Torrefaction for entrained-flow gasification of biomass Part III

Desk studies have identified entrained-flow gasification as a promising option for the large-scale production of syngas from biomass. This biosyngas then is a versatile feedstock for the production of transportation fuels, chemicals and electricity. However, an entrained-flow gasifier requires sub-millimetre sized feedstock particles and to reduce biomass to this size is known to be difficult and expensive because of the fibrous structure and the tenacity of many types of biomass, woody biomass in particular. Torrefaction may be an attractive biomass pre-treatment process, as it may largely improve the size reduction behaviour. Torrefaction essentially is a thermochemical pre-treatment at a temperature level of 200-3 00?°C in the absence of oxygen. It not only may improve the milling characteristics, but also may have a positive impact on transport and storage due to the hydrophobic nature of the torrefied biomass. As there was a lack of data and mechanistic knowledge on the chain of torrefaction - size reduction -particle feeding, the objective of the present work was to gain this knowledge and to generate design data with respect to this chain.

On the basis of an extensive set of lab- and bench-scale torrefaction experiments, it is believed that hemicellulose decomposition together with the depolymerisation of cellulose are the most important mechanisms reducing the fibrous structure and tenacity of biomass. Hemicellulose binds the cellulose fibres present in cell walls of plants. Destruction of this polymer leaves disoriented fibres and their mutual coherence disappears. Cellulose depolymerisation, subsequently, decreases the length of the fibres. These mechanisms improve the overall characteristics of size reduction strongly. The energy requirements can be reduced with 50-85% depending on the applied torrefaction conditions. The capacity can be increased with a factor of 2-6.5. Strikingly, these improvements have been observed for all evaluated woody biomass types (willow, larch, and beech).

Because of the shortening of the fibres, the particles resulting from the size reduction process become more spherical (i.e., decreased length-to-diameter ratio), which improves the fluidisation behaviour. In a conventional feeding system of a dry-feed entrained-flow gasifier, fluidisation needs to proceed in the smooth regime, which requires an A powder according to the well-known Geldart classification. The preparation of such a powder is considered only possible using torrefied biomass. A Geldart A powder is obtained for torrefied willow in the size range of approx. 30-400 |a,m. Fluidisation tests proved indeed that such a powder (willow torrefied at 270 ?°C, 30 min reaction time) can be fluidised smoothly, but only in a narrow range of fluidisation velocities. It is believed that further shortening of the cellulose fibres can widen this range. This may be possible through optimisation of the torrefaction conditions (i.e., further increase of temperature in the range of 270 to 300 ?°C).

The results on torrefaction reveal that the process produces a dry, more hydrophobic product with increased energy density. The observed order of reactivity for the evaluated biomass types is: larch < willow/beech < straw. The differences in reactivity are likely to be related to the differences in hemicellulose content between these biomass types: A higher hemicellulose content leads to higher torrefaction reactivity. In an operating window of 8 - 30 min reaction time and at 230 to 270 ?°C, the solid mass yield is in the range of 78% to 95% and decreases gradually with temperature. The energy retained in the solid product ranges from 83% to 97% (LHVdaf) and is typically 90%. On average, the energy yield also decreases with temperature, but unfortunately this cannot be statistically justified due to a relative large inaccuracy in the measurement of heating values (?± 1.5%). It is argued that mainly a higher dehydration with increased temperature explains the interrelation between both yields. Increasing the torrefaction temperature from 250 to 270?°C reduces the required reaction time from 15-30 min to 8-15 min.

In conclusion, applying torrefaction as a biomass pre-treatment process indeed may be expected to contribute largely to the technical and economic feasibility of entrained-flow biomass gasification. Prove for this should come from a techno-economic evaluation for which this work provides important input data. Further research focusing on torrefaction in the range of 270 -300 ?°C is recommended. Operation at such temperatures shortens the required reaction time and may further improve the fluidisation properties of torrefied wood.