Catalytic steam gasification of large coal particles 99

Catalytic steam gasification of large coal particles 99

Bibliography

ALLEN, T. 1997. Particle Size Measurement: Volume 2. 5

ed. Taylor and Francis Group.

pp.528.

ALLEN, T. and A. A. KHAN. 1970. Critical evaluation of powder sampling procedures. The Chemical Engineer. 238, pp.108-112.

BEND, S. L. 1992. The origin, formation and petrographic composition of coal. Fuel. 71, pp.851-870.

CHAPMAN, S. K. 1986. Working with a Scanning Electron Microscope. Kent: Lodgemark Press Ltd. pp.113.

CLOKE, M. and E. LESTER. 1994. Characterization of coals for combustion using petrographic analysis: a review. Fuel. 73(3), pp.315-319.

COLLOT, A.-G. 2006. Matching gasification technologies to coal properties. Coal Geology.

65, pp.191-212.

DAWES, S. B. and P. MAZUMBER. 2006. Method for preparing catalysts. US: 7,097,880.

DOMAZETIS, G., J. LIESEGANG, and B. D. JAMES. 2005. Studies of inorganics added to low-rank coals for catalytic gasification. Fuel Processing Technology. 86, pp.463-486.

ENGLAND, T., P. E. HAND, and D. C. MICHAEL et al. 2002. Coal Preparation in South Africa. Pietermaritzburg: Natal Witness Commercial Printers. pp.298.

EVERSON, R. C., H. W. J. P. NEOMAGUS, H. KASAINI, and D. NJAPHA. 2006. Reaction kinetics of pulverized coal-chars derived from inertinite-rich coal discards: Gasification with carbon dioxide and steam. Fuel. 85, pp.1076-1082.

FALCON, R. 1986. A brief review of the origin, formation, and distribution of coal in Southern Africa. Mineral Deposits of Southern Africa. 2, pp.1879-1898.

FALCON, R. M. S. and C. P. SNYMAN. 1986. An Introduction to Coal Petrography: Atlas of Petrographic Constituents in the Bituminous Coals of Southern Africa. Johannesburg: The Geological Society of South Africa. pp.27.

FRAISSARD, J. P. 1997. Physical adsorption: experiments, theory, and applications.

Dordrecht: Kluwer Academic Publishers. pp.619.

FRERIKS, I. L. C., H. M. H. VAN WECHEM, J. C. M. STUIVER, and R. BOUWMAN. 1981.

Potassium-catalysed gasification of carbon with steam: a temperature-programmed

desorption and Fourier Transform infrared study. Fuel. 60, pp.463-470.

Catalytic steam gasification of large coal particles 100 GALLAGHER JR., J. E. and C. A. EUKER JR. 1979. Catalytic coal gasification for SNG manufacture. In: 6th Annual International Conference on Coal Gasification, Liquefaction and Conversion to Electricity, 1979. Pittsburgh.

GASIFICATION TECHNOLOGIES COUNCIL. 2008. Gasification: Redifining Clean Energy.

Gasification. 1(1), pp.1-25.

GREGG, S. J. and K. S. W. SING. 1982. Adsorption, Surface Area and Porosity. 2

ed.

London: Academic Press Inc. pp.303.

GUZMAN, G. L. and E. E. WOLF. 1982. Kinetics of the K

CO

-Catalyzed Steam Gasification of Carbon and Coal. Ind. Eng. Chem. Process Des. Dev. 21, pp.25-29.

HABASHI, F. 1970. Principles of Extractive Metallurgy: Hydrometallurgy. New York: Gordon and Breach. pp.457.

HANAOKA, Y., E. BYAMBAJAV, and T. KIKUCHI et al. 2010. Steam Gasification of Low Rank Coals with Ion-Exchanged Sodium Catalysts Prepared Using Natural Soda Ash.

(Paper presented at Twenty Seveth Annual International Pittsburgh Coal Conference on 14 October 2010.) Instanbul, Turkey. 9p. (Unpublished.).

HANSON, S., J. W. PATRICK, and A. WALKER. 2002. Effect of coal particle size on pyrolysis and steam gasification. Fuel. 81, pp.531-537.

HARVEY, R. D. and R. R. RUCH. 1986. Mineral matter in Illinois and other U.S. coals. In:

Volume 301 of ACS symposium series, pp.10-40. Pennsylvania.

HATTINGH, B. B. 2009. The determination of the reaction mechanism involved in the CO

gasification of inertinite-rich, high ash coal. Potchefstroom.

HAYASHI, J. and K. MIURA. 2004. Pyrolysis of Victorian Brown Coal. In: C. LI, (ed).

Advances in Science of Victorian Brown Coal, Oxford: Elsevier, pp.134-217.

HIGMAN, C. and M. VAN DER BURGT. 2008. Gasification. 2

ed. Gulf Professional Publishing. pp.435.

HIPPO, E. J. and D. TANDON. 1996. Low temperature steam-coal gasification catalysts.

Preprints Fuel Division., pp.216-220.

HUANG, J. and A. P. WATKINSON. 1996. Coal gasification in a stirred bed reactor. Fuel.

75, pp.1617-1624.

HUTTINGER, K. J. and C. NATTERMAN. 1994. Correlation between coal reactivity and

inorganic matter content for pressure gasification with steam and carbon dioxide. Fuel. 73,

pp.1682-1684.

Catalytic steam gasification of large coal particles 101 ISHIDA, M. and C. Y. WEN. 1968. Comparison of kinetic and diffusional models for solid-gas reactions. American Institute for Chemical Engineering Journal. 14, p.311.

JAFFRI, G. and J. ZHANG. 2008. Catalytic gasification characteristics of mixed black liquor and calcium catalyst in mixing (air/steam) atmosphere. Journal of Fuel Chemistry and Technology. 4(36), pp.406-414.

JEFFREY, L. S. 2005. Characterization of the coal resources of South Africa. The Journal of The South African Institute of Mining and Metallurgy, pp.95-102.

JORDAAN, J. 1986. Highveld Coalfield. Mineral Deposits of Southern Africa. 2, pp.1985- 1994.

KARIMI, A., N. SEMAGINA, and M. R. GRAY. 2011. Kinetics of catalytic steam gasification of bitumen coke. Fuel. 90, pp.1285-1291.

KRUGER, D. J. 2008. The preparation of activated carbon from South African coal for use in PGM extraction. Potchefstroom.

LANG, R. J. 1986. Anion effects in alkali-catayzed steam gasification. Fuel. 65, pp.1324- 1330.

LANG, R. L. and R. C. NEAVEL. 1982. Behaviour of calcium as steam gasification catalyst.

Fuel. 61, pp.620-626.

LEE, W. J. and S. D. KIM. 1995. Catalytic activity of alkali and transition metal salt mixtures for steam char gasification. Fuel. 74(9), pp.1387-1393.

LESSARD, R. R. and R. A. REITZ. 1981. Catalytic Coal Gasification: An emerging technology for SNG. Texas.

LIU, Z. and H. ZHU. 1986. Steam gasification of coal char using alkali and alkaline-earth metal catalysts. Fuel. 65, pp.1334-1338.

LU, G. Q. and D. D. DO. 1992. A kinetic study of coal reject-derived char activation with CO

, H

O, and air. Carbon. 30, pp.21-29.

MAGEE, J. S. and G. E. DOLBEAR. 1998. Petroleum Catalysis in Nontechnical Language.

Tulsa: PennWell Publishing Company. pp.215.

MATSUKATA, M., E. KIKUCHI, and Y. MORITA. 1992. Migration of alkali and alkaline earth metal elements into carbon black. Fuel. 71, pp.705-707.

MCCORMICK, R. L. and M. C. JHA. 1995. Effect of Catalyst Impregnation Conditions and

Coal Cleaning on Caking and Gasification of Illinois No. 6 Coal. Energy & Fuel. 9, pp.1043-

1050.

Catalytic steam gasification of large coal particles 102 MCKEE, D. W. and D. CHATTERJI. 1978. The catalysed reaction of graphite with water vapour. Carbon. 16, pp.53-57.

MERCK CHEMICALS. 2011. 104928 Potassium carbonate. [online]. [Accessed 15 September 2011]. Available form World Wide Web: <www.merck-chemicals.com>

MINING TECHNOLOGY. 1997. Twistdraai Coal Mine, South Africa. [online]. [Accessed 20 October 2010]. Available form World Wide Web: <http://www.mining-technology.com>

MOLINA, A. and F. MONDRAGON. 1998. Reactivity of coal gasification with steam and CO

. Fuel. 77, pp.1831-1839.

MOULIJN, J. A., M. B. CERFONTAIN, and F. KAPTEIJN. 1984. Mechanism of the potassium catalysed gasification of carbon in CO

. Fuel. 63, pp.1043-1047.

NISHIYAMA, Y. 1991. Catalytic gasification of coals-Features and possibilities. Fuel Processing Technology. 29, pp.31-42.

OBERHOLZER, A. L. 2009. Characterisation and steam gasification of large, low grade coal particles using a specially designed thermo gravimetric analyser. Potchefstroom.

O'NEILL, F. (franceson@uis-as.co.za) 15 Aug. 2011. Discussion on error percentage on XRF analyses. E-mail to: Nel, S. (20282656@nwu.ac.za).

OSBORNE, D. G. 1988. Coal Preparation Technology. London: Graham & Trotman.

pp.1175.

PEREGO, C. and P. VILLA. 1997. Catalyst preparation methods. Catalysis Today. 34, pp.281-305.

PETERSEN, E. E. 1957. Reaction of Porous Solids. AIChE. 3(4), pp.443-448.

PINHEIRO, H. J. 1999. A techno-economic and historical review of the South African coal industry in the 19th and 20th centuries AND analyses of coal product samples of South African collieries 1998-1999 (In Bulletin 13. SABS:Pretoria. 97p.).

RANA, M. S., E. M. CAPITAINE, C. LEYVA, and J. ANCHEYTA. 2007. Effect of catalyst preparation and support composition on hydrodesulfurization of dibenzothiophene and Maya crude oil. Fuel. 86, pp.1254-1262.

SCHMAL, M., J. L. F. MONTEIRO, and J. L. CASTELLAN. 1982. Kinetics of coal gasification. Ind. Eng. Chem. Process. Des. Dev. 21, pp.256-266.

SCHOEMAN, L. (liezls@uis-as.co.za) 6 Dec. 2010. Discussion of XRF procedure used by

UIS Analytical Services. E-mail to: Nel, S. (20282656@nwu.ac.za).

Catalytic steam gasification of large coal particles 103 SCHOPF, J. M. 1966. Definitions of Peat and Coal and of Graphite that Terminates the Coal Series (Graphocite). The Journal of Geology. 74(5), pp.584-592.

SHADMAN, F. and D. A. SAMS. 1985. Interaction between Carbon and Alkali Metals during Catalytic Carbon Gasification. In: Proc. 17th Biennial Conf. Am. Carbon Soc., 1985.

Lexington:, pp.182-183.

SHADMAN, F., D. A. SAMS, and W. A. PUNJAK. 1987. Significance of the reduction of alkali carbonates in catalytic carbon gasification. Fuel. 66, pp.1658-1663.

SHARMA, A., T. TAKANOHASHI, and K. MORISHITA et al. 2008. Low temperature catalytic steam gasification of HyperCoal to produce H

and synthesis gas. Fuel. 87, pp.491-497.

SHETH, A., Y. D. YEBOAH, and A. GODAVARTY et al. 2003. Catalytic gasification of coal using eutectic salts: reaction kinetics with binary and ternary eutectic catalysts. Fuel. 82, pp.305-317.

SHUFEN, L. and S. RUIZHENG. 1992. Kinetic studies of lignite char pressurized gasification of CO

, H

and steam. Fuel. 73, p.413.

SMITH, D. and R. WHITTAKER. 1986. The Coalfields of Southern Africa: An Introduction.

Mineral Deposits of Southern Africa. 2, pp.1875-1879.

SOMMERFELD, D. A., J. JATURAPITPORNSAKUL, L. L. ANDERSON, and E. M. EYRING.

1993. Microscopic studies on the dispersion of iron/molebdenum bimetallic catalysts in hydrotreated Blind Canyon, Wyodak and Pittsburgh #8 coals. Fuel Processing Technology.

34, pp.197-206.

STIEGEL, G. J. and R. C. MAXWELL. 2001. Gasification technologies: the path to clean, affordable energy in the 21st century. Fuel Processing Technology. 71, pp.79-97.

SUN, Q., W. LI, H. CHEN, and B. LI. 2004. The CO2-gasification and kinetics of Shenmu maceral chars with and without catalyst. Fuel. 83, pp.1787-1793.

SU, J. L. and D. D. PERLMUTTER. 1985. Porosity effects on Catalytic Char Oxidation Part I:

A Catalyst Deposition Model. AlChE Journal. 31(6), pp.957-964.

SUZUKI, T., M. MISHIMA, and J. KITAGUCHI et al. 1984. The catalytic steam gasification of one Australian and three Japanese coals using potassium and sodium carbonates. Fuel Processing Technology. 8, pp.205-212.

SZEKELY, J. 1977. Reactions between porous solids and gases. In: L. LAPIDUS and N. R.

AMUNDSON, (eds). Chemical Reactor Theory: A Review, New Jersey: Prentice-Hall, Inc.,

pp.269-313.

Catalytic steam gasification of large coal particles 104 TAKARADA, T., Y. TAMAI, and A. TOMITA. 1986. Effectiveness of K

CO

and Ni as catalysts in steam gasification. Fuel. 65, pp.679-683.

TIEDT, L. R. 2011. Discussion regarding SEM photo's of raw and impregnated samples.

(Potchefstroom).

TIMPE, R. C., R. E. SEARS, and G. G. MONTGOMERY. 1987. Characterization of low-rank coal char used in the production of hydrogen. University of North Dakota Energy Research Center, Grand Forks. pp.1-9.

VAN DYK, J. C., M. J. KEYSER, and M. COERTZER. 2006. Syngas production from South African coal sources using Sasol-Lurgi gasifiers. International Journal of Coal Geology. 65, pp.243-253.

VAN NIEKERK, D., R. J. PUGMIRE, and M. S. SOLUM et al. 2008. Structural characterization of vitrinite-rich and inertinite-rich Permian-aged South African bituminous coals. International Journal of Coal Geology. 76, pp.290-300.

VERAA, M. J. and A. T. BELL. 1987. Effect of alkali metal catalysts on gasification of coal char. Fuel. 57, pp.194-200.

WAGNER, N. J. and B. HLATSHWAYO. 2005. The occurence of potensially hazardous trace elements in five Highveld coals, South Africa. International Journal of Coal Geology.

63, pp.228-246.

WANG, J., M. JIANG, Y. YAO, and J. CAO. 2009. Steam gasification of coal char catalyzed by K

CO

for enhanced production of hydrogen without formation of methane. Fuel (2009), doi:10.1016/j.fuel.2008.12.017.

WANG, J., M. JIANG, and Y. YAO et al. 2009. Steam gasification of coal char catalyzed by K

CO

for enhanced production of hydrogen without formation of methane. Fuel. 88, pp.1572-1579.

WANG, J., Y. YAO, J. CAO, and M. JIANG. 2010. Enhanced catalysis of K

CO

for steam gasification of coal char by using Ca(OH)2 in char preparation. Fuel. 89, pp.310-317.

WARD, C. R. 2002. Analysis and significance of mineral matter in coal seams. Coal Geology. 50, pp.135-168.

WCI. 2009. The Coal Resource: A Comprehensive Overview of Coal. [online]. [Accessed 5 March 2010]. Available form World Wide Web: <www.worldcoal.org>

WCI. 2009. The Coal Resource: A Comprehensive Overview of Coal. London.

Catalytic steam gasification of large coal particles 105 WU, S., J. GU, and L. LI et al. 2006. The reactivity and kinetics of Yanzhou coal chars from elevated pyrolysis temperatures during gasification in steam at 900-1200 C. Process Safety and Environmental Protection. 84, pp.420-428.

YE, D. P., J. B. AGNEW, and D. K. ZHANG. 1998. Gasification of a South African low-rank coal with carbon dioxide and steam: kinetics and reactivity studies. Fuel. 77, pp.1209-1219.

YEBOAH, Y. D., Y. XU, and A. SHETH et al. 2003. Catalytic gasification of coal using eutectic salts: identification of eutectics. Carbon. 41, pp.203-214.

YUH, S. Y. and E. E. WOLF. 1983. K

CO

-catalysed steam gasificationof supercritical extracted chars. Fuel. 62, pp.738-741.

YUH, S. J. and E. E. WOLF. 1984. Kinetic and FT-i.r. studies of the sodium-catalysed steam gasification of coal chars. Fuel. 63, pp.1604-1609.

ZARZYCKI, R. and A. CHACUK. 1993. Absorption: Fundamentals and applications.

Michigan: Pergamon Press. pp.638.

ZHANG, Y., S. HARA, S. KAJITANI, and M. ASHIZAWA. 2009. Modeling of catalytic gasification kinetics of coal char and carbon. Fuel (2009), doi:10.1016/j.fuel.2009.06.004.

ZHU, W., W. SONG, and W. LIN. 2008. Catalytic gasification of char from co-pyrolysis of

coal and biomass. Fuel Processing Technology., pp.890-896.

Catalytic steam gasification of large coal particles 106

Appendix A

Appendix A contains all relevant calculations regarding impregnation. This includes the procedure and calibration for ISE measurements, calculations done to determine the catalyst loading, and additional CT scans.

A.1. Procedure and calibration for ISE measurements

Procedure

The potassium ion (K

) concentration of the 0.5 M solution was calculated to be 39 098 ppm, which is outside the concentration range specified for this potassium ion specific electrode.

Therefore, the solution was diluted to ensure accurate measurements. For each concentration measurement, a 1 mL sample was taken from the original impregnation solutions, containing the various particle size coal samples. The 1 mL impregnation solution was diluted to 100 mL using deionised water, after which 2 mL potassium ion strength adjuster (ISA) was added. For every 50 mL solution, 1 mL ISA is required. The ISA is required for accurate ISE measurements, since it provides a constant background ionic strength for samples and standards. The ISA allows accurate measurements of potassium ion (K

) concentrations as low as 0.04 ppm. Glass beakers containing the diluted impregnation solution sample were covered and put in a water bath, set at 23 °C. This was done to ensure that the temperatures for the various diluted samples were uniform during ISE measurements. Once the temperatures of the diluted samples stabilised, the ISE measurements were taken with the potassium electrode. The diluted samples were stored in the water bath and multiple measurements were taken randomly. Therefore, an average catalyst loading was calculated for the ISE method, and the 95 % confidence interval was determined.

Calibration

The potassium electrode was calibrated before each ISE measurement to ensure accurate readings. During calibration of the ISE probe, it was found that the most accurate measurements were taken when the probe setting was adjusted to measure millivolts (mV).

In order to determine the concentration of the K

CO

impregnation solution, a calibration graph had to be constructed. Three calibration solutions were prepared with K

CO

concentrations of 0.4, 0.45 and 0.5 M. It was assumed that the decrease in solution

concentration would be in the range of 0.4 M to 0.5 M. The calibration curve is presented in

Figure A.1.

Catalytic steam gasification of large coal particles 107 Figure A.1: ISE calibration curve

As can be seen from Figure A.1, the calibration curve is constructed by plotting the ISE measurements (mV) against concentration. Numerous calibration measurements were performed, and a 95 % confidence interval was calculated. The 95 % confidence interval is illustrated by the error bars in Figure A.1. A trendline was fitted through the calibration data, in order to obtain a calibration equation. The following calibration equation was obtained, from which the concentration of K

CO

in the impregnation solution was calculated:

(Equation A.1)

A.2. Calculation of catalyst loading from ISE results

The calibration curves presented in Appendix A.1, were used to determine the final potassium concentration of the impregnation solutions. Once the final K

CO

concentration was determined, the difference in initial and final concentrations was determined:

[

]

( ) 52.75 ( ( )) 93.71

ISE measurement mV = × K CO M −

Catalytic steam gasification of large coal particles 108

2 3 2 3 2 3

[ K CO ] _decrease = [ K CO ] _i − [ K CO ] _{f (mol/L)} (Equation A.2)

It is assumed that the amount with which the potassium concentration in the impregnation solution decreases, is equal to the amount of potassium taken up by the coal samples.

Therefore, the potassium ion concentration is calculated from equation A.2:

2 3 2 3

[ K CO ] _decrease = [ K CO ] _adsorbed (mol/L) (Equation A.3)

2 3

[ K ⁺ ] _adsorbed = × 2 ([ K CO ] _adsorbed ) _(mol/L) (Equation A.4)

From the concentration of the adsorbed potassium, [ K

]

, the weight (g) of potassium adsorbed per weight unit (g) of coal can be calculated. Firstly, the [ K

]

is multiplied by the total volume of the final impregnation solution to calculate the total moles of potassium adsorbed:

[ ]

adsorbed adsorbed solution

Total mole K ⁺ = K + × Volume (mol) (Equation A.5)

The weight of potassium can be calculated by multiplying the total mol K

with the molecular weight of potassium:

(g) (Equation A.6)

In order to determine the weight percentage of potassium loaded onto the coal sample, on a coal basis, Mass K

is divided by the total mass of the coal sample:

.%

Mass K

100

Wt K

Mass coal sample

= × _(%) (Equation A.7)

A.3. Calculating K from K 2 O (XRF results)

The results obtained for the XRF analysis reported the potassium content in oxide form, K

O. In order to determine the potassium (K) loading obtained through impregnation, the elemental K had to be calculated from K

O, using the following calculation:

adsorbed adsorbed K

Mass K = Total mole K ⁺ × MW

Catalytic steam gasification of large coal particles 109 (%) (Equation A.8)

XRF analysis of the raw, un-impregnated coal indicated a potassium (K) content of 0.44 wt.%. This value was subtracted from the XRF results of the impregnated coal samples, in order to determine the amount of potassium added during impregnation.

The results presented on an ash basis are the results obtained from XRF analysis, as XRF analysis is an ash analysis. The XRF results (ash basis) were used to calculate the catalyst loading on a coal basis.

A.4. XRF results

Two different impregnated samples of each particle size was ashed and sent for XRF analysis, for repeatability. An average of the results obtained for each set of samples was used to calculate the potassium loading of the large coal particles. The repeatability values obtained from XRF are presented in Table A.1. It should be noted that these values are the original XRF results, and are presented on an ash basis.

Table A.1: XRF results in wt.% K

O

Particle size (µm) Sample A (wt.% K

O) Sample B (wt.% K

O) Average (wt.% K

O)

-212 µm 15.31 17.32 16.31

5 mm 7.78 8.08 7.93

10 mm 7.22 7.29 7.25

20 mm 8.08 6.22 7.15

30 mm 8.56 4.36 6.47

The average results shown in Table A.1 were used to calculate the potassium loading of the coal particles (wt.%, on a coal basis).

A.5. Catalyst loading calculations

The potassium loading was calculated with the wt.% K, calculated from the wt.% K

O results. In order to determine the amount of potassium added during impregnation, the initial

2

2

( .%)

( .%)

K O

K O wt MW

K wt MW

× ×

=

Catalytic steam gasification of large coal particles 110 potassium content of the raw coal was subtracted from the total potassium content, determined from the results in Table A.1.

(wt.% K

O/g ash) (Equation A.9)

The value for the initial potassium oxide (K

O) content of the raw coal is 0.53 wt.%, as presented in Section 4.5.2. From the previously mentioned value, the potassium (K) content is calculated to be 0.44 wt.%.

The potassium content, on a mass basis, can be calculated from K

.

%( )

( ) ( )

100 100

loaded mass

K Ash air dry basis

K = × (g/g coal) (Equation A.10)

The total wt.% potassium added during impregnation, on a coal basis, can be calculated as follows:

loaded mass 100%

K = K × (wt.% K/g (Equation A.11)

A.6. Procedure and data processing for CT scans

Sample Preparation

The coal samples were placed in florist foam blocks to secure the sample position. Separate CT scans were obtained for the different particle sizes, before and after impregnation.

Figure A.2 and Figure A.3 illustrate the positioning of the coal particles for CT scans.

2 loaded 2 total 2 initial

K O = K O − K O

Catalytic steam gasification of large coal particles 111 Figure A.2: Positioning of 5 mm particles for CT scans

Figure A.3: Positioning of 20 mm particle for CT scans

As can be seen from Figure A.2 and Figure A.3, the coal particles are securely positioned in florist foam to ensure accurate scans. The advantage of using florist foam is that the shape of the specific coal particle is indented into the foam block. The indents of the particles are used as guidelines to ensure that the same particle is positioned in the same manner for scans taken before and after impregnation. The process of comparing CT scans obtained before and after impregnation is simplified by maintaining a constant sample position. As observed from the figures above, the sample size of the different particle sizes vary.

Accurate CT scans can be obtained for the 5 mm and 10 mm particle sizes, using 2 to 4 particles per sample. However, in order to ensure the accuracy of the CT scans for the 20 mm and 30 mm particles, only one particle was used per sample. CT scans were obtained for all four particle sizes of the raw coal. After the scans were acquired the raw coal particles were impregnated, as described in section 5.3.3.1. After impregnation, CT scans were obtained for the impregnated coal particles.

Obtaining data for the 3D images involves 4 steps. During the first step, the coal sample is

rotated 360 degrees and high resolution digital radiographs are acquired every 0.5 degrees.

Catalytic steam gasification of large coal particles 112 The second step involves geometric and shading correction. During data correction, spatial and intensity irregularities are removed. Correction algorithms are also used to remove any distortions and to correct for beam hardening. The third step is known as reconstruction, where the individually corrected scans are combined using Cone Beam Back Projection to obtain a CT image. The CT image is made up of millions of pixels known as voxels. The final step is viewing the image results. X-Tek Graphical User Interface (XGUI) is used to acquire, reconstruct and view the images.

Data processing

The CT scans obtained with the HMXST CT system, were processed with VGStudio Max 2.1 software. This software is used for visualisation and processing of voxel data, obtained from CT scans. Although this software is very powerful and comprises of numerous functions, only the volume rendering and volume analyser functions were used in this study.

The volume rendering tool is used to manipulate the volume object in different ways. This is done by using the grey value histogram in the opacity manipulation area. The visibility of a certain object in a CT scan is dependent on the opacity/intensity of the voxels and the grey values of the elements which the object is comprised of. By using the volume rendering tool, the opacity of certain grey value ranges can be varied to enhance or eliminate their appearance. The interface of the volume rendering tool is illustrated in Figure A.4.

Figure A.4: Volume rendering tool interface

Catalytic steam gasification of large coal particles 113 As seen from Figure A.4 , the histogram in the opacity manipulation area has two distinguishable peaks. These peaks indicate that the CT scan has two discernable grey value ranges (densities). The first peak represents the air around the object, in this case the coal particle. The first step in processing the CT scans is to manipulate the opacity of the voxels which are included in the grey value range of air so that they are not visible. This removes the dullness and gives a brighter image. The second peak represents the second largest amount of voxels which fall into a certain grey value range. In this case the second peak symbolises voxels of the carbon element, since the majority of the coal particle is made up of carbon. The fact that only two peaks are visible on the histogram, does not indicate that the CT scan only comprises of air and carbon. This just implies that the amount of voxels depicting the two peaks is significantly more than the voxels falling into other grey value ranges. The first two peaks/intervals are disabled so that only the higher density materials, i.e. the minerals, are visible on the CT scans.

It is possible to see voxels in all grey value ranges, by selecting a different colour for each range. This is done by first dividing the histogram into various sections, known as intervals.

As seen from Figure A.4, the histogram is divided into three intervals. The first interval is

disabled, which means that the opacity of this grey value range is zero, and the air voxels

are not visible. The second interval represents the grey value range in which the carbon

element is obtained; while the third interval represents the grey value range in which all other

elements are obtained, such as minerals. In order to distinguish between the various

elements, a different colour can now be selected for each interval. As seen from Figure A.4,

the second interval is white (displays grey), while the third interval is red. The following CT

image was obtained when using the above-mentioned volume rendering settings:

Catalytic steam gasification of large coal particles 114 Figure A.5: CT scan obtained from volume rendering analysis

As observed from Figure A.5, the carbon in the coal particles has a grey colour, while the minerals in the coal particles are coloured red. The mineral bands in the coal particles are clearly distinguishable from the carbon. This shows that the voxels of carbon and minerals fall into different grey value ranges due to variation in intensity.

The second interval can also be disabled in order to eliminate the carbon from the image so

that only the minerals in the coal particles are visible, as shown in Figure A.6. A grey colour

was selected to enhance the visibility of the minerals.

Catalytic steam gasification of large coal particles 115 Figure A.6: CT scan illustrating minerals in coal particles

As seen from Figure A.6, only the minerals are visible in the CT scan. During volume rendering it is possible that the grey values of different voxels representing carbon and minerals overlap, and that some of the carbon is still included in scans showing only the minerals. The histogram is based on intensities of various elements, which cannot be directly related to the density of the elements. Therefore, in order to find the grey value range for a specific element, such as potassium, the image needs to be calibrated for water (with a known density), as well as the densities of the desired elements.

The volume analyser tool was used to determine if the amount of potassium added to the

coal during impregnation, is substantial enough to increase the mineral volume of the coal

sample. The volume of the coal sample and the minerals in the coal sample are analysed

using the data histogram, as illustrated in Figure A.7.

Catalytic steam gasification of large coal particles 116 Figure A.7: Volume analyser data histogram

The data histogram is used to analyse the properties of the voxel data within a selected range. The range can either be specified by moving the red slider from left to right, or by manually specifying the cursor range. The same grey value ranges used for the volume rendering analysis are used for the volume analysis. Firstly, the volume of the entire coal sample is determined by manually specifying the grey value range. This includes the carbon elements in the coal sample as well as the mineralogical elements. The range between the two red sliders is used to determine the volume, and the volume is given in mm

. Secondly, only the volume of the minerals in the coal sample is determined. The mineral volume is then used to calculate the volume percentage (vol.%) of minerals in the coal sample. This analysis is done for all particle sizes, before and after impregnation. The vol.% of minerals in the raw coal sample is compared to the vol.% of minerals in the impregnated coal sample, to determine if impregnation increases the mineral volume.

A.7. CT scans

A CT scan of four 5 mm raw particles is shown in Figure A.8. Only the minerals present in

the particles are illustrated in the scans.

Catalytic steam gasification of large coal particles 117 Figure A.8: CT scan of 5 mm raw coal particles

As seen from Figure A.8, the mineral distribution throughout the coal particles is irregular.

This clearly confirms the heterogeneity of coal, as well as the difference in composition between two particles of the same size. The CT scan of the impregnated 5 mm coal particles is presented in Figure A.9.

Figure A.9: CT scan of 5 mm impregnated coal particles

When the CT scan of the impregnated 5 mm particles (Figure A.9) is compared to the scan

of the raw coal particles (Figure A.8), a slight increase in minerals is observed as a result of

impregnation.

Catalytic steam gasification of large coal particles 118 The following images are CT scans of the 10 mm particles, before and after impregnation.

Figure A.10: CT scan of 10 mm raw coal particles

Figure A.11: CT scan of 10 mm impregnated coal particles

As can be seen from the CT scans presented in Figure A.10 and Figure A.11, a slight increase in mineral matter is observed for the particle on the left. However, no discernible difference exists between the mineral matter of the raw particle and that of the impregnated particle on the right.

The CT scans for the 10 mm and 20 mm coal particles are shown in this section.

Catalytic steam gasification of large coal particles 119 The subsequent images are slice view images of the 10 mm particles, before and after impregnation. Figure A.12 shows the clipping plane of the slice views for the 10 mm particles.

Figure A.12: Clipping plane of 10 mm particles

The slice views of the 10 mm particles are presented in Figure A.13 and Figure A.14.

Figure A.13: Slice view of raw 10 mm particles

Catalytic steam gasification of large coal particles 120 Figure A.14: Slice view of impregnated 10 mm particles

As seen from the slice views of the 10 mm particles, as shown in Figure A.13 and Figure A.14, an increase in minerals due to impregnation is not apparent. These results are similar to those found for the 5 mm particles presented in Section 5.4.4.2.

The following CT images presented are of the 20 mm coal particles, before and after impregnation.

Figure A.15: CT scan of raw 20 mm coal particle

Catalytic steam gasification of large coal particles 121 Figure A.16: CT scan of impregnated 20 mm coal particle

The CT scans of the 20 mm coal particles, as presented in Figure A.15 and Figure A.16, clearly show that the coal particle is comprised of numerous mineral bands. There is no discernible difference between the raw and impregnated 20 mm particles, as presented above. The clipping plane for the slice views of the 20 mm particles is shown in Figure A.17:

Figure A.17: Clipping plane of 20 mm particles

The slice view of the raw and impregnated coal particles, as shown in Figure A.18 and

Figure A.19, also illustrates the mineral band included in the coal particle.

Catalytic steam gasification of large coal particles 122 Figure A.18: Slice view of raw 20 mm coal particle

Figure A.19: Slice view of impregnated 20 mm coal particle

Catalytic steam gasification of large coal particles 123

A.8. Volume predictions

The volume analyser tool was used to measure the particle and mineral volume in Section 5.4.4.2. The accuracy of the volume analyser was validated by calculating the radius of the particles from the measured volume obtained from the volume analyser. The radius was calculated with the equation used to determine the volume of a sphere, since coal particles were selected to be spherically shaped:

(Equation A.12)

The volume prediction data is presented in Table A.1. The results presented in Table A.1 include the measured volumes obtained from the volume analyser, the radii calculated from the measured volumes, and the actual radii of the particles.

Table A.2: Validation of volume analyser measurements Particle size Measured volume

(mm

)

Calculated radius (mm)

5 mm 98.56 2.87

10 mm 318.47 4.24

20 mm 5129 10.7

30 mm 13462.82 14.76

As seen from the results presented in Table A.2, the radii calculated from the measured volume is a good prediction when compared to the intended radii of the particles. The particles were hand selected and sieved to obtain an average particle radius measurement.

A.9. ISE error predictions

As seen from the results in section 5.4.3.2, the 95 % confidence interval estimates a relatively large error. This can be attributed to the subtraction of two values, the initial and final impregnation solution concentration, each having an error. A summary of the initial and final concentrations, along with the respective errors calculated, is presented in Table A.3:

4 3

( ) 3

sphere

V = × Π × r

Catalytic steam gasification of large coal particles 124 Table A.3: Initial and final impregnation solution concentrations

Particle size Initial concentration (M) Final concentration (M) Difference (M)

5 mm 0.504 ± 0.007 0.44 ± 0.005 0.06 ± 0.01

10 mm 0.504 ± 0.007 0.46 ± 0.006 0.04 ± 0.01

20 mm 0.504 ± 0.007 0.46 ± 0.007 0.04 ± 0.01

30 mm 0.504 ± 0.007 0.49 ± 0.005 0.01 ± 0.01

As seen from the results, the errors for the difference in initial and final concentration is

higher than for the individual values alone. This error increases since two values, with

respective errors, are subtracted from each other.

Catalytic steam gasification of large coal particles 125

Appendix B

B.1. Conversion-time graphs

Figure B.1: Conversion-time graphs for 5 mm particles

Catalytic steam gasification of large coal particles 126

Figure B.2: Conversion-time graphs for 10 mm particles

Catalytic steam gasification of large coal particles 127

B.2. Experimental values for IM and VM and ash content

Table B.1: Experimental values for inherent moisture and volatile matter and ash contents Temperature (°C) Raw

(IM and VM)*

Catalysed (IM and VM)*

Raw (Ash)

Catalysed (Ash) 5 mm particles

800 0.29 0.25 0.16 0.17

0.29 0.29 0.14 0.09

825 0.30 0.27 0.14 0.14

0.32 0.27 0.13 0.13

850 0.30 0.27 0.14 0.13

0.30 0.25 0.15 0.17

875 0.29 0.30 0.14 0.12

0.29 0.26 0.13 0.15

10 mm particles

800 0.29 0.25 0.12 0.17

0.33 0.26 0.12 0.20

825 0.32 0.23 0.14 0.17

0.26 0.29 0.15 0.11

850 0.26 0.26 0.15 0.15

0.32 0.26 0.08 0.15

875 0.31 0.27 0.13 0.12

0.26 0.28 0.14 0.17

* (IM – Inherent moisture, VM – Volatile matter)

B.3. Experimental errors for average conversion runs

The errors for the average conversion plots were calculated using the following equation:

(Equation B.1)

( )

( )

=

−

 − 

 

 

=

∑

i 1 1,i 2,i

X X

X X

error % 2

N

Catalytic steam gasification of large coal particles 128 Where X

and X

are the i

carbon conversion of run 1 and run 2, and N is the amount of data point used for error determination.

Table B.2: Error % for average conversion runs

Sample 800 °C 825 °C 850 °C 875 °C

5 mm Raw 4 1 1 1

5 mm Cat 4 4 3 3

10 mm Raw 2 3 4 6

10 mm Cat 6 4 1 2

B.4. Particle size influence for 825 °C, 850 °C and 875 °C

Figure B.3: Influence of particle size on reactivity at 825 °C

Catalytic steam gasification of large coal particles 129 Figure B.4: Influence of particle size on reactivity at 850 °C

Figure B.5: Influence of particle size on reactivity at 875 °C

Catalytic steam gasification of large coal particles 130

B.5. Determination of reactivity, k

The slope from the –ln(1-X) vs. t plots were determined by fitting a linear line through the data, as presented in Figure B.7 and Figure B.7.

Figure B.6: Determination of reactivity, k, from slope (5 mm)

Catalytic steam gasification of large coal particles 131 Figure B.7: Determination of reactivity, k, from slope (10 mm)

B.6. Temperature profile during gasification

The following graph illustrates the temperature measurements acquired during a gasification

experiment conducted at 875 °C.

Catalytic steam gasification of large coal particles 132 Figure B.8: Temperature profile during gasification experiment at 875 °C

As can be seen from Figure B.8, the temperature does not vary considerably during

gasification. A 95 % confidence interval of ± 1 °C was calculated from the experimental

logged data.