Co-gasification of coal–biomass blended char with CO 2at temperatures of 900–1100°
C
Hyo Jae Jeong,Sang Shin Park,Jungho Hwang ⇑
Department of Mechanical Engineering,Yonsei University,50Yonsei-ro,Seodaemun-gu,Seoul 120-749,Republic of Korea
h i g h l i g h t s
Co-gasification of coal–biomass blended char with CO 2was conducted. Reactive synergy of the char was observed with the biomass blended char.
The volume reaction model,shrinking core model,and random pore model were used.
The activation energy and pre-exponential factor of the char gasification reaction were obtained.
a r t i c l e i n f o Article history:
Received 28February 2013
Received in revised form 23July 2013Accepted 7August 2013
Available online 23August 2013Keywords:
Co-gasification Coal Biomass Reactivity Synergy
a b s t r a c t
Co-gasified biomass and coal can be considered a potential fuel base for gasification and offers the advan-tage of a reduction in CO 2emissions.In addition,use of biomass could contribute to a reduction in fossil fuel dependency.In this study,a mixture of coal and biomass was placed in a lab-scale tube furnace and co-pyrolyzed to be transformed to a char under nitrogen atmosphere.Three mass ratios of coal and bio-mass,4:1,1:1,and 1:4,were tested,and then each char was co-gasified with CO 2after the furnace was isothermally maintained.Three isothermal conditions of 900,1000,and 1100°C were tested.In every test the carbon conversion ratio was calculated from the CO concentration measured at the exit of the fur-nace.The reactivity of char was improved with an increasing amount of biomass owing to the catalytic effect of the alkaline minerals included in the biomass.In addition,reactive synergy was observed with the biomass blended char and increased with the amount of biomass utilized.The volume reaction model (VRM),shrinking core model (SCM),and random pore model (RPM)were used to interpret the carbon conversion data.The overall fitting extent of the RPM was slightly better than that of the VRM and SCM,thus the RPM was adopted to derive reaction rate constants.For each coal–biomass ratio in the mix-ture,the activation energy and pre-exponential factor were determined using the Arrhenius equation.
Ó2013Elsevier Ltd.All rights reserved.
1.Introduction
The gasification process converts solid or liquid hydrocarbon feedstocks into synthesis gas that is suitable for use in electricity production or for the manufacture of chemicals,hydrogen,substi-tute natural gas (SNG),and transportation fuels.Gasification will be the heart of a new generation of energy plants for the capture of carbon dioxide.Coal gasification can therefore be regarded as clean coal technology (CCT)where the good thermodynamic per-formance of a power plant is joined with control of pollutant emis-sions (mainly CO 2emissions)[1,2].
Biomass is one environmentally friendly fuel.However,the sea-sonal nature of biomass availability makes energy production from biomass-only power plants difficult.Therefore,it might be prefer-able to utilize biomass in conjunction with coal for combustion and gasification [3].Co-gasified coal and biomass can be considered a potential fuel base for gasification and offers the advantage of a reduction in CO 2emissions,and even a net reduction,if CO 2cap-ture is incorporated as part of the process [4,5].In addition,use of biomass could contribute to the reduction of fossil fuel depen-dency [6].Gasification can be divided into two main stages:pyro-lysis and char gasification.Since char gasification determines the final conversion achieved in the process [5],the design of co-gasi-fication reactors requires an understanding of the reactivity of coal–biomass blended char.
Assuming there is no interaction between coal char and bio-mass char during the gasification process,carbon conversion at time (t )of blended char achieved through co-gasification could be predicted by an algebraic calculation using the blending ratio of biomass char in the blended char if the carbon conversion at time (t )of individual coal and biomass chars are known [3,7,8].If the experimental reactivity is higher than the calculated value,it could be said that synergy exists.
0016-2361/$-see front matter Ó2013Elsevier Ltd.All rights reserved./10.1016/j.fuel.2013.08.015
Corresponding author.Tel.:+82221232821;fax:+8223122821.
E-mail address:hwangjh@yonsei.ac.kr (J.Hwang).
For the past decade,studies on the co-gasification of coal and biomass blended char with CO2have been reported.Kajitani et al.gasified blended char derived from bituminous coal and cedar bark at a mass ratio of7:3with CO2using a thermal gravimetric analyzer(TGA)at850and950°C and using a drop tube furnace (DTF)at1200and1400°C.The blended char was prepared at a pyrolysis temperature of1400°C.Synergy was not observed at the high temperature conditions of1200and1400°C,while a little improvement in the char gasification reactivity of the mixtures was found at the low temperature conditi
ons of850and950°C [7].Gao et al.gasified blended chars derived from Genesee coal and sawdust at mass ratios of67:33,50:50,and33:67with CO2 using a TGA at800°C.Each blended char was prepared at four dif-ferent pyrolysis temperatures of700,1100,1250,and1400°C.It was found that the reactivity of the blended char increased com-pared to that of coal char when the pyrolysis temperature was 700°C.In addition,the reactivity increased with the amount of biomass.However,when the pyrolysis temperatures were1100, 1250,and1400°C,the reactivity of the blended char decreased when the proportion of biomass was larger than50%[4].Yuan et al.gasified blended chars derived from bituminous coal and rice straw at mass ratios of4:1,1:1,and1:4with CO2using a TGA at 1000°C.Each blended char was prepared at a pyrolysis tempera-ture of1200°C.Synergy was only observed during gasification of the4:1(coal:biomass)sample[3].
These previous studies reported the effect of co-gasification of blended char on reactivity.In the studies by Yuan et al.[3]and Gao et al.[4],the relationship between the reactivity and the amount of biomass was non-linear since relatively high pyrolysis temperatures were chosen for those cases.According to Yuan et al.[3],the char surface structure was crushed during pyrolysis at high temperature.This morphology change affected the gasifica-tion reactivity of the char.Therefore,selecting an appropriate pyro-lysis temperature is important when investigating the gasification reactivity of char.
In the present study,Shinhwa coal and pine sawdust were used as raw materials.Shinhwa coal was blended with pine sawdust at mass ratios of4:1,1:1,and1:4.Each blended sample was co-pyro-lyzed under a nitrogen atmosphere in a tube furnace reactor to be transformed to char when the reactor was gradually heated up to 1000°C.Then the char was co-gasified under a CO2atmosphere at different temperatures of900,1000,and1100°C.A real-time gas analyzer was used to measure the time variation of CO produc-tion for carbon conversion calculation.
Kinetic studies of coal–biomass blended char gasification were conducted,and to our knowledge,this is thefirst time kinetic con-stants have been obtained.The volume reaction model(VRM), shrinking core model(SCM),and random pore model(RPM)were used to interpret the experimental data.For each model,the acti-vation energy and pre-exponential factor of the coal–biomass blended char-CO2reaction were determined using the Arrhenius equation.
2.Kinetic modeling
For kinetic modeling,the carbon conversion ratio,X,is defined as the ratio of the gasified char at any time,t,to the initial char (on a dry),as given below.
X¼m0Àm
m0Àm a
ð1Þ
where m0is the initial mass of char,m a is the mass of ash in the ini-tial char,and m is the mass of char at time t.
The gasification rate is based on the main reaction of C+CO2?2CO.In this study,the CO production with time during the gasification process was continuously measured at a time interval of2s in order to estimate the weight loss of char.The car-bon conversion(X)at a certain reaction time,t n,can be defined by the following equation[9]:
Xðt nÞ¼
R t
½CO dt
R total
½CO dt
ð2Þ
where X(t n)is the carbon conversion at t n,and CO is the volumetric concentration of CO.
The rate of conversion(reactivity or reaction rate)is
dX
dt
¼kðT;p CO
2
ÞfðXÞð3Þ
where k is the rate constant based on the reaction temperature,T,
and the partial pressure of CO2,p
CO2
.
f(X)is a kinetic-model depen-dent function.Assuming that the partial pressure of CO2remains constant during gasification,the reaction rate of gasification can be represented by the Arrhenius equation as
k¼A expÀ
E
RT
ð4Þ
where A,R,and E are the pre-exponential factor(1/s),universal gas constant(8.314J/mol K),and activation energy(kJ/mol), respectively.
The gasification of char particles can be represented through a few models.In this study,the following three models were applied for studying the reactivity of char:volume reaction model(VRM), shrinking core model(SCM),and random pore model(RPM).Each model yields a different formulation of the term,f(X).The VRM as-sumes a homogeneous reaction throughout a char particle[10].In the VRM,the reaction rate is described as follows:
d X
¼k VRMð1ÀXÞð5ÞThe SCM assumes that the reaction initially occurs at the exter-nal surface of the char and gradually moves inside.At the interme-diate conversion of the solid,there is a shrinking core of non-reacted solid[11].The reaction is described as follows:
dX
dt
¼k SCMð1ÀXÞ2=3ð6ÞThe RPM considers the overlapping of pore surfaces,which re-duces the area that is available for reaction[12].The reaction is de-scribed as follows:
dX
¼k RPMð1ÀXÞ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
½1Àw lnð1ÀXÞ
q
ð7ÞThis model can predict the maximum value of reactivity as the reaction proceeds because it simultaneously considers the effects of pore growth during the initial stages of gasification and the destruction of pores due to the coalescence of adjacent pores. The RPM contains two parameters,k RPM and W,which is a param-eter that is related to the pore structure of the non-reacted sample. Given the variation of the carbon conversion with time,W can be obtained from the experimental carbon conversion values as fol-lows[12]:
w¼
2
max
ð8Þ
where X max represents the carbon conversion at which the reaction rate becomes the maximum.
466H.J.Jeong et al./Fuel116(2014)465–470
3.Experimental methods
3.1.Preparation of samples
Shinhwa coal and pine sawdust were used as coal and biomass, respectively.Shinhwa coal has commonly been used as a fuel in power plants in Korea,and pine sawdust has been used as a raw material for afluidized-bed gasifier at the Korea Institute of Indus-trial Technology(KITECH).The sizes of coal and biomass were60–70l m and1mm,respectively.In order to understand characteris-tics of the samples,proximate and ultimate analyses were con-ducted.X-rayfluorescence(XRF)analysis was also conducted to determine the mineral compounds present in the samples.Table1 shows the results of the proximate and ultimate analyses.Table2 shows the results of the XRF analysis.The lower calorific value of the sample(LHV)was calculated using Dulong’s formula[13]as follows:
LHV¼HHVÀ600ð9HþWÞð9Þwhere H and W are the weight percentages of hydrogen and mois-ture in the sample,respectively.The higher calorific value of the sample(HHV)was measured with an oxygen bomb calorimeter.
3.2.Experimental setup
A schematic of the experimental setup is shown in Fig.1.A fixed-bed reactor with an alumina tube with an ID of50mm and a total length of700mm was heated by a silicone electric heater. An R-type thermoc
ouple was inserted into the reactor for monitor-ing the gas temperature inside the reactor.
Product gases resulting from gasification were measured by a real-time gas analyzer(ABB2010,ABB)equipped with non-disper-sive infrared(NDIR)CO/CO2sensors,which has a response and good accuracy and saves data at a time-interval of2s.The gas ana-lyzer detects CO($30,000ppm)and CO2($50%)at resolutions of 1ppm and0.01%,respectively.
3.3.Pyrolysis process
Shinhwa coal was blended with pine sawdust at mass ratios of 4:1,1:1,and1:4.A mixture(500mg)was placed at the center of the reactor.Then,the sample was pyrolyzed by heating the mix-ture to1000°C at a heating rate of20°C/min and maintaining this temperature for60min to complete the pyrolysis.During the pro-cess,N2gas(99.999%)wasflowing into the reactor at a rate of5L/ min.According to Yuan et al.,biomass is much easier to crush to very small particles than coal during the pyrolysis process[3]. Therefore,to prepare blended char with original morphology,the morphology of the biomass should not be changed during the pyrolysis process.In a study by Dall’Ora et al.,a pyrolysis temper-ature lower than1100°C and a heating rate of10–20°C/min were recommended for preparation of pine wood char so that the char morphology might not change from its original state[14].In the study of Chen et al.,yiel
ds of char,tar,and gas products from coal pyrolysis were changed when the pyrolysis temperature was lower than900°C[15].Therefore,the pyrolysis temperature of1000°C was selected in this study.The mass of coal char obtained at the pyrolysis temperature of1000°C was313mg which was similar to that of the coal char(311.1mg)calculated by the results of prox-imate analysis.
3.4.Char gasification process
After the blended char was obtained by the pyrolysis process, the char was gasified under isothermal conditions at900,1000 and1100°C.When the desired gasification temperature was reached,theflow of N2(5L/min)was replaced by theflow of a mix-ture of N2(3L/min)and CO2(2L/min,99.999%).Both gases were premixed through the mixing chamber and delivered to the reac-tor.In every test,the carbon conversion ratio(X)was obtained at time(t)from the CO concentration measured in real time at the exit of the reactor.The VRM,SCM,and RPM were applied to the data of d X/d t to obtain the reaction rate constant.
An additive model was employed in order to evaluate the syn-ergy during the CO2co-gasification of blended char.According to the additive model,the co-gasification characteristics for all blend conditions are deducible from the gasification characteristics of each pure sample.In this study,the‘‘calculated’’car
bon conversion at time(t)of the blended char was obtained from the following equation:
X cal¼X CÂð1ÀF BiomassÞþX BÂF Biomassð10Þwhere X C and X B are the obtained carbon conversions at time(t)for coal char and biomass char,respectively.F Biomass is the mass frac-tion of biomass char in the blended char.
F Biomass¼
m pðBÞ
m pðCÞþm pðBÞ
ð11Þ
where m p(B)and m p(C)are the masses of biomass char and coal char, respectively.The masses can be obtained from the mass fractions of fixed carbon and ash in each sample according to the results of proximate analysis.
Table1
Properties of coal and biomass.
Coal Biomass
Proximate analysis(wt%,air-dried)Fixed
carbon
55.0416.59
Volatile31.9069.53 Ash7.18 2.80 Moisture 5.8711.09
Ultimate analysis(wt%,dry)C76.3647.73
H 4.52 6.06
O10.4141.24
N0.980
S0.100
Ash7.63 4.97
HHV/LHV(kcal/kg)7,171/
6,891
3,946/
3,552
Table2
XRF analysis of coal and biomass.
Compound Coal(%)Biomass(%)
SiO219 4.0
K2O 1.9 6.91
CaO33.955.3
TiO2 2.7 1.6
Cr2O30.42 2.2
MnO0.797.22
Fe2O330.1 2.7
NiO0.260.44
CuO0.540.67
Al2O3 5.1–
SO3 4.3–
SrO0.59–
ZrO20.20–
H.J.Jeong et al./Fuel116(2014)465–470
467
4.Results and discussion
Data indicating 100%carbon conversion for all coal–biomass mass ratios for the entire range of temperatures (900–1100°C)were acquired experimentally.Fig.2presents plots of the carbon conversion ratio (X )versus reaction time (t )for five mass ratios at 1000°C.The biomass char had much higher reactivity compared to the coal char,and the blended char also had higher reactivity compared to the coal char.Several studies have suggested that the reason for this finding is that some alkali metals inherent in biomass have an effect on the gasification reactivity of coal char [7].During gasification,the biomass char is able to adhere to the surface of the coal char,and gasification of the coal char might be influenced by the catalytic effect of residual minerals from the biomass char after the biomass char is converted to syngas [3].The alkali metals have been considered as effective catalysts for H 2O and CO 2gasification of carbon through the formation of a so-lid–solid
interface between potassium (K )and char [16].According to Huang et al.,alkali metal is inclined to form intercalation com-pounds with carbon,such as –KCK,which increase the interlayer distance and cause volume expansion.The C A C bonds existing be-tween layers are weakened and the gasification reaction is en-hanced [17].Therefore,it is expected that the appreciably higher reactivity of coal–biomass blended char in this study was due to the amount of alkali metal of biomass char.As can be s
een from the XRF results,which are listed in Table 2,the fraction of alkali
metals such as K in biomass char was higher than that in coal char.Hence,biomass is regarded as a major contributor to the enhanced reactivity of the co-gasification process.
In order to explain the gasification reaction,data for carbon conversion from X =0to X =0.95were used and applied to the VRM,SCM and RPM.Fig.3shows the plots of the VRM,SCM and RPM at 1000°C for five coal–biomass mass ratios.The correlation coefficients (R 2)of the three different models for the five coal–bio-mass mass ratios are listed in Table 3.In this study,the overall fit-ting extent of the RPM was slightly better than those of the VRM and SCM.Therefore,the RPM was adopted to derive the reaction rate constants based on the time variation results.
For application of the RPM,it was necessary to calculate the pore structure parameter,W .X max should first be determined for the calculation of W (see Eq.(8)).For example,Fig.4plots the experimental data of d X /d t versus X at the coal–biomass mass ratio of 1:4for three different gasification temperatures.For each gasifi-cation temperature,the value of X max was indicated where the plot peaked.Table 4shows five different values of W ,each of which represents the averaged value for the three different temperatures for each of the five coal–biomass mass ratios.
The study by Kim et al.[9]confirmed that the mass transfer,which includes both the diffusion inside the bed of coal char parti-cles and that inside the pores of the single particle,is negligible and the gasification rate is controlled by chemical reactions when the reaction rate is less than 0.0082s À1.In this study,the maximum reaction rate of co-gasification was 0.0038s À1at a coal–biomass mass ratio of 1:4and a gasification temperature of 1100°C.There-fore,the co-gasification reaction of blended char in this study was controlled via chemical reaction.In this study,the initial gasifica-tion rate,k RPM ;ð¼d X =d t j X ¼0Þwas obtained by linear regression of
the data of ð2=W Þ½f 1ÀW ln ð1ÀX Þg 1=2
À1 versus time (t )from X =0to X =0.95and was used as the index to represent the gasifi-cation reactivity of different coal–biomass mass ratios for quantita-tive comparison.Arrhenius plots for deriving the kinetic parameters of the reaction rate constant for the coal–biomass mass ratios are illustrated in Fig.5.The linear relationships between ln k RPM and 1/T indicated that the reaction followed the Arrhenius law,and R 2of each curve was higher than 98%.From the straight line fit of ln k RPM versus 1/T ,the activation energy E and the pre-exponential factor A were obtained for the five coal–biomass mass ratios.The activation energy and pre-exponential factor values are listed in Table 4.The activation energy of Shinhwa coal char ob-
Fig.1.Schematic of the experimental setup.
conversion ratio (X )vs.reaction time (t )at 1000°C.
tained in this study was 136.37kJ/mol,which is similar to the va-lue obtained with the SCM (156.8kJ/mol)[9].The activation en-ergy of biomass (pine)char was 100.60kJ/mol in this study,and this is consistent with values reported in previous works.Accord-ing to Blasi [18],the activation energy of biomass char was typi-cally within the range of 80.3–261kJ/mol.Seo et al.[19]reported that the activation energy of biomass (Pinus densiflora for.Multicau-lis )char was 134kJ/mol.As shown in Table 4,the activation energy decreased as the amount of biomass in the blended char increased.In the study by Yuan et al.,synergy was not observed only for coal–biomass mass ratios of 1:4and 1:1,while it was observed at a coal–biomass mass ratio of 4:1.They prepared the blended
Table 3
Correlations (R 2)with the VRM,SCM,and RPM.Temp.
Coal Coal:Biomass =4:1Coal:Biomass =4:1Coal:Biomass =1:4Biomass VRM
SCM RPM VRM SCM RPM VRM SCM RPM VRM SCM RPM VRM SCM RPM 900°C 0.920.980.980.940.99 1.000.96 1.00 1.000.990.990.990.900.970.991000°C 0.910.970.980.940.99 1.000.96 1.00 1.000.980.990.990.860.940.971100°C 0.980.990.990.960.99 1.000.960.99 1.000.960.990.990.850.920.95Average
0.94
0.98
0.98
0.95
0.99
1.00
0.96
1.00
1.00
0.98
0.99
0.99
0.87
0.94
0.97
Table 4
Kinetic parameters.
W
A (1/s)E (kJ/mol)Coal
2.337 2.69Â102
136.37Coal:Biomass =4:1 2.418 1.34Â103152.11Coal:Biomass =1:1 2.501 5.67Â102140.15Coal:Biomass =1:4 2.852 6.40Â102138.30Biomass
5.149
3.49Â101
100.60
reaction massH.J.Jeong et al./Fuel 116(2014)465–470469
版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。
发表评论