Maximizing the Competitive Advantages of Low-Cost Natural Gas in Carbon Fiber Production

Today's energy trends are shifting toward increased natural gas consumption, and when taken into account in the design of carbon fiber production facilities, they can yield advantages in terms of performance and economics within certain parameters. While the electrical supply is regarded as more stable and reliable, electricity is a more expensive energy source across most regions of the world. As the industry works to develop solutions for producing low-cost carbon fiber, all aspects of the production process are under review, and evaluating natural gas consumption can play an important role in solving the economics puzzle. This article will discuss innovations that can enable a transition to gas-fired units with improved controls and thermal stability, as well as hybrid gas/electric designs, and which also focus on reducing consumables spent in the thermal conversion process. Example scenarios will be presented to demonstrate the advantages.

A Universal Facility Example

Future carbon fiber facilities will need to be designed from the outset to operate at the lowest possible cost. While the industry has largely focused to date on reducing total grid electricity consumption, energy still represents 5 to 10 percent of total carbon fiber production costs [1]. To reach the lowest possible operating costs, carbon fiber facilities will need flexibility to use either natural gas or electrical energy, selecting whichever is cheaper in a given region. Unstable energy markets further complicate matters by causing this preferred energy source to change over time. For this reason, the ideal universal facility would have a hybrid design capable of running on either natural gas or electricity with the same equipment on a particular line. At the core of this hybrid technology are oxidation furnaces. Among all unit operations in the carbon fiber line, the oxidation system consumes the most energy, by a factor of approximately 1 to 4. The reason is the long fiber residence time required by the polyacrylonitrile (PAN) oxidation process relative to the carbonization steps, and the high process atmosphere support flow rates. Heating the oxidation furnace consumes 32 to 40 percent of the energy used in the carbon fiber line [2][3]. Any reduction in electricity cost here translates to a significant reduction in the total operating expenses of the carbon fiber line.

Universal Technology and How It Works in Practice

In the oxidation context, two to four equipment stacks are spread out, with typically four to eight thermal zones. Compare this to a carbonization furnace that can have four to ten zones within a single piece of equipment. Furnaces are simple, large enough to easily accommodate redundant heating devices, and are the components most likely to benefit from hybrid heating technology, given that the furnace process consumes ten times more electricity than low-temperature or high-temperature carbonization processes consume (Omnia, LLC, 2012). When all these factors come together, they allow hybrid furnaces to pass through a cost/benefit analysis that is not quite as clear-cut in a carbonization furnace. In a typical oxidation furnace zone, a circulation fan blows air through a series of electric heating elements to raise the temperature of the circulation air. The electricity supplied to these heating elements is controlled by a temperature control device specific to the method that maintains a set temperature within the fiber processing chamber. In a hybrid furnace, it would involve adding a gas-fired indirect heat exchanger in series with the electric elements. As shown in Figure 1, the setpoint of the first temperature control device (electric elements) can be manually set to a lower value than the second temperature control device (gas-fired heat exchanger). This allows the first heater to supply a certain amount of energy to raise the circulation air to the required process temperature, and the second heater can finish the job. In cases where gas is not needed (when electric heating is cheaper), the gas-fired heater can be set to zero and the electric elements can do all the work. In cases where electricity is not needed (when gas heating is cheaper), the electric heater can be set to zero and gas can do all the work. When some combination of heating sources is needed (for example, when the electric utility requests reduced consumption on peak summer days), the load can be shared between the two heaters. When the furnace needs to be heated as quickly as possible on initial startup, both heaters can be set to 100 percent efficiency to achieve heating much faster than either one could accomplish individually.

Overview of Energy Production and Consumption Trends

The outlook for the energy industry today differs significantly from projections made in recent years. New dynamics are influenced by oversupply on the market, partly from new supply discoveries and also from a general decline in demand. These forces have substantially altered energy dynamics in the global marketplace. Capitalizing on these geopolitical paradigm shifts can provide companies with sound strategic advantages for the future.

Recent Developments in Natural Gas

While naturally occurring gas has been known since ancient times, its commercial use extends to relatively recent history. Today, natural gas is a vital component of the world's energy supply. Taking the United States as an example, natural gas currently accounts for more than half of the energy consumed by residential and commercial customers and approximately 41 percent of the energy used by US industry.[5] In North America and elsewhere, recent discoveries have directed energy experts and policymakers to focus on natural gas as a driving force for pursuing a wide range of objectives (mitigating the impact of sudden spikes in energy prices, reducing dependence on foreign oil, addressing greenhouse gas emissions, and other priorities). Since we have information available, continuing to use the United States as an example, according to the Energy Information Administration (EIA), US net natural gas imports sustained an eight-year decline in 2014, falling 9 percent. As US dry natural gas production reached record high levels, declining domestic prices helped natural gas imports displace other sources. Net natural gas imports (imports minus exports) totaled 33.16 billion cubic meters in 2014, the lowest level seen since 1987. Crude oil, refined into gasoline or diesel, provides fuel for nearly all automobiles and trucks. Coal is used as the dominant fuel for electricity generation. However, as greenhouse gases continue to alter our environment, developing a roadmap toward a low-carbon economy will proceed very rapidly in North America and around the world. This transition will fundamentally transform our economy and the service infrastructure as we know it. Since the market remains largely fragmented and underdeveloped with still vast supply reserves, an uneven development landscape will see implementation and new policy applications define the market in the years ahead.

Theoretical Case Study

Effects of Hybrid Oxidation Technology on Operating Expenses:

In this section, we will address a theoretical analysis of three carbon fiber lines and the impact of different oxidation energy sources on operating expenses (OpEx). Line 1 – 3 meter – Current industry standard production scale Line 2 – 4 meter – Next production scale Line 3 – 5 meter – Future production scale We will review two different methods for supplying energy to these lines. Scenario 1 – Electric Oxidation Scenario 2 – Gas Oxidation These process parameters were used as inputs to Harper's standard process models to predict the energy expected to be consumed by heating the oxidation systems across our three lines. Since we have information available, using the United States as an example, these energy consumption values were multiplied by both the Industrial Sector costs for gas and electricity obtained from the US Energy Information Administration's Short-Term Energy Outlook.[9] The results are shown in Figure 2. In other parts of the world, heating with natural gas and electricity are much closer in cost. There are even times, such as off-peak periods when demand on the electrical grid is low, when heating oxidation furnaces with electricity is essentially cheaper than with natural gas.

Effects of Gas-Fired Low-Temperature Carbonization on Operating Expenses

To demonstrate the complexity of the issue, we will now evaluate the impact of choosing a gas-fired or electric low-temperature (LT) carbonization furnace. As process temperature increases, the efficiency of natural gas decreases [10]. When modeling electricity and gas energy consumption, a reasonably typical number of zones and temperature profile was assumed. Figure 4 shows the cost incurred to heat an LT furnace with 2014 electrical grid costs. Figure 5 shows the results of the same calculation, this time using 2008 electrical grid costs. In 2008, heating a low-temperature carbonization furnace with electricity could have been cheaper than heating it with gas. An anomaly occurred that year, with market forces temporarily inflating natural gas prices in the United States. The purpose of this analysis is to show that even in regions like the United States where natural gas is generally cheaper than electricity, the preference is not always clear-cut. Here the choice of heating source could be: Good – Electric Furnaces, Gas LT Better – Gas Furnaces, Gas LT Best – Hybrid Furnaces, Gas LT This good/better/best scenario comparing gas heating with electric heating applies to regions where natural gas is consistently the lower-cost energy source. The situation may be different in other parts of the world. For this reason, a hybrid facility design is ideal on a universal level.

Other Considerations Reliability

In a hybrid oxidation system, redundant heating sources can create greater reliability. Electric heating elements and gas-fired heat exchangers can be sized to handle full heating load, whether using all-electric or all-gas heating. In this way, simple redundancy-based reliability is achieved. If one heating source fails, the other cankeep the system running until the end of the operating period during which the faulty unit can be replaced.

Daily Operations

Electrical utilities typically structure customer contracts to apply expensive penalties for high consumption during peak demand times, such as midday during summer months. To avoid this, facilities that can effectively reduce their rhythm typically reduce production during these times. For example, it is common practice for industrial gas plants in summer to reduce the output of cryogenic air separation units during midday hours. During the operating cycle, this is not an option in carbon fiber lines where reducing output by, for example, reducing line speed could damage the fiber product by changing the time/temperature profile. The only way to reduce electricity consumption during operating hours is to divert part of the load to an auxiliary heating method (natural gas) during peak hours.

High-Temperature and Ultra-High-Temperature Furnaces

The final thermal processing steps in a carbon fiber line, high temperature (HT) and ultra-high temperature (UHT), are not discussed here. These processes operate between 1200°C and 2800°C. As process temperature increases, the efficiency of natural gas decreases. For this reason, natural gas is generally meaningful for oxidation furnaces (250°C) and may be meaningful for LT furnaces (800°C), but is definitely not a viable option for HT and UHT furnaces. The preferred technology in these processes is electric heating.

Summary

Natural gas will play an increasingly vital role in the world's energy infrastructure. As policymakers in countries with emerging natural gas supplies support the expansion of these supplies, the industry continues to invest in infrastructure. This will ensure the continuation of the trend of lower gas prices for industrial users, which will be significantly lower than electricity across all categories. Because carbon fiber oxidation furnaces and low-temperature furnace technology are energy-intensive, gas-fired technologies will securely carry investment decisions into the future. Harper International is at the forefront of adopting the most efficient and effective energy use scenario for market needs, with deep expertise in gas-fired, electric, and hybrid furnace technologies. James Fry / Application Engineer - Harper International

References

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3. Stry, B. (2013). How Mass Automotive Use Will Affect The Way Carbon Fiber Producers Must Think About Their Facility's Carbon Footprint. GoCarbonFiber. Seattle.

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