Processes for converting biomass to ethanol

First generation bioethanol

Bioethanol can be produced in several ways, depending on the type of biomass and the conversion technologies used.

·        Sugar crops : 1) fermentation - conversion of glucose to ethanol, 2) distillation - obtaining anhydrous ethanol  

·        Starch crops: 1) hydrolysis - conversion of starch to glucose, 2) fermentation, 3) distillation.

These feedstocks and conversion processes are used in what is commonly known as “first generation” bioethanol.

Second generation bioethanol

In the "second generation", bioethanol is produced from lignocellulosic feedstocks, which mainly consist of cellulose, hemicellulose and lignin. The conversion process is complex and there are four principal methods.

·        Biochemical conversion : 1) pre-processing (separating the components), 2) enzymatic hydrolysis - obtaining fermentable sugars from cellulose, 3) fermentation, 4) distillation  

·        Thermochemical conversion: 1) gasification - conversion of biomass into syngas in the presence of an oxidant (oxygen, steam, etc.), 2) purification, 3) catalytic conversion - conversion of syngas into ethanol

·        Mixed conversion : 1) gasification, 2) fermentation of syngas, 3) distillation

·        Chemical conversion : 1) decrystallisation, 2) diluted acid hydrolysis, 3) fermentation, 4) distillation

These technologies are more difficult to implement than those for converting sugar- or starch-based biomass. A few pilot plants already exist in Sweden and Canada. The industrialisation phase has begun in the US, where 6 pilot plants using biochemical and mixed technologies are being built.