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Silo Fires Require Specific Response Tactics

by Marcus
Silo Fires

Henry Persson discusses silo fires that require specific response tactics

The replacement of fossil fuels with renewable fuels has an impact on fire safety issues in many ways. In particular, the use of solid biofuels presents new risks and challenges for the industry and first responders alike. One common type of refined solid biofuel is wood pellets which are often stored in large silos after production or shipping. In the case of a silo fire, it is important to understand the nature of the fire and to use appropriate fire fighting response tactics.


In the past 10 years, the use of solid biofuels, and in particular wood pellets, has increased dramatically. In year 2000, the annual production of wood pellets in Europe and North America was about 1.5 million tons while the expected production for 2010 was in the range of 16 million tons. Sweden is the largest wood pellet consumer with a consumption of about 2.3 million tons 2010. The production of wood pellets in Sweden was about 1.65 million tons, while the remaining part is imported by ships, normally from North America and the Baltic states. However, pellets consumption is increasing dramatically in several other European countries as well, and as a consequence handling and storage of wood pellets is also increasing.

Research and experience

In order to improve our knowledge of fire development, detection and extinction techniques in silos, two main tests series have been conducted at SP Technical Research Institute of Sweden briefly described below. Experience from these projects has resulted in recommendations concerning proper extinguishing practices.

Silo extinguishing tests

The main purpose of the first project conducted in 2006 was to study fire extinction techniques in silos and to provide a basis for guidelines concerning the tactics to be used. The project also provided valuable information about the initial fire development of a simulated spontaneous ignition in the stored material and the possibility for early detection of such a fire.

The silo used for the tests was 1 m in diameter and 6 m high. Close to the base of the silo, a ventilation duct was installed, which was used both to provide ventilation to the silo during the “pre-burn” phase and for injection of inert gas during the extinguishing phase. The silo was filled with wood pellets up to a height of 5 m during the tests. Local auto-ignition was simulated using a coiled heating wire placed in the pellets, located centrally in the silo.

The experimental results from one of four tests are summarized in Figure 1. The extension of the pyrolysis zone was mainly downwards, towards the air inlet while a heat/moisture wave, with a temperature less than 100 C, slowly moved upwards, see photos figure 2. Although the distance from the point of ignition to the pellet surface was only about 2.5 m, it took about 20 hour before the fire could be detected by gas analysis in the top of the silo, clearly indicating the problems of early fire detection of smoldering fires in silos.

Silo fires require specific response tacticsSilo fires require specific response tacticsSilo fires require specific response tacticsSilo fires require specific response tacticsSilo fires require specific response tactics

Figure 1. Visualization of the recorded temperatures inside the silo during one of the fire tests.

Image: Silo fires require specific response tactics

Silo fires require specific response tactics Silo fires require specific response tactics

Figure 2. Photos taken during dismantling of the silo. Left – the pellets formed a congealed pile in the top part of the silo. Right – the pyrolysis zone about 0.5 m below the ignition source.

Image: Silo fires require specific response tactics

Gas filling tests

The purpose of the second project, conducted in 2008, was to investigate how nitrogen should be injected into a real silo during extinction to achieve optimal gas distribution. The experiments were performed in a 300 m3 steel silo with a diameter of 6 m and a height of 10.5 m and filled with about 260 m3 of wood pellets, see Figure 3. In total, five gas filling tests were conducted where the gas was injected from the centre of the base of the silo, or alternatively at one point along the silo wall. All tests were conducted in a “cold” silo (no fire) as the main focus was to study the gas distribution in the bulk material. The tests showed that the gas distribution was significantly influenced by the gas flow rate, the location of the inlet and the properties of the bulk, showing the need for several distributed gas inlets when inerting large diameter silos.

Silo fires require specific response tactics

Figure 3. Photo of the 300 m3 test silo used for the gas distribution tests.

Image: Silo fires require specific response tactics

Experience from real fire incidents

The results from the first silo project have successfully been applied to several real silo fires in in Sweden. In one fire incident, auto-ignition occurred in a silo, 47 m high and 8 m in diameter, filled to about 40 m with wood pellets. Elevated temperatures had been noted for some time and it was planned to empty the silo within the next few days. However, before such action could be taken, smoke was seen emerging from the top of the silo and the fire brigade was called. Initially, extinction was attempted using the application of liquid CO2 to the top volume of the silo. Approximately 35 tons of CO2 were applied intermittently over a period of approximately 18 hours. The application seemed to control the fire but it was not possible to verify how much of the gas penetrated into the bulk. Consequently, it was not possible to determine when a discharge operation could be safely started.

Nitrogen was therefore injected close to the silo base according to the recommendations from the silo experiments in 2006. In order to control the effect of the gas injection, temperatures and concentrations of CO, CO2 and O2 were measured in the top of the silo. In total, nitrogen injection continued for almost 65 hours without interruption until the silo content was discharged. Approximately 14 ton of nitrogen was used, which gives a total gas consumption of approximately 5.6 kg/m3, well in line with the recommendations from the research project.

Summarized guidelines

Based on both the results of the research projects and practical experience of real silo fires the following recommendations are given:

  • Make an initial risk assessment of the situation. Concentrations of carbon monoxide in indoor areas in the vicinity of the silo may be dangerously high. Further, consider the risk for dust and gas explosions in the silo and associated systems.
  • Close all openings to the silo and turn off ventilation so that air entrainment into the silo is minimized. A released hatch or similar in the silo top for gas and pressure relief should be present while still preventing any inflow of air.
  • Inject nitrogen close to the bottom of the silo. A large diameter silo will require several gas inlets. The nitrogen should be injected in gaseous phase, and an evaporator must be used. Assume an injection rate of 5 kg/m2 hour (cross-sectional area) and a total gas consumption of 5-15 kg/m3 (gross volume) of the silo.
  • If possible, measure the concentration of CO and O2 at the top of the silo during the entire extinguishing and discharge operation.
  • Do not begin discharging the silo until there are clear signs (low levels of CO and O2) that the fire is under control.
  • Be aware that the discharge capacity might be considerably reduced compared to a normal situation and that the discharge operation might take several days to complete.
  • The discharged pellets must be inspected for glowing or burning material and extinguished with water if necessary.
  • The gas injection should continue during the entire discharge process.

Important to remember!

  • Do not open the silo during the fire fighting operation. This will cause air entrainment which will increase the fire intensity and might cause dust and gas explosions and an escalation of the fire situation.
  • Do not use water inside a silo filled with wood pellets. Water application will cause considerable swelling of the pellets which could both damage the silo construction and cause significant problems for the discharge operation

Silo fires require specific response tactics
Henry Persson
+46 10 516 5198

Henry Persson is working as a Project Leader at the Fire Dynamics section at SP, the Swedish Technical Research Institute, mainly with testing and research in the area of, fire protection, fire extinguishing systems and extinguishing media. He has been working at SP since 1979 and is actively participating in the International and European standardisation work as the principal Swedish expert. The ISO group for fire fighting foams, ISO TC21/SC6/WG4, has been chaired by Mr Persson. Mr Persson has been project leader for a large number of BRANDFORSK projects and was the project leader at SP for the EU foam research project FOAMSPEX and has been participating in several other EU-projects.

Article from Brandposten magazine from SP Technology

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