Biomass experience and the dangers of dust

Photo: Studstrupværket

Based on extensive experience, notably at large Danish wood pellet handling and storage facilities, Ramboll has developed a holistic approach to fire safety at biomass fuelled power plants, based on a combination of detection and liquid nitrogen suppression, coupled with increased recognition of the risks posed by of dust.

As many power plants convert to biomass and establish ever larger biofuel storage facilities, the risk of the fuel catching fire increases. The fuel itself has inherent characteristics that contribute to the risk of self-heating, with wood pellets and dust from wood pellets prone to self-ignition. The size and configuration of storage facilities also play a major role.

A smouldering fire in a large storage facility is very difficult to detect, with the added challenge that it is also very complicated and hazardous to put out.

Ramboll has developed and has a patent pending on a fire fighting concept which is very well suited to large inventories and wood pellets. It involves injection of liquid nitrogen in combination with a control system monitoring the condition of the storage facility and the state of the fire.

The approach is in use at Studstrupværket and Avedøreværket – both Danish power plants firing wood pellets. The wood pellet storage facility at Avedøre is one of the largest fully automated wood pellet storage facilities in the world and experience there was instrumental in developing the new strategy combining surveillance and suppression.

Photo: Avedøreværket

Although in principle similar fire safety strategies can be adopted for different power plants employing various biomass fuels, the details need to be “tailor-made” based on the entire system and a specific risk assessment for individual sites and locations. The availability of nitrogen to be delivered to each location differs, for example.

The concept of combining detection systems with inert gas suppression is not a Ramboll invention – it is widely adopted in fire safety management – what is key to the Ramboll approach is the holistic, system wide, approach to appraisal of fire and explosion risks (not just to people but also to production and assets).

The hazards to be taken into account in the risk assessment are of course dependent on the nature of the fuel itself, but are also very much influenced by the design of the fuel handling/transport system and storage facility. The fire safety strategy has to be custom-made in each case.

For storage facilities, early detection is paramount, and needs to be designed for the specific case as there are different requirements for different fuels. Detection of a smouldering fire in a waste facility is different from detection in a large wood pellet silo or dome, and again detection systems for wood pellets stored in a dome would be different to those for wood pellets stored in A-frame storage facilities.

Over the years the approach has evolved, such that it is now understood that in a large wood pellet fuelled installation a fire will happen, although everything is done to minimise risks and consequences as much as is practical.

Focus on dust

Also, there is now an increased focus on dust in the fuel transport/handling systems upstream and downstream of storage facilities and on the fire and explosion risk that is presented by dust. This includes a greater emphasis on preventing dust from being produced in the first place and, if produced, preventing it from escaping to surroundings.

And when it does escape it is important to establish specific guidelines as to “how much is safe” (in terms of fire and explosion risk). The hazard has to be assessed and may result in hazardous area classification (ATEX), with requirements specified for equipment exposed to dust from pellets.

Image: DEM-CFD simulation of a biomass fuel transfer point with screen just below it

Several fire and explosion incidents experienced in the past could have been prevented if dust load had been kept to a minimum. Two key factors are important:

• Design the fuel transport and handling system so that it does not create and release dust. The illustration above shows a DEM-CFD (discrete element method – computational fluid dynamics) simulation by Ramboll of a biomass fuel transfer point with a screen just below it.

The simulation shows particle velocity, an important factor when designing a fuel transport system, where the aim is to reduce abrupt velocity changes/accelerations, avoid stagnation, minimise internal velocity gradients and align the belt and particle flow velocity vectors in the loading zone of the receiving belt. These taken together reduce generation and dispersion of dust. Furthermore, velocity alignment has a large impact on the performance of the inclined receiving conveyor.

• Dust will at some point escape, so be prepared. Bad quality pellets are occasionally received, or leaks can develop in enclosures. Sometimes the amounts of dust settling are greater than the operators can cope with, or the dust is building up in areas where housekeeping cannot be done while the fuel transport system is in operation. Therefore, it is of great value to have plant specific guidance that indicates safe dust layer thicknesses for the various locations within the power plant, indicating priorities for a more targeted, and therefore more effective, cleaning regime.

Author: Annette Mortensen, Ramboll