is a mixture of gases resulting from the degradation process of organic matter in the absence of oxygen, that is, in an anaerobic process. The main component of this mixture is methane, a gas with high energy potential, which makes biogas a renewable source of energy.
Any biodegradable organic matter can be added to the anaerobic biodigesters for the production of biogas and alternative energy, as follows:
• Animal production, waste, tailings and carcasses: swine farming, dairy farming and cutting, poultry farming, fish farming among others;
• Agricultural residues: bark, foliage, straw, grains and crop residues, among others;
• Industrial waste: bagasse, discards, effluents, fats, vinasse (by-product of sugar or alcohol), starch, glycerine (petroleum by-product) restaurant waste, industrial effluents with high organic load, among others;
• Municipal organic waste arising from human activity, such as: sewage, domestic organic waste, waste from maintenance of parks and gardens, among others.
Methane molecule (CH4): the main component of biogas.
The available methane gas (CH4) in the biogas is considered a gaseous fuel that has a very high energy content, a high calorific value, similar to that of natural gas. Since methane is the main constituent of biogas, it has no smell, colour, or taste, but biogas presents an unpleasant odour due to some gases present in its composition. Biogas is composed of short and linear hydrocarbons.
The biogas artificially produced in anaerobic biodigester plants, is composed of:
• Methane (CH4): 45-65% of the volume of the total volume;
• Carbon dioxide (CO2): 35-45% of the total volume;
• Traces of hydrogen (H2), nitrogen (N2), oxygen (O2) and hydrogen sulphide (H2S), among others.
Anaerobic digestion represents a delicately balanced ecological system, where each microorganism has an essential function. Methane production occurs in different natural environments such as marshes, soil, sediments of rivers, lakes and seas, as well as in the digestive organs of ruminant animals. But the optimal living conditions for anaerobic bacteria to produce biogas are:
Oxygen (O2) air is lethal to the methane anaerobic bacteria. If there is oxygen in the environment, the anaerobic bacteria paralyze your metabolism and stop developing.
Anaerobic bacteria produce methane. In a biogas plant, the biodigester (bioreactor) must be hermetically sealed against the entry of air (oxygen), otherwise biogas production does not occur because the anaerobic bacteria die; the biogas produced will then be rich in CO2 and not on methane. Thus, the biodigester must ensure complete anaerobiosis of the environment required for the metabolism of anaerobic bacteria.
The temperature inside the biodigester is an important parameter for the production of biogas. Bacteria that produce methane are very sensitive to temperature changes. Microbial growth can occur in three temperature ranges: thermophilic (45-70°C), mesophilic (20-45°C) and psychrophilic (0-20°C), but most anaerobic digesters (fermenters) have been projected in the mesophilic range, where the optimum temperature for better methane formation, occurs between 30 and 40°C, being the ideal 37°C. Thus, another role of the biodigester is also to ensure certain temperature stability for methane bacteria.
Changes in the pH of the medium sensibly affect the bacteria involved in the anaerobic digestion process. The range of operation of the biodigesters is between pH 6.0 and 8.0, and methane-producing bacteria (methane bacteria) have optimum growth in the pH range of 6.6 to 7.4. PH values below 6.0 and above 8.3 should be avoided as they may completely inhibit methane bacteria.
The presence of macronutrients (carbon, nitrogen, potassium, phosphorus and sulphur) and some micronutrients (minerals, vitamins and amino acids) are fundamental to the development of microorganisms (methane bacteria). For a good fermentation to take place inside a biodigester, the balance between nutrients is indispensable.
The knowledge of the chemical composition and the type of biomass used is very important, for example, animal wastes are rich in nitrogen; plant crop residues are rich in carbon; the mineral salts are present in animal waste and vegetable waste.
Biogas production, besides being an energetic alternative, being a low-cost fuel because it originates from a by-product, fits perfectly among the provisions presented by the World Bank for the sustainable use of renewable natural resources, to combat pollution and waste management, together with better management of waste as a foundation for sustainable development (ZANETTE, 2009).
Biogas, after purification, can be used as fuel gas for the generation of electric and heat energy in Otto Cycle engines powered by biogas. These motor-generators are specially developed to operate the biogas explosion.
In order for the produced biogas to be used in Otto cycle biogas engines, it is necessary to pass the biogas through an industrial process.
This process consists of:
• Extract the sulfuric gas (H2S) and the humidity (H2O) from the biogas.
• Eliminate possible undesirable gases within the biogas, such as ammonia (NH3), oxygen (O2), hydrogen (H2) and nitrogen (N2).
• The gas resulting from the previous process steps will directly feed the exclusive biogas engines.
These combined heat and power (CHP) units are known by the acronym CHP (combined heat and power), with many suppliers around the world (Caterpillar, GE, Cummings, Scania and others).
The Otto Cycle engine powered by biogas has an electricity generator coupled to its axis.
The torque of the motor rotates the generator, which in turn produces electricity.
Power generation is constant as long as the motor is powered and fully operational.
These motors are designed to run 8,000 hours in a year and usually achieve a superior energy efficiency of 91%. The electric efficiency of these engines’ ranges from 38-42%. The thermal efficiency of these engines, the capacity to generate thermal energy, would come from 40-45%.
As every Otto Cycle engine runs by explosion, a large amount of heat is released, by the engine exhaust from this explosion and by the block engine. Recovering this heat from the "exhaust" and "engine block" and utilizing them internally in productive processes of the biogas plant or the industry where the plant is installed, perfectly feasible from a financial and technical point of view. However, this study is specific to the conditions of each installation, making it difficult to quantify theoretical cases.
For example, the electric energy generated by 500 m3 per hour of biogas (being the biogas with 55% of CH4) is 1.13MWh. The total produced heat energy is 1.22MWh, with 0.84MWh produced by the engine "exhaust" and 0.38MWh produced by the engine block.
Some of the heat energy produced by the "engine block" will be used to heat the Biodigester and is not available for commercial use.
Biogas can be used as fuel gas instead of liquefied petroleum gas (LPG), extracted from non-renewable resources.
The biogas produced, with at least 50% methane gas (CH4), can be used directly in LPG burners with simple adaptations, after cleaned and extracted hydrogen sulphide gas (H2S) and humidity (H2O). That is, biogas is a direct and viable alternative to energy applications with LPG.
The calorific equivalence of a m3 of LPG with a m3 of biogas is equal to 0.45.
Unique solution for the treatment of agro-industrial residues.
Economically viable for the energy set
Electric energy management:
100% green and renewable
- Industrial residues
- Municipal organic residues
- Animal production