This paper summarizes the oil production from swine waste from our previous research and .... This factor should also be considered in selecting an upgrading.
RENEWABLE ENERGY FROM SWINE WASTE Bingjun He, University of Idaho, Moscow, ID 1 Yuanhui Zhang, Ted L. Funk, University of Illinois, Urbana, IL 2 Gerald L. Riskowski, Texas A &M University, College Station, TX 3
ABSTRACT: A thermochemical conversion (TCC) process was developed and researched to reduce waste and to produce renewable energy from swine manure. Experimental results showed that operating temperature and retention time were the key parameters. It is critical that a process gas, reducing or non-reducing gases but not water vapor, be added to the process in order to yield an oil product. The operating temperature ranged from 275°C to 350°C, corresponding operating pressures ranged from 5.5 to18 MPa. The initial pressures of process gases were 0.34-2.76 MPa (50 to 400 psi). Oil product was evaluated by elemental analysis, heating value, and benzene solubility. Oil viscosity and its change vs. storage time were also studied. Typical oil yield of the TCC process ranged from 60% to 65% on the input volatile solids. The average elemental composition of the raw oil products was 71.1% carbon, 8.97% hydrogen, 4.12% nitrogen, 0.2% sulfur, and 3.44% ash. The water content of raw oil varied from 11.3% to 15.8%, which was less dependent on the operating parameters. The average heating value of the oil products was 32,500 kJ/kg. These properties are comparable to those of liquefaction oils from wood sludge and other biomass. When CO was as the process gas, the oil viscosity was at the level of 0.5 Pa.s at operating temperatures of 315°C-350°C. The viscosity increased in the first 15d of storage, and did not substantially change over the rest of the 60d storage time. The TCC oil contains high sulfur and nitrogen content. Further investigation is necessary to reduce the high nitrogen and sulfur content before the full utilization of the oil product can be achieved.
Introduction The impact of large confinement operations of swine farms on the environment has caused increasing concerns from scientific communities and government agencies. Odor emission from the swine facilities has become a major concern of the republic. Regulations on manure management are likely to become stringent. Swine manure, once considered a nutrient-rich fertilizer, has become an expensive burden to the pork industry. On the other hand, swine manure is a plentiful source of biomass that has the potential to be converted into renewable energy through biological and/or chemical processes. A thermochemical conversion (TCC) process is a chemical reforming process in which the depolymerization and reforming reactions of ligno-cellulosic compounds occur in a heated and free oxygen-absent enclosure. Based on the characteristics and the products of a process, the TCC processes are further categorized as pyrolysis, gasification, and liquefaction. Among the TCC processes, direct liquefaction is the most widely studied biomass conversion process. Direct 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 Bingjun (Brian) He, assistant professor, Department of Biological and Agricultural Engineering, University of Idaho, Moscow, ID 83844. Phone: 208.885.7714, fax: 208.885.7908, email: . 2 Yuanhui Zhang, associate professor; Ted L. Funk, extension specialist and assistant professor; Department of Agricultural Engineering. 3 Gerald L. Riskowski, professor and head, Department of Biological and Agricultural Engineering.
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fuel from organic matter. The carbonaceous materials are converted to liquefied products through a complex sequence of changes in physical structure and chemical bonds (Chornet and Overend, 1985). Such liquefaction processes had been used to convert cellulosic wastes (Appell et al., 1980), municipal sewage sludge (Kranich, 1984), and many other high-moisture biomass feedstocks (Elliot et al., 1988) using carrier oils as reaction media. In most of the studies, livestock waste was not included as a major biomass resource for the direct liquefaction process, although it was used as feedstock in some pyrolysis studies (Kreis, 1979). Swine manure is a carbon-enriched biomass. It has the potential to be converted to a liquid oil product through a TCC process. To avoid the intense energy requirement of pre-drying of the swine manure as in pyrolysis, it is beneficial to process fresh manure directly without pre-drying. We explored the alternative means to treat swine manure from intensive production facilities, meanwhile, to produce renewable energy from swine waste, by a thermochemical conversion process, or direct liquefaction. This paper summarizes the oil production from swine waste from our previous research and detailed technical discussions have been reported elsewhere (He et al., 2000a,b; 2001a,b,c).
Materials and Methods TCC Reactor A TCC process using a batch TCC reactor was designed and developed. The reactor was made of T316 stainless steel with a capacity was 1.8 L (0.5 gal) (Parr Instruments Company, Moline, IL). It could operate at extreme operation conditions of 375°C (700°F) and 34.5 MPa (5000 psi). Two agitation propellers 80-mm (3 in) apart on a shaft were driven by a 200W magnetic drive. Operation of the reactor was controlled by a controller which featured a three-term Proportional-Integral-Differential temperature control with an accuracy of +_2°C. A high-pressure cable tubing connected to an inlet of the reactor for process gas introduction. The reactor unit was housed in an enclosed chamber where an exhaust fan provided a slightly negative pressure in the chamber to ensure any escaping gases from the process being exhausted outside of the operating room, as illustrated in figure 1. Controller4642fl~
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Figure 1. Illustration of the TCC process and control scheme.
Feedstock The feedstock, fresh swine manure, was collected from the partial slotted floor of a swine finisher room at the Swine Research Farm, University of Illinois at Urbana-Champaign. The total solids (TS), volatile solids (VS) contents, and pH were 27.4+1.4%wt, 87.3+1.3%wt, and 6.0+0.2, respectively. The elemental composition was, %wt dry basis (mean _+SD), C 45.67+1.12, H 6.45_+0.21, N 3.45+0.38, and S 0.038+0.006. Feedstock was prepared individually for each experiment by adjusting the total solids content with tap water to the desired level. Process Parameters
The parameters in the TCC process included the operating temperature (T), process gas initial pressure (Pini.), total solids content (TS), pH, and retention time (RT). The temperature range in this study was 275-350°C. The corresponding operating pressures were 5.5-~18 MPa (800-~2600 psi). The process gases included the reducing gases of carbon monoxide (CO) and hydrogen (H2) and nonreducing gases of nitrogen (N2), carbon dioxide (CO2), and air. The initial pressure of process gases ranged from 0.34 to 2.76 MPa (50 to 400 psi). The effect of pH on the fresh manure was studied at pH 4, 7, and 10. Retention time varied from 5 to 180 rain for different conditions. Because of the presence of abundant minerals and carbonates, no extra catalyst was added throughout the experiments.
Products Analysis The TS and VS of the feedstock were measured by following the procedures described in the Standard Methods for the Examinations of Water and Wastewater (Clesceri et al., 1989). The elemental analyses, including carbon, hydrogen, nitrogen (CHN), and sulfur (S), were performed on the oil product by a carbon-hydrogen-nitrogen analyzer (Model CE440, Exeter Analytical, Inc., N. Chelmsford, Mass.) and Inductively Coupled Plasma (Perkin Elmer Norwalk, Cont.), respectively. High heating values (HHV) of oil products were calculated using an equation based on the complete oxidation of carbon and hydrogen elements (He, 2000): HHV (kJ/kg) =f.(32,792.C + 142,900.H) + 9,275.S - 2,371.N
(1)
Where C, H, S, and N are the weight fractions of carbon, hydrogen, sulfur, and nitrogen in the raw oil products, respectively, and f is a correction factor of oxygen content on heating values (1 > f > 0). This equation gives smaller variations than the Dulong's equation (Sawayama et al., 1996; Selvig and Gibson, 1945). The low heating value (LHV) is based on the definition by Rick and Vix (1991) at the reference temperature of 25°C: LHV (kJ/kg) = H H V - 2 1 7 . 9 H - 24.42"W
(2)
where H is the hydrogen percentage by weight and W is the water percentage by weight. Benzene solubility content of the TCC oil product was performed referring to the ASTM standard method for petroleum products (ASTM, 1999a). The yield and benzene solubility of raw oil product are defined as follows: Oil solubility(%)- (1 -
Oil yield (%) =
solid residue(g) ) x 100% total oil sample(g)
total oil product (g) total volatile solids input (g)
x 100%
(3)
(4)
The water content in the raw TCC oil products was measured by solvent rectification following the procedure of ASTM Standard D95 (ASTM, 1999b) in a batch distillation assembly. Benzene (ACS Certified grade, Fisher Chemicals) was selected as the solvent. The water content of the oil products was calculated as follows:
Water content (%) - water collected in receiver (g) x 100% (5) weight of raw oil sample (g) Viscosity of the raw oil product was measured using a Synchro-Lectric viscometer (Brookfield Engineering Laboratories, Stoughton, Mass.) according to the ASTM Standard D5018 (ASTM, 1999c). The viscometer was calibrated with glycerin (Certified ACS grade, Fisher Chemicals) and the spindle speed was set at 20 rpm. Samples were measured at 65°C maintained by a water bath.
Results and Discussions Product Distribution The products after the TCC process were distributed into four different portions: raw oil product, gaseous product, solid product, and post-processed water. The amounts and composition of the different products varied according to the operating conditions. The input volatile solids distributed among all four products after the process, and the amount varied with the operating conditions. A product distribution based on the average of 90 experiments with CO as process gas is illustrated in figure 2.
(b)
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3.3%
~N~ oil / ~ 8.9%
gas 4.9%
In oil 62.3%
In postwater 13.0% ~_1
In
16.9%
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Figure 2. Product distributions. (a) total mass balance, and (b) volatile solids balance. Even though the percentages in each portion varied widely, the major portion of VS input was converted into raw oil product. The portion of VS in the gas product was mainly in the form of CO2. TCC Oil Production with CO as Process Gas The conversion process of swine manure to oil is similar to other biomass liquefaction processes and to some extent it is even easier as swine manure contains less lignin and the organic matter was finely "pre-processed" by the animal digestion process. On the other hand, less lignin means less energy content and results in a lower oil yield (Humphrey, 1979; Glasser, 1985). Swine manure has high oxygen to carbon ratio and low hydrogen to carbon ratio (Zahn et al., 1997; Hrubant et al., 1978). These characteristics affect the oil formation efficiency negatively. After the TCC process, swine manure slurry was completely converted into different products. Therefore, the conversion rate was not used as a parameter to characterize the TCC process as in other biomass conversion processes. Addition of a reducing gas is necessary in direct liquefaction. Preliminary test results showed that little or no organic carbon was converted to oil without the addition of a process. However, the process gas could be a reducing or non-reducing (see discussion on process gas effects below). Temperature had a substantial effect on the oil formation. Depolymerization reactions would not occur until the temperature reached the level where the activation energy is overcome. In this study, the preferred operating condition for
successful formation of TCC oil product was 285°C to 305°C, and the corresponding operating pressures were 6.8 to 11.5 MPa (1000 to 1650 psi). These conditions were much milder compared to the reported operating conditions for other biomass studies where the operating temperature and pressure were up to 400°C and 40 MPa (5800 psi) (Elliot et al., 1988; Kranich, 1984; Appell et al., 1980), respectively. The typical oil yield of the TCC process ranged from 60% to 65% on the volatile solids input. This was higher than that in wood and moist-biomass liquefaction where it was 25% to 35% (Figueroa et al., 1982; Elliott et al., 1988). The raw TCC oil was readily separated by gravity from the postprocessed water. Organic solvent extraction, which is usually needed for the recovery of liquefaction oils from some biomass conversion processes (Boocock et al., 1980; Elliott et al., 1988), was not necessary in the TCC process.
Elemental Composition and Heating Values It is necessary that biomass-derived oils be characterized in order to utilize them directly as fuel or for further upgrading. The chemical compositions of the biomass-derived oils are complicated and extensive numbers of compounds have been reported (Piskorz et al., 1988; Elliott et al., 1988; Pakdel and Roy, 1988; Maggi and Delmon, 1994; Sipila et al., 1998). The chemical and physical stability of the biomass-derived oils are also important for long-time storage of the oils (Adjaye et al., 1992). In this study, only the elemental composition was analyzed. The elemental composition, the benzene solubility, and high heating values did not vary as much as the raw oil product yield. The average carbon and hydrogen composition in the raw oil product was 71.1%_+4.5% and 9.0%_+0.5%, with the highest values of 77.9% and 9.8%, respectively. The average nitrogen content in the raw oil product was 4.1% with a standard deviation of 0.4%. About 3.4% of the raw oil product was ash. The oxygen content, calculated as the difference of the other mentioned elements and ash, averaged about 12%. Moisture content of the raw oil product ranged from 11.3% to 15.8% and the average benzene solubility of oil product was about 80%. Comparison of the above results to those of typical pyrolysis and liquefied oils is summarized in table 2. The elemental composition of TCC oils is very similar to that of liquefied oil obtained from kelp (Elliott et al., 1988). This means that both processes underwent a similar chemical mechanism except that the liquefied oil from kelp was obtained under much harsher operating conditions and the oil yield was much lower (about 20%). The TCC oils contained higher carbon and hydrogen content and lower nitrogen content than pyrolysis oils (Rick and Vix, 1991). The water and ash content of TCC oils were similar to those of pyrolysis oils. However, the sulfur content of TCC oils was much higher due to the high sulfur content in the swine manure, which was 0.38%. Further treatment is needed in order to utilize the TCC oil directly as fuel. This factor should also be considered in selecting an upgrading process because both nitrogen and sulfur in oils are major air pollution concerns.
Effects of Different Process Gases on the Process The process gas effects on elemental composition of the oil products are summarized in table 3. Based on the variance analysis of multiple-populations for the mean at 95% confidence level, the elemental compositions of carbon, hydrogen, nitrogen and sulfur between different process gases were not significantly different. The carbon content stayed at an average level of 72%. Although the difference between the carbon content of the oils with CO2 and H2 as process gases was 6.6%, it was insignificant because of the large variations of the measurements. When CO was added as the process gas, the hydrogen content was 9.6%, which was higher than when other process gases were used (about 8.8%). This 0.8%wt difference is very important to the energy content of oil products. Nitrogen was high in all measurements, ranged from 4.1% to 4.6%. It was slightly lower when CO was added, and
stayed at about the same level of 4.4% when other process gases were added. Sulfur content also stayed at about the same level (0.2%) with different process gases. When hydrogen was used as the process gas, the sulfur content appeared was lower. Due to the large standard deviation, however, this value did not significantly differ from the others based on variance analysis at 95% confidence level for the mean.
Table 2. Characteristics of the TCC oils and other biomass-derived oils. Properties Composition, %wt Carbon Hydrogen Nitrogen Sulfur Oxygen Ash Water, % Low heating value,
TCC Process Feedstock (1) TCC oil (2)
Pyrolysis oil (3)
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TCC operating temperature (°C) Figure 3. Effect of operating temperatures on oil viscosity. The oil samples were obtained with CO as the process gas and under the operating conditions of Pini- 0.69 M P a , T S - 20%, RT= 120 min, and feedstock pH - 6.1. The corresponding operating pressures were 7-18 MPa. Viscosities were measured at 65°C. In this study, the viscosity change was studied as an indicator of TCC oil stability under an ambient environment (about 22°C). The oil samples were obtained under operating temperature of
315°C with 0.69 MPa CO as the process gas. Six measurements were conducted to take an average. The viscosity of raw oil products increased from 9 Pa.s to about 13 Pa.s in the first 15 d of storage, and did not substantially change over the rest of the 60 d storage time which agreed with the observations of Adjaye et al. (1992). This change in viscosity should be considered during the TCC oil upgrading and utilization. The oil viscosities with different process gases differed considerably. Figure 4 shows the effect of alternative process gases on TCC oil viscosity. Each column represents the average of three to five different measurements and the error bars are standard deviations. Based on the statistical analysis of variance for multiple population means, the differences between the viscosities with the five process gases were significant at 95% confidence level. Under the given operating conditions, the lowest viscosity occurred when CO was applied as the process gas. The TCC oil with CO2 had the next lowest viscosity of 14.4 Pa.s, and, unexpectedly, the oil viscosity with hydrogen as the process gas was the highest (50 Pa.s).
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Figure 4-Viscosity
of oil products with different process gases. The viscosities were measured at 65°C. The TCC operating conditions were 305°C, Pini 0.69 MPa, RT = 120 min, TS = 20%, and feedstock pH = 6.1. The corresponding TCC operating pressure was 10.2 MPa. The error bars are standard deviations. Based on the statistical analysis of variance for multiple population means, the differences between the five process gases were all statistically significant at 95% confidence level. =
Conclusions The TCC process was successfully applied to the treatment of swine manure slurry reduce the waste strength and to produce liquid oil without any extra catalyst addition. The TCC oil products had similar properties as those of liquefaction oils from of other biomass. Since the TCC process of swine
manure originated as an alternative process for waste treatment, the utilization of the TCC oils as fuel to offset energy consumption is promising. However, utilization of the TCC oils as fuel will require further treatment to reduce the high nitrogen and sulfur content in the raw oil products. It is also necessary to upgrade the raw TCC oil to enhance its utilization value. A continuous-flow TCC process is desirable for producing composition-consistent oils from a scaled-up process. Further analyses, e.g. qualitative and quantitative chemical composition, are needed in order to fully explore other possible utilization of the TCC oils.
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