Environ. Sci. Technol. 2003, 37, 4517-4522
Recycling of Meat and Bone Meal Animal Feed by Vacuum Pyrolysis A. CHAALA AND C. ROY* De´partement de Ge´nie Chimique, Universite´ Laval, Sainte-Foy, Que´bec, Canada G1K 7P4
Due to the recent bovine spongiform encephalopathy (BSE) crisis in the European beef industry, the use of animalderived products to feed cattle is now severely restricted. Large quantities of waste animal meat and bone meal (MBM), also known as animal flour, have to be safely disposed of or transformed. One disposal option is pyrolysis. Vacuum pyrolysis of an animal flour sample has been performed in a laboratory reactor. The results obtained revealed that vacuum pyrolysis can be an attractive alternative to incineration and cement kilns. The process generated a combustible gas (15.1 wt %), a high calorific value oil (35.1 wt %), a solid residue rich in minerals (39.1 wt %), and an aqueous phase rich in organics (10.7 wt %). The gas and the aqueous phase can be used to provide heat to the vacuum pyrolysis reactor and the MBM drying unit. The oil can be used alone or mixed with petroleum products as a fuel in boilers or gas turbines. Conversion of animal waste by pyrolysis into fuels can contribute to the reduction of greenhouse gases. It is suggested to use the solid residue for agricultural soil enrichment in minerals and as a soil moisturizer.
Introduction The mad cow disease has had annoying repercussions in the animal feed industry. In fact, animal flour and byproducts therefrom, which are now produced in huge quantities, have to be safely disposed of. These products must be eliminated or safely recycled. In France alone, the total amount of animal residues which need to be eliminated is greater than 3 Mtons per year (1). This includes about 850000 tons of defated flour and 300000 tons of fatty products. Existing technologies can treat only 350000 tons of flour and the derived fatty byproducts. To avoid huge storage costs of animal flour and crude animal residues, new solutions have to be investigated. Incineration is a potential solution, but the fluffy nature of the flour is challenging as particles find their way in the emissions and the ash residue melts and stick inside the incineration chamber. It has been reported by McDonnell et al. (2) that, to effectively burn meat and bone meal, it is necessary to mix them with milled peat at a concentration of 35 wt %. A quick but transitory solution authorized by the French government for the elimination of animal flour is in the cement industry as a solid fuel (1). Since the cement industry in France is renowned for its suitability to use diverse types of fuel residues such as used lubricant oils, solvents, paints, used tires, and now animal flour, it has the potential to burn 450000 tons/year of animal flour after 2002. Currently only 16 out of 33 cement plants * Corresponding author phone: (418)656-7406; fax: (418)656-2091; e-mail:
[email protected]. 10.1021/es026346m CCC: $25.00 Published on Web 08/20/2003
2003 American Chemical Society
in France consume flours (1). According to the MIEFA (Mission interministe´rielle pour l’e´limination des farines animales), seven other cement factories which could consume 80000 tons/year of animal flour are required (3, 4). Some investigations have been performed on the possibility of making phenolic briquettes with a mixture of concrete and animal flour. This idea could not be realized at a large scale due to the high sensitivity of the briquettes to water. The briquettes decompose with time and can generate phenol derivative compounds (1). Animal flour contains a large amount of fats and other organic compounds. It can be converted by thermal degradation processes such as pyrolysis into transportable, storable, and workable fuels. Vacuum pyrolysis has proven to be applicable for the upgrading of a large variety of solid and semiliquid organic wastes (5-11). The process is commercialized by Pyrovac International Inc. under the trademark of Pyrocycling (7). The objective of this paper is to demonstrate the potential of the vacuum pyrolysis technology (Pyrocycling) for the treatment of animal flour. The results obtained at the laboratory scale and herein reported provide the background data for a technological development of this solution at the industrial scale.
Experimental Section Materials. The meat and bone meal (MBM) feedstock herein called animal flour was provided by Alex Couture Inc. (Charny, Que´bec, Canada). The material was received as a powder and had to be pelletized as its fluffy nature did not enable application of vacuum suction into the reactor. No binder was added during the extrusion process. The pellets had an average moisture content of 3.5 wt %. The size of the pellets varied between 5 and 20 mm with a diameter of 5 mm. The pellets were hard enough for treatment in a batch process. They would need to be further hardened however if processed in a continuous feed reactor. Analysis. Proximate analyses of the feedstock and the solid residue obtained were determined by means of a LECO TGA-601 analyzer, whereas the elemental composition (carbon, hydrogen, and nitrogen) was determined in a LECO CHN-2000 elemental analyzer. Physicochemical characteristics of the oils were determined according to appropriate ASTM methods. The gross calorific value was determined according to ASTM D-240 for solids and ASTM D-4809 for liquids. A SSC/5200 TG/DTG (220) microbalance from Seiko was used for the thermogravimetric tests. Small pellet samples weighing 7-8 mg and 1-2 mm in dimensions were heated from room temperature to 550 °C, under a nitrogen flow of 150 mL/min at a heating rate of 10 °C/min. The conditions for the thermogravimetric analyses were chosen near the conditions used in the pilot plant for the pyrolysis of similar residues such as urban sewage sludge, softwood bark pellets, and bagasse. The metal content of the solid materials was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a representative sample of ash dissolved in mineral acids. Organic liquids were analyzed by gas chromatography-mass spectrometry (GC-MS) using an HP-5890 gas chromatograph equipped with a 30 m × 0.25 mm i.d. J & W fused silica capillary column DB5 coupled to an HP5970 mass-selective detector. Pyrolysis Test. A series of laboratory tests were performed prior to fixing the best pyrolysis conditions. The feedstock as received was in a powder form. Its specific gravity was very low. During the pyrolysis test, in the extensive degradaVOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Vacuum pyrolysis laboratory-scale installation.
FIGURE 2. TG and DTG of the animal flour sample. tion process, a large amount of powder left the reactor with the vapors generated, causing the formation of plugs in the lines of the condensing system. To avoid this phenomenon, the feedstock was pelletized. Figure 1 illustrates the process schematics. The reactor used for the pyrolysis runs has a working volume of 1 L. The stainless steel reactor was installed in an electrically heated tubular furnace. The gas and vapors produced during the pyrolysis passed through a stainless steel trap and four cold traps connected in series. The stainless steel trap was maintained at room temperature. The last four traps made of Pyrex were maintained at -15, -40, -78, and -78 °C. The noncondensable gas was pumped into a stainless steel container, previously set under vacuum by means of a vacuum pump. The test temperature was measured using a thermocouple installed in the feedstock bed inside the reactor. Both the pressure and the temperature were measured and recorded every 30 s, by means of a data acquisition system. When the pressure of the recovered gases remained constant for 30 min, the pyrolysis of the feedstock was considered to be complete. The heating of the reactor was then terminated, and nitrogen was introduced to avoid any oxidation reactions. Once the reactor and the rest of the apparatus had cooled to room temperature, the pyrolysis products were recovered and weighed. 4518
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A sample of 516.5 g of pelletized animal flour was pyrolyzed at a temperature of 500 °C, a total pressure of approximately 4 kPa, and a heating rate of 15 °C/min. The batch pyrolysis test lasted 170 min. The end pyrolysis temperature was selected on the basis of a series of TG tests (Figure 2).
Results and Discussion Characterization of the Feedstock. The proximate analysis presented in Table 1 shows that the feedstock contains a high amount of volatile matters. The high ash content (24.8 wt %) is attributed to the presence of bones in the feedstock, and as a result this residue has a low calorific value (20 MJ/kg). The low fixed carbon (8.0 wt %) and the high oxygen (17.7 wt %) and nitrogen (8.9 wt %) contents are also noticeable. Thermogravimetric Analysis of the Feedstock. Figure 2 presents the TG (thermogravimetric) and DTG (differential thermogravimetric) curves of the animal flour sample. Three shoulders and one peak are observed on the DTG curve. The mass loss (3.25 wt %) represented by the first shoulder in the temperature range of 50-148 °C is attributed to the dehydration of the animal flour sample. The second shoulder, which is observed between 148 and 225 °C, may be due to the evaporation of low molecular weight compounds contained in the animal flour or/and the decomposition reactions
TABLE 1. Physicochemical Properties of the Feedstock and the Solid Residue properties moisture content, wt % proximate analysis (anhydrous basis, wt %) volatile matters ash fixed carbon elemental analysis (anhydrous basis, wt %) carbon hydrogen nitrogen oxygena gross calorific value, MJ/kg a
feedstock
solid residue
3.5
0.0
67.2 24.8 8.0
21.5 56.2 22.3
42.8 5.8 8.9 17.7 20
31.8 1.4 5.2 5.4 11.5
Determined by difference.
TABLE 2. Metal Content in the Feedstock and the Solid Residue (mg/kg)a element
feedstock
solid residue
element
feedstock
solid residue
Ca P Na K Mg Fe Zn Mn Cu Cr
67207 31842 8716 6339 1599 560.5 99.8 35.2 22.0 5.9
141167 72363 17872 13523 3381 883.8 183.9 74.1 41.5 5.9
Ti Ni Co Pb As Se Mo Al V Cd
3.5 2.9 0.4
