Cellulosic ethanol – The basics: Concepts and feedstocks

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Section: ADVANCED BIOFUELS
Series: Cellulosic ethanol
- The basics: Concept and feedstocks
- The basics: Conversion pathway – Biochemical
- The basics: Conversion pathway – Thermochemical
- Cellulosic ethanol biorefineries at commercial scale
Posts: CELLULOSIC ETHANOL

1. General concepts

Ethanol (also known as ethyl alcohol, grain alcohol, drinking alcohol or just alcohol) is a volatile, flammable, colourless liquid with the molecular formula C2H6O (it can be also written as CH3CH2OH and C2H5OH) and is often abbreviated as EtOH.

The European Renewable Ethanol Association (ePURE) states that the ethanol market can be broadly split into three segments:
(1) Fuel: Ethanol used as an additive in petrol or as an alternative fuel.
(2) Potable: Ethanol used to produce spirit drinks, as a food additive, for the extraction of aromas, for food preservation and the production of white vinegar.
(3) Industrial: Ethanol used as a renewable chemical component in various products as well as a renewable raw material for biobased chemical production.

Depending on its origin, the ethanol can be classified as:
- Renewable (bioethanol): It is produced from renewable biological resources (agricultural feedstock, agricultural waste, organic waste products, biomass, microorganisms).
- Synthetic (petroleum-derived ethanol): It is generated as a derivative from ethylene production using fossil-based raw materials.
Renewable and synthetic ethanol are chemically indistinguishable. The only difference between the two is the isotopic composition of the carbon atoms. Synthetic ethanol comes from fossil raw materials and renewable ethanol from “contemporary” materials.

Figure 1. General ethanol classification according to its origin

In turn, the renewable ethanol can be categorized as:
- Conventional (first generation, 1G): It is produced from starches or simple sugars.
- Cellulosic (second generation, 2G, advanced): It is produced from cellulosic feedstocks unsuitable for human consumption (agricultural and forest wastes, lignocellulosic crops).
Cellulosic ethanol and conventional ethanol are chemically identical, the isotopic composition is also the same. However, the first one is produced from different raw materials through more complex processes.

There are several technologies that can be used in conversion of biomass to cellulosic ethanol. The technologies can be grouped in two general pathways:
- Biochemical conversion (fermentation) through pretreatment and hydrolysis.
- Thermochemical conversion through gasification.

2. Feedstocks

Conventional ethanol production uses a fermentation process to convert starches or simple sugars contained in food crops.
- Simple sugars: sugar beet, sugar cane.
- Starches: wheat, corn barley, rye, triticale.
The vast majority of the world ethanol is produced from either corn or sugarcane. Currently, the most commonly used feedstocks in Europe are corn, sugar beet and wheat.

Cellulosic ethanol is produced from lignocellulosic biomass, which is primarily composed of cellulose, hemicellulose and lignin. Lignocellulosic biomass can be generally categorized as:
- Virgin biomass from naturally occurring plants (conventional logging residues, wood processing mills residues and removal of excess wood from forestlands).
- Waste biomass from industrial and agricultural by-products. Agricultural crop residues (rice straw, corn stover, sugarcane bagasse), manure, municipal solid waste and food/feed processing residues.
- Energy crops that are grown specifically for cellulosic ethanol production. Perennial grasses (switchgrass, Miscanthus) and short-rotation woody crops (poplar, willow, eucalyptus).

Figure 2. Two examples of lignocellulosic feedstocks: rice straw (left) and Miscanthus (left)

Cellulose is an important structural material for plants and is the most abundant biological molecule in the world. It works like a skeleton that allows them to stand upright and grow toward the sun, withstand environmental stresses and block pests. It is made up of many repeating glucose units (six carbon sugars, C6). Hemicellulose is a sugar polymer chain of xylose (five carbon sugars, C5). Lignin forms the hard plant cell walls. The production process for ethanol made from cellulosic feedstocks requires breaking down the feedstock into fermentable sugars (biochemical conversion). To achieve this, innovative technologies in the form of pretreatment and hydrolysis are used. Once these processes are completed, the remaining production steps are similar to those used to make ethanol from sugar crops. Lignin cannot be fermented into liquid fuels as can cellulose and hemicellulose.

3. Sustainability issues

The use of food crops for the production of 1G biofuels sparked the “food versus fuel” debate. On the one hand, biofuel critics claimed that farmers would sell their crops to higher-paying biofuel manufacturers instead of to their traditional purchasers and thus create food shortages and rapid price increases. On the other hand, supporters asserted that those effects could be attributed to rising costs of petroleum and not to biofuel production. The role of biofuels in global food price dynamics has been the subject of considerable discussion and media attention since 2007. During that debate, cellulosic ethanol emerged as an alternative to 1G ethanol, because it could use waste and non-food plants grown on lower-quality land.

Cellulosic ethanol is considered to provide better performance in terms of low risk of direct and indirect land-use change (ILUC) impacts. Because many cellulosic crops are perennial and roots are always present, they guard against soil erosion and better retain nitrogen fertilizer. Most cellulosic sources require much less intensive management than do grain crops, saving the fuel and carbon dioxide costs associated with field crop operations. The climate benefits of cellulosic biofuels derive from two sources: avoided petroleum use (the fossil fuel offset) and GHG mitigation during biofuel production, principally by soil C accumulation and avoided emissions.

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References
[1] Raphael Slade, Ausilio Bauen and Nilay Shah: “The greenhouse gas emissions performance of cellulosic ethanol supply chains in Europe”. Biotechnology for Biofuels 2(1):15, September 2009.
[2] Randy Schnepf (Coordinator): “Cellulosic Ethanol: Feedstocks, Conversion Technologies, Economics, and Policy Options”. Congressional Research Service, 7-5700, R41460, October 22, 2010.
[3] Kim Seung-Soo, Kim Jinsoo, Shin Seong-Cheol, Agblevor Foster A.: “Distinction between Bioethanol and Synthetic Ethanol in a Mixture of Gasoline Using Low Level Liquid Scintillation Counting”. Chemistry Letters 38(8):850-851, August 2009.
[4] G. Philip Robertson et al.: “Cellulosic biofuel contributions to a sustainable energy future: Choices and outcomes”. Science, 30 Jun 2017: Vol. 356, Issue 6345, eaal2324.
[5] Chen-Guang Liu et al.: “Cellulosic ethanol production: Progress, challenges and strategies for solutions”. Biotechnology Advances, Volume 37, Issue 3, May–June 2019, Pages 491-504.
[6] Monica Padella, Adrian O’Connell and Matteo Prussi: “What is still Limiting the Deployment of Cellulosic Ethanol? Analysis of the Current Status of the Sector”, Appl. Sci. 2019, 9, 4523; doi:10.3390/app9214523, 24 October 2019.
[7] Climate Technology Centre and Network (CTCN) - Cellulosic ethanol.
[8] Encyclopedia Britannica - Cellulosic ethanol.
[9] ePURE - What Is Renewable Ethanol?.
[10] ETIP Bioenergy - Cellulosic ethanol.

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