Fire safe storage of biomass and waste

Climate change and increased awareness of over exploitation of the earth's finite resources has already led to major changes in the community to increase the percentage of renewable energy and maximize resource recovery.

Den successiva övergången från fossila bränslen till olika typer av biobränslen/avfallsbränslen The gradual transition from fossil fuels to various types of biomass or waste fuels will lead to much more extensive handling and probably even larger stocks. Most likely, there will also be a strong preference that stocks should be placed as adjacent to an industry or a power plant as possible in order to minimize the need for transport.

It has in recent years been built up production facilities for different types of biofuels and systems to manage waste to be recycled or incinerated. In the case of biomass, wood chips have been used for quite some time but in recent years, the use of wood pellets has increased dramatically. The explanation probably lies in the pellet form, which makes fuel more homogeneous and manageable, and combustion systems have been developed both for small-scale use in residential boilers and for industrial systems for heat and power generation. Production of pellets is running throughout the year with buildup of large stocks that often reach maximum volume in the autumn and are then consumed during the winter months. Because of Sweden's forest resources, we are in an international perspective at the forefront of this development.

To optimize the use of resources in society the recycling of material needs to be increased. This represents an extensive sorting of waste for this to be used either as a source of raw materials or energy. According to the Decree on the landfill of waste (SFS 2001:512), combustible or organic waste shall not be put on landfills. Therefore, large stocks are built, both locally to get rational transportation but more important at recycling plants where waste from all or part of the country is collected. In recent years, this have created intermediate storages and logistic systems to handle all recycled waste that will either be recycled or incinerated. These storages shall, therefore, be temporary in the sense that the same waste cannot be stored for longer time periods. The continuous production/recycling process and variations in the recipient’s ability to dispose the waste can, however, periodically create relatively large intermediate storages. Since the energy consumption varies over the year, this means that also recycled waste for energy production needs to be stored. For example a mild winter reduces the energy consumption, resulting in even larger stocks. Temporary unplanned stops in energy production can also lead to increased stock. According to legislation the maximum storage time is three years for the waste if not to be considered a landfill. In reality this could be much longer for some parts of the storage, depending on how you define the timing for start of storage and start of consumption. It is, therefore, important that intermediate storage is done in a safe manner, both from an environmental and fire risk standpoint.

Overall, this means that very large production and handling facilities with large stocks are built up. Fire safety issues are not always handled in detail by the facility owners and relevant authorities. This is probably due to a combination of unawareness and lack of knowledge about how to solve these problems. If a fire should occur, they trust that the local fire rescuing services will have the resources and expertise to handle such a situation. However, knowledge of several aspects of fire safe storage is missing today. This has been reported in a study funded by the The Swedish Fire Research Board, Brandforsk. The study also suggested a research program to provide answers for many of the issues that exist today. Detailed knowledge is required in order to make qualified risk analysis, for example on ignition characteristics, fire development, fire spread, heat release rate and radiation levels, flame heights, emissions etc. Knowledge is also needed on impact of different configurations (e.g. heaps, silo storage, baled or stacked). To detect a fire at early stages, measurement for temperature monitoring/detection of various storage configurations needs to be further developed.

Storage

Wet biofuels can be handled and stored outdoors (sawdust, wood chips, straw, etc.) while dry fuels such as wood powder, pellets and briquettes are stored indoors in large stacks or in silos. Also the handling of the waste can be extensive with sorting, collection and reprocessing of waste in various purpose-built facilities for later use as a new raw material or in some cases fuel/ energy. Waste to be used for energy production is produced relatively evenly throughout the year and is usually stored outdoors. However, the need for heat and energy is greater during the winter months so very large stocks can therefore be formed. The storing is often made in large stacks, either loose (compacted or non-compacted) or in the form of stacked bales (round or square). The bales are in sizes of 1 m3 and are stacked in large, high stacks which may consist of several thousands of bales. Furthermore, alternative handling can be compacting and wrapping of waste into long "loafs", and in some cases, pelletized waste.

Ignition

Perhaps the most common cause of fires in large bulk storage is self-heating leading to spontaneous ignition. Self-heating is partly due to microbial activity, chemical oxidation and physical processes and can lead to spontaneous self-ignition within a few days, but more commonly after a few months or longer, depending on the circumstances. Today it is very difficult to distinguish a normal temperature increase from a "dangerous" temperature increase leading to spontaneous ignition. Spontaneous ignition has been studied within the research program CECOST (Centre for Combustion Science and Technology). Other natural fire causes can of course also occur in forms of various external ignition sources (fires in loading machines, thunder, grass fire, arson, etc.), which must also be considered.

Fire spread

As biofuels and different waste fractions are handled and stored in large quantities in industrial facilities (for example at bio-fuel producers, facilities for waste sorting and processing, power plants etc.) there is a great risk that an incipient fire in a fuel can spread quickly to the plant and thereby cause severe damage. Even an outdoor storage fire can spread in different ways to adjacent buildings and facilities. Again, knowledge of ignition and fire spread is necessary to take the right safety precautions. Fire Research has examined some of these factors in silo fires in previous studies.

A fire in a large fuel stocks may cause significant costs. The direct costs of the destroyed fuel, damaged plant parts, etc., and direct rescue service efforts can obviously be very extensive, but it also generates a lot of indirect costs which in many cases are not included in the total cost of damages. This may be downtime costs, both in the fire affected facility, but perhaps also for surrounding facilities that have to be closed due to smoke, etc. Residents in adjacent residential areas may be affected, and not least, there may be long-term costs due to pollution to air, land and water. A fire in the cold season could also affect third parties most significantly if the district heating would be switched off.

Fire extinguishing

There are high requirements on the local rescue service but today they are not organized and equipped for these types of fires. Storages also need to be built in such a way that rescue services can access potential fires and extinguishing efforts are facilitated. One very important aspect to consider is the safety associated with firefighting. SP Fire Technology has in several research projects studied extinguishment of silo fires and developed methods that have been used several times in real silo fires. Although the goal of the research is to create conditions to avoid or minimize fires, it is also important to be able to extinguish a fire in an effective manner if it would occur. Appropriate techniques and tactics can vary with the fire scenario. Suitable extinguishing agents, equipment and tactics for effective firefighting need to be evaluated with respect to different storage types. Various extinguishing agents and methods may also have different degrees of impact on the environment, which must also be considered.

Emissions

A fire often leads to serious problems with emissions (smoke production, odor, toxic gases and extinguishing water) to the surroundings. This is the case especially during fires in stocks of biomass or waste as they can be large and last for a long time and are often difficult to extinguish. Fire Research has studied emissions from fires in a number of projects. Emissions have been quantified in fire tests with electronic waste, tires and household waste (simulated deep fire), respectively. Furthermore, the distribution of emissions to air and water has been studied within a larger research project funded by the Swedish Rescue Services Agency (today called the Swedish Civil Contingencies Agency, MSB). More information about fire and the environment and emissions from other types of fires can be found here .

 

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