Many methods exist today for the disposal of organic solid wastes, yet each process has its own disadvantages. Furthermore, current environmental regulations limit the use of many methods. A long accepted practice is the conversion of organic solid wastes into clean gaseous and liquid fuels.
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There are three factors contributing to the present attraction to solid waste conversion:
(i) Municipal waste disposal problems including incineration pollution effects, a scarcity of landfill sites and continued increases in solid waste generation.
(ii) Conventional fuel prices have continued to rise, such that the conversion and use of solid waste fuels has become economically feasible.
(iii) The conversion of solid waste into gaseous or liquid fuels make it more economically attractive. One large conversion facility is necessary rather than solid waste processing plants at each application site.
The technology needed to effect solid waste conversion is already available and can be divided into three major processes:
(i) Anaerobic digestion – micro-organisms oxidise the organic solid wastes producing methane in an oxygen deficient atmosphere, using the oxygen atoms from wastes.
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(ii) Hydrogasification – methane is generated through the reaction of hydrogen with carbon containing compounds.
(iii) Pyrolytic conversion – gaseous and liquid fuels are the products of thermal decomposition of the organic solid wastes in an oxygen deficient atmosphere.
Other systems have been developed which would recover methane produced from existing landfills through wells sunk deep into the decomposing solid wastes.
However, the purpose of solid waste conversion is not to replace conventional fuel, but to supplement it and the entire energy network. Small percentage energy contributions from a number of sources can significantly lower the demand for fossil fuels, and these sources can be many and varied: solid wastes, wind, solar, geothermal, tides. In addition, solid waste conversion also effects an alternate waste disposal system, thus benefiting the earth’s environmental and energy cycles.
Anaerobic Digestion:
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Anaerobic micro-organisms digest organics in an oxygen deficient atmosphere yielding products of carbon dioxide and methane. A proposed project would utilise this process to provide 1500 ft3/day of methane gas.
Fig. 5.5 shows the Pfeffer process. Initially, the municipal solid waste passes through a shredder and is ground into particles of uniform size. A magnetic separator then removes the ferrous metals to be reclaimed, and the remaining waste is conveyed to an air classifier which divides the refuse into heavy and light fractions. The heavier wastes are trucked to landfills or further classified for resource recovery.
The light fraction which is high in paper content (i.e., cellulose) is mixed with sewage sludge and/or other chemical nutrients to produce a slurry. The chemical nutrients or sewage sludge must be introduced since they provide the required nitrogenous compounds for micro-organism growth, and solid waste usually is deficient in these compounds.
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This slurry then has its pH adjusted to 6.7 and temperature raised to 130-140°F and passed into a digester. These slurry parameters are for optimum micro-organism growth. The slurry circulates in the digester for 5 days. The gas generated by these micro-organisms is between 50 and 60 per cent methane and the rest carbon dioxide. This gas has a relatively low heating value, but can be used as is; however, to obtain a gas of pipeline quality this mixture must pass through a separator to produce methane and carbon dioxide. Fig. 5.6 shows the process flow plan of this separator.
The remaining residue in the digester or sludge is a mixture of lignin, plastics and some unreacted cellulose which, when it has its water content removed, results in a 75% volume reduction for the wastes entering the digester. The properties of this sludge are being examined for other industrial uses, but should these studies prove inconclusive the sludge will be dumped as stable landfill.
Hydrogasification:
Hydrogasification is a process in which hydrogen reacts with organics to generate a methane-rich gas product. This gas is then processed to separate the acid gases and carbon monoxide from the methane. The hydrogasification reaction is highly exothermic, which makes the additional heat unnecessary when solid wastes having high moisture contents are being converted.
Studies have shown that gas generated in this manner had a methane content of about 70% and a heating value of over 930 BUT/ft3. Cattle manure generates a gas product containing nearly 93% methane.
Cellulose is the major component of organic solid wastes. There are many types of cellulosic wastes including municipal solid wastes, agricultural wastes, sewage sludge, wood, lignin and animal wastes and they all have been converted to oil. The process involves the high pressure (4000 psig) reaction of cellulose, carbon dioxide and water with temperatures at 350-400°C.
Various catalysts and solvents are necessary to complete the reaction. Oil yields obtained have been 40-50% based on the conversion of over 90% cellulose. This oil is a brownish black liquid or a semisolid at 25°C and has a heating value of 15,000 BTU per pound. Table 5.15 shows the sulphur content of oil produced from the conversion of the various wastes listed.
The processes reactants are as follows:
(i) Cellulosic materials, including carbohydrates, wood wastes (cellulose and lignin), components of municipal solid wastes (cellulose, carbohydrates, proteins, fats, and other organic materials), sewage sludge, agricultural and animal wastes along with carbon monoxide and water can be converted to oil. The plastic content of the municipal solid waste is not expected to greatly affect the oil processing.
(ii) Carbon monoxide and water react with cellulose molecules to produce an oil. This process is usually carried out under temperatures of 250-400°C to reduce costly carbon monoxide consumption. It also eliminates water and some carbon dioxide and carbon monoxide to produce a black solid material known as char.
(iii) A solvent or liquid phase is necessary to prevent the formation of this solid char. It is also necessary as a vehicle for the addition of the reactants. Water seems to be the most effective vehicle, since catalysts readily dissolve in it as do some organic intermediates of the process and it is very inexpensive. Municipal solid wastes, sewage sludge and other substrates probably have water contents high enough such that no more need be added to the reaction.
The Conversion Reactions:
Cellulose (C6H10O5) is the major component of the cell walls of plants and trees, and therefore of paper and other wood products. It is made up of long chain glucose units. Starch, like cellulose, consists of glucose units and is widely distributed in polysaccharide food reserves stored in the seeds, roots, and fibres of plants.
The conversion of cellulose to oil is primarily based on the oxidation of the cellulose molecules and the formation of high hydrogen-to-carbon ratio molecules. Water and carbon dioxide are removed from cellulose and other carbohydrates with the addition of heat.
The removal of oxygen to form these high hydrogen-to-carbon molecules is effected by the addition of carbon monoxide to form carbon dioxide, by hydrogenation, by various disproportion reactions, and by combinations of these reactions. As a result of the large number of reactions possible, the oil produced is comprised of a complex mixture of different molecules.
Fig. 5.7 is a process flow diagram of a continuous operations unit to produce oil from organic solid wastes. System design calls for operation at maximum temperatures of 500°C for flow rates of 100 to 500 g/hr of solid waste slurry and carbon monoxide at a rate of 10 SCF/hr.
The carbon monoxide and waste slurry stream is pressurised and heated prior to entry into the bottom of the heated reactor. The liquid and gas products leave the top of the reactor and are separated in a high-pressure recovery process. A backpressure value continuously releases the gas products while the liquid is collected and discharged into low-pressure receivers.
Pyrolytic Conversion:
Pyrolytic conversion, or destructive distillation, causes the breakdown of materials under a high temperature and in the absence of air into three components – (i) a hydrogen, carbon monoxide and methane gas (ii) an oil-like liquid which includes acetic acid, acetone and methanol and (iii) a nearly pure carbon char. This method has generated the most interest and technological advances and has resulted in research by several major corporations.
The normal combustion process requires large amounts of air to complete the burning process and to remove the heat produced.
Using cellulose (C6H10O5) as the representative organic material the reaction will take place as follows:
C6H10O5 + 6O2 → 6CO2 + 5H2O
Air supply can be 400% above stochiometric requirements. As a result gas velocities are increased tremendously causing detrimental effects to air pollution control devices because flow rates through the device are greatly increased and large particulate loadings are generated from the air circulating through the burning wastes. These air pollution control devices make solid waste incineration very costly.
In pyrolysis heat must be added as the volatile compounds are distilled off the solid waste. The pyrolysis reaction is as follows –
C + H2O + Heat → H2 + CO
C + CO2 + Heat → 2CO
Carbon reacts with water and carbon dioxide in the presence of heat to produce carbon monoxide. Products of pyrolysis are dependent on time, temperature, pressure, the presence of catalysts, pyrolysis zones, and/or combustion zones within a particular system. However, the principle products of pyrolysis include hydrogen, carbon monoxide, carbon dioxide, methane and other hydrocarbons.
Different Pyrolysis Processes:
i. The Garrett Process:
Typical solid waste from San Diego is fed into a pyrolysis reactor. Table 5.16 shows an analysis of the San Diego solid waste used in this study.
The solid waste is first treated for the removal of glass and metals prior to the start of pyrolysis. The remaining solid waste material was used for analysis in the report using about 0.4 pounds of refuse per batch. The condensable liquids were passed through a series of cooling traps and condensed.
The gaseous products were captured in a gas balloon. The San Diego experiment ran pyrolysis tests at temperatures ranging from 900° to 1700°F. Gravimetrical analysis was performed to determine the quantities of char and condensable and the pyrolysis gases were measured by volume. Table 5.17 shows the amount of products produced at the various test temperatures.
Qualitative analyses were made on the condensable to determine the organic compounds present and it showed that in addition to water, the condensable phase included methanol, ethanol, isobutanol, n-pentanol, tert-pentanol, 1, 3-propaneidiol, 1-hexanol, and acetic acid. Table 5.18 shows the results obtained from an analysis of the char portion and Table 5.19 presents the result of gas chromatography analysis of the gas phase.
One of the most significant results obtained from the San Diego pyrolysis work was the measurement of the heating values of the products of pyrolysis and evaluation of these in terms of heat required to sustain the process. Fig. 5.8 is a graph of heat required to sustain the process recovery and utilisation from pyrolysis products.
To obtain a self-sustaining pyrolysis process and a thermal efficiency of 50%, the pyrolysis temperature should be 900°F. The char content could be adjusted to provide additional heat to the process. It should be noted however that these results are based on batch processes and that a continuous pyrolysis process is expected to have much better self-sustaining characteristics.
Process Description:
The Garrett process is a low-temperature flash pyrolysis. It converts ground up organic material into a high viscosity and high oxygen content fuel oil, combustible char and gas. Initially the municipal solid waste is shredded into rather coarse 3-inch particles requiring a shredder output of between 50-60 Hp-hours/ton.
From the primary shredder the refuse is transported to an air classifier and separated into a light fraction consisting of paper and plastics, and a heavy fraction consisting of glass, metals, wood and rock. This separation procedure reduces the inorganics in the light fraction to less than 4%.
The light solid waste fraction passes through a drying and screening process to reduce the inorganic content further and then is shredded to a very fine particle size (less than 24 mesh), requiring a shredder output of between 50-60 Hp-hours/ton. These fine organic solid waste particles enter the pyrolytic reactor and flash pyrolysis takes place at temperatures of around 900°F.
The heavy fraction passes through a magnetic separator for removal of ferrous metals and a mixed-colour glass cullet is removed by froth flotation in 99.7% purity. The remaining char is trucked and dumped at landfill sites. The heat required for pyrolysis is derived from combustion of the fuel gas and a fraction of the char produced. The Garrett system can produce over 1 barrel of oil/ton at 4.8 million BTU/BBL or 6,000 SCF of gas at 800 BTU/ft3 without requiring supplemental fuel sources to supply energy for the process. Fig. 5.9 is a block diagram of the Garrett process flow.
ii. Envirochem Landgard System:
Results from these operations enabled Envirochem to (i) demonstrate the continuous pyrolysis of typical unclassified municipal solid waste, (ii) acquire solid waste handling experience, (iii) gather the data necessary to design and construct large-scale facilities, and (iv) develop a solid waste residue resource recovery process.
Process Description:
The Envirochem Landgard process is the low-temperature pyrolysis of the municipal solid waste’s organic compounds through partial oxidation with air in a rotary kiln to produce char and combustible gases. Unclassified municipal solid waste is milled to produce a uniform particle size and conveyed to a bin. From the storage bin the shredded waste is fed continuously into the rotary kiln.
Shredded waste enters one end and fuel oil enters the opposite end of the kiln. The flow of gases and solids is counter-current to expose the feed to higher temperatures as it passes through the kiln. Thus, first drying and then pyrolysis occurs. The final residue is contacted with temperatures of 1800°F just before release from the kiln. In order to maximise pyrolysis action the kiln is designed to expose solid particles to a uniform temperature gradient.
The hot residue released from the kiln enters a water-filled quench tank and passes into a flotation separator. The material that floats off is a carbon char slurry. It is thickened and filtered to remove the water, and conveyed to a storage area before being trucked to landfill sites. The putrescible content of this residual char is about 0.1% which makes it ideal for landfill operations.
The heavy non-buoyant material is conveyed along the bottom of the flotation separator to a magnetic separator for removal of ferrous metals which are deposited in a storage area for railcar or truck shipment to scrap dealers. The balance of the heavy material, or glassy aggregate, passes through a screening process and then stored on-site for use in road construction.
Pyrolysis gases are drawn from the kiln to be piped to local utilities or industries for use as low BTU value gas to be combusted with oil, natural gas, or coal. Table 5.20 gives an analysis of gaseous fuel obtained from this process. Fig. 5.10 is a block diagram of the Landgard system gaseous fuel option.
iii. Union Carbide Oxygen Refuse Converter (Purox System):
The Purox system converts unclassified municipal solid waste into a clean burning fuel gas and an inert residue. It uses oxygen as a supplement to the pyrolysis reaction of the combustibles of the refuse to effect an environmentally acceptable fuel gas. It further provides high temperatures needed to form molten metal and slag from the non-combustible fraction of the solid waste. This resultant residue is inert and only 2% of the volume of the originally charged refuse.
Process Description:
The Purox process combines the rather unique advantages of pyrolysis (the generation of useful and valuable by-products) and high temperatures (the melting and fusing of the metals and glass). These are the effects of using oxygen rather than air in the conversion step.
Fig. 5.11 shows the vertical shaft furnace on which the process is dependent. Solid wastes enter the top of the furnace and oxygen is injected into the bottom. Char forms from the refuse and oxygen reacts with it generating high temperatures in the order of 2600°F to 3000°F. These temperatures are needed in the hearth to melt and fuse the metals and glass. This molten metal and glass sludge gravimetrically drains continuously into a water quench tank where it hardens into a granular material.
The reaction of oxygen and char generates hot gases which rise up counter to the flow of the falling refuse and pyrolyses the refuse as it cools. Near the upper portion of the furnace, the gases and refuse intermingle causing the gases to cool further and the refuse to be dried. The gases then exit the furnace at about 200°F. These gases consist of large amounts of water vapour, oil mist and small quantities of other constituents. The undesirable components are removed in the gas cleaning system.
The gas that leaves this cleaning process is a clean burning fuel comparable to natural gas with a heating value of about 300 BTU/cubic foot. The fuel characteristics are shown in Table 5.21. Its sulphur and nitrogen compounds are virtually non-existent as it can be used directly as a fuel supplement to any fuel consuming operation.
The emissions generated by combustion of this fuel are within air pollution regulations. Fly ash emissions have been measured at 0.008 grain/cubic foot or an order of magnitude below federal regulations. It can be combusted without the addition of costly modifications to the boiler system.
The Union Carbide process results in a net energy output. The final gas generated represents 83% of the fuel value of the unclassified municipal wastes entering the conversion system. A minimum amount of this fuel gas is used by the process to generate steam to heat the plant buildings, and to heat and maintain the auxiliary combustion chamber at operating temperatures.
Thus, the remaining fuel gas or approximately 75% of the fuel energy of the refuse is available for outside combustion sources. The usage of available energy from a 1000 ton per day refuse processing facility is shown in Table 5.22.
The residue that is left from the non-combustible fraction of the refuse is sterile and compact because it has been melted and fused to eliminate any biologically active material and to a minimum volume. Sanitary landfill techniques are unnecessary for its disposal. It is suitable for construction fill material.
Depending on the non-combustible content of the incoming municipal solid waste a final volume reduction of 97-98% can be effected. For the most efficient incinerators a volume reduction of 90% or less is considered good design. A process flow diagram for the Purox system is given in Fig. 5.12.
Fuel Gas:
The fuel gas generated by the Purox process can be used as a supplementary fuel in existing boilers or other fuel consuming operations. Table 5.23 compares the fuel gas of solid waste origin to some common hydrocarbons and carbon. The fuel gas has a lower heating value than methane (natural gas) or propane or butane. The problem arising from this relatively low heating value is the necessity of increased compression power for this fuel gas relative to methane. However, this is a minor economic factor because the pressures involved are relatively low.
Volume of combustion air required
Per SCF of fuel gas = 0.500/0.21 = 2.38 SCF
Per thousand BTU = (1000) (2.38)/286 (LHV) = 8.34 SCF
Volume of combustion products
Per SCF of fuel gas = 0.39 + 0.70 + 1.900 + 0.03* = 3.02 SCF
* Moisture content of fuel gas, assuming saturation @ 35 psig, 100°F
Per thousand BTU = (1000) (3.02)/286 (LHF) = 10.5 SCF
The combustion air requirements for the fuel gas per unit of heat release are less than the methane or the other fuel air requirements. Table 5.24 shows the combustion air requirements for the fuel gas. Therefore, the capacity of existing air blowers should be sufficient for the combustion of fuel gas without modifying the boiler.
The quantity of combustion products generated per unit of heat release is also lower for the fuel gas than for methane or the other fuels. This is due directly to the lower combustion air requirements for the fuel gas as seen in Table 5.23. Existing induced draft fans and air pollution control devices should not be adversely affected by the combustion of this fuel. This is important at plants that burn dirty fuels and sized pollution control equipment must remain effective.
Finally, the Purox process fuel gas combustion products release more heat per unit volume than the other fuels listed in Table 5.23, signifying that fuel gas flame temperature and heat transfer characteristics are similar to the other fuels.
In general the fuel gas of solid waste origin will be a small fraction of the total fuel requirements of a typical utility boiler. Thus, variations in production fuel gas flow and heating values can be absorbed through adjustment in the base fuel flow rate. This would require a relatively simple control system.
Landfill Gases:
Another potential source of clean gaseous fuels from organic solid waste are the gases generated at sanitary landfills. Should the tests show gas is available in quantities and qualities (1,000 ft3/min, 50% methane) of economically feasible proportions, NRG will install a molecular sieve purification station on the site to collect and then sell the gases. A landfill is economically feasible if it can produce 3.5 – 7 lb. of methane per pound of landfill and there is a nearby market for the gas.