These findings are in agreement with some previous studies but are important considering the significant amount of experimental data and the large number of coal chars investigated using a systematic approach. © 2012 American Chemical Society. Source

Li J.,KTH Royal Institute of Technology | Bonvicini G.,International Flame Research Foundation | Tognotti L.,International Flame Research Foundation | Tognotti L.,University of Pisa | And 2 more authors.
Fuel | Year: 2014

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 C at a rate of 104 C/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. © 2014 Elsevier Ltd. All rights reserved. Source

Li J.,KTH Royal Institute of Technology | Biagini E.,International Flame Research Foundation | Yang W.,KTH Royal Institute of Technology | Tognotti L.,International Flame Research Foundation | And 2 more authors.
Combustion and Flame | Year: 2013

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. © 2013 The Combustion Institute. Source

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