Process for treating biogas for subsequent compression

Abstract

The present invention relates to a process for treating biogas, enabling its subsequent storage in cylinders, transport and use in remote locations. According to the present invention, the process for treating biogas comprises adjusting the humidity, hydrogen sulphide (H₂S) and siloxane parameters of the biogas to enable its compression and subsequent storage in cylinders. The cylinders containing biogas are supplied to customers for use as an energy source, for generating heat or electricity, and as fuel.

Description

"PROCESS FOR TREATING BIOGAS FOR SUBSEQUENT COMPRESSION" FIELD OF THE INVENTION

[0001] The present invention relates to a process for treating biogas, enabling its subsequent storage in cylinders, transportation, and use in remote locations. BACKGROUND

[0002] The history of biogas utilization dates back to 1600, when the existence of a flammable substance in swamps was identified. In 1667, the scientist Thomas Shirley, in the United Kingdom, discovered that this substance was the result of the decomposition of organic matter.

[0003] However, the technique of using biogas only began to be explored at the end of the 19th and beginning of the 20th centuries. China and India were the first countries to use biogas as an energy source, for lighting and cooking.

[0004] In Brazil, the use of biogas began relatively recently, in the 1970s, during the oil crisis. The country's first biodigester was built at Granja do Torto, in Brasília, in 1979. The project was important because it demonstrated that it was possible to install a biogas production unit with simple and low-cost materials.

[0005] The Brazilian Federal Government's Energy Mobilization Program (PME), in 1982, encouraged the construction of biodigesters on rural properties. However, the lack of information and training meant that the system was inefficient, and many producers abandoned the technology.

[0006] In recent years, with advances in technology and improved technical knowledge on the subject, biogas is experiencing a new wave of strong expansion in Brazil and worldwide. FUNDAMENTALS

[0007] Currently, there are three main ways to commercially utilize biogas: Electric Power Generation:

[0008] Electricity generation is one of the main uses of biogas in Brazil. Many urban solid waste treatment plants, sewage treatment plants, and farms use biogas to power internal combustion engines or turbines that generate electricity. This electricity can be used for the facilities' own consumption or sold to the national electricity grid through the energy compensation system (distributed generation), encouraged by the Brazilian government.

[0009] Projects for generating electricity from biogas are especially popular in landfills and sewage treatment plants. In these applications, the biogas captured during the anaerobic decomposition of organic waste is used to generate energy continuously and stably. Thermal Power Generation

[0010] Biogas can also be used to generate heat for industrial processes. This heat can be used directly for heating water, drying grains, and other processes that require thermal energy. Industries such as ceramics, food and beverages, and agribusiness have benefited from the use of biogas as an alternative heat source, reducing costs associated with fossil fuels. Biomethane Production

[0011] The purification of biogas, which consists of removing CO2 and impurities (such as H2S and siloxanes), results in biomethane, a gas with characteristics / calorific value equivalent to natural gas. Biomethane is one of the most promising forms of biogas utilization in Brazil, especially as fuel for vehicles. Several projects in rural and industrial areas are converting biogas into biomethane to supply fleets of trucks, tractors, and buses, offering a cleaner alternative to diesel and gasoline.

[0012] Furthermore, biomethane can be injected directly into the natural gas network, replacing fossil gas in industrial and residential applications. In Brazil, this market is expanding, with increasing interest from gas distributors in integrating biomethane into their network, driven by policies incentivizing the use of renewable energies.

[0013] However, the biogas utilization technologies presented above have some disadvantages, such as:

[0014] Electricity production: energy generation is only viable if it is used on-site or injected into the electrical grid (on-grid system). The limiting factor is that there is not always sufficient energy demand at the location of the biogas plant. Similarly, there is not always an adequate electrical distribution network available in the region where the biogas plant is installed to inject any generated energy into the grid.

[0015] Thermal energy generation: just like electricity production, thermal energy production has the same limiting factor, that is, it is only viable to use thermal energy locally, in places where the biogas source exists.

[0016] Biomethane production: Biomethane production requires more complex and larger-scale equipment. This type of exploitation has been expanding in Brazil and worldwide; however, it is only economically viable for plants with relatively large biogas production. Smaller biogas sources, such as small landfills or sewage treatment plants, do not generate sufficient biogas flow to justify the installation of a biomethane production plant. OBJECTIVES OF THE INVENTION

[0017] Considering the context described above, it is an objective of the present invention to provide a process in which biogas can be compressed in high-pressure cylinders, transported via trucks, and used remotely for electricity or heat generation, as well as fuel.

[0018] An additional objective of the present invention is to make viable the exploitation of small biogas sources, such as small landfills or smaller sewage treatment plants.

[0019] An additional objective of the present invention is to determine the maximum permissible limits for a series of physical and chemical variables of biogas so that it can be compressed at high pressures without damaging the equipment, especially the compressors and cylinders. BRIEF DESCRIPTION OF THE FIGURES

[0020] Figure 1 illustrates a pressure-temperature phase diagram of carbon dioxide.

[0021] Figure 2 illustrates the molar fraction of CH4 in both phases calculated by different equation of state models.

[0022] Figure 3 illustrates the molar fraction of CH4 at different temperatures. DETAILED DESCRIPTION

[0023] A technical problem to be overcome by the present invention is to enable the compression of biogas without the need to process it to the level of biomethane, that is, without the removal of carbon dioxide (CO2).

[0024] Of particular relevance to this technical solution are the properties of CO2. The critical point of CO2 is 31.03 °C at a pressure of 73.8 bar. This means that if pure CO2 is compressed at a temperature below 31.03 °C (304.18 Kelvin), it will turn into a liquid at some pressure. This pressure will depend on the temperature, but will be less than 73.8 bar.

[0025] If the temperature is higher than 31.03 °C, CO2 will not liquefy during compression, but at pressures higher than 73.8 bar, it will transform into a special state called a supercritical fluid or dense phase.

[0026] Figure 1 illustrates the phase diagram of carbon dioxide.

[0027] It is possible to compress gases in conventional compressors as long as they are operating under gaseous or supercritical conditions.

[0028] Figure 1 illustrates the behavior of the system. The horizontal axis shows the mole fraction of methane (CH4) present. The mole fraction (times 100) represents the percentage of methane. The vertical axis provides the total pressure of the system. The unit in the original figure is MPa (megapascal). 1 MPa = 10⁻³ bar and 10 MPa = 10⁰ bar. Taking as an example a mixture of 45% CO2 and 55% CPU, then the mole fraction is 0.55, found on the rightmost part of the coordinate axis of the graph in Figure 2.

[0029] Assuming the compression of this mixture from 10 bar to 100 bar while maintaining a constant temperature of 250 K = -23 °C. Initially, this mixture is a gas, but upon reaching approximately 52 bar, the dew point (DP) for CO2 in the mixture is reached. Liquid CO2 will form, with some dissolved CPU. There will be a two-phase system, with a liquid and a gaseous phase. As the pressure continues to increase, the boiling point (BP) of CO2 is reached, and again there is a system with only a gaseous phase. Due to the formation of a liquid phase, such a system should be avoided.

[0030] If the gas mixture contains approximately 63% CH4 and the remainder CO2, then a different situation arises. According to Figure 2, the conditions under which a liquid phase is formed are not met. Instead, the gas is compressed and forms a supercritical fluid (also called a dense phase).

[0031] We can then use commercially available compressors for pure gases such as CO2, CH4 and H2 also for this mixture of 65% CH4 and 35% CO2, provided that no other substances are present, or that they are reduced to values ​​small enough not to interfere with the compression process.

[0032] With reference to Figure 3, at temperatures above -33 °C, liquid CO2 will not form if the CO2 concentration is below approximately 32% and therefore the CH4 concentration is greater than 68%.

[0033] In a process for compressing biogas according to the present invention, it was experimentally determined that the maximum limits of components typically found in the composition of biogas to allow compression and storage in cylinders without damage to the equipment should be: Moisture (H2O): maximum of 1% or 10 mg / m² 3 ; Hydrogen sulfide (H2S): maximum of 10 ppm; Siloxanes: maximum of 10 ppm.

[0034] To reduce the moisture content of the biogas to the specified limit, it is necessary to carry out the steps of cooling the biogas and separating the condensate formed, as described below.

[0035] The first step involves cooling the biogas until the moisture present in it condenses into liquid water. Biogas, when exiting an anaerobic digester or a landfill, is usually saturated with water vapor and at ambient or slightly higher temperatures, which favors the presence of moisture in the gas.

[0036] The biogas is then fed into a cooling system, such as a heat exchanger or a chiller. In this cooling system, the hot biogas comes into contact with surfaces cooled by a refrigerant fluid, such as chilled water or another refrigerant fluid.

[0037] Subsequently, the biogas is cooled to a temperature below its dew point, preferably between 3°C and 5°C, at which point the water vapor present in the gas will begin to condense into liquid. The choice of temperature depends on the operating conditions and the initial moisture level of the biogas, but temperatures in this range are generally sufficient to cause most of the water to condense.

[0038] As biogas is cooled, its ability to retain water vapor decreases. Therefore, the water vapor in the biogas begins to condense, transforming into droplets of liquid water. These droplets form on the internal surfaces of the heat exchanger, and throughout the process, the dried biogas continues its course while the condensed water is separated.

[0039] For the condensate separation stage after cooling, the biogas, now containing droplets of condensed water, is directed to a condensate separator. This device has the function of efficiently removing liquid water from the biogas stream, preventing it from returning to the gas system. The condensate separator allows the biogas to flow through it while the condensed water is collected and drained. The condensate separator may be known in the prior art as, for example, a gravity separator or a centrifugal separator.

[0040] As the biogas passes through the condensate separator, the condensed water is collected in a collection chamber of the separator and periodically drained manually or automatically.

[0041] To adjust the hydrogen sulfide (H2S) content of biogas, filtration is performed using iron-impregnated activated carbon. Activated carbon is a porous material with a high surface area, making it ideal for gas adsorption. In the H2S removal process, the activated carbon is impregnated with iron compounds, such as iron oxides or hydroxides. The biogas passes through a bed comprising the iron-impregnated activated carbon. As the H2S comes into contact with the material, a chemical reaction occurs between the H2S and the iron compounds. These compounds react chemically with the hydrogen sulfide in the biogas, transforming it into solid sulfur (S) or sulfated iron compounds, thus removing the H2S from the biogas stream, as shown in the reaction below. H2S + Fe(OH)3 Fe2S3 + H2O

[0042] Activated carbon continues to adsorb H2S until the iron compounds in the filter are saturated. At that point, the filter elements are replaced.

[0043] To adjust the siloxane content, an adsorption step is also used, which can be either chemical or physical adsorption.

[0044] In physical adsorption, siloxane molecules bind to the surface of activated carbon through van der Waals forces. This type of bond is relatively weak and reversible, allowing siloxanes to be desorbed under certain conditions, such as increased temperature.

[0045] In chemical adsorption, a stronger bond occurs between siloxane molecules and functional groups on the surface of the activated carbon. This type of adsorption is less common, but can occur depending on the chemistry of the siloxanes and the surface of the carbon.

[0046] The adsorption of siloxanes of the present invention is carried out in an adsorption column, in which activated carbon is contained in a vertical column, where biogas flows from bottom to top or from top to bottom. The column is filled with activated carbon which creates a filter bed.

[0047] Biogas containing siloxanes is directed to the column through a piping system. After passing through activated carbon, the treated biogas, now with most of the siloxanes removed, is directed to the outlet.

[0048] In addition, sensors and monitoring instruments are installed to measure the concentration of siloxanes in the biogas before and after the adsorption column, allowing for monitoring of the system's efficiency.

[0049] After the biogas is treated to achieve the desired parameters, it is compressed to a pressure between 200 and 250 bar and stored in cylinders.

[0050] The cylinders can be transported by land, for example, by truck. The cylinders are delivered to customers who will then use them as an energy source, for heat or electricity generation, and as fuel.

[0051] It will be readily understood by those skilled in the art that modifications can be made to the invention without departing from the concepts set forth in the preceding description. These modifications should be considered as included within the scope of the invention. Consequently, the particular embodiments described in detail above are merely illustrative and not limiting as to the scope of the invention, to which the full extent of the appended claims and any and all equivalents thereof should be given.

Claims

CLAIMS 1. Process for treating biogas for subsequent compression, characterized by the fact that it comprises the following steps: (i) adjusting the moisture content of the biogas, wherein the step of adjusting the moisture content further comprises the steps of: (a) convey the biogas to a cooling system; (b) cool the biogas to a temperature below its dew point, at which the water vapor present in the biogas will condense into liquid; (c) convey the biogas to a condensate separator, where the condensate separator removes the liquid water from the biogas; (ii) adjusting the hydrogen sulfide (H2S) content of the biogas, wherein the step of adjusting the H2S content further comprises the step of: (d) conveying the biogas through a bed of activated carbon impregnated with iron compounds, in which the H2S present in the biogas is adsorbed and transformed into solid sulfur (S) or sulfated iron compounds; (iii) adjusting the siloxane content of the biogas, wherein the step of adjusting the siloxane content further comprises the steps of: (e) to convey the biogas, through a piping system, to an adsorption column; (f) conduct the biogas through the adsorption column filled with activated carbon, where the siloxanes present in the biogas are adsorbed; (g) conduct the biogas to the outlet of the piping system; (iv) compress the biogas; (v) store the compressed biogas in cylinders.

2. Process according to claim 1, characterized in that the biogas content is: Moisture (H2O): maximum of 1% or 10 mg / m³ 3; hydrogen sulfide (H2S): maximum of 10 ppm; siloxanes: maximum of 10 ppm.

3. Process according to claim 1, characterized in that the biogas cooling temperature in step (b) is between 3°C and 5°C.

4. Process according to claim 1, characterized in that sensors are installed to measure the concentration of siloxanes in the biogas before and after the adsorption column.

5. Process according to claim 1, characterized in that, in step (iv), the biogas is compressed to a pressure between 200 and 250 bar.

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