Lower temperature operation with reduced sugar degradation is achieved by adding a small amount of mineral acid to the pretreatment process. The acid increases reaction rates at a given temperature and the ratio of hydroly­sis rate to the degradation rate is also increased.

A compromise between the reaction temperature and the reaction time exists for acid-catalyzed reactions. As for autohydrolysis, however, condi­tions explored range from several hours at 100°C to 10 seconds at 200°C with a sulfuric acid concentration of 0.5 to 4.0%. Acid catalysts have also been used in steam explosion systems with similar results. Xylose yields gener­ally range from 70 to 95%. However, sulfuric acid processes produce lignin that is more condensed (52% of the lignin extractable in dilute NaOH) than that produced by an autohydrolysis system. Sulfur dioxide has also been investigated as a catalyst to improve the efficiency of the pretreatments. Use of excess water increases energy consumption and decreases the concen­tration of xylose in the hydrolyzate, thus decreasing the concentration of ethanol that can be produced in the xylose fermentation step. In a study by Ojumu and Ogunkunle [51], production of glucose was achieved in batch reactors from hydrolysis of lignocellulose under extremely low acid (ELA) concentration and high-temperature condition by pretreating the saw­dust by autohydrolysis ab initio. The maximum glucose yield obtained was reported to be 70% for the pretreated sawdust at 210°C in the eighteenth minute of the experiment. This value is about 1.4 times the maximum glu­cose level obtained from the untreated sawdust under the nominally same condition [51].

The acid hydrolysis process has a long history of over 100 years. As an alternative to dilute acid hydrolysis, concentrated acid-based hydrolysis pro­cesses are also conceivable and available. However, these types of processes are generally more expensive to operate and render handling difficulties [52]. Sulfuric acid is the most common choice of catalyst; however, other mineral acids such as hydrochloric, nitric, and trifluoroacetic acids (CF3COOH) have also been used.