Care when baling sugarcane straw

With the increase in the collection and use of sugarcane straw to generate energy in plants, researchers are evaluating the impurities that may be present in the product during baling operations.

In Brazil, currently, around 90% of the nine million hectares of sugarcane are harvested mechanized and chopped during the harvesting process, according to data from the 3rd Conab harvest survey, December/2017, mainly in areas with slope of less than 12% and without natural obstacles. The adoption of the mechanized harvesting system for chopped sugarcane, when processing this entire plant mass, produces a large amount of vegetable waste: straw, which is separated from the sugarcane through the harvester's primary and secondary extractors.

Since the recovery of sugarcane straw has gained great importance in recent years for the sugar-energy sector, currently, one of the alternatives for collecting the remaining straw in the field is the adoption of the raking and baling operation a few days after harvest. . By burning it, there is the possibility of producing enough thermal and electrical energy to meet the plant's entire demand during the harvest and also generate surpluses for commercialization, being an important source of revenue that must be verified and validated.

In addition to the possibility of adding value to biomass, using this harvest residue to increase electricity generation with low greenhouse gas emissions, the presence of straw in the field provides benefits for agricultural production. Serving as soil cover, it brings advantages such as reducing erosion, conserving soil moisture and making nutrients available. However, in some areas, its accumulation can make it difficult for ratoon to sprout and encourage the proliferation of pests. In this way, part of the straw can be collected in the field and processed in the industry, while the rest remains in the soil for the aforementioned benefits. The quantity depends on several factors that must be analyzed according to certain climate, soil and sugarcane harvesting times.

Just like the amount of straw to be removed from the field, the quality of the material to be delivered to the plant is of fundamental importance for the maintenance and longevity of industrial equipment. This quality is measured based on the level of mineral impurities present in the straw. The equipment used to recover straw, the way it is operated and the soil and humidity conditions are the aspects that define the amount of impurities contained in the final product. Issues relating to collection costs and energy consumption are also relevant.

Mineral impurities, in addition to increasing the cost of transportation, considering that the earth adhered to the straw ends up making up the load and then needs to be returned to the field, also considerably increase the maintenance cost, especially of the equipment used to chop the straw in the field. and in industry. They also cause great damage by causing corrosion and scale in the tubes and components that exchange heat in the boilers, thus reducing their efficiency and useful life.

Among the advantages of baling, in terms of providing more favorable conditions for the use of straw in plants, is the possibility of collecting material with lower moisture content, increasing transport density and transforming straw into uniform units, which reduces the logistical costs of transportation, storage and processing.

However, this process of collecting straw in the field also presents bottlenecks that can impact the viability of using straw to generate bioelectricity. The baling process is made up of several activities (Figure 1), which involve operational costs and energy consumption, in addition to contaminating the raw material with impurities (earth). Among the operational stages of the bale route are: mechanized harvesting of sugar cane, harrowing, baling and loading and transport of bales. 

Studies on the use of straw to generate bioelectricity are being conducted by the Sucre Project, an initiative of the National Laboratory of Bioethane Science and Technology (CTBE), which is part of the National Center for Research and Energy in Materials (CNPEM), in Campinas ( SP). The project is managed by the United Nations Development Program (UNDP) and financed by the Global Environment Facility (GEF), and has been carrying out analyzes of the entire straw circuit at the plants since 2015. To evaluate the straw collection system via bales, analyzes of the content of mineral impurities were carried out in the various stages involving the straw collection process via bales, identifying the processes that most impact the variation in their contents. This will make it possible to propose strategies that improve straw quality. Mineral impurities are made up of the sum of ash that makes up straw and external contamination.

Raw material quality

The straw quality survey, in terms of mineral impurities, was carried out considering the steps described in Figure 1.

The tests were carried out in four plants, located in the state of São Paulo, throughout the 2015/2016, 2016/2017 and 2017/2018 harvests.

On average, the ash content of samples collected in the sugarcane field before sugarcane harvesting varied between 3% and 3,6% in all tests. The ash content takes into account mineral impurities, plus the ash constituents of straw. The sugarcane harvesting operation strongly contributed to the increase in the content of mineral impurities, as seen in Figure 2.

An increase of three to four times in the content of mineral impurities in straw is observed after mechanized sugarcane harvesting. This reinforces the need for correct control and regulation of the base cut for the quality of both the sugarcane and straw resulting from this process.

Harrowing is the second most preponderant operational factor for increasing the amount of soil in straw. According to assessments carried out by CTBE, within the Sucre Project, harrowing was responsible for increasing the content of mineral impurities in straw at Plant 2,5 by up to 3 times, as shown in Figure 3.

The raking operation allows the producer to group different amounts of straw to be collected, depending on the amount of biomass available and the strategy adopted by the plant/producer. Collecting larger amounts of straw potentially increases the hauling of soil to the windrows. The quality of the straw was also evaluated at different heights of the windrow formed (Figure 4).

It is observed that there is a tendency to increase the amount of land in the lower parts of the windrow. Regarding the amount of straw collected, worth up to 8t/ha, there is no difference in the average content of mineral impurities.

Finally, the amounts of soil in the bales were evaluated (Figure 5). The tests were carried out in several plants and different brands/manufacturers.

There was a wide variation in the levels of mineral impurities, with a minimum value below 6% in total ash and a maximum of almost 25%. Based on Figure 4, it can be assumed that the quality of straw in the bales is determined by the previous condition, windrow formation. When two brands were placed side by side, in different plants, with different operators, the ash obtained similar results. 

Field tests carried out to date have indicated the levels of mineral impurities that are commonly found in current practices. However, tests relating to the processing of baled straw at the plant and its burning in the boilers are still being carried out at some partner plants, in an attempt to determine limits for the mineral impurities in straw, and the impacts of other lesser-known contaminants (potassium, chlorine , sulfur) and less tested, in bagasse boilers burning more significant amounts of straw.

Fábio Makoto Okuno, Jorge Luís Mangolini Neves, Daniel Duft, CTBE and CNPEM

Article published in issue 183 of Cultivar Máquinas, April 2018.