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- Holmgren, Per, 1982-
(författare)
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Entrained flow studies on biomass fuel powder conversion and ash formation
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Reducing the global dependence on fossil fuels is of paramount importance in tackling the environmental challenges we face, not only tomorrow, but already today. Biomass offers a renewable supply of CO2-neutral raw material that can be converted into many different forms of fuels and valuable chemicals, making it a prime candidate for the technologies of tomorrow. However, the heterogeneous nature and distinctly different elemental composition of biomass compared to traditional fossil sources present new challenges to be solved. When it comes to thermochemical technologies, key issues concern fuel conversion efficiency, ash formation, ash/fuel interactions and ash/reactor material interactions.The objective of the present thesis was to provide new knowledge and insights into thermochemical fuel conversion, in particular its application in entrained flow technologies. A laboratory-scale reactor was constructed, evaluated and was used to study several aspects of high-temperature entrained flow biomass fuel conversion. Pulverized fuel particles from different biomass sources were used, and their physical and chemical interactions with the surrounding atmosphere, the concurrent ash element release, ash formation, and phase interactions were also studied in detail. In addition to the entrained flow reactor designed and constructed for this purpose, the main method for data collection was in situ optical studies of converting particles, either while entrained in the flow or when impacting upon surfaces. Elemental composition analysis of collected samples and gas analysis were also performed, allowing for a deeper understanding of ash element fractionation and interactions and thus explaining the observed properties of the resulting deposits or slag.The degree of conversion of fuels with very low ash content, such as stem wood, was well described and modeled by a novel method using optical data, offering a non-intrusive and non-destructive alternative to traditional techniques. Coupling computational fluid dynamics with optical data allowed for improved experimental data interpretation and provided improved accuracy for fuel particle residence time estimations, which is an important parameter when studying fast chemical reactions such as those taking place in reactors for entrained flow conditions. The results from studies on ash formation gave new insights into the feasibility of using dry-mixed K-rich additives for improving slag properties during gasification of Ca-rich and Si-rich fuels. Interpretations of the experimental results were supported by thermodynamic equilibrium calculations, and the conclusions highlight both possibilities and challenges in gasification with high fuel flexibility while at the same time producing a flowing slag. Applications and future implications are discussed, and new topics of interest are presented.
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| 4. |
- Göktepe, Burak
(författare)
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Entrained flow gasification of biomass: soot formation and flame stability
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Entrained flow gasification (EFG) is a well-proven, commercially available technology for large scale coal gasification processes, with a production of a high quality syngas (a mixture of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4) and other compounds). For biomass, the process is still under development and there are several hurdles that must be cleared before it can become commercial. In entrained flow gasification, solid fuel particles are milled to a size of a couple of hundred micrometers to ensure good heat and mass transfer with the surrounding hot gases, priorto be fed in a co-flow of oxidizer stream that can be either air or pure oxygen. The milled biomass particles have cohesive behavior and poor flowability, leading to serious challenges associated with consistent particle feeding and effective mixing. The pulverized fuel injector is a vital part of the gasification/combustion system and a well optimized fuel injector can help to promote the process efficiency by enhancing mixing, minimizing pollutant emission and fuel consumption. Biomass differs from coal not only in chemical composition (in terms of carbon, oxygen, volatile etc. contents) butalso in aerodynamic properties depending upon some factors, e.g. shape sphericity, aspect ratio, particle size, bulk density and particle cohesion force etc. One of the key challenges to implement biomass in entrained flow gasification is to ensure a good mixing of biomass particles with the oxidizer stream. A common concept is to impart swirling motion into the oxidizer stream, forming a recirculated hot gas flow that can participate in the gasification. The dispersion behavior of biomass particles in turbulentisothermal swirling flows has therefore been studied by using a two-phase particle image velocimetry technique. This technique provides simultaneous measurements of continuous (air) and disperse phase (pulverized pine particles) velocities. The results show that the addition of pulverized pine particles (with a size range of 112-160 μm) into turbulent air flow significantly affect the dispersion rate and velocity fields of thesuspending air flow in the burner near field, inducing a “blockage effect” where the air velocity is reduced along the jet core corresponding to a region of high particle concentration. It was also found that imparting swirling motion to the co-annular jet flow increased the particle dispersion due to strong centrifugal effects induced by the swirling motion. The entrained flow gasifier is operated at high temperatures to maintain high conversionand high cold gas efficiency, resulting in low tar yields, high oxygen demand and a viscous slag flow. High operating temperatures also favors soot formation that can be detrimental to the operation of the gasifier, e.g. clogging of flow passages, fouling on system components and reduced efficiency of gasification. A novel soot reduction method on the basis of forced dispersion of fuel particles has therefore been applied toa laboratory scaled entrained flow reactor. Pulverized pine particles with a size of 63- 112 μm were gasified in a sub-stoichiometric methane-air flame stabilized on a flat burner. Soot formation was measured along the reactor height in terms of volume fraction by a two-color laser extinction method. The results show that particle dispersion and inter-particle distance were enhanced by varying the flow velocity ratio between the particle carrier gas and the premixed flame. The soot volume fraction was found todecrease towards an asymptotic value with increasing inter-particle distance.There are other techniques to control particle dispersion and promote mixing, e.g. acoustic forcing or a synthetic jet flow. Both techniques induce a periodic motion to the gas phase flow that influences the motion of solid fuel particles. A synthetic jet actuator was used in both isothermal and reactive flows in a laboratory scale entrained flow reactor. It was found that the synthetic jet actuator formed local flows of dilute and dense gas particle suspensions via a convection effect induced by large scale flow structures. It was also shown that the synthetic jet actuator provided controlled particle dispersionin isothermal flows with respect to forcing amplitudes. The resulting flow field imposed significant effects on the amount of soot formed during gasification of pulverized pine particles.Acoustic forcing was applied to a 150 kW wood powder burner to excite one of thenatural system instabilities during combustion of wood powder particles. The effect of the instabilities on the flame shape and NOx formation were investigated at differentair/fuel ratios. The powder flame gave a quick response to external flow perturbations at 17 Hz showing irregular wobbling and increased NOx emission in the presence of acoustic excitation. Based on the experiences gained from the experiments, dispersion characteristics of particle-laden flow are of utmost importance to reliably predict and optimize pollutant emission. Controlled particle dispersion can be simply achieved by external forcing ofthe gas flow by a synthetic jet actuator without any need of a source of external fluid or time-consuming, expensive burner modifications.
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| 5. |
- Risberg, Mikael
(författare)
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Entrained flow gasification of biomass : On atomisation, transport processes and gasification reactions
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the energy consumption in the world is increasing together with an increase of greenhouse gas emission it is of importance to find alternatives to fossil fuels. Biomass is one of the alternative energy sources which is both renewable and CO2 neutral. However, due to the large variability of biomass and the influence from different types of contaminations it is important to find processes that can work with a range of biomass and preferably transform the energy in the biomass into higher value energy forms. Gasification is one of the most robust processes that can achieve this by transforming solid biomass (e.g. wood, bark or rice husks) or liquid byproducts from the forest products industry (black liquor) into a uniform synthesis gas that can be further upgraded into electric power or synthetic motor fuels. This thesis is focused on a specific class of gasifiers called entrained flow gasifiers that converts the biomass to syngas in a reactor where the fuel is entrained in a gas flow.In entrained flow gasification, small fuel particles are gasified together with an oxidizing medium like air or pure oxygen. The fuel particles are either formed a priori by milling of solid biomass or in situ by atomisation of a liquid fuel. In both cases the fuel is thoroughly mixed with a carrier gas and distributed into a hot gasification reactor. One type of entrained flow gasification that is of high relevance for countries with a significant pulp and paper industry is black liquor gasification. Black liquor is a by-product from pulping that is available in large quantities in chemical pulp mills. The energy in the black liquor is normally utilized for steam production in the mill but this steam can easily be produced from low grade biomass, thereby freeing the black liquor for other purposes. The most interesting process is when black liquor is gasified with pure oxygen at high temperature. This process creates a clean synthetic gas with very low concentration of tars that is suitable for catalytic conversion into transportation fuels, e.g. dimethyl ether or methanol.One of the key parts in a black liquor gasifier is the burner nozzle that is used to produce a spray of fine black liquor droplets inside the gasifier. This is a difficult task since black liquor has a very high viscosity. The black liquor atomisation process has therefore been studied with high speed photography to be able to visualize the process and thereby making it possible to optimize the burner nozzle so that it produces a spray with near uniform particle size and appropriate distribution in space. The results show that the fuel particles formed from the considered nozzle consisted of non-spherical and stretched ligaments that in some cases were further broken down into more spherical droplets. The experiments with black liquor were difficult and hazardous since the black liquor is caustic and hot since it needs to be preheated to around 120 °C before it can be atomised. It is therefore of interest to find non-hazardous substitute liquids that will have the same behaviour as black liquor in a nozzle. In a comparison between black liquor at 120 °C and a syrup/water mixture with equal viscosity and surface tension at room temperature it was found that the syrup/water mixture behaved nearly identical to the black liquor in a real burner nozzle.Connected to the atomisation studies, measurements of gas composition in a 3MW black liquor gasifier were made for different black liquor preheat temperatures. The results showed that preheating of black liquor had a significant influence on the syngas composition and the conclusion when this was combined with the results from spray visualization was that the main reason for the observed differences is the smaller droplet size that is achieved with higher preheating temperatures.In a large syngas plant where the goal is to catalytically convert the syngas into motor fuels or chemicals in a catalytic process, the raw syngas from the gasifier must be cleaned and conditioned in several steps. In all contemporary downstream processes the gas must be much colder than when it leaves the gasifier. Hence, gas cooling is an important unit operation in the syngas process. In order to optimize the overall efficiency of the syngas plant it is very important to recover the latent heat in the syngas at the highest possible temperature. One way to do this is to use a counter current condenser that cools the syngas and condenses most of the steam that is mixed with the syngas while at the same time steam that can be used by other processes is produced. The sizing of counter current condensers is therefore of high importance and one part of the thesis work was to develop a computational model that can be used for optimization of these units. In order to validate the code, measurements were carried out in the counter-current condenser in the 3 MW black liquor gasification pilot plant that was mentioned above. The predictions from the model were found to be in very good agreement with the temperature measurements from the pilot plant for the cases that were investigated.Another type of entrained flow gasification process is air-blown cyclone gasification where biomass powder is gasified in a cyclone shaped reactor. This gasification process can be used in combination with a gas engine to produce both heat and power that can be used in district heating applications or as prime mover and heat source in industrial processes where low grade biomass is available at low cost. This type of gasifier has the possibility to operate with ash rich fuels since it operates below the ash melting temperature and the majority of the ash is separated in the bottom of the cyclone.One of the objectives in this thesis was to evaluate the fuel flexibility of the cyclone gasifier by experiments with different fuels in a 500 kW pilot gasifier. From the gasification test it was found that torrefied spruce, peat, rice husk, bark and stemwood powder can be used as fuel to produce a syngas that can be used as fuel in a gas engine.To be able to understand the cyclone gasification process and be able to optimize different sizes of cyclone gasifiers a computational fluid dynamics model of the process has been developed and compared against experimental measurements, both in a 500 kW plant and a 4 MW plant. The results show that the model predicts the main gas species in the product gas and the amount of unconverted fuel reasonably well. It also predicts the effect of increased gasifier size and fuel power well. Therefore the model could be used as a tool for designing cyclone gasifiers in arbitrary sizes and to optimise operating parameters in existing gasifiers.
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