Evolution of an industry

The current dominant ethanol production process involves the fermentation of sugars and starches in food crops such as sugar cane, corn, potato and  cassava. These raw materials have short molecules which are quite easily digested by enzymes and are the feedstock for the first generation ethanol industry.

Major research has taken place over the last decade to find a way to be able to use agricultural waste products, such as the corn stover (stalks, leaves and other residue) and sugar cane bagasse (plant mass after the removal of sugar). The unutilized sugars contained in these types of biomass are made up of long molecules of polysaccharides which cannot easily be converted into ethanol by simple fermentation. In addition, cellulose and hemicellulose are entrapped in a matrix of lignin, preventing the enzymes’ access to the sugars and thus their transformation into ethanol.

The key is a pre-treatment phase to be integrated into the production process. Numerous pre-treatment process designs were developed using either acid or enzymatic hydrolysis or a combination of both; or thermo-mechanical technologies. The challenge was to move such technologies from small scale pilot and demonstration plant production to cost-efficient commercial scale production.

Cellulosic Ethanol Production: Nickel's role

A large share of the production equipment in any commercial scale ethanol plant is made from nickel-containing stainless steels such as Type 304(L) (UNS S30400 and S30403) and 316(L) (S31600 and S31603). They are the materials of choice for  numerous applications as they offer corrosion resistance coupled with good strength, ductility, toughness and ease of fabrication. The ethanol industry appreciates the long maintenance-free performance of production equipment made from stainless steels. 

For cellulosic ethanol production, the pre-treatment environments are harsh. This is especially the case in plants using acid hydrolysis, where the biomass is treated with diluted sulphuric acid at high  temperature and high pressure. This operating environment is highly corrosive and often nickel alloys such as Alloy C276 (UNS N10276) and other C-type nickel alloys must be used.

Many pre-treatment process technologies are based on enzymatic hydrolysis. Depending on the biomass feedstock, physical or chemical methods will be used. Some of the most cost-effective technologies may involve a combination of both acid and enzymatic hydrolysis.

Various stainless steel grades and nickel alloys are used in such operating environments: Types 316(L) and 316Ti (S31635), specialty stainless grades 904L (N08904), duplex 2205 (S32205), super duplex grades, such as 2507 (S32750), and super-austenitic 6% molybdenum alloys such as N08367 and S31254.

The bio-based chemicals industry

A cost-competitive bio-based chemical industry is emerging globally using bio-renewable feedstocks instead of fossil-based ones. The demand for  biochemicals is driven by both consumers and manufacturers concerned about long term sustainability of fossil fuels, as well as the environment.

Research activity is intense and some biobased chemical process technologies have reached commercial scale production. There is increasing demand for standard bio-based chemicals such as lactic acid, glycerin-based chemicals and bio-plastics and additionally for new ones such as biomethanol, bio-epichlorohydrin (ECH), biomonoethylene glycol  (MEG), bio-succinic acid and n-butanol/isobutanol.

With highly corrosive operating media, this industry will be relying on standard and specialty nickel-containing stainless steels for most production equipment. As an emerging industry with new process-related developments, appropriate material selection choices are crucial for economically viable commercial scale production and long term sustainability.

This article was published originally in Nickel Magazine VOL 29, NO2., 2014 - Game changers