2016 年 5 巻 2 号 p. 129-135
To investigate the influence of briquette size on both flaming combustion and char combustion of highly densified biomass briquette, combustion experiments were carried out with the briquette made of cypress sawdust. In this study, three sizes of cylindrical briquette of which the ratio of height to diameter is kept at 2 ((1) 30 mm in diameter and 60 mm in height, (2) 35 mm in diameter and 70 mm in height and (3) 40 mm in diameter and 80 mm in height) were used as fuel samples. The density of the briquette and the moisture content are kept constant at 1300 kg/m3 and 10 wt%, respectively. A single briquette is inserted into the furnace to which preheated air (673K, 1.42 m/s) is supplied and the mass loss rate and each combustion duration were measured. As a result, in the flaming combustion duration, it is observed that the average mass loss rate ((dM/dt)mean) of briquette per specific surface area (S/V, V: volume) is proportional to the surface area (S) of the briquette. In the briquette size used in the study, it is demonstrated by introducing Fourier number that the flaming combustion duration (tf) is proportional to the square of an inverse number of the specific surface area (tf ∝ (V/S)2). The normalized mass loss rate ((dM/M0)/(dt/tf)) in the normalized flaming combustion duration for all three briquette size can be expressed in a single curve. It is shown that the char combustion duration (tc) is also proportional to the square of an inverse number of the specific surface area (tc∝ (V/S)2). For char combustion, shrinking-core reaction model was applied to the cylindrical briquette used in the study and the oxygen diffusion coefficients through both the gas boundary layer (Kc) and the ash layer (Dc) were determined by the experimental results. The oxygen diffusion coefficient through the ash layer (Dc) is lower than that reported for lower density one and this may be attributed to the lower porosity of the highly densified briquette in this study. It is successfully demonstrated that the unburnt fraction in the normalized char combustion duration can be expressed by the model.