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: syredc@cf.ac.uk (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

Fuel 85 (2006) 1566–1578

www.fuelfirst.com

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.

References

[1] Kelley DW. Charcoal and charcoal burning. Bukinghamshire: Shire

Publications Ltd; 1986.

[2] Crumrin T. Fuel for the fires: charcoal making in the 19th century.

Chronicle of the early American industries association, vol. 47, June

1994; 1996, !http://www.connerprairie.org/historyonline/fuel.htmlO[3] Churchouse AH. Wood distillation. In: Heilbron IM, editor. Thorpe’s

dictionary of applied chemistry. London: Longman’s, Green and Co;

1954.

[4] Anon. Industrial charcoal making. Food and Agriculture Organization of

the United Nations; 1985.

C. Syred et al. / Fuel 85 (2006) 1566–15781578

[5] Sadakata M, Takahashi K, Saito M, Sakai T. Production of fuel gas and

char from wood, liginin and holocellulose by carbonization. Fuel 1987;

66:1667–71.

[6] Zaror CA, Pyle DL. The pyrolysis of biomass: a general review Wood

heat for cooking, Indian Academy of Sciences. London: Macmillian

Press; 1983.

[7] Fagbemi L, Khezami L, Capart R. Pyrolysis products from different

biomasses: application to the thermal cracking of tar. Appl Energy 2001;

69:293–306.

[8] Antal Jr MJ, Allen SG, Dai X, Shimizu B, TamMS, Gronli M. Attainment

of the theoretical yield of carbon from biomass. Ind Eng Chem Res 2000;

39(11):4024–31.

[9] Antal MJ. Biomass pyrolysis: a review of the literature part 1—

carbohydrate pyrolysis Advances in solar energy. Colorado: American

Solar Energy Society Inc; 1982.

[10] Antal Jr MJ, Gronli M. The art, science and technology of charcoal

production. Ind Eng Chem Res 2003;42:1619–40.

[11] Antal Jr MJ, Mochidzuki K, Paredes LS. Flash carbonisation of biomass.

Ind Eng Chem Res 2003;42:3690–9.

[12] Adouane B, Hoppesteyn P, De Jong W, Van Der Wel M, Hein KRG,

Spleithoff H. Gas turbine combustor for biomass derived LCV gas, a first

approach towards fuel-NOx modelling and experimental validation. Appl

Therm Eng 2002;22(8):959–70.

[13] Beer JM. Clean combustion in gas turbines: challenges and technical

responses-a review. J Inst Energy 1995;67:2–10.

[14] Beer JM, Garland. A coal fuelled combustion turbine cogeneration

system with topping combustion. J Eng Gas Turb Power 1997;119(1):

84–92.

[15] Hasegawa T. Study of low Nox in medium-btu fuelled 13008C—class gas

turbine combustor in IGCC. ASME International gas turbine and

Aeroengine congress and exibition, 98-Gt-331, Stockholm, Sweden;

1998.

[16] Hoppesteyn PDJ, De JongW, Andries J, Hein KRG. Coal gasification and

combustion of LCV gas. Bioresour Technol 1998;65(1–2):105–15.

[17] Nakata T, Sato M, Ninomiya T, Hasegawa T. A study of low NOX

combustion in LBG-fuelled 15008C class gas turbine. ASME J Eng Gas

Turb Power 1996;118(94-GT-218):534–40.

[18] Syred C, Griffiths AJ, Syred N. Turbine combustor with integrated ash

removal for fine particulates. ASME Turbo Expo, June 14–17, 2004,

Vienna, Austria; 2004.

[19] Syred C, Griffiths AJ, Syred N. Combustor development for dual fuels

with gas oil and biomass. 42nd Aerospace sciences meeting and exhibit

conference, AIAA, 5–9 January 2004, Nevada, USA; 2004.

[20] Syred C, Griffiths AJ, Syred N. Cyclone gasifier and cyclone combustor

for the use of biomass derived gas in the operation of a small gas turbine.

Fuel 2004;83(17–18):2381–92.