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Water for Energy and Fuel Production
will result in the final product HMF. Hydration of HMF by addition of a water molecule to the C 2 –C 3 olefinic bond of the furan ring leads to an unstable tricarbonyl
intermediate [18], which decomposes to the LA [13] and formic acid [8]. While the
intermediates proposed and some of the reaction steps identified by Sjostrom [13],
Klass [18], and Hayes et al. [8] are not completely proven, they were proposed by
Horvat et al. [19,20] based on the 13 C nuclear magnetic resonance (NMR) spectra of
the reaction mixture formed in the hydration of HMF. The reaction paths discussed
earlier indicate that both five- and six-carbon sugars can be converted to LA by the
appropriate hydrolysis process chemistry. This makes the process more attractive
than sugar fermentation process in which the conversion of five-carbon sugar by
enzymatic fermentation is problematic [8,21].
The actual hydrolysis process involves many degradation reactions producing
many intermediates. Some authors have estimated more than 100 such intermediates
[19,20]. These intermediates tend to cross-react and ultimately coalesce (partly by
a series of condensation reactions) to form an acid-resistant tar, which incorporates
many insoluble residues such as humins. The overall objective of the Biofine process
is to minimize the degradation and subsequent condensation reactions that produce
tar and increase the yield of LA. An improved reactor system and the use of polymerization inhibitors can provide LA yields of up to 70%–80% of the theoretical yield.
This means that a typical product distribution will have about 50% LA, 20% formic
acid, and about 30% tar for six-carbon sugars. The mass yield of furfural from fivecarbon sugars is about 50% of the original mass, the remainder being incorporated
in the Biofine char. These data are supported by the pilot-scale experiments from the
Biofine process at Glens Falls, New York [8]. The pilot plant that is in operation since
1996 has used numerous feedstock including paper sludges from the paper mill.
Biofine char contains ash and acid-insoluble ligneous materials. The properties
of char can be changed and optimized using high-temperature and high-pressure
cracking. Feedstock that contains high amount of extractive such as barks (that may
contain up to 25% fats, waxes, and terpenes) or a large amount of water-soluble carbohydrates will have those components largely end up in Biofine char [8,22]. While
these components reduce the overall yield per unit biomass processed, they improve
the heating value of char when the char is combusted [8,22].
The hydrothermal conversion of biomass to LA in the presence of homogeneous
acid catalysts was also examined by Galletti et al. [23]. They examined different
types of cheap raw materials such as poplar sawdust, paper mill sludge, tobacco
chops, wheat straw, and olive tree pruning. The yield of LA was improved by
optimization of the operating parameters such as the type and amount of acid catalysts, temperature, reactor residence time, biomass concentration, and electrolyte
addition. The catalytic performances were also improved by the use of microwave
radiation for heating the system. The microwave heating required less time for
heating and was more energy efficient. The hydrothermal conversion of inulin and
wheat straw was also examined in the presence of niobium phosphate catalyst.
The experimental data reported by Galletti et al. [23] showed that for both hydrochloric and sulfuric acid solutions in water, the favorable yields of LA were obtained.
For wheat straw with hydrochloric acid at 200°C and residence time of 1 h, the yields
for LA based on the cellulose content varied from 49% to 55% and the theoretical