Adsorption paration of carbon dioxide from
flue gas of natural gas-fired boiler by a novel
nanoporous b molecular basket Q adsorbent
Xiaochun Xu,Chunshan Song T ,Bruce G.Miller,Alan W.Scaroni
Clean Fuels and Catalysis Program,The Energy Institute,and Department of Energy and
Geo-Environmental Engineering,Pennsylvania State University,
209Academic Projects Building,University Park,P A 16802,USA
基数词
Abstract
A novel nanoporous CO 2b molecular basket Q adsorbent has been developed and applied in the paration of CO 2from the flue gas of a natural gas fired boiler.The nanoporous CO 2b molecular basket Q adsorbent was prepared by uniformly dispersing polyethylenimine (PEI)into the pores of mesoporous molecular sieve MCM-41.The u of MCM-41and PEI had a synergetic effect on the CO 2
adsorption.The rates of CO 2adsorption/desorption of PEI were also greatly improved.Adsorption paration results showed that CO 2was lectively parated from simulated flue gas and flue gas of a natural gas-fired boiler by using this novel adsorbent.The adsorbent adsorbed very little N 2,O 2and CO in the flue gas.Moisture had a promoting effect on the adsorption paration of CO 2from flue gas.The adsorbent simultaneously adsorbed CO 2and NO x from flue gas.The adsorbed amount of CO 2was around 3000times larger than that of NO x .The adsorbent was stable in veral cyclic adsorption/desorption operations.However,very little NO x desorbed after adsorption indicating the need for pre-removal of NO x from flue gas before capture of CO 2by this novel adsorbent.
D 2005Elvier B.V .All rights rerved.
Keywords:Adsorption paration;Carbon dioxide;Carbon questration;Flue gas;Natural gas-fired boiler;Mesoporous molecular sieve;Nanoporous
0378-3820/$-e front matter D 2005Elvier B.V .All rights rerved.doi:10.1016/j.fuproc.2005.01.002
T Corresponding author.Tel.:+18148634466;fax:+18148653248.
E-mail address:csong@psu.edu (C.Song).
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Fuel Processing Technology 86(2005)1457–
1472
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1.Introduction
In the transition towards a more sustainable energy economy,fossil fuels are likely to remain the main source of global energy supply for the foreeable future [1–3].However,related CO 2emissions are a major concern as increasing CO 2emissions has been identified as a contributor to global climate change,commonly known as the b greenhou effect Q .The continuous u of fossil fuels is dependent on the reduction of CO 2emissions.Improving the efficiency of energy utilization and increasing the u of low-carbon energy sources are considered to be potential ways to reduce CO 2emissions [4,5].Recently,CO 2capture and questration have been receiving significant attention and is being recognized as a third option [4,5].For carbon questration,the cost for CO 2capture is expected to compri about 75%of the total costs for geological or oceanic questration,with the other 25%costs attribute to transportation and injection [4].Therefore,the development of techniques for the cost-effective paration and capture of CO 2is considered to be one of the highest priorities in the field of carbon questration.
One of the major sources of CO 2emissions is the combustion of fossil fuels.In US,fossil fuel-fired electric power plants emit about 2billion tons of CO 2every year,of which constitutes about one third of the entire CO 2emission.With the discovery of incread number of natural gas fields,more electric power plants are converting to the u of natural gas.To date,all commercial CO 2capture pla
nts u process bad on chemical absorption with alkanolamine such as monoethanolamine (MEA)solvent.An example is the Fluor Econamine process.However,the liquid amine-bad process suffer from high regeneration energy,large equipment size,solvent degradation and equipment corrosion
[6].To overcome the disadvantages,veral other paration technologies,such as,adsorption,membrane and cryogenic paration have been studied [7–11].Becau of the low energy requirement,cost advantage,and ea of applicability over a relatively wide range of temperatures and pressures,adsorption paration attracts much interest.The key issue for adsorption paration is to develop an adsorbent with high CO 2adsorption capacity and high CO 2lectivity.
Recently,a new kind of high-capacity,high-lective CO 2adsorbents bad on a b molecular basket Q concept has been developed in our lab [10–12].The CO 2b molecular basket Q adsorbent can lectively b pack Q CO 2in a condend form in the mesoporous molecular sieve b basket Q and therefore exhibits a high CO 2adsorption capacity and a high CO 2paration lectivity.Adsorption paration of a simulated flue gas,which contains 14.9%CO 2,4.25%O 2and 80.85%N 2,showed that the CO 2adsorption capacity was 45ml (STP)/g-adsorbent,and the CO 2/O 2and CO 2/N 2paration
lectivity were 180and N 1000,respectively,at 758C.The CO 2adsorption capacity and the CO 2/O 2,CO 2/N 2paration lectivity of the b molecular basket Q adsorbent are all much higher than tho of the existing adsorbents,such as zeolites and molecular sieves [13–15].However,boiler flue gas contains many other gas,such as moisture,SO 2,NO x ,CO.Whether the gas will also be adsorbed by the b molecular basket Q adsorbent is not clear.On the other hand,the b basket Q material of MCM-41may not be stable under hydrothermal conditions
[16,17].The prervation of the structure of MCM-41is critically important for the CO 2adsorption paration performance of this novel adsorbent [10–12].In this paper,adsorption paration of CO 2from flue gas of natural gas-fired boiler,which contains 7.4–
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X.Xu et al./Fuel Processing Technology86(2005)1457–14721459 7.7%CO2,14.6%H2O,~4.45%O2,200–300ppm CO,60–70ppm NO x,and73–74%N2, by using the novel b molecular basket Q adsorbent is reported.The effect of other gas in the boiler flue gas,such as moisture,NO x and CO,on the adsorption paration performance and the stability of th
e CO2b molecular adsorbent Q is clarified.
2.Experimental
椰子的英语>法律解释方法2.1.Preparation and characterization of b molecular basket Q adsorbent
投资理财顾问b Molecular basket Q adsorbent(MCM-41-PEI-50)was prepared by loading50wt.% branched polyethylenimine(PEI,Aldrich,Mn=600)into the mesoporous molecular sieve MCM-41[10–12]and was ud as the adsorbent for the adsorption paration of CO2 from flue gas.In a typical preparation,100g PEI was dissolved in400g methanol under stirring for about15min,after which100g MCM-41was added to the solution.The resultant slurry was continuously stirred for about30min,and then dried at708C for16h under700mm Hg vacuum.
The CO2b molecular basket Q adsorbent before and after adsorption paration experiment was characterized by X-ray diffraction(XRD),N2adsorption/desorption, scanning electron microscopy(SEM)and pure CO2adsorption measurement.The XRD patterns were obtained on a Rigaku Geigerflex using Cu K a radiation.The N2 adsorption/desorption was carried out on a Quantachrome Autosorb1automated adsorption apparatus.The SEM images were photographed on a Hitachi S-3500N instrument.Pure CO2adsorption was measured using a PE-TGA7analyzer.The w
eight change of the adsorbent was followed to determine the CO2adsorption capacity [10,11].
2.2.Adsorption paration of CO2from flue gas of natural-gas-fired boiler
Flue gas from a natural-gas-fired boiler was generated from a D-type watertube boiler with a steam production capacity of15,000LB saturated steam/h.Particulates were removed from the flue gas by a ceramic filter before entering the adsorption bed.The temperature of the flue gas was reduced to about708C.The composition of the gas-fired boiler flue gas was7.4–7.7%CO2,14.6%H2O,~4.45%O2,200–300ppm CO,60–70 ppm NO x,and73–74%N2.
Adsorption paration of CO2from the flue gas of the natural gas-fired boiler was tested in a flow adsorption paration system[12,18].In a typical adsorption/desorption process,about30g adsorbent was placed in the central part of a stainless steel adsorption column(51mm O.D.;43mm I.D.).The particle size of the adsorbent was 0.5–1mm.The top and the bottom of the adsorption column were filled with a-alumina(~170g)to decrea the dead volume in the paration system.The adsorbent was heated up to1008C in helium atmosphere at a flow of5l/min and held at that temperature until there was no CO2detected in the effluent gas.The temperature was then adjusted to80F108C and the boiler flue gas mixture was introduced at6l/min. Generally,the adsorption was carried out for300–600s.After adsorption,the gas line
was switched to helium at a flow rate of 5ml/min to perform desorption at the same temperature.The time for desorption was 300–600s.The flow rate of the effluent gas was measured by a rotameter.The concentrations of CO 2,O 2,CO and NO x in the effluent gas were measured on-line using continuous emission monitors:model NGA 2000non-dispersive infrared CO 2analyzer;model NGA 2000paramagnetic oxygen analyzer;model NGA 2000non-dispersive infrared CO analyzer;and model NGA 2000chemiluminescence NO x analyzer (Romount Analytical).Moisture was esntially removed by two concutive cold traps before the effluent gas entered the analytic apparatus.Conquently,the measured gas concentrations were reported on a dry basis.The concentration of N 2was calculated by subtracting the concentration of CO 2,O 2,CO and NO x from the effluent gas mixture;therefore,N 2concentration during the desorption process was unknown.The analysis was carried out every 5–6s.Adsorption capacity in ml (STP)of adsorbate/g-adsorbent and desorption capacity in percentage were ud to evaluate the adsorbent.Adsorption/desorption capacity was calculated from the mass balance before and after the adsorption.The paration factor was defined as the mole ratio of the gas adsorbed by the adsorbent,(n i /n j )adsorbed ,over the mole ratio of the gas fed into the adsorbent bed,(n i /n j )feed (Eq.(1)).
a i =j ¼n i =n j ÀÁadsorbed n i =n j ÀÁfeed
:
ð1ÞBecau a -alumina slightly adsorbs the gas,a blank paration test with the column filled with the a -alumina (~210g)was carried out.This allowed for the adsorption/desorption capacity of the b molecular basket Q adsorbent to be calculated by subtracting the adsorption/desorption capacity between the paration experiment and blank experiment.
In order to investigate the effect of moisture in the flue gas and the effect of the particle size of the adsorbent on the adsorption paration of CO 2from flue gas,two experiments were performed using the simulated flue gas as adsorbate.In this adsorption/desorption process,about 2.0g of the adsorbent was placed in adsorption column (13mm O.D.and 9mm I.D.).The length of the adsorption bed was about 130mm.Before adsorption experiment,the adsorbent was heated up to 1008C in helium atmosphere at a flow of 50ml/min and held at that temperature overnight to ensure that there was no CO 2or moisture adsorbed prior to any experiment.The temperature was then adjusted to 75F 18C and the simulated dry or moist gas mixture (CO 2+N 2+O 2or CO 2+N 2+O 2+H 2O)was introduced at 10ml/min.Generally,the adsorption was carried out for 240min.After adsorption,the gas line was switched to 99.995%pure helium at a flow rate of 50ml/min to perform desorption at the same temperature.The time for desorption was 300min.The concentration of the gas in the gas mixture were measured on-line using a SRI 8610C Gas Chromatography (GC)with molecular sieve 5
A and Porapak T columns,and with TCD detector.The analysis is carried out every 5min or 15min when simulated dry flue gas or simulated moist flue gas was ud,respectively,as adsorbate.Gas flow rate was measured every 5min during the adsorption/desorption process.Adsorption/desorption capacity and paration factor were calculated from the method described above.
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3.Results and discussions
3.1.Preparation and characterization of b molecular basket Q adsorbent
XRD patterns of MCM-41before and after loading 50wt.%PEI are compared in Fig.1.The diffraction patterns of MCM-41did not change after PEI was loaded,which indicated that the structure of MCM-41was prerved.However,the intensity of the diffraction patterns of MCM-41changed.The intensity of the diffraction patterns of MCM-41decread after PEI was loaded,which was caud by the pore filling and indicated that PEI was loaded into the pores of MCM-41[10,11].The pore structure analysis by nitrogen adsorption/desorption confirmed that PEI was loaded into the pore channels of t
he MCM-41support.Completely degasd MCM-41showed a type IV isotherm.After loading the PEI,the mesoporous pores were completely filled with PEI resulting in a type II isotherm,restricting the access of nitrogen into the pores at the liquid nitrogen temperature.The surface area and pore volume of MCM-41before and after loading PEI are shown in Fig.2.The surface area,pore volume and average pore diameter of MCM-41were 1480m 2/g,1.0ml/g and 2.75nm,respectively.The residual pore volume was only 0.011ml/g,the surface area was estimated to be 4.2m 2/g and the average pore diameter was smaller than 0.4nm,for MCM-41-PEI-50.The morphology of MCM-41and MCM-41-PEI-50was photographed by SEM as shown in Fig.3.The particle size of the MCM-41support was 5–10A m.The
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2 Theta (degree)
小娃娃简笔画I n t e n s i t y MCM-41
MCM-41-PEI-50 (Before experiment)MCM-41-PEI-50 (After experiment)X 4
X 4
Fig.1.XRD patterns of MCM-41and MCM-41-PEI-50before and after adsorption paration experiment.X.Xu et al./Fuel Processing Technology 86(2005)1457–14721461
