Gorkem Kulah

Benchmarking grey particle approximations against nongrey particle radiation in circulating fluidized bed combustors

ABSTRACT Investigation of the effect of grey/nongrey particle property models on radiative heat fluxes and source terms is performed in the dilute zone of the lignite-fired 150 kW Middle East Technical University circulating fluidized bed combustor test rig. Predictive accuracy and computational economy of several grey particle models, geometric optics approximation (GOA) with average particle reflectivity (GOA2), GOA with Fresnel solution for particle reflectivity (GOA3), and Planck mean particle properties from spectral Mie solution are tested by benchmarking their predictions against spectrally banded solution of radiative transfer equation (RTE). Comparisons reveal that all grey models lead to accurate and CPU efficient radiative heat flux predictions. On the other hand, only GOA3 and Planck mean properties are in favorable agreement with the benchmark solution for both incident fluxes and source terms. These findings indicate that grey particle approximation with GOA3 is a more practical choice in solution of RTE as it eliminates the need for spectral calculations.

A Comprehensive Fluidized Bed Combustion Model Coupled with a Radiation Model

A freeboard model of an overall FBC system model where radiation between particle and gas is accounted for through Stefan-Boltzmann law was modified by incorporation of a radiation model based on method of lines solution of discrete ordinates method. The predictive accuracy of the coupled code was tested by comparing its predictions with measurements taken on a refractory-lined, hybrid, pilot scale FBC as well as with predictions of unmodified system model. Comparisons show good agreement particularly for the test with recycle of fine particles. Significance of using a rigorous radiation model was illustrated by validating its predictions against measurements available on an industrial scale fluidized bed boiler. It was concluded that coupling a rigorous radiation model with an overall FBC system model is essential for accurate prediction of the behaviour of industrial boilers characterized by low surface to volume ratio and relatively colder water walls.

CFD-DEM Simulation of a Conical Spouted Bed Operating with High Density Particles

The hydrodynamics of a spouted bed operating with high density zirconia particles was studied by CFD-DEM simulations. The model was validated by experimental data. Simulation results were used to obtain radial and axial profiles of the particle velocity, solid hold-up and flux in the bed at two bed heights. The gas jet velocity profile in the spout was also studied. For calculation of the solid circulation time in the bed, axial positions of 50 particles were traced in time. The mean solids circulation time in the bed was found to be 1.5 s.

Comparison Between Combustion Behavior of Solid Fuels and Their Chars Under Oxy-Fuel Conditions

The combustion characteristics of petcoke and imported coal and their respective chars under oxy-fuel (21% O2/79% CO2) and oxygen-enriched oxy-fuel (30% O2/70% CO2) environments were investigated by using non-isothermal thermogravimetric method coupled with a Fourier transform infrared spectrometer. Experimental results reveal that at elevated oxygen levels, profiles shift to lower temperature zone, peak and burnout temperatures decrease, weight loss rate increases significantly, and complete combustion is achieved at lower temperatures. The ignition, peak, and burnout temperatures of chars are found to be higher than those of their parents due to absence of volatile matter and higher ash content left in the char. An Arrhenius-type kinetic model was used to determine the values of the kinetic parameters of the combustion of the fuels and their chars. Values of activation energies and Arrhenius constants predicted by the model show that chars are less reactive than their parent fuels due to the absence of volatiles.

Assessment of gas radiative property models in the presence of nongray particles

ABSTRACT In this study, a radiation code based on the method of lines solution of the discrete ordinates method for the prediction of radiative heat transfer in nongray gaseous media is developed by incorporation of two different spectral gas radiative property models, banded spectral line-based weighted sum of gray gases (banded SLW) and gray wide band (GWB) approximation in the presence of nongray absorbing–emitting–scattering particles. The aim is to introduce an accurate and CPU efficient spectral gas radiation model, which is compatible with spectral fuel/ash particle property models. Input data required for the radiation code and its validation are provided from two combustion tests previously performed in a 300 kWt atmospheric bubbling fluidized bed combustor test rig burning low calorific value Turkish lignite with high volatile matter/fixed carbon (VM/FC) ratio in its own ash. The agreement between wall heat fluxes and source term predictions obtained by global and banded SLW models reveal that global SLW model can be converted to an accurate wide band gas model (banded SLW) which can directly be coupled with spectral particle radiation. Furthermore, assessment of GWB approximation by benchmarking its predictions against banded SLW model shows that GWB gives reasonable agreement with a higher CPU efficiency when the particle absorption coefficient is at least in the same order of magnitude with the gas absorption coefficient.

Effect of kaolin addition on alkali capture capability during combustion of olive residue

ABSTRACT The effect of kaolin addition on the combustion characteristics of olive residue was investigated by thermogravimetric analysis (TGA) combined with Fourier-transform infrared (FTIR) spectrometer in parallel with ash characteristics were studied by X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyzer. The experiments were carried out by burning olive residue with 2%, 4% and 8% by wt. kaolin at 900°C. Results indicate that the addition of kaolin led to a decrease in the rate of weight loss during char combustion and hence an increase in CO emission. This was due to a decrease in burning quality caused by the dilution effect of kaolin in the mixture. The increase in HCl(g) and SO2(g) concentrations with kaolin addition observed in FTIR results evaluated together with XRF and XRD results demonstrates the pathway of reactions for the capture of potassium by kaolin and formation of high-melting-temperature kalsilite crystals. Potassium retention in ash was found to increase up to 4% kaolin content; however, no significant improvement in potassium retention was observed with further addition of kaolin owing to the abundant amounts of Al and Si introduced via kaolin. The results show that 4% kaolin is the optimum content at which the alkali index decreases below unity, indicating reduced tendency of fouling in combustion systems.

Effect of limestone addition on radiative heat transfer during co-firing of high-sulfur content lignite with biomass in fluidized bed combustors

ABSTRACT In this study, influence of limestone addition on radiative heat transfer during co-firing of high-sulfur content lignite with olive residue in the freeboard of 300 kWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) is investigated by using an in-house developed radiation code based on Method of Lines (MOL) solution of Discrete Ordinates Method (DOM). The freeboard is treated as a 3D rectangular enclosure containing gray, absorbing, emitting gas with absorbing, emitting, anisotropically scattering particles surrounded by black/gray diffuse walls. Radiative properties of combustion gases and particles are evaluated by using Leckner’s correlations and Mie theory, respectively. Input data required for the model are provided from four combustion tests which were previously carried out sequentially for lignite only, lignite with limestone addition followed by 30 and 50 wt % shares of olive residue in the fuel mixture for the same Ca/S ratio. The results show that addition of limestone affects the radiative properties of the particles in the freeboard through the change of particle size distribution and load rather than optical properties and decreases the radiative wall heat fluxes along the freeboard. On the other hand, co-firing with biomass compensates this decrease as less limestone is utilized in almost direct proportion to the coal replacement.

Mathematical Modeling of a Bubbling Fluidized Bed Combustor Cofired with Lignite and Biomass

A comprehensive system model of fluidized bed combustor, previously developed and tested for the prediction of combustion behavior of fluidized bed combustors fired with lignite was extended for the modeling of cofiring of lignite with biomass by incorporating volatile release, char combustion, and population balance for biomass. The model predictions were validated against measurements taken on a Middle East Technical University 0.3 MWt Atmospheric Bubbling Fluidized Bed Combustor fired with lignite only, lignite with limestone addition, and about 50/50 lignite–olive residue mixture with limestone addition. Predicted and measured temperatures and concentrations of gaseous species along the combustor were found to be in good agreement. Introduction of biomass to lignite was found to decrease SO2 emissions but did not affect NO emissions significantly. The system model proposed in this study proves to be a useful tool in qualitatively and quantitatively simulating the processes taking place in a bubbling fluidized bed combustor burning lignite with biomass.