Swirling flows have been widely used for many years in engineering applications such as; chemical and mechanical mixing devices, separation units, spray drying technologies, turbo machinery and combustion systems. In practical combustion applications, swirl motion has been adopted to the incoming reactant flows in order to enhance the mixing of fuel and oxidizer and to improve flame stabilization and establishment, especially in regions of relatively low velocities, by recirculating hot product gas to the incoming reactants. At critical operating conditions the recirculation zone exhibit high sensitivity to flow disturbances leading to hydrodynamic instabilities. During combustion, these instabilities can interact with flame structures by modulating the rate of heat release, equivalence ratio, flame surface etc. As a result of these interactions, combustion instabilities can form in the systems. At initial state, combustion instabilities can stay unnoticed due to their relatively small amplitudes, but the amplitudes of the instabilities can increase when they couple with acoustical characteristics of any particular system elements. As a result, combustion systems can suffer from high amplitude noise, vibrations, flame flashback, local flame quenching, and even severe damages in system structures. This thesis provides insights into the interaction of acoustic waves with a swirl stabilized wood powder flame and its effects on flame structures. A high speed photography technique has been applied to wood powder flame under external forcing of the secondary air flow pattern to record spontaneous emission of radiant energy from the flame. Simultaneously, dynamic pressure signals were acquired with a data acquisition board in order to relate pressure data with radiant energy which has been assumed to be representative of heat release. In order to investigate the influence of the interactions on combustion, the resulting data were complemented with gas sampling measurements. From digital still images taken without external forcing, the wood powder flame was observed to expand to occupy the entire combustion chamber. In addition, the flame shape and size appears to be unchanged under a wide range of forcing frequencies, with one exception at a particular low frequency for which a resonant behaviour was observed. The critical frequency was 17 Hz independent of amplitude of the forcing frequency and at this forcing frequency dramatic changes in flame size and shape was observed. Instantaneous and phase averaged images have revealed the presence of large scale vortical structures that closely interacted with the flame surface. A fast Fourier transform of the point wise optical signal also shows that the flame is susceptible to instabilities at the acoustical forcing of 17 Hz. The existence of thermo-acoustically induced combustion instability has been investigated by a Rayleigh criterion which states that the amplitude of a sound wave will be amplified when heat is added less than 90 degrees out of phase with its pressure. In this study, the heat release extracted from high speed images recorded at 17 Hz is approximately 40 degrees out of phase with the pressure data which confirms the thermo-acoustic nature of the instability. Finally, from gas sampling measurements it was concluded that the acoustic oscillations at 17 Hz have increased the NOx emission level to around twice the level without forcing.

Den samlade erfarenhet som finns kring användandet av rena biobränslen för produktion av värme- och kraftvärme kommer till största del från rost- och fluidbäddpannor. Pulvereldningstekniken har traditionellt sett nyttjats mest för kolbränslen, och erfarenheter av rena biobränslen i pulvereldade anläggningar är därför begränsad. Med utgångspunkt i pulvereldningstekniken så har detta arbete fokuserat dels på att belysa för- och nackdelar med olika bränslekaraktäriseringsmetoder för rena biobränslen och dels på praktiska förbränningsförsök där askämnestransformationer och syrgasdopad förbränning studerats. Som bränslen har vetehalm och två skogsbaserade biobränslen, bark och stamved, nyttjats. Som ett stöd i analysen av de experimentella resultaten har kemiska jämviktsberäkningar använts.Resultatet visar att det finns ett behov att kunna utvärdera biobränslenas egenskaper under förhållanden som efterliknar dem som råder i den verkliga förbränningsprocessen, framför allt för pulvereldningsapplikationer. Mer specifikt bör en översyn av metoder för bedömning av malningsegenskaper, slagg- och påslagsbildning och devolatilisering och utbränning i pulverapplikationer göras. De experimentella sameldningsförsöken visar att vid pulverförbränning så främjas reaktioner mellan gasformiga askämnen före reaktioner där fasta eller flytande ämnen deltar. Försöken visar vidare att ett antagande om kemisk jämvikt endast kan motiveras i begränsade delar av processen samt att de olika askbildande huvudelementen dels separeras och dels koncentreras i olika delar av processen. Försöken där syrgasdopad förbränning studerades visar att denna teknik dels medför möjligheter till ökad kontroll på både processtemperatur och flammans karaktärstik och dels en ökad värmeöverföring i hela eldstaden.

No description available.

No description available.

The urban transformation that is taking place in Kiruna, a Swedish town well above the Arctic Circle in the sub-arctic climate, is due to the ground deformation caused by iron ore mining and it is affecting all the infrastructures of the town. This thesis focuses on the studies about the district heating for the town of Kiruna. In the field of district heating it is important to have a good knowledge about network behavior, especially if network structure is meshed, in order to understand how the flow is distributed in each pipe within the network. This thesis describes the development of a method for the simulation of district heating networks that makes it possible to analyze and study complex networks with meshed structure, something that has not been possible before. The thermal losses for each pipe type are required as a fundamental input for the simulation. A fictitious pipe series has been created so that its loss matches the losses in the real network. It has been found that the created series is close to the series with the highest losses that is manufactured today. When redesigning the network structure to deal with the urban transformation, an investigation is performed regarding how new low energy building can be heated. This investigation shows that the only heating system that fulfills the guidelines stated by the Swedish authorities about indoor climate is the floor heating. A techno-economical evaluation shows that the cheapest heating technology is an air to air heat pump. In order to make district heating attractive as a heating source for house owners a vigorous price reduction is needed.

Deposit formation in the grate-kiln process is a challenge for the iron ore pellet production industry. The deposits cause disturbances that affect the production capacity of the pelletizing plant. To prevent or mitigate these occurrences, it is important to understand the deposit formation mechanism during the process, which is the overall goal of this work. The results from this work can be used to enhance the understanding of deposit formation in the iron ore pelletizing industry.In this work, particle and deposit formations were studied both in a full-scale grate-kiln plant (40 MW) and in a pilot-scale pulverised coal-fired furnace (400 kW). The sampled particles and deposits were characterized with scanning electron microscopy equipped with energy dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), laser diffraction (CILAS) and X-ray fluorescence (XRF).In the first part of this work, the initiating step in deposit formation— i.e. particle formation mechanisms— was investigated. Particles were sampled from the transfer chute in a full-scale grate-kiln production plant during combustion of oil and coal in separate firings. The results showed that particles in the flue gas consisted principally of fragments from iron ore pellets and minor ashes from heavy fuel oil and pulverised coal combustion. Three categories of particle modes were identified: (1) a submicron mode consisting of condensed products from vaporized species that had relatively high concentrations of Na and K for both combustion cases, with high concentrations of Cl and S during heavy fuel oil combustion, and high concentrations of Si, Fe and minor P, Ca and Al during coal combustion (2) a first fragmentation mode consisting of both iron ore pellet fines and fly ash particles with a significant amount of Fe (>65 wt %) for both combustion cases, with high concentrations of Ca and Si during heavy fuel oil combustion and high concentrations of Si and Al during coal combustion (3) a second fragmentation mode consisting almost entirely of coarse iron ore pellet fines, predominantly Fe (~90 wt %). The particles in the flue gas were dominated by iron ore fines within the second fragmentation mode, which contributed >96 wt % of the total mass of collected particles.In the second part of this work, short-term deposits were collected at the same location in the grate-kiln as the collection of particles. They were characterized by their chemical composition and microstructure in order to obtain information about the deposit formation. Deposit sampling was carried out during separate combustion firings of oil and coal. A significant difference in the deposition behaviour was observed: deposition during oil firing was negligible compared with coal firing. The deposits from coal firing were mainly fine-grained iron oxide particles embedded in a molten (bonding) phase that comprised mainly of Si, Al, Fe, Ca and O. Moreover, it was found that the prevailing flue gas direction determines the formation of the deposits on the probe and that inertial impaction controls the deposition rate. However, this rate can also be affected significantly by the amount of entrained particles that were present in the kiln.In the third part of this work, two different coals were combusted both in a full-scale grate-kiln plant and in a pilot-scale pulverized coal-fired furnace (ECF). The ECF is designed as a scaled-down grate-kiln for combustion testing. Particle and short-term deposit samplings were carried out in both appliances. Dust originating from iron ore pellets was only present in the grate-kiln as there was no flow of iron ore pellets in the ECF. The results showed that Na, K and Cl contents in submicron mode were higher in the grate-kiln than in the ECF, due to alkali circulation in the grate-kiln. The coarse mode particles (2.6-4.2 μm) sampled from the grate-kiln contained significantly more Fe, which originated from the iron ore pellets. The presence of coarse particles (>6 μm) was substantial (>96 wt % of the total particle mass) in the grate-kiln but insignificant in the ECF. The short-term deposits from the grate-kiln consisted of a variety of particles from both iron ore pellets and coal ash particles embedded in an iron-rich silicate molten phase. Short-term deposits from the grate-kiln were harder and denser compared to the shortterm deposits from the ECF. Short-term deposits from the ECF were porous and consisted of coal ash particles embedded in a silicate molten phase. The molten phase in short-term deposits from the gratekiln had a higher Fe content and a higher CaO/(SiO2+Al2O3) ratio than the molten phase from the ECF short-term deposits. Thermochemical calculations showed that the molten phase in the short-term deposits from the grate-kiln had a lower viscosity compared to the molten phase in short-term deposits from the ECF.

The ambitious targets of the European Union in increasing the use of renewable energies to 20% of Europe’s energy needs, call for urgent changes, including in the biomass sector. The share of solid biomass for heating purposes could be further increased by replacing oil- and gas-fired furnaces with biomass boilers and by expanding the spectrum of biomass raw materials for small-scale combustion systems. The interest in using non-woody biomass fuels for heat production has been increasing in Europe due to two main factors. First, the market for fossil fuels is unstable and their prices are continuously rising. Second, the increase competition for woody biomass between the heating sector and other industries, have increased the price of wood. As a result, the interest for alternative biomass fuels is growing rapidly, covering woody materials of low quality, energy crops and forest residues.The present work aims at investigating the technical feasibility of using non-woody biomass fuels in existing small-scale combustion appliances developed for burning wood. Therefore, combustion tests with different non-woody biomass fuels and in different combustion appliances were performed in standard laboratory conditions and in households under real life conditions (field tests). The laboratory tests were performed using eight different fuels (straw, Miscanthus, maize, vineyard pruning, hay, wheat bran and Sorghum) while in the field tests straw, Miscanthus and maize were burned. The gaseous and particle emissions, the slag tendency and the efficiency of the combustion systems operated with non-woody biomass fuels were analysed and when possible compared with the legal requirements defined in FPrEN 303-5. The limitations of the investigated combustion appliances when operated with non-woody biomass fuels were analysed and discussed.Non-woody biomass fuels could be used for heat production in existing combustion appliances as long as the systems are adapted for burning high ash content fuels. Among the investigated fuels, Miscanthus, vineyard pruning and hay could be burnt in most of the tested combustion appliances while fulfilling the legal European requirements (defined in FprEN303-5) in terms of emissions and efficiency. The non-woody biomass fuels showed problems with ash accumulation and slag formation and could only be burned without unwanted shutdowns in combustion appliances adapted to manage high ash content fuels. Straw, wheat bran and maize were the most problematic fuels regarding slagging. The combustion appliances require appropriate technological developments to manage the strong variability in terms of chemical and thermal properties of the non-woody biomass fuels. The results of the laboratory tests were generally in agreement with the field test results.

Black liquor gasification at high temperature is a promising alternative to the conventional recovery boiler process used in chemical pulp mills today. Compared to a conventional recovery boiler a black liquor gasifier can increase the total energy efficiency of a chemical pulp mill and produce a synthesis gas that can be used for production of motor fuel. In Piteå, a 3 MW or 20 tons per day entrained flow pressurized black liquor gasification development plant has been constructed by Chemrec at Energy Technology Center. The plant has been run more than 12 000 h since 2005. One of the key parts in the gasification process is the spray burner nozzle where the black liquor is disintegrated into the hot gasifier as a spray of fine droplets. In this thesis the spray burner nozzle as characterized with high speed photography in order to visualize the atomization process of black liquor. The results showed that black liquor forms non-spherical and stretched ligaments and droplets with the considered nozzle. Comparison of the results with atomization of a syrup/water mixture showed that the results were qualitatively very similar which means that a syrup/water mixture can be used instead of black liquor for burner optimization experiments. This is a considerable experimental simplification. Also spatially resolved measurements of the gas composition in the development plant with a water cooled quench probe have been performed. From the gas composition measurement and the spray visualization it has been showed that the preheating of black liquor has a significant influence on the gas composition. Another important part in the gasification plant is the counter-current condenser where the gas is cooled and the water content in the gas is condensed away in vertical tubes that are cooled on the outside by a counter-current flow of water or steam. In this thesis a computational fluid dynamics model of the counter-current condenser have been developed. The model consists of a two-phase fluid model on the tube-side of the condenser and a single phase model of the shell side. Predictions from the model are in excellent agreement with temperature measurements from the condenser used in the 3 MW Black Liquor Gasification development plant. However, more validation data is necessary before a definite conclusion can be drawn about the predictive capability of the code, in particular with respect to scale up with about two orders of magnitude for commercial size gas coolers

The formation of sticky layers on bed particles has been considered as a prerequisite for bed agglomeration in fluidized bed combustion and gasification of woody fuels. In addition, the layer formed makes the bed particle less resistant against fragmentation. The fragments from quartz particles can result in deposition. The present investigation was undertaken to determine the layer formation process on quartz bed particles during combustion of wood-derived fuels and to determine the effect of quartz particle layers on deposit build-up in full-scale dual fluidized bed (olivine) steam gasification of logging residues. Bed material samples were collected from three different combustion appliances: bench-scale bubbling fluidized bed, full-scale bubbling fluidized bed and full-scale circulating fluidized bed. These were collected at different sampling times from start-up with fresh bed material. In addition, samples of deposits, bed material, coarse ash, fine ash, and fly coke from a dual fluidized bed gasification process were also collected. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were used to explore layer morphology, chemical composition and to gain information about crystalline phases of the layers. Significant differences in layer morphology and composition were found for quartz bed particles with different ages. For bed samples with operational durations of less than 1 day, only one thin Ca-, Si-, O- and K-rich homogeneous quartz bed particle layer with a relatively high K/Ca molar ratio was found. For quartz bed particles with an age of around 1 day to 2 weeks, an outer more particlerich coating layer was also found. During the initial days of this period, the layer growth rate was high but decreased over time, with decreasing K/Ca and increasing Ca/Si molar ratios in the inner bed particle layer. For bed particles with ages between 2 to 3 weeks, a much lower layer growth rate was observed. At the same time, the Ca/Si molar ratio reached high values and the K concentration remained at a very low level. In addition to these layer formation processes mentioned, an innerinner-/ crack layer was also formed simultaneously in the circulating fluidized bed (CFB) quartz bed particles along with the inner bed particle layer. By combining the experimental results on layer characteristics for samples with durations from 4 h to 23 days, with phase diagrams, thermochemical equilibrium calculations and a diffusion model, a mechanism of quartz bed particle layer formation was proposed. For younger bed particles (< around 1 day), the layer growth process is accelerated due to a high diffusion of calcium in a K-rich silicate melt. But with continuous addition of calcium into the layer, the amount of melt decreases and crystalline Ca-silicates starts to form. Ca2SiO4 is the dominating crystalline phase in the inner layer, while the formation of CaSiO3 and Ca3SiO5 are favored for younger and older bed particles, respectively. The decreasing amount of melt and formation of crystalline phases resulted in low diffusion rates of calcium in the inner layer and the layer growth process becomes diffusioncontrolled after around 1 day. The observation of formation of Ca3SiO5 in a thick bed layer after around two weeks may indicate substantially higher diffusion resistance and lower layer growth rate. The practical implication of the results from this work is that a low bed material renewal rate during wood-derived fired bubbling fluidized bed boilers using natural sand is recommended. The formation of high-melting Ca-silicates for older quartz bed particles protects the bed particle layer surface from further attack by potassium, leading to reduced agglomeration tendency. In dual fluidized (olivine) bed steam gasification, impurities, mainly composed of quartz particles brought into the fluidized bed with the feedstock, play a critical role for deposit formation in the post-combustion zone. Interaction between biomass ash and the quartz particles leads to formation of sticky potassium-rich silicate layers. Recirculation of coarse ash back into the combustion zone therefore leads to the enrichment of critical fragments. Improving the management of inorganic streams and controlling of temperature levels are therefore essential in operating with woody biomass fuels containing impurities (i.e. sand minerals).

The biomass pellet consumption, both in the industry and in residential appliances, has increased during the latest years and this is expected to continue. In order to handle the demand, more ash rich (>0,5 wt-%) raw materials have been introduced as energy wood and other forest based fuels. In connection with this, ash related problems as fouling, slagging and corrosion have occurred. However introduction of peat as a co-combustion fuel has turned out to have positive effects on these problems. Earlier research shows that introduction of peat into biomass results in reduced bed agglomeration, fouling and corrosion. Nevertheless the previous works has mainly focused on fluidized bed boilers with bed agglomeration and deposit formation in focus. Experimental work in small and residential combustion appliances is still scarce (e.g. in grate firing boilers and burner’s).In this work the influence of peat addition to biomass pellets on combustion characteristics here defined in terms of slagging characteristics, operational maintenance, gaseous- and particulate emissions in residential appliances, were determined. Six peat samples representing a broad variation in ash forming matter of Scandinavian peats were chosen in this study. They were co-pelletized with sawdust, energy wood, forest residue as well as wheat straw and combusted in a P-labeled underfed commercial pellet burner (15 kW) installed in a reference boiler.The NO and SO2 emissions were generally higher when introducing peat. However these emissions are both fuel and combustion specific and the increment has not been further investigated to conclude its origin. The particle emissions were in all cases totally dominated by fine (<1 μm) particles and contained high concentrations of K. When adding peat containing high amounts of Si and/or clay minerals into the wood derived fuels a clear reduction of emitted fine particles were shown. The likely mechanism is that the reactive Si and/or the clay minerals from the peat react with K vapor from the biomass forming K-silicates that will stay in the coarse ash fractions, hence capturing the K. The reduction was most obvious for the biomasses short in reactive Si as sawdust and forest residue. In fact a K reduction of up to 70 % was shown when adding a Si- or clay-rich peat to forest residue. However, when adding a peat with low ash content and high Ca content the reduction was diminished. It was further shown that addition of peat to the K and Si rich wheat straw did not generally affect the reduction of fine particle- and deposit forming K i.e. only a dilution effect of ash forming matter occurred. The slagging tendency was increased in all cases when peat was introduced hence also the operational maintenance was disturbed. However, great differences between different peat assortments existed in this aspect. High Si together with low Ca contents in the peat resulted in the most severe slagging, whereas a peat with higher Ca/Si ratio gave a significant lower slagging tendency. The sawdust was generally more affected by the peat addition than the other studied biofuels.In order to receive a considerable particle reduction in parallel with a manageable slagging tendency when using "problematic" biomass fuels in small scale burners and grate boilers, co-combustion with a carex based peat with a high ash content and relatively high Ca/Si ratio is suggested.