The need for energy accessibility and sustainability in Nigeria cannot be overemphasized. The US National Intelligence Council Report (2012) projected that projected by 2030, the global demand for energy would have increased by 50%. A common indicator Human Development Index (HDI) used as a goal for sustainable energy development, is a critical factor in determining the dimensions of the human well-being. A correlation of HDI value with energy production over the last three decades for Nigeria shows a negative slope as opposed to other developing/developed countries (Adediran, 2019).
This shows an urgent energy policy in our national development path. As reflected on the Sustainable Development Goal 7(affordable and clean energy) set by the UN towards 2030, this calls for a need in increase in accessibility to alternative energy sources which are renewable and sustainable as they play a major role in green technology; one of which is the production of bioethanol from biomass. Biomass feedstock can generally be divided into two sources: starch/food crops and lignocellulose materials.
The former, usually called the 1st generation biofuel, has shown low degree of sustainability due to their competition with world food supply. Hence, the 2nd generation biofuel emerged from lignocellulose materials. They are relatively cheap and do not compete with food. They are further divided into three categories: virgin biomass (e.g. trees, bushes, etc.), waste biomass (e.g. Sugarcane bagasse, waste newspaper, etc.) and energy crops (e.g. Elephant grass, switch grass, etc.). The population of Nigeria according to the United States Census Bureau (2019) is estimated to be over 178.
5 million people. Waste generation rate in Nigeria is estimated at 0.65-0.95 kg/capita/day which gives an average of 42 million tonnes of wastes generated annually (Ike at al., 2018). Waste biomass, a form of lignocellulose material constitutes about 18% of that and is emerging as an attractive option in the choice of raw materials for ethanol production due to reduction in feedstock cost as they constitute a bulk of wastes generated annually.
The cost of bioethanol production from lignocellulose biomass in current technologies is relatively high. Apparently, low yield is one of the main challenges contributing to the economic ineffectiveness of the process. The two major factors contributing to this is the choice of feedstock and the process route of production. Bioethanol production consists majorly of four processes: pre-treatment, hydrolysis, fermentation, and ethanol recovery and separation. The complexity and variability of lignocellulose biomass physicochemical structures hinder the hydrolysis of cellulose present in biomass to biofuels. Shadbadhr (2017) also reported that the hydrolysis of cellulose into fermentable sugars is still the main barrier in commercial-scale cellulosic ethanol production due to high cost of enzymes and low efficiency of the process caused by product inhibition. Various research studies have been conducted on specific processes in the production of bioethanol to increase efficiency of production on industrial scale. However, challenges are met during the choice of processes in production. While some technologies have interesting advantages, they are associated with certain drawbacks.
The choice of feedstock also is a major barrier to production efficiency. The purpose of the biomass feedstock is to obtain maximum amount of fermentable sugars from cellulose which is further processed to give ethanol. However, different biomasses have different physio-chemical structures including surface area, pure volume, etc. that affect the viability of sugar extraction from them. Lignocellulose materials which have high lignin content are not usually preferred because lignin is the structural backbone that surrounds the cellulose and hemicellulose and inhibits hydrolysis of the biomass. Also, availability of feedstock is a critical factor in determining the overall economics of the process when implemented.
Efforts have been made in modifying the production of bioethanol from lignocellulose materials considering the required capital and operating cost of running the production. However, low yield is unconventionally still a major problem that affect the production process. An optimal process configuration will be drawn out and justified with a cost analysis based on previous research carried out.
The aim of this project will be to develop a mathematical optimization model for the production of bioethanol from lignocellulose materials. Route decisions will be based on:
The selection of feedstock will be done using a specific process route. This is to know the optimal emerging feedstock that can be used. A model will be developed after determining the optimal process route for bioethanol production. This would be used for simulation work using DWSIM and Scilab to determine the best realistic conditions the process would undergo. Thereafter, an optimal process configuration will be selected based on economic analysis while also still considering realistic operational and capacity limits. Optimal operation and design variables will be determined to maximize ethanol production.
At the end of this project, an optimal process route for a specific feedstock will be developed to produce bioethanol from lignocellulose biomass with maximum effort to associate the output to realistic conditions on an industrial scale.
For the purpose of the project, the following will be obtained:
From a preliminary view, it may seem that the scope of this study is broad. As a result of that, a complete optimization and cost configuration of the whole process route may not be provided. As such, a specific unit operation that is critical in the overall economics of the process may be selected and a proper optimization and cost analysis will be carried out on that unit operation.
Sourcing of data may also be an issue as different research studies have been carried out at different conditions and feedstock. So, it may be rigid integrating these results into the required objective of this study.
This discusses the underlying theoretical framework of study, conceptual framework of study and reviews previous research work done based on this project. It outlines the gap(s) discovered in previous studies which this project seeks to focus on.
Ethanol (or ethyl alcohol) is a compound in the alcohol family i.e. contains the OH group. Its chemical formula is CH3 – CH2 – OH (C2H5OH). It is a primary alcohol and a volatile organic compound. Pure ethanol is a colorless, flammable liquid with a boiling point of 78.5℃ and melting point of -114.5℃ at 760 mmHg. It has a specific gravity of 0.789 at 20℃. It is commonly used as a solvent in pharmaceuticals because it is very soluble in water as a result of the presence of a polar – OH group. It is also used in alcoholic beverages and alternatively as a biofuel. It has a high bactericidal activity and it is commonly used as disinfectant. It is termed a renewable fuel due to its high octane rating, which when blended with gasoline up to 85% reduces emissions of greenhouse gases such as carbon monoxide (CO), Sulphur dioxide (SO2), etc. Bioethanol is a substitute to fossil fuels and has a wide range of applications in the energy sector of the world. When combusted, it can be used as a source of fuel for transportation, energy generation for electricity or as a source of heat. This makes ethanol an emerging option in the goal for sustainable and renewable energy incorporation in world’s development. Lignocellulose ethanol, an unfolding option is made from lignocellulose biomass by disintegrating cellulose in plant fibers. The cellulose is extracted and broken down to glucose and other sugars which can be fermented to give ethanol and carbon dioxide.
Feedstock, or biomass is an important element in the production of bioethanol. The concept of renewable energy development is solely based on the nature of feedstock such that it can be replenished as quickly as it is consumed. Bioethanol can generally be classified into 1st generation and 2nd generation biofuels. The 1st generation bioethanol is gotten from starch/food crops such as corn, wheat, potatoes, cassava while the 2nd generation bioethanol is derived from lignocellulose materials such as paper pulp, switch grass, sugarcane bagasse. The 1st generation bioethanol is produced based on the notion of generation of excess agricultural food crops. Subsequent statistics proved that the volume of excess food crops is low relative to the volume of demand for ethanol production. It was also evident that its shows a low degree of sustainability due to their competition with world food supply. Hence, the 2nd generation bioethanol production from lignocellulose materials came to play. A major advantage of the latter is its workable solution to the economics and quality of its feedstock. It is cheaper, easily available and does not compete with world food supply. Although, it has structural characteristics that makes hydrolysis more difficult than that of starch crops.
Lignocellulose biomass is usually derived from agricultural/forest residues, industrial waste and municipal solid wastes. Municipal solid waste seems to be the interest of this research as 42 million tonnes of wastes are generated annually in Nigeria of which 0.65-0.95 kg wastes/capita/day (Ike et al., 2019). The effective management of these wastes must be taken as a top priority to reduce the negative impact on the environment and subsequently convert them into valued products – bioethanol.
A lignocellulose material basically consists of cellulose, hemicellulose and lignin. Cellulose is a homopolymer of glucose with β,1-4 linkages. Hemicellulose is a heteropolymer that consists of hexose and pentose structures. Lignin is a highly irregular polymer with three dimensional structures built from oxygenated phenyl propane units interlinked via β,0-4 and α,0-4 aryl ether linkages (Thuong et al., 2019). The goal is to extract the cellulose and hemicellulose from the biomass. The exact sugar composition can vary depending on the nature of feedstock. Biomass with high lignin content are not preferable because of their complexity in fermentable sugar extraction. Shadbhar (2017) reported that lignin is a random aromatic compound that hinders cellulosic bioethanol production due to its strong linkages to cellulose and hemicellulose.
Fig. 2.1: Wang Q. et al. (n.d.). Structure of lignocellulose biomass. Fates of hemicellulose, lignin and cellulose in concentrated phosphoric acid with hydrogen peroxide (PHP) pretreatment (2018). RSC Advances.
Lignocellulose biomass majorly comprises of cellulose, hemicellulose and lignin. These three components all contribute to the overall yield of production. Their structures and physio-chemical properties will be discussed subsequently.
Cellulose (C6H12O6)n is a highly stable polymer consisting of thousands of long linear chains of glucose. It is the most important component of any basic lignocellulose material. From Table 1.1, it can be shown that cellulose is the most abundant component in lignocellulose biomass. Although, it has a wide range of applications in different fields such as pharmaceutical, food, cosmetics, textile industry, etc., this project is targeted to the application of cellulose in energy production. Cellulose is made up of a d-glucose unit at one end and a C4-OH group, the non-reducing end, while the terminating group is C1-OH, the reducing end with aldehyde structure. It basically consists of linear homopolysaccharide composed of β-D-glucopyranose units linked by β-1-4-linkages (Thuong et al., 2019). The repeating unit is composed of a dimer glucose known as cellobiose. It is produced by photosynthesis where plants utilize energy from the sun to convert water and carbon dioxide to long chains of glucose. The degree of polymerization is estimated up to 12,000 units in lignocellulose materials. It is stored in multiple chains compacted together in plants forming fibrils. Cellulose determines the strength of cell walls of plant. The structure of cellulose is important in determining the integrity of the cell walls. Cellulose structures can be classified into the following:
It is desirable to have cellulose microfiber structures in the feedstock for bioethanol production considering properties of all three types of structures. \n\n\n\n\n
Fig. 2.1: (n.d.). Structure of cellulose. Compound Summary of Cellulose (Accessed on March, 2020). National Center for Biotechnology Information, U.S. National Library of Medicine, Bethesda.
Hemicellulose is a branched heteropolymer consisting of hexose and pentose sugars. They are polysaccharides in cell walls of plant and contain xyloglucans, xylans, mannans and glucomannans and glucans. Hardwood and softwood contain mainly mannans while grasses and straws contains arabinans, galactan and xylans. Generally, hemicellulose is in relatively higher proportion in hardwood than softwood. Hemicellulose structures have relatively shorter chains (500 – 3000 sugar units) unlike glucose which have very long chains. They tend to attach tightly to the surface of cellulose micro-fibers. It has a relative bond with water to form a gelling agent. Hemicellulose have an erratic amorphous structure with very little strength. Because of this, it can easily be broken down in concentrated acidic or high temperature medium. Therefore, it is important to carefully choose the pretreatment conditions in order to prevent production of unwanted inhibitory by-products. \n\n\n\nFig. 2.2: Kulkarni, V. (n.d.). Structure of Hemicellulose. Natural polymers (2012). International Journal of Research in Pharmaceutical and Biomedical Sciences.
Lignin represents averagely 10 – 25% of biomass by weight as shown in Table 1.1. It is a highly irregular polymer with three dimensional structures built from oxygenated phenyl propane units interlinked via B,0-4 and a,0-4 aryl ether linkages (Thuong et al., 2019). It has a non-polar aromatic ring side chain and polar sulfonic acid group that contributes to its high stability. It forms a kind of adhesive matrix that encircles the cellulose and hemicellulose, providing strength and protection to the woody structure. It has a high molecular weight of more than 5000g/mol. Lignin has suitable properties for a wide range of applications: cement water reducer, paints, dyes, polyolefin, binders, emulsifiers, etc. Considered as a by-product in the production of bioethanol from lignocellulose, it is necessary to break down and remove lignin structures to access the cellulose and hemicellulose needed for fermentation. This is because residual lignin has a negative impact on the properties of cellulose. Although when removed, lignin can be utilized into potential useful applications such as valorization for biofuels, production of polymer building blocks and high molecular weight polymers.
The three major structures of lignin i.e. p-coumarylalcohol, coniferylalcohol and sinapylalcohol are shown below:
Fig. 2.3: H.V. Lee, S. S. (n.d.). Chemical Structures of Lignin. Conversion of Lignocellulose Biomass to Nanocellulose: Structure and Chemical Process (2014). Nanotechnology & Catalysis Research Centre(NANOCAT), University of Malaysia, Malaysia.
The choice of feedstock depends on its economics, quality and viability. Quality deals with the physio-chemical structure and complexity of the biomass. This affects the amount of fermentable sugars that can be extracted, playing a major role in the overall yield of production.