Trash and bagasse are two fibrous materials from sugarcane, each with its own specific characteristics. Although sugarcane trash and bagasse are fibers of the same origin, their physical and chemical characteristics may differ significantly. The sugarcane bagasse has smaller particle size because it has been milled in the juice extraction process. This results in finer particle size when compared with unpro­cessed trash (Olivares et al., 1998).

Of the two, bagasse is the more studied material, because it is essentially an industrial residue that has been used for many decades as a fuel in bagasse boilers. Although it is well known that biomass fuels should have lower moisture content, the drying of bagasse has not been considered a very profitable process, at least until recently. For this reason, bagasse has been used with its original moisture content of approximately 50 per cent wet basis.

Regarding particle size, bagasse is a very homogeneous material, at least when compared with trash. Therefore, bagasse burning is more predictable a process because the biomass is composed of fibers with more or less similar characteristics, like composition and size. Dirt is also a much easier problem to solve for bagasse than trash, at least in conventional low-pressure boilers (2.1 MPa).

There is no existing technology and experiments on bagasse pelletization and briquetting of sugarcane trash. Limited information is available on bagasse pelletization but commercial bagasse briquetting has not been reported in Brazil (Bezzon, 1994; Cortez and Silva, 1997). Some technical difficulties are associated with the long-term integrity of the bagasse briquette. The bagasse high moisture content, nearly 50 per cent w. b., is considered the most negative factor when briquetting is considered. Bezzon (1994) conducted experiments at UNICAMP heating up the bagasse up to 200-300° C before briquetting (1 cm diameter and 2 cm length). The applied pressure ranged from 20-25 MPa and yielded briquettes with densities from 1000 to 1240 kg/m3. The results were promising but experiments with larger briquettes were not conducted. It is known that heating the briquette can melt lignin resulting in a fiber-binding material.

Trash is essentially an agricultural residue that is now being seriously considered for energy purposes. Tops and leaves are the main components of sugarcane trash and they are removed at two different stages of the harvesting process. Top removal is the very first operation executed on standing stalks, before base cutting. Very little information is available in Brazil concerning trash recovery and use. Very few experiments and data are reported about its characteristics and how this influences equipment design and operation. Usina Sta. Elisa in Sao Paulo has conducted some experiments on trash recovery in cooperation with Dupont and Class. Also experi­ments are currently being conducted by Copersucar on trash recovery and its use in boilers baling trash with the Class and Case machines, but no conclusive reports are available so far.

The present harvesting technology has two main drawbacks in relation to top recovery. The first one is related to the topper not being able to reach the tops of nonstanding stalks. Efforts are being made to remove tops using the extractors at the second cleaning stage of the chopper harvester. This operation is highly inefficient and top removal is directly related to cane losses. The second one arises from the fact that after cutting and chopping, a low surface density of tops is left on the field for future collections, using adapted hay technology. Raking and windrowing of tops over a bare soil surface results in high dirt contamination. Field experiments indicated soil contents of 20 kg per ton of trash recovered using hay equipment (Copersucar, 1997). Harvester design should include trash recovering from the beginning. Two pieces of existing technology need to be incorporated into a cane harvester to make trash recovering more efficient.

Tops are green, high moisture residues that require field natural drying to improve biomass quality. After harvesting the green cane, trash may be left on the soil to dry for a few days. When the trash is nearly dry, with approximately 30 per cent moisture content, it can be recovered. If left in the fields, it may increase the risk of fire or may slow down ratoon sprouting. Thus it is generally accepted that at least part of the trash should be recovered. The specialists’ recommendations on how much trash should be recovered vary from 50 up to 90 per cent. It is believed that organic material left in the fields may bring some agronomic benefits helping to control weeds and increasing the long-term soil fertility. An experiment has been reported by Molina et al. (1995), using a roller type trash baler, which processed 5.7 t/h, recovering 83 per cent of trash with 30 per cent moisture content and obtaining low-density bales with 120kg/m3.

A series of operations are required to perform trash recovery starting with raking the trash into continuous windrows after natural drying in the field. In the seq­uence, the trash must be baled to make transportation economically feasible. The commercial balers will compact up to 150 and 200kg/m3 density. The final product is supposed to resist transportation and storage in adverse climatic conditions when stored in the field. The operating costs may be the determinant in making the trash recovery feasible. The costs reported by Molina et al. (1995) varied from US$ 7 to US$ 25.00/t, depending on local conditions such as topography, infrastructure and available technology. It is a general consensus, in Brazil, that it will be difficult to compete with bagasse, but not so difficult to compete with other alternative energy resources, such as natural gas from Bolivia which is being offered in the market at a cost between US$ 2 to 3 per MMBTU. The systems tested in the last 5 years by Copersucar include various alternatives and resulted in costs below US$ 1/MMBTU in at least two cases.

Particle size is certainly a major consideration with trash because it may affect the conversion residence time in the reactor. This may also be affected by the moisture content in the particles, some of which have more moisture than others. The heterogeneous characteristics of the trash are certainly a drawback, which requires a fuel preparation procedure. Dirt in the trash is another major problem since the dirt increases the ash content and may interfere with the ash melting point and formation of deposits in the heat exchanger walls when combustion reactions take place.

In short, the existing boilers installed in sugar factories throughout the world can handle and operate better using biomass similar to bagasse. Any large variation in particle size, moisture content and dirt content will negatively affect the reactor efficiency and operation. Most likely, the most appropriated procedure is to prepare the fuel to meet the equipment requirements or, if possible, design a reactor that can efficiently operate within a larger spectrum of fuel properties. At UNICAMP an unbaling system is being conceived to unbale and feed biomass (trash) into a boiler (see Figure 6.1). In this project, the aim is to develop a technology that allows a continuous supply of biomass up to the boiler distribution system.

Boiler

Conveyor

Unbahnq system

Chopper

9§&. Remover

Figure 6.1. The UNICAMP unbaling-feeding system for biomass (trash).

The main task of the unbaling system is to cut the bale and then feed the biomass by means of the feeding screw. A chopper located at the silo’s bottom performs the cutting. The remover helps by rotating the bale and placing it against the chopper. Figure 6.1 shows the unbaling system and the chopper. The system may also include a dryer to homogenize the material moisture content.

The costs involved in the biomass preparation are not negligible. Besides the necessary investments in equipment, there is also the need for capital for the system operation, particularly if a drying system is required and a storage facility is needed. This infrastructure and economics are being examined at UNICAMP and an in-factory system is being considered during the tests. An important drawback is that there is little information in the literature about large-scale biomass preparation and handling, except in forestry, and this information is essential in this kind of projects.

The economic use of trash will depend on investigating more cost-efficient technologies to handle, transport and use the material, transforming it into a more valuable commercial product. In this sense, more research is needed not only in the conversion for electricity, ethanol and fuel gas production but also in charcoal production. Charcoal has a well-established market in Brazil and its production is still based on very traditional technologies, based on low-efficiency furnaces and waste forest wood (Rosillo-Calle et al., 1996). The charcoal production from trash and bagasse could benefit both the sugar and the steel industries in Brazil, but no technology has yet been developed to adapt such by-products for this purpose.