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The underestimated explosion hazards of solid biofuels

30 October 2015

The growing trend of converting existing coal-fired plants to more environmentally-friendly fuels such as biomass requires operators to take a careful look at any potential increased risk of explosion and other potential safety issues. In this article, Johannes Lottermann and Francesca Vincenzi of Rembe GmbH Safety & Control analyse dust explosion risk in biomass plants and propose mitigation solutions. 

Wood can burn. This statement is obvious to anyone who has ever thrown a log on a fire. What is less known is that wood in the form of wood chips and pellets in modern power plants, furnaces or heat generators can offer some of the most hazardous operating conditions in industry in terms of explosion risk.

What makes wood materials and wood handling processes so special in terms of explosion hazards?

This question can be answered quite simply. The requirements of a dust explosion are:

·  combustible dust

·  air / oxygen

·  ignition source

·  dispersion of dust particles

Explosion pentagon
Explosion pentagon

·  confinement of dust cloud

Nearly all wood handling installations, including biomass power plants, have all five of these.

In biomass power plants, equipment such as mills, conveyors, cyclones, silos and dust collectors create and concentrate wood dust, creating potential explosive atmospheres. At the same time, potential ignition sources can be common, whether from mechanical moving parts, electrical equipment or other sources.

Several large explosions have already taken place in biomass plants, confirming the extent of this potential danger.

How to protect against explosion hazards in wood handling installations?

Table 1: Coal and wood explosion indices
Table 1: Coal and wood explosion indices

The explosion safety concept for such plants typically is made up of a combination of explosion prevention measures (to reduce the likelihood of explosion) and explosion protection measures (to reduce the effects of an explosion to an acceptable level).

Prevention Measures prevent the formation of explosive dust clouds as well as avoiding ignition sources by de-dusting, housekeeping, grounding, proper maintenance and/or installing spark extinguishers.

We know that even if all preventative measures are applied (especially with regard to the latter), this approach might lead to misapplication of spark extinguishers which

·  might not work if particles are large;

·  cannot suppress an explosion;

·  are only addressing the ignition risk arising from small, hot particles; and

·  do not prevent ignition sources from tramp metal or hot surfaces.

Overview of explosion protection measures []
Overview of explosion protection measures []

This is why Protective Measures also have to be applied in most wood handling installations. They typically apply one of three approaches:

·  Explosion Resistant Design (simple explanation: make equipment so sturdy it will withstand explosion overpressure of up to 10 bar)

·  Explosion Pressure Venting (simple explanation: pressure and flame relief by applying a predetermined breaking point on the installation)

·  Explosion Suppression (simple explanation: a rapid fire extinguisher that stops the flame )

To which can be added a fourth:

·  Explosion Isolation (simple explanation: Prevent flame and/or pressure propagation to down or upstream units).

The new VGB-Standard Fire and explosion protection in biomass power plants  VGB-S-018-00-2013-12-EN provides best practice approaches toward explosion safety as per the above mentioned principles.

Working principle of flameless venting devices
Working principle of flameless venting devices

The most common method of venting to the outside are through the use of explosion panels. The fact that these explosion panels can be combined with flame-trapping mesh materials allows various applications to be protected by so-called flameless vents such as Q-Rohr.

How to protect against explosion hazards in the milling area of a co-firing power plant

A new and innovative safety approach for coal/wood grinding plant has been developed based on the use of flameless venting devices and leaktight venting panels.

Roller mills are mostly located inside buildings. Conventional concepts are based on free and open venting devices which force operators, architects and engineers to incorporate enough space and special equipment allowing venting to the outside.

EGV venting panels on a storage silo in a biomass power plant
EGV venting panels on a storage silo in a biomass power plant

This is no longer required using flameless venting technology as this allows indoor venting and any changes to the building itself are no more required.

Due to the fact that the flameless explosion venting devices can be mounted inside the building the venting is done at the beginning of an explosion and close to its starting point. The explosion is thus  instantly vented and is less severe in comparison with conventional venting concepts using self-closing flaps.

This flameless venting approach has the advantage that in case of an explosion with a local cleaning procedure and the replacement of the busting disc, the device can be used again. This minimises loss of production.

To protect the downstream bag filter against excessive pressure in case of an explosion, bursting panels have to be applied on the bag filter and its inlet.

The actuation of bursting panels can be detected by using a signalling cable. The signalling device with an ultra-low current is intrinsically safe and operates on a simple, open-closed principle. The quantity of vents depends on the bag filter inlet size, the volume and the pressure resistance.

Explosion protection in a biomass power plant with venting panels and flameless venting
Explosion protection in a biomass power plant with venting panels and flameless venting

Due to the low vacuum condition present in the fine coal/wood bin, single-layer bursting discs with signalling units are the preferred solution for the explosion protection of the bins. As per the bag filter, the quantity of vents depends on the bin size (volume) and pressure resistance. In case of indoor installed bins, the application of venting with flame absorber is the right approach.

The full safety concept is based on purely passive systems, this eliminates false triggering risks, consequent recovery costs and regular maintenance by external professionals.

High levels of safety combined with an approach that overcomes some of the weaknesses of the classical explosion protection model make the described solution the most innovative safety concept in the explosion protection field for biomass power plants milling areas.

Considering all the above mentioned deliberations, economical explosion safety is an issue for highly specialised experts, providing an overall concept for all parts of a plant, considering plant-specific numbers and with extensive experience.

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