RENEWABLE NATURAL GAS-A STEP CHANGE TOWARDS CLEAN ENERGY
The world has shifted focus to reducing greenhouse emissions and introducing carbon-neutral and carbon-negative technologies. To accomplish this, scientists are developing new innovative tech such as electric immersion heaters. With a smaller environmental footprint and no loss in efficiency, electric heaters are driving these goals forward in all major industries. In large part, this is due to their compatibility and effectiveness with renewable energy options.
The agriculture industry is an especially important focus for cleaner energy. According to the United States Environmental Protection Agency (EPA), agriculture accounts for 11% of greenhouse emissions. Key agricultural emissions include livestock, manure management, landfill, wastewater treatment, and the burning of agricultural waste. These play a major role in the emissions of greenhouse gases. Greenhouses gases primarily consist of methane with:
- carbon dioxide
- carbon monoxide
Agriculture is not the only industry responsible for methane emissions. For instance, it is also extracted as part of the oil and gas exploration process and provides a valuable source of energy for domestic and commercial consumers. Energy use affects the emissions of most commercial and residential activities and processes. Taking the step towards a greener planet means moving towards clean energy through processes such as the purification of biogas.
Biogas, a byproduct of the agricultural emission processes, can be used to generate Renewable Natural Gas (RNG). New technologies allow this processing to remain energy efficient while still providing a smaller ecological footprint. As such, they are practical and easy to deploy within existing farms, wastewater treatment facilities, and landfill locations. This has led to the development and adoption of the Pressure Swing Absorption (PSA) process.
What is PSA?
Pressure swing Absorption (PSA) leverages the “affinity” that some gas molecules have to absorb onto certain solid absorbent material. The process occurs at high pressure. As the pressure reduces, the separation of the gas molecule from the solid surface occurs. Although this occurs at near ambient temperatures, temperature control is still necessary to provide high pressures for the absorption process.
The solid absorbent surface traps the target gas molecules on the porous surface of the absorbent, while other gas molecules travel through the absorbent bed unhindered. These gases are then available for extraction on the outlet or exhaust.
Absorbent materials, such as zeolite, have an affinity towards nitrogen at high pressure. Under low pressure, nitrogen separates from the zeolite resulting in regeneration of the absorbent. PSA technology physically binds gas molecules to the absorbent. The face of this binding depends upon various factors like partial pressure of the gas, operating temperatures, and the polarity of the gas molecule and the absorbent material. Electric immersion heaters play a crucial role in controlling these factors.
Temperature Swing Absorption (TSA) for Regeneration
After a certain number of cycles, the absorbent material loses its affinity to the gas molecules. This is because the pores are either filled with affinity gases. Another cause is the absorption of moisture when operating at near ambient temperatures. Moisture is a major factor in the degeneration of the absorbent bed.
Temperature Swing Absorption (TSA) helps regenerate the absorbent. The process increases the temperature from near ambient to 200 ˚C, evaporating the moisture. This coupled with low pressure PSA process will release the moisture and absorbed gases thus regenerating the absorbent bed.
Role of PSA in Biogas purification
PSA plays a pivotal role in the purification of biogas because of its high impurity content—which renders it useless for any energy generation activity. At high pressure, the biogas is introduced into the vessel filled with the bed of absorbent. The gas stream is purified when certain molecules (N2, O2, CO, H2S, and/or CO2) are absorbed by the absorbent bed. This results\ in 97%-98% pure methane extraction.
In the stream of biogas coming to the inlet of the vessel, highly volatile components of the gas like H2 (hydrogen) and CH4 (Methane) with low polarity are practically nonabsorbable in the absorbent bed packed with layers of zeolite (for nitrogen), activated carbon (for organic compounds), activated alumina (for H2S), silica gel (for water) and molecular sieves as opposed to the molecules of gases such as N2, O2, H2S, H2O and CO2 which are absorbed by the absorbents and pure methane is extracted at the outlet.
（Content from the wattco）