0 - INTRODUCTION
In the last 50 years, the increase in world population, together with the progressive industrialization of developing countries, has more than quadrupled the world's energy needs.
Fig. 0.1 - World energy consumption by source, estimated by Vaclav Smil for Energy Transitions: History, Requirements and Perspectives.
The high volatility in the price of fossil fuels, and the environmental issues related to their use have always fed, over the years, the interest to seek alternative with wider geographic distribution and lower cost, such as biomass.
For example, in 1970, Brazil initiated a program to replace gasoline with ethanol in order to reduce its dependence during politically and economically unstable periods; in this program, the sugar cane was chosen as raw material to produce ethanol and, consequently, agricultural and technological studies were greatly intensified, leading Brazil in a very favorable position in terms of energy supply1.
In the production of bioethanol, only approximately one third of the sugar cane is utilized: the remaining two thirds are composed of bagasse, which can be burned in cogeneration plants bioethanol - electricity, and the rest is waste material, like the trunk and the leaves; also note that the part of the plant that can be converted into ethanol by direct fermentation (i.e. juices, rich in monosaccharaides, obtained by squeezing) is the same as used in sugar production, thus generating competition between the food market and energy market2.
It is increasingly understood that first-generation biofuels (produced primarly from food crops such as grains, sugar beet and oil seeds) are limited in their ability to achieve targets for oil-product substitution, climate change mitigation, and economic growth. Their sustainable production is under review, as is the possibility of creating undue competition for the land and water used for food and fibre production. A possible exception that appears to meet many of the acceptable criteria is ethanol produced from sugar cane.
The cumulative impacts of these concerns have increased interest in developing biofuels produced from non- food biomass. Feedstocks from ligno-cellulosic materials include cereal straw, bagasse, forest residues, and purpose-grown energy crops such as vegetative grasses and short rotation forests. These second-generation biofuels could avoid many of the concerns facing 1G biofuels and offer greater cost reduction potential in the longer term24.
In order to increase the production of bioethanol, and to avoid decompensation in the food market, it is therefore necessary to develop technologies to convert polysaccharides contained in the lignocellulosic biomass into fermentable sugars3. The reason why 2G biofuels have not yet been taken up commercially, despite their potential advantages over 1G biofuels, is that the necessary conversion technologies production are estimated to be significantly higher than for many 1Gs at the moment.
Fig. 0.3 – Volumes of 2G versus 1G biofuels produced and estimates for the next years25.
The cell walls of plants are the source of the lignocellulosic biomass, a rich and chemically complex material, whose structure is represented mainly by the the physical-chemical interaction between cellulose, a linear polymer of glucose, hemicellulose, a polysaccharide of irregular composition and highly branched molecule, and lignin, a complex and heavy organic polymer.
The conversion of lignocellulosic biomass into monosaccharides and then, after, ethanol, can be made bio- chemically, through acid or enzymatic hydrolysis; the latter is an extremely complex multi-enzyme process, which requires that the biomass is pretreated, as the lignocellulosic materials are structured to resist the action of physical, chemical and biological external agents.
Pre-treatment is one of the key unit operations for the successful conversion of lignocellulosic materials to ethanol. This is due to the close association that exists among the three main components of the plant cell
wall (cellulose, hemicelluloses and lignin), which is by far the most determinant factor for the low accessibility of plant carbohydrates to biological processes such as enzymatic hydrolysis and fermentation.
Therefore, the main role of a pre-treatment method is to decrease the interaction between the main components of cell wall and make them susceptible to both saccharification and fermentation. Likewise, the best pre-treatment conditions must be defined as those in which the maximum recovery of water-soluble hemicellulose sugars is obtained, along with the production of the best possible substrate for enzymatic hydrolysis and fermentation; the pretreatment is therefore intended to expose as much as possible to the next action of cellulose enzymes, capable of breaking the chain of the cellulose obtaining glucose4.
Fig. 0.3 - Effect of pretreatment on the structure of lignocellulosic biomass.
This work reports a pretreatment test for the 2G ethanol conversion of lignocellulosic biomass, in this case spelt straw harvested in the region of Garfagnana, Italy.
All the experimental campaign was carried out at the Laboratorio Bioetanol, property of Chemistry Institute of the Federal University of Rio de Janeiro, part of the European Union – Brazil cooperation program ProEthanol2G.
0.1 – Objectives of Present Work
Spelt, common name used for three differents species of the genre Triticum, represent the oldest harvested wheat, utilized as food by mens since the Neolithic era. In Northern Tuscany, Garfagnana is a mainly agricultural region where this kind of wheat is widely diffused; the straw obtained from harvesting the spelt, is a readily available waste product, which finds (partly) an use only in animal feeding. The idea at the base of the this project is to investigate the possibility of turning an agricultural waste product like spelt straw into a biomass feedstock for production of second-generation ethanol.
Fig. 0.4 – A spelt field.
In Brazil, the research and development for second-generation biofuels is trying to fight the rising of prices in food market due to competition with first-generation ethanol. The straw, harvested in Italy, has been carried to the Bioethanol Laboratory of the Federal University of Rio de Janeiro, an excellence in the research of pretreatment and biomass conversion technologies for 2G ethanol.
The straw has been processed with LHW (liquid hot water) pretreatment (see Chapter 1 for details) and, with a statistical and experimental analysis, the process will be fine-tuned to work with this type of feedstock.
In this work, in particular, will be examined the pretreatment condition that allows the best glucose yield, a parameter directly correlated to the ethanol yield, without producing inhibitors dangerous for glucose to ethanol fermentation.
Fig. 0.4 – The equipment used for pretreatment.
A limited number of tests will be performed, testing five different values for each of the two pretreatment values (residence time and temperature), using a central composite experimental design. Then, the best condition will be selected analyzing glucose yield combined with low concentrations of fermentation inhibitors; to validate the model, a final test with the pretreatment condition thus identified, will be run;
moreover, a kinetic model for the pretreatment phase, will be determined.
After determining the best condition, mass and energy balance for an ideal operating 2G ethanol plant will be calculated, referring to the area of Garfagnana as potential source of biomass.
Will be also performed additional analyzes on biomass such as moisture content, ash content, content of impurities extracted with water and ethanol, and characterization of chemical composition of the biomass, cellulose, hemicellulose and lignin.