The drying of biomass is often a prerequisite for their further processing into fuels or products. The drying process – the removal of water by the direct or indirect use of heat – makes the dried material durable, mixable, grindable, or otherwise usable for further process steps such as pelletizing. However, as several recent dryer incidents bear witness to, drying also carries an inherent fire risk, as René Schwertfeger, Sales Manager at T&B electronic GmbH, explains.
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Founded in 1984, T&B electronic is a specialist industrial spark detection and extinguishing systems developer, provider, and a Verband der Sachversicherer (VdS) certified installer of such systems tailored for various industry sectors including wood pellet production.
According to René Schwertfeger, the company has installed around 7 000 systems around the world of which around 200 are in wood pellet plants.
Increased use of belt dryers
The first belt-dryer in Europe went into operation as long ago as 1907. Today they are used in a wide range of industrial drying applications – agriculture, biomass, pharmaceuticals, food and feed, plastics, and sludge to mention a few – and their popularity has increased significantly over the past few years, not least in wood pellet plants.
All these substances have one thing in common: they are combustible and, in combination with a belt-dryer, result in a high fire load, said René Schwertfeger.
Generally speaking, standard series belt dryers have a modular design made up of a product feeding module, a drying module, and a discharge module. The task of the product feeding module is to feed the supplied material continually and evenly onto the belt within the dryer.
The actual drying of the material takes place in the drying module by supplying preheated air which absorbs the moisture of the material to be dried and removes it from the system via extraction or exhaust air lines.
Residence times of up to 30 minutes are common, depending on the length of the dryer belt and the material properties.
At the end of the drying module, the product can have a residual moisture content as low as 3 percent. The discharge module transfers the material to the downstream conveyor without damaging the structure of the dried material.
Rise in fire incidents
The belt-dryer drying process has several advantages over other drying processes, which is why the number of belt dryers has risen sharply in recent years.
However, the increase in the number of belt dryers has also led to an increase in the number of fire incidents.
Numerous fire incidents in the past have shown that fully comprehensive fire protection in belt dryers is elementary, said René Schwertfeger, adding that a combination of several “unfavorable factors” is increasing the risk of fire risk in belt dryers dramatically.
On the one hand, we have relatively high drying temperatures, increasing dryness of the material and high dust content. On the other hand, the high proportion of material in such plants should not be underestimated, which is why the fire load must be considered very high. If sparks enter the belt-dryer from upstream processes, or if hot layers of material, created by deposits in the dryer, lead to pockets of members, a very rapid spread of fire is to be anticipated, Schwertfeger explained.
The increase of fire incidents in belt-dryers has also drawn the attention of leading insurance companies such as global insurance major HDI Global SE that amongst other things provide various coverages for industrial operations, including biomass plants.
HDI has its own Risk Guidelines in which they describe the risks in wood pelleting processes and which fire prevention system customers are obliged to provide in order to be able to get insurance coverage, said René Schwertfeger.
Amongst other things, HDI has presented a concept for extinguishing fires in belt dryers in one of its numerous fire protection guides.
Based on this guide T&B has created a certified fire protection concept tailored for belt-dryers in cooperation with VdS which, via VdS Inspection Services, is Europe’s leading expert organization for fire safety assessing over 21 000 fire safety systems worldwide annually.
Fire prevention approaches
Before elaborating on the T&B concept, it is useful to consider existing approaches to belt-dryer fire protection, for example, sprinkler systems.
A sprinkler system is only suitable to a limited extent for the protection of enclosed objects because, in the event of a fire in the dryer, the sprinkler is triggered and extinguishes the fire locally. However, the adjacent sprinklers are cooled by the water mist so that they are no longer triggered, even though the fire has most likely spread, so-called “sprinkler skipping”, Schwertfeger explained.
Water spray extinguishing systems are also used as an alternative. Unlike sprinkler systems, water spray extinguishing systems have an open nozzle network and can wet the dryer with a fine water spray pattern over the entire area when
triggered.
Activation can be pneumatic via thermal trigger elements or electrical via heat detectors or flame detectors.
There are uncontrollable general conditions that can lead to fire despite the installation of a water spray extinguishing system. On the one hand, sparking caused by upstream processes cannot be prevented and, on the other hand, pockets of embers in the discharge area cannot be detected at an early stage, which could result in a rapid spread of fire inside the dryer and fire spreading to downstream areas, said René Schwertfeger.
He added that the sluggish triggering behaviour of thermal trigger elements makes them unsuitable in a belt dryer application.
Whist thermal trigger elements and heat detection do make sense in the case of static monitoring, they are not the optimum solution when the process involves movement, since a temperature increase due to smouldering is very difficult to detect due to the permanent air exchange, he said.
Instead, by installing highly sensitive infrared (IR) spark detectors in the input module, sparks can be detected at an early stage and extinguished selectively, preventing potential ignition from upstream processes from entering the drying process in the first place.
In addition, IR spark detectors can also be implemented in the exhaust air ducts of the belt-dryer. The high exhaust air temperature is prevented by means of fibre optics, which means that the spark detectors can be used without any reservation even in this harsh environmental condition.
This way, the smallest sparks and hot particles which occur in smouldering can be detected as early as possible. To prevent fire from spreading to downstream plant areas, so-called combination detectors are installed in the output module.
These detectors combine the properties of conventional spark detectors and ember detectors and are thus able to detect both sparks and pockets of embers. In the event of detection, automatic extinguishing systems are also activated in the output module, which extinguishes the affected material selectively.
According to Schwertfeger, the special feature here is that automatic extinguishing systems are activated both below the detectors in the direction in which the material falls and above the detectors in the direction of the dryer belt.
Since belt-dryers are operated under negative pressure, the installation of the automatic extinguishing systems above the detectors has proven its worth in practice. Without these automatic extinguishing systems, it is very likely that sparks or hot particles will be sucked back towards the belt by the negative pressure in the belt-dryer and can have devastating consequences, René Schwertfeger said.
A certified combo system
On their own, neither the conventional water spray extinguishing system nor the classic spark extinguishing system provides fully comprehensive fire protection for belt-dryers.
T&B has used the advantages of both systems and combined them together with VdS in a new type of protection concept. The water spray extinguishing system provides comprehensive fire protection within the belt-dryer and the spark extinguishing system is used for preventive fire protection and the earliest possible activation of the extinguishing system explained Schwertfeger.
A special activation concept was developed which enables the spray extinguishing system to be triggered via the spark detectors or combination detectors in the input and output module as well as the spark detectors in the extraction lines.
This Schwertfeger says enables the fastest possible triggering of the water spray extinguishing system and minimizes damage to the belt dryer, thus reducing costly machine downtimes.
The VdS certification of this fire protection concept has now created a uniform standard that guarantees the greatest possible safety and machine availability and is also accepted by every property insurer. Time-consuming coordination and uncertainties about the acceptance of the fire protection implemented for belt-dryers are now a thing of the past, ended René Schwertfeger.
