Contribution of sugarcane to energy security and climate change mitigation

At the same time that, from the imperative need for energy security, the importance of the energy matrix relying on diverse sources arises, the planet urgently demands the replacement of fossil energy sources with renewable ones, in order to mitigate climate change which, in fact, has led to an increase in the frequency of extreme events, such as severe droughts which, among other damages, precisely cause energy crises. In this context, the immense value of taking advantage of all opportunities to increase the availability of renewable energy can easily be seen. Solar energy, wind energy, bioelectricity (electricity produced by burning biomass), biofuels (vehicle fuels are also energy commodities!), in short, every form of renewable energy is welcome and no opportunity should be wasted to increase its availability, to economically, socially and environmentally sustainable way.

In the Brazilian electrical system, thermoelectric plants powered by fossil fuels (natural gas, oil and coal) can even be turned off at times when demand is adequately supplied by other sources. With this in mind, it is worth noting that fossil fuel thermoelectric power is, by far, the most expensive among energy sources. Therefore, the increase in renewable energy production results in a lower risk of an energy crisis, a lower kWh price and, finally, a reduction in greenhouse gas emissions.

The energy of sugar cane

Of all the energy that sugarcane stores in its aerial part, resulting from photosynthesis, approximately a third is found in the sugars in the juice, a third in bagasse (the residue from juice extraction, rich in cellulose) and a third in leaves and tips (main constituents of the harvest residue known as “chaff”). For commercial sugars (brown, crystal, demerara, refined, etc.), the commercial value of sucrose has motivated sugarcane production in Brazil since 1532. Ethanol, today known as “first generation” (i.e., produced by the fermentation of sugars from broth), was treated as a by-product before the 1970s, as it is unfeasible to crystallize 100% of the sucrose in the broth, so ethanol production was motivated, before the “oil shocks”, mainly to take advantage of the residual sucrose in the molasses, and also the sugars glucose and fructose, present in the broth in smaller quantities than sucrose, which are fermentable, although they are not used for the production of commercial sugars.

Since Proálcool, the commercial importance of ethanol has grown, initially due to rising oil prices, but later the perception of the importance of renewable fuels for reducing greenhouse gas emissions became clearer. Given the low nutritional value of sugar and the high environmental value of ethanol, currently, among the products extracted from sugarcane juice, ethanol has the greatest social and environmental value, although sugar has a very strategic economic value for the stability of the productive sector , as the diversity of commercial products mitigates the risks arising from market fluctuations and cycles.

As for energy from sugarcane bagasse, until the end of the 2001th century it was used exclusively for the energy demands of industrial processes themselves. Today this seems absurd, but at that time there were no market structures through which plants could sell surplus energy. After the XNUMX energy crisis, improvements in boiler efficiency began to be encouraged, allowing plants to generate surplus energy and sell it (so-called “cogeneration”).

Finally, we come to chaff, harvest residue with almost a third of the energy of sugarcane. When sugarcane was harvested manually, the straw made the work of the cutters so unhealthy, painful and unproductive that it was necessary to burn it to prepare the sugarcane for harvesting. With the advent of mechanized harvesting, the practice of burning straw was abandoned, and sugarcane production began to use a new harvest residue, chaff, which is so valuable that it is more appropriate to call it a byproduct rather than residue.

Sugarcane straw has agronomic value when left in the soil, as it promotes great improvements in the productive environment and, consequently, better productivity, both of stalks (TCH) and sugar (TAH) per hectare, reduction of erosion, water loss through evaporation , increasing the organic matter content of the soil (which increases the efficiency of fertilization), recycling of nutrients (which in the long term reduces the need for fertilizer), improving the microbiological quality of the soil. In some annual agricultural crops, for example in the off-season soybean/corn succession system, there are strongly recommended agricultural practices, such as the intercropping of corn with brachiaria, in order to produce straw, such is the agronomic value of this by-product. Meanwhile, sugar cane has the privilege of producing very abundant straw on its own, around 140 kg of dry mass of straw for each ton of stalks produced.

Therefore, the straw left in the field is a valuable agricultural input, while the straw collected, carrying around a third of the energy of the aerial part of the sugarcane, can be raw material for the production of second generation ethanol (produced by the fermentation of glucose obtained by hydrolysis of cellulose) and/or for the production of bioelectricity. Both forms of energy are renewable and constitute valuable contributions to energy security and the reduction of greenhouse gas emissions.

In this context, questions arise about the best practices to optimize the use of this sugarcane by-product: how much straw to leave in the field to guarantee the agronomic benefits? Under what circumstances is it preferable to collect the straw completely, or partially collect it, or leave all the straw in the field? The questions seem simple, but the answers depend on several factors, such as soil and climate characteristics, incidence of diseases and pests (as the incidence of some pathogens and insects is favored by straw, while that of others is reduced), in addition to of the numerous aspects in which straw management influences the productive environment, requiring broad and transdisciplinary knowledge to support the best decisions.

Through research conducted at Embrapa Agropecuária Oeste, some recommendations along these lines have already been published, such as the preferred areas for collecting sugarcane straw in order to reduce diseases (Technical Communication 264); the recommendation, in the edaphoclimatic conditions of southern MS, to avoid the practice known as “desaleramento” or “de-rowing” (Technical Communiqué 248) 7. And there is other extensive work in progress in this line of research, studying various impacts of straw on the environment production and productivity of stalks and sugar cane, aiming at the continuous improvement of knowledge to support the best decisions, in order to optimize the use of the valuable agricultural input and energy raw material known as sugar cane chaff.

José Rubens Almeida Leme Filho, researcher at Embrapa Agropecuária Oeste