Introduction
Biomass is a biological material obtained from a living organism or a recently living organism. The word biomass is derived from two Greek words; bio meaning life, and maza meaning mass (Demirbas, “Fuels from Biomass” 43). Biomass means non-fossil and bio-decomposable organic matter (originating from plants, animals and microorganisms). The products, byproducts, residues and waste from agricultural processing, all constitute biomass. Bio-waste simply means non-fossil and biodegradable waste that is composed of organic matter. Bio-wastes include food and kitchen wastes, garden and park wastes, and wastes from food processing plants. Bio-waste is therefore an element of biomass.
Non-fossil wastes that are derived from various industrial and municipal solid wastes such as manure, agricultural residues, municipal sewage sludge, municipal solid waste, and industrial bio-sludge, are referred to as waste biomass. Waste biomass can be converted to animal feeds, alcohol fuels or industrial chemicals.
Today, biomass includes gases and liquids recovered from biodegradable organic matter, and are considered an important source of carbon -- in chemical industrial processes -- that can be used for producing various chemical products. Biomass provides a cheap source of renewable energy where solar energy is trapped in chemical form by photosynthesis (A.D. Little). Photosynthesis (the carbon fixation reaction used for reduction of atmospheric carbon dioxide) is thus, a fundamental process for the creation of bio energy. New technology has shifted the application of biomass for the generation of electricity to the production of industrial chemicals. Biomass processing involves chemical, physical, and microbial or enzymatic treatments (Güllü).
Chemical composition of biomass
Biomass is carbon based. It’s a mixture of organic molecules that contain hydrogen, atoms of oxygen, nitrogen, and small quantities of other atoms. The carbon in biomass is absorbed from the carbon dioxide in the atmosphere by plants through the process of photosynthesis. The plant materials are then eaten by animals, and the plant biomass is converted into animal biomass. If the plant materials are not eaten by animals, they are burned or broken down by micro-organisms. If burned, the carbon is restored to the atmosphere as carbon dioxide gas. If broken down by micro-organisms, the carbon is restored to the atmosphere as either carbon dioxide or methane, depending on the processes and conditions involved (BEC).
Difference between biomass and fossil fuels
Fossil fuels (coal, oil, gas, petroleum) are derived from biological materials that absorbed carbon dioxide from the atmosphere millions of years ago. They have high energy density; however, they must be burned in order to give out the energy. When the fuel is burnt, carbon is oxidized into carbon dioxide, and hydrogen into water. The combustion of fossil fuels returns carbon that was sequestered millions of years ago into the atmosphere. This results into an increase in the atmospheric carbon (BEC).
Biomass, on the other hand, absorbs carbon from the atmosphere as it grows, and returns it when burned. When managed on a sustainable basis, the biomass is harvested as part of a continually replenished crop (BEC). The new growing plants absorb carbon dioxide from the atmosphere at the same rate as the carbon dioxide is released by burning of the previous harvest. The end result is a closed carbon cycle without any net increase in the levels of atmospheric carbon dioxide.
Biomass feedstock
Because the biosphere contains huge amounts of energy of varying forms, biomass feed-stocks are largely varied. They can be listed as follows (Demirbas, “Fuels from Biomass” 45; Agrowal):
- Forest products such as wood, trees, shrubs, logging residues, wood residues, bark, sawdust, etc.
- Bio-renewable wastes such as agricultural wastes, crop residues, urban wood wastes, urban organic wastes, mill wood wastes, etc.
- Energy crops such as short-rotation woody crops, grasses, herbaceous woody crops, sugar crops, starch crops, oilseed crops, forage crops, miscanthus, switch grass, etc.
- Aquatic plants such as algae, water hyacinth, water weed, reed and rushes, etc.
- Food crops such as grains and oil crops
- Sugar crops such as sugar cane, sorghum, sugar beets, molasses, etc,
- Landfills such as hazardous waste, liquid waste, inert waste, non-hazardous waste, etc.
- Organic wastes such as municipal solid waste, municipal sewage and sledges, industrial organic wastes, etc
Biomass as a sustainable feedstock
Biomass provides an attractive feedstock for many chemical processes because it is a renewable resource. It offers positive environmental properties such as reduction of GHG emissions, NOx and SOx (it replaces the use fossil fuel by providing bio-energy and bio-fuel). Biomass is a cheaper source of energy compared to the fossil fuels whose prices are ever increasing. However, the processing biomass requires stringent environmental standards since the process liberates dioxins, polycyclic aromatic hydrocarbons, furans, VOCs and heavy metals (in case of traditional gasification) (Bauen).
Figure 1 below illustrates the conversion processes of various feedstocks.
Bio-refineries
Bio-refineries are the integrated biomass conversion technologies. They generate bio-power, bio-fuels, and/or bio products as an integrate plant process just like petroleum refineries that produce multiple fuels and products. They are highly flexible and they can process variety of biomass feed which makes them economically viable (Holtzapple)
Figure 2 illustrates the integrated biorefinery concept.
The Conversion of Biomass to Chemical Products
Chemically the biomass mainly consists of cellulose, lignin, hemicelluloses, extractives, lipids, proteins, simple sugars, starches, water, hydrocarbons, ash, and other compounds (Demirbas, “Fuels from Biomass” 49). Cellulose and hemicelluloses (holocellulose) particularly have greater industrial value because most of today’s biomass processing techniques are based on these two sugar units. The techniques convert the sugar units into materials such as ethanol, acetic acid, hydrogen etc. The lignin fraction consists of non-sugar macromolecules and mostly remains unconsumed except for the heat produced. Actually the average biomass consists of 60% wood and 40% non-wood materials. Therefore, due to the materials that it contains, conversion of the biomass into bio fuels and bio chemicals is technically feasible (Demirbas, “Biomass resource facilities”).
Bio-ethanol processing crop and non crop source
Bio-ethanol can be generated form wide variety of carbohydrates (CH2O)n by the enzymatic hydrolysis of sucrose followed by fermentation sugar units. Fermentation of sucrose is performed by use of commercial yeast such as Saccharomyces cerevisiae. Initially, the invertase enzyme from yeast catalyzes the hydrolysis of sucrose to convert it into glucose and fructose. Zymase, another enzyme that is also present in the yeast, converts the glucose and the fructose into ethanol. The presence of gluco-amylase enzyme is critical as it does the conversion of starch into D-glucose. As a result, enzymatic hydrolysis, fermentation, distillation and dehydration give anhydrous bio-ethanol. Corn which has 60–70% starch content is widely used in the “starch-to-bio-ethanol industry.” However, considering the huge amounts of fields required to grow corn, scientists started to search alternative methods to produce ethanol such as Lignocellulose.
Nowadays, Lignocellulosic perennial crops are commonly used; however, they are blended with corn for cost efficiency. In case of non-crop sources such as agricultural waste, the waste has to be treated in anaerobic digester (methane digester system) which decomposes them into simple organics and gaseous biogas products. Further digestion results in 50-50 mix of CH4 and CO2. This digestion runs for 15-16 days and produces 73–79% methane content. Pyrolysis of agricultural residues (especially the use of flash pyrolysis processes) can be used for the production of bio-oil (Demirbas, “Biomass resource facilities”).
Conversion of Waste Biomass to Chemicals
There are various processes which are employed to convert waste biomass (manure, agricultural residues, municipal sewage sludge, municipal solid waste, and industrial bio-sludge) into the respective useful end products rather than being dumped away. The complexity of these processes depends on the kind of technologies being used. Conversions of waste biomass into renewable energy forms play a major role in reducing the dependence on crude oil and thus promote economic growth. The process of waste biomass conversion to useful products has a positive effect on the environment because the renewable biomass recourse is effectively utilized, minimizing pollution and cost effects incurred in disposing these wastes. These wastes are considered important and the process of producing usable products from them enhances the concept of renewal energy. Biomass processing is environmental friendly and therefore it is highly recommended as the future industry of production especially energy production.
Chemical Processes of Waste Biomass Conversion
The chemical process of converting waste biomass in the form of agricultural residues such as straw, bagasse, and Stover to animal feed starts by mixing the waste with lime. The treatment of this biomass with lime is very essential because it makes it more digestible and can thus be fed to ruminant animals. The lime renders the agricultural waste biomass more digestible by the rumen microorganisms and thus can be used as a replacement for grain feeds. The use of these biomass products as feeds promotes saving since the waste products are renewed. It is a process that is cost effective as well as environmental friendly. The overall performance of animals’ health is higher as compared to the use of grain feeds since lime is an additive that promotes born formation (Holtzapple 1).
Alternatively, the agricultural residues treated with lime can be converted into industrial chemicals such as ketones, acetic, propionic, and butyric acids, or alcohols such as propanol, pentanol, and butanol. These conversions follow certain procedural steps; the first one involves the feeding of the lime-mixed waste to an anaerobic fermentation chamber. Inside this fermenter, the materials which are rendered more digestible are broken down to volatile fatty acids and salts such as calcium butyrate, propionate, and acetate (Holtzapple 1). Limestone is particularly applied in the fermentor to neutralize the volatile fatty acids produced in order to prevent the PH from going too low, and thus the production of the calcium VFA salts. Once the volatile fatty acids have been obtained, the second step involves concentration and then either of three steps namely, thermal conversion, hydrogenation or acidification can be undertaken to obtain either chemicals or fuels.
The acidification process of the concentrated volatile fatty acid salts yields butyric, acetic and propionic acid as the products. The second option involves thermal conversion of the concentrated volatile fatty acid salts at 430 degrees Celsius to release ketones such as Acetone, Methyl ethyl ketone, and Diethyl ketone (Holtzapple, 1). Alternatively, the concentrated volatile fatty acid salts may be exposed through the hydrogenation route whereby they are converted to alcohols such as propanol, butanol and pentanol. According to Holtzapple, the hydrogenation process is carried out with the aid of 200-g/L Raney nickel catalyst (3). The chemicals obtained after these processes such as the acidifying agents, and calcium salts can be readily recycled and used back in the conversion process, thus reducing any chances of generating wastes.
Figure 3 shows the conversion process
How the chemical technology intersects with society and human concerns
These conversion processes help to conserve the environment because the conversion of agricultural residue to animal feeds replaces the use of grain feeds which are cultivated with fertilizers. Cultivation of these feeds contributes to pollution of water sources through infiltration of the dissolved fertilizer in underground water, herbicides, as well as soil erosion (Holtzapple 3). Replacing these feeds with the lime-treated, easily digestible agricultural residues saves high costs of production, besides its environmental-friendly nature. In addition, Fertilizers are rich in elements of nitrogen and calcium which makes the soils to have better absorbance. Pollution is reduced to a higher extent if the use of these feeds is employe (Singh and Steven 235).
Production of Electricity from Waste Biomass
Waste biomass contains enormous amounts of electrical energy, which if properly harnessed may be used to supplement or replace the use of crude oil energy and save huge costs. These forms of energy which can be derived from waste biomass comprise of “process heat, steam, motive power, and electricity, as well as liquid fuels”, (Crocker and Crofcheck 1). Biomass is the perfect substitute for fossil fuels given its qualities of being a large natural renewable carbon resource and its cheap availability. Rather than disposing off the lignocellulosic biomass, it can be used as feed stocks for producing alternative energy forms such as biodiesel. Adoption of the microbial electrochemical technologies that utilize microbes as catalysts for various electrochemical reactions that yield electrical power provides a prime opportunity for harnessing the energy in waste biomass for constructive uses (Logan and Rabaey 1). The microbial fuel cells are examples of these reactions capable of generating electrical power.
The microbial electrochemical technologies involve the process of generating electricity using exoelectrogenic microorganisms with waste biomass acting as the fuel and oxygen as an oxidizer for the aerobic respiration of the bacteria. These microbes have the capacity of transferring “electrons outside the cell to insoluble electron acceptors like iron and other metal oxides, or to electrodes in bio-electrochemical systems” (Logan and Rabaey 1). Within the microbial fuel cells, the exoelectrogenic bacteria transfer electrons to the anode and protons into the solution, thus amounting to a negative potential at the anode of approximately -0.2 volts. This anode potential has a slightly higher voltage than that of the substrate’s half-cell reaction. On the cathode side, oxygen is usually used as an oxidizer (He et al. 4).
Combustion of the biomass is another process used to produce electricity and heat. The whole process involves the burning of biomass in a boiler with limited supply of oxygen to produce high-pressure steam. The produced steam is then directed through a number of turbine blades. Due to its high pressure, it makes the turbine rotate. This is in turn is joined to electricity generator which is made to turn by the steam and therefore produces electricity. This is one of the best ways of utilizing biomass wastes since production of electricity through this method is environmental friendly as compared to use of fossil fuels which emit carbon dioxide gases that are not friendly to the environment (He et al. 4).
Production of gasoline and Jet fuels from Biomass
The agricultural wastes from plants are converted to levulinic acid and formic acid through the process developed in the Wisconsin university. In this process, cellulose which is a large component of biomass is broken down by the use acids to form simple sugars (He et al. 4). Micro-organisms are then involved in the process of further converting the simple sugars to liquid fuels. Due to the use of acids in breaking down cellulose, the final product is the formation of levulinic acid and the formic acid. These chemicals are then combined through a chemical reaction which involves the use of energy and form a product known as gamma-valerolactone. This industrial chemical is then converted into butane by a catalyst made of silica and alumina. The butane gas is then converted to liquid hydrocarbons which are mainly the energy fuels that can be used as jet fuels. The process is said to be environmental friendly since it is possible to capture the carbon dioxide gas produced during the production of these fuels. This will in turn be used in other processes hence economical (Bridgwater and Boocock 12).
Hydrothermal gasification
This process involves the breaking down of lignin resulting in the formation of phenols and aromatics. Under this process, glycosidic bonds in cellulose tend to hydrolyze fast. With the use of catalysts, the final product can be a gas or liquid depending on other factors such temperature. If the temperatures are above the critical point, hydrolysis products are formed since the glucose decomposition is slowed. The formation of hydrolysis products enhances the liquefaction of the gas product (Crocker 212).
It must be noted that the production of hydrogen in biomass reactions is and endothermic process. The biomass content reacts in the presence of water under the gasification process to produce hydrogen gas and carbon dioxide. The production these gases is enhanced if the feed stocks being used are rich in hydrogen content. Water also reacts with the biomass contents and releases the hydrogen part of the molecule. The hydrogen produced can be used in formation of bombs and gas fuels. It can also be used in lifting weights since it is lighter than air (Paul et al. 367).
Biomass derived-intermediate compounds have been found to produces syngas which is a mixture of carbon monoxide gas and hydrogen gas. The reaction that produces this syngas takes place under the uncatalyzed conditions of gasification (Singh, Jasvinder and Gu 68). The cellulose and lignin are converted through intermediary reactions such as hydrolysis, thermal decomposition, methanation, steam reforming and water gas shift. The significance of this process is that it takes a shorter duration to produce hydrogen. It is also economical since it is endothermic requiring little energy. It is the maximization of the waste products such as sludge, animal wastes, manure, agricultural residues, municipal sewage sludge, municipal solid waste, and industrial bio-sludge that helps in production of hydrogen which has a wide range of applications (Bullis Para. 3.).
Lignin is the major component compound of biomass and its usage is wide and important in waste management. For instance, it has been noted that lignin decomposes in a hydrothermal environment. In this process, low molecular weight compounds are formed which include syringols, catechols and guaiacols. These are then converted to formaldehyde and alkylphenols through the process of condensation. It is then at this stage that hydrogen production is enhanced by the use of catalysts such as nickel and sodium hydroxide compounds. The low molecular weight compounds formed are easily converted to hydrogen as a result of catalysis. These processes are economical and can produce gases that can be used in a range of applications (Singh et al. 68).
Biomass processing world scenario
In 2006, renewable energy accounted for 7% of the world’s energy supply. Biomass accounts for 49% of renewable energy worldwide. Wood residues, forest residues, and wood waste feed stocks were widely used as biomass fuel and bio energy till 2005 (with overall contribution of 64%). It is followed by MSW, LFG, agricultural residues, bio-solids, corn and soybean oil which are largely used for production of bio fuels and other correlated products (overall contribution about 18%) (Beteta).
Future production and uses
The production and use of many different types of biomass feed stocks will increase significantly in the future. In 2005, US DOE and USDA convened an expert panel to evaluate whether the land resources of the United States can produce sustainable supply of biomass that is sufficient to replace 30% of the nation’s current petroleum consumption. Panel concluded that by the mid-21st century, the amount of feed stocks sustainably produced for bio energy each year could be increased nearly three times. Similarly, the amount of agricultural feed stocks sustainably harvested while continuing to meet national food and export demands each year could be increased three times. The potential increases in all of these biomass feed stocks can be achieved with relatively modest changes in agricultural and forestry practices, and land use including technological advances that increase feed stock yields, adoption of certain sustainable crop cultivation practices (e.g., no-till), and land use changes that allow production of large-scale perennial crops (Sheehan).
Conclusion
The chemical processes involved in production of energy, and other useful products from biomass are quite a number. They are environmental friendly and cost effective as well as efficient. Waste biomass is usually in the form of manure, agricultural residues, municipal sewage sludge, municipal solid waste, and industrial bio-sludge which are converted to animal feeds, alcohol fuels or industrial chemicals. The processes employed to convert this waste into the respective useful end products require skills and some knowledge but are very economical. The complexity of these processes depends on the kind of technologies being used. Conversions of waste biomass into renewable energy forms play a major role in reducing the dependence on crude oil and thus promote economic growth. The process of waste biomass conversion to useful products has a positive effect on the environment because the renewable biomass recourse is effectively utilized minimizing pollution and cost effects incurred in disposing these wastes. It is therefore important to note that wastes should be managed in order to avoid pollution. The chemical processes of converting biomass to useful products are easy and cost effective and therefore they should be employed in order to avoid pollution of the environment.
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