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Article

Evaluation of Different Oxygen Carriers for Chemical Looping Reforming of Toluene as Tar Model Compound in Biomass Gasification Gas: A Thermodynamic Analysis

by
Zhiqi Wang
*,
Jinzhi Zhang
,
Jingli Wu
,
Tao He
and
Jinhu Wu
Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
*
Author to whom correspondence should be addressed.
Atmosphere 2022, 13(6), 887; https://doi.org/10.3390/atmos13060887
Submission received: 22 April 2022 / Revised: 21 May 2022 / Accepted: 26 May 2022 / Published: 30 May 2022
(This article belongs to the Special Issue Syngas Production by Chemical Looping Gasification)
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Abstract

:
A thermodynamic study on a toluene chemical looping reforming process with six metal oxides was conducted to evaluate the product distribution for selecting an appropriate oxygen carrier with thermodynamic favorability towards high syngas yield. The results show that a suitable operation temperature for most oxygen carriers is 900 °C considering syngas selectivity and solid C formation whether the toluene is fed alone or together with fuel gas. The syngas selectivity of all oxygen carriers decreases with the increasing equivalence ratio, but the decrease degrees are quite different due to their different thermodynamic natures. With the increasing amounts of H2 and CO, the syngas selectivity for various oxygen carriers correspondingly decreases. The addition of CO2 and H2O(g) benefits reducing the solid C formation, whereas the addition of CH4 leads to more solid C being produced. Under the simulated gasification gas atmosphere, a synergetic elimination of solid C and water–gas shift reactions are observed. In terms of syngas selectivity, Mn2O3 possesses the best performance, followed by CaFe2O4 and Fe2O3, but NiO and CuO exhibit the lowest performance. BaFe2O4 presents a high H2 selectivity but a very poor CO selectivity due to the formation of BaCO3, which has a high thermodynamic stability below 1200 °C. Nevertheless, Mn2O3 is more likely to form solid C than feeding toluene alone and has a lower melting point. Considering syngas selectivity, carbon deposit and melting point, CaFe2O4 exhibits the highest performance concerning the tar chemical looping.

1. Introduction

Biomass energy utilization could result in reducing carbon emissions and in energy sustainability since biomass is considered a renewable energy and is abundant [1]. Generally, there is less ash in biomass, and its fixed carbon content is less than 20% by weight. This makes biomass gasification conversion technically and economically favorable, thereby facilitating the production of energy or chemicals [2]. Biomass gasification produces a gas mixture including not only fuel gas such as CO, H2, CO2 and CH4 but also impurities such as tar, H2S and NH3. Both power generation and liquid biofuel production require the limited content of such impurities, so the cleaning of gas mixture is necessary to obtain a fuel gas with an acceptable level of the contaminants [3]. Among various impurities, tar has been identified as the critical one, which condenses at temperature below 300–350 °C and obstructs downstream applications. Tar is a complex mixture of condensable hydrocarbons, which is mainly made of aromatic and polynuclear aromatic hydrocarbons, e.g., benzene, toluene, naphthalene, pyrene, and benzopyrene [4]. One- and two-ring aromatic hydrocarbons such as toluene and naphthalene are usually chosen as tar model compounds to deeply investigate tar conversion [5].
The removal or conversion of tar is the great technical challenge for developing a successful application of biomass gasification. There are different techniques, including wet scrubbing, in situ catalytic conversion in gasifier (catalytic gasification) and catalytic conversion outside gasifier (hot gas cleaning), for eliminating tar in the biomass gasification process [6,7]. Among them, hot gas cleaning is one of the most pertinent options because it can convert tar to valuable gases by catalysts and the sensible heat of the raw fuel gas from gasifier. However, the catalysts employed in the hot gas cleaning can be easily deactivated by rapid coke formation on the catalysts [8,9]. This is the major issue in the catalytic tar conversion outside gasifier. Chemical looping reforming (CLR), in which fuel is partial oxidized by oxygen carriers, is proposed as a promising scheme for eliminating the tar from downstream gas of biomass gasification [10].
The CLR process consists of two reactors, namely, reformer (fuel reactor) and regenerator (air reactor), and operates continuously. Fuel is partial oxidized by oxygen carriers in the endothermic reformer, the reduced oxygen carriers and the possible carbon deposit react with air in the exothermic regenerator [11,12]. By using the lattice oxygen of oxygen carriers, CLR avoids the deactivation caused by coke deposits in catalytic tar conversion through recycling oxygen carriers between the two reactors. The CLR over the past decade has been firstly developed to produce syngas or H2 mainly from CH4 [12,13,14,15,16,17,18,19]. Subsequently, its application for tar elimination of biomass-derived raw gas was proposed and demonstrated [20,21,22,23,24,25,26,27,28,29,30]. The CLR has the same basic principles as CLC (chemical looping combustion), but the target product of the CLR is syngas (H2 and CO). Therefore, to achieve the partial oxidation of fuels to syngas, the oxygen/fuel ratio is traditionally kept lower than the stoichiometric ratio to prevent the complete oxidation of the fuel [12,31]. However, the reforming reaction or partial oxidization of hydrocarbon fuels with metal oxides is usually a strongly endothermic process, and the oxygen carriers transport not only oxygen but also heat between the air reactor and fuel reactor. The amount of oxygen carriers required for heat management is difficult to match exactly with the requested proportion of the oxygen/fuel ratio for the CLR reaction. Furthermore, the rigid circulating rate of oxygen carrier particles between the two reactors can cause a lot of trouble to the operation. Therefore, in the CLR process, it is very important to select an oxygen carrier that is thermodynamically prone to partially oxidize fuels to synthesis gas rather than CO2 and H2O [31]. This can improve the operational flexibility of the CLR while ensuring that the fuel is partial oxidized by the oxygen carrier.
Transitional metal oxides are the popular choice for CLR oxygen carriers. Ni-, Cu-, Fe-, Mn-, Co- and perovskite-type oxides are widely studied as oxygen carriers for CH4 reforming, and the reactivity and selectivity for syngas production of those oxygen carriers have been extensively studied [12,13,14,15,16,17,18]. The progress in oxygen carrier development of methane-based chemical looping reforming has been reviewed in detail [19]. However, there are only limited studies on the tar CLR, especially the literature on the selectivity for syngas production of oxygen carriers is even less. Ni-, Cu-, Fe- and Mn-based oxygen carriers have been initially explored as oxygen carriers for tar or model compound reforming, but those studies mainly focused on the reactivity [20], carbon deposit [21,22], and conversion rate of tar or tar model compounds [23,24,25,26,27,28,29,30]. In our previous study [32], the syngas selectivity for CLR of toluene as a biomass tar model compound over two types of oxygen carriers (2CuO-2NiO/Al2O3 and CaFe2O4) were preliminarily evaluated in a TG-FTIR and a laboratory-scale fixed bed reactor; it was found that CaFe2O4 exhibited a good performance of partial oxidation of toluene to CO and H2, but 2CuO-2NiO/Al2O3 completely oxidized toluene to CO2. However, the evaluation experiments were carried out with toluene alone under the setting temperature and toluene’s feeding amount, and the effects of the parameter variations such as temperature, oxygen/fuel ratio and atmosphere (fuel gas) on the performance of the oxygen carriers also need to be investigated in details.
The thermodynamic analysis for a chemical reaction will provide a comprehensive understanding on the optimal condition, product distribution and heat balance [33,34,35,36]. The syngas selectivity of the reaction of an oxygen carrier with hydrocarbon fuels mainly depends on its molecular thermodynamic property. Thus, thermodynamic analysis can make a major contribution to identifying which oxygen carriers possess partial oxidation properties and can also provide additional insight into the thermodynamic equilibrium product distribution of an oxygen carrier reacting with tar in the raw gasification gas consisting of H2, CO, CO2, CH4 and H2O(g). However, a thermodynamic study on the partial oxidation of tar by CLR process is not available in the literature. In this paper, a comprehensive thermodynamic analysis is conducted to compare the difference in product distribution and tendency of CLR process in the reformer of toluene (tar model compound) reacting with CuO, Fe2O3, NiO, Mn2O3, CaFe2O4 and BaFe2O4 under different conditions. The oxidation reaction in the air reactor is thermodynamically well-defined and is not involved in this study. The analysis results can be used to predict the performances of the oxygen carriers, and provide the thermodynamic feasibility of the selection of a suitable oxygen carrier and guide the further experimental research for partial oxidation of tar by CLR process.

2. Materials and Methods

An algorithm to calculate the thermodynamic analysis and the equilibrium compositions of the CLR reaction of toluene with different oxygen carriers was carried out based on the minimization of the Gibbs free energy. The thermodynamic calculation was conducted on the modules of Equilibrium Compositions and Reaction Equations in HSC Chemistry version 6.0. HSC Chemistry has been applied to thermodynamic studies due to its friendly and powerful calculation methods for studying the effects of different variables on the chemical system at equilibrium [37]. This software enables the calculation of multi-component equilibrium compositions in heterogeneous systems easily. If the reaction system with its phases and species is specified and the amounts of the raw materials are given, the module will give the amounts of the products as a result at particular temperature–pressure conditions without reaction Equations being needed in the input. The calculations of the amounts of products at equilibrium are in isothermal and isobaric conditions.
In this study, the selected oxygen carriers included metal oxides such as CuO, NiO, Mn2O3 and Fe2O3, which are successfully used in chemical looping reforming studies [19,20,21,22,23,24,25,26,27,28,29,30], and metal ferrites such as CaFe2O4 and BaFe2O4, which exhibit a good selectivity of syngas production in chemical looping gasification [38,39,40,41]. The selected oxygen carriers, their reducible oxygen amount and reduced species considered, are listed in Table 1. The main reactions for toluene CLR process are summarized in Table 2. The possible products considered in the computation are H2, CH4, CO, CO2, H2O(g), solid C (coke) and reduced oxygen carriers (as shown in Table 1), as well as the unreacted oxygen carriers and toluene (C7H8). Additionally, the products of the CLR process of CaFe2O4 and BaFe2O4 also include CaCO3 and BaCO3, respectively. No other product formation is considered, and the reaction temperature range for this study is 600 to 1200 °C at 1 bar pressure. The input species are toluene, as well as oxygen carriers, or H2, CO, CH4, H2O(g) and CO2. In addition, 1 mol toluene is considered in all the calculations, but the amount of oxygen carriers varies according to the different equivalence ratios (oxygen/fuel ratio).
The equivalence ratio is defined as follows:
ER = nROA/nSOA
where nROA is the reducible oxygen amount provided by the selected oxygen carriers; nSOA is the stoichiometric oxygen amount of complete oxidation of 1 mol toluene, and the nSOA is 18 mol for 1 mol toluene.
The selectivity of CO and H2 for the oxygen carriers is defined by the following Equations:
SH2 = (neq,H2 − nin,H2)/(4nin,C7H8 + 2nin,CH4 + nin,H2O)∙100%
SCO = (neq,CO − nin,CO)/(7nin,C7H8 + nin,CH4 + nin,CO2)∙100%
where nin and neq are the initial amount of a certain substance, and the equilibrium amount of a certain substance, respectively.

3. Results

3.1. Effect of Temperature on Product Distribution and H2 and CO Selectivities

Operating temperature is one of the important thermodynamic parameters and has an important influence on the reaction products. The most desirable output of 1 mol toluene by chemical looping reforming with oxygen carriers is 7 mol CO and 4 mol H2. This requires the oxygen carriers to provide 7 mol of reducible oxygen, which means an equivalence ratio of 0.39. Hence, the effect of operating temperature on the product distribution was studied with an equivalence ratio of 0.39. Figure 1 illustrates the production of toluene CLR with the selected oxygen carriers at various temperatures. It can be observed from Figure 1a that the yield of H2 increases steadily with increasing temperature for all selected oxygen carriers, but slows down below 900 °C. Furthermore, the H2 yields of different oxygen carriers are different when the operating temperature is lower than 900 °C, while the difference in H2 production between different oxygen carriers above 900 °C is not pronounced. The moles of H2 produced for the toluene CLR reaction of various oxygen carriers at 900 °C range from 3.5 (CuO and NiO) to 3.85 (Mn2O3), and the H2 yields of all oxygen carriers can reach to about 4.0 mol at 1200 °C, which means about 100% of hydrogen in toluene is converted to H2. Accordingly, the amounts of the undesired hydrogen-containing products such as H2O(g) and CH4 (Figure 1a,c) decrease with the increasing temperature; moreover, the yields of H2O(g) and CH4 from different oxygen carriers vary greatly below 900 °C and slightly above 900 °C. As shown in Figure 1a,c, when the temperature is below 800 °C, CuO, NiO and Fe2O3 tend to produce H2O(g), while CaFe2O4, BaFe2O4 and Mn2O3 produce a small amount of CH4 in addition to H2O(g). Nevertheless, the yield of CH4 became negligible at temperatures above 900 °C (<0.05 mol).
The H2 and CO selectivities of all selected oxygen carriers were analyzed and are shown in Figure 1d. The H2 and CO selectivities of the selected oxygen carriers exhibit a similar nature as the yields of H2 and CO, the selectivities of H2 and CO increase with increasing temperature for all oxygen carriers at the temperature range of 600 to 1200 °C. The effect of temperature on the CO selectivity is greater than that of H2, i.e., the CO selectivity increases from below 10% to about 90% (except Mn2O3), whereas the H2 selectivity increases from about 45–65 to 95% as the temperature rises from 600 to 900 °C. The H2 selectivity is above 87% for all oxygen carriers, and the CO selectivity exceeds 76% for the four selected oxygen carriers except Mn2O3 and BaFe2O4 at 900 °C. Considering that the H2 and CO selectivities only increase slightly above 900 °C. as well as the lower melting point of Cu (1083 °C) and Mn2O3 (1080 °C) [42], the operating temperature is chosen as 900 °C in the next study. It should be noted that complete (100%) conversion of toluene for all the selected oxygen carriers occurs within the temperature range of 600 to 1200 °C and in the following cases, and the remaining amount of toluene is less than 10−17 mol in all of the considered cases.

3.2. Effect of Equivalence Ratio on Product Distribution and H2 and CO Selectivities

An ideal oxygen carrier should exhibit good partial oxidation performance in a wide range of equivalence ratios, which will facilitate the CLR operation and ensure the stability and continuity of the process. Figure 2 shows the effect of the equivalence ratio on the production of toluene CLR process at 900 °C for all selected oxygen carriers. As shown in Figure 2a, the H2 yields of the six oxygen carriers show different trends as the equivalence ratio increased from 0.39 to 1.0. The H2 yield of Mn2O3 does not change with the increase in the equivalence ratio and remains at 3.85 mol. On the contrary, the H2 yields of CuO and NiO decrease linearly with the increasing equivalence ratio, and decrease from 3.5 to 0 mol with the increase in the equivalence ratio from 0.39 to 1.0. The H2 yields of CaFe2O4 and BaFe2O4 decrease from 3.6 to 3.0 mol and 3.8 to 3.2 mol with the increase in the equivalence ratio from 0.39 to 1.0, respectively. Unlike others, the H2 yield of Fe2O3 decreases linearly as the equivalence ratio increases from 0.39 to 0.6, and remains at 2.4 mol when the equivalence ratio exceeds 0.6. Correspondingly, the undesired H2O(g) yield exhibits the opposite trend for the five oxygen carriers, except Mn2O3, with a constant H2O(g) yield as the equivalence ratio rises.
As shown in Figure 2b, the CO yields of the six oxygen carriers also show different trends with the increase in the equivalence ratio from 0.39 to 1.0. It is observed that the CO yield of BaFe2O4 slightly increases from 2.1 to 2.4 mol with the increasing equivalence ratio up to 0.6 and then starts to linearly decline to 0.7 mol at the equivalence ratio of 1.0. On the contrary, the CO yield of Mn2O3 increases linearly from 2.2 to 5.7 mol with the increasing equivalence ratio from 0.39 to 1.0. The production trends of CO for the other four oxygen carriers, CaFe2O4, Fe2O3, CuO and NiO, are similar to that of H2. The CO yields of CuO and NiO decrease from 5.3 to 4.4 mol as the equivalence ratio is increased from 0.39 to 0.6, and then they go down quickly until to zero at the equivalence ratio of 1.0; the CO yield of Fe2O3 decreases from 5.6 to 4.6 mol when the equivalence ratio increases from 0.39 to 0.6 and remains unchanged. The CO yield of CaFe2O4 changes a little at different equivalence ratios and declines from 5.4 to 4.7 mol as the equivalence ratio increases from 0.39 to 1.0, indicating that the equivalence ratio has little effect on it.
The CO2 yields of CuO and NiO increase linearly from 0.5 to 7.0 mol, and the CO2 yields of Fe2O3 increase from 0.5 to 2.3 mol with the increase in the equivalence ratio from 0.39 to 0.6 and then keeps this value. As expected, the CO2 produced from CaFe2O4 slightly increases as the equivalence ratio increases from 0.39 to 1.0: the increment is only 0.9 mol with a change from 0.4 to 1.3 mol. Those results correspond to the changes in CO production. However, the CO2 yields of Mn2O3 and BaFe2O4 are very small, and the maximum moles of CO2 from BaFe2O4 and Mn2O3 are only 0.2 and 0.1, respectively. On the other hand, the solid C formations from BaFe2O4 and Mn2O3 display the opposite trend (Figure 2c). The solid C yield of Mn2O3 drops linearly from 4.7 to 1.1 mol as the equivalence ratio increases from 0.39 to 1.0; the solid C yield of BaFe2O4 drops rapidly from 2.5 to 0.9 mol with an increase in the equivalence ratio from 0.39 to 0.6, and then the drop slows down from 0.6 to 0.2 mol as the equivalence ratio continuously increases to 1.0. The solid C produced from the other four oxygen carriers displays a similar trend, and quickly decreases from about 1.0 to 0.3 mol with an increasing equivalence ratio from 0.39 to 0.6, remaining unchanged as the equivalence ratio further increases. The CH4 yields of all the selected oxygen carriers decrease with the increasing equivalence ratio, as mentioned above that it is negligible at 900 °C (Figure 2c).
The effect of the equivalence ratio on the H2 and CO selectivities of selected oxygen carriers was also evaluated and is shown in Figure 2d. The H2 and CO selectivities exhibit different trends for different oxygen carriers with the increasing equivalence ratio, which exhibits a similar nature as the H2 and CO yields. The decreasing order of the selectivity of oxygen carriers towards H2 is as follows: Mn2O3 > BaFe2O4 > CaFe2O4 > Fe2O3 > CuO = NiO. The top three oxygen carriers have a H2 selectivity greater than 75% within the range of equivalence ratio from 0.39 to 1.0, indicating less H2O(g) and CH4 are produced. However, CO selectivity for the selected oxygen carriers cannot be ordered regularly. In general, the CO selectivity of CaFe2O4 is the highest and is almost unaffected by the equivalence ratio: it only changes from 78% to 70%. The CO selectivity of Fe2O3 is the second highest and decreases from 79% to 65% with the increasing equivalence ratio from 0.39 to 0.6 and then remains constant when the equivalence ratio is above 0.6. The CO selectivities of CuO and NiO fall quickly with the increasing equivalence ratio and decrease to 0 when the equivalence ratio is 1.0. The CO selectivity of Mn2O3 rises linearly from 32% to 82% as the equivalence ratio increases from 0.39 to 1.0. This is due to MnO being the main reduced species from Mn2O3 [43,44], and its reactivity is lower and does not react further with carbon and H2 below 1200 °C since the Gibbs free energy changes in their reactions are much higher than zero. This is also the reason why Mn2O3 produces higher solid C and H2 than the other oxygen carriers. In other words, it means that 1 mol of Mn2O3 can only provide 1 mol of oxygen thermodynamically below 1200 °C. The CO selectivity of BaFe2O4 is the lowest, its maximum value is only about 30% and it decreases as the equivalence ratio is above 0.6. This is attributed to the formation of more BaCO3, whose Gibbs free energy change based on the reaction of BaO and CO2 is −79.2 kJ/mol at 900 °C, and the higher yield of the solid C (as shown in Figure 2c). Considering the H2 and CO selectivities, CaFe2O4 and Fe2O3 possess the thermodynamic property of partial oxidation of toluene in a relative wide range of equivalence ratios.

3.3. Effect of Gasification Gas Component on Product Distribution and H2 and CO Selectivities

The tar content is relatively low compared with the concentrations of H2, CO, CO2, CH4 and H2O(g) in the raw biomass gasification gas. The reducing gas components, such as CO, H2 and CH4, may not only react with oxygen carriers but may also affect the equilibrium composition of chemical reaction products [45,46,47]. For the CO2 and H2O(g), they may not only affect the equilibrium composition of reaction products but may also react with the reduced species of the oxygen carriers. It is vital to investigate the impact of these gas components on the oxygen carriers’ CLR reaction. As shown above, the H2 and CO selectivities of the oxygen carriers exhibit a similar nature as the H2 and CO yields, but an opposite trend to the H2O(g) and CO2 yields. Therefore, the effects of the different gas components are mainly focused on the H2 and CO selectivities and the yields of solid C and CH4.

3.3.1. H2

Figure 3 shows the effect of H2 addition on the H2 and CO selectivities and products for the CLR reaction of the selected oxygen carriers with 1 mol of toluene at 900 °C with the equivalence ratios of 0.39 and 1.0, respectively. As shown in Figure 3a, at the equivalence ratio of 0.39, the H2 selectivity gradually decreases for all the selected oxygen carriers with the increasing amount of H2 addition, the decreasing trend and degree of H2 selectivity for all oxygen carriers are similar and the decrement is about 10% as the addition of H2 increases from 0 to 12 mol. However, at the equivalence ratio of 1.0, the H2 selectivities of different oxygen carriers show different decreasing trends and degrees. As the amount of added H2 increases from 0 to 12 mol, the H2 selectivities of Mn2O3, BaFe2O4 and CaFe2O4 decrease from 96% to 85%, 80% to 35% and 73% to 8%, respectively. The other three oxygen carriers display a greater decrement with respect to the H2 selectivity. The H2 selectivity of Fe2O3 drops from 61% to −51%, and the H2 selectivities of CuO and NiO fall from 0 to −102%. Notice that the negative value means the amount of H2 produced from toluene is less than the amount consumed. From Figure 3b, it can be observed that the CO selectivity almost remains unchanged and shows an imperceptible increase for all oxygen carriers at the equivalence ratio of 0.39. However, at the equivalence ratio of 1.0, the CO selectivities of Fe2O3 and Mn2O3 are almost unchanged and are 66% and 82%, respectively. CaFe2O4 and BaFe2O4 show an imperceptible increment towards CO selectivity, which increases from 70% to 76% and 11% to 14%, respectively. CuO and NiO display a greater increment towards CO selectivity, which rises from 0 to 55% with the increasing amount of added H2 from 0 to 12 mol.
From Figure 3c, one can see that the addition of H2 has little impact on the Solid C yields of all oxygen carriers at the equivalence ratios of 0.39 and 1.0. As shown in Figure 3d, there is little effect of H2 addition on the CH4 yields of the selected oxygen carriers, except Mn2O3. The CH4 yields of Mn2O3 exhibit a linear increase with the increase in H2 addition at the equivalence ratios of 0.39 and 1.0, but their values are still very low compared with other products. The results of Figure 3c,d indicate that the addition of H2 does not impact on the carbon formation (reactions (10)–(13)) and methanation (reactions (4)–(6)).

3.3.2. CO

Figure 4 illustrates the effect of the CO addition on the H2 and CO selectivities and the yields of solid C and CH4 for the CLR reaction of the selected oxygen carriers with 1mol toluene at 900 °C with the equivalence ratios of 0.39 and 1.0. From Figure 4a,b, it can be seen that the trend of the effect of CO addition on the CO selectivity for the selected oxygen carriers is similar to that of H2 addition on the H2 selectivity, and the effect of CO addition on the H2 selectivity for the selected oxygen carriers is similar to that of H2 addition on the CO selectivity whether the equivalence ratio is 0.39 or 1.0. As shown in Figure 4c, the yields of solid C for all oxygen carriers increase gradually with the addition of CO from 0 to 12 mol, indicating that the addition of CO promotes reaction (10). The CH4 yield of Mn2O3 decreases with the addition of CO from 0 to 12 mol, and the CH4 yields of other oxygen carriers are hardly affected by the addition of CO (Figure 4d).

3.3.3. CO2

As shown in Figure 5a,b, the H2 and CO selectivities exhibit different trends for different oxygen carriers with the addition of CO2 with the equivalence ratios of 0.39 and 1.0. At the equivalence ratio of 0.39, the H2 selectivities of BaFe2O4, CaFe2O4, CuO and NiO drop gradually with the increasing amount of CO2 addition. The H2 selectivity of Mn2O3 first remains constant and then declines after the addition of CO2 exceeds 5 mol, whereas the H2 selectivity of Fe2O3 first declines and then remains constant after the CO2 addition exceeds 6mol. The CO selectivities of the selected oxygen carriers increase first and then decrease or stay constant. The CO selectivity of Mn2O3 increases sharply from 32% to the maximum value of 98% as the added amount of CO2 increases from 0 to 5 mol, and then it decreases rapidly. The CO selectivity of BaFe2O4 increases sharply from 30% to 65% with increasing the added amount of CO2 from 0 to 3 mol and then remains at this value. At the equivalence ratio of 1.0, the H2 selectivities of Fe2O3 and CaFe2O4 remain almost unchanged with the addition of CO2 below 7 mol and then drops gradually, and so does the CO selectivity. For CuO and NiO, the H2 selectivity stays at about 0% and CO selectivity increases to the maximum with the increasing addition of CO2 from 0 to 2 mol and then drops rapidly. For BaFe2O4, the H2 selectivity has a slight decline, but the CO selectivity increases steadily with the increasing addition of CO2. The decrease in the CO selectivity is due to the additional CO2 that enlarges the feeding carbon and cannot convert to CO completely.
As shown in Figure 5c, the addition of CO2 is beneficial to the reduction in solid C yield. The decreases in solid C and CO2 mean that the additional CO2 promotes the reverse reactions (9) and (10). For Mn2O3, the additions of CO2 required for the CO selectivity to reach the maximum value (Figure 5b: 5 mol and 2 mol for the equivalence ratios of 0.39 and 1.0, respectively) and the beginning of the H2 selectivity to drop rapidly (Figure 5a) are consistent with those for the removal of carbon deposits (Figure 5c), implying that the additional CO2 reacts preferentially with solid C (revers reaction (10)) and then with H2 (reverse reaction (9)). Moreover, the addition of CO2 can suppress the CH4 yield (Figure 5d), which implies that reverse reaction (4) occurs with the addition of CO2.

3.3.4. H2O(g)

Figure 6 displays the effect of H2O(g) addition on the H2 and CO selectivities and the yields of solid C and CH4 for the CLR reaction of the selected oxygen carriers with 1mol toluene at 900 °C with the equivalence ratios of 0.39 and 1.0. It can be seen from Figure 6a,b that the trend of the effect of addition of H2O(g) on the H2 and CO selectivities of the selected oxygen carriers is similar to that of the CO2 addition. The only difference is that the CO selectivity of BaFe2O4 stays constant with the increasing H2O(g) addition from 0 to12 mol. The decrease in solid C yield (Figure 6c) means that the additional H2O(g) promotes the reverse reactions (11) and (12). For Mn2O3, the additions of H2O(g) required for the CO selectivity to reach the maximum value (Figure 6b: 5 mol and 2 mol for the equivalence ratios of 0.39 and 1.0, respectively) and the beginning of the CO selectivity to drop rapidly (Figure 6b) are consistent with those for the removal of solid C, implying that the additional H2O(g) reacts preferentially with solid C (reverse reactions (11) and (12)) and then with CO (reaction (9)). As shown in Figure 6d, the CH4 yield of Mn2O3 first increases and then decreases with the increasing H2O(g) addition. A possible reason for this is that the addition of a small amount of H2O(g) increases the yield of H2, thus promoting the methanation reaction; however, with the increase in the added amount of H2O(g), the reforming reaction of methane with steam (reverse reaction (5)) will occur, which is not conducive to the production of methane in terms of thermodynamics.

3.3.5. CH4

CH4 is a desired product for fuel gas but not for syngas. Generally, the content of CH4 in gasification gas is lower than that of H2, CO and CO2. From Figure 7a,b, it can be seen that the H2 selectivity increases slightly and then quickly reaches the maximum value of 97%, but the CO selectivity declines with the increasing addition of CH4 for all oxygen carriers at the equivalence ratio of 0.39. This means that the hydrogen in the added CH4 is substantially converted into H2 and the carbon is only partially converted to CO. However, at the equivalence ratio of 1.0, the H2 and CO selectivities gradually rise from about 70% to 80% for CaFe2O4 with the increasing addition of CH4, and they increase dramatically from 0 to 80% for CuO and NiO as the addition of CH4 increases from 0 to 7.0 mol. The H2 and CO selectivities of Fe2O3 remain unchanged with the added CH4 from 0 to 3.5 mol and then rise rapidly to about 75% by continually adding CH4 to 7.0 mol. The addition of CH4 has no effect on the H2 selectivity of Mn2O3, which keeps a high value of 97%, but has a negative impact on the CO selectivity of Mn2O3, which decreases from 83% to 45% as the addition of CH4 increases from 0 to 7.0 mol. The selectivities of H2 and CO for BaFe2O4 increase from 79% to 97% and 10% to 30%, respectively.
As shown in Figure 7c, the linear increase in the solid C yield and the decrease in the CO selectivity for all oxygen carriers at the equivalence ratio of 0.39 indicate that the added CH4 is mainly decomposed through reaction (13). However, the slight increases in the solid C yields at the equivalence of 1.0 and the solid C yields of the selected oxygen carriers with an equivalence ratio of 1.0 are much lower than that with an equivalence ratio of 0.39, implying that the added CH4 is consumed by the produced H2O(g) and CO2 (reverse reactions (4) and (5)). The relatively high yield of solid C for Mn2O3 at the equivalence of 1.0 are due to the inertness of the remanent lattice oxygen, such as MnO, which can neither react with toluene to form enough H2O(g) and CO2 nor with CH4, and CH4 is converted by the decomposition (reaction (13)). Thus, the solid C yield increases steadily, and CO selectivity decreases with the addition of CH4. From Figure 7d, one can see that the net yields of CH4 for all oxygen carriers increase with the increasing addition of CH4 at the equivalence ratios of 0.39 and 1.0, but the yields of CH4 are much smaller than the amount of CH4 addition. Finally, the values of the CH4 yield are less than 0.3 mol, which means that the added CH4 is almost converted completely.

3.4. Effect of Simulated Gasification Gas on Product Distribution and H2 and CO Selectivities

A single component of gasification gas cannot fully reflect the synergistic effect of the whole gas component on the CLR reaction of oxygen carriers with toluene. Therefore, a simulated fuel gas from biomass gasification with oxygen was adopted to evaluate the performance of the selected oxygen carriers towards toluene CLR under fuel gas atmosphere. According to our previous study [48], a typical composition of the raw fuel gas from biomass gasification with oxygen is as follows: H2, 12 mol; CO, 12 mol; CO2, 7 mol; H2O(g), 3 mol; CH4, 2 mol. Furthermore, 1 mol toluene in this simulated raw fuel gas corresponds to a tar content of 110 g/Nm3.
The effect of temperature on the performances of the selected oxygen carriers towards H2 and CO selectivity at the equivalence ratio of 0.39 under simulated gasification gas atmosphere is shown in Figure 8a. The H2 selectivity for all the selected oxygen carriers rises rapidly to a maximum with an increase in temperature from 600 to 900 °C, and then declines slightly with a further increase in temperature. The maximum values of the H2 selectivity of different oxygen carriers vary greatly, and the ranking of the maximum value of H2 selectivity for various oxygen carriers is in the following decreasing order: Mn2O3 (83%) > BaFe2O4 (72%) > CaFe2O4 (62%) > Fe2O3 = CuO = NiO (55%). However, it can be observed that the CO selectivity increases with increasing temperature for all oxygen carriers and also goes up dramatically with an increase in temperature from 600 to 900 °C and then grows slowly. Nevertheless, there is little difference in the CO selectivity for the selected oxygen carriers, except Mn2O3, which exhibits a much higher CO selectivity than the others. The descending order of CO selectivity at 900 °C is as follows: Mn2O3 (89%) > CaFe2O4 (74%) > Fe2O3 (71%) > CuO = NiO (70%) > BaFe2O4 (65%).
From the results of Figure 8b,c, one can see that as the temperature increases, the net yield of H2O(g) decreases rapidly and then increases slightly, while the net yield of CO2 decreases rapidly and then slows down. For all the selected oxygen carriers, the net yield of CH4 reduces quickly, and CH4 is consumed completely as the value at 900 °C is −2.0 mol, which is equal to the adding amount of CH4 (Figure 8d). Moreover, the yield of solid C declines sharply with a decrease in temperature from 600 to 900 °C, and is much higher than that of feeding toluene alone at lower temperature; the amount of solid C is increased by 2–3 times when the temperature is below 800 °C compared with feeding toluene alone. According to the above results as well as the negative and low values of the selectivities of H2 and CO at lower temperatures, it seems that reactions (10), (11) and (12) are domineering at lower temperatures due to their exothermic nature, whereas reverse reactions (9) and (10) are dominant at higher temperatures due to their endothermic nature and higher concentrations of H2 and CO2.
The effect of the equivalence ratio on the performances of the selected oxygen carriers under simulated gasification gas atmosphere at 900 °C is shown in Figure 9. The H2 selectivity decreases for all the selected oxygen carriers with the increasing equivalence ratio (Figure 9a). However, different oxygen carriers display different extents of decline. As the equivalence ratio varies from 0.39 to 1.0, the H2 selectivity decreases from 84% to 67% for Mn2O3, 73% to 54% for BaFe2O4 and 66% to 46% for CaFe2O4, respectively; unexpectedly, the H2 selectivity of Fe2O3 is the same as that of CuO and NiO at the range of 0.39 to 1.0, which decreases from 60% to 21%, 18% and 13%, respectively. As shown in Figure 9a, the trend of CO selectivity for all the selected oxygen carriers is similar to that of H2 selectivity. Nevertheless, the order of the CO selectivity for oxygen carriers changes. The CO selectivity decreases from 89% to 78% for Mn2O3, 75% to 60.0% for CaFe2O4, 72% to 41% for Fe2O3 and 71% to 37% for CuO and NiO, respectively. The CO selectivity of BaFe2O4 is the lowest and decreases from 66% to 36%. The lowest CO selectivity of BaFe2O4 is attributed to the thermodynamically favorable production of more BaCO3 due to the formations of more BaO and CO2 with the increasing equivalence ratio.
As expected, the net yields of H2O(g) and CO2 rise with the increase in the equivalence ratio because more reducible oxygen supplied by the oxygen carriers make complete oxidization more likely to occur. As shown in Figure 9b,c, the H2O(g) and CO2 yields and slopes of various oxygen carriers are different, and the orders of the oxygen carriers towards net yields and rising slopes of H2O(g) are the same as that of CO2. The net yields and slopes of H2O(g) and CO2 for CuO, NiO and Fe2O3 are almost identical and are the largest and steepest. Then, the following order is CaFe2O4, BaFe2O4 and Mn2O3. The net yields of CH4 of all the selected oxygen carriers are close to −2.0 mol as the equivalence ratio ranges from 0.39 to 1.0 (Figure 9d), which indicate that CH4 is completely converted. The solid C yields of the selected oxygen carriers are very low in all cases, and the maximum yield of solid C is below 0.55 mol, implying that the syngas atmosphere including H2O(g) and CO2 would alleviate the carbon deposit. Note that the most of net yield of CO2 are negative, which means that some of CO2 in syngas are consumed. The net yield of CO is above 9 mol (maximum theoretical conversion for 1mol toluene and 2 mol CH4), so the consumed CO2 is converted to CO for the selected oxygen carriers, except BaFe2O4, which convert some CO2 to BaCO3. However, only the net yield of H2O(g) of BaFe2O4 is negative, and the net yields of H2 of the other five oxygen carriers are below 8 mol (maximum theoretical conversion for 1mol toluene and 2 mol CH4), implying that not only the H2O(g) in the gasification gas is not converted but also some of H2 in the gasification gas is oxidized into water by these five oxygen carriers. In terms of the H2 and CO selectivities and solid C yield, Mn2O3 and CaFe2O4 are the suitable oxygen carriers for CLR of tar in the raw fuel gas from biomass gasification in a wide range of equivalence ratios.

4. Conclusions

A thermodynamic analysis has been conducted to evaluate the product distribution and tendency of toluene CLR process with six oxygen carriers based on the minimization of the Gibbs free energy. The effects of temperature, equivalence ratio, single fuel gas component and simulated gasification gas on the product distribution and syngas selectivity for the CLR process were investigated in detail. The results show that a suitable operation temperature for most oxygen carriers is 900 °C considering syngas selectivity and solid C formation and whether the toluene is fed alone or together with fuel gas. The syngas selectivity for all oxygen carriers decreases with the increasing equivalence ratio, but the decrease degrees are quite different due to their different molecular thermodynamic natures. As the additions of H2 and CO increased, the syngas selectivity for various oxygen carriers correspondingly decreased. The additions of CO2 and H2O(g) reduce the formation of solid C, whereas the addition of CH4 leads to more solid C being produced. Under the gasification gas atmosphere, synergetic solid C elimination and water–gas shift reactions are observed. In terms of syngas selectivity in the equivalence ratio range of 0.39 to 1.0, Mn2O3 possesses the best performance, followed by CaFe2O4 and Fe2O3, but CuO and NiO exhibit the worst performance; BaFe2O4 presents a high H2 selectivity but a very poor CO selectivity due to the formation of BaCO3, which has a high thermodynamic stability below 1200 °C. Nevertheless, Mn2O3 is most likely to form solid C as only toluene is fed and has a lower melting point. Considering the syngas selectivity, carbon deposit and melting point, CaFe2O4 is the priority in thermodynamics for CLR of biomass tar.
Although the thermodynamic analysis does not take into account the kinetics (rates) of the CLR process, the results of this study are still very useful for selecting materials with potential as an appropriate oxygen carrier and for finding the optimum reaction conditions and yields for further CLR experimental investigations without an expensive trial-and-error process.

Author Contributions

Conceptualization and original draft preparation, Z.W.; methodology, J.Z.; validation, J.Z. and J.W. (Jingli Wu); data curation, T.H.; review and editing, J.W. (Jinhu Wu). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 22178366, 22108287 and the Natural Science Foundation of Shandong Province of China, grant number ZR2020MB138.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Atmosphere 13 00887 g001 550
Figure 1. Effect of temperature on product distribution and H2 and CO selectivities of toluene CLR. (a) H2 and H2O yields; (b) CO and CO2 yields; (c) Solid C and CH4 yields; (d) H2 and CO selectivities.
Figure 1. Effect of temperature on product distribution and H2 and CO selectivities of toluene CLR. (a) H2 and H2O yields; (b) CO and CO2 yields; (c) Solid C and CH4 yields; (d) H2 and CO selectivities.
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Figure 2. Effect of equivalence ratio on product distribution and H2 and CO selectivities of toluene CLR. (a) H2 and H2O yields; (b) CO and CO2 yields; (c) Solid C and CH4 yields; (d) H2 and CO selectivities.
Figure 2. Effect of equivalence ratio on product distribution and H2 and CO selectivities of toluene CLR. (a) H2 and H2O yields; (b) CO and CO2 yields; (c) Solid C and CH4 yields; (d) H2 and CO selectivities.
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Figure 3. Effect of amount of H2 addition on the product distribution and H2 and CO selectivities of toluene CLR with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
Figure 3. Effect of amount of H2 addition on the product distribution and H2 and CO selectivities of toluene CLR with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
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Figure 4. Effect of amount of CO addition on product distribution and H2 and CO selectivities with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
Figure 4. Effect of amount of CO addition on product distribution and H2 and CO selectivities with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
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Figure 5. Effect of amount of CO2 addition on the H2 and CO selectivities and product distribution with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
Figure 5. Effect of amount of CO2 addition on the H2 and CO selectivities and product distribution with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
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Figure 6. Effect of amount of H2O(g) addition on the H2 and CO selectivities and product distribution with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
Figure 6. Effect of amount of H2O(g) addition on the H2 and CO selectivities and product distribution with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
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Figure 7. Effect of amount of CH4 addition on the H2 and CO selectivities and product distribution with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
Figure 7. Effect of amount of CH4 addition on the H2 and CO selectivities and product distribution with the equivalence ratios of 0.39 and 1.0. (a) H2 selectivity; (b) CO selectivity; (c) Solid C yield; (d) CH4 yield.
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Figure 8. Effect of temperature on H2 and CO selectivities and product distribution under simulated gasification gas atmosphere. (a) H2 and CO selectivities; (b) H2 and H2O yields; (c) CO and CO2 yields; (d) Solid C and CH4 yields.
Figure 8. Effect of temperature on H2 and CO selectivities and product distribution under simulated gasification gas atmosphere. (a) H2 and CO selectivities; (b) H2 and H2O yields; (c) CO and CO2 yields; (d) Solid C and CH4 yields.
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Figure 9. Effect of equivalence ratio on H2 and CO selectivities and product distribution under simulated gasification gas atmosphere. (a) H2 and CO selectivities; (b) H2 and H2O yields; (c) CO and CO2 yields; (d) Solid C and CH4 yields.
Figure 9. Effect of equivalence ratio on H2 and CO selectivities and product distribution under simulated gasification gas atmosphere. (a) H2 and CO selectivities; (b) H2 and H2O yields; (c) CO and CO2 yields; (d) Solid C and CH4 yields.
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Table
Table 1. The selected oxygen carriers’ reducible oxygen amount and reduced species.
Table 1. The selected oxygen carriers’ reducible oxygen amount and reduced species.
Oxygen CarrierReducible Oxygen Amount, mol/molReduced Products Considered
CuO1Cu, Cu2O
NiO1Ni
Fe2O33Fe, FeO
Mn2O33Mn, MnO
CaFe2O43CaO, Fe, FeO, CaCO3
BaFe2O43BaO, Fe, FeO, BaCO3
Table
Table 2. The possible reactions during toluene CLR process.
Table 2. The possible reactions during toluene CLR process.
Reaction DescriptionReaction Equation
Oxidation:C7H8(g) + 18MeO = 7CO2 + 4H2O + 8MeΔH298K < 0 *(1)
Partial oxidationC7H8(g) + 7MeO = 7CO + 4H2 + 18MeΔH298K < 0(2)
DecompositionC7H8(g) = 7C + 4H2ΔH298K = −50.0 kJ/mol(3)
Methanation2CO + 2H2 = CH4 + CO2
CO + 3H2 = CH4 + H2O(g)
CO2 + 4H2 = CH4 + 2H2O(g)		ΔH298K = −247.1 kJ/mol
ΔH298K = −206.0 kJ/mol
ΔH298K = −164.8 kJ/mol		(4)
(5)
(6)
Steam/CO2 reformingC7H8(g) + 7H2O(g)= 7CO + 11H2
C7H8(g) + 7CO2 = 14CO + 4H2		ΔH298K = 869.4 kJ/mol
ΔH298K = 1157.5 kJ/mol		(7)
(8)
Water gas shiftCO + H2O(g) = H2 + CO2ΔH298K = −41.2 kJ/mol(9)
Carbon formation2CO = CO2 + C
CO + H2 = C + H2O(g)
CO2+ 2H2 = C + 2H2O(g)
CH4 = 2H2 + C		ΔH298K = −131.3 kJ/mol
ΔH298K = −90.2 kJ/mol
ΔH298K = −172.5 kJ/mol
ΔH298K = 74.6 kJ/mol		(10)
(11)
(12)
(13)
* ΔH298K > 0 for CuO.
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Wang, Z.; Zhang, J.; Wu, J.; He, T.; Wu, J. Evaluation of Different Oxygen Carriers for Chemical Looping Reforming of Toluene as Tar Model Compound in Biomass Gasification Gas: A Thermodynamic Analysis. Atmosphere 2022, 13, 887. https://doi.org/10.3390/atmos13060887

AMA Style

Wang Z, Zhang J, Wu J, He T, Wu J. Evaluation of Different Oxygen Carriers for Chemical Looping Reforming of Toluene as Tar Model Compound in Biomass Gasification Gas: A Thermodynamic Analysis. Atmosphere. 2022; 13(6):887. https://doi.org/10.3390/atmos13060887

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Wang, Zhiqi, Jinzhi Zhang, Jingli Wu, Tao He, and Jinhu Wu. 2022. "Evaluation of Different Oxygen Carriers for Chemical Looping Reforming of Toluene as Tar Model Compound in Biomass Gasification Gas: A Thermodynamic Analysis" Atmosphere 13, no. 6: 887. https://doi.org/10.3390/atmos13060887

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