In this study, experiments on co-combustion of hard coal and solid recovered fuel (SRF) were performed in a circulating fluidized bed (CFB) reactor. With an inner diameter of 0.59 m, a total height of 8.6 m and a thermal load of 1 MWth, it is one of the largest pilot plants for CFBs in the world. The high flexibility of the test facility enables testing a variety of input materials under changing test conditions. During the pilot tests, the combustion of various fuel mixes ranging from 100% hard coal to 100% waste was tested to assess the impact of varying fuel ratios on the overall combustion process. It was found that stable operation could be maintained with increasing load shares of SRF. Even a firing rate of 100% SRF does not show a significant impact on the thermodynamic and hydrodynamic conditions in the reactor. To investigate the combustion conditions inside the reactor, three in-bed gas measurements were conducted, measuring the gas composition of CO2, O2, and CO as a horizontal profile through the reactor. The measurements were taken in the free board zone of the reactor at a constant height of 4.20m above the nozzle grid. The SRF fraction in the fuel mix at the time of the three in-bed measurements was 0%, 72%, and 100%. The influence of increased shares of SRF can clearly be seen by increased shares of CO and O2, while CO2 decreases at the same time. This finding indicates that the combustion zone is moved to upper regions of the reactor with increasing shares of SRF. The effect can be explained by the high amount of volatiles in SRF, which is roughly 2.5 times the share of volatiles in hard coal.