Sugarcane Bagasse
Sugarcane bagasse is the fibrous matter that remains after sugarcane stalks are crushed to extract their juice. For each 10 tonnes of sugarcane crushed, a sugar factory produces nearly 3 tonnes of wet bagasse. Since bagasse is a byproduct of the sugarcane industry, the quantity of production in each country is in line with the quantity of sugar produced. Also, in comparison to other agricultural residues, bagasse can be considered as a rich solar energy reservoir due to its high yields (about 80 t/ha in comparison to about 1,2, and 20 t/ha for wheat, other grasses and trees, respectively) and annual regeneration capacity [93].
The high moisture content of bagasse (typically 40 to 50%) is detrimental to its use directly as a fuel. In general, bagasse is stored prior to further processing. For electricity production, it is stored under moist conditions and the mild exothermic reaction that results from the degradation of residual sugars dries the bagasse pile slightly. For paper and pulp production, it is normally stored wet in order to assist in the removal of the short pith fibers, which impede the papermaking process, as well as to remove any remaining sugar.
Bagasse is an extremely inhomogeneous material comprising around 30-40% of "pith" fiber, which is derived from the core of the plant and is mainly parenchyma material, and "bast," "rind," or "stem" fiber, which comprise the balance and are largely derived from sclerenchyma material. Bagasse is often used as a primary fuel source for sugar mills; when burned in quantity, it produces sufficient heat energy to supply all the needs of a typical sugar mill, with energy to spare. To this end, a secondary use for this waste product is in cogeneration, the use of a fuel source to provide both heat energy used in the mill, and electricity, which is typically sold for the consumer electricity grid. The resulting CO2 emissions are equal to the amount of CO2 that the sugarcane plant absorbed from the atmosphere during its growing phase, which makes the process of cogeneration greenhouse gas neutral. In many countries, sugar factories significantly contribute "green" power to the electricity supply. For example, Florida Crystals Corporation, one of America’s largest sugar companies, owns and operates the largest biomass power plant in North America. The 140 MW facility uses bagasse and urban wood waste as fuel to generate enough energy to power its large milling and refining operations, as well as supply enough renewable electricity for nearly 60,000 homes.
Use in the paper industry is the second largest revenue stream from bagasse from sugar factories and is second only to electricity cogeneration. It is thought that bagasse has the added advantage over other forms of papermaking feedstock such as wood pulp. It generates a relatively small amount of greenhouse gases in the collection process compared to harvesting of wood chips, as the sugarcane has already been transported to the factory for extracting the sugarcane juice. Due to the ease with which bagasse can be chemically pulped, bagasse requires less bleaching chemicals than wood pulp to achieve a bright, white sheet of paper. The fibers vary in length depending on the country and cane variety, but are typically about 1.3 to 1.7 mm long. Bagasse fibers are well-suited for tissue, corrugating medium, newsprint, and writing paper. Other uses of sugarcane bagasse include making boards similar to plywood or particle board, namely bagasse board, and it is also made into cattle feed, whereby it is mixed with molasses. Sugarcane bagasse is very high in cellulose compared to many other feedstocks and the average analysis of bagasse is shown in Table 3.9 [94].
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Table 3.9 Average chemical composition of sugarcane
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Feedstocks for Cellulosic Ethanol Production 65