CTO Tests

All the char samples, as obtained in the HTWM reactor, were tested in the flow microreactor under isothermal conditions at four different temperatures: 673, 723, 773 and 823 K. Typical results of such combustion tests, in the case of cocoa shells char, are reported in Fig. 6 as the rate of change of the carbon conversion degree dX/dt (where X = (mo-m)/mo, m and mo being the current and the initial mass of carbon in the sample, respectively) as a function of X.

During CTO tests the separate contributions of CO and C02 formation to the carbon overall combustion were measured and the average values measured during each test are reported in Table 4 for all materials. In any case, the C0/C02 ratios remain practically constant along the whole carbon conversion and, as the test temperature changes from 673 to 823 K, slightly increases as expected [21-23]. However, in the case of the blends chars the values are higher than those exhibited from the two blend components separately and, in addition, first decrease and then increase (Table 4). This behaviour is quite strange since, as shown in the literature [21], the ratio should increase with the temperature. However, it is known that some oxides and salts may play as catalysts for the oxidation of carbonaceous materials [23-25]. Furthermore, it has been shown that the presence of catalysts during the carbonaceous material oxidation promotes the formation of surface (C-02) complexes giving rise to C02 so influencing the value of the amounts of CO and C02 evolved from the carbonaceous surface [23].

image200

Fig. 6. Comparison between CTO profiles of chars of cocoa shells at four different temperatures.

Table 4. Average CO/COi ratios during the CTO tests at four different temperatures of chars produced from various samples.

material

T, K

673

723

773

823

coal К char

0.35

0.4

0.45

0.6

cocoa shells char

0.065

0.066

0.12

0.14

wood chips char

0.22

0.25

0.24

0.23

coal K-cocoa shells blend char

0.48

0.41

0.45

0.6

coal К-wood chips blend char

0.6

0.45

0.52

0.6

Therefore, during the blend combustion the interaction of the inorganic matter of the blends components may give rise to the formation, if not present, or to the transformation, if previously present, of substances eventually acting as catalyst for combustion.

Constant temperature combustion tests allowed the determination of the kinetic parameters of the various char samples. All the evaluated kinetic parameters are summarised in Table 5. Here the values of the frequency factor (K0) and of the apparent activation energy (Ea), calculated at three values of X, are reported. The analysis of such results shows that: i) all the chars show an apparent activation energy that increases with carbon conversion; this could be ascribed to a possible non homogeneity of the chars that would lead to the faster combustion of the more reactive part and consequently to the slower combustion of the remaining less reactive part, or to a sort of annealing of the material because of the permanence at relatively high temperature; ii) cocoa and wood chars are very similar materials because their kinetic parameters are comparable; iii) coal char shows an apparent activation energy significantly higher than that of the biomass chars; iv) surprisingly, the apparent activation energy of the coal К-wood chips blend char is markedly higher than that of the coal К char although most (97-98%) of the blend char is actually coal char; v) this characteristic is not shown by the coal K-cocoa shells blend char, which also shows the higher constancy of the Ea with X; vi) cocoa and wood chars, obtained at 103 K/s and 104 K/s, show comparable apparent activation energy being almost constant with the carbon conversion (X) and with the heating rate of charring.

Table 5. Kinetic parameters of chars produced from various samples. Ко [min’1], Ea, [Kcal/mol],

X=0.3

X=0.5

X=0.7

Ко

Ea

Ко

Ea

Ко

Ea

Coal char

1.37* 107

29.63

1.05*107

29.80

12.98* 07

34.60

Cocoa char

0.29* 107

21.83

4.76*107

26.39

3.87*107

26.76

Cocoa char 104 k/s

9.13*107

21.95

4.27*107

23.33

1.37*107

25.55

Wood char

1.18*10®

22.28

4.02*106

24.61

3.36*106

25.16

Wood char 104 k/s

0.64*10®

20.48

2.09* 106

22.70

1.89*10®

23.42

Coal K-cocoa blend char

2.90*10®

33.88

2.01*10®

33.93

3.58*107

32.11

Coal К-wood blend char

3.64*10®

34.27

1.29*1010

40.45

1.22* 1010

41.32

It is worth noting that during the first tests for producing chars with the HTWM reactor very short sample residence times were used. That is 1 s when heating rates were 103 K/s and 0.1 s when it was 104 K/s. These times appeared enough for the biomasses charring but were insufficient for the coal К and coal К-derived blends. Indeed, such samples were characterised by low SSA (i. e. lower than that of the correspondent raw samples: see italic characters in Table 3) and activation energies and frequency factors strongly increasing with the carbon conversion, which give clear evidence for a bad charring process. Therefore, residence times of 4 seconds (1 s for heating and 3 for keeping the sample at the highest temperature) were employed when the sample contained coal K.

Altogether, results suggest that a role is likely played by the interaction of the minerals in the blend otherwise it is quite difficult to justify the higher reactivity (lower Ea) of the blend’s char with respect to that of the chars of pure samples.

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