The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Modelling and Experimental Study on Wood Chips Boiler System with Fuel Drying and with Different Heat Exchangers

Jukka Yrjölä

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Mechanical Engineering for public examination and debate in Micronova at Helsinki University of Technology (Espoo, Finland) on the 16th of November, 2006, at 12 noon.

Overview in PDF format (ISBN 951-22-8439-1)   [785 KB]
Dissertation is also available in print (ISBN 951-22-8438-3)

Abstract

This thesis introduces two types of boiler plants with a cross-flow warm air moving bed dryer. The first one dries wood chips for the use of the plant itself. The second one is also equipped with a sieve and it even produces fuel to the market, to the small-scale boilers, for example. This has notable technical benefits because the load of the boiler plant is more stable than that in the traditional district heating use. Good quality wood fuel also improves the operation and control of small-scale heating systems of customers, which can lead to cheaper furnaces with lower emissions.

There are interdependencies between the dryer, boiler and heat exchangers, which are the main equipments under study. In addition, the ranges of heating load, of moisture content of fuel and of outdoors air temperature are large. Therefore, models were developed for improved understanding and prediction of the thermal performance of the system and for technical optimisation of equipment. Six models are described, two for each equipment under study.

Energy and mass balance model and a deep bed model for drying were developed. The deep bed model bases on the shrinking core single-particle model. A model was developed to calculate the heat transfer rate and pressure loss of the convection section with tubes and another one for the convection section with a rectangular duct having an array of longitudinal rectangular fins for flue gas. Two heat exchangers are described and modelled, a pipe-bundle type flue gas heat exchanger and a flat plate type heat exchanger, which recovers heat from the drying exhaust gas. Case analyses for energy consumption and equipment dimensioning are described.

The models for a dryer were verified by the measurements on the laboratory test rig and other models on the 40 kWth pilot plant and on the 500 kWth demonstration plant and on three other boiler plants. Only the results of the model for convection section indicated an excessive difference between the measurements. All other models gave results in reasonably close accordance with the measurements.

This thesis consists of an overview and of the following 6 publications:

  1. Yrjölä J. Model and simulation of heat exchangers and drying silo in a new type of a boiler plant. Conference on Progress in Thermochemical Biomass Conversion. Tyrol, Austria, 2000. Bridgwater A.V (ed.); Blackwell Science Ltd, Cornwall, Great Britain, 2001: 678-692.
  2. Yrjölä J, Saastamoinen JJ. Modelling and practical operation results of a dryer for wood chips. Drying Technology, 2002; 20 (6): 1077-1099.
  3. Yrjölä J, Paavilainen J. Modelling and analyses of heat exchangers in a biomass boiler plant. International Journal of Energy Research, 2004; 28 (6): 473-494. © 2004 John Wiley & Sons. By permission.
  4. Yrjölä J. Production of dry wood chips in connection with a district heating plant. Thermal Science, 2004; 8 (2): 143-155. © 2004 VINČA Institute of Nuclear Sciences. By permission.
  5. Yrjölä J, Paavilainen J, Oravainen H. Modelling of the finned convection section of a biomass boiler. Energy Conversion and Management, 2006; 47 (15-16): 2552-2563. © 2006 Elsevier Science. By permission.
  6. Yrjölä J, Paavilainen J, Sillanpää M. Modelling and experimental studies on heat transfer in the convection section of a biomass boiler. International Journal of Energy Research, 2006; 30 (12): 939-953. © 2006 John Wiley & Sons. By permission.

Keywords: bed, bioenergy, chip, fin, fouling, heat transfer, wood chip

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© 2006 Helsinki University of Technology

Last update 2011-05-26