In recent years, rising natural gas prices, the second-hand engine market, and changes in the YEKDEM regulation have prompted investors to explore different maneuvers.
For instance, using an engine designed for natural gas with biogas or adapting a biogas engine to natural gas. Recently, due to the advantageous pricing of LPG compared to natural gas, using LPG instead of natural gas has become a consideration for operators of cogeneration plants that have unfortunately been out of operation for a long time.
There are two ways to operate a natural gas engine with LPG: either invest some money to run the engine at full power or run the engine as is at a lower load. Before diving into the details, it is beneficial to recall some common concepts related to this topic:
Engine Knock
Engine knock refers to the noise (or pinging) that occurs when the fuel-air mixture in the combustion chamber ignites spontaneously rather than being ignited by a spark plug. The resulting high-pressure wave propagates in the combustion chamber and reflects off metal surfaces. If knock is not prevented, it can lead to severe damage to components within the cylinder volume, making it a situation to be avoided. The most effective ways to prevent knock are to use knock-resistant fuel (gas) and to adjust the ignition timing with an electronic knock control system.
Compression Ratio
The second concept is the “compression ratio,” which represents the ratio of the maximum and minimum volumes within the cylinder during the piston’s movement. Depending on the knock tendency of the gas used in gas engines, the engine’s compression ratio can be designed to be higher or lower. As a general rule, a higher compression ratio is applied to achieve greater efficiency. However, the higher the compression ratio, the higher the tendency for the engine to knock.
Excess Air Coefficient
The third point is the “excess air coefficient,” which indicates the multiple of the air ratio in the fuel-air mixture. Different gases with varying thermal values require different air/gas mixture ratios. Therefore, the air-gas mixer unit varies accordingly for different gases.
Methane Number
The fourth concept is the “methane number,” which is a measure of a gaseous fuel’s resistance to knocking. Methane (CH₄) with a methane number of 100 is used as a reference fuel, while hydrogen (H₂) has a methane number of 0. A fuel gas with a methane number of 75 has the same knocking characteristics as a mixture of 75% methane and 25% hydrogen. The methane content in landfill gas and biogas is lower than that of natural gas (making it more resistant to knocking); therefore, they can operate with higher compression ratios without knocking.
Regarding LPG
LPG, which has a high flammability (more similar to hydrogen than methane), is considered a gas with a low methane number. Therefore, to avoid the risk of knocking, it can only be operated at lower compression ratios (for example, between 7.5:1 and 10:1).
If you wish to operate a gas engine (such as a cogeneration plant) originally designed for natural gas with LPG, you have two alternatives:
- Modifying the Engine for Maximum Power: This will require lowering the compression ratio, which means changing the pistons, and also replacing the gas mixer to ensure the air-gas mixture is suitable for LPG. Other necessary adjustments can be made by an authorized technician through the control system without changing components. Making these modifications will require a certain budget and time.
- Running at Low Load Without Modifications: In this option, the engine can be operated with LPG with just a few adjustments without incurring any extra costs. However, for safe operation, it is recommended to load the engine at a maximum of around 50%. Of course, the procedures that need to be performed on the engine should also be addressed according to the brand and model of the engine.
Conclusion
As long as natural gas prices do not decrease, the operation of natural gas engines with LPG is likely to remain a topic of discussion. If you would like to learn more about this issue, you can always contact us.
