Experimental study on SO2 recovery using a sodium-zinc sorbent based flue gas desulfurization technology☆

Yang Zhang ,Tao Wang ,Hairui Yang ,Hai Zhang ,*,Xuyi Zhang

1 Key Laboratory for Thermal Science and Power Engineering of Ministry of Education,Department of Thermal Engineering,Tsinghua University,Beijing 100084,China

2 National Engineering Laboratory of Coal-Fired Pollution Reduction,School of Energy and Power Engineering,Shandong University,Jinan 250061,China

Keywords:Flue gas desulfurization Waste treatment ZnSO3·2.5H2O pyrolysis Sodium-zinc sorbent based SO2 co-production

ABSTRACT A sodium-zinc sorbent based flue gas desulfurization technology(Na-Zn-FGD)was proposed based on the experiments and analyses of the thermal decomposition characteristics of CaSO3 and ZnSO3·2.5H2O,the waste products of calcium-based semi-dry and zinc-based flue gas desulfurization(Ca-SD-FGD and Zn-SD-FGD)technologies,respectively.It was found that ZnSO3·2.5H2O first lost crystal H2O at 100 °C and then decomposed into SO2 and solid ZnO at 260 °C in the air,while CaSO3 is oxidized at 450 °C before it decomposed in the air.The experimental results confirm that Zn-SD-FGD technology is good for SO2 removal and recycling,but with problem in clogging and high operational cost.The proposed Na-Zn-FGD is clogging proof,and more cost-effective.In the new process,Na2CO3 is used to generate Na2SO3 for SO2 absorption,and the intermediate product NaHSO3 reacts with ZnO powders,producing ZnSO3·2.5H2O precipitate and Na2SO3 solution.The Na2SO3 solution is clogging proof,which is re-used for SO2 absorption.By thermal decomposition of ZnSO3·2.5H2O,ZnO is re-generated and SO2 with high purity is co-produced as well.The cycle consumes some amount of raw material Na2CO3 and a small amount of ZnO only.The newly proposed FGD technology could be a substitute of the traditional semi-dry FGD technologies.

1.Introduction

Nowadays,FGD( flue gas desulfurization)technologies have been widely used in power generation and metallurgy industries.Based on the amount of water consumption,they can be classified as dry,semidry and wet types.Based on the sorbents,they can also be classified as calcium-based(CaO/Ca(OH)2),sodium-alkali-based(NaOH),zincbased(ZnO)and ammonia-based technologies.Among them,the calcium-based wet FGD is the most preferred technology for large scale coal- fired power plants because of its high desulfurization efficiency and vastly available sorbents[1-3].However,it requires high capital investment and operational cost,and often causes clogging and corrosion problems in the devices and pipe lines.

Compared with the wet FGD technology,calcium-based dry and semi-dry FGD(Ca-SD-FGD)technologies employ simpler processes,and therefore have less initial investments with less installation area required.They consume less water,and have fewer burdens of waste water treatment and less corrosion problems.The main concerns for these technologies are desulfurization efficiency and product post processing.A number of studies[4-13]have been conducted to improve the desulfurization efficiency,by controlling the operational parameters,including reaction temperature,Ca/S ratio,and sorbent characteristics with the laboratory experiments[4-6,13],pilot-reactors[7-9,11,12]and even practical applications[10].The studies showed that with carefuldesign and operation,the Ca-SD-FGD could have compatible desulfurization efficiency against the wet FGD.However,the Ca-SD-FGD product remains an intractable problem,a bottleneck of this technology[14-18].The reason is that the product generally contains 15%-25%calcium sulfite(CaSO3),and the calcium sulfite is easily oxidized into calcium sulfate(CaSO4)when in the atmosphere[19]or in the water[20],and then becomes unsuitable for building material.Conventionally,the product is classified as a waste.

Alternatively,zinc-based semi-dry FGD(Zn-SD-FGD)was proposed as a conditional substitute for Ca-SD-FGD,according to the early industrial testing carried out in metallurgical plants[21,22].The process can be generally described in two steps.In the first step,ZnO suspension is sprayed into a desulfurization tower to absorb SO2in the flue gas.The product of this step is the sediment mainly containing ZnSO3·2.5H2O.In the second step,the obtained sediment is converted into environment friendly final products[23-26].The sediment can be processed in three ways:(1)thermal decomposition(pyrolysis),(2)acid decomposition[25]and(3)oxidization[26].By acid decomposition and oxidization,ZnSO3·2.5H2O is converted into ZnSO4,a matter of higher solubility.However,a large amount ZnO is consumed since it is rather difficult to recycle ZnO from ZnSO4.By thermal decomposition,ZnO and ratherpure SO2gas are produced form the ZnSO3·2.5H2O sediment and the SO2gas can be re-used as a raw material for the sulfuric acid industry.With the rapid increase in the international price of sulfur,the thermal decomposition method has become more and more favorable.

The Zn-SD-FGD described above has high SO2removal efficiency,but traditional Zn-SD-FGD is not practicable mainly because of two reasons:(1)ZnO suspension is viscous,thus easy to cause clogging and(2)the sorbent ZnO is costly.

To resolve the first problem,an indirect zinc-based SO2removal method is proposed by placing the reaction ofZnOand SO2in a circulating tank instead of the desulfurization tower[24].In this method,a clear solution containing Zn(HSO3)2is spayed,then ZnO powder,instead of ZnO suspension,is added to the liquid in the circulating tank to react with SO2.By this arrangement,clogging problems in the pipes and valves could be avoided.However,because a large amount of H+exists in the absorbent liquid,the efficiency of the desulfurization reaction is limited,and it is difficult to produce stable Zn(HSO3)2solution.To resolve the second problem,thermal treatment is needed.However,the pyrolysis properties of Zn-SD-FGD waste have not previously been studied even though they are important for designing and controlling the overall Zn-SD-process.

Consequently,in the present study,experiments on the pyrolysis properties of both Ca-SD-FGD and Zn-SD-FGD wastes will be conducted.Based on the experimental results and analyses of the existing Zn-SD-FGD technology,a new sodium-zinc waste based FGD technology that can avoid the clogging problem and recycle ZnOand SO2is to be proposed and discussed.

2.Experimental Approaches

2.1.Samples

The sample of Ca-SD-FGD waste was obtained from a commercial power plant equipped with an FGD system.The sample of ZnSO3·2.5H2O was made in the laboratory.Fig.1 shows the schematics of the ZnSO3·2.5H2O sample preparation system.The main device is a glass reactor with 2 inlets for gas and ZnO additives,1 gas product outlet and 2 opening holes on the top for a pH meter and a stirrer.The volume of the reactor is 250 ml.High purity N2(＞99.99%)and SO2(＞99%)were used to obtain a reactant mixture with their flow rates individually controlled by flow meters.The pH value of the liquid in the reactor was detected by an electronic pH meter(OHAUS Starter 3C,USA,resolution:pH 0.1).

Fig.1.Schematics of the ZnSO3·2.5H2O sample preparation system.

The ZnSO3·2.5H2O sample preparation procedure has three steps:(1)Continuously inject a gaseous mixture of 5%SO2and 95%N2(1.5 L·m-1)into a solution of Na2CO3in 2%mass fraction in the reactor until the pH value of the liquid reaches 3-5.The simple composition of gaseous mixture and high SO2concentration are designed to obtain a pure sample of ZnSO3·2.5H2O.(2)Add ZnO powder into the reactor while continuously stirring the liquid until it becomes weakly alkaline with pH＞7.(3)Stop the stirring, filter to get the solid precipitate in the reactor and dry it at 50°C.

Note that all the above steps are not necessarily operated in an oxygen-free atmosphere.However,before the preparation,a pure N2stream was used to purge the reactor for more than 5 min.Table 1 lists the reactions during the ZnSO3·2.5H2O sample preparation process.

Table 1 Reactions in the ZnSO3·2.5H2O sample preparation process

2.2.Experimental apparatus

The thermo-gravimetric(TG)analyses were conducted by a TGA(model:Netzsch STA 409C)at a flow rate of 50 ml·min-1and a heating rate of 10 °C·min-1.Both air and N2were introduced as carrier gases in the experiment.The product gas was analyzed by a Fourier transform infrared spectroscopy(FT-IR)(model:THERMO NICOLET NEXUS670)with a resolution of8.0 cm?1.The crystallization of the samples was detected by the X-ray diffraction(XRD)system(Rigaku D/max-2500HB),using Cu Kαradiation in θ/2θ coupled mode with 0.02°step size.The X-ray tube was operated at 40 kV and 200 mA.Continuous scans were taken over a range of 2θ from 10°to 80°at a speed of 6°·min-1.The slit parameters were DS:1°,RS:0.3°and SS:1°.The XRD pattern shown in Fig.2 indicates that the prepared ZnSO3·2.5H2O sample had excellent crystal properties and with a small amount of ZnSO3·2H2O.It was also suspected that this sample contained some absorbed H2SO3and carbonate even though they could not detected by XRD.

3.Results and Discussion

3.1.TGA characteristics of Ca–SD-FGD wastes

Typical Ca-SD-FGD wastes contain CaSO3,CaSO4,Ca(OH)2,CaCO3and CaO,etc.Table 2 lists several majorthermal decomposition reactions of these components reported in the literature in the temperature range of 200-1250°C.

Fig.2.Crystal pattern of ZnSO3·2.5H2O sample detected by XRD(A:ZnSO3·2.5H2O;B:ZnSO3·2H2O).

Table 2 Reactions during the pyrolysis of Ca-SD-FGD waste

Fig.3 shows the TG curves of Ca-SD-FGD wastes in both N2and air atmospheres.In N2atmosphere(Fig.3a),there are three obvious stages of mass loss.The first stage happens under 450°C,corresponding to reactions(Ca1)and(Ca2).The second one happens in the temperature range of 600-700°C,corresponding to reaction(Ca4)and the third stage happens above 800°C,corresponding to reactions(Ca5)and(Ca6).However,in the air(Fig.3b),the TG curve largely differs in the temperature range of 450-750°C.The DTA curve indicates that an exothermic reaction happens and the sample mass increases at 450-600°C.This is due to reaction(Ca3),in which CaSO3is converted into CaSO4.When the temperature is higher than 600°C,the TG curve shows a sudden drop,as reaction(Ca4)occurs.When the temperature is over 750°C,the sample mass remains constant.The final sample mass in the air is higher than that in N2.The results indicate that reaction(Ca5)does not happen,and instead oxidization happens before pyrolysis when Ca-SD-FGD wastes are heated in the air.Consequently,the CaSO3pyrolysis process must be O2free if SO2recycling is desired.This makes the thermal decomposition treatment difficult for Ca-SD-FGD wastes.Other sorbents need to be considered.Thus,the zinc-based sorbent is directly come into mind.

Fig.3.Thermo-gravimetric and differential thermal(TG-DT)analyses of the Ca-SD-FGD waste in different atmospheres.

3.2.TGA characteristics of the ZnSO3·2.5H2O sample

Fig.4 shows the TG curves of the ZnSO3·2.5H2O sample in both N2and air atmospheres.The curve in the air is slightly above that in the pure N2.However,the slight difference in remaining mass(within 2%)is probably caused by slight oxidization,as shown by the DTA curves,when temperature is higher than 450°C.Nevertheless,the results indicate that ZnSO3·2.5H2O will not be oxidized before pyrolysis when it is heated up.This is a necessary condition for the thermal decomposition treatment of Zn-SD-FGD wastes.Fig.5 gives the XRD pattern of the final TGA solid product.Clearly,ZnOexists in an excellent crystal condition.A small amount of Zn(OH)2also exists but is negligible compared with ZnO.

Fig.4.Thermo-gravimetric and differential thermal analysis curves of the ZnSO3·2.5H2O sample in different atmospheres.

3.3.Pyrolysis process of the ZnSO3·2.5H2O sample

As shown in Fig.4,the pyrolysis process of the ZnSO3·2.5H2Osample can be divided into three temperature stages:100-130 °C,130-260 °C,and 260-400°C,respectively,marked as Stages 1,2 and 3.The gases produced during the pyrolysis process were analyzed by the FT-IR at three temperatures of 100 °C,180 °C and 300 °C,and results are shown in Fig.6.

Three species(H2O,CO2and SO2)are detected but the peaks in the infrared spectrum are different in the three temperature stages.H2O is the main product in Stage 1 with small amount of SO2detected as well.In Stage 2,the amount of H2O decreases and that of SO2increases but remains at a very low level.In Stage 3,a large amount of SO2is generated and the amount of H2O becomes negligible.In all stages,a small amount of CO2releasing is detected,indicating a slight decomposition of carbonates.The height of the absorbance peak of CO2increases with temperature but still remains at a low level.

Fig.5.XRD pattern of the final solid product of the TG analysis(A:ZnO;B:Zn(OH)2).

Fig.6.Fourier transform infrared spectroscopy analyses of the ZnSO3·2.5H2O sample during pyrolysis process[(a)100 °C;(b)180 °C;(c)300 °C)].

To further study the pyrolysis process of the ZnSO3·2.5H2O sample,quantitative analyses of the gaseous pyrolysis product were conducted.Using the peaks of wave numbers of H2O,CO2and SO2(at 1506.8 cm?1,2359.5 cm?1and 1373.9 cm?1respectively)as molar concentration reference,we can obtain the variations of molar concentration of H2O,CO2and SO2with temperature,as shown in Fig.7.Note that the value of molar concentration differs from the one in pure gaseous pyrolysis product since the testing gas is diluted by air and N2during the TGA process.It can be seen that most H2O is released at around 120°C(Stage 1)while SO2releasing is about one-third of H2O.Thus in Stage 1 the sample loses its crystalline water and H2SO3starts to decompose.In Stage 2(130-260°C),the release of H2O decreases to null,while the release of SO2remains at a low level.However,a rapid increase of SO2occurs when the temperature reaches 260°C in Stage 3.SO2molar concentration in the TGA exhaust grows 6 times from 260 °C to 270 °C and remains at a high level to 400°C.When temperature is higher than 400°C,both H2O and SO2are hardly detected.That means the decomposition reaction completes.CO2concentration increases with temperature in all three stages but less than 12 μl·L-1.

The results of Fig.7 indicate the ZnSO3·2.5H2O sample experiences two decomposition steps.The first one is the loss of crystal water,mainly occurring when temperature is under 260°C.The second one is the release of SO2when temperature is above 260°C.In addition,Fig.4 shows that the weight loss ratio of under and above 260°C is about 19.5%:26.5%=0.735.This value is close to the molecular weight ratio of H2O and SO2in ZnSO3·2.5H2O,which is 45:64=0.703.Thus,it is clear that the final solid pyrolysis product is ZnO.

Fig.7.Variations of H2O,CO2 and SO2 molar fractions with temperature and SO2 molar concentration in pure gaseous pyrolysis product when the ZnSO3·2.5H2O sample is pyrolyzed in air.

The carrier gases(N2,air)are removed and the SO2mole fraction in pure gaseous pyrolysis product is plotted versus temperature in Fig.7(b).As temperature increases,the SO2mole fraction first increases and then decreases.The SO2mole fraction in the pure gaseous pyrolysis productis higher than 95%in the temperature range of320-370°C,and is even higher than 97%in the temperature range of 340-365°C.This indicates that rather pure SO2can be obtained at around 350°C.

Based on the previous discussion,the key reactions and associated conditions are listed in Table 3.In the course of pyrolysis,low concentration SO2in flue gas is highly concentrated to a level that can be used directly for industry.Moreover,ZnO is the only final solid product.Therefore,ZnO experiences SO2absorption and re-generation procedures,forming an enclosed cycle.

In reactions(Zn1)and(Zn2),Na2CO3and Na2SO3solutions are used as SO2sorbents.Previous studies showed that these aqueous solutions work well in absorbing SO2in the flue gas[32,33].Compared with CaO and ZnO suspensions,Na2CO3and Na2SO3solutions can avoid the clogging problem since they are clear liquids,and more cost-effective than ZnO ones.Thus,based on reactions(Zn1)to(Zn11),a sodium-zinc sorbent based FGD(Na-Zn-FGD)technology with SO2co-production can be proposed.

4.The Na–Zn-FGD with SO2 Co-production

Fig.8 shows the schematics of the Na-Zn-FGD system with SO2coproduction.This system consists of four major devices:a desulfurization tower,a circulating tank,a drying tank and a decomposition tank.The reactions in each part are described as follows.

Table 3 Reactions during the pyrolysis of ZnSO3·2.5H2O sample

Fig.8.Schematics of a sodium-zinc sorbent based flue gas desulfurization system with SO2 co-production.

4.1.Desulfurization tower

When the process starts,Na2CO3solution is used to absorb SO2in the flue gas,corresponding to reaction(Zn1).The temperature of the flue gas entering the desulfurization tower is controlled at 150°C.Then,SO2is further absorbed by Na2SO3solution to produce NaHSO3by reaction(Zn2).Some dust particles in the flue gas such as SiO2and Al2O3.are removed from the bottom of the tower.When SO2concentration is sufficiently low,the treated flue gas is released from the tower.

4.2.Circulating tank

The circulating tank stores the NaHSO3solution released from the desulfurization tower.After aborting SO2,the pH value of the NaHSO3solution is about 3-5 and the temperature of the solution is cooled below 100°C before the solution enters the circulating tank.In the circulating tank,NaHSO3reacts with ZnO powders,producing ZnSO3·2.5H2O precipitate and Na2SO3solution,corresponding to reactions(Zn4)and(Zn5).For better reaction efficiency,the pH value inside the circulating tank should be maintained slightly above 7.The Na2SO3solution is returned to the desulfurization tower for the next SO2absorbing cycle,together with some gases(CO2,SO2,etc.).

4.3.Drying tank

The drying tank is used to remove moisture from the ZnSO3·2.5H2O precipitate from the circulating tank.The tank is maintained at about 240°C such that the free water and crystalline water are both removed corresponding to reactions(Zn6),(Zn7),(Zn8)and(Zn9),while ZnSO3does not decompose.The solid product left is relatively pure ZnSO3powder.The gases(H2O,SO2,etc.)generated in the drying tank are sent to the desulfurization tower.

4.4.Decomposition tank

The dried ZnSO3powder is decomposed into solid ZnO and gaseous SO2,according to reaction(Zn10)at about 350°C.The SO2with purity higher than 97%can be used as a raw material in the sulfuric acid industry and the ZnO powders are sent back to the circulating tank for the next cycle.

Note that when the process starts,Na2CO3solution is used as the absorbent material.After the system has been running for a sufficiently long time,the absorbent material automatically changes into Na2SO3solution.A small amount of Na and Zn loss is inevitable,so ZnO and Na2CO3are supplemented into the system to keep the system running in balance.

5.Conclusions

The experiments on the thermal decomposition of ZnSO3·2.5H2O and CaSO3in different atmospheres were conducted in TGA and the gaseous product of the thermal decomposition were measured by FTIR.It was found that ZnSO3·2.5H2O was decomposed at 260 °C in air without being oxidized,while CaSO3was oxidized at 450°C before decomposed in air.The main gaseous product of the thermal decomposition of ZnSO3·2.5H2O was rather pure SO2and the solid product ZnO.The experimental results suggest that Zn-SD-FGD technology is promising for SO2removal SO2recycling.

However,traditional Zn-SD-FGD technology is difficult to implement and commercialize since ZnO suspension is viscous and costly.Alternatively,a Na-Zn-FGD technology is proposed.In this new technology,Na2CO3is used to generate Na2SO3for SO2absorption,and the intermediate product NaHSO3reacts with ZnO powders,producing ZnSO3·2.5H2O precipitate and Na2SO3.Na2SO3is re-used for SO2absorption.By thermal decomposition,ZnO is re-generated and SO2with high purity is co-produced as well.The cycle only consumes small amount of raw material Na2CO3and ZnO.

The proposed Na-Zn-FGD technology with SO2co-production has several advantages.Na2SO3solution is used as the SO2absorbent instead of CaO or ZnO suspensions,avoiding the clogging problem in the desulfurization tower.In addition,the technology conserves raw material since both Na2SO3and ZnO are consumed and regenerated.Moreover,the final gaseous product is rather pure SO2that can be used in chemical industries to make the technology cost-effective and environmental-friendly.

More detailed studies on the newly proposed FGD technology are needed before it can be put into application.For example,it is needed to study operational parameters,including temperature to control completeness of SO2absorption and purity of SO2co-production.