Biogas Power Plant


By many years SMEA Engineering is a landmark in the field of design and consulting about biogas power plant, providing high quality projects customized solitions. The controlled anaerobic digestion is an important resource for the future, not only to provide electricity but also in order to produce biofuels. In order to avoid confusion, before extending the overview in this area is well to put immediately a clear distinction between the two terms used to describe the processes of this type of power plant:

  • Biomass: it is the material coming from the crops, and which is potentially transformable into gas and then into energy, such as corn, fodder, animal wastes, etc. The biomass and fuels coming from the crop, emit into the atmosphere, during combustion, an amount of carbon dioxide more or less corresponding to that which is absorbed previously by the plants during their growth process. The use of these energy purposes therefore limits the release of more carbon dioxide, the main cause of the greenhouse effect;
  • Biogas: this term means a mixture of various types of gas (mostly methane, 50-80%) produced by bacterial fermentation in anaerobic conditions (absence of oxygen) of organic residues from waste, decaying plant, rotting carcasses, manure, sewage sludge or waste agro-industry. The whole process sees the decomposition of organic material by some types of bacteria, producing carbon dioxide, molecular hydrogen and methane;


In the chart: Biogas Composition

  • Organic substrates: this term is commonly indicate the “input” of organic origin that can be introduced into the reactor for anaerobic digestion, and are diverse. First there are the cattle and pig slurry and poultry manure, particularly those of laying hens. There are then various residues of agricultural crops, as well as the “energy plants”, grown specifically for the production of energy (where, however, this does not involve subtraction of agricultural areas used for the production of food crops);

ciclo-biogasThe Biogas is considered a renewable energy source because, if reused, it can be placed directly in the environment and it comes from a natural processes of decomposition of organic matter. The inclusion under the category of clean sources ensures that the energy produced by their use is “salable” in the form of green certificates to industries that use fossil fuels, which are required by many laws to invest money in energy from alternative sources.



What we’ll try to do now, is to describe from the beginning to end the “cycle of biogas” in order to bring order to the terminology and operation of the different parts of the plant. Proceeding, we’ll describe also the possible construction alternatives specifying which of these can be chosen by the management of a company interested in investing.



The first step is to collect the biomass that will be introduced later in the biogas power plant system. The type of these is certainly the element that most affects the cost of the reaction and, for good measure, the profitability of agricultural plants; it is therefore essential to analyze all possible alternatives. Among the main agricultural biomass which can be used, we may mention the following:


  • Livestock manure. It is certainly of great interest because it ensure the intake to the plant as well as a variety of important microorganisms, with the function of inoculum (ie activation of the process), but also a whole series of trace elements which avoid the evident deficiencies of food with possible problems for the development of microflora;
  • Crop residues. Are wastes obtained from the agricultural production, particularly fodder, leachate silo, straw, fruit waste etc. Usually they are materials characterized by relatively high levels of dry (15-35%) that must be well assessed also from the point of view of their ability to bind to form, inside the digesters, floating layers difficult to deconstruct;
  • Organic fraction of municipal waste. It is a very large range of biomass: from sewage sludge to the organic fraction of municipal solid waste (OFMSW), to many other materials that do not present particular technical contraindications to their use in co-digestion;
  • Organic Waste. It’s about waste originated by the industrial process of agricultural products: both liquid products (such as whey), solid products as macro fruit (residue from the production of various juices) and slaughter waste. In particular the latters (blood, fat, stomach contents, etc.) are of great interest as co-substrates due to their high energy potential.
  • Energy crops. These crops are produced expressly for their use in co-digestion. The most-used are definitely corn, sorghum, triticale, wheat, rye etc.



Once stocked, vegetable residues are daily transported and discharged by a scraper in a separate mixing tank where where they undergo a pre-treatment and are finally mixed.
In order to optimize the plat yeld, or better, to improve the digestibility of the organic substance entered is possible to achieve specific preliminary treatment, obtaining a greater amount of energy per unit of biomass used, with evident energetic and environmental advantages; in this sense, we are developing an intense research activity in order to produce many updates in the future.
Nevertheless preventive mixing plays a fundamental role too, the second treatment which is carried out at this stage. To optimize the efficiency of the anaerobic process, as well as every biological process, it is necessary to ensure maximum contact between the substrate residues, silage, pomace and liquid which, after a short period of storage in the mixing tank, is transported by a pipe and fed into the digester. Even the manure will be fed into the digester, but separately from the substrate. This is because the external mixing would cause bad odors.


The substrates are then pumped into the digester, and this can be done in two ways:

  • Entering liquid fractions and solid separately. This assumption is used when the amount of vegetal biomass are well above the sewage available. While the latter are introduced via pump after homogenization in a special pre-tank capable of hosting the daily amount, the solid fraction of corn silage is fed from a metering hopper device through a system of augers which provides first to the biomass lifting and then to its dumping within the digester;
  • Entering with conveying by pumping. In a tank large enough to contain the power of one or two days, are collected substrates of pomace, vegetable residues and whey, and are thoroughly mixed to obtain a material sufficiently homogeneous and pumpable. The mixture then pass through a shredder and then pumped to the digester. The slurry, which are stored in a separate tank, are pumped into the pipe containing the mix of biomass to 10% of dry just before the entrance into the digester.


Is in the digester that begins the real methanogenic fermentation step of a biogas power plant, and therefore, the biogas production. You can also use two, one after the other, the so-called double. The substrates are placed in the first, where they remain for 60 days to ferment continually mixed, and then pass to the second, where they continue their fermentation under mixing for another 60 days.
The functionality of the system is strongly influenced by the various components characteristics. Is therefore necessary to consider, even for economic purposes, the possible variants of construction. The solutions are different, each with positive aspects and critical issues that must be considered in relation to the specific situation, to adopt the one that minimizes the negative aspects and gives the best guarantees of success. Among the different alternatives we may include:


  • Solution with concrete cast in place. It’s certainly the most widespread due to its easy adaptability to the demands of the various companies on the preparation of the holes and openings. It’s very appreciated by the installers that must not engage in any way to adapt their components to the masonry. By contrast, the quality of the manufactured article, in addition to the characteristics of the materials used and their dosages, is also influenced by the environmental conditions that lead to its variability;
  • Solution with prefabricated items. The use of prefabricated components allows to considerably shorten the times of realization; furthermore, being the materials constructed in the workshop, it ensures the construction quality constancy also in relation to the possibility of using very dense concrete, and then with an higher strength class, which is not practically possible with the jets in work in vertical formworks. On the other hand it is necessary prior coordination between manufacturer and an installer that requires a shared effort for a project adapting to the prefabrication constraints. Given this greater initial commitment, there is then a strong facilitation in planning future facilities;
  • Steel solution. Is maybe the more flexible and easily adaptable to different installation requirements situation. One aspect to consider is certainly the needing of protection against corrosion and to prevent any possible depression in the system with special attention; various options are possible: from the use of corrosion resistant material, to coatings of various nature. The advantages of this solution are due to the execution speed and to the ability to restore the sites to their original state without special costs thanks to the ferrous material residual value. Is essential to ensure the building insulation in order to avoid any thermal bridge.


Inside the digester of a biogas power plant, in order to optimize the anaerobic digestion efficiency, it’s necessary to ensure that there is the maximum contact between microorganisms and biomass fermentation: this is the aim of mixing. Biomass inside the digester is in fact continuously stirred. The difficulty to maintain optimum mixing conditions depends, in addition to the digester volume, also on the characteristics of the treated biomass and the solids content of the material. Here below some types of mixing patterns:

  • Hydraulics. It’s about mass mixing by recirculating the same through an external pump and its internal distribution through nozzles appropriately positioned and oriented. The interesting side of this method is the absolute lack of mechanical elements moving inside the digester, being the recirculation pump installed outside, in a position where it is easy to each operation of the control and handling;
  • Mechanical mixers. They are the most used solution due to their ease of application. The mixers can be both horizontal and vertical axis and the drive is electrical and external. In this way inside the digester are no moving parts but good reliability for which is really limited the possibility of unexpected breaks. Another variant consists of the mechanical mixers with motorization submerged, where are possible quick maintenance interventions (2-3 hours) keeping the digester in activity;


We said previously that it is important the biomass inside the digester remains at a temperature of 38-40 degrees. Is necessary therefore a heating system that copes with the heat losses due to the placing of the biomass and to the dispersions through the walls of the digester. In this regard there are two different technological settings:

  • The first avoids to engage the interior of the digester and provides for the recovery, through the pump, the material in the digestion to heat it in an external heat exchanger;
  • The second is about the installation, inside the digester, of a serpentine wall through which, thanks to the circulation of hot water, is the heat needed is provided;

A final description is deserved by the storage of biogas waiting to be used in cogeneration (CHP) or in flare:

  • In the first case, the realization of a double membrane is provided, the first is weather-resistant, the second (internal) has a gasometric function (thus in the gas measurement);
  • In the second case is used, only one elastic membrane rubber which allows to verify the quantity of gas available;
  • A third option is to install a gas meter steel with water motor. The rise and fall of the bell indicates the amount of accumulated gas. This solution is normally adopted when using a steel digester.


After fermentation the substrate is filtered by the coarse and fibrous parts before being conveyed in a container in reinforced concrete, taking the name of digestate, where it will remain for up to six / eight months. It will then be taken by tanker engaged in the distribution of the substance, of high nutritional value, on the fields, making it possible to fertilize and promote the growth of both the new crops of corn, triticale, sorghum, and cultures for the production of oils and wines. The solid filtered coarse-material, named “solid amendment” is used to make the ground and to fertilize it. It ‘also useful in the nursery and vegetable crops.


The strengths of a biogas power plant are numerous, and many are the reasons for investing:

  • with a biogas plant you can make money from the sale of energy produced, which is clean and renewable;
  • the gas generated from the anaerobic fermentation can be kept under control, channeled, stored and then reused at the right time;
  • Organic waste find their energy enhancement and you get a reduction of the costs of management of the installations from the point of view of consumption (in particular when the biogas produced is partly reused within the company as in the case of distilleries or dairies, the so called “combined power”);
  • Capturing the biogas you avoid the propagation in the atmosphere of methane contained in it (which is one of the “greenhouse gas” most dangerous for the environment: just think that 1 kg of methane is equivalent to 25 kg of CO2);
  • At the end of the production process carried out in biogas plants, the residual materials as digestate can be used for fertilization;
  • Carbon dioxide emitted by the combustion of the biogas is the same generated by plants during their life, and hence you don’t have an additional emission due to fossil fuels;
  • It gives the effect of containing the odor released from the organic waste and organic stabilization;
  • You get a sanitizing effect against pathogenic microorganisms, and are thus destroyed bacteria and viruses contained in the sludge;
  • You can use institutional contributions if biogas plants are installed, as in the case of biowaste disposal, the organic fraction of municipal solid waste (OFMSW);
  • The biogas is the most constant and reliable among renewable energy sources.


Is good to remember that when we talk about biogas power plants, we are talking about renewable energy, with a very low impact on the environment, and thus the health of citizens.
Even according to the Kyoto Protocol, has been stipulated that this type of energy does not contribute to the increase of CO2 in the atmosphere, because the one emitted by the combustion is equivalent to that which is captured and converted into oxygen during the cultivation cycle of the vegetable.
In addition, although they are already very low emission limits required by law, SMEA Engineering systems can fall further below of these impositions, as demonstrated in the table below relates to plants with thermal power within the 3 KW:


Please be aware that the securities issued by our plants, are also suitable for the most stringent regulatory requirements for plants greater than 3kW.
After this premise, it should be borne in mind that biogas plants although presents numerous advantages, one must always keep in mind the following critical issues:

  • The digesters temperature should be monitored especially in cold countries, because it should not fall below 15 ° C;
  • The plant should not be too far away from the inputs production areas, because for their eventual storage a high expense would be require;
  • You must avoid to subtract land intended for feeding and divert it to the production of energy, but use land that would otherwise be fallow or not exploited for the people’s livelihood.
  • Do not spray lands with heavy doses of fertilizers and pesticides knowing that the crops are intended for non-food use, thus saving the unnecessary pollution of the soil and groundwater beneath.

Thus, as usually common sense and a minimum of entrepreneurial vision is required.
The clean energy source provided by biogas, therefore, appears as extremely convenient, both from the economic, environmental and energetic point of view.

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