An evaluation of a new multi-sample Fischer Assay setup according to ISO/ASTM standards

and analysis of the tar fraction

6th International Freiberg Conference on IGCC & XtL Technologies - IFC2014 (19 - 22 May 2014)

Dresden Radebeul , Germany.

Leon Roets1, John R. Bunt1, 2, *, Hein W.J.P. Neomagus1 and Daniel van Niekerk2

1 Coal Research Group, Unit for Energy Systems, School of Chemical and Minerals Engineering, North-West University, Potchefstroom, 2520, South Africa. 20164200@nwu.ac.za

2 Sasol Technology (Pty) Ltd; P.O. Box 1, Sasolburg, 1947, South Africa

Article published in the Journal of Analytical and Applied Pyrolysis, (2014), 106 : 190-196.

Outline of Presentation

q Introduction

q Experimental

q Results and Discussion

q Concluding Remarks

q Acknowledgement

Introduction : q  As the global energy focus is shifting to cleaner technologies, more

focus will be extended to pyrolysis which is seen as one of the routes to utilize coal for clean coal technologies [1, 2].

q  Primary pyrolysis is the process by which tars, oils and hydrocarbon

gases evolve due to the heating of coal at temperatures ranging between 400 - 600°C [3-5]. Pyrolysis forms an important intermediate stage in all utilization processes of coal such as combustion, gasification, carbonization and liquefaction [6].

q  Considering that pyrolysis is of such importance for understanding the

conversion kinetics of coal [7] and in order to utilize products from coal optimally, it is important to ensure that the experimental methods that are used are accurate and effective.

Introduction :

q  The Fischer Assay originates from the low temperature retorting research of Franz Fischer and Hans Shrader in 1920 [8, 9]. It was formulated as ISO 647 in 1974 [9] and is described as a low cost method for the liquefaction/primary pyrolysis of coal that yields gas, tar and char [10].

q  In the ISO 647 method a coal sample is heated according to a heat

curve that specifies the temperature over intervals of 10 minutes for a total time of 80 minutes, up to 520°C [9]. During this process the coal converts to char, releasing volatiles which include tar and water that is condensed.

q  Water is removed by azeotropic distillation using a Dean and Stark

distillation method [11] with the use of toluene as solvent. The toluene can be removed by rotary evaporation to obtain the volatile matter. The char, volatile matter, moisture along with the gas content (determined by difference) constitute the mass of the original sample [9, 10].

Background :

q  The Fischer Assay method is still widely used in coal pyrolysis and liquefaction research [12-18]. There are, however, a few reservations with regards to this method:

Ø  it is regarded as a performance test, like the octane number of petroleum, but not as a quantitative test procedure due to variances in the process used [19];

Ø  it is only suitable for middle rank coals due to the set pyrolysis temperature of 520°C, which is too high for low rank coals and too low for high rank coals [20,21];

Ø  heat transfer lag may play a role due to the placement of the thermocouple on the side-wall [9] instead of in the coal bed; and

Ø  the operating temperature is limited by the thermal properties of aluminium, which is the material specified for the retort, and this thus limits the versatility of the standard [19].

Introduction :

q  Tar typically constitutes 50-80% of the volatile matter released during pyrolysis and analysis thereof gives a good account of the thermal breakdown of coal [22]. Therefore, it makes sense to analyse the tar fraction from various pyrolysis experiments in order to draw a meaningful conclusion of the processes occurring [5]. Due to the heterogeneous nature of tar (multi-component product), various analytical techniques are used to elucidate the composition [23-27].

q  Gas chromatography Mass spectrometry (GC/MS) has been shown to be effective for analysis of the low molecular mass fractions up to 450 µ [1, 5, 27], whilst size exclusion chromatography (SEC) has been proven to be accurate in the analysis of the higher molecular mass compounds [5, 25, 28, 29]. Simulated distillation (SimDist) provides an effective means to determine the boiling point distribution of coal tar and has been used in various studies of coal derived liquids [30-36].

Background :

q  It is the objective of this study to evaluate a newly designed Fischer Assay oven method (referred to as the NWU Fischer Assay setup), compared to the standard Fischer Assay method as specified by ISO 647. Comparison will be made based on product yields and analysis of the tar fraction.

Water separated from liquid yield by Dean-Stark method Toluene removal in Büchi Rotavapor

SANS 647:1974 / ISO 647:1974 Fischer Assay setup

Experimental :

q  A washed and air dried South African coal sample from the Greenside no. 5 Witbank coal seam was prepared to a size range of +212 - 1000 µm as specified by ISO 647 [9]. According to proximate analysis, this was a low ash (12.9%) coal sample containing 32.7% volatiles and 52.4% fixed carbon on a dry basis. Petrographic analysis showed that this was a vitrinite rich (56%) coal sample containing 9% liptinite and 28% inertinite (mineral matter free basis) [37].

q  The Fischer tars were prepared by the use of two methods:

Ø  according to the standard ISO 647 method over an open flame using aluminium retorts and temperature measurement via the retort sidewall thermocouple, and

Ø  the new NWU Fischer Assay setup oven method.

Experimental : The automated Fischer Assay setup (front view)

Electrically heated oven   Controller panel  

Receiver flasks   Ice baths  

Specifications for the NWU Fischer Assay method are: Ø  a 22 amp - 48kW rating over 220 V

oven; Ø  operation according to a pre-

programmed heat curve over 90 minutes based on the oven chamber temperature;

Ø  coal bed temperature as well as side wall temperature measurements;

Ø  stainless steel retorts manufactured according to the specifications of ISO 647;

Ø  a multi-sample method - with two retorts in the oven, making it possible to do two simultaneous Fischer Assay runs.

Experimental : The automated Fischer Assay setup (inside view)

Heating elements  

Oven wall  

Thermocouple on retort side-wall  

Stainless steel retorts  

Thermocouple in coal bed  

Experimental : The automated Fischer Assay setup showing gas recovery

Experimental :

q  The choice of stainless steel as retort material is motivated by the aim to provide a method which is more versatile and which is not limited by the retort material.

q  Aluminium, the retort material for ISO 647, has a melting point of

660 °C, whilst the annealing temperature can be as low as 343°C and the hot-working temperature is 250-510 °C [38]. These thermal properties thus limit the use of the method.

q  Therefore, the NWU Fischer Assay setup operates with stainless

steel retorts which means that pyrolysis experiments can be conducted at higher temperatures (<1000oC) for complete volatile matter removal of bituminous coals. Stainless steel does however have a lower heat transfer rate [38], and therefore the thermocouple placement in the coal bed is of utmost importance.

Experimental :

q  In both Fischer Assay methods, volatiles were condensed in a round-bottom flask immersed in ice and water. Water separation was conducted by Dean and Stark distillation with toluene as a solvent [11]. The toluene was removed by rotary evaporation using a Büchi Rotavapor R II with Büchi Vacuum pump V 700 at 60°C. All experiments were conducted according to procedure ISO 647 [9]. Similar studies were conducted by various research groups and are available in literature [12-18].

q  The ISO 647 standard provides statistical requirements with regards to the

repeatability and reproducibility of the Fischer Assay experiments [9]. These requirements were used to determine whether the differences observed between the Fischer Assay methods were statistically significant. Four successful duplicate runs were conducted for each experimental method. The tar fraction prepared for each run was analyzed individually, the error based on the standard deviation for duplicate runs and a 95% confidence interval were used to identify significant changes. The ISO 647 aluminium retort runs were regarded as basis/control.

Experimental :

q  Gas chromatography mass spectrometry

Ø  All oil samples were analyzed using a GC-FID (quantification) and a GC/MS (peak identification) fitted with PONA column (50m x 0.2 mm x 0.33 μm).

Ø  Samples for GC/MS analysis were prepared by dissolving 200 µg tar in 2 ml dichloromethane in PTFE vials. Approximately 1 μL of prepared sample was injected into the GC column with a split of 200.

Ø  The GC oven program was as follows: Initial temperature of 60 °C held for 5 minutes, ramp at 6 °C per minute to 240 °C and hold for 30 minutes (until all compounds have eluted).

Ø  Gas flow through column was 1.2 ml per minute (helium in GCMS and hydrogen in GC-FID).

Experimental :

q  Simulated distillation

Ø  Simulated distillation analysis was conducted according to ASTM D2887 standard. Simulated distillation was conducted using a high-temperature GC-FID fitted with an ARX 2887 Restek column (10m x 0.53 mm x 0.53 µm). A sample consisting of 0.2µL tar dissolved in dichloromethane was injected into the GC column per analysis. The initial temperature of the GC column was 40°C, with a ramp of 15°C per minute, and hold time of 10 minutes at the final temperature of 540°C was applied.

Ø  The SimDist results in this study were analysed based on an average boiling point calculation and evaluation of the mass loss curves, as used previously [33, 36, 39]. The average boiling point is calculated by use of equation 1 [33, 36, 47] :

                                       𝑨𝒗𝒆𝒓𝒂𝒈𝒆  𝒃𝒐𝒊𝒍𝒊𝒏𝒈  𝒑𝒐𝒊𝒏𝒕  = 𝑻↓𝟏𝟎   +  𝑻↓𝟑𝟎   +   𝑻↓𝟓𝟎   +   𝑻↓𝟕𝟎   +   𝑻↓𝟗𝟎 /𝟓 

where: T10, T30, T50, T70 and T90 is the temperature at which 10, 30, 50, 70 and 90% mass loss occurs.

Experimental :

q  Size-exclusion chromatography (SEC-UV)

Ø  The samples SEC-UV analysis were prepared by dissolving a small amount of tar in 1 ml 1-methyl-2-pyrollidone (NMP) and mixing it thoroughly. The tar prepared samples were diluted in NMP (100 μl tar and 1400 μl NMP) prior to the SEC analyses.

Ø  An Agilent 1100 high-performance liquid chromatograph (HPLC) was set at 80°C using a 300 mm long, 7,5 mm internal diameter PLgel mixed-E (Varian) GPC column for separation. HPLC grade 1-methyl-2-pyrollidone (NMP) from Merck Chemicals at a flow rate of 0.5 ml/min was used as eluent.

Ø  All chromatograms were collected using a ultra-violet (UV) detector with wavelengths of 280, 300, 350 and 370 nm. According to Alvarez et al., [40] and Trejo et al., [25], NMP is known to be opaque at a wave length of 254 nm, but becomes partially transparent at wavelengths higher than 270 nm.

Experimental : Size-exclusion chromatography (SEC-UV) calibration.

Calibration was done by dissolving SEC polystyrene standards (purchased from Varian) with molecular weights ranging from 162 - 19 640 g/mol in NMP [41]. Calibration was done at a wavelength of 260 nm, due to the transparency of NMP [25]. The calibration data was used to derive an equation from which the molecular weight of the molecules passing through the SEC columns could be determined by intra- and extrapolation. Size exclusion chromatography (SEC) results were area and peak-normalised according to the method described in previous studies [25, 41].

0.0

0.2

0.4

0.6

0.8

1.0

8 10 12 14 16 18 20 22 24

Peak

nor

mal

ised

inte

nsity

Elution time (min)

Std. 1 Std. 2 Std. 3 Std. 4 Std. 5 Std. 6 Std. 7 Std. 8 Std. 9 Std. 10

y = -0.2335x + 6.5474R² = 0.9888

2

2.5

3

3.5

4

4.5

8 10 12 14 16 18 20

Log(M

p)

Elution time (min)

Calibration points Calibration curve

Results and Discussion : Comparison of the heat curves for the automated Fischer Assay setup and ISO 647.

Ø Due to a lag in heat transfer between the chamber temperature of the NWU Fischer oven and the coal bed temperature, there is a significant difference in heating time and heating rate observed with the NWU Fischer Assay setup.

Ø The chamber temperature curve refers to the pre-programmed heat curve of the oven, whilst the bed temperature refers to the temperature of the coal bed inside the retort. The ISO curve is an experimental curve set up according to the standard specified in the ISO standard and is a side-wall temperature measurement.

Results and Discussion :

Ø  It is observed that the heating rate of the NWU Fischer Assay setup coal bed temperature is significantly lower at the start of the experiment than that measured using the ISO 647 method. The temperature difference observed between the two methods (ISO versus NWU) at the end of the experiment (80 minutes) was 42°C. It takes the NWU Fischer Assay setup coal bed temperature on average 115 minutes to reach the final temperature (520°C) as specified by ISO 647.

Ø  Kandiyoti et al. [5] and Taupitz [42] have confirmed earlier that final temperature (up to 600°C) and heating rate will affect the final tar yield and composition, therefore it has to be noted from the outset that this difference with regards to apparatus operation may influence the results obtained from these two methods under investigation.

Results and Discussion :

% Yield Error % % Yield Error % % Yield Error % % Yield Error % Automated 74.9 0.3 10.3 1.6 9.4 1.6 5.4 1.9 ISO 74.3 0.2 12.0 2.0 8.5 3.0 5.1 2.7

Table 1: Average Fisher Assay Results

Maximum acceptable differences between % yield results

[9]

ISO Automated

Tar 0.5 0.4 0.2 Water 0.4 0.4 0.1 Char 0.7 0.2 0.2

Table 2: Difference between Fischer Assay % yield results under the same experimental conditions

Char Tar Water Gas

The NWU experiments produced significantly lower tar and higher water fractions when compared with the ISO experiments; whereas the char and gas yields are within similar range for the two experimental methods.

A comparison of these values indicate that the differences observed in yields of all three of these components were smaller with the use of the NWU setup. From Table 1 it is clear from comparison of the percentage error values for each setup that the values calculated for the NWU Fischer Assay setup were smaller in the case of the tar, water and gas product yields, whilst the char yield show little difference. This along with the values reported in Table 2 confirms that the automated oven method is more repeatable than the standard ISO manually controlled temperature method.

The ISO standard [9] specifies maximum acceptable differences for runs conducted in the same apparatus under the same conditions

Results and Discussion : q  Fischer Assay

Ø  The increase in tar yield with an increased heating rate was previously attributed to the suppression of the retro-regressive re-polymerization reactions [5, 43-48], and the increase in bubble build-up within the particle that increases the internal pressure and forces the tar precursors out [39]. The heat transfer rate of aluminium compared to that of stainless steel between 20°C and 210°C is sixteen times larger [38]. It is therefore postulated that this lag affects the heating rate to a large extent and may therefore be responsible for differences in tar yield and composition.

Ø  It is difficult to account for the difference observed in the water yield for the two experimental setups. The longer run time of the NWU Fischer Assay setup runs and the increased initial heating rate in the aluminium retorts of the ISO experiments may also account for the observed differences. Longer reaction times may lead to increased thermal cracking of the coal and evolved tar (possibly mineral dissociation) resulting in the formation of more pyrolytic water [43].

Results and Discussion : GC/MS

Ø  GC/MS results were evaluated by division of the compounds present in the tar into specific families as reported in literature [49]. The molecular families are based on functionality (aliphatic compounds, alkyl benzene, alkyl indenes, alkyl naphthalene, alkyl phenol, mixed compounds, nitrogen containing compounds, poly-aromatic hydrocarbons and C4-aromatics).

Ø  Compounds that exhibited both aromatic and aliphatic mass peaks in the mass spectra were classified as mixed compounds. These are typically due to co-elusion of compounds during the GC analysis. The full GC/MS composition was normalized to only take into account the aromatic moiety of the tar (exclusion of the mixed and aliphatic compounds).

Ø  Unknown component fractions were omitted from the results, and the data was normalised.

Results and Discussion :

Ø  GC/MS results indicated that the families most common in the tar generated by the Fischer Assay experiments were: mixed compounds, alkyl-phenols, aliphatic compounds and alkyl-napthalenes (Table 4). Thus, by rejecting the mixed and aliphatic components, a large fraction of the tar sample is omitted for comparison. The GC/MS results shown in Tables 3 and 4 indicate the average yields shown at a 95% confidence limit interval range based on the standard deviation and percentage error as calculated between duplicate runs.

Results and Discussion :

Average % yield Average % yield range Automated ISO Automated ISO

Aliphatic 19.7 19.1 18.6 - 20.8 17.5 – 20.6 Alkyl-benzene 1.5 1.0 1.3 – 1.7 0.9 – 1.0 Alkyl-indene 0.2 0.1 0.2 – 0.2 0.1 – 0.1 Alkyl-naphthalene 3.8 4.3 3.2 – 4.3 1.1 – 7.4 Alkyl-phenols 31.3 22.0 27.4 – 35.1 20.4 – 23.5 Mixed hydrocarbons 42.6 50.4 39.9 – 45.2 47.5 – 53.3 Nitrogen-containing heteroatoms

0.2 0.1 0.1 – 0.2 0.1 – 0.1

C4-aromatics 0.4 2.2 0.2 – 0.5 2.0 – 2.3 Poly-aromatic 0.6 1.1 0.3 – 0.8 1.0 – 1.2

Table 3 : GC/MS Full spectrum results (95% confidence limit)

Comparison of the ISO and NWU methods (Table 3) indicates that the aliphatic components produced in both experiments overlap and therefore no significant difference is observed, although the values obtained for the aliphatic components produced via the NWU setup was higher on average. With regards to the mixed components, it can be seen from Table 3 that the tars from the NWU setup contained less mixed components when compared to that of the ISO setup. Due to the nature of these components this result is inconclusive. Therefore no significant differences with regards to this composition can be reported.

Results and Discussion :

Average % yield Table 4 : GCMS Benzene spectrum results (95% confidence limits)

Automated ISO Alkyl-benzene 3.0 – 4.9 2.4 – 3.7 Alkyl-indene 0.4 – 0.5 0.3 – 0.5 Alkyl-naphthalene 9.6 – 10.4 5.5 – 21.9 Alkyl-phenol 80.9 – 84.8 66.4 – 77.5 Nitrogen-containing heteroatoms

0.4 – 0.4 0.2 – 0.4

C4-aromatics 0.3 – 1.4 5.5 – 8.5 Poly-aromatic 0.6 – 2.4 3.3 – 3.9

Ø increase in alkyl-phenol groups and decrease in poly-aromatic and C4-aromatic components seen for the NWU runs, of which the latter makes a small contribution to the total tar yield. The alkyl-napthalene group also showed a great degree of variance in the ISO tar yield, and on average there is a large decrease in these compounds for the NWU runs.

Ø There seems to be a shift between the mixed components and the alkyl-phenolic compounds for the two setups. This may be due to further thermal cracking of tars due to the longer run time of the NWU Fischer Assay setup runs [43] as well as the lower heating rate, which is also responsible for the lower tar yield [43-48].

Results and Discussion : Recovered mass % with change in temperature (Simdist)

The recovered mass percentage (with change in temperature) for both the ISO and NWU experiments show on average, a good correlation between the two methods, but the NWU method does have a larger lighter molecular weight fraction present (<200°C), and a slightly lower higher molecular weight fraction between 430°C and 515°C. The average boiling point for the NWU runs was 330°C, whilst that of the ISO 647 run was 334°C. The average boiling points were found to be within the standard deviation limits of the ISO run, thus indicating correlation between the two methods.

Results and Discussion : Area normalized size-exclusion chromatography (SEC-UV) graph.

The change in distribution of light and high molecular mass compounds as measured by the SEC analysis based on area normalisation is shown in this Figure. From this representation it can be observed that there is a larger lighter molecular mass compound fraction present in the tars derived from the NWU Fischer setup. The peak normalised results confirmed the little difference seen in average boiling points for the derived tars as obtained by the SimDist results. From the SEC-UV results very little difference was observed between the tars produced by the two methods.

Concluding Remarks :

Ø  Due to heat transfer limitations of stainless steel compared to

aluminium, the heating rate of the NWU method was found to be lower than that of the ISO 647 method. This difference affects the final tar yield as well as the length of the experiment. The final tar yield obtained for the NWU setup was lower than that of the ISO 647 runs. The yields of tar, char and water (between duplicate runs) was found to be more repeatable in the case of the NWU setup when compared to duplicate runs of the ISO 647 setup. This confirms that the automated oven method is more repeatable than the standard ISO manually controlled temperature method.

Concluding Remarks :

Ø  With regards to tar quality produced by both methods, only minor differences can be reported. The SimDist and SEC-UV analysis indicated only small changes that cannot be regarded as significant. GC/MS analysis indicated a change with regards to the mixed, alkyl-phenolic, poly-aromatic and C4-aromatic group, whilst the alkyl-napthalene group showed a lot of variance for the ISO 647 runs. The role of the type of retort used in methods for Fischer tar preparation should be further studied, due to the compositional differences observed in these tars.

Concluding Remarks :

Ø  The NWU Fischer Assay method provides the advantage that it can be used for different coal ranks (not just the middle rank coals as is the case with the ISO 647 method). It is also not limited by the thermal properties of aluminium (close to 520°C) and therefore the temperature range at which experiments can be conducted is increased (up to 900°C). The added advantage of this method is that results are less operator dependant than for the standard ISO 647 method (proven by smaller differences in product yield and smaller variance in tar quality), due to a pre-programmed heat curve.

Ø  Due to the heterogeneous nature of coal derived liquids it is difficult to quantify differences and attribute them to definite factors. This study has however succeeded in providing an alternative method for ISO 647.

Acknowledgements

Thank You

The authors gratefully acknowledge the assistance from Sasol R&D personnel with regards to the analysis of the tar samples. Mr NC Bean is also acknowledged for the NWU oven design. This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation (NRF) of South Africa. (Coal Research Chair : Grant Number 86880).

Any opinion, finding or conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard.

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