Up-scaling Bio CNG Production by Down-scaling Gas Compression Rate

Introduction

Bio Gas available at low pressure limits its usage and plants which produce only Bio gas are not economical. It is required to purify the Biogas by removing corrosive gases like CO₂ and H₂S followed by its Compression. At high pressure its gas volume decreases and density increases, making it easier to store in compact containers, transport and use as a fuel. Biogas that has undergone Purification process and subsequently Compressed to 250 Bar is referred to as BIO-CNG.

Biogas is produced through the anaerobic digestion of organic materials such as agricultural residues, organic waste from households, and sewage sludge. The main components of biogas are methane (CH₄) and carbon dioxide (CO₂), with trace amounts of other gases.

The production of the biogas depends upon the anaerobic digestion process efficiency and feed stock availability through the plant’s operating time span.

Generally the Biogas generation process starts after around 20 to 22 days once the feedstock is fed to the digester. After 22 days at the initial stage the gas generation starts with a flow rate of 10 to 25%, it takes around further 2 months to ramp the Biogas generation from 25% to its full 100% capacity. During normal operation also one can see the drastic variations in bio gas generation.

Biogas generated at low pressure needs to be pressurised to facilitate the purification process as well as its further storage and distribution process.

In a Biogas Generation and BIO CNG Production plant various types of Compressors are utilised. Centrifugal, Screw and Reciprocating Compressors play a major role in such plants. Based on the specific characteristics these compressors are utilised for specific applications in Gas Purification and Gas compression train. Dynamic Compressors are well suitable for Low Pressure and high flow application; they cannot generate high discharge pressures. Positive displacement compressors i.e., reciprocating type compressors are used to compress gas up to 250 Bar. These compressors use a piston-cylinder arrangement to compress the gas.

Refer below comparison to understand the compressors briefly.

Criteria	Positive Displacement Compressors	Dynamic Compressors
Types	Reciprocating (Piston OR Diaphragm), Rotary - Screw	Centrifugal
Operating Principle	Reciprocating or rotary mechanisms	Rotating impellers or blades
Flow Rate	Low to Moderate flow rates	High flow rates
Pressure	Reciprocating can generate High discharge Pressures. Screw can generate low to moderate pressures	Can generate low to moderate pressures
BioGas compression	Suitable for Semi purified Biogas compression	Suitable for Semi purified Biogas compression
BioCNG application	Only Reciprocating Type compressors are suitable to generate 250 bar pressure, i.e. Bio-CNG application	Not suitable for Bio-CNG application

 

However, the positive displacement compressors struggle to operate at lower gas flow rates than its rated flow rates and require a 100% flow rate at its cylinder inlet. Hence most high pressure gas systems are not suitable to compress the gas at the initial stage or in an upset scenario. The delayed production of gas leads to the no compression situation hence leads to the monetary loss.

Generally in the Bio CNG industry gas is compressed if the Gas generation flow rate is above 50%. If gas is generating with flow rates i.e. less than 40/45% the current practice is to vent the gas, this is clear monetization loss.

The below table depicts the monetization period of the CBG plant.

Period	Production Flow rate	Compression Process	Current Practice
0 to 2 Months and Intermittent upsets if any	Around 0% to 25%	Not economical to compress	No compression. Production loss No Monetization
	Around 25% to 50%	Economical to compress. But unable to compress.	No compression. Production loss No Monetization
After 2 Months	50% to 100%	Compression resumes	Monetization starts.

 

The Plant uninterrupted Operation, Bio Gas Generation and Bio CNG Production Rate depends upon the Synchronization of various Systems involved in Plant. Every System of the plant shall be capable of adjusting themselves to variations in operating parameters. Failure in synchronization will lead to Production loss. Hence designing a gas compression system suitable to operate with variations in flow rates is critical from a Plant Economy point of view.

The limitations of Gas compression systems can be overcome by capacity control methods. It is essential for reciprocating compressors for several reasons, primarily related to optimizing their performance, meeting variable demand, and ensuring efficient operation.

The three major capacity control methods are:

• Variable Speed Drive (VSD) Control: This is a more advanced form of modulation control where the compressor motor speed is continuously adjusted using a variable speed drive. VSDs are highly efficient in matching the compressor output to the changing demand, resulting in energy savings.
• Gas Recycling: From High Pressure Discharge Side to Low Pressure Suction Side.
• Valve Unloading: Compressors can be equipped with multiple steps or stages, and the capacity is adjusted by activating or deactivating these stages. Each step corresponds to a different capacity level.

These control methods come with limitations. The below table depicts the limitations and not economical for CBG plant.

Capacity Control Method	Limitation
Compressor Speed Reduction with VFD Motors	Cannot go below the minimum operating speed of Compressor. Cannot achieve capacity reduction below 50%.
Gas Recycling from Compressor Discharge Side to Suction Side	The JT effect limits the use as the pressure difference is around 210 to 240 bar. Difficult to handle very low temperature gas which is at around -50°C. Leads to design a system considering cryogenic service and is not economical. Driver Power loss.
Valve Unloading - Converting Double Acting Cylinder to Single Acting Cylinder	Susceptibility to Torsional - Vibrational loads

 

However, by combining the Compressor Speed Reduction and Gas Recycling method the capacity control can be achieved, which is more economical and reliable.

By running the compressor to its minimum allowable operating speed, the reduction in flow rates is achieved from 100% to 65%. Further capacity reduction can be achieved by operating the Compressor with Gas Recycle mode. Further reduction in flow rates is achieved from 65% to 25%.

With careful and innovative design of the gas compression system and its components we can achieve the capacity reduction to 25%. With such a compression system around 25% gas which is generally flared/vented can be compressed and from initial stages only Plant can start the gas compression along with its monetization. During normal operation and in case of process upsets the compression process will keep on operating due to its capability of de-scaling the flow rates and adjusting itself to variations in gas generations.

Hope this would help in improving the efficiency and monetization of Gas generation, purification and compression Plant to a next level.

Read More