The agglomeration of bed material is a serious problem during biomass combustion. In this paper, an alternative bed material (porous alumina) was used to prevent defluidization and to improve efficiency during cedar pellet combustion. During the tests, we observed that porous alumina was more difficult to agglomerate than silica sand, because of its specific properties. The volatile matter from cedar pellets mostly burned in the dense bed when the porous alumina was employed, as porous alumina captured part of the volatile matter released from the fuel pellets. Therefore, the temperature of the alumina bed was higher than that of silica sand under the same experimental conditions (fuel feed rate and air feed rate), and the temperature gradient in the freeboard region using the former was larger than that using the latter. In addition, the temperature distribution in the horizontal direction was notably uniform. When silica sand was used as a bed material, the CO concentration sharply decreased with the height in the freeboard region. In the horizontal direction, the maximum CO concentration was found in the middle of the bed. By contrast, the height of the combustor had a minor influence on the CO concentration when porous alumina was used as the bed material. Our experimental results also showed that NOx emission was not affected by the type of bed material.