Cellulose is a polysaccharide that consists of a lot of D-glucose units combined. It is one of the major components of green plants. Cellulose is converted to biofuel as it is excreted by several bacteria. This conversion is carried out through series of different processes, all of which hold equal importance. Carrying out these processes is a hard task and requires thorough observations as a lot of hard work is put through all these tasks.
Industrial methodologies are incorporated for the smooth running of all the tasks as biofuel is considered one of the most important products to run the industries and different industrial processes. There are a lot of uses and applications of biofuels, strengthening their efficacy and efficiency.
Having the formula ‘(C6H10O5)n, an organic compound, known as cellulose, is a polysaccharide that consists of a linear chain of many hundred to many thousands of β(1→4) linked D-glucose units. The primary cell wall of oomycetes, various algae forms, and green plants have a significant structural component which is cellulose. It is secreted by some of the bacterial species for forming biofilms. On Earth, the most important and abundant organic polymer is cellulose. Cotton fiber's cellulose content is 90%, and wood's cellulose content is 40 to 50%, and almost 57% is the cellulose content of dried hemp.
Paper and paperboard can be produced by mainly using cellulose. One can convert smaller quantities of it into various derivative products like rayon and cellophane. Cellulose conversion into biofuels like cellulosic ethanol from energy crops is under development as a renewable source of fuel. Cotton and wood pulp are mainly used for attaining cellulose for industrial usage.
Some kinds of animals like termites and ruminants are capable of digesting cellulose with the aid of symbiotic micro-organisms living in their guts, for instance, Trichonympha. Cellulose is an insoluble dietary fiber's non-digestible constituent in human nutrition, which potentially aids in defecation and functions as a hydrophilicbulking agent for feces.
Cellulose is biodegradable and chiral. Cellulose is not soluble in water and most of the organic solvents. With 20-3- degrees of contact angle, cellulose is hydrophilic. Cellulose is odorless and has no taste too. In 2016, Dauenhauer et al. made pulse tests in which cellulose was seen melting at 467 degrees Celsius. Cellulose can be treated at high temperatures with the concentrated mineral acids to chemically break it down into its glucose units.
Derivation of cellulose
Scientists derive cellulose (a straight-chain polymer) from D-glucose units, and they condense through the β (1→4)-glycosidic bonds. No occurrence of branching or coiling and an extended and stiff rod-like conformation is adopted by the molecule unlike starch and this is helped by the glucose residues’ equatorial conformation.
Dependence of properties
Cellulose has various characteristics and most of them are determined by the degree of polymerization or length of its chain, or the number of glucose units making one polymer molecule. A typical chain length of 300-1700 units is possessed by the cellulose from the wood pulp; whereas a chain length of 800-10,000 units is possessed by the cotton, other plant fibers, and bacterial cellulose too.
Cellodextrins are molecules with extremely small chain lengths and that is a result of the breakdown of cellulose. They are soluble in organic solvents and water too. (C6H10O5)n is cellulose’s chemical formula and here, n displays the number of the groups of glucose and it is the degree of polymerization.
Components of cellulose
Fibrils with amorphous and crystalline regions make up cellulose. Cellulose pulp’s mechanical treatment individualizes these cellulose fibrils, which is often aided by enzymatic treatment or chemical oxidation, yielding semi-flexible cellulose nanofibrils of the length of 200 nanometres to 1 micrometer and it is determined by the intensity of the treatment.
Strong acid can be used to treat cellulose pulp for hydrolyzing the amorphous fibril regions, thus forming short rigid cellulose nanocrystals with a length of 100 nanometres. It is because of their usage as the Pickering stabilizers for emulsions, usage in nanocomposites with remarkable mechanical and thermal characteristics, formation of aerogels or hydrogels, and self-assembly into cholesteric liquid crystals, these nano cellulose hold high technological interest.
Currently, almost 96% of organic chemicals and 86% of energy being utilized in the world is supplied by fossil resources. One of the major issues right now is petroleum fuel's continuous depletion. Uneven geographic distribution, global warming, and availability are the other concerns that are associated with the large-scale usage of fossil fuels.
By 2050, there will be an expected increase of the global population by almost 3 billion people, which significantly raises the demand for fuels. It was indicated in one estimate that over the next 20 years, the world energy consumption will increase by 35 % for meeting the increasing demand of the industrialized countries and the fast development of the increasing economies. Fuels can be made from renewable biomass resources by developing new technologies as there will be a depletion of fossil fuels in the coming years.
Currently, biomass is responsible for the world’s 9.8% primary energy usage annually, out of which 70% is traditionally utilized (combustion for domestic heating with 15-20 MJ/kg of energy density) and 30% is utilized in modern forms (steam and liquid biofuel). The energy of biofuel is derived from biomass and biofuel is one of the fuel's types. Biofuel includes solid biofuels like a wood puck, wood cube, and wood pellets.
Example of gaseous biofuel is hydrogen, and example of liquid biofuels is butanol and ethanol. It is estimated that by 2030, the world's liquid biofuel production will increase to 5.9 million barrels per day from 1.9 million barrels per day in 2010. In the United States, it is expected that 136 billion litter biofuel will be produced by 2022, and out of that 136 billion, 61 million litter is formed from cellulosic materials.
Biomass has been used directly to produce electricity and heat. It was during World War II that the power regeneration engines were made to utilize biomass directly as an energy source. Although, several main problems come with this direct utilization and they are the following; when they are utilized as biofuel, biomass's uneven geographical distribution necessitate its transportation, biomass’s bulky nature (low heat value) results in complicated and costly transportation systems; engines using biomass have a high environmental effect and a poor efficiency (CO2 emission).
According to the first law of thermodynamics
It is stated in the first law of thermodynamics that energy will be consumed on any kind of biomass's chemical or biological conversion to biofuel, therefore during conversion, energy part stored in biomass will be lost, and in comparison to biomass (raw material), the biofuel will have lower energy ultimately. Converting biomass to biofuel comes with a lot of benefits that can outweigh the deficiencies that come with it.
As compared to biomass, biofuel's energy density (MJ/kg) will be higher, and the production of biofuel can eliminate combustion's common problems, for instance, superheater corrosion and fly ash disposal. The digested materials from the biorefinery can be utilized as a sustainable and remarkable fertilizer for crops and cultivation (a truly recyclable process). The process of the production of biofuel is more environment-friendly (less emission of carbon dioxide). In the sector of transportation, liquid fuel (ethanol) has a large demand.
Currently, first-generation biofuels are made from vegetable oils, starches, and sugars, but various issues are possessed by these products; 1) their contribution in saving fossil energy consumption and CO2 emissions are restricted by high energy input for their conversions and cultivations, and 2) their availability is restricted by per-hectare yield and soil fertility.
Although, these following facts make lignocellulosic biomass more attention-seeking; 1) There is no competition between it and food or food industries, 2) It is available in various countries locally, and 3) it is earth's most broadly spread renewable source available (annually, almost 6-7 times of total human energy consumption is the total chemical energy stored by the plants in biomass). Although, it is very difficult to convert woody biomass to fermentable sugars as compared to the agro-based biomass due to the presence of lignin and more hemicelluloses (not easily fermentable) and cellulose’s more crystallized and condensed structure in woody biomass.
Dissembling lignocellulosic biomass
We should firstly dissemble the lignocellulosic biomass for facilitating cellulose's isolation from the other constituents, for instance, hemicelluloses and lignin, for the production of the biofuel from the cellulose of biomass.
Cellulose macromolecules should be subsequently depolymerized as cellulose's biological and chemical conversions to biofuel are considerably improved by depolymerization. Then, biological treatments are used to convert the depolymerized cellulose like glucose into biofuel, and then at last the biofuel must be purified. In oxygen's presence, biomass must be deoxygenated as oxygen lessens the molecule's heat content and forms high polarity, and high polarity influences its blending with the current fossil fuels.
Pretreatment of biomass
The utilization of lignocellulosic biomass in the production of biofuel is difficult because of the complicated structure of lignocellulosic biomass. 15-20% lignin, 25-35% hemicelluloses, and 40-50% cellulose, generally make up Lignocellulosic raw materials. The plant cell wall can be dissociated with the help of the pretreatment stage for the improvement of the accessibility of microorganisms and/or chemicals to cellulose for the possible conversions. Lignin's removal is targeted by the pretreatment processes, which enhances cellulose's digestibility in the following hydrolysis process.
Lignocellulose's mechanical disruption is a part of physical pretreatment and it is an environmentally friendly process. Biomass's surface area is increased by this process and cellulose's crystallinity is decreased by this process, but no expensive mass loss is caused by it. Other physical methods like microwaves, electron rays, and gamma rays are used in irradiation for beaking the structure of the lignocelluloses. There have been appliances of microwave irradiation in various fields like food drying chemical synthesis and extraction.
Microorganisms generally have a restricted efficiency in forming biofuels. They also possess poor biomass/cellulose digestibility. There has been a common appliance of hydrolysis in the industrial scales for enhancing the efficiency of microorganisms in the production of biofuels before fermentation, and it depends on polysaccharides’ decomposition to the monosaccharides.
The fermentation processes to produce biofuels can be retarded by the process-induced and naturally occurring compounds, and they cause complications in the process of fermentation. Lignosulfonates, metal ions, furfural, acetic acid, and phenols are the inhibitors that can be removed biologically or chemically from the hydrolysates before the fermentation processes.
Over-liming and Boiling
Different studies have used over-liming and boiling extensively for lessening the concentration of the inhibitors. The concentration of volatile components was lessened by using boiling, for instance, furfural. Some insoluble inorganic salts capable of adsorbing inhibitors were created by the usage of over-liming.
If the concept of flocculation and adsorption phenomena is employed, the inhibitors can be alternatively removed. In this matter, the suitable choices can be the commercial adsorbents, like activated carbons, fillers, for instance, lime or calcium carbonate, or ion exchange resins. These inhibitors can be effectively removed by the adsorption/flocculation processes and they can also help in the enhancement of the performance of this process, on which the research is ongoing.
Annually, 7.5 billion gallons of ethanol was produced in the US by 136 ethanol plants in January 2008, and construction of 62 more plants is underway for increasing that amount by 13.3 billion gallons/year. Ethanol is currently dominating the biofuel market on a large scale (90% of the biofuel production of the world).
Although, sucrose (from sugarcane) or starch (from maize grains) is the source from where most of the ethanol is currently being produced today, for instance, first-generation biofuel. On other hand, lignocellulosic biomass displays more abundant feedstock for the production of ethanol, for instance, the second-generation biofuel. Although, making lignocellulosic ethanol is as costly as double the cost of production of corn-derived ethanol. Over $1 billion has been spent by the US department of energy (DOE) in this context, toward a realization of a goal that was made in 2012 of the production of lignocellulosic ethanol at $1.33 per gallon of a competitive cost.
Criticism on use of ethanol
In past, there have been critics regarding ethanol's production from biomass as Carbon dioxide is produced in large amounts as this fermentation process's by-product. Although, according to one study, if one considers the softwood (unspecified species and fermentation conditions) biomass as raw material, and FT-diesel were produced from lignin and ethanol was made from hemicelluloses and cellulose, then this integrated process can result in mass conversion efficiency of 54%, carbon conservation efficiency of 67%, but energy conversion efficiency of 88% is the most interesting.
Despite being known as a promising biofuel, ethanol has various drawbacks. Ethanol isn't an ideal biofuel due to its high water solubility, and some metallic components in the tanks can face corrosion while the plastics and rubbers utilized in the car engines will deteriorate, all due to ethanol's high concentration blends. In comparison with diesel fuel that's utilized in the automobile industry, ethanol has a much lower energy density as ethanol's heat value is 27 MJ/kg, whereas the FT-diesel's heat value is 42.7 MJ/kg. The incentives to attain a better alternative biofuel are consequently high.
In the future, one of the major energy carriers is considered to be hydrogen and that's because of its high energy content and that it can overcome global warming and air pollution. In order to produce hydrogen; the second generation biofuel, the suitable raw materials are the renewable carbohydrates (cellulose), because as compared to the hydrogen carriers like ethanol, methanol, biodiesel, and hydrogens, they are less expensive.
Use of glucose
A broadly used substrate to produce hydrogen is Glucose. $ 3.6 per GJ and $ 60/dry ton is the cost of production of the hydrogen from renewable biomass today and it is appealing. Transportation is hydrogen's most important energy application, particularly for light-duty vehicles. Although, there are some obstacles in the large-scale implementation of the hydrogen economy: safety concerns, fuel cell lifetime and cost, hydrogen distribution infrastructure, high-density hydrogen storage, and sustainable hydrogen production. Therefore, we should develop new strategies and technologies for making the production of hydrogen more industrially feasible and economically attractive.
Cellulose’s Biological conversion to hydrogen
Although, cellulose’s biological conversion to hydrogen takes place at much lower temperatures, implying that this process’s energy input is way lower as compared to the energy input of the chemical catalytic processes. In comparison, hydrogen production’s practical and theoretical yields through this method are low. Fermentation in principle can be used to produce up to 12 mol of hydrogen per mole of glucose and water.
Formation of furan-based biofuel
Cellulose's chemical conversion was done recently to introduce a new second-generation biofuel. Here, the initial production of the hydroxymethylfurfural (HMF) is from cellulose, and the conversion of HMF to 2,5-dimethylfuran (DMF) subsequently takes place. In comparison with ethanol and gasoline, the energy content of DMF is 31.5 MJ/l, 40% greater than the energy content of ethanol which is 23 MJ/l l, and similar to the energy content of gasoline which is 35 MJ/l. DMF is not miscible with water and is an interesting liquid biofuel. As compared to ethanol, DMF is less volatile as it has a bp of 92-94 C and ethanol has a bp of 78 C.
HMF from fructose
There was a proposal of a Two-solvent catalyst system of butanol/water at 180 celsius for the production of HMF from fructose. Here, the conversion of fructose/glucose occurs in the aqueous phase (with 83% HMF yield), and HCl functions as a catalyst there for the conversion of fructose to HMF. Moreover, NaCl is added to the system and HMF's transportation to the organic phase (butanol) from the aqueous phase is enhanced, preventing the HMF from any more degradation in the aqueous phase. Later, various distillation/separation units are used to purify HMF and butanol is recycled to the production reactor of HMF.
Cellulose's hydrothermal conversion to HMF (275-300 °C for less than 30 minutes) in homogenous systems in sulphuric acid’s presence alternatively led to a 20% Yield of HMF. Another research showed that in phosphoric acid’s presence, HMF’s yield was 65%, and it was conducted for less than 5 minutes at 228 °C.
Heating of cellulose
Cellulose can be alternatively converted into 5-chloromethyl furfural (CMF) by heating cellulose in LiCl’s presence in the concentrated HCl. The products must be subsequently extracted with 1,2-dichloroethane. In one study, 71% CMF was yielded by this process which was then converted to 5-ethoxy methyl furfural and DMF. 84% isolated DMF was yielded by this process but LiCl’s presence is one of this system's drawbacks. Stirring was done in the ethanol solution to alternatively convert the CMF to ethoxy methyl furfural (EMF).
235 °C of boiling point is possessed by EMF, and its energy density is the same as gasoline's energy density and 40% more than ethanol's energy density. We can also use EMF as a biofuel but this process faces a lot of challenges from the technological point of view and is very complicated.
DMF’s production cost is affected but the most significant parameters are the aforementioned process that are total purchased equipment cost, catalyst cost, by-product prices, production yield, and feedstock cost. Using the current technologies for the productions of CMF and DMF is not feasible. Despite the production of these chemicals at the laboratory scales, their commercialization is under serious complications because their processes of production demand numerous costly solvents that can’t be recycled easily. The final product's purification and separation from the solvents are also expensive.
Cellulose is one of the most important contents of green plants and is responsible for the formation of biofuel. The steps that go into the formation of biofuel are highly important and are carried out under great supervision to ensure the authenticity of these processes. However, biofuel is highly important to run the industries and daily uses of life.