Energy makes all life possible. However, after the industrial revolution, carbon dioxide () emissions, as a result of energy production, have risen dramatically. Since 1990, temperatures globally have risen by 0. 2 ? C and the concentrations in the atmosphere have increased from 354 parts per million (ppm) to 380 ppm and still increasing. If no action is taken, average temperatures globally could increase by 5. 8 ? C by 2100 and sea levels from 0. 09 to 0. 88 meters (Institution of Mechanical Engineers, 2009).
According to a report produced by World Energy Outlook (WEO) in 2009 “to avoid the most severe weather and sea-level rise and limit the temperature increase to about 2i??C, the green house-gas concentration needs to be stabilised at around 450ppm equivalent” (International Energy Agency, 2009). Similarly, a recent report by the Institution of Mechanical Engineers claimed that if no action is taken, sea levels will increase by 7 meters by 2250, flooding much of London, East Anglia and other coastal areas.
In the same report it is stated that “Our climate is changing so unless we adapt, we are likely to face a difficult future” (Institution of Mechanical Engineers, 2009). Moreover, the International Energy Agency, estimates that energy demand across the world will increase by 50% to 60%, between 2004 and 2030 (International Energy Agency, 2009). Because of these facts, concerns globally have increased, and people seek alternative methods of producing energy, more sustainable and less harmful for the environment and for all leaving organisms.
One such renewable energy resource in Biomass. Biomass, although its combustion emits like fossil fuels, its considered a renewable form energy. The reason for this is that the amount of that is released to the environment during the combustion was previously absorbed from the air, while the biomass plant was growing, over a short period of time. Thus, the biomass combustion system is considered to be Carbon Neutral. Furthermore, there is a major difference between biomass and fossil fuels.
In contrast, burning fossil fuels releases which had been locked up for millions of years in the ground, and will require millions of years to return back there. Replacing fossil fuel with a solid biomass fuel will typically reduce net emissions by over 90% (nearly carbon neutral) (Liu, 2010). However, it should be noted that, some emissions do occur, during its cultivation and combustion. 2. Defining Biomass According to Biomass Energy Centre (BEC), biomass can be defined as “the biological material derived from living, or recently living organisms.
In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material [… ] (It) is composed of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, often nitrogen and also small quantities of other atoms, including alkali, alkaline earth and heavy metals. These metals are often found in functional molecules such as the porphyrins which include chlorophyll which contains magnesium” (BEC (c) Crown Copyright, 2008).
However, biomass is not a new source of energy, on the contrary, is considered as one of the oldest sources. Burning wood, for example, to cook, produces energy from biomass. Humans have been using this source of energy for more than 400,000 years, and in some less economic developed countries, still do. As shown in Figure 1, biomass is one of the world’s most used alternative energy resource. Yet, a great part of its usage is unsustainable, contributing this way to the global warming (Thomas, 2008).
Biomass for use as fuels can be either gathered or grown. The main biomass fuel sources include come from (i) forests and processing of timber (ii) biodegradable waste (including municipal and agricultural waste) (iii) crop residues (such as straw) and (iv) energy crops, usually grown exclusively for generating energy. Biomass can be burned in suitably adapted traditional coal-fired power stations (also known as “co-firing”) or in specially designed biomass facilities for the production of electricity, heat, or combined heat and power (HELM, 2006).
In the United Kingdom, the use of biomass and energy crops is immature. In 2006, approximately 4,500 hectares of land was used for energy crop cultivation. However, this is likely to increase significantly in 2007. To date, the most practical experience had been gained with Short Rotation Coppice (SRC) Willow or, to a lesser extent, Poplar. Miscanthus (Elephant Grass) is also being cultivated as an energy crop and consideration is being given to the potential for Short Rotation Forestry (SRF) using native species (HELM, 2006).
Production of Biomass by Pyrolysis Pyrolysis, like biomass, dates back to ancient Egyptian times. It refers to the thermal decomposition of organic matter in the absence (or little presence) of oxygen for complete combustion. In the 1980’s, researchers concluded that the pyrolysis liquid yield could be increased by using fast pyrolysis, where a biomass feedstock is heated at a rapid rate and the vapors produced also condensed rapidly (Demirbas, 2009). Pyrolysis is the main process for converting biomass to useful fuel.
Biomass is heated or combusted with little or no oxygen at all in order to produce a gas mixture, rich in hydrocarbon, an oil liquid and a solid residue, rich in carbon. It usually occurs at a temperature range between 675-975 K and aims at producing a liquid fuel known as bio-oil or pyrolysis oil (Liu, 2010). These oils can be used as they are, or refined for higher quality. It can be carried out in the presence of little oxygen (gasification), water (steam gasification) and hydrogen (hydrogenation). Hydrogen is based on the following reactions (Demirbas, 2009):
The reactions taking place in pyrolysis are complex and thus, it results in non-equilibrium products and to unpredictable properties. In order to produce bio-oil, short reactor times and rapid cooling or quenching from the pyrolysis temperatures needs to take place. Thus, it is not considered as the product of thermodynamic equilibrium. From this process, a condensate is produced which is not at thermodynamic equilibrium as well, at storage temperatures. The chemical composition tends to thermodynamic equilibrium during its storage (Zhang, Chang, Wang, & Xu, 2006).
In general, properties depend on the (i) process temperature, (ii) the time of heating, (iii) the ambient conditions, (iv) the amount of oxygen (if any) present, (v) the nature of the feedstock and (vi) amount of water and other gases. Bio-oil is composed of a complex mixture of oxygenated hydrocarbons and its heating value is half of that of conventional oil. In many static applications like boilers or furnaces, it can substitute fuel oils or diesel for generating energy. Other chemicals can be extracted, including food flavorings, agrochemicals.
It is also possible, but not economic, to produce transportation fuels out of it (Bridgwater & Peacoke, 2000). Furthermore, bio-oils can be formed from a process known as Fast Pyrolysis. The term “fast pyrolysis” refers to a high temperature process in which biomass is heated rapidly without any oxygen. After this process, biomass is decomposed to generate mainly vapor, aerosols and some charcoal. After cooling and condensation bio-oil is formed. Pyrolysis is considered the simplest method for processing a fuel for the purpose of producing a better one.
It can be carried out, in the presence of low amounts of oxygen (gasification), water (steam gasification) and hydrogen (hydrogenation) (Demirbas, 2009). Figure 3 shows the typical properties of wood pyrolysis bio-oil and of heavy fuel oil: 4. Production of Biomass by Fermentation The term fermeration refers to the anaerobic biological process in which sugars (e. g. ) are converted to alcohol by the action of micro-organisms, usually yeast. Other raw material include sugar cane (Brazil), corn (USA), wheat (France) and other.
From this process, a liquid bio-fuel is produced known as ethanol. Bio-ethanol can be produced from biomass feedstock by the hydrolysis and sugar fermentation process (Liu, 2010). The following figure (Figure 4) shows the production process of bio-ethanol: Ethanol (also known as grain alcohol or ethyl alcohol) is a colorless liquid with a boiling point of 78i??C and its chemical formula is. In is pure form it can be used as a supplement to gasoline or as an alternative transportation fuel which will benefit the diminishing petroleum reserves.
This would benefit the environment excessively since a 10% ethanol blend (E10) would reduce the carbon monoxide emission by 25%-30%, the emissions by 6%-10% and nitrogen oxides by 20%. Additionally, it can substitute the Methyl Tertiary Butyl Ether (MTBE), a gasoline additive which is being eliminated due to environmental concerns (Datar, Shenkman, Cateni, Huhnke, & Lewis, 2004). As a gasoline additive it is used to increase octane and improve vehicle emissions.
Its heating value is approximately 27 MJ/kg while in normal petrol is approximately 40 MJ/kg (Liu, 2010). In order to produce sugars from biomass, the plant cell structure must be revealed. For this reason, biomass is pre-treated with acids and enzymes. Biomass contains a mixture of carbohydrate polymers from the plant cell walls known as cellulose, hemi-cellulose and lignin. The cellulose and semi-cellulose portions are hydrolyzed by enzymes or dilute acids into sugars that are then fermented into ethanol.
There are two reactions that taking place when biomass is converted into bio-ethanol (Liu, 2010): * Hydrolysis is the chemical reaction that converts the complex carbohydrates in raw feedstock into sugars. During this process, a molecule is divided into two parts by reacting with a molecule of water (H2O). One of the parts gets an OH- from the molecules of water, and the other gets an H+. In order to catalyze this reaction, acids and enzymes are used.
Enzymes are proteins, composed from hundred if amino-acids, which are produced from living organisms. Fermentation is a series of chemical reactions that convert sugars to ethanol. It is caused by yeast or bacteria, which are fed on the sugars. Ethanol and are produced while sugar is consumed. The chemical formulas of ethanol formation from and are (Datar, Shenkman, Cateni, Huhnke, & Lewis, 2004): From the previous two equations: There is an important issue with the ethanol production. This is the fact that it only uses a small portion of each plant, leaving a lot of biomass unused and wasted.
For example, in ethanol production from corn, the active agent is kernel and the rest (leaves, stalks and cobs) are left for waste. This raises the issue of energy balance of different bio-fuels. Nevertheless, the production of biodiesel requires vegetable oils (new or used) and animal fats chemically reacted with an alcohol, usually methanol, which creates little or no waste (Geyer, Chong, & Hxue, 2007). 5. Current Status and Concluding Remarks The main objective in the bio-energy research is the production of liquid bio-fuels in order to substitute crude oil products.
This report aimed at presenting and discussing the production of such liquid fuels from biomass by pyrolysis and fermentation. Its process is used for the production of different liquids; these are bio-oil and ethanol respectively. In general, the pyrolysis technology, compared with those of combustion and gasification, is in an immature state of development. This has as result, the development costs to be high and not well established. On the other hand, this also means that there is a considerable scope for cost reduction.
Fast pyrolysis technology, has been successfully demonstrated at small scale. However, on large pilot plants, demonstration projects which are in operation or on an advance level, face economic and other, non-technical barriers when trying to enter the market (Liu, 2010). The following figure (Figure 5) summarizes the applications of pyrolysis liquid products: The production of ethanol, by using the fermentation process, is commercially available and practiced around the world.
Global ethanol production has increased from ca. 7 billion litres in 2005, to 65. 4 billion litres in 2008 (Liu, 2010). In UK, the first commercial bioethanol plant started operating in Nov. 2007 in Norfolk. It is producing 70 million litres of bioethanol from 700 thousand tonnes of sugar beet per year. The 92% of the world’s total bioethanol production comes from the United Stated of America (52%), Brazil (37%) and China (3%). France and Canada follow the list of the top five ethanol producers. A total of 1% of the global liquid fuel demand is provided by bioethanol and biodiesel (Liu, 2010).
However, like pyrolysis, fermentation too is subjected to economic and other non-technical barriers. Ethanol production using this process is associated with high costs, low efficiencies, low yield, high wastes and uncertain economics. On the other hand, the cost of ethanol production from biomass has dropped after the increase in the oil prices. The increase in the yields of energy along with the increase in efficiency in ethanol plants and the efficient use of fertilizers has led to better energy ratios (Liu, 2010).