Drying Phenomenon in a Fixed Bed under the Bio Fuel Multi Stage Drying

Drying of solid particles is generally divided into three periods: a short initial heating period, a constant drying rate period, and a falling drying rate period. Drying in a fixed under various drying conditions was experimentally studied. The main goals of the measurements were to calculate the heat transfer coefficient based on the measured values, and to evaluate the validity of the constant drying rate period. The exact duration of the constant drying period concerning the entire bed is short. The calculated heat transfer coefficients seem to be somewhat reasonable. NOMENCLATURE A surface (m) cp specific heat (J/kgK) Deff effective diameter of particle (m) h specific enthalpy (J/kg) lv vaporisation heat of water (J/kg) Le Lewis number (-) m& mass flow rate (kg/s) M molar mass (kg/mol) po atmospheric pressure (=101.3 kPa) pv partial pressure of vapour (Pa) p'v saturation partial pressure (Pa) R gas constant (=8,314 J/molK) t,T temperature (C), (K) u moisture of sample (kg/kgd.m) V Volume (m) x absolute moisture of air (kg/kgd.a) Greek letters α heat transfer coefficient (W/mK) ρ density (kg/m) τ time (s) Subscript 0 initial a air d.a dry air d.m dry mass e evaporation in inlet out outlet s particle surface v vapour w water wb wet bulb INTRODUCTION In Finland bio fuel represents approximately 19 % of the total primary fuel consumed in forest industry. The most common bio fuels are bark, forest residues, and different kinds of waste wood. The moisture content of bio fuel typically varies between 50 and 63 w-% (water per total mass), considerably reducing the power production of the power plant. Compared to completely dry fuel the theoretical decrease in power production is 12 -21 % for the initial moistures 50 63 w-%, respectively. A multi stage drying system consists of several drying stages between which the same air flow is heated. Figure 1 illustrates in a Mollier diagram the change of the air moisture in multi stage drying having three drying stages compared to one stage drying. The change of air moisture is the same in both cases but the maximum air temperatures are lower in multi stage drying than in one stage drying. Because of the lower drying temperatures, it is possible to utilise lower temperature energy sources such as secondary heat flows in the heating of the drying air than in one stage drying. This increases the electricity production of the combined heat and power plant. Enthalpy (kJ/kgd.a) M oi stu re (k g/ kg d.a ) Saturation cuve Tperature( C)