Dynamics of the Removal of Elements of Peat Organic Matter during Thermal Decomposition
Abstract Generated abstract
This study examines how carbon, hydrogen, and oxygen from the organic matter of two West Siberian peats are redistributed into volatile products during thermal decomposition. Analytical samples of raised-bog and lowland peat were heated at a constant rate to selected temperatures between 150 and 500°C, after which the solid residues were analyzed for carbon and hydrogen and interpreted using elemental removal curves and Van Krevelen diagrams. The results show that peat type strongly affects the temperature dependence and mechanism of element removal: raised-bog peat loses carbon and hydrogen more readily at lower temperatures, with dehydration predominating from the start, while lowland peat shows a later maximum in carbon removal and predominantly decarboxylation up to about 300°C. Oxygen removal was more similar in the two peats, reaching its maximum rate near 270°C.
Full Text
Dynamics of the Removal of Elements of the Organic Matter of Peat During Thermal Decomposition
V. M. Ikrin, L. I. Petrovicheva, Ya. A. Belikhmaer
(Presented by the scientific-methodological seminar of the organic departments of the Faculty of Chemical Technology)
The study of the redistribution of elements among the decomposition products was carried out with two typical representatives of peats from deposits of the West Siberian Lowland, whose characteristics are presented in Tables 1 and 2.
Table 1
Characteristics of peats, %
| Sample no. | Type of peat | Degree of decomposition | $W^{a}$ | $A^{c}$ | $V^{r}$ | $C^{r}$ | $H^{r}$ | $(O+N+S)^{r}$ |
|---|---|---|---|---|---|---|---|---|
| 1 | Upland | 20 | 9.27 | 2.92 | 72.00 | 51.70 | 5.72 | 42.58 |
| 2 | Lowland | 35 | 10.70 | 10.90 | 68.42 | 53.80 | 5.21 | 40.99 |
Table 2
Group composition of peats
| Sample no. | Bitumens | Water-soluble and readily hydrolyzable | Humic acids | Fulvic acids | Nonhydrolyzable residue |
|---|---|---|---|---|---|
| 1 | 2.70 | 30.0 | 28.0 | 9.5 | 29.8 |
| 2 | 4.20 | 21.0 | 36.5 | 19.5 | 18.8 |
Thermal decomposition of an analytical peat sample weighing 1 gram was carried out in a quartz test tube 10 mm in diameter. The samples were heated in a tubular electric furnace connected to the programming unit of a derivatograph. The rate of temperature increase was kept constant and equal to 5 deg/min. Upon reaching the specified temperature, the test tube was removed from the furnace and rapidly cooled to room temperature. The cooled solid residue was examined for carbon and hydrogen content according to GOST 2408-49.
The choice of the temperature interval of the studies is connected with the results of derivatographic analysis, which showed that the main
the processes of thermal decomposition of peat take place in the temperature range 150–500°C.
As can be seen from Fig. 1, the amounts of C, H, and O passing into volatile products depend on the temperature and the type of peat. Thus, at 300°C, 17.1% C passes into volatile products from raised-bog peat, whereas only 4.5% C is removed from lowland peat.
The maximum rate of carbon removal is observed for raised-bog peat at 280°C and for lowland peat at 350°C. It should be noted that the difference in carbon content with increasing temperature not only does not decrease, but even increases. If in the initial lowland peat the C content is only 2.1% higher than in the raised-bog peat, then in the solid residue obtained at 450°C the C content is higher by 7.3%.
Fig. 1. Dynamics of removal of elements from the organic mass of peat as a function of temperature (a—carbon, b—hydrogen, c—oxygen). 1—raised-bog peat; 2—lowland peat.
The type of peat significantly affects the character of the hydrogen-removal curves. At 320°C, 50% H passes from raised-bog peat into volatile compounds, whereas lowland peat loses only 20% H. Such a difference in the dynamics of hydrogen removal can be explained by the occurrence of reactions of different types.
Construction of the Van Krevelen diagram in the coordinates of the H/C and O/C ratios showed that in raised-bog peat, dehydration reactions predominate from the very beginning of decomposition, whereas in lowland peat, decarboxylation reactions proceed predominantly up to 300°C. This is associated with the group composition of the peats, in particular with the higher content of water-soluble and readily hydrolyzable compounds in raised-bog peat.
Fig. 2. Van Krevelen diagram: a—demethanation reactions; b—dehydration reactions; c—decarboxylation reactions; 1—raised-bog peat; 2—lowland peat.
In the dynamics of O cleavage, considerably more similarities than differences are observed. In both peats the maximum rate of oxygen removal is reached at a temperature of 270°C.