AN ALTERNATIVE TO CHARCOAL FOR
REDUCING DEFORESTATION
Centre Technique Forestier Tropical
Department of Cirad
45 Bis, Avenue de la Belle Gabrielle
94736 Nogent Sur Marne cedex
FRANCE
ABSTRACT
Prospects for torrefied wood, with its
specific properties and high-energy yield, are potentially very promising:
| in industry, where its use as a reducer
should compare favorably with the use of charcoal, as the cost of low yields when
compounded by high wood prices is reflected in rising charcoal production costs. |
| in the domestic fuel sector, where there
is evidence to show that torrefied wood may be more competitive than charcoal in developed
countries and in the larger urban centers in arid zones where a correlative reduction in
pressures on threatened woodland ecosystems would ensue. |
At present 3 production techniques are
being successfully tested in France for industrial reliability:
| Continuous production of torrefied wood
by conduction: |
Plant producing 14,000 tons per year is
now operating at full capacity. This process uses maxi-chips and produces torrefied wood
of uniform quality being a close resemblance to charcoal. Production capacity for the
equipment under consideration is of 7, 000 t/year and more.
| Two discontinuous production systems by
convection |
Equipment now being brought into
industrial use has been developed to treat a wide range of particle sixes by partial
pyrolysis at more realistic levels of production.
The success of torrefied wood as a
reducer in electrometallurgy has been established by experimentation, and prospects for
its use as a barbecue fuel are promising. It
is now necessary to define prospects for its use in developing countries, in the domestic
fuel, and industrial sectors. These prospects will depend heavily on foreseeable
production costs as compared to those of locally produced wood-fuels, and on the extent of
local acceptability of torrefied wood as an alternative to charcoal.
A feasibility study undertaken by CTFT
in collaboration with A.I.I. (Swiss cooperation) at Rwanda has demonstrated the
techno-economic viability of a torrefied wood production project (6,000 tons per year) at
KI-GALI. The further acceptability study of this project will be completed by early 1990.
If torrefied wood proves to be widely acceptable in this context, together with its ease of transport, should open up possibilities for broadly based trade in wood fuels between developing countries.
1.0 Preamble
The contribution of
fuel wood to world energy supplies is considerable: in 1985 this amounted to 400 million
toe, as much as nuclear energy in the same year.
At present, however, the use of wood to supply energy is for the most part limited to
domestic heating and cooking needs and to the small scale craft-industry sector in
developing countries, and to rural areas in the industrialized timber-producing countries.
In France, for example, wood use for residential heating needs amounts to 7 million toe.
Industrial use of wood fuels is widespread only in a few cases - in Brazil for example,
where in the industries trial sector, iron and steel and cement manufacture consume some
100 million tonnes of charcoal per year.
In many countries,
maintaining wood as a significant contributor to national energy supplies appears to be
the best way of ensuring greater autonomy in this area, and also provides a cheap source
of energy that helps to promote development in many industrial sectors.
However, wood and
charcoal are solid fuels that are difficult to manage, given the wide seasonal variation
in their physical - chemical composition and their low energy content. Moreover, their use
generally requires some form of prior treatment as well as properly adapted energy
transformation equipment.
With long experience
in dealing with these constraints, the CTFT has contributed to research in the processing
of wood and more generally in the transformation of lignocelluloses material into
efficient solid, liquid or gaseous fuels.
Studies have been or
are now being carried out along several different lines:
| Gasification with air or oxygen (methanol
synthesizing) |
| High-temperature and high-pressure hydro
liquefaction |
| Water-based or compound ternary slurries:
ligneous fuel (torrefied wood or charcoal) + water + petroleum additive. |
| Torrefaction through controlled
pyrolysis. |
Among these processes,
torrefaction has been the first to attain industrial credibility with the production of an
efficient solid fuel. We shall now outline the present state of development of this
process.
2.0 Torrefied Wood
During the 1930's in
France, the qualities of what was by then known as torrefied wood warranted subsidies for
research into its production and use in gasifiers. The outcome of this appeared to be
favorable at the technical stage but, unlike gasifier technology to which it was linked,
torrefied wood did not find the market outlets to which it seemed suited at the time.
Research undertaken in
the early 1980's by ARMINES (1) in collaboration with the CTFT with the aim of updating
and defining knowledge in the field of pyrolysis has now brought the properties of
torrefied wood to the fore, and has in particular opened up new possibilities for this
product, which now has industrial trial scale market outlets in the field of
electrometallurgy.
2.2 Reaction Characteristics
Analysis of the
thermal evolution of wood in an inert atmosphere show that wood progressively gives off
constituents having greater and greater energy value. At the same time, two of its most
important characteristics, where energy production is concerned, are subjected to change
in a positive sense: hygroscopicity
Up to 160 C, wood loses
water and little else. Most of its physical and mechanical properties remain intact,
particularly its hygroscopic properties (it can reabsorb moisture).
Between 180, and 270
C, wood begins to brown and gives off moisture, carbon dioxide and large amounts of acetic
acid with some phenols, the latter in quantities, which increase in proportion as the
exothermal stage develops. These first emitted compounds have low energy value, and whilst
wood at this stage in torrefaction retains most of its energy potential, it loses its
hygroscopic properties and becomes more friable than untreated wood which however is
less friable than charcoal.
Torrefaction occurs
between 200 and 270 C and wood at this stage in the process acquires the properties, which
are specific to the substance known as torrefied wood:
| high-density energy content, as compounds
with high-energy value have not been lost. Energy conversion yields vary from 80-90%
depending on the reaction stage reached. |
| Hydrophobia: reabsorption of moisture is
practically nil when torrefied wood is stored, with moisture content stabilizing at about
3%. |
| Optimum durability/friability balance. |
The exothermic stage
(270 C and over) cannot as yet be controlled within an industrial production process. The
generic term of torrefied wood thus defines a group of products resulting from the
partially controlled and isothermic pyrolysis of wood or biomass occurring within a
temperature range of 200-230 C and 270-280 C. Each
given combination of temperature/processing time/wood species will result in a given
product, which can be reproduced with near-perfection.
3.0 Characteristics of Torrefied Wood
The results obtained by ARMINES (2) at pilot-laboratory stage, with beech sawdust and small pine chips, demonstrate the influence of final reaction temperatures and processing time on the quality of the end product, as is shown in Table 1 and Table 2.
The fixed carbon content increases with higher temperatures and with longer processing times. The latter relation, however, rapidly becomes asymptotic. On the basis of these results, the CTFT (3) carried out systematic torrefaction tests on Meranti (shorea) scrap in large chunks, with a 1 m# pilot kiln. For each time - temperature combination, the physical-chemical characteristics of the torrefied wood obtained were measured, as well as conversion yields in weight terms, on Dark and Light Red Meranti. Results are given in Table 3 and Table 4 .
Research has been
undertaken along two different lines at the pilot stage, and has resulted in the
development of industrial equipment of two types: for continuous processes by conduction
and for batch processes by convection.
4.1
Continuous processes
In the process
developed by PECHINEY Electrometallurgy, a heat transfer fluid circulates within the kiln
walls and over the raw material feeder screw. Heat transfer occurs through conduction as
wood particles come into contact with the heated surfaces.
With
this equipment, wood must first be partially dried, and torrefaction will only give even
results if materials of fairly small particle size are used (not more than 10 mm in
thickness).
Figure 1 shows the PECHINEY plant at LAVAL DE CERE in
diagram form.
Please also see the EC Brochure Wood roasting
unit
4.2
Fixed-bed batch processes
These are
discontinuous processes, where dehydration and torrefaction occur as an inert gas, whose
temperature increases with each stage in the process, is forced through a fixed bed of
material. With such processes where heat transfer occurs through convection, the drying
and torrefaction stages may be conducted separately, either in separate kilns or at
different times.
The process cycle can
also be modified to suit materials of different particle size without the need for
equipment modification: only reaction times and therefore production rates will change.
Two similar techniques are now being marketed by two French industrialists with fairly
different end-users in view. Both processes have been tested by the CTFT.
One process developed by Ets. FAGES HABERMANN, operates in a small-scale production unit where two reactors produce 500 t of torrefied wood annually for use as barbecue fuel and as firelighters. The reactor design is very simple, consisting of a kiln in which a basket is placed. Drying and torrefaction both take place in the kiln.
The second process,
developed by PILLARD, operates in a 2,000 t/year production unit. The reactor consists of
an oven into which tip trucks carry the raw material that has been partially dried prior
to loading. In both of these processes, hot gases are forced through the raw material
load.
Figure 2
shows the basic principles of the process in
diagram form.
5.0 Present & Future Applications
Torrefied wood is a
stable industrial product with constant and well-defined properties, it is easily packaged
and transported, and thus constitutes a most efficient fuel and reducer. Technically
speaking, its competitiveness is no longer open to doubt, and its future in the
applications we shall now outline will depend on its economic competitiveness.
With a 30 - 35 % fixed
carbon content, torrefied wood appeared to have interesting potential as a reducer. Experiments were carried out with a trial kiln for
the production of silicon metal, which requires an efficient, reactive and mechanically
more resistant reducer. These trials were successful: torrefied wood proved to be more
reactive and less electrically resistant than the traditional "charcoal and
wood" mixture.
These results led Ets
PECHINEY to take up the license for the ARMINES torrefaction process and to build the
first industrial unit, with EEC backing. This unit has been in operation since early 1987
and supplies over 10 000 t/y of torrefied wood for the reduction of silicon metal. This industrial and economic success opens up
market prospects for several hundred thousand tonnes per year.
5.2 Gasifier Fuel
Long-term gasification trials (4) have been carried out with a fixed bed countercurrent gasifier. The producer gas generated fed a PERKINS motor coupled to a 24 kW electrical generator. measurements were made continuously, comparing equal loads of 77 kg wood and torrefied wood. Results are summarized in Table 5
Torrefied wood as
compared to wood gave the following results:
| A much higher furnace temperature (over
1300 C) that increases carbon monoxide production but will entail the development of an appropriate firebox. |
| Much greater regularity in the
composition of gases during functioning. |
| Very clean and high quality gas. |
| Greater autonomy for the same load (over
30%). |
Thanks to a high
degree of reproducibility and standardization, torrefied wood facilitates the operation,
regulation and optimization of gasifiers.
5.3 Domestic Fuel
Barbecues
A survey carried out
among 180 consumers comparing torrefied wood and charcoal.
| Nature, appearance and cleanness were
considered to be satisfactory. |
| Kindling was considered to be easy or
very easy, in clear opposition to charcoal kindling. |
| Glowing embers were considered to form
more rapidly |
| The product appears to be more
appropriate to brisk cooking. |
| Absence of smoke
during cooking was noted almost unanimously |
The CTFT also carried
out standardized tests comparing Meranti wood in unprocessed, torrefied and charcoal form. The following characteristics were compared in
tests carried out at the Gembloux Agronomic Research Center Laboratory:
| duration of flame phase |
| duration of glowing ember phase |
| weight loss during flame phase |
| weight loss during ember phase |
| ash and unburned matter |
Measurements recapitulated in Table 6 show that:
| the total duration of combustion for
torrefied woods in similar to that of unprocessed wood. |
| the duration of the glowing ember phase
for torrefied woods is also similar to that of wood and 3 times shorter than for charcoal. |
In conclusion,
although barbecue users seem favorable to the use of torrefied wood, as is borne out by
the sale of some 500 tonnes of it for this purpose in France in 1987, objective
measurements show that it can be used in ember form for only a third of the time of
charcoal embers. This can be a handicap to success in this area.
Domestic use of
torrefied wood in developing countries:
A domestic cooking
test carried out with the Association Bois de Feu at their experimentation center led to
the rapid development of a cook stove suited to the use of torrefied wood and which gives
yields similar to those obtained with the best wood stoves.
It was found,
moreover, that rekindling is very rapid. Torrefied wood added to a fire kindles very
quickly, which makes stove management (control arid variation of heat output) much more
flexible than with wood and particularly with charcoal fires.
Principal results are given in Table 7 .
Residential heating / Boiler and wood stove tests
Boiler tests carried
out by the CETI&127;AT (5) in comparison with various competing fuels (wood chips,
wood pellets, straw, bark and wood mixtures) showed the excellent solid fuel qualities of
torrefied wood, which is very well suited to use in continuously functioning boilers
because of its high degree of homogeneity and its mechanical durability.
Measurement made
showed:
| excellent yields, attaining 76% and over |
| higher furnace temperature than with
other fuels |
| low gas emissions |
| very low carbon monoxide concentration
(low smoke pollution levels). |
Tests on wood stove
carried out by the CTFT with the LNE at the latter's experimentation center gave the same
results, with torrefied wood proving to be clearly superior to wood in the early stage of
the combustion cycle. Torrefied wood kindles and gives off heat very quickly, thus heating
a room within a short time. However, the high temperatures recorded on the glazed opening
and in the stovepipe are a potential hazard and demand careful management of combustion in
the boiler.
Early marketing trials
indicate that torrefied wood is appreciated as an aid to start an open-hearth fire.
5.4 Slurries
Pulverized mineral
coal in suspension with water or in a ternary mixture with petroleum additives can not
provide liquid fuel which can be stored and pump-fed for convenient use in driving motors
as well as for combustion purposes.
Studies are being
carried out on charcoal in suspension form. Torrefied wood” which can be crushed to a
Together with
different partners, the CTFT is now developing a research program in this field.
6.0 Some Economic Data
The two currently
operational industrial processes will produce torrefied wood at a cost price of 200 US$
Rotation kiln process
where heat transfer occurs through conduction is suited to higher production capacities
ranging from 7000 to 18000 t/year and more. Investment costs exclusive of building and
civil engineering costs amount to I50-170 US$ per tonne of finished product for
large-scale units.
The batch processes
where heat transfer occurs by convection are applicable where production will range from
2000-4000 t/year, although the initial investment costs required appear on first
assessments to be higher, for example, 200 US$ per tonne finished product.
A few years of
operation at nominal production levels should confirm these initial cost assessments.
A pre-feasibility
torrefaction project estimates carried out in RWANDA, on a 6000 t/year site using
plantation Eucalyptus bought on the local market (56 US$/ton of torrefied wood) found that
production costs for the torrefied wood obtained would range from 250-260 US$/t, depending
on the different elements taken into account and excluding the cost of the technical
assistance required at the start and for follow-up work at the initial production unit.
At this price,
torrefied wood is competitive in relation to charcoal and should find promising market
outlets provided that the acceptability study that is to be implemented confirms the
credibility of torrefied wood for potential users.
7.0 Conclusions & Prospects
The physical-chemical
characteristics of torrefied wood are of considerable interest, particularly as regards
homogeneity and seasonal regularity that are of particular value to potential industrial
users.
Furthermore, the
production techniques that have now been developed allow for considerable flexibility in
adapting the characteristics of torrefied wood to different market requirements. Tests
carried out by the
Taking into account
the socio-economic conditions that are specific to each country and to each tropical
region, the particular qualities of torrefied wood indicate it as an appropriate
substitute for charcoal in many cases, where favorable repercussions on the environment
can be outlined without requiring abrupt changes
in local customs. Whilst providing the same services as charcoal, torrefied wood used as a
substitute can result in a two or threefold reduction in wood consumption.
Moreover, in areas
with extensive but inefficiently or under-exploited woodland resources, torrefied wood
could have considerable impact on small and large-scale industries, as a fuel competing
with more conventional sources of energy which have to be imported.
References
(1) Armines (1984):
Proc-d Armines de torrefaction du bois.
(2) Guyonnet R., Bourgois J., (1985): Le bois torr-fi-: recherche sur les m-canismes r-actionnels.
(3)
Cazillac J.M., (1988): Etude sur la torr-faction du bois. Suivi indusa&127;iel
d'un projet pilote. M-moire de fin d'-tude.
(4) Corte P Etat du d-veloppement des
gazogenes. Action de 1'AFME.
(5)
Cetiat, (1984): La combustion du bois: -tude bibliographique des rejets dans
1'aunosph-re