Effects of Potassium Salts on Formaldehyde Decomposition in

Article pubs.acs.org/EF

Effects of Potassium Salts on Formaldehyde Decomposition in Supercritical Water Liang Zhao, Jun Zhang,* Hui Zhong, Changdong Sheng, and Qizhong Ding Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, Jiangsu, People’s Republic of China ABSTRACT: To explore the mechanism of the potassium effect on the biomass gasification process in supercritical water (SCW), formaldehyde, a typical intermediate formed in the process, was used as the feedstock and the experiments were carried out in a temperature range of 400−650 °C, a pressure range of 23−29 MPa, and a residence time range of 4−12 s, with KHCO3, K2CO3, KCl, and mixed potassium salts. The results showed that all potassium salts studied decreased the gasification efficiency and the yields of H2, CO2, and CO of formaldehyde. The inhibition level of gasification efficiency and hydrogen generation influenced by the potassium salts was on the order of mixed potassium salts > KHCO3 > K2CO3 > KCl. At the high temperatures (500−650 °C) and long residence times (8−12 s), the negative effects of the potassium salts on gaseous product generation were enhanced. The effects of the potassium salts on the gasification efficiency and hydrogen generation had slight dependence upon the pressure. At the high temperatures (500−650 °C), long residence times (8−12 s), and high pressures (25−29 MPa), each salt in mixed potassium salts had synergetic effects on the gaseous generation. Meanwhile, on the basis of the kinetic model, the kinetics analysis for the effects of the potassium salts was carried out. The results showed that the negative effects of the potassium salts on the yields of gaseous products were obtained by mainly hindering the HCHO direct decomposition reaction.

1. INTRODUCTION In comparison to pyrolysis and gasification technologies, biomass decomposition in supercritical water (SCW) has the features of high gasification efficiency (biomass can be entirely gasified1,2), high hydrogen content (half of gaseous products is hydrogen3,4), and low energy consumption (energy for drying wet biomass is conserved5). This technology has been considered as a potential way for hydrogen production by biomass thermochemical use. Understanding the conversion mechanism of biomass gasification in SCW is very important to develop this technology. Therefore, a lot of researchers5−7 have pursued the studies of the biomass conversion process in SCW and usually used the real biomass8,9 or biomass model compound10,11 as the object of study. These objects have big molecular weights, which makes the decomposition process complicated, leading to the difficulty in obtaining the detailed reaction mechanism. During the decomposition process with SCW, biomass first converts into big molecular intermediates. Second, the big molecular intermediates decompose into small molecular intermediates. Finally, the gaseous products are obtained by the further decomposition of the small molecular intermediates.12 Obviously, it is a good way to reveal the biomass decomposition mechanism by selecting some important small molecular intermediates. It has been confirmed that formaldehyde is a key intermediate during biomass decomposition.10,12 Osada et al.13,14 explored the formaldehyde decomposition process in SCW and proposed the elementary reaction pathways. Zhang et al.15 investigated the gaseous product distribution of formaldehyde decomposition at different temperatures (500−650 °C), pressures (25−30 MPa), residence times (14−34 s), and feedstock concentrations (6−14%). On the basis of the results by Osada et al.,13,14 Zhang et al.15 concluded the most likely reaction pathways of formaldehyde decomposition in SCW. Besides, Ohno et al.16 computationally revealed the direct © 2013 American Chemical Society

decomposition pathway of a single formaldehyde molecule using the quantum chemical program package. A better understanding about the formaldehyde decomposition mechanism was obtained by these studies above.13−16 However, different influence factors of formaldehyde decomposition, especially the inorganic compound existing in biomass, were not studied systematacially ever before. Potassium is generally considered as the main active inorganic compound in biomass, and its effect on biomass conversion in SCW has been shown with the real biomass9 or the model compound.11 In our previous study,15 the potassium additives, including K2CO3 and KOH, showed obvious effects on the gaseous products for formaldehyde decomposition. It could be noted that potassium is usually released in the form of KCl during biomass thermochemical conversion.17 Meanwhile, some related studies18,19 showed that the inorganic negative ions existing in biomass lixivium were mainly Cl−, CO32−, HCO3−, etc. Therefore, in this paper, KHCO3, K2CO3, and KCl were selected as the potassium model compounds existing in biomass. At different reaction temperatures (400−650 °C), pressures (23− 29 MPa) and residence times (4−12 s), the effects of a single species potassium salt (KHCO3, K2CO3, and KCl, respectively) or mixed potassium salts (KHCO3 + K2CO3 + KCl) on formaldehyde decomposition were considered.

2. EXPERIMENTAL SECTION 2.1. Materials. The concentration of formaldehyde (purity ≈ 37.0−40.0%, Xilong Chemical Reagent Co.) in the experiments was Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 26, 2013 Revised: September 10, 2013 Published: September 11, 2013 86

dx.doi.org/10.1021/ef401439k | Energy Fuels 2014, 28, 86−94

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0.1 mol/L. Potassium salts used as a substitute for potassium components in natural biomass were KHCO3 (purity ≥ 98.5%, Guanghua Fine Chemicals Co.), K2CO3 (purity ≥ 99.0%, Jiuyi Chemical Reagent Co.), and KCl (purity ≥ 99.5%, Qiangshun Chemical Reagent Co.). Deionized water was purchased from Southeast Pure Water Co. 2.2. Apparatus and Procedures. A schematic figure of the experimental system is shown in Figure 1. The design temperature and

Yi =

each gaseous product molar number formaldehyde weight

(1)

To compare the efficiency of formaldehyde conversion, the gasification efficiency of formaldehyde (GE, %) was defined as eq 2.

GE =

total gaseous product weight formaldehyde weight

(2)

3. RESULTS AND DISCUSSION In these experiments, the gaseous products detected by the gas chromatograph were comprised of hydrogen, carbon dioxide, and carbon monoxide. Methane and C2−C3 were not detected. The carbon mass balance analysis on formaldehyde decomposition was shown in Table 1. It can be seen that the results of Table 1. Carbon Mass Balance of Formaldehyde Decomposition in SCW ([HCHO]0 = 0.1 mol L−1, 600 °C, 25 MPa, snd 4 s) potassium

carbon content in gas (mg L−1)

carbon content in liquid (mg L−1)

carbon mass balance (%)

no K salts 1.5% KHCO3 1.5% K2CO3 1.5% KCl 1.5% mixed K salts

288.12 191.31 408.75 378.83 354.88

805.94 921.71 718.22 737.96 742.45

91.17 92.75 93.91 93.07 91.44

Figure 1. Schematic diagram of the experimental setup.

the carbon mass balance were always more than 91%. Because of the effumability of the small molecular components (formaldehyde, methanol, formic acid, etc.) in the liquid phase, there existed some errors in TOC analysis, especially the process of inorganic carbon removal coupled with blowing nitrogen. Thus, the results of carbon mass balance were reasonable. 3.1. Temperature. The yields of gaseous products in different temperatures were shown in Figure 2. It could be seen that increasing the reaction temperature from 400 to 650 °C promoted YH2, YCO, and YCO2 significantly. Some researches with real biomass or model compounds showed a promoting role of potassium salts for gaseous generation.20,21 However, for formaldehyde, one of the important small molecular intermediates in the SCW gasification process of biomass, all potassium salts studied here revealed inhibiting action on gaseous generation. The effect of the potassium salts was related to the temperature. When the temperature varied from 400 to 450 °C, the inhibition of the potassium salts was weak; above 500 °C, they made the yields of H2, CO, and CO2 decrease dramatically. Different potassium salts also showed different roles in the gaseous production. It could be noted from Figure 2b that the effect of KCl on the CO yield was much lower than that of K2CO3 and KHCO3, suggesting a different mechanism of them to the gasification process of formaldehyde in SCW. The objective of biomass decomposition in SCW is usually to produce hydrogen. The inhibition level of YH2 influenced by the potassium salts here was on the order of mixed potassium salts > KHCO3 > K2CO3 > KCl. The reason might be that the potassium salts, especially alkaline salts, were not favorable for some pathways, which were related to the gaseous generation during formaldehyde decomposition. Besides, Sinağ et al.11 found that the potassium salts increased YH2 and YCO2 during alcohols and acids decomposition, and the effects of potassium salts were related to formic acid by the formation of formate salt as eqs 3−6.11

pressure are 650 °C and 40 MPa, respectively. The experimental system is comprised of four parts, including the reactor, feeding−cleaning appliance, cooling unit, and sample collection device. The feedstock was fed into the reactor by a high-pressure pump (plunger type, made in China). The reactor was constructed using 316L tubes (7 mm inner diameter, 16 mm outer diameter, and 600 mm length) and heated by a 5 kW electric furnace. The temperature of the reactor was controlled using a K-type thermocouple installed at the center of the outer wall of the reactor. After leaving the reactor, the effluent was rapidly cooled in a double-pipe cooler and then successively flowed through two filters (50 and 5 μm). A back-pressure regulator was employed to reduce the pressure to atmospheric pressure. Finally, the effluent was separated into gas and liquid phases by a gas−liquid separator under ambient conditions. The gas flow rate was measured by the displacement method using graduated cylinders (10 or 100 mL) several times. Gaseous products were collected by the aluminum foil gas sample bag. Liquid products were preserved by the headspace bottle. For each experiment, the high-pressure pump provided a continuous supply of a formaldehyde solution with or without potassium salts (15 mg of K+/g of formaldehyde; single- or three-component mixed potassium salts) into the reactor. 2.3. Gaseous Analysis. An Agilent 6890N gas chromatograph (helium as the carrier gas) equipped with a Porapak Q column, a molecular sieve column, and a thermal conductivity detector (TCD) was used to determine hydrogen, carbon monoxide, carbon dioxide, methane, and C2−C3 in gaseous products. The column was initially held at 50 °C for 2 min. Then, the temperature increased to 100 °C at 5 °C/min and held at 100 °C for 13 min. The standard gas mixture was supplied by Nanjing Special Gas Factory Co. 2.4. Liquid Analysis. Using heated persulfate oxidation technology (200 g/L Na2S2O8 was selected as the oxidant), an OI Aurora 1030W total organic carbon (TOC) analyzer equipped with a 1088 rotary automatic sampler was used to determine the TOC content of the liquid products. 2.5. Data Interpretation. To analyze the gaseous products in the experiments, the yields of each product (Yi, where i = H2, CO, CO2, and others, mol/g) were defined as eq 1. 87


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Figure 2. Effects of potassium salts on Yi at different reaction temperatures (at 25 MPa and 8 s).

K 2CO3 + H 2O → KHCO3 + KOH

(3)

KOH + CO → HCOOK

(4)

HCOOK + H 2O → KHCO3 + H 2

(5)

2KHCO3 → K 2CO3 + CO2 + H 2O

(6)

However, the previous study by the authors22 found that K2CO3 decreased the formation of H2 and CO2 during formic acid decomposition. The reaction pathways of formate salt should be comprised of the decarboxylation reaction (HCOOK → KH + CO2) and the decarbonylation reaction (HCOOK → KOH + CO).16 The previous study22 showed that the reaction rate of the formate salt decarboxylation reaction was slower than that of formic acid (HCOOH → H2 + CO2). Despite the low yield of formic acid13,15 during formaldehyde decomposition, potassium formate can also be produced by eqs 3 and 4. Meanwhile, in eqs 3 and 4, a few amounts of CO were consumed. Because the potassium salts reduced the yields of all three gaseous products during formaldehyde decomposition, especially at above 500 °C, the total yields of gaseous products dropped when the potassium salts were added. As a result, the potassium salts significantly decreased GE (see Figure 3). It could be seen that the inhibition of the potassium salts to GE was also on the order of mixed potassium salts > KHCO3 > K2CO3 > KCl. GE with mixed potassium salts was lower than that with other potassium salts at above 500 °C, suggesting that each salt in mixed potassium salts had more obvious effects on decreasing the gaseous generation than their single potassium salt. The authors defined this result as the synergetic effects.

Figure 3. Effects of potassium salts on GE at different reaction temperatures (at 25 MPa and 8 s).

3.2. Pressure. Generally, the pressure had a little effect on Yi when it increased from 23 to 29 MPa, as Figure 4 shows. YH2 and YCO were obviously reduced with the addition of the potassium salts at different pressures. Besides, all potassium salts decreased YCO2 at the low pressures (23−25 MPa), and KHCO3 and K2CO3 promoted YCO2, while KCl and mixed potassium salts hindered CO2 generation at the high pressures (27−29 MPa). For hydrogen production at different pressures, the effects of the potassium salts also showed the same order with that at different temperatures discussed above. As seen from Figure 5, the variation of the pressure had unobvious effects on GE. All potassium salts restrained the gaseous products. The inhibition level of GE was still on the order of mixed potassium salts > KHCO3 > K2CO3 > KCl. At the same time, GE with mixed potassium salts was lower than 88


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Figure 4. Effects of potassium salts on Yi at different reaction pressures (at 600 °C and 8 s).

In the case of the short residence times (4−8 s) as Figure 7 shows, the potassium salts had a complex effect on GE. At the extended residence times (8−12 s), all potassium salts obviously decreased GE and the inhibition level was in a certain order (mixed potassium salts > KHCO3 > K2CO3 > KCl), while the synergetic effects of mixed potassium salts on gas generation could be seen.

4. KINETICS ANALYSIS In our previous study,25 a detailed kinetic model, including 10 main reaction pathways for formaldehyde decomposition in SCW, was set up, as eqs 7−16, and it was confirmed that this kinetic model predicted the products well. In this section, by means of this kinetic model, the influence mechanism of potassium salts on formaldehyde decomposition was discussed. Cannizzaro reaction

Figure 5. Effects of potassium salts on GE at different reaction pressures (at 600 °C and 8 s).

K1

2HCHO + H 2O → CH3OH + HCOOH

that with other potassium salts, especially at high pressures (25−29 MPa). 3.3. Residence Time. Experimental results of Yi obtained at different residence times (4−12 s) are shown in Figure 6. At the short residence time (4−8 s), the potassium salts had fluctuant effects on the yield of each gaseous product. Because of the short residence time (4−8 s), the instantaneous concentrations of formaldehyde and intermediate significantly varied.23,24 As a result, the impacts of the potassium salts on the formaldehyde decomposition did not show the same trends. At the extended residence time (8−12 s), all potassium salts decreased YH2, YCO, and YCO2. The inhibition level of YH2 was on the order of mixed potassium salts > KHCO3 > K2CO3 > KCl.

(7)

decarboxylation reaction K2

HCOOH → H 2 + CO2

(8)

decarboxylation reaction K3

HCOOH → H 2O + CO

(9)

HCHO direct decomposition reaction K4

HCHO → CO + H 2 89

(10)


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Figure 6. Effects of potassium salts on Yi at different residence times (at 600 °C and 25 MPa).

CH3OH dehydrogenation reaction K9

2CH3OH → CH3OOCH + 2H 2

(15)

esterification reaction K10

CH3OH + HCOOH ⎯→ ⎯ CH3OOCH + H 2O

(16)

According to the basic principles of chemical kinetics, the reaction rate equations of this kinetic model containing detailed pathways (eqs 7−16) were expressed as eqs 17−24 dC H 2 dt

Figure 7. Effects of potassium salts on GE at different residence times (at 600 °C and 25 MPa).

2 − K −5CCO2C H2 + 2K 9CCH 3OH

water-gas shift reaction K −5

(17)

dCCO = K3C HCOOH + K4C HCHO − K5CCOC H 2O dt

K5

CO + H 2O ←→ CO2 + H 2

= K 2C HCOOH + K4C HCHO + K5CCOC H 2O

(11)

+ K −5CCO2C H2 − K 7CCOC H2O

(18)

hydride transfer reaction K6

HCHO + HCOOH → CH3OH + CO2

dCCO2

(12)

dt

HCOOH synthesis reaction K7

CO + H 2O → HCOOH

+ K 6C HCHOC HCOOH (13)

K8

(19)

dC HCHO 2 = −2K1C HCHO − K4C HCHO − K 6C HCHOC HCOOH dt

aldol condensation reaction HCHO + 2CH3OH → CH3OCH 2OCH3 + H 2O

= K 2C HCOOH + K5CCOC H2O − K −5CCO2C H2

2 − K8C HCHOCCH 3OH

(14) 90

(20)


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Table 2. Reaction Rate Constant Ki (s−1) with or without Potassium Salts Ki

no K salts

add KHCO3

add K2CO3

add KCl

add mixed K salts

K1 K2 K3 K4 K5 K−5 K6 K7 K8 K9 K10

3.6448 6.3051 × 10−18 5.0441 × 10−18 0.1818 0.2065 0.0817 9.9185 × 10−15 6.5261 × 10−21 6.5977 × 10−21 7.9968 × 10−18 3.1218 × 10−16

5.6392 7.2430 × 10−17 1.1999 × 10−17 0.0921 1.5183 0.6008 8.4438 × 10−31 6.6456 × 10−21 6.7800 × 10−21 8.2798 × 10−18 3.2525 × 10−16

4.6789 7.7360 × 10−17 1.1944 × 10−17 0.1212 0.6392 0.2529 8.5075 × 10−31 6.7628 × 10−21 6.8469 × 10−21 8.5758 × 10−18 3.1105 × 10−16

3.6745 7.7930 × 10−17 1.1565 × 10−17 0.1212 1.0654 0.4216 8.6124 × 10−31 6.8402 × 10−21 7.0312 × 10−21 8.6402 × 10−18 3.2811 × 10−16

5.5819 8.495 × 10−17 1.1676 × 10−17 0.1191 1.2560 0.4970 8.8106 × 10−31 7.0741 × 10−21 7.3372 × 10−21 9.0832 × 10−18 3.2599 × 10−16

dCCH3OH dt

alkaline potassium salts) on the yields of gaseous products were obtained by mainly hindering the HCHO direct decomposition reaction. With Ki shown in Table 2, the gaseous products were calculated and the results with or without adding potassium salts were shown in Figure 8. It can be found that this kinetic model well-predicted gaseous distributions with or without potassium salts, except YCO in Figure 8d. The values of K4, K5, and K−5 with adding KCl (in Table 2) seemed imperfect. It should be optimized better using other proper mathematical methods, which were based on the experimental data in our following study. In Table 2, it can be found that gaseous products (H2, CO, and CO2) were mainly generated by the HCHO direct decomposition reaction and water-gas shift reaction. Other reaction pathways produced few gaseous products. To explore the effects of the potassium salts on the HCHO direct decomposition reaction and water-gas shift reaction with the residence time, the rates of formation and consumption for gaseous products based on this model were further analyzed with the experimental data at 600 °C and 25 MPa, and here, the results with adding KHCO3 as a representative were shown in Figures 9−11. It should be noted that the reactions of low formation or consumption rate were not shown in Figures 9−11. From Figure 9, it could be found that, with the increase of the residence time, the rate of H2 formation by HCHO direct decomposition always decreased and that by water-gas shift forward increased first and then decreased. When the residence time was short, the rate of H2 formation by HCHO direct decomposition was larger than that by water-gas shift forward, and this relation kept a smaller time interval with the addition of potassium. Potassium mainly affected the decomposition of HCHO, especially at the condition of the short residence times (4−8 s). Figure 10 indicated that, with the increase of the residence time, the rate of CO formation was the same as the rate of H2 formation by HCHO direct decomposition. The rate of CO formation by HCHO direct decomposition always decreased with the residence time. The rate of CO consumption was the negative rate of H2 formation by water-gas shift forward. The rate of CO consumption by water-gas shift forward decreased first and then increased. With the residence time, potassium affected the rates of CO formation and consumption by the decomposition of HCHO and water-gas shift forward, respectively, all of the time, especially at the short residence times (4−8 s).

2 = K1C HCHO C H2O + K 6C HCHOC HCOOH 2 2 − 2K8C HCHOCCH − 2K 9CCH 3OH 3OH

− K10CCH3OHC HCOOH

(21)

dC HCOOH 2 = K1C HCHO C H2O − K 2C HCOOH − K3C HCOOH dt − K 6C HCHOC HCOOH + K 7CCOC H2O − K10CCH3OHC HCOOH dCCH3OOCH dt

2 = K 9CCH + K10CCH3OHC HCOOH 3OH

dCCH2(OCH3)2 dt

2 = K8C HCHOCCH 3OH

(22)

(23)

(24)

where CH2 (mol/L) and others were calculated as the concentration of a given species in the reactor divided by the reactor volume. In section 3.3, the experimental results on the formaldehyde decomposition at different residence times (4−12 s) with or without potassium salts were obtained. According to these data, Ki was calculated by the least-squares method with the help of Micromath Scientist 3.026 and showed in Table 2. As seen from Table 2, the rate constant of the Cannizzaro reaction (K1) was larger than that of other pathways, indicating that this pathway was a fast reaction during formaldehyde decomposition. Besides, the rate constant of the water-gas shift reaction (K5 and K−5) at 600 °C (in this paper) was higher than that at 400 °C (the previous paper25), further demonstrating that the calculated results were reasonable. In Table 2, because the potassium salts facilitated the rate constant of the Cannizzaro reaction, the yields of CH3OH and HCOOH would be increased. As seen from K2, K3, and K9 in Table 2, CH3OH and HCOOH decompositions were not markedly promoted by the potassium salts, and their rate constants are very small. As a result, the yields of gaseous products were not obviously increased by CH 3OH or HCOOH conversion reactions as eqs 8, 9, and 15. Meanwhile, the HCHO direct decomposition reaction and water-gas shift reaction were also the fast reaction pathways. The potassium salts restrained the HCHO direct decomposition reaction but promoted the water-gas shift reaction. Thus, the negative effects of the potassium salts (especially 91


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Figure 8. Kinetic model prediction for Yi with and without potassium salts (0.1 mol/L HCHO, 600 °C, and 25 MPa).

Figure 9. Rates of H2 formation (at 600 °C and 25 MPa).

Figure 11 showed that water-gas shift forward was the main CO2 formation pathway. Few CO2 were consumed by water-gas

shift reverse. With the increasing residence time, potassium (KHCO3 here) notably promoted the rate of CO2 formation at 92


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Figure 10. Rates of CO formation/consumption (at 600 °C and 25 MPa).

Figure 11. Rates of CO2 formation/consumption (at 600 °C and 25 MPa).

of gaseous products were obtained by mainly hindering the HCHO direct decomposition reaction.

the short residence time and then rapidly decreased the rate of CO2 formation by water-gas shift forward.

■

5. CONCLUSION Potassium is the main active inorganic compound in biomass. Formaldehyde decomposition in SCW with or without potassium salts was studied to explore the effects of the potassium salts on the important micromolecule intermediate at different reaction temperatures (400−650 °C), pressures (23−29 MPa) and residence times (4−12 s). The main gaseous products of formaldehyde decomposition were H2, CO2, and CO. KHCO3, K2CO3, KCl, and mixed potassium salts decreased YH2, YCO2, YCO, and GE. The inhibition level of YH2 and GE influenced by the potassium salts was on the order of mixed potassium salts > KHCO3 > K2CO3 > KCl. At the high temperatures (500−650 °C) and the long residence times (8−12 s), the negative effects of the potassium salts on gaseous generation were stronger. The effects of the potassium salts on the gasification efficiency and hydrogen generation had a little dependence upon the pressure. At the high temperatures (500−650 °C), long residence times (8−12 s), and high pressures (25−29 MPa), in comparison to other potassium salts, mixed potassium salts further restrained the gaseous generation, suggesting that each salt in mixed potassium salts had synergetic effects on the gaseous generation. On the basis of the kinetic model, the kinetics analysis of the effects of the potassium salts was carried out. The kinetic model included 10 main reactions during formaldehyde decomposition in SCW. The results showed that the negative effects of the potassium salts (especially alkaline potassium salts) on the yields

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the support of the National Key Project for Basic Research of China (973 Program) under Project 2009CB220007 and the Opening Foundation of the State Key Laboratory of Multiphase Flow in Power Engineering at Xi’an Jiaotong University, Xi’an, China.

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