Residence time, coal particle

Fluidization and Fluid-Particle Systems Figure 2. Coal particle residence time vs. particle size. Hb at 1500 psi + 700°C. 

Figure 3 shows the effect of coal particle residence time on the fraction of carbon converted to liquid and gaseous products. Based on this correlation, the production of gaseous products appears to be less sensitive to coal residence time than the production of liquids. It also appears that the production of liquids is favored by shorter residence time. 

The feeder and injector produced a thin pencil-like p.c. stream which passed down through the hot zone. The total combustion air supplied was approximately 3 liters/min for the bituminous coals, giving between 10 and 25 percent excess air for p.c. feed rates of 0.24 to 0.28 g/min. The flow and heat transfer conditions were modeled using the methods described by Pigford (16) for conditions of superimposed natural and forced convection at very low mass flow rates. Particle residence times were calculated by summing the centerline gas velocity and terminal velocity using Stokes s law (17). The error introduced using this method should never have exceeded 10 percent, even when pyrite was tested and particle Reynold s numbers approached one. The residence times thus calculated were found to be between one and two seconds. 

The aim of the flash pyrolysis of coal is the production of smaller molecules from it in a shortest possible particle residence time. Therefore, the objective of studying the process chemistry of coal pyrolysis is to investigate the experimental parameters that permit this aim to be achieved and to establish the optimum conditions that produce a favorable product slate. The basic process parameters that influence the prodnct yields dnring flash pyrolysis of coal are (1) reaction temperature, (2) gas pressure, and (3) residence times of coal particles and ensuing tar vapors. In addition to these major process parameters, product yields can be influenced by other factors such as the nature of the pyrolysis gas and its partial pressure and the gas-to-coal ratio. 

In operating a fluidized bed reactor such as a fluidized bed coal gasifier, fine particles tend to be elutriated from the fluidized bed. The elutriated fines, if not recovered, represent a significant carbon loss and thus a significant loss of reactor efficiency. In actual industrial practice, the fines are recycled back to the fluidized bed for further consumption. The location of the fines reinjection point into the fluidized bed reactor is important in order to maximize the consumption of fines in each pass. Otherwise, the fines will build up in the recycle loop and increase the heat load of the reactor operation. The fines reinjection location is selected to maximize the fines residence time in the bed and to provide an conducive environment for consumption, such as high temperature and an oxidizing atmosphere. 

When coal or biomass is heated, many reactions including dehydration, cracking, isomerization, dehydrogenation, aromatization, and condensations take place. Products are water, carbon dioxide, hydrogen, other gases, oils, tars, and char. The product yields vary, depending on the particular feedstock composition, particle size, heating rate, solids and gas residence times, and the reactor temperature. 

The apparatus consisted of a flow reactor containing the coal and an oxygen absorber. A fixed bed reactor was used for studies on large coal particles, while a spout reactor was used for studies on small coal particles. Both reactors had a volume of about 400 cc. and were designed so that both wall and end effects were eliminated. Experimental residence time distributions indicated that the fixed bed reactor approximated a plug flow reactor, while the fluidized bed spout reactor had perfect mixing. 

The plasma jet provides a tool for rapidly heating coal and quenching the products. Its main disadvantages are that it is difficult to ascertain the temperatures and residence times to which the particles are subjected. 

Lasers and plasma provide means for investigating the rapid carbonization of coal at high temperatures. Owing to the short residence time of coal particles in a plasma jet, it is unlikely that thermodynamic equilibrium or even thermal equilibrium will be attained. Moreover the gaseous products will be heavily diluted by the carrier gas. Nevertheless the thermodynamics presented here provide a useful guide to the type of products which may be expected at various temperatures and their relative yields. 

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