Note: Descriptions are shown in the official language in which they were submitted.
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Torrefaction Process
The present invention relates to a process for treating a biomaterial, for
example, wood
chippings to produce a biofuel. In particular, the present invention provides
an improved
torrefaction process. Furthermore, the present invention provides an improved
fuel
suitable for use in processes such as energy production.
Torrefaction is a thermo-chemical treatment of biomass generally at 200 to 320
C. It can
also be described as a mild form of pyrolysis. Pyrolysis occurs above 400 C.
It is carried
out under atmospheric conditions and in the absence of oxygen. During
torrefaction the
biomass properties are changed to obtain a much better fuel quality for
combustion and
gasification applications. For example, the water contained in the biomass as
well as
superfluous volatiles are removed, and the biopolymers (commonly cellulose,
hemicellulose and lignin) partly decompose giving off various types of
volatiles.
The final product of torrefaction is the remaining solid, dry, blackened
material which is
referred to as "torrefied biomass" or "bio-coal". In order to create highly
efficient biomass-
to-energy chains, torrefaction of biomass in combination with densification
(pelletisation/
briquetting), can be used to overcome logistic economics in large scale green
energy
solutions. Torrefaction combined with densification leads to a very energy
dense fuel
carrier of 20-25 GJ/ton. Furthermore, this densification into pellets further
increases the
hydrophobic properties of the material making bulk storage in open air
feasible.
Torrefied biomass can be produced from a wide variety of raw biomass
feedstocks while
yielding similar product properties. This is because of the lignocelluloses
polymers
present in all biomass. In general (woody and herbaceous) biomass consists of
the
above three main polymeric structures: cellulose, hemicelluloses and lignin.
During the process, the biomass loses typically 20% of its mass (dry weight
basis), while
only 10% of the energy content in the biomass is lost. This energy (the
volatiles) can be
used as a heating fuel for the torrefaction process. An additional benefit of
torrefaction is
that all biological activity in the product is eliminated, reducing the risk
of fire and
stopping biological decomposition.
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Torrefaction of biomass leads to improved grindability of biomass. This leads
to more
efficient co-firing in existing coal fired power stations or entrained-flow
gasification for the
production of chemicals and transportation fuels.
Torrefaction processes have historically been long retention time processes.
There is
therefore a desire for an improved, preferably faster, process for treating a
biomass, or at
least a process that will mitigate some of the problems associated with the
prior art or
provide a useful alternative thereto. Furthermore, there is a desire for an
improved or
alternative fuel source.
Accordingly, in a first aspect, the present invention provides a process for
the torrefaction
of a biomass feedstock for the production of a biofuel, the process
comprising:
(i) torrefaction of a biomass feedstock in a toroidal bed reactor,
wherein the toroidal bed reactor comprises a reaction chamber with a
substantially circumferentially directed flow of fluid generated therein to
cause the
biomass feedstock to circulate rapidly about an axis of the reaction chamber
in a toroidal
band, and to heat the biomass feedstock, wherein the fluid comprises gas or
gases
introduced into the reaction chamber and wherein the chamber is maintained
under an
oxygen-depleted atmosphere.
According to a second aspect, the present invention provides a biofuel
obtainable
according to the process of the present invention.
The inventors have found that by using a toroidal bed reactor the process may
be
conducted in a significantly shorter time. That is, a toroidal bed reactor is
ideal for the
fast torrefaction of a biomass feedstock. Without wishing to be bound by
theory, it is
believed that the high turbulence in the reactor allows for express and more
precise
treatment of the feedstock through increased heat and mass transfer. This
minimises
undesirable oxygen contact which might lead to scorching. The ready throughput
allowed
by the reactor allows for a higher temperature to be used with even heating of
the
particles so that a surprisingly short residence time may be used.
The process of the present invention can provide an end product with a
calorific value,
greater than 20 GUmt. It is well know that conventional torrefaction process
can also
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achieve an end product with a calorific value of around 22 GJ/mt. However, the
retention
times required for this are long. For example:
1. Fluidized beds : 12-20 minutes
2. Plug flow kilns, screws: 30 minutes-2 hours
In contrast, example retention times and the consequent calorific values of
the product
produced by using a toroidal bed reactor in accordance with the present
invention are set
out in Table 1 below.
Table 1
Temperature [ C] Residence time LHV (GJ/mt)
(sec)
Softwood 340 80 22,83
Hardwood 340 80 22,50
Straw 345 60 20,69
Palm Shells 350 90 25,32
(LHV is the lower heating value, also known as net calorific value or net CV)
The inventors have also found that the method of the present invention allows
for the
operation of the process at a point which achieves coal-like properties with
minimal loss
of volatiles., i.e. minimal weight loss. Without wishing to be bound by
theory, it is
believed that the coal-like properties, such as brittleness and a breakdown of
fibrous
structure, arise from the cracking of hemi-celluloses in the feedstock.
Cracking of the
less reactive lignin is undesirable since it is this component that determines
the
compacting behaviour of the product. If the lignin is cracked then the treated
material
becomes less sticky and harder to compact.
The inventors have found that the toroidal bed reactor used in the present
invention is
uniquely capable of achieving this fine balance of cracking the hemi-
celluloses rather
than the lignin in the reaction times used. This fine balance allows for
minimal volatiles
losses of around 15-20% and the production of a product with around 22 GJ/mt
in very
little time. The inventors studies show that higher energy materials may be
produced,
such as 31 GJ/mt, although this is associated with a greater weight loss (60-
70%).
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In the following passages different aspects/embodiments of the invention are
defined in
more detail. Each aspect/embodiment so defined may be combined with any other
aspect/embodiment or aspects/embodiments unless clearly indicated to the
contrary. In
particular, any feature indicated as being preferred or advantageous may be
combined
with any other feature or features indicated as being preferred or
advantageous.
The invention will also be described with reference to the figures, provided
by way of
example, in which:
Figure 1 shows a toroidal bed reactor of the type disclosed in EP1 791632;
Figure 2 shows a flowchart of the process in accordance with the present
invention; and
Figure 3 shows ignition ability results of various materials.
An oxygen depleted atmosphere, as used in the present invention, has an oxygen
content of less than 10% oxygen by volume, still more preferably less than 2%.
Most
preferably there is substantially no oxygen present (less than 1 %, or less
than 0.5%).
This reduces the risk of the biomass feedstock spontaneously combusting.
Preferably
there is substantially no oxygen present. Preferably the gas or gases used in
the toroidal
bed reactor comprise super heated steam and/or an inert gas and/or a recycled
process
gas stream.
It is believed that the low residence times that are achievable by the
torrefaction process
of the present invention renders the undesirable presence of oxygen less
critical. While it
is desirable to minimize the contact of the feedstock with oxygen during
treatment, it is
believed that due to the relatively brief time that the feedstock is at high
temperature,
undesirable oxidation is avoided. Therefore, the process can be operated in
the
presence of some oxygen, whereas prior art methods require substantially no
oxygen to
be present.
Any biomass or biomaterial is suitable for use in the torrefaction process.
Biomass is
biological material derived from living, or recently living organisms. Biomass
is carbon
based and is composed of a mixture of organic molecules containing hydrogen,
usually
including atoms of oxygen, often nitrogen and also small quantities of other
atoms,
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including alkali, alkaline earth and heavy metals. Examples of biomass include
wood,
plant matter and waste (including sewage sludge and agricultural residues).
Wood
includes forest residues such as dead trees, branches and tree stumps, yard
clippings,
wood chips and process residues. Plant matter includes biomass grown from, for
5 example, miscanthus, switchgrass, hemp, corn, poplar, willow, sorghum or
sugarcane,
and includes straw and husks. Preferably the biomass treated has a solid form
and a
useful calorific value. If the calorific value is too high or too low then the
biomass may be
initially homogenized to provide a feedstock of substantially uniform
calorific value.
Preferably the biomass feedstock comprises wood or a wood derivative. Wood may
be
used in any form, but preferably in the form of chips (1 to 5 cm thick) or
chunks (5 to 10
cm thick). Wood derivatives include wood derived products such as MDF sheets
or
furniture, or processed wood composites. Preferably these are also in the form
or chips
or chunks. An initial pre-treatment step may be required, depending on the
feedstock to
obtain the desired particle size. In other embodiments other biomass may be
treated
such as palm shells and/or straw.
In one embodiment, the present process may be used as a method of recycling by
transforming waste wood products into a useful high calorific value biofuel.
The torrefaction step (i) preferably heats the biomass feedstock to a
temperature of from
280 to 400 C. More preferably the torrefaction step (i) heats the biomass
feedstock to a
temperature of from 320 to 350 C, and most preferably about 335 C. The heat
may be
supplied by using heated gas or gases, or by heating the furnace itself so
that the gases
within the furnace become heated, or by a combination of the two approaches.
Preferably this heat is provided by combustion of torrefaction gases produced
in the
process. These may be removed from the reactor and optionally treated, before
being
burnt to produce the heat energy required.
While conventional techniques using ovens and kilns heat the wood for from 10
to 12
minutes at 280-320 C, the process of the present invention may be performed in
around
1 minute at a higher temperature of preferably 320 to 350 C. Alternatively,
due to the
excellent mixing of the particles, lower temperatures (down to 280 C) may
still be used
and a short residence time is still sufficient to fully torrefy the biomass.
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Preferably the biomass feedstock has a residence time of less than 5 minutes
in the
toroidal bed reactor in step (i). This is to avoid scorching or over
decomposition of the
biomass material. It is preferred that the biomass feedstock has a residence
time of less
than 120 seconds, and more preferably less than 60 seconds, in the toroidal
bed reactor
in step (i), although at least 30 seconds residence time is preferred.
Residence time, as
used herein means the mean duration of time that an average piece of biomass
feedstock spends in the toroidal bed reactor at the set reaction temperature.
Preferably the flow of fluid within the reaction chamber has a horizontal and
a vertical
velocity component. This may be provided by using a chamber having a plurality
of
outwardly radiating inclined fluid inlets at or adjacent a base thereof, and
wherein fluid is
directed through the fluid inlets at the base of the chamber to generate the
circumferentially directed flow of fluid within the chamber. Preferably the
fluid directed
through said fluid inlets is given both horizontal and vertical velocity
components.
In an embodiment of the process, the process further comprises a
devolatilization step of
heating the biomass feedstock to between 200 and 280 C before performing step
(i).
This step may be performed in a conventional oven, for example, with the
biomass
passing on a conveyor belt through a heated chamber. Such high temperature
devolatilization techniques are well known in the art. Alternatively, the
devolatilization
may be performed in a further toroidal bed reactor. The heat for pre-
volatilization may be
obtained using heat exchange from the product or from energy obtained by
combusting
the torrefaction gases produced by the torrefaction process.
Preferably the biomass feedstock is subjected to at least one pre-treatment
step
selected from picking, milling, screening, mixing and blending. This
homogenizes the
biomass and ensures that even residence times can be achieved. Thus a constant
level
of torrefaction can be achieved and the calorific value of the product kept
substantially
constant.
Picking is the removal of outsized portions of the feedstock. Milling is the
grinding
reduction in the size of particles of the feedstock. Screening, blending and
mixing are
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steps used to ensure a homogeneous feedstock in terms of particle size,
consistency
and chemical make-up respectively.
The biomass feedstock may be dried at a temperature of from 100 to 200 C
before use.
This low temperature step may be used to reduce the moisture content of the
biomass
before treatment. This serves to reduce the energy requirement for heating the
treated
product. This may be performed in a conventional oven or a still further
toroidal bed
reactor. As with the optional devolatilization step, the energy for the drying
step may be
advantageously obtained using heat exchange.
Advantageously, the energy for performing the drying and/or the devolatilizing
step
and/or the torrefaction step may be obtained from combustion of the volatile
gas from the
biomass feedstock. The volatile gases are driven off during any heating of the
biomass
and may be collected and conserved for this purpose. The combustible volatile
gas
typically comprises one or more of carbon monoxide, hydrogen, and low
molecular
weight hydrocarbons, the proportions varying between different sources. The
combustion
of the volatile gases may be used to supplement other energy or heat sources.
Preferably the biomass feedstock has a water content of 20% or less, more
preferably
10% or less, before step (i). This minimizes the heating required and hence
accelerates
the speed of the torrefaction step (i).
Advantageously, the use of a toroidal bed reactor allows the process to be
operated as a
continuous process for the torrefaction of a biomass feedstock. This allows
for fast
product of a biofuel and for low energy costs since it does not need to be
performed in
batches with set-up and wind-down energy demands.
In step (i), the biomass feedstock may be fed into the toroidal bed reactor
via an airlock
system to prevent the loss of heat or the introduction of undesirable oxygen.
The process of the present invention is carried out in a toroidal bed reactor.
A toroidal
bed (TORBED (RTM)) reactor and process is described in EP 0068853, US 4479920,
and EP 1791632, the disclosures of which are incorporated here by reference.
In the
process, a material to be treated is embedded and centrifugally retained
within a
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compact, but turbulent, toroidally circulating bed of particles which
circulate about an axis
of the processing chamber. Specifically, the material forms particles within
the bed
which are circulated above a plurality of fluid inlets arranged around the
base of the
processing chamber. The fluid inlets are preferably arranged in overlapping
relationship
and the particles are caused to circulate around the bed by the action of a
processing
fluid, for example a gas injected into the processing chamber from beneath and
through
the fluid inlets. The fluid inlets may, for example, be a plurality of
outwardly radiating,
inclined vanes arranged around the base of the processing chamber.
By way of example, Figure 1 shows a schematic diagram of a toroidal bed
reactor 1. The
gaseous fluid (A) mixed with the feedstock enters through angled vents 9 in
the base of
the reaction chamber 3. The path of the turbulent flow in the reaction chamber
3 is
shown by the spiralling arrows marked (E). The dotted arrows show the
circulation
pathway (in 2 dimensions only) taken by the feedstock that is to be processed.
The toroidal bed reactor provides a rapidly mixing bed which can be used to
circulate
particulates toroidally through a zone in a process chamber where an
interaction occurs
with a gas stream.
Preferably a toroidal bed reactor for use in the present invention has a
reaction chamber
with a substantially circumferentially directed flow of fluid generated
therein to cause the
biomass feedstock to circulate rapidly about an axis of the reaction chamber
in a toroidal
band, and to heat the biomass feedstock, wherein the fluid comprises gas or
gases
introduced into the reaction chamber. Preferably the flow of fluid within the
reaction
chamber has a horizontal and a vertical velocity component. Preferably the
chamber
comprises a plurality of outwardly radiating inclined fluid inlets at or
adjacent a base
thereof, and wherein fluid is directed through the fluid inlets at the base of
the chamber
to generate the circumferentially directed flow of fluid within the chamber.
Preferably the
fluid directed through said fluid inlets is given both horizontal and vertical
velocity
components.
The biomass feedstock may be introduced into the reactor(s) by injecting it
through an
inlet under the influence of a compressed gas such as compressed air and/or an
inert
gas such as nitrogen, CFC and other noble/mono-atomic gases. In a preferred
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embodiment of the present invention, the inlet is located above the fluid
inlets at the base
of the chamber and the biomass feedstock is introduced into the chamber by a
gravity
feed mechanism, for example using an air lock device such as a rotary valve.
The
gravity feed mechanism may be provided in a vertical wall of the chamber.
It will be appreciated that the flow of fluid may be generated either before
or after the
biomass material is introduced into the chamber. Alternatively, the flow of
fluid may be
generated at the same time as the biomass material is introduced into the
chamber.
The flow of the fluid through the chamber may be generated in a manner as
described in
EP-B-O 382 769 and EP-B-0 068 853, i.e. by supplying a flow of fluid into and
through
the processing chamber and directing the flow by means of the plurality of
outwardly
radiating and preferably overlapping fluid inlets arranged in the form of a
disc and
located at or adjacent to the base of the processing chamber. The fluid inlets
are
inclined relative to the base of the chamber so as to impart rotational motion
to the
heating fluid entering the chamber, hence causing the heating fluid to
circulate about a
substantially vertical axis of the chamber as it rises. The fluid inlets may
comprise, for
example, a plurality of outwardly radiating vanes at or adjacent the base of
the chamber.
The vanes are typically inclined relative the base and preferably disposed in
overlapping
arrangement.
The solid char product produced by the process of the present invention is a
higher
calorific value per unit weight than the biomass feedstock. It also has a more
constant
calorific value. The processing has been found to provide a unique product
compared to
conventional methods in that, the product has improved ignitability (lower
temperature to
ignite) and increased hydrophobicity (and hence better potential for being
stored).
Without wishing to be bound by theory, it is believed that this arises by
virtue of an
increased surface area of the product that arises from the novel process.
Thus, it is an
excellent biofuel,
The biofuel may be the direct product from the torrefaction of the biomass.
Alternatively,
a further finishing step may be used to produce a granular, briquette sized,
or lump-sized
(preferably 5-10cm) biofuel. This may be achieved by introducing (for example,
mixing or
coating) a binder onto the torrefied biomass. This may occur either in the
reactor or in a
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finishing step, and can successfully agglomerate the torrefied biomass into a
desirable
size product. The term "agglomerate" as used herein refers to any process,
such as
pelletisation, that provides a consolidated fuel product. Suitable binders are
well known
in the art. In view of the outstanding homogeneity and increased calorific
value of the
5 produced biofuel, the biofuel is considered novel over such fuels made by
conventional
techniques.
In one embodiment of the process of the present invention, the process further
comprises a step of mixing the torrefied biomass with coal, preferably in an
amount of
10 from 90% to 75% coal by weight of the mixture of biomass and coal. That is,
to produce
a biofuel comprising from 10 to 25% by weight of the biofuel obtainable
according to the
process of the invention and the balance coal. In a preferred embodiment the
coal is
present in an amount of from 85 to 80% by weight, i.e. 15 to 20 % by weight of
torrefied
biomass. This is especially advantageous as this mixture allows for a decrease
in the
ignition temperature of the coal. For some coals their ignition temperature is
unfavorably
high and so may struggle to burn in a conventional reactor. The admixture of
the most
combustible torrefied material can be used to decrease the temperature at
which the
coal will stably burn. Preferably the mixture is with a coal having an
ignitability of less
than 6 kJ/kg coal C, and more preferably less than 5.5 kJ/kg coal C, such as
Kleinkopje
coal. These are typically "hard" coals.
The homogeneous nature of the biofuel produced by the present method
facilitates
mixing with the coal to provide a final fuel having substantially homogeneous
calorific
properties. The biofuel may be ground to allow easy mixing with the coal.
In one embodiment, the process comprises the torrefaction of a biomass
feedstock for
the production of a biofuel, the process comprising:
(i) torrefaction of a biomass feedstock comprising wood or a wood derivative
in a
toroidal bed reactor,
wherein the toroidal bed reactor comprises a reaction chamber with a
substantially circumferentially directed flow of fluid generated therein to
cause the
biomass feedstock to circulate rapidly about an axis of the reaction chamber
in a toroidal
band, and to heat the biomass feedstock, wherein the fluid comprises gas or
gases
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introduced into the reaction chamber and wherein the chamber is maintained
under an
oxygen-depleted atmosphere,
wherein the biomass feedstock is dried at a temperature of from 100 to 200 C
before
use. and wherein the process further comprises a devolatilization step of
heating the
biomass feedstock to between 200 and 280 C before performing step (i) to
provide the
biomass feedstock with a water content of 20% or less before step (i). This
combination
of processes has been found to be particularly efficient in the processing of
the wood
feedstock.
The process of the present invention will now be briefly summarized with
reference to
Figure 2. Figure 2 is a flow chart summarizing the steps of the present
invention. It
should be noted that several of the steps such as pre-treatment A', drying B',
heat
recycling with a combustor D', agglomerating and/or burning as a fuel, are
optional
steps.
Wood chippings 5 which serve as a biomass feedstock are subjected to an
initial
pretreatment A' in which they are picked and sized to ensure that a
substantially
homogeneously proportioned feedstock is used. The feedstock is then dried in
an oven
at 150 C for 30 minutes in a drying step B'.
The dried wood chippings 5 then pass into a toroidal bed reactor in a
treatment step C'.
Here the chippings 5 are subjected to a temperature of 335 C for on average 60
seconds. This produces a gas comprising volatiles 10 and a torrefied material
15.
The volatiles 10 are passed to a combustor D', where in combination with
additional
natural gas 20 they are combusted. This provides hot gases which are used to
provide
heat to both the drying step B' and the treatment step C'.
The torrefied material 15 is gathered from the toroidal bed reactor in a
hopper E'. The
material 15 may then be treated in one of various ways to produce a biofuel 30
which
can be combusted in, for example, a power station H' to produce energy.
Firstly, the
torrefied material 15 can be used as it is as the biofuel 30. Secondly, the
material 15 can
be subjected to a compaction process step G', optionally with a binder, to
produce even-
sized pellets (i.e. 5cm briquettes). Thirdly, the material 15 can be mixed
with another fuel
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such as a poorly ignitable coal before pelletisation in a compaction process
step Gto
provide a biofuel 30.
The product of the present invention will now be shown in the following non-
limiting
example in which the properties of beechwood CPMt" pellets, coal pellets and
blends of
these two materials were investigated. In particular, the following fuels/fuel
blends were
tested:
- Kleinkopje coal (South Africa)
- Beechwood CPMtm pellets
- Blend Kleinkopje and beechwood CPMt`" pellets 90%/10%
- Blend Kleinkopje and beechwood CPMtm pellets 80%/20%
The grinding behaviour of coals in power station milling plants with a
classification
system, as determined by energy demand and capacity of the milling plants,
does not
only depend on the hardness (solidity) of the coals. Rather it also depends on
the
particle size (fine grain fraction) of the shipment, the classifying behaviour
in the milling
plant, the compressive force in the mill and, in particular, on the expected
particle size of
the pulverised coal.
The results of the investigation are set out below in Tables 2a-d. These
demonstrate that
the wood can be ground faster (lower cycle factor), but takes more energy to
be ground.
Surprisingly the blends grind faster than either the coal or the wood alone
and the energy
cost for the grinding is comparable to that of the coal alone.
Table 2a
Kleinkopie
Results (Average results of the 2 last cycles)
generated particulate matter [%] 19.5
Cycle factor 4.17
Specific grinding energy [J/g sample] 6.94
Specific Comminution work:
[J/g sample] 28.98
[kWh/t sample] 8.05
force application angle beta 85.80
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Table 2b
Beechwood CPM pellets
Results (Average results of the 2 last cycles)
generated particulate matter [%] 22.08
Cycle factor 3.92
Specific grinding energy [J/g sample] 12.96
Specific Comminution work:
[J/g sample] 50.79
[kWh/t sample] 14.11
force application angle beta 87.73
Table 2c
Blend (Klein kop[e/Pellets (90%/10%))
Results (Average results of the 2 last cycles)
generated particulate matter [%] 22.79
Cycle factor 3.69
Specific grinding energy [J/g sample] 8.50
Specific Comminution work:
[J/g sample] 31.41
[kWh/t sample] 8.72
force application angle beta 86.53
Table 2d
Blend (Klein kopie/Pellets (80%/20%))
Results (Average results of the 2 last cycles)
generated particulate matter [%] 25.57
Cycle factor 3.30
Specific grinding energy [J/g sample] 9.27
Specific Comminution work:
[J/g sample] 30.52
[kWh/t sample] 8.48
force application angle beta 86.77
The ignition temperature parameter describes the ability of a fuel to form a
stable coal
flame. The manner in which a flame subsequently develops during the combustion
process depends on the combustion conditions in the boiler and the degree of
reactivity
the fuel has with regard to its basic organic matrix.
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When assessing coals and/or coal blends, the ignitability represents an
essential
parameter to characterise the formation and development of a coal flame. This
is based
on the assumption that a stable coal flame forms when volatile matter is
released from
the coal prior to the ignition point. The ignitability parameter, the quotient
of the ignition
potential and the ignition temperature, represents a comparative parameter for
the
ignition behaviour of different coals and coal blends.
Z WZ = NZ 500 kJ
rzis0 kgKohle HOC
The ignition potential (Nz500 in kJ/kg coal) is the chemical energy based on
the fuel mass
unit which is, up to a temperature of 500 C, contained in the volatile
decomposition
products (based on a water-free fuel). It therefore corresponds to the heat
content
developing in the first phase of the combustion process, i.e. immediately
after ignition.
Therefore the ignition potential is a measurand which allows an assessment of
the flame
stability after ignition.
The ignition temperature (tz150 in C) is the gas temperature at which a
pulverised coal
sample (m=0.2 g; d< 63 pm) is injected into a preheated muffle furnace under
defined
conditions, where it is ignited with a delay of 150 ms in air. The delay time
of 150 ms
corresponds to the maximum ignition distance of 3 m from the burner tip and a
pulverised coal-air blend with a flow velocity of 20 m/s.
The results are shown in Figure 3 which shows a plot of the ignitability of
various coal
types and fuel sources. From left to right, these are: Polen, Prosper-Haniel,
Appalachenkohle, Norweigan, Russland, US Steam 1,3%S, US Steam 2,0%S, Douglas,
Kleinkopje CPM-pellets 90/10 Blend, 80/20 Blend. The x axis shows the
ignitability
parameter. The y-axis shows ZWZ in kJ/Kg Coal C. Section A shows fuels that
burn
satisfactorily with no problems and a stable flame. Section B shows fuels with
unsatisfactory ignitibility or no ignition at all. As can be seen, the
blending of the coal with
CPM-pellets allows it to be increased in ignitibility and an 80/20 blend
provides a useful
fuel.
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When comparing the expected grinding behaviour of Kleinkopje coal to the one
of
beechwood CMP pellets, the clearly higher result of the pellets in connection
with the
specific comminution work is immediately noticeable. When grinding Kleinkopje
coal,
merely 8.05 kWh are required for each ton of generated pulverised coal. For
the pellets,
5 however, almost 1.8 times the energy is necessary. In this case, the result
is 14.11
kWh/t. However, with 4.17, the cycle factor for coal is slightly higher than
the one for the
pellets with 3.92. It should be noted that both results appear relatively high
in comparison
to earlier tests on fuels.
10 The test results show that approximately 1.9 times the energy (12.96 J/g
sample)is
required for the grinding of beechwood CPM pellets compared to Kleinkopje coal
(6.94
J/g sample).
Preparing blends from these two fuels affects the grinding process in a most
favourable
15 manner. Two blends were prepared for the test, which were composed as
follows:
Blend 1 Kleinkopje and beechwood CPM pellets 90%/10%
Blend 2 Kleinkopje and beechwood CPM pellets 80%/20%
Although the comminution work increases slightly in comparison to the pure
coal (-8%
(8.72kWh/t pulverised coal) in admixture with 10% pellets), this additional
energy
demand is not significant. When adding 20% biofuel to the coal, the
comminution work
increases by ^P5% (8.48 kWh/t pulverised coal) only and is therefore smaller
than with the
lower dosage.
The result for the cycle factor is very satisfactory. Compared to the initial
result of the
coal, this value drops by approximately 13% for Blend 1 and by about 26% for
Blend 2.
This means, the result of 3.30 for Blend 2 has reached a range which is by all
means
typical for imported and domestic coals.
Another positive aspect is the fact that the generated particulate matter of
19.50% for
Kleinkopje coal increases to 22.79% for Blend 1 and to as much as 25.57% for
Blend 2.
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However, an overall higher energy demand was noted for the grinding process.
This is
because the value of the specific grinding energy rises from 6.94 J/g sample
to 8.50 J/g
sample (Blend 1) and 9.27 J/g sample (Blend 2).
Coals with a high ignition potential reach higher temperatures immediately
after ignition
when burning in a pulverised coal flame. This causes such a pulverised coal
flame to
become "more stable". Due to the fact that:
= the better the ignition behaviour of a coal is, the greater its ignition
potential; and
= the worse the ignition behaviour of a coal is, the higher the ignition
temperature.
When comparing Kleinkopje coal to beechwood CMP pellets, the significant
difference
with regard to the ignitibility parameter is immediately noticeable.
Kleinkopje coal has a
ZWZ of only 5.79 kJ/kg coal C thus remaining below the limit of 6 kJ/kg coal
C.
Experience has shown that in order to ensure stable conditions in the furnace,
the value
should not drop below this limit. Otherwise, this could result in an
unsatisfactory
ignitability in the power plant. The pellets, however, have a relatively high
ZWZ of 11.36
kJ/kgcoal* C. For this reason, no problems are expected with regard to the
formation of
a stable flame when employing this fuel.
The favourable ignitability characteristics of the pellets are based on a very
low ignition
temperature and a very high ignition potential. With 556 C, the ignition
temperature of
the pellets is more likely to be attributed to the range of lignite coals.
Here, this hard coal
(Kleinkopje) has a clearly higher ignition temperature of 808 C.
Although the biofuel's ignition potential of 6.32MJ/kg is clearly higher than
the coal's with
4.68MJ/kg, it is not considered a particularly high value. The pellets are
expected to
ignite well, but the reaction will most likely not take place explosively in a
manner to
where it could cause problems in the burner.
If consideration is given to blending pellets to the coal for the purpose of
improving the
comparably inadequate ignitability, an addition of 10% would not necessarily
be sufficient
(5.55 kJ/kg coal*OC). This would depend on the coal. However, adding 20% of
pellets,
which equals 6.59 kJ/kg coal*OC in this case, would satisfy the requirements
of aZWZ
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17
larger than 6 kJ/kg coal*OC. It can therefore be assumed that, in this case,
no problems
would occur in the furnace.
Blend 1 (90% coal/ 10% pellets) has an ignition temperature of 785 C and an
ignition
potential of 4.36 MJ/kg. Blend 2 (80% coal/ 20% pellets) has an ignition
temperature of
698 C and an ignition potential of 4.60 MJ/kg.4.
The conducted tests confirm that the beechwood CPM pellets, up to a content of
20%,
influence the quality characteristics to a minor extent only. However, they
positively
affect the combustion-related parameters. This means:-
Ignition temperature:
The lignite-like CMP pellets cause the ignition temperature of the tested
blends to drop to
a temperature range which is relatively low for hard coals. This means that a
relatively
quick ignition can be expected. Given that the combustion technique is
appropriate, this
circumstance affects the burnout behaviour in a positive manner
Ignitability:
Selecting Kleinkopje coal as a hard coal meant selecting a coal quality which
is not
necessarily suitable for an unrestricted use in all types of firing systems.
This coal must
generally be ground much finer in order for the coal to meet the demands of
the power
plant with regard to its ignition and burnout behaviour. When assign the
pellets, the
ignitability and the formation of a stable flame are enhanced within a range
typical for
hard coals (with a dosage rate of 20%).
Grinding behaviour:
The required comminution work in AVT-like milling plants is considerable. We
do not
recommend the sole employment of pellets because the energy demand is
approximately 70% higher than for Kleinkopje coal which, in comparison, is
much harder.
However, blends frequently have characteristics which reflect the major
component of a
blend, and this applies to coal blends as well. The exact opposite applies to
the cycle
factor. It is reduced by approximately 20% when adding the pellets. Therefore
the mill
flow rate will most likely remain ensured.
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Although preferred embodiments of the invention have been described herein in
detail, it
will be understood by those skilled in the art that variations may be made
thereto without
departing from the scope of the invention or of the appended claims.
