BIOMETHANE – WHAT IT IS
Biomethane is a gaseous substance formed mostly by methane (about 95%), and is generated by renewable energy sources (such as biomass plants, which we will analyze later). A good part of the process for the production of this gas, is identical to that for the production of biogas, viewable in the appropriate page of our website: differs, however, because at a certain point of the process, biogas is extracted component of methane, draining it of CO2.
SMEA Engineering is one of the greatest exponents in the field of industrial design of this technology, we ensures professionalism and a very high technical know-how.
BIOMASS POWER PLANT FOR BIOMETHANE PRODUCTION – INTRODUCTION
Anaerobic digestion is a complex process of biological type, thanks to which, in the absence of air (oxygen) to the organic matter is converted into biogas which is collected, purified from moisture and CO2, and becomes methane. The biogas consists predominantly in a mixture of the latter gas (CH4) and carbon dioxide (CO2).
The amount percentage of methane in the biogas varies depending on the type of the organic substance used in the process conditions from a minimum of 55% to about 70%.
In order for the process to be possible, it is necessary the action of different groups of microorganisms, always present in the natural organic matrix, able to transform the starting organic substrate in intermediate compounds, the main of which are acetic acid (CH3COOH), carbon dioxide (CO2) and hydrogen (H2) in turn used by microorganisms called “methanogenic” that conclude the process of biological transformation producing methane.
The organic matrices, also called substrates, may have different origins such as corn silage, sewage farm, or even the Organic Fraction of Municipal Solid Waste (OFMSW) that we will use as an example below.
The advantage of anaerobic digestion is the conversion of organic matter in methane (CH4) and carbon dioxide (CO2), and then the final production of a renewable source of energy in the form of a biofuel with a high calorific value. The optimal reaction environment is around the neutral, the pH is around 7 – 7.5 and the process temperature is about 38-40 ° C, even if it may also happen in the thermophilic field at about 50 ° C.
Such process will allow simultaneously the treatment of waste through the reduction of the organic load, the digestate treatment with the aim of obtaining solid fertilizer and the treatment of the clarified liquid to be purified and discharged into the public sewer.
It should be noted that the plants designed by SMEA Engineering represent the best available technology in terms of efficiency of OFMSW treatment and all other possible substrates, combining the profitability of renewable bio-methane production with the digestate liquid fraction purification process energy costs.
FUNCTIONING OF A BIOMETHANE PRODUCTION PLANT – OFMSW
For the organic fraction of municipal solid waste treatment, is generally designed and built a prefabricated shed in reinforced concrete to receive and storage the OFMSW.
The trucks unload inside the shed and here a means (shovel) charge a hopper.
Inside the shed, the OFMSW is sent via a screw conveyor and a conveyor belt to two machines for shredding and separation of the plastic from the organic matter.
The OFMSW has an average dry substance percentage equal to about 22%, within the two machines is added water from the oxidation plant in order to bring the dry substance percentage to approximately 8-9%.
The resulting plastic materials are stored and sent to the recycling and recovery industry. The organic part, purified from plastic, is sent to a sedimentation basin (for sand and heavy substances separation) and from here to the mixing tank, where the material is homogenized to be sent to the anaerobic digestion.
The mixing and preparation tank of the raw material is made of stainless steel. From the mixing and preparation tank the product passes into the hydrolysis tub, well triturated and mixed with recirculating water coming from the oxidation section.
The hydrolysis tub acts as a pre-digester and as a storage tank and equalization.
This solution allows to obtain a product of excellent quality to digest which determines an important increase in the efficiency and therefore the amount of biogas produced.
The mixing system adopted in the design by SMEA Engineering, allows the total building maintenance from outside.
The primary digester is fed via a pump system with liquid product well homogenized, heated and hydrolyzed which will improve the yield of biogas, the digestibility and the rate of reaction.
The digestate then passes through the secondary digester and is recirculated to the first digester to ensure a stable operation of the apparatus while maintaining the optimum sludge concentration.
The digesters, for a good temperature control, have heating coils in which hot water circulates.
To stabilize the biogas flow rate is also carried a lung steel bell gasometric.
The middle phase of the process, is the biomethanation of biogas coming from the anaerobic digestion of the organic fraction of municipal solid waste (OFMSW, and is essentially constituted by:
- Biogas upgrading plant;
- Drying of biomethane;
- Odorizing plant.
Biogas coming from the digesters is pretreated with the systems described previously, with the aim of reducing the content of sulfur compounds and other contaminants, in order to adjust the biomethane to the expected use.
The pretreated Biogas is compressed to a value between 4 and 15 bar, variable as a function of its final use and of the maximum content of CO2 to be obtained.
The compressed biogas enters in a column for the CO2 selective absorption by washing in countercurrent with an aqueous solution of potassium carbonate (K2CO3); purified from CO2, it comes out as Bio-methane from the head of the absorber at a pressure variable between about 4 to 15 bar, depending on the initial compression’s pressure.
The system used thanks to the SMEA Engineering’s design of the biomethane palnt, provides an high-purity output, in fact it can lead to a CH4 content of 99.5%.
The solution which has absorbed the CO2 comes out from the bottom of the absorber and, after a flash treatment to recover any fractions of CH4 dissolved for solubility, feeds a regeneration column where the CO2 is released by stripping with steam produced by the same solution with an external source of heat (it is generated from a boiler heated water powered by biogas).
The CO2 is available from the head of the regenerator at an high purity level and therefore, after cooling and a condensate separation, can be downloaded to air or liquefied for other uses without the need of further treatment. The solution of Potassium Carbonate instead, freed from CO2, is cooled and then recycled to the absorber closing the cycle: from the cooling it is possible to recover 90% of the heat that will be used for heating the digesters.
The methane contained in biogas alimenting the purification plant is recovered almost entirely limiting the losses to less than 0.05%.
The selected machines are always the most reliable on the market with the objective of minimizing the causes of stationary, while the instrumentation and control system are perfectly interfaced with other units present in the site of the plant, and allow maximum flexibility of control and management.
The digestate in output, after being carefully treated, is ready to be used as fertilizer.
For the normal plant management it must be said that nonrenewable energy sources are not used: only in the starting phase of the plant, for heating the digesters, is usually used the heat produced by an oil boiler. When fully operational, you will use a hot water boiler fueled by biogas, producing heat for the biomethanation system and for maintaining the digesters temperature.
BIOMETHANE PRODUCTION ADVANTAGES
We analyze below the reasons why is convenient to invest in a biomethane plant:
- We replace fossil fuels as wished by many National Energy Plans;
- Production of biomethane from renewable waste;
- Employment benefits not only for those who will take up the plants management but also for the entire production line that contributes to the final result;
- Production of organic manures;
- Territory enhancement;
- OFMSW disposal (in case the plant is designed to have this material as input), methane production without emission of CO2 and additional decrease in indirect way the other greenhouse gases (in particular the CH4 released from uncontrolled landfills and in smaller part by the controlled ones);
- Possibility to reuse the water contained in the digestate;
- Reduction of the pollution and subsequent stabilization of the solid fraction (especially when it’s about waste);
- Elimination of pathogenic bacteria;
- Ability to regain and resell the CO2 extracted from the process.
Every country has different type of incentives for biomethane and OFMSW plants. For example in Italy, the current decree regulating the incentives for facilities to biomethane is the DM 5 December 2013 that, according to the destination, lists three types of grants::
- No specific usage. In the case in which the produced gas is simply inserted into the network without being intended for a particular use, a monetary incentive expressed in € / MWh is assigned, for a period of 20 years, given as the double of the annual average of the gas value, minus the price of the same in the relative month;
- Transport destination. If the gas is produced specifically for vehicles specific certificates are released (certified for consumption release), where each of them recognizes the production and networking of 10 Gcal;
- HEC Destination (high efficiency cogeneration). If biomethane is designed to fuel this type of system, then you benefits this special monetary incentives.