Hydrogen production from coal gasi?cation in supercritical water with a continuous ?owing system
Yongliang Li,Liejin Guo *,Ximin Zhang,Hui Jin,Youjun Lu
State Key Laboratory of Multiphase Flow in Power Engineering,Xi’an Jiaotong University,Xi’an 710049,China
a r t i c l e i n f o
Article history:
Received 23June 2009Accepted 7July 2009
Available online 31July 2009Keywords:Coal
Supercritical water Gasi?cation
Hydrogen production
a b s t r a c t
The technology of supercritical water gasi?cation can convert coal to hydrogen-rich gaseous product ef?ciently and cleanly.A novel continuous-?ow system for coal gasi?-cation in supercritical water was developed successfully in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF).The experimental device was designed for the temperature up to 800 C and the pressure up to 30MPa.The gasi?cation characteris-tics of coal were investigated within the experimental condition range of temperature at 650–800 C,pressure at 23–27MPa and ?ow rate from 3kg h à1to 7kg h à1.K 2CO 3and Raney-Ni were used as catalyst and H 2O 2as oxidant.The effects of main operation parameters (temperature,pressure,?ow rate,catalyst,oxidant,concentration of coal slurry)upon gasi?cation were carried out.The slurry of 16wt%coal t1.5wt%CMC was successfully transported into the reactor and continuously gasi?ed in supercritical water in the system.The hydrogen fraction reached up to 72.85%.The experimental results demonstrate the bright future of ef?cient and clean conversion of coal.
a2009Professor T.Nejat Veziroglu.Published by Elsevier Ltd.All rights reserved.
1.Introduction
Coal becomes more important both as an energy source and as the source of organic chemical feedstock in the 21st century [1].The clean and ef?cient use of coal has become one of the hot issues in energy research.Hydrogen is a clean energy with high heating value and zero emission,and will be ideal energy carrier in further.Taking into account the international oil prices unusual ?uctuations,a breakthrough is needed in the technology of hydrogen production in mass scale from coal.
The technology of coal gasi?cation in supercritical water is developed during the past two or three decades.The process takes the advantages of the outstanding dissolving capacity and mass transfer property of supercritical water to realize the rapid heating of coal,decrease of char formation and high gasi?cation ef?ciency.Coal can be converted to hydrogen-rich
gas in relatively low temperature compared with traditional gasi?cation process.Meanwhile compounds containing N,S are exhausted in liquid phase and the atmospheric pollution is greatly reduced [2].
In 1978,Modell in MIT [3]?rstly proposed that high heating value gas was produced from coal,wood and other solid or liquid organic material in supercritical water.In the 1980s and 1990s,researchers [4–6]mainly studied the extracting low-rank coal in supercritical water in the temperature of 647–773K and found that supercritical water was not only a reaction medium but also participated in the reaction.Until the beginning of this century,people began to apply super-critical water technology to the gasi?cation of coal for hydrogen-rich gas production.Wang et al [7]gasi?ed coal in supercritical water autoclave and found that the presence of Ca(OH)2facilitates the extraction of volatile matter from coal
*Corresponding author .Tel.:t862982663895;fax:t862982669033.
A v a i l a b l e a t w w w.s c i e n c e d i re ct.c o m
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /h e
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and the decomposition of the volatile matter to small mole-cule gases,with a decrease in the amount of residual char. Ca(OH)2increased the yield of hydrogen and?xed CO2.Lin et al.[8–11]gasi?ed pulverized coal and Ca(OH)2in autoclave, packed bed reactor,and continuous pipe reactor respectively. At the923K,carbon conversion of the coal in the continuous reaction of the coal/CaO mixture with steam was high as60–80%.H2fraction was higher than80vol%and only a little CH4 was produced.But Eutectic melting of Ca(OH)2/CaCO3was found to occur at973K,and this eutectic melting led to the growth of large particles of solid materials.That may cause plugging problems in the reactor.Bi[12]conversed Xiao-longtan lignite in sub-and supercritical water in an autoclave in the temperature range of623–823K and water density0–0.21g/cm3for0–60min under N2atmosphere.It was found that the key factor that affects the product distribution was reaction temperature.Yan[13]gasi?ed1wt%coal/CMC mixture in supercritical water in continuous system device with the present of0.1wt%NaOH.It was found that long resident time in the reactor favored complete gasi?cation but inhibited hydrogen production.Throughout the existing research works in the gasi?cation of coal in supercritical water for hydrogen production,the concentration of feedstock is low in most cases,and the experiment device is mainly an autoclave and unable to realize continuous and stable gas production.Focusing on this,a novel continuous-?ow system for coal gasi?cation in supercritical water was constructed in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF).Coal slurry with high concentration was successfully transported into the reactor and continuously gasi?ed in supercritical water in the system.The effects of main opera-tion parameters(temperature,pressure,?ow rate,catalyst, oxidant,concentration of coal slurry)upon gasi?cation were also carried out.
2.Apparatus and experimental procedures 2.1.Experimental apparatus
The experimental apparatus of continuous coal gasi?cation for hydrogen production in supercritical water is shown in Fig.1.The system includes reactor,heat exchanger,high pressure pumps,feeders,separator,cooler and so on.The reactor is fabricated from Hastelloy C276tubing with 17.15mm o.d.?10.85mm i.d.?1.24m length.And the reac-tor’s design temperature and pressure are800 C and30MPa, respectively.The reactor is heated to the set-point tempera-ture by six2kW electrical heaters that are around the outer surface of the reactor tube.The temperature of the reactor outer wall is measured by some?xed,type K thermocouples held on the outer wall of the reactor in good thermal contact. Two K-type thermocouples measure the?uid temperature of the inlet and the outlet of the reactor centerline.
The feeding system mainly contains a silo,one pump,two feeders and some valves.A movable piston was placed in each feeder,and water was separated from the feedstock by it. Water was pumped into the top of the feeder,and the piston’s moving forced the coal slurry into the reactor.When one feeder transported the slurry into the reactor,one feeder was under ambient pressure and biomass in the feedstock tank was pressured into the feeders by the pressured N2.The continuous feedstock transport was achieved by switching
Fig.1–Scheme of supercritical water gasi?cation system.1–water tank;2–?ush water tank;3–main?ow pump;4–auxiliary pump;5–?ush pump;6–cooler;7–preheater;8–mixer;9–reactor;10–charger;11–silo;12–?lter;13–back-pressure valve;14–gas-liquid separators;15–gas?ow meter;16–manometer;17–main?ow meter;18–auxiliary?ow meter.
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reactor.The other one is the ?ow of deionized water pres-surized to the cooler and preheater to the desired temperature and mixed with the ?ow of slurry.Flow rates of the coal slurry and pressurized water are measured by two mass ?ow meters (Endress tHauser and Micro Motion).After the mixing,the ?ow of slurry and deionized water ?ows into the reactor.The ?ow after reaction is cooled down in the cooler and ?ow through the ?lter to separate away the solid residual.At last gaseous product and liquid residue are separated in the gas–liquid separator.Flow rate of product gas is measured by the wet gas meter.
After the experiment,the reactor,preheater and cooler can be ?ushed from two opposite directions by two pumps in order to remove the residual coal slurry in the system.
2.2.Feedstocks
The feedstock Coal–Water–Slurry (CWS)is produced in Shandong Liangda CWS Company and the proximate and elementary analyzed results of the coal are shown in Table 1.The origin concentration of coal in the CWS is 67.39wt%and the CWS is diluted to desired concentration.Sodium carbox-ymethyl cellulose (CMC)is used as an additive to prevent the precipitation phenomenon caused by the dilution to guar-antee the continuous delivery at high pressure.Analytically pure anhydrous K 2CO 3is provided by Tianjin Baishi Chemical Company,and Raney-Ni is bought from Dalian Tongyong Chemical Company.30wt%analytically pure H 2O 2is purchased from Tianjin Jinbei Fine Chemical Company.
2.3.Analysis
The gas product is analyzed by a Hewlett-Packard model 6890gas chromatograph with thermal conductivity detectors.The carrier gas is Helium with the ?ow rate of 10ml/min.A Carbon-2000capillary column provided by CAS Lanzhou Institute of Chemical Physics is equipped,operating at 333K for 2min,followed by a 30K/min ramp to 423K,hold 2min.A standard gas mixture used for calibration is bought from and com-pounded by Beijing AP Beifen Gases Industry Limited Company.
The elemental analysis of the black particles and metal fragments is completed on a Bruker S4PIONEER X-ray ?uo-rescence spectrum (XRF).
3.Result and discussion
As for the main reactions in the gasi?cation process in supercritical water,it is commonly recognized that the main three reactions are concluded [14]:
C tH 2O /CO tH 2,
D H ?132kJ/mol (1)
CO tH 2O /CO 2tH 2,D H ?à41kJ/mol (2)
CO t3H 2/CH 4tH 2O,D H ?à206kJ/mol (3)
0.0
0.20.40.60.81.01.21.41.61.82.0Temperature/°C
Temperature/°C
G E H E o r C E /g ?g -1
0.040.050.060.070.08
YH2/(g ?g -1)
0G a s f r a c t i o n /%
a
b
Fig.2–The effect of temperature on gasi?cation result:(a)GE,HE,CE or YH 2;(b)Gas fraction (Feedstock,2wt%coal D 1.5wt%CMC D 1wt%K 2CO 3;Pressure,25MPa;Flow rate,5.0kg h -1).
0.0
0.20.40.60.81.01.2
1.4Temperature/°C Temperature/°C
G E H E o r C E /(g .g -1)
0.02
0.03
0.040.05
YH2/(g ?g -1)
0102030405060
70G a s f r a c t i o n /%
a
b
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The gaseous product in the experiment is analyzed to be mainly H 2,CO,CH 4,and CO 2,and small amount of C 2(C 2H 4and C 2H 6)are also detected.The content of the components (H 2,CO,CH 4,CO 2and C 2H x )in gaseous product is indicated by the volume fraction.GE (Gasi?cation Ef?ciency)and CE (Carbon gasi?cation Ef?ciency)were used to indicate the level of gasi?cation.YH 2(yield of hydrogen)and HE (Hydrogen gasi?cation Ef?ciency)are used to indicate the hydrogen production capacity of coal.ER (oxygen Equivalent Ratio)is used to express the amount of oxidant added.The relative expressions are de?ned as follows:GE ?
the total mass of mixture of gas by gasification (4)
CE ?
the mass of carbon in gaseous product the mass of carbon in feedstocks (5)
YH 2?mass of produced hydrogen the mass of feedstocks
(6)
HE ?
mass of element hydrogen in gaseous production the mass of element hydrogen in feedstocks
(7)
3.1.Effect of temperature
The effect of experimental result of hydrogen production by supercritical water gasi?cation of coal of 2wt%and 5wt%concentration with K 2CO 3as catalyst on temperature is shown as Figs.2and 3.As temperature increases,the gasi?cation ef?ciency,hydrogen gasi?cation ef?ciency,and carbon gasi?-cation ef?ciency increase sharply.In 800 C,the gasi?cation ef?ciency of slurry with 2wt%coal is 1.8099g g à1and the the data are 0.0452g g à1,0.5248g g à1and 0.2540g g à1,respectively.The CO fraction is low,less than 1%,the fraction of H 2is as high as 72.85%when concentration of coal is 2wt%,and 60%with the concentration 5%.It can be seen that high temperature has high bene?t on gasi?cation reaction.Because hydrogen is typical product from free-radical reactions;and high temperature favors free-radical reactions [15].It can be seen from reaction (1)that the gasi?cation reaction is endo-thermic,so high temperature favors gasi?cation process.What’s more,the residence time of coal particle in the reactor is limited and high temperature accelerates the gasi?cation and favors complete gasi?cation.
3.2.
Effect of pressure
The effects of pressure on the gasi?cation result of 2wt%and 5wt%coal are shown in Figs.4and 5respectively.It can be seen that pressure has no signi?cant effect on the gasi?cation of coal in supercritical water.As pressure increases,there is no obvious deviation in the gasi?cation ef?ciency and carbon gasi?cation ef?ciency.When the concentration of coal is 2wt%,hydrogen gasi?cation ef?ciency and the yield of hydrogen increase a bit.When the concentration of coal is 5%,hydrogen gasi?cation ef?ciency and the yield of hydrogen
decrease a bit.The fractions of CO and C 2are low,and the fraction of H 2is high.The fractions of H 2,CO 2and CH 4change negligibly as pressure increases.As reported by Kruse [15],hydrogen is typical free-radical product and as pressure increased,the ionic reaction is prohibited and free radical reaction is promoted.Therefore,high pressure inhibits hydrogen production.However high pressure also means high concentration of water and ionic product,so some reactions such as hydrolyze reaction can be enhanced.Due to the 23
24
25
26
27
0.2
0.40.60.81.01.21.41.61.8
Pressure/MPa
Pressure/MPa
G E H E o r C E /(g ?g -1)
YH2 /(g .g -1)
102030405060
70G a s f r a c t i o n /%
a
b
Fig.4–The effect of pressure on gasi?cation result:(a)GE,HE,CE or YH 2;(b)Gas fraction (Feedstock,2wt%coal D 1.5wt%CMC D 1wt%K 2CO 3;Temperature,7508C;Flow rate,5.0kg h -1).i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 35(2010)3036–3045
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3.3.Effect of ?ow rate
The effect of ?ow rate of the slurry upon the gasi?cation results of 2wt%coal is shown in Fig.6.As the ?ow rate increases,the hydrogen gasi?cation ef?ciency increases,and the gasi?cation ef?ciency and carbon gasi?cation ef?ciency change negligibly.The yield of hydrogen peaks when ?ow rate is 5kg h à1.The fraction of hydrogen is as high as 73.84%.The fractions of CH 4,CO 2and C 2increase as the ?ow rate
increases,and the fraction of CO is low.The ?ow rate affects the gasi?cation characteristics from two aspects.On one hand,the volume of the reactor is limited.Therefore,higher ?ow rate means shorter residence time in the reactor.It may lead to incomplete gasi?cation of the coal.On the other hand,low ?ow rate may cause bad heat transfer condition and tend to cause the sediment of coke on the inner wall of the reactor.So the suitable ?ow rate should be selected according to the reaction condition.
0.2
0.40.60.81.01.21.41.61.8Flowrate/(kg/h)G E H E o r C E /(g .g -1)
0.03
0.040.050.060.07
0.08YH2/(g .g -1)
1020304050607080G a s f r a c t i o n /%Flowrate/(kg/h)
a
b
Fig.6–The effect of ?ow rate on gasi?cation result:(a)GE,HE,CE or YH 2;(b)Gas fraction (Feedstock,2wt%coal D 1.5wt%CMC D 1wt%K 2CO 3;Temperature,7508C;Pressure,25
MPa).
0.0
0.20.40.60.81.01.2G E H E C E o r Y H 2/(g .g -1)
Different catalyst R-Ni K2CO3
102030405060G a s f r a c t i o n /%
Different catalyst
a
b
Pressure/MPa G E H E o r C E /(g .g -1)
YH2 /(g .g -1)
G a s f r a c t i o n /%Pressure/Mpa
a
b
Fig.5–The effect of pressure on gasi?cation result:(a)GE,HE,CE or YH 2;(b)Gas fraction (Feedstock,5wt%coal D 1.5wt%CMC D 1wt%K 2CO 3;Temperature,7508C;Flow rate,5.0kg h -1).
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3.4.Effects of catalyst addition
The effects of catalyst on the gasi?cation results are shown in Fig.7and it can be seen that K 2CO 3has better catalytic effect upon the gasi?cation reaction than that of Raney-Ni.The hydrogen yield is doubled when K 2CO 3is used as catalyst compared with that with Raney-Ni.Gasi?cation ef?ciency,carbon gasi?cation ef?ciency and hydrogen gasi?cation ef?-ciency with K 2CO 3are higher than those with Raney-Ni.The great catalytic effect may be due to the great enhancement of the water–gas shift reaction (2)in supercritical water.The
study of S ?nag ?[16]shows:alkaline compound through generating formate,catalyst water–gas shift reaction.What’s more,K 2CO 3is the salt that dissolves in water to form a uniform solution.Even though CMC is a stabilizing agent,Raney-Ni as a solid powder is not well distributed.
3.5.Effects of oxidant addition
As is shown in Fig.8,when oxidant is added,the gasi?cation ef?ciency and carbon gasi?cation ef?ciency increase,and the yield of hydrogen and the fraction of hydrogen are lower.
G E ,H E ,C E ,Y H 2(g .g -1)
ER
ER
G a s f r a c t i o n /%
a b
Fig.8–The effect of oxidant on gasi?cation result (a)GE,HE,CE or YH 2;(b)Gas fraction (Feedstock,2wt%coal D 1.5wt%CMC;Temperature,7508C;Pressure,25MPa;Flow rate,5.0kg h -1).
0.2
0.40.60.81.01.21.41.6
1.8Concentration/%G E H E o r C E /(g .g -1)
05YH2/(g .g -1)
2
4
6
8
10
12
14
16
G a s f r a c t i o n /%
Concentration/%
a
b Fig.9–The effect of concentration on gasi?cation result:(a)GE,HE,CE or YH 2;(b)Gas fraction (Feedstock,1.5wt%CMC D 1wt%K 2CO 3;Temperature,7508Pressure,25MPa;Flow rate,5.0kg h -1).
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Because hydrogen peroxide as oxidant reaction can in-situ heat up the reaction ?uid and temperature has been proved to be favorable for the gasi?cation reaction [2].Coal contains phenolic structures and phenols are dif?cult to be gasi?ed and are referred to the ‘‘last hurdle’’to get over for complete gasi?cation [17].Hydrogen peroxide is usually used as a source of free radicals to help decompose the phenolic compound [18,19].So the gasi?cation ef?ciency with the present of oxidant is relatively higher.Meanwhile the oxidant reacts with the combustible gaseous product,Therefore,the yield of hydrogen decreases as ER increases.When ER equals 0.3,the gasi?cation ef?ciency and carbon gasi?cation ef?ciency are 0.8346g g à1and 0.4079g g à1respectively,and the hydrogen gasi?cation ef?ciency and the yield of hydrogen are 1.0404g g à1and 0.0366g g à1.As ER increases,CH 4and C 2fractions decrease and CO 2fraction increases greatly.The yield of CO is negligible.
3.6.Effect of concentration
Fig.9shows the effect of concentration of coal on gasi?cation characteristics when K 2CO 3is used as catalyst,and the concentration of CMC is 1.5wt%.As the concentration of coal increases from 2wt%to 16wt%,the hydrogen gasi?cation ef?ciency and the yield of hydrogen decrease greatly and gasi?cation ef?ciency and carbon gasi?cation ef?ciency decrease as well.When the concentration is 16wt%,hydrogen gasi?cation ef?ciency is 0.7967g g à1,which is only 46%of the hydrogen gasi?cation ef?ciency when the concentration is 2wt%,and the yield of hydrogen is 0.022g g à1,only 36%of that when the concentration is 2wt%.The hydrogen fraction is more than 60%and the hydrogen fraction decreases when the concentration increases.The fractions of CH 4and CO 2increase as the concentration increases,while C 2and CO fractions decrease.When the concentration of coal was further increased to 20wt%,only part of the slurry could be pumped into the reactor and tens of liters of gas with the hydrogen fraction more than 60%was produced,and the experimental system can not be operated continuously.Because the mixture volume is limited and the production of char and tar in the process of incomplete gasi?cation causes blockage in the mixer.
Experimental result compared with that by Lin [9,10]and Yan [13]is shown in Table https://www.doczj.com/doc/bd4825788.html,pared with the experiment by Lin [9,10],the concentration of the feedstock is almost 3-
1020304050607080901002θ /degree 2θ /degree
07012001
SiO2(111)19.713SiO2(102)23.901SiO2(201)26.033
KAlSi3O8(-130)23.460KAlSi3O8(220)26.766KAlSi3O8(002)27.655KAlSiO4(110)34.792KAlSiO4(102)28.665KAlSiO4(101)22.382102030405060708090100
07012801
SiO2(200)23.861 SiO2(201)26.028 SiO2(210)26.724
KAlSi3O8(-130 )23.460KAlSi3O8(220)26.766 KAlSi3O8(002)27.655 KAlSiO4(101)22.382 KAlSiO4(102)28.665 KAlSiO4(110)34.792
a
b
1020304050607080
07041601
SiO2(100)19.876 SiO2(011)25.159 SiO2(112)47.149 NiO(101)37.249 NiO(012)43.287 NiO(110)62.854 C(111)26.611 C(010)43.455C(110)46.322
KAlSiO4(101)22.376 KAlSiO4(102)28.587 KAlSiO4(110)34.757 c
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fold,and the mass fraction of the catalyst in the slurry is comparatively less,only 1wt%.The gasi?cation ef?ciency of 16wt%coal is a little more than Lin when the concentration is 4.2wt%.The yield of hydrogen is only half of Lin’s,because Lin used CaO to ?x CO 2and promote water gas shift reaction.The hydrogen fraction is 77%in the present of CaO,and carbon dioxide fraction is only 2%.But the addition of CaO caused the blockage of the experiment system.CaO
regenerator makes the system complicated.Therefore,the experiment system in this paper comparatively is simple and easy to https://www.doczj.com/doc/bd4825788.html,pared with the work by Yan [13],the concentration is almost 15-fold.The yield of hydrogen is 0.0606g g à1when concentration is 2wt%,and hydrogen fraction is 68.9%,which is greatly higher than the result by Yan.
3.7.Solid residues
The exit of the reactor can be open after the experiment for maintain.Black char particles and metal fragments can be ?ushed out by water in atmospheric temperature and pres-sure.The char particles and metal fragments are collected,separated and analyzed.The elementary analysis results of black particles collected on three different experimental days compared with the original coal are shown as Table 3.It can be seen from Table 3that the elemental composition of the black particles and the original coal are different.Therefore,the assumption that the black particles are all the incompletely gasi?ed coal is excluded.The carbon and hydrogen in the char is less than original coal,while the oxygen elementary is more than original coal.Because the coal is not gasify average to every elementary and the ?rst reaction of coal in supercritical water is the extraction of fugitive constituent [5,6].It can be seen that sulfur elementary is nearly half of the original coal.It is also reported by [20]that supercritical water is an ef?cient medium for desulfurization of coal.
102030405060708090100070120032θ /degree
2θ /degree
NiCr2O4(311)35.892 NiCr2O4(511)57.559 NiCr2O4(440)63.204 Fe3O4(121)35.425 Fe3O4(103)35.537 Fe3O4(224)62.612
NiO(111)37.093NiO(200 )43.096 NiO(220 )62.586Ni(111)44.508 Ni(200)51.847 Ni(220)76.372 C(111)43.928C(220)75.292
NiCrFeO4(311)35.700 NiCrFeO4(400)43.385 NiCrFeO4(440)63.060 102030405060708090100
07012803Ni(111)44.494Ni(200)51.847Ni(220)76.378
NiO(220)62.997NiO(111)37.320NiO(200)43.363Fe3O4(220)30.362Fe3O4(440)63.166Fe3O4(311)35.765NiCr2O4(440)63.204NiCr2O4(511)57.559NiCr2O4(311)35.892NiFe2O4(220)30.298
NiFe2O4(440)63.023NiFe2O4(311)35.690a
b
1020304050607080
07041603
C(111)43.930 C(220)75.296
Fe3O4(121)35.425 Fe3O4(103)35.537 NiO(111)37.093 NiO(200)43.096 NiO(220)62.586
NiCrFeO4(440)63.060NiCrFeO4(400)43.385NiCrFeO4(311)35.700c
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XRD analysis results of the black particles are shown in Fig.10.The crystallographic orientation and position of the three strongest peaks are shown.It can be seen that the black particles may contain SiO2,KAlSiO4and KAlSi3O8.Char and NiO are also detected.SiO2,KAlSiO4and KAlSi3O8are ash residue produced in the process of coal gasi?cation.Char is due to the incomplete gasi?cation of coal.When Raney-Ni is not used as catalyst,NiO also can be detected in the reactor,which might be due to the corrosion of the reactor inner wall made of Hasterlloy C276.
The XRF analysis of the metal fragments is shown in Table 4.It can be seen that the metal fragment mainly contains Ni, O,Cr,Fe,Mo and Co,and the fraction of Ni amounts to as high as46%.In the experiment system,the reactor is made of Hastelloy C276while the reactor is made of1Cr18Ni9Ti.So the Ni fraction of46%is probably due to the corrosion of the reactor wall material Hastelloy C276.Antal encountered the same problems[21]using Hastelloy C276.
XRD analysis of the metal fragments is investigated and the crystallographic orientation and position of the three strongest peaks are shown in Fig.11.It can be seen that the metal fragments mainly contain NiO,Fe3O4and Ni,C.NiCr-FeO4,NiCr2O4and NiFe2O4are also detected.
4.Conclusions
A novel gasi?cation system of coal in supercritical water was manufacted in SKLMF.Coal slurry with high concentration was successfully transported into the reactor and continuously gasi?ed in supercritical water.The gasi?cation characteristics of coal were experimentally investigated in a tubular reactor and solid residues were analyzed.The conclusions are as follows,
(1)High temperature favors the gasi?cation of coal in super-
critical water,and the pressure has no signi?cant effect on the gasi?cation results.Flow rate has certain effect on gasi?cation results;optimal?ow rate should be selected according to the experiment condition.
(2)K2CO3has better catalytic effect than Raney-Ni.The
supercritical water partial oxidative gasi?cation of coal can improve the gasi?cation ef?ciency and carbon gasi?cation ef?ciency and decrease the production of char and tar,but the hydrogen fraction and yield of hydrogen decreases as well.
(3)As the concentration of the coal slurry increases,the
gasi?cation ef?ciency,hydrogen gasi?cation ef?ciency and carbon gasi?cation ef?ciency decrease.Continuous gasi?cation of16wt%coal and stable gas production are realized.The gasi?cation ef?ciency,carbon gasi?cation ef?ciency and hydrogen gasi?cation ef?ciency are
0.3172g gà1,0.1654g gà1and0.0220g gà1respectively. (4)Black particles collected from the reactor have less carbon,
hydrogen than the original coal,while the oxygen elementary is more than original coal.Supercritical water plays a desulfurization role in the coal gasi?cation process.Acknowledgements
This work is currently supported by the National Key Project for Basic Research of China through Contract No.2009CB220000 and the National High Technology Research and Development Program of China(863Program)through contract No. 2007AA05Z147.
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