Risk assessment of large scale storage of biopellets

Transcription

1 Risk assessment of large scale storage of biopellets Helene Olsson Danish Institute of Fire and Security Technology, Hvidovre, Denmark, Abstract: Large-scale storage of biopellets is most often done in flat storages or silos where the fuel can be stored protected from the weather (rain or snow). Large-scale storage of biopellets involves a major fire risk as the material is easily ignited, highly combustible and involves a risk of auto-ignition of the material. Auto-ignition occurs due to heat generated by biological / chemical processes in the material, and the heat cannot be led away fast enough compared to the rate of heating. There are many known instances of auto-ignition in stored biomass. Since heating usually takes place deep in the stack of material an incipient fire is extremely difficult to detect and therefore to fight and extinguish in its early stage. For storage in silos and flat storages a fire gives major challenges for firefighting and major damage in the form of operating losses and environmental pollution. Fighting a fire in large quantities of biopellets is a major challenge, as the material is hygroscopic and swells when in contact with water, is difficult to move, can explode due to dust and storage facilities are often located in major constructions in connection with other premises such as a port or power plant. A fire in biopellets also implies the possibility of re-ignition after long periods of time which complicates the rescue work. As a part of the project Large scale Utilization of Biopellets for energy Applications (LUBA) funded by the Danish Energy Agency under the ForskEl program, a generic, systematic risk assessment has been performed on storage and handling facilities for biopellets. A risk analysis determines that the major risks in regards to fire when handling and storing biopellets are dust explosion, fire, auto-ignition and lack of possibilities for egress. A "fishbone analysis" has been performed analyzing possible causes that can lead to fire and/or explosion finding that the major causes are the presence of combustible material, ignition sources, oxygen, and water and the biopellet quality. To detail the fishbone analysis further a root cause analysis has been performed revealing the cause for the presence of these risks. Determining the root cause of the risks gives the opportunity to establish mitigating and limiting actions in order to minimize risk of fire and loss in case of fire. The analysis concludes that actions should be focused on Restriction of combustible material in the plant in the form of accumulation of dust and biopellets and combustible materials in plant construction (conveyor and building). Layout must comply with the applicable requirements of technical regulations. Access to the plant parts that must be cleaned, maintained and serviced are to be optimal. Building and plant parts must be tight in order to protect the biopellets from the weather and minimize spill of wood dust and biopellets. The plant must be zone classified according to ATEX directive and components selected according to this classification.

2 Focus on plant safety and maintaining an improvement culture and safety culture among staff. Escape routes on the facility should comply with the requirements of max. 25 m to the nearest escape route and two independent escape routes. Installation of efficient fire detection systems in conveyor systems and buildings. Fire sectioning of the system to prevent fire spreading through the conveyor belts to storage facilities. Installation of pressurized irrigation systems on all conveyors. Use of inerting with gas in silos for firefighting. Consideration should be given as to how stocks should be evacuated from the facility, to where and how fast. A plan for training of plant personnel and firefighters must be prepared and implemented. 1. Introduction Large-scale storage of biopellets is most often done in flat storages or silos where the fuel can be stored protected from the weather (rain or snow). Large-scale storage of biopellets involves a major fire risk as the material is easily ignited, highly combustible and involves a risk of auto-ignition of the material. Auto-ignition occurs due to heat generated by biological / chemical processes in the material, and the heat cannot be led away fast enough compared to the rate of heating. There are many known instances of auto-ignition in stored biomass. Since heating usually takes place deep in the stack of material an incipient fire is extremely difficult to detect and therefore to fight and extinguish in its early stage. For storage in silos and flat storages a fire gives major challenges for firefighting and major damage in the form of operating losses and environmental pollution. Fighting a fire in large quantities of biopellets is a major challenge, as the material is hygroscopic and swells when in contact with water, is difficult to move, can explode due to dust and storage facilities are often located in major constructions in connection with other premises such as a port or power plant. A fire in biopellets also implies the possibility of re-ignition after long periods of time which complicates the rescue work. This article describes a generic risk assessment performed as a part of the project Large scale Utilization of Biopellets for energy Applications (LUBA) funded by the Danish Energy Agency under the ForskEl program. The risk assessment was performed in relation to the design, size and location of stocks, monitoring and fire detection in stocks and fire extinction in stocks. The risk and risk barriers are divided into categories according to the principle 3xP - Plant Process People: Plant includes the buildings, transportation equipment, and control systems including fire and gas detection systems and fire extinguishing equipment. Process includes the way products and materials are handled and procedures, guidelines and rules describing the work performed. People includes the personnel working at the plant, their competencies and how they are trained.

3 Figur 1: 3xP - Plant process people in relation to working with risk of fire The risks were identified and further analyzed in a "fishbone analysis" also known as an Ishikawa diagram where causes that can lead up to an event are collected. For each of the causes a 5 why analysis was performed. The analysis is an iterative question-asking technique used to explore the cause-and-effect relationships underlying a particular problem as described by Serrat [1]. The primary goal of the technique is to determine the root cause of a defect or problem. The 5 in the name derives from an empirical observation on the number of iterations typically required to resolve the problem. By letting the answer to one question form the basis of the next question ensures a logical flow to finding the root cause(s) to how and why an event can occur. In order to address the identified causes of a fire, preventive, mitigating and extinguishing barriers are found. 2. Risks The risks to the biopellet storage related to fire protection in this study are identified as: Fire. Auto-ignition. Dust explosion. Distance to escape routes / ingress routes. PLANT Risks of fire, dust explosion and auto ignition is present due to the large amount of combustible materials in the plant. The material is present as material transported through the entire plant from the quay via conveyor belts to warehouses and further on to the furnace and also in the form of deposits of dust and biopellets in the system due to dispersion of the material while handling. The materials of the plant such as the conveyor belts can also constitute a substantial fire load. In the plant and in connection with the operation of the plant there will be ignition sources which could ignite the dust and other combustible material and cause fire or dust explosion. The pellets themselves entail a risk of auto ignition. This process is probably caused by the chemical/biological processes in the pellets, which results in heating. As the pellets have good insulating properties, the temperature will increase to a point where the material may ignite

4 and start a smoldering fire. One of the known factors that can contribute to this process is moisture. Therefor it is important to protect the stock from moisture even in a small scale. In storage of biopellets a formation of CO can be seen. This is in fire terms no risk as the lower explosion limit for CO is 13%, which would be highly unlikely to achieve in the storage of biopellets. CO, however, is one of the gases, which may be an indicator of activity in the pellets with a risk of auto ignition as a result which is why the CO concentration is often monitored. Transportation distances for the biopellets can be very long on plants for storage and handling of biopellets. As the conveyor systems must be cleaned, inspected and maintained periodically, it is necessary to make sure that personnel performing these actions have means of escape within a reasonable distance. It is also necessary for the first responders to gain access to the plant within a reasonable distance in the event of a fire. PROCESS In regards to processes the risk assessment primarily has a focus on how to avoid an accumulation of dust in the plant, especially the conveyor systems, how to manage the use of ignition sources such as spark generating tools and how and when maintenance must be completed on critical equipment. Furthermore the emergency and contingency plan is an important part of the procedures describing how to manage the risks at hand and how to act in case of an unintended incident. In the cause analysis it became evident that the two major causes for risk of dust explosion and fire are: Use of mobile equipment. Lack of procedures. The term mobile equipment in this context covers loaders, various hand tools that can create sparks, cleaning devices and mobile electronic devices. The latter point is only relevant in relation to dust explosion. Mobile equipment must be regarded as a source of ignition in dust fire- and explosion scenarios. It is therefore important to keep focus on the use of mobile equipment, particularly in areas with dust concentrations above the lower explosion limits. The "lack of procedures" in this context covers a variety of procedural descriptions. This applies to procedures that for example describe the acceptable level of cleaning for specific areas, how to handle the combustible materials and how the organization relates to smoking. The analysis has shown that in regards to the risk of auto-ignition it can be assumed that the pellets storage time and the pellets quality / reactivity can have significant impact on the identified risks. This is a field where there has been and still is conducted a large amount of research. It has been shown that the species and origin of the wood used in the pellets has an impact. One of the goals of the research is to establish how the pellet quality and behavior during storage can be monitored in the large scale storage facilities in order to prevent autoignition incidents. In relation to the risk of escape routes being too long the analysis has not uncovered additional areas that can immediately be attributed to process parameters such as lack of procedures.

5 PEOPLE The people working in a facility with storage of biopellets have a great responsibility for the maintenance of fire safety at the facility in their daily work. Amongst the important tasks in relation to fire safety is particularly the cleaning, inspection and maintenance of the plant. There are several reasons to focus on dust and to keep the amount of dust at an acceptable level throughout the process. This is achieved partly by the plant structure (dust extraction, encapsulation, etc.), partly of the processes used in the handling and storage of biopellets (method of unloading, procedures for maintenance, inspection and cleaning, etc.). Ultimately, it is the staff's responsibility to follow up on the technical installations and follow the procedures developed for the processes. Training of the staff, the staff s assessment of what is an acceptable level, and the staff s knowledge about risks and how they are dealt with is an important piece to the puzzle for maintaining a fire safe operation. In case of fire, the staff also has an influence on the extent and consequences of the incident. All employees must be aware of the procedures to be followed in connection with a fire or similar event. Furthermore, they must be aware of the opportunities they have themselves to provide first response and thus contribute to limiting the damage and the consequences of the fire. This makes training a requirement, being for example drills and training courses in for example basic firefighting. In the category people risks in relation to fire primarily consist of: Lack of risk assessments in case of changes in operating conditions at the plant, for example in capacity changes. Lack of knowledge and / or focus on risks at the facility. Lack of training in response to risks. 3. Prevention / barriers Based on the above risks a number of barriers are defined. PLANT In the plant there must be focus on not accumulating combustible material and not handling more dust than necessary. This requires plant integrity so that no or minimal spillage and / or spread of dust will take place. The dust must be collected and contained so it does not spread in the plant. This can be done for example by dust removing devices in the conveyor system, where the dust in shunts is swirled up as a consequence of the material being moved from one conveyor belt to another. The dust should be removed entirely from the system and not added back into the transportation- and storage system at a later stage. The plant should preferably not in itself contribute to the fire and should be designed in noncombustible or flame-retardant materials that do not contribute to a possible fire and will not in connection with an ignition source be able to start a fire. This applies to for example conveyors and storage buildings. Buildings should be constructed in non-combustible or difficult combustible materials. The conveyor belt should be both flameproof and self-extinguishing. Conveyor belts should be tested and classified according to recognized standards such as: EN ISO 340 "Conveyor belts - Laboratory Scale characteristic flammability - Requirements and test methods" DS / EN12882 "Conveyor belts for general purpose use - Electrical and safety requirements for fire resistance."

6 FM Global's data sheet for conveyor belts (Data Sheet 7-11) and FM Global approval standard (FM Approvals, Approval Standard for Class 1, Conveyor belting, Class Number 4998, August 1995). Since a large part of the plant is classified as explosion risk areas due to the high concentration of dust, it is important that all components are risk assessed before mounting. The risk assessment of the installation must include risks during normal operation, cleaning and maintenance. The following data for wood dust is found: Table 1: Data for wooden dust from BGIA GESTIS-STAUB EX [2] Material Particle size (median) Moisture content Lower explosion limit Glowing temperatur e K st -value Ignition temperature Explosion class P max [mm] [weight %] [g/m 3 ] [ C] [ C] [bar m /s] - bar Wood, grinding dust (2008) Wood, grinding dust (2009) Wood, sawdust (5481) Wood, sawdust, drillshavings, cutting, grinding (5636) Wood (chips/dust) (26) 0, St 1 8,6 0, St 1 9,0 <0,063 2, St 1 8,9 0,032 1, St 1-0, St 1 8,6 The values given are based on general data from BGIA GESTIS-STAUB EX [2] Number in brackets in the name field indicates the material number in the database. The specifications of the dust will entail the following requirements to equipment:

7 Table 2: Requirements to equipment in wood dust environment. Zone Material group II Material category 1 1 or 2 1, 2 or 3 Type explosive atmosphere Max. temperature of equipment Dust cloud Dust layer D 273 C (2/3*T dust cloud) 215 C T dust layer - 75 o C Sealing class IP 6X IP 6X IP 6X (conductive dust) In order to minimize heat build-up from mechanical parts low-speed bearings could be considered, as they develop less heat. To monitor the operation of mechanical equipment parts, monitoring equipment for conveyor belts should be used to ensure that a belt that has come out of track is detected immediately as it can provide frictional heat with other system components and create heat. Roller guards should monitor the rollers in the conveyor system so that a defective roller will not be able to cause overheating. Plants must generally be designed so that the quantity of dust is limited in relation to dust explosions and the spread and accumulation of dust in installations and the surrounding environment. When handling pellets, plants must be designed so that the pellets are handled gently and not affected unnecessarily mechanically so as they will form dust. The plant must be designed with periodical cleaning, monitoring and maintenance activities in mind. Accessibility to for example surfaces that can collect dust and equipment that must be monitored and maintained must be ensured in order for the tasks to be completed safely, promptly and efficiently. The plant design must also ensure that the aids and tools for cleaning and maintenance can be carried/transported up to the heights where it will be used. The plant must be designed so that staff working at the facility with cleaning, inspection and maintenance has access to two independent means of escape within 25 m. Is there no way to establish an escape route within this distance, an alternative solution could be fire sectioning, ensuring that staff can seek safety in another fire section. Detection and alarm system whereby staff at an early stage can realize a hazard is also a possibility. In case of an emergency response the emergency personnel must be provided with access to the plant in order to fight the fire. On the plant there must be defined access/ingress routes for the emergency personnel, many of which will be identical to the escape routes. All enclosed plant parts such as conveyor belts and storages must be constructed so that in case of fire they can be smoke ventilated.

8 Detection As a barrier against the risk of fire, explosion and auto ignition different types of detection systems can be used. Detection of fire in facilities for handling and storage of biopellets is a difficult task due to several factors: The lack of tightness in the buildings. Dust from the pellets. Moist environment. The use of mobile equipment (i.e. loaders in manually operated stores). Another challenge in detecting fire is also that the typical initial fire in a biopellet storage is a smoldering fire, which may be located deep in the pellets stack, which means that signs of fire will not become evident before the fire has been going on for a while. The initial fire will be a smoldering fire which emits minimal heat to the outside. The combustion process will be characterized as a partial combustion, which releases relatively large amounts of carbon monoxide CO and decomposition products such as hydrogen (H 2 ) and methane (CH 4 ). There are several types of detection equipment and methods available in the commercial market. These are used for example for monitoring in the agricultural sector, various warehouses, and for monitoring pollution. Parameters such as humidity, CO, sound, gas, etc. are measured. These include: Thermal detectors. Optical detectors. Aspiration. CO detectors. H 2 detectors. Two criteria detectors - Thermal-Optical. Three criteria detectors - Thermal-Optical-Infrared. Four Criteria detectors - CO-Thermal-Optical Infrared. These monitoring systems can be divided into different types. Manual monitoring systems where personnel service is required, automatic monitoring systems that log the measured values, as well as automatic monitoring systems that initiate an alarm if a predetermined level is reached. In addition, the various types of surveillance systems are available with different power sources and some can transmit data wirelessly. Not all types of protection are equally suitable for the detection of fire in the biopellets.

9 Table 3: Suitability of detection principles in a plant for biopellets. Detection principle Place Conveyor belts Shunts Technical rooms Silo Flat storage Aspiration XX XX XX XX X CO-detection O O O X O Optical smoke detection O O X O O Multi criterion detection O O X O O Thermodetection XX X X XX 1 O Thermographic camera X XX XX O O XX = Good. X = Suitable. O = Not applicable. 1 In the stack. There is lack of operational experience with the various principles for detection storage of wood pellets why their suitability can be difficult to determine definitively. Spark Extinguishing systems In conveyor belts for biopellets can be installed a spark extinguishing system that by use of IR detectors can detect an ember in a material stream and subsequently extinguish with water spray through nozzles. For example, in a shunt from one conveyor belt to the next where the pellets can be exposed to a drop of approximately 1-2 meters the extinguishing system can detect an ember before it is passed on with less than 0,5 seconds from detection to addition of water spray. To detect a small glowing particle inside a material flow requires that the flow of material is free-falling and not too dense so the IR detector can "see" into the air / material stream. Spark extinguishing systems reduces the risk of explosion by extinguishing embers that could otherwise ignite a dust explosion in the shunt, which can be an explosive air / dust mixture. Firefighting equipment in the plant In plants for handling and storage of biopellets can be installed systems to fight a fire. This could for example be irrigation systems, using water to limit or extinguish a fire. The system can be automatically activated, as is known from conventional sprinkler systems where the system is activated at a given temperature or be manually activated on a control panel or by manual connection of water. The greatest effect is clearly achieved by the automated system, which will be activated as soon as a critical temperature is reached. The automatic type of system can involve a risk of faulty activation due to mechanical impact on the extinguishing system. For outdoor installations the system will be challenged by freezing temperatures.

10 In enclosed areas with high dust concentrations may be used explosion damping systems. This system will when a pressure rise is registered trigger a powder material which dampens the explosion. The previously mentioned, spark extinguishing systems with water spray, which is activated when a spark occurs could also be categorized as an extinguishing system. The ember is extinguished and cannot cause a fire or explosion in the further process. An inerting system can be installed in closed, confined spaces such as a silo. This type of system is operated manually. The fire is stifled by removing oxygen from the atmosphere surrounding the material, which is suspected of being or is visibly ignited. An inert gas such as nitrogen (N 2 ) or carbon dioxide (CO 2 ) can be used. CO 2 is extremely effective, as the gas is heavy and can cover the burning material. CO 2 is however more difficult to obtain than N 2. N 2 also has a good effect on inerting, but is somewhat more volatile than CO 2. The N 2 is often injected in the bottom of for example a silo as a gaseous gas and the gas is allowed to pass through the stored pellets thereby lowering the oxygen concentration in the stock itself. In 2011, experiments with inerting with liquefied nitrogen gas applied at the top of a silo were conducted at DONG s facility in Hvidovre, Denmark. It turned out to have great effect with a very rapid reduction of oxygen percentage and requires a significantly smaller amount of gas than using gaseous N 2. However, there is a need to demonstrate that the solution with inerting with liquid N 2 can stand alone. It is discussed whether inerting with liquid N 2 should be combined with inerting with gaseous N 2. In practice the consequences of continuous inerting with liquid N2 over several days are not known. It is most likely not a discussion of either / or, but more likely to find a way to combine the two solutions. By inerting with liquid N 2 a rapid inerting is achieved and with the use of gaseous N 2, a more controllable inerting is achieved. Limiting factor in the design of inerting systems is the evaporator capacity and the logistics of providing a sufficient amount of gas. The use of inerting sets high demands on the tightness of the storage facility, and means of isolating the openings to the outside such as explosion vents efficiently must be available. PROCESS Important precautionary barriers in regards to process, is the preparation of policies and procedures, which describes in detail how employees should react in certain situations or special events. Procedures to ensure uniform action must be drafted in a clear language, with good use of illustrations as a supplement to the written word. It is for example much easier to illustrate an acceptable level of cleaning with a photograph than with a written description. The procedures could also be used in the training of new employees and re-training of employees. In most industrial facilities there will always be an ongoing process of efficiency improvements in the facility for handling and storage of biopellets to ensure that production can be operated with the lowest possible cost. The plants are designed with assumptions of a production capacity and a certain level of maintenance based on available operating hours and specific maintenance frequencies. These assumptions will be challenged in the context of

11 streamlining processes in the production plants. There is a risk that the safety level of the plant at the time of design erodes over time because accidents fortunately rarely happen. Therefore, it is important to create a system for assessing the risks and consequences of changes in the form of for example production, restructuring and downsizing in personnel can have. Such an assessment must take into account the assumptions on which plants and processes are designed. It must be assessed whether the system components can handle the increased load for example the capacity of the dust extraction in the conveyor shunts. Increased production capacity of the plant must also increase the frequency for cleaning, inspection and maintenance. There should be a documented assessment of the consequences of a change so that risks are known and the organization can relate to these. PEOPLE Improving the efficiency of the plants operation and thus on the resources used to operate the plant in terms of finance and personnel, can cause the time allocated for training, exercises and generally to focus on and work with safety to also be cut down. The time allocated for these activities can easily become the first place to cut, as the effect of the time used for training and exercises are not visible on production capacity and quality. It is the responsibility of the plant s top management to communicate and pursue a policy that safety and security be put on the agenda and be prioritized. Understanding risks is important and prevention requires knowledge. This understanding and knowledge is not only achieved by preparing written procedures, but requires ongoing training, practice and discussion. The incorporation of an improvement culture in an organization is important to ensure understanding with the staff of the responsibility they have to sign in when they identify potential or actual safety problems. Similarly, a safety culture is of great importance to maintain the focus on safety in the plant. If safety is not put on the agenda, it will in busy situations and by virtue of the power of habit be at risk of being breached. 3. Conclusion Interaction between PLANT - PROCESS - PEOPLE In the analysis of risks, causes and barriers it appears that the three areas of plant, process and people cannot be kept completely separate. Barriers to the causes of a risk at the facility cannot always be solved simply by changing the system, but may need to be supplemented by implementing some new processes or modification of some existing processes. Similarly, the human factor (people) can be a barrier or lack thereof. The security at the facility will always be compromised by a poor process or a mishandling by the staff. Therefore, the three areas are intertwined with many interdependencies. Barriers to ensure fire safety The analysis concludes that actions should be focused on Restriction of combustible material in the plant in the form of accumulation of dust and biopellets and combustible materials in plant construction (conveyor and building).

12 Layout must comply with the applicable requirements of technical regulations. Access to the plant parts that must be cleaned, maintained and serviced are to be optimal. Building and plant parts must be tight in order to protect the biopellets from the weather and minimize spill of wood dust and biopellets. The plant must be zone classified according to ATEX directive and components selected according to this classification. Focus on plant safety and maintaining an improvement culture and safety culture among staff. Escape routes on the facility should comply with the requirements of max. 25 m to the nearest escape route and two independent escape routes. Installation of efficient fire detection systems in conveyor systems and buildings. Fire sectioning of the system to prevent fire spreading through the conveyor belts to storage facilities. Installation of pressurized irrigation systems on all conveyors. Use of inerting with gas in silos for firefighting. Consideration should be given as to how stocks should be evacuated from the facility, to where and how fast. A plan for training of plant personnel and firefighters must be prepared and implemented. References: [1] Serrat, O. (February 2009). The 5 Whys Technique. Retrieved February 2015 from adb.org: [2] Gestis Staub Ex. Retrieved November 2012 from