Novel Staged Free-Fall Reactor for the (Catalytic) Pyrolysis of Lignocellulosic Biomass and Waste Plastics

Abstract

Pyrolysis of lignocellulosic biomass and waste plastics has been intensely studied in the last few decades to obtain renewable fuels and chemicals. Various pyrolysis devices have been developed for use in a laboratory setting, operated either in batch or continuously at scales ranging from milligrams per hour to tenths of g per hour. We report here the design and operation of a novel staged free-fall (catalytic) pyrolysis unit and demonstrate that the concept works very well for the (catalytic) pyrolysis of pinewood sawdust, paper sludge, and polypropylene as representative feeds. The unit consists of a vertical tube with a pretreatment section, a pyrolysis section, a solid residue collection section, a gas-liquid separation/collection section, and a catalytic reaction section to optionally perform ex situ catalytic upgrading of the pyrolysis vapor. The sample is placed in a tube, which is transported by gravity through various sections of the unit. It allows for rapid testing with semicontinuous feeding (e.g., 50 g h^-1) and the opportunity to perform reactions under an (inert) gas (e.g., N₂) at atmospheric as well as elevated pressure (e.g., 50 bar). Liquid yields for noncatalytic sawdust pyrolysis at optimized conditions (475 °C and atmospheric pressure) were 63 wt % on biomass intake. A lower yield of 51 wt % (on a biomass basis) was obtained for the noncatalytic pyrolysis of paper sludge, likely due to the presence of minerals (e.g., CaCO₃) in the feed. The possibility of using the unit for ex situ catalytic pyrolysis (pyrolysis at 475 °C and catalytic upgrading at 550 °C) was also successfully demonstrated using paper sludge as the feed and H-ZSM-5 as the catalyst (21 wt % catalyst on biomass). This resulted in a biphasic liquid product with 25.6 wt % of an aqueous phase and 11 wt % of an oil phase. The yield of benzene, toluene, and xylenes was 1.9 wt % (on a biomass basis). Finally, the concept was also proven for a representative polyolefin (polypropylene), both noncatalytic as well as in situ catalytic pyrolysis using H-ZSM-5 as the catalyst at 500 °C. The liquid yield of thermal, noncatalytic plastic pyrolysis was as high as 77 wt % on plastic intake, while in situ catalytic pyrolysis gave a combined 7.8 wt % yield of benzene, toluene, and xylenes on plastic intake.

Figure 1

Schematic representation of the staged free-fall reactor.

Figure 2

Product yields (wt % on biomass intake) of pyrolysis of sawdust at different pyrolysis temperatures.

Figure 3

Comparison of the product yields obtained in the study (a) with representative examples reported in the literature (b, c, and d) for woody biomass in fluidized bed reactors. (b) Reproduced with permission from ref (43). Copyright 2005, American Chemical Society. (c) Reproduced with permission from ref (44). Copyright 2008, American Chemical Society. (d) Reproduced with permission from ref (34). Copyright 2010, Elsevier.

Figure 4

Elemental composition (dry basis) of the pyrolysis liquids obtained at different temperatures.

Figure 5

Temperature effect on the molecular weight distribution of the liquid phase for the pyrolysis of sawdust.

Figure 6

Mass balances for the thermal and in situ catalytic pyrolysis of PP (wt % on PP intake).

Figure 7

BTX yields (on PP intake) for thermal and in situ catalytic pyrolysis of PP using H-ZSM-5(23) (GC-FID with BTX calibration).