# Influence of Hydrothermal Carbonization on Catalytic Fast Pyrolysis of Agricultural Biomass
## 3 Results and Discussion
### Fast Pyrolysis Kinetics--Influence of HTC and CaO Addition  
It could be clearly seen from the Arrhenius plot (Figure 10) that HTC treatment exerted its influence on the kinetics of fast pyrolysis of miscanthus. In general, the inclination of the slope was higher for HTC-treated samples in comparison to the raw biomass. The effect was comparable to the effect made by CaO addition (as shown in Figure 11). Looking at Figure 11, the in situ addition of 20% of CaO catalyst allowed for achieving similar kinetics as for pyrolysis of the raw miscanthus. Other studies using CaO as an in situ catalyst in different types of fluidized beds and fixed beds had even higher mass shares of CaO, reaching Ca to biomass ratios as high as 2:1 [26]. However, the important difference is that the catalyst added in this study was initially cold and needed to be heated up after injection with biomass, locally decreasing the temperature at the beginning of the process. Nonetheless, despite such a limitation, activation energies determined within the scope of this study (Table 1)were much lower than the values reported for slow pyrolysis of miscanthus by [PERSON] et al. [109]. Figure 11: Arrhenius plot for pyrolysis of Miscanthus—influence of catalyst (dotted lines are trend lines). Figure 10: Arrhenius plot for pyrolysis of Miscanthus—influence of HTC (dotted lines are trend lines). [PERSON] et al. [109] reported activation energies (\(E_{a}\)) for slow pyrolysis of miscanthus, performed at heating rates of 10 to 50 \({}^{\circ}\)C/min, using TGA in a non-isothermal regime, for temperatures up to 600 \({}^{\circ}\)C. Reported \(E_{a}\) were 175.64 and 177.02 kJ/mol for samples milled using a knife mill (with a sieving screen of 425 \(\upmu\)m aperture) and roller mill, respectively [109]. [PERSON] et al. [110] reported the average \(E_{a}\) for pyrolysis of miscanthus, to be as high as 197.66 kJ/mol and 179.64 kJ/mol, using the KAS (Kissinger-Akahira-Sunose) and Starink methods, respectively. Heating rates ranging between 10 and 30 \({}^{\circ}\)C/min were used in this particular study [110]. [PERSON] et al. [111] investigated the pyrolysis kinetics of miscanthus, applying heating rates of 5, 10, and 20 \({}^{\circ}\)C/min, and for the degree of conversion <0.8 obtained \(E_{a}\) ranging between 141.7 and 202.3 kJ/mol using [PERSON]'s method [111]. For the Ozawa-Flynn-Wall method, the same study reported \(E_{a}\) ranging between 153.5 and 190.5 kJ/mol [111]. \(E_{a}\) values reported in this study (Table 1) were smaller than 101.10 kJ/mol for all the samples, indicating the importance of the heating rates of the process. Higher values of \(E_{a}\) were achieved for hydrochars and the CaO addition in comparison to pyrolysis of raw miscanthus. This is not surprising, as a part of the volatile matter is lost during the HTC process, as shown in Figure 6. Nonetheless, it is worth noting that the addition of 20% of CaO allowed for achieving almost the same value of \(E_{a}\) as for raw miscanthus (Table 1). The results obtained within the scope of this study are in good qualitative agreement with the work of [PERSON] and [PERSON] [112]. The work reported \(E_{a}\) for pyrolysis of miscanthus, ranging between 129-156 kJ/mol, increased after acid hydrolysis treatment, reaching 200-376 kJ/mol [112]. [PERSON] and [PERSON] [112] performed pyrolysis at heating rates of 2.5, 5, 10, 17, and 25 \({}^{\circ}\)C/min. Therefore, the importance of the heating rates should not be overlooked, especially if fast pyrolysis is a subject of consideration. Moreover, similar qualitative confirmation of the trend of increasing \(E_{a}\) with hydrothermal pretreatment could be found in the work of [PERSON] et al. [66], which showed that HTC pretreatment increased \(E_{a}\) in comparison to pyrolysis of untreated sewage sludge, with a heating rate of 10 \({}^{\circ}\)C/min [66].