Coal is today the fossil fuel which offers the greatest proven reserves. Due to increasingly stringent environmental legislation, coal fired combustion systems should be based on clean coal combustion technologies. For clean and efficient energy generation from coal reserves, the most suitable technology known to date is the &lsquo / Fluidized Bed Combustion&rsquo / technology. Applications of circulating fluidized bed combustion (CFB) technology have been steadily increasing in both capacity and number over the past decade for the utilization of this resource. Designs of these units have been based on the combustion tests carried out in pilot scale facilities to determine the combustion and desulfurization characteristics of the coals and limestones in CFB conditions. However, utilization of Turkish lignites with high ash, volatile matter and sulfur contents in CFB boilers necessitates adaptation of CFB combustion technology to these resources. Therefore, it has been the objective of this study to investigate combustion characteristics of an indigenous lignite in a circulating fluidized bed combustor. In this study, a 150 kWt Circulating Fluidized Bed (CFB) Combustor Test Unit was designed and constructed in Chemical Engineering Department of Middle East Technical University, based on the extensive experience acquired at the existing 0.3 MWt Bubbling Atmospheric Fluidized Bed Combustor (AFBC) Test Rig. Following the commissioning tests, combustion tests were carried out for investigation of combustion characteristics of &Ccedil / an lignite in CFB conditions and for comparison of the design of the test unit with experimental findings. The steady state results of the combustion tests reveal that &Ccedil / an lignite is fired with high combustion efficiency. Temperature profile along the riser is achieved to be almost uniform by good control of cooling system. Pressure drop through the dilute zone is found to be negligible because of low solid hold up in this zone. CO and NO concentrations within the flue gas are fairly lower, whereas N2O concentration is higher compared to the ones obtained in the bubbling AFBC test rig firing the same lignite. The deviation of particle size distributions of bottom ash and circulating ash among the tests are in line with the deviation of superficial velocity. In order to assess the validity and predictive accuracy of the pressure balance model, it was reapplied to the test unit utilyzing the revised input data based on the results of the combustion tests. Comparison of the model predictions with experimental results revealed that the predictions have acceptable agreement with the measurements. In conclusion, the performance of 150 kW CFBC Test Unit was found to be satisfactory to be utilized for the long term research studies on combustion and desulfurization characteristics of indigenous lignite reserves in circulating fluidized bed combustors.

Proton exchange membrane fuel cells are one of the most promising hydrogen energy conversion devices for portable, mobile and stationary applications. For wide spread usage to produce electricity platinum loading has to be decreased by using highly active electrocatalysts. Even 10 ppm carbon monoxide or higher than 30% carbon dioxide cause performance losses via deactivation which can be diminished by using binary catalysts. The aim of this thesis is to develop new platinum based electrocatalysts with high catalytic activity and to overcome the problems due to the deactivation. platinum and platinum-ruthenium based catalysts on different carbon supports have been prepared by supercritical carbon dioxide deposition and microwave irradiation methods. By using supercritical carbon dioxide deposition platinum on Vulcan XC72R (VXR), multi wall carbon nanotube (MWCNT) and Black Pearl 2000 (BP2000) catalysts were prepared and characterized by XRD, TEM and cyclic voltammetry (CV). XRD results showed that in catalysts prepared by using supercritical carbon dioxide deposition method, the particle sizes as low as 1-2 nm can be obtained. From the CV results the electrochemical surface areas obtained were Platinum/VXR&gt / Platinum/MWCNT&gt / PlatinumBP2000. By means of the oxygen reduction reaction (ORR), the number of electrons transferred per oxygen molecule was calculated as 3.5, 3.6 and 3.7 for Platinum/BP2000, Platinum/VXR and Platinum/MWCNT, respectively. The microwave irradiation was used to prepare platinum on VX, Regal and BP2000 and platinum-ruthenium on VX. The effects of microwave duration, base concentration, carbon support used and surfactant/precursor ratios were investigated. The particle sizes of the catalysts were ranging between 2-6 nm. The prepared catalysts were characterized by XRD, XPS, and then PEMFC tests were performed. The performance was ordered as Platinum/VX&gt / Platinum/Regal&gt / Platinum/BP2000. The power losses arising from carbon dioxide in hydrogen feed were decreased by using prepared platinum-ruthenium based catalysts.

Area averaging is one of the techniques used to solve problems encountered in the transport of momentum, heat, and mass. The application of this technique simplifies the mathematical solution of the problem. However, it necessitates expressing the local value of the dependent variable and/or its derivative(s) on the system boundaries in terms of the averaged variable. In this study, these expressions are obtained by the two-point Hermite expansion and this approximate method is applied to some specific problems, such as, unsteady flow in a concentric annulus, unequal cooling of a long slab, unsteady conduction in a cylindrical rod with internal heat generation, diffusion of a solute into a slab from limited volume of a well-mixed solution, convective mass transport between two parallel plates with a wall reaction, convective mass transport in a cylindrical tube with a wall reaction, and unsteady conduction in a two -layer composite slab. Comparison of the analytical and approximate solutions is shown to be in good agreement for a wide range of dimensionless parameters characterizing each system.

This study focuses on the molasses based complex medium design for benzaldehyde lyase production by recombinant E. coli BL21 and development of a feeding strategy based on the designed complex medium. For this purpose, firstly, the effects of molasses were investigated in laboratory scale bioreactors. As E. coli BL21 was not able to utilize sucrose, molasses was pretreated and hydrolyzed to fructose and glucose. Thereafter, effect of pretreated molasses concentration was investigated in the range of 16 to 56 kg m-3 by batch-bioreactor experiments / and the highest cell concentration and benzaldehyde lyase activity were obtained as CX=5.3 kg m-3 and A=1617 U cm-3, respectively, in the medium containing 7.5 kg m-3 glucose and 7.5 kg m-3 fructose. Then, different feeding strategies were developed to produce efficient cells with high concentration and BAL activity. In the first strategy, after 10 hours of batch-cultivation with molasses based medium having 7.5 kg m-3 glucose and 7.5 kg m-3 fructose concentration, based on the airflow rate, pretreated molasses was fed to the system. When air flow rate decreased considerably, fed was given to the system that results in increase in glucose and fructose concentration in the medium to 2.5 kg m-3. At the end of the process, the highest cell concentration obtained was CX=7.4 kg m-3. The maximum activity was reached at 20th hour as A=2360 U cm-3. On the other hand, as air flow variation only demonstrated the absence of glucose not fructose, a second strategy, based on the detection of the fructose and glucose concentrations during the process, was applied. In this strategy when glucose and fructose were depleted, fed was given to the system that results in increase in glucose and fructose concentration in the medium to 2.5 kg m-3 / and the highest BAL activity was obtained as 2370 U cm-3 at t= 26 h where the cell concentration was 7.5 kg m-3. At the last strategy, when glucose and fructose were depleted, fed was given to the system that results in increase in CGlucose=1.5 kg m-3 and CFructose=1.5 kg m-3 in the production medium to decrease the accumulation of acetic acid. By this strategy highest cell concentration was obtained as 8.04 kg m-3 at t=24 h and the highest BAL activity was 2315 U cm-3. These strategies could be accepted having the same BAL activity with little distinctions. However, cell concentration of the last one was higher than others and also the lowest amount of carbon source was used. Thus, last one could be chosen as the most favorable strategy.

In this study, the effects of pH on therapeutically important protein, recombinant human erythropoietin (rhuEPO), production by Pichia pastoris was investigated at pH=4.0, 4.5, 5.0, 5.5 and 6.0. rHuEPO production was started by methanol induction in fed-batch mode. The highest cell concentration was obtained at pH=4.5 as 81.4 g L-1. The co-substrate substrate sorbitol, which was added batch-wise, was consumed at t=15 h of the operations at pH=4.0, 4.5 and 5.0. However as the pH increases above pH=5.0 the sorbitol consumption rate decreases. The highest rHuEPO concentration was achieved at pH=4.5 as 0.158 g L-1 which was 1.43-, 1.24-, 1.95- and 1.23-fold higher than those obtained at pH=4.0, 5.0, 5.5, and 6.0, respectively. Also at pH=4.5 overall cell yield on substrate was 0.51 g g-1 and overall rHuEPO yield on substrate was 1.45 mg g-1. rHuEPO concentration was decreased in the last 3-6 hour of the operation due to proteolysis. Therefore extracellular protease concentrations in the medium were determined. As expected, since the investigated pH range was acidic, the amount of acidic proteases was found to be higher than neutral and basic proteases. Furthermore the total protease concentration increased linearly in the fermentation broth, having close values at different pH values. Thus, pH did not have a significant effect on extracellular protease activity. Alcohol oxidase (AOX) activities showed similar behavior at different pH. The highest specific AOX activity was attained at pH=4.5, at which the highest rHuEPO concentration was achieved, as 110.1 U g-1 CDW. Keywords:

NOx storage and reduction activities of Pt/BaO/CexZr1-xO2 (x= 1, 0.75, 0.5, 0.25, 0) catalysts were investigated by transient reaction analysis and Diffuse Reflectance Infrared Fourier Tramsform Spectroscopy (DRIFTS). Pt/BaO/Al2O3 catalyst was used as reference catalyst. Ce-Zr mixed oxides were synthesized by means of Pechini Method. Pechini method is a sol-gel like method based on mixing the related mixed oxide precursors with an &amp / #945 / -hydroxy carboxylic acid. 10% BaO (w/w) and 1% Pt (w/w) were deposited on synthesized mixed oxide and reference alumina support via incipient wetness impregnation. Prepared catalysts were characterized by BET, XRD and in-situ DRIFTS. Pt/BaO/CexZr1-xO2 catalysts (x= 0.75, 0.5, 0.25) exhibited higher surface than pure ceria and zirconia supported catalysts and highest surface was observed for Pt/BaO/Ce0.5Zr0.5O2. The reducibility of catalysts was tested by Temperature Programmed Reduction (TPR). It was observed that the reducibility of Pt/BaO/CexZr1-xO2 catalysts increases with incorporation of Zr into ceria lattice. Pt/BaO/Ce0.25Zr0.75O2 showed the highest reducibility. The Pt dispersions of catalysts were determined by CO chemisorption. The highest Pt disperison was observed in the presence of Pt/BaO/Ce0.5Zr0.5O2 catalyst. NOx storage and reduction efficiency of the catalysts were determined by transient reaction analysis in a home-built setup at 350oC. The reaction results revealed that all catalysts exhibit similar NOX storage and reduction performance. The NOX storage performances of all catalysts were differentiated via in-situ DRIFTS analysis. Pt/BaO/CexZr1-xO2 (x= 0.75, 0.5, 0.25) catalysts revealed higher performance than the ceria and zirconia supported catalysts. Pt/BaO/Ce0.5Zr0.5O2 catalyst absorbed highest amount of NO on the surface in the form of nitrate. The reduction behaviours of catalysts also were investigated by in-situ DRIFTS. It was observed that nitrates and nitrites were completely removed from the surface after reduction by H2 at 300oC.

It is desirable to increase the operation temperature of proton exchange membrane fuel cells above 100oC due to fast electrode kinetics, high tolerance to fuel impurities and simple thermal and water management. In this study / the objective is to develop a high temperature proton exchange membrane fuel cell. Phosphoric acid doped polybenzimidazole membrane was chosen as the electrolyte material. Polybenzimidazole was synthesized with different molecular weights (18700-118500) by changing the synthesis conditions such as reaction time (18-24h) and temperature (185-200oC). The formation of polybenzimidazole was confirmed by FTIR, H-NMR and elemental analysis. The synthesized polymers were used to prepare homogeneous membranes which have good mechanical strength and high thermal stability. Phosphoric acid doped membranes were used to prepare membrane electrode assemblies. Dry hydrogen and oxygen gases were fed to the anode and cathode sides of the cell respectively, at a flow rate of 0.1 slpm for fuel cell tests. It was achieved to operate the single cell up to 160oC. The observed maximum power output was increased considerably from 0.015 W/cm2 to 0.061 W/cm2 at 150oC when the binder of the catalyst was changed from polybenzimidazole to polybenzimidazole and polyvinylidene fluoride mixture. The power outputs of 0.032 W/cm2 and 0.063 W/cm2 were obtained when the fuel cell operating temperatures changed as 125oC and 160oC respectively. The single cell test presents 0.035 W/cm2 and 0.070 W/cm2 with membrane thicknesses of 100 &micro / m and 70 &micro / m respectively. So it can be concluded that thinner membranes give better performances at higher temperatures.

Titanium dioxide (TiO2), a semiconductor, has been used in many areas like heterogeneous photocatalysis. In the present study, the effect of hydrothermal treatment conditions and the use of carbon nanotubes on the photocatalytic activity of sol-gel synthesized titanium dioxide were examined. The anatase particles were transformed into layered trititanate particles with either nanotube or nanoplate structure by hydrothermal treatment under the alkaline conditions. Post hydrothermal treatment under neutral conditions was also applied and mesoporous particles were transformed into nanostructured, highly crystalline and ordered anatase particles. Photocatalytic activities of hydrothermally treated samples were determined against Escherichia coli under solar irradiation. Results showed that hydrothermal treatment under alkaline conditions improved the photocatalytic activity. However, although being highly crystalline, after post treatment, a limited activity was obtained because of dehydration of active (101) face of anatase. Nevertheless, TiO2&amp / #8217 / s initial inactivation constant rose from 0.6 to 2.9 hr-1 after regeneration of active sites in aqueous medium under solar irradiation. In order to enhance the surface area and improve activity, multi-walled carbon nanotubes were utilized during the synthesis of TiO2. The effect of calcination conditions and presence of sodium, iron and cobalt on the photocatalytic activity were also studied. For these samples, photocatalytic activities were tested with methylene blue solution under UV irradiation. It was observed that the utilization of CNTs enhanced both the surface area and the activity. Compositions with highest CNT content had better activities for their ability to delay charge recombination. While pure TiO2&amp / #8216 / s initial decomposition constant was 0.8 hr-1, with sodium doping the best value of 1.9 hr-1 was achieved.

Carbon nanotubes have received considerable attention since their discovery due to their novel properties. They have potential application areas in physics, chemistry and biology. Arc discharge, laser furnace, chemical vapor deposition and floating catalyst methods are the most commonly used methods to produce carbon nanotubes. Although carbon nanotubes have superior properties compared to other materials, they could not be used widely. The main reasons of this are that continuous and large scale production of carbon nanotubes could not be achieved and impurities have to be removed. To solve these problems more information about formation of carbon nanotubes has to be known. In this study concentration profiles of reactant and byproducts in a cylindrical reactor are investigated during carbon nanotube production. A special probe to collect gas samples along the reactor and samples loops to store the gas samples were designed and constructed. Gas samples were analyzed one by one in GC/MS. Experiments were done with and without catalyst at same experimental conditions. Thus, effects of catalyst on concentration profiles of chemicals were analyzed. To produce carbon nanotubes more acetylene was used compared to amount of acetylene used in pyrolysis. Increasing reaction temperature from 800&deg / C to 875&deg / C caused decomposing more acetylene and producing more carbon nanotubes. It is believed that data accumulation on the reactions involved in the gas phase will lead to large scale production and lower product costs with a large catalyst surface to be produced in the reactor.

The aim of this study was to modify the platinum particle size to observe the effects on the efficiency of a Dye Sensitized Solar Cell (DSCC). DSSC was prepared as follows: On the anode side, TiO2 was annealed on the transparent conducting oxide (TCO) which is SnO2:F coated and a cis-bis (isothiocyanato) bis (2,2&#039 / -bipyridyl-4,4&#039 / -dicarboxylato)&ndash / ruthenium(II) dye was adsorbed on the TiO2. On the cathode side, platinum was coated on TCO from an alcohol based solution of platinum (plasitol) by thermal decomposition method. Potassium iodide and iodine were dissolved in ethylene glycol to prepare the electrolyte. Four cathode surfaces were prepared by thermal decomposition method at 400 oC and 5 min. Cathode surface morphology was changed by changing the annealing conditions. Current-voltage measurements were performed for determining the cell efficiency. One cathode glass was used as such giving a cell efficiency of 2.36%. Three glasses were further thermally treated at 450 oC, 500 oC and 550 oC for 30 min. highest efficiency was measured with the counter electrode annealed at 550 oC for 30 min as 2.89%. SEM micrographs of the substrate which was SnO2:F coated TCO revealed a decrease in average surface particle size with an increase in annealing temperature. EDX mappings showed that as the annealing temperature increased, Pt particles segregated together to form porous patches. In this study, it was demonstrated that as the annealing temperature of cathode increased, DSSC efficiency increased. These results can be used to design cheaper DSSCs with higher efficiencies.