Rice straw gasification was carried out in a laboratory fluidized bed reactor system from 600 to 800 • C in order to well-understand the release and occurrence mode of alkali metals as a function of temperature during the gasification process. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was applied to analyze the original rice straw and obtained fly ash at different temperatures. The results show that the Water-Soluble, Ammonium acetate-Soluble, Hydrochloric acid-Soluble,

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... nd Aluminosilicate Combination-Soluble modes of the Na and K contents in rice straw decreased in sequence. The content of Water-Soluble salts of Na and K accounts for more than 50%, while the content of the Aluminosilicate Combination-Soluble mode is the lowest: less than 5%. The release rate of Na appears to be consistent but nonlinear, increasing with gasification conversion ranges between 50.2% and 70.8%, from which we can deduce that temperature is not the only factor that impacts Na emission. The release of K can be divided into two stages at 700 • C. At the first stage, the release rate of K is almost invariable, ranging from 23.3% to 26%. At the second stage, the release rate increases sharply: up to 55.9%. The concentration and the proportion of the Water-Soluble, Ammonium acetate-Soluble, and Hydrochloric acid-Soluble modes of Na in fly ash decrease with a temperature increase. The release of K can be explained as follows: one path is an organic form of K converted into its gaseous phase; the other path is a soluble inorganic form of K that is volatile at a high temperature. With a temperature increase, the Aluminosilicate Combination-Soluble mode of both Na and K increases. material for hydrogen production and other chemical productions [2, 3] . Unlike most fossil fuels, natural biomass is highly enriched in potassium (K), sodium (Na), and chlorine (Cl), especially when it derives from agricultural residues such as straw [4, 5] . During the thermal treatment of biomass in a gasifier, the alkali metals release to the gas phase and subsequently cause slagging, agglomeration, deposition, and corrosion in thermal fuel conversion systems [6] [7] [8] [9] [10] [11] [12] [13] . These problems seriously affect the safety of reactors' operation and reduce their thermal efficiency. However, on the other hand, the alkali metals play a catalytic role in the thermal utilization process, resulting in a rate increase of biomass decomposition, a yield increase of gas and char, and a yield decrease of tar [14] . During the gasification process, the alkali metals' residues on the char result in a decrease of the gasification temperature, an improvement in gasification efficiency, and a change in the products' composition [15] [16] [17] . Bouraoui et al. [18, 19] studied the effects of K and Si as mineral contents and the textural and structural properties of biomass on the CO 2 gasification reactivity of a biomass char using a thermogravimetric analysis. The results showed that when the conversion rate reaches 60%, the presence of K and Si becomes the major parameter influencing reactivity. Therefore, a good understanding of alkali metals' migration and transformation during the gasification process is highly significant for the resourceful utilization of biomass. There has been much research on the release of alkali metals during the biomass thermal treatment process. Joakim et al. [20] studied the release of critical ash-forming elements from the combustion of biomass containing K, Cl, and S. Keown et al. [21] conducted a pyrolysis of sugar cane bagasse and cane trash in a quartz fluidized-bed reactor, and found that less than 20% of the alkali metals volatilized from biomass samples at a slow heating rate (10 K/min), while 80% of the alkali metals volatilized at fast heating rates (>1000 K/s). There are four kinds of occurrence forms of alkali metals in biomass. The first one is the Water-Soluble form, in which alkali metals exist as ions and are soluble in water, ammonium acetate, and hydrochloric acid. The second one is the Ammonium acetate-Soluble form, in which alkali metals exist as carboxylate and coordination forms in nitrogen and oxygen functional groups. Ammonium acetate-Soluble alkali metals are insoluble in water, but are soluble in ammonium acetate and hydrochloric acid. The third one is the Hydrochloric acid-Soluble form, in which alkali metals attach to the ash surface in a non-crystalline form and are insoluble in ammonium acetate but soluble in hydrochloric acid. The last one is the Aluminosilicate Combination-Soluble form, in which alkali metals are insoluble in water, ammonium acetate, and hydrochloric acid. Generally, the first three forms are usually easy to evaporate while the last is difficult. The total amount of changes of alkali metals released during the thermal treatment process has been reported a lot by previous research, but the changes of alkali metals' forms, especially K's and Na's forms, have not been paid sufficient attention [22, 23] . If we can grasp the form change of alkali metals, then the behaviors of slagging, agglomeration, deposition, corrosion, and catalysis could be understood better. The objective of this paper is to obtain the release and transformation regularities of the occurrence forms of K and Na during the gasification process. The experiments were carried out by using a laboratory fluidized bed reactor. Rice straw was used as the raw material for biomass gasification. The gasification temperature ranged from 600 to 800 • C with 50 • C increments. The results in this paper could provide theoretical bases for the large-scale application of biomass gasification in the future.