Kinetic gasification reactor

These equations have further been coupled to the kinetics and transport relationships associated with the heterogeneous and homogeneous reactions of coal gasification. This coupled system of equations provides the theoretical basis of our computer model of coal gasification reactors. 

Research described in this paper focuses on the second step of the gasification process, and details the effects of temperature and residence time on product gas formation. Cellulose is used as a feedstock for pyrolytic volatiles formation. Earlier papers (JS.M) have discussed the effect of steam on cellulose pyrolysis kinetics. Two recent papers (1, 1 6) presented early results on pelletized red alder wood pyrolysis/gasification in steam. Future papers will discuss results using other woody materials, crop residues, and manures (17,1 ). Research to date indicates that all biomass materials produce qualitatively similar results in the gasification reactor described in the following section of this paper. Effects of pressure on the heat of pyrolysis of cellulose are also discussed as a prelude to future papers detailing the more general effects of pressure on reaction rates and product slates. 

Hla, S.S., Harris, D.J., and Roberts, D. (2006) CFD modelling for an entrained flow gasification reactor using measured intrinsic kinetic data. Proceedings of the Fifth International Conference in CFD in the Process Industries. 

Steam gasification and combustion kinetics of gingko nut shell in a Thermobalance Reactor... 

Source Adapted from various sources M. A. Elliot, Ed., Chemistry of Coal Utilization, 2nd Supplementary Volume, Wiley-Interscience, 1981 Cheremis-inoff, N, and P. Cheremisinoff, Hydrodynamics of Gas-Solid Fluidization, Gulf Publishing Co., 1984 Joaquin, R. H., Kinetic and Hydrodynamic Study of Waste Wood Pyrolysis for its Gasification in Fluidized Bed Reactor, European Foundation for Power Engineering, 2002 (http //www.efpe.org/papers.html). 

A set of 17 reactions was written to simulate the reactor events and they included 1 reaction for drying, 8 parallel reactions for devolatilization, 5 reactions for gasification and 3 reactions for combustion. The kinetic and thermodynamic parameters for these reactions were derived for a Wyoming subbituminous coal. 

Gasification. The only study on gasification kinetics of Texas lignite has been performed by Bass (23). Using a differential reactor, he obtained rate data at 700°C and for pressures ranging from 61.6 to 225.9 kPa. Rate equations as a function of steam partial pressure and carbon conversion were developed. 

In the 1970s Sawagushi et al. [21] presented resnlts and validated a kinetic approach to the intensity function in the case of steam gasification of PE in a fixed-bed reactor. These results are presented in Fignre 10.4. 

Many researchers have tried to correlate the kinetic rates of carbon gasification to the many physical and chemical properties of the different materials used. Despite of this a universal rate expression does not exist. Furthermore it is difficult to find data operational for reactor simulations in the relevant temperature and partial pressure ranges. In particular the variation of the reactivity with conversion due to structural variations is not dealt with, i.e. the structural profile is seldom explicitly given. 

Heterogeneous kinetics of straw pyrolysis and straw gasification are essentia data for reactor design. Pyrolysis is a relatively fast process. In view of the poor heat conduction of straw and straw char, the pyrolysis time is the time which is required to heat the center of the particle to the decomposition temperature. This is a rather simplified model, but allows a reasonable time estimate in view of the order of magnitude. 

A comprehensive experimental research program to investigate the effects of pressure on the products of steam gasification of biomass is currently underway. A stainless steel, tubular microreactor similar to the quartz reactor described earlier has been fabricated for the experimental work. The pyrolysis furnace used with the quartz reactor system has been replaced in the pressurized steam system by a Setaram Differential Scanning Calorimeter (DSC). The DSC provides for quantitative determination of the effects of pressure on pyrolysis kinetics and heats of reaction. 

All catalyst formulations, in fact, lead to the unselective formation of deep oxidation products and of syngas (H2 and CO). But cracking processes (here represented by simple dehydrogenation) may also occur at high temperature in the empty volumes of the reactor, with the formation of ethylene, methane, C4+ species, but also C (which is an issue when using a Pd-based catalyst, for instance). C gasification, as well as water-gas shift, CO and H2 post combustions, are usually also involved in the process surface kinetics. 

There are in general several steps of refinement to model a gasification system. Zero-dimensional models show the lowest complexity, and rely on empirical correlations or thermodynamic equilibrium calculations. The next step is a onedimensional model that usually requires kinetic expressions either to resolve the space or time coordinate using idealized chemical reactor models. Approaching two- or three-dimensional calculations provokes the use of computational fluid dynamics (CFD) that may incorporate either equiUbriiun or kinetics-based turbulence chemistry interactions. Each step of modeling adds significant complexity and calculation time. 

As it is nearly impossible to make any general kinetic description of the gasification processes as a practical design basis, constrained equilibrium calculations offer a useful tool for comparative studies [51]. If gasification reaction rates are considered as a rate of approach to chemical equilibrium, increasing residence times lead to gasification products near equilibrium [52]. This is especially true for fluidized-bed and entrained-flow processes. Because the equilibrium state does not depend on the path used to achieve it, a process simulator such as Aspen Plus [49] can use a hypothetical reactor to decompose coal into its elements. The subsequent equilibrium calculation can be carried out, including other feed streams. 

A gasification reaction is composed of various kinds of chemical processes such as pyrolysis of coal, decomposition of tar, oxidation of char, combustion of gas, shift reaction, and formation of various organic compoimds. In order to elucidate the reaction process, the method to delve into the composition of gas for information on the reaction state in the reactor needs to be established. As has been mentioned, kinetic and equilibrium theories are not available for this purpose. 

---