BIOGAS CLEANING

Origin

In the biogas production process, water is an intrinsic part of both the material to be digested (biomass) and the process itself. Furthermore, water is the medium in which biogas production takes place, regardless of whether the digestion process is dry or wet, the biogas produced will contain water vapour. The amount of water vapour contained in the biogas generated in the digester varies depending on the temperature at which the anaerobic digestion process takes place: psychrophilic, mesophilic and thermophilic and the pressure at which it is in the digester (reactor).

The biogas in the digester is saturated with water vapour. In general, the absolute humidity of the biogas moves in a range between 2.5 to 7% of its volume at room temperature.

Figure 1 shows the typical biogas production curve depending on the temperature and type of process, as well as the amount of H2O vapour that said biogas contains at the process temperature for each Nm₃ of biogas.

Figure 1. Biogas production curve as a function of the process temperature and amount of water vapour in the saturation condition. Taken from THEORETICAL-PRACTICAL GUIDE ON BIOGAS AND BIODIGESTERS

This water vapour that contains the biogas has a certain degree of acidity that is due, on the one hand, to intimate contact with acid gases such as H_2S,CO₂ and volatile organic acids (VFA) present in the biogas and, on the other, due to the chemical and thermodynamic equilibrium with the substrate (liquid phase) that gave rise to it. This liquid, being part of the digestion process (acidogenesis stage where different types of organic acids are produced) when in contact with them, increases its acidity.

Both the amount of water vapour contained in the biogas and its acidity are two aspects to take into account in a biogas installation to avoid damage and malfunction, as well as its possible use.

Effects

One of the effects of the presence of water vapour in biogas is that the LHV of biogas drastically decreases. For this reason, the energy performance of the equipment involved in its use as biofuel (engines, turbines, boilers, etc.) is affected, and thus the production of energy.

Figures 2 and 3 show the interrelation between temperature, water vapour content in biogas and its LHV. And how the moisture content affects the energy performance of the combustion engine (ƞ = 40%).

Figures 2. Variation of LHV of biogas vs temperature. figures 3. Variation of the power vs temperature of the biogas.

Another effect produced by the presence of water vapour in the biogas is the formation of condensates due to the cooling suffered by the flow of biogas downstream when it comes into contact with colder parts of the installation that are at ambient temperature. If these condensates are not eliminated, they accumulate. Besides, they produce the formation of plugs or hydraulic seals that obstruct the transport of biogas and, in case of low temperatures, it can even freeze, forming ice plugs

Figure 4 shows the formation of plugs or hydraulic seals that produces the continuous condemnation of water vapour in pipes due to the accumulation of condensates.

Figure 4. It shows the formation of condensate plugs due to steam condensation that takes place in the transport of biogas.

In turn, the formation and accumulation of condensate facilitates the formation of corrosive acids such as hydrogen sulfide (H₂Saq), which is produced when it comes into contact with acid gases such as hydrogen sulfide (H₂Sg) and CO₂ (also present in biogas). This, therefore, facilitates corrosion and deterioration of the: piping system, flow machine and power generation system.

Treatments

Therefore, it is necessary to eliminate water vapour to prevent the accumulation of condensate in the gas line, equipment and machines, and, thus, avoid clogging problems, corrosive acids formation and ice in low temperatures.

There are several methods of removing or reducing water vapour in biogas. These can be classified into two large groups. Condensation methods and drying methods. Table 1 shows the most common techniques to apply for the elimination of water vapour in biogas according to the method to be applied.

Table 1 shows the techniques for removing water vapour in biogas.

Method. Condensation technique

It is one of the most used techniques and is based on the principle of condensate removal by cooling the biogas below the dew point (above saturation and part of saturation). In this type of operation, both the variation in the amount of movement experienced by the flow of biogas during its transport (change of direction and dimension of the flow) and the temperature differential between the flow of biogas and that of the environment surrounding the piping system, are used.

Therefore, in the biogas line, equipment such as condensate pots, drip traps and hydrocyclones often appear at certain lengths. When taking advantage of the thermal exchange between the biogas flow circulating through the pipeline and the ambient temperature, the pipeline is installed with a certain inclination and condensate collection traps are placed at the lowest point. Thus, the condensate that is formed flows to the lower part and is collected in the drip trap that ensures its exit from the system. This procedure is valid both for pipes installed on the surface and for underground pipes, where the cooling is even greater. A condition for this operation (so that the biogas can be cooled) is that the gas pipe is long enough to produce said cooling.

When applying these techniques, one of the precautions in cold countries or countries with long winter seasons is to avoid freezing of the condensate in equipment, pipes, drip traps and condensate evacuation devices. Therefore, they must be easily accessible, their installation must be freeze-proof, and they must be emptied regularly.

Method. Drying technique

In general, biogas dehumidification is performed by cooling the gas by bringing it into contact with a cold surface and then removing the condensate. For this purpose, the gas is conducted through a shell-and-tube type heat exchanger bathed with a cooling fluid (usually water with glycol) to avoid freezing of the cooling medium when operating at temperatures below 0 ºC. In this equipment, the coolant flows in the opposite direction to the gas absorbing the heat present in the biogas and cooling it to a temperature, at which the water vapour contained in the gas condenses. This allows the water vapour to be removed from the biogas. In addition to water vapour, this technique also removes substances such as water-soluble gases (H₂S and NH₃), different types of hydrocarbons, D₄ and D₅ siloxanes (provided that the temperature reached in the biogas is below 4 ºC in the latter case).

Since this condensation operation usually involves three phases, it is advisable to link condensation techniques (such as condensate tanks and mist separators) to this system to ensure more efficient removal of water vapour from the biogas stream.

Due to its simplicity in operation and its low operating cost, the most used method is that of cooling systems (figure 5).

Other drying techniques are also commonly used to reduce the moisture content of the biogas before its use. These include compression or cooling of the biogas, adsorption on silica gel, molecular sieve or active alumina, or absorption of the biogas using glycol solutions or hygroscopic salts. However, given their complexity and high operating cost, these techniques have been displaced by the cooling technique.

Adsorption

Adsorption is a widely used technique in gas drying. This is because it is possible to achieve high drying efficiency and thus low dew points. This is the case, for example, with biomethane, which requires a dew point above -20 °C to be transported. However, this requires high pressures of 6 to 10 bar, which somewhat limits its use in biogas drying due to its operating cost. Silica gel, active alumina and molecular sieves are used as adsorbent materials. The adsorbents used are installed in a fixed bed and are operated alternately. Saturated adsorption materials can be regenerated hot or cold.

This type of drying technique is used, for example, to adjust the moisture content of biomethane after cooling drying to ensure that it reaches a quality equivalent to that of natural gas before it is injected into gas pipelines.

Absorption

Moisture removal by adsorption is a physical process where a hygroscopic solvent such as glycol is often used to absorb moisture from biogas when they come into contact inside a packing tower. In this operation, the biogas flows in an absorbent tower against the countercurrent of a glycol or triethylene glycol solution, with which both water vapour (moisture) and heavy hydrocarbons are removed from the raw gas. However, given its operational complexity, it is usually little currently applied in biogas.

Cooling drying. Case study

Figure 5 shows the BTS-Dryer technology developed by BGasTech for biogas drying, where various moisture removal methods are combined. In addition, it includes an energy recovery system with bubble washing of the biogas with its condensate. With this BTS-Dryer technology, not only absolute humidity is removed, but relative humidity is also reduced. This degree of reduction is in accordance with the equipment’s working temperature differential. This is of interest when the dry biogas passes to an adsorption stage on activated carbon.

Figure 5. TBTS-Dryer technology. WWTP Alcalá de Henares. Madrid. Courtesy of BGasTech

Condensation temperatures in a range of 2 to 4 ºC are the most common operating temperatures in these types of equipment, which allows, in addition to reducing absolute humidity, to eliminate other components such as cyclic chain siloxanes, mainly those of type D4 and D5. If the biogas is compressed before entering the cooling stage, its moisture removal performance will be even greater, due to the increase in the partial pressure of the water vapour in the biogas stream.

Table 2 shows the results achieved in its practical application in the drying of biogas in the WWTP located in the west of the city of Alcala (Madrid).

As can be seen from Table 2, this technique can achieve a reduction in absolute humidity below the< 1% and, in turn, reduce other types of undesirable components in the biogas.