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A clean, efficient system for producing Charcoal, Heat and Power (CHaP)
C. Syred a,*, A.J. Griffiths a, N. Syred a, D. Beedie b, D. James c
a Cardiff School of Engineering, Cardiff University, Queens Buildings, The Parade, Newport Road, Cardiff CF24 0YF, UKb BioEnergy Devices, Unit 28, St Theodores Way, Brynmenyn Industrial Estate, Bridgend CF32 9TZ, UK
c James Engineering Turbines Ltd, 5 St Johns Road, Clevedon, Somerset BS21 7TG, UK
Received 13 March 2005; received in revised form 12 October 2005; accepted 26 October 2005
Available online 5 December 2005
Abstract
There is a strong domestic and industrial market for charcoal in the UK and is still used in many developing countries for cooking and heating as
well as for many industrial applications. It is usually made in small-scale simple kilns that are very damaging to the environment, very inefficient
and labour intensive. The Charcoal, Heat and Power (CHaP) process offers a method for producing clean efficient charcoal under pressurised
conditions and uses the product gas from the carbonisation process to drive a small gas turbine to produce heat and power. The charcoal is
produced using waste forestry matter and other waste wood, including that from sustainably managed forests. The CHaP system can also be used
in developing countries where there is an excess of forestry waste and a shortage of fossil fuels.
The CHaP process was initially designed, developed and a prototype system built. This paper discusses the CHaP design and the various
components used, their separate development and integration into a system. Tests showed the process successfully produced a high quality
charcoal and the product gas effectively used to drive a gas turbine. The CHaP technology was proven and a new novel system of producing
charcoal under pressurised conditions was created coupled with a novel use of the product gas whose output was green heat and power. The initial
CHaP prototype showed the process was capable of producing low emissions and is virtually carbon neutral.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Charcoal; LCV wood gas; Combustor; Small gas turbine
1. Introduction
Long before its development as a fuel, charcoal was used as
a drawing medium by artists. Cave paintings made with
charcoal have been found, dated to 30,000 years BC. The
‘charcoal’ used here was more likely to be charred sticks from
a fire, rather than charcoal produced intentionally. The bronze
and iron ages, starting around 5500 years ago, are probably the
first use of charcoal as a fuel. Wood could not produce the high
temperatures needed to smelt, or reduce the ores, and then to
melt the resulting metal in order to cast it. Copper was first
reduced with charcoal around 3000 BC, starting the Bronze
Age, and around 1200 BC, the Iron Age began. It is possible
that the Egyptians also used charcoal in the early development
of glass. A by-product of producing charcoal, tar or pitch, was
used to waterproof wooden structures, in particular ships, as far
back as Roman times. In addition, the pyroligneous acid
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2005.10.026
* Corresponding author. Tel.: C44 29 2087 4318; fax: C44 29 2087 4317.
E-mail address: [email protected] (C. Syred).
(another by-product of charcoal manufacture) was used by the
Egyptians as an embalming material [1].
The production of charcoal involves burning the raw
material in an atmosphere free of oxygen (or air) and the
earliest method of charcoal production was probably with a pit
kiln, positioned in the forest, close to the point of wood
collection. This involved digging a shallow, level, pit and
stacking the timber to be used longitudinally along the bottom
of the pit. The complete pile was covered with vegetation,
straw and earth to make an airtight seal around the wood. The
wood was lit and the burning allowed to progress from one end
of the pit to the other, a process taking around 10–15 days [2].
Further developments led to the classical ‘forest kiln’, a
hemispherical woodpile built around a central shaft, which
acted as a chimney. Again, the woodpile was covered with soil
and turf to shut out the air, and lit by pouring several bucket
loads of hot embers down the chimney, which was then sealed.
Air supply to the heap was controlled by ensuring that any
cracks in the earth covering were repaired and opening or
closing purpose-made vents built at the base of the woodpile.
The charcoal burner had to attend to the kiln throughout the
burn to ensure that maximum charcoal was produced without
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C. Syred et al. / Fuel 85 (2006) 1566–1578 1567
the wood being burned to ashes, a process which would take
around 10 days. During this carbonisation process, the pile
would contract in size as the volatile matter was lost from the
wood. Average yield of charcoal from this type of kiln was
around 35–40 bushels of charcoal per chord of original wood
(i.e. around 35–45% of the original volume) depending on
operating conditions and wood-type. One major disadvantage
with this method of charcoal production was that a percentage
of the feedstock was burned to produce heat in order to power
the carbonisation process [1].
2. Charcoal production developments
Improvements to the traditional forest or pit kilns involved
building more permanent structures with bricks and more
recently, metal. This, however, presented the problem of
transporting large amounts of wood from the forest where it
was felled to the site of the carbonising facility. Initially, the
first development was to replace the forest kiln with a very
similar structure built with brick bases, in order that the tar and
pyroglineous acid could be collected in pits and put to further
use. Later, domed brick kilns were built, which were
themselves replaced with cast-iron retorts, where the wood to
be carbonised was held in a cylinder separate to the fuel used to
provide heat for the process. In this system, a brick-built
chamber incorporating a firebox remained hot while the cast-
iron cylinder holding the wood could be rapidly replaced,
saving time and heat energy. Quite a number of different
designs were produced using this basic design principle, with
additions for collecting the tar, acid and wood-gas by-products
of the process.
During the late 19th and the 20th century, much larger
industrial plants were built for larger quantities of charcoal to
be produced. Here, the wood and the final charcoal products
were held in railway-style wagons, which were pushed on
tracks into cast-iron tunnel retorts, and pulled out at the far side
when the process was complete. In some designs, the gases
produced by the carbonising wood were burned directly in the
furnace, reducing the fuel requirements of the system. A large
system was developed for refining and treating the by-products
of the carbonising process, similar in form to the plant used
today for refining oil. A number of large charcoal producing
plants were built, incorporating both retorts and refinery
processes, enabling both charcoal and many other products to
be produced, thus providing the raw materials for a wide range
of other processes [2,3].
Modern charcoal production methods have changed little
from the traditional forest kiln and remain inefficient, time
consuming and environmentally unfriendly with over 60% of
process energy loss. An extensive literature review found that
although some advances in charcoal production had been
made in the last century involving multiple batch loads, new
kiln designs, etc., these processes still remain inefficient and
time consuming. These modified processes are now no longer
used and UK production of charcoal has reverted back to the
more traditional kiln methods. Few references could be found
to work on charcoal production under pressurised conditions.
Charcoal in Europe is mainly used for the barbeque market,
although there are many other uses and the UK imports over
90% of its requirements. Interest is also growing in charcoal
as a ‘renewable’ fuel. Developing countries and those short of
fossil fuels however, use charcoal as their main cooking fuel
as well as for many industrial processes, such as smelting and
steel refining. Charcoal can also be ‘activated’ by further
refining and in this form is used in filters for water and air.
Charcoal can be used for medical purposes, both internally
and externally. It is used in sugar refining, agriculture,
horticulture, and as an ingredient in animal foodstuffs.
Specific charcoals (i.e. those resulting from particular wood
species) are used for gunpowder and fuse powders, and also
for artist charcoal.
The Charcoal, Heat and Power (CHaP) process discussed in
this paper offers a cheap, clean and efficient method of
producing charcoal with the waste energy being utilised in the
production of heat and power. This process can be used in
many situations both nationally and internationally. In the UK
the CHaP system could be used at forest management sites,
also with traditional and urban forestry. The completed system
uses wood sustainably derived either from ‘urban forestry’—
highway, amenity and domestic tree management operations—
or from revitalised deciduous woodlands. It could, if required,
utilise wood-chips from ‘energy plantations’ or waste from
conventional forestry. In developing countries, the CHaP
system could with modifications, use a range of different
biofuel and biomass materials. With increasing concerns over
climate change and the UKs commitment to increasing green
energy, reducing CO2 emissions, the process can make a useful
contribution to sustainability. The process can also use a
sawdust fed gasifier to provide heat to feed the carbonisation
process of the lumpwood. The hot gas (volatiles, tar, etc.)
driven off from the wood, combined with the gasifier gas, is
then fed into a combustor. This combustor then fires a small gas
turbine to produce green heat and power. The whole system is
operated under pressure. The CHaP system is thus an attempt to
improve the efficiency of the charcoal manufacturing process
by utilising available energy to generate electricity and heat
efficiently and economically whilst also reducing emissions.
The original system had a number of features in order to
achieve these aims:
1. As the carbonisation of wood is a cyclic process, a
modulated source of heat is required that can serve to
preheat the carbonisation vessel. This must be a gasifier to
avoid direct combustion of the lump wood intended for
charcoal production in the carboniser vessel. This can be
provided either by an available design of cyclonic gasifier
for sawdust or bio-oil. This means that none of the
feedstock intended for charcoal production is burned for
heat generation, whilst the evolved low calorific value fuel
gas can be blended with those from the gasifier and fed to a
special design of combustor. The combustor is used to fire
small gas turbine, thus the polluting gases resulting from
the carbonisation process will be burned cleanly and used to
produce heat and power.
C. Syred et al. / Fuel 85 (2006) 1566–15781568
2. The vessel for carbonising the wood will be operated at
moderate (gas turbine) pressures (3.2 bar absolute for the
pilot unit), higher pressures are envisaged later.
3. As the small robust gas turbine is direct fired to avoid
expensive gas cleanup systems, a special combustor had to
be evolved to deal with the variable mix of medium to low
calorific gases (LCV) from the carboniser and gasifier. This
incorporated novel vortex collector pockets (VCPs) to
remove and collect ash particles down to 5 mm without the
need for additional cyclone collectors in the system.
This paper describes the origins of charcoal use and
production, and the techniques for producing it. As CHaP
uses a pressurised system, results from the available literature
on the effects of temperature and pressure on the production of
charcoal are also discussed. The CHaP system, a clean,
efficient system for the production of charcoal, heat and power
is then described and a detailed discussion of the results from a
prototype system made.
3. Charcoal quality standards
Few standards exist which define the quality of charcoal,
particularly for the domestic market. Large industrial users
have a much tighter specification for the charcoal they can use,
particularly in the metal industry. In the domestic (barbeque)
market, no British standard exists whilst in Europe standards
exist in Germany, Belgium and France, i.e. the German DIN
51749, the French EP 846E and the Belgian NBN M11-001,
respectively. The German standard is quite specific on fixed
carbon content, giving a minimum of 78%, and quoting
maximum percentages of volatiles, ash and moisture. The
Table 1
Typical charcoal [4]
Wood species Production
method
Moisture
content (%)
Ash (%) Volatile
matter
(m.c./%)
Fixed
carbon
Dakama Earth pit 7.5 1.4 16.9 74.2
Wallaba Earth pit 6.9 1.3 14.7 77.1
Kautaballi Earth pit 6.6 3.0 24.8 65.6
Mixed tropical
hardwood
Earth pit 5.4 8.9 17.1 68.6
Mixed tropical
hardwood
Earth pit 5.4 1.2 23.6 69.8
Wallaba Earth
mound
5.9 1.3 8.5 84.2
Wallaba Earth
mound
5.8 0.7 46.0 47.6
Oak Portable
steel kiln
3.5 2.1 13.3 81.1
Coconut shells Portable
steel kiln
4.0 1.5 13.5 83.0
Eucalyptus
saligna
Retort 5.1 2.6 25.8 66.8
Key: (1) Guyana, (2) UK, (3) Brazil, (4) Fiji.
French and Belgium standards define the sizes of charcoal
pieces that can be sold to the public: the French quote 85% to
be in the 20–120 mm range and Belgium quotes a maximum of
10% below 20 mm and none over 160 mm. In Britain,
generally, charcoal is sold in pieces between 20 and 80 mm
in size at time of packing.
Proximate analysis results of a range of charcoal products
resulting from a number of woods, as manufactured by
traditional processes are shown in Table 1 [4]. Most charcoals
have a carbon content greater than 65% (with the exception of
the soft-burned sample) and a volatile matter content less than
26% (again, with the exception of the soft-burned sample).
Moisture content is generally below 8% and typically ash is
below 3% (although some exceptions exist). Moisture present
in the charcoal reduces the calorific or heating value of the
charcoal, since energy is required to heat and evaporate the
moisture.
For comparison, Table 2 shows the characteristics
demanded by a steel blast furnace plant in Brazil using
charcoal as a fuel. The table shows the range and yearly
averages of the charcoal used. The charcoal is a mixture of 40%
eucalyptus charcoal produced in company kilns and 60%
heterogenous natural wood charcoal manufactured by privately
operated kilns. The charcoal considered to be ‘good to
excellent’ is that produced from eucalyptus wood in company
kilns.
4. A qualitative description of the carbonisation process
The process of charcoal manufacture is known as the
destructive distillation of wood, and essentially involves
heating the wood to a temperature beyond 270 8C in an
(%)
Bulk density
(raw)
(kg/m3)
Bulk density
(pulverised)
(kg/m3)
Gross calorific
value (oven dry
basis) (kJ/kg)
Remarks
314 708 32,410 Pulverised fuel for
rotary kilns (1)
261 261 35,580 Pulverised fuel for
rotary kilns (1)
290 290 29,990 Pulverised fuel for
rotary kilns (1)
Low grade char-
coal fines (1)
Domestic charcoal
(1)
Well-burned
sample (1)
Soft-burned
sample (1)
32 500 (2)
30 140 (4)
(3)
Table 2
Characteristics of charcoal for a Brazilian blast furnace [4]
Chemical and physical
composition of charcoal
(dry basis) (by weight)
Max Min Yearly
average
Charcoal
considered
good to
excellent
Carbon (%) 80 60 70 75–80
Ash (%) 10 3 5 3–4
Volatile matter (%) 26 15 25 20–25
Bulk density—as received
(kg/m3)
330 200 260 250–300
Bulk density (dry) (kg/m3) 270 180 235 230–270
Average size—as received
(mm)
60 10 35 20–50
Fines content—as
received (!6.35 mm) (%)
22 10 15 Max 10
Moisture content—as
received (%)
25 5 10 Max 10
C. Syred et al. / Fuel 85 (2006) 1566–1578 1569
oxygen-free environment. This breaks down the complex
cellulose and hemicellulose molecules mainly into H2O, CO,
CO2, and char (solid carbon). The process of carbonisation is
generally described in terms of ‘Primary’ reactions and
‘Secondary’ reactions. Primary reactions are conversions of
the basic wood constituents to products including gases, liquid
tars and solid char, whereas secondary reactions reduce the
products of the primary reactions (in particular, the tars) to
lighter fractions and result mostly in gases.
5. Temperature–time characterisation
The production can be typically described as a three-stage
process:
1. Drying the wood to expel all remaining moisture.
2. Raising the temperature of the oven dry wood to 270 8C. At
this point, the wood begins to decompose, and an
endothermic reaction with spontaneous pyrolysis begins.
3. Final heating to 500–600 8C to drive off tar and increase the
fixed carbon content to an acceptable level.
These three stages can be further refined into the following
five stages:
(i) Temperature rises from 20 to 110 8C: Wood absorbs
heat energy, and releases water vapour.
(ii) Temperature will remain at or slightly above 100 8C
until all moisture is driven off (bone dry).
(iii) Temperature rises from 110 to 270 8C: Wood starts to
decompose, releasing some gases such as carbon
monoxide and carbon dioxide, and liquids, such as
acetic acid and methanol.
(iv) Temperature rises from 270 to 290 8C: Endothermic
reaction commences in the wood.
(v) Temperatures remain above 270 8C. This allows the
further breakdown of the wood to occur spontaneously,
provided that the temperature of the wood is not cooled
below 270 8C.
(vi) Temperature rises from 290 to 400 8C: Further break-
down of the wood allows a number of gases to be
released such as carbon monoxide, carbon dioxide,
hydrogen and methane, in addition to condensable
vapours such as water, acetic acid, methanol, and
acetone. Wood tars begin to predominate as the
temperature rises further.
(vii) Temperature levels around 400–600 8C: The main
process of carbonisation is complete, and the charcoal
is known as ‘soft-burned’. This type of charcoal can
contain up to 30% weight of tar, trapped in the internal
structure of the material. Further heating drives off
more of the tar and increases the fixed carbon content of
the final product.
6. The effects of temperature and pressure on the products
of carbonisation
Some research studies provided experimental results
detailing the effects of temperature and pressure on the
gaseous and liquid products of carbonisation, and are shown
in Fig. 1. Sadakata et al. [5] used mulberry wood in a
laboratory scale experiment, rapidly heating the wood at over
1000 8C minK1. Although the CHaP apparatus will not be
capable of heating the wood feedstock at this rate, their
results provide some useful trends. Fig. 1(a) shows the
temperature effects of the decomposition products during
wood carbonisation. In general, the graph indicates that the
gas products increase while the solid char products decrease.
The fraction of condensed liquids and tars appear to decrease
slightly with increased temperature, although this seems only
to have a significant effect when the temperature rises above
600 8C, above the operating temperature of the CHaP
carboniser, and therefore beyond the scope of this project.
Zaror and Pyle [6] collected data from a ‘slow’ pyrolysis
process (in contrast to the results shown by Sadakata et al.
[5]). Fig. 1(b) shows the effect of final pyrolysis temperature
on charcoal yield. The graph suggests a decrease of solid
charcoal production with increasing temperature, with a
corresponding increase in the carbon content of the solid
fraction. This supports the results shown in Fig. 1(a) where
increasing temperature causes an increase in the gaseous
products and therefore a corresponding decrease in the solid
products. The gas emitted from the wood during carbonis-
ation (termed ‘wood gas’) is a mixture of a number of
products. Fig. 1(c) shows the relationship between these
component gases and process temperature. Fig. 1(d) shows
the variation of the calorific value of the wood gas with
temperature, as given by two different sources [5,7] and
illustrates the range of wood gas calorific values that may be
expected at a specific temperature.
Antal et al. [8] used small amounts of biomass (w1 kg) in
experiments to determine the effects of pressure on the
charcoal process. The design of the CHaP process requires
that the carbonisation vessel operates at an elevated pressure of
3 bar absolute, and these results provide an indication of
Fig. 1. Temperature effects on charcoal process [5,7]; (a) temperature effect on wood, (b) final pyrolysis temperature on charcoal yield, (c) component gas
relationship with temperature, and (d) calorific value of wood gas with varying temperatures
C. Syred et al. / Fuel 85 (2006) 1566–15781570
the likely effect of pressure on charcoal production. Fig. 2(a)
shows a comparison of charcoal yield when operating a
carbonisation system at pressures 1 and 10 bar. The results
clearly show the charcoal yield is significantly increased with
pressure for all wood types. Fig. 2(b)–(d) shows the effect of
Fig. 2. Effect of pressure on various wood species [8]; (a) charcoal yield, (b
different pressures on charcoal volatile matter, fixed carbon
content and ash content respectively. Softwoods (Pine and
Spruce) showed a decrease in volatile matter, an increase
in fixed carbon content and an increase in ash content at the
elevated pressure. Hardwoods (Alder and Oak) showed
) volatile matter content, (c) fixed carbon content, and (d) ash content.
C. Syred et al. / Fuel 85 (2006) 1566–1578 1571
an increase of volatile matter, a decrease of fixed carbon
content and a decrease in ash content at the elevated pressure.
Birch wood, classified a hard wood, however does not follow
the trend of the other hardwoods and shows very small
decrease on volatile matter with elevated pressure with a
increase of fixed carbon content and a decrease of ash. Apart
from Antal et al. [8–10] very few studies have been found
which investigate the effects of elevated pressure on the results
of the carbonisation process. These results suggest that the
CHaP system will increase the yield of charcoal compared to
atmospheric processes whilst maintaining acceptable charcoal
quality.
A detail review of the production and properties of charcoal
is given by Antal and Gronli [10]. Antal et al. [11] also
investigated flash carbonisation of a fixed bed of biomass to
form charcoal and gas to utilise their green waste. This work is
ongoing and no literature could be found on modern techniques
for utilising the gas produced from the carbonisation process.
The CHaP project thus offers a very novel and efficient process
that can effectively utilise the process gas from carbonisation to
produce green heat and power.
It can be seen from the literature review that the Charcoal,
Heat and Power (CHaP) system is a further improvement in the
development of the charcoal manufacturing process. This
system offers the possibility of manufacturing charcoal with a
lower environmental impact, higher yield, as well as
simultaneously producing heat and electrical power. Charcoal
can be considered a renewable fuel, capable of producing the
high temperatures required of many industrial processes. It is
used in many parts of the world both for domestic cooking and
heating, as well as an industrial fuel.
Fig. 3. Schematic of t
7. CHaP design
The CHaP system uses four major subsystems and is shown
in Fig. 3 in schematic form.
(1) The carbonisation vessel and its ejector/flow recirculator.
(2) A secondary combustor capable of running on dual fuels,
oil (for start-up/shut-down or during certain operating
periods) and LCV gas.
(3) The combined support fuel-gas supply and carboniser heat
source.
(4) The gas-turbine based ‘turbo-alternator’ unit.
The turbine is initially spun by the alternator to a self-
sustaining speed. The combustors oil burner is then started and
compressed air is supplied to it from the turbine compressor,
through an air manifold and control valve 1 (CV 1), Fig. 3.
Additional secondary air is then supplied through CV 2.
Compressed air is then supplied through control valves 3, 4a,
4b, which is fed into the combustor pressure vessel and cools
the combustors surface. This air then mixes with the combustor
exhaust gas and reduces it temperatures so it is suitable for
firing into the turbine. Once full speed is reached, the
combustor is stabilised, and the turbine inlet temperatures are
reasonably constant then control valves 5 and 6 are open to
initially warm the carbonisation vessel and lump wood. After a
predetermined time, the gasifier is then turned on to provide
heat to the lump wood and start the carbonisation process. The
gasifier gas and carbonisation waste gas are then fed directly to
the combustor. As this gas enters the combustor, the combustor
oil burner flow rate is turned down automatically by a control
system.
he CHaP system.
Fig. 4. Schematic of the ejector-carboniser system showing the operating principle.
0
100
200
300
400
500
600
700
800
900
0 100 200 300
Time (min)
Tem
pera
ture
(°C
) inlet
Port 1
Port 2
Outlet
Centre ofcarboniser
Fig. 5. Temperature with time for carbonisation vessel when full of Lump wood.
C. Syred et al. / Fuel 85 (2006) 1566–15781572
The combustor is an integral part of the CHaP system and
must be capable of fully burning dual fuels, the LCV gas
produced from the carbonisation and gasification processes and
a range of supporting fuels (initially oil). The combustor must
have good heat storage capacity, produce low emissions and
fully burn out any tars remaining in the flow. Several studies
have been undertaken to develop LCV and dual fuel gas turbine
combustors. Problems encountered are numerous and include:
† Generation of non-premixed or diffusion flame to exclude
the danger of flashback.
† Maintaining high efficiencies whilst giving low NOx and
CO.
† The necessity of using larger fuel nozzles and swirlers to
handle the higher fuel gas volume.
† Issues of fuel quality restrictions such as hydrogen content,
particulates, alkalis, heavy metals, tars, fuel gas tempera-
ture, etc.
† The issue addressed in this paper are of redesigning the
combustor to avoid any drop in efficiency by essentially
increasing available residence time, whilst simultaneously
dealing with the contaminants in the LCV gas.
There is a wide range of work in this area as discussed in the
literature [12–17] where the issues raised above are more fully
discussed. These combustor designs are conventionally derived
from conventional gas turbine combustor systems fired on
conventional liquid fuels or natural gas. They are all designed
to be fired on cleaned bio-gas, this arises from the type of
turbine equipment used with sophisticated turbine blades
incorporating numerous fine cooling passages susceptible to
blockage. Conversely, CHaP addresses a different problem
involved with small-scale power systems. Here, gas turbine
systems are generally of simpler construction with un-cooled
turbine blades and can sustain modest levels of fine particulates
less than 5 mm in size. Indeed, some small turbine systems are
derived directly from turbochargers. Turbine inlet temperatures
are up to 900 8C. Low pressure drop across the system, low
emissions and good flame stabilisation are also necessary
requirements of the system.
The next section of this paper describes the design and
development of the CHaP process and its main components.
Initial tests performed on the system are also described.
7.1. Carbonisation vessel and its ejector/flow recirculator
The Carboniser vessel and ejector recirculator was a main
component of the CHaP system. The carbonisation vessel holds
lump wood under pressure in a flow of hot oxygen-deficient gas
(generated separately by a gasifier). Hot gas is recirculated
around the vessel by the use of an ejector and flow recirculator.
As the hot gas passes over the wood, the pyrolysis process
starts, volatiles are driven off, and charcoal forms. The volatiles
given off in this carbonisation process enrich the hot gas and
raise its calorific value. Fig. 4 shows the design of the
carbonisation and recirculator vessel. A tenth scale prototype
of the carboniser and ejector was initially built and isothermal
and hot gas tests performed under atmospheric conditions with
conditions representative of those under pressurised con-
ditions. The performance of the ejector was maximised by
testing isothermally and determining the optimum position of
the nozzle in relation to the carbonisation vessel outlet port
Fig. 6. Temperature profiles across combustor.
C. Syred et al. / Fuel 85 (2006) 1566–1578 1573
(Fig. 4, port 1), to achieve the maximum recirculation ratio.
The optimum position of the nozzle was in line with port 1 exit.
Tests showed the successful recirculation of the hot gas giving
a minimum recirculation ratio of 2 to 1. CFD modelling of the
system had been initially performed and matched the
experimental results closely. The LCV gasifier gas was
simulated using diluted natural gas and tests undertaken to
study the performance of the system for charcoal production.
Fig. 5 shows temperatures during a test with the carbonisation
vessel full of wood. The inlet temperatures are similar to those
expected from the gasification process [20] and thus are
appropriate to simulate conditions that would occur in the
complete CHaP system. Temperatures inside the carbonisation
vessel were seen to slowly rise through the process and peak
around 600 8C. This process follows the carbonisation process
described earlier in this paper, and tars and other volatiles
should be driven of by this peak temperature and an acceptable
level of fixed carbon achieved. Proximate analysis was
performed on the charcoal produced and the fixed carbon
content was 79.17%, moisture content 2.87%, volatiles 16.57%
and ash 1.37%. These results show the charcoal produced
corresponds to a good quality charcoal that is similar in
characteristics to that produced from more traditional methods,
Table 1.
Fig. 7. Particles collected in combustor for (a) conical bottom section and (b)
VCP.
7.2. Combustor
A cyclonic type combustor was chosen as previous research
showed robustness, stable flow and uniform outlet conditions
could be achieved through this type of design. A novel
tangential outlet would minimise pressure drop and create
uniform exit conditions. The combustor had two tangential
inlets besides the novel tangential outlet. An oil atomiser and
combustor can, originally used to fire the Rover gas turbine,
was attached to one of the inlet pipes and secondary air to the
cyclone combustion chamber was supplied through the second
inlet. The secondary inlet was also capable of supplying
product gas from a gasifier. A programme of testing was
carried out to characterise the combustor design and the oil
burner operating conditions. The viability of using LCV fuel
gas from the carboniser/gasifier (feeding the ejector) in the
combustor had also to be established, and the turn down ratios
with the various fuels determined. The final design was to
operate at a maximum output of 515 kW at a pressure of
3.2 bar. Thus atmospheric tests on the prototype combustor
were run up to an output of 200 kW with conditions
representative of those under pressurised conditions. The
combustor prototype was successfully matched to the Rover
gas turbine oil atomiser using kerosene. The tangential inlets
created a stable, strongly swirling flow that gives good mixing
and burn out rates. The combustor could be run over a range of
operating conditions from 50 to 200 kW, with varying air/fuel
ratios. Output from a gasifier was introduced to the combustor
and successfully operated with both fuels. The oil burner flow
rate could be turned down whilst keeping the gasifier flow rate
constant and maintaining a stable flame.
CFD modelling using the package Fluent 6 was initially
performed on the prototype combustor, inlet and exit
temperatures and emissions closely matched those measured
experimentally. The model was created with a vortex collector
pocket (VCP) at the outlet to collect fine particulates and a
central drop out pot for larger materials. The discrete phase
model was used to inject particles into the combustor to
simulate those occurring in the gas and investigate their
capture. The model showed the combustor was capable of
removing particles above 5 mm from the flow. This work also
supported this design process in that it identified the optimum
position for the tangential multi-fuel inlets as well as the
position of the VCP relative to the inlets and outlet. The
successful testing and modelling of the prototype combustor
confirmed the suitability of its general design and the capability
of burning dual fuels. The final design of the combustor was
tested against the Fluent predictions. This showed the
combustor ran well on LCV gas and oil as well as a
combination of these, producing a stable swirling flow with
good mixing and burnout, with early combustion initiated near
the inlets (Fig. 6). This was achieved with low pressure drop
across the combustor. A detailed discussion of the combustor
modelling is available in then literature [18,19]. The vortex
collector pocket (VCP) is positioned just before the outlet such
that the flow is drawn past the VCP as it is forced into the
tangential off-take that forms the exhaust. This mechanism
causes most fine particles to be projected into it. Fig. 7 shows
Fig. 8. Full-scale combustor.
C. Syred et al. / Fuel 85 (2006) 1566–15781574
particle trajectories inside the combustor. Larger particles are
collected in the bottom conical section of the combustor, and
smaller particles collected in the VCP. Most of the particles
that escape the combustor are less than 5 mm. This size of
particles in the exhaust gas is an acceptable value for direct
feed into the gas turbine.
A full-scale cyclonic combustor was therefore constructed
to the required specification and inlet conditions, Fig. 8. The
combustor was designed with three tangential inlets, an air
inlet, a high CV fuel inlet for support fuel and a low CV gas
inlet. The combustor was operated at a maximum thermal input
of 500 kW. The combustor was mounted vertically and with
the cone section at the base collecting larger particles from the
flow. The combustor was designed with a long chamber to
allow flame movement axially with varying thermal input and
quality whilst giving sufficient residence time for complete fuel
burnout and thus low emissions. The central section of the
combustor was refractory lined allowing substantial heat
storage capacity helping to create stable flames. The tangential
off take gives low system pressure drop whilst forces the
exhausting flow tangentially across a VCP aperture, hence
increasing separation capability. The combustor fires a Rover
Gas turbine operating at an inlet temperature of 800 8C, which
later will be extended to 900 8C. The exhaust gas of the
combustor was at higher temperatures than this, and was
diluted by a co-flowing air stream that was passed through a
jacket surrounding the combustor. This co-flow air lowers the
temperature of the exhaust gas and acts as a diffuser to the flow,
lowering the pressure drop across the combustor. The VCP
removes fine particle above 5 mm, which if carried through the
exhaust could damage the turbine. It also removes the need for
a separate cyclone separator to remove the particles, which
would increase the pressure drop across the system further.
Tests on the full-scale combustor were performed at
atmospheric conditions with inlet parameters representative
of those under pressure. The tests successfully proved the
combustor could run under a range of operating conditions
whilst maintaining a stable flame and uniform exit
conditions, as well as maintaining relatively low emissions.
The oil burner had a high turn down ratio (10 to 1) and
maintained stable conditions with varying equivalence ratios.
LCV gasifier gas combined with oil was successfully burnt in
the combustor producing a stable flame and uniform exit
conditions.
7.3. The combined support fuel-gas supply and carboniser heat
source
The last main component of the CHaP system is the support
fuel gas supply and carboniser heat source. A gasifier acts as a
source of support fuel-gas; this gas also acts as a heat-source to
drive carbonisation. Support fuel-gas is required to augment
the cyclically varying thermal output of the carboniser and
maintain a near-constant level of total gas thermal input to the
combustor and gas turbine. During the middle of the
carbonisation cycle, when carbonisation is occurring most
rapidly, carboniser-gas provides the main fuelling for the
combustor (the initial design point being 70% of total gas
thermal input). At the start and finish of the carbonisation
cycle, little energy is contained in the carboniser-gas and the
support fuel-gas provides the entire gas thermal input to the
combustor and gas turbine. The support fuel-gas also contains
the extra energy required to raise the temperatures of the
various thermal inertias within the gas generation system.
A pressurised design of an inverted, sawdust fired, cyclone
gasifier previously tested at Cardiff University [20] was to be
developed. However, due to the feedstock delivery problems
and time constraints other solutions had to be adopted, namely
a bio-oil gasifier. This produces the required hot oxygen-free
gas and is relatively much easier to engineer as fuel injection
and ignition may be accomplished by fairly standard fuel-oil
injection and ignition systems. Sub-automotive-grade bio-
diesel is a readily available, clean and carbon-neutral fuel-oil.
It is also a direct substitute for fossil-derived fuel-oil support
fuels for the main combustor. Thus a gasifier feedstock change
to fuel-oil had the beneficial side-effect of enabling elimination
of all requirements for fossil-derived fuels in the CHaP process.
The sawdust fired gasifier is being currently developed and will
eventually replace the oil fired version. The bio-diesel gasifier
is of similar configuration to the combustor, that is, a
tangential-inlet, tangential-outlet, single swirl chamber. It is
single-skinned and fully refractory-lined. The fuel oil is
injected into the gasifier using similar components to those
employed on the support fuel inlet to the combustor.
7.4. The gas turbine unit
A turbo-alternator system based on a kerosene-fired Rover
derived unit manufactured by Lucas for military auxiliary
powered purposes was supplied by James Engineering
C. Syred et al. / Fuel 85 (2006) 1566–1578 1575
Turbines (JET) Ltd. It had previously been demonstrated by
JET to run with an alternative external combustion system fired
by up to 50% sawdust augmented by kerosene.
Fig. 9. Complete CHaP system.
8. System analysis
Design ranges for thermal inputs to the combustor were
established (figures given for turbine at full load):
Effluent from carboniser: 0–70% of gas fuel (0–318 kW).
Gasifier gas (output): 30–100% of gas fuel (136–454 kW).
Combustor support fuel: !20% of total (100 kW).
These targets arose from the following considerations:
† Cyclonic sawdust gasifier thermal output is controllable in a
wide turndown range, expected to be approximately 100–
500 kW in CHaP conditions.
† The gasifier response is slow relative to the turbo-
alternator’s acceleration response to input energy vari-
ations.
† The carboniser energy output will contain both slow and
fast components: slow corresponding to the batch time-
scale; fast due to wood settling and wood fracturing
transients.
† It is preferable to run the turbo-alternator at maximum
power as this produces the highest electrical output.
† The input energy to the turbo-alternator must be regulated
to, and not exceed the maximum to prevent turbine over-
speed.
† Since the magnitude of carboniser output energy variation
is unpredictable, for precautionary purposes in this
first CHaP prototype, a fast response control on the turbo-
alternator input energy is needed of sufficient magnitude
to counter transient increases in carboniser output.
The Rover gas turbine’s original kerosene injection system
has a sufficiently fast response and an existing proven
control system based on turbine speed and jet pipe
temperature.
† A nominal power level of 100 kW by the support fuel oil
was the target. Modulation of this power level would
compensate for variation in the gasifier-gas power level
control.
† The maximum kerosene consumption of the original Rover
turbine’s combustor was 42.7 kg/h corresponding to
514 kW thermal (net CV basis). Additional estimated heat
fluxes are heat losses of gasifier and combustor (25 and
40 kW, respectively) and maximum power absorbed by
thermal inertia of wood (47 kW).
With the adoption of a bio-diesel gasifier to solve the fuel
feeding problems encountered with the sawdust gasifier, a
much lower turndown needed to be factored into the system
design. The effect was to transfer the main modulation
requirement to the combustor’s support fuel burner (to which
it is well suited). In a commercial CHaP system this would not
be required as multiple carbonisation retorts would be phased
so as to generate a near-constant production rate of effluent gas
energy, minimising support fuel requirement.
The full-scale CHaP rig was designed and manufactured as
shown in Fig. 9. The combustor was operated at a maximum
thermal input of 550 kW, with the gasifier rated at a maximum
200 kW. All components of the CHaP system were designed
and pressure tested to appropriate standards. The full-scale
combustor was placed in a pressure vessel that had cooling
inlets direct at the outer walls of the combustor to cool hot spots
identified during atmospheric tests. This cooling air then acts as
dilution air at the combustor exit. Testing at elevated pressure
occupied two phases. The first focussed on that part of the gas
circuit comprising the gas turbine and the combustor; the
second phase covered the complete gas circuit (including the
gasifier and carboniser). The first stage of the testing involved
the combustor coupled to the gas turbine. An isolation valve
was positioned just before the inlet for the LCV carboniser gas
which was closed during initial combustor/turbine tests. An
initial proving test with the gas turbine, prior to connecting it to
the new combustor, showed that the turbine was performing as
expected. Having installed the full-scale combustor, further
tests were carried out to check the control and instrumentation
systems. The turbine was started using its auxiliary motor and
the combustor then fired on gas oil and the stability of the
system monitored. Pitot tubes and thermocouples were position
throughout the rig and temperature, pressure drops and flow
rates were measured across the rig. The flow rate of fuel was
regulated by a control system.
8.1. Complete pressurised CHaP system testing
The complete CHaP cycle was tested initially with the
carboniser empty. The system had a compressor air manifold
(Fig. 3) with seven air outlets, two for the combustor primary
air and secondary air, three for the combustor cooling and
dilution air and two for the gasifier primary and secondary air.
An important consideration was the system instrumentation.
Mass flow measurements were taken at each of the compressor
manifolds air valves and the bell-mouth inlet of the compressor
as well as oil flow rates for both oil burners. Pressure
transducers were used across the ejector/carboniser, and the
Fig. 10. RPM and oil flow with time.
Fig. 11. Temperatures in CHaP cycle.
C. Syred et al. / Fuel 85 (2006) 1566–15781576
pressure difference measured across the compressor exit and
turbine inlet. Thermocouples were used to measure tempera-
tures in the compressor exit and manifold, the combustor, at the
system exhaust and turbine inlet as well as measurements in the
gasifier and carboniser.
8.2. CHaP testing with lump wood
The carboniser vessel was filled with wood and the CHaP
system was prepared for a full run. The gas turbine was
started by the auxiliary motor and then switched over to the
oil burner. The combustor was initially fired using the oil
burner to allow the combustors thermal mass to heat up.
After w12 min a steady combustor exit temperature was
reached of 850 8C and a turbine inlet temperature of 720 8C.
The turbine jet pipe exit temperature stabilised at 400 8C. At
this point the air valves from the compressor manifold to the
gasifier burners air inlet was opened allowing the compres-
sors hot air (at w200 8C) to circulate around the carboniser
vessel and heat up the lump wood. The LCV gas inlet valve
to the combustor was then opened. The combustor exit and
turbine inlet temperatures then stabilised at 800 and 700 8C,
respectively. The jet pipe exit temperature remained constant
at 400 8C. The system remained stable and the carboniser
inlet temperature increased to around 120 8C. After 35 min,
the gasifier was turned on to provide heat to the carboniser
vessel and the gasifier secondary air inlet opened. The
gasifier was turned down and the air valves closed to the
gasifier and combustor. The lump wood slowly pyrolised and
a LCV gas given off. This gas was fed into the combustor
and burnt. A stable turbine inlet temperature was maintained
by a control system that controlled the combustor oil inlet
flow. As more LCV gas entered the combustor the combustor
oil burner flow rate was decreased automatically. A stable
combustor exit temperature and turbine inlet temperature of
820 and 700 8C, respectively, was maintained. The combus-
tor oil burner flow rate was turned down from a full load of
10.5–1.5 g/s when the LCV gas was at its maximum safe
output. The output from the carboniser was maintained at a
stable rate by controlling the gasifier. To control the system
the gasifier needed to be switched on and off several times
throughout the cycle to achieve steady carbonisation and
control the amount of LCV gas produced to maintain stable
combustor conditions. After typically 3 h and 40 min, the
system was turned down when no further gas was produced
from the carboniser. This corresponded to previous calcu-
lations as to the length of the carbonisation process. (The
system is a batch process and for commercialisation a second
carboniser would be used with a switch over valve,
maintaining continuous turbine use.) The carboniser was
opened and examined, charcoal had been produced.
Proximate analysis of the charcoal was performed, and
showed the process made a high quality product.
The lump wood was weighed before and after carbonis-
ation and gave a yield of 38%. Overall mass balance results
from the various fuels used in the system showed the wood
produced a gas giving a calorific value of approximately
9.8 MJ/kg. Emissions from the turbine were monitored
throughout the tests. NOx levels remained at approximately
80 ppm and could have been reduced by detailed attention to
the secondary combustor. CO levels were generally low
w10 ppm throughout the process except when the combustor
oil burner and gasifier oil burner was being ignited or
extinguished. This is due to large fluctuations in flame
temperatures and air to fuel ratios resulting in momentary
incomplete combustion. As to be expected when the gasifier
was switched on and off these levels rose to several hundred
parts per million, and gradually dropped back to a low level
once the system had restabilised.
Fig. 10 shows the turbine speed and combustor oil flow rate
throughout the run. The turbine reaches it full speed after
2 min where it becomes stable. Throughout the run the turbine
speed was reasonably constant with small increases when
gasifier was modulated. The oil flow rate to the combustor
was monitored by a control system attached to the turbine.
This was controlled by the turbine inlet temperature. As LCV
wood gas was introduced into the combustor the oil flow rate
drops to maintain similar exit conditions, this is seen from the
fluctuations in oil flow rate in Fig. 10. Fig. 11 shows
temperatures in the CHaP cycle whilst running. The
compressor exit temperatures remain reasonably constant
(w200 8C) throughout the run. The temperatures of the
combustor products from the oil fired burner at inlet to the
main combustor was varied across the run (average 1350 8C)
Fig. 12. Pressure difference across CHaP system.
C. Syred et al. / Fuel 85 (2006) 1566–1578 1577
as this helps maintain reasonably uniform exit conditions. The
combustor exit temperature was reasonably constant around
800 8C. This exit gas was diluted with the cooling air inside
the combustor pressure vessel and reduces the turbine inlet
temperature to approximately 700 8C (this will be increased
later). Minimising pressure drop across the system was an
important design consideration as this has considerable impact
on the system efficiency and turbine performance. Fig. 12
shows the pressure difference between the compressor outlet
and turbine inlet. An average pressure drop of 0.2 bar is seen,
which is an acceptable value for the turbine rating. Due to
time limitations the system was run only for a few hours and
future testing and validation are ongoing.
9. Charcoal production
The dried wood was weighed before and after the CHaP
process. Successful carbonisation of the wood occurred and the
charcoal left gave a yield of 38%. Calculations from mass flow
of fuels into combustor and gasifier showed the combined hot
wood gas had a calorific value of approximately 9.8 MJ/kg.
Proximate analysis showed a high quality charcoal was
produced.
10. Effects of volatilised alkali salts
The effects of volatilised alkali salts from wood combustion
are well known [17,20] as are the problems of their
condensation on hot surfaces such as turbine blades. However,
in the system described, rugged turbine systems with uncooled
blades are used which are much more tolerant to deposition
than conventional turbines. The low inlet temperature of the
turbine also assists in this matter. Other work has shown that
typicallyO50% of the alkali salts condense on fine particulates
generated and are removed by the VCP and other particle
collectors in the system, this reducing turbine problems.
Compared to experiences with pressurised fluidised bed power
generation with coal [14] where direct firing of gas turbines is
also used with gas clean up via two stages of cyclone dust
separators, the fuel gases produced by this work appear to be no
less deleterious and thus suitable for direct firing of appropriate
gas turbines with uncooled turbines blades.
11. Conclusions
The main objectives of the project were to research a novel
energy technology and create a prototype machine for clean
and environmentally benign small-scale conversion of wood to
charcoal, heat and power. This new novel technology was
successfully researched, developed and commissioned in the
given time constraint and successfully demonstrating the CHaP
process and its commercial feasibility.
The CHaP project has developed a clean and efficient
system to produce charcoal heat and power. No research
could be found on pressurised charcoal production and
harnessing the energy produced in the associated LCV gas.
Similarly no existing, gas turbine combustors were capable
of efficiently burning the range of fuel inputs and or
simultaneously removing fine particulates from the gas
stream to minimise damage to the turbine. This research
will have a significant impact in producing efficiently low
cost charcoal and electricity, for the right application as well
as waste heat. There will be clear benefits both nationally
and internationally in producing a more sustainable
environment.
The next phase of the CHaP project is to attract the interest
of companies and commercialise the system. Several local
companies have shown interest in installing such a system.
Commercial systems will use much more modern designs of
gas turbines with un-cooled turbine blades to permit direct
firing. Because of the low pressure ratios of many designs of
small gas turbines a heat exchanger can be inserted between the
turbine outlet and compressor outlet to recover heat and
improve cycle efficiency by a predicted 8–10%. Development
will also be continued to produce a pressurised sawdust fed
gasifier to replace the oil gasifier.
As well as promoting the system nationally the CHaP
system will have great commercial benefits in developing
countries that have vast supplies of waste wood and
significant markets for charcoal. The system will therefore
be promoted both in Europe, North America and developing
countries where there are significant supplies of appropriate
wood.
Acknowledgements
The authors would like to thank EPSRC, contract
GR/N16587/01, and the DTI for their support in this project.
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