Ž.Fuel Processing Technology 642000271–284
r locate r fuproc
Effect of calcium oxide on pyrolysis of coal in a
fluidized bed
Zhu Tingyu ),Zhang Shouyu,Huang Jiejie,Wang Yang
participateState Key Laboratory of Coal Con Õersion,Institute of Coal Chemistry,Chine Academy of Sciences,
P.O.BOX 165,Taiyuan,Shanxi 030001,People’s Republic of China
Accepted 1January 2000
Abstract
The pyrolysis of Shenmu bituminous coal in both the prence and abnce of calcium oxide was studied under atmospheric pressure over a temperature range of 450–7508C in a fluidized bed react
or.Calcium oxide significantly increas the gas yield while decreasing the tar yield.Tars produced in both the prence and abnce of calcium oxide were analyzed and compared.The results indicated that tar yields reach a maximum at 6508C for coal pyrolysis without calcium oxide,in accordance with Tyler’s results.When calcium oxide is added,the temperature of tar maximum yield is reduced to 5508C.It is also demonstrated that the ratio of H r C;benzene,Ž.Ž.toluene,xylene BTX ;phenol,cresol,xylenol PCX and other light components in tar increa Ž.while the ratio of O q N q S r C and oxide compounds in tar decrea in the prence of calcium oxide.It is noted that the maximum composition of alkanes in tar is C –C in the prence of 1420calcium oxide,while it is C –C in the abnce of calcium oxide.It results from the fracture of 815alkanes with long chains to alkanes with short chains when calcium oxide is added.q 2000Elvier Science B.V.All rights rerved.
Keywords:Calcium oxide;Coal mild gasification;Fluidized bed英语沙龙下载
1.Introduction
The coal pyrolysis reaction is the first stage of coal thermal treatment.Rearch on coal pyrolysis reaction is of primary importance for the systematic study of the coal conversion process.The fluidized bed reactor is one of the most important types of reactors and is widely ud in the coal conversion process.To reduce noxious gas )Corresponding author.
0378-3820r 00r $-e front matter q 2000Elvier Science B.V.All rights rerved.
Ž.PII:S 0378-38200000075-8
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Z.Tingyu et al.r Fuel Processing Technology642000271–284
obamaemission and environmental pollution,some rearches on fluidized bed coal gasifica-tion and combustion have been carried out.Desulfurization by calcium oxide is also ud in the industry.
Ž
The conditions heating rate,temperature,catalyst,particle diameter,the type of coal
.
and the configuration of reactor of coal mild gasification greatly affect the yields and
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composition of products.Franklin1reported that the tar yield was decread when coal is pyrolyzed in the prence of calcium oxide,a low-cost additive.He further pointed
w x out that the hydroxy group in coal can be catalyzed by calcium oxide.Longwell2 found that calcium oxide greatly affected the degree and rate of aromatic compounds
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formation.Zhu Tingyu3investigated the activation energy of coal cracking in nitrogen by non-isothermal thermogravimetry and found that,in the prence of calcium oxide,it declined by34.5%and the temperature needed for the same weight loss rate is decread
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by608C.Literature data4showed that the prence of calcium oxide during pyrolysis significantly incread the yield of gas while the yield of liquid reduced slightly. Furthermore,the coal liquids produced were deoxygenated and desulfurized.
It can be noted that calcium oxide is widely investigated.But as to how calcium oxide affects the coal conversion process is still unclear.Therefore,this work investi-gated the fluidized bed coal mild gasific
ation with calcium oxide in detail.
2.Experimental
2.1.Experimental materials
The coal sample ud in this study was Shenmu coal,a Chine highly-volatile bituminous coal.The characteristics of the coal are summarized in Table1.Both coal and calcium oxide were ground and sieved to the size of y50q75mesh.Calcium oxide of AP grade was added to coal by direct mixing and then dried at1108C for10h before each experiment run.
2.2.Experimental procedure
Experiments were conducted at atmospheric pressure,over the temperature range of
Ž.
450–7508C and fluidizing gas nitrogen velocity of0.08m r s both in the prence and abnce of calcium oxide.The minimum fluidizing velocity of coal is0.019m r s,
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determined by experiment.The solid coal or coal r CaO feed rate of about3g r min was chon.The char,which was produced in the same experimental conditions,was loaded as bed material before feeding in every experiment.
Table1
Proximate and ultimate analysis of the Shenmu coal sample
Air-dry basis Dry ash-free basis Ash fusion temperature Ž.Ž.Ž.
wt.%wt.%8C
Moisture Ash Volatiles Fixed-carbon C H N S O D S F
T T T
3.80
4.683
5.5064.5081.85 5.360.980.3311.47120012401280
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Z.Tingyu et al.r Fuel Processing Technology642000271–284273
Fig.1.Schematic diagram of experimatal apparatus.1—Coal hopper,2—char hopper,3—screw feeder,4—preheater,5—feedpipe,6—electric furnace,7—fluidized bed reactor,8—gas distribution plate,9—heating control apparatus,10—filter,11—water cooler,12—char hopper,13—beaker,14—dry ice cooler.
Fig.1shows the flow sheet of the experimental system.The fluidized-bed reactor
Ž.
was made up of a48-mm i.d.=525-mm-long stainless steel pipe.The main parts of the reactor were the solid and gas feeding systems,gas preheater and external heating system,fluidized-bed reactor,gas–liquid paration,dust removal unit,the product
Ž.
collection unit and gas analysis system.Three thermocouples T1–T3and two pressure Ž.
gauges P1–P2were mounted in various parts of the system to monitor temperature and pressure.Reactor temperature of450–7508C was controlled with108C variation using temperature controllers.The overflow system was ud to maintain a constant solid level in the reactor.Plugging o
f the feed line resulted from caking or backflow of solids up
Ž.Ž. the feed line was minimized by means of i water cooling of the feed line,and ii nitrogen blowing.In order to minimize the effect of bed material on mild gasification,
Ž
throatthe product was sampled1h late after coal feeding calculation shows that99.9%of bed
.
material can be replaced by fresh feeding sample.The produced tar and water were parated by centrifugal dimentation.
2.3.Product analysis and characterization
After the removal of entrained dust and condensing of liquid from the volatile products,the gaous product was sampled and analyzed at once.Gaous samples were taken at5-min intervals and the tar,water and char were collected cumulatively for analysis and weighed after each run.
()Z.Tingyu et al.r Fuel Processing Technology 642000271–284
274Ž.CO and other gas H ,N ,CO,CH were analyzed parately by gas chro-2224matography with thermal conductivity probe.Methane and light hydrocarbon gas Ž.C –C were analyzed by Shimadzu GC-14B gas chromatograph with hydrogen flame 25ionization probe.In the end,gas composition was calculated through methane balance.The total yield of gas was calculated through nitrogen balance.The experimental results met the requirement of experimental error becau the nitrogen produced from coal in the experiment is only 0.07%of feeding nitrogen.Under each operation condition,the ultimate and proximate analys of the char composition were determined.Tar composi-Ž.tion was detected by VG QUATTRO gas chromatography-mass spectroscopy GC-MS .The char yield,water yield and tar yield on the ash and moisture-free ba were calculated by following the three equations:
W y W A char 0Char Yield s =100%W 1y A y M Ž.
0W water Water Yield s
=100%W 1y A y M Ž.0W y W y W y W 0char water gas
一线口语价格
Tar Yield s =100%W 1y A y M Ž.
0where W is sample weight before heating,A is ash content in sample;M is water 0content in sample,W is char weight after each operation;W is water weight after char water each operation.
3.Results and discussion
3.1.Distribution of products
The yields of pyrolysis products in both the prence and abnce of calcium oxide are shown in Table 2.It can be en that tar yields reached the maximum at 6508C with Table 2
Ž.Effects of CaO addition on the yield of mild gasification products for Shenmu bituminous coal wt.%daf coal Sample
讲座免费下载Temperature Gas yield Water yield Tar yield Char yield Ž.Ž.Ž.Ž.Ž.8C wt.%wt.%wt.%wt.%Coal 450
1.70 4.52
2.4391.35550
7.0 4.40 4.0784.53600
10.42 3.77 6.1379.68650
13.35 3.017.6376.01750
20.04 3.45 4.8971.62Coal q 12%CaO
450
2.04
3.86 2.1391.97550
10.04 4.12 5.1780.67600
13.66 4.10 5.0677.18650
20.44 2.17 3.5573.8475028.37 1.78 1.6968.16
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Z.Tingyu et al.r Fuel Processing Technology642000271–284275
meet是什么中文意思
chita
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the abnce of calcium oxide.It agrees with Tyler’s results5–7.When calcium oxide is added,the temperature of maximum tar yield is reduced to5508C.The tar yield decread while the gas yield incread with increasing the temperature.
Ž.
The process of coal pyrolysis can be divided two steps:1coal™primary product q Ž.Ž.
gas H,CH and so on q char;2primary product™cond product q gas.The 24
primary product results from the fracture of chemical bonds among function groups of coal.The primary composition and yield are related to the composition of coal and the condition of heating.Both steps are incread as temperature ris.Below5508C,step Ž.Ž.
1plays more a important role than step2for mild gasification with calcium oxide.
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As the temperature increas continuously,step2becomes increasingly important,so the distribution of products with calcium oxide is different from raw coal pyrolysis.
avnw x
Yeboah et al.8have pointed out that the difference between the prence and abnce of calcium oxide keeps constant with temperature.In this experiment,however,the tar yield incread as the temperature ro.It is becau the calcium oxide ud in literature is soaked whereas it is directly mixed here.
The prence of calcium oxide significantly decread the yields of water after the temperature reached6008C.This result is attributed to two reasons.The first reason is that calcium oxide can effectively catalyze the reactions of hydroxy group in coal.The cond reason is that calcium oxide affects the following reactions:
CO q H O m CO q H
222
CO q CaO™CaCO
23
Becau calcium oxide does not come into full contact with the primary product for the resistance of mass transfer,the above reactions were limited to below6008C.Thus, water yield is high.When temperature reaches6008C or above,the vaporizable of tar on coal surface accelerated and the above reactions occurred easily,so water yields decread.
Since calcium oxide was added to coal by direct mixing,it is believed that the first step of coal mild gasification is thermal pyrolysis to produce primary products,which include a lot of condend aromatic compounds.Becau the polarity of the active site of calcium oxide can affect the p-electron cloud’s stability of condend aromatic compounds,and calcium oxide itlf has a cracking active site on both inner and outer w x
surfaces9,it is assumed that calcium oxide may catalyze tho condary reactions of primary products to reduce the tar yield and increa the gas yield.
3.2.Influence of calcium oxide on gaous product yield and composition
Fig.2displays the total gas yield as a function of temperature.It is clearly found that the prence of calcium oxide significantly incread the total gas yield.Fig.3displays
the effect of temperature on H,CO and CO yields in the prence or abnce of
2
calcium oxide.Fig.4compares the CH,C H and C–C hydrocarbon yields in the
42425
prence or abnce of calcium oxide at different temperatures.
In the abnce of calcium oxide,CH and CO yields increa as temperature ris.
4
H yield is very low below6008C,but considerably incread above6008C.This is a
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