Table: problem with multicolumn

Question

I am writing a paper and I have the following issue:

I would like to observe the title "ultimate analysis" centered and above the columns that contain the values of C, H, N, S and O.

Similar for the title "proximate analysis", centered and above the columns which contain the values of VM, FC and Ash.

\documentclass[preprint,12pt]{elsarticle}
        % !TeX spellcheck = en_US 
        \usepackage{multirow} % para las tablas
        \usepackage{siunitx}% paquete para manejo de números y unidades
        \sisetup{detect-weight, detect-display-math}
        \sisetup{detect-inline-weight=math}
        \sisetup{mode=text,per-mode=symbol}% modo de unidades con barra en lugar de fracción de tamaño reducido
        \usepackage{booktabs}% para tablas
        \usepackage{graphicx}% para gráficos
        \usepackage{pgfplots}% para gráficos
        \usepackage{pdflscape}% para gráficos
        \pgfplotsset{compat=newest}% para gráficos
        \usetikzlibrary{plotmarks}% para gráficos
        \usetikzlibrary{arrows.meta}% para gráficos
        \usepgfplotslibrary{patchplots}% para gráficos
        \newlength\fwidth% definido en el código de Matlab
        \setlength{\fwidth}{0.8\textwidth}% defino valor de parámetro fwidth de gráficos
        \usepackage{tikzscale}% para gráficos
        \pgfplotsset{plot coordinates/math parser=false}% para gráficos
        \usepackage{overpic}% figuras
        \usepackage{tikz}% figuras
        \pgfplotsset{compat=1.15}
        \usepackage{float}
        \usepackage[margin=2.5cm]{geometry}% setea márgenes de hoja
        \usepackage{bicaption}
        \usepackage{makecell}
        \renewcommand\theadfont{\bfseries}
        \renewcommand\theadgape{}
        \usepackage[skip=1ex, labelfont=bf]{caption}
        \usepackage[skip=0.333\baselineskip]{caption}
        \usepackage{amssymb,amsmath}
        \usepackage{tabularx}
        \usepackage{url}
        \def\UrlFont{\normalfont}
        \newcolumntype{C}{>{\centering\arraybackslash}X}
        \setlength{\fwidth}{0.8\textwidth}
        \biboptions{sort&compress}
        \bibliographystyle{unsrt}
        \usepackage{subcaption}

        \journal{Applied Energy}

    \begin{document}
        \begin{table} [H]
            \setlength\tabcolsep{4pt}
            \caption{Characteristics of OS and biomass samples.}
            \label{Chemical properties of OS and the blends}
            \begin{tabularx}{\textwidth}{@{}l *{8}{C} @{}}
                \toprule
                Material
                & \multicolumn{4}{>{\hsize=\dimexpr4\hsize+8\tabcolsep\relax}C}{\textbf{Ultimate analysis}} 
                & \multicolumn{4}{>{\hsize=\dimexpr4\hsize+8\tabcolsep\relax}C}{\textbf{Proximate analysis}}\\ 
                & \multicolumn{4}{c}{(wt. \%)}
                & \multicolumn{4}{c}{(wt. \%)}\\
                \cmidrule(lr){2-6} \cmidrule(lr){7-9}
                & \textbf{C} &\textbf{ H} & \textbf{N} & \textbf{S}& \textbf{O}$^\gamma$ & \textbf{VM} & \textbf{FC} $^{\varphi}$ & \textbf{Ash}\\
                \midrule
                \textbf{OS} &9.3 &2.3 &0.2 &4.0 & 11.6 &24.0 &3.4 &72.6 \\
                \textbf{GP}&48.8 &6.0 &1.9 &0.2& 35.7 &72.7 &19.9 &7.4 \\
                \textbf{RH}&35.3 &4.7 &0.3 &0.1& 37.8 &64.4 & 12.8 &21.8 \\
                \textbf{EG}& 48.2&5.9 & $<$D.L&$<$D.L&45.7 &88.5 &11.3 &0.24\\
                \bottomrule
            \end{tabularx}
            \smallskip\footnotesize
            VM: volatile matter, FC: Fixed carbon, D.L: detention limit. All values are expressed in dry basis.\\ 
            $\varphi$: Determined by difference: $\text{FC}=100\%-\text{VM}\%-\text{Ash}\%$.\\
            All reported values are given with an error of 1\% of the value.\\
            $^\gamma$: Determined by difference: \%O = 100\% - C\% - H\% - N\% - S\% - Ash\%.\\
        \end{table}

\end{document}

Answer 1

The issue with your code is that you want to center 5 and 3 columns and not as you did in your code 4 and 4 columns:

            & \multicolumn{5}{c}{\textbf{Ultimate analysis}} % <========
            & \multicolumn{3}{c@{}}{\textbf{Proximate analysis}}\\ % <=====
            & \multicolumn{5}{c}{(wt. \%)} % <==========================
            & \multicolumn{3}{c@{}}{(wt. \%)}\\ % <========================

The complete corrected MWE

\documentclass[preprint,12pt]{elsarticle}
    % !TeX spellcheck = en_US 
    \usepackage{multirow} % para las tablas
    \usepackage{siunitx}% paquete para manejo de números y unidades
    \sisetup{detect-weight, detect-display-math}
    \sisetup{detect-inline-weight=math}
    \sisetup{mode=text,per-mode=symbol}% modo de unidades con barra en lugar de fracción de tamaño reducido
    \usepackage{booktabs}% para tablas
    \usepackage{graphicx}% para gráficos
    \usepackage{pgfplots}% para gráficos
    \usepackage{pdflscape}% para gráficos
    \pgfplotsset{compat=newest}% para gráficos
    \usetikzlibrary{plotmarks}% para gráficos
    \usetikzlibrary{arrows.meta}% para gráficos
    \usepgfplotslibrary{patchplots}% para gráficos
    \newlength\fwidth% definido en el código de Matlab
    \setlength{\fwidth}{0.8\textwidth}% defino valor de parámetro fwidth de gráficos
    \usepackage{tikzscale}% para gráficos
    \pgfplotsset{plot coordinates/math parser=false}% para gráficos
    \usepackage{overpic}% figuras
    \usepackage{tikz}% figuras
    \pgfplotsset{compat=1.15}
    \usepackage{float}
    \usepackage[margin=2.5cm]{geometry}% setea márgenes de hoja
    \usepackage{bicaption}
    \usepackage{makecell}
    \renewcommand\theadfont{\bfseries}
    \renewcommand\theadgape{}
    \usepackage[skip=1ex, labelfont=bf]{caption}
    \usepackage[skip=0.333\baselineskip]{caption}
    \usepackage{amssymb,amsmath}
    \usepackage{tabularx}
    \usepackage{url}
    \def\UrlFont{\normalfont}
    \newcolumntype{C}{>{\centering\arraybackslash}X}
    \setlength{\fwidth}{0.8\textwidth}
    \biboptions{sort&compress}
    \bibliographystyle{unsrt}
    \usepackage{subcaption}

    \journal{Applied Energy}

\begin{document}
\begin{table} [H]
\setlength\tabcolsep{4pt}
\caption{Characteristics of OS and biomass samples.}
\label{Chemical properties of OS and the blends}
\begin{tabularx}{\textwidth}{@{}l *{8}{C} @{}}
            \toprule
            Material
            & \multicolumn{5}{c}{\textbf{Ultimate analysis}} % <========
            & \multicolumn{3}{c@{}}{\textbf{Proximate analysis}}\\ % <=====
            & \multicolumn{5}{c}{(wt. \%)} % <==========================
            & \multicolumn{3}{c@{}}{(wt. \%)}\\ % <========================
            \cmidrule(lr){2-6} \cmidrule(lr){7-9}
            & \textbf{C} &\textbf{ H} & \textbf{N} & \textbf{S}& \textbf{O}$^\gamma$ & \textbf{VM} & \textbf{FC} $^{\varphi}$ & \textbf{Ash}\\
            \midrule
            \textbf{OS} &9.3 &2.3 &0.2 &4.0 & 11.6 &24.0 &3.4 &72.6 \\
            \textbf{GP}&48.8 &6.0 &1.9 &0.2& 35.7 &72.7 &19.9 &7.4 \\
            \textbf{RH}&35.3 &4.7 &0.3 &0.1& 37.8 &64.4 & 12.8 &21.8 \\
            \textbf{EG}& 48.2&5.9 & $<$D.L&$<$D.L&45.7 &88.5 &11.3 &0.24\\
            \bottomrule
        \end{tabularx}
        \smallskip\footnotesize
        VM: volatile matter, FC: Fixed carbon, D.L: detention limit. All values are expressed in dry basis.\\ 
        $\varphi$: Determined by difference: $\text{FC}=100\%-\text{VM}\%-\text{Ash}\%$.\\
        All reported values are given with an error of 1\% of the value.\\
        $^\gamma$: Determined by difference: \%O = 100\% - C\% - H\% - N\% - S\% - Ash\%.\\
\end{table}

and the result:

Answer 2

Since the headers are supposed to span 5 and 3 columns, respectively, and since no line breaks should be allowed in the header cells, you should replace

& \multicolumn{4}{>{\hsize=\dimexpr4\hsize+8\tabcolsep\relax}C}{\textbf{Ultimate analysis}} 
& \multicolumn{4}{>{\hsize=\dimexpr4\hsize+8\tabcolsep\relax}C}{\textbf{Proximate analysis}}\\ 
& \multicolumn{4}{c}{(wt. \%)}
& \multicolumn{4}{c}{(wt. \%)}\\

with

& \multicolumn{5}{c}{\textbf{Ultimate analysis}}
& \multicolumn{3}{c@{}}{\textbf{Proximate analysis}}\\
& \multicolumn{5}{c}{(wt. \%)}
& \multicolumn{3}{c@{}}{(wt. \%)}\\

If, on the other hand, you believe that the automatic line-breaking ability should be maintained for the header cells, you should employ the following code for the four lines in question:

& \multicolumn{5}{>{\hsize=\dimexpr5\hsize+8\tabcolsep\relax}C}{\textbf{Ultimate analysis}}
& \multicolumn{3}{>{\hsize=\dimexpr3\hsize+4\tabcolsep\relax}C@{}}{\textbf{Proximate analysis}}\\
& \multicolumn{5}{c}{(wt. \%)}
& \multicolumn{3}{c@{}}{(wt. \%)}\\

Either way, it's necessary to add @{} particles in the column definitions, as the overall tabularx specification suppresses whitespace along the right-hand edge of the table.

---

---

Addendum - If this were my table, I would not employ a tabularx environment, as there is neither a need nor a justification for allowing automatic line breaks in any of the cells. Instead, I would use a tabular* environment, with width set to \textwidth. I would also provide more structure to the header, and I'd get rid of all the bold-facing to avoid creating the impression of shouting at my readers. Importantly, I would employ the S column type (provided by the siunitx package) for all 8 data columns, to allow alignment of the numbers on their respective decimal markers.

\documentclass[preprint,12pt,numbers]{elsarticle}
% !TeX spellcheck = en_US
\journal{Applied Energy}

%% Commented out instructions of no relevance to the table at hand. 
%\usepackage{multirow}

\usepackage{siunitx}
\sisetup{detect-weight,
         detect-display-math,
         detect-inline-weight=math,
         mode=text,
         per-mode=symbol}
\usepackage{booktabs}
\usepackage{graphicx}

%\usepackage{pdflscape}
%\usepackage{pgfplots}
%\pgfplotsset{compat=newest}
%\usetikzlibrary{plotmarks}
%\usetikzlibrary{arrows.meta}
%\usepgfplotslibrary{patchplots}
%\usepackage{tikzscale}
%\pgfplotsset{plot coordinates/math parser=false}
%\usepackage{overpic}
%\usepackage{tikz}
%\pgfplotsset{compat=1.15}
%\newlength\fwidth
%\setlength{\fwidth}{0.8\textwidth}

\usepackage{float}
\usepackage[margin=2.5cm]{geometry}

%\usepackage{makecell}
%\renewcommand\theadfont{\bfseries}
%\renewcommand\theadgape{}

\usepackage{caption,subcaption}
\captionsetup{skip=1ex,labelfont=bf}
\usepackage{amssymb,amsmath,tabularx,url}
\def\UrlFont{\normalfont}

\biboptions{sort&compress}
\bibliographystyle{elsarticle-num} % or "unsrtnat"

\newcommand\mc[1]{\multicolumn{1}{c}{#1}} % handy shortcut macro

\begin{document}

\begin{table}[H]
\setlength\tabcolsep{0pt}  %
\caption{Characteristics of OS and biomass samples.}
\label{tab:characteristics}
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}}
             l
             S[table-format=2.1]
        *{3}{S[table-format=1.1]}
        *{3}{S[table-format=2.1]}
             S[table-format=2.2] }
\toprule
Material & \multicolumn{8}{c}{Type of Analysis}\\
\cmidrule{2-9}
& \multicolumn{5}{c}{Ultimate}
& \multicolumn{3}{c}{Proximate}\\
& \multicolumn{5}{c}{(wt.\ \%)}
& \multicolumn{3}{c}{(wt.\ \%)}\\
\cmidrule{2-6} \cmidrule{7-9}
& \mc{C} & \mc{H} & \mc{N} & \mc{S} & \mc{O$^\gamma$} 
& \mc{VM} & \mc{FC$^{\varphi}$} & \mc{Ash}\\
\midrule
OS & 9.3 &2.3 &0.2 &4.0 &11.6 &24.0 &3.4  &72.6 \\
GP &48.8 &6.0 &1.9 &0.2 &35.7 &72.7 &19.9 & 7.4 \\
RH &35.3 &4.7 &0.3 &0.1 &37.8 &64.4 &12.8 &21.8 \\
EG & 48.2&5.9 &{$<$D.L} &{$<$D.L}&45.7 &88.5 &11.3 &0.24\\
\bottomrule
\end{tabular*}

\medskip
\footnotesize
VM: volatile matter, FC: Fixed carbon, D.L: detention limit. All values are expressed in dry basis.

All reported values are given with an error of 1\% of the value.

$^\gamma$: Determined by difference: \%O = 100\% $-$ C\% $-$ H\% $-$ N\% $-$ S\% $-$ Ash\%.

$^\varphi$: Determined by difference: FC = 100\% $-$ VM\% $-$ Ash\%.

\end{table}

\end{document}