Unique and Essential Safety Features
Biogas is a great renewable energy source. It is created through natural biological processes, and the generation of biogas often results in a biologically stabilized waste product that can be reused and oftentimes sold as a commodity. The biogas industry has grown in the past two decades, gaining acceptance worldwide as a feasible renewable energy source. Along with that growth has come an increased need to address safety concerns.
There are some inherent dangers associated with biogas production plants. As more biogas plants are installed worldwide, there is increased potential for an accident that can have devastating effects on the environment, operations personnel and equipment. Statistically, even though accidents are rare, just one major accident can result in direct substantial costs and tarnish the reputation of all parties, including the biogas system supplier, the design engineer, the system owner and the renewable energy industry.
Besides risks that are typical of every production plant (fall hazards, chemical hazards, etc.) there are unique risks and potential dangers inherent in the characteristics of biogas itself. Biogas is composed of a large amount of methane (50 to 80 percent), carbon dioxide (20 to 50 percent), hydrogen sulfide and other gaseous or aerosol particles. Therefore, suffocation and asphyxiation from having no oxygen, toxic effects from hydrogen sulfide, and explosive or flammable hazards associated with the methane are a few potential dangers. There is also the potential for damage as a result of gas release due to a ruptured tank or piping.
Methods to mitigate and lower the potential for accidents include the following:
1. Apply the guidance given in the U.S. National Fire Protection Association 820 reference standard.
2. Use of warning signs at all potentially hazardous locations with respect to the conditions potentially present from the release of biogas.
3. Implement adequate equipment system redundancy and the multiple level safety net approach, in areas that have the greatest risk.
4. Follow sound engineering design practice for the collection and handling of biogas for existing and new facilities.
5. Provide detailed operations and maintenance information, standard operating procedures and operator training for the biogas systems.
Fire and Electrical Safety
The NFPA is the primary author of recommended standards for fire protection and electrical codes in the U.S. NFPA 820 is a set of fire safety standards for wastewater treatment and collection facilities. It sets standards for classifying areas (Class 1, Division 1, Class 1, Division 2, etc). These area classifications are then used by electrical installers of electrical components where an ignition might come in contact with flammable gas. NFPA 820 is not a code (law); however, some states have codified it in their state statutes. It is also primarily focused on municipal wastewater treatment facilities, but it contains some very good and practical recommendations for safety at industrial biogas plants and is followed by many biogas plant design engineers. Some suppliers and contractors may say that NFPA 820 is not code and is relevant only to municipal wastewater plants, so the recommendations do not need to be followed on an industrial project. The view of Symbiont Inc., a Milwaukee-based engineering and consulting firm, is that the NFPA 820 standard contains valuable design guidance information for protecting customers’ assets and personnel from fire and explosions at biogas plants and should be used as a design resource at every biogas facility. By applying the standards set forth in NFPA 820, all project affiliated parties are protecting themselves.
What is obvious to one employee may not be so obvious to others. A good practice in every plant is to include well-labeled, visible, and clearly understood signs that provide warning or reminders of the potential dangers in a particular area. Posted signs should include what personal protective equipment is needed to enter an area, or read, DO NOT ENTER.
Equipment component redundancy and a multilayered approach should be used in areas that pose a greater risk for biogas release. Some examples of the use of redundancy are pressure-vacuum relief (PVR) valves typically located on the biogas dome space of anaerobic digesters or on biogas storage systems. Typically, it is advisable to pair two PVR valves in parallel, with one being active and the other used as a standby. This is done so that the active PVR can be periodically switched to a standby status for maintenance while still protecting the tank by switching the standby unit to active. A multilayered safety approach is typically used when a failure could result in catastrophic damages or loss of life. These safety systems often involve both electronic programmable logic control (PLC) devices and mechanical systems so that if one system fails, the next system in the series will prevent an accident.
Another example of a multilayered safety approach is the safety systems used to prevent a tank from overflowing. Tanks usually contain a pressure transducer at the bottom of the tank or some other means of measuring the liquid level in the tank. In the first safety level, a sensor sends a signal to the PLC if the liquid level in the tank approaches a high level, and the PLC alerts operations personnel of the status. Most tank systems will also include a float switch at, or near the overflow level of, the tank that will alert operations personnel that the tank is approaching an overflow event—this is the second level. If the liquid level in the tank continues to rise unchecked, it will begin to flow out of the overflow pipe. The overflow pipe can be connected to another tank either upstream or downstream and temporarily flow into this tank instead of overflowing on the ground—the third safety level.
Finally, in the unlikely event that all secondary tanks become full and there is nowhere to go with the excess liquid, a berm system can be built around the tank system to prevent any overflow from reaching environmentally sensitive areas. The berm can also include a float switch that will alert personnel that the final safety level is being used, or is being breached.
Think Safety From the Beginning
Risk mitigation and safety begins at the conceptual stage and should be incorporated into the design of the biogas system from the very start. Good engineering practice involves more than just doing what is required; it involves a multistep thought process from the onset of design all the way through to commissioning of the system. Designers need to think forward when designing systems and place safety features where potential hazards might be present. An often-missed step in the design and commissioning process is thorough QA/QC, throughout the design process and prior to the release of the design for construction.
During biogas plant commissioning, every biogas system provider should offer thorough operator training. Most accidents are the result of human error, and personnel working with and operating the system on a daily basis will, statistically, have the tendency to make errors that cause an accident. Generally, operators are very conscientious of their work, but if they have not been adequately trained with the system, there is a greater chance of an unwanted incident. Designers need to make sure all staff understand the system and know how to operate it before the system supplier leaves the site. It should be stressed to operators that they need to think before they act, and apply the rules of cause and effect to each operation they perform.
Overall, it is important to understand that safety is a team effort that the supplier, designer and owner of the biogas system should be vested in from the beginning of the design throughout the entire life of the system. The mindset should be that a biogas plant is a long-term investment with safety being a high priority. If the safety techniques discussed above are applied, it is likely to result in a safe and profitable biogas plant.
Author: Michael O’Neil
Project manager and Senior Process Engineer, Symbiont