## 1 Introduction  
Besides solar and wind energy, biomass is considered to be a main renewable energy source, which can be used as adjustable, controlled energy in a renewable energy mixture of solar, wind, and biomass energy. The utilization of biomass is an effective solution for global warming and climate change. Unlike fossil fuels production, there are far less geographical restrictions for biomass production in most non-desert countries, so biomass utilization is also an important part of national energy security for non-OPEC (non-Organization of Petroleum Exporting Countries) countries. In recent decades, the energy supply derived from biomass keeps growing, while that from wind and solar is still at a low level [1]. As a key technology for biomass utilization, biomass gasification represents an efficient process for the production of power and heat and the production of hydrogen and second-generation biofuels. Gasification can theoretically convert all different types of biomass into syngas, which is mainly composed of hydrogen, carbon monoxide, carbon dioxide, and methane, for use as gas fuel and raw 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]. [PERSON] 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. [PERSON] et al. [20] studied the release of critical ash-forming elements from the combustion of biomass containing K, Cl, and [PERSON] 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.