Energy in agriculture in Brazil

Historically, agriculture has been a large contributor to the global green house gas emissions due to the use of fossil fuel-powered machinery during the production cycle. In recent years, there has been a pressing need to replace fossil fuel consumption given the increasing threats associated with climate change. This paper presents a review of the latest studies published in the field of energy in the agricultural sector, with a specific focus on Brazil. Based on the research trends observed over the last few years (2012-2020), findings tend to point towards improving current practices rather than studying alternatives to fossil fuel consumption, with recent publications (2020-2021) continuing this trend, primarily via marketand indicator-based studies. As reported in the reviewed literature, tobacco production has the largest energy demand, while wheat, sugarcane, and eucalyptus production have the greatest ethanol production potential. Both rapeseed and jatropha production share greater biodiesel production potential than soybean. Of the 14 crops considered in this study, four had a negative energy surplus: tobacco, orange, rice, and cotton.


INTRODUCTION
Over the last few decades, the global population has increased significantly. According to the most recent estimate of the United Nations (UN), there are now 7.7 billion inhabitants, and projections are set to increase to more than 9.7 billion by 2050. Most of the population increase is expected to take place in developing countries, specifically countries located in sub-Saharan Africa and Asia. On the other hand, developed countries, like most of the western European countries, will show a steady, if not declining, trend in population growth (FAO, 2012), as shown in (Figure 1).  (FAO, 2012) In addition to the anticipated population growth, the demand for resources will also increase. To satisfy the growing food demand, there will need to be increases in productivity and structural changes within the agricultural sector (DÁNIEL FRÓNA, 2019). Due to the rising food demand caused by population growth, agricultural land use has increased significantly across most continents over the last 20 years, primarily leading to a reduction in forest area.
The global land area is around 13.2 billion ha, of which 12% (1.6 billion ha) is currently being used for the cultivation of agricultural crops; further, 28% (3.7 billion ha) is covered by forest, and 35% (4.6 billion ha) comprises grasslands and woodland ecosystems (FAO; EARTHSCAN, 2011). According to the Food and Agriculture Organization (FAO) (FAO, 2020b), Asia's agricultural land is the largest when compared to the continent's extension, covering around 54% of its total surface. This is followed by Oceania with 45%, Africa with 38%, the Americas with 30% and, lastly, Europe with 21% of available agricultural land. Land use change varied among the different continents as well; Africa and the Americas reported a change in forest land to agricultural land over the past 20 years, contributing to the increased crop production needed to meet the demands across the continent, but at a higher environmental cost (TILMAN et al., 2011). Countries with the highest deforestation rate in terms of percentage of area were Paraguay (16%), Nicaragua (15%), and Cambodia (14%), converting most of their lands for other uses. In terms of area, Brazil registered the loss of 52 million ha of forest in favor of agricultural land, marking the highest amount of deforested area during this period.
Global population growth also entails increase energy demands, whether for heat, transportation, or electricity. The increase in energy demands around the world has been rising together with the population growth and development of numerous countries. According to data from the International Energy Agency (IEA), it was noted how by 2015, the global demand for energy was almost 10 million ktoe (29.6% in industrial, 28.6% in transport, 21.2% in residential, 2.1% for agricultural, and the remaining 18.5% used for commercial and other non-specified sectors). Traditionally, energy is generated by processing fossil fuels like coal, natural gas, and oil which, together, covered around 79% of the current global demand. During the transformation process, fossil fuels emit different pollutants like ash, sulfur oxides, nitrogen oxides, and volatile organic compounds into the atmosphere (CHMIELEWSKI, 2014). Furthermore, electricity production from fossil fuels not only pollutes the atmosphere, but also affects the water and soil during extraction (GAETE-MORALES et al., 2019). There are some traditional renewable sources that contribute to the generation of energy worldwide, the most notable among which are hydropower (covering about 2.5% of global demand) and biofuels (covering around 9.5% of the global energy demand). The latter is used mostly for heating purposes in developing countries that do not have access to modern cooking fuels (WORLD BIOENERGY ASSOCIATION, 2019). Wind and solar energy have recently shown rapid development around the world given their use for electricity generation; however, their share of the global market is still minimal.
Food and agriculture are two major sectors that have been growing in association with the increase in Energy in agriculture in Brazil the human population. This expansion has also affected global energy consumption through the industrialization of agriculture and large-scale production. As of 2018, the energy demand in the agricultural sector represented 2.1% of the total global energy demand, increasing from 149,194 ktoe worldwide in 2000 to 214,719 ktoe in 2018 (IEA, [s. d.]).
Land use for food crops growth is mostly determined by global diet and agricultural yields, meaning that the more yield the agricultural land has, the less land is required to produce certain crops. In this way, energy crops face competition for land use (POPP et al., 2014), limiting the availability of land and water for production. The dependency on fossil fuels in the agricultural food industry could be reduced by applying the circular economy approach, as some countries have been investigating methods that can convert waste generated during the harvesting of several crops as possible sources of energy for the agricultural sector (ZAPALOWSKA; BASHUTSKA, 2019).
Located East of South America, with an area of 8,515,759 km 2 , Brazil is the largest and most populated country in the continent. It is considered by the World Bank (THE WORLD BANK, 2020) as an upper-middle income country with an emerging economy. Furthermore, as stated by the OECD (OECD, 2018), Brazil's strong growth and social progress over the past decades have made it one of the world's leading economies. Its economy is mostly based on agriculture, industry, and a wide range of services. Considered as one of the most important countries in agriculture production, Brazil is the largest producer of sugarcane, soybeans, coffee and other crops. This is why its agricultural land comprises around 28.7% of the country's total surface (RAMOS; BALBUENO; PINHEIRO, 2017).
In a country like Brazil -which not only represents the most populated country in South America, but is also characterized by its rapidly developing economythere is a demand for a large quantity of energy supply. Considered as one of the countries with the highest renewable energy shares, Brazil's energy matrix is comprised of approximately 46.2% of renewable energies and 53.8% (INTERNATIONAL ENERGY AGENCY (IEA), 2020) of other sources, ranging from fossil fuel to nuclear energy. Among the country's renewable energy sources, biofuels/biomass and hydropower are the main sources of energy used, covering 25% and 12% of the total demand, respectively. Furthermore, Brazil's share of wind, solar, and other renewable energy sources has increased from 5.9% in 2017 to 6.7% in 2018 (EPE, 2018).
Given this information, the purpose of the present paper is to identify the theoretical energy balance of the primary crops in Brazil, while further determining different energy crops that may be important within the Brazilian context. The next sections present a review of recently published articles. Data on the energy used in the agricultural sector, as well as the possible energy contained in residues, are compiled from the reviewed articles and used to identify the theoretical opportunities for energy extraction within the agricultural sector.

Bioenergy in Brazil
Considered as a highly important country in the global biomass market due to its vast production of crops like sugarcane, soja, and others, which are used in the production of biofuels and biomass, Brazil has a strong presence and holds great potential to generate energy from agricultural and organic wastes (WELFLE, 2017). Brazil's Ministry of Agriculture estimates that the agricultural and feedstock land expansion will increase significantly in the coming years, converting natural savannas and pasturelands in the country for these purposes (MINISTRY OF AGRICULTURE LIVESTOCK AND FOOD SUPPLY, 2006). Biofuels already play an important role in the Brazilian energy market. Brazil has the most competitive program for the development and production of biofuels, and it has the second largest ethanol industry worldwide (WELFLE, 2017). The reserves of cultivable area (excluding forests) are significant enough to produce biomass as a source of vegetable oils, which could be converted into biodiesel, considerably expanding Brazil's production without affecting the environment.
Studies suggest that the agricultural value added, as well as the resulting CO 2 emissions, have short-and long-run bidirectional causality. This means that a change in agricultural production would have immediate effects on emissions due to the intensive fossil fuel consumption in the sector (BEN JEBLI; BEN YOUSSEF, 2017; LIU; ZHANG; BAE, 2017). Similar, short-run Granger causal relationships have been described from non-renewables to emissions and agriculture, from economic growth to agriculture, and from agriculture to renewable energy (LIU; ZHANG; BAE, 2017). These estimates seem to indicate that an increase in renewable energy consumption in agriculture decreases the overall CO 2 emissions. Short-run causality from agriculture to gross domestic product (GDP), as well as long-run causality from GDP to agriculture seem to indicate that an increase in agricultural production has immediate impacts on economic growth. In the long run, economic growth can improve the development of the agricultural sector (BEN JEBLI; BEN YOUSSEF, 2017).

MATERIAL AND METHODS
The present study is comprised of several parts. First, a literature review of relevant publications was conducted. Then, based on the findings from the literature, data on the energy opportunities and energy demands of the Brazilian agricultural sector were extracted and compiled, as shown in Table 1. The values and units were averaged and homogenized to have a base of comparison ( Table 2). The fuel production potential was converted to GJ/ha using the productivity of each crop, as well as the ethanol and bio diesel energy content values of 19.8 MJ/L and 34.5 MJ/L, respectively.
In the first part of this study, the primary subjects of interest identified in the field were identified from the literature, and included the following: agricultural residues (PORTUGAL-PEREIRA et al., 2015), ethanol, bio-refinery, bio-diesel (FAO, 2020a), crambe, sugarcane, bagasse, technical potential, boiler, co-generation, electrification (CARVALHO et al., 2020)  To identify the research trends over the last few years (see Figure 2), the fields of interest identified were cross-referenced with the 50 most cited publications per year during the period spanning 2012-2020. Publication selection was achieved through Scopus (SCOPUS, [s. d.]) and articles were chosen from peer-reviewed articles if they focused of agriculture, energy, and Brazil, and were published within the aforementioned time period. Throughout this period, the most frequently studied subjects that fell within the scope of this research were as follows: "sugar" (  "emissions" in 10.00%. "Ethanol" and "sustainability" were only mentioned in 6.40% of the articles, respectively. "Market" went from being the ninth most mentioned research topic in the period spanning 2012-2020, to being the sixth most mentioned, as it was featured in 7.00% of the 1,000 most recently published article.
"Sugar" was mentioned in 15.20% of the publications, "sugarcane" was mentioned in 13.00%, "agriculture" in 10.90%, "biomass" in 10.20% and Energy in agriculture in Brazil  Figure 7 present the data available on the crops reported in the reviewed literature to identify the keywords used to uncover research trends. With respect to ethanol (Figure 4), eucalyptus appears to have the greatest potential per hectare. This can be explained by the amount of biomass growth in 1 ha of land. Following eucalyptus, sugarcane and wheat have the highest ethanol production potential. Hemp is a crop not currently grown in Brazil, but has great potential for ethanol production with 80 GJ/ ha ethanol.
Currently, a large amount of the biodiesel produced in Brazil comes from soybean, despite the fact that its fuel potential per hectare is low, yet the amount harvested in Brazil is considerable enough to support production. The oil seeds crambe, rapeseed, and jatropha all have the potential for higher possible productivity per hectare, as well as higher biodiesel production potential ( Figure 5). Data on the energy requirements to prepare the field, as well as to seed, maintain, and harvest the crop were unavailable in the reviewed literature, as the articles exploring Brazil as a case study have paid little attention to these aspects.
Data on methane production were available for two crops (maize and hemp), only one of which is currently produced in Brazil. Of these two, maize has the biggest methane production potential, followed by hemp ( Figure 6). Maize also has higher crop productivity and lower energy requirements than hemp, making it preferable for methane production.
Only one study (SILVA NETO; GALLO, 2021) of those reviewed addressed the production of methane from vinasse, a byproduct of ethanol production. This was used to calculate the possible additional methane production resulting from the byproducts of ethanol produced per hectare (Figure 4) as presented in Figure 7. As this production is dependent on the amount of ethanol produced, the difference in the values from crop to crop is proportional to the difference of the crop's ethanol production potential.
Considering the total possible energy contained in the produced biofuel, the theoretical energy surplus per hectare of crop was calculated in Table 3. Among the crops considered here, only orange, tobacco, rice, and cotton have a negative surplus value, meaning that they require  4. Taken together, these findings seem to indicate that the Brazilian agricultural sector has the potential to generate more energy than is required by its crops for preparation, field maintenance, harvesting, and transportation. With the possible inclusion of energy crops, the agricultural sector could generate enough energy to operate with a greatly reduced amount of fossil fuels, or without them entirely. However, as illustrated in the trends identified in the literature, additional studies are needed to investigate the impacts associated with including energy crops in the Brazilian agricultural sector to further confirm this remark. more energy than can be produced with their residues. All other considered crops are able to produce greater energy than is needed.

1.
A review of the most recent publications on energy in the agricultural sector with a focus on Brazil was performed in this paper. The identified trends suggest that the focus of research over the last 8 years has been on sugarcane, emissions, and ethanol production. The most recent publications (i.e., published from 2020-2021) point to a continuation in this trend of research, with the marketsand indicator-based studies among the primary themes identified in the research; 2. Of the reviewed literature, tobacco has the highest energy demand, while wheat, sugarcane, and eucalyptus have the greatest ethanol production potential per hectare. Both rapeseed and jatropha were identified as having greater biodiesel production potential per hectare than soybeans; however, more studies on these seeds are needed to determine their energy requirements; 3. Of the 14 crops considered, only four had energy requirements that were greater than could be produced by their residues. Those crops were tobacco, orange, rice,