SCOPE OF THE INVENTION

This invention relates to a system and method for treating biogas, and more specifically to the removal of moisture from biogas and the subsequent heating of the biogas prior to its transference to external pipelines.

KNOWN TECHNOLOGY

Biogas is formed when organic matter decomposes in anaerobic conditions. On average, biogas contains 50-80% methane, 15-45% carbon dioxide, approximately 5% water and 0-2% hydrogen sulphide, as well as traces of chlorine and fluorine compounds. The composition of biogas is very close to that of natural gas, so biogas can in many cases be used as an alternative to natural gas. The main obstacle to this is the water content of biogas, which causes problems such as corrosion and operational faults in equipment designed to use natural gas. Various methods with related equipment and systems are used to dehydrate biogas.

The most common method to dehydrate biogas involves cooling down pressurised biogas whose temperature has risen due to pressurisation, which causes the moisture contained in the biogas to condense, after which it can be separated and removed from the system. One such method is described in patent publication JP 2004092466.

Another known method removes moisture from pressurised biogas through sorption, as described for example in publication GB 1349733.

Publication EP 0120547 A1 describes apparatus for drying pressurized air comprising two heat exchangers in which the wet pressurised air is cooled and both heat exchangers are arranged above each other in a common housing.

Patent publication US 20040042909 describes a system in which most of the moisture contained in pressurised biogas is removed by cooling, which

causes condensation and makes it possible to remove the moisture from the system. After this, the temperature of the biogas is raised again with coolant from a compressor. This is done to prevent the freezing of the biogas in the pipelines.

SHORT DESCRIPTION OF THE INVENTION

The known systems for treating biogas used in biogas plants are installed onsite, and the results are not always optimal.

The aim of this invention is to create a simple, reliable and easy-to-use system and related method for treating biogas in a pressurised form. This is done by using known technology in a new combination. The entire post- pressurisation biogas treatment system is constructed in a single framework at an engineering workshop, where the necessary adjustments, pressure tests and inspections by the authorities are also performed. Thus the system is ready for use as soon as it is delivered to the site, without the need for time-consuming installations and inspections. To remove the moisture from biogas, the pressurised biogas is cooled in two stages in two heat exchangers, on a section which is vertical in relation to the primary circuits. The vertical section is constructed so that the biogas flows from bottom to top through the heat exchangers' primary circuits, which causes the condensed fluid to run from top to bottom, again through the heat exchangers' primary circuits. The heat from the moist biogas in the first cooling phase is transferred to biogas which has already been dried and cooled down in the same system. This heats the dried and cooled biogas before it is removed from the system. In the second cooling phase, heat is transferred into a coolant coming from an external cooler. Moisture is removed from the cooled, partly moist biogas in a dehydrator, from which the condensation will run along a vertical pipe and through an anti-siphon valve into a receptacle from where it can be removed from the system.

SHORT DESCRIPTION OF THE FIGURE

The following figures are now referred to, to further explain the invention and to describe its application:

Figure 1 shows a system for treating biogas.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows, in diagrammatic form, a system for treating biogas, which system is constructed in a single framework and is used to dry moist biogas. Pressurised moist biogas is directed to heat exchanger 1 and then to heat exchanger 2, which both perform the cooling of the moist biogas and start off the condensation of the moisture. Heat exchangers 1 and 2 are installed vertically in relation to their primary circuits such that the moist biogas flows from bottom to top, and such that the moisture which is condensed in heat exchangers 1 and 2 runs from top to bottom through the same heat exchangers (1 and 2), into a receptacle (3). The secondary circuit of heat exchanger 2 transfers heat to biogas which has already been dried and cooled in the same system. In heat exchanger 2, heat is transferred into a coolant coming from an external cooler. The biogas which has been cooled in these two phases is conducted to a dehydrator (4), where the remaining moisture condenses and runs down a vertical pipe (5), through an anti-siphon valve (6) into the receptacle (3), where the level of liquid is maintained within allowable limits with the use of an automatic system to prevent the loss of pressure in the system. From this receptacle the condensed liquid is conducted away from the system. The anti-siphon valve (6) is also used to prevent pressure drops in the system. The dried and cooled biogas is conducted away from the dehydrator (4) towards heat exchanger 1 , where it is heated with the use of pressurised moist biogas, as described above, before being removed from the system.

Those skilled in the art will see that various kinds of modifications can be made to the application described above without deviating from the scope of the invention. Such modifications include the addition of measuring instruments and various equipment for controlling the biogas treatment process.

The invention also relates to a method for treating biogas in which pressurised moist biogas is cooled in two stages in heat exchangers 1 and 2, which are installed vertically in relation to their primary circuits, such that the moist biogas flows from bottom to top, and such that the moisture which is condensed in heat exchangers 1 and 2 runs from top to bottom through the same heat exchangers, 1 and 2, into a receptacle (3). In heat exchanger 1, the moist biogas is cooled with the use of already dried, cooled biogas, which is simultaneously heated in the secondary circuit of heat exchanger 1. In the second cooling phase in heat exchanger 2, heat is transferred into a coolant coming from an external cooler. The treated biogas is then conducted to a dehumidifier (4), where the condensed fluid runs down a vertical pipe (5), through an anti-siphon valve (6) into a receptacle (3) and is then removed from the system. In the receptacle (3), the level of liquid is maintained within allowable limits with the use of an automatic system to prevent the loss of pressure in the system. At the same time, the dried, cooled biogas which has been heated in heat exchanger 1 is removed from the system.