Upgrading of waste streams into fuel by means of drying with low temperature excess heat - a techno-economic evaluation

Tiina Myllymaa

Research output: ThesisDoctoral ThesisCollection of Articles


The use of industrial excess heat in drying of combustible waste streams enhances the energy efficiency of combustion process and reduces emissions. A suitable dryer for wastes is a belt dryer when low air temperatures are used. However, the total drying costs may vary considerably depending on how different design parameters are chosen. This dissertation studies the effects of the design parameters of the belt dryer on total drying costs when different waste streams are dried using low temperature excess heat. Temperature is usually lower than 100-120 °C. The waste materials included are woodchips, bark, and mixture of soot sludge and sawdust. The influence is assessed by changing the following parameters: bed height, drying air temperature, air velocity, and initial/final moisture contents of the material. In order to produce a techno-economic evaluation, a series of experimental tests have been conducted to gain information on drying kinetics of the streams in fixed beds. Based on the experimental data, an Excel-based programme was developed to determine the costs. Drying kinetics studies show that the air temperature has a great impact on the drying time for all materials; the lower the temperature, the longer the drying time needed. For example, for woodchips, the drying time is ~two times longer when the temperature decreases from 70 to 50 °C (300 mm). Results indicate that there is an optimum value for air velocity, after which increase of the velocity will no longer remarkably decrease the drying time. Results also indicate that the most optimal volume ratio for the sludge and sawdust is 50%:50% and the most optimal bed height is 200 mm. The economic calculations show that the highest bed height and the highest air temperature (bark: 250 mm, 110 °C; woodchips: 500 mm, 90 °C) yield the lowest drying costs, regardless of the change of moisture content, when the price of the heat is the same for every air temperature. For the sludge and sawdust mix, the lowest costs are gained with the highest air temperature (100 °C). The optimal bed height of the mix depends on the material and its initial moisture content. The higher the moisture content, the lower the optimal bed height is. When materials are dried, the availability of heat sources at different temperatures may be limited. In that case, it may be reasonable to use different air temperatures in successive drying blocks. On the basis of block studies, it seems to be more economic to use lower air temperatures in the first blocks when material is moist, and higher air temperatures in the last blocks when the moisture content of material is lower if the heat price is the same for both temperature levels. Air velocity should be the same over the entire dryer. In general, the results of this dissertation shows that it is important to pay attention to the main design parameters to optimize total drying costs.
Translated title of the contributionJätevirtojen jalostaminen polttoaineeksi kuivauksella käyttäen matalalämpötilaista ylijäämälämpöä - tekno-taloudellinen arviointi
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Ahtila, Pekka, Supervising Professor
  • Holmberg, Henrik, Thesis Advisor
Print ISBNs978-952-60-8833-4
Electronic ISBNs978-952-60-8834-1
Publication statusPublished - 2019
MoE publication typeG5 Doctoral dissertation (article)


  • drying costs
  • belt dryer
  • waste heat
  • woody biomass
  • bark
  • woodchips
  • sawdust
  • soot sludge


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