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STORAGE, HANDLING AND FLOW
OF BIOMASS MATERIALS
A Review
Mike Bradley
Rob Berry
0000
What is biomass?
 Organic

waste
Sewage sludge, shredded refuse
 “Coproducts”,
“Residues”

Waste but avoiding the “W” word!

From food, agricultural, or forest / timber / paper
processing

Straw, bits of tree, olive stones etc.
 Energy

crops
Chipped wood, cut elephant grass
Cereal coproduct:
wheatfeed pellets
Pellets
and
fines
Rounded
Like
any other
free-flowing
bulk solid
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Milled nuts
Sticky,
cohesive
Particles
roughly
rounded
Like
any other
cohesive bulk
solid
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Chopped straw
Long,
stringy
particles
Special
behaviour
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Miscanthus (elephant grass)
Long,
stringy
particles again
Special
behaviour
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Forest residue
Long,
stringy
particles again
Special
behaviour
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Features
Other examples
Horse
or poultry
bedding
Whole
Wet
pallets
leaves
Meat
waste
 Biomass
is not one
material!
 All
different handling
properties
 No
one handling system
can deal with all biomass
(economically)
 Special
characterisation
techniques available
Biomass macroeconomics
Renewable
energy costs more
 Currently
adds around £20 to the household
annual power bill
 Target
is to rise to £50 when about 20% of
our power from renewables
Sustainability
of biomass depends on
cross-subsidy from fossil fuels
The ROCs system
 The
Renewables Obligation
 ROC
= Renewables Obligation Certificate
 Granted
for a certain quantity of power
produced from certified renewables (1MWh =
1000 “units”)
 Each
generator must show a target proportion
of their output from renewables
 Proof
is in submission of ROCs at end of year
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ROC trading
 Generators
 Those
 The
with a surplus of ROCs can sell them
who haven’t earned enough can buy
market fixes the price
 Allows
“dirty” generator to carry on making cheap
electric from fossil fuels
 Whilst
they subsidise the “green” generators using
wind, biomass etc
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ROC banding
Or “When is a ROC
not a ROC?”
 More
costly renewables are given more ROCs
to subsidise them

Offshore wind 2

Solar PV 2
 Cheaper
ones given less ROCs

Biomass co-firing 0.5

Conversion of existing power plants from fossil to
biomass 1.5 but falling to 1
ROC rebanding
October
21 2011:
Proposed changes to how many ROCs
issued per MWh of generation:
http://www.decc.gov.uk/en/content/cms/ne
ws/pn11_85/pn11_85.aspx
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Current support, ROCs/MWh[1]
Renewable electricity technologies
Advanced gasification
2
Proposed ROC support/MWh[2]
2 in 2013/14 and 2014/15; 1.9 in
2015/16 and 1.8 in 2016/17
Other proposed changes
Proposed change to definition and merger with advanced pyrolysis
Call for evidence
Advanced pyrolysis
2
2 in 2013/14 and 2014/15; 1.9 in
2015/16 and 1.8 in 2016/17
Proposed change to definition and merger with advanced gasification
Call for evidence
Anaerobic digestion
2
2 in 2013/14 and 2014/15; 1.9 in
2015/16 and 1.8 in 2016/17
Biomass conversion
No current band but eligible to claim
1.5ROCs under current banding
arrangements
1
Call for evidence
Proposal for a new band.
Co-firing of biomass
0.5
0.5
Changes proposed to add fossil derived bioliquids.
Co-firing of biomass (enhanced)
No current band but 0.5 ROCs under
current banding arrangements
1
Call for evidence
Proposal for a new band.
Co-firing of biomass with CHP
1
1
Changes proposed to add fossil derived bioliquids, to exclude enhanced cofiring and to close this band to new accreditations from 1 April 2015.
Co-firing of energy crops
1
1
Changes proposed to the definition of energy crops and to exclude enhanced
co-firing.
Co-firing of energy crops with CHP
1.5
1.5
Call for evidence
Changes proposed to the definition of energy crops, to exclude enhanced cofiring and to close this band to new accreditations from 1 April 2015.
Dedicated biomass
1.5
1.5 to 31 March 2016
1.4 from 1 April 2016
Changes proposed to exclude biomass conversions and to add fossil-derived
bioliquids
Dedicated energy crops
2
2 in 2013/14 and 2014/15; 1.9 in
2015/16 and 1.8 in 2016/17
Changes proposed to the definition of energy crops and to exclude biomass
conversion.
Dedicated biomass with CHP
2
2 in 2013/14 and 2014/15
Changes proposed to add fossil derived bioliquids, to exclude biomass
conversion and to close this band to new accreditations from 1 April 2015.
Dedicated energy crops with CHP
2
2 in 2013/14 and 2014/15
Call for evidence
Changes proposed to the definition of energy crops, to exclude biomass
conversion and to close this band to new accreditations from 1 April 2015.
Energy from waste with CHP
1
0.5
Call for evidence
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Handling and storage issues with biomass
 Low
density – so large quantities
 Poor
flow – and all different
 Dust
emission
 Bio-activity
 Sensitivity
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– may self-heat
to water – need to keep dry
Energy density by volume
Wood pellets
Miscanthus (bale 25% MC)
Log w ood (stacked air dry: 20% MC)
Wood chips (30%
MC)
Coal
0
5000
10000
15000
20000
Volumetric energy density MJ/m3
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25000
30000
Bulk density of raw biomass fuels
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Govt of Ontario
Fundamental Issues;
Dust Emission
 Many
biomasses contain high dust levels
and have great potential to emit that dust
 “Surface
dry” at high moisture (even
20%)

Coal saturated at about 6%

Coal itself becomes more dusty when cohandled with biomass
 Also
have lower density and more
complex particle shapes
 Varies
within “same” material in
specification
Key dust hazards:
Health
Explosion
Mess
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risk
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


Aspergillus Mould
 Grows
freely on organic
matter at +14%
moisture approx.
 Commonly
present still
on materials that have
been dried
 Principal
pathogen for
Alveolitis
 Many
other hazardous
moulds found in
biomass
Wet cleaning and wet dust suppression
MUST NOT be used!
 E.g.
Spilt maize
attracted dampness
 Mouldy
in a few days
 Dusty
areas must be
kept dry
 Dust
must be kept
inside
Reducing dust emission
 Mainly
at transfer points
 Covers
required


Must be easy to open AND CLOSE! (For
maintenance)
Dust extraction NOT a panacea for inadequate
enclosure

Poor enclosure will greatly increase air volume to be
extracted
 Chute
designs
 Enclosure

of towers
Keeping draughts out!
Martin Engineering
“Hood and Spoon”
design
 Gentle
transfer to avoid
knocking dust out of
flow
 Design
curve to suit
particle trajectory
 Adjustment
needed on
site
 Definitely
sprays!
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NO wet
Primary explosions
initiated by ”ignition
sources”

Localised, small
 Secondary
explosions
initiated by primary
explosions.

Utter devastation
Diagram: Dave Price, Gexcon
The importance of
housekeeping
Dust layers
how much dust is too much?
1 mm dust layer
full ceiling height = 5 m, 
”ceiling height" = 1 m

100 g/m3,
500 g/m3.
Well into the explosible range!
Calc’s and diagrams by Dave Price, Gexcon
Secondary explosion
– utter devastation
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Design to aid good
housekeeping
 1mm
of dust?
 Open
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flooring
Sheet over “girts” or
add sloping dustshedder plates
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Dust refuges on top of cabinets
Again,
add shedders
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Keep heat sources clean
Or
risk
smouldering
fires
leading
to
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
“first in last out” discharge

“dead” regions of product

erratic discharge caused by
product on product shear during
emptying

central discharge channel

exaggerates segregation effects
of particles

hopper half angle shallower

poor stock rotation

Caking and biological spoilage

high storage capacity for a given
headroom – but often not all can
be discharged
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Core Flow:
Undesirable for
biomass in some
circumstances
Flow from top of
material
Static
material
Discharge
through
central flow
channel
0000
0000

“first in, first out” discharge

all storage capacity is “live”

consistent discharge encouraged by
the reduced levels of shear
generated as the product discharges
against relatively smooth wall
material - 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

Requires
interface
well
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designed
feeder
Mass flow:
Desirable for
biomass
All material in motion
during discharge
Shear
occurring at
walls of vessel
Sometimes the powder flow properties and the
equipment design are just not compatible!
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2004: Three distinct types of biomass
from a HANDLING point of view
 “CLASS

Pellets and dry powders (not too fine)
 “CLASS

1”: Rounded particles, free-flowing:
2”: Rounded particles, cohesive:
Milled nuts, wet powders; fine powders
 “CLASS
3”: Flaky/stringy particles – extreme shape:

At least one dimension much smaller than other two!

Chopped straw and grass;

Sawdust, wood shavings, chicken litter

Shredded paper or plastic;

Many wastes from sheet material

Herbaceous materials
Core flow bins with
non-free-flowing
materials
 Tendency
to rat-
hole
 Not
recommended
for class 2 or class 3
materials!
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Planetary screw
discharger
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Push floor (sliding frame) discharger
Hydraulic
cylinder
Reciprocating
framework
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Discharge
screw
Push floor discharger

1. Hydraulic Power
Pack
2. Rear Beam for
Cylinder Mounting
3. Hydraulic Cylinders
4. Push Elements
5. Levelling Screw
6. Discharge Head
7. Metering Discharge
Screw
8. Fall Chute
Picture:
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Problems with full-live-bottom for
flaky/stringy (“class 3”) biomass
 High
stress at base
 Creates
 High
forces on mechanism
 Severe

wear and screw breakage
In planetary or multiple screw machines
 Need

great strength in material
wide, shallow bunker
In sliding-frame type machine
 Great
expense to get ruggedness needed!
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Mass flow as a cheaper option
 Many
“class 3” biomasses
have moderate wall
friction


Especially against UHMWpolyethylene lining
Mass flow at reasonable
angle
 Low


stress at outlet
Flow occurs by relieving
stress, not by increasing it!
Reduced load on feeder
 Smaller
feeder
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Wedge-shaped mass flow bin
 Twin
screw
outlet
 Very
steep walls
for highly
frictional
material
 Most
cases do
not need such
steep walls!
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Pelletisation of biomass
Reducing
delivery cost
Reducing
handling challenges
 Reduced
variation in handling
properties
 Lower
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project cost
Case study:
pelletisation versus
chip
 10,000
tpa of wood
 Delivered
 Using
10 miles
8-wheel tipper (body 15m3)
 Assumptions:





Fuel consumption 6mpg fully loaded, 12mpg lightly loaded
Diesel £1.35 per litre
Driver cost £24/hr
Average speed 15 mph
Ignore purchase and depreciation
Pelletiser
5.5kW
motor
Produces
0.5 tph
Electricity
cost £0.09/kWh
 11kWh/tonne
x 10,000 tonnes x £0.09/kWh
= £9,900 pa electricity cost
Milling energy ignored
Transport costs - for 10 miles delivery
For a transport distance of
Transport costs:
Body capacity
Bulk density
Hence actual weight per delivery
Deliveries per annum
Miles light
Miles fully loaded
Diesel used pa
Cost of diesel
Driver hours per delivery
Driver hours pa
Driver cost per hour
Driver cost pa
Cost of pelletising
Total costs
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& proc. cost per tonne
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10 miles
pelletised
chip
m3
15
600
180
9
2.7
1111
3704
11111
74074
11111
2513
6173
£15,268
£37,500
0.67
0.67
741
2469
£24.00
£24.00
£17,778
£59,259
£9,900
£0
£42,946
£96,759
£4.29
£9.68
kg/m3
tonnes
deliveries
gallons
£ pa
£pa
Characterisation – more critical for biomass

Flow properties

Dustiness testing

Friction

Density

Caking

Moisture

Size and shape

Explosibility
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Choice of biomass cases for design
No
one system will handle all
biomass materials
But
experience shows that systems
designed for one biomass will soon
be burning another!
Consider
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this in design
Plan for the long term
Change
Allow
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in fuel portfolio is a given
space for retrofit
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Contents of the Guide
Coverage: Biomass handling and
biomass/coal co-handling
 Part
I: Basic problems
 Part
II: Guide to practice on existing
coal-handling installations gearing up for
biomass
 Part
III: Guide to practice for new
installations
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