Papers by Author: Mikko Hupa

Abstract: Recovery of energy from biomass and various waste–derived fuels by combustion has become important due to reduction of detrimental CO₂ emissions. Biomass does, however, release significant amounts of chlorine and alkali metals, as e.g. HCl(g), KCl(g), KOH(g) and NaCl(g), into the gas phase during combustion. The alkali chlorides may cause deposits on superheater tubes, which interfere with operation and can lead to corrosion and/or blockage of the gas path. To prevent and diminish the problems mentioned above, better and more detailed knowledge of the reactions between potassium chloride and the tube materials during combustion is needed. These materials commonly contain, among other metals, chromium, which is thought to protect the rest of the material since it forms a very dense but thin oxide layer on the surface of the tube material. It has been suggested that the reaction between solid or partly molten KCl and chromium oxide is the one responsible for starting the complex series of corrosion reactions. In this work, the overall reaction between potassium chloride and chromium was studied through partial reactions with compounds known to participate to the overall reaction or to be formed during it. The reactions were studied in synthetic air by heating sample mixtures in a DTA/TGA (Differential Thermal Analysis/ Thermogravimetric Analysis) apparatus. Selected samples were also studied and analyzed with a scanning electron microscope equipped with an energy dispersive x-ray analyzer (SEM/EDXA). Under the used conditions both potassium chloride and potassium chromate reacted with pure chromium and chromium oxide. In the case of chromium, chromium oxide was formed via the formation of potassium chromate. In reactions including chromium oxide as reactant also potassium dichromate was detected.

Abstract: The increasing use of biomass and waste derived fuels in combustion challenges the chemical durability of refractories. Durability of an alumina refractory was studied in a chemically aggressive environment. A mixture of potassium chloride and carbonate (molar ratio 1:9) was placed on the sample and heated at 700-1000°C in an electric laboratory furnace in air for one week. Cross-sections of the samples were studied by SEM-EDXA to determine penetration of potassium in the refractory. Potassium was found only in the silicate matrix phase of the alumina refractory. Penetration of potassium decreased steeply from the surface to 1 mm, after which the decrease was linear but varied with temperature. At 700 and 800°C the thickness of the matrix layer that had reacted with potassium was 3 mm, while the layer was thinner at 900 and 1000°C. At the higher temperatures a glassy layer consisting of K2O, Na2O, CaO and SiO2 formed on the refractory surface. At 900°C the thickness of the surface layer was of 10μm, while a 200μm layer was measured at 1000°C. The procedure used in this work can be used to develop a laboratory scale method to be used to study corrosion of refractories in biomass combustion devices.

Abstract: In this work we summarize the most important findings of the influence of glass composition, sample shape and fluid circulation on in vitro behavior of bioactive glasses in the system Na2O-K2O-MgO-CaO-B2O3-P2O5-SiO2. The sample shapes included plates, particulates, powdered glasses, glass fibers and sintered cones with interconnected porosity. The in vitro bioactivity was measured as the changes observed in the immersion solution, SBF, and the formation of reaction layers on the samples at 4 to 168 h immersion. A lower surface area to volume ratio gave smaller changes on the ion concentrations and pH of the immersion solution but thicker reaction layers on the glass surfaces. In particulate systems with circulating fluid the pH gradients in SBF were lower but surface layers more even than in static systems. The influence of glass composition on reaction layer formation as suggested by glass plates correlated with the in vitro behavior of glass particulates larger than 250 µm, porous cones and fibers when using similar SA/V ratio.

Abstract: A method for measuring the early stage ion release of glasses was developed in order to gain information on leaching kinetics and chemical resistance of glasses in aqueous environments. A continuous flow-through-reactor was designed in which the aqueous solution is fed through a bed of glass particles and the dissolved ions continuously recorded with a sensitive on-line analysis system. Experimental parameters, such as solvent flow rate and temperature, could easily be adjusted according to the needs. The flow-through-reactor system was tested with powdered samples of float and lead glasses, E-glass and bioactive glasses 45S5 and 1-98, all of which showed very different chemical durability in aqueous environments. The reactor was connected to inductively coupled plasma optical emission spectrometry (ICP-OES) and concentrations of the dissolved ions were measured simultaneously on-line every 30-40 seconds. In this work the initial stages of ion release were measured during the first 15 minutes of the leaching experiments at 40°C and 80°C. The results were compared with standard water durability test of the glasses. The dissolution of the glasses according to both methods showed similar behaviour.

Abstract: In vitro reactions of bundles of fibers with diameters 20-500 μm and crushed glasses of fractions 500-800 μm were compared with the reactions of plates of the same bioactive glasses. The samples were immersed in simulated body fluid (SBF) for 2-7 days. After immersion the changes on the surfaces of the samples were observed by SEM/EDXA. Layer formation on the glass surface was found to vary with glass composition, sample shape and local condition of single particle/fiber. However, only some fibers or particles formed similar in vitro reaction layers as the plates. The product form did not change the in vitro bioactivity of particles or fibers exposed to the bulk immersion solution. When the glasses were used as fiber bundles or particle beds, the packing degree and the flow of body fluids within the system interfered with the reactivity. Also a clear correlation between in vivo layer formation in bone and in vitro of the glass plates could be found.

Abstract: Factors controlling the antibacterial ability of three bioactive glasses were studied by comparing the changes in the SBF during immersion of the glasses with their response to four microorganisms. After immersion of 100 mg/ml fine powder (<45μm) of the glasses in the SBF for 1, 2, 4, 8, 16, 27 and 48 hours, the immersion solutions were filtered and the pH in the bulk solution was measured. Ionic concentrations of Na, K, Ca, Mg, P and Si ions in the immersion solutions were determined by ICP-OES. The antibacterial activity of the glasses showed good correlation with the changes of the pH values in the SBF solutions. No correlation was found between the ionic concentration and the antibacterial ability of the three glasses against the four tested microorganisms. The results suggest that, the antimicrobial effect of the glass powder against the microorganisms tested is mostly dependent on the increase of the pH in the solution to values detrimental for the bacteria growth.

Abstract: Implants with long lasting bioactivity and mechanical sustainability would be of interest in several novel clinical applications. By processing bioactive glass fibers and biodegradable polymers into 3D structures, bone formation ability of glasses and flexibility of polymers can be combined. In order to achieve desired physiological response, reactivity of bioactive glass fibers must be specified. Bundles of fibers within the range of bioactivity were soaked in the simulated body fluid at stationary conditions for several time intervals after which the cross-sectional surfaces of the fibers were studied with SEM-EDXA. The reaction layers and precipitations formed on the fiber surfaces suggest that the fibers react according to three mechanisms depending on the glass composition. Fibers with a high in vitro bioactivity showed the formation of distinct and thick silica –rich and calcium phosphate –rich layers already at one day’s immersion. Fibers of medium bioactivity did not show any clear silica –rich layer but a formation of calcium phosphate precipitations or layers at one day’s immersion. Slow glasses showed sporadic calcium phosphate precipitation only after the longest immersion times. The results indicate that the medium and slow glasses are interesting alternatives for applications where a long term mechanical durability suggested by their slow reactivity in combination with their osteoconductive tendency is desired.

Abstract: Antibacterial effects of three powdered bioactive glasses were compared by measuring the pH of the simulated body fluid in immediate contact with the glass powder particles and by cultivating four microorganisms in the powder-solution mixtures. After individual cultivation with the microorganisms the bioactive glasses showed ability to kill bacteria, but the effects were very dependent on glass composition and on the microorganism as well as on cultivation time. The results indicated that antimicrobial property of bioactive glasses correlates with their ability to change the pH of the body fluid in immediate contact with the glass. However, the increase in pH is not the only factor influencing the antimicrobial properties.

Abstract: The crystallization tendency for 30 experimental glasses in the system Na2O-K2O-MgOCaO-B2O3-P2O5-SiO2 was studied with thermal methods, DTA, HSM and XRD. The glasses were also immersed into simulated body fluid for 8 and 72 hours. The formation of the silica-rich gel and calcium phosphate layer on the glasses were analyzed with SEM. The in vitro behavior and crystallization tendency for heat-treated glasses were then related. This information is essential for choosing glass compositions that can be manufactured to desired products with controlled bioactivity for different applications. In general, glasses with low alkali content can tolerate heattreatment without crystallization but have less initial Si-gel formation ability and show less in vitro bioactivity than glasses with high alkali content.

Abstract: In vitro behavior of 30 new glasses in the system Na2O-K2O-MgO-CaO-B2O3 -P2O5-SiO2 was investigated by immersing them into a simulated body fluid for 4 to168 hours. This study involved the observation of both the changes in the properties of the immersion solution and on the glasses surface after immersion. In vitro reactivity was different for each experimental glass depending on its chemical composition. By comparing these glasses to four established bioactive glasses showing different in vivo bioactivity, a fast estimation of the bioactivity of glasses can be done; thereby the optimization of bioactive glasses for various clinical applications can be developed.