By Diane Rapaport, Freelance Editor
When a utility needs additional power during peak demand periods, standby and/or peaking units are placed into service. Although these units are idle until needed, they must start immediately when a utility calls for their services. Today, a significant number of utilities use aero-derivative gas turbines to generate peak power. These turbines are fueled by natural gas, diesel or #2 fuel oil. (The latter two fuels, known as middle distillate fuels, differ primarily in sulfur content.)
The choice of whether to use natural gas or other fuels is driven by cost. When natural gas prices spike, utilities switch to the middle distillates. These fuels, purchased when prices are low, are stored on-site in above ground storage tanks.
Unfortunately, a significant number of utilities are experiencing operational problems because middle distillate fuels are chemically dynamic. Within three weeks or less from the time of refining, liquid fuels begin to form solid particulates—insoluble gums and varnishes. The primary cause of the degradation is re-polymerization, a two-step process that includes the lengthening of the molecular chains and the sticking together or clotting of particles. Oxidation is only a secondary contributor.
The longer the fuel is stored after being refined, the worse the problem gets. As the re-polymerized molecular chain becomes longer it develops its own physical characteristics (stickiness). These longer sticky chains adhere to each other to form larger macroscopic particles that plug filters, smoke when burned and leave carbon residue throughout the systems. The particulates, which accelerate the formation of sludge in the storage tanks, increase the need for tank maintenance. Eventually the fuel in the tank can’t be pumped. It can also become noncombustible.
Changes in fuel distribution systems during the last decade also have a negative, if unintended impact, on the condition of the fuel at the time of delivery. Today, traders and brokers control when fuel ends up in local terminals. Without testing, utility users do not know how long it has been since the fuel was refined or its particulate content.
Switching to liquid fuels can be a problem for gas turbines. Photo Courtesy of Fuel Management Services
The major cause of the degradation of middle distillate fuels is the catalytic-cracking methods used to refine the crude oil. These methods result in fuels that are chemically dynamic. In a matter of weeks insoluble particulates are formed.
Prior to 1993, straight run distillation was used to produce fuels that were stable, and withstood heat, age and handling without producing significant particulates. Although oxidation was a problem, it could be treated with anti-oxidants. However, straight run distillation only converts about 55% of crude oil into middle and light distillates. To satisfy increasingly higher demands, refineries have turned to catalytic-cracking methods, which convert over 90% of a barrel of crude to middle and lighter distillates.
Since 1993, virtually no pure, straight run fuel has been available. The irony is that while the utility and other industries operate with some of the best and most efficient equipment being manufactured, these industries are losing equipment reliability when they use chemically active middle distillate fuels.
To deal with the instability of middle distillate fuels, and ensure that peak power is available when required, utilities have the following options:
- Use straight run fuels
- Use kerosene, which costs 1-10 cents more per gallon. However, kerosene is a lower quality fuel than other middle distillate fuels. In addition, kerosene is not always available to meet the needs of peak power.
- Dilute the particulates with new fuel oil
- Filter the fuel oil at a cost of between 15-25 cents/gallon
- Transfer the fuel to boiler stock and buy replacement fuel
- Use a fuel stabilization process at a cost of 1/2 to 1-cent per gallon
Today, a fuel stabilization process developed by George and Nancy Kitchen, founders of International Lubrication and Fuel Consultants (ILFC), Inc., makes middle distillate fuels more reliable. During development, the Kitchens found that the dynamic characteristics of catalytically cracked fuels posed a new set of problems for emergency back-up power systems. Existing laboratory test methods were found to be inadequate for determining the cause and rate of degradation. As a result they developed an accelerated aging test to measure fuel stability.
According to the Kitchens, the rate of polymerization is a direct function of the severity of the refining process (heat, pressure and catalysts), the crude stock, the blend stock, and the energy input into the fuel. Exposure of the fuels to heat and water accelerates re-polymerization and oxidation.
Over a period of years, IFLC developed several chemical additives, called “fuel inhibitors.” When added to fuels the additives are able to retard re-polymerization, disperse existing particulates, eliminate bacterial growth and prevent oxidation and tank corrosion. A stability test provides the information needed to specify which additives and what dosages are necessary to return the fuel to its original specification.
Today, the ILFC fuel stabilization process is the only one qualified for used by the United States Navy to help meet fuel specifications that ensure that the middle distillate fuels that power their ships meets their rigorous specifications. The Navy has used ILFC’s process for 10 years.
Case History: East Coast Power Company
Three years ago, a subsidiary of an East Coast utility discovered it had serious problems producing peak power when switching their F class gas turbines to #2 fuel oil. Black tar-like particulates plugged the main filters stations, restricting flow and causing frequent filter replacement and cleanup. Some of the gunk also passed through the filters and plugged up wire screen filter upstream of the gas turbine nozzles. When the fuel system became too gummed up the engines became fuel starved. The utility did not know they had a problem until they started having problems starting the gas turbines.
After determining that the failure stemmed from clogged filters, the plant carried out a root cause analysis. This included examining the plant’s operating procedures, equipment design and fuel instability. Plant management invited Ed Kitchen, president, Fuel Management Services (FMS), Inc. to make a presentation on the use of ILFC’s fuel stabilization process. During his presentation, Kitchen stated that the cost of the fuel stabilization process would be less than 1-cent per gallon. Further, with continual treatment and testing, the fuel would remain stable almost indefinitely. With this assurance, the plant decided to treat their fuel oil.
To determine the correct amount of inhibitor, the fuel was first sampled and analyzed. This analysis was necessary to identify the organic, inorganic and microbial contamination, the degree of repolymerization and oxidation, and the water and sediment content. Based on FMS’s analysis, the utility’s fuel was treated with prescribed dosages of fuel inhibitor to disperse existing particulates and inhibit repolymerization and biocide to inhibit bacterial growth.
Over a three-year period, the use of cleaner and more stable fuel has extended the life of the fuel oil filtering system. As a result, the utility has been able to operate their gas turbines more efficiently during periods of peak demand. In addition, the utility is now able to take advantage of low fuel prices without worrying about fuel degradation. Analysis of the fuel oil, which is carried out four times/year, is now part of the utility’s preventative maintenance program. When new fuel is delivered to the power plant a low dose of inhibitor is added.
Future Utility Applications
Despite the success of the program for stabilizing middle distillate fuels and eliminating existing particulates at the east coast power plant, other utilities have been slow to adopt fuel stabilization. Some of the reasons given by utilities for not using fuel stabilization inhibitors include:
- The utility already knows how to operate and maintain their equipment and is unaware that current refinery processes make fuels inherently unstable.
- They are not aware of fuel stabilization options for preventing repolymerization and making fuel stable.
- The utility assumes that, after refining, the fuel oil is delivered to the plant within a few days.
- The word “additive” does not always carry a positive connotation.
Additives first came into use to help cure problems caused by oxidation in fuels produced by straight run distillation. Unfortunately, many of the additives did not cure the primary cause of fuel degradation—repolymerization caused by cat-cracking. In addition, some additives contain inorganics that can create ash. Ash fouling of the turbine blades is a problem.
Analyzing and treating fuel when it is first delivered to the plant will ensure the reliability of middle distillate fuels. In addition to eliminating particulates and retarding repolymerization, fuel inhibitors also retard the growth of sludge, bacteria and fungus, thereby decreasing tank cleanouts.
The process used at the eastern power plant has been successfully used to clean fuel that had been stored in a tank that hadn’t been cleaned for 15 years. Once it had been tested and treated, the fuel was used without any problems. ILFC’s fuel stabilization process is not a cure for all fuel problems. When using any additive, users must regularly test the fuel to ensure the process is working efficiently. The most important component of analyzing the fuel is to understand how much repolymerization has occurred, what products need to be used, and in what dosages.
Fuel stabilization results in increased reliability of the fuels, filter life extension and reduced likelihood of startup failures when gas turbines are used for peaking. It also enables utilities to buy middle distillate fuels when prices are low. Once the high maintenance costs of dealing with unstable fuel are eliminated, power plants can be confident that their gas turbines will run smoothly when peak power is needed.
Diane Rapaport is a free-lance writer specializing in pollution prevention technologies.