WHEN IT COMES TO ON-SITE POWER, FUEL FLEXIBILITY IS KEY
One of the best ways to bring the efficiency and reliability benefits of on-site power to a wider range of applications and locations is to offer fuel flexibility. For decades, Capstone microturbines have provided such adaptability. The continuous combustion process of gas-turbine technology allows for a complete and controlled burn of a range of hydrocarbon fuels, so many Capstone customers have been able to use on-site fuels or re-purposed byproducts traditionally marked as waste. This is a characteristic that makes Capstone microturbines an ideal solution for a number of markets. However, configuring a system to use a different kind of fuel is not as simple as flipping a switch. Each microturbine application must be designed and engineered to effectively deliver the necessary fuel to the engine.
THE FUNDAMENTALS OF FUEL FLEXIBILITY
Here’s a mini-tech lesson in how a gas-turbine technology achieves fuel flexibility.
- Ambient air is pulled in through a filter and compressed. The amount of air compressed is much greater than what is needed for combustion, with a large portion of the air used to dilution, ensuring that the hot sections of the engine operates within their temperature limits.
- In recuperated models, which include Capstone’s microturbine products, the recuperator transfers waste heat from combustion exhaust to the compressed air. Energy recovered from the exhaust offsets fuel usage and greatly increases the system’s electrical efficiency.
- The pre-heated, compressed air enters the combustion chamber where fuel is added. A gas turbine’s combustion process is continuous and well controlled, rather than intermittent (as in a reciprocating engine).
- Additional energy from the combustion process is extracted during the power turbine stage, thus creating mechanical power that ultimately drives the compressor wheel and generator.
- The hot, post-turbine exhaust then passes through the recuperator, where it transfers some energy to the incoming compressed air stream (Step 2 above).
- The combustion products and residual heat are rejected through the exhaust stack, where they may be released to the ambient air or passed through additional equipment to recover additional energy.
Gas turbines require fuel to be compressed (or pumped, for liquid fuels) through various injectors into the engine to overcome pressures associated with the combustion chamber. Due to the level of the internal engine pressures achieved by air compression, a Capstone microturbine requires fuel at an elevated pressure for proper operation. For applications involving the use of low pressure pipeline natural gas, Capstone offers microturbines with an optional internal fuel gas booster. For other fuel types, Capstone offers various microturbine configurations to allow for their safe and reliable use, provided that the fuels are conditioned to Capstone’s specifications.
One concern to be aware of is that fuel contaminants can negatively impact turbine performance and life. Because contaminants in the fuel stream come in contact with the fuel compressor or pump, fuel valve, manifold or injectors, combustor, power turbine and recuperator, it is important to keep fuel containments within specified limits. This will allow the microturbine to remain consistent with baseline fuel performance and maintain integrity of the system over the project’s life. For example, to achieve reliable operation for a landfill gas fueled project in La Ciotat, France, the microturbines required gas compressors, water-glycol chillers, and heat exchangers for moisture removal. In addition, the system used activated carbon filtration to remove “siloxanes” from the fuel, which otherwise would lead to the formation of silica within the engine when burned.
OTHER CONFIGURATION NEEDS
Different fuel types can have varying combustion properties. Depending on the fuel’s constituents, which may consist of hydrocarbons and inert gasses, different fuels will have different physical properties. The impact of these properties must be taken into consideration, especially with respect to the gas turbine’s injectors, fuel control system, and the power turbine nozzle size.
Properties such as the fuel’s heating value will influence the amount of fuel consumed per kilowatt-hour of power generated without penalty to the microturbine’s fuel efficiency. This in turn affects the sizing of fuel control valves for full power production and idle operation. Software settings are influenced by fuel properties so that fuel system components can respond appropriately to changes in power demand. Likewise, internal components must be designed to maintain the correct conditions within the combustor to provide stable operation and minimize emissions.
Capstone microturbines are fuel flexible, but a single gas turbine cannot be used with just any fuel type. While some models can easily be modified to burn an alternate fuel type, other models require reconfiguration and/or ancillary equipment. In more complex or unconventional applications, the Capstone Applications Group can provide additional guidance on handling out-of-specification fuels.
One notable advantage of gas turbine engines over reciprocating engines is the gas turbines’ ability to burn very low-energy fuel sources, which are often found in landfills and occasionally in gas fields. While reciprocating engines experience a decrease in power output when burning low energy fuels, gas turbines do not. Capstone offers several configurations that are capable of low-BTU fuel operation.
Capstone microturbines are available in configurations that can operate on most commonly used fuel sources. Regardless of the fuel type used*, the microturbines retain all of Capstone’s energy-production benefits, such as high fuel efficiency, superior reliability, and low emissions.
This allows Capstone to provide key benefits to a range of applications and markets around the globe, especially as the world experiences the impacts of climate change.