Silo storage of biomass is growing but the material needs to be handled with a great deal of care.
Biomass handling has proved a useful additional material in dry bulk handling operations, as it has often compensated for falling volumes of other commodities.
However, storing biomass products requires particular attention. Silo fires and even explosions are, of course, nothing new. But with the growing demand for and supply of biomass, concerns are turning to keeping product safely stored before its eventual use in power generation.
These risks became apparent in April this year when a wood pellet silo at a storage and shipping terminal in Port Arthur, Texas, USA – one of five silos owned on-site by German Pellets Texas LLC (GPTX) and Texas Pellets Inc – began smouldering. On 21 April, GPTX retained global disaster recovery specialist Cotton Commercial USA to take the lead in extinguishing the fire, extracting the pellets, and restoring the site.
However, the silo eventually collapsed and, after a long battle to extinguish the fire, the city of Port Arthur filed a lawsuit against GPTX, citing the public nuisance the community had experienced over the calamity.
The challenges of storing biomass were on the agenda at the inaugural conference of the UK’s Association of Bulk Terminal Operators (ABTO), which took place in London this autumn.
“Biomass is not one material,” said Professor Mike Bradley, director of the Wolfson Centre at the University of Greenwich. “Different biomass products have different requirements,” he said, explaining that products can be made up of anything from organic residues, food waste, sewage, straw, cereal and olive stones, to chipped wood, elephant grass, wet leaves and paper.
“The key is to understand the properties of the particular range of materials involved, since no one handling system can deal with all types of biomass,” he said.
Highlighting some of the unloading and handling challenges faced by operators looking to adapt their terminals for biomass, he advised operators to keep a close eye on their quality control and safety procedures.
Biomass dust, he said, is a particular challenge. “Dust has caught people out in biomass handling,” he said. “It’s more mobile, it’s lighter, and will stay suspended for longer.” He added that there is a danger that, if not dealt with appropriately, it could result in terminal workers inhaling the dust and contracting ‘farmer’s lung’, the accumulation of mould spores in the lungs.
Bradley also said that the accumulation of biomass dust can increase the fire and explosion risks, particularly as some biomass cargoes are self-heating. “If you can write your name in the dust, you have an accident waiting to happen,” he warned.
This latter subject is particularly important in storage, specifically wood pellets. Dr Mi-Rong (Kimberly) Wu, project manager at TBA Netherlands, told delegates that biomass volume, rather than weight, has to be taken into account when considering silos and storage facilities.
She said that, because of its bulk density, more volume of solid biomass needs to be stored compared with coal, and this could require about 1.3 times more land to accommodate the higher volumes. Wu also said that solid biomass is sensitive towards degradation, and should not be stored for more than three months.
“Silos have been known to explode because of incorrect handling,” she told delegates. “For solid biomass products, temperature and carbon monoxide (CO) emissions must be constantly monitored.”
Summing up her presentation, she said: “Solid biomass properties are in a wide variation range. For solid biomass handling, the volumetric performance should be the main benchmark, rather than tonnage performance.”
For those terminal operators considering a transition from coal to biomass, she added, adjustments in terms of handling processes and storage requirements are necessary, along with in-depth investigations into logistics and material characteristics.
The propensity for biomass to auto-ignite is well known among specialists, but perhaps less so by entities engaged in handling it.
Industrial gases group BOC explains that bacterial and fungal activity generate heat in the pile, along with toxic and combustible gases. If not addressed, the insulating nature of the material causes the temperature to increase. When the rate of heat generation exceeds the rate of heat loss, the path to ignition and combustion is set.
A fire can occur at very low oxygen levels – as low as 3% oxygen can support a pyrolysis, says BOC.
There are a number of early indicators that storage facilities should watch for. These include rising CO concentration, followed by a temperature increase. But a problem with monitoring these factors is that auto-ignition can occur very deep within the material, such that, by the time it is apparent, “you already have an issue”.
Auto-ignition is made worst by the scale and geometry of the average storage silo, which affects the rate of heat loss. Very large silos can have a critical ignition temperature of as low as 50 degC, and this temperature can be reached in a short space of time.
A constant nitrogen purge through biomass can reduce the rate of bacterial oxidation, and assist in conditioning the material. At the same time, inerting the silo can reduce the rate of production of flammable gases, such as CO. This can help detect a fire more quickly, as the nitrogen gas will force combustion gases upwards to gas detection systems at the top of the silo.
But, if a fire starts, tackling it in a silo presents even more challenges than with an ordinary fire. For a start, the fire is hard to locate within the pile, and the onset is often not detected until it has grown to a substantial size. In addition to CO, it can also give off hydrogen, which, too, is highly flammable. Moreover, a minor gas explosion can cause a major dust explosion.
Fighting a biomass fire with water can cause serious complications and will not penetrate the pile, while the fire cannot be left to burn out, due to the amount of heat and the insulating properties of the wood. And if the fire is not completely extinguished, reignition can occur relatively easily. In 1998, a fire in Denmark burned for a year before it destroyed the silo.
North America has plenty of experience, not least from the huge grain silos that dot the Midwestern prairies.
John Arsenault, director of Quebec Wood Export Bureau’s wood pellet group, even says that it is not a question of if a fire occurs, but when. The realistic goal is not total elimination of fires at pellet mills and storage sites, according to Arsenault, but rather to reduce significantly the potential for risk while implementing sound damage mitigation tactics when a fire or explosion occurs.
The Wood Pellet Association of Canada is promoting safety through a new programme, urging terminal operators and silo engineers to build safety measures into the plant right from the start.
Adequate ventilation is critical. Condensation must be vented out, otherwise moisture will condense on the roof and come back into the pile as water, and moisture plays a major role in spontaneous combustion.
In Europe, too, the issue of temperature monitoring has been the subject of research. A 2012 study, led by the Swedish University of Agricultural Sciences, monitored temperatures in six large-scale (4,500t) silos for wood pellet storage over a seven-month period.
Each silo had 124 temperature sensors mounted on cables for an even distribution within the volume. During the study, silos were charged and discharged several times, creating different scenarios. Under certain circumstances, pellet temperatures increased vertically from bottom to top in an additive way, and temperatures around 65- 70 degC were reached at the top of the silos.
On some occasions, temperatures increased uncontrollably, and the silos were emptied due to the risk of fire. In an additive scenario, a maximum heat front velocity of 12 m3/hr and a maximum temperature increment for a specific sensor of 2.4 degC/hr was found. To avoid condensation of moisture from the ventilation air on stored pellets, it suggested a fan operation controlled by a dew point algorithm.
Fire prevention starts with a well thought-out design as each type of stored material requires its own considerations. Some systems are preventive, providing feedback that can help customers avoid a disaster. But some are shutdown systems, alerting customers to halt operations to avoid damage to or loss of equipment or product.
The most commonly recommended fire prevention technologies include temperature and humidity cables monitoring. A cable array hung from the roof monitors both, alerting customers when the environment ceases to be ideal. Gas detection systems monitor gases given off by biological products. This system provides information about an impending problem and, by acting quickly, a fire can be avoided. These systems tell users how aged some types of product are by analysing off-gas composition and quantities, and watching those levels provides clues that fire could soon be possible.
Point-level monitors identify the height of the product, and more specialized 3D monitors chart the surface of the pile and its shape. Heat detectors sense surface heat, while moisture meters can be installed on the inbound feed to detect if the product is off spec, failing to meet requirements for heat or moisture.
Spark detection recognises sparks when product is moved into the storage area, and metal detectors detect ferrous or non-ferrous tramp metal that could produce an ignition source.
Systems that monitor conveyors are especially important because many fires in bulk storage facilities are caused by the idlers on the belt conveyor not being properly maintained. If idler bearings begin to fail, and the belt continues moving anyway, the system can catch fire, with the conveyor carrying the flames right into the dome.
Similarly, bearing-temperature monitors analyse specific bearings to make sure operation is optimal. Dust collectors or misting stations can be also included to control fugitive dust.
Under the dome
Perhaps not surprisingly, Dome Technology, a dome manufacturer based in Idaho Falls, USA, argues that a dome is less likely to experience fires from stored wood pellets than a conventional silo because the interior is truss and support free, with no areas for dust to accumulate.
Even so, domes storing explosive materials are built with explosion panels at the apex. These panels are sealed to prevent moisture from entering the structure, but are rated to release in an explosion, or when internal air pressure is excessive. When the explosion panel is loosened, the shape of the structure channels energy through the openings, reducing the chance of the dome being compromised should an explosion occur, the company says.
At the end of the day, Dome Technology says the two most important items in a fire-prevention system are housekeeping and maintenance, neither of which can be engineered into the system.
It all comes down to training employees on the importance of housekeeping. Not all employees adopt the same level of cleanliness, so clearly explaining expectations regarding the worksite are essential. Showing employees how cleanliness correlates with safety can minimise the risk of catastrophe.
When equipment is well maintained, it is less likely to fail or cause sparks and heat, and when the facility is kept clean and free of considerable dust build-up, the potential for fire spread or secondary deflagrations is greatly reduced.
For facilities with combustible materials, the firm recommends having a plan for circulating product, especially in summer months. Operators must be aware of dwell time and know how often product needs to be moved to reduce hot spots and ageing in the pile, particularly for wood pellets and coal.
Another simple but highly important procedure is to check temperature on shipments into and out of the facility, ensuring all product is ‘on spec’ all the time. It is also important to have the ability to reject off-spec product before it enters storage.
A cleaning schedule should be developed. Some of Dome Technology’s customers opt for a full system clean after each shipment, from the top of the dome to the bottom of the tunnel, and a similar full clean of the entire outbound and inbound systems after truck, train, or ship departure.
Regular maintenance of all moving parts needs to be performed, including lubrication, following manufacturer recommendations. Worn items should be replaced when they are spent, rather than trying to eke out a little more production before discarding. To this end, Dome Technology recommends keeping spares on hand so that, when parts need to be replaced, extras are available. This allows production to continue without using materials that are defective or hazardous.