Reactors for catalytic pyrolysis

There are many forms of reactors used for the study, analysis and large scale semi­commercial testing in catalytic combustion. A full description of these is beyond the scope of this paper and the reader is referred to a number of papers. Samolada et al. have presented a typical laboratory set-up for gram quantities of biomass in a fixed bed reactor and this is very typical for small scale studies.133 Numerous pilot plant size (kg type quantities) reactors have also been studied and these are largely centred on fluidised bed type systems.141 These pilot stage systems will have hoppers and grinders for feeding real biomass samples (as particle size is critical in determining thermal transfer efficiencies), risers (for the fluidised bed generation), pyrolysis reaction chambers and systems for recovery and recycle of the catalyst. On the larger scale there are many variations of the methodology which allow product optimisation to different molecular weights as well as different products. The methods developed for commercial use and study have been summarised by Meng et al.142 These methods include, for example, catalytic steam pyrolysis where the addition of water promotes steam reforming reactions within the overall pyrolysis process. Quick contact cracking essentially involves a recirculating fluidised bed fast pyrolysis technique combined with a simple cracking catalyst which allows catalyst to be circulated in the fluidised bed and coke at the catalyst removed by oxidation during the recycle. The current development of large scale industrial plant is summarised by Dominov et al.143 The results of a commercial trial of catalytic pyrolysis technology with emphasis on the design and construction of plant were reported by Xie and Wang.144

Despite the complexity of these technologies and individual reactor designs, simple representation of the main methodologies can be made and these are represented in Fig. 14.1. In the simplest fixed bed system (Fig. 14.1A), catalyst and powdered biomass or other hydrocarbons are mixed and the composite placed in a tube (held in place by ceramic wool or sinter disks). The tube is externally heated to produce a high heating rate. The pyrolysis reaction and the catalytic cracking/reforming reaction can also be separated into two separate processes (Fig. 14.1B). This allows the cracking/reforming reactions to be run at different temperatures as well as secondary input of gas (water, hydrogen) to improve that catalytic process. Fluidised bed reactors can also be run with an in situ (i. e. in the pyrolysis reactor) catalytic process (Fig. 14.1c) or an ex situ process. One of the biggest advantages of an ex situ process is that the catalyst can be used in a fixed bed which prevents the impact damage that occurs in a fluidised bed which can severely limit catalyst lifetime. Catalysts can be periodically regenerated by oxidative treatment to remove coke using a twin reactor tube.