Evaluation of Sewage Sludge for Autothermal Pyrolysis Prior, to Pilot Test.

Operating pyrolysis in an autothermal mode is a means overcoming heat transfer barriers while simultaneously intensifying the pyrolysis process resulting in an exponential increase in throughput for a given size reactor. Therefore, the proposed 2 dry-ton/day autothermal pyrolysis system, which will be built and operated at a large municipal facility as part of a U.S. Department of Energy Grant is a significant step forward in the development of a pyrolysis technology suitable for application at any large wastewater treatment facility. Several alternative modes of operation for pyrolysis are depicted in Figure 1. Currently available pyrolysis technologies operate conventionally, in the absence of an oxidant (i.e. air/oxygen) and are therefore, allothermal, meaning that the thermal energy required is external to the process as depicted in Figure 2. Since the reactions of pyrolysis are endothermic, the absence of an oxidant requires that the energy demand be 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 930ËšF (500ËšC) 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 (Brown, 2021). Under these conditions, assuming a fixed diameter to length ratio, the reactor size scales to the square of the diameter, while the cost increases linearly with no economy of scale. For these reasons, 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 neither economically nor logistically feasible to scale to large applications. However, autothermal pyrolysis is. Autothermal pyrolysis refers to the special case of oxidative pyrolysis in which just enough oxidant (air) is introduced to oxidize enough of the products to provide the energy for pyrolysis within the reactor as depicted in Figure 3. 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) (Daugaard and Brown, 2003. The exothermic reactions of the oxidation of these products 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 of the reactor 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. The difference between the two operating modes is demonstrated with the following experimental results operating on red oak biomass (Pollin 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 a 3.2-fold increase. Figure 4 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 depends 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 5. Although autothermal pyrolysis is currently being demonstrated at Iowa State University at scales up to 50 dry-tons/day using feedstocks such as corn stover and red oak; the 2 dry-ton/day pilot to be operated at one of the Philadelphia Water Department (PWD) wastewater treatment plants will be the first system to operate on 100% sewage biosolids. Proximate, and ultimate analyses of the dried (<10% moisture) sewage sludge, is shown in Tables 1 & 2. The proximate analysis indicates total percentage greater than 100%. This is being investigate and one theory is that precipitation of siloxanes in the sludge to silica maybe a contributing factor. The enthalpy for pyrolysis will also be determined ahead of the research so that an accurate energy balance can be performed and included in this presentation.