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The PyroArc® Process Description
Overview
The PyroArc process recovers energy and material from all kinds of waste in a two-stage thermal process.
The first stage is a counter current melting gasifier. Solid waste materials are fed to the top of the gasifier and descend down the gasifier. The temperature at the top is 3-500°C and all moisture and volatile material evaporate. Pre heated air is injected in the bottom of the gasifier and the rest of the combustible material is gasified there at a temperature of about 1500°C. The incombustible materials like metal, glass and minerals melts and is tapped of as a metal alloy and a non-leaching slag.
Stage two is a plasma-powered reactor. Any gaseous or liquid waste to be treated is injected directly to the reactor and is mixed with the gas from the gasifier. The gas and liquid enters an air jet of high velocity and temperature (6-800 m/s and 3-5000°C) provided by a plasma generator. The air jet provides the necessary high temperature and immense dynamic power to decompose completely all hydrocarbons even halogenated hydrocarbons like PAH and dioxins.
In principal all types of materials can be treated in the process. The inorganic compounds are recovered as a non-leaching slag and a metal alloy. The non-leaching slag can be used as a construction material while the metal alloy can either be cast into sellable products or sent for refining. The organic materials are converted to a fuel gas consisting mainly of CO, H2, N2 and some CO2. The fuel gas can be use directly, in a gas engine to produce electric power or in a boiler to produce steam or hot water.
A new generation waste disposal system
Efficient waste handling is one of the great global challenges of our time, and one that needs to be solved on the local level. That is why there is a growing market for solutions, processes and technologies that allow for more efficient waste destruction in smaller and less costly plants.
Certainly, technologies exist on the market that comply with current emission regulations, but only for a well-defined waste (e.g. pre-treated, sorted MSW), or for systems with extensive cleaning and capturing facilities; however, with rather limited potential for co-generation. Usually these solutions apply to larger plants - typically beyond 30 000 tons per year for incineration, and even bigger for gasification concepts.
Drawbacks of traditional waste incineration processes are generally identified as highly leachable fly ash and bottom ashes, NOx-formation and incomplete dissociation of halogenated hydrocarbons like dioxins. Although some volume reduction is obtained (typically 70-75%), still a substantial volume of residues remains. The remaining 25-30% volume from such processes has to be disposed of - mostly on landfills. Since bottom ash from incineration usually contains dioxin, halogenated hydrocarbons and heavy metals, it is expected that the bottom ash from such processes will be classified hazardous in the future. Supplementary vitrification units might be required, although this would require substantial additional energy with less potential for energy recovery - if any. The drawbacks of fluidised bed technologies are much the same as for incineration although the combustion process is regarded better. However, the amount of ash may be higher due to contaminated sand from the process. The bottom slag and the fly ash is regarded more toxic than from incineration of MSW.
Figure 1:Typical volume of waste (left). With conventional technology the volume of residues corresponds to some 25-30% of the initial volume (mid column), whereas a complete volume reduction is obtainable for the PyroArc process (right).
Several processes are under development based upon gasification, pyrolysis and plasma. Efforts are being made to develop hybrid solutions for stabilising the residues (combining gasification-vitrification-and-melting technologies). All these processes involve some kind of gasification. Sometimes the gasifier is followed by a melting step for the non-combustible part of the waste. However, none of these processes have adopted the unique concept at the core of the PyroArc technology: A counter-current partial combustion combined with melting, vitrification and pyrolysis in one single unit.
The counter-current concept of one single reactor unit is important for obtaining high thermal efficiency (net 75 % plus). The logical choice, therefore, is a shaft furnace reactor known from the steelworks that guarantees a complete molten slag with good leaching resistance, with options for recovery of valuable metals. There are certainly some processes that utilise the shaft furnace concept, however, instead of the more efficient plasma treatment for the decomposition of hydrocarbons and other toxic gases, they all use post-combustion concepts.
The advantage of plasma technology is primarily the high temperatures of the plasma-jet. Therefore, this technology is applied in systems where high temperatures are required. The plasma-jet may be used for two different purposes: a) supporting gasification and the melting/vitrification of solid inorganic materials, and b) cracking of the complex pyrolysis gas into lighter hydrocarbon fractions.
Most plasma-based waste treatment processes lend themselves to melting and vitrification of solid materials - mostly metals and inorganic compounds. Such applications are known from the recovery of precious metals (alloys), and also from vitrification of contaminated soil. However, in some cases the plasma-jet is used for the gasification of solid particles. This is only efficient in cases where the mass and heat transfer restrictions are negligible - which is generally not the case. Some processes support gasification, for instance the complex processing of low-grade power coal. This way of using the plasma-jet for producing gas from solid waste generates significant amounts of dust that requires quite substantial gas cleaning. The only process that uses plasma jet in combination with gasification of waste for the purpose of cracking the pyrolysis gas is the unique proprietary designed PyroArc technology.
The PyroArc©Concept
The basic concept comprises a feed system, a shaft-furnace-gasification reactor, a plasma-augmented-high-temperature-cracker (i.e. a mixing-destruction reactor), a quencher, and a dust-collection-gas-cleaning system (for the capturing of volatile metals and alkaline salts). Consecutively, a downstream energy-recovery and gas-cleaning-filtering train is further attached.
The process involves three stages of conversion:
- gasification sustained by counter-current partial oxidation combined with vitrification and melting
- complete cracking (dissociation) of syngas combined with prevention of recombination by partial air intrusion
- energy recovery (heat and power -by air breathing engines or by combustion)
Core technology
The unique technology of the process is the plasma chamber, protected by intellectual law, and the way it is used in a plasma-mixing-destruction chamber for the cracking of syngas into a harmless fuel gas. This is quite essential for the complete decomposition of the syngas. Due to extreme temperatures from the plasma, two immediate advantages over any known alternative technology may be identified: 1) a complete decomposition of the highly toxic halogenated organic compounds, and 2) the higher heating value of the produced fuel gas.
The extreme temperature of the plasma-jet is used to resolve the tar problem associated with the syngas at the outset. This is explained by the decomposition reactions that crack the complex hydrocarbons into basic molecules. The absence of tar makes the down-stream gas cleaning easier and even more efficient. The long chains of hydrocarbons of the naphthalenes are simply broken by the high impact, and added to the fuel instead of being removed by the above filtering-scrubbing-catalytic techniques. Thereby, the process could increase the net electric conversion factor known from the current steam-based processes.
Brief process description
Waste material of various sources is charged into the shaft gasifier in which the organic components convert into a partly oxidised syngas, while the remaining inorganic species melt. Consisting mainly of carbon monoxide and nitrogen the syngas may be associated with a rather high content of tar-forming components (complex HC), and most often chlorinated hydrocarbons, hydrogen and water vapour. This gas is highly toxic and dangerous. Except for being burnt it cannot be used for any purpose without a thorough post treatment. Depending on the moisture of the feed, the temperature of the syngas will be around 400 - 700 °C when it leaves the gasifier. In the consecutive plasma-augmented gas cracking reactor the gas decomposes entirely due to the high temperature (3000 - 5000 °C) of the plasma-jet and its strong dynamic impact on the syngas, which provides a homogeneous temperature of more than 1200°C to all fraction of the syngas.
Figure 2: Concept image showing the feed system, the shaft-furnace-gasification reactor, the plasma-augmented-high-temperature-mixing-destruction reactor
Essential to waste treatment is that no material leaves the gas converter without being exposed to sufficiently high temperatures. The outcome is the fuel gas, leach resistant slag, molten metal and small amounts of secondary dust that may be subjected to recovery of materials like zinc, lead and mercury. About 65% of the heating capacity of the waste is retrieved in the form of fuel gas, and almost 30% is sensible heat. After conversion in a gas engine electricity is generated. Therefore, the process may be used for local co-generation based on a gas engine system.
Plasma generator and decomposition chamber
The foremost important features are the complete mixing and the instant initiation of the decomposition reactions accomplished in the plasma generator and decomposition chamber. This implies, as confirmed by testing:
- a complete dissociation of all hydrocarbons, even halogenated, with no indication of recombination
- no toxic nor carcinogenic organic compounds present in the produced fuel gas because no such compounds can survive at reactor temperature
- dioxin content of the fuel gas well below 1/10 of a nanogram per cubic meter of fuel gas
- NOx content within 10 - 30 ppm (even for nitrates)
The cracking of the syngas to a produced fuel gas is controlled by the intrusion of air to the mixing zone. The gas then diverts into the expansion zone followed by the equalising zone. The total residence time is 0.3 - 0.6 s. The purpose of the plasma generator is to:
- Provide a zone of high energy density
- Excite strong dynamic impacts to ensure a uniform temperature of the entire gas volume
- Supply heat in order to control the temperature level of the decomposition reactor
Figure 3: Image of the arc plasma showing in principal how the high temperature plasma is being generated.
Recordings of the dioxin level of the fuel gas from a test rig have been used to interpret the decomposition. All tests based on various materials from household waste to concentrated PCB oil, show that the dioxin level was kept well below 0.1 nanograms per cubic meter of fuel gas.
Figure 4: The functioning of plasma generator and decomposition chamber and secondary air intrusion. The heat required is 90-98% by partial oxidation and 2-10% electric power depending on the water content, typically 95% and 5% respectively.
The oxidation of the produced fuel gas is controlled by secondary air intrusion. The content of CO and CO2 of the produced gas is continuously recorded. The secondary air should be carefully controlled so that the oxidation ratio (as defined in Figure 4 is kept between 0.2 and 0.4. Experience shows that if the oxidation ratio becomes lower than 0.1, HCN may be formed. This should be avoided, especially in combination with wet gas cleaning. Vice-versa if the oxidation ratio exceeds 0.5, the formation of NOx may take place.
Shaft Gasifier
Owing to the extreme temperatures of the plasma jet the reaction rates in the decomposition reactor may become very high. For this reason a well premixed gas is required. For solid materials this can only be obtained by up-stream gasification. The gasifier is a counter-current-flow shaft reactor, known from the steel works, shown in Figure 5. The internal heat dissipation is controlled by the introduction of preheated blast air.
This type of shaft gasifier combines easy and rugged design with low thermal losses and long lining life. It is fully sealed so that no material can leave the unit without having been gasified or be completely melted.
The solid waste material is being fed into the shaft gasifier through a lock hopper system for shredded material, or an evacuated chamber for derived waste fuel. A short duct diverts the syngas from the top of the gasifier to the decomposition reactor.
As shown in Figure 5 the shaft gasifier is characterised by three distinct zones:
- evaporation
- carbonisation
- partial oxidation and vitrification
In the latter zone c), the charcoal descending from the carbonisation zone is mainly reduced to carbon monoxide by sub-stoichiometric reactions with preheated air. Owing to these exothermic reactions the temperature of this zone reaches 1450-1550°C. At this level all inorganic materials melt, and thus, leaving a homogeneous slag. Normally waste materials contain enough silica, which provides a glassy and leach resistant slag. Otherwise silica in the form of glass or a silica containing mineral must be added to obtain high leaching resistance.
Metals (contained by the waste) may be collected as molten metal from the shaft. The behaviour of metals appears to be slightly different:
- Metals with higher affinity to oxygen than iron (like Al, Ti, Mg, Ca) oxidise and dissolve in the slag
- Iron can be reduced to molten iron by adding coal and limestone to the charge
- Volatile metals like zinc, lead and mercury are evaporated and leave the gasifier as part of the syngas.
In the carbonisation zone (b) the solid material is being carbonised at a temperature of around 1000°C. The carbon-fix associated with most waste materials tends to form a low grade char with a carbon content of only 15-30%, the rest being inorganic material. In spite of the low carbon content the energy from this partial oxidation is sufficient to melt the inorganic components. To some extent, however, when the carbon-fix becomes too low, some coke may be added to secure the required temperature level.
All materials with some substantial vapour pressure, such as water and hydrocarbons, evaporate in the upper zone (a). The vapours leave the gasifier jointly with the syngas.
Gas Cleaning
The produced gas is quenched from 1200-1300 °C to 700 °C with recirculated fuel gas. Before the fuel gas enters the gas cleaning system the temperature is further reduced to about 150°C. A rather conventional down-stream gas cleaning system is used. The characteristic of the system may, however, depend on local conditions and specified emission level.
Compared to conventional technology some immediate differences become obvious:
- The volume of fuel gas to be cleaned amounts to only 30-40% of the corresponding volume of flue gas from an incineration process
- Practically no dioxins, and no chlorinated and halogenated hydrocarbons are present
- No components that form tar at low temperatures
- Less than 30 ppm NOx in the fuel gas
- Sulphur mainly appearing as metal sulphides
- Option for the capture of concentrated mercury, zinc and lead either as secondary dust or venturi sludge. (It should be noted that the emission of mercury depends more on the gas volume than on the total amount of mercury. Hence, a process offering low gas volume means substantially lower emission of mercury than conventional technology)
- Should the content of zinc and lead exceed 20-25% the recovery of these metals may become decisive for the economical viability of the plants.
Products
Fuel Gas
The fuel gas has a low calorific value (LCV) of about 4 MJ/Nm3. The content of carbon monoxide plus hydrogen amounts to 35-45%.
A rule of thumb approx. 65-70% of the energy content of the ingoing material (waste and or/biomass) is recovered in the fuel gas having a low calorific value (LCV) of about 4 MJ/Nm3, provided air is used as secondary and blast air. If oxygen enriched air is used the LCV can be increased to more then 8 MJ/m3 and the gas will contain 32% CO and 33% H2.
20-25% of the energy is in addition recovered as hot water or low pressure steam. This gives a unique flexibility with regard to utilisation of the recovered energy whereby the gas either can be used to replace other fossil fuels for production of electricity and steam.
Below is shown three different alternatives for energy recovery in the PyroArc©process.
Slag
The slag properties depend on the waste material. Normally the fraction of silica in the waste is sufficiently high to ensure a glassy structure with high leaching resistance. If not a suitable slag former (e.g. silica oxides or glass) can be added to influence the slag properties. The leaching of different components has been tested at a Dutch laboratory. The following table shows values obtained for the pilot PyroArc©process and normal bottom ash from municipal solid waste (MSW) to be compared to the thresholds given by the Dutch U1-standard. Contrary to the bottom ash, the values of the PyroArc©process clearly tell that there is no restriction of using its slag in buildings and for civil works.
Table of leaching resistance of slag:
Metals
Metals with a lower affinity to oxygen than iron can be recovered as molten metal, tapped separately from the shaft. This represents an interesting valorisation option for some kinds of waste like electronics and computer scrap. Especially copper alloys containing nickel, tin, silver and gold are easily retrievable.
The dust from zinc and lead can often be used for metal recovery. The alkaline salts usually extract the entire amount of chlorine present in the waste material. Depending on the local conditions and subject to proper treatment the alkaline salts could possibly be either as a liquid residue or as solid salt following a de-watering process.
Treatment tests
- Household waste (MSW)
- Impregnated wood
- Tires
- Car fluff
- Electronic waste
- Refrigerators
- Simulated hospital waste
- Asbestos
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- Chlorinated hydrocarbons
- PCB
- Freon
- Batteries
- Oil filter
- Paint, glue etc.
- Tannery waste
- Etc.
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