(524a) Smartwaste- Biomass Conversion into Low Oxygenated Hydrocarbons in Subcritical Water

Authors: 
Posmanik, R., Cornell University
Cantero-Tubilla, B., Cornell University
Tester, J. W., Cornell University

Sustainable
management of food wastes as well as animal manure and other agricultural
wastes is a major challenge facing food production worldwide. The dairy and
food industries generate approximately 19 and 36 million tons of waste per
year, respectively, only in the U.S. The majority of these residues, rich in
organic carbon and valuable nutrients, are nowadays disposed in landfills. The
current management strategies for manure and food waste are mainly based on
biological processes, converting organic wastes into biogas (anaerobic
digestion). Limited biogas conversion yield (40-50%) generates a large volume
of liquid effluent, containing an aqueous mixture of carbohydrates, proteins,
lipids, minerals and nutrients that should be further processed to exploit its
economic value. The SmartWaste approach is a process integration strategy
that uses a series of thermochemical processes to recycle multiple waste
streams, producing chemicals, fuels, and nutrients. Furthermore, these fast,
selective, and efficient waste-to-fuel processes can be easily integrated into
current bio-refineries flow-chart.

Among recycling approaches for high water
content waste feedstocks, SmartWaste proposed the hydrothermal biomass
conversion technology, using both sub- and supercritical water. The suggested
approach is based on three-stage processing of waste. First, the pretreatment
step focuses on blending different waste streams. Achieving and appropriate
feed composition is key to increase the bio-oil production yield in the second
stage. In the second step, hydrothermal liquefaction (HTL) of the blended waste
stream with subcritical water significantly reduces the oxygen content of the
feed to approximately 10% by dewatering and condensation reactions. In the
third step, the hydrogenation process, the produced bio-crude oil is further
upgraded, by reducing the oxygen content.

The SmartWaste process
depolymerizes organic waste into smaller structural units. The HTL step is
based on fast hydrolysis reactions, followed by dehydration and condensation of
sugar, lipids, proteins, and their degradation products, in a single step
reaction using subcritical water. This process benefits from the modification
in chemical behavior of supercritical water while approaching to the critical
point conditions (374 C and 22.1MPa). For instance, the
increase of ionic product (KW) favors ionic, and acid-base reactions
without the addition of a catalyst. Therefore, the optimum processing
temperature for this stage is expected to be between 250 and 350 C.
In terms of reaction time, these reactions are usually completed within 20-60
minutes. This represents a big step towards process intensification (reduction
of required reactor volume) compared to biochemical processes (enzymes) that
require few days.

The main challenges facing residual
biomass recycling are the highly complex polymeric composition, together with
the high oxygen content, making this feed low in energy content. The
flexibility of the hydrothermal technology allows the manipulation of process
conditions (reaction time, temperature, pressure) to adapt to different feed
compositions and achieve higher bio-oil yields (around 70% on a carbon basis),
with higher energy values (oxygen content of ~10% w/w). This oxygen content
represents a reduction of 70% with respect to the feed waste. As a consequence
of the dehydration and condensation reactions of waste compounds for the
generation of bio-oil, an aqueous-soluble carbon phase is also produced. In
addition, the solubilized-hydrolyzed amount of solids is not complete, being
bio-char being one the products of HTL. To complete the product distribution, a
gaseous phase is also generated. The relative amount of any of these four
phases depends on reaction conditions, apart from the presence of catalyst.

In this presentation, we will present experimental data for the
HTL compartment of the SmartWaste project. Carbon mass balances for a
wide range of biomass feedstocks will be discussed. This included modeling
compounds (polysaccharides, monosaccharides, proteins and long-chain fatty
acids) as well as real waste biomass (diary manure digestate collected from
anaerobic digestion in upstate NY, and carbohydrate rich food waste from
Cornell’s dining halls). An example for carbon mass balances is presented in
Fig. 1, showing the highest production of bio-crude oil from a mixture of polysaccharides,
proteins and lipids at 300 C
with negligible char production. The quality of the bio-crude
oil is showed in the Van-Krevelen diagram (Fig. 2), representing atomic rations
H/C vs. O/C. HTL significantly reduces the O/C and H/C ratios for all thermal
conditions studied.  During the HTL
process, the quality of the oil intermediate can be highly improved by choosing
the adequate biomass feed. For example, the alcoholic groups used for
condensation can be synthesized

directly in the reactor by lignin
hydrolysis, and high pH catalytic media can be achieved by proteins hydrolysis and
ammonia release.

The target product of the SmartWaste
approach is a stream of alkanes with minimal oxygen content. Further
reduction of the bio-oil oxygen content by hydrogenation on supported noble
metal catalysts will maximize the energy density of the final products.  The production of low polar stream of light
straight-chain alkanes (4 to 10 carbons) is achieved by functional reaction
routes including hydrolysis, hydrogenation, dehydration, and
hydro-deoxygenation steps. Thus, bifunctional acid/redox
catalysts are required for the hydrogenation step, where hydrolysis and
dehydration processes are carried out on solid acid sites and hydrogenation on
active metal sites of the catalyst. The SmartWaste produces carbon
streams that have similar characteristics to the streams out of an oil refinery.
Therefore, future production of biofuels, using small and selective
hydrothermal reactors and taking advantage of the available infrastructure for
oil refining, without large capital investments, is a reasonable and promising application.

In this research project it has
been demonstrated that the hydrothermal liquefaction of waste biomass is an
efficient process to convert low value material into enriched fuel and
chemicals precursors. The HTL process was able to reduce the oxygen content of
the waste streams in 70%. This represents the major oxygen reduction in the
whole process. It was also demonstrated that the composition of the biomass
reacting is a key factor to consider when designing the process. Adapting the
correct combination of sugars, proteins and lipids could increase the carbon oil
yields from 20-30% w/w up to 70% w/w.