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Apr 28, 2014 - Steam cogasification characteristics of petroleum coke with three coals of different ranks were investigated on a laboratory fixed-bed ...
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Steam Cogasification of Petroleum Coke and Different Rank Coals for Enhanced Coke Reactivity and Hydrogen-Rich Gas Production Sheng Huang,†,‡ Shiyong Wu,*,†,‡ Youqing Wu,†,‡ Yongdi Liu,§ and Jinsheng Gao†,‡ †

Department of Chemical Engineering for Energy Resources, ‡Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, §School of Resource and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Steam cogasification characteristics of petroleum coke with three coals of different ranks were investigated on a laboratory fixed-bed reactor to examine the effects of coal ratios and coal ranks on the gasification reactivity, gas yields, and compositions. Results show that the rise of the coal ratio leads to an increase in H2 yield and content but a decrease in CO yield and content in the producer gas, independently of coal rank. The cogasification reactivity of petroleum coke (PC) with XLT coal and PC with SF coal at the optimum coal contents of 40% and 80% are about 4.5- and 3.5-fold higher than that of PC gasification alone (XLT = Xiaolongtan lignite from Yunnan province, SF = Shenfu bituminous coal from Shaanxi province); meanwhile, H2 contents in the producer gas increased by 10.55% and 12.24%. The ordering for the cogasification reactivity of PC with different rank coals is as follows: PC/XLT > PC/SF > PC/GP, which is in accord with the chemical forms of catalytic elements present in the three coals (GP = Gaoping anthracite coal from Shanxi province). Obvious synergistic effects are observed during the cogasification of PC with SF coal and PC with XLT coal. The gases from cogasification of PC with different rank coals can be used as feed gas for the synthesis of DME, methanol, hydrocarbon fuel, and producing hydrogen, depending on coal contents and ranks. Under the present experimental conditions, the thermodynamic equilibrium of the water−gas shift reaction is never reached. The cogasification of PC with low rank coal, especially lignite, to enhance the gasification reactivity of PC and the production of hydrogen-rich gas (up to 64.12%) is a feasible and promising technology to utilize PC on a large scale.

1. INTRODUCTION Petroleum coke (PC) is a byproduct of delayed coking. With the development of crude oil refining technology and the continuous increase of crude oil supply worldwide, its output is increasing steadily.1−3 Thus, how to use PC (especially highsulfur PC) in a reasonable, efficient, and clean way becomes a subject worthy of being studied in depth. Gasification technology can convert a wide variety of carbonaceous materials (such as coal, biomass, and PC) into useful syngases (mainly CO and H2) with nearly zero pollution emissions.2−4 Therefore, it is considered to be an effective way to utilize PC for the production of syngases that can be used to satisfy the increasing demand of hydrogen by refineries and other applications.3,5 It has been reported that the porosity of PC is relatively low,2,6 and the carbon crystalline structure of PC is relatively ordered,2,6 resulting in its low gasification activity, which greatly restricts the applicability of PC as the feedstock of gasifier. Recognizing that the intrinsic reactivity of feedstock is one of the most influential parameters in the over gasification performance, some attempts have been made to improve the gasification reactivity of PC. It is well accepted that the gasification reactivity of carbonaceous materials can be enhanced in the presence of various alkali and alkaline earth metal compounds.3,7−9 Considering the low reactivity of PC and potential of the mineral matters present in coal, especially low rank coal, as a source of natural and inexpensive catalyst, coutilization of PC with coal would be a green alliance to improve the gasification reactivity of PC along with pursuing environmentally friendly technologies.10,11 Therefore, cogasification of PC with coal can realize the reasonable, efficient, and clean utilization of PC. © 2014 American Chemical Society

Cogasification based on different carbonaceous materials has received much attention in recent years, since it may compensate for their weaknesses of each other.9−16 It has also been reported that various synergy effects during cogasification may result in higher reactivity, higher fuel conversion, and lower tars production in product gases.12,13,17−19 As far as the cogasification is concerned, Pan et al.20 investigated the cogasification characteristics of blends of pine chips with a low-grade coal in a fluidized-bed reactor. They found that a blending content of no less than 20% pine chips could effectively improve the cogasification performance, thereby making the cogasification an attractive and economic technology for the use of low-grade coal. Fermoso et al.11 focused on the cogasification of a mixture of coal, PC, and biomass at atmospheric and high pressure in a fixed bed reactor. The results showed that the binary (coal−biomass) and ternary blends (coal−PC−biomass) have some synergetic effects that enhance gases production. Lee et al.21 investigated the gasification characteristics of coke and mixture with coal in an entrained-flow gasifier. The obtained results indicated that the gasification of mixtures resulted in a higher syngas heating value and cold gas efficiency because of the higher H2 and CO contents in syngases. Nemanova et al. 10 studied the cogasification of biomass and PC in an atmospheric bubbling fluidized bed and thermogravimetric analyzer. They found that the biomass ash in the blends had a catalytic effect on the Received: January 21, 2014 Revised: April 26, 2014 Published: April 28, 2014 3614

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Table 1. Proximate and Ultimate Analyses and Low Heating Value of Raw Materials proximate analysis (wt %)

ultimate analysis (wt %)

samples

Ad

Vdaf

FCdaf

Cdaf

Hdaf

Ndaf

St,d

LHV (MJ/kg)

PC XLT SF GP

0.15 13.13 6.50 22.08

9.09 52.14 37.66 11.13

90.91 47.86 62.34 88.87

90.47 68.75 80.53 90.08

3.49 3.93 4.80 4.22

1.23 1.94 0.89 0.69

4.94 1.22 0.33 0.23

32.80 19.16 26.43 24.80

Figure 1. Schematic diagram of a laboratory fixed-bed gasification setup. 1. Ar cylinder; 2. Mass flow meter; 3. Distillated water reservoir; 4. Constant flow pump; 5. Steam generator; 6. Electric furnace; 7. Quartz reactor; 8. Ice−water trapper; 9. Temperature-programmed cooler; 10. Online quadruple mass spectrometer. of samples (dry basis) was thinly spread on the sintered plate with a pore diameter smaller than 30 μm, which was placed at the central zone of the tubular reactor. The samples were heated at a heating rate of 10 K/min by electric furnace to a predetermined temperature under an argon flow of 500 mL/min. After the temperature was up to the predetermined temperature and leveled off for about 15 min, the steam cogasification started isothermally at the atmospheric pressure by sweeping steam (the partial pressure of steams was about 0.6) into the reactor. The outgoing gases passed through an ice−water trapper and then a cooler controlled at 243 K to prevent steam and tarry matters into the MS analyzer. The concentrations of H2, CO, CO2 and CH4 and Ar in the outgoing gases were quantitatively determined by an online quadruple mass spectrometer (HR20 Type MS, Hiden Co., England) with a detection limit up to ppb level. When all MS signals of H2, CO, CO2, and CH4 presented quite weak, the gasification experiment was considered ended. A steam gasification experiment included two stages: the pyrolysis of raw samples and the successive steam gasification of the residues from the pyrolysis of raw samples; the present study only made some investigations on steam gasification of the PC and coal blends. The detailed calculation processes of various gasification reactivity indexes, including the gasification carbon conversion (X), the instant gasification rate (R = dX/d, min−1), and the instant release rates (Ri, mmol·min−1·g−1sample, daf), total yields (Yi, mmol·g−1sample, daf), and total contents (CT (i), vol %) of various components in product gases were respectively calculated according to eqs 1−5.3

reactivity of PC during cogasification; the activation energy for pure PC was 121.5 kJ/mol, whereas for the 50/50 mixture it was 96.3 kJ/mol. Therefore, cogasification of different materials, such as low rank coal, PC, biomass, and wastes, can be attractive from economical, environmental, and social points of view in order to make use of possible synergistic effects via the production of fuel gas. To date, though a number of studies concerning cogasification of different fuels have been performed,13−23 the studies of cogasification of PC with coal are still rare, especially the hydrogen production characteristics during the cogasification of PC with coal. For these reasons, the present study is intended to study the cogasification characteristics of PC and coal blends on a laboratory fixed-bed reaction system with an online quadruple mass spectrometer. Three different rank coals, consisting of a lignite, a bituminous coal, and a anthracite coal, individually mixed with a PC, will be studied. The effects of coal contents and coal ranks on the gasification reactivity, hydrogen production characteristics, and other process parameters during the cogasification will be investigated in detail.

2. EXPERIMENTAL SECTION 2.1. Sample Details. In the present study, three Chinese coals of different ranks were selected, a Xiaolongtan lignite from Yunnan province (XLT), a Shenfu bituminous coal from Shaanxi province (SF), and a Gaoping anthracite coal from Shanxi province (GP). In addition, a PC from Jinshan petrochemical Co. Ltd. of China was selected for cogasification tests. All raw materials were ground to the particles with a size of less than 73 μm. The coal samples (XLT/SF/ GP) were individually mixed with PC by a wet method at coal contents of 0%, 20%, 40%, 60%, 80%, and 100% and stored as samples for cogasification experiments. The proximate and ultimate analyses and low heating values of all raw materials are presented in Table 1. 2.2. Steam Cogasification Experiments. The steam cogasification of PC with different rank coals was carried out on a laboratory fixed-bed reactor system, as shown in Figure 1. The reaction system was mainly composed of a steam generator and a vertically placed corundum tubular reactor (a diameter of 50 mm and a length of 1000 mm) which was heated by an electric furnace. For each run, about 1 g

t

X=

∫t = 0

500 × 10−3 × (CCO + CCO2 + CCH4)

0

22.4CAr

t

500 × 10−3 × (CCO + CCO2 + CCH4)

0

22.4CAr

∫t =g 0

× 12 × 12

500 × 10−3 × (CCO + CCO2 + CCH4)

dX R= = dt

3615

22.4CAr

× 12

t

500 × 10−3 × (CCO + CCO2 + CCH4)

0

22.4CAr

∫t =g 0

(1)

× 12

(2)

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Table 2. Ash Compositions Analyses of Three Coal Samples chemical compositions (wt %) samples

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

Na2O

K2O

SO3

XLT SF GP

11.63 25.29 52.37

7.79 11.26 30.43

12.83 12.89 3.57

44.29 34.62 6.52

3.93 3.91 0.80

0.39 0.90 1.30

0.13 1.60 0.61

0.34 0.71 1.15

18.79 7.47 2.27

Ri =

500Ci 22.4CAr t

500 × 10−3 × (CCO + CCO2 + CCH4)

0

22.4CAr

∫t =g 0

× 12

(i = CO, CO2 , CH4 , H 2) tg

Yi =

∫t = 0 0

500 × Ci 22.4CAr

t

500 × 10−3 × (CCO + CCO2 + CCH4)

0

22.4CAr

∫t =g 0

(3)

× 12

(i = CO, CO2 , CH4 , H 2) C T(i) =

Yi (i = CO, CO2 , CH4 , H 2) ∑ Yi

(4)

(5) Figure 2. XRD patterns of ashes from the three coals.

where C H 2 , CCO , C CO 2 , C CH 4 , and C Ar are respectively the concentrations (vol %) of H2, CO, CO2, CH4, and Ar in the outgoing gases at the time of t, t0 is the starting time of the gasification stage, and tg is the terminal of the gasification stage. 2.3. Characterization for Inorganic Minerals of Coals. In order to detect the inorganic elements as far as possibly, the ash samples from the three coals were prepared at 1088 K according to the standard of coal ash preparation (GB/T212-2001) and ASTM standards. All element contents present in the three coal ashes were measured by a sequential X-ray fluorescence spectrometer (XRF, XRF1800). An X-ray diffraction analyzer (XRD, JSM-6360LV, D/maxrA12 kW, 12 kW, 40 kV, 100 mA, Cu Kα radiation) was employed to determine the mineral components of three coal ashes, and major components were identified using the standard cards of JCPDS-ICDD (PDF-2 database Sets).

anhydrite (CaSO4, Card No: 37-1496), hematite (Fe2O3, Card No: 33-0664), forsterite ((Mg,Fe)2SiO4, Card No: 31-0795), and calcium aluminum iron oxide (CaAl4Fe8O19, Card No: 491586). In SF ash, the main components identified are quartz (SiO2, Card No: 65-0466), hematite (Fe2O3, Card No: 330664), anhydrite (CaSO4, Card No: 37-1496), and calcium aluminum oxide (Ca2Al2O5, Card No: 52-1722). In GP ash, quartz (SiO2, Card No: 65-0466) and grossite (CaAl4O7, Card No: 23-1037) are the identified crystalline phases. This result suggests that chemical forms of the mineral matters present in the three coals are partly similar to other coals reported in the literatures.23−26 The above results imply that some catalytic components contained in the three coals, such as calcium oxide, anhydrite, and hematite in XLT coal as well as anhydrite and hematite in SF coal, can promote the gasification reactions during the cogasification of PC with coal. 3.2. Effect of Coal Ratio on Steam Cogasification Characteristics of PC with SF Coal. 3.2.1. Cogasification Reactivity. Reactivity is one of the most important parameters determining fuel suitability for use in the gasification process at industrial scale.4,5 A comparative study of the reactivity of PC, SF coal, and their blends in the processes of steam gasification at constant temperature of 1173 K is presented in Figure 3. Figure 3 shows the carbon conversion X versus gasification time

3. RESULTS AND DISCUSSION 3.1. Mineral Components Analyses of Coal Samples. The ash compositions analyses of three coals at the ashing temperature of 1088 K is presented in Table 2. From Table 2, it is found that the inorganic elements contents are different in the three coal ashes, which are mainly composed of Si, Al, Fe, Ca, Mg, Na, K, etc. Besides, it is observed that the ashes of the three coals contain a certain amount of alkali metals (Na and K), alkaline earth metals (Mg and Ca), and transition metal (Fe), which can promote the gasification reactions to a certain extent depending on the chemical forms of these metals.9,20 The total contents of these catalytic elements (Na, K, Ca, Mg and Fe) in XLT, SF, and GP coal ashes are respectively up to 61.52%, 53.73%, and 12.65% (in the form of oxide); therefore, the total contents of these catalytic elements in XLT, SF, and GP coal are up to 8.08%, 3.49%, and 2.79%, as shown in Tables 1 and 2. The high contents of these catalytic elements in the three coal ashes make it feasible for use as gasification catalysts of low activity fuels, such as PC and oil shale.21−24 Figure 2 shows the X-ray diffraction patterns of ashes from the three coals at the ashing temperature of 1088 K. From Figure 2, it is observed that in the three ashes the compounds containing Si, Al, Ca, and Fe are presented predominantly, and these elements exist in different chemical forms. According to JCPDS-ICDD data files, the major crystalline phases identified in XLT ash are calcium oxide (CaO, Card No: 48-1467),

Figure 3. Effect of SF coal content on (a) carbon conversion versus gasification time, and (b) gasification rate versus carbon conversion during the steam cogasification of PC and SF coal blends at the gasification temperature of 1173 K. 3616

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Figure 4. Effect of SF coal content (a) 0%, (b) 20%, (c) 40%, (d) 60%, (e) 80%, and (f) 100% on the release rates of gaseous products from the cogasification of PC and SF coal blends at the gasification temperature of 1173 K.

that the cogasification rates of the blends increased slightly when the SF coal content is less than 40%. This is probably attributed to the contents of catalytic elements (Na, K, Ca, Mg, and Fe) are 0.70% and 1.40% in PC and SF blends at coal contents of 20% and 40%, and the catalytic effects of these elements are not obvious for steam gasification of the blends. The above results are in agreement with the observation that catalyst is activate only if the initial catalyst loading is larger than a threshold value.7,8 Furthermore, it is observed that the gasification rates of the pyrolysis residue of PC (second part of the gasification rate versus carbon conversion) during the cogasification of PC and SF blends are higher than that of PC gasification alone, which illustrates that the addition of SF coal to PC can promote the gasification of PC during the cogasification processes. The above results demonstrate that synergistic effects may exist during the cogasification of PC and SF coal, which can be mainly ascribed, on the one hand, to the catalytic effects of mineral components present in SF coal transferred from SF coal ash to PC and, on the other hand, to the free radicals produced from SF coal decomposition that can react not only with the organic matter in SF coal but may also react with the organic matter of PC.12 The synergetic effects during cogasification of different carbonaceous materials have been reported in literatures frequently.11−13,17−19 However, some studies have reported a lack of any synergetic effects during cogasification of different fuels that include a variety of coals, biomasses, PCs and wastes, etc.27−29 This apparent discrepancy

and the gasification rate (dX/dt) versus carbon conversion for steam cogasification of PC and SF blends with different SF coal ratios. From Figure 3a, it is observed that under the completed carbon conversion, the consumed gasification time for the cogasification of PC and SF blends decreased with increasing SF coal ratio, probably attributed to the higher reactivity of carbon in SF coal as well as the catalytic effects of mineral components present in SF coal (as shown in Table 2 and Figure 2). As is clearly presented in Figure 3b, the gasification rate versus carbon conversion patterns are different for PC, SF coal, and their blends. For pure PC and SF coal gasification (at SF coal contents of 0% and 100%), the gasification rates increased gradually as the carbon conversion increased at the beginning, then attained the maximum values at carbon conversions about 0.5, and finally, the gasification rates decreased until the gasification reactions completed. For steam cogasification of PC and SF blends, the gasification rates versus carbon conversions patterns can be divided into two parts: the first part of gasification rates versus carbon conversions X are before the carbon conversions of 0.11, 0.28, 0.50, and 0.75 during the cogasification of PC and SF blends with 20%, 40%, 60%, and 80% SF coal, and the second part of gasification rates are after the carbon conversions of 0.11, 0.28, 0.50, and 0.75, as the cogasification rate versus carbon conversion of PC and SF blends with 60% SF coal in Figure 3b. This is probably ascribed to the initially gasification of SF coal char, and then the gasification of pyrolysis residue of PC. Besides, it is observed 3617

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Figure 5. Effect of SF coal content on the (a) gas yields, and (b) gas composition from cogasification of PC and SF coal blends at the gasification temperature of 1173 K.

might depend on the operating parameters used, such as temperature, pressure, type of fuel, type of reactor, etc.17−19,27−29 In addition, it is found that the cogasification reactivity of PC and SF blends increased greatly when the SF coal content is larger than 40%. For instance, the reactivity indexes of the blends at SF coal contents of 60% and 80% are about 2- and 3.5-fold higher than that of PC gasification alone (as shown in Figure 7), which implies that the cogasification of PC and SF coal is an promising technology to improve the gasification activity of PC with low reactivity. 3.2.2. Gaseous Products Release and Distribution. Figure 4 shows the instant release rates of major gaseous products (H2, CO, CO2, and CH4) from steam cogasification of PC and SF blends at different SF coal ratios. During the high temperature gasification process, some incondensable hydrocarbons, such as C2H4, C2H6, C3H6, and C3H8, occur in a small quantity, so these minor gases were not detected in the present study. As can be observed from Figure 4, the instant release rates of H2, CO2, and CO for cogasification of PC and SF blends at different SF coal ratios are similar to the corresponding instant gasification rates with the gasification carbon conversion (as shown in Figure 3b). As can be seen, the instant release rates of H2, CO, and CO2 increased with SF coal ratio rises. When the SF coal content is less than 40%, the instant release rates of gaseous products increased slightly, especially for CO and CO2, which is in accordance with the effects of SF coal content on the gasification reactivity of PC and SF blends (as shown in Figure 3). However, it is also found that the increment of the instant release rates of H2, CO, and CO2 decreased when the SF coal content is larger than 80%. CO + H 2O → CO2 + H 2 ,

ΔH ° = − 45.2 kJ/mol

CH4 + H 2O → CO + 3H 2 ,

ΔH ° = 206.0 kJ/mol (II)

C + H 2O → CO + H 2 ,

ΔH ° = 118.9 kJ/mol

(III)

This reaction is a strong endothermic reaction, and it can be promoted by some catalysts.3,7,8,30 From the above discussion, it can be concluded that the catalytic components present in SF coal can also promote the steam reforming of methane, which is in accord with the results shown in Figure 5. Figure 5 shows the effects of SF coal content on the yields and compositions of major gaseous products in a dry and inert gas free basis from cogasification of PC and SF coal at the gasification temperature of 1173 K. According to Figure 5a, it is observed that the rise of the SF coal ratio leads to an increase in H2 yield (from 69.29 to 80.31 mmol/g-sample, daf) and CO2 yield (from 12.48 to 24.37 mmol/g-sample, daf) but a decrease in CO yield (from 51.33 to 23.67 mmol/g-sample, daf) until the SF coal content is 80%. According to Figure 5a, the total contents of H2, CO, CO2, and CH4 in the producer gas are also obtained, as shown in Figure 5b. With increasing SF coal content, H2 contents in the producer gas increased gradually from 51.88% (PC gasification alone) to 62.43% (cogasification of 20% PC with 80% SF coal), and that of CO decreased from 38.43% to 19.61%, resulting in a gradual decrease of H2 + CO contents. In comparison with SF coal, PC presents the relatively low H2 yield and content. The addition of SF coal to PC can improve H2 yields and contents during the cogasification processes, and H2 yield and content respectively reach the maximum values of 80.31 mmol/g-sample and 62.43% when SF coal content is 80%. Figure 6 shows the experimental and calculated H2 yields and contents based on the weighted average of separate gasification of PC and SF coal at gasification temperature of 1173 K. The results obtained clearly show that the experimental H2 yields and contents from cogasification of PC and SF coal blends are higher than those of calculated values based on the weighted average of separate gasification of PC and SF coal; other parameters such as gaseous products release rates, CO2 yields, and contents also obey the same rule. The above results verified further that the synergetic effects may occur during the cogasification of PC and SF coal under the present experimental conditions. From the above results, it can be concluded that the catalytic components present in SF coal can effectively promote the water−gas shift reaction (Reaction I), steam reforming of

(I)

When pure PC is gasified (Figure 4a), the release rate of CO is obviously larger than that of CO2. After the addition of SF coal to PC, the discrepancy of the release rates between CO and CO2 becomes smaller, which is probably because the catalytic components present in SF coal can promote the water−gas shift reaction (Reaction I) to produce more H2 and CO2, especially when the SF coal content is larger than 40%. Besides, it is observed that the release rates of CH4 are quite little, implying virtually no formation of CH4, especially for cogasification of PC and SF coal blends with high coal ratios. It is well accepted that steam reforming of methane (Reaction II) also occurs during the steam gasification of carbon:3,8 3618

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in coal, and the other is the activity of carbon in coal. The differences of the activities of carbons in different rank coals can be negligible during high temperature gasification of different coals, so it is suggested that the ordering for the catalytic effects of mineral components present in different rank coals is XLT > SF > GP. This is mainly ascribed to the calcium oxide, anhydrite, and hematite in XLT coal as well as anhydrite and hematite in SF coal, while no catalytic component detected in GP coal, as shown in Figure 2. Besides, it is observed that the cogasification reactivity of PC with XLT coal and PC with SF coal at the coal contents of 40% and 80% are about 4.5- and 3.5-fold higher than that of PC gasification alone, which indicates that cogasification is an effective way to utilize low gasification activity carbonaceous materials, such as PC. Furthermore, the variations of the cogasification reactivity of PC with XLT coal and PC with SF coal blends are very small when XLT and SF coal contents are up to 40% and 80%, and considering the effects of coal ratios on hydrogen production characteristics (as shown in Figures 4 and 5 and Figure 8)

Figure 6. Experimental and calculated H2 yields and contents from cogasification of PC and SF coal blends at the gasification temperature of 1173 K. ■ Experimental values; □ Calculated values.

methane (Reaction II), and carbon−water reaction (Reaction III) to enhance the gasification reactivity of PC and the production of H2-rich gas. Therefore, cogasification of PC with SF coal is an promising technology to utilize PC with low reactivity for the production of H2-rich gas to satisfy the increasing demand of hydrogen by refineries. 3.3. Effect of Coal Rank on Steam Cogasification Characteristics. 3.3.1. Cogasification Reactivity. For simplicity, a reactivity index Rs is introduced to quantify the averaged cogasification rate. It is defined as the reciprocal of the time used for a 50% conversion of carbon by steam gasification times 0.5, in a unit of min−1. Figure 7 shows the relationship

Figure 8. Effect of coal ranks on (a) H2, and (b) H2 + CO contents for the cogasification of PC and different rank coals at the gasification temperature of 1173 K.

during the cogasification processes, the optimum contents are 40% for XLT coal and 80% for SF coal during the cogasification of PC and coal blends at the present experimental conditions. 3.3.2. Gaseous Products Distribution. Figure 8 shows the effects of coal ranks on H2 and H2 + CO contents for the cogasification of PC with different rank coals at the gasification temperature of 1173 K. As can be observed, the addition of coal to PC leads to an increase in H2 production and a decrease in H2 + CO production. The H2 contents in the producer gas from the cogasification of PC with XLT coal and PC with SF coal blends increased with increasing coal contents until XLT and SF coal contents are up to 40% and 80%, while that from the cogasification of PC and GP blends increased slightly when GP content is larger than 40%. Besides, the ordering of H2 contents from cogasification of PC with different rank coals at the same coal ratio is PC/XLT > PC/SF > PC/GP, which is in accord with the results shown in Figure 7. Furthermore, it is worthy noticing that H2 contents in the producer gas greatly increased from 51.88% (PC gasification alone) to 64.12% (cogasification of 60% PC with 40% XLT coal). Generally speaking, getting from a hydrocarbon fuel to H2 via a conventional process route takes at least three steps: reforming or partial oxidation of fuel, water−gas shift reaction, and finally H2−CO2 separation. The water−gas shift reaction is generally accomplished via two steps: the first one operates at higher temperatures (623−723 K) using a Fe−Cr catalyst, and the second one does at lower temperatures (513−523 K) using a CuO-ZnO-Al2O3 catalyst.8,31,32 Rosen et al.31,32 reported that the energy consumption of water−gas shift reaction accounts for 4.4% of total energy consumption of the Koppers-Totzek

Figure 7. Relationship between the cogasification reactivity indexes and contents of catalytic elements in PC and coal blends with different coal contents under the constant gasification temperature of 1173 K.

between the cogasification reactivity indexes and contents of catalytic elements (Na, K, Ca, Mg, and Fe) in PC and coal blends with different coal contents under the gasification temperature of 1173 K. There are four points on each curve of the gasification reactivity indexes versus contents of catalytic elements in PC and coal blends between the gasification reactivity indexes of pure PC and coal, and with increasing catalytic elements contents in the blends, the corresponding coal contents of the four points are 20%, 40%, 60%, and 80%, respectively. As can be observed, coal in the blends can promote the gasification of PC to a certain extent depending on the coal ranks and ratios. It is clearly presented that the ordering for the cogasification reactivity of PC and three coals of different ranks at the same contents of catalytic elements in PC and coal blends is as follows: XLT/PC > SF/PC > GP/PC. The effects of coal ranks on the cogasification reactivity of PC and coal blends can be embodied from two aspects: one is the mineral compositions (chemical forms of the mineral matters) 3619

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against which to measure improvements. The extent of the water−gas shift reaction was defined according to following equation.22 PH2 × PCO2 extent of shift reaction ( − ) = PH2O × PCO

process for producing hydrogen from coal. Therefore, if the producer gas from the cogasification of PC and XLT coal is used for hydrogen production (lower CO content), the equipment capacity and energy consumption of water−gas shift reaction will sharply reduce.7,8,31−33 In a word, low rank coal, especially lignite, is conducive to improve the gasification reactivity of PC and to promote the hydrogen production to produce hydrogen-rich gas during the cogasification processes. It is well-known that the H2/CO molar ratio of the producer gas from the gasification of carbonaceous materials determines its uses.34 Therefore, the effects of coal ranks and coal ratios on H2/CO molar ratios of the producer gas from steam cogasification of PC with different rank coals were investigated, as shown in Figure 9. From Figure 9, it is observed that H2/CO

where PCO2, PH2, PCO, and PH2O are respectively the partial pressures of CO2, H2, CO, and steam in the exit gases. Figure 10 shows the change of the experimental and equilibrium extent of the water−gas shift reaction with

Figure 10. Experimental and equilibrium extent of the water−gas shift reaction with increasing coal contents at the gasification temperature of 1173 K. Figure 9. Effect of coal ranks on H2/CO molar ratios of the producer gas from steam cogasification of PC with different rank coals at the gasification temperature of 1173 K.

increasing coal contents. The equilibrium constant of the water−gas shift reaction can be obtained from the literature,22,33 and the extent of the water−gas shift reaction of the experiments can calculated from the final compositions of outgoing gases. The partial pressure of H2O in the exit gases was assumed to be that of the H2O input into the reactor; this was because there are many reactions, such as the water−gas shift reaction, steam reforming of methane and carbon−water reaction during the gasification process, so the H2O involved in only the water−gas shift reaction is hard to determine.34 As can be observed from Figure 10, the extent of the water−gas shift reaction in the equilibrium state is approximately 0.77, while the extent of the shift reaction under the present experimental conditions is lower than 0.77, which indicates that the thermodynamic equilibrium is never reached under the present experimental conditions. These results agree with those reported by Kumabe et al.19 who found that the experimental extent of the shift reaction was much lower than that of the equilibrium state during the cogasification of woody biomass and coal with air and steam at the gasification temperature of 1173 K. Chang et al.37 obtained that the equilibrium of the water−gas shift reaction will be reached at temperatures near 1350 K. Therefore, the producer gas composition under the present experimental conditions is different from the equilibrium composition because of the insufficient progress of the water−gas shift reaction at a low reaction temperature. Besides, it is observed that the extent of the shift reaction in the experiments increased with increasing coal contents and then leveled off at 40% XLT coal and 80% SF coal. 3.4. The Feasibility Analysis of H2-Rich Gas Production from Cogasification of Petroleum Coke with Coal. From all above results, it can be confirmed that the cogasification of PC with low rank coal, especially lignite, can enhance the gasification reactivity of PC and the production of hydrogen-rich gas. For instance, at the gasification temperature of 1173 K, the cogasification reactivity of 60% PC with 40% XLT coal is 4.5-fold higher than that of PC gasification alone,

molar ratios of the producer gas from steam cogasification of PC and GP are about 1, while those from steam cogasification of PC with SF coal and PC with XLT coal are in the range of 1−3 and 1−5. And H2/CO molar ratios of the producer gas from steam cogasification of PC with coal blends increased with increasing coal ratios until XLT and SF coal contents are up to 40% and 80%, while that from cogasification of PC and GP blends are almost unchanged. The H2/CO stoichiometric molar ratios required for the synthesis of DME (dimethyl ether), methanol and hydrocarbon fuel are 1, 2 and 2, respectively.34,35 Therefore, the gases from steam cogasification of PC with GP coal are favorable for DME synthesis, and those from steam cogasification of PC with SF coal are favorable for the synthesis of DME, methanol, and hydrocarbon fuel, depending on SF coal ratio in the blends. The H2/CO molar ratio of the producer gas from steam cogasification of PC with XLT coal is 1−5, and when XLT content is larger than 40%, the H2/CO molar ratio of the producer gas is up to 5, which is desirable for producing hydrogen for ammonia synthesis, fuel cell, and other applications.36 It is well documented that the water−gas shift reaction (Reaction I) greatly influences the compositions of the producer gas during the steam gasification of carbonaceous materials, particularly at high temperature.19,22 The theoretical and experimental equilibrium constants of the water−gas shift reaction were calculated to identify whether the gases from the reactor is in equilibrium or not.19,22 In any chosen chemical reacting system, the equilibrium studies are used to predict the maximum possible conversion. Besides, the theoretical and experimental equilibrium constants can serve to estimate how the producer gas compositions will change with the adjustments of the process operating parameters, to choose the suitable operating conditions and to provide an ideal goal 3620

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reactivity of PC with XLT and PC with SF coal at the optimum coal contents of 40% and 80% is about 4.5- and 3.5-fold higher than that of PC gasification alone; meanwhile H2 contents in the producer gas increased by 10.55% and 12.24%. The gases from the cogasification of PC with different rank coals can be used as feed gas for the synthesis of DME, methanol, hydrocarbon fuel, and producing hydrogen depending on coal ranks and contents. Under the present experimental conditions, the thermodynamic equilibrium of the water−gas shift reaction is never reached. The cogasification of PC with low rank coal, especially lignite, might be an excellent method to use lignite with high moisture content and PC with low reactivity.

and the H2 contents increase by 12.24% (from 51.88% to 64.12%). Besides, the inferior stability of PC−water slurry and low solid concentration (about 46%) of lignite-water slurry are the main problems during the slurry preparation processes of the two materials, and coslurrying of the two materials with superior slurry natures (61% solid concentration and enough stability) can satisfy the industrial application.38 Furthermore, the PC ash is mainly composed of vanadium and nickel.39 In the reducing conditions of gasification, vanadium and nickel mainly exist in the forms of V2O3 and Ni. V2O3 has a melting point of 2213 K,39,40 which is the most refractory minerals in PC ash under reducing atmosphere. V2O3 in PC ash does not form a molten slag at the typical gasification temperature, which would cause the slagging problems of PC ash and the erosion of refractory materials of gasifier. Woosung et al.40 reported that with increasing of anthracite contents from 10% to 20% in PC− coal blends, the temperature of critical viscosity decreased from 1594 K to 1584 K; they found that the anthracite ashes can be used as a medium to capture V2O3 and to maintain a continuous flow of slag. In the study of Wang et al.,39 a certain amount of V2O3 may be dissolved in the liquid slag of coal ash for V2O3 to form an equilibrium solid solution with Fe2O3 or Al2O3 under the reducing atmosphere,41 so the flow temperature of the mixed ash sample is stable when the V2O3 content was lower than 6%. Therefore, the low melting point mineral matters present in lignite can be used as carriers of vanadium and nickel compounds in PC to effectively alleviate the refractory brick erosion of gasifier under high temperature.5,38−41 Lignite takes 17% of all Chinese coal resources, and it is another fossil fuel which is not suited for use as a gasification raw material in most entrained-flow gasifiers because of its low calorific value and low solid concentration in slurry.42 Therefore, cogasification of PC with low rank coal, especially lignite, to produce H2-rich gas can not only realize the reasonable, efficient, and clean use of PC but also broaden the use of lignite. Cogasification is a feasible and promising technology to use PC with low reactivity for H2-rich gas (up to 64.12%) production on a large scale.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86-021-64253236. Address: 130 Meilong Road, Shanghai 200237, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Basic Research Program of China (No. 2010CB227003), the National High Technology Research and Development Program of China (No. 2012AA101810), and the State Natural Science Foundation of China (No. 20876050).

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