Chemical Looping Co-combustion of Sewage Sludge and Zhundong

Article pubs.acs.org/EF

Chemical Looping Co-combustion of Sewage Sludge and Zhundong Coal with Natural Hematite as the Oxygen Carrier Shouxi Jiang, Laihong Shen,* Xin Niu, Huijun Ge, and Haiming Gu Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, China ABSTRACT: Co-combustion of sewage sludge (SS) and Zhundong (ZD) coal can be an attractive way of disposing SS and using ZD coal. Chemical looping combustion (CLC) technology is an alternative solution applied to this co-combustion with capturing carbon dioxide and minimizing gaseous pollutants, especially NOx. The performance of chemical looping cocombustion of SS and ZD coal (SZ) was investigated in a 1 kWth continuous CLC reactor using natural hematite as the oxygen carrier in this work. The influences of the temperature in the fuel reactor (800−930 °C) were studied. The carbon conversion efficiency, carbonaceous gas conversion efficiency, and carbon capture efficiency for SZ increased with the increase of the FR temperature. In comparison to dewatered SS, SZ could obtain higher carbonaceous gas conversion efficiency among the temperature range. Although SZ obtained lower carbon conversion efficiency and carbon capture efficiency, both efficiencies of SZ reached an appropriate value at a high temperature and were higher than these for single ZD coal. The carbonaceous gas conversion efficiency and carbon conversion efficiency of SZ reached 91.3 and 86.5%, respectively, at 930 °C. After 5 h of operation, the reacted oxygen carrier showed similar reactivity compared to the fresh oxygen carrier, indicating that the hematite oxygen carrier possessed good long-term reactivity during the co-combustion process. Besides, although ZD coal had a higher content of alkali metal, sodium, there was no melting on the hematite and ash agglomeration occurred during continuous long time operation, which could be ascribed to the reduction of the sodium content in the ash and the generation of alkali metal compounds with high-temperature melting points, sodium aluminosilicate and sodium pyrophosphate aluminum.

1. INTRODUCTION Sewage sludge (SS) is the waste produced by wastewater treatments, and it contains a lot of water, organic matter, inorganic materials, toxic substances, and pathogenic agents. Thus far, SS has been a serious issue as a result of its huge volume and environmental problems that appeared during its disposal process with continuous wastewater treatment.1 SS is directly used as a fertilizer early on for its high content of nutrient elements, particularly phosphorus and nitrogen, but this has been decreasing because it brings many health and environmental problems as a result of its complicated compositions, such as toxic substances. Landfilling is also decreased as a result of the large demand of land and probably environmental problems. Thermal processing is believed as one of the most promising options of disposing SS as a result of reducing the volume and destructing the hazardous compounds in SS.2−5 In addition, SS is deemed to be an energy resource as renewable biomass; therefore, using SS as fuel can not only recover the energy content but also reduce the net emission of CO2.6 In addition, phosphorus in SS is enriched in the ash after thermal processing, so that phosphorus can be recycled comparatively easily, relieving the phosphorus crisis.7,8 The thermal processing technologies include mono-incineration, cocombustion, and an alternative process, such as gasification and pyrolysis.1 Many investigations on co-combustion of SS with coal showed that co-combustion with coal was a suitable option of disposing SS economically and environmentally safely.9−12 According to Strom et al., a synergetic promoting effect existed when the mixture of SS and coal was used as fuel.13 The investigations of Ninomiya et al. showed that co-combustion of SS and coal performed better than combustion of pure SS.14 © 2016 American Chemical Society

Therefore, co-combustion is a good selection for disposing SS. Zhundong coal (ZD coal) is produced from the biggest integrated coalfield of China, and its reserve can meet the need of China in the next century, whereas the utilization of ZD coal through conventional thermal technologies is facing serious ash-related problems, such as slagging and fouling, as a result of its high content of alkali metal, sodium.15 Therefore, finding some solutions of using ZD coal without ash problems is significant for relieving the energy crisis of China. Folgueras et al. reported that the fusion temperature of the ash from mixture SS and coal was related to the SS ash content in the total ash and the ash fusion temperature could acquire a high value when the ratio of SS ash in the ash from mixed fuel reached over 80%.16 Xu et al. also believed that co-combustion was an attractive option of using ZD coal, and Wang et al. found that SS could mitigate the ash problems resulting from alkali metals during the co-combustion process.17,18 Therefore, co-combustion of SS and ZD coal may be a significant option for not only disposing SS but also developing a new way of employing ZD coal. Because the content of nitrogen in SS is usually much higher than coal, a high emission of NOx is expected.19,20 It was found that an oxidation environment inevitably led to the generation of considerable quantities of NOx during cocombustion of SS and coal, even if the circulating fluidized Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 4, 2015 Revised: January 8, 2016 Published: January 8, 2016 1720

DOI: 10.1021/acs.energyfuels.5b02283 Energy Fuels 2016, 30, 1720−1729

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Energy & Fuels

comparatively superior rates of both reduction and oxidation for CLC.33 The disadvantage of iron oxides is poor oxygen transport capacity.34 Shen et al. found that the required recirculation of the oxygen carrier for transfer heat was much higher than that for carrying oxygen at a high temperature.35 Therefore, the weakness of iron oxides is not important. Lowcost natural iron ore received many investigations, and results showed that natural iron ore was a good selection for an oxygen carrier.33,36−39 In the present work, natural hematite is used as an oxygen carrier for CLC. Recently, CLC has been investigated to be a suitable combustion technology to dispose dewatered SS with some advantages; even the results show that the concentration of unconverted carbonaceous gas (CO and CH4) is high in the flue gas of FR.28,40 Moreover, the ash from SS can be effectively used as a low-cost oxygen carrier with appreciable reaction reactivity with high resistance to attrition and thermal sintering, and it can prevent deposit formation during CLC of coal.41,42 The investigations of CLC of coal illustrated that it was feasible to develop CLC with coal.43,44 The investigations on chemical looping co-combustion of SS and ZD coal are limited; therefore, it is important to obtain the basic knowledge of this co-combustion. Thus, the chemical looping co-combustion of SS and ZD coal is investigated in the present work. The present work investigated the effects of operation conditions on the chemical looping co-combustion behaviors using natural hematite as an oxygen carrier in a 1 kWth CLC unit. The characteristics of the used oxygen carrier and issues of ash from SS and ZD coal were also studied. This work may contribute to disposition of SS and utilization of ZD coal.

bed operated in a lower temperature averting the formation of NOx from atmospheric nitrogen.21,22 Besides, the potential of producing dioxin is also high because of the high content of chlorine in SS. The produced gaseous pollutants necessitate extensive flue gas cleaning to meet very strict emission limits normally imposed on waste incineration, which will increase the cost of SS disposition.1 Finding a clean combustion technology is significant for combustion of SS. Chemical looping combustion (CLC) technology has emerged as a novel combustion technology and can minimize the production of NOx as well as dioxin for the reducing atmosphere and lower operation temperature.23−26 CLC is also the most promising technology among technologies currently developed for carbon capture and storage (CCS) and becomes more interesting with the increasing pressure of global warming.27−29 When this technology is applied to cocombustion of SS and ZD coal, generated CO2 can be captured at a low cost, decreasing the emission of CO2. The schematic illustration of the CLC process is shown in Figure 1. CLC

2. EXPERIMENTAL SECTION 2.1. Fuel. Dry SS was used as solid fuel for the limiting of feeding in the present work, and the SS was supplied by a wastewater treatment plant located in Jurong, China. The particles of SS and ZD coal samples were crashed and then sieved to yield a size range of 0.1−0.3 mm. The two fuel particles were mixed in the weight ratio of 40:60 to obtain mixed ash with a high melting temperature. The proximate analysis of the SS and ZD coal is illustrated in Table 1. The content of carbon and lower heating value (LHV) are also shown in Table 1.

Figure 1. Schematic illustration of CLC.

consists of two reactors: an air reactor (AR) and a fuel reactor (FR). The operation of the system depends upon the circulation of oxygen carriers between AR and FR. The oxygen carriers transfer lattice oxygen and heat from the AR to FR for the indirect combustion of fuel, avoiding the direct mixing of fuel and oxygen in air. The oxygen carriers are usually solid metal oxides. In the FR, solid fuel is gasified by steam or carbon dioxide, producing syngas. The syngas reacts with the oxygen carrier generating CO2 and vapor. After cooling, pure CO2 can be obtained. Maybe the water contained in SS is able to be used as a gasification agent during the co-combustion process, averting extra equipment for drying SS by thermal processing methods and providing a gasification agent for CLC. The successful operation of the system lies on the circulating of the oxygen carriers. Therefore, the properties of the oxygen carriers are vital for the practice of the process of CLC. The important properties of oxygen carriers are high reactivity to both reduction and oxidation and high resistance to attrition and thermal sintering. Oxides of Fe, Cu, and Ni have been investigated as oxygen carriers.30−32 Although the oxidation and reduction activity of Ni is high, Ni is expensive and toxic. Cubased oxygen carriers are also not suggested for their low melting point. Oxides of Fe are investigated to be a suitable oxygen carrier with abundance, environmentally benign, and

Table 1. Proximate Analysis and Content of Carbon of SS and ZD Coal proximate analysis (wt %, ad)

SS ZD coal

moisture

volatile

fixed carbon

4.89 12.15

35.88 28.54

3.37 53.9

ash

content of carbon (wt %, ad)

LHV (MJ/kg)

55.86 5.41

21.55 65.69

10.31 23.56

2.2. Oxygen Carrier. In this work, natural hematite was used as the oxygen carrier, provided by the Nanjing Steel Manufacturing Company in China. The natural hematite is from the Rio Tinto Company of Australia. After the hematite was ground into small particles ranging from 0.1 to 0.3 mm, the hematite particles were calcinated in a muffle oven at 950 °C for 3 h to improve the mechanical properties. These hematite particles were sieved again and used as the fresh oxygen carrier in this study. Table 2 shows the elemental composition of hematite, expressed as oxides (wt %). 2.3. Experimental Setup. 2.3.1. 1 kWth Continuous Unit. Experiments were conducted in a 1 kWth continuous reactor of interconnected fluidized beds (shown in Figure 2). The CLC facility has been described in detail in our previous paper.40,45 This unit 1721


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m3/h. The inventory of oxygen carrier is about 2.5 kg. The range of the FR temperature was 800−930 °C, and the ambient temperature was about 10 °C. 2.3.2. Batch Fluidized Bed Reactor. Experiments were conducted in a bath fluidized bed reactor, as shown in Figure 3, to study the

Table 2. Elemental Composition of Hematite, Expressed as Oxides (wt %) Fe2O3

SiO2

Al2O3

P2O5

CaO

SO3

others

83.21

7.06

5.13

0.38

0.24

0.21

3.77

consists of a fast fluidized bed (AR), a cyclone, a spouted fluidized bed (FR), and a loop seal.

Figure 3. Schematic diagram of the batch fluidized bed reactor.

gasification rates of SS and mixture of SS and ZD coal (SZ), respectively. Fresh hematite (hematite 1) and hematite experienced about 5 h of operation in a 1 kWth unit using SZ as fuel (hematite 2) were also compared in reactivity. The reactor is composed of a straight stainless-steel tube with 32 mm inner diameter and 570 mm height, and it is heated electrically by a furnace. In the tube reactor, there is a porous distributor plate located at 450 mm from the bottom. In each test for investigation of the gasification rate, a sample of about 50 g of oxygen carrier particles was placed on the distributor plate and the reactor was heated to the setting temperature, 900 °C, in a N2 atmosphere. The inlet flow was switched to a mixture of steam and N2. When the temperature remained stable, the fuel (2 g) was added to the reactor from the top of the reactor. The outlet gas was sampled by a gas bag per 1 min for offline analysis until there was no carbonaceous gas detected in the exhaust gas of the reactor. Before test for comparison of oxygen carriers in reactivity, hematite 2 was oxidized to regenerate because the sample of hematite 2 was obtained from the FR. In each test, a sample of about 50 g of oxygen carrier particles was placed on the distributor plate and the reactor was heated to the setting temperature, 900 °C, in a N2 atmosphere. While the temperature was stable, the gas flow was changed to a mixture of N2 and CO with flow rates of 1.9 and 0.1 L/min [standard temperature and pressure (STP)], respectively. The outlet gas was sampled by gas bag per 1 min for offline analysis. Each experiment continued for 40 min. 2.4. Data Evaluation. The carbon conversion efficiency, ConC, represents the fraction of carbon in the solid fuel converted to carbonaceous gas in FR, and it is defined as

ConC =

Figure 2. Configuration for the 1 kWth continuous facility.

FC,FR FC,fuel

× 100% (1)

where FC,FR and FC,fuel are the molar flow of carbonaceous gas leaving the FR and total molar flow of carbon contained in the fed fuel, respectively. The carbonaceous-gas-producing ratio, P(t), is the ratio of the amount of produced carbonaceous gas in t min to total carbonaceous gas generated in 15 min. After 15 min, there is no carbonaceous gas produced

By means of a variable-speed screw feeder, fuel particles were pneumatically transferred to the bottom of the FR with a N2 stream in a volume flow controller. The amounts of other gas components in the flue gas of FR could be calculated according to the N2 stream. The flow of N2 was known. When each experimental run reached a relatively steady situation, the exhaust gases of FR and AR were sampled by gas bags for offline analysis, after separating the ash and cooling the water. The composition of the flue gas was analyzed by a NGA 2000 type gas analyzer (Emerson Company, Bloomington, MN). Operating conditions and primary parameters in this experiment were as follows: The air flow of AR was 0.84 m3/h. Two steam flows introduced to the loop seal and FR were 0.21 and 0.15 kg/h, respectively. The fuel feed rate was 100 g/h, corresponding to a thermal power of about 0.5 kW. Nitrogen for transferring fuel is 0.27

t

P(t ) =

∫0 FC dt FC,total

× 100% (2)

dt and FC, total are the moles of produced carbonaceous gas where ∫ from 0 to t min and total carbonaceous gas generated throughout the experiment, respectively. FC is the amount of carbonaceous gas produced in dt (dt = 1) min. It inflects the yield rate of carbonaceous gas. t 0 FC

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conversion efficiency rises with the increase of the FR temperature. Particularly, the conversion efficiency of SZ increases rapidly from 840 to 900 °C. The carbon conversion efficiency for SZ is much lower than that for SS at each temperature point in the range of temperatures. The carbon conversion efficiency of SZ reaches 82.5 and 86.5% at 900 and 930 °C, respectively. The carbon conversion efficiency of SS is 92.5% at 900 °C. The carbon conversion efficiency depends upon the gasification of fuel because gasification is the dominant way of converting the solid fuel into gas in the FR of CLC.39 Gasification consists of two successive processes: pyrolysis and char gasification (reactions R1 and R2).

The carbonaceous gas conversion efficiency, f CO2, can be presented as a fraction of total gaseous carbon oxidized to CO2 by the oxygen carrier in the flue gas of FR as follows:

fCO = 2

XCO2,FR XCO2,FR + XCO,FR + XCH4,FR

× 100% (3)

where Xi,FR (i = CO, CO2, and CH4) represents the concentrations of i in the flue gas of FR. The definition assumes that carbonaceous gases in the exhaust gas only contain CO2, CO, and CH4. The parameter shows the conversion degree of the produced carbonaceous gas from the solid fuel oxidized by the oxygen carrier in the FR. The carbon capture efficiency is defined as the ratio of carbonaceous gases in the FR to the total of those in two reactors ηCC =

FC,FR FC,AR + FC,FR

pyrolysis

× 100% (4)

fuel solid → char + volatile

where FC,AR is the molar flow of carbonaceous gas leaving the AR. The CO conversion efficiency, ConCO, is defined as the percentage of CO2 in the flue gas of the batch fluidized bed reactor ConCO =

XCO2 XCO + XCO2

(R1)

char gasification C + H 2O → CO + H 2

× 100%

(R2)

When solid fuel was carried into the FR, it was heated to the FR temperature in a moment for the intense exchange of heat and mass between the hot oxygen carrier and fuel and then the reactions R1 and R2 occurred. The pyrolysis and char gasification are endothermic, so that an increase of the temperature could greatly enhance the gasification of fuel, increasing the carbon conversion efficiency. The generated syngas was oxidized by the oxygen carriers, and the reaction rate of the oxidation reaction increased with increasing temperature. Then, the content of the syngas in the FR reduced, promoting the forward gasification reactions of fuel, especially above 850 °C.46−49 This may partly account for the rapid increase of carbon conversion from 840 to 900 °C. Because the gasification rate of solid fuel was lower, carbon in the fuel could hardly be converted into gas completely in the FR.39,50 A part of unconverted carbon in the fuel was transferred to the AR from the FR with external circulating of oxygen carriers, and the rest was carried out with ash by the flue gas from the FR, causing the loss of energy contained in carbon. As a consequence, a higher FR temperature is necessary to reduce the loss of carbon with ash and obtain higher carbon conversion efficiency. Figure 5 shows the carbonaceous-gas-producing ratio for SS and SZ. The carbonaceous gas is generated quickly at the beginning, and it reaches 81.5% at 2 min for SS. After that, the ratio increases slowly and the ratio of 100% is obtained at about 8 min. The ratio of produced carbonaceous gas rises linearly with time at the beginning of 4 min, and it keeps constant after 12 min for SZ. SS contains much less fixed carbon, about 16% of total carbon, and carbon in the SS is relatively higher reactive than that in coal; therefore, the carbonaceous gas could generate quickly during the gasification process.1 This may explain the higher carbonaceous-gas-producing ratio at 1 min and higher carbon conversion efficiency for SS. However, most carbon in ZD coal is fixed carbon, about 82%; therefore, producing carbonaceous gas from ZD coal in SZ was comparatively slow, and the ratio reached 80% at 4 min for SZ. Thus, SZ can be converted into gaseous fuel through gasification more easily than single coal in the FR of the CLC facility. To prove it, these values of carbon conversion efficiency obtained by previous work with only using ZD coal as fuel in the same CLC facility were used to compare to that obtained in the work shown in

(5)

where XCO and XCO2 are the concentrations of CO and CO2 in the flue gas, respectively.

3. RESULTS AND DISCUSSION 3.1. Effect of the FR Temperature. The operation parameter temperature in FR was influenced greatly on the reactions that occurred in FR for CLC. Consequently, influences of the temperature in FR were investigated in this section on carbon conversion efficiency, carbonaceous gas conversion efficiency, and carbon capture efficiency. These values obtained in this work were compared to those that were obtained by previous work with only using dewatered SS as fuel in the same reactor to understand better the co-combustion behaviors of SZ.40 The temperature was changed from 800 to 930 °C, and each temperature point was maintained for at least 1 h to reach a steady state in the present work. 3.1.1. Carbon Conversion Efficiency. Figure 4 shows the carbon conversion efficiencies for SZ and SS as a function of the FR temperature. The profiles of carbon conversion efficiency are similar in shape for SS and SZ. The carbon

Figure 4. Carbon conversion efficiencies of SZ and SS at different FR temperatures. 1723


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Figure 5. Carbonaceous-gas-producing ratio with time for SS and SZ at 900 °C.

Figure 7. Carbonaceous gas conversion efficiencies of SS and SZ at different FR temperatures.

Figure 6.51 It can be clear that the carbon conversion efficiency of SZ is higher than that of ZD coal from 870 to 930 °C.

SS and SZ. The carbonaceous gas conversion of SZ rises from 86.0% at 800 °C to 91.3% at 930 °C. The FR temperature has a lower effect on carbonaceous gas conversion efficiency for SZ compared to SS. It is clear that the carbonaceous gas conversion efficiency of SZ is higher than that of SS at each temperature point from 800 to 900 °C. As soon as the solid fuel was carried to the bottom of the FR, the carbonaceous gas was produced from the fuel and then reacted with oxygen carriers (reaction R3). The Fe-based oxygen carriers were identified to be in the form of Fe2O3 and Fe3O4 by X-ray diffraction (XRD) in the FR of this CLC facility.39,40,52 CmHnOx + Fe2O3 → CO2 + H 2O + Fe3O4

(R3)

In a spouted fluidized bed, the oxygen carriers and solid fuel particles were mixed at the bottom of FR and carried upward. However, carbon in the solid fuel could not be converted into gas completely in the FR, and unconverted carbon was retained as residue carbon before being carried to the upside of the FR, where the produced carbonaceous gas could not be converted into CO2 with insufficient fresh oxygen carriers, leading to the existence of CO and CH4 in the outlet gas of the FR.34,53 This means that the carbonaceous gas conversion efficiency depends upon the generation of carbonaceous gas from fuel near the surface of the bed. The carbonaceous gas conversion efficiencies for SS and SZ rise with the increase of the FR temperature. The increase of the carbonaceous gas conversion efficiency with the temperature can be explained by the increase of the gasification rate of fuel, producing more carbonaceous gas at a lower position of the FR, and the increase in the reaction rate between the oxygen carrier and carbonaceous gas. In comparison to SZ, lower carbonaceous gas conversion efficiency for SS, about 70% at 800 °C, indicates that a large part of carbonaceous gas generated quickly at the upper side of FR, and this part decreased largely with increasing temperature. The carbonaceous gas conversion efficiency of SZ is kept relatively high, even at a lower temperature, 800 °C, and its increasing rate with the temperature is comparatively small, which means that SS in SZ also obtains a high carbonaceous gas conversion. The generation of carbonaceous gas from SZ at a well-proportioned rate is also in favor of being oxidized by the oxygen carrier, as shown in Figure 5.54 Co-combustion is

Figure 6. Carbon conversion efficiencies of SZ and ZD coal at different FR temperatures.

Therefore, the presence SS in SZ is in favor of achieving higher carbon conversion efficiency in the FR compared to single ZD coal. The relatively slow increasing rate of carbon conversion efficiency with the increase of the FR temperature for SS can be attributed to its high ash content, 55.86%, with a rather low content of char, 3.87%.14 This characteristic of SS is resistant against the gasification reactions in the viewpoint of the chemical reaction, resulting in the char reacting with steam slowly and incompletely. The ash content in ZD coal is much lower, 5.56%. The difference of carbon conversion efficiency for both fuels decreases with the FR temperature. A higher temperature is positive for improving carbon conversion efficiency, especially for SZ. As a result, a higher FR temperature is important for SZ in viewing carbon conversion efficiency. 3.1.2. Carbonaceous Gas Conversion Efficiency. Figure 7 shows the carbonaceous gas conversion efficiencies versus the temperature in the FR with using SZ and SS. This conversion efficiency increases with the increase of the FR temperature for 1724


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resulting in the decrease of residue carbon in the FR and carbon transferred to the AR, which was responsible for the reduction of the CO2 concentration in the AR and the increase of carbon capture efficiency for SZ. The CO2 concentration of 0 in the AR with only using SS as fuel exhibited that none or a very few fuel particles were transferred to the AR. It meant that carbon in the AR was almost from ZD coal. The carbon capture efficiencies for SZ and ZD coal are shown in Figure 10. It is

beneficial to increase the carbonaceous gas conversion efficiency, especially at a lower temperature. 3.1.3. Carbon Capture Efficiency. The profiles of the CO2 concentration in the outlet gas of the AR for SZ and SS are shown in Figure 8. The CO2 concentration in the flue gas of the

Figure 8. Concentration of CO2 in the flue gas of the AR for SS and SZ at different FR temperatures.

AR decreases significantly with increasing the FR temperature for SZ. However, CO2 is not detected by the gas analyzer in the AR flue gas for SS and is kept at zero in the corresponding temperature range. Figure 9 shows the carbon capture efficiency as a function of the FR temperature for both fuels. The carbon capture

Figure 10. Carbon capture efficiencies of SZ and ZD coal at different FR temperatures.

clear that the carbon capture efficiency of SZ is higher than that of pure ZD coal. Therefore. co-combustion can contribute to a higher capture efficiency compared to pure ZD coal. The carbon capture efficiency increases with the temperature in FR for SZ, and the temperature above 900 °C can be suitable for SZ from the perspective of carbon capture efficiency. 3.2. Characteristics of the Reacted Oxygen Carrier. The combustion of fuel in CLC system depends upon the circulations of the oxygen carrier between the FR and AR; therefore, the properties of the oxygen carriers are vital for the operation of CLC and combustion of fuel. When solid fuel is used, the ash from fuel will deposit on the oxygen carriers, leading to some problems, such as sintering and change in reactivity of oxygen carrier particles.40,55 Thus, the oxygen carrier (hematite 2) obtained from the FR of the 1 kWth unit after about 5 h of continuous operation was characterized and compared to the fresh oxygen carrier (hematite 1). Figure 11 shows a detailed comparison of CO conversion with time between hematite 1 and hematite 2. On the beginning of experiments for hematite 1 and hematite 2, the conversion of CO is kept at 100%, and after some time, it decreases with time. At about 14 min for hematite 1 and 17 min for hematite 2, the conversion of CO starts to decrease gradually, and after 40 min, the CO conversions become 75.5% for hematite 1 and 85% for hematite 2. When the flow of CO entered the batch fluidized bed reactor, it reacted with the oxygen carriers immediately and then CO was oxidized to CO2 (see reactions R4−R6).

Figure 9. Carbon capture efficiencies of SZ and SS at different FR temperatures.

efficiency of SZ rises with the increase of the FR temperature, and it reaches 96.3% at 930 °C, while that of SS is kept at 100% among the temperature range. For the system of CLC, it could almost not be avoided that carbon in the solid fuel could not be converted into carbonaceous gas completely through gasification, and then part of them entered the AR with the external circulation of oxygen carriers. Carbon carried to the AR was oxidized to CO2 by oxygen from the air in the AR. With the increase of the temperature in the FR, the gasification process was enhanced, 1725

3Fe2O3 + CO = 2Fe3O4 + CO2

(R4)

3Fe3O4 + CO = 3FeO + CO2

(R5)

FeO + CO = Fe + CO2

(R6) DOI: 10.1021/acs.energyfuels.5b02283 Energy Fuels 2016, 30, 1720−1729

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Energy & Fuels

Figure 12. EDX analysis of hematite 2. Figure 11. CO conversion efficiency with time for hematite 1 and hematite 2.

ment effect of foreign ions, K+, Na+, and Ca2+, on reactivity of oxygen carriers can be explained by migration of the ions and the alkali-rich phase behaving as an active phase that can further promote the pore development and increase reactivity.58 Because ash from SS and ZD coal has a high content of alkali metals shown in Table 4, the SZ ash may have a promoted effect on the reactivity of oxygen carriers, but it is slight.41,51,59,60 The ash deposition may bring the sintering or agglomeration problems to the oxygen carriers.55,59 It was not found that SiO2 from the SS ash reacted with iron oxides in the oxygen carrier, producing low-melting compounds.41,40 Therefore, the high content of Si in ash, as shown in Table 4, has a negligible effect on the melting of oxygen carriers. Figure 13 shows the scanning electron microscopy (SEM) images of hematite 1 (fresh hematite) and hematite 2. The two images are taken at the same higher magnification of 10000×. There is no sintering and agglomeration on the surface of hematite 2, and hematite 2 shows much more small particles and pores than hematite 1, which facilitates the gas−solid reaction. According to the above, it can be concluded that hematite maintains a good reactivity after 5 h of operation and the SZ ash deposition on the oxygen carrier, which brings a slight impact on the oxygen carrier, appeared during the operation. 3.3. Characteristics of the Ash. The ZD coal, like biomass, contains a lot of alkali metal, especially sodium, shown in Table 4. Therefore, the utilizations of ZD coal through conventional combustion technologies are facing a significant challenge of ash deposition and slagging caused by alkali metal (sodium) on the heat-exchange surface, causing the deceasing of thermal efficiency and corrosion during the combustion process.15,61 The problem caused by the alkali metals can be mitigated using CLC for the relatively low reaction temperature (usually below 1000 °C) with small temperature gradients in the FR, and the low ratio of fuel ash to the total bed particles in the FR can alleviate the ash problem.45,53,62,63 In the continuous running process of co-combustion of SS and ZD coal, no problem of oxygen carrier melting and ash agglomeration appeared. The ash melting behavior, which is of crucial importance, particularly for slagging and fouling, is affected by the chemical composition of the ash, primarily the levels of alkali metals, phosphorus, chlorine, silicon, and calcium, as well as the chemical concentration of the compounds.64 Therefore, no ash problems appeared in the work, which could be ascribed to two reasons. First, the low ash content of ZD coal in SZ ash caused a negligible contribution

The reaction rate of reaction R4 is fast but the reaction rates of reactions R5 and R6 are much slower.56 Therefore, CO can almost be oxidized completely by the oxygen carriers at the phase from Fe2O3 to Fe3O4, and the slow reactions of reactions R5 and R6 may be responsible for the decrease of the CO conversion efficiency.40,55 When the outlet CO concentration reach 0.5−0.8 vol %, Fe2O3 contained in the oxygen carrier is deemed to be only reduced to Fe3O4.55 This corresponding time in this experiment is 20 min. It means that only reaction R4 occurred in the first 20 min in the present work. Additionally, there is almost no difference in CO conversion efficiency for hematite 1 and hematite 2 in the first 20 min. In the FR of the 1 kWth CLC facility, Fe2O3 can only be reduced to Fe3O4; therefore, the variation of hematite during operation may have a negligible effect on the performance of chemical looping co-combustion.40 The Brunauer−Emmett−Teller (BET) surface areas of reduced hematite 1 and hematite 2 were determined by a Micrometric ASAP 2020 and were 0.24 and 0.67 m2/g, respectively. The increase of the BET surface area may be caused by a positive effect of the ash.55 The elemental compositions of hematite 1 (fresh hematite) and hematite 2, expressed as oxides (wt %), are shown in Tables 1 and 3, respectively. The results were obtained by the analysis of Table 3. Elemental Composition of Hematite 2, Expressed as Oxides (wt %) hematite 2

Fe2O3

SiO2

Al2O3

P2O5

CaO

others

74.63

12.74

8.37

1.44

0.85

1.97

X-ray fluorescence (XRF). The discrepancy in element composition between hematite 1 and hematite 2 may be caused by the deposition of SZ ash. The energy-dispersive Xray (EDX) analysis of hematite 2 shows that a lot of phosphorus was found on the surface of the oxygen carrier, as shown in Figure 12. There was no phosphorus detected on the surface of the fresh oxygen carrier according to the EDX analysis result of the fresh oxygen carrier in the previous work.40 Phosphorus is rich in SZ ash. Therefore, it also indicates the existence of SZ ash on the surface of the oxygen carrier. Ash deposition is the challenge in the CLC process. The effects of ash on the oxygen carrier depend upon the ash kind and ash content.57 Bao et al. demonstrated that the enhance1726


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Energy & Fuels Table 4. Elemental Composition of ZD Coal Ash and SS Ash (wt %) ZD coal ash SS ash

Si

Al

Ca

Mg

Fe

P

Na

K

S

others

10.2 36.4

11.6 14.9

24.7 8.0

8.2 3.2

19.8 20.5

3.0 10.7

7.5 0

0.4 3.4

13.8 1.0

0.8 1.9

efficiency increased from 58 to 86.5%, the carbonaceous gas conversion efficiency increased from 86 to 91%, and the carbon capture efficiency increased from 83 to 96%. Therefore, a higher FR temperature is important for chemical looping cocombustion of SS and ZD coal. Moreover, experiments were conducted on a batch fluidized bed to evaluate the characteristics of the reacted oxygen carriers after 5 h of operation in a 1 kWth unit. The reacted oxygen carrier showed similar reactivity compared to the fresh oxygen carrier, indicating that the hematite oxygen carrier possessed good long-term reactivity. The ash from SZ was deposited on the oxygen carrier and had a slight effect on the reactivity of the oxygen carrier. In the present work, no ash-related problems appeared. It was found that sodium in ZD coal existed in the form of sodium aluminosilicate and sodium pyrophosphate aluminum with a high melting point in SZ ash. As a consequence, chemical looping co-combustion is a suitable combustion technology for disposing SS and using ZD coal.

Figure 13. SEM images of (left) hematite 1 and (right) hematite 2.

to the amount of SZ ash with the fuel weight ratio of 40:60, and the concentration of sodium from the ZD coal was reduced to about 1 in the total ash, according to the balance of sodium. In addition, as Table 4 shows, the SS ash contains a lot of Al2O3, SiO2, and P2O5, which can relieve the ash-related problems caused by the volatile of alkali metals by forming high-meltingpoint sodium compounds.17 Because the content of sodium in the SZ is very low, it is difficult to study the existence form of sodium. To investigate the existence forms of sodium in the SZ ash, an ash sample was obtained by gasification using the mixture of SS and ZD coal with a ratio of 20:80 in weight and was analyzed by XRD. Sodium aluminate silicon and NaAlP2O7 with high melting points were identified in the sample ash, as shown in Figure 14. These results indicate that chemical looping co-combustion can mitigate the ash problems caused by alkali metals.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-25-83795598. Fax: +86-25-83793452. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of this research work by the National Natural Science Foundation of China (Grants 51476029, 51276037, and 51406035) and the China Postdoctoral Science Foundation (2014M551489).

■ Figure 14. XRD analyses of the ash sample.

4. CONCLUSION The chemical looping co-combustion of SS and ZD coal was conducted in the 1 kWth continuous CLC unit. The CLC performance of SZ was investigated and compared to dewatered SS. In addition, the reacted oxygen carriers and ash were characterized. The effect of the temperature in the FR on the combustion performance was studied. The carbon conversion efficiency, carbonaceous gas conversion efficiency, and carbon capture efficiency for SZ increased with the FR temperature. In comparison to dewatered SS, SZ could obtain higher carbonaceous gas conversion efficiency among the temperature range. Even though SZ obtained lower carbon conversion efficiency and carbon capture efficiency, both efficiencies reached an appropriate value at a high temperature and higher than these for single ZD coal. As for SZ, the carbon conversion 1727

NOMENCLATURE SS = sewage sludge ZD = Zhundong CLC = chemical looping combustion CCS = carbon capture and storage AR = air reactor FR = fuel reactor LHV = lower heating value SZ = mixture of SS and ZD coal hematite 1 = fresh hematite hematite 2 = hematite experienced about 5 h of operation in a 1 kWth unit using SZ as fuel STP = standard temperature and pressure ConC = carbon conversion efficiency FC,FR = molar flow of carbonaceous gas leaving the FR FC,fuel = total molar flow of carbon contained in the fed fuel P(t) = carbonaceous-gas-producing ratio ∫ t0FC dt = moles of produced carbonaceous gas from 0 to t min FC,total = total carbonaceous gas generated throughout the experiment f CO2 = carbonaceous gas conversion efficiency Xi,FR = concentrations of i in the flue gas of FR (i = CO, CO2, and CH4) DOI: 10.1021/acs.energyfuels.5b02283 Energy Fuels 2016, 30, 1720−1729

Article

Energy & Fuels ηCC = carbon capture efficiency FC,AR = molar flow of carbonaceous gas leaving the AR ConCO = CO conversion efficiency XCO = concentration of CO in the flue gas of a batch fluidized bed XCO2 = concentration of CO2 in the flue gas of a batch fluidized bed CmHnOx = produced gas through gasification BET = Brunauer−Emmett−Teller SEM = scanning electron microscopy

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Chemical Looping Co-combustion of Sewage Sludge and Zhundong

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