Entrained Particle Reactors

Conditions similar to industrial entrained-flow gasifiers can be simulated in entrained particle reactors. The experimental units conform to the practical units in terms of heating rate (10 -10 K/s), temperature ( 1600 °C), pressure (1-70 bar), particle size ( 200 pm), and residence time ( Ss). Such reactors have a low feed rate (1-10 kg/h) and require external heating. Hence, they are not pilot-scale gasifiers with a self-sustaining process [68-72]. 

The operation consists of the injection of coal particles into a hot reactant gas flow and sampling of solid and gaseous products at several positions downstream from the injection point. The level of burnout must be estimated from ash tracing or product gas concentration. Char temperature and residence time also rely on estimates. Furthermore, the operating conditions are linked to mass-transfer and pore-diffusion limitations, which make it difficult to accomplish accurate Idnetic measurements [59]. 

Such apparatuses require considerable investment and operating costs. Also the interpretation of the results is elaborate and therefore subject to fundamental academic research. The obtained results have so far helped to improve models for entrained-flow gasifiers [72-74], but a broader database still needs to be developed for more general conclusions of industrial relevance. 

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In a fluidized bed reactor, entrained particles leaving in a dilute phase stream are conventionally and desirably either partially or wholly condensed into a bulk stream and returned to the bed via a centrifugally driven cyclone system. At equilibrium, or when steady state operation is attained, any particle loss rate from the cyclones, as well as the remaining bed particle size distribution, are functions of (a) the rate of any particle attrition within the system and (b) the smallest particle size that the cyclone system was designed to completely collect (i.e., with 100% efficiency), or conversely the largest size which the system cannot recover. These two functions result in an interdependency between loss rate and bed particle size distribution, eventually leading to an equilibrium state (Zenz Smith, 1972 Zenz, 1981 Zenz Kelleher, 1980). 

When a chemical reaction occurs in the system, each of these types of behavior gives rise to a corresponding type of reactor. These range from a fixed-bed reactor (Chapter 21-not a moving-particle reactor), to a fluidized-bed reactor without significant carryover of solid particles, to a fast-fluidized-bed reactor with significant carryover of particles, and ultimately a pneumatic-transport or transport-riser reactor in which solid particles are completely entrained in the rising fluid. The reactors are usually operated commercially with continuous flow of both fluid and solid phases. Kunii and Levenspiel (1991, Chapter 2) illustrate many industrial applications of fluidized beds. 

In a suspended bed or entrained flow reactor technology, the coal is crushed, dried, and then pulverized to fine powder in a crusher and mill. As Table 9.1 shows, the coal particles used in entrained flow reactors are very small. The pulverized coal is transported with air to the furnace (primary air), and secondary air is heated and fed into the combustor to ensure complete combustion. The residence time of the coal in the furnace is typically around 1-2 s, which usually suffices for complete combustion. However, not all coal burns completely, and fly ash will be generated (see Table 9.1). 

The Model. The physical system considered is an entrained bed reactor. Pulverized coal, carried by a gas stream, mixes with hot gas at the reactor entrance. As coal particles are carried their temperatures increase and devolatilization takes place. For practical purposes the particle size distribution was approximated by 10 discrete cuts. Since the devolatilization kinetics are highly temperature dependent and the temperature transient of a particle is affected by its size and mass separate account must be taken of each of the 15 reactions in each of the 10 different size particles. Without any detail on the derivation of the model, the equations can be summarized as ... 

The synthesis gas leaving the gasifier contains entrained particles of char and ash. Particulate removal is performed using cyclone separators and ceramic candle type hot gas filters. The coal gas is primarily comprised of H2, CO, C02, and H20. Since there is less than 0.1 mol% CH, reforming of the syngas is not necessary. However, in order to maximize hydrogen production, shift reactors will be needed to convert the carbon monoxide to hydrogen. 

The mass flow rate Gs of entrained solids per unit area leaving the fluidized-bed reactor is the sum of contributions from the entrainable particle size fractions (ut < u)... 

In order to achieve rapid heating of the biomass, it should be finely ground. The particle size of the biomass is dependent on the reactor type used. In particular, it was reported [17] that ablative reactors can pyrolyze large-size feedstocks, whereas the fluidized beds require smaller (below 3 mm) particles [16]. The difference is due to lower heat transfer through conduction in the fluidized beds, which is even lower in the entrained flow reactor, being in the range of 4% [17],... 

Existing entrained-flow reactors with solid particle flow can be retrofitted with monoliths in which the channel structure also works as a flow straightener, providing better plug-flow characteristics in large-diameter entrained-flow reactors, which suffer from backmixing of catalyst at the reactor wall (53-55). 

The entrained flow reactor utilized in this study is virtually the same unit described by Scaroni et al. ( ). In order to improve knowledge about, and control of, the time-temperature history of the coal particles, modifications have been made to the reactor and to the predictions of the time-temperature history of particles in the reactor (9,10). 

A schematic diagram of the entrained flow reactor is shown in Figure 1. At the top of the reactor, a screw feeder and semi-venturi system is used to entrain the ground coal particles in the cold primary gas stream. The coal is then injected into the reactor where it is entrained in, and heated by, the preheated secondary gas. The pyrolyzing coal particles fall in a thin stream through the reactor and are collected by a movable water-cooled collector probe. The time which the particles spend in the reactor is controlled by moving the collector probe up and down the reactor axis. The pyrolysis reactions are rapidly quenched in the collector probe, and the particles are separated from the gas stream by a cyclone in the collection system. 

In order to determine the total pyrolysis yield, another technique was used. Basically, the approach involves capturing the py-rolyzing coal particles in a crucible at the maximum residence distance in the reactor. The captured samples are then held in the reactor for 10 minutes to complete pyrolysis. It is thought that this technique gives a reasonable estimation of the total pyrolysis yield possible in an entrained flow reactor. The results of this study can be seen in Table V in which weight loss values for extended... 

The entrained flow reactor is usually used for biomass, but can be used for mixed plastics. A stoichiometric air-propane burner produces the inert hot flue gas that flows upwards through the vertical reactor and entrains the particles while heating. The Georgia Institute of Technology has developed an entrained flow reactor for biomass (6.4 m height. 

Within the context of a study aimed at the determination of the pyrolysis kinetics of polymers using a laminar entrained flow reactor (LEFR), Westerhout et al (1996) developed a comprehensive model that couples a single-particle con-... 

In 1979 Chem Systems initiated a program to develop a liquid-entrained catalyst reactor which would provide improved contacting of syngas with the catalyst in a three phase system (ref. 38). This reactor system uses much finer catalyst particles than the fluidized bed reactor, and the catalyst-liquid slurry circulates through the reactor. The syngas can be contacted with the catalyst-liquid slurry either counter currently or co-currently. It appears that this process is more efficient than the original fluidized bed process. However, a major problem with this type of three phase system will no doubt be the development of a suitable catalyst since it is unlikely that conventional co-precipitated Cu-ZnO-A Oj catalysts will have the desired characteristics, particularly mechanical strength. 

This is a well-established technology, and has been replaced today by the fluidization of fine particles (grains of sand, carbonization products, ash) using a controlled flow rate gas stream. As for entrained-bed reactors, they have not yet been employed to treat lignoceilulose wastes. 

In the entrained-flow reactor (Figure 18.11), the solid particles travel with the reacting fluid through the reactor. Such a reactor has also been described as a dilute or lean-phase fluidized bed with pneumatic transport of solids. 

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