Comparison on the steady smoldering characteristics of char and biomass rods

Abstract: To investigate the different characteristics of char and biomass rods in steady smoldering, experiments were carried out on the vertical downward smoldering of char, absolute dried biomass and air-dried biomass rods with different diameters (2-8 mm) made from elm-bark powder and cypress powder. The detailed production methods of three rod fuels are as follows: The elm bark powder, cypress powder and water were mixed uniformly at a mass ratio of about 1:1:4, and extruded into biomass rods with diameters of 3, 5, 8 mm. The rods were dried in the air (>48 h) to obtain air-dried biomass rod. The air-dried biomass rod was placed in electric drying oven and kept at 105 ℃ for 24 h to obtain absolute dried biomass rod. After absolute dried biomass rod was cooled, it was placed in drying basin for storage. The char rod was made by pyrolysis of air-dried biomass rod: The air-dried biomass rod was placed in quartz tube, and the nozzle was sealed with asbestos. It was placed in muffle furnace and heated from room temperature to 500 ℃ at a heating rate of 10 ℃·min-1. After holding for 2 h, the power supply was turned off to obtain a char rod. After it was cooled (> 12 h), it is stored in a sealed bag. The char and biomass rods were characterized by proximate analysis following GB/T 28731-2012. In the smoldering experiment, the tops of the respective char and biomass rods (about 15 cm in length) were ignited. After the smoldering got stabilized (about 4 minutes), smoldering propagation velocity was obtained by using the scale and timer to record the moving distance and time of reaction front, respectively. During the steady smoldering process of the rod, the temperature distribution of the reaction zone was taken using infrared thermal imager (ThermoProTM TP8), and the contours of flue gas layer around the rod were recorded by a self-made schlieren device. The rod fuel was then inserted into biomass ash to isolate oxygen and get extinguished, and the shapes of reaction zone were recorded by Leica microscope (M135C). The emissivity of each rod was measured as 0.93 in infrared image analysis. Moreover, the oxygen consumption rate of char and biomass rods was calculated by a self-written code. The results showed that 1) the char and biomass rods can self-adjust the shape of reaction zone to maintain steady smoldering, and the maximum temperature range of each rod fuel is between 620 and 770 ℃. 2) The smoldering propagation velocity of char rods is about 3.9 and 4.5 times that of absolute dried and air-dried biomass rods, respectively. Not endothermic pyrolysis in the smoldering process of char rods is observed, and the larger smoldering propagation velocity produces more heat during the same time, thus the maximum temperature of char rods is about 43 and 55 ℃ higher than that of absolute dried and air-dried biomass rods, respectively. The length of reaction zone of char rods is increased due to the greater propagation velocity and higher maximum temperature provide a larger reaction area to maintain steady smoldering. The length of the reaction zone of char rod is about 3.5 and 4.1 times that of absolute dried and air-dried biomass rods, respectively. The fuel consumption rate of char rods is about 2.4 times that of biomass rods. 3) The calculated and experimental contours of flue gas layer are in good agreement, and the oxygen consumption rate of char rods is about 3.1 and 3.6 times that of absolute dried and air-dried biomass rods, respectively. In the process of steady smoldering of char and biomass rods from the same source, when the external air conditions are certain, the size of reaction zone increases, so that the smoldering propagation velocity increases. In the application design, the size of reaction zone should be reasonably designed according to the selected materials and smoldering intensity. The finding can provide a theoretical reference to study further the steady smoldering mechanism and fuel options in the application.