ISSN print edition: 0366-6352
ISSN electronic edition: 1336-9075
Registr. No.: MK SR 9/7
Integration of biomass drying with combustion/gasification technologies and minimization of emissions of organic compounds
Karel Svoboda, Jiří Martinec, Michael Pohořelý, and David Baxter
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 02 Prague, Czech Republic
Received: 28 January 2008 Revised: 19 June 2008 Accepted: 4 July 2008
Abstract: Moisture content (MC) of green biomass or raw biomass materials (wood, bark, plants, etc.) commonly exceeds 50 mass % (wet
basis). The maximum possible MC of biomass fuel for big scale combustion (e.g. fluidized bed combustion with low external
heat losses) is approximately 60–65 mass %. Higher biomass MC generally causes operational problems of biomass combustors,
lower stability of burning and higher CO and VOC emissions. Gasification of biomass with higher MC produces fuel gas of lower
effective heating values and higher tar concentrations. In this review, various technological schemes for wood drying in combination
with combustion/gasification with the assessment of factors for possible minimization of emissions of organics from the drying
processes are compared. The simple direct flue gas biomass drying technologies lead to exhaust drying gases containing high
VOC emissions (terpenes, alcohols, organic acids, etc.). VOC emissions depend on the drying temperature, residence time and
final MC of the dried biomass. Indirect biomass drying has an advantage in the possibility of reaching very low emissions
of organic compounds from the drying process. Exhaust drying gases can be simply destroyed as a part of the total combustion
air (gas) in a combustion chamber or a gasifier. Liquid, condensed effluents have to be treated properly because they have
relatively high content of organic compounds, some of them accompanied by odor. Drying of biomass with superheated steam offers
more uniform drying of both small and bigger particles and shorter periods of higher temperatures of the dried biomass, particularly
if drying to the final MC below 15 mass % is required. In practical modern drying technologies, biomass (mainly wood) is dried
in recirculated gas of relatively high humidity (approaching saturation) and the period of constant rate drying is longer.
Drying of moist wood material (saw dust, chips, etc.) is required in wood pellet production. Emissions of organics in drying
depend on biomass properties, content of resins, storing time and on operational aspects of the drying process: drying temperature,
drying medium, final MC, residence time, and particle size distribution of the dried biomass (wood). Integration of biomass
drying with combustion/gasification processes includes the choice of the drying medium (flue gas, air, superheated steam).
Properties of the drying media and operational parameters are strongly dependent on local conditions, fuel input of the combustion/gasification
unit, cleaning of the exhaust drying media (gas, steam, wastewater), and on environmental factors and requirements.
Keywords: wood - drying - combustion - emissions - VOC - terpene
Full paper is available at www.springerlink.com.
Chemical Papers 63 (1) 15–25 (2009)