WET SCRUBBER STACK EMISSION TEST RESULTS

FROM THE NEW TRIPLE-PASS PULP DRYER

AT THE HJLLSBORO FACTORY OF AMERICAN CRYSTAL SUGAR COMPANY

Stanley J. Selle l, Michael D. Hohl2, F . A. (Tony) Heinbaugh\ Brad Carlson4 and Olivier Deur

lNorthwest Research, Inc. P.O. Box 5156, Grand Forks, ND 58206-5156

2Dakota Machine, Inc. 420 East Main Avenue, West Fargo, ND 58078

3American Crystal Sugar Company 101 North Third Street, Moorhead, MN 56560

4American Crystal Sugar Company RR2, Box 42, Hillsboro, ND 58045

5Maguin-Promill B.P. 239, F-28104 Dreux Cedex, France

INTRODUCTION

American Crystal Sugar Company (ACS) recently completed expansion of its Hillsboro factory in eastern North Dakota. As part of the expansion, the existing pulp dryer facility was rep laced. The new system included a Prom ill triple-pass dryer. Although widely used in Europe and elsewhere, this was the first application of this technology to sugar beet pulp in the USA. It is also the largest capacity Promill triple-pass sugar beet pulp dryer system in the world.

Dryer system components will be described here, along with a brief discussion of system performance. A key feature of the Pro mill design is provision for controlled emergency shutdown, utilizing a refractory guillotine to isolate the furnace exit from the dryer drum. Heat for drying is provided by a traveling grate furnace fIring a low-sulfur Montana subbituminous coal. Particulate emissions are controlled using a wet scrubber. The wet scrubber achieves zero discharge to wastewater systems by transferring blow down to the inlet of the dryer drum. The results of stack gas and particulate emission testing will be presented.

SYSTEM DESCRIPTION

A schematic overview of the new pulp dryer system at the Hillsboro factory is provided in Figure 1. High temperature gases enter the dryer directly from an overfeed stoker, traveling-grate furnace burning a Montana subbituminous coal. The original furnace was modified to provide the

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larger heat input required to meet the demands of the new, higher capacity dryer system. A mixture of inside and outside air is delivered to the furnace grate by the primary air (FD) fan. The primary air is preheated while passing through ductwork around the base of the emergency stack and outer cooling shell of the furnace. Primary air flow measurement allows control of the air­to-fuel ratio at the grate. A small quantity ofsecondary air is distributed across the front of the furnace, just above the coal gate, to ensure complete combustion and minimize CO and NOx production.

Figure 1. Overview, Hillsboro Dryer System.

Coal Bunker Stack(Recycle c I "HYDRONAT'OS=n1ttary!r J Pressed Pulp

1I .

~VJ~ ~ Furnace

nSurge

T Bin Scr~ll Primary 1 AirD C:J

L ~1

1...(_-+__•___.-___ Pulp is delivered directly from the presses or reclaim system to the pressed pulp surge bin.

The surge bin includes a set of short scrolls to discharge pulp from the center of this "live" bottom bin. The rpm ofthese short scrolls controls the pressed pulp feed rate to the dryer. The pressed pulp feed rate setpoint is programmed to respond to and maintain an operator-selected temperature at the dryer drum exit (or at the inlet of the product separation cyclones). Pressed pulp feed rate is measured by a weighbelt, which discharges into the dryer drum inlet isolation scroll.

The Promill dryer is a triple- Figure 2. Promill Triple-Pass Dry r Drum Schematic.

E~~i;=]c*i0=s~:~: ~""SOO ful~~:e; . area increases from inlet to outlet. The gas velocities and temperatures in each

pass are designed to optimize drying H~o~J.i~~:=5~~t

'~[I[iil~~affi.~JiliE~tll r-----;;:-=::--:--:--.---:-::---;-;-----------;-,,_:__--,

and transport of pulp particles. In the Gas first pass, surface moisture evaporation occurs quickly. The second and third sections are designed to provide longer treatment times to evaporate internal particle moisture. The Promill triple-pass dryer design ensures very rapid transfer of smaller, lighter particles to cooler sections of the drum. The average residence time is 8 minutes, compared with 20 to 30 minutes in a single-pass dryer. Design parameters for the Hillsboro dryer are provided in Table 1.

The Promill design emphasizes convective "in-flight" heat transfer. Pulp transport is a function of gas flow volume and drum rotational speed. More conventional, single-pass dryers are filled with metal surfaces designed to affect a tumbling and resuspension of solids to maximize exposure to hot gases. By contrast, the Promil1 dryer internals are virtually clear, containing only enough blades to maintain particle entrainment in the gas and optimize heat transfer to the pulp.

To

Wet Scrubber Sludge to Dryer (Slowdown)

148

The Promin dryer is a constant gas flow device. Therefore, dryer gas recycle plays a major role in optimizing perfoITIlance. Under low pulp feed conditions, the recycle rate can be increased up to 50% of the total dryer gas flow. This serves to maintain high dryer gas flow while reducing stack gas flow and heat loss. At high pulp throughput, recycle rates may be reduced to around 25%. In automatic control mode, the recycle flow is adjusted to maintain furnace static pressure.

The pulp dryer system at Hillsboro uti lizes two large cyclone collectors to separate dried pulp from dryer exhaust gas. The exit duct from each separation cyclone leads to an ID fan, as indicated in Figure 1. The internal surface of the upper inlet section ofeach cyclone was lined with ceramic tiles to resist abrasion. The major diameter of each

Dryer Drum: Diameter, feet Length, feet Rotational Speed, rpm

Rated Pressed Pulp Flow, Tph

Design Pressed Pulp H20, %

Dried Pulp Flow, Tph

Dried Pulp H20 , %

Evaporation, Tph: Pressed Pulp Sludge Return (Blowdown)

Total

Design Gas Temperatures, °c: Furnace Exit Dryer Drum Exit

Table 1. Promill Pulp Dryer Design Parameters.

24.54 77.08

2 to 2.5

95.27

76.65

24.44

9.00

70.83 -.l.]l 72.04

825 ± 25 122

cyclone is 15-Ii et, 8-inches, with an overall height from hopper bottom to roof of 41-feet, 4 Y:z-inches. The dried pulp is removed from the cyclone collection hoppers through rotary airlocks. A short chute leads to the top of the main dried pulp scroll, which can deliver product to any of three peliet mills; or to the dried pulp surge bin or dumping scroll.

The primary pulp dryer operating risk is fire. The Promill dryer design tends to minimize fire potential because of the following characteristics: .

• Rapid transport ofthe smallest and driest particles to the cooler regions ofthe dryer, • Minimal dryer gas oxygen content due to high recycle rates, • More consistent product moisture content, and • Lower dryer inlet temperatures.

In addition to a reduction in fire potential, an important feature of the Promill dryer system is provision for automatic emergency shutdown under designated conditions. In the event of high dryer exit temperature, high furnace exit temperature, loss of drum rotation, loss of ID fan, interruption of pulp feed, failure of separation cyclone airlocks, or system power failure, the Hillsboro dryer system enters a "fail safe" condition and shuts down immediately. The dryer operators can also initiate an emergency shutdown either by hitting a manual "fail safe" button on the control room wall or selecting the "fail safe" icon on the computer control screen. Immediately upon the initiation of "fail safe", the following actions occur:

,/ Refractory guillotine drops, isolating the dryer from the furnace, ,/ Water spray at the drum inlet through 4 nozzles for 20 or 25 seconds, ,/ Pressed pulp feed stops, ,/ ID fans stop,

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./ ID and recycle dampers close,

./ Furnace emergency stack opens,

./ Drum inlet feed scroll vent opens, II' Vacuum break valves on ID and recycle ducts open, ./ Coal grate stops, and ./ Primary and secondary air fans stop.

Restart ofthe system following a "fail safe" event is accomplished by reversing the actions listed above, provided that the instigating condition has been cleared. The viability and usefulness of this "fail safe" feature was demonstrated sev~al times during the shakedown and initial startup of the Hillsboro dryer. A number ofpotentially troublesome events were safely and conveniently dealt with and, most importantly, no fires occurred.

WET SCRUBBER Figure 3. Wet Scrubber Systems.

Particulate emissions controlled by a wet scrubber.

are The Recycle I Stack

scrubber design utilizes multiple nozzles spraying water at four levels. The system layout is illustrated in Figure 3. Either seal or industrial water may provide makeup water. Scrubber pH is monitored on the inlet side of the circulation pumps and controlled by addition of caustic from the main factory. It is expected that, under normal operating conditions, the operators will only be required to monitor sludge flow to the dryer inlet. The wet scrubber system specifications at design capacity are given in Table 2.

A cutaway view of the Promill Hydronat Table 2. Wet Scrubber Specifications .

wet scrubber is provided in Figure 4. The scrubber operates as a spray tower with fOUI levels of spray. The principle of operation is described

Scrubber Exit Gas Flow, Acfm

Scrubber Pressure Drop, in-H20

169, 100

2.5 5

as follows: l Scrubber Water Circulation: Flow, gpm 4,000

.. Dirty gas flows up through a colwnn Pressure, psig 72

ofwater sprays (B, c). While passing Makeup Water: through the sprays, particles are Flow, gpm 20 captured by liquid droplets . Pressure, psig 30

Subsequently, the water droplets and Sludge Return to Dryer: captured particulate are removed from Flow, gpm 5 the gas by high speed centrifugation Pressure, psig 15/45

Solids, % 3

t "HYDRONAT" Wet Scrubber

150

through a droplet separator fitted with radial blades (D). The cleaned gas then exhausts to the atmosphere through the stack (E). A droplet trap at the top of the stack completes the cleaning process by retaining any water droplets which might have formed in the stack.

~ The scrubber water is continuously recirculated (G).

~ To avoid saturating scrubber water with dust, which would obstruct the spray nozzles and decrease efficiency, scrubber liquid is continuously drained by means of a small pump (K) and returned to the dryer inlet pulp feed scroll. This blowdown is compensated for by continuous water make-up (1).

The vendor further describes a four stage particulate removal process: J

Stage 1: Precleaning zone. Figure 4. Promill Hydronat Wet Scrubber The gas enters tangentially at the bottom ofthe scrubber column, where HYDR01VATlarger particles are removed by the

WET SCRUBBER cyclonic action (A). The gas stream passing up into the first section of

A. Tangenti al Inlet spray nngs is then saturated with water (B). 13. Pre-washing Zoae

Stage 2: Cleaning zone. The E c. Washing Zone

gas exits the prec1eaning zone through D. Demisteran annular opening of decreased area,

formed by a conical plate placed E. Cleaned Air Exhaust where the vessel diameter increases.

F . Water Tank Gas acceleration and turbulence are thus created. The gas then passes into

G. Recirculation P ump an area of high intensity water spraying (high ratio of water to gas) c

H. Sludge Pump producing very intimate contact

I. Level Tank between gas and cleaning water (C).

J. W ater SUpplyStage 3: Drop separation.

Water droplets remaining in the gas K. Sludge Discharge stream are removed by high speed

L. Discharge centrifugat ion as it passes through a static radial bladed demister (D).

Stage 4: The gas stream then passes to atmosphere from the stack (E), at the very top of which is a circular droplet trap with a drain to the scrubber vessel.

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Proper performance at each ofthese four stages should allow removal ofpaI1icles within the 5 )lm to 10 )lm range and meet the performance guarantee. It is clear from this description that all regions of the Promill Hydronat wet scrubber contribute to the proper function of tills particulate collection device, including any cyclonic action in the stack.

COMPLIANCE TEST RESULTS

To meet State ofNorth Dakota permit requirements, compliance tests were performed on the Hillsboro dryer stack during December 1998. The tests were performed by Nova Consulting Group, Inc. (NOVA). An initial engineering test series indicated that the flow in the stack was more cyclonic than is acceptable when using the standard EPA Method 5 for particulate concentration in stack gas. They also found that the high stack gas moisture content precluded the measurement of PM IO concentrations.

Cyclonic flow is defined as gas flow which is not in a direction parallel to the axis of the stack. In order to detect the degree of nonparallel (cyclonic) flow, the angle between the plane perpendicular to the stack axis and a pi tot tube orientated to produce a null reading is determined at each sampling point in the stack. The EPA specifies that ifthe average of the absolute values of the angles is greater than 20 0

, the flow conditions are unacceptable? NOVA subsequently used a three­dimensional Pitot tube to verify the presence of cyclonic flow on the Hillsboro dryer stack. 3

Measurements during the compliance tests showed the average of the angles to be 62 .5 0 4 •

EPA Guidelines recommend three options when cyclonic flow is encountered:5

1. Find another sampling location, 2. Install flow straightening vanes upstream of the sampling location, or 3. Apply one of the modified sampling procednres.

The design characteristics ofthe Promill wet scrubber at Hillsboro make the first two options very difficult and expensive. The choices for modified sampling procedures are the alignment approach and a time-weighted alignment method.

The Alignment Approach. This approach involves turning the sampling nozzle into the direction ofgas flow assuming essentially tangential flow and ignoring the radial flow vector.

The Time-Weighted Alignment Method (Texas ACB Procedure).6 The tirne­weighted method is a refinement of the alignment approach in which compensation is made in the sampling time at each point by adjusting the dwell time proportional to the cosine of the angle of flow at that point. The purpose of the adj ustment is to account for the stratification ofparticulate matter caused by the centrifugal action of the cyclonic flow.

The biases inherent in the application ofeither modified sampling method appear to be in the direction ofmeasured particulate concentrations exceeding the actual concentrations in the gas. The

152

error, and the degree to which the particulate concentrations can be overstated, increases as particle .. 5

slzes mcrease.

Compli ance tests were performed on December 3-4, 1998. NOVA used the Time-Weighted Alignment Method (Texas ACB Proceduret for particulate measurements. Average system operating condilions during the compliance tests are summarized in Table 3. The water evaporation rate of 5 8 Tph is just over 80% of the maximum value of 72.04 Tph

Table 3. System Operating Parameters During specifi d by the vendor in Tab le 1. Compliance Tests, H illsboro Factory Dryer operation was very satisfactory

at these conditions compliance tests.

during the Parameter Average Value

Pressed Pulp Feed Rate, Tph 82.0

Compliance test results and Pressed Pulp Moisture , % 75.9 applicable emlSSIOn limits are Dried Pulp Moisture, % 8.5 reported in Table 4. The H illsboro pulp dryer system bas passed all applicable stack emjssion standards

Evaporated H20 , Tph

Dryer Gas Recycle, % of Total Gas

58.0

27.1

imposed by the State of North Cyclone Pressure Drop, in-H2O 5.9

Dakota. Wet Scrubber Pressure Drop, in-H2O 2.2

Wet Scrubber Water Supply, psig 59

Wet Scrubber Blowdown, % solids 2.5

Wet Scrubber pH 6.5

Stack Gas Analysis, % by volume: Moisture 43.3

02' dry 13.1 CO2, dry 6.9

Table 4. Allowable Emission Limits and Compliance Test Results, H illsboro Pulp Dryer.4

Air Contaminant Maximum Allowable Emission Limit Compliance Test, average

Particulate Matter/PM lO, Ib/hr 52 .0 46.114

Particulate MatterlPM10 , gr/dscf not applicable 0.081

S02, 1b/hr 63.3 11.55

S02, ppm not applicable 8.13

NOx' lblhr 100.0 46.91

NO , ppm not applicable 47.52x

Opacity, % 20 not measurable

290.41CO,lb/hr 461.0

481.24CO, ppm not applicable

18.12voe (THC) , lb/hr 92 .1

153

,- ­

REFERENCES

1. Beetpulp Dlying Unit Training and Operation Manual, Maguin Promill, September 1, 1998.

2. Peeler, l, Isokinetic Particulate Sampling in Non-Parallel Flow Systems - Cyclonic Flow, Entropy Environmentalists, Inc., Research Triangle Park, NC, 1977.

3. Tussey, L., Determination ofStack Gas Velocity and Volumetric Flo'vl' Rate Using a Directional Pitot Tube, Emission Measurement Center, Approved Altemative Method ALT-015, November 9,1988. .

4. Emissions Testing Report, Compliance Emissions Testing, American Crystal Sugar Company, Hillsboro Facility, Hillsboro, North Dakota, Nova Consulting Group, Inc., January 25, [999.

5. Westlin, P.R., Particulate Sampling in Cyclonic Flow, Emission Measurement Technical Information Center, EMTIC GD-008, October 3, 1989.

6. Stack Sampling Cyclonic Flow, Appendix H, Sampling Procedures Manual, Texas Air Control Board, Revised July 1985.

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