Intensification of Pyrolysis by Autothermal Operation

INTRODUCTION Pyrolysis was previously considered by the wastewater industry and is now experiencing a renewed interest. The reasons are: a) mounting evidence that PFAS are removed from the resulting biochar, b) the volume of the final material may be reduced by 90% and c) pyrolysis systems are easier to permit than incinerators. Pyrolysis is subdivided based on the primary product which is a function of heating rate and operating temperature. Slow pyrolysis favors the production of a solid fuel, such as charcoal, and occurs at lower heating rates and temperatures from 570 - 750 F. Fast pyrolysis involves high heating rates and temperatures of 900 F or higher and is often followed by rapid cooling to produce the oils from the condensable vapors. The conventional definition of pyrolysis is the thermal decomposition of solid organic substances, in the absence of oxygen, to produce char, non-condensable gases and condensable vapors, which when allowed to condense become oil, referred to as either py-oil or bio-oil. This definition is reasonable, since the primary product of fast pyrolysis is the bio-oil and any oxygen within the system would react with the vapors produced and thus decrease the bio-oil yield (Brown, 2021). A more accurate definition of pyrolysis is the 'thermochemical decomposition of biomass into a range of useful products, either in the total absence of oxidizing agents or with a limited supply that does not permit gasification to an appreciable extent.' (Basu, 2008) Fast pyrolysis of sewage sludge biosolids (SSB) is designed to remove PFAS from the resulting char and to reduce the volume of the SSB by 85 - 90% while minimizing external energy required to dry the material from 25 - 30% solids to the requisite 80 - 90% solids for pyrolysis. requirements. This is achieved by burning the non-condensable and condensable gaseous fuels in a thermal oxidizer and recovering the thermal energy (heat) for drying. See Figure 1. INTENSIFICATION OF PYROLSYIS FOR LARGE SCALE APPLICATIONs The reactions of pyrolysis are endothermic and thus require an input of energy. In the absence of oxygen, this energy demand is met by the transfer of external thermal energy (i.e. heat transfer) via either the reactor surface (auger reactor) or the media (fluidized bed reactor). However, providing this thermal energy at typical pyrolysis temperatures of approximately 930 F (500 C) can be challenging, especially as the reactor becomes large. Hot gases can be employed for this purpose using tubular heat exchangers, but the low specific heat of gases requires large volumetric flows to meet the thermal energy requirement. Granular media is more suitable for direct heating due to their refractory properties and high heat capacity. However, both heat carriers require ancillary equipment to heat and circulate them. As the reactor becomes larger the energy required for pyrolysis, which is a function of reactor volume, increases faster than the surface area for heat transfer, which makes heat transfer rate limiting in allothermal pyrolyzers. Under these conditions, assuming a fixed diameter to length ratio, the reactor size scales to the square of the diameter. The cost of the reactor also increases approximately with the square of the diameter cost increases are roughly linear with no economy of scale. This is the reason that the currently available pyrolyzers, for the treatment of SSB are restricted to relatively small quantities of sludge and therefore small wastewater treatment plants. Allothermal pyrolysis is not economically feasible to scale to large applications; however, autothermal pyrolysis is. Autothermal pyrolysis refers to the addition of a small amount of oxygen to partially oxidize some of the vapor products of pyrolysis to provide the energy for pyrolysis within the reactor. The amount of energy required to provide the energy for pyrolysis is a function of the feedstock and may be relatively small (7 - 10% of the energy content of the feedstock). The exothermic reactions of the oxidation of these vapors meets the energy demand of the endothermic reactions and the parasitic load of maintaining reactor temperature. The chemical reactions control the process and the capacity scales as the cube of the reactor diameter. Therefore, autothermal pyrolysis would provide a several fold increase in capacity over conventional pyrolysis in the same size reactor. APPARATUS AND RESULTS A schematic of the bubbling fluidized bed pyrolizer, shown in Figure 1, can operate as either a conventional or autothermal pyrolyzer. In this research the quality and quantity of the bio-oil produced as the throughput increased was evaluated. However, this does not have the same significance in the wastewater industry since the desired product is the thermal energy, recovered from the combustion of both the condensable and non-condensable gases in a thermal oxidizer, for drying the dewatered biosolids. The details for the bio-oil production and characteristics are discussed in detail in the paper by Polin et al. 2019. Operating in allothermal (conventional) mode, the pyrolyzer throughput was 253 lb/day (4.78 kg/hr). For autothermal operation, 11.4% of the stoichiometric oxygen requirement was introduced. The result was an increase in throughput to 814 lb/day (15.4 kg/hr), or 3.2-folds increase. Figure 2 shows the effect on throughout as the reactor diameter increases. Autothermal pyrolysis increases the throughput of a given reactor by the cube of the diameter (D3). Although there was a shift in the constituent products, the specific energy yield remained constant which indicates that the energy available for drying remains the same for either mode of operation. The second significant advantage of autothermal pyrolysis is that it provides a reduction in scale reducing capital costs, whereas allothermal pyrolysis depend on economies of scale. For autothermal pyrolysis capital costs of the reactor are a fraction of allothermal pyolyzers at the same throughput as seen in Figure 3. CONCLUSIONS AND RECOMMENDATIONS Autothermal pyrolysis is currently being demonstrated at scales up to 50 tons/day using feedstocks such as corn stover and red oak. The benefits of autothermal pyrolysis can be scaled for application at large wastewater treatment plants. The next step is a demonstration scale facility operating on one hundred percent sewage sludge biosolids at a large wastewater facility.