Article pubs.acs.org/EF
Characterization of Typical Biomass Ashes and Study on Their Potential of CO2 Fixation Wenlong Wang,* Yanli Zheng, Xin Liu, and Peng Wang National Engineering Laboratory for Coal-Fired Pollutants Emission Reduction, Shandong University, Jinan 250061, People’s Republic of China ABSTRACT: With the increasing use of biomass resources, the recycling or disposal of biomass ash should be given more attention. An idea is put forward in this paper to study the possibility of CO2 fixation with biomass ash through the bicarbonation reactions of carbonate minerals. Four typical biomass resources, corn stalk, wheat stalk, cotton branch, and poplar, in a rural area of northern China, were sampled. Their ashes were thoroughly characterized for their chemical and mineral compositions through the joint adoption of three analysis tools, X-ray diffraction, X-ray fluorescence, and thermogravimetric analysis. It was found that the percentages of carbonates, such as CaCO3, K2CO3, MgCO3, etc., could amount to 25−40% by weight in the ashes of wheat stalk and corn stalk and to about 70% in the cotton branch and poplar. The high carbonate contents give them a theoretically good potential to absorb CO2 through bicarbonation reactions. Experiments of bubbling CO2 into solutions made from the ashes showed that 65−97% by weight of the carbonates could, in practice, be transformed into bicarbonates. This enhanced method can make the biomass ashes fix much more CO2 than they would under natural absorption conditions. Therefore, proper use of biomass ash may help to mitigate climate change in a distributed way if the fertilization process is improved.
1. INTRODUCTION With the continuous rise of global energy consumption and increasing focus on global climate change, biomass, as a renewable, carbon-neutral energy source, is attracting more and more attention.1−3 It is estimated that the total amount of biomass in China is equal to almost 5 billion tons of standard coal, 0.7 billion tons of which may be exploited. Of all of the developable biomass, crop stalks make up over 50%.4,5 A great deal of research and application is being carried out for its direct combustion, gasification, or pyrolysis.6−13 No matter how the biomass is used, ash or residue will be generated. The development of biomass power plants in recent years increases the concern about the disposal of biomass ash.14−17 Although biomass ash can be used as polymer fillings, absorbents, etc., its use as a fertilizer in agriculture or forestry is the most common option because of its content of potassium.18−22 Potassium may partly take the form of carbonate, and biomass ash is usually alkaline. The alkalinity makes it possible for biomass ash to rapidly absorb CO2. The conversion from carbonate to bicarbonate may provide a considerable capability to fix CO2. Therefore, the possibility of CO2 fixation with biomass ash through the bicarbonation reactions of carbonate minerals is proposed in the present paper.23,24 If biomass ash does show potential to fix CO2, a small change in the way that it is used as a fertilizer would enhance the bicarbonation reactions beforehand to effect absorption of carbon and reduce its impact on climate change. Because the emission of CO2 comes from many kinds of human activities, its final capture and sequestration should also be accomplished through any possible means. To test the proposal stated above, four biomass ashes, i.e., corn-stalk ash, wheat-stalk ash, cotton-branch ash, and poplar ash, were prepared. By a combination of analysis tools, including X-ray diffraction (XRD), X-ray fluorescence (XRF), © 2012 American Chemical Society
and thermogravimetric analysis (TGA), the mineral compositions of the ashes were quantitatively determined. Subsequently, the theoretical amounts of possible CO2 absorption through bicarbonation were calculated for each type of ash. Then, the capacities for CO2 fixation of the four biomass ashes were studied experimentally. Through a comparison of experimental and theoretical results, the possibility of using biomass ash to fix CO2 was demonstrated.
2. EXPERIMENTAL SECTION 2.1. Characterization of Biomass Ash. Corn stalk, wheat stalk, cotton branch, and poplar are four typical biomass resources in rural areas of northern China. Each of them was sampled and burnt in a muffle furnace, in which each ash sample was kept at a temperature of 300 °C for over 1 h. The sampling process was designed carefully to avoid incomplete combustion as much as possible. To quantitatively determine the mineral composition of the biomass ashes, XRD, XRF, and TGA were jointly applied. The chemical characterization of the ashes was accomplished by synthesizing the results from the three analysis tools. Powder XRD was carried out on a D/MAX2500 V diffractometer at a reflection angle range of 2θ = 10−90°. Qualitative analysis of the XRD patterns identified the mineral phases present in the different ashes. A Quant’X XRF spectrometer was employed to determine the elemental contents of the ash samples. Subsequently, the content of each mineral was calculated through synthetically balancing the results of XRD and XRF. TGA was also conducted for the ash samples with a TGA/differential scanning calorimetry (DSC) simultaneous thermal analyzer. The analytical temperature was set from ambient temperature to 1200 °C under a nitrogen atmosphere, and the heating rate was 20 °C/min. The mass losses in different temperature ranges reflected the decomposition or volatilization of some of the carbonates Received: May 7, 2012 Revised: August 31, 2012 Published: August 31, 2012 6047
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HCO3−, could indicate how much carbonate had reacted with CO2 through bicarbonation reactions. To indentify the enhancement effect of CO2 bubbling, the CO2 absorption capability of the four ashes under natural conditions was also tested. A total of 2 g of each biomass ash was placed in an open beaker and kept moist by adding some drops of water every morning. After 30 days of exposure in the atmosphere, the biomass ashes were dissolved in 200 mL of distilled water, and then, the IC concentrations of the solutions were determined. By comparison of the results under natural and enhanced conditions, our initial idea of CO2 fixation was tested.
or alkali chlorides and could be used to verify or modify the mineral characterization results from XRD and XRF. 2.2. Experiments of CO2 Absorption with Biomass Ash. The experiments of CO2 absorption with biomass ash were carried out in the setup shown in Figure 1. A total of 2 g of biomass ash was
3. RESULTS AND DISCUSSION 3.1. Mineral Composition. Figure 2 shows the XRD patterns of the four kinds of biomass ash. It can be seen that potassium salts, carbonates, phosphates, and quartz are the main mineral phases in the biomass ashes. The potassium may take the form of KCl, K2SO4, or K2CO3. Phosphorus most likely exists as Ca2P2O7. In the patterns of corn- and wheat-stalk ashes, KCl and SiO2 show the strongest peaks, while the peaks of carbonates, phosphates, and sulfates are weak. On the contrary, the diffractive peaks of carbonates are dominant in the patterns of the other two ligneous ashes, cotton branch and poplar. CaCO3, MgCO3, and K2CO3 are the main carbonate phases in the cotton-branch ash, but K2Ca(CO3)2 takes the position of K2CO3 in the poplar ash. Table 1 gives the XRF analysis results. The XRD results provided necessary information to improve the accuracy of XRF analysis. For instance, chlorine took the form of KCl; the rest of the K and all of the other elements were calculated as oxides. It can be seen that corn- and wheat-stalk ashes are rich in SiO2 and KCl, accounting for over 50% of the total weight.
Figure 1. Experimental setup. dissolved in 50, 200, and 500 mL of distilled water in each experiment. The initial pH of each solution was measured with a pH-3c pH meter. Subsequently, CO2 was bubbled into the solutions. The flow rate of CO2 was 0.05 m3/h, and the partial pressure was 0.15 atm. The pH was measured consecutively. The bubbling was stopped when the pH had decreased to about 7. When the four types of ash were dissolved in 200 mL of water, air was bubbled into the final solutions to expel the excess CO2. For the four solutions of 200 mL of volume, the inorganic carbon (IC) concentrations were determined with a total organic carbon analyzer (TOC-V CPN) at the beginning and end of the CO2 bubbling. The increase of the IC concentration, which was due to the increase of
Figure 2. XRD patterns of four kinds of biomass ash. 6048
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Table 1. Chemical Compositions from XRF Analysis (%) corn-stalk ash wheat-stalk ash cotton-branch ash poplar ash
SiO2
KCl
K2O
CaO
MgO
SO3
P2O5
Al2O3
Fe2O3
others
total
34.34 46.58 10.04 4.30
19.71 16.67 3.45 0.99
10.86 2.89 12.55 7.70
8.52 10.90 25.39 30.56
5.44 3.07 6.69 12.00
0.51 2.45 1.37 1.18
2.24 0.4 5.49 9.22
1.43 0.83 1.31 0.46
1.31 1.52 1.69 0.91
15.64 14.69 32.02 32.67
100 100 100 100
Figure 3. TGA results for four kinds of biomass ash.
Table 2. Main Mineral Compositions of the Biomass Ashes (%) corn-stalk ash wheat-stalk ash cotton-branch ash poplar ash
SiO2
KCl
CaCO3
K2CO3
K2SO4
MgCO3
Ca2P2O7
K2Ca(CO3)2
others
total
34.34 46.58 10.04 4.30
19.71 16.67 3.45 0.99
11.13 18.75 35.38 31.17
15.06 0 16.06 0
1.21 5.34 2.98 2.65
11.01 6.21 13.54 24.28
4.53 0.8 11.07 18.61
0 0 0 15.78
3.01 5.65 7.48 2.22
100 100 100 100
content of KCl plus the CO2 that is combined with CaO and MgO. Therefore, when the analysis results of XRD, XRF, and TGA are combined, the mineral compositions of the four kinds of biomass ash can be determined. Table 2 gives the results. The contents of KCl and SiO2 are directly obtained from the XRF. MgO is sure to exist as MgCO3. All SO3 is assumed to combine with K2O and take the form of K2SO4, and the rest of K2O is considered to be K2CO3. CaO is considered first to combine with P2O5 in the form of Ca2P2O7, and then the rest takes the form of CaCO3. For poplar ash, K2CO3 should exist as K2Ca(CO3)2 according to the XRD result. The column “others” includes the other trace minerals: Al2O3, Fe2O3, etc. The results in Table 2 are in accordance with all of the XRD, XRF, and TGA results. 3.2. Potential of CO2 Absorption by Biomass Ash. As shown in Table 2, the carbonate contents in the four kinds of
CaO is the dominant constituent in the ashes of cotton branch and poplar. In Table 1, the column “others” contains mainly CO2, which is combined in the calcium, magnesium, or potassium carbonates, as well as other trace elements or oxides. Figure 3 shows the TGA results for the four kinds of biomass ash. In all cases, the main weight occurs in the temperature range of 400−950 °C, which can be attributed mostly to the volatilization of KCl and the decomposition of carbonates. MgCO3 is easy to decompose above 400 °C, and KCl will volatilize above 700 °C. CaCO3 usually decomposes to release CO2 at 850−950 °C, and the presence of alkali compounds can reduce its decomposition temperature. K2CO3 and K2SO4 may also make minor contributions to the total weight loss, but their decomposition temperatures are higher than that of KCl.25 The percentages of weight loss for all of the ashes at 400−950 °C are about 20−30%, which is in accordance with the total 6049
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represent the amounts of CO2 that can be absorbed by different carbonates through bicarbonation reactions. Clearly, cottonbranch ash and poplar ash have greater potential than the other two types in terms of CO2 fixation. A series of experiments were carried out at ambient temperature. Figure 4 shows the changes of pH during bubbling of CO2 into the solutions under different experimental conditions. In all cases, the pH declined. It can be seen that the decrease of pH is slower when the same mass of ash is dissolved in smaller volumes of water. It could well be that the low solubility of CaCO3 slows the bicarbonation when there is less water volume and that the reaction goes further only when transformation from CO32− to HCO3− changes the solubility equilibrium. Table 4 lists the initial and final pH values and the elapsed time when CO2 was bubbled into the 200 mL solutions. When
biomass ash are considerable. Thus, the biomass ashes should have great potential for absorbing CO2 by carbonates through bicarbonation reactions. The following bicarbonation reactions may take place: K 2CO3 + CO2 + H 2O ↔ 2KHCO3
(1)
CaCO3 + CO2 + H 2O ↔ Ca(HCO3)2
(2)
MgCO3 + CO2 + H 2O ↔ Mg(HCO3)2
(3)
Provided that all of the carbonates in biomass ash can react with CO2 to form bicarbonates, the maximum percentages of CO2 absorption by the biomass ashes can be worked out Table 3. Theoretical Percentages of CO2 Absorption by Each Biomass Ash (%) contribution of different minerals
corn-stalk ash
wheat-stalk ash
cottonbranch ash
poplar ash
K2CO3 CaCO3 MgCO3 total
4.80 4.90 5.57 15.27
0 8.25 3.14 11.39
5.12 15.57 6.85 27.54
2.92 16.63 12.28 31.83
Table 4. Initial and Final pH Values and the Elapsed Time When CO2 Was Bubbled into the Solutions
individually. Table 3 gives the results of calculations based on the following formula: MPCO2 = MPcarbonate × 44/Mcarbonate
items
corn-stalk ash
wheat-stalk ash
cotton-branch ash
poplar ash
initial pH final pH elapsed time(s)
10.07 7.11 140
9.58 6.9 40
10.54 7.12 210
10.34 6.91 160
the same volume of distilled water was added to the four kinds of biomass ash, the cotton-branch-ash solution had the highest initial pH, followed in order by poplar ash, corn-stalk ash, and wheat-stalk ash. This sequence is entirely accordant with the theoretical capacities for CO2 absorption listed in Table 3. It means that the alkalinities of the solutions are mostly attributable to the dissociation reactions of carbonates in the
(4)
where MPCO2 is the theoretical mass percentage of CO2 absorption by each carbonate in each biomass ash, MPcarbonate is the content of the carbonate (Table 2), and Mcarbonate is the molar mass of the carbonate. Therefore, the percentages
Figure 4. Changes of pH of the biomass solutions when the same kind of biomass ash is dissolved in different volumes of distilled water. 6050
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Table 5. Mass and Ratio of CO2 Absorption under Different Conditions with CO2 bubbled into the biomass-ash solution
corn-stalk ash wheat-stalk ash cottonbranch ash poplar ash
under natural absorption conditions
theoretical absorption capacity (%)
mass of absorbed CO2(g)
percentage of absorbed CO2 (%)
ratio of experimental absorption/theoretical absorption (%)
mass of absorbed CO2(g)
percentage of absorbed CO2 (%)
ratio of experimental absorption/theoretical absorption (%)
15.27
0.297
14.83
97.12
0.083
4.18
27.37
11.39
0.183
9.15
80.33
0.011
0.57
5.00
27.54
0.360
17.98
65.29
0.093
4.66
16.92
31.83
0.460
23.01
72.29
0.114
5.71
17.94
15.27, 11.39, 27.54, and 31.83% by weight for the ash of corn stalk, wheat stalk, cotton branch, and poplar, respectively. In the experiments in which CO2 was bubbled into the ash solutions, the percentages of CO2 absorption were 14.83, 9.15, 17.98, and 23.01% by weight for the ash of corn stalk, wheat stalk, cotton branch, and poplar, respectively, meaning that 65−97% of the carbonates could be transformed into bicarbonates. However, the bicarbonation ratio would be much lower under natural conditions, when the ashes only absorb the CO2 in the air. The potential for CO2 absorption investigated in the present study could help to mitigate climate change in a distributed way. For instance, the traditional irrigation method could be changed slightly by dissolving the biomass ash in water and bubbling CO2 into it. This measure could not only realize the CO2 fixation but also enhance fertilization by potassium contained in the biomass ash. The needed CO2 could be obtained from the exhaust gas of internal combustion engines, which are widely used in irrigation in rural areas of China. Over 0.7 billion tons of crop stalks can be produced in China annually, and 1 kg of biomass is equivalent to about 0.5 kg of standard coal. If 10% of the biomass ash from the crop stalks could be used as we propose, about 0.6−1.5 million tons of CO2 could be fixed. Because CO2 emission is not from a single source, its capture and use to mitigate the negative effects should also be developed in many distributed ways. Therefore, the approach proposed in the present paper is worthy of further development in the future.
biomass ashes. There were also differences in the rate of decrease in pH during the process of absorbing CO2. The pH of the wheat-stalk-ash solution decreased the fastest, followed by the corn-stalk-ash solution, poplar-ash solution, and cottonbranch-ash solution. The differences should also be attributable to the species and contents of carbonates in the biomass ashes. Through the determination of IC concentrations before and after the absorption of CO2, the practical capacities of the biomass ash solutions can be determined. Table 5 shows the CO2 absorption under enhanced CO2 absorption conditions and under natural absorption conditions. By comparison of the experimental CO2 absorption capacities to the theoretical capacities, it is found that the bicarbonation reactions under enhanced absorption conditions are much more effective than they are under natural conditions. When CO2 was bubbled into the solutions, 65−97% of the theoretical CO2 absorption capacities of the biomass ashes were exploited. However, when the biomass ashes were moistened and exposed to the atmosphere for 30 days, the percentages of transformation from carbonates to bicarbonates were much lower, with cornstalk ash having the highest (27.37%) and wheat-stalk ash having the lowest (5%). Under both enhanced and natural conditions, the corn-stalk ash showed the best CO2 fixation capability, probably because the percentage of K2CO3 in all of its contained carbonates is the highest and K2CO3 is much more easily dissolved than CaCO3 and MgCO3. It can be concluded that biomass ashes do have a certain potential to absorb CO2 through bicarbonation reactions. This characteristic could be employed to fix CO2 in some ways when biomass ashes are used.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-531-88399372-603. Fax: +86-531-88395877. E-mail:
[email protected].
4. CONCLUSION Corn stalk, wheat stalk, cotton branch, and poplar are four typical biomass resources in rural areas of northern China. However, it is hard to find any detailed reports on the mineral compositions of their ashes. Through the joint adoption of three analysis tools, XRD, XRF, and TGA, the ashes were thoroughly characterized for their chemical and mineral compositions. The results can provide helpful information when the corresponding biomass resources are used, and the synthetic quantitative analysis method may be applicable to the analysis of other complicated substances. It was found that the percentages of carbonates, such as CaCO3, K2CO3, MgCO3, etc., could amount to 25−40% by weight in the ashes of wheat stalk and corn stalk and about 70% in the cotton-branch ash and poplar ash. Because all of the ashes are alkaline, the high carbonate contents give them good potential to absorb CO2 through bicarbonation reactions. The theoretical CO2 absorption capacities were calculated to be
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the support of the National Natural Science Foundation of China (Grant 51106088) and the Program for New Century Excellent Talents in University (NCET-100529). We also thank the father and mother of the corresponding author for their help in collecting the biomass samples for this study.
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