5.18.1 Summary

Both fixed and fluid bed reactors have been operated for fast pyrolysis and disposal of wastes, with catalytic upgrading of the products in the case of the fluid bed work. The objective has been to carry out screening studies on feedstocks, products and particularly examine the production of noxious products that may mitigate against implementation.

5.18.2 Description

A 200 ml stainless steel fixed bed reactor has been used to pyrolyse tyres. The reactor is externally heated and nitrogen is used as a carrier gas. Heating rates up to 80°C/min and temperatures from 300-720°C (82). More recently, work using polystyrene as the waste has been carried out with secondary thermal cracking of the product vapours at 500, 600 and 700°C (83). Experiments were also carried out with a third catalytic cracker using ZSM-5 catalyst maintained at 400°C which lowered the proportion of styrene oligomers but increased the proportion of PAH.

Work has also been carried out on fixed bed catalytic upgrading of model compounds as a prelude to the pyrolysis of biomass. Model compounds such as furfural, cyclopentanone, anisole, ethylacetate and methanol were upgraded over 10 g ZSM-5 in a fixed bed using nitrogen as a carrier gas and a range of temperatures ranging from 300°C to 500°C, WHSV 4+0.5 (84). The results showed that the EHI [Effective Hydrogen Index] was important in determining the conversion to other products. As the EHI increases for each model compound, the degree of mass conversion also increases. The importance of this is that it was estimated that the products of wood pyrolysis would have a lower degree of mass conversion compared to liquids produced from RDF or rice husks. For biomass liquids with EHI number less than 1, a catalyst at 500°C would produce a liquid with the lowest oxygen content. For biomass liquids with a EHI greater than 1, a catalyst at 400°C would give optimal conversion.

Subsequently, a dual fluid bed was used to pyrolyse biomass. Biomass was pyrolysed in an externally heated 0.075 m diameter bed, 1 m high with nitrogen as the fluidising gas. This is shown in Figure 5,16. Part of the reactor freeboard was packed with ZSM-5 catalyst. Liquids were collected before and after pyrolysis for comparison. Selected results are given below for yield of oxygenated compounds from biomass pyrolysis.

Products

For both reactors, the liquids were recovered and analysed by GC/MS. Analyses for biomass derived liquids are shown in Table 5.18. Quantification of PAH was performed by GC/FID. The recovery of styrene form the polymer was 53.0 wt% ± 1% at cracking temperatures of 500 and 600°C decreasing to 34.0 wt% at 700°C. The effects of metal salts on the pyrolysis of cellulose has been studied in a fixed bed reactor and by TGA (85).

For tyres, the maximum yield of liquid was obtained at 80°C/min and 720°C with a yield of 58.8 wt % liquids, 14.8 wt% gas and 26.4 wt% char. Detailed analysis of the liquids by SEC, FTIR have been performed.

In zeolite cracking, the conversion of oxygenates in the pyrolysis vapours occurs at lower catalyst temperatures to H2O and to CO2 and CO at higher catalyst temperatures. Coke formation on the catalyst was typically 11.4-13.1 wt%, although the run times over which this occurred at not quoted.