Thermo-gravimetric analysis (TGA) was used to trace the fine dynamics of conversion (mass transfer) of small-size fuel particle (Bi < 0.1) at controlled temperature in thermally inert medium with hindered oxygen access (which prevent particle overheating and its premature burn-out) and mass transfer correlation having the value and sign of thermal effect. Experiments were performed at installation Q1500D (Hungary) according to standard procedure in air medium (ground fuel sample weight was 100 mg, inert medium charge — 400 mg, temperature rise at a speed of 0.3 K/ s, and final temperature 1000оС). The samples were wood particles, seeds and charcoal, products of their fast and slow thermal treatment by above described procedure and soot from ash box of pilot downdraft gas producer. Thermograms are shown in fig. 4.

Since the samples were actually dry, weight loss was mainly determined by coke-ash residue pyrolysis and oxidation effects. Overheating value and the sign of thermal effect were due to oxidizing exothermic processes in volatiles emitted by coke-ash residue (except the initial stage).

Steady heterogeneous burning of carbon of ground charcoal and coke-ash residue of bio fuels started at medium temperature above 350оС. For charcoal and wood particles this process is distinguished by appearance of specific temperature peak at 500оС. On having passed the peak, the burning of charcoal becomes uniform and finishes with some exposure at Т = 1000оС. Overheating curve for wood particles reproduces charcoal curve in shortened variant.

For seeds the pattern differs radically from above cited. In this case there is no overheating in the domain of volatile emission (which is weaker than with wood particles) which means that they behave like chemically inert substances. It is only at Т > 370оС the temperature of seed sample begins to exceed the ambient one. However, it exhibits its specific nature in this range too. Burnout curve for seeds has a low and extended (truncated) peak and a bit greater overheating in steady burning domain. Hence, the pyrolysis may be described as time extended process running in parallel with heterogeneous oxidation of coke-ash residue approximately up to 750оС.

After preliminary thermal treatment according to thermal shock scenario during 15-20 minutes at 400, 600, 800оС the following was found:

Wood after thermal treatment at heating rate of 200 К/20 min retains the peak of volatile emission at 342оС, but the first ("gas") preheating peak disappears, the height of the second peak (coke-ash residue burning) increases but the peak appears with a shift towards higher temperature domain (585оС); conversion process (weight loss) completes earlier (at 706оС instead of 850оС). Hence, the preliminary heating of fossil fuel improves its reactivity. Charcoal after thermal treatment in thermal shock conditions during 15-20 min at 400, 600, 800оС showed that increase of thermal treatment temperature resulted in the shift of coke — ash residue peak occurrence (at 496оС instead of 462оС), heating value and conversion rate were lower, process time and final temperature were rising and the fuel partially seized to burn.

Thus, in case of thermal treatment at 400 К / 20 min the moment of coke-ash residue burn­out coincide with the moment when maximum temperature is achieved in the plant (1000оС), whereas after thermal treatment at 800 К / 15 min the burning process finishes with incomplete burn out (unburned carbon of 9%) and much later after the furnace has been warmed up to maximum.

Charcoal after thermal treatment according to "heating simultaneously with furnace" scenario is characterized by still lower burn out rate and greater unburned carbon (14%) at the same final temperature values. The behavior of solid-phase volatile decomposition products settling in gas generator ash box (soot) is alike. Inert component content in these products (due to specific sampling conditions) reaches 50%, therefore the burn-out process finishes earlier which corresponds to 800оС.