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.

Biofuel Drying as a Concept to Improve the Energy Efficiency of an Industrial CHP Plant

Henrik Holmberg

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 Auditorium K216 at Helsinki University of Technology (Espoo, Finland) on the 13th of April, 2007, at 12 noon.

Overview in PDF format (ISBN 978-951-22-8649-2)   [1391 KB]
Dissertation is also available in print (ISBN 978-951-22-8648-5)


This thesis presents the results of a research project in which the enhancement of the CHP production by means of biofuel drying at an integrated pulp and paper mill has been explored. Instead of high temperature flue gas drying the use of low-temperature secondary heat as drying energy has been studied. The drying system considered in this thesis is classified as follows: Drying medium: Air Heat supply into the material: Convection Transport mechanism of the material inside the dryer: Conveyor Method to improve energy efficiency in the drying system: Multi-stage drying. In addition to secondary heat, the option of using steam as a drying energy has been included in the calculation models.

An optimization model for analyzing the integration of a multi-stage drying system into the CHP process has been developed. A simulation model based on the optimization model has been created to analyze the operation of the dryer under variable drying conditions. The simulation model can also be applied to the model-based control of the final fuel moisture content if there is a need to control it. Both models have been applied to a case study. Drying as a physical phenomenon has been studied experimentally in a small fixed bed dryer, and guidelines for dimensioning a continuous conveyor dryer in the case of multi-stage drying are presented. The energy efficiency of drying has also been analyzed using two evaluation methods: specific heat consumption and the irreversibility rate.

According to case study results, an optimally designed dryer uses only secondary heat, not steam, as a heat source, and earnings stem from decreased marginal fuel consumption, not increased power generation. Simulation calculations show that there are no economic grounds for control of the final fuel moisture content by adjusting heat inputs into the dryer. To control the final fuel moisture content the use of homogenous, and not just dry, fuel must have some positive influences on the operation of the boiler and/or power plant. Experimental tests show that critical moisture content is high for woody-based fuels and diffusion-based drying models must be used to determine drying times theoretically in a fixed bed. If the energy used in drying can be converted to mechanical work, the irreversibility rate is a more comprehensive method of comparing energy efficiency between different drying processes than specific heat consumption.

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

  1. Holmberg H, Ahtila P. Drying phenomenon in a fixed bed under the bio fuel multi stage drying. In: Oliveira A, Afonso C, Riffat S, editors. Proceedings of the 1st International Conference on Sustainable Energy Technologies, Porto, Portugal; 12-14 June, 2002. p. EES1 6-11. © 2002 University of Porto, Faculty of Engineering (FEUP). By permission.
  2. Holmberg H, Ahtila P. Comparison of drying costs in biofuel drying between multi-stage and single-stage drying. Biomass & Bioenergy 2004; 26: 515-530. © 2004 Elsevier Science. By permission.
  3. Holmberg H, Ahtila P. Optimization of the bark drying process in combined heat and power production of pulp and paper mill. In: Odilio AF, Eikevik TM, Strommen I, editors. Proceedings of the 3rd Nordic Drying Conference, Karlstad, Sweden; 15-17 June, 2005. © 2005 SINTEF Energy Research. By permission.
  4. Holmberg H, Ahtila P. Adjusting of temperature levels in multi stage drying system by means of outlet air measurements. In: Oliveira A, Afonso C, Riffat S, editors. Proceedings of the 1st International Conference on Sustainable Energy Technologies, Porto, Portugal; 12-14 June, 2002. p. EES2 1-5. © 2002 University of Porto, Faculty of Engineering (FEUP). By permission.
  5. Holmberg H, Ahtila P. Simulation model for the model-based control of a biofuel dryer at an industrial combined heat and power plant. Drying Technology 2006; 24: 1547-1557.
  6. Holmberg H, Ahtila P. Evaluation of energy efficiency in biofuel drying by means of energy and exergy analyses. Applied Thermal Engineering 2005; 25: 3115-3128. © 2005 Elsevier Science. By permission.

Keywords: biofuel drying, CHP production, secondary heat

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© 2007 Helsinki University of Technology

Last update 2011-05-26