How Plasma Converters Work

Currently, plasma plants aren't standardized. Many different companies are designing plasma facilities, and for the moment each facility is essentially custom-built. Still, most converters have the following components in common:

Conveyor system

In order to feed garbage into the converter, almost all plasma facilities have a conveyor system. Garbage is loaded on the conveyor and is pushed into the furnace (or pre-treatment system if the plasma facility uses one) by a plunger.

Pre-treatment mechanism

Although a plasma torch can break down waste without any special pre-treatment, most plasma facilities employ some sort of pre-treatment process to make the entire system more efficient. Some designs use grinders or crushers to reduce the size of the individual pieces of garbage before moving in to the furnace. The plasma torch can break down the smaller pieces faster.

Furnace

Here's where the magic happens. Furnaces have an airlock system to allow garbage to come in while preventing the hot gases in the furnace from escaping into the atmosphere. The furnace houses at least one plasma torch; many furnaces have multiple torches to break down all the matter. These torches are usually placed a little lower than halfway down the furnace. The furnace also features a drainage system to tap off the slag as it accumulates and a vent system to vent out the gases. In order to withstand the intense heat, furnaces are lined with refractory material and often have a water-cooling system as well.

Plasma torch

The plasma torches used in these facilities are custom-built. The amount of energy they consume, the lifespan of the electrodes it uses, the gas used for ionization (most torches just use ordinary air), the downtime it takes to replace an offline torch and the size of the plasma field it generates all depend on the specific manufacturer. Plasma torches are water-cooled.

In the next section we'll take a look at slag drainage and the afterburners. 

Molten slag pools at the bottom of the furnace and helps maintain the high temperature inside the gasification chamber. Occasionally slag must be drained from the furnace. Some furnaces have drains positioned at a certain height, others use a tap system. Either way, slag drains away from the furnace and cools in a separate chamber.

Gas ventilation

The furnace also has a vent system to allow gasified components to pass into another part of the system (either an afterburner or a gas cleaning chamber).

Afterburner

Gases can pass through a secondary chamber where natural gas flames combust any remaining organic material in the gases.

These extremely hot gases then pass through a Heat Recovery Steam Generator (HRSG) system, where they heat water to form steam. This steam then turns a steam turbine to create electricity.

Syngas cleaning

Alternatively, the gases from the furnace enter a chamber where they are cooled and scrubbed, usually by water. The gases pass through a spray of water, which scrubs the gases of pollutants and particulates. A filter system containing a base filter neutralizes acid gases. The acids in the gases and the bases in the filter combine to form inert salts. The cooled and clean gases continue through the system, which in most cases involves a gas turbine connected to an electricity generator. Some systems also harness the heat from these gases to generate steam, similar to the afterburner method mentioned above.

If the plant uses an afterburner, the remaining gases must be cleaned thoroughly to get rid of any hazardous material. Many designs include a dry scrubber system. In this system, powdered carbon is injected into the gases to strip away mercury, a poisonous element. Gases also pass through a fabric or bag filter to remove any other dangerous particulates, like lead. Once the gases have been cleaned they move to the stack, where they are released into the atmosphere.

There are three main byproducts that are a result of the plasma gasification process: synthetic gas (syngas), slag and heat. Let''s look at each of these byproducts in more detail.

Syngas is a mixture of several gases but mainly comprises hydrogen and carbon monoxide. It can be used as a fuel source, and some plants use it to both provide power for the plant and sell excess electricity to the power grid. Garbage contains a great deal of potential energy; the gasification process enables engineers to convert the potential energy into electrical energy.

How much gas is generated by a plasma converter? That depends on what you put into the furnace. If the garbage contains a lot of carbon-based material (in other words, organic waste), then you'll get more gas. Waste with a lot of inorganic material won''t yield as much gas. Because of this, some plasma facilities sort through garbage before feeding it into the system.

The solid byproduct from the gasification process is called slag. The weight and volume of the original waste material is dramatically reduced. According to Dr. Circeo of Georgia Tech's Plasma Department:

The slag can take different forms depending on how you cool it.

If slag is air-cooled, it forms black, glassy rocks that look and feel like obsidian, which can be used in concrete or asphalt. Molten slag can be funneled into brick or paving stone molds and then air cool into ready-to-use construction material.

If you were to blow compressed air through a stream of this molten material, you'd end up with rock wool. Rock wool has the appearance of gray cotton candy. It''s light and wispy, and according to Dr. Circeo, it has the potential to revolutionize the plasma waste treatment industry. Rock wool is a very efficient insulation material, twice as effective as fiberglass. It's also lighter than water, but very absorbent. Because of this, it could potentially be used to help contain and clean oil spills in the ocean. Cleanup crews could spread rock wool over and around an oil spill. The rock wool would float on the water while soaking up the oil, making collection a relatively easy process. Hydroponic growing systems can also use rock wool -- farmers can plant seeds in slabs or blocks of it.

Currently rock wool is produced by mining rocks, melting them down and then streaming the molten material onto spinning machines. The spinning machines fling strands of molten material in the air. Today, the price of rock wool is over a dollar a pound. Since rock wool would be a byproduct of the plasma gasification process, it could be sold for as little as 10 cents a pound. The price of insulation would decrease, efficiencies in energy-saving techniques would increase and plasma gasification plants would have another substantial source of income apart from selling electricity back to the grid.

Plasma technology experts, including Dr. Circeo, assert that the slag is virtually unleachable, meaning that any hazardous materials are inert and will not dissolve out of the slag.

The heat created by plasma facilities is considerable, measured in thousands of degrees Centigrade. Heat from the molten slag helps maintain the temperature within the furnace. Some of the heat from gases can be used to convert water into steam, which in turn can turn steam turbines to generate electricity.­

Waste treatment through gasification is unique in that it not only gets rid of garbage and generates electricity, it also produces byproducts that are valuable commodities themselves. In the next section, we'll talk about existing and future plasma plants and pioneering companies in this technology.

Currently, there are only two commercial plasma plants that process MSW, and they are both in Japan. In 1999, Hitachi Metals commissioned a pilot plant in Yoshii, Japan. This plant was modest, processing less than 30 tons per day of MSW. The successful operation of the plant spurred the development of two other plants within Japan. The pilot program ended in 2004, and Hitachi Metals decommissioned the plant.

The plant at Mihama-Mikata industrial park began operations in 2002. This plant can process up to 24 tons per day of MSW and four tons per day of wastewater treatment plant sludge. Because the plant is relatively small, it doesn't produce syngas for fuel. It does produce steam and hot water, however, which is used both for power and heat generation in the industrial park. The plant uses a water cooling system for the molten slag and separates the metal nodules to sell as scrap. The sand is mixed with concrete and used in paving stones.

The plasma gasification plant in Utashinai, Japan also began processing MSW in 2002. The original design of the plant factored in a capacity of around 170 tons per day of MSW and automobile shredder residue (ASR). Today the plant processes approximately 300 tons per day. The plant generates up to 7.9 megawatt-hours (MWh) of electricity, selling about 4.3 MWh back to the power grid.

Plasma gasification is also used for specialized waste handling projects. In Bordeaux, France, plants designed by Europlasma are used to melt asbestos or vitrify fly ash, particulates that are a result of using incinerators to destroy waste. Fly ash can contain hazardous materials and traditionally have been stored in specialized landfills. Using a plasma torch facility, Europlasma can convert the ash into slag, where the heavy metals and other hazardous materials are rendered inert.

Future Facilities

A demonstration facility Israel built by Environmental Energy Resources, Ltd. is scheduled to be converted into a commercial waste treatment facility. Russia has also expressed an interest in plasma gasification facilities, and currently uses plasma plants to treat low level nuclear waste in a plant outside of Moscow.

In the United States, Atlanta-based firm GeoPlasma is working with St. Lucie County in Florida to build and operate a plasma gasification plant. This plant would process all of the incoming waste for the county and begin to mine the existing landfill for waste. Once it is built, the facility will be able to process up to 1,000 tons of garbage per day and generate 67 MWh a day, with a net output of 33 MWh.

GeoPlasma has created a modular design for the plant, with two large plasma gasification chambers that will handle 500 tons per day. The modular design allows further expansion in the future – the proposed plan is to increase capacity to 3,000 tons of waste per day within a few years of operation. Engineers project that within 18 years, the existing landfill will be completely mined and treated. The electricity generated by the plant will be more than enough to power the 98,000 homes in the county.

Many areas across the nation are beginning to look into plasma gasification as a way to approach waste management. Several companies such as GeoPlasma, StarTech, Recovered Energy, Inc. and Plasco Energy Group are pioneers in bringing this technology into commercial use. Assuming the St. Lucie County project is a success, we may see more of these facilities commissioned across the nation soon.

Plasma arc technology has been used in various fields for decades. Experiments using plasma for waste management began in the 1980s. With all the benefits of plasma converters, why are we just now seeing these facilities being built? In the next section, we'll look at why it has taken decades for this technology to go from experimentation to implementation.

Plasma waste facilities have had several obstacles to overcome. First, they are a new technology. As Dr. Circeo points out, it can take many years for a new technology to go from discovery to commercial use. Sometimes this gap seems to coincide rather conveniently with the expiration of the initial patent on the idea. New technologies are also expensive; almost every plasma application requires a custom built facility. Until facility production can be standardized, costs will be high for plasma plants.

Aside from the cost of custom building the plant, other costs are a major factor. Until very recently, land costs were so low that it was cheaper to use landfills than it would be to design, build and maintain a plasma waste facility. Environmental concerns often take a back seat to economic realities, and it wasn't until tipping fees (the fee you have to pay to have garbage hauled to landfills) increased and landfill space decreased that plasma plants became economically feasible. Even in an ecologically-concerned culture, some companies don't focus on the environmental aspect for their business model. GeoPlasma, for example, positions itself as a power facility that uses a renewable resource for fuel. Dr. Hillestad of GeoPlasma asserts that by focusing on GeoPlasma's ability to produce electricity for low costs makes it a viable operation.

Waste management is big business. Any major revolution in waste management faces critics and opposition from those that benefit from the status quo. As environmental pressures increase (both from the perspective of waste management and that of renewable sources for fuel), city and county governments are more willing to explore alternate strategies to handle waste.

Making Plasma Plants Profitable

Plasma waste treatment facilities are becoming more cost effective, however. Because a plasma plant can generate revenue beyond tipping fees, they can competitively price tipping fees to make it cheaper to ship garbage to the facility than a landfill. As plasma facilities are standardized, tipping fees will continue to decrease.

With the right capacity, a plasma plant can generate enough syngas to run an engine or gas turbine and generate electricity. A 1,000 ton per day plant can generate enough electricity to power the plant itself and still have plenty of power to sell back to the grid.

The hot gases can be used to generate steam, which can turn steam turbines for electricity or be used to generate heat for the plant and other facilities.

Slag can be sold in any of its forms. The rock form can be used as gravel or molded into bricks. Sand can be mixed with concrete and used in various paving and construction projects. Rock wool can be used for insulation or to contain dangerous oil spills. The St. Lucie County plant will produce 12 tons per day of vitrified slag (from 1,000 tons of waste). If the molten slag is cooled by water, metal nodules can be separated from the slag and sold for scrap. The St. Lucie facility is expected to produce about 4 tons per day.

We'll look at what the future may hold for plasma gasification technology next.

Dr. Hillestad of GeoPlasma calls the present the "perfect storm" for plasma gasification technology. With focus increasing daily on mankind's environmental impact and the growing concern to look to renewable energy resources, plasma plants are well positioned to become an important part of how we generate power and deal with waste.

Potential uses for this technology (apart from new plasma waste treatment plants) include:

In-situ facilities

Dr. Circeo proposes the creation of a portable plasma gasification system to treat existing landfills without building an entire plant. Instead of a stationary furnace and gas treatment facility, he suggests boring holes into existing landfills, sticking a plasma torch into the hole, and capping the hole with a gas capture system. The landfill itself would act as the furnace vessel. Since plasma gasification is not a combustion process, the landfill contents would either gasify or vitrify, with no danger of fire.

Co-location with existing power plants

Another option that would significantly reduce the price of a plasma plant is the co-location of the plasma gasification chamber with a pre-existing power facility. Because the amount of gases produced by plasma plants is relatively small when compared to coal or oil-fired power plants, the power generators in plasma plants are smaller and less efficient (larger generators require much more gas). Coal and oil power plants use the same processes as plasma plants to treat gases and generate power. By connecting a line from a plasma gasification furnace to a coal or oil furnace, you eliminate the need for a plasma plant's gas treatment equipment, which make up approximately 50 percent of the overall cost of building a plasma waste treatment facility. The gases from the plasma furnace would combine with the gases in the coal or oil furnace. The relatively clean gases from the plasma furnace would help boost efficiency and reduce the amount of coal or oil needed to generate power.

Decontamination

The intense heat from plasma torches can completely neutralize the hazardous components found in diseased livestock or contaminated soil. Engineers could transport modular, portable plasma facilities to dispose of animal carcasses or treat soil on site. Incineration of such hazardous material doesn't always destroy all the contaminates, or produces ash that is also hazardous waste. Plasma gasification would destroy or render inert any harmful material.

To learn more about plasma waste converters, check out the links on the next page.