Health and safety aspects of solid biomass storage, transportation and feeding

Abstract

With the continuous growth in production and utilisationof solid biofuels, an increasing number of incidentshave taken place related to the handling of the biofuels,resulting in affected personnel due to injuries, infections,intoxications etc. resulting in illness or tragic loss oflife, or severe material damage due to dust explosions,fi res etc. This report has been written by various expertsthat are active in the IEA Bioenergy agreement, toprovide the reader with an understanding of the causes,effects and mitigation measures overview of the issues.The properties of a biomass material and the intended usedetermine how the material should be safely produced,transported, stored and used. While woody biofuels suchas pellets and chips from fresh or recycled wood dominatethe market in terms of volumes, other solid biofuelssuch as straw, biodegradable fuels used for anaerobicdigestion and municipal solid waste pose specifi c healthand safety challenges that need to be addressed.Self-heating processes may be due to biological metabolicreactions (microbiological growth), exothermic chemicalreactions (chemical oxidation) and heat-producingphysical processes (e.g. moisture absorption), and itmay occur both for dry and wet biofuels. It may becomeproblematic if a pile or silo is so large that the heatgenerated cannot be easily dissipated to the surroundings.While this is not the case for relatively small scaleinstallations as e.g. used by households, it needs attentionfor larger industrial storages. Several test methods areavailable for determining self-heating potential and selfignitionof materials on small scale, which can then beextrapolated to predict self-heating potential at largerscale. Apart from self-heating, biomass stock may beset on fi re through various external sources such as hotbearings, overheated electric motors, back-fi re, etc.Several conclusions can be drawn from the full scale silofi res that have already taken place. The most importantprevention measure to take is temperature monitoring ofthe storage at several different locations in the fuel bulk.For detection of any activity of the bulk, CO concentrationshould be measured in the air above the pellet surface.The fi rst sign of an on-going self-heating process is oftena sticky and irritating smell (probably from aldehydesand low molecular carboxylic acids). If this is sensed,there is already pyrolysis taking place somewhere in thefuel bulk and a fi re fi ghting operation has to be initiated.Such fi re fi ghting strategy needs to be determined case bycase and requires specialised fi re fi ghting equipment andtrained staff. Water should not be used in case of woodpellets, due to rapid expansion of the pellets (hinderingthe extinguishing operation and subsequent unloading ofthe silo) and the formation of explosive H2 through watergas shift reactions. A silo fi re is usually extinguishedby inerting the closed silo from the bottom, after whichfuel is discharged from an opening in the silo wall.Off-gassing is the process where volatile organiccompounds are released in the logistical chain. Onemechanism is the initial release of lipophilic compounds,yielding carbonyl compounds (aldehydes and ketones)and also complex terpenes. CO, CO2 and CH4 may alsobe released. The concentrations of aldehydes found indomestic sites and warehouses constitute a health hazardand require attention and preventative measures to betaken. Hexanal may enter the body by contact with skinor by inhalation and cause skin irritation, headaches, anddiscomfort on the eyes and nose. Other aldehydes such asmethanal and ethanal are suspected to be carcinogenicin high doses and may also have some short time effecton human health. There are several guidelines issued bygovernment offi cial institutes that describe the effectthese aldehydes have on human health depending onexposure time and level. Monoterpenes (particularlypresent in fresh raw material) cause eyes and respiratorysystem irritation. CO may be released from the autooxidation of lipophilic compounds. Related hazards arepredominantly poisoning, but it may also contribute toself heating or ignition processes. A combination of proper ventilation, gas meters and the use of self-containedbreathing devices is needed in areas where the levelsof CO might increase to poisonous concentrations.Dust clouds are a major cause of damage in thebioenergy sector. The combination of relatively smallparticle sizes and low minimum ignition energy resultsin a high ignition sensitivity. Signifi cant amounts offactory dust may stay be suspended in the air, so thatthe Minimum Explosible Concentration is easily reachedunder practical conditions if cleaning and ventilationare not done suffi ciently. It is therefore important tominimize the risk of dust explosions, by minimizingthe risk of sparks (e.g. due to electrostatic dischargethrough proper grounding) and good dust housekeepingthrough dust prevention and dust collection. Once anexplosion takes place, it needs to be properly contained,suppressed or vented. Compliance with ATEX Directivesand NFPA guidelines is essential in this respect.The health risks posed by biomass fuels in the form ofdusts and bioaerosols come from the both the physicalparticle and size effects. As particles become smallerthey pose a greater hazard. As a result limits on PM10and PM2.5 (particles less than 10 μm or 2.5 μmrespectively) are becoming more prevalent in nationalregulations. In addition the organic nature of biomassfuels may result in additional impacts through eitherallergenic or pathogenic routes. The most prevalentfeature will be the allergenic responses and the majorityof the effects will be minor and short lived; but increasingseverity of impact will also be linked to falling incidenceof response. In the same way pathogenic responses will bea rare occurrence, but potentially result in severe hazards.The biological materials released from biofuels are similarto the naturally occurring background levels and thehuman population are equipped with bodily responsesto deal with this natural environmental exposure. Thisnatural bodily response and the degree of variation inindividual sensitivity makes the determination of doseresponse relationships particularly diffi cult. Without thislevel of understanding it is diffi cult to ascribe defi nitelimits and regulations that are "safe" for the populationat large whilst also protecting the population fromhazards that derived from the alternatives to biofuel use(e.g. climate change, fossil fuel emissions, fuel poverty).Thus this area is generally governed by guideline valuesand recommendations to minimise exposure rather thandefi nitive limits that are derived from hard scientifi c data.The potential health impact of bioaerosols from wastemanagement processes is relatively low for wastecollection, transfer and sorting. Whilst it can be moderatefor open microbial processes such as composting(particularly during movement and unloading of material)due to pathogenic microorganisms and microbialconstituents or metabolic products, such as organic dustand endo-toxins, the risk is relatively low in the case ofanaerobic digestion where processes are contained andthe product is often hygienised. The risks are generallyairborne, and therefore likely to occur through inhalationof bioaerosols, but there are also ingestion risks, oftenthrough hand-mouth contact or from infection of wounds.Trauma and sharps injuries are the most commonrecorded cause of accidents and injury in the forestryand waste management sectors and we have assumedthat these also affect workers in the bioenergy andenergy from waste parts of these sectors. The seriousnature of some injuries and the number of injuriesmeans that there has been a lot of work to reduceinjuries and in most countries there are establishedobligations and guidance. Rather than comprehensivelyreviewing this legislation we have provided anintroduction with references, providing an insightinto the risk evaluation and mitigation methodologiesavailable at international and national level.

Keywords

Safe pellets

Published in

Publisher: IEA Bioenergy

SLU Authors

• Larsson, Ida

  • SP Technical Research Institute of Sweden

• Arshadi, Mehrdad

  • Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences
    

• Valencia-Reyes, Elizabeth

  • Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences
    

Associated SLU-program

Forest
Climate

UKÄ Subject classification

Chemical Process Engineering

Permanent link to this page (URI)

https://res.slu.se/id/publ/42336