Gasification Property of Direct Coal Liquefaction Residue with Steam

Gasification Property of Direct Coal Liquefaction Residue with Steam

0957–5820/06/$30.00+0.00 # 2006 Institution of Chemical Engineers Trans IChemE, Part B, November 2006 Process Safety and Environmental Protection, 84(...

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0957–5820/06/$30.00+0.00 # 2006 Institution of Chemical Engineers Trans IChemE, Part B, November 2006 Process Safety and Environmental Protection, 84(B6): 440– 445

www.icheme.org/psep doi: 10.1205/psep05020

GASIFICATION PROPERTY OF DIRECT COAL LIQUEFACTION RESIDUE WITH STEAM X. CHU 1

1,2

, W. LI

1

, B. LI

1

and H. CHEN

1

State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, China 2 Graduate School of the Chinese Academy of Sciences, Beijing, China

S

team gasification of char from Shenhua coal direct liquefaction residue (DLR) was investigated in a fixed bed reactor. The effect of temperature, mineral, liquefaction catalyst and heavy oil remained in the residue on the gasification property were examined. Compared with coal char, liquefaction residue char has lower gasification reactivity due to its main component of unreacted coal, heavy oil and condensation products during liquefaction process. The mineral matter has catalysis during the steam gasification of coal char and DLR char, the liquefaction catalyst remained in the residue is Fe12xS and has no remarkable effect on the steam gasification reactivity, because during the steam gasification process large amount of H2S is produced by the remained liquefaction catalyst (Fe12xS), which restrains the steam gasification of residue. After extraction of the heavy oil from the residue, the BET surface and pore structure of residue char increase but the steam gasification ability decreases. Keywords: steam gasification; direct coal liquefaction residue; mineral; liquefaction catalyst; heavy oil.

INTRODUCTION

boiling points like heavy oil and asphaltene, and another half of carbon-rich materials like reacted high polymer, unreacted coal, mineral and the remained liquefaction catalyst. The effective utilization of liquefaction residue is directly related with the economy of the liquefaction process. In a typical case, the unconverted coal and residue heavy oil are sent to a partial oxidation plant, along with the raw coal, to produce hydrogen (Comolli et al., 1988). In addition, the concept of partial liquefaction (Liu and Yang, 1998) was proposed for carrying out the liquefaction at mild conditions to convert only the hydrogen-rich portion of coal and to generate H2 from the carbon rich residue. Cui et al. (2001) investigated the possibility of liquefaction residue to produce hydrogen, he found the residue had good reactivity ability because the liquefaction catalyst and enriched mineral had catalysis on the gasification. The produced H2 could be used in liquefaction process, so it is necessary to understand the reactivity property of the residue. In the work of Cui et al. (2001), the residue was obtained in a tubing bomb batch reactor without solvent. It is known that the property of residue is closely related to the operation conditions of liquefaction process. Shenhua is the biggest coal liquefaction corporation in China. Now it is building a pithead direct coal liquefaction plant in its won coalfield. In this study, the steam gasification property of Shenhua direct liquefaction residue from a 100 kg day21 pilot plant was investigated, the gasification characteristic of direct liquefaction residue (DLR) char

Petroleum is the most important energy in the world. It is not only necessary for transportation and power generation, but also an essential material for chemical industry. The proved reserves of petroleum in China is 38 hundred million tons, account for 5.6% of the total proved fossil energy, of which 24% could be mining. The growth of petroleum demand in China is rapid, but the growth rate of domestic supply is comparatively slow, therefore, keeping the safety and stability of energy supply is an urgent mission for China. As for the method to solve the petroleum crisis, coal liquefaction is the best alternative solution because the coal reserve is abundant in China. The proved reserves of coal in china are 1145 hundred million tons, of which 93% could be mining. The coal liquefaction technology is a method of converting coal to petroleum through chemical reaction. Most coal liquefaction processes involve the dissolution of coal using a hydrogen-donor solvent and added gaseous hydrogen. So mass of hydrogen is needed in direct liquefaction process. After separation of the liquefaction products, about 15% is gases and 60% is mixed oil with the other 25% is residue. The residue contains half of organic matters with high  Correspondence to: Dr W. Li, State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China. E-mail: [email protected]

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GASIFICATION PROPERTY OF DIRECT COAL and coal char was studied. The effect of temperature, enriched mineral matter, heavy oil and liquefaction catalyst on the gasification reactivity of the residue was also examined. The results in this work will supply the basic gasification characteristics and reveal how the organic and inorganic matter remaining in DLR affect the gasification reactivity of Shenhua DLR, which will offer some help to the large-scale utilization of Shenhua DLR to produce hydrogen. METHODS AND MATERIALS

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demineralized using 38% HCl at 333 K for 1 h, and then reacted with 50% HF at 333 K for 1 h, finally treated by high concentration of HCl at 333 K for 1 h. Then the demineralized samples were washed with hot distilled water until there was no Cl2 was detected. The resulting samples were dried at 383 K for 24 h. The demineralized residue, termed as DDLR, was pyrolysed to produce char using the above procedure. For comparison, the demineralized raw coal, termed as DC, was also pyrolysed to form char to be gasified. The property of DC char and DDLR char is shown in Table 1.

The Samples The residue was obtained from a 100 kg day21 liquefaction pilot plant, termed as DLR in the later text. The operation conditions of liquefaction process are as follows: 17 MPa, 673– 733 K, hydrogenated heavy oil as solvent with a dispersed superfine iron as catalyst (Comolli et al., 1999). The property of raw coal and the liquefaction residue is given in Table 1. Char Preparation To effectively use the residue which contains 47.69% of volatile matter from Table 1, the residue should be pyrolysed and then gasified. The pyrolysis process could produce oil, asphaltene, gas and residue coke. The former two can be used as recycle solvent to match coal slurry (Tomita, 1991) and the char is gasified to produce hydrogen. The liquefaction residue was pyrolysed at 20 K min21 to 1223 K and held for 30 min under N2 atmosphere. After pyrolysis, the char samples were crushed and sieved to 60–100 mesh for gasification test. The raw coal char was prepared with the same procedure as that of the residue char. The property of all the samples is shown in Table 1. The volatile matter in samples was detected at 1173 K for 7 min. During char preparation the particle size of the sample is 60–100 mesh, however, in volatile matter test the sample was required to be further crushed to less than 80 mesh. That is to say, part of the particle size in char preparation is larger than that in volatile matter test. The remained volatile in larger particles will further release when they are crushed finely even the temperature of the volatile matter test is lower than that of char preparation. Hence, it is not strange that the char sample still has some volatile matter.

Gasification Tests The samples were gasified in a quartz tube reactor (F15 mm  580 mm) heated by an electric furnace. The quantity of steam was controlled by a micro flow control pump, which injects de-ionized water preheated by heating tape kept at 473 K. For each gasification test, approximately 1 g sample was placed in the sintered disk within the quartz tube. When the given temperature was reached with heating rate of 15 K min21, steam was introduced into the reactor with 40 ml min21 N2 as carrier gas. Prior to gasification test, the flow rate of steam from was changed from 800 ml min21 (at 473 K) to 1500 ml min21, the results show that the gasification rate of residue char has little change after the flow rate reaches 890 ml min21 at which the effect of external diffusion could be excluded and no steam concentration gradient across the sample bed. Hence, the steam flow rate is 890 ml min21 in all the gasification tests. Table 2 is the BET surface area and pore structure of coal char, DLR char and DDLR char, it was analysed at 77 K using nitrogen as adsorbate. It shows the surface area of the three char is small and coal char has higher BET surface area compared with DLR char. During gasification the gas was sampled by injector at the same time interval (5 min at 1173 K and 3 min at 1273 K) and analysed by gas chromatography. When the test is finished, the steam is stopped and the nitrogen still passes the reactor to cool the gasification residue to ambient temperature at which the residue is taken out and weighed. RESULT AND DISCUSSION Comparison of Gasification Reactivity Between Coal Char and DLR Char

Demineralization of the Samples its

To examine the effect of mineral matter in the residue on gasification reactivity, the residue were first

Figure 1 shows the char conversion during gasification of coal char and DLR char, respectively, at different

Table 1. Properties of the samples. Proximate analysis (wt%, ad) Samples Coal DLR Coal char DLR char DC char DDLR char CYHRC THFRC

Ultimate analysis (wt%, daf)

Moisture

Ash

Volatile

Fixed carbon

C

H

O

N

S

8.70 0.05 1.43 1.57 0.86 0.14 0.49 0.94

6.36 11.59 7.53 19.11 1.30 4.27 20.14 41.59

32.65 47.69 3.90 3.43 2.34 0.99 4.18 2.41

52.29 40.67 87.14 75.89 95.5 94.6 75.19 55.06

78.26 84.42 93.75 91.30 96.02 96.82 92.43 83.19

4.88 6.40 1.93 1.80 1.36 1.19 1.55 2.00

15.16 7.52 3.04 5.73 1.86 1.38 4.54 12.97

1.06 1.46 0.92 0.91 0.55 0.53 1.20 1.51

0.64 0.20 0.36 0.26 0.21 0.08 0.28 0.33

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CHU et al.

Table 2. BET surface and pore structure property of DLR and DDLR char. Samples Coal char DLR char DDLR char

BET surface area

Micro area

Micro volume

5.965 m2 g21 1.0368 m2 g21 0.8451 m2 g21

0.0691 m2 g21 0.0354 m2 g21 0.0652 m2 g21

0.000285 cm3 g21 0.000036 cm3 g21 0.000043 cm3 g21

temperatures and holding times. Compared with 1173 K, the conversion of the sample is higher at 1273 K during gasification of both coal char and residue char. In coal char gasification, at 1173 K after 45 min reaction, the final conversion of carbon in char is 88%; while at 1273 K after 21 min reaction, which is 92%. In DLR char gasification, at 1173 K after 45 min reaction, the final conversion of carbon is 73%; while at 1273 K after 21 min reaction, which is 77%. Compared with DLR char, coal char has higher conversion at the same time. Figure 2 is the formation rate of H2 and CO from gasification of DLR char and coal char. The formation rate of H2 and CO from DLR char gasification is lower than that from coal char, especially in the initial reaction stage. This suggests that coal char has better gasification reactivity than DLR char. It is known that DLR contains unreacted coal, condensation products, heavy oil, enriched minerals and liquefaction catalyst. To exclude the effect of mineral matter and liquefaction catalyst, DC char and DDLR char were gasified at 1273 K, the carbon conversion is shown in Figure 3. Compared with DDLR char, DC char has higher conversion at the same time. After 21 min gasification, the final conversion of carbon from gasification of DDLR char and DC char is 70% and 83%, respectively, suggesting the better gasification reactivity of DC char. Generally, low-rank coal char has higher reactivity than the high-rank coal char (Tomita, 1991). During direct liquefaction process the reaction temperatures is in range of 673– 733 K at which the active components in raw coal have been reacted, thus DLR is mainly composed of unreacted coal and condensation products resulted from

Figure 1. Carbon conversion of coal char and DLR char at different temperatures and holding times during steam gasification.

Figure 2. The formation rate of H2 and CO during gasification of coal char and DLR char with steam at 1273 K.

liquefaction, leading to the so called higher ‘rank’ of DLR and its lower gasification reactivity. Table 2 shows that compared with coal char, DLR char has lower surface. This is another important reason for the lower gasification reactivity of DLR char. Effect of Enriched Mineral and Liquefaction Catalyst on Gasification Reactivity of DLR Char During liquefaction process, the mineral matter in coal is enriched in residue. Table 3 is the ash composition analysis of Shenhua coal and DLR. After liquefaction, the content of ash is 11.99% while for the coal that is 6.36%. In order to study the effect of enriched mineral matter, the carbon conversion during gasification of DLR and DDLR char at 1273 K is compared in Figure 4. After acid wash, the

Figure 3. Carbon conversions of DC char and DDLR char during steam gasification at 1273 K.

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Table 3. The ash analysis of Shenhua coal and DLR. Samples Coal DLR

Ash

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

K2 O

Na2O

P2O5

6.36 11.59

0.94 2.80

0.68 0.40

1.08 3.78

1.74 2.31

0.23 0.27

0.04 0.10

1.48 1.65

0.03 0.04

0.09 0.16

0.01 0.01

char conversion is become lower during the whole gasification process, indicating that the enriched mineral matter has catalytic effect during gasification of DLR char. The enriched mineral matter inevitably includes the liquefaction catalyst which is converted to Fe12xS. In order to examine the effect of liquefaction catalyst on the gasification reactivity of DLR and exclude the effect of mineral matter in coal itself, the demineralized DLR (termed as DDLR) is mixed with the liquefaction catalyst and then pyrolysed to prepare char sample for gasification test. During pyrolysis the catalyst could decompose and Fe12x0 S could be generated. Figure 5 show the conversion during gasification of DDLR char with and without liquefaction catalyst. The carbon conversions of different samples have little difference, suggesting the less effect of liquefaction catalyst for gasification reaction. It is well accepted that the element Fe has positive catalysis during steam gasification of coal (Lemaignen et al., 2002; Tamai et al., 1977; Bernado and Trimm, 1979). However, in this experiment the liquefaction catalyst does not show obvious catalytic effect, there might be two reasons: Firstly, the catalytic effect of inorganic matter on char gasification reactivity may be related to the carboxylic and phenolic groups which hold the cations (Schafer, 1991), liquefaction process can destroy the functional groups of coal (Bohlmann et al., 1992), which also could be proved by the sharp decrease of oxygen content in DLR compared with that in raw coal shown in Table 1. Secondly, S is a harmful element during gasification, Matsumoto and Walker (1986) studied effect of H2S and COS during steam gasification of coal with K, Na, Ca and Fe as catalyst. The results show that 500 ppm of H2S and COS in the gas phase

obviously restrain the gasification ability, expecially in the steam gasification of coal during which S have strong restraining effect for the catalysis of Fe (Matsumoto and Walker, 1986). To further check the effect of liquefaction catalyst on gasifcation of DLR char, H2S emission during gasification of coal char and DLR char was compared below. Table 4 is the sulphur forms of coal and DLR. Compared with coal, DLR has higher content of S and most of the sulphur form is pyritic sulphur which comes from the catalyst used in liquefaction of raw coal. During liquefaction process the catalyst of Fe-S compound was translated as Fe12xS in presence of H2 and hydrogen donor solvent (Ikenaga et al., 1997; Matsuhashi et al., 1997). For the example of FeS2, the following reaction will be take place during liquefaction:

Figure 4. Carbon conversions of DLR char and DDLR char during steam gasification at 1273 K.

Figure 5. Carbon conversions of DDLR char with and without liquefaction catalyst during steam gasification at 1273 K.

FeS2 þ H2 ! H2 S þ Fe1x S

(1)

During pyrolysis of liquefaction residue for preparation of DLR char, the reaction (2) could take place (Liu et al., 2005): Fe1x S ! Fe1x0 S þ H2 S

(x . x0 )

(2)

Hence the main component of catalyst remained in DLR char is Fe12x0 S. Figure 6 is the concentration of H2S during steam gasification of coal char and DLR char. Compared with coal char, more H2S is produced during gasification of DLR char, which is mainly resulted from the hydrogenation of Fe12x0 S. With increasing time the concentration of H2S raises first, and then decreases for DLR char gasification.

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CHU et al. Table 4. The sulphur forms of coal and DLR.

Samples Coal DLR

St (wt%, ad)

Sp (wt%, ad)

Ss (wt%, ad)

So (wt%, ad)

0.64 1.82

0.05 1.37

0.05 0.27

0.54 0.18

St: total sulphur; Sp: pyritic sulphur; Ss: sulphate sulphur; So: organic sulphur.

At the beginning of gasification, the Fe12xS is covered by carbon which is consumed with the gasification going on and more Fe12xS could contract with H2, resulting in the increasing concentration of H2S. At the later stage of gasification few H2 is produced and less Fe12xS remained, leading to the low concentration of H2S formed. Effect of Remained Heavy Oil on Gasification Reactivity of DLR Char Heavy oil composed of oil, asphaltene and preasphaltene is believed to result in the high content of volatile matter in DLR. In order to investigate the effect of heavy oil on gasification of DLR, DLR was extracted by cyclohexane (CYH) and tetrahydrofuran (THF), then the residue was pyrolysed to produce char for gasification test. Cyclohexane could extract the oil in DLR, while THF could extract oil, asphaltene and preasphaltene in DLR. The remained residue is 87% and 53% after the extraction by CYH and THF, respectively. During char preparation the char yield is 57.8% and 86.3% for CYH and THF extracted sample, respectively. The proximate and ultimate analysis of the resultant char are listed in Table 1. Figure 7 is the SEM photograph of DLR char, CYH extracted char (CYHRC) and THF extracted char (THFRC). The char surface is incompact and micro-pore increases after extraction of heavy oil, especially extracted by THF. During solvent extraction process, solvent has swelling effect and may affect the gasification reactivity of residue. In order to

Figure 6. The concentration of H2S during steam gasification of coal char and DLR char at 1273 K.

Figure 7. The SEM photograph of DLR char (a), CYHRC (b) and THFRC (c).

elucidate the solvent effect on gasification, DLR was first immersed using solvent (CYH and THF), after the solvent was naturally volatilized in air, the sample was pyrolysed and then gasified. The gasification results of DLR char, cyclohexane immersed char (CYHIC), tetrahydrofuran immersed char (THFIC), CYHRC and THFRC at 1273 K are shown in Figure 8. Compared with the carbon conversion of DLR char, after immersion using CYH and THF, the conversion of CYHIC and THFIC has no much change; but after extraction using CYH and THF, the carbon conversion becomes lower. This indicates that the effect of solvent – DLR interaction on the gasification reactivity is negligible and solvent extraction does not affect the molecular nature of the sample. It is well accepted that nonpolar solvent extraction could destroy the non-covalent bonds in coal, but almost has no effect on the covalent bonds. DLR was obtained from high temperature and high pressure liquefaction process of raw coal, hence the non-covalent bonds are completely disappeared in DLR, leading to no variation of its macro-molecular structure during solvent extraction. After 21 min gasification, the final carbon conversion is 77%, 75% and 71% for DLR char, CYHRC and THFRC, respectively. This indicates the lower gasification reactivity of DLR char after solvent extraction. The BET surface area is 1.0368, 2.2974 and 12.1401 m2 g21 for DLR char, CYH and THF extracted

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REFERENCES

Figure 8. Carbon conversions of DLR char, CYHIC, THFIC, THFRC and CYHRC during steam gasification at 1273 K.

char, respectively. It is clear that after the solvent extraction, the surface of DLR char increases, especially extracted by THF. However, the gasification reactivity decreases after solvent extraction, suggesting the diffusion is not the rate-control factor under the present test conditions. The results also indicates that the gasification reactivity of char from heavy oil is higher than that from the unreacted coal in DLR. CONCLUSION The steam gasification of direct liquefaction residue (DLR) char was investigated using a fixed bed reactor. The results show the gasification reactivity of liquefaction residue char is increases with increasing temperature, Compared with DLR char, the coal char has better gasification reactivity but less H2S fromation. The enriched mineral has catalysis on the gasification of DLR char, but the liquefaction catalyst has no remarkably positive effect. After the solvent extraction of DLR, the BET surface of residue char increases, but its gasification reactivity is lowered, suggesting the higher activity of heavy oil than that of the unreacted coal in DLR.

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ACKNOWLEDGEMENT This work was financially supported by National Basic Research Program of China (2004CB217602). The manuscript was received 15 December 2005 and accepted for publication after revision 15 June 2006.

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