This article throws light upon the top four biological sources of energy. The sources are: 1. Biomass 2. Biogas 3. Bioethanol 4. Bio-Hydrogen.
Biological Source # 1. Biomass:
Biomass is organic material of biological origin that has ultimately been derived from the fixation of carbon dioxide and the trapping of solar energy during photosynthesis. If it is to be used as a fuel, the biomass can either be burned, releasing the chemical energy directly, or it can be converted to a liquid or gaseous fuel that is higher in energy per kilogram than the original biomass.
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Although unconverted biomass can be used as a fuel and, indeed, plays an important role in countries such as India—where the collection and burning of firewood is important to the rural economy—the large scale use of such fuel suffers from a number of disadvantages over fossil fuels such as coal and oil.
There are several potential disadvantages of using biomass as a fuel; list any that you can think of.
Firstly, biomass usually has a lower thermal content than fossil fuels.
Secondly, it often has a high moisture content, thus inhibiting easy combustion, resulting in a large energy loss on burning—mainly as latent heat of steam. The high moisture content also causes the material to be biodegradable so that it cannot be readily stored.
Thirdly, biomass tends to have a low density, in particular a low bulk density, which increases the size of equipment necessary for handling, storage and burning. Finally, the material is rarely in a homogeneous physical form and is not free-flowing, making automatic feeding of combustion plants difficult.
An intermediate stage to chemically or biologically converting the energy of biomass to a higher energy fuel is to extract the fuel from the biomass, usually as an oil or other hydrocarbon. There are many plants which accumulate oil—usually in their seeds—and these are often already exploited commercially as sources of edible oil. Such oils include sunflower, rapeseed, olive, peanut, linseed, soya bean and safflower.
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All these products are high in energy value and all are theoretically capable of being burned in boilers or used as diesel engine fuel. The main problem with the latter use is that the oils are often rather viscous and so do not inject easily into the combustion chamber of the engine. This problem can be overcome by esterifying the oils, although this will add a processing cost.
In addition to the plants used at present for edible oils, a number of species of plant have shown promise as producers of hydrocarbons for fuel use. These include the Euphorbias (spurges), the milkweeds (Asclepias sps.) and a tropical tree Copaifera.
An important feature of the first two examples is that they need little water to grow and, hence, can be grown in relatively dry environments where they will not compete for land used for food production.
The Euphorbias are related to the plants used to commercially produce rubber and produce a latex which is an emulsion containing 30% hydrocarbon in water. After the water has been removed, the resulting product is a liquid containing hydrocarbons of a lower molecular weight than present in petrol.
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The common Milkweed (Ascepsias speciosa) also contains around 30% hydrocarbons. Perhaps the best promise in this field is offered by the plant Copaifera multijuga which is found in Brazil.
This plant is a legume and, therefore, is able to fix nitrogen in its root nodules, which makes cultivation more economical as less fertilizer is needed. It grows to a tree of some 30 meters in height and, when tapped like a rubber plant or maple, produces a large volume of a liquid which has properties very similar to diesel oil.
The tapping of the plant can take place twice a year and so the potential for commercial exploitation is good. Another possible source of large scale production of hydrocarbons are certain freshwater and marine algae which are known to accumulate similar substances.
Although there is clearly potential for the production of biofuels by direct extraction from plants, at the present time (apart from small scale trials) these are only research projects or theoretical possibilities. There are, however, biofuels currently being produced by conversion of biomass to gaseous or liquid fuels.
Biological Source # 2. Biogas:
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Biogas is a mixture of gases—primarily methane and carbon dioxide which are produced when organic material is degraded under anerobic conditions. It is important that you remember the significance of anerobic conditions. Briefly distinguish the nature of the products produced by microorganisms growing aerobically from those of microorganisms growing anerobically.
Under strictly aerobic conditions the principle products are cellular material, carbon dioxide and water. The cellular material will itself be recycled when the cell dies—generating more carbon dioxide and water. Under anerobic conditions the end-products are far more varied. They include carbon dioxide and water but also many reduced organic compounds such as acids, alcohols and so on.
Polymeric materials are hydrolysed by microorganisms—such as the cellulolytic and proteolytic bacteria—to smaller molecules which then act as substrates for a second group of microbes, the acetogens.
These fermentative bacteria further degrade the higher fatty acids to low molecular weight fatty acids such as acetate and format, alcohols such as methanol, methylamines, and gases such as hydrogen and carbon dioxide.
Finally, these act as substrates for the methanogenic bacteria, which convert them to methane and carbon dioxide. The methanogenic bacteria are a group of strictly anerobic bacteria which requires highly reducing conditions (redox potential (Eh) < – 400 mV).
The types of energy yielding reactions which methanogenic bacteria as a group are capable of are shown in Table 29.1:
These microbes, unlike the fermentative acetogenic bacteria, are capable of anerobic respiration using carbon dioxide as the terminal electron acceptor and, in the process, produce methane. The resulting gas generated by the system—biogas—contains roughly 60% methane, 40% carbon dioxide and traces of hydrogen.
Biogas burns readily, although it does not have as high energy value as natural gas which is almost pure methane. It is suitable as a fuel for boilers and kilns and for internal combustion engines, although impurities in the gas can cause corrosion problems in the latter case.
For anyone to seriously consider harvesting biogas from a landfill site the major criterion—after safety—is, obviously, economic viability. List and discuss some of the factors which have to be considered before embarking on commercial exploitation. Read our response carefully.
The gas is initially collected by sinking a number of ‘gas wells’ into the top of the landfill. These are constructed of perforated or slotted piping which is then interconnected via a network of pipes to a gas pump or compressor. If an existing landfill is being harvested for gas then there will be no internal structure within the landfill.
With new sites which have been planned from the start for gas production, the gas wells are more complex and may have internal walls made of clay in order to trap the gas more effectively. The shape of these wells is the subject of research, but a number of different types have been tried including vertical wells, short horizontal, short vertical and bell-shaped.
From the compressor, the gas is passed to a chiller, which cools the biogas in order to remove water from it, which would render it less combustible and cause corrosion problems. There is also a flare stack which is used to burn-off excess gas when production exceeds demand, or when there is a problem further down the line. Finally, the gas is filtered and its composition is determined by monitoring prior to end use.
The process of gas abstraction at a landfill site is shown in Fig. 29.6:
These end uses are:
a. Direct combustion in boilers, kilns or furnaces;
b. Generation of electricity;
c. Upgrading and export or compression or liquefaction.
Of these alternatives, the first is the most economically feasible. The most effective use of the landfill gas is burning at a site close to the landfill itself, since building gas pipelines is expensive.
One of the most successful landfill gas schemes to date in the UK is that of the London Brick Company who are ideally positioned to exploit this technology since they have disused clay pits which can be used for landfill and they have a major use for the biogas in firing brick kilns produced near to the landfill.
Other examples of direct combustion of landfill gas include the heating of greenhouses, the firing of cement kilns and the production of steam for various industrial uses. If a local end-user of the gas is not available, a second alternative is to generate electricity from the biogas.
This can then be exported relatively cheaply by building electricity lines to connect in with the National Grid. At small scale, plants generating up to 1-2 MW, this usually involves the use of converted internal combustion gas engines, although larger schemes using gas turbines also exist.
The main problem with this approach is that landfill gas is not purely a methane/carbon dioxide mixture, but contains traces of many other gases some of which are corrosive to metal parts of engines.
This problem can be partially overcome by gas cleaning and by modifications to the generating engines and their lubricating oils. In the UK, in 1987, six out of the 20 landfill gas schemes in operation generated electricity, while, in the USA, the figure was 22 out of 54, with most new schemes opting for this approach.
The third alternative for landfill gas use is purification followed by export, either in a pipeline to a gas utility network or by bottling as compressed or liquefied gas. The process of gas clean-up to provide a suitably pure methane gas is complex and expensive and so this option is the least favoured of the three.
There are, however, a number of purification plants operating, mainly in the USA, and one scheme in Santiago, Chile, where landfill gas is used to supplement the city’s existing gas distribution system.
Biological Source # 3. Bioethanol:
The technology for the biological production of ethanol is as old as biotechnology itself since the production of alcoholic beverages by Man has been recorded for thousands of years. However, although the production of fuel ethanol has undoubtedly drawn upon this older technology, the processes involved are more complex. They are, however, less constrained since the product does not have to be potable.
The basic process involves conversion of the feedstock to a fermentable form, followed by fermentation and then distillation. With some feedstock’s, such as sugarcane, the initial stage can be omitted since the extracted substrate is readily fermentable. The unit operations involved in the production of ethanol from feedstock’s such as maize and sugar cane are shown in Fig. 29.7.
The standard technique currently used to process ethanol from the liquor leaving the fermenter (termed ‘beer’ in the industry) to the purity required for fuel ethanol is distillation. This is usually performed on a continuous basis, although batch distillation, as is performed in the whisky industry, is possible.
The process of distillation relies on the differing boiling points of ethanol and water (87°C and 100°C, respectively, in the pure state). When a mixture of water and ethanol containing less than 95% ethanol is heated to boiling, the vapour produced has a higher concentration of ethanol than the liquid phase and, if cooled, will condense to produce a distillate which is enriched in ethanol.
If a single volume of an ethanol-water mixture was heated the initial concentration of ethanol in the vapour would be relatively high, but the liquid phase would now be depleted in ethanol and so the vapour would gradually become richer in water and poorer in ethanol.
This means that a single batch distillation can only produce a limited quantity of enriched ethanol-water mixture. If, however, the initial distillate is then re-distilled, a further ethanol distillate could be obtained, although this would be a smaller volume than the first distillate.
For the future, a number of possibilities exist to make the bioethanol process more energetically feasible:
1. Recombinant DNA technology could be used to increase both the yield and ethanol tolerance of the yeast;
2. Procedures can be adopted to remove the ethanol from the fermenter before inhibition occurs. This could involve separating the yeast from the effluent by centrifugation and recycling the biomass to the fermenter.
Alternatively, ethanol could be removed continuously from the fermenter by vacuum distillation in which a partial vacuum is maintained in the headspace above the fermentation broth so that the ethanol evaporates rapidly at the operating temperature of the fermenter;
3. The separation of ethanol from water can be made more energy-efficient by using alternatives to distillation. These include reverse osmosis, where the ethanol is separated from the water by filtration through extremely small pores under high pressure; selective adsorption using solid adsorbents and the use of supercritical adsorbent.
Biological Source # 4. Bio-Hydrogen:
Hydrogen is an important energy carrier in many biological systems, particularly in anerobic fermentation processes. Hydrogen can be produced by anerobic fermentative bacteria such as Clostridia as an ‘electron sink’, thereby providing a method of removing NADH from the system.
When a substrate is oxidised during fermentative metabolism there must be a subsequent reduction and this usually generates NADH from NAD+.
The accumulation of NADH would upset the equilibrium and cause these oxidation reactions to stop and so the surplus NADH is channelled to hydrogen ions with the production of hydrogen, which is released from the cells:
NADH + H+ → NAD+ + H2
In anerobic environment, this hydrogen is an important source of energy. The methanogenic bacteria which can oxidise the hydrogen by using carbon dioxide as an electron acceptor to produce methane:
4H2 + CO2 → CH4 + 2H2O
If the hydrogen-producing bacteria are grown in appropriate conditions in the absence of hydrogen-utilizing bacteria then the hydrogen accumulates and can be collected. The feedstock for this fermentation could be cellulose, which can be hydrolysed to sugars and then fatty acids by certain anerobic bacteria providing a substrate for the hydrogen-generating Clostridia.
Some microscopic algae also produce hydrogen and there has been research into growing these organisms under sunlight in ‘photo bioreactors’, with the additional advantage of biomass production which could be used as animal feed. A more direct approach has also been suggested in which the photosynthetic apparatus of plants is linked biochemically to the hydrogen-producing system.
This possibility is shown in Fig. 29.8:
During photosynthesis the water molecule is split using light energy to yield hydrogen ions (protons), electrons and oxygen:
