Systematic investigation of SO2 adsorption and desorption by porous powdered activated coke: Interaction between adsorption temperature and desorption energy consumption

Jun Li ,Liqiang Zhang, *,Xiao Zhu ,Mengze Zhang ,Tai Feng ,Xiqiang Zhao ,Tao Wang,Zhanlong Song,*,Chunyuan Ma

1 National Engineering Laboratory for Reducing Emissions from Coal Combustion,Engineering Research Center of Environmental Thermal Technology of Ministry of Education,Shandong Key Laboratory of Energy Carbon Reduction and Resource Utilization,School of Energy and Power Engineering,Shandong University,Jinan 250061,China

2 College of Mechanical and Electronic Engineering,Shandong University of Science and Technology,Qingdao 266590,China

Keywords:Activated coke SO2 adsorption Desorption energy consumption Optimal adsorption temperature

ABSTRACT Porous carbon materials have been widely used for the removal of SO2 from flue gas.The main objective of this work is to clarify the effects of adsorption temperature on SO2 adsorption and desorption energy consumption.Coal-based porous powdered activated coke(PPAC)prepared in the drop-tube reactor was used in this study.The N2 adsorption measurements and Fourier transform infrared spectrometer analysis show that PPAC exhibits a developed pore structure and rich functional groups.The experimental results show that with a decrease in adsorption temperature in the range of 50-150 °C,the adsorption capacity of SO2 increases linearly;meanwhile,the adsorption capacity of H2O increases,resulting in the increase in desorption energy consumption per unit mass of adsorbent.The processes of SO2 and H2O desorption were determined by the temperature-programmed desorption test,and the desorption energies for each species were calculated.Considering the energy consumption per unit of desorption and the total amount of adsorbent,the optimal adsorption temperature yielding the minimum total energy consumption of regeneration is calculated.This study systematically demonstrates the effect of adsorption temperature on the adsorption-desorption process,providing a basis for energy saving and emission reduction in desulfurization system design.

1.Introduction

With the current rapid deterioration of the atmospheric environment,the development of new energy-saving and environmentally friendly technologies for reducing pollutant emissions is of great significance for the reduction of environmental pollution[1].Large-scale power plants and heating boilers fueled by fossil fuels such as coal are the main sources of SO2emissions[2],which lead to the presence of acid rain,haze,and other environmental problems [3].Although traditional wet flue gas desulfurization technology has exhibited high desulfurization efficiency,it is not suitable for application in environmental protection efforts because of its large water consumption and secondary pollutants[4].

Porous activated carbon (AC) materials have been found to demonstrate excellent SO2removal performance,because of their large surface area,microporous/mesoporous structure,and rich surface functional groups [5,6].Through a typical adsorption and desorption process,carbon adsorbent can be used to continuously desulfurize.In the presence of O2and H2O,the SO2adsorption mechanism is generally believed to proceed as follows: first,SO2is adsorbed on the carbon surface and converted into SO3under the catalysis of an active site and the oxidation of O2;then,it is eventually converted to H2SO4under hydration and stored in the pore structure.However,as the reaction progresses,the adsorbent is gradually deactivated because of the occupation of active sites and the blockage of pores.As a reverse reaction,saturated adsorbents can be restored by thermal regeneration,and H2SO4reacts with carbon at high temperatures to generate SO2,which is accompanied by the release of CO2,CO,and H2O [7,8].

To further improve the economy of desulfurization systems,methods aimed at increasing the SO2adsorption capacity and reducing desorption energy consumption have become the focus of research in recent years.For the SO2adsorption process,the influences of pore structure,surface functional groups,adsorption conditions,and precursor properties of carbon materials[9-12]on SO2adsorption capacity,as well as chemical impregnation and heat treatment processes to change physicochemical properties and improve the removal of SO2have been widely studied,presenting good results [13-15].In the desorption process,the main concerns are the gas desorption law and regeneration method[8,16,17].The influence of regeneration on the surface physicochemical properties and the performance of carbon adsorbent in the removal of SO2have also been widely studied [11,18,19].The focus has largely been on the single process of either SO2adsorption or desorption;consequently,the interaction between adsorption and desorption has been ignored,particularly from the energy-saving perspective.According to the literatures [20,21],the adsorption temperature has a great influence on the adsorption performance of SO2.Because SO2adsorption is an exothermic process,a lower adsorption temperature is favorable for the generation of a large sulfur adsorption capacity [20,22].Meanwhile,the resulting increases in the absorption of water in flue gas,thereby increasing the energy consumption of desorption caused by water evaporation,cannot be neglected.If emphasis is placed on desorption energy consumption,the reduction of the adsorption temperature has two important opposing effects in the treatment of a particular sulfur-containing flue gas.On the one hand,an increase in the adsorption capacity of SO2and H2O occurs,increasing the desorption energy consumption per unit mass.On the other hand,the total amount of adsorbent is reduced.By weighing the relationship between the adsorbent dose and the energy consumption per unit mass of the adsorbent,the optimal adsorption temperature can be determined to achieve the goal of energy savings and emission reduction for the two processes.Therefore,the adsorption capacity and desorption energy consumption can both be affected by the adsorption temperature;however,few studies have addressed this phenomenon.

To address this knowledge gap,the purpose of this study was to systematically explain the synergistic influence of the adsorption temperature on SO2adsorption and desorption energy consumption.The adsorption characteristics of carbon adsorbents for a series of temperatures and desorption properties of SO2and H2O were analyzed by SO2removal experiments and temperature-programmed desorption (TPD) tests,respectively.Furthermore,the desorption activation energies of SO2and H2O in different adsorption states were analyzed and calculated based on the variation of desorption peak temperatures for a series of heating rates.Finally,according to the composition of the simulated flue gas under experimental conditions,the optimal adsorption temperature was obtained by considering the relationship between the energy consumption per unit mass of adsorbent and the adsorbent dose to minimize the desorption energy consumption.

2.Materials and Methods

2.1.Sample preparation and characterization

A type of porous powdered activated coke (PPAC) prepared in O2and steam atmospheres (950 °C,6% (volume) O2,20% (volume)H2O,and 12% (volume) CO2) through a drop-tube reactor system was used in this study[23,24],as shown in Fig.S1(see Supplementary Material).The N2,O2and CO2were supplied by pressure cylinders (purity 99.999%,from Jinan Xusheng Gas Co.,Ltd,Jinan,China).The precursor was Mengdong lignite(obtained from Shenhua Shengli Power Plant,China).The raw material was dried at 85 °C for 10 h,then ground and sieved to obtain a particle size of 60-90 μm.

Proximate (SDTGA5000,Sundy,China) and ultimate analyzers(Vario EL cube analyzer,Elementar,Germany)were used to examine the samples.The pore structure of the PPAC was characterized by N2adsorption at -196 °C using an automatic surface analyzer(Autosorb1-C,Quantachrome,USA).The specific surface areas were calculated based on the Brunauer-Emmett-Teller (BET) method.The total pore and micropore volumes were calculated using the single-point adsorption method andt-plot method,respectively.The surface morphology of the PPAC samples was investigated by scanning electron microscopy (SEM,SUPARATM55,Carl Zeiss,Germany).The surface functional groups were characterized through Fourier transform infrared spectroscopy (FTIR,Nicolet 6700,Thermo Scientific,USA)with a resolution of 2 cm-1over the spectral range of 4000-400 cm-1.

2.2.SO2 adsorption and desorption tests

SO2adsorption and desorption tests were carried out in a labscale fixed-bed experimental system.A schematic diagram of the test system is shown in Fig.S2,consisting of a fixed-bed glass reactor (17 mm diameter,500 mm length),tube furnace,and water bath,with valves and a mass flow controller system.The fixedbed reactor contained a quartz mobile basket,which is convenient for weighing the sample mass.The outlet concentration of gases,including SO2and H2O,were continuously monitored by an online gas analyzer (Fourier transform infrared Gasmet Dx4000,Gasmet Company,Finland).The experimental settings for the adsorption and desorption of the PPAC sample are described as follows: (i)The dynamic adsorption temperature was controlled at 50,75,100,125,and 150°C.In a typical operation,1 g of PPAC was placed into the glass reactor,and simulated flue gas(4857 mg·m-3SO2,8%(volume)H2O,6%(volume)O2,N2balance,and a total flow rate of 500 ml·min-1) was introduced into the reactor after reaching the desired temperature.The SO2was supplied by pressure cylinders(purity 5% (N2balance),from Jinan Xusheng Gas Co.,Ltd,Jinan,China).The adsorption time was limited to 2 h each time.(ii)After adsorption,TPD analysis coupled with a Gasmet analyzer was applied to investigate the SO2-loaded PPAC regeneration.A 0.5 g SO2-loaded PPAC sample was placed in the reactor for each operation with 125 ml·min-1N2as the carrier gas.The TPD experiment was conducted in the temperature range of 20-850°C at a constant heating rate of 10 °C·min-1and maintained for 1 h at the highest temperature.The adsorption and desorption of PPAC capacities were obtained in units of milligram of SO2per gram of coke by the following equations:

whereQSO2is the SO2adsorption capacity (mg·g-1) of the PPAC sample;Qdesis the desorption amount (mg·g-1) of PPAC by TPD for SO2or H2O;m1andm2are the sample mass (g) for adsorption and desorption,respectively;CinandCoutare the concentrations of SO2(mg·m-3) at the reactor inlet and outlet,respectively;VadsandVdesare the total flow rates (ml·min-1) of the simulated flue gas and carrier gas,respectively;tadsandtdesare the time (min) of adsorption and desorption,respectively.It is considered that the adsorption and desorption of water are equal.

2.3.Activation energy and thermodynamic parameter calculation

The desorption peak temperature is related to the adsorption bond strength,and a stronger bond gives rise to a higher TPD peak temperature [25].To investigate the desorption processes of SO2and H2O at high-temperature,the activation energy(E)for desorption can be calculated according to the following equation [26]:

whereTmis the peak temperature (°C),β is the heating rate (°-C·min-1),Eis the activation energy for desorption (J·mol-1),Ris the gas constant (8.314 J·mol-1·K-1),andZis a constant that depends on the desorption kinetics.The TPD experiments were carried out in the temperature range of 20-850°C at a heating rate of 6-14 °C·min-1.The other thermodynamic parameters,namely,the pre-exponential factor (A),enthalpy (ΔH),Gibbs free energy (ΔG),and entropy (ΔS) were calculated using Eqs.(4)-(7) [27-29]:

whereKbandhrepresent the Boltzmann constant(1.381×10-23J·°C-1) and Plank constant (6.626 × 10-34J·s),respectively.

3.Results and Discussion

3.1.Sample characterization

The results of the ultimate and proximate analyses of the lignite precursor and PPAC sample are given in Table S1.After the hightemperature carbonization and activation process by H2O,CO2,and O2in the drop-tube reactor,the moisture and volatile contents in the PPAC were much lower,and the carbon and ash contents were much higher than those in the Mengdong lignite.This is mainly due to the rapid desorption of moisture and volatile matter during the rapid heating process (103-104°C·s-1) [30,31].Table 1 shows the pore structure characteristics of the PPAC sample,which has a relatively large pore volume and specific surface area coupled with a high proportion of micropores(54.39%Smi/SBET,and 39.63%Vmi/Vtot).It has been demonstrated that micropores are the primary hosts of SO2adsorption[9,38].Compared with other physical activation methods,the drop-tube reactor is relatively simple and efficient (Table 1).The surface morphology of the PPAC (Fig.1(a))showed an abundant pore structure,demonstrating the effective pore-formation caused by the prepared system.

The surface chemical properties of carbon adsorbents also have an important influence on the SO2adsorption process.Such as carboxyl and ketones can promote the adsorption and catalytic oxidation of SO2[10,11].The FTIR spectra of the PPAC sample are presented in Fig.1(b).Abundant types of surface functional groups on the sample surface can be intuitively seen from the FTIR spectra in the range from 400 to 4000 cm-1.From the derived spectra,five main regions can be identified [24,27,39].The peaks at 3600-3200 cm-1can be assigned to the OH stretching vibration of water,phenol,and alcohol;peaks at 3000-2800 cm-1are assigned to the aliphatic-CH group;peaks at 1800-1300 cm-1represent aromatic C=C,carboxylic acid C=O,and C-O groups;peaks at 900-1300 cm-1are from OH,C-O-C,and N-containing C-N groups;and peaks at 700-900 cm-1are characteristic of aromatic hydrocarbons.These surface functional groups have different acid bases and thermal stabilities.Some O-containing functional groups can be decomposed at high temperatures,releasing gases such as CO and CO2[40].In summary,the method of PPAC preparation using a drop-tube reactor system is feasible.

3.2.Dynamic adsorption of SO2 at different temperatures

To evaluate the adsorption performance of the PPAC sample,SO2dynamic adsorption experiments were performed at five grades of adsorption temperatures under the same simulated flue gas (Section 2.2).Fig.S3 shows the SO2breakthrough curves of the samples under different adsorption temperatures.The influence of the adsorption temperature on the breakthrough curve is evident,mainly reflected in two aspects.With a decrease in the adsorption temperature,the saturation adsorption time gradually increases from 10 min at 150 °C to 17 min at 50 °C;meanwhile,the rising rate of the SO2concentration at the outlet is reduced.By integrating the breakthrough curve over time,as in Eq.(1),the SO2adsorption capacity at different adsorption temperatures can be obtained.As seen in Fig.2(a),a remarkably good correlation(R2=0.983) between SO2capacity and adsorption temperature is evident,with the decrease of temperature,the SO2capacity increases from 61.53 mg·g-1at 150 °C to 98.85 mg·g-1at 50 °C(growth rate of 60.65%).The desulfurization process of AC is a coupling process of SO2physical adsorption and catalytic oxidation with the participation of oxygen and water vapor.From the point of view of gaseous molecular adsorption,the physical adsorption process is accompanied by the reduction of free energy and the decrease of enthalpy of the system,the physical adsorption is an exothermic process [21,41,42],so the increase of temperature is unfavorable to the physical adsorption.

Meanwhile,with a decrease in the adsorption temperature,the H2O adsorption capacity of the PPAC sample increases from 20.1 mg·g-1at 150 °C to 102.7 mg·g-1at 50 °C (growth rate of 410.99%),as shown in Fig.2(b).Through numerical curve fitting,the correlation between the H2O adsorption capacity and adsorption temperature shows a good polynomial curve relationship(R2=0.9904).With the decrease in the adsorption temperature,different trends of water adsorption were observed with respect to the temperature of 100 °C.When the adsorption temperature was lower than 100 °C,the adsorption of water rapidly increased,showing strong hydrophilicity;however,when the temperature was higher than 100 °C,this effect was significantly weakened.The detailed SO2/H2O adsorption capacities of the samples are listed in Table 2,showing that the decrease in adsorption temperature results in ann(H2O)/n(SO2)ratio greater than 1,and the value increases rapidly with the decrease of temperature.This indicates that in addition to the combining of SO2and O atoms to form H2SO4,excess water is adsorbed on the carbon surface;this trend gradually increases with the decrease in temperature.According to previous research [43,44],this is mainly because polar Ocontaining functional groups,such as carbonyl and,carboxyl [45],and newly formed H2SO4tend to adsorb water molecules by hydrogen bonding on the carbon surface and also easily to aggregate into groups to fill in the microporous structure under the action of hydrogen bonds.

Table 1 Pore structure characteristics data of PPAC and AC reported in some literature works

Fig.1.(a) SEM image and (b) FTIR spectra of PPAC sample.

3.3.TPD experiments

After adsorption,TPD analysis was used to explore the desorption gas release and determine the adsorption states of H2O and SO2.Although carbon is hydrophobic,this does not mean that carbon materials do not like water.In fact,the hydrophilicity of polar functional groups on the surface of carbon cannot be ignored under an atmosphere of relatively low humidity.Two conspicuous desorption peaks can be seen in the H2O-TPD profiles (Fig.3(a)).The first peak appears at approximately 230 °C in the center,which is related to the volatilization of hydrogen-bonded water.Hydrogen bonding increases the thermal stability of the desorption of water molecules [46];thus,the volatilization temperature of hydrogenbonded water is higher than the boiling point at atmospheric pressure.Fig.3(a) shows that the desorption peak strength increases rapidly with a decrease in the adsorption temperature,indicating that low temperature can strengthen the adsorption of hydrogenbonded water,which is consistent with the conclusion from the previous statement (Section 3.2,Table 2).The temperature of the second water desorption peak,located near 340 °C,is almost the same as that of the first SO2desorption peak (Fig.3(b)),which is caused by the dehydration of H2SO4.

Fig.3(b) shows the SO2-TPD profiles at different adsorption temperatures.There are two main SO2desorption peaks at 342 and 482 °C respectively,indicating that two different adsorption species also exist.Therefore,the first and most important peak occurs at 342 °C,which is related to the redox reaction of H2SO4and carbon in the pores [47].In this manner,the active sites and channels occupied by H2SO4can be released to restore the activity of the carbon adsorbent.The second peak is located at approximately 482 °C,and its strength is relatively small,which may be related to the decomposition of sulfates,especially Fe2(SO4)3[19,48].As can be seen the Fe 2p X-ray photoelectron spectroscopy spectra shown in Fig.S4,the content of Fe2(SO4)3increased after SO2loading,which is mainly due to the reaction of Fe2O3with H2SO4to form Fe2(SO4)3in the adsorption stage,but it will be decomposed into Fe2O3and SO2in the high temperature regeneration process.Therefore,some metal oxides in the carbon adsorbent ash or the metallic substanceviaimpregnation may react with SO2to form a stable sulfate in the adsorption process,which has a high decomposition temperature.As shown in Fig.3(b),the intensity of the first SO2desorption peak changes significantly with a decrease in the adsorption temperature,whereas the second SO2desorption peak changes only slightly.In general,the effect of adsorption temperature on SO2adsorption is mainly reflected in the reaction of H2SO4formation.

Moreover,we used the dimensionless SO2desorption rate (defined as×100%,0 ≤ti≤tdes)obtained by integrating the SO2desorption curve over time,as in Eq.(2),to characterize the regeneration rate over time,as shown in Fig.S5.The serial SO2desorption rate curves are basically consistent,indicating a similar SO2release law for the samples.When the temperature increased to 600 °C,the desorption rate of SO2was approximately 95%.Therefore,the later analysis was simplified such that complete regeneration could be realized when the temperature was raised to 600°C.

3.4.Kinetic study and thermodynamic parameters

To obtain the desorption activation energy of H2O and SO2at different adsorption species,five different heating rates were carried out in the TPD experiment.Peak Fit v4.12 from Systat Software was used to deconvolute the desorption profiles using the Gaussian-Lorentz method to distinguish different adsorption species.As described in Section 3.3,the desorption peaks of H2O and SO2are represented bya,bandc,d,respectively.As shown in Fig.S6,as the heating rate increases,the peak temperature gradually increases.

Based on the linear fit plot of 2 lnTm-ln βversus1/R(273+Tm)(Eq.(3)),the activation energy for the desorption of the SO2and H2O adsorption species on PPAC can be calculated,as shown in Fig.4 and Table 3.The slope calculated from the fitted line represents the strength of the activation energy.As shown in Fig.4,peakarepresents the volatilization of hydrogen-bonded water,with the smaller slope indicating a smaller activation energy,whereas peakdrepresents the decomposition of sulfate with a larger activation energy.In addition,the slopes of the fitted lines of peaksbandcare almost the same and overlap each other,indicating that peaksbandccorrespond to two products under the same reaction,in which H2SO4decomposes to release H2O and SO2simultaneously.

Fig.2.(a) SO2 and (b) H2O adsorption capacity at five grades of temperature.

Table 2 SO2 and H2O adsorption capacities and mass growth rate

Fig.3.(a) H2O-TPD and (b) SO2-TPD profiles at different adsorption temperatures.

Fig.4.Relationship between the maximum desorption temperature (Tm) and the heating rates (β) for SO2 and H2O.

The thermodynamic parameters (ΔH,ΔG,and ΔS) and kinetic parameterAwere calculated using Eqs.(4)-(7),and the obtained parameters are summarized in Table 3.Reaction chemistry can also be explained by the value obtained from the pre-exponential factorA[28].A lowA-value(＜109s-1)indicates a surface reaction;however,if the reactions do not depend on the surface area,the low value also demonstrates a closed complex.A high value (≥109s-1) indicates a simple complex [49].In this case,a preexponential factor of less than 104means that the activation energy value is relatively low;these lower values in activation energyEand pre-exponential factorAindicate a faster and more facile reaction.The change in enthalpy ΔHshows the energy differences between the activated complex and the reagents;the smaller the value,the more favorable it is to form an activated complex.The positive ΔHvalues calculated for all processes agree with the endothermic effect recorded during sulfur-containing PPAC regeneration.We determine that the difference between ΔHandEvalues is small at each peak conversion point,indicating that the potential energy barrier is lower,which is beneficial to the reaction.The change in Gibbs free energy ΔGreflects the increases in the total energy of the system as the reagents approach each other,forming the activated complex [49].The change in entropy ΔSexhibits negative values,indicating that the degree of disorder of the product formed by bond dissociation is lower than that of the initial reactant.Here,a high entropy value is observed,indicating that the sample is far from thermodynamic equilibrium.In this situation,the sample showed high reactivity and could react faster,thereby shortening the reaction time.

3.5.Optimization of adsorption temperature

Similarly,the deconvolution method was also used to separate the proportions of H2O and SO2at different adsorption temperatures.The area ratio α of different adsorption states was used to represent the proportion of each component,as shown in Table S2.The main effect of reducing the adsorption temperature on the adsorption capacity of H2O and SO2is to increase the proportion of peaka(hydrogen-bonded water) and peakc(adsorbed H2SO4).Specifically,combined with the adsorption capacity of H2O and SO2(Table 2),the amount of gas released at peaksbandccan be obtained,and the molar ratio ofis approximately 1,demonstrating that the two peaks are caused by the decomposition of H2SO4.

Table 3 Kinetic parameters of PPAC with a heating rate of 10 °C·min-1

According to the above analysis,under certain engineering conditions,reducing the adsorption temperature can affect the total desorption energy consumption in the following three ways: (i)increase the H2O adsorption capacity,(ii) increase the SO2adsorption capacity,and (iii) reduce the adsorbent dose.Evidently,the first two effects can increase the energy consumption of the unit adsorbent in the desorption process,whereas the third effect can directly reduce the energy consumption.Thus,on the basis of achieving the same purification effect,the total desorption energy consumption may be minimized by adjusting the adsorption temperature,which is of great significance for the optimization of the adsorption-desorption system in the effort to reduce operating costs.To analyze the consumption of desorption energy,the following assumptions are made:

(i) 2-h SO2adsorption capacity of PPAC is the working adsorption capacity.

(ii) adsorbed H2O and SO2are completely released at the desorption peak temperature.

(iii) desorption process begins at the corresponding adsorption temperature,and the temperature rises to 600 °C at a heating rate of 10 °C·min-1.

(iv) the influence of multi adsorption-desorption cycles is not considered in this study.

Based on the above analysis and assumptions,the energy consumption of desorption can be divided into four parts according to the adsorbent components.Details are as follows:

Part 1: Energy consumptionQ1(kJ·h-1) is equal to the heat absorbed by the temperature increase from the corresponding adsorption temperatureTadsof adsorbent to the final regeneration temperatureT4(600 °C).The calculation formula is as follows:

wherecp,cis the specific heat capacity of the PPAC sample,which is set ascp,c=0.00122T+0.868 kJ·kg-1·°C-1[50];qm,cis the mass of adsorbent (kg·h-1);Tadsis the adsorption temperature (°C);T4is the final regeneration temperature (°C).

Part 2: Energy consumptionQ2(kJ·h-1) is equal to the heat absorbed by the hydrogen-bonded water,as its temperature increases fromTadsto peakatemperatureT1(230.97 °C) and the water evaporates.The calculation formula is as follows:

whereT3is the peakdtemperature(°C);αdis the area ratio of peakdin SO2desorption (%);and ΔHdis the enthalpy of the sulfate decomposition (kJ·mol-1).

Therefore,the total energy consumption of the SO2-loaded adsorbent from the adsorption temperature to complete desorption can be expressed as the sum ofQ1toQ4.

According to the above description,the total energy consumption of desorption and the energy consumption of the components at different adsorption temperatures are calculated,as shown in Fig.5.The area of each part in the figure represents the energy consumption of desorption.Overall,the total desorption energy consumption decreased first and then increased as the adsorption temperature increased,a lower desorption energy consumption was achieved at 75°C.The main reason for this is that the adsorption capacity of SO2and H2O varies at different adsorption temperatures.As the adsorption temperature increases,the adsorbed SO2capacity gradually decreases (Fig.3(b)),causing an increase in the amount of adsorbent,which significantly increases the energy consumption of the carbon material during the heating process (energy consumption ofQ1increases from 7.03×105kJ·h-1for 50 °C to 9.69×105kJ·h-1at 150 °C).Meanwhile,a lower adsorption temperature simultaneously increases the H2O and SO2absorption capacities,resulting in increased energy consumptionQ2andQ3for H2O and H2SO4heating and further evaporation,respectively.Clearly,the energy consumption caused by water heating and evaporation increases rapidly (energy consumption ofQ2increases from 8.56×103kJ·h-1at 150 °C to 7.49×104kJ·h-1at 50 °C).In addition,the energy consumption value caused by the sulfate decomposition,represented byQ4,changes little with the adsorption temperature.Therefore,considering the influence of adsorption temperature on desorption energy consumption,it is effective to control the adsorption temperature near the optimal adsorption temperature of 75 °C.Meanwhile,the optimal adsorption temperature represents a balance between the adsorbent treatment capacity and the unit adsorbent energy consumption in the desulfurization system.

Fig.5.Desorption energy consumption of PPAC regeneration at different adsorption temperatures.

We express some additional thoughts and suggestions regarding energy saving in carbon material adsorption desulfurization.First,efforts should be made to improve the sulfur capacity of carbon adsorbents and greatly reduce the energy consumption by reducing the use of adsorbents.Second,the water adsorption capacity of the adsorbent should be selectively reduced,decreasing the energy consumption caused by water evaporation.Third,coal with lower ash content should be selected as the precursor for the preparation of carbon adsorbents,which can reduce the formation of sulfate in the adsorption process,thereby decreasing the maximum desorption temperature and energy consumption.

4.Conclusions

In conclusion,the effect of adsorption temperature on the adsorption and desorption processes of coal-based activated coke was systematically investigated.The main conclusions are summarized as follows:

1.The PPAC sample prepared in the drop-tube reactor system under an O2and steam atmosphere demonstrated a welldeveloped pore structure and abundant surface functional groups.

2.The adsorption temperature can greatly affect the performance of the adsorbent.With a decrease in the adsorption temperature,the adsorption capacity of SO2increased linearly from 61.53 mg·g-1at 150 °C to 98.85 mg·g-1at 50 °C.Meanwhile,the adsorption capacity of H2O increased from 20.1 mg·g-1at 150 °C to 102.7 mg·g-1at 50 °C.

3.Through TPD analysis,both SO2and H2O adsorbed on the surface of carbon exhibited two adsorption states.For H2O,the desorption peaks at 230.97 and 340.14 °C corresponded to the evaporation of hydrogen-bonded water and dehydration of liquid H2SO4,respectively.For SO2,the desorption peaks at 342.04 and 482.53°C corresponded to the reaction of H2SO4and C and decomposition of sulfate,respectively.

4.The desorption activation energies of the H2O adsorption species were 6.49 and 19.33 kJ·mol-1,which corresponded to hydrogen-bonded water and the dehydration of liquid H2SO4.The desorption activation energies of the SO2adsorption species were 20.39 and 39.13 kJ·mol-1,which corresponded to H2SO4and sulfate,respectively.

5.For a stable SO2-containing flue gas,the total desorption energy consumption decreased at first and then increased with an increase in the adsorption temperature,reaching a minimum value at 75 °C.With the increase in temperature,the increase in adsorbent treatment caused by the decrease in SO2adsorption capacity can significantly increase the energy consumption of regeneration.At a lower temperature,the energy consumption of water and H2SO4desorption gradually became significant,increasing the total energy consumption.Therefore,the optimal adsorption temperature represents a tradeoff between the adsorbent treatment and unit energy consumption.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2017YFB0602901).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.08.002.