1.1 Fuel structural transformation by pyrolysis

Pyrolysis causes significant changes of physical and chemical properties of fuel particles. Measurements showed a two-fold reduce of bio-fuel particle density in a narrow temperature range. Particle shrinks insignificantly, not more than 30% of its initial size. Since the change of volume does not exceed 20% of initial value, whereas the density decreases greatly, the porosity of particles increases. Due to pore opening the oxygen can reach new surface which was inaccessible earlier (fig. 2).

In the range from 200 to 700оС the specific surface area of wood chip greatly depends on rate of heating, i. e. in case of fast heating it will be one or two orders higher than area during slow heating. Fuel porosity curves (fig. 3) for test samples revealed three steady peaks at 5­50, 100-3000 и 10000-50000 nm in mezzo — and macro porosity domain (5-50000 nm).

The porosity of the first nanolevel is typical of dense fuel particles. For most coals the pores’ average diameter is within 4-10 nm, and more rarely in the range of 30-40 nm. Artificial materials with such pores are highly-scorched activated coals intended for absorption of large molecules such as organic dyes. When a particle enters the furnace the volume of pores of this class will greatly increase due to thermal decomposition of organic compound which is initiated by temperature rise and depressed with pressure increase being a natural regulator of gas formation process in material pores.

Nuclei of pore formation at nanolevel are thinner pores and cracks and it is within their volume that the detachment of gaseous "fragments" of splitting macro molecule of coal material occurs. Initial size of these pores is close to that of gas molecule diameter (0.4-1.0 nm). Cavity degasification process is retarded by molecular repulsion forces hindering the pass through "contracted" points in ultra microcracks and requiring great energy (activation) to overcome them which results in change of the state of dispersion phase. Materials with flexible structure (wood) form swollen-state colloidal systems resistive to both contraction and further expansion. In solid materials (cokes) structures similar to those observed in metals of interstitial compounds can be formed. The speed of gas diffusion from these pores depends on activation energy and temperature level. Gas molecule travel during typical in-furnace process time is compatible with the size of coal macro molecule.

Gas emission from numerous ultra micropores into larger ones acting as collectors continues during the entire particle burning period.

Significant flow resistance due to system porosity results in intra-pore pressure rise (up to saturation pressure) at initial destruction stage and in development of positive flow in the largest pores that hinders external gas inlet into particle pores. Simultaneously mechanical (rupture) stress may develop in the particle. With destruction process transit in its damping stage and intra-pore pressure reduction, pyrolysis gaseous products will be able to react with external oxidizing agent not only on the surface of the particle but inside the latter creating quite favorable conditions for homogeneous intra-pore burning.

As soon as degasification process is completed, free molecule diffusion (Knudsen diffusion) mode is established in nanolevel pores, coupled with convective Stefan’s flow. Based on numerous estimates, for particles from 10 to 1000 pm the degree of such porous space (with specific surface area Sp) participation in reaction insignificantly depends on particle size and at 600 °С it is for oxygen within the range of Sp / Scar < 0.1 (for fast heating cokes) and Sp / Scar < 0.03 (for slow heating cokes).

Pores of the second (medium) peak (dp = 0.1-3 pm) occur in the domain of transition from Knudsen mode to normal diffusion. They provide a better access for oxidant and can participate in reaction in larger volume. In pores of the third peak (dp > 10 pm) diffusion runs similar to that in unrestricted space. These pores constitute insignificant part of internal surface and their contribution to burnout rate is known to be negligible. However, their role is quite significant as they can deliver reagent to joined pores of first and second peaks.

The obtained data show that wood particles and pellets have low-porous structure (S0 < 2 m2/g), charcoal has mesoporous (S0 < 8.6 m2/ g) and seed has dense microporous structure (S0 < 0.01 m2/g). Specific surfaces vary quite significantly in original state but this difference tends to flatten out for products of their thermal treatment. It increases to the third order for

In seed which relates to bio fuels with the highest natural density and occupies intermediate place between wood and fossil coal the first peak pores dominate (more than 65% by volume). They are followed by the third peak pores (25%). Total volume of seed pores (0.045 cm3/ g) is 2,7 times less than that of pellet (0.12 cm3/g) and 27 times less than that of the wood (1.22 cm3/ g). Specific surface area of seeds is two orders lower than that of the wood. After thermal treatment the pore volume of the seed increased 10 folds and there appeared a second peak on the background of the first and the third peaks which is compatible with these two peaks, although their heights increased by one order.

In bio fuels with natural density (wood) the pores of second type dominate, whereas the pores of the first type have not been revealed and volume of the third type is insignificant. Thermal treatment of wood results in slight increase of total pore volume (twice), whereas its structure changes to form larger pores. The height of second peak reduced three times and the height of the third peak increased three times.

In pellet the structure of the wood subjected to sever mechanical processing (crushing, pressing) differs greatly from the original one, forming larger pores with drastic reduction of their original total volume (10 fold reduction). Pore distribution in pellet after thermal treatment is qualitatively identical to original one but the total volume increased 4 times and peaks became twice as high.

Comparison of porosity curves for various fuels shows that thermal treatment of bio fuels with different original structure will flatten out the difference with the formation of common transport pore structure for all fuels which may result in similar burn out rates by volume for their coke residues.