Exoglucanase that plays an important function in biomass degradation by catalyzing the hydrolysis of the non-reducing end beta-1,4-glucosidic linkages in cellulose and cellotetraose to release cellobiose. Hydrolyzes crystalline and amorphous cellulose but is inactive on hydroxyethyl cellulose, mannan, galactomannan, xyloglucan, arabinoxylan, arabinan, xylan, and pectin. Hydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose and cellotetraose, releasing cellobiose from the non-reducing ends of the chains. Optimum pH is 7. Optimum temperature is 45 degrees Celsius. Both N- and O-glycosylated. Belongs to the glycosyl hydrolase 6 (cellulase B) family.