The process

In the first step, wood chips are dried before they go through different stages during gasification. In this phase, the main components are the double fire gasifier reactors. Through a continuous process evaluation, the quality is checked and maintained stable. The process is completed by cleaning the wood gas, which is then converted into electricity in the cogenerator.

Sub-processes of gasification
Sub-processes of gasification
Flow diagram
Flow diagram

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GTS Syngas - Gasification power plants

GTS Syngas - Gasification power plants


Drying

Drying of wood chips

The drying of wood chips or shredded waste wood with a residual moisture content of about 10% bone dry usually occurs on a moving floor. In particularly wet basic material such as fresh forest wood, preliminary drying through the ventilation of the covered wood chips warehouse is reccomended. This warehouse ventilation is also required to prevent wood chips from overheating, and thus their following loss of quality. When using waste wood, this is not necessary, because the residual moisture content is normally between 20% and 30%, and the product is therefore storage-stable. The residual moisture content in waste wood mostly originates from the water added during the demolition of construction timber and during shredding (dust reduction). The wood moisture content also increases due to the outdoor storage.


Gasification

Process of thermochemical wood gasification


Drying zone (20 - 150 °C):

In the drying zone, wood chips are dried by a fuel moisture value from 10-15% bone dry to 0%. The water released remains in the process, and the chips are not subject to any chemical change. Depending on the heating rate, there are however micro-cracks and other similar physical changes.


Pyrolysis zone (150 - 500 ° C):

In this zone, under exclusion of air, the thermally induced pyrolytic decomposition of macromolecules (cellulose, hemicellulose and lignin) occurs. In a temperature range from 200 to 300 °C, macromolecules are broken up and thus irreversibly destroyed. Volatile gases (e.g. CO2, H2 and CH4) and organic vapours are released. These vapors are hydrocarbon compounds such as tar and aromatics, which condense at ambient temperature and pressure and precipitate as pyrolytic oil. At 400 to 500 °C, the pyrolytic decomposition of wood is largely completed. At this temperature, approximately 80% of material has been converted into gaseous and vaporous products. As residual matter, solid pyrolysis coke, which consists of carbon and ash, is left over.


Oxidation zone (500 - 1300 °C):

The heat energy required for gasification and pyrolysis is produced by partial combustion of pyrolysis products. This takes place where the gasifying agent (air) is supplied, namely in the middle and lower part of the reactor. The design of the hot zone must be as consistent as possible for the quality of gas, because the thermal cracking of long-chain hydrocarbons (tars) can take place only here.


Reduction zone (800 - 1100 °C):

The main part of the combustible components of the product gas is formed in the reduction zone. Water vapour and carbon dioxide are reduced by solid carbon to hydrogen and carbon monoxide. In this stage, in doing so, the Boudouard reaction and the heterogeneous water gas reaction go on. In addition, there are a number of other reactions, such as the heterogeneous reaction of methanogenesis or the water gas shift reaction. These are light weight reactions depending on pressure and temperature. Ensuring a sufficient heat in the reduction zone is essential, because the balances of the main reactions shift almost completely in the desired direction at temperatures around 900 °C. 


Reactors

Reactors as centerpieces

Reactor
Reactor
Reactor diagram
Reactor diagram

The double fire gasifier reactor is essentially structured as a DC reactor. However, it has a further oxidation zone at the height of the reactor grid. Thus, the wood chips combustion is optimized and the carbon content in the ash is below 5%, so that they can be disposed without any problems. Moreover, temperatures in the reduction zone can so be maintained in the optimum range. The reactor is always filled to the sluice. In the lower part there is ash, then charcoal and on the top of it wood chips. These subsequently flow through the various zones of the reactor. Gasifier reactors are designed as fixed-bed reactors with intermittent loading. They operate according to the double-fire principle with two oxidation zones. Thereby, the first step in the reactor is the pyrolysis of wood chips whose products then continuously enter the first oxidation zone. Then, the combustion gases pass through a reduction zone without air, and then again a - second - oxidation zone. In the process, carbon is completely converted into a lean gas, wood gas, which consists mainly of carbon monoxide, hydrogen and nitrogen from the supplied air, to be then converted into electricity in the cogenerator. Air is fed into the oxidation zone in exactly such a quantity that no coal is left, but only pure mineral ash.


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Reactor Sirion

Reactor Sirion 200kW


Wood gas purification

Wood gas purification

The raw gas formed during the thermochemical gasification does not meet the necessary requirements in terms of purity and temperature for use in a cogenerator. A three-stage gas treatment is used, which consists of the following units:

  • Raw gas cooler
  • Filter
  • Purified gas cooler (scrubber)

Torch operating with raw and purified gas
Torch operating with raw and purified gas

Torch operating with raw and purified gas. The outer burners show a blue flame. The raw gas in the middle has a clearly yellow-coloured flame.


Raw gas cooler
Raw gas cooler
Filter
Filter

Thanks to the optimized geometry of the series III reactor, the use of a cyclone for particle separation can be omitted. Consequently, there is also no cyclone ash to be disposed of. Overall, gas treatment occurs without using any condensate, and the only scrap material is the filter ash. The individual steps of gas processing are presented below.

The raw gas comes out of the reactor at a temperature of about 650 °C. Cooling to about 170 °C is carried out with a tube bundle heat exchanger. The major challenge lies in avoiding undesired condensation reactions in the heat exchanger.

Dust concentrations in raw gas are separated via a tissue filter, which is also used in flue gas purification. Acidic condensate droplets and hydrocarbon compounds that condense at a temperature above 160 °C may cause clogging at this point. Therefore, the specific fabrics are previously treated.  For the so-called precoating phase, a special dosing station combined with a recirculation system is arranged for the filter. As a precoating medium, either lime or sorbalit (a mixture of lime and activated carbon) are used. Depending on the differential pressure, purification occurs with pressure surges.


Purified gas cooling (scrubber)

Scrubber
Scrubber

Finally, the temperature of the filtered raw gas must be set to a level compatible with the motor. For the cogenerator gas motors, this temperature is 40 °C. Cooling occurs via a scrubber cooler. The inlet temperature is about 140 °C. Cooling can be obtained either with water or RME (rapeseed-oil methyl ester).

The advantage of wet cooling is the efficient separation of condensed components such as tar and water. These are then recirculated in the reactor. Thus, in this stage, no condensate must be disposed of. The recirculated condensate is gasified again in the reactor and exposed to the thermal effect, so positive for the gas quality, in the oxidation zone. Keeping the prescribed fuel moisture is essential for the function of the closed recirculation circuit, since the condensate amount would be too large should this level be exceeded.



Electricity and heat from wood

Electricity and heat from wood

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