Table 2. In one of the studies, Madsen et al. investigated the
residual water produced from the model compounds in different amounts of carbohydrate, lipid, lignin and protein at
335 °C (Madsen et al. 2016). The author quantified the
percentages of different compounds in aqueous phase by
preparing the calibration curves. A total of 67 compounds
were quantified, which include short-chain cyclic oxygenates, carboxylic acids, fatty acids, nitrogenates and
oxygenated aromatics as reflected in Fig. 12b.
The fatty acids containing ( ! C14) were found in higher
concentration due to the addition of alkali catalyst, while
cyclic oxygenates were probably derived from carbohydrates. Organic acids in the range of C12 are formed
specifically from the hydrolysis of proteins and carbohydrates; however, the oxygen containing aromatics mainly
originated from lignin. Surprisingly, the maximum concentration of acetic acid was formed from the proteins sample
that is probably due to deamination of glycine (Gai et al.
2015). Earlier studies from HTL of microalgae has indicated
a high concentration of acetamide, which is formed between
the reaction of acetic acid and ammonia (Maddi et al. 2016).
From nitrogenated compounds, especially pyrazines were
mainly formed from protein. The pyrazine is formed by the
secondary reactions of ammonia produced from deamination
reaction of amino acids. However, in sample 6 in Fig. 12b,
pyrazine increases 20-fold with the mixing of carbohydrates.
Moreover, the addition of slight quantity of lipid or lignin
significantly decreases the quantity of pyrazines that is due
to production of amides derivatives with carboxylic acids.
Gai et al. determined the presence of nitrogenous compounds in residual water from protein-rich feedstocks (Gai et
al. 2015a, b, c). The observed concentration of total nitrogen
(TN) contents in residual water was in the range of 11000 ±
306 to 31700 ± 1350 mg/l. The degradation of triglycerides
formed fatty acids. It is important to discuss that dicarboxylic
acids like succinic acid and glutaric acid existed in higher
amount in the mixtures of biomass that contain 45% protein
than in mixtures with lower protein. Villadsen et al. had used
HPLC-TOF-MS spectroscopic technique to detect fatty acids
in carbon number range (C14−C22) in residual water from
“Dried Distillers Grains with Solubles” (DDGS) (Villadsen
et al. 2012). The major fatty acids detected are octadecanoic
acid, tetradecanoic acid, hexadecanoic acid, icosanoic acid
(C20) and docosanoic acid (C22).
Besides the quantification of organic compounds, in one of
the studies from the literature Maddi et al. detected the inorganic elements from aqueous phase produced from
industry-based food residues, biomass affiliated to wastewater, municipal wastes and through ICP-OES (Maddi et al.
2017). The selected inorganic elements detected through
ICP-OES from different feedstocks are mentioned in Table 3.
For food waste four types of materials were used: Montepulciano grape pomace, Cabernet Sauvignon grape pomace,
sugar beet tailings, and grains. They are designated as F1W01,
F1W02, F1W03 and F1W04, respectively (Table 3).
From Table 1, Na and K showed the highest concentration. These elements existed in the form of chlorides, sulfates, nitrates, phosphates, etc. This is further validated by
the presence of high amounts of chlorides (57, 103,
73 ppm), sulfates (100, 231, 236 ppm), nitrates (65, 50,
67 ppm) and phosphates (24, 765, 62 ppm) for primary,
secondary and digested sludge, respectively. Sulfur was
detected in all aqueous phase streams of feedstock, which
could have originated from the hydrolysis and cleavage of
protein-containing sulfur. Similarly, phosphorus was found
in substantial amount in municipal organic and solid wastes
as well as other biomass derived from lignins (corn stover).
The higher concentration of phosphorus was detected in
the streams of residual water obtained from HTL of
municipal organic and solid waste residues. Intrinsically,
these wastes often carry phosphorus through industrial
run-offs, sewage, fertilizers etc. (Fytili and Zabaniotou
2008). However, for biological agents, the phosphorus salts
are used as feed on the media for fermentation of sugars
derived from corn stover. However, the global supply of
phosphorus is limited, and it is necessary to reuse phosphorus recovered from residual water generated from HTL
of municipal residual wastes (Neset and Cordell 2012).
The valorization of the aqueous phase as a precursor for
hydrothermal processing is an environmentally friendly
concept. Valorization of aqueous phase not only improves
the energy recoveries but also avoids the treatment cost of
the aqueous phase. This enormously decreases the process
cost and make the continuous HTL commercially viable for
large scale. Since HTL has not been used on a commercial
scale due to several complexities, therefore a new concept
has emerged as an integration of HTL unit with other
technologies, which are commercially available like gasification and anaerobic digestion. Thus, the option of integrated HTL with gasification plant has been suggested,
where aqueous phase from HTL could be utilized to produce
hydrogen gas that fulfills the hydrotreating requirement for
the bio-crude (Cherad et al. 2016). Few studies suggest the
integration of HTL with anaerobic digestion, which could
form the methane gas (Tommaso et al. 2015). Moreover,
apart from organic fractions, the aqueous phase is mostly
enriched with valuable elements like Na, K and Mg, which
paves another alternate way of utilizing aqueous phase as a
fertilizer for soil amendment and cultivating biomasses like
algae after the necessary treatment.
38
K. Sharma et al.
residual water produced from the model compounds in different amounts of carbohydrate, lipid, lignin and protein at
335 °C (Madsen et al. 2016). The author quantified the
percentages of different compounds in aqueous phase by
preparing the calibration curves. A total of 67 compounds
were quantified, which include short-chain cyclic oxygenates, carboxylic acids, fatty acids, nitrogenates and
oxygenated aromatics as reflected in Fig. 12b.
The fatty acids containing ( ! C14) were found in higher
concentration due to the addition of alkali catalyst, while
cyclic oxygenates were probably derived from carbohydrates. Organic acids in the range of C12 are formed
specifically from the hydrolysis of proteins and carbohydrates; however, the oxygen containing aromatics mainly
originated from lignin. Surprisingly, the maximum concentration of acetic acid was formed from the proteins sample
that is probably due to deamination of glycine (Gai et al.
2015). Earlier studies from HTL of microalgae has indicated
a high concentration of acetamide, which is formed between
the reaction of acetic acid and ammonia (Maddi et al. 2016).
From nitrogenated compounds, especially pyrazines were
mainly formed from protein. The pyrazine is formed by the
secondary reactions of ammonia produced from deamination
reaction of amino acids. However, in sample 6 in Fig. 12b,
pyrazine increases 20-fold with the mixing of carbohydrates.
Moreover, the addition of slight quantity of lipid or lignin
significantly decreases the quantity of pyrazines that is due
to production of amides derivatives with carboxylic acids.
Gai et al. determined the presence of nitrogenous compounds in residual water from protein-rich feedstocks (Gai et
al. 2015a, b, c). The observed concentration of total nitrogen
(TN) contents in residual water was in the range of 11000 ±
306 to 31700 ± 1350 mg/l. The degradation of triglycerides
formed fatty acids. It is important to discuss that dicarboxylic
acids like succinic acid and glutaric acid existed in higher
amount in the mixtures of biomass that contain 45% protein
than in mixtures with lower protein. Villadsen et al. had used
HPLC-TOF-MS spectroscopic technique to detect fatty acids
in carbon number range (C14−C22) in residual water from
“Dried Distillers Grains with Solubles” (DDGS) (Villadsen
et al. 2012). The major fatty acids detected are octadecanoic
acid, tetradecanoic acid, hexadecanoic acid, icosanoic acid
(C20) and docosanoic acid (C22).
Besides the quantification of organic compounds, in one of
the studies from the literature Maddi et al. detected the inorganic elements from aqueous phase produced from
industry-based food residues, biomass affiliated to wastewater, municipal wastes and through ICP-OES (Maddi et al.
2017). The selected inorganic elements detected through
ICP-OES from different feedstocks are mentioned in Table 3.
For food waste four types of materials were used: Montepulciano grape pomace, Cabernet Sauvignon grape pomace,
sugar beet tailings, and grains. They are designated as F1W01,
F1W02, F1W03 and F1W04, respectively (Table 3).
From Table 1, Na and K showed the highest concentration. These elements existed in the form of chlorides, sulfates, nitrates, phosphates, etc. This is further validated by
the presence of high amounts of chlorides (57, 103,
73 ppm), sulfates (100, 231, 236 ppm), nitrates (65, 50,
67 ppm) and phosphates (24, 765, 62 ppm) for primary,
secondary and digested sludge, respectively. Sulfur was
detected in all aqueous phase streams of feedstock, which
could have originated from the hydrolysis and cleavage of
protein-containing sulfur. Similarly, phosphorus was found
in substantial amount in municipal organic and solid wastes
as well as other biomass derived from lignins (corn stover).
The higher concentration of phosphorus was detected in
the streams of residual water obtained from HTL of
municipal organic and solid waste residues. Intrinsically,
these wastes often carry phosphorus through industrial
run-offs, sewage, fertilizers etc. (Fytili and Zabaniotou
2008). However, for biological agents, the phosphorus salts
are used as feed on the media for fermentation of sugars
derived from corn stover. However, the global supply of
phosphorus is limited, and it is necessary to reuse phosphorus recovered from residual water generated from HTL
of municipal residual wastes (Neset and Cordell 2012).
The valorization of the aqueous phase as a precursor for
hydrothermal processing is an environmentally friendly
concept. Valorization of aqueous phase not only improves
the energy recoveries but also avoids the treatment cost of
the aqueous phase. This enormously decreases the process
cost and make the continuous HTL commercially viable for
large scale. Since HTL has not been used on a commercial
scale due to several complexities, therefore a new concept
has emerged as an integration of HTL unit with other
technologies, which are commercially available like gasification and anaerobic digestion. Thus, the option of integrated HTL with gasification plant has been suggested,
where aqueous phase from HTL could be utilized to produce
hydrogen gas that fulfills the hydrotreating requirement for
the bio-crude (Cherad et al. 2016). Few studies suggest the
integration of HTL with anaerobic digestion, which could
form the methane gas (Tommaso et al. 2015). Moreover,
apart from organic fractions, the aqueous phase is mostly
enriched with valuable elements like Na, K and Mg, which
paves another alternate way of utilizing aqueous phase as a
fertilizer for soil amendment and cultivating biomasses like
algae after the necessary treatment.
38
K. Sharma et al.