Torrefaction solid product characteristics

Figure 4.2 presents the proximate analysis and the heating value of solid products at different conditions. The volatile combustible matter (VCM) decreased from 79.6% for raw RS to 44.5%, and 78.1% for raw CS to 48.1% at the conditions of 290 o_{C and 60} minutes, which was the most severe condition. Similar to the product yield shown in Figure 4.2, the decrease rate of the VCM from both biomass at different conditions were affected more by temperature than residence time. The decrease in the slope of the VCM percentages for RS was steeper than that for CS. One possible reason is because the xylan content of RS is four times higher than that of CS. Another reason can be the more reactive property of the rice straw. Wang et al., 2011 [163] found similar results with VCM degradation with wheat straw and cottons stalk. Also, it was observed that minimal changes in the VCM contents occurred between the conditions of 290-40 and 290-60 of both biomass. On the other hand, the relative amount of fixed carbon (FC) and ash contents to the VCM content increased. The decreased VCM content and increased FC content caused an enhanced high heating value (HHV, dry basis) of both products

MJ/kg for torrefaction at 290 oC for 60 minutes. This is an HHV augmentation of 50% for RS and 37% for CS from raw biomass.

(a) Rice straw

(a) Cotton stalk

Figure 4.2. Proximate analysis and its HHV of torrefied RS and CS products at the different temperature and residence time conditions. *average

The following empirical equations in terms of actual factors for HHV show how the conditions and heating value are related. The RS heating value can be determined depending largely on the effects of the temperature term and the second-order effect of temperature (modified quadratic model P-value < 0.0001), while both temperature and

time terms are considered linearly to determine the CS heating value (linear model p- value<0.0001).

RS HHV (MJ/kg) = 76.34 – 0.53 × Temp. + 0.055 × Time + 1.24E-3 × Temp.2 _(4.7) CS HHV (MJ/kg) = 8.62 + 0.045 × Temp. + 0.024 × Time (4.8)

An ultimate analysis was also conducted to find the content of C, H, N, S and O (by difference) of torrefied products, as shown in Table 4.3. With the higher temperature and the longer time, an increase in carbon content and a decrease in oxygen, hydrogen and nitrogen contents were found in both biomass. The removal of the hydroxyl radical group from the hemicellulose can be one reason for the decreased hydrogen and oxygen as the temperature increases as Nguila Inari et al., 2007 [164] reported. However, the RS and CS carbon content at 210 oC increased slightly along with the different residence times. This verifies a previous study that most hydrogen, C–C and C–O bonds form into volatile liquid or gas products between 200 oC and 250 oC [165].

A Van Krevelen diagram was used to understand the application of char as fuel and is shown in Figure 4.3. The lower the O/C and H/C ratio, the higher the energy of the products. The torrefaction process reduced the raw RS O/C atomic ratio from 0.81 to 0.24 (RS 290-60) and the H/C atomic ratio from 1.61 to 0.89 (RS 290-60), while the O/C atomic ratio of CS decreased from 0.87 to 0.21 (CS 290-60) and the H/C atomic ratio

at 290 oC can be used as a substitute for brown coal and peat as the torrefied products are located between them.

Table 4.3. Ultimate analysis of RS and CS solid products after torrefaction process at the different conditions (daf: dry ash free basis and * by difference).

daf Raw RS

210 o_{C 250} o_{C 290} o_C

wt% 20min 40min 60min 20min 40min ag 60min 20min 40min 60min C 45.0 53.6 54.0 54.4 54.7 60.1±1.3 60.7 60.4 65.3 71.5 H 6.03 6.32 6.22 6.04 6.15 5.9±0.06 5.94 5.86 5.95 5.30 O* 48.4 39.5 39.2 39.0 38.6 33.6 32.9 33.3 28.4 22.6 N 0.23 0.25 0.26 0.27 0.22 0.29±0.01 0.31 0.31 0.31 0.43 S 0.30 0.32 0.31 0.30 0.28 0.17±0.02 0.18 0.13 0.13 0.14 daf Raw CS 210 o_{C 250} o_{C 290} o_C

wt% 20min 40min 60min 20min 40min ag 60min 20min 40min 60min C 43.1 51.2 52.7 53.1 55.3 57.1±1.2 61.6 59.2 63.8 72.0 H 5.94 6.25 6.05 6.07 6.16 6.1±0.08 5.81 5.87 5.82 5.64 O* 49.9 41.4 39.9 39.4 36.9 35.1 30.7 33.1 28.3 19.9 N 0.90 1.00 1.17 1.13 1.38 1.31±0.008 1.52 1.42 1.73 2.04 S 0.23 0.20 0.24 0.27 0.27 0.31±0.25 0.36 0.33 0.39 0.45

Figure 4.3. Van Krevelen diagram in daf. (Peat, brown coal and peat adapted from E. Kurkela et al. [39]).

Fourier transform infrared spectroscopy (FTIR) was used to understand the chemical structure changes of torrefied products from raw biomass. The magnified FTIR spectra from 850 cm-1 to 1800 cm-1 of torrefied rice straw and cotton stalk is shown in Figures 4.4 (a) and (b).

(a) Rice straw

Some important peaks from previous studies [41,158,159,167] were used to explain the chemical changes of the torrefied char in the present study. Due to the different composition of both biomass, the FTIR spectra changes after the torrefaction treatment appear dissimilar. As the temperature condition increased, there were large decreases in the rice straw and cotton stalk bands at 3350 cm-1 (the O-H stretch) assigned to various polysaccharides and alcohols and at 1150-980 cm-1 of the C-O stretch of starch (1130-1050 cm-1 _{for intense polysaccharides), which was assigned to cellulose} [168]. Also, the reduced spectra at 897-898 cm-1 of the C-H deformation from both torrefied biomass was led by the cellulose decrease [41]. The FTIR band from 1680- 1600 cm-1 reported as a C=O stretch of carboxylic acid, the esterified pectin, was found to decrease after the torrefaction process of both products [168]. This was because pectin consists of many polysaccharides that easily decompose under heat treatment. On the other hand, several peaks related to the lignin of torrefied RS and CS increased; 1595 and 1510 cm-1 _{(a C=C stretch of aromatic skeleton vibrations), and 1215-1220 cm}-1 _(a strong vibration of C-C, C-O and C=O stretches) [169]. Also, the 1707 cm-1 peak increased, which is assigned to the C=O ester–including compounds attached to hemicellulose or to the carboxyl stretch of lignin at 1705-1720 cm-1_{). The augmented} lignin composition of the torrefied biomass resulted in an increased heating value and energy density because of the ether and C-C linkages in the lignin, which has a higher energy than any other bonding. This result is similar to the one found by Jae-Won Lee et al., 2012 [44]. The band at 1249 cm-1 assigned to the syringly ring and the C-O stretch in lignin and xylan was shifted to a lower wavenumber of 2000cm-1_{, similar to other bands}

at 1625 and 1735 cm-1. This means there were chemical changes around the functional groups after the torrefaction treatment [170,171]. The study by P. Rousset et al., 2011, [170] found a similar trend of spectrum changes in that 1720, 1599, 1511, 1451, and 1239 cm-1 _{bands increased after bamboo torrefaction treatment. FTIR bands of both} torrefied products from 1300-1480 cm-1 assigned to C-H deformation and C-O stretch for outer surface-suberin/cutin increased after torrefaction treatment [168]. Larger spectrum changes for cotton stalk in this range compared to that of rice straw indicate that cotton stalk contains relatively more outer surface-suberin/cutin composition.