TGA

Figure 5.1 presents the TG curves relating weight loss (%) and temperature (ºC) and Figure 5.2 presents the DTG curves, relating the weight loss rate (% min^-1) and temperature (ºC) for the three poplar biomass samples.

The overall trend is similar for the three biomass samples. In case of the DTG curves, the evolution shows two maxima weight loss rates (Wmax); the first maximum occurs during the second stage and the second maximum occurs during the third stage. Major differences in the results are found for the AF2 sample for temperatures above 400 ºC. In fact, AF2 reaches the second peak at lower temperature and with lower weight loss rate. Note that AF2 has a higher percentage of residual ash than the other samples.

Figure 5.1 - TG curves. Figure 5.2 - DTG curves.

The visual examination of both figures allows identifying three decomposition stages: (i) the heating and drying of the particle (SI); (ii) the devolatilization (SII); and (iii) the char oxidation (SIII). These three stages are considered in the analysis of the combustion process.

Temperature (^oC)

0 100 200 300 400 500 600 700 800 0

10 20 30 40 50 60 70 80 90 100

Weight loss (%)

AF8-P AF8-I AF2

0 5 10 15 20 25

DTG (%/min)

0 100 200 300 400 500 600 700 800

Temperature (^oC)

AF8-P AF8-I AF2

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Table 5.1 shows the combustion characteristics of each biomass obtained from the TG and DTG curves, namely the initial temperature (Tin), the burnout temperature (Tb), the maximum weight loss rate (Wi, max) and the temperatures where this rate occurred (Ti, max).

Table 5.1 - Combustion characteristics of each biomass fuel.

Stage Parameter AF8-I AF8-P AF2

SI T1, in (ºC) 38 39 31

SII

T2, in (ºC) 230 238 233

W2, max (% min^-1) 12.6 13.9 10.8

T2, max (ºC) 321 322 320

SIII

T3, in (ºC) 372 373 362

W3, max (% min^-1) 21.3 22.8 17.4

T3, max (ºC) 423 429 404

Tb (ºC) 433 438 437

The first stage (SI) begins at temperatures between 31 ºC and 39 ºC (T1, in). The total weight losses of AF8-P and AF8-I are similar (approximately 6.2% and 6.9%), but AF2 has a higher weight loss (approximately 9.3%).

The second stage (SII) starts at temperatures between 230 ºC and 238 ºC (T2, in). The maximum weight loss rate (W2, max) for each biomass takes place around a temperature of 320 ºC (T2, max), confirmed by the maximum in Figure 5.2. AF8-P presents the highest weight loss rate (W2, max) with 13.9 % min^-1. AF2 and AF8-I present 10.8 % min^-1 and 12.6 % min^-1, respectively. The percentages of total weight loss for AF8-P and AF8-I are similar (approximately 69%) and higher than that of the AF2 (approximately 63.5%).

The third stage (SIII) is defined as the point where weight loss rate is minimum (Figure 5.2), after the maximum of the second stage [6]. The corresponding temperatures range from 362 ºC to 373 ºC depending on the biomass. The maximum weight loss rate (W3, max) is observed at temperatures ranging from 404 ºC to 429 ºC (T3, max), in good agreement with the maximum observed in Figure 5.2.

The AF8-P sample has the highest weight loss rate of the whole combustion process during this stage, (cf. Table 5.1).

The burnout indicates the end of stage three. The burnout temperatures (Tb) are between 433 ºC and 438 ºC. The AF8-P has the highest burnout temperature and AF8-I the lowest. The total weight loss during this stage is lower for the AF2 sample (approximately 21.4%) when compared with the clones

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AF8 (the total weight loss for the AF8-P is approximately 23.1% and for the AF8-I is approximately 22.4%). The final solid residue (ash) at the end of the third stage is similar for the two clones AF8 (approximately 1.7%) and higher for the AF2 (approximately 5.8%).

The degradation overlap of the structural components creates difficulties in evaluating the influence of each component alone. In Figure 5.2, hemicellulose is identified by the curve’s shoulder and cellulose is identified by the curve’s maximum. Lignin slowly decomposes over a wider range of temperatures [6].

Table 5.2 shows the combustion characteristics of the second stage for woody energy crops samples, with similar chemical analysis, at 10 ºC min^-1 heating rates obtained from literature [9, 12]. Available parameters such as fuel, temperature interval (Ti, int), maximum weight loss rate (Wi, max) and temperatures where this rate occurred (Ti, max) are indicated for comparative purposes

Table 5.2 - Combustion characteristics of biomass fuels from the literature (second stage).

SII

Reference Fuel T2, int (ºC) T2, max (ºC) W2, max (% min^-1)

[9] Poplar 220 - 400 360 10

Willow 220 - 400 370 10

[12] Poplar 248 - 376 350 -

The initial decomposition temperatures in Table 5.1 are in the range of the initial decomposition temperatures in Table 5.2 (220 - 248 ºC). The stage ending temperatures in [9] are higher (up to 38 ºC with a maximum relative error of approximately 10%) than those obtained in the present experiments.

The stage ending temperature in [12] is similar to the stage ending temperature in Table 5.1 (T3, in).

The maximum decomposition temperatures in Table 5.2 (T2, max) are higher (up to 50 ºC) when compared to the same parameter in Table 5.1, leading to a maximum relative error of approximately 16% when compared to those obtained in the experiments.

The maximum decomposition rates in Table 5.1 (W2, max) are higher than the maximum decomposition rates in Table 5.2 (up to 3.9 % min^-1), leading to a maximum relative error of approximately 28% when compared to those obtained in the experiments.

Table 5.3 shows the combustion characteristics of the third stage for woody energy crops samples, with similar chemical analysis, at 10 ºC min^-1 heating rates obtained from the literature [9, 12].

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Table 5.3 - Combustion characteristics of biomass fuels from the literature (third stage) SIII

Reference Fuel T3, int (ºC) T3, max (ºC) W3, max (% min^-1)

[9] Poplar 340 - 550 457 6.3

Willow 340 - 520 488 6.7

[12] Poplar 391 - 505 450 -

The stage beginning temperatures in Table 5.1 are in the range of the initial decomposition temperatures in Table 5.3 (340 - 391 ºC). The burnout temperatures in Table 5.3 are higher than the burnout temperatures in Table 5.1 (up to 117 ºC and maximum relative error of approximately 27%

when comparing the values in Tables 5.1 and 5.3).

The maximum decomposition temperatures in Table 5.3 are higher than those reported in Table 5.1, with a maximum difference of 84 ºC, leading to a maximum relative error of approximately 17%.

The maximum decomposition rate values in Table 5.1 (W3, max) are higher than the maximum decomposition rates in Table 5.3 (up to 16.5 % min^-1), leading to a maximum relative error of approximately 72%.

The maximum relative errors reveal more accurate results for the second stage. Differences can be attributed to a number of factors. In references [9] and [12], initial sample weights are higher than in this study. Smaller samples allow faster heating rates and shorter analysis times, while larger samples can lead to combustion at higher temperatures. This may explain the overall differences in the temperatures. Non-standard methodologies can also alter the analysis. In fact, most studies do not explicitly indicate how the stage defining temperatures were chosen. As a result of this, the stage defining temperatures may eventually be different.

According to [8], TG and DTG are empirical tests, with results strongly depending on testing conditions (heat transfer, heating rate, particle size, sample mass and oxygen concentration) and also on apparatus and sample characteristics. Solid fuels are not homogeneous and different parameters can affect the results [12, 13], thus complicating literature comparisons. In order to be comparable, tests must be conducted under identical operating conditions and in the same testing apparatus [8].

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