生物质炭和富二氧化碳合成气制取二甲醚

徐 勇 颜世志 叶同奇 张 钊 李全新

(中国科学技术大学化学物理系,生物质洁净能源实验室,合肥230026)

生物质炭和富二氧化碳合成气制取二甲醚

徐 勇 颜世志 叶同奇 张 钊 李全新*

(中国科学技术大学化学物理系,生物质洁净能源实验室,合肥230026)

研究了一种利用富二氧化碳合成气和生物质炭联合制取二甲醚的方法,其过程包括两个步骤:富二氧化碳合成气调整以及调整后合成气合成二甲醚.在合成气调整过程中,利用生物质炭为原料在Ni/Al2O3催化剂上将富二氧化碳合成气调整为富一氧化碳合成气.经过800°C合成气调整后,合成气中CO2含量大幅降低而CO含量大幅提高,CO2/CO的摩尔比从原始合成气的6.33降至0.21.然后,分别用调整前后的合成气合成二甲醚,结果表明,经过调整后,C转化率得到很大的提高,二甲醚产率比调整前高4倍.本工作提供了一种可利用富二氧化碳生物质合成气制取燃料的途径,并且提供了一种新的利用生物质炭的方法.

生物质炭;二甲醚;生物质合成气调整;Ni/Al2O3催化剂;Cu-ZnO-Al2O3/HZSM-5催化剂

1 Introduction

Increasing concerns about the increasing energy demand, global climate changes,depletion of fossil fuel resources,and rise of oil price have pushed the renewable energy such as biomass energy to the hotspot topics in recent years.1,2Biomass is a rich,environmentally friendly and renewable resource which is globally available,and can be used as an alternative feedstock for energy source or chemicals.3,4As an only renewable carbon resource,biomass can be converted from solid phase into a wide range of liquid fuels(called as bio-fuels)or chemi-cals based on the thermochemical and biochemical processes, including bio-oil,bio-methanol,bio-ethanol,bio-diesel,liquid hydrocarbons(e.g.,gasoline,diesel,waxes),mixed alcohols, DME,acetic acid,and formaldehyde.5,6

To produce biofuels or chemicals from bio-syngas,the main procedures generally include the production of bio-syngas,syngas conditioning,and fuel synthesis.Bio-syngas can be produced from biomass gasification and bio-oil reforming.7,8However,these bio-syngases,generally,are rich in CO2.9In principle,the CO2-rich bio-syngas is possible for bio-fuels through CO2hydrogenation,but the fuel yield is much lower than that from CO-rich syngas.To obtain a higher fuel yield,the CO2-rich bio-syngas should be adjusted to meet the conventional fuel synthesis processes by decreasing CO2content and increasing CO content.

DME is a useful chemical intermediate for the production of many important chemicals such as dimethyl sulfate,methyl acetate,and light olefins.10As the oil resources are expected to deplete in near future,alternative fuels,such as biomass-derived DME and methanol,have been getting greater attention to minimize the emissions of global warming gases and hazardous components such as SOx,NOx,and particulate matter,and the application of DME as an alternative diesel fuel has been proposed recently.11In our previous work,attention has been paid to producing hydrogen,bio-syngas,and bio-fuels such as methanol and Fischer-Tropsch synthesis fuels from bio-oil.8,12-15Present work aims to convert the CO2-rich bio-syngas into CO-rich bio-syngas using biomass char and to efficiently produce DME from the bio-syngas.

2 Experimental

2.1 Catalyst preparation and characterization

Biomass chars were produced by fast pyrolysis of biomass in a fixed-bed reactor at a final pyrolysis temperature of 500°C. Some properties are listed in Table 1.The activated carbon(analytical reagent)was purchased from Sinopharm Chemical Reagent Co.,Ltd.in China.HZSM-5(with a SiO2/Al2O3molar ratio of 25)was provided by Nankai University.The Cu-ZnOAl2O3catalyst was purchased from Jingjiang Company in China.

Table 1 Main characteristic of the used husk char and the sawdust char

The NiO-Al2O3catalyst for the bio-syngas conditioning was prepared by the coprecipitation of the metallic nitrate mixture (Ni and Al)with K2CO3.The prepared process details were described elsewhere.16Cu-ZnO-Al2O3/HZSM-5 was prepared by physically mixing the powders(under150 meshes)of HZSM-5 and Cu-ZnO-Al2O3catalyst with a mass ratio of 1:2.17The contents of metallic oxide in the resulting samples were measured by inductively coupled plasma and atomic emission spectroscopy(ICP/AES,Atom scan Advantage of Thermo Jarrell Ash Corporation,USA).The Brunauer-Emmett-Teller (BET)surface area,pore volume,and average pore size were evaluated from the N2adsorption-desorption isotherms obtained at 77 K using a COULTER SA 3100 analyzer.X-ray diffraction(XRD)was performed to determine the bulk crystalline phases of catalyst.It was measured on an Xʹpert Pro Philips diffractrometer with a Cu Kαradiation(λ=0.15418 nm).

2.2 Reaction systems

The conditioning of CO2-rich bio-syngas was evaluated in a fixed-bed continuous-flowing quartz tube reactor at atmospheric pressure.The mixture of NiO-Al2O3catalyst and biomass char with a certain mass ratio was installed in the center of the reactor.Before reaction,the catalysts were reduced by hydrogen at 500°C for 2 h.The gases were fed into the reactor and controlled by a mass flow controller,and the effluent gases from the reactors were measured by flow display.Temperature was measured by the thermocouples inserted into the catalyst bed.DME synthesis was carried out in a fixed-bed continuousflowing stainless steel reactor.Generally,1.0 g catalyst diluted with a 2.0 mL Pyrex beads,was used for each test.Before reaction,the catalysts were reduced by hydrogen(10%(φ)in Ar)at 260°C for 6 h.Then,bio-syngas was conducted to the reactor for DME synthesis under a setup synthesis condition.H2,N2, CO,and CO2were detected by a gas chromatograph(Model: SP6890,column:TDX-01)with a thermal conductivity detector.DME,CH3OH,and hydrocarbons were detected by another gas chromatograph(Model:SP6890,column:PorapakQ-S, USA)with a flame ionization detector.Ultra-high-purity argon (99.999%)was used as the carrier gas.The performance of the bio-syngas conditioning and DME synthesis was evaluated by the following Eqs.(1-5):

where X represents DME,CH3OH,hydrocarbons,CO,CO2;m represents the carbon numbers of X;C,Y,and S represent conversion,yield,and selectivity,respectively.

3 Results and discussion

3.1 Catalyst characterizations

The nickel loading in the resulting NiO-Al2O3sample was about 18.6%(w)NiO,and CuO and ZnO loading in Cu-ZnOAl2O3sample were 63.1% and 27.3% respectively from ICP-AES results.The BET surface area,pore volume,and average pore size of the catalysts are listed in Table 2.

XRD patterns of different catalysts are shown in Fig.1.The diffraction peaks of NiO,Al2O3,and NiAl2O4were observed from the fresh catalyst.Three characteristic peaks at 2θ of 44.4°,51.9°,and 76.4°were identified from the used catalyst, corresponding to the diffractions of Ni.This indicates that NiO is reduced to the metallic Ni by H2.XRD patterns of the Cu-ZnOAl2O3catalyst are shown in Fig.1b.Phase due to CuO and its reflections at 2θ of 35.5°,38.6°,and 48.8°were observed in the fresh Cu-ZnO-Al2O3catalyst,the characteristic peaks of ZnO and Al2O3were also identified.After reduction,the characteristic peaks of Cu were identified at 2θ of 50.6°and 74.1°.In ad-dition,the typical XRD patterns from the fresh HZSM-5 and the reduced HZSM-5 suggested that the HZSM-5 structure was retained after reaction.

Table 2 BET surface area,pore volume,and average pore size of the catalysts

3.2 Effect of the mass ratio of catalyst to carbon (RC/C)on the CO2-rich bio-syngas conditioning

To further understand the effects of RC/Con the conditioning of CO2-rich bio-syngas over Ni/Al2O3,the present conditioning was carried out at various RC/Cvalues from 0 to 2.0,the reaction temperature and flow rate were kept constant at 700°C and 1200 L·kg-1·h-1,respectively.As shown in Fig.2,a further increase in RC/Cfrom 0 to 1.0 resulted in a significant increase in the CO2conversion and CO yield,but tended towards saturation when RC/Cwas over 1.0.The CO2conversion increased from about 25.4%to 58.4%when RC/Cincreased from 0 to 1.0. In particular,a slight increase in CO2conversion and CO yield was observed when RC/Cincreased from 1.0 to 2.0.The above results suggest that the performance of Ni/Al2O3catalyst for the CO2-rich bio-syngas conditioning can remarkably enhanced.The observed effects of Ni/Al2O3catalyst should be attributed to the dissociation of CO2on the surface of Ni.Ni/ Al2O3catalyst was widely used for the reforming of CH4with CO2to syngas.18-20Previous work21,22has proved the dissociation of CO2on surface of Ni(CO2(a)→CO(a)+O(a)).

3.3 Effect of temperature on the CO2-rich biosyngas conditioning

To investigate the effect of temperature on catalytic perfor-mance,the conditioning of CO2-rich syngas was carried out over Ni/Al2O3at various temperature(400-800°C),with a flow rate of 1200 L·kg-1·h-1and RC/Cof 1.Fig.3 presents the effects of temperature on the performance of the bio-syngas conditioning.It was observed that,by increasing the reaction temperature from 500 to 800°C at a constant RC/Cof 1.0,there was a sharp increase in the CO2conversion and CO yield.The CO2conversion remarkably increased from 36.2%at 500°C to 71.6%at 800°C,and CO yield increased from 23%to 90%. The main composition of the crude bio-syngas changed from V(H2)/V(CO)/V(CO2)=68.6/4.1/26.0 in the crude bio-syngas to V(H2)/V(CO)/V(CO2)=59.3/30.9/6.8 after the conditioning at 800 °C.In particular,the concentration of CH4decreases with increasing the temperature,and can not be detected over 700°C. The above results showed that the CO2conversion and CO yield were significantly enhanced by increasing temperature. The reasons are summarized as follows:firstly,both the Boudouard reaction(CO2+C=2CO)and reverse water gas shift(RWGS)reaction(CO2+H2=CO+H2O)involved in the bio-syngas conditioning are endothermic in thermodynamics.A recent work on the kinetics of char gasification with CO2claims that the increasing temperature shows a positive influence on the apparent activation energy,accompanied with an increase of gasification rate.23Accordingly,a higher temperature will accelerate the rates of Boudouard reaction and RWGS reaction,leading to an increase of converting CO2into CO in the bio-syngas conditioning.Secondly,high temperature may promote the dissociation of CO2on Ni,leading a further conversion of CO2to CO.

3.4 Effect of flow rate on the CO2-rich bio-syngas conditioning

To further understand the effect of flow rate on catalytic performance of NiO-Al2O3catalyst,the conditioning of CO2-rich bio-syngas was carried out at various flow rate(300-1500 L· kg-1·h-1),at 700°C and RC/Cof 1.0.As shown in Fig.4,at a flow rate of 300 L·kg-1·h-1),CO2conversion and CO yield were about 76.7%and 107%.With increasing the flow rate to 1500 L·kg-1·h-1,CO2conversion and CO yield decreased to 61%and 61%,respectively.The results suggest that an increase in flow rate leads to a decrease in CO2conversion and CO yield in the bio-syngas conditioning,which can be attributed to the residence time of biomass char and the CO2-rich bio-syngas in the catalyst bed is shortened by increasing flow rate.

3.5 DME synthesis

Fig.4 Effect of flow rate on CO2conversion(a),CO yield(b),gas composition(c),and n(CO2)/n(CO)(d)in the CO2-rich bio-syngas conditioningT=700°C,RC/C=1.0

For a comparison,the crude CO2-rich bio-syngas(V(H2)/ V(CO)/V(CO2)=68.6/4.1/26)and the conditioned bio-syngas V(H2)/V(CO)/V(CO2)=59.3/30.9/6.8)were tested for DME synthesis.The gas composition of the conditioned syngas was close to coal derived syngas,previous works have reported DME synthesis from coal syngas.24,25For DME synthesis from the crude CO2-rich bio-syngas,as shown in Table 3,CO2conversion increased from 5%to 15.1%with the increasing temperature from 240 to 320°C.An increase in the DME selectivity was observed when temperature increased from 240 to 280°C, but decreased with a further increasing temperature over 280°C. Table 4 presents the performance of the DME synthesis from the conditioned bio-syngas,CO conversion increased from 72.5%to 84.8%as temperature increased from 240 to 280°C, but a decreasing CO conversion was observed when temperature was higher than 280°C.The maximum conversion and DME yield were 84.8%and 65.5%respectively at 280°C.The above results indicated that the performance of the DME synthesis was significantly enhanced via the bio-syngas conditioning.

A generic accepted mechanism of DME synthesis from syngas over Cu-ZnO-Al2O3/HZSM-5 includes two sequential steps:methanol synthesis from syngas and the dehydration of methanol to DME,and the methanol synthesis reaction is the rate-limiting step in the direct DME synthesis from syngas.26Consequently,the different results of DME synthesis should be attributed to the different mechanism of methanol formation through CO hydrogenation and CO2hydrogenation.For CO hydrogenation,the mechanism of methanol formation over the Cu-based catalyst is“the formyl intermediate formation mechanism”,and the Cu crystallites in the catalyst have been identi-fied as the active catalytic sites.27Alternatively,the formation and hydrogenation of the formate intermediate are thought to be the key steps to produce methanol from CO2hydrogenation.28-30Another reason is water produced via the RWGS reaction in CO2hydrogenation may block the active sites on the catalyst surface.

Table 3 Effect of temperature on DME synthesis from the crude CO2-rich bio-syngas over the Cu-ZnO-Al2O3/HZSM-5 catalyst

Table 4 Effect of temperature on DME synthesis from the conditioned bio-syngas over the Cu-ZnO-Al2O3/HZSM-5 catalyst

Fig.5 (a)Catalysts stability during the bio-syngas conditioning,(b)catalysts stability during the DME synthesis from the conditioned syngas

3.6 Stability of catalysts

The stability of the Ni/Al2O3catalyst in bio-syngas conditioning process was tested by measuring the CO2conversion and CO yield as a function of the time on stream.As shown in

Fig.5a,no obvious changes are observed for about 50 h under the reaction conditions(T=800°C,RC/C=1.0,flow rate=1200 L· kg-1·h-1)in the conditioning process.It clearly shows that both the CO2conversion and CO yield remained essentially constant for the whole test period,which indicates that no noticeable deactivation of the catalyst is occurring.On the other hand,no noticeable deactivation of the Cu-ZnO-Al2O3/HZSM-5 catalyst was observed in DME synthesis process(Fig.5b).

4 Conclusions

This work reported a novel approach of DME synthesis from the CO2-rich bio-syngas,including two steps:the CO2-rich bio-syngas conditioning using biomass char and DME synthesis from the conditioned bio-syngas.For the bio-syngas conditioning,the CO2conversion and CO yield reached a level as high as 71.1%and 89%at 800°C,respectively,CO2/CO molar ratio significantly dropped from 6.33 to 0.21.For the DME synthesis,the maximal CO conversion and DME yield were 84.8%and 65.5%,which were much higher than those from the CO2-rich bio-syngas.Present work may provide a useful method for DME synthesis from the CO2-rich bio-syngas,and a novel utilization of biomass char.

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Dimethyl Ether Production from Biomass Char and CO2-Rich Bio-Syngas

XU Yong YAN Shi-Zhi YE Tong-Qi ZHANG Zhao LI Quan-Xin*
(Department of Chemical Physics,Anhui Key Laboratory of Biomass Clean Energy,University of Science and Technology of China,Hefei 230026,P.R.China)

We report on a novel approach toward dimethyl ether(DME)synthesis using crude CO2-rich bio-syngas and biomass char.The crude bio-syngas was derived from bio-oil reforming and was initially conditioned by catalytic conversion into CO-rich bio-syngas using biomass char over the Ni/Al2O3catalyst. The molar ratio of CO2to CO significantly decreased from 6.33 in the CO2-rich bio-syngas to 0.21 after bio-syngas conditioning at 800°C.The yield of dimethyl ether from the conditioned bio-syngas was about four times higher than that from the CO2-rich bio-syngas over the Cu-ZnO-Al2O3/HZSM-5 catalyst.This work potentially provides a useful approach toward producing biofuels and chemicals from bio-syngas and a novel utilization of biomass char.

Biomass char;Dimethyl ether;Bio-syngas conditioning;NiO-Al2O3catalyst; Cu-ZnO-Al2O3/HZSM-5 catalyst

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*Corresponding author.Email:liqx@ustc.edu.cn;Tel:+86-551-3601118.

The project was supported by the National Natural Science Foundation of China(50772107),National Key Basic Research Program of China(973) (2007CB210206),and National High-Tech Research and Development Program of China(863)(2009AA05Z435).

国家自然科学基金(50772107),国家重点基础研究发展规划项目(973)(2007CB210206)及国家高技术研究发展计划项目(863)(2009AA05Z435)资助