Alkaline Polyol Pulping –
Alternative Pathway for Chemicals from Lignocellulose
M. G. Schnitzleina, M. Hundtb, N. Engelb, D. Reina, K. Schnitzleinb
aMCI Management Center Innsbruck, Maximilianstr. 2, 6020 Innsbruck, AUSTRIA
bBrandenburg Technical University Cottbus, Burger Chaussee 2, 03044 Cottbus, GERMANY
The production of quality chemicals from biomass requires high standards on precursors for subsequent specialty products and biochemical processes. Purity and chemical compositions of these precursors, typically classified as cellulose, lignin and sugar fractions, need to match the tight requirements of biochemical pathways or catalytic conversion processes. Essentially all commercially introduced pulping processes and lignocellulose pre-treatment processes do not meet those general criteria as they were designed to focus on specific application requirements. The authors therefore conceptualized and developed a unique pulping process for producing suitable precursors for diverse chemical and biochemical production pathways with a focus on forest-based lignocellulose.
The process conceptualization employed an integrated product and process design approach, which included general quality criteria for possible precursors. Among others, the cellulose fraction needed to allow enzymatic hydrolysis with low enzyme concentrations through a significantly low lignin content and a low amount of undesired modifications of the cellulose fiber. For the lignin product, no hetero-atoms, like sulfur or chlorine, should be introduced to the product, which would compromise subsequent catalytic conversions and possible biochemical pathways. The sugar fraction was to be essentially free of furfural derivatives and to only contain mono- or oligomeric sugars with low molecular weights or sugar salts.
The process itself is perceived as a first stage in a distributed production scheme, being able to allow smaller economic entities to operate the fractionation plants and produce quality precursors as intermediates for larger chemical producers and customers. Subsequently, the process needed to build small, making large pretreatment vessels and high pressure operations prohibitive. Furthermore, bio-hazardous material was not allowed in the process to make operation in forest areas possible and safe. All those requirements were met by the presented process concept, which is using an alkaline polyol solution as a fractionation medium in the thermal conversion of soft- and hardwood lignocellulose. Fractionation is taking place at atmospheric pressure and temperatures of 200 °C and beyond, resulting in an essentially water-free thermochemical decomposition of the soft- and hardwood lignocellulose within minutes. With this the process could be designed using only continuous operating, small sized process equipment, thus allowing it to be implemented in mobile, even trailer based plants. As an important consequence, it is therefore possible to move the fractionation plant to the resources rather than feedstock to a centralized chemical operation. The term AlkaPolP has been coined to describe this fractionation principle using alkaline polyol in an essentially water-free solution.
Results from bench top laboratory work showed unbleached AlkaPolP cellulose fractions with softwood feedstock (pine) typically containing only 2–4 % w/w total lignin, with kappa numbers ranging from 7 to 10 and Klason lignin at about 1.5 %. Tests with softwood (pine, spruce) required fractionation residence times of 15 minutes and below, whereas those for hardwood (birch, common beech) could even be shorter to achieve similar fractionation results. Together with studies on a subsequent enzymatic hydrolysis, glucose recovery rates of up to 83 % of the feed stock glucose content were achieved. Furthermore, significantly lower enzyme concentrations were necessary for hydrolysis as compared to current lignocellulose ethanol processes. The lignin fraction obtained by precipitation from the black liquor showed average molecular weights of 1800-3500 and a PDI of 1.6 to 2.7 for pine and molecular weights of 1000-1500 and a PDI of 1.3 to 1.7 for beech. Due to its low hydrophobic characteristics, AlkaPolP lignin was successfully tested in the production of phenolic resin boards, substituting up to 25 % of phenol.
Based on laboratory data, various AlkaPolP process concepts were modeled using ASPEN Plus simulation software. Thermodynamic data for high alkaline solutions were calculated using an unsymmetric electrolyte model, allowing the evaluation of complex chemical equilibria. Precursors were priced based on current substitution opportunities in Western markets, leaving out possible future economical advantages due to improved product characteristics and new production pathways. Economics of various process schemes were studied which essentially differ in the way complete recycling of all process chemicals is achieved. The results are compared with published economical data for lignocellulose ethanol plants. Results show superior economic performance and substantiate a broad economic viability of the entire process concept. Future research strategies are also derived for the commercialization of AlkaPolP fractionation processes.
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