The objective of this research was to evaluate the chemically coagulated swine manure solids as biofuel and/or compost feedstock. Three coagulants, namely agricultural lime [CaCO3], hydrated lime [Ca(OH)2], and lime slurry [Ca(OH)2], were added to fresh swine manure to coagulate manure solids. Four levels, i.e., 0.00 (0.0X), 4.89 (0.5X), 9.77 (1.0X), and 19.77 (2.0X) gm Ca⋅liter-1, were tested, in triplicates. Increasing the coagulant concentration increased the total solids, ash content, and pH of solid manure samples, whereas it decreased their volatile solids, chemical oxygen demand, and heating value. At the coagulant level of 2.0X rate, heating values of samples coagulated by agricultural lime, hydrated lime, and lime slurry were 2.64, 4.48, and 4.54 MJ⋅kg-1, respectively. The heating value of raw manure solids was as high as 13.49 MJ⋅kg-1. Increasing the coagulant concentration increased the O/C atomic ratio for all the studied coagulants. Accordingly, the high coagulant concentrations might reduce the acceptability of the feedstock as a biofuel that can be co-combusted with other feed stocks. The C/N ratio and the pH values of the solid separated swine manure increased by increasing agricultural lime and hydrated lime concentrations. The former might increase satisfactoriness for composting these solids, whereas the latter might hinder their use in the composting process. The maximum coagulant concentrations that allowed pyrolyzing the final product, based on the net energy values, were 48.80 (2.0X), 18.06 (1.0X), and 18.06 (1.0X) gm⋅liter-1 for agricultural lime, hydrated lime, and lime slurry, respectively. The maximum acceptable coagulant concentrations that allowed composting the final product, based on the pH values, were 48.80 (2.0X), 0.00 (0.0X), and 9.03 (0.5X) gm⋅liter-1 for the same three coagulants.
The total number of pigs in the United States had reached 65.9 million head, as recently published in the quarterly inventory report by the USDA [
Traditionally, land application of swine manure is considered the most common practical and economical utilization method [
Typically, the moisture content and total solids of fresh swine manure are about 90% w.b. and 10% w.b., respectively [
Xiu et al. [
Reviewing the accessible literature reveals that there are no available data related to the energy contents and the thermal degradation behavior of the chemically coagulated swine manure solids. In addition, there are no data related to the maximum values of coagulants that will hinder the use of the final product as biofuel and/or compost feedstock. Consequently, swine producers, who practice manure-solid separation, should have interest in the determination of the optimum amount of coagulant additions and in the comparison between utilizing these solids as a biofuel or as a compost feedstock. Hence, the objective of this paper was to evaluate the chemically coagulated swine manure solids as a feedstock candidate for biofuel and/or compost.
Fresh swine manure was collected from a privately owned Arkansas farm. Three coagulants, namely agricultural lime [CaCO3], hydrated lime powder [Ca(OH)2], and lime slurry [Ca(OH)2], were used to coagulate solids from fresh swine manure. Lime slurry was prepared by mixing the coagulant with water at the ratio of 0.3:1.0 kg∙kg−1, which is the mix ratio lime slurries starts behaving less as a suspension and more as a paste [
Physical, chemical, and thermochemical characteristics of the solid separated swine manure were determined in triplicates. Moisture content, volatile solids and ash content were determined according to the standard methods [
Chemical oxygen demand (COD) was employed in this study as an indirect measurement of soluble and insoluble organic matter in the manure samples. Samples were digested using a HACH DRB200 Digester, Germany. Following, a HACH DR7200 Spectrophotometer, Germany, was used to measure the COD. The ultimate and ash analyses were performed in a specialized diagnostic laboratory (Huffman Laboratories Inc., Golden, Colorado, USA). Samples’ carbon, hydrogen, and nitrogen contents were determined according to standard ASTM D5373-14 [
The pyrolysis of raw manure, agricultural lime, hydrated lime, and chemically coagulated samples, was investigated using a thermogravimetric analyzer (Model TGA 4000, Perkin Elmer, Inc., Waltham, Massachusetts). To ensure that the decomposition was controlled by kinetics rather than diffusion, the particle size and the sample size were kept small. The dry samples were first milled and sieved to generate a subsample with a particle size less than 0.2 mm. The sample weight was maintained at 5 mg (±0.1 mg) in all the TGA tests. The analyzer was used to determine the relationship between the temperature and weight loss for the different samples under oxy-
Treatment | Level | Chemical1 mass added per liter of liquid manure (gm∙l−1) | Calcium mass added per liter of liquid manure (gm∙l−1) |
---|---|---|---|
Manure | Raw | - | - |
Agricultural Lime CaCO3 | 0.5X | 12.20 | 4.89 |
1.0X | 24.40 | 9.77 | |
2.0X | 48.80 | 19.55 | |
Hydrated Lime Ca(OH)2 | 0.5X | 9.03 | 4.89 |
1.0X | 18.06 | 9.77 | |
2.0X | 36.13 | 19.55 | |
Lime Slurry Ca(OH)2 | 0.5X | 9.03 | 4.89 |
1.0X | 18.06 | 9.77 | |
2.0X | 36.13 | 19.55 |
1Mass of CaCO3 and Ca(OH)2 only added assuming chemicals are 100% pure.
gen-free conditions. Nitrogen was used as the purge gas (50 mL∙min−1), at a heating rate of 5˚C∙min−1. Each sample was placed in a clean, inert alumina (Al2O3) crucible. A blank test was conducted with an empty crucible under the regular test conditions to quantify the buoyancy of the crucible. The experimental data was then corrected by subtracting the blank test results.
The results were analyzed using JMP® Pro software (version 11.0.0, SAS Institute Inc., Cary, North Carolina). Two-way ANOVA was used to analyze the impact of coagulant type and concentration on the characteristics of the feedstock.
Evaluation of the coagulated swine manure solids, as biofuel and/or composting feedstock, requires comprehensive analyses that determine the acceptable levels of the concentrations of each coagulant. The following section details the results of the lab analyses performed on the swine manure solids.
The average total solids of raw manure were found to be 32.1% ± 4.9%, as shown in
Treatment | Level | Total solids1 (%) | Volatile solids (%) | Ash (%) | Fixed carbon (%) |
---|---|---|---|---|---|
Manure | Raw | 32.1a ± 4.9 | 54.9a ± 1.8 | 30.5a ± 1.6 | 14.7a ± 0.4 |
Agricultural Lime | 0.5X | 56.7e ± 3.4 | 34.6b ± 4.1 | 46.8b ± 1.3 | 18.6b ± 3.2 |
1.0X | 66.2f ± 5.6 | 24.5d ± 2.6 | 50.7cde ± 1.9 | 24.8c ± 0.9 | |
2.0X | 68.0f ± 2.1 | 18.0f ± 1.4 | 52.7e ± 0.7 | 29.4d ± 0.7 | |
Hydrated Lime | 0.5X | 35.7bc ± 0.9 | 33.8b ± 3.3 | 48.6bcd ± 4.1 | 17.6ab ± 0.8 |
1.0X | 39.3cd ± 3.5 | 23.9de ± 1.2 | 57.3f ± 3.0 | 18.7b ± 2.0 | |
2.0X | 40.8d ± 1.3 | 21.1ef ± 1.6 | 60.3f ± 2.8 | 18.6b ± 4.3 | |
Lime Slurry | 0.5X | 33.1ab ± 4.2 | 35.5b ± 0.9 | 48.1bc ± 0.5 | 16.5b ± 0.4 |
1.0X | 35.6bc ± 1.5 | 30.3c ± 0.3 | 51.6de ± 1.1 | 18.2b ± 1.3 | |
2.0X | 39.1cd ± 0.2 | 29.9c ± 0.3 | 47.8bc ± 1.2 | 22.3c ± 3.0 |
1Averages within a column followed by different letter are significantly different at P ≤ 0.05 level and n = 3.
clear trend. These high values of the ash contents might hinder the suitability of these feedstocks being utilized in thermochemical conversion processes. A significant effect (P < 0.05) was observed between samples having various coagulant concentrations. Low concentration of hydrated lime or lime slurry addition (0.5X) did not show the significant difference as compared with manure samples. However, the addition of agricultural lime showed a significant difference from hydrated lime and lime slurry (P < 0.05). A wide range of the ash content was reported previously in the literature. Jørgensen and Jensen [
Conversely, for the present study, volatile solids concentrations in the manure samples decreased as the coagulant concentration increased, as shown in
Chemical oxygen demand of manure samples (
The highest heating value of 13.49 ± 0.42 MJ∙kg−1 was observed with the raw manure sample (
Treatment | Level | pH1 (-) | Chemical oxygen demand (mg∙g−1) | Higher heating value (MJ∙kg−1) |
---|---|---|---|---|
Manure | Raw | 7.1a ± 0.2 | 1777.5d ± 406.3 | 13.49f ± 0.42 |
Agricultural lime | 0.5X | 7.1b ± 0.0 | 1003.5abc ± 129.0 | 5.79cd ± 0.68 |
1.0X | 7.4c ± 0.2 | 950.0ab ± 140.0 | 4.51b ± 0.46 | |
2.0X | 7.5c ± 0.0 | 640.0a ± 17.3 | 2.64ba ± 0.68 | |
Hydrated lime | 0.5X | 12.3e ± 0.1 | 2916.7f ± 376.3 | 7.39e ± 0.38 |
1.0X | 12.5fg ± 0.0 | 1886.7d ± 118.5 | 5.81cd ± 0.33 | |
2.0X | 12.6g ± 0.0 | 1316.7bc ± 40.4 | 4.48c ± 0.70 | |
Lime slurry | 0.5X | 8.7d ± 0.1 | 2773.3ef ± 508.4 | 7.60 g ± 0.09 |
1.0X | 12.4ef ± 0.0 | 2343.3e ± 134.3 | 6.38f ± 0.07 | |
2.0X | 12.5g ± 0.0 | 1440.0cd ± 52.9 | 4.54a ± 0.04 |
1Averages within a column followed by a different letter are significantly different at P ≤ 0.05 level and n = 3.
of 14.3 MJ∙kg−1 for chemically pretreated swine manure solids followed by mechanical separation. The lower heating value in the present study might be due to the collection technique of the manure samples. Increasing the coagulant concentration decreased the samples’ heating value. The heating value dropped by 80.4%, 66.x%, and 66.3% corresponding to 2.0X concentration of agricultural lime, hydrated lime, and lime slurry, respectively.
The results of the ultimate analysis conducted on the manure samples amended by the three studied coagulants, i.e., agricultural lime, hydrated lime, and lime slurry, and the four levels of coagulant concentrations are shown in
The composition of the ash of the separated swine manure solids is presented in
Treatment | Level | C (%) | H (%) | N (%) | O (%) | S (%) | Empirical formula (-) | Molecular weight (g∙mol−1) |
---|---|---|---|---|---|---|---|---|
Manure | Raw | 34.46 | 4.60 | 3.86 | 23.35 | 0.83 | CH1.602O0.508N0.960S0.009 | 22.02 |
Agricultural lime | 0.5X | 22.38 | 2.21 | 1.44 | 26.35 | 0.39 | CH1.185O0.883N0.055S0.007 | 27.52 |
1.0X | 19.11 | 1.60 | 0.97 | 27.22 | 0.25 | CH1.105O1.068N0.044S0.005 | 30.25 | |
2.0X | 17.34 | 1.28 | 0.78 | 27.78 | 0.20 | CH0.886O1.202N0.390S0.004 | 32.25 | |
Hydrated lime | 0.5X | 23.23 | 2.83 | 1.28 | 23.64 | 0.42 | CH1.462O0.763N0.047S0.007 | 25.89 |
1.0X | 15.98 | 2.68 | 0.84 | 23.19 | 0.30 | CH2.013O1.088N0.045S0.007 | 31.65 | |
2.0X | 12.99 | 2.51 | 0.67 | 22.88 | 0.22 | CH2.319O1.321N0.044S0.006 | 35.66 | |
Lime slurry | 0.5X | 23.79 | 2.73 | 1.32 | 23.48 | 0.42 | CH1.377O0.740N0.048S0.007 | 25.43 |
1.0X | 20.62 | 2.53 | 1.22 | 23.80 | 0.37 | CH1.147O0.866N0.051S0.007 | 27.54 | |
2.0X | 14.68 | 2.44 | 0.83 | 33.67 | 0.25 | CH1.995O1.720N0.048S0.006 | 41.72 |
Treatment | Level | P2O5 (%) | K2O (%) | CaO (%) | MgO (%) | Na2O (%) | ZnO (%) | Fe2O3 (%) | CuO (%) | MnO (%) |
---|---|---|---|---|---|---|---|---|---|---|
Manure | Raw | 37.40 | 4.65 | 21.22 | 13.07 | 1.22 | NM | 2.29 | NM1 | 0.32 |
Agricultural Lime | 0.5X | 12.35 | 1.65 | 66.34 | 4.61 | 0.43 | 0.42 | 0.82 | 0.02 | 0.15 |
1.0X | 8.27 | 1.13 | 72.47 | 3.10 | 0.30 | 0.16 | 0.61 | 0.01 | 0.12 | |
2.0X | 6.42 | 0.88 | 75.51 | 2.55 | 0.23 | 0.12 | 0.52 | 0.01 | 0.10 | |
Hydrated Lime | 0.5X | 16.23 | 2.30 | 62.26 | 5.48 | 0.64 | 0.25 | 0.89 | 0.02 | 0.16 |
1.0X | 9.23 | 1.33 | 74.54 | 3.31 | 0.38 | 0.13 | 0.54 | 0.01 | 0.12 | |
2.0X | 6.39 | 0.98 | 82.21 | 2.46 | 0.27 | 0.10 | 0.40 | 0.01 | 0.10 | |
Lime Slurry | 0.5X | 17.52 | 2.42 | 61.30 | 5.64 | 0.72 | 0.28 | 0.90 | 0.03 | 0.16 |
1.0X | 11.70 | 1.61 | 69.68 | 3.98 | 0.45 | 0.20 | 0.70 | 0.02 | 0.14 | |
2.0X | 8.48 | 1.21 | 97.13 | 3.18 | 0.34 | 0.15 | 0.55 | 0.01 | 0.13 |
place between 220˚C and 350˚C. Devolatilization of raw manure reached its peak value of 0.392 %∙˚C−1 at the temperature level of 315.9˚C (
Amended manure samples using agricultural lime, hydrated lime, and lime slurry showed various levels of decomposition stages. Figures 2-4 show the weight loss and the weight-loss derivative curves of the manure samples amended by the three coagulants, i.e., agricultural lime, hydrated lime, and lime slurry. Two weight- loss peaks were detected during the pyrolysis of the manure samples amended with agricultural lime. The temperatures, corresponding to the decomposition peaks in the first stage as compared to those for raw manure, are listed in
Manure samples amended with hydrated lime and lime slurry showed three decomposition stages: two small peaks (220˚C - 420˚C) and a larger one that took place under the temperature range of 600˚C to 715˚C. These observations indicate that under extremely high temperature levels (650˚C - 750˚C) full decomposition of the coagulant is achievable. It is worth noting that, in the current study, the decomposition peaks did not follow a clear trend by increasing the coagulant concentration. This observation could be attributed to the fact that under pyrolysis conditions or any thermal treatment, the original feedstock matrix is transformed into a new structure biochar that might have its thermal properties.
Treatment | Level | Tp1 (˚C) | (dW/dt)p1 (mg∙s−1) | Energy required for pyrolysis (MJ∙kg−1) |
---|---|---|---|---|
Manure | Raw | 315.9 | 0.392 | 6.37 |
Agricultural lime | Raw | 415.4 | 0.144 | - |
Hydrated lime | Raw | 395.2 | 0.500 | - |
Agricultural lime | 0.5X | 316.0 | 0.222 | 3.16 |
1.0X | 311.8 | 0.277 | 2.56 | |
2.0X | 299.9 | 0.186 | 2.47 | |
Hydrated lime | 0.5X | 294.4 | 0.144 | 5.63 |
1.0X | 391.6 | 0.141 | 5.02 | |
2.0X | 396.7 | 0.158 | 4.80 | |
Lime slurry | 0.5X | 297.9 | 0.195 | 6.15 |
1.0X | 288.3 | 0.114 | 5.65 | |
2.0X | 293.6 | 0.0.06 | 5.05 |
In addition to the energy content of the solid separated swine manure and its thermal degradation, which were mentioned earlier, it was essential to compare the carbon, hydrogen and oxygen relationships of these feed stocks.
two types of coal and cottonwood for comparison. A similar diagram was developed by Van Krevelen [
It was essential to identify a unit function for the standardization of comparison. As a result, the unit function selected was 1 kg of the dried feedstock. It should be mentioned that this dry amount of feedstock started at different levels of the “as received” feedstock due to the variation in the initial moisture content levels. The total energy required to pyrolyze the wet feedstock could be obtained from the following equation:
where
Qtotal: is the total energy required for pyrolysis (MJ∙kg−1).
Qheating1: is the energy needed to heat the wet feedstock from 25˚C to 100˚C (MJ∙kg−1).
Qdrying: is the energy required to dry the feedstock (MJ∙kg−1).
Qheating2: is the energy needed to heat the dried feedstock from 100˚C to 550˚C (MJ∙kg−1).
Qpyrolysis: is the energy required to pyrolyze the dried feedstock (MJ∙kg−1).
The energy needed to heat the feedstock (Qheating1) was determined by adding the required energy to heat its two components, i.e., manure and coagulant solids and moisture. Each constituent of the energy was determined by multiplying its mass, specific heat, and the difference in the temperature between 100˚C and 25˚C. Specific heat capacity was assumed to be between 2.1 - 2.5 kJ∙kg−1∙˚C−1 for general organic materials [
The total energy required to pyrolyze raw manure reached 6.37 MJ∙kg−1, as shown in
coagulant concentration decreased the energy needed for pyrolysis. The maximum energy required for pyrolysis of 6.15 MJ/kg was achieved with manure sample coagulated by lime slurry of 0.5X, whereas the lowest energy needed for pyrolysis of 2.47 MJ∙kg−1 was achieved with manure sample coagulated by agricultural lime of 2.0X. The reduction of the energy required for pyrolysis can be attributed to the significant reduction of the energy required for drying. The net energy, as designated by the difference between the energy content of the feedstock and the energy needed for pyrolysis, could be also perceived from
As mentioned earlier,
An important step in the preparation process of composting the produced feedstock is to determine the amount of bulking material required to adjust the C/N ratio to reach the optimum range, i.e., 30. Bulking agents have low moisture content and high C contents [
where:
C/N: is the carbon to nitrogen ratio.
Q1: is the manure weight, kg.
Q2: is the bulking material weight, kg.
M1: is the manure moisture content, %.
C1: is the manure carbon concentration, g∙kg−1.
N1: is the manure nitrogen concentration, g∙kg−1.
M2: is the bulking agent moisture content, %.
C2: is the bulking agent carbon concentration, g∙kg−1.
N2: is the bulking agent nitrogen concentration, g∙kg−1.
As a result, by rearranging the previous equation, the amount of the bulking agent could be determined using the following equation.
Corn stalks were selected as a candidate feedstock with the assumption that its initial moisture, carbon and nitrogen contents are 11.00%, 41.18%, and 0.78%, respectively [
It was observable from
Treatment | Level | C/N1 (-) | CS2 (ton) | H/C1 (-) | O/C1 (-) |
---|---|---|---|---|---|
Manure | Raw | 8.93 | 1.65 | 1.60 | 0.51 |
Agricultural Lime | 0.5X | 15.54 | 0.75 | 1.18 | 0.88 |
1.0X | 19.70 | 0.42 | 1.00 | 1.07 | |
2.0X | 22.23 | 0.26 | 0.89 | 1.20 | |
Hydrated Lime | 0.5X | 18.15 | 0.34 | 1.46 | 0.76 |
1.0X | 19.02 | 0.23 | 2.01 | 1.09 | |
2.0X | 19.39 | 0.18 | 2.32 | 1.32 | |
Lime Slurry | 0.5X | 18.02 | 0.33 | 1.38 | 0.74 |
1.0X | 16.90 | 0.36 | 1.47 | 0.87 | |
2.0X | 17.69 | 0.25 | 1.99 | 1.72 |
1Atomic ratio (-); 2Corn stalks (ton).
From the experimental work described in this article, several important conclusions can be drawn:
・ Increasing the coagulant treatment level increased the total solids, ash content, and pH of manure samples, whereas it decreased their volatile solids and heating value.
・ The heating value of raw manure was 13.49 MJ∙kg−1, whereas it was 2.64 MJ∙kg−1 for agricultural lime, 4.48 MJ∙kg−1 for hydrated lime, and 4.54 MJ∙kg−1 for lime slurry at the coagulant 36.13 gm∙liter−1 rate.
・ Increasing the coagulant concentration decreased the acceptability of the solid separated swine manure as a biofuel source.
・ Based on the net energy, the maximum acceptable coagulant concentrations that allow pyrolyzing the final product were 48.80 (2.0X), 18.06 (1.0X), and 18.06 (1.0X) gm/liter for agricultural lime, hydrated lime, and lime slurry, respectively.
・ Considering only the C/N ratio increasing the coagulant concentration tended to increase the acceptability of the solid separated swine manure as a composting source.
・ Based on the pH values, the maximum acceptable coagulant concentrations that allow composting the final product were 48.80 (2.0X), 0.00 (0.0X), and 9.03 (0.5X) gm/liter for agricultural lime, hydrated lime, and lime slurry, respectively.
This manuscript is a part of the USDA-NIFA project No. 2010-04269 titled “Integrated Resource Management Tools to Mitigate the Carbon Footprint of Swine Production in the US”. The authors would like to thank the funding agency for their continued support. The authors also acknowledge the support of the Rice Research and Extension Center (RREC) in Stuttgart, Arkansas.
SammySadaka,Karl VanDevender, (2015) Evaluation of Chemically Coagulated Swine Manure Solids as Value-Added Products. Journal of Sustainable Bioenergy Systems,05,136-150. doi: 10.4236/jsbs.2015.54013