Yusuf Gogebakan

Investigation of ash deposition in a pilot-scale fluidized bed combustor co-firing biomass with lignite

This study presents the results from investigation of ash deposition characteristics of a high ash and sulfur content lignite co-fired with three types of biomass (olive residue, 49 wt%; hazelnut shell, 42 wt%; and cotton residue, 41 wt%) in 0.3 MW(t) Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) Test Rig. Deposit samples were collected on an air-cooled probe at a temperature of 500 degrees C. Samples were analyzed by SEM/EDX and XRD methods. The results reveal that co-firing lignite with olive residue, hazelnut shell and cotton residue show low deposition rates. High concentrations of silicon, calcium, sulfur, iron, and aluminum were found in deposit samples. No chlorine was detected in deposits. Calcium sulfate and potassium sulfate were detected as major and minor components of the deposits, respectively. High sulfur and alumina-silicate content of lignite resulted in formation of alkali sulfates instead of alkali chlorides. Therefore, fuel blends under consideration can be denoted to have low-fouling propensity.

Modeling of a Bubbling AFBC with Volatiles Release

A comprehensive model for continuous combustion of lignite with wide size distribution burning in its own ash in an atmospheric bubbling fluidized bed combustor (ABFBC) has been presented and used to correlate the data from a 0.3 MW ABFBC test rig. The model consists of submodels for hydrodynamics, volatiles release and combustion, char combustion, particle size distribution, entrainment and elutriation and is based on conservation equations for energy and chemical species. The volatiles release model is based on a particle movement model for estimation of portion of volatiles released in bed and a devolatilization kinetics model described by the distributed activation energy model for determination of time-resolved devolatilization profile of lignite particles. Uniform in-bed volatiles release is assumed. The overall model was applied to a 0.3 MW ABFBC test rig fired with lignite with VCM/FC ratio of 2.16. The fuel was fed 0.22 m above the distributor plate and the expanded bed height was I m. Predictions of the model were compared with measured concentration and temperature profiles and good agreement was obtained. Bed/freeboard combustion split predicted by the model was found to be 80/20 as opposed to the value 84/16 determined from measured species profiles.

Co-Firing of Olive Residue with Lignite in Bubbling FBC

The effect of biomass share on gaseous pollutant emissions from fluidized bed co-firing of various biomass fuels with high calorific value coals have extensively been investigated to date. However, effect of co-firing of olive residues with low calorific value lignites having high ash and sulfur contents has not been studied in bubbling fluidized bed combustors. In this study, experimental results of various runs pertaining to gaseous emissions (O2, CO2, CO, SO2, NO, N2O) from METU 0.3 MWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) test rig co-firing olive residue with indigenous lignite at different biomass shares are presented. The results reveal that co-firing increases combustion efficiency irrespective of the biomass share and that increase in biomass share reduces N2O and SO2 emissions considerably while increasing CO emission. O2, CO2 and NO emissions are not found sensitive to increase in biomass share. Olive residues are co-fired with high ash and sulfur containing lignite without any operational problems.

EFFECT OF RECYCLE ON FLUIDIZED-BED COMBUSTION AND EMISSION CHARACTERISTICS OF HIGH-SULFUR LIGNITE

The effect of recycle on gaseous pollutant emissions from fluidized-bed combustion of high-quality coals has extensively been investigated and is well documented. However, the effect of recycle on gaseous emissions from combustion of Turkish lignites with high ash, volatile matter, and sulfur contents has not been investigated to date. Recent trends in the utilization of indigenous lignites in fluidized-bed boilers necessitated investigation of pollutant emissions and adaptation of fluidized-bed combustion technology to these lignites. In this study, experimental results of various runs pertaining to gaseous emissions from the Middle East Technical University 0.3 MWt atmospheric bubbling fluidized-bed combustor test rig burning a typical indigenous lignite, Beypazari, at various recycle ratios, with and without limestone addition, are presented. Measurements show that the effect of recycle on already high combustion efficiency in once-through operation is not pronounced. As for the gaseous emissions, introduction of recycle reduces NOx and SO2 emissions but enhances CO emission.

Assessment of Catalyst Deactivation Model for Sulfur Retention in Fluidized Bed Combustors

Abstract An improved version of a combustion-desulfurization model for fluidized bed combustors burning high ash and sulfur content low quality lignite in its own ash with the addition of limestone is presented. The desulfurization model considers sulfur retention by both limestone and ash and is based on first order catalyst deactivation where the kinetics of desulfurization is described by two rate constants; one for the initial surface reaction, the other for the rate of pore plugging or deactivation. The accuracy of the complete model with experimentally determined deactivation rate constants was tested by applying it to a 0.3 MWt AFBC test rig and comparing its predictions with measurements. Comparisons show that (he model produces reasonably accurate predictions of sulfur retention provided that the rate constants are determined experimentally under the fluidized bed combustor operating conditions.

MODELING OF NOx EMISSIONS FROM FLUIDIZED BED COMBUSTION OF HIGH VOLATILE LIGNITES

A comprehensive model, previously developed and tested for prediction of behavior of continuous fluidized bed combustors is extended to incorporate NOx formation and reduction reactions and applied to the simulation of Middle East Technical University (METU) 0.3 MWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) burning lignites with high Volatile Matter/Fixed Carbon (VM/FC) ratios in their own ashes. Favorable comparisons are obtained between the predicted and measured temperatures and concentrations of gaseous species along the combustor. Results show that determination of partitioning of coal nitrogen into char-N and volatile-N, char combustion rate, and amount of volatile nitrogen released along the combustor are found to be the most important parameters that affect NO formation and reduction in bubbling fluidized bed combustors.

ASSESSMENT OF A MODEL WITH CHAR ATTRITION FOR A BUBBLING ATMOSPHERIC FLUIDIZED-BED COMBUSTOR

A comprehensive model, previously developed and tested for prediction of behavior of continuous fluidized-bed combustors, is extended to incorporate char attrition and is applied to the simulation of a 0.3 MegaWatt (MW) atmospheric bubbling fluidized-bed combustor burning lignites with high Volatile Matter/Fixed Carbon (VM/FC) ratios in their own ashes. The effect of this incorporation is assessed by comparing the predictions of the model with and without attrition with measurements. Favorable comparisons obtained between predicted and measured temperature and concentration profiles and carryover char particle size distributions imply that these parameters are not sensitive to char attrition for lignites and the operating conditions under consideration. However, comparisons between combustion efficiencies predicted with and without char attrition and those obtained from measurements show that consideration of char attrition is essential for accurate prediction of combustion efficiencies.

Effect of Recycle on Radiative Heat Transfer in the Freeboard of a Fluidized Bed Combustor

Effect of recycle on radiative heat transfer in the freeboard of a fluidized bed combustor is investigated by applying a previously developed 3-D radiation model to the prediction of incident radiative heat fluxes along the freeboard walls of lignite-fired 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig and comparing its predictions with measurements. Freeboard is treated as a rectangular enclosure containing gray, absorbing, emitting and isotropically scattering medium bounded by gray and diffuse walls. Radiative properties of the medium are calculated by using Leckner’s correlations for gas and Mie theory for polydisperse particle cloud. Radiative transfer equation for this system is solved by using Method of Lines (MOL) solution of Discrete Ordinates Method (DOM). Experimental data required for application and validation are generated from two runs in which parameters other than recycle ratio was held as nearly constant as possible. Comparisons between predicted incident radiative heat fluxes and measurements with and without recycle reveal that the agreement is excellent and that the effect of recycle on incident radiative heat fluxes is significant. A parametric study is also carried out to investigate the effect of particle load on fluxes. Predictions are found to be relatively insensitive to the particle load but strongly affected by the temperature profile.Copyright

Partitioning of Trace Elements During Fluidized Bed Combustion of High Ash Content Lignite

The behavior of 20 trace elements (As, B, Ba, Cd, Co, Cr, Cu, Hg, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Tl, V, Zn) and 8 major and minor elements (Al, Ca, Fe, K, Mg, Na, Si, Ti) during the combustion of high ash content lignite with and without limestone addition have been investigated in the 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig. Experiments were performed without fines recycle. Inert bed material utilized in the experiments was bed ash obtained previously from the combustion of the same lignite without limestone addition in the same test rig. Concentrations of trace elements in coal, limestone, bottom ash, cyclone ash and filter ash were determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Measurements show that the distribution of major and minor elements follows the ash split between the bottom ash and fly ash and that the major proportion of most of the trace elements (As, Ba, Cr, Hg, Li, Mo, Ni, Sn, V, Zn) are recovered in fly ash. Comparisons between the trace element partitioning of the runs with and without limestone addition reveal that addition of limestone shifts the partitioning of Ba, Cr, Hg, Mo, Ni, Sn, V, Zn from bottom ash to fly ash.Copyright

Co-Firing of Steam Coal With High Sulfur Content Lignite in a Bubbling Fluidized Bed Combustor

Combustion and emission behavior of 100 % steam coal (SET 1) and a mixture of 80 % by weight steam coal and 20 % by weight local lignite, characterized by high sulfur and ash contents, (SET 2) were investigated in the 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig. Experiments were performed with limestone addition at various Ca/S molar ratios with fines recycle. In both sets of experiments, parameters other than Ca/S molar ratio were held as nearly constant as possible. On-line measurements of O2 , CO2 , CO, SO2 , NOx emissions were carried out. Comparisons between the emissions show that lower NOx and SO2 emissions are obtained from combustion of steam coal/lignite mixture compared to those from steam coal only despite higher sulfur and almost equal nitrogen contents of the mixture. Calculated combustion efficiencies were found to be around 98 and 96 % for SET 1 and SET 2, respectively. As for the sulfur retention efficiencies, up to three times higher efficiencies were achieved when steam coal is co-fired with high sulfur lignite.Copyright