Kinetics of Gas Evolution During Thermal Decomposition of Peat
Abstract Generated abstract
The study examines gas evolution during the thermal decomposition of two Western Siberian peats, a sedge lowland peat and a complex raised peat, under inert conditions with linear heating and chromatographic analysis of gaseous products. It compares the temperature-dependent formation of CO2, CO, CH4, and H2, relating differences in product yields and rates to botanical and chemical composition. The results indicate that both peat types have similar patterns of thermal destruction and comparable temperature regions for gas formation, but differ in intensity and product distribution: raised peat yields more gaseous products and oxygen-containing gases, while lowland peat yields more solid residue, methane, and hydrogen. Kinetic analysis gives broadly similar activation energies for both peats, with first-order formation of oxygen-containing gases and higher apparent orders for methane and hydrogen formation.
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Kinetics of Gas Evolution during the Thermal Decomposition of Peat
S. I. Smolyaninov, Ya. A. Belikhmaer, V. M. Ikrin
(Presented by the scientific-methodological seminar of the organic departments of the Faculty of Chemical Technology)
The kinetics of gas evolution was studied on a derivatographic apparatus supplemented with chromatographic analysis of gaseous products under a linear temperature rise \((5^\circ\mathrm{C}/\mathrm{min})\). The medium was inert. The rate of withdrawal of gaseous products was \(25\ \mathrm{ml}/\mathrm{min}\), and the periodicity of sampling gas for analysis was 4 min.
The objects of study were two typical peats of Western Siberia: sedge lowland peat and complex raised peat. The characteristics of the objects of study are presented in Tables 1 and 2.
Table 1
Botanical composition and technical-analysis data, %
| Sample code | Botanical composition, % | Type of peat | Degree of decomposition | \(W^a\) | \(A^c\) | \(V^r\) |
|---|---|---|---|---|---|---|
| T | Sedge 70 Grass residues 15 Bogbean 10 Horsetail 5 |
Sedge lowland | 35 | 9,3 | 10,9 | 68,4 |
| B | Sph. angustifolium 30 Sph. polustria 25 Sph. cuspidatum 15 Cottongrass 20 Unidentified residues 15 |
Complex raised | 15 | 10,7 | 2,9 | 77,0 |
Table 2
Data of elemental and group chemical analysis, %
| Sample code | \(C^r\) | \(H^r\) | Bitumens (benz.) | WR and LG | Humic acids | Fulvic acids | Lignin | Cellulose |
|---|---|---|---|---|---|---|---|---|
| T | 53,8 | 5,2 | 4,2 | 21,0 | 36,5 | 19,5 | 14,4 | 4,5 |
| B | 50,0 | 5,7 | 2,7 | 40,0 | 20,0 | 21,0 | 9,0 | 7,3 |
The dynamics of the process of overall decomposition and formation of individual gas components is presented in Fig. 1.
Characterizing the process of peat destruction on the basis of the differential curve for the evolution of volatile products, one may note the presence of the range 0–500°C, as well as a range in which the rate of formation of volatile products is insignificant (500–1000°C).
Comparing high-moor and lowland peats, one may observe that up to 300°C the process intensity is higher for high-moor peat; up to 500°C the process intensity is higher for lowland peat; and in the high-temperature region they are very close.
At the first stage of decomposition, oxygen-containing products are formed mainly; their amount and rate of formation are greater for high-moor peat.
Fig. 1. Dynamics of peat pyrolysis. ○—○—○ peat G; +—+—+ peat B; a — rate of weight change; b — formation of CO₂; c — formation of CO; d — formation of CH₄; e — formation of H₂
If the intensity of carbon dioxide formation in the region of 500°C is greater for high-moor peat and further toward 800°C becomes equal to that for lowland peat, then the intensity of carbon monoxide formation in the region of 600–950°C is greater for lowland peat, and by 900°C they become equal. Processes of decomposition of the mineral part of peat (carbonates) and conversion of carbon dioxide may play an important role in the formation of oxygen-containing products in the high-temperature region.
The rate of methane formation has one maximum at 500°C, with both the intensity of formation and its amount being higher for lowland peat.
The same may be noted for the process of hydrogen formation, with the difference that two maxima are observed in the same temperature region, and the increase in the intensity of hydrogen formation is greater above 850°C. Characteristic of both peats is the identical temperature position of the regions of formation of the corresponding gases and of the characteristic portions of the curves (extrema). In addition, there is a temperature correspondence between the processes of formation of CO₂ and CO, H₂ and CH₄, which, probably, may be a consequence of the identical strength of the bonds whose destruction is responsible for them, or a consequence of an identical mechanism of formation.
Consequently, it may be stated that, regardless of the type of peat, its botanical and group chemical composition, the character of thermal destruction is similar. The intensity of the process and the quan-
the amount of the products formed depends on the chemical composition of the samples.
In lowland peat, a larger amount of solid product and a smaller amount of gaseous products are formed (Table 3).
For the liquid products the difference is small. With respect to the amounts of the individual gas components, it may be noted that during the thermal decomposition of high-moor peat more oxygen-containing gases and less hydrogen and methane are formed.
Table 3
Material balance of peat pyrolysis products, %
| Code | Solid | Liquid | Gaseous | CO₂ | CO | H₂ | CH₄ |
|---|---|---|---|---|---|---|---|
| T | 24.0 | 47.5 | 28.5 | 76.5 | 14.3 | 0.3 | 8.9 |
| B | 21.5 | 45.3 | 33.2 | 82.5 | 15.4 | 0.1 | 2.0 |
Kinetic analysis of the process of formation of volatile products for both peats is characterized by similar values of the activation energies: for the overall process—16–26 kcal/mol;
formation of CO₂—20–21 kcal/mol;
CO—15–20 kcal/mol;
CH₄—29–39 kcal/mol;
H₂—36–40 kcal/mol.
The order of the process for the formation of oxygen-containing gases is 1.0, and for the formation of methane and hydrogen it is 2–3, which is apparently associated with a more complex mechanism of formation of hydrogen and methane.