Fuel for Thought: Liquified Petroleum Gas (LPG)

April 3, 2023

If the past several years taught us anything about the energy landscape, it’s that having a range of fuel options for power generation is an important hedge against instability. Fortunately, fuel flexibility is a hallmark of Capstone microturbines, which can run off a variety of sources, from natural gas and propane to methane, hydrogen, and more.

In this newsletter, we’re turning the spotlight on Liquefied Petroleum Gas (LPG), a widely available fuel for power generation that is an excellent alternative to diesel and other expensive, “dirty” fuels. This primer will help you understand the different types of commercially available LPG and how they can be integrated with Capstone microturbine systems.

What is LPG?

Liquefied Petroleum Gas is characterized as a mixture of propane (C3), butane (C4), and small quantities of various other hydrocarbons, such as propylene and butylene.

Twenty-three Capstone C65 microturbines provide prime power to Southern California Edison's Avalon site on Catalina Island

LPG is transferred and stored as a pressurized liquid; however, its boiling point is such that it evaporates easily under ambient temperature and pressure. The molecular composition of LPG determines the dew point, heating value, density, and many other fuel properties, as well as the percentage of contaminants. These values are used to determine whether a fuel is acceptable for use in an engine or turbine. For this reason, it is important that the composition of the LPG is known prior to designing the fuel delivery system. Because the composition of LPG can vary significantly between fuel types, Capstone has enhanced the fuel capabilities of our C200 and C1000 series microturbines to use a wider variety of LPG.

The four most common commercially available types of LPG are Special Duty Propane (HD-5), Commercial Propane (HD-10), Propane-Butane Mixtures (PB Mix), and Commercial Butane. LPG can also be mixed with conditioned air in order to make an LPG/Air Mixture. The addition of air may serve to alter the overall fuel properties to a more desirable level for operation. Capstone’s microturbines have the ability to run using Special Duty Propane (HD-5), Propane-Butane Mixtures, or LPG/Air mixtures.

Propane storage tanks at Avalon site on Catalina Island

When comparing LPG to Natural Gas (NG), which is a mixture of primarily methane (C1), ethane (C2) and small quantities of other gases, it is important to note the difference in heating value. Natural Gas has an average heating value of 1,000 Btu/ scf. Special Duty Propane has an approximate heating value of 2,500 Btu/scf, and Commercial Butane is over 3,000 Btu/scf. Therefore, the heating value of LPG is roughly 2.5 to 3 times greater than NG. This means that LPG requires a much lower volumetric flow rate in order to achieve the same engine output. LPG is also stored as a liquid, which compresses the volume of the fuel 250:1. This is achieved without costly cryogenics required by Liquefied Natural Gas. These factors lead to a very small footprint for LPG, compared to the pipelines and large infrastructure required of NG, and LPG can be transported easily by vehicles and stored in tanks, making it a good replacement for diesel.

Using LPG in Microturbines

Generic Representation of a Propane Fuel-Delivery Train

Special Duty Propane (HD-5)

Special Duty or HD-5 Propane is defined as greater than 90 percent propane and less than 5 percent propylene. This grade of fuel is ideal for all types of engines and turbines due to the cleanliness of the burn and the low level of contaminants relative to diesel. Commercial Propane contains up to 10 percent propylene and cannot be used in reciprocating engines or turbines due to propylene’s tendency to polymerize under high heat.

All Capstone microturbine models have a version that can operate using HD-5 Propane. For anyone looking to install a new HD-5 Propane system, we suggest reviewing Capstone’s Turbine Talk article, Growing Interest in Propane Fuel [LINK TO ARTICLE], which highlights the use of HD-5 Propane in microturbine applications.

Propane-Butane Mixtures (PB Mix)

Propane-Butane Mixtures have no standard specification for their compositions and can therefore be a problem for gaseous fuel operation due to the low dew point of butane. The higher the concentration of butane, the lower the dew point falls, and the more heat tracing and insulation is needed with the fuel delivery system. This causes a higher risk of fuel condensation, which in turn may lead to engine problems. The LPG capable versions of the C200 and C1000 series microturbines were designed with a more versatile fuel system than their C30 and C65 counterparts. This includes internal heat tracing and insulation of fuel lines, which reduce the risk of condensing vapor due to heavier fuels. The goal of the heat tracing and insulation is to maintain the supplied inlet fuel temperature without the need to increase the fuel temperature or vaporize condensed liquids.

The LPG capable C200 and C1000 series microturbines are approved to operate using a Propane-Butane Mixture of up to 40 percent butane. This does not mean that PB Mixtures containing greater than 40 percent butane are automatically disqualified. Capstone applies the same limitations towards propylene, limited to less than 5 percent, as well as all other contaminants listed in the Special Duty Propane specification. Customers looking to use PB Mixtures are strongly encouraged to contact Capstone’s Applications department in order to verify the fuel specifications and to aid in design of the fuel delivery system.

LPG/Air Mixtures

Certain LPG types that are not suitable for operation in microturbines, may be approved when mixed with air. Alternatively, the mixture may attempt to match the properties of a more standard fuel, such as Natural Gas. LPG/Air mixtures are not standard and may require complex fuel delivery systems. The approval of these fuel types is dependent on the review of the fuel properties and composition. Detailed analysis would be needed in order to determine the feasibility for use in microturbines. Capstone’s Applications department has created several tools to aid in the analysis of fuels and should be contacted for all LPG/Air Mixture projects.

Real-World Application

In March 2023, a 600 kilowatt (kW), C600S, LPG-fueled system was commissioned at a remote food processing facility in Bamako, Mali. Like many land-locked countries, Mali relies on expensive, “dirty” fuels like diesel and heavy fuel oil, so this project was an important step in demonstrating the benefits of a system whose fuel is both less expensive and more environmentally friendly.

Official ribbon cutting ceremony at a remote food processing facility in Bamako, Mali, to celebrate the installation of a Capstone Green Energy C600S microturbine.

The new system also dramatically improves reliability, which addresses issues of load shedding and blackouts the facility previously experienced with utility power. And because the microturbines also require very little maintenance compared to other on-site power technologies like diesel generators, power availability and cost savings are also improved.

"The Mali project serves as a model for other industrial customers and power companies, showing they can quickly benefit from LPG as an alternative fuel," said Gorgui Ndoye, Business Development Director for Capstone Green Energy. "There is growing interest in LPG in many regions around the globe, especially in Africa, South America, the Caribbean, and some parts of Asia, where availability of the fuel has increased significantly in the past ten years."

Better for Business and the Environment

It’s difficult to underestimate the positive impact that added power reliability and fuel cost savings can have on a business’ bottom line. In many situations, the combination of LPG and microturbine technology offer so much upside—including cleaner fuel and lower emissions—it’s as close to a “no-brainer” operational decision as a business can get. What’s more, once a customer makes the decision to go with Capstone, we can fast-track and deploy the system nearly anywhere in the world within three months of order.

The world’s energy landscape isn’t likely to become more predictable any time soon. The smart power security decisions businesses make today will set them up to ride the ups and downs with confidence. An LPG-fueled microturbine system might just be the answer to a power-secure future.