(406f) Alkaline Polyol Pulping - A New Pathway to Chemicals From Biomass

Alkaline
Polyol Pulping –
Alternative Pathway for Chemicals from
Lignocellulose

M. G. Schnitzlein^a,
M. Hundt^b,
N. Engel^b,
D. Rein^a,
K. Schnitzlein^b

^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.