Silo Fires Require Specific Response Tactics
The replacement of fossil fuels with renewable fuels impacts 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 response tactics.
In the past 10 years, the use of solid biofuels, and in particular wood pellets, has increased dramatically. In 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 about 16 million tons. Sweden is the largest wood pellet consumer with a consumption of about 2.3 million tons in 2010. The production of wood pellets in Sweden was about 1.65 million tons, while the remaining part is imported by ships, typically from North America and in the Baltic states. Pellets consumption is increasing dramatically in several other European countries as well, however, and as a consequence, handling and storage of wood pellets is also increasing.
To improve the knowledge of fire development, detection and extinction techniques in silos, two main test series have been conducted at the SP Technical Research Institute of Sweden. 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 fires.
The silo used for the tests was 1 meter (3.28 feet) in diameter and 6 meters 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 infection of inert gas during the extinguishing phase. The silo was filled with wood pellets up to a height of 5 meters 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, where a heat/moisture wave, with a temperature less than 100 degrees Celsius (212 degrees Fahrenheit), slowly moved upward (see Figure 2). Although the distance from the point of ignition to the pellet surface was only about 2.5 meters, it took about 20 hours 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.
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-cubic-meter steel silo with a diameter of 6 meters and a height of 10.5 meters and filled with about 260 cubic meters of wood pellets. In total, five gas filling tests were conducted where the gas was injected from the center of the base of the silo, or alternatively at one point along the silo wall. All tests were conducted in a cold (no fire) silo 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.
Real Fire Experience
The results from the first silo project have successfully been applied to several real silo fires in Sweden. In one fire incident, auto-ignition occurred in a silo, 47 meters high and 8 meters in diameter, filled to about 40 meters with wood pellets. Elevated temperatures had been noted for some time and the plan was 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 carbon dioxide to the top volume of the silo.
Approximately 35 tons of carbon dioxide 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 order to control the effect of the gas injection, temperatures and concentrations of carbon monoxide, carbon dioxide and oxygen 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 tons of nitrogen was used, which gives total gas consumption of approximately 5.6 kilograms per cubic meter, well in line with the recommendations from the research project.
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 release hatch or similar opening 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 kilograms per cubic meter hour (cross-sectional area) and a total gas consumption of 5-15 kilograms per cubic meter (gross volume) of the silo.
• If possible, measure the concentration of carbon monoxide and oxygen 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 carbon monoxide and oxygen) 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 slowing or burning material and extinguished with water if necessary.
• The gas injection should continue during the entire discharge process.
Some important things to remember are:
• 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.
Author: Henry Persson
Project Leader Fire Dynamics Section, SP Swedish Technical Research Institute
+46 10 516 5198