# Gases produced by pyrolysis of cellulose

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.

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.

• Impressive. Do you make the tables by hand or is there some tool for making them? – CowperKettle May 9 '16 at 7:31
• @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 :) – ringo May 9 '16 at 8:02
• 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". – Ivan Neretin May 9 '16 at 8:11
• @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. – ringo May 9 '16 at 8:14
• Wow. Now that's very impressive indeed. – Ivan Neretin May 9 '16 at 8:19