Converting an Old Multiple Hearth Furnace to a New Pyrolyzer/Gasifier

Introduction At many water resource recovery facilities (WRRFs), when potential environmental impacts are viewed in their totality, thermally processing the sewage sludge remains an environmentally friendly and economically feasible option. Many of these plants have existing, older, multiple hearth furnaces (MHF). Often viewed as having greater emissions than the fluid bed reactors (FBR) which in recent years have gained a greater presence, that is only partially true. Both excess air and afterburner fuel demand can be significantly reduced by conversion of the MHF to a pyrolyzer/gasifier. The paper and presentation will explain how this is possible. History 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 these process modes is, however, very rare. On the other hand, FBRs have been more frequently operated in a gasification mode to process WRRF residuals. The first full-scale FBRs operating in a gasification mode were installed at the Hyperion WRRF in Los Angeles. The design capacity of each of the three (3) FBRs was 132.5 dry tons/day of a heat dried biosolids, called sludge derived fuel (SDF). Through heat recovery, superheated steam was produced, then converted to electricity in a condensing steam turbine. The SDF was gasified in the FBR with sub-stoichiometric air addition to produce a syngas. The hot syngas was then combusted in a multi-stage combustion afterburner. The gasification/staged-combustion approach provided significant reduction in nitrous oxide (NOx) emissions compared to conventional incineration. Operation began in 1986 and continued for about 10 years at part load. Problems with the upstream drying process limited throughput to about 100 dry tons/day of SDF. Unfortunately, the continuing problems with the sludge dryers, and a significant decrease in the cost of land disposal, made continued operation no longer economically viable. While Hyperion was a FBR, adaptation of these concepts to a MHF is also feasible. Aside from a MHF being able to produce a synthesis gas with lower particulate than a FBR, a MHF offers another advantage. A safe and stable combustion mode in a MHF can be achieved at a significantly lower exhaust temperature than a FBR, which is because of the reactions that occur in the freeboard that typically create a 200°F(+) increase in exit temperature. The counter-current flow of syngas and sludge feed make an excellent thermodynamic reactor. When operated in a true pyrolysis mode, a biochar would be produced. While production of an agricultural biochar can sometimes generate political currency, it must be balanced against the loss of BTU's that may have to be made up with auxiliary fuel. The use of biochar in a local market will depend on the soil, agricultural needs, and pyrolysis conditions used for producing the biochar. Using the pyrolysis gas from a pyrolysis process is possible but the removal of the tars, oils, and other condensables produced during pyrolysis before sending the pyrolysis gas to a commercial burner have heretofore defied a simple, straightforward, and reliable solution. Any project contemplating on recycling pyrolysis gas directly back to a burner will venture into somewhat uncharted territory. In the early 1980's, the Allegheny County Sanitary Authority (ALCOSAN) installed a dual-mode MHF system at its WRRF in Pittsburgh, PA. This system was designed to process 76.5 dry tons/day of raw sludge, and incorporated both starved-air combustion and conventional excess air combustion modes. When operating in the starved air mode, the furnace was divided into four zones: drying, starved air/gasification, carbon burnout and ash cooling. Burners were fired in the carbon burnout zone, but were rarely needed to any significant degree in the drying zone, as combustion air was introduced to the drying hearths to release a portion of the syngas heat to support the drying process. The carbon burnout zone was operated with excess air to keep temperatures within acceptable limits and yield an ash with less than 2% carbon. The ALCOSAN system sacrificed the production of biochar to provide the heat needed for the gasification process and the drying of the incoming feed. Syngas was combusted in the external afterburner and a waste heat recovery boiler was provided downstream of the afterburner to produce steam for plant and process heating. In the design concept of this paper, bioenergy recovery from the syngas produced in a MHF pyrolysis/gasification system would incorporate an external 'syngas oxidization chamber' equipped with a flame safety pilot burner. This refractory lined vessel is very similar to a separate vessel conventional afterburner commonly installed on MHFs (Figure 1). From the oxidizer, a portion of the heat would be used to heat the Multiple Hearth pyrolysis / gasification reactor and/or to pre-dry the incoming feed. Potential slag formation in the syngas oxidization chamber remains a challenge. This challenge has been meet using indirect heated, rotary kiln pyrolysis reactors. The challenge of slag control and combustion chamber design will be discussed in the paper /presentation. Heat and Material Balances The Heat and Material Balance remains at the heart of any thermodynamic process and the Laws of Thermodynamics: You Can't Beat the Game You Can't Even Break Even This principal remains inviolate and it is essential that any complex design of this nature begin with a thorough heat and material balance at each stage of the process. This enables the engineer to evaluate the interdependencies between each stage; to identify potential problem areas or design challenges; and to fully understand the overall system performance. When a thermal dryer is used ahead of the gasification/pyrolysis, the treatment of the water vapor, thermal or other, must be included in any analysis within the overall Thermodynamic System Boundary. Thermodynamics does not allow a 'free lunch' and the energy requirements for thermal drying are significant. In this basic concept, energy recovery is provided downstream of the external afterburner chamber in the form of hot thermal oil that can be used in an indirect thermal dryer system. This paper will discuss the process in greater detail and through a series of pictorial Heat and Material Balances for both pyrolysis and gasification systems. This will give the reader a better understanding of the process and hopefully prevent repeats of the failures of many previous gasification and pyrolysis ventures. Often these failed to recognize the true thermodynamic balance and the myriad of yet unsolved problems of removing the tars and oils from the pyrolysis gas. Figure 2 illustrates the output from a detailed heat and material balance around a MHF gasifier. Figure 3 shows the distribution of the syngas components from this system. Figures 4 and 5 illustrate the output from detailed heat and material balances around the external afterburner chamber and a subsequent thermal oil heat recovery unit, respectively. Figure 6 provides a summary of the heat and material balances around the entire thermodynamic system boundary. Additional diagrams will be included in the final paper for other optional design concepts. Conclusions The advantages of MHF gasification are readily apparent. The WRRF sludge can just be scalped (thermally dewatered) to 40% and the syngas will have a flame temperature of 1,844°F. Similarly, a 35% solids feed will yield a syngas with a flame temperature of 1,637°F in the secondary combustion chamber. If these flue gases are taken through a thermal oil heater and exhausted at 400°F, the thermal oil will have sufficient heat to dry a raw feed solids of 20% to the feed solids required for the system. Further modeling and evaluation will be included in the final paper to assess the feasibility of MHF pyrolysis. An existing MHF represents a major capital investment and developing means to exploit existing infrastructure and improve performance can have a significant positive impact on both asset management and operational economics at any plant. Converting an existing MHF into a pyrolyzer/gasifier with the addition of a secondary combustion chamber (afterburner) and thermal drying/dewatering system may prove to be a viable option that is worthy of evaluation. With interest growing in the use of gasification and pyrolysis as alternatives to conventional incineration, the options presented in this paper could be attractive in appropriate circumstances and the design approaches described in this paper can be invaluable in assessing the viability of implementing the conversion of an existing MHF to a new pyrolyzer/gasifier system.