Generation, Biofuels – Second Generation
Biofuels – Use
Biomass has the potential to supply a considerable portion of the energy needs of the world, but the conversion of biomass to energy is carried out in an unsustainable manner that there are many negative environmental consequences. If biomass is to supply a greater proportion of the energy needs in the future, the challenge will be to produce biomass and to convert and use it without harming the natural environment. Technologies and processes exist which, if used correctly, make biomass-based fuels less harmful to the environment than fossil fuels. Applying these technologies and processes on a site-specific basis in order to minimize negative environmental impacts is a prerequisite for sustainable use of biomass energy in the future.
Biodiesel and bioethanol are widely used in automobiles and freight vehicles. For example, in Germany, most diesel fuel on sale at gas stations contains a few percent biodiesel, and many gas stations also sell 100% biodiesel. Some supermarket chains in the UK have switched to running their freight fleets on 50% biodiesel, and often include biofuels in the vehicle fuels they sell to consumers, and an increasing number of service stations are selling biodiesel blends (typically with 5% biodiesel).
In Europe, research is being undertaken into the use of biodiesel as domestic heating oil. A blend of 20% biodiesel with 80% kerosene (B20) has been tested successfully to power modern high efficiency condensing oil boilers. Boilers needed a preheat burner to prevent nozzle blockages and maintain clean combustion. Blends with a higher proportion of biodiesel were found to be less satisfactory, owing to the greater viscosity of biodiesel than conventional fuels when stored in fuel tanks outside the building at typical (non-arctic) winter temperatures.
See also: Biodiesel, Biofuels, Ethanol, Methanol.
Biogas
Biogas, which is also known as biomethane, landfill gas, swamp gas, and digester gas, is a collection of gases (largely methane and carbon dioxide) produced by the anaerobic degradation of biomass (non-fossil organic matter) by various bacteria (Table B-11).
Thus, biogas is a combustible gas derived from decomposing biological waste under anaerobic conditions. Biogas typically refers to a biofuel gas produced by anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste, or any other biodegradable feedstock, under anaerobic conditions. Depending on where it is produced, biogas is also called swamp gas, marsh gas, landfill gas, and digester gas (Table B-9).
Table B-11 The composition of gas from various carbonaceous fuels.
| Composition: | Digester gas | Landfill gas | Sewage gas | Agri-waste |
|---|---|---|---|---|
| Carbon Dioxide (CO2) | 25-45% | 35-50% | 30-40% | 25-45% |
| Methane (CH4) | 45-65% | 40-45% | 55-65% | 55-75% |
| Nitrogen (N2) | 2-5% | 0-20% | 2-10% | 0-10% |
The primary component of biogas is methane gas, which comprises 50-90% by volume of biogas. Usually, biogas is 50% to 80% methane and 20% to 50% carbon dioxide, with the remainder trace gases such as hydrogen, carbon monoxide, and nitrogen. Methane is also the primary component of natural gas, but natural gas is normally recovered with more than 70% methane, along with other hydrocarbons (such as butane and propane) and traces of carbon dioxide and other chemicals. Natural gas is processed so that it is almost entirely, 98%, methane. Biogas is produced in a variety of low-oxygen natural environments with degradable organic matter, including swamps, marshes, landfills, agricultural and other waste (sewage sludge, manure, waste lagoons), aquatic sediments, wet soils, buried organic matter, as well as via enteric fermentation in some animal digestive tracts, notably in cattle.
Biogas normally consists of 50 to 60% methane and has a variable composition and energy content (Btu/ft3) depending upon the source (Table B-12).
Table B-12 Energy content of various gases.
| Coal seam gas (coalbed methane): 370-955 Btu/ft3 |
| Digester gas: 275-700 Btu/ft3 |
| Landfill gas: 275-700 Btu/ft3 |
| Manufactured gas (coal gas): 160-955 Btu/ft3) |
| Wood chip gas: 160-320 Btu/ft3 |
Biogas is produced by means of a process known as anaerobic digestion – a process in which organic matter is broken down by microbiological activity in the absence of air. Biogas is generated from concentrations of sewage or manure. These are usually in the form of slurry comprised mostly of water (almost 95%). The slurry is fed into a digester, this input can be continuous (usually the case with sewage) or in batches. The digestion continues from approximately 10 days up to weeks. The temperature in the digester should be kept at 35°C (95°F), and although the digestion itself produces heat, in colder climates, heat may be necessary.
There are two common technologies for the production of biogas. The first involves the fermentation of human and/or animal waste in specially designed digesters:
The second is a more recently developed technology for capturing methane from municipal waste landfill sites. The scale of simple biogas plants can vary from a small household system to large commercial plants. The digestion of animal and human waste yields several benefits: (i) the production of methane for use as a fuel and (ii) the waste is reduced to slurry which has a high nutrient content which makes an ideal fertilizer, and in some cases, (iii) the fertilizer is the main product from the digester and the biogas is merely a by-product. During the digestion process, bacteria in the manure are killed, which is a great benefit to environmental health.
Two popular simple designs of digester have been developed: (i) the floating cover digester and (ii) fixed dome digester. The digestion process is the same in both digesters, but the gas collection method is different in each. In the floating cover digester, the water-sealed cover of the digester is capable of rising as gas is produced and acting as a storage chamber, whereas the fixed dome digester has a lower gas storage capacity and requires good sealing if gas leakage is to be prevented.
The waste fed into the digester undergoes digestion in the digestion chamber and the temperature of the process is quite critical – methane-producing bacteria operate
