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\DOI{10.5802/crchim.240}
\datereceived{2023-03-30}
\daterevised{2023-05-16}
\dateaccepted{2023-06-30}
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\dateposted{2023-09-06}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Clean Processes for Sustainable Energy and Environmental Applications}
%\def\cdr@specialtitle@french{Mat\'eriaux et proc\'ed\'es propres pour des applications \'energ\'etiques et environnementales}
%\makeatother

\title{Slow pyrolysis of orange peels blended with agro-food wastes:
characterization of the biochars for environmental applications}

\author{\firstname{Sana} \lastname{Kordoghli}\CDRorcid{0000-0001-6609-1425}\IsEqualContrib}
\address{National School of Sciences and Advanced Technologies,
ENSTA-Borj Cedria, University of Carthage, Tunisia}
\address{Research Laboratory for Sciences and Technologies of
Environment LR16ES09, High Institute of Sciences and Technologies of
Environment, Carthage University, Borj Cedria, Tunisia}
\email[S. Kordoghli]{sana.kordoghli@enstab.ucar.tn}

\author{\firstname{Emna} \lastname{Fassatoui}\CDRorcid{0009-0006-2852-6890}\IsEqualContrib}
\addressSameAs{1}{National School of Sciences and Advanced Technologies,
ENSTA-Borj Cedria, University of Carthage, Tunisia}
\email[E. Fassatoui]{emna.fassatoui@enstab.ucar.tn}

\author{\firstname{Jean Fran\c{c}ois} \lastname{Largeau}\CDRorcid{0000-0002-2703-3668}}
\address{GEPEA-CNRS UMR 6144, IMT Atlantique, Nantes 44300, France}
\address{Icam, 35 rue du Champ de Man\oe{}uvres, 44470 Carquefou, France}
\email[J. F. Largeau]{jean-francois.largeau@icam.fr}

\author{\firstname{Besma} \lastname{Khiari}\CDRorcid{0000-0001-8251-853X}\IsCorresp}
\address{Laboratory of Wastewaters and Environment, Centre of Water
Researches and Technologies (CERTE), Technopark Borj Cedria, Touristic
road of Soliman, BP 273, 8020, Tunisia}
\address{National School for Engineering of Carthage, 45 Rue des
Entrepreneurs, Charguia II, 2035 Tunis, Tunisia}
\email[B. Khiari]{besmakhiari@yahoo.com}

\shortrunauthors

\begin{abstract}
The main objective of this research is to explore the viability of
adequate blends of orange peels with some abundant bio-wastes, such as
date pits, peanut shells, coffee grounds, and tea waste, to generate
bio-chars while applying a thermal pyrolysis process. It was revealed
that pure orange peels lead to a high biochar yield (32.05\%). The
different blends with orange peels did not show significantly increased
char output. However, the biochar yields were lower with coffee grounds
and date pits, which shows the positive effect of blending with orange
peels for these two biomasses as far as char is concerned. 
However, the blends appeared to favour syngas production
with yields as high as 56\% for the coffee grounds and orange peels
blend. FTIR and SEM characterizations indicated that recovered solids
could represent a viable source of carbon materials, biofertilizers, or
solid fuels.
\end{abstract}

\keywords{\kwd{Orange peels}
\kwd{Bio-waste}
\kwd{Valorization}
\kwd{Pyrolysis}
\kwd{Blends}}

\maketitle

\clearpage

\twocolumngrid

\end{noXML}

\section{Introduction}

A circular economy could be built by turning large amounts of agro-food
waste into valuable resources, thereby achieving sustainable
development goals. Waste valorization is the suitable strategy in order
to ensure a sustainable bioeconomy. Due to the massive generation of
waste, the environment has become a significant concern of society as a
result of industrialization and agricultural economic growth.
Currently, several wastes are not properly managed despite their
potential for energy and environmental applications. They are generally
discarded by landfilling and incineration. However, similar techniques
are not eco-friendly and fail to recover useful products/chemicals. For
instance, energy production and bio-based materials extraction routes
from agricultural residues could be part of a circular economy
approach~\cite{1}. Thermochemical conversion of agricultural residues
has the advantage of fully exploiting the entire organic matter of the
resource. However, depending on the procedure parameters, the
conversion of the residues also generates by-products that need to be
managed in different ways.

Pyrolysis and pyro-gasification are considered among the most effective
methods of biofuel production from biomass. These techniques allow
waste conversion into three value-added products: char, oil and gas,
thus facilitating its storage and transport. Depending on the
parameters of the process and the potential of the treated residues,
one form can be promoted: oils are favored by rapid pyrolysis, syngas
by pyro-gasification and~char by slow pyrolysis. This last solid
fraction is singled out thanks to its versatile applications as a fuel,
a soil conditioner, or as an adsorbent~\cite{2}.

Citrus waste is one biomass among those considered as excellent
candidates for the recovery of high-added value products. Indeed, the
deposits are important in many parts of the world~\cite{3}. The world
production of citrus fruits, all species combined, is estimated to 110
million tons per year, and is spread over a free area worth about 7.5
million hectares. Oranges account for almost 60\% of total world
production. 

In recent decades, several studies have been carried out on the
valorization of orange peels by pyrolysis. In this context, Aguiar 
\etal~\cite{4} conducted a comprehensive study to determine the
influence of temperature (300, 500 and 600~\textdegree C) and particle
size\break (300--800 mm) on the volatile products and the biochar yields
derived from the pyrolysis of orange peels in a fixed bed reactor. They
subsequently demonstrated that temperature has a more significant
effect than particle size on the products yields, the biochar
characteristics and the composition of the gaseous and the liquid
fractions. Miranda \mbox{\etal}~\cite{5} also pyrolyzed dried orange
peels and obtained an high bio-oil fraction (53.1\%). Authors noted
that such waste is capable of generously releasing volatile products
(78.9\% w/w). Ash rate was 2.94\% w/w while biochar was around 
21.1\% w/w. These results were similar to those obtained by Morales \etal~\cite{6} 
who were able to achieve a bio-oil yield of 77.64\%, a
biochar percentage of 20.93\% and a yield of gas as low as 1.43\%. 

On the other hand, the slow pyrolysis of dried citrus waste (lemon and
orange peels) was investigated by Volpe \etal~\cite{7} using a
fixed bed reactor for a temperature range from 400~\textdegree C to 
650~\textdegree C. They reported that the yield of bio-oil fluctuated between
36\% and 39\% w/w and the biochar rate varied between 37.2\% and
40.8\% w/w. 

More recently, S\'{a}nchez-Borrego \etal~\cite{8} used a vertical
tubular furnace at laboratory scale in order to pyrolyze orange waste
and pruning while varying the temperature (400--600~\textdegree C), the
heating rate (5--20~\textdegree C/min), as well as the inert gas flow
rate (30--300 mL of Argon per minute). Authors found that the optimized
conditions for biochar production from orange waste were: 400~\textdegree C, 
5~\textdegree C/min, and 150 mL/min. Despite their low
specific surface areas, these biochars were very effective when
removing sulfur from waste cooking oil. Selvarajoo \etal~\cite{9} 
have also slowly pyrolyzed citrus peels in the
temperature range of 300--700~\textdegree C. The results showed that an
increase in pyrolysis temperature led to~a low char yield (reduced from
53.62 to 22.01 wt\%). For energy purposes, the maximum HHV (High
Heating Value) and carbon content values were found in biochars formed
at 500~\textdegree C. 

Faster pyrolysis of orange peels was investigated by  Alvarez
\etal~\cite{10}. They have developed such technology to determine the
yields of this biomass derivative by varying the pyrolytic temperature.
They have revealed that the biochar yield fluctuates between 27\% and
33\% by weight in the temperature range of 425--500~\textdegree C.
Adeniyi \etal~\cite{11} have conducted a comparative study using the
ASPEN model in order to investigate the pyrolysis yields of five\break
different fruit peels, namely banana (\textit{Musa} spp.), orange
(\textit{Citrus sinensis}), sweet lime (\textit{Citrus limetta}), lemon
(\textit{Citrus limon}) and jackfruit (\textit{Artocarpus
heterophyllus}) peels. The obtained data showed that lemon and orange
peels, which have the greatest volatile matter values, delivered also
the maximum bio-oil yield. The jackfruit peel, on the other hand,
generated the maximum biochar percentage. Peels from bananas and sweet
limes exhibited comparable amounts of both oil and char. Lastly, based
on their research, the authors~have concluded that citrus peels,
including orange, sweet lime, and lemon peels, are excellent candidates
for liquid fuel generation~and/or for char formation.

Generally, the vast majority of the previous related works have focused
on the valorization of pure citrus waste, without mixing with other
wastes. Yet, although blending of two or more types of biomass is often
investigated as feedstock for pyrolysis, to the best of our knowledge,
mixing orange peels with other residues was not carried out in
pyrolysis tests. Thus, the novelty of this work is to extract high
added-value products, namely biochar, from orange peels blended with
different other food waste and to assess their performance in
comparison with the single biomass. More particularly, the present
study aims to fully characterize the pyrolytic solid by-products issued
from the blends of orange peels with several abundantly occurring
agro-food waste, including date pits, tea waste, coffee grounds, and
peanut shells.

\section{Materials and methods}

\subsection{Samples preparation}

Initially, selected biomasses were retrieved individually from specific
addresses, which were all located in the eastern region of Tunisia.
Orange peels (OP) were collected from the orange juice manufacturing
industries, while coffee grounds (CG) and tea waste (TW) were recovered
from tearooms and cafeterias. As for the date pits (DP), they were
collected by manufacturers dedicated to the production of date paste.
Finally, peanut shells (PS) were collected from farms based in the Cap
Bon region in Tunisia.

Prior to pyrolysis, samples were fed into a furnace for 24 h at a
temperature below 105~\textdegree C to ensure less than 10\%~moisture
content. Then, they \pagebreak were crushed by a ball mill and sieved in order to
have homogeneous products. Only one particle size was selected:
particles with a diameter between 1 and 1.5~mm. A second drying phase
was carried out in a specific oven for 3 days at 70~\textdegree C to
eliminate any residual moisture.

The established blends were then made with equal ratios based on orange
peels: 

B-OC: Blended Orange peels and Coffee grounds (50\%, 50\%)

B-OT: Blended Orange peels and Tea waste (50\%, 50\%)

B-OD: Blended Orange peels and Date pits (50\%, 50\%)

B-OP: Blended Orange peels and Peanut shells (50\%, 50\%)

B-OTC: Blended Orange peels, Tea waste and Coffee grounds (33\%, 33\%, 33\%).

\subsection{Experimental setup}

The reactor used for this study is a 40 mm high cylinder shaped with a
54 mm inside diameter. It is slid vertically in the center of a heating
collar covered by glass wool. This would avoid any thermal shift that
may occur between the actual temperatures reached by the reactor.
During pyrolysis test, 1 g of sample was placed in a crucible with 5mm
depth (Figure~\ref{fig1}).

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Schematic representation of the experimental setup.}
\end{figure*}

A stream of inert gas (N$_2$) with a flow rate of 50~mL/min was
introduced at the top of the reactor before launching the process in
order to ensure the reaction environment inerting.

Then the pyrolysis experiments were conducted at two different heating
rates (5~\textdegree C/min and 10~\textdegree C/min) for the single orange
peels and at 10~\textdegree C/min for the different blends. The pyrolysis
temperature was adjusted with a PID~controller to reach 650~\textdegree C
and then maintained for 1 h. During the experiment, a gas analysis
system (Micro GC FUSION analyzer) was installed at the output of the
reactor allowing the analysis of the chemical composition of the
released gases. Measurements were recorded every five minutes. A first
condenser was used to protect the analyzer from the water
vapour~released during the reaction and thus to prevent the penetration
of any type of oil or tar inside the Micro-GC.

For the reaction evolution monitoring, the temperature inside the
reactor was recorded during all experimentations.

After pyrolysis, the recovered bio-oils and biochars were weighed to
determine their percentages, and the biogas yield was obtained using
the subtraction method.


\subsection{Analytical methods}

Elemental analysis was performed using a CHNS elemental analyser (Flash
EA 1112 Series) to determine the elemental content of carbon (C),
hydrogen (H), nitrogen (N) and sulfur (S) present in the raw material.
The ash content was obtained by combustion of the sample in a furnace
at 900~\textdegree C for a period of 4~h~\cite{12}. However, the fixed
carbon percentage was estimated by the mass difference~\cite{13}.

The High Heating Value (HHV) was estimated based on the elemental
analysis data (CHN--O) using Sheng and Azevedo's correlation, as
below~\cite{14}:
{\begin{eqnarray}
\mathrm{HHV} &=& -1.3675 + 0.3137 \times \mathrm{C} + 0.7009\nonumber\\ 
&&\times\, \mathrm{H}+ 0.00318 \times \mathrm{O}\ (\mathrm{MJ}/\mathrm{kg})\label{eq1}
\end{eqnarray}}\unskip
Thermogravimetric analysis was carried out to characterize the
degradation behaviour of the different agro-food wastes using a TGA
analyzer (SETSYS Evolution 1750). The TGA tests were performed in an
inert atmosphere from ambient temperature to 700~\textdegree C at~the
heating rate of 10~\textdegree C/min.

In order to understand the chemical functional groups present in the
agro-waste, IR spectra were run taken from the Fourier Transform
Infrared Spectroscopy (Perkin Elmer, Spectrum 2). 

Atomic fluorescence spectroscopy, commonly known as X-ray fluorescence
spectrometry, was employed to identify the mineral composition of the
investigated samples.

The morphology of the agricultural waste was determined by using a
scanning electron microscope (SEM, JEOL-JSM-6400, LF-Ja). The SEM
analysis was performed to identify the agricultural waste based on
their morphological structure and to foretell their agglomeration
characteristics.

\section{Results and discussion}

\subsection{Characterization~of the raw materials}

\subsubsection{Proximate and elemental analyses}

The ultimate and proximate analyses of the raw materials are shown~in
Table~\ref{tab1}. The obtained values were compared with those reported in the
literature for the same materials as well as for other food wastes.

\begin{table*}
\caption{\label{tab1}Elemental and proximate analyses of the selected
biomasses\vspace*{-2pt}}
\fontsize{9}{10.8}\selectfont
\begin{tabular}{lcccccccccccc}
\thead
 & \multicolumn{5}{c}{Elemental analysis (wt\%)} && 
 \multicolumn{4}{c}{Proximate analysis (wt\%)} &
 \morerows{1}{\parbox[t]{1cm}{\centering HHV\break (MJ/kg)}}&  
 \xmorerows{1}{Ref.}\\
 \cline{2-6}
 \cline{8-11}
 & C & H & N & O$^*$ & S && 
 \parbox[t]{1.5cm}{\centering Moisture\break content db} & 
 \parbox[t]{1cm}{\centering Volatile\break matter} & 
 \parbox[t]{1cm}{\centering Fixed\break carbon}\vspace*{2pt}  &  Ash & 
  & \\ 
\endthead
\morerows{2}{\parbox{1cm}{Orange\break peels}} 
 & 41.05 & 6.10 & 0.91 & 51.94 & 0    && 7.65  & 73.87 & 16.18 & 2.3  & 15.95 & This work \\
 & 42.40 & 5.80 & 0.60 & 51.20 & 0    && 11    & 62    & 35    & 3    & --    & \cite{17} \\
 & 44.51 & 5.99 & 1.08 & 48.20 & 0.22 && 12.84 & 70.51 & 14.05 & 2.06 & 18.32 & \cite{18}\vspace*{5pt} \\ 
\morerows{1}{\parbox{1cm}{Coffee\break grounds}} 
 & 49.04 & 6.85 & 2.33 & 41.78 & 0 && 7.42 & 76.80 & 14.47 & 1.31 & 18.95 & This work \\
 & 54.61 & 6.59 & 3.97 & 34.83 & 0 && 1.31 & 77.51 & 19.83 & 1.35 & 22.74 & \cite{19}\vspace*{5pt} \\ 
\morerows{1}{Date pits} 
 & 45.86 & 6.73 & 1    & 46.41 & 0 && 5.71 & 69.45 & 23.86 & 0.98 & 17.88 & This work \\
 & 48.43 & 6.44 & 0.67 & 42.20 & 0 && 5.44 & 79.88 & 18.65 & 1.47 & 24.25 & \cite{20}\vspace*{5pt} \\ 
\morerows{1}{\parbox{1cm}{Peanut\break shells}} 
 & 48.52 & 5.80 & 0.71 & 44.97 & 0    && 6.33 & 76.69 & 11.28 & 5.7  & 18.06 & This work \\
 & 46.86 & 6.84 & 1.03 & 44.98 & 0.29 && 5.16 & 80.24 & 8.48  & 6.12 & 18.64 & \cite{21}\vspace*{5pt} \\ 
\morerows{1}{Tea waste} 
 & 45.72 & 6.04 & 2.90 & 45.34 & 0   && 7.86 & 80.04 & 8.08  & 4.02 & 17.35 & This work \\
 & 46.30 & 6.1  & 3.7  & 42.80 & 1.1 && 6.31 & 70.42 & 19.52 & 3.75 & --    & \cite{22}\vspace*{5pt} \\ 
\parbox{1cm}{Potato\break peels}\vspace*{5pt} & 43.49 & 6.08 & 2.78 & 47.20 & 0 && 8.07 & 68.80 & 14.38 & 8.75 & 17.36        & \cite{18}\vspace*{5pt} \\ 
\parbox{1cm}{Prickly\break pear}              & 34.70 & 5.3  & 1    & 37.90 & 0 && 7.2  & 79.90 & 16.43 & 0.95 & 14.60--15.30 &\cite{23}\vspace*{2pt} 
\botline
\end{tabular}
\tabnote{$^*$Obtained by difference; db: dry basis.\vspace*{-3pt}}
\end{table*}


The examined biomasses are characterized by a low amount of moisture
(ranging from 5.71~wt\% to 7.86 wt\%). Generally, for the pyrolysis
process, biomasses with low moisture contents (10\%) are often
preferred since pre-drying is an energy-consuming step~\cite{15}.

Orange peels have a high volatile content of 73.87\%, which contributes
to their strong reactivity and simple devolatilization during thermal
decomposition. The ash content is about 2.3\%, demonstrating thus its
applicability as a feedstock for the pyrolysis process. In fact, due to
its reduced ash percentage, such~raw material can result in fewer
operational complications such as slag deposition, internal pyrolyzer
corrosion, burning up rate, and pollution.

Elemental analysis shows that orange peels contain high amounts of
carbon (41.05\%), higher than other food residues such as prickly pears
and potato peel residues~\cite{18,23}. Moreover, this biomass consists
of an important proportion of~hydrogen (6.10\%) and oxygen (51.94\%)
contents. 

These values are nearly close to those found for the other studied
feedstock, which should favour the blending. It can be specifically
mentioned those\break referring to peanut shells (48.52\% C, 5.80\% H, and
44.97\% O) as well as coffee grounds (49.04\% C, 6.85\% H, and 41.78\%
O). It is commonly acknowledged that biomass with a high proportion of
organic materials is conducive to thermal degradation since
organic-rich residues are extremely reactive, simple to devolatilize
and generate a significant proportion of biochar via pyrolysis, mainly
with low O/C and high~H/C ratios. These ratios, an indicator of fuel
quality and type,~are particularly crucial if the biochar is going to
be used as an energy source alternative in fields, such as the
transportation sector, that require a pure and homogeneous fuel flow.

Moreover, orange peels and date pits are composed of low nitrogen
(0.91\% and 1\% respectively) and almost no sulfur which inhibits
hazardous gases (NOx, SOx)~\cite{16}, while coffee grounds and tea
waste consist of more nitrogen (2.3\% and, 3.8\% respectively) than the
other lignocellulosic biomasses.

\begin{table*}
\caption{\label{tab2}Mineral composition of the selected wastes$^*$\vspace*{-3pt}}
\begin{tabular}{lccccc}
\thead
  & Orange peels & Peanut shells & Coffee grounds & Date pits & Tea waste \\ 
 \endthead
Ca (\%) & 62.58  & 31.87  & 31.4\0 & 17.51  & 62.74 \\ 
K (\%)  & 30.30  & 47.54  & 54.88  & 55.42  & 11.82 \\ 
Mn (\%) & --     & --     & --     & --     & \07.75 \\ 
Fe (\%) & \00.93 & \02.23 & \01.79 & \04.35 & \04.07 \\ 
P (\%)  & \02.09 & \04.97 & \06.11 & \07.52 & \03.56 \\ 
Cl (\%) & \02.39 & \05.47 & --     & \07.62 & \03.46 \\ 
S (\%)  & \01.01 & \05.59 & \04.63 & \05.23 & \03.28 \\ 
Si (\%) & --     & \01.23 & --     & --     & \02.74 \\ 
Cr (\%) & --     & \00.74 & \01.12 & \01.41 & \00.55 \\ 
Cu (\%) & \00.37 & \00.34 & --     & \00.63 & -- \\ 
Sr (\%) & \00.32 & --     & --     & --     & -- \\ 
Zn (\%) & --     & --     & --     & \00.29 & -- 
\botline
\end{tabular}
\xxtabnote{$^*$Values are provided on dry basis.}
\vspace*{-3pt}
\end{table*}

The high heating values vary between 14.60 and 22.74 MJ/kg. These
measured results are in line with what was stated by previous
researchers \cite{17,18,19,20,21,22}. The calorific values of the
investigated residues are insufficient to allow their direct use as a
commercial fuel. As a result, these samples must be \mbox{enhanced} by the
pyrolysis process, which appears to be the best procedure for
generating biochar with increased calorific value, constituting thus a
novel\break fuel source.

Table~\ref{tab2} shows the different mineral contents of the examined
biomasses. Potassium (K) and calcium (Ca) are the most abundant
elements that subsequently constitute the basic mineral composition of
these raw materials. Concerning the distribution of the other elements
(Mn, Fe, Cl, Sr, etc.), small proportions are obtained that differ from
one biomass to another according to their composition. The presence of
inorganic minerals, and more specifically alkaline metals (K, Na, etc.)
and alkaline earth metals (Mg, Ca, etc.) have a direct effect on the
distribution of pyrolysis products~\cite{12}. In fact, potassium in the
mineral matter of biomass acts as a catalyst that promotes the
generation of biochar in favour of derived bio-oil~\cite{12}. In this
study, date seeds, peanut shells and coffee grounds contain the
greatest K proportions, suggesting they are adequate materials for
pyrolysis in order to recover solid residue.

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig02}
\caption{\label{fig2}TG and DTG curves of biomass samples at
10~\textdegree C/min: (a) OP, (b) DP, (c) PS, (d) TW, (e) CG.}
\vspace*{2pt}
\end{figure*}

\begin{table*}
\caption{\label{tab3}TG parameters of the raw biomasses and their
derivative blends in the different pyrolysis zones\vspace*{-1pt}}
\fontsize{9}{10.2}\selectfont
\tabcolsep=3.4pt
\begin{tabular}{llcccccccccccc}
\thead
& & & \multicolumn{5}{c}{Raw materials} && \multicolumn{5}{c}{Blends} \\
\cline{4-8}
\cline{10-14}
& & & {OP} & {TW} & {CG} & {PS} & {DP} && {B-OP} & {B-OC} & {B-OT} & {B-OD} & {B-OTC} \\ 
\endthead
& \morerows{5}{\parbox[t]{1.2cm}{Pyrolysis\newline zone}} &  
\morerows{2}{\textit{T}~(\textdegree C)} & 155 & 230 & 285 & 235 & 220 && 175 & 198 & 170 & 187 & 193 \\
&& & -- & -- & -- & -- & -- && -- & -- & -- & -- & -- \\
&& & 700 & 700 & 700 & 700 & 700 && 700 & 700 & 700 & 700 & 700 \\
\cline{3-14}
&& \morerows{2}{$X$ (\%)} & 26.6 & 27.8 & 23.5 & 25.8 & 22.1 && 25 & 21.3 & 24.2 & 22.3 & 20.6 \\
&& & -- & -- & -- & -- & -- && -- & -- & -- & -- & -- \\
&& & 94.6 & 91.5 & 90.8 & 93.8 & 95.1 && 96.7 & 93.2 & 94.5 & 92.5 & 93.2 \\
\hline

\morerows{16}{\raisebox{-20pt}{\parbox[t]{1.2cm}{Active\newline pyrolysis}}} 
& \morerows{7}{\raisebox{-10pt}{Zone 1}} & 
\morerows{2}{\textit{T} (\textdegree C)} 
& 155 & 230 & 285 & 235 & 220 && 175 & 198 & 170 & 187 & 193 \\

& & & -- & -- & -- & -- & -- && -- & -- & -- & -- & -- \\

&& & 293 & 275 & 398 & 473 & 330 && 265 & 240 & 298 & 279 & 294 \\

\cline{3-14}
& & \morerows{2}{\emph{X} (\%)} & 94.6 & 91.5 &90.8 & 93.8 & 95.1 && 96.7 & 93.2 & 94.5 & 92.5 & 93.2 \\

&& & -- & -- & -- & -- & -- && -- & -- & -- & -- & -- \\

&& & 60.2 & 81.2 & 43.7 & 34.7 & 50 && 69 & 76.5 & 74.2 & 75 & 78.9 \\
\cline{3-14}

&& {$T_{\mathrm{peak}\ 1}$ (\textdegree C)} & 210 & 210 & 320 & 350 & 300 && 235 & 221 & 240 & 230 & 243 \\
\cline{3-14}

& & $R_1$ (\%${\cdot}$s$^1$) & ${-}$0.084 & ${-}$0.015 &${-}$0.11 & ${-}$0.12 & ${-}$0.173 && ${-}$0.045 & ${-}$0.041 & ${-}$0.055 & ${-}$0.039 &${-}$0.042 \\
\cline{2-14}

&\morerows{7}{\raisebox{-10pt}{Zone 2}} & 
\morerows{2}{\textit{T} {(\textdegree C)}} & 293 & 275 & 398 & -- & 330 && 265 & 240 & 298 & 279 &294 \\

&& & -- & -- & -- & & -- && -- & -- & -- &-- & -- \\

&& & 420 & 440 & 500 & & 450 && 455 & 463 & 456 & 415 & 445 \\
\cline{3-14}

&& \morerows{2}{\emph{X} (\%)} & 60.2 & 81.2 & 43.7 & -- & 50 && 69 & 76.5 & 74.2 & 75 & 78.9 \\
&& & -- & -- & -- & & -- && -- & -- & -- & -- & -- \\
&& & 30.5 & 37.5 & 28.3 & & 35.1 && 34.5 & 33.4 & 40.2 & 41.5 & 41.4 \\
\cline{3-14}
& & $T_{\mathrm{peak}\ 2}$ (\textdegree C) & 342 & 380 & 488 & -- & 400 && 384 & 320 & 398 & 330 & 334 \\
\cline{3-14}
& & $R_2$ (\%${\cdot}$s$^{-1}$) & ${-}$0.065 & ${-}$0.033 &${-}$0.04 & -- & ${-}$0.049 && ${-}$0.085 & ${-}$0.071 & ${-}$0.062 & ${-}$0.084 &${-}$0.065 \\
\cline{2-14}
& & \parbox[t]{1.5cm}{\centering$R_M \times 10^3$ (\%${\cdot}$s$^{-1}{\cdot}\text{\textdegree}$C$^{-1}$)} & 0.6 & 0.2 & 0.4 & 0.3 & 0.7 && 0.4 & 0.41 & 0.38 & 0.42 & 0.37 \\
\hline
\morerows{5}{\parbox[t]{1.2cm}{Passive\newline pyrolysis}} & & \morerows{2}{\emph{T} (\textdegree C)} & 420 & 440 & 500 & 473 & 450 && 455 & 463 & 456 & 415 & 445 \\
&& & -- & -- & -- & -- & -- && -- & -- & -- & -- & -- \\
&& & 700 & 700 & 700 & 700 & 700 && 700 & 700 & 700 & 700 & 700 \\
\cline{3-14}
&& \morerows{2}{\emph{X} (\%)} & 30.5 & 37.5 & 28.3 & 34.7 & 35.1 && 34.5 & 33.4 & 40.2 & 41.5 & 41.4 \\
&& & -- & -- & -- & -- & -- && -- & -- & -- & -- & -- \\
&& & 26.6 & 27.8 & 23.5 & 25.8 & 22.1 && 25 & 21.3 & 24.2 & 22.3 & 20.6
\botline
\end{tabular}
\vspace*{-3pt}
\end{table*}

\subsubsection{Thermogravimetric analysis}\label{mark-3.1.2.}

The {TG} data, $T$ (temperature), X (mass loss), $T_{\mathrm{peak}}$
(peak temperature), $\mathrm{d}X$/$\mathrm{d}t$ (mass loss rate) and RM
(mean reactivity) are shown in Table~\ref{tab3}. The {DTG} curves
(Figure~\ref{fig2}) for biomass can be
divided into two main categories. The first type is the one
characterized by two degradation zones. These zones are discernible by
two peaks indicating that the volatiles are released in two stages
during the pyrolysis. The second type of DTG curve is distinguished by
a single peak indicating that the degradation of the hemicellulose~and
the cellulose occur simultaneously. As for lignin, which is the most
stable component, it always decomposes during the last pyrolysis phase
at a higher temperature.~In the present case and regarding the first
peak observed during the first stage of analysis 
(${<}$150~\textdegree C), the corresponding mass losses can be attributed not only
to the elimination of the residual moisture but also to the release of
the light molecular volatile species. The thermal decomposition of date
pits and orange peels can be dissociated into two stages: the first
step corresponds to the degradation of hemicellulose that starts at 
220~\textdegree C and ends at 330~\textdegree C with
${-}$0.173\%${\cdot}$s$^{-1}$ mass loss rate and between 
155~\textdegree C and 293~\textdegree C~with a mass loss rate of
${-}$0.084\%${\cdot}$s$^{-1}$ respectively. The following stage is
where cellulose decomposes and takes place between 330~\textdegree C and
450~\textdegree C with ${-}$0.049\%${\cdot}$s$^{-1}$ mass loss for date
pits and develops in the temperature range of 
293~\textdegree C--420~\textdegree C with ${-}$0.065\%${\cdot}$s$^{-1}$ loss rate in the case
of orange peels. These findings perfectly agree with results deducted
by several other researchers~\cite{12,18} whose biomass DTG profiles,
as well as the main degradation zones were almost identical to these
investigated samples. Concerning the tea waste decomposition, the first
stage of pyrolysis \mbox{(230--275~\textdegree C)} can be principally tied to
the hemicellulose degradation with a mass loss rate of
${-}$0.015\%${\cdot}$s$^{-1}$. The second phase occurs between 275 and
440~\textdegree C and can be assigned to the cellulose breakdown
(${-}$0.033\%${\cdot}$s$^{-1}$), which is consistent with the findings
of other studies~\cite{22,24}. The same behaviour is observed for
coffee grounds with slightly different zones and comparable mass loss
rates to that of tea waste. A similar result was reported by \mbox{Bejenari}
\etal~\cite{25}. The stages with important mass loss rates are
considered as active pyrolysis stages and radical devolatilization
reactions take place. Since the sample mass loss is negligible beyond
500~\textdegree C, it may be concluded that the thermal decomposition is
accomplished and the solid residue is formed at this temperature.
Unlike the previously cited biomasses, the DTG~curve of~peanut shells
showed one major peak at 350~\textdegree C, indicating that the
decomposition of hemicellulose and cellulose occurred in one single
phase, in agreement with the outcomes elicited by Cai \etal~\cite{26}.
In conclusion, the hemicellulose of these biomasses is more reactive
than the other two previous ones\break (Table~\ref{tab3}).

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}TG and DTG curves of orange based-blends at
10~\textdegree C/min: (a) B-OP, (b) B-OC,(c) B-OTC, \mbox{(d)~B-OD,} (e) B-OT.}
\end{figure*}

Regarding the DTG of the blends (Figure~\ref{fig3}), it was noted that the
orange peels have significantly impacted the degradation process of the
carried blends. The DTG profile of these samples in their pure form was
characterized by a single peak (in the case of peanut shells) or two
non-pronounced-peaks. After blending with orange peels, the DTG
profiles of our investigated biomasses became distinguished by two
visible peaks corresponding to the release of the volatile products
(Table~\ref{tab3}). This will be thereafter confirmed through the pyrolysis
tests, the results of which will be discussed in Section~\ref{sec3.2.1}.

In contrast to the pure orange peel pyrolysis, the solid residues
recovered from the pyrolysis of blends were substantially lower 
(Table~\ref{tab3}). This is likely attributed to the gaseous species emission due to
the thermal degradation of volatile matter and especially that of
cellulose, the degradation of which favours the production of biogas in
terms of quantity and\break quality.

\subsection{Pyrolysis of pure orange peels}

\subsubsection{Yields}\label{sec3.2.1}

After pyrolyzing orange peels at a heating rate of 10~\textdegree C/min,
the yield of the recovered solid residue was 32.05\%. This recorded
amount is in agreement with the previous works which position the yield
of generated biochar between 21\% and 57\%~\cite{4,7,9,10,27}.
Regarding the volatile products (gas $+$ liquid), the recovered gas
reached 39.88\% while the bio-oil yield was 28.07\%. This latter seems
rather low when comparing it to those obtained by other
studies~\cite{7,28,29} where they achieved mass yield values of 35 to
53\% after pyrolysis of citrus residues. However, overall, the
pyrolysis of such biomass under a heating rate of 10~\textdegree C/min
can be seen as a viable alternative, allowing for the simultaneous
valorization of the three types of derivative products (solid, liquid,
and gaseous) with significant amounts.

\subsubsection{Biochar~functional group analysis: FTIR\newline analysis}

The FTIR spectra of dried orange peels and their derived biochar are
presented in Figure~\ref{fig4}. Analysis of the pure biomass spectra indicates
the presence of characteristic bands corresponding to the existence of
the bonds and chemical groups of which they are formed, the
hemicellulose, cellulose and lignin. The largest band in the highly
energetic range (3100--3500 cm$^{-1}$) is allocated to the free and
intramolecular hydroxyl compounds associated with a significant
percentage of OH groups originating from carbohydrates, lignin,
together with both symmetric and asymmetric stretching vibrations
related to H$_2$O molecules~\cite{30}. The particular peak
detected at 2936 cm$^{-1}$ is attributed to the existence of stretching
vibrations C--H accompanied by two peaks around 2100 cm$^{-1}$ and 2300
cm$^{-1}$~\cite{31}, which can be assigned to C${\equiv}$C and
C${\equiv}$N respectively, giving thus the basic structure of these
lignocellulosic components. The band at 1915 cm$^{-1}$ corresponds to
aliphatic and/or unsaturated aromatic constituents~\cite{32}. The peak
observed at 1750 cm$^{-1}$ corresponds to C${=}$O vibrations in carboxylic
acid and/or carbonyl groups of esters, whereas the peak seen at 1632
cm$^{-1}$ is due to C${=}$C vibrations. In addition, S${=}$O elongations
contained in sulfates are distinguished~by bending vibrations at around
1415 cm$^{-1}$. Finally, the intense peak at 1050 cm$^{-1}$ corresponds
to the C--O bond present in primary alcohols and/or esters. Moreover, by
analyzing the spectra of the derived biochar, it can be determined that
the majority of the peaks previously detected in the pure sample appear
again but with less attenuated transmittances; this can be explained by
the rupture of the various covalent bonds that constitute the sample
during pyrolysis. In addition, by converting the pure biomass to its
biochemical form (biochar), the simple bonds are transformed into
aromatic bonds, and this has been well confirmed by the development of
new peaks in the wave zone of [600--1000] that correspond to the C--H
vibrations existing in the aromatic groups~\cite{30}.


\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}FTIR spectra of pure orange peels and their biochars.}
\vspace*{1.5em}
\end{figure*}

\subsection{Pyrolysis of blends}

\subsubsection{Yields}

Even if the goal of the blending was the char investigation in terms of
quantity and quality, the most striking observation concerns the gas
yield. Indeed, almost all blends, except the orange-tea mixture,
resulted in a significant increase in gas yields from 39.88\% to
44.63\%--56.04\% (Figure~\ref{fig5}). The exception of B-OT was ineffective for
boosting gas production with a proportion of 28.20\% but very suitable
for bio-oil generation (40.48\%) via the pyrolysis process.

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}Yields of different products derived from the
pyrolysis of the pure OP and its derivative blends at
10~\textdegree C/min.}
\end{figure*}


Compared to OP, the char yields of the blends did not substantially
change. Indeed, the percentages of biochars obtained from different
blends ranged from 28.66\% to 31.57\%. These values are close to the
values provided by Tariq \etal~\cite{33} when co-pyrolyzing orange
peels with oil palm and empty bunch fruit biomass. They showed that the
resultant char residues generated for both individual and biomass
blends varied from 28.6--36.8\%. However, compared to the char produced
by the pyrolysis of coffee grounds (27.83\%) and date pits (27.78\%),
it is noticed that their mixture with orange peels boosted the char
formation (29.36\%). Moreover, peanut shells as well as tea waste
seemed to have the highest solid residue yields of 31.57\% and 31.32\%,
respectively. It also follows that the other mixtures recorded liquid
yields, which range from 14.72\% to 26.01\%.

Finally, the reported outcomes have well confirmed that the
orange-peanut blend was particularly advantageous in terms of
generating biochar residue as well as enhancing gas yield, as compared
to pure orange peels or even the other carried blends. 

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}FTIR spectra of orange-based blends at
10~\textdegree C/min: (a) B-OP, (b) B-OC, (c) B-OD, (d) B-OT, 
\mbox{(e)~B-OCT}.}
\end{figure*}

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig07}
\vspace*{2pt}
\caption{\label{fig7}SEM images of the different biochars obtained at 10~\textdegree C/min.}
\vspace*{3pt}
\end{figure*}

\subsubsection{Biochar functional group analysis by FTIR}

Figure~\ref{fig6} represents the different FTIR spectra related to biochars
obtained by the various blends. There were strong similarities observed
between the biochars of the various samples performed, but there are
also some differences that can be highlighted. In the undulating area
1900--2400 cm$^{-1}$, several peaks are attributed to the C${\equiv}$C
and C${\equiv}$N stretchings, which correspond to the alkynes and
nitriles respectively. On the other hand, the peaks that are detected
in the range 1680--1760 cm$^{-1}$ are linked to C${=}$O elongation found
within the carbonyl, aldehyde, ester and carboxylic groups. The wide
band that is observed at wavelengths between 1000 and 1600 cm$^{-1}$
for orange-dates, orange-peanut, orange-coffee-tea and orange-tea
mixtures turns into more attenuated peaks in the spectra of the
orange-coffee blend, confirming that the structure of the molecules
\mbox{constituting} the functional~groups of the residual biochars is
distinct. Thus, the peaks detected at around 1500--1600 cm$^{-1}$ may
correspond to the C${=}$C bonds contained in alkenes, as well as~the N--O
stretching present in nitro components. The peaks in the 1200--1400
cm$^{-1}$ undulating zone can be attributed to S${=}$O vibrations
associated with sulfate groups. The largest peaks, visible between 1000
and 1200 cm$^{-1}$, are ascribed to the elongation of C--O bonds in
carboxylic groups, esters, ethers, or alcohols. Several compounds such
as cellulose, hemicellulose and lignin, can explain the presence of
these vibrations. Moreover, the wavelength range between 500 and 1000
cm$^{-1}$ is probably due to the \mbox{C--H} bonds that exist in the aromatic
groups as well as the alkenes. However, the spectra of the various
biochars show significant differences concerning the intensity of the
peaks detected in all the bands defined above. This fact seems quite
logical following the different chemical compositions, which makes the
uniqueness of each blend. This same observation (the attenuation of the
intensity of the peaks measured) is also accurate in relation to the
dissimilarities observed between biochars and their primary samples
(blends before pyrolysis) which do not differ too much from the
spectrum of pure orange peels.


\subsubsection{Biochar morphology characterization by SEM}

Scanning electron microscope (SEM) was used to examine morphological
changes in both pure orange peels and their derivative blends by
visualizing~the upper surface of the different resulting biochars.
Figure~\ref{fig7} displays the SEM images at various magnifications. The samples
noticeably show significant morphological differences between the raw
orange peels and the final combinations. Developing blends results in
partial morphological changes compared to the primary components of the
pure biomass sample. The pure sample's 5000-fold magnified image
depicts an extremely rich exterior surface on which the pores of the
biomass are clearly visible and different forms are observed such as
fluffy sponges, balls and simply small formless particles. These
structures are expected to benefit the water permeation and to
facilitate the uptake of pollutants from aqueous solutions. Before
pyrolysis, the SEM images of the different blends presents a
heterogeneous structure with a significant concentration of organic
matter conglomerates on the surface, which seems relatively smooth with
significant pores. The pyrolysis results in visible differences in
surface structure. Indeed, after pyrolysis, all the biochars develop a
fairly high porosity with a clear presence of spherical pores due to
the loss of moisture and volatile matter. The chars' surfaces appear to
be rougher, with less sharp forms on particle edges, fewer
conglomerated surface forms and thin walls randomly generated and
distributed with the presence of some porosity. This might be
attributed to the disintegration of the contained volatile organic
matter into soluble short sugar molecules. The orange-date waste
exhibits irregular coral-shaped structures accompanied by large amounts
of dense holes. As for the orange-coffee-tea blend char, the structure
is sponge-like, undefined and rough, with numerous pores on the
surface. It is also interesting to observe that the orange-coffee waste
produces smaller pores compared to the other blends. In fact, the
porous structure grows more pronounced in (B-OC), revealing its
viability as catalyst support and adsorbent. It is more likely to
contribute significantly to the improvement of soil quality when
applied as a soil conditioner. However, for the other blends, the
external pores are separated into fibres. This reflects a loss of
cohesion of structural blocks and a partial regeneration of biomass.
This phenomenon is probably due to the release of certain alkaline and
alkaline earth minerals initially present in the biomass, which
guarantees the binding of structural units (cellulose, fibres) using
electrostatic and Van der Waals~interactions~\cite{31}.



\section{Conclusion}

To enable the valorization of agricultural waste, developing
technologies to convert waste into value-added products is critical to
ensure environmental, social and economic sustainability. In this
context, the current study investigated the pyrolysis of single orange
peels but also and more importantly, the co-pyrolysis of orange peels
blended with other problematic agro-wastes, namely date pits, coffee
grounds, peanut shells as well as tea waste. All these biomasses could
be implemented as suitable candidates for the generation of interesting
biochars in terms of quantity and quality. More precisely, Orange
peels-Peanut shells blend (B-OP), as well as Orange Peels-Tea waste
blend (B-OT) were shown to be the optimal choices for generating
biochar with yields of 31.57\% and 31.32\%, respectively. Meanwhile,
the Orange peels-Date pits blend (B-OD), along with the Orange
peels-Coffee grounds blend (B-OC) exhibited the same biochar rate of
29.36\%. Even though this last rate is less than that of (B-OT) and
(B-OP), it should be noted that this is superior to the char outputs of
the sole coffee grounds (27.83\%) or the sole date pits (27.78\%)
before their blending with orange peels, indicating the positive effect
of the addition of this last biomass on the enhancement of their 
char\break yields. 

Interestingly, all the applied combinations (except B-OT) led also to
an improvement in the biogas production. For example, the biogas yield
increased by about 40\%, from 39.88\% w/w for pure orange peels to
56.04\% w/w for the Orange-Peanut blend (B-OP).

FTIR and SEM analyses showed that biochars from orange-based blends
exhibited a relatively high porous area, especially for the
orange-coffee mixture, proving its capacity to serve as a catalytic
support. It can also contribute effectively to the enhancement of soil
quality when applied as a soil conditioner. Until now, the lack of
information regarding pilot-scale systems and industrial transfer
represents one of the limitations of this study. Future studies would
help providing the proper solutions by using large devices to confirm
these promising results through testing more heating rates. These
findings will also be completed by future works on biogas generation to
ensure a sustainable blended orange by-products waste management and to
guarantee a circular economy within energy reliability for the
agri-food industry.

\section*{Conflicts of interest}

Authors have no conflict of interest to declare.

\section*{Acknowledgment}

We gratefully thank Mohamed Ouerhani for his assistance in carrying out
the experimental tests.

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