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Agroindustrial and forestry residues, which are by-products of key industrial and economical activities, stand out as potential raw materials for the production of renewable fuels, chemicals and energy. The use of wastes is advantageous as their availability is not hindered by a requirement for arable land for the production of food and feed. In addition, waste utilization prevents its accumulation, which is of great environmental concern due to its potential for contamination of rivers and underground water. In Brazil, the agroindustry of corn (13767400 ha), sugarcane (7080920 ha), rice (2890930 ha), cassava (1894460 ha), wheat (1853220 ha), citrus (930591 ha), coconut (283205 ha), and grass (140000 ha) collectively occupies an area of 28840726 ha (FAOSTAT, http://www. faostat.fao.org/site/567/default.aspx#ancor) and generates 597 million tons of residue per year. By itself, this scale of operation calls for new solutions aiming for the appropriate utilization of these valuable resources. However, innovative dealings must be environmentally and economically acceptable and, most importantly, have social meaning. Indeed, great social benefits could draw from novel yearround activities as alternatives for the typical seasonal jobs in agroindustry. Considering the production of biomass ethanol, the abundance of feedstock near the site of processing must be taken into account, as low-density biomass involves significant handling and transportation costs. Within this context, the crushed stalk of sugar cane (bagasse) and straw are obvious choices, although bagasse is often burned for the production of steam (heat) and power/electricity in sugar-ethanol mills and important amounts of straw are needed to keep the soil nutrients balance. Other agricultural by-products of importance in Brazil, such as corn straw, wheat straw, rice straw and rice hulls, grass and forestry materials and residues from citrus, coconut and cassava processing, also deserve attention as local feedstock for the development of new and profitable activities. As each type of feedstock demands the development of tailor-made technology, the diversity of the aforementioned raw materials could allow for new solutions for the production of chemicals, fuels and energy in accordance with the local availability of these materials.

BackgroundThe conditions for steam pretreatment of sugar cane bagasse and leaves were studied using CO2 as an impregnating agent. The following conditions were investigated: time (5 to 15 min) and temperature (190 to 220°C). The pretreatment was assessed in terms of glucose and xylose yields after enzymatic hydrolysis and inhibitor formation (furfural and hydroxymethylfurfural) in the pretreatment. Results from pretreatment using SO2 as impregnating agent was used as reference.ResultsFor sugar cane bagasse, the highest glucose yield (86.6% of theoretical) was obtained after pretreatment at 205°C for 15 min. For sugar cane leaves the highest glucose yield (97.2% of theoretical) was obtained after pretreatment at 220°C for 5 min. The reference pretreatment, using impregnation with SO2 and performed at 190°C for 5 min, resulted in an overall glucose yield of 79.7% and 91.9% for bagasse and leaves, respectively.ConclusionsComparable pretreatment performance was obtained with CO2 as compared to when SO2 is used, although higher temperature and pressure were needed. The results are encouraging as some characteristics of CO2 are very attractive, such as high availability, low cost, low toxicity, low corrosivity and low occupational risk.

The industrial production of sugar syrups from lignocellulosic materials requires the conduction of the enzymatic hydrolysis step at high-solids loadings (i.e., with over 15% solids [w/w] in the reaction mixture). Such conditions result in sugar syrups with increased concentrations and in improvements in both capital and operational costs, making the process more economically feasible. However, this approach still poses several technical hindrances that impact the process efficiency, known as the “high-solids effect” (i.e., the decrease in glucan conversion yields as solids load increases). The purpose of this review was to present the findings on the main limitations and advances in high-solids enzymatic hydrolysis in an updated and comprehensive manner. The causes for the rheological limitations at the onset of the high-solids operation as well as those influencing the “high-solids effect” will be discussed. The subject of water constraint, which results in a highly viscous system and impairs mixing, and by extension, mass and heat transfer, will be analyzed under the perspective of the limitations imposed to the action of the cellulolytic enzymes. The “high-solids effect” will be further discussed vis-à-vis enzymes end-product inhibition and the inhibitory effect of compounds formed during the biomass pretreatment as well as the enzymes’ unproductive adsorption to lignin. This review also presents the scientific and technological advances being introduced to lessen high-solids hydrolysis hindrances, such as the development of more efficient enzyme formulations, biomass and enzyme feeding strategies, reactor and impeller designs as well as process strategies to alleviate the end-product inhibition. We surveyed the academic literature in the form of scientific papers as well as patents to showcase the efforts on technological development and industrial implementation of the use of lignocellulosic materials as renewable feedstocks. Using a critical approach, we expect that this review will aid in the identification of areas with higher demand for scientific and technological efforts.