Optimisation of circulating fluidised-bed gasification for biomass/straw (OCIFF)

A comprehensive literature (Stoltze) search and quantum mechanical calculations suggest that alkali is bound as oxygen-alkali salt with a significant heat of bonding. This is in accordance with experiments made by Moilanen. The results suggest that chars from straws with a high alkali-salt to silicon ratio causes a steadily increasing catalyst/carbon ratio and thus increasing reactivity as a function of conversion. Furthermore, the internal area evolution as measured by the BET technique does not support a steeply increasing reactivity as a function of conversion caused by internal area evolution only. According to a simplified formulation the reactivity is proportional to the amount of independent water-soluble catalyst compounds weighed by coefficients characteristic for the individual compounds. Therefore when catalysts are catalysts with low catalytic coefficients, e.g., K_3PO_4 or reacted into non-soluble compounds like potassium calcium silicates or potassium aluminum silicates the reactivity is decreased as observed earlier. Experiments made in a H_20 and C0_2 atmosphere with very little H_2 and CO available suggests that two significantly different types of CO desorption step are important, one fast and one slow reaction pathway (Barrio). In accordance to Global Equilibrium Analysis (GEA) the strongest ash sintering was experimentally observed for wheat straws rich in silicon and with a low reactivity, while highly reactive straw rich in potassium but poor in silicon caused much less sintering, little slag formation but significant salt formation. In order to reduce the sintering tendency of fuels rich in potassium the use of kaolin and magnesium oxide additives had a great preventive effect on bed agglomeration and freeboard deposit formation in the VTT bench-scale bubbling fluidised-bed gasification tests carried out with straw. Calcium added in the form of ions in the form of calcium acetate to straw or a calcium hydroxide/calcium saccharate solution into straw pellets and calculations indicated that the major role of reactive calcium is to bind silicon. High-temperature diffraction measurements suggests that the reaction takes place rapidly after that the carbonaceous material (molasses/saccharate) is converted, i.e. at high degrees of conversion. A dissolved or activated calcium and phosphate additive mixture, 'CAP', apparently effectively binds corrosive potassium and sodium elements into very stable, less sintering and less-corroding compounds. Atmospheric fluidised bed gasification tests were made, where the addition of the CAP additive greatly reduced sintering and agglomeration. Additionally two tons of the pelletised additive-impregnated Wheat 2000+ straw could be gasified during a four days test-run in a two-stage gasifier with a dry straw conversion of 98 wt.%. During the test there were no signs of agglomeration or sintering in the char bed