Carbon fiber is a material that has, and will continue to revolutionize the products we use every day by making them stronger, lighter and more durable. Experts estimate that the specialty material is twice as stiff as steel and five times stronger, per unit of weight.
It is made from organic polymers which consist of long strings of molecules held together by carbon atoms. The manufacturing process is often unique to each supplier and can be as complex as the molecules themselves. Gases, liquids and other materials used in the manufacturing process create specific strengths, qualities and grades of carbon fiber. A series of ovens and furnaces also give strands their desired characteristics by varying temperatures, dwell time and oxygen levels. As you can imagine, the energy demand and heat input needed for the processing of carbon fiber is extensive.
When the industry first began, the cost of operation for the first carbon fiber conversion plants were not critical design parameters and neither were the environmental effects. As the demand for carbon fiber expanded, much has been done on the production side to increase efficiency while reducing energy demands of the oxidation and carbonization furnaces and ovens.
Abatement Trends and Techniques
Manufacturing carbon fiber can have serious environmental ramifications and pose immediate danger to humans if careful consideration is not given to emission control at the production phase of these materials. The primary air pollutants of concern are Hydrogen Cyanide (HCN) and Ammonia (NH3) which are immediately dangerous to human health in small quantities. Greenhouse Gases (GHGs) such as Carbon Monoxide (CO), Nitrogen Oxide (NOX) and Volatile Organic Compounds (VOCs) are also controlled pollutants, not only for their warming effects on our climate but also their direct correlation to energy consumption. The manufacturing process also generates tar residues and silicone which further complicates the destruction of pollutants.
As is the case with many industrial manufacturing operations, emissions are best destroyed through the use of thermal oxidation technologies via high temperature combustion - CnH2m + (n + m/2) O2 Þ n CO2 + mH2O + heat. However, very few industries use custom designed solutions like the carbon fiber processing industry does on these two abatement devices.
Regenerative Thermal Oxidizer (RTO)
The energy efficient operation and high destruction efficiency of the RTO make it a great emission control device for a wide variety of industrial processes. However, unique aspects of carbon fiber processing dictate several oxidizer modifications in order to maintain system effectiveness, reliability and safety.
Theoretically, emissions from the low and high temperature furnaces as well as the ovens can both be handled by the RTO; however it is used most frequently for the oven exhaust. The regenerative component to the RTO makes it ideal for handling high flow, low concentration streams. With achievable thermal efficiencies over 97% the RTO is capable of operating with little to no supplemental fuel use.
On most applications, airflow is typically pushed through an RTO in a forced draft configuration. This reduces main system fan size and the electrical consumption required for it to push process air into oxidizer. Due to the HCN found in carbon fiber processing, many RTOs on this application are supplied in an induced draft configuration. The main system fan must be oversized to pull air through the oxidizer but it will ensure that all HCN emissions are drawn into the oxidizer for destruction, protecting the company’s employees and neighborhood from a potentially lethal situation.
Whenever oxidizing siloxane vapors in an RTO, considerations must be made to minimize the impact of silica dust build up in the heat recovery beds. The formation of the inorganic silica particulate starts at elevated temperatures and tends to accumulate within the media beds. Heat recovery beds should be designed to prevent silica dust from collecting and clogging in the oxidizer without compromising the high thermal efficiency. If build-up does occur, the RTO media should accommodate periodic cleaning and potential replacement, if required.
Multi-Stage Direct-Fired Thermal Oxidizer (MS-DFTO)
Low and High Temperature (LT / HT) furnaces on carbon fiber processes generate emission streams heavily concentrated with pollutants. This makes it an ideal application for the DFTO, however this abatement technology may require a significant amount of supplemental fuel to achieve destruction temperatures. Subsequently, they can be a large source of Carbon Dioxide (CO2) and NOX which are greenhouse gases and contribute to global warming.
However GHGs are not necessarily a by-product of these air pollution control devices. In recent years the technology has evolved, incorporating multiple zones or stages that that minimize the effects of combustion. The MS-DFTO is being uniquely applied in the carbon fiber industry to ensure the production of these materials is environmentally sound.
During operation, furnace exhaust is pulled through the system with an induced draft fan configuration. Doing this increases safety by ensuring that all of the Hydrogen Cyanide emissions are drawn into the oxidizer for destruction, protecting the company's employees and neighborhood from the potentially lethal gas leaking out of flanges, instruments, etc. This also ensures that oxygen does not migrate back to the furnaces.
The furnace (LT & HT) exhausts are injected into the first stage of the DFTO where there is insufficient oxygen for complete combustion of the hydrocarbons. In this reducing environment, the substoichiometric operation causes the hydrocarbons, hydrogen cyanide and ammonia to dissociate resulting in free nitrogen. The free nitrogen and combustibles gases are competing for the limited oxygen available from the oxygenated hydrocarbons and combustion air from the burner. This keeps the nitrogen from being oxidized to NOX.
This first stage of combustion is designed in a chamber capable of continuous operation at very high temperatures for the required residence time. The second zone of the system takes the N2, H2O and residual hydrocarbons leaving the first zone and cools these gases. This chamber uses a venturi type design to maximize the mixing of the hot gases and cooling medium. The gases leaving the first zone are cooled to the point where auto-ignition of the remaining combustibles will still occur in the last zone. The proper cooling medium is required to adsorb a great deal of energy without substantially increasing the outlet gas volume.
The third and final zone reintroduces air into the stream so that oxidation of the residual hydrocarbons, carbon monoxide and hydrogen can occur. Fresh air is added to this section so that the outlet oxygen concentration is maintained at the discharge of the system, prior to the induced draft fan. This proper oxygen concentration and residence time at temperature ensures extremely high levels of destruction. Since NOX formation in this zone is to be prevented, the outlet temperature from this stage is limited by a predetermined temperature.
Today’s processors are taking a much more holistic approach when designing and optimizing their plants, from polymerization and spinning to oxidation, carbonization, surface treatment and pollution control. Physical connection techniques as well as operational interface between the process equipment and abatement devices are critical to a successful installation. Optimizing energy recovery from thermal oxidizers will improve overall plant efficiency by integrating the process heating equipment.