Simulation model for entrained bed gasification
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Contrary to impressions that might be gained from the intense interest now expressed in it, coal gasification is not a new or even a particularly novel concept. The technical principles underlying it were well understood before the middle of the Nineteenth Century. Some industrial gasification processes were in fact, operated as early as 1860. Several different types of reactors are used in coal gasification technology. For this work we consider entrained-bed gasification in which the coal is injected with the aid of a carrier gas into a hot flowing gaseous environment. The particles are entrained in the gas flow and heated up and react rapidly. Currently, available technology includes the Kopper-Totzek gasifier. The K-T unit operates at atmospheric pressure, generates no tars, and has a yield substantially of methane-free synthesis gas that typically contains 30-32 percent hydrogen and 55 percent carbon monoxide. The basic objective of this project was to obtain and test a reliable entrained bed gasification simulation model for use in a larger process simulation. Therefore, the model developed at the Department of Energy in Morgantown, West Virginia, by Larry A. Bissett for entrainment gasification, was obtained [Bissett, 1978]. They developed the model in order to assess the performance of entrained gasifiers. The model is based on energy and mass balances with the assumption of constant equilibrium temperature for water-gas shift reaction. For material balance purposes, the elements carbon, hydrogen, oxygen, nitrogen and sulfur, and the compounds carbon monoxide, carbon dioxide, steam, hydrogen sulfide, and methane were considered (no methane is formed in K-T gasifiers). For energy balance, the sensible heat of all streams, including the ash, and the combustion heat of the appropriate species were considered. Input .parameters were based on 100 pounds of coal, and including compositions, flows, and temperatures of entering streams. Since the model did not intrinsically calculate carbon conversions or heat losses, these parameters along with an assumed equilibrium temperature for water- gas shift reaction also had to be supplied. A computer program which has been developed for the model is modified in order to be applicable for K-T gasifers. The reaction temperature obtained by iteration using the Secant method, until the energy balance was satisfied. The Bissett model has weaknesses such as the need to specify parameters of heat losses and carbon conversion. But it can calculate the operating temperature and therefore gives the conditions which are necessary for production of medium BTU gases. In the K-T unit, the reaction temperature is in the range of 3300°F to 3500°F [Berkowitz, 1979]. To achieve such a high temperature, the flow of oxygen to the reactor should be high enough. Since the coal under consideration has a high percentage of moisture, no steam is fed to the reactor. Therefore, the only factor to control temperature is the amount of oxygen.
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