Liang_Bench-Scale-Testing-of-Zinc-Ferrite-Sorbent-for-Hot-Gas-Clean-up_2007

Liang_Bench-Scale-Testing-of-Zinc-Ferrite-Sorbent-for-Hot-Gas-Clean-up_2007
Liang_Bench-Scale-Testing-of-Zinc-Ferrite-Sorbent-for-Hot-Gas-Clean-up_2007

Journal of Natural Gas Chemistry16(2007)204–209

Article

Bench-Scale Testing of Zinc Ferrite Sorbent for Hot Gas Clean-up

Meisheng Liang1,2?,Hongyan Xu2,Kechang Xie2

1.College of Environmental Science and Engineering,Taiyuan University of Technology,Taiyuan030024,Shanxi,China;

2.Key Laboratory for Coal Science and Technology of Shanxi Province and Ministry of Education,

Taiyuan University of Technology,Taiyuan030024,Shanxi,China

[Manuscript received January5,2007;revised March13,2007]

Abstract:Advanced integrated gasification combined cycle(IGCC)power generation systems require the development of high-temperature,regenerable desulfurization sorbents,which are capable of remov-ing hydrogen sulfide from coal gasifier gas to very low levels.In this paper,zinc ferrites prepared by co-precipitation were identified as a novel coal gas desulfurization sorbent at high temperature.Prepara-tion of zinc ferrite and effects of binders on pore volume,strength and desulfurization efficiency of zinc ferrite desulfurizer were studied.Moreover,the behavior of zinc ferrite sorbent during desulfurization and regeneration under the temperature range of350?400℃are investigated.Effects of binders on the pore volume,mechanical strength and desulfurization efficiency of zinc ferrite sorbents indicated that the ad-dition of kaolinite to zinc ferrite desulfurizer seems to be superior to other binders under the experimental conditions.

Key words:zinc ferrite sorbent;binder;hot gas clean-up;sulfidation;regeneration

1.Introduction

New emerging technologies for highly efficient and clean generation of electric power from coal,such as integrated gasification combined cycle(IGCC)[1] demand that gas desulfurization undertake at high temperatures(600℃–700℃)and sulphur compound levels lower down to a few ppmv,for example,for feeding molten carbonate fuel cells(MCFC)[2].

Zinc ferrites exhibit a high reactivity in the sulfidation process at600?650℃,on which H2S con-centration in the outlet gas from a fixed-bed reactor decreases to only1?3ppmv,and sulfur capacity of sorbent at breakthrough is close to theoretical capac-ity(high sorbent efficiency).Additionally,these sor-bents have the advantage of easy regeneration in ox-idant atmosphere,and consequently,they are poten-tially suitable used as regenerable sorbents.However,in multi-cycle tests they show a progressive decay in the sulfidation reactivity[3],loss of sorbent efficiency [4]and degradation of the mechanical properties,es-pecially in fluidized-bed reactors[5].

The progressive decay in performance of zinc fer-rites is usually associated with an apparent lack of stability in sulfidation under the reducing power of coal gas,and incomplete conversion in regeneration, is due to the presence of sulphates and excessive ther-mal sintering accumulated in successive cycles.How-ever,the effect of these factors on the total sorbent decay has been studied only qualitatively.

In order to improve the sorbent performance in multi-cycle tests,the addition of some inorganic ox-ides has been proposed.The addition of TiO2may increase the stability of ZnO[6]through the forma-tion of different potential mixed oxides[7].How-ever,by using different physical techniques,recent

Journal of Natural Gas Chemistry Vol.16No.22007

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studies [8,9]have shown that this effect is mainly due to the formation of the spinel-structure compound Zn 2TiO 4.

IGCC has been regarded as one of national key foundation research program,that is “973”program in China and hot gas clean-up is one of key tech-nology of the whole process.The said “973”pro-gram demands that the sorbents have high desulfur-ization efficiency in temperature range of 300?550℃,and the high efficient desulfurization process should be adaptable to dust catcher operation temperature.Considering this new challenge,the sorbent should have high desulfurization efficiency at 350?550℃in sulfidation process and good regeneratible properties at above 600℃to lustrate sulfate in regeneration process.In this paper,zinc ferrites prepared by co-precipitation were identified as a novel coal gas desul-furization sorbent at high temperature.Preparation of zinc ferrite and effects of binders on pore volume,mechanical strength and desulfurization efficiency of zinc ferrite desulfurizer were studied.Moreover,the behavior of zinc ferrite sorbent during desulfuriza-tion and regeneration in the temperature range of 350?400℃is investigated.2.Experimental

2.1.Preparation of sorbent

The composition of the studied sorbents was des-ignated as ZF (1:1)(the molar ratio of component oxides in the correct order in parenthesis).Because the preparation of sorbent is the foundational part of the test and key technology for evaluating and investi-gating the properties of desulfurization sorbent,there are a number of sorbent manufacturing techniques in-volved,including spray drying,impregnation,crush-ing and screening,and granulation [10].It is gen-

erally considered that co-precipitation brings about the formation of catalytic active center between the reactant components through their combination with each other,so co-precipitation technique was selected to prepare the sorbents.The process is described as follows:

A:Making up solution.Zn(NO 3)2(AP)and Fe(NO 3)3(AR)in the calculated ratio of metal ox-ides were put into a beaker,then water was dropped into the beaker to prepare the saturated salt solution.

B:Preparation of co-precipitation powder.Satu-rated iron nitrate and zinc nitrate solution and am-monia (about 17vol%)were put into a specific vessel together with the same dropping velocity.The pH value was measured by acidity instrument every 10minutes in the process in order to maintain the fixed pH value about 9?10.

C:Co-precipitation powder management.The co-precipitation solution was dumped into a beaker and followed by boiling,aging,filtrating and drying at 120℃for 12h.

D:Shaping and calcining of Sorbent.Some binder was mixed with the co-precipitation powder and wa-ter was added to the powder mixture in order to ob-tain a paste with correct viscosity,and the paste was extruded in a syringe.The cylindrical sorbent extru-dates were approximately 3mm in diameter and 3mm in height.Then the sorbent was put into a muffle at 750℃for 8h in order to form a spinel structure and possess high reactivity and mechanical strength.

Six sorbents designated as ZFD4,ZFD5,ZFD6,ZFD7,ZFD8,ZFD9were prepared according to the above method.

2.2.Structural properties of the sorbent The chemical composition and structural proper-ties of zinc ferrite sorbent are shown in Table 1.

Table 1.The chemical composition and structural properties of zinc ferrite sorbent

ZnO:Fe 2O 3SiO 2:Al 2O 3Compressive strength

Pore volume Sample

(mole ratio)

(mole ratio)

Binder

(N ·cm ?1)

(ml ·g ?1)

206Meisheng Liang et al./Journal of Natural Gas Chemistry Vol.16No.22007

2.3.Preparation for reactant gas

The reactant gas of H2S was prepared by injecting some quantity of high-purity H2S gas into an evacu-ated cylinder together with high-purity N2.Other reactant gases such as H2,CO,CO2,N2,O2were supplied in cylinders.The reactant system was kept at about50℃.

2.4.Performance tests

Sorbent performances in successive sulfidation-regeneration tests were conducted in a quartz upflow fixed-bed reactor(19mm in diameter,about650mm in length).The sorbent was located in the isothermal region of the reactor and the reactant temperature was monitored using a type-k thermocouple located approximately at the center of the sample.Analy-sis of the outlet and inlet gas from the reactor was carried out using a gas chromatograph with a flame photometric detector(FPD)and for low concentra-tions sulfur compounds its sensitivity was higher than 5ppmv.

According to our“973”program,the reactant gas is a simulated Texaco gas,the reactant gas composi-tion in sulfidation and regeneration is shown in Ta-ble2.The nominal test condition is shown in Table3.

Table2.The reactant gas composition used in

sulf idation and regeneration

Gas composition Sulfidation(%)Regeneration(%)

Space velocity(h?1)20003000

Line speed(mm/s)35.444.2

Temperature(℃)350,400400?750 Desulfurizer volume(ml)2020

Size of desulfurizer(mm)φ3×3φ3×3 Pressure atmospheric pressure atmospheric pressure

In the process for sorbent activity evaluation, sulfur capacity=(mass of adsorbed sulfur on sorbent/mass of sorbent)×100%.Desulfurization efficiency=(sulfur concentration in inlet gas–sul-fur concentration in outlet gas)/sulfur concentration in inlet gas.

3.Results and discussion

3.1.Desulfurization reactivity of zinc ferrite sorbent

Figure1shows the ratio of H2S in outlet gas and inlet gas vs.time on stream for different sorbents. From Figure1,it is seen that the binder affects the breakthrough curve and the breakthrough time.It can be found that ZFD9sorbent has the shortest breakthrough time in all sorbents,and its first sul-fur capacity should be a minimum value.Table4gave the first sulfur capacity comparison of zinc ferrite sor-bents with the same content of active zinc ferrite and the addition of different

https://www.360docs.net/doc/8e13915859.html,parison of breakthrough curves of sorbents with dif ferent binder

Table4revealed that the sulfur capacity of ZFD7 is the highest,which is close to35%,and that of ZFD9 is the lowest of about3.2%.The former is ten times higher than the latter.It seems that the binder has great effect on the sulfur capacity of zinc ferrite sor-bent.As shown in Table1,ZFD4has the greatest

Journal of Natural Gas Chemistry Vol.16No.22007207

pore volume and ZFD7has less pore volume.These results suggest that chemical properties not physi-cal structure of the binder bring about greater effect on the reactivity of zinc ferrite sorbents.Therefore, there are much more research to be done in order to know more the correlation between the chemical properties and physical structure of binders and ac-tive substance.Our work will focused on the mecha-nism correlating these aspects in order to increase the reactivity of the sorbents.

Table4.The f irst sulfur capacity comparison of zinc ferrite sorbent with the same content of active zinc ferrite

and the addition of dif ferent binders

Item

123

208Meisheng Liang et al./Journal of Natural Gas Chemistry Vol.16No.22007

quence of the different preparation method used,as well as the calcinations at750℃for the fresh sor-bent and regeneration at650℃for the first regen-erated one.The trace FeS in the first regenerated one seems to catalysis the second sulfidation reaction. The structure is more advantageous to the desulfur-ization after high temperature regeneration.This is in good agreement with Pineda’s result[11].

https://www.360docs.net/doc/8e13915859.html,parison of breakthrough curves of H2S and COS

In performance tests of ZFD7sorbent,not only H2S but also COS was detected in the exit of the reac-tor,and COS concentration was approximately30.0% of that of H2S.H2S and COS breakthrough curves at 350℃and400℃were shown in Figure4and Fig-ure5,respectively.From these figures,it is seen that the breakthrough curves of COS were similar to that of H2S,and COS concentration in the outlet gas also increased when the desulfurization time extended,but the COS concentration in the outlet gas at400℃in-creased slowly.According to reference[11],there were two reactions to occur in the presence of ZnS as fol-lows:

Hydrogenolysis:

COS+H2?H2S+CO(1) Hydrolysis:

COS+H2O?H2S+CO2(2) On the basis of Equation(1)and Equation(2), Equation(3)can be deduced.

2COS+H2+H2O?2H2S+CO+CO2(3) Sasaoka et al.[12]conducted an experiment of H2S conversion to COS under simulated coal-derived gas (500ppm H2S,22%H2,50%CO,4%CO2,4.7%H2O, and N2balanced)and at400–700℃,it is realized that ZnS is not only a catalyst for COS hydrolysis reaction but also a catalyst for the reverse reaction of Equation (3).When the reaction temperature is low,reverse re-action of Equation(2)and Equation(3)could occur. The formation of COS at the beginning was deter-mined by the reverse reaction of Equation(2),but when H2S,CO and CO2had enough time to come into contact with each other,COS concentration in the outlet gas was determined by the reverse reaction of Equation(3).The result in reference[12]is con-sistent with that in Figure4and Figure5,

because Figure4.Breakthrough curves of ZFD7sorbent for H2S and COS at the temperature of

350℃

(1)H2S,(2)

COS

Figure5.Breakthrough curves of ZFD7sorbent for H2S and COS at the temperature of

400℃

(1)H2S,(2)COS

ZnFe2O4sorbent combined with H2S,and changed into ZnS,which could be a catalyst of reverse reaction of Equation(3).When H2S and ZnS have enough time to contact before the breakthrough under the experimental conditions,COS can be formed and its concentration decreased with the rise of the reaction temperature.

4.Conclusions

1)Zinc ferrite sorbents prepared by coprecipia-tion show high reactivity in three-cycle of sulfidation-regeneration test.The different performance exhib-ited by the freshly calcined and the first regenerated sorbents that can be extended up to the third cycle, is mostly due to textural difference derived from the

Journal of Natural Gas Chemistry Vol.16No.22007209

different preparation method used.

2)In the temperature range of350?400℃, zinc ferrite has a certain extent of desulfurization efficiency and reactivity as a novel coal gas desulfur-ization sorbent at high temperature.But the appear-ance of COS in the outlet gas in sulfidation as well as H2S must be accounted for in industrial applications.

3)Effects of binders on pore volume,strength and desulfurization efficiency of zinc ferrite sorbents in-dicated that the addition of kaolinite to zinc ferrite desulfurizer seems to be superior to other binders un-der the experimental conditions. Acknowledgements

This work was supported by the Ministry of Sci-ence and Technology of China under the Grant No. G2005CB221203and the Natural Science Foundation of China under Contract No.20576087.

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