1 Introduction

Ethanol fuel production has emerged as a critical component in the global transition towards renewable energy sources. Ethanol, a biofuel derived from biomass, offers a sustainable alternative to fossil fuels, contributing to energy security, reducing greenhouse gas emissions, and promoting rural economic development. The production of ethanol from various feedstocks, including corn, sugarcane, and cassava, has been extensively studied and implemented worldwide. Cassava, in particular, has shown significant promise due to its high starch content and adaptability to marginal lands, making it a viable candidate for large-scale bioethanol production (Sriroth et al., 2010; Jiang et al., 2015; García-Velásquez et al., 2020).

The history of corn ethanol production dates back to the early 20th century, with significant advancements occurring in the latter half of the century. Initially, ethanol was used as a fuel additive to improve octane ratings and reduce engine knocking. However, the oil crises of the 1970s spurred interest in ethanol as a renewable fuel alternative. The United States, in particular, invested heavily in corn ethanol production, leading to the establishment of a robust industry supported by government policies and subsidies. Over the decades, technological advancements have improved the efficiency and sustainability of corn ethanol production, making it a cornerstone of the biofuel industry (Han et al., 2011; Martinez et al., 2018; Jiao et al., 2019).

This study evaluates the entire process of corn ethanol fuel production, from agricultural cultivation to final energy application, examining key factors such as corn variety selection, agronomic practices, and processing techniques. The research also covers technological innovations, including the latest advancements in biocatalysts and fermentation technology, aiming to improve production efficiency and reduce energy consumption. The economic analysis assesses production costs, market trends, and policy impacts, exploring economic feasibility. The environmental impact and sustainability section uses life cycle assessment to analyze the effects of corn ethanol production on greenhouse gas emissions, land use, and water resource management. This study aims to provide valuable insights for academia, industry, and policymakers, promoting continuous improvement and development in corn ethanol fuel production, thereby enhancing energy security, environmental protection, and rural economic development.

2 Agricultural Cultivation of Corn

2.1 Selection of corn varieties for ethanol production

Selecting appropriate corn varieties is crucial for optimizing ethanol production. Varieties with high starch content are preferred as they yield more fermentable sugars, essential for ethanol production. Studies have shown that the amylose-to-amylopectin ratio in corn starch significantly affects the efficiency of ethanol fermentation. For example, dent corn and waxy corn, which have different amylose-to-amylopectin ratios, have been compared for their ethanol yields, demonstrating that the final ethanol concentration can vary based on the type of corn used (Pradyawong et al., 2018). Additionally, selecting corn varieties that can thrive on marginal lands with minimal agrochemical inputs can enhance the sustainability of ethanol production (Jiang et al., 2015).

2.2 Comparison of cassava starch and corn as feedstocks for bioethanol production

The study by Sarocha Pradyawong et al. (2018) explores the potential of cassava starch compared to corn starch in bioethanol production. The results indicate that the ethanol concentration from cassava starch is similar to that from corn and waxy corn (Table 1), making cassava a viable alternative feedstock for bioethanol production, particularly in Asian countries. The study highlights cassava's high fermentation rate and its efficiency in producing ethanol using both conventional and granular starch hydrolysis (GSH) processes.

Table 1 compares the characteristics of corn starch and cassava starch, focusing on key parameters for bioethanol production. The starch content of cassava (30%-35%) is lower than that of corn (64%-78%), but cassava has a larger average granule diameter (15 µm vs. 10 µm). The degree of polymerization for cassava (800) is lower than that for corn (3 000), and its gelatinization temperature range is also lower (65 ℃-70 ℃ vs. 75 ℃-80 ℃). These differences affect the ease of starch extraction and hydrolysis, with cassava being easier to process into bioethanol due to its lower protein and fat content. This table provides crucial insights into the structural differences affecting the efficiency of bioethanol production processes.

2.3 Agronomic practices for optimizing yield

2.3.1 Soil preparation

Proper soil preparation is fundamental for achieving high corn yields. Techniques such as soil plowing and the incorporation of organic fertilizers can significantly improve soil fertility and structure, thereby enhancing corn growth. The use of biochar, for example, has been shown to improve soil physicochemical properties, leading to higher crop yields (Wijitkosum and Sriburi, 2021).

2.3.2 Planting techniques

Effective planting techniques, including the selection of high-quality seeds and appropriate planting density, are essential for maximizing corn yield. Good planting practices ensure optimal plant spacing, which reduces competition for nutrients and sunlight, thereby promoting healthier and more productive plants (Sriroth et al., 2010).

2.3.3 Fertilization strategies

Fertilization strategies play a critical role in corn cultivation. The application of both organic and inorganic fertilizers can provide the necessary nutrients for corn growth. Studies have demonstrated that combining fertilizers with soil amendments like biochar can further enhance nutrient availability and uptake, leading to increased crop productivity (Wijitkosum and Sriburi, 2021).

2.3.4 Pest and disease management

Effective pest and disease management is vital for maintaining healthy corn crops. Integrated pest management (IPM) strategies, which include the use of resistant corn varieties, biological control agents, and appropriate chemical treatments, can help mitigate the impact of pests and diseases on corn yield. The development of disease-resistant corn varieties has also been highlighted as a key factor in ensuring sustainable corn production (Okudoh et al., 2014).

2.4 Harvesting and post-harvest handling

Timely and efficient harvesting is crucial to prevent losses and maintain the quality of corn for ethanol production. Mechanized harvesting techniques can improve efficiency and reduce labor costs. Post-harvest handling, including proper drying and storage, is essential to prevent spoilage and maintain the starch content of the corn. Ensuring that harvested corn is stored in optimal conditions can significantly impact the overall yield and quality of the ethanol produced (Nguyen et al., 2007).

3 Processing Steps of Corn Ethanol Production

3.1 Overview of corn processing steps

Corn ethanol production involves several key steps, from the initial processing of corn to the final production of ethanol. The primary stages include milling, pretreatment, enzymatic hydrolysis, fermentation, and distillation. Each step is crucial for converting the starch in corn into fermentable sugars and ultimately into ethanol. Studies have shown that while cassava starch yields a slightly lower final ethanol concentration compared to dent corn starch, corn still holds significant advantages in bioethanol production (Figure 1). By optimizing process conditions and enzyme usage, the utilization of corn starch and ethanol yield can be significantly improved. Additionally, corn, with its high starch content and lower gelatinization temperature, offers greater economic viability and operability in industrial production.

Figure 1 illustrates the efficiency comparison of corn and cassava as raw materials for bioethanol production and details the processes of traditional and GSH methods. In the traditional process, corn starch is liquefied at 60 ℃ using α-amylase, followed by saccharification at 30 ℃to produce ethanol via solid-state fermentation. In the GSH process, cassava starch is incubated at 48°C using GSH enzymes, followed by the same solid-state fermentation to produce ethanol. The bar chart in the figure shows that the final ethanol concentration from cassava starch is comparable to that from high-amylose corn starch, slightly higher than waxy corn starch, but lower than dent corn starch. This indicates that while corn has a slight edge in ethanol yield, cassava also holds considerable potential as a feedstock. By presenting the details of different processes and raw materials, the figure provides valuable insights for optimizing bioethanol production processes.

3.2 Dry milling vs. wet milling processes

Corn can be processed using either dry milling or wet milling methods. In the dry milling process, the entire corn kernel is ground into flour, which is then mixed with water to form a mash. This mash undergoes enzymatic hydrolysis to convert starches into sugars, which are then fermented to produce ethanol. The wet milling process, on the other hand, involves soaking the corn kernels in water and sulfur dioxide to separate the kernel into its component parts: starch, fiber, germ, and protein. The starch is then processed into ethanol. Wet milling is generally more complex and capital-intensive but allows for the production of multiple co-products, such as corn oil and animal feed (Han et al., 2011; Kang et al., 2014; García-Velásquez et al., 2020).

3.3 Pretreatment methods

Pretreatment is a critical step in ethanol production as it helps to break down the complex structure of the corn kernel, making the starches more accessible for enzymatic hydrolysis. Common pretreatment methods include physical, chemical, and thermal treatments. For instance, dilute acid pretreatment has been shown to be effective in breaking down lignocellulosic biomass, such as cassava stems, making the starches more accessible for subsequent enzymatic hydrolysis (Han et al., 2011; Ibeto et al., 2014; Rawaengsungnoen et al., 2018).

3.4 Enzymatic hydrolysis and fermentation

Enzymatic hydrolysis involves the use of enzymes like cellulase and β-glucosidase to convert the pretreated starches into fermentable sugars. The efficiency of this process can be influenced by factors such as enzyme loading and reaction conditions. For example, studies have shown that using 20 FPU/g cellulose of cellulase and 30 CbU/g of β-glucosidase can lead to a saccharification yield of 70% (Han et al., 2011). The resulting sugars are then fermented using yeast, typically Saccharomyces cerevisiae, to produce ethanol. The fermentation process can be optimized by controlling parameters such as temperature, pH, and yeast concentration (Han et al., 2011; Martinez et al., 2018; Aziz et al., 2020).

3.5 Distillation and dehydration processes

The final steps in ethanol production are distillation and dehydration. Distillation is used to separate ethanol from the fermentation broth, typically resulting in an ethanol concentration of around 10-12 vol%. This ethanol is then further purified using molecular sieves to remove any remaining water, achieving fuel-grade ethanol concentrations of 99.5% or higher. Energy-efficient distillation techniques, such as Very High Gravity (VHG) fermentation, can significantly reduce steam consumption and overall energy costs (Kang et al., 2014).

4 Technological Innovations in Ethanol Production

4.1 Advances in biocatalysts and fermentation technology

Recent advancements in biocatalysts and fermentation technology have significantly enhanced the efficiency and yield of ethanol production. The discovery and application of new enzymes have played a crucial role in this progress. For instance, the introduction of proteases and cellulases has improved the starch-to-ethanol conversion process, leading to higher ethanol yields and better downstream processing (Harris et al., 2014). Additionally, the use of solid-state fermentation technology to produce a biocatalyst containing a complex mix of enzymes has shown to significantly improve ethanol production performance, increasing the final ethanol concentration by 6.8% and accelerating the kinetics by 14 hours (Guillaume et al., 2019).

Moreover, the development of very high gravity (VHG) fermentation technology has emerged as a versatile method to save process water and energy. This technology allows for the fermentation of higher concentrations of sugar substrates, resulting in increased final ethanol concentrations and improved fermentation efficiency without major alterations to existing facilities (Puligundla et al., 2011). The use of hybrid membrane fermentation and heat integration through Pinch Analysis has also been evaluated, showing potential in reducing overall energy consumption and increasing surplus electricity generation (Lopez-Castrillon et al., 2013).

4.2 Integration of biotechnology in corn ethanol production

The integration of biotechnology in corn ethanol production has led to more sustainable and efficient processes. For example, the blending of commercial xylose-fermenting and wild-type Saccharomyces yeast strains has been shown to improve ethanol yields from deacetylated corn stover and sugarcane bagasse. This approach not only increased ethanol production by 6.5% but also reduced the residual xylose content and the amount of yeast required for inoculation (Wang et al., 2019).

Additionally, the incorporation of energy cane juice into the corn ethanol production process has demonstrated several benefits. Energy cane juice provides sugars that reduce the amount of corn and enzymes needed, decreases water usage, and significantly increases fermentation efficiency by providing minerals that support yeast growth (Sica et al., 2021). This integration makes the fermentation process more efficient and the production systems more sustainable.

4.3 Improvements in process efficiency and energy consumption

Improvements in process efficiency and energy consumption are critical for the economic viability of ethanol production. The integration of different technologies, such as the traditional dry-grind process with the gasification of corn stover followed by syngas fermentation or catalytic mixed alcohols synthesis, has shown to optimize energy use and reduce production costs. This integrated process requires only 17 MW of energy and 1.56 gallons of freshwater per gallon of ethanol produced, resulting in a production cost of $1.22 per gallon (Čuček et al., 2021).

Furthermore, the application of in situ ethanol removal during high solid fermentation has led to significant improvements in the fermentation process. This method allows for the complete fermentation of high solids slurries and faster fermentation rates, reducing the fermentation time by more than 50% and increasing ethanol yield by 88% compared to conventional processes (Kumar et al., 2018). The use of repeated fermentation with cell reuse has also been effective in improving fermentation efficiency and achieving high ethanol productivity from corncob residues (Fan et al., 2013).

5 Economic Analysis

5.1 Cost of corn production and processing

The cost of corn production and processing is a critical factor in the economic viability of corn ethanol fuel. The production costs are influenced by several factors, including the scale of the facility and the type of feedstock used. For instance, a study evaluating the economic feasibility of producing ethanol from various feedstocks found that the minimum ethanol selling prices (MESP) for a 50 million gallon per year facility ranged from $2.24 to $2.96 per gallon, which is comparable to gasoline prices. However, smaller facilities with a capacity of 1-2 million gallons per year had significantly higher MESPs, ranging from $5.61 to $7.39 per gallon, indicating that economies of scale play a crucial role in reducing production costs (Ro et al., 2019). Additionally, the integration of first- and second-generation ethanol production can potentially reduce downstream processing costs and improve overall economic feasibility (Joelsson et al., 2016).

5.2 Market trends and price volatility

The ethanol market is subject to significant price volatility, which is influenced by various factors including corn market uncertainty and oil price fluctuations. A study examining the impact of corn market uncertainty on US ethanol prices found that ethanol prices react positively to corn market volatility shocks, particularly when these shocks are positive (Dutta et al., 2018). Furthermore, the literature on the impacts of biofuels on agricultural commodity prices shows a wide range of estimates, with some studies indicating that a one billion gallon expansion of the US corn ethanol mandate could lead to a 3%-4% increase in corn prices (Condon et al., 2015). This price volatility poses challenges for the economic stability of ethanol production.

5.3 Subsidies and government policies impacting ethanol production

Government policies and subsidies have played a significant role in the growth of the corn ethanol industry. Policies such as the Volumetric Ethanol Excise Tax Credit (VEETC) and the Renewable Fuel Standard (RFS) mandate have been instrumental in promoting ethanol production. These policies have not only supported the expansion of ethanol production but have also contributed to reducing gasoline prices at the pump by up to 10% (Janda et al., 2022). Additionally, supportive biofuel policies have led to a significant increase in corn ethanol production in the USA, from 1.6 to 15 billion gallons between 2000 and 2019, contributing to a reduction in greenhouse gas emissions (Lee et al., 2021).

5.4 Economic feasibility and scalability

The economic feasibility and scalability of corn ethanol production are influenced by several factors, including facility size, feedstock costs, and technological advancements. Larger facilities benefit from economies of scale, resulting in lower production costs and higher economic viability (Ro et al., 2019). Techno-economic analyses of integrated biorefineries suggest that the production of hydrocarbon fuels and chemicals from corn ethanol can be economically competitive with conventional ethanol production, especially when considering the higher market value of hydrocarbon products (Wang et al., 2018). However, the scalability of these technologies requires further improvements in process yields, sustainable feedstock development, and catalytic reaction efficiencies (Tao et al., 2017). Overall, while the economic feasibility of corn ethanol production is promising, it is contingent on continued technological advancements and supportive government policies.

6 Environmental Impact and Sustainability

6.1 Greenhouse gas emissions and carbon footprint

Corn ethanol production has been a significant focus of biofuel policies aimed at reducing greenhouse gas (GHG) emissions. However, the environmental outcomes have been mixed. Studies indicate that while corn ethanol can reduce GHG emissions compared to gasoline, the reductions are often modest and sometimes negligible. For instance, the U.S. Renewable Fuel Standard (RFS) has not met its GHG emissions targets, with some analyses suggesting that the life-cycle GHG emissions of corn ethanol may be comparable to or even exceed those of gasoline (Hill, 2022; Lark et al., 2022). Conversely, other studies have shown that improvements in agricultural practices and ethanol production technologies have led to a decrease in the carbon intensity (CI) of corn ethanol over time, achieving a 23% reduction in CI from 2005 to 2019 (Lee et al., 2021). Additionally, double-cropping systems in Brazil have demonstrated significant GHG reductions when maize is grown as a second crop with soybeans (Moreira et al., 2020).

6.2 Land use and impact on biodiversity

The expansion of corn ethanol production has significant implications for land use and biodiversity. Increased corn cultivation often leads to the conversion of conservation lands and other ecosystems into agricultural use, which can reduce plant and animal biodiversity and disrupt ecosystem services (Hoekman and Broch, 2018). The intensification of corn farming, driven by biofuel policies, has also been linked to soil erosion, nutrient runoff, and other adverse environmental impacts (Hill, 2022). Moreover, the removal of corn stover for biofuel production can decrease soil organic carbon (SOC) levels, further exacerbating land degradation (Qin et al., 2018). However, strategies such as cover cropping and manure application can mitigate some of these negative effects by enhancing SOC and reducing overall GHG emissions (Qin et al., 2018).

6.3 Water usage and management

Water usage in corn ethanol production is another critical environmental concern. The increased frequency of corn cultivation and the removal of stover can lead to a decrease in soil water content and water yield, impacting local water resources (Zhao et al., 2020). Additionally, the intensive use of fertilizers in corn farming can lead to water quality issues, such as nitrate pollution, which poses risks to aquatic ecosystems and human health (Zhao et al., 2020; Lark et al., 2022). Effective water management practices are essential to mitigate these impacts and ensure the sustainability of corn ethanol production.

6.4 Strategies for improving sustainability in corn ethanol production

To improve the sustainability of corn ethanol production, several strategies can be implemented. Technological advancements in farming practices, such as precision agriculture, can reduce the intensity of fertilizer and fossil fuel use, thereby lowering the carbon footprint of corn ethanol (Scully et al., 2022). Enhancing the efficiency of ethanol refineries and adopting renewable energy sources for ethanol production can also contribute to GHG reductions (Lee et al., 2021; Scully et al., 2022). Additionally, land management practices that increase SOC, such as reduced tillage, cover cropping, and organic matter addition, can mitigate the negative impacts of stover removal and improve soil health (Qin et al., 2018). Policy measures that promote these sustainable practices and incentivize technological innovations are crucial for achieving the environmental benefits of corn ethanol production (Hoekman and Broch, 2018; Lark et al., 2022).

7 Energy Application of Corn Ethanol

7.1 Ethanol as a transportation fuel

Corn ethanol has been widely adopted as a transportation fuel due to its potential to reduce greenhouse gas (GHG) emissions and reliance on fossil fuels. The U.S. corn ethanol industry has seen significant growth, with production increasing from 1.6 to 15 billion gallons between 2000 and 2019, driven by supportive biofuel policies. This expansion has resulted in a 23% reduction in carbon intensity (CI) of corn ethanol, contributing to a total GHG emission reduction of 544 million metric tons of CO₂ equivalent from 2005 to 2019 (Lee et al., 2021). Additionally, ethanol's role as a biofuel is underscored by its ability to displace petroleum gasoline, thereby reducing overall GHG emissions (Lee et al., 2021).

7.2 Blending with gasoline and its implications

Ethanol is commonly blended with gasoline to create ethanol-gasoline mixtures such as E10 (10% ethanol) and E85 (85% ethanol). These blends have been shown to perform better than pure gasoline in terms of fossil fuel depletion and global warming potential. For instance, E10 and E85 blends reduce fossil fuel depletion by 6% and 64%-70%, respectively, and lower global warming potential by 1%-10% and 5%-113%, respectively (Liu et al., 2022). However, these blends also present challenges, such as increased ozone layer depletion, acidification, and eutrophication potential compared to gasoline (Liu et al., 2022). Additionally, dual-alcohol blends, which combine ethanol with other alcohols like methanol or iso-butanol, have been explored to optimize fuel properties and reduce evaporative emissions (Shirazi et al., 2019).

7.3 Comparison with other biofuels and fossil fuels

Corn ethanol is often compared with other biofuels and fossil fuels in terms of energy efficiency and environmental impact. While corn ethanol requires 29% more fossil energy than the ethanol fuel produced, it is still more favorable compared to other biofuels like switchgrass and wood biomass, which require 50% and 57% more fossil energy, respectively (Doe et al., 2019). Brazilian sugarcane ethanol, on the other hand, is more efficient, offsetting up to 86% of CO₂ emissions compared to oil use and providing a scalable solution to reduce CO2 emissions from the global transport sector (Jaiswal et al., 2017). Despite these benefits, some studies argue that corn ethanol's life-cycle GHG emissions are comparable to or even greater than those of gasoline, questioning its overall environmental advantage (Hill, 2022).

7.4 Future prospects and potential advancements

The future of corn ethanol as an energy source lies in improving production efficiency and integrating sustainable practices. Innovations such as the use of energy cane juice in corn ethanol production can enhance fermentation efficiency and reduce resource consumption, making the process more sustainable (Sica et al., 2021). Additionally, advancements in biorefinery technologies that co-produce ethanol and high-value chemicals from lignocellulosic biomass can lower production costs and increase the commercial viability of cellulosic ethanol (Rosales-Calderon and Arantes, 2019). As the industry evolves, continued research and policy support will be crucial in addressing the environmental and economic challenges associated with corn ethanol production and maximizing its potential as a renewable energy source.

8 Case Studies

8.1 Successful implementations of corn ethanol production

Corn ethanol production has seen significant advancements and successful implementations, particularly in the United States. From 2005 to 2019, the U.S. corn ethanol industry experienced a substantial increase in production, from 1.6 to 15 billion gallons. This growth was driven by supportive biofuel policies and resulted in a notable reduction in greenhouse gas (GHG) emissions. The carbon intensity (CI) of corn ethanol decreased by 23%, from 58 to 45 gCO2e/MJ, due to improvements in corn grain yield, ethanol yield, and energy efficiency in ethanol plants. These advancements led to a total GHG emission reduction benefit of 544 million metric tons of CO2e during this period (Lee et al., 2021).

In Brazil, the integration of energy cane juice into corn ethanol production has shown promising results. Energy cane juice can significantly enhance fermentation efficiency and reduce the amount of corn and water needed for ethanol production. This integration not only improves the sustainability of the production process but also supports the growth of the corn ethanol industry in regions like the Center-West of Brazil (Sica et al., 2021).

8.2 Comparative analysis of regional production systems

Comparative analyses of regional corn ethanol production systems reveal significant differences in energy efficiency and environmental impact. In the United States, the carbon intensity of corn ethanol has been actively researched and quantified, with recent life cycle analyses (LCAs) showing a central best estimate of 51.4 gCO2e/MJ, which is 46% lower than the average CI for neat gasoline (Fiture n1). The largest contributors to the total CI are ethanol production and farming practices, while land use change (LUC) is a minorcontributor (Scully et al., 2021).

In Brazil, the integration of sugarcane, corn, and grain sorghum in multipurpose plants has been evaluated. The environmental and energy performance of these integrated systems is greatly influenced by agricultural activities, with sugarcane cultivation playing a crucial role. However, the integration of starchy sources like corn and grain sorghum can increase the environmental impact, particularly in terms of climate change and human toxicity (Donke et al., 2016).

Figure 2 from Lee et al. (2021) illustrates the system boundaries and key parameters for the life cycle assessment of corn ethanol production. This includes four main stages: corn cultivation, ethanol production, ethanol transportation and distribution, and ethanol combustion. The figure details the greenhouse gas emissions at each stage, especially focusing on farm production inputs, corn growth, and the energy usage of biorefineries. By clearly indicating the carbon emissions at each stage, Figure 1 helps us understand the primary sources and influencing factors of greenhouse gas emissions throughout the corn ethanol production process, aiding in the development of strategies and measures to reduce emissions.

8.3 Lessons learned and best practices

Several lessons and best practices have emerged from the successful implementations and comparative analyses of corn ethanol production systems. Key lessons include:

1) Efficiency Improvements: Enhancing the efficiency of ethanol plants and agricultural practices can significantly reduce the carbon intensity of corn ethanol. For example, increasing corn grain yield and ethanol yield, along with reducing energy use in ethanol plants, has proven effective in the U.S. (Lee et al., 2021).

2) Integration of Alternative Feedstocks: Integrating alternative feedstocks like energy cane juice can improve the sustainability and efficiency of corn ethanol production. This approach has shown potential in Brazil, where energy cane juice enhances fermentation efficiency and reduces resource consumption (Sica et al., 2021).

3) Regional Adaptations: Tailoring production systems to regional conditions and integrating local agronomic factors can optimize energy productivity and sustainability. For instance, considering local agronomic factors in energy efficiency assessments can lead to significant improvements in net energy gain and energy return on energy invested (Arodudu et al., 2017).

4) Lifecycle Assessments: Conducting comprehensive lifecycle assessments (LCAs) is essential for understanding the environmental impact of corn ethanol production. These assessments help identify key areas for improvement and inform policy and market decisions (Scully et al., 2021).

9 Challenges and Future Directions

9.1 Technical and logistical challenges in production and distribution

Corn ethanol production faces several technical and logistical challenges. One significant issue is the energy intensity of the production process. Although there have been improvements, such as a 24% reduction in ethanol plant energy use from 2005 to 2019, the process still relies heavily on fossil fuels, which raises sustainability concerns (Lee et al., 2021). Additionally, the integration of alternative biomass sources, like energy cane, has shown potential to improve fermentation efficiency and reduce resource use, but the effects of such integrations are not fully understood and require further research (Sica et al., 2021). Logistically, the transportation and storage of ethanol pose challenges due to its corrosive nature, which necessitates specialized infrastructure that can be costly to implement and maintain (Eckert et al., 2018).

9.2 Regulatory and policy challenges

Regulatory and policy frameworks play a crucial role in the corn ethanol industry. In the United States, policies such as the Renewable Fuel Standard (RFS) have driven the growth of the industry by mandating the blending of ethanol with gasoline. However, these policies are subject to political and economic fluctuations, which can create uncertainty for producers (Lewandrowski et al., 2020). In Brazil, the RenovaBio policy aims to promote biofuels, including corn ethanol, but the country faces challenges in balancing the use of corn for ethanol production with its other agricultural demands (Eckert et al., 2018). Moreover, international markets are increasingly requiring biofuels to demonstrate significant greenhouse gas (GHG) emission reductions, which necessitates continuous improvements in production efficiency and sustainability (Lewandrowski et al., 2020).

9.3 Research and development needs

To address the challenges in corn ethanol production, ongoing research and development are essential. Key areas for future research include improving the energy efficiency of ethanol plants and exploring the use of alternative feedstocks, such as energy cane, to enhance sustainability (Sica et al., 2021). Additionally, advancements in fermentation technology and enzyme efficiency could further reduce the resource inputs required for ethanol production (Mohanty and Swain, 2019). There is also a need for comprehensive life-cycle analyses to better understand the environmental impacts of corn ethanol and to identify opportunities for reducing GHG emissions (Lewandrowski et al., 2020; Lee et al., 2021).

9.4 Future trends and potential breakthroughs

Looking ahead, several trends and potential breakthroughs could shape the future of corn ethanol production. The integration of advanced bioenergy crops, such as energy cane, into the production process could significantly enhance sustainability and efficiency (Sica et al., 2021). Technological innovations, such as improved fermentation processes and more efficient enzymes, are likely to play a critical role in reducing production costs and environmental impacts (Mohanty and Swain, 2019). Additionally, the development of policies that support the use of low-carbon biofuels and incentivize research into sustainable production methods will be crucial for the industry's growth (Lewandrowski et al., 2020). As global demand for renewable energy sources continues to rise, corn ethanol is poised to remain a key player in the biofuel sector, provided that these challenges are effectively addressed.

10 Concluding Remarks

Corn ethanol fuel production has been a significant focus of research and policy due to its potential as a renewable energy source. Key findings from the reviewed literature highlight several critical aspects. The carbon intensity (CI) of corn ethanol has decreased significantly over the past decades, primarily due to improvements in agricultural practices and ethanol production efficiency. Studies show a reduction in CI from 58 gCO2e/MJ to 45 gCO2e/MJ between 2005 and 2019, resulting in substantial greenhouse gas (GHG) emission reductions. The integration of first- and second-generation ethanol production, as well as the use of co-products like biogas and distiller’s dried grains with solubles (DDGS), can enhance the overall energy efficiency of ethanol plants. This integration can lead to a positive net present value and improved energy return on investment (EROI).

Regarding sustainability, adding energy cane juice to the corn ethanol production process can significantly improve fermentation efficiency and reduce the need for water and enzymes, making the production process more sustainable. However, the overall sustainability of corn ethanol is still debated, with some studies suggesting that the environmental benefits may not be as significant as previously thought. The economic feasibility of corn ethanol production is influenced by market conditions, including the prices of co-products and the cost of energy inputs. The production of hydrous ethanol, which requires less energy and water, can further reduce production costs and emissions.

Policymakers should continue to support research and development in corn ethanol production to further reduce its carbon intensity and improve energy efficiency. Policies should also encourage the integration of first- and second-generation ethanol production to maximize resource utilization and economic benefits. Ethanol producers should invest in technologies that enhance the efficiency of ethanol production, such as the use of energy cane juice and the production of hydrous ethanol. Additionally, considering the economic and environmental benefits of integrating biogas production and other co-products into the ethanol production process is important. The agricultural sector should adopt precision agriculture practices and other sustainable farming techniques to reduce the environmental impact of corn cultivation. This includes optimizing fertilizer use and conserving soil organic carbon to further lower the carbon intensity of corn ethanol. Researchers should focus on developing more accurate life cycle assessments (LCAs) that account for the latest advancements in corn ethanol production. Continued refinement of models to include co-products, land use changes, and conservation practices will provide more reliable data for policy and market decisions.

The future of corn ethanol fuel production looks promising, with ongoing advancements in technology and agricultural practices contributing to its sustainability and economic viability. However, it is crucial to address the environmental concerns associated with intensive corn farming and to explore alternative feedstocks and production methods that can complement corn ethanol. The integration of first- and second-generation ethanol production, along with the use of co-products, offers a pathway to a more efficient and sustainable biofuel industry. Stakeholders must work collaboratively to ensure that corn ethanol continues to evolve as a viable and environmentally friendly energy source.

Acknowledgments

The author extends sincere thanks to two anonymous peer reviewers for their feedback on the manuscript of this paper, whose evaluations and suggestions have greatly contributed to the improvement of the manuscript.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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