Revolutionizing Biomass Processing: The Design and Functionality of an Innovative Extruder for Sugarcane Bagasse Milling Pretreatment

Abstract

Milling pretreatment is a crucial step in the bioconversion of lignocellulosic biomass such sugarcane bagasse because it facilitates access to cellulose for subsequent chemical treatments. However, most experiments have been conducted at the laboratory scale, where it has been identified that high energy is required for the processing of biomass. For this reason, it is proposed to implement the screw extruder technique for the processing of cellulose. This article focuses on the characteristics, types, and applications of milling pretreatment for sugarcane bagasse, with a particular emphasis on its role in lignin removal and the milling design. Milling pretreatment reduces the particle size of lignocellulose biomass through compression shear and tearing mechanisms, which enhances the accessibility of cellulose and hemicellulose to enzymes and chemicals, thereby improving the efficiency of bioconversion processes. Innovative mathematical modeling, a mechanical design in a CAD application, and an FEA analysis of the milling pretreatment equipment are presented, providing insights into the design and optimization of milling pretreatment processes. This article presents an innovative potential system for milling pretreatment in sugarcane bagasse for the production of bioethanol, heat and power, and other value-added products, contributing to a more sustainable and circular economy.

1. Introduction

According to the World Bank, annually, 231 million tons of trash are produced, of which 52% is food waste, in Latin America and the Caribbean [1]. In recent years, research has been conducted on the use of these wastes, and several applications have been found, such as the production of sustainable energy. Lignocellulose biomass is composed of polysaccharides and lignin, which can be transformed into chemical products and bioenergy [2]. For the transformation of biomass, biorefineries are an interesting and environmentally friendly option. Biorefineries are responsible for processing, transforming, fractionating, and separating the biomass to produce chemical products and energy, among others [3]. Moreover, this technology integrates different chemical, physical, and biological processes, which are determined according to the biomass and the products to be obtained. It is important to highlight that biorefineries are tools that support economic, ecological, and social sustainability [3]. However, biomass has a complex and rigid structure that limits its availability in the process of obtaining it; for this reason, lignocellulose biomass is exposed to pretreatment. Lignocellulose biomass contains lignin and polysaccharides, which are wrapped in a lignin matrix and do not allow the entry of microorganisms. This is a problem when treating the biomass by hydrolysis. Therefore, when a pretreatment is applied to lignocellulose biomass, it can decrease the lignin content and increase the cellulose and hemicellulose resources for use in hydrolysis, improving the extraction yield [4].

Various studies have shown an improvement in the yield of the biomass when it is exposed to one or more pretreatments. There are chemical, biological, and physical pretreatments, among others. In the industry, physical pretreatment is the most well known and used. Different physical pretreatments, like ultrasonic baths, microwave, and milling, are generally used as the first step in the treatment of lignocellulose. They are intended to reduce the particle size and crystallinity index [5]. Milling is the most used because it is an effective and easily accessible process [6]. For example, ball milling was applied to palm biomass and improved the yield, with a reduction in the particle size for the subsequent treatments [7]. In fact, the selection of the pretreatment depends on the exploitation of the biomass components and the desired products. However, pretreatment carries great energetic demands for its operation; for this reason, extrusion has been studied as a milling pretreatment for lignocellulose biomass. Various papers have covered pretreatment approaches for lignocellulosic biomass milling, with an emphasis on how various milling procedures affect the biomass’s chemical and physical characteristics. Ball milling, hammer milling, knife milling, and colloid milling are some of the techniques examined; each has certain benefits and drawbacks [8]. Extrusion is a simple and cost-effective process that combines three very important stages: mixing, heating, and cutting [5]. Additionally, the application of pretreatment has been evidenced in different investigations, where the most common techniques have been implemented [9,10,11,12]; however, the extruder screw technique is beginning to be studied as a more complete and adaptable technique due to the combination of chemical and physical treatments in the same equipment. The aim of this article is to explore the development and application of an advanced extruder system tailored specifically to the milling pretreatment of sugarcane bagasse. In addition to detailing the design and functionality of the extruder, this article aims to provide a comprehensive overview of the milling pretreatment techniques, including their importance in biomass processing, the various methodologies employed, and the benefits that they offer in enhancing the biofuel and bioproducts’ production efficiency. By elucidating the different types of milling pretreatment approaches and their respective impacts on sugarcane bagasse, this article seeks to contribute to the advancement of sustainable biomass conversion technologies. Due to the existence of different milling techniques and functional equipment for treatment, it is necessary to perform a comparison with a technique that brings together several mechanisms, such as the extruder screw, as well as its combination with other chemical treatments to save energy and time, while also improving the accuracy of the results in a semi-continuous process.

2. Milling Pretreatment

Milling pretreatment is a physical pretreatment that reduces the particle size of the lignocellulose biomass through mechanisms such as cutting, shearing, compression, tearing, and breaking. This pretreatment has various types, where it is possible to apply one or more mechanisms, and they are selected depending on the type of biomass.

2.1. Mechanism

The reduction mechanisms in milling involve various methods that impact the size of solid particles; these are as follows [13].

• Cutting: Cutting involves the application of a force over a narrow area using a sharp cutting edge. This mechanism is used for fibrous and waxy substances and is carried out in cutter mills. For example, in pharmaceutical milling, cutting is employed to reduce the particle sizes of materials like fibrous and waxy substances using cutter mills.
• Compression: Compression occurs when the material is crushed between two surfaces by the application of pressure. This mechanism is suitable for soft materials and is carried out in roller mills. An example is the use of compression in roller mills to crush soft materials by applying pressure between two surfaces.
• Impact: Impact involves the contact of a material with a fast-moving part that imparts kinetic energy, causing internal stresses in the particle and breaking it. This mechanism is used for moderately hard and friable materials and is carried out in hammer mills and fluid energy mills. For instance, impact is utilized in hammer mills to break down materials through the forceful contact of hammers at high speeds.
• Attrition: Attrition occurs when the material is broken by rubbing it between two surfaces. This mechanism is suitable for brittle drugs and is carried out in fluid energy mills. An example is the use of attrition in fluid energy mills to break down brittle drugs by rubbing them between two surfaces.
• Shearing: Shearing involves the application of a force over a narrow area of material, resulting in shear forces that break the particles. This mechanism is used in cutting mills, where materials are cut by sharp blades. An example is the use of shearing in cutting mills to break down materials by applying shear forces over a narrow area.

A particle absorbs strain energy and is deformed under shear or compression until the energy exceeds the weakest flaw, and the strain energy required for fracture is proportional to the length of the crack formed. The size reduction process is carried out using various types of mills, including hammer mills, ball mills, cutter mills, end runner mills, fluid energy mills, and disk refiners [14].

2.2. Selection of Mill

The selection of a mill for the size reduction process depends on various factors related to the nature of the raw materials, such as the hardness, fibrousness, elasticity, stickiness, melting point, moisture level, particle nature, purity requirement, feed size to product ratio, bulk density, physical effect, and safety and economics. Below is a detailed explanation of each factor:

Hardness is a measurement of the resistance of a material to deformation or indentation. The hardness of the raw materials affects the choice of milling equipment. For example, harder materials require more energy and a more robust milling mechanism, such as impact milling, to reduce their size. In contrast, softer materials can be milled using compression or shearing mechanisms, such as roller milling or knife milling [15]. On the other hand, fibrous materials, such as wheat or cotton, require specialized milling equipment to reduce their size without damaging their fibrous structure. For example, in wheat milling, roller mills are used to crush the wheat kernels while preserving the bran and germ layers, which are rich in fiber and nutrients. In contrast, hammer mills or ball mills are not suitable for the milling of fibrous materials, as they can damage the fibrous structure and reduce the quality of the final product [16].

Elastic materials, such as rubber or polymers, can be challenging to mill due to their ability to deform and recover their original shape. Elastic materials require specialized milling equipment, such as cryogenic milling or ultrasonic milling, to reduce their size without generating excessive heat or damaging their properties [17]. Sticky materials, such as chocolate or honey, can adhere to the milling equipment and affect the efficiency and quality of the milling process. Sticky materials require specialized milling equipment, such as scraper blades or cooling systems, to prevent adhesion and ensure a consistent size reduction.

Materials with a low melting point, such as fats or waxes, can melt during the milling process, which affects the quality of the final product. Materials with a low melting point require specialized milling equipment, such as cooling systems or cryogenic milling, to prevent melting and ensure a consistent size reduction. The nature of the raw materials, such as their shape, size, and texture, affects the choice of milling equipment and the milling process. For example, irregularly shaped or large particles require more energy and a more robust milling mechanism, such as impact milling, to reduce their size. In contrast, regular or small particles can be milled using compression or shearing mechanisms, such as roller mills or knife mills [18]. The moisture level of the raw materials also affects the choice of milling equipment and the milling process. For example, materials with a high moisture level, such as clay or soil, require specialized milling equipment, such as hammer mills or attrition mills, to reduce their size without generating excessive heat or damaging their properties. In contrast, materials with a low moisture level, such as dry powders or granules, can be milled using compression or impact milling mechanisms, such as roller mills or ball mills [19].

The purity requirement of raw materials also affects the choice of milling equipment and the milling process. For example, materials with a high purity requirement, such as pharmaceuticals or food products, require specialized milling equipment, such as clean-in-place (CIP) systems or stainless-steel mills, to ensure a consistent size reduction and prevent cross-contamination [20]. The ratio of the feed size to the product size also affects the choice of milling equipment and the milling process. For example, materials with a large feed size and a small product size require more energy and a more robust milling mechanism, such as impact milling, to reduce their size. In contrast, materials with a small feed size and a large product size can be milled using compression or shearing mechanisms, such as roller mills or knife mills [21].

The bulk density and physical effect of the raw materials also affect the choice of milling equipment and the milling process. For example, materials with a high bulk density, such as metals or minerals, require more energy and a more robust milling mechanism, such as impact milling or ball milling, to reduce their size. In contrast, materials with a low bulk density, such as foams or fibers, can be milled using compression or shearing mechanisms, such as roller mills or knife mills [22]. Safety and economics are critical factors in the selection of milling equipment and the milling process. For example, the safety of the milling process, such as the risk of explosion or fire, affects the choice of milling equipment and the milling process. In addition, the economics of the milling process, such as the cost of energy and maintenance, affect the choice of milling equipment and the milling process.

As mentioned previously, there are different types of milling equipment, such as disk refiners, screw extruders, knife mills, hammer mills, roll mills, and centrifugal mills, which are operated with continuous regimes, and others, such as ball mills and roll mills, which are operated with discontinuous regimes [3].

Table 1. Different types of mills with their mechanisms and the products obtained [3].

Type of Mill		Products Obtained
Type of Mill	Reduction Mechanism	Feedstock	Product/Process	Yield	Source
Disk refiner	Shearing, tearing	Corn stover	Sugars	79% increase	[23]
Screw extruder (extrusion)	Compression, tearing, shearing	Sugarcane	Hydrolysis	68.2%	[24]
Knife mill	Shearing, cutting	Wheat straw	Sugars	24%	[25]
Hammer mill	Breaking	Wood chips	Sugars	4-fold over control samples	[26]
Roll mill	Compression, tearing	Different types of grass	Methane	474 mL/gVS (untreated: 33.9 mL/gVS)	[27]
Centrifugal mill	Cutting, breaking	Rice straw	Glucose	94% glucose conversion	[28]
Ball mill	Tearing, breaking	Napier silage	Methane	3608.6 mL CH4/L	[29]
Rod mill	Tearing, breaking	Wheat straw	Crystallinity index	Reduced to 11.59%, increased bio-oil yield to 46.16%	[30]

shows the different types of mills and their mechanisms of reduction in order to identify each one. The most used type of milling is ball milling (65%), followed by knife milling (13%), hammer milling (9%), rod milling (7%), centrifugal milling (4%), and roll milling (2%) [14].

2.3. Milling and Lignocellulosic Biomass

Milling pretreatment is the most used physical pretreatment method for lignocellulosic biomass due to its good accessibility and effectiveness in improving the biomass yield in subsequent processes. Milling reduces the total particle size of the biomass, which enhances the accessibility of cellulose for further treatment and enzymatic hydrolysis, ultimately leading to improved yields.

According to a study published in Biotechnology for Biofuels, milling is the most widely used physical pretreatment method in the industry. This is because milling is a cost-effective and scalable technology that can process large amounts of biomass. For instance, continuous pressurized MDR is a proven and scalable technology capable of processing up to 1500 tons of biomass per day, and it is generally run at lower temperatures and pressures than comparable technologies [14].

Milling reduces the particle size of the biomass, which increases the surface area and accessibility of cellulose, thereby enhancing the enzymatic hydrolysis process. This pretreatment method affects the cellulose structure by reducing the length and thickness of the crystallites, decreasing the crystallinity and degree of polymerization, and increasing the specific surface area of lignocellulosic biomass.

Studies have shown that milling pretreatment can significantly improve the enzymatic digestibility of biomass, with ball milling being a particularly effective method. One study found that the ball milling of willow resulted in a 90% reduction in the length of the crystallites, a 50% reduction in the thickness of the crystallites, and a 50% decrease in crystallinity [31]. Another study found that the ball milling of corn stover increased the specific surface area by 2.5 times and reduced the particle size by 90%, resulting in a 60% increase in enzymatic digestibility [32].

The time required for milling pretreatment can vary depending on the type of biomass and the desired level of particle size reduction. Generally, longer milling times result in smaller particle sizes and higher specific surface areas, which can lead to increased enzymatic digestibility. However, excessive milling can also cause physical and chemical changes in the biomass, such as the formation of fine particles and the degradation of hemicellulose, which can negatively impact the overall yield of the process [33]. Milling pretreatment does not cause significant chemical changes in the biomass, but it can disrupt the dense and complex physical structures of plant cell walls, reduce the size of the biomass particles, destroy the crystalline structure of cellulose, and increase the degree of exposure to cellulose through mechanical force [34].

3. Screw Extruder for Milling

Extrusion is a type of milling that is particularly effective for the pretreatment of lignocellulosic biomass, as it combines compression, tearing, and shearing mechanisms to disrupt the fibers and reduce the crystallinity and degree of polymerization of cellulose [35]. This results in the increased accessibility of the biomass to enzymes, which in turn leads to improved yields of sugars during subsequent hydrolysis processes [35].

The extruder is characterized by its screw configuration, which is the basis of its functionality. The screw is composed of transport screw elements (TSE) and reversed screw elements (RSE), which work together to crush and compress the biomass [35]. The RSE has threads whose pitch is opposite to that of the TSE, resulting in the accumulation and compression of the biomass fibers in the space between the TSE and RSE. The biomass is then crushed by the transport force and the reversed force generated by the TSE and RSE, respectively. The crushed biomass is forced to pass through the skewed slots of the RSE and enters the next section of the TSE and RSE to be further squeezed and crushed. This process is repeated, resulting in the continuous mechanical crushing of the biomass.

The extruder is particularly effective in reducing the moisture content of the biomass during the pretreatment process [14]. The high mechanical forces generated during the extrusion process cause the fibrillation and shortening of the biomass fibers, leading to an increased surface area and the greater accessibility of the biomass to enzymes. The heat generated during the extrusion process can also elevate the temperature to about 99 °C, which is ideal for the reaction of alkalis with the crushed biomass.

Extrusion is conducted in machines where both thermal and mechanical processes can take place. When using a thermal appliance, the biomass enters the tapered die section, where the remaining moisture is evaporated due to the high temperatures, intensifying the compression of the biomass residue materials, removing air and water to improve the penetration of solvents into the biomass fibers [36]. This high temperature and strong compression render the biomass material more suitable for burning and co-firing processes [37]. The extrusion process has several advantages over other densification methods. It can handle high solid loadings, making it suitable for industrial-scale applications. This process also improves the penetration of solvents into the biomass fibers, making it more appropriate for further processing, such as enzymatic hydrolysis or chemical treatments [38].

When only applying the extrusion process, lignocellulose biomass experiences few changes in its chemical composition; however, this process can be supplemented with chemical compounds to enable changes in the chemical composition of the lignocellulose biomass and combined with other chemical pretreatments [33,39]. The advantages of this technique are its low cost, the good monitoring of the variables, its easy adaptability to process modifications, and its continuous processing [38]. Other advantages are that it can accommodate short residency times, and its productivity is high. Moreover, it does not generate solid losses. Therefore, this technique is used for the bioethanol production from biomass, and its result is favorable; moreover, when combined with hydrolysis, sugars can be obtained [40].

In this process, single- or twin-screw extruders are used, which are essential for the particle size reduction of the lignocellulose biomass. However, there are other aspects that influence this process, such as temperature control, the screw speed, the residence time, and the mass transfer capabilities [41,42]. It is important to note that the feed to the system can be a solid material or a mixture of solids and liquids. For optimum performance, the interaction of the biomass components, the extruder screw design, and the operating conditions must be known. An extruder screw milling system can be designed with either single or twin screws, and each has different applications [43].

To optimize the design of an extruder for biomass pretreatment, several parameters need to be considered. These parameters can be categorized into the extruder design, biomass type, additives, and operating conditions.

The extruder design parameters include the barrel temperature, screw speed, screw configuration, and pressure. The barrel temperature and screw speed are critical in determining the degree of biomass degradation, while the screw configuration and pressure affect the mixing and shearing of the biomass. The optimal conditions for these parameters depend on the biomass type and the desired product [44]

The biomass type is an essential factor in extrusion pretreatment. Different biomass types have different physical and chemical properties, such as moisture content, particle size, and lignin content, which affect their extrusion behavior. For example, hardwoods have higher lignin content than softwoods, making them more challenging to extrude. Therefore, the extrusion conditions for hardwoods need to be adjusted accordingly [38].

Additives can also affect the extrusion process. For example, the addition of water or steam can enhance the extrusion process by increasing the moisture content of the biomass and reducing the viscosity of the extrudate. The addition of enzymes or chemicals can also improve the extrusion process by breaking down the biomass and enhancing the sugar recovery [45].

The operating conditions, such as the reaction time, biomass dry matter, and particle size, can also affect the extrusion process. The reaction time should be optimized to ensure that the biomass is adequately pretreated without over-degradation. The biomass’ dry matter and particle size should be adjusted to ensure that it flows smoothly through the extruder and is adequately mixed and sheared [46].

Based on the information from the sources provided, it is crucial to consider the characteristics of the biomass when determining the appropriate type of milling. When processing dry biomass, it is observed that the biomass incurs less damage compared to wet biomass during the milling process [47]. Dry milling helps to preserve the physical properties of the biomass, which is essential in maintaining its integrity and suitability for subsequent processes like enzymatic hydrolysis and fermentation.

The biomass chosen for milling pretreatment in this work is sugarcane bagasse due to its abundance and potential as a raw material for biofuel production. Cane bagasse is a lignocellulosic material derived from sugarcane processing—specifically, the fibrous residue left after extracting sugars from sugarcane. In regions like Brazil, which is a major sugarcane producer, there is a significant amount of bagasse generated as a byproduct of sugar production.

Cane bagasse is attractive for pretreatment processes like milling because it offers a sustainable and readily available source of biomass. By utilizing bagasse, which would otherwise be discarded or burned, as a feedstock for biofuel production, the overall efficiency and sustainability of the bioethanol production process can be enhanced. Additionally, bagasse has been found to be effective in bioethanol production due to its composition, rich in cellulose and hemicellulose, which can be converted into sugars for fermentation. Therefore, the choice of cane bagasse for milling pretreatment is driven by its abundance, sustainability, and potential as a valuable feedstock for biofuel production processes.

To demonstrate the practical application and effectiveness of the innovative extruder in improving the efficiency of biomass conversion, several experimental data and case studies can be considered. One such case study involved the twin-screw extrusion pretreatment of corn stover. Under experimental conditions using a twin-screw extruder at 99 °C, with a 325 rpm screw speed, 0.06 g/g biomass NaOH loading, and a biomass/liquid ratio of 1/2 (w/w), the results showed a sugar yield of 82%, lignin removal of 71%, and glucan and xylan conversion rates of 83% and 89%, respectively [35].

Another case study explored the extrusion pretreatment of various lignocellulosic biomasses, including sweet corn, barley straw, oil palm empty fruit bunches, and blue agave bagasse. Using a twin-screw extruder at temperatures ranging from 100 to 125 °C, screw speeds of 85–250 rpm, and a liquid/solid ratio of 3–4, the results demonstrated the significant destructuration of the lignocellulosic matrix and saccharification and fermentation yields comparable to those obtained at a laboratory scale, with a maximum of 85% from theoretical hexose sugars [38].

A third case study focused on alkaline twin-screw extrusion pretreatment for fermentable sugar production from corn stover. Using a twin-screw extruder at 140 °C, an 80 rpm screw speed, and a biomass/liquid ratio of 1/2 (w/w) without chemical additives, the results showed a sugar yield of 49%, glucan digestibility of 65.8%, and xylan digestibility of 65.6%. These case studies and experimental data collectively demonstrate the practical application and effectiveness of the innovative extruder in improving the efficiency of biomass conversion [48].

4. Sugarcane Bagasse

The principal chemical components of lignocellulose biomass are cellulose, hemicellulose, and lignin. Cellulose is a hexose polymer composed of monomers that occupies between 20 and 50% of lignocellulose biomass; hemicellulose is a heteropolymer composed of a mixture of pentoses and hexoses that occupies between 15 and 35% of the dry lignocellulose biomass. Moreover, it is composed of short branching sidechains; this leads to the greater uniformity of the lignocellulose structure. In addition, lignin is a biopolymer composed of three monomers, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, that occupies between 5 and 30%, and it acts as a barrier to the entry of microorganisms. The percentage depends on the type of biomass, source, and plant part, among others [49].

The biomass chosen for pretreatment with milling—specifically, cane bagasse—is suitable for this process due to the unique impact that extrusion has on its crystallinity. While extrusion generally increases the crystallinity of lignocellulosic biomass, sugarcane bagasse exhibits a decrease in crystallinity with extrusion. This is attributed to the specific temperature and conditions of the biomass during the process [50]. The reduction in crystallinity is beneficial for subsequent chemical treatments, as it enhances the accessibility of the biomass to enzymes and other chemical reagents. Additionally, the particle size of the biomass is reduced during extrusion, which is advantageous for further processing steps [49].

The sugarcane agro-industrial sector produced 23 million tons of cane during 2022, generating approximately 280 kg of bagasse for every ton of cane processed. Bagasse is a lignocellulosic material that is characterized by its large quantities of cellulose, which can be extracted for various applications, such as textile fibers. It is composed of short, long, and fine fibers [51]. One promising application of bagasse is in the production of bioethanol, which is a renewable and low-carbon fuel that can replace gasoline in transportation [52]. The dilute mixed-acid pretreatment of sugarcane bagasse has been shown to improve ethanol production, making it a promising approach for the commercial production of bioethanol from bagasse. Bagasse can also be used as a source of heat and power for the sugarcane industry. Bagasse can be burned to produce steam, which can be used to generate electricity for the mill and surrounding communities. This approach can help to reduce the industry’s reliance on fossil fuels and contribute to a more sustainable and circular economy.

In addition to these applications, bagasse can also be used as a feedstock for the production of bioplastics, which are biodegradable and environmentally friendly alternatives to traditional plastics. The cellulose and hemicellulose components of bagasse can be converted into bioplastics using various methods, including fermentation and chemical synthesis. Sugarcane bagasse is a lignocellulosic material and is characterized by large quantities of cellulose, which can be extracted for several applications, like textile fibers.

The fibers in sugarcane bagasse can be classified into three categories: short, long, and fine fibers [53]. The cellulose content in sugarcane bagasse is typically between 40 and 50%, making it a valuable source of cellulose for various applications [54]. The cellulose in sugarcane bagasse can be extracted and processed into various forms, including cellulose nanocrystals, which have high strength and stiffness, renewability, and environmental friendliness. These nanocrystals can be used in the synthesis of cellulose nanocrystal-based materials, which have been the subject of intense research in recent decades due to their potential applications in various industries, such as the manufacturing of composite materials and heavy metal adsorbents [53].

Sugarcane bagasse fiber and its cellulose nanocrystals have been used in the development of biocomposites, which are reinforced materials produced from the combination of a polymer matrix and a reinforcing agent, such as cellulose nanocrystals. Biocomposites produced from sugarcane bagasse fiber and its cellulose nanocrystals have shown improved mechanical properties, making them suitable for use in various applications, such as in the automotive and aerospace industries. Additionally, sugarcane bagasse fiber and its cellulose nanocrystals have been used in the development of heavy metal adsorbents, which are materials that can selectively bind and remove heavy metals from wastewater and other aqueous solutions [55]. The use of sugarcane bagasse fiber and its cellulose nanocrystals as heavy metal adsorbents has shown promising results, with the potential to contribute to the sustainable development of related products.

The twin-screw extrusion pretreatment parameters can be applied to other bioenergy source materials, but with some adjustments. The key considerations include adapting the power and screw design to suit the characteristics of the new biomass feedstock, as well as recalculating the optimal particle size needed for efficient processing.

The power required for the twin-screw extrusion process depends on the specific biomass feedstock and its properties. For example, if the new biomass has higher moisture content, more power may be needed to maintain the desired temperature and screw speed. Similarly, if the biomass has higher lignin content, more power may be needed to break down the lignin and achieve the desired particle size. The screw design should also be tailored to the specific biomass feedstock. For instance, if the biomass has higher fiber content, a screw with a higher compression ratio may be needed to effectively break down the fibers. Conversely, if the biomass has higher starch content, a screw with a lower compression ratio may be more suitable to avoid over-processing and preserve the starch.

The optimal particle size for twin-screw extrusion pretreatment can vary depending on the specific biomass feedstock. For example, if the new biomass has higher cellulose content, a smaller particle size may be needed to effectively break down the cellulose and achieve the desired saccharification yields. Conversely, if the biomass has higher lignin content, a larger particle size may be more suitable to avoid over-processing and preserve the lignin. Several case studies have demonstrated the effectiveness of twin-screw extrusion pretreatment for different biomass feedstocks. For instance, previous researchers used a bench-scale single-screw reactor to pretreat Miscanthus sacchariflorus under alkaline conditions, achieving significant improvements in biogas production [48] Similarly, other researchers combined extrusion with alkali pretreatment to increase the methane production from rice straw, demonstrating the effectiveness of this approach for different biomass feedstocks [48].

4.1. Chemical Composition

Sugarcane bagasse is composed of cellulose, hemicellulose, lignin, and others.

Table 2. Chemical composition of sugarcane bagasse.

Component	Percentage (%)
Cellulose	35–55	23.6–35	20	40.2	40.88	36.40	26–47	52.5
Hemicellulose	16–36	31.3–45	25	23.8	25.64	21.60	19–33	20.6
Lignin	14–26	13.4–30	42	25.2	23.42	19.60	14–23	25
Ash	1–5	1.7–6.2	-	-	-	-	1–5	1.9
Reference	[53,56]	[41]	[49]	[49]	[39]	[39]	[53]	[57]

shows the chemical composition of the initial biomass.

Cellulose has a crystalline structure, and its quantity depends on the source of the cellulose; on the other hand, hemicellulose has an amorphous structure that contains xylose and glucose, among others. Moreover, lignin gives greater rigidity to the biomass. In order to define the chemical properties of the biomass, a proximate and ultimate analysis must be conducted. The results found by other researchers can be seen in

Table 3. Chemical properties and elemental analysis for sugarcane bagasse.

Property	Result (%)
Inherent moisture	6.3 ± 0.1	-	-
Volatile matter	69.7 ± 0.7	-	74.82	84.54
Fixed carbon	13.1 ± 0.8	-	13.05	12.8
Carbon	49.79	41.67–58.14	43.35	45.57
Hydrogen	6.92	6.05–6.21	6.25	5.57
Nitrogen	0.42	0.37–0.69	0	0.305
Sulfur	0.08	-	0.05	0.04
Oxygen	42.78	34.57–41.33	45.79	45.135
Reference	[58]	[41]	[59]	[59]

.

4.2. Physical Properties and Mechanical Properties

The physical properties of biomass play a crucial role in the mechanical design of equipment for milling pretreatment. Understanding these properties is essential in optimizing the efficiency and effectiveness of the pretreatment process.

Table 4. Physical properties for sugarcane bagasse.

Physical Property	Result	Unit	Source
Tensile strength	170–290	MPa	[61]
Modulus of elasticity	15–27.1	GPa	[62]
Moisture	7.57 ± 1.9 × 10−1	%	[63]
Real density	0.1656 ± 9.9 × 10−4	g/cm3	[7]

provides key physical properties that are fundamental for the design of equipment tailored to the specific characteristics of the biomass being processed. These properties may include factors such as the particle size, density, moisture content, and other relevant parameters that impact the mechanical aspects of the milling process [60].

By considering the physical properties outlined in Table 4, engineers and researchers can design milling equipment that is well suited to handle the specific characteristics of the biomass feedstock. Factors such as the particle size distribution and density can influence the milling efficiency and energy consumption during the pretreatment process. Additionally, knowledge of these physical properties allows for the selection of appropriate equipment configurations, such as hammer mills, ball mills, or other types of milling machinery, to achieve the desired outcomes in terms of biomass particle size reduction and overall process effectiveness.

4.3. By-Products and Processes Favored by Milling

Various studies have been conducted to apply milling as a pretreatment to bagasse and sugarcane; depending on the desired process or by-products, different milling conditions are used. In order to understand the range and possible pretreatments of bagasse,

Table 5. Reports of sugarcane straw milling pretreatment.

Feedstock	Type of Milling	Conditions	Products Obtained/Process	Source
Sugarcane straw	Ball milling	90 min	Xylose/glucose: 77.6 and 56.8%. Reduction in cellulose crystallinity.	[64]
Sugarcane bagasse	Wet disk milling	5% solids, 15 L water, overnight soaking	Increased fibrillation, enhanced cellulolytic enzyme accessibility. Glucose/xylose: 49.3% and 36.7%.	[24,64]
Sugarcane bagasse	Disk milling	1–5% solids, passed between ceramic disks, multiple cycles	Small particle sizes, large specific surface areas, increased enzymatic hydrolysis yields.	[24]
Sugarcane bagasse	Ball milling/cut milling	10–120 min—room temperature	Increase in enzymatic saccharification.	[65]
Sugarcane bagasse	Liquid hot water/disk milling	140–180 °C for 10 min (20% w/w solid content) and then disk-milled	Improved glucose release: 41–177%.	[66]
Sugarcane bagasse	Wet disk milling	1800 rpm, 1 kg/15 L water, overnight, 20 cycles	Glucose/xylose: 78.7% and 72.1%.	[64]
Sugarcane bagasse	Ball milling	400 rpm in a 45 mL milling pot; cyclic mode of 10 min milling, followed by a 10 min rest	Glucose/xylose: 49.3% and 36.7%.	[64]

is presented.

It can be seen in Table 5 that there are no standardized conditions to obtain the best yields of sugars or valorized products form sugarcane. It can also be seen that ball milling and disk milling are the most used pretreatments with this specific biomass and that they can often be accompanied by other pretreatments, such as liquid hot water (LHW). It is worth noting that when adding other pretreatments, such as LHW, the process is no longer only physical (as with milling) and becomes a physicochemical mixed pretreatment. The use of milling leads mainly to the production of glucose/xylose, but mixed treatments can lead to other by-products. A milling design created specifically for the characteristics of sugarcane bagasse can improve the yield and the variety of products obtained with the pretreatment and optimize and standardize the process of biorefinery with sugarcane bagasse.

4.4. Colombian Sugarcane Bagasse Characterization

In order for the design to be applicable in the Colombian context, we performed a proximate, ultimate, and structural analysis of a Colombian sample of sugarcane bagasse. For the proximate analysis, the ash, volatile matter, and fixed carbon content were determined using the laboratory analytical procedures NREL/TP-510-42621 [67], “Determination of total solids in biomass and total dissolved solids in process liquid samples”, for moisture; NREL/TP-510-42622 [68], “Determination of ash in biomass”, for ash; and ASTM E872-82 [69], “Standard test method for volatile matter in wood fuel particulate analysis”, for volatile matter. Fixed carbon was calculated as 100 − (%moisture + %volatile matter + %ash). The ultimate analysis was based on ASTM-D5373 [70], “Standard Test Methods for Determination of Carbon, Hydrogen and Nitrogen in Analysis Samples of Coal and Carbon in Analysis Samples of Coal and Coke”. The structural analysis (cellulose, hemicellulose, and lignin) was based on the protocols of FNA-AOAC 973.18 [71], “Official methods of analysis, acid detergent fiber”; FND-AOAC 200.04 [72], “Official methods of analysis, neutral detergent fiber; and Lignin-AOAC 973.18 [71], “Official methods of analysis, lignin”.

The characterization results are presented in

Table 6. Analysis of sugarcane bagasse obtained in this study.

Property	Result (%)
Proximal analysis
Moisture	7.910 ± 0.07
Ash	1.555 ± 0.02
Volatile matter	80.93 ± 0.08
Fixed carbon	9.601
Elemental analysis
Carbon	45.22
Hydrogen	5.94
Nitrogen	0.29
Sulfur	0
Oxygen	48.56
Structural analysis
Cellulose	41.9
Hemicellulose	25.0
Lignin	8.1

.

The characterization data of sugarcane bagasse reveal the key properties that make it suitable for pretreatment via milling processes. The relatively low moisture content (7.910%) indicates that the bagasse is not excessively wet, which is favorable for milling processes. Excessive moisture can lead to operational challenges and reduced efficiency in milling equipment. The low ash content (1.555%) suggests that the bagasse has a relatively clean composition, which is beneficial for milling operations. High ash content can cause wear and tear on the milling equipment and affect the quality of the processed material. The high volatile matter (80.93%) content indicates that the bagasse is rich in combustible components, which can aid in the milling process by providing energy for size reduction and facilitating the breakdown of biomass components. The presence of fixed carbon (9.601%) contributes to the energy content of the bagasse, which can help to sustain the milling process and potentially enhance the efficiency of biomass breakdown.

Milling can be useful for this specific biomass by breaking down the complex structures of cellulose, hemicellulose, and lignin, making them more accessible for further processing and extraction. Milling can reduce the particle size of the biomass, increasing the surface area available for enzymatic hydrolysis or other chemical reactions, which can enhance the efficiency of the conversion process. Milling can also help to break down the lignin–carbohydrate complexes, making it easier to separate the individual components of the biomass.

5. Design of Device and Design Details

Designing an innovative extruder for sugarcane bagasse milling pretreatment is crucial in optimizing the bioethanol production process. Extrusion pretreatment is a critical step in biomass preparation, as it ensures efficient milling, drying, and sieving to produce a uniform substrate for further processing. An optimized extruder design allows for the precise control of the operating conditions, such as the temperature, residence time, and screw speed, which is essential in achieving optimal sugar recovery rates and minimizing the energy consumption. Additionally, sugarcane bagasse has poor flow capabilities, making it challenging to process. An innovative extruder design can incorporate additives or modifications to enhance the flowability of the substrate, reducing the risk of jamming and improving the overall processing efficiency.

Furthermore, an innovative extruder design can be scaled up for commercial applications, reducing the costs and increasing the efficiency of the overall bioethanol production process. This is particularly important for large-scale industrial operations. An optimized extruder design can also improve the sugar recovery rates, leading to increased bioethanol yields and reduced production costs. Moreover, an innovative extruder design can be optimized to minimize the energy consumption during the biomass preparation and extrusion process, reducing the overall environmental impact of the bioethanol production process. In summary, designing an innovative extruder for sugarcane bagasse milling pretreatment is crucial in achieving efficient biomass preparation, optimized operating conditions, improved flowability and scalability, enhanced sugar recovery, and reduced energy consumption.

5.1. Mathematical Model Applied

The single extruder screw is characterized by operating at low speeds, and, when a restriction is placed on the biomass flow, a profile of the temperature, filling, and pressure can be obtained, as shown in Figure 1. The restrictions of this type of screw are obtained by varying the diameter and pitch, among others [73]. For the design of the extruder screw and its components, the following parameters are considered.

• Mechanical power supply: The input power to the extruder screw is supplied directly by a geared motor, where the screw speed (N—rpm), torque (T—Nm), motor power (P—kW), and material feed flow (F—kg/h) are as shown in Equation (1) [73,74].

$$\mathrm{S}=\mathrm{ }{\displaystyle \frac{\mathrm{N}\mathrm{T}\mathrm{P}}{\mathrm{F}}}$$

(1)

• Residence time: This illustrates the path of the biomass in the extrusion process. It is very important in the process since it defines the particle sizes obtained and the products of the thermal processes. Its magnitude corresponds to the relationship between the flow, drag, and pressure of the material and the equipment. As the screw speed increases, the residence time decreases. The average residence time value is calculated using the equation [73,74]

$$\mathrm{A}\mathrm{R}\mathrm{T}={\displaystyle \frac{\mathrm{M}\mathrm{s}}{\mathrm{F}}}$$

(2)

where

• Ms: material mass (kg);
• F: total feed rate (kg/min).

• Conveyance and restrictions: In this section, the screw is defined by the pitch, channel depth, number, and thickness of the helices. To establish the depth of the channel and the thickness of the propellers, it is necessary to determine the diameter of the screw and the distance of the shafts. With Equation (3), the screw pitch (SP) can be calculated, and with Equation (4), the channel width can be calculated [73,74].

$$\mathrm{S}\mathrm{P}=\mathsf{\pi }\mathrm{D}\tan \mathsf{\theta }$$

(3)

$$\mathrm{W}={\displaystyle \frac{\mathrm{S}\mathrm{P}}{\mathrm{n}}}\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }-\mathrm{e}$$

(4)

where

• D: screw diameter;
• $\mathsf{\theta }$: flight helix angle;
• n: number of screw flights;
• e: screw flight thickness.

The drag flow rate (Qd) is also set with Equation (5). In addition, the drag flow correction factor (Fd) is obtained with Equation (6) [73,74].

$${\mathrm{Q}}_{\mathrm{D}}={\displaystyle \frac{\mathsf{\pi }}{2}}{\mathrm{F}}_{\mathrm{d}}\mathrm{W}\mathrm{H}\mathrm{D}\mathrm{N}\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\phi }$$

(5)

and

$${\mathrm{F}}_{\mathrm{d}}=1-{\displaystyle \frac{0.571\mathrm{H}}{\mathrm{W}}}$$

(6)

5.2. CAD Design Model

The design of an innovative extruder is crucial for efficient biomass processing. As stated in previous research [75,76,77], an innovative extruder design can enhance the efficiency of the milling process, allowing for the better processing of biomass materials. This is particularly important for sugarcane bagasse, which is a complex biomass material that requires specialized processing techniques to achieve optimal results.

The design of an innovative extruder should prioritize factors such as temperature control, the screw speed, and the residence time to ensure optimal processing conditions. As noted by previous work, the design of the extruder must take into account the specific requirements of the biomass material being processed [49]. By incorporating these design considerations, an innovative extruder can be obtained to revolutionize biomass processing by providing a more efficient and effective means of milling and pretreatment for biomass materials.

The need for innovative extruder designs is particularly pressing in the context of green energy. As the world transitions to renewable energy sources, biomass processing plays a critical role in the production of biofuels and other sustainable products. However, the efficiency of biomass processing is often limited by the effectiveness of the pretreatment methods. An innovative extruder design can help to address this challenge by providing a more efficient and cost-effective means of processing biomass materials.

Several studies have used different types of mills, such as ball mills, disk mills, and roller mills. The milling process has been implemented in different experimental cases [78,79,80] and is the most common and affordable process in the industry. However, there is a need to implement the extruder mill type as a milling alternative for the processing of large quantities of biomass with lower energy consumption. Moreover, it is important to note that this extruder mill design can be adapted with heaters and realize different chemical treatments at the same time as the milling process, enabling a semi-continuous process and functioning as a chemical reactor with added pressure and temperature meters. Finally, this design is innovative because it is intended for the processing of large quantities of biomass, in addition to the fact that it includes all three key mechanisms to achieve the complete processing of the biomass, thanks to its extruder screw design.

Applying the mathematical model, the drawing and design were obtained using Autodesk Inventor Professional 2023. These are presented in Figure 2 and are characterized as shown in Table 6.

It is important to note that the mill is designed to be loaded at 30% of the total capacity and the particle size will be reduced by 75% of the capacity to 3/4 (

Table 7. Characteristics of the mill.

Characteristic	Value
Capacity	14.7 ${\displaystyle \frac{f{t}^3}{hr}}$,
N	9.8 rpm
Motor power	1 HP
Length	4.5 ft
Torque	1050 lbin
Screw diameter	6 in
Screw pitch	Short and standard

).

Moreover, Figure 3 presents the different parts of the mill and

Table 8. List of the key parts of the designed mill for biomass pretreatment.

Number	Name
1	Screw
2	Channel
3	End Shaft
4	Drive Shaft
5	Shaft Seal
6	End Plate 2
7	End Plate 1
8	End Bearings
9	Motor
10	Flange Foot
11	Covers
12	Inlet
13	Discharge
14	End Bearings

presents their names. The most relevant part of the design is the screw, because it oversees the conveying, and, together with the channel, it exerts the necessary forces for the compression, tearing, and shearing mechanisms. In addition, the motor has an important function because it provides the necessary power to drive the screw, considering the weight of the screw and the transport of the biomass. It should be noted that the capacity of the equipment is given by the channel and the screw. The feeding and discharging parts are also important; they are designed to direct the biomass to the screw and channel.

In addition, the dimensions of the mill are shown in Figure 4.

5.3. Finite Element Analysis

The most critical part of the mill is the channel; therefore, a finite element analysis was performed for this part, highlighting the total deformation and the von Misses stress, as shown in Figure 5 and Figure 6.

The channel shows particle behavior in places with fixed supports. This is due to the weight for which it is designed; therefore, more fixed supports are required in the critical parts. For this reason, the greatest deformation is identified as occurring between the top and the center, as well as between the bottom and the center, represented by red, orange, yellow, and light green, as shown in Figure 5. However, it is evident that the largest deformation value corresponds to 0.00012779 m. It should also be noted that the deformation in 90% of the channel corresponds to the minimum.

Austenitic 304 stainless steel was selected for the design of the mill. It is characterized by low corrosion and oxidation. It is also suitable for cold working and is widely used in the food industry. Its physical properties, analyzed in [81], are shown in Figure 7.

Figure 6 shows the behavior of the channel with 304 steel with respect to the supported stress. It is observed that the maximum value is 25.7 MPa in a few areas of the channel, represented in red, followed by a stress value of 8.5 MPa in the central parts of the channel, represented in light blue; finally, in its largest region, a minimum stress level of 8675.9 Pa is found, represented in dark blue. This means that the selected material is suitable for the channel because its maximum stress value does not exceed the maximum stress value of the material.

6. Conclusions

In conclusion, milling pretreatment plays a crucial role in enhancing the efficiency of biomass conversion processes by reducing its particle size, increasing its surface area, and improving its accessibility to enzymes for subsequent hydrolysis and fermentation.

This pretreatment method has been shown to significantly improve the overall bioconversion efficiency of lignocellulosic biomass into biofuels and biochemicals, as well as the release of sugars.

The optimization of the milling parameters, such as the speed, time, and type of mill, is essential in maximizing the effectiveness of this pretreatment method and unlocking the full potential of biomass as a renewable resource for the production of bio-based products.

An extruder screw mill design is presented, designed specifically to operate with sugarcane bagasse. The designed screw extruder is designed for the milling of sugarcane bagasse with a capacity of 14.7 ${\displaystyle \frac{f{t}^3}{hr}}$, 9.8 rpm, power of 1 HP, a length of 4.5 ft, torque of 1050 lb-in, a diameter of 6 in, and a short and standard screw pitch. The designed screw extruder is to be loaded at 30% of the total capacity, and the particle size will be reduced by 75% of the capacity to ¾ inches.