Clara HUÉSCAR, Herodotos N. PHYLAKTOU, Gordon E. ANDREWS,
Bernard M. GIBBS
Energy Research Institute, SPEME, University of Leeds.
Determination of the minimum explosible and most reactive
concentrations for pulverised biomass using a modified
Hartmann apparatus.
Current trends of increasing use of pulverised biomass in power generation poses an
unquantified explosion risk on the storage, handling, processing and combusting equipment
and personnel. In compliance with safety regulations (DSEAR, ATEX) it is therefore necessary
to assess the explosibility characteristics of biomass powders and take appropriate protection
measures. However, the standard methods for measurement of such characteristics are not
suitable for testing fibrous woody biomass and this has led to a lack of reliable data for the
majority of biomass materials. This work reports measurements and procedures to be followed
for the determination of Minimum Explosible Concentrations (MEC) of biomass and torrefied
biomass powders using a modified Hartmann tube, which is similar to the standard set-up used
for the determination of Lean Flammability Limits (LFL) for gases. The modifications also
allow the reactivity of the powder (in terms of flame speed and rates of pressure rise) to be
charted against concentration (or equivalence ratio) and thus enable the quick identification of
the most reactive mixture for further reactivity measurements in the ISO 1m3 vessel. The
measurements of MEC indicated that pulverised biomass is more reactive than hydrocarbon or
coal fuels. The method was developed to give reproducible MEC measurements using small
samples and achieving rapid measurement compared to the method using the ISO 1m3 vessel.
1.
INTRODUCTION
The use of biomass for power generation is being incentivised by a number of policies
in the EU and in other countries worldwide (EC 2009) with the aim of promoting
electricity generation from renewable sources and reducing greenhouse gas emissions.
Electricity generation processes such as co-firing often use pulverised biomass as a
fuel (Sami et al. 2001), the same as in 100% pulverised biomass power plants. As in
many other industries in which powders are handled, there is a risk of creation of
explosive atmospheres. Safety regulations, like DSEAR and ATEX, are in place to
protect from potential fires or explosions. These regulations require an evaluation of
the explosibility characteristics of any potentially hazardous materials handled in a
workplace. Some of these characteristics are: maximum explosion pressure (Pmax),
deflagration index (Kst), Minimum Explosible Concentration (MEC), Minimum
Ignition Energy (MIE) or Limiting Oxygen Concentration (LOC). A series of
standards for the determination of explosion characteristics of dust clouds (BS-EN
2004; BS-EN 2004; BS-EN 2006; BS-EN 2006) specify the test apparatus to be used,
in most cases the ISO 1m3 vessel (ISO 1985).
1.1. Measurements of explosibility parameters of biomass powders
1.1.1. The ISO 1m3 vessel
Fibrous woody biomass cannot be properly tested in the 1m3 ISO vessel (Wilén and
Rautalin 1996). Due to the low bulk density of biomass the 5 L standard holder of the
powder in the 1m3 vessel is too small to contain the amounts needed for
characterisation and it is therefore necessary to use a bigger dust holder. Also fibrous
biomass cannot flow through the standard dispersion system. The ISO Standard notes
that different dispersion systems can be used but these have not been calibrated to
produce the same turbulence levels and therefore the Kst and Pmax values are often over
or underestimated (Garcia-Torrent et al. 1998; Wilén et al. 1999), and flammability
limits do not agree in many cases when using standard equipment (Wilén et al. 1999).
Torrefied biomass fuels are biomass fuels that have undergone a thermal pre-treatment
in order to improve its characteristics as fuels, such as transportation costs, heating
value or grindability properties, which approach those of low rank coals. Attractive as
these fuels are, there is also very little data about their explosion safety.
A recent series of fire and explosion incidents (see section 1.1.3) highlights the need
for appropriate data to further investigate and standardise methods of testing for
biomass powders (Wilén et al. 1999).
1.1.2. Alternative methods
The standard method for the determination of MEC of dusts clouds (1m3 vessel or 20
L sphere, although other methods can be used provided they give comparable results)
differs from that used for gases. The standard for the determination of Lean
Flammability Limits (LFL) of gases (BS 2003) uses a vertical tube of very similar
dimensions to the Hartmann tube that was originally used for the measurement of
explosibility of dust clouds (Hartman et al. 1943; Hartman and Nagy 1944; Jacobson
et al. 1961; Nagy et al. 1965; Nagy et al. 1968; Field 1983). The reason why the
preferred method for dusts is different is unclear. The European lean flammability
limit for methane (4.5%) would not be reproduced in a 1m3 ISO vessel, due to the 10
kJ ignitor used that creates an overpressure bigger than the overpressure that 4.5%
methane would create, and also because 300 mbar overpressure due to reaction has to
be recorded in order to conclude that an explosion occurred.
The Hartmann tube was originally used for the determination of explosive
characteristics of many different types of dusts. The method was abandoned since,
relative to the standard method (1m3 vessel), it has a number of disadvantages (Lee et
al. 1982; Eckhoff 2003):

very weak ignition source that might not be able to ignite flammable
mixtures,


a lower turbulence level,
results are only applicable at ambient conditions.
On the other hand the Hartmann tube is 1000 times smaller than the 1m3 vessel, and
therefore only a few grams of material is needed for each test instead of kilograms; the
dust is directly placed inside the vessel, avoiding the problem of dust delivery of
certain materials. Also, values found in the literature with Hartmann tubes are similar
to those measured with the standard method and, in any case, they are always smaller
and thus, safer (Maisey 1965; Field 1983; Eckhoff 2003).
There are two versions of the Hartmann tube, one with a vent at the top (Hartman et
al. 1943), in many cases the bursting of that top determines the existence of an
explosion (Dorsett et al. 1960), and the second version is the so called Hartmann
Bomb (Dorsett et al. 1960), which is an air tight vessel from which rates of pressure
rise, Pmax and Kst values can be derived. The effect of turbulence, moisture or particle
size on rates of pressure rise and Pmax was studied using the Hartmann method by
many researchers (Eckhoff 1977; Eckhoff and Mathisen 1977; Amyotte and Pegg
1989). The Hartmann tube method is currently used as the standard method for the
determination of Minimum Ignition Energy (MIE) (BS-EN 2002).
A technical issue that occurs when using a Hartmann tube with bursting vent is that
the criteria of existence of an explosion relays on visual observation. In the present
work a Hartmann tube was modified with the aim of developing a procedure and a
technique similar to the one used for gases that would allow the measurement of
Minimum Explosible Concentrations (MEC) of fibrous biomass and torrefied biomass
powders using small samples for rapid measurement compared to the legislated
method using the 1m3 ISO vessel; deriving fundamental combustion properties like
flame speeds and identifying the most reactive mixtures through the measurement of
flame speeds and rates of pressure rise.
1.1.3. Motivation and Objectives of this work
A recent series of fire and explosion incidents in plants handling biomass powders,
points to inadequate handling procedures and in turn to possible inadequacy of current
explosion safety data for these powders.




June, 2011 - explosion at the world’s largest pellet manufacturing facility in
Georgia, USA (Renewables-International-Magazine 2011).
July, 2011 – fire in Essex woodchip biomass processing plant (Materials
Recycling Weekly Magazine 2011)
October, 2011- fire at the Tyneside port which is intermediate biomass storage
facility for power plant (The-Journal 2011).
February, 2012 - fire at Tilbury power station in the biomass storage area
(The-Guardian 2012)
The above incidents plus a number of other unpublicised events have shaken the
confidence of the industry in the handling these new fuels.
In this paper we present part of the work at Leeds, aimed at addressing the uncertainty
and methodology issues in obtaining reliable and comparable explosion safety data for
raw and torrefied biomass powders. In particular we report measurements and
procedures to be followed for the determination of Minimum Explosible
Concentrations (MEC) of biomass and torrefied biomass powders using a modified
Hartmann tube.
2.
EXPERIMENTAL
2.1. Fuel analysis
The materials used for this study were torrefied wood and its corresponding raw wood
material, provided by Arigna Fuels Ltd. These were characterised using elemental
analysis, TGA-proximate, and gross calorific value analysis. The elemental analysis
was conducted using the Flash 2000 Thermo Scientific C/H/N/S Analyser.
Meanwhile, the proximate analysis was conducted using the TGA-50 Shimadzu
analyser. Measurement of the calorific value is performed in a Parr 6200 Oxygen
Bomb Calorimeter. Each sample was initially milled in a Retsch Cutting Mill SM100
with a 1000 μm bottom sieve, and separated in different particle size ranges: <38 μm,
38 μm-75 μm, 75 μm-150 μm, 150 μm-300 μm, 300 μm-500 μm, >500 μm using a
Retsch Sieve Shaker AS 200.
2.2. Modified Hartmann Tube
The apparatus used was originally a Chilworth Group A/B Flammability Screening
Apparatus comprising a 1L vertical Perspex tube with 322 mm length and 61 mm
internal diameter mounted on a base that contains the air dispersion control system.
The air system is connected to a line of compressed air supply. A remote control
handset operates the ignition arc and air dispersion. The constant arc is achieved from
a high voltage power supply.
Fig. 1. Modified Hartmann tube
Known masses of powder were loaded into the dispersion cup. The top of the tube was
always covered with a busting vent (20 μm thickness aluminium foil secured with a
locking ring). When the tube was securely positioned vertically onto the base;
compressed air was supplied to an internal 50 mL reservoir of air pressurised up to 7
bar (g). The constant arc is continually activated through the remote handset and the
dust was dispersed by allowing the compressed air supply also through the remote
control handset.
The modifications introduced to the apparatus included the fitting of a piezoelectric
Keller PAA-11 pressure transducer to record the pressure histories during each test;
three bare bead type-K thermocouples were mounted at 50 mm, 100 mm and 150 mm
above the ignitor in order to record the time at which the flame arrived to each of the
thermocouples; the air dispersion pressure was adjusted to 7 bar (g) to achieve better
repeatability of tests, by forcing a more even distribution of the powder within the
tube. Vent covers with different bursting pressures were investigated in order to
achieve a longer pressure history and better repeatability.
The modifications introduced facilitated the easier determination of ignition of the
powder at near limit mixtures, and allowed the measurement of rates of pressure rise,
and the determination of flame speeds between thermocouples. These additional data,
in turn, allowed the charting of reactivity with concentration and determination of
most reactive mixtures for different samples, for further assessment in the 1m3 vessel
An explosion was deemed to have taken place if the overpressure due to reaction was
≥100 mbar, or if the flame travelled to the thermocouple fitted at 100 mm distance
from the ignition source. This made the equipment similar to the EU gas Lean
Flammability Limit determination standard method (BS 2003).
The determination of MEC was achieved by testing decreasing masses of powder,
repeating each experiment three times, until a concentration was found at which no
ignition occurred. Consequently a curve representing probability of explosion against
concentration could be drawn (see Fig.4, for example). From such curve the MEC
could be found and defined as that concentration at which the probability of explosion
is 0%, which provided the safest values. MEC for 50% and 100% probabilities could
also be determined.
The initial pressure conditions in the vessel were found to be consistent when the air
injection pressure was set to 7 bar (g). At lower pressures the test repeatability was
reduced. The injection of compressed air increases the volume of air inside the vessel,
which was taken into account for the calculation of the concentration of dust inside the
vessel.
3.
RESULTS AND DISCUSSION
Table 1 shows the elemental and proximate analysis of the powders tested for MEC.
The torrefied material had higher carbon and ash contents, a higher heating value and
lower moisture and volatile content than the raw biomass. Due to the lower volatile
content, torrefied materials were expected to show a lower reactivity than raw biomass
(higher MEC). However, the moisture content played an important role as well.
Moisture decreases the reactivity of a dust. Heating and evaporation of moisture are
sources of heat sink, when the water vapour mixes with the pyrolysis gases the
mixture becomes less reactive (Bartknecht 1993). As a result, the high difference in
reactivities that would be predicted from the much lower presence of volatile matter of
the torrefied material might not be as large as expected, because of the higher moisture
content of the untreated powder.
The stoichiometric fuel to air ratio, expressed in grams of fuel per m3 of air, which is,
the specific mixture of fuel and air for which the oxidant is just enough to consume all
the fuel, was calculated from the elemental analysis, using the C, H and O contents of
each material. Consequently, in order to be able to compare different materials, the
concentrations of dust inside the vessel were expressed as equivalence ratios, which is
the ratio of actual fuel to air ratio to the stoichiometric fuel to air ratio.
3.1. Initial Test Conditions
The modified Hartmann tube used an aluminium foil vent cover that burst when an
explosion took place, and the pressure inside the vessel quickly decreased to
atmospheric pressure. Prior to a potential explosion, the initial conditions of pressure
had to be the same, not having any leakage of air from the vessel. The vessel remained
closed as long as the vent of aluminium foil was not ruptured; the introduction of
compressed air for dust dispersion created the same pressure increase inside the
vessel. Fig. 2 shows four different tests, in which it can be seen that the initial
conditions are the same inside the vessel prior to the ignition of dust. The injection
pressure in this case was always 6 bar (g), which introduced 350 mbar overpressure
into the vessel.
Table 1. Characterisation of Fuels
Sample
Raw Biomass
Torrefied Biomass
C (wt%)*
51.0
67.8
H (wt%)*
6.2
5.2
O (wt%)*
41.6
25.6
N (wt%)*
1.2
1.4
S (wt%)*
0.0
0.0
Moisture (wt%)
10.3
2.5
Ash (wt%)
3.0
5.1
Volatiles (wt%)
79.0
53.5
Fixed Carbon (wt%)
7.7
38.9
GCV (MJ/kg)**
17.9
22.5
Stoichiometric Fuel to Air Ratio,
(F/A)
191.0
139.5
*C, H, N, S contents on dry, ash free basis
**GCV on “as received” basis
It was observed that the behaviour of the aluminium foil vent (12 μm thickness) was
not consistent and sometimes it would rupture even when explosions did not take
place. A study was conducted to find a vent cover that could provide a higher bursting
pressure and a better consistency in order to be able to record a longer pressure trace
and achieve better repeatability of tests. As a result of the study a thicker aluminium
foil (20 μm) was chosen as the vent cover, since it could provide a higher resistance to
pressure build-up and repeatable results, see Fig. 3.
Fig. 2. Pressure Traces: Initial pressure conditions study
Table 2 shows the statistical study conducted for the 20 μm thickness aluminium foil.
Nine repeats of the same test (total ten tests) were performed, using a torrefied
biomass powder of <38 μm particle size range. 0.1g of powder were used on each test,
which corresponded to a concentration of 74.1g/m3, and a equivalence ratio of 0.4 in
this specific case in which the material had a stoichiometric fuel/air ratio of 182 g/m3.
The parameters studied were the maximum pressure at which the thicker aluminium
foil would burst (Pmax), the rate of pressure rise before the vent burst (dP/dt), and the
time in seconds between the time of injection of compressed air (t0) and the time
(tPmax) at which the pressure reached Pmax. The average values for those three variables,
their standard deviations and their coefficient of variation were calculated.
Maximum pressures inside the vessel and times to reach that maximum pressure were
fairly consistent, thus, the thicker aluminium foil was selected as the bursting vent
since it provided more repeatable results. The rate of pressure rise was less consistent.
The rate of pressure rise in a closed vessel is direct measure of the reaction rate and
therefore fairly sensitive to the actual local concentration of reactants. In dust
explosions one of the major challenges is the creation of homogeneous dust clouds. In
the Hartmann tube the distribution of the dust can be variable (Lee et al. 1982),
however, this will be the case for any experimental method for dust explosions. By
repeating the same test a number of times and increasing the pressure of air injection,
the error was reduced – see next section.
Table 2. Statistical study on repeatability of results for 20 μm aluminium foil
Test
Mass (g)
Concentration (g/m3)
Pmax (bar)
dP/dt (bar/s)
tPmax-t0 (s)
1
0.1
74.1
1.540
24.12
0.056
2
0.1
74.1
1.483
12.56
0.060
3
0.1
74.1
1.550
12.27
0.064
4
0.1
74.1
1.499
19.66
0.054
5
0.1
74.1
1.547
33.20
0.051
6
0.1
74.1
1.549
30.96
0.050
7
0.1
74.1
1.545
20.31
0.060
8
0.1
74.1
1.516
15.64
0.056
9
0.1
74.1
1.527
27.52
0.050
10
0.1
74.1
1.542
24.17
0.055
AVERAGE
1.530
22.0
0.055
STD. DEVIATION
0.023
7.3
0.005
Error (%)
1.5
33.1
8.2
Pmax
Test with no dust
dP/dt
Repeats of same test
t0
tPmax
Fig. 3. Pressure Traces: Study on repeatability of tests
3.2. Dust dispersion and injection pressure
The pressure at which the blast of compressed air is injected into the vessel to disperse
the dust had an effect over the dispersion of the dust. In order to find out the optimum
injection pressure a statistical study of repeatability of tests was conducted for
different injection pressures: 4 bar (g) and 6 bar (g). (Fig. 4)
By performing each test three times it was possible to draw a curve in which the
probability of explosion could be defined at different concentrations. This way it was
possible to map out the probability of explosions near the limit and express the MEC
of the dust as the last mixture that always explodes (MEC with 100% probability of
explosion, MEC100), or as the mixture with 50% probability of explosion (MEC50), or
as the first concentration that does not explode at all, with 0% probability of explosion
(MEC0). Currently, the MEC is defined as the minimum amount of dust suspended in
a volume of air capable of igniting and sustaining a flame (BS-EN 2006), the standard
for MEC determination of dusts requires starting the study with 500g/m3 of dust and
decrease the mass preceding by 50% until no explosion occurs, the first concentration
that does not ignite is the MEC, no repetition of tests is required (BS-EN 2006). For
gases, near the explosion limit, the variation of test substance content has to be
selected so that it is at the most equal to 10%, if explosion does not occur, then the test
has to be repeated 4 times more, and the mixture content would be further decreased if
in 5 tests the mixture does not prove to be not explosible (BS 2003). It was observed
with the modified Hartmann equipment that near the limits it is necessary to repeat the
tests since explosions can occur only sometimes and a step variation of 50% omits a
large range of concentrations that can be explosible or not, thus determining the wrong
limit. Using the Leeds modified Hartmann tube it is possible to repeat tests in the same
way that it is done for the determination of LFL of gases, which provides a more
accurate determination of the MEC relative to the 1m3 ISO vessel.
Fig. 4. Probability of Explosion vs. Equivalence Ratio: Study on air injection pressure
The study was conducted with Lycopodium powder. Ten tests were carried out for
each dust/air mixture (dust/air mixtures are expressed as equivalence ratios). The
result showed that the number of times that an explosion took place only sometimes
was narrowed when the injection pressure was higher. Therefore the distribution of the
dust proved to be better with higher air injection pressures and the MEC could be
more clearly identified. Eventually, the injection pressure adopted for the injection of
compressed air was 7 bar (g) which was the maximum pressure recommended by the
manufacturers of the Hartmann tube, and provided the most repeatable results.
3.3. MEC Determination
The MEC could be defined as the minimum concentration for which a 100% of the
tests performed showed ignition of the dust or a 50% probability of explosion or as the
mixture that showed 0% probability of explosion. The last one would be the safest
value. For the determination of MEC the procedure followed was to perform 3 tests
for each mixture, normally the starting mass to be tested was the stoichiometric
mixture and then decreasing the mass by 30% of the mass previously tested.
Consequently it was possible to plot a curve of probability of explosion against
equivalence ratio of the mixture, in the same way as in Fig. 4. It was thus possible to
determine the minimum explosible concentration of the dusts tested.
Figure 5 shows the MEC0 in terms equivalence ratio of the torrefied and raw biomass
materials and the particle size effect. The bigger the particle size the less reactive the
dust was, and therefore the higher the MEC. For smaller particle size ranges, the MEC
was practically independent of particle size, as has been reported in the literature
(Cashdollar and Hertzberg 1987). Raw biomass showed a slightly higher reactivity
than the torrefied material.
Minimum Explosible Equivalent Ratio
0.6
Torrefied Biomass
0.4
0.2
Raw Biomass
0.0
<38
38-75
75-150
150-300
Particle Size Range (m)
Fig. 5. Effect of Particle Size on MEC expressed as equivalence ratios
For fine biomass and torrefied biomass dusts, the lean flammability limit was around
0.2 times its stoichiometric fuel to air ratio. The lean flammability limit of
hydrocarbon dusts is typically 0.5-0.6 times its stoichiometric fuel to air ratio.
Therefore, as opposed to previous work (Wilén et al. 1999), biomass and torrefied
biomass powders show a higher reactivity than hydrocarbon and coal dusts. Table 3
shows values found in the literature that can be compared to values found for this
study.
In general the determinations made in this work with the much faster modified
Hartmann apparatus are comparable to the measurements made by other reputable
labs.
Table 3. MEC table comparison table (* Current study)
Raw Biomass (*)
Dust
Particle Size
(μm)
MEC100
MEC0
g/m
g/m
<38
74
38-75
Torrefied Biomass (*)
ER100
ER0
44
0.4
74
44
75-150
126
150-300
504
Dust
Wheat Starch
Cellulose
(Shen and Gu
2009)
Lycopodium
(Kubala et al.
1981)
Lycopodium
3
3
MEC100
MEC0
ER100
ER0
25
0.4
0.2
74
32
0.5
0.2
0.3
104
52
0.7
0.4
0.4
385
81
2.8
0.6
g/m
g/m3
0.2
52
0.4
0.2
63
0.7
74
2.6
Modified
Hartmann (*)
Hartmann
(Maisey 1965;
Field 1983)
3
1m3 or 20L
(Eckhoff 2003)
1m3
(NFPA 68 2007)
MEC0
ER0
MEC
ER
MEC
ER
MEC
ER
-
-
45
0.2
60
0.3
30
0.1
-
-
55
0.2
60
0.3
60
0.3
-
-
2025
0.1
3045
0.2-0.3
42
0.3
17
0.2
-
-
-
-
-
-
3.4. Determination of most reactive mixtures
The modified Hartmann tube made possible identifying most reactive mixtures using
the pressure and thermocouples readings. The pressure histories recorded allowed the
determination of rates of pressure rise, and thermocouple readings showed the times at
which the flame reached the thermocouples situated at known distances above the
source of ignition, therefore, the flame speeds were measured as well. Fig. 6 shows an
example with a torrefied material in the size range of 38 µm to 75 µm for which the
most reactive mixture was around 1.5 times its stoichiometric mixture. A good
correlation is suggested between the measured flame speeds and rates of pressure rise
which gives further validation of consistency of the methodology. The flame speeds
and rates of pressure rise can be further used to determine more fundamental
properties, such as burning velocities, of the explosible powder mixtures which can
then be used in combustion modelling of these powders.
30
25
Flame Speed (m/s)
2
20
1.5
15
1
10
0.5
5
0
Rate of Pressure rise (bar/s)
2.5
0
0
0.5
1
1.5
2
2.5
3
Equivalence Ratio
Fig. 6. Flame Speed and Rate of Pressure rise vs. Equivalence Ratio: Determination of most
reactive mixture
4.
CONCLUSIONS
For the quick determination of biomass MEC and most reactive concentration
mixtures, the standard Hartmann tube normally used for the simple determination of
whether of powder is explosible or not, has been modified by the addition of a
pressure transducer for rate pressure rise and maximum pressure recording and 3
thermocouples for recording the flame position and flame speed during a test. The set
up and methodology presented here is comparable to the more widely accepted
method for determination of Lean Flammability Limits for Gases. With a powder
injection pressure of 7 bar, and a standardised bursting vent of 20 μm thick aluminium
foil, it was shown to produce consistent reproducible MEC measurements using small
samples, comparable to other methods and achieving rapid measurement compared
with the legislated method using the ISO 1 m3 vessel. Also the most reactive mixtures
could be easily identified and eventually used in the 1m3 ISO vessel for the
measurement of Kst and Pmax. Furthermore, fundamental combustion properties such as
flame speeds which can, in turn, yield values for burning velocities can be derived
from the measurements, providing information for burner design. The biomass
measured MECs indicate that pulverised biomass is more reactive than hydrocarbon or
coal fuels.
5.
ACKNOWLEDGEMENTS
The authors are grateful to the Energy Programme (Grant EP/H048839/1) for financial
support. The Energy Programme is a Research Councils UK cross council initiative
led by EPSRC and contributed to by ESRC, NERC, BBSRC and STFC, and to Robert
Johnson from Arigna Fuels Ltd. for the supply of biomass and torrefied biomass
materials.
6.
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