Whether you are designing, constructing or operating a biogas plant, you need to identify health and safety risks and plan measures accordingly to either eliminate or mitigate them to make your facility as safe as possible. It is a crucial step to avoid injuries and deaths.
Frequently, however, human errors, equipment breakdowns or equipment misuse are the causes of accidents. Thus, accidents are often avoidable if you anticipate the risks or you plan rigorously your project.
The summary below identifies the major hazards associated with an AD facility, presents some necessary safety measures, shows schemas of an AD facility and gives a few examples of past accidents that occurred in the industry.
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.
Source: Institut National de l’Environnement Industriel et des Risques (INERIS) – Translated
All the following risks are easily mitigated if health & safety are taken into account at all phases of a biogas project development. The risks include, for example:
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. It can occur because of a gas leak, creation of an explosive zone, welding, clogged or frozen pipes or others.
To generate an explosive atmosphere, the following conditions are met simultaneously:
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.
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, even death.
The minimum regulatory oxygen content is 19%. The asphyxiants that are typical constituents of biogas are carbon dioxide (CO2) and methane (CH4).
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).
Someone that is exposed to H2S concentrations of over 50 parts per million can get serious injuries or die. Such concentrations also cause pipes or steel tanks corrosion or breakdowns of the biogas engine.
High-pressure gas or liquid leaks occur when pressure goes lower or high than normal in tanks. For example, a tank breakdown can cause a flooding of digestate on site. The digestate spreads on nearby lands if the flood is not contained on time.
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.
For example, a tank spilling into a slurry tank can create projections and aerosols containing microorganisms.
Source : Institut National de l’Environnement Industriel et des Risques (INERIS) – Translated
Regarding the fire hazards:
The flares are necessary in risk control on a biogas plant. If there are problems or the plant is on start-up phase, the flares have to treat the biogas. The biogas cannot be sent in the atmosphere.
Technical premises should be sufficiently ventilated.
Separate biogas and substrate piping. Biogas piping should be as well:
The operator and the plant designer have to take certain measures at every step of a project. The goal is to ensure safety and minimize risks.
This step is particularly important to ensure the safety of the biogas plant. The operator and the plant designer have to pay attention to:
Commissioning of a biogas plant can be the most dangerous step of a project.
Accidents that can happen include:
During this step, a lot of accidents and incidents happen. To avoid them, the operator must:
In Europe, there were about 800 accidents on biogas plants between 2005 and 2015. Fortunately, less than a dozen of them had consequences on humans. Here are some examples.
In 2018, an explosion followed by a fire occurred at the level of the gasometer in the post-digestor of the farm plant in Saint-Fargeau, France. The accident happens during the initial test for the operation of the agitator when its propeller was replaced.
In 2005, a large quantity of H2S leaks in the charging hall of the biogas plant of Rhadereistedt, Germany. The workers don’t follow the procedure to discharge the truck because of a breakdown of the lid covering the tank. The tank containing animal and dairy waste is left opened.
There is a number of guidelines on required safety measures for an anaerobic digestion plants. You can read them to get more information on this topic.
The document Safety rules for agricultural biogas plants (l’Institut national de l’environnement industriel et des risques de la France) (In French only) include information about the operation and maintenance of a biogas plant. It contains critical measures to prevent risks during the operation, reception, starting, testing and maintenance of biogas plants, to intervene in confined spaces, in the digestors, post-digestors and storage tanks.
Depending on your region, here are a few guides that you might find useful:
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