Iiiiiiiiii

500 700 900 1100 1300 1500

Temperature (K)

Figure 15.3. Experimental results of solid residue produced during cellulose pyrolysis.

Particle size and density are two important factors that affect pyrolysis since they determine the particle velocity and residence time in the CMHR. The effect of particle size and density was studied here using flow visualization. The flow visualization tests were performed on cellulose and surrogate solid waste particles as they travel downstream in the reaction zone of the CMHR. These tests provided the direct effect of temperature on residence time of particles in the reactor. Particle imaging was performed in the reactor with a charge coupled device (CCD) camera and frame grabber using Global Lab Image software and a computer. Particle images were taken under both nonburning and burning conditions. The flow inside the reactor was laminar. Near single particles were introduced into the furnace so that the equivalence ratio was low (fuel-lean). Sample images are shown in Figure 15.4. These images allow one to analyze the evolutionary behavior of solid particles, both temporally and spatially, as they travel through the test section of the reactor. The effect of particle size and density on the particle velocity, residence time, and transport behavior can be extracted from these images. It is found that, in general, larger cellulose particles move faster in the reactor and therefore have a shorter residence time as compared to the smaller size particles. The images also show a decrease in velocity as the particles travel downstream, in particular under the high-temperature environment of the reactor. As an example, at a temperature of 1286 K, the velocity of a 440-^m cellulose particle changes from 0.46 to 0.39 m/s within the viewing area of the camera. However, the velocity of a 400-^m particle is almost constant in the same viewing area at low ambient temperatures. Some scatter in the data (e.g., change in diameter for a given temperature or velocity for a given size) is attributed to the

Figure 15.4. Sample images of cellulose particles in the CMHR at different temperatures.

nonspherical nature of the particle. The solid particles are nonspherical so that the recorded image plane of the particle does not provide the true information on the particle size. Furthermore there may be some variation in particle density that will then affect the particle velocity. Significant differences are expected to exist between the particle and the surrounding gas that will alter the particle kinetics. In this study no attempt was made to determine the particle temperature. This effect is important for the kinetics and should be examined in the future.

The effect of moisture contained in the cellulose sample on gas product generation during pyrolysis was examined. A sample containing 75% cellulose and 25% water by weight was pyrolyzed in the CMHR. The results showed a decrease in CO production and an increase in CO2 and unburned hydrocarbons as seen from the results presented in Figure 15.5. This can be explained by the following water-gas shift reaction that enhances at elevated temperatures (above 1000 K):

This reaction affects the formation of CO and CO2 during pyrolysis since the carbon monoxide reacts with water to form hydrogen and carbon dioxide. The influence of intermediate radical species (such as OH) can have an influence on the above reaction. This influence as well as the influence of other species was not considered due to the complex interaction of various intermediate species. At high temperatures production of hydrogen occurs. This leads to new possibilities

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