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Atomization and Drying Characteristics of SewageSludge inside a Helmholtz Pulse CombustorDr. Wu Zhonghua a , Wu Long a , Li Zhanyong a & Arun S. Mujumdar ba College of Mechanical Engineering, Tianjin University of Science and Technology, Tianjin,Chinab Minerals, Metals and Materials Technical Centre, Department of Mechanical Engineering,National University of Singapore, SingaporePublished online: 19 Jun 2012.

To cite this article: Dr. Wu Zhonghua , Wu Long , Li Zhanyong & Arun S. Mujumdar (2012) Atomization and DryingCharacteristics of Sewage Sludge inside a Helmholtz Pulse Combustor, Drying Technology: An International Journal, 30:10,1105-1112, DOI: 10.1080/07373937.2012.683122

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Atomization and Drying Characteristics of Sewage SludgeInside a Helmholtz Pulse Combustor

Wu Zhonghua,1 Wu Long,1 Li Zhanyong,1 and Arun S. Mujumdar21College of Mechanical Engineering, Tianjin University of Science and Technology, Tianjin, China2Minerals, Metals and Materials Technical Centre, Department of Mechanical Engineering,National University of Singapore, Singapore

Sewage sludge of pasty consistency was atomized and drieddirectly using a high-temperature, highly turbulent gas flow in thetailpipe generated by a Helmholtz gas-fired pulse combustor (PC).Parametric studies were carried out to investigate the effect of dry-ing pipe length and sludge feed tube size on the PC sludge dryingprocess. Experimental results showed that the pulsating gas streamcan disperse the sludge into small particles with a narrow size distri-bution between 0.01 and 4mm and the granular sludge was driedrapidly due to the increased particle surface area and the high tem-perature of the drying medium. PC drying of sewage sludge wascompared to convective- and microwave-drying processes. Theapplication potential of the PC sludge-drying process is discussedbased on the experimental results.

Keywords Atomization; Pulse combustion; Sewage sludge;Thermal drying

INTRODUCTION

In 2011, over 20 million tons of sewage sludge wereproduced from wastewater treatment plants (WWTPs) inChina and the production of sludge is increasing continu-ously.[1,2] Worldwide the amount generated is severalfoldthis amount. One way to manage sewage sludge is to dryit, which can lower the water content below 5% dry solids(DS). This obviously reduces the mass and volume of sludgeand, consequently, the cost for storage, handling, andtransport. Dried sewage sludge can be employed in agricul-ture or forestry as organic matter and dumped in landfills.Drying also disinfects sludge for safe disposal. Waterremoval also increases the calorific value, transformingthe sludge into a combustible material. Furthermore, driedsludge becomes a pathogen-free, stable material due to high-temperature treatment in a pulse combustor (PC) tailpipe.

The operating cost and initial equipment investment aretwo key issues for sludge drying because sewage sludge hasa very low value. Different types of dryers are used

commercially for sludge drying; for example, fluidized bed,rotary, paddle, and belt dryers.[3–9] However, conventionalthermal drying systems are technically complex, requiringappreciable operating costs while imposing significantinitial investment. This solution is thus suitable for large-capacity WWTPs (over 100 tons sludge=day). It is estima-ted that the operating cost is about US$30–40=ton sludgeand an initial investment involves US$48,000–67,000=tonsludge for existing sludge drying projects in China. Forsmall and average size WWTPs, the initial investment iseven higher and thus conventional thermal drying systemsare not economically acceptable solutions. Consideringthat there are numerous small and average-size WWTPsin China, it is necessary to exploit a new drying technologyor equipment to economically manage sludge. This tech-nology, of course, is of global applicability.

Pulse combustion drying is a possible solution for smallor medium-size sludge plants. PC dryers have a simplestructure and require a low initial investment, making themeconomically acceptable for small WWTPs. In addition,PC dryers are highly energy efficient, with low pollutantemissions, and thus may result in low sludge drying costs.A typical PC dryer consists of a pulse combustor and a dry-ing chamber, and the configuration can vary.[10,11] Pulsecombustors provide an advantageous superposition ofunsteady gas flow and high-intensity sound waves. Such acombination not only accelerates the drying rate due toincreased heat and mass transfer but also allows processingof liquids, pastes, and agglomerated solids in a dispersedstate due to the aerodynamic effect from the high-intensitysound waves. Since the 1970s, PC drying has been exploitedfor drying of fruit and vegetables, sawdust, woodchips, kao-lin, coal, sludge from PC electroplate factories, etc.[12–17]

PC drying has also been tested for drying of sewage sludge;energy consumption of about 2,500 kJ=kg sewage sludgewith an 87% moisture content has been reported. However,no detailed technical information is available.[18] Literatureon PC sludge drying is limited and no reports oncommercial PC sludge drying are currently available.[19,20]

Correspondence: Dr. Wu Zhonghua, College of MechanicalEngineering, Tianjin University of Science and Technology,1038 Dagulan Road, Hexi District, Tianjin 300222, China;E-mail: wddhua@gmail.com

Drying Technology, 30: 1105–1112, 2012

Copyright # 2012 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2012.683122

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The research team members at Tianjin University ofScience and Technology have experience in pulse combus-tion and its industrial applications.[21–24] In 2010, aresearch project was launched to develop a PC sludge pro-cessing system that involves PC sludge drying and inciner-ation and aims to provide an economic sludge managementsolution for small and medium-size WWTPs, biotechnol-ogy factories, tanneries, etc. Experiments were conductedto study the atomization and drying characteristics of sew-age sludge injected in the tailpipe of a lab-scale Helmholtz-type pulse combustor to obtain relevant technical data andexperience for a pilot PC dryer design and to obtain furtherknowledge on PC sewage sludge drying.

MATERIALS AND EXPERIMENTAL METHODS

Materials

The sludge used in this experiment was obtained fromthe Tianjin Jizhuangzi Wastewater Treatment Plant locatedin the southern part of Tianjin City, China. It mainly pro-cesses wastewater from residences. The sludge had pre-viously undergone digestion and mechanical dewateringprocesses. The sludge had an initial moisture content ofabout 80% (wb) and the residual solid consisted of about52% organic matter, 2.84% total nitrogen, 2.71% totalphosphorus, and 1.17% total potassium. The initial moist-ure content of the sludge was measured using a standarddrying oven method. It was dried in a DL-101-3BS-type

electric drying oven (Tianjin Zhonghuan Electric DryingOven Company, Tianjin, China) at a temperature of105�C to constant weight; the initial moisture content ofthe sludge was calculated as 80% (wb). Viscosity wasmeasured at an ambient temperature of 20�C using anSNB-3-type rotation viscometer with an accuracy of�2.0% (Shanghai Gulei Measurement Equipment,Shanghai, China) and was found to be 7,982 Pa � S.Table 1 shows a comparison of the viscosity of municipalsludge with the viscosity of other liquids. From Table 1 itcan be seen that the viscosity of the sludge was about8� 106 times that of water, meaning that the sludge couldnot be atomized using conventional atomization methodssuch as nozzle or rotary disk atomizers without dilution.

Experimental Setup

Figure 1 shows the pulse combustion sludge dryingexperimental setup, which consisted of a pulse combustor,sludge feeder, and a cyclone. The pulse combustorconsisted of a short cylindrical combustion chamber(/100� 210mm) and a long tailpipe, one-way air and fuelvalve, spark plug, etc. Here is how the combustor works:air enters the combustion chamber where liquefied pet-roleum gas (LPG) is added; the fuel–air mixture is ignitedby a spark plug and explodes, creating a hot combustionproduct, pressurized to about 3 psi above ambient pressure.The hot gases rush out from the tailpipe. The air valve

FIG. 1. Pulse combustion sludge drying experimental setup: 1, electromagnetic valve; 2, control panel; 3, decouple room; 4, one-way gas valve; 5, com-

bustion chamber; 6, spark plug; 7, flame sensor; 8, one-way air valve; 9, K-type thermocouple; 10, fan; 11, half-infinite pipe; 12, pressure transmitter; 13,

data sampling system; 14, pressure sensor; 15, hopper; 16, orifice plate; 17, tailpipe; 18, cyclone.

TABLE 1Viscosity of sludge compared to other materials

Items Sludge Salad sauce Honey Tomato sauce Kerosene Water

Viscosity (mPa � S) 7,982� 103 200� 103 6� 103 3� 103 3� 100 1.005� 100

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reopens and allows the next air charge to enter. The fuelvalve then admits fuel, and the mixture explodes in thehot chamber. This cycle is repeated and pulse combustionis achieved in a cyclic manner, resulting in a strong oscillat-ing hot gas jet existing the tailpipe.

The pasty sludge was released using a screw feeder intothe tailpipe at preselected positions. An orifice plate wasinstalled at the exit of the feeder to control the diameterof the sludge feed, which varied in the range of 3–10mm.When the wet pasty sludge was injected directly into thetailpipe, the hot, high shear and intense pressure wavefronts, which result from the strong oscillating hot gasstream, instantly atomized the sludge into particulate mate-rials without the need for an atomizer. Downstream, thefinely dispersed sludge followed the movement of the fluegas and was dried instantaneously due to high temperatureof the flue gas. Finally, the sludge powder was retrievedusing standard collection equipment (a cyclone).

As shown in Fig. 1, the flow rate of the gaseous fuel(LPG) was measured and controlled using a rotameter.The instantaneous gas pressure and temperature in thepulse combustor were monitored using a PTX7517 press-ure transmitter and a K-type thermocouple with a maxi-mum measuring temperature of 1200�C. The sizedistribution of the dried sludge particles collected by thecyclone was measured using an Octagon Digital 2000vibrating sieve and the mean final moisture content wasmeasured using a standard drying oven method. The sludgefeed rate was determined by the rotation speed of the screwfeeder and the rotation speed was controlled by an electricfrequency converter.

PULSE COMBUSTOR OPERATIONALCHARACTERISTICS

In pulse combustion drying, the strong pulsating hot gasstream generated by the pulse combustor controls atomi-zation, drying, and the particle trajectories of the sludge;thus, the combustor design is critically important. It isnecessary to know the operating characteristics of thegas-fired PC used in this work. The pulse combustor isdesigned to operate at a frequency in the range of50–100Hz by varying the length of the tailpipe; its designedheat load is 40–60 kW.

Figure 2 show the periodical oscillations of the acousticpressure monitored inside the combustion chamber whenthe combustor operates on the gaseous fuel (LPG) at a flowrate of 2m3=h and has a tailpipe length of 3.5m. FromFig. 2 it can be also seen that the acoustic pressure oscil-lates from �6 to 10 kPa (the pulse amplitude is 8 kPa)and the pulse frequency is 49.6Hz. Figure 3 shows thegas pressure amplitude distribution along the tailpipe.The distribution curve is similar to the quarter-wave one.The pulse amplitude is maximum (8 kPa) at the beginningand minimum (0 kPa) at the exit of the tailpipe. Figure 4

shows three local gas temperatures monitored inside thecombustor. The flue gas temperature in the combustionchamber was measured at about 1050�C and the gastemperature in the tailpipe was about 600–800�C due toheat losses from the tailpipe wall. PC drying is a high-temperature drying process and hence requires very shortdrying times.

In 2008, Wu et al. carried out a computational fluiddynamics (CFD) simulation of the pulse combustion pro-cess in a gas-fired Helmholtz-type pulse combustor in orderto understand the flame structure, gas flow, and combus-tion characteristics in the burner and the resulting pul-sation phenomenon.[24] The simulated pulse combustorhad a combustion chamber (/92� 150mm) and a tailpipe(u36� 1,430mm) and operated on methane. Figures 5 and6 show the simulated gas pressure fluctuation inside thecombustion chamber and pressure amplitude distributionalong the combustor, respectively. Compared to Figs. 2–3,the pressure amplitude in the simulated combustor haddifferent values for various combustor designs tested.

FIG. 2. Measured oscillating gas pressure in the combustion chamber of

the gas-fired pulse combustor.

FIG. 3. Measured pressure amplitude distribution of the oscillating gas

stream along the tailpipe.

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However, it followed the same sinusoidal oscillation in thecombustion chamber and the quarter-wave distributionalong the tailpipe.

PULSE COMBUSTION DRYING OF MUNICIPALSLUDGE

Using the experimental setup shown in Fig. 1, PC dryingof sewage sludge was carried out under the following con-ditions: The flow rate of the LPG was 2m3=h. With a heat-ing value of the LPG of 105MJ=m3, the heat load of theburner was calculated as about 58 kW. The sludge feed ratewas preset to 24 kg=h. The diameters of the feeding tubeswere 3, 6, 8, and 10mm. The tailpipe length was 3.5m.The sludge feeding positions were set to 3, 1.5, 1.0, and0.5m distant from the tailpipe exit. Here, the distancebetween the feeding position and the tailpipe exit is definedas the drying pipe length. This drying pipe length is relatedto the residence time of the sludge material in the tailpipe.

Sludge Atomization

Figure 7 shows photographs of fresh sewage sludge andthe sludge after it was dispersed and dried in the pulse com-bustor. The fresh sludge was pasty and gray colored.PC-dried sludge is particulate=granular and it is black incolor. The color change from gray to black is due to oxi-dation of the organic matter in the sludge. Dried sludgeparticles were measured using an Octagon Digital 2000vibrating sieve and the sizes ranged from 0.01 to 4mm.The size and moisture distribution of the dried particleswere dependent on the PC drying conditions and are dis-cussed in detail in the next section.

The sludge atomization process was also visualized usinga 200-Hz StreamView LR high-speed camera (SouthernVision Systems). Figure 8 shows consecutive snapshots ofsludge particles as they exit the tailpipe exit. According tothe positions of the particles in two consecutive photos,the particle velocity was estimated to be about 6m=s as theyleft the tailpipe. Due to the high particle velocity, the sludgeparticles were observed to distribute over an area 8.2m inlength and 1.5m in width when directly exposed to theenvironment. A high particle velocity also means a short

FIG. 4. Computed oscillating pressure and fuel concentration in the

pulse combustor chamber (chamber length: 0.15m; tailpipe length:

1.43m; fuel: methane).

FIG. 5. Computed peak-to-peak amplitude of gas pressure and velocity

along the pulse combustor (chamber length: 0.15m; tailpipe length:

1.43m; fuel: methane).

FIG. 6. Computed local gas temperature distribution inside the pulse

combustor.

FIG. 7. Sludge status before and after pulse combustion drying: (a) pasty

sludge (before drying) and (b) particulate sludge (after drying).

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residence time inside the combustor tailpipe. For the 3-mdrying pipe, the particle residence time was about 0.5 s.

Sludge Particle Size Distribution

Figure 9 shows the size distributions of PC-dried sludgeparticulates under different PC drying conditions. It wasfound that the dried sludge particles had a size range of0.01–4mm. In this work, the particle size was classified intofive groups: 0.01–0.9, 0.9–1.6, 1.6–2.0, 2.0–2.8, and2.8–4mm. The mass percentage of smaller particles waslarger; that is, better sludge atomization was obtained.From Fig. 9 it can be seen that both drying tailpipe lengthand diameter of the feed have important effects on the par-ticle size distribution. When the sludge feed tube size was6mm, the mass percentage of 0.01–0.9mm particlesincreased from almost zero for a 0.5-m drying pipe to13.9% for a 3-m drying pipe; a long drying pipe increasesthe residence time of sludge in the tailpipe, resulting in alonger interaction time between the sludge and the pulsat-ing gas stream; thus, the sludge is atomized more fully.

Figure 10 shows the relationship between the Sautermean diameter of the sludge particles and the drying pipelength and feed tube size. From Fig. 8 it can be seen thatthe Sauter mean diameter of the dispersed particlesdecreased sharply when the length of the drying pipeexceeded 2.5m. It was speculated that when the length ofthe drying pipe was less than 2.5m, the sludge residencetime (less than 0.42 s) inside the combustor tailpipe onlyallowed primary atomization and therefore the pastysludge were atomized into relatively large particles. Whenthe drying pipe was longer than 2.5m, secondary atomi-zation occurred and larger sludge particles were generatedin a primary atomization stage and then further atomizedinto smaller particles. Thus, the mass percentage of smallerparticles increased sharply, as shown in Fig. 10a. There-fore, a low Sauter mean diameter of dried particles isexpected as the length of the drying pipe is increased.

Figures 9 and 10 describe a narrow particle size distri-bution obtained in PC drying of the pasty sludge. Thisphenomenon was also observed by Zbicinski for PC dryingof an NaCl solution.[16] Compared to a mean diameter of90.72mm for droplets atomized by a conventional nozzle-type atomizer, the mean diameter of droplets atomizedby a pulsation stream was reduced by almost 50%. Becausenozzles and rotary disks undergo wear in a conventionalspray dryer, the dynamics of atomization as well as the par-ticle size distribution changes. When a pulsating gas streamatomizes the fluid material, there are no mechanical partsto wear out; every droplet sees the same atomization energyand the same differential temperature. Thus, the dropletsatomized by a pulsating gas stream have a more consistentparticle size distribution over time. However, this is not acritical issue in sludge drying.

FIG. 8. Two sequential snapshots of sludge particles exiting the tailpipe:

(a) T and (b) Tþ 0.005 s (200Hz).

FIG. 9. Effects of sludge feed diameter and drying pipe length on the PC

dried sludge particle size distribution: (a) 3-m drying pipe length, (b) 2-m

drying pipe length, and (c) 0.5-m drying pipe length (color figure available

online).

FIG. 10. Relationship between sludge particle Sauter mean diameter and

drying pipe length.

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Sludge Drying Process

Figure 11 shows the final moisture content of driedsludge particles under various PC drying conditions. FromFig. 11 it can be seen that the smaller the dispersed sludgeparticles were, the lower their final mean moisture contentwas. This was expected because small sludge particles havea large surface area–to-volume ratio; heat is more easilytransferred into smaller sludge particles and thus the sludgewater is evaporated more rapidly. Figure 12 shows that thesludge final moisture content decreased with increased dry-ing pipe length and reduced feed tube diameter. A longercombustor tailpipe increases the residence time of thesludge particles and thus increases the drying time andreduces the final moisture content. In Fig. 12 it can be seenthat for a 3-m drying pipe and 6-mm feed tube diameter,the sludge moisture content decreased from initial 80% toabout 56%; that is, about 24% of the water was removedin just 0.5 s.

However, Fig. 12 also shows that the final moisturecontent was still high when particulate sludge left thepulse combustor tailpipe, indicating that only PC dryinginside the tailpipe could not reduce the sludge water con-tent to a low value. This was mainly due to the fact thatthe gas stream and particle velocity were high inside tail-pipe and there was not enough time to dry the sludgemoisture to a low value in the limited tailpipe length.Hence, a second drying stage was necessary. In this stagea spouted or a fluidized bed can be used to lower thesludge moisture content to preset value (10–15% or evenlower) because longer residence times can be provided toremove internal moisture.

Comparison of PC, Convective, and Microwave SludgeDrying

Both hot air convective and microwave drying studies ofsewage sludge were conducted in our laboratory. In the twodrying processes, the pasty sewage sludge was form into acylindrical sample with a diameter of 26mm and variousheights of 10, 20, and 30mm. Convective drying of sewagesludge was conducted using a hot air drying oven (modelGZX-9070MBE, Guangzhou Yongcheng ExperimentalEquipment Company, Guangzhou, China). Air velocitywas set at 0.3m=s and air temperature was set at 80, 100,120�C. Microwave drying of sewage sludge was conductedusing a domestic microwave oven (model G80F20CSL-B8,Guangdong Galanz Group Co., Guangdong, China) andthe microwave power was set at 567W.

Figures 13 and 14 show the drying curves obtained inthe convective and microwave drying processes. FromFig. 13 it can be seen that in the convective sludge drying

FIG. 11. Effect of sludge feed diameter and drying pipe length on the

sludge particle moisture content distribution: (a) 6-mm-diameter sludge

feed tube, (b) 8-mm-diameter sludge feed tube, and (c) 10-mm-diameter

sludge feed tube (color figure available online).

FIG. 12. Relationship between sludge final moisture content and drying

pipe length.

FIG. 13. Convective drying of sewage sludge (cylindrical sample:

D¼ 26mm, H¼ 10, 20, 30mm; air velocity¼ 0.3m=s) (color figure

available online).

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process, about 56, 90, and 110min were need to reduce themoisture content from an initial 80% to 5% for a10-mm-high sludge sample when the air temperatures were80, 100, and 120�C, respectively. Drying times were about18, 22, and 25min to decrease the moisture content from 80to 56% for all three cases. From Fig. 14 it can be seen thatin the microwave drying process about 40, 56, and 80 s wasrequired to decrease sludge moisture content from 80 to56% for a 10-mm-high sludge sample. In PC drying, thedrying time was only 0.5 s. Of course, it should be notedthat in PC drying the temperatures are high and the aver-age particle size is very small relative to sample sizes testedin hot air and microwave drying experiments. Although notests were conducted on the microbial load in the driedsludge, the operating conditions used in PC drying wouldmake the product safe from such contamination. The dry-ing time reported is only representative of what may beexpected in PC drying because this will depend on boththe sludge and the PC design and operating conditions.

Application Potential of PC Sludge Drying

The above experimental results show that the gas streamgenerated by the PC can atomize sludge into small particlesand instantly remove some part of sludge moisture in ashort time. Based on our experimental results, we believethat PC sludge drying technology is promising. For sludgelandfills or wet incineration, sludge is required be dried toabout 40–60% (wb). Only the use of a longer PC tailpipelength can meet this drying requirement without the needfor a second drying stage.

When dried sludge is used as fertilizer or for dry inciner-ation, a lower moisture content of about 10–15% (or evenlower) is required. In this case, sludge drying inside a PCtailpipe can be treated as the primary drying state. It hastwo functions:

1. Atomization of the pasty sludge into particles: This willgreatly increase the sludge surface area–to-volume ratioand intensify the heat and mass transfer between the

drying medium and sludge and thus speed up drying.Figure 15 shows the relationship between energy con-sumption and sludge moisture content during convectivedrying of a sewage sludge block. From Fig. 15 it can beseen that when the moisture content is in the range65–30%, sewage sludge becomes ‘‘plastic’’ due to volumereduction and crack formation.[25,26] This increases theresistance to heat and mass transfer, resulting in highenergy consumption, as shown in Fig. 15. Sludge atomi-zation can avoid the plastic stage and thus save energy.

2. Partial removal of particulate sludge moisture: In ourexperiments, it was observed that the surface water ofsludge particles is fully removed inside the tailpipe.These partly dried sludge particles do not agglomerateand deposit in the particle collection equipment. Partlydried sludge particles can be easily handled by theequipment used in the following stage of drying, suchas spouted or fluidized bed dryers.

CONCLUSIONS

A pulse combustion sludge drying setup was developedand sludge drying inside the PC tailpipe was studied exper-imentally. The following conclusions were drawn from thisstudy:

� The pulsating gas stream from the pulse combustorcan atomize sewage sludge into small particles.

� PC sludge drying is a fast drying process; 24% ofthe moisture was removed in 0.5 s under the con-ditions used in our experiment.

� PC pipe length and sludge feed tube size are twoimportant design parameters that should be care-fully chosen and optimized.

Further research and development is necessary todevelop a commercial PC sludge dryer. An assessment ofthe energy consumption, dryer scale-up, and emission testswill be conducted in the near future on a pilot-scale PCdryer.

FIG. 14. Microwave oven drying of sewage sludge (cylindrical sample:

D¼ 26mm, H¼ 10, 20, 30mm; microwave power¼ 567W).FIG. 15. Relationship between energy consumption and sludge moisture

content in convective sludge drying.

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ACKNOWLEDGMENTS

The authors are grateful for financial support from theChinese Service Center for Scholarly Exchange, ChinaMinistry of Education, and Tianjin Municipal EducationCommission (Project No. 20100416).

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