Damage control – biomass health and safety

20 August 2014

With the continuous growth in production and use of solid biofuels, an increasing number of handling-related incidents have taken place, which affect personnel through injuries, infections and intoxications, resulting in illness, tragic loss of life or severe material damage due to dust explosions, fires and more. Jaap Koppejan, task leader, IEA Bioenergy, examines the health and safety aspects of solid biomass storage, transportation and feeding.

The properties of a biomass material and the intended use determine how the material should be safely produced, transported, stored and used. While woody biofuels such as pellets and chips from fresh or recycled wood dominate the market in terms of volumes, other solid biofuels such as straw, biodegradable fuels used for anaerobic digestion, and municipal solid waste pose specific health and safety challenges that need to be addressed.

Self-heating processes may be due to biological metabolic reactions (microbiological growth), exothermic chemical reactions (chemical oxidation) and heat-producing physical processes (moisture absorption, for example), and it may occur for dry and wet biofuels. It may become problematic if a pile or silo is so large that the heat generated cannot be easily dissipated to the surroundings.

While this is not the case for relatively small-scale installations as used by households, for example, it needs attention for larger industrial storage. Several test methods are available for determining self-heating potential and self-ignition of materials on a small scale, which can then be extrapolated to predict self-heating potential at a larger scale. Apart from self-heating, biomass stock may be set on fire through various external sources such as hot bearings, overheated electric motors or back-fire, for example.

"The first sign of an ongoing self-heating process is often a sticky and irritating smell –probably from aldehydes and low molecular carboxylic acids."

Several conclusions can be drawn from the full-scale silo fires that have already taken place. The most important prevention measure to take is temperature monitoring of the storage at several different locations in the fuel bulk. For detection of any activity of the bulk, CO concentration should be measured in the air above the pellet surface.

Early indicators

The first sign of an ongoing self-heating process is often a sticky and irritating smell - probably from aldehydes and low molecular carboxylic acids. If this is sensed, there is already pyrolysis taking place somewhere in the fuel bulk and a firefighting operation has to be initiated. Such a firefighting strategy needs to be determined case by case, and requires specialised firefighting equipment and trained staff. Water should not be used in case of wood pellets, due to rapid expansion of the pellets - hindering the extinguishing operation and subsequent unloading of the silo - and the formation of explosive H2 through water gas shift reactions. A silo fire is usually extinguished by inerting the closed silo from the bottom, after which fuel is discharged from an opening in the silo wall. Off-gassing is the process where volatile organic compounds are released in the logistical chain. One mechanism is the initial release of lipophilic compounds, yielding carbonyl compounds (aldehydes and ketones) and also complex terpenes. CO, CO2 and CH4 may also be released.

The concentrations of aldehydes found in domestic sites and warehouses constitute a health hazard and require attention and preventative measures to be taken. Hexanal may enter the body via contact with skin or by inhalation and cause skin irritation, headaches, and discomfort on the eyes and nose. Other aldehydes such as methanal and ethanal are suspected to be carcinogenic in high doses and may also have some short-time effect on human health. There are several guidelines issued by government institutes that describe the effect these aldehydes have on human health depending on exposure time and level. Monoterpenes - particularly present in fresh raw material - cause eyes and respiratory system irritation. CO may be released from the auto-oxidation of lipophilic compounds. Related hazards are predominantly poisoning, but it may also contribute to self-heating or ignition processes. A combination of proper ventilation, gas meters and the use of self-contained breathing devices is needed in areas where the levels of CO might increase to poisonous concentrations.

Dust up

Dust clouds are a major cause of damage in the bioenergy sector. The combination of relatively small particle sizes and low minimum ignition energy results in high ignition sensitivity. Significant amounts of factory dust may be suspended in the air, so that the minimum explosible concentration is easily reached under practical conditions if cleaning and ventilation are not done sufficiently. It is therefore important to minimise the risk of dust explosions by minimising the risk of sparks - due to electrostatic discharge through proper grounding - and good dust housekeeping through dust prevention and dust collection.

Once an explosion takes place, it needs to be properly contained, suppressed or vented. Compliance with ATEX directives and NFPA guidelines is essential in this respect. The health risks posed by biomass fuels in the form of dusts and bioaerosols come from the physical particle and size effects. As particles become smaller they pose a greater hazard. As a result, limits on PM10.0 and PM2.5 (particles less than 10.0µm or 2.5µm respectively) are becoming more prevalent in national regulations. In addition, the organic nature of biomass fuels may result in additional impacts through either allergenic or pathogenic routes. The most prevalent feature will be the allergenic responses, and the majority of the effects will be minor and short lived; but, increasing severity of impact will also be linked to falling incidence of response. In the same way pathogenic responses will be a rare occurrence, but potentially result in severe hazards.

"The limitation of gas turbines can also be attributed to the emergence of renewable energy sources, which continue to attract sizable investment as international agencies look to bolster their green credentials."

The biological materials released from biofuels are similar to the naturally occurring background levels and the human population are equipped with bodily responses to deal with this natural environmental exposure. This natural bodily response and the degree of variation in individual sensitivity make the determination of dose response relationships particularly difficult. Without this level of understanding, it is difficult to ascribe definite limits and regulations that are "safe" for the population at large while also protecting the population from hazards that derived from the alternatives to biofuel use; for example, climate change, fossil fuel emissions and fuel poverty. Thus, this area is generally governed by guideline values and recommendations to minimise exposure rather than definitive limits that are derived from hard scientific data. The potential health impact of bioaerosols from waste-management processes is relatively low for waste collection, transfer and sorting. While it can be moderate for open-microbial processes such as composting - particularly during movement and unloading of material - due to pathogenic microorganisms and microbial constituents or metabolic products, such as organic dust and endo-toxins, the risk is relatively low in the case of anaerobic digestion where processes are contained and the product is often hygienised. The risks are generally airborne, and therefore likely to occur through inhalation of bioaerosols, but there are also ingestion risks, often through hand-mouth contact or from infection of wounds.

Trauma and sharp injuries are the most-common recorded cause of accidents and injury in the forestry and waste-management sectors, and it is assumed that these also affect workers in the bioenergy and energy from waste parts of these sectors. The serious nature of some injuries and the number of injuries means that there has been a lot of work to reduce injuries, and in most countries there are established obligations and guidance. Rather than comprehensively reviewing this legislation, IEA Bioenergy has provided an introduction with references, providing an insight into the risk evaluation and mitigation methodologies available at an
international and national level.

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