Gasification reactivity and ash sintering behaviour of biomass feedstocks

Char gasification reactivity and ash sintering properties of forestry biomass feedstocks selected for large-scale gasification process was characterised. The study was divided into two parts: 1) Internal variation of the reactivity and the ash sintering of feedstocks. 2) Measurement of kinetic parameters of char gasification reactions to be used in the modelling of a gasifier. The tests were carried out in gases relevant to pressurized oxygen gasification, i.e. steam and carbon dioxide, as well as their mixtures with the product gases H2 and CO. The work was based on experimental measurements using pressurized thermobalance. In the tests, the temperatures were below 1000 °C, and the pressure range was between 1 and 20 bar. In the first part, it was tested the effect of growing location, storage, plant parts and debarking method. The following biomass types were tested: spruce bark, pine bark, aspen bark, birch bark, forestry residue, bark feedstock mixture, stump chips and hemp. Thick pine bark had the lowest reactivity (instantaneous reaction rate 14%/min) and hemp the highest (250%/min); all other biomasses lay between these values. There was practically no difference in the reactivities among the spruce barks collected from the different locations. For pine bark, the differences were greater, but they were probably due to the thickness of the bark rather than to the growth location (see Ch. 3.4). For the spruce barks, the instantaneous reaction rate measured at 90% fuel conversion was 100%/min for pine barks it varied between 14 and 75%/min. During storage, quite large local differences in reactivity seem to develop. Stump had significantly lower reactivity compared with the others. No clear difference in the reactivity was observed between barks obtained with the wet and dry debarking, but, the sintering of the ash was more enhanced for the bark from dry debarking. Char gasification rate could not be modelled in the gas mixture of H2O + CO2 + H2 + CO, similarly as it can be done for coal. The reasons were assumed to be that in the carbon dioxide gasification, the gasification rate was negatively dependent on the CO2 pressure, the opposite of what is observed in steam gasification and the dependence of the gasification reaction rate on the conversion had three patterns. Normally it increases with the conversion, but it may also decrease or go through a minimum. According to the sintering tests, the ash residues were not totally sintered but they consisted of molten particles (spheres), unreacted char particles and powdery ash. The strongest sintering was observed for hemp, spruce bark obtained by dry debarking, and aspen bark. Increased pressure and CO2 resulted in intensified sintering, as has been observed in earlier studies.