In this work, a test rig is presented that is suitable for investigations on the equilibrium of high-temperature in situ desulfurization sorbents under real process conditions of a fluidized-bed gasifier. Gasification was performed using wood pellets as fuel with addition of a defined amount of CS2 to raise the content of sulfur without significantly changing the composition of permanent gases. The conversion of CS2 to H2S under gasification conditions ensures a high sulfur content in the produced gas in the range of 1200 ppmv, representing a synthesis gas derived from gasification of low-grade residual biomass. This type of gas is suitable for investigating the equilibrium of the desulfurization reaction under real process conditions, as the initial sulfur content is high enough to observe a desulfurization effect upon addition of potentially suitable sorbents. Furthermore, the low ash content of wood pellets ensures long-term stable gasification conditions. Investigations using lime as a sulfur sorbent were conducted in a systematic approach for system characterization with respect to overall conversion, sulfur balance, and residence time. Analytics were based on GC measurement of the synthesis gas covering all relevant volatile gaseous sulfur components such as COS, H2S, CS2, and CH3SH. The achieved desulfurization equilibria in the process application of in situ desulfurization proved to be in good accordance with theoretical values derived from a Factsage 6.4 simulation.