Over the last decade the upgrading of biogas to biomethane has become widespread in some countries where the production of biogas from collection systems in municipal waste landfills, sewage treatment plants of municipal wastewater and the digestion anaerobic biomass of agricultural and agro-industrial, was already established.
Currently biomethane is produced in 19 countries of which the 14 EU countries covers more than 70% of world production (Table 1).
The first plant that produced biomethane was built in Staten Island, in the state of New York, which starts his operations in 1981 and has a treatment capacity of 13,000 m3/h of biogas from landfill. In Europe, the record is held by the Netherlands with the plant, in operation since 1987, the Tilburg-De Spinder. (Purification capacity of 600 m3 of biogas/hour).
State of the art and prospects of development of the biofuel in the main European producer countries (ask eba updated data).
In December 2013 across Europe were operating more than 230 plants of purification and upgrading. Plant from biogas to biomethane, with a production of over one billion cubic meters per year. The production capacity reached and the year of entry into operation of the first plant, clearly demonstrate how the purification technology is now mature, widely tested and, therefore, no longer to be considered as a limiting factor.
Biomethane, turns out to be a bioenergetic vector with enormous potential. The opportunity to use the biomethane as replacement or supplemental of the natural gas in transportation networks and distribution comes from the implementation of European directives 2003/55 / EC and 28/2009 / eC, which attach particular importance to the exploitation of gas produced from renewable energy, attributing to biomethane the role of possible solution for achieving the objectives of the Kyoto Treaty to combat climate change.
These directives require member States to ensure that gas produced from biomass produced by fermentation, by thermochemical processes, as well as gas from other sources (resulting, for example, by the methanation of hydrogen from renewable sources), in compliance with the requirements of established quality, have non-discriminatory access to the transmission and distribution of natural gas, if they are conveyed and accumulated safely and that the end user can take advantage of them without any additional risk and with the respect of the environment. In the next chapter we will study in depth the situation in Germany, Sweden and the Netherlands, which are the first three nations in the list of the main producers of biomethane in Europe.
Germany is the European country where, in recent years, the biogas and biomethane chain has the biggest development. Currently they have 160 operating upgrading systems with a capacity of purification of approximately 90,000 Nm3 per hour with an average capacity of about 700 Nm3/hour of biomethane.
Of the 160 plants, 3% treated biogas from wastewater treatment plants; 9% treated biogas produced by anaerobic digestion of organic waste; 88% treated biogas derived from the fermentation of energy crops and animal wastes.
The goals that Germany has set itself are the production of 6 billions Nm3/year of biomethane by 2020 and the replacement of 10% of the total natural gas consumed in the country with biomethane by 2030.
As well as in the field of biogas, Sweden may be considered a forefront country also in upgrading biogas to biomethane and its subsequent use as a biofuel. The first plants were built in the early ’90s, but only since 1996 this type of fuel is used on a large scale both for private transport both for the public (bus). Today the number of upgrading plants has risen to 55, of which 69% use the technology of the “wash water”, 16% that of PSA, 12% chemical absorption and 3% the cryogenic treatment with a total capacity of 28,150 Nm3 / hour.
Thanks to the attractive incentives introduced by the Swedish government, biomethane is used mainly in the transport sector. The 85% of the annual production, is used as biofuel for transport, going to cover 70% of total consumption of natural gas (fossil and renewable). The remainder of biomethane is injected into the natural gas grid. Thanks to, this availability of biomethane and a growing number of service stations that use it, the sales of vehicles powered by natural gas is in constant and rapid growth.
Although the National development Plan of Renewable Sources of the Netherlands has not set a specific target for the use of biomethane, according to some studies it could replace from 15 to 20% of the natural gas consumed annually by 2030, that share could reach the 50% in 2050. Currently have 24 upgrading plants, the first of which began operation in 1987 to purify biogas from a landfill.
The Upgrading technologies
Biogas is composed byf 45-70 percent of methane (CH4). The second main component is carbon dioxide (Co2); it also contains, in small percentages, hydrogen sulfide (h2S), ammonia (NH3) and water vapor (H2O).
The natural gas of fossil origin contains, 85-98 percent of methane depending on the origin. To ensure that the quality of biomethane will be similar to the quality of the natural gas in the network is necessary to increase the percentage of Ch4 in the raw biogas. In figure 1 are schematically shown the stages of the process which basically are three.
Step 1 – The removal of impurities prevents corrosion or clogging of the components of the system and eliminates toxic substances and / or pollutants. It proceeds to a removal of water (biomethane to be compatible with injection in the network must be dry), hydrogen sulfide (coming from the organic material used), oxygen and nitrogen (is used a small amount of air in the previous phases), ammonia (not always necessary), silicone (found in MSW and sludge, are abrasive) and particulate (mechanical parts wear out).
Step 2 – There are several technologies available on the market for the removal of carbon dioxide.
PsA (for pressure swing adsorption)
The technology uses materials such as zeolites or activated carbon, which act as molecular sieves for retaining the carbon dioxide molecules on its surface, under certain conditions of pressure. The Co2 is then released during the depression.
PWs (Dry pressurized water)
The process is based on the water solubility of carbon dioxide. The gas is bubbled through a container of water under pressure. In addition to Co2, the process is able to remove even a certain percentage of ammonia and hydrogen sulfide, however, in the presence of high quantities of the latter, is required before desulfurization. At the end of the process it is necessary to proceed drying of the gas.
Also this upgrading process is based on a wash, however, is used, instead of water, a fluid with high capacity to retain Co2e h2S. This takes place by heating the fluid.
Even the amine washing is based on a chemical absorption. Unlike the washing technologies to water under pressure, the gases to be removed are absorbed through chemical reactions. In this way it is possible to significantly increase the load of the washing fluid.
Monoethanolamine (MEA) scrubbing
This washing process is advantageous when it is necessary only to remove the Co2, since it has low pressure requirements, while requiring a temperature of about 40° C. For this reason, it finds application where there is already a heat availability.
Diethanolamine (DEA) scrubbing
This technology is very similar to the MEA. The diethanolamine has more adsorption capacity than monoethanolamine, however, it is more polluting for the environment.
The process of membrane separation is based on the properties of the semipermeability of some polymers, that are not permeable by methane, but permeable by carbon dioxide. To obtain a good separation, it is necessary to drive the gas through the membrane at a pressure of from 25 to 40 bar. This technology it is continuously expanding and seems advantageous for smaller-scale plants. Fundamental for the life of the diaphragm is the prior removal of h2S and other impurities.
This technology is based on the fact that different gases have different temperatures for liquefaction. Requires significant amounts of energy to reach very low temperatures and high pressures, however, it allows to obtain large volumes of natural gas of high purity (99%) and CO2 adequate to commercial purity. It may be an option for large systems and in particular configurations that present availability of energy or cold (eg. In combination with regasification plants).
Step 3 – The post processing allows biomethane to reach the characteristics of natural gas in the network. We proceed to a conditioning (addition of propane to achieve the desired calorific value), odorisation (smell that allows you to feel any losses from the distribution system) and the regularization of the pressure (to adapt it to that of the distribution network). A European directive for the unification of standards for biomethane is in preparation.