Dual fluidized bed system for sulphur removal from biomass derived product gas

Paper in proceeding, 2011

The most elegant to clean raw gas from biomass gasification of tars is catalytic hot gas cleaning. However, most catalysts are poisoned after short operation due to carbon formation on their active sites. By circulating the catalyst in an interconnected fluidized bed system carbon poisoning can be prevented as producer gas is cleaned in one of the reactors and the catalyst is continuously regenerated in the other reactor. Theoretical calculations show furthermore that in addition to the tar reforming chemical looping has potential for continuous sulphur removal from biomass derived raw gas. The hypothesis was tested with a gas stream extracted from Chalmers biomass Gasifier while air mixed with nitrogen was used for regeneration. Experiments with a nickel magnesium aluminium oxide were conducted at 700°C and 800°C in the reformer part and 800°C in the regenerator. According to the equilibrium calculations for nickel, sulphide is formed below 550°C in reducing atmosphere corresponding to raw gas. In the regenerator the sulphide is oxidized to sulphate and released at temperatures above 800°C as sulphur dioxide. Due to the two different temperature windows it appears difficult to combine tar removal and simultaneous sulphur removal as tar reforming requires at least 700°C. However, the experimental results prove not only the principle idea, but show a strong decrease of the H2S concentration in a temperature range way above the predicted temperatures. At 700°C the tar concentration is almost unaffected by the contact with the oxygen carrier material. The most striking result from the experiment is actually the removal of 92% the H2S from the gas during that experiment. The corresponding SO2 signal revealed that only about 15-20% of the sulphur was regenerated at 800°C on the air side of the system. This disparity indicates an accumulation of sulphur in the material due to insufficient regeneration. In contrast to the low temperature experiment the reforming activity is more prominent at 800°C but only 44% of the H2S present in the gas was removed and about 30% of the removed sulphur was retained as SO2 in the flue gases of the regenerator. Even though the equilibrium calculations suggest running the gas cleaning part below 550°C as no sulphites are expected at elevated temperatures and neither sulphate under reducing conditions the experiments revealed sulphur transport at temperatures way above. A similar effect was observed in recent works on oxygen carrier materials, where these materials change their behaviour due to the formation of sulphates. The sulphate can probably be formed by remaining oxide in the particles as they due to continuous cycling, never only comprise one oxidation state like the equilibrium calculations presume.