THE WOLFSON CENTRE for Bulk Solids Handling Technology
STORAGE, HANDLING AND FLOW OF BIOMASS MATERIALS Update of experience
Mike Bradley Rob Berry
0000 Background
Several substantial major projects have come on stream in the last 2 years
Much has been learned – or confirmed about what we expected!
Mostly focusing on pellets
The Wolfson Centre for Bulk Solids Handling Technology
Overview
Projects
Dust
Flow behaviour
Fire
Explosion and other safety hazards
Ash handling
The Wolfson Centre for Bulk Solids Handling Technology Problems Successes
Dust Transfer point design
Self-heating and fire Large storage silos
Pellet quality Integrated means for dust control Asphyxiation/intoxication Integrated means for fire Flat stores and front loaders control
The Wolfson Centre for Bulk Solids Handling Technology Dust
Has proved to be the most persistent and difficult problem to combat
Even on well designed plants!
Dust levels can be high due to segregation and degradation
Even if clean pellets delivered!
Dust emission highly variable
Between shipments
Through a shipment Key dust hazards:
Health
Explosion risk
Mess
The Wolfson Centre for Bulk Solids Handling Technology Inhalation of Biomass Dust
More mobile than coal dust
Stays suspended better than coal dust
Much greater health danger than coal dust Danger of “Farmer’s Lung” (Alveolitis) Common in those handling biomass in agriculture Can stimulate allergic reaction Long term exposure can be debilitating
Reducing dust emission
Mainly at transfer points
Covers required
Must be easy to open AND CLOSE! (For maintenance)
Dust extraction required – careful integration, NOT a panacea for inadequate enclosure!
Poor enclosure defeats dust control
Chute designs
Enclosure of towers
Keeping draughts out! Martin Engineering Advantages of latest transfer point design
“Hood and Spoon” technology
Gentle transfer reduces dust pumping
Less damage to pellets
Design curve to suit particle trajectory
Combine with latest design of stilling chamber and some extraction Fair success in retro-fit of transfer chute improvements That’s a clean floor!!!!!
The Wolfson Centre for Bulk Solids Handling Technology Open conveyors or closed? Stilling chamber design
Depth and length, baffles
Extraction
Many pro’s and con’s of both!
The Wolfson Centre for Bulk Solids Handling Technology Heating in impact points
In impact zones after long drops
Biomass has shown up a unique ability in this regard!
Probably due to low thermal conductivity
Dangerously high temperatures recorded – well over 100C!
Options
Make impact more concentrated – to reduce area subject to friction
Make it more disperse – to disperse the heat
Slow the flow down earlier
Large – or very large silos for storage
Choice of size, including dome option
Mostly fairly successful for storage and discharge
Fire issues
Sizing of explosion vents
Safety substantially dependent on observing good management practices!
The Wolfson Centre for Bulk Solids Handling Technology Self-heating and fire
Can be unpredictable
Newer pellets = greater hazard
Mixing batches increases danger
The Wolfson Centre for Bulk Solids Handling Technology How not to fight a silo fire!
The Wolfson Centre for Bulk Solids Handling Technology The value of inert gas injection!
The Wolfson Centre for Bulk Solids Handling Technology Averdore wood pellet silo fire
Pellets started to self-heat in two silos
Large silo
Gas injection facilities
Tankers of nitrogen brought from all over Denmark and Netherlands
Self-heating successfully brought under control
Silo and fuel inventory saved!
Smaller (still large!) silo
No gas injection facility
Total loss of silo and contents Fire detection
Sophisticated gas analysers (not just CO trending) can sometimes detect self heating at early stage
Not always reliable!
BELIEVE any warning, even if nothing appears amiss externally!
The Wolfson Centre for Bulk Solids Handling Technology Integrated plan for silo fire protection
Heat detection on infeed
Storage time (self heating) – in relation to CORE FLOW or MASS FLOW
CO trending (not just level alarm) and gas analysis
Foam dry riser to combat surface fires
Ability to inert head space against dust explosion (<8% O2) Ability to inert interstitial gas in bulk solid to much lower level to smother the fire (<2% O2)
Need to measure O2 concentration in both – in several places Inlet and outlet sealed off to contain gases
Fire will take considerable time to cool – inerting must continue for some time (weeks)
Be VERY cautious about using water – it DOES NOT PENETRATE but can be used in VERY SHORT bursts to cool head space
Care about backdraft danger
See short course presentation on biomass for full recommendations How to size explosion vents for very large stores?
Use of “full volume” dust cloud gives vents too big to fit!
Unreasonable anyway
So what size dust cloud to use?
More research needed! Asphyxiation / intoxication accidents Reported fatal accidents as a result of CO intoxication and/or oxygen depletion in confined spaces loaded with forest products (2014-03-06) # # people people severely Pel- Wood Pulp Saw Green Year Place killed injured Circumstances Source of information lets Chips logs logs Lumber Rotterdam, The A stevedore entered a stairway between two cargo holds World Pellet Conference 2002, 2002 Netherlands 1 2 onboard MV Arrow Weaver Stockholm X Sailor entered a stairway next to a cargo hold onboard MV Transportstyrelsen, Swedish 2005 Gruvön, Sweden 1 Eken with pulp logs Maritime Administration Report X Stevedore was working in an open cargo hold onboard a Saga US East Coast Forest Carrier vessel with green lumber and was overcome 2005 port 1 by oxygen depletion Saga Forest Carrier X Port of Sailor entered a stairway between two cargo hold onboard Transportstyrelsen, Swedish 2006 Helsingborg, 1 1 MV Saga Spray with pellets Maritime Administration Report X Port of Sailor entered a stairway next to a cargo hold onboard MV Swedish Maritime Magazine, 2006 Skelleftehamn, 1 Noren with wood chips Issue 1-2007 X One sailor and the captain of the vessel MV Fembria Port of Timrå, enetered the stairway leading down to the cargo hold and 2007 Sweden 2 was overcome by oxygen depletion and CO VasaSecotidningen Arbetsskyddsdistrikt, June 2007 X X 2007 Finland 1 One person entering a pellet storage with 10 tonne VasaFinland Arbetsskyddsdistrikt, X 2008 Finland 1 One person entering a pellet storage with 10 tonne Finland X
Bornholm, Two crew members onboard MV Amirante loaded with Police Authority, Rönne, 2009 Denmark 2 pellets entered a stairway leading down to the cargo hold Bornholm X X 2010 Germany 1 One person entering a pellet storage with 150 tonne HealthPropellets, and SafetyAustria Authority, X 2010 Ireland 1 One person entering a pellet storage with 10 tonne Dublin, Ireland X One pregnant woman entered a storage with 100 tonne Neue Luzerner Zeitung 2011 Switzerland 1 pellets FebruaryFalköpings 9, Tidning, 2011 February 19, X 2014 Floby, Sweden 1 2 Entry in to pellets silo 2014 X Total 15 5 10 1 1 2 1 Staffan Melin Housekeeping, explosion and fire
The Wolfson Centre for Bulk Solids Handling Technology The importance of Primary explosions housekeeping initiated by ”ignition sources”
Localised, small
Secondary explosions initiated by primary explosions.
Utter devastation Diagram: Dave Price, Gexcon
Dust layers how much dust is too much?
1 mm dust layer full ceiling height = 5 m, 100 g/m3, ”ceiling height" = 1 m 500 g/m3.
Well into the explosible range!
Calc’s and diagrams by Dave Price, Gexcon Secondary explosion – utter devastation
The Wolfson Centre for Bulk Solids Handling Technology Design to aid good housekeeping 1mm of dust?
Open flooring The Wolfson Centre for Bulk Solids Handling Technology Housekeeping a tough challenge
Especially under belts
Secondary explosion hazard
The Wolfson Centre for Bulk Solids Handling Technology Keep heat sources clean
Or risk smouldering fires leading to The Wolfson Centre for Bulk Solidsignition Handling Technology Temperature rating of electrical equipment in dusty areas
The Wolfson Centre for Bulk Solids Handling Technology The Wolfson Centre for Bulk Solids Handling Technology Housekeeping continued
Cleaning is costly, unpleasant and adds no value
But a secondary explosion is devastating!
Minimise dust production
Contain dust as well as possible
Use extraction sparingly but where required
Design to facilitate DRY cleaning (never wet)
Built-in vacuum systems
Minimal horizontal surfaces
Eliminate draughts
Easy access to heat sources (motors, drives etc) Staff training and PPE
Cleaning staff must understand the value of their task
Make their job as easy as possible
Issue Powered Air- Purifying Respirators NOT dust masks
The Wolfson Centre 3M for Bulk Solids Handling Technology Don’t forget the ASH!
Biomass ash does not behave like coal ash!
More larger particles in cyclones and first field of ESP
Often will not convey in in dense phase pneumatic systems designed for coal fly ash!
Combustion may also have a strong influence on conveying properties
May require conversion to lean phase transport
Traditional fly ash bunkers (aerated discharge) often no good for biomass ash Ash blowing
Blowing hardware may be ok
Pipeline air flows need to be revised The less efficient alternative!
The Wolfson Centre for Bulk Solids Handling Technology Storage and Discharge Issues
Flow properties Different biomasses have VERY different flow properties Some exceedingly difficult and need special solutions Many spoil with time in static storage
Selection of appropriate flow pattern, and appropriate management of silo or bunker: In some silos, first-in-last-out discharge is ok Ensure regular emptying to avoid self heating In other cases, first-in-first-out is required for operational reasons Flow patterns in silos, and feeder interfacing
The Wolfson Centre for Bulk Solids Handling Technology 0000 Core Flow: “first in last out” discharge Often used in large “dead” regions of product
erratic discharge caused by product on storage silos product shear during emptying Flow from top of central discharge channel material exaggerates segregation of dust
hopper half angle shallower
No stock rotation – must be emptied Static COMPLETELY regularly material
Maximises danger of self heating
high storage capacity for a given headroom Discharge through central flow channel The Wolfson Centre for Bulk Solids Handling Technology 0000 “first in, first out” discharge Mass flow: all storage capacity is “live” consistent discharge encouraged by the reduced levels of shear generated as the product All material in motion discharges against relatively smooth wall material - during discharge not static product
degree of remixing during discharge minimises segregation effects
hopper half angle relatively steep – depends on biomass and surface
relatively lower storage volume for a given headroom - but all the product can be retrieved Shear occurring at Desirable for vessels not regularly emptied eg MILL FEED BUNKERS walls of vessel
Requires well designed feeder interface
The Wolfson Centre for Bulk Solids Handling Technology Importance of good feeder design
Constant capacity Progressive capacity
Creates compaction Allows dilation The Wolfson Centre Feederfor Bulk Solids Handling -Technologyto- bunker interface: How not to! Large ground area Low investment Labour intensive Fast build Flat store & Dust and safety Flexibility mobile plant Fire safety and No piling ATEX rating of machinery Cleaning
The Wolfson Centre for Bulk Solids Handling Technology Plan for the long term
Change in fuel portfolio is a given
Allow space for retrofit
The Wolfson Centre for Bulk Solids Handling Technology
The Wolfson Centre for Bulk Solids Handling Technology ATEX and DSEAR:
SHAPA guidance note
Available from Wolfson Centre
The Wolfson Centre for Bulk Solids Handling Technology Summary
WITH CARE many of the major safety and technical hazards can be avoided
Must be designed out at early stage
Dust remains a challenge
Explosion vent sizing on large silos
The Wolfson Centre for Bulk Solids Handling Technology Challenges going forwards
Achieving satisfactory safety levels
Change in fuel physical characteristics due to tightening of markets
Changes to regulatory policy
The Wolfson Centre for Bulk Solids Handling Technology THE WOLFSON CENTRE for Bulk Solids Handling Technology
University of Greenwich Medway School of Engineering Tel 020-8331-8646: Fax 020-8331-8647 www.bulksolids.com