Self-heating Hazards of Biomass Materials

Expansion of the biomass energy sector has driven the need for increased large-scale biomass storage capacity to ensure a stable supply of fuel. As storage requirements increase, so do the health and safety risks associated with storage of biomass—in particular, suffocation from off-gassing, fires (sometimes spontaneous) and dust explosions. As a result, the number of reported incidents at biomass facilities is steadily increasing. Online data indicates that although there were 65 reported incidents between 2000 and 2018, they are on the increase—nine incidents occurred in 2017 alone. Furthermore, 55 percent of the reported incidents involved fire, 27.5 percent of which were confirmed self-heating incidents. Additional fire incidents could have been due to self-heating, but not proven.
Biomass self-heating is a real problem. In 2017, a huge fire at the Advanced Agro-Power Plant facility occurred in Thailand. Police were quoted saying “a 500-tonne pile of biomass fuels caught fire, apparently because of accumulated heat.’’

Back to Basics
As we should all know by now, most bulk materials and powders that are handled in industry are combustible and under the right (wrong!) conditions can cause fire, flash fire or explosion hazards. A fire hazard will exist if three components are present in one location and at the same time—a combustible particulate solid, an oxidizing atmosphere (typically the oxygen in air), and a credible ignition source. This is commonly referred to as the fire triangle. In the case of dust cloud flash fire (deflagration) hazards, the following conditions must simultaneously be present: sufficient quantity of combustible dust to propagate a deflagration; oxidizing atmosphere credible suspension mechanism; and credible ignition source.

It is noteworthy that the first three conditions are usually expected at some point during any material/dust handling, transfer, processing, dust collection or packaging operations. And, of course, simultaneous existence of a credible ignition source will result in a dust cloud deflagration.  If a deflagration occurs in a confined or closed process vessel such as a conveyor or elevator or room/building, pressure that would be sufficient to rupture the confining enclosure, causing a dust explosion, can build.

From a regulatory point of view, in a facility where combustible powder/dust fire, deflagration, and explosion hazards exist, NFPA 652 Standard on the Fundamentals of Combustible Dust, 2019 edition, requires that the owner/operator of a facility shall be responsible for meeting the life safety, mission continuity, and mitigation of fire spread and explosions as follows:

• Reasonably protect occupants not in the immediate proximity of the ignition from the effects of fire for the time needed to evacuate, relocate, or take refuge.

• Reasonably prevent serious injury from flash fires and explosions.

• Reasonably protect adjacent properties and the public from the effects of fire, flash fire, or explosion.

• Limit damage to levels that ensure the ongoing mission, production, or operating capability of the facility to a degree acceptable to the owner/operator.

• Prevent or mitigate fires and explosions that can cause failure of adjacent buildings, compartments, enclosures, properties, storage, facility’s structural elements, or emergency life safety systems.

To meet the above criteria, NFPA 652 requires the owner/operator of the facility to complete the following tasks:

• Determine combustibility (fire) and explosibility hazards of materials.

• Conduct a dust hazard analysis (DHA), which is a systematic evaluation of potential dust fire, deflagration, and explosion hazards and recommendation of measures for their management.

• Manage identified fire, flash fire and explosion hazards.

• Establish written safety management systems.

A DHA is a systematic evaluation of potential dust fire, deflagration and explosion hazards in a process or facility, and recommendation of measures for their management. This involves identifying locations where combustible powder accumulations or explosible dust cloud atmospheres are or could be present during both normal and foreseeable upset conditions, and identifying potential ignition sources under normal and abnormal operating conditions.

For new construction, a DHA must be completed as part of the project. Existing processes and facilities must complete DHAs by Sept. 7, 2020.  Additionally, the DHA must be reviewed and updated at least every five years.  DHAs must be conducted by someone with proven expertise in hazards associated with handling and processing combustible particulate solids.

Depending on the type and nature of the powder and the process, credible ignition source(s) could include open flames, cutting and welding flames, friction heating or sparks, hot surfaces, sparks from electrical equipment, electrostatic discharges and self-heating.

For the biomass sector, there are other relevant industry- or commodity-specific NFPA standards that should also be consulted, depending on the nature of the biomass material. In this article, we have focused only on the requirements of the “top level” standard, NFPA 652.

Self-heating Hazards
When material is bulked in volume, subtle ignition sources associated with heating processes can sometimes present themselves, and perhaps the inherent instability of the material itself.  For example, bulk powders or powder layers can self-heat, smolder, and catch fire due to exothermic chemical reaction (exothermic decomposition or exothermic oxidation), or biological processes, which can also generate heat. Barns, hay fields, compost heaps and piles of wood chips have spontaneously caught fire in the past. In this case, it’s water that allows biological processes (respiration) to function and generate heat.

Once self-heating begins, there is the possibility of reaching a critical temperature at which the bulk material smolders and continues to accelerate in temperature rise. Bulked material serves as a good thermal insulator, inhibiting cooling and promoting the generation and buildup of heat energy from its core.

Self-heating is a complicated phenomenon consisting of both a heat-generating chemical reaction or biological process, and a heat-transfer (heat loss) process. In simple terms, when the rate of heat generation exceeds the rate of heat loss, temperature can quickly rise. If left unimpeded, material self-heating can result in smoldering. Without air, however, combustion cannot take place.  In this case, the bulk continues to superheat, but not catch fire, though the off-gasses produced can still provide an asphyxiation risk to operators.

When air is introduced, the bulk material can burst into flame. Even raking a pile of smoldering material can achieve this. If dust is then raised, there is a very real risk of dust explosion.
In all cases of spontaneous heating, time is an important factor, since some exothermic reactions take a while to get established. It is not unusual to have a fire or explosion days after a hopper or bunker has been filled.

Industrial operations that are prone to fires and possibly explosions due to self-heating include powder drying and heating processes. But heat can inadvertently be applied to materials by the sun, mechanical milling and grinding operations, or when a fugitive powder layer builds up on a hot surface, for example.  It is usually when the warm, processed material is allowed to build up as a bulk or layer in various locations within the process equipment, or ultimately, in hoppers, silos, big bags or smaller packages, that the problem becomes evident. Bulk storage of biomass material is the most common scenario for self-heating, and the larger the bulk, the greater the likelihood, with subsequent fire and explosions risk.

It should be noted that the onset temperature for self-heating is not an intrinsic property of the bulkier material. Factors such as composition, presence of impurities in the material, geometry, size of the accumulated powder, air/oxygen availability, and the duration of powder exposure to a given temperature can all affect the onset temperature of self-heating.

Powder Processing, Storage and Transportation
The first step in ensuring safety from self-heating fires and explosions is having a proper understanding of its self-heating properties (including its potential for gas generation).

Measurement of exothermic activity normally involves heating a sample under controlled conditions to determine the point at which its temperature starts to increase independently of the external heat source. For laboratory testing to provide usable indication of the hazards, the test sample must be representative of the powder in the process. Additionally, the laboratory tests must reasonably simulate the conditions that the powder experiences during processing and handling, and subsequent storage, packaging and transportation. Often, a screening test is initially performed, during which the test sample temperature is increased at a rate of 1 degree Centigrade per minute. If self-heating is observed at a temperature that is close to the process or storage temperature, or the process cycle is longer than the test period above the process temperature, then an isothermal self-heating test would also be required.

• Grewer Oven: Material (test sample) is heated up by means of a hot air stream that permeates through the sample. The surrounding temperature at which the sample temperature starts to rise faster than the inert reference sample is taken as the self-heating onset temperature of the sample.

• Isothermal basket test: Performed by heating the powder samples in cubical wire baskets of varying sizes (typically three sizes) in an oven to determine the minimum temperature at which each sample size self-heats. This test allows one to observe the effect of scale (that is, material size/quantity) on the material’s onset temperature for self-heating more precisely.

• Bulk powder test: Used to evaluate self-heating properties of materials in quantities not exceeding 1 ton in situations when it is heated in bulk form.

• Aerated powder test: Simulates conditions during heating operations of a material in quantities not exceeding 1 ton, during which a hot air stream flows through the bulking powder.

• Powder layer test: Simulates the conditions in which hot air passes above a layer or deposit of material in a dryer. Examples include tray dryers and material deposits on the internal surfaces of all dryer types.

For bulk storage of biomass materials, useful screening results are obtained from the above tests, but the basket tests can be used directly to model the effect of hopper/bunker storage volume. Remember, the bigger the storage volume, the lower the temperature at which self-heating will become a problem.

Precautions for Avoiding Hazards
• Keep the material temperature at a safe margin below the temperature for the onset for self-heating, obtained by appropriate laboratory test methods.

• Limit storage time.

• Facility and equipment design should avoid ledges, corners, dead zones, etc., where material could inadvertently build up inside process equipment.

• Avoid accumulation of hazardous levels of material deposits on the inside surfaces of process equipment.

• Measures to avoid other sources of ignition and fire and explosion protection must also be examined through a DHA.

Conclusion
Powders and bulked materials can smolder and catch on fire as a result of self-heating due to both chemical and/or biological processes.

Preventing self-heating and subsequent fires and explosions in bulk handling/processing operations requires proper understandingof the thermal instability characteristics of the material through specific and tailored laboratory tests, which should reasonably simulate the conditions experienced by the material during its processing and storage stages.


Author: Vahid Ebadat
CEO, Stonehouse Process Safety Inc.
609-455-0001
[email protected]