## 3 Results and Discussion  
### Characterization of Gasification Feedstocks  
Table 1 represents the proximate analysis, the ultimate analysis, the higher heating value and the structural characteristics of the raw fuels and their chars. As observed, the lignite, having lower carbon and hydrogen contents than barley straw but a 3-times higher ash content, presented a reduced higher heating value. The concentrations of nitrogen and especially sulfur were very low, revealing insignificant toxic emissions during the thermal processing of both materials. Upon pyrolysis, volatile compounds consisting mostly of hydrogen and oxygen species were released, leading to the enrichment of chars in carbon. This, in turn, resulted in a higher heating value of chars in comparison to raw fuels, and the effect was more pronounced in the case of barley straw, the volatile matter of which was double that of the lignite. Furthermore, the evolution of volatile matter caused an increase in pore volume, as can be seen from the table, and accordingly, about a 30-90-fold increase in the specific surface area of chars with respect to the raw samples. The enhancement of carbon stability, aromaticity and surface area after thermal treatment is very important for the reactivity of fuels [3; 27] and their gasification efficiency, as will be shown below. As gasification experiments were carried out up to 1000 \({}^{\circ}\)C, the fusibility behavior of ashes was investigated, and the data recorded by the heating microscope are compared in Figure 2. The initial deformation and softening temperatures of barley straw (1015 \({}^{\circ}\)C and 1091 \({}^{\circ}\)C) occurred about 160 \({}^{\circ}\)C and 190 \({}^{\circ}\)C earlier than those corresponding to lignite, however, both values imply deposition problems in boilers operating above 1000 \({}^{\circ}\)C. The fluid temperature of barley straw (1262 \({}^{\circ}\)C) was also lower than that of lignite (1395 \({}^{\circ}\)C). This behavior is attributed to the different mineral phases present in the two materials, melting at different temperatures. The XRD spectra of the ashes are illustrated in Figure 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Kardia Lignite (KL)** & \multicolumn{3}{c}{**Barley Straw (BS)**} \\ \hline  & **Raw** & **Char** & **Raw** & **Char** \\ \hline Volatile matter & 43.6 & - & 81.2 & - \\  & (19\% light gas) & - & (67\% light gas) & - \\ Fixed carbon & 23.1 & 50.6 & 7.3 & 55.0 \\ Ash & 33.3 & 49.4 & 11.5 & 45.0 \\ C & 36.0 & 41.7 & 46.9 & 71.4 \\ H & 2.9 & 1.3 & 7.1 & 1.9 \\ N & 1.2 & 1.1 & 1.0 & 0.7 \\ O & 25.9 & 6.2 & 33.5 & - \\ S & 0.7 & 0.3 & - & - \\ HHV (MJ/kg) & 12.0 & 15.3 & 21.3 & 22.1 \\ Specific surface area (25 \({}^{\circ}\)C, m\({}^{2}\)/g) & 1.5 & 42.1 & 2.0 & 176.0 \\ Pore volume \(\times\) 10\({}^{2}\) (25 \({}^{\circ}\)C, cm\({}^{3}\)/g) & 0.85 & 4.1 & 2.2 & 10.8 \\ Average pore size (Å) & 21.2 & 14.1 & 26.6 & 17.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Proximate analysis, ultimate analysis and structural characteristics of fuels (% dry). These ashes were produced at 550 \({}^{\circ}\)C, in order to detect any mineral phases before being transformed, or volatilized at higher temperatures. Thus, the relatively low initial deformation and softening temperatures of barley straw sample were attributed to the fusible forms of arcanite (MP 1070 \({}^{\circ}\)C), sylvite and fairchildite (MP 815 \({}^{\circ}\)C) [29] minerals, as Figure 3b shows. On the other hand, Kardia lignite ash was rich in calcium and silicon phases, mainly in the forms of calcite (MP > 1300 \({}^{\circ}\)C), anhydrite (MP > 1300 \({}^{\circ}\)C) and quartz (MP 1700 \({}^{\circ}\)C) [29], and contained considerable amounts of high-melting-point aluminum and iron minerals, while it had a lower content in alkali incorporated in clays and feldspars.