Fly Ash Treatment – Plasma Torches versus Graphite Electrodes
Plasma generating devices have to be designed to tolerate incredibly harsh conditions during operation and in particular very high heat loads and temperatures around the arc. However, when used in the vitrification of highly chlorinated materials the challenge is even greater than in other applications.
The combination of high general furnace temperatures, extremely high local temperatures near the arc, the presence of molten slags and metals and the circulation of chlorinated furnace gases and feed materials provide some of the most difficult operating conditions in any industrial process.
Under these circumstances, it is vital that plasma system providers chose the most appropriate plasma device in order to ensure long term safety and reliability of operation, without negatively affecting either the environmental or economic outcomes of the process.
Tetronics designs and supplies probably the widest range of plasma devices of any plasma system supplier in the world. These devices have been used for decades in many different industrial applications; the fact these robust designs have changed remarkably little over many years is also testament to the elegant and effective design of these devices.
This range of plasma devices include the following:
• Single DC Transferred Plasma Arc torches
o Cathode torches
o Anode torches
• Twin DC Transferred Plasma Arc torches
o Cathode and anode torch pairs
• DC Non-Transferred Plasma Arc torches
• Single DC transferred Plasma Arc graphite electrode
o Cathode electrodes
o Anode electrodes
• Twin DC Transferred Plasma Arc electrodes
o Cathode and anode electrode pairs
In addition to these choices above, some of these devices have also been used with AC electrical power.
The wide range of plasma devices available to Tetronics engineers and the extensive experience of their performance places Tetronics in a unique position to be able to select the most appropriate device for each application, rather than having to apply a ‘one size fits all’ approach to plasma device specification on each new project.
Plasma Device Comparisons
The most fundamental choice in this application is between transferred and non-transferred arc devices. Non-transferred arc devices are only available in the form of water-cooled plasma torches and are usually based on a vortex tube principle. These devices rely on a high-speed swirling gas (plus moving electrical fields in some cases) to move the arc roots around the anode and cathode electrodes, both of which are inside the torch. As a result, these devices tend to require very large flows of plasma gas. These high gas flows tend to increase levels of dust carryover from the furnace and also lead to higher losses of heat from the furnace in the form of heated gas. The fact that both ends of the plasma arc are within the plasma torch also leads to significant heat losses from the torch. The combined effect of these factors is that non-transferred arc torches tend to result in lower electrical efficiencies than other plasma devices. This leads to lower electrical and thermal efficiency, higher energy use and higher treatment cost than with other types of plasma device.
Another key difference between the different devices is between water-cooled torches and graphite electrodes. The key differences are listed below.
1. Graphite electrodes are widely used by Tetronics and others as plasma devices in harsh and demanding environmental applications. This provides the track record of many previous examples of their use in commercial plants in similar applications.
2. Graphite electrodes are very simple to use and result in no water cooling being inside the furnace. This means they cannot leak water into the furnace and require less complex supporting services and utilities. As a result, graphite electrodes are intrinsically safer than plasma torches, which provide a potential source of water leak and ultimately to steam explosions in the furnace.
3. Graphite electrodes require no maintenance. Graphite electrodes are consumed slowly by the process and are simply advanced into the furnace until a new section needs to added, when the plasma is switched off for a few minutes whilst a new section is screwed into the back of the existing electrode. This avoids the need to remove and dismantle a heavy, complex and potentially contaminated torch and then replace expensive components.
4. Graphite electrodes use typically no plasma gas. This reduces the physical carryover of untreated feed material, leading to secondary lower ash generation rates, and the loss of sensible heat in the gas, leading to higher thermal efficiency.
5. Graphite electrodes have no limit to their size or plasma power capability, compared with plasma torches which tend to have a maximum power output per torch. This provides a greater opportunity to scale the plasma technology up or down as required by each new application.
6. Graphite electrodes are extremely tolerant to the presence of corrosive gases, such as Cl, F, Br and S. Graphite is a material with an excellent intrinsic resistance to acid gases at all temperatures, especially when compared with the typical metals used to manufacture plasma torches. The corrosive effects of the acid gases are further enhanced with torches because the metal components are water cooled and tend to have cold outer surfaces, leading to condensation of acid gases on the surface and to aggressive attack of the material. Not only does this lead to shortened life of the torch components and frequent maintenance, but also water leaks into the furnace and possible steam explosions.
The third major difference between plasma devices is between DC and AC arcs. When using AC arcs, the arc restrikes many times per second, whereas a DC strikes once and then remains continuously on. The result of this is that DC arcs tend to have greater stability than AC arcs and are significantly quieter, both acoustically and electrically. The constant igniting and extinguishing of AC arcs tends to lead to electrical noise travelling back up into the supply grid. It also leads to constant motion of the magnetic fields around the plasma device, which can result in stray electrical currents in the plasma furnace structures.
On the basis of this knowledge and experience, Tetronics recommends using a DC plasma arc generated by a single graphite electrode (cathode) for the vitrification of chlorinated fly ash from waste to energy plants.
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