Production of Fischer Tropsch liquids

In cooperation with the biomass CHP in Guessing the first biomass-based Fischer-Tropsch trial plant in Austria was realized in Guessing by Vienna University of Technology (TUV) in the frame of the EC-project Renew and several national projects. A new laboratory scale Fischer-Tropsch-Reactor (slurry reactor) in a side stream of the existing allothermal fluidised bed gasifier at BKG was designed and installed. The new FT-synthesis plant operates in commercial environment and under permanent operation conditions. By this the long term performance and behaviour can be investigated. The catalysts used in the FT-slurry reactor are pre-commercial FT-catalyst, but also research FT-catalysts are studied.

The aim of this work is to develop a small scale BTL system, where in a polygeneration concept, not only BTL, but also electricty and heat is produced.

Description of the FT plant

The flow chart below shows the test rig for the Fischer Tropsch synthesis. The FT synthesis consists of the following main parts:

1. Gas drying by biodiesel scrubber
2. Additional atmospheric gas cleaning by activated charcoal
3. Compression of the gas to 20-30 bar
4. Various fixed bed reactors (HDS, ZnO, CuO, NaAlO 2 ) for gas cleaning
5. slurry FT reactor
6. offgas scrubber to remove waxes from the offgas
7. offgas cooler to remove liquid FT products from the offgas

During the experiments different combinations of the gas cleaning devices, different catalysts and operation parameters were tested.

The first step of drying is necessary, because the product gas has a water content of about 10%, which would condense in the gas compressions step. Here the gas is cooled down to about 3°C in direct contact with biodiesel, to remove the water content of the product gas.

The second step of activated charcoal is only used for some experiments, to investigate the removal of catalyst poisons at atmospheric pressure before the compression step (also to protect the gas compressor).

The compression of the gas consists of two steps, first a diaphragm pump to about 5 bars and then a piston compressor to 20-30 bars. The gas compression step caused most of the interruptions of the operation of the FT synthesis.

The gas cleaning consists of a hydrodesulphurisation step (HDS) to convert organic sulphur to H2S and then a ZnO adsorber and a CuO adsorber to remove the H2S.

After the gas treatment the clean gas is heated up to about 250°C and fed into the FT-reactor. The Fischer Tropsch reaction takes place in a slurry reactor (three phases; catalyst, gas, waxes) with a volume of 20 liters. The gas is leaving the reactor over sintered metal filters. After the FT-reactor the gas is expanded to normal pressure and the Fischer Tropsch products are separated from the gas stream with a scrubber (water/FT-products; ~80 °C). After the scrubber the gas is led through a heat exchanger and is cooled down to approximately 5°C. The off gas from the FT-plant is send back to the CHP-plant.

The liquid FT products are collected and distilled. The fraction up to 180°C is used as naphtha, from 180°C to 320°C is as diesel and the fraction above 320°C are waxes. The different liquid FT-products are delivered to the partners in the project.

Results of the experiments

The first experiments were used for investigating different FT-catalysts, gas cleaning methods and parameter variations. After variation of several parameters the conditions could be found to have stable operation of the FT-synthesis without any deactivation.

Typical temperature distribution over the FT reactor is shown in the following figure:

The FT-liquids produced during operation are collected in the offgas scrubber and in the offgas cooler. Also in the FT reactor inside there is a change of the hydrocarbons. The waxes used for starting are replaced with the time by the long chain hydrocarbons produced by the FT reactions.

In the following figure the distribution of hydrocarbons in the reactor is given:

In the following figure the distribution of hydrocarbons collected in the offgas treatment is given:

To determine the chain growth probability the mathematical equation of the stepwise chain growth concept according to Anderson, Flory, Schulz was used.

Wn mass fraction of species with carbon number n
n carbon number
a chain growth probability

In the following figure the logarithms of (Wn /n) is displayed against the carbon number. For the plot the sum of the gas analyses, simulated distillation of the condensed product and the simulated distillation results from the slurry in the reactor are used. The abnormal behavior of the plot at the low carbon numbers can be a reason that the condensation of the product in Güssing is not complete or the gas analyses are not sufficient. For the compounds with a carbon number from C10 to C30 an a of 0.87 can be determined.

Analysis of the Diesel fraction

The fraction from the raw FT product with a boiling range from 180-320°C was used as Diesel and analysed by the Institute of Petroleum Processing in Poland. Here only the results of the Cobalt catalyst are shown:

Till now only the raw FT Diesel, without any further treatment was produced and analysed. This Diesel consists mainly of paraffins and has therefore a excellent Cetane number, but poor cold behaviour. For the future the FT raw product will be also hydrotreated and isomerisised to get a high quality Diesel.

Conclusion and Future work

As a result of current concerns about both crude oil prices and CO2 -accumulation in the atmosphere, biofuels play a major role in tomorrow's energy supply. Synthetic biofuels that can be produced from biomass via gasification and subsequent catalytic conversion of the synthesis gas compounds CO and H2 are one promising option to meet the ambitious goals set by the European legislation.

While typically only the synfuel is regarded as the desired product and co-products such as electricity and district heat are of negligible interest, in this concept a different approach is introduced. In polygeneration plants that purposively sacrifice some synfuel yield to the advantage of power production, a high degree of flexibility is obtained that allows to design the product mix to the specific needs of the market or of other production facilities. The latter may be especially valuable for the wood processing industry, as synergies with a complementary “energy centre” can be achieved. Furthermore, the use of low temperature heat for district heating which is possible in the small scale of up to 100 MW fuel power not only adds to the viability of the process, but significantly improves the overall efficiency and thus maximizes the amount of CO2 -savings.

Not only did the results of this work prove the energetic advantage of such trigeneration facilities, but equally promising break-even points were attained. Thus, the risk of the implementation of the technologies in a larger scale is reduced, as not only diversification applies, but also dependence on the yet developing synfuel technology is abated.

Further research and development in this field will be directed towards the demonstration of the technologies in the larger scale in order to build the bridge between laboratory research and eventual industrial application. Moreover, efficiency improvements and cost reduction potentials ought to be sought for, along with optimization of the gas treatment, so as to efficiently integrate synthetic biofuel production plants from biomass into tomorrow's fuel supply.