Circulating fluidized beds pyrolysis

Reports on commercial implementation of pyrolysis are scarce, while there are several pilot-size units. For example, Dynamotive in Canada has built four fluidized-bed units with a total capacity of 200 tons of dry wood per day [14], Ensyn, also in Canada, has eight circulating fluidized-bed units with a total capacity of 100 dry tons per day [14], Biomass Technology Group from the Netherlands has designed rotating cone pyrolyzers with a total capacity of 2 tons of dry material per hour [14],... 

RCP [Recycled Clean Products] A high-temperature process for recycling sold wastes. It combines pyrolysis, smelting, and slag refining with postcombustion in a circulating, fluidized bed. Developed by Von Roll, Switzerland, and operated commercially in Germany since 1997. 

Kaczmarzyk, Grzegorz and Sciazko, Marek. Coal pyrolysis in a circulating fluidized-bed reactor for hybrid gas-steam power plants, Koks, Smola, Gas, vol. 35, No. 8, pp. 175-84 (1990). 

Flnidized-bed processes are either bnbbling or internally circulating. The fluidized-bed reactor is very versatile for the pyrolysis of polyolefins. Nevertheless one of the problems with fluidized-bed pyrolysis of post-consumer plastics relates to the stickiness of the sand particles (the fluidization medium) that becomes coated with fused plastic. In order to solve these problems, new reactors have been proposed, snch as the conical spouted bed, the conical rotary reactor, a sphere circnlation reactor and a reactor with mechanical particle stirring. 

Akzo Nobel is an important producer of chlorine, vinyl chloride and PVC, strongly interested in a process for feedstock recycling of MPW containing PVC. Since 1994 they have chosen a fast pyrolysis process in a circulating fluidized-bed reactor, based on the technique developed by the Battelle Memorial Institute (CO, USA) for biomass gasification. 

Milne et al. [42] used an internally circulating fluidized bed to study pyrolysis of low-density PE (LDPE) at temperatures ranging from 780 to 860° C and residence times of 400-600 ms. They achieved gas yields in excess of 90 wt% with total aUcene yields as high as 75 wt%, demonstrating a waste treatment potential for processes snch as flash pyrolysis, distinguished from classical pyrolysis by its high heating rates and low retention times, where these conditions can be maintained. 

The solid circulation rate is one of the basic operating parameters of a circulating fluidized bed since it influences not only the temperature difference between the different sections of the reactor but also the residence times of particles and therefore reaction and conversiem times. For the gasification of biomass for example high temperatures are required in the gasifier for the tar pyrolysis as well as for the efficiency of the catalysts used in the pilot plant. 

Hastaoglu M. A. Hassam M. S. (1995) Application of a general gas-solid reaction model to flash pyrolysis of wood in a circulating fluidized bed. Fuel, 74, 697-703. 

Bio-oils were produced m 1995-1998 from various wood species on fast pyrolysis pilot plants, which differed by their construction features NREL (ablative pyrolysis with hot gas filter, poplar wood, 1996), Aston FB (fluidized bed, poplar wood, 1995), IWC, TP 29 (fluidized bed, bamboo, beginning of 1998) BTG (rotating cone, pine wood, 1995), VTT (circulating fluidized bed, pine wood, 1997), IWC, E-filter (electrostatic precipitator, beech wood, end of 1998) and IWC Kl-FIO from different coolers (beech wood, end of 1998). 

Biomass Fast Pyrolysis in an Air-blown Circulating Fluidized Bed Reactor... 

Next, methane and carbon monoxide can be further converted into hydrogen by means of steam reforming or the water-gas shift reaction. The most important pyrolysis reactor types are ablative, fluidized bed, circulating fluidized bed and entrained flow reactors. Hydrogen yields can vary substantially with biomass type, facility size and process conditions... 

Figure 6. Circulating fluidized bed waste pyrolysis system [22] B. C. Crocker, R. C. Bailie, Incinerators, in Kirk-Othmer Encyclopedia of Chemical Technology, JohnWiley Sons, Inc. Reprinted by permission of John Wiley Sons Inc. All Rights Reserved.

The reactor is the core and is generally the most researched part of the pyrolysis technology. Extensive literature is available for catalytic pyrolysis that has been carried out at both bench/laboratory scale (ie, bubbling and circulating fluidized beds, auger reactors, and conical spouted bed reactors) and analytical scale reactors (ie, analytical pyrolysis or py-GC/MS either tubular quartz micro reactor or packed bed reactor). Specific reactor designs are not discussed in this work. Catalytic fast pyrolysis can be split into two different operation modes defined by the location of the catalyst in the process in situ and ex situ (Tan et al., 2013) (Fig. 14.2). 

Increase in thermal process temperature up to 685-715°C in a fluidized-bed system (Hamburg University Pyrolysis Process-HUPP) and application of a mixture of municipal plastic wastes resulted mainly in gaseous products, over 41 wt%, of which olefins constituted 15%, and aromatic (BTX)-containing liquid products [14], Considerably better results from the point of view of C2 and C3 olefins yield were obtained in other experiments. The application of steam as fluidization agent instead of circulation pyrolysis gas enabled an increase of C2-C3 olefins yield from 48 to 60%, accompanied by decrease in BTX yield from 24 to 11 wt% [15],... 

To recover a maximum of olefins and butadiene from recycling polyolefins, it is necessary to have a short residence time of the product gases in the fluidized bed zone to avoid no secondary reactions. The pyrolysis gas should not be circulated and used as fluidizing gas. For the experiments, steam was used as fluidizing gas [13, 14]. An easy separation of the hydrocarbon products is possible by condensation to water in a cooler. The results are shown in Table 17.6. As feedstock a light plastic fraction from household waste separation was used which contains 95.8% of PE and PP, 3% of PS, and 0.2% of PVC. 

In the two-bed pyrolysis system , fluidized fine solids circulate between a reactor to pyrolyze municipal refuse and an incinerator to burn up the produced char. 

Figure 1 indicates the schematic flow of two-bed pyrolysis plant. This plant consists of the pyrolysis reactor and the regenerator through which fluidized medium circulates, the precombustion burner, the raw material feeder, the cyclones, the heat exchangers, the vapor-liquid separator, the gas recycle blower, the air feeder, the gas cleaner and the produced-gas combustion device. The plant also includes various auxiliary equipment, the devices for measuring and recording flow rate, pressure and temperature, as well as the automatic analysis for gas. 

The plant was operated between 650°-750°C in the pyrolysis temperature under stable controls of pressure, temperature and sand circulation. Since the fluidized bed has an effect of heat accumulation, it is easy to control the operating temperature, though the heterogeneous refuse is fed to the bed. Consequently, no damage of reactor materials due to excessive temperature were recognized. 

In a dense-bed fluidized bed the preheated shale is further heated to and held at the retorting temperature for sufficient time to complete the pyrolysis reactions (Figure 3). The total inventory of shale in the retorting vessel is determined by the required residence time for complete kerogen conversion and the shale throughput. The retort heat requirements are supplied by ceramic balls which circulate in the inner loop. They are reheated in a separate vessel which may operate as a moving bed, raining pellet bed, or entrained flow heater. 

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