Ignition Behavior of Bio-Coke (Highly Densified Biomass Fuel) in High-Temperature Air Flows

Abstract

The ignition behavior of a newly developed biomass briquette, Bio-coke (BIC), is investigated. The fuel has unique features such as economical advantages for its versatility of biomass resources, high volumetric calorific value because of its high density (1300 kg/m³; twice or more than that of ordinary wood pellets) and high mechanical strength. The ignition characteristics of cylindrical BIC blocks (48 mm in diameter and 85 mm in length), important when using the fuel in actual combustion furnaces, are investigated in high temperature air flows (473-873 K, 550-750 NL/min.). In the experiments, preheated air is blown onto the bottom surface of BIC cylinders and the ignition behavior of the bottom surface is observed monitoring the surface temperature as well as the time dependent mass loss rates. The results show two ignition modes; (1) solid surface ignition preceding gas-phase ignition in high air temperature conditions (T≥598K), and (2) gas-phase ignition accompanied by simultaneous surface ignition occurring at relatively low air temperature conditions. The appearance of each mode depends on the preheated air supply condition in terms of the air temperature, flow velocity, and moisture content of the fuel. The rate of evolution of volatile gases is closely correlated with the temperature distribution inside the BIC briquette which depends on the heating rate, implying that variations in the temperature distribution inside the fuel could be one reason for the appearance of the observed ignition modes. It is suggested that the temperature distribution inside the fuel has to be taken into account in the control of the ignition behavior of BIC briquettes.