"Energy is not an end
but rather the means to achieve the goal
of sustainable human development"
Gasification is a thermal process converting dry biomass feedstock into a mixture of gases that can be burnt in internal combustion engines and gas turbines.
The basic principles of biomass gasification have been known since the late 18th century and commercial applications of the principle first recorded in 1830. By 1850, large parts of London had gaslights and there was an established gas industry manufacturing îproducer gasî from coal and biomass fuels. The use of producer gas to run internal combustion engine was first tried around 1881, and, it was referred to as 'suction gas', because the gas was sucked by the engine from the gasifier.
The advent of petroleum accelerated a decline in the need for producer gas, and it fell from popularity. Lately, the îenergy crisisî of the 1970s sparked a renewed interest in biomass gasification systems and, presently, environmental concerns are questioning the continued use of fossil fuels and the need for sustainable energy production has prompted new research into the possibility of gasification as a key source of energy production.
Gasification technologies can be chosen according to the feedstock nature to ensure better results and low environmental effects. Indeed, if well designed, gasification can allow a cleaner energy production with no emissions at all! Furthermore, gasification can help reducing the "waste disposal" constraints by using it as a feedstock to convert it into useful and valued product: fuel gas.
Environmentally, Biomass Gasification is a clean technology. It has the advantages of being one of the energy producing technologies free of CO2 emissions, if well designed, and of using a renewable energy source, which makes it a sustainable energy system. The disadvantages that can be pointed out from biomass gasification can successfully be mitigated both by "good practices" and engineering measures.
List of Content
Renewable Energy Sources
3. Biomass Energy Technologies
4. Biomass Gasification
4.1 Gasification History
4.2 Gasification Theory
4.3 Gasification Technology
4.4 Biomass for Gasification
4.5 Gasification and Sustainable Development
The potential of renewable energy sources is far enough to meet several times the world's energy demand. Renewable energy source (NES) such as biomass, wind, solar, hydropower and geothermal can provide sustainable energy services, based on the use of routinely available indigenous resources. The development and use of renewable energy sources can enhance diversity in energy supply markets, contribute for securing long term sustainable energy supplies, help reducing local and global atmospheric emissions and provide commercially attractive options to meet specific energy service needs, particularly in developing countries and rural areas, helping to create employment and well fare for the local communities (Hertzog et al).
Conventional energy sources based in oil,
coal and natural gas have proven to be highly effective drivers of
economic progress, but at the same time, promoting serious damage to
the environment and human health. Renewable energy sources currently
supply about 15-20% of global world's energy demand. This supply is
dominated by traditional biomass, mostly fuel wood used for cooking
and heating, especially in Africa, Asia and Latin America.
Biomass was the first household energy source used by human beings, and for nearly all human life history, wood has been our dominant energy source. According to Diebold (Solar Energy Research Institute), until the discovery of low cost petroleum and natural gas in the early 1900's, wood was the most significant energy and heat supplier. Combustion of wood in different scales of boilers produced steam for heating purposes, for power industrial machinery, and, even for transportation vehicles such as trains, ships and farm machinery. Present trends show that biomass will continue the most significant energy source for human kind in the future.
Biomass is the term used to name material
derived from plants, (grass, trees and crops) and animals. Plantal
biomass is mainly composed by carbon, oxygen and hydrogen (50:43:6%)
and traces of mineral elements such as nitrogen, potassium,
phosphorus, sulphur and some others. The predominant organic
compounds are cellulose and hemi-cellulose or just
carbohydrates polymers (75%) and
lignin (25%), the last, acting as "glue"
to hold the cellulose fibbers together (NREL).
Carbon dioxide from the atmosphere and water from the earth (hydrosphere) are combined through photosynthesis, driven by the solar energy, to produce carbohydrates that build the organic matter of vegetable biomass. The solar energy is transformed into chemical energy in the chemical bonds of the structural components of biomass. When burning biomass (extracting the energy stored in the chemical bonds) efficiently, oxygen from the atmosphere combines with the carbon from biomass to produce carbon dioxide and water. Both water and carbon dioxide are the basic compounds that, together with inorganic nutrients from the lithosphere, driven by the solar energy, build up new biomass "organism" through the process known as photosynthesis.
Thus, biomass is a renewable resource!
Biomass from animals comes from animal excreta (waste) which still contains carbon (or organic material) and, therefore, can be submitted to fermentation (anaerobic/aerobic digestion), converting it into biogas or light alcohols. In the other hand, the animal excreta, i.e., cow dung, can be dried and burnt directly as a solid fuel. The carbon present in these wastes comes principally from vegetable mater that composes the animal (herbivorous) diet. This could be the reason why biomass is mainly regarded just as from plants.
Environmental impacts of biomass production must be viewed in comparison to the likely alternative impacts without the bioenergy system in place. For instance, the relative impacts of producing bioenergy feedstock depends not only on how biomass is produced, but also on what would have happened otherwise. Through life cycle analysis (LCA) studies, it has been found that where biomass displaces fossil energy systems, there will be a reduction in the impact on global climate through a reduction in overall greenhouse gas (GHG) emissions, but for other types of emissions, (i.e., N2O, NOx, SO2) it depends upon the source of the biomass, technical details of the conversion process and fossil fuel being displaced (Hertzog et al).
Many bioenergy conversion technologies offer flexibility in choice of feedstock and the manner in which it is produced. In contrast, most agricultural products are subject to rigorous consumer demands in terms of taste, nutritional content, uniformity, organoleptic properties, etc. This flexibility makes it easier to meet simultaneous challenges of producing biomass energy feedstock and meeting environmental objectives. For instance, unlike the case with food crops, there are good possibilities for energy crops to be used to revegetate barren lands, to reclaim water logged or salinated soils, and to stabilize erosion prone land. Biomass energy feedstock when properly managed can both provide habitat and improve biodiversity on previous degraded land (Hertzog et al).
The externalities of bioenergy, which are
not accounted for its cost, are important to be considered as well
and can offer benefits compared to fossil fuels. Its carbon neutral
character is one of those externalities. Furthermore, biomass has a
very low sulphur content. It is available to most countries in the
world, while fossil fuels need to be imported from a limited number
of suppliers. Indigenous production of energy has a macroeconomics as
well as employment benefits: it can offer relatively large numbers of
unskilled jobs, which can be important for many developing countries.
Although there are environmental impacts related to bioenergy, it is
usually considerably more beneficial in terms of external costs than
coal, gas or oil (Hertzog
As a simple example,figure 6 shows the net
carbon emitted (as CO2) to produce 1 MJ energy output from different
fuels. From this picture, coal, diesel oil and natural gas emit
respectively about 2240%, 1630% and 1390% as much carbon as released
by biomass fuel (Short Rotation Coppice-Wood). It stresses that
biomass is far environmentally friendly fuel than fossil fuels.
3. Biomass Energy Technologies
The "primitive" biomass technology
conversion is the direct
combustion which still is widely used in
scales ranging from small scale (household uses for cooking or
heating) to industrial scale, for heat and/or electricity generation
in different scales up to hundreds of MWe.
Presently, biomass energy technologies consist of many other conversion technologies used to extract biomass energy and convert it into a more useful form. They can be divided principally in two main groups: thermochemical and bio-chemical (biological) conversion process. In Europe, another technology, the mechanic conversion process, is being used to extract oil from rapeseed, getting rapeseed cake, as a by-product.
Thermochemical processes are
Biochemical processes refer mainly to the conversion through
in the absence of air (oxygen) leading to a formation of biogas
(mixture of CO and methanol) and
in an aerobic environment, giving methanol and ethanol, as
The most dominant way of extracting biomass energy, still is the direct combustion. Nevertheless, direct combustion gives an energy low transfer efficiency since it depends in many factors including the reaction conversion rate. An intensive R&D is still underway to improve direct combustion energy efficiency.
Gasification is another thermochemical conversion process which converts dry biomass into a mixture of fuel gases that can be burnt in internal combustion engines and gas turbines. Actually, air gasification is a thermal process that takes place in a special sealed container in a poor oxygen environment.
Pyrolysis is the process that converts biomass into liquid fuel (bio-crude), solid and some gaseous fractions, in the total absence of air at relatively high temperature (about 500ºC).
According to Ferrero,
Energy production sources among the the two "distinct worlds" are
presently distributed as follows:
It should be noted however, that the picture
in developing countries does not include the biomass (especially
firewood and charcoal) used directly for cooking and heating purposes
in these countries, with emphasis at the household level. In fact, at
this level, biomass supplies about 80-90% of the total energy
4. Biomass Gasification
4.1 Gasification History
The basic principles of biomass gasification have been known since the late 18th century and commercial applications of the principle first recorded in 1830. By 1850, large parts of London had gas lights and there was an established gas industry manufacturing îproducer gasî from coal and biomass fuels. The use of producer gas to run internal combustion engine was first tried around 1881, and because the gas was sucked by the engine from the gasifier, it was referred to as 'suction gas'. Early gasifier designs show many features in common with more modern designs.
By 1920s, producer gas systems were being used to operate trucks and tractors in Europe. While it was demonstrated that it was possible to operate engines with producer gas, it was not convenient or reliable and producer gas systems for operating mobile or stationary engines did not gain acceptability. In the other hand, the advent of petroleum accelerated a decline in the need for producer gas, and it fell from popularity.
Biomass gasification systems reappeared with
a force in Europe, Asia, Latin America and Australia during World War
II as a result of the scarcity of petroleum fuels. In Europe alone,
almost a million gasifier-powered vehicles helped keep basic
transport systems running during the war. In most cases, the gasified
biomass fuels were either wood or charcoal. After World War II,
gasifier systems were generally abandoned, triggered by the
re-emergence of convenient and relatively inexpensive liquid fossil
The publishing of the 'Gengas' by the Swedish Academy of Engineering in the 1950, was a major break through in the promotion of gasification. This classic book on gasification outlines the scientific, technical and commercial information developed during World War II. Much of the information within this book remains relevant today, and is probably the singularly most important book published on gasification.
The îenergy crisisî of the 1970s sparked a renewed interest in biomass gasification systems. The technology was perceived as a relatively cheap indigenous alternative for small-scale industrial and utility power generation in those developing countries that suffered from high world market petroleum prices and had sufficient sustainable biomass resources (Mendis in Pyrolysis & Gasification).
4.2. Gasification Theory
Biomass energy has the potential to be
"modernised" world-wide, that is, produced and converted efficiently
and cost competitively into more convenient forms such as fuel gas
and liquids or electricity. A variety of technologies can convert
biomass into clean, convenient energy carriers over a range of scales
from household, village to large industrial. One of these
technologies is gasification through which biomass is converted into
fuel gas. It has been ages since gasification has been known, in the
late 18th century. Since then, it has proven to be capable of
enabling biomass to play a much more significant role in the future
than it does presently, especially in the developing countries. It is
a fact that raw biomass has several disadvantages as energy source.
It is bulky with low energy density (about 16-20 MJ/kg) and, direct
combustion is, generally, highly inefficient (as above mentioned) and
produces high levels of indoor and outdoor air pollutants.
The goal of modernised biomass energy technologies is to increase the fuel's density while decreasing it's emissions during production and use.
Gasification is, as stated before, a thermochemical conversion process that converts biomass into fuel gases. It can be classified in several categories, according to different reference items. Referring to the oxidant specie used for biomass oxidation, it can be regarded as air gasification, the common way of gasifying solid fuels, meaning that it uses oxygen from the air as oxidant; pure oxygen gasification and steam gasification, when using pure oxygen or steam as oxidants. Another alternatives are a mixture of air/oxygen and steam or (less used) carbon dioxide and hydrogen.
It can also, according to the energy source, be classified as autothermal, meaning that it gets energy from it's self oxidation phase to complete the process; or allothermal, meaning that energy must be supplied to "heat" the distillation phase, through pre-heating of the gasifying agent.
Air biomass gasification, the principal subject of this study, comprises three principal stages determined by chemical changes together with energy flows in form of heat. These three stages can be summarised by the following reactions:
Stage I: Oxidation
+ Heat (1)
H + O2 = H2O + Heat (2)
Stage II: Distillation (Pyrolysis)
+ Heat =
+ CO (3)
C6H10O5 + Heat = CnHmOy (4)
Stage III: Reduction (Gasification)
+ C + Heat = 2CO (5)
H2O + C + Heat = H2 + CO (6)
(Obs: equations not balanced)
Oxidation reactions, (1) and (2), are exothermic, which means that they release heat. The carbon (C) and Hydrogen (H) that get oxidised in this phase are from the organic molecules of the solid fuel (biomass). They are transformed into carbon dioxide and water vapour, respectively. Ash is also produced during combustion, as a result of non-combustible inorganic (mineral) compounds. The products from this phase, enter the last phase where, through reactions (5) and (6) are submitted to a reduction process into a still combustible species such as carbon monoxide and hydrogen. In parallel, there is a third phase, not necessarily related to reduction, occurring at high temperature (the energy for this process can be obtained from oxidation) to distillate the heavier biomass molecules into less heavier organic molecules and carbon monoxide, through reactions (4) and (5). In this phase, other than the products found from the chemical reactions, tar and char are also produced. tar is mainly gasified in phase III while char, depending upon the technology used, can be significantly "burned" through reactions (5) and (6), reducing the concentration of particulates in the product.
The net product of air gasification can be found by summing up the partial reactions, as follows:
+ CO +
+ Heat (7)
From this equation easily stands that air gasification is, at the end of the day, a starved (partial) combustion which partially oxidizes biomass into a still combustible mixture of gases.
"By-products" such as tar, char and ash, are of less importance as part of "combustible" product, and normally "washed out" from the so called producer gas. Indeed, producer gas has to be free of tar and particulates if it is to be burnt in an internal combustion engines. Therefore, the combustible content of producer gas is mainly carbon monoxide, with varying fractions of hydrogen and hydro-carbon gases (depending on the primary feedstock) and molecular nitrogen (N2). The combination of fuel gas, produced directly by gasification reaction with the nitrogen from the air contributes to make it's calorific value relatively low ( 4 - 6 MJ/m3) (BTG). This energy content, makes producer gas (from air gasification) suitable just for combustion in adjacent internal combustion engines, boilers or kilns and, due to the dilution promoted by nitrogen, not recommendable for being transported for medium/long distances (as it becomes economically negative). For combustion far away, pure oxygen or steam, should replace the air, as oxidant to gasify biomass. Then, the product is medium joule gas with relatively high energy density.
As already stated, through gasification, solid biomass can be an energy source for gas engines and turbines, for electricity production or for "internal combustion engines", even though its calorific value still relatively low. The product is a convenient and modernised fuel gas that can be used as the conventional fuel gas with the advantage of releasing less harmful emissions. Thus, gasification is referred as being the way of adding value to the solid biomass energy. According to De Montfort University, gasification converts biomass into combustible gas with 60-70% of initial energy content, and is a potential energy source for electricity production through combustion engines or turbines.
4.3. Gasification Technology
Gasification takes place inside a suitable vessel named gasifier which is characterised according to the design of fuel bed and the method in which the solid fuel is brought to contact with the oxidant (air, oxygen, steam, hydrogen, carbon dioxide or various mixture of previous species). According to the fuel gas end use, the gasifier system can be divided into Heat Gasifiers- used for fuelling external burners in boilers, kilns or dryers; and Power Gasifiers - coupled to internal combustion engines for shaft power producing. Additionally, apart from being auto/allothermal or Heat/Power, gasifiers can be classified as i)fixed bed; ii)fluidised bed and iii)entrained bed designs. Although fluidised and entrained bed gasifiers are robust and versatile in their operation, they're, at the same time, generally more difficult to design, build and operate, more expensive and not recommendable for small scale (<1MWe) applications. In the other side, fixed bed gasifiers are the most common, especially in the poor countries due to their simplicity on designing and construction, as well as low investment, operational and maintenance costs.
Fixed bed comprises typically four types, according to the air and feedstock flow directions. Such types are i)Down-draft (or co-current); ii)Updraft (or counter-current); iii)Crossdraft (or cross current) and iv)Open core (open current).
Gasifiers (FBG) have the advantage of
enabling a better mixing, optimised kinetics, particle/gas contact
and heat transfer as well as long residence time. These factors
contribute to a high carbon conversion rates and, consequently, high
FBG can be divided into Bubbling (BFBG) and Circulating (CFBG) gasifiers.
BFBG give a good temperature control and
high conversion rates, good scale-up-potential, possibility of in-bed
catalytic processing, are tolerant to particle size and to
fluctuations in feed quantity and moisture. Although their product
gas has low tar content, unhappily, it is rich in
CFBG are suitable for fuel capacity higher than 10 MWth. Compared to BFBG, they have the additional advantage of giving high gas quality.
4.4. Biomass for gasification
Conventional energy sources such as fossil fuels, nuclear power and large-scale hydro projects dominate the world energy picture. Other energy sources have yet to be developed to compete with these traditional supplies. The utilization of biomass energy has recently attracted much R&D entities and governments. Biomass energy comes in many forms, and can provide energy in many different ways.
In order to ensure reliable and efficient
operation, biomass fuels for gasification must meet certain
specifications. All gasifier types have fairly strict fuel
requirements with respect to size, moisture and ash content.
Inadequate fuel preparation is an important and frequent cause of
technical problems associated with gasification and therefore a
strict organization and control of fuel preparation procedures is of
For instance, according to data from UNDP/WB- Biomass Gasifier Monitoring Programme (cited by Mendis)at present, only wood, charcoal, rice husks and coconut shells are considered to be suitable biomass fuels for gasification, in general. Biomass fuels such as wood wastes, coconut shells and rice husks can generally be used with minor processing such as drying, sizing and screening. The typical characteristics of these biomass fuels can be summarised as presented in the Table #1.
Ash (% Dry)
Volatile Matter (% Dry)
Bulk density (kg/m3)
Average HHV (MJ/kg dry)
A generalised overview of the most
important fuel requirements for different type of gasifiers, are
presented in Table #2.
Gasification and Sustainable Development
Energy Outlook 2001 projections
indicate a continued growth in worldwide energy demand that is
supposed to increase by 59% the present World Energy Consumption by
2020 and, the emissions of carbon dioxide (one of the GHG that
contribute for Global Climate Change) in the same period are almost
going to double.
These forecasts are not just figures. Behind those numbers, there is a tremendous damages to the Earth's life support system. The human eagerness on easy profit and the selfish of the businessmen and politicians, evoking development targets, are taking the human specie straight to his early extinction. Unfortunately, the human specie is hauling all other species into death. This is an unsustainable development. The population fast growth that is being evoked to justify this "animalistic" development, is an unfair excuse, if we consider the disparity on benefits, as the present numbers and reality show is not the driving force for this unsustainable resources depletion and environmental damages.
Among other "development" sectors, the one playing a key role both for development and for "Earth damaging" is energy. Indeed, the present picture indicates that from the total of about 5.3 billion world inhabitants, 25% (1.3 billion) are using about 67% energy in the " economically powered" countries while the rest are being consumed in the poor countries to meet the needs of about 4 billion people . The powered world is using oil, coal and nuclear power to meet 62% of its needs, which corresponds to 233% as much as the oil, coal and nuclear usage in the poor countries (Engström). These numbers justify the present picture on CO2 emissions where the "contribution" from developed countries was, in 1999, 64% of the total world emissions or 3,898 million metric tons carbon equivalent. This is an authentic disaster for life and environment. The human still have in his hands solutions, not to reverse the situation (it seems to be late), but to stop the "way down". The solution is Sustainable Development which means Renewable Energy Sources, Cleaner Production and Equity. In this study, the main contribution is given in Renewable Energy Sources and Cleaner Energy Production.
As substantially explained before, biomass is, with no doubt, a renewable energy source that has to be well managed to fulfil forever its role natural as energy resource. Gasification, as an energy production technology, can be regarded as Cleaner Energy Production. Actually, gasification can be used to produce fuel gas in different technologies, according to feedstock characteristics and their potential emissions. While fossil fuels contaminate the atmosphere with sulphur dioxide, nitric oxides, and radioactive species (nuclear), using gasification instead of direct combustion, emissions can be significantly reduced almost to zero and, at the same time, the fuel gas quality can be improved to meet the requirements of different machinery (internal combustion engines included), to produce heat and electricity. This would allow the replacement of fossil fuels and radioactive power, the main responsible for the disaster we are already witnessing.
Gasification has the uniqueness characteristic of being such a technology that can even convert waste (from MSW to agricultural or crop residues, like coconut shells, rice husks and straw, wood residues, bagasse, etc.) in useful and high quality energy source. It is known how constraining is the disposal of any kind of waste, nowadays, due to environmental regulations and legislations. Gasification gives the advantage of separating the noxious substances from the fuel gas prior to combustion. Additionally, the internal combustion engines fuelled by fuel gas from gasification have the less emissions compared to petroleum derivates fueled engines. Sulphur dioxide and NOx are, normally, absent in fuel gas from biomass gasification.
But, of course, there are some disadvantages
that one should take care of, for a well succeeded biomass
gasification. These disadvantages are related to health hazards,
safety as well as the environment. These disadvantages can be listed
principally as odour, noise, fire/explosion risks, CO poisoning,
exhaust gases and wastewater (mainly from gas cleaning
Biomass gasification releases odorous gases like hydrogen sulphide, ammonia and carbon oxy-sulphide. Tar has also a characteristic pungent odour. The odour can also be emitted from gas leaks, the wastewater and condensate (tar) and the fly ash. The noise is a consequence of machinery while operating (feeding system, compressors, turbines). The explosion risks can be encountered when there are some leaks that release the fuel (flammable) gas or vapours into the atmosphere where it can form an explosive mixture with air, if an ignition source is present. The solid fuel storage, combustible dust, fuel drying and product gas are the main fire risks sources in biomass gasification. Carbon monoxide (an odourless and colourless gas) is tremendously toxic since it can combine with haemoglobin and form such a complex (carboxyhaemoglobin) that cannot capture oxygen. This causes asphyxia, when CO is inhaled.
The qualitative and quantitative composition of exhaust gases depend upon the primary feedstock. It can contain speciesthat are related to environmental effects like acid rain and soil contamination.
Nevertheless, biomass gasification still is, by far, a cleaner energy production technology and, all these effects can be mitigated with both engineering measures and operational "good practices".
In terms of performance, biomass gasification for power production can become more convenient if it is used for co-generation. In fact, a Combined Heat and Power (CHP) plant can produce power and heat in such a way that total energy efficiency can be about 80%. For instance, if a CHP plant is designed to produce 1MWe and 1MWth (heat to power ratio =1) in a peak operation of 7000 hours/year, the biomass consumption would be around 1600 ton/year, assuming biomass with 20% moisture. This can be achieved by using about 300 ha/year to grow a kind of biomass that gives 5 ton/year.ha (Ferrero).
Socially, both gasification and energy
biomass plantation are beneficial. In fact, the investment cost for
rural electrification based on classical centralised power plants, is
related to an erection of long electricity grids to connect the areas
to be electrified to the power plants, far away. Biomass
technologies, such as biomass gasification, that use a locally
available resources, would enable poor rural areas to access the
electricity produced in a decentralised power plants. It would bring
the opportunity to experience an economic and social development,
consequently offering more employment to the local people, more
opportunities for basic health care and, at the end of the day,
bringing welfare for the rural communities.
Rural sustainable development is a need to reduce disparities in basic life conditions between the people living in town (in one side) and the farmers and peasants (in the other side).
- Energy Information Administration (EIA), International Energy Outlook 2001; DOE/EIA-0484(2001), Washington DC, March 2001
- Ferrero Gian Luca, Biomass Gasification-version 1, Lior CD-R Collection (Renewable Energy Series), Lior International 2000
- Ferrero, G.L., Maniatis, K., Buekens, A. and Bridgwater, A.V. (ed), 1989, Pyrolysis and Gasification (Proceedings from an international Conference-Luxembourg, 1989), Elsevier Applied Science (UK), ISBN 1-85166-449-1
- Bridgwater, A.V.(ed), 1993, Advances in Thermochemical Biomass Conversion-Vol 1 (reviewed papers from the International Conference on Advances in Thermochemical Biomass Conversion-Switzerland, 1992), Blackie Academic & Professional (UK), ISBN 0-7514 0171 4
- Engström, F., Overview of Power generation from Biomass, 1999 (A paper presented at 1999 Gasification Technology Conference, S. Francisco, CA-USA)
- Hertzog, A.V., Lipman, T.E. and Kammen, D. M,(2000), Renewable Energy Sources (SES Material)
- Lior International web site
- Applied Sustainable Technologies Group (De Montfort University) web site
- US Department of Energy web site
- Biomass Technology Group (University of Twente) web site
- Thermogenics web site
- Dynamic Energy web site
- The website for Power Industry (IGCC power plant)
- Chandrakant Turare web site
- NREL (National Renewable Energy Laboratory-USDoE) in Nature's Renewable Storehouse of Solar Energy and Chemical Resources web site
- ARBRE Energy Ltd. (UK) web site
- Chandrakant Turare (University of Flensburg-Geramny) web site
- (All the websites were "consulted" in March/April 2001)