Efficient Exhaust Gas Treatment for Carbon Fibre Processes & Recycling

October 13, 2012 7:59 am

Ecopure VAR systems made by Durr provide carbon fibre manufacturers a huge savings on the fuel requirement for their exhaust gas treatment and significantly lower operating costs
The use of carbon fibre and composite materials has significantly increased in the past few years and will further increase mainly to reduce the weight of airplanes, cars and wind turbine blades in order to reduce fuel consumption and increase efficiencies. For the production of carbon fibre a reasonable amount of heat energy is required for fibre oxidation ovens and carbonisation furnaces, and substances such as hydrogen cyanide (HCN), Ammonia (NH3), carbon monoxide (CO), tar and other VOCs are discharged from these ovens and furnaces, which can cause serious environmental pollution as well as danger to human health if they are not treated properly.
A technology that has already been installed by Dürr in the early days of carbon fibre production in 1988, and which is still in permanent operation, is the Ecopure VAR technology, a direct fired thermal oxidiser system.
A VAR system consists of a refractory lined oxidation chamber, a burner unit and injection nozzles and lances, through which the exhaust gas from the carbon fibre oxidation ovens and carbonisation furnaces is directly injected into the oxidation chamber. By a turbulent mixing of these gases and heating them up to high temperatures with the burner, more than 99.9 per cent of these pollutants are destroyed and oxidised into harmless water and CO2 vapours. The hot purified gas leaving the oxidation chamber can then be used for the production of steam or for heating thermal oil, hot water or fresh air that can be used as energy source for ovens and furnaces.
VAR Based on these early experiences, in the 1990s Dürr developed and installed a special burner system and a special method for injecting the various exhaust gases such that 99.9 per cent destruction and all the relevant emission limits in Europe, the United States and Asia would be achieved at oxidation temperatures around 850 – 900° C, whereas typical systems today still require temperatures of more than 1,100° C to achieve these limits. Even the formation of NOx is reduced to a minimum in the Ecopure VAR, and the NOx concentration in the purified gas stays below 20 ppm.
With such a burner technology the Ecopure VAR systems made by Dürr provide carbon fibre manufacturers a huge savings on the fuel requirement for their exhaust gas treatment and significantly lower operating costs.
Dürr provides Ecopure VAR systems for treating exhaust gases from oxidation ovens and carbonisation furnaces in one combined unit as well as systems just for the treatment of exhaust from carbonisation furnaces.
Depending on the type of carbon fibre and pre-material sometimes also small amounts of silicone substances are released with the oven and furnace exhaust gas. These silicone substances oxidise into a solid that can block the heat recovery system if not properly considered in the design. For this reason Dürr also provides specially designed heat recovery systems to overcome such a blockage, or automatic cleaning systems that function during plant operation.
As such, many carbon fibre producers are cleaning their exhaust air with a Dürr Ecopure VAR unit and recover their heat energy for the process, such that the temperature of the purified air being finally discharged to the atmosphere is lower than the temperature of the incoming exhaust gases. The result and bargain for the carbon fibre producer is not just a green and environmentally friendly process, but also an energy requirement that is lower than it would be without exhaust gas treatment.
For all carbon fibre producers who cannot make use of all the possible recovered heat, Dürr offers the possibility to treat the exhaust gas from the oxidation ovens, or a part of it, in an Ecopure RTO Regenerative Thermal Oxidiser, a technology that Dürr has installed as early as 1996 for some carbon fibre makers.
RTOAn Ecopure RTO plant consists of its refractory lined regeneration heat exchanger tanks, oxidation chambers and associated burner system, as well as a duct system for direction of air streams. Each regeneration tank is packed with ceramic heat exchange material. The oxidation chambers include the burners and link the individual regeneration tanks to each other.
Tightly sealing, time controlled valves are integrated within the duct system for letting in exhaust gas and taking out purified gas, as required, to one of the regeneration tanks. The exhaust gas flows through the ceramic material from bottom to top so that it is preheated. At this stage a large proportion of the pollutants are already oxidised. In the oxidation chamber the exhaust gas is heated up to its full reaction temperature, completing the cleaning process. The purified gas then flows through the other tanks from top to bottom, transferring its heat to the ceramic material.
Dürr Ecopure RTOs are convincing because of their tremendously high level of thermal efficiency of up to 97 per cent, such that the temperature of the purified gas leaving the Ecopure RTO units is just around 30 – 40 °C higher than the temperature of the incoming exhaust gas. Therefore Ecopure RTO units can be operated autothermal, without any fuel requirement already at low pollutant concentrations in the exhaust gas, due to the thermal heat release during the oxidation process. Dürr Ecopure RTO units also achieve destruction efficiencies up to 99.9 per cent and are in compliance with all the relevant emission limits in all countries.
With the combination of Ecopure VAR for the treatment of the carbonisation furnaces exhaust gas and Ecopure RTO for the treatment of the oxidation oven exhaust gas the fuel consumption of the exhaust gas treatment system is close to zero, just a pilot flame is required. In addition heat can be recovered from both systems reducing the energy requirement for the production line to a minimum.