## 2 Experimental Setup and Method
### Experimental Setup  
The characteristics and consistent patterns of processes, developing during the ignition and combustion of composite fuel pellets (Table 1) were studied using an experimental setup, schematically represented in Figure 1. The radiant heating conditions in a high-temperature air medium were simulated in a tube muffle furnace Loiplf50/500-1200 (Laboratory Equipment & Instruments ZAO, Saint-Petersburg, Russia). The tube was 50 mm in the inner diameter and 500 mm in length; the temperature was varied in the range 20-1200 \({}^{\circ}\)C and measured with an in-built type K thermocouple. When FC-based pellets are burned under optimum conditions--in small-sized boiler furnaces of local heat supply with a small chamber volume--the velocity of pellets is somewhat different from that of the carrier medium (a mixture of air with combustion products). Thus, the radiant heat flux density significantly exceeds the convective heat exchange density. A set of 7-10 experiments were performed under identical initial conditions. In each of them the furnace was heated to a given temperature (\(T_{g}\)). Single fuel pellets, located on the holder (Figure 1), were fed into the furnace at a rate of 0.1 m/s along the symmetry axis of the tube up to its middle, where the given temperature \(T_{g}\) was maintained. Experimental samples of all the fuel compositions (No. 1-No. 8) were identical. They were cylinder-shaped pellets with a mass of 20 mg, 3 mm in diameter and 3 mm in height. The heating value of such fuel pellets was different (Table 1). However, this factor had a less significant effect on the difference in the ignition delay times of fuel compositions No. 1-No. 8 than the mass and dimensions of the fuel pellets. An SPSh20-23017/2000Z manipulator (Mechatronic Product Factory ZAO, Saint-Petersburg, Russia) moved the fuel sample holder and a Phantom V411 high-speed video camera (Vision Research, Wayne, NJ, USA) at the same time. The filming rate was 500 frames per second at resolution 800 \(\times\) 600 pixels. An LED projector was fixed on the muffle furnace opposite the video camera for better contrast of fuel pellet images during video recording (Figure 1). Tema Automotive software (Image Systems AB, Linkoping, Sweden) was used to process the video frames. Figure 1: Scheme of experimental setup. The ignition delay time (\(t_{d}\)) was calculated by how much time passed after the pellet started heating (on entering into the muffle furnace) up until the fuel ignited. The latter event (Figure 2a) was captured by the moment when controlled luminous intensity was achieved in the recording area, similarly to the methods in [35; 36; 37; 38; 39]. In the Tema Automotive software, the video recording processing algorithm monitored all the video frames in sequence. The values of the shades of gray (from 0 to 255, which corresponds to the range from black to white) were measured in each point (pixel) of the recording area (Figure 2a) for each video frame. The initial video recordings in color were converted into black and white. Thus, the interval of luminous intensity from 220 to 255 corresponded to the fuel combustion (luminance around the pellet as a result of a rapid exothermic reaction). The moment of ignition was recorded automatically by the values of the luminous intensity reaching or exceeding 220 (in the shades of gray) in any point around the pellet being monitored. For example, in Figure 2a the red cross indicates a point, in which the highest luminous intensity is reached in the shortest time, as compared with other points in the recording area. Once the ignition moment was determined, the video recording processing algorithm was used to monitor the development of the process by the growth of the mean luminous intensity value in the recording area, which indicated the fuel ignition sustainability. The fuel burnout duration was determined in a similar way (\(t_{b}\)). This is a time interval between the ignition moment and combustion completion moment of the fuel components.