Influence of air flow and radiative heating on oxidation of wood char at atmospheric pressure
Session chaired by Pr. Epaminondas Mastorakos
Influence of air flow and radiative heating on oxidation of wood char at atmospheric pressure Masaki NOAKIa, Michael A. DELICHATSIOS b,c, Kazuma Aokb, Hiroto Kohiyamab, Yoshifumi OHMIYAb aBuilding Research Institute, Japan, bTokyo University of Science, Japan, cNortheastern University USA ABSTRACT A series of experiments and analysis were carried out to comprehend the influence of air flow and external radiative heat flux on oxidation of wood char by using the Cone Calorimeter Test apparatus and a fan to generate the air flow. Two types of char specimens are compared, namely char produced from un-charred specimen following flaming or char produced from pre-charred specimen, in the experiment to grasp the influence of initial condition of specimen on char oxidation. Un-charred specimens were air-dried at atmospheric temperature whereas pre-charred specimens were charred in electric furnace with inner temperature500 oC before experiments. The main parameters were external heat flux and air flow velocity including cases without air flow. From the results we found that the weight of specimens after flame went out were about 20 % to 30% of the initial mass of wood. The heat of combustion (HC) during char oxidation is in the range of 25 to 35 kJ/g. The Heat Release Rate (HRR ranges from 20 to 45 kW/m2) and Mass Loss Rate (MLR ranges from 0.5 to 1.5 g/s.m2) of un-charred wood after flame went out and of pre-charred wood are approximately the same at the same external heat flux. The time to char oxidation of pre-charred wood is approximately inversely proportional to the square of external heat flux at values over 5 kW/m2 whereas pre-charred specimens at 5 kW/m2 external heat flux did not glow. The HRR and MLR increased with increasing air flow velocity. Pre-exponential factor A and activation energy E for the chemical reaction rate of char oxidation were calculated by using a model based on the balance of diffusion of oxygen in air and reactivity of char. The overall objective is to investigate how environmental conditions and the created process affect char reactivity, porosity and mode of oxidation (Regime III to Regime II). SUMMARY OF RESULTS The points of a special mention from results of the present research are the following: Mass of un-charred specimen at flame going out ranges from about 20 % to 30% of initial mass. Time to flame going out decreases with increasing external heat flux and is similar to the time at second peak of heat release rate (HRR). From the experiment using pre-charred specimen without air flow, time to char oxidation tends to be inversely proportional to square of external heat flux except at 5 kW/m2 of external heat flux where pre-charred specimen at 5 kW/m2 did not glow. From the results in the presence of air flow, higher velocity of air flow causes higher intensity of luminous of glowing. Note that a major part of ash layer covering on char surface was blown away over 1.6 m/s of air flow. In addition, the Mass Loss Rate (MLR) and surface temperature greatly increases with increasing air flow velocity. The Heat of Combustion (HC) of un-charred specimen remains at a constant level after ignition (about 12kJ/g) and shifts to another level (from 25 to 35 kJ/g) after flames went out. It is thought that the former one is caused by combustion of pyrolysis gases, while the latter one by char oxidation. On the other hand, the HC of pre-charred specimen kept approximately a constant level as high as the one of un-charred wood after flame went out. The ratios Xco/Xco2 in exhausted smoke of the present experiment ranging from 0.05 to 0.2 are approximately the same as the calculated values by using experimental empirical equations proposed in the literature. Furthermore, Heat of combustion (HC) calculated as heat release rate measured by oxygen consumption method divided by mass loss rate is approximately the same as the estimations based on thermo-chemical equation. The Pre-exponential factor A and activation energy E for calculating the chemical reaction constant of oxidation of wood char plate were estimated corresponding to reaction order n by using the model based on the balance of diffusion of oxidizer (e.g. oxygen in air) and reactivity of char (Regime III). From the results, estimated values of E and A are 47.7 kJ/mol, 7.68 m3/s.m2 , respectively at n=1.0. Calculated HRR using the estimated A and Ereproduce within 12.7 % of average relative error. In addition, the temperature of char surface in steady state is well estimated by the consideration of heat balance on char surface including the reaction rate relation obtained in this work. Note that modeling of ash influence on char oxidation is future work because knowledge and data regarding diffusivity of oxygen in ash and combustion catalysis of ash is necessary.
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