PILOT TESTING OF A VITRIFICATION SYSTEM FOR LOW-LEVEL RADIOACTIVE WASTES

Prent C. Houck
ATG Inc., Allied Technology Group (ATG)

ABSTRACT

ATG Inc., Allied Technology Group (ATG) is a technology development company committed to recognizing complex waste problems and providing technology for their solutions. The low-level dry active waste stream from nuclear power, R & D, and industry is amenable to thermal destruction. ATG is developing a commercial, multi-zone joule heated vitrification process system for thermal destruction of carbonaceous materials and radionuclide incorporation into glass prior to disposal. ATG conducted a series of pilot tests with a proto-type joule heated vitrification system. The results of DAW surrogate testing using wood, paper, cardboard, plastic, cloth, and rubber showed rapid depletion of oxygen with plastics and much lower oxygen consumption rates and processing rates for cellulosic materials and rubber as compared to plastic. These results provided the qualitative and quantitative data for the design basis of a full-scale multi-zone vitrification system. A full-scale system for thermally treating low-level radioactive wastes has been permitted and designed, and is in the final stages of construction and shake-down.

INTRODUCTION

ATG Inc., Allied Technology Group (ATG) is a low-level radioactive waste processing company that has developed an environmentally friendly, non-incinerator thermal treatment technology. Several alternatives were tested or evaluated over a five year period, and a multi-zone vitrification process was determined to be the most feasible process. This paper discusses the results of prototype tests undertaken to finalize the system design for a multizone joule heated vitrification process

Single zone joule heated vitrification systems have been used for vitrification of solids bearing liquids and organic ion-exchange resins. Joule heated vitrification systems have been employed in the glass industry for many years. Joule heating is a method that uses electrical energy to heat a molten pool of glass. Glass becomes electrically conductive about 1500°F. The electricity heats the glass by passing an alternating current between electrodes immersed in the molten glass. The electrical resistance of the glass dissipates the electrical energy as heat.

A single zone vitrification approach is not efficient for treatment of dry active waste that normally consists of wood, plastic, rubber, cellulose, clothing, and paper. In a multi-zone approach, the disintegration of the solid material and destruction of organic gas occur in two or more sections of a vitrification process chamber.

Pilot testing in a 100-125 lb/hr vitrification test process chamber was conducted in a series of four campaigns in 1996. These campaigns and their results are described in this paper.

System Description

The initial test system setup included a process air forced draft fan, induction draft fan, joule effect process chamber, caustic scrubber, power supply system, and cooling water system. Data was collected manually using a rotary selector switch to select between thermocouples. Power consumption for the joule heating system was read off the installed analog panels. A third party testing firm was contracted to measure air emissions and collect stack samples for analysis. The furnace draft and process air distribution adjustments were all manual. The materials processed consisted of premixed compositions of wood, cardboard, plastic, and rubber. The surrogate mixtures were put into plastic bags and packed into 15" x 15" x 20" cardboard boxes. The boxes were RAM fed into the process chamber. The purpose of the first test campaign was to verify system claims, understand how the system would react to the different waste streams, assess whether further testing should be conducted, and to determine the project viability.

A second campaign of tests was conducted to determine the maximum size of a bag of waste that could be introduced into the system without causing an upset. Feeding bags to the process chamber was determined to be a viable feed method. The system setup was the same as the first campaign of tests, except that the ram feeder was removed. Bags of surrogate materials were admitted to the process chamber by opening a port and tossing a bag into the chamber. Pure waste streams (wood, plastic, cloth, cardboard, and rubber) were bagged in plastic bags of various weights (1 lb, 2 lb, 5 lb, 8 lb, 10 lb, 15 lb, and 20 lbs). A third party analytical laboratory brought in instrumentation to collect data continuously for O₂, NO_x, CO, and CO₂ and to collect grab samples for further analysis.

After the first two test campaigns, the system was substantially modified over a 10 week period. The modifications included adding air injection ports, fabricating a crude lock-hopper batch feed system, addition of a continuous feed system, off gas cooling and filtration, continuous draft control, process air control, addition of primary control loops, and addition of process instrumentation for data collection. Two sets of tests were conducted using the modified system. A mass and energy balance program was created to model the process and interpret and predict the test results. This work tested the process control loops and improved feed systems, and was intended to identify problem areas for designing a commercial system.

Proof of Concept Test Results

A total mass of 1,230 lbs was processed during an 11.7 hour period. The systems capacity was determined to range from 68.4 - 133.6 lb/hr depending on the surrogate waste mixture (see Table I), a mass weighted average feed rate of 110.3 lb/hr was calculated. Several trends emerged from the test data. The processing rate is directly related to the bulk density of the materials, the energy value of the materials, the volatility of the organics, the mineral ash content of the materials, and oxygen supplied. The feed rates observed are estimates since the waste was fed by hand and visual observations were used to determine when the next batch could be started, but the trends were clear.

When the energy density of the material is examined, the control issues become obvious. Typical energy values and bulk densities for the materials are summarized in Table II along with their energy densities. Rubber has the highest energy density and lowest processing rate, and plastic has the lowest energy density and the highest processing rate. Wood and cardboard are mid range in energy density and feed rates. Additionally, plastic has a very high volatility compared to the other materials. The limiting factor for feeding plastics is the availability to supply oxygen and control the amount of heat released, which is critical in the event that the supplied bulk density were to increase significantly. For example, if the bulk density of the plastic approached the theoretical bulk density for polyethylene plastic (55-59 lb/ft³), the energy density would be 1,083,000 Btu/ft³. The volatility of materials is also a key factor in determining processing rates, since low volatility materials tend to carbonize and accumulate on the surface of the molten glass.

An analytical testing matrix was completed by a third party analytical lab. The test matrix is summarized in Table III and the results in Table IV. The air emissions were generally lower than expected with CO and Total Hydrocarbons (THC's) levels typically zero, except during periods of loss of process control (e.g. over feeding). Particulate emissions were high overall, resulting in the air emissions testing filters plugging up in periods ranging from a few minutes to 30 minutes. Normally these filters will last for 1-2 hours before plugging. NO_x and SO₂ emissions typically ranged from 35-50 ppm and 15-50 ppm respectively.

Particulates were analyzed for the mineral ash content to identify the substances. The content was predominantly SiO₂, Na₂O, K₂O, and P₂O₅. The ratios of the mineral ash results indicate that the particulate collected was mostly glass.

The first test campaign results were collected with an operating system that had no process control capability and with a crude caustic scrubber for off-gas treatment. Emissions data was collected upstream of the limestone caustic scrubber.

Several other key observations were made during the first tests: 1) oxygen levels below 8% generated conditions which produced smoke, particulates, CO, and THC's, 2) smoking conditions were associated with overfeeding conditions, and causes carbon and inorganics to accumulate rapidly on the molten glass surface, 3) increasing the volume of the process chamber above the glass surface would improve operations and increase the feed rate capacity, 4) rubber products produced a highly sticky particulate in the off-gas to the point that it began plugging up the air emissions sampling filters immediately, 5) some further development described below would clearly be required.

Capacity Testing Results

The system was configured exactly as the first test. Waste was packaged in smaller unit sizes and was not packaged into boxes. Waste was fed into the process chamber by opening a port and tossing a bag in. A third party independent laboratory was present to continuously monitor levels of O₂, CO, CO₂, moisture, and flow rate. The purpose of these tests was to determine the maximum batch size of a given waste type and to correlate furnace emissions data with process variations. Bags of surrogate materials of a single type were prepared in various sizes (1 lb, 2 lb, 3 lb, 5 lb, 10 lb, 15 lb, and 20 lb). The materials tested were wood, cardboard, polyethylene plastic, and rubber boots.

Typical test results showed a rapid depletion of oxygen to zero % in 20-60 seconds followed by recovery in a total of 3-4 minutes. Carbon monoxide levels increased to 100-2,000 ppm after 1Þ minutes. Results varied by waste types. The most rapid and complete oxygen depletion was caused by plastic followed by cardboard, wood, and rubber. The reaction rates and volatility of cardboard, wood, and rubber is significantly lower than plastic. This results in a much lower oxygen consumption. The oxygen level did not drop to zero. Carbon monoxide levels also did not rise as high nor as rapidly as for plastics.

The lower feed rates and oxygen consumption trends observed for cardboard, wood, and rubber were directly related to accumulations of carbon and minerals on the glass surface, and the lower surface area of these surrogate materials. Shredded and low density materials have more available surface area and react faster than unshredded materials. Cellulosic and rubber materials are less volatile than plastic and tend to reduce to a mass of carbon and mineral ash. The carbonized material and mineral ash accumulates on the glass surface in a pile. As the carbon and ash accumulates in a pile, it appears to restrict the gas solids contact required for reaction. While the mineral ash is soluble in molten glass, carbon is not. The result is that carbon forms a layer on the glass surface that hinders flow and absorption of mineral ash into the glass.

For this particular process chamber and configuration, a maximum batch size was found to be 3-5 lbs every 1-3 minutes. It was clear that improving the air distribution, mixing, and supply rate would increase the process feed rate while improving emissions. It was additionally observed that the accumulation of carbon and ash would be another limiting factor, and that some other means would be required to reduce the waste pile accumulations.

Pilot Scale Testing Results

The system was reconfigured to simulate the full scale commercial process system. The process added a quench system immediately after the process chamber. Oxygen was monitored by a continuous oxygen sensor in the process line upstream of the quench system. A baghouse was installed after the quench system and followed by a rotary lobe blower with speed control. Process air was supplied by a fan with 3.3 times the original capacity and with a control damper. Modifications were made to increase the number of air injection ports from 8 to 17. Additionally, a new manual air-lock batch feeder was fabricated and fitted to the side of the process vessel. A pressure transmitter was added to the process chamber to monitor and control the process vessel pressure. Attempts were also made to seal up numerous air infiltration points in the process chamber.

The third test campaign was initiated using the modified system for several hours. There were some initial problems with the quench system due to an initial loose fitting. Testing with the batch feeder through the side of the process chamber proved unsuccessful, but was attributable to the inability to modify the existing vessel ports in such a way that we could get the desired angle of entry. As a result, material accumulated at the mouth of the process vessel. The test was aborted and plans were made to modify the furnace for top entry feeding.

The problems with the quench system were resolved and modifications were made to the process vessel and the batch feeder that allowed us to use the air-lock batch feeder from the top of the process vessel. All the other controls and process instrumentation worked properly.

The fourth test campaign was started by feeding shredded plastic, cloth, wood, rubber, and cardboard sealed in polyethylene bags containing 1-3 lbs of a single type of material. The feed rates and quantities of each material processed during this campaign are summarized in Table V. The 1-3 lb bag sizes worked very well with no hang-ups in the batch feeder chute. An auger type feeder was also tested, but it clearly needed the surrogate material to be shredded to a smaller size and a different type of auger.

Feed rates ranging from 30 lb/hr (rubber) to 240 lb/hr (plastic) were achieved. A pile of carbon and mineral/ash formed directly underneath the feeder positions. Special air injection devices were used to distribute the waste pile. This had mixed results; the effect was extremely localized.

In contrast to previous campaigns, which relied on third party sampling and testing, stack emissions were monitored periodically with an ECOM-AC Model 69298 portable gas analyzer equipped with cells to measure O₂, CO, CO₂, NO, NO₂, and NO_x. Oxygen levels ranged from 3.8% to 19.9%, CO levels was 0.0 ppm throughout the tests, and CO₂ ranged from 0.6 to 3.5%. The NO_x measure ranged from 3 to 85 ppm with an average of 16.7 ppm with all of the contribution from NO.

This fourth test campaign was very successful. The only equipment that was not operated was the baghouse due to the wrong type of bags. The addition of process controls, draft control, and increase air supply and distribution resulted in increased feed rates and better emissions results. Carbon and mineral ash still accumulated, but a viable concept for resolving the problem was developed.

SUMMARY & CONCLUSIONS

Four campaigns of pilot tests were conducted during 1996 with the purpose of defining the areas of critical concern for a commercial a joule heated vitrification process for low-level radioactive wastes. Three methods (ram, batch, continuous) of feeding the process chamber were tested. Ram and batch feeding appeared to be viable if properly designed, but ram feeding was a considered a marginal method. The batch feeding apparatus was viable, but a considerably more robust design would be required. The auger feeding method has features that appear to make it the method of choice, however, a commercial system should have both methods for flexibility. The redesigned system used in the last two campaigns overcame the initial concerns regarding electrical complexity, process chamber size, accumulations of carbon and mineral/ash, and cost. The pilot system was capable of processing bags of surrogate wastes weighing from 1-3 lbs every 30 to 120 seconds depending upon the waste material. The process control loops tested increased the feed rates, reduced carbon accumulations and air emissions. The testing provided the basis for designing the air injection ports and process chamber sizing, as well as identifying critical equipment items. Air emissions testing results were excellent and indicated no environmental or regulatory areas of concern.

Control issues and feeding methods were resolved sufficiently to provide the design basis for a full scale commercial system. The full scale commercial system has been designed and is fully permitted at this time. Construction of the commercial system is nearly complete and shake-down testing has begun.