Upgrading of fast pyrolysis byproducts for material use with high value

Fast pyrolysis is a thermochemical conversion process for the production of liquid substances from solid organic fuels, be it a blend for energetic use (liquid fuel) or more specific chemical substances for material use. However, complete liquefaction of the biomass cannot be achieved and there is always a solid byproduct, i.e. char. In most industrial and/or pilot plants this char will be burnt within the process in order to supply the heat necessary to keep up the process. Depending on the process design this is not necessarily required and with an energetically optimized system, char could be recovered as byproduct. It can be marketed directly as solid fuel or – after upgrading – for material uses. The amount of char produced is comparatively low for woody biomass since the process is optimized for liquid yield and the potential of such an alternative char use as byproduct is low. However, high ash content of biomass tends to decrease the liquid yield in favor of higher solids and gas yields because secondary pyrolysis reactions are catalyzed and ash will accumulate in the solids. If ash-rich biomass is used for fast pyrolysis, solids will represent a more important byproduct with little more value than its heating value. ... mehrThis is the case for the bioliq® concept which aims at utilizing (wheat) straw as feedstock for fast pyrolysis. While the solid byproduct might be considered a drawback when using biomass residues with high ash content, this contribution aims at elaborating a value addition by producing a solid byproduct with high market value. More specifically, the upgrading to activated carbon is presented to achieve a material use of high value. The major problem which requires attention is the high ash content of the produced char, naturally. Very few inorganics are present in the liquid pyrolysis product(s) due to the low vapor pressure of most inorganic elements present in biomass. They are almost exclusively recovered with the char, which exhibits much higher ash content than the feedstock in consequence. In the bioliq® concept, wheat straw is one standard feedstock with an ash content of 7-10 wt.% leading to a char with an ash content of 30-40 wt.%. Hence, the experimental procedure of activation started with a demineralization step (in 1N H2SO4:HCl 1:1solution) yielding a demineralized char. With this demineralized char, both KOH at 700 °C and steam activation at 750 °C were tested. As expected, steam activation produced an activated carbon with much higher ash content due to the carbon burn off, which results in an enrichment of minerals. In consequence, the BET surface area of the steam activated carbon is comparably low (see Figure 1). Activation with KOH leads to a nearly complete removal of inorganic compounds and a BET surface area comparable to commercially available activated carbons. Notably, the ash content was only reduced to half by the demineralization step whereas it was reduced to <1 wt.% after activation with KOH (see Figure 1). Further experiments were conducted to investigate the demineralization step and it was found that it can be increased when the char is washed with HCl after soaking. Surprisingly, it was also found that ash removal can be achieved by direct activation with KOH without the demineralization step. Hence, combined demineralization and activation can be achieved in a single step, reducing the complexity significantly. In conclusion, two major benefits could be achieved. Firstly, it is shown that the byproduct of fast pyrolysis can be used as feedstock to produce an activated carbon representing a value addition to a marketable product. Secondly, a promising pathway is opened to produce activated carbon from biomass residues such as wheat straw instead of fossil resources.