Decomposition of tar derived from 4 biomass resources, i.e., cellulose, lignin (alkaline and dealkaline), pruned apple branch (PAB) and rice straw over calcined scallop shell (CS) and iron(Fe)-loaded CS were performed in a fixed bed reactor at different temperatures. It was found that alkali species such as potassium in the biomass could migrate to the surface of catalyst with the tar, and hence promote catalytic activity of the regenerated catalyst. In order to reveal this phenomenon, in this study a small amount of K was doped on CS and Fe-loaded CS and applied in the steam reforming of tar derived from cellulose and dealkaline lignin. The results indicated that the addition of K greatly enhanced hydrogen production rate. (C) 2012 Elsevier Ltd. All rights reserved.