Biomass can be a cost-effective form of renewable energy. As such, the number of power-generation projects adopting biomass is increasing significantly. It is estimated that by 2020 biomass could meet 20 per cent of the UK energy demand, with this figure more than doubling by 2030. Here, Chris Coffey, senior risk engineer of GexCon UK12229 gexcon uk.htm, analyses the risks associated.
Due to the organic dust generated at biomass facilities, the risk of an explosible dust cloud forming especially within an enclosed bulk storage volume is very likely.
Dust explosion risks prevailing in industrial facilities are dependent on a large variety of factors that include process parameters, such as pressure and temperature; equipment properties, such as the presence of moving elements, the mechanical strength of such equipment and the presence of effective dust handling equipment (known as local extract ventilation or LEV).
It also depends on the dust explosion characteristics (does it require a little energy or a lot of energy to ignite, does it propagate flame rapidly); and mitigating measures taken including ignition detection and suppression and constructive protective measures such as explosion relief venting combined with effective explosion isolation to mitigate the propagation of an explosion through interconnected process.
Biomass power plants frequently store biomass material (typically in pellet form) in very large storage silos. This allows a continuous power generation operation as there is always a large store of material available.
These silos can be built so large that they have volumes up to 100,000 m3. Biomass material is usually transported into the storage silos via conveyors, which drop the material in from the top of the silo roof. During filling or emptying operations, large dust clouds can potentially be created as a result of dust brought in with the pellets and even with clean pellets from attrition due to the pellets striking one another during free fall and on impact with the product pile.
Clouds of fine, dry biomass dust are highly explosive due to their large surface area for a given mass, and thus the presence of these clouds pose a significant safety risk should an effective ignition source be present inside the silo at the same time as a dust cloud having its concentration within explosible limits.
Explosion relief venting is a common concept used to reduce the potential consequences of dust explosions in silos and other vessels. Several venting panels are located on a silo, which yield at a predetermined low pressure. This allows relief of the pressure that is building up inside the silo during an explosion event, and therefore reduces the maximum pressure realised within the silo.
International standards for the design of explosion relief venting (NFPA 68 2013, EN 14491 2012, VDI 3673) were generally developed for smaller vessels with volumes less than 10,000 m3, with empirical calculations to determine the total vent area required to achieve explosion protection in the event of an internal explosion.
For the much larger silos commonly encountered on modern plants with volumes greater than 10,000 m3, there is no particular guidance or standard for reference. These empirical methods also do not allow details for the design to be assessed, as they do not include a detailed representation of the geometry. For, example, the effects of the layout of relief vents on such silos are not typically considered during design. In fact, The EN 14491 2012 standard states the following:
“If the enclosure is small and relatively symmetrical, one large vent can be as effective as several small vents of equal combined area. For large enclosures, the location of multiple vents to achieve uniform coverage of the enclosure surface to the greatest extent practicable is recommended.
“The required vent area can, in practical applications, be divided into several smaller areas as long as the total area equals the required vent area.”
GexCon’s experience, during various industrial projects and an ongoing research project, shows that the layout of the venting is critical as is a proper understanding and representation of structures or objects on the silo roof. In light of this, the above statement appears to be somewhat misleading and care must be taken in its interpretation.
As an alternative to the empirical formula presented in the standards, Computational Fluid Dynamics (CFD) codes may be used. FLACS-DustEx is a CFD tool developed to simulate dust explosions. A three-dimensional grid is made for the preferred simulation volume forming small grid cells (control volumes).
As an example, based on the geometry, fuel (dust type) and other key parameters, FLACS-DustEx calculates the pressure, temperature and dust concentration in each control volume at each time step. This solves the equations of mass balance within a Cartesian grid covering the whole process vessel domain inside and outside.
Hence, FLACS-DustEx can be used to determine the maximum pressures realised for any size and shape of silo containing any size and shape of dust cloud and the internal and external pressures, flame velocities, densities, drag forces on steel members. The far field pressures on external objects and buildings can also be evaluated in detail.
By using such refined analysis techniques based on a more realistic probable scenario, one may show that the vent area required to adequately protect a silo is significantly smaller than that predicted by EN 14491:2012, for example.
This can provide significant economic benefits. Reducing the explosion relief vent area to a minimum (lower CAPEX) while maintaining effective protection also has advantages over the working life time of the silo.
A reduced explosion relief vent area corresponds to a reduced chance of a mechanical failure of the panels and a reduced maintenance cost (lower OPEX). The probability of water ingress into the silo and the probability of a fire from an exothermic reaction occurring is also reduced (lower fire risk).
Savings made through this type of value engineering can be redistributed to afford other prevention methods within the process. For example, more robust dust control and ignition detection, which ultimately continues to drive a lower overall residual risk hence improving the safety of personnel and protecting the plant and business.
Dust and flames being emitted from explosion vent panels
during a silo explosion simulation
Pressures predicted on surrounding silos and buildings
during a silo explosion
The initial dust cloud defined inside a biomass storage silo
GexCon U.K.: Advanced Modelling Reduces Cost and Risk
Advanced modelling reduces cost and risk
Biomass can be a cost-effective form of renewable energy. As such, the number of power-generation projects adopting biomass is increasing significantly. It is estimated that by 2020 biomass could meet 20 per cent of the UK energy demand, with this figure more than doubling by 2030. Here, Chris Coffey, senior risk engineer of GexCon UK12229 gexcon uk.htm, analyses the risks associated.
Due to the organic dust generated at biomass facilities, the risk of an explosible dust cloud forming especially within an enclosed bulk storage volume is very likely.
Dust explosion risks prevailing in industrial facilities are dependent on a large variety of factors that include process parameters, such as pressure and temperature; equipment properties, such as the presence of moving elements, the mechanical strength of such equipment and the presence of effective dust handling equipment (known as local extract ventilation or LEV).
It also depends on the dust explosion characteristics (does it require a little energy or a lot of energy to ignite, does it propagate flame rapidly); and mitigating measures taken including ignition detection and suppression and constructive protective measures such as explosion relief venting combined with effective explosion isolation to mitigate the propagation of an explosion through interconnected process.
Biomass power plants frequently store biomass material (typically in pellet form) in very large storage silos. This allows a continuous power generation operation as there is always a large store of material available.
These silos can be built so large that they have volumes up to 100,000 m3. Biomass material is usually transported into the storage silos via conveyors, which drop the material in from the top of the silo roof. During filling or emptying operations, large dust clouds can potentially be created as a result of dust brought in with the pellets and even with clean pellets from attrition due to the pellets striking one another during free fall and on impact with the product pile.
Clouds of fine, dry biomass dust are highly explosive due to their large surface area for a given mass, and thus the presence of these clouds pose a significant safety risk should an effective ignition source be present inside the silo at the same time as a dust cloud having its concentration within explosible limits.
Explosion relief venting is a common concept used to reduce the potential consequences of dust explosions in silos and other vessels. Several venting panels are located on a silo, which yield at a predetermined low pressure. This allows relief of the pressure that is building up inside the silo during an explosion event, and therefore reduces the maximum pressure realised within the silo.
International standards for the design of explosion relief venting (NFPA 68 2013, EN 14491 2012, VDI 3673) were generally developed for smaller vessels with volumes less than 10,000 m3, with empirical calculations to determine the total vent area required to achieve explosion protection in the event of an internal explosion.
For the much larger silos commonly encountered on modern plants with volumes greater than 10,000 m3, there is no particular guidance or standard for reference. These empirical methods also do not allow details for the design to be assessed, as they do not include a detailed representation of the geometry. For, example, the effects of the layout of relief vents on such silos are not typically considered during design. In fact, The EN 14491 2012 standard states the following:
“If the enclosure is small and relatively symmetrical, one large vent can be as effective as several small vents of equal combined area. For large enclosures, the location of multiple vents to achieve uniform coverage of the enclosure surface to the greatest extent practicable is recommended.
“The required vent area can, in practical applications, be divided into several smaller areas as long as the total area equals the required vent area.”
GexCon’s experience, during various industrial projects and an ongoing research project, shows that the layout of the venting is critical as is a proper understanding and representation of structures or objects on the silo roof. In light of this, the above statement appears to be somewhat misleading and care must be taken in its interpretation.
As an alternative to the empirical formula presented in the standards, Computational Fluid Dynamics (CFD) codes may be used. FLACS-DustEx is a CFD tool developed to simulate dust explosions. A three-dimensional grid is made for the preferred simulation volume forming small grid cells (control volumes).
As an example, based on the geometry, fuel (dust type) and other key parameters, FLACS-DustEx calculates the pressure, temperature and dust concentration in each control volume at each time step. This solves the equations of mass balance within a Cartesian grid covering the whole process vessel domain inside and outside.
Hence, FLACS-DustEx can be used to determine the maximum pressures realised for any size and shape of silo containing any size and shape of dust cloud and the internal and external pressures, flame velocities, densities, drag forces on steel members. The far field pressures on external objects and buildings can also be evaluated in detail.
By using such refined analysis techniques based on a more realistic probable scenario, one may show that the vent area required to adequately protect a silo is significantly smaller than that predicted by EN 14491:2012, for example.
This can provide significant economic benefits. Reducing the explosion relief vent area to a minimum (lower CAPEX) while maintaining effective protection also has advantages over the working life time of the silo.
A reduced explosion relief vent area corresponds to a reduced chance of a mechanical failure of the panels and a reduced maintenance cost (lower OPEX). The probability of water ingress into the silo and the probability of a fire from an exothermic reaction occurring is also reduced (lower fire risk).
Savings made through this type of value engineering can be redistributed to afford other prevention methods within the process. For example, more robust dust control and ignition detection, which ultimately continues to drive a lower overall residual risk hence improving the safety of personnel and protecting the plant and business.
Dust and flames being emitted from explosion vent panels
during a silo explosion simulation
Pressures predicted on surrounding silos and buildings
during a silo explosion
The initial dust cloud defined inside a biomass storage silo
for a silo explosion simulation
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