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- Ström, Henrik, 1981, et al.
(författare)
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Experimental and Numerical Investigations of Ash Behaviour in Fixed-Bed Combustion of Woody Biomass Pellets
- 2017
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Ingår i: Proceedings of the Nordic Flame Days 2017, Stockholm, Sweden.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Combustion of woody biomass in a fixed-bed configuration could play an important role in the development of small-scale combined-heat-and-power solutions, such as externally-fired micro-gas turbines. However, many challenges still remain with regard to the complexity of maintaining high availability with an acceptable fuel flexibility, as ash transformations in the fuel bed can cause serious problems for the robustness of the process as well as for downstream components. In this work, fixed-bed combustion of two woody biomasses of different ash contents is studied experimentally and numerically in an attempt to advance the understanding of ash-related disturbances. A mathematical model for fixed-bed conversion of biomass is extended to account for ash transformations on both the single-pellet level and the bed level, and the predictions from this model are assessed against the experimental results. The agreement between simulations and experiments is good for low air-flow rates, whereas additional considerations are needed for air-flow rates closer to the stoichiometric limit. Although there was no catastrophic slagging observed in the experiments, the capabilities of the model to describe effects of such slagging, down to only slightly reduced combustion rates due to milder ash transformations, are confirmed by the simulations.
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| 2. |
- Boman, Christoffer, et al.
(författare)
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Development of innovative small(micro)-scale biomass-based CHP technologies
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- To enhance the overall efficiency of the use of biomass in the energy sector in Europe, the large electricity production potential from small-scale biomass heating systems should be utilised. So far, no technologically sound (in terms of efficiency and reliability) and economically affordable micro- and small-scale biomass CHP technologies are, however, available. Therefore, the present ERA-NET project (MiniBioCHP) aimed at the further development and test of new CHP technologies based on small-scale biomass combustion in the electric capacity range between some W and 100 kW. Within the project, an international consortium consisting of 12 partners from 4 countries, including university institutions, institutes and industry (both engineering and manufacturing), collaborated closely to perform high level R&D on three promising micro/small-scale biomass based CHP technologies which are covering a broad range of applications in the residential heating sector. The Austrian engineering company BIOS, coordinated the international project. The project was based on earlier basic research and development work related to these promising new technologies and aimed at the achievement of a technological level which allows a first (commercial) demonstration after the end of the project. The three CHP concepts included in the MiniBioCHP project were;1. Pellet stoves with a thermoelectric generator (TEG)2. Small-scale biomass boilers (10-30 kWth) with a micro-ORC process3. High temperature heat exchanger (HT-HE) for an externally fired gas turbine (EFGT)The Swedish part of the project was focused on the development of the concept of biomass based EFGT with dedicated R&D activities related to the development of the HT-HE system. The Swedish project consisted of the research partners Umeå University (project leader), Luleå University of Technology, Chalmers University of Technology and RISE Research Institutes of Sweden, together with the industrial partners Enertech AB/Osby Parca and Ecergy. The expertise of the Swedish partners regarding ash related problems, grate boiler combustion and modelling, deposit formation and high temperature corrosion, were combined with the know-how of a Polish partner regarding HT-HE design, construction, testing and optimisation.The HT-HE is the most crucial component in EFGT processes significantly influencing the investment costs, availabilities as well as the efficiencies that can be achieved. With a thermal capacity from several hundred kW up to 2-3 MWth) the CHP technology based on a biomass boiler and an EFGT is suitable for district heating systems, or process heat consumers. The electricity produced by the gas turbine (up to some 100 kWel) can be used to cover the own electricity consumption of a company and/or fed into the grid. Even though the concept of biomass based EFGT has been an interesting alternative for small-scale CHP production for some decades, and R&D activities have been undertaken, tackling both economic and technical aspects, only a few pilot-plants have been in operation and no initiative has so far reached the level of commercial implementation. Thus, the concept of EFGT fed with biomass is still considered to be in a rather early development stage and the main technical challenges are related to alkali deposit induced corrosion and thermal stress of the HT-HE material, turbine design/operation and system integration.Within the present project, a HT-HE prototype aimed for an EFGT system was therefore designed, constructed and successfully tested at flue gas temperatures up to 900°C. Thus, appropriate guidelines for a compact design of the HT-HE and recommendations have been worked out to minimize thermal stresses as well as ash related problems regarding ash deposit formation and high temperature corrosion in a biomass boiler system. Furthermore, different concepts for the overall biomass based EFGT system have been worked out and evaluated. The outcome of the project will hopefully be used in the further development work and form the basis for a first testing and demonstration plant within the coming years.
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| 7. |
- Niklasson, Fredrik, 1968, et al.
(författare)
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Estimation of the temperature in the bottom region of a fluidized-bed furnace burning biomass
- 2006
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Ingår i: Science in Thermal and Chemical Biomass Conversion. - 1872691978
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Konferensbidrag (refereegranskat)abstract
- The heat and mass balance of the bottom region of a fluidized bed boiler burning biomass is established with the aim of determining the furnace temperature. The influence on the temperature by the flow of solids in the upper part of the bottom region is investigated. Heat and mass balances of gas and fuel were applied over a control volume containing the bottom region, i.e. both the bed and the adjacent splash zone. The mass balance is based on given input flow rates of gas and fuel and concentrations in the gas leaving the control volume. The measured gas concentrations were used to determine the rate of fuel burnout inside the control volume. Pressure drop measurements along the height of the furnace estimate the solids concentration. Experiments were performed in a furnace with a cross-section area of 2.2 m2. The walls of the furnace are refractory lined up to a height of 2 m and, for simplicity, the control volume is chosen from the air distributor up to this height. Two fuel mixtures were used: one biomass and one of biomass and coal. The results show that the gas temperature at the top of the control volume strongly depends on the flow-rate of fine particles present in the fluid, even when the volume fraction of particles is small. The particles carry heat to the freeboard, where they transfer heat to the walls before returning to the bottom region. This process has previously been neglected in models of fluidized-bed combustion under bubbling conditions, overestimating the fluid temperature.
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| 8. |
- Niklasson, Fredrik, 1968
(författare)
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Heat Balance Modeling of a Stationary Fluidized-Bed Furnace Burning Biomass
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The influence of biomass fuel properties on the heat balance of the bottom region (bed and splash zone) of a commercial-scale bubbling fluidized-bed furnace is investigated theoretically and experimentally. The experiments have been carried out at the Chalmers 12 MWth research boiler, operated under non-circulating conditions during most of the experiments. The different parts of the work are summarized as follows. A two-phase flow model of the bed and the splash zone in a boiler is presented. The combustion rate in the bed is estimated by global kinetic expressions, limited by the gas exchange between oxygen-rich bubbles and a fuel-rich emulsion phase, whereas the combustion rate above the bed is determined from turbulence properties. A heat balance of the bottom region shows that the average temperature of the gas leaving the bottom region strongly depends on the flow rate of fine particles present in the fluid. The particles carry heat to the freeboard, where they transfer heat to the boiler walls before returning to the bottom region. A method is proposed for estimation of the effective lateral dispersion of fuel in a fluidized-bed combustor. By correlating the drying of the fuel particles and the measured moisture concentrations above the bed, the effective lateral dispersion coefficient of the fuel particles is determined. This coefficient was estimated to be on the order of 0.1m2/s, which is considerably higher than predicted by most of the expressions given in the literature. The local air ratios in the furnace are estimated by fluctuating signals from zirconia cell probes, which are compared to simultaneous gas concentrations of extracted gas samples. The time fraction during which the fluctuating zirconia cell signal shows oxidizing gas conditions is strongly correlated with the local air ratio of the gas. When this correlation is determined, the fluctuating signals from zirconia-cell sensors can be used to obtain the air ratio at different heights in the furnace. This provides a cheap and robust technique for on-line monitoring of the gas conditions in the furnace when, for example, optimizing the operation of a fluidized-bed boiler to reduce nitrous oxide emissions.
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| 10. |
- Niklasson, Fredrik, 1968-, et al.
(författare)
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On-line monitoring of agglomeration in fluidised bed boilers
- 2018
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Combustion in fluidized beds has several benefits, but a potential problem is bed agglomeration causing defluidisation. The most used counter measure is to regularly renew the bed material, inferring costs for new sand and deposition of spent material. For an adaptive optimization there is a need of a method which indicates when bed agglomeration is initializing, before it is too late to counteract.In this project, the conductivity of fluidized beds has been measured by a novel in-situ probe. The probe has been tested in a fluidized bed of sand and ashes at temperatures up to 1000°C. In addition, the probe has been tested in a fluidized bed while burning different fuels.The results show that the conductivity of the bed increases with temperature and concentration of ash. The conductivity varies strongly between different fuels. The signal from the probe reacts strongly to the onset of severe bed agglomeration, but it is hard to find any consistent tendencies that can be applied to predict it.
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