## 3 Results and Discussion
### Influence of Temperature on the Occurrence Form of the Alkali Metal  
The influence of gasification temperature on the occurrence forms of Na and K are shown in Figures 4 and 5, respectively. The horizontal coordinate represents the gasification temperature, and the vertical coordinates represent the percent of total amount and the concentration of each occurrence form, respectively. Additionally, the detailed forms of Na and K are listed in Tables 6 and 7, respectively. From Tables 4-6, it can been seen that compared to LTA, there was a significant reduction of the concentration and proportion of the WS of Na in the fly ash with an increasing temperature, which was reduced from 6.34 mg/g and 55.9% to 2.1 mg/g and 39%, respectively. The amount of WS, AA and HA decreased with an increase of temperature, but not linearly. At a lower temperature, the Na released mainly in form of Na\({}_{3}\)Cl\({}_{3}\) and Na\({}_{2}\)Cl\({}_{2}\). When the temperature was up to 800 \({}^{\circ}\)C, the Na released mainly as NaCl vapor [39]. The Na in the form of AC salts increased slightly with temperature, which is consistent with the research of [PERSON], [PERSON], and [PERSON] [34; 40; 41]. In addition to the Water-Soluble, Ammonium acetate-Soluble, and Hydrochloric acid-Soluble forms, an organic form also existed for Na in the fly ash. The reasons were proposed as follows. First, part of the Na combined with oxygen-containing functional groups and formed an intermediate substance O-Na+ group, which attached to the surface of char particles like pearls. The O-Na+ group is difficult to decompose due to the strong electron adsorption [16]. Second, part of the Na may also have combined with the char matrix to form a stable cross-linked structure [42; 43]. These two forms of Na attach on the char stably, but there is no enough energy to break the keys to evaporate Na in this experiment. As a result, organic Na still existed in the fly ash. Comparing Table 5 with Table 7 and Figure 5, the K in the form of silicate salts is difficult to react, and the concentration of the WS salts of K decreases from 69.642 to 55 mg/g, while that of the AA and HS forms of K both decrease to below 3 mg/g. It can be deduced that the K released before 700 \({}^{\circ}\)C was mainly in the form of organic salts of K. With the straw decomposing, the stronger evaporation of organic K was easy to evaporate due to its high thermal instability. It is generally considered that organic K begins to release at 200 \({}^{\circ}\)C with the release of volatile matter. When the temperature is higher than 700 \({}^{\circ}\)C, the concentration of K in the form of WS salts decreased from 69.642 mg/g and 20.733% to 7 mg/g and 20% compared with the LTA. In this period, the released K was in the form of water soluble inorganic salts that evaporated. [PERSON] et al. used chemical thermodynamic equilibrium (FactSage) to calculate the evaporation temperature, and obtained that the inorganic K of biomass began to evaporate at 627 \({}^{\circ}\)C. The reactions of K with S, Si, Al, and so on produced salts with low volatility when the temperature was lower than 627 \({}^{\circ}\)C [4]. This is coincident with the result that the inorganic K that evaporated divided into two stages at the cut-off temperature of 700 \({}^{\circ}\)C. [PERSON] et al. also obtained the result that the alkali metals that evaporated can be divided into two stages at the cut-off temperature of 500 \({}^{\circ}\)C [44]. So, the two stages of K release could be explained as follows: one path is organic K converted into its gaseous phase; the other path is soluble inorganic K volatilized at a high temperature. Specifically, with an increase in temperature, the number of AC salts of K increased. The possible reason is that the K reacted with the SiO\({}_{2}\), which accounts for 73.65% of fly ash, and produced a stable compound similar to K\({}_{2}\)O-4 SiO\({}_{2}\)[6; 21; 39; 43; 44]. So, the content of AC salts of K increased.