Experiences in Operating a Sulfur Furnace at Minn-Dak Farmers Cooperative

By

Brent Muehlberg

John Haugen

Upasiri Samaraweera

Jeffrey L. Carlson

265

Introduction

In the late 1990's, Minn-Dak began reviewing their S02 use. After the Union Carbide disaster in Bhopal, India, both OSHA and the EPA mandated new programs that were soon to be in place and operating for any company that had certain chemicals on site in sufficient quantities. Sulfur Dioxide is one of those chemicals. OSHA's new Process Safety Management Rules and the EPA's Risk Management Program required, among other things, tnat a trained Haz-Mat team be available, along with the equipment needed to do their jobs in the event of an accident. These programs are costly to implement and maintain.

Another reason to review our S02 use was to reduce the danger to, and discomfort of our employees. The liquid S02 lines ran through the inside of our main building in the area of the pulp presses and new diffuser. This area had been added on to several times in the past and the S02 lines had been re-routed to accommodate the construction. Pinhole leaks would cause lingering S02 on the beet end and made working there uncomfortable.

The third reason was the danger to the public. We had an aging 90-ton storage tank that could have posed a danger to the public miles away had there been a tank rupture. As a company, we had to consider that liability in the event of an accident.

Options

Options we considered were to discontinue using S02, and find an alternative chemical or chemicals, updating our liquid system, or installing a sulfur burning system.

No other suitable alternatives were found to replace S02. The other chemicals were more costly and not as functional as S02.

Updating the liquid system would have involved installing a new tank with a containment basin, adding a new building to house the S02 evaporators (The evaporators were located inside and were also a source of leaks· that caused discomfort to employees), and re-routing all liquid piping outdoors. It would also have involved installing new automated safety controls and shut-offs.

Even after this update, we would still be required to have a Risk Management Program, and we felt that even though reduced somewhat, our risk and liability of accident was still high. We felt that our company would feel impact even if an accident occurred at some other company or while the liquid S02 was being transported. An accident anywhere would at the least raise the price of S02, raise the cost of insurance, and add a public relations burden. At the worst, S02 might not be available, or be able to be transported, or the liability of an accident could put our company out of business.

A new sulfur burning system would have a higher initial cost, but would have a lower operating cost, no Risk Management Program would be needed, and the danger to employees and the public would be very much reduced. The estimated costs are compared in the following chart.

266

Liquid S02 Sulfur Stove Initial Cost $349,000 $912,000 Sulfur and Liquid S02 $110,000 $36,500 Emergency Response Equipment

$2,000 ($20,000 one time already spent but will need updating)

$0

Initial Emergency . Response training for 5 people

$2250 I expect a 5-person per year turnover of people

·$0

Annual Refresher training for 30 Emergency Response people

$11,400 (we have a total of 35 trained but 5 are getting the initial training)

$0

175 hours of Annual Training Drills for Emergency Response People

$2,800

Unloading Expense $22,000 (20 hours of unloading for 9 rail cars)

$5000 Unloading of Trucks of Sulfur

Risk Management Program and Mana~ement Expense

$3,200 to $5,200 annual expense

$0

Maintenance Expense $10,000? $20,000? Total Annual Expense $164,650 $61,500

Initial cost is the cost for equipment, controls, and installation, and was about 2Y:z times higher for the sulfur burning option.

The next line is the annual cost of equal amounts of S02. Not only is sulfur less expensive than S02, but you also get 2 tons of S02 for each ton of sulfur you burn.

Four out of the next 5 lines are estimated annual costs for maintaining the Risk Management Program, Haz-Mat team, and associated training and equipment. None of that is needed with the sulfur burning system.

Unloading expenses are also higher with the liquid S02 system because it takes longer and requires a trained employee to be constantly at the site.

Total annual expenses were expected to be about $100,000 a year less with the sulfur burning system.

We did consider a molten sulfur system. Sulfur melts at about 120°C and some systems receive, store, and use the sulfur in the liquid state. In this case, all tanks, lines, pumps, etc., must be continuously heated above 120°. We had experience with this type of system working with #5 fuel oil. The required temperatures for #5 are much lower that that of sulfur. We anticipated problems and felt it wasn't practical, especially in North Dakota.

Operation of New System

In the end we decided on a sulfur burning system that would burn sulfur pellets. The system was installed in the inter-campaign of 2001 and consists of the following as described below:

267

Figure 1: Unloading Hopper, Feeder, and Bottom of Bucket Elevator

• A sulfur pellet storage silo with unloading equipment. The silo can hold about 40 tons of sulfur pellets. The silo sets on load cells so that daily inventory and usage figures can be calculated. Sulfur is received in end dump trucks, similar to a grain truck. The sulfur is dumped into a hopper. A screw feeder pulls the sulfur pellets out of the hopper and feeds it to a bucket elevator, which lifts the pellets to the top of the silo. The unloading system has a dust fan that puts negative pressure on the elevator and screw conveyor to pull in any sulfur dust. The dust is collected in a bag house and dumped back into the sulfur pellet silo.

Figure 2: Top of Elevator and Sulfur Pellet Silo

268

• The system includes two sulfur furnaces with feeding systems. The sulfur is fed out of the silo with a cell lock type feeder, into a screw conveyor, through a diverter that switches the flow to one furnace or the other, and into one end of the furnace. Each furnace has a gate valve that closes off the inlet chute after the sulfur is fed, primary and secondary air valves, and a water-jacketed after­cooler.

Figure 3: Sulfur Furnace with Feeding System and Controls

• Because of the low pressures involved, absorption towers are required for the S02 to be absorbed into the juice or water. We installed one for thin juice and one for diffusion make-up water.

• The last part of the system is the induced draft fan . A fan sucks the S02 depleted gas out of each of the absorbers. This negative pressure sucks the S02 laden gas from the furnaces and creates the negative pressure necessary for the furnace to suck in combustion air through the primary air valve.

A nice feature is that the entire system is under negative pressure, including the furnaces, pipelines, and absorbers, so any leak will suck air in, but not let any S02 laden gas out into the atmosphere.

The amount of negative pressure in the furnace is controlled by the primary air valve. As the negative pressure increases, the valve opens to relieve the vacuum. As the negative pressure decreases toward 0, the valve closes to maintain the desired negative pressure.

The sulfur burns in a molten pool about 3" deep at the bottom of the furnace.

Three thermocouples measure the temperatures at about 2, 3, and 6" off the bottom, and are used to indicate and control the depth of sulfur.

The middle level thermocouple detects a low level by indicating a rise in temperature. The rise in temperature initiates a feed cycle. When the feed cycle is initiated, the inlet gate opens , the diverter gate diverts the sulfur flow to the correct furnace, and the feeder cell lock starts and runs for a predetermined time.

269

This feeds a predetermined amount of sulfur into the furnace where it drops into the molten pool, melts, and raises the level enough to cover the thermocouple, dropping the indicated temperature again.

Figure 4: Thin Juice Absorber

The 802 laden gas then goes through an ash drop-out zone in the furnace, then through an after-cooler to reduce the temperature of the gas to below 200°C, and then throu·gh a pipeline to the absorber.

The absorbers are vertical cylinders with conical bottoms. The 802 , and juice or water, flow counter current to each other. The 802 laden gas enters toward the bottom and is drawn up through the juice or water by action of the draft created by the 10 fan. Juice or water enters toward the top and into a pan with distribution holes that create a rain of juice or water down through the absorber. The body of the absorber below the distributor plate is filled with bent metal pieces that create a splashing, cascading effect. The 802 is readily absorbed leaving only 802

depleted gas at the top of the absorber to go to the JO fan.

Each of the 10 fans suck the 802 depleted gas from its respective absorber and blows it to the wet scrubber on the main boiler stack, where any remaining 802 is removed before it is discharged to the atmosphere. The 10 fans are driven by electric motors with VFO's (Variable Frequency Drives). The speeds of the 10 fans are controlled by the output of the respective controllers for either the diffusion make-up water pH or the thin juice 802 ratio.

Problems

When the system was engineered, the furnaces were sized to be able to run both absorbers on one furnace. That meant we had two 10 fans pulling 802 from one furnace. This caused control interaction because when one fan started to increase in speed, it sucked flow away from the other absorber, which caused an upset in the other process. We found that the control on both processes was much better when running each absorber and 10 fan on their own furnace.

270

Running each absorber on its own furnace solved the control interaction, but created another problem. Being the furnaces were sized to run both processes, when running only one absorber, each furnace was oversized.

This resulted in spotty (here and there) type burning. Sometimes there wouldn't be a flame near the control level thermocouple, and it would miss enough feed cycles to burn itself out and go empty, or at least require some manual operation to get the level back up to where it controlled automatically again.

In order to solve this problem, we reduced the size of the sulfur pool by making two hollow, SS blocks, which were installed one along the side and one at one end of the furnace burning zone. This contained the pool and flame to a smaller area to assure that there would always be a flame near the thermocouples. We still have enough capacity to run both absorbers on one furnace if we have the other furnace down, and were told later by the start-up consultant that the furnace's capacity turns up much better than it turns down.

We also changed the control sequence to provide a secondary feed cycle trigger that will initiate a feed cycle if the low level temperature gets higher than the control level temperature. This would happen if the level gets low enough that the middle thermocouple is out of the flame far enough that it starts to cool off from the combustion air and the low level thermocouple starts to get into the flame.

We also experienced some plugging problems in the after-coolers and the piping to the absorbers. This problem got a little better after reducing the size of the sulfur pool, most likely because of higher temperature and more efficient burning, but has not completely gone away.

Realizing that we may have to clean them from time to time, we installed some blind flanges and flush water lines to facilitate faster and easier cleaning.

We also have some ash build-up in the sulfur pool, which requires cleaning after about 120 days in order to keep the automatic feed controls working.

The last problem we encountered was not with the furnace, but with the absorbers. We found that too much water or juice flow through the absorber seems to cause a stoppage of S02 flow. We were already controlling the flow of thin juice through the thin juice absorber, because, in order to save space and money on the size of the thin juice absorber, we sized it for only about 1/3 of the thin juice flow. This flow was controlled by a flow controller and control valve. The rest of the thin juice bypasses the absorber and mixes with the flow from the absorber in the sulfitated thin juice tank before being filtered again. When the flow to the absorber was increased over a certain amount, in this case about 700 gpm, the ID fan would speed up to maximum speed and yet the flow of S02 wouldn't increase, but instead would drop off. By dropping the juice flow, the S02 flow would return to normal.

The diffuser water absorber is sized to take the whole flow. When a surge of diffuser water was needed, we lost control of the pH. Because of what happened with the thin juice absorber, we thought this loss of control might be because of too much flow through the absorber. To solve this, we installed an orifice plate in the water line just before the absorber and an overflow loop just before the orifice plate that bypasses excess water around the absorber directly to the Diffuser Make-up Water Supply Tank. The bore of the orifice and the height of the overflow is calculated so that when flow increases, the pressure upstream of the orifice raises

271

a column of water high enough to run over the bypass loop. This has solved the loss of control problem during high flow conditions.

It seems to work well to over-treat a portion of the juice or water, and then mix it with untreated juice or water, to end up with a final desired S02 concentration.

Conclusion

In conclusion, I can report that we have successfully operated the furnaces for 1:h campaigns with good control of S02 use.

The cost of sulfur has been reduced from 5.8 to 1.1 ¢ per ton beets, which for MD means an annual savings of about $100,000.

The Risk Management Program' has been discontinued because we now have no toxic chemicals on site in sufficient quantities to require one.

The danger to the employees and the public, and the risk to the company have both been greatly reduced.

272