Biomass Thermal Plant Precautions

Personnel working at commercial or industrial biomass heating plants should be mindful of everyday hazards.
By Sarah Ludwig | June 05, 2014

While operators of commercial or industrial biomass heating systems may become comfortable with daily regimes, several potential safety hazards loom amongst routine activities, dangers that could result in injury, equipment damage or worse. Main risk areas reside in fuel delivery, fuel storage and handling, and boiler and combustion areas, and potential for fires and explosions is always present. “They [hazards] are the most acute, the most deadly, the most traumatic,” says Chris Wiberg of Biomass Energy Laboratory. “Fire protection is the No. 1 thing we need to lock down. Most of the other risks are common to any work environment.”

 There are companies that specialize in detection, suppression, and fire management, but the real problem resides in the fact that putting out fires that occur in such biomass systems can be complex, and many fire departments simply aren’t trained to deal with them. “Let’s say the fire is burning in an open area that a firefighter can easily get to and he thinks, ‘OK, I can just go throw my hose on that flame on the floor.’ What he doesn’t necessarily know is that as soon as he throws the water on it, it causes a cloud of dust to come up, which becomes the explosion source,” Wiberg says. “So one of the problems in fighting this type of fire can be a secondary explosion. A fire department might say they understand how to put out that kind of a fire, but does the neighboring fire department know how to put out that kind of fire? How many people truly have experience putting out this type of fire? It’s relatively small.” And because fire departments are often volunteer, and these plants can be in very rural locations, he adds, it’s not a given that the firefighters are going to know and understand how to manage this kind of a fire. Wiberg hopes some kind of handbook can be developed on these types of fires and provided to local firefighters. “There are best practices, but they’re not defined yet. Eventually, what these initiatives hope to do is to better define the practices so that the people that will have to address them have a ready-made resource,” he says.

Rebecca Knecht, certified energy manager and project manager at Evergreen Engineering Inc., feels that attitudes toward safety have become better and more mainstream in the past several years. “Companies with an active workplace safety culture are lowering their workplace injury rates and incidents significantly and have increased buy-in among operators,” she adds.

Fuel Delivery
● Tilting trucks. “Fuel delivery to the site is normally handled by tractor-trailer rig, and it can be anything from a side dump to a full-tilt machine that literally tilts the entire truck up 90 degrees and shakes all the fuel out,” says Nicholas McGrew, project manager at Thermal Systems Engineering. Operators need to watch for personnel who may have gotten into the restricted area when a lift is occurring, says McGrew.

● Pedestrian and vehicular accidents with reversing trucks. Keeping a close and watchful eye on surroundings when backing up could prevent a collision. “Operators’ field of vision is key,” McGrew says.

● Mechanical equipment. “If it’s a site that already produces the biomass material, it’s normally just pushed around with track hoes or rubber-wheeled equipment,” says McGrew. Again, operators need to be aware of other personnel in the area. “Typically, there’s nothing to cordon off between piles of fuel and the walkway,” he says.

● Dust and explosion. “We have seen a significant shift to enclosing dump trucks to avoid dust contamination,” says Knecht. “Consequently, that dust is then subject to spontaneous combustion, as it is trapped inside the new structures.” Delivery by bags or material being blown into a silo also increases the risks of explosion.

● Falls into underground fuel storage spaces. “As they say, all accidents are preventable, and being tethered to prevent falls is paramount,” Knecht says.

Fuel Storage and Handling
● Fire. “Piles, if left alone and not managed (properly ventilated or turned over), can automatically ignite due to the interior temperatures,” Knecht says. “They can smolder for weeks or even months and, in some places, years. When the pile is eventually disturbed for use, oxygen is introduced and combustion results. Significant damage has occurred to facilities and sadly, workers have been burned or killed as a result. Fires can also start as a result of burn-back from the boiler or from a heat source within the storage area.

● Explosion. Wood pellets in good condition are not very susceptible to explosion, but once they have degraded, there is the potential for dust and therefore, the risk of an explosion in the right circumstances, such as a buildup of static electricity. Pneumatically transferred pellets offer the highest risk of dust explosion due to the potential for pellets to disintegrate during the delivery process. All delivery pipes should have a smooth internal bore, and bends should have a large radius to reduce the chances of pellet disintegration.

● Mechanical equipment. Some heating systems use live bottom feeders to convey fuel, and if not attentive to surroundings, it poses risks to personnel. “Live bottom feeders make me nervous as far as safety goes,” says McGrew. “If someone falls in there, there’s precious little time to get out.”

● Carbon monoxide, or off-gassing. “Wood doesn’t just off-gas automatically. Wood pellets, because there’s a whole bunch of them in a small area, have some kind of a natural biological process going on that’s causing the generation of carbon monoxide. In very small quantities, it doesn’t appear to be an issue, but certainly in huge industrial-type stores, we have examples of scenarios that have become fatal,” says Wiberg. “The off-gassing issues are only when you have quantities stored in the wrong environment and/or some period of time. It can creep up because maybe it’s not being watched like it should be.”

Boiler and Combustion
● Inability to extinguish immediately. A biomass boiler cannot be extinguished immediately because they have large thermal inertia caused by fuel burning on the grate and potentially also residual heat stored in the refractory. This presents a risk of excess temperature or pressure if the boiler must be shut down suddenly. This risk can be reduced by including a buffer vessel, an emergency heat dump or cooling loops in the design. The problem tends to be greater with wood chip boilers that are physically much larger than pellet boilers with similar outputs and hence have a larger thermal inertia

● Explosion. Occasionally, uncombusted, explosive gas mixtures can build up within a biomass boiler’s combustion chamber and flue, which are subsequently ignited and an explosion of some form can occur. This can happen in circumstances such as uncontrolled draft, excessive charging, delayed ignition, accidental or uncontrolled admittance of air to the combustion space. In order to avoid the consequentially severe explosion risks, it is vital that the boiler system is designed for the load, the correct controls are used for the boiler charging, and the manufacturer’s operating instructions are adhered to at all times.

ADDITIONAL SOURCE: Combustion Engineering Association, Health and Safety in Biomass Systems: Design and Operation Guide.

Author: Sarah Ludwig
Freelance journalist