I heated cotton in a sealed container (with a small hole) over a natural gas flame. Some gases and smoke were produced. What would they probably be? I can come up with some guesses based on the composition of cellulose: $\ce{CO2}$, $\ce{CH4}$ or possibly other hydrocarbons, $\ce{CO}$, $\ce{H2}$, $\ce{H2O}$, however I do not know which of those they are. Obviously, soot ($\ce{C}$) was also formed, due to the visible smoke particles.


1 Answer 1


During pyrolysis, organic compounds are thermally decomposed in the absence of oxygen. The pyrolysis products are classified into categories based on their physical state of existence: char (solid), bio-oil (liquid) and non-condensable gases (gas). The relative proportions of these three product fractions significantly vary depending upon the process conditions, as is shown in the table below.

$$ \small \begin{array}{lcc} \hline \text{Pyrolosis Technology} & \text{Residence Time} & \text{Heating Rate} & \text{Temperature} & \text{Char} & \text{Bio-Oil} & \text{Gases} \\ \hline \text{Conventional} & \text{5-30}\ \mathrm{min} & \text{<50} ^\circ \mathrm{C\ min^{-1}} & \text{400-600} ^\circ \mathrm{C} & \text{<35%} & \text{<30%} & \text{<40%}\\ \text{Fast Pyrolysis} & \text{<5}\ \mathrm{s} & \text{~1000} ^\circ \mathrm{C\ s^{-1}} & \text{400-600} ^\circ \mathrm{C} & \text{<25%} & \text{<75%} & \text{<20%}\\ \text{Flash Pyrolosis} & \text{<0.1}\ \mathrm{s} & \text{~1000} ^\circ \mathrm{C\ s^{-1}} & \text{650-900} ^\circ \mathrm{C} & \text{<20%} & \text{<20%} & \text{<70%}^{[1]}\\ \hline \end{array} $$

The exact compositions of the products of cellulose pyrolysis at different temperatures can be seen below.

$$ \small \begin{array}{lcc} \hline \text{Products} & \text{Peak Temp,}\ 500 ^\circ \mathrm{C} & \text{Holding Temp,}\ 400 ^\circ \mathrm{C} & \text{Peak Temp,}\ 750 ^\circ \mathrm{C} & \text{Peak Temp,}\ 1000 ^\circ \mathrm{C}\\ \hline \ce{CO} & \text{0.99%} & \text{0.25%} & \text{15.82%} & \text{22.57%}\\ \ce{CO2} & \text{0.3%} & \text{1.45%} & \text{2.38%} & \text{3.36%}\\ \ce{H2O} & \text{3.55%} & \text{6.49%} & \text{8.72%} & \text{9.22%}\\ \ce{CH4} & \text{0%} & \text{0%} & \text{1.11%} & \text{2.62%}\\ \ce{C2H4} & \text{0%} & \text{0%} & \text{1.05%} & \text{2.18%}\\ \ce{C2H6} & \text{0%} & \text{0%} & \text{0.17%} & \text{0.28%}\\ \ce{C3H6} & \text{0%} & \text{0%} & \text{0.70%} & \text{0.80%}\\ \ce{H2} & \text{0%} & \text{0%} & \text{0.36%} & \text{1.18%}\\ \ce{CH3OH} & \text{0.25%} & \text{0.21%} & \text{1.03%} & \text{0.98%}\\ \ce{CH3CHO} & \text{0.01%} & \text{0.05%} & \text{1.58%} & \text{1.7%}\\ \text{tar} & \text{16.37%} & \text{83.35%} & \text{59.92%} & \text{49.12%}\\ \text{char} & \text{83.63%} & \text{6.17%} & \text{3.32%} & \text{3.91%}\\ \text{other} & \text{0.19%} & \text{0.16%} & \text{2.14%} & \text{1.78%}\\ \text{total} & \text{105.25%} & \text{98.36%} & \text{98.8%} & \text{99.86%}\\ \hline \end{array} $$

The holding time for each of these reactions was $30\ \mathrm{s}^{[2]}$. As shown in the table, $\ce{CO}$, $\ce{H2O}$, and $\ce{CO2}$ are the major gaseous products, with $\ce{H2}$ and hydrocarbons being produced in considerably smaller proportion.

$^{[1]}$ Patwardhan, Pushkaraj Ramchandra, "Understanding the product distribution from biomass fast pyrolysis" (2010). Graduate Theses and Dissertations. Paper 11767.
$^{[2]}$ Hajaligol, M. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Product Compositions and Kinetics for Rapid Pyrolysis of Cellulose. Industrial & Engineering Chemistry Process Design and Development Ind. Eng. Chem. Proc. Des. Dev. 1982, 21, 457–465.

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    $\begingroup$ Impressive. Do you make the tables by hand or is there some tool for making them? $\endgroup$ May 9, 2016 at 7:31
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    $\begingroup$ @CopperKettle I did have to enter the data manually, but the formatting was taken from this meta post. Digging around the sandboxes can yield some really neat $\LaTeX$ tools :) $\endgroup$
    – ringo
    May 9, 2016 at 8:02
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    $\begingroup$ As usual, the most interesting part (nearly everything that gives color and smell) is also the hardest to analyze, so it is just swept under the rug and called "tar". $\endgroup$ May 9, 2016 at 8:11
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    $\begingroup$ @IvanNeretin On page 30 of the first source, there is a detailed analysis of exactly what this tar is. This question isn't about tar though, so I didn't mention it. $\endgroup$
    – ringo
    May 9, 2016 at 8:14
  • 1
    $\begingroup$ Wow. Now that's very impressive indeed. $\endgroup$ May 9, 2016 at 8:19

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