FDS User Manual: Vegetation pyrolyzate MW

Author: mcgratta

Manuals/Bibliography/FDS_general.bib

@@ -6421,6 +6421,15 @@ @ARTICLE{Trelles:JFPE2010
   year         = {2010}
 }
 
+@INPROCEEDINGS{Tripi:INTERFLAM2025,
+  author       = {A. Tripi and R. Greene and I. Leventon and K. McGrattan},
+  title        = {{Measurement of the Average Molecular Weights of Gaseous Pyrolyzates Produced by Combustible Solids}},
+  booktitle    = {Proceedings of the 16th International Conference and Exhibition on Fire Science and Engineering (Interflam~2025)},
+  month        = {June},
+  year         = {2025},
+  publisher    = {Interscience Communications Ltd.}
+}
+
 @TECHREPORT{Troup:1,
   author       = {Troup, J.M.A},
   title        = {{Large-Scale Fire Tests of Rack Stored Group A Plastics in

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -7340,12 +7340,19 @@ \subsection{Solid Phase}
 
 \FloatBarrier
 
-\subsection{Gas Phase}
+\subsection{Gas Phase Chemistry}
 \label{veg_pyrolysis_gas_phase}
 
-To estimate the heat of combustion of the gas phase reaction of fuel vapors generated by the pyrolysis of vegetation, the decomposition reaction given in Eqs.~(\ref{pyr_reac}) and (\ref{char_reaction}) is written as an equivalent set of reactions assuming that the Dry Vegetation is given by the effective formula C$_{\rm x}$H$_{\rm y}$O$_{\rm z}$A, where A represents the inorganic components that eventually form the Ash.
+The solid phase decomposition reactions given in Eqs.~(\ref{pyr_reac}) and (\ref{char_reaction}) can be written in terms of effective solid and gaseous molecules under the following assumptions:
+\begin{enumerate}
+\item The Dry Vegetation has the effective formula C$_{\rm x}$H$_{\rm y}$O$_{\rm z}$A, where A represents the inorganic components that eventually form the Ash.
+\item The char yield, $\nu_{\rm char}$, is taken as the fraction of the dry mass that remains after complete anaerobic pyrolysis.
+\item The ash yield, $\nu_{\rm ash}$, is taken as the fraction of the char mass that remains after complete oxidation.
+\item The mass of oxygen required to oxidize a unit mass of char, $\nu_{\rm O_2,char}$, as found in Eq.~(\ref{char_reaction}), is known. This parameter indirectly establishes the relative amount of carbon and oxygen in the char.
+\item The fuel gas, or {\em pyrolyzate}, is taken as a single composite gas species with an effective molecular weight, $W_{\rm pyr}$. Preliminary measurments~\cite{Tripi:INTERFLAM2025} suggest that the effective molecular weight of wood pyrolyzate is approximately 25~g/mol. 
+\end{enumerate}
 \begin{eqnarray}
-   \underbrace{\mathrm{C_xH_yO_zA \; (s)}}_{\rm Dry\ Vegetation} &\rightarrow& \underbrace{\mathrm{C_{x'}O_{z'}A \; (s)}}_{\rm Char} \; + \; \underbrace{\mathrm{C_{x-x'}H_yO_{z-z'} \; (g)}}_{\rm Fuel\ Gas} \\
+   \underbrace{\mathrm{C_xH_yO_zA \; (s)}}_{\rm Dry\ Vegetation} &\rightarrow& \underbrace{\mathrm{C_{x'}O_{z'}A \; (s)}}_{\rm Char} \; + \; \underbrace{\nu_{\rm pyr} \, \mathrm{C_{x''}H_{y''}O_{z''} \; (g)}}_{\rm Pyrolyzate} \\[.1in]
    \underbrace{\mathrm{C_{x'}O_{z'}A \; (s)}}_{\rm Char} \; + \; \underbrace{\mathrm{\nu_{\rm O_2} O_2 \; (g)}}_{\rm Oxygen} &\rightarrow& \underbrace{\mathrm{\nu_{\rm CO_2} \, CO_2 \; (g)}}_{\rm Carbon\ Dioxide} \; + \; \underbrace{\mathrm{A \; (s)}}_{\rm Ash} \label{char_chemistry}
 \end{eqnarray}
 \be
@@ -7355,16 +7362,22 @@ \subsection{Gas Phase}
 \ee
 \be
    {\rm x}' = \nu_{\rm CO_2}  \quad ; \quad
-   {\rm z}' = 2 \left(\nu_{\rm CO_2} - \nu_{\rm O_2} \right)
+   {\rm z}' = 2 \left(\nu_{\rm CO_2} - \nu_{\rm O_2} \right) \quad ; \quad
+   \nu_{\rm pyr} = \frac{ 12 ({\rm x-x'}) + {\rm y} + 16 ({\rm z-z'})}{W_{\rm pyr} }
+\ee
+\be
+   {\rm x}'' = ({\rm x-x'})/\nu_{\rm pyr}  \quad ; \quad
+   {\rm y}'' = {\rm y}/\nu_{\rm pyr}  \quad ; \quad
+   {\rm z}'' = ({\rm z-z'})/\nu_{\rm pyr} 
 \ee
 The ideal one step gas phase reaction of the Fuel Gas is as follows:
 \be
-   {\rm C}_{{\rm x}-{\rm x}'}{\rm H}_{\rm y}{\rm O}_{{\rm z}-{\rm z}'} + \nu_{\rm O_2}' \, {\rm O}_2 \rightarrow ({\rm x}-{\rm x}') \, {\rm CO}_2 + \frac{{\rm y}}{2} \, {\rm H_2O}  \quad : \quad
-   \nu_{\rm O_2}' = \frac{2({\rm x}-{\rm x}')+{\rm y}/2-({\rm z}-{\rm z}')}{2}
+   {\rm C}_{\rm x''}{\rm H}_{\rm y''}{\rm O}_{\rm z''} + \nu_{\rm O_2}' \, {\rm O}_2 \rightarrow {\rm x''} \, {\rm CO}_2 + \frac{{\rm y''}}{2} \, {\rm H_2O}  \quad : \quad
+   \nu_{\rm O_2}' = \frac{2{\rm x''} + {\rm y''}/2-{\rm z''}}{2}
 \ee
 The heat of combustion of the gas phase reaction (heat release per unit mass fuel gas consumed) can be estimated from the heat release per unit mass of oxygen consumed, $E=13.98$~MJ/kg, measured by Tihay et al.~\cite{Tihay:CF2014}:
 \be
-   \Delta h_{\rm c} \approx  \frac{W_{\rm O_2} \,  \nu_{\rm O_2}'}{12({\rm x}-{\rm x}')+{\rm y}+16({\rm z}-{\rm z}')} E   \label{Delta_h_pyr}
+   \Delta h_{\rm c} \approx  \frac{W_{\rm O_2} \, \nu_{\rm O_2}'}{W_{\rm pyr} \, \nu_{\rm pyr}} E   \label{Delta_h_pyr}
 \ee
 The total heat release rate of burning vegetation is the sum of both the gas phase combustion of pyrolyzed plant matter and the solid phase exothermic char oxidation reaction. The effective heat of combustion, $\Delta h_{\rm c,eff}$, is a weighted average of the two reactions. The heat of combustion of the pyrolyzed plant matter, $\Delta h_{\rm c}$, is approximately $17400$~kJ/kg according to Eq.~(\ref{Delta_h_pyr}). The heat of reaction for the char oxidation, $\Delta h_{\rm char}$ is approximately -$25000$~kJ/kg (the minus sign in this instance refers to an {\em exothermic} solid phase reaction). The effective heat of combustion is found from:
 \be