Thermal to 350°C), maximum weight loss of 78.2%

Thermal Gravimetric Analysis (TGA) was used
to characterize the thermal behaviour of biomass derived molecules. Results of
the thermal analysis of silica extracted from paddy straw powder has been
presented in Fig. 4a, which shows the TG (thermogravimetric) curve indicating
percent weight loss in relation to temperature of degradation. Herein, the separated
silica was heated at a temperature range of 25°C to 500°C in nitrogen
atmosphere. TGA curve (Fig. 4a) indicates that the amount of typical weight
loss was recorded in three stages during thermal decomposition: (i) moderate
weight loss (21.2%) in temperature range of 0°C to 120°C, which corresponds to
the presence of moisture adsorbed on the surface of hydroxyl groups of
nano-silica (Premaratne et al., 2013); (ii) weight loss in the temperature
range of 120°C to 330°C (22.9%), which corresponds to the reduction of silanol
groups of silicon dioxide (SiO2); (iii) third stage occurred above
330°C, indicating a sudden fall in the TG curve with mass loss of 20.5%,  that could be due to the process of
carbonization which led to the conversion of volatile components into gaseous
materials and tars (Mueller et al., 2003).

            Generally,
molecular composition of lignin consisting of aromatic rings with different
branching has led to a wide range of temperature stability (Yang et al., 2007).
In the present study, thermal degradation of the lignin samples was divided
into two phases as shown in Fig. 4b. In the first phase, the initial weight
loss occurred at 30°C to 130°C due to the loss of absorbed water in the lignin
structure with a mass reduction of 17.6%. In the second phase (130°C to 350°C),
maximum weight loss of 78.2% occurred that could be attributed to the
degradation of components of carbohydrates in the lignin samples, and further transformed
to volatile gases such as CO2 and CH4 (Watkins et al.,
2015). This may also be due to the fact that the degradation processes might
cause the breaking down of molecular chains of the lignin. It can also be
concluded from the present study that the thermal properties of lignin are
dependent on their source.

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