Formation of Dioxins during Energy Cogeneration by Burning Bagasse and Sugarcane Straw Fertilized with Chlorinated Compounds: State of the Art and Presentation of Alternatives
Abstract
Polychlorinated dibenzodioxins (PCDDs), also known as dioxins, are part of a group of organochlorine chemical compounds, highly persistent in the environment, with similar chemical structures. High levels of this compound are found in the burning ashes of the sugarcane and are attributed to the high concentration of chlorine present in bagasse and in the sugarcane straw that are used as a substrate for obtaining energy in the bioethanol production plants. This occurred due to the application of chlorine-containing fertilizers, such as potassium chloride, during the cultivation of sugarcane. Considering the high degree of toxicity of dioxins, the objective of this study was to conduct a literature review on the subject and research alternatives that can control the generation and emission of dioxins in bioethanol production plants, either through the application of appropriate technologies to control emission of dioxins generated during burning, or by substituting the chlorinated fertilizer used in the cultivation of sugarcane. From the analysis of different studies that researched the presence of dioxins in the ash of the bagasse burning boiler, there is a concentration that varies from 2, 2 to 190 picograms of dioxin equivalent toxicity (TEQ) per gram of ash. For the removal of these dioxins, the control systems commonly used in sugarcane plants have not proved to be efficient. In this context, the main alternatives would be the application of technologies for optimizing the combustion process combined with the treatment of end of pipe by means of a scrubber and bag filter or selective catalytic oxidation using NH3-SCR catalysts. Another option is the substitution of the potassium source, which presents itself as the most viable alternative, with the use of non-chlorine sources, such as Glauconitic Siltstone, Potassium Nitrate, Potassium Sulfate or vinasse instead of using chlorinated fertilizers such as Potassium Chloride.
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Introduction
Polychlorinated Dibenzodioxins (PCDDs), also known as Dioxins, comprise a group of organochlorine chemical compounds highly persistent in the environment, with similar chemical structures. The toxicity of these compounds varies according to the number and position of Chlorine atoms in these compounds (Ronald, 2011).
The 2,3,7,8-Tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), shown in Figure 1, is the most toxic molecule, of all dioxins, having four chlorine atoms. Such substance is considered as the most toxic, low molecular weight substance ever found by humans, proving to be only less toxic than some protein toxins, such as nicotine and sodium cyanide (Grossi, 1993). FIGURE 1: Chemical structure of 2,3,7,8-Tetrachlorodibenzo-p-dioxin molecule Dioxins are extremely stable and tend to accumulate within the food chain, with a half-life of 7 to 9 years in humans (ATSDR, 1998; Xu et al., 2016). In natural sediments, on the other hand, it is estimated that the half-life of dioxins is longer than 100 years (Sinkkonen & Paasiverta, 2000). In addition to being stable, they are practically insoluble in water and lipophilic (ATSDR, 1998; Ronald, 2011). Because they are highly resistant to chemical and microbiological degradation, in addition to accumulating within the environment, dioxins have been classified by the Stockholm Convention (2002) within the group comprised of 21 molecules known as “Persistent Organic Pollutants” (POPs) (Perlatti, 2012). Brazil is among one of the 170 countries that, at the Stockholm Convention (2002), adhered to the treaty to eliminate the sources of production of these POPs, being one of the regional leaders in Latin America for the control of POPs through the Company of Environmental Sanitation Technology (CETESB) (Perlatti, 2012).
The cultivation of sugarcane in Brazil is of paramount relevance for Brazilian agribusiness, considering that the country is considered the largest producer of sugarcane and the largest exporter of ethanol and sugar in the world. In the 2018/2019 harvest, the estimated production of sugarcane was 615.84 million tons and the harvested area was around 8.63 million hectares (CONAB, 2018).
Brazil has a consolidated trajectory in national and global ethanol production due to the fact that Brazilian ethanol has been produced since the 1970s, as a result of the National Alcohol Program, also known as “Próalcool.” The internal production of bioethanol has been increasingly growing, with an estimated production in the 2018/19 harvest equivalent to 32.31 billion liters of ethanol, which represents an increase of 18.6% compared to the previous harvest (CONAB, 2018). Brazilian ethanol is mostly produced through First Generation Ethanol (E1G), in which the production process occurs from the juice resulting from the sugarcane milling process and the energy cogeneration comes from the burning of sugarcane bagasse (Ansanelli et al., 2016).
In addition to E1G, another manner that tends to grow in the country in the coming years, due to the replacement of burnt straw by mechanized harvesting, is the process known as Second Generation Ethanol (E2G). This process differs from E1G in the type of substrate used during the energy cogeneration process, since in E2G both bagasse and straw are used to obtain electrical energy (Ansanelli et al., 2016).
Although ethanol production is already well consolidated in Brazil, the formation of dioxins during the process of burning sugarcane containing a high concentration of chlorine for power generation is a latent problem in ethanol production that occurs due to the use of fertilizers containing chlorine during the planting of this crop (Guevara et al., 2016). Therefore, the present study aims to analyze the state of the art on dioxin emissions in ethanol production plants and the intervening factors for the formation of these compounds, as well as to propose some measures that can be adopted to control the emission of these substances in order to make the process more sustainable.
Conclusion
This study shows that, among the intervening factors for the formation of dioxins, the presence of chlorine in sugarcane appears as one of the main precursors for the formation of these compounds. In addition to this, it has been found that the treatment systems used in sugarcane processing plants have low efficiency in the removal of dioxins, leading to the emission of these toxic substances that represent a risk to human health and environmental integrity.
The ineffectiveness of Venturi gas scrubbers, bag filters and electrostatic precipitators as methods of controlling the emission of dioxins to the environment requires modifications in the process of control or production, so that the emission of dioxins is effectively controlled. However, as shown in this study, the alternatives that can be adopted to increase the efficiency of emission control are expensive, as is the case with the adoption of selective catalytic oxidation, or are inefficient when sugarcane has a high content of chlorine, in the case of optimization of the combustion process combined with end-of-pipe treatment.
In this context, and considering that one of the main factors for the formation of dioxins in bioethanol production plants is the presence of chlorine in sugarcane, the most viable alternative, both environmentally and financially, is the replacement of the source of potassium used.
For this purpose, fertilizers containing a high chlorine content, such as Potassium Chloride (KCl), must be replaced with a non-chlorinated potassium source, as is the case with Potassium Nitrate, Potassium Sulfate and Glauconitic Siltstone. An economic analysis of these alternatives has pointed to Glauconitic Siltstone as the option with the best cost-benefit ratio, considering that this alternative has more competitive prices than Potassium Chloride, Potassium Nitrate and Potassium Sulfate.
Once Brazil has committed itself to eliminate sources of POPs production at the Stockholm Convention of 2002, it is necessary to establish a limit for the occurrence of dioxins for sources other than gas and products intended for animal feed, in addition to a continuous monitoring of boiler ashes, since the presence of dioxins in this residue has already been proven, as well as the replacement of potassium sources used in crops.