## 3 Results and Discussion
### Gasification Performance of Fuels and Their Mixture with Carbon Dioxide Reagent  
A comparison of the gasification profiles of the lignite and barley straw chars as a function of temperature, using carbon dioxide as the gasifying agent by conducting experiments in the TG-MS system, is made in Figure 9. Table 2 summarizes the characteristic parameters of the process, such as peak temperature and rate, reactivity, conversion and product gas composition. As can be seen, the lignite started to decompose earlier and its maximum (T\({}_{\text{max}}\)) was centered at 789 \({}^{\circ}\)C. On the other hand, barley straw char, although it presented a much higher peak temperature 865 \({}^{\circ}\)C, displayed a 3-times higher rate (R\({}_{\text{max}}\)) than the lignite, which resulted in enhanced reactivity, as expressed by the R\({}_{\text{max}}\)/T\({}_{\text{max}}\) ratio. Furthermore, the conversion of barley straw to carbon monoxide gas through the Boudouard reaction (6) was about 95%, in contrast to that corresponding to lignite char, which was only 62.9%. This low reactivity of lignite fuel is principally attributed to its elevated amount of ash (Table 1), serving as a physical barrier to the carbon dioxide reagent, as well as its reduced specific surface area (Table 1) available for reaction. Barley straw fuel, however, with a 4-times higher specific surface area and a significant amount of potassium, as previously presented (18% K\({}_{2}\)O in ash), known to catalyze the gasification process [32], exhibited a higher rate and final conversion. Figure 8: Higher heating value and syngas yield during gasification of fuels. Table 2 also shows that the product gas consisted mainly of carbon monoxide and small amounts of hydrogen, methane and hydrocarbons. The increased higher heating value of lignite gas is explained by its higher content in methane, which was most probably formed by the reaction of hydrogen, from the splitting of aromatic rings at high temperatures [22], with carbon in char. On the other hand, the yield of syngas produced, which was directly related to the conversion and gaseous composition, was greater in the case of barley straw (0.84 m\({}^{3}\)/kg versus 0.54 m\({}^{3}\)/kg of lignite). The TG/DTG gasification profiles of the lignite/barley straw char blends are illustrated in Figure 10. Comparing Figures 9 and 10, it can be observed that the blend curves lie between those of the component fuels. However, as also confirmed by Table 2, the inflection temperatures were shifted to higher values than those theoretically expected from the contribution of each component material, and the reaction rates were lower that the weighted average values. This, in turn, resulted in lower reactivity and final conversion, which did not present a linear relationship with the amount of blending, agreeing with earlier studies reporting a lower reaction rate of blends in comparison to component fuels [27]. Therefore, interactions occurred between the two fuels upon mixing, leading to synergistic effects. Considering that the experiments were carried out under exactly the same conditions, the reasons behind this behavior could be attributed to the physical structure and chemical composition of the two fuels. The high amount of lignite ash could have had a negative effect on the potassium catalytic effect of barley straw char.