Power Connect Newsletter

As more and more mobile and stationary generator applications shift away from liquid fuels like diesel and heavy fuel oil, there is an increasing demand for other portable or storage-friendly gaseous fuels where pipeline natural gas is not a logistical 
option. Additionally, for certain generator models, there are dual-fuel capabilities where natural gas and propane are both connected. The growth of gaseous, carbon-based fuels for generators has brought with it several challenges and inconsistencies in descriptions, definitions, and specifications. We’ll explore the topic of LPG in this white paper to provide some clarity around some terms that are commonly used in the industry. So first let’s start with some basic industry-accepted and used definitions:
 

• LPL: This is used to described propane in its liquid form, which is how it is stored in a tank and ready for usage as a fuel. This can be done via temperature or pressure; however, pressure is the normal process to liquify it, requiring about 175psig of pressure at 100°F to keep it in liquid state.

• LPV:  This is propane in its vaporous form, once it has been released from the storage tank. Due to its low boiling point, liquid propane will vaporize as soon as it is vented from its storage tank. Oftentimes generators will use additional vaporizers heated by engine jacket water to ensure that propane (and any other heavy hydrocarbons with it) will be in vapor form prior to entering the carburetor. This is common in oil and gas applications with high Btu wellhead gases.

• LPG: This acronym is used to describe liquid petroleum gas, which has no specific or controlled global definition regarding constituents and tolerances. The definition is representative of the source of these gases as a common byproduct of natural gas processing and petroleum refining. LPG gases are most heavily comprised of propane and butane, both iso-butane and normal-butane. These two gases can vary in LPG from nearly 0% to 100% composition and still be referred to as LPG. 

  • Some examples of LPG would be:
    • 50% propane, 25% iso-butane, 25% n-butane
    • 90% propane, 5% butane, 5% other
    • 80% butanes and 20% propane.
    • The exact composition (HD-5) of LPL or LPV also fit within this broad definition. More on this later.

• LP: This has been used to describe both LPL and LPV, but usually it is meant to represent LPV. If a document or specification lists only LP, it is always helpful to ask for clarification.

From a gas engine perspective, it is critical that it is provided with a consistent quality of supply, and it is also well understood that different hydrocarbons burn at different rates within the combustion chamber. That is why almost all gas engine manufacturers and therefore generator providers work with the well-defined and widely accepted HD-5 propane definition per ASTM D1835 and GPA 2140. In its simplest form the HD-5 
standard allows for:
 

1. Minimum of 90% propane content, 
2. Maximum of 5% propylene, (this is limited to minimize sticking in fuel system components)
3. Maximum of 2.5% butane and heavier, and
4. The remainder is fulfilled by other gases like inerts, methane, etc.

​All Generac SG and MG stationary gas generators that are approved for use on propane would be allowed only under the HD-5 specification, which is also documented in Generac specification A0000641396, latest edition. The MGG series of prime stationary and mobile gas generators are also built with a secondary fuel system that would require HD-5 propane.

Over the last two decades there has been a growing demand on a global scale to fuel gaseous generators with LPG fuels that do not meet the HD-5 definition. Market pull on LPG fuels has been seen from the Middle East to Africa, and from the Caribbean into South America. Each situation is unique as to why HD-5 propane is not available and that a non-conforming LPG fuel is preferred. In many countries, this type of LPG is  already available on the commercial market for other uses like vehicle fuel and cooking gas, and it may even have other names such as bottled gas, BBQ gas, calor gas, autogas and camping gas. In each case the content of butanes or propylenes does not allow for the fuel to comply with the engine manufacturer’s guidelines.

 

The specific problem with these fuels is that the content of higher chain hydrocarbons like butane and pentane drive the energy content (LHV or HHV in units of Btu/scft or MJ/Nm3) higher than what the engine was calibrated to run. High-energy content means that the fuel will be more reactive during combustion (lower Methane Number), causing a propane calibration or configuration to burn too quickly, leading to autoignition or detonation. This will destroy the engine over time. If we know that HD-5 propane is typically around 2350 Btu/scft LHV, some LPG fuels can reach 3100 Btu/scft, or approximately 30% more energy in the same volume of gas. The gas engine would need to be designed and developed specifically for this fuel, and the situation can be quite destructive on a highly turbocharged gaseous engine. Piston compression ratios and ignition timing are the two biggest levers that can be manipulated, and the sizing of the fuel system needs to be changed as lower overall fuel flow rates will be required.

So when applying a gaseous engine or generator with an LPG fuel, it is critically important to clarify the type of gaseous mixture that will be present at the engine flange. It is preferred based on the above to ensure that the fuel complies with the strict and common HD-5 definition. Other fuel blends may be considered and should be submitted through your technical support channel at Generac for additional consideration. The best situation is to obtain a gas chromatographic report from the gas supply utility or provider so that product application can be confirmed and any necessary derates can be applied.