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\DOI{10.5802/crchim.452}
\datereceived{2025-12-14}
\daterevised{2026-02-24}
\datererevised{2026-04-02}

\dateaccepted{2026-04-08}
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\section*{Declaration of interests}
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\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliations other than their research organizations.}

\dateposted{2026-06-24}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\title{Experimental real-world applications of products derived from
the hydrothermal carbonization of citrus waste, with quantitative
benchmarking}

\alttitle{Applications exp\'{e}rimentales en conditions r\'{e}elles des
produits issus de la carbonisation hydrothermale des d\'{e}chets
d'agrumes, avec \'{e}valuation comparative quantitative}

\author{\firstname{Besma} \lastname{Khiari}\CDRorcid{0000-0001-8251-853X}\IsCorresp}  
\address{Laboratory of Wastewaters and Environment -- Centre des
Recherches et Technologies de l'Eau - Rte Touristique de Soliman BP no
273 Technop\^{o}le Borj C\'{e}dria 8020 Nabeul, Tunisia}               
\address{National School for Engineering of Carthage-University of
Carthage, 45, avenue des Entrepreneurs, Charguia II, Tunis, Tunisia}   
\email{basma.khiari@enicar.ucar.tn}
\email[B. Khiari]{besmakhiari@yahoo.com}

\author{\firstname{Mejdi} \lastname{Jeguirim}\CDRorcid{0000-0003-2401-5824}}  
\address{Institute of Materials Science of Mulhouse University of
Strasbourg, University of Haute-Alsace Mulhouse, France}               

\author{\firstname{Wassim} \lastname{Slaimi}}  
\address{Tunisian Beverage Manufacturing Company, Tunis, Tunisia}                      
\address{University of Carthage, INSAT, Centre Urbain Nord, B.P. 676,
1080 Tunis, Tunisia}                      

\author{\firstname{Souhir} \lastname{Abdelmoumen}\CDRorcid{0000-0002-0820-6244}}  
\address{University of Carthage, INSAT, Research Laboratory EcoChimie
(LR21ES02), Centre Urbain Nord, B.P. 676, 1080 Tunis, Tunisia}         

\author{\firstname{Mohamed} \lastname{Zbair}\CDRorcid{0000-0003-4609-5159}}  
\addressSameAs{3}{Institute of Materials Science of Mulhouse University
of Strasbourg, University of Haute-Alsace Mulhouse, France}            

\author{\firstname{Salah} \lastname{Jellali}\CDRorcid{0000-0002-4095-4154}}  
\address{Centre for Environmental Studies and Research, Sultan Qaboos
University, Muscat, Al-Khoud 123, Oman}                      

\keywords{\kwd{Hydrothermal carbonization}\kwd{Citrus
waste}\kwd{Hydrochar}\kwd{Biofuel}\kwd{Process water}\kwd{Anaerobic
digestion}\kwd{Adsorption}}

\altkeywords{\kwd{Carbonisation hydrothermale}\kwd{D\'{e}chets
d'agrumes}\kwd{Hydrochar}\kwd{Biocombustible}\kwd{Eau de
proc\'{e}d\'{e}}\kwd{Digestion ana\'{e}robie}\kwd{Adsorption}}

\begin{abstract}
In recent years, hydrothermal carbonization (HTC) has transitioned from
a laboratory-scale process to a demonstrated technology for valorizing
citrus waste. This review synthesizes post-2015 experimental studies
and critical analyses, focusing on the real-world applicability of HTC
for citrus peels and pomace. The assessment covers energy carriers,
water treatment, soil amendment, aqueous phase valorization, and
advanced materials. Quantitative benchmarking reveals strong progress
in fuel pellet specification compliance and adsorption performance,
supported by pilot-scale continuous operation and life cycle assessment
(LCA). Key findings indicate that continuous HTC of orange peels can
produce hydrochar pellets meeting industrial fuel standards, with
improved ash behavior and higher heating value (HHV). Process water
recirculation enhances solid mass yield, while anaerobic digestion (AD)
of the aqueous phase achieves high chemical oxygen demand (COD) removal
and near-theoretical methane yields. However, gaps remain in
standardized reporting, field-scale agronomic trials, and
techno-economic analyses for adsorption applications. This synthesis
underscores the maturity of citrus-waste HTC for energy applications
and highlights the need for integrated system analyses to fully
capitalize on its biorefinery potential within a circular economy
framework.
\end{abstract}

\begin{altabstract}
Au cours des derni\`{e}res ann\'{e}es, la carbonisation hydrothermale
(HTC) est pass\'{e}e d'un proc\'{e}d\'{e} \`{a} l'\'{e}chelle du
laboratoire \`{a} une technologie d\'{e}montr\'{e}e pour la
valorisation des d\'{e}chets d'agrumes. Cette revue synth\'{e}tise les
\'{e}tudes exp\'{e}rimentales post\'{e}rieures \`{a} 2015 ainsi que les
analyses critiques, en mettant l'accent sur l'applicabilit\'{e} en
conditions r\'{e}elles de la HTC aux \'{e}corces et aux pulpes
d'agrumes.

L'\'{e}valuation couvre les vecteurs \'{e}nerg\'{e}tiques, le
traitement de l'eau, l'amendement des sols, la valorisation de la phase
aqueuse et les mat\'{e}riaux avanc\'{e}s. Une analyse comparative
quantitative met en \'{e}vidence des progr\`{e}s significatifs en
mati\`{e}re de conformit\'{e} des granul\'{e}s combustibles aux
sp\'{e}cifications industrielles et de performances d'adsorption,
soutenus par des op\'{e}rations continues \`{a} l'\'{e}chelle pilote et
des analyses du cycle de vie (LCA).

Les principaux r\'{e}sultats indiquent que la HTC continue des
\'{e}corces d'orange peut produire des granul\'{e}s d'hydrochar
r\'{e}pondant aux normes industrielles des combustibles, avec un
meilleur comportement des cendres et un pouvoir calorifique
sup\'{e}rieur (HHV) plus \'{e}lev\'{e}. La recirculation de l'eau de
proc\'{e}d\'{e} am\'{e}liore le rendement massique en solide, tandis
que la digestion ana\'{e}robie (AD) de la phase aqueuse permet
d'atteindre des taux \'{e}lev\'{e}s d'\'{e}limination de la demande
chimique en oxyg\`{e}ne (COD) et des rendements en m\'{e}thane proches
des valeurs th\'{e}oriques.

Cependant, des lacunes subsistent en mati\`{e}re de normalisation des
donn\'{e}es rapport\'{e}es, d'essais agronomiques \`{a} l'\'{e}chelle
du terrain et d'analyses technico-\'{e}conomiques pour les applications
d'adsorption. Cette synth\`{e}se met en \'{e}vidence la maturit\'{e} de
la HTC des d\'{e}chets d'agrumes pour les applications
\'{e}nerg\'{e}tiques et souligne la n\'{e}cessit\'{e} d'analyses
int\'{e}gr\'{e}es des syst\`{e}mes afin d'exploiter pleinement son
potentiel de bioraffinerie dans une logique d'\'{e}conomie circulaire.
\end{altabstract}

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\section{Introduction}\label{sec1}

According to the Food and Agriculture Organization (FAO) of the United
Nations, global citrus production has reached record highs in recent
years. In 2024, the total global production of citrus fruits was
approximately 152.2 million metric tons~\cite{1}. Oranges account for
the largest share, roughly 44.2\% of total production (${\sim}$67.2
million tons), while tangerines, mandarins, clementines are the
fastest-growing segment, accounting for roughly 34.2\% (${\sim}$52
million tons). Lemons and limes account for approximately 15.2\%
(${\sim}$23.2 million tons) and finally grapefruits and pomelos
contribute by 6.4\% with ${\sim}$9.8 million tons~\cite{1}.

While production volume is massive, it is subject to climatic
volatility. However, the market value is growing steadily. The
production volume compound annual growth rate (CAGR) (2018--2023) was
approximately of 2.3\%. 

Citrus production is heavily concentrated in the Northern Hemisphere,
specifically in Asia and the Americas. China accounts for 35\% of total
output and supplies 59\% of global mandarin demand. Brazil constitutes
34\% of the orange market, while India produces 22\% of the global
lemons.

These and other regions experience significant pollution resulting from
inadequately managed citrus waste~\cite{2}. Indeed, the citrus
processing industry (primarily for juice) generates massive amounts of
waste. It is estimated that for every ton of citrus processed, roughly
500--600 kg of waste (wet weight) is generated. This equates to
approximately 80--100 million metric tons of wet waste globally per
year. This waste is generally categorized as citrus peel waste or
citrus processing waste. The heterogeneous mixture of solid residues
commonly referred to as pomace represents the main by-product of juice
extraction. This pomace is primarily composed of peel, segment
membranes, seeds, and pulp residues. The peel itself consists of two
distinct layers: the outer colored skin, known as the flavedo, and the
inner white spongy layer, called the albedo. The flavedo is
particularly rich in essential oils, especially D-limonene, while the
albedo contains significant amounts of pectin and structural
polysaccharides. Together, these layers also contain valuable bioactive
compounds such as flavonoids, including hesperidin and naringin.

In addition to the peel, the segment membranes (often referred to as
the rag) form the fibrous network that holds the juice vesicles
together. This fraction is characterized by a very high content in
pectin and cellulose, making it structurally robust and particularly
interesting for fiber and biopolymer recovery. Seeds constitute another
important fraction of citrus waste. They are notable for their high oil
content, commonly referred to as citrus seed oil, and for their
significant protein levels. Seeds also contain limonoids, a class of
bioactive compounds with recognized functional properties. The pulp or
pulp wash residue, which remains after juice extraction and washing
steps, consists mainly of fibrous material rich in structural
carbohydrates.

From a chemical perspective, citrus waste is characterized by a high
moisture content, typically ranging between 70\% and 80\%, which
implies a substantial energy demand for drying if thermal valorization
routes are considered~\cite{3}. In dried citrus peel samples, moisture
content ranges from 5.34\% to 23.31\%, depending on drying methods and
citrus species~\cite{7}.\looseness=1

The organic fraction includes soluble sugars, such as glucose,
fructose, and sucrose, generally representing about 10--15\% of the
total mass~\cite{8,5}. In more detailed compositional analyses, dried
citrus peels have been shown to contain significant sugar
concentrations, with total soluble sugars ranging from 0.487 to 0.591 g
per gram of dry weight biomass~\cite{8}.\looseness=1

Dietary fibers, including cellulose, hemicellulose, and lignin, account
for approximately 10--20\%~\cite{5}. Compositional analysis of orange
waste has revealed specific percentages: cellulose at 69.096\%,
hemicellulose at 9.015\%, and lignin at 19.801\% of the total dry
matter~\cite{9}. These structural polysaccharides comprise the primary
components of the citrus peel's cell wall architecture and represent
important substrates for biorefinery applications.

Pectin is a particularly valuable component, representing about
15--30\% of the peel's dry weight, and is widely used as a gelling
agent in food and pharmaceutical applications~\cite{10}. Recent studies
confirm pectin yields ranging from approximately 11\% to over 34\%
under various extraction conditions, with optimal recoveries reaching
up to 45\% depending on extraction methodology and optimization of
parameters~\cite{11}.

Essential oils, predominantly D-limonene, are present at about
0.5--1.0\% of the wet waste weight. Lemon peel oil contains
approximately 1.5\% essential oil by weight, while orange peel yields
approximately 3.4\%. The concentration of limonene varies considerably
among citrus species, with orange peels typically containing higher
levels---reaching 98\% of the total essential oil composition. Although
highly valuable for green chemistry applications, D-limonene is also
highly flammable and can create operational and disposal challenges if
not properly managed~\cite{12}.

Finally, citrus residues contain phenolic compounds and flavonoids such
as hesperidin and naringin, which exhibit antioxidant properties and
enhance the potential for high-value biorefinery pathways~\cite{13}.
Hesperidin and naringin are the predominant flavonoids in most citrus
species, with hesperidin concentrations in citrus peels documented at
levels ranging from 27 to 35 mg/kg, while naringin ranges from 26 to 36
mg/kg of dry peel material~\cite{14}. These bioactive compounds have
demonstrated significant antioxidant, anti-inflammatory, and
health-promoting properties, making them increasingly valuable for
functional food development and nutraceutical applications~\cite{15}.

Currently, much of this waste is dumped or used as low-value animal
feed (due to high acidity and limonene, it can be toxic to livestock if
not treated)~\cite{16}. Traditional disposal methods, such as
incineration and landfilling, are costly and environmentally
unsustainable, prompting the search for innovative valorization
strategies~\cite{17}. Hydrothermal carbonization (HTC) has emerged as a
particularly promising thermochemical process for converting
high-moisture citrus residues into value-added products, including
hydrochar, bio-oil, and platform chemicals, under subcritical water
conditions (typically 180--300~\textdegree C)~\cite{16,18,19,20}.

Recent research has demonstrated the versatility of HTC for citrus
waste, with process optimization studies highlighting the influence of
reaction temperature, residence time, pH, and biomass-to-water ratio on
product yields and quality. HTC also enables the recovery of limonene
and other monoterpenes from orange peels, expanding the range of
marketable coproducts~\cite{19}. The process is energy-efficient for
wet biomass, eliminating the need for energy-intensive drying steps
required by conventional thermochemical treatments~\cite{21}.\looseness=1

Beyond fuel applications, hydrochars derived from citrus residues have
shown promise as adsorbents for environmental remediation, such as dye
and pharmaceutical removal from water, due to their high surface
functionality and tunable porosity~\cite{22,23}. Process water
recirculation during HTC can further enhance carbon recovery and reduce
water consumption, though it introduces new challenges related to the
management and valorization of the aqueous phase~\cite{24,25,26}.
Pilot-scale and industrial demonstrations have validated the technical
feasibility of HTC for citrus waste, but upscaling remains constrained
by reactor design, process water treatment, and economic
considerations~\cite{27,28,29}.

Since 2015, research has progressed from establishing fundamental
feasibility to demonstrating applications at pilot scale~\cite{30}.
This article consolidates evidence on the experimental real-world
applications of HTC for citrus wastes, providing a quantitative
benchmarking across different product streams and end-uses. It
critically evaluates the readiness of each application domain,
identifies persistent gaps, and offers a synthesis of operational
parameters and citrus-specific attributes that influence process
outcomes.

\section{HTC process for citrus residues}\label{sec2} 
\subsection{Operating parameters}\label{ssec21} 

Reported operating conditions for citrus waste HTC (Table~\ref{tab1})
generally align with canonical HTC ranges, with temperatures of
180--250~\textdegree C and residence times of approximately 1~h being
common for adsorption and recirculation studies~\cite{22,24}. Extended
reaction times at moderate temperatures (e.g., 210~\textdegree C for
180~min) have been shown to maximize hydrochar yield in orange peel
biorefinery contexts~\cite{16}. 

%tab1
\begin{table*}
\caption{\label{tab1}Reactor scale and operating modes}
\tabcolsep=2.5pt
\begin{tabular}{cccc}
\thead
Feedstock & Reactor/scale & Operating notes and & Ref. \\
\endthead

\parbox[t]{9.5pc}{\centering Orange peels (with olive pomace comparative)} &  
\parbox[t]{10pc}{\centering Continuous-flow pilot
(TORWASH$^{\text{\textregistered}}$)} &  
\parbox[t]{14.5pc}{\centering Mild HTC; 28 days continuous operation (orange peel)} & 
\cite{31}\vspace*{2pt} \\ 

Lemon peel waste & Lab batch &  
\parbox[t]{14.5pc}{\centering PW recirculation; 180/220/250~\textdegree
C, 60~min, 20 wt\% solid loading} &  \cite{24}\vspace*{2pt} \\

\parbox[t]{9.5pc}{\centering Orange peel (plus grape~skin)} &  Lab batch
& 180/220/250~\textdegree C, 1 h  & \cite{22}\vspace*{2pt} \\ 

Orange peel waste & Lab batch & 180--300~\textdegree C, 60--300 min &
\cite{16} \\ 

Orange pomace & Lab batch &  
\parbox[t]{14.5pc}{\centering Product characterization plus anaerobic
digestibility of process liquor} &  \cite{33}\vspace*{2pt} \\ 

Orange peel & Lab batch & 200~\textdegree C/2--16 h & \cite{34} \\ 

\parbox[t]{9.5pc}{\centering Orange peel-derived hydrochar
(urea-assisted)} &  Lab batch &  
\parbox[t]{14.5pc}{\centering Urea-assisted HTC, N-doping, KOH activation
600--800~\textdegree C} &  \cite{35}\vspace*{2pt} \\ 

Orange peels & Lab batch &  \parbox[t]{14.5pc}{\centering Thermal
activation/chemical activation with phosphoric acid} & 
\cite{23}\vspace*{2pt} \\ 

\parbox[t]{9.5pc}{\centering Citrus waste HTC aqueous organics
(bio-oil)} &  Pilot/bench AD reactor & 
\parbox[t]{14.5pc}{\centering High-loading hybrid anaerobic reactor} & 
\cite{36}\vspace*{2pt} \\ 

Orange juice &  \parbox[t]{10.5pc}{\centering Lab batch ${+}$\ post-HTC
activation and graphitization} & \parbox[t]{14.5pc}{\centering
180~\textdegree C, 6 h, KOH activation at 800~\textdegree
C/Graphitization} &  \cite{37}\vspace*{2pt}
\botline
\end{tabular}
\vspace*{-2pt}
\end{table*}

The unique chemistry of citrus waste, characterized by high pectin,
sugar, and acid content, specific ash composition (high K, Ca), and
essential oils, accelerates the HTC reaction and distinctly shapes
product distribution. Lower severity favors the formation of furans and
organic acids in the process water (PW), while higher severity
increases carbon densification but reduces mass yield and increases the
organic load of the PW~\cite{16,31}. 

The successful scale-up to continuous stirred/screw reactor systems
(TORWASH$^{\text{\textregistered}}$) over four weeks confirms the
robustness of the process for orange peels, with dewatering and
pelletization identified as integral downstream steps~\cite{31}.

Ugolini et al. assessed industrial-scale HTC environmental
impacts for orange peel among three wet residues and performed
cradle-to-gate LCA (ReCiPe) with Monte Carlo sensitivity, comparing
solid pellets and biogas outputs. While providing process-level LCA
data, they reported impact categories (climate change, PM,
acidification)~\cite{32}. 

\subsection{Comparisons to alternative thermochemical
routes}\label{ssec22}

For high moisture citrus residues, HTC avoids pre-drying and produces
hydrochars with improved hydrophobicity and handling relative to raw
peels. By contrast, pyrolysis and torrefaction typically require
pre-drying (10--20~MJ/kg moisture removal penalty) and yield
chars with higher aromaticity but lower mass yields~\cite{38,39,40}.
Activation processes further reduce solid yield while enhancing surface
area (Tables~\ref{tab2} and~\ref{tab3}).

%tab2
\begin{table*}
\caption{\label{tab2}Comparative performance of thermochemical
processes for citrus peel waste (orange, lemon, grapefruit, tangerine)}
\tabcolsep=3pt
\begin{tabular}{cccccccc}
\thead
Process & 
\parbox[t]{4pc}{\centering Temperature (\textdegree C)} & 
\parbox[t]{3pc}{\centering Mass yield (wt\%)} & 
\parbox[t]{3pc}{\centering Energy yield (\%)} & 
\parbox[t]{3pc}{\centering HHV (MJ/kg)} & 
\parbox[t]{3pc}{\centering Ash content (wt\%)} & 
\parbox[t]{6pc}{\centering Feedstock moisture requirement}
\vspace*{2pt}& Ref. \\ 
\endthead
HTC & 180--250 & 34--56 & 70--87 & 24.8--28.5 & 1.5--4.0 & 75--90\%
(wet process) & \cite{16,31,42} \\ 

Slow Pyrolysis & 400--500 & 21--41 & 45--65 & 26.3--31.0 & 2.0--9.0 &
${<}$15\% (requires drying) & \cite{4,43} \\ 

Fast Pyrolysis & 450--550 & 27--33 & 35--50 & 25--29 & 2.5--8.0 &
${<}$10\% (requires drying) & \cite{44} \\ 

Torrefaction & 200--300 & 52--94 & 71--99 & 19.9--27.7 & 3.4--9.8 &
${<}$10\% (requires drying) & \cite{45} \\ 

\parbox[t]{6pc}{\centering Activation (KOH/H\tsub{3}PO\tsub{4})} & 
400--800 & 10--30 & 20--40 &
28--32 & 1.0--5.0 & 
\parbox[t]{7pc}{\centering Pre-carbonization required} & \cite{44,46}
\vspace*{2pt}
\botline
\end{tabular}
\vspace*{5pt}
\end{table*}

%tab3
\begin{table*}
\caption{\label{tab3}Energy yield calculations: standardized basis for
citrus waste}
\begin{tabular}{ccccc}
\thead
Process & 
\parbox[t]{6pc}{\centering Gross energy yield (\%)} & 
\parbox[t]{4pc}{\centering Net energy yield (\%)$^{\mathrm{a}}$} & 
\parbox[t]{9pc}{\centering Drying energy penalty (MJ/kg feedstock)}
\vspace*{2pt}
& Ref. \\
\endthead
HTC (220~\textdegree C, PW recirculation) & 85 & 80--82 & 0 (wet feed)
& \cite{24,48} \\ 

HTC (250~\textdegree C, no recirculation) & 65--70 & 60--65 & 0 (wet
feed) & \cite{24,31,48} \\ 

Slow Pyrolysis (500~\textdegree C) & 45--55 & 35--45 & 3.2--3.8
& \cite{4,49} \\ 

Torrefaction (250~\textdegree C) & 90 & 55--65$^{\mathrm{b}}$ &
3.2--3.8 &~\cite{49,50} \\ 

Fast Pyrolysis (500~\textdegree C) & 40--50 & 30--40 & 3.5--4.0 &
\cite{44}
\botline
\end{tabular}
\tabnote{${}^{\mathrm{a}}$Net energy yield accounts for process energy
inputs and drying requirements.}
\tabnote{${}^{\mathrm{b}}$Torrefaction net yield assumes 80\% initial
moisture citrus peel requiring drying to ${<}$10\%.}
\end{table*}

Pilot evidence shows that HTC can upgrade citrus ash behavior (reduced
K/Cl, higher deformation temperature) to meet industrial pellet specs,
which is an advantage for combustion readiness of citrus
residues~\cite{31}, whereas direct combustion or torrefaction of peels
often faces slagging/fouling risks due to alkalis~\cite{41}.

In Table~\ref{tab2},
mass yield is the percentage of solid product
(hydrochar) recovered after HTC relative to the initial dry mass of the
feedstock (biomass/orange juice). The common formula (dry basis) used
is:
{\begin{equation*}
\mbox{mass yield } (\%) = \dfrac{\mbox{mass of dry
hydrochar}}{\mbox{mass of dry feedstock}} \times 100
\end{equation*}}\unskip
Also called the gross calorific value, HHV is the total amount of heat
released when a unit mass of the material (hydrochar or feedstock) is
completely combusted, with all water produced condensed back to liquid
(measured at constant volume, usually in MJ/kg or kcal/kg). It includes
the latent heat of vaporization of water formed during combustion.

Energy yield represents the percentage of the original energy content
of the feedstock that is retained in the hydrochar after HTC. It
combines the effects of mass loss and energy densification. The common
formula is:
{\begin{equation*}
\mbox{energy yield } (\%) = \mbox{mass yield} \times 
\dfrac{\mbox{HHV of hydrochar}}{\mbox{HHV of raw feedstock}}
\end{equation*}}\unskip
From a system-level energy and sustainability perspective, HTC is well
suited for wet residues, with energy yields up to ${\sim}$80\%, although
definitions and bases vary and must be standardized in
comparisons~\cite{21,38,39}. The favorable energy yield of HTC arises
from its ability to retain a larger fraction of the feedstock's carbon
in the solid phase under wet conditions, while simultaneously improving
ash quality. Conventional thermochemical methods (pyrolysis,
torrefaction, activation) can produce chars with higher aromaticity and
stability, but at the cost of lower mass and energy yields and higher
ash-related risks~\cite{47}. Thus, for wet citrus residues, HTC offers
a more sustainable pathway by avoiding drying penalties and producing
hydrochars compatible with combustion and pelletization standards.

Moreover, Catalkopru et~al.~\cite{48} demonstrated that recirculating
PW during HTC of orange pomace at 225~\textdegree C increased both mass
and energy yields through accumulation of organic acids that catalyze
dehydration and reduce solubilization of degradation products. Picone
et al. reported that PW recirculation at 180~\textdegree C increased
lemon peel hydrochar mass yield from 50.1\% to 55.9\% (first
recirculation) and energy yield proportionally, reaching up to
${\sim}$85\% energy retention in the solid phase. However, excessive
recirculation can accumulate inhibitory compounds, creating a trade-off
between yield and process stability~\cite{42}.

For orange peel, the optimal temperature window is 180--220~\textdegree
C. Below 180~\textdegree C, carbonization is incomplete; above
250~\textdegree C, excessive decarboxylation and dehydration reduce
mass yields to 40--45\% despite higher HHV. Satira et al.~\cite{16}
found that 210~\textdegree C and 180 min residence time maximized
orange peel hydrochar yield (30.1 wt\% on wet basis, ${\sim}$55--60\% on
dry basis), while higher temperatures favored liquid product formation
(5-HMF, furfural) over solid retention. Higher solid loading (e.g., 1:6
w/w orange peel:water) improves energy efficiency by reducing water
heating requirements, though excessive loading ({${>}$}1:4) can cause
incomplete heat transfer and uneven carbonization. Acidic conditions
(pH 3--4 using acetic or sulfuric acid) can increase hydrochar yields
by 50\% (from ${\sim}$20\% to 30\% for orange peel) by promoting
hydrolysis of hemicellulose and pectin while inhibiting excessive
degradation to soluble organics~\cite{16,31}.

In Table~\ref{tab3},
gross energy yield represents the total chemical
energy recovered in the solid fuel (hydrochar) relative to the energy
originally present in the raw biomass. It accounts only for the energy
content of the material itself, not the energy required to produce it.

It is calculated as: 
{\begin{equation*}
Y_{\mathrm{gross}} = \dfrac{M_{\mathrm{char}} \times
\mathrm{HHV}_{\mathrm{char}}} {M_{\mathrm{raw}} \times
\mbox{HHV}_{\mathrm{raw}}} \times 100
\end{equation*}}\unskip
where $M$ is the mass of the material and HHV is the higher heating
value. 

Net energy yield subtracts the external energy consumed during the
conversion process (electricity for reactors, heat for maintaining
pressure, etc.) from the energy stored in the final product. A positive
Net Yield means that the process produces more energy than it consumes
(energy-positive) while a negative one means the process consumes more
energy than the fuel it produces is worth (common in early-stage
laboratory setups).

The drying energy penalty refers to the specific energy cost required
to remove moisture from the material to make it a usable fuel. Because
citrus peels are approximately 80\% water, this is the hidden cost of
the process. There are two stages where this penalty typically occurs:
(i) pre-processing: drying raw peels before traditional thermal
treatment, (ii)~post-processing: drying the wet hydrochar after it comes
out of a hydrothermal reactor.

\section{Applications and product streams}\label{sec3}

The performance and applications of HTC outputs are detailed below and
summarized in Tables~\ref{tab4}--\ref{tab7}. Indeed, hydrochar derived
from citrus waste exhibits high surface area and porosity after
activation and a richness in functional groups (carboxyl, hydroxyl,
carbonyl), enhancing chemical reactivity. Tunability via HTC parameters
(temperature, time, pH) allows control over morphology and surface\break
chemistry.

%tab4
\begin{table*}
\caption{\label{tab4}Metal ion sensing applications (CD ${=}$\ carbon
dot, LOD ${=}$\ limit of detection, NP ${=}$\ nanoparticle)}
\tabcolsep 4pt
\begin{tabular}{cccc}
\thead
Target analyte & Precursor material & Key features/Performance & Ref. \\ 
\endthead
Fe\tsup{3+}/Fe speciation & 
\parbox[t]{8pc}{\centering Citrus-peel CDs/Grapefruit CDs} & 
\parbox[t]{16pc}{\centering Bimodal (colorimetric ${+}$\ fluorescence);
LOD 30--40 ${\upmu}$g/L; pH-dependent selectivity} & 
\cite{67,68,69}\vspace*{2pt} \\ 

Hg\tsup{2+} & 
Orange/Lemon-juice N-CDs & 
High selectivity; LOD ${\sim}$5.3 nM; biocompatible & 
\cite{70,71}\vspace*{2pt} \\ 

Cd\tsup{2+} & \textit{Citrus nobilis} CDs & 
\parbox[t]{16pc}{\centering LOD 0.12 ${\upmu}$g/mL; includes
antibacterial activity} & \cite{72}\vspace*{2pt} \\ 

Ag\tsup{+} & Mandarin-peel CDs &  \parbox[t]{16pc}{\centering Detection
via CD-mediated Ag-NP formation; LOD 0.6 ${\upmu}$M} & \cite{73}\vspace*{2pt} \\ 

Cr\tsup{6+} & Orange-pomace CDs & LOD 59.6 nM & \cite{74}
\botline
\end{tabular}
\end{table*}

%tab5
\begin{table*}
\caption{\label{tab5}Sensing of organic pollutants, antibiotics, and
biomolecules (CD ${=}$\ carbon dot, LOD ${=}$\ limit of detection, NP ${=}$\
 nanoparticle)}
\begin{tabular}{ccccc}
\thead
Category & Analyte & Precursor material & Performance/Range & Ref. \\ 
\endthead
\morerows{1}{Antibiotics/Drugs} & Oxytetracycline & Orange/watermelon
peel & Range: 0.25--100 ${\upmu}$M & \cite{45} \\
& Tetracycline & Lemon-peel CDs & LOD 50.4 nM & \cite{46}\vspace*{6pt} \\ 

\morerows{1}{Organic Pollutants} & 4-nitrophenol & Orange-pomace CDs &
LOD 14 nM & \cite{44} \\
& Tartrazine & Citrus-peel CDs & Detected alongside Fe\tsup{3+} &
\cite{37}\vspace*{6pt} \\ 

\morerows{1}{Biomolecules} & Dopamine & Orange peel N-CDs & 
\parbox[t]{8pc}{\centering Linear up to 300 ${\upmu}$M; ${\sim}$35\%
quantum yield} &  \cite{47} \\
& Ascorbic acid & Orange-peel carbon NPs & Coupled with Fe\tsup{3+}
detection & \cite{38}
\botline
\end{tabular}
\end{table*}

%tab6
\begin{table*}
\caption{\label{tab6}Applications and performance proxies by product
stream\vspace*{-3pt}}
\tabcolsep4pt
\begin{tabular}{cccc}
\thead
Product stream & Application type & Key performance proxies/results &
Ref. \\ 
\endthead
Hydrochar & Solid biofuel/pellets & 
\parbox[t]{14pc}{\centering Solid yield 31 wt\% (orange peel),
press cake dry matter 42\%; HHV increased; ash reduced; pellets met
industrial requirements} & \cite{31} \vspace*{3pt}\\ 

Hydrochar & 
\parbox[t]{10pc}{\centering Effect of PW recirculation on energy recovery} & 
\parbox[t]{14pc}{\centering PW recirculation increased solid mass yield; strongest at 180~\textdegree
C (${\sim}$6\% increase); TOC concentrated in liquid across cycles} & 
\cite{25} \vspace*{3pt}\\ 

Hydrochar & Soil amendment & 
\parbox[t]{14pc}{\centering Clay soil physical property changes and phytotoxicity assessed} & 
\cite{34} \vspace*{3pt}\\ 

Hydrochar (activated) & 
\parbox[t]{10pc}{\centering Water treatment (dye, emerging contaminants)} &
\parbox[t]{14pc}{\centering Adsorption isotherms, kinetics, thermodynamics; pH$_{\mathrm{PZC}}$ and
surface functionalities linked to performance} & \cite{22,23} \vspace*{3pt}\\ 

\parbox[t]{8pc}{\centering Porous carbon (post-HTC)} & 
VOC/halogen sorption & 
\parbox[t]{14pc}{\centering High performance for toluene and iodine adsorption after urea-assisted HTC and activation}
& \cite{35} \vspace*{3pt}\\ 

\parbox[t]{8pc}{\centering Porous carbon (post-HTC)} & 
Energy storage materials & 
\parbox[t]{14pc}{\centering High specific surface area (1725
m\tsup{2}/g) and porous microspheres suitable for supercapacitor
electrodes after KOH activation and graphitization} & \cite{37}
\vspace*{3pt}\\ 

Aqueous phase & Biological valorization (AD) & 
\parbox[t]{14pc}{\centering COD removals {${>}$}80\% for citrus HTC
liquid; methane yields near theoretical at
5~g$_{\mathrm{COD}}{\cdot}$L$^{-1}{\cdot}$d$^{-1}$} & \cite{33,36} \vspace*{3pt}\\ 

Aqueous phase & Recirculation within HTC & 
\parbox[t]{14pc}{\centering Increased mass yield; autocatalytic
benefits observed} & \cite{24} \vspace*{3pt}\\ 

Gas (non-condensable) & Internal use/venting &
\parbox[t]{14pc}{\centering Predominantly CO\tsub{2}; LCA considers
biogas co-product routes} & \cite{31,32}\vspace*{2pt}
\botline
\end{tabular}
\vspace*{-3pt}
\end{table*}

%tab7
\begin{table*}
\caption{\label{tab7}Applications and readiness across domains}
\tabcolsep4.5pt
\begin{tabular}{ccc}
\thead
Domain & Readiness indicators & Scale relevance \\ 
\endthead
\parbox[t]{6pc}{\centering Energy (solid fuel, pellets)} & 
\parbox[t]{17.5pc}{\centering Industrial pellet specs met; ash/K/Cl
reduced~\cite{31}; \Lbreak PW recirculation enhances yield at low
temperatures~\cite{24}; \Lbreak dual solid+chemical production
tunable~\cite{16}} & 
\parbox[t]{12pc}{\centering Pilot continuous validation~\cite{31};
\Lbreak LCA industrial-scale modeling~\cite{32}} \vspace*{5pt}\\ 

\parbox[t]{6pc}{\centering Water treatment/Sorption} & 
\parbox[t]{17.5pc}{\centering High activation-derived capacities for
VOCs/emerging contaminants; \Lbreak baseline hydrochar effective for
cationic dyes; pH$_{\mathrm{PZC}}$ control relevant~\cite{22,23}} &
\parbox[t]{12pc}{\centering Lab-scale characterization; post-HTC
processing required for high performance} \vspace*{5pt}\\ 

Soil/agriculture & 
\parbox[t]{17.5pc}{\centering Soil physical property changes;
phytotoxicity tests; needs field-scale validation~\cite{34}} & 
\parbox[t]{12pc}{\centering Lab-scale incubation/assays; actionable for
pilot field trials} \vspace*{5pt}\\ 

\parbox[t]{6pc}{\centering Aqueous phase valorization} & 
\parbox[t]{17.5pc}{\centering PW recirculation boosts yield~\cite{24}; 
\Lbreak high AD performance [COD removal {${>}$}80\%; theoretical methane
at high loadings~\cite{33}; digestibility of citrus HTC liquor
characterized~\cite{36}} & \parbox[t]{12pc}{\centering Lab-scale HTC;
bench/pilot AD reactor tested~\cite{36}} \vspace*{5pt}\\ 

\parbox[t]{6pc}{\centering Materials/energy storage} & 
\parbox[t]{17.5pc}{\centering High-surface-area porous carbons after
activation; electrode suitability~\cite{37}} &
\parbox[t]{12pc}{\centering Lab-scale materials development}
\vspace*{2pt}
\botline
\end{tabular}
\vspace*{2pt}
\end{table*}

The oxygenated surface chemistry of citrus hydrochar makes it suitable
for electrochemical sensing (detection of heavy metals, organic
pollutants, and biomolecules due to enhanced electron transfer), gas
sensing (functionalized hydrochar can detect volatile organic
compounds), and biosensing, as biocompatibility and functional groups
enable immobilization of enzymes or biomolecules for selective
detection~\cite{51}. Beyond sensing, hydrochar from citrus waste has
demonstrated potential in energy storage (serving as electrode material
in supercapacitors and batteries due to its porous structure and
conductivity~\cite{52}), in environmental remediation (adsorption of
dyes, heavy metals, and organic pollutants from wastewater), and in
catalysis (acting as a support for metal nanoparticles in heterogeneous
catalysis~\cite{53}).

\subsection{Energy applications}\label{ssec31}

The production of solid biofuels from citrus waste via HTC represents
the most mature application domain, substantiated by robust
quantitative benchmarking and successful scaling efforts. 

Hydrochar from citrus wastes shows improved energy density, combustion
reactivity, and hydrophobicity compared to raw biomass and torrefied
products~\cite{16,17,33,54}. For wet citrus wastes, HTC offers
more than 50\% energy savings over torrefaction~\cite{17}.

Continuous operation of HTC systems has been successfully demonstrated,
addressing crucial industrial scale-up challenges. Specifically, a
continuous-flow pilot HTC system (TORWASH$^{\text{\textregistered}}$)
processed orange peels stably for 28 days, successfully translating
batch performance to continuous operation. This pilot work included
integral steps like dewatering the product to press cakes (achieving
42\% dry matter) and subsequent pelletization~\cite{31}. The resulting
hydrochar pellets consistently met industrial fuel specifications. Key
improvements include an increased HHV and significantly enhanced ash
quality. Citrus peels are typically prone to slagging and fouling risks
during combustion due to high alkali content. In fact, the HTC
process increases the ash deformation temperature by removing
problematic elements like potassium (K) and chlorine (Cl). These
elements form low-melting compounds (such as potassium silicates or
chlorides) that drastically lower the deformation temperature. Reducing
them makes the \mbox{remaining} ash (richer in higher-melting components like
calcium, magnesium, or silica) more thermally stable.

The solid yield achieved during continuous operation was reported at 
31~wt\% on a dry basis. Furthermore, system-level sustainability is being
established through LCA at the industrial scale, which supports the
environmental framing for orange peel HTC to pellets and biogas,
particularly providing reliable assessments for impacts related to
climate change, particulate matter, and acidification~\cite{32}.

\subsection{Adsorption applications}\label{ssec32}

Baseline citrus hydrochars, derived from orange peels, effectively
adsorb cationic dyes. This performance is intrinsically linked to the
hydrochar's chemical properties, specifically its point of zero charge
(pH$_{\mathrm{PZC}}$) and oxygenated surface
functionalities~\cite{22,23,35}.

For applications targeting diverse or complex pollutants, such as
emerging contaminants or volatile organic compounds (VOCs) like toluene
and iodine, post-HTC activation and modification are generally
essential. Activation methods, including treatment with KOH or
H\tsub{3}PO\tsub{4}, or N-doping (e.g., via urea assistance),
substantially elevate performance, transforming the hydrochar into
high-surface-area porous carbons. For example, KOH activation of
orange-juice-derived hydrochar achieved specific surface areas up to
1725 m\tsup{2}/g and graphitized microspheres~\cite{37}. While
activation is necessary for advanced markets, future work should
clearly delineate the performance gains attributable to the canonical
hydrochar versus the subsequent post-HTC \mbox{processing.}\looseness=-1

\subsection{Agricultural applications}\label{ssec33}

As a soil amendment, orange peel hydrochar has been shown to modify the
physical properties of clay soil. Hydrochar enriches soils with Ca, Mg,
P, and can act as a slow-release fertilizer. Hydrochar addition
improves soil water retention, nutrient \mbox{retention,} and reduces
contaminant leaching~\cite{33,55}. \mbox{However,} phytotoxicity screening
remains a critical consideration. Enhanced water retention and nutrient
availability support plant growth~\cite{55}. The presence of residual
organics (e.g., essential oils or organic acids concentrated during
HTC) necessitates rigorous quality assurance/quality control (QA/QC)
and careful dosing management to define safe application windows before
transitioning to field\break trials~\cite{34}. 

\subsection{Synthesis of advanced materials}\label{ssec34}

A major high-value application of hydrochar is its use as a precursor
for advanced functional carbon materials. The chemical composition of
citrus waste, particularly its high carbohydrate and pectin content,
makes it highly suitable for producing hydrochar that can be further
converted into engineered carbon materials with tailored properties.

By tuning HTC conditions, citrus-derived hydrochar can serve as an
excellent carbon precursor for subsequent activation and graphitization
processes. For instance, hydrochar obtained from orange juice was
subjected to KOH activation followed by graphitization at
800~\textdegree C. This yielded porous carbon microspheres with a
remarkably high specific surface area of up to 1725
m$^{2}$/g~\cite{37}. These materials exhibited excellent
characteristics for electrochemical double-layer capacitors (EDLCs),
making them promising candidates for high-performance supercapacitor
electrodes.

In addition to energy storage applications, citrus hydrochar has also
been explored for the synthesis of carbon-based catalysts. For example,
acid-treated orange peel hydrochar has been successfully applied in
biodiesel production through esterification and transesterification
reactions~\cite{16}.

Furthermore, HTC conditions can be optimized for an integrated
biorefinery approach, simultaneously producing hydrochar (as a solid
precursor) and valuable platform chemicals such as
5-hydroxymethylfurfural (5-HMF), furfural, and levulinic acid in the
PW. Moderate-to-severe conditions (e.g., 210~\textdegree C, 180 min)
favor higher hydrochar yields, while milder conditions promote the
formation of furanic compounds.

\vspace*{-2pt}
\subsection{Functional carbon materials}\label{ssec35}
\vspace*{-2pt}

Hydrochars derived from citrus wastes represent particularly attractive
precursors for low-cost sensing and biosensing materials. Their
relevance stems from three key attributes:
\begin{itemize}
\item rich and tunable surface chemistry, dominated by oxygenated
functional groups (--COOH, --OH, C${=}$O) inherited from the high sugar
and pectin content of citrus biomass~\cite{23}
\item heteroatom doping capability, especially nitrogen incorporation
through urea-assisted hydrothermal carbonization (HTC)~\cite{35}
\item adjustable porosity and conductivity, achieved via chemical
activation (e.g., KOH, H\tsub{3}PO\tsub{4}) and post-thermal
treatments~\cite{37}.\looseness=1
\end{itemize}
These features, already well established in adsorption and
energy-storage applications, can be directly translated into
electrochemical, optical, and biosensing platforms.

\subsubsection{Structure--property relationships in citrus-derived
hydrochars}\label{sssec351}

Under moderate HTC conditions, citrus hydrochars exhibit carboxyl-rich
and highly oxygenated surfaces, which are particularly favorable for:
\begin{itemize}
\item metal ion coordination (e.g., Pb\tsup{2+}, Cd\tsup{2+}, Cu\tsup{2+})
through carboxylate and phenolic binding sites~\cite{30,56}
\item hydrogen bonding and ${\uppi}$--${\uppi}$ interactions with
organic pollutants~\cite{57,58}
\item post-functionalization reactions (e.g., amination, polymer
grafting) for sensor fabrication~\cite{59}.
\end{itemize}
Increasing HTC severity decreases O/C and H/C ratios, enhances aromatic
condensation, and improves electrical conductivity, although often at
the \mbox{expense} of surface functional group density~\cite{21,38,39}.
Achieving a balance between conductivity and surface reactivity is
therefore critical for sensing performance.
In citrus-based systems:
\begin{itemize}
\item Activated hydrochars from orange peels show strong affinity
toward emerging contaminants due to their oxygenated surface
chemistry~\cite{7}.
\item Urea-assisted porous carbons derived from orange peel demonstrate
high adsorption capacities for toluene and iodine, highlighting the
role of nitrogen doping in modulating surface interactions and
electronic structure~\cite{6}.
\item KOH-activated carbons produced from orange-juice-derived
hydrochar reach specific surface areas of approximately 
1700~m$^{2}{\cdot}$g$^{-1}$ with partially graphitized
domains~\cite{16}, providing high active-site density and improved
charge transfer properties.\looseness=1
\end{itemize}
Microporosity enhances analyte pre-concentration, while
meso-/macropores facilitate mass transport in thin films and electrode
coatings both essential for sensitive detection.

\subsubsection{Applications in sensing and biosensing}\label{sssec352}

Citrus-derived hydrochars can serve as electrode coatings in
voltammetric or amperometric systems. Their defect-rich domains promote
electron transfer, while oxygen- and nitrogen-containing groups enable
analyte pre-concentration. Nitrogen doping (e.g., via urea-assisted
HTC) further improves conductivity and catalytic activity, enhancing
heavy metal and pollutant detection~\cite{60,61}.

The strong adsorption capacity and tunable surface chemistry of
citrus-derived carbons enable their use in both chemiresistive and
optical sensing applications. High-surface-area activated carbons
obtained from orange-juice hydrochar, thanks to their hierarchical pore
structure and partial graphitization, are particularly promising for
chemiresistive detection of volatile organic compounds (VOCs), where
pollutant adsorption induces measurable changes in electrical
resistance~\cite{62,63}.

Citrus hydrochars, especially those rich in carboxyl groups from
pectin-containing residues, can serve as effective precursors for the
synthesis of fluorescent carbon dots (CDs) through additional
post-treatment. Nitrogen doping further improves their quantum yield
and enables tunable emission. These CDs act as effective fluorescent
probes for detecting metal ions and small organic molecules through
fluorescence quenching or electron-transfer mechanisms~\cite{62,63}.

Overall, citrus waste--derived hydrochars provide a structurally and
chemically versatile carbon platform. Their intrinsic oxygen-rich
chemistry, controllable porosity, and potential for heteroatom doping
position them as sustainable, low-cost materials for next-generation
environmental sensing and biosensing applications.

\subsection{Aqueous phase (process water)}\label{ssec36}

The management and valorization of the process water (PW) generated
during citrus HTC are critical for overall system efficiency and
sustainability, especially given the high load of total organic carbon
(TOC) and organic acids characteristic of citrus residues.
\looseness=-1

One successful operational strategy involves PW recirculation, which
increases the solid mass yield of the hydrochar. For lemon peel HTC,
recirculation achieved the largest increase in solid mass yield (up to
6\%) at lower severity conditions (180~\textdegree C), demonstrating an
autocatalytic benefit where accumulated organics accelerate the
carbonization process. While recirculation enhances solid yield, it
simultaneously concentrates TOC in the liquid, underscoring the
necessity for downstream PW management or polishing~\cite{24}.
\looseness=-1

Downstream biological valorization of citrus HTC liquors is highly
feasible. HTC process water (aqueous phase) from orange pomace supports
anaerobic digestion for methane/biogas generation~\cite{33}. Anaerobic
digestion (AD) achieved COD removal efficiencies exceeding 80\% at high
volumetric loadings (5 g$_{\mathrm{COD}}{\cdot}$L$^{-1}{\cdot}$d$^{-1}$) 
under mesophilic conditions. Methane yields approached theoretical
limits, corroborating earlier findings regarding the digestibility of
orange pomace HTC liquor. This robust performance validates the
feasibility of integrating HTC--AD systems, even despite the recognized
inhibitory risks posed by citrus terpenoids and organic
acids~\cite{36}.\looseness=-1

\subsection{Gas (non-condensable)}\label{ssec37}

No peer-reviewed publication specifically reports the exact gas
composition data for citrus hydrochar. However, the gaseous phase
generated during HTC of citrus wastes represents a minor but chemically
significant product stream. Unlike pyrolysis or gasification processes
that operate at higher temperatures and produce substantial gas yields,
HTC of citrus wastes probably generates gaseous products accounting for
only 2--5\% of the original biomass mass. The primary component of this
gaseous fraction is carbon dioxide (CO\tsub{2}), which constitutes
approximately 90--95\% of the total gas volume, accompanied by minor
quantities of hydrogen, methane, and light hydrocarbons~\cite{75,76}.

The dominance of CO\tsub{2} in the gaseous phase results probably from
the fundamental reaction mechanisms governing HTC. During the
hydrothermal treatment of citrus peel wastes at temperatures ranging
from 180 to 300~\textdegree C, extensive decarboxylation reactions
occur as the biomass undergoes dehydration and structural
rearrangement. These decarboxylation pathways release CO\tsub{2} as
citrus-derived compounds (including citric acid, pectin, cellulose,
hemicellulose, essential oils such as limonene, etc.) undergo thermal
decomposition in the subcritical water environment~\cite{31,32}.

The yield and composition of the gaseous phase from citrus waste HTC
are strongly dependent on process conditions, particularly temperature
and residence time. Research on orange peel waste demonstrates that
increasing reaction time from 1 to 3 h at 180~\textdegree C
progressively favors hydrochar formation while simultaneously
increasing gas generation and decreasing bio-oil production~\cite{36}.
This time-dependent shift in phase distribution indicates that extended
residence times promote secondary reactions that convert liquid
intermediates into gaseous products through further decarboxylation and
cracking reactions.

Temperature elevation generally enhances gas production across all
citrus waste types. Studies on lemon peel waste HTC conducted at 180,
220, and 250~\textdegree C revealed that higher temperatures accelerate
the degradation of biomass macromolecules and promote the formation of
organic acids, which subsequently undergo decarboxylation to generate
additional gaseous products~\cite{24}. At 250~\textdegree C, the PW
concentration of soluble organic compounds decreases as these species
participate in polymerization reactions or convert to gas-phase
products. 

The initial pH and solid-to-liquid ratio also influence gas evolution
during citrus waste HTC. Acidic conditions typical of citrus wastes
(due to inherent citric acid content) catalyze hydrolysis reactions
that release CO\tsub{2} during the breakdown of pectin and
hemicellulose structures. However, unlike other thermochemical
processes, the solid-to-liquid ratio has been found to have
insignificant effects on mass distribution between phases for citrus
wastes. 

Citrus wastes present unique characteristics that affect gaseous phase
composition compared to other lignocellulosic feedstocks. The high
moisture content (typically 80--96\%) and distinctive chemical
composition, rich in sugars, pectin, flavonoids, and D-limonene, create
specific degradation pathways during HTC~\cite{36}. Limonene, a
monoterpene abundant in citrus peels, poses particular challenges for
conventional AD due to its antimicrobial properties; however, HTC
effectively mitigates this inhibition by converting limonene into other
compounds or releasing it into the gas phase~\cite{36}.

The sugar-rich nature of certain citrus wastes (particularly
grapefruits and oranges with high fructose/glucose content) can lead to
anomalous mass yield trends with increasing temperature, as observed in
comparative studies with other fruit wastes~\cite{77}. These
carbohydrate-rich fractions may undergo Maillard reactions and
caramelization, influencing the partitioning of carbon between solid,
liquid, and gaseous phases.

While the gaseous fraction from citrus waste HTC is relatively small in
mass yield, its composition has important implications for the energy
balance and environmental footprint of the process. The high CO\tsub{2}
content makes this gas stream suitable for carbon capture and
utilization applications, particularly in greenhouse agriculture, where
citrus wastes are often generated. The minor hydrogen and methane
fractions, though present in small quantities, indicate potential for
integrating HTC with downstream AD of PWs to enhance overall biomethane
production~\cite{16}.

Recent research has explored the integration of citrus waste HTC with
AD systems, where PW (containing dissolved organic matter from the HTC
treatment) serves as a substrate for biomethane \mbox{production.} This hybrid
approach addresses the challenges of inhibition by limonene in direct
AD of citrus waste while maximizing energy recovery from both liquid
and gaseous process streams~\cite{36}.

\section{Limitations and gaps to address}\label{sec4}

Despite the rapid expansion of the scientific literature on HTC of
citrus residues, the translation of laboratory findings into robust,
real-world industrial applications remains limited. While numerous
studies demonstrate technical feasibility at bench scale, far fewer
provide evidence from pilot units, demonstration facilities, on-site
industrial trials, or field applications of HTC-derived products. 

This section critically evaluates the current state of real-world HTC
implementations for citrus wastes, with particular attention to four
interrelated dimensions: (i) process- and system-level constraints,
(ii)~methodological and reporting shortcomings, (iii)~product
performance and application-specific limitations, and (iv) comparative
and benchmarking deficiencies.

\subsection{Process- and system-level constraints}\label{ssec41}

Most studies that claim real-world relevance are still conducted at
small pilot scale, over short operating campaigns, and under highly
controlled feedstock and process conditions. Consequently, important
operational uncertainties remain unresolved. Dang et al. provided a
comprehensive overview of HTC research gaps, emphasizing the lack of
long-duration pilot campaigns, methodological inconsistencies, and
benchmarking deficiencies~\cite{18}. 

Indeed, a major limitation concerns the lack of long-duration
operational data. Very few investigations report continuous HTC
operation on citrus residues for periods exceeding three to six
months~\cite{78}. As a result, critical industrial issues such as
fouling, corrosion, and reactor plugging, particularly those associated
with pectin, essential oils, and inorganic scaling, are not well
quantified~\cite{79}. Maintenance requirements, downtime statistics,
and reliability indicators are almost entirely absent from the
literature. In the absence of such data, techno-economic analyses
(TEAs) and LCAs frequently rely on assumed plant availabilities and
uptime factors that may be overly optimistic.

Integration with existing citrus processing chains is also weakly
addressed. Experimental studies rarely consider how HTC could be
coupled with juice or essential oil extraction lines, for example by
treating peel after oil recovery rather than raw peel~\cite{80,81,82}.
Similarly, opportunities for heat cascading from existing boilers or
combined heat and power (CHP) units to supply HTC thermal demand are
seldom experimentally evaluated. On-site management and reuse of PW,
whether in washing, cleaning, or irrigation, are also poorly
documented. Consequently, many system-level studies effectively model
HTC as a standalone facility, which may overestimate capital intensity
while underestimating potential industrial synergies~\cite{82}.

Scale-up correlations are another critical gap. Hydrodynamic and
heat-transfer characteristics, such as mixing efficiency, heating
rates, and residence time distributions, are rarely characterized at
pilot scale. Scale-up is typically justified through geometric
similarity and nominally identical operating conditions (temperature,
residence time, solid loading), without accounting for differences in
heating profiles, non-uniform residence times in continuous or
semi-batch reactors, or altered reaction pathways~\cite{78}. This
undermines confidence in the transferability of laboratory-derived
yields and product properties to larger-scale systems.\looseness=1

Citrus residues are inherently heterogeneous and strongly seasonal, yet
these aspects are insufficiently treated in real-world studies. Most
investigations rely on a single, well-characterized feedstock and the
influence of citrus species and varieties (e.g., orange versus lemon or
mandarin), residue types (peel, pulp, seeds, whole pomace, mixed
sludges), and pre-processing steps (limonene extraction, washing,
enzymatic treatment) on HTC performance remains underexplored at pilot
scale~\cite{16,83}.

Many studies provide HHV data and mass yields for hydrochar, yet fail
to report actual thermal energy input, electrical consumption, or heat
recovery efficiency at pilot scale. Consequently, reported energy
yields often reflect only the feedstock-to-hydrochar conversion,
excluding parasitic loads such as pressurization, pumping, agitation,
and heat losses. This weakens the credibility of claims regarding net
energy gains or reductions in greenhouse gas emission~\cite{36}.

Management of PW is similarly underdeveloped. Citrus HTC liquors
contain high concentrations of dissolved organics such as sugars,
organic acids, phenolics, limonene derivatives, and inorganic
species~\cite{48}. Although COD, BOD (biochemical oxygen demand), and
ecotoxicity are sometimes characterized at laboratory scale,
pilot-scale treatment trials (e.g., anaerobic digestion, stripping,
advanced oxidation) remain rare~\cite{33}. For other wastes, some
studies quantify water reuse within the HTC process or surrounding
industrial operations, and comprehensive nutrient balances (N, P, K)
across solid, liquid, and gaseous phases, but none is found for citrus
waste. Water footprint indicators are entirely absent.

\subsection{Methodological and reporting shortcomings}\label{ssec42}

Substantial heterogeneity exists in experimental design, even among
studies presenting pilot-scale relevance~\cite{84}. Operating
conditions (temperature, residence time, and solid loading) vary
widely, sometimes without clear justification or optimization. Critical
parameters, such as heating and cooling rates, initial pH, and mixing
intensity, are frequently omitted, despite their strong influence on
the HTC of carbohydrate- and oil-rich citrus matrices~\cite{84}.
Reaction pathway analysis and kinetic modeling are also
limited~\cite{85}. 

Hydrochars and PWs derived from citrus HTC are often insufficiently
characterized for real-world deployment. For fuel or soil applications,
many studies report proximate and ultimate analyses alongside HHV
values~\cite{86}, but omit critical attributes such as ash composition
and speciation, soluble versus insoluble salts, chlorine and sulfur
content, heavy metals, organic micropollutants, leaching behavior, and
mechanical properties (e.g., attrition resistance, pelletization
performance). Without such data, compliance with fuel standards, soil
amendment regulations, or adsorption media specifications cannot be
reliably assessed~\cite{87,88}.

The liquid and gaseous phases are frequently treated as secondary
by-products~\cite{24}. However, environmental assessment requires
detailed speciation of dissolved organics, evaluation of
biodegradability, and quantitative gas composition analysis
(CO\tsub{2}, CH\tsub{4}, VOCs). The limited attention given to these
streams risks underestimating both environmental constraints and
valorization opportunities.

Replication at pilot scale is also rare due to cost and complexity,
resulting in limited uncertainty analysis. Variability in yields and
product properties is often underreported, and the sensitivity of
results to operational fluctuations is poorly understood~\cite{18}.
Furthermore, raw data, such as full mass balances, or energy and water
flow, are seldom made publicly available. This restricts
reproducibility and hinders meta-analysis and cross-study benchmarking.

\vspace*{-4pt}

\subsection{Application-specific limitations}\label{ssec43}

Citrus-derived hydrochars often exhibit elevated ash content and
potentially problematic ash chemistry, including alkali metals and
chlorine~\cite{23}. Empirical studies of slagging, fouling, and deposit
formation in industrial boilers are extremely limited~\cite{77}. Most
fuel suitability discussions rely on theoretical indices rather than
operational trials. Co-firing limits in coal or biomass blends remain
largely speculative~\cite{43}. Regulatory and market barriers are also
insufficiently explored. Compliance with solid recovered fuel (SRF) or
biofuel standards is nowhere explicitly tested, and issues such as
certification routes, off-taker requirements, and pricing structures
receive no attention.

Although many studies propose hydrochars as adsorbents, most tests are
conducted in simplified single-solute systems~\cite{77}, but cost per
unit of pollutant removed is almost never calculated. Besides,
competitive adsorption in realistic matrices (industrial effluents,
landfill leachate, agricultural runoff) has not been investigated yet,
nor has the regeneration performance or the disposal pathways for spent
hydrochar.\looseness=-1

Field trials assessing soil amendment performance are minimal. Most
available data derive from short-term pot experiments under controlled
conditions~\cite{34,55}. Long-term impacts on soil structure, microbial
communities, nutrient cycling, and crop yield remain uncertain.
Potential negative effects, such as phytotoxic residues, salinity, or
limonene-derived suppression of soil biota, are poorly addressed.
Claims regarding carbon sequestration frequently extrapolate from short
laboratory incubations or general biochar literature~\cite{30}, without
examining the specific chemical structure and stability of citrus HTC
chars under field conditions.

Citrus residues are distinguished by their high essential oil and
limonene content. Yet few pilot-scale studies quantify how residual
oils influence reaction kinetics, product distribution, emulsification
phenomena, or fouling in piping and heat exchangers. Integration of HTC
with prior oil extraction remains largely conceptual~\cite{18,53}.

Acidity presents another challenge. Organic acids formed during HTC can
intensify corrosion under high-temperature, high-pressure conditions.
However, systematic material compatibility studies under
citrus-specific HTC liquors are largely absent, leaving reactor
material selection insufficiently\break justified.

\subsection{Quantitative benchmarking deficiencies}\label{ssec44}

A central limitation across the literature is the absence of
standardized quantitative benchmarking frameworks, both within citrus
HTC and in comparison with alternative valorization routes.

Studies employ diverse metrics (mass yield, energy yield, carbon yield,
energy densification ratio, exergy efficiency) often calculated on
different bases (wet versus dry feedstock, per unit feed versus per
unit product) and with partial system boundaries that exclude parasitic
loads. This makes cross-comparison between pilot units or technologies
difficult.\looseness=-1

Economic and environmental key performance indicators (KPIs) are
frequently missing. Few studies report cost per tonne of citrus waste
treated, cost per unit of functional output, or greenhouse gas
emissions per tonne processed with clearly defined boundaries. When
LCAs are conducted, methodological assumptions vary considerably and
are seldom aligned with emerging guidelines for biochar and biomass HTC
systems~\cite{89}. Cavali et al. point out that much of the HTC
economic literature relies on simplistic payback or cost assumptions
(fixed char price), and that few TEAs incorporate realistic heat
integration, logistics, and multi-product allocation~\cite{90}.
\looseness=-1

HTC is often compared only with landfilling or incineration, while
alternative routes, such as anaerobic digestion, composting, direct
combustion, pyrolysis, or torrefaction, are not assessed under
consistent system boundaries and functional units~\cite{29,80}.
Multi-criteria decision analyses integrating economic, environmental,
and social indicators are rare. Consequently, claims that HTC is
optimal or \mbox{promising} are seldom supported by robust, quantitatively
benchmarked trade-off analyses.

\section{Conclusion}\label{sec5}

HTC offers a sustainable and flexible platform for the valorization of
citrus processing waste, with ongoing research focused on optimizing
operational parameters, expanding end-use applications, and overcoming
scale-up and environmental challenges.

More precisely, the field has progressed from demonstrating feasibility
and adsorption proxies to operational strategies for process water and
continuous fuel production, with sustainability lenses now applied. 
\looseness=-1

To enable the transition from promising experimental studies to robust
real-world implementation, several priority research directions emerge.
Long-duration pilot and demonstration campaigns should be conducted
under realistic feedstock conditions, with complete mass and energy
balances, systematic monitoring of fouling and corrosion, and
transparent reporting of downtimes and failures.

Standardized reporting protocols and harmonized performance metrics are
needed to enable meta-analysis and benchmarking. Integrated process
configurations, combining oil extraction, HTC, and downstream water or
biogas treatment, should be experimentally validated under industrial
constraints.
\looseness=-1

Application-focused qualification is essential, including boiler and
combined heat and power (CHP) trials with emission monitoring,
multi-solute adsorption tests with regeneration cycles, and multi-year
field studies evaluating agronomic performance and carbon
sequestration. Finally, comparative assessments using consistent TEA
and LCA frameworks, region-specific data, and decision-support tools
should guide stakeholders in determining when HTC represents a
genuinely preferable option relative to competing technologies.

Collectively, addressing these gaps is crucial for advancing citrus
waste HTC from technically promising yet often idealized case studies
toward robust, quantitatively benchmarked, and regionally optimized
real-world valorization pathways.

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