SAMPLING OF BIOGAS AND BIOMETHANE
FROM METHANIZATION

Biogas and biomethane from methanization

SOCLEMA offers solutions for sampling biogas and biomethane from methanization: sampling probes, cyclone separators, membrane separators, regulators, traced tubing, etc.

As a specialist in industrial sampling, SOCLEMA reminds you of the guidelines for biogas and bio-methane sampling.

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BIO-METHANE: GAS QUALITY AND MONITORING

Methanization is a technology based on the degradation of organic matter by micro-organisms, under controlled conditions and in the absence of oxygen, i.e. in an anaerobic environment.
This degradation produces:
- a moist product, rich in partially stabilized organic matter, known as digestate. The digestate is generally returned to the soil, possibly after a phase of maturation by composting.
- biogas, a gaseous mixture saturated with water at the digester outlet and composed of approximately 50% to 70% methane (CH4), 20% to 50% carbon dioxide (CO2) and a few trace gases (NH3, N2, H2S).

Biogas has a high calorific value and can be used in combustible form to generate electricity and heat, to produce fuel, or for injection into the natural gas network after purification.
Methanization is a widely used process in agriculture, but also in waste treatment, sewage sludge treatment ( STEP ) and certain industrial effluents . Biogas requires further purification before it can be used as a biofuel, fed into the gas distribution network, or transported and stored.
The process known as"upgrading biogas to biomethane" involves removing the CO2 and raising the methane content to around 98% by volume. The finished product, called bio-methane, has a higher quality and calorific value than biogas. It has chemical properties similar to those of natural gas, and is often injected directly into the natural gas network, where it is stored and distributed.

At European level, standards EN 16726, EN 16723-1 and EN 16723-2 set out the gas quality characteristics, parameters and their limits for assessing the quality of gas and bio-methane that is transmitted, injected into and from storage, distributed and used, guaranteeing conditions of safety and continuity of service, in accordance with current legislation.
Bio-methane standards describe:
- minimum chemical and energy characteristics of bio-methane,
- analysis and sampling methods,
- odorization,
- data connection and metrological characteristics of different measurement systems.

IDEAL IMPLEMENTATION OF RENEWABLE GAS SAMPLING

Gas sorption and surface treatment

Some constituents of renewable gases are highly susceptible to gas sorption effects associated with materials normally assumed to be inert. Material surfaces can become active as a result of contaminant build-up. It is therefore necessary to choose very carefully the materials and the surface condition of these materials that come into contact with the sample. Particular attention should be paid to sorption effects in sampling systems for the analysis of trace compounds. Sorption effects observed on certain materials can be modified and often reduced by surface treatment. Any clean, grease-free surface shows a lower adsorption capacity. Rough surfaces can form a core that favors adsorption and gas accumulation. Polishing techniques can also minimize the effects of adsorption and reduce the time required for conditioning, bringing sampling equipment into equilibrium.

Sampling materials

The selection of materials for sampling systems depends on the gas to be sampled. Due to the likely presence of small quantities of sulfur compounds, mercury, carbon dioxide, etc., in natural gas, all devices and connections should preferably be made of stainless steel or, for low-pressure gases, glass. Sampling moist, high-temperature gases, or gases containing hydrogen sulfide or carbon dioxide, presents additional material problems. This type of gas may require special materials and coatings for the sampling system.

Contamination

To avoid contamination, compounds must be carefully cleaned. Some contaminants are highly sorptive.

Sample condensation

All components of a sampling system must be kept at least 10°C above the dew point temperatures of possible gas mixtures. It is necessary to keep the temperature as constant as possible, as temperature fluctuations lead to different gas sorption effects. It may be necessary to use thermal insulation combined with heat tracing.

Flow disturbance

The most important thing is to avoid dead volumes in the sampling system, where mixing and stagnation occur.

Response times

System design must take into account the response time, which includes the internal volumes of the equipment. Response time should be kept to a minimum.

SAMPLING EQUIPMENT

The sampling equipment used for renewable gases from methanization is virtually identical to that used for natural gas. The equipment is proportioned according to pipe size (smaller) and gas flow rate (lower).

Sampling rods or probes

For liquid-free lines with temperatures well above the dew point, any type of rod is suitable. For lines where temperatures are close to the dew point, a specific rod is required to avoid problems associated with condensation and liquids entrained in the gas flow. To avoid altering the composition of the gas, liquids must be separated at source temperature and pressure.
The most basic sampling tube is a straight sampling tube. The other type of sampling tube frequently used in the gas industry is the expansion sampling tube. It is frequently used with continuous analysis systems. Expansion takes place at the end of the rod, through which the gas flow passes. The temperature drop is compensated by the thermal mass of the gas flow. We recommend installing the probe in the first third of the pipe. For pipes with a diameter of more than 30 cm, it is necessary to install the rod at least 10 cm from the wall. The sampling tube must be fitted with a suitable valve system, enabling the sampling line to be disconnected from the process line. It can be permanent or temporary.

Tubes for sampling lines

Sampling lines should be as short as possible and with the smallest possible diameter, without excessively reducing flow. The flow rate in the lines must be chosen so as to have a reduced response time.

Filters, membranes and separators

Filters and membrane separators may be necessary to obtain a clean, dry sample. It must be remembered that these systems must not alter the representativeness of the sampled composition. In general, separators are not recommended in sampling systems, but they can be used to ensure that no liquid enters the analyzer or sample cylinder. The pressure and temperature of these devices must match the pressure and temperature of the source to avoid altering the composition of the sample during the sampling process.

Safety valves

Service valves should be included at the inlet and outlet of the system; shut-off valves should be ball valves; needle valves are for flow control and regulation; relief valves should be installed to protect equipment and components. To minimize potential leakage points, the number of valves and fittings should be kept to a minimum. Elbows should be avoided; bent tubes are preferable. Valves with integral fittings are generally preferable.

Pressure regulators

In order to supply the analyzer with gas at the appropriate pressure, a pressure regulator is often required at the beginning or end of the line. Regulators are preferably made of stainless steel or PTFE. Due to the Thomson-Joule effect, the temperature drops by an average of 1°C every 2 bar, potentially creating condensation. It is usual to prevent this phenomenon by heating to compensate for the drop in temperature. The amount of heat required depends on gas composition, expansion, pressure and temperature, flow rate...

Heating units

Heating elements can be installed on the sampling probe or lines. They must be of the self-limiting type and comply with the electrical standards of the areas where they are used.