Mass pulse combustion

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

The term pulse combustion originates from intermittent (pulse) combustion of solid, liquid, or gaseous fuel in contrast to continuous combustion in conventional burners. Such periodic combustion generates intensive pressure, velocity, and to a certain extent, temperature waves propagated from the combustion chamber via a tailpipe (a diffuser) to the process volume (an applicator) such as a dryer, calciner, or incinerator. Because of the oscillatory nature of the momentum transfer, pulse combustion intensifies the rates of heat and mass transfer. 

Since transport phenomena in the pulse combustion chamber are beyond the scope of this book, the following sections will be restricted to momentum, and to heat and mass transfer between the combustion gases discharged through the tailpipe and the particles of a drying material or droplets of atomized liquid. 

Wu and Mujumdar (2006a) performed an extensive theoretical analysis of the effects of gas temperature, pulse frequency, amplitude and gas mass flow rate on the transient flow patterns, droplet trajectories and overall pulse combustion... 

Mujumdar, A. S., Wu, Z. H., 2004. Pulse combustion spray drying, in Topics in heat and mass transfer, (eds G. H. Chen, S. Devahastin, B. N. Thorat), IWSID-2004, Mumbai, India, pp. 79-91. 

Nomura, T., Nishimura, N., Hyodo, T., 1989. Heat and mass transfer characteristics of pulse-combustion drying process, in Drying 92, (ed. A. S. Mujumdar), Elsevier, pp. 51-57. 

Time-Resolved Laser-Induced Incandescence (by Prof. Alfred Leipertz et al.) introduces an online characterization technique (time-resolved laser-induced incandescence, TIRE-LII) for nano-scaled particles, including measurements of particle size and size distribution, particle mass concentration and specific surface area, with emphasis on carbonaceous particles. Measurements are based on the time-resolved thermal radiation signals from nanoparticles after they have been heated by high-energetic laser pulse up to incandescence or sublimation. The technique has been applied in in situ monitoring soot formation and oxidation in combustion, diesel raw exhaust, carbon black formation, and in metal and metal oxide process control. 

Fig. 11. Experimental setup for the in situ detection of chemisorbed CO during catalytic combustion of CO on Pt using optical infrared-visible sum frequency generation (SFG) and mass spectrometry. A mode-locked Nd YAG laser system is used to provide the visible laser beam (second harmonic 532 nm) and to pump an optical parametric system to generate infrared radiation (wir) tunable with a pulse duration of 25 ps. MC monochromator, PMT Photomultiplier, AES Auger Electron Spectrometer, LEED Low Energy Electron Diffraction Spectrometer, QMS Quadrupole Mass Spectrometers for CO Thermal Desorption (TD) and CO2 production rate measurements.

Coals were devolatilized at rates comparable with those encountered in combustion and gasification processes. Rapid pyrolysis was attained with pulse-heating equipment developed for this purpose. This technique permitted control of the heating time and the final temperature of the coal samples. Subbituminous A to low volatile bituminous coals were studied. All bituminous coals exhibited devolatilization curves which were characteristically similar, but devolatilization curves of subbituminous A coal differed markedly. The products of devolatilization were gases, condensable material or tar, and residual char. Mass spectrometric analysis showed the gas to consist principally of H2, CHh, and CO. Higher hydrocarbons, up to C6, were present in small quantities. 

Data generated using the experimental techniques described above are used to formulate hydrodynamic models that may be used to predict reactor performance. In this section, studies that employ chemical reactions to evaluate mass transfer and contacting efficiency are described. Selected references are shown in Table 7. Dry et al. have applied hot air pulses as a reacting tracer [87]. Chemical reactions used to probe gas phase hydrodynamics include thermal decomposition of sodium bicarbonate, ozone decomposition, coal combustion, and FCC coke combustion. 

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