Effects of Ca-Compounds on the Gases Formation Behavior during Molten Salts Thermal Treatment of Bio-Waste

: Bio-waste utilization is essential, and pyrolysis is a prominent way for its effective utilization. However, the gradual accumulation of ash compounds in the intermediate products probably affects the thermal conversion characteristics of bio-waste. In the present study, beech wood and disposable chopsticks were selected as bio-waste samples. The effects of typical ash components (Ca-compounds) on volatile formation behavior were investigated during the molten salts thermal treatment of bio-waste. Results demonstrated that about 80% mass of initial bio-waste was gasiﬁed into the volatiles at 300 ◦ C. The introduction of Ca-compounds in the molten salts slightly decreased the total yield of gaseous products. More speciﬁcally, Ca 2+ could improve the generation of CO 2 and suppress the generation of other gases (CO, H 2 , and CH 4 ), and this is accompanied by a reduction in the low heating value (LHV) of the gases. The possible reason is that Ca 2+ might act on the -OH bonds, phenyl C-C bond, methoxy bond and carboxylic acid -COOH bonds of the bio-waste to promote CO 2 release. In contrast, the introduction of CO 32 − and OH-tended to relieve the inhibition effect of Ca 2+ on the generation of H-containing gases. Meanwhile, the introduction of Ca 2+ can promote the conversion of bio-waste into liquid products as well as increase the saturation level of liquid products. Moreover, as a vital form of carbon storage, CO 2 was found to be abundant in the pyrolysis gases from molten salts thermal treatment of bio-waste, and the concentration of CO 2 was much higher than that of direct-combustion or co-combustion with coal. It’s a promising way for bio-waste energy conversion as well as synchronized CO 2 capture by using molten salts thermal treatment, while the introduction of small amounts of Ca-compounds was found to have no signiﬁcant effect on the change of CO 2 concentration.


Introduction
Energy utilization of bio-waste is one of the prominent ways to mitigate carbon dioxide emissions [1].However, low energy density and high cost in energy conversion process both restrict the development and application of bio-waste energy technology.Accordingly, many efforts have been made to convert bio-waste into high-valued energy products such as syngas and pyrolysis carbon [2,3].On the other hand, researchers are committed to reducing the energy consumption cost, to improve the economy of the utilization [4].During this process, many scholars have attempted to apply solar energy to the bio-waste thermal conversion, aiming at producing economical, high-value gases [5].Furthermore, in order to solve the stability problems caused by the fluctuation of solar energy [6], molten salts have been used as the reaction medium for heat storage to realize the stable utilization of bio-waste [7].It is noteworthy that bio-waste contains a certain amount of ash, which has been proved to change the physicochemical properties of molten salts.The accumulation of biomass ash in molten salts will lead to a decrease in the thermal diffusivity and thermal

Releasing Characteristics of Gases with the Addition of Different Ca-Compounds
The distribution characteristics of the pyrolysis (including conventional pyrolysis and molten salts thermal treatment) products from beech wood and disposable chopsticks is compared in Figure 1.It's obviously that more bio-waste was converted into pyrolysis gases and liquid products in the case of molten salts thermal treatment, which is consistent with previous studies [8,17].The mass ratios of volatiles from the two bio-wastes ranged from 79.4% to 89.3% during the molten salts thermal treatment, while the volatile yield was 18.3% to 20.5% in conventional pyrolysis.It suggests that molten salts probably had a catalytic effect on the release of volatiles during bio-waste pyrolysis, regardless of the type of the bio-waste.After the addition of various Ca-compounds, the distribution characteristics of the volatiles (including gaseous and liquid products) were changed, to a certain extent, ranging from 79.8% to 85.6% for beech wood and from 79.4% to 87.5% for disposable chopsticks.Moreover, the gas yield ranged from 54.6% to 62.3% for beech wood and from 55.4% to 63.3% for disposable chopsticks.
Catalysts 2022, 12, x FOR PEER REVIEW 3 of 14 bio-wastes, Ca(NO3)2, CaCO3, and Ca(OH)2 were added in the molten salts system, respectively.In detail, the gaseous and liquid products were detected using GC and GC-MS to further understand the thermal conversion behavior of the two bio-wastes.

Releasing Characteristics of Gases with the Addition of Different Ca-Compounds
The distribution characteristics of the pyrolysis (including conventional pyrolysis and molten salts thermal treatment) products from beech wood and disposable chopsticks is compared in Figure 1.It's obviously that more bio-waste was converted into pyrolysis gases and liquid products in the case of molten salts thermal treatment, which is consistent with previous studies [8,17].The mass ratios of volatiles from the two bio-wastes ranged from 79.4% to 89.3% during the molten salts thermal treatment, while the volatile yield was 18.3% to 20.5% in conventional pyrolysis.It suggests that molten salts probably had a catalytic effect on the release of volatiles during bio-waste pyrolysis, regardless of the type of the bio-waste.After the addition of various Ca-compounds, the distribution characteristics of the volatiles (including gaseous and liquid products) were changed, to a certain extent, ranging from 79.8% to 85.6% for beech wood and from 79.4% to 87.5% for disposable chopsticks.Moreover, the gas yield ranged from 54.6% to 62.3% for beech wood and from 55.4% to 63.3% for disposable chopsticks.In order to further explore the effect of Ca 2+ on bio-waste pyrolysis characteristics, different contents of Ca(NO3)2 were selected to study the gas generation characteristics during molten salts thermal treatment.As shown in Figure 2, compared with the conventional pyrolysis (salt-free), more gaseous products were generated from the selected biowaste in the molten salts pyrolysis.It is supposed that the catalysis of ions in molten salts greatly reduced the reaction activation energy during bio-waste pyrolysis [17].In addition, the gas yield was reduced with the addition of Ca(NO3)2, and the inhibition effect was more obvious with an increase in Ca(NO3)2 content.On the one hand, the introduction of Ca 2+ may change the physicochemical properties of molten salts [28].On other hand, Ca 2+ could promote the synthesis of small molecules, which then decreased the pyrolysis gas yield.In conventional pyrolysis (salt-free) process, the gaseous product is only CO2 at 300 °C, which is mostly produced from the decomposition of low-thermal-stability components in the bio-waste [33].From Figure 3, adding 10 g Ca(NO3)2 could promote the CO2 yield, while excess Ca(NO3)2 might suppress the formation of CO2.It is suggested that Ca 2+ promotes the release of CO2 by breaking the-OH bond, phenyl C-C, and methoxy and carboxylic acid-COOH bonds, which attach to the end of the bio-waste molecular structure [34].However, excess Ca 2+ may inhibit the transfer of NO3 − to NO2 − , which has been proved to play an important role for the bio-waste pyrolysis [32].Thus, the excess In order to further explore the effect of Ca 2+ on bio-waste pyrolysis characteristics, different contents of Ca(NO 3 ) 2 were selected to study the gas generation characteristics during molten salts thermal treatment.As shown in Figure 2, compared with the conventional pyrolysis (salt-free), more gaseous products were generated from the selected bio-waste in the molten salts pyrolysis.It is supposed that the catalysis of ions in molten salts greatly reduced the reaction activation energy during bio-waste pyrolysis [17].In addition, the gas yield was reduced with the addition of Ca(NO 3 ) 2 , and the inhibition effect was more obvious with an increase in Ca(NO 3 ) 2 content.On the one hand, the introduction of Ca 2+ may change the physicochemical properties of molten salts [28].On other hand, Ca 2+ could promote the synthesis of small molecules, which then decreased the pyrolysis gas yield.In conventional pyrolysis (salt-free) process, the gaseous product is only CO 2 at 300 • C, which is mostly produced from the decomposition of low-thermal-stability components in the bio-waste [33].From Figure 3, adding 10 g Ca(NO 3 ) 2 could promote the CO 2 yield, while excess Ca(NO 3 ) 2 might suppress the formation of CO 2 .It is suggested that Ca 2+ promotes the release of CO 2 by breaking the-OH bond, phenyl C-C, and methoxy and carboxylic acid-COOH bonds, which attach to the end of the bio-waste molecular structure [34].However, excess Ca 2+ may inhibit the transfer of NO 3 − to NO 2 − , which has been proved to play an important role for the bio-waste pyrolysis [32].Thus, the excess Ca(NO 3 ) 2 further reduces the gases yield, and will decrease the CO 2 concentration in pyrolysis gases.
Ca(NO3)2 further reduces the gases yield, and will decrease the CO2 concentration in pyrolysis gases.

S a l t f r e e
M o l t e n s a l t s   The gas yield of the examined two bio-wastes with the addition of different Cacompounds are shown in Figure 4a.The gas yield all decreased after the addition of Ca-compounds (Ca(NO3)2, CaCO3, and Ca(OH)2), which was consistent with previous work [23,35].Among those Ca-compounds, Ca(OH)2 seems to have had the strongest inhibitory effect on the pyrolysis gases generation, which decreased by 12.33% for beech wood and 12.37% for disposable chopsticks.It has been pointed out that OH − could inhibit decarbonylation (C-O) and decarboxylation (C-O-C) in hemicellulose [36].Another reason is that CO2 can be easily captured by Ca(OH)2 to form CaCO3(s).Moreover, the distribution of different gaseous products are presented in     The gas yield of the examined two bio-wastes with the addition of different Cacompounds are shown in Figure 4a.The gas yield all decreased after the addition of Ca-compounds (Ca(NO3)2, CaCO3, and Ca(OH)2), which was consistent with previous work [23,35].Among those Ca-compounds, Ca(OH)2 seems to have had the strongest inhibitory effect on the pyrolysis gases generation, which decreased by 12.33% for beech wood and 12.37% for disposable chopsticks.It has been pointed out that OH − could inhibit decarbonylation (C-O) and decarboxylation (C-O-C) in hemicellulose [36].Another reason is that CO2 can be easily captured by Ca(OH)2 to form CaCO3(s).Moreover, the distribution of different gaseous products are presented in The gas yield of the examined two bio-wastes with the addition of different Cacompounds are shown in Figure 4a.The gas yield all decreased after the addition of Ca-compounds (Ca(NO 3 ) 2 , CaCO 3 , and Ca(OH) 2 ), which was consistent with previous work [23,35].Among those Ca-compounds, Ca(OH) 2 seems to have had the strongest inhibitory effect on the pyrolysis gases generation, which decreased by 12.33% for beech wood and 12.37% for disposable chopsticks.It has been pointed out that OH − could inhibit decarbonylation (C-O) and decarboxylation (C-O-C) in hemicellulose [36].Another reason is that CO 2 can be easily captured by Ca(OH) 2 to form CaCO 3 (s).Moreover, the distribution of different gaseous products are presented in Figure 4b.Compared with the NaNO 3 -NaNO 2 system, the addition of Ca-compounds hardly changed the types of gaseous products during the pyrolysis process, while it did affect the yield of each gas.

Figure 4b
. Compared with the NaNO3-NaNO2 system, the addition of Ca-compounds hardly changed the types of gaseous products during the pyrolysis process, while it did affect the yield of each gas.

Effects of Ca-Compounds on Combustible Gases Release in Molten Salts
The change ratios of different small molecule gases (H2, CO2, CO, and CH4) produced during the thermal conversion of beech wood and disposable chopsticks are shown in Figure 5. Compared with NaNO3-NaNO2 salt, the conversion ratios of combustible gases (H2, CO, and CH4) decreased with the addition of Ca(NO3)2.It should be pointed out that the only distinction between the two molten salts is the existence of Ca 2+ , which could inhibit the decomposition of components, resulting in the yield decrease of gaseous products during the pyrolysis process [37].Notice that CO2 increased by 3.63% and 1.60%, respectively, after adding Ca(NO3)2 in the thermal conversion process of beech wood and disposable chopsticks.Ca 2+ will attach to the pyran ring of the bio-waste, and the ion effect will make it prone to ring-opening decarbonylation (C=O) at C1~C2, C1~O5, C3~C4, and C5~O5.Thus, more CO2 could be formed through the decarbonylation (C=O) reaction [38,39].It indicates that the introduction of carbon dioxide could dilute the calorific value of combustible gas in the mixture.

Effects of Ca-Compounds on Combustible Gases Release in Molten Salts
The change ratios of different small molecule gases (H 2 , CO 2 , CO, and CH 4 ) produced during the thermal conversion of beech wood and disposable chopsticks are shown in Figure 5. Compared with NaNO 3 -NaNO 2 salt, the conversion ratios of combustible gases (H 2 , CO, and CH 4 ) decreased with the addition of Ca(NO 3 ) 2 .It should be pointed out that the only distinction between the two molten salts is the existence of Ca 2+ , which could inhibit the decomposition of components, resulting in the yield decrease of gaseous products during the pyrolysis process [37].Notice that CO 2 increased by 3.63% and 1.60%, respectively, after adding Ca(NO 3 ) 2 in the thermal conversion process of beech wood and disposable chopsticks.Ca 2+ will attach to the pyran ring of the bio-waste, and the ion effect will make it prone to ring-opening decarbonylation (C=O) at C1~C2, C1~O5, C3~C4, and C5~O5.Thus, more CO 2 could be formed through the decarbonylation (C=O) reaction [38,39].It indicates that the introduction of carbon dioxide could dilute the calorific value of combustible gas in the mixture.
Compared to molten salts with Ca(NO 3 ) 2 , the introduction of CaCO 3 and Ca(OH) 2 provide OH − and CO 3 2− .From Figure 5, taking the NaNO 3 -NaNO 2 as the control, the ratios of total H-containing gases (H 2 and CH 4 ) were changed by −15.27%, −13.98% and 0.86% in those three molten salts systems for beech wood, and −3.70%, −2.33% and 1.78% for disposable chopsticks.It was concluded that OH − and CO 3 2− can relieve the negative effect of Ca 2+ on the generation of H-containing gases.The reason is that Ca(OH) 2 can provide more dissociative OH − , which can reduce the energy of C-H bond-breaking in cellulose and promote H-containing gas production [40,41].Moreover, CO 3 2− can significantly promote the cracking and deformation of C=C and C-H groups, and thus increase the formation of H-containing gases [42].
As shown in Figure 6, compared with conventional pyrolysis, more bio-waste is converted into the liquid volatile at 300 • C. The main reason is that NO 2 − in molten salts could play a prominent catalytic role, and the presence of Na + can lower the pyrolysis temperature.Meanwhile, NO 2 − can participate in the reaction to form more volatiles, leading to a higher liquid yield [32].Moreover, the addition of Ca-compounds promotes the yield of liquid products, which is in agreement with the previous work [13].The reason is that Ca-compounds may enhance the process of dehydroxyl, which is beneficial at forming liquid volatiles during the pyrolysis process.Furthermore, the promotion degree of different Ca-compound is in the order: CaCO 3 > Ca(NO 3 ) 2 > Ca(OH) 2 .Therefore, the introduction of Ca-compounds can promote the formation of macromolecular liquid volatiles from bio-waste and reduce the yield of gas products.Compared to molten salts with Ca(NO3)2, the introduction of CaCO3 and Ca(OH)2 provide OH − and CO3 2-.From Figure 5, taking the NaNO3-NaNO2 as the control, the ratios of total H-containing gases (H2 and CH4) were changed by −15.27%, −13.98% and 0.86% in those three molten salts systems for beech wood, and −3.70%, −2.33% and 1.78% for disposable chopsticks.It was concluded that OH − and CO3 2− can relieve the negative effect of Ca 2+ on the generation of H-containing gases.The reason is that Ca(OH)2 can provide more dissociative OH − , which can reduce the energy of C-H bond-breaking in cellulose and promote H-containing gas production [40,41].Moreover, CO3 2-can significantly promote the cracking and deformation of C=C and C-H groups, and thus increase the formation of Hcontaining gases [42].
As shown in Figure 6, compared with conventional pyrolysis, more bio-waste is converted into the liquid volatile at 300 °C.The main reason is that NO2 -in molten salts could play a prominent catalytic role, and the presence of Na + can lower the pyrolysis temperature.Meanwhile, NO2 -can participate in the reaction to form more volatiles, leading to a higher liquid yield [32].Moreover, the addition of Ca-compounds promotes the yield of liquid products, which is in agreement with the previous work [13].The reason is that Ca- In order to clearly explain the release characteristics of small molecule gas, the major organic composition of liquid products from the two bio-wastes were analyzed, as presented in Figure 7. CG-MS was used to test the composition and relative content of liquid products, and the major organic composition matter by functional group classification statistics.In this experiment, the liquid products were mainly divided into furan, aldehyde ketone, phenol, aromatic hydrocarbon and nitrogen-containing compounds.For the beech wood and disposable chopsticks, the introduction of Ca(NO 3 ) 2 , CaCO 3 , and Ca(OH) 2 inhibited the formation of furan and phenols, while it enhanced the formation of ketones/aldehydes.Ca-compounds will reduce the formation of oxygen-containing compounds (like phenols), because Ca-compounds can enhance the deoxygenation reaction of bio-waste pyrolysis [43,44].The catalytic action of Ca can promote the formation of ketones/aldehydes by ring opening and cracking of aromatic rings [45,46].compounds may enhance the process of dehydroxyl, which is beneficial at forming liquid volatiles during the pyrolysis process.Furthermore, the promotion degree of different Cacompound is in the order: CaCO3 > Ca(NO3)2 > Ca(OH)2.Therefore, the introduction of Ca-compounds can promote the formation of macromolecular liquid volatiles from biowaste and reduce the yield of gas products.In order to clearly explain the release characteristics of small molecule gas, the major organic composition of liquid products from the two bio-wastes were analyzed, as presented in Figure 7. CG-MS was used to test the composition and relative content of liquid products, and the major organic composition matter by functional group classification statistics.In this experiment, the liquid products were mainly divided into furan, aldehyde ketone, phenol, aromatic hydrocarbon and nitrogen-containing compounds.For the beech wood and disposable chopsticks, the introduction of Ca(NO3)2, CaCO3, and Ca(OH)2 inhibited the formation of furan and phenols, while it enhanced the formation of ketones/aldehydes.Ca-compounds will reduce the formation of oxygen-containing compounds (like phenols), because Ca-compounds can enhance the deoxygenation reaction of bio-waste pyrolysis [43,44].The catalytic action of Ca can promote the formation of ketones/aldehydes by ring opening and cracking of aromatic rings [45,46].In order to clearly explain the release characteristics of small molecule gas, the major organic composition of liquid products from the two bio-wastes were analyzed, as presented in Figure 7. CG-MS was used to test the composition and relative content of liquid products, and the major organic composition matter by functional group classification statistics.In this experiment, the liquid products were mainly divided into furan, aldehyde ketone, phenol, aromatic hydrocarbon and nitrogen-containing compounds.For the beech wood and disposable chopsticks, the introduction of Ca(NO3)2, CaCO3, and Ca(OH)2 inhibited the formation of furan and phenols, while it enhanced the formation of ketones/aldehydes.Ca-compounds will reduce the formation of oxygen-containing compounds (like phenols), because Ca-compounds can enhance the deoxygenation reaction of bio-waste pyrolysis [43,44].The catalytic action of Ca can promote the formation of ketones/aldehydes by ring opening and cracking of aromatic rings [45,46].Moreover, in order to explore the effect of Ca-compounds on the saturation level of liquid products, the ratios of C/H in liquid products in different molten salts systems were calculated, as shown in Figure 8.The curves of the C/H ratios in liquid products were calculated after removing the O atom for those molten salts systems.O atoms in liquid products could be removed according to the following routes.First, The -OH was removed due to the internal dehydration reaction.Secondly, CO 2 was produced by the cleavage/reforming of carboxyl (O-C=O) and carbonyl (C-O-C) groups.Thirdly, CO was mainly generated by cleavage of lignin units and diaryl ether bonds and ether bridges [47,48].In general, the saturation level of the liquid phase product was inversely proportional to the C/H ratio in liquid products, and the change of C/H ratio in liquid products was used to characterize the changing trend of the saturation level of the liquid product.A higher C/H ratio in liquid products corresponds to a lower saturation level.From Figure 8, it could be seen that the C/H ratio in liquid products corresponding to the highest content decreased with the addition of Ca-compounds, which indicates that Ca 2+ can promote the saturation level of liquid products.As mentioned above, compared to NaNO 3 -NaNO 2 system, the addition of Ca-compounds will decrease the yield of H-containing gases, and in turn promote the saturation level of liquid products.In addition, the degree of decrease for the C/H ratio in liquid products is as follows: Ca(NO 3 ) 2 > CaCO 3 > Ca(OH) 2 .Compared with Ca(NO 3 ) 2 , CO 3 2− and OH − were introduced in NaNO 3 -NaNO 2 salt for CaCO 3 and Ca(OH) 2 , which suggested that anions may relieve the inhibition effect of Ca 2+ on the saturation level of bio-waste pyrolysis products.
A higher C/H ratio in liquid products corresponds to a lower saturation level.From Figure 8, it could be seen that the C/H ratio in liquid products corresponding to the highest content decreased with the addition of Ca-compounds, which indicates that Ca 2+ can promote the saturation level of liquid products.As mentioned above, compared to NaNO3-NaNO2 system, the addition of Ca-compounds will decrease the yield of H-containing gases, and in turn promote the saturation level of liquid products.In addition, the degree of decrease for the C/H ratio in liquid products is as follows: Ca(NO3)2 > CaCO3 > Ca(OH)2.
Compared with Ca(NO3)2, CO3 2− and OH -were introduced in NaNO3-NaNO2 salt for CaCO3 and Ca(OH)2, which suggested that anions may relieve the inhibition effect of Ca 2+ on the saturation level of bio-waste pyrolysis products.

Application and Discussion
The combustible gases can be used in many ways, such as power generation, heating systems, and so on.In addition, the high concentration of CO2 is a benefit for its recycling, which is an effective way to reduce carbon emissions.The LHVs of bio-waste pyrolysis gases were calculated according to their yield and composition, and the results are listed in Figure 9.The LHVs of pyrolysis gases are marked with black dots, and red dots represent the LHVs without CO2.From Figure 9, the LHVs obviously varied with the addition of different calcium mixtures, while the LHVs of pyrolysis gases without CO2 changed little.The reason is that the Ca-compounds have the greatest impact on CO2 generation compared with other pyrolysis gases.In addition, for the pyrolysis gases of beech wood and disposable chopsticks, the LHVs (13.83 MJ/Nm 3 and 14.08 MJ/Nm 3 ) without CO2 were significantly higher than that (8.66 MJ/Nm 3 and 8.57 MJ/Nm 3 ) with CO2.

Application and Discussion
The combustible gases can be used in many ways, such as power generation, heating systems, and so on.In addition, the high concentration of CO 2 is a benefit for its recycling, which is an effective way to reduce carbon emissions.The LHVs of bio-waste pyrolysis gases were calculated according to their yield and composition, and the results are listed in Figure 9.The LHVs of pyrolysis gases are marked with black dots, and red dots represent the LHVs without CO 2 .From Figure 9, the LHVs obviously varied with the addition of different calcium mixtures, while the LHVs of pyrolysis gases without CO 2 changed little.The reason is that the Ca-compounds have the greatest impact on CO 2 generation compared with other pyrolysis gases.In addition, for the pyrolysis gases of beech wood and disposable chopsticks, the LHVs (13.83 MJ/Nm 3 and 14.08 MJ/Nm 3 ) without CO 2 were significantly higher than that (8.66 MJ/Nm 3   Combined with Figures 4b and 9, the LHVs of pyrolysis gases with the addition of Ca(NO3)2 and CaCO3 were lower than that in NaNO3-NaNO2 salt.The main reason is that the addition of the two Ca compounds would increase the proportion of CO2 in the pyrolysis gases.In addition, it was found that the LHVs of pyrolysis gases with the addition of Ca(OH)2 were the highest for those two bio-wastes in Figure 9.As mentioned in Section 2.2, although the total gas yield reduced in the presence of OH − , the proportion of the combustible gas increased in this circumstance.
The capture and utilization of CO2 can not only effectively reduce the pressure of Combined with Figures 4b and 9, the LHVs of pyrolysis gases with the addition of Ca(NO 3 ) 2 and CaCO 3 were lower than that in NaNO 3 -NaNO 2 salt.The main reason is that the addition of the two Ca compounds would increase the proportion of CO 2 in the pyrolysis gases.In addition, it was found that the LHVs of pyrolysis gases with the addition of Ca(OH) 2 were the highest for those two bio-wastes in Figure 9.As mentioned in Section 2.2, although the total gas yield reduced in the presence of OH − , the proportion of the combustible gas increased in this circumstance.
The capture and utilization of CO 2 can not only effectively reduce the pressure of greenhouse gas emissions, but also produce huge economic benefits [49].In Figure 10, the five-pointed stars and dots represent the CO 2 concentration after pyrolysis gas (including and excluding CO 2 ) combustion.As to the pyrolysis gases in different molten salts systems, the CO 2 concentrations was 23.6%, 25.2%, 24.1% and 22.6% for beech wood, respectively, and 24.1%, 24.5%, 23.8% and 22.5% for disposable chopsticks, respectively.From the results, the CO 2 concentration after pyrolysis gas combustion was much higher than that for biowaste direct-combustion and for coal-fired power plants.As for the bio-wastes studied in the paper, the introduction of Ca-compounds can increase the CO 2 concentration after pyrolysis gas combustion, which is a benefit for the CO 2 separation and capture [50].Combined with Figures 4b and 9, the LHVs of pyrolysis gases with the addition of Ca(NO3)2 and CaCO3 were lower than that in NaNO3-NaNO2 salt.The main reason is that the addition of the two Ca compounds would increase the proportion of CO2 in the pyrolysis gases.In addition, it was found that the LHVs of pyrolysis gases with the addition of Ca(OH)2 were the highest for those two bio-wastes in Figure 9.As mentioned in Section 2.2, although the total gas yield reduced in the presence of OH − , the proportion of the combustible gas increased in this circumstance.
The capture and utilization of CO2 can not only effectively reduce the pressure of greenhouse gas emissions, but also produce huge economic benefits [49].In Figure 10, the five-pointed stars and dots represent the CO2 concentration after pyrolysis gas (including and excluding CO2) combustion.As to the pyrolysis gases in different molten salts systems, the CO2 concentrations was 23.6%, 25.2%, 24.1% and 22.6% for beech wood, respectively, and 24.1%, 24.5%, 23.8% and 22.5% for disposable chopsticks, respectively.From the results, the CO2 concentration after pyrolysis gas combustion was much higher than that for bio-waste direct-combustion and for coal-fired power plants.As for the bio-wastes studied in the paper, the introduction of Ca-compounds can increase the CO2 concentration after pyrolysis gas combustion, which is a benefit for the CO2 separation and capture [50].Figure 11 shows that the main negative carbon reactions during molten salt pyrolysis, and CO 2 cycling would be accompanied by energy conversion during this process.In detail, for the energy conversion, solar energy is used for photosynthesis of plants and for heating low-temperature molten salts in this work.A large amount of combustible gas would be generated from waste bio-waste after molten salts thermal conversion.In addition, for CO 2 cycling, the combustible gas will generate CO 2 again, and then the CO 2 could be absorbed and stored by a capture system.
In particularly, ash matter would remain and change the physicochemical properties of molten salts during the thermal treatment process.In this paper, Ca(NO 3 ) 2 /CaCO 3 /Ca(OH) 2 were simulated as the main impurities to explore their effect on the pyrolysis characteristics of bio-waste in molten salts.Summarily, Ca 2+ can promote the CO 2 concentration of pyrolysis gases, while the accompanying anions (CO 3 2− , OH − ) weaken this promotional effect by increasing the concentration of H-containing gases.Overall, bio-waste thermal conversion could be a potential carbon capture method.
Figure 11 shows that the main negative carbon reactions during molten salt pyrolysis, and CO2 cycling would be accompanied by energy conversion during this process.In detail, for the energy conversion, solar energy is used for photosynthesis of plants and for heating low-temperature molten salts in this work.A large amount of combustible gas would be generated from waste bio-waste after molten salts thermal conversion.In addition, for CO2 cycling, the combustible gas will generate CO2 again, and then the CO2 could be absorbed and stored by a capture system.In particularly, ash matter would remain and change the physicochemical properties of molten salts during the thermal treatment process.In this paper, Ca(NO3)2/CaCO3/Ca(OH)2 were simulated as the main impurities to explore their effect on the pyrolysis characteristics of bio-waste in molten salts.Summarily, Ca 2+ can promote the CO2 concentration of pyrolysis gases, while the accompanying anions (CO3 2− , OH − ) weaken this promotional effect by increasing the concentration of H-containing gases.Overall, bio-waste thermal conversion could be a potential carbon capture method.

Materials
Two typical kinds of bio-waste, beech wood and disposable chopsticks were used in the present study.Beech wood is a common furniture material, which has the characteristics of stable wood property; hard material, impact resistance, and difficulty to deform.In addition, disposable chopsticks were selected to conduct pyrolysis experiments due to its huge abundance in China.Strips or tablets (1-2 cm) of feedstocks were dried at 105 °C for 24 h in an oven before the experiments.Analytical grade reactants, NaNO3-NaNO2 (SCR Co., Ltd., Shanghai, China) molten salts was physically mixed with a weight ratio of 56.3-43.7%.The three Ca-compounds, Ca(NO3)2-CaCO3-Ca(OH)2 (SCR Co., Ltd., Shanghai, China), were obtained with an analytical grade.It should be noted that Ca(NO3)2•4H2O was used to substitute for Ca(NO3)2.
The proximate/ultimate analysis results and three-components of beech wood and disposable chopsticks are listed in Table 1.

Materials
Two typical kinds of bio-waste, beech wood and disposable chopsticks were used in the present study.Beech wood is a common furniture material, which has the characteristics of stable wood property; hard material, impact resistance, and difficulty to deform.In addition, disposable chopsticks were selected to conduct pyrolysis experiments due to its huge abundance in China.Strips or tablets (1-2 cm) of feedstocks were dried at 105 • C for 24 h in an oven before the experiments.Analytical grade reactants, NaNO 3 -NaNO 2 (SCR Co., Ltd., Shanghai, China) molten salts was physically mixed with a weight ratio of 56.3-43.7%.The three Ca-compounds, Ca(NO 3 ) 2 -CaCO 3 -Ca(OH) 2 (SCR Co., Ltd., Shanghai, China), were obtained with an analytical grade.It should be noted that Ca(NO 3 ) 2 •4H 2 O was used to substitute for Ca(NO 3 ) 2 .
The proximate/ultimate analysis results and three-components of beech wood and disposable chopsticks are listed in Table 1.

Experimental Methods
In this experiment, 300 g of the mixed salt was added to the reactor (shown in Figure 12) and heated to 300 • C, and then the temperature was kept for at least 3 h to form a homogeneous liquid phase before experiments.In order to study the effect of Ca 2+ , 10 g or 30 g of Ca(NO 3 ) 2 were added to the molten salts system.In addition, to explore the effect of different Ca-compounds on the pyrolysis behavior of the bio-wastes, 10 g of Ca(NO 3 ) 2 , CaCO 3 , and Ca(OH) 2 were introduced into the molten salts, respectively.Considering the consumption of molten salts and the accumulation of impurities during the experiment, the number of experiments in each reactor was less than 12 groups.This ensures that the experimental results cannot be affected by the attenuation of the molten salts performance.In the experiments, about 1 g of the sample was added to the metal pocket placed above the molten salts, and nitrogen gas was constantly input to the reactor at 700 mL/min.Then, the nitrogen gas flowed for 3 min to evacuate the air in the reactor and pipeline.After that, the sample was put into the molten salts for 10 min thermal treatment.The U-shaped tubes were bathed in the liquid nitrogen to collect the liquid products.Meanwhile, the gaseous products were collected in a gasbag for subsequent experimental tests.After the pyrolysis process, the metal mesh pocket was lifted by connected rods from the molten salts to the upper part of the reactor, then cooled by nitrogen, and taken out at last. the number of experiments in each reactor was less than 12 groups.This ensures that the experimental results cannot be affected by the attenuation of the molten salts performance.In the experiments, about 1 g of the sample was added to the metal pocket placed above the molten salts, and nitrogen gas was constantly input to the reactor at 700 mL/min.Then, the nitrogen gas flowed for 3 min to evacuate the air in the reactor and pipeline.After that, the sample was put into the molten salts for 10 min thermal treatment .The U-shaped tubes were bathed in the liquid nitrogen to collect the liquid products.Meanwhile, the gaseous products were collected in a gasbag for subsequent experimental tests.After the pyrolysis process, the metal mesh pocket was lifted by connected rods from the molten salts to the upper part of the reactor, then cooled by nitrogen, and taken out at last.

Analytical Methods
An element analyzer (Vario Micro cube, Elementar, Langenselbold, Germany) was employed to determine the carbon, hydrogen nitrogen and sulfur content.The proximate analysis was measured according to GB/T28731-2012.The LHV of gaseous products was calculated through Equation (1): LHV (MJ/Nm 3 )=12.6×y CO +10.8× y H 2 +35.8× y CH 4 where, y CO , y H 2 and y CH 4 are the volume fractions (vol%) of carbon monoxide, hydrogen and hydrocarbons in the gaseous products respectively [51].
The water content of liquid products was examined with a Volumetric KF Titrator (V10S, Mettler Toledo, Greifensee, Switzerland).The chemical composition of tar was de-

Analytical Methods
An element analyzer (Vario Micro cube, Elementar, Langenselbold, Germany) was employed to determine the carbon, hydrogen nitrogen and sulfur content.The proximate analysis was measured according to GB/T28731-2012.The LHV of gaseous products was calculated through Equation (1): LHV (MJ/Nm 3 ) = 12.6 × y CO +10.8 × y H 2 +35.8 × y CH 4 (1) where, y CO , y H 2 and y CH 4 are the volume fractions (vol%) of carbon monoxide, hydrogen and hydrocarbons in the gaseous products respectively [51].
The water content of liquid products was examined with a Volumetric KF Titrator (V10S, Mettler Toledo, Greifensee, Switzerland).The chemical composition of tar was determined with using GC-MS (Agilent, 7890A/5975C, Santa Clara, CA, USA) with a capillary column (Agilent: HP-5MD, 19091s-433; 30 m × 0.25 mm i.d.× 0.25 µm d.f.).The injection volume of each sample was 1 µL, and the split ratio was 1:1.The gaseous products were analyzed with a GC (Agilent 7890B, USA).The compounds were identified using a National Institute of Standards and Technology library (NIST14.L).

Conclusions
In the present work, two bio-wastes (beech wood-disposable chopsticks) were selected to reveal the effects of Ca-compounds (Ca(NO 3 ) 2 , CaCO 3 , Ca(OH) 2 ) on the gas formation characteristics in a nitrates (NaNO 3 -NaNO 2 ) system.The main conclusions of this study are as following: the mass ratios of the volatiles from the two bio-wastes were much higher (86.4% and 85.6%) during molten salts thermal treatment than that in conventional pyrolysis (20.5% and 18.3%) at 300 • C. The main reason is that the ions in molten salts greatly reduced the reaction activation energy during the bio-waste pyrolysis process.As for the gaseous products, the addition of Ca-compounds in molten salts would slightly reduce the gas yield.Moreover, Ca(OH) 2 seems to have the strongest inhibitory effect on the pyrolysis gas generation, decreasing the gas production by 12.33% for beech wood and 12.37% for disposable chopsticks.From the perspective of ions, Ca 2+ can promote the production of CO 2 , while the release of other gases (CO, H 2 and CH 4 ) will be inhibited.At the same time, the OH − and CO 3 2− could relieve the effect of Ca 2+ on gas formation.For the liquid products, the introduction of Ca 2+ increases the saturation level of liquid products as well as increases the liquid yield.In particular, the addition of CaCO 3 contributed to an increase in liquid yield from 24.2% to 28.5% for beech wood and from 22.4% to 26.7% for disposable chopsticks.The addition of Ca-compounds in molten salts could increase the CO 2 concentration after the combustion of pyrolytic gases, which would benefit CO 2 separation and capture.Meanwhile, it is suggested that molten salts thermal treatment is a promising carbon negative technology to achieve carbon neutrality.

Figure 1 .
Figure 1.Products distribution in salt free/molten salts pyrolysis of the two bio-wastes.

Figure 1 .
Figure 1.Products distribution in salt free/molten salts pyrolysis of the two bio-wastes.

Figure 2 .
Figure 2. The generation properties of pyrolysis gases under different conditions.

Figure 2 .
Figure 2. The generation properties of pyrolysis gases under different conditions.

Figure 4 .
Figure 4.The distribution characteristics of gaseous products in molten salts pyrolysis.(a) Gas yield, (b) distributions of small molecules

Figure 4 .
Figure 4.The distribution characteristics of gaseous products in molten salts pyrolysis.(a) Gas yield, (b) distributions of small molecules.

Figure 5 .
Figure 5. Effects of Ca(NO 3 ) 2 /CaCO 3 /Ca(OH) 2 on the conversion of C or H of the small molecule gas.(a) C conversion rate of beech wood, (b) C conversion rate of disposable chopsticks, (c) H conversion rate of beech wood, (d) H conversion rate of disposable chopsticks.

Figure 6 .
Figure 6.Liquid yield of thermal conversion in molten salts with different Ca-compounds.

Figure 7 .
Figure 7. Liquid products of thermal conversion in molten salts.

Figure 6 .Figure 6 .
Figure 6.Liquid yield of thermal conversion in molten salts with different Ca-compounds.

Figure 7 .
Figure 7. Liquid products of thermal conversion in molten salts.Figure 7. Liquid products of thermal conversion in molten salts.

Figure 7 .
Figure 7. Liquid products of thermal conversion in molten salts.Figure 7. Liquid products of thermal conversion in molten salts.

Figure 8 .
Figure 8. Effects of Ca(NO 3 ) 2 /CaCO 3 /Ca(OH) 2 on the saturation level of the liquid products.

Figure 11 .
Figure 11.Main negative carbon reaction pathways of bio-waste pyrolysis in molten salts.

Figure 11 .
Figure 11.Main negative carbon reaction pathways of bio-waste pyrolysis in molten salts.

Figure 12 .
Figure 12.Schematic diagram of the molten salts pyrolysis reactor.

Figure 12 .
Figure 12.Schematic diagram of the molten salts pyrolysis reactor.

Table 1 .
Proximate and ultimate analysis on dry basis and three-components of the samples.
a Calculated by difference.b Not detected.