In late 2009, a study carried out by the Commission for Plant Safety of the German Federal Environment Ministry revealed critical defects in more than 60 percent of biogas facilities inspected. The study inspectors, which also included TÜV SÜD experts, detected weaknesses not only in the gas and ventilation systems, and regarding explosion protection, but also in component design, structural engineering and organizational measures.
This result confirms that stakeholders in practice still tend to underestimate the scope of required safety measures. Ensuring the safe operation of biogas facilities requires consideration of questions related to the gas, the electrical and the pressure systems. Other significant issues are related to fire safety and lightning protection, and to the layout and planning of escape routes and emergency response plans. Potential hazards to health and the environment also need to be limited.
Responsibility Rests with the Operator
Biogas plants process large quantities of combustible and toxic gases which pose increased fire, explosion or suffocation hazards in case of faults in design, materials or control. In the event of an incident at the plant, people may be injured, property damaged and the environment (air and water) polluted.
In this context, the operators of biogas plants have a high level of responsibility: Their duties include conducting the necessary inspections, ensuring safety and health documentation of sufficient explosion protection and expert training of employees. Operators violating these duties risk that the operation of their plants is no longer in compliance with the law, which may result in a shutdown of the plant and in restriction or even loss of insurance coverage.
Targeted Safety Assessment
Generally, agricultural biogas plants comprise a reception pit for collecting and preparing the slurry, a fermenter in which the biogas is produced, a final digestate storage tank and a combined-heat-and-power (CHP) unit in which the biogas is converted into electricity.
Biogas consists of methane (50 to 80 percent), carbon dioxide (20 to 50 percent), hydrogen sulphide (0.01 to 0.4 percent) and traces of ammonia, hydrogen, nitrogen and carbon monoxide. The constituents of ammonium and hydrogen sulphide are two aggressive chemicals that are constantly in contact with the tank walls, pipes and valves. Given this, the materials used for these components need to be highly resistant to chemicals and maintain this resistance over long periods.
The lower explosion limit (LEL) of methane is 4.4 percent, the upper explosion limit 16.5 percent. In combination with the oxygen in the air, methane concentration in this range can produce an explosive gas mixture. These explosions can cause severe ecological damage, serious injuries to people and damage to property. To ensure effective explosion protection, the gas sensors in the plant should be adjusted to 20 percent of the LEL, equivalent to methane concentration of 0.88 percent.
Carbon dioxide causes dizziness in concentrations between 1 and 5 percent, and rapidly leads to suffocation in concentrations of over 9 percent. People should not be exposed to concentrations higher than 30 to 100 parts per million. Hydrogen sulphide is particularly hazardous. It is perceived as disagreeable at a concentration of 50 milligrams per cubic meter (mg/m3). Concentrations of 150 mg/m3 cause irritation of mucous membranes. And at levels over 500 mg/m3, hydrogen sulphide causes olfactory paralysis and is fatal within minutes.
Apart from suffocation, fire and explosion hazards, leakage of fermentation substrates into water as a result of an incident in a biogas plant may cause severe environmental pollution. In view of the fact that the composition of liquid substrates is hard to control, operators face the challenge of having to dispose of the liquid digestate cost-effectively while also ensuring groundwater protection. As the digestate contains large quantities of water, transportation over long distances does not make good economic sense. Instead, local disposal should be given preference wherever possible.
The following applies to agriculture in particular: no two biogas plants are the same. As the responsibility rests with the operators, they must have precise knowledge of the specific requirements applying to their plants and must be able to assess possible hazards in accordance with the applicable laws, which in Germany include the Ordinance on Industrial Safety and Health, the Occupational Health and Safety Act and the Hazardous Substances Ordinance.
Operators must ensure systematic implementation of these occupational health and safety measures. The plant operators must also create an explosion protection document which comprehensively assesses the explosion hazards. An important factor in this context is that the room in which the plant is installed is considered an explosion hazard zone, unless the gas-carrying parts of the plant, including the gas extraction elements and the CHP unit, are permanently technically leak proof in service.
Gas storage tanks with flexible membrane roofs or storage bags must undergo direct leak testing. The pressure applied in this test should be at least 1.5 times the maximum operating pressure or equivalent to the preset value at which the pressure-relief valve opens, whichever of the two values is the higher. It is important that the gas storage tank is appropriately gas-tight and resistant to pressure, chemical media, ultraviolet radiation, temperature and weather influences.
Protection equipment (suitability, wiring) and the planning of the structure and technical systems (material selection and design) must be customized to the specific plant and inspected at regular intervals. Extraction systems, also those installed outside the biogas plant, reliably prevent incidents such as leaking of toxic gases.
Safety and Efficiency
Frequently, comprehensive hazard assessment also helps to uncover hidden potential for savings in the operation of a biogas plant. The objective is to realize the best possible plant design within the framework defined by ordinances, standards and technical rules. By doing so, operators can assess the efficiency and competitiveness of their existing plants more precisely on the one hand, while gaining valuable information for possible future extensions or modernizations on the other.
In this type of systematic assessment, organizational measures are increasingly joining aspects of technical safety in the focus of attention. However, in agricultural biogas plants, organizational measures have frequently not yet been given sufficient emphasis. In the case of an incident at the plant, weaknesses in escape and rescue routes and in the emergency preparedness and response plans of the plant in particular may jeopardize human life.
Emergency response plans first include basic rules on how to behave in the case of a fire (publicly displayed notice). Second, they must establish concrete instructions for all employees on site, addressing measures such as fire prevention and what to do in the case of a fire.
To ensure an effective alarm system, the sensors of automatic gas and fire detectors must be correctly positioned, calibrated, wired and serviced. Practical tests of the alarm systems and emergency drills with staff are imperative in this context. Ensuring that the alarm signals will actually reach all people on the premises is critical in this context.
When planning escape and rescue routes, special attention must be paid to the transition areas between rooms and buildings. Lockable doors in escape routes must be equipped with a specific mechanism ensuring that the door can be opened from the inside even if locked. Manually operated doors must always open in the direction of escape. In addition, steps must be taken to ensure that emergency lighting is both independent from the main supply and explosion-proof (in line with the relevant ATEX zone) and that emergency routes are sign-posted throughout.
Discussing and coordinating the rescue and escape plans with the local fire service is also highly advisable. During plant operation it is imperative that the escape routes are kept free from blockage by objects. This applies all the more as all material stored there may increase the fire loads.
In addition to a detailed and comprehensive occupational health and safety program, the operators of biogas plants must also increasingly focus on system-related and organizational safety measures. The task at hand is to find the ideal plant solution in terms of safety and cost-
effectiveness, while ensuring compliance with ordinances, laws and regulations. TÜV SÜD's experts have long-standing experience in the assessment and inspection of biogas plants and advise operators on plant optimization.
Authors: Johannes Steiglechner
Combustion Systems and Heat Engineering, TÜV SÜD Industrie Service GmbH
Biogas Centre of Competence, TÜV SÜD Industrie Service GmbH
+49 (0) 89 5190-1027
(UK implementation is under the DSEAR. See http://atexanddsear.co.uk/ for UK Gas Safety Regulations Compliance assistance. - Editor)