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The gasification of organic materials for the production of synthesis gas (syngas) has been in practice on an industrial scale for nearly 150 years. Streetlights in the late 1800’s were lighted with syngas derived from the gasification of coal. Syngas has been known by many names such as city gas, industrial gas, manufactured gas, water gas, coal gas and others.  It is a mixture of gases containing mainly carbon monoxide (CO) and hydrogen (H2). Simply stated gasification is the dissociation of atoms in a molecule. It involves heating an organic material to a temperature sufficient to break the molecular bonds holding carbon and hydrogen atoms to other atoms in the molecule and then reforming them. The energy required to dissociate any molecule is determined by Gibbs enthalpy of formation number and will be the same, regardless of the source of energy.

There are numerous technologies used for gasification of organic materials. Some are relatively new while others are only now being introduced to the industry. A proper review of these processes is necessary to avoid confusion when discussing their similarities and differences. When comparing the different processes it is important to conduct a comprehensive mass balance and energy balance on each process.

Gasification reactions are endothermic and thus require some sort of energy input to achieve temperatures and conditions sufficient to break the molecular bonds. There are five main differences to consider when comparing these processes:

1)     The efficiency of reaching reaction temperatures

2)     The inherent heat losses of the process

3)     The quality of the syngas generated

4)     The volume of the syngas generated

5)     Byproduct disposal, e.g. ash, char.

The following is a cursory overview of the most common variations of the gasification process.  Please note this is by no means all encompassing.

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    Conventional Gasifiers

    Conventional gasifiers rely on the partial combustion, or limited oxidation, of the material being gasified. This generally involves the introduction of air, which contains only 19% oxygen. (a pure oxygen plant is economically counterproductive because it is highly energy consumptive). The economies are further complicated by the introduction of nitrogen in those cases that use air as a source of oxygen. Air is about 78% nitrogen and this nitrogen will dilute the syngas, thus lowering the BTU value of the gas. It will also be a contaminant in the purity of the syngas. One example of a conventional gasifier is the “Fluidized Bed” gasifier. Here an inorganic medium such as sand is kept in a fluid state and acts as a heat sink. The material to be gasified is fed into the hot bed. One problem with fluidized bed gasifiers is the slagging temperature for the inorganic media is often very close to the temperature required for gasification. Thus it is possible to start agglomerating the fluidized media and have it drop, therefore interfering with the efficiency of the process.
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    Cupolas

    Cupolas have been used as gasifiers, but they require large amounts of combustion air plus fuel, such as petroleum coke or metallurgical coke, being added directly into the process,. The syngas produced is generally of a very poor quality due to the the volume of combustion air introduced. The waste must be pretreated by shredding, compacting and sizing to insure the waste is not expelled from the cupola by the air and gases in the system. In some cases plasma torches have been added to heat the burden to prevent its freezing and thus shutting down the cupola. Several cost factors make the cupola concept less desirable. These include: the cost of the coke, the cost of the plasma torches, and the cost of pretreatment of the waste.
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    Induction Furnaces

    These furnaces generate heat by electrical induction – that is electricity flows through conductive coils creating a magnetic field around a stationary, grounded bath. The result is very similar to what happens if you power an electric motor while preventing the rotor from turning. The conductive coils around the bath are usually water cooled to prevent their melting. This creates a significant heat loss and thereby a loss of energy efficiency. In addition to heat losses there are other problems inherent with the use of induction furnaces: 1) There is a limitation to the maximum temperature attainable 2) The replacement of coils is costly in terms of downtime, labor and capital 3) Induction baths are slow to reach desired temperatures 4) The baths are sluggish to respond to necessary changes in operating temperature.
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    Plasma Torches

    Plasma torches are electrical devices that have been used in small-scale reactors to generate the high temperatures necessary for gasification. However, there are several drawbacks to the use of plasma torches in an industrial scale gasification application.
    • Energy inefficient: Torches are a steel body structure housing metallic electrodes (usually copper). Plasma torches are energy inefficient due to as much as 45% total heat loss to the cooling water occurring in two areas of the torch
      • Outer steel body – This houses the torches working parts and is exposed to the hot gases in the furnace thus requiring water-cooling
      • Internal copper electrodes – These create the high temperature plasma arc and need to be cooled by a water circuit from the outer steel body.
    • Size limitations: The largest commercial plasma torches are about 1.5MW input power. A typical gasifier requires a net energy input of 0.6 MW -1.2MW per ton of waste
    • High capital cost: Plasma torch costs are in the $2 million per MW range. A viable commercial scale plant processing 10 tons per hour of waste would require 8 MW of net input power. This translates into 16 torches operating simultaneously
    • Torch gas is a diluent: Plasma torches require an arc gas to create and stabilize the arc. This arc gas dilutes the syngas, which lowers the BTU value
    • Maintenance cost: Torches are sensitive pieces of equipment requiring extensive man-hours from skilled laborers to maintain. The torch must be removed from the furnace to perform maintenance thus resulting in lost production time
    • Electrode costs: Torch electrode life is limited and each torch has two or three electrodes, which must be replaced periodically. Electrodes are expensive because they are made from high copper alloys machined to precise specs. This adds to operating costs.

    For these reasons the companies that began as plasma gasifiers using torches eventually switched to a mixed system where the plasma torch became secondary and not actually used for the gasification reaction per se. In some cases these companies employed a conventional gasification process, such as fluidized bed, followed by a plasma torch used mainly to re-heat the syngas. In other cases, an induction furnace is used to produce a molten metal bath, and a plasma torch is installed on top of the bath to provide the high temperatures for gasification.
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    Graphite Electrodes

    Another electrical method for creating a plasma arc uses graphite (pure carbon) electrodes. There are numerous advantages for using graphite arc plasma for gasification.
    • Proven technology: Graphite arc furnaces have been proven in large-scale industrial applications for more than one hundred years. They are used daily all over the world to melt scrap steel. When melting scrap steel, syngas is produced from the carbon contained in the scrap or the carbon additives used as reducing agents. This syngas is sometimes seen as a nuisance to the steelmaker. However, in some instances it is used to preheat the scrap; and in some European mills it is used to generate electricity
    • Energy efficient: The graphite electrodes are consumed in the process and do not need to be water cooled, thus avoiding heat loss to cooling water
    • Scalable: Graphite plasma systems typically operate in the 10 – 100 MW input power range
    • Relatively low capital costs: A graphite plasma system is an order of magnitude lower in cost than a plasma torch system. The cost per MW in a graphite system is less than $200,000. Also, the complete ArcSec Technologies system is less than half the cost of a similar sized conventional gasification system. This is due mainly to the reduced size of the plant. Because combustion air is not required, the plant is sized for less than one tenth the volume of gas handling of a combustion system
    • Relatively low operating costs: Graphite arc furnaces have been known to operate uninterrupted for as long as fourteen years. The electrodes are fed into the furnace automatically as they are slowly consumed. Also, the ArcSec Technologies system operates in a continuous mode and thus will not have the relative frequent (each hour) interruptions found in typical steel mill applications
    • Syngas purity: The ArcSec Technology is a sealed system operating under partial vacuum, thus eliminating the dilution effect from air. Also, the graphite plasma requires no gas to generate and stabilize the arc. Therefore there is no dilution or contamination of the syngas from an arc gas
    • Low electrode costs: Graphite electrodes are commodities sold to the steel industry from numerous suppliers on a regular basis. The carbon electrodes are consumed in the process and become part of the syngas product
    • No ash disposal: All of the input waste is converted into three product streams with no ash left to dispose
      • Organics are converted to syngas
      • In-organics form the intermediate slag layer and are tapped molten and spun into rockwool for commercial use
      • Metal units gather at the bottom of the furnace and are tapped molten to be cast into billets and sold as scrap to the steel industry.
    • No direct toxic emissions: All emission standards issued by the EPA and state governments are met or exceeded by the AST plasma arc process. In fact, the AST process has a distinct advantage over other treatment technologies. Graphite systems do not require the introduction of any extraneous air, as does a plasma torch system, or the compounded problem of excess air and low temperatures found in conventional gasification. Things such as sulfur dioxide cannot be formed due to the lack of excess oxygen. High temperatures found in the furnace, plus the highly reducing atmosphere, prevent the formation of dioxins, furans and other toxins. In addition, the AST process is capable of processing multiple streams without concerns about creating unwanted hazardous wastes. More specifically:
      • Gaseous emissions – There are no gas emissions per se, as the syngas is the “emission”. Of course, if the syngas is combusted, you will create the normal products of combustion, CO2 and H2O. Interestingly enough, due to the high amount of hydrogen found in syngas, its emissions are cleaner than even the combustion of natural gas. Due to its cooler flame temperature, there are less nitrogen oxides (NOx) formed than when you combust natural gas. Of course, control of NOx formation is well understood and easy to control
      • Liquid emissions – The only liquid emissions found in the AST process is from the “scrubber.” In a wet scrubber, the acidic portion of the raw syngas will be neutralized and turned into salts. For example, in the processing of MSW the main salt created is NaCl or table salt. As such, the dilution is low enough that the effluent from the scrubber may be directly discharged into the sewer system
      • Solid emissions – There is no solid discharge processing MSW. However, the treatment of waste materials with metals from steel mills or foundries presents an additional problem. In that case, those metals are recovered in the form of salts by the further processing of the scrubber effluent in a dedicated wastewater treatment plant that converts those metals to salts. Later they are dewatered in a filter press. Mercury and other heavy metals that may be present in the streams are not released into the atmosphere because they are captured in a conventional activated carbon filter (e.g. made by Nalco). The filters are removed periodically, as needed, to recover the mercury and other metals at their facilities.

By closely examining all parameters of existing gasification processes, it can be concluded that the ArcSec Technologies system has the lowest risks and the highest return on capital of any gasification technology. It is already a proven technology on a commercial scale. It can be adapted to process any and all types of waste, and the waste is converted to commercial products with no ash remaining for disposal. Finally, there are no direct emissions and any indirect emissions are dealt with in a manner that satisfies or exceeds both EPA and local state requirements.