Cellulose Degradation, Lignocellulose Conversion to Second-Generation Biofuels

Dear Colleagues,

With the continued pace of world economic growth, sustainable socioeconomic development will depend upon a supply of stock material inputs for agriculture, industry, and energy. Nowadays’ strong reliance on nonrenewable resources—especially fossil fuels—is constrained by economic, political, and environmental factors. The reliance on nonrenewable resources is accompanied by a heavy reliance on chemical and thermochemical processes; it is worth mentioning that the role of biological processes in the global economy is small, but growing fast.

The fermentation of sugar-based raw materials is referred to as “first-generation” bioethanol, whereas the use of lignocellulose raw materials is commonly called “second-generation” bioethanol.

Current technologies of biofuels are based on the transformation of glucose-based carbohydrates into bioethanol and vegetable oils into biodiesel fuel. At present, the main sources of carbohydrates for the fermentation into bioethanol are juices of sugarcane and sugar beet, as well as starches of corn, wheat, potatoes, and some other agricultural plants. For example, in Brazil 100% of ethanol is produced from the juice of sugarcane, and in the United States approximately 85% of the total ethanol production relies on corn grains. Therefore, the currently implemented “mature” technologies for the production of liquid biofuels are based on the transformation of food-grade carbohydrates into bioethanol. This approach results in a strong competition for agricultural land, which can cause the excessive consumption of water, an increase in food and feed prices, and even their deficits. Therefore, an alternative way to produce biofuels without competing with the food and feed industry is through the extensive and effective use of nonedible biomass, which involves softwood, forest residues, agricultural residues, as well as end or byproducts of the pulp and paper industry, etc. 

“Second-generation” bioethanol, with lignocellulose material as the feedstock, is a promising alternative to first-generation bioethanol.

A key chemical in the whole technology of second-generation fuel is undoubtedly cellulose, which is considered as the most widespread polymer that is endlessly renewable in nature. This polysaccharide is present in all plants and algae. It consists of long chains of β-glucose monomers gathered into microfibril bundles. Its special and complicated structure makes cellulose resistant to both biological and chemical treatments. Since lignocellulosic materials are very complicated, their pretreatment and saccharification are not simple either.

Once the lignocellulosic biomass is pretreated and hydrolysed, the released sugars can be fermented and the downstream process is similar to that of first-generation biofuels. Due to its dense structure, the barrier properties of lignin, and the nonproductive sorption of cellulolytic enzymes by hemicelluloses, the initial lignocellulosic biomass is highly recalcitrant to enzymatic hydrolysis. Therefore, some kind of pretreatment is usually applied to make the biomass more accessible to enzymes.

The major barriers for the commercialization of second-generation ethanol production are the high costs of the pretreatment, the enzymes used in the hydrolysis, and the conversion of 5C sugars into ethanol.

The first stage of this technology is the hydrolysis of cellulose into glucose. Such hydrolysis can be performed both by acids or cellulolytic enzymes. Enzymatic hydrolysis is preferable because this process is carried out at moderate temperatures and gives an increased yield of the sugar. The main objectives of the pretreatment are to loosen the physical structure of the biomass and eliminate noncellulosic components (lignin and hemicelluloses) in order to increase accessibility and the percentage of the cellulose fraction.

Therefore, the specific goal of this Special Issue is to publish both recent innovative research results and review papers on the problem of cellulose conversion into second-generation biofuels. Research papers are welcome, but if you would like to contribute a review paper please contact one of the editors to discuss the topic’s relevance before submitting the manuscript.

Dr. Andrzej Klasa
Dr. Andrzej Baryga
Guest Editors

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