Alkali-enhanced gasification of biomass : laboratory-scale experimental studies

Sammanfattning: Gasification seeks to break carbonaceous materials into synthetic gas (CO+H2) which can be subsequently upgraded into valuable products. Thus gasification can be utilized to convert low grade biomass stocks into carbon-neutral chemicals heat and power. Nonetheless, gasification produces tar and soot as a by-product, impurities which deposit on cold surfaces thereby risking operation downstream of the gasifier. Cleaning the syngas after the gasifier is a conventional way to attenuate the problem, yet a complex and expensive one. Thus, tar and soot should preferably be addressed already in the gasifier. Given that these impurities are non-equilibrium species they could be targeted by using some sort of catalytic material. Alkali elements have precisely shown to possess catalytic activity on char gasification, besides they have also been associated with a decrease in tar and soot. Yet, to design a functional alkali-catalysed gasification process we need to investigate in more detail on what exact products does alkali show an activity on, on what stage, under what circumstances and, on the measure that it is possible, the mechanism. This was investigated on the basis of experimental work that approached the topic from two opposite sides. On the one hand, we studied the effects of diluting the alkali content of a Na-rich black liquor (BL) by blending it with pyrolysis oil (PO), and on the other hand, we investigated adding various amounts of alkali on more conventional types of biomass fuels. Most of the experiments were conducted on a laminar drop tube furnace but the reactivity of BL chars was also studied through thermogravimetric analysis.Alkali was found to catalyse heterogeneous gasification reactions (e.g. char) and to lead to much lower yields of C2 hydrocarbons, heavy tars and soot, favouring the presence of lighter species over large aromatic clusters. Alkali was hypothesized to reduce the quantity of soot by inhibiting the formation and growth of PAH, key intermediates on the road to soot. Besides, it was found that the initial contact between the alkali and the organic matrix was not critical, neither for gas impurities nor regarding char conversion, suggesting that the activity of alkali was a gas-induced phenomenon. The latter implied the existence of a vaporization-condensation cycle that could supply alkali into the char. Nonetheless, the beneficial effects by alkali were impaired by the affinity of Si to capture K and form potassium silicates which are inert. This interaction effect was particularly noticeable on char conversion as the silicates are not only inert but also liquid and viscous and prompt to encapsulate the char particles, thereby limiting mass transfer.The experiments with blends of BL and PO showed that the concentration of alkali in BL could be decreased by 30% without any sign of a decrease in the catalytic activity on char gasification, thus indicating the existence a saturation threshold. Furthermore, adding PO into BL lead to a further reduction on the quantities of tar and soot, this finding was attributed to changes in the fuel composition unrelated to alkali. In any case the experiments with BL-based fuels showed lower amounts of tar and soot than those from alkali-impregnated biomass powder. The difference was partially attributed to the content of S in BL. The subsequent investigation targeting the role of S confirmed that S possessed a soot inhibiting role similar to that of alkali, yet unlike K, it did not show a catalytic effect on char gasification.