An industry-led investigation for safe storage at full scale of compressed wood pellets including self-heating and spontaneous combustion

• Main Author: Ashman, James Mark
• Other Authors: Jones, Jenny M. ; Williams, Alan
• Published: University of Leeds 2017
• Subjects: 662
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.718795

Large scale self-heating of wood pellets in storage is a major concern worldwide during long term storage due to the potential for fire with safety concerns for both personnel and plant. Additionally, any adverse publicity resulting from an incident involving biomass is likely to result in negative effects to the biomass industry as a whole. Self-heating could lead to serious accidental fires, causing enormous damage and danger to workers. The aim of the research in this thesis was to provide underpinning data and understanding in order to implement safe storage of wood pellets at Drax Power Ltd. Thus, the self-heating rates at different temperatures, and under different levels of inerting, were experimentally determined, and the thermal properties were measured for wood pellets produced for Drax Power Ltd. In addition, the thesis covers measurements at three scales; laboratory, temporary storage of approximately 40,000 tonnes through to largescale storage at approximately 280,000 tonnes spread equally over 4 domes… the largest storage of pellets ever investigated. The factors such as moisture content, pellet age and environment temperature were investigated and their impacts on the self-heating process were analysed. Moisture content has a significant effect on effective on self-heating but ventilation of the store was found to be more critical. Pellets age and environment temperature are two major factors impacting the self-heating and off-gassing process. The self-heating rate is significantly increased at higher a temperature and eventually will lead to a thermal runaway when the ambient temperature is high enough. Based on all measured properties, a self-heating model was developed to predict the self-heating process and thermal runaway in large wood pellet dome of ~120,000m3. This work contributed to the inerting of nitrogen gas to maintain an atmospheric (and therefore assumed within biomass) oxygen content less than 10% and monitoring protocols with the focus on carbon monoxide along with the understanding of temperature tracking for the large-scale dome storage of wood pellets, which is now used by Drax Power Ltd. Safety of all personnel working with biomass is paramount concern for Drax Power Ltd., which this thesis contributed to personal exposure monitoring and standard setting for gas exposure, mainly carbon monoxide and dioxide along with dust exposure. The handling of pellets through the supply chain and on site can cause the pellets to degrade and fine wood particles and dust is present within bulk pellets. Any release of wood dust into the atmosphere can pose a risk to health. Wood dust can cause serious health problems. Wood dust is a recognised respiratory irritant, sensitizer, asthmagen and for a limited number of species, a potential carcinogen (hard woods). Contact with wood dust can also lead to skin sensitisation and dermatitis. Drax Power Ltd, recognises it’s duties under Health and Safety and is committed to minimising the risk to health from wood dust in its premises. Both hardwood and softwoods dusts have a Workplace Exposure Limit (WEL) of 5mg/m3, which must not be exceeded. These are limits placed on the amount of dust in the air over an eight–hour shift. However the Control of Substance Hazardous to Health Regulations 2002 (as amended) requires exposure to wood dust to be “As low as reasonably practicable” (ALARP). Drax Power Ltd will manage exposure to wood dust to as low as reasonably practicable. The primary means of preventing exposure, is by containment of wood dust within the materials handling and processing plant. Dust control equipment such as Local Exhaust Equipment (LEV), will be used in conjunction with handling and processing plant to ensure that dust levels are kept to as low as is reasonably practicable. For certain operational and maintenance tasks it is necessary to use additional procedural controls to ensure that the risk of exposure to wood dust are managed effectively.