As a material, carbon fiber has fundamentally changed many of the products we use every day, 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.

Carbon fiber is made from organic polymers that consist of long strings of molecules held together by carbon atoms. The manufacturing process typically is

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. Thermal processing within a series of ovens or furnaces also give the 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 carbon fiber processing is extensive.

When the industry first began, the operating costs for the first carbon fiber conversion plants were not critical design parameters, and neither were the environmental effects. As the demand for carbon fiber expands, however, much has been done on the production side to increase efficiency and reduce energy the 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 even in very small quantities. Greenhouse gases (GHGs) such as carbon monoxide (CO), nitrogen oxide (NO_X) and volatile organic compounds (VOCs) also are controlled pollutants, not only for their warming effects on our climate but also due to their direct correlation to energy consumption. 

The manufacturing process for carbon fiber also generates tar residues and silicone that further complicate 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:

C_nH_2m + ( n + m/2 ) O₂ ⇒ n CO₂ + mH₂O + heat

Very few industries use custom-designed solutions like the carbon fiber processing industry does on two abatement devices: the regenerative thermal oxidizer (RTO) and the multi-stage direct-fired thermal oxidizer (MS-DFTO). A closer look at each will show how carbon fiber manufacturers are benefitting from advancements in their designs.

Regenerative Thermal Oxidizer

The energy-efficient operation and high destruction efficiency of regenerative thermal oxidizers make them useful emission control devices for many industrial processes. Unique aspects of carbon fiber processing, however, dictate several oxidizer modifications to maintain system effectiveness, reliability and safety.

Theoretically, emissions from low- and high-temperature furnaces as well as the ovens can be handled by the regenerative thermal oxidizer; however, it is used

most frequently for the oven exhaust. The regenerative component to the regenerative thermal oxidizer makes it suited for handling high-flow, low-concentration streams. With achievable thermal efficiencies of more than 97 percent, the regenerative thermal oxidizer is capable of operating with little to no supplemental fuel use.

On most applications, airflow typically is pushed through the regenerative thermal oxidizer in a forced-draft configuration. This reduces main system fan size — and the electrical draw required for it to push process air into the oxidizer.

By contrast, due to the hydrogen cyanide found in carbon fiber processing, many regenerative thermal oxidizers used in carbon processing applications are supplied in an induced-draft configuration. The main system fan must be oversized to pull air through the oxidizer, but this design helps ensure that all hydrogen cyanide emissions are drawn into the oxidizer for destruction. Such an approach helps protect the company’s employees and local neighborhoods from a potentially lethal situation.

Whenever oxidizing siloxane vapors in a regenerative thermal oxidizer, considerations must be made to minimize the impact of silica dust buildup in the heat recovery 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 buildup occurs, the layout of the regenerative thermal oxidizer’s media should accommodate periodic cleaning and even replacement if required.

Multi-Stage, Direct-Fired Thermal Oxidizers

Low- and high-temperature furnaces on carbon fiber processes generate emission streams heavily concentrated with pollutants. This makes carbon fiber processing well suited for direct-fired thermal oxidizers. The abatement technology may require a significant amount of supplemental fuel to achieve destruction temperatures, however. This can lead to the thermal oxidizer being a large source of carbon dioxide (CO₂) and NO_X — greenhouse gases that contribute to global warming.

However, greenhouse gases do not necessarily need to be a byproduct 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. Multi-stage, direct-fired thermal oxidizers are being applied during the carbon fiber production in ways that help ensure the production is environmentally sound.

For instance, 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. This helps protect the company's employees and the surrounding neighborhood from the potentially lethal gas leaking out of flanges, instruments and other vulnerable points. The induced draft also ensures that oxygen does not migrate back to the furnaces.

The low- and high-temperature furnace exhausts are injected into the first stage of the direct-fired thermal oxidizer, where there is insufficient oxygen for complete combustion of the hydrocarbons. In this reducing environment, the sub-stoichiometric operation causes the hydrocarbons, hydrogen cyanide and ammonia to dissociate, resulting in free nitrogen. The free nitrogen and combustible gases compete for the limited oxygen available from the oxygenated hydrocarbons and combustion air from the burner. This keeps the nitrogen from being oxidized to NO_X.

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 nitrogen gas, water 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 autoignition 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. Because NO_X formation in this zone is to be prevented, the outlet temperature from this stage is limited by a predetermined temperature.

Optimizing energy recovery from thermal oxidizers will improve overall plant efficiency by integrating the process heating equipment.