Using Biogas From Vinasse To Enable Sugarcane Bagasse Availability For Bioethanol Production

Recent environmental policies demand more and more biofuels, clean energy availability, and fewer pollutant emissions. In that sense, the ethanol and sugarcane industry stands out due to bioenergy production, especially in Brazil, where it produces not only bioethanol but bioelectricity as well, with a low net CO₂ emission. This is due to the burning of sugarcane bagasse in CHP (combined heat and power) cycles, which provides the sugarcane mill with heat and power for the process and which produces surplus electrical energy, which is sold to the national grid.

Not all of the sugarcane industry has a low carbon footprint, such as the ones that use sugarcane as a feedstock for ethanol production. For instance, corn-based ethanol uses natural gas, a fossil fuel, for power and heat generation, which accounts for high CO₂ emissions, whereas burning bagasse (which is a renewable fuel) does not.

The sugarcane mills are not free of environmental impacts, however. Burning bagasse generates particulate material, polluting the atmosphere and affecting neighboring regions; the production of ethanol produces a by-product called vinasse, a wastewater with high organic matter loads that is used for crop fertilizing (fertigation) due to its rich mineral content. Vinasse is a critical problem since it is produced in high amounts and presents high pollutant characteristic — its application on the soil results in several environmental impacts, such as soil contamination, GHG (greenhouse gases) production, among others. Environmental agencies in Brazil regulate the application of vinasse on the soil, and these regulations are becoming increasingly stricter.

The environmental issue of vinasse has led to research in other forms of valuation of this waste, among which vinasse bio-digestion stands out. By undergoing a microbiologic process called anaerobic digestion, the organic matter on vinasse is greatly reduced, while keeping its nutritional characteristics (thus, this effluent can still be used for fertigation).

As a result of this process, a gaseous fuel called biogas is produced. Biogas is mainly composed of methane (CH₄) and carbon dioxide (CO₂), with traces of hydrogen sulfide (H₂S). Thus, it is possible to burn this gaseous product and generate heat and power, while reducing the pollutant aspects of vinasse. Many studies on literature regard using biogas to provide environmental and energetic benefits for the sugarcane mill.

The next step in the ethanol industry is the so-called “second generation ethanol,” or 2G ethanol for short. This biofuel is any ethanol produced from a lignocellulosic material, such as wood, grasses or even sugarcane bagasse or corn stover. Basically, lignocellulosic matter is composed of lignin (a natural binding agent), cellulose and hemicellulose (natural sugar polymers).

By undergoing a pre-treatment process, which might be mechanical, chemical or thermic, the cellulose and hemicellulose are separated from the lignin. Then, the sugar polymers must be broken into the sugar monomers, which can be achieved via chemical or enzymatic processes. The resulting sugar-rich product is then fermented and distilled, much like the first generation ethanol process.

For the sugarcane industry, the main feedstock for 2G ethanol is the sugarcane bagasse. However, the bagasse is currently used for heat and power generation; using it as a feedstock for 2G ethanol would affect not only the mill energy matrix but also its financial incomes (since bioelectricity is a product of the sugarcane mill).

Currently, many authors in literature claim that the mill owner would have to choose between producing bioelectricity or 2G ethanol. Our objective is to show that, by introducing biogas produced from vinasse in the mill energy matrix, it is possible to produce 2G ethanol while keeping the heat and power production constant. To do so, we consider burning biogas instead of bagasse and make the biomass available for 2G ethanol production, promoting what we call an “energetic shift” of the sugarcane bagasse.

For burning biogas, we consider 10 different scenarios, which comprise of using different technologies for burning biogas and promoting the energetic shift. We study burning biogas in boilers (Rankine Cycle) and gas turbines (Brayton cycle), as well as combining boilers and gas turbines (combined cycle) in different operational conditions (low and high pressure). For the comparison between the scenarios, we study parameters such as total bagasse shift, increase in ethanol production, the efficiency of the cycles, reduction of organic load disposed at the soil and reduction of particulate material disposed at the atmosphere. For this work, economic aspects of the scenarios (such as NPV, IRR, payback period) were not estimated.

Our results show that an appreciative amount of bagasse can be shifted by introducing biogas in the mill energy matrix. The best results were obtained burning biogas in high pressure combined cycles. We also conclude that it is possible to displace all the bagasse available if bagasse straw is introduced in the mill energy matrix as well. We obtained significant environmental impacts reductions in the form of reduction of organic load to the soil and reduction of particulate material to the atmosphere. However, and economic analysis should be performed to assess whether the proposed process is viable or not, especially considering that 2G ethanol technology is still incipient and it is, as of yet, very costly.

These findings are described in the article entitled Energetic shift of sugarcane bagasse using biogas produced from sugarcane vinasse in Brazilian ethanol plants, recently published in the journal Biomass and Bioenergy. This work was conducted by Caio L. Joppert, Marilin M. dos Santos, Hirdan K. N. Costa, Edmilson M. dos Santos, and José R. Simões Moreira from the University of São Paolo.