biogas drying systems


What is biogas drying?

Biogas drying is an essential process in biogas production, which has grown exponentially in recent years and whose future looks promising. It is regarded as the main replacement for natural gas, both as fuel for vehicles or for injection into the natural gas network.

Renewable energy, including wind power, tidal energy, solar energy, biomass, and biofuel, is striving to become the leading form of energy production.

Biogas, the gas produced by the anaerobic (oxygen-free) decomposition of organic matter, is amongst the different energy types that have their origin in biomass. It is mainly composed of methane (CH4), which gives it the characteristic of a combustible gas.

Therefore, biogas from landfills, methanisation facilities and wastewater treatment plants (WWTPs) is a valuable material for the production of energy, biofuels and chemical products such as hydrogen and methanol.

From a technical perspective, biogas is a multi-component mixture of gases, with a basic composition (CH4, CO2, H2, O2, N2, water vapour, etc.) and a number of harmful components (NH3, siloxanes, halogenated and heavy hydrocarbons, H2S, etc.). Its composition is closely related to the input material of the anaerobic digestion process, the technology used for its production and the type of process selected.

Biogas quality must be enhanced for application and use in energy production and/or as a raw material in the manufacture of chemical products. Improving biogas quality involves reducing moisture content and removing all the harmful components indicated below.

This article provides a summarised description of drying/ dehumidification technology for the reduction of moisture content and the removal of harmful components from biogas.

Biogas drying, also known as biogas dehumidification, is understood to be the partial or total removal of the moisture, mainly water vapour, in the biogas stream.

Water vapour

Water vapour drastically reduces the net calorific value (NCV) of biogas, thereby affecting the energy performance of elements in which it is used as a biofuel (engines, turbines, boilers, burners, etc.). Therefore, prior to using biogas as energy, it is advisable to reduce its moisture content as much as possible by any means available. Such moisture reduction is also necessary to prevent the accumulation of condensates in the gas line and thus prevent the formation of corrosive acids, as well as the clogging of pipes. The water vapour content of the biogas is directly related to both the operating temperature of the biogas production system and the ambient temperature in general.

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Techniques for separation/removal of moisture (water vapour) from biogas

Many separation/removal techniques are applied to reduce moisture content, including, for example, adsorption with silica gel or dehumidification with glycol. Nonetheless, cooling and condensation are two biogas cleaning techniques/ operations which, in combination, offer very useful benefits. The main aim is to reduce moisture and also to remove, insofar as possible, components such as halogenated and heavy hydrocarbons as well as siloxanes, depending on the operating temperature.

Research on an industrial scale has demonstrated that the lower the cooling temperature used, the more effective the removal of moisture and other components such as siloxanes (mainly type D), and both halogenated and heavy hydrocarbons.

Condensation technique:

  • Droplet separator
  • Hydrocyclones
  • Steam traps
  • Water seals
  • Condensate pots

Drying technique:

  • Cooling
  • Adsorption with silica gel
  • Glycol
  • Heating

A biogas drying facility normally has the following elements:

  • Heat exchanger, generally of the shell and tube type
  • Condenser
  • Refrigeration unit
  • Accessories

A typical biogas drying facility is usually comprised of the following elements: condensate pot, heat exchanger in vertical position, droplet separator and refrigeration unit.

Some biogas drying facilities are equipped with an energy recovery unit/economiser for the purpose of minimising energy consumption required for the operation. Not only does this reduce operating costs, but it also enables lower fixed costs because less powerful refrigeration units can be used. The use of economisers depends on many factors, including the flow of biogas treated, temperature and the purpose of the facility. In 2008, Energy & Waste launched a heat exchanger/ economiser-cleaner for biogas cleaning. This system was the result of intensive R&D&i and affords efficient biogas cleaning combined with minimal energy consumption.

It is also possible to design biogas cleaning facilities featuring an economiser for the purpose of reducing moisture and removing siloxanes and hydrogen sulphide (H2S). The different equipment of which the technology is composed can be seen.

Chillers are at the very heart of biogas dehumidification processes. They supply the refrigerant, generally glycol water, to cool the heat exchanger where the biogas is cooled. Careful selection of the chiller is therefore vital to the correct operation and durability of the facility.

Current studies related to energy saving at such facilities (large facilities) focus on the use of adsorption chillers and research is being carried out into the use of exhaust gas from the CHP system to generate the refrigeration needed to cool the biogas, thereby closing a particularly interesting energy loop.

Types of biogas drying facilities

There are currently two basic types of biogas dehumidification facilities on the market, depending on the position of the heat exchanger unit: facilities with a horizontal arrangement and facilities with a vertical arrangement.

When possible, we always recommend to intall amvertical biogas drying/dehumidification facility, which offers the following benefits:

  • Smaller footprint
  • Enhanced evacuation of condensates
  • Lower probability of freezing in pipes when operating at low temperatures
  • Better heat transfer rates
  • More efficient moisture and contaminant removal

I need to dry my biogas

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