Experiments on the pyrolysis of cellulose were carried out in a controlled mixing history reactor (CMHR), which is a plasma-operated drop-tube furnace capable of operating at temperatures up to 2500 K. A schematic diagram of the CMHR is shown in Figure 15.1. The graphite core reactor tube is 2 inches in diameter and 60 inches long. The central test section of the reactor has two 24-inch long
Gas preparation chamber
Ceramic honeycomb flow straightener
Gas preparation chamber
Ceramic honeycomb flow straightener
Graphite reactor tube core
Water cooled sampling probe
Figure 15.1. A schematic diagram of controlled mixing history reactor (CMHR).
x |-inch wide diametrically opposite windows, which provide the desired optical access to the test section in the reactor. In addition it has two viewing ports of 1.25 inches in diameter located near the end of the test section. The reactedness of the graphite core under reducing conditions is negligible while that under oxidative conditions is less than 1% at the conditions examined. The reactor is heated with a nominal 40-kW nontransferred arc plasma torch. The desired gas temperature and composition within the reactor is achieved by diluting the high-temperature plasma gas with some dilution gas. The dilution gases used include argon, helium, hydrogen, nitrogen, air, carbon dioxide, or any mixtures of these depending on the type of environment (inert, oxidizing, or reducing) to be used. The temperature and chemical composition of gas in the test section of the reactor is therefore controlled. Particles of surrogate solid waste material are allowed to fall in the downward direction in the reactor. A transpiring wall water-cooled sampling probe, inserted from the bottom of the reactor, intercepts the particle stream after a desired residence time of particles in the high-temperature zone. Gaseous and particulate products are isokinetically sampled at various axial positions in the reactor. The reactor is allowed to move in a vertical direction relative to the fixed position of the sampling probe. The residence time of the particles in the high-temperature zone of the reactor is therefore controlled. The reactor has the capability to examine a wide range of materials and particles sizes exposed to different residence times, chemical environments, and temperatures.
A hot gas preparation chamber located at the top of the reactor allows for a thorough mixing of the plasma gases with inert gases to produce a uniform temperature and composition of the carrier gas. Varying the amount of gas (such as, N2, Ar, O2) to the gas preparation chamber provides control of gas composition inside the reactor. A water-cooled feed probe is used to feed the surrogate solid waste into the reaction chamber. This is located at the centerline of the hot gas preparation chamber. The waste particles are therefore kept isolated from the high-temperature environment until they are exposed in the test section for their thermal destruction. The particle feeder is essentially of a fluidized-bed type. The solid waste particles in the bed are elutriated with the carrier gas (argon) and flow through the bed. The design enables good control over the particle flow rate.
The transpiring wall water-cooled sampling probe design allows one to collect the gases and solid residue material under isokinetic conditions after the material has been exposed to high temperatures for a prescribed residence time. The solid products enter the probe where they are further quenched with argon gas flowing radially inward and then downward through the probe. This freezes the chemical composition of the incoming material instantly.
A cascade impactor is attached to the sampling probe exit to collect the solid residue for subsequent analysis. This provides information on weight loss as a function of residence time of the solid material in the high-temperature zone. The cascade impactor has six stages that collect solid material according to bin sizes down to 0.2 ^m. The cascade impactor therefore separates out the particles into a number of size bins. The separation of the particles is accomplished by passing the collected gases (containing the particles) through orifices of successively smaller size diameter. Larger particles are inertially collected on the first collection plate while the smallest size on the last collection stage. The residue gases are pumped using EPA method 23 sampling apparatus and then analyzed using on line gas analyzers and a gas chromatograph.
The thermal destruction behavior of both cellulose and surrogate solid waste has been examined. The surrogate waste stream represents conditions wherein 90% of the food waste has been removed from the waste stream for pulping prior to thermal processing. The chemical constituency of the waste stream, although somewhat simplified, is quite realistic. The food waste entering the thermal destruction facility represents nonpulpable items such as corn cobs, bones, and food residue contaminated with metal, glass, and paper fraction of the waste. The steel component is mostly tin cans, and this has been simplified to pure iron. This simplification, although unacceptable for the slag chemistry, is reasonable for examining the thermal destruction behavior of the wastes. The glass fraction of the waste has also been simplified. The small amount of aluminum in the waste has also been omitted. The alkali content (Na and K) was generalized to Na2O while the alkali earth content (Ca and Mg) was generalized to CaO.
After the bulk of the food waste (90%) has been removed from the given solid waste, the waste material will have the following composition: 6.75% food material (30% bone representing 75% hydroxyapatite and 25% organics, and 70% food representing 50% organics and 50% water), 62.22% paper (consisting
Table 15.5 Elemental Composition of Surrogate Solid Waste (SSW) and Its Excursions
Without SSW Paper
Without Without Without Steel Aluminum Glass
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