At many water resource recovery facilities (WRRFs), thermally processing of sewage sludge, primarily via incineration, remains an environmentally friendly and economically feasible option. Many plants have existing, older, multiple hearth furnaces (MHFs). Often viewed as having greater emissions than more modern fluid bed reactors (FBRs), this is only partially true. Both excess air and afterburner fuel demand of a MHF can be significantly reduced by converting it to a pyrolysis/gasification system. This paper will explain how this is possible. Operation of a MHF in pyrolysis or gasification mode is not a new concept. MHF systems have been used in carbon regeneration and activated carbon manufacturing for decades. Historical applications using MHF systems to process WRRF residuals in any of these process modes are, however, very rare. On the other hand, FBRs have more frequently been operated in a gasification mode to process WRRF residuals. Due to its design, a MHF can produce a synthesis gas with lower particulate than a FBR and offers the added advantage that a safe, stable combustion mode can be achieved at a lower exhaust temperature than a FBR. Further, the counter-current flow of syngas and sludge feed inherent in a MHF makes it an excellent thermodynamic reactor. When operated in a true pyrolysis mode, a biochar would be produced, along with a mixture of pyrolysis gas and pyrolysis oil. While biochar production may be attractive, it must be balanced against the loss of BTU’s that may have to be made up with auxiliary fuel. Using the pyrolysis gas is relatively straightforward; however, use of the pyrolysis oil can pose significant challenges that must be addressed to take advantage of the energy value of this mixture of tars, oils, and other condensables. In the design concept of this paper, bioenergy recovery from the syngas produced in a MHF pyrolysis/gasification system would incorporate an external oxidization chamber. From the oxidizer, heat would be used to meet thermal loads of the MHF reactor and/or to pre-dry the incoming feed. To evaluate the feasibility of converting an existing MHF in this manner, it is essential to begin with a thorough heat and material balance at each stage of the process. This enables the engineer to evaluate interdependencies between each stage; to identify potential problem areas or design challenges; and to fully understand overall system performance, including the parasitic thermal demands of pre-drying the feed. This paper will discuss these processes in greater detail through pictorial heat and material balances for both pyrolysis and gasification systems, and diagrams illustrating the application of these processes to real world situations. This will provide a better understanding of the process and help to avoid repeating the failures of many earlier pyrolysis/gasification ventures. An existing MHF represents a major capital investment and developing means to exploit this infrastructure and improve performance can have a significant positive impact on asset management and operational economics at any plant. Converting an existing MHF into a pyrolysis/gasification system may prove to be a viable option that is worthy of consideration.
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