Syngas production by chemical-looping gasification of waste activated carbon with iron-based oxygen carrier

China Abstract Waste activated carbon (WAC), as a typical solid waste, can be utilized by chemical looping gasification (CLG) technology with an iron-based oxygen carrier to produce valuable synthesis gas. A series of experiments on WAC of the CLG process have been carried out in a fixed-bed reactor. The operation parameters involving the OC/WAC mole ratio, steam flow rate and reaction temperature during CLG reactions have been investigated in detail. Further, the cyclic performance within 10 cycles has been also discussed. Fresh and cyclic reaction oxygen carrier samples have been analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In order to obtain high-quality syngas with high carbon conversion, the optimal of OC/WAC mole ratio, steam flow rate and reaction temperature are 0.15, 0.10 and 950 °C, respectively. The iron-based oxygen carrier exhibits a stable cyclic performance during the multiple tests, following the reaction path of Fe 2 O 3 → Fe 0.98 O in the individual reduction process. Moreover, the iron-based oxygen carrier could be oxidized almost to its initial state after 10 redox tests. No obvious sintering and agglomeration phenomena are observed. The WAC of CLG presents a new approach for the comprehensive utilization and disposal of solid waste, especially with low volatile feedstock.

The oxygen carrier is continuously transported and circulated between two interconnected chambers. The oxygen required for gasification is provided by lattice oxygen released from the OC, which lead to the lower cost without air separation.
Moreover, the lattice oxygen is capable of avoiding combustion reactions by controlling the mole ratio between fuels and OCs. The schematic diagram of CLG process for synthesis gas production is exhibited in Fig. 1.   Fig. 1 Schematic of a fixed bed reactor.
The OC is the crucial backbone in the chemical looping process. Recently, a large number of oxygen carriers have been reported in CLG processes, such as Fe (Huang et al. 2016), Ni (Medrano et al. 2015), and Mn (Yin et al. 2018). The iron-based OC is the most common option, due to the inexpensive, durable, stable, moderately reactive, nonpoisonous, and resistant to sintering or attrition Liu et al. 2018). Huseyin et al. (2014) tested biomass CLG using Fe2O3/Al2O3 as an oxygen carrier in a 10 kWth interconnected fluidized bed reactor. The OC performed good stability and reactivity during 60-h continuous runs. In particular, the Fe2O3/Al2O3 demonstrated excellent performance in the utilization of waste resources, such as sewage sludge, waste water and municipal solid waste ). Huang et al. (2017) used natural hematite as an oxygen carrier in sewage sludge disposal. The results showed that the carbon conversion of fuel had increased by 36.29%. The hematite also exhibited good performance during continuous tests for 23 h. Qin et al. (2018) investigated Fe2O3/Al2O3 using the CLG process of organic waste water to produce high H2/CO ratio syngas. The Fe2O3/Al2O3showed an otable performance and achieved auto-thermal feasibility in this CLG process. The highest syngas yield of 3.325 L/g was obtained with almost 100 % carbon conversion.
The resource utilization of WAC to produce synthesis gas would be achieved by CLG with opening a broader means for treatment of carbonaceous solid wastes. The purpose of this work is to verify the feasibility of iron-based OCs for the treating of WAC in CLG, focusing on macroscopic research combining various analytical methods. A series of experiments were carried out to investigating the effect of parameters including the mole ratio between the OC and WAC, reaction temperature and steam flow on the gasification process. Afterward, the stability of the OC during multiple redox reactions was also discussed, and 10-cycle experiments were conducted. Finally, the fresh and used OC were collected and analyzed with X-ray diffraction (XRD) and scanning electron microscopy (SEM) to better reveal the crystalline phase transformation and surface morphology changes, respectively.

Materials
A typical waste activated carbon was collected from sugar factory in Shandong Province, China. The samples were dried at 90°C for 2h and then ground and sieved into an approximate size of 80~200 μm. The proximate and ultimate analyses, and lower heating values of the WAC are provided in Table 1.

Oxygen carrier preparation
The Fe2O3/Al2O3 oxygen carrier was prepared using the impregnation method. A given amount of Fe(NO3)3•9H2O was dissolved in deionized water for preparation of the saturated solution. Here, Al2O3 powder was added into the saturated solution as an inert support. Then, the mixture solution was stirred and heated at 80 °C until the water was evaporated. After that, the residue was dried at 120 °C for12 h. Then, it was transferred into a muffle furnace and calcined at 900°C for 3 h under an air atmosphere. Finally, the obtained precursors were crushed and sieved into ~100 μm.

Experimental procedure
To simplify the CLG reaction process of WACs, a series of experiments were implemented in a fixed-bed reactor to test the feasibility of WAC disposal using CLG.
The schematic diagram is illustrated in Fig.2. The proposed apparatus are composed of a quartz tube reactor, a mass flow controller, a steam generator section, a cooling system, an exhaust gas sampling container, and an analysis system. The tube reactor was heated by an electronic furnace. The temperature was controlled by a thermocouple surrounding the tube. The steam generator was composed of a constant flow pump for steam generation through preheating by a furnace at 300°C.
In the reduction stage of each test, a mixture of WAC and oxygen carrier with various OC/WAC ratios (0, 0.1, 0.15, 0.2, and 0.25) was initially loaded in a quartz boat and placed on the cool zone of the tube. High-purity argon was used as the purge gas at 200 mL/min for 15 min in advance to remove the air. When the reactor reached the desired reaction temperature (750, 800, 850, 900, and 950 °C), argon was replaced by a mixture stream of different steam flow rates (0, 0.05, 0.10, 0.15, and 0.20 mL/min). Next, the argon was used to flush the gasification reactor for 5 min. After that, the quartz boat was rapidly moved into the heating zone, and the gasification time was 60 min, which was sufficient time for the reaction. The exit gas was passed through for cooling, purifying and drying. Finally, the generated gases were collected via a group of sampling bags at 5 min intervals. During the oxidation period, the reduced OC was oxidized by air at a volumetric flow rate of 200 mL/min in a tubular furnace. The oxidation time was set to 60 min. Since the reaction system was a batch system, the OC particles were alternatively exposed to oxidation and reduction stages to simulate the circulating experiments. The compositions of the flue gas were measured by off-line gas chromatography with a TCD detector. After the reactor cooled to ambient temperature, the selected oxygen carrier particles were collected for characterization analysis.

Oxygen carrier Characterization
To better understand the reaction mechanism, a series of characterization methods were employed. The crystalline phase of the oxygen carrier samples were confirmed with X-ray diffraction using Cu-Kα radiation with 40 kV and a current of 40 mA. The samples were scanned in the 2θ range of 5-80°with a step range of 0.02°.
Moreover, scanning electron microscopy (SEM) using a Hitachi S4800 was used to evaluate the morphology surface features of the oxygen carrier samples at the different stages.

Data processing
(1) In each part, the relative gas content (Ci) of each component in the dry basis outlet gas can be evaluated as follows: Where yi represents the actual volume fraction of gas species i (H2, CO, CH4, and CO2) in the dry outlet gas; v denotes the volume flow rate of the dry basis flue gas in the outlet.
(2) The lower heating value (LHV, MJ/Nm 3 ) of the generated gas products can be calculated as follows: LHV= 126CCO+108CH2+388CCH4 (2) (3) The gas yield (Gv, Nm 3 /kg) is the volume of dry basis gas product yielded from the unit mass of WAC sample, calculated as follows: (4) The carbon conversion efficiency of the WAC (ηc,%) is the proportion of the carbon fraction of carbon-containing gas in the outlet gas to the carbon fraction of the WAC fed into reactor, where CC% is the carbon content of the WAC: ηc= 12( co + co 2 + CH 4 )· v 24.45· %· WAC (4) (5) The effective gas content (Yg, %) is defined as the ratio of the volume of effective gases (H2, CO, CH4) to the total amount of gas produced by WAC: The syngas yield (Nm 3 /kg) is the addition of effective gas yield: 3 Results and discussion

Effect of (OC/WAC) ratio
The OC was displayed as an oxygen source and heat carrier. A series of comparison experiments were conducted to obtain the optimal OC/WA C value. In addition, without active role, Al2O3 was used as the bed material for the blank experiment for comparison. 3a. H2 is consumed at first, due to the low activation energy as shown in the reaction (7). Thus H2 content gradually decreases with the increase of OC/WAC. Mainly ascribed to the supplement of suitable lattice oxygen put rightward the reaction (10), CO concentration firstly increases when OC/WAC is lower than 0.1. However, CO declines sharply while the CO2 inclines monotonously as the OC/WAC increases from 0.10 to 0.25. This can be explained that excess lattice oxygen promoted the reactions shown in reaction (7) and (8). The combustible gases were consumed to more CO2 and H2O generation, indicating that gasification process gradually was replaced by combustion process in this case. CH4 content declines slightly with an increasing OC/WAC, on account of the reaction (9) Fig.3b. Due to the promoting the WAC conversion, carbon conversion gradually increases from 32.72% to 75.53% as the increasing of OC/WAC from 0 to 0.25.
Meanwhile, the syngas concentration and syngas yield are up to 67.41% and 1.12 Nm 3 /kg, respectively. It resulted that the syngas generation were evidently enhanced with the addition of OC. Because the OC was prone to achieve the partial oxidize the WAC with producing the target gaseous products . Besides, the OC enhanced the water-gas shift reaction to generate more H2 ).
However, when the OC/WAC was more than 0.1, syngas concentration and syngas yield drastically decreased. This mainly attributed to a part of combustible gas was completely oxidized by the excess lattice oxygen of OC, which was adverse to syngas generation. In general, the higher OC/WAC meant the more OC added in reactor, which significantly promoted WAC conversion but sacrificed synthesis gas yield.
Aiming at the target of high-efficiency resource utilization of WAC, the OC/WAC ratio was determined at 0.15, where the highest LHV value of 7.67MJ/Nm 3 was obtained. The reactions associated with OC are listed as followed: Owing to the addition of H2O, the H2 generation and CO consumption were facilitated with the water-gas shift reaction (12) shifting toward the right. It was noteworthy that the CO content presented a fluctuation trend, possibly because of the competition effects between reactions (11), (12) and (13). Because the fixed carbon of WACs preferentially reacted with steam to generate CO rather than undergoing solid-solid reactions between OC and WAC particles. The CO2 concentration decreased overall, and it was much lower than that of without steam. This result suggested that an appropriate steam addition significantly improves the syngas yield, especially H2 generation, which contributed to enhance the quality of synthesis gas.
Additionally, the effect of steam flow rate on carbon conversion, syngas concentration and syngas yield is displayed in Fig.4b. Carbon conversion presents an increasing tendency and then decreases slightly as the increasing of steam flow rate with the maximum value of 60.3% at 0.10 mL/min. The syngas concentration first decreases and then gradually increases to 66.9% at 0.20 mL/min. The syngas yield presents an overall uptrend with the first growing rapidly and then moderates. This may have resulted from the reduction reactions being close to chemical equilibrium.
On the one hand, the steam could evidently promoted carbon conversion and improved the contact between solid reactants as an oxygen source. Accordingly, solid-solid reactions between the OC and WAC were improved (Huang et al. 2016).
On the other hand, the more steam facilitated the water-gas shift reaction to the right to generate more H2. Generally, the steam actually accelerated the syngas yield, especially H2 yield. However, it was notable that the excessive steam (>0.10 mL/min) had no apparent impact on carbon conversion. At the same time, the more steam would shorten the residence time between the gas phase and solid phase. The reactions between the WAC and steam were particularly inhibited. Thus, the WAC conversion and gas yield were restricted within high steam amounts. The flow rate of 0.10 mL/min was selected as the proper steam flow rate, where the carbon conversion of 60.3% and LHV of 7.15 MJ/Nm 3 were achieved.

Effect of reaction temperature
Reaction temperature was a key parameter for the fuel conversion and OC performance in the CLG processing (Taba et al. 2012 The elevated temperatures favored the endothermic reactions (11), (12) and (13), which generated the more combustible gases. In addition, the endothermic reaction (14) was inhibited by a high temperature, which led to the decreasing of CO2. This indicated that high temperatures were favorable for generating syngas with high calorific values. In addition, the OC presented poor reactivity under low temperatures, leading to the limited of carbon conversion and syngas products. Accompany the temperature rising, the more lattice oxygen was released. So the WAC conversion was enhanced. Generally, a high reaction temperature was necessary for WAC thermal conversion and high-quality synthesis gas yields. Thus, 950 °C was the optimum reaction temperature for the CLG process of WAC. At this temperature, the carbon conversion, syngas yield, and LHV attained maximum values were 71.54%, 1.47 Nm 3 /kg, and 7.39 MJ/Nm 3 , respectively.

Effect of numbers of cycles
In order to evaluate the reactivity stabilization of OCs in the CLG process, the multi-cycle tests were carried out in a fixed-bed reactor. The reaction temperature, OC/WAC, steam flow rate, and reaction time were fixed at 900°C, 0.15, 0.10 mL/min and 60 min, respectively.
As illustrated in Fig. 6,   Fig.7XRD patterns of fresh and used OC samples Scanning electron microscopy (SEM) was used to evaluate the morphology of OC samples. As illustrated in Fig. 8a, the surface structure of the fresh sample is tough and loose with a porous structure. It was beneficial for the diffusion and reactions of reactants (Das et al. 2018). After the 1 st cycle, the greater porosity structure was observed than that of fresh sample. It was ascribed to gas escaping from the interior of OC particles . In this case, the structure was conducive for the penetration of gaseous reactants into the core of the OC particles.
Therefore, the performance of OC was increased in the former cycles. Finally, as shown in Fig. 8c, agglomeration is observed in the 10 th sample. Some small granules merge into larger granules. It was adverse to the reaction between the OC and other reactants. This may have resulted from the lattice transfer and continuous thermal stress at high temperatures during the multiple tests. Some WAC ash might block the pore and cause a negative change to the surface structure as well. However, it still remained porous. These results indicated that iron-based oxygen carriers were a promising candidate in the WAC CLG process.

Conclusions
This work focused on verifying the feasibility of WAC in the CLG process using an iron-based oxygen carrier to produce a syngas product. The iron-based oxygen carrier showed favorable reactivity for syngas generation and WAC conversion. The effects of operation parameters were examined on a bench-scale fixed-bed reactor.
The following conclusions are drawn based upon the results: (1) The optimal OC/WAC mole ratio is determined at 0.15, where the highest yield of syngas product of 1.12Nm 3 /kg and the acceptable carbon conversion of 60.31% are obtained based on the trade off between WAC conversion and syngas yield.
(2) An adequate amount of steam introduction would promote WAC conversion and hydrogen generation. The relatively high carbon conversion of 60.31% and the LHV of 6.95 MJ/Nm 3 are reached at the steam flow rate of 0.10 mL/min .
(3) When the reaction temperature is 950 °C, the carbon conversion, syngas yield and LHV are maximized.
(4) The cyclic performances of the OC during 10-time cycle tests are evaluated.
Over the long run, no obvious change occurs in the crystalline phase and morphology structure based on the XRD and SEM results. The iron-based oxygen carrier exhibits good cycling performance.
(5) The oxygen carrier follows the reaction path of Fe2O3→Fe0.98O in the individual reduction process. Then it could be oxidized to its initial state after 10 redox cycle tests. In general, the comprehensive disposal of WAC by the CLG process with the iron-based oxygen provides a highly efficient and environmentally friendly way.