Choren Process

The following reaction equation simplifies the gasification process:

(-CH2-)Biomass + H2O –> CO  + 2H2

The gasification process is the reverse of the one that occurs in the Fischer-Tropsch synthesis. In the gasification, a synthetic gas is produced from one hydrocarbon. In the FT synthesis, a hydrocarbon is produced from a synthetic gas. The difference is that, in the Fischer-Tropsch, a different hydrocarbon than before is produced. A liquid fuel is then produced from a biomass, e.g. wood, by means of gasification and FT synthesis.

The gasification is a process between pyrolysis (no oxygen present) and combustion (sufficient oxygen present). It is made up of the following steps:

· Drying (vaporisation of water; T < 200°C)
· Pyrolysis (decomposition of the biomass and tar formation; 200°C < T < 500°C, zone of highest tar formation at 350°C to 400°C)
· Oxidation (oxidation of carbon hydride and hydrogen to cover heat requirement of reaction; 500°C < T < 2,000°C)
· Reduction (reduction of the oxidation products CO2  and H2O to CO  and H2; 500°C < T < 1,000°C)

The gasification can be described by the following equilibrium reactions:

I.  C + CO2 <–> 2CO
II.  C + H2O <–> CO  + H2
III. CO  + H2O <–> CO2  + H2
IV.  C + 2H2 <–> CH4

As the temperature rises, the reaction balances I. and II. move to the right in the direction of CO  or CO  and H2, while the balance reactions III. and IV. move to the left in the direction of CO  and H2O or C and H2.

The oxygen required for the oxidation is used in pure form. In an air separation plant, the air oxygen is separated from the nitrogen. This allows the volume of the reactors to kept considerably smaller. Also there is no later separation of the nitrogen oxide from the synthetic gas.

The diagram shows the process for the gasification technique used by Choren with an entrained-flow gasification system. The entrained-flow gasification is part of the multi-stage process in which the biomass has to be pre-treated: a low-temperature carbonisation gas and coke need to be produced from the biomass in the so-called low-temperature gasifier before it can be converted into a synthesis gas in the high-temperature gasifier.


Prestage of the Choren entrained-flow gasifier

The low-temperature carbonisation of the biomass occurs in the low-temperature gasifier, in a horizontal container at 400°C to 500°C producing tar-rich gas and tar-free coke. To allow continuous operation and to balance the temperature in the container, it is equipped with a horizontal mixer shaft with a stirring attachment. The gasification agents (O2, CO2, H2O) flow through the container from underneath. The necessary heat is created by the oxidation of parts of the biomass.

The entrained-flow gasifier is a vertical container into which the solid or liquid fuel is always blown in from above. The emitted heat can be used to create vapour by water-cooled walls. The gasification of the tar-rich gas with oxygen to CO2  and H2O has so far occurred at a pressure of five bar and a temperature of 1,400°C to 1,600°C at the top of the entrained-flow gasifier. The ground coke is fed in with an inert gas in the lower section of the entrained-flow gasifier. In this section, CO2  and H2O that continues to flow from above is reduced to CO  and H2 with the help of the coke. At the same time, this causes the gas to be called to the approx. 900°C that is required. It is operated above the ash-fusing temperature. This means that the otherwise solid ash is liquid at these temperatures. All anorganic components of the biomass flow as liquid ash down the walls into a slag bath. The great advantage of this is that these non-required components can effectively be removed from the synthesis gas during gasification. For example, biomass with a high content of ash, like straw, can be used without any problems. Furthermore the molten slag protects the walls of the gasifier against corrosion.

Advantages: The pre-treatment allows solid biomass to be used for producing synthesis gas. An entrained-flow gasifier is particularly advantageous due to the low formation of methane (CH4), the small amounts of tar (“tar-free”) and the high degree of carbon conversion (> 99%). Furthermore many of the entrained-flow gasifiers are suitable for ash-rich biomass. Finally also the concept of the separate blowing in of the coke is an advantage. Other methods needed to cool the high temperatures of 1,400°C to 1,500°C with water. This is costly and wastes valuable energy. The conversion of the carbon also cools the synthesis gas. However, a large part of the energy is bound in the synthesis gas in the form of chemical energy.

All of this helps increase the efficiency of the SunFuel production and reduce costs. In this SunFuel process, an efficiency of approx. 45 - 50% and manufacturing costs of approx. €0.70 is normal. The process is still full of potential for optimisation meaning SunFuel production costs or 50 euro cents seem very realistic in the future. This means that from a crude oil price of approx. $75/barrel of the SunFuel manufacturing process can compete with the current fuel production costs in a refinery. At present, a barrel of crude oil costs around $60! The environmentally friendly SunFuel is also attainable from an economic viewpoint.