By J.A. Cole, D.W. Hansell, N.C. Widmer, Wm. R. Seeker, Energy and Environmental Research Corporation, Irvine, California (EER has a Canadian office in Markham, Ontario); K. J. Wilson, T.P. Parr and K.C. Schadow, Naval Air Warfare Center, Weapons Division, China Lake, CA
As the US Navy moves into the 21st century, both international and domestic regulations will drive changes to more environmentally responsible operation of Naval ships. MARPOL1 and the Environmentally Sound Ships program are examples of regulations that will eventually ban such common practices as at-sea dumping of waste paper, food, scrap, and human wastes.
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| USS Constellation ready for action. US Navy Photo |
Unfortunately, naval ships, unlike vacation cruise ships, are designed to maximize their primary function, which is to protect national interests at sea and in extreme circumstances engage in warfare. This means that there is little room available for installation of incinerators and associated waste handling and processing equipment. This incongruity between desirable waste handling practices and the primary mission of naval ships is driving the development of more compact hardware and advanced technologies for at-sea treatment of shipboard-generated wastes.
Incineration, or more politically, waste thermal treatment, is a preferred method for waste handling. It can accomplish several of the goals of at-sea treatment of shipboard wastes, including volume reduction, sterilization, and detoxification. It is also considered to be the most cost-effective approach available and among the safest, requiring little specialized personnel training.
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Unfortunately neither land-based nor existing seaworthy incinerator designs can meet the Naval requirements of compactness and light weight. This has led to the exploration of novel approaches, such as the use of forced acoustics to improve heat transfer, turbulent mixing, and firing density in order to reduce the size and increase the throughput of incineration systems.
There are several types of periodic phenomena that occur in combustion systems. Acoustic resonance in large-scale combustion systems, and slugging in fluidized bed combustion can cause catastrophic damage to equipment. On the other hand, pulse combustors and Rijke tube combustors are used to in crease heat transfer in many practical systems. Reciprocating internal combustion engines rely on periodic combustion for normal operation, while gas turbine combustors are designed to avoid resonance phenomena that can damage turbine blades and bearings. Forced acoustic controlled combustion can also increase firing density (measured in MW/m³) beyond that attainable in normal diffusion-controlled combustion.
In those applications where acoustic resonances are desirable, the application of forced acoustics in combustion provides a decided advantage over naturally-occurring acoustics. This is be cause forced acoustics lends itself to active closed-loop feedback control. Con sider for example a combustion system where both the quality and quantity of fuel vary, and these variations are for all practical purposes outside the control of the system operator. This situation actually exists with two-stage waste incinerators and landfill gas combustion systems. In the case of incineration, the first stage of the combustion system is a starved-air pyrolysis unit, while the second stage is an afterburner intended to destroy the gaseous products of waste pyrolysis.
In our example incinerator, both the rate of pyrolysis product production and the composition of those products depend on the current state of the system and the nature of the waste being processed. The afterburner must be designed so that under the worst case scenario there will be no significant release of undesirable products of incomplete combustion released to the atmosphere.
Depending on local regulatory jurisdiction, this means that the minimum temperature at the exit of the afterburner must be from 1700 to 2000°F (925 to 1I00°C) and the minimum residence time 1 – 2 s. This sets strict physical limits on the volume and weight of the afterburner. By coupling forced acoustic controlled combustion with a closed-loop feedback control system able to respond in millisecond time frames, these requirements could be drastically altered, with the result being a much smaller, lighter, more robust afterburner having better emission characteristics than conventional systems.
EER and the Naval Air Warfare Center (NAWC) are part of a project team that is examining applications for forced acoustics in waste thermal treatment systems for use on US Navy warships. Two specific applications are under study. The first is an advanced afterburner, and the second is a sludge incinerator. approaches for the two
systems are different, but both use forced acoustics as a means to improve specific aspects of the system operation.
Background
Blackwater sludge is the term for raw sewage that has been processed through a macerator pump to produce a slurry containing about 98 percent water and slightly less than two percent solids. Presently, there are about 30 US Navy ships using blackwater sludge incinerators. These incinerators, illustrated in Figure 1, are based on a 1950s technology and were first commissioned by the Navy in the mid 1960s.
Sewage is introduced through a coarse hollow-cone spray nozzle at one end of the incinerator. The majority of the spray droplets impact on the inconel walls of the unit where they evaporate and pyrolyze and the resulting gases are incinerated. A tangentially-mounted burner, fueled with JP-5, induces a vortical flow in the incinerator and heats the walls. Exhaust is removed through a large air cooled port located on axis at the end opposite the sludge nozzle.
At present this sludge incinerator design is capable of treating only blackwater sludge and has a throughput capacity of 30 gph, its emission characteristics are not well known, and there are no regulations covering its operation. The US Navy wants to change the operation of this sludge incinerator in a number of ways to make it more compatible with current and planned ship board operations.
The Navy is presently developing polishing technologies to clean up grey water, that is water collected from showers and sinks, and possibly galleys. This water is being cleaned up to comply with Clean Water Act standards so it will not introduce pollutants to the oceans when dumped overboard. Along a similar vein, the Navy has developed oil separation systems to remove oil and sol vents from bilge water so it too can be dumped overboard. The residuum from these clean up operations is targeted for treatment in the blackwater sludge incinerator.
On top of these efforts, the Navy is also looking into systems for removing half of the water from the blackwater sludge, that is, doubling the solids concentration. After all, one of the main functions of the blackwater sludge incinerator is to evaporate water, which is a very energy-intensive process. Removing half the water will increase the solids concentration to nearly four per cent, which will change the spray characteristics considerably. Finally, the Navy wants to increase the throughput of blackwater sludge by 30 to 50 percent.
Possible effects of these changes include increased paniculate matter production, increased alkali in ash, introduction of chlorinated solvents with concomitant corrosion issues, and reduced residence time in the combustion chamber. Forced acoustics is being considered as one part of a suite of technologies used to address the Navy’s needs to adapt the blackwater sludge incinerator to the new process stream compositions. The combination of increased heat and mass transfer induced by a high sound pressure level (SPL) acoustic field can both increase droplet evaporation rates and enhance the firing density of the primary fuel.
The controlled vortex afterburner has been under development by NAWC for several years. Their efforts have yielded a number of accomplishments including derivation of scaling criteria and the discovery that NOx and CO emissions could be simultaneously reduced by proper control of their system. The basic operation of the controlled vortex afterburner is illustrated in Figure 2.
Starting on the left of the figure we have a central air jet issuing past a dump plane. This jet induces the periodic formation of annular vortices. Fuel can be introduced through an annular opening in the surface of the dump plane coaxial with the air jet so that it is entrained into the vortices and mixed with the vortex gases through strain-enhanced diffusion and convection. If the system is ignited, the vortex will consist of air mixed with hot reacting combustion products. As fuel is added to the vortex its ignition will be strain delayed, resulting in reduced NOx emissions (as a result of lower peak combustion temperatures) and reduced products of incomplete combustion (as a result of improved mixing and elimination of cold pockets). The behavior of the system is enhanced dramatically if acoustic forcing is used on both the central air jet and the annular fuel stream. The production of vortices by the central air jet is periodic with a natural frequency. However, if the intervals between successive vortex shedding events are measured and recorded, a probability density function (PDF) can be constructed, as shown on the left of Figure 3, that shows a broad distribution of time intervals centered around the natural frequency.
By forcing the jet at or near its natural frequency the PDF is collapsed to a narrow spike, illustrated on the right of Figure 3, allowing the vortex shedding events to be accurately predicted and controlled. Likewise, the annular fuel jet can be modulated with acoustic forcing. By setting the two frequencies equal and adjusting the phase angle between the two forcing functions the fuel can be introduced at the optimal point for entrainment and mixing with the vortices.
Further improvement to this system was achieved by introduction of a secondary air stream introduced annularly and coaxially with the fuel stream. By modulating the secondary air rather than the fuel it is possible to gate the introduction of the fuel, and enhance performance even more. The work of Parr, el at., showed that DREs for benzene increased from about 3-nines to better than 5-nines (detection limit), CO emissions were reduced from 2900 to as low as 2 ppm, and significant reductions in both NOx and unburned hydrocarbons were observed. The basic system design has been scaled up in stages from about 4.7 kW to 300 kW with some of the tests conducted in open air, but with most work done using an open-end tubular enclosure. The combined effort by EER and NAWC extends this work to an enclosed system to simulate the environment in an actual incinerator afterburner.
Blackwater sludge incinerator
The blackwater sludge incinerator effort is in the design stage. A full-scale mockup of the incinerator, shown in Figure 4, has been constructed ac cording to design criteria that include matching all physical dimensions and reactant flow rates, plus simulation of the thermal boundary conditions of the original unit. Ports have been added both for optical access and to permit gas and particulate sampling, and the incinerator has been instrumented with thermocouples to monitor temperatures and heat fluxes through the walls.
EER has also engaged the services of Hersh Acoustical Engineering, Inc. to design an acoustic driver system for the blackwater sludge incinerator. An array of acoustic ports will be installed on the spray nozzle end of the incinerator according to the layout shown in Figure 5. This array of eight loud speakers will permit the simultaneous excitation of multiple acoustic modes in the incinerator so as to in duce large SPLs and SPL gradients throughout the combustion chamber volume.
Tests scheduled on the blackwater incinerator simulator will first include establishment of a baseline performance characterization. This will include operation of the system at 30 gph of two percent solids sludge. Measurements will be made of gaseous and solids emissions, gas and surface temperatures, and visual observation of operational characteristics. Once the acoustic array is added to the system a series of tests will be conducted in which both the burner firing rate and the sludge feed rate are increased to the point of system failure. System failure occurs when sludge accumulates in the bottom of the incinerator and exhaust temperatures rise due to reduced sludge boil-off. The load and firing rate at system failure will be noted and then the tests repeated with longitudinal acoustic excitation at 1000 Hz and 150 dB SPL. It is the objective of these brute-force tests to demonstrate and ob serve any increase in system throughput attainable through the use of acoustic forcing.
Future plans for the blackwater sludge incinerator simulation unit include first the study of acoustic modes other than the longitudinal mode. The eight-loudspeaker array is designed to take advantage of spinning and rotating acoustic modes through phase delay sequencing of the driver signals to the individual speakers. Depending on the magnitude of effects noted with alternate acoustic modes, alternate system geometries may be employed. For example, to take full advantage of the acoustic impacts on heat transfer, it will be desirable to eliminate the existing hollow-cone spray nozzle in favor of a solid cone nozzle that will produce smaller droplets with a lower droplet velocity. This will give the droplets more time to evaporate, while simultaneously reducing their evaporation time. This, of course, means changing fundamentally the way in which the incinerator operates and may necessitate further geometric changes to accommodate the differences.
Other plans include blending of simulated bilge water sludge with the blackwater sludge, working with a more concentrated blackwater sludge, and possibly changing the burner geometry to an axial firing configuration.
Controlled vortex afterburner
The final design of the full-scale controlled vortex afterburner tested at EER is shown in Figure 6. This device was attached to a 24-inch inside diameter cold wall package boiler in the configuration shown in Figure 7. Parametric tests were then conducted to determine whether the work of Parr could be extended to closed systems, and how enclosure would impact performance. In these initial tests CO was used as a surrogate for system performance. Although unburned hydrocarbons were monitored for DRE, in all cases where acoustic excitation was used the measured levels were below the detection threshold of the instrumental hydrocarbon analyzer and so were of limited use in the parametric studies.
The existence of an optimal frequency for acoustic excitation is illustrated by Figure 8 which shows the effect of driving frequency on CO emission levels at two different system stoichiometries. For the most fuel-lean case very little effect of driving frequency is evident because CO levels are already low even in the absence of acoustic forcing. However, at Phi = 0.8, where CO concentrations of 420 – 550 ppm were measured in the absence of acoustic forcing, a very distinct minima in CO was detected at 241 Hz. This is very near the calculated optimum of 220 Hz. The difference may be due to un certainties in jet velocities, or may be related to changes in the Jet fluid properties due to the radiant heat loading from the heated refractory in the pack age boiler. Once this optimal frequency was identified, it was used for the majority of further tests.
Figure 9 shows the impact of oxygen level on CO emissions in the absence of acoustic excitation. The system behaves similarly to any combustion system in that the CO levels de crease as oxygen increases. However, the magnitude of the CO levels suggests that, in the absence of acoustic excitation, mixing of fuel and air in this system is poor. When the secondary air flow rate is increased from 2 to 45 scfm the CO levels increase even further to nearly 900 ppm (dry, 7% O,) in the worst case shown in Figure 9. However, as acoustic power is added to the system the CO levels drop dramatically as shown in Figure 10. The power levels shown in the figure are estimates based on linearization of the manufacturer’s laboratory test data on the Ling acoustic transducer. However, even if the power levels shown are not highly accurate, the results of these tests suggest that an acoustic power level on the order of 100 W is sufficient to reduce CO levels to about 40 – 50 ppm. It should also be noted that while the worst unforced emission levels were obtained using a secondary air flow rate of 45 scfm, with acoustic forcing the CO emissions were comparable to the best results obtained.
Preliminary data have now been collected in the EER system with phase synchronous acoustic excitation of the secondary air and the fuel stream. Similar to open-air tests conducted at China Lake, modulating the fuel has resulted in even lower emissions. Analytical equipment is also being upgraded to pro vide for NOx emissions data, as well as benzene detection limits at least two orders of magnitude lower than the present instrumentation. These additional data will help to provide a more comprehensive picture of the promise of acoustic afterburners for closed-loop active control of emissions in waste thermal processing systems.
Acknowledgements
Financial support for this work is provided by the Strategic Environmental Research and Development Program of the Office of Defense Research and Engineering under contract N00014-96-C-0105. Dr. Klaus C. Schadow is the Project Technical Monitor.
1MARPOL refers to three international treaties: The Convention for the Prevention of Marine Pollution by Dumping from Ships and Aircraft, adopted at Oslo on 15 February 1972; The International Convention for the Prevention of Pollution from Ships, 1973, adopted at London on 2 November 1973; and The Protocol of 1978 Relating to the International Convention for the Prevention of Pollution from Ships, 1973, adopted at London on 17 February 1978.
1Energy and Environmental Research Corporation, Irvine, California. EER has a Canadian office in Markham, Ontario
2Naval Air Warfare Center, Weapons Division, China Lake, CA