Reminder of the risks and safety measures for anaerobic digestion plants

By | 2018-01-10

Safety Precautions for Anaerobic Digestion Systems

A number of important safety issues and potential risks for humans and the environment exist when constructing and operating a biogas plant : explosion, fire, asphyxiation, poisoning, surface water leakages, etc. Taking proper precautions and safety measures have the aim of avoiding any risks and contribute to ensuring a safe operation of the plant.

Plant designers and operators must work hand in hand to identify health and safety risks and take actions to mitigate them.

Depending on your region, here are a few guides that you might find useful:

The summary below identify the major hazards associated with an AD facility and presents some necessary safety measures.

1    Biogas

The biogas produced by an anaerobic digestion plant is composed of combustible gases methane (50-75%), carbon dioxide (25-50%), water (H2O), nitrogen (N2), oxygen (O2 ), hydrogen sulfide (H2S), ammonia (NH3), and trace elements (organo-halogenated, siloxanes, etc.). H2S, CO2, and water make the biogas very corrosive.

The composition can vary according to the nature of the incoming substrates and the operating conditions.

Properties of Biogas

 

2    Health & safety risks associated with a biogas plant

All the following risks are easily mitigated if health & safety are engraved within all phases of a biogas project development.

  • Fire
  • Explosion
  • Confined space hazards
  • Gas poisoning (H2S, NH3)
  • Hydraulic discharge
  • High-pressure gas or liquid leaks
  • Rotating mechanical equipment
  • Pathogens (diseases)
  • Electrical system hazards

Biological hazards (pathogens)

Wastes of animal and human origin, used as AD feedstock, contain various pathogenic bacteria, parasites and viruses. Pathogenic species that are regularly present in animal manures, slurries and household waste are bacteria (e.g. Salmonellae, Enterobacter, Clostridiae, Listeria), parasites (e.g. Ascaris, Trichostrangylidae, Coccidae), viruses and fungi.

Chemical hazards

Due to their toxicological properties, ammonia, hydrogen sulphide or carbon dioxide expose operators to safety hazards. To reduce the risks, it is essential to maintain the threshold limit value (TLV).

 

Methane exposure levels and effects

Exposure level (ppm)        Effect or symptom

1000                                        NIOSH 8-hours TLV*

50,000 to 150,000               Potentially explosive

500,000                                 Asphyxiation

Asphyxiation

Biogas generation, transportation and flaring can lead to oxygen-deficient atmospheres. The biogas accumulation in a confined space can significantly reduce the level of oxygen (anoxia) and result in poisoning or asphyxiation symptoms and even death.

The minimum regulatory oxygen content is 19%. Following are the various asphyxiants that are typical constituents of biogas:

• Simple asphyxiants – carbon dioxide and methane
• Chemical asphyxiants – ammonia and hydrogen sulfide

Fire and explosion

Under certain conditions, biogas in combination with air can form an explosive gas mixture. The risk of fire and explosion is particularly high close to digesters and gas reservoirs.

To generate an explosive atmosphere, the following conditions are met simultaneously:

  • Presence of a combustible gas: methane (CH4)
  • Presence of an oxidant: oxygen from the air
  • Presence of an ignition source
  • Concentration of the combustible gas included in its explosive range (LIE-LSE)
  • Presence of a confinement.

For a biogas composed of 60% methane and 40% carbon dioxide, the range of biogas concentrations in the air necessary to reach the explosive range is between 8.5 and 20.7.

 

Explosion triangle for biogas

Figure: Explosion triangle for biogas

In Europe, explosion safety measures are stipulated in the European Directive 1999/92/EC. Explosion hazardous places are classified in terms of zones, based on frequency and duration of the occurrence of an explosive atmosphere.

Overview of hazards in biogas plants (German Biogas Association)

 

Safety precautions (French National Institute for Environmental Technology and Hazards)

General

  • Use corrosion-resistant material to water or sulphur products;
  • Equipment should be designed not to let biogas in or out;
  • Electrical installations must comply with standards and regulations.

Regarding the fire hazards:

  • The constituent materials, in particular digesters or the combustion unit, must be non-combustible;
  • The installations must comply with the provisions of Article R4216 of the Labor Code: provide specific extinguishing systems inside the premises of the detection systems;
  • Storage inside the premises must be avoided;
  • Petrolium products storage should respect the Decree as of July 1, 2014 setting up the technical rules and applicable security for storage of petroleum products.

Digesters, post-digesters and biogas storage tanks

  • Install a device to protect against risks of explosion;
  • Safety valve: digesters and biogas storage tanks must be equipped with safety devices (safety valves) that prevent excessive vacuum or overpressure;
  • Sluice gates: have 2 sluice gates for one digester in the case one of them breaks down. Moreover, there should be a possibility to lock one of the sluice gates manually;
  • Mixer: get a backup power supply.

Biogas desulphurization through the injection of air

  • Inside the digester or post-digester, air metering pump should be set so that the air flow does not exceed 8% of biogas volume produced at the same period;
  • Digester inlet piping should be equipped with a check valve that will prevent the biogas from flowing back.

Combustion unit

Set up the following safety devices:

  • Stop button located outside the premises to allow emergency stop of the motor;
  • Pneumatic shut-off valves;
  • Flexible anti-vibration couplings.

Flares

  • Gas flares must be equipped with a flame arrester;
  • It must have a ventilation device.

Condenser

  • Make sure that during steam dump no biogas leaks outside;
  • Install detectors of high and low levels connected to the pump stop.

Digestate storage tank

  • If the tank is covered, it is important to ensure that it is sufficiently ventilated;
  • Install the detector of high level connected to the digestate supply.

Technical premises

Technical premises should be sufficiently ventilated.

Piping

Separate biogas and substrate piping. Biogas piping should be as well:

  • Airtight and tested before first use;
  • Fluid, pressure and corrosion resistant;
  • Easily accessibe and preferably placed on the surface;
  • Should consist of welded sections and not to go out of confined spaces, especially the premises;
  • Provide the slopes to evacuate the corrosives and condensate;
  • Piping holding the substate should be protected against freezing;
  • Security valves should be placed upstream of the installations intended for production, storage, treatment and use of biogas;
  • Systems that trigger security valves should be installed in easy to access places.