The quantities of animal wastes generated have been steadily increasing with the population growth of farm animals necessary to feed the ever growing human population. Animal wastes make up a large portion of the total organic solid waste produced. These solid organic wastes have a fuel oil and fuel gas potential that is equivalent to 15% of the oil or 38% of the natural gas demands.
There are three major processes for conversion of animal wastes to synthetic fuels:
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1. Hydrogenation.
2. Pyrolysis.
3. Bioconversion.
Hydrogenation and pyrolysis are available processes and it is expected that they will probably be commercialised within this decade. Bioconversion has received very little attention.
Method # 1. Hydrogenation:
In hydrogenation, organic waste and about 5% alkaline catalyst (i.e., sodium carbonate) are mixed in a vessel with carbon monoxide and steam at initial pressures of 100-250 atmospheres and temperatures of 240-380°F for about an hour or less.
Under optimum operating conditions, as much as 99% of the carbon content can be expected to be converted to oil or nearly 85 gallons per ton of dry waste. Normally, in practice, a conversion rate of approximately 85% can be obtained. In order for this reaction to be self-sustaining, a portion of this converted oil is used to provide heat and carbon monoxide for the reaction. This results in a net yield of about 50 gallons per ton of dry animal waste.
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The animal waste oil is a heavy paraffinic oil with an oxygen content of about 10% and a nitrogen content of nearly 5%. Sulphur content is relatively low, about 0.4%, below the sulphur content limit of most cities.
The heating value of this fuel oil is about 15,000 BTU per pound. In comparison, the widely used No. 6 fuel oil has a combined oxygen and nitrogen content of about 2% and a heating value of about 18,000 BTU per pound. The original heating value of the raw animal wastes varies from 3000 to 8000 BTU per pound.
Another method for production of synthetic fuels from animal wastes is pyrolysis. The major disadvantage of pyrolysis for small scale operations is the production of at least three different types of fuel – gas, oil and char; thus increasing handling and marketing problems. However, pyrolysis is performed at atmospheric pressure making construction and operating costs lower than those of hydrogenation.
Method # 2. Pyrolysis:
The pyrolysis of animal wastes involves the initial heating of the wastes to about 1000°F in a heat exchanger with no oxygen. For every ton of animal wastes, pyrolysis produces about 40 gallons of oil, 160 pounds of char and varying amounts of a gas with a heating value of 400 to 500 BTU per scf. The gas and a portion of the char are utilised by the process to make it self-sustaining.
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The oil produced has an oxygen content of about 33%, nitrogen content of less than 1% and a sulphur content of less than 0.3%. This high oxygen content results in an oil with a heating value of only about 10,500 BTU per pound. The heating value of this oil is actually about 75% that of No. 6 fuel oil because the animal waste oil has a specific gravity of about 1.3 as compared to 0.98 for the No. 6 fuel oil.
Research into many processes to generate a synthetic fuel from animal wastes is in various stages of development. Some have shown promising results and it is expected that oil conversion of animal wastes will eventually involve large-scale conversion plants. These plants will be centrally located and receive animal wastes from large areas and in large quantities. Large amounts of oil could then be produced to make such a plant economically feasible.
Methane Production:
There are many small scale installations of manure to methane plants. Farmer have used manure methane to heat barns, silos and other facilities. Others have been more enterprising and run cars on this methane.
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The applications prove that methane production from animal wastes is technically feasible. However, small-scale applications do not provide for complete utilisation of the animal waste resource. Large animal waste processing plants may in the future contribute to a solution to the natural gas shortage problem by generating methane.
Methane may actually develop into a renewable fuel. Plant life has always utilised energy from the sun through the process of photosynthesis and stored it in organic carbon bearing compounds. Death of the plant brought anaerobic bacteria to consume the decaying plant cells and through their metabolic process generated methane, or natural gas. Natural gas is found in underground reservoirs, or at the earth’s surface due to escaping gas from faulted rock formations.
Natural gas has for years been a by-product of sewage treatment plants. These plants usually can operate pumps, compressors, and/or heating facilities with this gas. The anaerobic process of a sewage treatment plant is very similar to that of a domestic septic tank or that of an animal waste treatment plant.
A slurry of sewage and water is conveyed to an oxygen deficient tank and natural bacterial action completes the process. The operations can become complex, since the gas must be continuously collected and the sludge removed and the tank temperature maintained at 95°F for optimum process conditions.
Methane can be effected by pyrolysis. Bioconversion of animal wastes can produce large amounts of natural gas under the proper operating conditions.
Method # 3. Bioconversion:
Bioconversion of the animal wastes is a highly complex microbiological process. Generally, the process involves the anaerobic digestion of the wastes to generate carbon dioxide and methane. The organic solids of the animal wastes are initially solubilised by enzymes into compounds that can be utilised by anaerobic bacteria. Cellulose and starch are broken down to the simple sugars and the proteins are broken down to amino acids. Only the fatty acids are not affected by these enzymes.
In the anaerobic environment the bacterial metabolic rate is limited and is dependent on the chemical oxygen in the organic matter being decomposed. The chemical reactions of the metabolic process results in the formation of acids.
The high acid concentration retards bacterial growth and the system approaches biological equilibrium. The system then develops a new bacteria type to utilise these organic acids. These bacteria metabolise the organic acids into carbon dioxide and methane. Metabolism of the amino acids liberates ammonia which neutralises some of the remaining acids.
As the pH rises bacteria growth increases under the more favourable conditions producing a large methane bacteria population. These methane bacteria permit the further degradation of the more complex organic compounds. These organic compounds are finally broken down to methane and carbon dioxide by direct metabolism. A schematic diagram of the metabolic degradation of animal wastes is shown in Fig. 5.30.
Operating problems of the digester are kept to a minimum provided a balanced bacterial population of acid formers and methane formers is maintained. The organic portion of the animal wastes upon addition to the digester will then be quickly converted to methane and carbon dioxide.
Alternatives to the traditional animal waste disposal method of fertiliser land spreading have been distinctly directed toward resource recovery, particularly energy recovery.
Processes to generate an energy source from these materials have been developed and include the following works:
(i) The Bureau of Mines utilises anaerobic degradation of animal wastes to produce methane.
(ii) The Bureau of Mines has produced methane and char with another process.
(iii) The Bureau of Mines has also produced oil from bovine manure.
(iv) A southwestern company is building a biogasifier plant to produce methane from cattle manure.
(v) A major corporation has a process for producing single cell protein from manure for use as animal feed supplement.
The products recovered from animal wastes normally include methane gas, oil, char and soil stabilisers. Market conditions can fluctuate to eventually make the production and sale of these substances more economically feasible. The animal wastes can then be utilised effecting another viable energy source and eliminating a pressing disposal problem.