Steam Explosion- How it works….

( the Chemistry behind it all )

The main components of fibrous materials are cellulose and lignin.  The lignin acts like glue and binds the cellulose chains together. In order to utilize these fibers, it is necessary to separate the lignin from the cellulose. In the past, it has only been possible to break the bonding between the cellulose and the lignin through very expensive and highly polluting chemical pulping processes.  The steam explosion process can break the bond between the cellulose and the lignin using only high pressure steam.  Steam Explosion Technology has application in pulping of non-woody material, cellulose derivatives, and fractionation.

Continuous Steam Explosion technology provides industry with a low cost and environmentally friendly route to such products as pulp for paper, cellulose derivatives for food application, and specialty chemicals . The continuous process uses high pressure steam to free the cellulose from materials such as wood, straw and sugarcane bagasse. The high temperature of the process eliminates or greatly reduces the requirements for using chemicals associated with current industrial practices.

Steam explosion is an autohydrolysis process, meaning water is used in the reaction to break molecules into their fragments, in our case wood, peanut shells, and other biomass. Its effect on biomass include:

1)  the splitting of accessible glycosidic bonds,

2)  the splitting of alpha-ether linkages of lignin,

3)  the splitting of lignin-carbohydrate complex bonds, and

4)  minor chemical modification of lignin and carbohydrates.

Glycosidic bonds are how carbohydrates attach to one another. The hydrolysis of a glycosidic bond consumers water which is needed for the process to work.

By splitting ligin at the alpha position it increases the polarity of lignin. If enough linkages are split it will eventually get dissolution of the polymer causing some depolymerization of ligin.

The major carbohydrate found in wood is glucose, the building block of cellulose. This process tries to break the hydrogen and possible covalent bonds between the cellulose, hemicellulose, and lignin to be used as industrial raw materials.

In order to obtain cellulose, hemicellulose, and lignin from the exploded fibers, fractionation, or separation, must be carried out.

One consequence of the steam explosion on the structure and behavior of lignocellulosic materials is the solubility of the biomass in neutral and alkali solvents. When exposed to a neutral solvent the lignin softens such that when the pulp processed under higher temperatures it tends to allow the lignin to spread amongst the fibers. The lignin then hardens stiffening the sheet. An alkaline solvent could cause the lignin to dissolve onto the pulp fibers, like the Kraft process.

Normally, steam assisted fractionation of biomass by steam explosion produces two useful polymer fractions.  First is a partially hydrolyzed cellulose fraction which normally results in a 40-50% yield.  Second is a pure, alkali-soluble, lignin fraction which results in a 15-25% yield.  (Yield depends primarily on the severity of the steam explosion process performed.)