Nanhang Dong,Ruiqiang Huo,2,Ming Liu,2,Lisheng Deng,Zhengbing Deng,Guozhang Chang,Zhen Huang,Hongyu Huang

1 School of Energy and Power Engineering,Northeast Electric Power University,Jilin 132012,China

2 Key Laboratory of Renewable Energy,Chinese Academy of Sciences(CAS),Guangzhou Institute of Energy Conversion,Guangzhou 510640,China

3 Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China

4 China United Engineering Corporation Limited,Hangzhou 310052,China

5 State Key Laboratory of High-efficiency Utilization of Coal and Green,Chemical Engineering,Ningxia University,Yinchuan 750021,China

6 Dalian National Laboratory for Clean Energy,CAS,Dalian 116023,China

Keywords:Chemical looping gasification(CLG)Copper slag NiO Sludge Oxygen carrier(OC)

ABSTRACT Chemical looping gasification(CLG)provides a novel approach to dispose the sewage sludge.In order to improve the reactivity of the calcined copper slag,NiO modification is considered as one of the good solutions.The copper slag calcined at 1100°C doped with 20 wt%NiO(Ni20-CS)was used as an oxygen carrier(OC)in sludge CLG in the work.The modification of NiO can evidently enhance the reactivity of copper slag to promote the sludge conversion,especially for sludge char conversion.The carbon conversion and valid gas yield(Vg )increase from 67.02%and 0.23 m3·kg?1 using the original OC to 78.34%and 0.29 m3·kg?1 using the Ni20-CS OC,respectively.The increase of equivalent coefficient(Ω)facilitates the sludge conversion and a suitable Ω value is determined at 0.47 to obtain the highest valid gas yield(0.29 m3·kg?1).A suitable steam content is assigned at 27.22%to obtain the maximum carbon conversion of 87.09%,where an acceptable LHV of 12.63 MJ·m?3 and Vg of 0.39 m3·kg?1 are obtained.Although the reactivity of Ni20-CS OC gradually decreases with the increase in cycle numbers because of the generation of NiFe2 O4-δ species,the deposition of sludge ash containing many metallic elements is beneficial to the sludge conversion.As a result,the carbon conversion shows a slight uptrend with the increase of cycle numbers in sludge CLG.It indicates that the Ni20-CS sample is a good OC for sludge CLG.

1.Introduction

Sewage sludge,the by-product of sewage treatment plants is a complex mixture composed of inorganic and organic substance,bacterial microbes,and toxic pollutants[1,2].The production of municipal sludge increases dramatically year by year around the world,and the safe and effective treatment of sludge has become a hot issue.The reduction,harmlessness and resource recovery is viewed as the main objective of sludge treatment[3].The thermochemical treatment is considered as an important approach to achieve the objective because high temperature can significantly reduce the volume of sludge,kill pathogenic microorganisms,and recycle the energy in the sludge[4].Among the thermochemical methods,chemical looping gasification (CLG) is viewed as an attractive and promising technology for sludge treatment because of some potential merits.

Compared with the traditional gasification technologies(e.g.air gasification,steam gasification,and oxygen gasification),CLG is viewed as a novel gasification technology because it uses the lattice oxygen in oxygen carrier(OC)instead of gaseous molecular oxygen as an oxidized medium[5,6].The CLG process includes two interconnected reactors(i.e.Fuel Reactor(FR)and Air Reactor(AR)),where a metallic oxide as an OC circulated between the FR and the AR provides the required heat and oxygen sources for fuel gasification[7,8].The solid fuel(CHxOy)is gasified to the high quality syngas while the OC(MeO)is reduced in the FR,as shown in reaction(1),and then the reduced OC(Me)is regenerated to its initial state in the AR,as shown in reaction(2).The required heat of fuel gasification is provided by the oxidization of reduced OC,thus achieving to the self-heating operation of CLG process.Hence,the overall reaction of CLG is equal to the oxygen gasification,as shown in reaction(3).

Since the production of pure oxygen is not required,the synthesis gas with low cost can be produced in CLG process.Meantime,the high quality synthesis gas with relatively high heating value and low tar in CLG also can be produced because the N2dilution is avoided and the OC plays a role of a catalyst to catalytically crack tar[9,10].In addition,CLG technology demonstrates a characteristic of low NOxemission.The N2avoidance and relatively low temperature(<1000°C)inhibit the formation of thermal and rapid NOx[11].NOxprecursors(i.e.NH3and HCN),easily generated under reducing atmosphere are prone to form N2in the presence of OC,thus restraining the generation of fuel NOx[12].Hence,the CLG technology is used as an innovation approach for sewage sludge disposal based on the mentioned above advantages.

The OC is the cornerstone of the successful implementation of CLG.Besides the excellent redox activity and regenerability,a good OC candidate needs to have the characteristics of anti-sintering,low cost and environmental friendliness [13,14].Until now,more than 1000 kinds of OCs have been reported,including transition metal oxides,non-metal oxides,minerals and waste materials [15–17].Among them,the iron-based oxide is considered as one of the suitable OCs for the industrial application of CLG due to its oxygen carrying capacity,low cost and environmentally friendly[18,19].The presence of Fe2O3OC evidently promoted the conversion of microalgae and the gasification efficiency increased from 61.65% without OC to 81.64%with OC[20].The Fe2O3/Al2O3OC presented a good reactivity in CLG of biomass during 60 h continuous operation in a 10 kWthdual circulation fluidized bed reactor[21].Natural iron ore was also often used as an OC in CLG because of its abundance and lower cost.The biomass CLG using hematite as an OC was discussed in detail in a single bubbling fluidized bed[9,22].The hematite was similar to the water vapor,which provided the oxidizing medium for biomass conversion,while it also can act as a catalyst to remove biomass tar.The reactivity of natural hematite as an OC in CLG of rice husk was investigated via a 25 kWthdual circulation fluidized bed reactor,it was found that the gas yield was obtained the maximum of 0.74 m3·kg?1at 860 °C [23].The natural iron ore OC showed a good cycling stability during continuous 23 h in CLG of sewage sludge[24].

The sewage sludge generally contains a large amount of inorganic ash,easily causing the loss of OC particles during the ash discharge.Therefore,it is very important for the cost of OC in sludge CLG.Copper slag,as a byproduct of copper smelting plant,is very cheap and widely sourced[25].Generally,the yield of copper slag attains to 2.2-3 ton for every ton of refined copper[26].The main components of copper slag are fayalite(Fe2SiO4)and magnetite(Fe3O4)because the vast majority of copper element is extracted during the copper ore smelting [27],which is easily formed the Fe2O3species(the active component of OC)under high-temperature via the reactions(4) and(5)[28].Thus,the calcined copper slag can be used as an OC in CLG.

The calcined copper slag played dual functions of oxidation and catalysis in the gasification of biomass and garbage[29,30].Although the copper slag calcined at 1100°C showed a good cycling stability in the CLG of sewage sludge,the carbon conversion was relatively low and it was only 79.78%even at a high mass ratio of OC/sludge under steam atmosphere[28].It indicated that the calcined copper slag OC showed a relatively low reactivity.Thus,some approaches need to be adopted to improve the reactivity of calcined copper slag.It is a good solution to enhance the reactivity of the copper slag by doped with foreign ions(e.g.transition metals (Ni/Cu/Mn) and alkali and alkaline earth metals)[31–33].Among these foreign ions,the transition metal Ni shows an excellent performance in CLG.The doping of NiO evidently improved the reactivity of natural iron ore and the carbon conversion attained to 55.60%in CLG of biomass char[34].The reactivity of ilmenite OC significantly enhanced even the addition of a small amount of NiO [35].Therefore,it can conclude that the compound OC,prepared by NiO doped copper slag possesses an excellent performance in CLG.Actually,Fe-Ni bimetallic OC not only can greatly improve the reactivity of Fe-based OC,but also can overcome the high cost and environmental hazard of Ni-based OC[31].

In order to enhance the conversion of sewage sludge,benefiting its disposal and treatment,the NiO-doped copper slag used as an OC in CLG of sludge is a good solution.Hence,this work is to explore the performance of NiO-doped copper slag OC in CLG of sewage sludge.In the present work,the performance of calcined copper slag doped with different mass ratio was first evaluated and a suitable ratio was screened out.Then,the gasification performance of the selected OC with sewage sludge was investigated on a fixed bed,and the effects of various parameters including the oxygen sources,equivalent coefficient(Ω),steam content and cycle numbers on the sewage sludge gasification were discussed in detail.Finally,the characterizations of the fresh and used OCs were analyzed to clarify the evolutions.

2.Experimental

2.1.Sewage Sludge

The sewage sludge used in this work was collected from Shunde Wastewater Treatment Plant in FoShan city.It was first dried at 105°C for 48 h until a constant mass,and then crushed and sieved to 80 mesh (0.18 mm).The proximate and ultimate analyses of sewage sludge are listed in Table 1.

2.2.Preparation of OCs

The NiO-doped copper slag OC was prepared by the incipient-wetness impregnation method.The original copper slag sample,collected from Yunnan Copper Industry was first calcined in a muffle furnace at 1100 °C for 3 h to achieve its complete oxidation.The sample was crushed into the powder to obtain the original OC(1100CS).According to the mass ratio of NiO to 1100CS,given at 3 wt%,5 wt%,10 wt%,20 wt%and 50 wt%,a corresponding amount of Ni(NO3)3·6H2O powder (Aladdin company,chemically pure) was dissolved at a certain amount of deionized water to obtain the solution.Then,the calcined copper slag powder was poured into the solution,and it was transferred to a beaker with a magnetic stirrer and kept at 40°C for 24 h to obtain the slurry.Moreover,it was dried at 105°C in an oven for 48 h to obtain the mixture.Finally,the dried mixture was calcined at 950 °C for 3 h to decompose the nitrate,and then it was crushed into the powder to obtain the different OC samples,which are named as Ni3-CS,Ni5-CS,Ni10-CS,Ni20-CS and Ni50-CS,respectively.

2.3.Characterization of OCs

The crystal structure of the OC samples was analyzed using an X-ray diffractometer(XRD,X'Pert Pro MPD)equipped with Cu Karadiation(λ=0.15406 nm).The operating voltage and current were set at 40 kV and 40 mA,respectively.Moreover,the diffraction angle (2θ) of the samples was scanned from 10° to 80° at a rate of 2(°)?min?1.Theelemental composition of the OC samples was analyzed by an X-ray fluorescence spectrometer(XRF,AXIOSmAX-PETRO).The H2-temperature programmed reduction(H2-TPR,CPB-1)and the thermo-gravimetric analyzer(NETZSCH,STA-449-F3)were used to compare the reactivity of different OCs.In the H2-TPR tests,about 100 mg of sample was placed in a Utube reactor and then reacted with 10.0 vol%H2balanced with He(The flow rate of gas mixture was controlled at 60 ml·min?1).The sample was heated from room temperature to 900 °C at a heating rate of 10°C·min?1and then maintained at a constant temperature(900°C)for 1.0 h.In the TG tests,about 30 mg of sample with the mass ratio of sewage sludge to OC at 5:5,was heated from room temperature to 900°C at a heating rate of 15°C·min?1and then kept at a constant temperature for 0.5 h under an argon atmosphere.The carrier gas and shielding gas were high purity argon and their flow rates were set at 40 ml·min?1and 20 ml·min?1,respectively.

Table 1 The proximate and ultimate analyses of sludge(*:by difference)

2.4.Fixed bed

The performance of the CLG of sewage sludge was conducted on a fixed bed.The schematic layout of the fixed bed and the experimental procedures were mentioned in the previous work[24].During the reduction stage,1.0 g sewage sludge stored in the fuel reservoir was promptly dropped into the reactor while the steam was introduced into the reactor if it was required after attaining the desired temperature.The carrier gas of argon was fixed at 50 ml·min?1and the reaction time was set at 1.0 h.The inert gas(Ar)was switched to the air during the oxidation period.The flow rate of air was fixed at 150 ml·min?1and the oxidation time was set at 0.5 h.The inert gas (Ar) was kept for 15 min between reduction and oxidation period to prevent the gas mixing.The reaction temperature was fixed at 900°C.The mass of OC is determined by an equivalent coefficient value(Ω),which is defined the mole ratio of the actual oxygen supply in the feeding OC to the theoretical oxygen demand for complete combustion of unit mass of sludge(1.0 g).After purification and drying process,the entire gas product was collected by a sampling bag for the analysis of gas chromatograph(SHIMADZUA Gas Chromatograph,GC-2014).The used OC samples were cooled to room temperature under an inert atmosphere and then collected for the later analysis.

2.5.Data processing

The parameters involved in the work,including the relative concentration of gas products (Ci,%),carbon conversion of sewage sludge(ηc,%),lower heating value of syngas(LHV,MJ·m?3) and valid gas yield(Vg,m3·kg-1)were calculated through referring to the previous work[28].

3.Results and Discussion

3.1.Performance of different OCs

The elemental compositions of the different OCs were analyzed by an XRF and the results are shown in Table 2.The OC samples withdifferent Ni-doped(X)is represented by NiX-CS.The main elements of original OC include Fe,O,Si and a small amount of Ca,Zn,Al,Mg and Cu.More and more Ni element appears in the modified OCs with the increase of the Ni loading.It is worth noting that the Cu content in the copper slag sample is very low.

Table 2 XRF analysis of different OCs

The XRD patterns of different OCs are shown in Fig.1.A large amount of Fe2O3species and a small amount of Fe3O4and SiO2species,derived from the decomposition of the fayalite at high temperature are observed in the 1100CS sample.Besides the species of Fe2O3,Fe3O4and SiO2,two new species of NiO and NiFe2O4appear in the modified OC.The formation of a small amount of NiFe2O4species is attributed to the chemical reaction(6),where the NiO species reacts with Fe2O3species to generate NiFe2O4species at high temperature[34].Furthermore,the intensity of diffraction peak of NiO and NiFe2O4species gradually increase with the increase in Ni loading.

The results fully indicate that the modification of NiO evidently changes the crystalline structure of the copper slag OC sample.Hence,it can infer that the modified OC should present the different reactivity compared with the original OC.

The H2-TPR and TG experiments were conducted to compare the reactivity of different OCs and the experimental results are shown in Figs.2 and 3.A distinct H2reduction peak,located at around 760°C is observed for the 1100CS sample,which is associated with the stepwise reduction of Fe2O3species (i.e.Fe2O3→Fe3O4→FeO →Fe) [34].In addition to the H2reduction peak located at a relatively high temperature(～750°C),another small shoulder peak of H2reduction located at a relatively low temperature(～600°C)is also observed in the modified OC samples and it is more and more evident with the increase in Ni loading.Besides these two reduction peaks,it is also found that another sharp H2reduction peak lies at a lower temperature(～450°C)in the last three modified OC samples.The first H2reduction peak in the last three modified OC samples is associated with the reduction of NiO species and the peak intensity shows an uptrend with the increase of the amount of Ni loading due to the formation of more NiO species.The second H2reduction peak(a small shoulder peak)corresponds to the slight reduction of NiFe2O4species,where the partial Ni element is firstly divorced from the spinel structure [34].The third H2reduction peak with a wide span is closely related to the stepwise reduction of Fe2O3and NiFe2O4-δ(i.e.the partially reduced NiFe2O4species) species.It is noteworthy that the third H2reduction peak gradually shifts to the low temperature with the increase of the amount of Ni loading,which is ascribed to the formation of more NiFe2O4species at high Ni doping ratio,thus enhancing the reactivity of OC samples.

Fig.1.The XRD patterns of different OCs.

Fig.2.The H2 -TPR curves of different OCs.

The TG experiments on OC reacted with sewage sludge were conducted to further evaluate the reactivity of different OCs,and the results are shown in Fig.3.

The mass loss of mixtures(different OCs and sewage sludge)gradually increase with the increase in Ni-doping ratio from the TG curves.Furthermore,it is observed that the sewage sludge conversion is divided into three stages from the DTG curves.The first stage is ascribed to the removal of a small amount of moisture in the sewage sludge below 150°C.The second stage is associated with the devolatilization of sewage sludge between 150°C and 550°C,where a large amount of volatile is quickly removed and the stage is quite dramatic.The third stage locates above 550°C(the weightlessness peak locats at about 900 °C),which is attributed to the chemical reaction(7)of residual char reacted

with OC particles.Furthermore,it is found that the DTG curves of all the mixtures are almost overlapped at the former two stages,which means that the former two stages of sewage sludge conversion are not related to the OC particles.It is ascribed to these two stages being not the kinetic control stages,thus the moisture and volatiles in sewage sludge can be rapidly escaped before contacting with the OC particles.The third stage,where the solid–solid reactions between the sludge char and OC particles mainly occur[24],is the kinetic control process of sludge conversion.In this stage,the OC provides the lattice oxygen for sludge char conversion,thus a distinct difference is observed from the DTG curves because of the difference in the reactivity of OC particles.The initial temperature gradually shifts to the low temperature and the temperature span of mass loss gradually increases with the increase in the Nidoping ratio.Because of the reactivity of NiO or NiFe2O4species obviously higher than that of Fe2O3species [36],thus the formation of NiO and NiFe2O4species in modified OC can evidently improve the reactivity of original OC(1100CS).In other words,the higher the loading of NiO implies the stronger the interreaction between the OC particles and the sludge char,thus the conversion of sludge is more thorough.As a result,the mass loss of mixture gradually increases with the increase in Ni-doping ratio.In the blank experiment[28],where the OC sample was replaced by the inert carrier(Al2O3),the third mass loss peak was very small because the inert Al2O3cannot provide lattice oxygen for char conversion.

The TG results are in line with the H2-TPR results,in which the oxidizability of calcined copper slag is significantly enhanced by NiO modification.However,it is well known that the NiO material is relatively expensive and harmful to the environment.Moreover,the metallic nickel is easy to cause sintering of OC particles because of the thermodynamic limitations [37].Therefore,an appropriate NiO-doping ratio is very important for the modified copper slag OC.The composite Fe-Ni bimetallic OCs not only can overcome the thermodynamic limitations,high cost and environmental harmfulness of Ni-based OCs,but also can significantly improve the oxidizability of Fe-based OCs.Hence,it is viewed as a good choice to use of the Fe-Ni bimetallic OC for sewage sludge conversion.The modified copper slag with Ni-doping ratio of 20 wt% is selected as an OC for CLG of sewage sludge in this work.

3.2.Comparative experiments

The comparative experiments on CLG of sludge using two different OCs (Ni20-CS and 1100CS)were investigated to further highlight the role of Ni in modified OCs,and the results are shown in Fig.4 and Table 3.During the experiments,the mass of OCs was fixed at～3 g(Ω=0.47).

Fig.3.TG-DTG curves of different OCs with sewage sludge.

Fig.4.Effect of different oxygen resources on the gas yield.

Table 3 Gasification characteristics of sewage sludge under different oxygen sources

The huge differences in sludge gasification are presented because of the different performance of two OCs.In the baseline experiment,where the 1100CS was used as an OC,the gas yields of CH4,CO2,CO,and H2are lower than those in the experiment with Ni20-CS OC,but the gas yield of ChHk(h ≥2)is higher than that in the experiment with Ni20-CS OC.The results indicate that the modification of nickel in OC is beneficial to the conversion of macromolecular hydrocarbons (i.e.ChHk,h ≥2)to small molecular compounds(e.g.H2/CO).This is ascribed to the Ni doped OC has the better oxidation and catalytic performance,which promotes the conversion of sludge char and tar.Accordingly,the carbon conversion and valid gas yield increase from 67.02% and 0.23 m3·kg?1with 1100CS OC to 78.34% and 0.29 m3·kg?1with Ni20-CS OC,respectively.However,the LHV of gasification gas decreases from 15.82 MJ·m?3with 1100CS OC to 12.27 MJ·m?3with Ni20-CS OC,which is attributed to a fact that the improvement of OC reactivity(i.e.doping of Ni)facilitates to more combustible gases being converted to CO2and H2O.Thus,the lower relative concentrations of combustible gases with Ni20-CS OC leads to the decline of LHV of gasification gas.

In a word,it once again indicates that the modification of metallic Ni can evidently enhance the reactivity of original OC(i.e.1100CS),which is in line with the H2-TPR and TG-DTG experiments.

3.3.Effect of Ω

The Ω value is a crucial parameter to evaluate the sewage sludge gasification.Effect of Ω on the gasification characteristics of sludge was investigated and the results are shown in Fig.5.

Apparently,the concentrations of combustible gases(i.e.H2,CH4,CO,and ChHk)show a downtrend with the increase in Ω,which is ascribed to the chemical equilibrium (8).The increase of Ω facilitates the reaction(8)proceeding towards the right because more lattice oxygen is provided at high Ω value.As a result,the concentrations of combustible gases decrease and the CO2concentration increases.Accordingly,the low heating value (LHV) of crude gas shows a downtrend with the increase in Ω value.

It is well known that the Al2O3carrier is an inert carrier and it cannot provide the oxygen source required for the sludge gasification.Thus,only a devolatilization process occurs for the sludge conversion with Al2O3carrier.In this work,the active OC is replaced by the inert Al2O3carrier when the Ω value is kept at zero.Consequence,the sewage sludge is only pyrolyzed at high temperature because of the absence of oxygen source and a large amount of organic matter is contained in the char and tar at the Ω value of zero.As a result,the carbon conversion of sludge is low(54.45%).

According to the TG results,the OC can provide active lattice oxygen for sludge gasification,thus facilitating more C and H elements in sludge to be converted into gas products.Since more lattice oxygen is provided at a high Ω value,the carbon conversion of sludge evidently increases and it shows an uptrend with the increase in Ω value.Additionally,a high Ω value implies the increase in bed height of OC particles,prolonging the residence time to further promotes the sludge conversion.As a result,the carbon conversion increases from 54.45%at Ω of 0 to 87.62%at Ω of 1.09.Interestingly,the valid gas yield(Vg)exhibits a“parabolic change”in the range of Ω value increasing from zero to 1.09.At the low Ω value,the C/H elements in sludge are prone to generate CO/H2because of the lack of lattice oxygen;however,the C/H elements in sludge are more easily to generate CO2/H2O in the presence of sufficient oxygen source when the Ω value exceeds 0.47.As a result,the valid gas yield shows a trend of first increasing and then decreasing with the increase in Ω value and it attains to the maximum at Ω value of 0.47.

The purpose of sludge CLG is to convert sludge into high-quality syngas to realize the resource utilization.At the same time,the reduction of sludge also should be taken into account.Hence,a suitable Ω value was selected at 0.47 in this work,where the highest valid gas yield(0.29 m3·kg?1) and a relatively high carbon conversion (78.34%) were obtained.

3.4.Effect of steam

It is well known that the steam can provide oxygen sources for sludge conversion.What's more,the steam can be used as a fluidizing agent in the industrialization of fluidized bed,which is very crucial for the industrial operation of CLG technology.Consequently,the effect of steam content on the CLG of sludge was investigated on the fixed bed,and the experimental results are shown in Fig.6.During the experiment,the reaction temperature is fixed at 900 °C and the Ω value is set at 0.47.Additionally,the change of steam content was achieved by changing the flow rate of injected water while maintaining the carrier gas flow rate at 50 ml·min?1.

According to the Le Chatelier's principle,the increase of reactants is beneficial to the chemical equilibrium proceeding towards the right.Thus,the chemical equilibriums (9)-(12) proceed towards the right with the increase of steam content.Accordingly,the H2and CO2yields increase and CH4and CnHmyields decrease with the increase of steam content.The CO yield also shows a slight downtrend,which decreases from 143 L·kg?1of 0.00%to 135 L·kg?1of 42.75%because of the balance between the reaction(9)and the reactions(10)-(12).Based on the evolutions of relative gas concentrations,the LHV of gasification gas shows a downtrend with the increase in steam content.

Fig.5.Gasification characteristics of the sewage sludge under different Ω value.

On the one hand,the increase of steam facilitates the conversion of volatiles and char,thus more C and H elements in sludge are converted into gas products.As a result,the carbon conversion shows an uptrend with the increase of steam content.On the other hand,the addition of excess steam would lead to the decrease of the temperature in the effective reaction space and the increase of superficial velocity,which are not conducive to the conversion of sludge because of the decline of reaction temperature and the decrease of residence time[9].Thus,the carbon conversion shows a downtrend due to the introduction of excess steam.According to the combination of these two aspects,it can be concluded that there should be a suitable steam content during sludge gasification,where the carbon conversion attains to the maximum.In this work,the carbon conversion firstly increases from 78.34%at steam content of 0.00%to 87.09%at steam content of 27.22%,and then it slightly decreases to 84.58%at steam content of 42.75%.Additionally,the generation of steam is an energy-intensive process and excess steam would significantly increase the operating costs.Hence,a suitable steam content is assigned at 27.22%to obtain the maximum carbon conversion of 87.09%,where an acceptable LHV of 12.63 MJ·m?3and Vgof 0.39 m3·kg?1are obtained.

3.5.Cyclic performance of OC

OC is alternately exposed to a reduction–oxidation atmosphere to achieve the high-efficient conversion of sludge.Hence,the recyclability of the OC is a key property in the sludge CLG.In the work,a continuous operation of 12 cycles was carried out on the fixed bed to evaluate the recyclability of the Ni20-CS OC.The reaction temperature,Ω value and steam content were fixed at 900°C,0.47 and 27.22%,respectively.

The H2-TPR experiments of the different OCs and the sludge ash were firstly performed to explore the reactivity of OC after multiple cycles,and the experimental results are shown in Fig.7.There are two apparent reduction peaks in the H2-TPR profiles of OCs.The first peak locates at about 450°C,and the intensity becomes weaker and weaker with the cycle numbers.The second peak is found at about 700 °C,and the intensity shows an uptrend after multiple cycles.Additionally,it is further observed that the second peak moves slightly towards the low temperature zone with the increase of cycle numbers.The first reduction peak is ascribed to the reduction of NiO species and the second reduction peak is assigned to the stepwise reduction of Fe2O3and NiFe2O4-δspecies according to the section‘Performance of different OCs’.With the increase of cycle numbers,more and more NiFe2O4-δspecies are formed in the OCs through the reaction(13)because the OC particles are exposed to a reducing-oxidizing atmosphere for a long

Fig.6.Effect of steam content on the sludge gasification.

Fig.7.H2 -TPR curves of different OCs and sludge ash.

time at high temperature(900°C),thus the content of NiO species in the OCs gradually decreases with the cycle numbers.Hence,it can be inferred that the first reduction peak becomes weak gradually and the second reduction peak becomes strong gradually with the increase of cycle numbers.Additionally,the order of oxidizability of three species is NiO>NiFe2O4-δ>Fe2O3[36],thus the second peak moves to the left.Furthermore,it can be concluded that the reactivity of OC particles shows a downtrend with the increase in cycle numbers because of the formation of more NiFe2O4-δspecies.At the same time,it is also observed that the sludge ash has a certain oxidation capacity because it contains many metal oxides.

The results of the effects of cycle numbers on sludge CLG are shown in Fig.8.The decrease of OC reactivity would be adverse to the conversion of combustible gas.In other words,the combustible gas yields(i.e.H2,CO,CH4and ChHk)would increase and CO2yield would decrease if the reactivity of OC decreases.It is observed that the gas yields of H2,CO and CO2show an uptrend with the increase of cycle numbers,however,the gas yields of CH4and ChHkdisplay a downtrend after 12 cycles.This is attributed to the fact that some hydrocarbons(i.e.CH4and ChHk)are catalytic reformed to produce H2,CO and CO2via the reactions(14)-(15)because the sludge ash contains some alkali and alkaline earth metal oxides(e.g.K,Ca,and Mg),which are the good catalysts for the reforming of hydrocarbons[38,39].As a result,the LHV of the syngas decreases from 12.89 MJ·m?3to 11.79 MJ·m?3after 12 cycles.

Fig.8.Cyclic performance of Ni20-CS OC in sludge CLG.

The conversion of sludge in CLG is codetermined by the reactivity of OC and the gas mean residence time.The H2-TPR experiments indicate that the reactivity of OC shows a downtrend after 12 cycles.However,it is observed that the carbon conversion shows a slight uptrend(from 87.47%at the 1st cycle to 88.94%at the 12th cycle)with the increase of cycle numbers.Hence,it can be inferred that the gas mean residence time evidently prolongs with cycle numbers to obtain a high carbon conversion.With the increase of cycle numbers,the ash,accounting for～50 wt%of the sludge gradually accumulates on the bed of reactor,which can evidently prolong the gas mean residence time.A long gas mean residence time facilitates the sufficient contact between gas/solid and solid phases,thus more carbon element in sludge is converted into gas products,promoting the conversion of sludge.Additionally,the sludge ash has the oxidation-catalytic properties because it contains some alkali and alkaline earth metals (e.g.K and Ca) and transition metals(e.g.Fe),thereby further enhancing the conversion of sludge.Hence,more tar and char generated from sludge pyrolysis are converted to gas products with the increase of cycle numbers due to the presence of sludge ash.

3.6.Characterizations of OCs

Fig.9.XRD patterns of different OCs.

The XRD analysis of Ni20-CS OC particles in different states was also performed and the results are shown in Fig.9.The fresh OC particles is composed of a large number of Fe2O3and NiO phases and a small number of NiFe2O4-δ,Fe3O4and SiO2phases.A large number of Fe(Ni)alloy phase is generated in the reduced OC particles since a large amount of lattice oxygen is used as the oxidation agent of sludge gasification.Comparing with the fresh OC particles,there is no significant change of species in the reoxidized OC particles,however,the peak intensity of NiFe2O4-δspecies in the reoxidized OC becomes stronger and it becomes much stronger with the increase of cycle numbers.This is mainly ascribe to the reaction (13),where the Fe(Ni) alloy phase is oxidized into NiFe2O4-δphase at high temperature (900 °C) and more and more NiFe2O4-δphase is generated with the cycle numbers.After 12 cycles,the NiO phase replaced by the NiFe2O4-δphase becomes the main active component containing Ni in the OC particles.Thus,the reactivity of OC particles shows a downtrend after multiple cycles,which is in line with the H2-TPR experiments(Fig.7).

As shown in Table 4,the XRF analysis shows that the sludge ash contains a variety of active metal elements(e.g.Fe,Ca,K and Mg).Hence,the sludge ash has a certain oxidation-catalysis capacity,which facilitates the sludge conversion.As a result,the carbon conversion shows an uptrend with the cycle numbers in sludge CLG.What is more,although the sludge ash contains a large number of mineral phases,it is observed that the mineral elements are difficult to react with OC particles,thus inhibiting the inactivation of OC particles.Consequently,the sludge ash only adheres to the surface of OC particles.

Because of the sludge ash only adhering to the surface of OC particles,thus it is easy to separate the ash from OC particles according to their differences in density and size through a fluidized bed coupling with a cyclone,which is often used as the industrial equipment of sludge gasification.However,the main purpose of this work is to explore the effects of modification of NiO on the calcined copper slag OC in the sludge CLG,thus a fixed bed reactor can meet the requirements and the ash separation was not considered in the work.Comparing with the original OC(1100CS)[28],the use of Ni20-CS OC can evidently improve the sludge conversion and the amount of OC particles can be reduced by about half to obtain the same carbon conversion(～80%)in the sludge CLG.Although the reactivity of Ni20-CS OC gradually decreases with the increase in cycle numbers,the deposition of sludge ash can effectively alleviate the shortcoming.In fact,the reactivity of NiFe2O4-δspecies is much higher that of Fe2O3species during the CLG,therefore,the reactivity of Ni20-CS OC after 12 cycles is still higher than that of the original OC(1100CS).As a result,the Ni20-CS sample is a good OC for long-term service in sludge CLG.

Table 4 The elemental composition of sludge ash

4.Conclusions

CLG of sewage sludge using calcined copper slag as an OC was investigated in the previous work and it was found that the reactivity of original copper slag(1100CS OC)was not enough good.In order to improve the reactivity of 1100CS OC,the original OC was modified by NiO in the present work.A series of modified OCs with different mass ratio of NiO were prepared and it is found that the presence of NiO can evidently improve the reactivity of OC to promote the sludge conversion.TGA results further indicate that the modified OC facilitates the sludge char conversion.Although the increase in NiO content enhances the reactivity of modified OC,a doping ratio of 20 wt%NiO copper slag(Ni20-CS)was selected as an OC candidate according to the thermodynamic limitations,cost and environmental friendliness.The effects of various parameters including oxygen source,equivalent coefficient (Ω),steam content and cycle numbers on sludge conversion in CLG were discussed in detailed on a fixed bed.Compared with the 1100CS OC,the use of Ni20-CS OC can significantly increase the carbon conversion and valid gas yield(Vg),which increase by 17%and 26%,respectively.The increase of equivalent coefficient(Ω)promotes the sludge conversion and a suitable Ω value is found at 0.47 to obtain the highest valid gas yield(0.29 m3·kg?1)and an acceptable carbon conversion of 78.34%.Appropriate steam facilitates the sludge conversion and an optimal steam content is determined at 27.22%,where the maximum carbon conversion of 87.09% and an acceptable LHV of 12.63 MJ·m?3and Vgof 0.39 m3·kg?1are obtained.More and more NiFe2O4-δspecies originated from the oxidation of Fe(Ni)alloy species is formed with the increase in cycle numbers,thus the reactivity of Ni20-CS OC shows a downtrend after 12 cycles.However,the sludge ash contains some alkali and alkaline earth metals and transition metals,facilitating the sludge conversion.Additionally,the accumulation of sludge ash is beneficial to the increase of the gas mean residence time,further promoting the sludge conversion.Thus,the deposition of sludge ash contributes to the sludge conversion.As a result,the carbon conversion slightly increases with the increase of cycle numbers.The separation of sludge ash can be easily achieved based on the differences in density and size because of the sludge ash only physically adhering to the surface of OC particles.Hence,the CLG using Ni20-CS as a modified OC is a promising solution for sludge disposal and the Ni20-CS OC shows an acceptable sustainability in the long time running.

Acknowledgements

The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China(51776210),the National Key Research and Development Program of China(2018YFB0605405),the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(GML2019ZD0108),DNL Cooperation Fund CAS(DNL180205),Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2018-K15),and the Youth Innovation Promotion Association CAS(2018384).