Moisture management is an essential part of biogas process design. If overlooked, it can lead to corrosion, equipment failures and unplanned downtime. A drying investment should therefore be evaluated not only by its installation cost, but also by the maintenance and downtime costs it helps prevent.
Why does moisture management matter?
Biogas leaving the digester is generally saturated with water vapour. As the gas cools in the pipework, water vapour condenses. H₂S and CO₂ dissolving in this condensate can create an acidic environment that accelerates corrosion in pipelines and equipment. Accumulated condensate can restrict gas flow and increase pressure loss. In cold conditions, it can freeze and block the line. Liquid water carried into engines and combined heat and power (CHP) systems can also cause operating problems and equipment damage.
Moisture management should be designed alongside the H₂S removal process. The aim is to prevent condensation while maintaining the operating conditions required by the treatment equipment. Since a certain humidity range supports H₂S removal with some activated carbon grades, making the gas as dry as possible at every stage is not necessarily the right design objective.
If the gas is to be upgraded to biomethane, its water content must meet the requirements of the upgrading process and the end use. BioLNG production requires deep dehydration to prevent ice and hydrate formation during cryogenic cooling.
Drying and moisture management methods
Condensation and cooling
This is the most common first step. Condensate forming in correctly sloped pipework is removed through collection points and condensate traps. Active gas chillers cool the gas to condense part of the water vapour, and the resulting droplets are then removed by separators. This is a widely used approach for CHP gas supply and pre-drying before further treatment. The dryness achieved depends on the cooling temperature and operating pressure.
Adsorption (desiccant beds)
Materials such as silica gel, activated alumina and molecular sieves adsorb water. Depending on the material and system design, very low water dew points can be achieved. Adsorption can be used to meet gas grid quality requirements, while molecular sieves are generally preferred for the deep dehydration required before BioLNG production. Continuous operation is maintained by alternating beds between drying and regeneration.
Absorption (glycol)
A liquid desiccant, such as triethylene glycol, absorbs water and is then regenerated using heat. This method can be considered particularly for high-flow gas processing applications. Its suitability for biomethane plants depends on flow rate, pressure, target water content and process integration. The operating and maintenance requirements of the regeneration unit and glycol circulation system should also be considered during selection.
Comparing the methods
| Method | Drying performance | Scale considerations | Capital expenditure (CAPEX) | Main operating expenditure (OPEX) drivers | Typical applications |
|---|---|---|---|---|---|
| Condensation and cooling | Limited dew point reduction | Applicable across a wide range of flow rates | Generally low–medium | Cooling energy, maintenance | CHP, pre-drying |
| Adsorption | Low or very low water dew point | Sized according to flow rate and required dryness | Generally medium–high | Regeneration energy, desiccant material, gas consumption or losses | Meeting gas grid quality requirements, deep dehydration before BioLNG production |
| Absorption (glycol) | Design-dependent dew point reduction | Can be considered particularly at high flow rates | Depends on process scope; generally medium–high | Regeneration heat, pumps, glycol losses | Suitable gas processing and biomethane applications |
These comparisons indicate general trends. Flow rate, pressure, inlet moisture content and required dryness can change the outcome. The reference pressure must always be specified when comparing water dew points.
How dry does the gas need to be?
The required dryness is determined by the end use and equipment specifications. For gas used exclusively in CHP systems, condensate drainage, active cooling and droplet separation may be sufficient in many applications. However, the final design must follow the engine manufacturer’s gas quality requirements and account for the lowest operating temperature in the gas line.
Biomethane intended for grid injection must meet the relevant grid’s water dew point requirement at the specified operating pressure, or its water content limit. The drying method is selected according to these requirements and the biomethane production process used.
For BioLNG production, the water content must be reduced to the very low level required by the liquefaction process. Molecular sieve dehydration is generally used for this purpose. Insufficient drying can cause blockages in cryogenic equipment due to ice and hydrate formation.
In cold regions, condensate drainage points and pipe sections where water can accumulate may need protection against freezing. Insulation and heat tracing are applied where required.
How should moisture be monitored during operation?
Moisture management does not end with dryer selection. The target water dew point and reference pressure should be defined, while outlet conditions, condensate drains and dryer performance should be monitored throughout operation. In adsorption systems, bed saturation and regeneration performance should also be monitored.
Where necessary, reheating after cooling reduces the gas’s relative humidity and helps manage the risk of condensation in downstream pipework. However, reheating alone does not remove water from the gas; temperatures along the line must remain above the gas’s water dew point.
How should a drying investment be evaluated?
Evaluating drying solely by installation cost can be misleading. Inadequate moisture management can increase total operating costs through corrosion, unplanned downtime and shorter equipment life. Excessive drying, on the other hand, can add unnecessary capital expenditure and energy consumption.
The right approach is to select a system that reliably delivers the dryness required by the end use.
Uestco has been developing turnkey solutions for biogas and natural gas process applications since 2006. By considering moisture management alongside H₂S removal, compression and BioLNG production, Uestco assesses drying requirements according to each plant’s process conditions and end use.
Frequently asked questions
Why does biogas need to be dried?
Water vapour in the gas leaving the digester can condense as the gas cools. The resulting liquid water can cause corrosion, flow problems and equipment damage. Drying and condensate drainage are used to control these risks.
What level of drying is sufficient for CHP?
In many applications, condensate drainage, active cooling and droplet separation may be sufficient. The required conditions should be determined according to the engine manufacturer’s gas quality specifications and the lowest operating temperature in the gas line.
Is adsorption mandatory for biomethane grid injection?
The key requirement is compliance with the relevant grid’s water dew point or water content limit. Adsorption is one available method; the choice depends on the gas conditions and process design.
Why does BioLNG require deep dehydration?
During cryogenic cooling, residual water can form ice and hydrates that block equipment. The water content must therefore be reduced to the very low level specified by the liquefaction process. Molecular sieves are generally used for this purpose.
We can assess your drying requirements together, based on your process conditions and the gas’s end use.
