A typical PEM™ system includes the following components:

The picture below shows many of these components on the Process Test System. However, not all subassemblies are present on this test system.

PEM™ PROCESS CHAMBER

The process chamber is the heart of the PEM™ system, and is the location where waste materials are processed producing hydrogen-rich synthesis gas, a glass product and metal.

Plasma (an electrically conducting gas) heating provides radiant power, which heats the waste in a reducing environment consisting of steam and inert gas such that the organic constituents in the waste are gasified while the metals and mineral contents are melted to form a molten bath. The gasified organic compounds are reacted with steam and oxygen to produce a hydrogen-rich synthesis gas (syngas). The chamber is designed to operate safely and reliably and have sufficient residence time and temperature such that organics, including biological agents and pathogens, contained in the waste are completely destroyed, while maintaining low levels of particulate and metals carryover. The chamber has capability to receive additives/fluxes that mix with the molten metals and inorganic components of the waste to produce a leach resistant glassy product.

Two sources of energy are utilized to process the waste: the DC (direct current) arc plasma zone, and the AC (alternating current) joule-heated zone. The DC arc plasma is created by applying a DC potential across the graphite arcing electrodes with a single electrode at one polarity and the other electrodes at the opposite polarity. A stable plasma arc is then formed between the molten bath and arcing electrodes. The second source of energy to the process chamber is supplied directly to the molten glass via joule-heating electrodes submerged in the melt. A three-phase AC potential is placed across the joule-heating electrodes, which results in current flow through the glass. The molten glass acts as an electrical resistor such that electrical power is converted to heat supplied directly to the molten glass.

The process chamber is a water-jacketed steel vessel lined with refractory. The process chamber encompasses inductively heated side and bottom drain for metal removal. The process chamber proper consists of two zones. One zone is the melt tank, which contains the molten glass, and the metal phase, which lies on the bottom of the melt tank. The second zone is the plenum, or vapor space above the melt. The chamber lining is composed of several different types of refractory and insulating materials. These materials serve to reduce energy losses to the water jacket as well as to contain the molten glass and metal phases. The plenum area of the process vessel is lined with both insulating materials and material to protect the steel shell from corrosive gases and vapors formed by thermal dissociation of the waste.

The process flow for the process chamber is based on the receipt of waste and glass former from the waste feed system. Waste enters the process chamber and falls through the high temperature plasma to the molten glass surface, forming a pile where the waste material continues to be exposed to energy from the DC plasma plus the joule-heated glass. Steam is injected into the process chamber plenum, in the region of the waste pile, to steam reform organic constituents. Organic constituents in the waste are dissociated to their elements, in the absence of oxygen, and reformed to produce CO, H2, HCl, N2 and H2S. Inorganic oxides dissolve into the glass phase. Metals present in the waste melt and settle to bottom of the tank.

As described above, power is input into the process from two sources, the DC plasma and AC joule heating of the molten glass. The relative fraction of power from these two sources is controlled by the operator, and may be varied to accommodate a variety of wastes being processed. Wastes that contain a high percentage of organic material require a higher percentage of power from the DC plasma, while wastes that contain a high fraction of inorganic material will be processed with a higher fraction of processing power from the joule-heated glass tank. Wastes that are consistent in their relative proportion of organic content, as is typical of medical waste, are, of course, processed with a constant proportion of power from the plasma and the joule heaters. The refractory temperature and the plenum temperature will also determine the amounts of power from each source.

The DC electrodes enter the process chamber through penetrations in the top of the PEM™. The AC electrodes enter the chamber at the perimeter of the melt tank and are equally spaced. They extend down through the plenum, protected by Inconel alloy sheaths, into the molten glass. The joule-heating electrodes are protected from gases in the process chamber plenum by maintaining a low volume flow of nitrogen through the annular space between the electrodes and the sheath. The DC electrodes also enter the process chamber through the top of the PEM™, but near the center of the chamber. The DC electrodes can be continuously fed into the chamber and replacement of both DC and AC electrodes can be carried out without removal of the barriers that prevent mixing of air and the gases in the Process Chamber. Additional nitrogen is injected into the PEM™ processing chamber through the DC electrode penetrations in the lid of the process chamber.

The diagram below illustrates the relative locations of the AC resistive heating and DC plasma-arc heating electrodes, and the areas in which the two heating types take place.

The DC plasma power is controlled via its current and voltage. The plasma voltage is proportional to the length of the arc, which is controlled by adjusting the physical distance between the tips of the electrodes and the molten glass surface. System operators will set the desired plasma voltage and manually adjust the current to achieve the desired power. After these power parameters have been set, the computerized control system will automatically adjust the vertical position of the electrodes to maintain the plasma voltage set point.

Adjusting the AC current flowing through the molten glass bath controls the joule-heating power. The resistance of the molten glass determines the corresponding potential, and power added. The resistance of the glass is controlled by its composition. Therefore, the operator will set the desired power as a set-point, and the AC power supply will achieve this power input by automatically adjusting AC current to achieve that set point. In addition to passing AC current between the joule-heating electrodes, the electrical configuration will enable operators to pass AC current to the floor of the melt tank. The current can be transferred to the bottom of the furnace if the temperature of the lower melt needs to be increased; to assist in pouring material through the bottom drain for example.

Process syngas, consisting of H2 and CO is removed from the chamber via the off-gas removal vent, and subsequently treated by the process gas scrubber system. Indicators monitor temperature, pressure and volumetric flow rate of the process off-gas, respectively. The process chamber is maintained under a slight vacuum by the off-gas vent system to assure that no process gases or vapors escape untreated from the chamber.

The injection of nitrogen into the plenum at the joule-heating electrode penetrations, the plasma electrode penetrations, and the steam injection penetrations, maintains inert conditions in the process chamber ports. At each of these nitrogen injection points, the overall nitrogen flow rate and pressure of the nitrogen supply header is automatically monitored.

The external surfaces of the process chamber are water cooled via the vessel jackets. The water-cooling jacket is made up of multiple distinct circuits in the process vessel lid, the process vessel walls, the process vessel floor, and the over-flow section. The inlet temperature of the process water as well as the outlet temperatures for each cooling water circuit are continuously monitored. In addition to the water cooling of the process chamber vessel, a high-purity process water stream cools the electrical connections to both the plasma and joule-heating electrodes.

Top of page

WASTE FEED SUBSYSTEM

The design of the waste feed system is dependent upon the nature and type of waste to be processed. For medical waste systems, the waste feed system is most likely to be designed in a manner to accommodate introduction of waste packages. Systems designed for processing municipal wastes can be designed for processing of bulk or baled waste. IET works closely with companies that are highly experienced in the design and construction of waste feed systems to assure that these subassemblies are economical, practical, fully functional and compatible with the types of waste to be processed and the PEM™ systems being installed.

The following illustration depicts a typical bulk feed setup, where the waste is fed into a hopper and is metered into the Process Chamber via an auger mechanism.

 

Top of page

GLASS FORMER MATERIALS SUBSYSTEM

When required in order to assure that the vitrified portion of the products of PEM™ processing meets leachability requirements, a subassembly for the introduction of glass formers will be included in the PEM™ system. IET teams with companies highly experienced in the design, engineering and construction of these subassemblies to assure reliability of operation.

 

Top of page

GLASS AND METAL RECOVERY SUBASSEMBLIES

The function of the glass and metal recovery subassemblies is to discharge the molten product from the process chamber and to provide confinement during product recovery. Molten glass is discharged via the side drain while metals are discharged through the bottom drain of the process chamber.

Vitrified product, with typical temperatures of 1200° to 1400°C, is poured from the process chamber via the side or bottom drains, into steel receiving canisters. Typically, 55-gallon (200-liter) drums are used for receiving the molten glass. The product handling system for the side and bottom drains operate similarly. Poured product from either drain is sealed from the surrounding atmosphere via a stainless steel jumper that mates the drain-line with an enclosed stainless steel chamber that contains the receiving canister. A flexible metal bellows is incorporated in the connecting jumper to allow for thermal expansion. The bellows enables the mating assembly to be raised and lowered when full canisters are removed and new canisters loaded. Additionally, the bellows enables load cell measurements of each canister during pouring.

The following illustration depicts a glass draining arrangement.

Molten metals are poured from the bottom of the process chamber. The following illustration depicts a sample metal removal arrangement.

The key control devices for each product handling system (side and bottom drain) are the canister load cells, the clamping canister mating assembles, and the isolation gate valves. The load cells enable continuous weight measurements of the canisters alerting operators when a canister is full and needs to be changed. The clamps on the containment chamber mating assemblies enable operators to attach and detach the containment chambers from the sealed drain assembly during canister changes. The isolation slide-gate valve closes to maintain a seal on the discharge pipe during canister changes.

Top of page

PROCESS GAS SCRUBBER SUBASSEMBLY

The gas cleaning subassembly is designed to be compatible with the wastes to be processed. The volume of off-gas that is produced is very small, typically around 10% of that of an incinerator. The low amount of off-gas allows for high efficiency gas cleaning systems that ensure the effective removal of volatile heavy metals such as mercury.

Typically, the gas cleaning system is comprised of several gas cleaning operations. The first system is generally a partial quench followed by a particulate removal baghouse. Next is a full quench and a wet packed column scrubber, used to remove the remaining particulate and acid gases, if present. After the wet scrubber additional filtering may be required to remove any remaining trace levels of particulate and mist carry over from the wet scrubbing system. Specialized carbon filters may also be used to capture any mercury that might be in the feed.  The final element of the gas cleaning system is a blower to provide a continuous draw of gas from the PEM™ process chamber. In systems with energy recovery units the gas from the blower is directed into the energy recovery device (i.e. generator set).

The wet scrubber is typically a packed bed column. The PEM™ system has been demonstrated to have very low particulate carryover relative to other thermal treatment systems; therefore, the particulate accumulation in the scrubber is minimal. The majority of the particulate matter that is removed by the baghouse is simply recycled back to the process chamber. The packed bed scrubber can be designed to remove acids of chlorine, fluorine, bromine, and sulfur with efficiencies high enough to exceed MACT release standards. In systems incorporating product recovery such as hydrochloric acid and sulfur recovery, additional unit operations are added to accomplish those separation tasks.

Top of page

POWER CONVERSION SUBASSEMBLY

The power conversion system, when incorporated, is comprised of a modified diesel or reciprocating natural gas engine, gas turbine or fuel cell power generation system. The internal combustion engines and gas turbines can accept the cleaned gas directly from the PEM™ system and efficiently convert the chemical energy value of the PEM™ produced gas into clean electrical energy.

Fuel cells can also be an excellent match to PEM™ systems.  The syngas can be shifted to produce a pure hydrogen stream using catalysts beds and hydrogen purification systems.


Top of page