Applied Energy 87 (2010) 1710–1715

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Applied Energy

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Catalytic effect of black liquor on the gasification reactivity of petroleum coke

Xiuli Zhan, ZhiJie Zhou *, Fuchen WangKay Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e i n f o

Article history:Received 23 July 2009Received in revised form 27 October 2009Accepted 28 October 2009Available online 25 November 2009

Keywords:Petroleum cokeBlack liquorGasificationAlkali metals

0306-2619/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.apenergy.2009.10.027

* Corresponding author. Tel.: +86 021 64252974; faE-mail address: zzj@ecust.edu.cn (Z. Zhou).

a b s t r a c t

CO2 gasification of petroleum coke using black liquor as a catalyst was performed in a thermogravimetricanalyzer (TGA) under temperatures 1223–1673 K at ambient pressure to evaluate the effect of blackliquor loading on petroleum coke gasification. It was found that the gasification reactivity of petroleumcoke was improved greatly by black liquor. The gasification reactivity was affected by different loadingmethods in the order: wet grinding > dry grinding > physical impregnation > dry mix. The catalytic activ-ity of black liquor was higher than that of pure alkali metal. The effect of temperature on the gasificationreactivity of petroleum coke was changed by black liquor. The reactivity reaches its maximum at 1573 K.The reactivity of petroleum coke was found higher than that of Shenfu coal when black liquor loading is5 wt.% (of petroleum coke), clearly demonstrating that black liquor could be an effective catalyst forpetroleum coke gasification.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Gasification technology can convert a wide variety of resourcessuch as coal, biomass and petroleum coke into useful syngas(CO + H2) with near-zero pollution emissions [1–3]. In recent years,preliminary efforts [4–6] have been put into petroleum coke gasi-fication. The low reactivity of petroleum coke, derived from its car-bon structure, low combustible volatile and low ash content, is thekey problem in the application of petroleum coke as the feedstockof entrained bed gasifier. In consequence, it is critical to improvethe gasification of petroleum coke.

The experience and theory of coal gasification could be used asreferences to study petroleum coke gasification. Alkali and alkalineearth metallic (AAEM) species containing abundant sodium actingas effective catalysts for coal combustion and gasification [7–12] atappropriate range of temperatures would be effective catalysts forpetroleum coke.

However, it is expensive and economically unfeasible in gasifi-cation to use pure AAEM species as catalysts. The recovery andtreatment are still difficult after gasification, so it is urgent tosearch a cheaper and more effective catalyst, such as black liquorto replace the pure alkali species. Black liquor, which is a majorwaste from the pulp and paper making industry, contains pulpingchemicals (most alkaline salts) and organic compounds. The organ-ic compounds that have been extracted out from the wood afterthe pulping process with a high fuel value are mainly composedby lignin along with some cellulose and hemicellulose fibers.

ll rights reserved.

x: +86 021 64251312.

Hence, it is imperative to recover both chemicals and energy fromthe black liquor [11]. Recently, the effect of black liquor on coalgasification has been investigated [11,13]. The results showed thatthe sodium compounds in black liquor had a catalytic effect on coalgasification. There are also some reports [14–17] about black liquorgasification as an attractive powerhouse recovery cycle technology.But there is no reported research on the catalytic effect of black li-quor on petroleum coke gasification.

The aim of the research is to investigate the feasibility of mak-ing black liquor as a catalyst for petroleum coke gasification at dif-ferent temperatures, loadings and loading methods.

2. Experimental

2.1. Samples preparation

The petroleum coke sample came from Qi Lu Refinery Plant. BL(black liquor) is obtained from a local pulp mill. Shenfu coal fromthe largest coalfield of China (Shanxi Province) is chosen to com-pare petroleum coke gasification reactivity, because it was usedas raw material in most Chinese coal–water slurry gasificationindustry.

The solid additive is derived by drying BL for 12 h at 378 K.Tables 1 and 2 show the analysis of petroleum coke and BL. Table1 gives the results that petroleum coke is high in calorific value andfixed carbon, low in ash, volatile and moisture when comparedwith Shenfu coal and black liquor. Table 2 shows the ash analysisof black liquor on dried basis. There are plenty of sodium and mag-nesium in the ash. Sodium and magnesium are effective catalystsfor carbonaceous materials.

Table 1Proximate analysis and ultimate analysis.

Sample Proximate analysis (wt.%) Ultimate analysis (wt.%) HHV (MJ kg�1)

Mad Ad Vd FCd Cd Hd Nd Sd Od

Coke 0.64 0.30 9.57 90.13 89.70 3.75 0.95 1.84 3.46 36.20Shenfu coal 7.17 6.82 37.51 55.67 69.23 4.72 0.86 0.49 17.88 28.36Solid BL 79.23 39.46 41.15 19.39 38.95 3.38 0.50 9.40 8.31 16.63

Subscript statements: ad refers to air dried basis; d refers to dried basis.Abbreviations statements: M refers to moisture; A refers to ash; V refers to volatile matter; FC refers to fixed carbon; HHV refers to high heating value.

Table 2Sequential X-ray fluorescence spectrometer analysis of BL ash.

Component

SO3 Na2O MgO K2O Cl CaO SiO2 Fe2O3 P2O5 Cr2O3 CuO

Content (wt.%) 47.74 32.88 13.77 2.05 1.92 0.63 0.32 0.31 0.27 0.06 0.05

X. Zhan et al. / Applied Energy 87 (2010) 1710–1715 1711

The methods of BL loading are including dry mix, physicalimpregnation, dry grinding and wet grinding. Firstly, petroleumcoke was dried for 1 h at 378 K. The loading of BL is ranging from1 wt.% to 20 wt.% of petroleum coke on dried basis. The loadingmethods are briefly introduced as below:

(1) Dry mix: The dried BL and dried petroleum coke were simplyground by mortar and pestle and then sieved to 74–165 lm.

(2) Physical impregnation: Petroleum coke (74–165 lm) wasimpregnated in the deionized water (in which BL has beendissolved) for 24 h first and then dried at 378 K for 1 h.

(3) Dry grinding: Grinding process was carried out in a plane-tary ball-mill (XQM, Nanjing Kexi Instrument Institute)with stainless steel container (1070 ml) and balls. Fortybig balls (1.93 mm diameter) and 200 small balls(0.93 mm diameter) were used together in grinding pro-cess. All tests were run at 280 rpm. Petroleum coke anddried BL were ground together for 0.5 h to generate gasifi-cation samples (74–165 lm).

(4) Wet grinding: In wet grinding, petroleum coke and the deion-ized water which dissolved additive were ground togetherfor 0.5 h just like the method of dry grinding. After grinding,the products were dried at 378 K for 1 h to generate gasifica-tion samples (74–165 lm).

For physical impregnation and wet grinding, petroleum coke ac-counted for 65 wt.% of the slurry. The samples are all sieved to 74–165 lm before gasification.

2.2. Gasification procedure

Evaluation of gasification reactivity was performed in a TA-SDT-Q 600 thermogravimetric analyzer by the isothermalmethod.

The gasification procedure is described as follows: a weighedprepared sample (around 8 mg) was heated under N2 stream upto the desired temperature at 50 K min�1. Obviously, the pyrolysisof BL and petroleum coke will take place during temperatureincreasing. In order to separate petroleum coke gasification fromthe pyrolysis, N2 was replaced by CO2 at the flow rate of100 ml min�1 after the desired temperature was reached and noevident weight loss was observed. Only the gasification processwas calculated and analyzed in this paper.

The system recorded automatically the weight loss by a highlysensitive analytical balance located in the casing of apparatus. Thepurity of N2 and CO2 is 99.99 wt.% and 99.8 wt.%, respectively.

2.3. Calculation of conversion

BL is mainly composed of gasifiable hydrocarbon, which couldbe gasified together during gasification. The weight loss of the BLwas synchronous as that of petroleum coke [13]. Then, the conver-sion for pure petroleum gasification is defined as below:

x ¼ Dm�m0 � ð1� aÞ � bm0 � a� FCd

ð1Þ

where Dm is the weight change of samples during gasification timeinterval; m0, the initial sample weight when the gasification start-ing; a, the petroleum coke weight/[petroleum coke weight + BL (so-lid weight)]; b, the weight change percent of pure BL duringgasification time interval, wt.%; and FCd is the fixed carbon in thesample, wt.%.

2.4. X-ray diffraction analysis

The crystal structure of the sample was investigated by a RigakuX-ray diffractometer with Cu Ka radiation.

2.5. BET surface area and pore volume analysis

The BET surface area and pore volume analysis were investi-gated by an ASAP 2020 Physical adsorption instrument.

3. Results and discussion

3.1. The effect of BL loading method on petroleum coke gasification

Fig. 1 shows the gasification reactivity of petroleum coke load-ing 5 wt.% BL in CO2 at 1273 K by different loading method. Theloading method affects the gasification reactivity in the order:wet grinding > dry grinding > physical impregnation > dry mix.

Wet grinding and dry grinding are better than the other twomixed method owing to the mechanical grinding process [6]. Table3 shows the BET surface area of the samples. It can be seen that theBET surface area is in the order: wet grinding > dry grind-ing > physical impregnation > dry mix. The surface area of samplesby wet grinding and dry grinding is ten times bigger than that ofphysical impregnation and dry mix.

The crystal structure was measured by XRD. Fig. 2 shows thecomparison of XRD pattern. A 0 0 2 band of carbon was observedat around 25�. The peak of samples by wet grinding was muchweaker than that of pure petroleum coke, indicating that the car-bon of the former was less graphitic. Obviously, the graphitic struc-ture was reduced by grinding.

0 50 100 150 2000.0

0.2

0.4

0.6

0.8

1.0

x

c

onve

rsio

n, %

(4)(3)

(2)

t min

(1)1273K

pure petroleum coke

Fig. 1. Variation of x with t at 1273 K for petroleum coke with 5 wt.% BL by differentloading ways: (1) dry mix; (2) physical impregnation; (3) dry grinding and (4) wetgrinding.

10 20 30 40 50 60 70 80

Inte

nsity

/ C

PS

wet grinding

2θ / deg.

pure petroleum ckoe

Fig. 2. XRD analysis of the samples.

1712 X. Zhan et al. / Applied Energy 87 (2010) 1710–1715

So, the mechanochemical interaction in grinding process willlead to the decrease in graphitic structure and the increase in spe-cific surface area. Zou et al. [6] also found that the decrease in gra-phitic structure was deduced by mechanochemical interaction. Alower ordering of carbon layers and a higher specific surface areamay be the reason for wet grinding and dry grinding are betterthan the other two mixed method.

Secondly, the gasification reactivity of petroleum coke after wetgrinding is superior to that by dry grinding. When additive isloaded, the contact between additive and petroleum coke particlesresulting from the two activation methods is quite different. In drygrinding, the newly generated interface has much higher free en-ergy, which will make petroleum coke particles aggregate sponta-neously, thereby resisting the contact with additive. According tothe oxidation–reduction mechanism for catalytic carbon gasifica-tion, gasification rate is controlled by the content of active metalspecies and its existing form. So, the relative looser contact will re-sult in the volatilization of active metal species in additive duringgasification. While in wet grinding, the newly generated interfacewill be wrapped by solution instantaneously, and the active metalcations of additive dissolved in the ion form will be adsorbed onthe surface of particles. So, the active metal species have a strongercontact with petroleum coke and are not easily volatilized in theprocess of gasification and retain a high catalytic reactivity [6].

Dry mix is the loading method in which BL shows the lowestcatalytic activity of all mainly on account of its less effective uni-formity and dispersion. Dried BL does not exist in ion form but inparticle with the least contact to petroleum coke. So, the volatiliza-tion of Na is notable resulting in the low catalytic activity. Physicalimpregnation is better than dry mix, because the Na ions solved inthe water and have a better dispersion in petroleum coke. So, theloading method of BL is another factor influencing the sodium cat-alytic activity.

0.6

0.8

1.015wt%

20wt%10wt% 5wt%

2wt%

nver

sion

, %

1wt%

1273KShenfu coal

3.2. The catalytic effect of BL

As shown in Fig. 3, petroleum coke shows the much lower gas-ification reactivity compared to Shenfu coal because of its morerefractory structure and low ash content.

Table 3BET surface area of the samples.

Samples BET surface area (m2/g) Pore volume (m3/g)

Wet grinding 4.71 0.011Dry grinding 4.02 0.009Physical impregnation 0.37 0.0009Dry mix 0.35 0.0010Pure petroleum coke 1.92 0.05Black liquor 0.25 0.0012

BL could significantly catalyze petroleum coke gasification. Themethod of BL loading is wet grinding. It can be seen that the x at0 wt.%, 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.% and 15 wt.% BL loading inpetroleum coke under 1273 K increases with t and BL loading.However, when BL loading increases to 20 wt.% of petroleum coke,the catalytic effect is not increasing. There appears visible cross-over between the loading content at 15 wt.% and 20 wt.% implyingthat the blocking of pores in petroleum coke, caused by the excesscatalyst loading, restricts the access of CO2 to the surface ofpores in petroleum coke.

The catalytic effect of BL depending on its loading and disper-sion does not appear excellent when the ratio of Na+/C is low,but it is getting stronger with the ratio of Na+/C increasing. BLassembles when the loading is too high. And the active sites ofpetroleum coke are covered by the components of BL, so the reac-tivity of petroleum coke gasification declined.

Petroleum coke with 5 wt.% BL loading shows better reactivitythan Shenfu coal. So, BL with this loading is appropriate to catalyzepetroleum coke, otherwise the heating value will be too low if toomuch BL was loaded. In order to show and compare the catalyticaction of BL with pure alkali species, analysis of BL catalytic effectis shown in Fig. 4 on the basis of the same sodium loadings in allthe samples. The additives are BL, BL ash and Na2CO3. The so-dium/petroleum coke in this case is about 0.5% according to thesample (petroleum coke with 5% BL), and the mixing method iswet grinding. Fig. 4 shows that the catalytic activity is in the order:BL > Na2CO3 > BL ash, the differences among the three samples arenotable. Valenzuela-Calahorro et al. [18] also found that the

0 20 40 60 80 1000.0

0.2

0.4

x

c

o

t min

0%

Fig. 3. Variation of x with t at different loadings of BL by wet grinding.

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

t min

x

Con

vers

ion,

% (1)(2)

(3)

1273K

Fig. 4. The catalytic component analysis of petroleum coke: (1) petroleumcoke + BL ash; (2) petroleum coke + Na2CO3 and (3) petroleum coke + BL.

X. Zhan et al. / Applied Energy 87 (2010) 1710–1715 1713

catalytic effect of BL is slightly higher than NaOH or KOH. In thegasification reaction, an intermediate activated complex will be

0.0

0.2

0.4

0.6

0.8

1.0

cokex

c

onve

rsio

n, %

t min

1223K

coke+5wt%BL

a

1223K

0.0

0.2

0.4

0.6

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1.0

x

c

onve

rsio

n, %

coke+ 5wt%BL

t min

1373K

coke

c

1373K

0.0

0.2

0.4

0.6

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1.0

x

c

onve

rsio

n, %

coke+5wt%BL

t min

1573K

coke

0 20 40 60

0 20 40 60

0 20 40 60

e

1573K

Fig. 5. Variation of x with t a

formed, and then it was decomposed. The formation of the inter-mediate activated complex is an endothermic process. Thoughthe formation of the activated complex is higher with BL than withNaOH or KOH, such a complex is more unstable with BL, and itsdecomposition rate is the main factor in controlling the specificrate of gasification of the carbon. So, BL showed higher catalyticactivity than NaOH or KOH. Because the catalyst loadings on thepetroleum coke were carried out by wet grinding using BL, NaOHor KOH aqueous solutions, differences in chemical–physical prop-erties of the petroleum coke water slurry (e.g., in viscosity) mightinfluence the degree of sodium dispersion and hence its catalyticactivity as the gasification of the carbon proceeded. Obviously,the further investigation on the mechanism of the reason why BLshows higher catalytic activity is needed.

3.3. The temperature effect on BL catalytic activity

Fig. 5 shows the catalytic activity of BL on petroleum coke gas-ification by CO2 at different temperatures. BL is 5 wt.% of petro-leum coke by wet grinding. The gasification reactivity ofpetroleum coke is greatly affected by temperatures ranging from

0.0

0.2

0.4

0.6

0.8

1.0x

con

vers

ion,

%

coke+5wt%BL

t min

1273K

coke

b

1273K

0.0

0.2

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x

c

onve

rsio

n, %

coke+5wt%BL

t min

1473K

coke

d

1473K

0 20 40 60

0 20 40 60

0 20 40 600.0

0.2

0.4

0.6

0.8

1.0

x

c

onve

rsio

n, %

coke+5wt%BL

t min

1673K

coke

f

1673K

t different temperatures.

1714 X. Zhan et al. / Applied Energy 87 (2010) 1710–1715

1223 K to 1373 K, while for 1473–1673 K, the temperature effectsare weakened. The reaction rate is so fast from 1473 K to 1673 Kthat the conversion approaches invariability within 20 min.

Because of the high ratio of C/H, the petroleum coke is com-posed by polycyclic aromatic hydrocarbons and rich in the aromat-ics with lots of rings [19]. The compounds in petroleum coke arevarious according to the different amount of rings in aromatics.The polycyclic aromatics of low reactivity would not be gasifiedat low temperatures. It is concluded from Fig. 5 that the big aro-matics ring systems start gasification rapidly from 1473 K. Tem-perature is one of the most important factors influencing thecatalytic activity of BL. The difference between the reactivity ofpetroleum coke with BL and without BL becomes less and less withtemperature increasing.

Obviously, sodium lost at high temperature is a significant fac-tor influencing petroleum coke gasification reactivity. Li and VanHeiningen [20] found that the explanation for the sodium emissionis that part of the sodium salts mainly the carbonate reacts withcarbon at higher temperatures to produce metallic sodium vaporand the presence of CO and CO2 in the atmosphere inhibits thatreaction. After pyrolysis at 948 K or higher, all the sodium in blackliquor char is in the form of inorganic salts. In the nonisothermalslow-pyrolysis data, the sodium in the BL residue decreases aspyrolysis proceeds after 948 K. And the sodium emission is negligi-ble during gasification of black liquor char with CO2 below 1073 K.The sodium loss during the pyrolysis is also seen in the data ofFrederic and Hupa [21]. They found that a substantial quantity ofsodium fume is formed from black liquor during pyrolysis. Theypresent experimental findings which indicate that the total fumeyield during devolatilization is at least ten times higher than dur-ing other stages of combustion.

Temperatures reached in this work are much higher than 948 K,so there must be a lot of sodium loss. The sodium loss is the mainreason for the difference between the reactivity of petroleum cokewith BL and without BL becoming less with temperature increas-ing. Although the sodium loss is getting more and more significant,BL still shows catalytic activity under temperatures from 1223 K to1573 K. It means that there is still some sodium remaining inpetroleum coke until 1573 K. Investigation is needed about howmuch sodium is left when the isothermal temperature is reachedand the CO2 gasification is about to start.

It is seen that BL could improve the reactivity of petroleum coketo different extent from 1223 K to 1573 K, but it also decreases thereactivity of petroleum coke at 1673 K. To avoid experiment errorand the quality of the observation, each sample was tested twice inTGA. The reactivity of sample at 1573 K is always higher than thatat 1673 K. It is unusual for temperature effect on the reactivity. Be-cause the reactivity of pure petroleum coke is improved by increas-ing temperature from 1223 K to 1673 K, obviously the reason forthat unusual phenomenon is not the sodium loss.

Table 4 shows the pore structure of petroleum coke with BLwhen x = 0.3. The specific surface area and pore volume increasewith temperature increasing before 1473 K, after reaching theirmaximum, they decrease with temperature increasing when thetemperature is higher than 1473 K. The values of specific surfacearea and pore volume are particularly low at 1673 K.

Table 4The pore structure of petroleum coke with BL when x = 0.3.

Samples BET surface area (m2/g) Pore volume (m3/g)

1123 4.71 0.0171273 5.06 0.0251373 6.37 0.0391473 7.35 0.0501573 1.92 0.0451673 0.24 0.0011

Zhou et al. [22] also found that the pore of coal specific surfacearea and pore volume increase with temperature increasing before1600 K and decrease with temperature increasing after 1600 K.Tang et al. [23] found that when temperature reaches to ash sort-ing temperature, the mineral granule developed to agglomerationfrom random distribution with increasing temperature.

So, the agglomeration of smelting BL ash attached petroleumcoke pore surface will significantly aroused pore plugging at hightemperatures. Hence, the gas diffusing resistance increased [24].And the reactivity decreased. So, change of surface area and porevolume of petroleum coke at high temperatures by black liquorash is the main reason for the reactivity of petroleum coke withBL lower than that of petroleum coke without BL at 1673 K.

4. Conclusions

Black liquor is an effective catalyst for petroleum coke gasifica-tion under temperatures from 1223 K to 1573 K and at appropriateloadings from 1 wt.% to 15 wt.%. The gasification reactivity was af-fected by different loading methods in the order: wet grind-ing > dry grinding > physical impregnation > dry mix. Thecatalytic activity of black liquor was higher than that of Na2CO3.The gasification reactivity of petroleum coke with 20 wt.% black li-quor loading is not higher than that of petroleum coke with15 wt.% black liquor loading. Temperatures, loadings and loadingmethods are great factors influencing the catalytic activity of BL.The reactivity of petroleum coke with 5 wt.% BL reaches its maxi-mum at 1573 K and was found higher than that of Shenfu coal,clearly demonstrating that black liquor could be an effective cata-lyst for petroleum coke gasification. More work should be under-taken to investigate the mechanism of the reason why blackliquor shows higher catalytic activity. And the kinetics of petro-leum coke catalytic gasification is needed.

Acknowledgements

The research is supported by National Key State Basic ResearchDevelopment Program of China (973 Program, 2004 CB 217700),Program for Changjiang Scholars and Innovative Research Teamin University (PCSIRT-IRT 0620), Shanghai outstanding academicleaders Subsidy Scheme (08 XD 1401300), Sinopec Ningbo Engi-neering Company Limited and 863 colleges and universities sub-ject of chemical heat recovery application of new technologies incoal gasification research.

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