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uilu # 99-7021
Paper No.
994062
An ASAE Meeting Presentation
| B. J. He | Y. Zhang | Y. Yin | G. L. Riskowski | T. L. Funk |
| Research Assistant ASAE Student Member |
Associate Professor ASAE Member |
Visiting Scholar | Professor ASAE Member |
Assistant Professor ASAE Member |
Summary: A thermochemical conversion (TCC) process was applied to the treatment of swine manure slurry for energy production and waste reduction. The objectives of this first stage study were to explore the feasibility of oil production from swine waste and to determine the waste reduction rates through this process. A bench TCC reactor was developed and tested. The operating temperature ranged from 250°C to 305°C. The retention time was two hours. A reducing agent, CO, was added to promote oil product conversion. Operating pressure ranged from 6.9 to 10.3 MPa and pH was monitored but not controlled. The oil product was evaluated through element analysis, heating value, and benzene solubility. The waste reduction rate was evaluated by chemical oxygen demand (COD) before and after the TCC process. The operating temperature and CO addition were the important factors affecting oil yield and quality. At temperatures of 285ºC or above with CO addition, the carbon content was typically 65% to 68%, and hydrogen content 8% to 10%. The oil yield was as high as 63% of the initial volatile solids in the input. The benzene solubility of the oil product was as high as 90%. The average heating value of the oil product was 30,500 kJ/kg. The COD reduction rate was as high as 72% through this TCC process.
Keywords Swine manure, thermochemical conversion, liquefaction, renewable energy, biomass
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ABSTRACT.A thermochemical conversion (TCC) process was applied to the treatment of swine manure slurry for energy production and waste reduction. The objectives of this first stage study were to explore the feasibility of oil production from swine waste and to determine the waste reduction rates through this process. A bench TCC reactor was developed and tested. The operating temperature ranged from 250°C to 305°C. The retention time was two hours. A reducing agent, CO, was added to promote oil product conversion. Operating pressure ranged from 6.9 to 10.3 MPa and pH was monitored but not controlled. The oil product was evaluated through element analysis, heating value, and benzene solubility. The waste reduction rate was evaluated by chemical oxygen demand (COD) before and after the TCC process. The operating temperature and CO addition were the important factors affecting oil yield and quality. At temperatures of 285°C or above with CO addition, the carbon content was typically 65% to 68%, and hydrogen content 8% to 10%. The oil yield was as high as 63% of the initial volatile solids in the input. The benzene solubility of the oil product was as high as 90%. The average heating value of the oil product was 30,500 kJ/kg. The COD reduction rate was as high as 72% through this TCC process.
KEYWORDS. Swine manure, thermochemical conversion, liquefaction, renewable energy, biomass
Swine manure management for large confinement operations is a major concern of both the public and pork industry. Many researchers have initiated studies to explore effective and efficient ways to solve this problem. Application of a thermochemical conversion process for swine manure treatment is one of the possible solutions (He et al., 1998). Studies on livestock waste conversion processes were mainly conducted during the 1970's and concentrated on pyrolysis and/or gasification of cattle manure to produce combustible gases. Swine manure was rarely the feedstock for the thermochemical conversion process. However, swine manure is a biomass rich in cellulose and lignin. It has the potential to be converted to a liquid oil product.
Ligno-cellulosic wastes can be converted to various forms of energy through numerous thermochemical conversion processes, depending upon the characteristics of the raw materials and the type of energy desired. Among the thermochemical conversion processes, liquefaction was one of the most studied and widely used (Minowa et al., 1995; Datta and McAuliffe, 1993; Gharieb et al., 1993; Chornet and Overend, 1985; Kranich, 1984; Kaufman and Weiss, 1975). Liquefaction was historically linked to hydrogenation and other high-pressure thermal decomposition processes that employ reactive hydrogen or carbon monoxide (CO) to produce a liquid fuel from organic matter. In the liquefaction process, the carbonaceous materials are converted to liquefied products through a complex sequence of changes in physical structure and chemical bonds (Chornet and Overend, 1985). In this study, a thermochemical conversion process, liquefaction, was applied to the treatment of swine manure slurry for energy production and waste reduction. The objectives of this study are to (1) explore the feasibility of oil production from swine waste, (2) determine the waste reduction rates through this process, and (3) examine the operation parameters that affect the swine manure liquefaction process.
The process setup and control scheme of the thermochemical conversion (TCC) reactor were described by He et al. (1998). The TCC processor was upgraded to operate at an extreme operating condition of 375°C and 35 MPa for an extended investigation range. A high-pressure cable tubing connects the CO cylinder to the inlet of the TCC processor. The reactor was housed in a closed chamber. The chamber was under a slightly negative pressure and exhausts out to ensure no CO leaks into room.
The collection of the feedstock, fresh swine manure, was from the same source and follows the same processing procedures as in the preliminary research (He et al., 1998). The chemical analysis results show that the characteristics of the swine manure, such as carbon and hydrogen content, volatile solids, and pH value were consistent from batch to batch.
Temperature was determined as the most important parameter in the process, because the equilibrium was established between water vapor and liquid water in the closed system, and water vapor contributes to the pressure increase and the gas production. The total operating pressure is coupled with operating temperature and changes during the course of the feedstock decomposition. CO serves as a reducing agent in the process. The amount of CO affects the oxygen content in the depolymerized products, or the quality of the oil product. In the closed system, the initial pressure of CO is proportional to the initial amount of CO added, therefore, after determining the amount of feedstock (total volatile solids), the CO initial pressure determines the initial ratio of CO to total volatile solids.
Solids content is another major parameter affecting the TCC process. Based on preliminary tests, about 87% of the total solids are volatile. Since the volatile solids are the fraction of the manure that can be converted to oil products, high volatile solids content is desirable. However, manure with 25% (by weight) or more total solids content is difficult to pump. Manure with less than 10% total solids is easier to pump, but may not be economical. Initially, the level of total solids content was chosen to be 20% in this study.
Retention time is a kinetic parameter of the TCC process. It affects the organic conversion rate or product yields. Inadequate retention time of the reactants will lead to an incomplete conversion process. However, excessive retention time may result in over-reacting of the oil products and formation of char. The retention time was set to 120 minutes in this study based on preliminary work. The natural pH (6.5 ± 0.3) of the fresh manure was monitored but not controlled.
The TCC processor was operated in a batch mode. After introducing the initial CO, the reactor was heated up to a pre-set temperature. The rates of temperature increase were 5-10°C/min. It took 40-50 minutes to reach this pre-set temperature. The highest temperature increasing rate occurred between 220°C to 250°C, when exothermic reactions start. The temperature and pressure responses of the TCC process were discussed by He et al. (1998). After each run, the reactor was cooled down to about 150°C or lower at which time the reactions all terminated in 5 minutes. When the reactor was further cooled to ambient temperature, the temperature and residual pressure were recorded for gas product estimation. The agitation is assumed to have a minor effect on the TCC process. The agitation was set at a constant speed of 200 rpm and kept constant for all experiments in the study.
The outputs from this TCC process include gases, liquid oil, post-processed water, and solid products. Gases were released after the run. Gas samples were collected in 100-ml serum bottles for laboratory analysis. The oil product was readily separated from the post-processed water after the run, since it is lighter and floats to the top of the post-processed water. The solid product consists of inert materials and char particles. The char particles are so fine that they remained suspended in the liquid. The separation of solids and post-processed water was conducted using vaccum filtration with a glass fiber filter (pore size 1.2 µm. HACH company, Loveland, Co.).
The analysis of gas product composition was performed by a gas chromatography (GC) designed for simple gas analysis (Model 580, GOW-MAG Instrument Company, Bridgewater, NJ). The GC is equipped with a column of porous polymer and molecular sieve 5 Å in the size of 80~100 meshes. The column is 8 centimeters in diameter and 1.2 meters in length. The amount of gaseous product was estimated by using the Starling modified Benedict-Webb-Rubin gas equation of state (Starling, 1971). This equation was mainly used for the thermal property calculations of hydrocarbons. The error range is 0.5%~2.0% for light hydrocarbons, CO2, H2S, and N2. In the preliminary study of this TCC process, the major component of gas product was carbon dioxide. When the equation was tested with known CO2 data, the results showed a very good prediction with an error less than ± 1% as pressure ranged up to 4 MPa.
The elemental analyses were performed on the oil product and post-processed water. These included carbon, hydrogen and nitrogen (CHN) analysis, and metallic element analysis using carbon-hydrogen-nitrogen analyzer (CE440 by Exeter Analytical, Inc. N. Chelmsford, MA) and Inductively Coupled Plasma (Plasma II by Perkin Elmer Norwalk, CT), respectively.
The quality of the TCC oil product was evaluated by the carbon and hydrogen content, the heating values, and the benzene solubility. The carbon and hydrogen content were obtained from the CHN analysis. Heating values of oil product were estimated based on the complete oxidation of carbon and hydrogen elements and considering the oxygen content in the oil product. A set of calculations on more than ten known heating value compounds showed that the errors of the estimated heating values were within 5%. Solubility of the oil products in benzene solvent was also conducted, which is another accepted method to evaluate pyrolysis oils (Appell et al., 1980).
The procedures of chemical oxygen demand (COD), volatile solids and total solids, and pH values measurements were the same as in the preliminary study (He et al., 1998).
The procedures of chemical oxygen demand (COD), volatile solids and total solids, and pH values measurements were the same as in the preliminary study (He et al., 1998).
The solubility of oil product, oil product yield, and COD reduction rate are defined as following, respectively:
(1)
(2)
(3)
The standard errors were within 5% for solids measurements, oil solubility, elemental analysis, and total mass balance in this study. The standard error for the COD measurement was within 8%.
The focus of this study is on the TCC oil product and waste strength reduction of swine manure. The process yields four products: oil, the post-processed water, gases, and solid residues. The results of ten example experiments are summarized in Table 1.
The conversion process of swine manure to oil is similar to other biomass liquefaction processes. The biomass conversion in this study is even easier in the sense that swine manure contains less lignin, which is very difficult to decompose. The biomass has been "pre-processed" by the pigs to such a uniform size that it is suitable for liquefaction. On the other hand, less lignin means less energy content (Humphrey, 1979; Glasser, 1985) resulting in less oil yield. Swine manure has a high oxygen to carbon ratio and low hydrogen to carbon (Zahn et al., 1997; Hrubant et al., 1978). These affect the oil formation efficiency negatively because high oxygen content in organic matter implies low heating value. According to preliminary test results, there was little organic carbon converted to oil without the addition of a reductant. The oil yield was less than 8% (by weight) (He et al., 1998). In a liquefaction process, some sort of reductive chemical reagent, e.g., hydrogen or CO, is needed to increase the oil production rate (Datta and McAuliffe, 1993; Appell et al., 1980). In this study, CO was employed as the reductant. The experimental results showed that the addition of the CO improved the organic carbon conversion to oil product.
Temperature had a substantial effect on the oil formation, as expected. The depolymerization reactions of organic matter could not occur until the temperature reached such a degree that the activation energy was overcome. It was observed from preliminary work that the reactions initiated from 160°C, but the depolymerization reactions did not occur until 220(C. The depolymerization reaction did not proceed to completion and some of the feedstock remained intact at 220°C or below. In this study, the reaction temperature range was 250°C to 305°C. The corresponding pressures were 5.5 MPa to 11 MPa. These were lower than those in liquefaction processes of wood sludge where temperature was as high as 400°C and pressure was as high as 24 MPa (Meier & Rupp, 1991). Within this temperature range, the volatile solids conversion rates to oil product ranged from 11% to 63% (Table 1). It was obvious that operating temperature substantially affected the conversion process, though there is a variation in the oil product yields. One of the notable phenomena was that at 275°C or below, the oil product did not form successfully in every run. This was presumably due to the complexity of the swine manure composition and the slight variation from batch to batch.
The quality of TCC oil product is evaluated by an element analysis, its benzene solubility, and heating value. Based on the chemistry principle that "like dissolves like", benzene solubility is another parameter to characterize the oil product quality. The more the oil product dissolves in benzene, the more oil-like components it contains, thus the better quality of the oil. The oil product samples at 285°C to 305°C had a benzene solubility of 81.6% to 89.8%. The oil products at 270°C or below had a low solubility as the result of incomplete depolymerization. Portions of the feedstock remained un-reacted, but contained in the oil layer. Therefore, from the oil product yield and quality point of view, the operating temperature needs to be 285°C or higher.
The contents of carbon and hydrogen are important indicators of the quality of the TCC oil product. For high quality oil, the content of carbon and hydrogen must be sufficiently high and oxygen content should be as low as possible. Besides carbon and hydrogen, the oil product also contains many other elements, such as oxygen, nitrogen, sulfur, and minerals. The carbon content in the study ranged from 63% to 71%, the hydrogen 8% to 10%, and nitrogen 3.8% to 4.6%. The average ratio of carbon to hydrogen is 7.4:1 (by weight). A review of the literature shows the carbon and hydrogen content is equivalent to or better than the pyrolysis oil made from wood sludge liquefaction, where the carbon content ranged from 50% to 67%, hydrogen from 7% to 8%, and nitrogen 8% to 10% (Rick and Vix, 1991). Oxygen content was not analyzed, but it was assumed to be a major component besides the carbon and hydrogen. The TCC oil product has a relatively high nitrogen content, about 4-4.5%wt as a result of the high nitrogen content in the feedstock. A typical analysis of dry raw swine manure sample showed that the nitrogen content was 3.74%wt, while the carbon and hydrogen contents were 47%wt and 6.5%wt, respectively. Based on the C:H ratio and assuming the remainder is oxygen, the heating value of the TCC oil product was estimated as 30,500 kJ/kg.
The waste strength in the post-processed water was substantially reduced in the TCC process. The feedstock swine manure slurry processed in this study was controlled at 20% (by weight) of total solids, of which approximately 85% to 88% was volatile solids. The COD of this feedstock was 237,400(1,200 mg/L. After the TCC process, the COD range of the post-processed waters was from 66,400 mg/L to 124,300 mg/L and the sample mean and standard deviation were 96,020 and 20,610 mg/L, respectively. The corresponding volatile solids contents in the post-processed water ranged from 3.27% to 6.76%. The COD of the post-processed water were 28% to 50% of those in the original manure slurry. More than 50% of the organic matter was converted to oil product that can be readily separated from the liquid.
It was also observed that the runs with higher oil yields also had higher COD reduction rates. The runs with operating temperature of 285°C or higher had an average oil product yield of 46.7%. The average COD for the runs was 82,080 mg/L with a standard deviation of 12,300 mg/L. The corresponding COD reduction rate was 65.4%. This was 15% more COD reduction rate than that of the runs with operating temperature lower than 285°C, of which the average oil product yield was 31.5%. This is because less organic matter remained in the post-processed water when the oil conversion rates were high. Therefore, temperature is the most important operating parameter that affects the oil production and waste reduction.
In the study, the nutrient content, nitrogen (N), phosphate (P), and potassium (K), of the post-processed water were measured for some runs. The NPK concentrations in the post-processed water were basically constant regardless of the operation conditions. This is because the minerals from the feedstock remained essentially in aqueous solution. The major portion of the nitrogen in the feedstock was in nitrate form that dissolved in the aqueous solution. The NPK value is still too high to be discharged to a wastewater treatment system. If diluted, it could be used for irrigation under some specific conditions.
Carbon dioxide (CO2) was the sole gas by-product in the TCC process. The GC analysis showed no methane or other gases. Carbon dioxide was formed when the depolymerization occurred and the carbonyl groups were thermally cleaved. CO2 was also released as the result of decarboxylation reactions. It was observed that more CO2 was produced and more char formed if no CO was added as a reductant at high operating temperatures. The CO addition consumption becomes an indication of oil product formation, i.e., CO reduced the feedstock, eliminating elemental oxygen and releasing carbon dioxide.
The results showed that 76.7% of the CO was converted to CO2 in Run #5. It had about the same oil product quality as Run #6 but 5.6% less oil yield. The CO conversion rate, however, was 50% higher than that in Run #5. This was because four times more CO added in Run #6. In terms of consumption of CO per unit weight of volatile solids input, the values were 0.125 g CO/g VS and 0.095 g CO/g VS for the runs #6 and #5, respectively, excluding the amount consumed by the free oxygen in the head space. The difference of these values was not as significant as CO conversion rate only. Although excessive CO addition resulted in a better oil yield, it is not economical in terms of the operation cost.
Solid product was only a small portion of the total input, usually less than 5% (by weight). It contained inert materials, such as dust, and some char formed. Depending on the operation conditions, the volatile solids content in the solid product ranged from 30% to 70% by weight.
Determining the role of each operating parameter is very important for the optimization of the TCC process. The effects of operating parameters on the oil formation, waste reduction, and oil product quality were investigated systematically. The optimum operation conditions were determined through a set of orthogonal experiments. The results are discussed in separate paper (He et al., 1999).
The TCC process was applied to the treatment of swine manure slurry to produce liquid oil and reduce the waste strength. The oil yield was as high as 63% of the total volatile solids of the feedstock. The COD in the post-processed water after the TCC process had a reduction rate as high as 72%. The TCC oil product had a similar quality to that of pyrolysis oil from wood sludge. The average heating value of the oil product was estimated a 30,500 kJ/kg. It was concluded that the TCC processing of swine manure is feasible. The application of the TCC process to the treatment of swine manure not only can reduce the waste strength, but also can produce useful energy in the form of liquid fuel. Further research is needed in improving the oil conversion efficiency and utilization of the oil product.
The Illinois Council on Food and Agricultural Research is acknowledged for providing financial support for this preliminary study. Thanks are extended to Mr. Peter G. Stroot, a research engineer of the Department of Agricultural Engineering, University of Illinois at Urbana-Champaign, for his great thoughts and discussions. The authors would like to thank Dr. Joanne Chee-Sanford of the Department of Animal Sciences, University of Illinois at Urbana-Champaign, for her kind assistance in gas analysis.
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