A biomass briquette, which are highly densified biomass raw material and a type of renewable energy are expected to suppress the global warming. To effectively utilize biomass briquettes, it is important to know their combustion characteristics. It is reported that the flaming combustion duration is proportional to the product of briquette density and inverse square of specific surface area, and the char combustion duration is proportional to the product of fixed carbon density and inverse square of specific surface area, with a single cylindrical briquette of which aspect ratio is 2 in the constant hot air condition. Each proportional constant, Cf for flaming combustion and Cc for char combustion, can be considered as the indicators of the biomass briquette combustion rate, may change with the briquette's aspect ratio. Previous studies have reported that the flaming combustion duration and the char combustion duration are both affected by the aspect ratio. However, the extent and tendency of each influence and the reason why such influences occur have not been clarified. In the present study, combustion experiments of a single cylindrical briquette were conducted to clarify the trends and extent of the influence of the briquette aspect ratio on both combustion durations.
This study aims to clarify the effect of inner-diameter differences on the delayed onset of flashback, where the H2/air premixed flame, after being trapped near a wire mesh, passes through the mesh and flashback occurs with a time delay. In the experiment, as the inner diameter decreased, the flashback range decreased, and the time from the flame trap near the mesh to the flashback increased. This is because the heat loss to the tube wall increased as the inner diameter decreased. On the other hand, the mesh temperature just before the start of flashback decreased as the inner diameter decreased. This is because heat recirculation is greater as the inner diameter decreases due to the longer time from the flame trap near the mesh to the flashback and the low flow rate of the mixture. Additionally, flame-front position was calculated using image analysis, and flame propagation behavior was examined in terms of the apparent flame speed. As a result, the findings of this study clarified the effects of heat loss and heat recirculation differences due to variations in the inner diameter on flame behavior near the mesh. This study provides knowledge on flame behavior near the flame arrester under flow conditions and contributes to the safe use of hydrogen.
When using carbon-free fuels (H2/NH3), achieving total heat recovery via an economizer-specifically complete recovery including latent heat-is desirable to compensate for their low calorific values LHV (kJ/mol). To the best of the authors' knowledge, this is the first numerical study on economizers under H2/NH3 combustion exhaust gas conditions. Analysis of heat transfer around the heat transfer tube placed in a combustion exhaust gas flow of H2/NH3 yielded the following results: (1) A liquid film formed downstream of the tube, creating a region of locally high heat flux. (2) The heat flux into the tube increased with the gas velocity at the economizer inlet, and the heat flux upstream of the tube wall increased significantly. The contribution of latent heat to the heat transfer downstream of the tube wall exhibited a similar moderately increasing trend along the tube wall starting from the vicinity of the 70-degree position, regardless of the inlet gas velocity. (cf. stagnation point: 0 degrees) (3) The effects of the inlet gas temperature on condensation phenomena in the boundary layer and the recovery rates of sensible and latent heat were clarified. This knowledge will facilitate the preliminary design of a multi-tube structure based on the local gas temperature at each tube row of the economizer.
