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\title{\vspace{-1.0cm}Optical Chemiluminescence Diagnostics for Syngas Composition and Combustion State}
\author{Robin Clark}
\date{}

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\section*{Abstract}

This note proposes a non-intrusive optical method for assessing syngas composition and combustion quality using flame chemiluminescence. 
By observing emission from excited radicals (OH*, CH*, and C$_2$), it is possible to infer air--fuel ratio, combustion stability, and the presence of carbon-rich species. 
The approach is intended as a fast, low-cost diagnostic layer rather than a replacement for conventional gas analysis. The concept is well suited to structured investigation as a PhD topic.

\section*{1. Motivation}

Syngas composition varies significantly depending on feedstock and gasifier conditions, typically comprising mixtures of H$_2$, CO, CH$_4$, CO$_2$, and N$_2$.
 Conventional measurement techniques (e.g. NDIR, TCD, lambda probes) are often:

\begin{itemize}
\item intrusive or require gas sampling,
\item relatively slow,
\item costly in industrial environments.
\end{itemize}

There is therefore value in a real-time, in-situ diagnostic method based on combustion behaviour.

\section*{2. Principle}

During combustion, short-lived excited radicals emit light at characteristic wavelengths:

\begin{center}
\begin{tabular}{l l l}
\textbf{Species} & \textbf{Wavelength} & \textbf{Interpretation} \\
\hline
OH* & $\sim$310 nm & Oxidation zone / flame front \\
CH* & $\sim$430 nm & Hydrocarbon breakdown \\
C$_2$ (Swan bands) & $\sim$516 nm & C--C chemistry / soot precursors\\
\end{tabular}
\end{center}

These emissions arise from reaction~kinetics and flame~chemistry rather than bulk temperature alone, making them sensitive to both mixture and fuel composition.

\section*{3. Core Measurement Concept}

The approach is based on measuring intensity ratios:

\begin{align}
R_1 &= \frac{\text{OH}^*}{\text{CH}^*} \quad \text{(air--fuel ratio)} \\
R_2 &= \frac{\text{C}_2^*}{\text{CH}^*} \quad \text{(hydrocarbon richness)} \\
R_3 &= \frac{\text{C}_2^*}{\text{OH}^*} \quad \text{(soot tendency)}
\end{align}

In addition, temporal behaviour provides diagnostic information:

\begin{itemize}
\item Standard deviation of OH* intensity $\rightarrow$ flame stability
\item Cross-correlation between bands $\rightarrow$ regime transitions
\end{itemize}

\section*{4. Role of C$_2$ Chemiluminescence}

The inclusion of C$_2$ (Swan bands) is key to extending the method beyond conventional OH*/CH* sensing.~\footnote{C$_2$ chemiluminescence (Swan bands, $\sim$516~nm) is typically observed in fuel-rich or locally oxygen-limited regions of hydrocarbon flames. In these conditions, oxidation of carbon fragments is inhibited and radical recombination pathways dominate, leading to formation of C$_2$ via reactions such as C + CH $\rightarrow$ C$_2$ + H. Excited C$_2^*$ species emit banded radiation as they relax, producing the characteristic green Swan bands. The presence of C$_2$ is therefore indicative of carbon--carbon bond formation and is closely associated with the early stages of soot precursor development.} 

\subsection*{4.1 Physical Significance}

C$_2$ emission is associated with:

\begin{itemize}
\item presence of C--C bonds,
\item locally fuel-rich regions,
\item formation of soot precursors.
\end{itemize}

\subsection*{4.2 Diagnostic Value}

\begin{center}
\begin{tabular}{l c c c l}
\textbf{Condition} & OH & CH & C$_2$ & \textbf{Interpretation} \\
\hline
H$_2$-rich gas & High & Low & $\approx$0 & Clean combustion \\
CO/H$_2$ mix & Moderate & Low & $\approx$0 & Typical syngas \\
CH$_4$ present & Moderate & High & Low--mod & Methane content \\
Heavy HC / tar & Lower & High & High & Soot risk / contamination \\
\end{tabular}
\end{center}

Thus, C$_2$ provides sensitivity to carbon chemistry and enables discrimination between different syngas compositions.

\section*{5. Additional Spectral Features}

Other emissions of potential interest include:

\begin{itemize}
\item CN bands ($\sim$388 nm): nitrogen-containing species
\item Na/K lines ($\sim$589 nm): contaminants or ash
\item Continuum emission: soot radiation and incomplete combustion
\end{itemize}

A multi-band or spectrally resolved approach may allow further discrimination using statistical or machine learning techniques.

\section*{6. Implementation}

A practical system could consist of:

\begin{itemize}
\item Photodiodes with narrow bandpass filters (310 nm, 430 nm, 516 nm)
\item Transimpedance amplifiers (TIA front-end)
\item ADC and embedded processing (e.g. STM32 class device)
\end{itemize}

Signal processing would include averaging, ratio calculation, and temporal analysis.

\section*{7. Applications}

Potential applications include:

\begin{itemize}
\item Gasifier monitoring
\item Industrial burner optimisation
\item Detection of:
  \begin{itemize}
  \item flame instability,
  \item soot formation,
  \item poor mixing or fuel variation,
 \item inference of calorific value (subject to calibration against gas composition)
  \end{itemize}
\end{itemize}

\section*{8. Research Opportunity}

Key open questions suitable for PhD investigation include:

\begin{itemize}
\item Calibration of optical signals against known gas compositions
\item Sensitivity to temperature and pressure variations
\item Robustness under optical fouling
\item Extension to spectrally resolved measurement
\end{itemize}

\section*{9. Conclusion}

Multi-band chemiluminescence sensing offers a promising route to fast, non-intrusive diagnostics for syngas combustion. 
The addition of C$_2$ emission provides a potentially valuable link to fuel composition, 
extending the method beyond simple air--fuel ratio measurement. 
The concept warrants structured experimental validation and is well suited to academic--industrial collaboration.

\section*{10. Proposed Work (Outline)}

\begin{itemize}
\item Controlled combustion experiments with known gas mixtures
\item Multi-band optical measurement and calibration
\item Development of regression models for composition and calorific value
\item Validation under real-world conditions (optical fouling, turbulence)
\end{itemize}

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