QUANTITATIVE XRD ANALYSIS OF DIFFERENT COAL … · Quantitative XRD analysis appears to be better...

QUANTITATIVE XRD ANALYSIS OF DIFFERENT COAL SAMPLES

TO UNDERSTAND THEIR ASHING PROCESS DURING THE INITIAL

STAGES OF COMBUSTION.

N. Tshiongo-Makgwe1

University of Johannesburg, Faculty of Engineering and the Built Environment, School of

Mining, Metallurgy and Chemical Engineering, Department of Metallurgy, South Africa,

POBOX 526, Wits 2050, South Africa

ABSTRACT

Coals in general have very complex chemistry and depending on the intended

usage, further challenges are experienced. Such challenges vary from the methods

mining and cleaning that should be employed to yield the best quality coal for the

specified usage and the final products which are produced from coal use. It is

therefore very important to understand the chemistry of raw coal and the ashing

properties of such coals and the information from this work can also assist in

highlighting the environmental and health aspects that are linked to processing and

usage of coal. This work focused on a qualitative studies using XRD to analyze five

different coal samples and the chemistry change during the ashing process which

takes place during the coal combustion process. An ashing process was conducted

on the five samples at 350⁰C, 500⁰C, 750⁰C and 1000⁰C and their results where

compared to the composition of the raw coal. The results from this work showed that

there are up to 40 major phase-mineral compositions found in the samples analyzed,

of which some of them are one phase-mineral composition that appears as different

phases/forms in the coal ash.

Key words: ash composition, XRD analysis, combustion, phase-mineral

1. INTRODUCTION Coal in South Africa is dominantly utilised in electricity generation followed by the

metallurgical industry and a smaller percentage in synthetic fuel. For these different

intended usages, there are requirements from the user that coal producers should

know and ensure that their beneficiation processes for effectively production of coals

that meets those specifications. Before designing a coal processing and

beneficiation method or the proper usage, it is very important to determine the

1 Corresponding author. Tel.: +27(0) 11 559 6168 cell +27(0) 84 714 2162 E-mail address:

[email protected]

mailto:[email protected]

different minerals contained in the coal body and their effects on the utilization

processes. In so doing, coal producers should also be mindful of environmental

pollution that are associated with the production and usage of coal due to its mineral

composition. Such environmental pollutions include the solid and effluents wastes

generated in the beneficiation process, the waste produced in coal usage such as

the fly ash, bottom ash and the off gases that are released (Winburn et al. 2000). All

this requires a good understanding of the formation, mineral compositions and

properties of coal associated with both beneficiation and utilization process. Previous

research have been done on the compositions and properties of solid combustion

waste from various coal fired power stations around the world (Vassilev et al. 2005),

and emphasis was made on the importance of knowledge relating to the origin,

utilization, environmental, technological problems and chemical and phase-mineral

compositions associated with coal.

There are different analytical methods used to determine the mineral composition of

coal and numerous authors who have studied these. Of these methods, powder X-

ray diffraction (XRD) has been the most utilised (van Alphen 2007)(Winburn et al.

2000)(Saikia, Boruah, and Gogoi 2007)(Mahadevan 1940) (Ritz and Klika 2010).

Scanning electron microscopy with XRF combination (SEM-XRD) is another method

used widely (Ritz and Klika 2010) for determination of elemental individual minerals.

There are also other sophisticated methods like Computer-controlled SEM

(CC_SEM)as stated in (van Alphen 2007) (Ritz and Klika 2010)and automatic image

analysis SEM (AIA_SEM) (Ritz and Klika 2010) and simple methods based on the

recalculation of chemical analysis to normative minerals.

There are various limitations that are linked to the methods listed above, and such

include XRD which has low detection limit for minerals with low ash content (Ritz and

Klika 2010). Quantitative XRD analysis appears to be better suited for characterising

complex materials (Winburn et al. 2000) like coal and coal ash which has many

phase compositions.

Analysis of the waste products from coal combustions are just as important to study

as they are similarly complex in composition depending on the coal. The ash is

formed from Inorganic matter in coal which includes (Ritz and Klika 2010) (Vassilev

et al. 2005)(Zhang, Han, and Xu 2003) compounds dissolved in pore solutions in

coal, chemical elements bonded to organic parts, and crystalline or amorphous form

of minerals. Determination of qualitative and quantitative determination of organic

minerals is impacted greatly by crystalline minerals (Ritz and Klika 2010).

Inorganic elements in coal is comprised of three group; major elements group (more

than 1000 ppm concentration), minor elements group (between 100 and 1000 ppm

concentration) and trace elements group (less than 100ppm concentration) (Zhang,

Han, and Xu 2003). Of the latter elements groups; elements like Al, Si, Fe, Ca, Mg,

Na, K, Cl and S are the typical major elements, elements such as Ba, P and Ti are

the most common minor elements found in coal,(Zhang, Han, and Xu 2003) and

there is a wide range of trace elements in coal and ash waste. The major elements

are highly responsible for slag formation and environmental pollutions associated

with coal combustion. Minor and trace elements do not have proven impact on the

environment.

The current paper is aiming at studying ash products of coal samples at different

temperatures to assess the improvement that can be made in the detection limit of

the studied samples. Objectives of this work is to characterise and understand

phase-mineral and chemical compositions of the different South African coal, and to

predict potential possible environmental concerns related to ash formed from this

coals.

2. EXPERIMENTAL

2.1 Sample preparation

Five different coal samples from around South Africa where used for this study. The

samples where ground to powder form. The samples where split using a rotary

splitter and packaged, and samples of approximately 1g were taken for the different

tests performed. Moisture content, volatile matter, ash composition and calorific

value studied by proximate analysis.

2.2 Combustion process A temperature controlled furnace was used to ash the samples at the different

temperatures. Preheated furnace to a stabilised required temperature. A crucible

containing the samples was placed in the furnace for 90 minutes. The samples were

left to cool down in ambient temperature overnight. The samples were analysed

using XRD in the in-house laboratory.

3. RESULTS AND DISCUSSIONS

Organic and inorganic matter in coal can be identified easily because of the colour

difference even with the naked eye. Figure 1 depletes difference in reflection and

dullness in colour, at the same time indicating complexity in composition of a single

coal particle.

Figure 1: Coal Samples with mineral matter and different maceral compositions.

3.1 Characterisation of coal samples

Proximate analysis conducted on the initial coal samples for percentage moisture,

volatile matter (VM), ash composition and calorific value (CV). This was conducted

five times on the same conditions and the average results are tabulated below.

Table 1: Proximate analysis results for the five samples considered.

Sample %Moisture % Vol Matter

% Ash Fixed carbon

CV

1 6.15 22.32 20.70 50.82 23.63

2 2.86 17.15 38.75 41.24 18.23

3 7.35 22.39 24.60 45.66 20.91

4 4.29 23.04 31.29 41.37 20.30

5 7.35 22.12 24.36 46.17 20.59

From the proximate analysis all of the samples had displayed similar results except

for sample 2 with less than 3% moisture content while the rest are between 4% and

8%, they also have between 22%-23% volatile matter while sample 2 has only 17%

volatile matter. In terms of percentage ash, sample two is still the odd one out at

above 38% while the rest are between 20-31%. The same behaviour is displayed on

CV and the only composite of coal that displayed a different behaviour even though

still the lowest fixed carbon was closer to sample 4.

Proximate analyses provide basic coal properties and characteristics, but XRD

analyses provides in-depth minerals and phase mineral composition. From

proximate analyses percentage ash content is obtained, but the compositional

minerals in the ash can require more advance analyses.

3.2 Evaluations of minerals in the samples by XRD

XRD analysis yielded 40 inclusive phase-minerals which were observed in the

samples. During the combustion process the first stage is characterised by

decomposition of coal organic components followed by the release when ignited as

per the combustion process. This work was conducted to understand the

volatilization or release of major elements at different temperature and their modes

of occurrence, concentration, physical changes and chemical reactions that takes

place during the combustion process. This was assessed by the amount of phase

minerals detected on the raw coal as compared to what is detected on various ash

compositions analysed by XRF.

Quantitative analysis of different mineral phases using XRD as an effective tool has

been proven by not giving individual diffraction patterns for the individual minerals

content (Ward, Nunt-Jaruwong, and Swanson 2005). This was confirmed that it can

be done for South African coals, and results are tabled in Table 2-6 and diffraction

patters for phase mineral compositions listed on Figure 2-6 for the five samples that

were considered for this study.

Table 2: Mineralogy of quantitate phase minerals percentage in weight assessed using XRD at different temperatures for sample 1.

Mineral phase names Chemical formula

Raw 350°c 500°c 750°c 1000°c

Aluminium Iron ( Al0.25 Fe0.75 ) 2.77

Anatase Ti O2 2.974 0.738 1.739 3.161

Aragonite Ca ( C O3 ) 3.434 3.245

Calcium carbonate Ca C O3 3.092 1.567 1.633

Calcium Iron Oxide Ca Fe O2 2.634

Calcium Manganese CaMn4 0.898

Calcium Silicon Oxide Ca Si O3 2.993

Calcium Sulphate Ca S O4 1.127

Carbon C 2.37

Dolomite Ca Mg ( C O3 )2 2.794

gypsum Ca S O4 ·2 H2 O 3.145 3.01 3.135

Gypsum, syn Ca ( S O4 ) ( D2 O )2 3.137

Hematite HP,Iron(3) Fe2 O3 1.53 3.053 2.702

Iron diiron(3) oxide Fe3 O4 2.694

Iron sulfate FeSO4 3.279

Iron sulfide Fe S 1.671 3.139

Kaolinite Al4 ( O H )8 ( Si4 O10 ) 1.015 1.72 1.791 3.227

Lime, syn Ca O 3.101 2.986

Magnesiorferrite, syn Fe2 Mg O4 2.856

Magnesite, HP Mg ( C O3 ) 3.156

Magnetite High Hp Mg ( C O3 ) 2.683

Monticellite, HP Ca Mg Si O4 3.32

plumbojarosite Pb Fe6 ( S O4 )4 ( O 3.183 3.457 3.196 2.924 3.177

Pyrite Fe S2 3.245 1.711

Quartz, low ,alpha -Si Si O2 0.557

Quartz, syn Si O2 0.418 0.85 0.743

Quartz,low HP, syn Si O2 0.373

Titanium Oxide Ti O 1.42

% of minerals at different temparatures

Sample 1

The abundance of minerals in sample 1 is shown in Table 1, where it can be seen

that many phase minerals that are detected in different phase using XRD as a tool

can be analysed for on ash samples that have been produced from different

temperatures. Some phase minerals like Aluminium Iron, Calcium Iron Oxide,

Calcium Sulphate, Dolomite, Iron diiron oxide, synthetic Magnesioferrite and Quartz

containing low alpha silicon are observed only on ashes formed at higher

temperatures, whereas some phase minerals like Iron sulfate, Magnesite (in all

phases) and some low HP Quarts can be formed at lower temperatures. Calcium

manganese is the only phase mineral that is observed on the raw coal sample only,

whereas other phase minerals are observed in raw and in the ash samples at

different temperatures.

Figure 2: Chemical profiles as analysed using XRD indicating phase mineral peaks

at different temperatures for sample 1

XRD peaks of the different phase minerals indicate the results on Table 2, and it can

be seen that most peaks and frequencies observed in the raw and temperatures less

than 500°C have decreased on higher temperatures as indicated by less peaks on

the XRD patterns.

Table 3: Mineralogy of quantitate phase minerals percentage in weight assessed

using XRD at different temperatures for sample 2.

Figure 3: Chemical profiles as analysed using XRD indicating phase mineral

peaks at different temperatures for sample 2

Sample 2 was not different as compared to sample 1, as there are more phase

minerals observed on the low temperature ashes than the higher temperatures. Even

though there was the difference in the initial coal quality for the different samples,

sample 2 behaved the same way in terms of phase minerals observed by XRD.


Raw 350°c 500°c 750°c 1000°c

Anatase, syn Ti O2 2.985 3.006

Anhydrite Ca ( S O4 ) 3.138

Chlorite-llb-4

( Mg11.06 Fe0.94 ) ( ( Si5.22

Al2.78 ) O20 ( O H )16 ) 1.384

Chlorite-serpentine

( Mg , Al )6 ( Si , Al )4 O10 ( O

H )8 1.148

Graphite-2H,syn C 1.607

gypsum Ca S O4 ·2 H2 O 2.815 3.231 3.178 3.24 1.998


kaolite-1A Al2 Si2 O5 ( O H )4 1.031 1.221

Kaolite-1Ad Al4 ( O H )8 ( Si4 O10 ) 1.878

Lime, syn Ca O 1.765 2.124 1.506

muscovite K Mg Al Si4 O10 ( O H )2 1.782


Quartz, alpha low Si O2 0.744 0.454

Quartz, syn Si O2 1.048

Quartz,low HP, syn Si O2 0.39 0.489

Rutile,syn Ti O2 3.057 3.299

Vaterite Ca C O3 3.039

Vaterite, syn Ca C O3 3.004 3.014 1.491

Sample 2


Table 4: Mineralogy of quantitate phase minerals percentage in weight

assessed using XRD at different temperatures for sample 3.

Analyses of sample 3 revealed the same results as the other samples. In addition,

these results indicated that sample 3 only had one phase of Quartz present and this

was detected at all temperatures at a very low rate of less than 1% in weight. This is

also corroborated by the XRD patterns in Figure 4 where the peak for this phase

mineral was observed to be lower than that observed from sample 1 and 2 where

three phases of quarts where observed.


Raw 350°c 500°c 750°c 1000°c

Alstonite Ba Ca ( C O3 )2 1.565

Anatase Ti O2 2.716

Anatase, syn Ti O2 3.018 1.972 1.471 1.427

Anhydrite Ca ( S O4 ) 1.882 2.95 1.46 3.065

Corundum (Cr-doped)Al2 O3 1.929

gypsum Ca S O4 ·2 H2 O 2.843 3.125 1.683 3.22 2.96

Hematite HP,Iron(3) Fe2 O3 1.835 3.385

Hexagonal anhydrite CaSO4 3.094

Kaolinite Al4 ( O H )8 ( Si4 O10 ) 0.871

kaolite-1A Al2 Si2 O5 ( O H )4 1.031

Lime, syn Ca O 2.999 3.416

Magnesite, HP Mg ( C O3 ) 1.703 3.111 1.893

Magnetite High Hp Mg ( C O3 ) 2.962 2.282

Mikasaite, syn Fe2 ( S O4 )3 1.766 3.061

Moissanite- 8H,syn Si C 1.27

Mullite Al2 ( Al2.5 Si1.5 ) O9.75 1.861 3.069


Pyrite Fe S2 2.937 3.092 1.883 2.85 3.433

Pyrite, cuprian, syn ( Cu0.4 Fe0.6 ) S2

Pyrrhotite high,syn Fe S 3.023 2.009

Quartz,low HP, syn Si O2 0.927 0.518 0.765 0.681 0.714

Vaterite Ca C O3 2.764 2.928

Vaterite, syn Ca C O3 3.096 1.866

Wustite,syn Feo3 1.681

Sample 3



peaks at different temperatures for sample 3.

Although sample 4 had phase minerals like quartz in two different forms, the amount

of other minerals were higher with Gypsum higher than 3% for sample 4. It was also

indicated by the XRD patterns that were observed. When phase mineral weight

compositions are higher, this also affects the peaks of the XRD patterns.




Raw 350°c 500°c 750°c 1000°c

Anatase Ti O2 1 522


Anhydrite Ca ( S O4 ) 1.442

Aragonite Ca ( C O3 ) 1.95 2.141

Corundum (Cr-doped)Al2 O3 1.878 1.783

Dolomite Ca Mg ( C O3 )2 3.23 1.442 3.135 1.734

gypsum Ca S O4 ·2 H2 O 3.076 2.854 1.657 2.875 3.127


kaolite-1A Al2 Si2 O5 ( O H )4 0.796 1.236

Lime, syn Ca O 3.045 1.62 1.79 1.258

Magnesite, HP Mg ( C O3 ) 2.931 1.247

Magnetite High Hp Mg ( C O3 ) 1.902

Mikasaite, syn Fe2 ( S O4 )3 1.566

Mullite Al2 ( Al2.5 Si1.5 ) O9.75 1.733 2.939 3.35


Pyrite Fe S2 1.915 3.455 3.402

Quartz, syn Si O2 0.987

Quartz,low HP, syn Si O2 0.716 0.524 0.678 0.906

Rutile,syn Ti O2 2.014

Vaterite Ca C O3 2.937

Vaterite, syn Ca C O3 1.642 3.212

Sample 4


For sample 4, most phase minerals were observed on the ash formed in higher

temperatures as shown in Table 5. Phase minerals like Magnesite and Magnetite are

observed only at the raw coal sample and ashes formed at the highest temperature.

Small but measurable amounts of quarts have been observed in two phases, but

generally higher amounts observed on other phase minerals.



Results from this work indicates that profiling the sample using XRD is required

because all even though the samples can be classified in the same group, when it

comes the quantitative minerals contained and how they are formed in different

temperatures different results are obtained. Better understanding of the sample

would be made with such profiling.



Sample 5 has high mineral concentrations in weight percentages on the raw and

lower temperatures and lower on the high temperature ashes. The XRD patters

showed the same with lower peaks at the higher temperatures and high peaks at

lower temperatures.




Raw 350°c 500°c 750°c 1000°c


Anhydrite Ca ( S O4 ) 2.031 1.043 1.365

Aragonite Ca ( C O3 ) 3.209 0.922

Dolomite Ca Mg ( C O3 )2 3.207 1.106

Graphite-2H,syn C 0.556

gypsum Ca S O4 ·2 H2 O 1.79 1.784 2.977 1.092


Kaolinite Al4 ( O H )8 ( Si4 O10 ) 0.651

kaolite-1A Al2 Si2 O5 ( O H )4 1.068

Lime, syn Ca O 2.988 1.721 1.289

Magnesite, HP Mg ( C O3 ) 1.648 0.781

Mikasaite Fe2 ( S O4 )3 1.818 1.362

Mullite Al2 ( Al2.5 Si1.5 ) O9.75 2.903 1.463 1.505 2.846 0.82

muscovite K Mg Al Si4 O10 ( O H )2


Pyrite Fe S2 2.85 1.604 2.85 2.85 0.921

Quartz, syn Si O2 1.546 0.757

Quartz,low HP, syn Si O2 1.088 1.337

Rutile,syn Ti O2 2.887 0.862

Vaterite, syn Ca C O3 1.429

Sample 5


Coal properties depends mainly on its physical and chemical structure and variety in

the organic and inorganic matter (Saikia, Boruah, and Gogoi 2007) (Saikia and

Boruah 2008), and the properties of South African coals are no different as the

phase minerals contained would determine the properties. Properties of coal will

determine the usage of coal, the changes and phase transformation during

combustion process. The current work indicates that to be able to determine the

later, qualitative and quantitative analysis and profiling of coal is required. The

mineral matter in coal affects coal usages (Tshiongo and Mulaba-bafubiandi 2013).

XRD is a powerful profiling tool in coal characterisation and acceptance for the

intended usage.

From the overall results, aluminium appears as part of two-phase minerals

(Aluminium Iron and Kaolinite). From the two phase minerals there are no traces of

Aluminium Iron detected on raw coal and those combusted at lower temperatures

from (350-750○C), the only observation of this mineral is at 1000○C. When compared

to Kaolinite, smaller content where detected at lower temperatures and a higher

amount at 1000○C.

Silicon appeared in four phase minerals, where Calcium Silicon oxide is at higher

temperature ashes and Monticellite, HP at lower temperature but the other

occurrence being Kaolinite and Quarts in all temperatures. Calcium Silicon oxide is

formed from reactions of coal compositions and this reaction of formation from Lime

and Quartz according to the following reaction (Wang et al. 2016);

CaO + SiO2 = CaSiO3

Figure 7 indicates the relationship between Monticellite and Calcium Silicon oxide,

where they exist together but one decreases in weight percentage as the other

increases. This explains the results were either one is found at lower temperatures

and the other at high temperature respectively.

Figure 7: Phase equilibrium diagram of the join W–D of the ternary system

SiO2–CaO–MgO (Diba et al. 2014)

From the phase equilibrium above, it is evident that most of the SiO2-CaO-MgO

phase minerals exist together and this is proven by the obtained results. Their

compositions even at the low ash temperatures investigated in this work is always

vice-versa for example where compositions of Monticellite is higher there is lower

composition in %weight of Calcium Silicon Oxide . This behaviour is noted by the

results were either one will be observed at any temperature.

4. SUMMARY

From the proximate analysis, sample 2 is the least desirable as it has higher ash

content, low volatiles and lower CV as compared to the other samples. Chemical

composition of coal varies greatly even if the proximate analyses classify it as the

same type or rank. This is despite the region of the samples, and the current work

proved this. The variety in composition in coal and its heterogeneity is due to the

phase mineral compositions.

Coal quality has no impact on the detection limit of the raw and ash samples, this is

shown by the similarities of the peaks and the detection of different mineral phases

as indicated by the results. It is therefore recommended that before utilising any coal,

it is better to profile the samples to know the phase minerals that are formed at the

temperature that coal will be subjected to. An amount of individual elements presents

can be estimated using these results using the recalculation methods. Reactions that

are taking place during the combustion process, together with the products formed

from the different reactions can be identified.

This would give information that is useful to identify the mineral formed at these

temperatures and ways to deal with the hazards they pose to humans and the

environment when such coal are combusted. The total summative 40 phase minerals

observed from the samples considered therefore confirm coal being a

heterogeneous matter. It is therefore not adequate to characterise coal on just the

proximate and ultimate analysis without deeper understanding of the coal in

question. XRD is therefore an adequate and reliable way of quantitative analyses of

minerals present in the coal and ash samples and this analytical technique can be

used to profile the different coal sample and their ashes formed.

5. ACKNOWLEDGEMENTS

I would like to thank Bongumusa Mhlanga for his diligent work in the laboratory. I will

also like to express gratitude to the University of Johannesburg, department of

Metallurgy and the support staff who assisted in the work.

6. REFERENCES

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Diba, Mani, Ourania-Menti Goudouri, Felipe Tapia, and Aldo R. Boccaccini. 2014. “Magnesium-Containing Bioactive Polycrystalline Silicate-Based Ceramics and Glass-Ceramics for Biomedical Applications.” Current Opinion in Solid State and Materials Science 18 (3): 147–67. doi:10.1016/j.cossms.2014.02.004.

Mahadevan, C. 1940. “Studies in Coal by X-Ray Diffraction Methods.”

Ritz, M., and Z. Klika. 2010. “Determination of Minerals in Coal by Methods Based on the Recalculation of the Bulk Chemical Analysis.” Acta Geodyn.Geomater 7 (4): 453–60.

Saikia, Binoy K, and Rajani K Boruah. 2008. “Structural Studies of Some Indian Coals by Using X-Ray Diffraction Techniques” 16: 89–94.

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Tshiongo, N, and A Mulaba-bafubiandi. 2013. “South African Coal and Its Abrasiveness Index determination:An Account of Challanges.” In Southern African Universities Power Engineering Conference (SAUPEC).

Vassilev, Stanislav V., Christina G. Vassileva, Ali I. Karayigit, Yilmaz Bulut, Andres Alastuey, and Xavier Querol. 2005. “Phase–mineral and Chemical Composition of Composite Samples from Feed Coals, Bottom Ashes and Fly Ashes at the Soma Power Station, Turkey.” International Journal of Coal Geology 61 (1): 35–63.

doi:10.1016/j.coal.2004.06.004.

Wang, Fanmao, Jijun Wu, Wenhui Ma, Min Xu, Yun Lei, and Bin Yang. 2016. “Removal of Impurities from Metallurgical Grade Silicon by Addition of ZnO to Calcium Silicate Slag.” Separation and Purification Technology 170: 248–55. doi:10.1016/j.seppur.2016.06.060.

Ward, Colin R., Sorawit Nunt-Jaruwong, and Jeni Swanson. 2005. “Use of Mineralogical Analysis in Geotechnical Assessment of Rock Strata for Coal Mining.” International Journal of Coal Geology 64 (1–2): 156–71. doi:10.1016/j.coal.2005.03.014.

Winburn, Ryan S, Stephanie L Lerach, Bryan R Jarabek, Marissa A Wisdom, Dean G Grier, and Gregory J Mccarthy. 2000. “Quantitative XRD Analysis of Coal Combustion by-Products by the Rietveld Method. Testing with Standard Mixture.” International Centre for Diffraction Data ( ICDD), Advances in X-Ray Analysis. 42 (C): 387–96.

Zhang, Jun, Chun Li Han, and Yi Qian Xu. 2003. “The Release of the Hazardous Elements from Coal in the Initial Stage of Combustion Process.” Fuel Processing Technology 84 (1–3): 121–33. doi:10.1016/S0378-3820(03)00049-3.

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