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  • 1. Sand, U.
    et al.
    Sandberg, J.
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Larfeldt, J.
    Bel Fdhila, Rebei
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Numerical prediction of the transport and pyrolysis in the interior and surrounding of dry and wet wood log2008In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 85, p. 1208-1224Article in journal (Refereed)
    Abstract [en]

    The numerical simulation of the pyrolysis process of a dry and wet birch wood log in a cylindrical heating chamber is preformed. The model includes the flow inside and outside the porous wood log and accounts for convective, conductive and radiative heat transfer. A two-step pyrolysis reaction scheme is used to model the conversion from wood to tar. gas and char. The results of the simulations compare well with the authors experimental data which are presented in terms of radial temperature distribution and mass reduction, for both dry and wet cases. Our transient simulations provide us with the detailed flow field inside and outside the wood log. It clearly shows not only the existence but also the structure of the Pyrolysis gas plumes leaving the wood. These plumes have only been visualised experimentally by few authors [Brackmann C et al. Optical and mass spectroscopy study of the pyrolysis gas of wood particles. Appl Spectros 2003:57(2):216-22, [12]] without any quantitative Measurements and the present investigation gives a realistic estimation that we presently use to evaluate its impact on the heat and mass transfer, and on the momentum balance and the particle dispersion in a near future work. The gas plumes have a maximum velocity magnitude ranging between 0.1 and 0.2 m s(-1) and vanish when all the wood gas is produced. It is shown that increasing the convective flow around the wood log do not significantly modify the pyrolysis gas plume structure and seems to have small effect on the overall heating and the pyrolysis process which are mainly controlled by the thermal radiation from the hot surrounding walls.

  • 2.
    Sand, Ulf
    et al.
    Mälardalen University, Department of Public Technology.
    Sandberg, Jan
    Mälardalen University, Department of Public Technology.
    Bel Fdhila, Rebei
    Mälardalen University, Department of Public Technology.
    Two-phase transport model for the pyrolysis process of a vertical dry wood cylinder, including the surrounding flow field2006In: International Journal of Green Energy, ISSN 1543-5075, Vol. 3, no 1, p. 63-78Article in journal (Refereed)
    Abstract [en]

    This paper describes a mathematical model for the pyrolysis of a small dry pine wood cylinder. The computational domain is axisymmetric and involves the heating chamber, with the wood cylinder vertically situated in the centre of the chamber. The model simulates the laminar flow around the particle and the laminar flow inside the wood/char matrix by applying a two-phase transport model where the solid wood/char matrix acts as one phase and the various gases produced from the pyrolysis process is assembled in the other phase.

    Convective, conductive and thermal radiation transfer modes are included in the model. A two-step pyrolysis reaction scheme is used for the modelling of the conversion from wood to tar and gas. Both the thermal conductivity and the permeability of the wood/char matrix are modelled anisotropically in order to capture the directional differences in heat and mass transport, existing in real wood.

    Results from simulations are compared with measurements from literature for the centre core solid temperature and the conversion from wood to char, tar and pyrolysis gas in the particle during heating. The results show very good agreement with the measured temperature profile. The simulated conversion profile shows an overall good agreement with the measurements, however with discrepancies in the early stage of the process. Besides the successful validation with the experimental data, it provides us with all the details of the distribution of the migrating pyrolysis gas and tar, the temperature, the velocity flow field and pressure in the wood/char cylinder.

  • 3.
    Sandberg, Jan
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Fouling in biomass fired boilers2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis describes a detailed investigation into fouling in biomass fired boilers according to fuel mixture, combustion conditions, transportation of particles by the flue gas and the probability of particles impinging and sticking onto heat transfer tubes. The effects of fouling on overall boiler performance and the efficacy of soot blowing are also investigated.

    Both theoretical simulations and practical experiments on a 157 MW circulating fluidized bed boiler are presented.

    The deposit thickness on and around a heat exchanger tube is shown to be mainly dependent on the ash particle size, as particles larger than 10 µm (Stokes number larger than 0.1) mainly impinge on the windward side of tubes. The study also shows that fuel containing small amounts of chlorine and zinc – common elements in recycled wood – may cause both higher deposit growth rates and rapid increases in corrosion rates. These elements (chlorine and zinc), together with alkali metals from the biomass have the potential to form sticky compounds that increase the deposit growth rate.

    Reducing deposits by soot blowing is very effective at removing loose deposits but the hard sintered part of the deposits is almost unaffected. The use of recycled wood has a larger impact on the deposit growth rate than the soot blowing interval.

    Numerical simulations show that deposits on the superheater tubes redistribute the heat transfer rate from the superheaters to reheater 1 and partially redistribute turbine power from the high pressure turbine to the intermediate pressure turbine

  • 4.
    Sandberg, Jan
    Mälardalen University, Department of Public Technology.
    Fouling in biomass fired boilers2007Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In order to reduce the discharge of the greenhouse gas CO2, the use of biomass is nowadays promoted as fuel in boilers. Compared to boilers fired with coal and oil the biomass-fired boilers have more complications related to both fouling and corrosion on the heat transfer surfaces. After the combustion, unburned inorganic matter in state of vapour, melts and solid particles are transported in the flue gas and may form deposits on heat transfer surfaces.

    Deposits on the heat transfer surfaces may result in both increasing corrosion and decreasing boiler efficiency as the heat transfer rate to the superheaters and reheaters decrease by deposits.

    In order to understand the process of deposit build-up, the whole combustion and transport process had to be analysed including aspects such as, boiler design, fuel properties and combustion environment, followed by particle transport phenomena and the probability for particles to get stuck on the heat transfer tubes.

    In this thesis numerical simulation of particle trajectories has been conducted as well as measurements of deposits on a special designed deposit probe followed by investigation of on-site measurements of deposit depth on the super-heater tubes in a circulating fluidised bed in Västerås, Sweden.

    Numerical simulations of particle trajectories in the vicinity of two super-heater tubes were conducted in an Eulerian-Lagrangian mode considering the flue gas and ash particles phase. Particle impingements on the tubes were investigated for different particle sizes. The results from the particle trajectory simulations show that particle larger than 10 µm will mainly impinge on the windward side of the first tube but, however also on the sides of the second tube in the flue gas flow direction. In theory as well as from observations and measurements two tubes can merge together by the deposit build-up. Smaller particles are usually more dispersed due to turbulence and thermophorectic forces, resulting in a more even impingement distribution on the whole surface of the tubes.

    Probe measurements reveal that the deposit layer growth rate have a significant temperature and time dependence. After the initial deposit build-up a sintering process occurs and sintering is also proven to be dependent on temperature and exposure time.

    Soot-blowing is the most common method to reduce the effect of deposits on the heat transfer tubes. In the present thesis the soot boiling efficiency is therefore also investigated. The soot-blowing show a strong positive effect on the heat transfer rate in a short time (hours) perspective after a soot-blowing cycle is completed. This positive effect is much weaker when considering a time period of three years. This is an effect of fact that soot-blowing mostly remove the loose part of the deposit material leaving the hard sintered part unaffected.

    The subject of deposit build up on superheater tubes in large scale boilers involves multi-discipline knowledge and historically, the related research is mostly conducted as measurements and experiments on operating plants. Possibly in the future, theoretical simulations will have a bigger part of research on deposit build-up where the calculations are to be calibrated through measurements on real sites plants.

  • 5.
    Sandberg, Jan
    et al.
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Fdhila, Rebei Bel
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Dahlquist, Erik
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Avelin, Anders
    Mälardalen University, School of Business, Society and Engineering.
    Dynamic simulation of fouling in a circulating fluidized biomass fired boiler2011In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 88, no 5, p. 1813-1824Article in journal (Refereed)
    Abstract [en]

    A dynamic model is presented for a biomass-fired circulating fluidized bed boiler. The model is based on energy and mass balances for the components in the boiler and on a combustion model for the fluidized bed. The main purpose of the model is to simulate how deposits affect the boiler efficiency and performance. The model is verified against the municipal circulating fluidized bed boiler in Vasteras, Sweden, which produces 157 MW. The distribution of deposits on the surfaces in the boiler is well known from inspections. These observations are used as inputs to the model to simulate their effects on boiler performance. The heat exchanger most affected by fouling is Superheater 2, which is the first heat exchanger in the flue gas channel. Deposits typically reduce the heat transfer rate by half over a season despite soot blowing. This and other fouling scenarios are simulated and presented in this article. The simulations show that fouling on superheaters redistributes the heat transfer rate from the superheaters to Reheater 1 and partially redistributes turbine power from the high pressure turbine to the intermediate pressure turbine. If the boiler is running at maximum load, water injection to Reheater 1 has to increase to maintain temperatures below the permitted limit. The dynamic effects of fouling are small and the total efficiency of the boiler is only marginally affected. Fouling on evaporating surfaces has major dynamic effects and dramatically decreases the boiler efficiency. A decrease in fuel rate flow is needed to maintain temperatures in the fluidised bed and in the flue gas channel within acceptable limits.

  • 6.
    Sandberg, Jan
    et al.
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Karlsson, Christer
    Mälardalen University, School of Sustainable Development of Society and Technology.
    Fdhila, Rebei Bel
    Mälardalen University, School of Sustainable Development of Society and Technology.
    A 7 year long measurement period investigating the correlation of corrosion, deposit and fuel in a biomass fired circulated fluidized bed boiler2011In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 88, no 1, p. 99-110Article in journal (Refereed)
    Abstract [en]

    The present investigation involves a unique, 7 year (2001-2007) long study of corrosion and deposits on superheater tubes in a biomass fired circulated fluidized bed boiler. These measurements are correlated against the different fuels used over this period. In the earlier years, the boiler was run with a mixture of different biomass fuels and peat. In later years, recycled wood was introduced into the fuel mix. The deposit growth rate approximately doubled when the recycled wood content of the fuel was increased to 10-20%. Small amounts of chlorine and zinc were found both in the recycled wood and in the deposit layer. These elements together with alkali metals from the biomass, have the potential to form sticky compounds that increase the deposit growth rate. The corrosion rate of the superheater tubes varied over the study period. A number of possible explanations for this phenomenon are discussed.

  • 7.
    Sandberg, Jan
    et al.
    Mälardalen University, Department of Public Technology.
    Sand, Ulf
    Mälardalen University, Department of Public Technology.
    Bel Fdhila, Rebei
    Mälardalen University, Department of Public Technology.
    Long time investigation of the effect of fouling on the super-heaters in a circulating fluidised biomass boiler2006In: International journal of energy research (Print), ISSN 0363-907X, E-ISSN 1099-114X, Vol. 30, no 13, p. 1037-1053Article in journal (Refereed)
    Abstract [en]

    The present investigation involves measurements and theories on the mechanisms of the forming of deposit layers on super-heater tubes in a biomass-fired CFD boiler. The deposit layer thickness and the soot-blowing frequency effect on the super-heaters heat transfer are the main subject of the study that has been conducted over a 3-year period. The measurements show a deposit growth rate on the super-heaters of approximately 4 g m−2 h−1. The distribution of the deposit material varies significantly between the windward and the leeward side of the tubes, with the thickest layers on the windward side. Further down stream of the first super-heater, the fouling problem on the super-heater and re-heater tubes are not so severe. A theoretical model shows that a deposit layer of 20 mm will decrease the heat transfer rate of the first super-heater by nearly 40%. The soot-blowing system shows a strong positive effect on the heat transfer rate of the super-heater a few hours after a soot-blowing sequence has been completed. However in the long run, the varied soot-blowing frequency does not have a significant influence on the deposit layer growth rate.

  • 8.
    Sandberg, Jan
    et al.
    Mälardalen University, Department of Public Technology.
    Sand, Ulf
    Bel Fdhila, Rebei
    Measurements, theories and simulations of particle deposits on super-heater tubes in a CFB biomass boiler2006In: International Journal of Green Energy, ISSN 1543-5075, E-ISSN 1543-5083, Vol. 3, no 1, p. 43-61Article in journal (Refereed)
    Abstract [en]

    The present investigation involves theories, simulations and experiments on deposit layers on super-heater tubes in a circulating fluidised bed in Vdsteras in Sweden. Simulation of particle trajectories in the vicinity of two super-heater tubes is conducted in a Eulerian-Lagrangian mode for the flue gas and the ash particles from the combustion process. Particle impingements on the tubes are investigated for different particle sizes. Measurements of the buildup of deposit layers in the super-heater environment are conducted using a deposit probe. Deposit layer growth and growth rate is analysed for different probe temperatures, as well as the aspect of sintering on the probe ring surface. Analysis of the probe deposit material and deposits from the super-heaters and from textile filters are chemically analysed. The temperature dependence of the deposit materials viscosity is predicted from the chemical analysis of the samples. A model is included to simulate the effect of the deposit layer thickness on the tube heat exchange. The results from the particle trajectory simulations show that particle larger than 10 mu m will mainly impinge on the front of the first tube and that smaller particles are more dispersed due to turbulence and thermophorectic forces, enabling a more even impingement on the whole surface of the tubes. The probe deposit layer growth measurements show significant temperature dependence. The deposit material sintering and distribution is proven to be dependent on; temperature, particle size and exposure time. The stickiness of the deposit material is shown to be dependent on the SiO2 and alkali relation in the samples, estimated through a viscosity model.

  • 9.
    Sandberg, Jan
    et al.
    Mälardalen University, Department of Public Technology.
    Sand, Ulf
    Bel Fdhila, Rebei
    Numerical simulation of fouling on super-heater tube walls2002In: Proceedings of the 10th workshop on two-phase flow predictions, Merseburg, April 9 - 12, 2002Conference paper (Refereed)
  • 10.
    Skvaril, Jan
    et al.
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Avelin, Anders
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Sandberg, Jan
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Dahlquist, Erik
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    The experimental study of full-scale biomass-fired bubbling fluidized bed boiler2014In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 61, p. 643-647Article in journal (Refereed)
    Abstract [en]

    This paper presents experimental data concerning combustion characteristics of full-scale biomass-fired bubbling fluidized bed (BFB) steam boiler with a thermal output of 31 MW. The purpose of the experimental measurements is to show how the values of selected combustion parameters vary in reality depending on measurement position. Experimentation involves specifically a determination of combustion gas temperature and concentration of gas species i.e. O2, CO2, CO and NOX at different positions in the furnace and the flue gas trains. Character of results from the furnace indicates the intermediate stage of thermochemical reactions. Increased levels of CO close to the wall have been found, this may be indicating reducing atmosphere and thereby increased corrosion risk. Results from flue gas trains demonstrate that behavior there is related to the fluid dynamics and heat transfer, the temperature is too low for further combustion reactions. Results show great variations among measured values of all measurands depending on a distance along the line from the wall to the center of the boiler. The measurements from permanently installed fixed sensors are not giving value representing average conditions, but overall profiles can be correlated to online measurements from fixed sensors.

  • 11.
    Skvaril, Jan
    et al.
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Kyprianidis, Konstantinos
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Avelin, Anders
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Sandberg, Jan
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Erik, Dahlquist
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Experimental investigation of part load operation of a full-scale biomass-fired fluidized bed boilerManuscript (preprint) (Other academic)
  • 12.
    Thorin, Eva
    et al.
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Sandberg, Jan
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center.
    Yan, Jinyue
    Mälardalen University, School of Business, Society and Engineering, Future Energy Center. KTH, Sweden.
    Combined Heat and Power2015In: Handbook of Clean Energy Systems, John Wiley & Sons, 2015Chapter in book (Refereed)
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