18.4.1 Feedstocks

In Brazil, sugarcane-derived bioethanol is one of the most successful examples of large scale biofuel production, distribution and use. The use of ethanol in transportation and for the production of electricity from sugarcane residues accounts for nearly 16% of the total energy supply [1], making it the second primary source after oil. Current ethanol production is based on first generation (1G) technology, fermenting sugars extracted from sugarcane stalks. However, sugar only represents approximately one third of the energy content of sugarcane. The other two thirds are composed of straw that is either burned on the field or left as mulch, and bagasse, the fibrous material left from the juice extraction process, which is mostly used as fuel for process heat and electricity generation at the mill. Cellulosic ethanol, also known as second generation (2G) ethanol, can be produced from what is currently considered agricultural and industrial residues (straw and bagasse).

Bagasse is readily available at the plant site without collection and transportation costs, in the shredded form and with low ash content. However, two trends might put pressure on bagasse availability for cellulosic ethanol production: the declining fiber content of new sugarcane varieties — a target of most crop breeding programs — and the increase in surplus power generation, especially in the new mills. On the other hand, there are commercially available technologies to reduce process steam consumption — the main energy demand at the plant — thus reducing bagasse consumption and increasing its availability for cellulosic ethanol production.

Pre-harvest burning has been a conventional practice to facilitate sugarcane manual harvest. Due to environmental and socioeconomic reasons, there are ongoing burning phase out programs in the main sugarcane-growing regions in Brazil, with the gradual replacement of manual harvest with burning by mechanized harvest without burning leaving 10-20 tons (dry matter) of straw per hectare on the field. This lignocellulosic material has also been considered as a feedstock for cellulosic ethanol production in Brazil. However, the task of collecting, transporting and pre-treating this material presents important challenges that need to be overcome before it can be used on a commercial basis. The low mass and energy density, and the distribution throughout extensive land areas are limitations due to transportation costs. Collection methods such as bailing can deteriorate the quality of the feedstock by increasing the ash content to a level that requires a pretreatment to bring the values to acceptable levels [2]. There is ongoing research to develop mechanical harvesters that can efficiently handle both sugarcane stalks and straw, being capable of separating and conditioning the straw with sufficient load density for low-cost transportation as well as maintaining adequate quality for industrial use.

There are also potential benefits of leaving crop residues on the field, such as protection against erosion, nutrient cycling, soil carbon sequestration, weed suppression and soil moisture retention. On the other hand, considering the large quantities of residue generated and their high carbon-to-nitrogen ratio and fiber content, it is likely that removing part of the straw will still secure most environmental and agronomic benefits. Those benefits are site specific, so the amount of straw that can be removed sustainably should be calculated considering climate, topography, soil, and crop variables.

Sugarcane is a semi-perennial crop with a plant crop and successive regrowth crops, known as ratoons. After five or six harvests on average, it is necessary to replant the crop, and there is usually a period of a few months in which there is a fallow period or cover crops, usually legumes. Sweet sorghum (Sorghum bicolor L. Moench) has been evaluated as feedstock for both first and second generation ethanol production [3]. Currently, there is preliminary research in Brazil for cropping sweet sorghum in the short period between sugarcane cycles, providing supplementary feedstock for ethanol mills in a period of low sugarcane availability.

The growing interest in bioenergy crops led to the development of cane varieties with high stalk and leaf fiber and lower sucrose content, called “energy cane” [4]. Since the primary energy content per unit of cropped area is higher than with conventional sugarcane, energy cane has the potential to become another alternative feedstock for cellulosic ethanol production in Brazil.