During hydrolysis, water molecules react with the glycosidic bonds in the structure of cellulose and hemicellulose and degrade them to sugar units such as glucose, xylose, etc. Free sugar units can be obtained from lignocellulosic material by either thermochemical processes or a combination of thermochemical and biochemical processes.

The chemical processes are divided into two general types, one using high acid concentration in the hydrolysis step, one example is called the Concentrated Hydrochloride Acid Process (CHAP), and one using dilute acid in the hydrolysis step, one example was developed in cooperation between Canada, America and Sweden (called CASH).

CHAP process

The Concentrated Hydrochloride Acid Process (CHAP) is based on the hydrolysis of lignocellulosic feedstock by concentrated hydrochloric acid at low temperature. The process was developed for cellulose rich raw material since a high concentration of the acid may cause the degradation of pentose in hemicellulose to furfural derivatives. The ethanol yield is typically about 35%. The concentrated acid is corrosive and the process needs higher capital investment due to more expensive materials. The dangers associated with the recovery of the concentrated acid make this method less attractive. In addition, during combustion of lignin which is contaminated with hydrochloric acid there is some risk for dioxin emissions. Due to the corrosive problems with hydrochloric acid, focus has moved to concentrated sulphuric acid however, the major problem of recovering the acid remains unsolved so far.

CASH process

The Canada, America and Sweden Hydrolysis (CASH) process was developed in cooperation between Canada, America and Sweden. In this method, hydrolysis occurs with dilute sulphuric acid at a temperature of around 200°C (pressure 8-25 bar). Previous studies have shown that by using SO2 and dilute sulphuric acid in two steps, this increases the sugar and also ethanol yield since the amounts of inhibitors such as furfural are decreased. The process was developed for woody biomass. The ethanol yield is around 20% of the energy content in the raw material, however, up to 40% of the energy content of the biomass is bound in the hydrolysis residue, mostly the insoluble and condensed lignin, but also a large portion of unreacted cellulose. The reason why there is cellulose left in the hydrolysis residue is due to the reaction kinetics of the hydrolysis compared to the kinetics of the sugar break down reactions. At the end of the reaction, only the very stable form of the cellulose is left, making the hydrolysis reaction very slow, but at the same time, the sugar concentration has increased, making the breakdown reactions faster. At one point, the breakdown of the carbohydrates is faster than their formation and thereby the sugar concentration declines. The hydrolysis residue can be used as a solid biofuel in boilers or directly in powder fuel turbines or pelletised.