HYBRID STAGED THERMOLYSIS TO VALORISE BIOMASS

The need for a renewable ‘green’ chemistry is adamant because of the adverse effects of the increasing use of fossil fuels on our society, like global warming and the depletion of fossil fuel resources. Therefore, the use of ligno-cellulosic (woody) types of biomass as a renewable source for chemicals and energy is increasingly becoming important. Unfortunately, the heterogeneity of biomass presents a major obstacle to chemical utilization. The main constituents hemicellulose, cellulose and lignin are strongly interconnected by a variety of physico-chemical bonds that makes it difficult to extract individual chemicals in high yields. So an efficient and cost-effective fractionation technology to cleanly split the biomass into its main constituents is a valuable asset. It opens up the possibility to treat each constituent separately, using dedicated conversion technologies to get specific target chemicals. Thermolysis is a heat-treatment option to convert the biomass into chemicals according to differences in thermochemical stability between the main biomass constituents. However, since these thermochemical stabilities –at least partially- overlap, a careful selection of thermolysis process conditions is necessary to ensure a selective degradation of the chosen biomass constituent. At the same time a premature degradation of the other fractions should be prevented. Main parameters are temperature, heating rate, residence times, reaction medium and the application of catalysts. At present there is a lack of integrated processes that valorise the whole of the biomass in a cost-effective manner. The synergistic combination of aqua-thermolysis (heat-treatment in water at elevated pressure) and pyrolysis (thermal degradation in the absence of oxygen) is a promising thermolysis option in which the fractionation of the lignocellulosic biomass is integrated with the production of valuable chemicals. Preliminary non-catalytic experiments with beech, poplar and spruce wood and wheat straw have indicated the potential of this combination of aqua-thermolysis and pyrolysis to valorise lignocellulosic biomass. The relatively low-temperature (150 – 250°C) aqua-thermolysis focusses on the production of e.g. furfural from exclusively the hemicellulose fraction of the biomass. In a subsequent (fast) pyrolysis of the hemicellulose-free residu at slightly higher temperatures (300 – 500°C), the cellulose fraction can be selectively depolymerised into e.g. levoglucosan, a promising chemical building block for a variety of products. Finally, the resulting refractory lignin-residu (char) may be used as fuel, soil-improver or converted into monomeric phenols by an oxidative pyrolysis. Up to 7 weight percent (dry base) of furfural, 10 weight percent (dry base) of levoglucosan and 20 weight percent (dry base) of char have been obained in a two-step process consisting of a batch-type aqua-thermolysis in an autoclave and a bubbling fluidised bed (fast) pyrolysis. The use of specific catalysts is considered to boost these yields. Another important issue that will be addressed, is the separation of the target chemicals from the crude product mixture. Finally, to better understand the relations between the chemical changes in the biomass during the course of the integrated process and type and amount of the resulting chemical products that are formed an extensive C13-solid state NMR study has been conducted