H2S removal from biomass gasification syngas using high alkali molten salts

Biomass energy is currently the fourth largest energy source in the world. The S content in plants is usually between 0.05% and 0.29%. Biomass gasification technology is one of the important and preferred methods for efficient biomass utilization. During the gasification process, S in the biomass is converted into an S-containing compound represented by H2S into the crude syngas. It is necessary to remove H2S in the biomass gasification crude syngas to meet the needs of the follow-up synthesis process. In order to study the removal characteristics of acid-polluted gases from biomass gasification syngas by high alkali molten salts, experiments were carried out in a small fixed-bed reactor with 8.3% Na2CO3-91.7% NaOH to remove H2S from simulated crude syngas. And then model of H2S removal was built by ignoring H2S concentration gradient in molten liquid. The results showed that high alkali molten salts had excellent effect on H2S removal. In most case, more than 99.9 % of H2S was absorbed under the experiment conditions. The H2S content in the purified syngas could meet the requirements of the conventional synthetic process. When superficial gas velocity variation between 0.87×10-3- 4.3×10-3 m/s, and the reaction temperature increased from 350 to 500℃, H2S absorption rate remained higher than 99.96%. Molten salts temperature, superficial gas velocity had no significant effect on H2S removal process. While molten salts static liquid height, bubble size increased the residence time of bubbles in the molten salts and the mass transfer efficiency between the molten salts, and the bubbles had greater effect on H2S removal. When the molten salt static liquid heights increased from -70 mm to 40 mm, the H2S absorption rate increased from 99.32% to 99.99%. After been absorbed, S stabilized in molten salts mainly in forms of Na2S, Na2SO3 and Na2SO4 with a uniform radial distributions and a slight increase in the concentration from the top layer to the bottom layer in the axial direction which may be due to the coarse aeration of the molten salt through the molten syngas. By ignoring H2S concentration gradient in gas bubble, mathematical model between H2S removal rate and reaction condition was established from gas-liquid mass transfer equation from spherical coordinate system. The model showed that when the physical property of molten salts and syngas were fixed value, bubbles diameter, molten salts static liquid height were the main factor that affects H2S absorption. Function of H2S concentration in gas bubble was in inverse proportion to the cube of the diameter, and proportional to the height of the static liquid heights of molten salts. This model can reliably predict the H2S removal process, and it can provide a theoretical basis for the practical application of H2S removal from crude synthesis gas using molten salt. In principle, the model can also be used for calculation in other similar chemical processes.