The efficiency of electricity production in biomass-fired circulating fluidized bed (CFB) boilers needs to be enhanced, which may increase the risks of high-temperature corrosion (HTC) as a side effect derived from the higher steam temperature. In this study, coal bottom ash (CBA) obtained from a pulverized coal-fired power plant was used to replace all the regular bed materials (quartz sand) in a biomass-fired CFB boiler, attempting to solve its HTC problem. Two kinds of mature deposits on the high-temperature superheater with regular bed materials and with CBA as a bed material were obtained and analyzed in detail. The deposit formation mechanisms with regular bed materials were discussed. Our results show that CBA can capture 22% of potassium during biomass combustion in the CFB boiler. However, CBA cannot effectively decrease the deposition of potassium chloride on high-temperature superheater, therefore leaving a serious HTC problem. The low effect of CBA for potassium capture may derive from its low chemical reactivity.