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.

since 2004 now R&D is done on this synthesis by Bioenergy2020+ together with his scientific partner Vienna University of Technology. Here a gas treatment and a slurry FT reactor were developed, so that in combination with the steam gasification, an economic production of FT raw product can be done.

The technical steps are gasification, gas cleaning and treatment, the FT synthesis itself and the Hydroprocessing of the raw FT products, as shown in Figure below

In the gasifier, the solid feedstock is converted into synthesis gas, which consist mainly of hydrogen and carbon monoxide. In the next step all impurities like sulfur or chlorine components are removed and in the gas conditioning, the correct pressure and H2:CO ratio are adjusted. In the FT synthesis reactor, H2 and CO react on the catalyst to a mixture of hydrocarbons, mainly paraffin's with carbon numbers from C1 to C60. The raw product from the FT synthesis consists mainly of waxes, but also here already a diesel fraction can be separated by vacuum distillation. The wax fraction is finally converted by hydroprocessing, which is a typical refining process, into diesel and kerosene.

So 2 different types of fuels are produced in a FT synthesis:

Both products from the FT synthesis produce very high quality blending component for fossil diesel or kerosene. For this reason not only the biomass conversion to FT products, but also a cooperation with a refinery/fuel distributor is recomended on the long term, where the refinery makes the final hydroprocessing, the blending and also the distribution to the consumer.

Description of the FT plant

At the location of Güssing a lab-scale plant for the conversion of 5Nm³/h of product gas is available. On this lab-scale plant first a reliable gas treatment to remove catalyst poisons and afterwards the slurry FT reactor itself was developed. This lab scale plant uses the synthesis gas from the biomass CHP Güssing after removal of particles and heavy tars, as shown in the figure below.

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. 1st gas cleaning by activated charcoal
  3. Compression of the gas to 20-30 bar
  4. 2nd gas cleaning by various adsorbers (ZnO, CuO, NaAlO 2 )
  5. slurry FT reactor
  6. separation of FT raw product from tailgas

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 removes the main amount of sulphur and other poisons (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 second gas cleaning consists of different adsorbers like ZnO or CuO to remove all catalyst poisons to below 10bbp.

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 Ft product together with the tailgas is cooled down in several steps to room temperature. Here the condenssation and separation of the raw FT product takes place.

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 different projects.

Selected Results of 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 liquid separators. 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 typical 0.9 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 based catalyst are shown:

Properties

Unit

EN 590:2004

World Wide Fuel Charter, category 4

Method applied

Results of FT Diesel
Results of HPFT Diesel

 

 

min

max

min

max

 

Cetane number

-

51,0

-

55

-

EN ISO 5165

75-85
65-80

Density at 15 o C

kg/m3

820

845

820

840

EN ISO 12185

770-790
770-780

Polycyclic aromatic hydrocarbons

%(m/m)

-

11

-

2,0

EN 12916

< 1
n.a.

Total aromatics content

%(m/m)

-

-

-

15

EN 12916

< 1
n.a.

Sulphur content

mg/kg

-

50

-

sulphur free (5)

EN ISO 20884

< 5
n.a.

Flash point

o C

>55

-

>55

-

EN 2719

87 to 91
80

Carbon residue

%(m/m)

-

0,30

-

0,20

EN ISO 10370

< 0,03
n.a.

Ash content

%(m/m)

-

0,01

-

0,01

EN ISO 6245

< 0,0015
n.a.

Water content

mg/kg

-

200

-

200

EN ISO 12937

200 to 300
n.a.

Total contamination

mg/kg

-

24

-

10

EN 12662

2 to 4
n.a.

Copper strip corrosion

(3h at 50 °C)

rating

class 1

 

class 1

 

EN ISO 2160

class 1 a
class 1 a

Oxidation stability

g/m3

-

25

-

25

EN ISO 12205

< 5
n.a.

Lubricity, corrected wear scar diameter

m m

-

460

-

400

ISO 12156

340 to 360
n.a.

Viscosity at 40oC

mm2/s

2,00

4,50

2,00

4,00

EN ISO 3104

2.3 to 2.5
2.0 to 2.2

Oxidation stability

g/m3

-

25

-

25

EN ISO 12205

< 12
n.a.

Cold Filter Plugging Point, (CFPP)

o C

-

-20

-

-20

EN 116

-5 to 0
-50 to -60

Both direct FT Diesel, without any further treatment, but also diesel from hydroprocessing was produced and anaylsed. The direct FT diesel consists mainly of paraffins and has therefore a excellent Cetane number, but poor cold behaviour. the HPFT diesel from hydroprocessing has a high share of iso-paraffins and and excellent cold behaviour and still a very good Cetane number of about 70. Both diesel fractions were used for blending with fossil diesel and tested in diesel engines. As well known the FT diesel improves the properties of fossil diesel and emissions are reduced,as shown in the figure below.

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.

The next step will be a pilot plant in the scale of 1 barrel / day, where the project will start in beginning of April 2015.