Development of a Bench-Scale High-Pressure Fluidized Bed Reactor and Its Sequential Modification for Studying Diverse Aspects of Pyrolysis and Gasification of Coal and Biomass

Abstract

This paper reviews the design, operation, and sequential modifications of a bench-scale high-pressure fluidized bed reactor system, capable of diverse pyrolysis and gasification applications, all based on the same hardware platform. The reactor is small: 34 mm i.d. and 504 mm high. The mechanical design is relatively simple and inexpensive, featuring direct electrical heating to avoid the use of a separate furnace. The reactor body is made of a high-strength alloy, with 1000 h creep resistance at maximum design conditions (1000 °C and 3.0 MPa). The design thus obviates the use of a “cold” pressure casing. The system can be operated by a single person and has demonstrated ability to generate fuel reactivity and product distribution data rapidly and cheaply, using a wide range of solid fuels. In batch operation, conversions achieved during coal pyrolysis and gasification experiments were comparable with results from a high-pressure wire-mesh reactor, indicating nearly single-particle behavior. The system has been modified for continuous feeding (∼3 g min −1), to study the effect of reaction conditions on the fate of fuel-N, during the gasification of coal and sewage sludge (to 3.0 MPa). The work showed that steam plays a primary role in the formation of NH 3 from both volatile-N and char-N. HCN concentrations were found to depend strongly on the residence time at temperature and on the extent of contact with heated bed solids. A quartz lining allowed the determination of extents of trace element emissions during gasification. Above 900 °C, enhanced depletion of Ba, Pb, and Zn in bed solids was accompanied by enrichment of fines, collected in a downstream filter. The system was subsequently modified to enable a slug of coal to be injected and bed solids discharged (1−3.0 MPa) from the fluidized bed under controlled conditions and precise solid residence times in the bed. The reactivities of the chars were measured as a function of “residence time at temperature” and were observed to decrease with increasing temperature, time, pressure, and particle size. At 1000 °C, coal char reactivities were found to diminish by nearly a factor of 4 within 10 s. The study confirmed and extended prior work carried out in a high-pressure wire-mesh reactor. The system was subsequently configured for pyrolyzing waste plastic material at low temperatures. Altering the electrode positions enabled changing the position of the heated zone, enabling trouble-free injection of polymer samples at temperatures up to 600 °C. The aim of these experiments was to investigate the potential to produce liquid fuel precursors from pure and mixed waste plastics. In its latest incarnation the system has been further modified for performing oxy-fuel gasification experiments.